Optical time domain reflectometer based on high-gain optical pulse transceiving

Abstract

The invention discloses an optical time domain reflectometer based on high-gain optical pulse transceiving, and relates to the technical field of optical cable testing devices. The reflectometer comprises a reflection echo digital processor and a control module, and the reflection echo digital processor is bidirectionally connected with the control module; an excitation pulse signal output end of a drive pulse generator in the reflection echo digital processor is connected with a pulse signal input end of an excitation light pulse drive circuit, and a test laser pulse output end of the excitation light pulse drive circuit is connected with a test signal input end of a transmit-receive isolator. A test signal output end of the transmit-receive isolator is connected with one end of a tested optical cable, a reflection echo signal output end of the tested optical cable is connected with a reflection echo signal input end of the transmit-receive isolator, and a reflection echo signal output end of the transmit-receive isolator is connected with a signal input end of the APD pre-amplification and photoelectric conversion circuit. The signal output end of the APD bias circuit is connected with the bias signal input end of the APD pre-amplification and photoelectric conversion circuit. The reflectometer has the advantages of high sensitivity, accurate test and the like.

Description

Technical Field

[0001] The invention relates to the technical field of optical cable testing devices, and in particular to an optical time domain reflectometer based on high-gain optical pulse transceiver. Background Art

[0002] The transmission characteristics of optical cables mainly refer to the distance loss characteristics and total loss characteristics, echo reflection, etc. The fault characteristics of optical cables mainly refer to the reflection loss of active connectors when the optical cables are connected, the distance position of Fresnel reflection pulses at the breakpoints of the optical cables, etc. These characteristics are mainly obtained by injecting periodic optical pulses of specified pulse width into the optical cables to drive and stimulate the generation of reflection echoes, then receiving the reflection echo optical signals, and finally performing photoelectric conversion and digital analysis on the reflection echo optical signals.

[0003] Optical fiber communication systems are not only used in long-distance communication trunk lines, but also in various applications in the last mile of optical fiber, such as home and office automation systems. Optical time domain reflectometer is an important measurement device for testing the performance of optical fiber networks and locating fault points. The optical time domain reflectometer measures optical fiber joints, loss, length, reflectivity and other parameters by detecting the corresponding relationship between the intensity and time of backward Rayleigh scattering and Fresnel reflected light signals in optical fibers. The optical time domain reflectometer in the prior art has low sensitivity and high power consumption. Summary of the invention

[0004] The technical problem to be solved by the present invention is how to provide an optical time domain reflectometer based on high-gain optical pulse transmission and reception with high sensitivity and accurate testing.

[0005] In order to solve the above technical problems, the technical solution adopted by the present invention is: an optical time domain reflectometer based on high-gain optical pulse transceiver, including a reflection echo digital processor and a control module, the reflection echo digital processor and the control module are bidirectionally connected, the data processed by the reflection echo digital processor is transmitted to the control module for processing and analysis, the fault location of the optical cable is obtained, and the control module is controlled by the control module; the excitation pulse signal output end of the driving pulse generator in the reflection echo digital processor is connected to the pulse signal input end of the excitation optical pulse driving circuit, the test laser pulse output end of the excitation optical pulse driving circuit is connected to the test signal input end of the transceiver isolator, and the receiving The test signal output end of the transmitting isolator is connected to one end of the optical cable under test, the reflected echo signal output end of the optical cable under test is connected to the reflected echo signal input end of the transceiver isolator, the reflected echo signal output end of the transceiver isolator is connected to the signal input end of the APD preamplifier and photoelectric conversion circuit, the signal output end of the APD preamplifier and photoelectric conversion circuit is connected to the reflected echo signal input end of the reflected echo digital processor after passing through the program-controlled amplifier module and the A / D conversion module in sequence, and the signal output end of the APD bias circuit is connected to the bias signal input end of the APD preamplifier and photoelectric conversion circuit, so as to input a bias signal to the APD preamplifier and photoelectric conversion circuit.

[0006] A further technical solution is that, during the reflectometer test, under the control of the reflected echo digital processor, a periodic test light pulse with a specified pulse width is sent to the optical cable under test. During the transmission of the test light pulse in the optical cable, a Rayleigh reflected light echo signal is generated due to the inherent manufacturing defects of the optical cable. If the test light pulse encounters a fault point, the refractive index of the fault point changes, and the test light pulse generates a Fresnel reflected light pulse at the fault point. The Fresnel reflected light pulse and the Rayleigh reflected light echo signal are transmitted back to the APD preamplifier and photoelectric conversion circuit for preamplification, and then A / D conversion is performed. After the sampling data is processed, it is uploaded to the control module, and the main control programmer reconstructs and analyzes the characteristics of the reflected echo. According to the waveform characteristics and propagation time of the reflected echo and the reflected pulse in the optical cable, the optical cable loss and the fault point position are calculated and displayed, so as to realize the detection of optical cable transmission characteristics and fault location.

[0007] A further technical solution is that the excitation light pulse driving circuit includes a capacitor C2', one end of the capacitor C2' is the pulse signal input end of the excitation light pulse driving circuit, and the other end of the capacitor C2' is divided into three paths, the first path is grounded through a resistor R4', the second path is connected to the gate of the field effect tube V2', and the third path is connected to the gate of the field effect tube V3'. The source of the field effect tube V2' and the source of the field effect tube V3' are grounded, and the drain of the field effect tube V2' is connected to the drain of the field effect tube V3' and then divided into three paths, the first path is connected to the resistor R3' and The capacitor C1' is grounded, the second path is connected to one end of the resistor R2', and the third path is connected to one end of the potentiometer RP'; the other end of the resistor R2' is divided into two paths, the first path is connected to one end of the resistor R1', and the second path is connected to one end of the temperature control potentiometer RT'; the other end of the potentiometer RP', the other end of the resistor R1' and the other end of the temperature control potentiometer RT' are connected to each other and then connected to the negative electrode of the single-mode continuous wave laser V1', the positive electrode of the single-mode continuous wave laser V1' is connected to the power supply VCC, and the single-mode continuous wave laser V1' outputs an optical pulse signal.

[0008] A further technical solution is that the APD bias circuit includes a MAX1932 chip U1, wherein the pin 1 of the U1 is connected to one end of a resistor R12, and the other end of the resistor R12 is a clock signal input end; the pin 2 of the U1 is connected to one end of a resistor R13, and the other end of the resistor R13 is a data signal input end; the pin 12 of the U1 is connected to one end of a resistor R11, and the other end of the resistor R11 is a chip select signal input end; the EP pin of the U1 is grounded; the pin 8 of the U1 is grounded in turn through a resistor R9 and a capacitor C6, and the node of the resistor R9 and the capacitor C6 is connected to the resistor The first end of the resistor R8 is connected to the first end of the resistor R9, the second end of the resistor R10 is connected to the first end of the resistor R11, the fourth end of the resistor R11 is connected to the first end of the resistor R12, and the fifth end of the resistor R13 is connected to the first end of the resistor R14. The first end of the resistor R14 is connected to the first end of the resistor R15. The second end of the resistor R15 is connected to the first end of the resistor R16, the fourth end of the resistor R17 is connected to the first end of the resistor R18, and the fifth end of the resistor R19 is connected to the second end of the resistor R20. The other end of the resistor R3 is connected to one end of the capacitor C4, and the other end of the capacitor C4 is divided into two paths, the first path is connected to the 5th pin of the U1, and the second path is connected to the other end of the resistor R4; the other end of the resistor R5 is divided into two paths, the first path is connected to the ground after passing through the potentiometer RP and the resistor R6 in sequence, and the second path is grounded through the temperature control potentiometer RT; the 10th pin of the U1 is connected to the gate of the field effect tube V2, the source of the field effect tube V2 is grounded, and the drain of the field effect tube V2 is divided into two paths, the first path is connected to one end of the inductor L1, and the second path is connected to the positive electrode of the diode V1. The other end of the inductor L1 is connected to the positive electrode of the diode V1. The end is divided into four paths, the first path is connected to the 11th pin of U1, the second path is connected to the power supply VCC, the third path is grounded through the capacitor C1, and the fourth path is grounded through the capacitor C2; the cathode of the diode V1 is divided into four paths, the first path is grounded through the capacitor C3, the second path is grounded through the resistor R1, the third path is connected to the 4th pin of U1, and the fourth path is connected to one end of the resistor R2; the other end of the resistor R2 is divided into four paths, the first path is connected to the node of the resistor R4 and the capacitor C4, the second path is grounded through the capacitor C5, the third path is grounded through the single-mode continuous wave laser V4, and the fourth path is the bias voltage output end of the bias circuit.

[0009] The beneficial effect of adopting the above technical solution is that when the reflectometer described in the present application is working, first, under the control of the reflection echo digital processor, the optical pulse driving circuit is stimulated to send a periodic test optical pulse of a specified pulse width to the optical cable. During the transmission of the optical cable, the test optical pulse generates a Rayleigh reflection light echo signal due to the inherent manufacturing defects of the optical cable (water ripples, tiny bubbles, impurities, etc. that appear in the optical cable when the optical cable is produced). If the test optical pulse encounters a fault point, such as reflection loss due to poor optical cable connection, disconnection of the optical cable, or bending of the optical cable, the test optical pulse generates a Fresnel reflection light pulse at the fault point due to the change in the refractive index of the fault point. The Fresnel reflected light pulse and Rayleigh reflected light echo signal are transmitted back to the APD preamplifier and photoelectric conversion circuit of the tester for preamplification with high sensitivity, and then digital processing is performed. These digital processing are completed by the reflected echo digital processor, including sampling control and A / D conversion control, signal-to-noise ratio enhancement and filtering, etc. After the sampling data processing is completed, it is uploaded to the control module of the optical time domain reflectometer, and the main control programmer reconstructs and analyzes the reflected echo. According to the waveform characteristics and propagation time of the reflected echo and the reflected pulse in the optical cable, the optical cable loss and the fault point position are calculated and displayed, thereby realizing the detection of optical cable transmission characteristics and fault location. The reflectometer described in this application has the advantages of high sensitivity, low power consumption, and accurate testing. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments.

[0011] Figure 1 is a principle block diagram of a reflectometer according to an embodiment of the present invention;

[0012] Figure 2 is a schematic diagram of the working timing of the reflectometer according to the embodiment of the present invention;

[0013] Figure 3 is a circuit schematic diagram of an excitation light pulse driving circuit in a reflectometer according to an embodiment of the present invention;

[0014] Figure 4 is a circuit schematic diagram of an APD bias circuit in a reflectometer according to an embodiment of the present invention;

[0015] Figure 5a is a reflection echo image reconstructed before the APD high-sensitivity gain adjustment in an embodiment of the present invention;

[0016] Figure 5b is a reflection echo image reconstructed after the APD high-sensitivity gain is adjusted in an embodiment of the present invention;

[0017] Figure 6 It is a schematic diagram of linear region position identification in an embodiment of the present invention. DETAILED DESCRIPTION

[0018] The following is a clear and complete description of the technical solutions in the embodiments of the present invention in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0019] In the following description, many specific details are set forth to facilitate a full understanding of the present invention, but the present invention may also be implemented in other ways different from those described herein, and those skilled in the art may make similar generalizations without violating the connotation of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0020] Overall, such as Figure 1 As shown, an embodiment of the present invention discloses an optical time domain reflectometer based on high-gain optical pulse transceiver, and the optical time domain reflectometer includes a reflection echo digital processor and a control module. The reflection echo digital processor and the control module are bidirectionally connected, and the data processed by the reflection echo digital processor is transmitted to the control module for processing and analysis to obtain the fault location of the optical cable, and is controlled by the control module; the excitation pulse signal output end of the driving pulse generator in the reflection echo digital processor is connected to the pulse signal input end of the excitation light pulse driving circuit, the test laser pulse output end of the excitation light pulse driving circuit is connected to the test signal input end of the transceiver isolator, and the test signal output end of the transceiver isolator is connected to one end of the optical cable under test, and the optical cable under test is connected to the test signal input end of the optical cable under test. The reflected echo signal output end is connected to the reflected echo signal input end of the transceiver isolator, the reflected echo signal output end of the transceiver isolator is connected to the signal input end of the APD preamplifier and photoelectric conversion circuit, the signal output end of the APD preamplifier and photoelectric conversion circuit is connected to the reflected echo signal input end of the reflected echo digital processor after passing through the programmable amplifier module and the A / D conversion module in sequence, and the signal output end of the APD bias circuit is connected to the bias signal input end of the APD preamplifier and photoelectric conversion circuit, and is used to input a bias signal to the APD preamplifier and photoelectric conversion circuit.

[0021] The working sequence of the optical time domain reflectometer is as follows Figure 2When the optical time domain reflectometer is testing, first, under the control of the reflection echo digital processor, the optical pulse driving circuit is stimulated to send a periodic test optical pulse of a specified pulse width to the optical cable. During the transmission of the test optical pulse, the Rayleigh reflection light echo signal is generated due to the inherent manufacturing defects of the optical cable (water ripples, tiny bubbles, impurities, etc. that appear in the optical cable during production). If the test optical pulse encounters a fault point, such as reflection loss due to poor cable connection, cable disconnection, or cable bending, the test optical pulse generates a Fresnel reflection light pulse at the fault point due to the change in the refractive index of the fault point. The Fresnel reflection light pulse and the Rayleigh reflection light pulse are The reflected light echo signal is transmitted back to the APD preamplifier and photoelectric conversion circuit of the tester for high-sensitivity preamplification, and then digitally processed. These digital processes are completed by the reflected echo digital processor, including sampling control and A / D conversion control, signal-to-noise ratio enhancement and filtering. After the sampling data is processed, it is uploaded to the control module of the optical time domain reflectometer. The main control programmer reconstructs and analyzes the reflected echo. According to the waveform characteristics and propagation time of the reflected echo and the reflected pulse in the optical cable, the optical cable loss and fault point location are calculated and displayed, thereby realizing the detection of optical cable transmission characteristics and fault location.

[0022] Excitation light pulse driving method:

[0023] like Figure 1 As shown, when an optical time domain reflectometer performs optical time domain analysis on an optical cable, in order to obtain the reflected echo of the optical cable, a periodic optical pulse with a specified pulse width must be injected into the optical cable for test drive excitation. The optical pulse described in this application is generated by pulse driving using a low-power continuous wave laser.

[0024] like Figure 3The schematic diagram of the excitation light pulse driving circuit is shown, and the excitation light pulse driving circuit includes a capacitor C2', one end of the capacitor C2' is the pulse signal input end of the excitation light pulse driving circuit, and the other end of the capacitor C2' is divided into three paths, the first path is grounded through a resistor R4', the second path is connected to the gate of the field effect tube V2', and the third path is connected to the gate of the field effect tube V3'. The source of the field effect tube V2' and the source of the field effect tube V3' are grounded, and the drain of the field effect tube V2' is connected to the drain of the field effect tube V3' and then divided into three paths. The first path is connected to the gate of the field effect tube V3' in sequence through the resistor R3 ' and capacitor C1' are grounded, the second path is connected to one end of the resistor R2', and the third path is connected to one end of the potentiometer RP'; the other end of the resistor R2' is divided into two paths, the first path is connected to one end of the resistor R1', and the second path is connected to one end of the temperature control potentiometer RT'; the other end of the potentiometer RP', the other end of the resistor R1' and the other end of the temperature control potentiometer RT' are connected to each other and then connected to the negative electrode of the single-mode continuous wave laser V1', the positive electrode of the single-mode continuous wave laser V1' is connected to the power supply VCC, and the single-mode continuous wave laser V1' outputs an optical pulse signal.

[0025] like Figure 3 As shown, the driving electric pulse acts on the gates of the dual-gate field effect transistor pair V2' and V3' through the DC blocking capacitor C2', and pulse drives the single-mode continuous wave laser V1' to output light pulses. The load resistors are R1', R2', and R3'. The capacitor C1' is a filter circuit, RP' is an output power adjustment potentiometer, and RT' is a temperature control potentiometer.

[0026] Single-mode continuous wave laser V1' has a threshold current. The driving current must be greater than the threshold current for the laser to output laser (coherent light). When the driving current is lower than the threshold current, the laser outputs incoherent light. The threshold current of the laser increases with the increase of temperature, thereby reducing the output power of the laser. In order to offset the influence of the change of the threshold current of the continuous wave laser with temperature on the output pulse power within the full temperature range, the driving current and threshold current of the laser must be temperature controlled.

[0027] Technical requirements for continuous wave lasers:

[0028] 1) The average power of the continuous wave output by the single-mode continuous wave laser is required to be no less than 7mW;

[0029] 2) The limiting breakdown current of a single-mode continuous wave laser: not less than 400mA / 1μs drive pulse (25°C) (or the technical requirements specified by the laser manufacturer). If the limiting breakdown current is low, it will limit the pulse drive current of the laser, resulting in a reduction in the pulse power of the laser.

[0030] 3) Threshold current of single-mode continuous wave laser: no more than 10mA (25°C) (or the technical requirements specified by the laser manufacturer);

[0031] 4) Change in threshold current of single-mode continuous wave laser over the entire temperature range: not more than 50% (compared with the threshold current at 25°C) (or the technical requirements specified by the laser manufacturer).

[0032] Requirements and methods for continuous wave laser pulse output control:

[0033] 1) Pulse drive mode: such as Figure 3 As shown, use medium power (single tube pulse drive current is not less than 0.68A) dual-gate field effect tube single tube or multiple tubes parallel drive or other drive methods;

[0034] 2) Pulse drive circuit parameter design: The pulse drive circuit parameters are designed based on the laser's limiting breakdown current, the laser's quantum efficiency, and the drive pulse width specified when performing optical time domain analysis on the optical cable.

[0035] Since the ultimate breakdown current of a continuous wave laser is relatively low (i.e., the ultimate breakdown damage power is low), when performing optical time domain analysis on optical cables over medium and short distances (characteristic analysis is not less than 60km, fault detection is not less than 120km), from the perspective of safety and reliability design, the upper limit of the drive pulse width should be 3μs.

[0036] If the limit breakdown current of the continuous wave laser is not less than 400mA / 1μs driving pulse, according to the equivalent limit breakdown energy of the laser, when the pulse width is 3μs, the limit breakdown current of the laser is 1 / 3 of the 1μs pulse width driving pulse, that is, the driving pulse current is 133mA. The optical pulse output power is obtained based on the product of the quantum efficiency of the laser and the driving current. A set of driving parameter examples is shown in Table 1.

[0037] Table 1 Laser drive parameters

[0038]

[0039]

[0040] The operating current of the excitation light pulse driving circuit is calculated according to the following formula:

[0041] I LD(25℃) =(V CC(25℃) -V LD(25℃) ) / (R1′+R2′+Rds / 2)

[0042] Where: I LD(25℃) The peak value of the pulse drive current of the excitation light pulse drive circuit at 25°C;

[0043] Vcc(25℃) It is the working voltage of the excitation light pulse driving circuit at the reference working temperature of 25°C;

[0044] V LD(25℃) It is the voltage drop of the excitation light pulse driving circuit at the reference operating temperature of 25°C;

[0045] R1' is the resistance value of bias resistor R1';

[0046] R2' is the resistance value of bias resistor R2';

[0047] Rds / 2 is the internal resistance of the driver, which is calculated by connecting field effect transistors V2' and V3' in parallel.

[0048] Temperature control method: Use a temperature-controlled digital potentiometer and a series-parallel resistor network to control the temperature of the load resistor of the pulse drive circuit, so that the pulse drive circuit is within the specified full temperature range. The drive current of the laser changes with the threshold current of the laser, and the range of change is designed according to the parameters specified by the laser product.

[0049] Pulse drive safety measures: The power supply of the pulse driver has a current limiting function. When the driver current is close to the limit breakdown current of the continuous wave laser, the power supply circuit has an overcurrent protection function to avoid damage to the laser.

[0050] 1) Continuous wave single-mode lasers usually refer to single-mode lasers used in communication equipment and measurement equipment. They are used in large quantities and have a wide source. They have good temperature characteristics and stability. The average output power of continuous wave single-mode lasers is not more than 10mW, and the threshold current is small, generally lower than 10mA. Continuous wave single-mode lasers are generally integrated with isolators to improve the stability of the laser. When a back detector is integrated (to sample the output power of the laser), it is convenient to realize APC automatic power control of the laser output power.

[0051] 2) The temperature-controlled potentiometer is an integrated digitally controlled potentiometer. The temperature-controlled potentiometer has multiple storage units, which are addressed by temperature value to store resistance values ​​corresponding to different temperatures, that is, they are divided according to the full working temperature range. Different temperature ranges correspond to one resistance value. When the temperature changes, the resistance value of the potentiometer corresponds to the current temperature range storage and the corresponding resistance value. Temperature-controlled potentiometers with different resistance values ​​and temperature series can be selected according to design requirements. Major component manufacturers include MAX and other companies in the United States.

[0052] Reflected echo optical signal receiving and processing method:

[0053] like Figure 1As shown, after being driven by the laser pulse, the reflected echo signal is transmitted back to the preamplifier through the transceiver isolator. The preamplifier and photoelectric converter described in this application are APD avalanche diodes. Since the reflected signal is extremely weak, an APD with avalanche gain is used, and the photoelectric conversion is implemented in a high-gain reverse bias working mode to maximize the detection signal-to-noise ratio.

[0054] APD needs to be reverse biased to a specified working voltage when working normally (APD product manual will provide typical values ​​of reverse bias working voltage), but conventional bias voltage cannot effectively increase APD gain to meet low-noise and high-gain amplification of weak signals. This method proposes a visual APD high-gain bias adjustment method to increase APD gain and achieve low-noise amplification and photoelectric conversion of reflected echoes.

[0055] APD is a temperature-sensitive device. When the APD is biased with a fixed operating voltage, its avalanche gain decreases with increasing temperature and increases with decreasing temperature. The relationship between the APD high gain bias voltage and the operating temperature is shown in formula (2), which is the control basis for temperature compensation when the APD high gain bias voltage changes with temperature.

[0056] V Rr(25℃+ΔT) =V Rr(25℃) (1+β×ΔT) (2)

[0057] Where: V Br(25℃+ΔT) is the high gain bias voltage of the APD avalanche diode when it deviates from the reference operating temperature by ΔT;

[0058] V Br(25℃) It is the high gain bias voltage of the APD avalanche diode at the reference operating temperature of 25°C;

[0059] β is the temperature coefficient of the APD avalanche diode;

[0060] ΔT is the temperature change of the APD avalanche diode when it deviates from the reference operating temperature of 25°C.

[0061] The APD avalanche gain is controlled by reverse bias voltage. In order to keep the APD avalanche gain constant within the specified operating temperature range, the APD bias voltage must be temperature controlled. When the temperature rises, the APD bias voltage is increased to compensate for the decrease in APD gain to avoid a decrease in the signal-to-noise ratio of the preamplifier signal. When the temperature drops, the APD bias voltage is reduced to compensate for the increase in APD gain to avoid saturation of the preamplifier signal. The control elements are a temperature-controlled digital potentiometer and a series-parallel resistor network.

[0062] Technical requirements for APD avalanche diodes:

[0063] 1) Avalanche gain and reverse bias voltage:

[0064] Typical gain value: not less than 10;

[0065] Typical value of reverse bias voltage: not less than 40V;

[0066] 2) Dark current: typical value is no more than 100pA;

[0067] 3) Breakdown voltage:

[0068] Greater than the typical reverse bias voltage value;

[0069] The difference between the breakdown voltage and the typical reverse bias voltage is required to be greater than 5V. This voltage difference provides a high gain control voltage range. The larger the voltage difference, the larger the adjustment range.

[0070] 4) Gain variation over the entire temperature range: not more than 50% (or the technical requirements specified by the laser manufacturer).

[0071] Requirements and methods for APD bias, temperature control and adjustment:

[0072] Bias Control:

[0073] APD bias voltage is controlled using a dedicated integrated circuit, such as Figure 4 The peripheral circuit of the controller is the device ( Figure 4 The typical circuit and specified parameters of resistors R1~R16, capacitors C1~C5, field effect transistors V1~V3, and inductor L1 are shown in the figure. The APD bias voltage is obtained by the output voltage divider circuit (Formula 3).

[0074] like Figure 4As shown, the APD bias circuit includes a MAX1932 chip U1, wherein the pin 1 of the U1 is connected to one end of a resistor R12, and the other end of the resistor R12 is a clock signal input end; the pin 2 of the U1 is connected to one end of a resistor R13, and the other end of the resistor R13 is a data signal input end; the pin 12 of the U1 is connected to one end of a resistor R11, and the other end of the resistor R11 is a chip select signal input end; the EP pin of the U1 is grounded; the pin 8 of the U1 is grounded via a resistor R9 and a capacitor C6 in turn, and the node of the resistor R9 and the capacitor C6 is connected to one end of a resistor R10. The other end of the resistor R10 is connected to the positive electrode of the diode V3, and the 9th pin of the U1 is connected to the negative electrode of the diode V3 as the overcurrent alarm signal output end of the bias circuit; the 6th pin of the U1 is connected to one end of the resistor R7, and the other end of the resistor R7 is divided into five paths, the first path is connected to one end of the resistor R8, the second path is connected to the 7th pin of the U1, the third path is connected to one end of the resistor R3, the fourth path is connected to one end of the resistor R4, and the fifth path is connected to one end of the resistor R5; the other end of the resistor R8 is the bias voltage monitoring signal output end, and the other end of the resistor R3 is connected to the negative electrode of the diode V3. One end is connected to one end of capacitor C4, and the other end of capacitor C4 is divided into two paths, the first path is connected to pin 5 of U1, and the second path is connected to the other end of resistor R4; the other end of resistor R5 is divided into two paths, the first path is connected to ground after passing through potentiometer RP and resistor R6 in sequence, and the second path is grounded through temperature control potentiometer RT; pin 10 of U1 is connected to the gate of field effect tube V2, the source of field effect tube V2 is grounded, the drain of field effect tube V2 is divided into two paths, the first path is connected to one end of inductor L1, and the second path is connected to the positive electrode of diode V1, and the other end of inductor L1 is divided into There are four paths, the first path is connected to the 11th pin of U1, the second path is connected to the power supply VCC, the third path is grounded via the capacitor C1, and the fourth path is grounded via the capacitor C2; the cathode of the diode V1 is divided into four paths, the first path is grounded via the capacitor C3, the second path is grounded via the resistor R1, the third path is connected to the 4th pin of U1, and the fourth path is connected to one end of the resistor R2; the other end of the resistor R2 is divided into four paths, the first path is connected to the node of the resistor R4 and the capacitor C4, the second path is grounded via the capacitor C5, the third path is grounded via the single-mode continuous wave laser V4, and the fourth path is the bias voltage output end of the bias circuit.

[0075] The bias voltage of the APD avalanche diode is obtained by the output voltage divider circuit:

[0076] V BK =V REF (R5+RP+R6) / (R4+R5+RP+R6) (3)

[0077] Where: V BK is the high gain bias voltage of the APD avalanche diode;

[0078] V REF It is the reference output voltage of the controller of APD avalanche diode, and the reference output voltage can be programmable;

[0079] R4, R5, R6 are the resistance values ​​of bias circuit resistors R4, R5, R6 respectively;

[0080] RP is the resistance value of the high gain bias voltage output trimmer potentiometer of the APD avalanche diode.

[0081] Temperature control method:

[0082] Method 1: Figure 4 As shown, a temperature-controlled digital potentiometer RT is used to control the voltage divider ratio of the APD reverse bias voltage, and the reverse bias voltage of the APD is temperature-regulated in the full temperature range;

[0083] Method 2: If Figure 4 As shown, when the output of the bias controller has a digital control function, in the case of a temperature sensor, a temperature control driving voltage table is designed in the software, and the output voltage of the bias controller is controlled by looking up the table through the temperature, thereby realizing temperature control of the APD bias.

[0084] Visualize high gain adjustment methods, such as Figure 4 and Figure 5a-5b As shown in the figure: Set the output voltage of the APD bias controller to the typical reverse bias voltage value specified for the APD product; start the echo reflection test to obtain the logarithmic characteristic curve of the reflected echo; change the reverse bias voltage of the APD to exceed the typical value and be lower than the breakdown voltage value; synchronously adjust RP and the controller output, as shown in the figure. Figure 5a-5b As shown, the reflected echo is maximized and meets the specified linearity value requirement (such as the specified linearity is 0.1 dB) in the logarithmic linear region of the reflected echo.

[0085] Linear region and linearity determination:

[0086] The linear region is the test value region of the optical cable loss in the optical cable reflection echo. The higher the linearity, the more accurate the measurement value. This requires that the reflection echo must be linearly amplified. If the amplifier is severely saturated, the linearity is reduced, and the error of the measurement value of the optical cable loss using the linear region increases. The determination of the linear region and linearity is one of the software functions of the optical time domain reflectometer. Linearity is the allowable value of the linearity of the reflection echo specified by the user of the optical time domain reflectometer during use. The linearity is defined by the user.

[0087] The key to software detection is to automatically identify the starting position of the linear region. Figure 6 As shown in the figure, when the falling edge of the near-end or middle reflected pulse is tangent to the linear zone extension straight line, the starting evaluation point that meets the specified linearity requirements is defined as the starting position of the linear zone.

[0088] The control module identifies the starting position of the linear region of the reflected echo signal by the following method:

[0089] 1) The linear region evaluation expression is defined as:

[0090]

[0091] Where: n is the linear region evaluation value corresponding to the distance position n, dimensionless;

[0092] n is the distance position of the reflected echo, dimensionless;

[0093] y n is the echo reflection value corresponding to the distance position n, in dB;

[0094] y (n+p) is the echo reflection value corresponding to the distance position (n+p), p is a constant related to the current distance range of the optical time domain reflectometer, and the unit is dB;

[0095] y (n+4) is the echo reflection value corresponding to the distance position (n+4), in dB;

[0096] y (n+p+4) is the echo reflection value corresponding to the distance position (n+p+4), in dB.

[0097] 2) Linearity is defined as: γ n =(1-ρ n );

[0098] 3) Four consecutive judgments and satisfy γ n ≤|±Linearity|When required, confirm initial γ n The n value that meets the linearity requirement is the starting position of the linear region.

Claims

1. An optical time domain reflectometer based on high-gain optical pulse transceiver, characterized in that: The invention comprises a reflection echo digital processor and a control module, wherein the reflection echo digital processor and the control module are bidirectionally connected, and data processed by the reflection echo digital processor is transmitted to the control module for processing and analysis, so as to obtain the fault position of the optical cable and accept the control of the control module; an excitation pulse signal output end of a driving pulse generator in the reflection echo digital processor is connected to a pulse signal input end of an excitation optical pulse driving circuit, a test laser pulse output end of the excitation optical pulse driving circuit is connected to a test signal input end of a transceiver isolator, and a test signal output end of the transceiver isolator is connected to one end of the optical cable to be tested. The reflected echo signal output end of the optical cable under test is connected to the reflected echo signal input end of the transceiver isolator, the reflected echo signal output end of the transceiver isolator is connected to the signal input end of the APD preamplifier and photoelectric conversion circuit, the signal output end of the APD preamplifier and photoelectric conversion circuit is connected to the reflected echo signal input end of the reflected echo digital processor after passing through the programmable amplifier module and the A / D conversion module in sequence, and the signal output end of the APD bias circuit is connected to the bias signal input end of the APD preamplifier and photoelectric conversion circuit, so as to input a bias signal to the APD preamplifier and photoelectric conversion circuit.

2. The optical time domain reflectometer based on high-gain optical pulse transceiver as claimed in claim 1, characterized in that: During the test of the reflectometer, a periodic test light pulse with a specified pulse width is sent to the optical cable under the control of the reflected echo digital processor. During the transmission of the optical cable, the test light pulse generates a Rayleigh reflected light echo signal due to the inherent manufacturing defects of the optical cable. If the test light pulse encounters a fault point, the test light pulse generates a Fresnel reflected light pulse at the fault point due to the change in the refractive index of the fault point. The Fresnel reflected light pulse and the Rayleigh reflected light echo signal are transmitted back to the APD preamplifier and photoelectric conversion circuit for preamplification, and then A / D conversion is performed. After the sampling data is processed, it is uploaded to the control module, and the main control programmer reconstructs and analyzes the characteristics of the reflected echo. According to the waveform characteristics and propagation time of the reflected echo and the reflected pulse in the optical cable, the optical cable loss situation and the fault point position are calculated and displayed, so as to realize the detection of optical cable transmission characteristics and fault location.

3. The optical time domain reflectometer based on high-gain optical pulse transceiver as claimed in claim 1, characterized in that: The excitation light pulse driving circuit includes a capacitor C2', one end of the capacitor C2' is a pulse signal input end of the excitation light pulse driving circuit, and the other end of the capacitor C2' is divided into three paths, the first path is grounded through a resistor R4', the second path is connected to the gate of the field effect tube V2', and the third path is connected to the gate of the field effect tube V3'. The source of the field effect tube V2' and the source of the field effect tube V3' are grounded, and the drain of the field effect tube V2' is connected to the drain of the field effect tube V3' and then divided into three paths. The first path is connected to the gate of the field effect tube V3' in sequence through the resistor R3' and the capacitor C1'. The first path is connected to the ground, the second path is connected to one end of the resistor R2', and the third path is connected to one end of the potentiometer RP'; the other end of the resistor R2' is divided into two paths, the first path is connected to one end of the resistor R1', and the second path is connected to one end of the temperature control potentiometer RT'; the other end of the potentiometer RP', the other end of the resistor R1' and the other end of the temperature control potentiometer RT' are connected to each other and then connected to the negative electrode of the single-mode continuous wave laser V1', the positive electrode of the single-mode continuous wave laser V1' is connected to the power supply VCC, and the single-mode continuous wave laser V1' outputs an optical pulse signal.

4. The optical time domain reflectometer based on high-gain optical pulse transceiver as claimed in claim 3, characterized in that: The operating current of the excitation light pulse driving circuit is calculated according to the following formula: I LD(25℃) =(V CC(25℃) -V LD(25℃) ) / (R1′+R2′+Rds / 2) Where: I LD(25℃) The peak value of the pulse drive current of the excitation light pulse drive circuit at 25°C; V cc(25℃) It is the working voltage of the excitation light pulse driving circuit at the reference working temperature of 25°C; V LD(25℃) It is the voltage drop of the excitation light pulse driving circuit at the reference operating temperature of 25°C; R1' is the resistance value of bias resistor R1'; R2' is the resistance value of bias resistor R2'; Rds / 2 is the internal resistance of the driver, which is calculated by connecting field effect transistors V2' and V3' in parallel.

5. The optical time domain reflectometer based on high-gain optical pulse transceiver as claimed in claim 1, characterized in that: The APD bias circuit includes a MAX1932 chip U1, wherein the pin 1 of the U1 is connected to one end of a resistor R12, and the other end of the resistor R12 is a clock signal input end; the pin 2 of the U1 is connected to one end of a resistor R13, and the other end of the resistor R13 is a data signal input end; the pin 12 of the U1 is connected to one end of a resistor R11, and the other end of the resistor R11 is a chip select signal input end; the EP pin of the U1 is grounded; the pin 8 of the U1 is grounded via a resistor R9 and a capacitor C6 in turn, and the node of the resistor R9 and the capacitor C6 is connected to one end of a resistor R10 , the other end of the resistor R10 is connected to the positive electrode of the diode V3, and the 9th foot of the U1 is connected to the negative electrode of the diode V3 as the overcurrent alarm signal output end of the bias circuit; the 6th foot of the U1 is connected to one end of the resistor R7, and the other end of the resistor R7 is divided into five paths, the first path is connected to one end of the resistor R8, the second path is connected to the 7th foot of the U1, the third path is connected to one end of the resistor R3, the fourth path is connected to one end of the resistor R4, and the fifth path is connected to one end of the resistor R5; the other end of the resistor R8 is the bias voltage monitoring signal output end, and the other end of the resistor R3 is connected to the negative electrode of the diode V3. One end is connected to one end of capacitor C4, and the other end of capacitor C4 is divided into two paths, the first path is connected to pin 5 of U1, and the second path is connected to the other end of resistor R4; the other end of resistor R5 is divided into two paths, the first path is connected to ground after passing through potentiometer RP and resistor R6 in sequence, and the second path is grounded through temperature control potentiometer RT; pin 10 of U1 is connected to the gate of field effect tube V2, the source of field effect tube V2 is grounded, the drain of field effect tube V2 is divided into two paths, the first path is connected to one end of inductor L1, and the second path is connected to the positive electrode of diode V1, and the other end of inductor L1 is divided into There are four paths, the first path is connected to the 11th pin of U1, the second path is connected to the power supply VCC, the third path is grounded via the capacitor C1, and the fourth path is grounded via the capacitor C2; the cathode of the diode V1 is divided into four paths, the first path is grounded via the capacitor C3, the second path is grounded via the resistor R1, the third path is connected to the 4th pin of U1, and the fourth path is connected to one end of the resistor R2; the other end of the resistor R2 is divided into four paths, the first path is connected to the node of the resistor R4 and the capacitor C4, the second path is grounded via the capacitor C5, the third path is grounded via the single-mode continuous wave laser V4, and the fourth path is the bias voltage output end of the bias circuit.

6. The optical time domain reflectometer based on high-gain optical pulse transceiver as claimed in claim 5, characterized in that: The control basis of temperature compensation when the high gain bias voltage of the APD avalanche diode changes with temperature is shown in the following equation: V Rr(25 ℃ +ΔT) =V Rr(25 ℃ ) (1+β×ΔT) Where: V Br(25 ℃ +ΔT) is the high gain bias voltage of the APD avalanche diode when it deviates from the reference operating temperature by ΔT; V Br(25 ℃ ) It is the high gain bias voltage of the APD avalanche diode at the reference operating temperature of 25°C; β is the temperature coefficient of the APD avalanche diode; ΔT is the temperature change of the APD avalanche diode when it deviates from the reference operating temperature of 25°C.

7. The optical time domain reflectometer based on high-gain optical pulse transceiver as claimed in claim 5, characterized in that: The bias voltage of the APD avalanche diode is obtained by the output voltage divider circuit: V BK =V REF (R5+RP+R6) / (R4+R5+RP+R6) Where: V BK is the high gain bias voltage of the APD avalanche diode; V REF It is the reference output voltage of the controller of APD avalanche diode, and the reference output voltage can be programmable; R4, R5, R6 are the resistance values ​​of bias circuit resistors R4, R5, R6 respectively; RP is the resistance value of the high gain bias voltage output trimmer potentiometer RP of the APD avalanche diode.

8. The optical time domain reflectometer based on high-gain optical pulse transceiver as claimed in claim 1, characterized in that: The control module identifies the starting position of the linear region of the reflected echo signal by the following method: 1) The linear region evaluation expression is defined as: Where: n is the linear region evaluation value corresponding to the distance position n, dimensionless; n is the distance position of the reflected echo, dimensionless; y n is the echo reflection value corresponding to the distance position n, in dB; y (n+p) is the echo reflection value corresponding to the distance position (n+p), p is a constant related to the current distance range of the optical time domain reflectometer based on high-gain optical pulse transmission and reception, and the unit is dB; y (n+4) is the echo reflection value corresponding to the distance position (n+4), in dB; y (n+p+4) is the echo reflection value corresponding to the distance position (n+p+4), in dB. 2) Linearity is defined as: γ n =(1-ρ n ); 3) Four consecutive judgments and satisfy γ n ≤|±Linearity|When required, confirm initial γ n The n value that meets the linearity requirement is the starting position of the linear region.

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