The field of high speed semiconductor lasers has undergone substantial advances since the first demonstration of a laser with modulation bandwidth beyond 10GHz. Much of the present understanding on the high speed modulation of semiconductor lasers comes from a small signal analysis of the laser rate equations. The present state of the art is exemplified by the demonstration of a 16GHz bandwidth (-3dB) contricted mesa laser and a 22GHz bandwidth vapor phase regrown buried heterostructure laser, both at a wavelength of 1.3μm. On a parallel front, there has been substantial progress in using laser diodes for picosecond pulse and microwave/millimeter wave signal generation. Picosecond optical pulse sources are major tools for studying ultrafast physical and chemical phenomena. A compact and reliable semiconductor picosecond laser source is highly desirable, especially for instrumentation applications such as picosecond optical sampling for non-invasive characterization of high speed electronic curcuits. Demonstrated methods of picosecond pulse generation from a laser diode include mode-locking and gain/Q switching. Gain switching is a very simple technique of producing short pulses from a laser diode, although the pulse width seldom falls much below 20ps unless a saturable absorber is introduced into the cavity. Factors affecting gain-switched pulse width will be examined, along with Q-switching by means of an intracavity modulator. Presence of a saturable absorber in a laser diode can also cause repetitive Q-switching, or self-pulsation, whose frequency is tunable with injection current. Previous efforts on passive mode-locking has led to pulses of a few picosecond wide. Recent efforts concentrated on the use of high speed semiconductor lasers and to modulate/mode-lock the laser at frequencies above the relaxation oscillation frequency[4,5,6]. Such high frequency mode-locking technique was first demonstrated at 17GHz, in which 12ps pulse were generated, and recently similar configurations have succeeded in generating subpicosecond optical pulses and at 40GHz. The mode-coupling technique has been considered for narrow band modulation of the diode at millimeter wave frequencies. Recent studies have shown that it is possible to mode lock semiconductor lasers at millimeter wave frequencies approaching and beyond 100GHz. The mode-locked output usually takes the form of sinusoidal modulation, and can be regarded for practical purposes as a highly efficient means of directly modulating an optical carrier in a narrow band at millimeter wave frequencies.
© 1989 Optical Society of AmericaPDF Article
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