A single-sided aperture microstructure fiber laser tunable filter based on ionic liquid filling

Abstract

A tunable fiber laser filter based on a single-sided aperture microstructure filled with an ionic liquid is disclosed. The fiber cross-section has a core and a single-sided aperture structure, with the aperture filled with a photosensitive ionic liquid material having a refractive index higher than that of a quartz substrate. When the axially transmitted light in the fiber satisfies the phase matching condition between the core mode and the liquid column mode within the aperture, a resonant coupling phenomenon occurs between the core mode and the liquid column mode at a specific wavelength. This results in a series of resonant loss peaks in the fiber transmission spectrum that exhibit high sensitivity to changes in the refractive index of the liquid within the aperture. Furthermore, by applying external laser illumination to change the refractive index of the ionic liquid, dynamic tuning of the resonant wavelength of the microstructure fiber transmission spectrum can be achieved. This invention has advantages such as high wavelength tuning sensitivity, non-contact tuning method, good compatibility with existing fiber optic systems, and strong functional scalability, and can be applied to fields such as fiber optic communication, sensing, lasers, and integrated optics.

Description

Technical Field

[0001] This invention belongs to the field of optical fiber communication and optical fiber sensing technology, and relates to liquid filling technology of microstructured optical fibers. Based on the photosensitivity of ionic liquids and the resonant coupling effect of microstructured optical fibers, a tunable filter with a single-sided aperture microstructured optical fiber is obtained. Background Technology

[0002] Microstructured optical fiber (MOF) is a novel type of optical fiber based on photonic crystal theory. Due to the periodic micropores in its cross-section, MOF offers greater structural flexibility compared to traditional optical fibers. It also possesses a series of novel optical properties unmatched by traditional fibers, including cutoff-free single-mode transmission, large / small mode field size, high nonlinearity, and controllable dispersion. In recent years, MOF has become a research hotspot in the field of fiber optics, greatly promoting the development and progress of fiber optic communication and fiber optic sensing.

[0003] With the rapid development of optical fiber communication technology and optical fiber sensing technology, tunable optical fiber filters have been widely used, such as the temperature-tunable optical fiber filter disclosed in publication number CN 115097658 A (publication date: September 23, 2022). This application proposes a temperature-tunable fiber filter, which can achieve tunable filtering by changing the ambient temperature. However, when the ambient temperature is 30℃, the spectral bandwidth of this filter near 1017nm is only 8.2nm. The literature Chen Yufang et al, “All-Optical Tunable Fiber Filter Based on Phosphate Glass Microspheres”, Chinese Journal of Lasers, 2021, 48(01), 114-122, proposes an all-optical tunable fiber filter based on phosphate glass microsphere cavity. The diameter of the phosphate glass fiber used in this fiber filter is between 200 and 500μm. During the fabrication, it is necessary to meet two conditions: the transfer temperature matching of the cladding glass and the core glass and the matching of the thermal expansion coefficients of the cladding glass and the core glass. The fabrication is difficult and costly, so its practical application is limited. In addition, the literature Li Xuan et al, “Study on Spectrum Properties of Ultra-narrow Band Fiber BraggGratings for Optical Fibers”, Chinese Journal The paper ofLasers, 2023, 50(14):02, analyzes the fabrication conditions of ultra-long and ultra-narrowband fiber Bragg grating filters. The study found that the fabrication process is complex, costly, and relatively wavelength-dependent, which limits the practical application of such filters. Meanwhile, the paper Shen Xiaoyue, Zheng Laifang, and Li Yun, "Design of High-Precision Large-Range Tunable Filters Based on Fiber Bragg Gratings," Electronic Technology and Software Engineering, 2022(22):4, proposes a high-precision large-range tunable filter based on fiber Bragg gratings and has applied for a related national invention patent (a microstructure fiber broadband polarization filter with tunable filtering direction, patent number: CN 112859234 B, authorization announcement date: April 29, 2022). This patented solution achieves tunable optical filtering by changing the ambient temperature and stress conditions around the fiber Bragg grating, but its operating wavelength range is 30-50nm, thus limiting its practical application. Summary of the Invention

[0004] The purpose of this invention is to address the problems commonly found in existing tunable fiber optic filters, such as limited tuning range, low tuning sensitivity, and poor tuning linearity. It provides a microstructured fiber laser tunable filter based on photosensitive ionic liquid filling. This filter integrates the refractive index response characteristics of the ionic liquid under external laser irradiation with the resonant coupling effect of the microstructured fiber, achieving tunable filtering by adjusting the power of the external illumination laser. This tunable filter can operate simultaneously across multiple wavelength bands and features high wavelength tuning sensitivity, a wide operating wavelength range, non-contact tuning, simple and easy tuning methods, compact structure, good compatibility with existing fiber optic systems, strong functional scalability, and high spectral stability. It can be applied to fields such as fiber optic communication, sensing, lasers, and integrated optics, and can also be used for sensing laser power and sample refractive index.

[0005] Technical solution of the present invention

[0006] A single-sided-aperture microstructured fiber laser tunable filter based on photosensitive ionic liquid filling is proposed. The microstructured fiber has a core and a single-sided-aperture structure. By filling the side aperture of the microstructured fiber (MOF) with a photosensitive ionic liquid (1-pentyl-3-methylimidazolium iodide) with a refractive index of 1.5461 without laser illumination, resonant coupling occurs between the core mode and the various liquid column modes within the side aperture, forming a quasi-dual-core microstructured fiber structure that combines the functions of a liquid column waveguide and a core waveguide. Under the condition of phase-matched coupling, i.e., Δβ=β... c -β l =0,β c β l These are the transmission constants for the core mode and the liquid column mode, respectively. This fiber structure has multiple sets of resonant peaks with different laser intensity response characteristics in multiple bands, enabling tunable optical filtering based on laser intensity tuning, and sensing of laser power and the refractive index of the sample in the side hole based on wavelength demodulation.

[0007] The MOF substrate material used in this invention is pure quartz, with circular cladding apertures. The core diameter is 8.74 micrometers, and the cladding diameter is 125 micrometers. The distance between the center of the aperture and the center of the core is 32.051 micrometers, and the radius of the filled aperture is 20 micrometers. The refractive index of the quartz cladding at 1550 nm is 1.444, which is 0.005 lower than that of the core.

[0008] The working principle of this new type of optical fiber structure is as follows:

[0009] Based on different optical mechanisms, microstructured optical fibers can be divided into refractive index-guided fibers and bandgap-guided fibers. In refractive index-guided fibers, the core refractive index is usually greater than the cladding refractive index, thus allowing the formation of fiber waveguides based on total internal reflection. Bandgap-guided fibers, on the other hand, confine the light field to a core region with a lower refractive index than the cladding based on the photonic bandgap effect. These fibers can be used in low-loss optical transmission and other fields. Based on the principle of total internal reflection, low-frequency optical transmission (LP) can be simultaneously transmitted within the core of the microstructured fiber. 01 Both fundamental and higher-order modes are considered, but the energy of the higher-order modes in the core is negligible compared to the fundamental mode; therefore, only the effect of the core fundamental mode is taken into account. Since the refractive index of the ionic liquid filling is 1.5461 without laser illumination, which is much greater than that of the quartz substrate, multiple localized higher-order liquid column modes can be excited in the liquid-filled region under the condition of total internal reflection transmission. Theoretical calculations show that the liquid column mode LP exists in the 1430-1640 nm wavelength range. 6,13 LP 8,11 LP 7,11 LP 3,13 LP 10,10 and LP 4,12 It can couple with the fiber core mode, resulting in the appearance of multiple resonant peaks in the fiber transmission spectrum. Due to the resonant coupling between the fiber core fundamental mode and different higher-order liquid column modes, these resonant peaks have different response characteristics to the external laser intensity. This characteristic can be used to realize optical filters based on laser intensity tuning, and it can also be applied to simultaneously sense the external laser power and the refractive index of the liquid sample filled in the side hole.

[0010] Simulation results show that this structure has six resonant peaks in the wavelength range of 1430-1640 nm. Compared with other reported types of fiber optic intensity sensors, this single-sided aperture microstructure fiber laser tuned filter based on photosensitive ionic liquid filling has excellent characteristics such as high tuning sensitivity, high resonant tuning linearity, simple implementation, good spectral stability, and good compatibility with existing fiber optic systems.

[0011] Advantages and beneficial effects of the present invention:

[0012] This filter device, based on a non-contact tuning method using external laser illumination, exhibits high linearity and stability in wavelength tuning characteristics near multiple resonant wavelengths. While possessing high wavelength tuning sensitivity, it leverages the portability of the fiber optic platform to achieve high-precision detection of external laser intensity. Furthermore, the filter's main advantages lie in its ability to integrate the refractive index response characteristics of ionic liquids under external laser irradiation with the resonant coupling effect in microstructured optical fibers. It boasts advantages such as a wide operating wavelength range, high tuning sensitivity, simple and easy tuning methods, and good compatibility with fiber optic systems. In addition, it has significant potential applications in multi-parameter sensing, wavelength division multiplexing / demultiplexing, all-optical tuning systems, and integrated photonics. Attached Figure Description

[0013] Figure 1 This is a schematic diagram of the cross-section of the microstructured optical fiber in this invention, where 1 represents the fiber core and 2 represents the side hole filled with photosensitive ionic liquid. The core diameter and cladding diameter of this optical fiber are 8.74 μm and 125 μm, respectively. The distance between the center of the side hole and the center of the core is 32.051 μm, and the radius of the side hole is 20 μm. The refractive index of the quartz cladding at 1550 nm is 1.444, which is 0.005 lower than that of the core.

[0014] Figure 2 The curves show the distribution of effective refractive index as a function of wavelength for the core mode and different higher-order liquid column modes when the refractive index of the liquid filling the side hole varies in the range of 1.5455-1.5461 with a step size of 0.0002. The inset shows the mode field distribution of the corresponding higher-order liquid column modes.

[0015] Figure 3 for Figure 2 The curves and linear fitting results of the wavelength of each resonance peak as a function of the refractive index of the ionic liquid filling the side holes.

[0016] Figure 4 The results show the response curves and linear fitting results of the ionic liquid's refractive index as a function of increasing and decreasing irradiation laser power.

[0017] Figure 5 for Figure 2 The drift curves and linear fitting results of the wavelengths of each resonance peak as a function of laser power.

[0018] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings. Detailed Implementation

[0019] This invention fills the cladding side holes of a single-sided aperture microstructure optical fiber with a photosensitive ionic liquid. Under the condition of satisfying phase-matched coupling, this optical fiber simultaneously exhibits multiple sets of resonant peaks in multiple wavelength bands. Compared with other light intensity sensors, this invention has excellent characteristics such as high wavelength sensitivity, high resonant tuning linearity, low insertion loss, simple implementation, and good stability.

[0020] The single-sided aperture microstructure optical fiber used in this invention consists of a quartz substrate and a single-sided circular aperture, wherein the aperture is filled with a photosensitive ionic liquid (1-pentyl-3-methylimidazolium iodide). The characteristic of the filled optical fiber structure is that LP (phosphoric acid liquid) can exist in the fiber core region. 01 The fundamental mode and higher-order modes: multiple higher-order modes can exist in the liquid column region. When the mode fields of the fiber core fundamental mode and the liquid column higher-order modes overlap, a resonant coupling effect of optical field energy will occur. That is, part of the energy in the fiber core will be coupled to the liquid column region for transmission. Under the condition of phase matching coupling, the transmission spectrum of the optical fiber will show a resonant coupling loss peak at a specific wavelength position.

[0021] Figure 1 This is a schematic diagram of the cross-section of the microstructured optical fiber in this invention, where number 1 represents the fiber core and number 2 represents the side aperture filled with photosensitive ionic liquid. The core diameter and cladding diameter of this fiber are 8.74 μm and 125 μm, respectively. The distance between the center of the side aperture and the center of the core is 32.051 μm, and the radius of the filling aperture is 20 μm. The refractive index of the quartz cladding at 1550 nm is 1.444, which is 0.005 lower than that of the core. This microstructured optical fiber simultaneously supports both the core mode and the liquid column mode within the side aperture, and resonant coupling occurs between the two at specific wavelengths that meet the matching conditions. During the coupling process between the core mode and the liquid column mode, the optical field energy of the microstructured optical fiber exhibits a loss resonance peak in the fiber transmission spectrum due to the absorption of light by the liquid. The wavelength position of this peak is highly sensitive to changes in the refractive index of the ionic liquid within the side aperture.

[0022] Figure 2 The effective refractive index distribution curves of the core mode and different liquid column modes are shown as a function of wavelength when the refractive index of the liquid filling the side hole varies in the range of 1.5455-1.5461 in steps of 0.0002. The inset above the curves shows the mode field distribution of each order of liquid column mode. It can be found that the dispersion curves of the core mode and each order of liquid column mode intersect. Therefore, the fiber transmission spectrum will exhibit narrowband resonance peaks due to coupling from the core mode to the liquid column mode at the intersection of the two types of dispersion curves. Figure 2 As can be seen, when the refractive index of the filling liquid is 1.5461, the core LP of the microstructured optical fiber... 01When the refractive index matching coupling condition is met, the fundamental mode can resonate and couple with the higher-order liquid column mode near 1448.5 nm, 1460.7 nm, 1502.0 nm, 1552.1 nm, 1582.7 nm and 1622 nm, respectively, resulting in a series of corresponding resonant loss peaks in the fiber transmission spectrum.

[0023] Figure 3 for Figure 2 The curves showing the change in wavelength of each resonance peak with the refractive index of the ionic liquid filling the side holes are presented. By comparing the wavelength shift of all resonance peaks in the 1430-1640 nm range, it can be seen that the wavelength of each resonance peak shifts linearly towards longer wavelengths with the increase of the refractive index of the ionic liquid, and the linear fitting degree can reach above 0.99. Furthermore, a wavelength shift of LP near 1552.1 nm can be observed. 3,13 The wavelength shift sensitivity of the resonant peak is the highest, reaching 9765.39 nm / RIU.

[0024] Figure 4 The figure shows the response curve of the ionic liquid refractive index to the laser power. It can be observed from the figure that for different processes of gradually increasing and decreasing laser power, the refractive index of the ionic liquid exhibits a linear and monotonically decreasing relationship with the laser power, with a linear fit of 0.999.

[0025] Figure 5 According to Figure 3 and Figure 4 The refractive index variation relationship was used to obtain the wavelength shift curves of each resonance peak as a function of laser power. It can be observed that the resonance wavelength exhibits a blue shift with increasing laser power and a red shift with decreasing laser power. The resonant wavelength positions corresponding to the same irradiation laser power during both the increase and decrease of laser power show good agreement, and the wavelength tuning sensitivity is similar, indicating that the tuning process has good restitution. Linear fitting results show that the laser power sensitivity varies at each resonance wavelength because the resonance peaks in different bands originate from the coupling between the fiber core fundamental mode and different higher-order modes in the liquid core region. Furthermore, the linear fitting degree of each resonance wavelength drifting with laser power can reach above 0.99.

[0026] The single-sided aperture microstructure fiber laser tunable filter proposed in this invention has advantages such as a wide operating wavelength range, compact structure, non-contact optical control tuning method, good spectral stability and high wavelength tuning sensitivity. In addition, since the position of the resonant wavelength and the wavelength tuning sensitivity depend on the type of ionic liquid material, wavelength tuning of different resonant peaks can be achieved by selecting the type of ionic liquid material. At the same time, the external laser power can be sensed by demodulating the wavelength of different bands.

Claims

1. A single-sided aperture microstructure fiber optic tunable filter based on ionic liquid filling, characterized in that... By filling the side holes of a microstructured optical fiber with 1-pentyl-3-methylimidazolium iodide, a photosensitive ionic liquid with a refractive index of 1.5461 when no laser illumination is applied, resonant coupling occurs between the fiber core mode and the various liquid column modes within the side holes. When the refractive index matching coupling condition is met, i.e., Δβ=β c -β l =0, β c β l The transmission constants for the core and liquid column modes are respectively. The optical fiber transmission spectrum exhibits multiple resonance peaks with different laser power response characteristics in multiple bands, enabling tunable filtering based on laser intensity tuning. The core diameter of the microstructured optical fiber is 8.74 micrometers, and the cladding diameter is 125 micrometers. The distance between the center of the side hole and the center of the core is 32.051 micrometers, and the radius of the filled side hole is 20 micrometers. The refractive index of the quartz cladding at 1550 nm is 1.444, which is 0.005 lower than that of the core.

2. The tunable filter according to claim 1, characterized in that... The substrate material of the microstructured optical fiber used is pure quartz, and the shape of the cladding edge hole is circular.

3. The tunable filter according to claim 1, characterized in that... The tunable filter has multiple sets of resonant peaks sensitive to laser intensity in multiple bands in its transmission spectrum, which can be used to sense laser power based on wavelength demodulation.

4. The tunable filter according to claim 1, characterized in that... The resonance peaks appearing in the transmission spectrum of the tunable filter are the result of the resonant coupling between the core mode and the side hole liquid column mode. Under different power laser illumination, multiple sets of resonance peaks with laser intensity response characteristics can appear simultaneously in different wavelength bands. Therefore, based on wavelength demodulation, high-sensitivity optical filtering based on laser intensity tuning and laser power sensing based on wavelength demodulation can be realized.

5. The tunable filter according to claim 1, characterized in that... The tunable filter can simultaneously perform laser tuning on multiple resonant peaks in the wavelength range of 1430-1640 nm, and has high tuning linearity.

6. The tunable filter according to claim 1, characterized in that... The tunable filter is based on a non-contact tuning method using laser illumination, which has no impact on the sensing structure itself and is highly compatible with existing fiber optic systems.

7. The tunable filter according to claim 1, characterized in that... The tunable filter can form a resonant coupling between the core mode and the liquid column mode by filling the side hole with a liquid sample with a refractive index higher than that of the substrate, thereby causing the appearance of a resonant peak in the transmission spectrum of the microstructured optical fiber. Therefore, it can achieve high-sensitivity sensing of the sample refractive index based on wavelength demodulation.

Images