A mode field coupling based off-center photonic crystal fiber

Abstract

This invention provides a polarized photonic crystal fiber based on mode field coupling, relating to the fields of photonic crystal fiber and supercontinuum light source technology. It includes at least two independent pump regions and periodically distributed air holes surrounding each pump region. Different pump regions correspond to and transmit different pump modes. Each pump region consists of a main coupling region, or a combination of a main coupling region and several auxiliary coupling regions connected to it, and the main coupling regions of each pump region have the same region size. The advantages are that it solves the problem of untunable supercontinuum light source bands, eliminates the need for additional coupling efficiency optimization, and offers a simple structure and strong stability.

Description

Technical Field

[0001] This invention relates to the field of photonic crystal fiber and supercontinuum light source technology, and particularly to a polarized photonic crystal fiber based on mode field coupling. Background Technology

[0002] Supercontinuum light sources broaden narrowband pump light into a broadband spectrum by exciting various nonlinear effects such as self-phase modulation, cross-phase modulation, and four-wave mixing in a nonlinear medium with strong pump light. Due to their advantages such as wide wavelength coverage, good coherence, and high brightness, they have significant application value in many fields, including optical measurement, biological imaging, optical communication, and precision spectral detection.

[0003] Existing supercontinuum light sources are mostly based on a single fiber core structure design. Due to the constraints of the fiber core structure, mode field, and the inherent characteristics of the nonlinear medium, their output spectral bands are relatively fixed, making it difficult to achieve flexible switching between multiple bands according to application requirements. Some tunable supercontinuum light sources require complex fiber core structure optimization or external control methods (such as temperature and pressure regulation) to achieve band fine-tuning, which has problems such as complex structure, poor stability, and low coupling efficiency.

[0004] Photonic crystal fiber, as a type of microstructured fiber with flexible structural design freedom, allows for precise control of mode field distribution, nonlinear characteristics, and coupling efficiency through the rational design of the core and cladding air hole distribution. This provides a feasible technical approach to solving the problem of tunable supercontinuum light source bands. Current supercontinuum light source designs based on photonic crystal fiber mostly focus on the nonlinear optimization of a single core or achieve parallel transmission through multi-core structures. However, there is a lack of designs that utilize off-core multi-region combined pumping combined with mode field coupling theory to achieve multi-band supercontinuum light source switching. Furthermore, the coupling efficiency of existing multi-core photonic crystal fibers often relies on complex structural parameter optimization, increasing fabrication difficulty and cost, and limiting their practical engineering applications.

[0005] Therefore, developing a photonic crystal fiber with a simple structure, requiring no additional optimization of coupling efficiency, and capable of flexible switching of multi-band supercontinuum light sources has become an urgent need in the field of supercontinuum technology. Summary of the Invention

[0006] To address the problems existing in the prior art, the present invention provides a polarized photonic crystal fiber based on mode field coupling, comprising at least two independent pump regions and air holes periodically distributed around each pump region, wherein different pump regions correspond to and transmit different pump modes. Each of the pumping regions consists of a main coupling region, or a combination of a main coupling region and several auxiliary coupling regions connected thereto, and the main coupling regions of each pumping region have the same region size.

[0007] Preferably, when the pumping region is composed of a main coupling region, both the main coupling region and the surrounding air holes are regular hexagons.

[0008] Preferably, when the pumping region consists of a main coupling region and a plurality of auxiliary coupling regions connected thereto, the main coupling region and each of the auxiliary coupling regions are connected in series sequentially.

[0009] Preferably, the dimensions of the main coupling region and each of the auxiliary coupling regions decrease sequentially in series, and the main coupling region has the largest region size.

[0010] Preferably, the main coupling region and each of the auxiliary coupling regions are based on a regular hexagon. In a series sequence, one side of the previous regular hexagon is stretched outward in a direction perpendicular to it while its length decreases until it reaches the side length of the regular hexagon connected with it. The stretching direction of the main coupling region and each of the auxiliary coupling regions is the same. The air holes adjacent to the main coupling region and each of the auxiliary coupling regions are also adaptively stretched based on a regular hexagon to fit the area size of the main coupling region and each of the auxiliary coupling regions.

[0011] Preferably, the main coupling regions of each of the pump regions have the same region size as the regular hexagonal reference before stretching.

[0012] Preferably, there are three pumping regions, namely a first pumping region, a second pumping region, and a third pumping region; The first pumping region includes only the first region as the main coupling region; The second pumping region includes a second region and a third region connected in series, wherein the second region is the main coupling region and the third region is the auxiliary coupling region; The third pumping region includes a fourth region, a fifth region, and a sixth region connected in series. The fourth region is the main coupling region, and the fifth and sixth regions are the auxiliary coupling regions.

[0013] Preferably, the pumping region is a silicon dioxide substrate, and the air pores are in a vacuum state.

[0014] The above technical solution has the following advantages or beneficial effects: 1) Solve the problem of untunable supercontinuum light source bands: By combining different pumping regions and combining mode field coupling theory, the output of supercontinuum light sources with significant spectral differences can be realized. The bands can be flexibly switched according to actual needs to adapt to different application scenarios. 2) No additional optimization required for coupling efficiency: Based on the design that the main coupling regions of each pumping region have the same area size, the structural symmetry is used to ensure the coupling efficiency of the mode field, avoiding complex parameter optimization processes and reducing the difficulty and cost of fabrication; 3) Simple structure and strong stability: The overall structure consists of a pump region and periodically distributed air holes. There are no complex doping or external control components. The fabrication process is mature, the optical transmission loss is low, the mode field is stable, and it is suitable for engineering applications. Attached Figure Description

[0015] Figure 1 A schematic diagram of a polarized photonic crystal fiber based on mode field coupling is shown in a preferred embodiment of the present invention. Figure 2 In embodiments 1 and 2 of the present invention, supercontinuum spectra were generated for three different sizes of fiber cores; Figure 3 The dispersion curves of three different fiber core sizes are shown in Embodiments 1 and 2 of the present invention. Detailed Implementation

[0016] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments. The present invention is not limited to this embodiment; other embodiments that conform to the spirit of the present invention may also fall within the scope of the present invention.

[0017] In a preferred embodiment of the present invention, based on the above-mentioned problems existing in the prior art, a polarized photonic crystal fiber based on mode field coupling is provided, such as... Figure 1 As shown, it includes at least two independent pumping regions and air holes periodically distributed around each pumping region. Different pumping regions correspond to and transmit different pumping modes. Each pumping region consists of a main coupling region, or a combination of a main coupling region and several auxiliary coupling regions connected to it, and the main coupling regions of each pumping region have the same region size.

[0018] Specifically, such as Figure 1 As shown, the cross-sectional structure of the mode-field coupled eccentric photonic crystal fiber of the present invention consists of a pump region in white area and air holes in periodically distributed black area. The pump region is a silica matrix, preferably made of high-purity fused silica, which features low loss, high nonlinearity, and strong chemical stability, precisely meeting the requirements for supercontinuum generation. The air holes are in a vacuum state, and their periodic distribution effectively constrains the mode field.

[0019] In a preferred embodiment of the present invention, when the pumping region is composed of a main coupling region, the main coupling region and the surrounding air holes are all regular hexagons, which can ensure uniform mode field distribution, improve mode field confinement effect, and reduce optical transmission loss.

[0020] In a preferred embodiment of the present invention, when the pumping region consists of a main coupling region and several auxiliary coupling regions connected thereto, the main coupling region and each auxiliary coupling region are connected in series to ensure smooth connection between them, thereby achieving efficient mode field coupling and providing a structural basis for spectral difference modulation.

[0021] In a preferred embodiment of the present invention, the size of the main coupling region and each auxiliary coupling region decreases sequentially in series, and the main coupling region has the largest region size. By perturbing the mode field distribution through size differences, nonlinear effects of different intensities and types are excited, thereby realizing the output of supercontinuum light sources with different spectra.

[0022] In a preferred embodiment of the present invention, the main coupling region and each auxiliary coupling region are based on a regular hexagon. In a series sequence, one side of the previous regular hexagon is stretched outward in a direction perpendicular to it while its length decreases until it reaches the side length of the regular hexagon connected with it. The stretching directions of the main coupling region and each auxiliary coupling region are the same. The air holes adjacent to the main coupling region and each auxiliary coupling region are also adaptively stretched based on a regular hexagon to fit the area size of the main coupling region and each auxiliary coupling region, ensuring the periodicity and integrity of the air hole cladding, further improving the mode field constraint effect and reducing optical transmission loss.

[0023] In a preferred embodiment of the present invention, the main coupling regions of each pumping region have the same region size as the regular hexagonal reference before stretching. This symmetrical design can ensure the mode field coupling efficiency in each pumping mode without the need for additional optimization of structural parameters such as air hole spacing and aperture, thus simplifying the structural design and fabrication process.

[0024] Example 1 like Figure 1 As shown, there are three pumping regions: the first pumping region, the second pumping region, and the third pumping region. These three independent pumping regions correspond to three different pumping modes, enabling the output of supercontinuum light sources with three significantly different spectra, thus meeting the needs of different application scenarios. The first pumping region only includes the first region 1 as the main coupling region; The second pumping region includes a second region 2 and a third region 3 connected in series. The second region 2 is the main coupling region and the third region 3 is the auxiliary coupling region. The third pumping region includes a fourth region 4, a fifth region 5, and a sixth region 6 connected in series. The fourth region 4 is the main coupling region, and the fifth region 5 and the sixth region 6 are auxiliary coupling regions.

[0025] Specifically, such as Figure 1 As shown, the preferred dimensions of each region are as follows: Region 1 is 6μm, Region 2 is 6μm, Region 3 is 3μm, Region 4 is 6μm, Region 5 is 3μm, and Region 6 is 0.5μm. Air holes form a periodic cladding structure around each region, with the air holes arranged in a periodic hexagonal pattern. The hole diameter and spacing are designed to fit the dimensions of each pumping region. In this embodiment, the cladding air hole diameter is 2μm, and the center-to-center distance between adjacent air holes is 4μm, ensuring stable mode field confinement performance in each pumping mode and reducing optical transmission loss.

[0026] Furthermore, the first pumping region only includes region 1 (6μm) as the main coupling region. Region 1 and the surrounding air holes are all regular hexagons, which can be directly used for pumping as independent fiber cores to excite nonlinear effects and generate supercontinuum light sources.

[0027] The second pumping region includes region 2 (6μm) and region 3 (3μm) connected in series. Region 2 is the main coupling region and region 3 is the auxiliary coupling region. The size of the two regions decreases in series (6μm>3μm). The main coupling region (region 2) has the largest region size. At the same time, both regions are connected in series according to a preset stretching method based on a regular hexagon. The adjacent air holes are synchronously and adaptively stretched, which can realize efficient mode field coupling between the main and auxiliary coupling regions.

[0028] The third pumping region comprises regions 4 (6μm), 5 (3μm), and 6 (0.5μm) connected in series. Region 4 is the main coupling region, while regions 5 and 6 are auxiliary coupling regions. The dimensions of the three regions decrease sequentially in series (6μm > 3μm > 0.5μm), with the main coupling region (region 4) having the largest region size. All three regions are connected in series with a regular hexagonal base and the same stretching direction. The adjacent air holes are synchronously and adaptively stretched to ensure smooth mode field coupling and to excite more complex nonlinear effects.

[0029] It should be noted that the dimensions of each region mentioned above refer to the corresponding hexagonal reference dimensions before stretching, i.e., regions 1, 2, and 3 are all 6μm. The three regions have good structural symmetry, and during the model field coupling process, there is no need to optimize parameters such as aperture and spacing to ensure stable and efficient coupling efficiency, which significantly simplifies the structural design and fabrication process.

[0030] Example 2 In this embodiment, a pulsed laser with a center wavelength of 1064 nm, a pulse width of 100 fs, and a repetition frequency of 80 MHz is selected as a unified pump source. By focusing the pump light onto different pump regions in Example 1, three different pump modes are achieved, thereby outputting three supercontinuum light sources with significantly different spectra, specifically including: 1. Standalone pump mode The pump light is focused on region 1 (6μm silicon dioxide region) of the first pump region. Region 1 serves as an independent main coupling region, and the air holes around it are regular hexagonal, which can achieve strong mode field confinement. Under the excitation of the pump light, various nonlinear effects such as self-phase modulation and four-wave mixing are excited in region 1. The pump light energy is fully utilized, and the first supercontinuum light source is finally output.

[0031] Spectroscopic analysis revealed that the supercontinuum light source has a spectral coverage range of over 500nm-1700nm, making it suitable for conventional applications such as optical measurement and biological imaging.

[0032] 2. Dual-zone combined pumping mode The pump light is focused into the second pump region (a combination of region 2 and region 3). Region 2 (the main coupling region) and region 3 (the auxiliary coupling region) are smoothly connected in a series structure. After the pump light is injected, efficient mode field coupling occurs between the two regions, achieving energy superposition. Due to the size difference between the two regions (6μm and 3μm), the mode field distribution exhibits asymmetry, which significantly enhances the intensity of nonlinear effects compared to the single pump mode. This alters the types and intensities of the excited nonlinear effects, ultimately resulting in the output of a second supercontinuum light source.

[0033] This light source has a significant difference in wavelength compared to the first type of supercontinuum light source. Both the spectral coverage and peak wavelength have been adjusted, making it suitable for applications such as higher-precision spectral detection.

[0034] 3. Three-zone combined pumping mode The pump light is focused into a third pump region (a combination of regions 4, 5, and 6). Regions 4 (the main coupling region), 5 (the auxiliary coupling region), and 6 (the auxiliary coupling region) are connected in series according to the core scheme, with their sizes decreasing sequentially. Cooperative mode field coupling occurs among the three regions. Region 4 provides basic energy transfer and nonlinear excitation, region 5 assists in enhancing the mode field coupling effect, and the ultrathin structure (0.5 μm) of region 6 further perturbs the mode field distribution, exciting more complex nonlinear effects, ultimately outputting a third type of supercontinuum light source.

[0035] like Figure 2 As shown, the three curves correspond to the supercontinuum spectra of three different sizes of fiber cores pumped by femtosecond light sources at high duty cycle PCF, clearly demonstrating the band differences and coverage. Figure 3In the figure, the three curves correspond to the dispersion curves of high duty cycle PCFs with three different fiber core sizes.

[0036] In this embodiment, by combining three different pumping regions and using mode-field coupling theory, three supercontinuum light sources with significantly different spectra are output, allowing for flexible switching of wavelengths to suit different application scenarios. The design utilizes the identical dimensions of silicon dioxide structures in regions 1, 2, and 4, leveraging structural symmetry to ensure efficient mode-field coupling, avoiding complex parameter optimization processes, and reducing fabrication difficulty and cost. The overall structure consists of silicon dioxide and air holes, without complex doping or external control components, resulting in a mature fabrication process, low optical transmission loss, and strong mode-field stability, making it suitable for engineering applications. The differentiated design of regions 3 (3μm) and 6 (0.5μm) precisely controls the mode-field distribution under the combined pumping mode, further expanding the spectral differences of the three supercontinuum light sources and improving the tunable wavelength range.

[0037] The above description is merely a preferred embodiment of the present invention and does not limit the implementation and protection scope of the present invention. Those skilled in the art should realize that any equivalent substitutions and obvious changes made using the content of this specification and illustrations should be included within the protection scope of the present invention.

Claims

1. A mode field coupling based off-center photonic crystal fiber, characterized in that, It includes at least two independent pumping regions and air holes periodically distributed around each of the pumping regions, with each pumping region corresponding to and transmitting a different pumping mode; Each of the pumping regions consists of a main coupling region, or a combination of a main coupling region and several auxiliary coupling regions connected thereto, and the main coupling regions of each pumping region have the same region size.

2. The off-center photonic crystal fiber according to claim 1, wherein, When the pumping region is composed of a main coupling region, the main coupling region and the surrounding air holes are both regular hexagons.

3. The polarized photonic crystal fiber according to claim 1, characterized in that, When the pumping region is composed of a main coupling region and several auxiliary coupling regions connected thereto, the main coupling region and each of the auxiliary coupling regions are connected in series sequentially.

4. The polarized photonic crystal fiber according to claim 3, characterized in that, The dimensions of the main coupling region and each of the auxiliary coupling regions decrease sequentially in series, and the main coupling region has the largest region size.

5. The off-core photonic crystal fiber according to claim 4, characterized in that, The main coupling region and each of the auxiliary coupling regions are based on a regular hexagon. In a series sequence, one side of the previous regular hexagon is stretched outward in a direction perpendicular to it while its length decreases until it reaches the side length of the regular hexagon connected with it. The stretching direction of the main coupling region and each of the auxiliary coupling regions is the same. The air holes adjacent to the main coupling region and each of the auxiliary coupling regions are also adaptively stretched based on a regular hexagon to fit the area size of the main coupling region and each of the auxiliary coupling regions.

6. The off-core photonic crystal fiber according to claim 5, characterized in that, The main coupling regions of each pump region have the same region size as the regular hexagonal reference before stretching.

7. The polarized photonic crystal fiber according to claim 1, characterized in that, The pumping regions are three, namely the first pumping region, the second pumping region, and the third pumping region; The first pumping region includes only the first region as the main coupling region; The second pumping region includes a second region and a third region connected in series, wherein the second region is the main coupling region and the third region is the auxiliary coupling region; The third pumping region includes a fourth region, a fifth region, and a sixth region connected in series. The fourth region is the main coupling region, and the fifth and sixth regions are the auxiliary coupling regions.

8. The polarized photonic crystal fiber according to claim 1, characterized in that, The pumping region is a silicon dioxide substrate, and the air holes are in a vacuum state.

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