The semiconductor laser is very small in size and appearance. It is similar to a transistor and has the operation like LED but the output beam has the characteristics of laser light. The material which often used in semiconductor laser is the gallium Arsenide, therefore semiconductor laser is sometimes known as Gallium Arsenide Laser. It is also called Injection Laser.
The semiconductor is made in unique manner for the semiconductor laser.
In reality a semiconductor laser is simply a semiconductor diode, because its active medium is the junction of the forward biased P-N diode, shown as
Here the metal contacts shown are used to connect the P-N material to the DC power supply. The junction shown is few micrometers thick. At the junction light is emitted when electrons or current pass from N to P type material. In other words, current is injected into the junction between N and P type materials. It is why we use to semiconductor laser the name of Injection Laser also.
Since we know that a minimum current density (similar to the gain threshold) is necessary for the occurrence of lasing. So when the minimum current density is reached then increasing the current density across the junction region will increase the output of the laser.
Unlike other lasers, semiconductor laser does not need mirrors to obtain the reflectivity needed to produce feedback mechanism. Reflection from the cleaved ends of the semiconductor chip is enough to produce lasing.
The reflectivity of the interface between the semiconductor material and air is approximately 36% which is enough to provide adequate feedback as well as serve as the output coupler. The beam although exist from both ends. If a beam is desired from one end of the laser only then opposite ends can be coated to reflect higher amounts of light.
The temperature has great effect on the output of the semiconductor laser. When the temperature increases then significant power losses occurs within the laser. That is why the semiconductor laser is sometimes cooled by liquid nitrogen or some other cooling system. However these lasers can be operated at room temperature if the losses are acceptable and current density is high enough.
Pulsing the current leads us to very much improved performance. Gallium Arsenide lasers are usually operated in pulsed mode, with duty cycle less than 1% because higher duty cycles cause an increased temperature, which greatly affect the output characteristics. The gallium arsenide laser produces light in near infrared spectrum ranging from 845nm to 905nm. The lasing medium of the semiconductor laser is short and rectangular. Therefore the output beam does not have the same dimension in both vertical and horizontal axis. Hence the beam profile has an unusual shape. The beam divergence of semiconductor lasers is much greater than most of the lasers, depending on temperature, therefore ranging from 125 to 400 milli radians.
In spite of the fact that semiconductor lasers do not produce a beam with characteristics as desired in other types of lasers, their small size, low power consumption and relatively high efficiency make them very useful device.
Application / Uses of Semiconductor Lasers
- The semiconductor laser can be pulsed at varying rate and pulse widths. Therefore this laser is a natural transmitter of digital data.
- Semiconductor laser is well suited for interface with fiber optic cables used in communication.
Advantages of Semiconductor Lasers
- Smaller size and appearance make them good choice for many applications.
- From cost point of view the semiconductor lasers are economical.
- Semiconductor lasers construction is very simple.
- No need of mirrors is in semiconductor lasers.
- Semiconductor lasers have high efficiency.
- The low power consumption is also its great advantage.
Disadvantages of Semiconductor Lasers
- Due to relatively low power production, these lasers are not suited to many typical laser applications.
- Semiconductor laser is greatly dependent on temperature. The temperature affects greatly the output of the laser.
- The lasing medium of semiconductor lasers is too short and rectangular so the output beam profile has an unusual shape.
- Beam divergence is much greater from 125 to 400 milli radians as compared to all other lasers.
- The cooling system requirement in some cases may be considered its disadvantage.