FIO/ LS Technical Digest @ 2011 OSA
Tapered As2S3 chalcogenide photonic crystal fiber for broadband mid-infrared supercontinuum generation Amine Ben Salem, Rim Cherif, and Mourad Zghal University of Carthage, Engineering School of Communication of Tunis (Sup’Com), Cirta’Com Laboratory, Ghazala Technopark, 2083 Ariana, Tunisia
[email protected]
Abstract: We design an As2S3 tapered photonic crystal fiber for mid-infrared supercontinuum generation. More than three octave spanning spectrum is generated in 8 mm-long taper with low input pulse energy of 100 pJ at 4.7 µm. 1. Introduction Highly nonlinear chalcogenide glasses are of great interest for low power and mid-IR applications. Chalcogenide glasses based on As2S3 have been the subject of intense investigations due to their high Kerr nonlinearity n2 (100500 times larger than of silica glass), low linear loss and low two photon absorption (TPA) coefficient [1]. Recently, Hudson et al. [2] have shown the generation of one octave spanning supercontinuum (SC) from 970 to 1990 nm in a 1.3 µm waist diameter and 50 mm long-tapered As2S3 standard fiber with low input pulse energy of 77 pJ. This broadening was achieved at a wavelength of 1.55 µm in the anomalous dispersion regime. In this paper, we show that we can move the zero dispersion wavelength (ZDW) beyond 2 µm toward mid-IR region by tapering As2S3 photonic crystal fibers. We also report on the generation of more than three octave spanning SC extending from 3140 nm to 6330 nm in 8 mm-long As2S3 tapered photonic crystal fiber (TPCF) having a waist diameter of 1.3 µm. The As2S3 TPCF is designed to generate broadband SC with very low input pulse energy of 100 pJ and short interaction distance of few millimeters at a pump wavelength of 4.7 µm. 2. Simulations and results
Chromatic dispersion [ps/nm.km]
We designed a TPCF with red-shifted ZDW so that pumping in the anomalous dispersion regime at the mid-IR region can be possible. In order to find the minimum value of chromatic dispersion at a pump wavelength of 4.7 µm, we study four different As2S3 TPCF structures having a hole diameter d of 0.3 µm and a pitch varying from 0.5 µm to 0.8 µm with a step size of 0.1 µm. Fig. 1(a) shows the calculated chromatic dispersions of the four As2S3 TPCF obtained by means of a full vectorial finite element method (FEM). 300
µm µm µm µm
200 100 0 -100
p = 4.7 µm
-200 3,2
3,6
4,0
4,4
4,8
Wavelength [µm]
(a) (b) Fig. 1. (a) Chromatic dispersion calculated for tapered As2S3 photonic crystal fibers (d=0.3 µm). (b) Poynting vector calculated for the As2S3 TPCF with core diameter of 1.3 µm (=0.8µm) at p=4.7 µm.
We notice that by increasing from 0.5 µm to 0.8 µm, we are able to shift the ZDW toward the mid-IR region. The As2S3 TPCF with a core diameter of 1.3 µm (i.e. a pitch value of 0.8 µm) shows a ZDW of about 4.31µm and the lowest value of chromatic dispersion (32.6 ps/nm.km) at a pump wavelength of 4.7 µm. Under these conditions, high nonlinearities and large mid-IR supercontinua are expected to be generated [3]. Fig. 1(b) depicts the Poynting vector calculated in the 1.3 µm diameter As2S3TPCF at p = 4.7 µm. We evaluate the effective mode area Aeff to be equal to 13.58 µm² and deduce a nonlinear coefficient = 393.8 (W.km)-1. To simulate the nonlinear pulse propagation, we solve the nonlinear Schrödinger equation (NLSE) given by [4]:
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FIO/ LS Technical Digest @ 2011 OSA
j m1 m m A A A m! t m z 2 m0
j j 2 Aeff 2
1 j A( z, t ) R(t ' ) A( z, t t ' ) 2 dt' 0 t
(1) where A(z,t) is the slowly varying envelope, α is the loss coefficient set to be 1 dB.m-1 [1], and βm the mth-order dispersion coefficients. α2 is the TPA coefficient of the As2S3 taken as 6.2×10-15 m/W and 0 is the pump central frequency. The nonlinear response function R(t ) (1 f R ) (t ) f R hR (t ) includes the fractional contribution fR and the delayed Raman response function hR(t) of the As2S3 material [1]. The NLSE is solved by the symmetrized split step Fourier method. We consider the injection of a chirped hyperbolic-secant input pulse having a duration of 160 fs from a mid-IR tunable laser from 3.3 µm to 10 µm [5]. In order to optimize the SC generation with low input pulse energy of 100 pJ in the As2S3 TPCF with a core diameter of 1.3 µm, we select a pump wavelength of 4.7 µm operating in the anomalous dispersion regime and set the chirp to be equal to -1.2. Fig. 2(a) depicts the temporal evolution of an initial prechirped 160 fs pulse with input energy of 100 pJ. We find that the 160 fs-input pulse undergoes soliton-self compression, so that, a 45.2 fs temporal compressed pulse is generated in only 8 mm As2S3 taper length. Thus, a compression factor of 3.54 is achieved. The corresponding generated SC shows more than 3 octave spanning extending from 3140 nm to 6330 nm and maximum spectral broadening is achieved after 8 mm propagation distance, as seen is Fig. 2(b). This broadening is mainly attributed to self phase modulation (SPM) and nonlinearly phase-matched parametric processes as dominant mechanisms since the soliton fission length is found around 10 mm. 0
Peak power [kW]
1,6
Normalized intensity [dB]
(a) input (z = 0) output (z = 8 mm)
1,2 45.2 fs
0,8
(b) -10 -20 -30 -40
0,4 0,0 -300
-200
-100
0
100
200
300
Time [fs]
Fig. 2. (a) Temporal evolution of the 160 fs input pulse undergoing soliton self-compression at low input energy of 100 pJ. (b) Spectral evolution: the white dashed line shows the maximum spectral broadening at a distance of 8 mm.
3. Conclusion The obtained results show that by tapering As2S3 photonic crystal fiber, chromatic dispersion can be engineered and shifted toward mid-IR wavelength region. Generating broadband supercontinua in the long wavelength side is now possible which is very promising for mid-IR low-power devices and applications. References [1] C. Xiong, E. Magi, F. Luan, A. Tuniz, S. Dekker, J. S. Sanghera, L. B. Shaw, I. D. Aggarwal, and B. J. Eggleton, “Characterization of picosecond pulse nonlinear propagation in chalcogenide As2S3 fiber,” Appl. Opt. 48, 5467-5474 (2009). [2] D. D. Hudson, S. A. Dekker,E. C. Mägi, A. C. Judge, S. D. Jackson, E. Li, J. S. Sanghera, L. B. Shaw, I. D. Aggarwal, and B. J. Eggleton, “Octave spanning supercontinuum in an As2S3 taper using ultralow pump pulse energy,” Opt. Lett. 36, 1122-1124 (2011). [3] J. M. Dudley, and S. Coen, “Coherence properties of supercontinuum spectra generated in photonic crystal fiber and tapered optical fiber,” Opt. Lett. 27, 1180-1182 (2002). [4] G. P. Agrawal, Nonlinear Fiber Optics, 4th Edition, Academic, Elsevier (2006). [5] F. Seifert, V. Petrov, and M. Woerner, “Solid-state laser system for the generation of midinfrared femtosecond pulses tunable from 3.3 to 10 µm,” Opt. Lett. 19, 2009-2011 (1994).
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