In-line fiber optic interferometers have attracted intensive attention for their potential sensing applications in refractive index, temperature, pressure and strain measurement, etc. Typical in-line fiber-optic interferometers are of two types: Fabry-Perot interferometers and core-cladding-mode interferometers. It’s known that the in-line fiber optic interferometers based on single-mode fibers can exhibit compact structures, easy fabrication and low cost. In this paper, we review two kinds of typical in-line fiber optic interferometers formed in single-mode fibers fabricated with different post-processing techniques. Also, some recently reported specific technologies for fabricating such fiber optic interferometers are presented.

A birefringent single-layer coating photonic crystal fiber biosensor based on surface plasmon resonance is proposed to realize high sensitivity, which is easy to implement, in that only gold is deposited externally. The birefringent nature of the structure provides the sensor with high sensitivity. The results show that the biosensor can obtain the wavelength sensitivity of 15180 nm/refractive index unit (RIU) and high linearity with the analyte RI range of 1.40–1.43, corresponding to the resolution of $5.6818\times {10}^{-6}\mathrm{RIU}$. Owing to the high sensitivity and simple structure, the proposed sensor can find important applications in biochemical and biological analyte detection.

A novel photonic crystal fiber (PCF) with an elliptical tellurite core is proposed to realize high birefringence and high nonlinearity simultaneously as well as low confinement loss at the wavelength of 1.55 μm. The guiding properties, such as the birefringence, the nonlinearity, and the confinement loss, have been investigated by using the full vectorial finite element method. The results show that the birefringence and the nonlinear coefficient can be up to $7.57\times {10}^{-2}$ and $188.39{\mathrm{W}}^{-1}{\mathrm{Km}}^{-1}$, respectively, and the confinement loss can be only ${10}^{-9}\mathrm{dB}/\mathrm{m}$. The proposed PCF can find potential applications in optical fiber sensing, polarization-maintaining transmission, and super-continuum generation.

Due to its ability to reconfigure the optical channel, intelligent reflecting surface (IRS) has revealed its great potential for enhancing the performance of visible light communication (VLC) systems. In this paper, we apply a mirror array-based IRS in typical multiple-input multiple-output based VLC (MIMO-VLC) systems and optimize the IRS configuration to minimize the system bit error rate (BER). By utilizing the tight analytical BER upper bound, a BER minimization problem is formulated subject to the IRS configuration constraints. To solve this problem, an optimization algorithm is proposed which decomposes the original problem into three sub-problems, and the complexity of the algorithm is analyzed. The superiority of the obtained optimal IRS configuration scheme over the baseline scheme in terms of BER performance has been successfully verified through simulations in an indoor IRS-aided $4\times 4$ MIMO-VLC system applying different MIMO techniques.

All-optical controlling provides burgeoning, precise and compact access in photonics and optoelectronics with tunable performance. Through tuning the physical parameters of optical-controlled devices, such as effective index, bandwidth and dispersion, the output characteristics of free-running laser can be manipulated for widespread applications in spectroscopy and laser biology. Among which the flexible optical-thermal tunability of graphene introduces an advantageous avenue for the enhancement of photosensitivity of optical-controlled devices to efficiently regulate laser properties. Here, we fabricate an optical-controllable fiber filter based on a graphene-assisted micro-fiber Bragg grating, whose wavelength can be linearly tuned by controlling pump light, and a tuning step of 3.1 pm is achieved. By cooperating it with a fiber laser operating at a Q-switched mode-locked state, the lasing wavelength is all-optically controlled with a linear tuning sensitivity of 23.5 pm/mW. Considering the optical controlling performances, such a wavelength-tunable fiber laser may find potential applications in all-optical computing, all-optical logic gating and generation of microwave source.

A polarization splitter based on dual-core photonic crystal fiber with ${{\rm Ge}_{20}}{{\rm Sb}_{15}}{{\rm Se}_{65}}$ glass is proposed to realize ultrashort length and high extinction ratio simultaneously at the common wavelength of 1.55 µm. The characteristics of the polarization splitter have been investigated by the finite element method. By tailoring the structure parameters on the performance of the splitter, the optimized parameters are obtained to analyze the birefringence, the coupling length, the normalized power transfer, and the extinction ratio. A comparison with other designs demonstrates the advantages of our polarization splitter, which can find potential applications in optical communication systems, optical fiber sensing, etc.