Single Photon Counters

Figure 1: Photon Detection Probability as a function of Photon Wavelength is shown. The SPCM is sensitive to photons within the white shaded region only.

Features

• Low Dark Counts
  • SPCM20A(/M): 25 Hz (Typical)
  • SPCM50A(/M): 150 Hz (Typical)
• Two Detector Sizes
  • SPCM20A(/M): Ø20 µm Active Area
  • SPCM50A(/M): Ø50 µm Active Area
• Active Quenching
• Temperature Stabilized
• USB Interface
• Pulse Output
• TTL Gating/Trigger Input
• Compact Size: 68 mm x 85 mm x 25 mm

Applications

• Spectroscopy with Single Molecules
• Spectro-Photometrical Measurements
• Flow Cytometry
• Photon Correlation Spectroscopy
• Lidar

Thorlabs' Photon Counter Modules use a silicon avalanche photodiode to detect single photons. The SPCM counters are sensitive to photons emitted in the 350 to 900 nm range with the maximum sensitivity at 500 nm (see Fig. 1). They work by converting an incoming photon into a TTL pulse in the detector, which is counted by the internal 31-bit counter. An additional SMA connection offers a direct output pulse signal from the module that can be viewed on an oscilloscope or connected to an external counter. Please see the Tutorial tab for details about the functionality of this photon counter.

An integrated Peltier element stabilizes the diode's temperature below the ambient temperature to reduce the dark count rate. The two available models, SPCM20A and SPCM50A, have low typical dark count rates of 25 and 150 counts per second, respectively, which allows them to detect power levels down to 0.14 fW.

The active quenching circuit integrated into the diode of the SPCM enables high count rates. Its high speed allows users to count a photon every 35 - 45 ns, depending on the model chosen. The SPCM20A provides an active area of Ø20 µm and the SPCM50A offers Ø50 µm.

Software

The SPCM includes a software package with GUI for out-of-the-box operation. The following operating modes can be set by the software:

• Manual Mode for manual operation
• Free Running Timer Counter for counting incident photons for a certain number of "Time Bin Lengths"
• Externally Triggered Timer Counter for triggering the timer start for counting incident photons for a certain time period
• Externally Triggered Counter for starting and stopping the counter by an external trigger
• External Gating for activating the counter and the APD externally

For more details about the software and its operation modes, please see the Software tab.

Click to Enlarge

Software

The SPCM includes a software package with GUI for out-of-the-box operation. The following operating modes can be set by the software:

Manual Mode:
The counter is started and stopped manually by pressing the Start/Stop button (toggle function). The timer will be reset at each start.

Free Running Timer Counter:
Both the number of time bins (i.e, the number of measurements) as well as the minimum interval between two subsequent bins can be set.

Externally Triggered Timer Counter:
In this mode, the timer is started by an external trigger signal and counts incident photons during the set time bin length. The active trigger slope (rising or falling) can be selected.

Externally Triggered Counter:
In this mode, the external trigger signal will start and stop the counter.

External Gating:
The counter and the APD are activated externally.

Measurement Settings:
In the array mode, each data value is recorded to an array. In the continuous mode, the measurement is restarted after the preset number. Both modes can be saved as a .txt file. The measurement results can be represented as a bar (XY bar with counts vs. number of measurements), graph (curve), table (numeric) or alignment (numeric with additionally information) display. The number of measurements can be defined, and the measurements can be repeated.

Download Page for the latest version of Thorlabs software package for Single Photon Counter Modules:

• Software: Software package for Windows with driver and graphical user interface for operating the device in standard applications.

Download Page

Operating Principle of Single Photon Counters

Avalanche photodiodes operated in the Geiger Mode have the ability to detect single photons. This single photon sensitivity can be achieved by biasing the APD above the breakdown voltage (Point A in Fig. 1). The APD will remain in a metastable state until a photon arrives and generates an avalanche (Point B). This avalanche is quenched by an active quenching circuit inside the APD (Point C), which lowers the bias voltage below the breakdown voltage (labeled V_BR in Fig. 1).

Current Voltage Characteristics

Figure 1: Current Voltage Characteristics of an Avalanche Photodiode Operated in Geiger Mode

Afterwards the excess bias voltage can be restored. During this time, which is known as the pulse dead time of the diode, the APD is insensitive to any other incoming photons. Spontaneously triggered avalanches are possible while the diode is in a metastable state. If these spontaneous avalanches occur randomly, they are called dark counts. If the spontaneously triggered avalanches are correlated in time with a pulse caused by a photon, it is called an afterpulse. To block such afterpulses in the measurement, an additional pulse dead time can be set in the software, which will cause the internal counter of the SPCM to ignore all pulses occurring during this pulse dead time.

Definitions

Geiger Mode:
In this mode, the diode is operated slightly above the breakdown threshold voltage. Hence, a single electron-hole pair (generated by absorption of a photon or by a thermal fluctuation) can trigger a strong avalanche.

Dark Count Rate:
This is the average rate of registered counts in the absence of any incident light and determines the minimum count rate at which the signal is dominantly caused by real photons. The false detection events are mostly of thermal origin and can therefore be strongly suppressed by using a cooled detector.

Active Quenching occurs when a fast discriminator senses the steep onset of the avalanche current and quickly reduces the bias voltage so that it is below breakdown momentarily. The bias is then returned to a value above the breakdown voltage in preparation for detection of the next photon.

Dead Time is the time interval the detector spends in its recovery state. During this time, it is effectively blind to incoming photons. The dead time fraction, which is an inherent feature of an active quenching circuit, may be defined as the ratio of missed to incident events.

Afterpulsing:
During an avalanche, some charges can be trapped inside the high field region. When these charges are released, they can trigger an avalanche. These spurious events are called Afterpulses. The life of those trapped charges is on the order of a few tenths of a microsecond. Hence, it is likely that an afterpulse occurs directly after a signal pulse.