Laser Diode

A laser diode is a small semiconductor gadget that produces strong and precise light emissions through a cycle called stimulated emission. These gadgets track down wide applications because of their proficiency and minimal size. When electric current flows through the p-n junction, the gain is generated through it. A laser diode is a semiconductor device that is identical to a light-emitting diode(LED) and converts electrical energy into light. In this article, we’ll learn about their development, working, types, and applications, and how these standardized gadgets work.

What is a Laser Diode?

A laser diode is a semiconductor device that transmits coherent and highly focused light through a process called stimulated emission. It comprises a p-n junction, where electrons and holes combine, releasing energy as photons. This coherent light is delivered when photons stimulate further photon emission, making a concentrated and monochromatic laser beam. It produces coherent radiation with same frequency , which can be visible or infrared spectrum.
This process is spontaneous and produces light at same frequency and phase. Laser diodes are broadly utilized in different applications, including media communications, laser pointers, optical capacity gadgets, clinical instruments, and modern gear because of their productivity, compact size, and accuracy in conveying intense light beam emissions.

Laser Diode

Characteristics of Laser Diodes

• Coherence: Laser diodes emit coherent, meaning the transmitted photons have a similar frequency and are in same phase, creating highly focused and intense beam.
• Monochromatic: It means one color. Laser diodes are monochromatic because it emits light of one color of a particular wavelength. This characteristic is used in the field like fiber optics.
• Well-directed: The light will be directed into a narrow beam in this case. It is simple to deploy over optical fiber.
• Compact Size: Laser diodes can be incorporated into small systems and devices due to their small size and lightweight.
• Threshold Value: It is the most important characteristic of the laser diode. It operates only when the power is more than the threshold value is applied. This is because emissions are weaker at the lower energy. The graph given below explains this phenomenon.

Threshold Value

Types of Laser Diodes

The types of laser diode are as follows:

• Single-Mode Laser Diodes: The diodes produces the cross-type laser lights. Single-Mode laser Diodes provides bright and efficient light. It finds its application in the fields like communication, metrology and many more.
  
• Multi-Mode Laser Diodes: It produces the laser lights which have multiple transverse modes. These modes helps in producing the low and broad beams. It’s application are in the fields like data processing, solid-state lasers, and many more.
• Vertical-Cavity Surface-Emitting Laser (VCSEL) Diodes: These diodes are used in transmitting the light perpendicular to the semiconductor’s surface. It is used in short-range information correspondence, optical interconnects and 3D detecting in facial recognition technology.
• External Cavity Diode Lasers (ECDL): These diodes are used in incorporating the external optical cavity, which can provide additional control over the laser outputs. External Cavity Diode Lasers allow for a narrow linewidth, tunable, and more stable laser beams, which makes the ECDLs suitable for many applications which have the requirements of precise wavelength control.
  
• High-Power Laser Diodes: It produces concentrated and intense laser beams with significantly higher optical output power than low-power or standard laser diodes. The applications of High-Power laser diodes are welding, laser cutting, and handling of material. They have the capacity to emit visible and infrared wavelengths.

Other laser diodes are Double Heterostructure Laser Diode : Quantum Well Laser Diode, Separate Confinement Heterostructure Laser Diode, and so on.

Construction of Laser Diode

Semiconductors like gallium arsenide (GaAs) or indium gallium arsenide (InGaAs) are used to build a laser diode. It comprises of a few layers:

Laser Diode Construction

• P-N junction: It is the fundamental structure of lasthe er diode. A p-N junction is formed when the p-type and n-type regions meet. P-type and N-type regions are often made of materials such as gallium arsenide (GaAs) grown epitaxially on a substrate. The p-n junction creates a barrier that prevents the carriers (holes and holes) from moving electrons between two regions.
• Active Region: The laser action takes place here. It is located within the p-n junction. It is a thin layer of semiconductor material usually made of different compounds such as GaAs or InGaAs.In the active area, charge carriers (electrons and holes) recombine, releasing energy in the form of photons. This area is also known as the enhancement medium because it is responsible for amplifying the light from stimulated emission.
• Mirrors: Each end of a laser diode has two mirrors, one of which is completely reflective and the other of which is only partially reflective. Highly reflective mirrors are created through the process of cleaving. This glass is necessary to create optical gaps that allow photons to bounce back and forth, making light visible.
• Metal Contact : The input terminals are attached to the metal plates which is between the n -type and p- type layers .

Working of Laser Diode

Basically , a laser emits a beam of electromagnetic nature which is coherent in nature .The working of the laser diode is based on:

• Absorption
• Spontaneous emission
• Stimulated emission

Absorption

The laser diode has a p-n junction with holes and electrons in it. In absorption, electrons jump to a higher energy level by absorbing the energy when a certain voltage is applied. This means that the electron jumps from the valence band to the conduction band as seen in the figure below. This transition is called absorption.

Spontaneous emission

After the lifetime of the excited electrons in the upper state ends, they recombine in the hole. The electrons move to the lower energy level to achieve stability. While coming down to the lower energy level, electrons will release the energy ‘hv’ as shown in the figure below. This energy is the difference between the two levels which is used to determine the frequency of emission photons. The energy released is in the form of light and thus photons will be emitted. This process is known as spontaneous emission.

Stimulated Emission

In stimulated emission, electrons are hit by the photons with a high energy and photons are produced by external light. The electron absorbs the energy and recombines with the hole when photon reaches the electron, this lead to the emission of more number of photons. Therefore, an incident photon causes the release of another photon. Therefore it is called stimulated emission.

Working Principle

How Laser Beam are Formed?

Laser beams are formed through a process called stimulated emission within a laser cavity. This process creates a highly concentrated, coherent, and well-defined beam of light. The steps of laser beam formation is given below:

• Stimulated Emission: In stimulated emission, electrons are hit by the photons with a high energy and photons are produced by external light. The electron absorbs the energy and recombines with the hole when photon reaches the electron, this lead to the emission of more number of photons. Therefore, an incident photon causes the release of another photon. Therefore it is called stimulated emission.
• Population Inversion: When more number of atoms or molecule are in the excited state than in the ground state, the phenomena is referred as population inversion. Due to this condition, we are able to operate lasers.
• Feedback: The stimulated emission leads to photon amplification, but to develop a well-defined beam, optical feedback is required.
• Laser threshold: When the light intensity in the cavity reaches the threshold value, a laser beam begins to form.
• Laser Beam Emission: When a light beam reaches the laser’s threshold, part of the mirror allows some photons to pass, forming a laser beam.

Voltage-Current Characteristic of Laser Diode

The V-I characteristic of laser diodes is non-linear and exhibits unique behavior. It is important to understand the performance and limitations of the laser diode. Their V-I characteristic is like other semiconductor diodes.

V-I Characteristic

According to the figure shown above, the current starts flowing only above a certain threshold voltage (in this case threshold voltage is 1.6V). The threshold voltage depends on the material used. When the threshold voltage is crossed current rises rapidly with the increase in the voltage.

Laser diodes generally do not operate by applying a fixed voltage because the current flowing depends on the applied voltage and could also be affected by the temperature of the device.

L-I Characteristics of Laser Diode

L-I characteristic of laser diode shows on varying current, and how the light output varies. According to the graph given, following properties have been observed:

• On increasing the laser current after a certain threshold value, the energy of light increases, but it is dependent on temperature.
• The laser current rises exponentially when the temperature is increased.
• The laser diode must be operated after the threshold value is crossed for reliable operation.

L-I Characteristic

Advantages and Disadvantages of Laser Diode

There are some advantages and disadvantages of laser diode given below :

Advantages

• Efficiency: Laser diodes are generally energy-efficient, converting a significant portion of electrical input power into optical output power.
• Stability: Laser diodes have low power and wavelength fluctuations over time and good output stability.
• Versatility: Laser diodes track down applications in diverse fields, including broadcast communications, clinical gadgets, standardized identification scanners, laser pointers, cutting and etching frameworks.
• Low Divergence: Laser diodes are useful for applications that require precise targeting or focusing due to their low divergence and relatively narrow beam.

Disadvantages

• Temperature Sensitivity: Laser diodes can perform differently depending on the temperature, and they are frequently affected by changes in temperature.
• Cost: It is expensive when it is used for explicit frequencies or high-power applications.
• Safety Concerns: Laser diodes can produce extremely concentrated light that could be harmful. Without proper safety measures, it can cause eye injury or skin burns.
• Overvoltage and Over-current Vulnerability: Laser diodes are delicate to overcurrent and overvoltage conditions, which can lead to harm or decreased execution.

Laser Diode Applications

• Telecommunications: Laser diodes are utilized in optical fiber which helps to communicate information over significant distances.
• Laser Printing: Laser diodes are fundamental parts in laser printers and scanners for excellent printing.
• Consumer electronics: DVD and Blu-ray players use laser diodes to peruse and compose information on optical plates.
• Clinical Instruments: Laser diodes are utilized in clinical gadgets for medical procedures like dental methodology, and painless therapies.
• Scientific instrumentation : It is used in scientific instruments for spectroscopic analysis. It allows researchers to analyze the chemical composition of materials.
• Industrial Application: It is used in industrial areas in cutting , welding, etc. in which it uses high intensity laser beam.

Conclusion

Laser diodes are semiconductor gadgets that produce coherent and highly focused light through stimulated emission. They offer various benefits, like compact size, effectiveness, and flexibility, making them crucial in various fields. However, they likewise have constraints, including temperature sensitivity and safety concerns.

Laser diodes work when electron-hole recombination takes place inside a p-n junction, resulting in the stimulated emission in an optical cavity. This cycle helps in producing the laser light, useful in applications going from broadcast communications to clinical gadgets and materials handling.

FAQs on Laser Diode

1. What makes the laser diode more efficient than the other lasers?

Laser diodes are more efficient because without the use of external pumping source, they convert electrical energy directly into coherent light.

2. What is the threshold current of a laser diode?

It is the minimum current which is required by the diode to achieve stimulated emission to produce the laser light. When the current is less than threshold value, laser diode behave as a LED.

3. Can laser diodes be modulated at high speeds?

Yes, many laser diodes can be modulated at high frequencies. It make them suitable for applications in telecommunications and data transmission.