Semimonolithic resonant structures external to lasers have been proposed for use in optical frequency-conversion applications - for example, doubling the frequencies of laser beams. These structures would offer most of the advantages, without most of the disadvantages, of monolithic resonant cavities.

A Semimonolithic Cavity would cost less than does a monolithic cavity of equal capability. Unlike a monolithic cavity, it would be somewhat adjustable.

In contradistinction with discrete external cavities (that is, external cavities that are assemblies of discrete optical components), monolithic external cavities offer advantages of lower overall intracavity losses, no dispersion-induced mismatches, mechanical stability with consequent frequency stability, compactness, and lower overall cost. The disadvantages of monolithic cavities include vulnerability to manufacturing errors, lack of any degree of freedom for alignment, potential for difficulty in changing cavity lengths for resonance frequency locking, and, in some applications, unavailability of nonlinear crystals that are large enough.

A typical semimonolithic structure ("cavity") of the proposed type would include a nonlinear optical crystal fitted with optical components of glass or other suitable linear optical material on both ends (see figure). The glass chosen for this application must have an index of refraction as close as possible to that of the nonlinear crystal. The curved end mirrors of the cavity ("cavity mirrors," for short) would not be fabricated on the nonlinear crystal as in a monolithic cavity; instead, the cavity mirrors would be fabricated on the glass endpieces. The second-harmonic output could be extracted from the cavity through one of the cavity mirrors or through a dichroic beam splitter.

In a monolithic cavity, if any error occurs in fabrication of the cavity-mirror surfaces, or if the apices of these curved surfaces are not exactly coaligned, then the entire piece of nonlinear material must be discarded or completely reworked. In a semimonolithic cavity like the one proposed here, one could replace the glass endpieces or make small corrections on them. The polishing and coating characteristics of optical glasses and the techniques for fabricating mirror surfaces on them are well known. Thus, the fabrication of cavity mirrors on glass for the proposed cavity could be accomplished more reliably and cheaply than can fabrication of the same mirrors on an exotic nonlinear crystal for a monolithic cavity.

In the semimonolithic cavity, there would be some limited freedom to adjust the alignment of the cavity mirrors by slightly adjusting the positions and orientations of the glass endpieces before bonding them in place. The effective optical length of the cavity could be adjusted by applying an electric field to the nonlinear material, provided that the material was accessible in the position and orientation suitable for that purpose. Alternatively, prior to entering the resonant cavity, the laser beam could be bounced off a translation mirror that was piezoelectrically or otherwise adjustable. In thus freeing the cavity designer from limitations on the available size of the nonlinear crystal, the semimonolithic cavity would offer a major advantage.

One disadvantage of the semimonolithic cavity would lie in the potential for optical losses that occur at the multiple surfaces traversed by the laser beam. Some of these surfaces could be antireflection-coated to reduce losses. The polished surfaces at the ends of the nonlinear crystal and the facing surfaces of the glass blocks could be formed at the Brewster angle to reduce losses further.

This work was done by Hamid Hemmati of Caltech for NASA's Jet Propulsion Laboratory and funded under the AITP Program. For further information, access the Technical Support Package (TSP) free on-line at under the Physical Sciences category.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to

Technology Reporting Office
Mail Stop 122-116
4800 Oak Grove Drive
Pasadena, CA 91109
(818) 354-2240

Refer to NPO-19789, volume and number of this NASA Tech Briefs issue, and the page number.

Photonics Tech Briefs Magazine

This article first appeared in the August, 1998 issue of Photonics Tech Briefs Magazine.

Read more articles from the archives here.