April 2004 DiMarzio & McKnight, Northeastern University 10464-19-1

ECEG287 Optical Detection Course Notes

Part 19: Conclusion

Profs. Charles A. DiMarzio

and

Stephen W. McKnight

Northeastern University, Spring 2004

April 2004 DiMarzio & McKnight, Northeastern University 10464-19-2

Electromagnetic Spectrum (by λ)

1 μ 10 μ 100 μ = 0.1mm

0.1 μ10 nm =100Å

VIS=

0.40-0.75μ

1 mm 1 cm 0.1 m

IR=

Near: 0.75-2.5μ

Mid: 2.5-30μ

Far: 30-1000μ

UV=

Near-UV: 0.3-.4 μ

Vacuum-UV: 100-300 nm

Extreme-UV: 1-100 nm

MicrowavesX-Ray Mm-waves

10 Å1 Å0.1 Å

Soft X-Ray RFγ-Ray

(300 THz)

April 2004 DiMarzio & McKnight, Northeastern University 10464-19-3

What is Optical Detection?

• The goal is to get information from light.– Usually we look for variations in the amount of light

over• space...• or time...• or spectrum...• or some combination of these.

• Generally the output is an electrical signal.– It may be digitized for use in a computer.– We need to measure this signal in the presence of noise.

April 2004 DiMarzio & McKnight, Northeastern University 10464-19-4

Course Overview

2. Sources andRadiometry

2-5. Detectors

3. Noise

6. Circuits7. Coherent Detection8. Signal Statistics9. Array Detectors

April 2004 DiMarzio & McKnight, Northeastern University 10464-19-5

Some Detection Issues

• Optics– Radiometry, Beam Shaping, and Filters

• Detector Physics– Converting Optical Energy to Electrical

• Receiver Circuit– Matching to Detector, Proper Biasing

• Interpretation of Data– Dealing with Noise and Signal Statistics

April 2004 DiMarzio & McKnight, Northeastern University 10464-19-6

General Detector Issues

• Spectral Response• Modulation Response• Responsivity• Noise (NEP)• Damage Level• Sensitive Area• Circuit Considerations• Device-Specific Issues

• Filtering– Angle, Position,

Wavelength

• Packaging– Window Transmission,

Position

• Power Requirements• Cooling/Vacuum

Requirements

April 2004 DiMarzio & McKnight, Northeastern University 10464-19-7

Square-Law Detector

April 2004 DiMarzio & McKnight, Northeastern University 10464-19-8

NoiseSignal+ NoisePs

Ps

Pn

April 2004 DiMarzio & McKnight, Northeastern University 10464-19-9

Noise Issues

• Optical Signal Power (Watts)– Normally Related to Some Desired Quantity

(Reflectivity, Temperature, Distance, Magnetic Field, Scattering, Absorption, etc.)

• NEP (Watts per root Hertz)– Can be Related to “NEX”

• Example: NET

April 2004 DiMarzio & McKnight, Northeastern University 10464-19-10

Two Basic Detection Concepts

• Thermal Detectors • Photon Detectors

e-

h

Photon Energy: E=h=hc/Total Energy: PtPhoton Count: np=Pt/hElectron Count: ne=qPt/hElectron Rate: ne/t=qP/hCurrent: ene/t=(qe/h)P

Absorber

HeatSink

Power: PHeating: (dT/dt)H = CPCooling: (dT/dt)C =(T-Ts)Steady State: (T-Ts)/C = P

i/P

Stopped Mon 5 Jan 04

April 2004 DiMarzio & McKnight, Northeastern University 10464-19-11

Detector Types

• Thermal– Characteristics

• Wide Bandwidth

• Accuracy

– Examples• Thermocouple

• Thermopile

• Pyroelectric

• Photon– Characteristics

• Speed• Sensitivity

– Examples• Photoemissive• Photoconductive -

intrinsic & extrinsic• Photovoltaic -

intrinsic & extrinsic

April 2004 DiMarzio & McKnight, Northeastern University 10464-19-12

Course Overview (1)

• 3 - Noise and Photon Detectors

• - Materials Considertations – (4) in Photoemissive Detectors– (5,6) in Semiconductor Detectors

• 7 - Types of Semiconductor Detectors

• 8 - P-N Junction Effects & Other Detectors

• 9,10 - Detectors as Circuit Elements

April 2004 DiMarzio & McKnight, Northeastern University 10464-19-13

Course Overview (2)• 11,12 - Coherent Detection

• 13 - Semiconductor Photoconductive Detectors

• 14 - Signals and Noise

• 15 - Intro to Arrays & a bit about color

• 16 - Gain & BW in Semiconductor Dets.

• 17 - Array Detectors

• 18 - Odds and Ends

April 2004 DiMarzio & McKnight, Northeastern University 10464-19-14

SNR Layout for Coherent Detection

Ps

PBKG

BPF

Preamp AmpPLO Filter?