Question

a. mention and explain the characteristics of the laser beam that distinguishes it from general light
b. write a summary of the working principles of the laser according to the assignment about the laser you have made

Answer 1

a. Laser radiation has the following important characteristics over ordinary light source. They are: i) monochromaticity, ii) directionality, iii) coherence and iv) brightness.

1. Monochromaticity : Monochromatic light means a light containing a single colour or wavelength. The photons emitted from ordinary light sources have different energies, frequencies, wavelengths, or colours. Hence, the light waves of ordinary light sources have many wavelengths or colours. Therefore, ordinary light is a mixture of waves having different frequencies or wavelengths.

2. Directionality: In normal light photons travels in random directions, in laser, all photons will travel in the same direction. Therefore, the laser emits light only in one direction. This is called the directionality of laser light. The width of a laser beam is extremely narrow. Hence, a laser beam can travel to long distances without spreading.

3. Coherence: In laser, the electron transition occurs artificially. In other words, in laser, electron transition occurs in a specific time. All the photons emitted in laser have the same energy, frequency, or wavelength. Hence, the light waves of laser light have a single wavelength or colour. Therefore, the wavelengths of the laser light are in phase in space and time. In laser, a technique called stimulated emission is used to produce light.

4. Brightness/intensity: In laser, the light spreads in a small region of space and a small wavelength range. Hence, laser light has greater intensity when compared to the ordinary light. If you look directly along the beam from a laser (caution: don’t do it), then all the power in the laser would enter your eye. Thus, even a 1 Watt laser would appear many thousand times more intense than 100 Watt ordinary lamp.

b. The principle of a laser is based on three separate features: a) spontaneous emission , b) population inversion of electronics and c) an optical resonator.

i) stimulated emission within an amplifying medium : Stimulated emission is defined as the process in which the electrons in the excited atoms are released on their own from their higher state to the ground state.

ii) population inversion : Population inversions can be produced in a gas, liquid, or solid, but most laser media are gases or solids. Typically, laser gases are contained in cylindrical tubes and excited by an electric current or external light source, which is said to “pump” the laser. Similarly, solid-state lasers may use semiconductors or transparent crystals with small concentrations of light-emitting atoms.

iii) Optical resonator: An optical resonator is needed to build up the light energy in the beam. The resonator is formed by placing a pair of mirrors facing each other so that light emitted along the line between the mirrors is reflected back and forth. When a population inversion is created in the medium, light reflected back and forth increases in intensity with each pass through the laser medium. Other light leaks around the mirrors without being amplified. In an actual laser cavity, one or both mirrors transmit a fraction of the incident light. The fraction of light transmitted—that is, the laser beam—depends on the type of laser. If the laser generates a continuous beam, the amount of light added by stimulated emission on each round trip between the mirrors equals the light emerging in the beam plus losses within the optical resonator.