RU2214583C1 - Optical reflectometer - Google Patents

Abstract

FIELD: measurement technology. SUBSTANCE: optical reflectometer includes pulse former, radiation source, coupler, controlled fiber- optical path, two photodetectors, analog commutator, comparator, attenuator, pulse stretcher, analog-to-digital converter, two microcontrollers, indicator, control desk, programmable permanent storage connected in series. It is supplemented with two electron keys, electron analog key, delay line and programmable frequency divider. EFFECT: raised accuracy and resolution of measurements. 4 dwg

Description

 The invention relates to the field of measurement technology, communication technology and optoelectronics and can be used in the electrical industry, communications industry in the production of optical fibers and fiber optic cables, as well as in the laying and operation of fiber optic paths.

 A device for diagnosing fiber optic paths, including a pulsed optical radiation source, an optical directional coupler, a photodetector, an amplifier for measuring signals, a processing device, an oscillographic recorder [1].

 The operation of the device lies in the fact that a short optical pulse is fed to the input of the controlled fiber optic path and the parameters of the pulse signals are measured at the output of the photodetector installed at the output of the coupler in such a way that this photodetector receives an optical backscattering signal. The magnitude of the reflected optical power from the inhomogeneities is determined by the amplitude of the pulses of the optical signal reflected from their location. The distance to the heterogeneity can be judged by the magnitude of the delay of the optical signal. The shorter the measuring pulse, the more accurately you can determine the location of a local heterogeneity. On the other hand, with a decrease in the duration of the probe pulses, their frequency spectrum increases. With an increase in the broadband of the device nodes, the dynamic range of measurements decreases and the complexity and cost of the device increases. The device has a small dynamic measurement range and low accuracy of measuring distances to local inhomogeneities.

A known device for diagnosing fiber optic paths, the closest in its technical essence to the invention [2]. The device allows you to measure the value of local inhomogeneities, the distance to local inhomogeneities and reflection loss. The device contains:
- two microcontrollers;
- pulse shaper;
- semiconductor radiation source;
- optical coupler;
- two optical connectors;
- two photodetectors;
- switch;
- programmable attenuator;
- comparator;
- pulse expander;
- analog-to-digital converter;
- Remote Control;
- digital indicator;
- reprogrammable read only memory.

The disadvantage of this device is: low resolution and accuracy, limited by the time it takes to perform one operation of the microcontroller that detects a signal at the output of the comparator. The presence of a signal at the output of the comparator is determined by the following sequence of instructions:
PR1: SNB $ 06.0; test bit 0 (port "B"), skip the following instructions, if there is a signal;
JMP PR1; Go to label PR1.

 This sequence of instructions takes three machine cycles. If we use a Scenix Semiconductor SX18AC microcontroller operating at a frequency of 150 MHz with pipelined execution of instructions as a microcontroller that detects a signal reflected from inhomogeneities, then the ambiguity in determining the moment of arrival of the next pulse reflected from the inhomogeneity will be the execution time of three machine cycles, then there are 3 • 6.7 ns = 20.1 ns. Taking into account the refractive index of the core of the optical fiber 1.47, the ambiguity in determining the distance to the location of the heterogeneity will be about 4 m.

 The proposed device solves the problem of increasing the resolution and accuracy of measuring the distance to the location of local inhomogeneities in the fiber optic path.

 The essence of the invention lies in the fact that two electronic digital keys, one electronic analog key, a delay line, a programmable counter frequency divider are introduced, the data bus of a programmable counter frequency divider connected to the outputs of the first microcontroller, a control bus with the outputs of the second microcontroller, a counting input with the output of the comparator, an additional output is connected to the control input of the analog key, the main output is connected to the input of the delay line, to the input of the second microcontroller and to the output the first electronic digital key, the input of which is connected to the output of the delay line and the input of the second electronic digital key, the output of which is connected to the input of the pulse shaper, and the control inputs of the first and second electronic keys are connected to one of the terminals of the second microcontroller, an analog electronic key is connected between the output of the attenuator and the input of the pulse expander, the inputs of the comparator and attenuator are connected to the output of the analog switch.

 The drawing (see figure 1) shows the structural diagram of the reflectometer.

 The device consists of sequentially installed pulse former 1, a semiconductor radiation source 2, an optical coupler 3, an optical connector 4 connected to the measured fiber optic path 5, a second optical connector 6, optically coupled to a photodetector 7, a second photodetector 8, mounted on the second output coupler 3, analog switch 9, the inputs of which are connected to the outputs of photodetectors 7 and 8, and the output is connected to the inputs of the comparator 10 and programmable attenuator 11, dv microcontrollers 12 and 13, a programmable counter-frequency divider 14, two digital keys 15 and 16, an analog key 17, a pulse expander 18, a delay line 19, an analog-to-digital converter (ADC) 20, an indicator 21, a control panel 22, a programmable constant storage device 23.

 OTDR operates as follows.

 The OTDR measures the reflection loss, reflected optical power and the distance to the location of the local heterogeneities of the fiber optic path. To measure reflection losses, calibration is required first. In calibration mode, optical power is measured at the output pole of the measured fiber optic path. The measuring signal in this case is a sequence of optical pulses generated by a pulse shaper 1 and a semiconductor radiation source 2 from the meander generated by the microcontroller 12.

 In the measurement mode, the microcontroller 12 generates a single optical pulse using a former 1 and a semiconductor radiation source 2, which is introduced into the fiber optic path 5. Optical pulses reflected from the local inhomogeneities of the path are directed through the connector 4 and the coupler 3 to the optical input of the photodetector 8. A photodetector converts optical pulses into electrical ones. Due to the positive feedback, the auto-generation mode is set. Positive feedback consists of the following circuit: optical connector 4, coupler 3, photodetector 8, switch 9, comparator 10, counter-divider 14, delay line 19, electronic key 16, driver 1, radiation source 2. Depending on the path length, the pulse can be delayed in the delay line n times. This is necessary so that the new measuring pulse does not overlap the reflected pulses from the previous measuring signal. The frequency of self-oscillations depends on the propagation time of the pulse along the fiber optic path and constant delays at the nodes of the OTDR and uniquely determines the distance to the location of the inhomogeneity. The main advantage of the proposed device compared to the prototype device is the absence of a microcontroller in the feedback circuit, introducing uncertainty in detecting the occurrence of a measuring pulse.

The amplitude value of the pulse measuring signals is proportional to the power reflected from the inhomogeneities with a constant conversion coefficient. In addition to the distance to the location of the inhomogeneities and the magnitude of the reflected optical power, the reflectometer can calculate the reflection loss from the selected heterogeneity according to the formula:
α _o = -10 • logP _i / P _o (dB), (1)
where α _o - reflection loss from the selected heterogeneity,
P _i - power reflected from the i-th local heterogeneity,
P _about - the power of optical radiation in the output pole of the fiber optic path obtained during calibration.

 When measuring only reflected optical power from local inhomogeneities and the distance to the location of the inhomogeneities, the calibration mode is not used.

 OTDR nodes are controlled, the heterogeneity to be measured, the number of repetitions of the fixed delay, the calculation of the desired characteristics is carried out using two microcontrollers: a high-speed microcontroller 12 and a slower one, but having a much wider set of commands (instructions) of the microcontroller 13.

 Instructions (user programs) are written to the resident memory of microcontroller programs. When you turn on the power of the OTDR, the instructions recorded in the memory of the programs of microcontrollers 12 and 13 begin to be executed. The main program algorithm of the microcontroller 12 is shown in FIG. 2a, the external interrupt processing routines are shown in FIG. 2b, the main program of the microcontroller 13 is shown in FIG. 3a, the internal interrupt processing algorithm is shown in FIG. 3b. Plots of signals in various nodes of the OTDR in the calibration mode are presented in Fig. 4a, in the measurement mode - in Fig. 4b.

 The microcontroller 12 is faster. He is the first to complete the initialization. During initialization, the microcontroller adjusts its outputs (ports) to input or output information, adjusts external interrupt parameters and special registers. After initialization, the controller goes into standby mode of commands from the microcontroller 13.

The microcontroller 13 after initializing the working and special registers, setting up ports for input or output, carries out a dialogue with the operator. The operator sets the following measurement parameters:
- one of the following operating modes:
measuring the distance to the location of inhomogeneities and reflection losses (in this mode, both poles of the fiber optic path should be accessible);
measuring the distance to the location of inhomogeneities and the magnitude of the reflected optical power (in this mode, it is enough to have access to one pole of the fiber optic path);
- refractive index of the core of the optical fiber (by default, the refractive index of fused silica is 1.44778);
- the maximum value of the length of the fiber path, if the path length is unknown, then the default length is set (the maximum possible path length);
- the range of local heterogeneities whose characteristics must be measured (by default, the maximum possible number of heterogeneities is set);
- the number of averages (the default is the number of averages corresponding to the measurement time, not exceeding three minutes).

 After entering the parameters and connecting the monitored fiber-optic path to the optical connectors, the operator enters the "CONDUCT MEASUREMENT" command.

 After the microcontroller 13 memorizes the measurement parameters and receives the command “MEASURE MEASUREMENT”, it sets the built-in timer for a certain time, depending on a given number of averagings.

If calibration mode is necessary, then microcontroller 13:
- sets a code on the data bus of the counter-divider frequency to highlight the second heterogeneity;
- connects using the analog switch 9 of the output of the photodetector 7 to the inputs of the comparator 8 and the attenuator;
- transmits a calibration command to the microcontroller 12;
- prohibits interruptions from the built-in timer;
- performs a subroutine for reading the result of the conversion of the ADC, which includes the following operations:
ADC start-up;
delay for the conversion time;
analysis of the result of the conversion;
increase attenuator attenuation if the signal is above normal;
attenuator attenuation reduction if the signal is below normal;
resetting the pulse expander;
if the signal was lower or higher than normal, then return to the beginning of the subprogram, if the signal is within the normal range, then exit the subprogram;
- produces a hardware reset of the microcontroller 12;
- allows interrupts from the built-in timer and proceeds to refine the measurement mode.

In measurement mode:
- sets the code of the first heterogeneity from a given list of measured inhomogeneities on the data bus of the counter-frequency divider;
- connects using the analog switch 9 of the output of the photodetector 8 to the inputs of the comparator and attenuator;
- calculates the repetitions of a fixed delay and transmits commands and data to the microcontroller 12;
- proceeds to waiting for the signal "START" from the microcontroller 12.

The microcontroller 12, taking the calibration command, performs the following actions:
- writes the code set by the microcontroller 13 to the counter-frequency divider 14, allows the counting and formation of overflow pulses;
- Loops, generating 1 meander at the input of the pulse shaper;
- upon receipt of a hardware reset signal, it repeats the initialization and waits for commands and data from the microcontroller 13.

Accepting the command "PERFORM MEASUREMENT" microcontroller 12:
- receives and stores in the repetition register the value of the number of repetitions of a fixed delay;
- writes the code set by the microcontroller 13 to the data bus of the counter-divider into the counter-frequency divider 14;
- opens the key 15 and closes the key 16;
- allows the count and signal overflow counter-divider frequency 14;
- generates a single pulse at the input of the pulse shaper, which at the same time is the "START" signal for the microcontroller 13;
- expects the appearance of a signal at the main output of the counter-divider frequency 14;
- after detecting the signal at the output of the counter-divider 14:
blocks the counter-divider frequency 14;
checks the contents of fixed delay retry registers;
if the repeat register has not been reset, the microcontroller decrements it, proceeds to wait for the signal at the output of the delay line, after the signal arrives, returns to the analysis of the contents of the repeat register;
if the repeat register is reset, the microprocessor opens the key 16, closes the key 15,
allows the count and overflow signals of the counter-divider 14;
increments drive registers;
again proceeds to waiting for the signal at the output of the counter-divider 14.

Receiving the signal "START", the microcontroller 13 performs the following actions:
- starts the built-in timer;
- Performs the ADC reading routine;
- analyzes the presence of the "STOP" command from the operator; if a command is received, it returns to the dialogue mode to adjust the measurement parameters;
- if the “STOP” command is absent, checks the flag for the end of the cycle, if the flag is cleared, returns to the analysis of the “STOP” command;
- if the flag is set, it is reset;
- analyzes whether all the heterogeneities specified by the operator are processed, if the parameters of all the heterogeneities are measured, then performs a hardware reset of the microcontroller 12, proceeds to the processing of the measured information base, calculates the reflection loss, the reflected power value, the distance to the location for all heterogeneities, goes to dialogue with the operator and displaying information on the indicator;
- if not all the specified heterogeneities are processed, it generates new data for the microcontroller 12, sets a modified code at the inputs of the counter-divider frequency 14;
- produces a hardware reset of the microcontroller 12.

When the timer works out the specified time interval, the corresponding flag is set, causing the main program of the microprocessor 13 to be interrupted. In this case, the microprocessor:
- generates an external interrupt signal of the microcontroller 12;
- reads the accumulative registers of the microcontroller 12, which stores the number of pulses reflected from a given heterogeneity, accumulated over a specified time interval;
- sets the flag to end the cycle;
- restores the contents of the registers of the built-in timer;
- returns to the main program.

The microprocessor 12 upon receipt of an external interrupt signal proceeds to its processing subroutine:
- blocks the counter-divider frequency 14;
- transfers the contents of the accumulation registers to the microcontroller 13;
- goes into low power mode.

 Plots of signals in the nodes of the device are presented in figa (for calibration mode) and 4b (for measurement mode). In calibration mode, microprocessor 12 forms a meander directed to the input of pulse former 1, which shortens the leading edge and forms a short pulse of the pump current of the semiconductor radiation source 2. Short optical pulses passing through coupler 3 are introduced using connector 4 into the fiber optic path (BOT ) and with the help of connector 6 they are output to the optical input of the photodetector 7. The photodetector converts them into electrical impulses, which, through the analog switch 9, arrive simultaneously at the comparator OP 10 and attenuator 11. The analog switch 9 is switched by a microcontroller 13. The comparator 10 converts the pulse level to a standard value (CMOS or TTL) and sends them to the input of the frequency divider 14. The frequency divider is a set of binary counters with inputs for preliminary recording of data and triggered by a decrease in voltage at the counting input. The main output of the frequency divider counter is the overflow output of the last binary counter. An additional output of the counter-divider 14 is the output of the multi-input element "And" (included in the counter-divider), connected to the binary counters in such a way that its output is activated one pulse before the overflow signal appears. In calibration mode, a code is written to binary counters, to data registers, in which an overflow signal appears every second counting pulse. An additional output of the counter-divider 14 is connected to the control input of the analog switch 17. As follows from the diagrams in Fig. 4a, in the calibration mode, the switch 17 emits every second pulse that is fed to the input of the pulse expander from the photodetector 7. The pulse expander reduces the speed requirements of the analog a digital converter 20, converting the amplitude of the pulse into a digital code. The amplitude of the pulses is proportional to the optical power at the output pole of the fiber optic path (taking into account the attenuation introduced by the attenuator 11). The power value at the output pole is necessary for calculating reflection losses using formula (1).

In the measurement mode using the microcontroller 12, the shaper 1 and the radiation source 2, a single optical pulse is generated (the "START" signal), which is sent to the HER. The backscattering radiation from the fiber optic path using a coupler 3 is directed to the optical input of the photodetector 8. Using the analog switch 9, the microcontroller 13 connects the output of the photodetector 8 to the inputs of the comparator 10 and attenuator 11. An example is considered in the diagrams shown in Fig. 4b, in which reflected pulses are received from the input end face, two local inhomogeneities, and from the output end of the BOT to the photodetector 8. The counter-divider 14 is configured to highlight the third order pulse (taking into account the fact that the first pulse is always reflected from the input end of BOT, the pulse is allocated from the second local inhomogeneity). The additional output of the counter-divider 14 is connected to the control input of the analog key 17. As follows from the diagrams, the key is opened by the trailing edge of the pulse from the previous heterogeneity and is closed by the trailing edge of the pulse from the measured heterogeneity. In this case, the signal reflected from the measured inhomogeneity is passed through the key to the input of the pulse expander 18. At the output of the pulse expander using the ADC 20, its amplitude is measured. The pulse at the main output of the counter-divider frequency is detected by the microcontroller 12 for three clock cycles. After detecting the pulse at the main output of the counter-divider, the microcontroller blocks the counter-divider frequency 14, inhibiting the counting mode. Then it analyzes where to direct this impulse. If other pulses from the inhomogeneities of the fiber optic path are possible caused by the “START” signal, then this pulse is delayed in the delay line 19, for this, key 16 must be closed and key 15 open. The number of repetitions of the passage of the delay line 19 is determined by the contents of the repetition register, which is set by the microcontroller 13, and depends on the BOT length (should be greater than twice the propagation time of the signal along the fiber optic path). After the required delay time interval is reached, the microcontroller 12:
- opens the key 16 for the passage of the signal to the input of the pulse shaper 1 to form a new measuring optical pulse;
- allows the counting mode of the counter-divider frequency 14 to receive the next batch of reflected pulses;
- produces an increment of the registers of pulse accumulators from a given heterogeneity of the BOT;
- goes into standby mode of a new pulse at the main output of the counter-divider frequency 14.

So, the process of measuring the fiber optic path consists of the following stages:
1. Tuning for local heterogeneity from a given range.

 2. Initialization of the self-oscillating process. The self-oscillation period is determined by the quasi-constant delay of the measuring signal in the device nodes: photodetector 8, switch 9, comparator 10, counter-divider 14, delay line 19 (n times), key 15 (n-1 times), key 16, pulse shaper 1, the radiation source 2, in the input optical shoelaces connecting the output of the radiation source 1, the coupler 3, the optical input of the photodetector 8 and the optical connector 4 and a variable value of the delay associated with the propagation time of the measuring optical signal to the loc Flax heterogeneity. The period of the self-oscillating process, thus, determines the distance to the location of the heterogeneity.

 3. In each period of the self-oscillating process, a signal is measured at the output of the pulse expander 19, which determines the Fresnel reflection signal from the location of the inhomogeneity.

 4. After the timer is activated, which determines the accumulation period, the accumulation registers contain information about the location of the heterogeneity. The ADC output determines the reflection loss from the heterogeneity and the level of reflected optical power.

 5. If the specified heterogeneity range is not fully processed, then the next heterogeneity from the specified range is tuned.

 6. If all the heterogeneities are processed, then the information is displayed on the indicator in dialogue with the operator.

 We evaluate the characteristics of the proposed device.

As noted earlier, the period of the self-oscillating process during the processing of each heterogeneity consists of two components:
T = t _y + 2 • τ, (2)
where t _y is the delay of the optical signal in the nodes of the device;
τ is the delay of the optical signal in the measured BOT.

It is known that the delay of the optical signal in the fiber determines its length according to the formula:
L = c • τ / n _c , (3)
where c is the speed of light in vacuum;
n _c is the refractive index of the fiber core of the optical fiber cable.

Подставляя формулу (4) в (3), получим:

Таким образом, по периоду следования импульсов автоколебаний можно рассчитать расстояние до места неоднородности. Эту задачу решает микроконтроллер 13.We rewrite formula (2) as follows:

Substituting the formula (4) in (3), we obtain:

Thus, by the period of the self-oscillation pulses, the distance to the place of heterogeneity can be calculated. This problem is solved by the microcontroller 13.

The initial data for calculating the period of self-oscillations are the following data:
- the number of accumulated pulses in the accumulation registers of the microcontroller 12 at the time of operation of the timer built into the microcontroller 13;
- the period of time for which the timer T _n .

During the accumulation time T _n , for which the timer is configured, N pulses will be registered.

The pulse repetition period can be found from the formula:
T = T _n / N (6).

.In view of (6), we rewrite formula (4) as follows:

.

С учетом (3) разрешение по расстоянию можно найти из формулы

где с - скорость света в вакууме;
n_c - показатель преломления сердцевины оптического волокна;
Т_н - период накопления;
N - количество накопленных импульсов.Let us evaluate the resolution of measuring the distance to the location of the heterogeneity. The delay increment of the optical signal corresponding to the increase in the number of accumulated pulses per pulse can be found from the formula:

Taking into account (3), the distance resolution can be found from the formula

where c is the speed of light in vacuum;
n _c is the refractive index of the core of the optical fiber;
T _n - accumulation period;
N is the number of accumulated pulses.

We substitute the parameter values into formula (9) for a specific example. To do this, we set the following values:
- the length of the optical fiber L = 2 km;
- the delay time of the signal in the nodes of the device should be more than double the value of the propagation time of the signal along the fiber optic path, let t _y = 25 μs;
- the refractive index of the core of the optical fiber n _with = 1.4561;
- accumulation time T _n = 30 s.

Let the refractive index of the core of the optical fiber be n _c = 1.4561, then the delay of the optical signal in the fiber will be τ = 2L / (s • n _c ) = 4000 / (3 • 10 ⁸ • 1.5) = 9.15688025 μs.

The pulse repetition period T = (9.15688025 + 25) μs = 34.15688025 μs. During the time T _n = 30 s, N = T _n / T = 878300 pulses will be accumulated.

If the drive registers register one more pulse, i.e. 878301 pulses, then this will correspond to the repetition period equal to T = T _n / N = 34.15685511 μs. Hence, the delay increment of the signal in the optical fiber will be Δτ = 0.00002514 μs, which corresponds to a fiber length of 0.0528 m. Thus, the resolution of the proposed meter with an optical fiber length of 2000 m will be 5.28 cm.

Отсюда можно выделить выражение для погрешности времени задержки оптического сигнала в оптическом волокне:

Определим значения параметров для рассмотренного выше примера, задаваясь стабильностью кварцевых тактовых генераторов δ=10^-8 и нестабильностью задержки в микросхемах, равной 0,2 нс. Будем также учитывать отсутствие корреляционной связи между нестабильностью кварцевых генераторов микроконтроллеров 12 и 13 и нестабильностью задержек в узлах рефлектометра. Можно показать, что для рассмотренного случая погрешность измерения времени задержки составит 0,2 нс, что соответствует длине оптического волокна 40 см.The accuracy of measuring the distance to the location of optical inhomogeneities will be determined by the stability of the refractive index along the length of the optical fiber, the stability of quartz clock oscillators and signal delays at the nodes of the reflectometer. Taking into account possible parameter errors, formula (7) for calculating the delay of an optical signal in an optical fiber can be represented as follows:

From this we can single out the expression for the error in the delay time of the optical signal in the optical fiber:

We determine the parameter values for the example considered above, setting the stability of the crystal oscillators δ = 10 ^-8 and the instability of the delay in microcircuits equal to 0.2 ns. We will also take into account the absence of a correlation between the instability of the crystal oscillators of microcontrollers 12 and 13 and the instability of delays at the nodes of the OTDR. It can be shown that for the case under consideration, the error in measuring the delay time will be 0.2 ns, which corresponds to an optical fiber length of 40 cm.

Sources of information
1. Method and device for optical reflectometry of fibers in the time domain. GB Patent 2167261 A, G 01 S 17/10, G 02 B 6/00, 11.16.84.

 2. A positive decision on the application 2000127248 from 10.31.2000.

Claims (1)

 An optical reflectometer containing a pulse shaper, a radiation source, a coupler, one output of which is connected to the optical input of the photodetector, and the second with a connector - to the input optical pole of the monitored fiber optic path, and a second photodetector optically connected by the optical connector to the output pole of the monitored fiber optic path, analog switch, the inputs of which are connected to the outputs of photodetectors, a comparator, attenuator, will expand pulse pulse, analog-to-digital converter, two microcontrollers, indicator, control panel, reprogrammable read-only memory, the control inputs of the switch and attenuator connected to the outputs of the first microcontroller, the attenuator output connected to the input of the pulse expander, the control input of which is connected to the first microcontroller, and output - with an analog input of an analog-to-digital converter, the digital outputs of which are connected to the inputs of the first microcontroller, one of the outputs of the second micro the controller is connected to the input of the pulse shaper, the microcontrollers are connected by a data bus, the indicator, a control panel, and a reprogrammable read-only memory device are connected to the terminals of the first microcontroller, characterized in that, in order to increase the resolution and accuracy of measurements, two electronic digital key, electronic analog key, delay line, programmable counter frequency divider, and the data bus programmable counter divider frequent you are connected to the outputs of the first microcontroller, the control bus with the outputs of the second microcontroller, the counting input with the output of the comparator, the additional output is connected to the control input of the analog key, the main output is connected to the input of the delay line, to the input of the second microcontroller and to the output of the first electronic digital key, input which is connected to the output of the delay line and the input of the second electronic digital key, the output of which is connected to the input of the pulse shaper, and the control inputs of the first and second electronic the keys are connected to one of the terminals of the second microcontroller, an analog electronic key is connected between the output of the attenuator and the input of the pulse expander, the inputs of the comparator and attenuator are connected to the output of the analog switch.

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