optical apd rx1

Description: Many types of LEDs exhibit gain effects similar to those of an avalanche photodiode when reverse-biased. This phenomenon has been observed in the voltage range of 25-300 volts, with the voltage, spectral response, and overall sensitivity/gain varying based on the LED's construction and material. Some circuits can be adapted for experimentation with LEDs to explore this gain mechanism. The extent of this effect and the voltage at which it occurs can vary significantly from one device to another, as LEDs are not designed to operate in this mode. The stability and long-term reliability of using LEDs in this manner should also be considered. Despite the variability in performance, certain LEDs can function effectively as gain-enhanced detectors in this mode, making them appealing for building optical transceivers that utilize a single diode for both transmission and reception. However, tests indicate that the sensitivity of a standard LED in this mode is generally inferior to that of a well-designed optical detector using standard PIN photodiodes, particularly for baseband (audio) applications. Nevertheless, the avalanche effect can be effectively utilized to create an optical transceiver that uses an LED for both transmission and detection, resulting in a compact package with reasonable performance. There is a lack of "ultimate sensitivity" comparisons between PIN photodiode-based receivers and avalanche LED receivers for subcarrier (ultrasonic and higher) operations. The circuits detailed on this page can serve as the foundation for an optical receiver that leverages the avalanche effects. For applications requiring higher bandwidth, albeit with reduced weak-signal sensitivity, a standard transimpedance amplifier circuit may be employed with either a genuine avalanche photodiode or a reverse-biased LED. An article discussing the avalanche gain effect in LEDs can be found linked at the bottom of this page under "Optical Receiver Operation With High Internal Gain of GaP and GaAsP/GaP Light-emitting Diodes." The Avalanche Photodiode (APD) functions by electrically responding to light that strikes its active surface through the photoelectric effect, converting varying light levels into corresponding electrical currents. Unlike standard photodiodes, which lack gain, APDs can amplify the signal due to their intrinsic ability to produce a cascade of electrons from a single photon event, with the number of electrons proportional to the APD's gain and the applied voltage. At low light levels, the currents generated by standard photodiodes can be exceedingly small, making them susceptible to noise from various sources, including thermal noise from connected circuits and noise from components within the circuitry. In such low-light conditions, these noise sources can easily mask the desired signal. The APD mitigates this issue through its internal amplification mechanism, allowing a single photon to generate multiple electrons. This amplification increases the output signal from the APD, enabling it to surpass much of the noise generated by subsequent components and amplifier stages.

The exploration of using LEDs as gain-enhanced detectors opens new avenues for optical communication systems. The unique characteristics of LEDs, when reverse-biased, can lead to innovative circuit designs that take advantage of their avalanche gain effects. In practical applications, the design of a transceiver utilizing these principles would require careful selection of LED types based on their voltage response and gain characteristics. Circuit designs may integrate feedback mechanisms to optimize performance, ensuring that the receiver remains sensitive to low-level signals while maintaining bandwidth requirements.

For implementation, the circuit may include a combination of passive and active components, such as resistors to limit current, capacitors for filtering, and operational amplifiers configured as transimpedance amplifiers to convert the photogenerated current back into a voltage signal. Additionally, attention should be paid to the layout and shielding of the circuit to minimize external noise interference, ensuring that the signal integrity is preserved. The versatility of LEDs in this context not only provides a compact solution for optical transceivers but also promotes the exploration of novel applications in sensing and data transmission, leveraging the unique properties of these devices in ways that were previously underutilized.As it turns out, many types of LEDs appear to exhibit gain effects akin to those of an avalanche photo diode when reverse-biased - an effect that has been observed in the 25-300 volt area with the voltage, spectral response, and overall sensitivity/gain depending on the LED`s construction and material. It is possible to adapt some of the circuits on this page to be used with an LED for experimentation of this gain mechanism. Not surprisingly, the degree to which this effect occurs and at what voltage tends to vary wildly from device to device as this is not a mode in which LEDs were intended to operate! The stability and long-term viability of operating an LED this way is another factor that should be considered.

While the results do vary, some LEDs have been noted to offer reasonable performance as gain-enhanced detectors when operated in this mode making it practical to have at least moderate performance as a detector - a particular attraction if one wants to build an optical transceiver that uses just one diode for both receive and transmit. While an interesting phenomenon, tests thusfar seem to indicate that, at least for baseband (audio) use, the ultimate sensitivity of a typical LED operated in this mode is worse than that of a well-designed optical detector using standard PIN photodiodes.

Having said this, this avalanche effect can be used to good effect to make an optical transceiver that uses the LED for both transmitting and detection and yield a single, compact package with reasonable performance. I`ve not seen "ultimate sensitivity" comparisons with PIN photodiode-based receivers and avalanche LED receivers for subcarrier (ultrasonic and up) operation.

The circuits described on this page can form the basis of an optical receiver utilizing the avalanche effects. If higher bandwidth is desired (with an acceptance of poorer weak-signal sensitivity) then a typical transimpedance amplifier circuit may be employed with either a "real" APD or a reverse-biased LED.

An article describing the use of the avalanche gain effect exhibited by LEDs may be found at the bottom of this page in the article "Optical Receiver Operation With High Internal Gain of GaP and GaAsP/GaP Light-emitting diodes" linked at the bottom of this page. The Avalanche Photo Diode (APD) is, as its name implies, a diode that electrically responds to light that impinges on its active surface using the photoelectric effect, converting varying amounts of light to correspondingly varying electrical current.

"Normal" photodiodes also do this, but have no "gain" in doing so - that is, it takes the action of more than a single photon to cause the movement of a single electron. As one might imagine, at very low light level - where there are relatively few photons - the currents produced by a standard photodiode can be very small indeed!

APDs have intrinsic to them the ability to effectively amplify the signal due to the fact that one "event" can loose a barrage (or "avalanche") of electrons, the number being related to the gain of the APD itself which is also related to the voltage applied to the APD. The tiny currents produced at very low light levels have to compete with currents (noise) from other sources such as the thermal noise from the circuits attached to it, noise contribution by components in that circuitry and other noise sources related to the diode and transistors themselves.

As one can imagine, at these very low light levels these other noise sources could easily dilute or drown out the desired signal! The APD helps combat the problem of these noise sources by having its own internal amplification mechanism meaning that, on average, a single photon can cause the movement of many electrons and by having this additional amplification, the signal coming out of the APD can be high enough to overcome much of the noise of the following components and amplifier stages.

As with any circuit, the