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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 15, Iss. 25 — Dec. 10, 2007
  • pp: 16690–16695
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Impact of nonlinear spectral broadening in ultra-long Raman fibre lasers

V. Karalekas, J. D. Ania-Castañón, P. Harper, S. A. Babin, E. V. Podivilov, and S. K. Turitsyn  »View Author Affiliations


Optics Express, Vol. 15, Issue 25, pp. 16690-16695 (2007)
http://dx.doi.org/10.1364/OE.15.016690


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Abstract

We present an experimental study of the impact of FWM-induced nonlinear spectral broadening on the effective reflectivity of ultra-long Raman fiber laser cavities of diverse lengths and fiber bases. We observe an exponential decay of the effective reflectivity with growing power. In standard single-mode fiber, effective reflectivity drops of up to 50% for shorter cavity lengths are observed, while the longest cavity length of 82.4km displays power leakage amounting to an effective reduction of reflectivity of approximately 30%. Using different types of fiber we examine the effect of chromatic dispersion on the Stokes wave broadening.

© 2007 Optical Society of America

1. Introduction

Stimulated Raman scattering (SRS) finds a very important application in the development of Raman fibre lasers (RFL). A typical RFL is a fiber resonator in which SRS shifts the spectrum of the radiation from a pump or series of pumps propagating through an optical fibre, towards lower frequency (~14 THz lower) Stokes components, which can be trapped in the fibre with the help of fibre Bragg grating (FBG) reflectors. The Stokes radiation is sometimes used as a Raman pump itself in a multi-cavity cascading process [1

1. E.M. Dianov, M.V. Grekov, I.A. Bufetov, S.A. Vasiliev, O.I. Medvedkov, V.G. Plotnichenko, 1 V.V. Koltashev, A.V. Belov, M.M. Bubnov, S.L. Semjonov, and A.M. Prokhorov, “CW high power 1.24 µm and 1.48 µm Raman lasers based on low loss phosphosilicate fibre,” Electron. Lett. 33, 1542–1544 (1997) [CrossRef]

]. RFLs are photonic devices with great commercial potential in a variety of practical cases, as their tunability, compactness, and capability for multi-wavelength operation makes them attractive CW sources in applications such as long-distance remote sensing [2

2. Y. -G. Han, T. V. A. Tran, S. -H. Kim, and S. B. Lee, “Development of a multiwavelength Raman fiber laser based on phase-shifted fiber Bragg gratings for long-distance remote-sensing applications,” Opt. Lett. 30, 1114–1116 (2005) [CrossRef] [PubMed]

], optical tomography [3

3. P. -L. Hsiung, Y. Chen, T. Ko, J. Fujimoto, C. de Matos, S. Popov, J. Taylor, and V. Gapontsev, “Optical coherence tomography using a continuous-wave, high-power, Raman continuum light source,” Opt. Express 12, 5287–5295 (2004) http://www.opticsinfobase.org/abstract.cfm?URI=oe-12-22-5287 [CrossRef] [PubMed]

], supercontinuum generation [4

4. S.V. Smirnov, J.D. Ania Castanon, T.J. Ellingham, S.M. Kobtsev, S. Kukarin, and S.K. Turitsyn, “Optical spectral broadening and supercontinuum generation in telecom applications,” Opt. Fiber Technol. 12122–147 (2006) [CrossRef]

] or as high power pump sources for distributed Raman amplified telecommunication systems, (see [5

5. “Raman amplification in Fiber Optical Communication Systems,” C. Headley and G.P. Agrawal (Eds.), Academic Press (2005)

] and references therein), to name but a few.

An important characteristic of RFL sources is that the spectrum of the generated Stokes wave typically exhibits four-wave mixing (FWM)-induced nonlinear broadening [6

6. J.-C. Bouteiller, “Spectral modeling of Raman fiber lasers,” IEEE Photon. Technol. Lett. 15, 1698–1700 (2003) [CrossRef]

] due to its propagation through the fibre cavity. In ideal circumstances, the generated radiation would be perfectly trapped inside the cavity formed by the two FBGs positioned at both of its ends. In most practical cases, though, the radiated spectrum is broadened beyond the bandwidth of the FBGs, resulting in leakage of laser power from the cavity which has an impact on the RFL’s performance, degrading its quality. In particular, as it was demonstrated in [7

7. S.A. Babin, D.V. Churkin, and E.V. Podivilov “Intensity interactions in cascades of a two-stage Raman fiber laser,” Opt. Commun. 226, 329–335 (2003) [CrossRef]

], the “effective” reflectivity of the FBGs decreases proportionally with increasing total power in the cavity, and hence with the amount of broadening of the first-Stokes wave.

2. Experimental setup and procedure

Fig. 1. Experimental Setup of the proposed system

The basic design of an ultra-long Raman laser cavity is schematically depicted in Fig. 1. The system consists of two equal-power depolarized pumps centred at 1365nm and launched from both ends of a standard-single mode fibre (SMF) span. Two FBGs with high reflectivity (~99%), centred at 1455nm and with a bandwidth of ~1nm, are positioned at either ends of the fibre span forming a high-Q cavity trapping the 1455nm generated waves. When the total power from the primary pumps is above the required threshold for SRS-induced gain to overcome fibre attenuation at the Stokes wavelength, the cavity starts lasing at 1455 nm. In order to study the dependence of effective reflectivity with the injected primary pump power and cavity length, different fibre spans were considered. In particular, SMF fibre reels with lengths of 6.6, 10.1, 21, 40, 50.8 and 82.4km of SMF where utilized.. The intra-cavity generated first-Stokes wave was measured via two 99:1 splitters placed between the first reflector and the fibre span, as it is illustrated in Fig. 1. The 1% splitter outputs were positioned facing opposite directions in order to allow for the simultaneous monitoring of both the incident and reflected first-Stokes wave components via ports (I) and (II) respectively, with the use of a two-port switch. In this way we ensured that the experimental parameters were not affected by the switching between monitoring directions. The 1455nm spectra corresponding to the waves ‘incident to’ and ‘reflected from’ the grating were captured using a high resolution (~0.01nm) optical spectrum analyzer, and the powers were measured with a power meter for high accuracy.

3. Results and discussion

Fig. 2. (a) Total 1455nm intra-cavity power as a function of the total 1365nm input pump power for all of the studied lengths. (b) Spectral evolution of the 1455nm Stokes spectrum in a 21km cavity for the input power of 288, 504 and 1140mW. The inset plot presents the reflected (inner) and incident (outer) spectra for 504mW of pump power.

Fig. 3. (a) Effective cavity reflectivity as a function of the total 1365nm pump power for 6.6, 10.1, 21, 40.4, 50.8, 82.4km cavities. (b) Numerical contour plot for a 10km cavity, showing iso-lines of identical total 1455nm power vs. pump power and effective reflectivity. The dots represent superimposed experimental results.

A possible solution to reduce the impact of nonlinear broadening would be to use fibre Bragg gratings with a wider reflection band (e.g. chirped fibre Bragg gratings). Although a wider grating bandwidth would in principle allow for even further broadening of the Stokes wave, it should also prove to be more resistant to effective reflectivity degradation, hence reducing power leakage, with the additional advantage of potentially improving signal gain flatness over a certain bandwidth [13

13. J.D. Ania-Castañón, V. Karalekas, P. Harper, and S.K. Turitsyn, “Simultaneous spatial and spectral transparency using ultra-long fibre laser transmission”, in Proceedings of the European Conference on Optical Communications 2007, paper 1.4.3, Berlin, Germany, 2007

].

Fig. 4. Effective reflectivity as a function of total 1455 Stokes power for 10km of TrueWave-RS, LS-LEAF, and SMF. Solid lines represent exponential fit.

Another very important factor that must be taken into account when designing an ultra-long laser for maximum efficiency is fiber dispersion, more specifically the proximity of the Stokes wavelength to the zero-dispersion wavelength (ZDW) of the fiber. This is illustrated by the results in Fig. 4, which compares the exponential decay of the effective reflectivity as total intra-cavity power increases, for three similar lengths (~10 km) of different fiber bases (standard SMF, LEAF and Truewave fiber). Despite all three fibers having a comparable nonlinear coefficient, reflectivity drops much faster with power for the Truewave fiber laser, for which the lasing wavelength lies very close to the ZDW of 1454 nm, but still in the anomalous dispersion regime. Modulation-instability induced nonlinear broadening, coupled with the enhanced FWM processes thanks to more efficient phase matching, greatly reduce the effective reflectivity at high powers. On the other hand, LEAF and SMF, for which the lasing wavelength lies more than 100 nm away from the ZDW, show responses that are nearly identical to each other, with much lower nonlinear broadening than Truewave.

The strong dispersion-dependent response of the Stokes wave broadening in the cavity stresses the need for a careful choice of the most convenient fiber for each different possible application of the URFL. In the case of broadband transmission, an appropriate combination of fibre dispersion and grating bandwidth could provide, for example, a moderate nonlinear broadening of the Stokes wave for increased gain flatness, while still preserving an adequate laser efficiency. On the other hand, when dealing with nonlinear signal processing applications, for which the choice of fiber dispersions at the signal wavelength will be constrained, the use of specifically tailored, dispersion-engineered fibers could provide an additional degree of freedom to optimize system performance.

4. Conclusion

We have experimentally demonstrated the power dependence of spectral broadening in ultra-long fibre Raman lasers with cavity lengths up to 82.4 km. Broadening of the radiation beyond the bandwidth of the cavity gratings leads to power leakage out of the fibre resonator and to additional power-dependent loss that can formally be treated as a reduction (approximately exponentially dependent on power) of the effective reflectivity of the FBGs. Even when operating at Stokes wavelengths well separated from the fiber ZDW, we have shown that this effective reflectivity can be reduced to about 50% for gratings with a ~1nm bandwidth in the case of short URFL cavities and up to 70% for a longer 82.4 km cavity. Nonlinear pump broadening thus has to be carefully taken into account when designing URFL for nonlinear optical processing applications, in which the combination of short cavity lengths and high pump powers could lead to important degradation on the cavity quality. The impact of nonlinear broadening is less likely to have a dramatic effect on the performance of longer URFL with applications in long-distance communications, but it should be considered as a potentially deleterious factor nonetheless.

If the laser Stokes wavelength falls close to the ZDW of the cavity fiber, nonlinear broadening is greatly enhanced, and effective reflectivity deteriorates much faster with power, being reduced to just 10% at maximum pump power in a 10 km Truewave fiber cavity.

Acknowledgments

J. D. Ania-Castañón and S. K. Turitsyn would like to acknowledge the support the Engineering and Physical Sciences Research Council (EPSRC). Paul Harper would like to acknowledge support from The Nuffield Foundation Grant Ref: NAL/32796.

References and links

1.

E.M. Dianov, M.V. Grekov, I.A. Bufetov, S.A. Vasiliev, O.I. Medvedkov, V.G. Plotnichenko, 1 V.V. Koltashev, A.V. Belov, M.M. Bubnov, S.L. Semjonov, and A.M. Prokhorov, “CW high power 1.24 µm and 1.48 µm Raman lasers based on low loss phosphosilicate fibre,” Electron. Lett. 33, 1542–1544 (1997) [CrossRef]

2.

Y. -G. Han, T. V. A. Tran, S. -H. Kim, and S. B. Lee, “Development of a multiwavelength Raman fiber laser based on phase-shifted fiber Bragg gratings for long-distance remote-sensing applications,” Opt. Lett. 30, 1114–1116 (2005) [CrossRef] [PubMed]

3.

P. -L. Hsiung, Y. Chen, T. Ko, J. Fujimoto, C. de Matos, S. Popov, J. Taylor, and V. Gapontsev, “Optical coherence tomography using a continuous-wave, high-power, Raman continuum light source,” Opt. Express 12, 5287–5295 (2004) http://www.opticsinfobase.org/abstract.cfm?URI=oe-12-22-5287 [CrossRef] [PubMed]

4.

S.V. Smirnov, J.D. Ania Castanon, T.J. Ellingham, S.M. Kobtsev, S. Kukarin, and S.K. Turitsyn, “Optical spectral broadening and supercontinuum generation in telecom applications,” Opt. Fiber Technol. 12122–147 (2006) [CrossRef]

5.

“Raman amplification in Fiber Optical Communication Systems,” C. Headley and G.P. Agrawal (Eds.), Academic Press (2005)

6.

J.-C. Bouteiller, “Spectral modeling of Raman fiber lasers,” IEEE Photon. Technol. Lett. 15, 1698–1700 (2003) [CrossRef]

7.

S.A. Babin, D.V. Churkin, and E.V. Podivilov “Intensity interactions in cascades of a two-stage Raman fiber laser,” Opt. Commun. 226, 329–335 (2003) [CrossRef]

8.

J.D. Ania-Castañón, “Quasi-lossless transmission using second-order Raman amplification and fibre Bragg gratings,” Opt. Express 12, 4372–4377 (2004) http://www.opticsinfobase.org/abstract.cfm?URI=oe-12-19-4372 [CrossRef] [PubMed]

9.

T.J. Ellingham, J.D. Ania-Castañón, R. Ibbotson, X. Chen, L. Zhang, and Sergei K. Turitsyn, “Quasi-lossless optical links for broad-band transmission and data processing,” IEEE Photon. Technol. Lett. 18, 268–270 (2006) [CrossRef]

10.

J.D. Ania-Castañón, T.J. Ellingham, R. Ibbotson, X. Chen, L. Zhang, and Sergei K. Turitsyn, “Ultralong raman fiber lasers as virtually lossless optical media,” Phys. Rev. Lett. 96, 023902 (2006) [CrossRef] [PubMed]

11.

S.A. Babin, V. Karalekas, P. Harper, E.V. Podivilov, V.K. Mezentsev, J.D. Ania-Castañón, and S.K. Turitsyn, “Experimental demonstration of mode structure in ultralong Raman fiber lasers,” Opt. Lett. 32, 1135–1137 (2007) [CrossRef] [PubMed]

12.

S.A. Babin, D.V. Churkin, A.E. Ismagulov, S.I. Kablukov, and E.V. Podivilov, “FWM-induced turbulent spectral broadening in a long Raman fiber laser,” J. Opt. Soc. Am. B 24, 1729–1738 (2007) [CrossRef]

13.

J.D. Ania-Castañón, V. Karalekas, P. Harper, and S.K. Turitsyn, “Simultaneous spatial and spectral transparency using ultra-long fibre laser transmission”, in Proceedings of the European Conference on Optical Communications 2007, paper 1.4.3, Berlin, Germany, 2007

OCIS Codes
(140.3550) Lasers and laser optics : Lasers, Raman
(190.5650) Nonlinear optics : Raman effect

ToC Category:
Nonlinear Optics

History
Original Manuscript: September 10, 2007
Revised Manuscript: October 19, 2007
Manuscript Accepted: October 20, 2007
Published: December 3, 2007

Citation
V. Karalekas, J. D. Ania-Castañón, P. Harper, S. A. Babin, E. V. Podivilov, and S. K. Turitsyn, "Impact of nonlinear spectral broadening in ultra-long Raman fibre lasers," Opt. Express 15, 16690-16695 (2007)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-25-16690


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References

  1. E.M. Dianov, M.V. Grekov, I.A. Bufetov, S.A. Vasiliev, O.I. Medvedkov, V.G. Plotnichenko,1 V.V. Koltashev, A.V. Belov, M.M. Bubnov, S.L. Semjonov, and A.M. Prokhorov, "CW high power 1.24 µm and 1.48 µm Raman lasers based on low loss phosphosilicate fibre," Electron. Lett. 33, 1542-1544 (1997) [CrossRef]
  2. Y. -G. Han, T. V. A. Tran, S. -H. Kim, and S. B. Lee, "Development of a multiwavelength Raman fiber laser based on phase-shifted fiber Bragg gratings for long-distance remote-sensing applications," Opt. Lett. 30, 1114-1116 (2005) [CrossRef] [PubMed]
  3. P. -L. Hsiung, Y. Chen, T. Ko, J. Fujimoto, C. de Matos, S. Popov, J. Taylor, and V. Gapontsev, "Optical coherence tomography using a continuous-wave, high-power, Raman continuum light source," Opt. Express 12, 5287-5295 (2004) http://www.opticsinfobase.org/abstract.cfm?URI=oe-12-22-5287 [CrossRef] [PubMed]
  4. S.V. Smirnov, J.D. Ania Castanon, T.J. Ellingham, S.M. Kobtsev, S. Kukarin, S.K. Turitsyn, "Optical spectral broadening and supercontinuum generation in telecom applications," Opt. Fiber Technol. 12122-147 (2006) [CrossRef]
  5. "Raman amplification in Fiber Optical Communication Systems," C. Headley and G.P. Agrawal (Eds.), Academic Press (2005)
  6. J.-C. Bouteiller, "Spectral modeling of Raman fiber lasers,"IEEE Photon. Technol. Lett. 15, 1698-1700 (2003) [CrossRef]
  7. S.A. Babin, D.V. Churkin, E.V. Podivilov "Intensity interactions in cascades of a two-stage Raman fiber laser," Opt. Commun. 226, 329-335 (2003) [CrossRef]
  8. J.D. Ania-Castañón, "Quasi-lossless transmission using second-order Raman amplification and fibre Bragg gratings," Opt. Express 12, 4372-4377 (2004) http://www.opticsinfobase.org/abstract.cfm?URI=oe-12-19-4372 [CrossRef] [PubMed]
  9. T.J. Ellingham, J.D. Ania-Castañón, R. Ibbotson, X. Chen, L. Zhang, SergeiK. Turitsyn, "Quasi-lossless optical links for broad-band transmission and data processing," IEEE Photon. Technol. Lett. 18, 268-270 (2006) [CrossRef]
  10. J.D. Ania-Castañón, T.J. Ellingham, R. Ibbotson, X. Chen, L. Zhang, SergeiK. Turitsyn, "Ultralong raman fiber lasers as virtually lossless optical media," Phys. Rev. Lett. 96, 023902 (2006) [CrossRef] [PubMed]
  11. S.A. Babin, V. Karalekas, P. Harper, E.V. Podivilov, V.K. Mezentsev, J.D. Ania-Castañón and S.K. Turitsyn, "Experimental demonstration of mode structure in ultralong Raman fiber lasers," Opt. Lett. 32, 1135-1137 (2007) [CrossRef] [PubMed]
  12. S.A. Babin, D.V. Churkin, A.E. Ismagulov, S.I. Kablukov, E.V. Podivilov, "FWM-induced turbulent spectral broadening in a long Raman fiber laser," J. Opt. Soc. Am. B 24, 1729-1738 (2007) [CrossRef]
  13. J.D. Ania-Castañón, V. Karalekas, P. Harper and S.K. Turitsyn, "Simultaneous spatial and spectral transparency using ultra-long fibre laser transmission", in Proceedings of the European Conference on Optical Communications 2007, paper 1.4.3, Berlin, Germany, 2007

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