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Laser emits tunable blue light

02 Sep 2008

A solid-state laser that offers practical output powers and a tuning range of 425 to 489 nm is unveiled by researchers in Spain.

Performing intracavity frequency doubling in a continuous wave (CW) singly resonant OPO (SRO) creates a high power, singlemode and widely tunable source of blue laser light, say researchers at ICFO in Barcelona, Spain. Using this approach, the team has generated nearly 450 mW over a tuning range of 425 to 489 nm. (Optics Letters 33 1228)

"To our knowledge, there are no other practical solid-state laser technologies that can provide tunable blue radiation," Goutam Samanta, a researcher in the nonlinear optics group at ICFO told optics.org. "The single-mode and high output power, possibility of power scaling and expansion of the wavelength range makes our approach exceptionally promising."

Generating tunable blue light at useful powers and with desirable output characteristics using solid-state laser technology remains challenging. In the CW regime, one common approach is to frequency double a 946 nm Nd:YAG laser. Although this produces power levels of a few Watts at 473 nm, the major drawback is the lack of tunability.

CW SROs have been widely used to generate light in the 1.5 to 4.5 micron range using PPLN as the nonlinear material. Generating shorter wavelengths however requires a pump source emitting in the green at 532 nm. The problem here is that PPLN suffers from photorefractive damage when subjected to visible radiation.

The key to the result is a new nonlinear material, MgO-doped stoichiometric lithium tantalate (MgO:sPPLT), which is used as the gain medium in the OPO.

Samanta and colleague Majid Ebrahim-Zadeh place the MgO:sPPLT in a ring cavity comprising two concave reflectors, two plane mirrors and a BIBO crystal for frequency doubling. A CW Nd:YVO4 laser emitting at 532 nm pumps the cavity.

By varying the temperature of the MgO:sPPLT from 71°C to 240°C, the SRO signal beam could be continuously tuned from 978 to 850 nm. The corresponding second harmonic wavelengths from 489 to 425 nm are generated by varying the internal angle of the BIBO crystal from 163.8 to 155.2 degrees.

Samanta and Ebrahim-Zadeh measured powers varying from 45 mW at 425 nm to 300 mW at 489 m, with as much as 448 mW at 459 nm.

"We used an MgO:sPPLT crystal with a single grating period (7.97 µm)," explained Samanta. "If we used grating periods in the range of 11-13 µm, we will be able to generate signal radiation down to 600 nm, which will in turn make it possible to generate frequency doubled radiation down to 300 nm in the UV."

The researchers believe that a 10W pump laser at 532 nm would generate more than 1W of tunable single-frequency radiation over the 300-530 nm range with excellent beam quality.

"The next step is to increase the blue power to more than 1W, optimize output beam quality, implement active frequency and power stabilization using electronic control, and demonstrate fine frequency tuning," Samanta and Ebrahim-Zadeh conclude.

Jacqueline Hewett is editor of Optics & Laser Europe magazine.

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