Optical detectors fall into two classes, thermal detectors and quantum detectors. Thermal detection devices include thermocouples, bolometers, and pyroelectric detectors, all of which measure the heat transferred to the detector by the light absorbed by the detector. Some temperature-sensitive quantity, such as voltage, current, or resistance is monitored.
Quantum detectors include photodiodes, photoconductors, phototransistors, charge-coupled detectors (CCDs), and photo-multiplier tubes. Quantum detectors measure the rate at which individual photons interact with the detector.
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The photodiode (PD) is a variation on the familiar silicon photocell used in solar power conversion. But the photodiode is constructed and operated for optimum sensitivity and accuracy, while the solar cell is optimized to generate maximum electrical power. The photodiode is an ordinary diode in which the diode junction is exposed to light. Photons with sufficient energy generate free electron-hole pairs in the diode junction, or "depletion layer". The free electrons travel through the N-layer and the holes move through the P-layer, thus generating a backward photocurrent. |
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| The PD can be connected directly to an ammeter, permitting the most sensitive measurement of the light power. The diode can also be reverse biased to measure higher powers at faster speeds. |
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In either mode of operation, the photocurrent I is proportional to the power P of the light illuminating the photodiode. I = KPD P, where KPD is the responsivity of the photodiode. KPD depends on the diode construction, the wavelength of the light, and on the temperature. The photodiode is a quantum device. Almost every incident photon generates an electron-hole charge pair. We therefore have I = e N Q, where Q is the quantum efficiency and e is the magnitude of the electron's charge. Q is less than or equal to 100 %. The optical power is equal to the number of photons per second, N, times the energy per photon, E = hc/λ. P = N E = Nhc/λ The PD responsivity therefore is given by KPD = Qeλ/(hc). The responsivity of a typical Hamamatsu model S2386 silicon photodiode is shown on the right. The value of the responsivity is KPD = 0.43 A/W at 632.8 nm wavelength. The quantum efficiency at 632.8 nm is Q = KPDE /e = 84 % . Links:
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| The photomultiplier tube (PMT) also responds to individual photons, but it has built-in amplification which delivers thousands to millions of electrons to the measuring circuit for each photon detected. Because of this amplification, PMTs are suitable for measuring low light levels and for "photon counting" in which photons are recorded one at a time as individual current pulses. |
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A photon absorbed by the photocathode can liberate a free electrons into the vacuum tube, via the photoelectric effect. The quantum efficiency Q of a PMT is usually 25% or less, so not every photon liberates an electron. Since the photocathode is biased with a large negative voltage, HV = 500–2000 V, the electron is accelerated toward a second electrode called a "dynode." The electron gains speed and therefore kinetic energy while traveling toward the first dynode. It strikes the first dynode hard enough to generate about 10 new free electrons, which are accelerated to the second dynode, where each of these electrons generates 10 more electrons, and so on. The final electrode, the anode, collects the electrons and delivers them to an external circuit for measurement. In a nine-dynode PMT the ten-fold multiplication by each dynode delivers one billion electrons to the external circuit for each photon detected. |
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The voltage divider network shown in the figure on the right carries a steady current (–HV/Rtot) which supplies the electrons for multiplication at each dynode. The desired signal is the current generated at the anode, I = NQeG where N is the number of photons hitting the photocathode per second, Q is the cathode quantum efficiency and G is the photomultiplier gain, which is 109 in the example above. The minimum detectable light level is much lower for the PMT than for the PD. The output anode current of the PMT an be measured directly with a pico-ammeter, or it can be determined by measuring the voltage across a load resistor connected from the anode to ground. Link:
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