Application Cases

Experiment Title: Study on the Electroluminescence Radiation Characteristics of Avalanche Photodiodes

Testing Purpose:Avalanche photodiodes (APDs) are high-speed, highly sensitive diodes. Compared to conventional photomultiplier tubes, APDs have high quantum efficiency and stability, and they are highly adaptable to external environments. Therefore, APDs have been widely used in weak light field measurements and photon counting statistics.

Testing Equipment:Voltage amplifier, single-photon detector, digital oscilloscope, monochromator, oscilloscope, computer, etc.

Experimental Process:

Figure 1: Experimental Setup

The experimental setup is shown in Figure 1. A single-photon detector was used to record the photons radiated by the APD. The reverse operating voltage applied to the APD was provided by a voltage amplifier. The output signal of the voltage amplifier was controlled by an external trigger signal. When a square wave signal was used to trigger the voltage amplifier, the amplifier would output a high-voltage square wave signal to provide the reverse operating voltage for the APD. A digital oscilloscope was used to monitor the operating current of the APD. Lens L1 was used to collect the radiated photons to improve collection efficiency. The collected light was filtered by a monochromator with a grating of 120 grooves/mm. The filtered light was focused onto the SPCM (Single-Photon Counting Module) by lens L2. The logic pulses output by the SPCM were collected by a digital counter. To reduce the influence of ambient light on the measurement results, both the APD and the SPCM were placed in a light-shielding device. A computer was used to control the wavelength selection of the monochromator and communicate with the SR620.

Experimental Results:

Figure 2: Experimental Results

When a periodic reverse voltage was applied to the APD, the transient characteristics of photon radiation from the APD could be observed. To improve collection efficiency, the APD and the SPCM were placed face-to-face with their photosensitive areas as close as possible. The operating current of the APD and the count values of the SPCM were recorded simultaneously, and the results are shown in Figure 2.

The period of the high-voltage square wave signal was 100 s. In Figure 2(a), the low level was 124 V, which was below the avalanche voltage of the APD. At this voltage, the APD was not operating, and its operating current was almost zero. The count value of the SPCM corresponded to the dark count. This period corresponds to the curves before the rising edge and after the falling edge in Figure 2. The high level in Figure 2(a) was 176 V, slightly above the avalanche voltage of the APD. This voltage was sufficient to trigger the APD to operate in the avalanche state, resulting in a significant increase in the count values output by the SPCM. As the voltage increased, the count values of the SPCM increased instantaneously and reached a maximum value. Subsequently, the count values gradually decreased and stabilized. The transient curve of photon radiation could be well fitted by an exponential decay curve with a time constant of τ = 5.15 s. The sampling interval T in Figure 2(d) was 0.216 s, and the sampling duration was 0.1 s. Figure 2(c) shows the operating current of the APD, which had almost the same variation trend as the count values of the SPCM. During this process, the instantaneous maximum value of the SPCM was 759 Kcps, which was 2.4 times the stable count value (314 Kcps); the instantaneous maximum current of the APD was 10.24 mA, which was 3.9 times the stable value (2.64 mA). By comparison, it can be seen that APDs have a high probability of radiating photons at the moment of power-on.

Figure 3: Relationship Between the Operating Current of the APD and the Count Values of the SPCM

The relationship between the operating current of the APD and the count values of the SPCM is shown in Figure 3, which is a linear relationship. Considering the effect of the spatial solid angle, the linear proportion of the number of photons radiated by the APD to the current applied to the APD was 44 Mcounts/(s·mA·sr). The operating current of the APD is the dominant factor for photon radiation.

Recommended Voltage Amplifier: ATA-2161

Figure: ATA-2161 High-Voltage Amplifier Specifications

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