Optical Time Domain Reflectometer Based on High-Gain Optical Pulse Transceiver

Abstract

The present invention discloses an optical time domain reflectometer based on high-gain optical pulse transceiver, which relates to the technical field of optical cable testing devices. The reflectometer includes a reflected echo digital processor and a control module, and there is a two-way connection between the reflected echo digital processor and the control module; the excitation pulse signal output end of the drive pulse generator in the reflected echo digital processor is connected to the pulse signal input end of the excitation optical pulse drive circuit, the test laser pulse output end of the excitation optical pulse drive circuit is connected to the test signal input end of the transceiver isolator, the test signal output end of the transceiver 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 output end of the reflected echo signal of the transceiver isolator is connected to the signal input end of the APD preamplification and optoelectronic conversion circuit, and the signal output end of the APD bias circuit is connected to the bias signal input end of the APD preamplification and optoelectronic conversion circuit. The reflectometer has the advantages of high sensitivity and accurate testing, etc.

Description

Technical Field

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

[0002] The transmission characteristics of an optical cable mainly refer to the distance loss characteristics, total loss characteristics, echo reflection, etc. of the optical cable. The fault characteristics of the optical cable mainly refer to the reflection loss of the active connector during optical cable connection, the distance position of the Fresnel reflection pulse at the optical cable break point, etc. These characteristics are mainly obtained by injecting a periodic optical pulse with a specified pulse width into the optical cable for test drive excitation to generate a reflected echo, then receiving the reflected optical signal, and finally performing optoelectronic conversion and digital analysis processing on the reflected optical signal.

[0003] Optical fiber communication systems are not only applied to long-distance communication trunk lines, but also to various applications in the last mile of optical fiber, such as home and office automation systems. An optical time domain reflectometer is an important measuring device for testing the performance of an optical fiber network and locating fault points. The optical time domain reflectometer measures parameters such as optical fiber joints, losses, lengths, and reflectivities by detecting the correspondence between the intensity of the backward Rayleigh scattering and Fresnel reflection optical signals in the optical fiber and time. The sensitivity of the optical time domain reflectometer in the prior art is low, and the power consumption is high. 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 transceiver with high sensitivity and accurate testing.

[0005] To solve the above technical problems, the technical solution adopted by the present invention is as follows: An optical time domain reflectometer based on high-gain optical pulse transceiver, including a reflected echo digital processor and a control module. The reflected echo digital processor and the control module are bidirectionally connected. The data processed by the reflected echo digital processor is transmitted to the control module for processing and analysis to obtain the fault location of the optical cable, and it accepts the control of the control module. The excitation pulse signal output end of the drive pulse generator in the reflected echo digital processor is connected to the pulse signal input end of the excitation optical pulse drive circuit. The test laser pulse output end of the excitation optical pulse drive circuit is connected to the test signal input end of the transceiver isolator. The test signal output end of the transceiver 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 output end of the reflected echo signal of the transceiver isolator is connected to the signal input end of the APD preamplification and optoelectronic conversion circuit. The signal output end of the APD preamplification and optoelectronic conversion circuit is sequentially connected to the reflected echo signal input end of the reflected echo digital processor through a programmable gain module and an A / D conversion module. The signal output end of the APD bias circuit is connected to the bias signal input end of the APD preamplification and optoelectronic conversion circuit for inputting a bias signal to the APD preamplification and optoelectronic conversion circuit.

[0006] Furthermore, the technical solution lies in that when the reflectometer is testing, it sends a periodic test optical pulse with a specified pulse width to the optical cable under test under the control of the reflected echo digital processor. During the transmission of the test optical pulse in the optical cable, due to the inherent manufacturing defects of the optical cable, Rayleigh reflected optical echo signals are generated. If the test optical pulse encounters a fault point, due to the change in the refractive index at the fault point, the test optical pulse generates a Fresnel reflected optical pulse at the fault point. The Fresnel reflected optical pulse and the Rayleigh reflected optical echo signal are both transmitted back to the APD preamplification and optoelectronic conversion circuit for preamplification, and then A / D conversion is performed. After the sampling data processing is completed, it is uploaded to the control module. The main control program reconstructs and analyzes the characteristics of the reflected echo. Based on 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 location are calculated and displayed, realizing the detection of the optical cable transmission characteristics and the fault location.

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

[0008] Further technical solution lies in that the APD bias circuit includes a MAX1932 type chip U1. One end of the 1st pin of the U1 is connected to one end of a resistor R12, and the other end of the resistor R12 is the clock signal input terminal; one end of the 2nd pin of the U1 is connected to one end of a resistor R13, and the other end of the resistor R13 is the data signal input terminal; one end of the 12th pin of the U1 is connected to one end of a resistor R11, and the other end of the resistor R11 is the chip select signal input terminal; the EP pin of the U1 is grounded; one end of the 8th pin of the U1 is grounded through a resistor R9 and a capacitor C6 in sequence. The node of the resistor R9 and the capacitor C6 is connected to one end of a resistor R10, and the other end of the resistor R10 is connected to the positive electrode of a diode V3. The 9th pin of the U1 is connected to the negative electrode of the diode V3 and serves as the overcurrent alarm signal output terminal of the bias circuit; one end of the 6th pin of the U1 is connected to one end of a resistor R7. The other end of the resistor R7 is divided into five paths. The first path is connected to one end of a resistor R8, the second path is connected to the 7th pin of the U1, the third path is connected to one end of a resistor R3, the fourth path is connected to one end of a resistor R4, and the fifth path is connected to one end of a resistor R5; the other end of the resistor R8 is the bias voltage monitoring signal output terminal. The other end of the resistor R3 is connected to one end of a capacitor C4. 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 grounded through a potentiometer RP and a resistor R6 in sequence, and the second path is grounded through a temperature control potentiometer RT; the 10th pin of the U1 is connected to the gate of a field effect transistor V2. The source of the field effect transistor V2 is grounded. The drain of the field effect transistor V2 is divided into two paths. The first path is connected to one end of an inductor L1, and the second path is connected to the positive electrode of a diode V1. The other end of the inductor L1 is divided into four paths. The first path is connected to the 11th pin of the U1, the second path is connected to a power supply VCC, the third path is grounded through a capacitor C1, and the fourth path is grounded through a capacitor C2; the negative electrode of the diode V1 is divided into four paths. The first path is grounded through a capacitor C3, the second path is grounded through a resistor R1, the third path is connected to the 4th pin of the U1, and the fourth path is connected to one end of a 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 a capacitor C5, the third path is grounded through a single-mode continuous wave laser V4, and the fourth path is the bias voltage output terminal of the bias circuit.

[0009] The beneficial effects of adopting the above technical solutions are as follows: When the reflectometer of the present application is working, first, under the control of the reflected echo digital processor, the excitation optical pulse driving circuit sends a periodic test optical pulse with a specified pulse width to the optical cable. During the transmission of the test optical pulse in the optical cable, due to the inherent manufacturing defects of the optical cable (such as water ripples, microbubbles, impurities, etc. that appear in the optical cable during the production of the optical cable), a Rayleigh reflected optical echo signal is generated. If the test optical pulse encounters a fault point, such as a reflection loss due to poor optical cable connection, the optical cable is disconnected, or the optical cable is bent, due to the change in the refractive index at the fault point, the test optical pulse generates a Fresnel reflected optical pulse at the fault point. The Fresnel reflected optical pulse and the Rayleigh reflected optical echo signal are both transmitted back to the APD pre-amplification and optoelectronic conversion circuit of the tester for pre-high-sensitivity amplification, and then digitized. These digitized processes are completed by the reflected echo digital processor, including sampling control, 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 characteristics of the reflected echo. Based on 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 location of the fault point are calculated and displayed, thereby realizing the detection of the optical cable transmission characteristics and the fault location. The reflectometer of the present application has the advantages of high sensitivity, low power consumption, and accurate testing. Description of the Drawings

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

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

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

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

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

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

[0016] Figure 5b is the reconstructed reflected echo diagram after the APD high-sensitivity gain adjustment in the embodiment of the present invention;

[0017] Figure 6 is the schematic diagram for identifying the linear region position in the embodiment of the present invention. Specific Embodiments

[0018] Combined with the accompanying drawings in the embodiments of the present invention, the technical solutions in the embodiments of the present invention are clearly and completely described. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative work fall within the protection scope of the present invention.

[0019] In the following description, many specific details are set forth in order to provide a thorough understanding of the present invention. However, the present invention may be implemented in other ways different from those described herein. Those skilled in the art can make similar generalizations without departing from the connotation of the present invention. Therefore, the present invention is not limited by the specific embodiments disclosed below.

[0020] Generally, as Figure 1 shown, an optical time domain reflectometer based on high-gain optical pulse transceiver is disclosed in the embodiments of the present invention. The optical time domain reflectometer includes a reflected echo digital processor and a control module. A two-way connection is established between the reflected echo digital processor and the control module. The data processed by the reflected echo digital processor is transmitted to the control module for processing and analysis to obtain the fault location of the optical cable, and it accepts the control of the control module. The excitation pulse signal output terminal of the drive pulse generator in the reflected echo digital processor is connected to the pulse signal input terminal of the excitation optical pulse drive circuit. The test laser pulse output terminal of the excitation optical pulse drive circuit is connected to the test signal input terminal of the transceiver isolator. The test signal output terminal of the transceiver isolator is connected to one end of the optical cable under test. The reflected echo signal output terminal of the optical cable under test is connected to the reflected echo signal input terminal of the transceiver isolator. The output terminal of the reflected echo signal of the transceiver isolator is connected to the signal input terminal of the APD preamplification and optoelectronic conversion circuit. The signal output terminal of the APD preamplification and optoelectronic conversion circuit is sequentially connected to the reflected echo signal input terminal of the reflected echo digital processor through a programmable gain module and an A / D conversion module. The signal output terminal of the APD bias circuit is connected to the bias signal input terminal of the APD preamplification and optoelectronic conversion circuit for inputting a bias signal to the APD preamplification and optoelectronic conversion circuit.

[0021] The working timing of the optical time domain reflectometer is as Figure 2As shown in the figure. When the optical time domain reflectometer (OTDR) performs detection, first, under the control of the reflected echo digital processor, the excitation optical pulse driving circuit sends a periodic test optical pulse with a specified pulse width to the optical cable. During the transmission of the test optical pulse in the optical cable, due to the inherent manufacturing defects of the optical cable (such as corrugations, micro-bubbles, impurities, etc. that appear in the optical cable during the production of the optical cable), Rayleigh reflected optical echo signals are generated. If the test optical pulse encounters a fault point, such as a reflection loss due to poor optical cable splicing, the optical cable is disconnected, or the optical cable is bent, due to the change in the refractive index at the fault point, the test optical pulse generates a Fresnel reflected optical pulse at the fault point. The Fresnel reflected optical pulse and the Rayleigh reflected optical echo signal are both transmitted back to the APD pre-amplification and optoelectronic conversion circuit of the tester for pre-high-sensitivity amplification, and then digital processing is performed. This digital processing is 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 program reconstructs and analyzes the characteristics of the reflected echo. Based on 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 location of the fault point are calculated and displayed, thereby realizing the detection of the optical cable transmission characteristics and fault location.

[0022] Excitation optical pulse driving method:

[0023] As Figure 1 shown in the figure, when the optical time domain reflectometer performs optical time domain analysis on the optical cable and needs to obtain the reflected echo of the optical cable, it is necessary to inject a periodic optical pulse with a specified pulse width for test drive excitation. The optical pulse described in this application is generated by pulse driving a low-power continuous wave laser.

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

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

[0026] The 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 in temperature, thereby reducing the output power of the laser. In order to offset the influence of the change in the threshold current of the continuous-wave laser with temperature on the output pulse power within the full temperature range, it is necessary to implement temperature control on the driving current and threshold current of the laser.

[0027] Technical requirements for the continuous-wave laser:

[0028] 1) The average power of the continuous wave output by the single-mode continuous-wave laser is required to be not less than 7 mW;

[0029] 2) The ultimate breakdown current of the single-mode continuous-wave laser is not less than 400 mA / 1 μs driving pulse (25 °C) (or the technical requirements specified by the laser manufacturer). If the ultimate breakdown current is low, it will limit the pulse driving 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: not greater than 10 mA (25 °C) (or the technical requirements specified by the laser manufacturer);

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

[0032] Requirements and methods for pulsed output control of continuous-wave lasers:

[0033] 1) Pulsed driving method: As Figure 3 shown, use a medium-power (single-tube pulsed driving current not less than 0.68 A) dual-gate field-effect transistor, single-tube or multi-tube parallel driving, or other driving methods;

[0034] 2) Design of pulsed driving circuit parameters: The pulsed driving circuit parameters are designed based on the laser's ultimate breakdown current, the laser's quantum efficiency, and the driving pulse width specified for optical time domain analysis of 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 an optical cable over a medium to short distance (not less than 60 km for characteristic analysis and not less than 120 km for fault detection), it is advisable to set the upper limit of the driving pulse width to 3 μs from the perspective of safety and reliability design.

[0036] If the ultimate breakdown current of the continuous-wave laser is not less than 400 mA / 1 μs driving pulse, based on the equivalent ultimate breakdown energy of the laser, when driving with a 3 μs pulse width, the ultimate breakdown current of the laser is 1 / 3 of that with a 1 μs pulse width, i.e., the driving pulse current is 133 mA. The optical pulse output power is obtained from the product of the laser's quantum efficiency and the driving current. An example of a set of driving parameters is shown in Table 1.

[0037] Table 1 Laser driving parameter table

[0038]

[0039]

[0040] The operating current of the excitation optical 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] In the formula: I LD(25℃) is the peak value of the pulsed driving current of the excitation optical pulse driving circuit at 25 °C;

[0043] Vcc(25℃) is the operating voltage of the excitation optical pulse drive circuit at the reference operating temperature of 25°C;

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

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

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

[0047] Rds / 2 is the internal resistance of the driver, calculated as the parallel connection of the field effect transistors V2' and V3'.

[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 in the specified full temperature range, the drive current of the laser changes with the threshold current of the laser, and the change range 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 drive current of the driver approaches the limit breakdown current of the continuous wave laser, the power supply circuit has an overcurrent protection function to avoid damaging the laser.

[0050] 1) Continuous wave single-mode lasers usually refer to single-mode lasers used in communication equipment and measurement equipment. They have a large usage volume, wide sources, good temperature characteristics and stability. The average output power of continuous wave single-mode lasers is not greater than 10mW, the threshold current is small, generally less than 10mA. Isolators are generally integrated in continuous wave single-mode lasers to improve the stability of the lasers. When an integrated back detector (sampling the output power of the laser) is integrated, it is convenient to implement APC automatic power control for the output power of the laser.

[0051] 2) The temperature-controlled potentiometer is an integrated digital control potentiometer. The temperature-controlled potentiometer has multiple storage units, addressed by temperature value, to store the resistance values corresponding to different temperatures, that is, divided according to the full operating temperature range, and a resistance value corresponds to different temperature ranges. When the temperature changes, the resistance value of the potentiometer stores the resistance value corresponding to the current temperature range. Temperature-controlled potentiometers with different resistance values and temperature series can be selected according to design needs. The main device manufacturers include companies such as MAXIM in the United States.

[0052] Method for receiving and processing the reflected optical signal:

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

[0054] When the APD operates normally, it requires a reverse bias of a specified operating voltage (the product manual of the APD will provide the typical value of the reverse bias operating voltage). However, the conventional bias voltage cannot effectively increase the APD gain to meet the low-noise high-gain amplification of weak signals. This method proposes a visual APD high-gain bias adjustment method to increase the APD gain and achieve low-noise amplification and optoelectronic conversion of the reflected echo.

[0055] The APD is a temperature-sensitive device. When the APD is biased at a fixed operating voltage, its avalanche gain decreases with the increase in temperature and increases with the decrease in temperature. The relationship between the APD high-gain bias voltage and the operating temperature is shown in Equation (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] In the formula: V Br(25℃+ΔT) is the high-gain bias voltage of the APD avalanche diode when the temperature deviates from the reference operating temperature by ΔT;

[0058] V Br(25℃) 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 when the APD avalanche diode deviates from the reference operating temperature of 25°C.

[0061] The APD avalanche gain is controlled by the reverse bias voltage. To keep the APD avalanche gain constant within the specified operating temperature range, the bias voltage of the APD must be temperature-controlled. When the temperature rises, the bias voltage of the APD is increased to compensate for the decrease in the APD gain and avoid a decrease in the signal-to-noise ratio of the preamplified signal; when the temperature decreases, the bias voltage of the APD is decreased to compensate for the increase in the APD gain and avoid saturation of the preamplified signal. The control components are a temperature-controlled digital potentiometer and a series-parallel resistor network.

[0062] Technical requirements for the APD avalanche diode:

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

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

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

[0066] 2) Dark current: typical value not greater than 100pA;

[0067] 3) Breakdown voltage:

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

[0069] It is required that the difference between the breakdown voltage value and the typical reverse bias voltage value: 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) Variation of gain over the full temperature range: not exceeding 50% (or the technical requirements specified by the laser manufacturer).

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

[0072] Bias control:

[0073] Use an application-specific integrated circuit to control the bias voltage of the APD, as Figure 4 shown. The peripheral circuit of the controller is the typical circuit and specified parameters of the devices ( Figure 4 resistors R1~R16, capacitors C1~C5, field effect transistors V1~V3, inductor L1 in it). The APD bias voltage is obtained from the output voltage dividing circuit (Equation 3).

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

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

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

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

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

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

[0080] RP is the resistance value of the fine-tuning potentiometer for the high-gain bias voltage output of the APD avalanche diode.

[0081] Temperature control method:

[0082] Method 1: As Figure 4 shown, use the temperature-controlled digital potentiometer RT to control the voltage division ratio of the APD reverse bias voltage, and perform temperature adjustment on the APD reverse bias voltage in the full temperature range;

[0083] Method 2: As Figure 4 shown, when the output of the bias controller has a numerical control function, in the case of having a temperature sensor, design a temperature control drive voltage table in the software, look up the table through temperature, and control the output voltage of the bias controller, so as to achieve the temperature control of the APD bias.

[0084] Visual high-gain adjustment method, as Figure 4 and Figures 5a - 5b shown: 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 APD reverse bias voltage to exceed the typical value and be lower than the breakdown voltage value; Synchronously adjust RP and the controller output, as Figures 5a - 5b shown, so that the reflected echo is the largest and meets the specified linearity value (such as the specified linearity is 0.1 dB) requirement in the logarithmic linear region of the reflected echo.

[0085] Determination of the linear region and linearity:

[0086] The linear region is the region of the optical cable loss test value in the optical cable reflected echo. The higher the linearity, the more accurate the measurement value. This requires that the reflected echo must be linearly amplified. If the amplifier is severely saturated, the linearity decreases, and the measurement error 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 specified reflected echo linearity defined by the user of the optical time domain reflectometer, and the linearity is defined by the user.

[0087] The key to software detection is to automatically identify the starting position of the linear region. Automatically identifying the starting position of the linear region is as Figure 6 shown. When the falling edge of the proximal or middle reflection pulse is tangent to the extended straight line of the linear region, the starting evaluation point that meets the specified linearity requirement is defined as the starting position of the linear region

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

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

[0090]

[0091] In the formula: ρ 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, with the unit of dB;

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

[0095] Iy (n+4) is the echo reflection value corresponding to the distance position (n + 4), with the unit of dB;

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

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

[0098] 3) When it is judged four times continuously and satisfies the requirement of γ n ≤|±linearity|, confirm that the n value when the initial γ n 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: It includes a reflected echo digital processor and a control module. There is a two-way connection between the reflected echo digital processor and the control module. The data processed by the reflected echo digital processor is transmitted to the control module for processing and analysis to obtain the fault location of the optical cable and accepts the control of the control module. The excitation pulse signal output terminal of the drive pulse generator in the reflected echo digital processor is connected to the pulse signal input terminal of the excitation optical pulse drive circuit. The test laser pulse output terminal of the excitation optical pulse drive circuit is connected to the test signal input terminal of the transceiver isolator. The test signal output terminal of the transceiver isolator is connected to one end of the optical cable under test. The reflected echo signal output terminal of the optical cable under test is connected to the reflected echo signal input terminal of the transceiver isolator. The output terminal of the reflected echo signal of the transceiver isolator is connected to the signal input terminal of the APD preamplification and optoelectronic conversion circuit. The signal output terminal of the APD preamplification and optoelectronic conversion circuit is sequentially connected to the reflected echo signal input terminal of the reflected echo digital processor through a programmable gain amplifier module and an A / D conversion module. The signal output terminal of the APD bias circuit is connected to the bias signal input terminal of the APD preamplification and optoelectronic conversion circuit for inputting a bias signal to the APD preamplification and optoelectronic conversion circuit; When the reflectometer is testing, it sends a periodic test optical pulse with a specified pulse width to the optical cable under test under the control of the reflected echo digital processor. During the transmission of the test optical pulse in the optical cable, due to the inherent manufacturing defects of the optical cable, a Rayleigh reflected optical echo signal is generated. If the test optical pulse encounters a fault point, due to the change in the refractive index at the fault point, a Fresnel reflected optical pulse is generated at the fault point. The Fresnel reflected optical pulse and the Rayleigh reflected optical echo signal are transmitted back to the APD preamplification and optoelectronic conversion circuit for preamplification, and then A / D conversion is performed. After the sampling data processing is completed, it is uploaded to the control module. The main control program reconstructs and analyzes the characteristics of the reflected echo. Based on 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 location are calculated and displayed to achieve the detection of the optical cable transmission characteristics and the fault location; The preamplification and optoelectronic converter is an APD avalanche diode. The control basis for temperature compensation when the high-gain bias voltage of the APD avalanche diode changes with temperature is shown as follows: V Rr (25 °C + ΔT) = V Rr (25 °C)(1 + β × ΔT) Where: V Rr(25 °C +ΔT) is the high-gain bias voltage of the APD avalanche diode when the deviation from the reference operating temperature is ΔT; V Rr(25 °C ) is the high-gain bias voltage of the APD avalanche diode at the reference working temperature of 25 °C; β is the temperature coefficient of the APD avalanche diode; ΔT is the temperature change amount when the APD avalanche diode deviates from the reference operating temperature of 25°C; The preamplification and optoelectronic converter is an APD avalanche diode. The bias voltage of the APD avalanche diode is obtained from the output voltage dividing 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 is the controller reference output voltage of the APD avalanche diode, and the reference output voltage is programmable; R4, R5, and R6 are the resistance values of the bias circuit resistors R4, R5, and R6 respectively; RP is the resistance value of the high-gain bias voltage output trimming potentiometer RP of the APD avalanche diode.

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

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

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

5. The optical time domain reflectometer based on high-gain optical pulse transceiver according to 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: ρ n is the evaluation value of the linear region 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, with the unit of 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 transceiver, unit is dB; y(n + 4) is the echo reflection value corresponding to the distance position (n + 4), unit is dB; y(n + p + 4) is the echo reflection value corresponding to the distance position (n + p + 4), unit is dB; 2) The linearity is defined as: γ n = (1 - ρ n ); 3) Continuously judge four times and satisfy γ n ≤ |±linearity| requirement, then confirm the initial γ n The n value that satisfies the linearity requirement is the starting position of the linear region.

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