All-optical automatic gain control in loop erbium-doped fiber amplifiers

All-optical automatic gain control in loop erbium-doped fiber amplifiers is investigated using single Bragg reflecting fiber grating. Reflecting back part of the filtered amplified spontaneous emission to the loop mirror generates a lasing signal. The lasing wavelength is selected at 1525 nm, which is outside the amplifier bandwidth, to avoid interference between the signal and laser oscillation, and away from the amplifier surveillance wavelength at 1510 nm. This technique avoids tapping part of the amplifier output signal for generating the lasing signal. The loop amplifier has 20 nm flat gain spectrum with peak to peak gain variation less than 0.5 dB. Adjusting the lasing threshold using variable optical attenuator placed before the fiber Bragg grating controls the achieved gain clamping. An input signal consists from 8 channels picked up on the ITU frequency grid in the range from 1540 nm to 1560 nm and two laser diode pumps at 980 nm each with 60 mW pumping power are used in the experiments. The power excursion when adding or dropping any number of the 8 channels with -25 dBm input power per channel is less than 0.3 dB. The transient response time of the surviving channel(s) is measured to less than 200 mu s.