Refractive index sensor based on grating-prism system and its measurement method

Abstract

The present invention discloses a refractive index sensor and measurement method based on a grating-prism system, belonging to the field of optical interferometry. The sensor comprises a sequentially arranged parallel light, a plane grating, a sample cell, a converging component, and a spectral analysis system. The sample cell is provided with a prism and a sample to be measured. The parallel light propagates through the plane grating, which splits the parallel light into multiple sub-beams. The sub-beams are then converged by the converging component and arrive at the spectral analysis system for refractive index measurement. The sensor uses a plane grating to periodically block the incident parallel light, splitting it into sub-beams. Each level of diffraction light in each sub-beam is considered a waveband. After the sub-beams propagate through the prism and the sample to be measured, the same-order sub-bands of any two adjacent sub-beams have the same phase delay difference. By selecting the same-order sub-bands to construct multi-beam interferometry, the sensor can simultaneously achieve low-cost, high-sensitivity, and high-resolution refractive index measurement.

Description

Technical Field

[0001] The invention relates to a refractive index sensor based on a grating-prism system and a measurement method thereof, belonging to the technical field of optical interference measurement. Background Art

[0002] Optical refractive index sensors based on interferometry have a wide range of applications in physical, chemical, and biological measurement. Reported refractive index sensors achieve high-precision refractive index measurements, sometimes through ultra-high sensitivity or ultra-high resolution. However, their reliance on a single, high-sensitivity or high-resolution metric for refractive index measurement leads to high costs for these refractive index measurement systems.

[0003] A refractive index sensor can be designed by using a transmissive step grating with a staircase structure to split the wavefronts of parallel light. The characteristics of wavefront interference make the refractive index sensor have high sensitivity; multi-beam interference can compress the half-width of the interference fringes, thereby achieving higher resolution. Therefore, this type of sensor has both high sensitivity and resolution. In turn, it is possible to achieve high measurement accuracy with a lower-performance detection system. However, the difficulty in manufacturing transmissive step gratings with a staircase structure has restricted the promotion of this type of refractive index sensor. A considerable number of people in the existing technology have been searching for alternatives to transmissive step gratings. How to design an optical interferometry device for refractive index sensing that is easy to manufacture and easy to promote while achieving high sensitivity and high resolution while maintaining or even surpassing the performance of existing transmissive step gratings has become an urgent need. Summary of the Invention

[0004] In view of the deficiencies in the prior art, the present invention aims to provide a refractive index sensor based on a grating-prism system and a measurement method thereof, which solves the problems encountered in the prior art.

[0005] The refractive index sensor based on the grating-prism system described in the present invention includes parallel light, a plane grating, a sample cell, a converging component, and a spectral analysis system arranged in sequence. A prism and a sample to be measured are provided in the sample cell. The parallel light propagates through the plane grating, which splits the parallel light into multiple sub-beams. Each level of diffracted light in each sub-beam is regarded as a wave band. After the sub-beams propagate through the prism and the sample to be measured, the same-level sub-wave bands of any two adjacent sub-beams have the same phase delay difference. The sub-beams are then converged by the converging component and reach the spectral analysis system for refractive index measurement.

[0006] As a further solution of the present invention, the plane grating is a transmissive amplitude-modulated grating, and the plane grating is formed by a series of light-transmitting and light-impermeable structures arranged periodically.

[0007] As a further solution of the present invention, a multi-beam interference system is constructed using the plane grating and a prism.

[0008] As a further solution of the present invention, the parallel light is broadband light or laser.

[0009] As a further solution of the present invention, the sample cell has two light-transmitting surfaces at the front and rear, and the parallel light passes through the front and rear surfaces in sequence.

[0010] As a further solution of the present invention, the spectrum analysis system is a spectrometer or an optical power meter.

[0011] As a further solution of the present invention, an aperture is provided in the spectrum analysis system, and the aperture cooperates with the converging component to select the diffracted light order of the plane grating.

[0012] As a further solution of the present invention, the converging component is a converging lens.

[0013] The measuring method of the refractive index sensor based on the grating-prism system of the present invention comprises the following steps:

[0014] S1: Parallel light propagates through a plane grating and is split into multiple sub-beams by the plane grating. Each level of diffraction light of each sub-beam is regarded as a wave band.

[0015] S2: The sub-beam propagates through the prism and the sample to be measured; the same-order sub-bands of any two adjacent sub-beams have the same phase delay difference;

[0016] S3: The transmitted light from the sample cell is converged by the converging component, and the transmitted light from the plane grating is selected by the aperture, and these transmitted lights are made to undergo multi-beam interference. As the refractive index of the sample changes, the interference fringes drift;

[0017] S4: Use the spectrum analysis system to detect the drift of the interference fringes and measure the refractive index of the sample to be tested.

[0018] Compared with the prior art, the present invention has the following beneficial effects:

[0019] The refractive index sensor based on a grating-prism system and its measurement method described in the present invention use a plane grating to periodically block incident parallel light, splitting the parallel light into sub-beams. Each order of diffracted light of each sub-beam is regarded as a wave band. After the sub-beams propagate through a prism and a sample to be measured, the same-order wave bands of any two adjacent sub-beams have the same phase delay difference. These sub-beams are converged by a converging lens, and the diffracted light orders of the plane grating are selected by an aperture to construct multi-beam interference, thereby achieving low-cost, high-sensitivity, and high-resolution refractive index measurement.

[0020] The present invention utilizes a combination of a planar grating and a prism, effectively replacing a staircase-like transmissive echelle grating. While significantly reducing production costs compared to staircase-like transmissive echelle gratings, it still achieves high-sensitivity, high-resolution refractive index sensing, promoting the application and widespread adoption of high-precision refractive index sensors in the fields of physics, chemistry, and biomass measurement. This solves problems encountered in existing technologies. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 A schematic diagram of the connection of a refractive index sensor based on a grating-prism system according to the present invention;

[0022] Figure 2 A propagation path diagram of the refractive index sensor based on the grating-prism system of the present invention;

[0023] Figure 3 This is a schematic diagram of the optical path difference generated when diffracted light propagates through a prism and a sample to be measured in a refractive index sensor based on a grating-prism system of the present invention;

[0024] Figure 4 This is an interference fringe pattern when the refractive index sensor based on the grating-prism system of the present invention measures the refractive index of salt water;

[0025] Figure 5 This is a graph showing that when the refractive index sensor based on the grating-prism system of the present invention measures the refractive index of salt water, as the refractive index of the sample increases, the interference fringes drift toward shorter wavelengths;

[0026] In the figure: 1. Parallel light; 2. Plane grating; 3. Sample cell; 4. Sample to be measured; 5. Prism; 6. Converging component; 7. Spectral analysis system. DETAILED DESCRIPTION

[0027] The present invention will be further described below with reference to the accompanying drawings and embodiments:

[0028] Example 1:

[0029] like Figure 1 As shown, the refractive index sensor based on the grating-prism system described in the present invention includes parallel light 1, a plane grating 2, a sample cell 3, a converging component 6 and a spectral analysis system 7 arranged in sequence. A prism 5 and a sample to be measured 4 are provided in the sample cell 3. The parallel light 1 propagates through the plane grating 2, which divides the parallel light 1 into multiple sub-beams. Each level of diffraction light of each sub-beam is regarded as a wave band. After the sub-beams propagate through the prism 5 and the sample to be measured 4, the same-level sub-wave bands of any two adjacent sub-beams have the same phase delay difference, and then converge through the converging component 6 to reach the spectral analysis system 7 for refractive index measurement.

[0030] In this embodiment, the plane grating 2 is a transmissive amplitude-modulated grating, and the plane grating 2 is formed by a series of light-transmitting and light-impermeable structures arranged periodically.

[0031] In this embodiment, the plane grating 2 and the prism 5 form a multi-beam interference system.

[0032] In this embodiment, the parallel light 1 is broadband light or laser.

[0033] In this embodiment, the sample cell 3 has two light-transmitting surfaces at the front and rear, and the parallel light 1 passes through the two surfaces in sequence.

[0034] In this embodiment, the spectrum analysis system 7 is a spectrometer or an optical power meter.

[0035] In this embodiment, the spectrum analysis system 7 is provided with an aperture, which cooperates with the converging component 6 to select the diffracted light order of the plane grating 2 .

[0036] In this embodiment, the converging component 6 is a converging lens.

[0037] Figure 2 The light propagation path diagram of the grating-prism system in the present invention is as follows: parallel light 1 is incident on the plane grating 2. As a preferred embodiment, Figure 2 Taking the zero-order diffraction light of a plane grating as an example, the working principle of the grating-prism system is introduced.

[0038] from Figure 2 It can be seen that the zero-order diffracted light from the slits at different positions of the plane grating 2 has different optical path lengths in the prism 5 and the sample 4 to be measured, and therefore has different phase delays after passing through the prism 5 and the sample 4 to be measured.

[0039] Figure 3 This is a schematic diagram illustrating the optical path difference generated when analyzing the zero-order diffracted light in two adjacent beams of light propagating through a prism and a sample under test. The grating constant of the plane grating in the figure is d. When the zero-order diffracted light enters the prism, the refraction angle is θ1. In prism 5, the distance between the centers of the two adjacent zero-order diffracted light beams is d1, and the refractive index of the prism is n1. When the zero-order diffracted light enters the hypotenuse of the prism, the incident angle is θ2, and the refraction angle is θ3. In the sample under test, the distance between the centers of the two adjacent zero-order diffracted light beams is d2, and the refractive index of the sample under test is n2.

[0040] from Figure 3 It can be seen that for the zero-order diffraction light of any two adjacent sub-beams, when they propagate through the prism-to-be-tested sample, the optical path difference generated can be expressed as formula (1):

[0041] n1d1(tanθ1+tanθ2)-n2d2tanθ3(1)

[0042] Similar to the Fresnel half-wave band approximation method, the present invention regards the zero-order diffraction light at all positions of a slit as a whole, which is equivalent to a wave band. The wave band propagates through the prism 5 and the sample to be measured 4 in sequence, and is finally superimposed to form the resultant electric field of a single wave band.

[0043] It can be seen from formula (1) that the resultant electric field of any two adjacent wavebands has the same phase delay difference.

[0044] Therefore, the system composed of the plane grating 2 and the prism 5 functions similarly to a transmissive echelle grating with a stair-like structure.

[0045] Assuming that the effective period number N of the plane grating is 30, the period d is 37 μm, and the slit width a is 18.5 μm, preferably, a prism is made of quartz, and the prism is an isosceles right-angle prism with both acute angles being α=45°.

[0046] When the sample to be tested is salt water with a volume fraction of 0.2%, the multiple beams of light emitted from the sample to be tested are converged by a converging lens and the high-order diffraction light is filtered out by the aperture. The interference fringes obtained are as follows: Figure 4 As shown in I(λ), it can be seen that the interference fringes have a small half-height width, so that good peak wavelength resolution can be obtained.

[0047] Furthermore, since the period of the plane grating 2 is very uniform, the fringes formed by the interference of the transmitted light of the grating-prism system have a good signal-to-noise ratio, making the peak wavelength resolution of the interference fringes better than 0.01nm. When the salinity of the salt water changes by 1%, the interference fringes are as follows Figure 5 As shown by II(λ) in the figure, the interference fringes shift 12.5nm to shorter wavelengths with the increase of sample salinity (refractive index). The estimated refractive index measurement sensitivity is about 1.042×10 4 nm / RIU.

[0048] Therefore, the refractive index sensor based on the grating-prism system designed by the present invention has a refractive index measurement limit of less than 10 due to its high sensitivity and high resolution. -6 RIU.

[0049] Example 2:

[0050] The method for measuring a refractive index sensor based on a grating-prism system according to the present invention comprises the following steps:

[0051] S1: Parallel light 1 propagates through plane grating 2 and is split into multiple sub-beams by plane grating 2. Each diffraction level of each sub-beam is regarded as a wave band.

[0052] S2: The sub-beam propagates through the prism 5 and the sample 4 to be measured; the same-order sub-bands of any two adjacent sub-beams have the same phase delay difference;

[0053] S3: The transmitted light from the sample cell 3 is converged by the converging component 6, and the transmitted light from the plane grating 2 is selected by the aperture, and these transmitted lights are made to undergo multi-beam interference. As the refractive index of the sample changes, the interference fringes drift;

[0054] S4: Detect the drift of the interference fringes using the spectrum analysis system 7 to measure the refractive index of the sample 4 to be measured.

[0055] Based on Example 1, this example further describes a measurement method for a refractive index sensor based on a grating-prism system. This method achieves high-resolution, high-sensitivity, and wide-dynamic-range refractive index measurement at a low cost, resolving the problems encountered in the prior art.

[0056] The basic principles, main features, and advantages of the present invention are shown and described above. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The above embodiments and descriptions are merely illustrative of the principles of the present invention. Various changes and modifications may be made to the present invention without departing from the spirit and scope of the present invention. Such changes and modifications are intended to fall within the scope of the present invention. The scope of protection claimed in the present invention is defined by the appended claims and their equivalents.

Claims

1. A refractive index sensor based on a grating-prism system, characterized in that: The invention comprises parallel light (1), a plane grating (2), a sample pool (3), a converging component (6) and a spectrum analysis system (7) arranged in sequence, wherein a prism (5) and a sample to be measured (4) are provided in the sample pool (3), the parallel light (1) propagates through the plane grating (2), and the plane grating (2) divides the parallel light (1) into multiple sub-beams, and each level of diffracted light of each sub-beam is regarded as a wave band. After the sub-beams propagate through the prism (5) and the sample to be measured (4), the same-level sub-bands of any two adjacent sub-beams have the same phase delay difference, and the sub-beams are then converged by the converging component (6) and arrive at the spectrum analysis system (7) for refractive index measurement; the plane grating (2) and the prism (5) constitute a multi-beam interference system.

2. The refractive index sensor based on the grating-prism system according to claim 1, characterized in that: The plane grating (2) is a transmissive amplitude modulation grating, and the plane grating (2) is formed by a series of light-transmitting and light-impermeable structures arranged periodically.

3. The refractive index sensor based on the grating-prism system according to claim 1, characterized in that: The parallel light (1) is broadband light or laser.

4. The refractive index sensor based on the grating-prism system according to claim 1, characterized in that: The sample pool (3) has two light-transmitting surfaces at the front and rear, and the parallel light (1) passes through the two surfaces in sequence.

5. The refractive index sensor based on the grating-prism system according to claim 1, characterized in that: The spectrum analysis system (7) is a spectrometer or an optical power meter.

6. The refractive index sensor based on the grating-prism system according to claim 1, characterized in that: The spectrum analysis system (7) is provided with an aperture, which cooperates with the converging component (6) to select the diffracted light order of the plane grating (2).

7. The refractive index sensor based on the grating-prism system according to claim 1, characterized in that: The converging component (6) is a converging lens.

8. A method for measuring a refractive index sensor based on a grating-prism system according to any one of claims 1 to 7, characterized in that: The method comprises the following steps: S1: Parallel light propagates through a plane grating and is split into multiple sub-beams by the plane grating. Each level of diffraction light of each sub-beam is regarded as a wave band. S2: The sub-beam propagates through the prism and the sample to be measured; the same-order sub-bands of any two adjacent sub-beams have the same phase delay difference; S3: The sub-beams are converged by the converging component, and the transmitted light of the plane grating is selected by the aperture, and these transmitted lights are made to undergo multi-beam interference. As the refractive index of the sample changes, the interference fringes drift; S4: Use the spectrum analysis system to detect the drift of the interference fringes and measure the refractive index of the sample to be tested.

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