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

Although diode lasers are almost ideal sources for ultrahigh-speed data communication systems, system performance remains critically dependent on the quality of the optical pulses that they generate. Uniquely among lasers, the output power can be modulated directly by modulating the diode current. However, this leads to relaxation oscillations and a roll-off at high frequencies that is superimposed on the frequency response of the drive circuit. This is limited by parasitics and maximum modulation frequencies range from 1 to 70?GHz, depending on the type of laser and its packaging. The higher values are obtained with short cavity lengths and tight optical confinement, the highest frequencies being achieved in vertical-cavity devices. Modulation bandwidth is usually limited by circuit parasitics, device heating and the maximum power-handling capability of the laser facets. The generation of picosecond and femtosecond pulses demands special techniques and three - gain switching, Q-switching and mode locking - are discussed in detail, with their relative advantages and disadvantages compared. Very short pulses inherently contain a significant spread of wavelengths and their generation requires the optical gain in the laser medium to extend across that wavelength range. While diode lasers satisfy this criterion better than many other types, the effect of gain non-linearities and carrier-transport effects prevent the Fourier-transform limit from being achieved in practice. As a result, external pulse-compression techniques, which exploit the detailed temporal and spectral properties of the laser pulses, such as frequency chirping, self-phase modulation and group velocity dispersion, are becoming more important, and diode lasers are increasingly challenged as primary sources by compact, efficient, diode-pumped solid-state lasers. The paper summarizes the levels of maximum pulse power and minimum duration that have been achieved using the various techniques.

https://iopscience.iop.org/article/10.1088/0034-4885/63/12/203/pdf

Fast phenomena in semiconductor lasers

Ultrashort laser pulses are finding increasing application in numerous branches of science and technology. This article is an attempt to provide a comprehensive review of the whole field of ultrashort pulse generation, covering both the experimental and the theoretical aspects of the subject. By far the commonest method of generating pulses in the picosecond and subpicosecond regime is the technique known as 'mode-locking' and it is with this that the review is therefore largely concerned. After introductory remarks and a simple treatment of some of the general principles, the central sections of the review deal with the main generic techniques, namely spontaneous mode-locking, active mode-locking (including mode-locking by synchronous pumping) and passive mode-locking. Within each section the material is organised into three main categories, namely (a) general background, (b) historical survey (mainly relating to experimental work and densely packed with references to published work) and (c) theoretical methods (presented with a strongly pedagogical flavour). A major section at the end deals with a mass of important (and mostly recent) work that does not fit naturally into the main framework of the review. This includes USP generation in semiconductor lasers, a number of special methods such as injection mode-locking (and others where two or more mode-locking techniques are combined), as well as chopping and switching techniques.

http://iopscience.iop.org/article/10.1088/0034-4885/46/8/001/pdf

The generation of ultrashort laser pulses

Microwave oscillations produced by continuously operating GaAs stripe geometry junction lasers are reported here for the first time. These oscillations have been measured both in the light output of the diodes and in the dc current applied to the laser connections. The frequency of the oscillation lies in the 0.5- to 3.0-GHz range and depends strongly on current and temperature. The frequency typically increases with increasing current at a rate of 15 to 20 MHz/mA and decreases with increasing temperature at 100 to 150 MHz/°K. Introduction of an external microwave signal locks the frequency of the oscillation and reduces the oscillation width from 10 MHz to the external signal width. The frequency of oscillation agrees with a theory of intensity fluctuations in lasers based on the rate equations.

Continuous microwave oscillations in GaAs junction lasers

We find that a tube of pure Ne20, excited by a discharge and placed within the cavity of a He3–Ne20 laser operating at 6328 A can cause a stable, continuous train of short optical pulses to be produced.

USE OF NEON AS A NONLINEAR ABSORBER FOR MODE LOCKING A He–Ne LASER

Generation of Ultrashort Light Pulses with a GaAs Semiconductor Laser

The results are given of an investigation of the time characteristics of an electron-beam-pumped GaAs sealed-enclosure laser. Spatial scanning of the electron beam across the GaAs crystal made it possible to vary the power, shape, and repetition frequency of 10−10–10−6 sec laser output pulses. The scanning was achieved by a wide-band system of deflecting plates and a grid modulation electrode.

Generation of controllable light pulses in an electron-beam-pumped laser

A high-speed photographic method was developed for measuring the spatial and temporal characteristics of pulsed laser radiation in the middle infrared range. An investigation was made of the dynamics of the angular divergence of a beam from a cryogenically cooled electron-beam-controlled CO laser with different types of resonator. The main factors limiting the angular divergence of the radiation from such lasers were found to be inhomogeneity of the cooling of the active medium and gasdynamic perturbations arising from a nonuniform distribution of the pump energy.

Investigation of the dynamics of the angular divergence of a radiation beam from a pulsed electron-beam-controlled CO laser

An electron-beam-pumped cadmium sulfide laser was developed in the form of a sealed unit cooled with liquid nitrogen. The electron energy was up to 60 keV, the density of the electron-beam current up to 10 A/cm2, and the laser radiation output up to 40 W. The duration of the output pulses could be varied from 10–6 to 10–10 sec by modulation and scanning the crystal with the electron beam. The pulse repetition frequency of the electron beam was up to 200 Hz and the scanning frequency of the beam was up to 1 GHz.

Yu. A. Drozhbin

Papers

Generation of Ultrashort Light Pulses with a GaAs Semiconductor Laser

Generation of controllable light pulses in an electron-beam-pumped laser

Investigation of the dynamics of the angular divergence of a radiation beam from a pulsed electron-beam-controlled CO laser

Kgp-2: an electron-beam-pumped cadmium sulfide laser

Measurement of the divergence of pulse-laser radiation by the focal spot method using a mirror wedge