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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 4 — Feb. 25, 2013
  • pp: 4405–4410
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Raman-free nonlinear optical effects in high pressure gas-filled hollow core PCF

M. Azhar, G. K. L. Wong, W. Chang, N. Y. Joly, and P. St.J. Russell  »View Author Affiliations


Optics Express, Vol. 21, Issue 4, pp. 4405-4410 (2013)
http://dx.doi.org/10.1364/OE.21.004405


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Abstract

The effective Kerr nonlinearity of hollow-core kagomé-style photonic crystal fiber (PCF) filled with argon gas increases to ~15% of that of bulk silica glass when the pressure is increased from 1 to 150 bar, while the zero dispersion wavelength shifts from 300 to 900 nm. The group velocity dispersion of the system is uniquely pressure-tunable over a wide range while avoiding Raman scattering—absent in noble gases—and having an extremely high optical damage threshold. As a result, detailed and well-controlled studies of nonlinear effects can be performed, in both normal and anomalous dispersion regimes, using only a fixed-frequency pump laser. For example, the absence of Raman scattering permits clean observation, at high powers, of the interaction between a modulational instability side-band and a soliton-created dispersive wave. Excellent agreement is obtained between numerical simulations and experimental results. The system has great potential for the realization of reconfigurable supercontinuum sources, wavelength convertors and short-pulse laser systems.

© 2013 OSA

1. Introduction

2. Experimental details

We used a kagomé HC-PCF with 18 µm core diameter, permitting λ0 to be pressure-tuned from ~300 to ~900 nm. The experimental set-up consisted of a 28 cm length of the kagomé HC-PCF with gas cells at each end. The pump laser was an amplified Ti:sapphire laser system oscillating at a center wavelength of 800 nm, delivering pulses of duration 140 fs at a repetition rate of 250 kHz. Typical launch efficiencies were ~30% and the maximum pulse energy launched into the core was 450 nJ. The fiber was enclosed in a steel tube joining the two gas cells, which had the advantage that the pressure was equalized inside and outside the PCF, thus avoiding possible structural distortion. The system was capable of withstanding pressures up to 150 bar. The diagnostics included a CCD camera and an optical spectrum analyzer, and the input pulse was characterized using a frequency resolved optical gating (FROG) system. At a pressure of 90 bar, λ0 coincides with the pump laser wavelength. At this pressure the nonlinear refractive index n2 is only one order of magnitude lower than that of pure silica. By varying the gas pressure we were able to observe soliton fission, supercontinuum generation, dispersive wave emission and modulational instability (MI) at relatively low pulse energies (~250 nJ).

3. Group velocity dispersion

4. Results

Figure 2
Fig. 2 Experimental and numerical evolution of the output spectra with launched pulse energy for five different gas pressures. The black and white dashed vertical lines indicate the location of λ0. A and N denote anomalous and normal dispersion regimes. The pump frequency is kept constant at 0.375 PHz (800 nm). The arrows show the soliton recoil.
shows the experimental and theoretical output spectra with increasing input power at five different gas pressures. The expected zero dispersion wavelengths are calculated using Eq. (1) and are represented by the vertical black and white dashed lines. Good agreement is reached between experiment and numerical simulations using the unidirectional field propagation equation [9

9. W. Chang, A. Nazarkin, J. C. Travers, J. Nold, P. Hölzer, N. Y. Joly, and P. St. J. Russell, “Influence of ionization on ultrafast gas-based nonlinear fiber optics,” Opt. Express 19(21), 21018–21027 (2011). [CrossRef] [PubMed]

]. At 25 bar (Fig. 2(a)), when the pump wavelength is far from λ0, the results are similar to those reported in [2

2. N. Y. Joly, J. Nold, W. Chang, P. Hölzer, A. Nazarkin, G. K. L. Wong, F. Biancalana, and P. St. J. Russell, “Bright spatially coherent wavelength-tunable deep-UV laser source using an Ar-filled photonic crystal fiber,” Phys. Rev. Lett. 106(20), 203901 (2011). [CrossRef] [PubMed]

].

The soliton order is ~27 for 450 nJ pulse energy at 25 bar (γ ≈1.34 × 10−5 W−1m−1). Under these circumstances the solitons break up and phase-match to higher frequency dispersive waves in the normal dispersion regime. The emission of dispersive waves causes soliton recoil to lower frequencies, thus conserving energy [11

11. N. Akhmediev and M. Karlsson, “Cherenkov radiation emitted by solitons in optical fibers,” Phys. Rev. A 51(3), 2602–2607 (1995). [CrossRef] [PubMed]

]. Although the spectrum analyzer was unable to detect the dispersive waves directly (the simulations show that they should appear at ~300 nm), the experimental measurements show an accompanying soliton recoil at ~940 nm (~0.32 PHz), denoted by the arrows in Fig. 2(a). The asymmetric extension toward shorter wavelengths can be explained by the frequency-dependence of γ [12

12. J. C. Travers, “Blue extension of optical fibre supercontinuum generation,” J. Opt. 12(1130011), 1–18 (2010).

, 13

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

].

As the pressure is increased to 50 bar, the nonlinearity increases and λ0 moves closer to the pump wavelength (the intrinsic weak anomalous dispersion of the empty core is increasingly cancelled by the normal dispersion of the gas). Consequently spectral broadening appears at a much lower pump power. Numerical simulations show the appearance of multiple solitons at ~1000 nm. In the experiments there is as expected no evidence of any Raman-induced self-frequency shift with increasing pump power. The generated soliton remains fixed in a narrow wavelength band (indicated by the arrow on Fig. 2(b)). The spectral broadening is greater than an octave, because the low and flat dispersion profile of the gas-filled kagomé HC-PCF (compared to solid-core systems) significantly reduces group-velocity walk-off between different frequency components and ensures long interaction lengths. This enhances the effectiveness of four-wave mixing as a broadening mechanism, resulting in the generation of a cascade of sidebands, allowing it to dominate the spectral broadening process.

By varying the gas pressure, a great variety of different spectral broadening regimes, governed by processes such as soliton fission and MI, can be accessed without changing the pump laser or the fiber. Such flexibility is unique to the PCF-based system.

On increasing the pressure to 75 bar (Fig. 2(c)), λ0 moves even closer to the pump wavelength and two distinct MI sidebands can be seen at pulse energy of ~200 nJ. It is curious that these sidebands appear to be asymmetrically distant from the pump frequency. This is caused by phase-matching of the solitons to dispersive waves [10

10. G. Agarwal, Nonlinear Fiber Optics, 4th ed. (Academic, 2006)

,14

14. M. Droques, B. Barviau, A. Kudlinski, M. Taki, A. Boucon, T. Sylvestre, and A. Mussot, “Symmetry-breaking dynamics of the modulational instability spectrum,” Opt. Lett. 36(8), 1359–1361 (2011). [CrossRef] [PubMed]

], which appear only in the normal dispersion regime, creating asymmetry in the observed spectrum. A simple analysis based on matching the propagation constants of solitons and linear waves confirms that the dispersive wave band appears at a higher frequency than the high frequency MI side-band [5

5. J. C. Travers, W. Chang, J. Nold, N. Y. Joly, and P. St.J. Russell, “Ultrafast nonlinear optics in gas-filled hollow-core photonic crystal fibers,” J. Opt. Soc. Am. B 28(12), A11–A26 (2011).

]. This is also seen in Figs. 3(a)
Fig. 3 Numerical X-FROG traces at 75 bar. (a) MI bands at 170 nJ launched pulse energy and (b) asymmetric spectrum at 250 nJ pulse energy when the dispersive wave band overlaps with the high frequency sideband of the MI. The black and white dashed lines indicate the position of the zero dispersion frequency c/λ0. The red curve indicates the mismatch in propagation constant β between the solitons and dispersive waves at a given frequency. The β mismatch is zero at the white circle for a soliton at the pump frequency, resulting in generation of a dispersive wave in the normal dispersion regime. The red arrow marks the pump frequency.
and 3(b), which show the results of a numerical X-FROG analysis of the signal after 28 cm of propagation for launched energies of 170 and 250 nJ at 75 bar. At the lower pulse energy (Fig. 3(a)), where the dispersive wave contribution is small, the MI sidebands are relatively symmetric in spacing and intensity. As the energy is increased, however, the broad dispersive wave band overlaps with the high frequency MI side-band, overwhelming it (Fig. 3(b)) and producing strong asymmetry between the side-bands. Recently Droques et al. studied the interaction between a MI side-band and a dispersive wave in a solid core fiber [13

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

]. To avoid Raman perturbations, however, they were forced to work at low CW power levels—a limitation that is entirely absent in our PCF-based system. Of course, if required, a Raman-active gas such as hydrogen can be used if Raman effects are needed, offering yet another degree of freedom compared to existing systems.

At 90 bar and 250 nJ pulse energy, λ0 coincides with the pump wavelength and the sidebands are symmetric in frequency (Fig. 2(d)). The X-FROG trace in Fig. 4(a)
Fig. 4 Numerical X-FROG traces for 250 nJ launched pulses at 90 bar. (a) Symmetric MI spectrum after 28 cm of propagation. Arrow denotes the asymmetric SPM due to higher order dispersion. (b) after 58.5 cm. Dotted lines have same meaning than for Fig. 3.
also shows a pair of distinct symmetric MI sidebands at zero delay. The self-phase modulation (SPM) trace in the X-FROG is distorted at a delay of ~110 fs, an effect that we attribute to higher order dispersion, which becomes important close to λ0. Indeed, when higher order dispersion is switched off in the simulations, this feature vanishes. Using numerical simulations, we followed the propagation of the pulse over a longer length (58.5 cm). Both the dispersive wave band (overlapping with the high frequency MI band) and the soliton regime (anomalous dispersion) are clearly seen in Fig. 4(b). At 110 bar (γ ≈5.65 × 10−5 W−1m−1, Fig. 2(e)), the dispersion is normal and spectral broadening due to SPM is observed.

5. Concluding remarks

Kagomé-style HC-PCF, filled with noble gases at high pressure, provides a unique and highly versatile system for exploring Raman-free nonlinear optics. Regimes of normal and anomalous dispersion can be readily accessed with a fixed-frequency laser merely by tuning the gas pressure. Additionally, the dispersion changes much more slowly with wavelength than in silica fibers, which is vital for the success of phase-matched nonlinear processes over broad wavelength ranges. High energies to be guided without optical damage or photo-darkening—serious problems in fibers with solid glass cores. Our experience shows that experiments can be run over many months with no sign of deterioration—provided of course there are no gas leaks.

It is interesting to note that argon enters the supercritical state for pressures higher than 48 bar, which is the critical pressure for argon (the critical temperature is ~150 K). Since the experiments were conducted at room temperature, the gas is actually in the supercritical state for many of the measurements (Figs. 2(b)-2(e)). However, since the operating temperature is well above the critical temperature, the argon density (and hence the nonlinearity level) still depends linearly on pressure, as confirmed in the experiments and the numerical modeling.

References and links

1.

P. St. J. Russell, “Photonic-crystal fibers,” J. Lightwave Technol. 24(12), 4729–4749 (2006). [CrossRef]

2.

N. Y. Joly, J. Nold, W. Chang, P. Hölzer, A. Nazarkin, G. K. L. Wong, F. Biancalana, and P. St. J. Russell, “Bright spatially coherent wavelength-tunable deep-UV laser source using an Ar-filled photonic crystal fiber,” Phys. Rev. Lett. 106(20), 203901 (2011). [CrossRef] [PubMed]

3.

P. Hölzer, W. Chang, J. C. Travers, A. Nazarkin, J. Nold, N. Y. Joly, M. F. Saleh, F. Biancalana, and P. St. J. Russell, “Femtosecond nonlinear fiber optics in the ionization regime,” Phys. Rev. Lett. 107(20), 203901 (2011). [CrossRef] [PubMed]

4.

M. F. Saleh, W. Chang, P. Hölzer, A. Nazarkin, J. C. Travers, N. Y. Joly, P. St. J. Russell, and F. Biancalana, “Theory of photoionization-induced blueshift of ultrashort solitons in gas-filled hollow-core photonic crystal fibers,” Phys. Rev. Lett. 107(20), 203902 (2011). [CrossRef] [PubMed]

5.

J. C. Travers, W. Chang, J. Nold, N. Y. Joly, and P. St.J. Russell, “Ultrafast nonlinear optics in gas-filled hollow-core photonic crystal fibers,” J. Opt. Soc. Am. B 28(12), A11–A26 (2011).

6.

M. Azhar, G. Wong, W. Chang, N. Joly and P. St.J. Russell, "Nonlinear optics in hollow-core photonic crystal fiber filled with liquid argon," in CLEO: Science and Innovations, OSA Technical Digest (online) (Optical Society of America, 2012), paper CTh4B.4.

7.

E. Marcatili and R. Schmeltzer, “Hollow metallic and dielectric waveguides for long distance optical transmission and lasers,” Bell Syst. Tech. J. 43, 1783–1809 (1964).

8.

J. Nold, P. Hölzer, N. Y. Joly, G. K. L. Wong, A. Nazarkin, A. Podlipensky, M. Scharrer, and P. St. J. Russell, “Pressure-controlled phase matching to third harmonic in Ar-filled hollow-core photonic crystal fiber,” Opt. Lett. 35(17), 2922–2924 (2010). [CrossRef] [PubMed]

9.

W. Chang, A. Nazarkin, J. C. Travers, J. Nold, P. Hölzer, N. Y. Joly, and P. St. J. Russell, “Influence of ionization on ultrafast gas-based nonlinear fiber optics,” Opt. Express 19(21), 21018–21027 (2011). [CrossRef] [PubMed]

10.

G. Agarwal, Nonlinear Fiber Optics, 4th ed. (Academic, 2006)

11.

N. Akhmediev and M. Karlsson, “Cherenkov radiation emitted by solitons in optical fibers,” Phys. Rev. A 51(3), 2602–2607 (1995). [CrossRef] [PubMed]

12.

J. C. Travers, “Blue extension of optical fibre supercontinuum generation,” J. Opt. 12(1130011), 1–18 (2010).

13.

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

14.

M. Droques, B. Barviau, A. Kudlinski, M. Taki, A. Boucon, T. Sylvestre, and A. Mussot, “Symmetry-breaking dynamics of the modulational instability spectrum,” Opt. Lett. 36(8), 1359–1361 (2011). [CrossRef] [PubMed]

15.

J. G. Rarity, J. Fulconis, J. Duligall, W. J. Wadsworth, and P. St. J. Russell, “Photonic crystal fiber source of correlated photon pairs,” Opt. Express 13(2), 534–544 (2005). [CrossRef] [PubMed]

OCIS Codes
(190.4370) Nonlinear optics : Nonlinear optics, fibers
(060.5295) Fiber optics and optical communications : Photonic crystal fibers

ToC Category:
Nonlinear Optics

History
Original Manuscript: October 25, 2012
Revised Manuscript: December 19, 2012
Manuscript Accepted: December 30, 2012
Published: February 13, 2013

Citation
M. Azhar, G. K. L. Wong, W. Chang, N. Y. Joly, and P. St.J. Russell, "Raman-free nonlinear optical effects in high pressure gas-filled hollow core PCF," Opt. Express 21, 4405-4410 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-4-4405


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References

  1. P. St. J. Russell, “Photonic-crystal fibers,” J. Lightwave Technol.24(12), 4729–4749 (2006). [CrossRef]
  2. N. Y. Joly, J. Nold, W. Chang, P. Hölzer, A. Nazarkin, G. K. L. Wong, F. Biancalana, and P. St. J. Russell, “Bright spatially coherent wavelength-tunable deep-UV laser source using an Ar-filled photonic crystal fiber,” Phys. Rev. Lett.106(20), 203901 (2011). [CrossRef] [PubMed]
  3. P. Hölzer, W. Chang, J. C. Travers, A. Nazarkin, J. Nold, N. Y. Joly, M. F. Saleh, F. Biancalana, and P. St. J. Russell, “Femtosecond nonlinear fiber optics in the ionization regime,” Phys. Rev. Lett.107(20), 203901 (2011). [CrossRef] [PubMed]
  4. M. F. Saleh, W. Chang, P. Hölzer, A. Nazarkin, J. C. Travers, N. Y. Joly, P. St. J. Russell, and F. Biancalana, “Theory of photoionization-induced blueshift of ultrashort solitons in gas-filled hollow-core photonic crystal fibers,” Phys. Rev. Lett.107(20), 203902 (2011). [CrossRef] [PubMed]
  5. J. C. Travers, W. Chang, J. Nold, N. Y. Joly, and P. St.J. Russell, “Ultrafast nonlinear optics in gas-filled hollow-core photonic crystal fibers,” J. Opt. Soc. Am. B28(12), A11–A26 (2011).
  6. M. Azhar, G. Wong, W. Chang, N. Joly and P. St.J. Russell, "Nonlinear optics in hollow-core photonic crystal fiber filled with liquid argon," in CLEO: Science and Innovations, OSA Technical Digest (online) (Optical Society of America, 2012), paper CTh4B.4.
  7. E. Marcatili and R. Schmeltzer, “Hollow metallic and dielectric waveguides for long distance optical transmission and lasers,” Bell Syst. Tech. J.43, 1783–1809 (1964).
  8. J. Nold, P. Hölzer, N. Y. Joly, G. K. L. Wong, A. Nazarkin, A. Podlipensky, M. Scharrer, and P. St. J. Russell, “Pressure-controlled phase matching to third harmonic in Ar-filled hollow-core photonic crystal fiber,” Opt. Lett.35(17), 2922–2924 (2010). [CrossRef] [PubMed]
  9. W. Chang, A. Nazarkin, J. C. Travers, J. Nold, P. Hölzer, N. Y. Joly, and P. St. J. Russell, “Influence of ionization on ultrafast gas-based nonlinear fiber optics,” Opt. Express19(21), 21018–21027 (2011). [CrossRef] [PubMed]
  10. G. Agarwal, Nonlinear Fiber Optics, 4th ed. (Academic, 2006)
  11. N. Akhmediev and M. Karlsson, “Cherenkov radiation emitted by solitons in optical fibers,” Phys. Rev. A51(3), 2602–2607 (1995). [CrossRef] [PubMed]
  12. J. C. Travers, “Blue extension of optical fibre supercontinuum generation,” J. Opt.12(1130011), 1–18 (2010).
  13. J. M. Dudley, G. Genty, and S. Coen, “Supercontinuum generation in photonic crystal fiber,” Rev. Mod. Phys.78(4), 1135–1184 (2006). [CrossRef]
  14. M. Droques, B. Barviau, A. Kudlinski, M. Taki, A. Boucon, T. Sylvestre, and A. Mussot, “Symmetry-breaking dynamics of the modulational instability spectrum,” Opt. Lett.36(8), 1359–1361 (2011). [CrossRef] [PubMed]
  15. J. G. Rarity, J. Fulconis, J. Duligall, W. J. Wadsworth, and P. St. J. Russell, “Photonic crystal fiber source of correlated photon pairs,” Opt. Express13(2), 534–544 (2005). [CrossRef] [PubMed]

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