Optics Chapter 7. Applied Optics
Recommended Post : 【Physics】 Physics Table of Contents
1. Generation of Electromagnetic Waves
2. Control of Electromagnetic Waves
3. Transmission of Electromagnetic Waves
4. Detection of Electromagnetic Waves
1. Generation of Electromagnetic Waves: Light Source
⑴ Type 1. Radiant Sources : Also known as continuous sources
① Utilizes blackbody radiation : Emits radiation across a wide wavelength range uniformly
② Used for measuring object’s absorption spectrum, reflection spectrum, etc.
③ 1-1. Tungsten Lamp : Incandescent lamp using a tungsten filament
○ Filament is heated up to 3,000 K
○ Due to high temperature, tungsten filament metal gradually vaporizes, causing lamp’s light to dim
○ Emits visible light in the range of 320 to 2,500 nm, as well as near-infrared light
④ 1-2. Mercury (Hg) vapor, Xenon (Xe) gas discharge lamp
○ Emits ultraviolet and visible light
⑤ 1-3. Electric discharge lamp filled with Hg vapor, Xe gas
○ Emits ultraviolet and visible light
⑥ 1-4. Globar
○ Current passed through silicon carbide rod to heat up to 1500 K
○ Emits infrared radiation in the range of 5000 to 200 cm-1
⑦ Standard Light Sources
○ Standard Light Source A : Tungsten incandescent lamp in gas-filled state. 2854 K
○ Standard Light Source B : Light source imitating average solar radiation. 4870 K
○ Standard Light Source C : Light source imitating direct sunlight on a clear sky. 6740 K
○ Standard Light Source D : Complement to Source C, with adjustable color temperature. Includes D65, D75, etc.
○ Standard Light Source F : Standard for fluorescent lamps. Includes F1, F2. Approximately 4000 K
⑵ Type 2. Light Emitting Diode (LED)
① Emit light due to recombination of carriers near the junction when forward bias is applied
Figure. 1. Principle of LED
A is p-type semiconductor and B is n-type semiconductor
② N-type semiconductors have lower energy levels than P-type semiconductors, but N-type’s conduction band is higher than P-type’s valence band
③ LEDs emit light corresponding to the band gap size
④ Recombination states can occasionally break for some reason, leading to continuous recombination and continuous emission of light
⑤ (Note) Most diodes convert energy into heat, not light
○ Silicon (Si) semiconductor, Germanium (Ge) semiconductor : Convert energy into heat, not light
○ Gallium Arsenide (GaAs) semiconductor, Gallium Phosphide (GaP) semiconductor : Emit light
⑥ High efficiency : Energy savings of up to 90% possible
⑶ Type 3. Lasers : One or a few wavelengths. Line source
① 3-1. Gas lasers
Figure. 2. Principle of Gas Laser
○ Vibrational frequency
○ 1st. Electron in ground state E1 is excited to E3 by optical pumping
○ 2nd. Atoms constituting the laser medium have many electrons in metastable state E2 from E3
○ Metastable state : Electrons that should be in the ground state E1 are maintained in the excited state E2
○ 3rd. Monochromatic light A is incident on the atoms constituting the laser medium, inducing emission of A and B
○ 4th. Left side of the laser reflects all light, while the right side transmits only part of it
○ 5th. Reflective light induces further emission of light
Figure. 3. Internal structure of a Gas Laser device
② 3-2. Semiconductor lasers
⑷ Type 4. Acceleration of Charged Particles
① Poynting vector : Acceleration of charged particles generates electromagnetic waves
② 4-1. Radio wave generation
③ 4-2. Bremsstrahlung : Visible light is emitted when electrons decelerate. Also known as cyclotron radiation, synchrotron radiation, etc.
⑸ Type 5. Fluorescence
⑹ Type 6. Cherenkov Radiation
⑺ Type 7. Bremsstrahlung
⑻ Type 8. Fluorescence due to High Z Metal
① 8-1. Gamma fluorescence
② 8-2. Beta fluorescence
③ 8-3. Characteristic X-ray
⑼ Type 9. Surface Plasmon Resonance (SPR)
① Definition : Free electrons resonate when light is incident on the interface of materials with positive permittivity (air, water, etc.) and negative permittivity (metal)
② Electromagnetic waves parallel to the surface are generated due to electron oscillation
③ Highly sensitive sensor as resonance occurs under specific conditions
④ Examples of metals : Silver, gold, copper, titanium, chromium
⑽ Considerations when choosing light sources
① Must emit sufficient radiant energy
② Energy distribution by wavelength should be stable over time
2. Control of Electromagnetic Waves: Wavelength Selector (e.g., Prism, Diffraction Grating, Optical Filter)
⑴ Diffraction Grating
① Definition : Grooves in a polished metal surface resembling saw teeth
○ Reflective diffraction gratings used for ultraviolet and visible regions have 300 to 6,000 grooves per millimeter
○ Reflective diffraction gratings used for infrared region have 10 to 200 grooves per millimeter
○ Grooves must be uniform in size, parallel, and equidistant
② Echellette Type Diffraction Grating
○ When collimated light hits the reflective surface of the echellette, reflection occurs at each facet
○ Interference occurs between reflected rays
○ Reinforcement interference occurs when the difference in travel distance between adjacent rays is an integer multiple of the wavelength of the ray **
Figure. 4. Reinforcement Interference Conditions of Echellette Type Diffraction Grating
⑵ Optical Filter
① Interference Filter
○ Consists of thin, transparent dielectric layer sandwiched between two semitransparent films
○ Bragg diffraction : With thinner dielectric layer and larger incident angle, the wavelength of the radiation increases
Figure. 5. Bragg Diffraction
○ Bragg diffraction becomes the basis of X-ray diffraction (XRD)
○ Characteristic : Narrow effective wavelength width (FWHM)
② Absorption Filter
○ Generally weakens incident light across the spectrum
○ Characteristic : Wide effective wavelength width (FWHM)
○ Examples : UV cut-off filter, NIR absorption filter
3. Transmission of Electromagnetic Waves
⑴ Type 1. Optical Fiber
Figure. 6. Structure of Optical Fiber
① Definition : Bundle of glass, fused silica, or plastic threads capable of transmitting radiation for hundreds of meters or more
② Diameter ranges from 0.05 μm to 0.6 cm
③ Structure : Core, Cladding
④ Core material has higher refractive index than cladding material
⑤ Real-world application
○ Multiple fibers used together : Fiber is too thin for geometrical optics approximation (Snell’s law) to work
○ Overlapping different frequencies of light to transmit information : Unlike electrons where Pauli’s exclusion principle applies, light can overlap
○ Bending of fibers leads to irregular reflection angles, causing blurring of the image → graded index is used to prevent this
○ ~0.2 dB/km loss
⑥ Classification According to Material
○ Glass or Plastic : Visible Light, Near Infrared Region
○ Fused Silica : Ultraviolet Region to Near Infrared Region
⑦ Classification According to Purpose
○ Reflective Probe
○ Transmissive Probe
○ Dip Probe
4. Detection of Electromagnetic Waves
⑴ Characteristics of Detectors
① Characteristic 1: Signal-to-Noise Ratio (S/N ratio)
② Characteristic 2: Noise Equivalent Power (NEP)
○ Definition : Minimum detectable intensity of incident radiation by the detector
○ Represents the intensity of the signal that produces the same output as the noise
○ N : Noise Voltage or Current (RMS)
○ S : Signal Output Voltage or Current (RMS)
○ Ee : Intensity of Incident Radiation (Wcm-2)
○ A : Photosensitive Area of the Detector (cm2)
○ Δf : System’s Frequency Bandwidth (Hz)
③ Characteristic 3: Detectivity : Denoted as D
○ Definition : Measure of the minimum detectable intensity of radiation
○ D = 1 / NEP
④ Characteristic 4: Detection Ability : Denoted as D*
○ Definition : S/N ratio for a system reference bandwidth Δf = 1 Hz when 1 W of radiation is incident on a unit area of the detector
○ D* = A0.5 / NEP
⑤ Characteristic 5: Spectral Photosensitivity or Radiant Sensitivity : Denoted as σ
○ Ratio of signal voltage or current RMS value to the rms value of the intensity of light incident on the detector
○ Absolute Spectral Responsivity Curve : Graph of absolute spectral responsivity at each wavelength λ
○ Relative Spectral Responsivity Curve : Curve normalized so that the maximum value of absolute spectral responsivity is 1
⑵ Type 1: Internal Photoelectric Effect in Photodetectors : Increase in electrical conductivity upon absorption of radiation
① Type 1-1: Photoconductive Detector or Photoconductive Cell
○ Definition : Variable resistance that changes with light intensity. No polarity
○ Sometimes denoted by λ
○ Photogain Coefficient : Negative values result in resistance decrease with increasing light intensity
○ Generally use semiconductors where resistance decreases upon light absorption
○ 1st. Light generates electron-hole pairs : Light in near infrared region (750 nm ~ 3000 nm)
○ 2nd. Photoconductive Effect : Increase in electrical conductivity of regions where electron-hole pairs are generated
○ 3rd. Increased electrical conductivity leads to higher current
○ 4th. Detect light intensity by measuring current variation
○ Examples : CdS, CdSe, PbS, PbSe (800 nm ~ 2000 nm), Ge:Au, HgCdTe, Hg1-xCdxTe
○ Example : Cadmium Sulfide Cell
Figure. 7. Cadmium Sulfide Cell
○ Advantages : High sensitivity. Compact. Inexpensive. High power capacity. Resistant to noise. Can operate in AC. Relatively high output
○ Disadvantages : Slow response time (10 ~ 100 ms). Low light sensitivity. Susceptible to ambient light, leading to significant hysteresis
○ Dark Resistance : Approximately 200 kΩ
○ Light Quantity of Theater Audience (10 lux) : Approximately 10 kΩ
○ Excessive Light Quantity : Resistance becomes very low, resulting in excessive current
② Type 1-2: Silicon Diode Detector (also known as Photodiode)
○ Definition : Device that converts light energy into electrical energy
○ Reverse Bias Circuit
○ Incident light in depletion region generates electron-hole pairs, leading to current flow. Similar to Photoconductive Effect
○ Essentially, electrons in the p-type semiconductor transition to the n-type semiconductor’s conduction band, generating both holes and free electrons
○ Acts as a sensor: Photocurrent proportional to light quantity, not dependent on reverse bias voltage (because photocurrent is proportional to the number of electrons, which is proportional to light intensity)
○ Increase depletion region for better sensitivity (i.e., raise threshold)
○ Example 1: CD Players, Fire Alarms, Remote Control Receivers
○ Example 2: Solar Cells
Figure. 8. Circuit of a Solar Cell
ⓐ : Direction of Electrons, ⓑ : Direction of Current, X is n-type Semiconductor
○ Example 3: Image Sensor in Digital Cameras (CCD)
Figure. 9. Image Sensor in Digital Cameras [Footnote: 6]
○ Light Path : Lens → CCD → Conversion to Current Signal → Extract Brightness, Color, Coordinate Info based on Intensity and Position of Detected Light
Threshold frequency should be lower than visible light frequency
○ Example 4: Multi-channel Photodetectors (Photodiode Array Spectrophotometers)
○ Simultaneously measures dispersed radiation for different wavelengths by rotational motion
○ Typically employs an array of 1024 or 2048 silicon diode detectors
○ Advantages : Fast speed, excellent reproducibility, simultaneous measurement at multiple wavelengths
○ Disadvantages : Low resolution (1 ~ 3 nm) (0.1 nm achievable for dispersive), affected by light source intensity and detector sensitivity
○ Used in real-time spectrophotometers
○ Example 5: PN Photodiode, PIN Photodiode, Avalanche Photodiode, Phototransistor, PSD, 1D·2D Array
③ Type 1-3: Compound Types
○ Photointerrupters : LED-Phototransistor, etc.
○ Photocouplers : LED-Photodiode, etc.
⑵ Type 2: External Photoelectric Effect in Photodetectors : Emitting electrons upon absorption of radiation, resulting in photocurrent
① Type 2-1: Phototube
② Type 2-2: Photomultiplier Tube (PMT)
○ Cathode Surface : Emits electrons upon absorbing radiation
○ Diodes : Emit far more secondary electrons than received from cathode surface. Several diodes are used
○ Anode : Collects electrons emitted from the diode
○ Advantages: Low noise, high sensitivity, good responsiveness, linear output current
○ Disadvantages: Mechanically weak, complex power supply
③ Type 2-3: X-ray Photoelectron Spectroscopy (XPS)
⑶ Type 3: Thermal Detectors
① Overview
○ Utilizes temperature increase caused by absorbing radiation
○ Sealed in a vacuum to minimize heat transfer
○ Primarily used for detecting low-energy infrared radiation
② Type 3-1: Pyroelectric Detectors : LiTaO3, PbTiO3, PVF2, etc.
③ Type 3-2: Thermocouple
○ Seebeck Effect : Also known as Thermo-electric Effect
○ Definition : Generation of electromotive force when two different conductors or semiconductors are joined at one end and subjected to a temperature difference
○ Similar Concepts : Peltier Effect, Thomson Effect
○ Cause : Even with the same temperature difference, the potential difference at the junctions of the materials may vary
○ Each material can be considered as a cell in a battery with a different voltage value → Generates a net current
○ (Note) Materials with low resistance might have significant potential differences (or not)
○ Discovered by Thomas S. Seebeck in 1821
○ Often used with one end placed in ice water (0 ℃) as a temperature sensor
○ Thermoelectric power’s magnitude and polarity aren’t affected by conductor thickness or length
○ Seebeck Coefficient : Thermoelectric power per 1 ℃ temperature difference
○ Types
○ Uses materials like Bi and Sb thin films, single crystal silicon, etc.
○ K type : chromel and alumel
○ Others : E, J, N, B, R, S
○ Too weak for practical use as a power source
Figure. 10. Example of a Thermocouple
○ Thermocouples have a wider temperature range compared to thermistors
Figure. 11. Comparison of Thermocouples and Thermistors
④ Type 3-3: Bolometer
○ Absorbs incident infrared radiation and heats up, causing a change in object’s resistance
○ Type 1: Metallic : Platinum or nickel wires (RTD, resistance temperature diode)
○ Type 2: Semiconductor
○ Type 2-1: Thermistor : Semiconductor variable resistor that changes resistance with temperature
Figure. 12. Representation of a Thermistor
○ Type 2-2: Silicon
○ Type 3: Superconductor
Input : 2020.04.01 17:19