Supercontinuum generation in photonics crystal fibers (PCFs) pumped by CW lasers yields high spectral power density
and average power. However, such systems require very high pump power and long nonlinear fibers. By on/off
modulating the pump diodes of the fiber laser, the relaxation oscillations of the laser can be exploited to enhance the
broadening process. The physics behind the supercontinuum generation is investigated by sweeping the fiber length, the
zero dispersion wavelength, and the fiber nonlinearity. We show that by applying gain-switching a high average output
power of up to 30 W can be maintained and the spectral width can be improved by 90%. The zero dispersion wavelength
should be close to but below the pump wavelength to achieve the most visible light. By increasing the nonlinearity the
fiber length can be reduced from 100 m to 25 m and the efficiency of visible light generation is improved by more than
200%.
High-power fiber lasers and amplifiers have gained tremendous momentum in the last 5 years. Many of the traditional manufacturers of gas and solid-state lasers are now pursuing the fiber-based systems, which are displacing the conventional technology in many areas. High-power fiber laser systems require reliable fibers with large cores, stable mode quality, and good power handling capabilities-requirements that are all met by the airclad fiber technology. In the present paper we go through many of the building blocks needed to build high-power systems and we show an example of a complete airclad laser system. We present the latest advancements within airclad fiber technology including a new 100 μm single-mode polarization-maintaining rod-type fiber capable of amplifying to megawatt power levels. Furthermore, we describe the novel airclad-based pump combiners and their use in a completely monolithic 350 W cw fiber laser system with an M2 of less than 1.1.
The frequency-doubled radiation of an Erbium-doped fiber laser is used for supercontinuum generation in a small-core
microstructured fiber with two zero-dispersion wavelengths. Average powers up to 49 mW are launched
into the highly nonlinear photonic-crystal fiber. The generated supercontinuum shows a short-wavelength peak
centered around 670 nm and a long-wavelength peak centered around 1100 nm. More than 35 mW is contained
in the short-wavelength peak. We use the anomalous dispersion of a SF10 prism compressor to compress the
short-wavelength peak of the spectrum. The compressed pulse has a central wavelength of 670 nm and a duration
of 27 fs.
We demonstrate electrical tunability of a fiber laser using a liquid crystal photonic bandgap fiber. Tuning of the laser is
achieved by combining the wavelength filtering effect of a liquid crystal photonic bandgap fiber device with an
ytterbium-doped photonic crystal fiber. We fabricate an all-spliced laser cavity based on a liquid crystal photonic
bandgap fiber mounted on a silicon assembly, a pump/signal combiner with single-mode signal feed-through and an
ytterbium-doped photonic crystal fiber. The laser cavity produces a single-mode output and is tuned in the range 1040-
1065 nm by applying an electric field to the silicon assembly.
We demonstrate suppression of amplified spontaneous emission at the conventional ytterbium gain wavelengths around
1030 nm in a cladding-pumped polarization-maintaining ytterbium-doped solid core photonic crystal fibre. The fibre
works through combined index and bandgap guiding. Furthermore, we show that the peak of the amplified spontaneous
emission can be shifted towards longer wavelengths by rescaling the fibre dimensions. Thereby one can obtain lasing or
amplification at longer wavelengths (1100 nm - 1200 nm) as the amount of amplification in the fibre is shown to scale
with the power of the amplified spontaneous emission.
High-power fiber lasers and amplifiers have gained tremendous momentum in the last five years, and many of the
traditional manufactures of gas and solid-state lasers are pursuing the attractive fiber-based systems, which are now
displacing the old technology in many areas. High-power fiber laser systems require specially designed fibers with large
cores and good power handling capabilities - requirements that are all met by the airclad fiber technology. In the present
paper we go through many of the building blocks needed to build high-power systems and we show an example of a
complete airclad laser system. We present the latest advancements within airclad fiber technology including a new 70
μm single-mode polarization-maintaining rod-type fiber capable of amplifying to MW power levels. Furthermore we
describe the novel airclad based pump combiners and their use in a completely monolithic 350 W CW fiber laser system
with an M2 of less than 1.1. Finally, we briefly touch upon the subject of photo darkening and its origin.
Photonic crystal fibers consisting of a solid fused silica core surrounded by a regular array of sub-micron air-holes have been shown to operate with single mode core well above 30 μm in active laser geometries as well as passive beam delivery fibers. Novel designs based on the combination of stress applying elements that are index matched to the holey cladding have recently been emerged. In this report we summarize the properties of these polarization maintaining photonic crystal fibers. Beside the characterization of the polarizing window and birefringence, high power laser and amplifier configurations using these fibers are demonstrated and first experiments concerning the temperature sensitivity of the polarizing properties are presented.
Hollow core photonic crystal fiber (HCPCF) amplifiers, in which Er3+- or Yb3+- doped glass acts as the gain medium, are proposed as a means of achieving high power pulse amplification. Double-clad configurations are identified which capture multimode pump light up to an NA of around 0.33. The nonlinear and breakdown properties of a HC-PCF amplifier with a mode area of approximately 50μm2 are predicted to be comparable to those of a solid core fiber amplifier with a mode area of 1000μm2. Mode competition effects within the HC-PCF amplifier strongly degrade the output signal unless the net gains of the unwanted guided modes are below that of the signal mode. This can be achieved if the ratio of amplifier gain to scattering loss is larger for the signal mode than any of the undesired guided modes. Assuming loss is dominated by hole interface roughness scattering, and an even doping profile produces the gain, the ratios for the unwanted guided modes of a typical HCPCF geometry are calculated to be similar to that for the signal carrying mode. The mode competition also places a lower bound on the active fiber length, typically implying a longer length is required than in a solid core fiber amplifier. This adversely affects the device efficiency due to scattering loss of the pump field incurred at the air/glass interfaces. To achieve a clean mode output and acceptable efficiency, alternative designs for the HC-PCF will need to be developed.
Fiber lasers deliver excellent beam-quality and high efficiency in a robust and largely maintenance-free format, and are now able to do so with output powers in the kilowatt regime. Consequently, fiber lasers have become an attractive alternative to solid-state and gas lasers for e.g. material processing like welding, cutting and marking.
The all-glass air-clad photonic crystal fibers (PCFs) combine large mode-field diameters (currently up to 40 μm), high numerical aperture (typically in the 0.6-0.65 range), high pump absorption (30 dB/m demonstrated in ytterbium) and excellent high-power handling (kW CW and mJ pulses demonstrated). These properties have made this fiber type one of the most promising candidates for the future high-power fiber laser and amplifier systems that are expected to replace many of the traditional systems in use today.
To utilize the high numerical aperture and large mode-field diameters of the air-clad PCFs, special care must be taken in the system integration. In this paper, we will show examples of how these fibers can be integrated in laser and amplifier sub-assemblies with standard fiber pump-interfaces for use with single-emitter diodes or diode-bar pump sources. Moreover, we report on the most recent advances in fiber design including rod-type fibers and broadband polarizing ytterbium-doped large-mode-area air-clad fibers. Finally, we will review the latest results on PCF-based amplifier and laser configurations with special focus on high-power CW systems and high-energy pulsed configurations.
We report on the latest development within active photonic crystal fibers for high power lasers and amplifiers with special focus on how the fibers can be improved with both polarization-maintaining and polarizing properties. We describe rod-type fibers for which a record-high power extraction of 250W/m is achieved. Moreover, we describe how active characterization is used to optimize fibers for laser and amplifier sub-assemblies with respect to beam quality, efficiency and robustness. Finally, we illustrate how the fibers can be integrated with high NA tapers and passive air-clad fibers containing Bragg grating to form an all-fiber, alignment-free, high-power fiber laser subassembly.
Light sources with a broad spectral output and diffraction limited beam quality have a wide variety of present and future applications. A few of particular interest are hyperspectral laser radar for environmental monitoring, active hyperspectral imaging for detection and identification of objects, and speckle-free illumination. With the exception of systems based on amplified femto- or picosecond lasers, which are large and extremely complicated, pulse energies from supercontinuum laser sources have been limited to <10 microJoules which is generally not sufficient for the applications listed above. We present a simple technique to generate broadband light spanning several hundred nanometers in the near infrared with pulse energies of ~1 mJ, an improvement of approximately two orders of magnitude. The system is comprised of a Q-switched Nd:YAG laser and a very large mode area photonic crystal fiber. A combination of cascaded stimulated Raman scattering, four wave mixing, and self-phase modulation is responsible for the spectral broadening. Possibilities of scaling the output to the ~10 mJ level as well as extending the spectral coverage to the visible and mid-infrared will also be discussed.
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