Photodetectors — Key Components of Fiber Optic Sensors

In Fiber Optic Sensors, photodetectors are the key devices that convert optical signals into electrical signals. This article briefly introduces the main characteristics and selection principles of several commonly used photodetectors.

1. Semiconductor photodetectors

HN photodiodes, avalanche photodiodes (APD), PIN-FET miniature modules and others are commonly used photodetectors in fiber optic sensors. Although their structures differ, the detection process is essentially the same: incident photons generate electron-hole pairs, which are separated by a strong electric field and produce a photocurrent. Thus, they function as photon counters.

Silicon photodiodes are inexpensive, perform well (quantum efficiency up to 90% or higher, response time below 1 ns, dark current in the 10^-10 A range), and are convenient to use, making them ideal photodetectors. Their best response wavelength is near 0.8–0.9 μm (the operating band of GaAs LD and LEDs). Alloy photodiodes suit the near-infrared band. For longer wavelengths, Ge photodiodes or III-V ternary/quaternary compounds may be used. Main noise sources include quantum noise, dark current noise, surface leakage noise and leakage current noise; the intrinsic fluctuation of photogenerated carriers limits ultimate sensitivity.

Avalanche photodiodes (APD) provide internal gain that increases system sensitivity, typically by 10 to 100 times and in some cases up to 1000 times. Their bias circuits require temperature drift compensation because gain strongly depends on temperature. Gain is also sensitive to the electric field, so the bias must remain stable, often within tens of millivolts. Avalanche gain is stochastic and produces excess noise (characterized by the noise factor F(M)=M2/M^2). Reach-through APDs have low excess noise; for example, with gain of 100 the excess noise factor can be about 5. Response time is around 1 ns and dark current can be near 10^-11 A. Si-APDs can approach high quantum efficiency with modest excess noise. Ge-APDs operate in the 1.2–1.6 μm band but may exhibit higher excess noise at typical gains.

PIN-FET miniature modules combine a small-area low-capacitance diode with a high-output-impedance FET preamplifier. Their low input capacitance and high input impedance reduce thermal noise and are advantageous for longer wavelength detection.

2. Photomultiplier tubes

Photomultiplier tubes are the most sensitive photodetectors and can detect single photons. Unlike APDs, gain in photomultipliers is well controlled and does not introduce excessive excess noise. The spectral response from near infrared to near ultraviolet depends chiefly on the photocathode material.

3. Selection principles for photodetectors

Key criteria for choosing a photodetector include achievable signal strength, background light level, and the required signal-to-noise ratio. If the required SNR is high, the detector should meet these conditions:

1. High sensitivity within the operating wavelength band;

2. Minimal detector-introduced noise, so choose devices with low dark current, leakage and parallel conductance;

3. Good stability and high reliability.

Silicon photodiodes have a small temperature coefficient and are relatively reliable and stable. APDs and photomultiplier tubes have gains that vary with bias and temperature; APDs require highly stable bias supplies and temperature compensation. The detector should also be compact, easy to assemble and couple to optical fibers, require reasonable bias/offset currents, and be cost effective.

Atonm fiber optic sensors are easy to install and operate.

DIN rail mounting

1. Align the DIN rail as shown in the figure; push the slot at the bottom of the unit in the direction of arrow 1 while pushing in the direction of arrow 2.

2. To remove the sensor, push the unit forward and lift up in the direction of arrow 3.