COMPOSITE MATERIAL HOLLOW ANTIRESONANT FIBERS

We study novel designs of hollow core antiresonant fibers comprising multiple materials in their core boundary membrane. We show that this type of fiber still satisfies an antiresonance condition and compare their properties to those of an ideal single-material fiber with an equivalent thickness and refractive index. As a practical consequence of this concept, we discuss the first realization and characterization of a composite silicon/glass based hollow antiresonant fiber. © 2017 Optical Society of America

After almost two decades of improvement in fiber technology, the development of Hollow Core optical fibers (HCs) [1,2] is still very active. In particular, the use of hollow fiber structures comprising only a limited number of detached glass tubes within an outer glass jacket is gaining significant interest [3,4]. The use of this fiber type has resulted in HCs with relatively low attenuation in the visible [5,6], near [7,8,9] and mid-infrared spectral ranges [4], combined with ultra-large transmission bandwidths [5,8,9]. Possible applications of these hollow Anti-Resonant Fibers (ARFs) range from high power laser delivery [7] to gas based laser sources [10] and telecommunications [9].
Several modifications of this basic fiber design have been proposed [5,[11][12][13][14][15] in order to further reduce their level of leakage loss or increase their birefringence. However, all of the previous designs and fabrication proposals have concerned ARFs made of a single material (typically silica even though other glass materials have also been considered [16,17]). In this paper we seek to explore a novel form of this ARF type, by using, for the first time (to our knowledge), multiple materials for the fiber cladding. We numerically study the transmission properties of these Composite Material Anti-Resonant Fibers (CM-ARFs), by correlating the composite material membrane properties to that of a "single equivalent membrane", with an identical effective thickness and refractive index. We apply this concept to the design of a novel form of antiresonant fiber by adopting a hybrid semiconductor/glass core boundary membrane. Similarly to previous works on semiconductor optical fibers [18,19], the inclusion of a semiconductor is desirable in order to realize functionalized devices. For example, a light induce refractive index change may be adopted for all optical fiber modulation [20]. We numerically demonstrate that, in contrast to previous structures with silicon in the cladding area [18], this novel type of Silicon Anti-Resonant Fiber is predicted to have very low attenuation (<0.1dB/m). Finally, we report on the first fabrication and characterization of a silicon/borosilicate based ARF. Fig. 1 shows a reproduction of a silica based ARF (n=1.41 at λ=2.7μm) already fabricated in [4] with 10 cladding tubes and an original thickness of the cladding tubes t=2.4μm (in white). The core radius is 47μm. An additional internal membrane tA (in red) is added corresponding to a material with refractive index n2=2. where c is t membrane thick ndex. Therefore he frequency sp educed by a fa g. 2 of a di ence of leaka wn in Fig.1 at a he anti-resona g, we will refer al membrane to the minimu Fig. 1 The established analogy between a composite material and a conventional single material ARF suggests that only the overall optical path travelled by light at the core boundary is relevant for antiresonance guidance. This opens up the possibility to exploit the properties of additional materials deposited on the basic optical fiber matrix in order to activate and functionalize its behavior, for example, via the free carrier plasma-dispersion effect, in which the change of refractive index and absorption resulting from a change in the concentration of free carriers by photo-excitation can be used in silicon-based all-fibre integrated modulators to achieve intensity or phase modulation [20]. As a first step of this "active hollow core waveguide" concept, we have investigated the feasibility of a particular form of CM-ARF, in which the core boundary membrane is made of a composite hybrid semiconductor/glass material. Semiconductor optical fibers [19] have received great attention in the recent years because of the prospective for integration of the existing optical fibre infrastructure with the novel silicon photonics platform [22]. Since the first inclusions of semiconductors within optical fibres [23] progress in the area has seen some demonstrations of optical devices [19]. Although potentially these structures may provide unique characteristics, their use is currently limited by the very high attenuation observed to date in these fibres (of order 1dB/cm). Here we show that this problem can be mitigated by filling semiconductors within ARFs. Fig. 4 shows a typical section of a borosilicate based ARF [17] filled with amorphous hydrogenated silicon (a-Si:H) by using a High Pressure Chemical Vapor Deposition (HPCVD) method [23].
The CM-ARF of Fig. 4 (a) was obtained after a deposition of 48 hours at a temperature of 400 °C and has 3 layers of material (Silicon (white)/Borosilicate (gray)/Silicon (white). The Si layer thickness was measured using the SEM to be close to 300nm. We then tested several types of ARF by using the same HPCVD technique and modifying the temperature profile along the fiber samples as well as the filling time. The fiber shown in the inset of Fig. 4(b) (core diameter d=60μm and glass layer thickness of 1μm) was coiled in a furnace at a temperature of 450 °C and filled for only 4 hours in order to obtain a very thin a-Si:H layer thickness. A length of 35 cm of this fibre was obtained and tested. We could reveal the presence of a thin layer of a-Si:H by means of Raman Spectroscopy. Figure 4(b) shows the Raman Shift spectrum taken using the sample in the inset (shown in its longitudinal section (A) and transversal section (B)) and using an optical pump at a wavelength of 0.63 μm. The Raman shift at the points centred on the external and internal side of the cladding tubes confirm the presence of a-Si:H [24].
The spectrum of the signal transmitted through the considered 35cm long sample is shown in Fig. 5 together with the near field intensity profile recorded by an Infrared Camera. In the considered spectral range (0.7μm to 1.6 μm) the CM-ARF has two transmission windows (just above 0.8 μm and around 1.2 μm) and presents several peaks probably related to the coupling of the fundamental-like mode (in the inset) to the cladding modes, in both the high refractive index (~3.6) of the a-Si:H. In Fig. 6(A) we have compared this measured transmission spectrum (blue solid line) with that of 7 meters of the same ARF without any silicon filling (green solid line, glass thickness is 1μm and glass dispersion is taken into account) and the calculation of the leakage loss for both the filled CM-ARF (black dotted line) and the unfilled ARF (red dotted line). Our simulations show how the addition of the internal and external silicon layer to the original ARF results in a red-frequency shift of the entire transmission spectrum. Fig. 6(B) shows what would be the effect on the leakage loss of a variation of the coating thickness of ±10nm, proving that the fiber attenuation levels would be kept similar but there would be a shift of the resonant wavelength (±0.05μm).
In order to show the impact of the optical absorption of the a-Si:H layers on the fibre performance we have compared in Fig. 7 the attenuation components of the CM-ARF. The fiber attenuation related to the presence of a glass (red line) and silicon (green line) material absorption are obtained as the product between the optical mode overlap on each material and their intrinsic absorption. Concerning this last quantity, we have measured the