Shawn Redmond

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.

Coherent combination of high-power, zigzag slab lasers

We demonstrate a scalable architecture for a high-power, high-brightness, solid-state laser based on coherent combinations of master oscillator power amplifier chains. A common master oscillator injects a sequence of multikilowatt Nd:YAG zigzag slab amplifiers. Adaptive optics correct the wavefront of each amplified beamlet. The beamlets are tiled side by side and actively phase locked to form a single output beam. The laser produces 19 kW with beam quality <2x diffraction limited. To the best of our knowledge, this is the brightest cw solid-state laser demonstrated to date.

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.

Multi-kW near-diffraction-limited single-frequency Nd:YAG laser

Northrop Grumman is developing a laser architecture that can scale to >100 kW with a near-term goal of a 25 kW demonstration. The near-term 25 kW design is based on two chains of four slab amplifiers that produce average power of 12.5 kW each. Adaptive optics sense the output wavefront and piston relative to a reference, then adjust the phase of the master oscillator input to each chain to keep the wavefronts of each chain uniform and in phase. To reach the goal of 12.5 kW per chain, Northrop has demonstrated power scaling of individual amplifiers by extracting 4.5 kW form a single amplifier using a multimode resonator. This is well above the minimum needed to achieve 12.5 kW from a four-amplifier chain.

High efficiency coherent beam combining of semiconductor optical amplifiers

We demonstrate 40 W coherently combined output power in a single diffraction-limited beam from a one-dimensional 47-element array of angled-facet slab-coupled optical waveguide amplifiers at 1064 nm. The output from each emitter was collimated and overlapped onto a diffractive optical element combiner using a common transform lens. Phase locking was achieved via active feedback on each amplifiers drive current to maximize the power in the combined beam. The combining efficiency at all current levels was nearly constant at 87%.

High speed, high power one-dimensional beam steering from a 6-element optical phased array

Beam steering at high speed and high power is demonstrated from a 6-element optical phased array using coherent beam combining (CBC) techniques. The steering speed, defined as the inverse of the time to required to sweep the beam across the steering range, is 40 MHz and the total power is 396 mW. The measured central lobe FWHM width is 565 μrad. High on-axis intensity is maintained periodically by phase-locking the array via a stochastic-parallel-gradient-descent (SPGD) algorithm. A master-oscillator-power-amplifier (MOPA) configuration is used where the amplifier array elements are semiconductor slab-coupled-optical-waveguide-amplifiers (SCOWAs). The beam steering is achieved by LiNbO(3) phase modulators; the phase-locking occurs by current adjustment of the SCOWAs. The system can be readily scaled to GHz steering speed and multiwatt-class output.

19-kW Phase-locked MOPA Laser Array

We have developed a scalable architecture of phase-locked Nd:YAG master oscillator power amplifiers. In cw operation a 2×1 array emitted 19.0 kW with 30% optical efficiency and average beam quality of 1.73 × diffraction-limited.

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.

Slab-coupled optical waveguide lasers and amplifiers

Slab-coupled optical waveguide lasers (SCOWLs) and amplifiers (SCOWAs) are inherently low-confinement structures with large nearly-circular modes that are easily coupled to optical fibers or collimated for free-space applications. Recently SCOWL powers have increased to 3 W by increasing the cavity length to 1 cm and improving the heat removal. SCOWAs are coherently combined using active phase control to achieve a very high-brightness source. Our coherent beam combining system consists of single-pass amplifiers with angled-facet SCOWAs that suppress feedback. Single-pass, 5-mm long, SCOWAs have now been demonstrated with 1.5 W CW output with only 50 mW seed power. Arrays of 47 SCOWAs have demonstrated a raw power of 57 W with 50 mW of seed power per element. A coherent beam combining demonstration is currently being assembled.