Engineering Medical Optics BME136/251 Winter 2018 Monday/Wednesday 2:00-3:20 p.m. Beckman Laser Institute Library, MSTB 214 (lab) *1/17 UPDATE Wednesday, 1/17 Optics and Photonic Devices III: homework 1 handout Monday, 1/22 Laser Microscopy: homework 1 due Wednesday, 1/24 Optical Fibers: homework 2 handout Monday, 1/29 Endoscopy: homework 2 due Wednesday, 1/31 Midterm Monday, 2/5 OCT I Wednesday, 2/7 Spectroscopy I: homework 3 handout Monday, 2/12 Spectroscopy II: homework 3 due Wednesday, 2/14 Spectroscopy III Monday, 2/19 President s day Wednesday, 2/21 Tissue Optics I: homework 4 handout Monday, 2/26 Tissue Optics II: homework 4 due Wednesday, 2/28 Tissue Optics III: homework 5 handout Monday 3/5: homework 5 due
Photonic Devices: Detectors Photon Source Light -Tissue Interaction Photon Detector Optical System for Light Delivery and Collection
Optical Detection Transduction: convert radiant (optical) power into electrical signal Single element detectors Multichannel detectors Performance: Sensitivity, linearity, dynamic range, SNR f(λ), photon arrival rate, Quantum efficiency (η)
Transducer Characteristics Performance Responsivity: R(λ) Rms sig out X(voltage, current, charge)/rms incident radiant power Φ Sensitivity: Q(λ) Q(λ) = dx/dφ Spectral Response R(λ) vs. λ; Q(λ) vs. λ RMS = Peak/sqrt 2
Transfer Function Dynamic Range: total range of incident radiant power where transducer responds LDR= linear dynamic range, typically in power of 10 X(λ) Slope = sensitivity (Q = dx/d Φ) Responsivity (R = X/ Φ) at given incident power Φ (λ) Other X factors: temperature, bias voltage
Optical Responsivity and Quantum Efficiency The optical response of a photodetector is characterized by also quantum efficiency, η (and is related to responsivity) Quantum efficiency can be external, η ext or internal, η int. External is the number of electrons of current per incident photon. Internal is the number of electrons of current per absorbed photon Responsivity, R, is the photocurrent per unit incident power (amps per watt) R = I ph Φ inc = η exte!ω Proportional to QE
Transducer Temporal Response Stability: constancy of Q or R vs. time Degradation, hysteresis Response speed Time constant: τ = 1/2πf c f c = 3-db roll off frequency, i.e. at when R (Voltage) has fallen to 1/ 2 = 0.707 of max value for sinusoidal input (db=20log(v1/v2)) 3-db Power: when power has dropped to 50% of peak (db=10log(p1/p2)) Rise time: time for output to rise from 10-90% of final value for instantaneous increase in radiant power
Detector Limits Dark Signal Electrical output in absence of radiation Thermal + shot + Dark current (no light!) I V Noise Shot noise: randomness of photon arrival and electron excitation, charge carrier movement, voltage applied, current flows, Poisson Johnson-Nyquist (thermal) noise, P = kt f: Charge carrier movement, no voltage applied (dark), Gaussian NEP (noise equivalent power, Φ n ): rms radiant power (W) from sinusoidally modulated source that give rise to rms signal equal to rms dark noise (specify for f, BW, detector area, λ).
Detector Limits Detectivity, D: Measure of minimum detectability: D = 1/Φ n D* = normalized for area (cm 2 ) and BW (Hz) (=DA 1/2 ( f) 1/2 ; 10 10 (photodiode) -10 17 (PMT)
Photon Detectors Photobiological Human Eye Photoemissive Photomultiplier tubes (PMT) Multi-anode arrays Pn-junction devices Photodiodes, photovoltaics 1 and 2-d arrays
Photon Detectors: The Eye Pixels: ~6M Rods; ~120M cones Contrast range: static, spatial frequency dep: ~10 3 Dynamic Range ~10 6-9 (chemical, Purkinje) Min response time:~5-20 ms Scanning: Saccades (~20-200 ms) http://www.astro.virginia.edu/class/oconnell/astr130/human-eye.html https://commons.wikimedia.org/w/index.php?curid=25597295
Photon Detectors: The Eye R, B, G: retinal + photopsins Pupil Response Rhodopsin retinal + opsin Purkinje shift http://cnx.org/content/col11496/1.6/, Jun 19, 2013., CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=30148001
Photo Multiplier Tube (PMT) Photoemissive cathode, easily ionized: Sb-Cs; K-Cs-Sb Absorbs photons, emits electrons QE = #e- released/#photons absorbed; 0.2-0.4; f(λ) Cathode: neg. bias vs. anode, high voltage: 400-2500 V DC Dynode: electron multiplication/gain
PMT Voltage Divider Photoemissive cathode: ejects photoelectron Strikes dynode 1: 2-5 secondary electrons released (MgO, GaP) Electrons accelerated by field between dynodes; each releases 2-5 electrons Each dynode down chain biased ~+100V, multiplication until anode reached;
PMT Each photoelectron at first dynode produces large charge packet (several ns) at anode Gain: f(hv); ave. number of electrons per anode pulse ~ 10 4-10 7 E.g. for gain = 10 6 : 1.6 x 10-19 C/electron x 10 6 = 1.6 x 10-13 C For 5 ns pulse at anode, i = 32 µa during pulse (C = 1 amp-s) 2 modes of operation: average current, photon counting (# of anode pulses per unit time)
PMT Typical performance: (HV, gain dependent) D* 10 12-10 17 (dark current from photocathode) R(λ) 10-10 5 A/W Linearity: overall, 10 6 (varies w/ HV) Spectral range: 110-1000 nm Rise time: ~1-10 ns Output: current (pa-µa); counts (10-10 6 )
Impact of Photocathode
Photodiodes Absorption of light by pn-junction diode Promote electrons from valence to conduction band Form electron-hole pairs in depletion zone If rate of light-induced charge carrier production exceeds thermal, current proportional to incident radiant power
(Reminder) PN Junctions: LEDs, lasers, solar cells, photodiodes, diodes + V - - V + + V - P N P N P N I I I I I I V V V Forward-bias (LED/laser) Reverse-bias (photodetector) Solar cell
Si Photodiode Silicon lattice p layer: infuse with Group III elements, e.g. Ga, Al; n layer: Group V, e.g. P, Sb Best operation: reverse bias mode, no current flows. Photocurrent is linear and proportional to photon flux. High linear dynamic range
Photodiode Features Typical performance D* 10 10-10 12 (dark current from pn junction) R(λ) 0.05-0.5 A/W Linearity: 10 7 Spectral range: 250-1100 nm Rise time: 1-10 ns Output: current (na-µa)
Photodiodes Gain: Avalanche photodiode Fast response, high sensitivity APD operates in reverse breakdown region of pn junction (~10 5 V/cm) Internal gain of 10-1000 Avalanche where photo-induced e-hole pairs accelerated and create additional charge carriers Fast response times (high BW), high sensitivity, reduce linear dynamic range
Avalanche Photodiode (APD) Impact ionization causes an exponential growth of carriers in the depletion region. 1 impact ionization event à 2 electrons à 4 electrons à 8 http://clivetec.0catch.com/diodes.html
Avalanche Photodiode (APD) Gain vs Reverse Bias Voltage Hamamatsu S9251 Si APD Amplification is used to improve the signal-to-noise ratio However, noise is also amplified (including avalanche-related noise) Require high reverse bias voltages Temperature sensitive
Multichannel Detectors Photodiode Arrays Charge Coupled and Charge Injection Devices (CCDs) Analog output CMOS sensors Each pixel contains photodetector and amplifier Digital output Can be inexpensive when made in volume (cell phone cameras, webcams)
Charges generated by photons stored in MOS capacitors MOS initially reverse biased +V to metallic electrode Creates depletion region in Si below electrode (potential well) Photons produce e-hole pairs, electrons stored in wells (10 6 ) in proportion to I and int. time; 30 MP consumer cameras (>5k x 5k) in 35mm chip Charge shifted horizontally to Fast shift register, and down to readout preamp: serial row by row scan of stored charge
CCD features High full well capacity Pixels: ~2-20 µm, 512x512 to ~10kx10k Noise: Dark noise + read noise dominated by read noise, faster = noisier Frame transfer is noisiest Slow read rates and reduce noise by t 1/2 Dark noise f(temp): cool to reduce, can be 10s of electrons Blooming: overfill wells with electrons Linear Dyn. Range LDR = Full well capacity - Noise electrons Cooled, slow scan: 16-18 bit
Full color imaging Bayer mask Prism and three CCDs