APD in Free Space Optics (FSO) Applications
Avalanche photodiode 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) is almost always a photon-scarce application, requiring low light detection technology – to ensure link availability and enhance range. FSO/Laser Links have to operate in variety of “Atmospheric Fade” conditions including Fog, Dust Storms, Snow, Ice, Rain and Smog.
For low-light detection in the 200 to 1150 nm range, the designer has two basic detector choices – the silicon PIN detector, or the silicon avalanche photodiode (APD). APDs are widely used in instrumentation and aerospace applications, offering a combination of high speed and high sensitivity unmatched by PIN detectors.
Silicon Avalanche photodiodes are commercially available that span the wavelength range from 300 to 1700 nm. Silicon APDs can be used between 300 to 1100 nm, germanium between 800 and 1600 nm and InGaAs from 900 to 1700 nm.
Although significantly more expensive than germanium APDs, InGaAs APDs are typically available with significantly lower noise current, exhibit extended spectral response to 1700 nm, and provide higher frequency bandwidth for a given active area. A germanium APD is recommended for environmental applications in high electromagnetic interference (EMI), where amplifier noise is significantly higher than the noise from an InGaAs APD, or for applications where cost is primordial consideration.
APD Performance for Free Space Optics / Optical Wireless
Typical demonstrated APD performance includes:
Noise equivalent power (NEP) of Detection of 100-photon, 20 ns pulses with standard APDs
Detection of 10-photon, 20 ns pulses with special APDs
BER data communications at 810 nm at 60 Mb/s with only 39 photons/bit
Photon-counting detection efficiencies > 70% at 633 nm; dark counts of only 1 count/s on 150 µm diameter APDs
Detection and resolution of low energy, 1 to 30 keV X-rays
In order to understand why more than one APD structure exists, it is important to appreciate the design trade-offs that must be accommodated by the APD designer. The ideal APD would have zero dark noise, no excess noise, broad spectral and frequency response, a gain range from 1 to 106 or more, and low cost. More simply, an ideal APD would be a good PIN photodiode with gain! In reality however, this is difficult to achieve because of the need to trade-off conflicting design requirements. What some of these trade-offs are, and how they are optimized in commercially available APDs, are listed below.
Consider the schematic cross-section for a typical APD structure shown in Figure 1. The basic structural elements provided by the APD designer include an absorption region A, and a multiplication region M. Present across region A is an electric field E that serves to separate the photo-generated holes and electrons, and sweeps one carrier towards the multiplication region. The multiplication region M is designed to exhibit a high electric field so as to provide internal photo-current gain by impact ionization. This gain region must be broad enough to provide a useful gain, M, of at least 100 for silicon APDs, or 10-40 for germanium or InGaAs APDs. In addition, the multiplying electric field profile must enable effective gain to be achieved at at field strength below the breakdown field of the diode.
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)
An APD differs from a PIN photodiode by providing internal photo-electronic signal gain. Therefore, output signal current from and APD equals M * RO* PS, where RO is the intrinsic responsivity of the APD at a gain M=1 and wavelength l, M is the gain of the APD, and PS is the incident optical power. The gain is a function of the APDs reverse voltage, VR, and will vary with applied bias. A typical gain-voltage curve for a silicon APD is shown in Figure 2.
One of the key parameters to consider when selecting an APD is the detector’s spectral noise. Like other detectors, and APD will normally be operating in one of two noise-limited detection regimes; either detector noise limited at low power levels, or photon shot noise limited at higher powers. As an APD is designed to be operated under a reverse bias, sensitivity at low light levels will be limited by the shot noise and the APDs leakage current. Shot noise derives from the random statistical fluctuations of the dark current, (or signal current). Dark current or shot noise in a PIN differs for an APD however, as bulk leakage current is multiplied by the gain, M, of the APD.
In addition, the avalanche process statistics generate current fluctuations, and APD performance is degraded by an excess noise factor (F) compared to a PIN.
At higher signal light levels, the detector transitions to the photon shot noise limited regime where sensitivity is limited by photon shot noise on the current generated by the optical signal. Total noise from the APD in illuminated conditions equals the sum of the detector noise plus the signal shot noise.
Signal to Noise Radio (SNR) of APD for FSO
In the absence of other noise sources, an APD therefore provides a signal-to-noise ratio (SNR) which is worse than a PIN detector with the same quantum efficiency. An APD, however, can produce a better overall system signal-to-noise ratio than a PIN detector in cases where the APD internal gain boosts the signal level without dramatically affecting the overall system noise.
Noise equivalent power (NEP) cannot be used as the only measure of a detector’s relative performance, but rather detector signal-to-noise (SNR) at a specific wavelength and bandwidth should be used to determine the optimum detector type for a given application. Note that optimum signal-to-noise occurs at a gain M where total detector noise equals the input noise of the amplifier or load resistor. The optimum gain depends in part on the excess noise factor, F, of the APD, and ranges from M=100 to 1000 for silicon APDs and is 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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