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Abstract
Herein, a high pressure-sensitive and stable fiber Fabry-Perot (FP) interferometer with nano-diaphragm assembled by H-O catalysis bonding is proposed and demonstrated. In order to assemble a nano-diaphragm-based fiber FP interferometer by H-O catalysis bonding technique, a SiO2 film, introduced as a bridging layer on the nano-diaphragm, can be regarded as a solid adhesive to bridge hollow-core fiber end-face and nano-diaphragm. As thus, by depositing bonded layers on different diaphragm materials, this H-O catalysis bonding technology can be used to for assembling FP interferometer with different materials nano-diaphragms. Experimentally, Si nano-diaphragm is transferred to hollow-core fiber end-face to build a stable fiber FP interferometer without polymeric adhesive. Experimental results reveal that this Si nano-diaphragm-based fiber FP interferometer has a high (79.6 pm/kPa) pressure sensitivity and a low (17.3 pm/°C) temperature sensitivity. Besides that, different materials nano-diaphragm also can be assembled by using this H-O catalysis bonding technique, and the functional FP interferometer can be realized by using functional nano-diaphragm material. Thus, a Pd nano-diaphragm is successfully assembled to build a FP interferometer with a hydrogen concentration measurement capacity. Further investigation will focus on exploitation of multi-material nano-film patterning transfer and different nano-film integration by using this H-O catalysis bonding transfer.
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1. Introduction
Fiber-optic sensors have attracted much attention due to their distinct advantages over the conventional sensors, such as immunity to electromagnetic interference, high resolution, high accuracy, small in size, long-distance sensing [1–4]. So far, various types of fiber optic sensors have been successfully explored and widely applied for health and safety monitoring of composite materials, large civil engineering structures, quantitative chemical process, and structural health monitoring. Among the many optical fiber sensors, fiber Fabry-Perot (FP) interferometric sensors, especially film-based FP interferometric sensors, have been attractive for a wide range of applications, because they have many distinctive advantages over traditional FP interferometers or fiber sensors, such as more sensitive to slight variations in pressure (such as sound [5]). For a film-based FP interferometer, the thin film is selected as a reflective diaphragm and a pressure-sensitive component, and the integration between thin film and optical fiber is a key step in film-based FP interferometer preparation. Recently, the thin film can be integrated with optical fiber by using various methods [6,7]. However, the structure (film thickness) and type (material) of thin film and the integration method between it and fiber can directly affect the response characteristics, stability and complexity of the fabrication process of thin-film-based fiber FP interferometer, thus, the thin film transfer and assembly technique, may be a crucial factor to influence the response characteristics, detection capability and other inherent characteristics of thin-film-based fiber FP interferometer.
Traditionally, the thin-film transfer and assembly methods can be principally classified as physical method and chemical method. The chemical transfer includes surface-energy-assisted transfer [8], Surface-functionalization-medicated transfer [9], H-O catalysis bonding technique [10] and so on. The physical transfer involves laser-induced forward transfer [11], Van der Waals force transfer [12,13], viscoelastic stamping transfer [14], adhesive bonding transfer [15,16]. In all the above methods, H-O catalysis bonding transfer, adhesive bonding transfer and Van der Waals force transfer are demonstrated to be available for fiber FP interferometer preparation [10,12,15]. However, the robust bonding between film and fiber end-face can only be achieved by using H-O catalysis bonding technique and adhesive bonding transfer. But the adhesive bonding transfer usually relies on the polymer adhesive, which will result in the heterogeneity of the final structure, and it is hard to avoid the adhesive deterioration. Obviously, H-O catalysis bonding transfer seems to be a very potential thin-film transfer method to build a high-stable fiber FP interferometer. Initially, H-O catalysis bonding is used to splice fused silica components in optical systems [17,18], which is a high-precision joining technique. Subsequently, thin film transfer technology based on H-O catalysis bonding is coming through. The traditional H-O catalysis bonding is usually used to assembly thick film (micron or more), and the assembly of nanometer thin film devices based on H-O catalysis bonding is very rare. But since the H-O catalysis bonding technique has high-precision alignment as well as good adhesive properties, it is very attractive for the assembly and splicing of fiber-optic FP structures with nanoscale film materials.
Herein, a high pressure-sensitive and stable fiber-optic FP interferometer with nano-diaphragm assembled by H-O catalysis bonding is proposed and demonstrated, and it is experimentally verified that this H-O catalysis bonding technology can be used to for assembling FP interferometer with different materials nano-diaphragms. During H-O catalysis bonding-based nano-diaphragm assembly, a SiO2 nano-film, introduced as a bridging layer on the target nano-diaphragm, can be regarded as a solid adhesive to bridge hollow-core fiber end-face and nano-diaphragm by chemical reactions. After bonding reaction between SiO2 bridging layer and fiber end-face, the assembled of the nano-diaphragm to fiber end-face is achieved by dissolving support substrate of the nano-diaphragm, and the robust bonding is realized. Experimentally, Si nano-diaphragm transfer to hollow-core fiber end-face is demonstrated by using this proposed nano-film bonding method, which offers a new solution to build fiber FP interferometer without polymer material to avoid material degradation and ambient environmental influence. Meanwhile, we demonstrate the feasibility of bonding Pd nano-diaphragm to hollow-core fiber tips using H-O catalysis bonding, which can give fiber FP interferometer the capacity to detect changes in hydrogen concentration.
2. Principle of H-O catalysis bonding technology
As a chemical bonding method, H-O catalysis bonding is widely used for precisely connecting optical elements and establishing (micro-) opto-electro-mechanical systems with high-stability [19]. For H-O catalysis reaction, an aqueous hydroxide solution or silicate solution as the bonding solution should be introduced in between two oxide-surfaces, while hydroxide ions in the solution act as a catalyst and etch the two oxide surfaces. This process will release silicate ions,
Due to the instable chemical property of the silicate ion, it will not decompose only in a solution environment with a higher PH value. As the hydroxide ions in the bonding solution are consumed, PH of the solution decreases to PH < 11, the silicate ions decompose to produce a solution with supersaturated Si(OH)4 [20],
At room temperature, since the solution with supersaturated Si(OH)4, Si(OH)4 single molecules begin to polymerize to form Si(OH)4 precipitation. Due to that the hydroxide ions in the bonding solution are constantly consumed, the reaction continues in the positive direction. Ultimately, Si(OH)4 between the two surfaces can be dehydrated and condensed to siloxane chains [21], and as the lengthening siloxane chains, these chains form three-dimensional network to connect two silica surfaces together. Subsequently, the undehydrated Si-OH groups will be dehydrated and the water molecules should be evaporated or removed by heating to avoid an unstable bonding. After H-O catalysis reaction, the two silica surfaces are connected together by a stronger and more stable silicate network. Since H-O catalysis reaction process relies heavily on reaction conditions, the duration of the H-O catalysis reaction can be greatly shortened by changing the type and concentration of H-O catalysis bonding solution, reaction temperature and humidity. Generally, the whole bonding process will need several weeks under room temperature and pressure, and after bonding, the joint interface also has high-strength and high-stability. Thus, the time required for the chemical bond strength to reach its maximum can be shorten by increasing reaction temperature [22].
3. Experimental results and discussion
3.1 Si nano-diaphragm-based fiber FP interferometer and high-sensitive pressure sensing
For a high pressure-sensitive and stable fiber FP interferometer with Si nano-diaphragm assembled by H-O catalysis bonding, in order to verify its feasibility, a Si nano-diaphragm is experimentally transferred onto hollow-core fiber end-face by using the H-O catalysis bonding transfer method. Figure 1 shows the Si nano-diaphragm-based fiber FP interferometer preparation sequence using H-O catalysis bonding. Firstly, Si/SiO2 deposition. A 30 nm Si nano-diaphragm is deposited on a polished water-soluble KBr substrate by using a magnetron sputtering coater (Quorum Q300T+), and subsequently, a 35 nm SiO2 nano-film, functions as a bridging layer, is deposited on the Si nano-diaphragm by using a commercial high vacuum coating unit (JGP-450 magnetron sputtering system). Secondly, H-O catalysis bonding. For a catalysis bonding process, the crucial factor is the H-O catalysis reaction, which depends on the type and concentration of H-O catalysis bonding reaction solution. Since the chemical process is dominated by OH−, and to make sure that the entire chemical bonding process can occur at room temperature, a dilute Na2O·3.3SiO2 solution is used as catalyst and bonding solution to achieve this Si nano-diaphragm transfer, the 34% Na2O·3.3SiO2 in aqueous solution and H2O is proportion in a 1:4 ratio. The PH value of the diluted bonding solution is measured to be 11.95. Compared to using NaOH solution as catalyst and bonding solution, the siloxane network formed by using sodium silicate solution exhibits a larger range and higher bonding strength [23]. Before assembling Si nano-diaphragm to hollow-core fiber end-face, the used hollow-core fiber is spliced with a single-mode fiber by using a commercial fusion splicer (Fujikura, FSM-50S), moreover, the diameter of hollow-core fiber is equal to single-mode fiber, which is 125 µm, and the inner diameter of the hollow-core fiber is 40 µm. Then, the hollow-core fiber is cleaved to its desired length (85 µm) by using a fiber cleaver under a microscope. Since the bonding target interface is hollow-core fiber end-face, while the prepared bonding solution is introduced in between hollow-core fiber end-face and SiO2 layer, a three-dimensional silicate network is formed due to chemical reaction between SiO2 and sodium silicate solution, and it can bond the hollow-core fiber end-face and Si/SiO2 together. Thirdly, Si/SiO2 composite nano-diaphragm transfer. The final step to complete the transfer is striping Si/SiO2 composite nano-diaphragm from KBr substrate. Since the water solubility of KBr substrate, it is very easy to strip Si/SiO2 composite nano-diaphragm from KBr substrate, which can be realized by dissolving KBr substrate. After dissolving KBr substrate, the hollow-core fiber with transferred Si nano-diaphragm is heated at 150°C for 4 hours to remove water molecules. Finally, a Si nano-diaphragm is successfully transferred to the hollow-core fiber end-face by using H-O catalysis bonding. Additionally, due to its versatility and the applicability to multi-material thin film transfer, the nano-diaphragm-based FP interferometer can be endowed with a specific function or specific response characteristics by selecting different thin film materials by using this nano-diaphragm assembly method.
Fig. 1. Si nano-diaphragm-based fiber FP interferometer preparation sequence using H-O catalysis bonding. (a) Si/SiO2 deposition. (b) H-O catalysis bonding. (c) Si/SiO2 transfer.
Figure 2(a) shows the scanning electron microscopic image of SiO2 nano-film which functions as a bridging layer and is deposited on Si nano-diaphragm. Significantly, during bonding reaction, if SiO2 layer on Si surface is too thin, ions in H-O catalysis bonding solution will reach Si surface, and it can react with Si to form hydrogen, and then it prevents the chemical bonding formation and leads to failed robust bonding. After experimental verification, it is found that the thinnest SiO2 layer to achieve the robust bonding is about 35 nm in our experiments. Thus, a 35 nm SiO2 nano-film is introduced as a bridging layer on Si nano-diaphragm, it can be regarded as a solid adhesive to bridge Si nano-diaphragm and hollow-core fiber end-face together by chemical reactions. Figure 2(b)-(d) show the optical microscope photographs of hollow-core fiber end-face with a bonded Si/SiO2 film. As shown in Fig. 2(b)-(d), it is demonstrated that Si nano-diaphragm can be bonded onto hollow-core fiber end-face by using H-O catalysis bonding method.
Fig. 2. Interface morphology. (a) Thicknesses of SiO2. (b)-(d) Optical microscope images of Si/SiO2 composite film bonded onto hollow-core fiber facet.
As shown in the above experiments, H-O catalysis bonding technique can be used for the assembly of high-quality suspended nano-film. Thus, as a result of the characteristics of H-O catalysis bonding technique, we may introduce it to the fields of optical fiber sensing, in which the existing optical fiber FP fabrication methods can be optimized and refined by using H-O catalysis bonding technique, in order to construct highly stable fiber FP interferometers. Thus, we used this Si nano-diaphragm bonding technique to assemble a Si/SiO2 composite nano-diaphragm-based fiber extrinsic FP interferometer. For this FP cavity, hollow-core end-face is sealed by the suspended Si/SiO2 composite nano-diaphragm bonding, which kept an air-gap remained between single-mode fiber-hollow-core fiber spliced face and Si/SiO2 composite nano-diaphragm. Finally, the spliced face and the suspended film form the FP cavity. Herein, a Si/SiO2 composite nano-diaphragm is transferred and bonded to the hollow-core fiber facet as a suspended film by using this Si nano-diaphragm bonding technique. Since H-O catalysis bonding is a chemical reaction-based bonding method, the bonding between Si/SiO2 composite nano-diaphragm and hollow-core fiber facet is glue-free, which can indicate that FP cavity has the characteristics of heterogeneity-free and high-stability.
As this fiber extrinsic FP interferometer with Si/SiO2 composite nano-diaphragm, when light is incident from single-mode fiber, it is partially reflected (I1) by single-mode fiber-hollow-core fiber spliced face, the rest light is injected into FP cavity. When it arrives at the suspended Si/SiO2 composite nano-diaphragm, it is reflected (I2) back into the fiber core. Then, light I1 interferes with light I2, and this FP interferometer with low fineness can be modeled as a two-light interference process, for which the reflected light intensity can be expressed as [24–26],
When the FP interferometer is subjected to external pressure perturbations, the wavelength shift is
For this Si/SiO2 composite nano-diaphragm-based fiber FP interferometer, the external pressure perturbation will cause changes in the surface morphology of the suspended Si/SiO2 composite nano-diaphragm, which leads to changes in FP cavity length, and these changes will be accurately reflected in interference spectrum evolution. It is well-known that nano-diaphragm-based fiber extrinsic FP interferometer is highly sensitive to pressure, hence, we will verify the high-pressure sensitivity of this FP interferometer. While the external pressure is applied to the suspended Si/SiO2 composite nano-diaphragm, the suspended film will deform, which resulting in a change in FP cavity length. According to the deformation model, the center deflection of the suspended film is equal to the change of FP cavity length, which can be expressed as
Thus, the wavelength shift caused by external pressure perturbations is
Figure 3 depicts the experimental setup for pressure testing. A wide-band source (BBS) sends light to the FP interferometer through a circulator, the reflection spectrum from the FP interferometer is detected and recorded an optical spectrum analyzer (OSA). The FP interferometer is put into a sealed adjustable pressure chamber. Subsequently, the pressure response characteristics of the fiber FP interferometer with Si/SiO2 composite nano-diaphragm assembled by H-O catalysis bonding is experimentally verified. Figure 4 shows the reflection spectrum of Si/SiO2 composite nano-diaphragm-based FP interferometer with an 85 µm cavity length prepared by this H-O catalysis bonding technique. In order to verify the pressure response of this FP interferometer, a static pressure test is performed, and the pressure changes in a small range (0-16 kPa). As the pressure increases, it can cause the deformation of the suspended Si/SiO2 composite nano-diaphragm as well as the change of FP cavity length, resulting in a shift of the reflection spectrum of this FP interferometer. According to Eq. (8), the FP cavity length (wavelength shift) is dependent on pressure, the applied pressure can be detected and evaluated by measuring the resonance wavelength shift. As shown in Fig. 5(a), it can be observed that the reflection spectra of this FP interferometer shift toward short wavelengths with the increasing pressure. In addition, Fig. 5(a) illustrates that the reflection spectra shift linearly as the increasing pressure, which is coincident with Eq. (8). As shown in Fig. 5(b), when the pressure changes between 0 kPa and 16 kPa, from the linear fit, the pressure sensitivity is 79.6 pm/kPa, and the linearity is as high as 0.99.
Fig. 4. Reflection spectrum of a Si/SiO2 composite nano-diaphragm-based FP interferometer fabricated by H-O catalysis bonding.
Fig. 5. Static pressure response, (a) Blue shift of the spectra as pressure increases, (b) Wavelength shift versus pressure change.
After verifying the pressure detection principle and this sensor’s pressure detectability, we will investigate the dynamic response, repeatability and reversibility. Thus, the dynamic pressure response of FP interferometer is investigated through two different dynamic pressure measurement processes at room temperature. Figure 6(a) and 6(b) show time-dependent pressure response and reversibility with a reciprocating pressure evolution and straight pressure evolution, respectively. As seen in Fig. 6, the detected respective quasi-stable pressure values at a positive-going cycle are in accord with their corresponding nominated values, and they are also in accord with the results measured when recovering from high to low pressure. From Fig. 6, the dynamic responses of this FP interferometer reveal that this Si/SiO2 composite nano-diaphragm-based FP interferometer prepared by this H-O catalysis bonding technique has good repeatability and stability.
Fig. 6. Dynamic pressure response, (a) Reciprocating pressure evolution, (b) Straight pressure evolution.
Because stability is an important performance indicator in practical applications, we investigate the stability of this FP interferometer, and the result reveals that the wavelength fluctuation is less than 15 pm over 10 min, as shown in Fig. 7. The measurement error caused by central wavelength fluctuation is much lower than 0.189 kPa. Additionally, from Fig. 7, it can be observed that the wavelength shifts regularly toward long wavelengths, which indicates that the pressure in the pressure chamber decreases, thus, we deduce that the wavelength fluctuation is caused by the defects of the tightness of the pressure chamber.
H-O catalysis bonding technique, as a chemical reaction-based bonding method, possesses a high possibility of being a glue-free and heterogeneity-free bonding. The Si/SiO2 composite nano-diaphragm-based FP interferometer is prepared by this H-O catalysis bonding technique, which can avoid the introduction of polymer-based adhesion agent during preparation and eliminate the large deviation and instability caused by temperature dependence of adhesion agent. Herein, in order to verify the temperature dependence, the temperature response of this FP interferometer must be also investigated. Figure 8 illustrates the temperature response of this Si/SiO2 composite nano-diaphragm-based FP interferometer. Figure 8(a) shows the reflection spectrum evolution of this Si/SiO2 composite nano-diaphragm-based FP interferometer with the increasing temperature. As shown in Fig. 8(b), temperature gradually increases from 45 °C to 75 °C with an increment of 10 °C, the resonance wavelength shifts towards longer wavelength. Figure 8(c) illustrates that a 17.3 pm/°C average temperature sensitivity is achieved. Experimental results demonstrate that this FP interferometer prepared by this H-O catalysis bonding technique has the similar temperature sensitivity characteristic compared with the all-fiber FP interferometer [27].
Fig. 8. Temperature response, (a) Red shift of the spectra as temperature increases, (b) Wavelength shift versus temperature change, (c) Reflection spectrum evolution with the increasing temperature.
3.2 Pd nano-diaphragm-based fiber FP interferometer and high-sensitive hydrogen sensing
In order to verify that H-O catalysis bonding technique can be applied to the assembly of functional material nano-diaphragms to the end face of fibers and to broaden the functions and application scope of nano-diaphragms-based fiber external FP interferometers, the Pd nano-diaphragm is successfully transferred onto hollow-core fiber end-face by using H-O catalysis bonding technique. Since Pd is a typical hydrogen-sensitive material, this Pd nano-diaphragm-based fiber FP interferometer can be used as a hydrogen sensor. The detailed hydrogen sensing mechanism is described in our previous publication [28–30]. Figure 9(a) and (b) shows the surface morphology of hollow-core fiber end-face with a transferred 22 nm Pd nano-film. Figure 9(c) shows the reflection spectrum of the hydrogen sensor with a cavity length of 63.5 µm, fabricated using the H-O catalysis bonding technique. From Fig. 9(d), the hydrogen concentration is dynamically cycled from 0.5% to 3.5% and then back to 0.5% (in steps of 0.5%). As the hydrogen concentration increases, the Pd lattice expands, resulting in a decrease in the FP cavity length and the reflection spectrum shifts towards short wavelength. By comparing the hydrogen response during positive and negative cycles of dynamic concentration, it can be observed that the hydrogen sensor fabricated using H-O catalysis bonding technique exhibits good stability and reversibility. As shown in Fig. 9(d), while hydrogen concentration changes 3.5%, the wavelength shift is 0.375 nm.
Fig. 9. (a) and (b) Surface morphology of hollow-core fiber end-face with a transferred 22 nm Pd nano-film, (c) Reflection spectrum of a Pd-based hydrogen sensor fabricated by H-O catalysis bonding, (d) Time dependence and reversibility of the hydrogen-sensitive response.
Figure 10(a) shows the reflection spectrum evolution of the Pd nano-diaphragm-based hydrogen sensor with different hydrogen concentrations. As the hydrogen concentration increases, the resonance wavelength shifts towards shorter wavelength. The relationship between resonance wavelength and hydrogen concentration is shown in Fig. 10(b), the hydrogen sensitivity coefficient is estimated by linear fitting, the hydrogen sensitivity is 84.9 pm/1% with a linearity of 0.98. Because time response is an important performance indicator, the time response of this hydrogen sensor is investigated, and Fig. 10(c) show the response time and recovery time of this Pd nano-diaphragm-based hydrogen sensor under different hydrogen concentrations. From Fig. 10(c), it can be found that the recovery time and response time are influenced by hydrogen concentration. At low concentrations, the recovery/response time is relatively long and shortens as the hydrogen concentration increases. Therefore, the 3.5% hydrogen concentration corresponds to a minimum recovery time as low as 9.9 s and a minimum response time as low as 40 s. The experimental results demonstrate that the feasibility of bonding Pd nano-diaphragm to hollow-core fiber using H-O catalysis bonding, which can give fiber FP interferometer the capacity to detect changes in hydrogen concentration.
Fig. 10. (a) Reflectance spectra of Pd-based hydrogen sensors at different hydrogen concentrations, (b) The wavelength in a linear fit with increasing hydrogen concentration, (c) Response time and recovery time
4. Conclusion
In this paper, we proposed and demonstrated a high pressure-sensitive and stable fiber FP interferometer assembled with nano-diaphragm by H-O catalysis bonding technology, and experimentally verified that different materials nano-diaphragm can be assembled based on this H-O catalysis bonding technique, which can endow the FP interferometer with different functions. The H-O catalysis bonding technique is a technological innovation for nano-diaphragms-based FP interferometer fabrication. The use of this nano-diaphragm transfer method provides a new solution for the construction of fiber FP interferometers without heterogeneous materials, avoiding material degradation and the influence of the surrounding environment. The experimental results reveal that the Si nano-diaphragm-based fiber FP interferometer fabricated by using this H-O catalysis bonding transfer technology has a high (79.6 pm/kPa) pressure sensitivity and a low (17.3 pm/°C) temperature sensitivity, the low temperature-sensitive characteristics is quite close to that of an all-fiber FP interferometer. And beyond that, we further assembled the Pd nano-diaphragm with the hollow-core fiber tips by using H-O catalysis bonding technique, which provided the optical fiber FP interferometer with the ability to detect the change of hydrogen concentration. In certain respects, the experimental results demonstrate that H-O catalysis bonding technology has a wide range of potential applications for the construction of multi-species nanofilm fiber-optic FP interferometers. Further investigation will focus on the exploitation of multi-material nano-film patterning transfer and different nano-film integration technology.
Funding
National Natural Science Foundation of China (61727816, 62171076, 62275039); Fundamental Research Funds for the Central Universities (DUT21RC(3)021, DUT21RC(3)080); State Key Laboratory of Advanced Optical Communication Systems and Networks (2022GZKF001).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper can be obtained from the authors upon reasonable request.
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