Patent ID: 6031648
Filing Date: 2000-02-29
Classification: H04B

Abstract:
Apparatus for free-space optical transmission of information comprising:a transmitter for optically transmitting, via free space, said information together with pilot information of fixed strength said transmitter including:a first source of light having a first frequency which is amplitude-modulated with said information,a second source of light having a second frequency and constant amplitude in order to represent said pilot information, said first and second frequencies being different from one another, anda combiner for launching said light from said first and second sources onto a substantially common optical path through said free space; anda receiver for receiving from said free space said information and said pilot information, said receiver including:a detector for producing an output indicative of the strength of said pilot information as received by said receiver, said detector including a first photodetector assembly which is responsive to light having substantially only said first frequency for producing a first electrical output signal indicative of said information as received by said receiver, a second photodetector assembly which is responsive to light having substantially only said second frequency for producing a second electrical output signal indicative of the strength of said light of said second frequency as received by said receiver,a reference signal source for producing an output of constant amplitude,an amplifier for producing an output indicative of an amount by which said output of said second detector differs from said reference signal source output, anda variable gain amplifier for amplifying said information as received by said receiver by an amount influenced by said output indicative of said amount by which said output of said second detector differs from said reference signal.