RU66522U1 - PHOTO RECEIVER - Google Patents

Abstract

A photodetector comprising a series-connected avalanche photodiode, an amplifier, a low-pass filter, a comparator, a pulse duration discriminator, as well as an adjustable power source, the output of which is connected to an avalanche photodiode, and the first and second inputs are connected respectively to the first and second outputs of the pulse duration discriminator, signal evaluation unit, the input of which is connected to the amplifier output, a reference voltage source, the output of which is connected to the second input of the comparator, a high-frequency gene a radiator whose output is connected to the second input of the pulse duration discriminator, an optical radiation source optically coupled to the avalanche photodiode and a synchronization unit, the first output of which is connected to the optical radiation source and the synchronizing input of the signal estimation unit, and the second input is connected to the third input of the pulse duration discriminator characterized in that in order to reduce distortion during image formation in infrared laser scanning devices with the summation of the signals of two topriemnyh devices, a reference voltage source is adjustable within the image line, and its control input coupled to the third output of the synchronization unit.

Description

The utility model relates to high-speed meters of optical radiation power and can be used in optoelectronic systems that form an image. The result of using the utility model is the reduction of distortions during image formation in optoelectronic systems with the summation of the signals of two photodetector devices.

Known photodetector on an avalanche photodiode (US Patent No. 4015118, USC1. 250 / 211J), containing serially connected avalanche photodiode, amplifier, bandpass filter, comparator, adjustable power source, the output of which is connected to the avalanche photodiode, and also a voltage reference source, the output of which is connected to the second output of the comparator, a signal estimation unit, the input of which is connected to the output of the amplifier, an optical radiation source optically coupled to an avalanche photodiode, and a high-frequency generator, the oscillation frequency and which lies in the passband of the band-pass filter, and the output is connected to an optical radiation source. In this device, the avalanche photodiode is stabilized at a constant coefficient of avalanche multiplication, which leads to a narrow dynamic range of the received signals and limits the possibility of using the photodetector in optoelectronic systems.

Closest to the utility model is a photodetector (Russian Patent No. 1679212, MKI G01J 1/44, Bull. No. 35, 1991) containing a series-connected avalanche photodiode, amplifier, low-pass filter, comparator, pulse duration discriminator, and adjustable power supply output

which is connected to the avalanche photodiode, and the first and second inputs are connected respectively to the first and second outputs of the pulse duration discriminator, a signal estimation unit, the input of which is connected to the amplifier output, a reference voltage source, the output of which is connected to the second input of the comparator, a high-frequency generator, the output of which connected to the second input of the pulse duration discriminator, an optical radiation source optically coupled to an avalanche photodiode, a synchronization unit, the first output of which is connected n with an optical radiation source and a clock input of the signal estimation unit, and the second output is connected to the third input of the pulse duration discriminator.

In this device, a 10-20-fold adjustment of the multiplication factor of the avalanche photodiode is carried out by adjusting the voltage on the avalanche photodiode, which allows you to increase the upper power limit of the recorded optical signals.

Photoreceiving devices on silicon avalanche photodiodes are the most sensitive devices that receive radiation in the near infrared range. Of the variety of optical-electronic systems that form the image, the most interesting results are obtained from the use of a photodetector in laser systems with line scanning (W.F. Matthews, R. F. Jung. Laser line scanning sensors. Optical Engineering, vol. 14, No. 2, No. 2, 1975).

Flint avalanche photodiodes FD-115L (B) are some of the best domestic devices (Gulakov IR, Shunevich SA Photon counting by avalanche photodiodes. Instruments and experimental equipment, No. 4, 1987). To increase the size of the input window, FD-115L (B) avalanche photodiodes are available with a focusing cone (focon) with an input diameter of 2.2 mm. Fokon - an optical fiber, uses the effect of total internal reflection and is designed to reduce the area of the light flux. Aperture angle of radiation input into the focon FD-115L (B)

about 30 ° and they can be used in line-by-line laser systems only if the optical summation of signals from two off-axis parabolic mirrors is replaced by the electrical summation of the signals of two photodetector devices, each of which receives radiation from its own parabolic mirror (Russian Patent No. 2027202, MKI G02B 26/10, Bull. No. 2, 1995).

An infrared laser scanning device (Patent of Russia No. 2027202) is used at night to form an image of the area under the aircraft. The image is scanned perpendicular to the direction of flight due to the optical-mechanical scanning, in which the image lines are formed. Sweep along the second coordinate, in the direction of flight, is carried out due to the flight of the aircraft. The laser beam of the scanning device is reflected by various sections of the terrain under the aircraft, and after focusing with off-axis parabolic mirrors, the received radiation is incident on two photodetector devices on avalanche photodiodes. After processing the received signals in photodetectors, they are added to the analog adder and fed to the registration unit, which forms an image of the area. With a 10-fold change in flight altitude (the received signal is inversely proportional to the square of the flight altitude), the transmittance of the atmosphere in two directions of the laser radiation can vary by 2-3 times. Thus, the received optical signal from the brightest areas (ρ = 0.4-0.5), which are displayed as white areas in the image, can vary 200-300 times. In each of the photodetectors of the infrared laser scanning device, the maximum signal level during the image line varies not only depending on the flight altitude, but also depending on the scanning angle, when the effective working area of off-axis parabolic

mirrors. When the scan angle changes from -φ _max to + φ _max (φ _max ≈60 °, viewing angle 2φ _max ≈120 °), the effective area of off-axis parabolic mirrors changes by about three times.

In the prototype (Russian patent No. 1679212), when used in an infrared laser scanning device (two photodetectors are used simultaneously), the image line is divided into two intervals: T ₁ - reception of reflected laser radiation, T ₂ - adjustment of the multiplier of the avalanche photodiode. The reception cycle is synchronized by a synchronization unit, at the two outputs of which, during T ₂ , a first pulse of duration τ _{1 is formed} , and a pulse of τ ₂ following it. During the first pulse τ ₁ , the avalanche photodiode is illuminated with a reference radiation by an optical radiation source. If during T _{1 the} power of the received radiation is less than the power of the reference radiation during the pulse τ ₁ , then in this case the stabilization of the multiplication coefficient of the avalanche photodiode is carried out according to the power of the reference radiation. In the avalanche photodiode, the avalanche multiplication coefficient M ₀ , equal to

M _o = V _op / P _and S _o R _in K K _y

where V _op - voltage at the output of the reference voltage source;

P _and - the pulse power of the exposure of the avalanche photodiode by the source of optical radiation;

S _o - current sensitivity of the avalanche photodiode without avalanche multiplication;

R _in - input impedance of the amplifier;

K _y - the gain of the amplifier voltage.

The adjustment of the avalanche multiplication coefficient is carried out during the pulse τ ₂ . Outside the interval τ ₂ , the logical states are generated at the outputs of the duration discriminator by the pulse, which maintain during the (T ₁ + τ ₁ ) constant the avalanche multiplication coefficient of the avalanche photodiode.

If during interval T ₁ at time interval τ _1/2 power of the signal exceeds the power of pulse illumination optical radiation source _and P and P is equal _to, the voltage at the output of the controlled power source changes in such a way that the multiplication factor of the avalanche photodiode becomes equal to M

M = V _op / P _with S _o R in _x K _y

Thus, in the prototype, the multiplication factor of the avalanche photodiode at low power of the signals received during the string is maintained at a constant value of M _o - close to optimal, providing the best signal-to-noise ratio. With a large signal power, the multiplication factor of the avalanche photodiode is adjusted depending on the power of the signals received during the line.

The disadvantage of the prototype is that when using the device in scanning systems with the summation of the signals of two photodetector devices, distortions may occur during image formation due to differences in the sensitivity of these two photodetector devices. In a scanning system, using the summation of the signals of two photodetectors, the effective lens area of each photodetector (off-axis parabolic mirrors), depending on the scanning angle φ, can vary by 2-3 times. The signal power from the brightest sections of the terrain P _with which M is stabilized can also vary by a factor of 2–3 if the bright sections

The terrain is located on the edge of the line of sight. That is, when the multiplication coefficient of the avalanche photodiode M is stabilized by the received signal, due to the difference in the effective area of the lenses during the image line, the sensitivity of the two photodetectors may differ by 2-3 times, which will lead to image distortion.

The essence of the technical solution in the utility model is aimed at reducing distortions when forming images in scanning devices with the summation of the signals of two photodetector devices.

This goal is achieved by the fact that in the photodetector (in the infrared laser scanning device - Patent of Russia No. 2027202 uses two photodetector devices described in this utility model), the reference voltage source is adjustable during the image line, and its control input is connected to the third output block synchronization.

Figure 1 shows the block diagram of the photodetector, figure 2 shows the receiving part of the optical circuit of an infrared laser scanning device with the summation of the signals of two photodetectors, figure 3 shows the effective area of each of the two off-axis parabolic mirrors of the infrared laser scanning device scan.

The device comprises a series-connected avalanche photodiode 1, an amplifier 2, a low-pass filter 3, a comparator 4, a pulse duration discriminator 5, and an adjustable power supply 6, the output of which is connected to the avalanche photodiode 1, and the first and second inputs are connected respectively to the first and second the outputs of the discriminator of the pulse duration 5, the signal evaluation unit 7, the input of which is connected to the output of the amplifier 2, the reference voltage source 8, the output of which is connected to the second input of the comparator 4, a high-frequency generator OP 9, the output of which is connected to the second input of the discriminator of the pulse duration 5, the source 10 of the optical

radiation optically coupled to the avalanche photodiode 1 and the synchronization unit 11, the first output of which 12 is connected to the optical radiation source 10 and the synchronizing input of the signal estimation unit 7, and the second output 13 is connected to the third input of the pulse duration discriminator 5, and the voltage reference source 8 is made adjustable during the image line, and its control input is connected to the third output 14 of the synchronization unit 11.

The photodetector operates as follows. When used in infrared laser scanning systems (see figure 2), the device receives periodically repeating signals (image lines). The scanning system uses two photodetectors - the first photodetector 15 and the second photodetector 16, which receive laser signals reflected by the side faces of the tetrahedral scanner 17 and focused by the first 18 and second 19 off-axis parabolic mirrors. After processing the received signals in photodetectors, they are added to the analog adder 20 and fed to the registration unit 21, which forms the image of the area. In essence, each of the photodetectors works independently of the other photodetector.

The image line (reception cycle) is divided into two intervals: T ₁ - reception of optical radiation, T ₂ - adjustment of the multiplication factor of the avalanche photodiode. The reception cycle is synchronized by the synchronization unit 11, at the output 12 of which, in the interval T ₂ , a first pulse with a duration of τ _{1 is formed} , and a second pulse with a duration of τ ₂ following it at the output 13. During the first pulse, the avalanche photodiode 1 is illuminated by the reference radiation of the optical radiation source 10. If during the interval T _{1 the} power of the received radiation is much less than the power of the reference radiation during the pulse τ ₁ , then in this case, stabilization

the multiplication factor of the avalanche photodiode is carried out by the power of the reference radiation. This light pulse of duration τ ₁ is converted by an avalanche photodiode 1 into a current pulse, the amplitude of which is proportional to the multiplication factor of the avalanche photodiode 1. At the input resistance of amplifier 2, the current pulse is converted into a voltage pulse, which is amplified by amplifier 2 and fed through low-pass filter 3 to comparator 4 The signal at the output of the comparator 4 is equal to the logical "1" if the signal at its input exceeds the reference voltage from the source 8 of the reference voltage. During T _{2, the} reference voltage is V _op . The discriminator of the pulse duration 5 checks whether the logical “1” from the comparator 4 exceeds for τ _{1 the} duration of τ _1/2 . If the duration of the logical “1” from the comparator 4 exceeds τ _1/2, then the discriminator of the pulse duration 5 during τ ₂ reduces the voltage at the output of the regulated power source 6, and reduces the coefficient of avalanche multiplication. If the duration of the logical “1” from the comparator 4 is less than τ _1/2 , then the discriminator of the pulse duration 5 during τ ₂ increases the voltage at the output of the regulated power supply 6 and increases the coefficient of avalanche multiplication. Thus, in the avalanche photodiode 1, the avalanche multiplication coefficient M _{0 is} maintained, which is equal to

M _o = V _op / P _and S _o R in _x K _y

where V _op - voltage at the output of the reference voltage source;

P _and - the pulse power of the exposure of the avalanche photodiode by the source of optical radiation;

S _o - current sensitivity of the avalanche photodiode without avalanche multiplication;

P _in - input impedance of the amplifier;

K _y - the gain of the amplifier voltage.

Outside the interval τ ₂ in the device during (T ₁ + τ ₁ ) is maintained constant multiplication factor of the avalanche photodiode 1.

If during the interval T ₁ , at the output of the comparator 4, a logical “1” is formed during the total time τ _1/2 , then the multiplication coefficient of the avalanche photodiode will be less than M ₀ . During the interval T ₁ (during the image line), the voltage at the output of the reference voltage source 8 is regulated - it is equal to V _op ^* (during T ₂ it is equal to V _op ). In a laser scanning system with the summation of the signals of two photodetectors (see figure 2), the effective area of off-axis parabolic mirrors 18, 19 depends on the scanning angle. In fact, the effective area of parabolic mirrors 18, 19 is determined by what part of the mirrors works efficiently - it is “illuminated” by the received light fluxes reflected from the side mirror faces of the tetrahedral scanner 17. At each time t during T ₁ both photodetectors 15, 16 receive luminous fluxes from the same area. After adding the signals from the outputs of the photodetectors 15, 16 in the analog adder 20, the image of the underlying terrain is recorded in the registration unit 21.

For the first parabolic mirror at the beginning of the line, the effective area is maximum and then decreases (change in the scanning angle from -φ _max to + φ _max ), and for the second parabolic mirror at the beginning of the line, the effective area is minimum and then increases. The effective areas of two mirrors in the middle of the line T ₁ are equal to each other (φ = 0), and during the line they vary within the same boundaries. Graphs of the dependence of the effective areas of the first and second parabolic mirrors on the scan angle φ are shown in figure 3 and have the following mathematical dependencies:

S _first = S _max cos (45 + φ / 2) S _second = S _max cos (45-φ / 2)

The voltage of the source 8 repeats the dependence of the effective area of parabolic mirrors S _eff on the scan angle φ - time during the line T ₁ (for the first photodetector S _eff = S _first , for the second photodetector S _eff = S _sec ) Choose V _op ^* - voltage the output of the source 8 of the reference voltage so that during T ₁ V _op it would be equal to:

V _op ^* = V _op S _eff / S _max

V _op ^{* is} regulated during T ₁ , and during T ₂ V _op ^* = V _op . The synchronization and control of the regulated sources of reference voltage 8 is carried out due to the fact that its control input is connected to the third output 14 of the synchronization unit 11, and as the output 14, a data bus about the scanning angle φ (time from the beginning of the line - t during the reception time can be used) optical signals T ₁ ). The simplest embodiment of an adjustable reference voltage source 8 is a serial connection of a programmable read-only memory and a digital-to-analog converter (DAC), while the scan angle bus φ is used as the control signal 14.

Using an adjustable reference voltage source 8 makes it possible to make the comparator 4 operate only depending on the reflection coefficients of the laser radiation from the terrain, to balance the sensitivity of two photodetector devices, and thus reduce distortion in the formation of images in devices with the summation of the signals of two photodetector devices.

Claims (1)

A photodetector comprising a series-connected avalanche photodiode, an amplifier, a low-pass filter, a comparator, a pulse duration discriminator, as well as an adjustable power source whose output is connected to an avalanche photodiode, and the first and second inputs are connected respectively to the first and second outputs of the pulse duration discriminator, signal evaluation unit, the input of which is connected to the amplifier output, a reference voltage source, the output of which is connected to the second input of the comparator, a high-frequency gene a radiator whose output is connected to the second input of the pulse duration discriminator, an optical radiation source optically coupled to the avalanche photodiode and a synchronization unit, the first output of which is connected to the optical radiation source and the synchronization input of the signal estimation unit, and the second output is connected to the third input of the pulse duration discriminator characterized in that, in order to reduce distortion during image formation in infrared laser scanning devices with the summation of the signals of two photodetectors, the reference voltage source is made adjustable during the image line, and its control input is connected to the third output of the synchronization unit.