Laser light (.lamda..sub.L) within a spectrum range is generated (51.sub.g) and filtered (29.sub.g). Thereby the spectral location of the filter characteristic (.lamda..sub.F) is shifted in a controlled manner (60) to establish a desired characteristic of output laser light as for considering a temperature depended shift (.DELTA.N) of the spectrum range generated by the laser source (51.sub.g).