A novel fiber Michelson interferometer (FMI) based on parallel dual polarization maintaining fiber Sagnac interferometers (PMF-SIs) is proposed and experimentally demonstrated for temperature sensing. The free spectral range (FSR) difference of dual PMF-SIs determines the FSR of envelope and sensitivity of the sensor. The temperature sensitivity of parallel dual PMF-SIs is greatly enhanced by the Vernier effect. Experimental results show that the temperature sensitivity of the proposed sensor is improved from −1.646 nm/°C (single PMF-SI) to 78.984 nm/°C (parallel dual PMF-SIs), with a magnification factor of 47.99, and the temperature resolution is improved from ±0.03037°C to ±0.00063°C by optimizing the FSR difference between the two PMF-SIs. Our proposed ultrasensitive temperature sensor is with easy fabrication, low cost and simple configuration which can be implemented for various real applications that need high precision temperature measurement.

Light detection and ranging (LiDAR), as a hot imaging technology in both industry and academia, has undergone rapid innovation and evolution. The current mainstream direction is towards system miniaturization and integration. There are many metrics that can be used to evaluate the performance of a LiDAR system, such as lateral resolution, ranging accuracy, stability, size, and price. Until recently, with the continuous enrichment of LiDAR application scenarios, the pursuit of imaging speed has attracted tremendous research interest. Particularly, for autonomous vehicles running on motorways or industrial automation applications, the imaging speed of LiDAR systems is a critical bottleneck. In this review, we will focus on discussing the upper speed limit of the LiDAR system. Based on the working mechanism, the limitation of optical parts on the maximum imaging speed is analyzed. The beam scanner has the greatest impact on imaging speed. We provide the working principle of current popular beam scanners used in LiDAR systems and summarize the main constraints on the scanning speed. Especially, we highlight the spectral scanning LiDAR as a new paradigm of ultrafast LiDAR. Additionally, to further improve the imaging speed, we then review the parallel detection methods, which include multiple-detector schemes and multiplexing technologies. Furthermore, we summarize the LiDAR systems with the fastest point acquisition rate reported nowadays. In the outlook, we address the current technical challenges for ultrafast LiDAR systems from different aspects and give a brief analysis of the feasibility of different approaches.

A highly-sensitive temperature sensor with controllable sensitivity based on Vernier effect by cascading a tunable extrinsic Fabry-Perot interferometer (FPI) and a fixed reflective Lyot filter (RLF) is theoretically investigated and experimentally demonstrated. The temperature sensitivity can be tuned by modulating the cavity length of the extrinsic FPI and online monitoring the envelope of superimposed spectrum with optical spectrum analyzer (OSA). The FPI works as the reference arm to tune the temperature sensitivity of the sensing system, while the RLF with 1-meter polarization maintaining fiber (PMF) acts as the sensing probe. Experimental results prove that by changing the cavity length of the FPI, the sensitivities of −3.82 nm/°C, −8.33 nm/°C and −14.63 nm/°C can be achieved. Compared with the single sensing element, the sensitivities are magnified by 3.78, 8.25 and 14.49 times. The proposed temperature sensor is feasible to be applied practically in scenarios which require different temperature sensitivities in demanded temperature detection ranges.

An optical fiber coupler is a simple and fundamental component for fiber optic technologies that works by reducing the fiber diameter to hundred nanometers or several micrometers. The microfiber coupler (MFC) has regained interest in optical fiber sensing in recent years. The subwavelength diameter rationales vast refractive index (RI) contrast between microfiber “core” and surrounding “cladding”, a large portion of energy transmits in the form of an evanescent wave over the fiber surface that determines the MFC ultrasensitive to local environmental changes. Consequently, MFC has the potential to develop as a sensor. With the merits of easy fabrication, low cost and compact size, numerous researches have been carried out on different microfiber coupler configurations for various sensing applications, such as refractive index (RI), temperature, humidity, magnetic field, gas, biomolecule, and so on. In this manuscript, the fabrication and operation principle of an MFC are elaborated and recent advances of MFC-based sensors for scientific and technological applications are comprehensively reviewed.

High repetition rate ultrafast fiber lasers are essential for both scientific research and industrial applications. Here, we report the first, to the best of our knowledge, all-fiber ring laser with a fundamental repetition rate exceeding 1 GHz, by integrating all necessary functions into one single device with commercial active fiber pigtails. Low-threshold self-starting mode-locking is achieved by using a carbon nanotube (CNT) saturable absorber that directly deposits on the Er-doped fiber connector. The laser operates at 1562 nm, delivering 682 fs pulses with 1.028 GHz fundamental repetition rate with good stability, low intensity/phase noise, and high signal-to-noise ratio. The proposed compact source is promising for high repetition rate applications and provides a reference for fiber lasers with different mode-locking mechanisms or in various wavelength ranges. We also provide in-depth discussions on the key challenges and future directions for fiber lasers mode-locking at GHz rates using real saturable absorbers.

We present for the first time, to the best of our knowledge, the pump-power-controlled, all-polarization-maintaining (all-PM), all-fiber configured, wavelength-tunable mode-locked fiber laser in the L-band (1565 to 1625 nm). A tuning range over 20 nm (1568.2 to 1588.9 nm) is attained simply by varying the pump power between 45 and 115 mW. Our work represents the first demonstration of wavelength tuning in all-PM configured nonlinear polarization evolution (NPE) lasers. The non-mechanical and electrically controllable tuning method offers ease of use and cost efficiency within an advanced all-PM, all-fiber design, indicating promising adaptability to diverse wavelength bands.