WHAT IS LASER DIODE?

                         Laser diode

Laser diodes play an important role in our everyday lives. They are very cheap and small. Laser diodes are the smallest of all the known lasers. Their size is a fraction of a millimeter.
Laser diodes are also known as semiconductor lasers, junction lasers, junction diode lasers or injection lasers. Before going into laser diodes, let us first look at diode itself.

What is a p-n junction diode?

A p-n junction diode is a semiconductor device that allows the flow of current in only one direction.

The p-n junction diode is made of two types of semiconductor materials namely p-type and n-type semiconductor. The p-type semiconductor is joined with the n-type semiconductor to form a p-n junction. The device that results from the joining of a p-type and n-type semiconductor is called a p-n junction diode.

The p-n junction diode allows electric current in forward bias condition whereas it blocks electric current in reverse bias condition.

If the positive terminal of the battery is connected to the p-type semiconductor and the negative terminal of the battery is connected to the n-type semiconductor, the diode is said to be forward biased.

When a forward bias voltages is applied to the diode, free electrons start moving from the negative terminal of the battery to the positive terminal of the battery similarly holes start moving from the positive terminal of the battery to the negative terminal of the battery.

Because of these flow of charge carriers (free electrons and holes), electric current is generated in the p-n junction diode.

In ordinary p-n junction diodes, the electrons moving from n-type to p-type will recombines with the holes in the p-type semiconductor or junction. Similarly, the holes moving from p-type to n-type will recombines with the electrons in the n-type semiconductor or junction.

We know that the energy level of free electrons in the conduction band is high as compared to the holes in the valence band. Therefore, the free electrons will release their extra energy(non-radiative energy) while recombining with the holes.

In the  (LEDs) or laser diodes, the recombination takes place in a similar manner. However, the free electrons in LED’s or laser diodes release energy in the form of light while recombining with the holes.

What is a laser diode?

A laser diode is an optoelectronic device, which converts electrical energy into light energy to produce high-intensity coherent light. In a laser diode, the p-n junction of the semiconductor diode acts as the laser medium or active medium.

The working of the laser diode is almost similar to the light emitting diode (LED). The main difference between the LED and laser diode is that the LED emits incoherent light whereas the laser diode emits coherent light.

Laser diode construction

The laser diode is made of two doped gallium arsenide layers. One doped gallium arsenide layer will produce an n-type semiconductor whereas another doped gallium arsenide layer will produce a p-type semiconductor. In laser diodes, selenium, aluminum, and silicon are used as doping agents.

P-N junction

When a p-type layer is joined with the n-type layer, a p-n junction is formed. The point at which the p-type and n-type layers are joined is called p-n junction. The p-n junction separates the p-type and n-type semiconductors.

For the construction of laser diodes, gallium arsenide is chosen over silicon. In silicon diodes, the energy is released during recombination. However, this release of energy is not in the form of light.

In gallium arsenide diodes, the release of energy is in the form of light or photons. Therefore, gallium arsenide is used for the construction of laser diodes.

N-type semiconductor

Adding a small percentage of foreign atoms into the intrinsic semiconductor produces an n-type or p-type semiconductor.

If pentavalent impurities are added to the intrinsic or pure semiconductor, an n-type semiconductor is produced. In n-type semiconductors, free electrons are the majority charge carriers whereas holes are the minority charge carriers. Therefore, free electrons carry most of the electric current in n-type semiconductors.

P-type semiconductor

If trivalent impurities are added to the pure semiconductor, a p-type semiconductor is produced. In p-type semiconductors, holes are the majority charge carriers whereas free electrons are the minority charge carriers. Therefore, holes carry most of the electric current in p-type semiconductors.

Main steps required for producing a coherent beam of light in laser diodes

The main steps required for producing a coherent beam of light in lasers diodes are: light absorption, spontaneous emission, and stimulated emission.

Absorption of energy

Absorption of energy is the process of absorbing energy from the external energy sources.

In laser diodes, electrical energy or DC voltage is used as the external energy source. When the DC voltage or electrical energy supplies enough energy to the valance electrons or valence band electrons, they break bonding with the parent atom and jumps into the higher energy level (conduction band). The electrons in the conduction band are known as free electrons.

When the valence electron leaves the valence shell, an empty space is created at the point from which electron left. This empty space in the valence shell is called a hole.

Thus, both free electrons and holes are generated as a pair because of the absorption of energy from the external DC source.

Spontaneous emission

Spontaneous emission is the process of emitting light or photons naturally while electrons falling to the lower energy state.

In laser diodes, the valence band electrons or valence electrons are in the lower energy state. Therefore, the holes generated after the valence electrons left are also in the lower energy state.

On the other hand, the conduction band electrons or free electrons are in the higher energy state. In simple words, free electrons have more energy than holes.

The free electrons in the conduction band need to lose their extra energy in order to recombine with the holes in the valence band.

The free electrons in the conduction band will not stay for long period. After a short period, the free electrons recombine with the lower energy holes by releasing energy in the form of photons.

Stimulated emission

Stimulated emission is the process by which excited electrons or free electrons are stimulated to fall into the lower energy state by releasing energy in the form of light. The stimulated emission is an artificial process.

In stimulated emission, the excited electrons or free electrons need not wait for the completion of their lifetime. Before the completion of their lifetime, the incident or external photons will force the free electrons to recombine with the holes. In stimulated emission, each incident photon will generate two photons.

All the photons generated due to the stimulated emission will travel in the same direction. As a result, a narrow beam of high-intensity laser light is produced.

How laser diode works?

When DC voltage is applied across the laser diode, the free electrons move across the junction region from the n-type material to the p-type material. In this process, some electrons will directly interact with the valence electrons and excites them to the higher energy level whereas some other electrons will recombine with the holes in the p-type semiconductor and releases energy in the form of light. This process of emission is called spontaneous emission.

The photons generated due to spontaneous emission will travel through the junction region and stimulate the excited electrons (free electrons). As a result, more photons are released. This process of light or photons emission is called stimulated emission. The light generated due to stimulated emission will moves parallel to the junction.

The two ends of the laser diode structure are optically reflective. One end is fully reflective whereas another end is partially reflective. The fully reflective end will reflect the light completely whereas the partially reflective end will reflect most part of the light but allows a small amount of light.

The light generated in the p-n junction will bounce back and forth (hundreds of times) between the two reflective surfaces. As a result, an enormous optical gain is achieved.

The light generated due to the stimulated emission is escaped through the partially reflective end of the laser diode to produce a narrow beam laser light.
All the photons generated due to the stimulated emission will travel in the same direction. Therefore, this light will travel to long distances without spreading in the space.

Advantages of laser diodes

1. Simple construction
2. Lightweight
3. Very cheap
4. Small size
5. Highly reliable compared to other types of lasers.
6. Longer operating life
7. High efficiency
8. Mirrors are not required in the semiconductor lasers.
9. Low power consumption

Disadvantages of laser diodes

1. Not suitable for the applications where high powers are required.
2. Semiconductor lasers are highly dependent on temperature.

Applications of laser diodes

1. Laser diodes are used in laser pointers.
2. Laser diodes are used in fiber optic communications.
3. Laser diodes are used in barcode readers.
4. Laser diodes are used in laser printing.
5. Laser diodes are used in laser scanning.
6. Laser diodes are used in range finders.
7. Laser diodes are used in laser absorption spectrometry.