Detection Questions & Answers

What’s an MPPC? Is it similar to an SiPM?

MPPC stands for Multi-Pixel Photon Counter, the product family that includes our SiPMs. Since we believe the future of Geiger-mode gain detectors includes more than just silicon-based devices, we’ve adopted the term MPPC to cover all the various materials (like InGaAs) that can be used to catch those shifty little photons!

What’s the maximum amount of light that I can put into my MPPC, photodiode, or APD?

If your detector is or has the potential to be exposed to strong light, try to keep the detector’s average current below 100 mA. The detector will be non-linear well before this current range, and exceeding it may cause damage—especially in devices with a wirebond connection. While the detector won’t explode or disintegrate (well, maybe disintegrate if it’s reaaaaalllyyy strong laser light), the wirebond will disconnect and create an open circuit. This destroys a detector’s ability to send current outside of its package.

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.

When looking at your PMTs, I see that there are some “P” types and regular types. What’s the difference?

Well, I’m glad you asked. The P (like in R1924P) stands for “photon counting” type. When producing PMTs, you can get a typical production distribution for a number of different PMT characteristics. Two characteristics that are specifically important for photon counting applications (where you use a photon counting circuit and a counter) are gain and dark counts. The “P” type PMTs are selected out of the distribution for higher than average gain and lower than average dark counts. The high gain helps you set your threshold level for the lower level discriminator in the photon counting circuit. The low dark counts help you achieve good signal-to-noise ratio (SNR) even at low photon levels (see the equation in a previous Q&A). So really, the difference is that “P” types are selected out of the distribution for good photon counting characteristics.

Thanks for the info. What about the PMT modules? I heard that there’s a P type module and even a PA type module. What’s the deal?

We have a few different types of PMT modules.

  • Current Output Type: This is a PMT and a voltage divider circuit with a high voltage transformer. So it only needs low voltage (5-15 V) for operation. These can come in “P” type where the PMT inside is selected for photon counting as mentioned above. These can be used with a photon counting circuit.
  • Voltage Output Type: These modules are the same as above and also have a current-to-voltage amplifier. Because there’s an amplifier already built-in and you need specific amplifiers for photon counting, we typically do not have “P” designations for them.
  • Photon Counting Type: These PMT modules are the whole kit and caboodle! They have the PMT (P Type) with the voltage divider socket and the photon counting circuit. The output of these modules are logic pulses which can go right into the counter! The added bonus is that the gain/voltage of the PMT is already set for perfect photon counting. No adjustment is needed; you can just start counting!

Wait a minute, what about the PA type I hear so much about?

Man, you’re sharp! For some modules, specifically modules with a GaAsP/GaAs photocathode, we have what’s called “PA” type. The P still stands for photon counting as above. The extra A stands for AWESOME. Just kidding, the extra A is a designation that the protection circuit in the module has been changed. The protection circuit monitors the average signal current from the PMT. The GaAsP/GaAs photocathodes are particularly sensitive to high light levels. The protection circuit is put in place so that when the signal output current reaches a specific value (which relates to the amount of light on the photocathode), the PMT voltage shuts down or an alarm goes off. However, we find for some applications (like multiphoton microscopy), it is OK for there to be sudden (and few) high peaks of current. In these cases, we increase the protection circuit current value of the PMT module from 2 uA to 50 uA. We do this because while some sudden pulses may be high, the average current is still low, and therefore the PMT is still safe. So the H7422PA-40, for example, is a photon counting PMT module with an increased protection circuit from 2 uA to 50 uA.

Also, these modules with protection circuits can be modified to not include the protection circuit at all. Please inquire with our engineers to learn more about these options!

I see Hamamatsu offers associated components for photomultiplier tubes (PMTs). In particular, what is the difference between D-type and DP-type socket assemblies for PMTs?

D-type sockets contain a resistive divider network needed to distribute the applied high voltage across the PMT dynodes. DP-type sockets also contain this same resistive divider network, in addition to a high voltage power supply built into the socket that transforms low voltage to the high voltage required for the PMT. This allows users to operate the PMT using 5 or 15 volts depending on the model.

Can I use any socket assembly with my PMT?

Not every socket is the same! Each PMT has corresponding compatible socket assemblies. The matching socket assembly, if available, for any PMT can be found listed in our photomultiplier tubes catalog. Also, each PMT may have multiple socket assemblies available for a desired characteristic: for example, the divider may be different for high pulse linearity vs. DC linearity, etc. If you need any help selecting, please feel free to give us a call.

OK, so Hamamatsu offers PMTs and also corresponding sockets to pair with them. Are there any offerings that include a PMT and these associated components all in one package?

Why yes! Hamamatsu also offers PMT assemblies and modules. A PMT assembly integrates both a Hamamatsu PMT and resistive divider network (D-type socket assembly) into one package. They can often come with HA coating, magnetic shielding, or cases or connectors which house the PMT and socket assembly together.

All PMT modules will contain, at least, a PMT, resistive divider network, and high voltage power supply in one package. In addition, there are some module offerings that may also include amplifiers, thermoelectric cooling, and PC interfacing.

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

Dino Butron is an Applications Engineer, specializing in all of our point detectors and signal-to-noise simulations. He and his photonic sidekick (his cat Bash) tackle technical issues daily.

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

Kate Pritchard has gone where no applications engineer has gone managing the newly formed University Support Group. The group is dedicated to supporting our research and academic customers through technical support, collaborations, and face time. Traveling at the speed of light to explore strange and exciting new worlds for Hamamatsu! (See what I did there...)

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