CN112857745A

CN112857745A - High-spatial resolution OTDR device and method based on high-speed complementary Gray code

Info

Publication number: CN112857745A
Application number: CN202110194743.7A
Authority: CN
Inventors: 张建国; 王洋; 赵彤; 潘通; 申志儒; 王安帮
Original assignee: Taiyuan University of Technology
Current assignee: Taiyuan University of Technology
Priority date: 2020-12-16
Filing date: 2021-02-21
Publication date: 2021-05-28
Anticipated expiration: 2041-02-21
Also published as: CN112857745B

Abstract

The invention belongs to the field of optical fiber time domain reflection measurement; the high-speed complementary Gray code is used as a modulation signal, and a chaotic laser which is wide in output band, stable and free of obvious periodicity is directly modulated and distributed by a driving circuit to be used as a detection light, so that the high-precision diagnosis of the optical fiber fault is realized; the ADC speed is reduced by using a low-speed analog-to-digital converter, a good detection effect is achieved, and the cross-correlation is quickly calculated by using a quick Hadamard conversion module; the designed chaotic OTDR has simple structure, stable performance and low cost, is beneficial to realizing integration and productization, and can meet the requirements in practical engineering application.

Description

High-spatial resolution OTDR device and method based on high-speed complementary Gray code

Technical Field

The invention relates to the technical field of optical fiber time domain reflectometry, in particular to a high-spatial resolution OTDR device and a high-spatial resolution OTDR method based on high-speed complementary Gray codes.

Background

Fiber optic fault detection is a fundamental requirement of fiber optic transmission systems. Typically, commercial fiber optic fault location systems are based on Optical Time Domain Reflectometry (OTDR). An Optical Time Domain Reflectometer (OTDR) is an optical fiber fault detection method recommended by the international telecommunication union, and information such as attenuation and breakpoint of a measured optical fiber is obtained by using a backscattered light signal generated by rayleigh scattering and fresnel reflection when a probe light is transmitted in the optical fiber. The OTDR can be used for measuring the attenuation of the optical fiber, the loss of a joint, the positioning of a fault point of the optical fiber, knowing the loss distribution condition of the optical fiber along the length and the like, and is an essential tool in the construction, maintenance and monitoring of the optical cable. However, this technique must compromise between spatial resolution and dynamic range. Because an expensive and complicated ultrashort pulse laser is not used, the traditional OTDR adopts a laser single pulse as detection light, the technology is mature, the structure is simple, but the spatial resolution is reduced along with the increase of the measurement distance, and can only reach several meters or even dozens of meters. The resolution determines the minimum distance between two points which can be distinguished by the OTDR, and the higher the resolution is, the more accurate the fault point is positioned. The signal-to-noise ratio determines the maximum distance that the OTDR can measure at a certain average number of times. The resolution of the measurement depends on the pulse width. On the other hand, the peak power of the laser is limited, and only increasing the optical pulse width is necessary to increase the energy of the incident optical pulse, which in turn causes a decrease in resolution, so that the resolution and the signal-to-noise ratio of the OTDR using a single pulse are mutually restrictive.

With the rapid development of multi-node and miniaturized fiber networks, OTDR needs to realize higher spatial resolution to meet the fault location of the dense fiber network. Therefore, OTDRs based on ultra-short optical pulses, photon counting and pseudo-random codes are proposed in succession. The ultra-short optical pulse technique can improve the spatial resolution of the OTDR, but has a short measurement range, a low signal-to-noise ratio (SNR), and an expensive and complicated apparatus; the photon counting OTDR increases the dynamic range on the premise of ensuring the spatial resolution, but the testing time is long, and the testing data needs to be corrected, so that the application of the photon counting OTDR in practice is limited; pseudo-random code modulated OTDR increases the measured distance by increasing the code length without affecting the spatial resolution, however, the limited code length limits further increase of its dynamic range or measured distance.

Chaotic light time domain reflectometry has advantages over the above methods because of its ranging capability of about several kilometers with a resolution of centimeters. The chaotic optical time domain reflectometer (COTDR, chaos OTDR) uses continuous broadband chaotic laser as a detection signal, and can realize high-precision optical fiber fault positioning irrelevant to a detection distance by performing cross-correlation processing on a reference signal and an echo signal. However, the optical feedback chaotic laser generating devices are all composed of separate optical devices, and have large size and unsatisfactory output chaotic light quality. The reason is as follows: on one hand, the laser is very sensitive to the intensity and polarization state of feedback light as a nonlinear optical device, so that the generated chaotic light is easily interfered by external factors and is difficult to continuously and stably output; on the other hand, due to the existence of the delay feedback cavity, the chaotic laser generated by the structure has weak periodicity, namely, the chaotic laser has periodic side lobes on a relevant curve, and misjudgment is brought to measurement.

Disclosure of Invention

The invention overcomes the defects of the prior art and provides a high-spatial resolution OTDR device and a method based on high-speed complementary Gray codes.

In order to achieve the above object, the present invention is achieved by the following technical solutions.

The OTDR generating device with high spatial resolution based on the high-speed complementary Gray code comprises an external clock source, a Multiplexer (MUX), a high-speed complementary Gray code generator, a driving circuit, a distributed feedback laser (DFB), a coupler, a circulator, a tested optical fiber, a first Avalanche Photodetector (APD), a second Avalanche Photodetector (APD), a frequency divider, a sampling clock generator, a first tracking and holding circuit module, a second tracking and holding circuit module, an analog-to-digital converter (ADC) and a fast Hadamard conversion module (FHT); the output port of the external noise source is respectively connected with the N-level frequency divider, the Multiplexer (MUX) and the high-speed complementary Gray code generator; the output port of the multiplexer is respectively connected with the high-speed complementary Gray code generator and the frequency divider; the input interface of the sampling clock generator is connected with the output interface of the frequency divider, and the output interface of the sampling clock generator is respectively connected with the first track-hold circuit module and the second track-hold circuit module; the input port of the driving circuit is connected with the output port of the high-speed complementary Gray code generator, and the output interface of the driving circuit is connected with the input interface of the distributed feedback laser (DFB); an input interface of the coupler is connected with an output interface of a distributed feedback laser (DFB), and an output interface of the coupler is respectively connected with the circulator and the first Avalanche Photodetector (APD); the circulator is connected with the optical fiber to be tested and a second Avalanche Photodetector (APD); an output interface of a first Avalanche Photodetector (APD) is connected with a first tracking and holding circuit module; the output interface of the second Avalanche Photodetector (APD) is connected with the second tracking and holding circuit module; the output interfaces of the first track-hold circuit module and the second track-hold circuit module are connected with the input interface of the analog-to-digital converter; and an input interface of the fast Hadamard transform module (FHT) is connected with an output interface of the analog-to-digital converter, and outputs the cross-correlation graph.

Furthermore, the down-sampling unit comprises a frequency divider, a sampling clock generator, a first track-and-hold circuit module and a second track-and-hold circuit module; the method comprises the steps that a clock signal of an external clock source is input into a sampling clock generator after being subjected to frequency division through a frequency divider, and an output signal of the sampling clock generator carries out down-sampling on electric signals generated by conversion of a first Avalanche Photodetector (APD) and a second Avalanche Photodetector (APD).

The adjustable frequency and speed of the high-speed complementary Gray code generator are realized by adopting the N-level frequency divider and the MUX. The high-speed complementary Gray code is adopted, the cross-correlation curve with zero side lobe is realized, and compared with the traditional single pulse technology, the cross-correlation curve has relatively better performance and realizes high spatial resolution.

A high-spatial resolution OTDR generating method based on high-speed complementary Gray codes comprises the following steps:

1) the frequency of a high-speed complementary Gray code generator is controlled by a clock signal of an external clock source through an N-level frequency divider and a multiplexer to realize speed regulation, the high-speed complementary Gray code is used as a modulation signal, a distributed feedback laser is directly modulated through a radio frequency modulation port of a laser driving circuit to output chaotic laser, the output chaotic laser is divided into reference light and detection light through an optical fiber coupler, the detection light is injected into a tested optical fiber through an optical circulator, and an echo signal and the reference light reflected at an optical fiber fault position are converted into corresponding electric signals through two identical avalanche photoelectric detectors.

2) The down-sampling unit divides the frequency of a clock signal of an external clock source through a frequency divider and then inputs the frequency divided clock signal into a sampling clock generator, and an output signal of the sampling clock generator performs down-sampling on an electric signal generated by converting two identical avalanche photodetectors; the down-sampled signals are converted into digital signals from analog signals through an analog-to-digital converter, the two paths of digital electrical signals are synchronously acquired and correlated through a fast Hadamard conversion module, and the position of a fault point is determined according to the delay time of the peak value of the obtained correlation curve.

Compared with the prior art, the invention has the following beneficial effects:

the synthesis of the zero sidelobes is performed by summing the auto-and cross-correlations of a binary count sequence or a subset of sequences, such as a cyclic shift or quadratic residue code of the M-sequence. The invention uses high-speed complementary Gray code as modulation signal, and directly modulates the chaotic laser with wide band, stability and no obvious periodicity output by a Distributed Feedback (DFB) laser through a driving circuit to be used as detection light, thereby realizing the high-precision diagnosis of optical fiber faults. The peak value of the autocorrelation function of one of the golay complementary sequences is equal to the number of codes involved, but there are many side lobes, which are about 10% of the peak value. When the two groups of autocorrelation results are added, the peak value is increased by two times, and the side lobe completely disappears. It has relatively better performance and achieves high spatial resolution relative to conventional single-pulse techniques. The low-speed analog-to-digital converter (ADC) is used for reducing the ADC speed and achieving a good detection effect, and a Fast Hadamard Transform (FHT) module is used for fast calculating the cross-correlation. The down-sampling unit is adopted to sample the electric signal, the ADC speed is reduced, a good detection effect is achieved, and the device has the advantages of being simple in structure and low in cost.

The FHT module is adopted to realize the quick cross-correlation calculation, save time and obtain a good calculation result. The FHT is essentially implemented by the Hadamard transform and the butterfly fast method, and the Hadamard transform is characterized by:

1. and (3) real number transformation: the signal used is typically a real signal, the Hadamard transform of which is still a real sequence, which avoids the complex operations necessary for the FFT.

2. The transformation involves only addition and subtraction: as can be seen from the structure of the Hadamard matrix, the transform involves only addition and subtraction, avoiding the multiplication necessary for the FFT.

3. The inverse transformation is simple: the positive and negative transformations of the transformation differ by only a factor 1/N, which allows the inverse transformation to be calculated with the help of the positive transformation.

4. There are fast algorithms: since the Hadamard matrix can be factorized, a butterfly fast method exists for this transform.

FHT can greatly reduce the time of correlation processing.

In summary, the apparatus for generating high spatial resolution OTDR of high speed complementary gray code of the present invention has the following advantages:

1. the invention adopts the N-level frequency divider and the MUX to realize the adjustable frequency and speed of the high-speed complementary Gray code generator.

2. The invention adopts high-speed complementary Gray codes, realizes the cross-correlation curve with zero side lobe, has relatively better performance and realizes high spatial resolution compared with the traditional single pulse technology.

The method for generating the high-spatial-resolution OTDR of the high-speed complementary Gray code has the following beneficial effects:

1. the invention adopts the down-sampling unit to sample the electric signal, reduces the ADC speed, achieves good detection effect, and has the advantages of simple structure and low cost.

2. The invention adopts the FHT module to realize the quick cross-correlation calculation, saves time and obtains a good calculation result.

Drawings

FIG. 1 is a general schematic of the present invention;

FIG. 2 is a schematic diagram of the operation of a down-sampling unit in an embodiment of the present invention;

in the figure: the device comprises an external clock source 1, a Multiplexer (MUX) 2, a high-speed complementary Gray code generator 3, a driving circuit 4, a Distributed Feedback (DFB) laser 5, a coupler 6, a circulator 7, a tested optical fiber 8, a first Avalanche Photodetector (APD) 9, a second Avalanche Photodetector (APD) 10, a frequency divider 11, a sampling clock generator 12, a first track-and-hold circuit module 13, a second track-and-hold circuit module 14, an analog-to-digital converter 15 and a Fast Hadamard Transform (FHT) module 16.

Detailed Description

In order to make the technical problems, technical solutions and advantageous effects to be solved by the present invention more clearly apparent, the present invention is further described in detail with reference to the embodiments and the accompanying drawings. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. The technical solution of the present invention is described in detail below with reference to the embodiments and the drawings, but the scope of protection is not limited thereto.

The invention aims to provide a high-spatial resolution OTDR generating device based on high-speed complementary Gray codes. A high-spatial resolution OTDR generating device based on a high-speed complementary Gray code is shown in figure 1 and comprises an external clock source 1, a Multiplexer (MUX) 2, a high-speed complementary Gray code generator 3, a driving circuit 4, a Distributed Feedback (DFB) laser 5, a coupler 6, a circulator 7, a measured optical fiber 8, a first Avalanche Photodetector (APD) 9, a second Avalanche Photodetector (APD) 10, a frequency divider 11, a sampling clock generator 12, a first tracking and holding circuit module 13, a second tracking and holding circuit module 14, an analog-to-digital converter (ADC) 15 and a fast Hadamard conversion (FHT) module 16.

The output port of the external noise source 1 is respectively connected with an N-level frequency divider, a Multiplexer (MUX) 2 and a high-speed complementary Gray code generator 3; the output port of the multiplexer 2 is respectively connected with the high-speed complementary Gray code generator 3 and the frequency divider 11; an input interface of the sampling clock generator 12 is connected with an output interface of the frequency divider 11, and output interfaces thereof are respectively connected with the first track-and-hold circuit module 13 and the second track-and-hold circuit module 14; an input port of the driving circuit 4 is connected with an output port of the high-speed complementary Gray code generator 3, and an output interface of the driving circuit is connected with an input interface of a Distributed Feedback (DFB) laser 5; an input interface of the coupler 6 is connected with an output interface of the Distributed Feedback (DFB) laser 5, and output interfaces of the coupler are respectively connected with the circulator 7 and the first Avalanche Photodetector (APD) 9; the circulator 7 is connected with a tested optical fiber 8 and a second Avalanche Photodetector (APD) 10; the output interface of the first Avalanche Photodetector (APD) 9 is connected to the first track and hold circuit module 13; the output interface of the second Avalanche Photodetector (APD) 10 is connected to the second track and hold circuit module 14; the output interfaces of the first track and hold circuit module 13 and the second track and hold circuit module 14 are both connected with the input interface of an analog-to-digital converter (ADC) 15; an input interface of the Fast Hadamard Transform (FHT) module 16 is connected to an output interface of the analog-to-digital converter (ADC) 15 and outputs the cross-correlation pattern.

The down-sampling unit includes a frequency divider 11, a sampling clock generator 12, a first track-and-hold circuit module 13, and a second track-and-hold circuit module 14. The down-sampling unit divides the frequency of the clock signal of the external clock source 1 by the frequency divider 11 and inputs the divided frequency into the sampling clock generator 12, and the output signal of the sampling clock generator 12 down-samples the electrical signals generated by the conversion of the first Avalanche Photodetector (APD) 9 and the second Avalanche Photodetector (APD) 10.

The working flow of the device for generating the high-spatial resolution OTDR based on the high-speed complementary Gray code is as follows: a clock signal of an external clock source controls the frequency of a high-speed complementary Gray code generator through an N-level frequency divider and a MUX, the high-speed complementary Gray code is used as a modulation signal, a Distributed Feedback (DFB) laser is directly modulated by a radio frequency modulation port of a laser driving circuit to output broad-band, stable and chaotic laser without obvious periodicity, the output chaotic laser is divided into two paths through a 99:1 Optical Fiber Coupler (OFC), 1% of light is used as reference light, 99% of light is used as detection light, the detection light is injected into a measured optical fiber through an Optical Circulator (OC), and an echo signal and the reference light reflected at an optical fiber fault position are converted into corresponding electric signals through two identical Avalanche Photodetectors (APDs). The down-sampling unit divides the frequency of a clock signal of an external clock source through a frequency divider and then inputs the frequency divided clock signal into a sampling clock generator, and an output signal of the sampling clock generator down-samples an electric signal generated by converting two identical Avalanche Photodetectors (APDs). The down-sampled signals are converted into digital signals from analog signals through an analog-to-digital converter, the FHT module is used for synchronously acquiring and correlating the two paths of digital electric signals, and the position of a fault point is determined according to the delay time of the peak value of the obtained correlation curve.

The second objective of the present invention is to provide a method for generating high spatial resolution OTDR based on high-speed complementary gray code, comprising the following steps:

the frequency of a high-speed complementary Gray code generator is controlled by a clock signal of an external clock source through an N-level frequency divider and a MUX to realize speed adjustment, the high-speed complementary Gray code is used as a modulation signal, a Distributed Feedback (DFB) laser 5 is directly modulated through a radio frequency modulation port of a laser driving circuit to output broad-band, stable and chaotic laser without obvious periodicity, the output chaotic laser is divided into reference light and detection light through an Optical Fiber Coupler (OFC), the detection light is injected into a tested optical fiber through an Optical Circulator (OC), and an echo signal and the reference light reflected at an optical fiber fault position are converted into corresponding electric signals through two same Avalanche Photodetectors (APDs).

The down-sampling unit divides the frequency of a clock signal of an external clock source through a frequency divider and then inputs the frequency divided clock signal into a sampling clock generator, and an output signal of the sampling clock generator down-samples an electric signal generated by converting two identical Avalanche Photodetectors (APDs). The down-sampled signals are converted into digital signals from analog signals through an analog-to-digital converter, the FHT module is used for synchronously acquiring and correlating the two paths of digital electric signals, and the position of a fault point is determined according to the delay time of the peak value of the obtained correlation curve. The peak value of the autocorrelation function of one of the golay complementary sequences is equal to the number of codes involved, but there are many side lobes, which are about 10% of the peak value. When the two groups of autocorrelation results are added, the peak value is increased by two times, and the side lobe completely disappears. It has relatively better performance and achieves high spatial resolution relative to conventional single-pulse techniques. The low-speed analog-to-digital converter (ADC) is used for reducing the ADC speed and achieving a good detection effect, and a Fast Hadamard Transform (FHT) module is used for fast calculating the cross-correlation.

The down-sampling unit includes a frequency divider 11, a sampling clock generator 12, a first track-and-hold circuit module 13, and a second track-and-hold circuit module 14. Fig. 2 is a schematic diagram of the operation of the down-sampling unit, and the embodiment of the present invention is described in detail with an external clock source of 1 GHZ: clock signals passing through an external signal source 1 of 1GHZ and N frequency dividers 2 are transmitted to a high-speed complementary Gray code generator, and then the clock signals passing through the 10 frequency dividers 2 are used for sampling by a down-sampling unit.

The FHT is essentially implemented by the Hadamard transform and the butterfly fast method, and the Hadamard transform is characterized by:

1. and (5) real number transformation. The signal used is typically a real signal, the Hadamard transform of which is still a real sequence, which avoids the complex operations necessary for the FFT.

2. The transformation involves only addition and subtraction. As can be seen from the structure of the Hadamard matrix, the transform involves only addition and subtraction, avoiding the multiplication necessary for the FFT.

3. The inverse transformation is simple. The positive and negative transformations of the transformation differ by only a factor 1/N, which allows the inverse transformation to be calculated with the help of the positive transformation.

4. Fast algorithms exist. Since the Hadamard matrix can be factorized, a butterfly fast method exists for this transform.

FHT can greatly reduce the time of correlation processing.

While the invention has been described in further detail with reference to specific preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.

Claims

1. High spatial resolution OTDR generating device based on high-speed complementary Gray code is characterized in that: the device comprises an external clock source (1), a multiplexer (2), a high-speed complementary Gray code generator (3), a driving circuit (4), a distributed feedback laser (5), a coupler (6), a circulator (7), a tested optical fiber (8), a first avalanche photodetector (9), a second avalanche photodetector (10), a frequency divider (11), a sampling clock generator (12), a first track-and-hold circuit module (13), a second track-and-hold circuit module (14), an analog-to-digital converter (15) and a fast Hadamard conversion module (16); the output port of the external noise source (1) is respectively connected with the N-level frequency divider, the multiplexer (2) and the high-speed complementary Gray code generator (3); the output port of the multiplexer (2) is respectively connected with the high-speed complementary Gray code generator (3) and the frequency divider (11); an input interface of the sampling clock generator (12) is connected with an output interface of the frequency divider (11), and an output interface of the sampling clock generator (12) is respectively connected with the first tracking and holding circuit module (13) and the second tracking and holding circuit module (14); an input port of the driving circuit (4) is connected with an output port of the high-speed complementary Gray code generator (3), and an output interface of the driving circuit (4) is connected with an input interface of the distributed feedback laser (5); an input interface of the coupler (6) is connected with an output interface of the distributed feedback laser (5), and an output interface of the coupler (6) is respectively connected with the circulator (7) and the first avalanche photodetector (9); the circulator (7) is connected with the measured optical fiber (8) and the second avalanche photodetector (10); the output interface of the first avalanche photodetector (9) is connected with a first tracking and holding circuit module (13); the output interface of the second avalanche photodetector (10) is connected with a second tracking and holding circuit module (14); the output interfaces of the first track and hold circuit module (13) and the second track and hold circuit module (14) are connected with the input interface of the analog-to-digital converter (15); and an input interface of the fast Hadamard transform module (16) is connected with an output interface of the analog-to-digital converter (15) and outputs the cross-correlation graph.

2. A high-speed complementary golay code based high spatial resolution OTDR generating device according to claim 1, characterized in that: the down-sampling unit comprises a frequency divider (11), a sampling clock generator (12), a first track-and-hold circuit module (13) and a second track-and-hold circuit module (14); the clock signal of the external clock source (1) is divided by a frequency divider (11) and then input into a sampling clock generator (12), and the output signal of the sampling clock generator (12) carries out down-sampling on the electric signals generated by the conversion of the first avalanche photodetector (9) and the second avalanche photodetector (10).

3. An OTDR generating method of a generating device according to claim 1 or 2, characterized by comprising the steps of:

1) the frequency of a high-speed complementary Gray code generator is controlled by a clock signal of an external clock source (1) through an N-level frequency divider and a multiplexer (2), so that the speed regulation is realized, the high-speed complementary Gray code is used as a modulation signal, a distributed feedback laser (5) is directly modulated and distributed through a radio frequency modulation port of a laser driving circuit to output chaotic laser, the output chaotic laser is divided into reference light and detection light through an optical fiber coupler, the detection light is injected into a tested optical fiber through an optical circulator, and an echo signal and the reference light reflected at the fault position of the optical fiber are converted into corresponding electric signals through two identical avalanche photoelectric detectors;

2) the down-sampling unit divides the frequency of a clock signal of an external clock source through a frequency divider (11) and inputs the frequency divided signal into a sampling clock generator (12), and an output signal of the sampling clock generator (12) down-samples an electric signal generated by converting two same avalanche photodetectors; the down-sampled signals are converted into digital signals from analog signals through an analog-to-digital converter (15), a fast Hadamard conversion module (16) is used for synchronously acquiring and correlating two paths of digital electric signals, and the position of a fault point is determined according to the delay time of the peak value of the obtained correlation curve.