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Optics Letters

| RAPID, SHORT PUBLICATIONS ON THE LATEST IN OPTICAL DISCOVERIES

  • Editor: Alan E. Willner
  • Vol. 38, Iss. 13 — Jul. 1, 2013
  • pp: 2283–2285
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530 W, 1.3 mJ, four-channel coherently combined femtosecond fiber chirped-pulse amplification system

Arno Klenke, Sven Breitkopf, Marco Kienel, Thomas Gottschall, Tino Eidam, Steffen Hädrich, Jan Rothhardt, Jens Limpert, and Andreas Tünnermann  »View Author Affiliations


Optics Letters, Vol. 38, Issue 13, pp. 2283-2285 (2013)
http://dx.doi.org/10.1364/OL.38.002283


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Abstract

We report on a femtosecond fiber laser system comprising four coherently combined large-pitch fibers as the main amplifier. With this system, a pulse energy of 1.3 mJ and a peak power of 1.8 GW are achieved at 400 kHz repetition rate. The corresponding average output power is as high as 530 W. Additionally, an excellent beam quality and efficiency of the combination have been obtained. To the best of our knowledge, such a parameter combination, i.e., gigawatt pulses with half a kilowatt average power, has not been demonstrated so far with any other laser architecture.

© 2013 Optical Society of America

In recent years, significant progress has been made regarding scaling of the performance of femtosecond fiber systems. However, there are issues that currently limit both the achievable peak and the average power of a linear amplifier chain. For example, the onset of mode instabilities [1

1. T. Eidam, C. Wirth, C. Jauregui, F. Stutzki, F. Jansen, H. Otto, O. Schmidt, T. Schreiber, J. Limpert, and A. Tünnermann, Opt. Express 19, 13218 (2011). [CrossRef]

] sets an upper limit to the average power that can be obtained from a single fiber amplifier. Typically, the power threshold of this effect lies in the range of some hundreds of watts up to the kilowatt level, strongly depending on the properties of the fiber and other system parameters. Furthermore, the pulse energy is ultimately limited by the extractable energy of the fiber, nonlinear pulse distortions, and damage issues [2

2. L. Zenteno, J. Lightwave Technol. 11, 1435 (1993). [CrossRef]

].

Different theoretical models have been investigated in order to understand the physical origin of mode instabilities [3

3. C. Jauregui, T. Eidam, H. Otto, F. Stutzki, F. Jansen, J. Limpert, and A. Tünnermann, Opt. Express 20, 12912 (2012). [CrossRef]

5

5. A. Smith and J. Smith, Opt. Express 19, 10180 (2011). [CrossRef]

], and advanced fiber designs, such as large-pitch fiber (LPF) [6

6. J. Limpert, F. Stutzki, F. Jansen, H.-J. Otto, T. Eidam, C. Jauregui, and A. Tünnermann, Light Sci. Appl. 1, e8 (2012). [CrossRef]

], chirally coupled core (CCC) fiber [7

7. H.-W. Chen, T. Sosnowski, C.-H. Liu, L.-J. Chen, J. R. Birge, A. Galvanauskas, F. X. Kärtner, and G. Chang, Opt. Express 18, 24699 (2010). [CrossRef]

], and distributed mode filtering (DMF) fiber [8

8. T. T. Alkeskjold, M. Laurila, L. Scolari, and J. Broeng, Opt. Express 19, 7398 (2011). [CrossRef]

], have been developed to increase the average power threshold. In order to increase the achievable pulse energies, spatial and temporal scaling techniques are employed to reduce the peak intensity inside the fiber. Examples of those techniques include the chirped-pulse amplification (CPA) concept [9

9. D. Strickland and G. Mourou, Opt. Commun. 56, 219 (1985). [CrossRef]

] and the use of large-mode-area fibers. Furthermore, nonlinear pulse distortions can be either reduced by employing circular polarization [10

10. D. Schimpf, T. Eidam, E. Seise, S. Hädrich, J. Limpert, and A. Tünnermann, Opt. Express 17, 18774 (2009). [CrossRef]

] or compensated by an active shaping of the spectral phase [11

11. V. V. Lozovoy, I. Pastirk, and M. Dantus, Opt. Lett. 29, 775 (2004). [CrossRef]

]. With these techniques, in systems with a single main amplifier, femtosecond pulses have been demonstrated at average powers of up to 830 W [12

12. T. Eidam, S. Hanf, E. Seise, T. V. Andersen, T. Gabler, C. Wirth, T. Schreiber, J. Limpert, and A. Tünnermann, Opt. Lett. 35, 94 (2010). [CrossRef]

] and pulse energies of up to 2.2 mJ [13

13. T. Eidam, J. Rothhardt, F. Stutzki, F. Jansen, S. Hädrich, H. Carstens, C. Jauregui, J. Limpert, and A. Tünnermann, Opt. Express 19, 255 (2011). [CrossRef]

].

Despite the vast progress that has been made in scaling the average output power and the pulse energy, the performance of a single laser amplifier (independently of its architecture) will always be limited. Therefore, in spite of all the progress, the ultimate performance of a single laser amplifier might still not be sufficient to reach the desired laser parameters required for high-intensity physics at high repetition rates [14

14. U. Keller, IEEE Photon. J. 2, 225 (2010). [CrossRef]

,15

15. S. Hädrich, M. Krebs, J. Rothhardt, H. Carstens, S. Demmler, J. Limpert, and A. Tünnermann, Opt. Express 19, 19374 (2011). [CrossRef]

]. Hence, additional approaches for scaling of the performance should be considered. One example is the coherent combination of spatially separated amplifiers. In this concept, pulses emitted from a preamplifier are split into a number of channels; they are amplified in each channel independently and then, finally, a recombination of the pulses into one takes place. With this approach, an improvement of the average power and pulse energy by a factor equaling the total number of channels is possible in the ideal case. While coherent combination is especially suitable for fiber lasers due to their very high and reproducible beam quality, it might also be adapted to other laser architectures, such as innoslab [16

16. P. Russbueldt, T. Mans, J. Weitenberg, H. Hoffmann, and R. Poprawe, Opt. Lett. 35, 4169 (2010). [CrossRef]

] or thin disks [17

17. C. Saraceno, F. Emaury, O. Heckl, C. Baer, M. Hoffmann, C. Schriber, M. Golling, T. Südmeyer, and U. Keller, Opt. Express 20, 23535 (2012). [CrossRef]

].

This concept has been applied to systems working in continuous-wave operation for some time [18

18. R. Uberna, A. Bratcher, and B. Tiemann, IEEE J. Quantum Electron. 46, 1191 (2010). [CrossRef]

,19

19. C. Yu, S. Augst, S. Redmond, K. Goldizen, D. Murphy, A. Sanchez, and T. Fan, Opt. Lett. 36, 2686 (2011). [CrossRef]

], but it was only quite recently applied to the ultrashort-pulse regime [20

20. E. Seise, A. Klenke, J. Limpert, and A. Tünnermann, Opt. Express 18, 27827 (2010). [CrossRef]

,21

21. L. Daniault, M. Hanna, L. Lombard, Y. Zaouter, E. Mottay, D. Goular, P. Bourdon, F. Druon, and P. Georges, Opt. Lett. 36, 621 (2011). [CrossRef]

]. So far, by combining two rod-type fibers, pulse energies and, therewith, peak powers exceeding the single-amplifier limit can already be demonstrated [22

22. A. Klenke, E. Seise, S. Demmler, J. Rothhardt, S. Breitkopf, J. Limpert, and A. Tünnermann, Opt. Express 19, 24280 (2011). [CrossRef]

]. The usability of the coherent combination approach depends strongly on the achievable combination efficiency, especially when the number of channels becomes large. This efficiency is determined by the quality of the beam overlap, as well as by the spectral phase and amplitude differences of the pulses, which can be caused by dissimilar nonlinear effects and dispersion in the different channels. However, according to theoretical investigations with realistic assumptions of these differences, it should be possible to increase the number of channels with only a modest drop in total efficiency [23

23. A. Klenke, E. Seise, J. Limpert, and A. Tünnermann, Opt. Express 19, 25379 (2011). [CrossRef]

,24

24. G. Goodno, C. Shih, and J. Rothenberg, Opt. Express 18, 25403 (2010). [CrossRef]

]. Furthermore, the first experiment regarding the combination of four single-mode fibers at low power has recently been demonstrated [25

25. L. Siiman, W. Chang, T. Zhou, and A. Galvanauskas, Opt. Express 20, 18097 (2012). [CrossRef]

]. However, in the experiment described herein, we increase the number of channels from two to four and demonstrate a combination of very high average powers and pulse energies to show the viability of the coherent combination concept at these parameters. A schematic picture of the experimental setup is shown in Fig. 1. A mode-locked solid-state oscillator generates femtosecond pulses with a repetition rate of 40 MHz, a bandwidth of 6 nm, and an average power of 150 mW. These pulses are stretched in a grating stretcher to about 2 ns. The stretcher uses 1740lines/mm gratings and imposes a spectral hard cut of 7 nm. The system includes three preamplifiers, whereby the last two are based on an LPF design [6

6. J. Limpert, F. Stutzki, F. Jansen, H.-J. Otto, T. Eidam, C. Jauregui, and A. Tünnermann, Light Sci. Appl. 1, e8 (2012). [CrossRef]

]. The output of the preamplifiers provides sufficient pulse energy and average power to seed all four main amplifiers. In our experiment, the seed power for every main amplifier is about 1 W. Additionally, a phase shaper is included in the setup to compensate for spectral phase distortions that may occur due to nonlinearities or uncompensated higher-order dispersion. This way, the phase shaper improves the pulse quality at the output of the system. Two acousto-optic modulators (AOMs) are used to reduce the repetition rate to the desired value. The main-amplification stage comprises four LPF fibers in a parallel configuration with a mode-field diameter of 59 μm and a length of 1.2 m each. These fibers are especially suitable for coherent combination due to the high stability and quality of the output beam at high average powers. Waveplates are placed in front of each fiber in order to achieve the required polarization direction after amplification. The splitting of the seed beam is realized with polarization-dependent beamsplitter cubes in a cascaded setup. A delay line (Δφ in Fig. 1) consisting of a piezo-mounted mirror is inserted at the output of one of the ports of each cube in order to actively stabilize the path lengths of the different channels and to compensate for fluctuations due to air movement, vibrations, and thermal changes. On the other hand, combination of the output beams of the amplifiers can no longer be realized with polarization-dependent beamsplitter cubes due to the very high average powers causing thermal lensing on these elements. The issue is not inherent to coherent combination, but occurs while handling very high average powers. To mitigate this problem, we employed thin-film polarizers (TFPs) and other low absorbing components, i.e., specifically selected lenses. Behind every TFP, an antireflection coated laser window is inserted. This way, a small reflected fraction of the beam is routed to a Hänsch–Couillaud detector [26

26. T. W. Hänsch and B. Couillaud, Opt. Commun. 35, 441 (1980). [CrossRef]

], which can detect the polarization state of the combined beam. This detector is then connected to the corresponding delay line using an electronic control device. The regulator in this device stabilizes the linear polarization state of every combined laser beam at every combination step. This allows for further combination steps and for the compression of the final beam with polarization-dependent dielectric gratings. The advantage of this stabilization technique is that no phase modulation is necessary to calculate an error signal for the piezo-electric actuators. The total stabilization system has a bandwidth close to 1 kHz, which turned out to be more than enough to stabilize almost all typical fluctuations present in such a system [27

27. E. Seise, A. Klenke, S. Breitkopf, J. Limpert, and A. Tünnermann, Opt. Lett. 36, 3858 (2011). [CrossRef]

]. In the referenced publication, the short- and long-term stability of a coherently combined laser system were thoroughly investigated. Because we use a stabilization scheme based on the same technique, a similar behavior is expected for this system. After combination, the pulses are recompressed in a grating compressor that shares one of its gratings with the stretcher. The compressor efficiency was measured to be around 80%.

Fig. 1. Schematic overview of the experimental setup.

Fig. 2. Autocorrelation trace of the combined pulse at an average power of 530 W.
Fig. 3. Spectrum of the combined beam with a band width of 2.7 nm.
Fig. 4. Caustic of the beam for the M2 measurement at 530 W.

In conclusion, we have demonstrated a femtosecond fiber CPA system consisting of four coherently combined amplifiers. It delivers an average power of 530 W and pulse energies of 1.3 mJ. With the aid of coherent combination, it is possible to achieve system parameters that are currently not achievable with a serial amplifier system. These experiments demonstrated an excellent beam quality and a very high combination efficiency of the total system of 93%. Additionally, we have already seen that the system can be used together with hollow-core compression as a driver for high-harmonic generation. Depending on the fiber type, we expect to be able to improve the performance of this system even further. We think that with the coherent combination concept and further progress in fiber laser technology, average powers in the range of 1 kW and pulse energies of 10 mJ are realistic parameters in the future.

This work has been partly supported by the German Federal Ministry of Education and Research (BMBF) under contract 13N12082 “NEXUS” and the European Research Council under the European Union’s Seventh Framework Program (FP7/2007-2013)/ERC grant agreement no. 240460 “PECS”. A. K. and M. K. acknowledge financial support by the Helmholtz-Institute Jena.

References

1.

T. Eidam, C. Wirth, C. Jauregui, F. Stutzki, F. Jansen, H. Otto, O. Schmidt, T. Schreiber, J. Limpert, and A. Tünnermann, Opt. Express 19, 13218 (2011). [CrossRef]

2.

L. Zenteno, J. Lightwave Technol. 11, 1435 (1993). [CrossRef]

3.

C. Jauregui, T. Eidam, H. Otto, F. Stutzki, F. Jansen, J. Limpert, and A. Tünnermann, Opt. Express 20, 12912 (2012). [CrossRef]

4.

B. Ward, C. Robin, and I. Dajani, Opt. Express 20, 11407 (2012). [CrossRef]

5.

A. Smith and J. Smith, Opt. Express 19, 10180 (2011). [CrossRef]

6.

J. Limpert, F. Stutzki, F. Jansen, H.-J. Otto, T. Eidam, C. Jauregui, and A. Tünnermann, Light Sci. Appl. 1, e8 (2012). [CrossRef]

7.

H.-W. Chen, T. Sosnowski, C.-H. Liu, L.-J. Chen, J. R. Birge, A. Galvanauskas, F. X. Kärtner, and G. Chang, Opt. Express 18, 24699 (2010). [CrossRef]

8.

T. T. Alkeskjold, M. Laurila, L. Scolari, and J. Broeng, Opt. Express 19, 7398 (2011). [CrossRef]

9.

D. Strickland and G. Mourou, Opt. Commun. 56, 219 (1985). [CrossRef]

10.

D. Schimpf, T. Eidam, E. Seise, S. Hädrich, J. Limpert, and A. Tünnermann, Opt. Express 17, 18774 (2009). [CrossRef]

11.

V. V. Lozovoy, I. Pastirk, and M. Dantus, Opt. Lett. 29, 775 (2004). [CrossRef]

12.

T. Eidam, S. Hanf, E. Seise, T. V. Andersen, T. Gabler, C. Wirth, T. Schreiber, J. Limpert, and A. Tünnermann, Opt. Lett. 35, 94 (2010). [CrossRef]

13.

T. Eidam, J. Rothhardt, F. Stutzki, F. Jansen, S. Hädrich, H. Carstens, C. Jauregui, J. Limpert, and A. Tünnermann, Opt. Express 19, 255 (2011). [CrossRef]

14.

U. Keller, IEEE Photon. J. 2, 225 (2010). [CrossRef]

15.

S. Hädrich, M. Krebs, J. Rothhardt, H. Carstens, S. Demmler, J. Limpert, and A. Tünnermann, Opt. Express 19, 19374 (2011). [CrossRef]

16.

P. Russbueldt, T. Mans, J. Weitenberg, H. Hoffmann, and R. Poprawe, Opt. Lett. 35, 4169 (2010). [CrossRef]

17.

C. Saraceno, F. Emaury, O. Heckl, C. Baer, M. Hoffmann, C. Schriber, M. Golling, T. Südmeyer, and U. Keller, Opt. Express 20, 23535 (2012). [CrossRef]

18.

R. Uberna, A. Bratcher, and B. Tiemann, IEEE J. Quantum Electron. 46, 1191 (2010). [CrossRef]

19.

C. Yu, S. Augst, S. Redmond, K. Goldizen, D. Murphy, A. Sanchez, and T. Fan, Opt. Lett. 36, 2686 (2011). [CrossRef]

20.

E. Seise, A. Klenke, J. Limpert, and A. Tünnermann, Opt. Express 18, 27827 (2010). [CrossRef]

21.

L. Daniault, M. Hanna, L. Lombard, Y. Zaouter, E. Mottay, D. Goular, P. Bourdon, F. Druon, and P. Georges, Opt. Lett. 36, 621 (2011). [CrossRef]

22.

A. Klenke, E. Seise, S. Demmler, J. Rothhardt, S. Breitkopf, J. Limpert, and A. Tünnermann, Opt. Express 19, 24280 (2011). [CrossRef]

23.

A. Klenke, E. Seise, J. Limpert, and A. Tünnermann, Opt. Express 19, 25379 (2011). [CrossRef]

24.

G. Goodno, C. Shih, and J. Rothenberg, Opt. Express 18, 25403 (2010). [CrossRef]

25.

L. Siiman, W. Chang, T. Zhou, and A. Galvanauskas, Opt. Express 20, 18097 (2012). [CrossRef]

26.

T. W. Hänsch and B. Couillaud, Opt. Commun. 35, 441 (1980). [CrossRef]

27.

E. Seise, A. Klenke, S. Breitkopf, J. Limpert, and A. Tünnermann, Opt. Lett. 36, 3858 (2011). [CrossRef]

OCIS Codes
(060.2320) Fiber optics and optical communications : Fiber optics amplifiers and oscillators
(140.7090) Lasers and laser optics : Ultrafast lasers
(140.3298) Lasers and laser optics : Laser beam combining

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: April 5, 2013
Revised Manuscript: May 24, 2013
Manuscript Accepted: May 27, 2013
Published: June 25, 2013

Virtual Issues
July 1, 2013 Spotlight on Optics

Citation
Arno Klenke, Sven Breitkopf, Marco Kienel, Thomas Gottschall, Tino Eidam, Steffen Hädrich, Jan Rothhardt, Jens Limpert, and Andreas Tünnermann, "530 W, 1.3 mJ, four-channel coherently combined femtosecond fiber chirped-pulse amplification system," Opt. Lett. 38, 2283-2285 (2013)
http://www.opticsinfobase.org/ol/abstract.cfm?URI=ol-38-13-2283


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References

  1. T. Eidam, C. Wirth, C. Jauregui, F. Stutzki, F. Jansen, H. Otto, O. Schmidt, T. Schreiber, J. Limpert, and A. Tünnermann, Opt. Express 19, 13218 (2011). [CrossRef]
  2. L. Zenteno, J. Lightwave Technol. 11, 1435 (1993). [CrossRef]
  3. C. Jauregui, T. Eidam, H. Otto, F. Stutzki, F. Jansen, J. Limpert, and A. Tünnermann, Opt. Express 20, 12912 (2012). [CrossRef]
  4. B. Ward, C. Robin, and I. Dajani, Opt. Express 20, 11407 (2012). [CrossRef]
  5. A. Smith and J. Smith, Opt. Express 19, 10180 (2011). [CrossRef]
  6. J. Limpert, F. Stutzki, F. Jansen, H.-J. Otto, T. Eidam, C. Jauregui, and A. Tünnermann, Light Sci. Appl. 1, e8 (2012). [CrossRef]
  7. H.-W. Chen, T. Sosnowski, C.-H. Liu, L.-J. Chen, J. R. Birge, A. Galvanauskas, F. X. Kärtner, and G. Chang, Opt. Express 18, 24699 (2010). [CrossRef]
  8. T. T. Alkeskjold, M. Laurila, L. Scolari, and J. Broeng, Opt. Express 19, 7398 (2011). [CrossRef]
  9. D. Strickland and G. Mourou, Opt. Commun. 56, 219 (1985). [CrossRef]
  10. D. Schimpf, T. Eidam, E. Seise, S. Hädrich, J. Limpert, and A. Tünnermann, Opt. Express 17, 18774 (2009). [CrossRef]
  11. V. V. Lozovoy, I. Pastirk, and M. Dantus, Opt. Lett. 29, 775 (2004). [CrossRef]
  12. T. Eidam, S. Hanf, E. Seise, T. V. Andersen, T. Gabler, C. Wirth, T. Schreiber, J. Limpert, and A. Tünnermann, Opt. Lett. 35, 94 (2010). [CrossRef]
  13. T. Eidam, J. Rothhardt, F. Stutzki, F. Jansen, S. Hädrich, H. Carstens, C. Jauregui, J. Limpert, and A. Tünnermann, Opt. Express 19, 255 (2011). [CrossRef]
  14. U. Keller, IEEE Photon. J. 2, 225 (2010). [CrossRef]
  15. S. Hädrich, M. Krebs, J. Rothhardt, H. Carstens, S. Demmler, J. Limpert, and A. Tünnermann, Opt. Express 19, 19374 (2011). [CrossRef]
  16. P. Russbueldt, T. Mans, J. Weitenberg, H. Hoffmann, and R. Poprawe, Opt. Lett. 35, 4169 (2010). [CrossRef]
  17. C. Saraceno, F. Emaury, O. Heckl, C. Baer, M. Hoffmann, C. Schriber, M. Golling, T. Südmeyer, and U. Keller, Opt. Express 20, 23535 (2012). [CrossRef]
  18. R. Uberna, A. Bratcher, and B. Tiemann, IEEE J. Quantum Electron. 46, 1191 (2010). [CrossRef]
  19. C. Yu, S. Augst, S. Redmond, K. Goldizen, D. Murphy, A. Sanchez, and T. Fan, Opt. Lett. 36, 2686 (2011). [CrossRef]
  20. E. Seise, A. Klenke, J. Limpert, and A. Tünnermann, Opt. Express 18, 27827 (2010). [CrossRef]
  21. L. Daniault, M. Hanna, L. Lombard, Y. Zaouter, E. Mottay, D. Goular, P. Bourdon, F. Druon, and P. Georges, Opt. Lett. 36, 621 (2011). [CrossRef]
  22. A. Klenke, E. Seise, S. Demmler, J. Rothhardt, S. Breitkopf, J. Limpert, and A. Tünnermann, Opt. Express 19, 24280 (2011). [CrossRef]
  23. A. Klenke, E. Seise, J. Limpert, and A. Tünnermann, Opt. Express 19, 25379 (2011). [CrossRef]
  24. G. Goodno, C. Shih, and J. Rothenberg, Opt. Express 18, 25403 (2010). [CrossRef]
  25. L. Siiman, W. Chang, T. Zhou, and A. Galvanauskas, Opt. Express 20, 18097 (2012). [CrossRef]
  26. T. W. Hänsch and B. Couillaud, Opt. Commun. 35, 441 (1980). [CrossRef]
  27. E. Seise, A. Klenke, S. Breitkopf, J. Limpert, and A. Tünnermann, Opt. Lett. 36, 3858 (2011). [CrossRef]

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