A dual-core photonic bandgap fiber (PBGF) is demonstrated by infusing a high-index liquid into a dual-core air-silica photonic crystal fiber (PCF). Extremal couplings have been experimentally observed. The temperature tunable characteristics of the dual-core PBGF’s bandgap guiding and dual-core coupling are experimentally and numerically investigated. When we rise temperature, the dual-core PBGFs’ bandgaps have been changed: compression of bandwidth, blue-shift and depression of the guiding band. Especially, the dual-core coupling is temperature tunable because of the tunability of the infusion liquid’s index. We find that the rise of temperature increases the coupling length which results in the blue-shift of the resonant peak wavelengths with a speed of 1.9nm/°C, for a 20mm dual-core PBGF.
© 2008 Optical Society of America
Photonic crystal fibers [1, 2] (PCF) have drawn much interest because of their wide range of improved optical properties and the ability of flexibly manipulating these properties. Accordingly, many PCF devices have been demonstrated such as PCF gratings, PCF filters, and PCF couplers [3–7]. As an important device for fiber communication and fiber sensing, fiber couplers have achieved many applications and drawn increasing research interest. Particularly, by introducing two defects, two cores, into a PCF, dual-core in-fiber PCF coupler can be realized. In their study, K. Saitoh et al. found that PCF coupler can significantly shorten the fiber length for multiplexer-demultiplexers to only few millimeters [3, 4]. Recently, by introducing high-index rods into dual-core PCFs to replace the air-holes, Z. Wang, et al. have fabricated all-solid dual-core photonic bandgap fibers (PBGF) and experimentally observed extremal coupling and de-coupling .
On the other hand, we notice that there are heating studies on material infused PCFs in recent years. Tunable devices based on air-silica PCFs, such as fiber filters and fiber gratings have been proposed by infusing active materials into their air-holes [8, 9]. What’s more, PBGFs can be implemented by infusing the index-guided air-silica PCFs with high-index materials, whose bandgap can be shifted by varying the index of the material. For instance, by filling liquid polymer or liquid crystal (LC) into single-core air-silica PCFs’ air-holes, tunable PBGFs and PBGF devices have been demonstrated [10–17]. In these fibers, the guidance is due to the anti-resonant scattering which can be explained by the ARROW regime [18, 19].
Considering these merits of infusion in PCFs, we aim to fabricate a novel kind of dual-core PBGFs in this study based on the infusion in a dual-core PCF, which hasn’t yet been studied to our best knowledge. The dual-core PBGF is realized by infusing a high-index temperature-responsive liquid into a dual-core air-silica PCF. The temperature tuned bandgap guiding and dual-core coupling have been experimentally and numerically investigated. The experimental results have good agreement with the theoretical results.
2. Fabrication and bandgap transmission of the dual-core PBGF
The dual-core air-silica PCF we adopted in the experiment is presented in Fig. 1(a), wherein, the pitch Λ (hole-to-hole distance) is about 3.7µm, the air-hole’s diameter D is averaged to 2.6µm, and the core-to-core distance Dc is 7.4µm.
The liquid material (Certified Refractive Index Liquids produced by Cargille Laboratories) we employed for infusion in this study has a refractive index of 1.650 at room temperature (25°C). Its refractive index is tunable typically between 20°C and 80°C with a temperature coefficient of dn/dT=-0.000465°C-1. The liquid can be easily infused into the dual-core PCF by capillary force and thus transform the dual-core PCF into a dual-core PBGF. Figure 1(b) is the sectional image of the dual-core PBGF with the infusion liquid, in which, we can find supermodes in the liquid cylinders in cladding. Without any difficulty, we realized infusion of centimeters as shown in Fig. 1(c). Since there is possibly no bandgap guiding in red light band, the red light from the super-continuum source can not be confined in the silica core. Therefore, the dual-core PBGF turns to be red (the bright line in Fig. 1(c)).
We measured the spectral transmission of the dual-core PBGF with a fiber based super-continuum source. The input light was coupled into one of the two cores and the output light from each of the two cores was measured by an optical spectrum analyzer. The transmission spectrum in Fig. 2 presents five bandgaps, which is in good agreement with the numerical results. The bandgap edges are calculated by the plane-wave-expansion method (PWEM) using the MIT plane-wave package [20, 21] and the dispersion curves are calculated by the finite element method (FEM) using the FEMLAB™ full vector finite element method package. The bandwidths of the bandgaps are about 140nm from 1235nm to 1375nm (Bandgap-1), 60nm from 975nm to 1035nm (Bandgap-2), 55nm from 915nm to 970nm (Bandgap-3), 80nm from 750nm to 830nm (Bandgap-4) and 50nm from 650nm to 690nm (Bandgap-5), respectively.
For dual-core fibers, there is always coupling between the two cores. The coupling length Lc is the distance along the fiber in which there is total transfer of power from one core to the other: Lc=π/|βeven-βodd|, where βeven and βodd are the propagation constants of the even and odd supermodes at wavelength of λ, respectively. And the output power P is the power output from the same core the light was originally inputted:
where L 0 and P 0 are the fiber length and total power, respectively. Generally, the coupling length is wavelength-dependent and the output power from either of the two cores will vibrate along wavelength because of the dual-core coupling according to Eq. (1). Accordingly, as shown in Fig. 2, the resonant peak wavelengths (in the bansgaps) correspond to the maximum or minimum couplings. The length of the dual-core PBGF is about 20mm.
We fabricated another dual-core PBGF with a fiber length of about 60mm. The experimental transmission spectra in Bandgap-1 are presented in Fig. 3. The wavelength distances between adjacent peaks are much smaller than those in Fig. 2, as a result of the longer fiber length. What’s more, at wavelengths around 1270nm and 1355nm as shown in Fig. 3(a), extremal couplings are observed by presenting either maximum or minimum coupling lengths [5, 6]. Generally, at wavelengths around extremal couplings, the coupling length varies slowly along wavelength, which results in slower variation of transmission. Therefore, the slower variation of the coupling length (along wavelength) will lead to larger distance between the adjacent peak wavelengths. In Fig. 3(a), the either maximum or minimum coupling lengths are proved by presenting much larger peak-to-peak distance between the adjacent peaks at wavelengths around 1270nm and 1355nm. As expected, the two extremal couplings are both very close to bandgap cutoffs, which is in good agreement with the results in Ref. 7. This is due to the influence of bandgap cutoffs, which shapes dispersion curves at wavelengths close to them and therefore results in maximum, minimum coupling lengths and de-couplings. The extremal couplings are anticipated to have potential applications for broadband couplers’ design.
3. Temperature tuning properties of the dual-core PBGF
The dual-core PBGF’s temperature dependence of bandgap-1, bandgap-2 and bandgap-3 has also been measured as shown in Fig. 4. As for the liquid we adopted in this study, changing the temperature leads to a large change of the refractive index for the infused liquid (dn/dT=-0.000465°C-1). Therefore, the drop of the liquid’s refractive index, which is resulted by rising temperature, will decrease the index contrast between the infused liquid and silica. And according to the ARROW regime described by N. M. Litchinitser, et al., the transmission spectrum of the PBG structure is largely determined by the index contrast [18, 19]: when the index contrast decreases, there are obvious blue-shift of bandgaps and compression of the bandgaps’ width, as seen in Fig. 4. For bandgap-1, the blue-shift speed is averaged to 1.5nm/°C, as shown in Fig. 4(a). When temperature rises to 90°C, bandgap-1’s bandwidth is about 115nm from 1145nm to 1260nm, which means a blue-shift of 90nm and a compression of 25nm compared with that under temperature of 30°C. The numerical results calculated by PWEM have also been demonstrated. The bandgap edges’ blue-shift speed is averaged to 1.4nm/°C for the numerical results, which is in good agreement with the experimental sensitivity of 1.5nm/°C. In Fig. 4(b), the rise of temperature also results in blue-shift of bandgap-2. But for bandgap-3, we can see obvious depression when temperature rises from 70 °C to 90°C. At the temperature of 90°C, bandga-3 has been totally depressed.
When temperature rises, the bandgaps undergo blue-shift because of the drop of infusion liquid’s refractive index. Besides, the drop of the liquid’s refractive index also changes the dual-core coupling, typically the coupling length Lc. The dual-core air-silica PCF has a typical coupling length of about 205mm at wavelength of 1240nm. However, when infused with the liquid which changes the dual-core PCF into a dual-core PBGF, the coupling length has been remarkably reduced to 10-1mm scale, according to the numerical results as shown in Fig. 5. At wavelengths range from 1237nm to 1247nm, the coupling lengths will be increased when temperature rises. The increase speed of coupling length at wavelength of 1240nm is averaged to 2.6µm/°C (for Y-polarization) and 4.3µm/°C (for X-polarization). Up to now, we haven’t carried out a method for the measurement of coupling length, it is too much small (10-1mm scale). However, according to the experimental results as shown in Fig. 6 and Fig. 7, we believe the temperature tuning of coupling length is effective.
According to Eq. (1), the change of coupling length will directly lead to the variation of the output transmission: the shift of the maximum or minimum coupling wavelengths (the resonant peak wavelengths). In Fig. 6, the experimental transmission spectra have been presented for a 20mm dual-core PBGF. When temperature rises from 40°C to 40.5°C, 41°C, 41.5°C, and 42°C, the peak wavelength at 1242.7nm has been moved to 1241.6nm, 1240.7nm, 1239.8nm, and 1238.9nm, respectively. The blue-shift speed is averaged to 1.9nm/°C, which is very close to the numerically calculated speed of 2.0nm/°C (for Y-polarization) and 1.7nm/°C (for X-polarization). What’s more, we also measured the dual-core coupling’s temperature dependence at wavelength of 1310nm, as shown in Fig. 7. Along with the rise of temperature from 20°C to 40°C, the transmission intensities in both two cores undergo periodic variation. The peaks and valleys in Fig. 7 correspond to the maximum or minimum couplings, which is controlled by temperature tuning. The results indicate very effective tuning of the dual-core coupling which could find promising applications in tunable directional couplers and multiplexer-demultiplexers.
In this study, the proposed dual-core PBGFs’ loss is generally larger than 5dB. We believe, by using dual-core PCFs with more rings of air-holes and using materials with less absorption, we could possibly realize dual-core PBGFs with less loss. And on the other hand, the coupling loss is averaged to 4dB at one splicing point. The coupling loss is expected to be much smaller if we use certain dual-core PCFs with larger core-diameter and core-to-core distance, or if we adopt certain matched-fiber to better match the model fields’ coupling.
As a conclusion, we have experimentally demonstrated a dual-core PBGF by infusing a temperature-responsive liquid into a dual-core air-silica PCF. Extremal couplings have been experimentally observed. The experimental and numerical investigations indicate that the dual-core PBGF’s bandgap guiding is temperature tunable. When we rose temperature, the dual-core PBGFs’ bandgaps have been changed: compression of bandwidth, blue-shift and depression of the guiding band. Furthermore, temperature tuning of dual-core coupling has also been investigated, experimentally and numerically. We found that the rise of temperature increased the coupling length which results in the blue-shift of the peak wavelengths with a speed of 1.9nm/°C, for a 20mm dual-core PBGF. The proposed dual-core PBGF has potentials for applications as fiber sensors and optical communication devices, such as temperature sensing, tunable directional couplers and multiplexer-demultiplexers.
The work is supported by the National Natural Science Foundation of China under Grant No. 10674074, 10774077 and 60736039, the National Key Basic Research and Development Program of China under Grant No. 2003CB314906, the Tianjin Natural Science Foundation under Grant No. 06YFJZJC00300, and the National High-Technology Research and Development Program of China under Grant No. 2006AA01Z217. We thank Qing Shi, Qiang Fang, Xiaosong Zhang et al. from Institute of Modern Optics, Nankai University, for the help in the experiment, and Yangtze Optical Fibre and Cable Company Ltd. for providing us the fiber.
References and links
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