1556 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 27, NO. 11, JUNE 1, 2009 Dispersion-Engineered Photonic Crystal Fibers for CW-Pumped Supercontinuum Sources Alexandre Kudlinski, Géraud Bouwmans, Marc Douay, Majid Taki, and Arnaud Mussot Abstract—We report recent advances on the spectral control of continuous-wave-pumped supercontinuum sources. We show that the generated infrared SC spectrum can be tailored by using photonic crystal fibers with two zero-dispersion wavelengths. The dynamics of the spectral broadening is studied, and we show that slightly different nonlinear mechanisms occur as the zero-dispersion wavelengths are brought closer to each other. We also report the generation of a visible continuous-wave-pumped supercontinuum by using dispersion engineered photonic crystal fibers in which the zero-dispersion wavelength slightly decreases as a function of length over 200 m. The resulting supercontinuum source spans from 650 nm to 1380 nm with an average output power of 19.5 W. The nonlinear mechanisms producing this spec- tacular effect are carefully investigated with support of numerical simulations. We show that the generation of visible wavelengths is due to the trapping of dispersive waves by powerful red-shifting solitons. Index Terms—Dispersive waves, photonic crystal fibers, solitons, supercontinuum generation. I. INTRODUCTION T HE DRAMATIC spectral broadening undergone by an optical field traveling through a nonlinear material is re- ferred as supercontinuum (SC) generation [1]. The interest in research about SC generation has reborn since the development of photonic crystal fibers (PCFs) in the late nineties [2]. By tai- loring the geometry of the photonic crystal cladding and the core size, it is indeed possible to profoundly alter the group velocity dispersion (GVD) of the guided modes [3]–[5] and to significantly increase the nonlinear coefficient as compared to conventional fibers [6], which are key points for SC gen- eration. SC-based sources are potentially useful for many ap- plications, such as optical coherence tomography (OCT) [7], [8], confocal microscopy or characterization of optical com- ponents among others. Two-octave-spanning SC have been re- Manuscript received October 31, 2008; revised February 10, 2009. Current version published May 13, 2009. This work was supported in part by the “Con- seil Régional Nord Pas-de-Calais”, the “Fonds Européens de Développement Economique des Régions” and the “Centre Nationale de la Recherche Scien- tifique” (CNRS). It has also been performed within the framework of the GDR PhoNoMi2, the european COST 299 and the IAP. The authors are with the Université des Sciences et Technologies de Lille, IRCICA, FR CNRS 3024, Laboratoire PhLAM, UMR CNRS 8523, 59655 Villeneuve d’Ascq Cedex, France (e-mail: alexandre.kudlinski@univ-lille1.fr; geraud.bouwmans@univ-lille1.fr; geraud.bouwmans@phlam.univ-lille1.fr; marc.douay@univ-lille1.fr; Abdelmajid.Taki@univ-lille1.fr; arnaud.mussot@ univ-lille1.fr). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JLT.2009.2015966 ported by pumping a few meters length of PCF with pulsed lasers delivering a few kilowatts peak power, from femtosecond to nanosecond pumping regimes [9]. An alternative approach to high pump peak power requirements consists in using contin- uous-wave (CW) lasers (at a multi-watt level) combined with longer nonlinear interaction lengths (i.e., longer fibers). This latter approach presents the benefits of delivering a higher spec- tral power density with a substantial lower intensity noise than their pulsed counterparts. Moreover, SC sources based on CW pumping present an extremely low coherence length, which is of particular interest for submicrometer resolution OCT for in- stance. Additionally, PCFs can be spliced to recently developed CW fiber lasers, preserving the all-fiber format. First experimental SC with CW lasers have been obtained with Raman fiber lasers launched in standard telecommunica- tion fibers [10]–[12]. More impressive performances have re- cently been obtained by using powerful Ytterbium (Yb) fiber lasers emitting around 1 m launched into highly nonlinear PCFs [13]. A spectral power density as high as 50 mW/nm was reported experimentally over a spectral width of 400 nm in the near infrared [13], [14]. But one drawback of these configura- tions is that the bandwidth of the SC spectrum is not easily ad- justable to the one required for a particular application. As a con- sequence the pump power budget is not optimized if the appli- cation requires a given spectral power density over a particular spectral range. Quite recently, it has however been demonstrated numerically [15] and experimentally [16], [17] that a simple scheme based on a PCF with two-zero dispersion wavelengths (ZDWs) allows a tailoring of the SC spectrum extent. One of the particular features of the SC in the configurations described above is that the spectrum was broadened only on the long wave- length side of the pump, due to the Raman-nature of the SC generation process. The lack of short wavelengths generation with pumping at 1 m has been an unresolved issue for a long time. A short-wavelength extension based on four-wave mixing and trapped dispersive waves has been reported very recently in quasi-CW pumping conditions [18] and by using an industrial class laser of 400 W [20]. In these experiments, the minimal equivalent pump power necessary to reach visible wavelengths was as high as 100 W. At the same time, some of us reported the generation of a visible CW-pumped SC by using an engi- neered PCF pumped with a substantially lower pump power of only 13.5 W and a total output power of 9.5 W [21]. In the first part of this paper, we go further into the exper- iments reported in [16], [17] in which it is demonstrated that the generated infrared SC spectrum can be tailored by using PCFs with two ZDWs. We present a detailed dynamics of the nonlinear mechanisms involved in the spectral broadening in 0733-8724/$25.00 © 2009 IEEE