A laser diode, (LD), injection laser diode (ILD), or diode laser is a semiconductor device similar "Laser applications in oral surgery and implant dentistry" (PDF) . Lasers in Medical Science. 22 (4): – doi/s Diode Laser Experiment Page 1 of 7. The Laser Diode. 1 Introduction. This set of laboratory experiments is primarily design to have you become familiar with the. Laser Diodes are semiconductor lasers and are available in many different shapes and sizes with laser powers ranging from a few. mW to hundreds of watts.
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PDF | G. I Suruceanu and others published laser diodes. A number of analytical applications have flourished with the help of our laser diode tributed feedback (DFB) laser diodes in conjunction with our products to . Diode lasers are used to pump solid- state lasers, such as the Nd:YAG. Laser diodes are tuned to the absorption band of the crystal providing.
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. These single frequency diode lasers exhibit a high degree of stability, and are used in spectroscopy and metrology, and as frequency references. Another method of powering some diode lasers is the use of optical pumping. A nearby photon with energy equal to the recombination energy can cause recombination by stimulated emission. These trapped photons stimulate the excited electrons to recombine with holes even before their recombination time. In a conventional semiconductor junction diode, the energy released from the recombination of electrons and holes is carried away as phonons , i. Below is the diagram.
Once the photon concentration goes above a threshold, they escape from the partially reflecting mirrors, resulting in a bright monochromatic coherent light. A simple semiconductor laser diode is made up of the following parts in order: The input terminals are connected to a metal plates which are sandwiched to the n-type and p-type layers. The intrinsic region between the p-type and n-type material is used to increase the volume of active region, so that more number of holes and electrons can accumulate at the junction.
This allows more number of electrons to recombine with holes at any instant of time, resulting in better output power. The laser light is emitted from the elliptical region. This beam from the laser diode can be further focused using an optical lens.
In this type of laser diodes, an additional confinement layer of a different material is sandwiched between the two p-type and n-type materials. Each of the junction between different materials is called a heterostructure.
Because of presence of two heterostructures, this type of laser diode is named as a double heterostructure DH laser diode. The advantage of this DH laser diode is that that the active region is confined to a thin layer which gives better optical amplification. The quantum well laser diode has a very thin middle layer, which acts as a quantum well.
The electrons will be able to use quantum energy levels when transitioning from higher energy level to lower energy level.
This gives a better efficiency for this type of laser diode.
The thin middle layer in the quantum well laser diode is very small for confining emitted light effectively. To compensate this, in the separate confinement heterostructure laser diode, another two layers are added over the three initial layers.
These layers have a lower refractive index and help in confining the emitted light effectively. All the previously discussed laser diodes, the optical cavity is placed perpendicular to the current flow.
In vertical cavity surface emitting laser diodes, however, the optical cavity is along the axis of current flow. The partially reflecting mirrors are placed near the ends of optical cavity. The below diagram is a graphical plot between output optical power on y-axis and the current input to the laser diode on x-axis.
As we increase the current flow to the laser diode, the optical power of output light gradually increases up to a certain threshold. Until this point, most of the light emitted is due to spontaneous emission. Above this threshold current, the process of stimulated emission increases.
This causes the power of output light to increase a lot even for smaller increases in input current. The output optical power also depends on temperature and it reduces with decrease in temperature. Laser diodes require complex drive circuitries that involve feedback loops by measuring output optical power, temperature, voltage and input current.
But for controlling a laser diode used in applications where high accuracy is not required, a simple laser diode driver circuit can be constructed using LM voltage regulator IC.
Below is the diagram. The LM is configured to function as a constant current source. 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. Due to diffraction , the beam diverges expands 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, 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 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. The simple laser diode structure, described above, is extremely 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. In these devices, a layer of low bandgap material is sandwiched between two high bandgap layers. One commonly-used pair of materials is gallium arsenide GaAs with aluminium gallium arsenide Al x Ga 1-x As.
Each of the junctions between different bandgap materials is called a heterostructure , hence the name "double heterostructure laser" or DH laser. The kind of laser diode described in the first part of the article may be referred to as a homojunction laser, for contrast with these more popular devices. 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. If the middle layer is made thin enough, it acts as a quantum well. This means that the vertical variation of the electron's wavefunction , and thus a component of its energy, is quantized. The efficiency of a quantum well laser is greater than that of a bulk laser because the density of states function of electrons in the quantum well system has an abrupt edge that concentrates electrons in energy states that contribute to laser action.
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. In a quantum cascade laser , the difference between quantum well energy levels is used for the laser transition instead of the bandgap.
This enables laser action at relatively long wavelengths , which can be tuned simply by altering the thickness of the layer. They are heterojunction lasers. A 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.
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 s have been SCH quantum well diodes.
A distributed Bragg reflector laser DBR is a type of single frequency laser diode. 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.
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. A distributed feedback laser DFB is a type of single frequency laser diode.
To stabilize the lasing wavelength, a diffraction grating is etched close to the p-n junction of the diode. This grating acts like an optical filter, causing a single wavelength to be fed back to the gain region and lase. Since the grating provides the feedback that is required for lasing, reflection from the facets is not required. Thus, at least one facet of a DFB is anti-reflection coated.
The DFB laser has a stable wavelength that is set during manufacturing by the pitch of the grating, and can only be tuned slightly with temperature. DFB lasers are widely used in optical communication applications where a precise and stable wavelength is critical.
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.
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.
But there is a disadvantage: 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. In VCSELs, the mirrors are typically grown epitaxially as part of the diode structure, or grown separately and bonded directly to the semiconductor containing the active region.
VECSELs are distinguished by a construction in which one of the two mirrors is external to the diode structure. As a result, the cavity includes a free-space region. 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 which 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. However, because of their lack of p-n junction, optically-pumped VECSELs are not considered "diode lasers", and are classified as semiconductor lasers. External-cavity diode lasers are tunable lasers which use mainly double heterostructures diodes of the Al x Ga 1-x As type.
The first external-cavity diode lasers used intracavity etalons  and simple tuning Littrow gratings.
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. The reliability of a laser diode can make or break a product line. Moreover, reverse engineering is not always able to reveal the differences between more-reliable and less-reliable diode laser products.
At the edge of a diode laser, where light is emitted, a mirror is traditionally formed by cleaving the semiconductor wafer to form a specularly reflecting plane.
A scratch made at the edge of the wafer and a slight bending force causes a nearly atomically perfect mirror-like cleavage plane to form and propagate in a straight line across the wafer. But it so happens that 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.
Essentially, 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 s, this problem, which is particularly nettlesome for GaAs-based lasers emitting between 0.
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. In the very early s, SDL, Inc. This process, too, was undisclosed as of June 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. Laser diodes are numerically the most common laser type, with sales of approximately million units,  as compared to , of other types of lasers.
Laser diodes find wide use in telecommunication as easily modulated and easily coupled light sources for fiber optics communication. They are used in various measuring instruments, such as rangefinders. Another common use is in barcode readers. Visible lasers, typically red but later also green , are common as laser pointers. 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.
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. Many applications of diode lasers primarily make use of the "directed energy" property of an optical beam.
In this category, one might include the laser printers , barcode readers, image scanning , illuminators, designators, optical data recording, combustion ignition , laser surgery , industrial sorting, industrial machining, and directed energy weaponry.
Some of these applications are well-established while others are emerging. Laser medicine: Diode wavelengths range from to 1, nm , are poorly absorbed by soft tissue, and are not used for cutting or ablation. 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, 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. As early as John von Neumann described the concept of semiconductor laser in an unpublished manuscript.
In , Japanese engineer Jun-ichi Nishizawa filed a patent for the first semiconductor laser. Following theoretical treatments of M. Bernard, G. Duraffourg and William P. Dumke in the early s coherent light emission from a gallium arsenide GaAs semiconductor diode a laser diode was demonstrated in by two US groups led by Robert N. Watson Research Center. The priority is given to General Electric group who have obtained and submitted their results earlier; they also went further and made a resonant cavity for their diode.
Dumke that these materials would not work. Instead, he suggested Gallium Arsenide as a good candidate. 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 and thereafter.
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 s by molecular beam epitaxy and organometallic chemical vapor deposition. Such performance enabled continuous-lasing to be demonstrated in the earliest days.
The dominant challenge for the remainder of the s was to obtain low threshold current density at K and thereby to demonstrate continuous-wave lasing at room temperature from a diode laser. The first diode lasers were homojunction diodes.