We report the fabrication and characterization of an AsSe microtaper with a protective cladding made of PolyMethyl MethAcrylate (PMMA). The AsSe core of the microtaper provides an ultrahigh nonlinearity up to γ = 133 W−1m−1 whereas the polymer cladding provides mechanical strength for normal handling of the device and reduces sensitivity to the surrounding environment.
©2010 Optical Society of America
Highly nonlinear waveguides are raising a lot of interest for their use in nonlinear devices such as mode-locked lasers, supercontinuum sources, optical time-domain demultiplexers, and wavelength converters. Nonlinear waveguides are characterized by their waveguide nonlinearity parameter γ = 2πnˉ 2/(λAeff), where nˉ 2 is the effective material nonlinearity, λ is the signal wavelength, and Aeff is the effective area . In such devices, the nonlinear effect cumulates following a phase-shift ϕNL = γPL, where P is the power of the propagating pulse and L is the waveguide length . The most desirable property of highly nonlinear waveguides is a waveguide nonlinearity parameter that is maximized to reduce both the required waveguide length and required peak pulse power. Such waveguides improve the device compactness and reduce the power consumption. A logical approach to maximize the waveguide nonlinearity parameter is to make the nonlinear waveguide out of a material with a large material nonlinearity and to ensure that the guided mode is strongly confined thereby minimizing the effective area.
Chalcogenide glasses are of particular interest for nonlinear device fabrication as they exhibit one of the largest material nonlinearity that is up to three orders of magnitude greater than that of silica, a low two photon absorption, and a short response time <100 fs . In addition to using a material with a large material nonlinearity, waveguide structures with minimized effective area such as microtapers also provide a significant increase in the waveguide nonlinearity parameter [4,5]. Not only restricted to this major benefit, microtapers also provide a group-velocity dispersion that is broadly variable [6,7].
Combining both a large material nonlinearity and a small effective area, a wire made of AsSe fiber tapered down to ~1 μm in diameter was reported with a waveguide nonlinearity parameter of γ = 93 W−1m−1 [8,6]. Although this microtaper provides one of the highest waveguide nonlinearities ever reported, its practical use is questionable due to mechanical and optical limitations. Mechanically, the few cm long and ~1 μm wire is extremely fragile and can hardly be taken out of its tapering apparatus without rupture. The unprotected microtaper is also subject to surface damage and contamination. Finally, the traveling wave is also sensitive to the medium surrounding the AsSe wire since a non negligible fraction (9%) of the fundamental mode power propagates outside the optimally nonlinear wire. This represents a drawback of the unprotected microtaper in view of device insensitivity to the environment.
Chalcogenide glasses have been combined to materials with a compatible softening temperature such as polymers and tellurite glass to enable simultaneous heat and stretch processes. Chalcogenide-polymer Bragg fibers with a polymer core and a cladding composed of alternating chalcogenide-polymer layers have been fabricated to guide CO2 laser light [9–11]. More recently, a photonic crystal fiber combining a chalcogenide core with a holey tellurite cladding has been fabricated to enable a waveguide nonlinearity γ = 9.3 W−1m−1 and supercontinuum generation .
In this paper, we report the fabrication and optical characterization of the first hybrid chalcogenide-polymer microtapers. With this material combination, the chalcogenide wire induces a large Kerr effect whereas the polymer coating provides sufficient mechanical robustness and flexibility to the assembly for normal handling as well as limiting the evanescent interaction with the environment.
2. Microtaper design
Figure 1 presents a schematic of the hybrid microtaper. It comprises a wire section where most of the nonlinear effects occur and a transition region between the single mode fiber and the wire section. The microtaper is made of an AsSe single mode fiber surrounded by a polymer coating made of PolyMethyl MethAcrylate (PMMA). This polymer was chosen because its softening temperature is compatible with that of AsSe which is TAsSe = 190°C. Determination of the optimal microtaper desing involved an analysis of the field propagating in the wire section of the microtaper, leading to values of waveguide nonlinearity parameter and the chromatic dispersion parameters.
Microtapers have a large refractive index contrast at the core-cladding interface. The weak guidance approximation is thus expected to be misleading, and a full-vectorial analysis is required [1,13,14]. Taking into account the discontinuity of the radial component of the electric field at the AsSe-PMMA interface, the vectorial nature of the electric field, and the different material composition of the microtaper, the effective material nonlinearity and effective area are given by [1,14]15,16], n 2 is the material nonlinearity (n 2,AsSe = 1.1 × 10−17 m2W−1, n 2,PMMA = −8 × 10−19 m2W−1) [3,17], k 0 is the wavenumber, E and H are the electric and magnetic fields, respectively, ε 0 and μ 0 are the electric permittivity and the magnetic permeability of free space, respectively, z is the direction of propagation and A is the transverse surface area. Figure 2 shows the waveguide nonlinearity parameter versus the AsSe wire diameter at a wavelength of 1550 nm. The absolute theoretical maximum of the waveguide nonlinearity parameter reaches γ max = 185 W−1m−1 with an AsSe wire diameter of 0.47 μm.
For chromatic dispersion calculations, the wavelength dependence of the refractive index for AsSe and PMMA is calculated using the Cauchy relation n 2(λ) = A + B/λ 2 + C/λ 4 where A, B, and C are the Cauchy coefficients for the material of interest and λ is the wavelength in μm. For AsSe, A = 7.56, B = 1.03 μm2, and C = 0.12 μm4 in the range of 0.9 μm < λ < 1.7 μm , and for PMMA, A = 2.149, B = 0.028 μm2, and C = −0.002 μm4 in the range of 0.6 μm < λ < 1.6 μm . The propagation constant β and the effective refractive index neff = β/k 0 of the fundamental mode are calculated by solving the characteristic equation of the waveguide with the refractive indexes given above. The chromatic dispersion is then given from Dc = -(λ/c)(d 2 neff/dλ 2) . Figure 2 also shows the chromatic dispersion of the hybrid microtaper as a function of wire diameter.
To simulate pulse propagation in the microtaper, a split-step Fourier method based on the generalized nonlinear Schrodinger equation is used 2,3]. In the simulations, the pulse was propagated in the SMF fiber as well as in the hybrid microtaper, transition region and wire section, each with appropriate values of γ and Dc. No higher order of β than β 3 was required to ensure a good agreement between experiment and theory.
Linear losses in the hybrid microtaper arise from various origins: butt-coupling losses, material absorption losses, and adiabaticity losses. Butt-coupling losses occur at the SMF-AsSe fiber interfaces due to mode mismatch and Fresnel reflection (0.5 dB per interface). Material losses in the wire section are derived from α dB hybrid = ΓAsSe × α dB AsSe + ΓPMMA × α dB PMMA, where the confinement factor Γi = Pi/Ptot with Pi being the power fraction of the mode in layer i and Ptot the total power of the mode. The attenuation coefficients in AsSe and PMMA at a wavelength of 1550 nm are α dB AsSe = 0.0085 dB/cm  and α dB PMMA = 0.5 dB/cm , respectively. Finally, adiabaticity losses may occur in the transition regions where the mode from the single mode AsSe fiber is converted into a wire mode, and back into a single mode AsSe fiber mode .
3. Fabrication and characterization
The microtaper was fabricated using a heat and stretch technique specifically adapted to the hybrid configuration. It involved the use of an AsSe fiber that was assembled at CorActive HighTech from high-purity glass using double-crucible process . The 7 μm core was made from As39Se61 and the 170 μm cladding was made from an AsSe glass with slightly reduced As content, leading to a numerical aperture of NA = 0.2. Linear and nonlinear absorption coefficients of this fiber were α = 0.23 m−1 and α 2 = 2.5 × 10−12 m/W, respectively [3,8]. The AsSe fiber was pre-stretched to reduce the core diameter down to 5.6 μm and ensure single-mode propagation. The uniform waist section of the pre-stretched fiber was cleaved, inserted into a PMMA cylinder with internal/external diameter of 230/1000 μm, butt-coupled and secured to standard silica fibers using UV cured epoxy. The assembly was heated at 160°C to uniformly collapse the polymer rod over the modified chalcogenide fiber.
With this uniform perform, the temperature was then raised to 190°C and the assembly was stretched adiabatically until the AsSe wire section of the hybrid microtaper reached the target diameter. Two samples are reported in this paper. Hybrid microtaper #1 was stretched down until the wire section reached a length of 7.0 cm, the AsSe wire then had a diameter of 1.8 μm whereas the polymer diameter was 5.4 μm. Figure 3 shows a microscope picture of the hybrid microtaper #1. For hybrid microtaper #2, the length of the wire section was 9.7 cm, the AsSe wire had a diameter of 0.8 μm whereas the polymer diameter was 2.4 μm. In both samples, the polymer diameter was sufficiently large to allow handling of the hybrid microtaper without damage.
Figure 4 shows the setup used to characterize the linear and nonlinear properties of the samples. A mode-locked laser sent pulses of 330 fs full-width at half-maximum at a repetition rate of 20 MHz and at a central wavelength of λ = 1552.4 nm. The power from the pulses was then adjusted using a variable attenuator and an in-line power meter before injection in the microtaper. The peak power reaching the AsSe wire section of the microtaper could be varied up to a maximum of 50 W. Light from the microtaper output was sent to an optical spectrum analyzer and a powermeter.
4. Results and discussion
Figure 5 shows the optical spectrum of pulses at increasing peak power levels at the output of the hybrid microtaper #1. The split-step Fourier method is used to fit the experimental data with a good agreement and lead to γ wire = 22 W−1m−1, Aeff = 1.4 μm2, Dc = −950 ps/nm-km (β 2 = 1210 ps2/km), β 3 = 2.2 ps3/km. In the wire section of this microtaper, ~100% of the power is propagating in the AsSe wire and no significant fraction in the PMMA, thus leading to a linear attenuation coefficient of α dB hybrid = 0.0085 dB/cm. The value provided for γ wire represents the value in the wire section of the microtaper, where 93% of the nonlinear phase-shift accumulates. The 7% remaining is accumulated in the transition regions of the microtaper near the wire section.
Figure 6 shows the output spectra of the hybrid microtaper #2. In this case, a supercontinuum is being observed with a 20 dB spectral width greater than 500 nm. The split-step Fourier method with γ wire = 133 W−1m−1, Dc = −160 ps/nm-km (β 2 = 205 ps2/km), β 3 = 3.8 ps3/km, Aeff = 0.34 μm2 and a loss of α dB hybrid = 0.018 dB/cm are used to simulate pulse propagation in the microtaper as shown in Fig. 7 .
Measuring the linear losses of both samples above led to 10.5 and 12 dB, respectively. The device loss before and after stretching were equal and thus we conclude that the mode compression/dilatation at the input/output transition sections of the microtaper are adiabatic. The loss induced at each facet is inferred by comparing the nonlinear spectral broadening taken with the signal propagating in either direction in the device.
We reported the first hybrids AsSe-PMMA microtapers providing an ultrahigh waveguide nonlinearity, sufficient mechanical robustness for normal handling, and reduced sensitivity to the surrounding environment. Hybrid microtapers having an AsSe wire section of 1.8 μm and 0.8 μm were fabricated leading to waveguide nonlinearity parameters γ = 22 W−1m−1 and γ = 133 W−1m−1 respectively. From theory, the maximum waveguide nonlinearity parameter could even be increased to up to γ = 185 W−1m−1. With such a large waveguide nonlinearity parameter, a 7 cm hybrid microtaper could replace a conventional highly non-linear silica fiber (γ~0.01 W−1m−1) of 1.0 km. The polymer coating made of PMMA has a softening temperature compatible with AsSe and enables stretching of both materials simultaneously. Hybrid AsSe-PMMA microtapers of a few cm, with their high waveguide nonlinearity parameter and group-velocity dispersion that is broadly variable while keeping a large waveguide nonlinearity, are foreseen as a key element in any Kerr-based nonlinear device.
References and Links
1. S. Afshar V and T. M. Monro, “A full vectorial model for pulse propagation in emerging waveguides with subwavelength structures part I: Kerr nonlinearity,” Opt. Express 17(4), 2298–2318 (2009). [CrossRef] [PubMed]
2. G. P. Agrawal, Nonlinear Fiber Optics (Academic press, 2007), 4th ed.
3. R. E. Slusher, G. Lenz, J. Hodelin, J. Sanghera, L. B. Shaw, and I. D. Aggarwal, “Large Raman gain and nonlinear phase shifts in high-purity As2Se3 chalcogenide fibers,” J. Opt. Soc. Am. 21(6), 1146–1155 (2004). [CrossRef]
4. P. Dumais, F. Gonthier, S. Lacroix, J. Bures, A. Villeneuve, P. G. J. Wigley, and G. I. Stegeman, “Enhanced self-phase modulation in tapered fibers,” Opt. Lett. 18(23), 1996–1998 (1993). [CrossRef] [PubMed]
6. D.-I. Yeom, E. C. Mägi, M. R. E. Lamont, M. A. F. Roelens, L. Fu, and B. J. Eggleton, “Low-threshold supercontinuum generation in highly nonlinear chalcogenide nanowires,” Opt. Lett. 33(7), 660–662 (2008). [CrossRef] [PubMed]
8. E. C. Mägi, L. B. Fu, H. C. Nguyen, M. R. E. Lamont, D. I. Yeom, and B. J. Eggleton, “Enhanced Kerr nonlinearity in sub-wavelength diameter As(2)Se(3) chalcogenide fiber tapers,” Opt. Express 15(16), 10324–10329 (2007). [CrossRef] [PubMed]
9. S. D. Hart, G. R. Maskaly, B. Temelkuran, P. H. Prideaux, J. D. Joannopoulos, and Y. Fink, “External reflection from omnidirectional dielectric mirror fibers,” Science 296(5567), 510–513 (2002). [CrossRef] [PubMed]
10. B. Temelkuran, S. D. Hart, G. Benoit, J. D. Joannopoulos, and Y. Fink, “Wavelength-scalable hollow optical fibres with large photonic bandgaps for CO2 laser transmission,” Nature 420(6916), 650–653 (2002). [CrossRef] [PubMed]
11. T. Engeness, M. Ibanescu, S. Johnson, O. Weisberg, M. Skorobogatiy, S. Jacobs, and Y. Fink, “Dispersion tailoring and compensation by modal interactions in OmniGuide fibers,” Opt. Express 11(10), 1175–1196 (2003). [CrossRef] [PubMed]
12. M. Liao, C. Chaudhari, G. Qin, X. Yan, C. Kito, T. Suzuki, Y. Ohishi, M. Matsumoto, and T. Misumi, “Fabrication and characterization of a chalcogenide-tellurite composite microstructure fiber with high nonlinearity,” Opt. Express 17(24), 21608–21614 (2009). [CrossRef] [PubMed]
13. V. Tzolov, M. Fontaine, N. Godbout, and S. Lacroix, “Nonlinear modal parameters of optical fibers: a full-vectorial approach,” J. Opt. Soc. Am. B 12(10), 1933–1941 (1995). [CrossRef]
14. S. Afshar V, W. Q. Zhang, H. Ebendorff-Heidepriem, and T. M. Monro, “Small core optical waveguides are more nonlinear than expected: experimental confirmation,” Opt. Lett. 34(22), 3577–3579 (2009). [CrossRef] [PubMed]
15. G. Boudebs, S. Cherukulappurath, M. Guignard, J. Troles, F. Smektala, and F. Sanchez, “Linear optical characterization of chalcogenide glasses,” Opt. Commun. 230(4-6), 331–336 (2004). [CrossRef]
16. J. Swalen, R. Santo, M. Tacke, and J. Fischer, “Properties of polymeric thin films by integrated optical techniques,” IBM J. Res. Develop. 21(2), 168–175 (1977). [CrossRef]
17. F. D'Amore, M. Lanata, S. M. Pietralunga, M. C. Gallazzi, and G. Zerbi, “Enhancement of PMMA nonlinear optical properties by means of a quinoid molecule,” Opt. Mater. 24(4), 661–665 (2004). [CrossRef]
18. M. G. Kuzyk, Polymer Fiber Optics: Materials, Physics, and Applications (CRC press, 2007).
19. J. D. Love and W. M. Henry, “Quantifying loss minimization in single-mode fibre tapers,” Electron. Lett. 22(17), 912–914 (1986). [CrossRef]
20. R. Mossadegh, J. S. Sanghera, D. Schaafsma, B. J. Cole, V. Q. Nguyen, R. E. Miklos, and I. D. Aggarwal, “Fabrication of single-mode chalcogenide optical fiber,” J. Lightwave Technol. 16(2), 214–217 (1998). [CrossRef]