Double-core optical fiber integrated MZ interferometer based on thermal diffusion

Abstract

The invention discloses a double-core optical fiber integrated MZ interferometer based on thermal diffusion. The interferometer is mainly characterized in that a core structure of the interferometer is formed by sequentially welding an input optical fiber (1), a first double-cladding transition optical fiber (2), a double-core optical fiber (3), a second double-cladding transition optical fiber (4) and an output optical fiber (5). Thermal diffusion treatment is performed by long-term high-temperature heating in the welding regions (A) and (B). The thermal diffusion treatment enables fiber core doping elements to be diffused, a transition area is formed in the double-clad optical fiber, the coupling efficiency between the single-mode optical fiber and the double-core optical fiber is remarkably improved, and compared with a traditional tapering structure, the thermal diffusion area is higher in mechanical strength. The interferometer device is compact in structure, high in integration level and excellent in interference contrast, can realize high-precision phase sensitive sensing, is suitable for distributed measurement of physical quantities such as temperature, bending and refractive index, and can be widely applied to optical fiber sensing.

Description

Technical Field

[0001] The present invention relates to the technical field of optical fiber interferometers, and in particular to an optical fiber integrated MZ interferometer based on thermal diffusion. Background Art

[0002] A Mach-Zehnder interferometer is an optical device based on the principle of two-beam interference. Its operating principle is that incident light is split into two light waves by a beam splitter and transmitted along different spatially separated paths. These different transmission paths result in an optical path difference between the light waves, which in turn creates a phase difference. When the two light waves converge and superimpose in a beam combiner, this phase difference determines the interference effect, forming periodic interference fringes with a specific intensity distribution at the output.

[0003] The core advantages of the Mach-Zehnder interferometer lie in its extremely high phase sensitivity, strong resistance to electromagnetic interference, compact structure and easy full-fiber integration. These characteristics make it an ideal tool in the fields of precision sensing and optical signal processing. In terms of sensing, it is widely used to measure small changes in physical quantities with high precision, such as temperature, bending, pressure, refractive index, vibration and sound waves. It has extremely high sensitivity and is particularly suitable for distributed fiber optic sensing systems in harsh electromagnetic environments. In the field of optical devices, it is often used as a key component in optical filters, optical switches, optical modulators and optical wavelength division multiplexers / demultiplexers. It plays an important role in optical fiber communications and optical signal processing systems by precisely controlling the optical path difference or phase of the interferometer arms to achieve effective control of the amplitude, phase or wavelength of the optical signal.

[0004] Traditional fiber-based MZ interferometers have significant technical shortcomings in terms of coupling architecture and functional implementation. While biconical structures offer the advantage of simple fabrication, this structure deforms the fiber, and the long fiber length between the biconical segments results in reduced sensitivity in sensing scenarios, limiting their practicality. Replacing biconical structures with long-period fiber gratings (LPFGs) significantly increases costs due to their reliance on specialized grating writing equipment, and the centimeter-scale grating size makes it difficult to meet the demands of miniaturized deployment. MZ interferometers constructed with specialized optical fibers (such as polarization-maintaining fibers and photonic crystal fibers) are not only expensive to purchase, but also prone to introducing additional losses during the splicing process, significantly increasing the complexity of the process. There are other multi-core fiber coupling methods, such as spatial lens assemblies, three-dimensional waveguides, melt cones, fan-out devices, etc., which have inherent defects such as high insertion loss, large device size, and complex manufacturing process. After coupling and integrating the interferometer, they still need to face derivative problems such as suppressing inter-core signal crosstalk and regulating the matching of coupling structure and interference mode. The defects of these two types of technical links are superimposed on each other, which not only restricts the improvement of the core performance of the interferometer, but also further increases the R&D and mass production costs due to the complexity of the process, making it difficult to adapt to the technical demand of the optical sensing field for "low cost, high integration, and high reliability".

[0005] Patent publication number CN103267999A proposes an MZ interferometer based on a dumbbell-shaped fiber structure, achieving beam splitting and combining through an expanded core region and a tapered region. However, this structure requires a millimeter-scale tapered length, and the exposed tapered region is susceptible to external stress and temperature disturbances, resulting in reduced interferometric stability.

[0006] Patent publication number CN216206430U proposes an integrated interferometer that relies on discrete components such as beam splitters, Faraday plates, and mirrors, which restricts the miniaturization of the structure. In addition, UV glue is used to encapsulate the optical interface. Deformation of the glue layer under temperature-changing environments may cause the optical path to shift.

[0007] Patent publication number CN119024583A proposes a fiber optic structure and Kerr fiber interferometer based on the Kerr effect, which uses a Kerr effect liquid medium to control the optical path. However, it requires etching a microcavity in the fiber core and integrating multiple sets of electrodes, which poses risks to packaging reliability and process complexity.

[0008] Patent publication number CN120064211A proposes a Mach-Zehnder interferometer fiber optic sensor based on an arc-shaped cavity single-mode optical fiber. Multi-point monitoring is achieved by processing the arc-shaped cavity with a femtosecond laser. However, the cavity depth reaches 70% of the cladding, which seriously weakens the mechanical strength, and the femtosecond laser equipment is expensive.

[0009] The present invention discloses a dual-core fiber integrated MZ interferometer based on thermal diffusion, which solves the problems existing in previous fiber interferometers, such as large mode field mismatch loss, fragile mechanical structure, complex process and high cost, low integration and insufficient interference signal contrast. Through the design of fully fused interface-free structure and thermal diffusion gradient refractive index coupling zone, high-precision phase-sensitive sensing can be achieved, which is suitable for distributed measurement of physical quantities such as temperature, bending, and refractive index. Summary of the Invention

[0010] The purpose of the present invention is to provide a dual-core fiber integrated MZ interferometer based on heat diffusion technology and a double-clad fiber transition structure.

[0011] The object of the present invention is achieved like this:

[0012] The double-core fiber MZ interferometer is composed of an input fiber (1), a first double-clad transition fiber (2), a double-core fiber (3), a second double-clad transition fiber (4) and an output fiber (5). In the interferometer, the input fiber (1) is used to input an optical signal, the first double-clad transition fiber (2) has its input end fused to the output end of the input fiber (1), the double-core fiber (3) has its input end fused to the output end of the first double-clad transition fiber (2), and the second double-clad transition fiber (4) has its input end fused to the output end of the double-core fiber (3). ) is fused at the output end of the first double-clad transition fiber (2) and the output end of the second double-clad transition fiber (4), and the output fiber (5) is fused at its input end to the output end of the second double-clad transition fiber (4). The output fiber (5) is used to output an optical signal, wherein the fusion region (A) between the first double-clad transition fiber (2) and the dual-core fiber (3) and the fusion region (B) between the dual-core fiber (3) and the second double-clad transition fiber (4) are processed using a thermal diffusion technology; and in the interference region (C) section of the dual-core fiber (3), the two interference arms form an asymmetric sensing structure through the fiber opening processing (D) section.

[0013] In the dual-core fiber integrated MZ interferometer based on thermal diffusion, the input fiber 1 is composed of a single-mode fiber, the input end of which is connected to the light source in the thermal diffusion device, and the output light of the light source is transmitted from the core in the single-mode fiber.

[0014] The dual-core fiber integrated MZ interferometer based on thermal diffusion, wherein the dual-core fiber 3 is a symmetrical dual-core fiber, and its two symmetrical cores serve as the reference arm and transmission arm of the interferometer respectively. When light is transmitted in the TCF, coupling occurs between the two cores to form interference.

[0015] In the dual-core fiber integrated MZ interferometer based on thermal diffusion, the core mode field diameters of the first double-clad transition fiber 2 and the second double-clad transition fiber 4 are between the mode field diameters of the input fiber 1 and the output fiber 5 and the single-core mode field diameter of the dual-core fiber 3, so that thermal diffusion can smoothly transition the mode field size of light in this region from a size close to that of a single-mode fiber to a size close to that of a single-core mode field of a dual-core fiber, thereby minimizing coupling loss.

[0016] The dual-core fiber integrated MZ interferometer based on thermal diffusion, wherein the output fiber is composed of a single-mode fiber, the output end is connected to a spectrometer, and the spectral characteristics are monitored in real time. When the spectrometer collects a characteristic interference spectrum, it indicates that the mode coupling between the dual cores has met the phase matching requirements of the MZ interferometer.

[0017] The dual-core fiber integrated MZ interferometer based on thermal diffusion, wherein the thermal diffusion treatment is performed using laboratory-made thermal diffusion equipment, which can accurately control the heating temperature and duration of thermal diffusion.

[0018] The dual-core fiber integrated MZ interferometer based on thermal diffusion, wherein the thermal diffusion processing area is the fusion joint area between the first double-clad transition fiber 2 and the dual-core fiber 3, and the fusion joint area between the dual-core fiber 3 and the second double-clad transition fiber 4. The flame head of the thermal diffusion equipment needs to be aligned with the center position of the two fusion joints to apply uniform thermal diffusion.

[0019] The dual-core fiber integrated MZ interferometer based on thermal diffusion, wherein the thermal diffusion treatment, performed at a relatively low temperature (1100°C-1300°C) for a long time (60-90 minutes), promotes deep, uniform, and slow diffusion of dopants (primarily Ge) in the inner cladding / core of the double-clad fiber and the core / cladding of the dual-core fiber. The refractive index of the optical fiber depends on the local dopant concentration. Under the condition of uniform radial diffusion of the heating temperature, the diffusion coefficient D can be considered a constant. Since the heated area is much larger than the fiber diameter, axial and azimuthal diffusion can be ignored. The process then follows Fick's diffusion law: Here, the local dopant concentration C is a function of the radial position r and the heating time t, and the thermal diffusion coefficient D describes the temperature dependence within a certain temperature range.

[0020] Thermal diffusion changes the concentration distribution of dopants in the optical fiber. For germanium-doped optical fiber, the refractive index distribution and the dopant concentration distribution satisfy a linear relationship: Among them, n cl is the cladding refractive index, n co is the core refractive index.

[0021] The gradient refractive index distribution formed by thermal diffusion optimizes the propagation characteristics of the light field in the coupling region, making the mode field size of the light transition smoothly and significantly reducing the coupling loss. The coupling efficiency is closely related to the mode field matching degree and can be expressed as: Where w1 is the mode field radius of the input fiber, and w2 is the mode field radius of the output fiber. Thermal diffusion ensures that w1 ≈ w2 in the coupling region, at which point the coupling efficiency approaches the theoretical maximum. This efficient coupling ensures 50:50 splitting and low-loss combining of the optical signals in the two arms of the interferometer, improving interferometric contrast.

[0022] The sensing mechanism of the interferometer is based on the phase modulation principle of dual-beam interference: when the external physical quantity acts on the dual-core optical fiber 3, the two interference arms produce an optical path difference due to the asymmetric sensing structure, resulting in a phase difference of the output interference light signal. The change is manifested as a wavelength shift of the characteristic interference spectrum; by demodulating the spectrum shift The linear relationship between the physical quantity and the environmental parameters can be realized by high-precision distributed measurement.

[0023] The beneficial effects of the present invention are:

[0024] 1. The transition from double-clad fiber solves the mode field mismatch problem between single-mode fiber and symmetrical dual-core fiber. Thermal diffusion promotes the deep diffusion of dopants (such as Ge) to form a graded refractive index distribution, optimizes the light field coupling efficiency, and further optimizes the mode conversion efficiency. The overall insertion loss is much lower than that of traditional fusion splicing.

[0025] 2. The entire device is based on fusion connection, and the key coupling area is formed by modifying the material itself without fragile structure, so it has high mechanical strength and long-term ambient temperature stability.

[0026] 3. The coupler function is directly implemented in the welding point area, without the need for additional discrete coupling components. The device structure is extremely compact and has strong integrity.

[0027] 4. The thermal diffusion process parameters (temperature, time, heating area size / position) are highly controllable, making the characteristics of the formed coupling zone (size, refractive index distribution gradient) highly controllable and repeatable, thereby ensuring the consistency of coupling efficiency between devices, which is crucial for mass production and application.

[0028] 5. The thermal diffusion coupling zone at the input end can achieve a near-ideal (e.g., 50:50) and stable splitting ratio, while the thermal diffusion coupling zone at the output end can achieve efficient and low-crosstalk beam combining. Combined with the good symmetry and low-loss transmission of the two arms of the symmetrical dual-core fiber, this ensures high contrast of the interferometer output signal. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 It is a structural diagram of a dual-core fiber integrated MZ interferometer based on thermal diffusion.

[0030] Figure 2 It is the operation and running process of the dual-core fiber integrated MZ interferometer heat diffusion system based on heat diffusion.

[0031] Figure 3 It is the placement location of the heating tank and optical fiber heat diffusion area of ​​the heat diffusion system of the dual-core optical fiber integrated MZ interferometer based on heat diffusion.

[0032] Figure 4 This is a schematic diagram of the structure of a dual-core fiber integrated MZ interferometer based on thermal diffusion after the first stage of thermal diffusion.

[0033] Figure 5 This is a schematic diagram of the propagation of optical signals after the dual-core fiber integrated MZ interferometer based on thermal diffusion completes two stages of thermal diffusion.

[0034] Figure 6 This is a spectrum simulation diagram of a dual-core fiber integrated MZ interferometer based on thermal diffusion. DETAILED DESCRIPTION

[0035] The present invention will be further described below with reference to specific embodiments.

[0036] The structure of the dual-core fiber integrated MZ interferometer based on thermal diffusion proposed by the present invention is as follows: Figure 1 As shown, the interferometer is constructed by sequentially fusion-splicing an input fiber 1, a first double-clad transition fiber 2, a dual-core fiber 3, a second double-clad transition fiber 4, and an output fiber 5. Heat diffusion treatment is performed using a laboratory-made heat diffusion device at the fusion region between the first double-clad transition fiber 2 and the dual-core fiber 3, as well as at the fusion region between the second double-clad transition fiber 4 and the dual-core fiber 3, ultimately completing the interferometer packaging.

[0037] Example 1 The embodiment of the present invention provides a working method of a dual-core fiber integrated MZ interferometer based on thermal diffusion, which includes the following two stages: (1) Preparation process The dual-core fiber 3 is a symmetrical dual-core fiber with a single core diameter of 8.5 μm, a core spacing of 55 μm, and a length of 5 cm. Its two cores serve as the interferometer's interference arms. The double-clad transition fiber 2 and double-clad transition fiber 4 are both the same type of double-clad fiber, with an outer cladding diameter of 125 μm, an inner cladding diameter of 77 μm, a core diameter of 11 μm, and a length of 1-2 cm. Their core mode field diameters are between the mode field diameters of the input fiber 1 / output fiber 5 and the single core mode field diameter of the dual-core fiber 3.

[0038] like Figure 2 As shown, for the ends to be fused, between the first double-clad transition fiber 2 and the dual-core fiber 3 (corresponding to fusion region A), and between the dual-core fiber 3 and the second double-clad transition fiber 4 (corresponding to fusion region B), after removing the coating, a fiber cleaver was used to cut 12.5mm lengths of bare fiber from each end. The cleaved double-clad fiber and the dual-core fiber were coaxially aligned and fused using a fiber fusion splicer, resulting in a total joint length of 25mm (12.5mm double-clad fiber segment + 12.5mm dual-core fiber segment). This length matches the 25mm heating tank dimensions of the homemade heat diffusion equipment, ensuring that the heating tip precisely aligns with the center of the fusion joint and achieving uniform temperature field during the heat diffusion process.

[0039] Connect the input end of input fiber 1 to the light source output of the thermal diffusion device. Then, fuse the output end of input fiber 1 to the input end of the first double-clad transition fiber 2, forming an initial joint without thermal diffusion. Leveraging the fiber's inherent refractive index profile, this achieves a low-loss mode transition from single-mode fiber to double-clad fiber.

[0040] The pre-treated first double-clad transition fiber 2 is fused with the input end of the dual-core fiber 3 to form region A. Region A is then placed in a heating tank of a thermal diffusion device so that the coating surfaces on both sides of region A are flush with the edges of the heating tank. The output end of the dual-core fiber 3 is connected to a beam analyzer through a fiber adapter for thermal diffusion treatment. The thermal diffusion process is as follows: Figure 3 shown.

[0041] like Figure 4 As shown, while the thermal diffusion device performs thermal diffusion at a set temperature, a beam profiler monitors the optical field distribution and power changes at the dual-core fiber output end 2 in real time. Thermal diffusion in region A is complete when the optical signal exhibits balanced dual-core mode coupling. After thermal diffusion, region A undergoes deep diffusion of Ge dopants from the inner cladding / core of the double-clad fiber to the core / inner cladding of the dual-core fiber, forming a graded-index coupling region. This optimizes the mode conversion efficiency of light from DCF to TCF.

[0042] Repeat the above operation to fuse the output end of the dual-core fiber 3 with the input end of the second double-clad transition fiber 4 to form area B. After cleaning the surface of area B, fuse the output end of the second double-clad transition fiber 4 with the input end of the output fiber 5. Then, connect the output end of the output fiber 5 to the spectrum analyzer to form the following: Figure 1 The optical link is shown.

[0043] Place area B in the heating tank of the thermal diffusion equipment and repeat the thermal diffusion process. However, the stopping condition of the thermal diffusion is determined by the interference spectrum characteristics of the spectrometer: when the spectrum analyzer collects the interference light with periodic light intensity modulation in the wavelength domain, such as Figure 6 The spectrum simulation pattern shown in FIG5 shows that the thermal diffusion of region B is completed and the annealing operation of the thermal diffusion equipment is performed. Figure 5 The optical signal shown is input from the input optical fiber, split into two paths, and finally converges into the MZ interference structure of the output optical fiber.

[0044] (2) Measurement process After the above fabrication process, the two ends of the thermal diffusion-based dual-core fiber integrated MZ interferometer were fixed to a precision translation stage, with the dual-core fiber segment (TCF) suspended between two rigid supports. The input and output ends were connected to a broadband light source and a spectrum analyzer, respectively. Light emitted from the broadband source, after passing through the first thermal diffusion coupling segment (A), is split into two beams within the fiber. The light then propagates through the two interferometer arms of the dual-core fiber. Finally, the beams are combined in the second thermal diffusion coupling segment (B), where they interfere and propagate to the spectrum analyzer. A fixed-step vertical displacement is applied, and the spectrum analyzer measures the spectral changes within the bending range. The bending displacement is determined by the shift in the interferogram valley.

[0045] Output light intensity The phase difference between the two interferometer arms Determines the phase difference Determined by the optical path difference between the two arms, the interference light intensity can be described as: in, is the incident light intensity, is the intensity of the interference line, is the optical path difference between the two interferometer arms.

[0046] When the translation stage causes the interferometer to bend during movement, the bending displacement Causes the phase difference to change: in, The bending sensitivity coefficient is calibrated by experiment. The distance between adjacent peaks / valleys in the interference spectrum is defined as FSR, which is determined by the optical path difference. When bending displacement occurs, the interference spectrum moves. It can be described as:

Claims

1. A dual-core fiber integrated MZ interferometer based on thermal diffusion, characterized by: It consists of an input optical fiber (1), a first double-clad transition optical fiber (2), a dual-core optical fiber (3), a second double-clad transition optical fiber (4) and an output optical fiber (5). In the interferometer, the input optical fiber (1) is used to input an optical signal, the first double-clad transition optical fiber (2) has its input end fused to the output end of the input optical fiber (1), the dual-core optical fiber (3) has its input end fused to the output end of the first double-clad transition optical fiber (2), and the second double-clad transition optical fiber (4) has its input end fused to the output end of the dual-core optical fiber (3). The output optical fiber (5) is fused, and its input end is fused with the output end of the second double-clad transition optical fiber (4). The output optical fiber (5) is used to output an optical signal, wherein the fusion region (A) between the first double-clad transition optical fiber (2) and the dual-core optical fiber (3) and the fusion region (B) between the dual-core optical fiber (3) and the second double-clad transition optical fiber (4) are processed using a thermal diffusion technology; and in the interference region (C) section of the dual-core optical fiber (3), the two interference arms form an asymmetric sensing structure through the optical fiber opening processing section (D).

2. The interferometer according to claim 1, wherein: The input optical fiber (1) and the output optical fiber (5) are both single-mode optical fibers, and are used to transmit the optical signal input to the interferometer and output the optical signal after interference.

3. The interferometer according to claim 1, wherein: The dual-core optical fiber (3) is a symmetrical dual-core optical fiber, and the dual-core spacing d needs to satisfy d>3w, where w is the single-core mode field radius of the dual-core optical fiber, so as to avoid crosstalk caused by evanescent field coupling between cores during thermal diffusion.

4. The interferometer according to claim 1, wherein: The core mode field diameters of the first double-clad transition optical fiber (2) and the second double-clad transition optical fiber (4) are between the mode field diameters of the input optical fiber (1) and the output optical fiber (5) and the single core mode field diameter of the dual-core optical fiber (3).

5. The interferometer according to claim 1, wherein: The thermal diffusion treated area is the fusion joint area between the first double-clad transition optical fiber (2) and the dual-core optical fiber (3) and the dual-core optical fiber (3) and the second double-clad transition optical fiber (4).

6. The interferometer according to claim 1, wherein: The heat diffusion-treated coupling region achieves a beam splitting ratio close to 1:1 and low-crosstalk beam combining, ensuring high contrast of the interference signal.

7. The interferometer according to claim 1, wherein: The asymmetric sensing structure is to perform a hole opening process at the corresponding position of the core on one side of the (D) section of the dual-core optical fiber (3), the hole depth penetrates the cladding to the edge of the core, and the aperture range is 5-15um.

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