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New laser system for metrology and other applications

Development of a laser system in the visible spectral region for numerous scientific and technical applications, including laser-based metrology, printing and biotechnology.

Compact and efficient coherent sources, namely diode lasers and diode-pumped solid-state lasers, presently exist mainly in the near-infrared spectral range. Nonlinear optical mixing, in particular Second Harmonic Generation (SHG), can be used to convert the wavelength of these attractive sources into the red, green and blue spectral range. Second harmonic generators are mainly based nowadays on very few selected organic materials, such as LiNbO3 and KTiOPO4 (KTP). The application of these materials for generating visible wavelengths is rather limited owing to the requirement of phase-matching between the interacting waves: both the fundamental and second harmonic waves must propagate with the same phase velocity. Normally, phase matching is not satisfied, owing to the dispersion of the material. To overcome this problem, birefringent phase matching is used, i.e. specific angles of propagation, which are polarizations of the optical waves, are selected so that the refractive indices of the two waves are identical. However, in many cases, there is no combination of propagation angles and polarizations that satisfies this requirement. For example, blue light cannot be produced by birefringently phase-matched second harmonic generation, neither in LiNbO3 nor in KtiOPO4. Even if the phase matching condition is satisfied, it is often at very high or very low temperatures that are not practical in a commercial device and in polarization angles in which the nonlinear crystal is not efficient. An additional limitation of nonlinear optical frequency converters is their rather low efficiency. For example, the frequency doubling efficiency of the above-mentioned crystals under optimal focusing conditions is only ~ 0.2 %/Wcm. This means that for typical crystal lengths of 1 cm and pump power of 1 Watt, only 2 mW of the second harmonic will be generated. As a result of these limitations, there is currently a lack of compact and efficient sources in large parts of the visible spectrum. In the proposed project we plan to overcome the problems of phase matching and conversion efficiency. For the first problem, we will employ a different method of phase matching, called Quasi Phase Matching (QPM). In QPM, rather than relying on the material's natural properties of dispersion and birefringence, the phase matching condition is satisfied by periodic modulation of the sign of the material's nonlinear coefficient at a fixed period, which is typically several microns long. This method has been known for many years, but practical methods have only been developed very recently for the sign modulating of the nonlinear coefficient at the required resolution and accuracy. We will rely on the technique of electric field poling of KTiOPO4 crystal, which was recently developed in ISRAEL. In this method, a periodic electrode is put on one side of the crystal using photolithographic techniques. An applied electric field reverses the sign of the nonlinear coefficient and the crystal can be used for nonlinear frequency conversion. The QPM technique has inherent advantages over the traditional birefringent phase matching, in particular the ability to phase match any interaction within the crystal's transparency range. Second harmonic generation of red, green and blue wavelengths can be achieved by selecting appropriate periods for the sign modulating of the nonlinear coefficient. In contrast to the poor efficiencies obtained in single-pass schemes, we plan to use resonant frequency doubling. In this method, the crystal is put into an optical cavity and the laser is locked to the cavity resonance frequency. Owing to the high circulating intensity inside the cavity, much higher second harmonic power can be obtained. For example, in a recent experiment performed at TEL-AVIV UNIVERSITY, frequency doubling efficiencies up to 69.4 % were been obtained by doubling a continuous-wave 1064 nm Nd:YAG laser, and the power level in the green light was up to 268 mW. Similar conversion efficiencies are also expected in the blue and red by using Nd:YAG lasers operating at 1340 and 946 nm. Resonant doubling requires a continuous-wave single-longitudinal mode laser that can be frequency tuned at a fast rate. Lasers with these features are currently produced by INNOLIGHT. RAICOL CRYSTALS commercially produces various kinds of periodically poled KTiOPO4 crystals. Hence, the proposed project will provide a synergetic combination between two companies specialising in laser sources and nonlinear frequency converters, and will enable a new family of coherent, powerful sources in the visible range to be provided. Keywords: lasers, optical mixing, quasi phase matching.
Acronym: 
RGB LASER SOURCES
Project ID: 
2 871
Start date: 
01-05-2002
Project Duration: 
30months
Project costs: 
1 200 000.00€
Technological Area: 
Optical Technology related to measurements
Market Area: 

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