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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 18, Iss. 16 — Aug. 2, 2010
  • pp: 16733–16738
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Supercontinuum spectrum control in microstructure fibers by initial chirp management

Rodislav Driben and Nickolai Zhavoronkov  »View Author Affiliations


Optics Express, Vol. 18, Issue 16, pp. 16733-16738 (2010)
http://dx.doi.org/10.1364/OE.18.016733


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Abstract

Experiments and numerical simulation were performed for verification of the role of femtosecond pulse chirp for supercontinuum generation in photonic crystal fiber. We demonstrate that injection of high power negatively chirped pulses near zero dispersion point brings an advantage over positively chirped pulses resulting in additional collision between solitons and in development of a significantly broader spectrum. Coupling between Raman induced solitons and dispersive waves generated by higher order dispersion was proven to be the key mechanism behind the results.

© 2010 OSA

1. Introduction

2. Experimental setup

A Ti:sapphire laser provided 25-fs pulses centered at 790 nm with spectral width of 45 nm FWHM was used in the experiment. The whole experimental setup is shown in Fig. 1
Fig. 1 Experimental setup: NGVD, the mirror pair with negative dispersion; PCF, the photonic crystal fiber; MO, the microscopic objective; CL, condensing lens; OSA, the optical spectrum analyzer ANDO AQ-6315A; PM, the power meter. Insert: Cross section of the 2.5-μm core diameter PCF with zero dispersion wavelength 790 nm.
.

The spatial properties of the output laser beam were found to be excellent, demonstrating symmetrical Gaussian profile with M2 close to 1. Mirrors introducing negative dispersion of about – 40 fs2/bounce (Layertec GmbH) were used to manage the chirp of the pulses. The mirrors were specially designed as a pair with the aim to compensate the oscillation of GVD and to minimize the introduced third order dispersion (TOD). The beam could experience up to 12 bounces within the single NGVD mirror-pair. To increase the amount of introduced negative dispersion another mirror-pair could be inserted into the beam path. The laser radiation was focused by a microscope objective (40X, NA = 0.65) into the photonic crystal fiber (PCF) of 38 cm length with the diameter of the central rod of 2.5 μm and zero dispersion wavelength of λD = 790 nm. The throughput of the PCF exceeds 60%, what was used to calculate peak power insight the PCF. The highly divergent output radiation from the PCF was collimated by an aspheric condensing lens with a 4.6 mm focal length (NA = 0.55). The input pulse duration and phase were measured by Spider (APE Berlin). The original laser pulses showed a positive chirp after passing through the focusing optics with the pulse duration extended from initial 25 fs up to 150 fs. This excess of positive dispersion can be to compensated by introducing 24 reflections from NGVD mirror. By inserting or removing one NGVD mirror-pair the pulse duration could be stretched up to 60 fs. The chirps in these two cases manifest the opposite signs with additionally introduced GVD of about ± 480 fs2. The spectra of the radiation after propagation in the PCF were recorded for different values of the input pulse chirp as a function of input pulse power. Comparison of the spectra was performed for equal pulse energies at the level of 20 dB, while the peak power decreased as the chirp grew. Surprisingly, we observed a drastic reduction of the spectral bandwidth for pulses with positive chirp compared to unchirped or negatively chirped pulses. At the same time the spectra for the unchirped and negatively chirped pulses demonstrated very similar dynamics showing development of fundamental solitons at much lower input powers compared to the pulses with the positive chirp. From here on we will analyse the pulses with the same duration of 60 fs and peak intensity, but with the opposite chirps, to clarify only the impact from different sign of chirp on SC dynamics.

3. Prechirped pulse propagation through PCF and supercontinuum generation

A comparison between experimental data (solid curve) and numerical simulations (dashed curve) for the input peak powers of 2, 10, 50, 100 kW’s are presented in Fig. 4
Fig. 4 (Color online) Comparison between spectra observed in the experiment (solid red curve – negatively prechirped pulse, solid green curve – positively prechirped pulse) and the calculated numerically (dashed red curve – negatively prechirped pulse, dashed green curve – positively prechirped pulse).
.

The experiments reveals an about three times lower threshold for formation of Raman soliton for the pulses with negative chirp (input pulse peak power P0 = 1.2 kW) as compared with the positively chirped pulses (input pulse peak power P0 = 3.4 kW). We can clearly see stronger broadening for the pulses with negative chirp at 50 and 100 kW input power, compared with the positively chirped pulses with the same input power. Although the calculated spectra differ slightly from the experimental ones, a good consistency can be recognized in the spectrum width and shape. Figure 5(a)
Fig. 5 (Color online) (a) Spectral broadening at 0.01 for positively and negatively prechirped 60 fs pulses with different input power (experimental data + numerics). Output spectrum for negatively prechirped (solid red curve) and positively prechirped (dashed green curve) 60 fs pulse in hypothetical models, (b) excluding the Raman term, (c) excluding the dispersions terms higher than 3rd order, with input pulse peak power P0 = 100 KW.
demonstrates the spectral bandwidth measured at 10−2 in the experiment and calculated by the numerical modeling as a function of increasing input peak power. From the input peak power exceeding 20 kW the difference in spectral broadening between negatively and positively prechirped pulses starts to manifest itself clearly and grows up to 130 nm for P0 = 100 kW.

As already mentioned above, the SC generation is a complicated process originated from the combined action of SPM, SRS, SS and high order dispersion (HOD). According to [7

7. Z. Zhu and T. G. Brown, “Effect of frequency chirping on supercontinuum generation in photonic crystal fibers,” Opt. Express 12(4), 689–694 (2004). [CrossRef] [PubMed]

, 10

10. A. Fuerbach, C. Miese, W. Koehler, and M. Geissler, “Supercontinuum generation with a chirped-pulse oscillator,” Opt. Express 17(7), 5905–5911 (2009). [CrossRef] [PubMed]

] the positively chirped pulses launched within the anomalous dispersion region undergo initial compression and more rapid spectral broadening than the negatively chirped pulses. In our experiments λZD = 790 nm is located in the middle of the input pulse spectrum, thus the dispersive compression operates in a same way for both chirp signs. The simulations support this assumption as can be seen in Fig. 2(a), 2(c) and Fig. 3(a), 3(c) after 2-3 cm pulse propagation into PCF. To evaluate the contribution from the different phenomena we simulated SC generation by omitting in turn the above effects Fig. 5(b), 5(c). Interplay between Raman-induced solitons and radiation created by HOD [17

17. A. V. Yulin, D. V. Skryabin, and P. S. Russell, “Four-wave mixing of linear waves and solitons in fibers with higher-order dispersion,” Opt. Lett. 29(20), 2411–2413 (2004). [CrossRef] [PubMed]

] was found as the dominant mechanism responsible for the difference in the two chirp signs spectra. We can clearly observe almost identical spectra broadening for both signs of prechirp if Raman term (fR = 0) [Fig. 5(b)] or higher order dispersion from 4th order [Fig. 5(c)] are neglected.

4. Conclusion

In conclusion, we have studied experimentally and numerically an effect of initial chirp sign of input pulses on pulse dynamics and spectral broadening in PCF. Various input peak power ranging from few kW up to 0.1GW were examined. For the inputs pulse power in a range of 2-10 kW the pulse dynamic is similar for both signs of initial chirp, resulting in the similar bandwidth of output spectrum. However, when the pulses with higher input power injected in PCF, the negative chirp gives rise to stronger red-shifted Raman solitons, colliding with each other and broadening the spectrum in a more efficient way. The effect of pulse prechirping influences of dynamics through the entire propagation length, even on the latest stages of propagation where the highly complicated multiple interactions occur between Raman induced red-shifted solitons and radiation waves.

Acknowlegements

We gratefully acknowledge Prof. Fedor Mitschke for helpful discussions. Special appreciation is addressed to reviewer 2 for helpful suggestions that led to a clarification of the discussion.

References and links

1.

R. R. Alfano and S. L. Shapiro, “Observation of self phase modulation and small-scale filaments in crystals and glasses,” Phys. Rev. Lett. 24(11), 592–594 (1970). [CrossRef]

2.

J. C. Knight, T. A. Birks, P. St. J. Russell, and D. M. Atkin, “All-silica single-mode optical fiber with photonic crystal cladding,” Opt. Lett. 21(19), 1547–1549 (1996). [CrossRef] [PubMed]

3.

J. M. Dudley, G. Gentry, and S. Coen, “Supercontinuum generation in photonic crystal fiber,” Rev. Mod. Phys. 78(4), 1135–1184 (2006). [CrossRef]

4.

D. V. Skryabin and A. V. Gorbach, “Looking at a soliton through the prism of optical supercontinuum,” Rev. Mod. Phys. 82(2), 1287–1299 (2010). [CrossRef]

5.

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(1), 25–27 (2000). [CrossRef]

6.

K. L. Corwin, N. R. Newbury, J. M. Dudley, S. Coen, S. A. Diddams, K. Weber, and R. S. Windeler, “Fundamental noise limitations to supercontinuum generation in microstructure fiber,” Phys. Rev. Lett. 90(11), 113904 (2003). [CrossRef] [PubMed]

7.

Z. Zhu and T. G. Brown, “Effect of frequency chirping on supercontinuum generation in photonic crystal fibers,” Opt. Express 12(4), 689–694 (2004). [CrossRef] [PubMed]

8.

X. Fu, L. Qian, S. Wen, and D. Fan, “Nonlinear chirped pulse propagation and supercontinuum generation in microstructured optical fibre,” J. Opt. A, Pure Appl. Opt. 6(11), 1012–1016 (2004). [CrossRef]

9.

H. Zhang, S. Yu, J. Zhang, and W. Gu, “Effect of frequency chirp on supercontinuum generation in photonic crystal fibers with two zero-dispersion wavelengths,” Opt. Express 15(3), 1147–1154 (2007). [CrossRef] [PubMed]

10.

A. Fuerbach, C. Miese, W. Koehler, and M. Geissler, “Supercontinuum generation with a chirped-pulse oscillator,” Opt. Express 17(7), 5905–5911 (2009). [CrossRef] [PubMed]

11.

K. J. Blow and D. Wood, “Theoretical description of transient stimulated Raman scattering in optical fibers,” IEEE J. Quantum Electron. 25(12), 2665–2673 (1989). [CrossRef]

12.

G. P. Agrawal, Nonlinear Fiber Optics, 4th ed. (Academic Press, 2007).

13.

J. Herrmann, U. Griebner, N. Zhavoronkov, A. Husakou, D. Nickel, J. C. Knight, W. J. Wadsworth, P. St. J. Russell, and G. Korn, “Experimental evidence for supercontinuum generation by fission of higher-order solitons in photonic fibers,” Phys. Rev. Lett. 88(17), 173901 (2002). [CrossRef] [PubMed]

14.

A. Efimov, A. V. Yulin, D. V. Skryabin, J. C. Knight, N. Joly, F. G. Omenetto, A. J. Taylor, and P. Russell, “Interaction of an optical soliton with a dispersive wave,” Phys. Rev. Lett. 95(21), 213902 (2005). [CrossRef] [PubMed]

15.

F. M. Mitschke and L. F. Mollenauer, “Discovery of the soliton self-frequency shift,” Opt. Lett. 11(10), 659–661 (1986). [CrossRef] [PubMed]

16.

F. Luan, D. V. Skryabin, A. V. Yulin, and J. C. Knight, “Energy exchange between colliding solitons in photonic crystal fibers,” Opt. Express 14(21), 9844–9853 (2006). [CrossRef] [PubMed]

17.

A. V. Yulin, D. V. Skryabin, and P. S. Russell, “Four-wave mixing of linear waves and solitons in fibers with higher-order dispersion,” Opt. Lett. 29(20), 2411–2413 (2004). [CrossRef] [PubMed]

OCIS Codes
(060.5295) Fiber optics and optical communications : Photonic crystal fibers
(320.6629) Ultrafast optics : Supercontinuum generation

ToC Category:
Ultrafast Optics

History
Original Manuscript: May 25, 2010
Revised Manuscript: June 30, 2010
Manuscript Accepted: July 7, 2010
Published: July 23, 2010

Citation
Rodislav Driben and Nickolai Zhavoronkov, "Supercontinuum spectrum control in microstructure fibers by initial chirp management," Opt. Express 18, 16733-16738 (2010)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-16-16733


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References

  1. R. R. Alfano and S. L. Shapiro, “Observation of self phase modulation and small-scale filaments in crystals and glasses,” Phys. Rev. Lett. 24(11), 592–594 (1970). [CrossRef]
  2. J. C. Knight, T. A. Birks, P. St. J. Russell, and D. M. Atkin, “All-silica single-mode optical fiber with photonic crystal cladding,” Opt. Lett. 21(19), 1547–1549 (1996). [CrossRef] [PubMed]
  3. J. M. Dudley, G. Gentry, and S. Coen, “Supercontinuum generation in photonic crystal fiber,” Rev. Mod. Phys. 78(4), 1135–1184 (2006). [CrossRef]
  4. D. V. Skryabin and A. V. Gorbach, “Looking at a soliton through the prism of optical supercontinuum,” Rev. Mod. Phys. 82(2), 1287–1299 (2010). [CrossRef]
  5. 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(1), 25–27 (2000). [CrossRef]
  6. K. L. Corwin, N. R. Newbury, J. M. Dudley, S. Coen, S. A. Diddams, K. Weber, and R. S. Windeler, “Fundamental noise limitations to supercontinuum generation in microstructure fiber,” Phys. Rev. Lett. 90(11), 113904 (2003). [CrossRef] [PubMed]
  7. Z. Zhu and T. G. Brown, “Effect of frequency chirping on supercontinuum generation in photonic crystal fibers,” Opt. Express 12(4), 689–694 (2004). [CrossRef] [PubMed]
  8. X. Fu, L. Qian, S. Wen, and D. Fan, “Nonlinear chirped pulse propagation and supercontinuum generation in microstructured optical fibre,” J. Opt. A, Pure Appl. Opt. 6(11), 1012–1016 (2004). [CrossRef]
  9. H. Zhang, S. Yu, J. Zhang, and W. Gu, “Effect of frequency chirp on supercontinuum generation in photonic crystal fibers with two zero-dispersion wavelengths,” Opt. Express 15(3), 1147–1154 (2007). [CrossRef] [PubMed]
  10. A. Fuerbach, C. Miese, W. Koehler, and M. Geissler, “Supercontinuum generation with a chirped-pulse oscillator,” Opt. Express 17(7), 5905–5911 (2009). [CrossRef] [PubMed]
  11. K. J. Blow and D. Wood, “Theoretical description of transient stimulated Raman scattering in optical fibers,” IEEE J. Quantum Electron. 25(12), 2665–2673 (1989). [CrossRef]
  12. G. P. Agrawal, Nonlinear Fiber Optics, 4th ed. (Academic Press, 2007).
  13. J. Herrmann, U. Griebner, N. Zhavoronkov, A. Husakou, D. Nickel, J. C. Knight, W. J. Wadsworth, P. St. J. Russell, and G. Korn, “Experimental evidence for supercontinuum generation by fission of higher-order solitons in photonic fibers,” Phys. Rev. Lett. 88(17), 173901 (2002). [CrossRef] [PubMed]
  14. A. Efimov, A. V. Yulin, D. V. Skryabin, J. C. Knight, N. Joly, F. G. Omenetto, A. J. Taylor, and P. Russell, “Interaction of an optical soliton with a dispersive wave,” Phys. Rev. Lett. 95(21), 213902 (2005). [CrossRef] [PubMed]
  15. F. M. Mitschke and L. F. Mollenauer, “Discovery of the soliton self-frequency shift,” Opt. Lett. 11(10), 659–661 (1986). [CrossRef] [PubMed]
  16. F. Luan, D. V. Skryabin, A. V. Yulin, and J. C. Knight, “Energy exchange between colliding solitons in photonic crystal fibers,” Opt. Express 14(21), 9844–9853 (2006). [CrossRef] [PubMed]
  17. A. V. Yulin, D. V. Skryabin, and P. S. Russell, “Four-wave mixing of linear waves and solitons in fibers with higher-order dispersion,” Opt. Lett. 29(20), 2411–2413 (2004). [CrossRef] [PubMed]

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