Short Pulse Generation and microwave/millimeter wave Narrowband Modulation in Semiconductor Lasers

Abstract

The field of high speed semiconductor lasers has undergone substantial advances since the first demonstration of a laser with modulation bandwidth beyond 10GHz[1]. 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)[2] contricted mesa laser and a 22GHz bandwidth vapor phase regrown buried heterostructure laser[3], 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[4], in which 12ps pulse were generated, and recently similar configurations have succeeded in generating subpicosecond optical pulses[5] and at 40GHz[6]. The mode-coupling technique has been considered for narrow band modulation of the diode at millimeter wave frequencies. Recent studies[7] 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.