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.