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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 17, Iss. 15 — Jul. 20, 2009
  • pp: 12785–12793
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Cavity dispersion management in continuous-wave supercontinuum generation

Sonia Martin-Lopez, Pedro Corredera, and Miguel Gonzalez-Herraez  »View Author Affiliations


Optics Express, Vol. 17, Issue 15, pp. 12785-12793 (2009)
http://dx.doi.org/10.1364/OE.17.012785


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Abstract

Supercontinuum generation using continuous-wave pumping is usually obtained by pumping a suitable fiber with a high-power fiber laser. Whereas many studies have concentrated in optimizing the dispersion characteristics of the nonlinear medium (the fiber) used to obtain the spectral broadening, very few have actually concentrated in optimizing the pump laser characteristics, and in particular, the dispersion in the cavity. In this paper we experimentally demonstrate that the fiber laser cavity dispersion has a strong influence in Raman fiber laser-pumped continuous-wave supercontinuum generation. We show that anomalous dispersion in the cavity favors spectral broadening over normal dispersion, since large, high-contrast intensity noise appears at the output of the laser. Additionally, we find that there is an optimum value of chromatic dispersion coefficient to obtain the most efficient broadening.

© 2009 Optical Society of America

1. Introduction

Continuous-wave supercontinuum (CW-SC) generation in optical fibers has attracted much attention in the past few years for the possibility of developing compact, high-quality sources for ultrahigh resolution optical coherence tomography [1

1. Pei-Lin Hsiung, Yu Chen, Tony Ko, J. Fujimoto, C. J. S. de Matos, S. Popov, J. R. 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/oe/abstract.cfm?URI=oe-12-22-5287 [CrossRef] [PubMed]

]. Among their good properties, these sources exhibit very low coherence lengths (allowing resolutions of only several micrometers), high-power spectral densities (normally in the order of several milliwatts per nanometer), and lower values of relative intensity noise than their pulsed counterparts. Additionally, nonlinear pump spectral broadening of continuous-wave (CW) beams has been demonstrated as an effective tool to develop spectrally flattened Raman amplifiers[2

2. S. Martin-Lopez, M. Gonzalez-Herraez, P. Corredera, M.L. Hernanz, and A. Carrasco, “‘Gain-flattening of fiber Raman amplifiers using non-linear pump spectral broadening,” Opt. Commun. , 242, 463–469 (2004) [CrossRef]

].

The seed that starts the broadening process of the CW beam is the incoherence of the source used as the pump [3

3. F. Vanholsbeeck, S. Martin-Lopez, M. Gonzalez-Herraez, and S. Coen, “The role of pump incoherence in continuous-wave supercontinuum generation,” Opt. Express 13, 6615–6625 (2005) http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-13-17-6615 [CrossRef] [PubMed]

, 4

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

]. The quasi-CW input beam develops into a train of subpicosecond pulses induced by the modulation instability (MI). These subpicosecond pulses lead to the formation of optical solitons with inherently random parameters, which self-frequency shift differently depending on their characteristics. The resulting supercontinuum spectrum is hence the average of many different soliton spectra, which have suffered different frequency shifts.

In this paper we investigate experimentally the dependence of the supercontinuum spectrum on the dispersion in the fiber laser cavity. We show that the cavity dispersion plays potentially a key role in enhancing CW-SC development. In particular, we show that having anomalous dispersion in the cavity favors the appeareance of fast intensity instabilities with high contrast, which in turn favor the broadening process. Additionally, we show that there is an optimum value of anomalous dispersion coefficient that yields the most efficient broadening.

2. Experimental setup

The experimental setup used in this work is depicted in Fig. 1. The setup is basically comprised of a home-made Raman fiber laser and a dispersion-shifted fiber that acts as the nonlinear medium. The output of both the laser and the supercontinuum are characterized temporally by means of an autocorrelator and spectrally by means of an optical spectrum analyzer (OSA). The power of the laser and the supercontinuum is also monitored in all the cases by means of an integrating sphere radiometer.

Fig. 1. Experimental setup for the SC generation in a 11 km of DSF fiber, pumped by a Raman Cavity Laser. RFL: Raman fiber laser, WDM: wavelength division multiplexer (port R: 1450–1480 nm, port P: 1528–1563 nm, port C: all wavelengths), PC: polarization control, OSA: optical spectrum analizer.

A typical design of a Raman fiber laser consists on a pump laser and a nested linear cavity of fiber Bragg gratings [9

9. R. K. Jain, C. Lin, R. H. Stolen, and A. Ashkin, “A tunable multiple Stokes cw fiber Raman oscillator,” Appl. Phys. Lett. 31, 89 (1977) [CrossRef]

, 10

10. G. P. Agrawal “Nonlinear Fiber Optics” (Academic Press, 1995), Chap. 5.

, 11

11. E. M. Dianov, I. A Bufetov, V. M. Mashinsky, A. V. Shubin, O. I. Medvedkov, A. E. Rakitin, M. A. Melkumov, V. F. Khopin, and A. N. Guryanov “Raman fibre lasers based on heavily GeO2-doped fibres,” Quantum Electron. 35435–441 (2005) [CrossRef]

]. In our case, the design of the cavity is slightly modified to optimize the laser efficiency. One end of the cavity consists on a loop mirror designed with a 50/50 optical coupler and a polarization controller. Due to the short length of the fiber coupler arms (less than 1 m) and the identical coupler relation, this mirror works as a perfect mirror with 100%reflectivity. In the other end of the cavity we introduce a conventional fiber Bragg grating. The grating has the maximum reflectivity at 1554.28 nm, with a spectral width at half maximum of ~1 nm. The maximum reflectivity of the grating is 80%. Between the fiber Bragg grating and the fiber loop we introduce some kilometers optical fiber which act as the Raman gain medium. The fiber in the cavity is changed in the different tests, but the rest of the experimental setup remains the same.

To pump this cavity we use a commercial continuous-wave Raman fiber laser, with the center wavelength at 1455 nm and a maximum output power of 2.4 W. The spectral width at 20 dB from the peak is ~1 nm. This laser is fed into the cavity through an optical circulator. The value of relative intensity noise (RIN) of the laser is -110 dBc/Hz.

To generate the supercontinuum, we use 11 km of dispersion-shifted fiber after the Raman laser. The zero-dispersion wavelength of this fiber is at 1553.2 nm, and the dispersion slope is 0.056 ps/nm/km. Since the laser emission is centered in 1555 nm, the propagation is, in all cases, in the region of small anomalous dispersion of the fiber.

3. Results

3.1. Normal or anomalous dispersion regime in the cavity

First of all we choose two different fibers (F1 and F2) which are tested in the Raman laser cavity. Their characteristics appear in table 1.

Fibers F1 and F2 are chosen so as to have the same product γLeff (within the uncertainty of our measurement of γ). This ensures that (1) the nonlinear phase shift is equal for the same input pump powers (2) the threshold is very similar for both fibers and (3) very similar values of PStokes are obtained for the same input pump power. The main difference between them is that they exhibit a different dispersion regime at the lasing wavelength (1555 nm). F1 exhibits anomalous dispersion at 1555 nm while F2 shows normal dispersion. In Figs. 2(a) and (b) we depict the spectra at cavity output for these two fibers. In Figs. 2(c) and (d) we depict the

Table 1. Characteristics of the different fibers used in the experiments. D is the dispersion coefficient at the lasing wavelength

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autocorrelation traces at the cavity output for these two fibers. The insets 2(c) and (d) are a representation of the typical intensity output obtained for these two cases. These traces have been acquired with a digital oscilloscope with 2.5 GHz bandwidth and a fast InGaAs detector with a bandwidth of 1.5 GHz. The output power of the cavity is around 400 mW for all the cases.

Fig. 2. Spectra (a) and autocorrelation traces (b) of the output of the Raman cavity for Fiber F1 and Fiber F2.

Fig. 3. Output spectra of the supercontinuum generated by pumping 11 km of DSF with the output of the Raman cavities built with Fibers F1 and F2.

Considering that fast, high-contrast intensity oscillations appear at the output of the laser for F1 and not for F2, we should expect that the supercontinuum generated with F1 in the cavity would be much broader than with F2 in the cavity for the same laser power. The results of supercontinuum generation with F1 and F2 in the cavity are plotted in Fig. 3, both for 400 mW of Stokes power and 2.1 W of pump power. As expected, the supercontinuum obtained with F1 in the cavity is much broader, both on the red and blue-shifted parts. We may distinguish which is the main driving process behind the supercontinuum by estimating the soliton order of the pulses injected in the fiber. Considering a peak power of the intensity oscillations of 800 mW (twice the mean, consistent with the autocorrelation trace), and the duration of the pulses estimated from the autocorrelation trace, we can conclude that we are injecting in the fiber pulses with soliton order well over 1, and therefore soliton fission is the main motor of the supercontinuum in this case. On the contrary, for F2 the main driving force must be plain MI, since the intensity noise is not of high-contrast.

Fig. 4. Spectra (a) and autocorrelation traces (b) of the output of the Raman cavity for Fiber F1 and Fiber DCF.

3.2. Optimization of the dispersion value

It seems clear that the efficiency in SC generation is considerably higher when the cavity dispersion is anomalous. This is due to the appearance of intensity oscillations in the laser output. There is a second question to solve now, which is whether we can optimize the value of the cavity dispersion so as to maximize the broadening efficiency. The reason is clear: by varying the cavity dispersion in the anomalous dispersion regime we can modify the parameters of the output intensity oscillations. If the dispersion coefficient is small, the cavity should deliver shorter oscillations with higher mean frequency, while for larger dispersion, the oscillations should be longer and with lower mean frequency.

To see what values of dispersion make the optimum broadening, we perform the same super-continuum experiment with Fibers F3, F4 and F5 in the cavity. The characteristics are shown in table 2.

Table 2. Characteristics of the different fibers used in the experiments. D is the dispersion coefficient at the lasing wavelength

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Again, they are chosen so as to have roughly the same product γLeff and different dispersion coefficients at 1550 nm. For the experiments with these three fibers the output power of the cavity is ~400 mW too. The spectra at the laser output are shown in Fig. 5.

Fig. 5. Output spectra of the fiber laser with fibers F3, F4 and F5 in the cavity

Fig. 6. Autocorrelation traces of the fiber laser with fibers F3 (trace a), F4 (trace b) and F5 (trace c) in the cavity.
Fig. 7. Output spectra of the supercontina generated pumping 11 km of DSF with fibers F3, F4 and F5 in the Raman fiber laser cavity

To get some more insight into this result, we measure the autocorrelation traces of the super-continua obtained with F3, F4 and F5 in the cavity. The temporal widths at full width at half maximum (FWHM) obtained with F3, F4 and F5 in the cavity are 176 fs, 164 fs and 288 fs respectively. As we can see, the shortest feature corresponds to the wider supercontinuum, but this shortest feature does not correspond to the shortest trace at the fiber input.

The results can be explained by the CW supercontinuum dynamics [3

3. F. Vanholsbeeck, S. Martin-Lopez, M. Gonzalez-Herraez, and S. Coen, “The role of pump incoherence in continuous-wave supercontinuum generation,” Opt. Express 13, 6615–6625 (2005) http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-13-17-6615 [CrossRef] [PubMed]

, 4

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

]. When the light signal from a partially coherent beam propagates in an optical fiber in the anomalous dispersion regime, the intensity instabilities of the pump lead (normally through modulation instability) to soliton fission if they are energetic enough. However, if they are not powerful enough, the instabilities simply suffer dispersion. In our case, the intensity fluctuations at the output of the lasers built with F3 and F4 in the cavity are relatively long (several ps), but energetic enough to lead to soliton fission. For Fiber F5, the laser fluctuations are shorter and the peak power is the same, and therefore the fluctuations are less energetic, so they cannot lead to further soliton fission and pulse compression in the supercontinuum fiber. Thus, in this case, the intensity fluctuations are simply broadened because of dispersion at the fiber output, while they were strongly shortened for the cases of F3 and F4.

Qualitatively speaking, the rule to optimize the cavity dispersion seems to be roughly the same rule as in conventional dispersion management for supercontinuum generation [5

5. J. Nathan Kutz, C. Lynga, and B. Eggleton, “Enhanced supercontinuum generation through dispersion-management,” Opt. Express 13, 3989–3998 (2005) http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-13-11-3989 [CrossRef] [PubMed]

]: overall, the dispersion should be decreasing all along the propagation (including the cavity), and the dispersion steps should not be too high. This should ease soliton fission and also favor pulse compression along the fiber.

4. Conclusion

In conclusion, we have shown the importance of the cavity dispersion in continuous-wave Raman fiber laser-pumped supercontinuum generation. It is shown that anomalous dispersion in the cavity favors the broadening over normal dispersion, since the intensity oscillations at the output of the laser are considerably larger. Additionally, we find that there is an optimum dispersion to obtain the most efficient broadening.

Acknowledgements

We acknowledge financial support from theMinisterio de Educacion y Ciencia through projects TEC2006-09990-C02-01 and TEC2006-09990-C02-02, the support from CSIC through project MeDIOMURO and the I3P Post-Doc program, and the support from the Comunidad Autonoma de Madrid through the projects FUTURSEN S-0505/AMB/000374 and FACTOTEM S- 505/ESP/000417. We also acknowledge fruitful discussions with Dr. Arnaud Mussot (University of Lille) in the framework of COST Action 299.

References and links

1.

Pei-Lin Hsiung, Yu Chen, Tony Ko, J. Fujimoto, C. J. S. de Matos, S. Popov, J. R. 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/oe/abstract.cfm?URI=oe-12-22-5287 [CrossRef] [PubMed]

2.

S. Martin-Lopez, M. Gonzalez-Herraez, P. Corredera, M.L. Hernanz, and A. Carrasco, “‘Gain-flattening of fiber Raman amplifiers using non-linear pump spectral broadening,” Opt. Commun. , 242, 463–469 (2004) [CrossRef]

3.

F. Vanholsbeeck, S. Martin-Lopez, M. Gonzalez-Herraez, and S. Coen, “The role of pump incoherence in continuous-wave supercontinuum generation,” Opt. Express 13, 6615–6625 (2005) http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-13-17-6615 [CrossRef] [PubMed]

4.

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

5.

J. Nathan Kutz, C. Lynga, and B. Eggleton, “Enhanced supercontinuum generation through dispersion-management,” Opt. Express 13, 3989–3998 (2005) http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-13-11-3989 [CrossRef] [PubMed]

6.

T. Sylvestre, A. Vedadi, H. Maillotte, F. Vanholsbeeck, and S. Coen, “Supercontinuum generation using continuous-wave multiwavelength pumping and dispersion management,” Opt. Lett. 31, 2036–2038 (2006) http://www.opticsinfobase.org/abstract.cfm?URI=ol-31-13-2036 [CrossRef] [PubMed]

7.

L. Abrardi, S. Martin-Lopez, A. Carrasco-Sanz, F. Rodriguez-Barrios, P. Corredera, M. L. Hernanz, and M. Gonzalez-Herraez, “Experimental study on the role of chromatic dispersion in continuous-wave supercontinuum generation,” J. Lightwave Technol. 27, 426–435 (2009) http://www.opticsinfobase.org/abstract.cfm?URI=JLT-27-4-426 [CrossRef]

8.

S. Martin-Lopez, A. Carrasco-Sanz, P. Corredera, L. Abrardi, M. L. Hernanz, and M. Gonzalez-Herraez, “Experimental investigation of the effect of pump incoherence on nonlinear pump spectral broadening and continuous-wave supercontinuum generation,” Opt. Lett. 31, 3477–3479 (2006) http://www.opticsinfobase.org/abstract.cfm?URI=ol-31-23-3477 [CrossRef] [PubMed]

9.

R. K. Jain, C. Lin, R. H. Stolen, and A. Ashkin, “A tunable multiple Stokes cw fiber Raman oscillator,” Appl. Phys. Lett. 31, 89 (1977) [CrossRef]

10.

G. P. Agrawal “Nonlinear Fiber Optics” (Academic Press, 1995), Chap. 5.

11.

E. M. Dianov, I. A Bufetov, V. M. Mashinsky, A. V. Shubin, O. I. Medvedkov, A. E. Rakitin, M. A. Melkumov, V. F. Khopin, and A. N. Guryanov “Raman fibre lasers based on heavily GeO2-doped fibres,” Quantum Electron. 35435–441 (2005) [CrossRef]

12.

S. A. Babin, D. V. Churkin, A. E. Ismagulov, S. I. Kablukov, and E. V. Podivilov, “Four-wave-mixing-induced turbulent spectral broadening in a long Raman fiber laser,” J. Opt. Soc. Am. B 24, 1729–1738 (2007) http://www.opticsinfobase.org/abstract.cfm?URI=josab-24-8-1729 [CrossRef]

13.

J. Schroeder, D. Alasia, T. Sylvestre, and S. Coen, “Dynamics of an ultrahigh-repetition-rate passively mode-locked Raman fiber laser,” J. Opt. Soc. Am. B 25, 1178–1186 (2008) http://www.opticsinfobase.org/abstract.cfm?URI=josab-25-7-1178 [CrossRef]

14.

A. S. Gouveia-Neto, A. S. L. Gomes, J. R. Taylor, B. J. Ainslie, and S. P. Craig, “Cascade Raman soliton fiber ring laser,” Opt. Lett. 12, 927–929 (1987) http://www.opticsinfobase.org/abstract.cfm?URI=ol-12-11-927 [CrossRef] [PubMed]

OCIS Codes
(190.4370) Nonlinear optics : Nonlinear optics, fibers
(190.4380) Nonlinear optics : Nonlinear optics, four-wave mixing
(190.5530) Nonlinear optics : Pulse propagation and temporal solitons
(060.3510) Fiber optics and optical communications : Lasers, fiber

ToC Category:
Nonlinear Optics

History
Original Manuscript: April 14, 2009
Revised Manuscript: June 19, 2009
Manuscript Accepted: June 19, 2009
Published: July 13, 2009

Citation
Sonia Martin-Lopez, Pedro Corredera, and Miguel Gonzalez-Herraez, "Cavity dispersion management in continuous-wave supercontinuum generation," Opt. Express 17, 12785-12793 (2009)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-17-15-12785


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References

  1. Pei-Lin Hsiung, Yu Chen, Tony Ko, J. Fujimoto, C. J. S. deMatos, S. Popov, J. R. 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/oe/abstract.cfm?URI=oe-12-22-5287 [CrossRef] [PubMed]
  2. S. Martin-Lopez, M. Gonzalez-Herraez, P. Corredera, M.L. Hernanz, and A. Carrasco, "Gain-flattening of fiber Raman amplifiers using non-linear pump spectral broadening," Opt. Commun. 242, 463-469 (2004). [CrossRef]
  3. F. Vanholsbeeck, S. Martin-Lopez, M. Gonzalez-Herraez, and S. Coen, "The role of pump incoherence in continuous-wave supercontinuum generation," Opt. Express 13, 6615-6625 (2005). http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-13-17-6615 [CrossRef] [PubMed]
  4. J. M. Dudley, G. Genty, and S. Coen, "Supercontinuum generation in photonic crystal fiber," Rev. Mod. Phys. 78, 1135 (2006) [CrossRef]
  5. J. Nathan Kutz, C. Lynga, and B. Eggleton, "Enhanced supercontinuum generation through dispersion-management," Opt. Express 13, 3989-3998 (2005). http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-13-11-3989 [CrossRef] [PubMed]
  6. T. Sylvestre, A. Vedadi, H. Maillotte, F. Vanholsbeeck, and S. Coen, "Supercontinuum generation using continuous-wave multiwavelength pumping and dispersion management," Opt. Lett. 31, 2036-2038 (2006). http://www.opticsinfobase.org/abstract.cfm?URI=ol-31-13-2036 [CrossRef] [PubMed]
  7. L. Abrardi, S. Martin-Lopez, A. Carrasco-Sanz, F. Rodriguez-Barrios, P. Corredera, M. L. Hernanz, and M. Gonzalez-Herraez, "Experimental study on the role of chromatic dispersion in continuous-wave supercontinuum generation," J. Lightwave Technol. 27, 426-435 (2009). http://www.opticsinfobase.org/abstract.cfm?URI=JLT-27-4-426 [CrossRef]
  8. S. Martin-Lopez, A. Carrasco-Sanz, P. Corredera, L. Abrardi, M. L. Hernanz, and M. Gonzalez-Herraez, "Experimental investigation of the effect of pump incoherence on nonlinear pump spectral broadening and continuous-wave supercontinuum generation," Opt. Lett. 31, 3477-3479 (2006). http://www.opticsinfobase.org/abstract.cfm?URI=ol-31-23-3477 [CrossRef] [PubMed]
  9. R. K. Jain, C. Lin, R. H. Stolen, and A. Ashkin, "A tunable multiple Stokes cw fiber Raman oscillator," Appl. Phys. Lett. 31, 89 (1977) [CrossRef]
  10. G. P. Agrawal "Nonlinear Fiber Optics" (Academic Press, 1995), Chap. 5.
  11. E. M. Dianov, I. A Bufetov, V. M. Mashinsky, A. V. Shubin, O. I. Medvedkov, A. E. Rakitin, M. A. Melkumov, V. F. Khopin, and A. N. Guryanov "Raman fibre lasers based on heavily GeO2-doped fibres," Quantum Electron. 35435-441 (2005) [CrossRef]
  12. S. A. Babin, D. V. Churkin, A. E. Ismagulov, S. I. Kablukov, and E. V. Podivilov, "Four-wave-mixinginduced turbulent spectral broadening in a long Raman fiber laser," J. Opt. Soc. Am. B 24, 1729-1738 (2007). http://www.opticsinfobase.org/abstract.cfm?URI=josab-24-8-1729 [CrossRef]
  13. J. Schroeder, D. Alasia, T. Sylvestre, and S. Coen, "Dynamics of an ultrahigh-repetitionrate passively mode-locked Raman fiber laser," J. Opt. Soc. Am. B 25, 1178-1186 (2008). http://www.opticsinfobase.org/abstract.cfm?URI=josab-25-7-1178 [CrossRef]
  14. A. S. Gouveia-Neto, A. S. L. Gomes, J. R. Taylor, B. J. Ainslie, and S. P. Craig, "Cascade Raman soliton fiber ring laser," Opt. Lett. 12, 927-929 (1987). http://www.opticsinfobase.org/abstract.cfm?URI=ol-12-11-927 [CrossRef] [PubMed]

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