Compact Nd:YAG Laser Generates 1.25 W of Blue Light
Composite YAG rod enables fundamental-power scaling.
Compact blue lasers have many potential applications, including optical displays and data storage, medical diagnostics and underwater communication. Recently, researchers at Tianjin University in China demonstrated a small, internally frequency-doubled Nd:YAG laser that generates 1.25 W at 473 nm. They believe that the 15.2-W, fundamental-wavelength output of their laser at 946 nm sets a record for diode-pumped Nd:YAG, and that several straightforward improvements could result in the conversion of significantly more of this output to the blue second harmonic.
The laser is based on the quasi-three-level 4F3/2 to 4I9/2 transition in neodymium. The researchers previously achieved 8.3 W of 946-nm radiation from this transition but were limited at that level by severe thermal effects in their conventional (noncomposite) Nd:YAG laser rod.
To achieve the current 15.2 W, they turned to a composite rod that had two undoped 4-mm-long end caps and a 4-mm-long central section doped with 1.1 atomic percent neodymium. They end-pumped the rod with a fiber-coupled, 808-nm diode laser (Figure 1). A spherical output mirror and a reflective coating on the pumped end of the Nd:YAG rod formed a flat-convex laser resonator that was 13 mm long. To suppress the competing 1.064-μm line, they ensured that both resonator reflectors had less than 50 percent reflectivity at that wavelength.
Figure 1. The resonator mirrors, on the back surface of the Nd:YAG rod and on the spherical output mirror, had greater than 50 percent reflectivity at 1.064 μm to prevent that line from reaching threshold. Images reprinted with permission of Optics Letters.
The rod’s thermal back focal distance measured only several centimeters at the higher pumping levels (~25 W at 808 nm). To minimize the adverse effects of thermal focusing, the researchers focused the pump radiation to a spot considerably larger than the intracavity beam waist. They obtained the optimum 946-nm output of 15.2 W from an incident pump power of 40.2 W, with a slope efficiency of 45 percent. Taking into account the absorption coefficient of Nd:YAG at 808 nm, they calculated that the slope efficiency was 60.3 percent with respect to absorbed pump power.
But the mismatch between the pump beam and the laser beam brought a consequence of reduced beam quality: At 15.2 W of output, the M squared was 13.1. The investigators obtained better beam quality but lower 946-nm power by focusing the pump to a tighter spot, which more nearly matched the laser’s fundamental mode.
Rod temperature also was an important parameter, so they wrapped the rod with indium foil and clamped it in a cooled copper heat sink to reduce its temperature. They observed more than a 10 percent increase in output power when they cooled the rod from 18 to 3 °C.
Although the blue second-harmonic output of the laser may be its most interesting aspect from a commercial perspective, the technically challenging part is generating 946-nm output in the first place.
Having achieved what they believe is a record 946-nm output, the scientists inserted a 14-mm-long LBO crystal between the laser rod and the output mirror (Figure 2). They replaced the output coupler with a dichroic mirror (R∼0.998 at 946 nm, R<0.04 at 473 nm and R<0.1 at 1.064 μm) that focused sharply into the nonlinear crystal and observed as much as 1.25 W of blue, 473-nm light emerging through the dichroic mirror.
Figure 2. The researchers inserted the doubling crystal between the laser rod and the output mirror. The second-harmonic output probably could have been increased by polarizing the laser and by recovering the second-harmonic light generated traveling back toward the laser rod.
Although the back facet of the laser rod (the pump facet) was coated for high reflectivity at the second harmonic, it is likely that dispersion and polarization scrambling in the rod negated any positive effect of this coating. The 1.25 W of observed second harmonic was probably half the total second harmonic generated in both directions in the laser.
A folded resonator or a dichroic coating (high reflectivity at the second harmonic) on the front facet of the rod could result in a significant enhancement of blue output. Moreover, although the LBO crystal was aligned for type I phase-matching, no attempt was made to polarize the laser, so presumably at least half the intracavity fundamental power was not available for conversion to the second harmonic.
Optics Letters, June 15, 2006, pp. 1869-1871.
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