Laser Diode: Working Principle, Construction, Types, Application

A laser diode is a small semiconductor device that emits powerful and precise light using a process known as stimulated emission. These devices are capable of producing an intense laser ray with uniformly sized light waves. This characteristic makes laser beams extremely bright and concentrated. These diodes have a high power-to-size ratio and generate electrically efficient laser light. Different semiconductor components and layer architectures can be used to generate different wavelengths. Diode lasers are frequently employed in conjunction with different lasers to serve as an optical pump. Diode lasers have wide applications in a variety of sectors such as telecommuting, manufacturing, and medicinal treatments.

What is a Laser Diode?

A laser diode is a small, solid-state equipment that uses semiconductor material to produce continuous light. Materials such as gallium nitride (GaN) or gallium arsenide (GaAs), among others, are used to create them. The laser can be made up of a single diode or a combination of many diodes. It can also include a wide range of other optical parts. When power is applied, the diode laser’s power usually increases linearly. Diode lasers are highly favored due to their high electrical-to-optical efficiency, which means that they are excellent at converting current flow into laser power. Typically, this ratio is 50%; however, in ideal conditions, it might reach 80%.

How Does Laser Diode Work?

To operate, laser diodes must induce photon emission at a semiconductor junction. Emissions from a laser diode can be classified into three categories based on how they are stimulated.

Stimulated Absorption

A laser diode consists of the p-n junction where both electrons and holes are involved. An excess of negatively charged carriers, or electrons, is produced by the n-type area, and an excess of positively charged carriers, or holes, is produced by the p-type. At the p-n junction, electrons absorb energy and move to a higher energy level in response to an applied voltage. Furthermore, holes develop at the stimulated electron’s initial location. During this stimulated state, electrons can stay without interacting with the hole for a brief period, which is called “upper-state life” or “recombination time.” Laser diodes can recombine in milliseconds or less, on average.

Spontaneous Emission

Highly stimulated electrons recombine with holes at the end of their upper-state duration. When electrons go from a higher to a lower energy level, the energy difference is converted into photons or electromagnetic radiation. The same mechanism produces light in LEDs. The emitted photon’s frequency is determined by the disparity between the two levels of energy.

Stimulated Emission

The reflective layers at the ends of the diode laser’s structure form an “optical cavity.” Internal reflection of photons results in coherent, narrow-band light, which is further increased by optical feedback. Photons from an external light source strike electrons with a higher energy level in a process known as stimulated emission. In addition, stimulated emission happens when a photon interacts with a stimulated electron and releases another photon as a result. When these photons reach the electrons, they absorb energy, recombine with the holes, and emit another photon.

How is Laser Diode Constructed?

Gallium arsenide (GaAs) or indium gallium arsenide (InGaAs) semiconductors are used to build laser diodes. Let’s take a look at the fundamentals of laser diode’s construct:

P-N Junction

The connection that occurs between the p-type and n-type regions is known as the p-n junction. Materials like gallium arsenide (GaAs) grown epitaxially on a substrate are generally used to create P-type and N-type regions. Electrons cannot go between the two regions due of the resistance formed by the p-n junction between the holes of the two regions.

Active Region

Laser activity takes place in the active region and this region is situated in p-n junction. It is a thin layer of semiconductor typically made of InGaAs or GaAs compounds among others. Charge carriers, such as electrons and holes, recombine in the active region and discharge energy through the emission of photons. Because it is in charge of enhancing the light from stimulated emission, this region is often referred to as the enhancement medium.

Mirrors

A laser diode comprises two mirrors on each of its ends, with one of that being fully reflecting and the other moderately reflective. Cutting provides mirrors with a high degree of reflection. To produce the optical gaps that enable photons to bounce back and forth and produce visible light, this glass is essential.

Metal Plates

Situated between the n-type and p-type layers, the input terminals are fastened to the metal plates.

Properties of Laser Diodes

There are different properties of laser diodes some of which are discussed briefly here:

It is Monochromatic

Monochromatic means composed of a single color. Because they only emit one color of light on a specific wavelength, laser diodes are monochromatic. This feature is applied in fields such as fiber optics.

High Power Efficiency

Because these lasers convert energy into laser power with minimum waste, they have a higher power efficiency than other types of lasers.

Lasers Are Coherent

When coherent light—defined as photons that are transmitted at the same frequency and phase—is produced by laser diodes, it results in incredibly concentrated, powerful light beams.

Lasers Are Well-Directed

Laser diodes emit light in the form of a narrow beam that is efficiently transmitted through an optical fiber.

It Requires High Energy

It is the laser diode’s most important feature. It only works when more electricity is applied than the threshold. It’s due to lower emission levels at lower energies.

Wavelength Flexibility

Diode lasers are capable of producing light in visible, near-infrared (NIR), and ultraviolet (UV) wavelengths. In numerous applications, the class is perfect due to its wavelength selectivity.

Laser Can Compact

Due to the compactivity of the laser, it can be integrated into small systems and equipment.

Categories of Laser Diodes

Laser diodes can be divided into two basic categories. Here we will have a brief look at both of these categories of laser diodes:

Injection Laser Diodes

This is similar to LEDs, except that laser diodes use narrow semiconductor channels while LEDs use broad ones. In this situation, the diode works as a waveguide, and the light beam passes through it. The light beam intensifies due to the constant stimulated emission.

Optical Pumped Semiconductor Laser Diodes

In this case, the injection laser diode acts as an external pump. Semiconductor materials of the III & V group serve as the basis. The process of amplification is achieved by stimulating emission. It has various benefits, such as reducing interference caused by the structure of the electrode. Additionally, it offers a good range of wavelengths.

Types of Laser Diodes

There are different types of laser diodes we are going to discuss some of the important types in this section, let’s dive into the details:

Double Heterostructure Laser Diode

• Within the laser diode family, there is a particular type known as double heterostructure (DH) lasers, which feature a heterostructure to enhance efficiency.
• In comparison to traditional homojunction designs, DH lasers have a lower current threshold, higher efficiency, and higher output power.
• These devices find widespread application in optical data devices, laser printing, telephony, and measurement equipment that are facilitated by lasers.
• Their excellent efficiency, minimal threshold currents, and substantial output are extremely beneficial in long-distance fiber-optic telecommunications.

Quantum Well Laser Diode

• The group of technologies called quantum well diode lasers are made up of components with improved optical and electrical qualities due to their quantum excellent architecture.
• Compared to simpler devices, they achieve greater wavelength control, higher power effectiveness, and reduced threshold current.
• The thin semiconductor wafers with a tighter bandgap are stacked and encased in higher bandgap layers to facilitate the construction of these devices.
• Narrow linewidth output is a well-known feature of quantum well diode lasers.
• Telecom, optical storage of data, laser printing, and clinical diagnostics are sectors that greatly benefit from this kind of equipment.

Separate Confinement Heterostructure Laser Diode (SCH)

• The heterostructure construct for separate confinement heterostructure (SCH) laser is applied to enhance their optical and electrical features.
• This contributes to decreased optical losses, greater transport confinement, along overall enhanced efficiency over typical homojunction lasers.
• SCH lasers generate an extra complex heterostructure through the addition of numerous semiconductor wafers with different band gaps.
• The depletion layer is sandwiched by broader bandgap layers. Such complexity enables more effective confining for both carriers and optical modes.
• They are ideal for a wide range of applications, including telecoms, optical data storage, laser printing, optical sensing, and laser-based research, however are additionally especially well-suited to more hostile environments and fiber-optic systems for communication.

External Cavity Diode Lasers (ECDLs)

• External Cavity Diode Lasers (ECDLs) are a type of equipment that amplifies as well as controls a laser’s emission using an external cavity, typically with external reflectors or grating.
• In comparison to other laser diode designs, ECDLs offer significantly enhanced flexibility, thinner linewidth, and accurate wavelength control.
• Their construction is comparable to that of conventional diode lasers, with a forward-biased p-n junction featuring an active region where photons are released
• An external cavity that provides optical input is installed in a laser, enabling accurate personalizing of emission frequencies.
• A reflector, grating, or other optical device is housed in this cavity, and it reflects a part of the beam back into it.
• The placement or incoming angle of the exterior mirror or grating is capable of being carefully adjusted to precisely control the laser wavelength.
• This feature of ECDLs enables several sophisticated uses in metrology, atomic and molecular physics, and spectroscopy.

Quantum Cascade Laser Diodes (QCLs)

• The quantum cascade laser (QCL) uses quantum cascade transitions between various levels of energy across multiple semiconductor junctions as its laser source.
• Multiple quantum wells are used for constructing QCLs, while different bandgap layers of semiconductors serve as barriers.
• Applying a forward bias current allows the electrons and holes to move through several quantized levels of energy, effectively producing photons at every alteration.
• They can emit a wide range of wavelengths within the electromagnetic spectrum’s mid-infrared and Terahertz regions.
• The layer thicknesses and biased voltage can be readily adjusted to tune their emission wavelength, which makes them perfect for spectroscopic analysis purposes that need to use many different wavelengths.
• In addition, they are employed in mapping, medical diagnostic systems, free-space communication, and observing the environment.

Vertical Cavity Surface Emitting Laser Diode (VCSEL)

• At times, Vertical Cavity Surface Emitting Devices (VCSED) are referred to as VCSELs, or vertical cavity surface emitting lasers.
• This is a class of semiconductor laser diodes that discharge laser beams through the chip’s top layer in a direction perpendicular to the chip’s surface.
• A p-n junction wafer that has a vertical cavity made up of two dispersed Bragg reflector reflectors is the foundation of VCSELs.
• Using this kind of laser, the active zone is frequently populated with quantum wells or comparable gain-inducing structures.
• The ability to produce VCSELs at wafer size leads to reduced cost of production alongside greater uniformity over manually made devices, which is one of its primary benefits.
• They have widespread application in optical interconnects, fiber-optic networks, as well as high-speed data transfer technologies.
• Additionally, they find utility in a broader range of optical and sensor technologies including optical mouse, laser-based printers, and 3D scanners, along with in 3D mapping in identifying faces and depth sensing in mobile devices.

Vertical External Cavity Surface Emitting Laser Diode (VECSEL)

• A specialized kind of laser device known as a vertical external cavity surface emitting laser (VECSEL) combines the advantageous aspects of external cavity diode lasers (ECDLs) and vertical-cavity surface-emitting lasers (VCSELs). 
• High output power, wavelength tunability, and superior beam quality are the distinctive properties that result from this combination.
• Photon is discharged perpendicular to the chip surface by VECSELs since their laser cavity is positioned vertically.
• Effective transfer of heat as well as precise control over the output beam are made possible by this vertical arrangement.
• They have an exterior cavity configuration made up of extra reflecting surfaces positioned outside the chip body. It also allows energy scaling with beam shape and wavelength adjustment.
• VECSELs provide a consistent beam contour with a small divergence angle, which are attributes of high-quality output, through the employment of precisely constructed external cavities.
• They are useful for high-data-rate optical transmission, laser spectroscopy, laser cooling, atom-trapping and manipulation, laser ablation, and other precision applications.

Interband Cascade Lasers (ICLs)

• Interband cascade lasers, or ICLs, function by monitoring the interband transition between several electronic bands that are present in the active zone.
• In the mid-infrared wavelength range, they provide effective and high-performance functioning.
• Interband transitions between energy bands inside a wafer are advantageous to ICLs, which can be utilized to achieve higher optical yield and laser emission by taking advantage of cascaded transitions between several phases or quantum wells.
• More restricted interband transitions are the basis for conventional diode lasers. Typically, their design produces radiation in the mid-infrared range, which spans from 3 to 12 micrometers.
• Gas detecting, analysis of chemicals, monitoring the environment, manufacturing process oversight, and free-space optical communications are among the fields in which ICLs are used. Certain contaminants can be detected and measured using mid-infrared radiation.

Distributed Bragg Reflector Laser Diode

• Devices named distributed Bragg reflectors (DBRs) have an incorporated dispersed Bragg reflector within the amplification chamber.
• This feature enables accurate emission frequency adjustment as well as narrower screening for high spectrum clarity and selectivity.
• Refractive index materials alternating in layers serve as a wavelength-selective mirror in the Bragg grating.
• The intended radiation can pass through the amplification cavity whereas all non-selected wavelengths of light are reflected by this design.
• This design provides precise wavelength selection and allows for tweaking of the radiated wavelength over a spectrum by modifying the grating duration or refractive index pairs.
• It also makes it easy to adapt and integrate with a wide range of applications, including OCT (optical coherence tomography) and wavelength-division multiplexing (WDM) systems.
• Telecom, fiber-optic sensing, metrology, spectroscopy, and optical coherence tomography are among the fields that employ these lasers.

Distributed Feedback Laser Diode

• The design of DFB (distributed feedback) lasers is comparable to that of conventional semiconductor lasers.
• The use of a periodic grating structure inside the active zone, commonly known as the external waveguide, is distinctive to this type.
• The distributed feedback grating modifies the refraction index of the waveguide region, therefore periodically modulating the gain profile.
• This serves as a feedback mechanism, requiring optical feedback/amplification at a specific wavelength while blocking other modes. Thus, these devices generate photons with a specified wavelength, great spectral purity, and an extremely narrow line width.
• Applications such as sensors, high resolution (HR) spectroscopy, metrology, and high data rate fiber-optic communications are all well suited for this.

Advantages of Laser Diodes

There are some advantages of laser diodes over other types of lasers. In this part, we will discuss some major advantages of laser diodes:

Low Power Consumption

• They have a higher power efficiency than other laser kinds because they transform electricity into laser power with a lesser amount of waste.

Wavelength Flexibility

• Options for design provides wavelength versatility. Diode lasers can produce light in the ultraviolet (UV), visible, and near-infrared (NIR) areas. The category’s wavelength selectability makes it suitable for a wide range of applications.

High-Speed Operation

• They provide precise control over the laser power and quick operation. High-efficiency energy may be used for drilling, engraving, and cutting thanks to quick reaction times and accurate beam focusing.

Long Operational Life

• Diode lasers have a dependable and lengthy working life. They can thus be used for extended periods of continuous operation.

Compact Size

• It is easier to work with laser diodes because of its compact size. They are lightweight, and small which makes them portable and easy to handle. They are also portable due to their size.

Stability

• Laser diodes feature excellent power stability, minimal power consumption, and gradual wavelength variations.

Versatility

• Laser diodes find uses in a variety of areas, including broadcast communications, healthcare devices, standard identity scanners, laser pointers, and cut and etch frames.

Highly Reliable

• With their relatively narrow beam and minimal divergence, laser diodes are useful for applications requiring precise concentration or targeting.

Disadvantages of Laser Diodes

There are some disadvantages to using laser diodes over other types of lasers. Let’s have a brief look at the disadvantages:

Expensive

• When employed for high-power or specific frequency applications, it is costly. The production of high infrared (IR) or ultraviolet (UV) diode-lasers can be costly and challenging.

Lower Beam Quality

• Usually, the multi-mode beam quality is worse than that of solid-state or gas lasers. Instruments featuring a single mode provide higher beam quality.

Sensitive to Temperature Fluctuation

• Temperature variation can greatly affect diode laser performance. Stabilization requires efficient mechanisms for managing heat and moderating temperature.

High Voltage Vulnerability

• Due to their sensitivity to an overvoltage and overcurrent, laser diodes may be damaged or perform less well.

Safety Issues

• Laser diodes have the potential to emit dangerously intense light. Poor safety precautions may result in burning to the epidermis or injuries to the eyes.

Applications of Laser Diodes

Diode lasers are useful in different sectors with their beneficial features. In this part, we will briefly discuss the major applications of laser diodes.

Telecommunications

• Fiber-optic communication systems make use of diode lasers. They serve as a source of energy for data transmission.
• Signal amplification is essential in long-distance fiber-optic systems in order to compensate for loss of signal. Because of this, erbium-doped fiber amplifiers, or EDFAs, are used.
• Additional applications for diode lasers include data transfer via the air and wavelength division multiplexing (WDM), which boosts the capacity of optical communication networks by sending out several signals at different wavelengths at the same time.

Medical Instruments

• Because of their small size, strength, and adaptability, diode lasers serve a purpose in an array of clinical applications.
• Hair removal, skin treatments, soft tissue surgery, photodynamic therapy (PDT), endovenous laser treatment (EVLT) of varicose veins, and low-level laser therapy (LLLT) are just a few of the clinical applications of such lasers.
• Diode lasers, for instance, are employed in LLLT in the repair of tissues and relief from pain. The laser permeates the surrounding tissue, promoting cellular metabolism, lowering swelling, and easing discomfort.

Laser Printing

• Numerous industries use laser diodes for printing and print-related applications. Printers that use lasers are primarily powered by laser diodes.
• They serve as the print process’ light sources, scanning a photoreceptive surface to produce an electrostatic picture that attracts toner.
• Additionally, they are utilized in printers that print barcodes and QR codes on surfaces by locally heating materials that are susceptible to heat.
• Diode lasers are also used in the following applications: printing banknotes, passports, and official documents with security features like holograms, and microtext.

Industrial Applications

• Since laser diodes are small, powerful, but electrically efficient, they are frequently employed in materials processing operations.
• Laser diodes are increasingly being used in laser cutting systems to automatically cut a variety of materials. A highly concentrated beam with a high energy density can be produced by laser diodes. This makes cutting a variety of materials quick and precise.
• They are also frequently employed in welding applications, where a concentrated beam melts and merges materials to create a fusion. The jewelry, electronics, and automobile industries are finding greater and greater use for laser welding.
• When used to make tiny holes in metals, ceramics, and semiconductors, drilling and micromachining employ the laser diodes’ highly focused beam.

Scientific Instruments

• Because laser diodes are so well suited for spectroscopy, substances such as chemicals can be precisely and sensitively analyzed.
• Using laser light to illuminate a subject, Raman Spectroscopy analyzes the light which scatters back to learn more about the material’s composition and structural characteristics.
• Laser diodes with adjustable Raman shifts allow for targeted stimulation and detection.
• Moreover, laser diodes are employed as the sources of excitation in fluorescence spectroscopy, which lights a sample and measures the amount of fluorescence released to identify compounds.
• In addition, there are further uses for cavity ring-down spectroscopy (CRDS), laser-induced breakdown spectroscopy (LIBS), laser-induced fluorescence (LIF) spectroscopy.

Sensors

• Lasers with diodes are extensively utilized in sensors since coherent light makes it easier to observe fluctuations in the target’s reflecting or transferred frequency or phases.
• Location as well as distance calculations can be performed with the use of laser diodes. To ascertain a target’s positioning or the distance, laser triangulation sensor shoot a laser beam toward it.
• These sensors find applications in automation, metrology, and robotics. Laser doppler velocimetry (LDV) systems, light detection and ranging (LiDAR) systems, and flow and water sensing are further applications.

Video Reference

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