Method and apparatus for repetition rate synchronisation of mode-locked lasers

Abstract

A method and apparatus for passively synchronising the repetition rate of two or more mode-locked lasers is described. The method and apparatus involve forming a first synchronising optical field () by separating a portion of an output field of a first mode-locked laser () and thereafter redirecting this synchronising optical field to form a driving signal for a second mode-locked laser (). Employing these techniques results in systems with timing jitter of less than 1 fs. The method is independent of the wavelength and polarisation at which the mode-locked lasers operate and so is not limited to use with any particular type of mode-locked laser. Since the technique is passive it does not require the employment of electronics, variable time delay paths or additional non-linear optical crystals. Therefore, the method and apparatus are significantly less complex than those known in the art and are not power limited by additional non-linear optical processes. Part of the output () of the first mode-locked laser () is redirected via a beam splitter () and beam steering mirrors () and a half-wave plate () to a polariser () in the beam line of the second mode-locked laser (). The seeding and synchronising signal from the first mode-locked laser () may be perpendicularly polarized with respect to the polarization of the second mode-locked laser () and may have a different wavelength.