(EN) A first optical port of vapour cell 1 receives a probe laser beam which leaves the cell via a second optical port. A first portion of the emerging probe beam is directed to optical modulator 3 which applies a frequency modulation, according to a first electrical signal, to form a frequency-modulated pump beam. This is directed back into the cell via the second optical port to superpose with the probe beam within a vapour. A second portion of the emerging probe beam is directed to optical modulation detector 5 to generate a second electrical signal. Signal comparator 4 compares the first and second signals to provide a comparison electrical signal for stabilising the frequency of a laser. The modulator may comprise a piezo-actuated retroreflector, or electro-optical modulator providing a path of varying refractive index. A beam splitter may direct the portions of the emerging probe beam to the modulator and to the detector. A volume of the apparatus may be less than 14 cm3 or less than 5 cm3.