Photodetectors Essay

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Optical detectors are components that convert incoming optical energy into electrical signals. Many photonics applications, such as monitoring laser power levels and laser metrology, require the usage of these detectors.1-3 Although there are many different types of optical detectors available covering parts of the electromagnetic spectrum: the ultraviolet, visible and infrared, optical detectors are divided into two main categories: photon and thermal detectors. Photon detectors generate one electron for an incoming photon, which is detected by an electronic circuit, while thermal detectors convert optical energy to heat, which then generates an electrical signal.3
Herein, two types of photodetectors are examined: a PD and a PMT. Although
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The sensitivity, S, is defined as the measure of the average current, I, generated over the input power, P, as shown in (1).

S=I/P (1)

Shot noise is a type of noise that appears in photon counting optical devices as a consequence of the particle nature of light. Shot noise results from fluctuations in the packet size of photon energy. Schottky’s formula (2) expresses the root mean square (RMS) shot noise current:

I_N=〖(2geIB)〗^□(1⁄2) (2)

in which I is the average current at the anode, g is the gain of the PMT, and B is the electrical bandwidth of the detection circuit. Gain is a measure of the signal amplification and it is expressed as the ratio of charge q to electron charge e (3). g=q/e (3)

Gain can also be determined from (4):

I=ηϕeg (4)

where η is the quantum efficiency and ϕ is the photon flux. For monochromatic light, the photon flux is given by
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Schematic of PMT Setup.

Figure 3. Addition of an External Capacitor in Parallel with PMT Detector Circuit.
To investigate shot noise dependence of the PMT, measurements were taken in the absence of a capacitor as well as in the presence of three different capacitors CDM33±5%, CDM180±5%, and CDM510±5% pF. To vary the bandwidth, the capacitors were introduced in the system as presented in Figure 3. The control voltage was set at 0.700 V. The shot noise for the three capacitors and without capacitor was measured at five different optical power levels achieved with the aid of ND filters of 6.5, 6.7, 6.9, 7.1 and 7.3 OD.
Gain measurements were taken by recording dark current pulses on the digital oscilloscope while the PMT was covered. This was done five times without capacitor in line at control voltages of 0.750 V and 0.850 V, five times with the CDM100±5% capacitor at control voltages of 0.750, 0.850, and 0.950 V, and five times with the CDM180±5% capacitor at control voltages of 0.750 and 0.850

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