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We report on experimental studies of a SESAM mode-locked Er-doped fiber laser completely spliced from polarization-maintaining fiber components. With the fiber mode spot properly aligned to the SESAM, the laser operates harmonic mode-locking (HML) available in a whole range of pump powers up to ∼355mW with the supermode suppression level (SSL) less than 25 dB. Importantly, the HML laser generates the pulses in a linear polarization state and enables the pulse repetition rate (PRR) up to ∼1145 MHz. In our experiments we show that an optical injection of an external continuous wave (CW) into the laser cavity improves the stability of the laser operation in HML regime resulting in an increase of the SSL by 20–30 dB. Besides, the CW injection is able to expand the range of pump powers available with HML laser providing an increase of the maximal laser PRR up to ∼2195 MHz. Importantly, the implemented optical injection does not affect high purity of the laser polarization state. The presented numerical simulations enable qualitative explanations of the effects observed experimentally.

We have performed the experimental and numerical studies exploring dynamical processes in a harmonically mode-locked fiber laser responsible for birth and annihilation of solitons in the laser ring cavity. The injection of external continuous wave (CW) allows one-by-one change of the number of solitons in the laser cavity thus enabling the fine-tuning of the pulse repetition rate. We present new experimental observations of the laser transition dynamics associated with the changes of the soliton numbers and give clear insight into the possible physical mechanisms responsible for these effects.

Moving differential and dynamic window moving averaging are simple and well-known signal processing algorithms. However, the most common methods of obtaining sufficient signal-to-noise ratios in distributed acoustic sensing use expensive and precise equipment such as laser sources, photoreceivers, etc., and neural network postprocessing, which results in an unacceptable price of an acoustic monitoring system for potential customers. This paper presents the distributed fiber-optic acoustic sensors data processing and noise suppression techniques applied both to raw data (spatial and temporal amplitude distributions) and to spectra obtained after the Fourier transform. The performance of algorithms’ individual parts in processing distributed acoustic sensor’s data obtained in laboratory conditions for an optical fiber subjected to various dynamic impact events is studied. A comparative analysis of these parts’ efficiency was carried out, and for each type of impact event, the most beneficial combinations were identified. The feasibility of existing noise reduction techniques performance improvement is proposed and tested. Presented algorithms are undemanding for computation resources and provide the signal-to-noise ratio enhancement of up to 13.1 dB. Thus, they can be useful in areas requiring the distributed acoustic monitoring systems’ cost reduction as maintaining acceptable performance while allowing the use of cheaper hardware.

A multimode optical fiber supports excitation and propagation of a pure single optical mode, i.e., the field pattern that satisfies the boundary conditions and does not change along the fiber. When two counterpropagating pure optical modes are excited, they could interact through the stimulated Brillouin scattering (SBS) process. Here, we present a simple theoretical formalism describing SBS interaction between two individual optical modes selectively excited in an acoustically isotropic multimode optical fiber. Employing a weakly guiding step-index fiber approach, we have built an analytical expression for the spatial distribution of the sound field amplitude in the fiber core and explored the features of SBS gain spectra, describing the interaction between modes of different orders. In this way, we give a clear insight into the sound propagation effects accompanying SBS in multimode optical fibers, and demonstrate their specific contributions to the SBS gain spectrum.

This work studies the application of low-cost noise reduction algorithms for the data processing of distributed acoustic sensors (DAS). It presents an improvement of the previously described methodology using the activation function of neurons, which enhances the speed of data processing and the quality of event identification, as well as reducing spatial distortions. The possibility of using a cheaper radiation source in DAS setups is demonstrated. Optimal algorithms’ combinations are proposed for different types of the events recorded. The criterion for evaluating the effectiveness of algorithm performance was an increase in the signal-to-noise ratio (SNR). The finest effect achieved with a combination of algorithms provided an increase in SNR of 10.8 dB. The obtained results can significantly expand the application scope of DAS.

The spectrally flat supercontinuum generation in the wavelength range of 900–2400 nm is demonstrated in silica-based fibers of variable core diameter and dispersion. It is shown that, in comparison with standard optical fibers of the same length, supercontinuum spectra 200 nm wider can be realized in the samples under study. The significant difference between the spectral and temporal transformations of radiation depending on the direction of propagation is demonstrated in the researched fiber samples.

We report on new techniques enabling both the mitigation of supermode laser noise and highly precise setting of the pulse repetition rate (PRR) in a soliton harmonically mode locked (HML) fiber laser employing nonlinear polarization evolution (NPE). The principle of operation relies on resonant interaction between the soliton pulses and a narrow-band continuous wave (CW) component cooperatively generated within the same laser cavity. In contrast to our recent findings [Opt. Lett. 46, 5747 (2021) and Opt. Lett. 46, 5687 (2021)], the new methods are implemented through the specific adjustment of the HML laser cavity only and do not require the use of an external tunable CW laser source.