Microstructure core photonic crystal fiber for blue extension of supercontinuum generation Xinben Zhang a , Xian Zhu a , Ruixian Xing a , Xiaobo Yang b , Fagang Jiang b , Haiqing Li a,b , Jinggang Peng a,b , Nengli Dai a,b , Jinyan Li a,n a Wuhan National Laboratory of Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, China b Union Hospital, Tongji Medical College, Huazhong University of Science and Technology, Wuhan 430022, China article info Article history: Received 4 December 2012 Received in revised form 6 February 2013 Accepted 8 February 2013 Available online 22 February 2013 Keywords: Supercontinuum Photonic crystal fiber Group-velocity matching abstract We investigate photonic crystal fibers (PCFs) with nanosize airholes (NAHs) in the core for the blue extension of supercontinuum generation. The principle behind the design is to enhance the evanescent wave in the IR part of the supercontinuum. When the zero-dispersion wavelength is fixed around 1 mm, the infrared wavelength of 2500 nm of the proposed fiber is able to group-velocity match to the short- wavelength of 403 nm, which is about 60 nm shorter than that of a conventional high-D PCF. Simultaneously, the nonlinearity is enhanced about three times. The dependence of PCF characteristics on NAHs is also discussed. Simulated results confirm the possibility of increasing the blueshift of the generated supercontinua in designed PCFs. & 2013 Elsevier B.V. All rights reserved. 1. Introduction Supercontinuum sources, which are characteristic of good spatial coherence, high brightness, and broad bandwidth, have found numerous applications in diverse fields such as fluores- cence microscopy, broadband spectroscopy, fluorescence lifetime imaging, and optical coherence tomography [1–4]. In recent years, photonic crystal fibers (PCFs), which have high nonlinear coeffi- cients and flexible control of dispersion properties, are exten- sively employed for supercontinuum generation. The system that consists of short-pulse Yb-doped fiber lasers, which are well- established and commercially available, and highly nonlinear PCFs enables the supercontinuum spanning from visible to infra- red frequencies [1,5–16]. Especially, supercontinuum sources covering the visible wavelength region are of interest in biome- dical imaging, since many fluorescent molecules are excited in a wavelength range from  600 nm down to  350 nm [1]. It is well known that long- and short-wavelength edges of a superconti- nuum are linked through the group-velocity matching relation, that is they have the same group-velocity [12]. However, the group-velocity curve on the short-wavelength side is steep, and the soliton is hard to continue to redshift after  2500 nm since the strong OH absorption and the large dispersion. As a result, the supercontinuum is difficult to cover the violet region, which is far detuned from the pump wavelength. Numerous robust solutions based on single-fiber have been developed. One straightforward method is adopting high-D PCFs [8,12,16], in which group-velocities of long-wavelength increase. Alternative techniques using zero-dispersion wavelength (ZDW) decreasing PCFs [1,14] are more efficient and more complex. For this fiber, the group-velocity profile that determines the output spectrum is provided by the end of the PCF, where the ZDW needs not in the vicinity of the pump wavelength. Nevertheless it requires careful design the evolution of the ZDW. Another route is manipulating the group-velocity profile for shorter wave- lengths by changing the glass composition of the fiber [17]. The simulation results show that it is possible to increase the blueshift of the generated supercontinuum about 20 nm. In this paper, we examine a PCF with a microstructure core (MC-PCF) to extend the blue edge of the supercontinuum by modifying the group-velocity profile and increasing the nonli- nearity. The structure is shown in Fig. 1, where d is the airhole diameter and L is the spacing between the centers of adjacent airholes called pitch. Though this structure-like PCF with nanosize airholes (NAHs) had been demonstrated previously [18–22], most of them barely focused on the flattened dispersion and/or the high nonlinearity. Here, we show the potential performance of this fiber in violet-light supercontinuum generation. To avoid the complexity of the problem, ZDWs of considered fibers are set to around the pump wavelength, that is  1 mm, spectra then can sufficiently broaden [23]. 2. Theory Usually, a supercontinuum is generated by pumping a high nonlinearity fiber in the anomalous dispersion regime in the vicinity of the ZDW for efficient solitons. In that case, this Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/optcom Optics Communications 0030-4018/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optcom.2013.02.004 n Corresponding author. Tel./fax: þ86 2787559463. E-mail addresses: ljy@mail.hust.edu.cn, lijinyan203@gmail.com (J. Li). Optics Communications 298–299 (2013) 191–195