Simple and low-cost technique for wavelength division multiplexing channelmonitoring

Operation and management of wavelength division multiplexing (WDM) systems require the monitoring of optical channel frequency and power. We propose a simple and low-cost solution for tracking the frequency of WDM channels, b ased on the thermooptic tuning of single or coupled-cavities Fabry-Perot si licon optical filters. The fabricated structures are single-cavity filters exhibiting, thanks to a suitable coating stack on both cavity sides, high f inesse and narrow bandwidth. Moreover, the free spectral range is large com pared to the channel spacing, enabling the monitoring of one carrier freque ncy at a time. By means of a heater, we change the cavity refractive index and move the transmission peaks, thus scanning the WDM frequency set. In pa rticular, we demonstrate the possibility to resolve up to seven 50-GHz-spac ed channels with a crosstalk of -10 dB at wavelengths around 1550 nm. Bette r performances, in terms of resolvable channels and crosstalk can be obtain ed by using two coupled cavities, having a common resonance peak and differ ent free spectral ranges. The global optical transfer function of such a ca scade shows only one transmission peak in a frequency range of about 30 nm, and can be thermally tuned in this range. (C) 2000 Society of Photo-Optica l Instrumentation Engineers. [S0091-3286(00)01506-3].