>Sagnac ring miniature optical fiber coupler for water pollution detection
This paper presents a chloride ion sensor based on a miniature fiber optic coupler with a Sagnac ring. The miniature optical fiber coupler is manufactured by using a flame brush coating method to pull two single-mode optical fibers twisted together. This structure is highly sensitive to changes in external refractive index because the microfiber coupler has a small diameter, allowing most of the evanescent field around the microfiber to interact with the external environment, while the Sagnac ring is based on a clockwise and counterclockwise beam Multi-beam interference. Can the sensor be used to measure the chloride ion concentration with high sensitivity in real time? It has an important role in monitoring the quality of water resources, detecting marine chloride ion concentration, and monitoring environmental conditions.
出的氯 离子浓度传感器的示意图。 Fig. 1 shows a schematic diagram of the proposed chloride ion concentration sensor. It consists of a miniature fiber optic coupler and a Sagnac ring.
Figure 2 shows a microscopic image of a miniature fiber optic coupler with a diameter of 10 μm. A coupler is formed by tapering two single-mode optical fibers (Corning SMF-28) together using a flame brushing method. First, the fibers are tangled together and fixed on a translation stage of a fiber cone system. The fiber is then heated with a hydrogen flame while precisely controlling the translation phase to move in the opposite direction. Finally, the two outputs of the coupler are fused together to form a Sagnac ring. The input light is a supercontinuum broadband light source (SBS) with a spectral range of 1250-1650nm. The output spectrum is recorded using a spectral analyzer (OSA) with a resolution of 0.2nm.
Figure 1 Experimental device for measuring chloride ion concentration
Figure 2 A microscope with a 10 μm diameter coupler
In the traditional Sagnac ring structure, the main function of the fiber coupler is to split the light into two beams, and these two beams propagate in the fiber loop in the opposite direction. The two beams accumulate phase differences in the fiber loop and recombine at the coupler, causing interference. The transmission equation of a Sagnac ring based on a standard commercial coupler is as follows:
） 2 (1) T (λ) = (1-2 ) 2 (1)
Where r is the coupling radius of the coupler. The coupling coefficient rM of the miniature fiber coupler is expressed as
Where V is the normalized frequency and is expressed by
Where a is the diameter of a microfiber, n1 and n2 represent the refractive index of the fiber cladding and the external environment, respectively. From equations (2) and (3), it can be observed that r M is related to the input wavelength λ, the diameter 2a of the miniature fiber coupler, and the refractive index of the external environment n2. For a given miniature fiber-optic coupler, the diameter is fixed, so only the input wavelength and changes in the external environment need to be considered.
When the light of amplitude Ein is incident on one of the input ports, the output amplitude of the output port is expressed as follows:
Where L is the coupling length. When ports 3 and 4 are connected together to form a Sagnac ring, the light propagates backwards in the ring and causes interference in the coupler. In this case, the light from ports 3 and 4 is incident light, so the transmission equation for port 2 is
For standard commercial couplers, the input light Ein is divided into E3 and E4, and its expression is:
From equations (4), (5), (7) and (8), the coupling radius of the proposed sensor can be obtained:
Combining all the above formulas, the transfer equation of the structure is obtained as follows:
For microfiber couplers that have been manufactured, any change in the external refractive index has a large effect on the coupling coefficient. When the external environment is a chloride ion solution, the following expression is: . Therefore, the sensor can be used to measure the chloride ion concentration , Its sensitivity can be expressed as
Can be calculated from equation (2) . At a certain temperature, the relationship between the two is linear and can be inferred . It is easy to find S> 0, which means that as the refractive index of the external environment increases, the interference spectrum shifts to longer wavelengths.
Figure 3 Optical transmission spectrum simulation of different ports
Figure 4 Simulation of optical transmission spectra at different chloride ion concentrations
Based on the above equation and the manufacturing parameters of the miniature fiber coupler, we performed a theoretical simulation. Figure 3 shows the interference spectra of different output ports. The solid black line indicates the interference spectrum of the output light from port 3 or port 4, which is transmitted only in the miniature fiber coupler. The solid red line is the interference spectrum of the output light of port 2. It is transmitted in the miniature fiber coupler and reflected by the Sagnac ring. In miniature fiber couplers, mode coupling occurs between two single fibers. As described in coupled-mode theory, two tapered fibers work together as a composite system to propagate even and odd modes. Due to the interference between the odd and even modes, a periodic exchange of guided mode power is caused, so that the transmission spectrum exhibits a periodic resonance behavior at the output. After connecting the Sagnac rings formed by ports 3 and 4, the two beams from the microfiber coupler will propagate backwards in the loop and will again interfere when they reenter the microfiber coupler. Therefore, as shown by the red line in FIG. 3, the output transmission spectrum of port 2 is enhanced. Figure 4 shows that as the ambient concentration increases, the wavelength shifts red.
The experimental setup for measuring the chloride ion concentration is shown in Figure 1. Light is emitted from a supercontinuum broadband light source (SBS), providing input light from 600 to 1700 nm and entering port 1. The light then enters the Sagnac ring formed by connecting ports 3 and 4 through the micro-fiber coupler and travels in the loop in the opposite direction. Finally, the light passed through the micro fiber coupler again and entered port 2. Port 2 was connected to an OSA (Yokogawa AQ6370C) with a resolution of 0.2 nm, and the transmission spectrum was recorded.
Figure 5 Spectrum of ports 3 and 4
When the input light only passes through the micro-fiber coupler, the output spectra of the light from ports 3 and 4 are shown in Fig. 5, respectively. Due to mode interference between the low-order symmetrical mode and the antisymmetric mode, the output spectrum oscillates. The power of the two output ports is equal, and finally a 50:50 split ratio is achieved. This is because the energy is incident from a fiber in the uncoupled region and is concentrated in the core.
Figure 6 Transmission spectrum with and without Sagnac rings
When the light beam propagates into the tapered region, the normalized frequency decreases as the core and cladding become thinner, and the energy gradually diffuses outside the core. It can also be seen from FIG. 6 that the power distribution at the output port is different along different optical paths. When the input light passes through the miniature fiber coupler and propagates to the Sagnac ring, the output transmission spectrum of port 2 is shown by the red line in FIG. 6. Figure 6 shows that the free spectral range (FSR) of the micro-fiber coupler is about 26 nm, while the FSR of the micro-fiber coupler with Sagnac ring, as shown in Figure 6, is about 50 nm, which means that the FSR has micro-fibers with Sagnac ring The coupler is almost half of a separate microfiber coupler. It can be observed from equation (5) that the power of the output light is equal to the sum of the light powers at ports 3 and 4, that is, the Sagnac ring enhances the interference of light.
Figure 7 Evolution of the transmission spectrum of the sensor under different chloride ion concentrations
A miniature fiber coupler with a Sagnac ring was used to study the characteristics of the sensor. The miniature optical fiber coupler has a diameter of 10 μm and a length of 1 mm. The manufactured sensor is used to detect changes in the external chloride ion concentration. First, solutions with different chloride ion concentrations were prepared by mixing deionized water with a certain mass fraction of a 20% NaCl solution. The concentration range increased from 20 ‰ to 60 ‰ with an increment of 10 ‰. It is worth noting that the measurement process should be performed at a constant ambient temperature without wind interference. In addition, the fiber should be kept straight to avoid measurement errors due to bending. The transmission spectrum of the Sagnac ring of the miniature fiber coupler based on different chloride ion concentrations is shown in FIG. 7. Figure 7 shows that all resonance peaks shift with increasing chloride ion concentration. This result occurs because most of the evanescent field extends into the medium around the tapered waist of the microfiber coupler, which strongly affects the interference conditions of light entering the two ports of the Sagnac ring. The change in ion concentration causes a change in the coupling coefficient of the micro-fiber coupler, which results in a change in the interference spectrum of the proposed structure. Therefore, the observed change in the transmission spectrum may be related to the value of the chloride ion concentration. Here, three formants with different wavelengths are selected for further research and analysis. The results are shown in Figure 8.
Figure 8 Fitting plots at wavelengths of 1390, 1478, and 1595 nm, respectively
Figure 8 is a fitting graph showing curves corresponding to different chloride ion concentrations; from (A) to (C), these curves represent data recorded at wavelengths of 1390, 1478, and 1595 nm, respectively. It is worth noting that as the chloride ion concentration increases, all corresponding tilt angles in the curve shift to longer wavelengths, and the maximum sensitivity is obtained when the chloride ion concentration is 60 ‰. According to the derived fitting equation, the maximum chloride sensitivity at wavelengths of 1390, 1478, and 1595 nm were 338, 367, and 423 pm / ‰, respectively, and the determination coefficients R2 were 0.986, 0.987, and 0.980, respectively. Table 1 lists the comparison of different fiber chloride ion concentration sensors. In this experiment, the accuracy of the wavelength shift measurement is about 0.2 nm, and the detection limit of the chloride ion concentration is as low as 0.447%. Higher sensitivity can be achieved by measuring at longer wavelengths or by reducing the diameter of the miniature fiber coupler.
Table 1 Comparison of different optical fiber chloride ion concentration sensors
This paper introduces a fiber optic coupler that uses Sagnac rings to detect water pollution, especially chloride ion concentrations. The characteristics of transmission spectrum under different chloride ion concentrations were studied. As the chloride ion concentration increases, the resonance peak shifts to longer wavelengths. At a wavelength of 1595nm, the sensitivity reached a maximum of 423pm /%, and the detection limit of the chloride ion concentration was 0.447%. These research results show that the proposed structure has great application potential in the fields of water quality monitoring, biomedicine and oceanography.
From <Microfiber coupler with a Sagnac loop for water pollution detection>