We propose a hybrid integrated self-injection locked (SIL) narrow linewidth laser with adjustable feedback intensity. An adjustable Sagnac loop reflector (ASLR) coupled to a high- Q micro-ring resonator is introduced to adjust and optimize the optical feedback intensity of the SIL laser. The experimental results show that the linewidth and tuning performance of the SIL laser are closely related to the optical feedback intensity. After optimizing the reflectivity of ASLR, more excellent frequency noise suppression capability and larger locking range can be obtained compared with the conventional SIL lasers with fixed feedback intensity. The intrinsic linewidth is reduced to 60 Hz, and the locking range without phase compensation is extended to 6.3 GHz. This work has important application value in the field of coherent detection and precision metrology.

Time-to-digital converter (TDC) is the key technology to realize accurate time delay measurement in high-precision optical fiber time-frequency transmission and synchronization, optical sensing and many scientific applications. The performance of FPGA-TDC based on the carry chain is sensitive to the operating temperature. This paper presents a parallel multichain cross segmentation method, without multitime measurements, which merges multichain into an equivalent chain, achieving low temperature coefficient and maintaining high precision. The equivalent chain breaks the limit of the intrinsic cell delay of a single carry chain, improves the precision and reduces the impact of temperature variation significantly. A two-channel TDC based on parallel multichain cross segmentation method is implemented in a 28 nm fabrication process Kintex-7 FPGA. The results show that the performance of TDC is improved with the increase of the number of chains. The 10-chain TDC with 1.3 ps resolution, 4.6 ps single-shot precision performs much better than the plain TDC with 11.4 ps resolution, 8.7 ps single-shot precision. The resolution is stable with 0.0002 ps/°C temperature coefficient under an operating temperature range from 25 °C to 70 °C. Moreover, the proposed method reduces the complexity of the circuit and the resource usage.

A frequency-stabilized narrow-linewidth external cavity diode laser (ECDL) at the wavelength of the R18 carbon dioxide (CO ₂ ) absorption line for space-borne CO ₂ detection LIDAR is reported. The ECDL module based on the fiber Bragg grating (FBG) is integrated in a compact butterfly package, emitting at the wavelength of 1572.02 nm and provides continuous tunability of 22 GHz. A polarization-maintaining all-fiber ring resonator is used as a transfer cavity to transfer the long-term stability of a metrological frequency reference at 1550.12 nm to the target wavelength. The combination of narrowing linewidth technique and frequency stability transfer technique enables lasing with an intrinsic Lorentzian linewidth of 15 kHz as well as a relative frequency stability on the order of $10^{-12}$ for averaging time from 1 to 1000 s (minimum of $2.1\times 10^{-12}$ at 2 s).

An absolutely consistent fiber-optic phase synchronization scheme based on fixed-phase-reference optical active compensation is proposed. By measuring the time interval of time pulse and the phase difference of frequency signal, and then using the optical delay line to compensate the phase change in real time according to a fixed phase reference, the phase period ambiguity and the phase-lock control initial randomness are eliminated, realizing a stable and repeatable fixed phase difference between different sites. Then the phase synchronization performances of stability and absolute consistence have been demonstrated through experiments in different scenarios. The phase difference fluctuation within 10000 s is only ±0.0006 rad, showing an ultra-high stability. Under the total of 5 different operations, including the signal source restart, system restart, adding 20 m additional fiber, adding 1 km additional fiber, and adding 2 km additional fiber, the maximum inconsistency of the mean phase difference is about 1% of one full cycle. The proposed and verified absolutely consistent phase synchronization method can well maintain the coherence in transferring frequency signal, having important application prospects in radar network and other time-frequency-phase synchronization-based coherent detection.

The low-earth-orbit (LEO) satellite communication has been proposed to support the next generation 6G space-airground integrated network owing to its large coverage area and seamless connectivity. To overcome the challenge of long-distance transmission for the free space optical (FSO) links, satellite communication systems often rely on high power transmissions to achieve high signal-to-noise ratios (SNRs) and high throughput, albeit at the expense of increased nonlinearity. In this paper, we propose a Q-learning inspired geometric shaping (QLI-GS) optimization scheme for amplitude phase shift keying (APSK) constellation to mitigate nonlinear effects and maximize the transmission distance. A pairwise optimization (PO) algorithm inspired by Q-learning is employed to achieve QLI-GS optimization for the APSK constellation according to an enhanced nonlinear interference noise model (ENINM) with effective memory mechanism, which ensures fast convergence and suppresses nonlinear impairments. Our results show the iteration time of the QLI-GS-APSK is reduced by approximately 99.89% compared to the generalized mutual information (GMI) maximized GS algorithm, and it also offers greater nonlinear tolerance than normally multi-ring APSK and traditional POAPSK. By leveraging a fiber-optical sampled bidirectional dualterminal FSO equivalent link, the QLI-GS-32APSK can experimentally achieve a data rate of 100 Gbit/s over a 230 km distance based on a single-wavelength system. A maximum distance of 580 km at a data rate of 60 Gbit/s using QLI-GS8APSK has been successfully demonstrated under 7% hard decision-forward error correction (HD-FEC) threshold of 3.8e-3, with a corresponding link budget of 62 dB. Additionally, the robustness of QLI-GS optimization under different atmospheric turbulence conditions has been further validated. We demonstrate single carrier 100Gbit/s QLI-GS-32APSK rate transmission over a 4.6 km outdoor FSO link with an Fried's coherence length of 11 cm, achieving 0.9 dB shaping gain over 32APSK. The results highlight the promising potential of the proposed QLI-GS-APSK scheme in addressing nonlinear challenges in satellite communications.

Ultra-stable lasers serve as the backbone for the most advanced optical metrology scientific experiments, enabling the ability to laser interferometry or atomic spectroscopy at the highest levels of precision, thanks to the rapid development of laser wavelength stabilization techniques based on optical or electrical feedback from an external reference such as absorption line or optical cavity. With the increasing interest in making these scientific systems portable and applying it outside of the laboratory, it still remains an open question as to how to realize a laser source that can reach the high wavelength stabilization and still remain sufficiently compact and portable for field use. Here, we describe a wavelength stabilization fiber laser with intracavity reference mechanism, by utilizing the beat note of dual modes inside the laser cavity as the laser wavelength drift reference signal. A laser wavelength drift less than 5 kHz and ADEV 10 ⁻¹³ level at integration time scales from 0.01s to 1000s is achieved, which is to-date the best results for the laser wavelength stabilization without the external frequency reference. The outcome of this work presents a new way to achieve an ultra-stable laser with a simpler, lower cost feature. It offers new insights to promote the highest precision optical metrology scientific application outside of the laboratory, and also gives the possibilities of applying the ultra-stable laser source into a wider range of industrial application.