Abstract

We introduce a new semiconductor structure and demonstrate its use as an intracavity femtosecond mode locker. The structure consists of an AlAs/AlGaAs quarter-wave dielectric stack grown by molecular-beam epitaxy. We insert a single quantum well into a quarter-wave layer, as shown in Fig. 1(a). The reflectivity spectrum [Fig. 1(b)] shows a characteristic Bragg reflection with a small dip at the exciton wavelength of 850 ran. This structure provides high reflectivity, low loss, and adjustable saturation intensity, with no further processing required after MBE growth. We refer to it as the saturable Bragg reflector (SBR). With the Ti:sapphire laser cavity as shown in Fig. 2, we obtain 8-fs pulses for operation in a purely-Kerr-lens-mode locking regime. When we incorporate the SBR by focusing with a 20-cm-radius mirror, we obtain self-starting transform-limited pulses of 90-fs duration. Figure 3 shows a typical autocorrelation trace, and Fig. 1(b) shows the mode locked spectrum.

© 1995 Optical Society of America