Wideband fiber-optic signal processor

Abstract

In the transmission electrical signals using an optical carrier the signal power carried by an optical carrier is proportional to the optical power. High optical power levels are desired for optical carriers that are modulated by weak electrical signals, however, significant unmodulated power is left in the original carrier after processing. To maximize the radio frequency (RF) signal power generated by a given (maximized) photodetector current (for a given input power), in this invention, the optical carrier power is reduced. This is accomplished by the addition of a narrowband optical filter, such as a Fabry-Perot filter, to reduce the average optical carrier power without reducing the modulation sidebands, which results in an increased modulation depth. Therefore, greater RF and microwave power is generated by a photodetector with the same photocurrent. With a laser source exhibiting a beam whose linewidth is narrow compared to the filter bandwidth, the filter is tuned, or frequency locked, to match the filtering resonance with the optical carrier wavelength. With the high finesse filter and narrow-linewidth laser beam, the frequency range of operation is extended from the filter half-bandwidth to the next resonance of the filter. Thus, broadband effective gain results without introducing additional noise.