Steven J. Augst

Coherent beam combining and phase noise measurements of ytterbium fiber amplifiers

Phase noise characterization of a 10-W Yb fiber amplifier is presented, and we demonstrate phase locking of two fiber amplifiers with near-perfect fringe contrast at power levels of as much as 10 W per fiber. Coherent beam combining is maintained during the turn-on transient as well as in thermal steady-state operation.

Coherent combining of a 4 kW, eight-element fiber amplifier array.

Commercial 0.5 kW Yb-doped fiber amplifiers have been characterized and found to be suitable for coherent beam combining. Eight such fiber amplifiers have been coherently combined in a tiled-aperture configuration with 78% combining efficiency and total output power of 4 kW. The power-in-the-bucket vertical beam quality of the combined output is 1.25 times diffraction limited at full power. The beam-combining performance is independent of output power.

Wavelength beam combining of ytterbium fiber lasers.

Wavelength beam combining of five ytterbium fiber lasers is demonstrated in a master-oscillator power-amplifier configuration at combined powers up to 6 W. The combined beam profile has an M2 value of 1.14, which is equal to that of an individual fiber. Beam steering in one dimension over 140 resolvable spots is also demonstrated.

Diffractive coherent combining of a 2.5 kW fiber laser array into a 1.9 kW Gaussian beam

Five 500 W fiber amplifiers were coherently combined using a diffractive optical element combiner, generating a 1.93 kW beam whose M(2)=1.1 beam quality exceeded that of the inputs. Combining efficiency near 90% at low powers degraded to 79% at full power owing to thermal expansion of the fiber tip array.

Active coherent beam combining of diode lasers

We have demonstrated active coherent beam combination (CBC) of up to 218 semiconductor amplifiers with 38.5 W cw output using up to eleven one-dimensional 21-element individually addressable diode amplifier arrays operating at 960 nm. The amplifier array elements are slab-coupled-optical-waveguide semiconductor amplifiers (SCOWAs) set up in a master-oscillator-power-amplifier configuration. Diffractive optical elements divide the master-oscillator beam to seed multiple arrays of SCOWAs. A SCOWA was phase actuated by adjusting the drive current to each element and controlled using a stochastic-parallel-gradient-descent (SPGD) algorithm for the active CBC. The SPGD is a hill-climbing algorithm that maximizes on-axis intensity in the far field, providing phase locking without needing a reference beam.

External cavity beam combining of 21 semiconductor lasers using SPGD

Active coherent beam combining of laser oscillators is an attractive way to achieve high output power in a diffraction limited beam. Here we describe an active beam combining system used to coherently combine 21 semiconductor laser elements with an 81% beam combining efficiency in an external cavity configuration compared with an upper limit of 90% efficiency in the particular configuration of the experiment. Our beam combining system utilizes a stochastic parallel gradient descent (SPGD) algorithm for active phase control. This work demonstrates that active beam combining is not subject to the scaling limits imposed on passive-phasing systems.

High-power coherent beam combination of semiconductor laser arrays

We have coherently combined a total power of 7.2 W CW using an individually addressable 10-element-array of 980-nm slab-coupled optical waveguide lasers (SCOWLs). The robust, passive phase-locking is accomplished using a fractional Talbot external cavity.

Wavelength beam combining of ytterbium fiber lasers in a MOPA configuration

Summary form only given. There is an increasing interest in combining large numbers of relatively low-power lasers to produce much more powerful, high-brightness laser beams. Coherent combining and wavelength (spectral) combining are the two main approaches being studied. We have been concentrating on wavelength beam combining with fiber lasers and have previously demonstrated this concept at low power. Here we discuss recent work to extend this idea to a master-oscillator power-amplifier (MOPA) configuration by adding a power amplification stage to each oscillator fiber and demonstrating recombination at higher powers.

Coherently combined diode laser arrays and stacks

We have coherently combined up to 7.2 W CW using an individually addressable 10-element-array of 960-nm Slab-Coupled Optical Waveguide Lasers (SCOWLs). We are currently scaling the phase-locked output power to 100 W using SCOWL stacks.

Coherent and spectral beam combining of fiber lasers

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.