Laser diode
This article's lead section may be too short to adequately summarize the key points. (November 2016) |
InGaN green-blue laser; became widely available in mid-2018. | |
Type | semiconductor, light-emitting diode |
---|---|
Working principle | semiconductor, carrier generation and recombination |
Invented | Robert N. Hall, 1962; Nick Holonyak, Jr., 1962 |
Pin configuration | Anode and cathode |
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.[1]: 3
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, in the form of an emitted photon is generated. 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
Theory
This section needs additional citations for verification. (July 2011) |
A laser diode is electrically a
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
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.[2][3] 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.[4]
Generation of spontaneous emission
When an electron and a hole are present in the same region, they may
Direct and indirect bandgap semiconductors
The difference between the photon-emitting semiconductor laser and a conventional phonon-emitting (non-light-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
Generation of stimulated emission
In the absence of stimulated emission (e.g., lasing) 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 (about a nanosecond for typical diode laser materials), before they recombine. A nearby photon with energy equal to the recombination energy can cause recombination by
Optical cavity and laser modes
As in other lasers, the gain region is surrounded with an
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 light, then the waveguide can support multiple
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. Note that these lasers may still support multiple longitudinal modes, and thus can lase at multiple wavelengths simultaneously. The wavelength emitted is a function of the band-gap 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 band-gap 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 Perot (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.
Formation of laser beam
Due to
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
Other teams at
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 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 utilized 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 amperes per square centimeter. 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
For their accomplishment and that of their co-workers, Alferov and Kroemer shared the 2000 Nobel Prize in Physics.
Types
The simple laser diode structure, described above, is inefficient. Such devices require so much power that they can only achieve pulsed operation without damage. Although historically important and easy to explain, such devices are not practical.
Double heterostructure lasers
In these devices, a layer of low
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.
Quantum well lasers
If the middle layer is made thin enough, it acts as a
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 mode.
Further improvements in the laser efficiency have also been demonstrated by reducing the quantum well layer to a quantum wire or to a sea of quantum dots.
Quantum cascade lasers
In a
Interband cascade lasers
An Interband cascade laser (ICL) is a type of laser diode that can produce coherent radiation over a large part of the mid-infrared region of the electromagnetic spectrum.
Separate confinement heterostructure lasers
The problem with the simple quantum well diode described above is that the thin layer is simply too small to effectively confine the light. To compensate, another two layers are added on, outside the first three. These layers have a lower refractive index than the centre layers, and hence confine the light effectively. Such a design is called a separate confinement heterostructure (SCH) laser diode.
Almost all commercial laser diodes since the 1990s have been SCH quantum well diodes. [citation needed]
Distributed Bragg reflector lasers
A distributed Bragg reflector laser (DBR) is a type of single frequency laser diode.[11] It is characterized by an optical cavity consisting of an electrically or optically pumped gain region between two mirrors to provide feedback. One of the mirrors is a broadband reflector and the other mirror is wavelength selective so that gain is favored on a single longitudinal mode, resulting in lasing at a single resonant frequency. The broadband mirror is usually coated with a low reflectivity coating to allow emission. The wavelength selective mirror is a periodically structured diffraction grating with high reflectivity. The diffraction grating is within a non-pumped, or passive region of the cavity . A DBR laser is a monolithic single chip device with the grating etched into the semiconductor. DBR lasers can be edge emitting lasers or VCSELs. Alternative hybrid architectures that share the same topology include extended cavity diode lasers and volume Bragg grating lasers, but these are not properly called DBR lasers.
Distributed-feedback lasers
A
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 onephase-shift (1PS) or multiple-phase-shift (MPS) in the uniform Bragg grating.[12] 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.
Vertical-cavity surface-emitting laser
Vertical-cavity surface-emitting lasers (VCSELs) have the optical cavity axis along the direction of current flow rather than perpendicular to the current flow as in conventional laser diodes. The active region length is very short compared with the lateral dimensions so that the radiation emerges from the surface of the cavity rather than from its edge as shown in the figure. The reflectors at the ends of the cavity are dielectric mirrors made from alternating high and low refractive index quarter-wave thick multilayer.
Such dielectric mirrors provide a high degree of wavelength-selective reflectance at the required free surface wavelength λ if the thicknesses of alternating layers d1 and d2 with refractive indices n1 and n2 are such that n1d1 + n2d2 = λ/2 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, 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.
Vertical-external-cavity surface-emitting-laser
Vertical external-cavity surface-emitting lasers, or
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 junction, optically pumped VECSELs are not considered diode lasers, and are classified as semiconductor lasers.[citation needed]
Electrically pumped VECSELs have also been demonstrated. Applications for electrically pumped VECSELs include projection displays, served by
External-cavity diode lasers
External-cavity diode lasers are tunable lasers which use mainly double heterostructures diodes of the AlxGa(1-x)As type. The first external-cavity diode lasers used intracavity etalons[13] and simple tuning Littrow gratings.[14] Other designs include gratings in grazing-incidence configuration and multiple-prism grating configurations.[15]
Reliability
This section may be too technical for most readers to understand.(July 2011) |
Laser diodes have the same reliability and failure issues as light-emitting diodes. In addition they are subject to catastrophic optical damage (COD) when operated at higher power.
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
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.
As a result, 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 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 positive feedback, and the result can be melting of the facet, known as catastrophic optical damage, or COD.
In the 1970s, this problem, which is particularly nettlesome for GaAs-based lasers emitting between 0.630 µm and 1 µm wavelengths (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
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 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.,
In the mid-1990s, IBM Research (Ruschlikon, Switzerland) announced that it had devised its so-called E2 process which conferred extraordinary resistance to COD in GaAs-based lasers. This process, too, was undisclosed as of June 2006.
Reliability of high-power diode laser pump bars (used to pump solid-state lasers) remains a difficult problem in a 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.
Applications
Laser diodes are numerically the most common laser type, with 2004 sales of approximately 733 million units,[16] as compared to 131,000 of other types of lasers.[17]
Telecommunications, scanning and spectrometry
Laser diodes are widely used in
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 an optical beam. In this category, one might include the
Medical uses
Laser medicine: medicine and especially dentistry have found many new uses for diode lasers.[18][19][20][21][22][23] The shrinking size and cost[24] of the units and their increasing user friendliness makes them very attractive to clinicians for minor soft tissue procedures. Diode wavelengths range from 810 to 1,100 nm, are poorly absorbed by soft tissue, and are not used for cutting or ablation.[25][26][27][28] Soft tissue is not cut by the laser's beam, but is instead cut by contact with a hot charred glass tip.[27][28] The laser's irradiation is highly absorbed at the distal end of the tip and heats it up to 500 °C to 900 °C.[27] Because the tip is so hot, it can be used to cut soft-tissue and can cause hemostasis through cauterization and carbonization.[27][28] Diode lasers when used on soft tissue can cause extensive collateral thermal damage to surrounding tissue.[27][28]
As laser beam light is inherently coherent, certain applications utilize 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,
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.
Maskless photolithography
Laser diodes are used as a light source for maskless photolithography.
Common wavelengths
Visible light
- 405 nm: Blu-ray Disc and HD DVDdrives
- 445–465 nm: data projectors
- 488 nm: InGaNgreen-blue laser; became widely available in mid-2018.
- 505 nm: InGaNbluish-green laser; also became widely available in mid-2018.
- 510–525 nm: OSRAM for laser projectors.[29]
- 635 nm: AlGaInPbetter red-laser pointers, same power subjectively twice as bright as 650 nm
- 650–660 nm:
- 670 nm: AlGaInPbar-code readers, first diode-laser pointers (now obsolete, replaced by brighter 650 nm and 671 nm DPSS)
Infrared
- 760 nm: AlGaInP gas sensing: O
2 - 785 nm: GaAlAs compact discdrives
- 808 nm: DPSS Nd:YAG lasers(e.g., in green laser pointers or as arrays in higher-powered lasers)
- 848 nm: laser mice
- 980 nm: Yb:YAGDPSS lasers
- 1,064 nm: DPSSlaser pump frequency
- 1,310 nm: InGaAsP, InGaAsNfiber-optic communication
- 1,480 nm: InGaAsPpump for optical amplifiers
- 1,512 nm: InGaAsP gas sensing: NH
3 - 1,550 nm: InGaAsP, InGaAsNSbfiber-optic communication
- 1,625 nm: InGaAsPfiber-optic communication, service channel
- 1,654 nm: InGaAsP gas sensing: CH
4 - 1,877 nm: GaInAsSb gas sensing: H
2O - 2,004 nm: GaInAsSb gas sensing: CO
2 - 2,330 nm: GaInAsSb gas sensing: CO
- 2,680 nm: GaInAsSb gas sensing: CO
2 - 3,030 nm: GaInAsSb gas sensing: C
2H
2 - 3,330 nm: GaInAsSb gas sensing: CH
4
See also
- Collimating lens
- Laser safety
- List of laser articles
- Superluminescent diode
References
- ^ ISBN 978-1-118-14817-4.
- ^ 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.
- ^ Coherent white paper (2018-05). "Advantages of Optically Pumped Semiconductor Lasers – Invariant Beam Properties"
- .
- doi:10.1063/1.1777371. Archived from the original(PDF) on 2011-05-03.
- ^ Oral History Transcript — Dr. Marshall Nathan, American Institute of Physics
- ^ "After Glow". Illinois Alumni Magazine. May–June 2007.
- ^ "Nicolay G. Basov". Nobelprize.org. Retrieved 2009-06-06.
- ISBN 978-0-07-026215-7.
- ^ ISBN 0-07-027738-9.
- ^ Bouchene, M.M.; Hamdi, R.; Zou, Q. (2017). "Theorical analysis of a monolithic all-active three-section semiconductor laser". Photonics Letters of Poland. 9 (4): 131–3.
- PMID 19680331.
- .
- ISBN 0-12-222695-X.
- ^ Steele, Robert V. (2005). "Diode-laser market grows at a slower rate". Laser Focus World. 41 (2). Archived from the original on 2006-04-08.
- ^ Kincade, Kathy; Anderson, Stephen (2005). "Laser Marketplace 2005: Consumer applications boost laser sales 10%". Laser Focus World. 41 (1). Archived from the original on June 28, 2006.
- PMID 16366049.
- PMID 16358809.
- PMID 28509563.
- .
- S2CID 23606690.[permanent dead link]
- ISBN 978-3-031-43338-2.
- ^ Feuerstein, Paul. "Cuts Like A Knife". Dental Economics. Retrieved 2016-04-12.
- ISBN 9780721640075.
- ISBN 9780824777111.
- ^ PMID 24571504.
- ^ a b c d Vitruk, PP (2015). "Oral Soft Tissue Laser Ablative and Coagulative Efficiencies Spectra". Implant Practice US. 7 (6): 19–27.
- S2CID 114572097.
Further reading
- Van Zeghbroeck, B.J. "Principles of Semiconductor Devices". (for direct and indirect band gaps)
- Saleh, Bahaa E.A.; Teich, Malvin Carl (1991). Fundamentals of Photonics. Wiley. ISBN 0-471-83965-5. (For Stimulated Emission)
- Koyama, F.; Kinoshita, S.; Iga, K. (1988). "Room temperature cw operation of GaAs vertical cavity surface emitting laser". IEICE Transactions (1976-1990). 71 (11): 1089–90. (for VCSELS)
- Iga, Kenichi (2000). "Surface-emitting laser—Its birth and generation of new optoelectronics field". IEEE Journal of Selected Topics in Quantum Electronics. 6 (6): 1201–15. . (for VECSELS)
- ISBN 978-1-4822-6106-6. (For external cavity diode lasers)
External links
- An Introduction to Laser Diodes
- Overview of available single mode diode lasers
- Video showing laser bar assembly process
- Sam's Laser FAQ by Samuel M. Goldwasser
- Driving Diode Lasers. EuroPhotonics, 08/2004
- Britney Spears Guide to Semiconductor Physics Edge-emitting lasers
- Application and technical notes explaining current and temperature control of laser diodes
- Application explaining how to design and test laser driver [1]