We present a hybrid photonic crystal fiber in which a guided mode is confined simultaneously by modified total internal reflection from an array of air holes and antiresonant reflection from a line of high-index inclusions. Experimental results demonstrate that this fiber shares properties of both index-guided and photonic bandgap structures.
© 2006 Optical Society of America
Photonic Crystal Fibers (PCFs) are optical fibers which wavelength-scale morphological microstructure running down their length. PCFs have been intensively explored in recent years, in an effort to understand and exploit their broad potential. They have been demonstrated to guide light by two different mechanisms depending on the photonic crystal formed in their cladding and the type of core used .
In index-guiding PCFs, light is confined to a solid core by total internal reflection from a reduced-effective-index cladding material formed from glass shot through with an array of air holes. Some of the remarkable properties of this kind of PCF are endlessly single-mode behavior , high birefringence , ultrahigh nonlinear coefficient , large mode-area  and efficient dispersion management, including ultra-flattened  and strongly anomalous dispersion .
On the other hand, in photonic bandgap PCFs, light can be confined to a low-index region - even a large air hole - by reflection from the photonic crystal cladding. Light with propagation constants β corresponding to the cladding bandgaps cannot escape the core and is therefore guided along the fiber with low loss. Although some of the most spectacular properties of photonic bandgap fibers (PBGFs) are found in fibers with hollow cores , photonic bandgap guidance can also be observed when the core is solid .
Although photonic bandgaps are often associated with large refractive index contrasts, all-solid PBGFs with an index contrast of only 1% have recently been demonstrated, in which the pure silica core in surrounded by an array of Ge-doped silica rods  . The key features of such fibers are low attenuation, easy fabrication, guidance in a set of finite frequency windows and unusual dispersion characteristics. Another fascinating feature of this kind of PCF is that it is possible to match the group indices at widely different wavelengths because of the presence of multiple bandgaps. Furthermore, the spectra of these fibers can be tuned by changing the refractive index of the inclusions, so they can be used as tunable devices .
This paper describes a hybrid photonic crystal fiber that confines and guides light in a solid core by both of these mechanisms simultaneously. We show for the first time that total internal reflection (TIR) and antiresonant reflection , which is central to the photonic bandgap effect in fibers, can act together to confine light in a fiber guided mode.
2. Hybrid PCF concept
The hybrid PCF is composed of air holes and germanium-doped silica rods disposed around an undoped silica core, as shown in Fig. 1(a). The air holes are arranged in a hexagonal pattern as in index-guiding PCFs, whereas the high-index rods replace a single row of air holes along one of the PCF axes.
Along the y-axis the core has a higher refractive index than the effective refractive index of the cladding , so along this axis light can be guided by total internal reflection from two half-spaces of ”holey” silica, as in the index-guiding PCFs. On the other hand, along the x-axis TIR is not possible, since the cladding rods have a higher index than the core. In this case, confinement only occurs in restricted bands of wavelength, which coincide with the “photonic bandgaps”, which occur at the antiresonant conditions of the high-index inserts. According to the Antiresonant Reflecting Optical Waveguide (ARROW) model , the high-index rods in the cladding permit light to couple out from the core if they are in resonance, but they reflect it back into the core if they are antiresonant. The relevant effective indices are demonstrated in Fig. 1(b).
The stack-and-draw procedure has been used to fabricate hybrid PCFs. A standard multimode fiber preform with a plain silica cladding and a Ge-doped core was stacked with hollow silica capillaries and a single solid silica rod to form the structure of interest. We used two different multimode preforms: the first (preform A) had a quasi-parabolic index profile and a maximum index contrast Δn = 1%. The second preform had an almost step-index profile with Δn = 2.03%. Fibers were drawn from stacks incorporating both preforms, as shown in Fig. 2.
The Ge-doped regions show clearly on the scanning electron micrographs. The second structure shown in Fig. 2(b) has a stronger confinement due to a larger index contrast on both axes. However, computations based on the multipole method suggest that lowest confinement losses will be very low (less than 1dB/km) in both cases.
4. Hybrid PCF based on the preform A
The full-vectorial finite element method has been used to model and analyze the fabricated structures . For the pure silica background we used nsi=1.45 and we approximated the gradual variation of the rod permittivity by means of a staircase function .
For the fiber shown in Fig. 2(a), the rods support several resonances/antiresonances within the transparency window of silica. Figure 3(a) presents an effective index analysis of this hybrid PCF. The red line with triangles is the fundamental mode computed for a simple index-guiding PCF formed by a pure silica core surrounded by holey silica (not shown) . The black line with squares shows the effective index of the holey cladding material, below which guided modes are cut off. The other three lines represent the modes computed for the hybrid structure. In the antiresonant wavelength ranges, the modes resembles that of a standard index-guiding PCF, and tracks along the red line with triangles until coming into resonance with the next mode of the high-index rods. The pattern of the modes can be construed as arising when the more dispersive modes of the high-index rods intersect with the mode of a standard index-guiding PCF, leading to anti-crossings.
In order to measure the photonic bandgaps a supercontinuum source, pumped at 1064nm, has been coupled to the hybrid fiber using a 40× objective lens. The output was analyzed using an optical spectrum analyzer. Figure 3(b) shows the transmitted spectrum recorded with a fiber sample of 2.0m length. As predicted in the simulations, there are 3 antiresonant regions, which are represented by the low-attenuation windows in the transmitted spectrum. The first such region stands out from the background by about 30dB.
Near the edges of the low-loss ranges the confined hybrid modes adopt the character of the corresponding high-index rod mode. Figure 4 presents near-field images of the fiber output recorded by a CCD camera. The wavelengths have been selected by using a monochromator at the fiber input. These show the lowest-order semi-infinite bandgap at 1010nm (4(i)), the anticrossing with the LP 11 mode in the spectral region 800–935nm (4(f) – 4(h)), the confined mode in the second bandgap at 725nm (4(e)), the anticrossing with the LP 02 mode around 600nm, and the confined mode in the third bandgap at 575nm (4(b)).
5. Hybrid PCF based on the preform B
This PCF has a higher refractive index step (Δn = 2.03%). Figure 5(a) presents the effective index analysis for this fiber. There are 4 low-attenuation regions between 400 and 1700nm. Figure 5(b) shows the transmission spectrum of this fiber, with 4 low-attenuation regions in good agreement with the antiresonant regions predicted by the simulations.
The waveguide dispersion of PBG fibers is expected to be very different from that of total internal-reflection-guiding fibers. The dispersion of the proposed fiber has been measured using low-coherence interferometry. Figure 6 shows the simulated and measured group-velocity dispersion of the guided mode in the third bandgap. Note that the longer-wavelength part of this transmission band corresponds to guidance in a higher-order mode.
As in other bandgap fibers, the group index curve in each band drops to a minimum before rising again. In hollow-core  fibers the minimum (which corresponds to the zero in group velocity dispersion) is shifted toward the short-wavelength edge of the bandgap as a result of the effects of anomalous waveguide dispersion, while in solid-core fibers formed from silicate glass  it is shifted to the long-wave length edge by the material dispersion. In these silica-core fibers the dispersion of the core material increasingly becomes strongly normal at short wavelengths, shifting the minimum point of the group index curve to the long-wavelength side of the transmission band . The zero-dispersion wavelength occurs at λ = 1.0μm.
Moreover, the dispersion is large and normal (about -2800ps/km/nm) at the short wavelength edge of the bandgap and starts to increase as we approach the long wavelength edge. The same behavior is expected for the other 3 bandgaps, as in the traditional photonic bandgap fibers . Therefore, the chromatic dispersion can be potentially engineered in the fibers by exploiting the properties of both propagation mechanisms.
We have demonstrated that photonic bandgap and total internal reflection waveguiding can be observed simultaneously in an optical fiber, provided three different refractive indices are used. In some ways these structures are very similar to those which have been studied extensively using planar photonic crystal technology. The losses in this case are far lower, and propagation over many metres of fiber is possible. The guided modes can be considered as arising between the anticrossings between the modes of the high-index cladding rods and the modes of a solid core index-guiding PCF. The dispersion of the guided modes has overall properties determined by these anticrossings, but within the low-loss windows is influenced by presence of the air holes. We believe that this work offers insight into the nature of the two different guidance mechanisms.
This work was partly funded by the U.K. E.P.S.R.C. JCK is a Fellow of the Leverhulme Trust. The authors would like to thank John Roberts for evaluating the confinement losses.
References and links
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