Optical orthogonal frequency division multiplexing (OFDM) utilizes orthogonal sub-carrier channels whose symbol rates are equal to their frequency spacing. Optical OFDM is effective in increasing the spectral efficiency of optical communication, and is applicable for highly spectral-efficient and adaptive optical networks including an elastic network as well as point-to-point transmission. As transmission capacity is varied depending on the traffic and transmission distance in these adaptive optical networks, we need to develop an OFDM signal demultiplexer with high-speed processing and low-power consumption, which can flexibly deal with various symbol rate and number sub-carrier channels in the optical domain. We previously reported an integrated-optic demultiplexer for variable optical OFDM signals, which is composed of an array of variable optical attenuators (VOAs) before a slab star coupler-type optical discrete Fourier transform (DFT) circuit. However, this demultiplexer showed large loss variation (several dB) when changing its characteristics in response to various symbol rate OFDM sub-carriers.
In this presentation, I propose and report a tunable optical OFDM demultiplexer that consists of an array of VOAs, optical DFT circuit, and an array of Mach-Zehnder interferometer-type tunable couplers. The newly added array of tunable couplers after the optical DFT circuit could efficiently utilize output lightwave and decrease the loss variation (intrinsically zero). I report the operating principle of the proposed tunable demultiplexer, its characteristics evaluation, and preliminary experimental results of the tunable demultiplexer fabricated with silicon waveguide technology. The size, processable channel number and symbol rate of the demultiplexer were 1 mm x 8 mm, 2 to 8 and 10 to 40 Gsymbol/s, respectively.