Jin-Long Xiao

Thin-film lithium niobate electro-optic modulators exhibit outstanding advantages such as large bandwidth, low insertion loss, and low half-wave voltage, demonstrating broad application prospects. However, due to internal defects in lithium niobate crystals, modulators exhibit electro-optic relaxation phenomena, with the relaxation time of thin-film structures being reduced by more than two orders of magnitude compared to bulk materials. In this study, we fitted and simulated the electro-optic relaxation behavior of thin-film lithium niobate modulators based on RC circuit model, effectively explaining their time-domain response characteristics under low-frequency conditions. By comparing thin-film modulators with and without silica cladding structures, the fitting results indicate that the relaxation time of modulators with cladding is approximately 11.9 ms, showing positive DC drift, whereas the relaxation time of modulators without cladding is significantly shortened to about 88.6 μs and exhibits negative DC drift. Additionally, the enhancement of optical intensity alters the photoconductivity of the material, thereby affecting its low-frequency electro-optic response behavior. This research provides important ideas for the design and optimization of next-generation integrated lithium niobate photonic modulators with high stability and controllability.

Using parallel chaotic sources for simultaneous multi-channel random bit generation is an effective approach to increase the generation rate. We propose utilizing multiple longitudinal modes of a self-chaotic semiconductor microcavity laser as parallel chaotic sources to generate random bits, and found that retaining the least significant bits (LSBs)—a post-processing step typically required for random bit generation from chaotic semiconductor lasers—can dramatically reduce the correlation between different longitudinal modes. As long as no significant correlation remains between the longitudinal modes after retaining the LSBs, they can be used as parallel chaotic sources for random bit generation. In the case of 8-bit raw data, the correlation is generally eliminated when 5 or fewer LSBs are retained. Furthermore, without additional post-processing beyond LSB retention, the resulting random bit stream can typically retain up to 5 LSBs while still passing the statistical tests specified in the National Institute of Standards and Technology Special Publication 800-22. This implies that the correlation requirements for parallel chaotic sources can be significantly relaxed. We collected signals generated by three longitudinal modes of a self-chaotic laser at a sampling rate of 10 GSa/s, achieving a random bit generation rate of 130 Gb/s. The use of a solitary multi-longitudinal-mode self-chaotic microcavity laser not only simplifies the system but also further enhances the rate of random bit generation.

Stabilized narrow-linewidth laser sources and soliton microcombs based on self-injection locking have driven revolutionary advancements from metrological to astronomical applications. Recently, coupled cavities have shown critical significance in optical frequency comb generation, particularly in achieving high conversion efficiency and enabling the formation of ultra-broadband soliton microcombs. In this work, we have developed a methodology for investigating self-injection locking in a laser diode-coupled-cavity system with Rayleigh backscattering. We have derived a set of rate equations to describe the system, obtained the analytical expression for effective detuning, and analyzed the critical coupling conditions and locking range for the coupled cavities. The analytical and numerical simulation results demonstrate that when the coupled-cavity structures induce mode splitting, the self-injection locking system exhibits dual-mode locking behavior. Furthermore, the auxiliary cavity introduces additional phase and losses to the coupled cavity, and the physical explanation for resonance splitting arises from the phase-balancing effect of pump detuning on the additional phase. On the other hand, the losses introduced by the auxiliary cavity can be effectively compensated by tuning the coupling strength between the waveguide and the main cavity. Therefore, we have also derived an analytical expression for the critical coupling coefficient in such laser diode-coupled-cavity systems. The proposed method can be extended to investigate self-injection locking in multi-coupled-cavity systems and provides a theoretical foundation for the realization of stabilized multi-frequency narrow-linewidth laser sources and high-efficiency, broadband soliton microcombs.

Chaos correlation optical time domain reflectometry (OTDR), which has the advantage of no trade-off between resolution and dynamic range compared with conventional OTDR, can be used for fiber fault detection. This paper proposes a bit-quantization method for OTDR by quantizing the time-domain signal collected by the oscilloscope into a bit-signal amplitude. The experimental results show that the spatial resolution of quantized data of different bits is not degraded, and the dynamic range is only slightly reduced by comparing the cross-correlation functions (CCF) calculated based on experimental data and bit data. The OTDR spatial resolution of 1-bit data is 3 cm, and the dynamic range is reduced by less than 1 dB compared to conventional OTDR. It theoretically validates the feasibility of using simple sampling devices such as 1-bit data samplers for OTDR in the future without significantly degrading OTDR performance.

We experimentally demonstrate spontaneous period-one (P1) oscillation in a solitary semiconductor microlaser. By engineering degenerate-mode splitting in a deformed square microcavity with two circular vertices, two near-degenerate modes with opposite symmetries can interact nonlinearly via the carrier effect. When the mode-frequency interval is tuned close to the intrinsic relaxation-oscillation frequency, the internal-mode interaction gives rise to spontaneous P1 oscillation. In a deformed square microcavity laser with a side length of 26 μm and a circular vertex radius of 12 μm, the P1 oscillation yields comb-like optical spectra featuring approximately 10 lasing lines. Microwave signals corresponding to the oscillation frequency can be generated by optical heterodyne detection or directly extracted from the electrode of the microlaser. By adjusting the injection current of the laser, the microwave frequency is tunable in the ranges from 6.62 to 7.66 GHz and from 9.24 to 10.06 GHz. Our findings offer a new, to the best of our knowledge, way to generate stable periodic oscillations in a compact-size solitary microlaser.

We propose and experimentally demonstrate a coupled twin-waveguide semiconductor laser featuring high-order slotted surface gratings to achieve a variety of nonlinear dynamics. The surface gratings serve as active distributed Bragg reflectors for selecting the lasing modes, while the nonlinear dynamics arise from mode coupling and carrier diffusion between the two identical waveguides. The coupling coefficient is designed to obtain coupled modes with a splitting frequency exceeding ten gigahertz, slightly higher than the relaxation oscillation frequency, thereby enhancing the nonlinear interaction between the modes due to the carrier density oscillation. By adjusting the mode frequency difference and the relaxation oscillation frequency through injection currents, we successfully achieve various controllable dynamic states, including periodic, quasiperiodic, and chaotic oscillations. A chaotic optical signal with inherent randomness is generated, reaching a maximum effective chaotic bandwidth exceeding 11 GHz with a flatness of 8.4 dB. Our approach offers a novel light source that can be fabricated using photolithography and regrowth-free processes, paving the way for mode engineering aimed at tailoring lasing actions in semiconductor lasers to generate nonlinear dynamics on-chip.

A coherent optical frequency comb (OFC) with suppressed linewidth is demonstrated based on the spectral broadening of a directly modulated semiconductor microcavity laser with self-injection locking. Due to the high electro-optical response, the directly modulated microcavity laser provides a 10-GHz-spaced seeding OFC with 9 comb teeth in a 10 dB flatness window. Besides, an optical feedback fiber loop with two sub-loops is introduced to reduce the linewidth of the microlaser and suppress the undesired longitudinal modes. Self-injection locking is realized at the feedback power of 72 µW, and the laser linewidth is narrowed from 10 MHz to 14.5 kHz. Moreover, spectral broadening and pulse compression are used to broaden the optical spectrum and compress the pulse width, which results in an OFC with 36 comb lines in a 10 dB power deviation and a 1.8 ps optical pulse. Owing to the high coherence of the directly modulated OFC, all comb lines feature narrow linewidths in the range of 14.5∼18.2 kHz.

Unidirectional emission, one of the key technologies to achieve large-scale photonic integration, plays an important role in optical antennas, high-performance grating couplers, and energy-efficient lasers. At the same time, high chirality has demonstrated its potential application in highly sensitive optical sensors and coherent light sources with miniaturized sizes. Here, we designed a deformed circular-sided pentagonal microresonator (CPM) with low refractive index material for the combination of unidirectional light emission and high chirality. Two typical materials of polymer and silica are studied. For the polymer CPM, unidirectional emission with a divergence angle as small as 3° is predicted by constructing a notch at the vertex, and the chirality is 0.831. For the silica CPM, a divergence angle for unidirectional emission down to 10° and chirality as high as 0.9623 is predicted by introducing a channeling waveguide. The far-field emission pattern and chirality change drastically upon the attachment of a nanoparticle, predicting an ideal platform for a low-cost, portable, and highly sensitive nanoparticle detector.

A method for achieving the single-mode and efficient unidirectional emission of a whispering gallery mode (WGM) semiconductor laser is presented herein. Hybrid square-rectangular lasers (HSRLs) and hybrid square/rhombus-rectangular lasers (HSRRLs) consisting of a Fabry–Pérot (FP) cavity and a square or rhombus cavity microcavity are described. In addition, a transmitter optical subassembly (TOSA) based on an HSRRL chip was fabricated, which has a wide and continuous wavelength tuning range. Wavelength channels from 1555.75 nm to 1568.15 nm with a spacing of 50 GHz were demonstrated with a good side mode suppression ratio (SMSR) and good output power. These devices have the potential to meet the typical requirements of optical communication networks.

A Brillouin-assisted 80-GHz-spaced dual-comb source with a reconfigurable repetition frequency difference ranging from 48 MHz to 1.486 GHz is demonstrated. Two pairs of dual-pump seeds with an interval offset produce the corresponding dual Brillouin lasers in two fiber loops, and then the Brillouin lasers give rise to dual combs via the cavity-enhanced cascaded four-wave mixing effect. The repetition frequency difference is determined by the interval offset of the dual-pump seeds, which is induced by the Brillouin frequency shift difference between different fibers in a frequency shifter. Each comb provides 22 lasing lines, and the central 10 lines in a 20-dB power deviation feature high optical signal-to-noise ratios exceeding 50 dB. The linewidths of the dual-comb beating signals are less than 300 Hz, and the absolute linewidths of the comb lines are around 1.5 kHz. The dual-comb source enables substantial repetition frequency differences from 48 MHz to 1.486 GHz by changing the pluggable fibers in the frequency shifter.