Frequency Doubling of CW And Pulsed CO 2 Lasers Using Diffusionbonded, Quasi-Phase-Matched Gaas Stacks
- PDF / 2,590,856 Bytes
- 8 Pages / 414.72 x 648 pts Page_size
- 62 Downloads / 150 Views
ABSTRACT We describe the fabrication, characterization and frequency doubling properties of first order stacks of diffusion bonded 2" diameter GaAs wafers. Near IR imaging through the stacks indicated that excellent bonding was obtained over 40%-70% of the central area of the wafers. A power spectrum analysis of the spectral noise (due to interface reflections) appearing in the transmission data is shown to be a quantitative diagnostic tool useful for determining interface quality and for accurately estimating the thickness of the bonded layers in a stack. The conversion efficiency for a four layer stack at 10.6 microns was found to be 0.03% or 3 mJ for a 10 mJ pulse with a 100 nanosecond pulse length. The corresponding efficiency for cw SHG was found to be 0.0002%. INTRODUCTION In 1962, Armstrong et. al.' suggested that is would be possible to use a stack of wafers of a semiconductor such as GaAs with proper crystal orientations and thicknesses to efficiently convert the wavelength of a pump laser via the nonlinear processes of second harmonic generation or optical parametric oscillation. The high value of the second order nonlinear optical susceptibility, the laser damage threshold and the thermal conductivity of GaAs make it a suitable candidate for frequency conversion of multiwatt lasers. The wafer thickness should be an odd multiple of the coherence length of the pump wavelength which for GaAs is in the range of 103107 microns. 2,3 The orientation should be such that the E vector will be parallel to the [1,1,1] direction in the crystal. Most recent work has utilized a stack of the form (100),( 10), -(110), (110), -(110) ....... (100) where the (100) wafers are inactive end caps. The efficiency increases as the square of the number of wafers in the stack and it is inversely dependent on the stack order. In 1996 Szilisagi et. al. and Thompson et. al. successfully demonstrated the viability of this approach. Szilsagi et. al. used a 3rd order stack of 5 (110) wafers aligned at the Brewster Thompson et. al. used a 1st order stack of nine angle to eliminate Fresnel reflection losses. (111) wafers also aligned at the Brewster angle. Note that to attain conversion efficiencies above 50%, ninety or more 100 micron thick wafers are required. As the 1st order stack wafer thickness is nominally 1/5 the thickness of a conventional wafer, the wafers are quite fragile and free standing wafers stacks did not seem to be practical. Additionally, tolerance to error for large n is almost- non-existent. However, in 1993 Gordon et. al. proposed diffusion-bonding GaAs wafer stacks as an approach to overcome some of these limitations. In principle, this approach could make quasi-phase matched GaAs stacks competitive with and potentially a replacement for the two state-of-the-art nonlinear optical crystals, ZnGeP2 and AgGeSe 2 and the emerging materials, CdGeAs 2 and AgGaTe 2.6 Pragmatically, this approach deserves a serious assessment and the results of one such assessment will be described in this paper. The fabrication process for
Data Loading...
