Investigation of GaAs modulators at 10.6 and 337 microns
Epton, Paul Joel
Wilson, William L.
Master of Science
An attempt was made to electrooptically modulate the 1.6 |i,m light from a COj laser. The modulator was a 13 m thick epitaxial layer of GaAs grown on a degenerately doped GaAs substrate. The modulator was situated in a microwave cavity and DC biased into Limited Space-Charge Accumulation (LSA) mode oscillation. The electric field for the electrooptic effect was provided by the DC bias pulse and the microwave oscillation. Bias pulse heating of the GaAs substantially changed its infrared transmission and prevented observation of any electrooptic modulation that occurred. A study was also carried out of wide bandwidth amplitude modulation of the 337 p,m light from an HCN laser. The modulator was a 1 thick epitaxial layer of GaAs grown on an insulating GaAs substrate. The doping density of the epilayer was ND = 4.61x1 cm and NA 2.45x1^3 cm. The modulator was immersed in liquid helium, freezing out the free carriers on the donor impurities. Pulsed microwave radiation was used to impact ionize the impurities for modulation, and the transmission of the GaAs increased as the neutral donors were ionized. A peak in the modulation index of 29% was obtained at 2 mW of absorbed microwave power for the GaAs situated in a terminated waveguide. A 1% modulation index with better coupling of the 337 pm light and poorer microwave coupling was obtained for a microwave cavity configuration. From a measurement limited by the detector bandwidth, the modulator risetime was found to be less than 15 ns.