Laser Diode

A semiconductor laser that is used for signal transmission in optical fibre is the laser diode, where LASER stands for Light Amplification by Stimulated Emission of Radiation.

A laser diode is like a light-emitting diode that emits a high-powered light through a glass lens to reduce signal loss.

A laser diode is also called an injection laser diode. It comprises the following components:

• Photodiode

• Laser

• Glass lens at the front

• A fibre

Common types of laser diodes used in telecommunication are:

• Double heterostructure

• Quantum well

• Quantum cascade

• Separate confinement heterostructure

• Distributed feedback

• VCSEL

• VECSEL

• External cavity

Laser Diode 

Laser diodes are used as light sources in optical communication. The first laser was built by Theodor H. Maiman at Hughes Research lab in 1960.

We can use the two classes of laser diode - one produces light emissions by itself and another one uses an external source.

A laser diode has the following components about which we will discuss in detail by the construction of the laser diode:

Construction of Laser Diode

• A laser diode is made of two semiconductor layers viz: P-type and N-type semiconductor. These semiconductors are made of Gallium arsenide and they are doped with Selenium, Aluminium, or Silicon.

The construction of laser diodes is similar to LED, the only difference is that an active layer is made between these two semiconductors. This active layer often consists of a Quantum well and is made of an intrinsic semiconductor (intrinsic semiconductor means undoped or pure or natural semiconductor).

• A depletion layer is between the two semiconductors because by applying a voltage to the diode, the electron-hole combination occurs.

• The front is completely polished; this surface acts as a reflecting surface or a mirror while the side view is partially polished which acts as a partially reflecting surface.

• The front face is attached with metal contacts to allow biassing in the semiconductor. Biasing means supplying power to the diode.

• Laser diodes provide a light of the same frequency.

Now, let’s talk about its working:

Working Principle of a Laser Diode

A laser diode works on the principle of stimulated emission and so emissions occur in three types:

• Stimulated absorption

• Stimulated emission

• Spontaneous emission

Generally, an electron while migrating to the lower energy level releases energy equal to the gap between them and chances are they come back after absorbing energy.

However, a laser diode works on stimulated emission. In this process, we strike a  photon  with an electron to get the light that we can see in the image below:

(Image will be uploaded soon)

So, what happens here is when we strike a photon (in the form of light energy) with the electron in the conduction band (CB). Already one electron is reaching the valence band; however, because of striking with a photon, an extra electron also reaches the valence band. In this way, we get two photons emitting out in the form of light.

Characteristics of Laser Diode

A laser has several scientific, commercial, and medical applications. The laser has the following characteristics:

• Highly directive - Light emitted by laser diodes is directed into a narrow beam that can be easily launched through an optical fibre.

• Monochromaticity - Laser diodes emit a narrow light containing a single colour.

• Lasers have the same frequency and wavelength.

• A laser is a high-intensity beam.

• Lasers are coherent - Light emitted is of a single wavelength.

• Lasers have the same phase and frequency.

• Lasers can travel large distances.

Advantages of Laser Diode

We find various advantages of using laser diodes; let’s have a look at these:

• Low power consumption.

• Low cost of manufacturing and operations.

• Operable for long hours.

• Easily portable due to small size and internal architecture.

• Highly reliable.

• Generates highly-efficient light.

Disadvantages of Laser Diode

Just like the coin has two sides, a laser diode also does, it has a few advantages with a big list of advantages, as nothing is perfect. So let’s look at a few disadvantages:

• Expensive as compared to other light-emitting devices.

• Lasers are harmful to the eyes.

• A laser diode is a temperature-dependent device, and its operations get affected by the rising temperature.

• Laser diodes are not suitable for high-power operations.

More about Laser Diode

A laser diode (LD) is a semiconductor gadget like a light-emanating diode in which a diode-syphoned straightforwardly with an electrical flow can actuate lasing conditions at the diode's intersection.

The doped p–n-transition, which is voltage-driven, permits an electron to recombine with a hole. Radiation in the form of an emitted photon is created as a result of the electron's decrease from a higher energy level to a lower one. This is known as spontaneous emission. Stimulated emission occurs when the procedure is repeated and produces light with the same phase, coherence, and wavelength.

The wavelength of the output beam is determined by the semiconductor material used, which in today's laser diodes ranges from infrared to UV. Laser diodes are the most common type of laser manufactured, having several applications such as fibre optic communications, barcode readers, laser pointers, CD/DVD/Blu-ray disc reading/recording, laser printing, laser scanning, and light beam lighting. Laser diodes may be utilised for general lighting by using a phosphor similar to that found in white LEDs.

Theory

Electrically, a laser diode is a PIN diode. The laser diode's dynamic locale lies in the characteristic (I) area, and the transporters (electrons and openings) are syphoned there from the N and P districts, individually. While early diode laser research was done on simple P–N diodes, all current lasers employ the double-hetero-structure approach, which confines the carriers and photons to enhance their possibilities of recombination and light production. A laser diode, unlike a conventional diode, aims to recombine all carriers in the I region and create light. As a result, laser diodes are made with direct band-gap semiconductors.

The laser diode epitaxial structure is produced using one of the crystal growth processes, typically beginning with an N doped substrate and progressing through the I doped active layer, P doped cladding, and the contact layer. The active layer is often made up of quantum wells, which have a lower threshold current and better efficiency.

Pumping by Electrical and Optical Means

Laser diodes are a subset of the broader category of semiconductor p–n junction diodes. Forward voltage bias across the laser diode enables the two charge carrier species – holes and electrons – to be "injected" into the depletion area from opposing sides of the p–n junction. 

Holes are injected from the p-doped semiconductor, while electrons are injected from the n-doped semiconductor. (A depletion area, empty of charge carriers, occurs as a result of the electrical potential difference between n- and p-type semiconductors whenever they come into physical contact.) Because most diode lasers are controlled by charge infusion, this sort of laser is in some cases known as "infusion lasers" or "infusion laser diodes" (ILD). Diode lasers can alternatively be categorized as semiconductor lasers because they are semiconductor devices. Diode lasers are distinguished from solid-state lasers by nomenclature.

Optical pumping is another technique of powering certain diode lasers. Optically syphoned semiconductor lasers (OPSL) utilise an III-V semiconductor chip as the addition medium and another laser (frequently one more diode laser as the syphon source. OPSL has different advantages over ILDs, most remarkably frequency choice and the shortfall of obstruction from inward cathode structures. One more element of OPSLs is that the bar properties - uniqueness, shape, and pointing - stay consistent when syphon power (and consequently yield power) differs, even over a 10:1 result power proportion.

The Production of Spontaneous Emission

At the point when an electron and an opening are in a similar region, they can recombine or "obliterate," bringing about unconstrained outflow — that is, the electron can repossess the energy state of the hole, releasing a photon with the energy difference between the electron's initial state and the hole's state.

(In a traditional semiconductor junction diode, the energy released by electron-hole recombination is transported away as phonons, or lattice vibrations, rather than photons.) The features of spontaneous emission below the lasing threshold are comparable to those of an LED. Although spontaneous emission is required to commence laser oscillation, it is one of the numerous reasons for inefficiency once the laser has begun to oscillate.

Semiconductors with Direct and Indirect Band Gaps

The difference between a photon-emitting semiconductor laser and a standard phonon-emitting (non-light-emitting) semiconductor junction diode is the kind of semiconductor employed, which has a physical and atomic structure that allows photon emission. Photon-emitting semiconductors are known as "direct bandgap" semiconductors. Silicon and germanium, both single-element semiconductors, have bandgaps that do not align in the way required for photon emission and are hence not termed "direct."

Other materials, known as compound semiconductors, have almost similar crystalline structures to silicon or germanium but violate the symmetry by alternating arrangements of two distinct atomic species in a checkerboard-like pattern. The key "direct bandgap" feature is created by the transition between the materials in the alternating pattern. Gallium arsenide, indium phosphide, gallium antimonide, and gallium nitride are all compound semiconductor materials that may be utilised to make light-emitting junction diodes.

Stimulated Emission Generation

In the absence of stimulated emission (e.g., lasing) circumstances, electrons and holes can dwell in close proximity to one another for a limited duration, referred to as the "upper-state lifespan" or "recombination time" (approximately a nanosecond for common diode laser materials), before recombining. By stimulated emission, a nearby photon with the same energy as the recombination energy can trigger recombination.

This produces another photon with the same frequency, polarisation, and phase as the first, travelling in the same direction as the first.This implies that invigorated outflow causes gain in an optical influx (of the right frequency) in the infusion region, and the increase develops as the quantity of electrons and holes injected across the junction grows. Because the spontaneous and stimulated emission processes in direct bandgap semiconductors are substantially more efficient than in indirect bandgap semiconductors, silicon is not a frequent material for laser diodes.

Modes of Optical Cavity and Laser

To construct a laser, the gain zone is enclosed by an optical cavity, like in conventional lasers. In the most basic kind of laser diode, an optical waveguide is formed on the surface of the crystal, limiting the light to a relatively narrow line. The crystal's two ends are cleaved to generate absolutely smooth, parallel edges, resulting in a Fabry–Pérot resonator. 

Photons emitted into a waveguide mode will travel down it and be reflected numerous times from each end face before exiting. Light is enhanced by stimulated emission as it goes through the cavity, but the light is also lost owing to absorption and incomplete reflection from the end facets. Finally, if the amplification exceeds the loss, the diode begins to "lase."

The shape of the optical cavity determines certain critical features of laser diodes. As a rule, the light is bound inside an exceptionally dainty layer, and the construction just backings a solitary optical mode opposite to the layers. If the waveguide in the transverse direction is wider than the wavelength of light, the waveguide can support many transverse optical modes, and the laser is referred to as "multi-mode." These transitionally multi-mode lasers are reasonable for applications that require a lot of force yet not a restricted diffraction-restricted TEM00 bar, like printing, enacting synthetics, microscopy, or syphoning different sorts of lasers.

When a tiny focussed beam is required, the waveguide must be narrow, on the order of the optical wavelength. As a result, only a single transverse mode is sustained, resulting in a diffraction-limited beam. Single spatial mode devices of this type are employed in optical storage, laser pointers, and fibre optics. It should be noted that these lasers may still support numerous longitudinal modes and hence can lase at several wavelengths at the same time. The wavelength emitted is determined by the semiconductor material's band-gap and the modes of the optical cavity. In general, photons with energies slightly over the band-gap energy will gain the most, and the modes closest to the gain curve's peak will lose the most powerfully.

Depending on the operating circumstances, the breadth of the gain curve will decide the amount of extra "side modes" that may additionally lase. Fabry Perot (FP) lasers are single spatial mode lasers that can support numerous longitudinal modes. Within the gain bandwidth of the lasing medium, an FP laser will lase at several cavity modes.The quantity of lasing modes in a FP laser is commonly temperamental, fluctuating inferable from varieties in current or temperature.

Single longitudinal mode diode lasers can be created. These high-stability single-frequency diode lasers are utilised in spectroscopy, metrology, and as frequency references. Single-frequency diode lasers are classified as distributed feedback (DFB) or distributed Bragg reflector (DBR).

Laser Beam Formation

The beam diverges (expands) fast after exiting the chip due to diffraction, often at 30 degrees vertically by 10 degrees laterally. A lens is required to create a collimated beam like that produced by a laser pointer. Cylindrical lenses and other optics are employed when a circular beam is required. Because of the disparity in the vertical and lateral divergences, the collimated beam for single spatial mode lasers utilising symmetrical lenses is elliptical in form. With a red laser pointer, this is plainly visible.

In recent years, the basic diode described above has been substantially changed to fit current technology, resulting in a variety of laser diodes, as shown below.

Laser Diode Applications

The laser diode has the following applications:

• Laser printers, CD and DVD players, and fibre optic transmission are examples of consumer electronics.

• Industrial uses: Laser diodes are favoured for industrial applications because they are a source of a high-intensity laser beam and may be used for cutting, drilling, welding, and other tasks.

• Medical applications: Laser diodes are utilised in the removal of undesirable tissues and malignancies, as well as in dental medicine.

• Scientific instrumentation: Laser diodes can be used to power instruments such as spectrometers, range finders, and contact-less measurements.