We demonstrate fabrication of robust, low-loss silica photonic wires using tapered microstructured silica optical fiber. The fiber is tapered by a factor of fifty while retaining the internal structure and leaving the air holes completely open. The air holes isolate the core mode from the surrounding environment, making it insensitive to surface contamination and contact leakage, suggesting applications as nanowires for photonic circuits. We describe a transition between two different operation regimes of our photonic wire from the embedded regime, where the mode is isolated from the environment, to the evanescent regime, where more than 70% of the mode intensity can propagate outside of the fiber. Interesting dispersion and nonlinear properties are identified.
©2004 Optical Society of America
Silica-based optical fibers have long been deployed in telecommunications and sensing applications. Refinements in optical fiber fabrication techniques and fiber design have culminated in today’s low loss single mode fiber (SMF). However, while standard SMF has revolutionized data transport, it is less suitable for use in ultra compact integrated photonics which require much smaller dimensions for both the waveguide structure and the mode which propagates in it.
A new class of low-loss small-mode area silica waveguide has recently been reported [1–3], fabricated by tapering  conventional single-mode optical fiber to much smaller diameters than had previously been thought possible . Centimeters of uniform, smooth submicron silica wires, or silica nanowires, with diameters drawn down to 50 nm, have been demonstrated. These flexible nanowires are able to guide light at visible and infrared wavelengths with low bend loss. They suggest a fiber platform for easily fabricated nano-photonic devices. Such sub-micrometer waveguides will also exhibit nonlinear properties at relatively low power, due to the combination of tight mode confinement and strong waveguide dispersion [3,5,6,7].
Although impressive, the nanowires initially described in ref.  required a two-step taper process in order to maintain uniformity of the diameter. The initial flame drawing process was used to bring the fiber diameter down to one micrometer, after which a heated sapphire tip was used for even heat transfer to the fiber which was then drawn down still further, to a diameter of 50–500 nm. Since one end of the fiber was cleaved for the second step of the process, evanescent coupling was used to send light into the nanowire. A more recent paper  has demonstrated fabrication of similar nanowire devices, with diameters to 300 nm, using only a well-controlled single-step fiber taper process and allowing for experimentally simpler coupling through fusion splicing to the untapered fiber. The lack of a cladding layer causes these nanowires to be susceptible to both physical contact leakage as well as scattering induced environmental losses, as a significant proportion of the intensity of the guided mode travels outside the wire. This environmental sensitivity can be desirable for fiber-optic sensing [1,2,6], but is likely to be detrimental for applications in nonlinear optics or in mode coupling.
More recently Leon-Saval et al.  reported a novel approach to fabricating waveguides that possess similar optical properties to the nanowires reported in . It has been shown that photonic crystal fibers (PCFs) can be tapered while retaining the cross-section profile . By tapering a PCF they scaled the core diameter by a factor of 10, down to 300 nm, resulting in enhanced optical nonlinearities and modified dispersion properties without the high coupling losses typical of small core PCFs. They then demonstrated optical continuum generation in this nanowire using lower pump intensities than previously reported using conventional untapered microstructured fibers .
In this paper, we demonstrate a new type of silica nanowire, using a technique similar to one that was previously described in Ref.  in the context of fabrication of dispersion-engineered devices. We start with a microstructured fibre designed to support mode propagation compatible with standard single-mode fibre, but in which the primary cladding layer is further surrounded by large air holes and a secondary silica-capping layer, shown schematically in Fig. 1. By tapering the fiber [4,11] we reduce the dimensions by a factor of 50, and fabricate 40 mm long single-mode silica waveguides with an inner diameter as small as 700 nm, where the inner diameter is defined by the width of the silica region bounded by the air holes. For larger outer diameters of the waist, the outer silica capping layer prevents interaction with contaminants or other environmental influences as the air holes confine the propagating optical mode. The resulting silica nanowires are insensitive to the environment and are flexible, yet exhibit strong mode confinement and small effective area in similar manner to the devices described in Refs. [1,2] suggesting applications as nanowires for photonic circuitry, with low environmental sensitivity. Furthermore the untapered fibre permits single mode optical coupling to conventional fibre with extremely low loss, while the adiabatic taper shape gives loss-less energy transfer through the device. For smaller diameters the mode propagates in both the internal air holes and the surrounding space around the fibers and the devices are of possible utility as localized optical fiber sensors, similar to the devices of refs. [1,2].
We characterize the behavior of our devices both experimentally and via beam-propagation method modeling. At small diameters, these tapers exhibit strong waveguide dispersion which, when combined with the high effective nonlinearity due to tight mode confinement, suggests device applications in nonlinear switching, parametric gain, and low power white-light continuum generation [3,9,12,13].
Figure 1 summarizes the tapered MOF photonic wire concept. The microstructured optical fiber (MOF) chosen as the starting medium was fabricated at the Optical Fiber Technology Centre, University of Sydney and is similar in structure to the fiber described in refs. [10,14]. It has an 8 µm germanium doped core at the centre of a silica inner cladding region of diameter 36 µm. This guiding structure is surrounded by six 40 µm air holes, which in turn are surrounded by a secondary silica capping layer as illustrated in Fig. 1, resulting in an overall diameter of approximately 130 µm for the structure. The guided mode of the untapered MOF matches the guided mode of a standard single-mode fiber, since the central waveguide region has optical properties almost identical to a standard fiber but with less cladding material, consequently they can be spliced with less than 0.1 dB loss .
In our fabrication apparatus [4,11,15], shown in Fig. 2, a traveling nozzle brushes a 3 mm wide butane flame across the stretched MOF while the ends are uniformly pulled apart with stepper motors, producing tapered structures. The nanowire proportions and taper profile are tailored by the appropriate choice of flame brushing profile, where the rate of elongation is computed using a simple volume-conservation algorithm . The flame temperature and rate of elongation are optimized such that the fiber viscosity and transverse tension are counter-balanced. The pull rate is increased to minimize the time during which the glass has low viscosity, so that the internal microstructure is maintained by the gas pressure in the holes. For outer diameters below ~2.5 µm, the holes begin collapse due to surface tension effects, though this could be improved with better flame control. We have tapered the MOF down to 1.5 µm outer diameter with the holes still partially open, as shown by the scanning electron micrograph (SEM) images of Fig. 3, though the holes no longer provide significant guidance for this structure. For the experiments presented here, the tapers were made using a~1 m length of MOF. However, one could use a much shorter (<1 cm) length of fiber, spliced between two pieces of SMF so that a higher gas pressure could be maintained in the air holes, thus delaying the onset of hole collapse and allowing one to achieve smaller fiber dimensions [8, 16].
3. Modal properties of MOF photonic wire
We produce our photonic wires using a simple single-step process and do not observe a significantly increased loss at smaller diameters due to effects of diameter variation, unlike refs. [1,6]. The more recent reports [2,3] have also described photonic wire fabrication using a well-controlled flame-brushing process, showing flame control and stability to be the most critical parameters in smooth taper formation. For devices fabricated using standard single-mode fibre, surface contamination should be expected to contribute an additional source of loss. This is a much less important effect for our devices since the propagating modes are largely protected by the external silica capping layer. The taper slopes of our devices are adiabatic, ensuring that the fundamental mode in the germanium core evolves into the fundamental mode of the tapered photonic wire without exciting higher order modes. This further contributes to low device insertion loss and therefore accurate loss measurement.
The propagation properties through the tapered photonic wire depend strongly on the ratio of outer diameter OD and the propagating wavelength λ. Figure 4 illustrates cross-sections of the mode profiles for different regimes of operation as calculated with the beam propagation method, in dimensionless units of the ratio OD/λ. For each OD/λ value we have plotted for comparison the simulated mode profile of a pure silica photonic wire with the same OD and a photonic wire of size corresponding to the inner portion of the tapered MOF (embedded nanowire). At more moderate diameter reductions the fundamental mode is tightly confined by the air holes and behaves like an air-clad waveguide. The mode remains well isolated from the outside region of the fiber and the device can be viewed as a nanowire embedded within a hollow core fiber. Figure 4(a) (left) shows an example of this regime, for a ratio of OD/λ=3.75. At a wavelength of λ=633 nm this example corresponds to OD=2.3 µm, and the diameter of the central core nanowire region is approximately 725 nm. The right side of Fig. 4(a) illustrates the simulated mode profile for the corresponding 725 nm nanowire. The two modes at 633 nm are similar and have similar effective indices. In this regime, as well as having a mode isolated from the external environment, the mode is tightly confined and has a small effective mode area, much like the silica nanowire reported in refs. [1–3]. By contrast, when the silica nanowire length scale factor OD/λ is further reduced, either by further tapering of the nanowire or through coupling longer wavelength light, the mode begins to leak out of the air-holes, as illustrated in Fig. 4(b). This transition occurs at a value of approximately OD/λ=1.6. Finally, at yet smaller values of OD/λ, the mode fills the whole fiber and most of the mode intensity propagates outside the fiber; for a ratio of OD/λ=0.6, 70% of the intensity is outside the fiber. The evanescent field makes the photonic wires more sensitive to the external environment than a solid silica wire with the same OD, suggesting such MOF photonic wires could be used to create robust highly sensitive detectors.
Loss measurements for our tapered nanowires are acquired using a simple in-situ set up, shown in Fig. 2. Single-mode fiber is spliced onto each untapered end of the photonic wire device. The adiabatic taper slope ensures that only the fundamental mode travels through the taper. Calibrated loss measurements are made at 1550 nm by attaching one end of the device to a narrow-band fiber-coupled laser and the other end to an optical multimeter. We measure losses of 0.03 dB/mm for an OD=3 µm (900 nm inner diameter) taper of length 40 mm at λ=1550 nm, a considerably lower loss than reported in .
To verify that the embedded nanowires are insensitive to the external environment, wavelength-dependent loss measurements are conducted with and without index-matching fluid applied uniformly to the outside of a tapered device with an outside diameter of 3.5 µm. These measurements are conducted by attaching one single-mode spliced-spliced end to a broadband coupled-coupled source (Agilent EELED 83437A) emitting from 1250 nm to 1700 nm, while the other end is attached to an optical spectrum analyzer. When wavelengths are launched in the embedded nanowire regime, the guided mode is confined by the air holes so it does not interact with the fluid and propagates with no additional loss. Conversely, in the evanescent field regime, the propagating mode is no longer confined and the presence of the index matching fluid results in radiation loss. Figure 5 illustrates the experimental and theoretical wavelength-dependent increase in the loss associated with the transition from low loss in the tightly confined embedded regime at λ=1250 nm to higher loss at λ=1700 nm, as the mode extends into the cladding towards the evanescent field regime, where it is absorbed by the index-matching oil. The measured loss is given relative to the transmission through the structure with no oil surrounding it. The theoretical plot is obtained by solving for the core mode of taper waist using the beam propagation correlation method and calculating the loss from the imaginary part of the effective index. The inset shows the corresponding calculated mode shape.
4. Nonlinearities in MOF photonic wires
Nonlinear effects in silica fibers are governed primarily by group velocity dispersion (GVD), particularly in phase-matched processes, and by the nonlinear coefficient γ=n2ω0/cAeff, where n2 is the nonlinear coefficient for the intensity-dependent index of refraction of silica n=n0+n2I, ω0 is the center frequency of the intense beam, c is the speed of light, and Aeff is the effective area of the mode at ω0. Since a significant portion of the core mode in a MOF photonic wire propagates in air, and n2air<<n2silica, in order to get a accurate estimate of γ for our structure, we modify the equation for Aeff given in Ref  to integrate only the mode intensity lying in silica,
where s(x,y) is a step function equal to 1 in silica and 0 everywhere else. One can see from Fig. 4 that this value will vary quite a bit with OD/λ. Figure 6 shows that the optimum occurs when the outer diameter-wavelength ratio OD/λ=2.73 (e.g., a 2.1 µm photonic wire, at the Ti:Sapphire laser wavelength of λ=800 nm). The figure plots the normalized quantity 2πλ2/Aeff (silica) vs. OD/λ for our fiber, where Aeff(silica) is calculated from Eq. (1). The normalized expression is valid for all OD and λ for our MOF. At larger diameters than the optimum the mode is less tightly confined so the beam intensity is lower, while at a smaller diameter a significant part of the beam energy is no longer propagating in the solid material. In each case, the power required for a given nonlinear phase change is higher than the optimum. We obtain the nonlinear coefficient γ by multiplying the normalized expression by n2/λ3, with n2=2.6×10-16 cm2/W . For λ=800 nm, γ=466 W-1 km-1, which is a factor of ~5 larger than for many commercially available highly non-linear MOFs  and ~100 times larger than for standard SMF. This translates into a π nonlinear phase shift using a modest 1 kW peak power pulse through only 7 mm of MOF photonic wire.
We can also tailor the GVD of our devices to be near zero for OD/λ close to 2.73, where we obtain optimum γ, providing further enhancement for nonlinear phase matched processes. Figure 7 plots the GVD for various values of OD, where the curves are calculated from the real part of the mode index obtained from beam propagation as described above. These calculations do not account for the effect of the downtaper and uptaper regions of the fiber, which can have quite different dispersion from both the taper waist and the full fiber. However, the nonlinear effects in this structure occur primarily in the waist, and so phase-matched nonlinear processes are most strongly affected by the GVD in the waist. For an improper choice of OD/λ, where the modal intensity may be larger in the downtaper than in the waist, one would have to give more weight to considering the dispersion in this region. The GVD zero crossing wavelength is very sensitive to diameter and can be effectively engineered by appropriate device design [6, 7]. Zero GVD is obtained at the wavelength around 800 nm for an outer diameter of approximately 2.5 µm, or OD/λ=3.13. The near zero dispersion at λ=800 nm coupled with high nonlinear efficiency could be used for benefit in four-wave mixing, nonlinear switching and low-power continuum generation applications [3,12,13,19,20]. The MOF photonic wires also exhibit very large normal GVD when tapered down to sub-micron outer diameters, and this could be of use in compact pulse compression applications.
In conclusion, we demonstrate a new approach to fabricating embedded silica nanowires using a standard flame-brushing process. We taper the fibers by a factor of fifty with minimal distortion of the internal structure. The resulting embedded nanowires are robust and exhibit very low propagation losses. We are able to couple light directly into the nanowire with low loss using adiabatic tapering, allowing for precise loss measurement. Our devices have low sensitivity to the environment, but continue to exhibit tight mode confinement. Several nanowires can thus be closely packaged and bent sharply without incurring large losses. The nanowires can exhibit very high effective nonlinearities and can be tailored to have either very large or zero group velocity dispersion. Smaller taper diameters without hole collapse could be achieved with better flame control, using a shorter length of MOF, or using a MOF with larger air holes in the cladding.
This work was produced with the assistance of the Australian Research Council under the ARC Centre of Excellence program. CUDOS (the Centre for Ultrahigh-bandwidth Devices for Optical Systems) is an ARC Centre of Excellence. Yannick Lizé wishes to recognize financial assistance from the Canadian Institute for Photonic Innovations (CIPI).
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