Circular Dichroism Detection Device, Preparation Method Thereof and Detection Method

Abstract

The present invention discloses a circular dichroism detection device, a preparation method thereof, and a detection method thereof. The circular dichroism detection device is a micro - spiral fiber grating, which comprises a plurality of completely identical micro - spiral fiber grating segments with a period of P. Each micro - spiral fiber grating segment comprises a plurality of gratings with a reference grating period of Λ0, wherein the period P and the reference grating period Λ0 satisfy: Λ0 = λ0 / Δn, P = λ 0* Λ 0* s / |λ w ‑λ0|, λ w is the working central wavelength required for circular dichroism detection, λ0 is the reference central wavelength of the input left - and right - circularly polarized light, s is the waveguide dispersion correction factor of the original optical fiber for making the micro - spiral fiber grating, and Δn is the refractive index difference of the coupling mode. When the circular dichroism detection device of the present invention is immersed in a solution of a left - or right - handed chiral substance, the power changes of the left - or right - circularly polarized light with a specific wavelength passing through the detection device are different, and thus the type of chiral substance in the solution can be detected. The circular dichroism detection device of the present invention is simple to manufacture, has a fast detection speed, and high stability.

Description

Technical Field

[0001] The present invention relates to the technical field of optical detection, and in particular, to a circular dichroism detection device, a preparation method thereof, and a detection method thereof. Background Art

[0002] Some biological and chemical molecules have the property of non - coincidence with their mirror images, showing different chiral structures, and being enantiomers of their mirror images. For example, amino acids on the earth are mainly in the left - handed chiral structure (left - handed conformation), while sugars are mainly in the right - handed chiral structure (right - handed conformation). When substances with such structures interact with light, their properties for left - and right - circularly polarized light are different, showing optical activity (circular birefringence) and circular dichroism. Such substances are also called "optically active substances". Among them, the optical activity (circular birefringence) refers to the phenomenon that when linearly polarized light passes through an optically active substance, due to the different refractive indices of the left - and right - circularly polarized lights that make up the linearly polarized light during propagation in the chiral substance, the synthesized linearly polarized light rotates in a certain direction; circular dichroism refers to the phenomenon that an optically active substance has different transmittance (absorbance) rates for left - and right - circularly polarized lights in a specific wavelength region, as reflected in the Figure 1 circular dichroism spectrum shown. The chirality of optically active drugs is closely related to their therapeutic properties, biological activities, toxicity, etc. Therefore, in the preparation process of drugs, the determination of chirality, that is, its circular dichroism, is essential. The existing quantitative calculation of circular dichroism mainly uses circular dichroism spectra. Common circular dichroism spectrum measurement methods based on Raman optics require a high - output - power laser light source, as well as high - quality optical devices such as filters, lenses, beam splitters, polarizers, and wave plates. The measuring device is complex and expensive, and the measured signal is extremely weak. Generally, it takes several hours (sometimes several days) to accurately measure the circular dichroism spectrum of the test sample. In addition, most of the existing artificial chiral materials with optical activity are metal nano - materials. Such materials are difficult to combine with the optical system to achieve practical measurement of circular dichroism spectra, and the random movement or arrangement of metal nano - particles in solution will affect the stability of circular dichroism spectrum measurement. Summary of the Invention

[0003] Object of the Invention: Aiming at the problems existing in the prior art, the present invention provides a circular dichroism detection device with simple production, fast detection speed, and high stability, as well as a preparation method and a detection method thereof.

[0004] Technical Solution: The circular dichroism detection device described in the present invention is specifically a micro - spiral fiber grating. The micro - spiral fiber grating includes several completely identical micro - spiral fiber grating segments with a period of P. Each micro - spiral fiber grating segment includes several gratings with a reference grating period of Λ0, where the period P and the reference grating period Λ0 satisfy:

[0005] Λ0 = λ0 / Δn, P = λ0*Λ0*s / |λ w -λ0|

[0006] In the formula, λ w is the working center wavelength required for circular dichroism detection, λ0 is the reference center wavelength of the input left and right circularly polarized light, s is the waveguide dispersion correction factor of the original optical fiber for fabricating the micro-helical fiber grating, and Δn is the refractive index difference of the coupling mode. Among them, the calculation method of the refractive index difference of the coupling mode is:

[0007] Δn = ((n FL - n HL ) + (n FR - n HR )) / 2

[0008] In the formula, n FL , n FR , n HL , n HR are the effective refractive indices of the left and right circularly polarized fundamental modes, and the left and right circularly polarized higher-order azimuthal or radial modes coupled with them, respectively.

[0009] As an alternative, the grating included in the micro-helical fiber grating section is a phase modulation grating, and the reference grating period of the nth grating is Λ0, and the period Λ n is:

[0010]

[0011] In the formula, Λ0 = λ0 / Δn represents the reference grating period of the micro-helical fiber grating, represents the refractive index phase value after refractive index phase modulation of the nth grating in the period P of the micro-helical fiber grating, n = 1, 2,..., N, N is the number of gratings in the period P, and α1, β1 represent the optimization parameters of the refractive index phase value .

[0012] As another alternative, the grating included in the micro-helical fiber grating section is a DC modulation grating, and the reference grating period of the nth grating is Λ0, and the DC refractive index Δn DC (z) is:

[0013] Δn DC (z) = λ0 * α2 * cos(2 * π * n * z / P + β2) / P

[0014] In the formula, z is the zth point in the propagation direction, n = 1, 2,..., N, N is the number of gratings in the period P, and α2, β2 represent the optimization parameters of the DC refractive index Δn DC (z).

[0015] The present invention provides a method for preparing a circular dichroism detection device, including the following steps:

[0016] (1) Fix the fixator and the rotator on translation stage 1 and translation stage 2 respectively, and place both translation stage 1 and translation stage 2 on translation stage 3;

[0017] (2) Obtain an original optical fiber, fix one end of the original optical fiber on the fixator and the other end in the rotator;

[0018] (3) Fabricate a grating with a period of Λ n : Heat the optical fiber by a laser or a hydrogen-oxygen flame. When the optical fiber is heated to the molten state, by controlling the moving speed v n of translation stage 3 and the rotation rate w n of the rotator, a grating with a period of Λ n is obtained, where the moving speed w n and the rotation rate v n satisfy v n / w n = Λ n ;

[0019] (4) Repeat step (3) such that n traverses any number between 1 and N, where N is the number of gratings within the period P, to obtain a micro-spiral fiber grating section including N gratings with periods of Λ n respectively;

[0020] (5) Repeat step (4) to obtain a phase-modulated micro-spiral fiber grating including several micro-spiral fiber grating sections with a period of P.

[0021] Further, the original optical fiber is specifically any one of a single-mode optical fiber, a few-mode optical fiber, a photonic crystal fiber, a ring-core optical fiber, or a multi-core optical fiber. The reference grating period Λ0 of the micro-spiral fiber grating << 100 μm; for the angular order l and the radial order v of the coupled high-order modes, generally l + v >> 10 is required.

[0022] The present invention provides another method for fabricating a circular dichroism detection device, including the following steps:

[0023] (1) Fix the fixator and the rotator on translation stage 1 and translation stage 2 respectively, and place both translation stage 1 and translation stage 2 on translation stage 3;

[0024] (2) Obtain an original optical fiber, fix one end of the original optical fiber on the fixator and the other end in the rotator;

[0025] (3) Fabricate a grating with a period of Λ0 and a DC refractive index Δn DC(z) Grating: The optical fiber is heated by a laser or a hydrogen-oxygen flame. When the optical fiber is heated to the molten state, by controlling the moving speed v0 of the translation stage 3 and the rotation rate w0 of the rotator, a grating with a period of Λ0 is obtained, where the moving speed w0 and the rotation rate v0 satisfy v0 / w0 = Λ0; then, any one of the methods of directly irradiating the grating with a period of Λ0 by a laser, the arc method, mechanical extrusion, or acoustic wave modulation is used to form a direct current refractive index Δn DC (z);

[0026] (5) Repeat step (4) to obtain a direct current modulated micro-spiral fiber grating including several micro-spiral fiber grating segments with a period of P.

[0027] The method for detection using a circular dichroism detection device provided by the present invention includes the following steps:

[0028] (1) Place the circular dichroism detection device into a solution containing a chiral substance (left-handed or right-handed chiral structure molecule);

[0029] (2) Input left-circularly polarized light with a resonant wavelength λ L , and use a optical power meter to measure the change in optical power ΔP of the light passing through the circular dichroism detection device L ;

[0030] (3) Input right-circularly polarized light with a resonant wavelength λ R , and use a optical power meter to measure the change in optical power ΔP of the light passing through the circular dichroism detection device R ;

[0031] (4) Compare the power changes ΔP L and ΔP R of the left and right circularly polarized lights at two resonant wavelength points to detect the chiral polarity of the chiral molecules in the solution. When ΔP L > ΔP R , it is determined that the solution mainly contains right-handed chiral structure molecules; when ΔP L < ΔP R , it is determined that the solution mainly contains left-handed chiral structure molecules. According to the power change values at the two resonant wavelength points, the concentration of the chiral structure molecules can be further quantitatively calculated.

[0032] Advantageous effects: Compared with the prior art, the present invention has the remarkable advantages that the manufacturing method of the present invention is simple and can be realized on traditional optical fiber processing equipment, overcoming the problem that traditional chiral fiber gratings require optical fibers with special-shaped cores such as spiral cores or rectangular cores and are difficult to be produced on a large scale and at low cost; compared with traditional metal nanomaterials with chiral detection characteristics, the polarization characteristics of this method are stable and it is easy to be integrated with an optical measurement system; compared with Raman optical activity spectroscopy, this method has the advantages of low cost, simple system, high measurement sensitivity, and fast detection speed. Brief Description of the Drawings

[0033] Figure 1 It is a circular dichroism characteristic diagram of chiral molecules;

[0034] Figure 2 It is a schematic diagram of an experimental bench for preparing micro - helical fiber gratings;

[0035] Figure 3 It is a schematic structural diagram of the un - modulated micro - helical fiber grating provided by the present invention;

[0036] Figure 4 It is a schematic structural diagram of the phase - modulated micro - helical fiber grating provided by the present invention;

[0037] Figure 5 It is a schematic structural diagram of the DC - modulated micro - helical fiber grating provided by the present invention;

[0038] Figure 6 It is a schematic diagram for circular dichroism detection using a circular dichroism detection device provided by the present invention;

[0039] Figure 7 It is a transmission spectrum diagram of a phase - modulated micro - helical fiber grating with a tunable working wavelength range realized by using different phase - modulation periods P;

[0040] Figure 8 It is for λ w = λ0 = 1224nm, the transmission spectrum diagram of the micro - helical fiber grating working in this wavelength region;

[0041] Figure 9 It is for λ w = 612nm, the transmission spectrum diagram of the micro - helical fiber grating working in this wavelength region. Detailed Embodiments

[0042] The embodiments of the present invention will be introduced in detail below with reference to the drawings.

[0043] Embodiment 1

[0044] This embodiment provides a circular dichroism detection device, which is specifically a micro - helical fiber grating realized by phase modulation. The micro - helical fiber grating includes several completely identical micro - helical fiber grating segments with a period of P. Each micro - helical fiber grating segment includes several gratings with a reference grating period of Λ0, where the period P and the reference grating period Λ0 satisfy:

[0045] Λ0 = λ0 / Δn, P = λ0*Λ0*s / |λ w - λ0|

[0046] λ wis the working central wavelength required for circular dichroism detection, λ0 is the reference central wavelength of the input left and right circularly polarized light, s is the waveguide dispersion correction factor of the original optical fiber for fabricating the micro-spiral fiber grating, and Δn is the refractive index difference of the coupled mode.

[0047] Among them, the calculation method of the refractive index difference of the coupled mode is:

[0048] Δn = ((n FL - n HL ) + (n FR - n HR )) / 2

[0049] In the formula, n FL , n FR , n HL , n HR are the effective refractive indices of the left and right circularly polarized fundamental modes, and the left and right circularly polarized higher-order angular or radial modes coupled with them respectively.

[0050] Among them, the grating included in the micro-spiral fiber grating section is a phase modulation grating, and the period Λ n of the nth grating is:

[0051]

[0052] In the formula, Λ0 = λ0 / Δn represents the reference grating period of the micro-spiral fiber grating, represents the refractive index phase value after refractive index phase modulation of the nth grating in the period P of the micro-spiral fiber grating, n = 1, 2,..., N, N is the number of gratings in the period P, and α1, β1 represent the optimization parameters of the refractive index phase value . The original optical fiber is specifically any one of single-mode optical fiber, few-mode optical fiber, photonic crystal fiber, ring-core optical fiber or multi-core optical fiber.

[0053] The smaller the reference grating period of the micro-spiral fiber grating, the better. Generally, Λ0 << 100um; the higher the angular order l and radial order v of the coupled higher-order modes, the stronger the interaction effect between the light wave and the chiral substance, and the larger the separation interval Δλ LR of the coupled wavelengths. Generally, it is required that l + v >> 10.

[0054] The present invention designs a circular dichroism detection device based on a phase-modulated micro-helical fiber grating. The circular birefringence (i.e., circular dichroism) in the fiber grating is achieved through the micro-helical structure, and a polarization-independent fiber helical grating is used to realize the coupling loss of left and right circularly polarized lights at different separated wavelength positions while maintaining the characteristics of circular birefringence. Although the helical fiber grating has been widely studied at home and abroad, there is no report on using a micro-periodic helical structure fiber to achieve circular dichroism and the effective separation of the coupling loss wavelengths of left and right circularly polarized lights for chiral substance detection. Although the chiral fiber grating also separates left and right circularly polarized lights at different coupling wavelengths by using a helical structure fiber, it is different from the present invention in that the chiral fiber grating uses two fiber gratings with different chiral structures to realize different coupling characteristics of left and right circularly polarized lights respectively, and special-shaped core fibers such as helical cores or rectangular cores are required. The present invention is made of original fibers such as traditional circularly symmetric single-mode fibers, few-mode fibers, photonic crystal fibers, ring-core fibers, and multi-core fibers, without the need for special-shaped core fibers, and uses a fiber grating with the same chiral structure to realize different coupling characteristics of left and right circularly polarized lights; the present invention proposes a unique method for adjusting the measurement wavelength region through a phase-modulated grating, which can meet the requirements of any measurement wavelength range; a method of introducing a grating is proposed to separate left and right circularly polarized lights at different wavelengths; the circular dichroism detection device based on the phase-modulated micro-helical fiber grating proposed by the present invention is entirely composed of fibers, and has the advantages of low cost, low insertion loss, small volume, high sensitivity, strong adaptability, and simplicity and practicality, and can fully realize practical application.

[0055] Embodiment 2

[0056] This embodiment provides another circular dichroism detection device. The difference between this embodiment and Embodiment 1 is that this embodiment is realized by DC modulation. The grating included in the micro-helical fiber grating section of this embodiment is a DC modulation grating, and the reference grating period of the nth grating is Λ0, and the DC refractive index Δn DC (z) is:

[0057] Δn DC (z) = λ0 * α2 * cos(2 * π * n * z / P + β2) / P

[0058] In the formula, z is the zth point in the propagation direction, n = 1, 2,..., N, N is the number of gratings within the period P, and α2 and β2 represent the optimization parameters of the DC refractive index Δn DC (z).

[0059] Embodiment 3

[0060] This embodiment provides a method for preparing a circular dichroism detection device, which uses Figure 2 the experimental platform shown. The experimental platform includes a translation stage 1 (Stage1), a translation stage 2 (Stage2), a translation stage 3 (Stage3), a fixture (Clamp), and a rotator (Rotator). The fixture and the rotator are respectively fixed on the translation stage 1 and the translation stage 2, and the translation stage 1 and the translation stage 2 are both placed on the translation stage 3. It may also include a broad light source (ASE) and an optical spectrum analyzer (OSA) for observing the characteristics of the helical optical fiber during production, such as loss characteristics, etc. This experimental bench can synchronously control the heating, movement, and rotation of the optical fiber, and synchronously move and rotate the optical fiber heated to the molten state to form a helical fiber grating with a specific designed helical structure. The period length Λ of the formed helical fiber grating is determined by the moving speed v (mm / s - millimeters per second) of the optical fiber controlled by the translation stage 3 (Stage 3) and the rotation speed w (turn / s - turns per second) controlled by the rotator (Rotator), and is expressed as Λ = v / w (mm / turn - millimeters per turn).

[0061] The circular dichroism detection device prepared in this embodiment is the phase-modulated micro-helical fiber grating in Embodiment 1, and the preparation method specifically includes the following steps:

[0062] (1) Fix the fixture and the rotator on the translation stage 1 and the translation stage 2 respectively, and place the translation stage 1 and the translation stage 2 on the translation stage 3;

[0063] (2) Obtain an original optical fiber, specifically a single-mode optical fiber (Fujikura Inc, -SR15E). One end of the original optical fiber (Optical fiber) to be processed is fixed, and a weight (Weight) is used to keep the original optical fiber straight during processing. The other end is fixed and loaded into the rotator. In other embodiments, the original optical fiber can also be any one of a few-mode optical fiber, a photonic crystal fiber, a ring-core optical fiber, or a multi-core optical fiber;

[0064] (3) Prepare a grating with a period of Λ n : To ensure the uniformity of the heating zone, a sapphire tube (Sapphiretube) is used to form the heating zone, and an optical switch (Shutter) is used to control the on / off of a carbon dioxide laser (CO2 laser). The optical fiber is heated by the carbon dioxide laser. When the optical fiber is heated to the molten state, by controlling the moving speed v n of the translation stage 3 and the rotation rate w n of the rotator, a grating with a period of Λ n is obtained, where the moving speed w n and the rotation rate v n satisfy v n / wn = Λ n ; In other embodiments, other lasers such as ultraviolet lasers or femtosecond lasers, or hydrogen-oxygen flames can also be used to achieve heating;

[0065] (4) Repeat step (3) so that n traverses any number between 1 and N, where N is the number of gratings within the period P, to obtain a micro-spiral fiber grating section including N gratings with periods of Λ n respectively;

[0066] (5) Repeat step (4) to obtain a phase-modulated micro-spiral fiber grating including several micro-spiral fiber grating sections with a period of P. The number of sections M of the micro-spiral fiber grating section generally takes a value greater than 3. The larger the M value, the fewer the harmonic components in the transmission spectrum of the grating, and the higher the quality of the grating.

[0067] For the prepared circular dichroism detection device, if the working wavelength is the same as the reference wavelength (i.e., λ w = λ0), then the value of each period is Λ0. At this time, there is no need to adjust the working wavelength, and its structure is as Figure 3 shown. If the working wavelength is not the same as the reference wavelength (i.e., λ w is not equal to λ0), then the value of each period is different, and its structure is as Figure 4 shown.

[0068] Due to the influence of the micro-periodic spiral structure, the left and right circular polarization modes in the optical fiber have different reflection and propagation paths due to the spin Hall effect, that is, different effective refractive indices for transmission are formed in the phase-modulated micro-spiral fiber grating prepared in this embodiment. When the angular or radial order of the optical fiber mode is higher, the center of the optical energy flow of the relevant mode is farther away from the center of the optical fiber, resulting in a greater separation of the effective refractive indices of the left and right circular polarization modes. Finally, an obvious separation of the coupling wavelengths is achieved under the action of the grating; when the period of the micro-periodic spiral structure optical fiber is smaller, the spin Hall effect is enhanced, and the refractive index separation between the left and right circular polarization modes of the relevant mode will be further greater. The separation degree of the final coupling wavelength depends on the product of the spiral grating period and the coupling refractive index difference between the left and right circular polarization modes. Therefore, in the experiment, by reducing the spiral period Λ0, and selecting the high-order angular order l or radial order v mode to couple with the fundamental mode, an effective separation of the resonance wavelengths of the left and right circularly polarized light can be achieved.

[0069] Example 4

[0070] This embodiment provides a preparation method for the circular dichroism detection device of Example 2. This method is completed based on the experimental bench shown in Figure 2 , and specifically includes the following steps:

[0071] (1) Fix the fixer and the rotator on the translation stage 1 and the translation stage 2 respectively, and place both the translation stage 1 and the translation stage 2 on the translation stage 3;

[0072] (2) Obtain an original optical fiber, fix one end of the original optical fiber on a fixture and the other end in a rotator.

[0073] (3) Fabricate a grating with a period of Λ0 and a DC refractive index Δn DC (z): Heat the optical fiber by a laser or a hydrogen-oxygen flame. When the optical fiber is heated to the molten state, control the moving speed v0 of the translation stage 3 and the rotation rate w0 of the rotator to obtain a grating with a period of Λ0, where the moving speed w0 and the rotation rate v0 satisfy v0 / w0 = Λ0; then use any one of the methods of directly irradiating with a laser, arc method, mechanical extrusion or acoustic wave modulation on the grating with a period of Λ0 to form a DC refractive index Δn DC (z);

[0074] (5) Repeat step (4) to obtain a DC-modulated micro-helical fiber grating including several micro-helical fiber grating segments with a period of P.

[0075] The structure of the circular dichroism detection device prepared in this embodiment is as Figure 5 shown.

[0076] Example 5

[0077] This embodiment provides a method for circular dichroism detection using a circular dichroism detection device, as Figure 6 shown, including the following steps:

[0078] (1) Place the circular dichroism detection device into a solution containing a chiral substance;

[0079] (2) Input left-circularly polarized light with a resonant wavelength of λ L , and use an optical power meter to measure the change in optical power ΔP L passing through the circular dichroism detection device. ΔP L = P0 - P L , where P0 is the input power and P L is the output optical power of the fiber grating after inputting left-circularly polarized light with a wavelength of λ L ;

[0080] (3) Input right-circularly polarized light with a resonant wavelength of λ R , and use an optical power meter to measure the change in optical power ΔP R ; ΔP R = P0 - P R , and P R is the output optical power of the fiber grating after inputting right-circularly polarized light with a wave λ R ;

[0081] (4) Compare the power changes ΔP L and ΔP R of the left and right circularly polarized lights at two resonant wavelength points to detect the chiral polarity of chiral molecules in the solution. When ΔP L > ΔP R , it is determined that the molecules in the solution are mainly right-handed chiral structure molecules; when ΔP L < ΔP R , it is determined that the molecules in the solution are mainly left-handed chiral structure molecules. According to the power change values at the two resonant wavelength points, the concentration of chiral structure molecules can be further quantitatively calculated.

[0082] The present invention is experimentally verified below.

[0083] The tuning effect of the period P on the working wavelength is as Figure 7 shown. In the figure, L is the total length of the fabricated micro-spiral fiber grating. As Figure 7 shown, when the period P = L / 8 and P = L / 4, near the reference center wavelength λ0, the reference working center wavelengths of the left and right circular polarizations are λ 0L and λ 0R . With the wavelength intervals Δλ = |λ w - λ0| ≈ λ0 / 8 and Δλ = |λ w - λ0| ≈ λ0 / 4, coupling peaks appear, indicating that the working center wavelength is tuned to λ w ≈ λ0 ± Δλ. The working center wavelengths of the left and right circular polarizations are λ wL and λ wR .

[0084] For the micro-spiral fiber grating with the structure shown in Figure 3 and a working wavelength of 1224 nm, its transmission spectrum is as shown in Figure 8 (a). When the test solution contains left-handed or right-handed chiral structure substances, the formed transmission spectrum diagrams are respectively as shown in Figure 8 (b) and 8(c). For the micro-spiral fiber grating with the structure shown in Figure 4 or 5 and a working wavelength of 612 nm, its transmission spectrum is as shown in Figure 9 (a). When the test solution contains left-handed or right-handed chiral structure substances, the formed transmission spectrum diagrams are respectively as shown in Figure 9 (b) and 9(c). It can be seen that in different left / right-handed chiral substance solutions, the left / right circularly polarized lights show different changes in coupling depth. Therefore, it is possible to determine whether the solution contains left-handed or right-handed chiral structure substances according to the changes in the coupling depth of the left / right circularly polarized lights. According to the power change values at the two resonant wavelength points, the concentration of chiral structure molecules can be further quantitatively calculated.

[0085] The above-disclosed is only a preferred embodiment of the present invention, and it cannot be used to limit the scope of the rights of the present invention. Therefore, equivalent changes made according to the claims of the present invention still fall within the scope covered by the present invention.

Claims

1. A circular dichroism detection device, characterized in that: The device is specifically a micro - spiral fiber grating, which includes several identical micro - spiral fiber grating segments with a period of P. Each micro - spiral fiber grating segment includes several gratings with a reference grating period of Λ0. Among them, the period P and the reference grating period Λ0 satisfy: Λ0 = λ0 / Δn, P = λ0 * Λ0 * s / |λ w - λ0| where λ w is the working central wavelength required for circular dichroism detection, λ0 is the reference central wavelength of the input left and right circularly polarized light, s is the waveguide dispersion correction factor of the original optical fiber for fabricating the micro-spiral fiber grating, and Δn is the refractive index difference of the coupled mode; The fiber Bragg grating segments of the micro spiral fiber grating include phase modulation gratings, and the reference grating period of the nth grating is Λ0, and the period Λ n is: Λ n =Λ 0* (1 + Δφ n ) where Λ0 = λ0 / Δn represents the reference grating period of the micro - helical fiber grating, and Δφ n = φ n - φ n-1 , φ0 = 0, φ(n)=α 1* sin(2 * π * n / N + β1) represents the refractive index phase value after refractive index phase modulation of the n - th grating in the period P of the micro - helical fiber grating, n = 1, 2, …, N, N is the number of gratings in the period P, and α1, β1 are the optimization parameters of the refractive index phase value φ(n); The calculation method of the refractive index difference of the coupling mode is: Δn=((n FL -n HL ) +(n FR -n HR )) / 2 where n FL , n FR , n HL , n HR are the effective refractive indices of the left and right circularly polarized fundamental modes, and the left and right circularly polarized higher-order azimuthal or radial modes coupled thereto, respectively.

2. A method for preparing the circular dichroism detection device according to claim 1, characterized in that It includes the following steps: (1) Fix the fixator and the rotator on the translation stage (1) and the translation stage (2) respectively, and place both the translation stage (1) and the translation stage (2) on the translation stage (3); (2) Obtain an original optical fiber, fix one end of the original optical fiber on the fixator and the other end in the rotator; (3)The preparation period is Λ n of the grating: Heating the optical fiber by a laser or a hydrogen-oxygen flame. When the optical fiber is heated to the molten state, by controlling the moving speed v n of the translation stage (3) and the rotation rate w n of the rotator, a grating with a period of Λ n is obtained, where the moving speed v n and the rotation rate w n satisfy v n / w n = Λ n ; (4) Repeat step (3) so that n traverses any number between 1 and N, where N is the number of gratings within the period P, to obtain a micro - helical fiber grating segment including N gratings with periods Λ n respectively; (5) Repeat step (4) to obtain a phase - modulated micro - spiral fiber grating including several micro - spiral fiber grating segments with a period of P.

3. The method for preparing a circular dichroism detection device according to claim 2, wherein: The original optical fiber is specifically any one of a single - mode fiber, a few - mode fiber, a photonic - crystal fiber, a ring - core fiber, or a multi - core fiber.

4. The method for preparing a circular dichroism detection device according to claim 2, characterized in that: The reference grating period Λ0 of the micro-spiral fiber grating << 100 μm; the angular order l and the radial order v of the coupled high-order mode satisfy l + v >> 10.

5. A circular dichroism detection device, characterized in that: The device is specifically a micro - spiral fiber grating, which includes several identical micro - spiral fiber grating segments with a period of P. Each micro - spiral fiber grating segment includes several gratings with a reference grating period of Λ0. Among them, the period P and the reference grating period Λ0 satisfy: Λ0 = λ0 / Δn, P = λ0 * Λ0 * s / |λ w -λ0| where λ w is the working center wavelength required for circular dichroism detection, λ0 is the reference center wavelength of the input left and right circularly polarized light, s is the waveguide dispersion correction factor of the original optical fiber for fabricating the microhelical fiber grating, and Δn is the refractive index difference of the coupling mode; The grating included in the micro spiral fiber grating section is a DC modulation grating, and the reference grating period of the nth grating is Λ0, and the DC refractive index Δn DC (z) is: Δn DC (z)= λ0*α2*cos(2*π*n*z / P+β2) / P where z is the z-th point in the propagation direction, n = 1, 2, …, N, N is the number of gratings within the period P, and α2, β2 represent the optimization parameters of Δn DC (z); The calculation method of the refractive index difference of the coupling mode is: Δn=((n FL -n HL ) +(n FR -n HR )) / 2 where n FL , n FR , n HL , n HR are the effective refractive indices of the left and right circularly polarized fundamental modes, and the left and right circularly polarized higher-order azimuthal or radial modes coupled thereto, respectively.

6. A method for preparing the circular dichroism detection device according to claim 5, characterized in that It includes the following steps: (1) Fix the fixator and the rotator on the translation stage (1) and the translation stage (2) respectively, and place both the translation stage (1) and the translation stage (2) on the translation stage (3); (2) Obtain an original optical fiber, fix one end of the original optical fiber on the fixator and the other end in the rotator; (3) Preparation period is Λ0, DC refractive index is Δn DC (z) Grating: Heat the optical fiber with a laser or a hydrogen-oxygen flame. When the optical fiber is heated to the molten state, by controlling the moving speed v0 of the translation stage (3) and the rotation rate w0 of the rotator, a grating with a period of Λ0 is obtained, where the moving speed v0 and the rotation rate w0 satisfy v0 / w0 = Λ0; then, any one of the methods of directly irradiating the grating with a period of Λ0 by a laser, the arc method, mechanical extrusion, or acoustic wave modulation is used to form a DC refractive index Δn DC (z); (4) Repeat step (3) to obtain a DC - modulated micro - spiral fiber grating including several micro - spiral fiber grating segments with a period of P.

7. The method for preparing a circular dichroism detection device according to claim 6, wherein: The original optical fiber is specifically any one of a single - mode fiber, a few - mode fiber, a photonic - crystal fiber, a ring - core fiber, or a multi - core fiber.

8. A method of detection using the circular dichroism detection device according to claim 1 or 5, characterized in that It includes the following steps: (1) Place the circular dichroism detection device into a solution containing chiral substances; (2)Input resonant wavelength λ L The left circularly polarized light is used, and the change in optical power ΔP passing through the circular dichroism detection device is measured using an optical power meter L ; (3) Input resonant wavelength λ R The right-circularly polarized light is used, and the change in optical power ΔP passing through the circular dichroism detection device is measured using an optical power meter R ; (4) Compare the power changes ΔP of the left and right circularly polarized lights at two resonant wavelength points L and ΔP R to detect the chirality polarity of chiral molecules in the solution. When ΔP L > ΔP R , it is determined that the molecules in the solution are mainly right-handed chiral structure molecules; when ΔP L < ΔP R, , it is determined that the molecules in the solution are mainly left-handed chiral structure molecules.

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