Thin-disk mode-locked oscillators: Progress and future prospects

Abstract

The average output power and peak power scalability are the key motivation of the development of ultrafast laser systems, enabling them for a wide range of applications. The thin-disk ultrafast lasers stand out due to their advantageous geometric design. The SESAM and Kerr-lens thin disk mode-locked oscillators represent the foremost configurations available in this field. They facilitate the direct generation of high average power and high peak power ultrafast pulses at high repetition rates. These compact tabletop systems offer a compelling alternative to bulky amplifier setups and have recently achieved > 0.5 kW average power and > 100 MW peak power at a megahertz repetition rate. With the continuous pursuit of shorter pulse durations and high peak power, the significance of these oscillators is rapidly expanding. This review focuses on the recent advancements and operational trade-offs in different parameters of thin-disk mode-locked oscillators. With this, we will delve into their capabilities for generating few-cycle pulses and achieving gigawatt-level peak powers, particularly investigating their suitability for post-pulse compression. Furthermore, we will explore the potential of these laser systems for generating broadband few-cycle mid-infrared laser sources and optical parametric chirped amplification. The review will also highlight the application of thin disk oscillators for efficient high-harmonic generation and discuss the possible implementation of dual comb TD-oscillators to enable dual comb spectroscopy in less explored spectral regions such as UV and XUV.

Graphical abstract

Keywords

ultrafast lasers / Kerr-lens / SESAM / HHGs / spectroscopy

1 Introduction

In recent years, significant progress in ultrafast laser pulse sources has accelerated scientific research, enabling the direct observation of some of the fastest natural phenomena. High peak powers ultrafast lasers find widespread application in many fields, including higher-harmonic generations (HHG) [1], X-ray lasers [2], nuclear photonics [3, 4], high energy-density plasma physics, astrophysics, particle acceleration [5, 6], and industrial processing [7]. The commercial availability of laser diodes (LDs) [8] has significantly propelled the emergence of various diode-pumped solid-state laser (DPSSL) technologies, including fiber amplifiers [9, 10], InnoSlab amplifiers [11−16], and thin-disk lasers (TDLs) [17−19]. These technologies have emerged as powerful sources for generating and amplifying ultrafast laser pulses with average power ranging from tens to multi-hundreds of watts. Each technology employs unique geometric configurations to address heat extraction challenges, enabling them to surpass the kilowatt (kW) average power threshold. However, in long gain mediums such as slab lasers and fiber, energy scaling requires very complex and expensive techniques like chirped pulse amplification (CPA) [15, 20, 21] coherent beam combining (CBC) [22, 23] or divided pulse amplification (DPA) [24] to avoid the acclamation of detrimental nonlinearities. In contrast, diode-pumped TDLs, leveraging their advantageous geometry, facilitate the generation of high-energy, high-peak-power ultrafast pulses in a more cost-effective and compact configuration.

The TD concept was invented in 1994 by a research team led by A. Giesen at the Institut für Strahlwerkzeuge (IFSW) of the University of Stuttgart, Germany. The TDL architecture employs a thin disk-shaped gain medium as its core component [18]. The disk is polished with an anti-reflection coating on its front face and a highly reflective dielectric coating on its rear surface for both pump and laser wavelengths. It is thermally coupled to a water-cooled heat sink, typically made of materials such as gold-tin, indium-tin, or diamond. This configuration enables highly efficient thermal management, predominantly allowing for one-directional heat transfer. Consequently, temperature gradients primarily occur along the laser beam axis and within the disk plane while maintaining relatively uniform temperatures in the radial direction for homogeneously pumped disks [25]. Owing to the large surface-to-volume ratio, it can support very high pump power densities >10 kW/cm². The smaller interaction length mitigates the nonlinear effects during amplification; however, this advantage comes with lower single-pass gain (10%−15%). This limitation can be improved by multi-pass configuration or by combining multiple disks in series. Further, a short beam path inside the disk-crystal also results in low pump power absorption. Thus, the pump beam is reflected multiple times to the disk for efficient absorption via the pumping system, usually consisting of parabolic mirrors and prism-retro-reflectors. The pump power absorption >95% can be achieved with such a configuration. The TDL geometry also offers a compelling approach for power scaling with minimal thermal lensing and aberrations, where power and energy scaling are achieved by simply increasing the laser and pump beam diameters on the disk surface while increasing the pump power [25−27]. TD technology is an outstanding performer in scaling output power to multi-kW level at high beam quality in both continuous and pulsed regimes. Fig.1 illustrates the evolution of the TDL concept from its inception to the current state-of-the-art kW-level ultrafast amplifiers. Theoretically, in continuous wave (CW) operation, up to 40 kW power per disk can be extracted, with further limits imposed by amplified spontaneous emission (ASE) [17, 19]. In 2021, TRUMPF reported a 24 kW (TruDisk 24001) CW thin-disk laser achieved through polarization coupling of two individual thin-disk, each with a power of 12 kW (TruDisk 12001) [28]. The TD geometry also provides substantial advantages for ultrafast-pulsed operation via regenerative amplifier [29−34], multipass amplifier [35−40], and mod-locked oscillators [41−48]. While amplifier chains have demonstrated impressive performance, achieving >2 kW average power in multi-pass, multi-stage configurations [40], and ~1 kW in regenerative amplification [34], mode-locked oscillators have emerged as particularly promising, exceeding the 500 W threshold at sub-picosecond pulse durations in a single, compact tabletop setup [42].

In the following, we will focus on TD technology as a promising source for the direct generation of high-power ultrafast pulses via mode-locking. This review aims to provide a comprehensive overview of recent advancements in mode-locked TD oscillators, critically examining the key parameters and trade-offs for achieving optimal performance. We will delve into the significance of generating few-cycle pulses and their pivotal role in enhancing HHG efficiency. The review will provide potential applications for broadband mid-infrared (MIR) laser sources and optical parametric chirped pulse amplification (OPCPA). Finally, a comprehensive analysis of TD oscillators as promising sources of dual-comb spectroscopy (DCS) in ultraviolet, deep ultraviolet, and extreme ultraviolet regions is also provided. We believe this study will serve as a valuable resource for researchers seeking to understand and further advance the field of thin-disk mode-locked oscillators.

2 Mode-locking

Mode locking is the mechanism of generating ultrafast laser pulses, particularly in the femtosecond regime. In this technique, the randomly oscillating cavity modes are locked in a defined phase relation by introducing a loss modulator, contracting energy into extremely short pulses much shorter than cavity roundtrip time. Two mode-locking mechanisms exist: (i) active mode-locking, where electronically driven loss modulations are externally introduced via electro-optic or acoustic-optic effects. These loss modulations show sinusoidal behavior with a period equal to cavity roundtrip time. A net gain occurs around a minimum of loss modulation in a steady state situation, which results in pulses shorter than cavity round trip time, (ii) passive mode-locking, here a saturable absorber (SA) produces loss modulation via intracavity laser radiations (i.e., low losses for intense pulses). While passing through SA, a pulse with Gaussian distribution sees fewer losses at the peak where intensity is higher than its wings. Thus, a pulse with a shorter duration has high intensity and saturates the SA more strongly, while low-intensity radiations get absorbed. In active mode-locked lasers, the full exploitation of available bandwidth of gain medium is limited by the frequency/amplitude modulation of optical switches. In contrast, in passive mode-locked lasers, the fast response of SA generates the pulses down to bandwidth limit of gain medium [54]. In the earlier stage, ultrafast solid-state lasers relied on dye-based laser technology or active mode-locking. The realization of a stable, passively mode-locked diode-pumped solid-state laser (DPSSL) with a regular pulse train remained difficult till the invention of the semiconductor saturable absorber mirror (SESAM) in 1992 [55]. Further, the emergence of diode-pumped solid-state laser technology improves the reliability and compactness of the passive mode-locked regime.

Amongst all LD-pumped SSL technologies, the TD lasers with short laser beam interaction offering very small nonlinearities are well-suited for the direct generation of ultrafast radiations. In the last two decades, two passive mode-locking techniques, SESAM and Kerr-Lens mode-locking (KLM), have shown unsurpassed performances in terms of pulse energy and average power with TD. Where the first technique offers a more robust cavity design structure and supports efficient laser operation at very high power, the latter one supports the generation of ultrafast pulses beyond the bandwidth limit. In the following section, we will discuss both mechanisms in detail and highlight their performance matrices with TD.

3 TD-SESAM mode-locked oscillators

3.1 SESAM

SESAMs have become key mode-locking tools for different types of lasers, such as solid-state lasers, semiconductor lasers, and fiber lasers. It is a semiconductor saturable absorber integrated on a mirror structure [56], and its operation relies on the capacity to modify optical absorption characteristics in response to incident light, thus effectively serving as a rapid optical switch. Structurally, it constitutes a semiconductor single quantum well (QW) or multiple QWs saturable absorber layers integrated into a Bragg mirror (bottom reflector) with alternative layers of aAs/AlAs (28−32 layers). Semiconductor materials such as GaAs, InGaAs, and GaInNAs are employed as saturable absorber layers for operation at 0.8, 1, and 1.3 μm wavelength regimes, respectively [57]. The top of SESAM usually has three interface designs: air interface, i.e., non-top-coated (NTC), semiconductor top-coated (SCTC), or dielectric top-coated (DTC) [58, 59]. The latter two provide a high damage threshold for high-power operations. The key optical parameters that characterize the SESAM are wavelength range where it absorbs, saturation fluence F_{\mathrm{s}\mathrm{a}\mathrm{t}}, modulation depth \mathrm{\Delta }R:(R_{\mathrm{n}\mathrm{s}}-R_{\mathrm{l}\mathrm{i}\mathrm{n}}), non-saturable losses \mathrm{\Delta } R_{\mathrm{n}\mathrm{s}}:( 1-R_{\mathrm{n}\mathrm{s}}), recovery time, and rollover fluence F_2 [59]. All these parameters are custom-designed over a wide range of pulse operations and have an influence on the realization of mode lock operation.

Saturation fluence F_{\mathrm{s}\mathrm{a}\mathrm{t}} represents the pulse energy fluence required for SESAM absorption saturation [57]. It is crucial to initiate a stabilized mode-lock operation. The intracavity pulse fluence in mode-locked laser is typically 3−10 times of F_{\mathrm{s}\mathrm{a}\mathrm{t}} [60]. For pulse fluence \ll F_{\mathrm{s}\mathrm{a}\mathrm{t}}, the SESAM operates at a high absorption state; as the pulse fluence exceeds the F_{\mathrm{s}\mathrm{a}\mathrm{t}}, the SESAM shifts to a low-absorption state, generating ultrashort pulses. Modulation depth \mathrm{\Delta }R represents the maximum reflectivity corresponding to the complete absorption saturation (i.e., the reflectivity difference between fully saturated and unsaturated SESAM [60]. Employing the SESAM with a larger modulation depth drives to achieve shorter pulses. However, this approach imposes an upper limit due to the onset of Q-switch instabilities [61]. Conversely, a lower modulation depth suppresses the Q-switching instabilities [57] but weakens the self-starting tendency of the mode-locking process. Recovery time τ represents the required to return in a higher absorption state. A shorter recovery time of several picoseconds or less is critical for generating pulses in the sub-100-fs range [62]. However, achieving a shorter recovery time remains challenging without compromising the non-saturable losses and crystal quality. This is because the semiconductor absorbers layers for fast recovery time are grown at lower temperatures, which introduces the neutral As antisites mid-gap defects and increases the losses, ultimately leading to increased optical losses [63, 64]. Although the soliton concept relaxes this limitation, fast recovery dynamics still support shorter pulses. With recovery time τ ≥ T_R (cavity roundtrip time), the mode-locking tends to operate in pure Q-switch mode [56]. Non-saturable losses \mathrm{\Delta } R_{\mathrm{n}\mathrm{s}}: fraction of unsaturable reflectivity. These losses arise due to scattering, internal defects, auger recombination [63], etc., offsetting the reflectivity from the ideal value. These losses should be as small as possible (ideally <0.1%). Inverse saturation absorption (ISA)/rollover fluence F_2: At a certain point, reflectivity is saturated, and decreases (1/e) with further increases in fluence characterized by rollover fluence F_2, and the effect is termed ISA [65]. This effect intends to destabilize the pulsed regime via the onset of multiple pulsing. ISA is directly related to two photon absorption (TPA) in SESAM, which occurs mainly in spacers and end Bragg layers [66]. The TPA induces the rollover effect and prevents an increase in the pulse energy.

For high power operation, a dielectric top-coated SESAM structure is more suited to increase the damage threshold and F_{\mathrm{s}\mathrm{a}\mathrm{t}}. This basically reduces the electric field in QWs and thus increases F_{\mathrm{s}\mathrm{a}\mathrm{t}}. It is also important to remember that when optimizing the SESAM, the product of F_{\mathrm{s}\mathrm{a}\mathrm{t}}. \mathrm{\Delta }R is an intrinsic constant that represents the total energy required to saturate the absorber completely, which is proportional to the number of states of the absorber. These parameters cannot be simultaneously decreased or increased. However, for high-power ultrafast operation, SESAMs with higher F_{\mathrm{s}\mathrm{a}\mathrm{t}} and large \mathrm{\Delta }R are required. Alternately, quantum dots (QDs)-SESAM are favored over QWs-SESAMs because, in QD structure, the dot density is proportional to the absorber’s density of states, which can be manipulated to tune both F_{\mathrm{s}\mathrm{a}\mathrm{t}} and \mathrm{\Delta }R together for better performance [67]. QDs-SESAM offers additional freedom to tune modulation depth for better performance while keeping the F_{\mathrm{s}\mathrm{a}\mathrm{t}} constant. A SESAM structure designed for high power operation > 500 W is shown in Fig.2(a), whereas the impact of incident fluence on SESAM reflectivity can be seen in Fig.2(b).

3.2 TD-SESAM performance analysis

The TDL concept, with its exceptional heat dissipation capabilities when integrated with SESAM, results in a power-scalable ultrafast technology. In TD-mode-locking, SESAM is usually applied as an end mirror, but comparable results can also be achieved by relocating it to any other position inside the cavity. The first TD-SESAM mode-locked oscillator was demonstrated in 2000 at ETH Zurich [51]. Here, 730 fs pulses at 16.2 W average output power were obtained with Yb:YAG, using output coupling of 5.5%. A SESAM with F_{\mathrm{s}\mathrm{a}\mathrm{t}}~100 μJ/cm² and modulation depth of ~0.5% allowed using a relatively larger spot size of ~600 μm and a stabilized laser operation. In earlier research, the TD-SESAM mode oscillators reached > 60 W average power, while the pulse energy remained limited to < 2 μJ at the sub-picosecond level [44]. This limitation persists due to the emergence of strong non-linear effects stemming from the nonlinear refractive index of air and other cavity components at high pulse energy. Controlling the nonlinearities makes it evident that these oscillators can approach a 100 MW peak power level when operated in a vacuum or inert gas atmosphere [41−43, 68−71]. Further, in pursuit of high average power and high energy, a three-pair DTC SESAM with 30-pair AIAs/GaAs distributed Bragg reflector, F_{\mathrm{s}\mathrm{a}\mathrm{t}} >100 μJ/cm² and ΔR with ≥1% was designed. Clara et al. [71] demonstrated a high pulse energy of 80 μJ with an average output power of 242 W at 3.03 MHz in MPC, while Francesco et al. [43] scaled the average output power to 350 W with a pulse duration of 940 fs at 8.8 MHz of PRR in 2019. In power scaling, a breakthrough was achieved in 2024 with Yb:YAG TD-SESAM achieving a record 550 W average power [42]. The oscillator provides 852 fs pulses at the repetition rate of 5.5 MHz, thus reaching 100 μJ class with an optical-to-optical conversion efficiency of 35%. Multiple factors were considered to make this possible, including an improved multi-pass scheme, low air pressure, and a specially designed SESAM. The multiple pass schemes consist of 24 passes per round trip through TD. It differs from the previous set-up as laser cavity mode has only one tight focus, reducing the amount of SPM. This design produces a Gouy phase of 5.7π (corresponds to 133 mrad roundtrip B-integral), much lower than the previously reported record power, which uses a 4f cavity design and has a much higher Gouy phase of 9.6π. Similarly, a specially designed SCTC thinner SESAM with only three QW layers was preferred over DTC as silicate bonding of SESAM to superstrate requires SCTC rather than DTC. Further, the whole setup was operated in a vacuum at 33 mbar. A complete growth mechanism and reason for the selection of SCTC-SESAM are given in Ref. [42]. This achievement paves the route toward kW-level ultrafast mode-locked oscillators. All the results mentioned above are obtained with Yb:YAG gain medium. Fig.3 shows the experimental configuration and mode radius evaluation along the cavity length.

Although Yb:YAG, for its favorable thermo-optical characteristics and large-scale easy growth mechanism, is the material of choice for high-power TD-SESAM mode-locked oscillators and has approached multi-100 W power levels, however, the pulse duration remains limited to > 500 fs. Efforts to decrease the pulse duration to < 500 fs with Yb:YAG remained challenging due to its limited gain bandwidth and spatial hole burning effects (SHB), and only a slight decrease in pulse duration has been observed from ~1 ps fs down to 583 fs. Thus, a milestone for TD-SESAM to extend the higher power in sub-100 fs regime is linked with the broadband gain medium. In this regard, apart from Yb:YAG, various other Yb-doped gain materials with broad emission bandwidth were investigated concurrently, such as Yb:SSO [72], Yb:CALGO [73−76], Yb:YCOB [77], Yb:KYW [78, 79], Yb:LuScO₃ [80−82], Yb:ScYLO [83], Yb:LuO [84−88], and Yb:CaF₂ [89−91]. The pulses as short as 142 fs have been achieved with the Yb:LuO TD oscillator [85], while sub-100 fs pulses were only achieved with ultrabroad bandwidth Yb:LuScO and Yb:CALGO at average power ≤ 5 W. This is due to their low damage threshold and the strong thermal effects, which make it challenging to scale output power while maintaining the shortest possible pulse duration. Further, in the 2.1 μm domain, the holmium-doped YAG Ho:YAG emitting wavelength has also been tested in this mode-locked mechanism, delivering an impressive 50 W of average output power with 1.13 ps pulse duration [92]. Further, with the development of next-generation SESAMs optimized for this spectral range, the authors anticipate scaling the pulse energy > 10 μJ, which could enable the generation of powerful broadband mid-infrared (MIR) laser sources, see Section 5.2.2. Tab.1 presents the summary of some state of the art TD-SESAM oscillators with respect to SESAM parameters.

In summary, although much shorter pulses have been achieved with broader gain bandwidth mediums, the average power remained limited to a 100 W regime. Thus, It is believed that Yb:YAG will remain as a dominating gain medium to keep the place to derive for kW TD oscillators for its favorable thermos-optical properties. Further, optimized SESAMs with larger saturation fluences, high damage threshold, and low non-saturable losses are perfectly suitable for power and energy scaling TD mode-locked oscillators [59, 65].

4 TD Kerr-lens mode-locked oscillators

4.1 Kerr lens mode-locking

The KLM [96] is a preferred technique for generating fs pulses beyond the gain bandwidth limit. In this approach, the intense pulses are subjected to self-focusing while passing through a non-linear optical medium (Kerr effect). The intensity-dependent non-linear refractive index causes a slight change in the spatial intensity distribution of laser mode inside the resonator. Thus, traversing through mediums with n₂ > 0, the center part of the Gaussian pulse experiences a higher refractive index than the wing due to high intensity and introduces the lens type effect. For a symmetrical beam with radius {\omega }_x={\omega }_y=\omega and a pulse of peak power P_0, the focal length of the KM lens in a cavity of length L is approximately f_{\mathrm{K}\mathrm{M}}^{-1}=\frac{4n_2{d}_{\mathrm{K}\mathrm{M}}{P}_0}{\pi {\omega }^4} [97], where d_{\mathrm{K}\mathrm{M}} is the thickness of KM. When the Kerr effect is combined with an intracavity aperture, it introduces an intensity-dependent loss element and induces self-amplitude modulation (SAM), which promotes pulsed operation over CW operation [98]. The aperture can be a physical hard aperture element added inside the cavity or a soft aperture form by a careful overlap of pump and laser modes. For realization KLM, the resonator is operated near one of the stability edges to increase the mode sensitivity of the resonator. The cautious optimization of amplitude modulation, in turn, introduces an ultrafast artificial SA that supports fs-pulse generation beyond the gain bandwidth limit [96, 99, 100]. Due to the ultrafast SA, the intracavity dispersion management and gain bandwidth of the active medium are primarily the only limitations to the pulse duration

4.2 Factors influencing TD-KLM operation

In typical bulk oscillators, the laser medium alone provides the Kerr-lens effect and power amplification [99]. However, in TD geometry, an extra KM is introduced inside the cavity to achieve the required nonlinear effect, since large laser mode size on TD and a small interaction length could not initiate sufficient Kerr lensing effects. This separation of gain and KM presents several benefits, including better optimization of round trip gain and the ability to choose suitable material and thickness for the KM. In TD, the realization of efficient KLM depends on several parameters. These parameters include the amount of group delay dispersion GDD, KM thickness, KM position relative to the focus point, mode size on the KM, size, location of the hard aperture, and OC rate.

Group delay dispersion GDD: It is very crucial to optimize the negative GDD. Typically, a flat GDD profile over a broader spectrum is necessary to reduce the pulse duration, as seen in Fig.4(a). The pulse duration for a given gain spectrum decreases almost linearly with GDD down to the point where the laser is no longer mode-locked. As the pulse duration decreases from 84 to 27 fs, GDD decreases from −6000 to −900 fs² [101]. Further decreasing GDD causes the pulses to become unstable. Conversely, increasing GDD from the optimal value will increase the pulse energy and pulse duration [102]. Thus, the peak power remains almost constant Fig.4(b). KM-thickness: A particular thickness of KM is required for SAM, which is critical for initiating and stabilizing KLM operation. The intracavity peak power scales inversely with Kerr thickness. This is because a thicker KM increases the induced SPM and smoothens the pulse-building process but lowers the mode-lock threshold and provides less peak power. Conversely, a thin KM has a high threshold, allowing for high peak power pulses; however, the mode-locking process with a thin KM becomes unpredictable, less reproducible, and often leads to damage to intracavity elements [103]. Mode size on KM and relative position: In TD-KLM, increasing the mode size on KM while keeping other parameters constant increases the peak power. The mode size scaling can be done by replacing focusing mirrors of large ROC. Further, shifting the KM away from the focus improves the peak power. Still, it increases the challenge of initiating mode-locking due to the increased prominence of nonlinear effects in ambient air, such as self-focusing and SPM. However, once mode-locking has started with focus inside the KM, moving the KM further from the focus point can increase the average output power proportionally to the increase in pumping power [101].

AS, TD-KLM is implemented by operating the resonator near the stability edge to optimize the effects of SAM, thereby enhancing sensitivity to thermal lensing within the resonator. The thermal lensing caused by KM may significantly affect the behavior of the oscillator by expanding and shifting the resonator stability zone [53]. The thermal lensing can move the ML working point towards the stability zone, thus reducing the Kerr sensitivity and modulation depth. This could result in a decrease in output spectrum bandwidth and longer pulse duration. Thus, a KM with higher thermal conductivity is preferred. Fused silica (FS), a commonly used material in KM, exhibits considerable thermal lensing when the intracavity power exceeds 0.5 kW [104]. Alternatives with high thermal conductivity, such as YAG plates, sapphire, and crystalline quartz, offer viable replacements for FS. Notably, a sapphire plate can endure up to 1.2 kW of power within the resonator, ensuring stable operation. [104]. Tab.2 presents the thermal conductivities and nonlinear refractive indices of various KMs.

4.3 TD-KLM performance analysis

Although the basic concept of combining TD and KLM was presented much earlier, the practical realization was not achieved until 2011. Here, we will limit ourselves to some state-of-the-art performance of TD-KLM and highlight the possibility of self-starting mechanisms. In the first-ever demonstration of TD-KLM, nearly gain-bandwidth limited pulses of 200 fs (exploiting > 90% emission bandwidth of Yb:YAG) were generated at an average output power of 17 W using 5.5% OC [109]. These results represented the shortest pulses obtained with the Yb:YAG TD oscillator at that time. Later, the same group explored the positive group delay dispersion (GDD) regime, a scheme previously applied in TD-SESAM to scale the pulse energy [110]. They achieved 190 fs ultrafast pulses with 17 W of output power [111], although the mode-locking could not be realized in a hard aperture (HA) and was achieved with a soft aperture module in conjunction with an SESAM. The operation with positive intracavity GDD reduces the peak power drastically inside the cavity due to pulse chirping; thus, a lower GDD value is required for stabilized operation than in a negative GDD regime [112]. For the given value of GDD, energy scaling in a positive regime is superior to negative GDD and allows higher pulse energies [111]. It also tends to prefer single pulse formation, but mode-locking start-up becomes challenging with positive GDD since pulse formation occurs in only limited window SPM and GDD. Later, in pursuit of high power, Brons et al. [53] implemented the straightforward geometric scaling of TD-KLM and obtained 330 fs ultrafast pulses at 270 W power. Here, the output power and extraction efficiency are maximized via a multi-pass gain trip and by optimizing the oscillator mode to pump spot overlap ratio. Further, the author successfully increases the pulse energy to 14 μJ (38 MW peak power) by simply enlarging the mode size inside the KM and optimizing the TD and HA distance from KM. These are the highest results obtained from Yb:YAG laser regarding average power and pulse energy in the air with this technology. More recently in 2023, a low pressure of 300 mbar was used to produce 202 W with 140 fs pulses at 15.4 MHz of PRR [113].

In TD-mode-locked oscillators, the maximum operating regime typically centers around ≤ 50 MHz repetition rates. Achieving significantly higher repetition rates (PRR >100 MHz) in TD-mode-locked oscillators necessitates a significant reduction in cavity length. However, the geometrical constraints imposed by the TD laser head design, particularly the physical dimensions of the gain medium and the associated pumping infrastructure, inherently limit the achievable cavity length. Furthermore, the short cavity comes at the expense of a smaller cavity mode size since the mode size scales with cavity length L as {\omega }_x=\sqrt{\lambda L/\pi }. Thus, operation at short cavity length tends to deteriorate the mode overlap and hence decrease the optical conversion efficiency. Zhang et al. [114] reported a 75 W TD-KLM oscillator based on Yb:YAG, delivering 215 fs pulses using a shorter cavity corresponding to 260 MHz PRR with an optical conversion efficiency of 21.4%. Here, to provide a larger mode size on TD in a short 577 mm cavity, a combination of a 150 mm concave mirror and a 75 mm lens was used, which improves the mode-matching. In 2023, Liu et al. [115] realized sub-100 fs pulses from ring cavity TD KLM with an average output power of 6.2 W. The same group, in 2024, further scaled the average power to 100 W level at a repetition rate of 65.3 MHz [116]. This stands out to be the highest average power ever achieved with TD-KLM in the ring cavity. Similar to TD-SESAM, various other Yb-doped broadband materials have also been employed with TD-KLM to achieve shorter pulses. In the KLM, by optimizing OC and dispersion, pulse duration up to 35 fs was obtained by fully harnessing the emission bandwidth of Yb:LuO [102]. Other materials, like Yb:CALGO deliver the shortest pulse of 30 fs with an average power <1 W [117].

4.4 Self-started TD-KLM

TD-KLM oscillators face a significant challenge in achieving stable operation, as mode-locking does not typically initiate spontaneously. An external perturbation is usually required to transition the laser from continuous-wave (CW) operation to the mode-locked regime. Later, following the progress of self-starting in bulk KLM, a self-started TD-KLM was also demonstrated. In 2023, Yang et al. [119] explored the idea of self-start TD-KLM by investigating the traditional KLM self-start condition based on the equation: kP>T_R\{\mathrm{l} \mathrm{n}(m_{i)}{\tau }_c}. Where P is intracavity power, T_R cavity roundtrip, {\tau }_c correlation time of axial modes, and k is the loss coefficient, which plays a role in determining the SAM and depends upon HA size, beam diameter, and spot size variation factor. They achieved self-started KLM operation while achieving the k = (1.26−1.68) × 10⁻⁷/W satisfying the self-starting condition. The maximum output power achieved in this experiment was 49 W at a repetition rate of 46.5 MHz, corresponding to a pulse energy of 1.05 μJ. This represents the highest pulse energy reported to date for a self-starting TD-KLM oscillator. In the same study, a novel approach to inducing mode-locking was introduced, utilizing optical perturbation. Instead of relying on mechanical perturbations, this method introduces feedback from the laser light itself through an extended cavity. To implement this, one of the high reflectors (HRs) in the laser cavity is replaced with a second output coupler (OC) with a low transmission (≤ 1%), which minimally affects the intracavity power. A second HR is then placed at a certain distance from the second OC and mounted on a rotatable stage. The laser light reflected from this second HR is directed back towards the main cavity, providing the necessary optical perturbation. By carefully adjusting the distance and rotation angle of the second HR, spontaneous mode-locking can be initiated. These advancements in self-starting techniques significantly enhance the robustness and ease of operation of TD-KLM oscillators, paving the way for further development and broader applications of these high-power ultrafast laser sources.

5 Energy scaling challenges and milestones

For energy scaling in TD-mode locked oscillators, steps such as controlling nonlinearities, increasing cavity roundtrip time (repetition rate), and increasing average power are critical. Here, we will discuss the associated challenges with these steps for energy scaling and the most recent developments towards reaching pulse energy of ≥100 μJ milestone.

5.1 First step: Controlling the nonlinearities

In TD-mode locked oscillators, gain medium, Brewster plate/KM and air atmosphere are three key factors contributing to total nonlinearities. Comprehensive details about the nonlinearities of TD oscillators are given in Ref. [120]. The nonlinearities that arise from TD are negligible due to its small thickness and large mode size in a single pass. However, it becomes significant for the large number of passes requiring high gain. The nonlinearities arise from KM and Brewster plates of TD-KLM, SPM, and TD-SESAM, respectively. Further, the contribution of air toward nonlinearities can be neglected for shorter cavities >10 MHz as the refractive index of air is much smaller than that of solid nonlinear mediums. However, the contribution of air non-linearities increases as the Rayleigh range length between two focusing mirrors increases in TD-KLM. For large cavities approaching a few MHZ repetition rates and high intracavity peak power, the SPM contribution from the air becomes significant and cannot be ignored further. Therefore, oscillators need to operate in a noble gaseous atmosphere or under reduced air pressure at high peak power. Noble gas atmosphere: Operating with the noble gas atmosphere is crucial to keep γ and –GDD to the minimum possible value, as the refractive index of noble gases is multi-times smaller than air. For example, the refractive index of He-gas (~4 × 10⁻²⁴ m²/W) is 8 times smaller than air (3 × 10⁻²⁴ m²³/W) [121]. Thus, pulse energy can be scaled by replacing the air with He-gas. The pulse energy of the Yb:YAG TD SESAM oscillator was scaled up to 5.1 μJ at a 12.3 MHz repetition rate by introducing He-gas inside the cavity [122]. However, it is important to note that the noble-gas content inside the box also affects the stable laser operation. Low air pressure: To scale pulse energy, a more viable alternative is using low air pressure inside the chamber to keep the nonlinearities under control. Since the nonlinear refractive index of air is pressure-dependent, thus air parasitic nonlinearities decrease with a decrease in air pressure. The mode-locking operation in low-pressure facilitates the high-energy ultrafast pulses by reducing the likelihood of air ionization, plasma formation, and nonlinearity effects. Low air pressure supports the control of nonlinearities, but in the process of decreasing air pressure, one has to be careful with turbulences that arise with low air pressure. This approach also results in carbon deposition on vacuum-operated optical elements at high intensities, particularly in the case of KLM. Therefore, it is necessary to strike a balance for the optimal air pressure value at which SPM and dispersion due to air become insignificant compared to other intracavity elements. For a comprehensive analysis of the impact of air pressure or other gases within the resonator, see Refs. [48, 120].

5.2 Second step: Increasing cavity length and power scaling

The pulse energy scaling is directly related to repetition rate and average output power (E_{\mathrm{p}\mathrm{u}\mathrm{l}\mathrm{s}\mathrm{e}}=P_{\mathrm{o}\mathrm{u}\mathrm{t}}/f_{\mathrm{r}\mathrm{e}\mathrm{p}}). The decrease in repetition rate involves the increase in resonator length. For this purpose, using an intracavity passive multi-pass cell (MPC) is the method of choice while keeping the oscillators compact. Here, the cavity length is extended by a large number of back-and-forth reflections between two mirrors of MPC within the cavity. Integrating MPC within the existing cavity can decrease the repetition rates from 100 MHz to a few MHz regime at the given average output power [123]. This configuration provides the unity transformation of Gaussian beam q-parameters and keeps the mode size invariant. In this case, the resonator stability remains intact, which otherwise, for any other cavity length extension, can lead to a mismatch between laser modes and cause the resonator to be unstable or inefficient. However, these extended cavity lengths pose a challenge of excessive nonlinearities from long air interaction lengths inside the cavity. A significant increase in pulse energy >10 μJ was realized by extending the cavity length via Herriot-type MPC and operating the oscillator in the He-atmosphere [124]. On the same line, another approach to scaling the pulse energy is an increase of output power. This involves the utilization of a large OC, which also helps to keep nonlinearities at a minimum by decreasing intracavity power. However, for efficient operation with a large OC, a higher gain per roundtrip is critical in TD mode-locked oscillators to keep the conversion efficiency high. It is usually obtained through multiple passes from the gain medium, where a beam passes through the gain medium multiple times in one round-trip using reflecting optics. Nevertheless, spatial limitations may impede the adaptation of multi-pass configuration. Further, maintaining a consistent mode size for consecutive reflections in multi-pass geometry is essential for optimizing power extraction efficiency. With an increasing number of passes, an optimized cavity design can resolve the challenges of growing differences in subsequent mode sizes. An imaging technique where beam traversing the laser beam in horizontal and vertical planes in a compact setup called Active multi-pass cell (AMC) [69] is usually adopted to multiply the number of passes through the disk with minimal effect on the cavity mode size. This technique has been successfully adopted to both KLM and SESAM, TD mode-locked oscillators [69, 70]. This simultaneously allows the extension of the cavity length and enables the use of large OC (> 50%) for high average output power extraction. It is important to note that AMC with a larger OC greatly reduces the ratio of intracavity pulse energy to external pulse energy and, eventually, nonlinearities. Thus, it also allows for energy scaling in an ambient atmosphere [70]. Using the AMC concept, a maximum pulse energy of 41.4 μJ was generated in ambient air with TD-SESAM. Similarly, the AMC concept has been successfully implemented with TD-KLM in 2019. The integration of AMC allowed the use of up to 50% OC and achieved the pulse energy of 13.2 μJ at an intracavity peak power of 80 MW [125] in the air. Despite larger OC, the large number of passes in ambient air can still induce strong SPM, which necessitates a significant amount of negative GDD, as in Ref. [93], a total −346500 fs² GDD is required to balance the SPM. Consequently, a large number of dispersive mirrors (DMs) are required to balance SPM, which causes severe thermal effects. An operative approach to cancel the excessive SPM is by utilizing cascaded quadratic nonlinearities (CQN) [94]. This approach enables the pulse energy of 19 μJ and the highest output power of 210 W of any TD-SESAM oscillator operated in the air. It paves the way for high-power laser operation in the air.

Following these developments, one can move towards further pulse energy scaling by combining the Herriott-type MPC, multi-pass gain round trip, and low atmospheric pressure. Thus, combining these effects, 80 μJ pulse energy at 3.03 MHz was achieved at a low atmospheric pressure of 1 mbar [71]. In the most recent 2024 [42], a record of 100-μJ pulse energy was demonstrated with a TD-SESAM mode-locked oscillator, breaking the long-standing record of 80 μJ achieved in 2019. This breakthrough is achieved by scaling the average power to 550 W at a pulse repetition rate of 5.5 MHz. The system was operated in low air pressure with a multipass gain scheme. Fig.5 indicates the progress of energy scaling of TD mode-locked under the implementation of noble gas, multipass (either via MPC or multi-pass through the gain medium and low air pressure conditions.

6 Towards few-cycle pulses and GW peak power

The minimal achievable pulse duration depends on the mode-locking technique type and the gain medium’s emission bandwidth. However, it does not solely set the upper limit for the shortest achievable pulses in anomalous dispersion. According to the complex Ginzburg−Landau equation representing the mode-locking performance [127], sufficiently low linear losses and high nonlinear modulation depth can overcome the gain bandwidth limitation to shorten the pulse duration. The linear losses can be reduced by employing a low OC, which results in low output power, and mode-locked laser operation at low power supports the generation of gain bandwidth-limited pulses. Similarly, the accessible gain bandwidth, determined by the gain per roundtrip, also contributes to the pulse duration reduction. The gain bandwidth tends to be narrower in laser geometry for a smaller gain per roundtrip. Therefore, pulse duration limits are always towards the longer end in TD geometry, compared to bulk for a given type of material and mode-locking mechanism [128]. In TD-SESAM, the primary factor for achieving shorter pulses is the gain bandwidth; however, optimized SESAM parameters significantly influence pulse duration. Larger modulation depths and faster recovery times of SESAM are advantageous for pushing the pulse duration to the limits of the emission bandwidth of the material. At a given modulation depth of SESAM, the minimum pulse duration scales with pulse energy [126]. Further, the pulse duration can be optimized by controlling the amount of non-linearity (SPM) via inserting the Brewster plate inside the cavity and positioning it close to OC along the beam axis [81]. The SPM is inversely proportional to the cross-sectional area of the laser beam in the Brewster plate. In TD geometry, KLM is the method of choice to produce shorter pulses than the SESAM, where the pulse duration is towards the higher end of the bandwidth limit.

In TD-KLM pulse duration optimization, reducing the OC along with aperture size is critical, as it maximizes the modulation depth of SAM and favors stable mode-locking [102, 118, 129]. However, this comes at the cost of output power and efficiency. Further, in the pulse shortening, the broader bandwidth pulses exceeding the emission bandwidth of the laser medium can be obtained by utilizing the non-linear effects inside the cavity to enhance the SAM coefficient. The strength of SAM prescripts the pulse formation and pulse duration. To enhance SAM, apart from KM, a separate set of non-linear plates is placed inside the cavity in the arm where the beam diameter is relatively small. These nonlinear plates act as distributed Kerr mediums (DKLM), which induce the distributed Kerr effect, enhance the overall SAM, and eventually broaden the pulse spectrum. This is an effective way to generate ultrashort pulses where the extent of spectral broadening can be controlled by varying the number of crystals, their thickness, and the type of material. In 2022, using six nonlinear plates of variable thickness as distributed KMs, the final mode-locked spectrum exceeds the emission bandwidth of Yb:YAG by ~4 fold, yielding 47 fs pulses at an average power of 3.5 W with η_o−o = 3.5% [118]. The low power is attributed to a large number of losses due to reflection from multiple non-linear plates, and the scheme was further improved by replacing six separate plates with a single plate of high non-linear coefficients [130]. Although DKLM shows the potential for pulse shortening, a clear trade-off exists between peak power and pulse duration for pulses approaching the 50 fs regime. For this, a promising alternative approach towards sub-100 fs at high peak power is realized where additional frequencies are generated inside the spectrum by excessive SPM in the KM inside the laser cavity. In Ref. [101], 37 fs pulses at 17.7 W and 84 fs pulses at 69 W are generated by operating in a strongly SPM-broadened regime while optimizing the introduced negative GDD. The oscillator operates at 17 MHz repetition, and a maximum peak power of 42 MW was obtained with 84 fs pulse duration. For a strongly broadened SPM regime, the author designed the cavity with a double intracavity focus and placed the setup under a vacuum at 100 bar. This mechanism is further optimized for high-power operation, and eventually, the highest peak power of > 100 MW was achieved at a pulse duration of 52 fs [48]. These pulses stand out as the shortest ever obtained directly in any TD mode-locked oscillators for ≥ 100 W. Fig.6(a) and (b) show the experimental set-up of oscillators operating with DKLM and strongly broad SPM regime. Further understanding of the interplay between nonlinearities (SPM) and dispersion control (group delay dispersion) inside resonators is critical to achieving shorter pulses.

6.1 Post-pulse compression

As of today, the motivation for ultrafast femtosecond laser systems is to reach the shorter pulses of few-cycle down to a single cycle simultaneously at high peak power for their potential applications in ultrafast science. However, the trade-off between pulse duration and average power is formidable to avoid as one tries to tune the TD oscillator towards shorter pulses. The inherited narrow bandwidth of TD-laser gain medium limits the few-cycle pulse generation from high-power oscillators. Thus, to overcome this intrinsic limitation, a potential route is the utilization of external nonlinear pulse broadening and compression. This mechanism shortens the pulses multifold to reach a few cycles without losing much power. Here, the spectrum of output pulses is nonlinearly broadened via SPM and is subsequently compressed in the time domain. TD-oscillators providing transform-limited soliton pulses with excellent temporal shape and high beam quality at high repetition rates are excellent sources of non-linear pulse compression. In this techniques, Kangomé hollow-core-photonic crystal fibers (HC-PCFs) [132, 133] and multi-pass cells (MPCs) [131, 134] are the two most commonly employed schemes for ultrafast pulse compression having energies in the multi-μJ range. These techniques provide excellent power transmission efficiency with high-power laser sources. Following these approaches, the TD-oscillators have compressed up to sub-50 fs, reaching >100 MW peak power with a maximum transmission efficiency of 88% [132, 133, 135]. With the multiple operating challenges associated with HC-PCFs, such catastrophic damage due to the self-focusing of the Kerr medium (spatial variation of the Kerr effect) for peak power exceeding the self-focusing threshold of material, the compression techniques based on MPC have shown more reliability and compactness. The MPC-based post-pulse compression, with favorable characteristics such as high compression factor, high transmission efficiency, and applicability to a wide range of pulse energies, has recently attracted much attention. In 2016 [136], a technique based on the dielectric MPC-bulk was first proposed in the Innoslab amplifier for post-pulse compression. In MPC, spectral broadening can be achieved using a solid medium or noble gas as a non-linear medium [137, 138].

Considering the advantages of progress in MPCs post-pulse compression techniques, a route to obtain few-cycle pulses with TD oscillators is achieved by cascading multiple MPC compression stages. In 2019 [139], 534 fs pulses were compressed down to 27 fs by two-stage pulse compression, MPC-bulk + multi-plate scheme reaching the peak power of 166 MW. In this work, transform-limited 5 fs pulses were also demonstrated by cascading an extra multiple plate setup. However, no further details about beam parameters were provided. Most recently [131], cascading multi-stage compression, the TDL oscillator was compressed to few-cycle pulses, delivering GW peak power. A 120 fs TD-KLM oscillator was compressed by factor 15 down to 8 fs in two-stage MPC at 100 W of average power and 14 MHz repetition rate. The oscillator reaches a record of 0.9 GW peak power without any pre-amplifications. The compressed pulses at high peak powers pave the way for efficient HHG with high photon flux. A road map to cascaded post-pulse compression can be seen in Fig.6(c). Tab.4 represents the performances of TD oscillators integrated with post pulse compression mechanism.

7 Characteristics of gain material for ultrafast TDLs

The spectrum of solid-state gain media is extremely widespread for ultrafast laser operation, encompassing crystals doped with rare earth elements (Nd³⁺, Yb³⁺, Tm³⁺, Ho³⁺) and transition metals (Ti³⁺, Cr²⁺) [143]. In TD geometry, it is always advantageous to use materials with good thermal conductivity, small quantum defects (1- {\lambda }_p/{\lambda }_l), high doping concentration, large gain bandwidths, pumping wavelengths supporting high-power LD, high fluorescence lifetime, and high resilience to thermally induced stress fractures. These factors are crucial for generating high-power ultrafast lasers. As different material characteristics are linked to laser geometry, for instance, if the doping concentration of active ions is too high, it can increase the pump absorption coefficient but decrease the thermal conductivity. On the other hand, if the doping level is too low, the required thickness of the disk may increase, leading to a rise in the maximum temperature. Therefore, a material that allows optimal doping levels while maintaining high thermal conductivity is highly desirable for TD ultrafast geometry. In TD, higher effective pump power density favors the adoption of quasi-three-level materials (e.g., Yb³⁺-doped) that would have otherwise necessitated relatively high pump power densities to surpass the transparency threshold to operate the laser efficiently [19, 50]. Thus, TDs are largely mode-locked with Yb-doped gain mediums. The rare earth Yb³⁺-doped materials are highly suitable for short pulse generation. Their quasi-three-level nature increases the overall laser efficiency in TD geometry, which allows the realization of smaller active volumes with efficient heat removal [144]. In contrast to other rare earth ions, relatively strong electron−phonon coupling broadens the host’s absorption and emission spectra [145], allowing pulses up to sub-100 fs depending on host materials and mode-locking technique. For sufficient absorption of pump light, the large doping concentration of Yb³⁺ ion can be realized, bringing the advantage of realizing a thinner disk, which improves the cooling effects and reduces the transparency threshold density [146]. Theoretically, no concentration quenching occurs in Yb-doped materials, but migration of excitation energy between Yb-ion becomes faster at high doping concentration. A part of the energy portion may transferred to the impurities (Er³⁺, Tm³⁺, Ho³⁺, etc.), resulting in radiative and non-radiative de-excitation and eventually decreases the Yb-fluorescence lifetime, thermal conductivity, and increases the threshold [147, 148]. A decrease in fluorescence lifetime leads to an elevated pump threshold, subsequently intensifying the thermal load within the crystal. Additionally, the reduction in thermal conductivity can be accounted for the mass imbalance between Yb³⁺ ions and those of the host crystal [148−150]. In the following section, we will discuss all the thermal and spectral properties of all Yb-doped gain mediums applied in TD mode-locked oscillators and highlight the possible future material.

7.1 Yb-doped gain mediums

The Yb:YAG material significantly improves energy and power in mode-locked TDLs. It offers better thermal conductivity at 11 W·m⁻¹·K⁻¹, decreases to 7 W·m⁻¹·K⁻¹ with higher doping concentration, and has a large gain bandwidth of ~ 8 nm FWHM. It has two strong absorption peaks at 940 nm and 969 nm (ZPL) with an absorption bandwidth of 12.5 nm FWHM and 2.5 nm FWHM, respectively. Pumping at ZPL reduces heat by 32% compared to pumping at 940 nm and permits significantly higher pumping intensities in the same environment [151, 152]. Notwithstanding these advantageous characteristics, the initial selection of Yb:YAG was not motivated by its capability to facilitate the generation of femtosecond pulses. Simple growth mechanism at the commercial level in excellent optical quality and large crystal size, it has become the preferred best-developed gain material. However, limited bandwidth of Yb:YAG allows for the generation of approximately 700 fs pulses in an efficient high-power operation [153]. Therefore, the emphasis was redirected towards investigating broadband Yb-doped materials that can efficiently support sub-100 fs at a power level of 100 W in a TD geometry. For shorter pulse duration, considerable interest has been shifted towards Yb-doped cubic sesquioxides (Yb³⁺:Re₂O₃), especially Sc₂O₃ (Scandia), Y₂O₃ (Yttria), and Lu₂O₃ (Lutetia) because of their potential to facilitate efficient ultrafast laser operation due to larger emission bandwidth. Their isotropic thermo-mechanical properties cause uniform thermal expansion, avoiding the stress inside the crystal [83]. Among the sesquioxides family, Yb:LuO (Yb:Lu₂O₃) has shown more promising results, with a broader bandwidth of ~13 nm supporting sub-100 fs pulses. Unlike other materials, it exhibits higher thermal conductivity ~11 W·m⁻¹·K⁻¹ even at doping level >2% because of the similar size and mass of Yb³⁺ (173u) and Lu³⁺ (175u) ions, and doping only slightly reduces the heat conductivity [83]. For example, in the case of Yb:YAG, the significant mass difference between Y-ions (89u) and Yb-ions (173u) leads to a substantial reduction in thermal conductivity. In contrast, Yb:LuO experiences only a slight reduction in thermal conductivity due to comparable masses of Yb (173u) and Lu (175u) [83, 150]. This characteristic favors higher doping levels without negatively influencing the thermal conductivity. Moreover, at equivalent Yb-densities, a thinner Yb:LuO disk can be realized instead than Yb:YAG. Other Yb-doped materials, such as Yb-scandium silicon oxide (Yb:SSO), Yb-tungstates (Yb:KYW, and Yb:KLuW), Yb-fluorides (Yb:CaF₂), Yb:YCOB, and Yb:CALGO, providing broader gain bandwidth, have also been integrated with this geometry. Despite high thermal conductivity and broader spectral bandwidth, the utilization of most these materials remained limited due to extreme challenges in efficient crystal growth. Growing these crystals efficiently using the conventional Czochralski method on a large scale is difficult due to their high melting point of about 2430 °C, which negatively affects their crystal quality and optical efficiencies [148]. However, samples grown by the heat exchange method (HEM) have shown improved performance [154, 155].

8 Applications of femtosecond TD-oscillators

8.1 Octave spanning few cycle MIR femtosecond pulses

In recent times, the demand for robust broadband femtosecond mid-infrared (MIR) laser sources (500–5000 cm⁻¹) is rapidly growing. Such sources are applied over a wide range of spectroscopic applications, particularly in the field of strong-field physics, high-fidelity molecule detection (mostly molecules exhibit fundamental vibrational modes in the range between 2−20 μm) [156], and cold tissue ablation [157]. The generation of coherent, broadband MIR sources relies on frequency down conversion via optical parametric process (OPOs, or OPAs) [142, 158] or intra-pulse difference frequency generation (IPDFG) [141, 159−161] of NIR radiation lasers.

IPDFG offers a simplified scheme where frequency down conversion is achieved in a single beam pass structure, eliminating the complicated temporal phase matching issue compared to OPA. However, with the limited power scalability of the driving source, higher energies are realized at the expense of low repetition rates. For more discussion on techniques for generating and amplifying ultrafast MIR sources, please refer to the review article [162]. In DFG, the choice of driving laser and nonlinear crystal is critical for power scalability at high repetition [163]. For power scalability, high power, high repetition rate TD-mode locked lasers are advantageous over Ti:Sa systems. Such systems avoid multiphoton absorption in nonlinear crystals and enable high-power operation in IPDFG. In 2015, Pupeza et al. [159] demonstrated the first IPDFG-based high-power, few-cycle MIR laser source driven by TD-oscillator. For few-cycle MIR generations, the deriving source was a standard 1 μm TD-KLM oscillator generating 90 W with 250 fs pulses at a 100 MHz repletion rate. As shown in Fig.7(a), the pulses were compressed down to 19 fs with 50 W average power in a subsequent nonlinear pulse compression stage based on PCF. An LGS (LiGaS₂) crystal with a damage threshold of 1 × 10⁻¹² W/cm² is ideally suited for power-scalable DFG. The crystal was rotated to evenly distribute the p-polarized beam for Type-I matching. The generated MIR and driving beam propagate collinearly and are separated by a Ge filter, see Fig.7(c). Finally, a broadband few-cycle MIR spectrum spanning from 6.7 to 18 μm with power 103 mW at 100 MHz was achieved. Further, to derive IPDGF via ~2 μm rather than conventional ~0.8 or 1 μm, near-IR pulses provide higher conversion efficiency due to a two times lower quantum defect. The MIR~2 μm driving lasers are more suitable for pumping the non-oxide nonlinear crystals, which are more readily available than LGS and have no two-photon absorption. In 2018, Zhang et al. [141] realized a CEP stabilized broadband MIR laser spanning from 500 to 2250 cm⁻¹ with an average output power of 24 mW and 77 MHz of repetition rate via IPDGF. A Ho:YAG TD-KLM mode-locked oscillator operating at 2 μm was built; the oscillator delivers 18.7 W of average output power at 260 fs pulse duration and 77 MHz pulse repetition rate. For the anticipated advantage of a 2 μm driven source, the output pulses are spectrally broadened and self-compressed via 12 μm long LMA-PCF to 15 fs (2.1 optical cycles). Here, non-oxide crystal GaSe is the material of choice to generate MIR due to high thermal conductivity, low parasitic effects, large non-linear coefficient, and large transparency windows over a board wavelength range (0.65−20 μm). For MIR generation, the self-compressed ~2 cycle pulses are focused on GASe at a peak intensity of 45 GW/cm². Type-I phase matching was achieved by rotating the crystal and HWP to distribute polarized beams equally on the ordinary and extraordinary axis of the crystal. After IPDGF, a CEP stabilized continuous spectrum spanning from ~5−20 μm at >24 mW of average power was detected. Further, OPAs are favorable to expand spectrum all the way from deep UV top THz regime and is assisted by interaction with nonlinear mediums. The generation of multiple-watt average power MIR radiations as a powerful tool for femtosecond spectroscopy was also realized by pumping OPA with a high-power NIR TD-mode locked oscillator [163]. The TD-KLM oscillator emitting 50 W of average power at 230 fs pulse duration and 37.5 MHz of repetition rate applied directly pump to OPA and femtosecond MIR pulses with ~5 W of average power at 4.1 μm and 1.3 W at 8.5 μm were generated. These above-mentioned results demonstrate the suitability of TD-mode locked oscillators for the generation of MIR – femtosecond pulses with high average power and repetition rate. These laser sources can further enable the MIR frequency comb to expand the application to MIR spectral region.

8.2 TD-oscillators driven field resolved spectroscopy

The broadband MIR sources, when combined with carrier-envelope phases (CEP) and few-cycle pulses, enable electro-optical sampling (EOS) based field resolved spectroscopy (FRS) [142, 164], which provides a promising alternative to time-integrating spectroscopies such as Fourier Transform spectroscopy. TD-mode locked oscillators, providing controlled frequency comb [46], waveform stabilized CEP, happen to be versatile tools for FRS domains. A review [165] briefly discusses the early application of the TD-oscillator as a deriving source for MIR-FRS. The state-of-the-art high repetition rate (multi-MHZ), high average power, and NIR femtosecond TD mode-locked oscillators enabling the temporally coherent radiations have leveraged the FRS with unparalleled sensitivity and detection range for fingerprinting of complex molecular structures [164]. Further, to develop the FRS to its potential, highly sensitive detection techniques for measurement, waveform control, and stability are essential. Later, in 2024, Hussain et al. [142] demonstrated the fluctuation measurement of a few-cycle MIR waveform by EOS. The gated pulses for EOS are obtained by deriving the IPDFG in LGS crystal via 16 fs pulses generated from a spectrally broadened NIR TD-KLM oscillator. Here, EOS measurements confirm sub-attosecond waveform jitter stability and validate the aptness of TD-mode locked oscillators in such high precision measuring and detection techniques as a compact and powerful deriving source of fey cycle NIR pulses.

8.3 OPCPA

Optical parametric amplification (OPA) leads to instantaneous photon amplification with a very high single pass while avoiding gain narrowing and parasitic lasing [166]. Combining the concept of OPA [167] and CPA [168] (with a different objective from typical CPA), known as OPCPA, has opened new pathways for generating ultra-broadband solid-state laser sources. One of the most important properties of OPCPA is its simultaneous scalability in peak and average power. For efficient OPCPA, a high-energy broadband seed source with a CEP-stabilized waveform and a pumping source with intense ~1 ps are ideal [169]. The intense picosecond pulses facilitate the realization of thinner non-linear crystals, which results in a large amplification bandwidth while keeping the same gain level [170]. The recent developments in TD-mode locked oscillators, reaching 100 μJ pulse energy and peak power of >100 MW at MHz repetition rate with sub-ps pulse duration, places them as the best suitable candidates for OPCPA pumping. Earlier, in 2012, Fattahi et al. [170] demonstrated a 11.5 MHz, 8 W OPCPA pumped by 1 ps Yb:YAG TD-SESAM oscillator.

Further, in a proof of principle, the authors [171] demonstrated TD-oscillator driven OPCPA, where seed and pump pulses are generated from the same TD-mode locked oscillator. In this scheme, the TD pulse energy is focused on a nonlinear crystal for frequency doubling and is reflected by the dichroic mirror for pumping. The remaining fundamental pulse energy is spectrally broadened and compressed to a shorter pulse duration to derive supercontinuum generation (SCG) for seed, which is subsequently amplified in a single-stage OPCPA. Further, the authors [172] proposed that broadband seed pulses with well-behaved spectral phase and CEP stabilization are also preferable for developing power-scalable OPCPA. The current developments in TD-KLM oscillators towards high peak power and CEP stabilization [45] hold a promising aspect for providing a NIR femtosecond continuum seed for OPCPA. Recently, CEP stabilization of over 100 MW level based on TD KLM mode-locked oscillator was demonstrated [173]. The mode-locked pulses from TD-KLM are spectrally broadened and compressed down to < 40 fs, which are further used to generate light with broad spectral range of 550−1400 nm. Such CEP stabilized NIR continua exhibiting a well-behaved spectral phase based on few-cycle TD oscillators could be an ideal seed for third-generation OPCA to reach multi-TW-level peak power [172].

9 High harmonic generation (HHG) and attosecond science

One of the key motives in attosecond science [114] is to enable high-harmonic-generation (HHG) sources (XUV/EUV) with high photon flux based on a compact laser system. The high harmonics (XUV/EUV) are used to investigate the ultrafast phenomena at the atomic and molecular levels and are generated by focusing the fs-pulses with peak intensities >10¹³ W/cm² into the gas target [174]. To enable accurate spectroscopic data and resolve ultrafast dynamics of a matter, an intense light source with MHz reparation rate and fs pulse duration is critical. In recent years, there has been a major effort to scale (HHG) systems with high photon flux at MHz pulse repetition rates [175] to speed up imaging and pump-probe measurements and broaden the application range such as frequency metrology, time-resolved spectroscopy and high spatial resolution microscopy [176, 177]. A high repetition rate and high photon flux HH are valuable rather than high energy to avoid the space charge effect for high-resolution photoelectron spectroscopy. Thus, a driving laser source with a combination of a high repetition rate, high peak power >100 MW, and shorter pulse duration <100 fs is desired for efficient HHGs [178]. In this regard, two directions for HHG have been extensively investigated. The first one is the utilization of femtosecond enhancement cavity (fsEC), whose potential for generating HHG at MHz repetition was demonstrated in 2005 [179]. Here, the pulse enhancement factor scales the intracavity peak power sufficiently to a higher level at 100s of MHz repetition rate. This intracavity enhancement reduces the power requirement of driving the laser. Based on these systems, the photon flux up to multi 100 μW in a given harmonic of generated XUVs has been reached [180]. However, it has a very complex experimental configuration and requires high phase stability. Further, the fsECs are very sensitive to the losses, and to reach a higher enhancement factor, the total roundtrip losses have to be lower than 1%, including OC losses [181], which largely compromises the output efficiency. The second direction is to increase the average power and repetition rate of ultrafast Yb-doped driving sources. These sources include CPA fiber amplifiers [182], Innoslab amplifiers [183, 184] and TD amplifiers [135]; however, again, these systems relied on multi-stage amplification together with non-linear post-pulse compression for higher peak intensities, resulting in a large footprint and complex configuration. A more compact and integrated solution is the implementation of TD oscillators as the deriving laser source for HHG, as they simultaneously provide high peak power, high repetition rate, and ultrashort pulses.

9.1 TD-oscillators for HHGs

The TD-mode locked oscillators are capable of generating high peak power with MHz repetition rate and better signal-to-noise ratio. Further, they provide transform-limited soliton pulses with excellent temporal shape and high beam quality for non-linear pulse compression schemes to reach a few-cycle regimes. This whole package ranks the TD oscillators as the best-suited, simple, and compact single-stage source for HHG. A discussion about earlier work on TD-based HHG is provided in review articles [165, 185]. We will further discuss some recent work related to HHGs and their efficient extraction.

The earlier work related to external HHG and intracavity HHG driven by linear cavity TD oscillators has achieved some promising results with high photon flux and up to 0.4 μW average power in 25th harmonic (30 eV) [135, 181, 186, 187]. It is believed that with proper pressure handling of target gas and increasing the peak power of driving photons, the photon flux and average power can further be increased. While having enough peak power, It is important to note that in linear standing wave cavity, forward and backward propagation of NIR pulses poses a challenge. The both way propagating pulses could generate HHs in both directions, dividing the converted power into parts and with wastage of generated power in the return path. At the same time, this could also affect the stable mode-locking. The solution to such issues lies in TD oscillators operating with ring cavities. The intense fs-pulses generated in a TD-mode-locked ring cavity allow one-way propagation of NIR wave for intracavity HHG and result in highly concentrated converted output power. In Ref. [178], HHG was investigated with TD-KLM in the ring cavity. XUV radiation was generated in argon with an average power level of 47 nW in 17^th harmonics (20 eV) at a 3.1 MHz repetition rate. In this work, the author also proposed a new route for the generation of MHz coherent XUV. He used multiport HHG in a 100 m long ring cavity of TD mode-locked oscillator Fig.8(a). Here, two-intracvity HHG ports were introduced at the tight focus for the generation of XUV. Two gases, He and Ne, are simultaneously injected for the generation of the corresponding XUV at port 1 and port 2, respectively. The output spectra of both HH pulses are shown in Fig.8(c). The author compares the spectra of both simultaneously generated HH with those generated with only single port HH operation Fig.8(b). The HHG spectrum of Ne obtained with two ports gas target was similar to that of single port HHG. The mode-locked operation remained stable during multi-port operation. This approach happened to be more advantageous as it does not require frequency stabilization. Further, the two different XUV beams at MHz repetition rate and femtosecond pulse duration generated from a single ring cavity also enable ultrafast XUV spectroscopy with an XUV pump and XUV probe measurements. The author suggests that further improvements can be realized in terms of power and efficiency by using a more powerful light source with a much shorter pulse duration. Recently, in 2024 [116], another Yb:YAG TD-KLM ring cavity oscillator was demonstrated, which simultaneously delivers high intracavity peak power > 100 MW at a 65.3 MHz repetition rate with 175 fs pulse duration. This high peak power and high repetition rate with sub-200 fs pulse duration ring cavity TD-oscillator has the potential to generate multiport coherent XUVs, which will further improve the time-resolved spectroscopy measurements.

Another important issue related to TD-intracavity-driven HHG is their efficient extraction. As the HHs are generated collinearly along the driving laser, the efficient out-coupling of generated XUV without wastage becomes a critical task. The author in Ref. [187] further improved XUV extraction efficiency by applying a TD-oscillator with high peak power and a coated grazing incident plate (GIP) instead of a conventional Sa-plate at Brewster’s angle. With the combination of coated GIP and improved driving laser source [187], an average power of up to 5.4 μW at 21st harmonics (25 eV) was achieved. Although coated GIP improves the out-coupling efficiency of XUV, the pronounced thermal lensing effect arising from GIP still limits the performance, and the peak power of the driving source has to be decreased to 950 MW for non-coated GIP to around 500 MW for coated GIP. This limitation can further be rectified by optimizing the coated GIP structure, including incident angle design, the material of the top coated layer, and the change of substrate from Sa to FS [187].

In summary, the conversion efficiency of HHG with TD oscillators can be improved by shortening the pulse duration and, hence, increasing the peak power. In this regard, the TD-mode KLM oscillators take the lead over TD-SESAM. The longer pulse duration of TD-SESAM oscillates could lead to high degrees of ionization of gas before reaching the pulse peak, affecting the phase matching of generated harmonics and eventually causing low conversion efficiency [178]. Consequently, post-pulse compression is typically employed with TD-SESAM to shorten the pulses. On the other hand, the TD-KLM, with its instantaneous fast response and high damage threshold of cavity components, supports much shorter pulses and higher peak powers without requiring post-pulse compression. These combined factors make TD-KLM more promising for HHG. Furthermore, ring cavity TD oscillators are more favorable than linear cavity oscillators for the generation of high photon flux. Future development of high power, sub-100 fs ring cavity TD-oscillators based on broadband gain mediums, such as Yb:LuO and Yb:CALGO, could further enhance HH conversion efficiency. On the line, with the realization of 500 W TD-oscillator and post-pules compression techniques being matured, GW peak power is achievable by cascading the multiple compression stages to derive ~ 50 μJ HH.

9.2 TD-UV/XUV dual comb spectroscopy

Since the invention of laser frequency combs, they have been considered an attractive tool for spectroscopy [188]. The emergence of a new version called DCS [189] integrates the advantages of both broadband and tunable laser spectroscopy and takes lead in molecular fingerprinting over single comb spectroscopy. To date, DCS is the most widely followed form of direct comb spectroscopy and has outpaced other spectroscopy techniques in terms of spectral range, spectral resolution, robustness, and short measurement time [190]. The basic DCS concept relies on two-frequency comb laser sources with slightly different repetition frequencies. Such frequency combs are often generated from mode-locked laser systems. To realize DCS, the outputs of these two individual frequency comb laser systems are combined to generate a rf comb. For spectroscopy, the sample is introduced in the optical path of a single or both beams. A detailed description of the DCS principle is provided in the review articles [188, 190, 191]. The high spectral resolution and precision measurements of such systems primarily depend on the frequency stability and low phase noise of both mode-locked lasers. A subsequent pulse amplification for high power may introduce the noise, which affects the sensitivity of the signal. Thus, an active stabilization of repetition rates and carrier-envelope offset frequency f_{\mathrm{C}\mathrm{E}\mathrm{O}} is required. However, such stabilization of the pulse train adds complexity to the system [46] and hinders its practical applications. A potential alternative is the generation of two frequency combs with a slight difference in repetition rate from a single cavity mode-locked system. Such single cavity dual comb systems are more compact and simpler. One of the key advantages of these systems is that both frequency combs share most of the cavity elements and have common noise, which effectively cancels out during the beating process. Different mode-locked systems have demonstrated dual combs in a single cavity [192−194]. Amongst these, the TD-mode locked oscillator stands out as an ideal dual-comb source. As they provide much higher average output power and peak power, and no subsequent amplification is required; this eliminates the possibility of noise addition to the signal. Thus, a cumbersome, complex phase locking system can be avoided, which further simplifies the system. The dual comb laser systems with high average power based on single cavity TD-KLM oscillators have recently been realized [46, 195], which directly enables the near IR DCS.

While DCS is widely used in various spectral regions like NIR and MIR, research on ultraviolet(UV)-DSC and deep ultraviolet (DUV)-DCS remains nascent due to the absence of direct dual comb sources in this region [196, 197]. High-resolution UV-spectroscopy provides deep, insightful information of electronic transitions in atoms, astronomical observations, and absorption characteristics of the greenhouse which have large absorption cross-sections in the UV spectral range [191]. The basic concept of UV-DCS is same as described above. The UV combs are produced via frequency-up conversion driven by the NIR frequency comb source. The extension of DCS toward UV region depends upon the peak power of dual comb laser sources, as the peak intensities of order 10¹³−10¹⁴ W/cm² are required for HHG. TD-mode locked oscillators with their peak power of multi tens of MW have the capability of efficient frequency-up conversion into UV regions. In 2023, Hofer [198] realized a frequency-doubled dual comb for visible-DCS from a single cavity dual comb TD oscillator. It was comprehended that the high output peak power of TD dual comb oscillator could further facilitate nonlinear frequency conversion and pave the way towards UV and DUV-DCS via third harmonic generation (THG) and fourth harmonic generation (FHG). Following this approach and concept given in Ref. [191], a possible road map to extend the domain of NIR dual comb TD oscillators for the realization of UV/DUV-DCS and XUV-DCS is illustrated in Fig.9.

As a UV/DUV-dual comb can be established through third and fourth harmonics generation, the further extension of the spectral range to an XUV-dual comb can only be achieved by HHG from the gas target. However, its realization faces challenges regarding the phase noise behavior of HHG. The phase noise increases with harmonic order as well as can be transferred to HHG from deriving frequency comb source and density fluctuation of the target gas. This largely influences the sensitivity of the spectrometer. As f_{\mathrm{C}\mathrm{E}\mathrm{O}} stabilization of TD-oscillators can be achieved with intracavity HHG in gas [186], and the single cavity dual frequency comb TD-mode locked oscillators with their inherited capabilities of high peak power and low noise, are ideally suited for dual combs XUV. The XUV-DCS has the potential to investigate 3D structures on the nanometer scale.

10 Conclusion and outlook

Thin-disk mode-locked oscillators have emerged as a compelling platform for generating high-power ultrafast laser pulses. Recent advancements have pushed the boundaries of these systems, reaching remarkable performance levels with sub-kilowatt-level average powers and hundred-megawatt peak powers at megahertz repetition rates from a single, tabletop system. This review has provided an overview of recent advancements in mode-locked TDLs, focusing primarily on Yb-doped gain media. SESAM and Kerr-lens mode locking are the two prominent techniques employed in TD oscillators, both demonstrating remarkable performance with tens of microjoules of pulse energy and several hundred watts of average power. Notably, TD-SESAM oscillators have exceeded the 500 W average power threshold in the ultrafast regime, while TD-KLM oscillators have achieved >100 MW peak powers and few-cycle pulse generation. These achievements position TD oscillators as potential replacements for bulky amplifier systems, offering significant advantages in terms of size, cost, and complexity.

The pursuit of high peak power and high average power simultaneously drives future research directions by triggering the nonlinear frequency conversion. Integrating TD oscillators with multi-pass amplification configurations, such as active multi-pass cells (AMCs) and multi-pass cells (MPCs), in a controlled environment holds significant promise for achieving further power scaling and shorter pulse durations, respectively. Further, exploring this concept with alternative gain media having broader bandwidths and larger emission cross-sections, such as Yb:YLF, Ho:YAG, and Yb:CALGO, offers exciting avenues for performance enhancement. However, the availability of high-quality TD crystals with larger dimensions and excellent surface quality is crucial for realizing the full potential of these advanced configurations, as wavefront distortions can accumulate during multiple passes. Moreover, TD-oscillators delivering transform-limited soliton pulses with excellent temporal and spatial beam quality are ideal sources for nonlinear post-pulse compression techniques, enabling the generation of gigawatt-level few-cycle pulses. These high-peak-power sources are not only crucial for HHG with efficiency of 10⁻⁷−10⁻⁶ and attosecond science but also find significant applications in industrial processes such as extreme ultraviolet (EUV) lithography. Furthermore, these oscillators hold significant promises (i) as pump sources for subsequent amplification stages for optical parametric chirped pulse amplification (OPCPA) and (ii) as deriving laser for the generation of broadband mid-infrared (MIR) laser sources. Moreover, as dual-frequency comb has been successfully demonstrated with a single-cavity TD oscillator, which enables the DCS in the IR and visible regions, thanks to their high peak power, this technique can be further extended to less or unexplored UV/DUV and XUV spectral regions.

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