Optical fiber sensors based on an interferometer structure play a significant role in monitoring physical, chemical, and biological parameters in natural environments. However, sensors with high-sensitivity measurement still present their own challenges. This paper deduces and summarizes the methods of sensitivity enhancement in interferometer based fiber optical sensors, including the derivation of the sensing principles, key characteristics, and recently-reported applications.The modal coupling interferometer is taken as an example to derive the five terms related to the sensitivity: (1) the wavelength-dependent difference of phase between two modes/arms $\partial {\varphi }_d/\partial \lambda$, (2) the sensor length $L_{w,A}$, (3) refractive index difference between two modes/arms $\mathsf{\Delta }{n}_{eff,A}$, (4) sensing parameter dependent length change $\alpha$, and (5) sensing parameter dependent refractive index change $\gamma$. The research papers in the literature that modulate these terms to enhance the sensing sensitivity are reviewed in the paper.

A bidirectional pumping Brillouin random fiber laser based on a non-uniform fiber is proposed and demonstrated for truly random number generation with enhanced bit rate. Numerical simulation indicates that, thanks to the unique distributed Brillouin gain profile along the non-uniform fiber, random lasing oscillation within a broadband Brillouin gain can be established by the mutual effect of Brillouin scattering and enhanced Rayleigh scattering, providing physical entropy source for truly random number generation. The poof-of-concept experiments demonstrate a truly random number with a bit rate of 71 Mbps, which significantly breaks through the limitation of the intrinsic Brillouin bandwidth (~10 MHz) in communication fibers.

We demonstrate an approach for high-sensitivity temperature and strain measurement in a dual-core As ₂ Se ₃ PMMA taper with an As ₂ Se ₃ core diameter of 2.5 μm. To improve the sensitivity of temperature and strain measurement, we designed the dual-core As ₂ Se ₃ -PMMA fiber with inorganic As ₂ Se ₃ cores and organic PMMA cladding and also the even and odd modes to build an interferometer. The variation of the difference between phases of the two modes with respect to wavelength (∂φd(λ)/∂λ) decreases with the As ₂ Se ₃ core diameter increasing, and consequently, thermally induced and straininduced change of the difference between phases of the two modes φd(λ) leads to a large wavelength shift indicating enhancement of the temperature and strain measurement sensitivity. Furthermore, thermally induced longitudinal and transverse forces on the As ₂ Se ₃ cores further enhance the temperature measurement sensitivity. High sensitivities of 436 pm/°C, -6.23 pm/με and 572 pm/°C, and -3.63 pm/με from the transmission spectra of axis-1 and axis-2 in the dual-core As ₂ Se ₃ -PMMA taper are obtained.

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.

Speckle is a persistent disease that damages image quality and needs to be suppressed for optical imaging. One effective method is to use light sources with low spatial coherence. In this article, a low spatial coherence and high brightness random laser based on TiN/Graphene self-assembly structures is proposed. The physical mechanisms of TiN/Graphene self-assembly structures enhancing the radiation properties of random lasers are analyzed in detail. Additionally, the speckle noise is significantly suppressed by the random laser in chaotic fluid environments. The speckle constant is as low as 0.04, making it difficult to observe with the naked eye. Due to its low speckle noise, the image quality illuminated by the random laser is more than 3 times better than that illuminated by a conventional laser. Furthermore, the random laser's high brightness and narrow spectral peaks enable the visualization of finer structures compared to using a halogen lamp. This study paves a new way for random lasers to be used in a variety of applications in chaotic fluid environments, such as speckle-free imaging, bioimaging, projection display, and free-space optical communication.

We propose and demonstrate an approach for 10 kHz to 34 MHz ultrasound detection based on a dual-core As₂Se₃-PMMA taper. We fabricate dual-core As₂Se₃-PMMA tapers with a Young’s modulus 24 times smaller than that of silica fibers, which enhances the acoustic response. Dual-core As₂Se₃-PMMA fiber tapers show high sensitivities to both shear and longitudinal waves due to the dual-core structure, low Young’s modulus, submicrometer dimension of the core, and the high-contrast interference pattern by the even and odd modes. A dual-core As₂Se₃-PMMA taper with a core diameter of 0.6 µm detects acoustic waves in the frequency range from 10 kHz to 34 MHz, which are excited by three piezoelectric transducers with optimal operating frequencies of 100 kHz, 3.65 MHz, and 6.8 MHz, respectively.