Mode-locking of a CW laser

Short pulses are useful in a number of applications in optics, e.g., time resolved spectroscopy, sampling of fast electronics, and communications. This has led to a number of mode-locking techniques to obtain such pulses, for example acousto-optic, injection, synchronous-pump, collidingpulse mode-locking, and Kerr-lens mode-locking. Using the latter two techniques, pulses of less than 100 fs duration can be readily obtained. With particular attention to the design of each intracavity element, pulses as short as 5 fs (which corresponds only to two optical cycles) have been obtained [1]. In this experiment you will mode-lock a continuous wave laser to generate a high repetition rate train of short pulses. The technique used will be a variation of injection mode-locking. A fraction of the laser’s output beam will be shifted in frequency by one longitudinal mode and re-injected into the laser, causing locking of the adjacent longitudinal mode, the next mode, and so forth. This yields an equally spaced phase-locked comb of longitudinal modes across the gain curve (in the frequency domain), the Fourier transform of which is a train of short pulses (ideally of length approximately equal to the inverse of the gain bandwidth and at a repetition rate equal to the inverse of the longitudinal mode spacing.) The main goals of the experiment will be as follows: