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.

The demand for greenhouse gas measurement has increased dramatically due to global warming. A 1.57-μm airborne double-pulse integrated-path differential absorption (IPDA) light detection and ranging (LIDAR) system for CO₂ concentration measurement was developed. The airborne field experiments of this IPDA LIDAR system were conducted at a flight altitude of approximately 7 km, and the weak echo signal of the ocean area was successfully received. The matched filter algorithm was applied to the retrieval of the weak signals, and the pulse integration method was used to improve the signal-to-noise ratio. The inversion results of the CO₂ column-averaged dry-air mixing ratio (XCO₂) by the scheme of averaging after log (AVD) and the scheme of averaging signals before log were compared. The AVD method was found more effective for the experiment. The long-term correlation between the changing trends of XCO₂ retrieved by the IPDA LIDAR system and CO₂ dry-air volume mixing ratio measured by the in-situ instrument reached 92%. In the steady stage of the open area (30 km away from the coast), which is almost unaffected by the residential areas, the mean value of XCO₂ retrieved by the IPDA LIDAR system was 414.69 ppm, with the standard deviation being 1.02 ppm. Compared with the CO₂ concentration measured by the in-situ instrument in the same period, bias was 1.30 ppm. The flight path passed across the ocean, residential, and mountainous areas, with the mean value of XCO₂ of the three areas being 419.35, 429.29, and 422.52 ppm, respectively. The gradient of the residential and ocean areas was 9.94 ppm, with that of the residential and mountainous areas being 6.77 ppm. Obvious gradients were found in different regions.

The spaceborne double-pulse integrated-path differential absorption (IPDA) light detection and ranging (LIDAR) system was found to be helpful in observing atmospheric CO₂ and understanding the carbon cycle. The airborne experiments of a scale prototype of China’s planned spaceborne IPDA LIDAR was implemented in 2019. A problem with data inversion caused by the detector module nonlinearity was found. Through many experiments, the amplifier circuit board (ACB) of the detector module was proved to be the main factor causing the nonlinearity. Through amplifier circuit optimization, the original bandwidth of the ACB was changed to 1 MHz by using a fifth-order active filter. Compared with the original version, the linearity of optimized ACB is improved from 42.6% to 0.0747%. The optimized ACB was produced and its linearity was verified by experiments. In addition, the output waveform of the optimized ACB changes significantly, which will affect the random error (RE) of the optimized IPDA LIDAR system. Through the performance simulation, the RE of more than 90% of the global area is less than 0.728 ppm. Finally, the transfer model of the detector module was given, which will be helpful for the further optimization of the CO₂ column-averaged dry-air mixing ratio (XCO₂) inversion algorithm.

We demonstrate a compact fiber optic accelerometer (FOA) utilizing a push-pull spring-mass structure with a size of 25 mm × 25 mm × 25 mm, showcasing the excellent performance of strong sensitivity, minimal self-noise, and wide dynamic range. The presented sensing probe is a Michelson interferometer (MI) that employs the 3 × 3 coupler demodulation method to extract the information of vibration. Both arms of the MI are fiber coils wrapped around a movable push-pull folding spring structure, serving as the sensing arm for detection, which results in twice the sensitivity compared to a single-sensing arm accelerometer. The theoretical design and finite element analysis are validated experimentally, confirming that the proposed FOA demonstrates a mean sensitivity of 48.73 dB re rad/g over the frequency band of 5 Hz-1 kHz, closely matching the simulated results. It also achieves a large dynamic of 161.12 dB at 50 Hz. Noise evaluation experiment results reveal a low average noise level of 15.85 ng/Hz1/2 of the proposed sensor, indicating that it is one of the fiber optic interferometric accelerometers with the lowest noise reported to date.

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.

We demonstrate a fiber-optic micro-electron-mechanical systems (MEMS) accelerometer based on a symmetric push-pull spring structure made by micromachining on a silicon substrate, which combines the advantages of high sensitivity, low self-noise, and large operation bandwidth compared with existing fiber-optic MEMS accelerometers. A balanced Michelson interferometer based on a 3×3 fiber coupler is employed as a displacement transduction system, and an improved elliptic fitting algorithm (EFA) is proposed for phase demodulation within this system, addressing the limitations of the traditional EFA under small-signal condition. Experimental results validate the theoretical design and finite-element analysis, demonstrating that the proposed sensor achieves an average sensitivity of 48.17 dB re rad/g within its operational bandwidth of 5–80 Hz and a resonant frequency of 139 Hz, both of which closely align with the simulated results. The sensor's transverse crosstalk is less than 1.53%. The results of the self-noise evaluation experiment verify that the proposed sensor attains a low average noise of 40 ng/Hz1/2, proving that it has nano-g resolution in acceleration detection applications.