CN102332956B - A Dispersion Compensation Method for Broadband Light Source - Google Patents

Abstract

The invention discloses a dispersion compensation method for a broadband light source. A polarization-preserving fiber polarization coupling testing system based on a Michelson interferometer is formed by the broadband light source, and the Michelson interferometer compensates the optical path difference between an excitation mode and a coupling mode in the polarization-preserving fiber, and a detector acquires an interference signal; specifically, the method comprises the following steps of: intercepting the initial data acquired by the detector through a window function, and taking out the interference data Imain between the excitation modes and the interference data Icoupling between the excitation mode and the coupling mode; respectively executing Hilbert transformation and Gaussian fitting on the Imain and the Icoupling, obtaining envelopes (I)main and (I)coupling of the interference signal, getting a birefringence dispersion coefficient Delta D according to a ratio Eta of the width of the (I)main to the width of the (I)coupling at a 1/e part, obtaining a phase factor needed for the dispersion compensation, and then multiplying the phase factor by a nonlinear frequency spectrum function with dispersion information to eliminate nonlinear phase items causing the widening of the interference signal envelopes; and finally, obtaining the dispersion compensated interference signal Icomp by executing Fourier inversion on the obtained linear frequency spectrum function.

Description

A Dispersion Compensation Method for Broadband Light Source

technical field

The invention relates to a polarization-maintaining optical fiber polarization coupling test technology, in particular to a dynamic dispersion compensation method for a polarization-maintaining optical fiber polarization coupling test system built by a broadband light source, and belongs to the technical field of high-precision measurement.

Background technique

A broadband light source refers to a low-coherence light source with a certain spectral width and a short coherence length. Because the light source has a wide spectral width, the interference system built by it is often called a white light interference system. In commonly used photoelectric interferometry systems, the most commonly used light sources are various single-mode or narrow-band high-coherence lasers. Under precise measurement conditions, the system can obtain nanometer-level measurement accuracy, but the single-value dynamic range of this type of measurement system is generally small, and can only perform relative measurement of physical quantities and cannot achieve absolute measurement. If the output power of the light source changes during the measurement process , the measurement of the measurand change information will be affected. At the same time, this type of system has strict requirements on the external environment such as temperature, humidity, and pressure, and has a complex structure and high cost. In contrast, white-light interferometers using broadband light sources can address some of these challenges. The interferometric method has an interference main maximum value at zero optical path difference, which can perform absolute measurement of the physical quantity to be measured; the measurement of the physical quantity to be measured is realized by detecting the relative difference of interference fringes, and the sensitivity to the external environment is also very low. , and the difficulty of signal detection and processing is not high; it has the advantages of large dynamic range of measurement, high resolution and simple structure. However, due to the large spectral width of the broadband light source, when the light passes through the nonlinear dispersion medium, the phase of the light wave will be distorted, which will cause the interference envelope to broaden and reduce the measurement resolution. Therefore, when using a broadband light source for interferometric measurement , the second-order dispersion term related to the wavelength of light will become a problem that cannot be ignored. Therefore, the compensation of the second-order dispersion term caused by the broadband light source has become one of the key technologies to improve the measurement accuracy of the white light interferometry system.

At present, the compensation for dispersion is mostly concentrated in the field of communication technology. In addition, dispersion compensation technology is especially common in the measurement of optical coherence tomography. Patent 200710199953 proposes a chromatic dispersion compensation optical fiber, which compensates the dispersion of optical fiber lines in a communication system by combining positive and negative dispersion optical fibers. Patent 200710107461.9 proposes an electrical method to process the acquisition signal, which can realize compensation for optical fibers with any dispersion value. Patent 200610052463.8 proposes a dispersion compensation method in optical coherence tomography, which is realized by adding a blazed grating parallel to the original blazed grating to the original single-grating fast-scanning delay line. The above dispersion compensation methods mostly use dispersion compensating fiber or electrical and optical phase modulation compensation methods. These methods can only compensate for optical fibers with fixed dispersion values, or due to factors such as their own limited frequency bands, they cannot Realize effective compensation for the dispersion of the ultra-broadband optical signal, or the implementation device is complicated, and noise and errors are easily introduced. However, in view of the real-time performance of the low-coherence polarization-maintaining fiber polarization coupling test system and the arbitrariness of the birefringence dispersion value, there is an urgent need for a fast and real-time compensation method for any dispersion value.

Contents of the invention

The object of the present invention is to propose a dispersion compensation method for a broadband light source through a polarization-maintaining optical fiber polarization coupling detection system based on a Michelson interferometer constructed by a broadband light source and in combination with the characteristics of the system interference signal. Fast and real-time compensation for the dispersion introduced when the system uses a broadband light source, so as to realize quasi-distributed sensing of stress, position, temperature, etc. by polarization-maintaining optical fibers.

The dispersion compensation method of the broadband light source provided by the present invention uses a broadband light source to construct a polarization-maintaining optical fiber polarization coupling test system based on a Michelson interferometer, and the light emitted by the light source is coupled into the polarization-maintaining optical fiber along a certain characteristic axis of the optical fiber to form an excitation mode. When a certain point of the fiber is subjected to a force, part of the energy of the excited mode will be coupled to the other characteristic axis of the fiber to form a coupled mode. The optical path difference between the excited mode and the coupled mode in the polarization-maintaining fiber is compensated by Michelson interferometer, and the interference signal is collected by the detector. The specific compensation methods include:

1. The original data collected by the detector is intercepted through the rectangular window function to intercept the interference data I _main of the excited mode and the excited mode and the interference data I _coupling of the excited mode and the coupled mode;

其中，c为真空中光速，Δλ和λ₀分别为光源光谱的谱宽和中心波长，L为光纤耦合点距光纤出射端的距离。因为当存在双折射色散时，干涉包络在1/e处宽度将以

速率展宽；Step 2. Hilbert transform and Gaussian fitting are performed on the I _main and I _coupling intercepted in the first step respectively to obtain the ratio of the width of the envelope <I> _main and <I> _coupling of the interference signal at 1/e η, the measured birefringent dispersion coefficient ΔD of the optical fiber,

Among them, c is the speed of light in vacuum, Δλ and _λ0 are the spectral width and central wavelength of the light source spectrum, respectively, and L is the distance between the fiber coupling point and the fiber output end. Because when there is birefringent dispersion, the interference envelope width at 1/e will be given by

rate widening;

ω为光波频率，ω₀为光波的中心频率。3. Obtain the dispersion phase compensation factor from the birefringence dispersion coefficient ΔD of the optical fiber in,

ω is the frequency of the light wave, and ω ₀ is the center frequency of the light wave.

式中，Δβ(ω)为保偏光纤快慢轴的传播常数差，Δn_b为位相双折射，ΔN_b为群双折射，ΔΦ(ω)中第三项关于ω的非线性项即为引起干涉包络展宽的色散相位项。Step 4. Multiply the dispersion phase compensation factor obtained in step 3 and the excitation mode intercepted in step 1 with the spectral function of the coupled mode interference data I _coupling to eliminate the interference signal envelope caused by the phase ΔΦ(ω) of I _coupling The broadened nonlinear phase term yields a linear spectrum signal; where,

In the formula, Δβ(ω) is the propagation constant difference between the fast and slow axis of the polarization maintaining fiber, Δn _b is the phase birefringence, ΔN _b is the group birefringence, and the third non-linear term about ω in ΔΦ(ω) is the interference Dispersive phase term for envelope broadening.

Step 5. Finally, inverse Fourier transform is performed on the linear spectrum signal obtained in Step 4 to obtain the dispersion-compensated interference signal I _comp .

Advantage and positive effect of the present invention:

1. The present invention aims at the characteristics of high test accuracy, good real-time performance and uncertain birefringence dispersion value required by the polarization-maintaining fiber polarization coupling test system based on Michelson interferometer, and designs a broadband light source applied to the dispersion medium The method of dynamically compensating the dispersion caused by the measurement can realize the compensation of the dispersion value of any coupling position of the polarization-maintaining fiber, and improves the accuracy of the long-distance polarization-maintaining fiber coupling strength test.

2. The dispersion compensation method is realized by the analysis and processing of the original signal by the computer, which has a high degree of automation and has the advantages of fast compensation speed, simple and easy operation, and good real-time performance.

3. The dispersion compensation method of the present invention is also widely applicable to long-distance polarization-maintaining optical fiber gyroscope measurement, polarization-maintaining optical fiber extinction ratio test and other test systems using broadband light sources.

Description of drawings

Figure 1 is a schematic diagram of a low-coherence polarization-maintaining fiber polarization coupling test system.

Figure 2 is a schematic diagram of the system collecting interference signals.

Fig. 3 is a functional block diagram of the system dispersion compensation.

Figure 4 is the interference data of the coupling point at 400m of the polarization-maintaining fiber, (a) experimental data, (b) data after compensation.

Fig. 5 is the interference data of the coupling point at 1000m of the polarization maintaining fiber, (a) the data collected by the experiment, (b) the data after compensation.

Figure 6 shows the interference data of coupling points at 399.9m and 400m of the polarization maintaining fiber, (a) experimental data, (b) data after compensation.

In the figure, 1 light source, 2 optical isolator, 3 polarizer, 4 polarization maintaining fiber, 5 beam expander collimator lens, 6 half-wave plate, 7 Glan prism, 8 beam splitter, 9 reflector, 10 guide rail, 11 scanning mirrors, 12 converging lenses, 13 detectors, 14 acquisition cards, 15 computers, 16 motors, 17 motors.

The present invention will be further described below in conjunction with accompanying drawing.

Detailed ways

Figure 1 shows the schematic diagram of the low-coherence polarization-maintaining fiber polarization coupling test system. In the polarization coupling test experiment, the supercontinuum light source 1 emits supercontinuum light, passes through the optical isolator 2, and passes through the polarizer 3 along the polarization-maintaining One of the main axes of the fiber 4 is coupled into the polarization-maintaining fiber, and then collimated by the beam expander collimator lens 5. The half-wave plate 6 changes the polarization state of the light and combines with the Glan prism 7 to project the light waves of the fast and slow axes of the polarization-maintaining fiber to 45°. direction, the light projected to the same polarization state enters the adjustable Michelson interferometer, and the projected light is divided into two beams by the beam splitter 8, one beam reaches the mirror 9, and the other beam reaches the movable beam fixed on the guide rail 10 The scanning mirror 11 moves the scanning mirror to compensate the optical path difference so that the excitation mode and the coupling mode interfere, and the interference light enters the detector 13 through the converging lens 12, and then the acquisition card 14 converts the optical signal collected by the detector into a digital electrical signal and sends it to into the computer 15 for processing. The rotation of the half-slide and the movement of the guide rail in the experimental device are realized by the signal from the computer to the acquisition card, and the acquisition card controls the motor 16 and the motor 17. Since the polarization maintaining fiber has the characteristic of maintaining the polarization state of light, when the fiber is subjected to external force or has defects, the linearly polarized light transmitted along one axis will be coupled to the other axis, resulting in the coupling of the light propagation mode. The position of the final coupling point and the intensity of the coupling point can be obtained by demodulating the collected interference light signal.

The specific steps of the dispersion compensation method of the broadband light source provided by the present invention include:

1. The original data collected by the detector is intercepted through the rectangular window function to intercept the interference data I _main of the excited mode and the excited mode and the interference data I _coupling of the excited mode and the coupled mode;

其中，c为真空中光速，Δλ和λ₀分别为光源光谱的谱宽和中心波长，L为光纤耦合点距光纤出射端的距离。因为当存在双折射色散时，干涉包络在1/e处宽度将以

速率展宽；Step 2. Hilbert transform and Gaussian fitting are performed on the I _main and I _coupling intercepted in the first step respectively to obtain the ratio of the width of the envelope <I> _main and <I> _coupling of the interference signal at 1/e η, the measured birefringent dispersion coefficient ΔD of the optical fiber,

Among them, c is the speed of light in vacuum, Δλ and _λ0 are the spectral width and central wavelength of the light source spectrum, respectively, and L is the distance between the fiber coupling point and the fiber output end. Because when there is birefringent dispersion, the interference envelope width at 1/e will be given by

rate widening;

其中，

ω为光波频率，ω₀为光波的中心频率。3. Obtain the dispersion phase compensation factor from the birefringence dispersion coefficient ΔD of the optical fiber

in,

ω is the frequency of the light wave, and ω ₀ is the center frequency of the light wave.

式中，Δβ(ω)为保偏光纤快慢轴的传播常数差，Δn_b为位相双折射，ΔN_b为群双折射，ΔΦ(ω)中第三项关于ω的非线性项即为引起干涉包络展宽的色散相位项。Step 4. Multiply the dispersion phase compensation factor obtained in step 3 and the excitation mode intercepted in step 1 with the spectral function of the coupled mode interference data I _coupling to eliminate the interference signal envelope caused by the phase ΔΦ(ω) of I _coupling The broadened nonlinear phase term yields a linear spectrum signal; where,

In the formula, Δβ(ω) is the propagation constant difference between the fast and slow axis of the polarization maintaining fiber, Δn _b is the phase birefringence, ΔN _b is the group birefringence, and the third non-linear term about ω in ΔΦ(ω) is the interference Dispersive phase term for envelope broadening.

Step 5. Finally, inverse Fourier transform is performed on the linear spectrum signal obtained in Step 4 to obtain the dispersion-compensated interference signal I _comp .

As shown in Figure 3, the excitation mode and excitation mode interference data 20 and the excitation mode and coupled mode interference data 21 are respectively intercepted from the collected raw data 18 through the window function 19, and then through Hilbert transform 22 and Gaussian approximation Combining 23 to obtain the envelope of two sets of interference data, the bifold dispersion coefficient 24 of the polarization-maintaining fiber is measured from the two envelopes at the 1/e width ratio, so as to obtain the dispersion phase compensation factor 25 required to compensate the dispersion value at the coupling point, And multiply it with the nonlinear spectral function 27 obtained by the Fourier transform 26 of the data envelope at the coupling point, eliminate the nonlinear term that causes the envelope to broaden in the nonlinear spectral function, obtain the linear spectral function 28, and finally obtain Inverse Fourier transform 29 is performed on the linear spectrum function of the dispersion compensation to obtain the interference signal 30 after dispersion compensation.

In the detection process of the present invention, the supercontinuum light source uses SUPERLUM IRELAND light source model P4-0114, the spectral density of the light source is Gaussian distribution, the central wavelength is 1315nm, the spectral width is 30.08nm, and the beat length is 2.6mm. The measured polarization-maintaining fiber adopts the polarization-maintaining fiber of the Forty-six Research Institute of China Electronics Technology Group Corporation with a working wavelength of 1310nm and a cut-off wavelength of 1208.10nm. The detector is PDA10CS-EC from THORLABS company, the detection wavelength range is 700nm-1800nm, and the USB6251 data acquisition card of NI company is used to collect data for subsequent processing.

然后将其与耦合点数据的傅里叶变换后得到的包含色散信息的非线性频谱信号相乘后即可消除双折射色散引起的非线性效应，得到线性频谱信号，最后将线性频谱信号进行傅里叶反变换就可以得到色散补偿后的干涉信号。During the experiment, the polarization-maintaining fiber used for testing was connected to the system. After starting all the equipment, the rotation of the half-wave plate was first controlled by a stepping motor, so that the two beams of light transmitted along the polarization-maintaining fiber were projected to the direction of 45°, and then Set the fiber length to be scanned for data acquisition. At this time, the stepping motor will drive the scanning mirror to perform optical path compensation, and the signal detected by the detector will be collected by the acquisition card and sent to the computer for analysis and processing. Due to the axis error between the optical fiber and the polarizer, in the collected interference signal, the maximum position of the interference between the excitation mode and the excitation mode is the position of zero optical path difference, which corresponds to the exit end of the optical fiber. When there is a large coupling point at a certain point in the fiber, the interference data related to the position and size of the coupling point formed by the interference between the excited mode and the coupled mode will appear in the corresponding position of the collected interference signal, as shown in Figure 2. Since the alignment error between the optical fiber under test and the polarizer joint will cause a large optical mode coupling, it can be seen in the collected signal that there will be a large coupling point here. The method mentioned in 3 extracts the envelope of the interference data of the excited mode and the excited mode and the excited mode and the coupled mode, divides the width of the two at 1/e and calculates the bifold dispersion coefficient ΔD, and then obtains the dispersion compensation factor

Then multiply it with the nonlinear spectrum signal containing dispersion information obtained after the Fourier transform of the coupling point data, and then the nonlinear effect caused by birefringence dispersion can be eliminated, and the linear spectrum signal is obtained, and finally the linear spectrum signal is processed The interference signal after dispersion compensation can be obtained by inverse Liye transform.

1. Implementation Example 1

得出所测光纤的双折射色散系数为ΔD＝0.0116×10^-9ps/(nm·km)，将其带入第3步的

中求得色散补偿因子的值并与第一步截取的激发模与耦合模干涉数据I_coupling的频谱函数相乘，即可消去I_coupling的频谱中引起干涉信号包络展宽的非线性相位项得到线性频谱函数，最后由第5步进行傅里叶反变换后得到色散补偿后的干涉信号。实验采集数据后利用软件处理数据的方法，流程如图3所示，即可快速实时地补偿400m保偏光纤的色散。补偿结果如图4所示，补偿后激发模与耦合模干涉数据的包络在1/e处的宽度减小到27.36um，与没有色散的激发模与激发模干涉数据的包络宽度26.10um接近。Connect the 400m polarization-maintaining optical fiber into the system, and it can be measured that there is a large coupling point at the joint between the 400m optical fiber and the polarizer. The data collected by the detector is intercepted by window function, Hilberta transform and After Gaussian fitting, the width of the envelope of the interference data of the excited mode and the coupled mode at 1/e is 122.69um, and the width of the envelope of the interference data of the excited mode and the coupled mode at 1/e is 26.10um. The ratio can be η=4.70, and then by the formula in the second step

It is obtained that the birefringent dispersion coefficient of the measured optical fiber is ΔD=0.0116×10 ^-9 ps/(nm km), which is brought into the third step

The value of the dispersion compensation factor is obtained in , and multiplied by the spectral function of the excitation mode and coupled mode interference data I _coupling intercepted in the first step, the nonlinear phase term that causes the interference signal envelope to broaden in the spectrum of I _coupling can be eliminated to obtain The linear spectrum function, and finally the interference signal after dispersion compensation is obtained after the inverse Fourier transform is performed in the fifth step. The method of using software to process the data after collecting data in the experiment, as shown in Figure 3, can quickly and real-time compensate the dispersion of the 400m polarization-maintaining fiber. The compensation result is shown in Figure 4. After compensation, the width of the envelope of the interference data between the excitation mode and the coupled mode is reduced to 27.36um at 1/e, and the envelope width of the interference data between the excitation mode and the excitation mode without dispersion is 26.10um. near.

2. Implementation example two

The 1310nm polarization-maintaining fiber used in the experiment generally has a very small birefringence dispersion coefficient, but when the fiber length is long, the birefringence dispersion of the fiber will make the interference light signal broaden more seriously. Therefore, in the experiment, a 1000m polarization-maintaining optical fiber was connected to the system. As described in Example 1 of the embodiment, it can be measured that there will be a large coupling point at 1000m. The envelope of the coupled-mode interference data is subjected to dispersion compensation. The results before and after compensation are shown in Figure 5. It can be seen that the width of the envelope of the excitation-mode and coupled-mode interference data at 1/e is reduced from 373.07um to 39.93um after compensation.

3. Implementation example three

In order to further illustrate the importance of dispersion compensation when using a broadband light source as the light source of the test system, a 400m polarization-maintaining fiber is connected to the system, and an external force is applied at a distance of about 10cm from the joint between the 400m fiber and the polarizer. Case 1 and the principle of polarization coupling test, there will be a coupling point at 399.9m and 400m respectively, due to the long transmission distance, the envelopes of the interference data of the excited mode and the coupled mode at the two coupling points will widen and overlap to distinguish The interference data envelope at the two coupling points after dispersion compensation is about 4.7 times narrower than that before compensation, so the two close coupling points can be easily distinguished. The results before and after compensation are shown in Figure 6.

Claims (1)

1. A dispersion compensation method for a broadband light source, using a broadband light source to build a polarization-maintaining fiber polarization coupling test system based on Michelson interferometer, the light emitted by the light source is coupled into the polarization-maintaining fiber along a certain characteristic axis of the fiber to form an excitation mode, when When a point of the fiber is subjected to a force, the excitation mode will have part of its energy coupled to the other characteristic axis of the fiber to form a coupled mode; the optical path difference between the excitation mode and the coupling mode is compensated by the Michelson interferometer, and the interference signal is detected by the detector , characterized in that the compensation method includes: 1. The original data collected by the detector is intercepted through the rectangular window function to intercept the interference data I _main of the excited mode and the excited mode and the interference data I _coupling of the excited mode and the coupled mode; Step 2. Hilbert transform and Gaussian fitting are performed on the I _main and I _coupling intercepted in the first step respectively to obtain the ratio of the width of the envelope <I> _main and <I> _coupling of the interference signal at 1/e η, the measured birefringent dispersion coefficient ΔD of the optical fiber, Among them, c is the speed of light in vacuum, Δλ and _λ0 are the spectral width and central wavelength of the light source spectrum, respectively, L is the distance from the fiber coupling point to the fiber output end, because when there is birefringence dispersion, the width of the interference envelope at 1/e will be given by

rate widening; 3. Obtain the dispersion phase compensation factor from the birefringence dispersion coefficient ΔD of the optical fiber

in,

ω is the light wave frequency, and ω ₀ is the center frequency of the light wave; Step 4. Multiply the dispersion phase compensation factor obtained in step 3 and the excitation mode intercepted in step 1 with the spectral function of the coupled mode interference data I _coupling to eliminate the interference signal envelope caused by the phase ΔΦ(ω) of I _coupling The broadened nonlinear phase term yields a linear spectrum signal; where,

In the formula, Δβ(ω) is the propagation constant difference between the fast and slow axis of the polarization maintaining fiber, Δn _b is the phase birefringence, ΔN _b is the group birefringence, and the third non-linear term about ω in ΔΦ(ω) is the interference The dispersion phase term of the envelope broadening; Step 5. Finally, inverse Fourier transform is performed on the linear spectrum signal obtained in Step 4 to obtain the dispersion-compensated interference signal I _comp .

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