Ivan A. Lobach

Broad-range self-sweeping of a narrow-line self-pulsing Yb-doped fiber laser

The effect of broad-range (16 nm) self-sweeping of a narrow-line (less than 1 pm) Yb-doped fiber laser has been demonstrated experimentally. It is found that the effect arises from the self-sustained relaxation oscillations. As a result, the sweeping rate increases as square root of the laser power and decreases with increasing cavity length. Based on these results we propose a model describing dynamics of the laser frequency. The model takes into account the effects of gain saturation at the laser transition and spatial hole burning in the self-pulsing regime.

Application of a Self-Sweeping Yb-Doped Fiber Laser for High-Resolution Characterization of Phase-Shifted FBGs

Application of a self-sweeping self-pulsing laser for high-resolution spectral analysis of π-shifted FBGs has been demonstrated for the first time. It is shown that the self-sweeping laser generates nearly single-frequency radiation during a single pulse. This fact allows us to characterize the shift-induced dip of 14 MHz width in the FBG reflection spectrum. Nonlinear deformation of the measured ultranarrow spectral structure with increasing incident power is observed. Sampling interval during the scan is shown to be limited by pulse to pulse frequency shift, which is equal to the cavity mode spacing of 6 MHz. The main feature of the applied technique is its simplicity due to the absence of special elements for frequency scanning.

Narrowband random lasing in a Bismuth-doped active fiber.

Random fiber lasers operating via the Rayleigh scattering (RS) feedback attract now a great deal of attention as they generate a high-quality unidirectional laser beam with the efficiency and performance comparable and even exceeding those of fiber lasers with conventional cavities. Similar to other random lasers, both amplification and random scattering are distributed here along the laser medium being usually represented by a kilometers-long passive fiber with Raman gain. However, it is hardly possible to utilize normal gain in conventional active fibers as they are usually short and RS is negligible. Here we report on the first demonstration of the RS-based random lasing in an active fiber. This became possible due to the implementation of a new Bi-doped fiber with an increased amplification length and RS coefficient. The realized Bi-fiber random laser generates in a specific spectral region (1.42 μm) exhibiting unique features, in particular, a much narrower linewidth than that in conventional cavity of the same length, in agreement with the developed theory. Lasers of this type have a great potential for applications as Bi-doped fibers with different host compositions enable laser operation in an extremely broad range of wavelengths, 1.15–1.78 μm.

Mechanism of mode coupling in multicore fiber lasers

We present what is believed to be the first experimental demonstration of a new mechanism of mode coupling in multicore fibers (MCFs) based on their indirect interaction inside the fiber via intermediate mode, analogous to the Bragg mode, which is very sensitive to bending of the fiber. Very strong coupling between the core modes regardless of large spacing (approximately 28 microm) between them has been demonstrated in the MCF laser as well as in the probe beam schemes. 70% of power conversion from one core to another with beating length of tens of centimeters in 4-core MCF is measured.

Femtosecond Laser Inscription of Long-Period Fiber Gratings in a Polarization-Maintaining Fiber

The method of femtosecond laser inscription by slit-apertured beam is applied for fabrication of long-period fiber gratings in polarization-maintaining Panda-type fiber. Polarization properties as well as ambient refractive index sensitivity are investigated for the inscribed gratings. It is shown that the method allows us to create high-quality element for the use as spectral filter of high-power fiber laser as well as high-temperature sensor.

Single-frequency Bismuth-doped fiber laser with quasi-continuous self-sweeping

Generation of regular pulses of linearly polarized radiation with periodic self-induced laser line sweeping by ~10 nm near central wavelength of ~1460 nm has been demonstrated for the first time in an all-fiber Bismuth laser without any tuning element. It has been shown that the radiation of each pulse is single-frequency, and the pulse-to-pulse frequency shift is as low as 1 MHz corresponding to one intermode interval in 100-m long laser cavity. The measured intra-pulse frequency chirp is below 1 MHz while the pulses are long (~10 μs) and overlapping. Thus the sweeping is nearly continuous in frequency and time domains.

Dynamics of ultra-long Brillouin fiber laser

We report on experimental studies of random lasing realized in optical fibers with the use of Brillouin amplification and Rayleigh backscattering employed as a distributed feedback instead of a cavity mirror. In our experiment 25-km-long high quality standard telecom single-mode fiber was employed for Rayleigh reflection uniformly distributed over all fiber length. We have observed a clear competition between a classical Brillouin scattering and Brillouin lasing. Presence of extended fluctuation-free fragments in the recorded oscilloscope traces highlights Stokes power statistics typical for laser radiation rather than for Brillouin process. The results of the experiments are in a perfect agreement with the model of Brillouin - Rayleigh cooperative process in long fibers.

Real-time high-resolution heterodyne-based measurements of spectral dynamics in fibre lasers.

Conventional tools for measurement of laser spectra (e.g. optical spectrum analysers) capture data averaged over a considerable time period. However, the generation spectrum of many laser types may involve spectral dynamics whose relatively fast time scale is determined by their cavity round trip period, calling for instrumentation featuring both high temporal and spectral resolution. Such real-time spectral characterisation becomes particularly challenging if the laser pulses are long, or they have continuous or quasi-continuous wave radiation components. Here we combine optical heterodyning with a technique of spatio-temporal intensity measurements that allows the characterisation of such complex sources. Fast, round-trip-resolved spectral dynamics of cavity-based systems in real-time are obtained, with temporal resolution of one cavity round trip and frequency resolution defined by its inverse (85 ns and 24 MHz respectively are demonstrated). We also show how under certain conditions for quasi-continuous wave sources, the spectral resolution could be further increased by a factor of 100 by direct extraction of phase information from the heterodyned dynamics or by using double time scales within the spectrogram approach.

Fourier synthesis with single-mode pulses from a multimode laser.

Short pulses are generated by mode-locking techniques: amplitude modulation in time domain or frequency modulation in frequency domain. Direct Fourier synthesis of radiation from several single-frequency sources offers an opportunity to generate arbitrary waveforms. Here we report on a new technique of short-pulse synthesis in the Fourier domain. Instead of independent laser sources, we use a single multimode laser with retrieval of its individual cavity modes into a time sequence coherently combined in an external cavity. Combination of 20 consequent single-mode pulses has been performed, demonstrating a new way for arbitrary waveforms synthesis.

New mechanism of the mode coupling in multi-core fiber lasers

We present the first experimental demonstration of strong coupling between the core modes in multi-core fibers (MCF) regardless of large spacing (~28μm) between them. The effect is very sensitive to bending of the fiber and is observed in the MCF laser as well as in the probe beam schemes. We explain the observed effect by a mechanism of the mode coupling based on their indirect interaction inside the fiber via intermediate cladding mode, analogues to the Bragg mode. 70% of power conversion from one core to another with beating length of tens of centimeters in 4-core MCF is measured.