A high-precision optical fiber strain sensing system using broadband light drive and its demodulation method

Abstract

The invention discloses a high-precision optical fiber strain sensing system driven by broadband light. The system is composed of a broadband light source module, a passive optical fiber sensor module, a circulator, a phase modulation module, a delay module, a data processing module, a photoelectric conversion module and a phase-locked amplifier module. The passive optical fiber sensor module is provided with an optical fiber ring resonator. Broadband light is emitted from the light source and enters the first port of the circulator, and is emitted from the second port and enters the ring resonator. It passes through the phase modulator module and the delay module and returns to the ring resonator again. The output light then enters the photoelectric conversion module unidirectionally through the circulator for photoelectric conversion. The system avoids various noises caused by lasers, has higher measurement accuracy, and also makes the use of broadband light sources make the system cost low. At the same time, due to the use of a self-reference sensing mechanism, the system does not need a complex and precise feedback frequency locking system, which significantly reduces the complexity of the system.

Description

Technical Field

[0001] The present invention relates to the field of optical fiber sensing technology, in particular to a high-precision optical fiber strain sensing system driven by broadband light and a demodulation method thereof. Background Art

[0002] Compared with traditional electronic sensors, fiber optic sensors have the following advantages, such as: the main material of optical fiber is silicon dioxide, which has stable chemical properties, corrosion resistance, high temperature, and high pressure, and is compatible with communication optical fiber to achieve integrated transmission and sensing, etc. It has been widely used in many military, civilian, and scientific research fields such as aerospace, biomedicine, resource exploration, and geological monitoring. In the field of fiber optic sensing, most high-precision fiber optic sensors are generally based on high-Q fiber resonant cavities, and usually use narrow linewidth laser light sources and frequency-locked demodulation systems. For example, for high-precision fiber Fabry-Perot interferometer sensors, the use of frequency-stabilized lasers and frequency-locked demodulation technology can even achieve a strain measurement accuracy of 10-15 levels. The measurement accuracy is often limited by the frequency noise of the laser.

[0003] Although the use of laser-driven optical fiber sensors has achieved very high measurement accuracy, such sensors are complex in structure and expensive, so their applications are very limited. For example, if you want to achieve the strain measurement accuracy of skin strain, the required laser line width is about the order of 100 Hz, and the price of such a high-performance laser is very expensive. In addition, the use of such a high-performance laser for high-precision measurement requires a complex optical frequency feedback tuning system. The high cost of the light source and the complex optical frequency feedback tuning system greatly limit the practical application of such sensors. In addition to the problems of cost and complexity, there are also problems such as the narrowband of the laser light source is incompatible with the wavelength division multiplexing required by the sensing system, and the various parasitic noises of the laser affect the measurement accuracy. The present invention provides a high-precision optical fiber strain sensing system driven by broadband light and a demodulation method thereof. Summary of the invention

[0004] The purpose of the present invention is to provide a high-precision optical fiber strain sensing system driven by broadband light and a demodulation method thereof, so as to solve the problems raised in the above background technology: 1. The laser-driven optical fiber sensor has a complex structure and is expensive; 2. The narrow band of the laser light source is incompatible with the wavelength division multiplexing required by the sensing system, and various parasitic noises of the laser affect the measurement accuracy.

[0005] To achieve the above object, the present invention provides the following technical solutions:

[0006] A high-precision optical fiber strain sensing system driven by broadband light, the system consists of a broadband light source module, a passive optical fiber sensor module, a circulator, a phase modulation module, a delay module, a data processing module, a photoelectric conversion module and a phase-locked amplifier module. An optical fiber ring resonator is arranged in the passive optical fiber sensor module. Broadband light is emitted from the light source and enters the first port of the circulator, and is emitted from the second port and enters the ring resonator, and then returns to the ring resonator through the phase modulator module and the delay module. The output light then enters the photoelectric conversion module unidirectionally through the circulator for photoelectric conversion. The data processing module extracts a strain change rate signal from the electrical signal output by the photoelectric conversion module, and performs 1 / f filtering. The demodulated signal reflects the strain information applied to the optical fiber ring cavity, and the strain information is the output of the sensing system.

[0007] The fiber ring resonator is composed of a high splitting ratio coupler and an optical fiber. Two groups of high splitting ratio couplers are provided. The fiber ring resonator is formed by fusion splicing the high splitting ratio coupler with the optical fiber.

[0008] In the composition of this system, an acousto-optic frequency shift module can be added to realize the frequency shift of broadband light, which is used to debug the optimal working point of the system. In actual work, the acousto-optic frequency shift module can be removed to simplify the system. At the same time, this sensor system does not need to perform frequency locking operation, and the system itself can always work stably at the optimal working point in the open-loop state;

[0009] To further illustrate, the demodulation method is as follows:

[0010] S1. Using the optical fiber resonant cavity as the strain sensing unit, the low-coherence light emitted by the broadband light source passes through the resonant cavity for the first time, passes through the delayed optical fiber, passes through the optical fiber resonant cavity for the second time, and is received by the photodetector;

[0011] S2. According to S1, after the low-coherence light passes through the optical fiber resonant cavity for the first time, the spectrum of the transmitted light wave is modulated into the comb spectrum of the resonant cavity. After phase modulation, the light wave passes through the optical fiber resonant cavity for the second time. The transmission spectrum of the optical fiber resonant cavity is shifted due to the effect of the measured parameter, resulting in a change in the efficiency of the light wave passing through the optical fiber resonant cavity and a change in the optical power reaching the photodetector.

[0012] S3. By collecting the changes in the optical power of the photodetector, it is demodulated using the phase-locked amplification technology to restore the signal to be measured applied to the sensor.

[0013] According to the above demodulation method, it is further described that, in S1, the broadband light source is a wide-spectrum light source, the fiber resonant cavity is a high-fineness ring resonant cavity, and the fiber resonant cavity can be any one of a ring resonant cavity and a linear Fabry-Perot resonant cavity.

[0014] In S3, the sensor is a four-port fiber ring resonator, two groups of ports are transmission ports, and the other two groups of ports are optical power collection ports. The transmission spectrum between the transmission ports presents an upward convex resonance peak.

[0015] In S2, the modulation method of the light wave before entering the resonant cavity for the second time can be either a lithium niobate waveguide or a piezoelectric transducer.

[0016] Compared with the prior art, the present invention has the following beneficial effects:

[0017] 1. This system avoids various noises caused by lasers and has higher measurement accuracy. For example, a strain resolution of 120fε can be achieved at 1kHz using a 2-meter-long FRR sensor.

[0018] 2. The system uses broadband light sources, making the system cost low;

[0019] 3. Since this system uses a self-reference sensing mechanism, the system does not require a complex and precise feedback frequency locking system, which significantly reduces the system complexity. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is the overall architecture diagram of the system in the present invention;

[0021] Figure 2 The broadband optical resonance curve and the frequency discrimination curve obtained when the acousto-optic frequency shifter of the present invention is linearly swept;

[0022] Figure 3 is the strain power spectrum density of the sensor output signal in the present invention.

[0023] Figure 1 Among them, 1- broadband light source, 2- circulator, 3- PZT, 4- fiber ring resonator, 5- acousto-optic modulator, 6- phase modulator, 7- delay module, 8- photodetector, 9- data processing module. DETAILED DESCRIPTION

[0024] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0025] Example

[0026] See also Figure 1-3 , the present invention provides a technical solution:

[0027] A high-precision optical fiber strain sensing system driven by broadband light, comprising a broadband light source, an optical circulator, a photodetector, a data processing unit, an acousto-optic frequency shift modulator (AOM), a phase modulator, a time-delay optical fiber, a fiber ring resonator FRR and a strain application module;

[0028] The fiber ring resonator FRR is composed of a high-splitting ratio coupler and an optical fiber. There are two groups of high-splitting ratio couplers. The fiber ring resonator FRR is formed by fusion splicing the high-splitting ratio coupler with the optical fiber. The high-splitting ratio coupler is a single-mode 99:1 coupler with a free optical spectrum of 91.15MHz, a cavity length of 188.5cm, a resonance peak half-width of 1.54MHz, and a finesse of ±60.

[0029] The photodetector model is Thorlabs PDA10CS-EC, which has tunable gain and bandwidth;

[0030] The delay fiber length is 10 km, corresponding to a delay of τ≈100 s;

[0031] The strain applying module is a piezoelectric ceramic transducer, on which the optical fiber is wound to apply strain;

[0032] The data processing unit uses a phase-locked amplifier, acquisition card and subsequent data processing operations;

[0033] The workflow of the above system specifically includes the following steps:

[0034] Ⅰ. First, perform a frequency sweep operation. The AOM performs a linear frequency shift on the optical signal, and at the same time, records the optical power at the PD end. Thus, the relationship curve between the optical power at the PD end and the AOM frequency shift amount is obtained, that is, the resonance curve of the broadband light, such as Figure 2 (a)

[0035] The specific expression is derived as follows: After the broadband light passes through an FRR, it enters the FRR for the second time after a delay fiber, and then reaches the PD. The optical power expression reaching the PD is:

[0036]

[0037] Where τ is the delay between two passes of the broadband light through the FRR, F FRR (λ) is the power transmission function of FRR, F FRR (λ τ ) is the power transmission function of FRR after the delay τ. Obviously, is the delayed autocorrelation function of FRR, defined as R τ , the above formula (1) can be simplified as:

[0038] P OUT =GPin R τ ;

[0039] Considering that the coupler spectral coefficient of FRR is constant in the transmission spectrum expression of FRR, the wavelength-related parameters λ0 and B only change the integration range. τ The influence of is also small, so R τ Only the length change ΔL of FRR in time τ τ Directly related, this formula and Figure 2 (a) It can be seen that when the sensor is subjected to strain, the optical power at the PD end will change, which is also the working principle of this strain gauge;

[0040] Ⅱ. Due to the left-right symmetry of the broadband optical resonance peak, we cannot determine the direction of the length change rate only by it. Therefore, we added a phase modulator (PM) to the system to apply fast perturbations, and applied PDH technology to obtain a centrally symmetric discrimination curve. The phase adjustment frequency applied in the experiment was 500kHz, and the obtained PDH discrimination curve is as follows: Figure 2 (b)

[0041] It should be noted that when the FRR is not subject to strain, this broadband optical interference system will always be at the peak of the broadband optical resonance peak, so this system does not need AOM to perform frequency locking operation. The system itself can always work stably at the best working point in the open-loop state, that is, within the central linear region of the PDH curve. The purpose of introducing AOM here is only to debug the phase modulation and demodulation system of this system, obtain intermediate test data and set the modulation and demodulation parameters. When the system actually conducts a sensing experiment, AOM can be removed to simplify the system.

[0042] Ⅲ. During the experiment, the AOM is removed and the output of the lock-in amplifier after demodulation and filtering of the signal is collected. After 1 / f filtering and calibration, this is the output of the sensor system;

[0043] IV. In the sensing experiment, we glued a part of the optical fiber cavity of the FRR sensing unit to a PZT with glue, and applied a sinusoidal strain signal with a frequency of 1000Hz and a peak amplitude of 2nε. We placed the FRR sensor in a sealed box for vibration isolation, and placed the delay optical fiber on the experimental platform without any vibration isolation treatment. In the experiment, we collected the signal output by the phase-locked amplifier, calculated its voltage power spectrum density after 1 / f filtering, and obtained the following Figure 3 The curve in

[0044] from Figure 3 It can be read that the strain resolution of this strain measurement system at 1kHz reaches

[0045]

[0046] In summary, the self-referenced optical fiber sensing system proposed in this patent reduces the cost of the light source by adopting a broadband light source, and adopts a self-reference method to avoid a complex frequency-locking system, so that the system can stably perform open-loop measurements, greatly reducing the complexity of the system;

[0047] Since laser frequency noise is avoided, this system can improve the strain measurement accuracy by increasing the delay fiber, increasing the length of the sensing unit or increasing the fineness. Due to the use of a broadband light source and the simplicity of the demodulation system, this system can easily realize the arraying of sensors in a wavelength division multiplexing manner. Although the embodiments of the present invention have been shown and described, it is understood by those of ordinary skill in the art that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the present invention, and the scope of the present invention is defined by the attached claims and their equivalents.

Claims

1. A high-precision optical fiber strain sensing system using broadband light drive, characterized in that: The system is composed of a broadband light source module, a passive optical fiber sensor module, a circulator, a phase modulation module, a delay module, a data processing module, a photoelectric conversion module and a phase-locked amplifier module. The passive optical fiber sensor module is provided with a fiber ring resonator. The broadband light is emitted from the light source and enters the first port of the circulator, and is emitted from the second port and enters the ring resonator. It passes through the phase modulator module and the delay module and returns to the ring resonator again. The output light then enters the photoelectric conversion module unidirectionally through the circulator for photoelectric conversion. The data processing module extracts the strain change rate signal from the electrical signal output by the photoelectric conversion module and performs 1 / f filtering. The demodulated signal reflects the strain information applied to the fiber ring cavity, and the strain information is the output of the sensing system. The fiber ring resonator is composed of a high-splitting ratio coupler and an optical fiber. The high-splitting ratio coupler is provided with two groups. The fiber ring resonator is formed by fusing the high-splitting ratio coupler with the optical fiber.

2. The demodulation method of a high-precision optical fiber strain sensing system driven by broadband light according to claim 1, characterized in that: The demodulation method is as follows: S1. Using the optical fiber resonant cavity as the strain sensing unit, the low-coherence light emitted by the broadband light source passes through the resonant cavity for the first time, passes through the delayed optical fiber, passes through the optical fiber resonant cavity for the second time, and is received by the photodetector; S2. According to S1, after the low-coherence light passes through the optical fiber resonant cavity for the first time, the spectrum of the transmitted light wave is modulated into the comb spectrum of the resonant cavity. After phase modulation, the light wave passes through the optical fiber resonant cavity for the second time. The transmission spectrum of the optical fiber resonant cavity is shifted due to the effect of the measured parameter, resulting in a change in the efficiency of the light wave passing through the optical fiber resonant cavity and a change in the optical power reaching the photodetector. S3. By collecting the changes in the optical power of the photodetector, it is demodulated using the phase-locked amplification technology to restore the signal to be measured applied to the sensor.

3. The demodulation method of a high-precision optical fiber strain sensing system driven by broadband light according to claim 2, characterized in that: In S1, the broadband light source is a wide-spectrum light source, and the fiber resonant cavity is a high-fineness ring resonant cavity.

4. The high-precision optical fiber strain sensing system and demodulation method thereof using broadband light drive according to claim 3, characterized in that: The optical fiber resonant cavity is any one of a ring resonant cavity and a linear Fabry-Perot resonant cavity.

5. The high-precision optical fiber strain sensing system and demodulation method thereof using broadband light drive according to claim 2, characterized in that: In S3, the sensor is a four-port fiber ring resonator, two groups of ports are transmission ports, and the other two groups of ports are optical power collection ports. The transmission spectrum between the transmission ports presents an upwardly convex resonance peak.

6. The high-precision optical fiber strain sensing system and demodulation method thereof using broadband light drive according to claim 2, characterized in that: In S2, the modulation method of the light wave before entering the resonant cavity for the second time adopts either a lithium niobate waveguide or a piezoelectric transducer.

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