Field of the Invention
The present invention relates to harmonic generation of laser beams in nonlinear optical (NLO) materials including but not limited to KBBF/RBBF crystals and, in particular, harmonic generation over a substantial range of wavelengths with good efficiency.
Related Art
Alkali fluorberyllium borates such as in the form of potassium beryllium fluoroborate (KBe2BO3F2 or KBBF) and beryllium boron rubidium fluoride (RbBe2BO3F2 or RBBF) crystals for frequency multiplying of an input laser beam frequently rely on direct or indirect optical coupling with prisms or optical slabs. To generate higher harmonics the beam must propagate with certain angle (phase matching angle or PMA) relative the optical axis of the crystal. This strongly depends on wavelength of the beam. Discussion of PMA can be found at H. Zhang, G. Wang, L. Guo, A. Geng, Y. Bo, D. Cui, Z. Xu, R. Li, Y. Zhu, X. Wang, C. Chen, 175 to 210 nm widely tunable deep-ultraviolet light generation based on KBBF crystal, Applied Physics B, Vol. 93, Issue 2-3, pp. 323-326; C. T. Chen, L. L. Wang, X. Y. Wang, Z. Y. Xu, Deep-UV nonlinear optical crystal KBe2BO3F2—discovery, growth, optical properties and applications, Applied Physics B, Vol. 97, Issue 1, pp, 9-25; and C. Chen, et al, Improved Sellmeier Equations and Phase-Matching Characteristics in Deep-Ultraviolet Region of KBe2BO3F2 Crystal, IEEE Journal of Quantum Electronics, Vol. 44, Issue 7, pp. 617-621, each of which is incorporated by reference herein.
Examples of prism-based systems can be seen at U.S. Pat. No. 6,959,305 and published U.S. Patent Application US2014/0071519, both of which are incorporated by reference herein. Another example, and its use in a system applied to photoemission spectroscopy, is described in R. Jiang, D. Mou, Y Wu, L. Huang, C. McMillen, J. Kolis, H. G. Giesber III, J. J. Egan, and A. Kaminski, “Tunable Vacuum Ultraviolet Laser Based Spectrometer for Angle Resolved Photoemission Spectroscopy”, Review of Scientific Instruments 85, 033902 (2014) (available on-line at http://dx.doi.org/10/1063/1.4867517), also incorporated by reference herein.
However, coupling the incoming or outgoing beam is efficient for only a small range of wavelengths because the faces of such prisms are cut at predefined angles. For example, the range of effective acceptance angles for PMA may be on the order of 5 to 10° relative to the prism face and this tends to limit the wavelengths that can effectively be utilized.
Additionally, a variety of factors can affect or influence the conversion efficiency from the incoming source laser to the generated harmonic beam. Using angle tilting for PMA requires components and techniques to vary the orientation of the crystal (and its optical axis) to the incoming beam. Utilization of optical components (including prisms and optically clear slabs) at the crystal can involve losses at the respective interfaces.
Similar issues can exist with other NLO materials. Therefore, there is room for improvement in this technical art.