Optical Devices

Slide 1

Electromagnetic spectrum

insert wavelengths of blue to red. 6.071 Optoelectronics

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Slide 2

Electromagnetic spectrum

E = hν = kTe

E - Energy k - Plank’s constant ν - frequency k - Boltzman’s constant Te - equivalent temperature °K 6.071 Optoelectronics

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Slide 3

Electromagnetic Spectrum slide deleted

Slide 4

Light-emitting diodes (LEDs) cathode

anode

anode

p

n

cathode

When electrons and holes combine in the depletion region the excess energy is given up in the form of a photon. This energy is characteristic of the materials from which the junction is constructed and thus the photon always has the same wavelength. To emit photons the junction must be forward biased, and the wavelength as typically red, green, yellow, or in the infrared. The LED sits in a reflector and is covered by a plastic diffuser. 6.071 Optoelectronics

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Slide 5

Light-emitting diodes (LEDs)

simple LED,red, green, yellow, orange, or infrared.

blinking LED, blinks 1-6 times per second. Can be used as a trivial oscillator.

Simple means of creating numbers and characters. For numbers drive with 74HC4511. 6.071 Optoelectronics

Tricolor LED, green one way, red the other.

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Slide 6

LEDs specs viewing angle LED type degrees color NTE3000 indicator 80 red NTE3010 indicator 90 green NTE3026 tristate 50 red/gree n NTE3130 blinker 30 yellow NTE3017 infrared

intensity

MCD 1.4 1 1.5/0.5 3

forward reverse max DC voltage break- current drop down V V mA 1.65 5 40 2.2 5 35 1.65/2.2 70/35 5.25 1.28

0.4 6

max power mW 80 105 200

20 100

175

Notice that the forward voltage drop is not the 0.6 V we associate with a silicon diode, and that the reverse breakdown voltage is quite small.

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Slide 7 LED operation

Graphs produced with Mathematica™

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Slide 8 LED Datasheet

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Slide 9 LED Datasheet 2

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Slide 10

voltage level sensor

Some LED applications logic probe Vcc Vin

LED lights when the breakdown voltage of the zener is exceeded. The source must provide the current for the LED. 6.071 Optoelectronics

A logic high turns on the Darlington pair and the LED. This is a high impedance measurement. 10

Slide 11 Tristate polarity indicator Ig

A tristate diode indicates the direction of DC current flow, or if the current is AC.

Vin R1

Ir R2

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The diode protects for reversed voltage breakdown and the resistors protect the tristate diode from voltage breakdown.

positive current

red

negative current

green

AC current

yellow 11

Slide 12

Photoresistors insert figs and diagrams from data sheets.

A photodiode is a light controlled resistor that operates much like a thermistor. In an intrinsic semiconductor, photons can promote an electron to the conduction band leaving a hole in the valence band. This increase in carriers leads to a reduction in the resistance. Photoresistors are non-linear, slow (~100 ms) but cheap.

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Slide 13 Photoresistor Specifications

material CdS CdSe PbS PdTe PdSe

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energy gap (eV) 2.42 1.73 0.37 0.29 0.27

wavelength (Å) 5130 7176 33664 42811 45982

Graphs produced with Mathematica™ 13

Slide 14

Photoresistor applications Two simple circuits to switch a relay by the action of a photoresistor. Which one is on in the dark?

on in the dark

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on in the light

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Slide 15 Photodiodes

diode current

electric field

diode voltage 6.071 Optoelectronics

When a photon of sufficient energy hits the semiconductor then an electron/hole pair is created. The minority carrier then diffuses to the junction and an electric field is created. This electric field across the depletion zone is equivalent to a negative voltage across the unbiased diode. At left shows the increased negative offset as a function of increased light intensity. 15

Graphs produced with Mathematica™

Slide 16 Characteristics of photodiodes Idiode P where P is the incident photon power per unit area. Typically S~2 µA/mW/cm2.

The sensitivity of photodiodes is defined as,

S=

Silicon peaks at 900 nm in the infrared, and the sensitivity at 600 nm is down by ~60%. Diodes are faster than photoresistors, and a reverse bias speeds up the minority carrier diffuse and thus the response time (~ 1µs). R Idiode

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Vout = RIdiode

Vout

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24 17-18 Go to the manufacturer’s web site to obtain a datasheet of their product. Please follow these steps: 1. Go to the web site for Fairchild Semiconductor: http://www.fairchildsemi.com/ 2. View the conditions of use for the web site by following the link on the home page called Site Terms & Conditions, or by following this link: http://www.fairchildsemi.com/legal/index.html 3. Return to the home page. 4. In the search box, enter the product number QSE773 into the search box, select “Product Folders and Datasheets” and click “go”. You want the datasheet for Plastic Silicon Pin Photodiode. 5. You will be presented with several options (download PDF, email for example). Select how you would like to receive this datasheet.

Slide 19 Phototransistors, emitter p A phototransistor can be an npn bipolar transistor with a large base that does not have a lead. When photons hit the base they create electron/hole pairs, the electrons are drawn to the collector and the holes are filled with electrons from the emitter. Thus there is a current from the collector to emitter.

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n collector

The dark current is typically ~0.1 µA with a light current of ~1 mA. 19

24 20-21 Go to the manufacturer’s web site to obtain a datasheet of their product. Please follow these steps: 1. Go to the web site for Fairchild Semiconductor: http://www.fairchildsemi.com/ 2. View the conditions of use for the web site by following the link on the home page called Site Terms & Conditions, or by following this link: 3. http://www.fairchildsemi.com/legal/index.html Return to the home page. 4. In the search box, enter the product number BPW36 or BPW37 into the search box, select “Product Folders and Datasheets” and click “go”. You want the datasheet for Hermetic Silicon Phototransistor. 5. You will be presented with several options (download PDF, email for example). Select how you would like to receive this datasheet.

Slide 22

Uses of Phototransistors optical receiver

Light reaching the phototransistor modulates the base of the bipolar transistor. The capacitor blocks current from the DC optical field. 6.071 Optoelectronics

tachometer

Once each rotation the slot in the disk permits light to reach the phototransistor which then shorts the output resistor. The output is counted for a fixed period of time to determine the rotation frequency. 22

Slide 23

Optoisolators Optoisolators can be used to provide isolation between components, to avoid ground loop problems, to control floating electronics, and to provide DC shifts.

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Slide 24

Optoisolators 5V 1k

5V

5 - 12 V 4.7 k

1k

Vout

5 - 12 V

Vout 4.7 k

Which acts as an inverter?

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