a semiconductor laser having a predetermined frequency noise spectrum and a predetermined tuning response, said semiconductor laser further having tuning means and operating at an optical frequency tuneable by said tuning means, said laser providing an output signal;

a low noise current source operatively connected to said laser for supplying current thereto, said current source being particularly adapted to prevent a significant degradation of the frequency noise spectrum of the laser;

an optical frequency discriminator for establishing a frequency reference for said laser, said optical frequency discriminator having a predetermined frequency response and being operatively connected to a portion of the output signal of said laser, the optical frequency discriminator providing an error signal representative of a noise in the frequency of said laser with respect to said frequency reference;

control means operatively connected to said error signal and to said laser tuning means, said control means providing a frequency feedback signal to the tuning means in response to said error signal and comprising:

distinct compensation networks for compensating the error signal with respect to the frequency response of said frequency discriminator, the frequency noise spectrum of said laser and the tuning response of said laser, respectively; and

sequencing means for enabling an automatic frequency locking of said laser on the frequency reference of said optical frequency discriminator, the sequencing means controlling a power-up sequence of said frequency locking; and

an enclosure for enclosing said frequency discriminator to isolate said frequency discriminator from external perturbations.