An apparatus and to a method of monitoring an interferometer, comprising the steps of: coupling a first optical signal into the interferometer and into a wavelength reference element, detecting a first resulting interference signal being a result of interference of parts of the first optical signal in the interferometer, detecting a resulting reference signal of the wavelength reference element, the resulting reference signal being a result of interaction of the first optical signal with the wavelength reference element, and comparing the first resulting interference signal with the resulting reference signal to detect a drift of the interferometer, if any.