Detection Questions & Answers

Which detector gives me the best signal-to-noise ratio (SNR)?

Which photodetector has the best SNR varies greatly with light level, speed, and support electronics. It’s usually best to determine the light level and which noise source limits you: your signal shot noise, your electronics/amplifier noise, or your detector’s dark noise.

For high light levels where amplifier noise is negligible and you are signal shot noise limited, detectors with the highest quantum efficiency (QE) will have the best SNR.

For lower light levels where the readout or amplifier noise dominates, detectors with high internal gain are required for good SNR. The detector’s internal gain reduces the importance of the amplifier noise, so you are then limited by dark or signal shot noise.

For the lowest light levels, detectors with the lowest dark noise will have the best SNR.

If you’re not sure, we can help! Please contact us to get an SNR calculation for your unique situation.

For more info about choosing a detector, read our Guide to detector selection article.

What is photon counting?

Photon counting is being able to measure and observe a single incident photon. It is typically used with a photon counting specific circuit that discriminates electronics noise out from the observation and removes the excess noise factor, which is noise from the detector’s intrinsic gain mechanism. Using this circuit and because the light levels are so low, we are typically limited by the dark noise of the detector.

What detector options are available for photon counting?

For photon counting, detectors with intrinsic gain are necessary. A commonly used detector is a photomultiplier tube, which uses dynodes to provide intrinsic gain to the detector. Photodiodes do not have intrinsic gain which means 1 incident photon will, at most, allow the flow of 1 electron, which is too small of a signal to overcome the noise. Avalanche photodiodes (APD) have intrinsic gain, but the gain is too small to overcome noise in most cases, unless the APD is run in Geiger mode. Single-photon avalanche diode (SPAD) and Multi-Pixel Photon Counters (MPPC) are based on APDs in Geiger mode, so these two types of detectors have enough intrinsic gain to show single photon pulses.

How can I improve my signal-to-noise ratio (SNR) if I am barely able to differentiate my signal from noise in an MPPC?

If you are limited by the dark noise, it benefits you to try and decrease the dark noise as much as you can to see if you can detect your single photons. One improvement can be using a cooled detector. Examples of a cooled MPPC are the S13362-3050DG and S13362-1350DG. Cooling will help reduce the number of dark counts and dark noise at the cost of needing more power to drive the detector with cooling. If a module is preferred, we do have a variety of cooled modules including the new C14455-GA and C14456-GA series, which have peak sensitivities at 600 nm. If cooling is not preferable for the MPPC and other aspects of the signal/noise cannot be changed, switching the detector family to PMTs may be preferred since PMTs have lower dark counts per unit active area. Please contact Hamamatsu’s Technical Support team if you would like additional information about the tradeoffs and options.

How else can I improve my SNR when I’m photon counting?

Increasing the measurement time can improve your SNR by a factor of the square root of your integration time. Meaning if your current measurements is only 1 second and now you measure the number of signal counts and dark counts for 4 seconds, you will approximately double your SNR!

Here’s the equation for photon counting SNR:

SNR=Detected counts - Dark counts * Measurement TimeDetected counts+2 *Dark Counts

For more information, view a presentation about low light detection.

What is an APD?

APD is short for avalanche photodiode. Similar to other photodetectors, an APD converts light energy (photons) into an electrical current. A silicon APD has spectral sensitivity from the UV to NIR range, and InGaAs APDs extend into the NIR range. An APD is basically a PIN photodiode with the advantage of providing gain. Gain is achieved through a process called impact ionization or the avalanche effect. Applying a large reverse voltage (100 to 500 V) to the APD creates a strong electric field across the PN junction (depletion region) of the APD. The electric field will accelerate electron-hole pairs from within the structure of the APD toward the depletion region. Upon collision with the lattice structure in the depletion region, additional electron-hole pairs are created. The typical gain for an APD is 40 to 100.

What are the advantages and disadvantages of an APD?

Advantages:

  1. The APD has very high quantum efficiency, so it’s highly sensitive. This feature is advantageous for detecting low light levels.
  2. The APD is a high-speed device, meaning it has a very fast rise time on the order of 150 ps.

Disadvantages:

  1. To maximize the gain of the APD, a relatively high reverse voltage is required. The range of the reverse voltage (bias voltage) is from 100 to 500 V.
  2. The APD is more sensitive to changes in temperature compared to other photodetectors. An increase in temperature will cause a decrease in gain. The parameter used to reference temperature sensitivity is called “temperature coefficient of breakdown voltage,” and its units are given in V/degrees C.

What are the different types of silicon APDs offered by Hamamatsu?

Short wavelength type:
This type of APD has a peak response near 600 nm. This group is broken down into two additional categories: low bias operation type and low terminal capacitance type. The low bias type APDs require bias voltages that are 300 V less than the low terminal capacitance type. The low terminal capacitance types are made for higher speed of operation. If the capacitance of the APD decreases, the rise time also decreases (frequency response increases). The rise time will increase if the effective photosensitive area increases. Typical rise time values for the low capacitance type range from 0.5 to 32 ns.

Near-infrared type:
There are 4 types of near-infrared (NIR) APDs: low bias, low capacitance, 900 nm band/low capacitance, and TE-cooled. The low bias and low capacitance types have similar characteristics as the short wavelength types. The 900 nm band/low capacitance type offers sensitivity near 900 nm along with a high-speed response. The applications for this APD include optical rangefinders and automotive LiDAR. The TE-cooled type APD has a built-in TE-cooler, which keeps the ambient temperature constant. This feature helps to maintain gain stability.

If you’ve got a technical question you’d like to see answered on this page, email us.

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Meet the engineers

Eric Mesa is an Applications Engineer out of NJ, who understands the intricacies of signal-to-noise comparisons. He likes baseball and most other sports, and when it comes to detector selection, he always hits a home run!

Neil Patel enjoys the majesty of narwhals and photons. He glides through technical issues just as the unicorn of the sea glides through the water. Fun fact: The narwhal’s tusk is actually a protruding tooth.

Peter Lopez is an Applications Engineer in Hamamatsu’s San Jose, CA, office. He received his Electrical Engineering degree from the University of Michigan, and he has worked in many different industries including semiconductor, infrared detection, microscopy, and medical devices. His past positions have provided a good background for the many different types of markets served by Hamamatsu’s component products. Although originally from Michigan, he’s enjoyed living and working in California for the past 25 years. In his spare time, he enjoys sailing, playing golf, and attending San Jose Sharks hockey games.

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