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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 15, Iss. 12 — Jun. 11, 2007
  • pp: 7528–7536
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10 kHz water-cooled Ti :Sapphire femtosecond laser

Guillaume Matras, Nicolas Huot, Emmanuel Baubeau, and Eric Audouard  »View Author Affiliations


Optics Express, Vol. 15, Issue 12, pp. 7528-7536 (2007)
http://dx.doi.org/10.1364/OE.15.007528


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Abstract

We demonstrate operation of a simple and reliable water-cooled femtosecond laser running at 10 kHz suitable for industrial micromachining applications. A laser geometry involving only a regenerative amplifier and delivering 3.5 W average power 60-fs pulses is compared to a more conventional architecture using an additional multi-pass amplifier. Both laser systems require a moderate pumping laser of ~30 W average power and deliver high-quality beams (M2<1.2).

© 2007 Optical Society of America

1. Introduction

The great quality improvement brought by the use of femtosecond lasers in the field of microprocessing of materials is now well established [1

1. C. Momma, B. N. Chichkov, S. Nolte, F. von Alvensleben, A. Tunnermann, H. Welling, and B. Wellegehausen, “Short pulse laser ablation of solid targets,” Opt. Commun. 129, 134–142 (1996). [CrossRef]

, 2

2. S. Valette, R. Fortunier, E. Audouard, R. Le Harzic, N. Huot, and P. Laporte, “Heat affected zone in aluminium single crystals submitted to femtosecond laser irradiations,” Appl. Surf. Sci. 239, 381–386 (2005). [CrossRef]

]. Considering a typical averaged ablation threshold of ~100 mJ.cm-2 in the femtosecond regime, it appeared that the use of >100 μJ-energy pulses was convenient for femtosecond micromachining. 1-kHz repetition rate 1mJ-range femtosecond laser systems based on Ti:Sapphire have then been extensively used both in laboratories and in micromachining companies. Processing durations may benefit from an increase in the repetition rate while maintaining the output energy of the laser source about constant.

We propose to use the simplest cooling method, that is to say water cooling, to develop a reliable, compact 10 kHz femtosecond laser source whose performances are suitable for industrial micromachining applications.

2. Cavity design

The main difficulty to be overpassed to design a high average-power Ti: sapphire amplifier is thermal lensing. Indeed, in a first approximation, by neglecting stress and thermal expansion effects, the focal length of the induced thermal lens in the Ti: Sapphire crystal may be expressed as :

fth=2πr2KηPP(dndT)
(1)

where r is the pump radius, PP is the pumping power, n is the refractive index of the crystal, η is wavelength-dependent and is the fraction of the pump power that is dissipated into heat. K is the heat conductivity [11

11. W. Koechner, “Thermal lensing in a Nd:YAG laser rod,” Appl. Opt. 9, 2548–2553 (1970). [CrossRef] [PubMed]

]. In practice, using water-cooling, the focal length of the thermal lens becomes shorter than 10 cm at a repetition rate of 10 kHz.

In the case of V or Z-type cavities where the laser crystal is placed between two concave mirrors in the vicinity of the beam waist, such a short thermal focal length makes impossible to obtain a large mode size in the crystal and to avoid optical damage in the Pockels cell or on the end mirror. Moreover, the mode size in the crystal has to be kept small enough to reduce the dependence on the fluctuations of thermal lensing in the crystal. Consequently, this involves low average pumping power and moderate power extraction.

We developed a new design of cavity, referred to as Laser 1, that enables to overpass these limitations: the Ti:Sapphire crystal is placed in a V-type cavity at a distance of 2 mm from the end mirror whose curvature is adapted to the focal lens of the thermal lens (Fig. 1). Consequently, both mode diameter and size of the stability zone become independent on the thermal lensing. Only the relative position of the stability zone varies and is centered on the value of the thermal focal length.

Fig. 1. Schematic of Laser 1.

Stability and sensitivity diagrams are plotted in Fig. 2 for cavity parameters detailed in Table 1. The stability curve exhibits a wide stability zone corresponding to a large mode diameter of 290 μm in the crystal. The sensitivity to a cavity misalignment is defined as the inverse of the maximum tilt angle of one cavity mirror that still maintains the laser mode within the pumping area in the crystal. The sensitivity curve keeps well below 1000 rad-1 nearly all over the stability zone, which ensures an easy alignment of the regenerative cavity.

Table 1. cavity parameters for the design of the regenerative amplifier referred to as Laser 1. See Fig. 1 for schematic.

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Fig. 2. Stability and sensitivity curves as a function of the inverse of the thermal focal length. The calculations were conducted with parameters of Table 1. A mode diameter of ~300 μm together with a weak sensitivity to a cavity misalignment are obtained over the major part of the stability zone.

One last difficulty for the development of high average power Ti:Sapphire femtosecond sources is to get a reliable high average power high beam quality green laser. With the amplifier design depicted in section 2, we point out that a large mode size in the crystal increases drastically the tolerance on the pump laser beam quality, which makes the design of a pumping laser much easier, thus reducing the cost and improving reliability.

3. Experimental set-up

All the Ti:Sapphire crystals involved in these experiments are water-cooled. They are wrapped in an indium foil to optimize the heat contact with a copper mount.

Laser 1 is described in Fig. 3. 20-fs 4-nJ pulses at a repetition rate of 75 MHz from a self mode-locked oscillator (Femtosource) are stretched to 250 ps in an Öffner stretcher using a 1200 lines/mm reflection grating. 3,1-mJ 40-ns pulses at a repetition rate of 10 kHz from an intracavity frequency doubled M2=20 diode-pumped Q-switched Nd:YAG laser (Thalés Laser) are focused down to 2w~350 μm inside a 10×10×15 mm3 Ti:sapphire crystal. The cavity design corresponds to the one detailed in section 2. Optical switching for regenerative amplification is ensured by a water-cooled KD*P Pockels cell and two dielectric polarizers with reverse wedge. Two 1500 lines/mm reflection gratings are used in the compressor. The beam diameter on the gratings is 1 cm and the overall compressor efficiency reaches 70 %.

Fig. 3. Experimental set-up of Laser 1: RR: retroreflector; Ti:Sa: Ti:Sapphire crystal (10×10×15 mm3); G1: 1200 lines/mm grating; G2: 1500 lines/mm grating; M1: flat dichroic mirror; M2: R=8 cm convex mirror; M3: R=50 cm concave mirror; M4: flat mirror; M5: R=275 mm convex mirror; M6: R=550 mm concave mirror; L1: f’=20 cm converging lens; L2: f’=50 cm converging lens; L3: f’=15 cm converging lens; L4: f’=-10 cm diverging lens; L5: f’=50 cm converging lens; TFP: thin film polarizer; KD*P: KD*P Pockels cell; FR: Faraday rotator; P: photodiode monitoring.

As a comparison, we developed another amplification geometry, referred to as Laser 2, also using water cooling, and based on commonly used laser architectures. Indeed, Laser 2 involves a regenerative followed by a 2-pass amplifier (Fig. 4). The regenerative amplifier consists in a conventional “V-type” cavity in which the brewster-cut Ti:Sapphire crystal (ϕ6×20 mm) is placed between 2 concave mirrors (M2 and M3) in the vicinity of the beam waist. In this geometry, a small mode size (2w~200 μm) is required in the crystal as detailed in section 2. Consequently, the pump power has to be reduced down to 15 W. Moreover, the position of the crystal together with the beam overlap inside the crystal are still critical. Optical switching for regenerative amplication is the same for Laser 1 and Laser 2. 19 W pumping power are then focused down to 2w~330 μm inside a second water cooled Ti:Sapphire crystal (15×15×10 mm3). A second pass in this amplifier is ensured after reflection on a R=5 cm convex mirror whose curvature compensates for converging thermal lensing in the crystal.

Fig. 4. Amplification set-up of Laser 2. Ti:Sa: Ti:Sapphire crystal; M1: flat dichroic mirror; M2: R=10 cm concave mirror; M3: R=50 cm concave mirror; M4: flat mirror; M5: R=5 cm convex mirror; TFP: thin film polarizer; KD*P: KD*P Pockels cell; FR: Faraday rotator; L1: f’=40 cm converging lens; L2: f’=28.6 cm converging lens; L3: f’=100 cm converging lens; L4: f’=25 cm converging lens; P: photodiode monitoring.

4. Results

Laser 1 delivers up to 5 W after amplification and 3.5 W after compression at a repetition rate of 10 kHz, which is, to our knowledge, the highest value ever obtained in a reliable, compact and water-cooled geometry. This value is to be compared to the 3.5 W output power delivered by Laser 2 after amplification (table 2), which keeps close to those described in reference [5

5. J. Squier, G. Korn, G. Mourou, G. Vaillancourt, and M. Bouvier, “Amplification of femtosecond pulses at 10-kHz repetition rates in Ti :Al2O3,” Opt. Lett. 18, 625–627 (1993). [CrossRef] [PubMed]

] for comparable pumping power. Indeed, the pumping values indicated in Table 1 are measured at the output of the laser, and not on the Ti:Sa crystals. The amplification geometry used in Laser 1 appears to be highly efficient, despite water cooling.

Table 2. Characteristics of the two amplification geometriesreferred to as Laser 1 and Laser 2.

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A shot-to-shot rms stability of 1.5% limited by the stability of the pump is obtained after amplification in Laser 1 at 5 W average output power.

Pulse duration, deduced from an autocorrelation trace (Fig. 5), is 60 fs and is still above the Fourier limit corresponding to the spectral width of 20 nm measured after regenerative amplification. This spectral width value results from a spectral narrowing in the regenerative amplifier. Indeed, the 60 nm-wide amplifier input spectrum is affected by the spectral transmission of the amplifier, which is measured to be 35 nm-wide in free-running condition. Anyway, a pulse duration of 60 fs is very well adapted to micromachining applications, as the ultra-short laser-material interaction regime is typically reached below a few hundreds of femtoseconds [1

1. C. Momma, B. N. Chichkov, S. Nolte, F. von Alvensleben, A. Tunnermann, H. Welling, and B. Wellegehausen, “Short pulse laser ablation of solid targets,” Opt. Commun. 129, 134–142 (1996). [CrossRef]

].

Fig. 5. Autocorrelation trace of compressed pulses of Laser 1, corresponding to a 60-fs pulse width.
Fig. 6. Measurement of the M2 parameter of Laser 1 output beam running at 3.5 W output power after compression. Beam parameters of Mx 2=1.12 and MY 2=1.18 are deduced respectively in both transverse directions X and Y, typical for a high quality beam.

The pulse contrast reaches 3000:1. The beam quality is estimated by a measurement of the M2 parameter. For doing this, a f=40 cm converging lens is used and a CCD camera is translated in the focal region. The corresponding beam size measurements are plotted in Fig. 6. Beam parameters of Mx 2=1.12 and MY 2=1.18 are deduced respectively in both transverse directions X and Y, typical for a high quality beam.

We also mention that Laser 1 is still able to deliver 4.6 W before compression when pumped by a 35-W average power M2=30 green laser, which highlights its ability to be pumped with low beam quality lasers.

Fig. 7. Optical microscope photographs of grooves machined in steel by 10 scans with a 60-fs pulse duration and a-b): 1.68J/cm2, 1.75mm/s, 1kHz or c-d): 1.84 J/cm2, 17.5mm/s, 10kHz. ac): surface of the sample and b-d): bottom of the groove.

The ability of this compact water-cooled 10 kHz femtosecond laser source to perform micromachining has been evaluated by machining grooves on steel samples Fe/Cr18/Ni10/Mo3). The samples are translated in front of the focal spot at a translation speed v and two repetition rates are used: v=17.5 mm.s-1 for 10 kHz and v=1,75 mm.s-1 for 1 kHz using an electronic Pockels-clock divider. All other experimental parameters are the same: pulse duration of 60 fs, beam parameter M2=1.15, focusing lens of f=25 cm focal length, laser fluence of ~1,7 Jcm-2 and 10 scans. Optical microscope images in Fig. 7 show the same machining quality. Only the groove width slightly differs, which is due to a small variation in the focal spot diameter and in the laser fluence between 1 and 10 kHz running conditions. A groove width of 85 μm and 100 μm is obtained at 1 and 10 kHz respectively for both repetition rates with similar depth of around 11.5 μm and the processing time is reduced by a factor of 10 when using this compact and reliable 10 kHz source compared to standard 1 kHz femtosecond lasers. Consequently, this source is suitable for high speed industrial femtosecond laser processing applications.

A detailed study of femtosecond processes in the 10-kHz range is currently under progress.

5. Conclusion

We demonstrated a compact reliable water-cooled femtosecond all-solid-state laser running at a repetition rate of 10 kHz. Average power of 3.5 W is obtained after compression using a moderate pumping laser delivering 31 W. The amplification geometry is highly efficient despite water-cooling, and the relatively large mode size in the Ti:Sapphire crystal enables to increase the tolerance on the pump laser beam quality. The pulse duration is 60 fs and the beam is nearly diffraction limited (M2<1.2). We showed that this amplification geometry is more compact and more efficient than conventional designs involving “V or Z-type” architectures followed by multi-pass amplification. The linear lowering of the time process with repetition rate from 1 up to 10 kHz has been demonstrated, opening the way to reliable high speed industrial femtosecond micromachining.

References and links

1.

C. Momma, B. N. Chichkov, S. Nolte, F. von Alvensleben, A. Tunnermann, H. Welling, and B. Wellegehausen, “Short pulse laser ablation of solid targets,” Opt. Commun. 129, 134–142 (1996). [CrossRef]

2.

S. Valette, R. Fortunier, E. Audouard, R. Le Harzic, N. Huot, and P. Laporte, “Heat affected zone in aluminium single crystals submitted to femtosecond laser irradiations,” Appl. Surf. Sci. 239, 381–386 (2005). [CrossRef]

3.

S. Ito, H. Nagaoka, T. Miura, K. Kobayashi, A. Endo, and K. Torizuka, “Measurement of thermal lensing in a power amplifier of a terawatt Ti:Sapphire laser,” Appl. Phys. B 74, 343–347 (2002). [CrossRef]

4.

M. Zavelani-Rossi, F. Lindner, C. Le Blanc, G. Chériaux, and J. P. Chambaret, “Control of thermal effects for high-intensity Ti :sapphire laser chains,” Appl. Phys. B 70, S193–196 (2000). [CrossRef]

5.

J. Squier, G. Korn, G. Mourou, G. Vaillancourt, and M. Bouvier, “Amplification of femtosecond pulses at 10-kHz repetition rates in Ti :Al2O3,” Opt. Lett. 18, 625–627 (1993). [CrossRef] [PubMed]

6.

Y. Nabekawa, T. Togashi, T. Sekikawa, S. Watanabe, S. Konno, T. Kojima, S. Fujikawa, and K. Yasui, “All-solid-state high-peak-power Ti :Sapphire laser system above 5-kHz repetition rate,” Appl. Phys. B 70, S171–179 (2000). [CrossRef]

7.

D. M. Gaudiosi, A. L. Lytle, P. Kohl, M. M. Murnane, H. C. Kapteyn, and S. Backus, “11-W average power Ti :Sapphire amplifier system using downchirped pulse amplification,” Opt. Lett. 29, 2665–2667 (2004). [CrossRef] [PubMed]

8.

I. Matsushima, H. Yashiro, and T. Tomie, “10 kHz 40W Ti:Sapphire regenerative ring amplifier,” Opt. Lett. 31, 2066–2068 (2006). [CrossRef] [PubMed]

9.

M. Hentschel, Z. Cheng, F. Krausz, and Ch. Spielmann, “Generation of 0.1-TW optical pulses with a single stage Ti:Sapphire amplifier at a 1-kHz repetition rate,” Appl. Phys. B 70, S161–164 (2000). [CrossRef]

10.

N. Zhavoronkov and G. Korn, “Regenerative amplification of femtosecond laser pulses in Ti:Sapphire at multikilohertz repetition rates,” Opt. Lett. 29, 198–200 (2004). [CrossRef] [PubMed]

11.

W. Koechner, “Thermal lensing in a Nd:YAG laser rod,” Appl. Opt. 9, 2548–2553 (1970). [CrossRef] [PubMed]

OCIS Codes
(140.0140) Lasers and laser optics : Lasers and laser optics
(140.3280) Lasers and laser optics : Laser amplifiers
(140.3390) Lasers and laser optics : Laser materials processing
(140.7090) Lasers and laser optics : Ultrafast lasers

ToC Category:
Lasers and Laser Optics

History
Original Manuscript: January 8, 2007
Revised Manuscript: April 19, 2007
Manuscript Accepted: April 20, 2007
Published: June 4, 2007

Citation
Guillaume Matras, Nicolas Huot, Emmanuel Baubeau, and Eric Audouard, "10 kHz water-cooled Ti :Sapphire femtosecond laser," Opt. Express 15, 7528-7536 (2007)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-12-7528


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References

  1. C. Momma, B. N. Chichkov, S. Nolte, F. von Alvensleben, A. Tunnermann, H. Welling, and B. Wellegehausen, "Short pulse laser ablation of solid targets," Opt. Commun. 129, 134-142 (1996). [CrossRef]
  2. S. Valette, R. Fortunier, E. Audouard, R. Le Harzic, N. Huot, and P. Laporte, "Heat affected zone in aluminium single crystals submitted to femtosecond laser irradiations," Appl. Surf. Sci. 239, 381-386 (2005). [CrossRef]
  3. S. Ito, H. Nagaoka, T. Miura, K. Kobayashi, A. Endo, and K. Torizuka, "Measurement of thermal lensing in a power amplifier of a terawatt Ti:Sapphire laser," Appl. Phys. B 74, 343-347 (2002). [CrossRef]
  4. M. Zavelani-Rossi, F. Lindner, C. Le Blanc, G. Chériaux, and J. P. Chambaret, "Control of thermal effects for high-intensity Ti :sapphire laser chains," Appl. Phys. B 70, S193-196 (2000). [CrossRef]
  5. J. Squier, G. Korn, G. Mourou, G. Vaillancourt, and M. Bouvier, "Amplification of femtosecond pulses at 10-kHz repetition rates in Ti :Al2O3," Opt. Lett. 18, 625-627 (1993). [CrossRef] [PubMed]
  6. Y. Nabekawa, T. Togashi, T. Sekikawa, S. Watanabe, S. Konno, T. Kojima, S. Fujikawa, and K. Yasui, "All-solid-state high-peak-power Ti :Sapphire laser system above 5-kHz repetition rate," Appl. Phys. B 70, S171-179 (2000). [CrossRef]
  7. D. M. Gaudiosi, A. L. Lytle, P. Kohl, M. M. Murnane, H. C. Kapteyn, and S. Backus, "11-W average power Ti :Sapphire amplifier system using downchirped pulse amplification," Opt. Lett. 29, 2665-2667 (2004). [CrossRef] [PubMed]
  8. I. Matsushima, H. Yashiro, and T. Tomie, "10 kHz 40W Ti:Sapphire regenerative ring amplifier," Opt. Lett. 31, 2066-2068 (2006). [CrossRef] [PubMed]
  9. M. Hentschel, Z. Cheng, F. Krausz, and Ch. Spielmann, "Generation of 0.1-TW optical pulses with a single stage Ti:Sapphire amplifier at a 1-kHz repetition rate," Appl. Phys. B 70, S161-164 (2000). [CrossRef]
  10. N. Zhavoronkov and G. Korn, "Regenerative amplification of femtosecond laser pulses in Ti:Sapphire at multikilohertz repetition rates," Opt. Lett. 29, 198-200 (2004). [CrossRef] [PubMed]
  11. W. Koechner, "Thermal lensing in a Nd:YAG laser rod," Appl. Opt. 9, 2548-2553 (1970). [CrossRef] [PubMed]

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