An apparatus (8, 14) for wavelength shifting light from a first frequency .nu..sub.1 to a second frequency .nu..sub.2 is provided in the form of a semiconductor intersubband laser (8) lasing at its intersubband frequency .nu..sub.3. The output light is wavelength shifted to a frequency .nu..sub.2=.nu..sub.1+n.nu..sub.3, where n is a non-zero integer ( . . . , -2, -1, 1, 2, . . . ). The wavelength shifted light .nu..sub.2 may be amplitude modulated and/or frequency modulated to impart information upon it. The wavelength shifting is a coherent process allowing for the possibility of coherent communication techniques to be used.