One Gigabit Wideband Modulator

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Abstract
An exploratory development of a 1 Gbps single-pass single-unit (unmultiplexed) modulator for use with a mode-locked and frequency-doubled neodymium YAG laser in a space laser communication system was conducted for the Air Force Avionics Laboratory. The modulator was to operate at data rates up to 1 Gbps with a 0.53 micrometer laser input. The performance goals were (1) a static extinction ratio of 100 to 1 or greater, (2) a worst case dynamic extinction ratio of 30 to 1 or greater with a 1 Gbps pseudorandom code input, (3) 100% depth of modulation, and (4) at least 80% transmission of the beam input power statically. This effort consisted of a 1 Gbps 0.53 micrometer modulator and driver in breadboard form used as a laboratory tool to demonstrate the feasibility of this modulator design concept. The modulator development utilized lithium tantalate crystals in four different configurations. The modulator drivers were designed with both thin film hybrid and discrete component input stages with the dual output stage using only discrete components. Various modulator and modulator driver combinations were used in the dynamic performance evaluations. The performance of the best modulator and modulator driver, met most of the design goals. The modulator driver had 20 volts ±1 volt outputs with timing inaccuracies of ≤ ±175 ps including PN generator timing inaccuracies of ≤ ±70 ps. The driver had rise times between 400 ps and 500 ps. The modulator had two 10mm long lithium tantalate crystals tapered in the c-axis in order to reduce the switching voltage by 20% which resulted in a dc half-wave switching voltage of 19.8 volts at 0.53 pm wavelength. This modulator had a static extinc-tion ratio of 90:1 and a worst case dynamic extinction ratio of ≥ 22.4:1 with a 1 Gbps pseudorandom code. The depth of modulation was 100% since the driver applied full half-wave switching voltage to the modulator. The modulator (crystals only - single pass) transmitted 81.5% of the beam input power statically.

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