Abstract

In order to use a cascade of erbium-doped fiber amplifiers (EDFAs) in a wavelength-division multiplexing (WDM) system, the power output of the signal channels must be equalized.¹ We demonstrate a system for power equalization that consists of two single-stage acousto-optical tunable filters (AOTFs) [Fig. 1(A)]. The first AOTF places notches of the correct depths at the measured frequencies to equalize the optical powers and the second AOTF monitors the optical spectrum providing feedback to the servo loop. To our knowledge, this is the first use of the AOTF as an optical spectrum analyzer in a WDM system. The monitor AOTF is driven by a frequency- swept RF signal dithered at 100 kHz with a 20-kHz modulation depth. The monitoring optical signal is measured by phasesensitive detection of the spectrum to obtain a derivative signal such that the zero crossings define the optical wavelengths and the amplitudes of the signal relate to optical powers. The integral of the detected signal is the convolution of the optical spectrum and the AOTF filter function. A computer controls the AOTF drive system, data acquisition from a phase-sensitive detector, and performs the equalization control. This system equalizes the output peak powers of two optical channels whose input powers were −3.3 dBm and −11.0 dBm, respectively. With an insertion loss of about 13.6 dB, the equalized output was −24.6 dBm as shown by the dynamic response of the system for each channel (Fig. 2). After one servo loop iteration, the two optical powers are within 0.6 dB of each other. In a long EDFA chain, the servo loop iteration time is not critical. However, iteration timing is very critical in a laboratory recirculating loop. Using the AOTF as the spectral measurement device [Fig. 3(A)] allows complete flexibility and tolerance of the optical wavelengths in the system. A feedback servo loop proportionally and derivatively controls the equalizing notch depths, which are then applied to the equalizing stage at the measured resonant frequencies by an arbitrary function generator. The equalizing stage of the AOTF functions as a wavelength addressable analog switch, which depletes each wavelength channel by the amount of power required to equalize the output powers² [Fig. 3(B)].

© 1996 Optical Society of America