We present a novel method based on optical frequency-modulated continuous-wave interferometry that can realize simultaneous measurement of gas absorption spectra and optical path lengths (OPLs) in the widely used multipass cells (MPCs). This method involves a Fourier transform (FT) and an inverse FT, by which the gas absorption spectrum and the OPL are retrieved in frequency and time domains, respectively. Various OPLs are achieved by retrieving gas absorption spectra associated with different reflection positions in the MPC. As a demonstration, absorption spectra of acetylene around 1520 nm are measured using a commercial White type MPC, achieving a noise equivalent absorbance of 0.01 and a spatial resolution of 290 μm over 22.5 m OPL. Our proposed method shows advantages for MPC-based gas sensing applications in the significant relaxation of OPL calibration demands and flexible extension of measurement dynamic range.

Strong scattering random media can localize light and extend photon lifetime through multiple scattering, which offers opportunities for stabilizing random lasers. Here, we demonstrate a frequency stabilized Brillouin random laser with high coherence enabled by photon localization in random fiber grating array (RFGA). Photon trapping is realized due to wave interference in multi-scattering Fabry–Pérot (FP) cavities between random fiber gratings enabling light localization to prolong photon lifetime. The formation of the high finesse peaks of RFGA suppresses multi-longitudinal modes, which offers single-mode operation at high pump power. The RFGA distributed feedback-based Brillouin random fiber laser (BRFL) maintains a small frequency drift with the pump laser (a phase-locked laser with a linewidth of 100 Hz) at 51 kHz/s for a total change of 620 kHz over 12 s. Note there is no locking between the two lasers, and the beat frequency is measured by the optical heterodyne method. The correlation coefficient change of the measured optical beat frequency is maintained at 4.5%. This indicates that the BRFL is capable of maintaining a small optical frequency difference with the phase-locked pump laser over 12 s thanks to the RFGA capable of trapping photons in the same path, which is a remarkable feature for a random fiber laser. Furthermore, we confirm the single-mode lasing with a long lifetime in the stabilizing BRFL by the replica symmetry behavior and ultralow intensity noise at high pump power. Our findings explore a new approach to stabilize the frequency of Brillouin random lasers passively without commonly used active phase locking laser themes, which makes a simple and cost-effective system.

Distributed optical fiber strain sensing significantly increases the number of sensing points compared with FBG sensor, which makes it an excellent candidate for shape sensing. Theoretical analysis indicates that the spatial resolution of strain measurement is crucial to the performance of shape sensing, so a shape sensor based on differential pulse-width-pair Brillouin optical time-domain analysis (DPP-BOTDA) is proposed to improve the spatial resolution and shape sensing performance. The sensing fiber is attached on the both sides of a steel strip substrate, which enables the measurement of Brillouin frequency shifts (BFSs) of both the sides to suppress temperature crosstalk. In experiment, firstly, the dependence of BFS variation on the curvature of fiber is measured, the result of which agrees well with theory. Then the reconstruction of three shapes are demonstrated, the spatial resolution of which is 10 cm.

The distributed high temperature measurement of an optical fiber subjected to electric arc discharges based on optical frequency-domain reflectometry is experimentally demonstrated. The distributed temperature profile is attained in an open glow regime of a few milliamps with maximum detectable temperature up to 2100 ± 20 °C. The discharge arc-induced softened length of the fiber and mechanical stress are measured and statistically analyzed in terms of the correlation of the Rayleigh spectra. The large wavelength scanning range of OFDR enables much higher accuracy for the delay time measurement with a minimum measured delay of 40 fs. The delay shift over the entire heating range for a single discharge duration is statistically calculated by using a temporal correlation method. The reliability of the thermal sensitivity coefficient as 10 pm/°C for telecom single mode fiber (SMF, @1550 nm) is quantitatively analyzed and evaluated by the correlation coefficient. Lastly, a spectral mapping method is employed in spectrum monitoring for discharge dynamic impact on the optical path length (OPL) and local Rayleigh scatter.

The distributed temperature profile of hydrogen flame based on optical frequency-domain reflectometry (OFDR) was experimentally demonstrated for the first time. Spatial temperature field at different flow rate of H₂ flame was monitored by OFDR via a telecom fiber (Corning SMF-28, CPC6) inside the flame over seconds, and the highest temperature is on the sides of center flame separated by ∼1.4mm with difference of 140∼190°C over the flame dimension of 2.5 mm above 900°C. Uniformity level of temperature is studied by varying the distance between fiber and tube entrance, and the largest uniform region over 1-millimeter length of fiber is obtained. Rayleigh scatters correlation coefficient decreases with temperature to 90% around 400°C, further reduces to 70% about 800°C, and 50% roughly at 1000°C. It indicates that a nonlinear thermal sensitivity of SMF is expected for temperature higher than 400°C with OFDR measurement. The durability of single-mode fiber under H₂ flame is studied via decorrelation time at various temperature. It maintains 20s at 880°C with correlation coefficient around 68% and drops to 50% decorrelation at 1000°C over 20s. This information is important for high temperature measurement using telecom fiber over 800°C based on OFDR. A maximum temperature of 1100°C was measured by OFDR, and it is possible for higher temperature measurement beyond of 1100°C with quicker system response time (<1s).

Frequency fluctuation is a major problem in high-precision metrology as real-time optical frequency measurement is not available with commercial photodetectors; alternatively, frequency-stabilized lasers as a reference are also not accessible in most laboratories. In this study, we propose and demonstrate a polarization-maintaining random fiber grating ring (PM-RFGR) resonator in a PM Brillouin random fiber laser (BRFL) to achieve sub-MHz frequency drift, which is measured by the optical beat of the random laser and the pump laser. Experimental results show that longitudinal modes are suppressed in the BRFL owing to the feedback of the RFGR resonating with one longitudinal mode of the random laser. The BRFL shows mode-hopping-free operation over 14.9 s due to the self-adjustment of random modes with small frequency difference to thermal and acoustic variations and self-injection locking through RFGR. As a result, a small frequency drift of ∼340 kHz with single-longitudinal mode is achieved in the BRFL enabled by the RFGR, which offers an all optical locking mechanism for optical frequency stabilization.