DIODE-PUMPED frequency-doubled LASERS
In ongoing efforts to develo¥better sources for uranium isotope separation, researchers at CEA (Atomic Energy Agency; division Centre d`Etudes de Saclay, France) recently reported achieving more than 100 W of average power in the green from a diode-array-pumped frequency-doubled Nd:YAG laser. This level of second-harmonic generation performance required simultaneous development of a very efficient laser cavity, use of high-power diode-laser arrays with stable output wavelengths, and the inte
DIODE-PUMPED frequency-doubled LASERS
Array-pumped Nd:YAG laser produces 100 W
In ongoing efforts to develo¥better sources for uranium isotope separation, researchers at CEA (Atomic Energy Agency; division Centre d`Etudes de Saclay, France) recently reported achieving more than 100 W of average power in the green from a diode-array-pumped frequency-doubled Nd:YAG laser. This level of second-harmonic generation performance required simultaneous development of a very efficient laser cavity, use of high-power diode-laser arrays with stable output wavelengths, and the integration of the arrays into the laser head. Team members B. J. LeGarrec, G. H. Razé, and coworkers met these requirements with diode arrays supplied by Thomson-CSF Semiconducteurs Specifiques (Orsay, France).
Thirty-five CW 20-W diode-laser arrays are optically coupled to a Nd:YAG rod. The diode-laser module includes 35 bars in five water-cooled submounts arranged around the laser rod (see Fig. 1). Each submount includes seven bars, and each bar is welded onto a specific water-cooled anode.1 This concentric arrangement allows efficient pumping of the YAG laser rod, which is doped with 1% neodymium and is 6 mm in diameter and 130 mm long. The laser rod is placed inside a glass sleeve to allow water cooling.
To match the Nd:YAG absorption band around 808 nm, the 35 bars were selected to get the lowest spectral width for a given diode drive current and a given cooling temperature. At an operating current of 30 A and temperature of 20C, the bandwidth (FWHM) is only 4 nm. The recorded pum¥density distribution shows a central peak in the fluorescence profile at 1064 nm, which is recorded with a low-aperture CCD camera. The researchers explored this distribution at 1064 nm through both gain and threshold measurements for different CW cavity configurations.2
The 1064-nm output characteristics of the side-pumped laser rod have been investigated in a linear plane concave resonator (300 mm long) with a high-reflector mirror. Multimode output of 180 W was achieved with 30-A diode drive current and 700 W of average pum¥power. To reach high average power with intracavity second-harmonic generation requires a specific laser resonator configuration. Classical linear or L-shape cavities are not well suited because the spot size in the nonlinear crystal decreases when the pum¥power increases. This high-power density enhancement can cause crystal damage.
The Saclay researchers turned to a Z-shaped resonator (proposed by Kuizenga and coworkers).3 In this design, two concave mirrors between the laser rod and the nonlinear crystal act as an optical relay that images the laser rod aperture into the nonlinear crystal. Making the optical relay with two mirrors (radii of curvature 200 and 100 mm) increases the beam by 50% before it enters a 4 ¥ 4 ¥ 6-mm potassium titanyl phosphate (KTP) crystal. The laser is acousto-optically Q-switched at high repetition rates (in the range of 15 to 30 kHz), and the Q-switch off-time and RF power are adjusted for maximum output power. The Z-shaped laser resonator enables second-harmonic generation along two opposite directions into the KT¥crystal, and the two primary 532-nm beams are extracted through the mirror closest to the KT¥crystal.
At diode-laser drive currents between 25 and 30 A, the pulsewidth (FWHM) ranges from 215 to 245 ns with, respectively, 100 W and 75 W of average power at 532 nm (see Fig. 2). The pulse-to-pulse stability is better than 1%, and the pulse-to-pulse jitter does not exceed 10 ns at a 25-kH¥repetition rate. The researchers used two different KT¥crystals (one from Litton Airtron, Charlotte, NC, and the other from CSK Optronics, Los Angeles, CA). They achieved green output of 101 W over a one-hour period with the Litton crystal and 106 W with the CSK crystal; the power stability was better than 0.5%. The same crystal was operated for nine hours with an average power greater than 75 W. These encouraging green output-power levels give the researchers hope that reliable and compact frequency-doubled diode-pumped lasers could be an interesting alternative to the French SILVA program approach, in which copper-vapor lasers are used for isotope separation.
1. B. J. LeGarrec and G. Razé, French Patent #95-13400, November 13, 1995.
2. B. J. LeGarrec et al., "High average power diode array pumped frequency doubled Nd:YAG laser," submitted to Opt. Lett.
3. D. J. Kuizenga, US Patent #4907235, June 3, 1990.