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The present disclosure relates to lasers and laser systems that generate different wavelengths by nonlinear sum or difference frequency conversion and, in particular, compensation for the spatial walk-off phenomenon associated with critical phase matching of a nonlinear crystal in the production of harmonic laser output at peak power.
Laser wavelength converters are in widespread use in many industrial applications. For example, laser systems performing wavelength conversion to generate green and ultraviolet (UV) laser output have been used in laser micromachining systems. Two conventional ways of accomplishing harmonic generation entail intracavity and extracavity harmonic conversion. Harmonic generation using intracavity harmonic conversion is advantageous in that it produces with high efficiency laser output with good beam quality. Power degradation and nonlinear crystal damage control are, however, of special concern in high power applications operating at shorter wavelengths. Harmonic generation using extracavity harmonic conversion is beneficial in that it extends the lifetime of the nonlinear crystal, but the harmonic conversion efficiency is lower, especially for a lower peak power laser.
A tightly focused beam, such as, for example, a laser beam with a 100 μm diameter spot size, contributes to achieving higher conversion efficiency. There are, however, competing factors affecting harmonic conversion efficiency. On one hand, the crystal length is limited because of the effect of a spatial walk-off phenomenon resulting from critical phase matching of the nonlinear crystal, and, on the other hand, the smaller beam focus, the larger the beam divergence angle in the nonlinear crystal. Moreover, the beam spot size is limited by the damage threshold of the nonlinear crystal. Improving harmonic conversion efficiency is, therefore, a challenging endeavor.
It is known that nonlinear conversion efficiency is proportional to the length of the nonlinear crystal and the square of the peak power. To achieve high conversion efficiency of nonlinear harmonic conversion for an extracavity configuration, a small spot size is used but is limited by two major factors. The first limitation is anti-reflective (AR) coating and bulk crystal material damage caused by high peak power intensity. The second limitation is that the small spot size imposes on the nonlinear crystal the spatial walk-off phenomenon, which limits the harmonic conversion efficiency and laser beam quality.
Investigations have been carried out to improve harmonic generation efficiency. U.S. Pat. No. 7,016,389 of Dudley et al. states that there are conversion efficiency benefits stemming from the high power circulating in the laser cavity in intracavity nonlinear frequency generation.
One approach used in extracavity harmonic generation to increase harmonic conversion efficiency by keeping a high peak power intensity without damage concern and using longer crystals with reduced walk-off effect entails imparting with a cylindrical lens an elliptical shape to the laser beam so that the major axis of the elliptical laser beam is in the walk-off plane of the nonlinear crystal. There are, however, certain disadvantages with this approach. They include the apparent need for additional components to shape the laser beam. Alignment difficulties increase with addition of at least two cylindrical lenses to the optical system because a second cylindrical lens is needed to reshape the elliptical beam to a round beam.
A method of performing sum or difference frequency mixing of laser beams achieves efficient harmonic conversion in the production of high peak power laser output. The method entails use of a birefringent crystalline frequency conversion medium having an entrance facet, an interior, and a length. First and second laser beams propagating along respective first and second propagation paths are directed for incidence at an entrance angle on the entrance facet of the frequency conversion medium. The first laser beam has a first wavelength and first spot shape, and the second laser beam has a second wavelength and a second spot shape. The birefringence of the frequency conversion medium contributes to divergence and overlap for an effective interaction length of the first and second propagation paths of the respective first and second laser beams as they propagate within the interior and along the length of the frequency conversion medium.
Integral birefringence compensation of the frequency conversion medium is effected by setting the entrance angle to a value that imparts ellipticity to the first and second spot shapes. This causes, in comparison to a value of the entrance angle representing normal incidence of the first and second laser beams on the entrance facet, formation of a greater effective interaction length of overlap of the first and second elliptical spot sizes of the respective diverging first and second laser beams propagating within the frequency conversion medium to perform sum or difference frequency mixing in the production of harmonic laser output at high peak power.
The birefringent crystalline frequency conversion medium is a critical phase-matched nonlinear crystal preferably of Type I or Type II. Setting the entrance angle forms a wedge-faceted nonlinear crystal that acts as a cylindrical lens to impart ellipticity to the beams propagating in the nonlinear crystal and thereby reduce the effect of the walk-off phenomenon. The wedge-faceted nonlinear crystal can be used in both intracavity and external cavity configurations of sum frequency or difference frequency generation with improved conversion efficiency. The nonlinear crystal material can be any one of LBO, BBO, KTP, CBO, CLBO, KDP, KBBF, LiNbO3, KNbO3, GdCOB, and RBBF. The wedge-faceted nonlinear crystal can be used for harmonic generation to get shorter wavelengths or with an optical parameter oscillator (OPO) to get longer wavelengths. The harmonic generation can be second, third, fourth, and fifth harmonic generation.
Additional aspects and advantages will be apparent from the following detailed description of preferred embodiments, which proceeds with reference to the accompanying drawings.
Critical phase matching is a technique used to obtain phase matching of the nonlinear process in nonlinear crystal 30. The interacting input beams 34 and 36 are aligned at an angle relative to the axes of the refractive index ellipsoid. There is a restricted range of beam angles (called “acceptance angle”) at which critical phase matching works. Commercially available nonlinear crystals have for critical phase matching operation a nominal entrance angle that is very close to normal to the entrance surface of the crystal. Crystal phase matching is, therefore, an angular adjustment of the crystal or beam that is used to find a phase-matching configuration. Moreover, a normal incident (i.e., specified for normally incident light) AR coating 40 dictates the extent (i.e., ±10°) to which the entrance angle can depart from the surface normal before an onset of appreciable incident light reflection results in significant light transmission loss. IR beam 34 and green light beam 36 propagate parallel to each other and are incident on AR coated-entrance facet 38 at nearly normal (i.e., (90°±5°) entrance angle to achieve critical phase matching at a specified temperature.
Spatial walk-off is a phenomenon in which the intensity distribution of a beam propagating in a birefringent crystal drifts away from the propagation direction of the beam. Spatial walk-off is directly related to the acceptance angle of critical phase matching. Phase matching becomes incomplete when tightly focused beams are used, having a large beam divergence.
The smaller overlap of circular spot shapes 50 and 52 of laser beams exiting nonlinear crystal 30 produces higher order laser modes resulting in laser output that departs from a Gaussian shape. The greater overlap of elliptical spot shapes 54 and 56 results in laser output more closely of Gaussian shape.
An experiment comparing the disclosed and prior art methods of harmonic conversion was performed using two LBO crystals of 20 mm length. One of the LBO crystals was of the conventional rectangular shape shown in
Harmonic frequency conversion implemented with a wedge-faceted nonlinear crystal can be configured in either external cavity structure or intracavity structure. The nonlinear crystal material can be of Type I or II, and the frequency conversion can be either sum frequency or difference frequency. The nonlinear crystal can be any one of BBO, LBO, CBO, CLBO, KBBF, RBBF, KTP, LiNbO3, KNbO3, GdCOB, and BIBO.
It will be obvious to those having skill in the art that many changes may be made to the details of the above-described embodiments without departing from the underlying principles of the invention. The scope of the present invention should, therefore, be determined only by the following claims.