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Periodically Poled MgO:LiNbO3 Generates High-Power Green

Photonics Spectra
Jun 2004
Breck Hitz

Although green powers in excess of 1 W have been generated with periodically poled LiNbO3 (PPLN), the results have been obtained only when the nonlinear crystal is heated to temperatures significantly above 100 °C, so as to avoid deleterious photorefractive effects in the crystal.

Scientists in Japan now report using a different crystal, periodically poled MgO:LiNbO3 (PPMgLN), which has generated green power in excess of 1 W from a room-temperature crystal. They believe that this is the highest power reported from a room-temperature periodically poled crystal, and that their technique can be refined to produce compact and efficient visible sources for applications such as optical displays, materials processing and optical communications.

The scientists, from the Institute for Molecular Science in Okazaki and from Matsushita Electric Industrial Co. Ltd. in Osaka, started with an Nd:GdVO4 resonator in a twice-folded Z-configuration that produced 6.8 W at 1.06 µm when pumped with 15 W from an 809-nm diode laser (Figure 1). The output beam was 98.8 percent polarized and had a bandwidth of approximately 0.25 nm, and its power fluctuated by ±1.1 percent over a period of several hours.

Periodically Poled MgO:LiNbO<SUB>3</SUB> Generates High-Power Green

Figure 1. The periodically poled MgO:LiNbO3 (PPMgLN) crystal generated 1.38 W of second-harmonic (green) power in a single pass of the fundamental beam from the Nd:GdVO4 laser. A short lens focused the fundamental power into the nonlinear crystal, and a dichroic mirror separated the unconverted fundamental power from the second harmonic.

One of two PPMgLN crystals, 10 or 25 mm in length, served as the frequency doubler. The researchers polished the end faces of the crystals and antireflection coated them for the 1.06-µ;m fundamental wavelength but not for the 532-nm second harmonic. They placed the crystal on an aluminum plate, and its temperature was held constant at 29.5 °C to within 0.1 °C.

They measured the second-harmonic output and the conversion efficiency as a function of 1.06-µm power incident on both of the nonlinear crystals (Figure 2) and observed a maximum green power of 1.18 W from the 25-mm crystal. Taking into account the Fresnel reflection at the crystal's exit face, they calculated the internal green power to be 1.38 W and the internal conversion efficiency to be 19.6 percent.

Periodically Poled MgO:LiNbO<SUB>3</SUB> Generates High-Power Green
Figure 2. The researchers measured the second-harmonic power and the conversion efficiency with the 10-mm (top) and 25-mm (bottom) PPMgLN crystal. The solid curve shows the theoretical quadratic dependence of second-harmonic power on the fundamental.

Because the acceptance bandwidth of the 25-mm crystal was 0.22 nm, somewhat less than the laser's 0.25-nm spectral width, the researchers believe that higher second-harmonic powers could be achieved by reducing the width of the laser line.

But, significantly, the green output power was steady over a multihour period, indicating that the photorefractive effects typically observed with PPLN are not present in PPMgLN. The slight roll-off in conversion efficiency at higher power levels results from thermal nonuniformity caused by absorption of both fundamental and second-harmonic power in the nonlinear crystal.


GLOSSARY
optical communications
The transmission and reception of information by optical devices and sensors.
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