Illustration of Resonant Modulation

Former chapters in Part I of this book discussed present understanding of direct modulation properties of laser diodes with particular emphasis on modulation speed. A quantity of major significance in the small-signal modulation regime is the -3 dB modulation bandwidth, which is a direct measure of the rate at which (primarily baseband) information can be transmitted by intensity modulation of the laser. However, one can obtain a large modulation optical depth (in fact, pulse-like output) at repetition rates beyond the -3 dB point by driving the laser with sufficient RF drive power to compensate for the drop-off in the modulation response of the laser. This technique is very useful in generating repetitive optical pulses from a laser diode at a high repetition rate, although the repetition rate itself has no significance in terms of information transmission capacity of the laser. A means to reduce the RF drive power required for modulating the laser to a large optical modulation depth at high repetition rate is the technique of “mode locking.” The laser diode is coupled to an external optical cavity whose round-trip time corresponds to inverse of the modulation frequency applied to the laser diode. The modulation frequency in this scenario is limited to a very narrow range near the “round-trip frequency” (defined as inverse of the round trip time) of the external cavity. An example of this approach used a LiNbO₃ directional coupler/modulator to produce optical modulation at 7.2 GHz [80]. Another example involved coupling the laser diode to an external fiber cavity [81] which produced optical modulation up to 10 GHz. Chapter 4 describes experimental work which extended the small-signal -3 dB direct modulation bandwidth of a solitary laser diode to ~12 GHz using a “window” buried heterostructure laser fabricated on semi-insulating substrate (BH on SI) [27].

This chapter describes results of modulation of this “window BH on SI” laser at frequencies beyond the −3 dB point, in both the small signal and large signal regimes. It will be described below that lasers operating in this mode can be used as a narrowband signal transmitter at frequencies beyond the −3dB point (or relaxation oscillation frequency), with a reasonably flat response over a bandwidth of up to ~1 GHz. The response of the original solitary laser at this frequency range is substantially lower than that in the baseband range (i.e., at frequencies < relaxation resonance) and consequently high power RF drivers are necessary to attain a sufficient optical modulation depth for communications purpose. It was found that a weak optical feedback from an external optical cavity can boost the response by a substantial amount over a broad frequency range around the round-trip frequency of the external cavity. A strong optical feedback produces a sharp spike in the response of the laser at the round-trip frequency of the external cavity (hereafter called “on-resonance”). Under this condition, picosecond optical pulses can be generated by applying a strong current modulation to the laser on resonance, which can be interpreted as active mode locking [82] of the longitudinal modes of the composite cavity formed by the coupling of the laser diode and the external cavity.