Differential quadrature phase shift keying (DQPSK) modulation is attractive in high-speed optical communications because of its resistance to fiber nonlinearities and more efficient use of fiber bandwidth compared to conventional intensity modulation schemes. Because of its wavelength conversion ability and phase preservation, semiconductor optical amplifier (SOA) four-wave mixing (FWM) has attracted much attention. We experimentally study wavelength conversion of 40 Gbit/s (20 Gbaud) non-return-to-zero (NRZ)-DQPSK data using FWM in a quantum dash SOA with 20 dB gain and 5 dBm output saturation power. Q factor improvement and eye diagram reshaping is shown for up to 3 nm pump-probe detuning and is superior to that reported for a higher gain bulk SOA.
All-optical high-speed turbo-switches can effectively increase the switching speed using cascaded semiconductor optical amplifiers (SOAs). The overall recovery time or the bandwidth of turbo-switch was numerically analyzed with time-domain and frequency-domain SOA models. The turbo-switch was explored from the fundamental carrier dynamics in SOAs for the purpose of further increasing its operation speed. An integrated turbo-switch was also been proposed and demonstrated, where a phase adjustable Mach-Zehnder interferometer (MZI) was applied as an optical band-pass filter between SOAs. Wavelength conversion was first demonstrated at 84.8 Gbit/s using the integrated turbo-switch.
In this paper, we demonstrated a novel physical mechanism based on the well-barrier hole burning enhancement in a quantum well (QW) semiconductor optical amplifier (SOA) to improve the operation performance. To completely characterize the physical mechanism, a complicated theoretical model by combining QW band structure calculation with SOA’s dynamic model was constructed, in which the carrier transport, interband effects and intraband effects were all taken into account. The simulated results showed optimizing the thickness of the separate confinement heterostructure (SCH) layer can effectively enhance the well-barrier hole burning, further enhance the nonlinear effects in SOA and reduce the carrier recovery time. At the optimal thickness, the SCH layer can store enough carrier numbers, and simultaneously the stored carriers can also be fast and effectively injected into the QWs.
Silicon micro-ring resonators (MRRs) are compact and versatile devices whose periodic frequency response can be exploited for a wide range of applications. In this paper, we review our recent work on linear all-optical signal processing applications using silicon MRRs as passive filters. We focus on applications such as modulation format conversion, differential phase-shift keying (DPSK) demodulation, modulation speed enhancement of directly modulated lasers (DMLs), and monocycle pulse generation. The possibility to implement polarization diversity circuits, which reduce the polarization dependence of standard silicon MRRs, is illustrated on the particular example of DPSK demodulation.
An effective theoretical analysis method is presented to analyze different linear optical signal processing functions with optical filters reported in literatures. For different applications, the optical filters are supposed to operate on the analog or digital part of the signal separately, namely analog spectrum conversion and digital spectrum conversion. For instance, the return-to-zero (RZ) to non-return-to-zero (NRZ) format conversion for intensity or phase modulated signals are based on the analog spectrum conversion process, while the (N)RZ to (N)RZ phase-shift-keying (PSK) format conversion, logic NOT gate and clock recovery for RZ signals are based on the digital spectrum conversion process. Theoretical analyses with the help of numerical simulation are used to verify the reported experimental results, and all the experimental results can be effectively analyzed with this analytical model. The effect of the transmission spectrum of the filter on the performance of the converted signal is investigated. The most important factor is that the theoretical analysis provides an effective way to optimize the optical filter for different optical signal processing functions.
Wavelength conversion based on degenerate four-wave mixing (FWM) was demonstrated and compared between silicon nanowire and microring resonator (MRR). 15 dB enhancement of conversion efficiency (CE) with relatively low input pump power (5 mW) was achieved experimentally in an MRR. The impacts of bus waveguide length and propagation loss were theoretically analyzed under the effect of nonlinear loss.
In this paper, we presented switching dynamic investigations on an InP photonic-crystal (PhC) nanocavity structure using homodyne pump-probe measurements. The measurements were compared with simulations based on temporal nonlinear coupled mode theory and carrier rate equations for the dynamics of the carrier density governing the cavity properties. The results provide insight into the nonlinear optical processes that govern the dynamics of nanocavities.
In this paper, we experimentally demonstrate an all-optical continuously tunable fractional-order differentiator using on-chip cascaded electrically tuned microring resonators (MRRs). By changing the voltage applied on a MRR, the phase shift at the resonance frequency of the MRR varies, which can be used to implement tunable fractional-order differentiator. Hence fractional-order differentiator with a larger tunable range can be obtained by cascading more MRR units on a single chip. In the experiment, we applied two direct current voltage sources on two cascaded MRRs respectively, and a tunable order range of 0.57 to 2 have been demonstrated with Gaussian pulse injection, which is the largest tuning range to our knowledge.
In this paper, we propose an on-chip all optical transistor driven by optical gradient force. The transistor consists of a single micro-ring resonator, half of which is suspended from the substrate, and a bus waveguide. The free-standing arc is bent by optical gradient force generated when the control light is coupled into the ring. The output power of the probe light is tuned continuously as the transmission spectrum red-shift due to the displacement of the free-standing arc. The transistor shows three working regions known as cutoff region, amplified region and saturate region, and the characteristic curve is tunable by changing the wavelength of the control light. Potential applications of the all optical transistor include waveform regeneration and other optical computing.
The mode characteristics are investigated for the rectangular microresonators with an output waveguide connected to the midpoint of the long side for wide and continuous wavelength tuning. Through adjusting the aspect ratio of the rectangular microresonator, the mode Q factors can be greatly enhanced. Furthermore, the large mode interval between the high-Q modes makes the rectangular microresonators suitable for tunable lasers. As a special case, single-mode operation is achieved with a continuous tuning range of 9.1 nm for a square microlaser with the side length of 17.8 mm and the output waveguide width of 1.8 mm.
In this paper, we reviewed the fabrications of functional microcavity lasers in soft materials such as polymer and protein by femtosecond laser processing. High-quality (Q) microdisks with a laser dye (Rhodamine B, RhB) acting as gain medium were fabricated that produced whispering-gallery-mode (WGM) lasing output. We also obtained unidirectional lasing output with a low lasing threshold in a deformed spiral microcavity at room temperature. Photonic-molecule (PM) microlasers were prepared to investigate the interaction and coupling effects of different cavities, and it was found that the distance between the two disks plays an important role in the lasing behaviors. Single-mode lasing was realized from a stacked PM microlaser through Vernier effect. Furthermore we adopted the biocompatible materials, RhB-doped proteins as a host material and fabricated a three-dimensional (3D) WGM microlaser, which operated well both in air and aqueous environment. The sensing of the protein microlasers to Na2SO4 concentration was investigated. Our results of fabricating high-Q microlasers with different materials reveal the potential applications of femtosecond laser processing in the areas of integrated optoelectronic and ultrahigh sensitive bio-sensing devices.
100-GHz cross-cascaded arrayed waveguide gratings (AWGs)-based wavelength selective optical switching optical cross-connects (OXCs) modules with Mach-Zehnder interferometer (MZI) thermo-optic (TO) variable optical attenuator (VOA) arrays and optical true-time-delay (TTD) line arrays is successfully designed and fabricated using polymer photonic lightwave circuit. Highly fluorinated photopolymer and grafting modified organic-inorganic hybrid material were synthesized as the waveguide core and cladding, respectively. The one-chip transmission loss is ~6 dB and the crosstalk is less than ~30 dB for the transverse-magnetic (TM) mode. The actual maximum modulation depths of different thermo-optic switches are similar, ~15.5 dB with 1.9 V bias. The maximum power consumption of a single switch is less than 10 mW. The delay time basic increments are measured from 140 to 20 ps. Proposed novel module is flexible and scalable for the dense wavelength division multiplexing network.
Silicon photonics has become very popular because of their compatibility with mature CMOS technologies. However, pure silicon is still very difficult to be utilized to obtain various photonic functional devices for large-scale photonic integration due to intrinsic properties. Silicon-plus photonics, which pluses other materials to break the limitation of silicon, is playing a very important role currently and in the future. In this paper, we give a review and discussion on the progresses of silicon-plus photonics, including the structures, devices and applications.
Directional couplers (DCs) have been playing an important role as a basic element for realizing power exchange. Previously most work was focused on symmetric DCs and little work was reported for asymmetric directional couplers (ADCs). In recently years, silicon nanophotonic waveguides with ultra-high index contrast and ultra-small cross section have been developed very well and it has been shown that ADCs based on silicon-on-insulator (SOI) nanophotonic waveguides have some unique ability for polarization-selective coupling as well as mode-selective coupling, which are respectively very important for polarization-related systems and mode-division-mulitplexing systems. In this paper, a review is given for the recent progresses on silicon-based ADCs and the applications for power splitting, polarization beam splitting, as well as mode conversion/(de)multiplexing.
Liquid crystal photonic bandgap (LCPBG) fibers provide a versatile and robust platform for designing optical fiber devices, which are highly tunable and exhibit novel optical properties for manipulation of guided light. We review the research progress on design, fabrication and development of integrated LCPBG fiber devices.
Phase shifter is one of the key devices in microwave photonics. We report a silicon microring resonator with coupling modulation to realize microwave phase shift. With coupling tuning of the Mach-Zehnder interferometer (MZI) coupler to change the resonator from under-coupling to over-coupling, the device can realize a p phase shift on the incoming microwave signal with a frequency up to 25 GHz. The device can also realize 2.5p continuous phase tuning by manipulating the three DC bias voltages applied on the MZI coupler.
In this paper, we proposed and experimentally demonstrated a route-asymmetrical light transmission scheme based on the thermal radiative effect, which means that forward and backward propagations of an optical device have different transmittances provided they are not present simultaneously. Employing a fiber-chip-fiber optomechanical system, our scheme has successfully achieved a broad operation bandwidth of at least 24 nm and an ultra-high route-asymmetrical transmission ratio (RATR) up to 63 dB. The route-asymmetrical device has been demonstrated effectively with not only the continuous-wave (CW) light but also 10 Gbit/s on-off-keying (OOK) digital signals. Above mentioned unique features can be mostly attributed to the significant characteristics of the thermal radiative effect, which could cause a fiber displacement up to tens of microns. The powerful and significant thermal radiative effect opens up a new opportunity and method for route-asymmetrical light transmission. Moreover, this research may have important applications in all-optical systems, such as the optical limiters and ultra-low loss switches.
GaAs-based polarization modulators (PolMs) exhibit the unique characteristic of simultaneous intensity and complementary phase modulation owing to the linear electro-optic (LEO) effect determined by crystallographic orientations of the device. In this paper, we reviewed the principle of operation, the design and fabrication flows of a GaAs-based PolM. Analytical models are established, from which the features of a PolM are derived and discussed in detail. The recent advances in PolM-based multifunctional systems, in particular the PolM-based optoelectronic oscillator (OEO) are demonstrated with an emphasis on the remarkable development of applications for frequency conversion, tunable microwave photonic filter (MPF), optical frequency comb (OFC), arbitrary waveform generation (AWG) and beamforming. Challenges in practical implementation of the PolM-based systems and their promising future are discussed as well.
In this paper, the key technologies and research progress of chaotic optical communication are reviewed. We first discuss the chaos generation methods based on different nonlinear components. Then we focus on the frontiers of chaotic optical communications, including how to improve the security, and the development about the transmission capacity and distance of chaotic optical communication in laboratory and field. At last, we discuss limitations and potentials of chaotic optical communications and draw a conclusion.
Emerging applications based on optical beams carrying orbital angular momentum (OAM) will likely require photonic integrated devices and circuits for miniaturization, improved performance and enhanced functionality. This paper reviews the state-of-the art in the field of OAM of light, reports recent developments in silicon integrated OAM emitters, and discusses the applications potentials and challenges in silicon integrated OAM devices which can be used in future OAM based optical communications systems.
An in-band optical signal-to-noise ratio (OSNR) monitoring technique with high resolution and large measurement range is demonstrated based on low-bandwidth coherent receiver and a tunable laser. The measurement range of OSNR is from 10 to 25 dB and the resolution can be controlled about ±1 dB.