ECEG 287 Optical Detection Course Notes Part 10

ECEG 287 Optical Detection Course Notes Part 10: More on the Detector as a Circuit Element Profs. Charles A. Di. Marzio and Stephen W. Mc. Knight Northeastern University, Spring 2004 February 2004 Charles A. Di. Marzio, Northeastern University 1

Topics • PMT Dynode Circuits – Conventional Voltage Divider – Cockroft-Walton • Dynode Modulation – Application – Technique February 2004 Charles A. Di. Marzio, Northeastern University 2

PMT Concept February 2004 Charles A. Di. Marzio, Northeastern University 3

PMT Dynode Chain Currents (1) Assumption is made that all V/R are much larger that corresponding Gic. V 0=-HV February 2004 ic G 12 ic G 13 ic G 14 ic R 01 R 12 R 23 R 34 V 3 V 1 V 2 Charles A. Di. Marzio, Northeastern University 4

PMT Dynode Chain Currents (2) At high cathode currents, higher dynode currents modify dynode voltages. V 0=-HV February 2004 ic G 12 ic G 13 ic G 14 ic R 01 R 12 R 23 R 34 V 3 V 1 V 2 Charles A. Di. Marzio, Northeastern University 5

Cockroft-Walton Divider Emitter-follower-like circuit maintains the dynode voltages without high power consumption. February 2004 Charles A. Di. Marzio, Northeastern University 6

DOT: Using Modulated Light Demod Laser P P t February 2004 t Charles A. Di. Marzio, Northeastern University 7

Physical Reason for Dispersion Imaginary part of diffusive wavenumber increases with frequency Easy to understand in terms of m 100574 a. mmultiple paths. February 2004 Charles A. Di. Marzio, Northeastern University 8

DOT Instrumentation at MGH TECHNOLOGY • Near-infrared light • Fiber optics • Computed Tomography ADVANTAGES • Optical contrast • Portable - bedside, ambulance • Continuous • Inexpensive • DISADVANTAGES • Resolution From David A. Boas - MGH NMR Center • Depth penetration Charles A. Di. Marzio, Northeastern University February 2004 9

Functional Imaging of a Neonate 4 cm 6 cm Mid-line At Rest Passive movement of right arm From David A. Boas - MGH NMR Center February 2004 Detectors Sources Charles A. Di. Marzio, Northeastern University Data Set I - 98 -05 -14 10

Demodulation Techniques 300. 00 MHz 10 k. Hz Ref DC Out Transmitter 10 k. Hz Sig LO Laser 300. 01 MHz 300 MHz Sig P P t February 2004 Charles A. Di. Marzio, Northeastern University 11 t

PMT Time Delay and Bandwidth • Dynode Paths • Transit Time • Variations February 2004 Charles A. Di. Marzio, Northeastern University 12

Long and Short Paths in PMT February 2004 Charles A. Di. Marzio, Northeastern University 13

Mixing at the Dynode ic V 0=-HV R 01 G 12 ic G 13 ic G 14 ic R 12 R 23 R 34 V 3 V 1 V 2 February 2004 Charles A. Di. Marzio, Northeastern University t 14

G 13 , Two-Stage Gain PMT Dynode Modulation Example 10 8 6 4 2 0 -900 February 2004 Operating Point m 10464_10 a. m -850 -800 -750 V 2 , Dynode Voltage, Volts Charles A. Di. Marzio, Northeastern University -700 15