Laser properties of continuous-grown Nd:GdVO4/GdVO4 and Nd:YVO4/YVO4

composite crystals under direct pumping

Xudong Li*, Xin Yu, Fei Chen, Renpeng Yan, Junhua Yu, Deying Chen

National Key Laboratory of Tunable Laser Technology, Harbin Institute of Technology, 150080, Harbin, China

*Corresponding author: kevin2025@163.com

Abstract: We present the laser properties of continuous-grown Nd:GdVO4/GdVO4 and Nd:YVO4/YVO4 composite crystals under LD direct pumping . The maximum CW output powers of 20W with a slope efficiency of 74.9% to absorbed pump power and 32W with 82.7% were obtained in Nd:GdVO4/GdVO4 and Nd:YVO4/YVO4 lasers, respectively. To our knowledge, this is the highest slope efficiency obtained in Nd

3+ lasers.

Furthermore, at the repetition rate of 100kHz, the minimum pulse width of 15.0ns and 12.9ns, the peak power of 11.8kW and 22.4kW were obtained for Nd:GdVO4/GdVO4 and Nd:YVO4/YVO4 lasers, respectively.

2009 Optical Society of America

OCIS codes: (140.3580) Lasers, solid states; (140.3530) Lasers, neodymium; (140.3480), Laser, diode-pumped; (140.3540) Laser, Q-switched

References and links

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2. Y. F. Chen, C. F. Kao, T. M. Huang, C. L. Wang, and S. C. Wang, “Influence of thermal effect on output power optimization in fiber-coupled laser-diode end-pumped lasers,” IEEE J. Quantum Electron 3, 29–34 (1997).

3. Y. Sato, T. Taira, N. Pavel, and V. Lupei, “Laser operation with near quantum-defect slope efficiency in Nd:YVO4 under direct pumping into the emitting level,”Appl. Phys. Lett. 82, 844–846 (2003).

4. V. Lupei, N. Pavel, Y. Sato, and T. Taira, “Hihgly efficient 1063-nm continuous-wave laser emission in Nd:GdVO4,” Opt. Lett. 28, 2366–2368 (2003).

5. M. Tsunekane, N. Taguchi and H. Inaba, “High power operation of diode-end-pumped Nd:YVO4 laser using composite rod with undoped end,” Electron. Lett. 32, 40–42 (1996).

6. R. Weber, B. Neuenschwander, and H. P. Weber, “High-power end-pumped composited Nd:YAG rod,” in Conference on Lasers and Electro-Optics Europe, Technical Digest (Optical Society of America, 1996), paper CMA4.

7. Huai-Chuan Lee, Patrick L. Brownlie, Helmuth E. Meissner, and Edward C. Rea, Jr, “Diffusion-bonded composite of YAG single crystals,” Proc. SPIE 1624, 2–10, (1991).

8. T. Ogawa, Y. Urata, S. Wada, K. Onodera, T. Imai, H. Machida, M. Higuchi, and K. Kodaira, “Efficient 879nm-LD pumped Nd:GdVO4 laser and its thermal characteristics,” in Advanced Solid-State Photonics, Technical Digest (Optical Society of America, 2004), paper CThJJ6. http://www.opticsinfobase.org/abstract.cfm?URI=CLEO-2004-CThJJ6

9. N. Pavel and T. Taira, “High-power continuous-wave intracavity frequency-doubled Nd:GdVO4-LBO laser under diode pumping into the emitting level,” IEEE. J. Sel. Top. Quant. 11, 631–636 (2005).

10. X. Li, X. Yu, J. Gao, F. Chen, J. Yu, and D. Chen, “Laser operation at high repetition rate of 100 kHz in Nd:GdVO4 under 879nm diode-laser pumping,” Appl. Phys. B 92, 199–202 (2008).

11. R. H. Senn and L. E. Record, “Multiform crystal and apparatus for fabrication,” US Patent, 5394420 (1995). 12. A. K. Gardner, M. Staninec, and D. Fried, “The influence of surface roughness on the bond strength of

composite to dental hard tissues after Er:YAG laser irradiation,” Proc. SPIE 5687, 144–150 (2005). 13. Xudong Li, Xin Yu, Jing Gao, Jiangbo Peng, Fei Chen, Junhua Yu, and Deying Chen, “Laser operation of LD

end-pumped grown-together Nd:YVO4/YVO4 composite crystal,” Laser Phys. Lett. 5, 429–432 (2008). 14. Jérǒme Goujon, and Olivier Musset, “Comparison between 2 different composite Nd3+:YVO4 crystals in a fibre

coupled diode pumped laser,” in Advanced Solid-State Photonics, Technical Digest (Optical Society of America, 2008), paper WB5. http://www.opticsinfobase.org/abstract.cfm?URI=ASSP-2008-WB5

15. T. Ogawa, T. Imai, K. Onodera, H. Machida, M. Higuchi, Y. Urata, and S. Wada, “Efficient pulse operation of Nd:GdVO4 laser with AO Q-switch,” Appl. Phys. B 81, 521–524 (2005).

#111203 - $15.00 USD Received 11 May 2009; revised 29 Jun 2009; accepted 4 Jul 2009; published 13 Jul 2009

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16. Y. F. Chen, T. M. Huang, C. C. Liao, Y. P. Lan, and S. C. Wang, “Efficient high-power diode-end-pumped TEM00 Nd:YVO4 laser,”IEEE Photonic Tech. L. 11, 1241–1243 (1999).

17. Yoichi Sato and Takunori Taira, “The studies of thermal conductivity in GdVO4, YVO4, and Y3Al5O12 measured by quasi-one dimensional flash method,” Opt. Express 14, 10528–10536 (2006).

18. Y. Sato and T. Taira, “Thermo-optical and –mechanical parameters of Nd:GdVO4 and Nd:YVO4,” in Conference on Quantum Electronics and Laser Science, Technical Digest (Optical Society of America, 2007), paper JWA87. http://www.opticsinfobase.org/abstract.cfm?URI=PhAST-2007-JWA87

19. V. V. Kochurikhin, K. Shimamura, and T. Fukuda, “Czochralski growth of gadolinium vanadate single crystals,” J. Cryst. Growth, 151, 393–395 (1995).

1. Introduction

Diode-laser (LD) end-pumped solid-state lasers (DPSSLs) have widely applications on laser science and engineering community for their compactness, good beam quality, and high energy conversion efficiency. However, because of the limitation on thermal fracture of laser crystals due to the thermal effects, output power from being scaled to a higher power in an end-pumped geometry is relatively difficult [1,2]. Some methods was induced to reduce the thermal effects of the laser crystal, such as direct pumping [3,4] and composite crystal [5,6,7], which were proved to be very efficient means.

Direct pumping has been proved to have the potential to be the most efficient pumping scheme for a four-level laser because it reduces the quantum defect and eliminates the loss induced by the nonunity quantum efficiency. The pumping of the Nd:GdVO4 and Nd:YVO4 crystals at 879nm or 880nm instead of 808nm leads to the reduction of quantum defect ratio from 0.24 to 0.17 in the case of

4F3/2→

4I11/2 emission, which reduces the thermal loading by

about 28% at the laser wavelength of 1063nm. There is a significant reduction of thermal loading when pumping at high power. Takayo Ogawa et al reported an effective Nd:GdVO4

laser with a slope efficiency of 78%, which was pumped by 879nm diode-laser [8]. Y. Sato realized a near quantum-defect slope efficiency in Nd:YVO4 laser under direct pumping [3]. 80% and 75% slope efficiency were obtain under Ti:sapphire and LD pumping at 880nm, respectively. A maximum output power of 8.9W with 60% slope efficiency in absorbed power of 879nm pumped 1.0at.% Nd:GdVO4 laser were obtained by N. Pavel [9]. Our group reported an efficient Nd:GdVO4 laser under 879nm pumping in 2008, a maximum continuous-wave (CW) output power of 13.3W with the slope efficiency of 74.6% in absorbed power was obtained [10]. Increase of output power and slope efficiency could be expected because of the reduction of quantum defect and thermal loading owning to direct pumping into the emitting level.

The adoption of composite crystals is the other effective way to depress the thermal effect of laser crystal. The concept of combining doped and undoped components (composite crystals) for improving thermal uniformity is applied to solid-state laser rods or slabs. Due to the undoped pumped end, the surface contribution to the thermal lens caused by rod expansion is almost eliminated and it’s favorable to compensate for the thermal lens [5]. Composite crystals can be mainly fabricated by optical bonding, fusion bonding or diffusion bonding [7,11]. Taking the advantages of no adhesives, less distortion on the bonding interface and flexible manufacture, diffusion bonding was accepted and developed widely. However, a high quality composite crystal made by diffusion bonding depends on crystal’s surface process [12]. Any impurities and scratches on bonding surfaces can bring additional scatter losses and cause the failure in diffusion bonding. In addition, for anisotropic materials, it is sufficiently necessary to align the orientation of both crystals’ optical axes very delicately before diffusion bonding. Taken the two aspects above for account, composite Nd:GdVO4 and Nd:YVO4

crystals with high quality are relatively difficult to fabricate by diffusion bonding. To the best of our knowledge, we didn’t find any reports about composite Nd:GdVO4 crystal until now. Recently a technique is developed to manufacture the composite crystal [13]. The principle is to make grow the crystal normally doped then to suspend the growth from the solution containing the laser impurities and to replace this bath by an undoped solution. It’s a continuous-grown process. Jérǒme Goujon and Olivier Musset made some detailed

#111203 - $15.00 USD Received 11 May 2009; revised 29 Jun 2009; accepted 4 Jul 2009; published 13 Jul 2009

(C) 2009 OSA 20 July 2009 / Vol. 17, No. 15 / OPTICS EXPRESS 12870

comparisons between two different composite Nd:YVO4 crystals fabricated by diffusion bonding and continuous-growth in LD pumped lasers [14], respectively. Continuous-grown Nd:YVO4/YVO4 composite crystal laser compared with diffusion bonding one, the output power was better and the quality of the beam and its stability were improved. They considered that the continuous-grown composite crystals could represent a very good alternative face to the diffusion bonding one. However, we didn’t find further reports about the laser properties of continuous-grown composite crystal, especially for composite Nd:GdVO4/GdVO4 crystal.

In this paper, for the first time to our knowledge, we studied the laser properties of continuous-grown Nd:GdVO4/GdVO4 and Nd:YVO4/YVO4 composite rods under direct pumping. With the aid of the efficient heat dissipation due to the composite crystal and the reduction of quantum defects due to the direct pumping, extremely high-efficient CW and pulsed lasers used Nd:GdVO4/GdVO4 and Nd:YVO4/YVO4 composite rods were obtained, respectively. The maximum CW output powers of 20W with a slope efficiency of 74.9% to absorbed pump power and 32W with 82.7% were obtained in Nd:GdVO4/GdVO4 and Nd:YVO4/YVO4 lasers, respectively. At the repetition rate of 100kHz, the minimum pulse width of 15.0ns and 12.9ns, the average output power of 17.7W and 28.9W, the peak power of 11.8kW and 22.4kW were obtained for Nd:GdVO4/GdVO4 and Nd:YVO4/YVO4 lasers, respectively.

2. Experimental setup

Fig. 1. Experimental setup of laser system.

An end-pumping scheme was used to demonstrate efficient laser emission at 1.06µm. The experimental setup is shown schematically in Fig. 1. The crystal rods used in our experiments are specified in Table 1. All crystal rods are fabricated by Beijing Ke-Gang Electro-optics Company in China. Two facets of all crystal rods were antireflection-coated at both the pump and laser wavelength. A 100mm long laser cavity was constructed by two plano-plano mirrors M1 and M2. M1 coated antireflection at pump wavelength and high reflectivity at laser wavelength. M2 with a transmissivity at 1064nm of 35% was served as the output coupler. A fiber-coupled LD (NL-LDM-120-879, made by nLIGHT Inc.) with center wavelength locate at 879nm is the pumping source, which has a top-hat intensity distribution and the full width at half maximum (FWHM) is about 2.2nm. The pump light of LD was imaged a spot of 533µm diameter into the crystal rod through two achromatic lenses. The composite crystal rod was wrapped with indium foil and mounted in a copper heat-sink with microchannel structure, which had been proved to have more capability of thermal dissipation.

Table 1. Specifications of crystal rods used in our experiments

Crystal rods Total dimensions (mm×mm×mm)

Undoped rod long (mm)

Nd3+ concentration (at.%)

1 Nd:GdVO4/GdVO4 3×3×10 2 0.3

2 Nd:YVO4/YVO4 3×3×10 2 0.3

3 Nd:GdVO4 4×4×8 — 0.5

4 Nd:YVO4 4×4×8 — 0.5

#111203 - $15.00 USD Received 11 May 2009; revised 29 Jun 2009; accepted 4 Jul 2009; published 13 Jul 2009

(C) 2009 OSA 20 July 2009 / Vol. 17, No. 15 / OPTICS EXPRESS 12871

3. Experimental results and discussions

Prior to the research on the laser properties of continuous-grown composite crystals, a comparative study of conventional Nd:GdVO4 and Nd:YVO4 crystal lasers by direct pumping was carried out under the same conditions.

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Fig. 2. CW output power versus absorbed pump power.

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Fig. 3. Average output power versus absorbed pump power at the repetition rate of 100kHz.

Highly efficient laser systems for Nd:GdVO4 and Nd:YVO4 were achieved under direct pumping at CW and A-O Q-switch mode as shown in Fig. 2 and Fig. 3, respectively. The compressional-mode A-O Q-switch (39041-50DSFPS, made by Gooch and Housego Inc.) had antireflection coating at 1063-nm on both sides and had a centre frequency of 41MHz and a radio-frequency power of 50W. The absorbed pump power was beyond 35W, we didn’t take a risk of increasing the pump power further considering the safety of the laser rods. As shown in Fig. 2, the maximum multi-mode CW output power of both Nd:GdVO4 and Nd:YVO4 lasers were beyond 22W. The optical-optical efficiency of CW output power to absorbed pump power was about 69% and the slope efficiency achieved about 75% for both Nd:GdVO4 and Nd:YVO4 lasers. Fig. 3 shows the average output power as a function of the absorbed pump power at the repetition rate of 100kHz. More than 20W average output power were obtained and the slope efficiencies of average output power were equal and about 67.6% for both Nd:GdVO4 and Nd:YVO4 lasers. The output ratio of Q-switching to free running at 100kHz was reached at about 91%, which was much higher than ~60% and ~83.3% obtained in [15,16]. The reasons had been described in our former report [10], which were attributed to good mode-match, direct pumping and compressional-mode AO Q-switch. In this experiment, we did not find the obvious difference on laser output characteristics between Nd:GdVO4 and Nd:YVO4 lasers. It’s revealed that both Nd:GdVO4 and Nd:YVO4 are efficient laser crystal for diode-directly-pumped solid-state lasers.

In order to evaluate the laser properties of the continuous-grown composite crystals, Nd:GdVO4/GdVO4 and Nd:YVO4/YVO4 rods took the place of the Nd:GdVO4 and Nd:YVO4

rods, respectively. The CW multimode laser output was measured for the existing output coupler transmissivity of 35% nearly to the optimized one. Fig. 4 shows the CW output power as a function of absorbed pump power at diode-laser direct pumping. As shown in Fig. 4, the maximum output powers of 20W with a slope efficiency of 74.9% to absorbed pump power and 32W with 82.7% were obtained in Nd:GdVO4/GdVO4 and Nd:YVO4/YVO4 lasers,

respectively. The M2

factors at the maximum CW output power were measured as M2~1.9

for Nd:GdVO4/GdVO4 laser and M2~2.5 for Nd:YVO4/YVO4 laser, respectively. The

maximum absorption efficiencies to pump power are about 46.8% and 55.5% for Nd:GdVO4/GdVO4 and Nd:YVO4/YVO4 crystals, respectively. We’re surprised by the slope efficiency to absorbed pump power of 82.7%, which is almost equal to the quantum limit.

Four reasons are thought to be contributed to it. First is uniform absorption to pump power in the whole crystal rod due to the lower absorption efficiency; second is the reduction of thermal loading due to direct pumping; third is excellent thermal dissipation of continuous-grown composite crystal; fourth is good match between LD’s emitting linewidth and crystal’s

#111203 - $15.00 USD Received 11 May 2009; revised 29 Jun 2009; accepted 4 Jul 2009; published 13 Jul 2009

(C) 2009 OSA 20 July 2009 / Vol. 17, No. 15 / OPTICS EXPRESS 12872

absorption linewidth, which will be described in detail later. It’s a pity that we did not obtain the same excellent result in Nd:GdVO4/GdVO4 laser. Besides the slight lower thermal conductivity than that of Nd:YVO4 [17,18], the quality of Nd:GdVO4/GdVO4 crystal is likely to be responsible to it. Nd:GdVO4, as a new crystal, however, it was difficult to grow high-quality crystals due to a serious interface instability problem and scattering [19], especially in addition to the continuous-growth. Nevertheless, it’s anticipated that the excellent laser properties can be also obtained by improved Nd:GdVO4/GdVO4 crystal.

Fig. 5 shows the optical-optical efficiency to absorbed pump power (ηo-o) at different absorbed pump power. As shown in Fig. 5, ηo-o increases by exponential decay's form as the absorbed pump power increases. When the absorbed pump power is beyond about 20W, ηo-o

begins to increase slowly as the absorbed pump power increases. We think the uniform absorption is accomplished in the whole crystal rod at this time. When the incident pump power increases further, the absorption efficiency will slight increase to no longer increase because the LD’s emission wavelength shifts to go beyond the absorption linewidth of the crystal. As a result, ηo-o will increase no longer obviously. The maximum ηo-o of 70.6% and 72.4% were obtained at the absorbed pump power of about 27W and 39W for Nd:GdVO4/GdVO4 and Nd:YVO4/YVO4 lasers, respectively. Extremely high ηo-o indicates that continuous-growth Nd:GdVO4/GdVO4 and Nd:YVO4/YVO4 composite crystals show attractive prospect for construction of highly efficient CW Nd lasers scaling up to high powers.

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Fig. 4. CW output power versus absorbed pump power.

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Fig. 5. Optical-optical efficiency to absorbed pump power versus absorbed pump power.

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Fig. 7. Emitting wavelength of LD at different incident pump power.

Prior to the explanation to the extremely high slope efficiency obtained above, we measured the upper lasing level absorption spectra of the composite Nd:GdVO4/GdVO4 and

#111203 - $15.00 USD Received 11 May 2009; revised 29 Jun 2009; accepted 4 Jul 2009; published 13 Jul 2009

(C) 2009 OSA 20 July 2009 / Vol. 17, No. 15 / OPTICS EXPRESS 12873

Nd:YVO4/YVO4 crystals in the relevant spectral regions by spectrophotometer (UV-3101PC), respectively. As shown in Fig. 6, the peak absorption to the upper lasing level appeared around 879.4nm and 880.0nmm for Nd:GdVO4/GdVO4 and Nd:YVO4/YVO4 crystals, respectively. The FWHM of both are about 2.0nm, which are convenient for diode pumping. Meanwhile, since the emitting wavelength of LD depends on current, we also measured the emitting wavelength of LD at different incident pump power by means of a fiber spectrometer (HR4000, Ocean Optics Inc.). As shown in Fig. 7, the center emitting wavelength of LD increases as the incident pump power increases and arrives at about 880nm with a FWHM of about 2.2nm at the incident pump power of about 80W. Consequently, the highly effective energy extraction has been realized due to effective match between LD’s emitting linewidth (2.2nm) and crystal’s absorption linewidth (2nm), combination with a good top-hat intensity distribution of pump light.

The efficient pulse operations at the repetition rate of 100kHz of Nd:GdVO4/GdVO4 and Nd:YVO4/YVO4 composite crystal were also presented with the A-O Q-switch inserted into the resonator. Fig. 8 and Fig. 9 show the pulse width and average output power as a function of absorbed pump power for Nd:GdVO4/GdVO4 and Nd:YVO4/YVO4 lasers, respectively. At the repetition rate of 100kHz, the minimum pulse width of 15.0ns and 12.9ns, the average output power of 17.7W and 28.9W, the peak power of 11.8kW and 22.4kW were obtained for Nd:GdVO4/GdVO4 and Nd:YVO4/YVO4 lasers, respectively. Experiment results also indicate that both continuous-grown Nd:GdVO4/GdVO4 and Nd:YVO4/YVO4 composite crystals have promising applications on highly efficient A-O Q-switch operation.

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Fig. 9. Pulse width and average output power versus absorbed pump power.

4. Conclusion

In conclusion, laser properties of continuous-grown Nd:GdVO4/GdVO4 and Nd:YVO4/YVO4

composite crystals under LD direct pumping were presented. The maximum CW output powers of 20W with a slope efficiency of 74.9% to absorbed pump power and 32W with 82.7% were obtained in Nd:GdVO4/GdVO4 and Nd:YVO4/YVO4 lasers, respectively. The achievement of near quantum-limit slope efficiency is attributed to the uniform absorption to pump light in the whole crystal rod, the reduction of thermal loading due to direct pumping; excellent thermal dissipation of continuous-grown composite crystal and good match between LD’s emitting linewidth and crystal’s absorption linewidth. At the repetition rate of 100kHz, the minimum pulse width of 15.0ns and 12.9ns, the average output power of 17.7W and 28.9W, the peak power of 11.8kW and 22.4kW were obtained for Nd:GdVO4/GdVO4 and Nd:YVO4/YVO4 lasers, respectively. It’ concluded that both continuous-grown Nd:GdVO4/GdVO4 and Nd:YVO4/YVO4 composite crystals are efficient laser crystal for diode-directly-pumped solid-state lasers, and show attractive potential in the construction of highly efficient CW and pulsed Nd lasers scaling up to high power with improved quality of composite crystal and optimal laser scheme.

#111203 - $15.00 USD Received 11 May 2009; revised 29 Jun 2009; accepted 4 Jul 2009; published 13 Jul 2009

(C) 2009 OSA 20 July 2009 / Vol. 17, No. 15 / OPTICS EXPRESS 12874