The series D photodetectors from Martin, Froeschner & Associates are designed to detect weak high-speed signals on unpredictably varying strong backgrounds. Frequency response is from true DC to as high as 10 GHz, with usable gain out to 20 GHz. Overall optical gain ranges from 200 to 2500 V/W. The photo-diodes are directly coupled to the input fiber end and have spectral ranges of 450 to 870 nm for GaAs or 850 to 1700 nm for InGaAs. The fully enclosed modules may be connected directly to a scope or incorporated into a system. They can be powered by battery or electricity.