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AVALANCHE PHOTODIODES & HIGH SPEED MEASUERMENTPresented By:Renu Verma (162517)Rohit Kumar (162519)ME- I&C (R)Presented To:Dr. (Mrs.) Lini MathewAssociate ProfessorNITTTR, Chandigarh

IntroductionAnavalanche photodiode (APD)is a highly sensitive, high speed semiconductor light sensors, electronic device that exploits the photoelectric effect to convert light to electricity. APDs arephotodetectorthat provide a built-in first stage ofgainthrough avalanche multiplication.By applying a high reverse bias voltage (100200 V insilicon), APDs show an internal current gain effect due toimpact ionization.The higher the reverse voltage, the higher the gain.

Principle of Avalanche Multiplication

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Working of Avalanche PhotodiodeWhen the electron-hole pairs are generated in the depletion layer of a photodiode with a reverse voltage applied to the PN junction.

The electrons drift towards the N+ side while the hole drift towards the P+ side due to electric field developed across the PN junction.The thickness of /I-region is large as compared to other region so that the incoming light is quickly passes through n+ and p-layers and spend major time in this region creating large electrons and hole pairs.

Characteristic of APDOperated with much higher reverse bias, which allows each photo generated carrier to be multiplied by avalanche breakdown.

Dark Current Vs Reverse VoltageThe dark current is mainly created by diffusion, generation-recombination and tunneling.In an APD dark current is generated both from leakage at the surface of the diode and also from electron holes thermally generated within the bulk of the silicon which are then multiplied in the gain region.

By increasing the gain of the APD, by increasing the external bias, also increases this dark current.

Advantage:APD includes greater level of sensitivity.Fast response time.High performance.Disadvantage:The APD multiplication process also produces an additional noise component, known as "excess noise.APD include much higher operating voltage may be required.

ApplicationAPDs are widely used in applications such as distance measurement, data transmission ( over fibre or through free space ), range finding, high speed industrial inspection ( including colour measurement ) and in various other medical and scientific instrumentation

HIGH SPEED MEASUERMENT

Introduction High precision speed measurement techniques are a very important subject in development of Physics. It impacts also in the applied, industrial and commercial fields around the world. It is required to have standards that allow keeping congruent measurements in different parts of the world.

Device for indicating the angular (rotary) speed of a rotating shaft. Either by registering the total no. of revolution during the period of contact or by indicating the number of revolution per min.

Tachometer

What Are the Different Types of Tachometers?Tachometers can be classified 1.On the basis of data acquisitionContact Non contact types2. Classified as data type Analog Digital 3. On the basis of power .MechanicalElectrical

ComparisonContact TypeThe tachometer has to be in physical contact with the rotating shaft.Preferred where the tachometer is generally fixed to the machine.Generally, optical encoder / magnetic sensor is attached to shaft of tachometer.Non Contact TypeThe tachometer does not need to be in physical contact with the rotating shaft.Preferred where the tachometer needs to be mobile.Generally, laser is used or an optical disk id attached to rotating shaft and read by a IR beam or laser.

How To Choose a Tachometer?AccuracyPrecisionRangeAcquisition TimeContact type / Non Contact typePortable / FixedDigital / AnalogCost

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Optical Technique For High Speed Measurement

Optical techniques are most suitable for speed measurement of fast moving projectile, particularly when the projectile is of small size and speed is to be measured in the vicinity of the projectile. Optical techniques have distinct advantage of being non-contact, non-destructive in nature and are suitable for practically all sizes of objects.

Working

Transmitting Part:Laser Gun emits laser light Laser beam falls on collimating(minimize divergence) optics lanceOptical Screen:Its have a rotating projectileLaser beam cut by this projectileReceiving Part:When laser beam fall on Photodiode, produce pulsesDetector count number of pulses passes

Doppler Effect Therelativistic Doppler effectis the change infrequency(andwavelength) oflight, caused by the relative motion of the source and the observer (as in the classicalDoppler effect), when taking into account effects described by thespecial theory of relativity.

Conti. Wavelength , fs frequency , v Velocity of observer, c Velocity of light

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Due to the relativistic time dilation, the observer will measurethis time to be

is the Lorentz factor. The corresponding observed frequency

The ratio

is called Doppler factor of the source to the relative operator. The resulting redshift can be written as:

Laser Doppler Anemometer

The Laser Doppler Anemometer is a very high-tech anemometer, so it is usually more accurate.Laser Doppler anemometers use a beam of light from a laser that is divided into two beams, with one propagated out of the anemometer. The air particles caught in the beam are reflected back into the anemometer where its measured.When the particles are in great motion, they produce a Doppler shift for measuring wind speed in the laser light, which is used to calculate the speed of the particles, and therefore the air around the anemometer.

Advantage & DisadvantageAdvantage:The device does not disturb the flow during measurementThe device measure velocity directlyVolume of sensing part can be very smallHighly accurateCan be used to measure flow of both gases and liquids.Disadvantage:Requires transparent channelsCannot be-used on clean flows

ApplicationsWind tunnel studiesBlood flow measurementSensing of wind velocityUsed in the field of combustion

High Accuracy Speed Measurement Using GPS (Global Positioning System)

Typical approach to using GPS for speed measurement today is to consider a series of track points that record position estimates(latitude and longitude) determined by the GPS at regular time intervals. The difference between the known satellite carrier frequency and the frequency determined at the receiver is known as a Doppler shift. This Doppler shift is directly proportional to velocity of the receiver along the direction to the satellite, regardless of the distance to this satellite.

Fiber Coupling High Speed Measurement

This method is also similar like previous method. Here fiber coupling are use to communicate between transmitter and receiver.

Drive CircuitLight SourceOpticalFiberDetector

Train Speed Measurement

A critical safety issue is correct timing for crossing gates at railroad highway grade crossings. If the warning signals and crossing gates are activated too soon, impatient motorists may try to cross in spite of them which may result in a train-car collision. If the system is activated too late, again a serious accident may occur. Train presence and speed detection can also be performed by measuring how the light signal passing through the rail-bonded fiber sensor is affected by a train passing over the rail.

Advantages & DisadvantageAdvantages:Optical Fiber Cables can run massive distances like 40 KM or much more (Single Mode Fiber Cables) without having to repeat the signal anywhere in-between.Optical Fiber Cables are flexible and can be laid both within the buildings (Indoor Fiber Cables) and outside the buildings (Shielded Fiber Cables).Disadvantage:Fiber splicing is a complicated procedure and requires skilled manpower to achieve.

Reference High-speed optical frequency-domain imaging S. H. Yun, G. J. Tearney, J. F. de Boer, N. Iftimia and B. E. Bouma Harvard Medical School and Wellman Laboratories for Photomedicine, Massachusetts General Hospital 50 Blossom Street, BAR-7, Boston, Massachusetts 02114 syun@bics.bwh.harvard.edu.High accuracy speed measurement using GPS (Global Positioning System)by Tom J. Chalko, MSc, PhD*Senior Scientist, Scientific Engineering Research P/L, Mt Best, Vic 3960, Australia.International Journal Of Engineering Sciences & Research Technology Speed Violation Detection System : A Review Sukhdeep Singh*1, Rishma Chawla2, Harpal Singh3.

International Congress of Engineering Physics IOP Publishing High precision speed measurement by using inter ferometric techniques M A Rodrguez vila, R Ochoa Valiente, L A Garca Trujillo Faculty de Ciencias Fsico Matemticas. Universidad Autnoma deCoahuila. Saltillo, Coahuila de Zaragoza, Mxico.FIBER OPTICAL SENSORS FOR HIGH-SPEED RAILAPPLICATIONS IDEA Programs Transportation Research Board500 Fifth Street, NWWashington, DC 20001.Reference

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