Victor Khitrov

High-power single-polarization and single-transverse-mode fiber laser with an all-fiber cavity and fiber-grating stabilized spectrum

Conventionally, large-mode-area (LMA) fiber lasers use free-space polarizing components to achieve linear polarization output. External components, however, significantly limit laser robustness and power scalability. We demonstrate, to the best of our knowledge, the first high-power all-fiber cavity single-polarization single-transverse-mode LMA fiber laser, without the use of free-space polarizing components. This has been achieved by using tightly coiled high-birefringence 20 microm core LMA fiber. The lasing spectrum at 1085 nm has been stabilized by a fiber grating, spliced at one end of a LMA fiber. Up to 405 W of single-polarization output with a polarization extinction of >19 dB with a narrow spectrum (1.9 nm FWHM) and in a single-transverse mode (M2 < 1.1) has been demonstrated. The simplicity of a monolithic-cavity approach is highly beneficial for a number of applications, including the use of a fiber laser for nonlinear wavelength conversion and for coherent and spectral beam combining.

kW level narrow linewidth Yb fiber amplifiers for beam combining

Beam combining of fiber lasers has attracted much interest as a practical means to power scale fiber laser/amplifiers beyond the limitations of a single mode output from an individual fiber [1]. Almost all of the high power demonstrations to date that deliver good beam quality after the combing process (coherent and spectral) require some linewidth control for efficient combining, typically less than 10GHz [2,3,4]. Previously we demonstrated single mode, Yb-doped LMA fiber amplifiers operated with around 7GHz linewidth at 1kW output power [5]. In this paper, the latest generations of these amplifiers, based on the latest developments in LMA Yb-doped fiber technology demonstrate the capability to operate with linewidths around 3GHz at the 1kW power level. We present the latest data on optical properties of these new Yb-doped amplifiers and the SBS threshold as a function of input seed laser linewidth and discuss the technologies being developed to operate at higher power levels and narrower linewidths.

Damage mechanisms in components for fiber lasers and amplifiers

In this paper we review the damage mechanisms that need to be considered when building high power fibre lasers. More specifically we look at thermal issues, optically induced coating damage, bulk and surface damage thresholds of the host glass. We also discuss the reliability of tapered fibre bundles and Bragg gratings at these power densities.

Large-mode-area double-clad fibers for pulsed and CW lasers and amplifiers

The advent of double clad fiber technology has made high power lasers and amplifiers possible. However, the scalability of output powers can be limited by amplified spontaneous emission and nonlinear processes such as stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS). These limitations can be overcome by using low numerical apertures (NAs), large-mode areas (LMAs), novel index profiles and high dopant concentrations. This paper discusses advances made in design and fabrication of highly efficient, large-mode area double clad fibers. Experimental and modeling results pertaining to changes in mode area, resultant power density and nonlinear threshold with changing core size are discussed. In addition, the mechanical reliability of the LMA fibers is discussed.

Er-doped high-aspect-ratio core rectangular fiber producing 5 mJ, 13 ns pulses at 1572 nm

We have produced 5 mJ, 13 ns pulses using a very large-mode-area rectangular fiber containing a high-aspect-ratio core with dimensions of ~30 μm×580 μm. The rectangular fiber design retains a thin cladding dimension ~0.5 mm, which is compatible with a compact coiled package. We have developed a theoretical model that achieves good agreement with the experimental data. This model indicates that an optimized fiber design can be scaled to pulse energies of several tens of millijoules. We also discuss a strategy for maintaining good beam quality in both transverse dimensions of this fiber.

50W single-mode linearly polarized high peak power pulsed fiber laser with tunable ns-μs pulse durations and kHz-MHz repetition rates

We report our recent progress in the design and fabrication of a completely monolithic linearly-polarized pulsed Yb-doped fiber laser, with >10kW peak power, tunable 2ns-0.2μs pulse duration, tunable 50kHz-50MHz repetition rate and 50W average power in a diffraction-limited, linearly polarized and stabilized 0.8nm line-width output beam operating at 1064nm. The innovative all-fiber design of the laser is desirable for deployment in industrial applications. A wide range of independently-tunable pulse durations and repetition rates make this laser capable to address a large variety of laser applications, including high-power nonlinear wavelength conversion processes, LIDAR, etc.

1 kW, 15 μJ linearly polarized fiber laser operating at 977 nm

A linearly-polarized, 977 nm pulsed laser capable of 1 kW, 15 μJ output has been demonstrated. The laser is based on Yb3+-doped fiber technology, is core pumped and has a monolithic, all-fiber design. A 13 dB polarization extinction ratio was observed at the maximum measured output power. The output performance of the laser is pump-limited and shows no sign of non-linear effects at the demonstrated output powers. The laser emission is inherently near-diffraction limited due to the single-mode nature of the fibers used.

400W monolithic high efficiency 2 μm MOPA

Progress is being made developing monolithic, all-fiber 2μm wavelength devices that operate robustly at higher power levels. This development includes the critical Tm-doped LMA fiber technology, compatible components such as pump combiners and couplers, along with the optimization of high brightness, high efficiency 790nm pump diodes. In this paper we present recent CW power scaling results and demonstrate a monolithic MOPA system operating at 400W output power with around 20% E-O efficiency.

Frequency Doubling of Tm-Doped Fiber Lasers for Efficient 950nm Generation

We demonstrate a robust, highly efficient pulsed Tm-doped fiber laser systems operating at 1908nm and producing 6W average power. Using PPLN crystal we demonstrate 60% conversion efficiency to 954nm.

Linearly polarized high-power fiber lasers with monolithic PM-LMA-fiber and LMA-grating based cavities and their use for nonlinear wavelength conversion

We report our recent progress in designing and manufacturing new, completely monolithic, linearly polarized, continuous wave (CW) fiber lasers that provide more than 300W of output power in a near diffraction limited, single transverse mode, spectrally stabilized output beam having a narrow line-width. The demonstrated design is simple and practical: the monolithic laser cavity may consist of only a coil of polarization maintaining (PM), large mode area (LMA) active fiber having a fiber Bragg grating (FBG) at one end and a fiber cleave at the other end. Proper selection of the coil diameter enables gain in only one polarization mode so as to provide the linearly polarized output. Fiber lasers built using this novel technique do not require any external polarizing components or the use of polarizing fiber. Such compact and robust fiber lasers are suitable for a variety of applications, such as multi-kW power scaling through coherent beam combining, nonlinear wavelength conversion processes using a variety of nonlinear materials, etc.