Radiant energy – Photocells; circuits and apparatus – Temperature control of photocell
Reexamination Certificate
2001-03-21
2003-05-27
Allen, Stephone (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Temperature control of photocell
C250S2140RC
Reexamination Certificate
active
06570149
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a photodetector which is capable of detecting extremely weak light by using a photodiode, and more particularly to a photodetector which is applicable to environmental measurements, such as measurement of water droplets and dust particles, analyses of gases and trace materials in industrial fields, medical diagnoses, etc.
2. Description of the Related Art
As a photodetector of this kind, the present assignee has already developed a photodetector
31
shown in FIG.
3
. This photodetector
31
is configured such that an extremely weak near-infrared light can be detected by using an avalanche photodiode
11
as a photodetector element, and includes a cooling block
2
constructed such that the avalanche photodiode
11
can be mounted on a pedestal arranged therein. Further, the photodetector
31
includes a power supply block
13
for generating bias voltage applied to the avalanche photodiode
11
, a voltage detection block
32
for detecting the bias voltage of the avalanche photodiode
11
to output the detection signal indicative of the sensed bias voltage to the power supply block
13
, a temperature sensor
33
attached to the pedestal in the cooling block
2
together with the avalanche photodiode
11
, a temperature sensor block
34
for approximately detecting an temperature of the avalanche photodiode
11
based on a signal from the temperature sensor
33
, a cooler
15
for cooling the pedestal in the cooling block
2
to thereby maintain the temperature of the avalanche photodiode
11
detected by the temperature sensor block
34
at a predetermined temperature, and a sensor block
36
for detecting an amount of an incident light impinging on the avalanche photodiode
11
based on a detection signal from the avalanche photodiode
11
.
According to the photodetector
31
constructed as above, first, the cooler
15
cools the avalanche photodiode
11
to the predetermined temperature, and the power supply block
13
maintains the bias voltage of the avalanche photodiode
11
at a predetermined voltage. In this state, the temperature sensor block
34
detects a temperature of the avalanche photodiode
11
based on the signal from the temperature sensor
33
, and the cooler
15
cools the avalanche photodiode by feedback control such that the temperature of the avalanche photodiode
11
detected by the temperature sensor block
34
becomes equal to the predetermined temperature. At the same time, the power supply block
13
is feedback-controlled such that the bias voltage detected by the voltage detection block
32
becomes equal to the predetermined voltage, thereby maintaining the bias voltage of the avalanche photodiode
11
at the predetermined voltage. Then, a signal light is blocked from impinging on the avalanche photodiode
11
, and in this state, the sensor block
36
detects the amount of noise based on a dark current flowing through the avalanche photodiode
11
. Next, a signal light is permitted to impinge on the avalanche photodiode
11
, and in this state, the sensor block
36
detects the amount of signal light incident on the avalanche photodiode
11
. Then, the sensor block
36
causes the amount of noise contained in the detected amount of the signal light incident on the avalanche photodiode
11
, and the amount of noise detected when the signal light is blocked from impinging on the same, to cancel each other, thereby detecting the amount of the incident signal light itself.
As described above, in the photodetector
31
, feedback control is carried out such that operating conditions, such as the cooling temperature and the bias voltage of the avalanche photodiode
11
become constant, and at the same time the amount of noise is eliminated by cancellation from the detected amount of the incident signal light to thereby reduce an error in the detection of the amount of the signal light.
However, the photodetector
31
has room for improvement in the following points: The avalanche photodiode
11
has detection characteristics very sensitive to changes in the cooling temperature in units of {fraction (1/100)}° C. and changes in the bias voltage even in units of mV. On the other hand, heat enters the cooling block
2
from outside by way of an optical fiber cable connected to the avalanche photodiode
11
, a cable for use in supply of the bias voltage, a cable for use in detecting the bias voltage, and a cable for use in detecting the temperature. In this case, if the amount of heat entering the cooling block
2
is constant, it is possible to hold the cooling temperature of the avalanche photodiode
11
constant to some extent by using the cooler
15
, whereas if the ambient temperature outside the cooling block
2
changes, the amount of heat entering the cooling block
2
varies with this change. This causes as light change in the cooling temperature of the avalanche photodiode
11
. Further, the sensitivity of the temperature sensor
33
per se varies with the lapse of time due to heat cycle etc. In addition, it is physically or mechanically difficult to bring the avalanche photodiode
11
into direct contact with the temperature sensor
33
, and therefore, thermal resistance between them cannot be reduced to 0. As a result, the amount of change in temperature of the avalanche photodiode
11
, and the amount of change in temperature detected by the temperature sensor
33
do not necessarily agree with each other. In view of the above problems, it is very difficult to control the temperature of the avalanche photodiode
11
itself to the order of accuracy of {fraction (1/100)}° C.
Further, although the bias voltage of the avalanche photodiode
11
is feedback-controlled such that the same becomes equal to a predetermined voltage, it is very difficult to control the bias voltage such that it is not changed even in units of mV. Therefore, according to the photodetector
31
, there can be an error in detection of the amount of an incident signal light or the amount of noise due to a slight change in the operating conditions of the avalanche photodiode
11
, so that even if the amount of noise is cancelled out, there remains an error in the detected amount of the incident signal light. Further, since the voltage detection block
32
is connected to the avalanche photodiode
11
, there is a fear that mixing of noise from the voltage detection block
32
degrades the detection accuracy of the photodetector
31
. Therefore, there is a demand for enhanced detection accuracy on the photodetector
31
.
Additionally, in the photodetector
31
, it is required to feedback-control the bias voltage and the cooling temperature of the avalanche photodiode
11
with very high accuracy, so that the voltage detection block
32
and the temperature sensor
33
are required to have a high-precision detecting capability. This makes the photodetector
31
itself very expensive, and increases the size of the photodetector
31
against the demand of downsizing thereof. Therefore, there is also a demand for improvement in these points on the photodetector
31
.
SUMMARY OF THE INVENTION
The present invention has been made in view of the above problems, and therefore, an object thereof is to provide a photodetector which is capable of attaining an enhanced detection accuracy and at the same time permits reduction in size and manufacturing costs thereof.
To attain the above object, the invention provides a photodetector including a photodiode for detecting an incident light, in a state of a predetermined bias voltage set thereto, and a cooler for cooling the photodiode to a predetermined cooling temperature, wherein an amount of the incident light on the photodiode is detected based on a detection signal from the photodiode.
The photodetector according to the invention is characterized by comprising a control block that adjusts at least one of the bias voltage and the predetermined cooling temperature such that a value of the detection signal from the photodiode generated in a state of the incident li
Kudo Makoto
Maruyama Tomoyuki
Narusawa Fumio
Allen Stephone
Greenblum & Bernstein P.L.C.
Hioki Denki Kabushiki Kaisha
Spears Eric J
LandOfFree
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