What is Photonics?
Photonics is the science and technology of light (photons) — how we generate it, control it, transmit it, and detect it.
It is the modern extension of optics and plays a major role in communication, medicine, computing, and sensing systems.
Light behaves as both a wave and a particle. Photonics uses this dual nature to design devices such as lasers, optical fibers, modulators, and detectors.
Photon Energy
Every photon has energy:
E = hν = hc/λ
Where:
- h = Planck’s constant
- ν = frequency
- λ = wavelength
Shorter wavelengths (blue, UV) → higher energy
Longer wavelengths (red, IR) → lower energy
Light Sources in Photonics
- Lasers
- LEDs
Lasers (Light Amplification by Stimulated Emission of Radiation)
Lasers are the backbone of photonics. They produce light that is:
- Monochromatic (single wavelength)
- Coherent (waves in phase)
- Directional (low spread)
- Highly intense
How do lasers work?
- Pumping: Energy supplied to atoms
- Population inversion: More excited atoms than ground-state atoms
- Stimulated emission: A photon causes another identical photon to be emitted
- Optical cavity: Mirrors reflect light back and forth
- Output mirror: Lets part of the beam escape → laser beam
Types of lasers
- Semiconductor lasers (in DVD players, communication devices)
- Gas lasers (He–Ne)
- Solid-state lasers
- Fiber lasers
LEDs (Light-Emitting Diodes)
LEDs (Light-Emitting Diodes) are semiconductor light sources widely used in photonics.
When a p–n junction LED is forward biased:
- Electrons from the n-region recombine with holes in the p-region
- Each recombination releases energy as a photon (light)
- This process is called spontaneous emission
Key features of LEDs
- Broader spectrum than lasers (not perfectly monochromatic)
- Incoherent light (waves are not in phase)
- Lower intensity but highly efficient and long-lasting
- Color (wavelength) depends on the band gap of the semiconductor
- Available from infrared to ultraviolet
Applications of LEDs in Photonics
- Optical communication (short-range links, plastic fibers)
- Display technologies (TVs, monitors, mobile screens)
- General lighting (LED bulbs, panels)
- Optical sensors and remote controls
LEDs vs Lasers in Photonics
LEDs:
- Simpler, cheaper, lower intensity
- Used for indicators, displays, short-distance optical links
Lasers:
- Coherent, highly directional, high intensity
- Used for long-distance communication, precision measurement, cutting, medical surgery
Optical Fibers
Optical fibers transmit light over long distances with extremely low loss.
Total Internal Reflection
Light stays inside the fiber due to total internal reflection (TIR) between the core and cladding.
Numerical Aperture (NA)
Where
- n1= refractive index of core
- n2 = refractive index of cladding
Acceptance Angle
θmax = sin−1(NA)
Types of Fibers
- Single-Mode Fiber (SMF): Long-distance, high bandwidth (telecom)
- Multi-Mode Fiber (MMF): Short-distance, lower bandwidth
Why telecom uses 1310 nm and 1550 nm?
These wavelengths have minimum signal attenuation.
Important Terms in Photonics
- Responsivity: Current produced per unit optical power
- Quantum Efficiency: Fraction of photons successfully detected
Photodetectors
Photodetectors are devices that convert light to electrical signals.
The common types of Photodetectors are:
- PIN Photodiode
- Avalanche Photodiode (APD)
Optical Communication
Photonics forms the backbone of the global internet.
Basic Communication System
- Transmitter: Laser source
- Medium: Optical fiber
- Receiver: Photodetector
- Repeaters: Boost the signal for long distances
Multiplexing
- WDM: Multiple wavelengths in one fiber
- DWDM: Dense WDM → hundreds of channels
Used in:
5G networks, undersea cables, data centers
Integrated Photonics
Photonics on a chip — similar to electronic integrated circuits, but using light.
Technologies
- Silicon photonics
- Photonic crystal waveguides
- Nanophotonics
- Plasmonics
Applications:
- Ultrafast computing
- On-chip communication
- Optical signal processing
Quantum Photonics
Quantum photonics uses individual photons for secure communication and computing.
Key Concepts
- Single-photon sources
- Entanglement
- Quantum key distribution (QKD)
Used in quantum networks and future quantum computers.
Applications of Photonics
Photonics powers modern technologies:
- Fiber-optic internet
- Laser surgery
- LiDAR in self-driving cars
- Barcode scanners
- Medical imaging (OCT)
- LED and display technologies
- Holography
- Optical sensors
- Space communication
Glossary of Key Terms
Recap of the Key Terms in Photonics
Attenuation: Loss of optical power during transmission.
Cladding: Outer lower-index region that keeps light confined within the core.
Core: Inner high-refractive-index region of an optical fiber where light travels.
Dispersion: Spreading of light pulses as they travel through a fiber limiting bandwidth.
DWDM (Dense WDM): High-capacity version of WDM with very closely spaced wavelengths.
Integrated Photonics: Photonic circuits fabricated on chips similar to electronic integrated circuits.
Laser: Device that emits highly coherent monochromatic and directional light through stimulated emission.
LEDs (Light-Emitting Diodes): Semiconductor light sources widely used in photonics.
Multi-Mode Fiber (MMF): Carries many modes; used for shorter distances.
Numerical Aperture (NA): Measure of how much light an optical fiber can accept; determines acceptance angle.
Optical Fiber: A thin flexible waveguide that transmits light using total internal reflection.
Photodiode: A semiconductor device that converts light into electrical current.
Photon: A particle of light carrying energy E=hν.
Photonics: The science and technology of generating, controlling, transmitting, and detecting photons.
Plasmonics: Use of electron oscillations at metal surfaces to confine light at sub-wavelength scales.
Population Inversion: Condition where more atoms are in excited states than in ground state — necessary for laser action.
Quantum Efficiency: Fraction of incident photons that successfully generate electrons in a photodetector.
Quantum Photonics: Application of single photons for secure communication and quantum computing.
Responsivity: Electrical output per unit optical input of a photodetector.
Single-Mode Fiber (SMF): Fiber that carries only one propagation mode; used for long-distance high-bandwidth communication.
Stimulated Emission: Process where an incoming photon causes an excited atom to emit another identical photon.
WDM (Wavelength Division Multiplexing): Technique using different wavelengths to send multiple signals through one fiber.
Quiz
Recap the concepts you have learnt. Try to answer the questions. You can find the answer to any question by clicking on the icon.
What is photonics?
Science and technology of generating, controlling, transmitting, and detecting photons.
What makes laser light different from ordinary light?
Laser light is coherent, monochromatic, directional, and highly intense.
What is stimulated emission?
An excited atom emits a photon identical to the incoming one in energy, phase, and direction.
What is population inversion?
A condition where more atoms are in excited states than in the ground state.
What principle allows light to stay confined inside an optical fiber?
Total Internal Reflection (TIR).
What is Numerical Aperture (NA) of an optical fiber?
A measure of how much light the fiber can accept; it determines the acceptance angle.
Why is single-mode fiber used for long-distance communication?
It has very low dispersion and supports high bandwidth.
What is the purpose of the cladding in an optical fiber?
It has a lower refractive index to ensure total internal reflection in the core.
What does a photodiode do in an optical communication system?
It converts received light into an electrical signal.
What is responsivity of a photodetector?
The electrical output generated per unit optical input power.
What is meant by optical attenuation?
Loss of optical power as light travels through a fiber.
Why are 1310 nm and 1550 nm wavelengths preferred in fiber optics?
They have minimal attenuation, allowing long-distance transmission.
What is Wavelength Division Multiplexing (WDM)?
Using multiple wavelengths to transmit different signals simultaneously through one fiber.
What is Dense WDM (DWDM)?
A high-capacity version of WDM with very closely spaced wavelengths.
What is integrated photonics?
Use of photonic circuits on chips for processing and transmitting light signals.
What is plasmonics?
Study of electron oscillations at metal surfaces that confine and control light at nanoscale dimensions.
What is a single-photon source?
A device that emits one photon at a time, used in quantum photonics.
What is Quantum Key Distribution (QKD)?
A secure communication method using quantum properties of photons to generate encryption keys.
Why is laser coherence important in communication systems?
It minimizes dispersion and maintains the signal’s integrity over long distances.
What is dispersion in optical fibers?
Spreading of light pulses during transmission, which can cause overlap and detection errors.
How does an LED produce light?
An LED emits light when electrons recombine with holes across a forward-biased p–n junction, releasing energy as photons (spontaneous emission).