As the demand for optical fiber communications bandwidth grows, the implementation of signal processing functions using all-optical techniques becomes increasingly attractive. In recent years, a number of methods have been used to perform functions such as wavelength conversion for WDM systems, gated mixing for TDM multiplexing and demultiplexing, spectral inversion for dispersion compensation, and all-optical switching. Three-wave mixing in c(2) media is an attractive approach, presenting a combination of low pump power, wide bandwidth, and negligible degradation of signal to noise ratio. In this paper, we describe optical frequency mixers implemented using annealed proton exchanged waveguides in periodically poled lithium niobate. These devices have been used in a variety of system experiments. We present several WDM demonstrations, including wavelength conversion, dispersion compensation by mid-span spectral inversion, and compensation of Kerr nonlinearities. We also discuss TDM demonstrations such as efficient all-optical gating and multiplexing/demultiplexing of high bit-rate data streams.