There is discussed an optical system comprising a laser device that outputs a divergent light beam. A first portion of the divergent light beam, including a central portion, passes through an etalon device, which acts as a wavelength discriminator, and then the central portion is incident on a first monitor photodiode, which generates a wavelength-dependent detection signal. A second portion of the divergent light beam is incident on a second monitor photodetector, without passing through the etalon device, to generate a wavelength-independent detection signal. A processor processes the wavelength-dependent detection signal and the wavelength-independent detection signal to determine a control signal for controlling the wavelength of the laser device. By accurately positioning a photodetector at the central fringe of the divergent light beam following transmission through the etalon device, a compact and cost-effective wavelength locking arrangement is provided.