This invention relates generally to the use of non-linear optical mixing crystals, and, more specifically, to the use of such crystals to double the frequency of coherent optical radiation.
Non-linear optical crystals are used in many ways in optical technologies, a principal application being doubling of the frequency of an incident laser beam through generation within the crystal of a second harmonic. Another application is to generate radiation which has a frequency equal to the sum or difference of the frequencies of two incident radiation beams. There are many materials that have been used or suggested over the years for use as a mixing crystal, such as KTP (KTiOPO.sub.4), lithium tantalate (LiTaO.sub.3), and lithium niobate (LiNbO.sub.3). It is common to use such crystals to double the frequency output of a laser. This allows the use of long wavelength lasers, such as those in the infrared region of the spectrum, in a system that generates light in the green or blue portion of the spectrum with such a mixing crystal being used as a second harmonic generator (SHG). Such a SHG crystal is also used with a tunable laser, thereby to provide a frequency double coherent radiation source which is tunable over some range. One application of such a tunable system is in spectroscopy, where the interaction with a material sample with a coherent radiation source scanned over a predetermined frequency range is desired.
As is well known, the phases of the incident radiation and the radiation generated within the crystal are desirably matched to maximize the efficiency of its operation. This provides a maximum output intensity for a given input intensity. Phase matching is required since the incident and generated radiation travel at different speeds through the crystal. The refractive index of the crystal is different for the two radiation waves. This results in destructive interference of the two waves within the crystal if nothing is done about it. One phase matching technique used with a birefringent type of crystal involves polarizing the incident wave such that the orthogonally polarized second harmonic wave sees the same refractive index. The cause of the destructive interference is then eliminated between the two polarized waves, and the efficiency of operation of the crystal is optimized.
A second type of non-linear mixing crystal utilizes quasi-phase-matching (QPM). The relative phase of the incident and generated waves are corrected within the crystal at regular intervals by forming the crystal to have a structural periodicity along a direction of travel of the waves. One type of periodic structure modulates the sign or magnitude of the non-linear coefficient through the material. In a specific form, the sign of the non-linear coefficient is alternated along the length of the crystal at intervals related to the coherence length of the radiation. In ferroelectric crystals like LiNbO.sub.3, LiTao.sub.3 and KTP, the regions of opposite sign are correlated with the direction of the ferroelectric domain, the act of creating a region of opposite sign is termed "domain inversion", and the structure as a whole is termed a "periodically poled" device. A dimension of each region in the direction of radiation propagation is usually made to be equal to one coherence length of the interference in the crystal. The principles underlying operation of such QPM crystals are given in a paper, Fejer et al., "Quasi-Phase-Matched Second Harmonic Generation Tuning and Tolerances," IEEE Journal of Quantum Electronics, Vol. 28, No. 11, November, 1992, pps. 2631-2654, which is incorporated herein by this reference.
More recently, QPM crystals have been formed by the use of integrated circuit processing technology. A film is lithographically delineated on the surface of the crystal into a grating with a periodicity designed to achieve quasi-phasematching for a set of desired conditions. In one class of fabrication techniques, the film is used to facilitate the spatially periodic application of a high voltage electric field that induces periodic domain inversion. In another class of fabrication methods, the film is diffused into the surface of the crystal and the act of diffusion, combined with appropriate thermal processing, induces domain inversion. Specific forms of such devices are described in U.S. Pat. No. 5,036,220 of Byer et al. (1991), which is incorporated herein by this reference. The specific material processing resulting in domain inversion is not critical to operational aspects of this patent.
It is a general object of the present application to operate non-linear mixing crystals at their maximum conversion efficiency.
It is a more specific object of the present invention to provide a system for controlling an operating parameter of a non-linear mixing crystal in order to maximize its conversion efficiency under varying operating conditions.
It is another object of the present invention to provide a combination of a tunable laser and a non-linear crystal which operates at a maximum conversion efficiency, as a second harmonic generator, over the tunable frequency range of the laser.
It is a further object of the present invention to operate a non-linear crystal at a maximum conversion efficiency when being used to generate a radiation beam having a frequency that is either a sum or differences of the frequencies of two input laser beams.
It is yet another object of the present invention to provide an improved structure of a non-linear mixing crystal that is more easily controllable to operate at its maximum conversion efficiency.
It is yet a further object of the present invention to provide structures of QPM crystal units that are easy to fabricate for use as the non-linear crystal in such systems.