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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 24 — Nov. 21, 2011
  • pp: 24280–24285
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Coherently-combined two channel femtosecond fiber CPA system producing 3 mJ pulse energy

Arno Klenke, Enrico Seise, Stefan Demmler, Jan Rothhardt, Sven Breitkopf, Jens Limpert, and Andreas Tünnermann  »View Author Affiliations


Optics Express, Vol. 19, Issue 24, pp. 24280-24285 (2011)
http://dx.doi.org/10.1364/OE.19.024280


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Abstract

We present a fiber CPA system consisting of two coherently combined fiber amplifiers, which have been arranged in an actively stabilized Mach-Zehnder interferometer. Pulse durations as short as 470 fs and pulse energies of 3 mJ, corresponding to 5.4 GW of peak power, have been achieved at an average power of 30 W.

© 2011 OSA

1. Introduction

In recent years intense laser pulses have found application in various industrial and scientific areas. Significant progress has been made over the last few years in improving the performance, meaning the pulse energy and average power, of laser systems emitting ultrashort pulses at high repetition rates. For this purpose, different amplification architectures such as slab [1

1. P. Russbueldt, T. Mans, J. Weitenberg, H. D. Hoffmann, and R. Poprawe, “Compact diode-pumped 1.1 kW Yb:YAG Innoslab femtosecond amplifier,” Opt. Lett. 35(24), 4169–4171 (2010). [CrossRef] [PubMed]

], thin disk [2

2. C. R. Baer, Ch. Kränkel, C. J. Saraceno, O. H. Heckl, M. Golling, R. Peters, K. Petermann, Th. Südmeyer, G. Huber, and U. Keller, “Femtosecond thin-disk laser with 141 W of average power,” Opt. Lett. 35(13), 2302–2304 (2010). [CrossRef] [PubMed]

,3

3. U. Buenting, H. Sayinc, D. Wandt, U. Morgner, and D. Kracht, “Regenerative thin disk amplifier with combined gain spectra producing 500 μJ sub 200 fs pulses,” Opt. Express 17(10), 8046–8050 (2009). [CrossRef] [PubMed]

] and fiber [4

4. T. Eidam, S. Hanf, E. Seise, T. V. Andersen, Th. Gabler, Ch. Wirth, Th. Schreiber, J. Limpert, and A. Tünnermann, “Femtosecond fiber CPA system emitting 830 W average output power,” Opt. Lett. 35(2), 94–96 (2010). [CrossRef] [PubMed]

,5

5. T. Eidam, J. Rothhardt, F. Stutzki, F. Jansen, S. Hädrich, H. Carstens, C. Jauregui, J. Limpert, and A. Tünnermann, “Fiber chirped-pulse amplification system emitting 3.8 GW peak power,” Opt. Express 19(1), 255–260 (2011). [CrossRef] [PubMed]

] have been developed. All of these concepts can deliver average powers in the kW-range and pulse energies in the mJ-range. However, all of them suffer from their specific limitations, e.g. thermal and nonlinear effects, when pushed to even higher performance levels. Spatially separated amplification followed by coherent combination offers a new path to scale the performance of laser systems in spite of the mentioned limitations. Additionally, the amplification does not necessarily have to take place in an active medium, but optical parametric amplifiers can be used instead. Recently the combination of pulses coming from two parametric amplifiers in an OPCPA system has been demonstrated. In this case, the bandwidth of the combined pulses could be increased significantly compared to a single OPCPA system to support single-cycle pulses [6

6. S.-W. Huang, G. Cirmi, J. Moses, K.-H. Hong, S. Bhardwaj, J. R. Birge, L.-J. Chen, E. Li, B. J. Eggleton, G. Cerullo, and F. X. Kartner, “High-energy pulse synthesis with sub-cycle waveform control for strong-field physics,” Nat. Photonics 5(8), 475–479 (2011). [CrossRef]

]. Combining a large number of fiber lasers has also been proposed to reach peak-powers in the exawatt level [7

7. “The Extreme Light Infrastructure European Project,” http://www.extreme-light-infrastructure.eu/reports.php

].

The viability of the coherent combining approach to scale the pulse energy and average power of laser systems has already been demonstrated by combining two LMA fiber amplifiers, resulting in femtosecond pulses with a pulse energy of 0.5 mJ at an average power of 88 W. A combining efficiency as high as 90% has been achieved [8

8. E. Seise, A. Klenke, S. Breitkopf, J. Limpert, and A. Tünnermann, “88 W 0.5 mJ femtosecond laser pulses from two coherently combined fiber amplifiers,” Opt. Lett. 36(19), 3858–3860 (2011). [CrossRef] [PubMed]

]. It should be mentioned that a different group carried out successfully a similar experiment using a different combining technique and stabilization method based on a lock-in amplifier [9

9. L. Daniault, M. Hanna, L. Lombard, Y. Zaouter, E. Mottay, D. Goular, P. Bourdon, F. Druon, and P. Georges, “Coherent beam combining of two femtosecond fiber chirped-pulse amplifiers,” Opt. Lett. 36(5), 621–623 (2011) [CrossRef] [PubMed]

]. Furthermore, the impact of the B-Integral and of the fiber length mismatch on the combining efficiency in such amplifying interferometers has been studied in [10

10. A. Klenke, E. Seise, J. Limpert, and A. Tünnermann, “Basic considerations on coherent combining of ultrashort laser pulses,” Opt. Express (submitted to).

]. According to this study, the combination of ultrashort pulse amplifiers can be efficiently carried out even under realistic experimental conditions, i.e. at high B-Integrals (~10 rad) and with a mismatch of the dispersive lengths between the amplification channels of the order of a few cm.

2. Experimental setup

The experimental setup is shown in Fig. 1
Fig. 1 Schematic setup of the coherently combined chirped-pulse fiber amplifiers. SLM: spatial-light modulator, AOM: Acousto-optic modulator, HC: Hänsch-Couillaud detector
. It is based on a strongly modified version of the experimental setup presented in [12

12. F. Röser, T. Eidam, J. Rothhardt, O. Schmidt, D. N. Schimpf, J. Limpert, and A. Tünnermann, “Millijoule pulse energy high repetition rate femtosecond fiber chirped-pulse amplification system,” Opt. Lett. 32(24), 3495–3497 (2007) [CrossRef] [PubMed]

]. A mode-locked oscillator delivers pulses with a bandwidth of about 7 nm at a repetition rate of 40 MHz and an average power of 150 mW. The center wavelength is 1030 nm. The pulses are stretched in a fiber stretcher consisting of 100 m single-mode fiber with a core diameter of 6 µm. After amplification in the first preamplifier, the pulses are stretched in a grating stretcher with 1740 lines/mm gratings and a spectral hard-cut of 7 nm to a pulse length of about 2 ns. Then, a spatial light modulator (SLM) is used to compensate for any residual spectral phase (including accumulated nonlinear phase due to SPM in the amplifiers) that may still exist behind the compressor. This SLM is controlled by an active pulse shaping algorithm based on MIIPS [13

13. V. V. Lozovoy, I. Pastirk, and M. Dantus, “Multiphoton intrapulse interference. IV. Ultrashort laser pulse spectral phase characterization and compensation,” Opt. Lett. 29(7), 775–777 (2004) . [CrossRef] [PubMed]

]. With a combination of two acousto-optic modulators (AOM) and a second single-mode fiber preamplifier, the repetition rate can be reduced to 10 kHz, while still maintaining sufficient average power to seed the third preamplifier. This one uses an ytterbium-doped photonic crystal fiber with a core-diameter of 40 µm and a high numerical aperture pump-cladding of 200 µm. It can provide average powers of about 500 mW while still withstanding output pulse energies of up to 50 µJ.

The two main-amplifiers are coherently combined in a Mach-Zehnder type interferometer. The splitting and combining of the beams is realized using polarization dependent beam splitter cubes, i.e. employing the polarization combining technique described in [14

14. R. Uberna, A. Bratcher, and B. Tiemann, “Coherent Polarization Beam Combining,” IEEE J. Quantum Electron. 46(8), 1191–1196 (2010). [CrossRef]

]. A half-wave plate in front of the input cube is used to split the incoming beam into two with an intensity ratio of 1:1 to get the same seed power for both main amplifiers. An actively controlled delay line is used to match the optical path lengths of the two channels to one another. This delay line is realized with a piezo-mounted mirror set on a manual translation stage. While the translation stage helps to achieve a coarse path length match, the piezoelectric actuator is responsible for the fine corrections of the path length mismatch in a range of just several wavelengths. With a double pass through a quarter-wave plate positioned at an angle of 45°, the polarization of the reflected beam at the splitting cube, i.e. of the beam going through the delay line, is rotated by 90° and the beam is then transmitted straight through the cube.

The main amplifiers are two 80 cm long polarizing rod-type PCF fibers with a mode-field diameter of 75 µm. Up to a power level of 20 W the emitted beam quality is very close to diffraction-limited with a very high pointing stability. Higher average powers lead to slight mode deformations which are detrimental for the coherent addition. Both amplifiers are pumped by 915 nm laser diodes. This pump wavelength is chosen to obtain a higher inversion level compared to the standard 976 nm pumping of ytterbium-doped fibers. As a result, higher pulse energies can be extracted before saturation-induced pulse shaping reduces the temporal and spectral width of the amplified chirped pulse.

DOLP=PmaxPminPmax+Pmin and systemefficiency=PlinP1+P2
(1)

One should note that this definition of the system efficiency includes loses at the combining cube due to non-perfect polarization of the amplifier emission. To make sure that the polarization of the combined beam is not just optimized at one particular distance from the combining cube, the polarization dependency of the compressor gratings is exploited in using it as the analyzer. This ensures an excellent overlap of the beams over a distance of at least 10 m.

3. Experimental results

The spectrum in Fig. 2 a
Fig. 2 (a) Spectra and (b) autocorrelation traces of the single amplifiers and of the combined beam at a combined pulse energy of 2.1 mJ.
) shows the spectrum of the signal after amplification. The spectral bandwidth is slightly narrowed from an initial value of 4 nm, delivered by the pre-amp system, to 3 nm at the output. This was mainly due to saturation effects which cause the leading edge of the pulse (red) being more amplified than the tail (blue). Nevertheless, a good match of the amplified spectra coming from the two channels can be observed in Fig. 2, which allowed obtaining a high system efficiency. After compression, a duration of the autocorrelation trace of 780 fs (Fig. 2 b) was measured which, using a deconvolution factor of 1.68 calculated with the transform-limited pulse, corresponds to an estimated pulse duration of 465 fs. Again, a very good match between the characteristics of the pulses coming from the channels and the combined pulses could be observed. The combined pulse energy was 2.6 mJ directly after the output polarization cube and 2.1 mJ behind the compressor. The B-Integral in this configuration for the two combined amplifiers was calculated to have a value of 7 rad, whereby the SLM reduced the pulse quality degradation due to the presence of nonlinearity.

Finally, the output power of the channels was raised again to a compressed 16.3 W per channel, resulting in a combined and compressed pulse energy of 3 mJ (Fig. 3
Fig. 3 Autocorrelation traces of the single amplifiers and of the combined beam at combined pulse energies of (a) 2.8 mJ and (b) 3 mJ.
). The B-Integral in this case was calculated to be 9 rad. Again, this value is for the two combined amplifiers only. However, in this configuration, the DOLP dropped to 79% and the system efficiency to 84%. Difficulties with the excitation of the fundamental mode in the few-mode fibers of the main amplifier prevented obtaining better values and/or higher power levels. Nevertheless, pulse durations as short as 470 fs could be achieved, corresponding to peak powers as high as ~5.4 GW.

The characterization of the stability of the setup was done by recording the voltage applied to the piezo in the delay line. With an additional calibration, the compensated optical path length difference (OPD) can be calculated. Two of these traces are shown in Fig. 4
Fig. 4 Compensated OPD (a) just after the system was switched on at 2.1 mJ compressed pulse energy and (b) after a couple of minutes running at a compressed pulse energy of 3 mJ.
for a timescale of 450 s. The first one was recorded right after switching on the amplifiers while, in the second case, the system was already running for a couple of minutes. Hence, a drift of the OPD can clearly be seen at the beginning of the first trace, before the system reaches a stable state. This drift can be explained by thermal effects that have an influence on the optical path lengths of the channels. In the second trace a peak-to-peak fluctuation of 2.3 rad could be measured. This high stability of the setup was achieved by placing the whole setup into a housing and using water cooled modules for the two main amplifiers, therefore reducing thermal drifts.

4. Conclusion

In conclusion, it has been shown that with two coherently combined amplifiers in a femtosecond fiber CPA system, pulse energies can be achieved that are higher than the currently reported record value of 2.2 mJ for a single amplifier [5

5. T. Eidam, J. Rothhardt, F. Stutzki, F. Jansen, S. Hädrich, H. Carstens, C. Jauregui, J. Limpert, and A. Tünnermann, “Fiber chirped-pulse amplification system emitting 3.8 GW peak power,” Opt. Express 19(1), 255–260 (2011). [CrossRef] [PubMed]

]. Furthermore, the extraction of the presented results from a single amplifier stage would overstress current fiber laser technology. Thus, the 3 mJ of compressed energy reported here would require 3.8 mJ out of the single fiber amplifier. This, assuming a stretched pulse duration of 2 ns, an active fiber as used in the setup (75 µm MFD, length = 80 cm), would lead to a B-integral as high as 12 rad, peak powers as high as 2 MW, a fluence of 86 J/cm2 and a peak intensity of 45 GW/cm2. Hence, the extraction of the energy levels presented herein would be very close to or even beyond the well-known limitations of fiber based amplification of ultrashort pulses.

The coherent combining approach makes it possible to circumvent those limitations. At the same time, total combining efficiencies of up to 89% could be achieved, which are close to the values previously reported at significantly lower pulse energies. The stability tests also revealed that a carefully setup system lowers the requirements on the compensation of OPD fluctuations. These results make us confident that further performance scaling to pulse energies beyond 10 mJ is possible by using a larger number of amplifiers in a similar configuration. It should also be emphasized that this concept suits any amplification architecture of ultrashort laser pulses, including parametric amplifiers.

Acknowledgments

This work has been partly supported by the German Federal Ministry of Education and Research (BMBF) and the European Research Council (ERC), SIRG 240460-PECS. A. K. acknowledges financial support by the Helmholtz-Institute Jena. E. S. acknowledges financial support by the Carl Zeiss Stiftung Germany.

References and links

1.

P. Russbueldt, T. Mans, J. Weitenberg, H. D. Hoffmann, and R. Poprawe, “Compact diode-pumped 1.1 kW Yb:YAG Innoslab femtosecond amplifier,” Opt. Lett. 35(24), 4169–4171 (2010). [CrossRef] [PubMed]

2.

C. R. Baer, Ch. Kränkel, C. J. Saraceno, O. H. Heckl, M. Golling, R. Peters, K. Petermann, Th. Südmeyer, G. Huber, and U. Keller, “Femtosecond thin-disk laser with 141 W of average power,” Opt. Lett. 35(13), 2302–2304 (2010). [CrossRef] [PubMed]

3.

U. Buenting, H. Sayinc, D. Wandt, U. Morgner, and D. Kracht, “Regenerative thin disk amplifier with combined gain spectra producing 500 μJ sub 200 fs pulses,” Opt. Express 17(10), 8046–8050 (2009). [CrossRef] [PubMed]

4.

T. Eidam, S. Hanf, E. Seise, T. V. Andersen, Th. Gabler, Ch. Wirth, Th. Schreiber, J. Limpert, and A. Tünnermann, “Femtosecond fiber CPA system emitting 830 W average output power,” Opt. Lett. 35(2), 94–96 (2010). [CrossRef] [PubMed]

5.

T. Eidam, J. Rothhardt, F. Stutzki, F. Jansen, S. Hädrich, H. Carstens, C. Jauregui, J. Limpert, and A. Tünnermann, “Fiber chirped-pulse amplification system emitting 3.8 GW peak power,” Opt. Express 19(1), 255–260 (2011). [CrossRef] [PubMed]

6.

S.-W. Huang, G. Cirmi, J. Moses, K.-H. Hong, S. Bhardwaj, J. R. Birge, L.-J. Chen, E. Li, B. J. Eggleton, G. Cerullo, and F. X. Kartner, “High-energy pulse synthesis with sub-cycle waveform control for strong-field physics,” Nat. Photonics 5(8), 475–479 (2011). [CrossRef]

7.

“The Extreme Light Infrastructure European Project,” http://www.extreme-light-infrastructure.eu/reports.php

8.

E. Seise, A. Klenke, S. Breitkopf, J. Limpert, and A. Tünnermann, “88 W 0.5 mJ femtosecond laser pulses from two coherently combined fiber amplifiers,” Opt. Lett. 36(19), 3858–3860 (2011). [CrossRef] [PubMed]

9.

L. Daniault, M. Hanna, L. Lombard, Y. Zaouter, E. Mottay, D. Goular, P. Bourdon, F. Druon, and P. Georges, “Coherent beam combining of two femtosecond fiber chirped-pulse amplifiers,” Opt. Lett. 36(5), 621–623 (2011) [CrossRef] [PubMed]

10.

A. Klenke, E. Seise, J. Limpert, and A. Tünnermann, “Basic considerations on coherent combining of ultrashort laser pulses,” Opt. Express (submitted to).

11.

D. Schimpf, T. Eidam, E. Seise, J. Limpert, and A. Tünnermann, “Model-based phase-shaping for SPM-compensation in mJ-pulse-energy fiber CPA-systems,” in Advanced Solid-State Photonics, OSA Technical Digest Series (CD) (Optical Society of America, 2010), paper AWB14.

12.

F. Röser, T. Eidam, J. Rothhardt, O. Schmidt, D. N. Schimpf, J. Limpert, and A. Tünnermann, “Millijoule pulse energy high repetition rate femtosecond fiber chirped-pulse amplification system,” Opt. Lett. 32(24), 3495–3497 (2007) [CrossRef] [PubMed]

13.

V. V. Lozovoy, I. Pastirk, and M. Dantus, “Multiphoton intrapulse interference. IV. Ultrashort laser pulse spectral phase characterization and compensation,” Opt. Lett. 29(7), 775–777 (2004) . [CrossRef] [PubMed]

14.

R. Uberna, A. Bratcher, and B. Tiemann, “Coherent Polarization Beam Combining,” IEEE J. Quantum Electron. 46(8), 1191–1196 (2010). [CrossRef]

15.

T. W. Hänsch and B. Couillaud, “Laser frequency stabilization by polarization spectroscopy of a reflecting reference cavity,” Opt. Commun. 35(3), 441–444 (1980). [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:
Lasers and Laser Optics

History
Original Manuscript: August 29, 2011
Revised Manuscript: October 21, 2011
Manuscript Accepted: October 27, 2011
Published: November 14, 2011

Citation
Arno Klenke, Enrico Seise, Stefan Demmler, Jan Rothhardt, Sven Breitkopf, Jens Limpert, and Andreas Tünnermann, "Coherently-combined two channel femtosecond fiber CPA system producing 3 mJ pulse energy," Opt. Express 19, 24280-24285 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-24-24280


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References

  1. P. Russbueldt, T. Mans, J. Weitenberg, H. D. Hoffmann, and R. Poprawe, “Compact diode-pumped 1.1 kW Yb:YAG Innoslab femtosecond amplifier,” Opt. Lett.35(24), 4169–4171 (2010). [CrossRef] [PubMed]
  2. C. R. Baer, Ch. Kränkel, C. J. Saraceno, O. H. Heckl, M. Golling, R. Peters, K. Petermann, Th. Südmeyer, G. Huber, and U. Keller, “Femtosecond thin-disk laser with 141 W of average power,” Opt. Lett.35(13), 2302–2304 (2010). [CrossRef] [PubMed]
  3. U. Buenting, H. Sayinc, D. Wandt, U. Morgner, and D. Kracht, “Regenerative thin disk amplifier with combined gain spectra producing 500 μJ sub 200 fs pulses,” Opt. Express17(10), 8046–8050 (2009). [CrossRef] [PubMed]
  4. T. Eidam, S. Hanf, E. Seise, T. V. Andersen, Th. Gabler, Ch. Wirth, Th. Schreiber, J. Limpert, and A. Tünnermann, “Femtosecond fiber CPA system emitting 830 W average output power,” Opt. Lett.35(2), 94–96 (2010). [CrossRef] [PubMed]
  5. T. Eidam, J. Rothhardt, F. Stutzki, F. Jansen, S. Hädrich, H. Carstens, C. Jauregui, J. Limpert, and A. Tünnermann, “Fiber chirped-pulse amplification system emitting 3.8 GW peak power,” Opt. Express19(1), 255–260 (2011). [CrossRef] [PubMed]
  6. S.-W. Huang, G. Cirmi, J. Moses, K.-H. Hong, S. Bhardwaj, J. R. Birge, L.-J. Chen, E. Li, B. J. Eggleton, G. Cerullo, and F. X. Kartner, “High-energy pulse synthesis with sub-cycle waveform control for strong-field physics,” Nat. Photonics5(8), 475–479 (2011). [CrossRef]
  7. “The Extreme Light Infrastructure European Project,” http://www.extreme-light-infrastructure.eu/reports.php
  8. E. Seise, A. Klenke, S. Breitkopf, J. Limpert, and A. Tünnermann, “88 W 0.5 mJ femtosecond laser pulses from two coherently combined fiber amplifiers,” Opt. Lett.36(19), 3858–3860 (2011). [CrossRef] [PubMed]
  9. L. Daniault, M. Hanna, L. Lombard, Y. Zaouter, E. Mottay, D. Goular, P. Bourdon, F. Druon, and P. Georges, “Coherent beam combining of two femtosecond fiber chirped-pulse amplifiers,” Opt. Lett.36(5), 621–623 (2011) [CrossRef] [PubMed]
  10. A. Klenke, E. Seise, J. Limpert, and A. Tünnermann, “Basic considerations on coherent combining of ultrashort laser pulses,” Opt. Express (submitted to).
  11. D. Schimpf, T. Eidam, E. Seise, J. Limpert, and A. Tünnermann, “Model-based phase-shaping for SPM-compensation in mJ-pulse-energy fiber CPA-systems,” in Advanced Solid-State Photonics, OSA Technical Digest Series (CD) (Optical Society of America, 2010), paper AWB14.
  12. F. Röser, T. Eidam, J. Rothhardt, O. Schmidt, D. N. Schimpf, J. Limpert, and A. Tünnermann, “Millijoule pulse energy high repetition rate femtosecond fiber chirped-pulse amplification system,” Opt. Lett.32(24), 3495–3497 (2007) [CrossRef] [PubMed]
  13. V. V. Lozovoy, I. Pastirk, and M. Dantus, “Multiphoton intrapulse interference. IV. Ultrashort laser pulse spectral phase characterization and compensation,” Opt. Lett.29(7), 775–777 (2004) . [CrossRef] [PubMed]
  14. R. Uberna, A. Bratcher, and B. Tiemann, “Coherent Polarization Beam Combining,” IEEE J. Quantum Electron.46(8), 1191–1196 (2010). [CrossRef]
  15. T. W. Hänsch and B. Couillaud, “Laser frequency stabilization by polarization spectroscopy of a reflecting reference cavity,” Opt. Commun.35(3), 441–444 (1980). [CrossRef]

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