B. Moslehi et al., OPTICAL AMPLIFIERS AND LIQUID-CRYSTAL SHUTTERS APPLIED TO ELECTRICALLY RECONFIGURABLE FIBER OPTIC SIGNAL PROCESSORS, Optical engineering, 32(5), 1993, pp. 974-981
We demonstrate the application of optical amplifiers and polymer-dispe
rsed liquid-crystal (PDLC) shutters to electrically reconfigurable fib
er optic delay line signal processors. Two 8-tap finite impulse respon
se (FIR) fiber filter modules were fabricated. These two modules can a
lso be interconnected in a cascaded or parallel configuration to imple
ment 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 rela
tively inexpensive PDLCs were used to realize electrically reconfigura
ble analog tap weights. The individual fiber filters were then evaluat
ed for their impulse and frequency responses. The fabricated filters u
sed single-mode fibers and fiber components and were polarization inde
pendent to within 0.5 dB. The sampling frequency was about 200 MHz, wh
ich can easily be upgraded into the gigahertz range. The tapping extin
ction ratio was about 13 dB with subkilohertz tunability speed. The am
plified spontaneous emission noise can limit the filtering performance
unless appropriate spectral filtering is included before detection. T
hese optically amplified electrically reconfigurable fiber signal proc
essors have the potential to lead to the realization of complex progra
mmable and adaptive optical systems.