A method and assembly for frequency stabilization of an optical signal especially from a laser, the assembly (11) comprising beam splitter (15), a passive frequency discriminator (16), and a pair of photodiodes (17 & 18). The beam splitter (15) in use receives a collimated optical beam (B) and diverts a sample beam (B.sub.1) which is directed towards the discriminator (16) which transmits a portion (B.sub.2) of said sample beam to one of the photodiodes (17) and reflects a second portion (B.sub.3) of said sample beam to the other photodiode (18). A control signal (S.sub.3) is derived from the respective intensities of the transmitted and reflected portions of the sample beam, and utilized to operate the electronic control to adjust the frequency of the laser output signal, if required, to maintain a specified frequency.