Alexey G. Kuznetsov

Q-switched fiber laser with spectral control for frequency doubling

A-Q switched pulsed ytterbium-doped fiber laser is created and optimized for frequency doubling and tripling. The spectral width of the laser radiation (less than 0.13 nm) is controlled using a Bragg grating. At a pulse repetition rate of 1 kHz, the peak power is greater than 13 kW and the pulse duration is 30 ns.

Spectral broadening of incoherent nanosecond pulses in a fiber amplifier

Amplification of incoherent light pulses in a relatively short active fiber is treated. Spectral broadening due to the self-phase modulation effect at negligibly small dispersion has been studied theoretically. The expression for the shape of the output spectrum has been obtained for input pulses of arbitrary temporal and spectral shape at various gain coefficients. The expression is found to be simplified in case of a hyperbolic secant temporal shape. The calculated shapes have been compared with the experimentally measured spectra for Q-switched fiber laser nanosecond pulses amplified in Yb-doped fiber, demonstrating excellent agreement of theory and experiment. The spectrum of the output pulse is shown to be composed of two different-scale structures: a narrow central part copying the initial shape and broad exponential tails that grow with increasing output power.

Frequency doubling and tripling in a Q-switched fiber laser

The frequency doubling and tripling in a Q-switched all-fiber laser are studied. It is demonstrated that the main limitations on the efficiency of the harmonic generation are related to the random polarization that is nonuniform with respect to axes, the asymmetric pulse shape with a flat trailing edge, and the significant spectral broadening of the multimode radiation of the fiber master oscillator in the fiber amplifier. The methods to increase the efficiency are proposed. For an IR pulse energy of about 0.3 mJ, duration of about 40 ns, and repetition rate of 1 kHz, the second- and third-harmonic pulse energies are greater than 60 and about 10 μJ, respectively.

Actively Q-switched Raman fiber laser

A new scheme providing actively Q-switched operation of a Raman fiber laser (RFL) has been proposed and tested. The RFL consists of a 1 km single-mode fiber with a switchable loop mirror at one end and an angled cleaved output end. An 1080 nm pulse with microsecond duration is generated at the output by means of acousto-optic switching of the mirror at ~30 kHz in the presence of 6 W backward pumping at 1030 nm. In the proposed scheme, the generated pulse energy is defined by the pump energy distributed along the passive fiber, which amounts to 30 μJ in our case. The available pump energy may be increased by means of fiber lengthening. Pulse shortening is also expected.

Comparison of temperature distribution measurement methods with the use of the Bragg gratingsand Raman scattering of light in optical fibers

Two types of fiber-optical measurement systems are compared: a line with a large number of point sensors based on fiber Bragg gratings (FBGs) interrogated by a tunable continuous fiber laser and a distributed system based on optical time-domain reflectometry (OTDR) of the Raman scattering of radiation of a pulsed fiber laser. Methods for increasing the measurement accuracy with the use of additional calibration of the Bragg wavelength shift over a fiber interferometer in the FBG system and spectral filtration of the Stokes and anti-Stokes components of the Raman scattering with the use of spectral-selective fiber couplers in the OTDR system are proposed and implemented. Physical effects on system parameters are analyzed, compared, and optimized for applications with monitoring of the temperature distribution in turbogenerators and oil wells.

Fifty-ps Raman fiber laser with hybrid active-passive mode locking.

Actively mode locked Raman lasing in a ring PM-fiber cavity pumped by a linearly polarized Yb-doped fiber laser is studied. At co-propagating pumping, a stochastic pulse with duration defined by the AOM switching time (~15 ns) is generated with the round-trip period. At counter-propagating pumping, one or several sub-ns pulses (within the AOM switching envelope) are formed. It has been found that the formation of such stable multi-pulse structure is defined by the single-pulse energy limit (~20 nJ) set by the second-order Raman generation. Adding a NPE-based saturable absorber in the actively mode locked cavity, results in sufficient shortening of the generated pulses both in single- and multi-pulse regimes (down to 50 ps). A model is developed adequately describing the regimes.

Q-switched fiber laser with controllable output spectrum

This paper reviews studies of the possibility of controlling the output spectrum of Qswitched fiber lasers. Various laser configurations for producing output radiation with characteristics optimized for specific applications, such as high-power pulses for micromachining of materials, probe pulses in fiber-optic sensor systems, etc., are considered. The mechanism of broadening of the lasing line is elucidated, and methods for controlling the output spectrum in Q-switched all-fiber lasers are described. Frequency tuning in the amplification line and generation of higher harmonics in nonlinear crystals are considered.

Random distributed feedback Raman fiber lasers

In this chapter we briefly review the basic principles of Raman fiber lasers operating via random distributed feedback, including details of feedback mechanism, various cavity designs, and corresponding power and spectral characteristics, as well as their statistical properties. We also compare performances of the random Raman fiber lasers (RRFLs) with that for conventional Raman fiber lasers (RFLs) with linear cavity based on two reflectors. The performance analysis includes polarization management, optimization of conversion efficiency, cascaded generation of higher Stokes orders, opportunities for short-wavelength generation via direct pumping by high-power laser diodes, or frequency doubling of random Raman fiber laser radiation. Pulsed operation of random Raman fiber lasers via active or passive Q-switching is also analyzed. The analysis shows that the output characteristics of Raman fiber lasers with random distributed feedback reached to the moment already outperform in many aspects those for conventional Raman fiber lasers. The unique performance of random fiber lasers opens the door to their application in advanced technologies, such as long-distance amplifier-free transmission and remote sensing, low-coherence IR, and visible sources for bio-imaging and others.

Comparison of Raman and Fiber Bragg Grating-Based Fiber Sensor Systems for Distributed Temperature Measurements

Two types of distributed fiber sensor systems for temperature measurements have been developed: the first one is multi-point Fiber Bragg Grating based system with interrogation by CW tunable laser and nonlinearity compensation by reference interferometer. The second device is Raman scattering system based on optical time domain reflectometry (OTDR) with a pulsed laser providing spatial resolution of several meters and efficient spectral filtering of the Stokes and anti-Sokes signals by means of WDM couplers. Physical effects important for the systems operation are analyzed and their parameters are compared and optimized for applications in oil-gas industry and turbogenerator temperature monitoring.