Mechanism and sensing applications of antiresonant reflecting guidance in an alcohol-filled simplified hollow-core (SHC) photonic crystal fiber (PCF) are demonstrated. By filling one air hole in the air cladding of the PCF with alcohol, anti-resonant reflecting guidance of light can be achieved and energy leakage of the core modes can be induced at resonant wavelengths of the Fabry-Pérot (F-P) resonator formed by the alcohol-filled layer combined with the silica cladding in the cross-section of the PCF. The proposed structure exhibits periodic lossy dips in the transmission spectrum, of which the visibilities are sensitive to the refractive index of surrounding medium due to the reflectivity variation of the F-P resonator. Water level sensing is experimentally realized with this principle and the lossy dip exhibits a linear decrease against water level with a sensitivity of 1.1 dB/mm. The sensor is also sensitive to environmental temperature and a temperature sensitivity of −0.48 nm/°C is obtained between room temperature and 60 °C.
© 2013 Optical Society of America
Hollow core photonic crystal fibers (HC-PCFs) are optical fibers which guide light in a hollow core surrounded by a microstructured cladding formed by periodic arrangement of air holes in silica [1,2]. In HC-PCFs, light is confined in the air core by the photonic bandgap (PBG) effect , where the holey photonic crystal cladding acts as a mirror, and reflects light at wavelengths within the bandgap. HC-PCFs have been used to build up sensors for applications such as strain and temperature measurement , hydrostatic pressure sensing , and gas sensing . Due to its strong confinement of optical field in the air core, mode field can hardly reach beyond the claddings of such fibers. To measure the refractive indices (RI) of surrounding environments, the analyte has to be contacted with the core field of the HC-PCF, and special procedure has to be taken such as drilling a hole into the fiber core with femtosecond laser , which causes a lot inconvenience in its practical applications. Long period gratings (LPGs) have been fabricated in PCFs for RI sensing, however, LPGs made in HC-PCFs may not be suitable, since the higher order cladding modes can hardly be excited to expand to the surrounding media [8,9]. Recently, leaky mode resonance (LMR) has been realized in capillary waveguides or pipe waveguides. The core modes can oscillate and radiate through the cladding since the core has a refractive index less than the cladding. Guiding mechanism of these waveguides has been described as the anti-resonant reflecting guiding , which is similar to the anti-resonant reflecting optical waveguide (ARROW) model [11,12]. The cladding of the capillary waveguide is considered as a F-P etalon, where wavelengths that satisfy the resonate condition of the etalon take no reflection at the core-cladding interface and radiate through the cladding (leaky modes), and the wavelengths under the anti-resonant condition of the cladding are confined in the air core (guided core modes). The presence of leaky modes in the cladding makes these waveguides sensitive to the surrounding environments, and various sensing applications have been proposed like bio-sensing , film sensing , powder and liquid vapor sensing . Most of the above mentioned works are based on pipes or tubes, and the pumping and detecting are based on free space coupling, which makes the system complicated and costly. So an all-fiber configuration is expected to be more compact and convenient for in field sensing applications.
In this paper, we report the mechanism and sensing applications of anti-resonant reflecting guidance in alcohol-filled SHC PCF. By infiltrating alcohol into one air hole in the air cladding of the fiber, a double-layered F-P resonator can be formed by the alcohol-filled layer combined with the silica cladding in the fiber cross-section. The infiltrated alcohol causes the reduction of light confinement in the core and core mode field expansion, and leaky mode resonance can be achieved at resonant wavelengths of the double layered F-P resonator, which results in lossy dips in the transmission spectrum of the fiber. The visibilities of the dips are sensitive to surrounding medium since the reflectivity of the double layered F-P resonator can be affected by the refractive index of surrounding medium. The proposed fiber structure can be used for water level sensing and the lossy dips exhibit linear decay against the increasing of water level and a sensitivity of 1.1 dB/mm is obtained. The sensor is also sensitive to environmental temperature, and the wavelength of the lossy dips blueshift with temperature increasing and exhibits a sensitivity of −0.48 nm/°C between room temperature and 60 °C. The compact device can be an attractive candidate for physical, biological and chemical sensing.
2. Fabrication and guiding mechanism of the proposed structure
The cross section view of the fiber (produced by the YOFC Ltd.) is given in Fig. 1(a), which is composed of a hollow hexagonal core and a layer of air hole cladding [7,16]. The diameter of the outer cladding, the air holes cladding, the air core and thickness of the struts is 140 μm, 70 μm, 22 μm and 340 nm, respectively. To make the proposed structure, a selective infiltration method similar to that reported in  was used. However, glue sealing was used here in the blocking process since the silica struts would collapse during the arc fusion procedure. First of all, a section of SHC PCF was cleaved at both ends, and one end of the fiber was sealed with glue, as can be seen in Fig. 1(b). The depth of the glue at the fiber end was less than 30 μm. After drying of the glued end, femtosecond laser micromachining was carried out to selectively open the air holes. A femtosecond (fs) laser that produces pulses with duration of 35 fs at 800 nm wavelength and a repetition rate of 1 kHz was used. Fs laser pulses with pulse energy of 10 μJ were focused onto the fiber end through a 10 × microscope objective with an exposure time of 3 seconds. The selectively opened air hole can be seen in Fig. 1(c). Then the drilled fiber end was immersed into alcohol to be infiltrated by capillary effect. After the infiltration process, the fiber was cleaved at both ends, and was spliced to SMFs with a Fujikura 80S fusion splicer. Arc parameters were optimized to avoid air holes collapsing in the SHC PCF. It should be noted that the alcohol at the fiber ends was evaporated due to the high temperature during the arc fusion process. The schematic construction of the proposed device is given in Fig. 1(d).
The SHC PCF we used is categorized as the Kagomé-lattice fiber [18,19], the nature of guidance is mainly driven by the silica struts surrounding the hollow core, playing the role of ARROW . Before the fiber is infiltrated with alcohol, light is well confined in the air core of the SHC PCF due to the weak interaction between the core and cladding modes . This can be explained by the strong transverse field mismatch between the modes, which leads to the washing out of the overlap with the core field distribution .
With the presence of alcohol in the air holes of SHC PCF, the interaction between the core and cladding modes can be greatly enhanced due to better phase matching, which leads to the degradation of light field confinement in the air core. Fields of the core mode will radiate through the silica ring surrounding the air core, and reaches the outer cladding. Figure 2(a) gives the sketch of the optical path of the beams at the alcohol-filled area and the outer silica cladding. The alcohol-filled air hole combined with the outer silica cladding can be considered as a double-layered F-P resonator . For the wavelengths that satisfy the resonate condition of the resonator, constructive interference occurs, which means that the F-P resonator is highly transparent for these wavelengths, and light can not be reflected and will leak out of the cladding. On the other hand, for the anti-resonant wavelengths (i.e. the wavelengths that do not satisfy the resonate condition), destructive interference take place, and light can be well reflected by the F-P resonator, which is confined in the air holes of the fiber as the guided modes of the waveguide. The position of the non-transmission wavelengths can be described by the following equation Eq. (1) we can see that the resonant wavelengths are dominated by the thickness and material index of alcohol and the outer silica cladding. Considering d1 of about 23 μm (the value of d1 is variable, here we consider its maximum), d2 of 33 μm, and the refractive index of alcohol and silica of 1.354 and 1.46 respectively, the non-transmission wavelengths can be predicted to be at 1532.97 nm, 1554.57 nm, 1576.77 nm, and 1599.6 nm within the band range of our light source with Eq. (1). Two samples with filling length of 6 mm and 17 mm were made, and the transmission spectrum were collected with an ASE light source (ALS-1550-20) and an optical spectrum analyzer (Yokogawa AQ6370B), which are plotted in Fig. 2(b). The locations of the transmission dips of the two samples are in good agreement with the theoretical predictions with slight deviations. In fact, since the value of d1 is variable for different radial directions (as can be seen from the cross section view of the fiber in Fig. 1(a)), the predicted non-transmission wavelengths would be variable as well. From Fig. 2(b) we can also see that the visibilities of the attenuation dips of the 6-mm- and 17-mm-filled sample are 10 and 20 dB, respectively, which means that the energy leakage accumulates along the alcohol-filled fiber length at resonant wavelengths. To further understand the variation of the non-transmission wavelength location, we made another sample in which the six air holes of the air cladding were all infiltrated with alcohol, to make comparison with the sample filled only one air hole (Fig. 3(a)). The spectrum of the 1-hole- and 6-hole-filled samples are plotted in Fig. 3(b), from which we can see that 6-hole-filled sample has several non-transmission wavelength regions rather than some non-transmission dips of the 1-hole-filled sample, and each region includes several peaks as well. This phenomenon indicates that the transmission spectra of the 6-hole-filled sample results from the overall contribution of the anti-resonant reflecting effect in all of the 6 filled holes, where each of the filled holes exhibit different non-transmission wavelengths due to geometrical differences.
3. Water level and temperature sensing
As has been discussed in section 2, the light is guided in the hollow core fiber waveguide by the anti-resonant reflection guiding mechanism, which means that the reflected power from the double-layered F-P resonator can be equivalent of the transmission power of the waveguide. By calculating the reflections of the F-P resonator, the transmission of the fiber waveguide can be obtained. Figure 2(a) illustrated the multiple beam interference within the double-layered F-P etalon, with the notations of the amplitude of the incident light to be Ai, the reflection coefficient of the incident light at the air core-alcohol interface and the silica cladding-surrounding medium interface to be r and r’, and the transmission coefficient incident in and out of the alcohol-filled area to be t and t’, respectively. The reflection and transmission at the alcohol-silica cladding interface is ignored in this case to simplify the calculation. The complex amplitude of the reflection light of the F-P resonator can be written asEqs. (2) and (3) into (4), we have Eq. (5), and we get Eq. (6) we can see that the intensity of the resonant wavelength is influenced by the reflective coefficient at both boarders of F-P resonator. In this case, if the surrounding media of the alcohol-filled fiber changes, the reflective coefficient at the outmost surface of the cladding (r’) will be changed, which will change the transmission power at the resonant wavelengths.
When the fiber is under normal atmosphere, the r and r’ have close absolute value, and Tresonant is calculated to be a small value by Eq. (6), which correspond to the very low transmission power at the resonant wavelengths, as we have observed in our experiment. However, when the alcohol-filled PCF is immersed into water, the refractive index of the surrounding media will change from 1.000 to 1.336, and the reflection coefficient at the outmost surface (r’) will thus drop to a much smaller value. In this case, the transmitted power of the resonant wavelengths (Tresonant) will be much higher compared with the case in air. So according to Eq. (6), if the whole PCF is immersed under water, the transmission dips at the resonant wavelengths should almost disappear. Of course, the degradation of the transmission dips is determined by the immersing depth in water, since the part of fiber which remains above water (in air) still contributes to the transmission losses at the resonant wavelengths. We carried out an experiment to investigate the water level response of the proposed sensor structure using a setup like that in . The sensor was fasten to a vertically placed translation stage, and gradually immersed into water by adjusting the height of the stage. We used the broadband light source and the optical spectrum analyzer to collect the device spectra simultaneously. The visibility of the transmission dips were found to degrade with immersing depth (see Fig. 4(a)). Note that the degradation started only when the alcohol-filled part of the SHC PCF was immersed into water, which could be defined as the effective sensing head. By the time when the effective sensing head was totally under water, the attenuation dips disappeared completely, which means that the length of the effective sensing head is also the measurement range of the sensor. The wavelength of the attenuation dips did not change much during the immersing process, as seen in Fig. 4(a). Figure 4(b) shows the liquid level response of a sample with filling length of 1 cm in one air hole, the loss of the attenuation dips exhibits approximately linear decrease response to water level, with a sensitivity of 1.1 dB/mm. Assuming a light intensity resolution of 0.01 dB, the water level resolution could be estimated to be about 10 μm for the proposed sensor.
The temperature response of the sensor was investigated with a column oven (LCO 102). A sample with filling length of 1.2 cm in one air hole was heated from room temperature to 60 °C with an increment of 10 °C. The measurement range is limited below the boiling point of alcohol (78.37 °C) to avoid its evaporation. This situation can be improved by using a liquid with higher boiling point like ethylene glycol. The attenuation dips were found to shift towards shorter wavelengths with temperature increasing, as can be seen in Fig. 5(a). The wavelength shifting can be explained by the thermal-induced refractive index change of alcohol and silica material. By differentiate Eq. (1) with respect to temperature (T), we get the temperature sensitivity as Eq. (7). It should be noted that in Eq. (7) we only considered the change of n1 (dn1=αdT, α is the thermo-optic coefficient of alcohol), since silica has much smaller thermo-optic coefficient than alcohol, which can be ignored in this case.Eq. (7). The wavelength shift versus temperature in our experiment is plotted in Fig. 5(b), and the temperature sensitivity was obtained to be −0.48 nm/°C.
In conclusion, we demonstrated an alcohol-filled SHC PCF sensor which works on the antiresonant reflecting guidance mechanism. Attenuation dips are observed in the transmission spectra of the structure, which originate from the energy leakage from the core to outer cladding at the resonant wavelengths of the double-layered F-P resonator formed by the alcohol-filled layer combined with the silica cladding in the cross-section of the PCF. Attenuation dips are observed in the transmission spectra of the structure, which originate from the energy leakage from the core to outer cladding at the resonant wavelengths of the double-layered F-P resonator formed by the alcohol-filled layer combined with the silica cladding in the cross-section of the PCF.
This work was supported by the National Natural Science Foundation of China under Grant Nos. 61108016, 61138006, 11074085 and 60925021 and also by ”the Fundamental Research Funds for the Central Universities,” HUST2013QN022.
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