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.

What is claimed is: 
     
       1. A signal processor comprised of: means for generating optical signal;   means for generating a first electrical signal;   means for modulating the optical signal with said first electrical signal to produce a modulated optical signal;   a narrow-band optical notch filter for use in both the transmission and reflection modes having a finesse in the 30-250 range with a bandwidth from approximately 50 MHz to 400 GHz and a free spectral range from 10 GHz to 12,500 GHz to sharply filter the modulated optical signal;   means for transmitting the filtered modulated optical signal; and   means for converting the filtered modulated optical signal into a second electrical signal.   
     
     
       2. A signal processor, as in claim 1, wherein said means for generating an optical signal is a continuous-wave laser. 
     
     
       3. A signal processor, as in claim 1, wherein said narrowband notch optical filter is a fiber optic ring resonator. 
     
     
       4. A signal processor, as in claim 1, wherein said narrowband notch optical filter is a Fabry-Perot filter. 
     
     
       5. A signal processor, as in claim 1, wherein said means for converting the filtered modulated optical signal into a second electrical signal is a photodetector. 
     
     
       6. A signal processor comprised of: a light source for generating an optical signal;   a first electrical signal:   said first electrical signal and optical signal being applied to an external optical modulator wherein said optical signal is modulated by said first electrical signal to produce a modulated optical signal;   a narrow-band optical notch filter for use in both the transmission and reflection modes having a finesse in the 30-250 range with a bandwidth from approximately 50 MHz to 400 GHz and a free spectral range from 10 GHz to 12,500 GHz to increase the modulation depth of the optical signal thereby producing a modulated optical signal;   a predetermined length of optical fiber receiving the modulated optical signal from the filter; and   a detector on the opposing end of the length of optical fiber from the filter converting the modulated optical signal to a second electrical signal.   
     
     
       7. A method for processing an optical signal comprised of the steps of: generating a continuous wave optical signal;   modulating the continuous wave optical signal with a first electrical signal producing a modulated optical signal;   filtering the modulated optical signal with a narrow-band optical notch filter for use in both the transmission and reflection modes having a finesse in the 30-250 range with a bandwidth from approximately 50 MHz to 400 GHz and a free spectral range from 10 GHz to 12,500 GHz to filter to sharply increase the modulation depth of the modulated optical signal, and   converting the filtered modulated optical signal into a second electrical signal.