A method for preparing a stepped waveguide microcavity refractive index sensor

Abstract

The present invention discloses a method for preparing a stepped waveguide microcavity refractive index sensor, comprising the following steps: (1) using femtosecond laser writing technology to write a straight single-mode waveguide in a transparent substrate material; and (2) using micromachining technology to prepare a stepped microcavity on one side of the waveguide region. The stepped microcavity structure consists of two rectangular microcavities of different lengths and widths, the length directions of the two microcavities being along the waveguide transmission direction, and the two rectangular cross-sections perpendicular to the waveguide transmission direction each covering a portion of the waveguide mode field, while the sum of the two rectangular cross-sections does not cover the entire waveguide mode field. Compared with existing waveguide microcavity interferometric sensors, this method can overcome the FSR limitation on the refractive index detection range, thereby accurately reading the actual refractive index value of the liquid within a larger refractive index variation range.

Description

Technical Field

[0001] The invention belongs to the field of optical sensing, and in particular relates to a method for preparing a stepped waveguide microcavity Mach-Zehnder sensor. Background Art

[0002] Waveguide microcavity Mach-Zehnder interferometry sensors (hereinafter referred to as waveguide microcavity sensors) are a sensor structure that has gained increasing attention in recent years. This structure etches a portion of a waveguide into a microcavity, splitting the signal into two parts. The two parts are transmitted through the microcavity and the remaining waveguide, and then converge at the output end, thereby achieving interference and detecting the refractive index changes of the material in the microcavity. Advantages of this sensor include compact structure and high sensitivity. This structure can be integrated into both planar waveguides and optical fibers. In recent years, the development of waveguide microcavity sensors based on optical fiber structures has been particularly noteworthy.

[0003] The greatest advantage of waveguide microcavity sensors lies in their extremely high sensitivity. However, because the basic principle of this type of sensor is two-beam interference, this high sensitivity is often accompanied by a smaller refractive index detection range. This is because the transmission spectrum of this type of sensor is a simple periodic oscillation spectrum. Whenever the spectral drift of the resonance peak reaches an integer multiple of the free spectral range (FSR), the spectrum will overlap with the original spectrum, making it impossible to determine the exact drift of the resonance peak.

[0004] However, in practical applications, an excessively large FSR is unrealistic for the following reasons: First, it makes it difficult to capture the reference resonance peak within the emission bandwidth of the broadband light source; second, it increases the 3dB bandwidth of the interference peak, resulting in a decrease in measurement resolution. In other words, in actual detection, this type of sensor can accurately measure refractive index changes within a narrow refractive index range, but cannot accurately measure the refractive index of any liquid being tested. This is a significant limitation that restricts the practical application of this type of sensor. Summary of the Invention

[0005] To overcome the shortcomings of existing technologies, the present invention provides a design and fabrication method for a stepped waveguide microcavity sensor. This sensor structure can be integrated into planar waveguides and optical fibers. Compared to existing waveguide microcavity sensors, it significantly expands the refractive index measurement range, enabling accurate measurement of the refractive index of liquids.

[0006] In order to solve the above technical problems, the present invention provides the following technical solutions:

[0007] A method for preparing a stepped waveguide microcavity interferometric refractive index sensor comprises the following steps:

[0008] (1) Using femtosecond laser writing technology, straight single-mode waveguides are written in transparent substrate materials;

[0009] (2) Micromachining technology is used to prepare a stepped microcavity on one side of the waveguide area. The stepped microcavity structure consists of two rectangular microcavities with different lengths and widths. The length directions of the two microcavities are along the waveguide transmission direction. The two rectangular cross-sections perpendicular to the waveguide transmission direction each cover a part of the waveguide mode field, and the sum of the two rectangular cross-sections does not cover the entire waveguide mode field.

[0010] Preferably, the stepped microcavity structure needs to penetrate the outer surface of the waveguide so that the space inside the cavity is exposed to the external environment.

[0011] Preferably, the length direction of the stepped microcavity is parallel to the waveguide transmission direction, wherein the rectangular microcavity with a longer length is located outside the waveguide, and the rectangular microcavity with a shorter length is located inside the waveguide.

[0012] Preferably, in step (1), when the transparent substrate material is replaced with a single-mode optical fiber, there is no need to additionally write a straight waveguide.

[0013] The application of the microcavity interferometer obtained by the above method in the preparation of optical fiber sensors.

[0014] Compared with the prior art, the present invention has the following beneficial effects:

[0015] Since the resonance peaks in the interference spectrum become easier to identify, compared with the existing waveguide microcavity interferometer sensor, it can break through the FSR's limitation on the refractive index detection range, so that the actual refractive index value of the liquid can be accurately read within a larger refractive index change range. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 Schematic diagram of the three-dimensional structure of the stepped waveguide microcavity sensor of the present invention.

[0017] Figure 2 This is a top view schematic diagram of the stepped waveguide microcavity sensor of the present invention, where arrows indicate the respective transmission paths of the three beams of light involved in the interference.

[0018] Figure 3 This is a typical transmission spectrum of the stepped waveguide microcavity sensor of the present invention (D1=2.5 μm, D2=3.5 μm, L1=120 μm, L2=48 μm, and the refractive index of the liquid is 1.35).

[0019] Figure 4 The schematic diagram of the sensing effect of the stepped waveguide microcavity sensor of the present invention is shown in FIG. Figure 3 same;

[0020] Figure 5The schematic diagram of the structure of the quartz glass waveguide microcavity refractive index sensor based on the traditional scheme;

[0021] Figure 6 This is a typical transmission spectrum of a quartz glass waveguide microcavity refractive index sensor based on a traditional solution (L1=120 μm, D1=4.5 μm, liquid refractive index of 1.35);

[0022] Figure 7 Schematic diagram of the sensing effect of the quartz glass waveguide microcavity refractive index sensor based on the traditional scheme, where the structural parameters are Figure 6 same.

[0023] Figure 8 The schematic diagram (top view) of a YAG crystal dual-waveguide microcavity refractive index sensor includes: 1. Waveguide region; 2. Waveguide sample substrate or fiber cladding; 3. Microcavity; 4. Writing region. DETAILED DESCRIPTION

[0024] It should be noted that the following detailed descriptions are illustrative and intended to provide further explanation of the present application. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which the present application belongs.

[0025] Embodiment 1:

[0026] The preparation method of the optical fiber step-type microcavity refractive index sensor comprises the following steps:

[0027] (1) A section of single-mode quartz optical fiber with a cladding diameter of 125 μm was stripped of its coating and its surface was cleaned. A microcavity interference structure was constructed based on the existing optical fiber core without the need for additional waveguide preparation.

[0028] (2) Using femtosecond laser writing technology, a microcavity 3 was written in the core of a quartz optical fiber. The dimensions of the microcavity were: D1 = 2.5 μm, D2 = 3.5 μm, L1 = 120 μm, and L2 = 48 μm. The laser writing parameters used were as follows: the central wavelength of the femtosecond laser was 520 nm; the repetition rate was 200 kHz; the pulse energy was 0.6 μJ; the scanning speed was 200 μm / s; and the numerical aperture of the microscope objective was 0.75.

[0029] (3) The microcavity area written by the femtosecond laser was corroded using a 10% HF aqueous solution. The chemical etching conditions were: room temperature, immersion of the quartz glass in the HF solution for 10 minutes, and assisted by ultrasonic oscillation.

[0030] Example 2:

[0031] The preparation method of the quartz glass stepped waveguide microcavity refractive index sensor comprises the following steps:

[0032] (1) Using femtosecond laser writing technology, a straight waveguide was written at a depth of 80 μm inside the quartz glass material. The laser writing parameters used were as follows: the central wavelength of the femtosecond laser was 520 nm; the repetition rate was 200 kHz; the pulse energy was 0.6 μJ; the scanning speed was 200 μm / s; and the numerical aperture of the microscope objective was 0.75.

[0033] (2) Using femtosecond laser writing technology, a microcavity 3 was written on the surface of the quartz glass material. The dimensions of the microcavity were: depth 80 μm, D1 = 2 μm, D2 = 3 μm, L1 = 120 μm, and L2 = 48 μm. The laser writing parameters used were as follows: the central wavelength of the femtosecond laser was 520 nm; the repetition rate was 200 kHz; the pulse energy was 0.6 μJ; the scanning speed was 200 μm / s; and the numerical aperture of the microscope objective was 0.75.

[0034] (3) The microcavity area inscribed by the femtosecond laser is etched using a 10% HF aqueous solution. The chemical etching conditions are: room temperature, immersion of the quartz glass in the HF solution for 10 minutes, and assisted by ultrasonic oscillation. To prevent the chemical etching process from damaging the waveguide structure, in actual operation, the waveguide writing process in step 1 can also be adjusted to be carried out after steps 2 and 3.

[0035] Example 3:

[0036] The preparation method of the YAG crystal stepped waveguide microcavity refractive index sensor comprises the following steps:

[0037] (1) Using femtosecond laser writing technology, a double-line waveguide was written at a depth of 80 μm inside the YAG crystal material. The center distance between the double-line waveguides was 20 μm. The laser writing parameters used were as follows: the central wavelength of the femtosecond laser was 520 nm; the repetition rate was 20 kHz; the pulse energy was 0.3 μJ; the scanning speed was 50 μm / s; and the numerical aperture of the microscope objective was 0.5.

[0038] (2) Using femtosecond laser writing technology, a microcavity 3 was written on the surface of the YAG crystal material. The dimensions of the microcavity were: depth 80 μm, D1 = 7 μm, D2 = 8 μm, L1 = 120 μm, and L2 = 48 μm. The laser writing parameters used were as follows: the central wavelength of the femtosecond laser was 520 nm; the repetition rate was 20 kHz; the pulse energy was 0.3 μJ; the scanning speed was 50 μm / s; and the numerical aperture of the microscope objective was 0.5.

[0039] (3) The microcavity region inscribed by the femtosecond laser is etched using a 10% HF aqueous solution. The chemical etching conditions are: room temperature, immersion of the crystal in the HF solution for 6 minutes, and assisted by ultrasonic oscillation. To prevent the chemical etching process from damaging the waveguide structure, in actual operation, the waveguide writing process in step 1 can also be adjusted to be carried out after steps 2 and 3.

[0040] Its principles and features are:

[0041] like Figure 2 As shown by the three arrows, the stepped microcavity design is equivalent to splitting the incident waveguide power into three parts: the first portion of the power travels through a cavity of length L1+L2, the second portion travels through a cavity of length L1, and the third portion travels through the remaining waveguide. Therefore, the sensor's interference effect stems from the interference of the three beams, modulating the transmission spectrum and making the individual interference peaks easier to identify. Furthermore, since the effective refractive index of the first two portions of power changes with the measured liquid at the same amplitude during microcavity transmission, the overall transmission spectrum drifts while the spectral patterns of the individual interference peaks remain unchanged during changes in the liquid's refractive index. Therefore, the drift of a characteristic interference peak can be identified over a wide range, enabling accurate readings of the liquid's refractive index.

[0042] For the quartz glass waveguide, when D1=2.5 μm, D2=3.5 μm, L1=120 μm, and L2=48 μm, the transmission spectrum of the prepared step-type microcavity sensor is calculated theoretically under the condition of a liquid refractive index of 1.35, as shown in the attached figure. Figure 3 As shown in Figure 2, the resonance peak near 1440 nm has a very high degree of recognition. When the refractive index of the liquid changes, the corresponding change in the transmission spectrum is as follows: Figure 4 As shown, it can be seen that when the refractive index of the liquid changes over a large range from 1.34 to 1.36, the actual refractive index value of the liquid can still be accurately measured by reading the position of the characteristic resonance peak.

[0043] For comparison, the structure and effect diagram of the quartz glass waveguide microcavity interference sensor based on the traditional solution are listed as a comparison. Its basic structure is as follows Figure 5 As shown. Here, L1 is still set to 120 μm, and D1 is set to 4.5 μm. Under the condition of liquid refractive index of 1.35, its transmission spectrum is as follows Figure 6 As shown in the figure, the interference spectrum shows periodic oscillation characteristics, and the shapes of the resonance peaks have no obvious differences and do not have identifiable characteristics. Therefore, it can only measure the refractive index changes in a small range and cannot measure the actual refractive index of any liquid sample. The reason is that when the drift of the resonance peak increases by one FSR, the interference spectrum will overlap with the original spectrum, making it impossible to identify the exact spectral drift. For example, Figure 7As shown in FIG, when the refractive index of the liquid changes from 1.345 to 1.355, the drift of the resonance peak exceeds the FSR, which makes it impossible to accurately distinguish the peak position drift, that is, the actual size of the refractive index of the liquid cannot be accurately measured.

Claims

1. A method for preparing a stepped waveguide microcavity refractive index sensor, characterized in that: Here are the steps: (1) Using femtosecond laser writing technology, straight single-mode waveguides are written in transparent substrate materials; (2) Micromachining technology is used to prepare a stepped microcavity on one side of the waveguide area. The stepped microcavity structure consists of two rectangular microcavities with different lengths and widths. The length directions of the two microcavities are along the waveguide transmission direction. The two rectangular cross-sections perpendicular to the waveguide transmission direction each cover a part of the waveguide mode field, and the sum of the two rectangular cross-sections does not cover the entire waveguide mode field.

2. The preparation method according to claim 1, wherein The stepped microcavity structure needs to penetrate the outer surface of the waveguide so that the space inside the cavity is exposed to the external environment.

3. The preparation method according to claim 1, wherein The length direction of the stepped microcavity is parallel to the waveguide transmission direction, wherein the rectangular microcavity with a longer length is located outside the waveguide, and the rectangular microcavity with a shorter length is located inside the waveguide.

4. The preparation method according to claim 1, wherein In step (1), when the transparent substrate material is replaced with a single-mode optical fiber, there is no need to additionally write a straight waveguide.

5. Use of the microcavity interferometer obtained according to the method of any one of claims 1 to 4 in the preparation of optical fiber sensors.

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