Laser diode
A laser diode is a semiconductor device similar to a light-emitting diode in which a diode pumped directly with electrical current can create lasing conditions at the diode's junction.
Driven by voltage, the doped p–n-transition allows for recombination of an electron with a hole. Due to the drop of the electron from a higher energy level to a lower one, radiation is generated in the form of an emitted photon. This is spontaneous emission. Stimulated emission can be produced when the process is continued and further generates light with the same phase, coherence, and wavelength.
The choice of the semiconductor material determines the wavelength of the emitted beam, which in today's laser diodes range from the infrared to the ultraviolet spectra. Laser diodes are the most common type of lasers produced, with a wide range of uses that include fiber-optic communications, barcode readers, laser pointers, CD/DVD/Blu-ray disc reading/recording, laser printing, laser scanning, and light beam illumination. With the use of a phosphor like that found on white LEDs, laser diodes can be used for general illumination.
Theory
A laser diode is electrically a PIN diode. The active region of the laser diode is in the intrinsic region, and the carriers are pumped into that region from the N and P regions respectively. While initial diode laser research was conducted on simple P–N diodes, all modern lasers use the double-hetero-structure implementation, where the carriers and the photons are confined in order to maximize their chances for recombination and light generation. Unlike a regular diode, the goal for a laser diode is to recombine all carriers in the I region, and produce light. Thus, laser diodes are fabricated using direct band-gap semiconductors. The laser diode epitaxial structure is grown using one of the crystal growth techniques, usually starting from an N-doped substrate, and growing the I active layer, followed by the P-doped cladding, and a contact layer. The active layer most often consists of quantum wells, which provide lower threshold current and higher efficiency.Electrical and optical pumping
Laser diodes form a subset of the larger classification of semiconductor p–''n junction diodes. Forward electrical bias across the laser diode causes the two species of charge carrier – holes and electrons – to be injected from opposite sides of the PIN junction into the depletion region. Holes are injected from the p''-doped into the undoped semiconductor, and electrons vice versa. Due to the use of charge injection in powering most diode lasers, this class of lasers is sometimes termed injection lasers, or injection laser diodes. As diode lasers are semiconductor devices, they may also be classified as semiconductor lasers. Either designation distinguishes diode lasers from solid-state lasers.Another method of powering some diode lasers is the use of optical pumping. Optically pumped semiconductor lasers use a III-V semiconductor chip as the gain medium, and another laser as the pump source. OPSLs offer several advantages over ILDs, particularly in wavelength selection and lack of interference from internal electrode structures. A further advantage of OPSLs is invariance of the beam parameters – divergence, shape, and pointing – as pump power is varied, even over a 10:1 output power ratio.
Generation of spontaneous emission
When an electron and a hole are present in the same region, they may recombine or annihilate producing a spontaneous emission — that is, the electron may re-occupy the energy state of the hole, emitting a photon with energy equal to the difference between the electron's original state and hole's state. Spontaneous emission below the lasing threshold produces similar properties to an LED. Spontaneous emission is necessary to initiate laser oscillation, but it is one among several sources of inefficiency once the laser is oscillating.Direct and indirect bandgap semiconductors
The difference between the photon-emitting semiconductor laser and a conventional phonon-emitting semiconductor junction diode lies in the type of semiconductor used, one whose physical and atomic structure confers the possibility for photon emission. These photon-emitting semiconductors are the so-called "direct bandgap" semiconductors. The properties of silicon and germanium, which are single-element semiconductors, have bandgaps that do not align in the way needed to allow photon emission and are not considered direct. Other materials, the so-called compound semiconductors, have virtually identical crystalline structures as silicon or germanium but use alternating arrangements of two different atomic species in a checkerboard-like pattern to break the symmetry. The transition between the materials in the alternating pattern creates the critical direct bandgap property. Gallium arsenide, indium phosphide, gallium antimonide, and gallium nitride are all examples of compound semiconductor materials that can be used to create junction diodes that emit light.Generation of stimulated emission
In the absence of stimulated emission conditions, electrons and holes may coexist in proximity to one another, without recombining, for a certain time, termed the upper-state lifetime or recombination time, before they recombine. A nearby photon with energy equal to the recombination energy can cause recombination by stimulated emission. This generates another photon of the same frequency, polarization, and phase, travelling in the same direction as the first photon. This means that stimulated emission will cause gain in an optical wave in the injection region, and the gain increases as the number of electrons and holes injected across the junction increases. The spontaneous and stimulated-emission processes are vastly more efficient in direct bandgap semiconductors than in indirect bandgap semiconductors; therefore, silicon is not a common material for laser diodes.Optical cavity and laser modes
As in other lasers, the gain region is surrounded by an optical cavity to form a laser. In the simplest form of laser diode, an optical waveguide is made on that crystal's surface, such that the light is confined to a relatively narrow line. The two ends of the crystal are cleaved to form perfectly smooth, parallel edges, forming a Fabry–Pérot resonator. Photons emitted into a mode of the waveguide will travel along the waveguide and be reflected several times from each end face before they exit. As a light wave passes through the cavity, it is amplified by stimulated emission, but light is also lost due to absorption and by incomplete reflection from the end facets. Finally, if there is more amplification than loss, the diode begins to lase.Some important properties of laser diodes are determined by the geometry of the optical cavity. Generally, the light is contained within a very thin layer, and the structure supports only a single optical mode in the direction perpendicular to the layers. In the transverse direction, if the waveguide is wide compared to the wavelength of the light, then the waveguide can support multiple transverse optical modes, and the laser is known as multi-mode. These transversely multi-mode lasers are adequate in cases where one needs a very large amount of power, but not a small diffraction-limited TEM00 beam, such as in printing, activating chemicals, microscopy, or pumping other types of lasers.
In applications where a small, focused beam is needed, the waveguide must be made narrow, on the order of the optical wavelength. This way, only a single transverse mode is supported and one ends up with a diffraction-limited beam. Such single-spatial-mode devices are used for optical storage, laser pointers, and fiber optics. These lasers may still support multiple longitudinal modes, and thus can lase at multiple wavelengths simultaneously. The wavelength emitted is a function of the bandgap of the semiconductor material and the modes of the optical cavity. In general, the maximum gain will occur for photons with energy slightly above the bandgap energy, and the modes nearest the peak of the gain curve will lase most strongly. The width of the gain curve will determine the number of additional side modes that may also lase, depending on the operating conditions. Single-spatial-mode lasers that can support multiple longitudinal modes are called Fabry-Pérot lasers. An FP laser will lase at multiple cavity modes within the gain bandwidth of the lasing medium. The number of lasing modes in an FP laser is usually unstable and can fluctuate due to changes in current or temperature.
Single-spatial-mode diode lasers can be designed so as to operate on a single longitudinal mode. These single-frequency diode lasers exhibit a high degree of stability, and are used in spectroscopy and metrology and as frequency references. Single-frequency diode lasers are classed as either distributed-feedback lasers or distributed Bragg reflector lasers.
Formation of laser beam
Due to diffraction, the beam diverges rapidly after leaving the chip, typically at 30 degrees vertically by 10 degrees laterally. A lens must be used in order to form a collimated beam like that produced by a laser pointer. If a circular beam is required, then cylindrical lenses and other optics are used. For single-spatial-mode lasers, using symmetrical lenses, the collimated beam ends up being elliptical in shape, due to the difference in the vertical and lateral divergences. This is easily observable with a red laser pointer. The long axis of the ellipse is at right-angles to the plane of the chip.The simple diode described above has been heavily modified in recent years to accommodate modern technology, resulting in a variety of types of laser diodes, as described below.
History
Following theoretical treatments of M.G. Bernard, G. Duraffourg, and William P. Dumke in the early 1960s, coherent light emission from a gallium arsenide semiconductor diode was demonstrated in 1962 by two US groups led by Robert N. Hall at the General Electric research center and by Marshall Nathan at the IBM T.J. Watson Research Center. There has been ongoing debate as to whether IBM or GE invented the first laser diode, which was largely based on theoretical work by William P. Dumke at IBM's Kitchawan Lab in Yorktown Heights, NY. The priority is given to the General Electric group, who submitted their results earlier; they also went further and made a resonant cavity for their diode. It was initially speculated, by MIT's Ben Lax among other leading physicists, that silicon or germanium could be used to create a lasing effect, but theoretical analyses convinced William P. Dumke that these materials would not work. Instead, he suggested gallium arsenide as a good candidate. The first visible-wavelength laser diode was demonstrated by Nick Holonyak, Jr. later in 1962; he used gallium arsenide phosphide.Other teams at MIT Lincoln Laboratory, Texas Instruments, and RCA Laboratories were also involved in, and received credit for, their historic initial demonstrations of efficient light emission and lasing in semiconductor diodes in 1962 and thereafter. GaAs lasers were also produced in early 1963 in the Soviet Union by the team led by Nikolay Basov.
In the early 1960s, liquid-phase epitaxy was invented by Herbert Nelson of RCA Laboratories. By layering the highest-quality crystals of varying compositions, it enabled the demonstration of the highest-quality heterojunction semiconductor laser materials for many years. LPE was adopted by all the leading laboratories worldwide and was used for many years. It was finally supplanted in the 1970s by molecular-beam epitaxy and organometallic chemical vapor deposition.
Diode lasers of that era operated with threshold current densities of 1000 A/cm2 at 77 K temperatures. Such performance enabled continuous lasing to be demonstrated in the earliest days. However, when operated at room temperature, about 300 K, threshold current densities were two orders of magnitude greater, or 100,000 A/cm2, in the best devices. The dominant challenge for the remainder of the 1960s was to obtain low threshold current density at 300 K and thereby to demonstrate continuous-wave lasing at room temperature from a diode laser.
The first diode lasers were homojunction diodes. That is, the material of the waveguide core layer and that of the surrounding clad layers were identical. It was recognized that there was an opportunity, particularly afforded by the use of liquid-phase epitaxy using aluminum gallium arsenide, to introduce heterojunctions. Heterostructures consist of layers of semiconductor crystal having varying bandgap and refractive index. Heterojunctions had been recognized by Herbert Kroemer, while working at RCA Laboratories in the mid-1950s, as having unique advantages for several types of electronic and optoelectronic devices, including diode lasers. LPE afforded the technology of making heterojunction diode lasers. In 1963, he proposed the double heterostructure laser.
The first heterojunction diode lasers were single-heterojunction lasers. These lasers used aluminum gallium arsenide p-type injectors situated over n-type gallium arsenide layers grown on the substrate by LPE. An admixture of aluminum replaced gallium in the semiconductor crystal and raised the bandgap of the p-type injector over that of the n-type layers beneath. It worked; the 300 K threshold currents went down by 10× to 10,000 A/cm2. Unfortunately, this was still not in the needed range, and these single-heterostructure diode lasers did not function in continuous-wave operation at room temperature.
The innovation that met the room temperature challenge was the double-heterostructure laser. The trick was to quickly move the wafer in the LPE apparatus between different melts of aluminum gallium arsenide and a third melt of gallium arsenide. It had to be done rapidly since the gallium arsenide core region needed to be significantly under 1 μm in thickness. The first laser diode to achieve continuous-wave operation was a double heterostructure demonstrated in 1970 essentially simultaneously by Zhores Alferov and collaborators of the Soviet Union, and Morton Panish and Izuo Hayashi working in the United States. However, it is widely accepted that Alferov and team reached the milestone first.
For their accomplishment and that of their co-workers, Alferov and Kroemer shared the 2000 Nobel Prize in Physics.