Pump-limited kW-class operation in a multimode fiber amplifier using adaptive mode control was achieved. A photonic lantern front end was used to inject an arbitrary superposition of modes on the input to a kW-class fiber amplifier to achieve a nearly diffraction-limited output. We report on the adaptive spatial mode control architecture which allows for compensating transverse-mode disturbances at high power. We also describe the advantages of adaptive spatial mode control for optical phased array systems. In particular, we show that the additional degrees of freedom allow for broader steering and improved atmospheric turbulence compensation relative to piston-only optical phased arrays.
We investigate high brightness pumping of multi-kW fiber amplifier in a bi-directional pumping configuration. Each pump outputs 2 kW in a 200 μm, 0.2 NA multi-mode fiber. Specialty gain fibers, with 17 μm MFD and 5-dB/meter pump absorption, have been developed. The maximum fiber amplifier output power is 2550 W, limited by multi-mode instability, with 90% O-O efficiency and M2 < 1.15. The fiber amplifier linewidth is <12 GHz. We also present kW fiber amplifier results using gain fiber with metalized fiber coating.
Advanced, high-brightness photoinjectors are required for the next generation of linear accelerators and free-electron
lasers. Current photoinjector lasers suffer from complexity due to the use of multiple amplifier stages to achieve the
desired pulse energy and have issues with power scaling. In this work, we used a liquid-nitrogen-cooled, Yb:YAG,
mode-locked laser master-oscillator/power-amplifier as a high-average-power laser source for laser photoinjector applications.
Such a laser can provide average powers of 93 W with repetition rates of 1 MHz (93 μJ per pulse). The 10-ps
pulses from the oscillator were amplified in a four-pass cryo-Yb:YAG amplifier.
State-of-the-art diffraction-limited fiber lasers are presently capable of producing kilowatts of power. Power levels
produced by single elements are gradually increasing but beam combining techniques are attractive for rapidly scaling
fiber laser systems to much higher power levels. We discuss both coherent and spectral beam combining techniques for
scaling fiber laser systems to high brightness and high power. Recent results demonstrating beam combination of 500-W
commercial fiber laser amplifiers will be presented.
A cryogenically cooled Yb:YLF laser with 224-W output power at 995 nm, linearly polarized along the c-axis, has been
demonstrated, and laser oscillation has also been obtained polarized along the a-axis. The beam quality had an M2 ~ 1.1
at 60-W output and M2 ~ 2.6 at 180-W output for c-axis polarization. This level of average power is approximately two
orders of magnitude higher than demonstrated previously in cryogenic Yb:YLF. A cryogenic Yb:YLF mode-locked
oscillator is under development, which will be used to as the input to a Yb:YLF amplifier to generate a short pulses at
high average power.
We have been developing a high power, high brightness semiconductor diode laser concept, the Slab-Coupled
Optical Waveguide Laser (SCOWL). This laser concept is based upon slab coupling, in which a large, multimode
waveguide is converted to a large, single mode waveguide by means of slab coupling of the higher order waveguide
modes. SCOWL devices feature large, nearly circular mode sizes (≈4 x 4 &mgr;m and larger) and low modal loss, leading
to low gain per unit length, allowing for the construction of long (≈1 cm cavity length) devices. These characteristics
allow for high single mode output power. For 980-nm AlGaAs/InGaAs/GaAs-based SCOWL devices, we have
demonstrated > 1 W CW output power in a single spatial mode, with brightness levels of > 100 MW/cm2-str. We have
constructed high power arrays of SCOWL devices with bar widths of 1 cm and cavity lengths of 3 mm, and have
demonstrated > 90 W under CW operation. By using the technique of wavelength beam combining (WBC), which is
analogous to wavelength division multiplexing in optical communications, we have been able to combine the outputs
from the elements of a SCOWL array to obtain 50 W peak power (30 W CW) with nearly diffraction-limited beam
quality. These SCOWL arrays combined by WBC have demonstrated record single bar brightness levels, 3.6 GW/cm2-
str. The WBC SCOWL approach is inherently scalable, and offers a route to obtaining kW-class, nearly diffraction
limited output from an all-diode laser source. We have also recently extended single SCOWL devices to the multi-Watt
regime, demonstrating 2.8 W CW output power from a 980-nm SCOWL with a novel design.
Spectroscopic and thermo-optic properties of laser crystals are needed for solid-state laser performance modeling. Here we present a brief report on the measurement of key thermo-optic properties: thermal conductivity (κ), coefficient of thermal expansion (α), and thermal coefficient of refractive index (dn/dT). κ was measured using laser-flash method. α was measured using a 632.8-nm He-Ne Michelson laser interferometer. dn/dT was determined at 1064 nm, using measured values of α and the thermal coefficient of the optical path length. An extended paper containing more detailed results will be submitted to the Journal of Applied Physics.
Significant progress has been recently reported on beam combining of arrays of fiber and semiconductor lasers using both coherent (phased array) and wavelength (spectral) techniques. The choice between these two classes of beam-combining techniques has implications for the degree of control required on the array elements and on the optical system that is using the beam-combined array. Coherent beam combining imposes more stringent requirements on array-element control (spectrum, phase, amplitude, and polarization) than wavelength combining. Also, wavelength-combined systems should degrade more gracefully than coherent systems and require fewer changes to optical systems as the number of elements is varied.
A diffraction-grating based demultiplexer is made to have low polarization dependence and high diffraction efficiency properties. The device is made is made of a Si grism working in reflection and having optimised grove profile easily manufactured by standard crystallographic etch of Si surface.
The development of high-average-power solid-state lasers with good beam quality has been limited primarily by thermo- optic distortions in the laser gain medium. Cooling Yb:YAG gain media to cryogenic temperatures promises to significantly reduce thermo-optic distortions relative to 300 K Nd:YAG. Preliminary results have been obtained for a diode-pumped, cooled Yb:YAG laser operating at 1.03 micrometers , and the experiments to date verify a large reduction in thermo-optic effects.
This paper describes recent developments in modelocking techniques for ultrashort pulse generation in solid stat lasers. Studies in Ti:Al2O3 provide a model system for examining different modelocking approaches. Special emphasis is placed on the use of additive pulse modelocking for achieving passive modelocking using a fast saturable absorber-like mechanism. Different models of APM are described and its extensions to other solid state laser materials are discussed.