A method and an apparatus for optical frequency measurement, using one or two frequency-stabilized mode-locked laser combs operating at different repetition rates to mix with a laser under measurement respectively so as to generate two beat frequencies. The first section provides a low-selective interferometric filtering together with an amplification of a light wave resonating in the laser structure. The functional test procedure creates random cache access sequences that cause varying loads of traffic in the cache memory using a test pattern based on a random seed. With the use of the liquid coating method to form the filter, productivity is increased, manufacturing costs are minimized, and scratch resistance is provided. A digital to analogue converter 27 is coupled to receive the digital representation of a radio frequency signal 26 comprising multiple radio frequency bands, and adapted to convert the digital representation of a radio frequency signal 26 into an analogue radio frequency signal 28 comprising multiple radio frequency bands. The central waveguides are positioned such that the grating axis passes through the port associated with one of the central waveguides or passes between the ports associated with the central waveguides.