Device to detect frequency converted signals with high efficiency

Abstract

The purpose of the invention is to separate and detect a weak frequecy converted signal light beam (f.sub.s +f.sub.p) which is in the presence of a much stronger pump light beam (f.sub.p) at a slightly different frequency. Such a device is useful in laser radars and laser transmitted communication systems where the information is transmitted as a light beam of frequency f.sub.s but more conveniently selected at another frequency f.sub.s tf.sub.p. A specific example would be laser communication system using a carbon dioxide laser (10.6 micron wavelength 3.times.10.sup.13 Hz frequency). In some applications a cooled detector (required for most efficient detection at this frequency) may not be feasible and a frequency conversion detector, operating with uncooled detectors, would be preferred. The device comprises a nonlinear crystal which first receives a properly phased matched pump light beam (f.sub.p) and a signal light beam (f.sub.s) and produces a third beam (f.sub.s +f.sub.p) at a different frequency. The output of the nonlinear crystal passes through a filter which greatly attenuates the beam f.sub.p and passes the beam f.sub. s +f.sub.p with little or no attenuation. The beams leaving the filter are than passed through a prism where the residual beam f.sub.p is separated from the beam f.sub.s +f.sub.p and directed onto a bi-cell. The beam f.sub.p is directed with substantially one half of the beam on one cell and the other half on the other cell of the bicell. With the beam f.sub.s turned off the intensity of the two cells of the bi-cell are balanced and subtracted to produce a minimum output signal. The beam f.sub.s is then again turned on and the change in the output is an indication of the intensity of the beam f.sub.s +f.sub.p.