Methods for direct modulation of semiconductor lasers are reviewed with the objective of indicating the advantages and limitations of each method. Techniques for producing amplitude, pulse, and frequency modulation of the optical wave are included. The modulation capabilities of present pulsed lasers are analyzed with special attention given to their operation at room temperature. In addition, several ways of producing analog position or width modulation of microwave-rate optical pulses are described, and the capabilities of optical frequency modulation by acoustic waves are reviewed. A new way of obtaining mode-locked optical pulses with a semiconductor laser is also suggested.

Direct modulation of semiconductor lasers

An external cavity coupled to a conventional Fabry-Perot GaAs diode laser operating continuously has been found to cause modulation of the light output at a frequency within the range 05 to several GHz The modulation depth is close to 100 percent and the linewidth can be made as narrow as 180 kHz The modulation is thought to be stimulated by the intensity noise fluctuations, which peak at the well-known spiking frequency f r  The oscillations are strongly enhanced by a frequency locking action of the external cavity, being efficient when the external cavity round-trip time 2L/c , or a multiple thereof, corresponds to the inverse of the spiking frequency Since the latter is dependent on both pump current and temperature, the system can simply be tuned by adjusting the pump current For a fixed resonator length, the narrow-band oscillations occur in a small current range, in which an increase in frequency with increasing current at a rate of 400 kHz/mA is observed A small-signal analysis based on simple rate equations shows the influence of the external cavity on the intrinsic resonance frequency f r  It demonstrates that self-modulation can only occur for small values of the coupling coefficient e between the laser diode and the external cavity

Microwave self-modulation of a diode laser coupled to an external cavity

We review recent developments in directly modulated lasers (DMLs) with low operating energy for datacom and computercom applications. Key issues are their operating energy and the cost for employing them in these applications. To decrease the operating energy, it is important to reduce the active volume of the laser while maintaining the cavity Q-factor or photon lifetime in the cavity. Therefore, how to achieve high-reflectivity mirrors has been the main challenge in reducing the operating energy. In terms of the required output power from the lasers, the required input power into the photodetector and the transmission distance determine the lower limit of laser active volume. Therefore, the operating energy and output power are in a trade-off relationship. In designing the lasers, the cavity volume, quantum well number, and optical confinement factor are critical parameters. For reducing the cost, it is important to fabricate a large-scale photonic integrated circuit (PIC) comprising DMLs, an optical multiplexer, and monitor photodetectors because the lower assembly cost reduces the overall cost. In this context, silicon (Si) photonics technology plays a key role in fabricating large-scale PICs with low cost, and heterogeneous integration of DMLs and Si photonics devices has attracted much attention. We will describe fabrication technologies for heterogeneous integration and experimental results for DMLs on a Si substrate.

Low-operating-energy directly modulated lasers for short-distance optical interconnects

A cw single‐mode tunable GaAs diode laser has been locked to an external passive cavity by making use of the dependence of the output frequency on the injection current. The resulting long‐term stability of the laser optical frequency was checked by reference to a microwave frequency standard, through the light‐shift effect in a cesium vapor. The laser frequency was found to track the drift of the cavity bandpass with residual variations less than 300 kHz rms. Servo‐controlled frequency scanning was achieved over 5 GHz.

Frequency‐controlled cw tunable GaAs laser

In the past decade the semiconductor laser—also known as the diode or junction laser—has become a key device in optical electronics because of its pure output spectrum and high quantum efficiency. Not coincidentally, its output can be modulated at very high speeds; this property and its compact size make it useful in a vast range of applications, from fiber‐optic communications to optical radar.

Advances in Semiconductor Lasers

Spectrum change of diode laser output light by millimeter-wave amplitude modulation is observed. The spectrum consisting of the sideband by modulation showed apparent dependence on the resonance condition of the diode dimensions. A spectrum of less than 50 GHz modulation is observed when the diode dimensions are well designed.

F. Kitasawa

Papers

Amplitude modulation of diode laser light in millimeter-wave region