Single photon avalanche detector (SPAD) difficulties with photometer

In summary, the conversation discusses the development of a SPAD based photometer with specific features and limitations. The user has experienced issues with the SPAD going blind at high rates or continuous illumination and is looking for possible causes and solutions. They have also tested the single photon deadtime experimentally and found that the problem was due to operating too near the SPAD breakdown voltage. The photometer is being used for optical SETI purposes and the user is continuing to work on improving its sensitivity.
  • #1
Benschu
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TL;DR Summary
The SPAD and circuitry have been developed over a substantial time. However, the output drops to zero whenever a non-pulsed optical signal is applied.
I have been developing a SPAD based photometer having a 10 ns deadtime, near-zero dark counts, near-zero afterpulsing, and that is temperature independent (within a reasonable range). It works well with photon level pulsed optical signals, (10ns to several microsecond pulse widths at rates up to about 50 kHz), but at higher rates or whenever the illumination is continuous (at any level), the SPAD goes completely blind. Early on I've experienced the same issue with ordinary APDs. The ECL based detector circuitry is happy up to 30 MHz, so not believed to be a factor here.

There is, of course, difficulty observing the SPAD-detector junction because an extra picofarad or two throws measurements completely off.
I am concerned there may be something strange going on there, but at a loss as to how it might be checked. The bias resistors at the SPAD junction are carbon film and I plan to replace them with metal film resistors in the next day or so. Any ideas out there about possible causes for this limitation will be appreciated.
Ben
 
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  • #2
Benschu said:
whenever the illumination is continuous (at any level), the SPAD goes completely blind
What does that mean? A very dim continuous illumination should look like individual photons arriving at random times.

Did you test the single photon deadtime experimentally with two pulses separated by x ns?
 
  • #3
mfb said:
What does that mean? A very dim continuous illumination should look like individual photons arriving at random times.

Did you test the single photon deadtime experimentally with two pulses separated by x ns?
Hi mfb,
Yes, using short driving pulses (<15ns), an NIR LED doesn't begin emitting until nearly 15 ns. Also, it is not difficult to approximate the number of photons expected from an LED given the power, distance, beam divergence, efficiency, etc.
I have been using photomultipliers for years with routine single-photon detection sensitivity. SPADs and APDs a bit new to me. The problem with SPAD blindness at high count rates has been found. I have been trying to operate too near the SPAD breakdown voltage. So, that problem is mostly solved. Many compromises are required.

The use for these detectors is for optical SETI searching for pulsed laser signals. If one does all the calculations, it is likely that, for my telescope, a received signal with be less than 100 photons and maybe a lot less. So, the work continues.

Ben.
 

1. What is a single photon avalanche detector (SPAD)?

A single photon avalanche detector (SPAD) is a type of photodetector that is capable of detecting and measuring the arrival of individual photons. It operates by amplifying the signal from a single photon, allowing for extremely sensitive measurements of light intensity.

2. What are some common difficulties with SPADs?

One common difficulty with SPADs is their high dark count rate, which refers to the number of false detections of photons in the absence of actual light. This can lead to inaccurate measurements and a decrease in sensitivity. Another difficulty is their sensitivity to temperature changes, which can affect the performance of the detector.

3. How does a SPAD compare to other types of photodetectors?

SPADs have several advantages over other types of photodetectors, including their high sensitivity and fast response time. They are also compact and can be easily integrated into various systems. However, they have a limited dynamic range and are susceptible to noise, making them less suitable for certain applications.

4. What are some methods for reducing SPAD difficulties?

One method for reducing SPAD difficulties is to cool the detector to decrease the dark count rate. This can be achieved through thermoelectric cooling or liquid nitrogen cooling. Another method is to use a coincidence detection technique, where multiple SPADs are used to confirm the detection of a single photon, reducing the chances of false detections.

5. What are some potential applications of SPADs?

SPADs have a wide range of applications, including in quantum cryptography, fluorescence lifetime imaging, and time-of-flight measurements. They are also used in lidar systems for remote sensing and in medical imaging for better visualization of biological tissues. As technology advances, SPADs are being used in more diverse and innovative ways.

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