lasers and optical fibres

• Optical fibres are used extensively in communication systems due to their low signal loss and high bandwidth.
• LASER stands for Light Amplification by Stimulated Emission of Radiation
• LASER is used to produce a unidirectional, extremely intense, and coherent beam of light
• Characteristics/Properties of Laser Light:

  • Unidirectionality
  • Monochromaticity
  • High Intensity
  • Coherence
• In LASER, photons are emitted in one direction along the optical axis of the system due to stimulated emission
• Laser beam consists of photons of almost the same wavelength, giving a single color to the light
• LASER gives out light in a narrow beam with energy concentrated in a small region
• Light waves emitted by the laser source are in phase and of the same frequency, making it highly coherent
• Interaction of Radiations with Matter:

  • Induced absorption
  • Spontaneous emission
  • Stimulated emission
• Induced Absorption:

  • Absorption of incident photon by an atom, leading to excitation to a higher energy state
• Spontaneous Emission:

  • Emission of a photon by an atom from a higher energy state to a lower energy state without external energy
• Stimulated Emission:

  • Emission of a photon by an atom in the excited state under the incidence of a passing photon of the right energy
• Boltzmann relation for the ratio of population of two atomic states:

  • Population inversion
  • Expression for energy density of radiation in terms of Einstein’s coefficients
• Conditions for Laser Action:

  • Rate of stimulated emission should dominate spontaneous emission and induced absorption
• Requisites of a Laser System:

  • Active medium
• Three requisites of a laser system:

  1. Active medium:

  • Material medium where population inversion and lasing action occur
  • Provides energy levels for atomic transitions
  • Chosen based on possessing metastable states for achieving population inversion
  • Types of active mediums: gas lasers, solid-state lasers, semiconductor lasers, liquid lasers
• Classification of lasers based on active medium:

  • Gas lasers: mixture of gases (e.g., He-Ne laser, CO2 laser)
  • Solid-state lasers: crystals (e.g., Ruby laser, YAG laser)
  • Semiconductor lasers: semiconductors (e.g., Gallium Arsenide laser)
  • Liquid lasers: chemicals (e.g., Dye lasers)
• 1. Pumping:

  • Process of raising atoms from lower to higher energy states
  • Types of pumping techniques: optical pumping, electric discharge, direct conversion, chemical reactions
• 1. Resonant cavity:

  • Necessary for generating coherent and amplified light output
  • Consists of two parallel mirrors with active medium in between
  • One mirror is 100% reflecting, the other is partially reflecting
  • Maintains standing wave pattern with distance between mirrors equal to n(λ/2) for resonance condition
• Three Level Pumping Scheme:

  • Excites atoms to a higher energy state than the upper laser state
  • Achieves population inversion for stimulated emission
  • Requires high pumping power and produces light in pulses
• Four Level Pumping Scheme:

  • Moves population from lowest state to highest fourth level
  • Attains population inversion between third and second levels for lasing
  • Operates in continuous wave mode with less power needed for population inversion
• Helium-Neon Laser:

  Construction:

  • Gas laser with Helium and Neon gases in a 10:1 ratio
  • Resonant cavity in a sealed quartz tube with mirrors and electrodes for excitation
• Working:

  • Electrical discharge excites He and Ne atoms
  • Helium atoms in metastable state transfer energy to Neon atoms
  • Achieves population inversion between Neon states for lasing
  • Emits light at three wavelengths due to radiative transitions
• Salient Features of He-Ne laser:

  • Uses Four level pumping scheme
  • Active centres are Neon atoms
  • Operates in continuous wave mode
• Semiconductor Laser:

  Principle:

  • Based on electron-hole recombination in a direct band gap semiconductor
• Semiconductor laser principle:

  • Based on electron hole recombination in a direct band gap semiconductor, resulting in photon emission
• Construction of a semiconductor laser:

  • Consists of a p-n junction with heavily doped p- and n- regions
  • Laser sides are about 1 mm, with a p-n junction layer width of ~1µm
  • Top and bottom faces have metallic contacts for current passage
  • Front and rear faces are polished parallel to each other and perpendicular to the junction plane, forming the optical resonator
  • Other two opposite faces are roughened to prevent lasing action in that direction
• Working of a semiconductor laser:

  • When forward biased, electrons move to p-region and holes to n-region
  • Electrons and holes recombine in the junction region, emitting photons
  • At low current, spontaneous emission occurs; at threshold current, population inversion happens
  • Forward bias acts as a pumping source, triggering stimulated emission and producing a laser beam
  • Emission wavelength depends on doping and threshold current
• Advantages of semiconductor lasers:

  • Compact size
  • Lightweight
  • Good reliability
  • Long service life
  • Low power consumption
  • Safe operation
  • Low maintenance cost
• Applications of semiconductor lasers:

  • Optical communication
  • Optical data storage
  • Laser pointers
  • CD and DVD writing/reading
  • Metrology
  • Spectroscopy
  • Material processing
  • Pumping of other lasers
  • Medical treatments
• LIDAR (Light Detection and Ranging):

  • Remote sensing method using pulsed laser light to measure ranges to the Earth
  • Active remote sensing system for measuring vegetation height across wide areas
  • Provides precise, three-dimensional information about Earth's shape and surface characteristics
  • Components include a laser, scanner, and specialized GPS receiver
  • Used in topographic and bathymetric applications
• Holography:

  • Technique for recording 3D images on a 2D surface using interference
  • Requires highly coherent laser light
  • Involves recording and reconstructing holographic images
  • Used for various applications like data storage, stress detection, and archival records in museums
• Optical fibre construction:

  • Consists of two main parts:
  • Core: Inner cylindrical structure with refractive index n1
  • Cladding: Outer concentric cylinder with refractive index n2 (n2 < n1)
  • Encapsulated in an elastic, abrasion-resistant plastic material jacket called sheath
• Working principle:

  • Transmission of light through optical fibre is based on total internal reflection
  • Total Internal Reflection:
  • Occurs when light travels from denser to rarer medium and the angle of incidence is greater than the critical angle for the pair of media
  • Light gets reflected back in the denser medium, known as total internal reflection
• Critical angle:

  • The angle of incidence in the denser medium for which the angle of refraction in the rarer medium is 90 degrees
  • Calculated by Snell’s law of refraction: n1 sin θc = n2 sin 90 degrees, therefore θc = sin-1 (n2/n1)
• Acceptance angle (θo):

  • Maximum allowed angle that the incident ray can make with the fibre axis for the light ray to pass through the fibre
• Numerical Aperture (NA):

  • Measure of the light-gathering capacity of the optical fibre
  • Defined as the sine of the acceptance angle: NA = sin θo
• Expression for numerical aperture (NA):

  • NA = √(n1^2 - n2^2) / n0, where n0 is the refractive index of the surrounding medium (e.g., air)
• Condition of propagation:

  • Light ray will propagate through the fiber if the angle of incidence is less than or equal to the acceptance angle: θi ≤ θo
• Fractional index change (Δ):

  • Ratio of the refractive index difference between core and cladding to the refractive index of the core of an optical fibre: Δ = (n1 - n2) / n1