1. Introduction

The spectral region of 3 to 5 μm, in the mid-infrared (mid-IR), is currently of growing interest because the development of a new generation of laser sources promises to open this spectral window for applications in the near future. Many gases exhibit strong molecular absorption at these wavelengths, especially in the wavelength range of 3 to 3.5 μm (e.g. CH₄ has a strong absorption band around 3.3 μm). Thus, the availability and performance of single-mode fibers for this spectral range is of great interest. In bulk silica the material loss above 3 μm is greater than 60 dB m^-1 [1] (a curve for the Suprasil F300 (Heraeus) used in this study is shown in Fig. 1) and would be even higher in a traditional single-mode fiber. For practical purposes this means that standard silica fibers are unusable in this wavelength range. As a result, the search for useful fibers for the mid-IR has focused on other materials, such as chalcogenide glasses, which have been developed and demonstrated in both bulk and fiber form [2]. However, this has required the development of novel processing routes for purification and fiber drawing (compared to the more established silica based technology) and to achieve theoretical losses further improvements in these technologies may be required [2]. An alternative technology is inner-surface-coated hollow-core fibers [3,4], which are typically highly multimode and bend-sensitive, but have excellent power handling characteristics, capable of delivering over 1 kW continuous wave power at 10.6 μm [4], making them suitable for many applications where beam quality is not an issue.

 

Fig. 1. Bulk attenuation of dry silica (Suprasil F300) used for the HC-PCF fabrication [1].

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Hollow core photonic crystal fibers [5] (HC-PCF’s) are a new form of optical fiber waveguide with unique properties, which are currently being investigated for a variety of applications such as high power and ultrashort-pulse delivery [6–9] and gas sensing [10]. The guided mode in these fibers is strongly confined within a hollow core, greatly reducing the effect of the solid material on the fiber’s optical properties and liberating its performance from the material constraints. For example, the damage thresholds of single-mode hollow-core fiber are above those of their conventional solid-core counterparts [6,7]. In another possible application area, light-gas interactions, hollow-core fibers provide an outstanding interaction volume (the entire core) when compared to more traditional evanescent field configurations [10,11].

It has been predicted that low-loss guidance in a hollow-core fiber is possible if the fiber is formed from soft glass which is IR-transparent [12,13]. The material losses of soft glasses in the region around 3 μm are routinely less than 1 dB m^-1, far lower than for silica (see Fig. 1), providing careful purification techniques are employed. However, the precursor materials are relatively expensive and toxic compared to silica, making cost and handling an issue. As an alternative, we demonstrate in this paper that silica based hollow core photonic crystal fibers could be useful for many mid-IR applications. Due to the low overlap of the guided light with glass, which can be less than 1% [14], the effect of the relatively high material loss of silica at these wavelengths is minimized, giving a loss of 2.6 dB m^-1 in the wavelength range 3.1 to 3.2 μm in an effectively single-mode fiber. This level of loss means that the current fiber is suitable for applications where short lengths of fiber are required. However, with improvements in fiber design lower losses around 0.5 dB m^-1 are predicted and such fibers will be suitable for a much wider range of applications.

2. Fiber fabrication

The HC-PCF was fabricated using the stack and draw technique in which thin walled silica tubes (Suprasil F300, Heraeus) are drawn down to form capillaries and then stacked to form a close packed array. Nineteen capillaries were omitted from the centre of the stack to form the core and the whole stack was jacketed with a silica cladding and drawn down to the final fiber dimensions. A scanning electron micrograph of the fiber is shown in Fig. 2. The fiber core diameter is 40 μm and the overall outside diameter is 150 μm. The nearest-neighbor hole spacing, or cladding structure pitch, is around 7 μm.

 

Fig. 2. Micrograph of the HC-PCF designed for operation in the mid-IR region.

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The use of a large (19-cell) core and the choice of a relatively thick core wall are the result of a prior optimization procedure which reduced the overlap of the guided mode with the silica [14]. Some distortion of the structure around the hollow core is apparent in the micrograph, and we expect that the fiber performance will improve with refinement of the fabrication parameters. However, the cladding structure and the core features mentioned above already represent a relatively advanced fiber design for mid-IR bandgap guidance.

3. Demonstration of infrared bandgap guidance

All the characterization of the fiber was carried out using a Bentham TM300 Monochromator, with a 300 lines mm^-1 grating (dispersion: 10 nm mm^-1) and a 2×2 mm pyroelectric detector. A tungsten halogen bulb was used as a broadband light source where the hot envelope of the bulb acts as a black body radiator. The experimental set-up for spectroscopy measurements is shown in Fig. 3. Light was coupled into the fiber using a 37 mm focal length ZnSe lens. This configuration does not represent the optimum coupling arrangement for the fiber but allowed adequate signal for both spectroscopy and attenuation measurements. The fiber output end was positioned in the plane of the “wide open” input slit of the monochromator. The input and output ends of the fiber were held securely in v-grooves located on microblocks for accurate and repeatable positioning. The output slit, set at 1.5 mm, determines the wavelength resolution of the scans since the input slit width is essentially the fiber core diameter in this arrangement. Measured data were normalized against the lamp spectrum recorded through the complete optical train consisting of ZnSe lens, spectrograph and detector.

 

Fig. 3. Experimental set-up for spectroscopy measurements.

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3.1 Position of bandgap

With the arrangement shown in Fig. 3, using 2.7 m of the HC-PCF, a series of 20 scans using a slit width of 1.5 mm (resolution 15 nm) and a step size of 2 nm were performed and averaged. The wavelength range scanned was from 2.9 to 3.5 μm. The spectrum shown in Fig. 4 clearly demonstrates a low-loss transmission window between 3 to 3.2 μm with a peak around 3.14 μm, well above the cut-off for transmission in a silica core fiber (considered to be around 2 μm for practical purposes). The attenuation of a standard silica fiber of this length and at this wavelength is expected to be greater than 180 dB, while the attenuation of the fundamental capillary mode should be far greater [15]. No additional transmission bands were observed outside the region shown in Fig. 4. The structure within the low-loss window (3 to 3.2 μm) can be attributed to surface-mode anticrossings, as observed in similar fibers at shorter wavelengths [16].

 

Fig. 4. Average of 20 spectra (normalized against the lamp spectrum) demonstrating location of the bandgap with a peak at 3.14 μm. The attenuation at 3.14 μm (2.6 dB m^-1) is indicated.

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3.2 Confinement in core

We have verified that the light guided in the low-loss transmission window was confined to the core using a knife-edge scanning technique. The fiber was brought up to the monochromator until the output face was virtually coincident with the plane of the entrance slit. We scanned the output face of the fiber across the edge of the monochromator entrance slit, and measured the transmitted signal at the peak wavelength (3.14 μm) of the low-loss transmission band.

 

Fig. 5. Signal recorded as a knife edge is scanned across the output face of the fiber. The zero position represents one edge of the air core. The solid line shows a calculated Gaussian mode profile with an e^-2 width of 15μm.

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A plot of the intensity at 3.14 μm as a function of distance across the fiber face is shown in Fig. 5. The signal increased from zero to a maximum as the fiber was scanned over a distance of approximately 40 μm, equivalent to the core diameter of the fiber. The solid line represents a Gaussian mode profile with an e^-2 width of 15 μm, the calculated mode shape based on the structure shown in Fig. 2. The similarity between the measured and calculated profiles (Fig. 5) suggest that the transmitted light is well confined to the core region of the fiber. Given that the fiber structure is very similar, other than in scale, to those used previously within the silica low-loss window, we believe that the transmitted light is in the single low-loss fiber mode. We anticipate that the large material loss will cause a substantial differential modal attenuation which will strongly discriminate against other guided modes.

3.3 Attenuation measurement

A cut-back technique was used to measure the attenuation in the fiber at the bandgap peak wavelength, 3.14 μm. In order to evaluate error margins in these measurements, intensities were measured 9 times for each fiber length, each time with a freshly cleaved fiber end. After each cleave 4 spectra, scanning through the peak of the bandgap, were taken and averaged. This was done to quantify the repeatability associated with removing the output end of the fiber from the monochromator, cleaving and then placing back in the v-groove. The launch end of the fiber was not moved throughout the measurement for both the long and short piece.

Initial measurements were made on a fiber of length 2.58 m, followed by a further 9 measurements on the “cut back” length of 0.92 m. The signal strength measured at 3.14 μm for 2.58 m was (2.04 ± 0.23)×10^-6 nA; and for 0.92 m was (5.39 ± 0.15)×10^-6 nA. Therefore attenuation for the HC-PCF at 3.14 μm was calculated to be 2.6 ± 0.3 dB m^-1.

This clearly demonstrates the feasibility of using silica based hollow core photonic crystal fibers for mid-IR applications. Based on an ideal, undistorted, fiber structure and using a figure of 70 dB m^-1 for the bulk loss of silica [1] with a 0.6% overlap of the light with glass it is predicted that we should be able to fabricate a HC-PCF with an attenuation of less than 0.5 dB m^-1 at 3.1 μm [5]. The measured attenuation (2.6 dB m^-1) is higher than the predicted attenuation (0.5 dB m^-1) because the fiber structure (Fig. 2) is imperfect. We expect that the light is guided single-mode, giving excellent beam quality which could be advantageous for many applications, especially where in combination with coherent sources. Future work will characterize fiber performance in greater detail, including an analysis of the power-handling capability of such designs.

3.4 Bend loss

Other hollow-core fiber designs suffer from losses when the fiber is bent, which is frequently a limitation on their applications. In contrast, silica hollow-core photonic bandgap fibers, like those described here but fabricated for shorter wavelengths within the transparency window of silica, have no discernable bend loss, even when the fiber is bent to just short of the fracture point. In our experiments we have no measurable reduction in the transmitted signal at a bend radius of 12.5 mm, which we discovered to be the fracture point. Smaller bend radii were not possible partly because of the absence of a protective polymer coating.

4. Conclusion

We have described the demonstration of a silica based hollow core photonic crystal fiber with a low loss (2.6 ± 0.3 dB m^-1) transmission window in the mid-IR. We predict that attenuation of future fibers will fall to less than 1 dB m^-1 at all wavelengths out to about 3.5 μm, greatly extending the operating wavelength range of single-mode optical fibers formed from silica. Although it is possible that hollow core photonic crystal fibers fabricated from novel chalcogenide glass materials (such as As-S and As-Se [13]) may surpass the losses achieved with silica based fibers, silica based technology offers some advantages in terms of cost, ease of fabrication and toxicity. This makes the silica based HC-PCF’s ideal candidates for many mid-IR applications.