We report the results of a systematic experimental and theoretical study of 1.06 µm pumped supercontinuum generation in a range of holey fibers with different flattened dispersion profiles. Clear differences in terms of the underpinning mechanisms emerge depending on the spacing between the two fiber zero-dispersion wavelengths. By examining the phase matched wavelength range of the corresponding fiber dispersions, one can predict the maximum achievable supercontinuum bandwidth.
©2006 Optical Society of America
Supercontinuum (SC) generation in holey fibers (HFs) is an established technique for producing broadband light sources [1, 2]. This has revolutionized the field of frequency metrology and has opened up a host of new and significant applications in areas such as medical imaging, telecommunications and sensing.
Typically, bulky and expensive Ti:Sapphire-based femtosecond pump sources operating at wavelengths around 780 nm have been used for investigations of SC generation and, therefore, most HFs are designed for use at this pump wavelength. However, there is rapidly growing interest in the development of more practical and efficient pump lasers to facilitate more widespread deployment of SC technology . High average power excitation of HFs can be realized by employing Yb-doped fiber lasers operating near 1.06 µm, which offer high optical efficiencies and the capability to generate pulses with durations ranging from <100 fs through to the CW regime. Moreover, the holey fiber can be spliced directly to the laser output to make a robust fully-integrated fiber-based SC source. There is thus a great demand for HFs with optimized dispersion profiles for SC generation in the vicinity of this wavelength.
From earlier work performed at wavelengths around 780 nm it is clear that pumping in the anomalous dispersion regime and controlling the position of the zero-dispersion wavelength(s) (ZDW) relative to the pump wavelength can be used to tailor the extent of the spectral broadening, as well as the detailed spectral shape of the SC radiation [4–9]. In this paper we report the fabrication of HFs with flattened dispersion characteristics around 1.06 µm and investigate both experimentally and theoretically the SC generation characteristics through a combination of self-phase modulation (SPM), soliton formation, Raman soliton self-frequency shifting (SSFS), and parametric processes of these fibers. In particular, we investigate the differences in both the dominant physical mechanisms and the shape of the generated spectra depending on the spacing between the ZDWs and the corresponding phase matching conditions for four-wave mixing (FWM) processes. Our results highlight the flexibility for tailoring SC properties, which can be achieved by specifically designed dispersion flattened holey fibers . This shows the feasibility of all-fiber SC sources with specific spectral behaviour, e.g., maximum spectral coverage, high spectral flatness, high spectral coherence, tolerance to source wavelength drift etc.
2. Experiment and results
We employed a mode-locked Nd:glass pump laser operating at 1.06 µm. The laser operated at a repetition rate of 80 MHz, a pulse width of 380 fs, and was able to generate a maximum average output power of 250 mW. A 60× microscope objective was used to couple the light into various holey fibers with two ZDWs of 1 m length, with input coupling efficiencies of 13–30%.
We measured the spectra generated in six fibers (A to F) at various pump power levels. Sample spectra for fibers A, E, and F at maximum power (739pJ, 675pJ and 425pJ in the fibers respectively) are depicted in Fig. 1 (top). The calculated dispersion profiles are shown in Fig. 2.
We established the HF designs based on a hexagonal stacking geometry offering dispersion flattened characteristics in the vicinity of 1.06 µm using a finite-element-method based fiber design tool, and fabricated the fibers according to these designs using a standard stack and draw process. All fibers were drawn from a single preform under slightly different drawing conditions. The fibers are of standard hexagonal symmetry with cylindrical holes and a small core diameter of ~2 µm. An SEM picture of Fiber C is shown in the inset of Fig. 2. From SEM pictures of the fibers we measured the average pitch, Λ, and air-filling fraction, d/Λ, for the first 6 rings as Λ=1.482 µm, d/Λ=0.456 (Fiber A), Λ=1.463 µm, d/Λ=0.437 (Fiber B), Λ=1.457 µm, d/Λ=0.426 (Fiber C), Λ=1.453 µm, d/Λ=0.423 (Fiber D), Λ=1.435 µm, d/Λ=0.399 (Fiber E), and Λ=1.434 µm, d/Λ=0.384 (Fiber F), respectively.
In order to enhance our understanding of the nonlinear processes during pulse propagation in the fibers, we performed numerical simulations of the corresponding generalized nonlinear Schrödinger equation [11,12] using a standard split-step Fourier tool , which takes into account the full dispersion profile as well as nonlinear Kerr and Raman effects . The dispersion profiles in Fig. 2, nonlinear refractive index n2 of 2.5×10-20 m2W-1 and effective mode area Aeff of 4 µm2 were used as an input to these simulations. Simulated spectra for fibers A and E are shown in Fig. 1 (bottom). In all cases we found good agreement between the predicted and observed spectra. For the interpretation of the observed spectra, we also investigated the properties of FWM processes. Pumping the fibers in the anomalous dispersion regime can lead to the phase-matched generation of long and short wavelength bands via FWM . We calculated the generated components directly from the dispersion profiles. The phase matching curves at low pump power for fibers A to E are shown in Fig. 3.
From Figs. 2 and 3 we see that increasing the separation of the two ZDWs, i.e., the width of anomalous dispersion region, also increases the resulting phase matched wavelength range. Here, the phase matched range is taken from the longest and the shortest wavelengths generated by FWM processes when pumped inside the anomalous dispersion regime. For example, for fiber A the longest Stokes band is around 1990 nm for pumping at 1080 nm, and the shortest anti-Stokes band is around 630 nm for pumping at 910 nm, which implies a phase matched range of ~1360 nm. Note also that the phase matching curves allow us to interpret some pairs of peaks found in the SC spectra of Fig. 1 by FWM.
We have measured the SC bandwidths at maximum pump power for fibers B, C, D, E, and F. All bandwidths were taken at the 20 dB level in order to include all wavelength components independent of experimental power constraints. The results are shown along with the phase matched range in Fig. 3 (right). We find good agreement for fibers B, C, D, and E. For fiber F, no phase-matched range exists and the observed broadening is due to other mechanisms. For completeness we have included the data for fiber A in Fig. 3 (right). However, for this fiber the experimental measurement of the SC bandwidth was limited to a maximum value of ~1000 nm by the 1650 nm long wavelength cutoff of the spectrum analyzer rendering the comparison with theory somewhat meaningless.
We have measured SC spectra for fibers A to F at various pump power levels and compared the results with numerical simulations to identify the relevant nonlinear mechanisms. Taken together, these results provide a consistent picture of SC generation in dispersion flattened fibers, which we will discuss in the following using the examples of the fibers with the longest anomalous dispersion range (fiber A) and the shortest range (fiber E), as shown in Figs. 4 and 5, respectively.
Fiber A exhibits a wide anomalous dispersion region with a separation of the two ZDWs of ~470 nm. Here, SC generation at low powers starts with the formation of fundamental solitons which are subsequently shifted to longer wavelengths by SSFS, as observed by the soliton peak at ~1200 nm in Fig. 4 at 147 pJ. Numerically, we found that this leads to nearly linearly broadening of the SC spectrum with increasing pump power below 300 pJ. At pump powers of 300–400 pJ, these soliton effects shift power into most of the anomalous dispersion wavelength range, and thus FWM processes start to generate frequency components outside this range, see Fig. 4 at 409 pJ. Fig. 5 (top) shows the measured SC bandwidth together with results of simulations with constant anomalous dispersion, where no phase-matched FWM processes can occur . In this case only the linear broadening due to SSFS is observed but no sudden increase for higher powers, which confirms our interpretation. For pump powers exceeding ~400 pJ, the measured SC bandwidth appears approximately constant at ~1000 nm (Fig. 4, 739 pJ, and Fig. 5), although this measurement is again limited by our OSA.
Different mechanisms can be observed in our fibers with narrower anomalous dispersion regions, e.g, for fiber E which exhibits a narrow anomalous dispersion region with a spacing of the two ZDWs of ~230 nm. This fiber generates a symmetrical spectrum with sharp edges, see Fig. 1. The measured SC bandwidth as a function of pump power in this case is shown in Fig. 5(bottom) together with corresponding simulation results. For this fiber, the spectra show no evidence of soliton formation at any power level. Instead, SC generation is dominated by SPM and FWM processes . At low power levels, the spectrum is slightly broadened by SPM. At input pulse energies of ~200 pJ, FWM processes set in, which leads to a sudden increase in bandwidth to a near maximum value. We observe two main phase-matched wavelength components at ~800 nm and ~1400 nm together with the residual pump peak at ~1060 nm. At maximum input power, the spectrum becomes more uniform and the flatness improves to ~10 dB. This has to be compared to a flatness of ~20 dB for fiber A. The maximum span of the spectrum of fiber E is narrower than that of fiber A, but much lower pump energies are required to reach the maximum bandwidth. Therefore, the spectral shape of the SC is determined primarily by the fiber characteristics and is essentially independent of the specific characteristics of the pump laser itself .
The different behavior of supercontinuum generation in fibers with narrow and broad anomalous dispersion regions, respectively, can be understood by the following considerations. The fibers with broad anomalous dispersion exhibit comparably large dispersion at the pump wavelength of 1060 nm. The dispersion length L D  is thus short enough that dispersion effects play a significant role in the pulse dynamics within the 1 m length of our fibers, a requirement for soliton formation. Simultaneously, the large dispersion and the large wavelength difference between the frequency components generated by FWM (1990 nm and 720 nm for a 1060 nm pump in fiber A) lead to spatial walk-off within 1 cm of propagation. This effect strongly suppresses the FWM gain. For fibers with narrow anomalous dispersion regions, on the other hand, the dispersion at the pump wavelength is small. Therefore, nonlinear effects dominate over dispersion effects and no solitons can be formed within 1 m of fiber. Moreover, spatial walk-off between the FWM components at 1380 nm and 860 nm created from the 1060 nm pump in fiber E occurs only after 20 cm of propagation and therefore the FWM gain is much larger in this fiber and the sudden increase in SC bandwidth, as shown in Fig. 5, occurs at lower pump powers.
Finally, we launched the pump pulses into a fiber with wholly normal dispersion (fiber F). In this case, no solitons can be formed and no phase-matched FWM processes can occur. The observed spectrum, see Fig. 1, shows some residual broadening due to SPM, but no supercontinuum is observed. However, the resulting spectrum has a flatness of <5dB due to the absence of Modulation Instability in the normal dispersion regime .
We have systematically investigated the generation of supercontinuum light in a range of small-core holey fibers with flattened dispersion profiles, pumped with 380 fs pulses at 1.06 µm. We have calculated the phase matched wavelength range for each fiber and have shown that this accurately predicts the maximum SC bandwidth . By measuring the SC bandwidths at varying pulse power and comparing the results with numerically simulated spectra, we have been able to identify the different nonlinear processes which are dominating the SC generation in each parameter regime. Wider anomalous dispersion regions give rise to broader supercontinua, however, at the cost of reduced flatness and higher pump powers. The most uniform spectra with improved stability against laser power and frequency fluctuations are found in fibers with two closely spaced zero-dispersion wavelengths.
References and Links
1. J. K. Ranka, R. S. Windeler, and A. J. Stentz, “Visible continuum generation in air-silica-microstructure optical fibers with anomalous dispersion at 800 nm,” Opt. Lett. 25, 25 (2000). [CrossRef]
2. A. Rulkov, M. Vyatkin, S. Popov, J. Taylor, and V. Gapontsev, “High brightness picosecond all-fiber generation in 525–1800nm range with picosecond Yb pumping,” Opt. Express 13, 377 (2005). [CrossRef] [PubMed]
3. J. M. Harbold, F. Ö. Ilday, F. W. Wise, T. A. Birks, W. J. Wadsworth, and Z. Chen, “Long-wavelength continuum generation about the second dispersion zero of a tapered fiber,” Opt. Lett. 27, 1558 (2002). [CrossRef]
6. K. M. Hilligsøe, T. V. Andersen, H. N. Paulsen, C. K. Nielsen, K. Mølmer, S. Keiding, R. Kristiansen, K. P. Hansen, and J. J. Larsen, “Supercontinuum generation in a photonic crystal fiber with two zero-dispersion wavelengths,” Opt. Express 12, 1045 (2004). [CrossRef] [PubMed]
7. T. V. Andersen, K. M. Hilligsøe, C. K. Nielsen, J. Thøgersen, K. P. Hansen, S. R. Keiding, and J. J. Larsen, “Continuous-wave wavelength conversion in a photonic crystal fiber with two zero-dispersion wavelengths,” Opt. Express 12, 4113 (2004). [CrossRef] [PubMed]
8. M. H. Frosz, P. Falk, and O. Bang, “The role of the second zero-dispersion wavelength in generation of supercontinua and bright-bright soliton-pairs across the zero-dispersion wavelength,” Opt. Express 13, 6181 (2005). [CrossRef] [PubMed]
9. P. Falk, M. H. Frosz, and O. Bang, “Supercontinuum generation in a photonic crystal fiber with two zerodispersion wavelengths tapered to normal dispersion at all wavelengths,” Opt. Express 13, 7535 (2005). [CrossRef] [PubMed]
10. M. L. V. Tse, P. Horak, F. Poletti, N. G. R. Broderick, J. H. V. Price, J. R. Hayes, and D. J. Richardson, “A systematic study of supercontinuum generation at 1.06 micron in holey fibers with dispersion flattened profiles,” Optical Fiber Communication Conference (OFC), OThQ5,(2006).
11. G. P. Agrawal, Nonlinear Fiber Optics, 3rd ed. (Academic Press, San Diego, CA, USA,2001).
12. K. J. Blow and D. Wood, “Theoretical Description of Transient Stimulated Raman Scattering in Optical Fibers,” IEEE J. Quantum Electron. 25, 2665 (1989). [CrossRef]
13. R. Paschotta, R P Photonics Consulting GmbH, Zurich, Switzerland.
14. J. M. Dudley, L. Provino, N. Grossard, H. Maillotte, R. S. Windeler, B. J. Eggleton, and S. Coen, “Supercontinuum generation in air-silica microstructured fibers with nanosecond and femtosecond pulse pumping,” J. Opt. Soc. Am. B 19, 765 (2002). [CrossRef]
15. T. Schreiber, T. V. Andersen, D. Schimpf, J. Limpert, and A. Tünnermann, “Supercontinuum generation by femtosecond single and dual wavelength pumping in photonic crystal fibers with two zero dispersion wavelengths,” Opt. Express 13, 9556 (2005). [CrossRef] [PubMed]