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.
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 M<sup>2</sup> ~ 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.
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.
Optical interconnects offer advantages over electrical interconnects in terms of clock skew, crosstalk, and RC delay for ULSI (Ultra Large Scale Integrated-Circuit) silicon technology. Optical interconnects are also applicable in optical communications where compact optical devices are fabricated and incorporated in an on-chip integrated optical system. Polycrystalline silicon (polySi)/SiO<SUB>2</SUB> is an attractive waveguiding system that offers significant advantages in both applications with its compact size and compatibility with multilevel CMOS processing. Based on the process optimization that led to a low-loss polySi material, we have fabricated compact waveguide bends and splitters that were microns in size. To study the modal behavior in bending and splitting, we compared multi-mode and single-mode waveguides that were used in fabricating bends and splitters. Two waveguide cross-section dimensions, 0.5 micron X 0.2 micron and 2 microns X 0.2 microns, were used for single- mode waveguide and multi-mode waveguide, respectively. Micron- sized bending was realized with a low loss of a few dBs. Single-mode bends showed less than 3 dB loss for a bending radius of 3 microns, which was lower than that for multi-mode bends. Two different types of splitters, single-mode Y- splitters and multi-mode Y-splitters were fabricated and characterized in terms of their splitting uniformity. One X four and 1 X 16 optical power distribution systems were built based on different splitting schemes and their output power uniformity was compared. Due to the high dielectric contrast of our polySi/SiO<SUB>2</SUB> waveguide system, the smallest 1 X 16 optical power distribution was realized in an area smaller than 0.0001 cm<SUP>2</SUP>.