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With double frequency to deeper blue

July 01, 2003

Light transports data at high speed over fibre-optic networks. Light measures and creates images of large and small objects in scanners and microscopes. Light writes information onto all sorts of recording media and surfaces. The best-quality light is produced by lasers, which have meanwhile become an indispensable tool - not only in medicine and chemical analysis. Although there are many materials capable of generating beams of laser light, few of them operate at the shorter wavelengths. Scientists from the Fraunhofer Institute for Physical Measurement Techniques IPM are presenting a laser that produces wavelengths of down to 370 nanometers - which is the near ultraviolet range.

This extremely short wavelength is achieved by means of a frequency-doubling technique. Andreas Hofmann, who leads the IPM's laser imaging research group, makes use of more familiar imagery to explain the basic principle: "Overtones - mostly the double of the fundamental frequency - are well known in the field of acoustics. Accomplished guitar players are able to coax such tones out of their instruments by deliberately damping with their finger the fundamental vibration at certain points of the string." The researchers have managed to achieve a similar selective effect by applying electric fields to crystals of lithium niobate, a common laser source.

Using an IPM-patented drive system, they have been able to achieve an output power of several milliwatts for the first time. Frequency doubling offers the distinct advantage of allowing commercial laser diodes to be used as the pumping device. They emit light in the visible to near-infrared region of the spectrum, at wavelengths between 740 and 1,100 nanometers. At an output power of 100 milliwatts, they can indeed produce beams of sufficiently high quality to enable the short-wavelength main laser to be used in imaging, analysis and diagnosis instruments.

What's more, the beam can be directed and modulated. The scientists achieve this flexibility in shaping the beam by deliberately modifying the molecular structure of the lithium niobate crystal. Metal electrodes are placed on the two surfaces of the crystal slice using established photolithography techniques. When an electric field is applied, the crystal axis between the two electrodes switches permanently to the opposite sense. This method, known as domain inversion, produces lasing areas of a defined geometry within the crystal. This technique is prerequisite to the volume processing of such crystal wafers.

Fraunhofer-Gesellschaft