CN107063433B - Phase-sensitive optical time domain reflectometer and its implementation based on Golay complementary series - Google Patents

Abstract

The present invention relates to a kind of phase-sensitive optical time domain reflectometer based on Golay complementary series, which includes that input light generates unit (1), polarization-maintaining beam splitter (2), coded sequence modulation unit (3), circulator (4), testing fiber (5), homodyne frequency shifter (6), polarization control unit (7), probe unit (8) and signal demodulation unit (9)；The method that phase-sensitive optical time domain reflectometer is realized based on Golay complementary series is also disclosed simultaneously.Probe unit of the present invention has used I/Q demodulating system, can restore whole field informations especially absolute phase information of true backward Rayleigh scattering light effectively in real time with the structure of refining；Its local oscillator gain of light makes the pole weak signal for demodulating optical fiber tail-end become possibility, so this system facilitates the distance sensing of extension system.

Description

Phase-Sensitive Optical Time Domain Reflectometer Based on Golay Complementary Sequence and Its Implementation method

technical field

The invention relates to a phase-sensitive optical time-domain reflectometer, belonging to the field of optical fiber measurement and sensing, and more particularly, the invention relates to a new phase-sensitive optical time-domain reflectometer based on Golay complementary sequence, and a new phase-sensitive optical time domain reflectometer based on Golay complementary sequence A method for implementing a phase-sensitive optical time domain reflectometry sequence.

Background technique

Distributed Optical Fiber Sensing (DFOS) is widely used in modern society, especially in the fields of oil transmission, power monitoring, large-scale structural safety, and border security. The basic principle of distributed optical fiber sensing is that changes in external environmental factors, such as temperature, vibration, etc., lead to changes in the characteristics of the optical fiber itself, such as the refractive index, which in turn affects the optical field transmitted in the optical fiber and changes the scattered light field in the optical fiber. Therefore, by detecting the characteristics of scattered light (Rayleigh scattered light, Brillouin scattered light or Raman scattered light), the affected position of the fiber can be obtained, and even the external temperature field or vibration field can be restored.

Phase-sensitive optical time domain reflectometry (Φ-OTDR) is one of the most rapidly developing branches in the field of distributed optical fiber sensing, not only because of its high precision, high spatial resolution, robustness, and It has also attracted wide attention because of its refined structure, single-ended measurement, and its ability to restore the external vibration field. One of the most important applications of Φ-OTDR is the Distributed Acoustic Sensing (DAS) system, that is, distributed monitoring based on the ability of Φ-OTDR to restore the external vibration field. application prospects.

The principle of Φ-OTDR is as follows: when the narrow pulse output by the coherent light source enters the fiber to be tested, the Rayleigh scattered light (Rayleigh light) along the fiber continuously returns to the input end (point 0); Rayleigh light with half pulse width coherent at 0 point, and finally output the system; if the external vibration field changes, its frequency and amplitude are the same as the two points of Rayleigh light (one point is located before the disturbance, and the other point needs to pass through the disturbance position). It is proportional to the phase difference between them; therefore, the external vibration field can be restored by demodulating the phase of the sensing system. From the principle of Φ-OTDR, it can be seen that the spatial resolution of this system is half the pulse width; and the system is highly real-time, which can realize dynamic measurement by continuously injecting pulses.

However, existing Φ-OTDRs have certain limitations. Higher input pulse peak power generally leads to a higher signal-to-noise ratio of the sensing signal. However, high pulse peak power will cause fiber nonlinearity and cause signal damage. Therefore, the peak power of the input pulse needs to be limited. Limitation; and the system must not make the pulse width too large to achieve high spatial resolution. The above two defects are the key problems to be solved to realize the real-time measurement of long-distance Φ-OTDR. Increasing the average number of times can improve the signal-to-noise ratio of the sensing signal to a certain extent, thereby extending the sensing distance; however, if the average number of times is too large, the real-time performance will be sacrificed, and the maximum detectable frequency will decrease, which highlights the optical pulse coding technology. Necessity of application in long-distance Φ-OTDR system.

Golay complementary sequence is a kind of correlation code, which is widely used in the field of communication and sensing. The characteristic of this code is to use the correlation between the codewords to find the response to a single impulse. The Golay complementary sequence is a bipolar code, that is, it consists of "+1" and "-1"; the Golay complementary sequence contains two lines, and the codeword length N of each line needs to be a positive power of 2. Let two rows of Golay complementary sequences be A and B. The Golay complementary sequence conforms to the relevant properties shown in formula (1).

"⊕" in the formula (1) represents the autocorrelation operation, N is the length of a single-line codeword; k represents the number of shift bits when performing the correlation operation, and |k|≤N. δ(y) represents the unit impact function, and its expression is as described in equation (2).

It can be seen from equation (1) that the multiples of the unit shock function will be obtained after the correlation operation of the two lines of the Golay complementary sequence; and because the convolution of the delta function with any function is equal to the function itself, if a certain system can be written as Convolutional form, that is, a linear system, this system can be encoded using Golay's complement.

The advantages of Golay complementary sequences in long-distance Φ-OTDR applications. Compared with single pulse, Golay first has high coding gain, and N-bit Golay coding has higher signal-to-noise ratio than the average N times of single pulse demodulation. Since the unipolar Golay complementary sequence is always 4 lines, the measurement time required by the Golay is only the same as the measurement time of the single pulse averaged 4 times. For example, with 128-bit Golay complementary sequence (N is 128), the signal-to-noise ratio of the coded and demodulated signal is improved by about 5.6dB compared to the average of 128 single pulses, but the measurement time is 1/32 of the time used for 128 averages. Therefore, compared with the single-pulse average, the Golay complementary sequence can not only save the measurement time, but also improve the signal-to-noise ratio of the measurement signal. Compared with other coding methods, the biggest advantage of Golay is that its measurement time does not increase with the increase of the number of coding bits, and the measurement time it needs is always the time required for the average of 4 single pulses. For example, compared with another Simplex coding method widely used in distributed optical fiber sensing systems, the measurement time required for N-bit Simplex coding is the time required for averaging N times of a single pulse. Therefore, the measurement time required to compare the Golay complementary sequences of some other coding methods will not increase with the increase of the coding length, which will ensure the response speed of the system in the long-distance Φ-OTDR system.

The coding on the coding strength compared to the phase can realize the return-to-zero codeword, so that the intersymbol interference can be prevented. Specifically, the adjacent code words must be separated by at least one pulse width distance so as not to cause the signal responses of the two code words to overlap at a certain position, and the return-to-zero code of intensity modulation can meet this requirement as long as it is set properly condition. Since the phase encoding cannot realize the return-to-zero code, there will be the problem of inter-symbol crosstalk. Therefore, the system of the present invention adopts the method of intensity coding.

SUMMARY OF THE INVENTION

In order to overcome the technical problem of limited peak power and spatial resolution of the prior art phase sensitive optical time domain reflectometer, the present invention provides a phase sensitive optical time domain reflectometer based on the Golay complementary sequence. The reflectometer can quickly and efficiently restore the frequency and amplitude information of the external vibration field, breaking through the limitation of peak power and spatial resolution in the phase-sensitive optical time domain reflectometer; at the same time, the invention also discloses a phase based Golay complementary sequence. The realization method of sensitive optical time domain reflectometer.

For solving the above technical problems, the technical scheme adopted in the present invention is as follows:

A phase sensitive optical time domain reflectometer based on Golay complementary sequence, the phase sensitive optical time domain reflectometer includes an input light generating unit, a polarization-maintaining beam splitter, a coding sequence modulation unit, a circulator, an optical fiber to be measured, and a homodyne shifter. frequency converter, polarization control unit, detection unit and signal demodulation unit;

in,

The input light generating unit, the polarization maintaining beam splitter, the homodyne frequency shifter, the polarization control unit, the detection unit and the signal demodulation unit are signally connected in sequence;

The polarization-maintaining beam splitter is further connected to the coding sequence modulation unit, the circulator and the fiber to be tested in sequence, and the circulator and the detection unit are signal-connected to each other.

Compared with the prior art, the beneficial effects of the present invention are: the detection unit of the present invention uses an I/Q demodulation system, which can effectively restore all the light field information of the real back Rayleigh scattered light in real time with a refined structure, especially is absolute phase information; at the same time, its local oscillator optical gain makes it possible to demodulate the extremely weak signal at the end of the fiber, so this system helps to extend the sensing distance of the system.

At the same time, based on the above-mentioned phase-sensitive optical time domain reflectometer based on the Golay complementary sequence, the present invention also discloses a realization method of the phase sensitive optical time domain reflectometer based on the Golay complementary sequence, and the method includes the following steps:

1) Insert the intensity-modulated 4-row Golay complementary sequence suitable for the unipolar system into the fiber to be tested to receive its backward Rayleigh scattered light signal;

2) After collecting the optical fiber to be tested, the Rayleigh scattered light signal is used as the system response of each line, and the system response of the above-mentioned 4 lines of Golay complementary sequences is subjected to autocorrelation operation decoding, and the single-impulse phase response is recovered by recovery;

3) The phase information of the obtained single pulse is obtained by unwrapping operation, and the phase difference between the two sampling points of the single pulse is obtained; periodically enter the Golay complementary sequence to obtain the phase difference between the two sampling points over time. According to the phase difference is proportional to the frequency and amplitude of the external vibration, the frequency and amplitude of the external vibration can be recovered.

In step 2) of the above method, the recovery method of the monopulse phase response is as follows:

Assume that the 4-row Golay complementary sequences for the unipolar system inserted into the fiber to be tested are A ₁ , A ₂ and B ₁ , B ₂ , and the corresponding 4-row sequence system responses are R _A1 (t), R _A2 (t) and R _B1 (t), R _B2 (t);

The Rayleigh scattered light signal response based on the 4-line sequence will recover the single codeword (single pulse) system response, and the recovery formula is shown in expression (3).

In the formula,

⊕ represents the cross-correlation operation;

W(t) represents the single-impulse response of the phase-sensitive optical time domain reflectometer.

Preferably, the Golay complementary sequence is coded with an intensity return to zero.

Preferably, when the Golay complementary sequence whose intensity is returned to zero is actually set, the length of the return to zero between the codewords needs to be greater than the length of one codeword.

Based on the above method, the present invention only uses 4 times the time measured by a single pulse, and achieves a better effect than the average N (N is the length of a single-line codeword of the Goley complementary sequence), ensuring the real-time performance of the Φ-OTDR system; The corresponding spatial resolution is half of the width of a single codeword, so this method improves the average power of the input signal while ensuring the spatial resolution, which can make the signal transmit farther, which is beneficial to the long-distance sensing. ; Compared with other coding methods, the measurement time of the Golay complementary sequence will not increase with the increase of the number of coding bits, thus ensuring the response speed of the entire system and the fast dynamic measurement of the system.

Description of drawings

Fig. 1 is the schematic diagram of the phase sensitive optical time domain reflectometer of the linear system;

2 is a schematic diagram illustrating that the return-to-zero length between code words cannot be less than the length of one code word;

Fig. 3 is the system structure block diagram of utilizing the Golay complementary sequence to realize the phase sensitive optical time domain reflectometer;

Figure 4 is an experimental rendering of the Rayleigh scattered light intensity response AC component obtained by inserting 4 rows of Golay complementary sequences;

Fig. 5 is the experimental effect diagram of the change of phase difference with time after adding a disturbance of a specific frequency to the fiber to be tested;

Fig. 6 is the frequency domain diagram of demodulating external disturbance signal after adding the disturbance of a specific frequency in the fiber to be measured;

Labels in the figure: 1. Input light generating unit; 2. Polarization maintaining beam splitter; 3. Code sequence modulation unit; 4. Circulator; 8. Detection unit; 9. Signal demodulation unit.

Detailed ways

The present invention will be further described below in conjunction with the accompanying drawings. Embodiments of the present invention include, but are not limited to, the following examples.

First, the phase-sensitive optical time-domain reflectometer (Φ-OTDR) for phase demodulation is described as a linear system. The linear system is the basis and premise that the coding can be implemented. Let the codeword repetition period (bit rate) be the same as the sampling time interval (sampling rate); let the φ-OTDR system response of a pulse with an amplitude of 1 and an initial phase of 0 be r(t); let the length be S+1 The arbitrary coding sequence of is Assuming that the total number of points sampled on the optical fiber is H+1, the response of the monopulse system is r(t)=[r ₀ exp(jρ ₀ ), . . . , r _H exp(jρ _H )] ^T . Figure 1 briefly illustrates the response of the system when S=2, H=1. The code word in FIG. 1 is [1, 1, 1] ^T , and the curves labeled ①, ② and ③ correspond to the system responses of the 1st, 2nd, and 3rd codewords, respectively. As can be seen from Figure 1, the length of the system's response R(t) to the encoding should be S+H+1, that is

R(t)=[R(t ₀ ), . . . , R(t _S+H )] ^T . The general situation can be deduced from Figure 1. The response of the φ-OTDR system to any coding sequence can be represented by equation (4).

It can be known from the definition of convolution that the matrix representation of equation (4) can be equivalent to the convolution operation, as shown in equation (5).

R=A*r(5)

In formula (5), * represents a convolution operation. Since the response of the φ-OTDR system to the encoding can be expressed as a convolution operation, the φ-OTDR system for phase demodulation is a linear system.

It will be explained later that the length of the return to zero between the codeword and the codeword needs to be greater than the length of one codeword. If two adjacent code words enter the fiber, the Rayleigh scattered light received by the receiver at 0 can be as shown in Figure 2: the time when the first (second) code words completely enter the fiber is set to αΔt(βΔt ); the area marked ①(②) indicates the relationship between the position where the signal received by the first (two) codewords at the receiving end originates from the fiber to be tested and the sampling duration. As can be seen from Figure 2, if the region ① and the region ② do not overlap, α and β need to satisfy the relationship shown in Equation (6).

β≥α+1 (6)

It can be seen from Figure 2 and equation (6) that the return-to-zero length between two adjacent codewords must be greater than the length of one codeword Δt, otherwise for a certain position, the signals of the two codewords will be aliased , so that the system loses linearity.

Based on the above content, the phase-sensitive optical time domain reflectometer based on the Golay complementary sequence is described below with reference to Figure 3:

A phase sensitive optical time domain reflectometer based on Golay complementary sequence, the phase sensitive optical time domain reflectometer includes an input light generating unit 1, a polarization-maintaining beam splitter 2, a coding sequence modulation unit 3, a circulator 4, and an optical fiber to be measured 5. The homodyne frequency shifter 6, the polarization control unit 7, the detection unit 8 and the signal demodulation unit 9; wherein, the input light generating unit 1, the polarization maintaining beam splitter 2, the homodyne frequency shifter 6, the polarization control unit The unit 7, the detection unit 8 and the signal demodulation unit 9 are connected in sequence; the polarization-maintaining beam splitter 2 is also connected to the code sequence modulation unit 3, the circulator 4 and the fiber to be tested 5 in sequence, and the circulator 4 and the detection unit 8 are signal-connected to each other.

In the above phase sensitive optical time domain reflectometer, the polarization-maintaining beam splitter 2 includes one input end a, two output ends b, and output ends c, and the circulator 4 includes one input end a' and two output ends b' , the output terminal c'.

The input light generating unit 1 generates an optical signal as the input light; the polarization maintaining beam splitter 2 divides the output optical signal of the light generating unit input from the input end a into two channels without changing the polarization state of the input optical signal, and the output end b outputs One of the signals is used as the signal light input into the fiber to be tested, and the other output from the output end c is used as the local oscillator light required for coherent demodulation; the code sequence modulation unit 3 modulates the signal light output from port b of the polarization maintaining beam splitter 2 into Golay complementary sequence; the circulator 4 inputs the modulated sequence from the input end a' to the fiber to be tested connected to the output end b'; the backward Rayleigh scattered light of the fiber to be tested 5 is output from the output end b' of the circulator 4 to the output end c'; the homodyne frequency shifter 6 performs frequency shift processing on the optical signal output by the output end c of the polarization maintaining beam splitter 2 as the local oscillator, so that its frequency is the same as the frequency of the backward Rayleigh scattered light; The control unit 7 adjusts the polarization state of the continuous light output from the homodyne frequency shifter, so that the detection unit has an optimal local oscillator light gain; the detection unit 8 uses the I/Q demodulation technology to The backward Rayleigh scattered light (that is, the signal to be measured) is mixed with the eigenlight output by the polarization control unit, and finally the Golay complementary sequence with the local oscillator light gain is outputted as a backward Rayleigh scattered light field; the signal demodulation unit 9 Demodulate the backward Rayleigh scattered light field with local oscillator gain output from the detection unit to obtain a single impulse response; then calculate the phase of the backward Rayleigh scattered light to obtain the phase difference between sampling points, Thereby, the amplitude and frequency of the external vibration field are restored.

When the above-mentioned phase sensitive optical time domain reflectometer is used, the input power of the input light generation unit 1 is adjusted first, that is, the peak power of the input code word is then input to the code sequence modulation unit through the output end b of the polarization maintaining beam splitter 2. A Golay complementary sequence is generated, and then input to the fiber to be tested through the output end b' of the circulator 4. Afterwards, the Rayleigh scattered light passes through the output end c' of the circulator 4, and is input to the detection unit 8 together with the local oscillator light optimized by the homodyne frequency shifter 6 and the polarization control unit 7 for I/Q modulation, so that the signal is The light is mixed with the intrinsic light and input into the Rayleigh scattered light field. The detection signal finally enters the signal demodulation unit 9, demodulates the single impulse response, then demodulates the signal phase through the unwrapping algorithm, and restores the frequency and amplitude of the external vibration field according to the phase difference.

In the following, the usage of the phase-sensitive optical time domain reflectometer will be described with reference to specific embodiments.

The length of the fiber to be tested is 2km; the duration of a single codeword (single pulse) is 80ns, and the corresponding spatial resolution is about 8m; the repetition period of the codeword is 40μs; the length of a single line of the Golay complementary sequence is 32 bits.

A 90-degree optical mixer and two detectors with a bandwidth of 10M are used to form a detection unit 8 for I/Q demodulation. A piezoelectric ceramic (PZT) wound with a 10m fiber was added at the end of the fiber to simulate the external vibration field, and the applied vibration frequency was 355Hz.

Figure 4 shows the back Rayleigh scattered light intensity (removing the DC response) of the 4-line codeword output from the detection unit after entering the Golay complementary sequence suitable for the unipolar system. Figure 4(a), (b), (c) and (d) respectively show that the first, second, third and fourth lines of the Golay complementary sequence are detected at 0 after the codewords are inserted into the sensing system The received back Rayleigh scattered light intensity. It can be seen from Figure 4 that the response of each line of codewords has the following trend: the intensity in the middle is larger and the intensity at both ends is lower. The reason for this phenomenon is that the front-end codewords have not yet "entered" completely, and some codewords at the back-end have already "walked out" of the fiber. Since the greater the intensity, the better the demodulation signal-to-noise ratio will be brought, and it can be seen from Figure 4 that the Golay complementary sequence is significantly helpful for the signal intensity enhancement.

5 and 6 respectively show the time domain and frequency domain signals of the external vibration field demodulated by the signal demodulation unit. Figure 5 shows the relationship between time and phase difference; the demodulation curve in Figure 5 is sinusoidal, echoing the signal applied to the tail end PZT. Fig. 6 shows the frequency domain signal corresponding to the demodulated time domain signal shown in Fig. 5. It can be seen from Fig. 6 that the demodulated signal obtains a peak value near 355 Hz and exhibits a high signal-to-noise ratio. Figures 5 and 6 prove that this system can restore the external vibration field to a high degree.

The above are the embodiments of the present invention. The foregoing are various preferred embodiments of the present invention. If the preferred embodiments in each preferred embodiment are not obviously self-contradictory or are premised on a certain preferred embodiment, each preferred embodiment can be used in any combination. The examples and the specific parameters in the examples are only for the purpose of clearly describing the inventor's invention verification process, not for limiting the scope of patent protection of the present invention. The scope of patent protection of the present invention is still based on the claims. Equivalent structural changes made in the contents of the description and drawings of the invention shall be included within the protection scope of the present invention.

Claims (3)

1. The method for realizing the phase-sensitive optical time domain reflectometer based on the Golay complementary sequence comprises an input light generating unit (1), a polarization-preserving beam splitter (2), a coding sequence modulating unit (3), a circulator (4), an optical fiber to be tested (5), a homodyne frequency shifter (6), a polarization control unit (7), a detection unit (8) and a signal demodulating unit (9); the input light generating unit (1), the polarization-maintaining beam splitter (2), the homodyne frequency shifter (6), the polarization control unit (7), the detection unit (8) and the signal demodulation unit (9) are sequentially in signal connection; the polarization-preserving beam splitter (2) is also sequentially connected with the coding sequence modulation unit (3), the circulator (4) and the optical fiber (5) to be detected, and the circulator (4) and the detection unit (8) are in signal connection with each other;

the method is characterized by comprising the following steps:

1) 4 rows of Golay complementary sequences which are subjected to intensity modulation and are suitable for a unipolar system are driven into an optical fiber (5) to be detected, and backward Rayleigh scattering light signals are received;

2) collecting backward Rayleigh scattering optical signals of an optical fiber (5) to be detected as the system correspondence of each row, performing autocorrelation operation decoding on the system responses of the 4 rows of Golay complementary sequences, and recovering to obtain single pulse phase responses;

the recovery method of the single-pulse phase response is as follows:

the complementary sequence of intensity modulation Golay for a unipolar system is A, assuming 4 rows of fibers (5) to be tested₁、A₂And B₁、B₂The corresponding obtained 4-row sequence system response is respectively R_A1(t)、R_A2(t) and R_B1(t)、R_B2(t)；

The rayleigh scattered light signal response based on the 4-row sequence will recover a single codeword, i.e. the system response of a single pulse, with the following formula:

in the formula,representing a cross-correlation operation;

w (t) represents the single pulse response of the phase-sensitive optical time domain reflectometer;

3) the obtained monopulse phase response is subjected to unwrapping operation to obtain phase information, and a phase difference between two sampling points of the monopulse is obtained; and periodically pumping Golay complementary sequences to obtain the variation of the phase difference between two sampling points along with time, and recovering the frequency and amplitude of the external vibration according to the direct proportion relation between the phase difference and the frequency and amplitude of the external vibration.

2. The method of claim 1, wherein the Golay complementary sequence is encoded using intensity nulling.

3. The method of claim 2, wherein the length of the zero-return between codewords is larger than the length of one codeword when the Golay complementary sequence with zero-return strength is actually set.