Hans W. Bruesselbach

Low-heat high-power scaling using InGaAs-diode-pumped Yb:YAG lasers

We report to our knowledge the highest to date quasi-CW output power, 600 W and pulse energy, >1 J, for an InGaAs diode-pumped Yb:YAG laser. In separate preliminary results, we have also obtained 225 W of average output power under true CW diode pumping. This performance was obtained using a laser head designed to be part of a master oscillator power amplifier (MOPA) operating at 3 kW. We summarize why the diode-pumped Yb:YAG crystal laser is ideal for scaling to high average powers and the different approaches being pursued. We also report our latest results for side-pumped rod devices.

Power-scalable phase-compensating fiber-array transceiver for laser communications through the atmosphere

We report laboratory experiments demonstrating a phase-compensating 70-mm-diameter aperture transceiver that comprises a hexagonally close-packed array of seven 23-mm-diameter fiber collimator subapertures. Other than the collimators, the transceiver uses only fiber optics, connected as a master oscillator-multiple amplifier. The master oscillator is a fiber-coupled 1.5-µm diode laser, which is split and fed to 1-W fiber amplifiers before it exits the collimators. To obtain a phase-coherent far field we control each subapertures phase by adjusting the current to its amplifiers pump diodes in a multidither arrangement, maximizing the signal at the receiver. We achieve a diffraction-limited coherent beam combination in the far field that produces 1.4 W of power in the main lobe, in agreement with theory.

69-W-average-power Yb:YAG laser

We report what is to our knowledge the highest quasi-cw output power to date, 150 W, with average output power of 69 W and pulse energy of 250 mJ, for an InGaAs diode-pumped Yb:YAG laser. This performance was obtained by use of a laser head designed to be part of a master oscillator power amplifier operating at 3 kW.

200 W self-organized coherent fiber arrays

We report producing 200 W coherent fiber laser arrays without active control. This outcome is obtained via self-organization using a non-fiber coupler for two- to ten-laser arrays.

Self-organized coherence in fiber laser arrays

Self-organized coherence between fiber lasers has been reported both via all-fiber 2x2 directional coupler trees and in spatially multi-core fibers. We have taken this a major step forward, coupling together a number of independent fiber lasers to obtain a spatially and spectrally coherent far field, with no active length, polarization, or amplitude control. The near field output comes from a spatial array rather than from a single fiber, making this approach scalable to extremely high power.

A 2.65-kW Yb:YAG single-rod laser

We report a continuous-wave average output power of 2.65 kW from a single Yb:YAG laser rod pumped with 9000 W from 940 nm InGaAs laser diodes. To the best of our knowledge, this is the highest average output power ever reported from a single Yb:YAG gain element. The optical-to-optical efficiency (i.e., output power to raw laser diode optical power) was 28%. We also obtained 860 W with an M/sup 2/ of 2.1 when pumping with 6000 W, obtaining 14% optical-to-optical efficiency.

High-power Yb:YAG rod oscillators and amplifiers

We report 0.95 kW average output power from a single cw-diode pumped Yb:YAG power oscillator. The 3-mm diameter solid-state laser rod is side pumped by three sets of cw diode arrays each of which has an electrical-to-optical efficiency of up to 50%. Our phase-conjugate master oscillator, power-amp architecture will incorporate this pump cavity as one of the power amplifiers for multi-kW average power, good beam quality laser applications.

Detection of 532-nm frequency-doubled Nd:YAG radiation in an active rubidium atomic resonance filter.

We report the detection of frequéncy-doubled Q-switched Nd:YAG laser radiation at 532.24 nm by an active rubidium atomic resonance filter. The Rb 5p level was populated by using a diode laser operating at 795 nm. The 532-nm laser excited atoms to the 10s level; these decay through p levels to the ground state. Signals were observed at 420/422 nm and 359 nm, corresponding to fluorescence from states 6p and 7p. Fluorescence noise was observed only on the 420/422-nm transition; the 359-nm signal had no detectable noise.

A range-resolved Doppler imaging sensor based on fiber lasers

We have developed a unique eyesafe (1.5 /spl mu/m) laser with a pseudorandom pulse output. The laser is based on a mode-locked pulse format; the pulse pattern repeats over a time consistent with the cavity round-trip period but appears random within that period. The pseudorandom pulse format occurs when we overdrive a passive mode-locking mirror in a long fiber laser. This laser is used for generating range-resolved Doppler images.

Coherent combining of fiber lasers

It seems straightforward to combine the latest high power fiber lasers together to scale up power. Yet every approach, including HRLs self-organization architecture, has proved more challenging than expected at high power. In practice power scaling elicits several undesirable phenomena whose consequences inhibit coherent combining. Stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS), for example, can together turn the relatively benign power fluctuations associated with mode beating into fiber-damaging pulses. Coherence and polarization integrity are inevitably impaired by glass-related nonlinear index changes (n2). HRL has demonstrated two to four lasers coherently combined to the 200 Watt level, using free space coupling with beamsplitters. In addition, nine independent lasers were locked in an all-fiber coupler