A switchable multi-wavelength erbium-doped fiber laser (EDFL) based on parallel dual Lyot filters is experimentally demonstrated in this paper. A new type of parallel-structure fiber filter is designed and its spectrum is simulated. The structure is simple. The spectra of the filter changes when adjusting the polarization states of the polarization controllers (PCs). Due to the characteristics of the filter spectra and wavelength competition, the output wavelengths can be single wavelength, dual wavelengths and three wavelengths. When the output is single wavelength, the wavelength range is tunable with a tuning range of ~13 nm. The wavelength interval of dual wavelengths can be adjusted from 3.03 nm to 17.15 nm, which is a relatively wide range. The interval of three wavelengths also changes to some extent. Moreover, the optical signal-to-noise ratios (OSNRs) of the output wavelengths can up to 47 dB.

A novel scheme for liquid level detection using an all-fiber Mach–Zehnder interferometer, based on two up-tapers, is proposed and experimentally investigated. The effective refractive indices of the axisymmetric modes ${\mathrm{LP}}_{0n}$ are analyzed while it is partly immersed in liquid. The sensitivity of the sensor is greatly dependent on the fiber length between the two up-tapers, as well as the order of the cladding modes through the calculation. The experimental results show that the sensitivity of the Mach–Zehnder interferometer reaches a 22.5 nm wavelength shift per meter of level change with the middle fiber length of 14.3 cm. The proposal might be attractive since the up-tapered liquid level sensor could easily be fabricated and used for a large measuring range.

A novel individually switchable, widely tunable and interval-adjustable multi-wavelength erbium-doped fiber laser (EDFL) is experimentally demonstrated in this paper. It is based on the cascaded structure of SMF-TMF-SMF (STS) filter and Lyot filter with the assistance of nonlinear polarization rotation (NPR). The homemade STS filter is a comb filter that formed by splicing a section of two-mode fiber (TMF) and two segments of single-mode fiber (SMF) with core-offset structures. Compared with other multimode interferences, the spectrum of STS filter is more uniform. When inserted into the laser cavity, this comb filter functions as the wavelength selector with wavelength interval of ~0.8 nm. The insertion loss of the filter is only ~0.4 dB. By properly adjusting the polarization controllers (PCs) in the laser cavity, individually switchable wavelengths with different flexible lasing states can be obtained, such as tunable adjacent single-, dual-, triple-, quadruple- and quintuple-wavelength outputs. The tuning range can up to 30 nm, from 1530 nm to 1560 nm. When the number of output wavelengths is 1~3, the tuning process can keep the step size of 0.8 nm or 1.6 nm. The non-adjacent dual- and triple-wavelength outputs, as well as adjacent sextuple- and septuple-wavelength outputs can also be achieved. The output characteristics, such as flexible switchability and tunability, are difficult to implement by other methods.

A photonic approach to generate triangular waveform signals is proposed and analyzed. With active bias control, two sub-MZMs (MZ-a and MZ-b) of a dual-parallel Mach–Zehnder modulator (DP-MZM) operate at minimum transmission point, leaving the main MZM (MZ-c) at quadrature transmission point. Triangular waveform can be observed by a parameter setting of modulation index. The proposal is first analyzed and then validated by simulation. The key significance of the scheme is that it is capable of generating triangular waveform signals via a sinusoid local oscillator.

A fiber extrinsic Fabry-Pérot interferometer (EFPI) assisted with iron V-groove for temperature measurement is proposed and investigated by means of both numerical simulation and experiment for the first time to our best knowledge. The main temperature sensing component is acted by the iron V-groove whose coefficient of linear thermal expansion (CLTE) is much higher than that of the silica glass. Two fibers are stuck to the V-groove with two glued points, respectively. Maximum sensitivity of 260.7 nm/°C, which is the highest value for a fiber interferometric sensor up to now, has been achieved experimentally. It is worth noting that the temperature sensitivity of this sensor can be improved limitlessly via implementing a smaller gap size of the EFPI, longer distance between the two glued points or material with higher CLTE of the V-groove, theoretically.

An optical channel distortion equalization method based on silicon photonic reservoir computing (PhRC) structure with particle swarm optimization (PSO) algorithm is proposed. The weights training scheme of the photonic readout layer of PhRC is trained by PSO algorithm towards a lower bit error rate (BER) and a better eye diagram, simultaneously. The self-organizing evolutionary PSO algorithm enables fast convergence and iterative optimization for training optical weights. Without the necessary the reservoir signal states obtained from the monitors, it eliminates the noise introduced by the optical monitors. We implement a system-level simulation, including RC structure, training algorithm, and fiber communication construction. Because of the excellent performance of the PSO training algorithm, a BER of 9.15 × 10 ⁻⁵ is obtained with the 25 Gb/s on-off keying (OOK) input signal in the case of 25 km single-mode transmission. This BER result is three orders of magnitude lower than that before the equalization. This equalizer mitigates the processing bandwidth limit and provides a distorted signals equalization method in the optical domain with simple and effective training strategy. This method shows much potential in high speed optical communication channel equalization in the future.