Bandwidth-Limited Picosecond Pulse Generation in an Actively Mode-Locked GaAlAs Diode Laser

Abstract

While the first actively mode-locked CW diode lasers [1] produced 20 ps pulses, residual reflectivity at the cleaved facets interfered with the mode-locking process and the pulses contained temporal sub-structures. As is well known [2] from experience with other types of mode-locked lasers, elimination of spurious reflections inside the laser resonator is necessary to obtain reliable mode-locking with the generation of bandwidth-limited pulses. The complete suppression of internal mode-structure in a diode laser operating in an external cavity has recently [3] been obtained by anti-reflection coating a GaAs/GaAlAs DH laser with the stripe contact making an angle of 5° to the normal of the diode facets. In this manner a smooth lasing spectrum of bandwidth ~lÅ (FWHM) was obtained for operation at 10% above the lasing threshold. We now report the generation of the first bandwidth-limited pulses from an actively mode-locked CW diode laser. A streak camera driven continuously in synchronism [4,5] with the laser pulse train facilated optimization of the mode-locking process and gave direct measurement of the pulse shapes and durations. With second-harmonic intensity correlation measurements unambiguous pulse shapes are not obtained and it is not possible to detect low-intensity but long-lasting sections of the laser output containing a substantial proportion of the laser energy outside the ultra-short pulses themselves, as was found with the first synchronously pumped mode-locked dye lasers [4]. Excess background energy content within the mode-locked pulse train would be deleterious for applications in optical electronics.