All-optical signal processing for application in telecommunication networks has been studied extensively over the last decade, because signal processing in the optical domain offers a number of advantages. Using optical processing makes opto-electrical / electro-optical conversion in the network nodes unnecessary, which can potentially be more cost-effective. Moreover, the bandwidth of the signal can be significantly larger, compared to electrical processing, because the optical signal processor can work at higher bit rates.
Optical signal processing can make use of a number of phenomena and devices to perform certain processing functions. These nonlinear optical processes are for example four wave mixing (FWM), self phase modulation (SPM), cross phase modulation (XPM), sum frequency generation (SFG), difference frequency generation (DFG), and second harmonic generation (SHG). Various optical materials and devices can be utilized for optical signal processing, such as highly nonlinear fibers (HNLFs), silicon waveguides, chalcogenide waveguides, periodically poled lithium niobate (PPLN) waveguides, semiconductor optical amplifiers (SOAs), and photonic crystals or photonic crystal fiber. An additional advantage of optical compared to electrical signal processing is that optics can make use of all four dimensions of wavelength, polarization, amplitude and phase. Recently, nonlinear effects in multimode fibers are investigated, which could add an additional degree of freedom by utilizing the optical mode.
Examples of functions that can be performed with nonlinear devices are:
- Wavelength conversion
- Optical multiplexing and demultiplexing
- Fast / discrete Fourier transform
- Digital-to-analog (D/A), and analog-to-digital (A/D) conversions
- Optical logic gates (AND, OR, XOR, etc.)
- All-optical tunable delays