We demonstrated a fiber optic distributed acoustic sensor based on a double Sagnac interferometer, using two wavelengths separated by CWDM modules. A mathematical model of signal formation principle, based on a shift in two signals analysis, was described and substantiated mathematically. The dependence of the sensor sensitivity on a disturbance coordinate and frequency was found and simulated, and helped determine a low sensitivity zone length and provided sensor scheme optimization. A data processing algorithm without filtering, appropriate even in case of a high system noise level, was described. An experimental study of the distributed fiber optic sensor based on a Sagnac interferometer with two wavelengths divided countering loops was carried out. An accuracy of 24 m was achieved for 25.4 km SMF sensing fiber without phase unwrapping.

In this paper, we propose a computational approach for description of radiation transfer in a quasi-ordered medium and study the impact of scattering on electromagnetic wave propagation and image formation. It combines finite-difference time-domain method, Monte Carlo simulations, and radiative transfer theory. Using as an example terahertz imaging, we analyze modulation transfer function (MTF) of imaging system operated at 0.25 THz for scattering material layers placed between the object and the imaging plane. We experimentally study imaging of bar-pattern test-objects through a quasi-ordered scattering medium. Both numerical and experimental results are in good agreement and demonstrate an impact of quasi-ordered scatterers on quality of THz images, i.e. particular combination of the electromagnetic wavelength and parameters of scattering materials could enhance MTF compared with ones with random particle structures.

In this review paper, we describe the current state of the art to stabilize the output radiation of ultrashort-pulse (USP) fiber lasers and analyze passive methods to reduce the magnitude of fluctuations in the amplitude–frequency noise of output radiation. Regarding main noise characterization in mode-locked fiber lasers, we further consider the influence on laser operation of primary generation regimes starting up in cavities, such as solitons, stretched pulses, similaritons, and dissipative solitons. Then, we proceed to analyze the external and internal factors that affect the stability of the output radiation characteristics depending on the mode-locking mechanism and the resonator scheme.

We have studied the ultrafast saturation behavior of a high-density well-aligned single-walled carbon nanotubes saturable absorber (HDWA-SWCNT SA), obtained by a high-pressure and high-temperature treatment of commercially available single-wall carbon nanotubes (SWCNTs) and related it to femtosecond erbium-doped fiber laser performance. We have observed the polarization dependence of a nonlinear optical saturation, along with a low saturation energy level of <1 fJ, limited to the detector threshold used, and the ultrafast response time of <250 fs, while the modulation depth was approximately 12%. We have obtained the generation of ultrashort stretched pulses with a low mode-locking launching threshold of ~100 mW and an average output power of 12.5 mW in an erbium-doped ring laser with the hybrid mode-locking of a VDVA-SWNT SA in combination with the effects of nonlinear polarization evolution. Dechirped pulses with a duration of 180 fs were generated, with a repetition rate of about 42.22 MHz. The average output power standard deviation was about 0.06% RMS during 3 h of measurement.

An Er-doped all-fiber ultrashort pulse laser with positive total net-cavity group-velocity dispersion is demonstrated based on a hybrid mode-locking mechanism ensured by single-walled carbon–boron–nitrogen nanotubes with coaction of the nonlinear polarization evolution effect. The generation regime with a similariton-like spectrum is obtained. The spectrum width is ~31.5 nm, and the minimal pulse duration is ~294 fs at full width at half maximum. The average output power is ~3.2 mW, corresponding to 0.376 nJ pulse energy and 1.25 kW peak power. The fundamental pulse repetition rate is ~8.5 MHz, with a signal-to-noise ratio of 60 dB. The standard deviation of average output optical power stability, measured for 12 h, is about ~1% RMS, and the maximum level of relative intensity noise (RIN) does not exceed <−120 dBc/Hz in the 30 Hz–1 MHz frequency range. To prove the similariton-like regime generation, we also studied numerically and experimentally the pulse evolution during propagation through a laser resonator and output single-mode fiber with anomalous dispersion.

In this article, we report on experimental studies of the influence of several laser radiation parameters, such as the duration of the laser pulse, the radiation wavelength, and the pulse energy, on the efficiency of the destruction of urinary calculi. The study used a laser lithotripter based on a fiber Tm laser generating at a wavelength of 1940 nm with pulses with a duration of about 1800 μs and pulse energy of up to 6 J, as well as a femtosecond solid-state Yb laser generating at a wavelength of 1032 nm with a pulse duration of about 250 fs and pulse energy of up to 400 μJ. A comparative analysis was carried out according to such criteria as the productivity of lasers when removing a unit mass of images and the amount of sample displacement resulting from the retropulsion effect. The results obtained in this work demonstrated that the femtosecond laser loses approximately two times its efficiency in terms of sample material removal. However, this shows the absolute advantage of the photoionization mechanism of femtosecond laser ablation in the study of retropulsion and thermal heating, which were completely absent in this case.