Chapter 7

Chapter 7 - Click to edit Master subtitle style 5/13/10 THE...

Info iconThis preview shows pages 1–17. Sign up to view the full content.

View Full Document Right Arrow Icon

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: Click to edit Master subtitle style 5/13/10 THE FIBER FORUM JOSEPH C. PALAIS PRESENTED BY 5/13/10 Joseph C. Palais 7.1 22 Chapter 7 Light Detectors 5/13/10 Joseph C. Palais 7.1 33 Section 7.1 Photodetection Mechanisms Detectors convert light signals into electrical signals. Internal Photoelectric Effect Generation of mobile charge carriers in semiconductors by absorption of photons. 5/13/10 Joseph C. Palais 7.1 44 Section 7.1 Photodetection Mechanisms Devices: pn photodiode PIN photodiode avalanche photodiode (APD) 5/13/10 Joseph C. Palais 7.1 55 External Photoelectric Effect Generation of free electrons when photons strike the surface of a metal. Electrons are emitted from the surface. Devices: Vacuum Photodiode Photomultiplier tube LIGHT DETECTORS 5/13/10 Joseph C. Palais 7.1 66 PHOTODETECTION MECHANISMS Important Detector Properties 1. Responsivity: Power Optical Input Current Output = P i Optical Power Photodetector Electrical Current / i A W P (7.1) 5/13/10 Joseph C. Palais 7.1 77 2. Spectral Response: Range of optical wavelengths over which the detector is useful. It is often displayed as a curve of responsivity versus wavelength. An example appears on the next slide. PHOTODETECTION MECHANISMS 5/13/10 Joseph C. Palais 7.1 88 PHOTODETECTION MECHANISMS 0.5 0.7 0.9 1.1 0.5 ( m) ( / ) A W Silicon Photodiode Response 5/13/10 Joseph C. Palais 7.1 99 3. Speed of Response: Range of modulation frequencies over which the detector is useful. As before, if tr is the rise time, the bandwidth is (approximately) r dB t f 35 . 3 =- ( 7 .2 ) P i 10% 90% tr PHOTODETECTION MECHANISMS Input Output 5/13/10 Joseph C. Palais 7.1 1010 Ot he r Im port a nt Prope rt ie s: Size , t e m pe rat ure se nsit ivit y, gain, life t im e , circuit com ple xit y, and cost . PHOTODETECTION MECHANISMS Click to edit Master subtitle style 5/13/10 THE FIBER FORUM JOSEPH C. PALAIS PRESENTED BY 5/13/10 Joseph C. Palais 7.2 1212 Section 7.2 Vacuum Photodiode And Photomultiplier Vacuum Photodiode v hf Electrons Cathode Anode + i RL V-- + 5/13/10 Joseph C. Palais 7.2 1313 VACUUM PHOTODIODE AND PHOTOMULTIPLIER The work function a is defined to be: Energy required to liberate an electron from the metal cathode. Units of energy: Joules In order to free an electron: the photon energy must equal, or exceed, the work function. 5/13/10 Joseph C. Palais 7.2 1414 VACUUM PHOTODIODE AND PHOTOMULTIPLIER hc (7.3) hf hc Thus, the incoming photon frequency must satisfy: 5/13/10 Joseph C. Palais 7.2 1515 VACUUM PHOTODIODE AND PHOTOMULTIPLIER The longest wavelength that can be detected is called the cutoff wavelength . It is given by: 24 . 1 (7.4) where c is in m, and in eV. For detection we require: hc c = or 24 . 1 = c 5/13/10 Joseph C. Palais 7.2 1616 VACUUM PHOTODIODE AND PHOTOMULTIPLIER Current flows through the load resistor RL during the time that electrons travel from the cathode to the anode. At the anode, the electrons and positive charges drawn there through the circuit neutralize each other. The current stops when the electrons reach the anode. 5/13/10...
View Full Document

Page1 / 75

Chapter 7 - Click to edit Master subtitle style 5/13/10 THE...

This preview shows document pages 1 - 17. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online