The conversion of light fields in photonic crystal fibers (PCFs) capitalizes on the dramatic enhancement of several optical nonlinearities. We present here spectrally smooth, highly broadband supercontinuum radiation in a short piece of high-nonlinearity soft-glass PCF. This supercontinuum spans several optical octaves, with a spectral range extending from 350 nm to beyond 3000 nm. The selection of an appropriate propagation-length determines the spectral quality of the supercontinuum generated. Experimentally, we clearly identify two regimes of nonlinear pulse transformation: when the fiber length is much shorter than the dispersion length, soliton propagation is not important and a symmetric supercontinuum spectrum arises from almost pure self-phase modulation. For longer fiber lengths the supercontinuum is formed by the breakup of multiple Raman-shifting solitons. In both regions very broad supercontinuum radiation is produced.
©2006 Optical Society of America
The tale of supercontinuum (SC) generation in photonic crystal fibers (PCFs) has been underscored with spectacular success and promise of further scientific advancement across the basic and applied Sciences. More generally, PCFs have been a catalyst for studies of the wide array of nonlinear interactions between light and matter because of their unique optical properties. Habitually, these fibers possess an array of holes of variable and precisely controllable diameters (ranging anywhere from ~25 nm to ~50 µm) running along the fiber length that act as optical barriers or scatterers and trap light within a central core which can either be hollow or made of solid glass. The very large air-glass refractive index difference opens up many new possibilities that are not routinely available in standard fibers. The design and positioning of the air holes in PCFs allows to uniquely and precisely engineer the dispersion properties of the waveguide offering unprecedented efficiency in the enhancement of nonlinear effects as well as providing the opportunity to operate in a predefined physical regime . Furthermore, the air/glass geometry allows the production of guiding structures out of any single optically transparent material that can be suitably drawn without the need for bulk doping (in contrast to conventional optical fibers).
The well known physical processes associated with nonlinear optics and nonlinear optics in fibers/waveguides, have undergone a research renaissance thanks to the ease with which these enhanced effects are obtainable in photonic crystal fibers. Familiar and loved phenomena like self-phase modulation, optical solitons and pulse transformation, multimode phase-matching, and supercontinuum generation, to name a few, have been thoroughly revisited and re-examined in this modern setting fueled by the convenience of PCFs. In spite of the fundamental physics being well established [2, 3], the importance of practical availability of highly nonlinear optical phenomena cannot be overestimated and occupies a position of prominence in the development of present and future applications.
As far as modern day supercontinuum generation goes, of great interest are PCFs fabricated with materials that have a higher nonlinear response than the widely used fused silica, by far the most popular PCF constituent to date. Recently, photonic crystal fibers have been realized in Schott SF6 glass  by extrusion of a glass rod through a suitably shaped perform die. Generation of SC in these PCFs has been demonstrated by several groups by coupling 1550 nm wavelength ultrafast pulses in moderate lengths of this fiber (ranging from 30 cm to ~1 m) [5, 6]. We have previously investigated the nonlinear processes  in this type of fiber by launching ultrafast pulses at a central wavelength of 1550 nm into a 75-cm segment of SF6-PCF. The supercontinuum radiation was observed to be quite broad, potentially extending beyond the sensitivity limits of the detection apparatus used, leaving open the question on its ultimate spectral range.
The generation of broadband light is certainly of interest for numerous spectroscopic applications, especially at longer (λ>2 µm) wavelengths where there is a considerable and largely unmet demand for coherent and high-brightness light sources. The quality of the radiation produced, however, remains an issue of crucial importance [7–9]. The optical nonlinearities that are germane to supercontinuum generation provide considerable challenges for its control. Typically, supercontinuum traces exhibit several spectral features that can be precisely related to the cascaded nonlinear processes that occur during propagation. The ensemble of nonlinear dynamics that come into play in the transformation of an ultrashort pulse in SC has been the focus of intense research aimed at more efficient and controlled supercontinuum generation and optimization.
Generally speaking, the majority of the processes involved are resonant in nature and require certain wave-matching conditions to be satisfied. Previous studies have identified white-light generation in PCFs to be caused by the fission of high-order solitons into redshifted solitons accompanied by the generation of blueshifted non-solitonic radiation [10, 11]. Further attention has been devoted to the relationship between SC radiation and the input pulse chirp  the mode field diameter dependence , and the input pulse polarization [14–16] to name a few. Also, the generation of new spectral components through the interaction between (the linear) dispersive waves and (the nonlinear) solitons in PCF has been thoroughly investigated [17–19]. It has been previously established [1, 20] that controlling the dispersion D(λ)=∂/∂λ 1/vG=-2πc/λ2 β2(ω) is a crucial step in defining the physical regime that affects pulse evolution in PCFs.
2. Experimental results
The experimental results presented here indicate that in highly nonlinear regimes such as the ones encountered in PCFs, the propagation distance is an equally crucial parameter to define the physical regime that affects pulse transformation. This is not too surprising (and very widely investigated) from a physical standpoint. What is, however, remarkable is the spectral extent of the supercontinuum generated in a regime where only self-phase modulation dominates and without the occurrence of (multiple) soliton fission. We address here both the spectral breadth of the generated SC and the control of its spectral features by examining different lengths of SF6-PCF.
A scanning-electron microscope image of the cross-section of the actual PCF sample used in the experiments is shown in Fig. 1, which also illustrates the experimentally measured dispersion curve for the SF6-PCF used in these measurements. The physical description of nonlinear propagation of ultrashort pulses and their transformation is, to first order, governed by self-action effects which are quantified through the intensity (I=(n0c/8π) |E|2) dependence of the index of refraction of the material n(I)=n0+n2I, where n0 is the linear index of refraction of the material, n2=12π2/n02c χ (3) is its nonlinear index of refraction and χ(3) is its third order nonlinear optical susceptibility. The reported value for Schott SF6 glass, n2=2.2 10–-19 m2/W, is almost an order of magnitude higher than the n2 for silica. This high value of n2, in combination with the fiber’s dispersion properties, make SF6-PCF particularly appealing for operation in the near infrared at wavelengths greater than ~1300 nm, a spectral window of particular relevance given the availability of compact fiber sources in this region and its relevance for information processing applications.
The experimental setup consists of an optical parametric oscillator (OPO) laser source that provides tuneable femtosecond pulses in the near-IR with pulse duration of 110 fs at a repetition rate of 80 MHz. The experiments presented here were conducted at an operating wavelength of 1550 nm. Control over the input coupling of laser light into the fiber is achieved by a sequence of waveplates and polarizers that provide variable attenuation and can also adjust the polarization vector of the input E-field to be aligned along the direction of the eigenaxes of the PCF. The polarization state of the input is especially important for the generation of guided high-order harmonics in PCFs  but has been observed to have little or no effect on the SC generation process in this case. The SF6-PCF used in these experiments has a 2.6 micron core diameter and, correspondingly, a zero dispersion wavelength at 1300 nm. This implies that the laser pulses that are propagating in the SF6-PCF are well within the anomalous dispersion region of the fiber (Fig. 1.). Detection of the radiation generated in the SF6-PCF is achieved by using an optical spectrum analyzer (OSA), which covers the wavelength range from 350 nm to 1750 nm, and a second spectrometer equipped with a cryogenically cooled HgCdTe detector which covers the mid-infrared wavelength region from 2000 to 3000 nanometers. In order to accurately measure the longer wavelength end of the spectrum, the SF6-PCF output is sent through a mechanical chopper and coupled into the spectrometer after dispersing the fiber output through a prism in order to block the shorter wavelengths and rule out the possibility of detecting some spurious signal due to diffraction order overlap. Lock-in detection provides a dynamic range in excess of four orders of magnitude in the 2000–3000 nm region, which is comparable to the dynamic range of the OSA used in the 350–1750 nm spectral window. The SF6-PCF is carefully cleaved to very short lengths (<1 cm) and mounted on custom machined fittings which secure the fiber in place on commercial XYZ fiber-coupling stages and allow the positioning of the short-working distance coupling (NA=0.6) and recollimating microscope objectives. The spectral output from a Z=5.7 mm piece of SF6 PCF is illustrated in Fig. 2, which combines on the same graph the data obtained from the OSA and the data obtained from the second spectrometer . Supercontinuum radiation is found to be present over the whole detection range of the apparatus, covering a spectral region that extends from 350 nm to past 3000 nm. The average power of the SC pulses detected at the output of the fiber is 70 mW. This places the pump pulse energies close to 1 nJ (875 pJ assuming negligible losses in the fiber) and implies that the coupling efficiency into the SF6-PCF is around 30% . A striking characteristic of this unprecedented broad supercontinuum radiation is the obvious absence the typical spectral structure that generally accompanies the nonlinear transformation of short laser pulses in the SC generation process. This desirable spectral quality, however, is lost when the same experiment is repeated in longer pieces of the same SF6 photonic crystal fiber.
For direct comparison, spectral data was acquired as a function of input power in the 350–1750 nm range for different lengths of the same SF6-PCF. Special attention was devoted to keeping identical coupling and detection conditions (i.e. collimating optics and detector interface-see inset of Fig. 4). The results of these measurements are shown in Fig. 3.
In the longer piece of PCF, the experimental data shows a “more traditional” spectral signature of soliton fission accompanied by the Raman-induced soliton self frequency-shifting, which can be clearly identified in the contour plots by the presence of several spectral branches that originate at the pump wavelength and shift towards the longer wavelength region of the spectrum shown in Fig. 3(b). The physical pattern of multiple soliton fission leading to SC generation has been convincingly explained by several groups [11, 24, 25]. In the short piece of PCF shown in Fig. 3(a), however, the spectral branches that are associated with the soliton fission and its subsequent red-shift do not appear and the spectral evolution is strongly suggestive of a process entirely dominated by self-phase modulation, with two spectral lobes appearing symmetrically around the pump wavelength. In spite of the absence of well defined higher order nonlinear contributions, the nonlinear pulse transformation over this short distance is robust enough to generate >2500 nm of supercontinuum, as displayed in the results of Fig. 2. The observed spectral behavior is consistent with the fact that, in this case, the fiber length (5.7 mm) is considerably shorter than the dispersion length (LD=τ2/β2(ω) which is estimated to be ~40 cm at λ=1550 nm for this fiber) and would not easily account for the formation of stable optical solitons, in contrast to what happens for the longer piece of PCF. As fiber lengths approach a few centimeters, significant structure starts to appear revealing more intricate nonlinear dynamics. We have previously been successful in accurately describing the nonlinear behavior of pulse evolution in PCFs through careful simulations based on a generalized nonlinear Schroedinger equation (NLSE) . Despite our best efforts, we cannot accompany these experimental results, as we customarily do, with accurate modelling of the process since, to the best of our knowledge, the Raman response function for SF6 glass, (which includes instantaneous electronic and delayed Raman contributions), is not available in the literature making appropriate parameterization of the NLSE impossible.
This series of experiments has revealed very interesting behavior from a short piece (Z=5.7 mm) of SF6 photonic crystal which can be summarized in (a) the observation of dramatically broad supercontinuum radiation spanning from 350 nm to beyond 3000 nm and (b) the absence of soliton fission in the process of ultrabroad supercontinuum generation thereby providing the spectral smoothness of this broad supercontinuum.
These results underscore, once more, the extreme importance of operating at appropriate lengths in such highly nonlinear waveguides and confirm that the defining physical interaction that underpins very broad SC generation occurs in the very first instances of propagation in these fibers and is largely independent of high-order nonlinear effects. Furthermore, short fiber lengths offer further advantages since length-dependent absorption is reduced, allowing for a broader spectrum to be generated, and chromatic dispersion is minimized, thereby lessening the temporal broadening that accompanies SC generation.
This extremely broad and smooth supercontinuum radiation promises to be of further interest for many applications and is likely to continue to impact several areas of research across the Sciences from biomedical imaging and diagnostics (such as optical coherence tomography), to precision spectroscopy and frequency metrology to name a few. Furthermore, the greater extent of the spectrum generated offers possibilities in fiber-based sources of longer wavelength coherent light. Finally, the pump wavelengths in the λ>1300 range make these PCFs ideal for all-fiber ultrabroad SC sources, bringing the versatility of nonlinear optics to reduced and convenient dimensions.
References and links
1. W. H. Reeves, D. V. Skryabin, F. Biancalana, J. C. Knight, P. S. Russell, F. G. Omenetto, A. Efimov, and A. J. Taylor, “Transformation and control of ultra-short pulses in dispersion-engineered photonic crystal fibres,” Nature 424, 511–515 (2003). [CrossRef] [PubMed]
2. R. Alfano, The Supercontinuum Laser Source (Springer, NY, 1989).
3. G. P. Agrawal, Nonlinear Fiber Optics (Academic Press, San Diego, CA, 2001).
4. V. Kumar, A. K. George, W. H. Reeves, J. C. Knight, P. S. Russell, F. G. Omenetto, and A. J. Taylor, “Extruded soft glass photonic crystal fiber for ultrabroad supercontinuum generation,” Optics Express 10(25), 1520–1525 (2002).
5. V. L. Kalashnikov, E. Sorokin, S. Naumov, I. T. Sorokina, V. V. R. Kumar, and A. K. George, “Low-threshold supercontinuum generation from an extruded SF6PCF using a compact Cr4+: YAG laser,” Appl. Phys. B-Lasers and Optics 79, 591–596 (2004). [CrossRef]
6. H. Hundertmark, D. Kracht, D. Wandt, C. Fallnich, V. Kumar, A. K. George, J. C. Knight, and P. S. Russell, “Supercontinuum generation with 200 pJ laser pulses in an extruded SF6 fiber at 1560 nm,” Optics Express 11, 3196–3201 (2003). [CrossRef] [PubMed]
7. N. Nishizawa and T. Goto, “Widely wavelength-tunable ultrashort pulse generation using polarization maintaining optical fibers,” IEEE J. Sel. Top. Quantum Electron. 7, 518–524 (2001). [CrossRef]
8. T. Hori, J. Takayanagi, N. Nishizawa, and T. Goto, “Flatly broadened, wideband and low noise supercontinuum generation in highly nonlinear hybrid fiber,” Optics Express 12, 317–324 (2004). [CrossRef] [PubMed]
9. T. Hori, N. Nishizawa, T. Goto, and M. Yoshida, “Experimental and numerical analysis of widely broadened supercontinuum generation in highly nonlinear dispersion-shifted fiber with a femtosecond pulse,” J. Opt. Soc.Am. B 21, 1969–1980 (2004). [CrossRef]
10. A. V. Husakou and J. Herrmann, “Supercontinuum generation, four-wave mixing, and fission of higher-order solitons in photonic-crystal fibers,” J. Opt. Soc. Am. B 19, 2171–2182 (2002). [CrossRef]
11. J. Herrmann, U. Griebner, N. Zhavoronkov, A. Husakou, D. Nickel, J. C. Knight, W. J. Wadsworth, P. S. J. Russell, and G. Korn, “Experimental evidence for supercontinuum generation by fission of higher-order solitons in photonic fibers,” Phys. Rev.Lett.88 (2002). [CrossRef] [PubMed]
12. G. Sansone, G. Steinmeyer, C. Vozzi, S. Stagira, M. Nisoli, S. De Silvestri, K. Starke, D. Ristau, B. Schenkel, J. Biegert, A. Gosteva, and U. Keller, “Mirror dispersion control of a hollow fiber supercontinuum,” Appl.Phys. B 78, 551–555 (2004). [CrossRef]
13. B. Kibler, J. M. Dudley, and S. Coen, “Supercontinuum generation and nonlinear pulse propagation in photonic crystal fiber: influence of the frequency-dependent effective mode area,” Appl. Phys. B 81, 337–342 (2005). [CrossRef]
14. A. Proulx, J. M. Menard, N. Ho, J. M. Laniel, R. Vallee, and C. Pare, “Intensity and polarization dependences of the supercontinuum generation in birefringent and highly nonlinear microstructured fibers,” Opt. Express 11, 3338–3345 (2003). [CrossRef] [PubMed]
15. L. Tartara, I. Cristiani, V. Degiorgio, F. Carbone, D. Faccio, M. Romagnoli, and W. Belardi, “Phase-matched nonlinear interactions in a holey fiber induced by infrared super-continuum generation,” Opt. Commun. 215, 191–197 (2003). [CrossRef]
16. S. Coen, A. H. L. Chau, R. Leonhardt, J. D. Harvey, J. C. Knight, W. J. Wadsworth, and P. S. J. Russell, “Supercontinuum generation by stimulated Raman scattering and parametric four-wave mixing in photonic crystal fibers,” J. Opt. Soc. Am.B 19, 753–764 (2002). [CrossRef]
17. A. Efimov, A. V. Yulin, D. V. Skryabin, J. C. Knight, N. Joly, F. G. Omenetto, A. J. Taylor, and P. Russell, “Interaction of an optical soliton with a dispersive wave,” Phys. Rev. Lett.95, (2005). [CrossRef] [PubMed]
18. F. Biancalana, D. V. Skryabin, and A. V. Yulin, “Theory of the soliton self-frequency shift compensation by the resonant radiation in photonic crystal fibers,” Phys. Rev. E70 (2004). [CrossRef]
20. N. Y. Joly, F. G. Omenetto, A. Efimov, A. J. Taylor, J. C. Knight, and P. S. Russell, “Competition between spectral splitting and Raman frequency shift in negative-dispersion slope photonic crystal fiber,” Optics Commun. 248(1–3), 281–285 (2005). [CrossRef]
21. F. G. Omenetto, A. Efimov, A. J. Taylor, J. C. Knight, W. J. Wadsworth, and P. S. J. Russell, “Polarization dependent harmonic generation in microstructured fibers,” Opt. Express 11, 61–67 (2003). [CrossRef] [PubMed]
22. For the specific set of measurements presented here we are not able to include data in the 1750–2000 nm region. The presence of supercontinuum in this spectral region has been previously verified in this PCF by the authors .
23. The total average power through the fiber is determined using a thermal head power meter, with NIST traceable calibration and sensitivity in the 250nm-10 µm range (+/- 5% accuracy).
24. J. M. Dudley, X. Gu, L. Xu, M. Kimmel, E. Zeek, P. O’Shea, R. Trebino, S. Coen, and R. S. Windeler, “Cross-correlation frequency resolved optical gating analysis of broadband continuum generation in photonic crystal fiber: simulations and experiments,” Opt. Express 10, 1215–1221 (2002). [PubMed]
25. A. V. Husakou and J. Herrmann, “Supercontinuum generation of higher-order solitons by fission in photonic crystal fibers,” Physical Review Letters8720, (2001).