An optical filter for providing frequency dependant filtering of an optical input signal, the filter including: an input wave guide; an output waveguide; a polarisation separation means; a variable polarisation rotation means, the variation being dependant on a first control input signal; and a birefringent element having variable birefringence, the variation being dependant on a second control input signal; wherein an optical input signal emitted from the input waveguide passes through the polarisation separation means, the variable polarisation rotation means and the birefringent element to the output waveguide so as to produce at the output waveguide an optical output signal having a frequency dependant output characteristics determined by one of the control input signals.