Abstract

Conditions for the frequency locking of cw lasers and the behavior of the laser frequency in the vicinity of the locking-range are well known. These effects are of importance for seeding lasers and for the (undesired) frequency locking of the two counterpropagating fields in laser gyros. To study the respective behavior in femtosecond dye lasers we used the experimental setup as shown in Fig. 1. The counterpropagating pulses of a modelocked ring dye laser (τ_p ≈ 100fs) exhibit different oscillation frequencies, which can be measured as beat note (usually ≤ 1 MHz) if the pulses overlap a photo-diode.² The output of one propagation direction through mirror M3 is coupled into the other propagation direction through mirror M1. The prism P is moved with a constant velocity so as to Doppler-shift the seed pulse. The beat note is then measured as function of the position of P and for various strength of the feedback. Figure 2 shows the beat frequency versus the position of P, i.e., versus the relative delay of the pulse circulating in the cavity and the seed pulse. Three different regimes can be distinguished in order of increasing advance of the seed pulse. (A) The beat frequency decreases continuously starting from the value v„ of the unseeded laser. (B) The frequencies of the two counterpropagating pulses lock, which manifests itself in the detection of the Doppler frequency. (C) For not too strong a feedback the beat frequency gradually increases until a stationary value is reached, which is smaller than v_u. Here the seed pulse exists as a satellite pulse in the main cavity. The onset of the frequency locking shifts toward earlier times (smaller advance of the seed pulse) with increasing feedback, as depicted in Fig. 3. Also, the locking range becomes narrower as the feedback and/or prism velocity increases.

© 1994 IEEE