A new amorphous Si20Te80 glass alloy with high optical transparency has been synthesized and investigated. A modulator based on the amorphous Si-Te acousto-optic cell and operating in the mid-IR region has been manufactured and tested using CO2 (10.6 µm), He-Ne (3.39 µm) gas lasers and 3.3 µm (based on InGaAsSb/InAsSbP double heterostructure), and 1.87 µm (based on GaInAsSb) diode lasers. The modulator response time appeared to be not more than 0.3 µs which together with the high modulation efficiency (up to 90%) make possible use in many applications, e.g. for modulation of CW laser radiation, for optical gas sensor applications, for diode laser spectroscopy to make mode selection of wavelength selection within a lasing pulse.