Laser|Working Principle, Applications

Laser light

The Laser

Laser, which is the acronym for light amplification by stimulated emitted radiation, is a device that amplifies (generated) light, as the name suggests, but they are rarely used for this purpose in practice. Lasers are used mainly as an optical oscillator with light bouncing back and forth in an optical cavity. One end of the cavity is made to have almost 100% reflection while the other is significantly less to allow the emission of monochromatic light (not exactly single wavelength but a very narrow band of wavelengths, typically 0.1–5 nm).

Working Principle of a Laser

In the working of a laser, certain vital conditions are required to be met before lasing can take place. These prerequisite lasing concepts are discussed below and together they explain the principle behind the operation of a laser.

In laser, the conversion process is fairly efficient compared to LEDs and it has a high output power. In comparison, for an LED to radiate 1 mW of output power, up to 150 mA of forwarding current is required, whereas for a laser diode to radiate the same power only 10 mA or less of current is needed.

Stimulated Emission

In order to describe the basic principle of stimulated emission, a two-level atomic system will be considered. In this simple system, an atom in the upper-level E2 can fall into level E1, as shown in Figure below, in a random manner giving off a photon with energy (E2 − E1) in the process called spontaneous emission.

Stimulated emission in Laser
Stimulated emission

Another possibility, one which is of greater interest here, is when an external source with energy hv = E2 − E1 is incident on the atom in level E2. This forces the atom in the upper-level E2 to transit to the lower level E1. The change in the energy involved in this process is then given off or emitted as a photon that has the same phase and frequency as the incident (exciting) photon. And so the stimulated radiation is coherent. This describes stimulated emission, a process that underpins the working of a laser. The impinging photon (or external source of energy) stimulates the emission of another photon (note that Ep = hv = hc/λ).

In other words, an external stimulus is required to achieve stimulated emission; this can be in the form of an optical excitation as just described or in the form of carrier injection through forward bias in the case of laser diodes. This property of the laser ensures that its spectral width will be very narrow compared to LEDs. In fact, it is quite common for laser diodes to have 1 nm or less spectral width at both 1300 nm and 1550 nm.

Population Inversion

In the two-energy-level atomic system under consideration, under thermal equilibrium, the number of atoms N1 in the lower level E1 is greater than the number of atoms N2 in the upper level E2 as stipulated by the Boltzmann statistics given by

\frac{N_{1}}{N_{2}}=\frac{g_{1}}{g_{2}}\exp (\frac{E_{1}-E_{2}}{kT})

To attain optical amplification necessary for lasing to take place, we require a nonequilibrium distribution of atoms such that the population of the E1 level is lower than that of the upper level E2, that is, N2 > N1. This condition is called population inversion. By using an external excitation, otherwise called ‘pumping’, atoms from the lower level are excited into the upper level through the process of stimulated absorption.

A two-level atomic system is not the best in terms of lasing action as the probability of absorption and stimulated emission are equal, providing at best equal populations in the two levels E1 and E2. A practical laser will have one or more meta-stable levels in between. An example is the four-level He–Ne laser illustrated in Figure below.

Population Inversion in Laser
Population Inversion

To attain population inversion, atoms are pumped from the ground-state level E0 into level E3. The atoms there then decay very rapidly into a metastable level E2 because they are unstable in level E3. This increases the population at level E2 thereby creating population inversion between E2 and E1. Lasing can then take place between E2 and E1.

The Structure of Common Laser Types

Fabry–Perot Laser

The simplest laser structure is that of Fabry–Perot laser (FPL); the structure is essentially a resonating cavity of the type shown in Figure below. The cavity mirrors are formed by the boundary between the high refractive index semiconductor crystal and the lower refractive index air. One facet is coated to reflect the entire light incident on it from within the cavity (gain medium) while the other facet is deliberately made to reflect less in order to emit the light generated within the cavity.

Fabry–Perot Laser Diode, FPL Diode
Fabry–Perot Laser Diode
  • Lasers are oscillators operating at optical frequency. The oscillator is formed by a resonant cavity providing selective feedback.
  • The cavity is normally a Fabry-Perot resonator i.e. two parallel plane mirrors separated by distance L, light propagating along the axis of the interferometer is reflected by the mirrors back to the amplifying medium providing optical gain.
  • The dimensions of the cavity are 25-500 µm longitudinal, 5-15 µm lateral, and 0.1-0.2 transverse.
  • The two heterojunctions provide the carrier and optical confinement in a direction normal to the junction. The current at which lasing starts is the threshold current. Above this the output power increase sharply.

Distributed Feedback Laser

To reduce the spectral width, we need to make a laser diode merely radiate in only one longitudinal mode. The distributed feedback (DFB) laser, which is a special type of edge-emitting lasers, is optimized for single-mode (single-frequency) operation. The single-mode operation is achieved by incorporating a periodic structure or a Bragg grating near the active layer as depicted in the figure below.

Distributed Feedback  Diode, DFB diode
Distributed Feedback Laser Diode

In DBF laser the lasing action is obtained by periodic variations of refractive index along the longitudinal dimension of the diode.

Light source linearity

  • In order to have a distortion-free output signal, the modulation must be confined to the linear region to the characteristics.
  • Two important nonlinear effects are harmonic and intermodulation distortions. The nonlinear distortions in LED are due to effects depending on the carrier injection level, radiative recombination, and other mechanisms.
  • The nonlinearities are as a result of inhomogeneities in the active region of the device also because of power switching between dominant lateral modes in the laser. These situations are called as kinks.
Kink on Modulation Range
Kink on Modulation Range

Application of Laser

  1. Laser diodes are performed where high radiance is required. High radiance is generated due to the amplifying effects of stimulated emission. The laser diode can supply optical power in milliwatts.
  2. Where narrow linewidth of the order of 1 nm (10 A0) or less is required. The narrow linewidth is useful in minimizing the effects of material dispersion.
  3. Laser diodes are preferred where modulation is needed in the gigahertz range.
  4. Whare temporal coherence is required.
  5. Where good spatial coherence is required, it allows the output to be focused by a lens into a spot that has a greater intensity than dispersed unfocussed beam.

Advantages of Laser

  1. Simple economic design
  2. High optical power
  3. Production of light can be precisely controlled
  4. Can be used at high temperature
  5. High coupling efficiency
  6. Low spectral width (3.5 nm).
  7. High radiance compared to LED.
  8. Modulation capabilities upto GHz range.
  9. Ability to maintain the intrinsic layer charecterics over long periods.
  10. Ability to transmit optical output powers between 5 and 10 mW.

Disadvantages of Laser

  1. At the end of the fiber, a speckle pattern appears as two coherent light beams add or subtract their electric field depending upon their relative phase.
  2. The laser diode is extremely sensitive to overload currents and at high transmission rates.

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