OSA's Digital Library

Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 23 — Nov. 7, 2011
  • pp: 22575–22581
« Show journal navigation

Propagation-length independent SRS threshold in chirally-coupled-core fibers

Xiuquan Ma, I-Ning Hu, and Almantas Galvanauskas  »View Author Affiliations


Optics Express, Vol. 19, Issue 23, pp. 22575-22581 (2011)
http://dx.doi.org/10.1364/OE.19.022575


View Full Text Article

Acrobat PDF (1331 KB)





Browse Journals / Lookup Meetings

Browse by Journal and Year


   


Lookup Conference Papers

Close Browse Journals / Lookup Meetings

Article Tools

Share
Citations

Abstract

Both analytical study and numerical simulations show that the propagation-length independent Stimulated Raman Scattering (SRS) threshold can be achieved by Stokes wave suppression in optical fibers. We propose a specific design based on Chirally-Coupled-Core (CCC) fibers with spectrally-tailored wavelength-selective transmission to suppress the Stokes wave of Raman scattering. Fibers with length-independent nonlinearity threshold could be particularly advantageous for high power lasers and fiber beam delivery for material processing applications.

© 2011 OSA

1. Introduction

Though optical nonlinearity is useful in a number of applications, it is sometimes detrimental. In later cases it is critical to avoid the onset of nonlinear effects by resorting to relatively short signal-propagation distances, since, as universally known, at a fixed optical peak power the nonlinearity threshold is inversely proportional to nonlinear interaction length. This constitutes a general technological constraint in a wide range of applications, such as long-range signal transmission in optical fiber communications, and generation or propagation of high energy and high power signals in fiber lasers. For example, in broad-band continuous-wave and pulsed fiber lasers, where the fiber length is usually limited by the Stimulated Raman Scattering (SRS) [1

1. R. G. Smith, “Optical power handling capacity of low loss optical fibers as determined by stimulated Raman and brillouin scattering,” Appl. Opt. 11(11), 2489–2494 (1972). [CrossRef] [PubMed]

,2

2. D. J. Richardson, J. Nilsson, and W. A. Clarkson, “High power fiber lasers: current status and future perspectives [Invited],” J. Opt. Soc. Am. B 27(11), B63–B92 (2010). [CrossRef]

], but where increasing fiber length is needed for more effective heat removal from a fiber at high pumping [2

2. D. J. Richardson, J. Nilsson, and W. A. Clarkson, “High power fiber lasers: current status and future perspectives [Invited],” J. Opt. Soc. Am. B 27(11), B63–B92 (2010). [CrossRef]

], this trade-off limits the maximum achievable output power.

In this paper, using analytical and numerical-simulation methods, we show that such trade-off can be overcome and that length-independent SRS threshold can be achieved in optical fibers with the Stokes-wave loss. It is important to note that SRS suppression in optical fibers using Stokes-wave loss has been previously proposed and demonstrated with several different techniques [3

3. F. Benabid, G. Bouwmans, J. C. Knight, P. St. J. Russell, and F. Couny, “Ultrahigh efficiency laser wavelength conversion in a gas-filled hollow core photonic crystal fiber by pure stimulated rotational Raman scattering in molecular hydrogen,” Phys. Rev. Lett. 93(12), 123903 (2004). [CrossRef] [PubMed]

9

9. T. Taru, J. Hou, and J. C. Knight, “Raman gain suppression in all-solid photonic bandgap fiber,” in European Conference and Exhibition on Optical Communication 2007, Berlin (Sep. 2007), paper 7.1.1.

]. However, to the best of our knowledge, it has not been recognized before that it is possible to achieve length-independent SRS threshold when Stokes-wave experiences significant loss. The importance of this finding is that it conceptually enables an entirely new avenue of effectively suppressing nonlinearity compatible with very long fiber lengths.

To practically implement the length-independent SRS threshold for high peak power SRS-free propagation, one needs a fiber structure with a distributive and large Stokes-wave loss, characterized by a spectral profile with a sharp cut-off at short wavelengths to achieve strong discrimination between signal and Stokes waves. Survey of the previously reported methods of SRS suppression through Stokes-wave loss indicates that these techniques are not well suited for achieving length-independent SRS suppression. Indeed, the implementation with long period grating [4

4. D. Nodop, C. Jauregui, F. Jansen, J. Limpert, and A. Tünnermann, “Suppression of stimulated Raman scattering employing long period gratings in double-clad fiber amplifiers,” Opt. Lett. 35(17), 2982–2984 (2010). [CrossRef] [PubMed]

] is not distributive, and, therefore, in principle is not compatible with length-independent SRS suppression. The bending of conventional [5

5. P. D. Dragic, “Suppression of first order stimulated Raman scattering in erbium-doped fiber laser based LIDAR transmitters through induced bending loss,” Opt. Commun. 250(4-6), 403–410 (2005). [CrossRef]

] and W-type [6

6. J. Kim, P. Dupriez, C. Codemard, J. Nilsson, and J. K. Sahu, “Suppression of stimulated Raman scattering in a high power Yb-doped fiber amplifier using a W-type core with fundamental mode cut-off,” Opt. Express 14(12), 5103–5113 (2006). [CrossRef] [PubMed]

] fibers does not provide with sufficiently sharp short-wavelength cut-off thus making it difficult to differentiate between signal and Stokes waves. As a result these two techniques lead to relatively mild SRS suppression. The dual-hole-assisted fibers [7

7. L. A. Zenteno, J. Wang, D. T. Walton, B. A. Ruffin, M. J. Li, S. Gray, A. Crowley, and X. Chen, “Suppression of Raman gain in single-transverse-mode dual-hole-assisted fiber,” Opt. Express 13(22), 8921–8926 (2005). [CrossRef] [PubMed]

], filter fibers [8

8. J. M. Fini, M. D. Mermelstein, M. F. Yan, R. T. Bise, A. D. Yablon, P. W. Wisk, and M. J. Andrejco, “Distributed suppression of stimulated Raman scattering in an Yb-doped filter-fiber amplifier,” Opt. Lett. 31(17), 2550–2552 (2006). [CrossRef] [PubMed]

] and all-solid photonic bandgap fibers [9

9. T. Taru, J. Hou, and J. C. Knight, “Raman gain suppression in all-solid photonic bandgap fiber,” in European Conference and Exhibition on Optical Communication 2007, Berlin (Sep. 2007), paper 7.1.1.

] have sharp wavelength edges, but their suppression magnitudes are also relatively small (usually below 1dB/m). The common limitation for all these techniques caused by the small Stokes-wave loss is that length-independent SRS suppression is weak and, consequently, should occur only in very long fibers according to our estimate - starting from approximately hundreds of meters), which is perhaps the main reason why this length-independent nature of Stokes-wave suppression has not been recognized before. Furthermore, the other common limitation for reported techniques is that they are difficult to implement in large-core fibers, thus making them of limited use in high power laser technology. We show that specially designed large-core Chirally-Coupled-Core (CCC) fibers, which by itself provides improved SRS threshold with a much larger mode field diameter than single mode fibers, can provide with distributive and >10dB/m Stokes-wave loss with sharp short-wavelength cut-off, which makes it capable of sustaining length-independent SRS-free operation at tens-of-kW optical powers starting from few meters of fiber length. Such fibers could facilitate practical implementation of high-power fiber laser and high-power fiber delivery for material processing applications.

2. Length independent SRS threshold with Stokes-wave suppression

Pcr30Aeffg0Leff.
(4)

From both Eqs. (3) and (4) one can see that when pump and Stokes losses are equal the SRS threshold is indeed inversely proportional to the propagation length.

However, when the Stokes wave experiences larger loss than the pump Δαs = αsαp > 0, which one can refer to as Stokes-wave suppression, following the same path as described above but with αsαp, one can arrive at the following expression for the SRS threshold in large mode area fibers (30µm 0.06NA step-index, for example):

Pcr30Aeffg0L+ΔαsAeffg0.
(5)

This leads to an important result: for sufficiently large Stokes suppression Δαs and long enough propagation length L, the second term becomes dominant in Eq. (5), so the critical power Pcr becomes propagation-length independent. Indeed, with increasing L the first term in Eq. (5) decreases, and eventually becomes much smaller than the second term, so the SRS threshold becomes solely determined by the magnitude of the Stokes suppression Δαs:

Pcr|LΔαsAeffg0.
(6)

Even though this result might be a bit unexpected, the basic physics behind it is rather straightforward. Let’s rewrite Eq. (2) in a slightly more convenient form:

Ps(0)exp[ΔαsL+Pcrg0L/Aeff]=Pcr.
(7)

Since the gain term ePcrg0L/Aeff and the suppression term eΔαsL in Eq. (7) have the same dependence over the length L, the competition between Stokes wave generation and attenuation becomes indeed length-independent. Thus, once Δαs exceeds Pcrg0/Aeff, as shown by Eq. (6), SRS would never reach threshold regardless of the propagation length L.

All the above analysis of SRS threshold is based on certain simplifying assumptions, the most important one of which is that we discarded frequency dependence of both the SRS gain and Stokes-wave loss (apart, of course, from taking into account effective SRS gain bandwidth, when calculating Stokes-wave seed power Ps(0) for substitution in Eq. (2). In reality, however, Raman gain spectrum in fused silica has a certain spectrally dependent profile shown in the left-down-corner inset of Fig. 1 [10

10. G. P. Algrawal, Nonlinear Fiber Optics, 3rd ed. (Academic Press, 2001).

]. Therefore, to better quantify the conclusions on the length-independent SRS threshold, it is necessary to consider a more comprehensive model with the entire Raman Stokes gain spectrum, which due to the complexity can be only done numerically. Thus, we can use the coupled intensity equations for Raman scattering [10

10. G. P. Algrawal, Nonlinear Fiber Optics, 3rd ed. (Academic Press, 2001).

]:
{dIpdz=ωpωsgR(Ω)IpIs,dIsdz=gR(Ω)IpIsαsIs,
(8)
where gR(Ω) is the Raman Stokes gain spectrum as a function of frequency shift. By assuming the flattop Stokes suppression across the Raman Stokes gain spectrum at 0, 15, 40 and 100 dB/m levels, we can numerically solve Eq. (8) and simulate the Raman scattering process. The simulation results for SRS threshold are shown as symbol points in Fig. 1. We can see that, the numerical simulation with coupled intensity equation in Eq. (8) agrees very well with the analytical equation in Eqs. (5)(7), and both of them show that SRS threshold becomes propagation-length independent when Stokes wave suppression is present.

3. SRS suppression in specially designed large-core CCC fibers

The practical challenge of implementing the Stokes wave suppression to achieve length-independent SRS threshold is associated with (i) designing a medium (e.g. an optical fiber) with a precise transmission spectrum to match the required Stokes-wave gain spectrum of an optical fiber (see the left-down-corner inset of Fig. 1), and (ii) achieving sufficiently high Stokes-wave loss so that length-independent SRS suppression would start occurring at practically short fiber lengths. Indeed, to the best of our knowledge, none of existing techniques have demonstrated >10dB/m Stokes-wave suppression. In fact, the all-solid photonic crystal fiber only shows ~1dB/m suppression and the dual-hole-assisted fiber only shows ~0.1dB/m suppression. Thus, according to the plot of 1dB/m suppression in Fig. 1, only a mild increase of the SRS threshold can be achieved, and length-independent nature of such SRS suppression becomes manifest only starting from approximately hundred meter long fibers. In addition, none of the existing techniques have demonstrated the capability to provide large-mode-area effective single-mode performance, which is critical for high power fiber laser and also helpful to increase the SRS threshold intrinsically.

Generally speaking, by properly designing the parameters of CCC fibers, one can control the wavelength position and loss magnitude of the fundamental mode’s loss resonance to precisely match the required Stokes wave loss spectrum for the purpose of achieving length-independent SRS threshold. From another perspective, if given a specific fabricated CCC sample such as the one shown in the up-right-corner inset of Fig. 2, one can choose the right pump wavelength to be suppressed for the Raman Stokes-wave gain. In this particular sample (with core size of 30μm), the pump wavelength at 1085nm can be suppressed. In Fig. 2, we plot the Raman Stokes gain of pump wavelength at 1085nm as a function of wavelength with blue solid line and vertical axis on the left, and we also plot the transmission spectrum at 1085nm~1245nm range of this CCC fiber sample with red solid line and vertical axis on the right. It indeed shows that this CCC fiber sample can be used to suppress the Raman stokes wave at the pump wavelength of 1085nm.

4. Discussion and conclusion

In this paper, we have demonstrated the concept of propagation-length-independent SRS threshold using both analytical and numerical analysis. As one suggested practical implementation of this concept, we demonstrate possibility to design CCC-geometry large-core fibers with spectrally tailored fundamental mode transmission, so that length-independent SRS threshold can be achieved at a certain signal wavelength. The measured transmission spectrum of a fabricated CCC sample appears to match the theoretically required Stokes-wave suppression spectral profile. Such approach could be very useful in high power fiber lasers or especially for high power delivery fibers in industrial processing applications.

Acknowledgments

This work was supported by US Army Research Office grant W911NF051057.

References and links

1.

R. G. Smith, “Optical power handling capacity of low loss optical fibers as determined by stimulated Raman and brillouin scattering,” Appl. Opt. 11(11), 2489–2494 (1972). [CrossRef] [PubMed]

2.

D. J. Richardson, J. Nilsson, and W. A. Clarkson, “High power fiber lasers: current status and future perspectives [Invited],” J. Opt. Soc. Am. B 27(11), B63–B92 (2010). [CrossRef]

3.

F. Benabid, G. Bouwmans, J. C. Knight, P. St. J. Russell, and F. Couny, “Ultrahigh efficiency laser wavelength conversion in a gas-filled hollow core photonic crystal fiber by pure stimulated rotational Raman scattering in molecular hydrogen,” Phys. Rev. Lett. 93(12), 123903 (2004). [CrossRef] [PubMed]

4.

D. Nodop, C. Jauregui, F. Jansen, J. Limpert, and A. Tünnermann, “Suppression of stimulated Raman scattering employing long period gratings in double-clad fiber amplifiers,” Opt. Lett. 35(17), 2982–2984 (2010). [CrossRef] [PubMed]

5.

P. D. Dragic, “Suppression of first order stimulated Raman scattering in erbium-doped fiber laser based LIDAR transmitters through induced bending loss,” Opt. Commun. 250(4-6), 403–410 (2005). [CrossRef]

6.

J. Kim, P. Dupriez, C. Codemard, J. Nilsson, and J. K. Sahu, “Suppression of stimulated Raman scattering in a high power Yb-doped fiber amplifier using a W-type core with fundamental mode cut-off,” Opt. Express 14(12), 5103–5113 (2006). [CrossRef] [PubMed]

7.

L. A. Zenteno, J. Wang, D. T. Walton, B. A. Ruffin, M. J. Li, S. Gray, A. Crowley, and X. Chen, “Suppression of Raman gain in single-transverse-mode dual-hole-assisted fiber,” Opt. Express 13(22), 8921–8926 (2005). [CrossRef] [PubMed]

8.

J. M. Fini, M. D. Mermelstein, M. F. Yan, R. T. Bise, A. D. Yablon, P. W. Wisk, and M. J. Andrejco, “Distributed suppression of stimulated Raman scattering in an Yb-doped filter-fiber amplifier,” Opt. Lett. 31(17), 2550–2552 (2006). [CrossRef] [PubMed]

9.

T. Taru, J. Hou, and J. C. Knight, “Raman gain suppression in all-solid photonic bandgap fiber,” in European Conference and Exhibition on Optical Communication 2007, Berlin (Sep. 2007), paper 7.1.1.

10.

G. P. Algrawal, Nonlinear Fiber Optics, 3rd ed. (Academic Press, 2001).

11.

K. Okamoto, Fundamentals of Optical Waveguides, 2nd ed. (Academic Press, 2006).

12.

J. M. Fini, M. D. Mermelstein, M. F. Yan, R. T. Bise, A. D. Yablon, P. W. Wisk, and M. J. Andrejco, “Distributed suppression of stimulated Raman scattering in an Yb-doped filter-fiber amplifier,” Opt. Lett. 31(17), 2550–2552 (2006). [CrossRef] [PubMed]

13.

X. Ma, “Understanding and controlling angular momentum coupled optical waves in chirally coupled core fibers,” PhD thesis.

14.

X. Ma, C.-H. Liu, G. Chang, and A. Galvanauskas, “Angular-momentum coupled optical waves in chirally-coupled-core fibers,” (submitted to Opt. Express).

OCIS Codes
(060.2310) Fiber optics and optical communications : Fiber optics
(140.3510) Lasers and laser optics : Lasers, fiber
(190.0190) Nonlinear optics : Nonlinear optics

ToC Category:
Frequency Conversion, Combs and Nonlinear Waveguides

History
Original Manuscript: September 6, 2011
Revised Manuscript: October 20, 2011
Manuscript Accepted: October 20, 2011
Published: October 25, 2011

Virtual Issues
Nonlinear Optics (2011) Optical Materials Express

Citation
Xiuquan Ma, I-Ning Hu, and Almantas Galvanauskas, "Propagation-length independent SRS threshold in chirally-coupled-core fibers," Opt. Express 19, 22575-22581 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-23-22575


Sort:  Author  |  Year  |  Journal  |  Reset  

References

  1. R. G. Smith, “Optical power handling capacity of low loss optical fibers as determined by stimulated Raman and brillouin scattering,” Appl. Opt.11(11), 2489–2494 (1972). [CrossRef] [PubMed]
  2. D. J. Richardson, J. Nilsson, and W. A. Clarkson, “High power fiber lasers: current status and future perspectives [Invited],” J. Opt. Soc. Am. B27(11), B63–B92 (2010). [CrossRef]
  3. F. Benabid, G. Bouwmans, J. C. Knight, P. St. J. Russell, and F. Couny, “Ultrahigh efficiency laser wavelength conversion in a gas-filled hollow core photonic crystal fiber by pure stimulated rotational Raman scattering in molecular hydrogen,” Phys. Rev. Lett.93(12), 123903 (2004). [CrossRef] [PubMed]
  4. D. Nodop, C. Jauregui, F. Jansen, J. Limpert, and A. Tünnermann, “Suppression of stimulated Raman scattering employing long period gratings in double-clad fiber amplifiers,” Opt. Lett.35(17), 2982–2984 (2010). [CrossRef] [PubMed]
  5. P. D. Dragic, “Suppression of first order stimulated Raman scattering in erbium-doped fiber laser based LIDAR transmitters through induced bending loss,” Opt. Commun.250(4-6), 403–410 (2005). [CrossRef]
  6. J. Kim, P. Dupriez, C. Codemard, J. Nilsson, and J. K. Sahu, “Suppression of stimulated Raman scattering in a high power Yb-doped fiber amplifier using a W-type core with fundamental mode cut-off,” Opt. Express14(12), 5103–5113 (2006). [CrossRef] [PubMed]
  7. L. A. Zenteno, J. Wang, D. T. Walton, B. A. Ruffin, M. J. Li, S. Gray, A. Crowley, and X. Chen, “Suppression of Raman gain in single-transverse-mode dual-hole-assisted fiber,” Opt. Express13(22), 8921–8926 (2005). [CrossRef] [PubMed]
  8. J. M. Fini, M. D. Mermelstein, M. F. Yan, R. T. Bise, A. D. Yablon, P. W. Wisk, and M. J. Andrejco, “Distributed suppression of stimulated Raman scattering in an Yb-doped filter-fiber amplifier,” Opt. Lett.31(17), 2550–2552 (2006). [CrossRef] [PubMed]
  9. T. Taru, J. Hou, and J. C. Knight, “Raman gain suppression in all-solid photonic bandgap fiber,” in European Conference and Exhibition on Optical Communication 2007, Berlin (Sep. 2007), paper 7.1.1.
  10. G. P. Algrawal, Nonlinear Fiber Optics, 3rd ed. (Academic Press, 2001).
  11. K. Okamoto, Fundamentals of Optical Waveguides, 2nd ed. (Academic Press, 2006).
  12. J. M. Fini, M. D. Mermelstein, M. F. Yan, R. T. Bise, A. D. Yablon, P. W. Wisk, and M. J. Andrejco, “Distributed suppression of stimulated Raman scattering in an Yb-doped filter-fiber amplifier,” Opt. Lett.31(17), 2550–2552 (2006). [CrossRef] [PubMed]
  13. X. Ma, “Understanding and controlling angular momentum coupled optical waves in chirally coupled core fibers,” PhD thesis.
  14. X. Ma, C.-H. Liu, G. Chang, and A. Galvanauskas, “Angular-momentum coupled optical waves in chirally-coupled-core fibers,” (submitted to Opt. Express).

Cited By

Alert me when this paper is cited

OSA is able to provide readers links to articles that cite this paper by participating in CrossRef's Cited-By Linking service. CrossRef includes content from more than 3000 publishers and societies. In addition to listing OSA journal articles that cite this paper, citing articles from other participating publishers will also be listed.

Figures

Fig. 1 Fig. 2 Fig. 3
 

« Previous Article  |  Next Article »

OSA is a member of CrossRef.

CrossCheck Deposited