What is basic requirement of semiconductor laser? Draw its label diagram and explain its working with necessary theory. Write down the applications of semiconductor laser. OR
Explain the term
(1) Spontaneous emission
(2) Optical pumping
Describe the construction and working of semiconductor laser. Describe various application lf semiconductor laser.
How do pumping and population inversion are achieved in a semi-conductor laser? What are advantages of using hetrojunction over homojunction in semi-conductor lasers ?
What do you understand by population inversion? With help of energy level diagram explain how population inversion is achieved to He-Ne laser.
Give the reasons for the following basic properties of a laser :
1. High intensity
2. High directionality
Ans.: Population inversion
Under ordinary conditions of thermal equilibrium the number of atoms in higher energy level is considerably smaller than the number in higher energy level so that there is very little stimulated emission compared to absorption. Hence under ordinary condition an incident photon is more likely to be absorbed rather than emission. Hence laser action will not take place. If, however, the larger number of atoms are made available in the higner energy level than stimulated emission will take place easily. This process of achieving the larger number lf atoms in the higher energy level than the lower energy level is known as population inversion. The term population inversion describes an assembly of atoms in which the majority are in energy levels above the ground state . Theprocess of achieving population inversion is known as "pumping" of atoms. Most commonly used methods are as follows : 1. Optical pumping (Used in Ruby Laser). 2. Electric discharge (Used in Helium-Neon Laser). 3. Inelastic-atom-atom collisions. 4. Direct conversion (Used in Semi-conductor Laser). 5. Chemical reaction (Used in CO2 Laser).
Helium Neon Laser is a four-level laser and was built by Ali javan, W. Bennett and D. Herriot in 1961.
Construction : The schematic of a typical He-Ne laser is shown in fig. It consists of a long discharge tube of length about 50 cm and diameter 1 cm. The tube contains a mixture of about 10 parts of helium and 1 part of neon at a low pressure. At both ends of the tube are fitted optically plane and parallel mirrors, one of them being partially silvered. The spacing of the mirrors is equal to an integral number of half0wavelengths of the laser light. In discharge. An electric discharge is produced in the gas by means of electrodes outside the tube connected to a source of high-frequency alternation current.
Working : When the power is switched on, the electrons from the discharge collide with and "pump" the He and Ne atoms to metastable states 20.61 e V and 20.66 e V respectively above their ground states. Some of the excited He atoms transfer their energy to ground-state Ne atoms in collision, with the 0.05 e V of additional energy being provided by the kinetic energy of the atoms. Thus, the purpose of the He atoms is to help in achieving a population inversion in the Ne atoms. When an excited Ne atom passes, from the metastable state at 20.66 e V to an excited state of 18.70 e V, and it emits a photon of wavelength 6328 A. This photon travels through the gas-mixture, and if it is moving parallel to the axis of the tube is reflected back and forth by the mirror-ends until it stimulates an excited Ne and causes it to emit a fresh 6328 A. photon in phase with the stimulating photon. This stimulated transition from 20.66 e V level to 18.70 e Vlevel is the laser transition. This process is continued and when a beam of coherent radiation becomes sufficiently intense, a portion of it escapes through the partially silvered end. In He-Ne laser, the power needed for excitation is less than that in a three-level laser. A He-Ne laser operates in continuous wave mode.
Semiconductor laser : Semiconductor laser has remarkably small size, exhibits high efficiency and can be operated at low temperatures. A semiconductor laser is made by forming a hunction between p-type and n-type materials. The basic mechanism includes, when the current is passed through this p-n junction diode in forward bias holes move from p-region to n-region and electrons move from n-region to p-region. These electrons and holes are recombined in the junction region and emit photons due to the transition of electrons from the conduction band to the valence band resultion in stimulated radiation coming from a very narrow region near the junction. As the applied current is gradually increased, a stage is reached when spontaneous emission changes into stimulated emission thereby laser beam is emitted. This process is enhanced by polishing the surfaces of the p-n junction to act as mirrors. Initially Ga-As junction diode has been used for emission of laser with a frequency. Later on p b-s, In etc. have been used for laser action. Presently, semiconductor laser is made of an active layer of gallium arsenide of thickness 0.2 microns. This is sandwiched in between a n-type Ga As Al layer as shown in figure. The resonant cavity is provided by polishing opposite faces of the Ga As crystal and the pumping occurs by applying the electric field. This type of laser beam has wavelengths range of 7000 A - 25000 A.