A high resolution measurement method and apparatus for tracking wavelength transients in tunable lasers. The apparatus comprises a Mach-Zehnder interferometer (MZI) which is used to generate a self-heterodyne signal between the wavelength transient to be measured, which is effectively the laser signal passed along the time-delayed arm of the MZI, and the laser wavelength after the tuning transient has subsided, which is effectively the same laser signal passed along the direct arm of the MZI. The heterodyne signal is detected on a receiver, and can then be measured with the frequency resolution typical of electronic measurements, such as by means of an oscilloscope. The only laser required is the laser under inspection. The wavelength measurement accuracy is up to twice the laser linewidth, and is only effectively limited by the laser phase noise. The method can be used to implement an automatic frequency control system for tunable lasers.