Visible lasers are difficult to make. As the effective wavelength becomes shorter, the effort required to operate a laser increases exponentially. Therefore, lasers working at the longer near-IR (NIR) and IR wavelengths are more common, since they are typically more efficient than visible lasers. However, efficient high-power visible lasers can be made indirectly through nonlinear conversion.1 This involves focusing high-power NIR lasers into nonlinear crystals whose atoms interact with the intense laser light to convert some of the NIR energy to visible light. Shorter wavelengths may result as harmonics of the fundamental NIR wavelength similarly to how a guitar string may be made to play a higher harmonic pitch.
Nonlinear conversion is only efficient at very high intensities (~ tens of megawatts/cm2), which represents part of the difficulty in converting NIR to visible light. To achieve these intensities, we use external enhancement cavities to repeatedly reflect the output in a resonant fashion within a confined space, so that the beam overlaps itself dozens of times.2 When an appropriate nonlinear crystal is placed within the resonating cavity (see Figure 1), it can produce the desired visible output power with efficiencies approaching 100%.3 The Pound-Drever-Hall phase-discriminant tracking method can be employed to maintain the conversion cavity's resonant condition.4,5
Figure 1. Green laser light at 532nm is generated by coupling a near-IR (NIR) fiber laser at 1064nm to a resonant-enhancement cavity containing a nonlinear crystal (XTAL). DFB: Distributed feedback. VIS: Visible.
Maintaining constant output power and high beam quality in visible lasers can also prove challenging. We have developed lasers that exhibit robust and reliable performance under varying environmental conditions. We have improved the long-term performance of our lasers by manufacturing them in a clean room and hermetically sealing the conversion cavities (see Figure 2).
Figure 2. Hermetically sealing the conversion cavity increases laser survivability for industrial and commercial applications.
Until recently, only two types of high-power NIR fiber lasers have been available: ytterbium lasers, centered around 1.06μm, and erbium-based light sources, centered around 1.5μm. These two fundamental wavelengths determine the red (631nm), green (532nm), and blue (448nm) wavelengths of the lasers that may be produced through second-harmonic generation (SHG) or sum-frequency mixing (SFM). Red and green are generated in single cavities using one or two NIR fundamental lasers. Blue is produced by coupling the output of an SHG cavity from one NIR laser into an SFM cavity for mixing with another NIR source.
We recently developed high-power fiber lasers using thulium,6 centered around 2μm. This new NIR source provides a straightforward method for achieving useful lasers at wavelengths that before could not be easily achieved or were even available. For example, yellow (589nm) ‘guide-star’ lasers can now be readily produced. Guide-star lasers are used to create artificial stars in the upper atmosphere for detecting and correcting unwanted image distortions introduced into astronomical observations by atmospheric effects. To date, achieving such guide-star lasers has been difficult. However, we now propose to create yellow lasers simply by substituting thulium-doped fiber lasers operating at 1908nm into our established blue-laser architecture (see Figure 3). Many other useful wavelengths (413, 465, 488, 514, 647nm, etc.) can be produced by combining ytterbium, erbium, and thulium sources using our ‘paint-box’ architecture.
Figure 3. A yellow 589nm guide-star laser can be produced using existing blue-laser architecture. Er: Erbium. Tm: Thulium. Yb: Ytterbium. SHG: Second-harmonic generation. SFM: Sum-frequency mixing.
Currently, nearly 50 Evans & Sutherland red, green, and blue lasers are installed and running continuously in over a dozen locations worldwide in a variety of environmental conditions. Each laser provides up to 18W of continuous-wave visible-laser power, and is compact and light enough to be shipped by air in instrument cases. Visible output powers are limited only by the available NIR laser power. We are actively investigating other UV, visible, and NIR applications for our fiber-based technology for which no practical, high-power continuous-wave solutions currently exist.
Jesse Anderegg, Forrest Williams
Evans & Sutherland
Salt Lake City, UT
Jesse Anderegg is an optical engineer. He has over 12 years of experience in the fields of optics, fiber optics, and optical design and manufacturing. He received his master's of physics instrumentation degree from the University of Utah in 2000.
5. R. W. P. Drever, J. L. Hall, F. V. Kowalski, J. Hough, G. M. Ford, A. J. Munley, H. Ward, Laser phase and frequency stabilization using an optical resonator, Appl. Phys. B 31, pp. 97, 1983.