Q-switched Nd:YAG optical vortex lasers D. J. Kim,1 J. W. Kim,1,* and W. A. Clarkson2 1
Department of Applied Physics, Hanyang University, Ansan, Gyeonggi-do 426-791, South Korea 2 Optoelectronics Research Center, University of Southampton, Southampton, SO17 1BJ, UK * [email protected]
Abstract: Q-switched operation of a high-quality Nd:YAG optical vortex laser with the first order Laguerre-Gaussian mode and well-determined helical wavefronts using a fiber-based pump beam conditioning scheme is reported. A simple two-mirror resonator incorporating an acousto-optic Qswitch was employed, along with an etalon and a Brewster plate to enforce the particular helicity of the output. The laser yielded Q-switched pulses with ~250 μJ pulse energy and ~33 ns pulse duration (FWHM) at a 0.1 kHz repetition rate for 5.1 W of absorbed pump power. The handedness of the helical wavefronts was preserved regardless of the repetition rates. The prospects of further power scaling and improved laser performance are discussed. ©2013 Optical Society of America OCIS codes: (080.4865) Optical vortices; (140.0140) Lasers and laser optics; (140.3070) Infrared and far-infrared lasers; (140.3530) Lasers, neodymium; (140.3540) Lasers, Q-switched.
References and link 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.
A. M. Yao and M. J. Padgett, “Orbital angular momentum:origns, behaviour and applications,” Adv. Opt. Photon. 3(2), 161–204 (2011). L. Allen, M. J. Padgett, and M. Babiker, “The orbital angular momentum of Light,” Prog. Opt. 39, 291–372 (1999). Q. Zhan, “Cylindrical vector beams:from mathematical concepts to applications,” Adv. Opt. Photon. 1(1), 1–57 (2009). S. Franke-Arnold, L. Allen, and M. Padgett, “Advances in optical angular momentum,” Laser & Photon. Rev. 2(4), 299–313 (2008). D. G. Grier, “A revolution in optical manipulation,” Nature 424(6950), 810–816 (2003). J. E. Curtis, B. A. Koss, and D. G. Grier, “Dynamic holographic optical tweezers,” Opt. Commun. 207(1-6), 169–175 (2002). N. P. Robins, C. Figl, M. Jeppesen, G. R. Dennis, and J. D. Close, “A pumped atom laser,” Nat. Phys. 4(9), 731– 736 (2008). A. Jesacher, S. Fürhapter, S. Bernet, and M. Ritsch-Marte, “Spiral interferogram analysis,” J. Opt. Soc. Am. A 23(6), 1400–1409 (2006). G. Molina-Terriza, J. P. Torres, and L. Torner, “Twisted photons,” Nat. Phys. 3(5), 305–310 (2007). J. Hamazaki, R. Morita, K. Chujo, Y. Kobayashi, S. Tanda, and T. Omatsu, “Optical-vortex laser ablation,” Opt. Express 18(3), 2144–2151 (2010). K. Toyoda, F. Takahashi, S. Takizawa, Y. Tokizane, K. Miyamoto, R. Morita, and T. Omatsue, “Transfer of Light helicity to Nanostructures,” Phys. Rev. Lett. 110(14), 143603 (2013). V. Sizyuk, A. Hassanein, and T. Sizyuk, “Hollow laser self-confined plasma for extreme ultraviolet lithography and other applications,” Laser Part. Beams 25(01), 143–154 (2007). C. Paterson and R. Smith, “Higher order Bessel waves produced by axicon-type computer-based holograms,” Opt. Commun. 124(1-2), 121–130 (1996). M. W. Beijersbergen, R. Coerwinkel, M. Kristensen, and J. P. Woerdman, “Helical-wavefront laser beams produced with a spiral phase plate,” Opt. Commun. 112(5-6), 321–327 (1994). M. W. Beijersbergen, L. Allen, H. E. L. O. van der Veen, and J. P. Woerdman, “Astigmatic laser mode converters and transfer of orbital angular momentum,” Opt. Commun. 96(1-3), 123–132 (1993). Y. J. Huang, P. Y. Chiang, H. C. Liang, K. W. Su, and Y. F. Chen, “High-power Q-switched laser with highorder Laguerre–Gaussian modes: application for extra-cavity harmonic generations,” Appl. Phys. B 105(2), 385– 390 (2011). M. Okida, Y. Hayashi, T. Omatsu, J. Hamazaki, and R. Morita, “Characterization of 1.06 μm optical vortex laser based on a side-pumped Nd:GdVO4 bounce oscillator,” Appl. Phys. B 95(1), 69–73 (2009). S. P. Chard, P. C. Shardlow, and M. J. Damzen, “High-power non-astigmatic TEM00 and vortex mode generation in a compact bounce laser design,” Appl. Phys. B 97(2), 275–280 (2009).
#197360 - $15.00 USD Received 9 Sep 2013; revised 29 Oct 2013; accepted 18 Nov 2013; published 21 Nov 2013 (C) 2013 OSA 2 December 2013 | Vol. 21, No. 24 | DOI:10.1364/OE.21.029449 | OPTICS EXPRESS 29449
19. Y. Zhao, Z. Wang, H. Yu, S. Zhuang, H. Zhang, X. Xu, J. Xu, X. Xu, and J. Wang, “Direct generation of optical vortex pulses,” Appl. Phys. Lett. 101(3), 031113 (2012). 20. J. W. Kim, J. I. Mackenzie, J. R. Hayes, and W. A. Clarkson, “High power Er:YAG laser with radially-polarized Laguerre-Gaussian (LG01) mode output,” Opt. Express 19(15), 14526–14531 (2011). 21. J. W. Kim and W. A. Clarkson, “Selective generation of Laguerre–Gaussian(LG0n) mode output in a diode-laser pumped Nd:YAG laser,” Opt. Commun. 296, 109–112 (2013). 22. D. Lin, J. M. Daniel, and W. A. Clarkson, “Single frequency Nd:YAG laser with LG01 donut mode output,” presented at 2013 SPIE Photonics West, San Francisco 2–7 Feb., 8599–51 (2013). 23. C. Tamm, “Frequency locking of two transverse optical modes of a laser,” Phys. Rev. A 38(11), 5960–5963 (1988). 24. R. L. Phillips and L. C. Andrews, “Spot size and divergence for Laguerre Gaussian beams of any order,” Appl. Opt. 22(5), 643–644 (1983). 25. W. Koechner, Solid State Laser Engineering (Springer-Verlag, 1992), Chap. 7–8. 26. D. Lin, J. M. O. Daniel, and W. A. Clarkson, “Controlling the handedness of directly excited Laguerre Gaussian modes in a solid-state laser,” presented at CLEO-Europe-IQEC Munich 12–16 May, CA10.2 (2013). 27. W. A. Clarkson, N. S. Felgate, and D. C. Hanna, “Simple method for reducing the depolarization loss resulting from thermally induced birefringence in solid-state lasers,” Opt. Lett. 24(12), 820–822 (1999).
1. Introduction Optical vortex beams with a ring-shaped intensity profile and helical wavefronts have been attracting much interest due to their possession of orbital angular momentum (OAM) [1–4] and are used in a number of application areas such as optical trapping and manipulation of particles [5–7], optical imaging [8], quantum optics [9] and laser material processing [10–12]. Recently, Omatsu et al. investigated laser ablation using high-energy optical vortex laser pulses and succeeded in fabricating chiral metal nanoneedles with a tip curvature of 99.8%) at the lasing wavelength (1064 nm) and high transmission (T>95%) at the pump wavelength (808 nm) and a plane output coupler (OC) with 10% transmission at the lasing wavelength. A plano-convex lens of a 300 mm
#197360 - $15.00 USD Received 9 Sep 2013; revised 29 Oct 2013; accepted 18 Nov 2013; published 21 Nov 2013 (C) 2013 OSA 2 December 2013 | Vol. 21, No. 24 | DOI:10.1364/OE.21.029449 | OPTICS EXPRESS 29451
200
200
150
150
100
100
50
50
0
0
10
20
30
40
50
120 0
8
100 0
Peak power (kW)
250
Pulse width (ns)
250
6
600
4
400 2 0
0
800
200 0
Repetition rate (kHz)
10
20
30
40
50
0
Average power (mW)
Pulse energy (μ J)
focal length with anti-reflection-coated faces at the pump and lasing wavelengths was placed inside the resonator to ensure that the resonator was stable over the full range of the pump powers used in our experiments. We used an anti-reflection coated 1.0 at% Nd:YAG crystal with a length of 5 mm as the gain medium and mounted it in a water-cooled aluminum heatsink maintained at 19 °C. This was positioned in close proximity to the pump in-coupling mirror. Using this arrangement, the TEM00 mode radius on the input coupler surface was calculated to be ~275 μm, and hence the ring-shaped pump beam waist outer radius was adjusted to be ~325 μm for the LG01 mode generation [21]. The confocal parameter (2π n w 2/ λ M 2) for the pump beam inside the Nd:YAG crystal was calculated to be 17.9 mm satisfying the condition for robust mode selection that the ring-shaped pump beam profile was preserved over the gain region [20]. Under this configuration, the laser was operated in a multi-
Repetition rate (kHz)
(a)
(b)
1 kHz, 232 μJ
5 kHz, 159 μJ
20 kHz, 51 μJ
(c) Fig. 3. (a) Q-switched pulse energy, pulse width (FWHM), peak power and average output power versus a pulse repetition rate, (b) the single Q-switched pulse and its pulse train at the repetition rate of 5 kHz, and (c) the output beam profiles at the repetition rate of 1 kHz, 5 kHz, and 20 kHz for an absorbed pump power of 5.1 W.
longitudinal mode without any particular helical wavefronts [22]. In order to force the LG mode output to have one particular helical wavefronts, the LG0n mode should be composed of
#197360 - $15.00 USD Received 9 Sep 2013; revised 29 Oct 2013; accepted 18 Nov 2013; published 21 Nov 2013 (C) 2013 OSA 2 December 2013 | Vol. 21, No. 24 | DOI:10.1364/OE.21.029449 | OPTICS EXPRESS 29452
degenerate HGn0 and HG0n modes with a locked frequency [23]. Therefore an etalon and a Brewster plate were inserted into the cavity for fixing the operating laser frequency. The continuous-wave (cw) laser output power as a function of absorbed pump power is shown in Fig. 2. The laser yielded 1.5 W of the linearly-polarized output at 1064 nm for an absorbed pump power of 5.7 W. The corresponding slope efficiency with respect to absorbed pump power was 32%. Laser performance was also evaluated with a 5% transmission output coupler. In this case, the laser yielded a maximum output power of 1.1 W for the same maximum pump power corresponding to a slope efficiency of 23%. Thus, we used the output coupler with 10% transmission for the Q-switched laser. The near-field output beam profile was monitored at the relay-imaged position as a function of laser output power with the aid of a silicon CCD camera (Spiricon BS-USB-SP620). The output beam had an axially-symmetric ring-shaped intensity profile at all power levels, the inset in Fig. 2. The beam propagation factor (M2) was measured to be ~2.01 with the aid of a Beam Profiler (Thorlabs) and the silicon CCD camera, which is in close agreement with the theoretical value for a pure LG01 mode, 2 [24], and hence the excited mode was indeed the LG01 mode. Q-switched operation of the Nd:YAG laser was achieved using a TeO2 acousto-optic (AO) Q-switch. The AO Q-switch was carefully placed and adjusted to preserve the spatial beam characteristics while maximizing the diffraction efficiency. The average output power as a function of absorbed pump power in Q-switched mode of operation is shown in Fig. 2. Figure 3 shows the dependence of the pulse energy and the corresponding peak power on the repetition rate for an absorbed pump power of 5.1 W including the oscilloscope traces of the Q-switched pulse and its pulse train for a repetition frequency of 5 kHz. At the pulse repetition frequency of 50 kHz, the pulse energy was ~22 μJ with a relatively long pulse duration of ~194 ns corresponding to a peak power of ~113 W. The pulse energy increased along with the decrease of the pulse width at lower repetition rates due to the increase of the stored energy between the pulses and hence the higher gain. Therefore, at the low pulse repetition frequencies (
Department of Applied Physics, Hanyang University, Ansan, Gyeonggi-do 426-791, South Korea 2 Optoelectronics Research Center, University of Southampton, Southampton, SO17 1BJ, UK * [email protected]
Abstract: Q-switched operation of a high-quality Nd:YAG optical vortex laser with the first order Laguerre-Gaussian mode and well-determined helical wavefronts using a fiber-based pump beam conditioning scheme is reported. A simple two-mirror resonator incorporating an acousto-optic Qswitch was employed, along with an etalon and a Brewster plate to enforce the particular helicity of the output. The laser yielded Q-switched pulses with ~250 μJ pulse energy and ~33 ns pulse duration (FWHM) at a 0.1 kHz repetition rate for 5.1 W of absorbed pump power. The handedness of the helical wavefronts was preserved regardless of the repetition rates. The prospects of further power scaling and improved laser performance are discussed. ©2013 Optical Society of America OCIS codes: (080.4865) Optical vortices; (140.0140) Lasers and laser optics; (140.3070) Infrared and far-infrared lasers; (140.3530) Lasers, neodymium; (140.3540) Lasers, Q-switched.
References and link 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.
A. M. Yao and M. J. Padgett, “Orbital angular momentum:origns, behaviour and applications,” Adv. Opt. Photon. 3(2), 161–204 (2011). L. Allen, M. J. Padgett, and M. Babiker, “The orbital angular momentum of Light,” Prog. Opt. 39, 291–372 (1999). Q. Zhan, “Cylindrical vector beams:from mathematical concepts to applications,” Adv. Opt. Photon. 1(1), 1–57 (2009). S. Franke-Arnold, L. Allen, and M. Padgett, “Advances in optical angular momentum,” Laser & Photon. Rev. 2(4), 299–313 (2008). D. G. Grier, “A revolution in optical manipulation,” Nature 424(6950), 810–816 (2003). J. E. Curtis, B. A. Koss, and D. G. Grier, “Dynamic holographic optical tweezers,” Opt. Commun. 207(1-6), 169–175 (2002). N. P. Robins, C. Figl, M. Jeppesen, G. R. Dennis, and J. D. Close, “A pumped atom laser,” Nat. Phys. 4(9), 731– 736 (2008). A. Jesacher, S. Fürhapter, S. Bernet, and M. Ritsch-Marte, “Spiral interferogram analysis,” J. Opt. Soc. Am. A 23(6), 1400–1409 (2006). G. Molina-Terriza, J. P. Torres, and L. Torner, “Twisted photons,” Nat. Phys. 3(5), 305–310 (2007). J. Hamazaki, R. Morita, K. Chujo, Y. Kobayashi, S. Tanda, and T. Omatsu, “Optical-vortex laser ablation,” Opt. Express 18(3), 2144–2151 (2010). K. Toyoda, F. Takahashi, S. Takizawa, Y. Tokizane, K. Miyamoto, R. Morita, and T. Omatsue, “Transfer of Light helicity to Nanostructures,” Phys. Rev. Lett. 110(14), 143603 (2013). V. Sizyuk, A. Hassanein, and T. Sizyuk, “Hollow laser self-confined plasma for extreme ultraviolet lithography and other applications,” Laser Part. Beams 25(01), 143–154 (2007). C. Paterson and R. Smith, “Higher order Bessel waves produced by axicon-type computer-based holograms,” Opt. Commun. 124(1-2), 121–130 (1996). M. W. Beijersbergen, R. Coerwinkel, M. Kristensen, and J. P. Woerdman, “Helical-wavefront laser beams produced with a spiral phase plate,” Opt. Commun. 112(5-6), 321–327 (1994). M. W. Beijersbergen, L. Allen, H. E. L. O. van der Veen, and J. P. Woerdman, “Astigmatic laser mode converters and transfer of orbital angular momentum,” Opt. Commun. 96(1-3), 123–132 (1993). Y. J. Huang, P. Y. Chiang, H. C. Liang, K. W. Su, and Y. F. Chen, “High-power Q-switched laser with highorder Laguerre–Gaussian modes: application for extra-cavity harmonic generations,” Appl. Phys. B 105(2), 385– 390 (2011). M. Okida, Y. Hayashi, T. Omatsu, J. Hamazaki, and R. Morita, “Characterization of 1.06 μm optical vortex laser based on a side-pumped Nd:GdVO4 bounce oscillator,” Appl. Phys. B 95(1), 69–73 (2009). S. P. Chard, P. C. Shardlow, and M. J. Damzen, “High-power non-astigmatic TEM00 and vortex mode generation in a compact bounce laser design,” Appl. Phys. B 97(2), 275–280 (2009).
#197360 - $15.00 USD Received 9 Sep 2013; revised 29 Oct 2013; accepted 18 Nov 2013; published 21 Nov 2013 (C) 2013 OSA 2 December 2013 | Vol. 21, No. 24 | DOI:10.1364/OE.21.029449 | OPTICS EXPRESS 29449
19. Y. Zhao, Z. Wang, H. Yu, S. Zhuang, H. Zhang, X. Xu, J. Xu, X. Xu, and J. Wang, “Direct generation of optical vortex pulses,” Appl. Phys. Lett. 101(3), 031113 (2012). 20. J. W. Kim, J. I. Mackenzie, J. R. Hayes, and W. A. Clarkson, “High power Er:YAG laser with radially-polarized Laguerre-Gaussian (LG01) mode output,” Opt. Express 19(15), 14526–14531 (2011). 21. J. W. Kim and W. A. Clarkson, “Selective generation of Laguerre–Gaussian(LG0n) mode output in a diode-laser pumped Nd:YAG laser,” Opt. Commun. 296, 109–112 (2013). 22. D. Lin, J. M. Daniel, and W. A. Clarkson, “Single frequency Nd:YAG laser with LG01 donut mode output,” presented at 2013 SPIE Photonics West, San Francisco 2–7 Feb., 8599–51 (2013). 23. C. Tamm, “Frequency locking of two transverse optical modes of a laser,” Phys. Rev. A 38(11), 5960–5963 (1988). 24. R. L. Phillips and L. C. Andrews, “Spot size and divergence for Laguerre Gaussian beams of any order,” Appl. Opt. 22(5), 643–644 (1983). 25. W. Koechner, Solid State Laser Engineering (Springer-Verlag, 1992), Chap. 7–8. 26. D. Lin, J. M. O. Daniel, and W. A. Clarkson, “Controlling the handedness of directly excited Laguerre Gaussian modes in a solid-state laser,” presented at CLEO-Europe-IQEC Munich 12–16 May, CA10.2 (2013). 27. W. A. Clarkson, N. S. Felgate, and D. C. Hanna, “Simple method for reducing the depolarization loss resulting from thermally induced birefringence in solid-state lasers,” Opt. Lett. 24(12), 820–822 (1999).
1. Introduction Optical vortex beams with a ring-shaped intensity profile and helical wavefronts have been attracting much interest due to their possession of orbital angular momentum (OAM) [1–4] and are used in a number of application areas such as optical trapping and manipulation of particles [5–7], optical imaging [8], quantum optics [9] and laser material processing [10–12]. Recently, Omatsu et al. investigated laser ablation using high-energy optical vortex laser pulses and succeeded in fabricating chiral metal nanoneedles with a tip curvature of 99.8%) at the lasing wavelength (1064 nm) and high transmission (T>95%) at the pump wavelength (808 nm) and a plane output coupler (OC) with 10% transmission at the lasing wavelength. A plano-convex lens of a 300 mm
#197360 - $15.00 USD Received 9 Sep 2013; revised 29 Oct 2013; accepted 18 Nov 2013; published 21 Nov 2013 (C) 2013 OSA 2 December 2013 | Vol. 21, No. 24 | DOI:10.1364/OE.21.029449 | OPTICS EXPRESS 29451
200
200
150
150
100
100
50
50
0
0
10
20
30
40
50
120 0
8
100 0
Peak power (kW)
250
Pulse width (ns)
250
6
600
4
400 2 0
0
800
200 0
Repetition rate (kHz)
10
20
30
40
50
0
Average power (mW)
Pulse energy (μ J)
focal length with anti-reflection-coated faces at the pump and lasing wavelengths was placed inside the resonator to ensure that the resonator was stable over the full range of the pump powers used in our experiments. We used an anti-reflection coated 1.0 at% Nd:YAG crystal with a length of 5 mm as the gain medium and mounted it in a water-cooled aluminum heatsink maintained at 19 °C. This was positioned in close proximity to the pump in-coupling mirror. Using this arrangement, the TEM00 mode radius on the input coupler surface was calculated to be ~275 μm, and hence the ring-shaped pump beam waist outer radius was adjusted to be ~325 μm for the LG01 mode generation [21]. The confocal parameter (2π n w 2/ λ M 2) for the pump beam inside the Nd:YAG crystal was calculated to be 17.9 mm satisfying the condition for robust mode selection that the ring-shaped pump beam profile was preserved over the gain region [20]. Under this configuration, the laser was operated in a multi-
Repetition rate (kHz)
(a)
(b)
1 kHz, 232 μJ
5 kHz, 159 μJ
20 kHz, 51 μJ
(c) Fig. 3. (a) Q-switched pulse energy, pulse width (FWHM), peak power and average output power versus a pulse repetition rate, (b) the single Q-switched pulse and its pulse train at the repetition rate of 5 kHz, and (c) the output beam profiles at the repetition rate of 1 kHz, 5 kHz, and 20 kHz for an absorbed pump power of 5.1 W.
longitudinal mode without any particular helical wavefronts [22]. In order to force the LG mode output to have one particular helical wavefronts, the LG0n mode should be composed of
#197360 - $15.00 USD Received 9 Sep 2013; revised 29 Oct 2013; accepted 18 Nov 2013; published 21 Nov 2013 (C) 2013 OSA 2 December 2013 | Vol. 21, No. 24 | DOI:10.1364/OE.21.029449 | OPTICS EXPRESS 29452
degenerate HGn0 and HG0n modes with a locked frequency [23]. Therefore an etalon and a Brewster plate were inserted into the cavity for fixing the operating laser frequency. The continuous-wave (cw) laser output power as a function of absorbed pump power is shown in Fig. 2. The laser yielded 1.5 W of the linearly-polarized output at 1064 nm for an absorbed pump power of 5.7 W. The corresponding slope efficiency with respect to absorbed pump power was 32%. Laser performance was also evaluated with a 5% transmission output coupler. In this case, the laser yielded a maximum output power of 1.1 W for the same maximum pump power corresponding to a slope efficiency of 23%. Thus, we used the output coupler with 10% transmission for the Q-switched laser. The near-field output beam profile was monitored at the relay-imaged position as a function of laser output power with the aid of a silicon CCD camera (Spiricon BS-USB-SP620). The output beam had an axially-symmetric ring-shaped intensity profile at all power levels, the inset in Fig. 2. The beam propagation factor (M2) was measured to be ~2.01 with the aid of a Beam Profiler (Thorlabs) and the silicon CCD camera, which is in close agreement with the theoretical value for a pure LG01 mode, 2 [24], and hence the excited mode was indeed the LG01 mode. Q-switched operation of the Nd:YAG laser was achieved using a TeO2 acousto-optic (AO) Q-switch. The AO Q-switch was carefully placed and adjusted to preserve the spatial beam characteristics while maximizing the diffraction efficiency. The average output power as a function of absorbed pump power in Q-switched mode of operation is shown in Fig. 2. Figure 3 shows the dependence of the pulse energy and the corresponding peak power on the repetition rate for an absorbed pump power of 5.1 W including the oscilloscope traces of the Q-switched pulse and its pulse train for a repetition frequency of 5 kHz. At the pulse repetition frequency of 50 kHz, the pulse energy was ~22 μJ with a relatively long pulse duration of ~194 ns corresponding to a peak power of ~113 W. The pulse energy increased along with the decrease of the pulse width at lower repetition rates due to the increase of the stored energy between the pulses and hence the higher gain. Therefore, at the low pulse repetition frequencies (
