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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 5 — Feb. 27, 2012
  • pp: 5319–5324
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TEM00 mode content of a two stage single-frequency Yb-doped PCF MOPA with 246 W of output power

Malte Karow, Chandrajit Basu, Dietmar Kracht, Jörg Neumann, and Peter Weßels  »View Author Affiliations


Optics Express, Vol. 20, Issue 5, pp. 5319-5324 (2012)
http://dx.doi.org/10.1364/OE.20.005319


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Abstract

Gravitational wave detectors require linearly polarized single-frequency laser sources with a high fractional TEM00 mode content. We investigated the modal decomposition of a polarization maintaining photonic crystal fiber with a mode field diameter of 29 µm, operating in a single-frequency master-oscillator power-amplifier scheme, with respect to the TEMnm modes. Low degradation of the beam quality with increasing pump power could be observed, while a maximum power in the TEM00 mode of 203 W was achieved.

© 2012 OSA

1. Introduction

In the past years high-power single-frequency laser sources have found great interest in a variety of applications, such as Doppler-Lidar, coherent beam combining for power scaling and frequency conversion. Our primary area of interest is the use of such systems as laser sources in interferometric gravitational wave detectors (GWD). Currently, the lasers for the 2nd generation of Laser Interferometer Gravitational Wave Observatories (LIGO) are being installed. These laser systems consist of a solid-state high-power ring oscillator, which is injection-locked to an amplified nonplanar ring oscillator (NPRO) to achieve single-frequency operation. It delivers 220 W of output power at the wavelength of 1064 nm [1

1. L. Winkelmann, O. Puncken, R. Kluzik, C. Veltkamp, P. Kwee, J. Poeld, C. Bogan, B. Willke, M. Frede, J. Neumann, P. Wessels, and D. Kracht, “Injection-locked single-frequency laser with an output power of 220 W,” Appl. Phys. B 102(3), 529–538 (2011). [CrossRef]

]. After filtering the output beam with a non-confocal ring cavity, a pure TEM00 mode with an output power of 168 W was obtained. Still one major limitation of the interferometer sensitivity at high frequencies is shot noise, which decreases proportionally to the square root of the laser power. Therefore, the 3rd generation of these interferometric GWDs will most likely require single-frequency TEM00 laser sources at 1064 nm with output powers in the kW range [2

2. N. Mavalvala, D. E. McClelland, G. Mueller, D. H. Reitze, R. Schnabel, and B. Willke, “Lasers and optics: looking towards third generation gravitational wave detectors,” Gen. Relativ. Gravit. 43(2), 569–592 (2011). [CrossRef]

]. To achieve this power level while maintaining single-frequency operation, the master oscillator power amplifier (MOPA) scheme is a promising approach. As the interferometer requires a stable linear polarization state, the laser should have a constant high degree of polarization.

Fiber-based MOPA systems have made great progress in terms of output power over the past years. Double-clad fibers allowed the utilization of low-brightness pump sources, which are capable of delivering the necessary pump power to amplify signals up to several 100 W. In the 1 µm wavelength range, Yb-doped fiber amplifier systems deliver CW output power levels of several kW [3

3. C. Wirth, O. Schmidt, A. Kliner, T. Schreiber, R. Eberhardt, and A. Tünnermann, “High-power tandem pumped fiber amplifier with an output power of 2.9 kW,” Opt. Lett. 36(16), 3061–3063 (2011). [CrossRef] [PubMed]

]. However, when amplifying kHz-linewidth signals, one of the major obstacles is the onset of stimulated Brillouin scattering (SBS).

To increase the threshold of this nonlinear power-limiting effect, different approaches have been proposed. Especially in the field of fiber development a lot of progress has been made over the past years. The current power records for different polarization maintaining fiber designs employed in single-frequency master oscillator power amplifier systems are 402 W for a step-index large mode area (LMA) PM fiber [4

4. Y. Jeong, J. Nilsson, J. K. Sahu, D. N. Payne, L. M. B. Hickey, and P. W. Turner, “Power scaling of single-frequency ytterbium-doped master-oscillator power-amplifier sources up to 500 W,” IEEE J. Sel. Top. Quantum Electron. 13(3), 546–551 (2007). [CrossRef]

], 511 W for a chirally coupled core (CCC) fiber [5

5. C. Zhu, I. Hu, X. Ma, and A. Galvanauskas “Single-frequency and single-transverse-mode Yb-doped CCC fiber MOPA with robust polarization SBS-free 511 W output,” Advanced Solid-State Photonics, OSA Technical Digest (CD) (Optical Society of America, 2011), paper AMC5.

], and 494 W for an acoustically segmented photonic crystal fiber (PCF) [6

6. C. Robin and I. Dajani, “Acoustically segmented photonic crystal fiber for single-frequency high-power laser applications,” Opt. Lett. 36(14), 2641–2643 (2011). [CrossRef] [PubMed]

]. All these fiber designs were customized, and are not yet commercially available. They were primarily tested for their SBS threshold, while the beam quality has been investigated in terms of M2 measurements. Even though the output beams were found to be near diffraction limited, with M2 values between 1.1 [4

4. Y. Jeong, J. Nilsson, J. K. Sahu, D. N. Payne, L. M. B. Hickey, and P. W. Turner, “Power scaling of single-frequency ytterbium-doped master-oscillator power-amplifier sources up to 500 W,” IEEE J. Sel. Top. Quantum Electron. 13(3), 546–551 (2007). [CrossRef]

] and 1.3 [6

6. C. Robin and I. Dajani, “Acoustically segmented photonic crystal fiber for single-frequency high-power laser applications,” Opt. Lett. 36(14), 2641–2643 (2011). [CrossRef] [PubMed]

], a conclusion about the TEM00-mode content cannot be drawn. In fact, M2 values of <1.1 still allow for the fraction of higher order mode beam content to be as high as 30% [7

7. S. Wielandy, “Implications of higher-order mode content in large mode area fibers with good beam quality,” Opt. Express 15(23), 15402–15409 (2007). [CrossRef] [PubMed]

].

2. Experimental setup

We chose a polarization maintaining PCF (DC-400-40-PZ-Yb) by NKT photonics, containing a 40 µm Yb-doped silica core with a NA of 0.03 for our experiments. The resulting mode field diameter (MFD) of 29 µm of this fiber is the largest commercially available one. It was specified as operating transversely single-mode at a signal wavelength of 1060 nm. The pump-cladding diameter and NA were 400 µm and 0.46, respectively, leading to a cladding absorption of 2.4 dB/m at 976 nm. Furthermore, the fiber had an airclad and an outer silica glass cladding with a diameter of 700 µm, which was finally coated with a high temperature acrylate. Two boron stress rods induced high birefringence to ensure linear-polarization operation, if the seed signal polarization was launched in the slow fiber axis.

A fiber length of 6.8 m was used in the experiments. It was partly coiled on a 40 cm diameter metal spool, but not actively cooled. Only the pump end of the fiber was placed in a water-cooled copper V-groove. The fiber ends were angle-polished, after the airholes had been collapsed to seal the end facets.

The setup is depicted in Fig. 1
Fig. 1 Experimental setup. PR: partial reflector, DCM: dichroic mirror.
. A nonplanar ring oscillator (NPRO) delivered the low-noise ~1 kHz linewidth seed signal with an output power of 500 mW at 1064 nm. This seed signal was preamplified in a step-index PM fiber with a 10 µm core (PLMA-YDF-10/125 by Nufern), which was chosen to assure single-mode seeding of the main-amplifier. For improved long term reliability, an all-fiber set-up was chosen for the pre-amplification stage. It was pumped in a co-propagating scheme to protect the pump diodes. The maximum available power in front of the main amplifier was about 8 W. Two 30 dB isolators protected the preamplifier from backreflections of the main amplifier. A partial reflector was used to sample the backscattered light from the PCF amplifier stage, while the transmitted pump light was separated from the signal beam by a dichroic mirror (DCM). The seed signal was mode-matched to the fundamental mode of the main amplifier fiber by a spherical lens with f = 100 mm and an axial gradient glass lens with f = 10 mm.

A counter-propagating pump scheme was used for the PCF amplifier stage. The pump light from a temperature stabilized laser diode module emitting at 976 nm was delivered by a multimode fiber with a fiber diameter of 600 µm and a NA 0.22. It was collimated by a commercial water-cooled high-power collimator, and subsequently launched into the fiber through an f = 100 mm spherical lens and an axial gradient glass lens with f = 15 mm. A pump coupling efficiency of approximately 85% could be achieved. After separation from the pump light by a dichroic mirror, the amplified signal passed through a silica plate, which sampled part of the beam for diagnostic purposes.

3. Experimental results

3.1 General amplifier characterization

The amplifier output power versus the absorbed pump power is plotted in Fig. 2(a)
Fig. 2 (a) Signal output power at 1064 nm (squares) and fractional TEM00 mode content (triangles) versus absorbed pump power at 976 nm. (b) Output spectrum at 246 W of output power. The resolution bandwidth of the optical spectrum analyzer was set to 0.5 nm.
. The differential optical-to-optical efficiency with respect to the absorbed pump power was 80%. At an absorbed pump power of 363 W and a corresponding output power of 294 W parasitic laser processes occurred, which limited the power scaling of the amplifier. The parasitic laser processes became manifested in additional laser spikes in the forward optical spectrum. Possible sources of a parasitic resonator are backreflections from optical components, even though the amplifier gain was only about 16 dB. In addition, the lasing could have occurred on a higher order mode due to transverse spectral hole burning [10

10. Z. Jiang and J. R. Marciante, “Impact of transverse spatial-hole burning on beam quality in large mode area Yb-doped fibers,” J. Opt. Soc. Am. B 25(2), 247–254 (2008). [CrossRef]

]. In this case, the mode would have experienced a significantly higher gain. No further amplifier characterization at this power level was done to prevent the amplifier fiber and other optical components from being damaged. However, the ASE suppression was >50 dB and therefore still excellent at an output power of 246 W, as can be seen in optical output spectrum (Fig. 2(b)).

To verify that the amplifier was operating below the SBS threshold, the forward relative intensity noise and high resolution backward optical spectra were monitored. No indication of stimulated Brillouin scattering could be observed at the highest power level of 294 W. The polarization extinction ratio (PER) of the system was 23 dB at an output power of 11 W. At 246 W, even a PER of 27 dB could be achieved.

Increasing the seed power should allow for further power scaling, which would require a modified preamplifier allowing for a higher output power. However, the aim of this experiment was to identify the limit of this simple two-stage setup with a truly single-mode all-fiber seed for the higher power final amplifier stage and to measure the fractional TEM00-mode content of the beam at the maximum achieved output power.

3.2 Mode content measurements

Further mode scans were carried out at different power levels. The corresponding measured fractional TEM00 mode contents of the core light are also shown in Fig. 2(a) (blue triangles). At an amplifier output power of 33 W the higher order mode content was found to be already 7.2%. Using a 3 m long sample of a PCF with a slightly smaller core diameter (38 µm) and a different geometry of the stress rods, the higher order mode content of the output beam was less than 5% even at an amplifier output power of 121 W. Anyhow, with that fiber, increasing the output power led to the onset of mode-instabilities [12

12. T. Eidam, C. Wirth, C. Jauregui, F. Stutzki, F. Jansen, H.-J. Otto, O. Schmidt, T. Schreiber, J. Limpert, and A. Tünnermann, “Experimental observations of the threshold-like onset of mode instabilities in high power fiber amplifiers,” Opt. Express 19(14), 13218–13224 (2011). [CrossRef] [PubMed]

], so that the mode scanning experiments with that fiber sample were stopped at this power level. For the single-stage amplifier presented in Ref [8

8. M. Hildebrandt, M. Frede, P. Kwee, B. Willke, and D. Kracht, “Single-frequency master-oscillator photonic crystal fiber amplifier with 148 W output power,” Opt. Express 14(23), 11071–11076 (2006). [CrossRef] [PubMed]

]. the output beam contained only 2% higher order modes at 28 W of output power. Nevertheless, the beam quality decreased to a higher order mode content of 7.4% at the maximum output power of 148 W.

4. Conclusion

Acknowledgment

This work was conducted in the framework of the Cluster of Excellence “Centre for Quantum-Engineering and Space-Time Research” (QUEST), funded by the German Research Foundation (DFG).

References and links

1.

L. Winkelmann, O. Puncken, R. Kluzik, C. Veltkamp, P. Kwee, J. Poeld, C. Bogan, B. Willke, M. Frede, J. Neumann, P. Wessels, and D. Kracht, “Injection-locked single-frequency laser with an output power of 220 W,” Appl. Phys. B 102(3), 529–538 (2011). [CrossRef]

2.

N. Mavalvala, D. E. McClelland, G. Mueller, D. H. Reitze, R. Schnabel, and B. Willke, “Lasers and optics: looking towards third generation gravitational wave detectors,” Gen. Relativ. Gravit. 43(2), 569–592 (2011). [CrossRef]

3.

C. Wirth, O. Schmidt, A. Kliner, T. Schreiber, R. Eberhardt, and A. Tünnermann, “High-power tandem pumped fiber amplifier with an output power of 2.9 kW,” Opt. Lett. 36(16), 3061–3063 (2011). [CrossRef] [PubMed]

4.

Y. Jeong, J. Nilsson, J. K. Sahu, D. N. Payne, L. M. B. Hickey, and P. W. Turner, “Power scaling of single-frequency ytterbium-doped master-oscillator power-amplifier sources up to 500 W,” IEEE J. Sel. Top. Quantum Electron. 13(3), 546–551 (2007). [CrossRef]

5.

C. Zhu, I. Hu, X. Ma, and A. Galvanauskas “Single-frequency and single-transverse-mode Yb-doped CCC fiber MOPA with robust polarization SBS-free 511 W output,” Advanced Solid-State Photonics, OSA Technical Digest (CD) (Optical Society of America, 2011), paper AMC5.

6.

C. Robin and I. Dajani, “Acoustically segmented photonic crystal fiber for single-frequency high-power laser applications,” Opt. Lett. 36(14), 2641–2643 (2011). [CrossRef] [PubMed]

7.

S. Wielandy, “Implications of higher-order mode content in large mode area fibers with good beam quality,” Opt. Express 15(23), 15402–15409 (2007). [CrossRef] [PubMed]

8.

M. Hildebrandt, M. Frede, P. Kwee, B. Willke, and D. Kracht, “Single-frequency master-oscillator photonic crystal fiber amplifier with 148 W output power,” Opt. Express 14(23), 11071–11076 (2006). [CrossRef] [PubMed]

9.

C. Gréverie, A. Brillet, C. N. Man, W. Chaibi, J. P. Coulon, and K. Feliksik, “High power fiber amplifier for Advanced Virgo,” Conference on Lasers and Electro-Optics, OSA Technical Digest (CD) (Optical Society of America, 2010), paper JTuD36.

10.

Z. Jiang and J. R. Marciante, “Impact of transverse spatial-hole burning on beam quality in large mode area Yb-doped fibers,” J. Opt. Soc. Am. B 25(2), 247–254 (2008). [CrossRef]

11.

P. Kwee, F. Seifert, B. Willke, and K. Danzmann, “Laser beam quality and pointing measurements with an optical resonator,” Rev. Sci. Instrum. 78(7), 073103 (2007). [CrossRef]

12.

T. Eidam, C. Wirth, C. Jauregui, F. Stutzki, F. Jansen, H.-J. Otto, O. Schmidt, T. Schreiber, J. Limpert, and A. Tünnermann, “Experimental observations of the threshold-like onset of mode instabilities in high power fiber amplifiers,” Opt. Express 19(14), 13218–13224 (2011). [CrossRef] [PubMed]

13.

C. Jauregui, T. Eidam, J. Limpert, and A. Tünnermann, “The impact of modal interference on the beam quality of high-power fiber amplifiers,” Opt. Express 19(4), 3258–3271 (2011). [CrossRef] [PubMed]

14.

A. V. Smith and J. J. Smith, “Mode instability in high power fiber amplifiers,” Opt. Express 19(11), 10180–10192 (2011). [CrossRef] [PubMed]

OCIS Codes
(060.2320) Fiber optics and optical communications : Fiber optics amplifiers and oscillators
(140.3295) Lasers and laser optics : Laser beam characterization
(060.5295) Fiber optics and optical communications : Photonic crystal fibers

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: January 18, 2012
Revised Manuscript: February 9, 2012
Manuscript Accepted: February 13, 2012
Published: February 17, 2012

Citation
Malte Karow, Chandrajit Basu, Dietmar Kracht, Jörg Neumann, and Peter Weßels, "TEM00 mode content of a two stage single-frequency Yb-doped PCF MOPA with 246 W of output power," Opt. Express 20, 5319-5324 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-5-5319


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References

  1. L. Winkelmann, O. Puncken, R. Kluzik, C. Veltkamp, P. Kwee, J. Poeld, C. Bogan, B. Willke, M. Frede, J. Neumann, P. Wessels, and D. Kracht, “Injection-locked single-frequency laser with an output power of 220 W,” Appl. Phys. B102(3), 529–538 (2011). [CrossRef]
  2. N. Mavalvala, D. E. McClelland, G. Mueller, D. H. Reitze, R. Schnabel, and B. Willke, “Lasers and optics: looking towards third generation gravitational wave detectors,” Gen. Relativ. Gravit.43(2), 569–592 (2011). [CrossRef]
  3. C. Wirth, O. Schmidt, A. Kliner, T. Schreiber, R. Eberhardt, and A. Tünnermann, “High-power tandem pumped fiber amplifier with an output power of 2.9 kW,” Opt. Lett.36(16), 3061–3063 (2011). [CrossRef] [PubMed]
  4. Y. Jeong, J. Nilsson, J. K. Sahu, D. N. Payne, L. M. B. Hickey, and P. W. Turner, “Power scaling of single-frequency ytterbium-doped master-oscillator power-amplifier sources up to 500 W,” IEEE J. Sel. Top. Quantum Electron.13(3), 546–551 (2007). [CrossRef]
  5. C. Zhu, I. Hu, X. Ma, and A. Galvanauskas “Single-frequency and single-transverse-mode Yb-doped CCC fiber MOPA with robust polarization SBS-free 511 W output,” Advanced Solid-State Photonics, OSA Technical Digest (CD) (Optical Society of America, 2011), paper AMC5.
  6. C. Robin and I. Dajani, “Acoustically segmented photonic crystal fiber for single-frequency high-power laser applications,” Opt. Lett.36(14), 2641–2643 (2011). [CrossRef] [PubMed]
  7. S. Wielandy, “Implications of higher-order mode content in large mode area fibers with good beam quality,” Opt. Express15(23), 15402–15409 (2007). [CrossRef] [PubMed]
  8. M. Hildebrandt, M. Frede, P. Kwee, B. Willke, and D. Kracht, “Single-frequency master-oscillator photonic crystal fiber amplifier with 148 W output power,” Opt. Express14(23), 11071–11076 (2006). [CrossRef] [PubMed]
  9. C. Gréverie, A. Brillet, C. N. Man, W. Chaibi, J. P. Coulon, and K. Feliksik, “High power fiber amplifier for Advanced Virgo,” Conference on Lasers and Electro-Optics, OSA Technical Digest (CD) (Optical Society of America, 2010), paper JTuD36.
  10. Z. Jiang and J. R. Marciante, “Impact of transverse spatial-hole burning on beam quality in large mode area Yb-doped fibers,” J. Opt. Soc. Am. B25(2), 247–254 (2008). [CrossRef]
  11. P. Kwee, F. Seifert, B. Willke, and K. Danzmann, “Laser beam quality and pointing measurements with an optical resonator,” Rev. Sci. Instrum.78(7), 073103 (2007). [CrossRef]
  12. T. Eidam, C. Wirth, C. Jauregui, F. Stutzki, F. Jansen, H.-J. Otto, O. Schmidt, T. Schreiber, J. Limpert, and A. Tünnermann, “Experimental observations of the threshold-like onset of mode instabilities in high power fiber amplifiers,” Opt. Express19(14), 13218–13224 (2011). [CrossRef] [PubMed]
  13. C. Jauregui, T. Eidam, J. Limpert, and A. Tünnermann, “The impact of modal interference on the beam quality of high-power fiber amplifiers,” Opt. Express19(4), 3258–3271 (2011). [CrossRef] [PubMed]
  14. A. V. Smith and J. J. Smith, “Mode instability in high power fiber amplifiers,” Opt. Express19(11), 10180–10192 (2011). [CrossRef] [PubMed]

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