Laser Characteristics and Applications

Laser Characteristics

• LASERS stand for Light Amplification by Stimulated Emission of Radiation.
• Lasers are light amplifying devices, but they are difficult to use as pure amplifiers.
• Lasers are mainly used as optical oscillators.
• Semiconductor lasers are the most popular type of laser due to their low power, small size, and affordability.
• Semiconductor lasers are also known as injection lasers or laser diodes.
• They are made using heavily doped p⁺-n⁺ junction direct band gap materials.
• Semiconductor lasers typically operate in the infrared region.
• To achieve optical light amplification, more atoms must be in the higher energy level (E₂) than the lower energy level (E₁).
• This condition is known as population inversion.
• The optical cavity of a laser is formed using a pair of mirrors, one of which reflects the light, and the other allows some light to escape as the laser beam.
• The emitted photons in a laser have similar phase and polarization, ensuring coherence.

Semiconductor Laser (Laser Diode LD)

• Semiconductor lasers operate based on the same principle as general lasers.
• The mirror structure is formed by cleaving the semiconductor crystal plane, creating a Fabry-Perot resonator.
• When forward-biased, both spontaneous and stimulated emission occur in semiconductor lasers.
• When the current applied exceeds the threshold current, a high coherent beam of light is emitted.
• The characteristics of a laser diode are observed using a power meter and a current meter.
• Spatial emission characteristics are measured by observing the emitted pattern on a diffusing screen at different distances from the diode.
• Spectral characteristics are obtained by plotting power vs current characteristics while maintaining a fixed bias voltage.

Optical Detector

• Optical detectors are devices that detect light.
• They are classified into two types based on internal gain:
  • Without Internal Gain:
    • p-n photodiode
    • p-i-n photodiode
  • With Internal Gain:
    • Avalanche Photodiode (APD)
• p-i-n photodiodes operate in reverse bias.
• The current generated in a reverse-biased p-i-n photodiode is due to both thermally-generated minority carriers and photo-generated carriers.
• The reverse-biased current is proportional to the light intensity.
• A photodiode circuit typically consists of a photodiode connected to a resistance and two voltmeters.
• The current is measured using an ammeter.
• The current flowing in the circuit when no light is incident is called the dark current.
• When light is incident, the current varies with the light intensity.

Study of Different Connectors

• Optical fiber connectors are developed to meet various requirements in optical fiber communication.
• Connectors should be easily attached, have low cost, and low loss.
• Connectors are usually plug-like in structure and designed to align two optical fibers.
• Common connector types include:
  • ST
  • SC
  • SMA
  • FCD
  • FC
  • D4
  • DIN
  • Biconic
• Connectors are typically pre-attached to the fiber end, but some are available separately and need to be attached on-site.
• The insertion loss of a connector is typically 1 dB per connector.

Splice Available Optical Fiber

• Splicing is used to permanently join two optical fibers.
• There are two types of splicers:
  • Fusion Splicer: permanently fuses two fibers by applying an electric arc.
  • Mechanical Splicer: aligns two fibers using epoxy resin or V-groove.
• Fusion splicing is commonly used for fiber optic splicing in OSP and premises, compatible with single-mode and multimode fibers.
• Two types of core alignment are used in fusion splicing:
  • Core alignment (uses optical microscope)
  • Cladding alignment (uses stationary V-grooves)
• Mechanical splicing is often used for splicing ribbon fibers.

This quiz explores the fundamental concepts of lasers, including their definitions, types, and operational principles. Key topics such as semiconductor lasers, population inversion, and the optical cavity are covered to enhance understanding of this technology. Test your knowledge on how lasers amplify light and their common uses.