OSA's Digital Library

Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 15, Iss. 8 — Apr. 16, 2007
  • pp: 4781–4786
« Show journal navigation

High power scaling of a passively modelocked laser oscillator in a bounce geometry

D. J. Farrell and M. J. Damzen  »View Author Affiliations


Optics Express, Vol. 15, Issue 8, pp. 4781-4786 (2007)
http://dx.doi.org/10.1364/OE.15.004781


View Full Text Article

Acrobat PDF (602 KB)





Browse Journals / Lookup Meetings

Browse by Journal and Year


   


Lookup Conference Papers

Close Browse Journals / Lookup Meetings

Article Tools

Share
Citations

Abstract

In this paper we present the first operation and power scaling of a modelocked Nd:YVO4 bounce laser oscillator at 1064nm. We obtain up to 16.7W of average output power from 38W of pump power, in a continuous-wave modelocked pulse train with 30ps pulses at a repetition rate of 78MHz. We then use a Master Oscillator Power Amplifier (MOPA) configuration utilising another bounce amplifier, to achieve 60W of modelocked output power.

© 2007 Optical Society of America

1. Introduction

There is increasing interest in multi-watt solid-state, picosecond laser sources for use in industrial applications, as well as basic scientific applications, as the high peak powers attainable in a short pulse format mean they offer advantages for material processing with reduced heat damage, and provide efficient harmonic conversion to the useful visible and ultraviolet spectral regions. Many of these systems are based on passive modelocking with Semiconductor Saturable Absorber Mirrors (SESAM) as they are simple to implement, and modern growth technologies mean that their properties can be tailored for particular implementations [1

1. U. Kelleret al, “Semiconductor Saturable Absorber Mirrors (SESAM’s) for Femtosecond to Nanosecond Pulse Generation in Solid-State Lasers,” IEEE J. Sel. Top. Quantum Electron. 2, 435–453 (1996). [CrossRef]

,2

2. S. Tsudaet al, “Mode-Locking Ultrafast Solid-State Lasers with Saturable Bragg Reflectors,” IEEE J. Sel. Top. Quantum Electron. 2, 454–464 (1996). [CrossRef]

]. Further, many of these SESAM laser systems are based on diode end-pumped solid state geometries [3–5

3. Jing-Liang Heet al, “4-ps passively mode-locked Nd:Gd0.5Y0.5VO4 laser with a semiconductor saturable-absorber mirror,” Opt. Lett. 29, 2803–2805 (2004). [CrossRef] [PubMed]

], and whilst this technique offers the possibility of achieving very high beam quality, the power scaling potential is limited by thermal issues arising from the high optical intensities at the pump face. Side-pumped slab geometries have been investigated to try and overcome this issue by distributing the pump power over a larger volume [6

6. T. Grafet al, “Multi-Watt Nd:YVO4 laser, mode-locked by a semiconductor saturable absorber mirror and side-pumped by a diode-laser bar,” Opt. Commun. 159, 84–87 (1999). [CrossRef]

,7

7. G. J. Spühleret al, “Diode-pumped passively mode-locked Nd:YAG laser with 10-W average power in a diffraction-limited beam,” Opt. Lett. 24, 528–530 (1999). [CrossRef]

]. This gives better power scaling potential, but comes at the cost of generally reducing the attainable beam quality and efficiency due to the asymmetry between the pumped volume and the laser mode.

2. Modelocked oscillator

The experimental set-up for the modelocked bounce oscillator is shown in Fig. 1. The oscillator consists of a slab crystal of Nd:YVO4 with 1.1 at.% doping and dimensions 20 × 5 × 2 mm. The crystal was pumped on its 20 × 2 mm face by a single 40 W diode bar at 808 nm and the face was anti-reflection coated at this wavelength. The diode employed fast axis collimation and the emission was brought to a line focus on the crystal face by a vertical cylindrical lens (VCLD) of focal length 30 mm. The end faces of the slab crystal were anti-reflection coated at the lasing wavelength of 1064 nm and the cavity was formed by a SESAM designed for operation at 1064 nm, and an output coupling plane mirror coated to give a transmission of 50% at the lasing wavelength. The SESAM used is a commercially available device from BATOP [12] and has a modulation depth of 3%, a recovery time of less than 10 ps, non-saturable losses of less than 0.3% and a saturation fluence of 70 μJ/cm2. Two spherical mirrors with radii of curvatures 50 cm and 25 cm were positioned to re-image an intracavity beam waist onto the SESAM, with a reduction in the mode area by a factor of 4. Two vertical cylindrical lenses (VCL1 and VCL2) of focal length 50 mm and a horizontal cylindrical lens (HCL) of focal length 100 mm were used inside the cavity to facilitate mode-matching between the gain region and the TEM00 cavity mode.

Fig. 1. The experimental set-up for the modelocked Nd:YVO4 bounce oscillator. The dotted box indicates the position of the back mirror when the cavity is operated in a compact, CW-running form.

With this cavity design, we obtain a CW modelocked pulse-train with a repetition rate of 78 MHz, and a background free intensity auto-correlation, based on a Michelson interferometer, was used to determine the pulse duration as 30ps. The spectral bandwidth of the laser emission could be determined only to be greater than 10 GHz by use of an available Fabry-Perot etalon with a free spectral range of 10 GHz. Figure. 2 shows the pulse train on two different timescales, one short, demonstrating clean pulses, the other significantly longer, demonstrating CW modelocking with no large scale variations in the pulse amplitude.

Fig. 2. The pulse train from the modelocked oscillator. On the left, a short timescale showing artefact-free pulses. On the right, a longer timescale showing no modulation in the pulse amplitude.

The output average power for the cavity as a function of diode pump power is shown in Fig. 3. We obtain a maximum output power of 16.7 W with 38 W of optical pumping, representing an overall optical-optical efficiency of 44%. The slope efficiency was 55% and the output beam is TEM00 at all power levels.

Fig. 3. Average output power vs input pump power for the modelocked oscillator (red triangles), and the compact CW-running cavity (black squares).

To assess how well the oscillator is performing, we performed a comparison of the modelocked cavity with a bounce oscillator configured for CW running in a compact form. In this case, the back section of the laser cavity which contains the SESAM, is replaced by a 100% reflecting mirror placed in the position indicated in Fig. 1, and we compare the power curves obtained from each. The resultant output average power for the compact CW cavity is also shown in Fig. 3. For the CW cavity, we obtain a maximum output of 18.1 W in a TEM00 beam from 38 W of optical pumping. Considering the insertion losses of the SESAM, this demonstrates that the modelocked cavity is operating very efficiently close to ideal. The two cavities operate with very similar slope efficiencies and it is observed that the output beam profiles are also almost identical, further evidence of the integrity of the modelocked cavity. It is noted further that the lasing threshold for both cavities is similar (~3 W optical pump power), and that the threshold for the onset of modelocking is at approximately 7 W of optical pumping.

3. Modelocked master oscillator power amplifier

With this result and configuration we are approaching the damage limits of the SESAM structure and so a further increase in the attainable modelocked power would require a redesign of the cavity. This would then have to be the case at another power level when the damage limit is once again reached. A fixed cavity design is thus not scaleable to an arbitrarily high power and a more flexible approach to achieving high power is to use a Master Oscillator - Power Amplifier (MOPA) arrangement (Fig. 4.) [14

14. A. Agnesiet al, “Amplification of a low-power picosecond Nd:YVO4 laser by a diode-laser, side-pumped, grazing-incidence slab amplifier,” IEEE J. Quant. Elec. , 42, 772–776 (2006). [CrossRef]

,15

15. Y. Ojimaet al, “Over 10-watt pico-second diffraction-limited output from a Nd:YVO4 slab amplifier with a phase conjugate mirror,” Opt. Express 13, 8993–8998 (2005). [CrossRef] [PubMed]

].

Fig. 4. Experimental set-up for modelocked MOPA

For this, we use another high gain bounce geometry module as the power amplifier, with the previously described modelocked cavity as the master oscillator. For the amplifier, we use a 25 × 5 × 2 mm, 1.1 at.% doped Nd:YVO4 crystal and an internal bounce angle of 7°. The crystal is pumped by a 100 W diode bar brought to a line focus by a 25 mm focal length VCL. Again, we use two VCLs with focal lengths 50 mm for mode matching in the vertical. With a master oscillator average power of 10.4 W, and varying the pump power of the amplifier, we obtain the power curve shown in Fig. 5. With maximum amplifier pumping of 104 W, we obtain a maximum modelocked output power of 60 W, which represents an amplifier extraction efficiency of 48%. The beam quality is maintained as TEM00 over the full pumping range of the amplifier which demonstrates the high power scaling potential of the bounce geometry as an amplifier solution. The MOPA power curve shows no sign of saturation and so it should be possible to scale the output power further without degrading the beam, by simply increasing the amplifier pumping. With this further power scaling potential we expect to obtain a modelocked output of greater than 100 W.

Fig. 5. MOPA average output power vs amplifier pump power.

4. Conclusion

In conclusion, we have demonstrated CW modelocked operation of the bounce amplifier geometry and attained a modelocked oscillator that gives a constant amplitude pulse train with pulses of duration 30 ps. Modelocking can be maintained over a wide range of pump powers and we achieve a maximum output power of 16.7 W with overall optical-optical conversion efficiency of 44% and a slope efficiency of 50%. We then implemented a further bounce power amplifier in a MOPA configuration, and using a 10.4 W master oscillator achieved an amplified modelocked output power of 60 W, which represents an extraction efficiency from the amplifier of 48%. No sign of saturation was visible and we thus anticipate that further power scaling of the modelocked output should be possible to the 100 W level by simply increasing the amplifier pump power.

Acknowledgments

The Authors acknowledge support from the Engineering and Physical Sciences Research Council (UK) under grant number GR/T08555/01.

References and links

1.

U. Kelleret al, “Semiconductor Saturable Absorber Mirrors (SESAM’s) for Femtosecond to Nanosecond Pulse Generation in Solid-State Lasers,” IEEE J. Sel. Top. Quantum Electron. 2, 435–453 (1996). [CrossRef]

2.

S. Tsudaet al, “Mode-Locking Ultrafast Solid-State Lasers with Saturable Bragg Reflectors,” IEEE J. Sel. Top. Quantum Electron. 2, 454–464 (1996). [CrossRef]

3.

Jing-Liang Heet al, “4-ps passively mode-locked Nd:Gd0.5Y0.5VO4 laser with a semiconductor saturable-absorber mirror,” Opt. Lett. 29, 2803–2805 (2004). [CrossRef] [PubMed]

4.

Y. F. Chenet al, “Diode-end-pumped passively mode-locked high-power Nd:YVO4 laser with a relaxed saturable Bragg reflector,” Opt. Lett. 26, 199–201 (2001). [CrossRef]

5.

J. Konget al, “Diode-pumped passively mode-locked Nd:GdVO4 laser with a GaAs saturable absorber mirror,” Appl. Phys. B 79, 203–206 (2004). [CrossRef]

6.

T. Grafet al, “Multi-Watt Nd:YVO4 laser, mode-locked by a semiconductor saturable absorber mirror and side-pumped by a diode-laser bar,” Opt. Commun. 159, 84–87 (1999). [CrossRef]

7.

G. J. Spühleret al, “Diode-pumped passively mode-locked Nd:YAG laser with 10-W average power in a diffraction-limited beam,” Opt. Lett. 24, 528–530 (1999). [CrossRef]

8.

A. Minassianet al, “Ultrahigh-efficiency TEM00 diode-side-pumped Nd:YVO4 laser,” Appl. Phys. B 76, 341–343 (2003). [CrossRef]

9.

A. Minassianet al, “High-Power Scaling (>100 W) of a Diode-Pumped TEM00 Nd:GdVO4 Laser System,” IEEE J. Sel. Top. Quantum Electron. 11, 621–625 (2005). [CrossRef]

10.

A. Minassianet al, “Ultrahigh repetition rate Q-switched 101W TEM00 Nd:GdVO4 laser system,” in Proceedings of Conference on Lasers and Electro-Optics (CLEO) Europe, Munich, 2005, paper CA3-2-Tue.

11.

G. Smithet al, “High Power-Scaling of Self-Organising Adaptive Lasers with Gain Holography,” in Proceedings of Conference on Lasers and Electro-Optics (CLEO), Long Beach, Calif., paper CFM-1-Fri.

12.

http://www.batop.de

13.

C. Hönningeret al, “Q-switching stability limits of continuous-wave passive mode locking,” J. Opt. Soc. Am. B 16, 46–56 (1999). [CrossRef]

14.

A. Agnesiet al, “Amplification of a low-power picosecond Nd:YVO4 laser by a diode-laser, side-pumped, grazing-incidence slab amplifier,” IEEE J. Quant. Elec. , 42, 772–776 (2006). [CrossRef]

15.

Y. Ojimaet al, “Over 10-watt pico-second diffraction-limited output from a Nd:YVO4 slab amplifier with a phase conjugate mirror,” Opt. Express 13, 8993–8998 (2005). [CrossRef] [PubMed]

OCIS Codes
(140.3580) Lasers and laser optics : Lasers, solid-state
(140.4050) Lasers and laser optics : Mode-locked lasers

ToC Category:
Lasers and Laser Optics

History
Original Manuscript: November 29, 2006
Revised Manuscript: February 6, 2007
Manuscript Accepted: February 11, 2007
Published: April 4, 2007

Citation
D. J. Farrell and M. J. Damzen, "High power scaling of a passively modelocked laser oscillator in a bounce geometry," Opt. Express 15, 4781-4786 (2007)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-8-4781


Sort:  Author  |  Year  |  Journal  |  Reset  

References

  1. U. Keller et al, "Semiconductor Saturable Absorber Mirrors (SESAM’s) for Femtosecond to Nanosecond Pulse Generation in Solid-State Lasers," IEEE J. Sel. Top. Quantum Electron. 2, 435-453 (1996). [CrossRef]
  2. S. Tsuda et al, "Mode-Locking Ultrafast Solid-State Lasers with Saturable Bragg Reflectors," IEEE J. Sel. Top. Quantum Electron. 2, 454-464 (1996). [CrossRef]
  3. Jing-Liang He et al, "4-ps passively mode-locked Nd:Gd0.5Y0.5VO4 laser with a semiconductor saturable-absorber mirror," Opt. Lett. 29, 2803-2805 (2004). [CrossRef] [PubMed]
  4. Y. F. Chen et al, "Diode-end-pumped passively mode-locked high-power Nd:YVO4 laser with a relaxed saturable Bragg reflector," Opt. Lett. 26, 199-201 (2001). [CrossRef]
  5. J. Kong et al, "Diode-pumped passively mode-locked Nd:GdVO4 laser with a GaAs saturable absorber mirror," Appl. Phys. B 79, 203-206 (2004). [CrossRef]
  6. T. Graf et al, "Multi-Watt Nd:YVO4 laser, mode-locked by a semiconductor saturable absorber mirror and side-pumped by a diode-laser bar," Opt. Commun. 159, 84-87 (1999). [CrossRef]
  7. G. J. Spühler et al, "Diode-pumped passively mode-locked Nd:YAG laser with 10-W average power in a diffraction-limited beam," Opt. Lett. 24, 528-530 (1999). [CrossRef]
  8. A. Minassian et al, "Ultrahigh-efficiency TEM00 diode-side-pumped Nd:YVO4 laser," Appl. Phys. B 76, 341-343 (2003). [CrossRef]
  9. A. Minassian et al, "High-Power Scaling (>100 W) of a Diode-Pumped TEM00 Nd:GdVO4 Laser System," IEEE J. Sel. Top. Quantum Electron. 11, 621-625 (2005). [CrossRef]
  10. A. Minassian et al, "Ultrahigh repetition rate Q-switched 101W TEM00 Nd:GdVO4 laser system," in Proceedings of Conference on Lasers and Electro-Optics (CLEO) Europe, Munich, 2005, paper CA3-2-Tue.
  11. G. Smith et al, "High Power-Scaling of Self-Organising Adaptive Lasers with Gain Holography," in Proceedings of Conference on Lasers and Electro-Optics (CLEO), Long Beach, Calif., paper CFM-1-Fri.
  12. http://www.batop.de
  13. C. Hönninger et al, "Q-switching stability limits of continuous-wave passive mode locking," J. Opt. Soc. Am. B 16, 46-56 (1999). [CrossRef]
  14. A. Agnesi et al, "Amplification of a low-power picosecond Nd:YVO4 laser by a diode-laser, side-pumped, grazing-incidence slab amplifier," IEEE J. Quantum Elec.  42, 772-776 (2006). [CrossRef]
  15. Y. Ojima et al, "Over 10-watt pico-second diffraction-limited output from a Nd:YVO4 slab amplifier with a phase conjugate mirror," Opt. Express 13, 8993-8998 (2005). [CrossRef] [PubMed]

Cited By

Alert me when this paper is cited

OSA is able to provide readers links to articles that cite this paper by participating in CrossRef's Cited-By Linking service. CrossRef includes content from more than 3000 publishers and societies. In addition to listing OSA journal articles that cite this paper, citing articles from other participating publishers will also be listed.


« Previous Article  |  Next Article »

OSA is a member of CrossRef.

CrossCheck Deposited