APD in Free Space Optics (FSO) Applications

Avalanche photodiode (APD) detectors are used in many diverse applications such as Free Space Optics, laser range finders and photon correlation studies.

Ideal for Low Light Detection to overcome thick Fog and other Fade effects

Free Space Optics (FSO) often operates in photon-scarce environments, and use low light detection technology to function reliably under challenging conditions like fog, dust storms, snow, rain and smog.

For low-light detection in the 200-1150 mm range, designers choose between the silicon PIN detector or the silicon avalanche photodiode (APD), with APDs preferred for their superior sped and sensitivity.

Silicon APDs cover the wavelength range 300-1700 nm, germanium APDs cover 800-1600 nm, and InGaAs APDs cover 900-1700 nm.

While InGaAs APDs are more expensive, they offer lower noise, extended spectral response to 1700 nm, and higher bandwidth. Germanium APDs are more cost-effective and better suited to high electromagnetic interference (EMI) environments where amplifier noise is significantly higher.

APD Performance for Free Space Optics/ Optical Wireless

Typical demonstrated APD performance includes:

• Detection of 100-photon, 20 ns pulses with standard APDs, and 10-photon, 20 ns pulses with special APDs.
• BER data communications at 810 nm at 60 Mbps with only 39 photons/bit.
• >70% photon-counting efficiency at 633 nm with dark counts of 1 count/s on 150 µm diameter APDs.
• Detection and resolution of low energy X-rays (1-30 keV).

An ideal APD would combine zero dark noise, no excess noise, broad spectral and frequency response, a gain range from 1 to 106 or more, and low cost- functioning like a PIN diode with gain. However, real-world designs must trade-off the conflicting design requirements listed below.

APDs include an absorption region A, and a multiplication region M, as shown in Figure 1. An electric field E across region A separated photo-generated holes and electrons, and sweeps one carrier towards region M. The multiplication region uses a high electric field to provide internal photo-current gain by impact ionisation. This gain region must be broad enough to provide a gain of at least 100 for silicon APDs and 10-40 for germanium or InGaAs APDs. The electric field must be strong enough for gain but below the diode’s breakdown limit.

Figure 1 shows the reach-through structure which offers the best available combination of high speed, low noise and capacitance and extended red response.

APD versus PIN diode Performance for FSO (Free Space Optics)

APDs differ from PIN photodiodes by providing internal photo-electronic signal gain. The output signal current is given by:

I = M * RO * PS

where:

• M = gain (this is a function of the reverse voltage, VR, and varies with applied bias)
• RO = intrinsic responsivity at gain M = 1
• PS = incident optical power

A typical gain-voltage curve for a silicon APD is shown in Figure 2.

A key parameter to consider when selecting an APD is the detector’s spectral noise. APDs operate in two noise-limited regimes: detector noise- limited at low power levels, or proton shot noise- limited at higher powers. As APDs operate under a reverse bias, sensitivity at low light levels is limited by shot noise and the APS;s leakage current. Shot noise arises from random fluctuations in dark current (or signal current). Unlike PIN diodes, the bulk leakage current is multiplied by the gain, M, of the APD. The avalanche process statistics also generate current fluctuations, so APD performance is degraded by an excess noise factor (F) compared to a PIN.

At higher signal light levels, an APD operates in the photon shot noise-limited regime, where sensitivity is constrained by photon shot noise from the optical signal’s current. The total noise in illuminated conditions is the sum of the detector noise and the signal shot noise.

Signal to Noise Radio (SNR) of APD for FSO

While APDs generally have a lower signal-to-noise ratio (SNR) than PIN detectors with equal quantum efficiency due to internal noise, they can still achieve better overall system SNR in low-light conditions by amplifying weak signals without significantly increasing system noise.

Noise Equivalent Power (NEP) alone is not sufficient for comparing detectors. Instead, SNR at a specific wavelength and bandwidth is a more accurate metric. Optimum SNR occurs when the total detector noise equals the amplifier or load resistor’s input noise. The optimum gain depends on the excess noise factor (F) of the APD, with M = 100 to 1000 for silicon APDs and limited to M = 30 to 40 for germanium and InGaAs APDs.

Summary: Optical Receivers for Free Space Optics/ Laser Links

In summary, an optical receiver using an APD offers superior sensitivity than PIN at medium-to-high bandwidths. APD receivers offer maximum range and link availability/reliability especially in poor weather conditions. A choice of Silicon, Germanium or InGaAs offers operation at a variety of wavelengths used in Free Space Optics.

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