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We reported a dual-wavelength laser with a ceramic Nd:YAG as laser material and Cr:YAG as frequency selector and saturable absorber. Continuous-wave output power was achieved to be as high as 6.19 W at 1052 nm. With Cr:YAG, the laser has dual-wavelength at 1052 and 1064 nm. The shortest pulse width, maximum pulse energy and highest peak power were 4.8 ns, 103.2 µJ, and 21.5 kW. This pulsed laser is possible to be used as a new source to generate terahertz radiation.
©2009 Optical Society of America
1. Introduction
Nowadays, dual-wavelength pulsed laser has attracted much attention due to their promising applications in the generation of ultrahigh repetition rate pulses by optical beating [1,2] and coherent THz radiation by difference frequency generation [3,4]. In the pulsed laser regime, investigations have revealed the excellent thermal and laser properties of Nd:YAG crystal and this material has been commercial. Due to the stark split [5], the Nd:YAG crystal laser with the wavelengths in the range of 1.05 to 1.44 µm has been achieved [6], which identified that Nd:YAG crystal is a promising gain material for the generation of pulsed dual-wavelength laser.
For a Nd:YAG crystal, the long crystal-growth time and low Nd-ions doping level take obstacle to its application. In order to overcome these disadvantages, in the 1950s, Coble [7] developed the translucent ceramic, and since then the studies on Nd:YAG laser ceramic has been paid more and more interests. In 2002, Lu et al demonstrated the 1.46 kW high-power Nd:YAG laser [8], and in 2006, 67 kilowatt average laser output power has been achieved with this ceramic [9]. A lot of researches on this material have shown that this ceramic has almost similar properties with Nd:YAG crystal. Up to now, only Nd:YAG ceramic lasers with the wavelength of 1.06 [8], 1.3 [10] and 0.94 µm [11] were reported, and no other wavelength laser was achieved. It this letter, for the first time to our knowledge, we demonstrated a Nd:YAG ceramic continuous-wave (cw) laser at 1052 nm and passively Q-switched dual-wavelength laser at 1052 nm and 1064 nm with Cr:YAG as frequency selectors and saturable absorbers. By appropriately suppressing the oscillation at 1064 nm, the cw ceramic Nd:YAG laser at 1052 nm was achieved over than 6 W. By simply inserting a Cr:YAG in the cavity, the passively Q-switched dual-wavelength laser at 1052 and 1064 nm was obtained. The ratio of the two components in this laser can be tuned by the pump power and/or initial transmission of Cr:YAG. In these operations, the shortest pulse width, maximum pulse energy, and highest peak power were 4.8 ns, 103.2 µJ, and 21.5 kW.
2. Experiments
The experimental setup shown in Fig. 1 was based on a simply plano-concave resonator. The pump source employed in the experiment was a fiber-coupled laser-diode (LD) with a central wavelength around 808 nm. Through focusing optics (N. A.=0.22), the output of the source was focused into the laser crystal with a spot radius of 0.256 mm. The input mirror M1 was a concave one with a curvature radius of 200 mm, AR coated at 808 nm on the flat face, and high-transmission (HT) coated at 808 nm, high-reflection coated at 1052 and 1064 nm, on the concave face. The output coupler (OC) M2 was a flat mirror. In order to obtain the laser at 1052 nm, the OC with a transmission of 21% at 1052 nm and HT coated (T=85.6%) at 1064 nm was used. For comparison, an OC with T=21.5% at 1064 nm were used. The laser output power was measured by a power meter (EPM 2000. Melectron Inc.) and temporal behaviors of the Q-switched laser were recorded by a 500-MHz digital oscilloscope (TDS 3050, Tektronix Inc.). With a spectrum analyzer (HR 4000CG-UV-NIR, Ocean Optics Inc.), the spectra were also achieved.
Initial transmissions T0 of the two Cr:YAG samples used in experiments were 91% and 77% at 1064 nm, and 89% and 75% at 1052 nm, respectively, with AR coatings at 1.06 µm on their end-faces. The Nd:YAG ceramic with Nd concentration of 2 at.% were cut with dimensions of 3 mm×3 mm×4.96 mm. Its end-faces were polished and not coated. To remove the heat generated from Nd:YAG and Cr:YAG under high pump power levels, the ceramic was wrapped with indium foil and mounted in a water-cooled copper block, and Cr:YAG were attached on a copper block without cooling water. The temperature of cooling water was controlled to be 10 °C.
3. Results and discussions
Removing Cr:YAG from the cavity, we achieved the cw ceramic Nd:YAG laser operation at 1052 nm firstly with the OC of T=21% at 1052 nm (T=85.6% at 1064 nm). The cavity was tuned to be as short as 1.5 cm. The output power with the increase of incident pump power (Pin) was shown in Fig. 2. The obtained maximum output power was 6.19 W under the incident pump power of 17.25 W with optical conversion efficiency of 35.8%. The threshold was measured to be 0.75 W and slope efficiency was 39%. In all the cw laser operations, the laser has a wavelength of 1052 nm and no other wavelength appeared. The spectrum of the laser at 6 W was shown in Fig. 3. With the OC of T=21.5% at 1064 nm, the laser at this wavelength was also achieved. The threshold was 0.44 W, and maximum output power was 7.31 W at the incident pump power of 17.25 W with the optical efficiency of 42.3%. The higher output power and efficiency were generated by the larger emission cross-section at 1064 nm than that at 1052 nm [5]. Tuning the cavity to be 2.5 cm and inserting Cr:YAG in it, the passively Q-switched laser can be obtained. The output power with the increase of incident pump power was also shown in Fig. 2. Firstly, we used the Cr:YAG with T0=91% at 1064 nm. The threshold was 2.63 W and maximum output power was 2.82 W with optical conversion efficiency of 16.3%. As recorded by the optical spectrum analyzer in this operation, only the mode at 1052 nm oscillated at threshold. The mode at 1064 nm appeared when the incident pump power and output power were over than 3.79 W and 0.25 W, respectively. Figure 4 (a)~(d) have shown the spectra under the incident pump power of 2.63 W, 3.79 W, 13 W and 17 W, respectively. It can be found that the component at 1064 nm rose with the increase of pump power. When the pump power was over 17 W, the mode at 1064 nm became comparable and even higher than that at 1052 nm. Replacing Cr:YAG at T0=91% at 1064 nm to be that with T0=77%, the mode at 1064 nm oscillated firstly. The mode at 1052 nm appeared at the incident pump power over than 7.02 W. Figure 4 (e)~(h) have shown spectra under the incident pump power at 4.55 W, 7.02 W, 13 W, and 17 W, respectively. It can be found that the component at 1052 nm also increased with the increase of pump power, and in all these operations, the component of this mode was smaller than that at 1064 nm. The output power with the increase of incident pump power was also shown in Fig. 2. The maximum output power was achieved to be 1.81 W under the incident pump power of 17.25 W, with optical conversion efficiency of 10.5%. In all the passive Q-switching, the pulse width decreased with the increase of incident pump power. When the incident pump power increasing from threshold to 17.25 W, the width varied from 11.6 to 8.9 ns for T0=91%, which is 7.9 to 4.8 ns for T0=77%. The shortest pulse width and largest pulse energy were obtained with T0=77% at 1064 nm under the incident pump power of 17.25 W, which were 4.8 ns and 103.2 µJ, respectively, corresponding the peak power of 21.5 kW. The pulse with the width of 4.8 ns was shown in Fig. 5.
In all the passive Q-switching operations, the repetition rate almost linearly increased with the increase of pump power. In order to confirm that there is no time jitter in the dual wavelengths, a stable sum-frequency laser at about 529 nm was achieved with a type-I cut BiBO crystal placed outside the cavity.
Considering the saturable absorption properties of Cr:YAG, the absorption cross-section at 1052 nm is larger than that at 1064 nm and the absorptions at the two wavelength become much smaller, when Cr:YAG is “bleached” by one or both of the two wavelength laser, than that at the starting of the pulse. Moreover, the population inversion density and gain in the laser material when the laser operates in a passive Q-switching mode are much larger than that in cw laser operation. When T0 was large, the loss of 1052 nm mode were much smaller than that of 1064 nm and 1052 nm mode oscillated firstly. As the increase of pump power, the larger population inversion density in Nd:YAG and small absorption by Cr:YAG at 1064 nm, especially when the saturable absorber was bleached by the 1052 nm mode, can generate the 1064 nm mode and the two modes oscillated simultaneously. When T0 became small, due to the larger absorption of Cr:YAG at 1052 nm and much larger emission cross-section at 1064 nm of Nd:YAG ceramic, the threshold of 1064 mode was reached firstly and this mode oscillated before that of 1052 nm. Because of the larger round trip loss in the cavity, the threshold was much higher than that when T0 was larger. As the pump power increasing, the saturable absorber was “bleached” by that of 1064 nm, and the two modes can also oscillated simultaneously. As the analysis above, the two wavelength mode have similar repetition rate with no time jitter as observed in the experiments, which agree with the experimental results.
The processes of the passive Q-switching with Cr:YAG as frequency selector and saturable absorber can also be analyzed based on the Q-switching models developed by Degnan [12] and Zhang [13]. From the coupled rate equations, the initial population inversion density ni at the start of Q-switching and threshold of the pulsed laser can be given by:
where R is the reflectivity at the laser wavelength, L is the remaining round-trip dissipative optical loss which is assumed to be 8% according the reflective loss on the uncoated ceramic surfaces, σ is the emission cross-sections of Nd:YAG at 1052 nm (σ1052=1.51×10-19 cm2) and 1064 nm (σ1064=4.58×10-19 cm2) [5], and l=4.96 mm is the length of the ceramic.
When the Cr:YAG with T0=91% at 1064 nm was used, ni was calculated to be 3.55×1018 cm-3 at 1052 nm and 4.88×1018 cm-3 at 1064 nm, which means that the threshold at 1052 nm is much smaller than that at 1064 nm, and the laser at 1052 nm oscillated firstly. When T0=77% at 1064 nm, ni=5.84×1018 cm-3 at 1052 nm and 5.57×1018 cm-3 at 1064 nm, and the laser at 1064 nm oscillated firstly.
When T0=91% at 1064 nm and the Cr:YAG was “bleached” by the laser at 1052 nm, the transmission of Cr:YAG becomes the saturable one. At the same time, if the residual population inversion density nr reached the threshold of the laser at 1064 nm, the dual-wavelength can be obtained. The residual population inversion density nr can be given by:
where Ts is the saturated transmission of Cr:YAG. With the parameters of Cr:YAG given by Burshtein et. al [14], nr was calculated to be 4.57×1018 cm-3 and the total population inversion density was 8.12×1018 cm-3, which means that only in the high pump power, the dual-wavelength laser could oscillated. When T0=77% at 1064 nm, the nr for 1052 nm was calculated to be 3.17×1018 cm-3 and the total population inversion density was 8.74×1018 cm-3, which indicated that only in higher pump power, the dual-wavelength laser could oscillated.
With a Nd:YAG crystal, Smith [15] and Marking [6] have demonstrated the laser performance at 1052 nm by using a dispersive prism and thin solid etalon, respectively. With a ceramic Nd:YAG, we have obtained an over than 6 W cw laser at 1052 nm pumped by a LD, for the first time to our knowledge. Recently, with Cr:YAG as saturable absorber and frequency selector, a dual-wavelength Nd:YAG crystal laser of 1052 and 1064 nm has been reported [16]. By optimizing the cavity and saturable absorber, the dual-wavelength Nd:YAG ceramic laser was achieved here, which showed an obvious improvement compared with the crystal laser [16]. We also found that the ratio of components in the dual-wavelength laser at 1052 and 1064 nm can be tuned by changing of the pump power and/or with saturable absorbers with different initial transmissions. It can be proposed that, with an appropriate nonlinear optical crystal, the THz radiation should be generated by using this pulsed dual-wavelength ceramic Nd:YAG laser as source.
4. Conclusion
In conclusion, with Cr:YAG as frequency selectors and saturable absorbers, a passively Q-switched dual-wavelength ceramic Nd:YAG laser at 1052 and 1064 nm was demonstrated. The maximum cw output power at 1052 nm was achieved to be 6.19 W. We found that the radio of the two mode in the dual-wavelength laser can be tuned by the pump power and/or initial transmission of Cr:YAG. The shortest pulse width, maximum pulse energy and highest peak power were achieved to be 4.8 ns and 103.2 µJ and 21.5 kW.
Acknowledgement
This work is supported by the National Natural Science Foundation of China (No. 50872070, 50702031, 50721002), the 973 Program of China (G2004CB619002, 2007CB613302), and the Program of Introducing Talents of Discipline to Universities in China.
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