Fiber Optics and Laser Instrumentation PDF - Lecture Notes

Fiber Optics and Laser Instrumentation (ECD 409) Dr. Swati Rajput Assistant Professor Department of Electronics Engineering Indian Institute of Technology (Indian School of Mines) Dhanbad Fiber Optics and Laser I...

Fiber Optics and Laser Instrumentation (ECD 409) Dr. Swati Rajput Assistant Professor Department of Electronics Engineering Indian Institute of Technology (Indian School of Mines) Dhanbad Fiber Optics and Laser Instrumentation: Course Goals and Key Takeaways Course Objective To present an introduction to Fiber optics and laser instrumentation. It emphasizes on understanding of the basic knowledge, how fiber optic will be used for communication as well as sensing applications. It will also give an idea how Laser will be used in instrumentation and measurement to meet the demand of industry. Learning Outcomes Upon successful completion of this course, students will: Have a broad understanding of optical fiber as a transmission line as well as a sensor. Have a high-level understanding of different types of fiber optics sensor. Be able to design fiber optic sense network to measure the different type of physical parameter. Be able to know the different application of Laser in the field of instrumentation and measurement which will be helpful to full fill the requirement of industry, medicine, and society. 08-02-2025 2  Fiber Optics and Laser Instrumentation : Key Topics at a Glance Unit No. Topics to be Covered Lecture Hours Learning Outcome 1. Introduction: Introduction to optical fibers; Overview of an optical fiber 07 This will help student to understand the basic of optical communication; Transmission characteristics of optical fiber. fiber and its application in high speed communication system. 2. Optical Fiber Sensors: Intrinsic, extrinsic, and interferometric fiber optic 10 This unit will help student in understanding how optical sensors for the measurement of strain, temperature, pressure, fiber can be used as modern sensor which has advantage displacement, velocity, acceleration, acoustic sensors, sensors for over conventional electronic/electrical sensor. measurement of magnetic field and current, humidity, pH, rotation, gyroscope. 3. Optical Sensors for Remote Detection: Magnetic and electric field 04 This unit will help student to design fiber optics sensor measurements based on –Intensity, Phase, Polarization, Frequency, system to measure and control some physical as well as Wavelength modulation. electrical parameter from remote distance. 4. Optical Devices and Equipment: Optical source and detector, Optical 07 This unit will help student in understanding the Time Domain Reflectometer (OTDR), Optical Spectrum Analyzer, Optical construction working and operation of basic optical Power Meter. devices and equipment used in instrumentation and measurement. 5. Laser Instrumentation: Applications of Laser for the measurement of 07 This will help student how the optical components like distance, velocity, acceleration, current and voltage. Medical applications laser can be used in commercial application. of lasers. 6. Industrial Applications of Laser: Application of Laser in material 04 This will help student how the optical components like processing and design. laser can be used for industrial applications. Textbook: Reference Books: 1. Senior J.M, “Optical Fiber Communications- 1. Kesier G, “Optical Fiber Communication”, Tata McGraw Hill, New Delhi. Principle and Practice”, 2. John F. Read, “Industrial Applications of Laser”, Academic Press. 08-02-2025 3. Monte Ross, “Laser Applications”, Tata McGraw Hill. 3  Fiber Optics and Laser Instrumentation : Weightage Weightage of Different Components 1. Quiz/ Assignment 10 Marks 2. Mid Semester Examination 30 Marks 3. Quiz/ Assignment 10 Marks 4. End Semester Examination 50 Marks Total Marks = 100 08-02-2025 4  Semiconductor Optical Sources External Quantum Efficiency of Light Emitting Diode What fraction of internally generated power comes out of the device. In LED, certain fraction of electrical power is converted into optical power and the optical power which is generated inside the device is Pint. Only a fraction of internally generated power is available as the device output. But because of the structure of the LED; index contrast between different layers only a fraction of internally generated power comes out. And therefore, there is a finite external quantum efficiency of a Light Emitting Diode. 5  Semiconductor Optical Sources External Quantum Efficiency of Light Emitting Diode Medium of semiconductor and where light source is kept in a point source which sends light in all the directions. We have all the possible angles of incidence at semiconductor air interface. Angles which is greater than the critical angle at the semiconductor air interface they are reflected back via the process of total internal reflection. Then the light only in this cone which has semi vertical angle 𝜃𝑐 will be refracted➔ will come out of the semiconductor block. Rest of the light will be totally internally reflected and Fraction of light that comes out will come back into the semiconductor. This contributes towards Finite External Quantum Efficiency of an LED. Total light which is incident External quantum efficiency can be calculated as the; we have the transmission coefficient from semiconductor to air for refracted light T(𝜃) 6  Semiconductor Optical Sources External Quantum Efficiency of Light Emitting Diode Fraction of light that comes out Total light which is incident T(𝜃) : Fresnel Transmission Coefficient Fraction is purely due to index contrast between the 7 semiconductor material and air.  Semiconductor Optical Sources P-i Characteristics of Light Emitting Diode If you supply some forward biased drive current (i) to a LED ➔ then how much power would be generated Response of the device for a given input Response if output power and input is injection current 8  Semiconductor Optical Sources Example: InGaAs LED which emits a wavelength of 1310 nm; 𝝉𝒓 = 𝟑𝟎 𝒏𝒔𝒆𝒄 𝒂𝒏𝒅 𝝉𝒏𝒓 = 𝟏𝟎𝟎 𝒏𝒔𝒆𝒄 I =35 mA. If the refractive index of the material is 3.5, find the power emitted from the device. 𝜂𝑖𝑛𝑡 = 0.77 𝑃𝑜𝑢𝑡 = 0.36 𝑚𝑊 9  Semiconductor Optical Sources LED Characteristics: Output Spectrum ❖ Output spectrum of an LED➔ occupies a certain spectral range ❖ Although it is an LED at 860 nm wavelength, but it is not strictly an LED at 860 nm. It extends beyond this on either side. ❖ An LED has an spectral width of 30 to 40 nm. Full Width Half Maximum 10 Semiconductor Optical Sources LED Characteristics: Temperature Dependence ❖ With change in the temperature, the output power will decrease ❖ Decrease is more prominent In Edge Emitting LED than Surface Emitting LED 11  Semiconductor Optical Sources LED : Radiation Pattern Bit more directional as compared to surface emitting LED Emits in all the direction Pattern is Lambertian 12  Semiconductor Optical Sources LED : Modulation So, the time in which the optical power will rise to 90% of the maximum then it is called rise time. Fall time in an LED refers to the time it takes for the light output to decrease from 90% to 10% of its maximum intensity after the driving current is turned off. 13 Semiconductor Optical Sources LED : Modulation Bandwidth 14  Semiconductor Optical Sources : LASER Diode ❖ High Power ❖ Highly Directional ❖ Monochromatic ❖ Coherent Requirement for Lasing ❖ Let us consider a material system, out of which we are making a Laser ❖ Upper laser level is metastable so that we can have stimulated emission ❖ N1 is number of atoms per unit volume in lower Laser level ❖ N2 is number of atoms per unit volume in upper Laser level ❖ The very first requirement for Lasing is Population Inversion: ∆𝑁 = 𝑁2 − 𝑁1 > 0 ➔ more number of atoms in upper state than lower state so that there would be downward transition and we have radiative optical output. ❖ This is obtained by Pumping ➔ Optical or Electrical ❖ ❖ So basically, we have to lift the atoms from bottom to top using a pump. ❖ In diode lasers it is electrical pumping ➔ it is by injection current ❖ The gain of the system is given by ❖ We have attained the population inversion, if we input a signal then it would be amplified via the process of stimulated 15 emission. Semiconductor Optical Sources : LASER Diode 16  Semiconductor Optical Sources : LASER Diode ❖ So, by population inversion and pumping ➔ we had the amplifier but Laser is a self sustained device ❖ Feedback-➔ But we need to convert this amplifier to oscillator for that we need to make a resonator cavity. ❖ Amplifier amplifies an externally injected signal by passing it through a pumped gain medium. ❖ Oscillator incorporates an optical cavity (resonator) with mirrors that reflect light back and forth. ❖ The feedback allows the light to build up from spontaneous emission into a coherent beam. ❖ For resonator cavity we have to include the laser material in between two mirrors so that we can provide feedback and make it a resonator cavity. ❖ Lasing will start when the round trip gain > 1 because the light is going back and forth between the two mirrors and in one round trip if the net gain is greater than 1 then we will have Lasing ❖ Net gain➔ There is a gain and also there is losses so the gain should be able to compensate for losses to have oscillations or lasing 17  Semiconductor Optical Sources : LASER Diode Resonator Cavity ❖ For the resonant cavity, we have the active medium and let us say that the length of this active medium is d, Initial Intensity Intensity before hitting R2 ❖ One round trip Intensity before hitting R1 18  Semiconductor Optical Sources : LASER Diode Resonator Cavity ❖ We should provide the injection current and when the injection current is low then the population inversion is not sufficient, then the gain coefficient is not sufficient to overcome the losses and lasing does not start. ❖ When we slowly increases this current then as soon as the gain provided overcomes the losses ➔ lasing starts. 19 Semiconductor Optical Sources : LASER Diode Resonator Cavity 𝟏 𝟏 𝟏 𝑱𝒕𝒉 = [𝜶 + 𝐥𝐧 ] 𝜸 𝟐𝑳 𝑹𝟏 𝑹𝟐 20  Semiconductor Optical Sources : LASER Diode Laser Diode ❖ In typical Laser Diode the pumping is by injection current. ❖ And when the current is greater than the threshold current or the gain provided is greater than the threshold gain then the lasing starts. 21  Semiconductor Optical Sources : LASER Diode Laser Diode : P-i Characteristics ❖ Initially when the current is low then we have a very small power coming out. ❖ If we increase the current the power would increase, but still, it would not be very high. ❖ As soon as the current crosses the threshold current value, the power rises sharply ➔ Laser Action Power produced per unit injection current Photons produced per unit injected electrons Efficiency would be lesser than 1 because not all the injected electrons will result in radiative recombination. 22  Semiconductor Optical Sources : LASER Diode Laser Diode : Temperature Dependence ❖ With the increase in the temperature the threshold current increases 23 Semiconductor Optical Sources : LASER Diode Laser Diode : Output Spectrum ❖ Equidistant sharp peak and the power level is much higher ❖ This peaks corresponds to multi longitudinal modes of the cavity➔ interference pattern in the cavity 24 Semiconductor Optical Sources : LASER Diode Laser Diode : Longitudinal modes 25 Semiconductor Optical Sources : LASER Diode Laser Diode : Longitudinal modes 26  Semiconductor Optical Sources : LASER Diode Single Longitudinal Mode LASER ❖ We have as laser but it has got several lines; ideally, we should have a LASER which has only one wavelength or line width as narrow as possible. ❖ One way to obtain single mode LASER is to increase the line separation so that only one mode falls in the gain bandwidth. ❖ Line separation can be increased by decreasing d but that reduces the volume of the gain medium ➔ so I have to increase the threshold current. 27  Semiconductor Optical Sources : LASER Diode Distributed Bragg Reflector and Distributed Feedback LASERS ❖ Another way to have single wavelength LASER is ➔ Use the mirrors that are highly wavelength selective ❖ Bragg Grating reflects a particular wavelength 28  Semiconductor Optical Detectors Information Electrical Optical Optical Fiber Source Transmit Source Cable Optical Electrical Destination Detector Receive The photodetector is an essential component in an optical receiver that converts the incoming optical signal into an electrical signal. Performance and Compatibility Requirements for Photodiodes High sensitivity at the operating wavelength Minimum noise introduced by the detector Large electrical response to the received optical Stability of performance characteristics signal Small size Short response time to obtain a suitable High Reliability bandwidth Low bias voltages and cost 29  Semiconductor Optical Detectors Photoelectric Effect ❖ We have a metal and we incident optical energy on it (incident photons) ➔ if the energy of the photons is sufficient then it will knock out the electrons from the metal and then these electrons are available for the conduction. ❖ Photoelectric effect occurs in metals where you have free electrons in the conduction band and these electrons although we call them free but they are attached to the material with certain energy which is known as work function (W). ❖ Emax is maximum kinetic energy 30  Semiconductor Optical Detectors Photoelectric Effect ❖ In a semiconductor, we have a valence band and a conduction band and in between the bands we have energy bandgap. ❖ Now if we want to knock out an electron from a semiconductor, then we will have to incident a photon of energy which can overcome this bandgap ➔ so that this electron can go into the conduction band. ❖ Electron Affinity➔ Energy difference between the vacuum level and the bottom of the conduction band. ❖ For electron to come out from the material ➔ it need to overcome the electron affinity. 31  Semiconductor Optical Detectors Internal Photo Effect ❖ In Internal Photo Effect, we incident a photon onto the semiconductor material and depending upon the energy of the photon: ✓ Photon can be absorbed ✓ Photon can be transmitted (we can see a semiconductor material as a transparent material) ❖ Absorption of photon results in the generation of the electron-hole pair. ❖ These photoexcited carriers they remain within the material. ❖ Application of the electric field to the material results in the transport of the electron and hole and consequent flow of the electric current in the electric circuit of the detector. Devices based on Internal Photo Effect ❖ P-n junction based semiconductor Photodiode ❖ P-i-n Photodetector ❖ Avalanche Photodetector (APD) 32  Semiconductor Optical Detectors EC Incident Photon Eg Semiconductor 𝒉𝝂 EV v 𝒉𝝂 < 𝑬𝒈 v Initial Optical W 𝒉𝝂 = 𝑬𝒈 Intensity 𝑰𝟎 𝒉𝝂 > 𝑬𝒈 Source: Semiconductor Physics and Devices Book by Donald A. Neamen Optically generated electron hole pair formation in a semiconductor 𝑑𝐼 Condition for Optical Absorption Change in optical intensity w.r.t. width ➔ ∝ −I 𝑑𝑊 𝑑𝐼 ℎ𝑐 = −𝛼𝐼 ⇒ 𝐼 = 𝐼0 exp −𝛼𝑊 ; 𝑑𝑊 𝐸𝑔 = ℎ𝜈(𝐸𝑛𝑒𝑟𝑔𝑦 𝑜𝑓 𝑃ℎ𝑜𝑡𝑜𝑛) = = 𝜆𝑔 𝛼 𝑖𝑠 𝐴𝑏𝑠𝑜𝑟𝑝𝑡𝑖𝑜𝑛 𝐶𝑜𝑒𝑖𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑡 6.63×10−34 Photon Intensity after traversing W; I = 𝐼0 exp −𝛼𝑊 𝑒𝑉𝑠 ×3×108 𝑚𝑠 −1 1.24 1.6 ×10−19 = Transmitted Optical Power 𝑃𝑡𝑟 = 𝑃𝑖𝑛 exp(−𝛼𝑊) 𝜆𝑔 (µ𝑚) 𝜆𝑔 (µ𝑚) 33  Semiconductor Optical Detectors p-n Junction based Photodiode Zero Bias p-type S/C W n-type S/C -- ++ Electron -- ++ Hole -- ++ -- ++ -- ++ No Bias W Reverse Bias p-type S/C n-type S/C -- -- ++ ++ -- -- ++ ++ -- -- ++ ++ -- -- ++ ++ -- -- ++ ++ - + Source: Semiconductor Physics and Devices Book by Donald A. 34 Reverse Bias Neamen 34  Semiconductor Optical Detectors p-n Junction based Photodiode Reverse Bias W p-type S/C n-type S/C 𝑞𝑉 - - - - ++ ++ 𝐼= 𝐼𝑜 (𝑒 𝐾𝑇 − 1) - - - - ++ ++ Electron-hole - ++ ++ - - - generated pairs - - - - ++ ++ - - - - ++ ++ 𝒕𝒅𝒊𝒇𝒇 𝒕𝒅𝒓𝒊𝒇𝒕 𝒉𝝂 = 𝑬𝒈 - + Source: Semiconductor Physics and Devices Book by Donald A. Neamen Photocurrent (𝐼𝑝 ) Reverse Bias Photocurrent (𝐼𝑝 ) Photogeneration of electron-hole pair ℎ𝑐 1.24 Current which arises in conjunction with the absorption of light. 𝐸𝑔 = ℎ𝜈 = = 𝜆𝑔 𝜆𝑔 (µ𝑚) Dark Current 𝑞𝑉 Dark current is leakage current that flows when a bias voltage is 𝐼= 𝐼𝑜 (𝑒 𝐾𝑇 − 1) applied to a photodiode 35  Semiconductor Optical Detectors p-n Junction based Photodiode Photocurrent (𝐼𝑝 ) Absorption of photons in a photodiode produces carrier pairs and thus giving rise to photocurrent Responsivity (R) Measures photocurrent per unit optical power incident on a photodetector 𝐼𝑝 ∝ 𝑃𝑖𝑛 ⇒ 𝐼𝑝 = 𝑅𝑃𝑖𝑛 Also referred as a sensitivity of a photodetector; Unit: A/W Quantum Efficiency (𝜂) Fraction of incident photons which are absorbed by the photodetector and generated electrons which are collected at the detector terminals: 𝑁𝑢𝑛𝑏𝑒𝑟 𝑜𝑓 𝐸𝑙𝑒𝑐𝑡𝑟𝑜𝑛𝑠 𝐺𝑒𝑛𝑒𝑟𝑎𝑡𝑒𝑑 𝐸𝑙𝑒𝑐𝑡𝑟𝑜𝑛 𝐺𝑒𝑛𝑒𝑟𝑎𝑡𝑖𝑜𝑛 𝑅𝑎𝑡𝑒 (𝑟𝑒 ) 𝜂= = 𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝐼𝑛𝑐𝑖𝑑𝑒𝑛𝑡 𝑃ℎ𝑜𝑡𝑜𝑛𝑠 𝑃ℎ𝑜𝑡𝑜𝑛 𝐼𝑛𝑐𝑖𝑑𝑒𝑛𝑡 𝑅𝑎𝑡𝑒 (𝑟𝑝 ) 𝑂𝑝𝑡𝑖𝑐𝑎𝑙 𝑃𝑜𝑤𝑒𝑟 𝑃𝑖𝑛 𝑟𝑝 = = 𝑃ℎ𝑜𝑡𝑜𝑛 𝐸𝑛𝑒𝑟𝑔𝑦 ℎ𝜐 36  Semiconductor Optical Detectors p-n Junction based Photodiode 𝐼𝑝 Τ𝑞 𝑅ℎ𝜐 𝑅ℎ𝑐 𝜂= = = 𝑃𝑖𝑛 Τℎ𝜐 𝑞 𝑞𝜆 Long-Wavelength Cut-off 𝜂𝜆(𝜇𝑚) 𝑅= Threshold for detection 1.24 𝑐 ℎ𝑐 ℎ𝜐 ≥ 𝐸𝑔 ⇒ ℎ ≥ 𝐸𝑔 ⇒ 𝜆𝑐 = 𝜆𝑐 𝐸𝑔 Why does responsivity increase with wavelength ? ℎ𝑐 ℎ𝑐 𝐸𝑔 = ⇒ 𝜆𝑐 = 𝜆𝑐 𝐸𝑔 Semiconductor Optical Detectors p-n Junction based Photodiode Semiconductor Optical Detectors p-n Junction based Photodiode  Semiconductor Optical Detectors p-n Junction based Photodiode Performance Metrics and Design Considerations 𝑷𝒂𝒃𝒔 𝑷𝒊𝒏 Absorbing Region 𝑷𝒕𝒓 W Transmitted Optical Power 𝑃𝑡𝑟 = 𝑃𝑖𝑛 exp −𝛼𝑊 Absorbed Optical Power 𝑃𝑎𝑏𝑠 = 𝑃𝑖𝑛 − 𝑃𝑡𝑟 = 𝑃𝑖𝑛 [1 − exp −𝛼𝑊 ] Each Absorbed Photon creates an electron hole pair: 𝑃𝑎𝑏𝑠 𝑃𝑖𝑛 [1−exp −𝛼𝑊 ] 𝜂= = Ideally, 𝜂 𝑠ℎ𝑜𝑢𝑙𝑑 𝑏𝑒 1; 𝑖𝑛 𝑡ℎ𝑎𝑡 𝑐𝑎𝑠𝑒 𝛼𝑊 ≫ 0 𝑃𝑖𝑛 𝑃𝑖𝑛 𝑰𝒑 Τ𝒒 𝑹𝒉𝝊 𝑹𝒉𝒄 𝜂 = 1 − exp(−𝛼𝑊) 𝜼= = = 𝑷𝒊𝒏 Τ𝒉𝝊 𝒒 𝒒𝝀 𝜼𝒒 𝟏 − 𝐞𝐱𝐩 −𝜶𝑾 𝒒 [𝟏 − 𝐞𝐱𝐩 −𝜶𝑾 ]𝝀(𝝁𝒎) 𝑹= = = 𝒉𝝊 𝒉𝝊 𝟏. 𝟐𝟒  Semiconductor Optical Detectors p-n Junction based Photodiode Performance Metrics and Design Considerations 𝜂𝑞 1 − exp −𝛼𝑊 𝑞 [1 − exp −𝛼𝑊 ]𝜆(𝜇𝑚) 𝑅= = = ℎ𝜐 ℎ𝜐 1.24 For higher responsivity, the width of the absorbing region should be large In case of p-n junction photodiode, there comes a limitation Current-Voltage Characteristics of p-n junction Diode  Semiconductor Optical Detectors Speed of the Response of the Photodetector Time constant incurred by the capacitance of the photodiode with its load(RC) capacitance of the photodiode Cd is that of the junction together with the capacitance of the leads 𝜀𝑠 𝐴 𝐶𝑗 = 𝑊 where εs is the permittivity of the semiconductor material and A is the diode junction area. small depletion layer width w increases the junction capacitance capacitance must be minimized ➔ reduce the RC time constant which also limits the detector response time  Semiconductor Optical Detectors Speed of the Response of the Photodetector Drift time of carriers through the depletion region(𝑡𝑑𝑟𝑖𝑓𝑡 ) Longest transit time, tdrift, is for carriers which must traverse the full depletion layer width W 𝐷𝑒𝑝𝑙𝑒𝑡𝑖𝑜𝑛 𝐿𝑎𝑦𝑒𝑟 𝑊𝑖𝑑𝑡ℎ (𝑊) 𝑡𝑑𝑟𝑖𝑓𝑡 = 𝐷𝑟𝑖𝑓𝑡 𝑉𝑒𝑙𝑜𝑐𝑖𝑡𝑦 (𝑉𝑑 ) A field strength above 2 × 104 V cm−1 in Si➔ maximum (saturated) carrier velocities of approximately 107 cm s−1. ∴ transit time through a depletion layer width of 10 μm is around 0.1 ns = 100 psec In conventional p-n junction photodiode, the diffusion time Diffusion time of generated carriers outside the depletion region(𝜏𝑑𝑖𝑓𝑓 ) is greater than the drift time of the generated carriers..!!! Time taken, tdiff, for carriers to diffuse a distance d 𝑑2 𝑡𝑑𝑖𝑓𝑓 = 𝐷𝑐 ➔ Carrier Diffusion Constant 2𝐷𝑐 For example, the hole diffusion time through 10 μm of silicon is 40 ns whereas the electron diffusion time over a similar distance is around 8 ns  Semiconductor Optical Detectors p-i-n Photodetector To increase the light absorption area To reduce the diffusion time of the carrier generated To reduce the junction capacitance Introduction of the intrinsic region between p and n region Reduction of the width of the p and n region W p-type Intrinsic Region n-type Source: Optical Fiber Comm. By John M. Senior Wp Wn Key Features of p-i-n Photodetector By reducing the width of p-type and n-type High response speed region and increasing the width of the intrinsic High Sensitivity region, we try to make the drift carrier time and Low Dark Current diffusion carrier time approximately equal..!!!  Semiconductor Optical Detectors p-i-n Photodetector The bandwidth of a photodetector refers to the range of frequencies or wavelengths of light that it can effectively detect. Measures how quickly the photodetector can respond to changes in incident light and convert them into electrical signals. The ultimate bandwidth of the device is limited by the drift time of carriers through the depletion region tdrift 1 𝑣𝑑 𝐵𝑚 = = 2𝜋𝑡𝑑𝑟𝑖𝑓𝑡 2𝜋𝑊 Width of the Absorbing Region 𝜂𝑞 1 − exp −𝛼𝑊 𝑞 [1 − exp −𝛼𝑊 ]𝜆(𝜇𝑚) 𝑅= = = ℎ𝜐 ℎ𝜐 1.24 For higher responsivity we want a wider absorbing region ➔ results in longer carrier transit time. Trade-off between Responsivity and Bandwidth….!!!! Semiconductor Optical Detectors p-i-n Photodetector Semiconductor Optical Detectors p-i-n Photodetector  Semiconductor Optical Detectors Avalanche Photodetector A photodiode with an internal gain. High sensitivity Operates under very strong reverse bias This has a more sophisticated structure than the p–i–n photodiode in order to create an extremely high electric field region (approximately 3 × 105 V cm-1) Therefore, as well as the depletion region where most of the photons are absorbed and the primary carrier pairs generated, there is a high-field region in which holes and electrons can acquire sufficient energy to excite new electron–hole pairs. This process is known as impact ionization and is the phenomenon that leads to avalanche breakdown in ordinary reverse-biased diodes. It often requires high reverse bias voltages (50 to 400 V) in order that the new carriers created by impact ionization can themselves produce additional carriers