A wavemeter for determining a wavelength of an incident optical beam comprises four optical components, each being arranged in the incident optical beam or in a part of it, providing a path with a respective effective optical length, and generating a respective optical beam with a respective optical power depending on the wavelength of the incident optical beam. The optical powers oscillate periodically with increasing wavelength, and a phase shift of approximately pi/2 is provided between two respective pairs of the four optical components. Respective power detectors are provided, each detecting a respective one of the optical powers. A wavelength allocator is provided for allocating a wavelength to the incident optical beam based on the wavelength dependencies of the detected first, second, third, and fourth optical powers.