Peter A. Thielen

Active phase and polarization locking of a 1.4 kW fiber amplifier

A three-stage Yb-fiber amplifier emitted 1.43 kW of single-mode power when seeded with a 25 GHz linewidth master oscillator (MO). The amplified output was polarization stabilized and phase locked using active heterodyne phase control. A low-power sample of the output beam was coherently combined to a second fiber amplifier with 90% visibility. The measured combining efficiency agreed with estimated decoherence effects from fiber nonlinearity, linewidth, and phase-locking accuracy. This is the highest-power fiber laser that has been coherently locked using any method that allows brightness scaling.

Diffractive-optics-based beam combination of a phase-locked fiber laser array

A diffractive optical element (DOE) is used as a beam combiner for an actively phase-locked array of fiber lasers. Use of a DOE eliminates the far-field sidelobes and the accompanying loss of beam quality typically observed in tiled coherent laser arrays. Using this technique, we demonstrated coherent combination of five fiber lasers with 91% efficiency and M2=1.04. Combination efficiency and phase locking is robust even with large amplitude and phase fluctuations on the input laser array elements. Calculations and power handling measurements suggest that this approach can scale to both high channel counts and high powers.

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.

Suppression of stimulated Brillouin scattering in single-frequency multi-kilowatt fiber amplifiers

Previous research has shown that temperature gradients along a fiber can broaden the Stimulated Brillouin Scattering (SBS) gain profile and thereby increase the SBS threshold. However, within practical temperature ranges this method has been limited to SBS thresholds of a few hundred Watts. It is also well known that strain gradients applied to a fiber can broaden the SBS resonance. To suppress the SBS threshold to kW levels in fiber amplifiers of length ~5 m requires broadening of the SBS resonance width to ~1 GHz, which can be achieved with a strain of 1 - 2%. Although tensile strain is generally limited by fiber failure to less than ~1%, compressive strain has been employed to the level of many percent in a number of applications in the tuning of fiber Bragg gratings. We demonstrate the effect of SBS gain broadening and suppression by strain gradients at high power (~ 190 W) for the first time to our knowledge, and explore scaling of this method to kW output levels.

Scalable Coherent Combining of Kilowatt Fiber Amplifiers Into a 2.4-kW Beam

We demonstrate coherent beam combining of multiple Yb-doped non polarization-maintaining fiber amplifiers operating at individual power levels above 1 kW. A 2.4-kW, M2 = 1.2 output beam was generated by combining three amplifiers with a diffractive optical element (DOE). A single low-power sample of the combined output beam provided error signals for active phase and polarization locking of all three fibers. The beam combining efficiency of 80% was nearly constant with amplifier power, indicating the absence of significant thermal or nonlinear effects. Modeling anchored by precise measurements of the beam characteristics indicates potential for this architecture to scale to higher fiber counts, higher combining efficiency, and higher power.

Two-dimensional diffractive coherent combining of 15 fiber amplifiers into a 600 W beam

We demonstrate coherent beam combining using a two-dimensionally patterned diffractive optic combining element. Fifteen Yb-doped fiber amplifier beams arranged in a 3×5 array were combined into a single 600 W, M²=1.1 output beam with 68% combining efficiency. Combining losses under thermally stable conditions at 485 W were found to be dominated by spatial mode-mismatch between the free space input beams, in quantitative agreement with calculations using the measured amplitude and phase profiles of the input beams.

Coherent Combination of Fiber Lasers with a Diffractive Optical Element

An actively phase-locked array of five fiber lasers is coherently combined using a diffractive optical element with 91% efficiency and M2=1.04. Calculations and power handling measurements suggest this approach is scalable to high powers.

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.

Coherent Combining of a 1.26-kW Fiber Amplifier

A 1.26-kW, multi-stage Yb fiber MOPA was coherently combined using active polarization and phase control with 94% visibility to a second fiber amplifier, consistent with estimated decoherence effects from fiber nonlinearity, linewidth, and phasing accuracy.

Diffractive Beam Combining of a 2.5-kW Fiber Laser Array

Five 500-W fiber amplifiers were coherently combined with 79% efficiency using a diffractive optical element (DOE) combiner, generating a single beam whose M2 = 1.1 beam quality exceeded that of the inputs.