Spectral broadening of femtosecond pulses in a ... - OSA Publishing

Spectral broadening of femtosecond pulses in a single-mode As-S glass fiber D.-P. Wei and T. V. Galstian* Department of Physics, Université Laval, Québec, Canada G1K 7P4 [email protected]

I. V. Smolnikov and V. G. Plotnichenko Fiber Optics Research Center, General Physics Institute of the Russian Academy of Sciences, 38 Vavilov st., Moscow, 117756, Russian Federation [email protected]

A. Zohrabyan *Photintech Inc., 2740, Rue Einstein, Québec, Canada G1P 4S4 [email protected]

Abstract: We report a strong spectral broadening of femtosecond pulses propagating in a single-mode As-S glass fiber of 1.5 m length. The pump pulse spectrum is broadened by a factor of five when the input power is grown up to 16.4 mW. The broadened spectra are nearly symmetric and self-phase modulation is believed to be the dominant nonlinear effect responsible for this process. © 2005 Optical Society of America OCIS codes: (060.2400) Fiber properties; (190.4370) Nonlinear optics, fibers.

References and links 1.

T. Cardinal, K. A. Richardson, H. Shim, A. Schulte, R. Beatty, K. Le Foulgoc, C. Meneghini, J. F. Viens, and A. Villeneuve, “Nonlinear optical properties of chalcogenide glasses in the system As-S-Se,” J. NonCryst. Solids 256&257, 353-360 (1999). 2. S. Spalter, G. Lenz, R. E. Slusher, H. Y. Hwang, J. Zimmermann, T. Katsufuji, S-W. Cheong, and M. E. Lines, “Highly nonlinear chalcogenide glasses for ultrafast all optical switching in optical TDM communication systems,” in Proceedings of IEEE Conference on Optical Fiber Communication (Institute of Electrical and Electronics Engineers, Baltimore, Maryland, 2000), pp. 137-139. 3. J. Harbold, F. Wise, and B. Aitken, “Se-based chalcogenide glasses 1000 times more nonlinear than fused silica,” in Proceedings of IEEE Conference on Lasers and Electro-Optics (Institute of Electrical and Electronics Engineers, Baltimore, Maryland, 2001), pp. 14-15. 4. M. Asobe, T. Kanamori, and K. Kubodera, “Applications of highly nonlinear chalcogenide glass fibers in ultrafast all-optical switches,” IEEE J. Quantum Electron. 29, 2325-2333 (1993). 5. R. E. Slusher, G. Lenz, J. Hodelin, J. Sanghera, L. B. Shaw, and I. D. Aggarwal, “Large Raman gain and nonlinear phase shifts in high-purity As2Se3 chalcogenide fibers,” J. Opt. Soc. Am. B 21, 1146-1155 (2004). 6. L. B. Shaw, P. A. Thielen, F. H. Kung, V. Q. Nguyen, J. S. Sanghera, and I. D. Aggarwal, “IR supercontinuum generation in As-Se photonic crystal fiber,” in Proceedings of OSA Topic Meeting on Advanced Solid State Photonics (Optical Society of America, Vienna, Austria, 2005), paper: TuC5. 7. E. M. Dianov, V. G. Plotnichenko, Yu. N. Pyrkov, I. V. Smol’nikov, S. A. Koleskin, G. G. Devyatykh, M. F. Churbanov, G. E. Snopatin, I. V. Skripachev, and R. M. Shaposhnikov, “Single-mode As-S glass fibers,” Inorganic Materials 39, 627-630 (2003). 8. M. Asobe, H. Kobayashi, H. Itoh, and T. Kanamori, “Laser-diode-driven ultrafast all-optical switching by using highly nonlinear chalcogenide glass fiber,” Opt. Lett. 18, 1056-1058 (1993). 9. F. M. Mitschke and L. F. Mollenauer, “Discovery of the soliton self-frequency shift,” Opt. Lett. 11, 659-661 (1986). 10. D.-P. Wei, T. V. Galstian, A. Zohrabyan, and L. Mouradian, “Tunable femtosecond soliton generation in a Ge-doped fiber,” Electron. Lett. 40, 1329-1330 (2004). 11. P. Petropoulos, H. Ebendorff-Heidepriem, V. Finazzi, R. C. Moore, K. Frampton, D. J. Richardson, and T. M. Monro, “Highly nonlinear and anomalously dispersive lead silicate glass holey fibers,” Opt. Express 11, 3568-3573 (2003), http://www.opticsexpress.org/abstract.cfm?URI=OPEX-11-26-3568.

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Received 8 February 2005; revised 17 March 2005; accepted 18 March 2005

4 April 2005 / Vol. 13, No. 7 / OPTICS EXPRESS 2439

12. G. Genty, M. Lehtonen, H. Ludvigsen, J. Broeng, and M. Kaivola, “Spectral broadening of femtosecond pulses into continuum radiation in microstructure fibers,” Opt. Express 10, 1083-1098 (2002), http://www.opticsexpress.org/abstract.cfm?URI=OPEX-10-20-1083.

1. Introduction Chalcogenide glasses are composed of chalcogen elements S, Se or Te typically along with such elements as As, Ge, Ga or Sb. These materials allow the fabrication of low-loss fibers. Due to the wide infrared transmission window and high nonlinear refractive index, chalcogenide glass fibers attract great attention for numerous potential applications in the field of infrared power delivery, biochemical sensing, imaging and nonlinear optics. In addition, rare earth ions can be incorporated into chalcogenide glasses for applications such as fiber lasers and amplifiers. It was previously reported that the nonlinear refractive index n2 of chalcogenide glasses could be made hundreds even 1000 times the value of fused silica [1-3]. Single-mode As2S3based glass fibers with the n2 value of 4.0×10-14 cm2/W, which is two orders of magnitude higher than that of silica fibers, have been fabricated and used in ultrafast all-optical switching [4]. In [5], Slusher et al. recently reported high-purity As2Se3 chalcogenide fibers with n2 values nearly 1000 times higher than those for silica fibers. Raman gains approximately 800 times that of silica fibers were also measured for these fibers [5]. More recently, As-Se photonic crystal fiber has been fabricated, and supercontinuum generation from 2100 nm to 3200 nm has been achieved with the fiber length of only 1 m [6]. Compared to their silica counterparts, highly nonlinear chalcogenide glasses or fibers enable applications of all-optical switching and all-optical processing with more compact devices and lower operation power. In this letter, we study the nonlinear propagation of femtosecond pulses in a single-mode As-S glass fiber. The pump pulse source is a passively mode-locked fiber laser with the central wavelength around 1558 nm, which is far away from the zero-dispersion wavelength of the chalcogenide fiber used and lies in the normal dispersion regime. When the fiber-input power is increased, the output spectra from a 1.5 m chalcogenide fiber are strongly broadened almost symmetrically with a 15-dB bandwidth of 310 nm. The symmetric spectral broadening shows that self-phase modulation (SPM), along with the normal group velocity dispersion (GVD) of the chalcogenide fiber, plays an important role in the pulse evolution process. 2. Experimental results The experimental setup is schematically depicted in Fig. 1. A passively mode-locked Erdoped fiber laser (EXFO MLFL-100) was used as the pulsed pump source in our experiment. The fiber laser generates 100 fs FWHM pulses at a repetition rate of 20 MHz and with the maximum average output power of 26 mW. It has a three-peak spectral structure with peak wavelengths located at 1532 nm, 1558 nm and 1580 nm, respectively. The temporal form of the fiber laser pulse, measured with an autocorrelator, shows a Gaussian-like shape. 1.5 m chalcogenide fiber

Optical spectrum analyzer

100 fs fiber laser Gallium Fig. 1. Experimental setup.

The pump pulses were coupled into a 1.5 m long single-mode As-S glass fiber, which was fabricated from high-purity glasses with the double-crucible method [7]. This fiber has a core diameter of 4 µm and a cladding diameter of 125 µm. Its mode field diameter at 1550 nm was #6571 - $15.00 US

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measured to be 6.2 µm. The compositions of the core and the cladding are As40S60 and As38S62, respectively. The numerical aperture of the fiber is approximately 0.16 at wavelengths near 1550 nm. Fig. 2 shows the transmission loss spectrum of the fiber. The transmission loss is 660 dB/km at 1558 nm, but decreases down to around 100 dB/km in the wavelength range of 2100 - 2200 nm. To our knowledge, this fiber has a very low transmission loss for single-mode As-S glass fibers, as compared in [7]. The absorption peak at 1940 nm is due to the absorption of hydroxyl groups in fiber cladding. The fiber has a cutoff wavelength of 1000 nm with a bending strength around 1000 Mpa [7]. The zerodispersion wavelength of the fiber is around 4900 nm, which is far away from the central wavelength of the pump used in the present work. In order to eliminate cladding modes in the chalcogenide fiber, we removed the coating of the two portions (~ 10 cm in length) near two fiber ends, and immersed the non-coated portions into liquid gallium, as shown in Fig. 1. The output spectrum of the fiber was measured with an optical spectrum analyzer (ANDO AQ6317B).

Fig. 2. Transmission loss spectrum of the single-mode chalcogenide glass fiber with core/cladding diameters of 4/125 µ m.

Injection of the fiber laser light into the chalcogenide fiber was achieved by butt coupling with a maximum coupling efficiency around 63%, which is rather high. Two reasons may be responsible for the high coupling efficiency. First, the mode field diameters of the chalcogenide fiber and the laser output silica fiber are 6.2 µm and 6.7 µm respectively, which match very well. Secondly, some residual cladding modes inevitably exist in the cladding of the chalcogenide fiber although the two fiber ends were immersed into liquid gallium. The launched powers to the chalcogenide fiber were varied by adjusting the coupling efficiency, and were estimated by measuring the output powers from the chalcogenide fiber taking into account the fiber losses and Fresnel losses at the output end of the fiber.

Fig. 3. Nonlinearly broadened output spectra from the 1.5 m chalcogenide fiber for fiber-input powers of (a) 7.5 mW, (b) 11.9 mW, and (c) 16.4 mW. The dashed curve shows the initial pump pulse spectrum.

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After passing through the 1.5 m chalcogenide fiber, the spectrum of the pump pulse was strongly broadened when the fiber-input power was increased. In Fig. 3, we show the output pulse spectra measured when the fiber-input powers are (a) 7.5 mW, (b) 11.9 mW, and (c) 16.4 mW, respectively. The initial pump pulse spectrum is also shown in Fig. 3 (dashed curve) for comparison. It is worth noting that all curves in Fig. 3 are to the same scale but displaced vertically for the sake of clarity. With the maximum available fiber-input power of 16.4 mW, the output pulse spectrum was strongly broadened with a 15-dB bandwidth of 310 nm, which was measured at -15 dB level from the spectral peak. In Fig. 4, the 15-dB bandwidths of the broadened spectra are plotted along with the corresponding fiber-input powers. We find that there is no evidence of a saturation effect for the spectral-broadening in the 1.5 m chalcogenide fiber from all available fiber-input powers in our experiment. For a certain fiber-input power, by adjusting the polarization direction of the launched pump pulse, we found that the output spectra were slightly polarization-dependent. This polarization dependence should be related to the random birefringence of the chalcogenide fiber and can be eliminated by use of polarization maintaining fibers.

Fig. 4. 15-dB bandwidths of the broadened spectra versus the input power.

3. Discussion and summary When the launched power into chalcogenide fiber is relatively low, it was observed that the pump pulse spectrum is broadened symmetrically with a multipeak substructure correlated with its initial three-peak spectral profile. This indicates that SPM plays a dominant role at this stage. With further increasing the fiber-input power, the pump pulse spectrum is strongly broadened and the multipeak substructure nearly disappears, as shown by solid curves in Fig. 3. However, the broadened pump pulse spectra always keep nearly symmetric shapes. Thus, we believe that SPM is the dominant nonlinear effect contributing to the spectral broadening in the chalcogenide fiber. In Fig. 3, from the broadened spectra it seems that a little bit more energy may be found at the longer wavelength side of the pump, which should be owing to stimulated Raman scattering (SRS). The small spectral peaks around 1750 nm are artificial ones caused by the ANDO optical spectrum analyzer. We also investigated the spectral broadening of femtosecond pulses in a longer chalcogenide fiber with the length of 2.2 m. The fiber type is the same as above. When the fiber-input power is increased, we found that the pump spectrum evolution behavior is essentially similar to the one that we have observed with the 1.5 m chalcogenide fiber. Compared to the 1.5 m fiber, however, the longer fiber of 2.2 m seems not to be able to significantly enhance the spectral broadening effect. It was previously estimated that GVD value of As2S3-based glass fibers is as large as 660 ps / nm ⋅ km at 1300 nm and 410 ps / nm ⋅ km in the 1550-nm region [8]. Therefore, due to large material dispersion in the #6571 - $15.00 US

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chalcogenide fiber, the input femtosecond pulse, which has a broad bandwidth, should be broadened very quickly when the pulse propagates along the fiber. In addition, SPM leads to an enhanced rate of pulse broadening since the pump wavelength is located in the normal dispersion regime of the chalcogenide fiber. The spectral broadening, which is caused by intensity-dependent SPM, should be much stronger at the initial propagation stage in the chalcogenide fiber and become weaker along with further propagation. Thus, due to the presence of large GVD value in chalcogenide fiber, it may not be an efficient way to enhance the spectral broadening effect by using longer fibers above a certain optimal length. However, as shown in Fig. 4, it is expected to obtain larger spectral broadening when higher input powers are used. For comparison, we launched the pump pulses into 1.5 m and 2.2 km standard single-mode fibers (SMF-28), respectively. The launched pump light power was around 22 mW in both cases. The measured output spectra are shown in Fig. 5, also compared to the initial pump pulse spectrum (dashed curve). When the 1.5 m SMF-28 was used, the output spectrum was broadened slightly. The multi-peak spectral structure shows a typical SPM pattern, as shown in Fig. 5(a). SRS should be responsible for the slightly asymmetric broadening of the pump spectrum. When the length of SMF-28 was increased to 2.2 km, the output spectrum was broadened to about twice of the initial pump spectrum, as shown in Fig. 5(b). An obvious spectral component can be observed at the longer wavelength side of the pump, which should belong to the wavelength-shifted pulse resulting from the soliton self-frequency shift effect in the anomalous dispersion regime of SMF-28 [9, 10].

Fig. 5. Nonlinearly broadened output spectra from SMF-28 with the lengths of (a) 1.5 m, and (b) 2.2 km, respectively. The dashed curve shows the initial pump pulse spectrum.

In summary, we have demonstrated nonlinear propagation of infrared femtosecond pulses in a 1.5 m single-mode As-S fiber. The observed strong spectral broadening is very attractive since highly nonlinear chalcogenide glass fibers may be promising candidates for the design of compact wideband sources with many potential applications. It is worth noting that microstructure fibers also attract great interest recently since the small core design and tailorable dispersion properties enhance the fiber nonlinearities with various applications [11, 12]. The spectral broadening mechanism in the As-S fiber here is similar to that in microstructure fibers when the pump is located in the normal dispersion regime [12]. Acknowledgments The authors would like to thank Photintech Inc and the government of Québec for their financial support.

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