CN105403257A - Distribution type Brillouin time domain analysis system - Google Patents

Abstract

Distributed Brillouin optical time-domain analysis has been widely concerned, but due to the problems of pulse distortion and high power in Raman amplification, it has not been well applied. The invention combines the distributed Brillouin amplification technology with the frequency comb technology and applies it to a long-distance distributed Brillouin optical time domain analysis system. Firstly, it solves the intensity noise transfer and nonlinear effect caused by distributed Raman amplification, and secondly, because the input optical power is very low, it provides a layer of guarantee for system reliability; secondly, the frequency comb is used to mix the incident pulsed light The method to restore the pulse shape avoids the distortion of the pulse, thereby solving the problem of the deterioration of the spatial resolution caused by the pulse distortion, and can weaken the transient gain saturation to a certain extent.

Description

A Distributed Brillouin Optical Time Domain Analysis System

technical field

The invention belongs to the field of optical fiber technology, and specifically proposes a method for improving the performance of a Brillouin optical time domain analysis system by combining distributed Brillouin amplification technology and frequency comb technology.

Background technique

Distributed optical fiber sensing was proposed in the 1970s. It not only has the advantages of no radiation interference, good anti-electromagnetic interference, and stable chemical properties, but also can feedback the spatial distribution of the measured field on the optical fiber path and the time-dependent change information. Based on its unique advantages, distributed optical fiber sensors have been widely used in the safety detection of major structures and equipment such as pipeline bridges, mining areas, geology, optical/cables, etc.

Based on the distributed Brillouin optical time domain analysis system, it can have high measurement accuracy, measurement range and spatial resolution, so this technology has been widely concerned at present. Because the optical fiber sensing system requires a long sensing distance, that is, the pulsed light needs to be distributed flat and strong enough in the optical fiber path. To achieve this purpose, an erbium-doped fiber amplifier can be used to amplify the pulse before the pulsed light enters the fiber under test. The optical method, but this method has the problem that the farther the pulse light propagates along the fiber, the lower the power. The pulse light is only stronger in the front section of the fiber, and it will be attenuated by the loss of the fiber and the continuous detection light at the back end. The sharp drop will seriously affect the measurement effect of the back end.

Therefore, the distributed Raman amplification technology is used in the optical time-domain analysis system. Raman amplification is used to perform distributed amplification on the pulsed light, which can make the pulsed light sufficiently strong and evenly distributed, and obtain better measurement results at the end. But there will be the following problems in this technology:

1. Distributed Raman amplification requires very high incident light power, which can reach several watts, so it will affect the reliability of the entire system. For example, the optical fiber connector may be burned out, and the detector and other devices may be damaged. burn out.

2. High incident optical power will lead to the generation of nonlinear effects and the transfer of intensity noise.

3. The problem of the distortion of the outgoing pulse has not been solved, and the distortion of the pulse will lead to the deterioration of the spatial resolution and the saturation of the transient gain.

Contents of the invention

Problem to be solved by the present invention has two points:

1. On the premise of ensuring long-distance, high spatial resolution and high precision of the sensing system, use low incident optical power to perform distributed amplification of pulses;

2. Solve the distortion problem of the outgoing pulsed light.

The present invention solves the technical problem proposed like this:

A distributed Brillouin optical time-domain analysis system that uses distributed Brillouin amplification to mix frequency combs with Brillouin pump light.

Further, the distributed Brillouin optical time domain analysis system includes lasers, couplers, acquisition systems, optical pulse encoding and decoding systems; also includes electro-optic modulators, frequency comb sources, microwave sources, fiber amplifiers, acousto-optic modulators , Brillouin amplification system.

Further, the laser 1 is connected to the first coupler 2, and the upper end of the first coupler 2 is connected to the second coupler 3, the first polarization controller 4, the first electro-optical modulator 5, the circulator 12, the fiber amplifier 13, and the Dimming attenuator 14; the upper left end of the first electro-optic modulator 5 is sequentially connected to the electric amplifier 9, the mixer 8, and the first microwave source 7; the upper right end of the first electro-optic modulator 5 is connected to the first DC source 10; the frequency comb Source 6 is connected to mixer 8 .

Further, the fiber amplifier is an erbium-doped fiber amplifier.

The combination of distributed Brillouin amplification technology and frequency comb technology is applied to the distributed Brillouin optical time domain analysis system. While improving the uniformity of pulse light distribution in the entire sensing fiber, the incident light power is greatly reduced, and it can The outgoing pulse shape is restored to the incoming pulse shape by a frequency comb.

Beneficial effects of the present invention: the present invention combines the distributed Brillouin amplification technology with the frequency comb technology and applies it to a long-distance distributed Brillouin optical time domain analysis system. Firstly, it solves the intensity noise transfer and nonlinear effect caused by distributed Raman amplification, and secondly, because the input optical power is very low, it provides a layer of guarantee for system reliability; secondly, the frequency comb is used to mix the incident pulsed light The method to restore the pulse shape avoids the distortion of the pulse, thereby solving the problem of the deterioration of the spatial resolution caused by the pulse distortion, and can weaken the transient gain saturation to a certain extent.

Description of drawings

Figure 1 is a structural diagram of a distributed Brillouin optical time-domain analysis system based on the combination of frequency comb distributed Brillouin amplification and optical pulse coding technology;

Figure 2 is the relationship curve of Brillouin detection wave frequency shift-time (space)-power measured by experiment at a distance of 50km;

Figure 3 is a diagram of the relationship between the outgoing pulsed light and the frequency comb

Figure 4 is the temperature distribution map extracted after Lorentz fitting

Figure 5 is a sketch of the principle of distributed Brillouin amplification, υ _b is the inherent Brillouin frequency shift of the tested fiber, υ ₁ , υ ₂ and υ ₀ are the continuous detection light frequency, pump light frequency, pulse light frequency, Δ _υ is the frequency shift of the acousto-optic detector.

Among them: laser 1, first coupler 2, second coupler 3, first polarization controller 4, first electro-optic modulator 5, frequency comb source 6, first microwave source 7, mixer 8, electrical amplifier 9 , the first DC source 10, the first fiber Bragg grating 11, the circulator 12, the first erbium-doped fiber amplifier 13, the adjustable optical attenuator 14, the third coupler 15, the first polarization scrambler 16, the isolator 17 , second polarization controller 18, second electro-optic modulator 19, second microwave source 20, second DC source 21, circulator 22, second fiber Bragg grating 23, second erbium-doped fiber amplifier 24, adjustable optical attenuation device 25, acousto-optic modulator 26, third DC source 27, second polarization scrambler 28, third erbium-doped fiber amplifier 29, detector 30, computer 31, acquisition card 32, adjustable optical attenuator 33, circulator 34. A third fiber Bragg grating 35, a fourth erbium-doped fiber amplifier 36, a circulator 37, and an optical fiber 38 to be tested.

detailed description

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

1. Specific working methods:

As shown in Figure 1: the Brillouin optical time-domain analysis system of the present invention includes a laser 1, a first electro-optic modulator 5, a first fiber Bragg grating 11, a first erbium-doped fiber amplifier 13, and a first polarization scrambler 16 , the second polarization controller 18, the second electro-optic modulator 19, the second fiber Bragg grating 23, the second erbium-doped fiber amplifier 24, the acousto-optic modulator 26, the second scrambler 28, the third erbium-doped fiber amplifier 29 , a detector 30, an acquisition card 32, a third fiber Bragg grating 35, a fourth erbium-doped fiber amplifier 36, an optical fiber 38 to be tested, a distributed Brillouin amplification system, and an optical pulse encoding and decoding system. Wherein, the laser 1 is a distributed feedback semiconductor laser, which is used to generate the laser light used in the experiment, with a power of 6 dBm. The beam splitting ratio of the coupler 2 is 1/9, wherein 10% of the light passes to the coupler 3, and the beam splitting ratio of the coupler 3 is 1/1, wherein one path passes through the first electro-optic modulator 5 to generate two sidebands, Because the electro-optic modulator is quite sensitive to polarization, a polarization controller 4 is added before the first electro-optic modulator 5, where the adjusted frequency comb is mixed with the high-frequency components. Afterwards, the low-frequency components are filtered out by the circulator 12 and the fiber Bragg grating 11, and the remaining high-frequency components are used as pump light for distributed Brillouin amplification. The pump light power required here is different from that required for distributed Raman amplification. A few watts, here only a few milliwatts are required. The other path split by the coupler 3 enters the second electro-optic modulator 19, as above, a polarization controller 18 needs to be added before, and two sidebands are generated by the second electro-optic modulator, and then there is a circulator 22 and a fiber Bragg grating 2 The high-frequency components are filtered out, and the remaining low-frequency components are used as continuous detection light. Ninety-one percent of the way split by the coupler 2 is transmitted to the acousto-optic modulator 26, and the required pulsed light is generated by the acousto-optic modulator, where the optical pulse code is added, and then enters the second scrambler 28 , The third erbium-doped fiber amplifier 29. Finally, it enters the first port of the circulator 37, emerges from the second port of the circulator 37, enters the optical fiber under test 38, and encounters the counterpropagating distributed Brillouin amplified pump light and the continuous detection light to generate Brillouin interaction. The Brillouin amplification is shown in Figure 5.

Two, Brillouin optical time domain analysis system temperature/strain sensing method of the present invention, comprises the following steps:

a. Inject laser light into the fiber;

b. Mixing the frequency comb with the pump light;

c. Accept the amplified Brillouin detection light;

d. Sweep the frequency of the second microwave source 20, and use the data acquisition and analysis system to obtain the distribution of strain and temperature along the optical fiber;

Specifically, after the pulsed light is emitted from the second port of the circulator 37, it meets the pumping light and the continuous detection light, and the frequency difference between the pulsed light and the pumping light is set as the inherent Brillouin frequency shift of the optical fiber, and through the Brillouin During the process, the pump light performs distributed amplification on the pulsed light, so that the pulsed light is amplified, the power is evenly distributed in the fiber, and the end can also maintain the same linear noise ratio, spatial resolution and measurement accuracy. Subsequently, the pulsed light and the continuous detection light perform Brillouin interaction, so that the detection light carries sensing information, and finally exits from the third port of the circulator 37 and enters the numerical processing system.

3. Verification of experimental results

Figure 2 is the relationship curve of Brillouin detection wave frequency shift-time (space)-power measured by experiment at a distance of 50km;

Among them, the inherent Brillouin frequency shift of optical fiber is 10850MHz. The fiber ends were placed in a temperature-controlled box at 56°C. The Brillouin frequency shift-Brillouin gain curve presents a Lorentzian distribution. From the three-dimensional diagram of the Brillouin gain of the entire fiber to be tested, the stimulated Brillouin signal maintains a high signal-to-noise level in the 50km fiber. Compared with that, there is a significant ~36MHz Brillouin frequency shift change at the measurement point near 50km.

Figure 3 is a diagram of the relationship between the outgoing pulsed light and frequency combs. It can be seen from the figure that when the frequency combs are too small, obvious pulse distortion will appear. If there are enough frequency combs, when the number of frequency combs reaches 12, the pulse distortion has been completely eliminated.

Figure 4 is the temperature distribution map extracted after Lorentz fitting. It can be seen that the whole process has a very good signal-to-noise ratio, and there is a very obvious temperature drift at the end, which meets the set experimental conditions and the spatial resolution has reached 3.2m. It has been verified by experiments that this method can indeed perform Brillouin optical time-domain analysis with only a few milliwatts of power under the conditions of long-distance, high signal-to-noise ratio and high precision.

Finally, it should be noted that: the above is only a preferred embodiment of the present invention, and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, for those skilled in the art, it still The technical solutions recorded in the foregoing embodiments may be modified, or some technical features thereof may be equivalently replaced. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included within the protection scope of the present invention.

Claims (4)

1. A distributed Brillouin optical time-domain analysis system that uses distributed Brillouin amplification to mix frequency combs with Brillouin pump light. 2. A kind of distributed Brillouin optical time domain analysis system according to claim 1, is characterized in that, comprises laser device, coupler, acquisition system, optical pulse encoding and decoding system; Also comprises, electro-optical modulator, frequency Comb source, microwave source, fiber amplifier, acousto-optic modulator, Brillouin amplification system. 3. A kind of distributed Brillouin optical time-domain analysis system according to claim 1, wherein the laser 1 is connected with the first coupler 2, and the upper end of the first coupler 2 is connected with the second coupler 3, The first polarization controller 4, the first electro-optic modulator 5, the circulator 12, the fiber amplifier 13, the adjustable optical attenuator 14; A microwave source 7 ; the upper right end of the first electro-optic modulator 5 is connected to the first DC source 10 ; the frequency comb source 6 is connected to the mixer 8 . 4. A distributed Brillouin optical time-domain analysis system according to claims 1-3, characterized in that the optical fiber amplifier is an erbium-doped optical fiber amplifier.