Abstract

Significant progress has been made recently in operating semiconductor laser diodes at very high frequencies. The development of ultrafast semiconductor laser diodes [1] have extended the -3dB direct modulation bandwidth to above 10GHz. It will be shown in this paper that in addition to being useful as optical transmitters of microwave signals up to X-band frequencies, the recently developed ultra-high speed laser can be used to generate picosecond pulses at a frequency of 18GHz. The laser used is a window buried heterostructure laser on semi-insulating substrate[1]. The presence of the window near the end facet alleviates the problem of catastrophic damage and enable the laser to operate at very high optical power densities. The tight optical and electrical confinement along the length of the laser cavity (except at the window region) enables maximum interaction between the photon and electrons to take place and results in a very high direct modulation bandwidth. The -3dB direct modulation bandwidth of this device operating at 14mW is 12GHz. The intrinsic modulation response at 16GHz is approximately 13dB below the midband value. This loss in modulation efficiency can be compensated by coupling the laser to an external cavity of the appropriate length. The external cavity is composed of a short length of standard graded index multimode fiber of 50 μm core diameter, with a high refractive index hemispherical lens attached to one end of the fiber to facilitate coupling. The far end of a fiber is cleaved and is butted to a gold mirror. This induces a very sharp resonance in the modulation response of the laser. When the laser is driven on resonance by a microwave source whose power output is > 10dBm the optical modulation depth approaches unity and the optical waveform becomes pulse-like. Using an optical second harmonic generation autocorrelator the optical pulse width is found to be 12ps(FWHM). This, in effect, is active mode-locking of a laser diode at a repetition rate of 17.5GHz.

© 1985 Optical Society of America