Laser-diode (LD) pumped self-frequency doubling (SFD) Nd:GdCa4O(BO3)3 (Nd:GdCOB) miniature laser was demonstrated. The output power as high as 1.35 W was achieved which is over than five times than previous values for Nd:LnCa4O(BO3)3 (Ln = Y or Gd) SFD lasers and becomes the highest continuous-wave output power in this field to our knowledge. The maximum optical conversion efficiency is 17%. By comparison, we found that the cutting direction along its optimal phase-matching direction out of the principal planes is the determining factor resulting in this watt-level efficient output power. Different with previous studies, the emission wavelength is centered at about 545 nm. We believed that this efficient laser will become the most competitive one in the existing commercial green lasers, especially in the laser display, medical treatment and spectroscopic analysis etc.
© 2010 OSA
In recent years, miniature green laser sources have demonstrated their promising applications, such as optical data storage, laser printing, undersea communications, medicine and displays etc . As we know, the simplest way to obtain those miniature sources is diode-pumped self-frequency-doubling (SFD) lasers. In these lasers, the key material is a SFD crystal which performs both functions of laser emission and nonlinear optical conversion in a single crystal. Nd or Yb doped YAl3(BO3)4 (YAB), Nd:LiNbO3 and LnCa4O(BO3)3 (Ln = Y or Gd) are three most promising series SFD crystal. To our knowledge, the highest-power SFD laser was achieved to be 1.1 W up to now with a Yb:YAB . However, this crystal should be grown by a flux method and it is difficult to grow this crystal in large size. Compared with other SHG crystals, Nd: LnCa4O(BO3)3 (Ln = Y or Gd) crystals can be grown by the Czochralski method in large size, and have high nonlinear coefficients, damage threshold and optical gain at 1.06 µm . With Nd:YCa4O(BO3)3 (Nd:YCOB) and Nd: GdCa4O (BO3)3 (Nd:GdCOB), the highest SFD lasers were achieved to be 245 mW  and 225 mW , respectively.
From the previous studies of our group on this family crystals, due to their low symmetrical structure (monoclinic structure, with space group of Cm), their highest effective nonlinear coefficients was not along the phase-matching (PM) direction in the principal planes and the crystal cut along the PM direction out of the principal planes is more efficient [5–7]. The 4F3/2→4I11/2 band of Nd ions in the LnCa4O(BO3)3 was also changed with the temperature, which induce the emission wavelength changed with the pump power . Based on the analysis by Lucas-Leclin et. al , at low pump power, the laser wavelength was centered at 1061 nm, and when the pump power was high, the wavelength was changed to be 1091 nm. Also different with the other Nd-doped crystals (808 nm), the absorption wavelength of these crystal was centered at 811 nm with small absorption cross-section . Beside those above, the Nd ions also have absorption at about 530 nm, which corresponds the SFD laser wavelength of Nd:GdCOB . Maybe due to the complexities of this crystal listed above, high output power has not been achieved in past decade.
In this letter, we report the laser-diode (LD) pumped continuous-wave (cw) SFD yellowish green miniature laser output with a Nd:GdCOB crystal. The highest output power was 1.35 W with a center wavelength of about 545 nm. Combining its advantages, it indicates that this crystal should be a promising material in the SFD laser field.
This Nd:GdCOB crystal was grown by the Czochralski method with Nd concentration of about 8 at.% for compensating its small emission cross-section, and cut along their type I PM direction out of its principal planes with the fundamental wavelength of 1091 nm and almost similar with that shown in the Ref. 7 (Ө = 66.38 ° and φ = 134.48 °, walk-off angle = 0.892 ° ). The dimensions of Nd:GdCOB is 3 mm × 3 mm × 8 mm. Its faces were polished and the front one was anti-reflection (AR) coated at 808 nm and high-reflection (HR, R>99.8%) coated at 1060—1091 nm and 532—545 nm. In order to maximizing the absorption, the end face was HR coated at 808 nm and 1060—1091 nm, and high-transmission (HT) coated at 530—545 nm.
The experimental configuration was exhibited in Fig. 1 . The pump source employed in the experiment was a fiber-coupled LD with a central wavelength around 808 nm. The core size of the fiber is 100 μm in radius with a numerical aperture of 0.22. The pump light was focused into the Nd:GdCOB crystal by an imaging unit with a beam compression ratio of 1:1. To remove the heat generated from Nd:GdCOB under high pump power levels, this SHG crystal was wrapped with indium foil and mounted in a water-cooled copper block. In order to accurately measuring the output power of green lasers, a mirror was all-reflection at 1060-1091 nm and HT coated at 530—545 nm (transmission of 78%). The output power was measured by a power meter (EPM 2000. Molectron Inc.). With an optical spectrum analyzer (HR4000CG-UV-NIR, Ocean Optics Inc.), the laser spectra were achieved.
3. Results and discussions
The output power with the increase of incident pump power was presented in Fig. 2 . The threshold (Pth) was measured to be 0.29 W, and highest output power was 1.35 W under the pump power of 7.94 W with the optical conversion efficiency of 17%. With a Glan-Thomson prism, the SFD laser was found to be linearly polarized. It can also be found that the SFD green power rose with the increase of the pump power with a quadratic curve. By analyses, it can be found that the fundamental power should be linearly increased with the incident pump power. However the second-frequency-generation (SHG) laser should be quadratic increased with the fundamental laser power. Therefore, it can be easily to understand the SFD laser quadratic increasing with the incident pump power. Removing F, the total output power of fundamental and SFD lasers can be obtained which is also shown in Fig. 2. Under the pump power of 7.94 W, the total output power was achieved to be 1.36 W. It can be seen that, with the increase of the incident pump power, the ratio of the SFD to fundamental output powers raised. The stability of the SFD laser was also measured under the pump power of about 1.03 W which is shown in Fig. 3 . Observed for half an hour, the instability is found to be less than ± 1%. Under this power, the mode size was studied with the knife edge method. The radius is about 2.44 mm with the length of 22.5 cm behind the output face of the SFD crystal, and the M2 value of the SFD laser beam is estimated to be 4.6.
By the optical spectrum analyzer, it can also be found that the SFD laser was centered at about 545 nm corresponding the SHG at 1091 nm. The laser spectrum was shown in Fig. 4 . From this figure, it can be found that, no other laser existed except that centered at 545 nm in the SFD lasers, which means that the output power measured in Fig. 2 was the pure laser centered at 545 nm. Considering the analysis by Lucas-Leclin et. al , with the increase of the pump power, the temperature of Nd:GdCOB crystal increased, which induced the transitions between stark levels of Nd ions centered at 1091 nm generated. The 1091 nm emission originates from the upper sublevel of the 4F3/2 manifold, and its population is favored at high temperature, which induced the 1091 nm mode dominating and generated the SFD yellowish green laser centered at 545 nm.
Compared with the previous highest-power SFD (245 mW) laser with its isomorph Nd:YCOB , we believe that this more excellent result is generated by the cutting direction which has much larger effective nonlinear coefficient than that in the principal planes . Although Wang et. al  have achieved the SFD laser using the same cutting-direction Nd:GdCOB crystal with a Ti:Sapphire laser as the pump source, the output power (225 mW) and conversion efficiency (14.4%) were much lower, and the output wavelength was different (530.5 nm). This SFD laser was also higher than the previous highest SFD laser achieved with a Re:YAB (Re = Yb, Nd) crystal [2,11–13] which should be grown using flux-method with much long growth process (about 50 days) and small boule size (about 2 cm × 2 cm × 2cm). Because of the miniature structure and high efficiency of this SFD laser, and the rapid grown process, large size and low cost of this Nd:GdCOB crystal, we believed that this compact, efficient and low-cost, and high-power miniature laser should have promising applications in the green laser field. Considering the peak absorption of Nd:GdCOB locating at about 812 nm, it can be proposed that the SFD laser will be improved with a LD emitting at the center of 812 nm as the pump source.
In conclusion, the SFD yellowish green miniature laser was achieved with a Nd:GdCOB crystal cut along the optimal PM direction out of the principal planes. The output power quadratic increased with the incident pump power. The generated 545 nm of this SFD laser was qualitative analyzed according the wavelength of fundamental lasers. Considering the advantages of this Nd:GdCOB crystal and SFD laser, we believed that this efficient laser would become the most competitive one in the existing commercial green lasers, especially in the laser display, medical treatment and spectroscopic analysis etc.
This work is supported by the National Natural Science Foundation of China (Nos. 50721002, 50590401/E01) and the Grant for State Key Program of China (2010CB630702).
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