The Twin Head Efficient Oscillator (THEO) concept uses a pair of smaller, identical laser pump modules, oriented to remove asymmetrical thermo-optical effects typical in single-slab lasers such as HOMER (High Output Maximum Efficiency Resonator), MLA (Mercury Laser Altimeter), LOLA Lunar Orbiter Laser Altimeter, and GLAS (Geoscience Laser Altimeter), while simultaneously increasing efficiency and lifetime. This creates 100+ mJ pulses in an oscillator-only design, with reduced risk of optical damage, record efficiency, high stability, long life, and high TEM00 beam quality typical of much smaller rod-based cavities. Near-field-beam quality is critical to efficient second harmonic generation (SHG 532 nm), which is typically poor in slab-based Nd:YAG lasers.

The new THEO design uses HOMER’s “floating” optical bench idea and flight-quality optical mounts to maintain precise alignment with minimum mass, while being thermomechanically isolated from induced stresses the enclosure may experience. The result is a highly optimized, oscillator-only laser system, promising even higher efficiencies, improved beam quality, longer lifetimes, and over four times the pulse energy than was recently demonstrated with HOMER, which has the highest efficiency and lifetime reported for any laser of its type and class.

This innovation includes a dual-head gain module design, which removes the thermal lensing effects and allows for possible compensation by setting the proper curvature of the high reflective end mirror and fine-tuning each of the two laser diode array pump parameters. The unstable resonator cavity, in combination with the Gaussian reflective output coupler, enables aperture-free, high-power, single spatial mode laser beams in an oscillator-only zigzag slab design. This is not possible with stable cavities. The oscillator-only design produces a very highly efficient design, unmatched by earlier works or published MOPA (masteroscillator- power-amplifier) laser systems.

The cavity in this innovation requires far fewer optical components, a full third that of the GLAS lasers of equal pulse energy, which directly translates to reduced mass, footprint, cost, complexity, and risk. The dual-head feature also allows compensation for beam quality tenability, should individual diode arrays or diode bars begin to drop prematurely. As bars reduce in performance over the life of the system, the electrical drive parameters of each head can be fine-tuned to maintain excellent beam quality, and to maintain total system BOL (beginning- of-life) performance parameters.

Finally, the long-term test was completed after the THEO laser passed 2 billion shots with zero measurable decay, and a pulse energy of 108 mJ. At the time of this reporting, THEO has been packaged and initially tested ready to advance officially to TRL-6 (technology readiness level) with proper modeling and environmental testing, but awaiting a NASA mission to fund the effort.

This work was done by Donald Coyle and Robert Frederickson of Goddard Space Flight Center; Paul Stysley, Richard Kay, and Demetrios Poulios of American University; and Kenneth Cory and Gordon Blalock of Sigma Space Corp. GSC-16362-1

Photonics Tech Briefs Magazine

This article first appeared in the January, 2014 issue of Photonics Tech Briefs Magazine.

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