A simple in-fibre technique for the measurement of small frequency separations is described. A sinusoidal frequency modulation, applied to the source, is converted into phase modulation of an optical carrier, thereby enabling measurement of the frequency modulation amplitude. The source must be continuously tunable, which makes the technique particularly suitable for measurement of frequency shifts produced by injection current modulation of solid state lasers. The technique is demonstrated by incorporation into an electronic speckle pattern interferometer (ESPI) surface contouring system.