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.
Monolithic, all-fiber PM optical amplifiers have been investigated regarding components performance,
amplifier design and narrow line-width amplification. PM-based components performed very well, the
pump/signal combiner in particular handling > 300W pump power and still maintaining the PER of the
input signal. A co-pumped amplifier configuration was chosen based on reliability and potential SBS
mitigation (i.e. fiber length after the active fiber output). This system based on PLMA-YDF - 25/400 fiber
appeared to give the best output power (155W-177W, depending on the length of passive delivery fiber),
PER (~ 17 dB) and excellent beam quality (M2= 1.1). Altering the fiber temperature and length were
necessary to provide the best results.
We report on the recent progress in the design and development of completely monolithic linearly-polarized pulsed
fiber amplifiers seeded by Q-switched fiber laser oscillators. We demonstrate near diffraction limited beam quality with
~ 20 kW peak power (1mJ pulse energy, ~ 45 nsec) pulses and an average power ~ 20 W at 20 kHz repetition rate with
linearly polarized (> 17dB PER) output from a simple MOPA design. The laser produces spectrally narrow pulses with
~ 0.5 nm linewidth centered at 1064nm, suitable for various non-linear applications including generation of visible and
UV light. The simple MOPA design consists of a monolithic fiber amplifier based on an optimized coil of polarization
maintaining large mode area (PM-LMA) fiber with 30 &mgr;m core and low power Q-switched fiber oscillator. Excellent
output beam quality is achieved through the mode selectivity of the coiled PM-LMA fiber in the amplifier stage. Such
compact and robust fiber lasers are suitable for a variety of applications, such as nonlinear wavelength conversion
processes using a variety of nonlinear materials, laser radars, etc.
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.
In this paper we present advances made in the development and fabrication of highly efficient, large-mode area fibers for eye-safe wavelengths (1.55 μm, 2.0 μm). LMA Er/Yb co-doped and Tm doped fibers have been successfully fabricated, with 25 μm core and 250 to 300 μm clad diameters, that are suitable for nanosecond pulsed amplification in LIDAR applications as well as high power CW amplification. Manufacturing challenges for these novel fibers are discussed. Measured and modeled data, for both types of fibers, are presented. The development of non-PM and PM-LMA fibers for eye-safe applications is expected to spur rapid progress in power scaling at these wavelengths, similar to that witnessed by the industry at 1.06 μm.
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.
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.
Fiber lasers have a number of distinct advantages over their more conventional solid state laser alternatives. These advantages include size, reliability, wavelength selectivity, heat dissipation, wall plug efficiency and operational cost. Furthermore they can be operated without the need for active cooling or optical alignment. Consequently the market for these more traditional laser sources are beginning to be eroded by the emergence of fiber lasers. In 1999 high power fiber lasers became a reality, with the world's first single-mode fiber laser exhibiting in excess of 100W cw output. However it was soon recognized that conventional small core, high NA fiber designs were not appropriate to applications requiring further scaling of the output power. More specifically it was found that the maximum achievable output power in such fibers were restricted by a fundamental susceptibility to optical nonlinearities, including stimulated Raman scattering (SRS), stimulated Brillouin scattering (SBS) and self-phase modulation. In order to overcome the limitations imposed by these parasitic nonlinear processes, it has been necessary to develop fibers with high rare-earth dopant concentrations in relatively large core, low numerical aperture fibers. These so-called large mode area (LMA) fibers are directly responsible for the recent explosion in demonstrated diffraction-limited beam quality output powers, now approaching the kW-level from a single fiber. To further scale the output power it is necessary to combine the output of several fiber lasers. Indeed, for a number of industrial and military applications it is desirable to scale the total output power to between several and hundreds of kW's. It is therefore advantageous to be able to coherently combine the beams from multiple fibers and this it turns makes it desirable for the fiber to also be polarization maintaining. This provides yet another layer of complexity to the fiber design but such fibers are now a commercial reality. In this paper we review the recent and ongoing advantages in fiber design that is facilitating the development and production of lasers and amplifiers with ever increasing output powers.
Although fiber amplifiers have been employed in communications systems for many years, until very recently the fiber laser was little more than a scientific curiosity. However the fiber laser format has a number of intrinsic advantages over lamp and diode pumped YAG lasers including size, reliability, wavelength selectivity, heat dissipation, wallplug efficiency and operational cost; and with kiloWatt output powers now possible fiber lasers are beginning to replace lamp and diode pumped YAG lasers in many industrial applications. In this paper we review the recent and ongoing advances in fiber design that have facilitated this revolution.
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.
Fibers for high-power laser and amplifier applications require large claddings with high numerical apertures for efficiently coupling pump energy. In addition, such fibers should have high rare-earth dopant concentrations in relatively large cores, with low numerical apertures, to reduce non-linearities. Furthermore, polarization maintaining double-clad fibers (PM-DCF) are needed for coherently combining the outputs of several lasers/amplifiers to achieve output powers in excess of 100 kW for military and industrial laser applications. In this paper, we report the progress made towards fabricating PM double-clad fibers, with a variety of fiber characteristics, to facilitate development and production of high-power lasers and amplifiers. In particular, a Panda-type PM-DCF with a 0.06 NA, 30 micron diameter, Yb-doped core is reported. We also discuss various criteria that are critical for designing these PM double clad fibers.
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