Abstract: An external cavity semiconductor laser light source which comprises: an external cavity semiconductor laser light source unit; an optical filter for selecting an output beam of the external cavity semiconductor laser light source unit in a single mode; a drive unit for varying a wavelength of a transmitted light or a wavelength of a reflected light, from the optical filter; a wavelength control unit for controlling the drive unit; an optical frequency reference light source for generating a light having a reference optical frequency; an optical frequency/voltage conversion unit for generating a signal which corresponds to a difference in optical frequency between the light outputted from the optical frequency reference light source and the output beam of the external cavity semiconductor laser light source unit; a low pass filter for transmitting a low-frequency component of the signal of the optical frequency/voltage conversion unit; a switch for feeding the signal into the external cavity semiconductor ligh

Abstract: A wavelength tunable semiconductor laser light source, which has an external resonator semiconductor laser source unit; an optical filter for selecting an output beam of the external resonator semiconductor laser source unit in a single mode; a drive unit for changing wavelength of a transmitted beam or of a reflected beam, from the optical filter; and a control unit for controlling the drive unit; a first beam splitting device for receiving the output beam of the external resonator semiconductor laser source unit as one of incident beams and for outputting it into two branches; an etalon for receiving one of output beams from the first beam splitting device; a measuring unit for etalon transmittance, for measuring a transmittance of the etalon to transmit it to the control unit; and a wavemeter for specifying a wavelength of the output beam of the semiconductor laser source unit with an accuracy in a free spectral range of the etalon, on the basis of the output beam of the semiconductor laser source unit or

Abstract: A wavemeter apparatus for a precise determination of a wavelength of a target light beam and its measuring error is presented. The apparatus is an interferometer type apparatus which computes fractional part of the interference fringes. The apparatus includes a beam splitter (1) to divide the target light beam into two beams which are injected into a fixed mirror (2) and a moving mirror (3) and the reflected beams are inputted into a photoreceiver (4) to generate interference fringes. The photoreceiver (4) generates a first pulse signal (21) for every fringe spacing given by .lambda./2 and the distance measuring device (5) generates a second pulse signal (22) for each distance d travelled by the moving mirror given by a resolution capability of the distance measuring device (5). A computation section (6) computes count values M and N(M), for the first pulse signals (21) and the second pulse signals (22), respectively. The fractional part of the count .epsilon.M is computed by using a relation M.lambda.