Optical amplifiers and liquid-crystal shutters applied to electrically reconfigurable fiber optic signal processors

We demonstrate the application of optical amplifiers and polymer-dispersed liquid-crystal (PDLC) shutters to electrically reconfigurable fiber optic delay line signal processors. Two 8-tap finite impulse response (FIR) fiber filter modules were fabricated. These two modules can also be interconnected in a cascaded or parallel configuration to implement a 16-tap fiber FIR filter. An erbium-doped fiber amplifier with a peak gain of about 20 dB was used to compensate for the large optical losses involved in the filters due to the large tap numbers. The relatively inexpensive PDLCs were used to realize electrically reconfigurable analog tap weights. The individual fiber filters were then evaluated for their impulse and frequency responses. The fabricated filters used single-mode fibers and fiber components and were polarization independent to within 0.5 dB. The sampling frequency was about 200 MHz, which can easily be upgraded into the gigahertz range. The tapping extinction ratio was about 13 dB with subkilohertz tunability speed. The amplified spontaneous emission noise can limit the filtering performance unless appropriate spectral filtering is included before detection. These optically amplified electrically reconfigurable fiber signal processors have the potential to lead to the realization of complex programmable and adaptive optical systems.