An injection laser driver regulates the bias and modulation depth by means of two feedback control loops, one deriving its feedback control signal from a measure of the mean optical output of the laser, and the other deriving its feedback control signal from a measure of the spectral purity of that optical output. The spectral purity measure is conveniently effected by measuring the coherence function at a specific value of differential delay using a Mach-Zehnder interferometer configuration with a passive quadrature optical network.