A method for preparing a waveguide microcavity Mach-Zehnder interferometer refractive index sensor

Abstract

This invention discloses a method for fabricating a waveguide microcavity Mach-Zehnder interferometer refractive index sensor. The method comprises the following steps: using femtosecond laser inscription technology, a waveguide and a microcavity are inscribed in a transparent substrate material. The waveguide consists of a linear waveguide and branched linear waveguides; the microcavity's cross-section covers the branched linear waveguide's mode field; the linear waveguide must maintain structural integrity and have no spatial overlap with the microcavity. Compared to traditional microcavity Mach-Zehnder sensors, this method offers higher sensitivity and is simpler to fabricate. The resulting device exhibits improved mechanical strength and process tolerance.

Description

Technical Field

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

[0002] Waveguide microcavity Mach-Zehnder refractive index sensors are typically created by etching a microcavity into a portion of a waveguide. The fundamental mode power of the waveguide is split into two, transmitting one signal in the microcavity and the other in the remaining waveguide. When the two signals reunite at the waveguide's output, interference occurs. When a broadband light source is used as the incident signal, changes in the refractive index of the liquid in the microcavity cause a shift in the peak position of the transmission spectrum at the output, enabling refractive index sensing.

[0003] Currently, microcavity Mach-Zehnder sensors are widely used in fiber optic sensing. Their high refractive index sensitivity is typically higher than that of common interferometric sensors such as micro-nano fiber optic sensors and fiber FP cavity sensors. However, traditional microcavity Mach-Zehnder sensors still have some shortcomings, mainly manifested as follows: 1. Because the fiber core is located at the bottom of the microcavity, it is usually difficult to ensure that the sidewalls of the microcavity at this location are steep and have low surface roughness, making it difficult to guarantee the extinction ratio and quality factor of the transmission spectrum; 2. Because the power ratio of the two signals is heavily dependent on the depth and bottom morphology of the groove, adjusting the extinction ratio is difficult, and the repeatability of device fabrication is poor; 3. Due to the limitations of the interference principle, the refractive index sensitivity of this type of sensor is essentially determined by the refractive index of the waveguide material and the liquid, and cannot be further improved; 4. Grooving near the fiber core often leads to a significant decrease in the mechanical properties of the fiber, making it brittle and easily breakable. Summary of the Invention

[0004] To overcome the shortcomings of existing technologies, the present invention provides a novel design and fabrication method for a microcavity Mach-Zehnder interferometer sensor. The sensor structure described in this solution can be integrated into planar waveguide devices and optical fibers. Compared to traditional microcavity Mach-Zehnder sensors, it exhibits higher sensitivity and is simpler to fabricate. The resulting device exhibits improved mechanical strength and process tolerances.

[0005] In order to solve the above technical problems, the present invention provides the following technical solutions:

[0006] A method for preparing a waveguide microcavity Mach-Zehnder interferometer refractive index sensor comprises the following steps: using femtosecond laser writing technology to write a waveguide and a microcavity in a transparent substrate material, wherein the waveguide consists of a linear waveguide and a branched linear waveguide; the cross section of the microcavity covers the mode field of the branched linear waveguide; the linear waveguide must maintain structural integrity and have no spatial overlap with the microcavity.

[0007] Priority solution: When the transparent substrate material is replaced with a single-mode optical fiber, no additional linear waveguide inscription is required.

[0008] Preferred solution: The branch linear waveguide consists of three parts: a front-end bridging waveguide, a branch linear waveguide, and a rear-end bridging waveguide. The branch linear waveguide runs parallel to the linear waveguide, and the front-end bridging waveguide and the rear-end bridging waveguide are used to connect the linear waveguide and the branch linear waveguide.

[0009] Preferred solution: The bridging regions at both ends of the branched linear waveguide are S-shaped curved waveguide structures or linear waveguide structures.

[0010] The Mach-Zehnder interferometer obtained by the above preparation method is used in the preparation of optical fiber sensors.

[0011] Compared with the prior art, the present invention has the following beneficial effects:

[0012] 1. When used for liquid refractive index sensing, it can achieve higher detection sensitivity than traditional microcavity Mach-Zehnder sensors;

[0013] 2. In the present invention, since the microcavity completely penetrates the branch waveguide, the waveguide (or optical fiber) mode energy will pass through the relatively steep sidewalls of the microcavity and stay away from the bottom of the microcavity. Therefore, there is no need to precisely control the depth of the microcavity, which has better process tolerance and repeatability. In addition, it is also easy to obtain lower loss and larger interference extinction ratio.

[0014] 3. When this structure is used in optical fiber, the required microcavity etching depth is relatively small (within 30 μm), while the etching depth required for traditional microcavities is about 62 μm. Therefore, it can reduce the damage to the mechanical properties of the optical fiber, making the entire optical fiber sensor structure more stable and less prone to breakage. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 Schematic diagram of the structure of the ultra-high-sensitivity microcavity Mach-Zehnder sensor of the present invention;

[0016] Figure 2 This is the expected effect (transmission spectrum) of the ultra-high-sensitivity microcavity Mach-Zehnder sensor of the present invention, which is obtained using the double-beam interference theory;

[0017] Figure 3 Schematic diagram of the structure of a coreless optical fiber microcavity Mach-Zehnder interferometer sensor;

[0018] Among them: 1. Transparent substrate material; 2. Linear waveguide; 3. Front-end bridge waveguide; 4. Branched linear waveguide; 5. Back-end bridge waveguide; 6. Microcavity; 7. Single-mode optical fiber; 8. Optical fiber core. DETAILED DESCRIPTION

[0019] 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.

[0020] Example 1

[0021] Preparation method of microcavity Mach-Zehnder interferometer sensor in quartz glass substrate:

[0022] Step A: Cut the quartz glass into substrates of 10 (x) × 10 (y) × 2 (z) mm, and perform optical polishing on two xz planes and two xy planes;

[0023] Step B: Using femtosecond laser writing technology, a microcavity 6 was written on the surface of the quartz glass substrate. The dimensions of the microcavity were: z-depth 110 μm, y-cavity length 150 μm, and x-cavity width 30 μm. The laser writing parameters used were as follows: femtosecond laser central wavelength 520 nm; repetition rate 200 kHz; pulse energy 0.6 μJ; scanning speed 200 μm / s; and numerical aperture of the microscope objective lens 0.75.

[0024] Step C: Etching the microcavity area inscribed by the femtosecond laser using a 10% HF aqueous solution. The chemical etching conditions are: room temperature, soaking the quartz glass in the HF solution for 10 minutes, and assisting with ultrasonic vibration.

[0025] Step D: Using femtosecond laser writing technology, a linear waveguide 2 and branch waveguide structures (including a front-end bridge waveguide 3, a branch linear waveguide 4, and a rear-end bridge waveguide 5) are inscribed. All waveguides are located 80 μm below the surface. The linear waveguide 2 and the branch linear waveguide 4 extend in the y-direction, and the branch linear waveguide 4 is interrupted by the microcavity 6. The spacing between the branch linear waveguide 4 and the linear waveguide 2 is 140 μm. The angle between the two bridge waveguides and the linear waveguide 2 is 2°. The length of the branch linear waveguide 4 is 500 μm.

[0026] The femtosecond laser writing parameters are as follows: central wavelength 520 nm; repetition rate 500 kHz; pulse energy 130 nJ; scanning speed 200 μm / s; and an oil immersion objective with a numerical aperture of 1.25.

[0027] Example 2

[0028] Preparation method of microcavity Mach-Zehnder interferometer sensor in single-mode optical fiber:

[0029] Step A: Strip the coating of a 125 μm diameter single-mode silica optical fiber and clean the surface.

[0030] Step B: Using femtosecond laser writing technology, a microcavity 6 was written on the surface of the single-mode optical fiber cladding. The microcavity dimensions were: 30 μm depth in the z-direction, 150 μm length in the y-direction, and 30 μm width in the x-direction. The laser writing parameters used were as follows: a central wavelength of the femtosecond laser of 520 nm; a repetition rate of 200 kHz; a pulse energy of 0.6 μJ; a scanning speed of 200 μm / s; and a numerical aperture of the microscope objective of 0.75.

[0031] Step C: Etching the microcavity region inscribed by the femtosecond laser using a 10% HF aqueous solution. The chemical etching conditions are: room temperature, soaking the optical fiber in the HF solution for 5 minutes, and assisting with ultrasonic vibration.

[0032] Step D: Using femtosecond laser writing technology, write the branch waveguide structure (including the front bridge waveguide 3, the branch linear waveguide 4, and the rear bridge waveguide 5); wherein the branch linear waveguide 4 extends along the y-direction and is interrupted by the microcavity 6; the distance between the branch linear waveguide 4 and the fiber core is 40 μm, the angle between the two bridge waveguides and the fiber core is 2°, and the length of the branch linear waveguide 4 is 1000 μm;

[0033] The femtosecond laser writing parameters are as follows: central wavelength 520 nm; repetition rate 500 kHz; pulse energy 130 nJ; scanning speed 200 μm / s; and an oil immersion objective with a numerical aperture of 1.25.

[0034] Example 3

[0035] like Figure 3 As shown, the preparation method of the coreless optical fiber microcavity Mach-Zehnder interferometer sensor comprises the following steps:

[0036] Step A: Remove the coating of a coreless quartz optical fiber with a diameter of 125 μm and clean the surface. Align the two ends and fuse them to a single-mode quartz optical fiber 7 with a diameter of 125 μm.

[0037] Step B: Using femtosecond laser writing technology, a microcavity 6 was written on the surface of the coreless optical fiber cladding. The microcavity dimensions were: z-depth 30 μm, y-cavity length 150 μm, and x-cavity width 30 μm. The laser writing parameters used were as follows: femtosecond laser central wavelength 520 nm; repetition rate 200 kHz; pulse energy 0.6 μJ; scanning speed 200 μm / s; and numerical aperture of the microscope objective lens 0.75.

[0038] Step C: Etching the microcavity region inscribed by the femtosecond laser using a 10% HF aqueous solution. The chemical etching conditions are: room temperature, soaking the optical fiber in the HF solution for 5 minutes, and assisting with ultrasonic vibration.

[0039] Step D: Using femtosecond laser writing technology, write the linear waveguide 2 and branch waveguide (including the front bridge waveguide 3, the branch linear waveguide 4, and the rear bridge waveguide 5) structure; wherein the branch linear waveguide 4 extends along the y direction and is cut off by the microcavity 6; the distance between the branch linear waveguide 4 and the optical fiber core 8 is 40 μm, and the length is 1000 μm; the angle 8 between the front and rear bridge waveguides and the optical fiber core is 2°, and each bridge waveguide adopts an S-shaped bend structure, which is formed by splicing two circular arcs, so that the two ends of the bridge waveguide are tangentially connected to the linear waveguide 2 and the branch linear waveguide 4;

[0040] The femtosecond laser writing parameters are as follows: central wavelength 520 nm; repetition rate 500 kHz; pulse energy 130 nJ; scanning speed 200 μm / s; and an oil immersion objective with a numerical aperture of 1.25.

[0041] The working mode of the sensor involved in the present invention is:

[0042] Using fiber or objective lens coupling, the signal from a broadband light source is coupled into the device from the front end of a linear waveguide. The waveguide mode signal is then collected at the rear end of the linear waveguide and input into a spectrometer. By immersing the liquid under test in the microcavity, the refractive index of the liquid can be determined by observing the peak position of the interference spectrum in the spectrometer.

[0043] Its specific working principle and characteristics are:

[0044] The optical path difference of the sensor involved in the present invention depends on the sum of the contributions of the microcavity and the waveguide. It is easy to draw the following conclusion: the refractive index sensitivity S of the device can be expressed as:

[0045]

[0046] in,

[0047] In the above two formulas, λ is the wavelength of the reference peak in the transmission spectrum; n eff is the effective refractive index of the waveguide (or fiber core); n s is the refractive index of the liquid to be measured; ΔL is the geometric length difference between the branch waveguide (including the aforementioned front-end bridge waveguide, branch linear waveguide, and rear-end bridge waveguide) and the main waveguide (the aforementioned linear waveguide); L2 is the length of the microcavity.

[0048] In contrast, the sensitivity S of the traditional waveguide microcavity Mach-Zehnder interferometer sensor is determined only by the optical path difference change of the microcavity part, and its value is:

[0049]

[0050] It has nothing to do with the specific design parameters and cavity length of the waveguide structure.

[0051] By comparison, it can be seen that the sensor scheme proposed in this application can obtain higher refractive index detection sensitivity than traditional microcavity sensors by reasonably selecting device structural parameters (such as the separation angle of the branch waveguide, the distance from the straight waveguide, the length of the microcavity, etc.).

[0052] For example, the sensitivity of a conventional optical fiber microcavity sensor near a refractive index of 1.33 is approximately 12,500 nm / RIU (observed near 1,500 nm). However, the novel microcavity sensor proposed in this application, when the cavity length is designed to be 150 μm, the separation angle of the two bridge waveguides relative to the main axis is 2 degrees, and the distance between the branch straight waveguide and the straight waveguide is 140 μm, has a sensitivity near a refractive index of 1.33 of approximately 20,620 nm / RIU (observed near 1,500 nm). This is a sensitivity improvement of approximately 65% compared to conventional microcavity sensors. The specific effect (transmission spectrum) is shown in the attached figure. Figure 2 shown.

Claims

1. A method for preparing a waveguide microcavity Mach-Zehnder interferometer refractive index sensor, characterized by: The steps are as follows: using femtosecond laser writing technology, writing a waveguide and a microcavity in a transparent substrate material, wherein the waveguide consists of a linear waveguide and a branched linear waveguide; the cross section of the microcavity covers the mode field of the branched linear waveguide; the linear waveguide must maintain structural integrity and have no spatial overlap with the microcavity; The branch linear waveguide is composed of three parts: a front-end bridge waveguide, a branch linear waveguide, and a rear-end bridge waveguide. The branch linear waveguide runs parallel to the linear waveguide, and the front-end bridge waveguide and the rear-end bridge waveguide are used to connect the linear waveguide and the branch linear waveguide. The bridging regions at both ends of the branch linear waveguide are S-shaped curved waveguide structures or linear waveguide structures.

2. The preparation method according to claim 1, wherein: When the transparent substrate material is replaced with a single-mode optical fiber, there is no need to additionally write a linear waveguide.

3. Use of the Mach-Zehnder interferometer obtained according to the preparation method according to any one of claims 1-2 in the preparation of optical fiber sensors.

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