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Optics Chapter 7. Applied Optics

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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

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.

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

1-2. Mercury (Hg) vapor, Xenon (Xe) gas discharge lamp

○ Emits ultraviolet and visible light

1-3. Electric discharge lamp filled with Hg vapor, Xe gas

○ Emits ultraviolet and visible light

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

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

Type 3. Lasers : One or a few wavelengths. Line source

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

3-2. Semiconductor lasers

Type 4. Acceleration of Charged Particles

Poynting vector : Acceleration of charged particles generates electromagnetic waves

4-1. Radio wave generation

4-2. Bremsstrahlung : Visible light is emitted when electrons decelerate. Also known as cyclotron radiation, synchrotron radiation, etc.

Type 5. Fluorescence

Type 6. Cherenkov Radiation

Type 7. Bremsstrahlung

Type 8. Fluorescence due to High Z Metal

8-1. Gamma fluorescence

8-2. Beta fluorescence

8-3. Characteristic X-ray

8-4. Pair production and annihilation

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

Actual light sources



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

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

Characteristic 1: Signal-to-Noise Ratio (S/N ratio)

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)

Characteristic 3: Detectivity : Denoted as D

○ Definition : Measure of the minimum detectable intensity of radiation

○ D = 1 / NEP

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

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

Type 1: Internal Photoelectric Effect in Photodetectors : Increase in electrical conductivity upon absorption of radiation

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

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

Type 1-3: Compound Types

○ Photointerrupters : LED-Phototransistor, etc.

○ Photocouplers : LED-Photodiode, etc.

Type 2: External Photoelectric Effect in Photodetectors : Emitting electrons upon absorption of radiation, resulting in photocurrent

Type 2-1: Phototube

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

Type 2-3: X-ray Photoelectron Spectroscopy (XPS)

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

Type 3-1: Pyroelectric Detectors : LiTaO3, PbTiO3, PVF2, etc.

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

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

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