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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 3 — Jan. 31, 2011
  • pp: 1854–1859
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All-fiber based amplification of 40 ps pulses from a gain-switched laser diode

Sebastian Kanzelmeyer, Hakan Sayinc, Thomas Theeg, Maik Frede, Joerg Neumann, and Dietmar Kracht  »View Author Affiliations


Optics Express, Vol. 19, Issue 3, pp. 1854-1859 (2011)
http://dx.doi.org/10.1364/OE.19.001854


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Abstract

Amplification of a gain-switched laser diode is demonstrated in an all-fiber based setup. The amplified spontaneous emission between two consecutive pulses was investigated quantitatively in the time domain. A maximum pulse energy of 13 µJ at a repetition rate of 1 MHz and a pulse duration of 40 ps was extracted, corresponding to a peak power of 270 KW. To the best of our knowledge, this is the highest extracted pulse energy from a laser system seeded by a gain-switched laser diode. Temporal pulse deformation due to intrapulse Raman scattering was observed in the reported system.

© 2011 OSA

1. Introduction

Laser systems delivering pulses in the picosecond regime have a broad range of applications in material processing, nonlinear microscopy and biomedical research [1

1. S. Nolte, C. Momma, H. Jacobs, A. Tünnermann, B. N. Chichkov, B. Wellegehausen, and H. Welling, “Ablation of metals by ultrashort laser pulses,” J. Opt. Soc. Am. B 14(10), 2716–2722 (1997). [CrossRef]

,2

2. A. Bülter, R. Erdmann, and T. Schmitt, “Amplified picosecond diode lasers simplify biomedical research,” Biophot. Int. 12, 38–41 (2005).

]. Typically, the required pulse durations and peak powers for these applications are achieved by laser systems based on mode-locked lasers. A modelocked laser was amplified in a three stage fiber amplifier chain to an average power of 96 W at a repetition rate of 478 MHz and pulse durations of 16 ps by S.P. Chen et al [3

3. S.-P. Chen, H.-W. Chen, J. Hou, and Z.-J. Liu, “100 W all fiber picosecond MOPA laser,” Opt. Express 17(26), 24008–24012 (2009). [CrossRef]

]. Recently, a compact laser system consisting of a fiber amplified microchip laser capable of producing 85 ps pulses with a peak power of 3 MW was demonstrated [4

4. D. Nodop, O. Schmidt, J. Limpert, and A. Tünnermann, “105 kHz, 85 ps, 3 MW microchip laser fiber amplifier system for micro-machining applications,” in Conference on Lasers and Electro-Optics/Quantum Electronics and Laser Science Conference and Photonic Applications Systems Technologies, OSA Technical Digest (CD) (Optical Society of America, 2008), paper CThL1.

].

Gain-switched laser diodes are reliable laser sources for producing sub 100 ps pulses with excellent pulse to pulse stability and a very low timing jitter [5

5. K. Lauritsen, S. Riecke, M. Langkopf, D. Klemme, C. Kaleva, C. Pallassis, S. McNeil, and R. Erdmann, “Fiber amplified and frequency doubled diode lasers as a highly flexible pulse source at 532nm,” Proc. SPIE 6871, 68711L, 68711L-9 (2008). [CrossRef]

,6

6. P. Dupriez, A. Piper, A. Malinowski, J. K. Sahu, M. Ibsen, B. C. Thomsen, Y. Jeong, L. M. B. Hickey, M. N. Zervas, J. Nilsson, and D. J. Richardson, “High average power, high repetition rate, picosecond pulsed fiber master oscillator power amplifier source seeded by a gain-switched laser diode at 1060 nm,” IEEE Photon. Technol. Lett. 18(9), 1013–1015 (2006). [CrossRef]

]. The driving electronics allows for control of the repetition rate and the pulse release time. This enables for synchronization to other systems leading to a high versatility. Due to the low output power compared to other solid-state lasers, amplification becomes necessary to reach aplicable power levels. The combination of a gain-switched laser diode and an all-fiber based amplifier therefore leads to a compact and stable laser source which can be adapted for many applications [6

6. P. Dupriez, A. Piper, A. Malinowski, J. K. Sahu, M. Ibsen, B. C. Thomsen, Y. Jeong, L. M. B. Hickey, M. N. Zervas, J. Nilsson, and D. J. Richardson, “High average power, high repetition rate, picosecond pulsed fiber master oscillator power amplifier source seeded by a gain-switched laser diode at 1060 nm,” IEEE Photon. Technol. Lett. 18(9), 1013–1015 (2006). [CrossRef]

8

8. F. Gonthier, L. Martineau, N. Azami, M. Faucher, F. Seguin, D. Stryckman, and A. Villeneuve, “High-power All-Fiber components: the missing link for high-power fiber lasers,” Proc. SPIE 5335, 266–276 (2004). [CrossRef]

]. A fiber amplifier allows for a very high gain, while an excellent beam quality is maintained over a wide power range. This was demonstrated by the amplification of 20 ps pulses from gain-switched laser diodes in several stages of ytterbium doped fiber (YDF) amplifiers by Dupriez et al [6

6. P. Dupriez, A. Piper, A. Malinowski, J. K. Sahu, M. Ibsen, B. C. Thomsen, Y. Jeong, L. M. B. Hickey, M. N. Zervas, J. Nilsson, and D. J. Richardson, “High average power, high repetition rate, picosecond pulsed fiber master oscillator power amplifier source seeded by a gain-switched laser diode at 1060 nm,” IEEE Photon. Technol. Lett. 18(9), 1013–1015 (2006). [CrossRef]

] and Chen et al [7

7. K. K. Chen, J. H. Price, S. U. Alam, J. R. Hayes, D. Lin, A. Malinowski, and D. J. Richardson, “Polarisation maintaining 100W Yb-fiber MOPA producing µJ pulses tunable in duration from 1 to 21 ps,” Opt. Express 18(14), 14385–14394 (2010). [CrossRef] [PubMed]

]. An average power of 321 W at a repetition rate of 1 GHz [6

6. P. Dupriez, A. Piper, A. Malinowski, J. K. Sahu, M. Ibsen, B. C. Thomsen, Y. Jeong, L. M. B. Hickey, M. N. Zervas, J. Nilsson, and D. J. Richardson, “High average power, high repetition rate, picosecond pulsed fiber master oscillator power amplifier source seeded by a gain-switched laser diode at 1060 nm,” IEEE Photon. Technol. Lett. 18(9), 1013–1015 (2006). [CrossRef]

] and an average power of 100 W at a repetition rate of 56 MHz [7

7. K. K. Chen, J. H. Price, S. U. Alam, J. R. Hayes, D. Lin, A. Malinowski, and D. J. Richardson, “Polarisation maintaining 100W Yb-fiber MOPA producing µJ pulses tunable in duration from 1 to 21 ps,” Opt. Express 18(14), 14385–14394 (2010). [CrossRef] [PubMed]

] could be achieved. Applications such as material processing require high pulse energy and peak power preferable at repetition rates in the MHz or kHz regime because of the limited speed of scanning and switching devices. Recently, a gain-switched laser diode was amplified at a repetition rate of 1 MHz in a fiber based system resulting in an average output power of 3.5 W and a pulse duration of 90 ps [9

9. H. Liu, C. Gao, J. Tao, W. Zhao, and Y. Wang, “Compact tunable high power picosecond source based on Yb-doped fiber amplification of gain switch laser diode,” Opt. Express 16(11), 7888–7893 (2008). [CrossRef] [PubMed]

]. However, no information on pulse energy and peak power was given. The low output power of gain-switched laser diodes requires the consideration of the effects of amplified spontaneous emission (ASE) generated in an unsaturated fiber amplifier. A high amount of ASE leads to inefficient amplification of the signal and a continuous-wave (cw) power background. If the amount of ASE is unknown, reliable information about the pulse energy cannot be extracted from the average output power and the repetition rate.

In this work, all-fiber based amplification of a gain-switched laser diode with detailed investigation of the signal to ASE ratio is demonstrated. Therefore, the emission of the system was analyzed with respect to average pulse and ASE power with an acousto-optic modulator (AOM) at a repetition rate of 1 MHz. From these measurements, reliable values of the pulse properties such as energy, peak power and optical spectrum could be obtained. Furthermore, temporal deformation of the pulses due to nonlinear effects was observed and is discussed.

2. Experimental setup

The experimental setup of the laser system is shown in Fig. 1
Fig. 1 Experimental setup of the laser system with fiber amplifier stages.
. For amplification of the low power gain-switched laser diode (PicoQuant GmbH, Berlin, Germany), a three stage ytterbium fiber amplifier was realized. All amplification stages were completely fiber based and directly spliced together allowing for efficient power coupling. Thus, free space losses between the amplifier stages were avoided and high mechanical stability was guaranteed, which was crucial for stable operation of the system. Usually power scaling in fiber amplifiers is limited by nonlinear effects such as stimulated Raman scattering (SRS) and self-phase modulation (SPM) [10

10. G. P. Agrawal, Nonlinear Fiber Optics, 3rd Ed. (Academic Press, 2001).

]. To minimize these effects, reduction of the optical power density within the fiber was beneficial by increasing the fiber core diameter while maintaining single-mode propagation [11

11. A. Galvanauskas, G. C. Cho, A. Hariharan, M. E. Fermann, and D. Harter, “Generation of high-energy femtosecond pulses in multimode-core Yb-fiber chirped-pulse amplification systems,” Opt. Lett. 26(12), 935–937 (2001). [CrossRef]

]. Thus, a successive power scaling from stage to stage was realized by increasing the mode-field diameter of each amplification stage to operate each amplifier under saturated conditions as far as possible and keep the ASE on a low level.

The seed laser pulses had durations of 37 ps at a central wavelength of 1040 nm with a bandwidth of 20 nm. Repetition rates from 31.25 kHz up to 80 MHz could be selected by the associated driving electronics. A Faraday isolator protected the seed laser from back reflections. Pulse energies of about 20 pJ corresponding to an average power of 20 µW at a repetition rate of 1 MHz were launched into the first amplification stage. A wavelength division multiplexer (WDM) enabled pump light coupling of a cw laser diode (LD) with up to 600 mW average power at a wavelength of 976 nm into the core of an ytterbium doped single-clad fiber with a mode-field diameter of 4.4 µm and a length of 60 cm. The output of the first amplification stage was directly spliced to the second pre-amplification stage consisting of an 80 cm long YDF with a core diameter of 10 µm. A self made mode-field adaptor (MFA) allowed for efficient power coupling from the smaller core diameter fiber into the 10 µm core diameter fiber. The second YDF was also co-directionally core-pumped by a cw laser diode with up to 600 mW average power at a wavelength of 976 nm. The single clad pre-amplification stages were protected from back reflection by a fiber coupled isolator.

The final amplifier stage consisted of an all-fiber multimode pump combiner with signal feed through, spliced to a double clad large mode area (LMA) YDF with a core diameter of 30 µm, a cladding diameter of 250 µm and a length of 2.5 m. Again the mode-field of the 10 µm fiber was matched to the 30 µm fiber by a mode-field adaptor. The power amplification stage was co-directionally pumped by multimode laser diodes operating at a wavelength of 976 nm. Overall the amplifier was completely fiber based enabling for a maximum of mechanical stability and efficiency in terms of power coupling.

3. Results

The achieved output power of the complete system after the last amplification stage with a core diameter of 30 µm is shown for a repetition rate of 1 MHz in Fig. 3
Fig. 3 Average output power vs. absorbed pump power at 1 MHz repetition rate and corresponding beam profile.
. The M-square value was measured to be about 1.23 at the horizontal and 1.25 at the vertical beam axis. The corresponding measurement is depicted as inset in Fig. 3. The maximum extracted output power was 14.9 W at an absorbed pump power of 25.3 W, whereas about 3 W of the pump light was transmitted through the fiber.

At the repetition rate of 1 MHz the launched average power of the seed source was 20 µW. At these low power levels saturation of the first YDF with a mode-field diameter of 4.4 µm was not achieved, since the calculated saturation power for this fiber is about 4 mW. Therefore, the first amplification stage determined the performance of the whole laser system in terms of ASE emission and self-lasing, since the subsequent stages were all operated in a saturated regime. To ensure an efficient and stable regime of operation, the ratio of signal to ASE had to be quantified. For seed sources with narrow spectral distribution, information on the signal to ASE ratios can be obtained from the optical spectrum of the amplifiers output radiation. Due to the broadband nature of the used seed source and the overlap of the optical spectra with the ytterbium emission spectrum a distinction between ASE and signal was difficult. However, ASE results in a cw emission allowing for a determination of the signal to ASE ratio in the temporal domain. Therefore, the output radiation was analyzed with respect to pulse and cw power utilizing an AOM for gating. As shown in Fig. 4a
Fig. 4 Setup (a) and time scheme (b) of the AOM measurement for pulse analysis.
), the output radiation was reflected by a dichroic mirror for removal of the residual pump light. A part of the light was reflected by a glass plate and coupled into the AOM. Its first order deflection was set to a time window of about 40 ns containing the pulses. Afterwards, the time window was inverted as sketched in Fig. 4b). The average power was measured. The pulse duration of about 40 ps was much smaller than the AOM time window. So the amount of ASE radiation in the 40 ns time window had to be accounted for as well. At a repetition rate of 1 MHz this resulted in about 4% additional ASE power, which was considered in the calculations. Reliable information about the signal to ASE ratio could be extracted from these measurements.

The ASE ratios and the average power versus pump power of the main amplification stage were measured. The pulse energy can be calculated from the average power, if the signal to ASE ratio is taken into account. The ASE ratios and corresponding pulse energies are shown in Fig. 5
Fig. 5 (a) Signal to ASE ratios with corresponding pulse energies and (b) pulse duration with corresponding peak power at 1 MHz repetition rate.
. The highest extracted pulse energy was 13 µJ at 14.9 W average output power with a signal to ASE ratio of 88%.

The pulse duration of the laser system was measured with the fast photodiode, while the peak power of the pulses was calculated from the corresponding pulse energies and pulse durations. For increasing pulse energy, the full width at half maximum (FWHM) pulse duration broadened from 37 ps to 46 ps at 13 µJ. This evolution is shown in Fig. 5b). It was accounted for pulse broadening in the calculation of the pulse peak power, which led to a maximum pulse peak power of 270 kW being, to the best of our knowledge, the highest achieved peak power by fiber based amplification of a gain-switched laser diode. The normalized temporal pulse shapes at pulse energies of 0.5 µJ and 13 µJ are shown in Fig. 6a
Fig. 6 Temporal pulse shapes (a) and optical spectra (b) after AOM filtering of the pulses at a pulse energy of 0.5 µJ and 13.1 µJ corresponding to 17.3 kW and 270 kW peak power.
). The pulses were detected on a sampling oscilloscope triggered by a synchronization signal of the seed laser electronics, which allowed for a precise determination of the temporal delay of the pulses. It is obvious that in the temporal domain power is shifted into the leading edge of the pulses. The corresponding optical spectra are shown in Fig. 6b). For increasing pulse energies spectral power was shifted from 1040 nm to longer wavelengths due to intra-pulse Raman scattering, which is assumed to be the dominant nonlinear effect [10

10. G. P. Agrawal, Nonlinear Fiber Optics, 3rd Ed. (Academic Press, 2001).

]. The negative peaks at 1155 nm and 1185 nm were caused by the spectral transmission of the dichroic mirror placed at the output of the power amplification stage for the separation of transmitted pump light.

The optical spectra in Fig. 6b) furthermore prove stable pulsed operation of the laser system even at energies of 13.1 µJ, since no peaks resulting from narrow bandwidth cw laser operation were evident. Further power scaling of the system was not limited by damage of the fiber or self lasing. Due to the temporal and spectral broadening further scaling of the pulse peak power became inefficient, since higher pulse energies resulted in stronger temporal broadening of the pulses.

4. Conclusion

Acknowledgements

This work was funded by the German Federal Ministry of Education and Research (BMBF) under FKZ: 13N9685.

References and links

1.

S. Nolte, C. Momma, H. Jacobs, A. Tünnermann, B. N. Chichkov, B. Wellegehausen, and H. Welling, “Ablation of metals by ultrashort laser pulses,” J. Opt. Soc. Am. B 14(10), 2716–2722 (1997). [CrossRef]

2.

A. Bülter, R. Erdmann, and T. Schmitt, “Amplified picosecond diode lasers simplify biomedical research,” Biophot. Int. 12, 38–41 (2005).

3.

S.-P. Chen, H.-W. Chen, J. Hou, and Z.-J. Liu, “100 W all fiber picosecond MOPA laser,” Opt. Express 17(26), 24008–24012 (2009). [CrossRef]

4.

D. Nodop, O. Schmidt, J. Limpert, and A. Tünnermann, “105 kHz, 85 ps, 3 MW microchip laser fiber amplifier system for micro-machining applications,” in Conference on Lasers and Electro-Optics/Quantum Electronics and Laser Science Conference and Photonic Applications Systems Technologies, OSA Technical Digest (CD) (Optical Society of America, 2008), paper CThL1.

5.

K. Lauritsen, S. Riecke, M. Langkopf, D. Klemme, C. Kaleva, C. Pallassis, S. McNeil, and R. Erdmann, “Fiber amplified and frequency doubled diode lasers as a highly flexible pulse source at 532nm,” Proc. SPIE 6871, 68711L, 68711L-9 (2008). [CrossRef]

6.

P. Dupriez, A. Piper, A. Malinowski, J. K. Sahu, M. Ibsen, B. C. Thomsen, Y. Jeong, L. M. B. Hickey, M. N. Zervas, J. Nilsson, and D. J. Richardson, “High average power, high repetition rate, picosecond pulsed fiber master oscillator power amplifier source seeded by a gain-switched laser diode at 1060 nm,” IEEE Photon. Technol. Lett. 18(9), 1013–1015 (2006). [CrossRef]

7.

K. K. Chen, J. H. Price, S. U. Alam, J. R. Hayes, D. Lin, A. Malinowski, and D. J. Richardson, “Polarisation maintaining 100W Yb-fiber MOPA producing µJ pulses tunable in duration from 1 to 21 ps,” Opt. Express 18(14), 14385–14394 (2010). [CrossRef] [PubMed]

8.

F. Gonthier, L. Martineau, N. Azami, M. Faucher, F. Seguin, D. Stryckman, and A. Villeneuve, “High-power All-Fiber components: the missing link for high-power fiber lasers,” Proc. SPIE 5335, 266–276 (2004). [CrossRef]

9.

H. Liu, C. Gao, J. Tao, W. Zhao, and Y. Wang, “Compact tunable high power picosecond source based on Yb-doped fiber amplification of gain switch laser diode,” Opt. Express 16(11), 7888–7893 (2008). [CrossRef] [PubMed]

10.

G. P. Agrawal, Nonlinear Fiber Optics, 3rd Ed. (Academic Press, 2001).

11.

A. Galvanauskas, G. C. Cho, A. Hariharan, M. E. Fermann, and D. Harter, “Generation of high-energy femtosecond pulses in multimode-core Yb-fiber chirped-pulse amplification systems,” Opt. Lett. 26(12), 935–937 (2001). [CrossRef]

12.

L. Thévenaz, “Slow and fast light in optical fibres,” Nat. Photonics 2(8), 474–481 (2008). [CrossRef]

13.

K. Lee and N. Lawandy, “Optically induced pulse delay in a solid-state Raman amplifier,” Appl. Phys. Lett. 78(6), 703–705 (2001). [CrossRef]

OCIS Codes
(060.2320) Fiber optics and optical communications : Fiber optics amplifiers and oscillators
(140.3280) Lasers and laser optics : Laser amplifiers

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: September 17, 2010
Revised Manuscript: October 28, 2010
Manuscript Accepted: January 11, 2011
Published: January 18, 2011

Citation
Sebastian Kanzelmeyer, Hakan Sayinc, Thomas Theeg, Maik Frede, Joerg Neumann, and Dietmar Kracht, "All-fiber based amplification of 40 ps pulses from a gain-switched laser diode," Opt. Express 19, 1854-1859 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-3-1854


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References

  1. S. Nolte, C. Momma, H. Jacobs, A. Tünnermann, B. N. Chichkov, B. Wellegehausen, and H. Welling, “Ablation of metals by ultrashort laser pulses,” J. Opt. Soc. Am. B 14(10), 2716–2722 (1997). [CrossRef]
  2. A. Bülter, R. Erdmann, and T. Schmitt, “Amplified picosecond diode lasers simplify biomedical research,” Biophot. Int. 12, 38–41 (2005).
  3. S.-P. Chen, H.-W. Chen, J. Hou, and Z.-J. Liu, “100 W all fiber picosecond MOPA laser,” Opt. Express 17(26), 24008–24012 (2009). [CrossRef]
  4. D. Nodop, O. Schmidt, J. Limpert, and A. Tünnermann, “105 kHz, 85 ps, 3 MW microchip laser fiber amplifier system for micro-machining applications,” in Conference on Lasers and Electro-Optics/Quantum Electronics and Laser Science Conference and Photonic Applications Systems Technologies, OSA Technical Digest (CD) (Optical Society of America, 2008), paper CThL1.
  5. K. Lauritsen, S. Riecke, M. Langkopf, D. Klemme, C. Kaleva, C. Pallassis, S. McNeil, and R. Erdmann, “Fiber amplified and frequency doubled diode lasers as a highly flexible pulse source at 532nm,” Proc. SPIE 6871, 68711L, 68711L-9 (2008). [CrossRef]
  6. P. Dupriez, A. Piper, A. Malinowski, J. K. Sahu, M. Ibsen, B. C. Thomsen, Y. Jeong, L. M. B. Hickey, M. N. Zervas, J. Nilsson, and D. J. Richardson, “High average power, high repetition rate, picosecond pulsed fiber master oscillator power amplifier source seeded by a gain-switched laser diode at 1060 nm,” IEEE Photon. Technol. Lett. 18(9), 1013–1015 (2006). [CrossRef]
  7. K. K. Chen, J. H. Price, S. U. Alam, J. R. Hayes, D. Lin, A. Malinowski, and D. J. Richardson, “Polarisation maintaining 100W Yb-fiber MOPA producing µJ pulses tunable in duration from 1 to 21 ps,” Opt. Express 18(14), 14385–14394 (2010). [CrossRef] [PubMed]
  8. F. Gonthier, L. Martineau, N. Azami, M. Faucher, F. Seguin, D. Stryckman, and A. Villeneuve, “High-power All-Fiber components: the missing link for high-power fiber lasers,” Proc. SPIE 5335, 266–276 (2004). [CrossRef]
  9. H. Liu, C. Gao, J. Tao, W. Zhao, and Y. Wang, “Compact tunable high power picosecond source based on Yb-doped fiber amplification of gain switch laser diode,” Opt. Express 16(11), 7888–7893 (2008). [CrossRef] [PubMed]
  10. G. P. Agrawal, Nonlinear Fiber Optics, 3rd Ed. (Academic Press, 2001).
  11. A. Galvanauskas, G. C. Cho, A. Hariharan, M. E. Fermann, and D. Harter, “Generation of high-energy femtosecond pulses in multimode-core Yb-fiber chirped-pulse amplification systems,” Opt. Lett. 26(12), 935–937 (2001). [CrossRef]
  12. L. Thévenaz, “Slow and fast light in optical fibres,” Nat. Photonics 2(8), 474–481 (2008). [CrossRef]
  13. K. Lee and N. Lawandy, “Optically induced pulse delay in a solid-state Raman amplifier,” Appl. Phys. Lett. 78(6), 703–705 (2001). [CrossRef]

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