Athermal Solid-State Lasers


S.R. Bowman

Optical Sciences Division

Introduction: Conventional solid-state lasers are exothermic. The processes of optical excitation and stimulated emission can never yield unit quantum efficiency. The residual energy from this quantum defect results in waste heat generation within the lasing medium. In high-power solid-state lasers, heat loads are often in excess of 500 W/cm3. This magnitude heat load severely distorts laser beam quality and restricts the average power of laser systems.

In pursuit of higher power, NRL is investigating a new mode of laser operation called Radiation Balanced Lasers (RBLs). The basic concept of the RBL is to use anti-Stokes fluorescence to offset the quantum defect of the stimulated emission.1,2 Under ideal conditions, the absorbed power balances the radiated power at each point within the laser material and waste heat is eliminated. The radiative balance is maintained by controlling wavelengths and intensities within the cavity. Operation in the RBL mode removes thermal restrictions on the laser medium, allowing increased laser volume and average power.

Fig 4

FIGURE 4
The prototype Radiation Balanced Laser included eight high-brightness diodes that end-pumped a small Yb:KGW crystal (red slab at the center).

The Prototype: A prototype device has been demonstrated at NRL. Material studies suggested that the protoytpe take the form of a Yb3+ doped KGd(WO4)2 resonator.3 This device used eight, 25-W laser diodes to end-pump a 1 x 2 x 10-mm slab of Yb:KGW (Fig. 4). The diode pumps were centered at 1001 nm, and the Yb laser operated at 1040 nm. The room temperature fluorescence of the Yb:KGW is centered at 992 nm. This laser operated at 3.3% wall plug efficiency with only 0.42% heat generation in the KGW crystal. Current efforts to optimize materials and lasing conditions should reduce this heat load even further. Scaling of this RBL concept to the kilowatt level is currently underway.

[Sponsored by ONR and NAVSEA]
References

1 S.R. Bowman, "Lasers Without Internal Heat Generation," IEEE J. Quantum Electron. 35, 115-122 (1999).
2 S.R. Bowman, "Non-Exothermic Quasi-Two Level Laser," U.S. Patent #6,370,172, issued April 2002.
3 S.R. Bowman, N.W. Jenkins, B. Feldman, and S. O'Connor, "Demonstration of a Radiatively Cooled Laser," Conference on Lasers and Electro-Optics, Long Beach, CA, June 2002.