Benjamin Pulford

Modal instability-suppressing, single-frequency photonic crystal fiber amplifier with 811 W output power

An acoustic- and gain-tailored Yb-doped polarization-maintaining photonic crystal fiber is used to demonstrate 811 W single-frequency output power with near diffraction-limited beam quality. The fiber core is composed of 7 individually doped segments arranged to create three distinct transverse acoustic regions; including one region that is Yb-free. The utility of the Yb-free region is to reduce coupling between the LP01 and LP11 modes to mitigate the modal instability. The application of thermal gradients is utilized in conjunction with the transverse acoustic tailoring to suppress stimulated Brillouin scattering. To the best of our knowledge, the 811 W output represents the highest power ever reported from a near diffraction-limited single-frequency fiber laser.

Coherent combination of high power fiber amplifiers in a two-dimensional re-imaging waveguide

Four actively phase-locked beams produced by fiber amplifiers in a master oscillator power amplifier (MOPA) configuration were coherently combined in a glass capillary re-imaging waveguide producing more than 100 W of coherent output with 80% combining efficiency and excellent beam quality. The beam combiner components maintained a temperature below 30 degrees C with no external cooling at >100 W of combined 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 beam combining of fiber amplifiers in a kW regime

Single-frequency coherent beam combination (CBC) of 16 fiber lasers with kW class output power is presented. In addition, kW scale CBC of three Photonic Crystal Fiber (PCF) amplifiers in a filled aperture configuration is reported.

400-W near diffraction-limited single-frequency all-solid photonic bandgap fiber amplifier

An ytterbium-doped large-mode area photonic bandgap fiber is used to demonstrate 400 W of single-frequency output at 1064 nm with excellent beam quality and minimal stimulated Brillouin scattering. The fiber possesses all-solid microstructures embedded in the cladding and a core composed of phosphosilicate with a diameter of ∼50  μm. As the signal power is pushed beyond 450 W, there is degradation in the beam quality due to the modal instability. We briefly discuss techniques to alleviate this problem in future designs. To the best of our knowledge, the 400-W single-frequency near diffraction-limited output far exceeds the current state-of-the-art from such type of fiber amplifier.

Multi-kilowatt power scaling and coherent beam combining of narrow-linewidth fiber lasers

We report results from two ~1.5 kW Yb-doped fiber amplifiers with comparable optical to optical efficiencies and linewidths. One amplifier utilized a fiber with a core diameter of 25 μm while the core diameter of the fiber utilized in the other amplifier was 20 μm. Stimulated Brillouin scattering (SBS) suppression in both cases was achieved through pseudo-random bit sequence (PRBS) phase modulation. While the power generated in the larger core fiber was modal instability (MI) limited, no sign of MI was observed in the smaller core fiber. This may allow us to utilize the higher MI threshold fiber to scale further while maintaining sufficiently narrow linewidth for beam combining. Furthermore, in a demonstration of the utility of applying a thermal gradient in conjunction with phase modulation to suppress SBS further, we report on a 1 kW amplifier driven at a PRBS clock rate of 2 GHz. Finally, we compare the coherent beam combining properties of amplifiers seeded with PRBS phase modulated sources to those seeded with white noise sources.

Polarizing ytterbium-doped all-solid photonic bandgap fiber with ~1150µm 2 effective mode area

We demonstrate an Yb-doped polarizing all-solid photonic bandgap fiber for single-polarization and single-mode operation with an effective mode area of ~1150µm(2), a record for all-solid photonic bandgap fibers. The differential polarization mode loss is measured to be >5dB/m over the entire transmission band with a 160nm bandwidth and >15dB/m on the short wavelength edge of the band. A 2.6m long fiber was tested in a laser configuration producing a linearly polarized laser output with a PER value of 21dB without any polarizer, the highest for any fiber lasers based on polarizing fibers.

Large-Mode-Area All-Solid Photonic Bandgap Fibers for the Mitigation of Optical Nonlinearities

There is still significant need for power scaling of fiber lasers. Large-mode-area fibers are a key for the mitigation of optical nonlinearities. In recent years, mode instability has shown itself to be an additional significant limiting factor for single-mode power scaling in the regime of a few hundred watts to kilowatts. It is better appreciated now that further power scaling requires significant high-order-mode suppression in addition to a large effective mode area in a fiber. In recent years, we have shown that all-solid photonic bandgap fibers are a superior approach due to their unsurpassed higher-order-mode suppression in large-mode-area designs, making them well suited for applications at high average powers. We will review of some of the recent progress, challenges, and prospects of all-solid photonic bandgap fibers in this invited paper.

Single-frequency Yb-doped photonic crystal fiber amplifier with 800W output power

A novel acoustic and gain tailored Yb-doped photonic crystal fiber is used to demonstrate over 800 W single-frequency output power with excellent beam quality at 1064 nm. The large mode area fiber core is composed of 7 individually doped segments arranged to create three distinct acoustic regions and preferential gain overlap with the fundamental optical mode. This design leads to suppression of both stimulated Brillouin scattering and modal instability. To the best of our knowledge, the output power represents the highest power ever reported from a near diffractionlimited single-frequency fiber laser. Furthermore, we show that by using a broadband seed, 1.22 kW of output power is obtained without the onset of the modal instability.

Phasing of High Power Fiber Amplifier Arrays

We report locking the phase of a five element 725-W amplifier array and in addition we report phase locking off the backscatter from a remote object. The rms phase error was measured to be ?/60.