Abstract: Described herein is a system and method of controlling an optical heterodyne measurement system (1). The measurement system (1) has a tunable laser (9) for generating a local oscillator signal, an optical input (5) for receiving an input optical signal (7) and a mixing module (13) for mixing the local oscillator signal with the input optical signal to generate an output optical measurement signal. One embodiment provides a method including the steps of: a) receiving an input electrical drive signal for driving the tunable laser (9) to produce a laser output having a spectral linewidth and peak central frequency; b) coupling the input electrical drive signal with an electrical linewidth control signal (54) to selectively broaden the spectral linewidth; and c) during a measurement period, selectively tuning the central frequency of the laser in a stepwise manner across a predetermined frequency spectrum at predefined tuning increments.