Photonics Notes

Photonics

Introduction

• Photonics is the science dealing with the production, control, and detection of photons.
• It combines optics and electronics, utilizing electromagnetism and quantum mechanics.
• Photonics has applications in communication, data processing, transportation, medicine, lighting, etc.
• Photons in photonics have a similar role to electrons in electronics, but offer advantages due to the high speed of light.
• Photonic transmission allows information to travel long distances quickly.
• Optical fibers require devices for generating, switching, and amplifying light for long-distance transmission.
• Solid State Lighting (SSL), mainly using Light Emitting Diodes (LEDs), offers higher efficiency and reliability compared to incandescent lighting.

Light Emitting Diode (LED)

• LEDs are heavily doped pn junctions that emit light when forward biased.
• In forward bias, electrons move from the n-region to the p-region, and holes move from the p-region to the n-region.
• Electron-hole recombination occurs on either side of the pn junction, releasing energy as light.
• The energy of emitted photons is nearly equal to the band gap energy.
• Light intensity increases with forward current up to a maximum, then decreases.
• The color/wavelength of emitted light depends on the band gap energy.
• Wavelength of emitted light is given by: $\lambda = \frac{hc}{E_g}$, where:
  • $h$ is Planck's constant.
  • $c$ is the speed of light in free space.
  • $E_g$ is the band gap energy.
• Different materials are used for different colors (red, green, blue, yellow, orange, etc.).
• Semiconductor diodes produce radiation during electron-hole recombination, but it's often absorbed by the material itself.
• LEDs use wide band gaps and are constructed to allow radiation to escape.
• Semiconductor materials for LEDs should have a band gap energy of about $2eV$.
• LEDs are encapsulated with a transparent cover to emit light.
• LEDs typically emit nearly monochromatic colored light.
• White light can be produced from color LEDs using:
  • Phosphor conversion (using phosphor to convert blue or UV light into white light).
  • RGB systems (mixing light from red, green, and blue LEDs).
  • Hybrid methods (combining phosphor-converted and monochromatic LEDs).
• Isamu Akasaki, Hiroshi Amano, and Shuji Nakamura developed the blue LED using Gallium Nitride (GaN) in 1994, winning the 2014 Nobel Prize in Physics.
• Blue LEDs can be used with phosphor to create green, yellow, and red light, which humans perceive as white light for general illumination.
• Indium Gallium Nitride (InGaN) is used for violet, blue, and green LEDs.
• Aluminum Gallium Indium Phosphide (AlGaInP) is used for green, yellow, orange, and red LEDs.
• Other materials include Zinc Selenide $(ZnSe)$, Aluminum Gallium Phosphide $(AlGaP)$, Gallium Arsenide Phosphide $(GaAsP)$, and Aluminum Gallium Arsenide $(AlGaAs)$.
• Organic Light Emitting Diodes (OLEDs) and Polymer Light Emitting Diodes (PLEDs) are also available.
• Quantum dot LEDs (QLEDs) are a newer advancement, using quantum dots between n-type and p-type semiconductors, producing light upon electron-hole recombination. QLEDs offer reliability, energy efficiency, low cost and tunable wavelengths.

LED Characteristics

• The junction voltage-current characteristic of an LED is similar to that of a diode, but with different knee voltages.
• Silicon diodes have a knee voltage of about $0.7V$, while LEDs range from $1.5V$ to $2.2V$, depending on the material.
• A series-connected resistor is needed when operating an LED from a DC voltage source greater than the LED's forward voltage to prevent damage.
• LEDs emit light when the forward voltage reaches a certain level, and intensity increases with increasing forward voltage.
• LEDs do not emit light when reverse biased, and reverse operation at high voltage can quickly destroy them.

Advantages of LED

1. Energy saving.
2. Cost reduction.
3. Low voltage and current requirements.
4. Low power output (less than $150$ milliwatts).
5. Fast response time (about $10$ nanoseconds).
6. No heating or warm-up time needed.
7. Miniature size and lightweight.
8. Rugged construction, able to withstand shock and vibrations.
9. Long lifespan (more than $10$ years).

Disadvantages

1. Sensitivity to voltage or current excess.
2. Higher initial price compared to conventional light sources.
3. Decreased efficiency with increasing electric current.

Applications of Light Emitting Diodes

1. Light sources in optical fiber communication systems with photodiodes.
2. Digital displays in modern electronic devices.
3. Bulbs in homes and industries.
4. Lights in motorcycles and cars.
5. Message displays in mobile phones.
6. Traffic light signals.

Photo Detector

• A photodetector converts light signals into voltage or current.
• They are used at the receiving end of optical communication links.
• Requirements include high sensitivity, high reliability, short response time, low bias voltage, and high electrical response.
• Examples include photodiodes and phototransistors.
• Solar cells are similar devices that absorb light and convert it into electrical energy.
• LEDs are the inverse of photodiodes, converting voltage/current into light.

Photo Diode

• A photodiode is a light detector that converts light into current or voltage.
• Response time decreases with increasing surface area.
• They are similar to regular semiconductor diodes but transparent enough to allow light to reach the device.

Types of Photodiode

1. PN Photodiode
2. Schottky Photo Diode
3. PIN Photodiode
4. Avalanche Photodiode

Junction Photodiode

• It is a reverse-biased pn junction in a clear plastic medium.
• When exposed to light, the current varies linearly with the light flux.
• The unit is very small (approximately $1/10$ inch).

Construction

• Formed by diffusing lightly doped P region into heavily doped N region.
• Depletion region exists between P and N regions.
• Active area is coated with antireflection coating.
• Non-active area is coated with a thick layer of $\text{SiO}_2$.
• Thickness of the non-active area controls the response and speed.

Working

• Connected in a circuit in reverse bias.
• Reverse saturation current (leakage current or dark current) flows due to thermally generated minority carriers.
• Incident light releases electrons from the valence band, creating electron-hole pairs (photocarriers).
• Photocarriers produce photocurrent in addition to dark current.
• Dark current should be minimized for increased sensitivity.
• Wider depletion region absorbs more light.
• Resulting output current or voltage can be measured.

Applications of Photodiode

1. Used in scintillators, charge-coupled devices, photoconductors, and photomultiplier tubes.
2. Used in consumer electronics like smoke detectors, CD players, TVs, and remote controls.
3. Used for precise measurements of light intensity.
4. Used in medical fields for analyzing samples, computed tomography detectors, and blood gas monitors.
5. Used for lighting regulation and optical communications due to their speed and complexity.

PIN Photodiode

• A photodiode with a lightly doped intrinsic semiconductor between heavily doped P and N regions to improve sensitivity.
• The intrinsic region is wider (10-200 microns) than the depletion region of a normal PN junction.
• Reverse bias extends the space charge region throughout the intrinsic region.
• Light excites electrons, producing a large number of electron-hole pairs in the intrinsic layer.
• The intrinsic layer absorbs more photons, increasing photocurrent, efficiency, speed, and sensitivity compared to pn junction photodiodes.

Applications of PIN Diode

1. Used in RF and microwave switches and attenuators due to low capacitance.
2. Used for fiber optic network cards and switches.
3. Used to detect X-rays and gamma rays.

Solar Cell

• Devices using the photovoltaic effect to convert solar energy into electrical energy.
• Generates electric potential when irradiated by optical radiation.
• Works in photovoltaic mode with zero bias.
• Sunlight is trapped inside to produce a voltage (photovoltaic effect).
• A single solar cell produces about $0.6V$.
• Connecting cells in series increases the output voltage.
• A solar panel/module is an array of connected solar cells.
• Materials used include single crystal Silicon $(E<em>g = 1.11eV)$, GaAs $(E</em>g = 1.40eV)$, CdSe $(E_g = 1.74eV)$, etc.

Construction

• Heavily doped p-n junction with a thin top layer (n region).
• The p-n junction is very narrow (around $20nm$) due to high doping levels.
• Large surface area to receive ample light.
• Anode connection from the bottom (p layer) and cathode from the top (n layer).
• Antireflection coating on the top layer.

Working

• Light knocks electrons out of the n region; electrons travel to the p region through an external load, creating electric current.

I-V Characteristics of a Solar Cell

• I-V measurements characterize solar cells.
• Open circuit voltage $(V_{oc})$ is produced without current when there is no load.
• Short circuit current $(I_{sc})$ flows without voltage when terminals are shorted.
• Output power is maximum for a specific load resistance.
• $V<em>{max}$ and $I</em>{max}$ are voltage and current at the maximum power point $(P_{max})$.
• Load resistance is chosen to maximize output power.

Efficiency of a solar cell

• Ratio of total power converted to total power available for energy conversion.
• $\eta = \frac{\text{maximum output electrical power}}{\text{input optical power}} = \frac{P_{max}}{\text{light intensity} \times \text{area of the solar cell}}$

Fill factor

• Fraction of the product of open circuit voltage and short circuit current that equals the maximum output power.
• $f = \frac{\text{maximum output power}}{V<em>{oc} \times I</em>{sc}}$
• The fill factor ranges from $0.65$ to $0.8$. A higher fill factor means a greater power output.

Advantages

1. No fuel usage, making them safe.
2. Years of use without maintenance costs.
3. No atmospheric and noise pollution when operating.

Disadvantages

1. Delicate and brittle.
2. Slow response compared to photodiodes.
3. Large space consumption.
4. Requires periodic cleaning.

Applications

1. Power sources in satellites and rockets.
2. Telecommunications in remote areas.
3. Defense equipment like remote instrumentations and radars.
4. Rural electrification, water pumping, domestic supply, healthcare, lighting, ocean navigation aids.
5. Pocket calculators, watches, torches, garden lights, portable fans, radios, toys, street lights, traffic signals, electric fences.

Fibre Optics

• Conventional long-distance communication uses radio waves $(~10^6 Hz)$ and microwaves $(~10^9 Hz)$.
• Light beams can carry more information simultaneously.
• One telephone conversation occupies $10^3 Hz$.
• Lasers can handle $10^9$ simultaneous connections.
• Unguided open space communication is limited to a few tens of kilometers due to atmospheric absorption.
• Guided transmission led to the development of optical fiber.

Optical fibres

• A thin cylindrical transparent dielectric material surrounded by another dielectric material with a lower refractive index.
• The inner cylinder is the core, and the outer cylinder is the cladding.
• Light propagates through the core by total internal reflection at the core-cladding boundary.
• Materials used are high content silica glass, multicomponent glass, and plastic.
• Plastic fibers are more flexible but have higher attenuation than glass fibers.
• Total internal reflection is achieved by a one-step difference or gradually diminishing steps in refractive index between core and cladding.
• Classified as step index fiber and graded index fiber.

Step Index Fibre

• Characterized by a core with a constant refractive index $(n<em>1)$ throughout its bulk, surrounded by cladding with a lower refractive index $(n</em>2)$.
• Refractive index decreases abruptly at the core-cladding boundary.
• Light undergoes repeated total internal reflections at the core-cladding boundary.

Graded Index Fibre

• A core made of thin layers of transparent material with gradually diminishing refractive index, surrounded by a cladding with a lower, constant refractive index.
• Light undergoes gradual bending towards the axis and reflects back at the core-cladding interface.
• The cladding prevents light from escaping.

Numerical Aperture

• Measure of the light-gathering capacity of an optical fiber.

• Defined as the sine of the maximum acceptance angle.

• $N.A. = \sin \theta_m$

• $\theta_m$ is the acceptance angle.

Theory

• Refractive index of core w.r.t air:
  $n<em>0 \sin \alpha = n</em>1 \sin \theta$
• For critical rays, $\alpha = \alpha<em>m, \theta = \theta</em>c$:
  $n<em>0 \sin \alpha</em>m = n<em>1 \cos \theta</em>c$
• Refractive index of core w.r.t. cladding:
  $\frac{n<em>1}{n</em>2} = \frac{1}{\sin \theta<em>c}$$\sin \theta</em>c = \frac{n<em>2}{n</em>1}$
  $\cos \theta<em>c = \sqrt{1 - \sin^2 \theta</em>c} = \frac{\sqrt{n<em>1^2 - n</em>2^2}}{n_1}$
• $N.A. = \sin \alpha<em>m = \frac{\sqrt{n</em>1^2 - n<em>2^2}}{n</em>0}$
• If the fiber is in the air $(n<em>0 = 1)$, then: $N.A. = \sin \alpha</em>m = \sqrt{n<em>1^2 - n</em>2^2}$

Fibre Bundles

• Large numbers of fibers put together.
• Three categories:
  • Aligned bundle (coherent bundles).
  • Fused bundle.
  • Unaligned bundle.

Aligned Bundle

• Fibers have the same coordinates at both ends.
• Used in image transferring coherent bundles, like fiber optic endoscopes used inside the human body.

Fused Bundle

• Flexible bundle with fibers fused for high packing efficiency and surface quality.

Unaligned Bundle

• Fibers are randomly positioned and used for conducting light around corners.

Advantages of Optical Fibres

• Replacing conventional telecommunication networks due to large information carrying capacity and cost-effectiveness.
• Low transmission loss, allowing greater distances between repeater stations.
• Small diameter and silica/glass composition reduces volume and weight.
• Immunity to electromagnetic interference.
• Usable in explosive and high voltage environments.
• Made of abundant and cheap silica and glass.
• Used in computer links, space vehicles, industrial automation, process control, etc.

Application in Communication System

• Replacing wire transmission lines.
• The system includes an optical transmitter, fiber optic transmission line, and optical receiver.

Components

1. Subscriber's Telephone: Converts sound to electrical signals.
2. Encoder: Converts continuous electrical signals into coded digital pulses.
3. Optical Transmitter: Miniature semiconductor laser or LED that modulates light with the signal.
4. Optical Fibre Link: Transmits the encoded optical signal.
5. Photo detector: Converts optical signal back into electric pulses.
6. Decoder: Converts digital pulses into analogue signal.
7. Subscriber's Telephone: Reproduces the sound.

Applications of optical fibre

Industrial and Technological Applications

1. Sensors for displacement, pressure, temperature, flow rate, liquid level, chemical composition, etc.
2. Security alarm systems, electronic instrumentation systems, industrial automation.
3. Monitoring atmospheric pollution and suspended particles.
4. Remote monitoring and surveillance.
5. Cable TV, CCTV, LAN, WAN.
6. Signaling and decorative purposes.
7. Transfer of infrared energy.
8. Defense communication systems, ships, aircrafts, submarines, missiles.
9. Intranet and Internet connections.
10. Fiber optic communication systems have a large bandwidth, so they can accommodate a large number of channels and are suitable for the transmission of digital data generated by computers.
11. Fiber are used to send a large number of telephone signals without any inferences.

Medical Applications

1. Biosensors to measure and monitor temperature, blood pressure, blood flow, oxygen saturation levels, and hemoglobin proportion.
2. Testing tissues and blood vessels below the skin.
3. Examining the heart, pancreas, etc.
4. Endoscopes visualize internal parts without surgery.
   • (a) Gastroscope examine the stomach.
   • (b) Bronchoscope see upper passages of lungs.
   • (c) Orthoscope see the small spaces within joints.
   • (d) Peritoneoscope tests the abdominal cavity, lower parts of liver and gall bladder.
   • (e) Cytoscope used to tumors, inflammation and stones in the urinary blad-der.
   • (f) Couldoscope is used to test female pelvic organs.

Fibre Optic Sensors

• Used for sensing weak fields like acoustic fields, magnetic fields, current, rotation, acceleration, strain, pressure, and temperature.
• Consists of a light source, optical fiber sensor element, and light detector.
• Different types include intensity modulated, phase modulated, and polarization modulated.

Intensity Modulated Sensor

• The measured causes a change in the intensity of the received light.

Pressure Sensor

• Consists of an LED, photodetector, optical fiber, and reflecting diaphragm.
• Pressure changes move the diaphragm, modulating the intensity of light.
• Measures pressure changes up to $6$ megapascals accurately.
• Useful for monitoring pressure in arteries, bladder, urethra, and chemical industries.

Sound Wave Detector

• Sound waves vibrate the fiber optic sensor element, causing transverse misalignment.
• This leads to coupling loss, modulating the intensity of the transmitted signal.
• Sufficient sensitivity to detect deep sea noise levels and displacement of a few angstroms.

Phase Modulated Sensor

• External perturbation causes a change in the phase of light passing through it, measured by interferometric techniques.
• Light from a laser is split and sent through sensing and reference fiber arms.
• The sensing arm is in direct contact with the measurand (e.g., temperature).
• Phase difference is detected by a Mach Zhender interferometer.