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A laser diode ( LD, also injection laser diode or ILD or semiconductor laser or diode laser) 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 electron 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 (IR) to the ultraviolet (UV) spectra. Laser diodes are the most common type of lasers produced, with a wide range of uses that include fiber-optic communications, , , 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 , laser diodes can be used for general illumination.
Another method of powering some diode lasers is the use of optical pumping. Optically pumped semiconductor lasers (OPSL) use a III-V semiconductor chip as the gain medium, and another laser (often another diode laser) as the pump source. OPSLs offer several advantages over ILDs, particularly in wavelength selection and lack of interference from internal electrode structures.Arrigoni, M. et al. (2009-09-28) "Optically Pumped Semiconductor Lasers: Green OPSLs poised to enter scientific pump-laser market", Laser Focus World "Optically Pumped Semiconductor Laser (OPSL)", Sam's Laser FAQs. A further advantage of OPSLs is invariance of the beam parameters – divergence, shape, and pointing – as pump power (and hence output power) is varied, even over a 10:1 output power ratio. Coherent white paper (2018-05). "Advantages of Optically Pumped Semiconductor Lasers – Invariant Beam Properties"
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 mode, 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 laser 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 (FP) 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 (DFB) lasers or distributed Bragg reflector (DBR) lasers.
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.
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 (LPE) 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 (and thus the bandgap) 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 (formed from heterostructures) 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 ( p- and n-type) 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 (including Dmitri Z. Garbuzov) 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.
The advantage of a DH laser is that the region where free electrons and holes exist simultaneously—the active region—is confined to the thin middle layer. This means that many more of the electron-hole pairs can contribute to amplification—not so many are left out in the poorly amplifying periphery. In addition, light is reflected within the heterojunction; hence, the light is confined to the region where the amplification takes place.
Lasers containing more than one quantum well layer are known as multiple quantum well lasers. Multiple quantum wells improve the overlap of the gain region with the optical waveguide normal mode.
Further improvements in laser efficiency have also been demonstrated by reducing the quantum well layer to a quantum wire or to a sea of .
Almost all commercial laser diodes since the 1990s have been SCH quantum well diodes.
The threshold current of this DFB laser, based on its static characteristic, is around 11 mA. The appropriate bias current in a linear regime could be taken in the middle of the static characteristic (50 mA). Several techniques have been proposed in order to enhance the single-mode operation in these kinds of lasers by inserting a one-phase-shift (1PS) or multiple-phase-shift (MPS) in the uniform Bragg grating. However, multiple-phase-shift DFB lasers represent the optimal solution because they have the combination of higher side-mode suppression ratio and reduced spatial hole-burning.
Such dielectric mirrors provide a high degree of wavelength-selective reflectance at the required free surface wavelength if the thicknesses of alternating layers and with refractive indices and are such that , which then leads to the constructive interference of all partially reflected waves at the interfaces. But there is a disadvantage: because of the high mirror reflectivities, VCSELs have lower output powers when compared to edge-emitting lasers.
There are several advantages to producing VCSELs when compared with the production process of edge-emitting lasers. Edge-emitters cannot be tested until the end of the production process. If the edge-emitter does not work, whether due to bad contacts or poor material growth quality, then the production time and the processing materials have been wasted.
Additionally, because VCSELs emit the beam perpendicular to the active region of the laser as opposed to parallel as with an edge emitter, tens of thousands of VCSELs can be processed simultaneously on a three-inch gallium arsenide wafer. Furthermore, even though the VCSEL production process is more labor- and material-intensive, the yield can be controlled to a more predictable outcome. However, they normally show a lower power output level.
One of the most interesting features of any VECSEL is the small thickness of the semiconductor gain region in the direction of propagation, less than 100 nm. In contrast, a conventional in-plane semiconductor laser entails light propagation over distances of from 250 μm upward to 2 mm or longer. The significance of the short propagation distance is that it causes the effect of antiguiding nonlinearities in the diode laser gain region to be minimized. The result is a large-cross-section single-mode optical beam that is not attainable from in-plane ("edge-emitting") diode lasers.
Several workers demonstrated optically pumped VECSELs, and they continue to be developed for many applications, including high-power sources for use in industrial machining (cutting, punching, etc.) because of their unusually high power and efficiency when pumped by multi-mode diode laser bars. However, because of their lack of p– n junctions, optically pumped VECSELs are not considered diode lasers, and are classified as semiconductor lasers.
Electrically pumped VECSELs have also been demonstrated. Applications for electrically pumped VECSELs include projection displays, served by frequency doubling of near-IR VECSEL emitters to produce blue and green light.
Many of the advances in reliability of diode lasers in the last 20 years remain proprietary to their developers. Reverse engineering is not always able to reveal the differences between more-reliable and less-reliable diode laser products.
Semiconductor lasers can be surface-emitting lasers such as VCSELs, or in-plane edge-emitting lasers. For edge-emitting lasers, the edge facet mirror is often formed by cleaving the semiconductor wafer to form a specularly reflecting plane. This approach is facilitated by the weakness of the 110 crystallographic plane in III-V semiconductor crystals, such as GaAs, InP, GaSb, etc. compared to the other planes.
The atomic states at the cleavage plane are altered compared to their bulk properties within the crystal by the termination of the perfectly periodic lattice at that plane. Surface states at the cleaved plane have energy levels within the otherwise forbidden bandgap of the semiconductor. Thus, when light propagates through the cleavage plane and transits to free space from within the semiconductor crystal a fraction of the light energy is absorbed by the surface states, where it is converted to the heat by phonon-electron interactions. This heats the cleaved mirror. In addition, the mirror may heat simply because the edge of the diode laser—which is electrically pumped—is in less-than-perfect contact with the mount that provides a path for heat removal.
The heating of the mirror causes the bandgap of the semiconductor to shrink in the warmer areas. The bandgap shrinkage brings more electronic band-to-band transitions into alignment with the photon energy, causing yet more absorption. This is thermal runaway, a form of the positive feedback, and the result can be melting of the facet, known as catastrophic optical damage - COD.
In the 1970s, this problem, which is particularly nettlesome for GaAs-based lasers emitting between 0.630 μm and 1 μm (less so for InP-based lasers used for long-haul telecommunications, which emit between 1.3 μm and 2 μm), was identified.
Michael Ettenberg, a researcher and later Vice President at RCA Laboratories' David Sarnoff Research Center in Princeton, New Jersey devised a solution. A thin layer of aluminum oxide was deposited on the facet. If the aluminum oxide thickness is chosen correctly, it functions as an anti-reflective coating, reducing reflection at the surface. This alleviated the heating and COD at the facet.
Since then, various other refinements have been employed. One approach is to create a so-called non-absorbing mirror (NAM) such that the final 10 μm or so before the light emits from the cleaved facet are rendered non-absorbing at the wavelength of the interest.
In the very early 1990s, SDL Inc. began supplying high-power diode lasers with good reliability characteristics. CEO Donald Scifres and CTO David Welch presented new reliability performance data at, e.g., SPIE Photonics West conferences of the era. The methods used by SDL to defeat COD were considered to be highly proprietary and were still undisclosed publicly as June of 2006.
In the mid-1990s, IBM Research - Ruschlikon, Switzerland announced that it had devised its so-called E2 process, which conferred extraordinary resistance to the COD in GaAs-based lasers. This process also was undisclosed as of June 2006.
Reliability of high-power diode laser pump bars (used to pump solid-state lasers) remains difficult problem in the variety of applications, in spite of these proprietary advances. Indeed, the physics of diode laser failure is still being worked out, and research on this subject remains active, if proprietary.
Extension of the lifetime of laser diodes is critical to their continued adaptation to a wide variety of applications.
Both low- and high-power diodes are used extensively in the printing industry, both as light sources for scanning (input) of images and for very-high-speed and high-resolution printing plate (output) manufacturing.
Infrared and red laser diodes are common in CD players, , and DVD technology. Violet lasers are used in HD DVD and Blu-ray Disc technology. Diode lasers have also found many applications in laser absorption spectrometry (LAS) for high-speed, low-cost assessment or monitoring of the concentration of various species in gas phase.
High-power laser diodes are used in industrial applications such as heat treating, cladding, seam welding, and for pumping other lasers, such as diode-pumped solid-state lasers.
Uses of laser diodes can be categorized in various ways. Most applications could be served by larger solid-state lasers or optical parametric oscillators, but the low cost of mass-produced diode lasers makes them essential for mass-market applications. Diode lasers can be used in a great many fields; since light has many different properties (power, wavelength, spectral and beam quality, polarization, etc.), it is useful to classify applications by these basic properties.
Many applications of diode lasers primarily make use of the directed energy property of the optical beam. In this category, one might include
Some of the above applications are well-established, while others are emerging.
Diode wavelengths range from 810 to 1,100 Nanometre, are poorly absorbed by soft tissue, and are not used for cutting or ablation. Soft tissue is not cut by the laser's beam, but is instead cut by contact with a hot charred glass tip. The laser's irradiation is highly absorbed at the distal end of the tip and heats it up to 500–900°C. Because the tip is so hot, it can be used to cut soft tissue and can cause hemostasis through cauterization and carbonization. Diode lasers when used on soft tissue can cause extensive collateral thermal damage to surrounding tissue.
As laser beam light is inherently coherent, certain applications use the coherence of laser diodes. These include interferometric distance measurement, holography, coherent communications, and coherent control of chemical reactions.
Laser diodes are used for their narrow spectral properties in the areas of range-finding, telecommunications, infra-red countermeasures, spectroscopic sensing, generation of radio-frequency or terahertz waves, atomic clock state preparation, quantum key cryptography, frequency doubling and conversion, water purification (in the UV), and photodynamic therapy (where a particular wavelength of light would cause a substance such as porphyrin to become chemically active as an anti-cancer agent only where the tissue is illuminated by light).
Laser diodes are used for their ability to generate ultra-short pulses of light by the technique known as mode-locking. Areas of use include clock distribution for high-performance integrated circuits, high-peak-power sources for laser-induced breakdown spectroscopy sensing, arbitrary waveform generation for radio-frequency waves, photonic sampling for analog-to-digital conversion, and optical code-division-multiple-access systems for secure communication.
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