Lasers and Fibre Optics: Principles, Devices, and Applications

TAKEAWAYS:

• Lasers generate coherent light via stimulated emission in a gain medium (e.g., He-Ne, semiconductor), requiring population inversion and an optical cavity.
• Fibre optics transmit data via total internal reflection in thin glass fibres, with attenuation minimized by cladding and doping.
• Applications: Lasers in barcode scanners (supermarkets), fibre-optic internet (Ncell/NTC), and medical surgery (eye correction); fibre optics in long-distance telecom (Google’s undersea cables).
• Key differences: Lasers amplify light; fibre optics guide it. Both rely on wave interference and quantum principles.
• Exam focus: Draw energy-level diagrams for lasers, explain total internal reflection with Snell’s law, and list real-world uses (e.g., Daraz’s logistics tracking via fibre optics).
• Maths: Use Fresnel equations for reflection/transmission at interfaces and Beer-Lambert law for attenuation in fibres.

1. Lasers: Principles and Construction

1.1 What is a Laser?

A laser (Light Amplification by Stimulated Emission of Radiation) produces coherent, monochromatic, and directional light via stimulated emission. Unlike LEDs, lasers use an optical cavity (mirrors) to sustain oscillations.

graph LR
    A["Photon Emission"] -->|"Spontaneous"| B["Random Photon"]
    B -->|"Stimulates"| C["Electron Drop"]
    C -->|"Releases"| D["Identical Photon"]
    D -->|"Feedback"| E["Optical Cavity"]
    E -->|"Amplifies"| F["Laser Beam"]

1.2 Key Components of a Laser

1. Gain Medium: Atoms/molecules (e.g., He-Ne gas, semiconductor crystals) that amplify light via population inversion.
2. Pumping Mechanism: Energy input (electric discharge, light, or chemical) to excite electrons.
3. Optical Cavity: Two mirrors (one fully reflective, one partially reflective) to create standing waves and sustain oscillations.

1.3 Energy Level Diagram and Working

Process:

1. Helium atoms are excited by electric discharge to a high-energy state.
2. Collisional transfer excites neon atoms to a metastable state (long lifetime).
3. Spontaneous emission produces a photon; stimulated emission amplifies it.
4. The photon bounces between mirrors, stimulating more emissions (light amplification).
5. A partially reflective mirror outputs the coherent beam at 632.8 nm (red).

Worked Example: He-Ne Laser Power Output

• Given: Pump power = 10 mW, quantum efficiency = 20%, mirror reflectivity = 99%.
• Output power = Pump power × efficiency × (1 − reflectivity) = 10 mW × 0.2 × 0.01 = 0.2 mW.

1.4 Types of Lasers

1.5 Applications of Lasers

• Communication: Fibre-optic lasers (e.g., Ncell’s 4G/5G backhaul).
• Medicine: LASIK surgery (excimer lasers reshape corneas).
• Industry: CD/DVD burning (semiconductor lasers).
• Military: Target designation (e.g., Nepal Army’s rangefinders).
• Everyday Life: Barcode scanners (supermarkets), laser pointers.

2. Fibre Optics: Principles and Working

2.1 What is Fibre Optics?

Fibre optics transmits light signals through thin glass/plastic fibres via total internal reflection (TIR), enabling high-speed data transfer with minimal loss.

2.2 Total Internal Reflection (TIR)

Conditions for TIR:

1. Light travels from denser to rarer medium (core to cladding).
2. Angle of incidence > critical angle (${\theta }_c={\sin }^{-1}(n_2/n_1)$).
   • $n_1$: Refractive index of core (~1.46).
   • $n_2$: Refractive index of cladding (~1.44).

Worked Example: Critical Angle in Fibre

• Given: $n_1=1.46$, $n_2=1.44$.
• Critical angle ${\theta }_c={\sin }^{-1}(1.44/1.46)=76.3^{\circ }$.
• Acceptance angle (for input light): ${\theta }_a={\sin }^{-1}(\sqrt{n_1^2-n_2^2})=86.4^{\circ }$.

2.3 Types of Optical Fibres

2.4 Attenuation and Dispersion

• Attenuation: Loss of signal strength due to:
  • Absorption (impurities in fibre).
  • Scattering (Rayleigh scattering in glass).
• Dispersion: Signal spreading due to:
  • Modal dispersion (multimode fibres).
  • Chromatic dispersion (different wavelengths travel at different speeds).
• Solution: Use single-mode fibres and repeaters (amplifiers).

Worked Example: Signal Loss Calculation

• Given: Fibre length = 10 km, attenuation = 0.5 dB/km.
• Total loss = 10 km × 0.5 dB/km = 5 dB.
• Power after loss = $P_0\times 10^{-5/10}=P_0\times 0.316$ (31.6% of original).

2.5 Fibre-Optic Communication System

sequenceDiagram
    participant Transmitter
    participant Fibre
    participant Receiver
    Transmitter->>Fibre: Encodes data as light pulses (LED/Laser)
    Fibre->>Fibre: Transmits via TIR (minimal loss)
    Fibre-->>Receiver: Light pulses detected (photodiode)
    Receiver->>Receiver: Decodes to electrical signal

Components:

1. Transmitter: LED/Laser diode (converts electrical → light).
2. Fibre: Core/cladding (guides light).
3. Receiver: Photodiode (converts light → electrical).
4. Repeaters: Amplify signal every ~80 km.

2.6 Applications of Fibre Optics

• Telecommunications: Ncell/NTC’s internet backbone (low latency, high bandwidth).
• Medical: Endoscopes (flexible fibres for internal imaging).
• Aerospace: Aircraft wiring (lightweight, immune to EMI).
• Military: Secure communication (tapped-resistant).
• Everyday Life: Internet cables, CCTV, car braking systems.

## In the Real World

1. eSewa/Khalti Payments

   • Fibre optics enable real-time transaction processing between banks and eSewa’s servers. Without single-mode fibres, delays would occur due to high data volume.
   • Lasers in QR code scanners (semiconductor lasers) read payment details instantly.

2. Ncell’s 4G/5G Network

   • Fibre-optic cables connect cell towers to the core network, transmitting data at terabits per second.
   • Laser-based repeaters amplify signals every 80 km to maintain strength.

3. LASIK Eye Surgery (Nepal Eye Hospital)

   • Excimer lasers (193 nm UV) precisely reshape the cornea to correct vision. The laser’s coherence ensures micrometer-level accuracy.

4. Daraz Logistics Tracking

   • Fibre-optic sensors monitor package temperature/humidity during transit, using total internal reflection for real-time data.

5. Nepal Stock Exchange (NEPSE) Trading

   • High-frequency trading relies on low-latency fibre-optic links between brokers and the exchange server. A 1 ms delay can cost millions in trades.

## Exam Tip

1. For Lasers:

   • Must-draw: Energy-level diagram for He-Ne laser (label metastable states, transitions, and output wavelength).
   • Common mistakes: Forgetting population inversion or optical cavity in explanations.
   • Applications: Link to medicine (LASIK), communication (fibre lasers), and everyday tech (barcode scanners).

2. For Fibre Optics:

   • Must-know: Total internal reflection conditions and critical angle formula.
   • Worked examples: Always show attenuation calculations and acceptance angle problems.
   • Real-world tie: Relate to Ncell/NTC networks or eSewa’s backend infrastructure.

3. Short Notes (2×5):

   • Semiconductor laser: Mention p-n junction, recombination, and 650 nm emission.
   • Fibre-optic advantages: Immunity to EMI, high bandwidth, lightweight.

4. Numerical Problems:

   • Expect questions on:
     • Laser power output (given pump efficiency).
     • Fibre attenuation (dB/km to total loss).
     • Critical angle (using Snell’s law).