A fiber-optic refractive index sensor based on vernier effect

Abstract

The application discloses a fiber refractive index sensor based on a vernier effect, which comprises a broadband light source, an isolator, a first fiber ring resonant cavity, a second fiber ring resonant cavity, a refractive index sensitive unit, a spectrum analysis module and a data processing unit; the input end of the isolator is connected with the output end of the broadband light source; the input end of the first fiber ring resonant cavity is connected with the output end of the isolator; the input end of the second fiber ring resonant cavity is connected with the output end of the first fiber ring resonant cavity; the first fiber ring resonant cavity and the second fiber ring resonant cavity constitute a cascaded fiber ring resonant cavity; the refractive index sensitive unit is connected into the second fiber ring resonant cavity; the input end of the spectrum analysis module is connected with the output end of the second fiber ring resonant cavity; and the input port of the data processing unit is connected with the output port of the spectrum analysis module. The application has the characteristics of high sensitivity, simple structure, high demodulation precision and suitability for mass production.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of refractive index monitoring, and particularly relates to a fiber refractive index sensor based on vernier effect. BACKGROUND

[0002] At present, fiber sensing technology has been applied to monitoring environment, biomedical treatment, diagnosis, material biochemical sensing analysis and other application fields. The important advantage of fiber sensing technology is that the sensor is small in size, light in weight, high in precision, resistant to high temperature, corrosion and electromagnetic interference, high in resolution, and integrated with sensing and transmission, so that the measurement problem that cannot be completely performed by conventional electrical sensors can be solved.

[0003] Refractive index is an important characteristic of optical materials, and refractive index measurement is widely applied to various fields, such as environmental pollution monitoring, river and seawater pollution monitoring, and organic matter concentration detection. In addition, it also has many applications in thin films, polymers, crystals, mammalian tissues and the like. Many refractive index sensors based on optical fibers have been proposed, but the design of sensors with high sensitivity and high precision is still a challenging problem in the field of optical technology. At present, there are various methods for manufacturing sensors, and various structures of sensors are emerging, but their sensitivity is not very high. For example, the refractive index sensor based on Fabry-Perot interferometer has a relatively low refractive index sensitivity due to the complex and random manufacturing process of the cavity; the refractive index sensor based on a tapered structure has a simple structure and low cost, but the biggest problem is that the structure is very fragile; the refractive index sensor based on photonic crystal fiber has high sensitivity, but the cost of photonic crystal fiber is high; the above-mentioned fiber refractive index sensors based on mode interference have a large spectral bandwidth, resulting in insufficient precision in wavelength demodulation. For the same structure of the sensor, the sensitivity of the sensor has a limit, and it is difficult to further improve. SUMMARY

[0004] In view of the above problems, the purpose of the present application is to provide a fiber refractive index sensor based on vernier effect, which aims to further improve the measurement sensitivity and improve the measurement precision. The vernier effect based on the cascaded fiber ring resonator cavity can widen and effectively adjust the free spectral range, and has a great improvement in sensitivity compared with other sensors, and has higher demodulation precision.

[0005] The technical scheme of the present application is: a fiber refractive index sensor based on vernier effect, comprising:

[0006] a broadband light source;

[0007] an isolator, an input end of which is connected with the broadband light source;

[0008] The first fiber ring resonator cavity comprises a first coupler, a polarization controller and a second coupler, port 1 of the first coupler is connected with the output end of the isolator, port 2 of the first coupler is connected with port 5 of the second coupler, port 3 of the first coupler is connected with port 4 of the second coupler, and the polarization controller is arranged on the optical fiber connected between port 3 of the first coupler and port 4 of the second coupler;

[0009] The second fiber ring resonator cavity comprises a third coupler, a fourth coupler and a refractive index sensitive unit, port 8 of the third coupler is connected with the refractive index sensitive unit and port 11 of the fourth coupler in sequence, and port 9 of the third coupler is also connected with port 10 of the fourth coupler; the first fiber ring resonator cavity and the second fiber ring resonator cavity are connected through port 6 of the second coupler and port 7 of the third coupler to form a cascaded fiber ring resonator cavity;

[0010] The spectrum analysis module is connected with port 12 of the fourth coupler at the input end;

[0011] The data processing unit is connected with the output end of the spectrum analysis module at the input end.

[0012] Further, the coupling coefficients of the first coupler and the third coupler are 0.88-0.9, the coupling power from port 2 to port 3 of the first coupler is 88-90%, and the coupling power from port 8 to port 9 of the third coupler is 88-90%.

[0013] Further, the coupling coefficients of the second coupler and the fourth coupler are 0.9-0.99, the coupling power from port 4 to port 5 of the second coupler is 90-99%, and the coupling power from port 10 to port 11 of the fourth coupler is 90-99%.

[0014] Further, the wavelength resolution of the spectrum analysis module is 0.01-0.1 nm.

[0015] Further, the refractive index sensitive unit is any one of a cladding-etched single-mode fiber, a cladding-ground single-mode fiber, a hollow-core fiber or a thin-core fiber.

[0016] Further, the first coupler, the second coupler, the third coupler and the fourth coupler are all 1x2 couplers.

[0017] Further, the difference between the cavity lengths of the first fiber ring resonator cavity and the second fiber ring resonator cavity is 0-1 cm.

[0018] The process of deducing the refractive index of the measured substance by the above optical fiber refractive index sensor is as follows: the first optical fiber ring resonant cavity and the second optical fiber ring resonant cavity are designed to have different lengths L1 and L2, and L2 = l + l0, wherein l is the total length of the optical fibers in the optical fiber ring except the refractive index sensitive unit, and l0 is the length of the optical fiber of the refractive index sensitive unit;

[0019] The free spectral ranges FSR1 and FSR2 of the first optical fiber ring resonant cavity and the second optical fiber ring resonant cavity are represented as:

[0020]

[0021]

[0022] wherein n eff is the effective refractive index of the sensitive unit, n is the effective refractive index of the single-mode optical fiber, and λ is the selected operating wavelength;

[0023] In the hybrid cascade structure, the output spectrum of the cascaded optical fiber ring resonator is equal to the product of the respective output spectra, and therefore the resonance peak of the final output spectrum is the resonance peak common to the optical fiber rings of the first optical fiber ring resonant cavity and the second optical fiber ring resonant cavity. The final free spectral range FSR is related to the FSR1 and FSR2 of the first optical fiber ring resonant cavity and the second optical fiber ring resonant cavity alone, and is specifically represented as:

[0024]

[0025] wherein M is an amplification factor, representing the ability to amplify the sensitivity by optical vernier effect;

[0026] In the initial state, the length L1 of the first optical fiber ring resonant cavity is controlled, and the length L2 of the second optical fiber ring resonant cavity includes the total length of the connecting optical fibers and the sensitive unit optical fiber, so that L1 < L2. According to the formulas (1) and (2), FSR1 > FSR2. When the refractive index liquid to be measured is added on the refractive index sensitive unit through the dropper, the external environment changes from air to the refractive index liquid to be measured, which will cause the effective refractive index n eff of the refractive index sensitive unit in the second optical fiber ring resonant cavity to increase. According to the formula (2), FSR2 decreases. According to the formula (3), the FSR of the transmission spectrum decreases after the light passes through the two rings. According to the above analysis and the formulas (1)-(3),

[0027] wherein l0 is the length of the refractive index sensitive unit, and k is a proportional coefficient;

[0028] The change of n eff is demodulated by the change of in the transmission spectrum. eff, and further deduce the refractive index of the external to-be-measured substance, and the demodulation part is realized through a spectrum analysis module and a data processing module.

[0029] Compared with the prior art, the present application has the beneficial effects that:

[0030] (1) The optical fiber refractive index sensor based on the vernier effect provided by the present application converts the detected refractive index signal into optical signal propagation, which has the characteristics of small propagation loss, easy long-distance transmission, strong anti-electromagnetic interference ability, corrosion resistance, high temperature resistance, simple structure, low cost, and suitability for mass production, compared with the electrical sensor in the prior art.

[0031] (2) The optical fiber refractive index sensor based on the vernier effect provided by the present application can adjust the amplification coefficient by controlling the difference between the lengths of the two rings, thereby realizing the adjustment of sensitivity amplification. Compared with general single-structure optical fiber refractive index sensors, the present application has high sensitivity and can realize adjustable sensitivity.

[0032] (3) In the optical fiber refractive index sensor based on the vernier effect provided by the present application, by appropriately selecting the coupling coefficient of the coupler in the optical fiber ring, a very sharp resonance peak can be obtained, the bandwidth of the output spectrum is very narrow, and the wavelength demodulation accuracy is relatively high.

[0033] (4) The optical fiber refractive index sensor based on the vernier effect provided by the present application uses the relative value FSR for demodulation, instead of the absolute value change of a single wavelength as in most mode interference-based optical fiber refractive index sensors, thereby avoiding the error caused by temperature cross-influence and improving the measurement accuracy. BRIEF DESCRIPTION OF DRAWINGS

[0034] Figure 1 is the system block diagram of the present application;

[0035] Figure 2 is the overall structure schematic diagram of the present application;

[0036] Figure 3 is the refractive index sensitive unit schematic diagram in the embodiment of the present application.

[0037] Among them, 1 is a broadband light source, 2 is an isolator, 3 is a first optical fiber ring resonant cavity, 4 is a second optical fiber ring resonant cavity, 5 is a spectrum analysis module, 6 is a data processing unit, 7 is a first coupler, 8 is a polarization controller, 9 is a second coupler, 10 is a third coupler, 11 is a fourth coupler, 12 is a refractive index sensitive unit, 13 is a single-mode optical fiber cladding, and 14 is a single-mode optical fiber core. DETAILED DESCRIPTION

[0038] The specific embodiments of the present application are described in detail below with reference to the accompanying drawings, but the protection scope of the present application is not limited by the specific embodiments.

[0039] Embodiment: a fiber refractive index sensor based on vernier effect, the system block diagram is as shown in Figure 1 , including broadband light source 1, isolator 2, first fiber ring resonator 3, second fiber ring resonator 4, spectrum analysis module 5 and data processing unit 6.

[0040] In this embodiment, the broadband light source is used to send optical signals; the input end of the isolator is connected to the output end of the broadband light source, which is used to prevent the influence of back-reflected light on the broadband light source; the input end of the first fiber ring resonator is connected to the output end of the isolator; the input end of the second fiber ring resonator is connected to the output end of the first fiber ring resonator; the first fiber ring resonator and the second fiber ring resonator constitute a cascaded fiber ring resonator for measuring refractive index; the input end of the spectrum analysis module is connected to the output end of the sensing unit for spectral analysis; the input port of the data processing unit is connected to the output port of the spectrum analysis module for demodulation operation on the data analyzed by the spectrum analysis module.

[0041] Based on the above-mentioned fiber refractive index sensor based on vernier effect, the test architecture of the embodiment is as shown in Figure 2 , wherein the first fiber ring resonator 3 includes a first coupler 7, a polarization controller 8 and a second coupler 9; wherein the port 1 of the first coupler 7 is connected to the output end of the isolator 2, the port 2 of the first coupler 7 is connected to the port 5 of the second coupler 9, and the port 3 of the first coupler 7 is connected to the port 4 of the second coupler 9; the polarization controller is installed on the optical fiber connected between the port 3 of the first coupler 7 and the port 4 of the second coupler 9; the second fiber ring resonator 4 includes a third coupler 10, a fourth coupler 11 and a refractive index sensitive unit 12, the port 8 of the third coupler 10 is connected to the refractive index sensitive unit 12, and then connected to the port 11 of the fourth coupler 11, the port 9 of the third coupler 10 is connected to the port 10 of the fourth coupler 10;

[0042] The first fiber ring resonator 3 and the second fiber ring resonator 4 are connected through the port 6 of the second coupler 9 and the port 7 of the third coupler 10 to form a cascaded fiber ring resonator; the liquid to be measured refractive index is added by a dropper on the platform where the refractive index sensitive unit is placed.

[0043] In this embodiment, the coupling coefficients of the first coupler 7 and the third coupler 10 are 0.88, and ports 1 to 3 are straight-through, i.e., the power of port 1 entering port 3 is 12%, and the power of port 7 entering port 9 is 12%; the greater part of the energy oscillates in the ring; the coupling coefficients of the second coupler 9 and the fourth coupler 11 are 0.99; i.e., the power coupled from port 4 into port 5 is 99%, and the power entering port 6 is 1%; the power coupled from port 10 into port 11 is 99%, and the power entering port 12 is 1%. The coupling coefficients of the four couplers are optimized to make the output spectrum of the cascaded fiber ring resonator more precise, i.e., the linewidth is narrower, so that clearer resonance peaks are obtained.

[0044] The transmission spectrum of the fiber ring resonator cavity (taking the first fiber ring resonator cavity 3 as an example) can be expressed as:

[0045]

[0046] wherein A1 = [(1-γ1)(1-γ2)] 1 / 2 , γ1 and γ2 are the intensity loss coefficients of the first coupler 7 and the second coupler 9 in the ring respectively, k1 and k2 are the coupling coefficients of the first coupler 7 and the second coupler 9 respectively, t1 = exp(-αL1) is the transmission loss of the first fiber ring resonator cavity 3, α and β are the absorption coefficient and the propagation coefficient of the optical fiber respectively, and the precision of the output spectrum of the first fiber ring resonator cavity 3 is:

[0047]

[0048] wherein, k 2r is the intensity coupling coefficient of the second coupler 9 corresponding to the resonance;

[0049] It can be concluded from formula (4) and formula (5) that the intensity and the precision of the output spectrum of the fiber ring are related to the coupling coefficients of the couplers, and therefore the couplers with coupling coefficients of 0.88 and 0.99 can achieve higher output intensity and greater precision.

[0050] In this embodiment, a spectrum analysis module with a wavelength resolution of 0.02 nm is selected to achieve higher precision demodulation. As shown in Figure 3 , the length L of the ground two single-mode fiber claddings 13 and the middle single-mode fiber core 14 is 1-3 cm, and a too short length will result in insufficient sensitivity to the change of the external refractive index, and a too long length will increase the insertion loss and deteriorate the transmission spectrum signal. The thickness d of the ground cladding is 30-60 microns, and similarly, a too small d value will result in insufficient sensitivity to the external environment, and a too large d value will increase the loss.

[0051] Specifically, the two fiber ring resonators are designed to have different lengths L1 and L2, and L2 = l + l0, where l is the total length of the fiber segments other than the refractive index sensitive unit 12, and l0 is the length of the refractive index sensitive fiber.

[0052] The FSR1 and FSR2 of the first fiber ring resonator 3 and the second fiber ring resonator 4 can be expressed as:

[0053]

[0054]

[0055] where n eff is the effective refractive index of the sensitive unit, n is the effective refractive index of the single-mode fiber, and λ is the selected operating wavelength.

[0056] In the hybrid cascade structure, the output spectrum of the cascaded fiber ring resonator is equal to the product of the respective output spectra, so the resonance peak of the final output spectrum is the resonance peak common to the first fiber ring resonator 3 and the second fiber ring resonator 4, and the final FSR is related to the FSR1 and FSR2 of the first fiber ring resonator 3 and the second fiber ring resonator 4 individually, and can be expressed as:

[0057]

[0058] As can be seen from equation (2), when the FSRs of the two fiber ring resonators are close but not equal, the cascaded fiber ring resonator produces an optical vernier effect, and the final output spectrum forms a periodic envelope; the closer the FSRs of the two fiber ring resonators, the greater the FSR of the envelope, and M is the magnification factor, which represents the ability to amplify the sensitivity of the optical vernier effect.

[0059] In the initial state, the length L1 of the first fiber ring cavity is controlled, and the length of the second fiber ring cavity (including the total length of the connecting fiber and the sensitive unit fiber) is controlled to be L2, so that L1 is slightly smaller than L2, and the smaller the control ring length difference is, the better. L1 < L2, and from equations (1) and (2), FSR1 > FSR2 can be obtained. When the refractive index liquid to be measured is added to the sensitive unit through the dropper, the external environment changes from air to the refractive index liquid to be measured, which will cause the n eff of the refractive index sensitive unit 12 in the second fiber ring resonator to increase, and from equation (2), FSR2 decreases. From equation (3), the FSR of the transmission spectrum after the light passes through the two rings decreases, and from the above analysis and equations (1)-(3), it can be obtained that where l0 is the length of the refractive index sensitive element, and k is a proportional coefficient. The change in n can be demodulated from the change in the transmission spectrum eff . effThe refractive index of the external substance to be measured is derived, and the demodulation is realized by the spectrum analysis module 5 and the data processing module 6.

[0060] The specific models of the electronic components are not specially specified, and common products on the market can be selected as long as the use requirements of the application are met.

[0061] The above specific embodiments further illustrate the purposes, technical solutions and beneficial effects of the application. It should be understood that the above description is only a specific embodiment of the application and does not limit the application. Any modification, equivalent replacement, improvement, etc. within the spirit and principles of the application is included in the protection scope of the application.

Claims

1. A fiber-optic refractive index sensor based on the Vernier effect, characterized in that, The application relates to a fiber-optic refractive index sensor, which comprises the following components: a broadband light source (1); an isolator (2) connected with the broadband light source (1); a first fiber-optic ring resonator (3) comprising a first coupler (7), a polarization controller (8) and a second coupler (9), wherein port 1 of the first coupler (7) is connected with the output end of the isolator (2), port 2 of the first coupler (7) is connected with port 5 of the second coupler (9), port 3 of the first coupler (7) is connected with port 4 of the second coupler (9), and the polarization controller (8) is arranged on an optical fiber connected between port 3 of the first coupler (7) and port 4 of the second coupler (9); a second fiber-optic ring resonator (4) comprising a third coupler (10), a fourth coupler (11) and a refractive index sensitive unit (12), wherein port 8 of the third coupler (10) is connected with the refractive index sensitive unit (12) and port 11 of the fourth coupler (11) in sequence, and port 9 of the third coupler (10) is also connected with port 10 of the fourth coupler (10); the first fiber-optic ring resonator (3) and the second fiber-optic ring resonator (4) are connected with each other through port 6 of the second coupler (9) and port 7 of the third coupler (10) to form a cascaded fiber-optic ring resonator; the difference between the cavity lengths of the first fiber-optic ring resonator (3) and the second fiber-optic ring resonator (4) is 0-1 cm; a spectrum analysis module (5) connected with port 12 of the fourth coupler (11); the wavelength resolution of the spectrum analysis module (5) is 0.01-0.1 nm; a data processing unit (6) connected with the output end of the spectrum analysis module (5); the coupling coefficients of the first coupler (7) and the third coupler (10) are 0.88-0.9, the coupling power from port 2 to port 3 of the first coupler (7) is 88-90%, and the coupling power from port 8 to port 9 of the third coupler (10) is 88-90%; the coupling coefficients of the second coupler (9) and the fourth coupler (11) are 0.9-0.99, the coupling power from port 4 to port 5 of the second coupler (9) is 90-99%, and the coupling power from port 10 to port 11 of the fourth coupler (11) is 90-99%; the refractive index sensitive unit (12) is a single-mode optical fiber with a corroded cladding; The process of the optical fiber refractive index sensor for deriving the refractive index of the measured substance is that the first optical fiber ring resonant cavity (3) and the second optical fiber ring resonant cavity (4) are designed to have different lengths , , wherein is the total length of each section of the optical fiber in the optical fiber ring except the refractive index sensitive unit (12), is the optical fiber length of the refractive index sensitive unit (12). Free spectral range of the first fiber ring resonator (3) and the second fiber ring resonator (4) and is represented by: （1）； (2)； wherein, neff is the effective refractive index of the sensitive unit, neff is the effective refractive index of the single mode optical fiber, λ0 is the selected operating wavelength; In the hybrid cascade structure, the output spectrum of the cascaded fiber ring resonator is equal to the product of the respective output spectrum, so the resonance peak of the final output spectrum is the resonance peak common to the first fiber ring resonator cavity (3) and the second fiber ring resonator cavity (4) fiber ring, and the final free spectral range related to the first fiber ring resonator cavity (3) and the second fiber ring resonator cavity (4) of the cascade alone and Specifically expressed as: （3）； wherein is the magnification factor, representing the ability to amplify sensitivity as an optical vernier effect; In the initial state, the length of the first fiber ring resonator (3) is controlled , and the length of the second fiber ring resonator (4) is , The total length of the connecting fiber and the sensing unit fiber is included, so that According to the formulas (1) and (2) When the liquid with the refractive index to be measured is added on the refractive index sensitive unit (12) through the dropper, the external environment changes from air to the liquid with the refractive index to be measured, which will cause the effective refractive index of the refractive index sensitive unit (12) in the second fiber ring resonator (4) to increase According to the formula (2) According to the formula (3), after the light passes through the two rings, the FSR of the transmission spectrum decreases, and according to the above analysis and the formulas (1)-(3) ; wherein is the length of the refractive index sensitive unit (12), is the proportional coefficient; The change of the refractive index of the substance is demodulated by the change of the transmission spectrum , and the refractive index of the substance is derived. The change of the refractive index of the substance is demodulated by the change of the transmission spectrum , and the refractive index of the substance is derived. The demodulation part is realized by the spectrum analysis module (5) and the data processing module (6).

2. A fiber-optic refractive index sensor based on the Vernier effect as claimed in claim 1, wherein, the first coupler (7), the second coupler (9), the third coupler (10) and the fourth coupler (11) are all 1x2 couplers.

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