The high available bandwidth of optical fibres has led to a large interest in wavelength division-multiplexed (WDM) networks. Such systems rely on the functionality provided by tunable sources, receivers and filters. Components must be quickly tunable over a large range, have a narrow-band channel response, low insertion loss and be robust and repeatable in their operation. Various wavelength filtering techniques have been previously proposed. These have been based on liquid crystal birefringent slabs [1], Fabry-Perot cavity filters [2], acousto-optic surface wave effects [3] and semiconductor resonant amplification [4]. A method of space-wavelength switching using a mechanically rotatable grating has also been demonstrated [5]. Such methods are generally analogue in operation and require complex calibration or control techniques.