RAPID COMMUNICATIONS PHYSICAL REVIEW A 81, 031803(R) (2010) Widely tunable femtosecond solitonic radiation in photonic crystal fiber cladding Jiahui Peng, 1,* Alexei V. Sokolov, 1 F. Benabid, 2 F. Biancalana, 3 P. S. Light, 2 F. Couny, 2 and P. J. Roberts 4 1 Department of Physics and Institute for Quantum Studies, Texas A&M University, College Station, Texas 77843, USA 2 Department of Physics, University of Bath, Bath, BA2 7AY, United Kingdom 3 School of Physics and Astronomy, Cardiff University, CF24 3 AA Cardiff, United Kingdom 4 DTU Fotonik, Danish Technical University, Lyngby, DK-2800, Denmark (Received 3 April 2009; published 18 March 2010) We report on a means to generate tunable ultrashort optical pulses. We demonstrate that dispersive waves generated by solitons within the small-core features of a photonic crystal fiber cladding can be used to obtain femtosecond pulses tunable over an octave-wide spectral range. The generation process is highly efficient and occurs at the relatively low laser powers available from a simple Ti:sapphire laser oscillator. The described phenomenon is general and will play an important role in other systems where solitons are known to exist. DOI: 10.1103/PhysRevA.81.031803 PACS number(s): 42.81.Dp, 42.65.Re, 42.65.Ky, 42.70.Qs Ultrashort pulsed lasers have now become an essential tool in fundamental and applied science. The most common technique for generating ultrashort optical pulses relies on laser mode-locking [1]. The spectral location of these pulses is typically restricted to the near IR because of the available gain media. Widely tunable ultrashort optical pulses may be obtained with optical parametric amplifiers, but those systems are bulky and the total efficiency is low [2]. Additional degrees of flexibility are offered by solitons in fibers. Indeed, propagation dynamics of higher order solitons in optical fibers provides an efficient means for compression and red-shifting of ultrashort pulses [3]. With the advent of photonic crystal fibers (PCFs), there has been a revived interest in solitonic dynamics because of its relevance in supercontinuum generation [4]. Here, we report on a PCF-based technique that exploits efficient generation of resonant dispersive waves (RDWs) emitted from a soliton formed in a Kagome-PCF cladding [5]. It allows us to generate femtosecond pulses at wavelength ranges not covered by the conventional mode-locked lasers and with almost an octave-wide tuning bandwidth spanning from ∼380 to ∼700 nm. Furthermore, provided a judiciously engineered dispersion is used, this mechanism of RDW-based ultrashort pulse generation can be generalized to other media such as mode-locked fiber lasers or ultra-fast optical switchers, to mention just a few. This technique can also be used to transfer the temporal and spectral structure of a soliton to another spectral region. Solitons have been the subject of intense theoretical and experimental studies in many different fields, including hydro- dynamics, nonlinear optics, plasma physics, and biology [6]. Temporal optical solitons [7,8] are widely applied in optical communications and ultrashort laser systems. When there is a perturbation acting on a soliton (e.g., by a localized loss in the fiber or modified parameters in the laser cavity), a soliton will reshape and shed excess energy into RDWs [9]. So far, the interaction between a soliton and a dispersive wave has been considered to be a parasitic effect and the prior work on RDW per se was kept marginal relative to the soliton dynamics. Indeed, in all the soliton-related applications, the RDW gen- * Corresponding author: Jiahui.Peng@nrc-cnrc.gc.ca eration is a source of dissipation of the soliton energy, which affects the performance of soliton-based telecommunication systems [10] and limits the pulse duration in soliton lasers [11]. It is well known that nanojoule femtosecond light pulses can generate supercontinua in PCFs [12]. However, in many cases instead of a supercontinuum, narrower band femtosecond pulses tunable over an extremely large spectral range are needed [13]. One of the many applications which would benefit from widely tunable femtosecond optical pulses is Raman microspectroscopy, where low-energy ultrashort pulses are preferred [14]. One of the advantages of PCF is the capability of engineer- ing and control the fiber’s group velocity dispersion (GVD) [15]. As described in the following, it is this dispersion control that enables our RDW technique. As in convention fibers, opti- cal solitons can be formed inside our PCF with negative GVD and Kerr nonlinearity at play [10]. The ultrashort soliton evo- lution inside the fiber can be described by the standard scalar generalized nonlinear Schr¨ odinger equation (GNLSE) [10] i∂ z A(z, t ) + ˆ D(i∂ t )A(z, t ) + γA(z, t ) ×  t −∞ R(t − t ′ )|A(z, t ′ )| 2 dt ′ = 0, (1) where A(z, t ) is the complex envelope of the electric field, γ = n 2 ω 0 /cA eff is the nonlinear coefficient, A eff is the effective mode area, n 2 is the Kerr coefficient, ω 0 is the pump frequency, ˆ D(i∂ t ) = ∑ m2 i m m! β m ∂ m t is the dispersion operator which takes into account the full complexity of the fiber GVD, and β m is the mth order dispersion coefficient. R(t ) is a function describing the Kerr and the Raman contributions to the nonlinearity, and its explicit expression can be found in many textbooks [10]. The self-steepening term has been neglected as the linear fiber group velocity always dominates over the nonlinear one by several orders of magnitude [10]. As is well known, new frequencies will be generated by the soliton due to the perturbation resulting from higher order dispersion [9]. In the common case of a third-order dispersion perturbation, there is only one phase-matched frequency (i.e., only one RDW), but in the case of fourth-order dispersion (4OD), there are two symmetric frequency-detuned RDWs: one on each side of the soliton [9]. Recently, femtosecond 4OD RDW generation was observed in a Ti:sapphire laser, and tunability was achieved 1050-2947/2010/81(3)/031803(4) 031803-1 ©2010 The American Physical Society