JPS592849B2 - How to measure weak light using an avalanche photodiode - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for measuring the weak amount of incident light by counting avalanche current pulse noise of an avalanche photodiode APD. The purpose of this is to improve the linearity between the pin and the pin.

The present inventors cooled an avalanche photodiode and investigated the (N/sec) of the avalanche current pulse noise over the entire wave height range. As a result, the (N/sec) value was found to be It was revealed that there is a maximum value at high wave heights. Moreover, we found that information about weak light is abundantly contained exclusively in the former region of low wave height. The present invention utilizes this fact to clearly measure weak light.
A detailed explanation will be given below by illustrating a specific example. Figure 1a shows the measuring device.

The avalanche photodiode APD is cooled to a constant temperature, and a reverse bias voltage Va is applied to keep the dark current Id in the avalanche region at a constant value. The constant cooling temperature is achieved by cooling with a refrigerant 1 such as LN2 and controlling energization of the heater 2 using a temperature detection element 3 in contact with the APD, as shown in FIG. Ib. Note that the APD includes, for example, N
A light amount Pin is input from the e-Ne gas laser 4 through the glass fiber 5, and the pulse current obtained at this time is passed through the load resistor RL. Furthermore, this is amplified by a distributed amplifier 6 connected to this, and then counted by a counter 8 through a single channel analyzer □. The analyzer 7 detects a certain section ΔV of the pulse height obtained from the distributed amplifier 6.
This is a so-called differential wave height discriminator, ie, a window, which sends only the signal to the counter 8. The window can be shifted in the direction of pulse height. The capacitor C connected in parallel to APD and RL in FIG. 1a is provided to smooth the extremely sharp pulse noise waveform and match it to the response speed of the analyzer T. Further, 9 is used for waveform observation with a synchroscope. Power measurement of the incident light amount Pin becomes extremely difficult as the input power 25 decreases, but as shown in Figure Ib, a pre-calibrated neutral density filter (ND filter) 10 is connected to the laser oscillator 4 and the glass fiber 5. This is done by inserting it between.

30 Figure 2 shows sample A (product name APD200B) at 190°C.
℃, the window of the pulse height discriminator is set to △V■O, 1 (V), and the pulse height is shifted sequentially from zero to 3 (V) in (N/sec). The value is plotted on the vertical axis.

As is clear from Figure 35, Pin is 10-12 to 10-1
When the wave height is 3 (W) or less, the maximum value of (N/sec) appears at two locations, one where the wave height is relatively high and one where the wave height is relatively low. In this case, the maximum point of (N/Sec) at a high wave height moves to a lower wave height as the light amount Pin increases, but at a low wave height of approximately 0.2 (V), the maximum point of (N/Sec) /Sec), as the light amount Pin increases, only the value of (N/Sec) increases proportionally, and the peak value where the maximum point exists does not move.

In other words, the maximum value that occurs at high wave heights is thought to be due to microplasma (N/Sec) at the PN junction,
The maximum value of (N/Sec) at low wave heights is considered to be excessive noise due to optical information. The present invention provides a window of the pulse height discriminator between the pulse height zero and the first maximum value.
That is, a differential type discriminator is set to improve the linearity between the weak light amount (Pin) and the pulse count rate (N/Sec).

Figure 3 A, C, and C are the results for three materials, and the horizontal axis (P
It can be seen that there is a proportional relationship between the vertical axis (N/Sec) and the vertical axis (N/Sec).

[Brief explanation of the drawing]

FIG. 1a is a circuit diagram for implementing the measurement method of the present invention,
b is an explanatory diagram of the cooling device and light intensity input device, FIG. 2 is a graph showing the relationship between the pulse height of pulse noise measured with the circuit of FIG. 1a and the pulse count rate (N/Sec), and FIG. Light intensity for avalanche photodiode (Pin)
It is a graph which shows the linearity of the pulse count rate (N/Sec) with respect to FIG. APD...Avalanche photodiode, 6...
Distributed amplifier, 7... Single channel analyzer (differential wave height discriminator), 8... Counter.

Claims (1)

[Claims] 1. A method for measuring weak light, which is characterized in that when an avalanche photodiode is cooled and light is applied, a region in which the pulse height of the avalanche current pulse noise is relatively low is extracted and counted using a pulse height discriminator.