Propagation characteristics of photonic crystal fibers selectively filled with ionic liquid

Photonic crystal fibers (PCFs) have good-quality microfluidic channels throughout their length, being able to hold a sample volume of a few nanoliters per centimeter of fiber.¹ With the help of selective filling technology, selectively filled PCFs have attracted considerable interest in optical-fiber device fabrication, and they offer a number of potential applications in environmental monitoring, biological detection, and sensing, among others.^2,3

Ionic liquids (ILs), which have been known about for a century, have received extensive interest only recently because of their unique characteristics.⁴ ILs are used widely nowadays as solvents and reaction media in many diverse applications such as carbon dioxide sensors,⁵ electrospinning,⁶ catalysis,^7,8 energy storage,⁹ and nanostructure synthesis.^10,11

In this paper, we investigate three types of PCF selectively filled with IL (1-butyl-3-methylimidazolium iodine): one with all air holes filled, one with an IL-filled air hole in the second ring, and one with an IL-filled air hole in the third ring. The transmission properties of these selectively filled PCFs are analyzed numerically and experimentally.

The PCFs (L = 8.0 cm) used in our experiment were fabricated by Yangtze Optical Fiber and Cable Company Ltd., China. Each PCF has five rings of air holes running along its axis, as shown in Fig. 1(a). The average diameter of the air holes is 3.9 μm, and the average interhole spacing is 5.75 μm. The IL used was 1-butyl-3-methylimidazolium iodine, produced by Shanghai Chengjie Chemical Co., Ltd., China, and the refractive index is 1.572 at 532 nm. The cross sections of the three types of IL-filled PCF are shown in Figs. 1(b)–1(d), respectively. In the first PCF type, IL infiltrated all the air holes under capillary force [Fig. 1(b)], whereas the second and third PCF types were developed by infiltrating IL selectively into one air hole in the second and third ring, respectively [Figs. 1(c) and 1(d)]. The filled hole can be considered as being a liquid waveguide.

The experimental setup used to measure the rates of change of the spectra of the first and second types of IL-filled PCF is shown in Fig. 2(a). The range of the broadband light source (BLS; Keysight 8164B lightwave measurement system) was 600–1700 nm. The IL-filled PCFs were placed in a temperature chamber, and an optical spectrum analyzer (OSA; Yokogawa AQ6370C; operation wavelength range: 600–1700 nm) with a maximum resolution of 0.02 nm was used for analysis.

Figure 2(b) shows the experimental setup used measure how the intensity of the resonant dips changed with the polarization of the light source. The range of the BLS was 1455–1640 nm, and a polarization synthesizer (PS; Keysight N7786B) was used. The third type of IL-filled PCF was placed in the temperature chamber, whereupon an optical power meter (OPM; Keysight N7744A; operation wavelength range: 1455–1640 nm) with a maximum resolution of 2 pm was used.

First, we investigate numerically the characteristics of the three different types of filled PCF. All the calculations are based on the full-vector finite-element method and plane-wave expansion method. The parameter values were set to those of the actual PCFs as given above. Figure 3 shows the room-temperature bandgap distribution for the fully filled PCF shown in Fig. 1(b). The normal index-guided PCF was transformed into a photonic bandgap fiber because of the higher refractive index of the IL rods in the cladding compared with that in the air holes. There are two obvious bandgaps, namely, 1100–1200 nm (bandgap 1) and 1200–1500 nm (bandgap 2), in which the fundamental mode LP₀₁ of the central solid core is confined; this is formed by the super mode of the IL-filled rods in the cladding.

Figure 4 shows the dispersion characteristics for LP₀₁ in the central solid core and the higher-order modes in the IL-filled core in the second ring of the PCFs. LP₀₁ couples with LP₁₂ and LP₃₁, and the anti-cross area is around the wavelengths of 1200 and 1300 nm. Moreover, LP₀₁ couples with LP₀₂ and LP₂₁ around the longer wavelength of 1800 nm.

For the third type of PCF, the IL is in the third ring, separated from the central solid core by two rings of air holes. Therefore, we consider the solid core and the IL core as being two separate waveguides.¹² The dispersion characteristics for LP₀₁ and the higher-order modes are shown in Fig. 5. The resonant wavelength between LP₀₁ and LP₂₁ is around 1690 nm. LP₀₁ and LP₀₂ couple at longer wavelength, and the resonant wavelength is around 1780 nm, which is similar to that shown in Fig. 4.

In the experiment, selective filling¹³ was used to fill each PCF with IL as shown in Figs. 1(b)–1(d). In each type of filled PCF, the length filled with IL was nearly 8.0 cm, and the air holes at either end were left to reduce the splice loss when spliced with a standard single-mode fiber (SMF; Corning SMF-28). During the experiment, the filled PCFs were placed in a temperature chamber to change the effective refractive index of the IL.

Figure 6(a) shows the transmission spectrum of the fully filled PCF shown in Fig. 1(b). The insertion loss is less than 12 dB, due mainly to the mode field mismatch of splicing between the PCF and SMF. There are three clear attenuation gaps at the wavelengths of 780–830 nm (BG1), 1000–1100 nm (BG2), and 1280–1500 nm (BG3). The positions of BG2 and BG3 are similar to the simulation results, as shown in Fig. 3. With increasing temperature, the entire spectrum moves to shorter wavelengths. We measured the movement of dip A; with increasing temperature from 27 °C to 60 °C, the sensitivity is around −2.9 nm/°C as shown in Fig. 6(b).

Figure 7 shows the propagation characteristics of the second type of PCF shown in Fig. 1(c). Unlike that in Fig. 6(a), this spectrum contains no bandgap, but Fig. 7(a) shows some obvious resonant dips. According to the simulation results, the dips at around 1080 and 1260 nm should be due to the coupling between LP₁₂ and LP₀₁ and between LP₃₁ and LP₀₁, respectively. The coupling between LP₀₂ and LP₀₁ and between LP₂₁ and LP₀₁ could not be measured because the upper measurement wavelength of the OSA was 1700 nm.

Next, we analyze the temperature tuning characteristics of this type of PCF. Figure 7(a) shows the transmission spectrum at 30 °C, 39 °C, and 48 °C. With increasing temperature, each dip in the resonant region experienced blue shift. The shapes of the resonant dips for LP₁₂ and LP₀₁ changed minimally with temperature, which is why we measured only the resonant dips for LP₃₁ and LP₀₁. The relationship between resonant wavelength and temperature maintains good linearity, being around −1.89 and −1.95 nm/°C as measured from dips B1 and B2, respectively, in Fig. 6(b).

Next, we study the third type of IL-filled PCF shown in Fig. 1(d). With the setup illustrated in Fig. 2(b), the polarization angle (PA) from 0° to 90° was divided into 40 equally spaced values. Similar to the details shown in Fig. 6(a), the spectrum from 1450 to 1640 nm contains some resonant dips, as shown in Fig. 8(a). These dips could be formed by the coupling between LP₂₁ and LP₀₁ in this wavelength range. Moreover, the coupling between the modes in this type of PCF is weaker because the distance between the silica core and the IL-filled core in the third ring is greater than that in the second ring, and so the resonant dips are narrower.

Figure 8(a) shows the transmission spectrum of the resonant dips under different PAs at a temperature of 37°C. The intensity of the coupling between the different modes changes with the polarization of the input light. If the polarization of the two coupling modes is similar, then the dips in the spectra deepen, and vice versa. Figure 8(b) shows that the intensity of dip C changes linearly as the PA increases from 52.5° to 62.5° and becomes shallow when the PA rises to 57.5°. The sensitivity is around −0.79 dB/PA.

Next, we analyze the temperature response of this type of PCF. The PA of the input light was set to 10° because the transmission dips of this type of PCF are sensitive to the PA. Moreover, the effective refractive index of the IL changed slightly with temperature, and the dip intensity tended to be shallow under the same PA. Therefore, the temperature was increased by only 0.1 °C each time to reduce the effects of temperature and polarization on the spectrum shape, as seen in Fig. 8(c). The sensitivity of dips D1 and D2 is around −2.26 and −3.51 nm/°C, respectively [Fig. 8(d)].

We developed three types of IL-filled PCF and investigated their bandgap distribution, coupling mode, and propagation characteristics numerically and experimentally. The PCF with an IL-filled air hole in the third ring was the most sensitive to temperature, with a sensitivity of around −2.9 nm/°C. The intensity of the resonant dip was sensitive to the PA; the rate of change of PA intensity was around −0.79 dB/PA. The utility of IL-filled PCFs can be controlled by changing the filling position flexibly, thereby allowing IL-filled PCFs to be used under flexible conditions and controllable temperatures to create a compact PA sensor.

This work was supported partly by the National Natural Science Foundation of China (Grant Nos. 11804171, 11674177, and 61775107), partly by the Natural Science Foundation of Tianjin, China (Grant No. 16JCZDJC31000), and partly by the Self-made Experiment Teaching Instrument Project of Nankai University 2018 (Grant No. 2018NKZZYQ04).

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

 Xiaoqi Liu received M.S. and Ph.D. degrees in optical engineering and precision instruments and machinery from Harbin Engineering University, Harbin, China, in 2008 and 2014, respectively. She is currently an Experimenter at the College of Electronic Information and Optical Engineering, Nankai University, Tianjin, China. Her research interests include opto-fluids in microstructure fibers and fiber sensors.

 Yange Liu received an M.S. degree in optics from Nankai University, Tianjin, China, in 1998, and a Ph.D. degree in optical engineering from Tianjin University, Tianjin, in 2001. She is currently a Professor at the Institute of Modern Optics, Nankai University. Her research interests include micro/nanostructured optical devices, optical fiber communication and sensing technology, and optical fiber lasers and amplifiers. She has published more than 100 peer-reviewed journal papers that have been cited more than 2000 times.

 Zhi Wang received B.S. and M.S. degrees from Tianjin University, Tianjin, China, in 1998 and 2001, respectively, and a Ph.D. degree in optics from Nankai University, Tianjin, in 2005. He is currently a Professor at the Institute of Modern Optics, Nankai University. His research interests include microstructured optical fiber devices, optical fiber communication and sensing technology, optical fiber lasers, and tunable photonic devices.