A Laser Diode is a semiconductor device similar to a light-emitting diode (LED). It uses a p-n junction to emit coherent light in which all the waves are at the same frequency and phase. This coherent light is produced by the laser diode using a process termed as “Light Amplification by Stimulated Emission of Radiation”, which is abbreviated as LASER. And since a p-n junction is used to produce laser light, this device is named a laser diode. Before we learn more about the working process of a laser diode, let’s look at how laser light is different from other types of light, and its advantages.

          The light from sunlight or from most of the artificial light sources contains waves of multiple wavelengths and they are out of phase with each other. The light waves from monochromatic light sources like incandescent bulbs also are not in phase with each other. In contrast to the previous light sources, laser diodes produce a narrow beam of laser light in which all the light waves have similar wavelengths and they travel together with their peaks lined up. This is why laser beams are very bright and can be focused over a very tiny spot.

          Of all the devices that produce laser light, laser diodes or semiconductor lasers are the most efficient and they come in smaller packages. So they are widely used in various devices like laser printers, barcode readers, security systems, Autonomous vehicles (LIDAR), Fiber optic communications, etc.

How does a Laser Diode work?
The working of a laser diode takes place in three main steps:

Energy Absorption

          The laser diode consists of a p-n junction where holes and electrons exist. (Here, a hole means the absence of an electron). When a certain voltage is applied at the p-n junction, the electrons absorb energy and transition to a higher energy level. Holes are formed at the original position of the excited electron. The electrons stay in this exciting state without recombining with holes for a very small duration of time, termed as “recombination time” or “upper-state lifetime”. The recombination time is about a nanosecond for most laser diodes.

Spontaneous Emission

          After the upper-state lifetime of excited electrons, they recombine with holes. As the electrons fall from a higher energy level to a lower energy level, the difference in energy is converted into photons or electromagnetic radiation. This same process is used to produce light in LEDs. The energy of the emitted photon is given by the difference between the two energy levels.

Stimulated Emission

          We need more coherent photons from the laser diode than the ones emitted through the process of spontaneous emission. A partially reflecting mirror is used on either side of the diode so that the photons released from spontaneous emission are trapped in the p-n junction until their concentration reaches a threshold value. These trapped photons stimulate the excited electrons to recombine with holes even before their recombination time. This results in the release of more photons that are in the exact phase with the initial photons and so the output gets amplified. Once the photon concentration goes above a threshold, they escape from the partially reflecting mirrors, resulting in a bright monochromatic coherent light.

Construction of a Laser Diode
A simple semiconductor laser diode is made up of the following parts in order:
 Metal Contact
 P-type Material
 Active/Intrinsic Region (N-type Material)
 N-type Material
 Metal Contact

          The input terminals are connected to a metal plate that is sandwiched between the n-type and p-type layers. This type of laser diode is also called a “Homojunction Laser Diode”. The intrinsic region between the p-type and n-type material is used to increase the volume of the active region so that more holes and electrons can accumulate at the junction. This allows more number 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. This entire PIN diode (P-type, Intrinsic, N-Type) arrangement is enclosed normally in a metal casing.

Types of Laser Diodes

Double Heterostructure Laser Diode

          In this type of laser diodes, an additional confinement layer of different materials is sandwiched between the two p-type and n-type materials. Each of the junctions between different materials is called a heterostructure. Because of the presence of two heterostructures, this type of laser diode is named a double heterostructure (DH) laser diode. The advantage of this DH laser diode is that the active region is confined to a thin layer which gives better optical amplification.

Quantum Well Laser Diode

          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 levels to lower energy levels. This gives a better efficiency for this type of laser diode.

Separate Confinement Heterostructure Laser Diode

          The thin middle layer in the quantum well laser diode is very small for confining emitted light effectively. To compensate for 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.

Vertical Cavity Surface Emitting Laser Diode (VCSEL)

          For 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 the optical cavity.

Other Types

• Quantum Cascade Laser Diode
• Interband Cascade Laser Diode
• Distributed Bragg Reflector Laser Diode
• Distributed Feedback Laser Diode
• External Cavity Diode Laser
• Vertical External Cavity Surface Emitting Laser Diode (VCSEL)

Laser Diode P-I Characteristics
The below diagram is a graphical plot between output optical power on the y-axis and the current input to the laser diode on the 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 a decrease in temperature.

Laser Sensor Ratings

Ratings of any electronic components provide us with the specifications which are discussed below.

• The supply voltage is 5V (DC)
• Current is 30 Ma
• The wavelength is 650 nm
• The wavelength color is red

Advantages and Disadvantages
The advantages of this sensor include the following

1. The range of measurement is large
2. Operating distance is large
3. Resolve capacity less than one micron at a decreased price

The disadvantages of this sensor include the following

1. The accuracy of the sensor can be affected by dust or other materials
2. Damages eyesight

Applications of a Laser Diode

Laser Diode modules are used in all major areas of electronics ranging from

• Consumer Electronics: CD/DVD players, Laser printers, Fiber Optic Communication, Barcode Readers, etc.
• Medical Machines: Laser diodes are used in machines used to remove unwanted tissues, eliminate cancer cells, non-invasive and cataract surgeries, etc.
• Autonomous Vehicles: Laser diode technology is used in making LIDAR systems implemented for autonomous driving
• Scientific Instrumentation: Lasers are used in devices used for remote contactless measurements, spectrometry, range finders, etc.
• Industrial Applications: Laser Diodes are used as a source of high-intensity laser beams for the precise cutting of materials. They are also used in 3D printing to soften the substrate.