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.