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Microstructured optical fiber photonic wires with subwavelength core diameter

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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

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Figures (7)

Fig. 1.
Fig. 1. Schematic summarizing the MOF photonic wire concept. The guided mode of the untapered MOF with germanium core propagates through the taper into the single-moded embedded nanowire. A simulated mode profile for λ=1200 nm and outside nanowire diameter of 2.7 µm is shown. Inset: Scanning electron microscope image for such a photonic wire which retains its structure and proportions. The size of the embedded silica nanowire is 800 nm.
Fig. 2.
Fig. 2. Setup for tapering and loss measurement using a broadband source (BBS) 1250′ 1700nm, and an optical spectrum analyzer (OSA). The mode matching between the SMF and the MOF allows for very low loss splice. Adiabatic tapering ensures single mode transmission for the whole spectral range, thus accurate loss measurement.
Fig. 3.
Fig. 3. SEM images of the fiber cross-section for outer diameters of a) 5.0 µm, b) 2.7 µm, c) 2.3 µm, and d) 1.5 µm.
Fig. 4.
Fig. 4. Top. Simulation of the mode profiles intensity with the corresponding index profiles for (left) the MOF photonic wires, (centre) pure silica photonic wires and (right) silica nanowires corresponding to the inner core of the MOF photonic wire. The propagation regimes in dimensionless units of ratio outer diameter OD to wavelength λ are: (a) OD/λ=3.75 - the mode is confined to the inner core and isolated from the environment: (b) OD/λ=1.8 - a transition regime and (c) OD/λ=0.6 - where 70% of the intensity propagates outside the inner core compared to 25% for the pure silica photonic wire.
Fig. 5.
Fig. 5. Increased loss of a 3.5 µm outside diameter MOF photonic wire as a function of wavelength when index-matching fluid (n=1.515) covers the device. The solid curve shows the theoretical loss in this device due to radiation, while the dots show the experimental results.
Fig. 6.
Fig. 6. Calculated effective nonlinearities for our MOF in normalized units of 2πλ2 /Aeff(silica) vs. OD/λ, where Aeff(silica) is the effective area of the mode which lies inside silica.
Fig. 7.
Fig. 7. Group velocity dispersion for outer diameters of 750 nm, 1.5 µm, 2 µm, 2.5 µm and 3 µm. Very high normal GVD is obtained in the transition from the embedded nanowire regime to the evanescent regime and zero dispersion can be obtained at Ti-sapphire wavelengths for a 2.5 µm MOF photonic wire.

Equations (1)

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A eff ( silica ) = ( E 2 d y d x ) 2 s ( x , y ) E 4 d y d x


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