CN102410990A - High-sensitivity micro-nano optical fiber refractive index sensor and preparation method thereof - Google Patents

Abstract

The invention discloses a high-sensitivity micro-nano optical fiber refractive index sensor and a preparation method thereof, wherein the sensor comprises a broadband light source, an optical fiber annular mirror and a spectrum analyzer which are sequentially connected along an optical transmission path, wherein the optical fiber annular mirror comprises an optical fiber coupler, a birefringent micro-nano optical fiber and a polarization controller which are connected along the optical transmission path; after light emitted by the broadband light source enters the optical fiber ring mirror, the formed two light propagating in opposite directions generate polarization phase difference through the birefringent micro-nano optical fiber, form polarization interference after passing through the polarization controller, and finally are detected and output by the spectrum analyzer. The invention adopts the birefringent micro-nano optical fiber for sensing, the optical fiber has a rectangular or quasi-rectangular double symmetrical structure, and the polarization interference spectrogram obtained by utilizing the unique birefringence and the birefringence scattering effect changes along with the change of the surrounding refractive index, thereby obtaining the ultrahigh sensing sensitivity.

Description

A high-sensitivity micro-nano optical fiber refractive index sensor and its preparation method

technical field

The invention relates to the technical field of optical fiber refractive index sensors, in particular to a high-sensitivity micro-nano optical fiber refractive index sensor and a preparation method thereof.

Background technique

Refractive index sensing is used to detect changes in the refractive index of the external environment. Since the refractive index is a basic property of matter, it is an important parameter in the fields of physics, biology, chemistry and other disciplines. Therefore, the refractive index sensor is very important for environmental monitoring, food Safety, pharmaceutical development, clinical testing and other related fields have important significance and applications. The optical fiber refractive index sensor overcomes the traditional sensor with its advantages of high sensitivity, strong anti-electromagnetic interference ability, fast response speed, anti-biochemical corrosion, small size, light weight, non-toxicity, flexible and convenient operation, and long-distance transmission capability with low energy loss. The methods include grazing incidence method, diffraction grating method, broadband absorption spectroscopy and so on, and their development has always been paid attention to by people. Many fiber-optic refractive index sensors have emerged, and their realization principles include surface plasmons, long-period gratings, intermode interference, and microfluidic porous fibers.

In recent years, micro-nano fiber has attracted people's research interest. Due to the strong evanescent field effect of micro-nano fiber, it has inherent advantages in refractive index sensing. The proposed implementation methods include micro-nano fiber ring junction, Bragg grating, long-period grating, Transmission energy detection, etc., but the sensing sensitivity of these methods is still greatly limited, for example, the literature "Fei Xu, Valerio Pruneri, Vittoria Finazzi, Gilberto Brambilla.An embedded optical nanowire loop resonator refractometric sensor. Optics Express, 2008, 16 (2): 1062-1067." It is proposed to tie the micro-nano optical fiber into a ring junction and encapsulate it with a polymer. This method has a complicated manufacturing process, and the theoretically obtained sensitivity is 700nm/RIU (refractive index unit). Another example is the document "X.Fang, C.R.Liao, D.N.Wang. Femtosecond laser fabricated fiber Bragg grating inmicrofiber for reactive index sensing. Optics Letters, 2010, 35(7): 1007-1009." Write the Bragg grating to make the refractive index sensor, and the measured sensitivity is up to 234.1nm/RIU. Another example is the document "Haifeng Xuan, Wei Jin, and Shujing Liu. Long-period gratings in wavelength-scale microfibers. Optics Letters, 2010, 35(1): 85-87." Using femtosecond laser writing technology in micro-nano fibers Writing long-period gratings, the measurement sensitivity reaches 1900nm/RIU, the sensitivity is slightly lower and the femtosecond laser is expensive. In addition, the femtosecond laser writing technology has a destructive effect on the fiber structure.

Contents of the invention

The purpose of the present invention is to overcome the shortcomings and deficiencies of the prior art, to provide a high-sensitivity micro-nano optical fiber refractive index sensor, which utilizes the unique birefringence and dispersion effect of the birefringent micro-nano optical fiber to obtain ultra-high sensitivity.

Another object of the present invention is to provide a method for preparing a high-sensitivity micro-nano optical fiber refractive index sensor, so that it has higher refractive index sensing sensitivity, and can realize real-time sensing and rapid detection of micro-variable refractive index in the external environment. detection.

In order to achieve the above object, the present invention adopts the following technical solutions:

The high-sensitivity micro-nano optical fiber refractive index sensor of the present invention includes a broadband light source, an optical fiber loop mirror and a spectrum analyzer sequentially connected along the optical transmission path, wherein the optical fiber loop mirror includes an optical fiber coupler connected along the optical transmission path, a birefringent micro-nano optical fiber and the first polarization controller; after the light emitted by the broadband light source enters the fiber optic loop mirror through the fiber coupler, two beams propagating in opposite directions are formed, one of which passes through the first polarization controller and the birefringent micro-nano fiber in turn, and the other One beam passes through the birefringent micro-nano fiber and the first polarization controller in turn. The two beams produce a polarization phase difference, and after being combined by a fiber coupler, a polarization interference spectrum is formed, and finally the output is detected by a spectrum analyzer.

The birefringent micro-nano optical fiber is usually on an optical fiber with a rectangular or quasi-rectangular double symmetrical structure cladding, using traditional optical fiber melting and drawing technology to heat and melt the optical fiber at high temperature, so that the longest side dimension of the cross section Less than 10μm, the two ends of the standard optical fiber are fused to connect various optical fiber instruments.

Preferably, the fiber optic loop mirror also includes a second polarization controller and a polarization-maintaining fiber, and the fiber coupler, the first polarization controller, the birefringent micro-nano fiber, the second polarization controller and the polarization-maintaining fiber are along the light transmission The paths are connected in sequence, the second polarization controller is used to adjust the density of the interference spectrum, and the polarization-maintaining fiber is used as a reference fiber to improve the sensitivity of the sensor.

Preferably, the polarization-maintaining fiber is a birefringent micro-nano fiber, a panda-type polarization-maintaining fiber, a bow-tie-type polarization-maintaining fiber, an elliptical-type polarization-maintaining fiber or a polarization-maintaining photonic crystal fiber.

Preferably, the light splitting ratio of the fiber coupler is usually 50%:50%.

Preferably, the cross-section of the birefringent micro-nano optical fiber is a rectangle or a double-symmetric structure similar to a rectangle.

The birefringent micro-nano optical fiber includes an optical fiber core and an optical fiber cladding surrounding the optical fiber core.

The refractive index of the optical fiber core is higher than that of the optical fiber cladding.

In order to achieve the above another purpose, the present invention adopts the following technical solutions: a method for preparing a high-sensitivity micro-nano optical fiber refractive index sensor, comprising the following steps:

(1), select the birefringent micro-nano fiber, the cross-sectional structure of the birefringent micro-nano fiber is a double symmetric structure of a rectangle or a rectangle;

(2), using a fiber coupler, connect the two ports on the same side of the fiber coupler to the broadband light source and the spectrum analyzer respectively;

(3) Two ports on the other side of the fiber coupler are connected to a birefringent micro-nano fiber and a polarization controller to form a closed optical path, thereby forming a fiber loop mirror including a micro-nano fiber.

Compared with the prior art, the present invention has the following advantages and effects:

1. The present invention proposes a high-sensitivity micro-nano optical fiber refractive index sensor, which adopts a birefringent micro-nano optical fiber with a double symmetrical structure, and utilizes its unique birefringence and birefringence dispersion characteristics to realize ultra-high sensitivity refractive index transmission. Sensitivity, the present invention breaks through the sensitivity limitation of the existing scheme, and its sensitivity can reach 5000-100000nm/RIU.

2. Compared with the traditional optical refractive index sensing method, the present invention has the advantages of small size, light weight, compatibility with optical fiber systems, and remote monitoring.

3. Compared with other optical fiber-type refractive index sensing methods, the present invention has the advantages of smaller size, simple structure, easy integration, fast response speed and the like.

4. Compared with the existing micro-nano optical fiber refractive index sensing technology, the present invention has higher sensitivity (the sensitivity can be increased by more than one order of magnitude), and the measurement stability is good.

5. The present invention avoids loading the substance to be measured into the micropore structure of the optical fiber, and directly places the sensing fiber in the substance to be measured for sensing, thereby realizing fast and accurate sensing, so it has great application potential.

Description of drawings

Figure 1 is a micro-nano fiber optic refractive index sensor based on polarization interference;

Fig. 2 is a cross-sectional schematic diagram of a birefringent micro-nano fiber;

Fig. 3 is a schematic diagram of the interface between the sensing fiber and the substance to be measured;

Fig. 4 is the measurement data and the theoretical calculation curve of the refractive index solution using the sensor of the present invention;

Fig. 5 is the improved micro-nano fiber optic refractive index sensor;

Fig. 6 is the relationship between the wavelength and the refractive index corresponding to the transmission spectra of different reference fiber lengths.

Detailed ways

The present invention will be further described in detail below in conjunction with the embodiments and the accompanying drawings, but the embodiments of the present invention are not limited thereto.

Example 1

As shown in Figure 1, a high-sensitivity micro-nano optical fiber refractive index sensor includes a broadband light source 1 sequentially connected along the optical transmission path, an optical fiber coupler 2 with a splitting ratio of 50%:50%, a birefringent micro-nano optical fiber 3, and A polarization controller 4 and a spectrum analyzer 5, wherein the fiber coupler 2, the birefringent micro-nano fiber 3 and the first polarization controller 4 form a fiber optic ring mirror; the light emitted by the broadband light source 1 enters the fiber ring through the fiber coupler 2 After the mirror, two light beams propagating in opposite directions are formed, one of which passes through the first polarization controller 4 and the birefringent micro-nano fiber 3 in sequence, and the other passes through the birefringent micro-nano fiber 3 and the first polarization controller 4 in sequence , the two beams of light produce a polarization phase difference. By adjusting the first polarization controller 4, the polarization interference spectrum is formed after the optical fiber coupler 2 is combined. Calculate the refractive index of the substance to be measured, calculate the wavelength shift of the interference spectrum, and then deduce the change of the refractive index of the substance to be measured.

Fig. 2 is the schematic cross-sectional view of birefringent micro-nano optical fiber 3 of the present invention, and it comprises germanium-doped optical fiber core 6 and the optical fiber cladding 7 that is made of silica material, and optical fiber core 6 is circular structure, and optical fiber cladding The layer 7 is a rectangular structure, which is arranged around the fiber core 6 and surrounds the fiber core 6; the longest side dimension of a typical fiber cladding is not more than 10 μm. The refractive index of the fiber core is higher than that of the fiber cladding.

In Figure 3, the interface between the sensing fiber and the substance to be measured is shown, 8 represents the micro-nano optical fiber sensing section, 9 represents the substance to be measured, 10 represents the interface between the substance to be measured and the external environment, and 11 represents the optical fiber packaging position .

Figure 4 shows the relationship between the wavelength and the refractive index corresponding to the trough position of the outgoing spectrum obtained from experimental measurements. In this example, the micro-nano optical fiber is placed in a sucrose solution, and the refractive index of the solution is changed by adjusting the sucrose concentration. In Figure 4 , the circle represents the example measurement data points, the solid line represents the numerical fitting curve obtained by the following equation (1), it can be seen that the measured value can be well consistent with the theoretical value; in the measurement, the wavelength increases with the refractive index In the vicinity of the aqueous solution, the refractive index is about 1.333. The experimentally obtained sensitivities are 12511nm/RIU, 13441nm/RIU, 18987nm/RIU, 18677nm/RIU, and the maximum sensitivity reaches 18987nm/RIU.

Example 2

The high-sensitivity micro-nano optical fiber refractive index sensor of the above-mentioned embodiment 1 can be improved as follows, as shown in Figure 5, the improved refractive index sensor is different from Figure 1 in that one side of the micro-nano optical fiber is sequentially connected through a standard optical fiber A second polarization controller 12 and polarization-maintaining fiber 13, this polarization-maintaining fiber 13 is as a reference fiber, in this example is a bow-tie type polarization-maintaining fiber, by adjusting the second polarization controller 12 can adjust the degree of density of the interference spectrum, We adjust the interference spectrum to a sparse state, corresponding to the situation where two birefringent fibers cause polarization phase difference subtraction, as shown in equation (3) below, and at the same time adjust the first polarization controller 4 to enhance the extinction ratio of the interference spectrum Easy to measure.

Figure 6 shows the relationship between the wavelength and the refractive index corresponding to the position of the trough of the exit spectrum measured with different bow-tie fiber lengths, where the discrete points represent the measurement results, and the solid line represents the result of the quadratic equation fitting. The data measurement and theoretical fitting with reference fiber and without reference fiber are shown. The results show that: near the wavelength of 1550, the refractive index sensitivity of the sensor without reference fiber is 7151nm/RIU, while the length of polarization-maintaining fiber is 4.7cm and At 10.8cm, the corresponding refractive index sensitivities are 26065nm/RIU and -16196nm/RIU, respectively. It can be seen that the absolute sensitivity is greatly improved after adding the reference fiber, which is consistent with the theoretical prediction.

The preparation method of the high-sensitivity micro-nano optical fiber refractive index sensor of the present invention comprises the following steps:

(1), select the micro-nano optical fiber, the cross-sectional structure of the micro-nano optical fiber is a birefringent micro-nano optical fiber with a rectangular or rectangular double symmetric structure, and the length ratio of the longest side and the shortest side passing through the center of the optical fiber cross section is 1.05～5.0, their size is 10nm～10μm; the micro-nano fiber is on the optical fiber with rectangular or quasi-rectangular double symmetrical structure cladding, using conventional optical fiber melting and drawing technology to heat and melt the optical fiber at high temperature Manufactured; the two ends of the micro-nano optical fiber are fused with standard optical fibers to connect various optical fiber instruments.

(2), the birefringent micro-nano fiber of step (1) is fused to the fiber optic loop mirror, specifically: using a fiber coupler (the splitting ratio is usually 50%: 50%), two ports on the same side of the fiber coupler The broadband light source and the spectrum analyzer are respectively connected, and the birefringent micro-nano fiber and the polarization controller are connected in sequence between the two ports on the other side to form a closed optical path, thus forming a fiber loop mirror containing the micro-nano fiber, which is the micro-nano fiber Refractive index sensor, due to the birefringence effect of the birefringent micro-nano fiber, the two polarization states of the light wave transmitted in the fiber produce an optical path difference, adjust the state of the polarization controller, and the polarization interference spectrum can be obtained in the spectrum analyzer.

In the above-mentioned fiber optic loop mirror, the transmission spectrum of the polarization interference due to the birefringence effect is expressed as T=sin ² (Φ/2), where Φ is the polarization phase difference, expressed as:

Φ＝(2π/λ)BL (1)

Among them, λ is the wavelength of light, B=n _i -n _j is the birefringence of the micro-nano fiber, n _i and n _j are the effective refractive indices of the two polarization modes of the waveguide respectively, and L is the length of the micro-nano fiber. In order to theoretically calculate the change rate of wavelength with the refractive index, in equation (1), assuming that the phase difference Φ is constant, the microvariable Δn of the refractive index will cause the birefringence to change, so that the wavelength will drift, and the birefringence dispersion effect of the fiber is considered at the same time Differentiate the equation (1), and the sensitivity formula is obtained as:

$\mathrm{<span\; class="notranslate">\; S</span>}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; \&Delta;\&lambda;</span>}}{\mathrm{<span\; class="notranslate">\; \&Delta;\; n</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}<span\; class="notranslate">\; \&Center\; Dot;</span><span\; class="notranslate">\; \&PartialD;</span>\mathrm{<span\; class="notranslate">\; B</span>}<span\; class="notranslate">\; /</span><span\; class="notranslate">\; \&PartialD;</span>\mathrm{<span\; class="notranslate">\; no</span>}}{\mathrm{<span\; class="notranslate">\; B</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; \&lambda;</span>}<span\; class="notranslate">\; \&PartialD;</span>\mathrm{<span\; class="notranslate">\; B</span>}<span\; class="notranslate">\; /</span><span\; class="notranslate">\; \&PartialD;</span>\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}{\mathrm{<span\; class="notranslate">\; G</span>}}\frac{<span\; class="notranslate">\; \&PartialD;</span>\mathrm{<span\; class="notranslate">\; B</span>}}{<span\; class="notranslate">\; \&PartialD;</span>\mathrm{<span\; class="notranslate">\; no</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span>$

是折射率导致的双折射变化率，G是光纤的群双折射，由方程(2)可知灵敏度的大小除了与波长λ和双折射变化率

相关外，还与群双折射G的大小相关，而G的大小与双折射色散值

相关，当G趋近于零时，器件灵敏度将变得非常大。in,

is the birefringence change rate caused by the refractive index, and G is the group birefringence of the fiber. From equation (2), it can be known that the sensitivity is not related to the wavelength λ and the birefringence change rate

In addition to correlation, it is also related to the size of the group birefringence G, and the size of G is related to the birefringence dispersion value

Related, when G approaches zero, the device sensitivity will become very large.

(3) Place the birefringent micro-nano optical fiber of the above sensor in the substance to be measured, the interference spectrum of the device will drift due to the change of the refractive index of the substance, and the change of the refractive index of the substance can be detected by measuring the amount of wavelength shift.

On the basis of the above scheme, in order to further improve the sensing sensitivity, the following improvements can be made to the micro-nano optical fiber refractive index sensor. A polarization controller and a section of ordinary polarization-maintaining optical fiber are sequentially connected to one side of the birefringent micro-nano optical fiber through a standard optical fiber As a reference fiber, the reference fiber itself does not participate in sensing, it plays a role in modulating the refractive index sensing sensitivity of the device. The reference fiber can be a high-birefringence micro-nano fiber, or a traditional panda-type polarization-maintaining fiber, bow-tie type Polarization-maintaining optical fiber or elliptical polarization-maintaining optical fiber, etc., may also be polarization-maintaining photonic crystal optical fiber. Since there are two birefringent fibers in the interference system, the phase difference expression (1) becomes:

Φ＝(2π/λ)(B ₁ L ₁ ±B ₂ L ₂ ) (3)

Among them, "+" and "-" represent two birefringent fibers causing polarization phase difference phase increase and phase subtraction respectively, which can be realized by adjusting the polarization controller between the fibers, B ₁ and L ₁ correspond to the first birefringent fiber Refractive micro-nano fiber, B ₂ and L ₂ correspond to the second polarization-maintaining fiber. Due to the change of B1 of the first optical fiber due to the change of the external refractive index, it can be seen from the equation (3) that the wavelength changes, and the change of the wavelength causes the _B2 of the second optical fiber to change. Similarly, the equation (3) is Differential operation, the refractive index sensitivity formula can be obtained as:

$\mathrm{<span\; class="notranslate">\; S</span>}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; \&Delta;\&lambda;</span>}}{\mathrm{<span\; class="notranslate">\; \&Delta;n</span>}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; \&lambda;<figure-callout\; id="1"\; label="L"\; filenames="HDA0000080261430000011.png,HDA0000080261430000021.png"\; state="\{\{state\}\}">L</figure-callout></span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; \&Center\; Dot;</span><span\; class="notranslate">\; \&PartialD;</span>{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="1"\; label="B"\; filenames="HDA0000080261430000011.png,HDA0000080261430000021.png"\; state="\{\{state\}\}">B</figure-callout></span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; /</span><span\; class="notranslate">\; \&PartialD;</span>\mathrm{<span\; class="notranslate">\; no</span>}}{{\mathrm{<span\; class="notranslate">\; G</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="1"\; label="L"\; filenames="HDA0000080261430000011.png,HDA0000080261430000021.png"\; state="\{\{state\}\}">L</figure-callout></span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; \&PlusMinus;</span>{\mathrm{<span\; class="notranslate">\; G</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 4</span>}<span\; class="notranslate">\; )</span>$

Among them, G ₁ and G ₂ represent the group birefringence of the first birefringent micro-nano fiber and the second polarization-maintaining fiber, as long as |G ₁ L ₁ ± G ₂ L ₂ | tends to zero, which can greatly improve the sensing sensitivity of the device.

The above-mentioned embodiment is a preferred embodiment of the present invention, but the embodiment of the present invention is not limited by the above-mentioned embodiment, and any other changes, modifications, substitutions, combinations, Simplifications should be equivalent replacement methods, and all are included in the protection scope of the present invention.

Claims (8)

1. A high-sensitivity micro-nano optical fiber refractive index sensor is characterized by comprising a broadband light source, an optical fiber ring mirror and a spectrum analyzer which are sequentially connected along an optical transmission path, wherein the optical fiber ring mirror comprises an optical fiber coupler, a first polarization controller and a birefringent micro-nano optical fiber which are sequentially connected along the optical transmission path; light emitted by a broadband light source enters the optical fiber ring mirror through the optical fiber coupler to form two light beams which are transmitted in opposite directions, one light beam sequentially passes through the first polarization controller and the birefringent micro-nano optical fiber, the other light beam sequentially passes through the birefringent micro-nano optical fiber and the first polarization controller, the two light beams generate polarization phase differences, the polarization interference spectrum is formed after wave combination through the optical fiber coupler, and finally the polarization interference spectrum is detected and output by the optical spectrum analyzer.

2. The high-sensitivity micro-nano optical fiber refractive index sensor according to claim 1, wherein the optical fiber ring mirror further comprises a second polarization controller and a polarization maintaining optical fiber, the optical fiber coupler, the first polarization controller, the birefringence micro-nano optical fiber, the second polarization controller and the polarization maintaining optical fiber are sequentially connected along an optical transmission path, the second polarization controller is used for adjusting the density degree of an interference spectrum, and the polarization maintaining optical fiber is used as a reference optical fiber.

3. The high-sensitivity micro-nano optical fiber refractive index sensor according to claim 2, wherein the polarization maintaining optical fiber is a birefringent micro-nano optical fiber, a panda type polarization maintaining optical fiber, a bow-tie type polarization maintaining optical fiber, an elliptical type polarization maintaining optical fiber or a polarization maintaining photonic crystal optical fiber.

4. The high-sensitivity micro-nano optical fiber refractive index sensor according to claim 1, wherein the fiber coupler splitting ratio is 50% to 50%.

5. The high-sensitivity micro-nano optical fiber refractive index sensor according to claim 1, wherein the birefringent micro-nano optical fiber comprises an optical fiber core and an optical fiber cladding surrounding the optical fiber core.

6. The high-sensitivity micro-nano optical fiber refractive index sensor according to claim 5, wherein the cross section of the optical fiber cladding is a rectangular or rectangular-like double symmetrical structure.

7. The high-sensitivity micro-nano optical fiber refractive index sensor according to claim 5, wherein the refractive index of the fiber core is higher than that of the fiber cladding.

8. The preparation method of the high-sensitivity micro-nano optical fiber refractive index sensor according to any one of claims 1 to 7, which is characterized by comprising the following steps of:

(1) selecting a birefringent micro-nano optical fiber, wherein the cross section structure of the birefringent micro-nano optical fiber is a rectangular or quasi-rectangular double symmetrical structure;

(2) connecting two ports on the same side of the optical fiber coupler with a broadband light source and a spectrum analyzer respectively by adopting the optical fiber coupler;

(3) and the birefringence micro-nano optical fiber and the first polarization controller are sequentially connected between the two ports on the other side of the optical fiber coupler to form a closed optical path, so that the optical fiber annular mirror containing the birefringence micro-nano optical fiber is formed.

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