Passively mode-locked C-band fiber laser employing carbon nanotubes as saturable absorbers

Abstract

Passively mode-locked fiber lasers find extensive applications in communications, ultrafast science, and materials processing. In this study, carbon nanotubes (CNTs) were employed as saturable absorbers in a passively mode-locked fiber laser. We systematically explored the influence of different dispersion characteristics on the output of mode-locked pulses by precisely adjusting the cavity length. The experimental results clearly indicate that when the dispersion is altered from −0.25 to −0.12 ps², the pulse width can be effectively reduced from 858 to 645 fs. In addition, we comparatively analyzed the effects of CNT-SA aqueous solution and CNT-PVA-SA on the pulse output of the fiber laser. It was discovered that the morphology of the material exerts a significant impact on the mode-locking threshold, pulse width, and stability of the laser. This discovery offers a crucial theoretical foundation for future material selection in the field of passively mode-locked fiber lasers, facilitating more optimized designs and enhanced performance in related applications.

Graphical abstract

Keywords

laser / saturable absorber / doped fiber / carbon nanotubes

1 Introduction

Ultrashort optical pulses are widely used in spectroscopy, nonlinear optics, and industrial processing due to their excellent monochromaticity, directionality and coherence [1, 2]. Mode-locked fiber lasers are mainly realized by both active mode-locking and passive mode-locking. Active mode-locking is achieved by controlling the parameters through an external modulator to achieve longitudinal mode-locking [2], while passive mode-locking is achieved by using a saturable absorber (SA) to regulate the light intensity, and when the light intensity exceeds a threshold, the absorption of the SA is weakened, which permits part of the light to pass through to form a stabilizing pulse, thus achieving mode-locking [1]. Compared with active mold clamping, passive mold clamping has the advantages of small size, low cost, and simple structure. SA is one of the effective methods to generate passive mold locking pulses [3, 4]. SAs can be categorized into artificial SAs and real SAs, each of which has its own unique advantages [5, 6]. Artificial SAs modulate pulses through optical polarization interference or fast saturable absorption effects (e.g., NPR, NALM, NL-MMI) [7−9], which provide high flexibility, while real SAs [10] utilize material nonlinearities (e.g., SESAM, graphene, SWNT, black phosphorus, transition-metal sulfides, and MXenes) [11−17] to convert a continuous wave into an optical pulse, which has excellent nonlinear absorption properties and has a clear advantage in generating ultrashort pulses. However, these materials differ in modulation depth, stability, and preparation process [18, 19].

Carbon nanotubes [20] have attracted much attention because of their broad wavelength coverage, fast recovery, low saturation, insensitivity to polarization and easy fabrication. In 2003, Set et al. [21] successfully applied carbon nanotube technology to fiber lasers to achieve mode-locked pulse output, which was the first application of carbon nanotubes in the field of fiber lasers. In 2019, Pawliszewska et al. [22] constructed a thulium-doped fiber laser using a metallic single-walled carbon nanotube film, and succeeded in obtaining soliton pulses with pulse widths of 212 fs and pulse energies up to 3.79 nJ. In 2023, Apandi et al. [23] demonstrated that single-walled carbon nanotubes-polyethylene oxide (SWCNT-PEO) films irradiated by γ rays were used as SA to generate solitons and trapped soliton passively mode-locked erbium-doped fiber lasers (EDFL), and the unirradiated films generated soliton mode-locked pulses with a pulse width of 0.60 ps². Although CNTs have been widely studied in fiber lasers, the output of mode-locked lasers based on dispersion to CNT-SAs and the output of fiber lasers based on the morphology of CNT-SAs have not been carefully investigated.

In this study, an erbium-doped fiber passive mode-locked laser with CNT as a SA was constructed. The nonlinear saturable absorption characteristics of CNT and its composite with PVA were measured by the double-balance probing technique with modulation depths of 3.44%, 11.6% and 3.56%, respectively. The effect of different net dispersion conditions on the mode-locked pulse output was investigated and analyzed, and it was found that the pulse width was shortened from 858 fs to 645 fs as the net dispersion in the cavity was reduced from −0.25 to −0.12 ps². The effect of CNT-SA aqueous solution on the pulse output was further compared with that of CNT-PVA-SA film, and the results showed that the mode-locking threshold of CNT-PVA-SA (1:1) was 52.3 mW, which is lower than the mode-locking threshold of CNT-PVA-SA (3:1) and CNT-SA aqueous solution. The study verifies the potential application of CNT-PVA-SA in fiber lasers and provides an experimental basis for the optimization of novel saturable absorption materials.

2 Experimental setup

As shown in Fig.1, the LD is a laser diode with a wavelength of 976 nm, the EDF is an erbium-doped fiber, the PC is a polarization controller, the WDM is a WDM, the PI-ISO is a polarization-independent isolator, and the OC is a 3:7 optical coupler. In the resonant cavity, the 976 nm pump light output from the LD is transmitted to the 0.6 m long EDF (Er110-4/125, LIEKKI) through the 980/1550 nm WDM in the triplex device, which has a peak absorption coefficient of 110 dB/m at 1530 nm and a dispersion parameter of −36 ps/nm/km. The PI-ISO in the triple unit ensures unidirectional operation of the cavity and prevents the EDF from burning holes. PC regulates optical pulse polarization and birefringence to ensure stable mode-locked output; and CNT-SA is placed between two fiber patch cords and secured by a flange to ensure good contact and saturation absorption. The coupling ratio of the OCs in the 3-in-1 device is 30:70, where 30% of the light output is used for monitoring and 70% of the light is returned to the cavity to form feedback to maintain the laser stability. The experimental fiber cavity is mainly composed of a single mode fiber (SMF-28) with a dispersion parameter of 17 ps/nm/km to keep the whole cavity in negative dispersion.

3 Preparation of CNT thin films

There are various ways to make films, and each way has its own unique characteristics and scope of application [24, 25]. The fabrication procedure is shown in our previous report [26]. In order to test the nonlinear characteristics of CNT-SA and CNT-PVA-SA, an experimental setup as shown in Fig.2(a) was utilized. The experimental setup consists of two main parts: a seed source and an erbium-doped fiber amplifier (EDFA). The seed source is a homemade fiber laser with a central wavelength set to 1560 nm, a pulse width of 650 fs, and a fundamental repetition frequency of up to 27 MHz. power meter 1 (Pm1) is used to measure the output power of the pulsed laser in the unobstructed case, while power meter 2 (Pm2) is used to measure the output power of the pulsed laser after interacting with the CNT-SA or CNT-PVA-SA. In addition, in order to precisely control the laser output and coupling, we used an optical coupler (OC) with a coupling ratio of 50:50. The experimental results are shown in Fig.2(b)−(d), where the transmittance tends to grow with the increase of pump power intensity. However, when the power intensity continues to increase, the transmittance stabilizes and forms a straight line, which marks the saturation of the material [27]. The experimental data were fitted with T = 1 − ΔT/(1 − I/I_s) − α_ns (where T is the transmittance of the SA, ΔT is the modulation depth parameter, I is the input optical intensity, I_s is the saturated optical intensity, and α_ns is the unsaturated loss). As shown in Fig.2(b), the modulation depth of CNT-SA is 3.44% and the unsaturated loss is 46.8%. Further analyzing Fig.2(c), it can be found that the lower concentration of CNT-PVA-SA shows higher modulation depth of 11.60% and unsaturated loss of 12.3%. From Fig.2(d), the higher concentration of CNT-PVA-SA shows a modulation depth of 3.56% and an unsaturated loss of 27.6%. Both CNT and CNT-PVA-SA show applicability to pulsed fiber lasers, with CNT-PVA-SA showing a higher modulation depth at lower concentrations, which provides new possibilities for the design and optimization of pulsed fiber lasers. This provides new possibilities for the design and optimization of pulsed fiber lasers.

4 Experimental results and discussion

In a negative dispersion environment, the pulse exhibits more pronounced nonlinear effects as the pump power increases. When these nonlinear effects reach a dynamic equilibrium with the intrinsic dispersion in the resonant cavity [28], the resonant cavity can stably output regular soliton pulses. In the experiment, the length of the single-mode fiber was shortened to regulate the dispersion magnitude in the cavity. Dispersion is a key factor in soliton pulse transmission, which will affect the phase, amplitude, spectrum, and time domain characteristics of the pulse, and may lead to pulse broadening and distortion. Dispersion changes the propagation speed of different frequency components, resulting in phase change, uneven amplitude, and spectrum broadening, thus affecting the transmission effect of signals. However, under appropriate conditions, soliton pulse can maintain its stability through the interaction of nonlinear effect and dispersion [29, 30]. In the experiments, it was observed that the fiber laser produced stable mode-locked pulses at dispersions of −0.25 ps², −0.19 ps², and −0.12 ps², and pump powers of 110.8 mW, 81.5 mW, and 93.2 mW, respectively. With the gradual increase of pump power, we observed that the initial continuous light gradually transited to a stable locking mode. At low pump power, the laser output is a stable continuous light without pulse, and the output light intensity has no obvious fluctuation in time domain. When the pump power is increased, a series of stable short pulses in time domain are formed, that is, the mode-locked state is entered [31]. Fig.3 illustrates the typical output characteristics of a fibre laser, including oscilloscope traces, spectra. Fig.3 (a)−(c) show the output spectra of the fiber laser under three different cavity lengths with central wavelengths of 1564.5 nm, 1563.1 nm, and 1562.9 nm and corresponding 3 dB bandwidths of 5.95 nm, 6.06 nm, and 6.33 nm, which result in a decrease in peak intensity, a shift in the central wavelength, and an increase in the bandwidth of the fiber laser output spectra. The difference of cavity length leads to the decrease of peak intensity, the shift of central wavelength and the increase of output spectral bandwidth of fiber laser. These changes are caused by the influence of cavity length on gain distribution, mode competition, fiber dispersion characteristics and nonlinear effects [32−35]. At both ends of the spectrum, we observe distinct Kelly sidebands, which are typical of fiber lasers operating in a soliton mode-locked state [36]. Fig.3(d)−(f) show the pulse sequence plots observed by the oscilloscope, and the time intervals between the pulses are 61.34 ns, 47.7 ns, and 32.61 ns under the three cavity length conditions, which correspond to the time for the laser to run one revolution inside the cavity, confirming that the laser cavity is in the soliton pulsed operation state. The time interval of these pulses is exactly equal to the time required for the optical signal to propagate through the laser once. This time matching indicates that the optical signal in the laser cavity is effectively fed back and enhanced after each pulse emission. Therefore, this phenomenon proves that the laser cavity is in a single-pulse operating state, thus ensuring the stability and regularity of the pulse output [37].

Fig.4(a)−(c) illustrate the output pulse autocorrelograms for the three cavity lengths. According to the measurements, the corresponding pulse widths are 0.858 ps, 0.707 ps, and 0.645 ps. The time bandwidth products (TBPs) are 0.515, 0.504, and 0.501, respectively, which are larger than the values of the transform-limited sech² pulses, indicating that the pulses have a slight chirp. By further analyzing the RF plots (d)−(f), we conclude that the signal-to-noise ratios (SNRs) of the pulses are 54.5 dB, 57 dB, and 55.1 dB, respectively, which proves that the proposed fiber laser operates stably with excellent mode-locking performance. Different cavity lengths produce different nonlinear effects and affect the pulses differently, and the SNR of the final output pulse changes accordingly [38].

From the above experimental results, as the length of the intracavity single-mode fiber decreases, the intracavity net negative dispersion gradually decreases, the spectral width gradually increases, the spectral fringes become smoother, and the pulse width gradually becomes narrower, which suggests that the dispersion plays a crucial role in the generation and evolution of the soliton pulse. Chromatic dispersion, together with nonlinear effects, determines the formation and stability of solitons, affecting the shape, speed and energy distribution of the pulse, which in turn affects transmission efficiency and accuracy [39, 40]. The conventional soliton Kelly sidebands are generated by the periodic amplification and attenuation of the soliton pulse in a mode-locked fiber laser. However, the presence of sidebands affects the stability of the laser and reduces the pulse energy, thus limiting the narrowest pulse width that can be achieved by the pulse. Effectively adjusting the sidebands of conventional soliton pulses not only significantly improves the performance of the fiber laser, but also optimizes the output characteristics of the laser to meet the stringent requirements in various fields such as precision measurement, communication, and scientific research, which has a broad application prospect.

While keeping the resonant cavity unchanged, the CNT-SA in the cavity was replaced with CNT-PVA-SA. After inserting the CNT-PVA-SA, a mode-locking pulse was obtained by adjusting the pump power and the position of the polarization controller (PC). When the the cavity was CNT-PVA-SA (1:1), a mode-locking pulse was observed when the pump power was increased to 52.3 mW. When the pump power was further increased to 58.1 mW, a stable output of clamping pulse was obtained. The output results are shown in Fig.5(a−c). The red curve in Fig.5(a) shows its output spectrum, whose center wavelength is located at 1562.3 nm and the spectral width is 3.6 nm. It is worth noting that there are obvious sidebands on both sides of the spectrum, which is a common feature of soliton pulses when they are transmitted in fiber lasers. Fig.5(b) shows the fitting results of the autocorrelation curve with a pulse width of 0.816 ps. The TBP is 0.360, which is larger than the Sech² transformed limit value of 0.315, indicating that the pulse has a slight chirp characteristic. Fig.5(c) demonstrates that the time interval between pulses is 61.34 ns, which corresponds to a repetition frequency of 16.13 MHz, in line with the total cavity length of 12.6 m set in the experiment. In addition, the SNR is as high as 46 dB, which proves that the mode-locked pulse has good stability.

Keeping the rest of the resonant cavity unchanged, CNT-PVA-SA (1:1) in the cavity is replaced by CNT-PVA-SA (3:1) and its effect on the mode-locked pulse of the fiber laser is observed. When the pump power is increased to 56.4 mW, the same mode-locked pulse output as the previous CNT-PVA-SA (1:1) is observed. When the pump power is further adjusted to 58.1 mW, a stable mode-locking pulse is obtained. The experimental results are shown in Fig.5(a), (d) and (e), respectively. From the black curve in Fig.5(a), the center wavelength of the output spectrum is located at 1564.5 nm, and the spectral width reaches 3.9 nm. The clearly visible sidebands on both sides of the spectrum are typical characteristics of the soliton pulses generated when they are transmitted in a fiber laser. Fig.5(d) shows the fitting results of the autocorrelation curve. The pulse width is 0.783 ps and the TBP is 0.374, which is larger than the Sech² transform limit value of 0.315, indicating that the output pulse has some chirp characteristics. Fig.5(e) demonstrates the time interval between pulses and the repetition frequency. The experimental results show that the time interval between pulses is 61.31 ns, which corresponds to a repetition frequency of 16.13 MHz. This repetition frequency coincides with the total cavity length of 12.9 m. The signal is characterized by a high SNR of 0.315, which is the same as that of the chirped signal. In addition, the SNR of the signal is 50.5 dB, further confirming the stability of this mode-locked pulse output.

By analyzing the experimental results, it is found that CNT-SA and CNT-PVA-SA exhibit different mode-locking performances in fiber lasers. Comparing the pulse widths and spectral widths of the two, it is found that CNT-PVA-SA produces very similar pulse widths and spectral widths at different ratios, but CNT-PVA (3:1) is slightly superior in performance. This finding provides a valuable reference for optimizing the output characteristics of fiber lasers by adjusting the ratio of SA. It is worth noting that the output conventional soliton pulses exhibit certain chirp characteristics whether CNT-SA or CNT-PVA-SA is used. This is mainly due to the sharp increase of the peak pulse power during the amplification of the pulse through the erbium-doped fiber. In the laser cavity, due to the presence of nonlinear effects, the pulse gradually accumulates chirp during transmission, and in the remaining negative dispersion fiber, these broadened pulses undergo the process of compensation and compression, and ultimately output soliton pulses with chirp. This finding not only reveals the difference in mode-locking performance between CNT-SA and CNT-PVA-SA in fiber lasers, but also provides insights into the transport and evolution mechanisms of soliton pulses in fiber lasers. It is of great significance for further optimizing the performance of fiber lasers and achieving high-quality pulse output.

The change in laser output with increasing pump power was recorded and the experimental results were obtained as shown in Fig.6. Fig.6(a) shows the spectral evolution of the CNT-SA laser when the pump power is increased from 110.8 to 134.2 mW. When the pump power is increased to 128.3 mW, a small short streak appears at the upper edge of the spectrum, which indicates that the spectrum has been modulated and the soliton splitting starts. This splitting phenomenon is due to the accumulation of nonlinear effects caused by the increase in pump power, which breaks the balance between dispersion and nonlinearity and leads to pulse splitting [41]. Fig.6(b) depicts the spectral evolution of the CNT-PVA-SA (1:1) laser when the pump power is increased from 58.1 to 81.5 mW. When the pump power reaches 64 mW, the pulse begins to split, like the case of the CNT-SA laser. At high pump power, the accumulation of nonlinear effects leads to different propagation velocities of different frequency components in the pulse. This difference causes phase distortion within the pulse, which leads to pulse broadening and shape distortion [42]. Fig.6(c), on the other hand, demonstrates the spectral evolution of the CNT-PVA-SA (3:1) laser at different pump powers. It is noteworthy that the center wavelength of the output pulse remains almost constant as the pump power increases, showing that the laser has good wavelength stability when the pump power is varied. Fig.6(d) presents the pulse sequence when the pump power is varied. The pulse train always remains stable without splitting as the pump power increases. This indicates that the fiber laser can produce stable mode-locked pulse output over a wide range of pump power. Fig.6(e) demonstrates the relationship between output power and pump power. Through linear fitting, the output power was found to be linearly proportional to the pump power level with a slope efficiency of 2.9%. This result further confirms the direct influence of the pump power on the output power in fiber lasers and provides us with a reference for optimizing the output power. In order to verify the temporal stability of the proposed mode-locked fiber laser, we recorded the spectra every 10 min and plotted the results in Fig.6(f). As can be seen from the figure, the Kelley sidebands of the spectrum do not change and the center wavelength does not shift with time. The stability of the Kelly sidebands indicates that there is no significant change in the gain characteristics and optical conditions in the laser cavity, which shows the superiority of the laser design that can effectively resist the influence of changes in the operating environment. Meanwhile, the stability of the center wavelength also reflects the frequency stability of the laser, i.e., the laser does not drift during long-term operation, which indicates that the locking mechanism or feedback control system of the laser operates effectively to ensure the stability of the output optical frequency [43]. As shown in Tab.1, the data of SA-based mode-locked lasers in recent years are compared.

5 Conclusion

This paper discusses the effect of different dispersions on the pulse output of a fiber laser and the effect of different saturable absorber states on the pulse output of a fiber laser. The effects of different net dispersions on the mode-locked pulse output in the annular cavity are investigated and analyzed, and it is found that the pulse width is shortened from 858 to 645 fs when the net dispersion of the cavity is reduced from −0.25 to −0.12 ps². Moreover, the effects of CNT-SA aqueous solution and CNT-PVA-SA film on the output pulses of the fiber laser were compared. The results show that CNT-PVA-SA (1:1) has the lowest mode-locking threshold of 52.3 mW, while CNT-PVA-SA (3:1) has a more stable pulse output, and the CNT-SA aqueous solution has the narrowest pulse width of 0.858 ps. It is believed that passive mode-locked lasers with low mode-locking thresholds, narrow pulse widths, and high stabilities can be achieved by using saturable absorbers with different fabrication methods. Finally, this study not only validates the potential application of CNT-PVA-SA in fiber lasers, but also provides a valuable experimental basis for exploring and optimising novel saturable absorber materials.

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