Etches-based refractive index sensors and their measurement methods

Abstract

This invention discloses a refractive index sensor based on a stepped-grating and its measurement method, belonging to the field of optical interferometry. It includes an incident light, a sample cell, a converging lens, and a spectral analysis system arranged sequentially. The sample cell is located behind the incident light and contains the sample to be measured and the stepped-grating. The incident light, after passing through the sample and the stepped-grating, is converged by the converging lens before reaching the spectral analysis system for refractive index measurement. This invention uses a stepped-grating to split the incident parallel light into many parts, filtering out diffracted light, constructing multi-beam interference, and designing a refractive index sensor to achieve high-sensitivity, high-resolution refractive index measurement. This solves the problems existing in the prior art.

Description

Technical Field

[0001] This invention relates to a refractive index sensor based on a stepped grating and its measurement method, belonging to the field of optical interferometry technology. Background Technology

[0002] Optical interferometers offer high measurement accuracy and have wide applications in precision measurement. By converting the information to be measured into changes in refractive index and detecting this change using refractive index sensors based on optical interferometers, numerous sensors have been designed for the detection of physical, chemical, and biological quantities. However, refractive index sensors that simultaneously possess high resolution and high sensitivity are currently rare. Achieving high-precision refractive index measurement solely through high resolution or high sensitivity places high demands on the performance of the detection equipment, resulting in high costs.

[0003] (1) Some refractive index sensors have extremely high resolution but low sensitivity. For example, resonant interferometers output very narrow fringe half-widths, with spectral resolution less than 0.1 pm, or even smaller. Therefore, they can detect minute shifts in interference fringes caused by changes in refractive index, thus achieving high-precision refractive index measurement. However, the extremely high spectral resolution necessitates very expensive light sources and photoelectric detection equipment, resulting in high sensor system costs. Furthermore, the refractive index measurement sensitivity of this approach is low, with a theoretical maximum sensitivity of 10 pm. 3 The actual sensitivity is usually much lower than the theoretical value, which limits the accuracy of refractive index detection.

[0004] (2) Some refractive index sensors have high sensitivity but poor resolution. Designing a mode interferometer based on micro / nano fiber optics, or fabricating a Mach-Zehnder interferometer on fiber using femtosecond lasers, can achieve resolutions greater than 10. 4 The refractive index measurement sensitivity is 10 nm / RIU; the dual-path Mach-Zehnder interferometer can even achieve 10 6 The sensitivity is nm / RIU. However, these interferometers have wide fringes, with a free spectral range greater than tens to hundreds of nanometers, resulting in low spectral resolution and hindering the achievement of high detection accuracy. Furthermore, the ultra-high sensitivity also leads to a small dynamic range for the refractive index sensor, making it unsuitable for measuring large-scale refractive index variations. Therefore, improving the design of existing refractive index sensors has become a pressing technical problem. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the present invention aims to provide a refractive index sensor based on a stepped-grating and its measurement method. The stepped-grating is used to split incident parallel light into many parts, filtering out diffracted light, constructing multi-beam interference, and designing a refractive index sensor to achieve high-sensitivity, high-resolution refractive index measurement. This solves the problems existing in the prior art.

[0006] The refractive index sensor based on a stepped grating of the present invention includes an incident light, a sample cell, a converging lens, and a spectral analysis system arranged in sequence. The sample cell is located behind the incident light. The sample cell contains a sample to be tested and a stepped grating. The incident light passes through the sample to be tested and the stepped grating, and is then converged by the converging lens before reaching the spectral analysis system for refractive index measurement.

[0007] Furthermore, an aperture is provided between the converging lens and the spectral analysis system, the aperture being used to filter out diffracted light.

[0008] Furthermore, the incident light is parallel.

[0009] Furthermore, the incident light is broadband light or laser light.

[0010] Furthermore, the step grating has a staircase-like structure and is a transmissive step grating, with the light-transmitting surface of the step grating being larger than the cross-section of the incident light.

[0011] Furthermore, the converging lens is one or more of the following: a Green lens, an aspherical lens, and a conventional convex lens.

[0012] Furthermore, the sample to be tested is either a liquid or a gas.

[0013] Furthermore, the sample cell has two light-transmitting surfaces, front and back, through which the incident light passes sequentially.

[0014] Furthermore, the spectral analysis system is a spectrometer or an optical power meter.

[0015] This invention relates to a measurement method for a refractive index sensor based on a stepped grating, comprising the following steps:

[0016] S1: Parallel light is incident on the sample and the grating, and then transmitted. Depending on the phase delay, the transmitted light is divided into many beams. Each beam contains non-diffractive light. The phase difference between any two adjacent non-diffractive transmitted beams is the same, and this phase difference is proportional to the difference between the refractive index of the sample and the refractive index of the grating material.

[0017] S2: The transmitted light from the eddy grating is converged by the converging lens, and the diffracted light is filtered out by the aperture. The remaining non-diffracted light interferes. As the refractive index of the sample changes, the interference fringes will drift.

[0018] S3: The interference light is transmitted to the spectral analysis system, which is used to detect the drift of the interference fringes and thus measure the refractive index of the sample under test.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] The refractive index sensor and its measurement method based on a stepped grating described in this invention use a stepped grating to divide the incident parallel, thereby constructing multi-beam interference and compressing the half-width at half-maximum (WHM) of the interference fringes, which can greatly improve the resolution of refractive index measurement. By selecting a material with a small difference in refractive index from the sample to be measured to make the stepped grating, high refractive index measurement sensitivity can be obtained. After selecting the material of the stepped grating, by reducing the step width of the stepped grating, the free spectral range of the interference fringes can be increased, thereby obtaining a large dynamic range for refractive index measurement.

[0021] The refractive index sensor based on a stepped grating described in this invention can achieve high-resolution, high-sensitivity, and wide dynamic range refractive index measurement, thus solving the problems existing in the prior art. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the refractive index sensor based on a stepped grating according to the present invention;

[0023] Figure 2 This is a structural diagram of the echelle grating in the refractive index sensor based on the echelle grating of the present invention;

[0024] Figure 3 The interference fringes are those used in the grating-based refractive index sensor of this invention when measuring the refractive index of a water-based sample using a grating.

[0025] Figure 4 The interference fringe pattern of two-beam interference when measuring water-based samples;

[0026] Figure 5 This invention relates to a grating-based refractive index sensor, which measures the refractive index of a water-based sample using a grating. The interference fringes shift towards shorter wavelengths as the sample's refractive index increases.

[0027] In the figure: 1. Incident light; 2. Etches; 3. Converging lens; 4. Spectral analysis system; 5. Aperture; 6. Sample to be tested; 7. Sample cell. Detailed Implementation

[0028] The present invention will be further described below with reference to the accompanying drawings and embodiments:

[0029] Example 1:

[0030] like Figure 1 As shown, the refractive index sensor based on a stepped grating of the present invention includes an incident light 1, a sample cell 7, a converging lens 3, and a spectral analysis system 4 arranged in sequence. The sample cell 7 is located behind the incident light 1. The sample cell 7 contains a sample to be tested 6 and a stepped grating 2. After the incident light 1 passes through the sample to be tested 6 and the stepped grating 2, it is converged by the converging lens 3 and then reaches the spectral analysis system 4 for refractive index measurement.

[0031] To further illustrate the above embodiments, an aperture 5 is provided between the converging lens 3 and the spectral analysis system 4, and the aperture 5 is used to filter out diffracted light.

[0032] To further illustrate the above embodiment, the front and rear order of the stepped grating 2 and the sample 6 can be interchanged. The sample cell 7 has two light-transmitting surfaces, front and rear, through which the incident light 1 passes sequentially.

[0033] To further illustrate the above embodiments, incident light 1 is parallel light.

[0034] To further illustrate the above embodiments, the parallel light can be broadband light or a laser with good monochromaticity.

[0035] To further illustrate the above embodiments, such as Figure 2 As shown, the stepped grating 2 has a stair-shaped structure and is a transmissive stepped grating. The light-transmitting surface of the stepped grating 2 is larger than the cross-section of the incident light 1. Preferably, the stepped grating 2 is made of fused silica, but the present invention is not limited to stepped gratings made of fused silica material. Transmissive stepped gratings with a stair-shaped structure made of any transparent material are applicable to the present invention.

[0036] To further illustrate the above embodiments, the converging lens 3 is one or more of a Green lens, an aspherical lens, and a conventional convex lens.

[0037] To further illustrate the above embodiments, if the transmitted light from the converging lens 3 propagates to the spectral analysis system 4 via air, then the aperture 5 is a circular aperture. If the converging lens 3 has a single-mode pigtail, then the transmitted light from the converging lens 3 propagates to the spectral analysis system 4 via a single-mode optical fiber, and the numerical aperture of the single-mode optical fiber is equivalent to an aperture.

[0038] To further illustrate the above embodiments, the sample 6 to be tested is a liquid or a gas.

[0039] To further illustrate the above embodiments, the spectral analysis system 4 is a spectrometer or an optical power meter. If the parallel light is broadband light, then the spectral analysis system 4 is a spectrometer; if the parallel light is monochromatic laser, then the spectral analysis system 4 is an optical power meter.

[0040] The specific application of this embodiment is as follows:

[0041] Figure 2 Here is a simplified diagram of a step grating, where l is the step width and h is the step height. Assume the number of periods in step grating 2 is N, the refractive index of the material used to fabricate the step grating is n1, and the refractive index of the sample is n2. Figure 1 As shown, if parallel light is incident perpendicularly on the grating from left to right, the intensity of the interference light received by the spectral analysis system 4 can be expressed as:

[0042]

[0043] Where A is the amplitude of transmitted light at each step of the stepped grating, and λ is the wavelength of light.

[0044] If a transmission-type grating is fabricated using quartz, assuming the number of steps N = 20 and the step width l = 18 μm, the interference fringes obtained when measuring a water-based sample would be as follows: Figure 3 As shown. An in-line Mach-Zehnder interferometer, fabricated using a femtosecond laser to etch grooves on a single-mode fiber, satisfies two-beam interference. When the width of the fiber groove is 18 μm, the interference fringes are as follows. Figure 4 As shown. Comparison Figure 3 and Figure 4 It can be seen that a grating with N=20 steps can compress the half-width at half-maximum (FWHM) of the interference fringes by more than 10 times compared to two-beam interference. Due to the narrower interference fringes, the resolution of refractive index measurements can be significantly improved. Furthermore, because the step width is small, only 18 μm, from... Figure 3 It can be seen that the resulting interference fringes have a large free spectral range, which is beneficial for obtaining a large dynamic range for refractive index measurement.

[0045] The m-th order bright fringe received by the spectral analysis system 4 satisfies the following equation:

[0046] (n1-n2)l=mλ (2)

[0047] The refractive index sensing sensitivity of the m-th order bright fringe can be expressed as:

[0048]

[0049] If the refractive index n1 of the material used to make the stepped grating is greater than the refractive index n2 of the sample, it can be seen from formula (3) that as the refractive index of the sample increases, the interference fringes will drift toward the short-wavelength direction.

[0050] like Figure 5 As shown, near a light wavelength of 1550 nm, when the refractive index n2 of the water-based sample increases by 10... -3 At that time, the interference fringes shift blue by 13.3 nm towards the shortwave direction. The calculated sensitivity of the refractive index sensor is 1.33 × 10⁻⁶. 4 nm / RIU enables high-sensitivity refractive index sensing.

[0051] In summary, the refractive index sensor based on a stepped grating described in this invention can achieve high-resolution, high-sensitivity, and large dynamic range refractive index measurement, solving the problems existing in the prior art.

[0052] Example 2:

[0053] The measurement method of the refractive index sensor based on the stepped grating described in this invention includes the following steps:

[0054] S1: Parallel light is incident on the sample 6 and the grating 2, and then transmitted. Depending on the phase delay, the transmitted light is divided into many beams. Each beam of transmitted light contains non-diffractive light. The phase difference between any two adjacent non-diffractive transmitted beams is the same, and this phase difference is proportional to the difference between the refractive index of the sample and the refractive index of the grating material.

[0055] S2: The transmitted light from the eddy grating 2 is converged by the converging lens 3, and the diffracted light is filtered out by the aperture 5. The remaining non-diffracted light interferes, forming interference fringes with a narrow half-width at half-maximum. As the refractive index of the sample changes, the interference fringes will drift.

[0056] S3: The interference light is transmitted to the spectral analysis system 4, and the drift of the interference fringes is detected by the spectral analysis system 4 to realize the measurement of the refractive index of the sample 6 under test.

[0057] Based on Example 1, this embodiment uses the measurement method based on the grating-based refractive index sensor described in Example 1 to achieve high-resolution, high-sensitivity, and large dynamic range refractive index measurement.

[0058] In the description of this invention, it should be noted that the terms "upper", "lower", "left", "right", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0059] The refractive index sensor and its measurement method based on a stepped-grating, as described above in conjunction with the accompanying drawings, utilizes a stepped-grating to divide incident parallel light into multiple parts. After filtering out diffracted light, multi-beam interference is constructed, and a refractive index sensor is designed to achieve high-sensitivity, high-resolution refractive index measurement. This solves the problems existing in the prior art. However, the present invention is not limited to the described embodiments. Variations, modifications, substitutions, and modifications made to the embodiments without departing from the principles and spirit of the present invention still fall within the protection scope of the present invention.

Claims

1. A refractive index sensor based on a stepped grating, characterized by: The application relates to a refractive index measuring device, which comprises sequentially arranged incident light (1), a sample cell (7), a converging lens (3) and a spectrum analysis system (4), wherein the sample cell (7) is located behind the incident light (1), a sample to be measured (6) and a stepped grating (2) are arranged in the sample cell (7), the incident light (1) passes through the sample to be measured (6) and the stepped grating (2), is converged by the converging lens (3) and reaches the spectrum analysis system (4) to perform refractive index measurement; a diaphragm (5) is arranged between the converging lens (3) and the spectrum analysis system (4), and the diaphragm (5) is used for filtering out diffracted light.

2. The step-index grating-based refractive index sensor of claim 1, wherein: The incident light (1) is parallel light.

3. The step-index grating-based refractive index sensor of claim 2, wherein: The incident light (1) is broadband light or laser.

4. The step-index grating-based refractive index sensor of claim 1, wherein: The stepped grating (2) is a stair-like structure and is a transmission type stepped grating, and the light transmission surface of the stepped grating (2) is larger than the cross section of the incident light (1).

5. The step-index grating-based refractive index sensor of claim 1, wherein: The converging lens (3) is one or more of a Green lens, an aspherical lens and a conventional convex lens.

6. The step-index grating-based refractive index sensor of claim 1, wherein: The sample to be measured (6) is a liquid or a gas.

7. The step-index grating-based refractive index sensor of claim 1, wherein: The sample cell (7) has front and rear light transmission surfaces, and the incident light (1) passes through the front and rear surfaces in sequence.

8. The step-index grating-based refractive index sensor of claim 1, wherein: The spectrum analysis system (4) is a spectrometer or an optical power meter.

9. A measurement method of a refractive index sensor based on a stepped grating, applied to the refractive index sensor based on a stepped grating according to any one of claims 1 to 8, characterized by, The method comprises the following steps: S1: parallel light is incident on the sample to be measured (6) and the stepped grating (2) and then is transmitted, according to the different phase delay amounts, the transmitted light is divided into many beams, each transmitted light beam contains non-diffracted light, the phase difference of any two adjacent transmitted non-diffracted light beams is the same, and the phase difference is proportional to the difference between the refractive index of the sample and the refractive index of the stepped grating material; S2: the transmitted light of the stepped grating (2) is converged by the converging lens (3), the diffracted light is filtered out by the diaphragm (5), the remaining non-diffracted light is interfered, and with the change of the refractive index of the sample, the interference fringes are shifted; S3: the interference light is transmitted to the spectrum analysis system (4), the shift of the interference fringes is detected by the spectrum analysis system (4), and the refractive index of the sample to be measured (6) is measured.

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