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This paper reports on the operation of lithium niobate electro-optic waveguide modulators at temperatures down to 15°K. Commercial and laboratory fiber pigtailed devices have successfully been cooled without any increases in insertion loss from temperature induced stresses in device packaging. Three x-cut devices exhibited a linear increase in Vg voltage of 8 %5:1% when cooled from room temperature to ,-20°K. The broadband frequency response improved at lower temperatures. A velocity-matched experimental modulator has shown increased bandwidth when cooled to liquid nitrogen temperature.

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Low temperature experiments have been conducted to characterize the performance of high speed photodetectors and LiNbO3 optical modulators in cryogenic environments down to 4.2°K. Metal-semiconductor-metal (MSM) photodiodes fabricated from GaAs and InGaAs have been characterized. Results demonstrate that both the responsivity and bandwidth depend on temperature. Specific modulator parameters quantified at cryogenic temperatures include bandwidth, Vn (half wave voltage), optical loss and package stability. The successfull operation of MSM photodiodes and LiNbO3 modulators at cryogenic temperatures enables a high sensitivity fiber optic approach to superconducting ciruit interfaces.