VISIBLE SOLID-STATE LASERS

A high-power all-solid-state visible-output laser has been introduced by Spectra-Physics Lasers (SPL, Mountain View, CA). The Millennia delivers more than 5 W of diffraction-limited CW green output power from an intracavity doubled neodymium-doped yttrium vanadate (Nd:YVO4) laser that is pumped by fiber-coupled diode bars (see photo). A noncritically phase-matched lithium triborate (LBO) crystal is used to convert the 1.064-µm output of the Nd:YVO4 laser to 532 nm.

VISIBLE SOLID-STATE LASERS

DPSS laser seen as argon-ion alternative

Stephen G. Anderson

A high-power all-solid-state visible-output laser has been introduced by Spectra-Physics Lasers (SPL, Mountain View, CA). The Millennia delivers more than 5 W of diffraction-limited CW green output power from an intracavity doubled neodymium-doped yttrium vanadate (Nd:YVO4) laser that is pumped by fiber-coupled diode bars (see photo). A noncritically phase-matched lithium triborate (LBO) crystal is used to convert the 1.064-µm output of the Nd:YVO4 laser to 532 nm.

The system, which is the first commercial green-output solid-state laser at this power level, is intended to provide an alternative to small-frame water-cooled argon-ion lasers, according to SPL marketing manager, Bruce Craig. The laser is more compact than existing ion lasers yet requires only a standard 110-V outlet and no external cooling water. And because many of the specifications, such as optical noise and pointing stability, are better than specifications for most argon-ion lasers, Craig expects users to see performance benefits in many of the Millennia`s intended applications. These include pumping other devices, such as Ti:sapphire and dye lasers, as well as spectroscopy, medicine, and reprographics.

Solving the "green problem"

The optical noise performance of the Millennia, specified as <0.1% RMS, is, says Craig, "an order of magnitude lower than that of an ion laser." This performance results from a new approach to solving the so-called "green problem" commonly associated with frequency-doubled diode-pumped devices. To date, such lasers have not been successful in applications requiring low-noise green sources because chaotic amplitude fluctuations--caused by nonlinear coupling of the few axial modes of the laser--occur on the output beam and cannot be removed using straightforward feedback control.

Although several approaches have been tried that overcome this noise problem--single-frequency operation to eliminate interaction between multiple axial modes, for example--none has proven reliable and cost effective enough for large-scale use. Now, however, SPL claims to have solved the problem with a newly patented technique called quiet multiaxial mode doubling.

In this approach, the laser cavity is set u¥such that a large number of axial modes (nominally 100) oscillate, thereby ensuring no single mode is able to reach a peak power high enough to induce a significant nonlinear loss for the other axial modes, which is the effect that caused the noise problem described above. The result is a high-quality output beam with very low noise.

Two 20-W CW diode-laser bars end-pum¥the Nd:YVO4 laser and achieve a conversion efficiency greater than 50%. The diode bars are located in the laser power supply and fiber-bundle-coupled to the laser head. According to Craig, the diode bars are derated to maximize their operating lifetime. Although the new laser will be priced somewhat higher than an equivalent small-frame ion laser, Craig notes that the Millenia`s efficiency is such that it operates from a standard 110-V single-phase service and dissipates only about 100 W of waste heat, which should make it attractive in situations where water and power are at a premium and when environmental concerns are an issue.

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