Cavity enhanced optical spectroscopy with a cavity having a predetermined deviation from a mode degeneracy condition
Improved ease of mode matching to a passive optical cavity is provided by selecting a cavity design that has a predetermined deviation from a reference cavity design having high transverse mode degeneracy. This predetermined deviation tends to be small, so that the first overlap of high-order transverse modes with the lowest order transverse mode in frequency occurs at relatively high transverse mode numbers. Coupling to high-order transverse modes is thereby reduced, since high-order transverse modes having relatively high transverse mode numbers tend to be more difficult to couple to, and tend to have high loss. During assembly of such a cavity, it can be useful to apply a perturbation to the cavity to further optimize mode matching. For example, the length of an enclosed cavity can be adjusted by altering the number and/or length of spacers in the cavity housing.
1. A method of making an apparatus for cavity enhanced optical spectroscopy, the method comprising:
providing a source of optical radiation capable of emitting source radiation;
providing an optical resonator having a lowest order resonator transverse mode and having multiple other resonator transverse modes, wherein the source radiation is coupled to the lowest order resonator transverse mode;
providing a detector capable of receiving radiation emitted from the optical resonator;
providing a reference optical resonator design having mode frequencies spaced by a small integer fraction of a free spectral range of said reference optical resonator, wherein said small integer fraction is selected from the group consisting of: ⅙, ⅕, ¼, ⅓, ½, 1, 0, ⅖, ⅔, ⅗, ¾, ⅘, and ⅚;
deriving a design for said optical resonator by making a predetermined and non-zero deviation away from the reference optical resonator design.
2. The method of claim 1 , further comprising selecting said predetermined deviation according to a method comprising:
defining FSR as a free spectral range of said optical resonator;
defining doFSR as a transverse mode frequency spacing of said reference optical resonator design;
defining daFSR as a transverse mode frequency spacing of said design of said optical resonator;
defining Δd=d a −d 0 to be said predetermined deviation;
defining d n , and d d to be nonnegative integers having no common factor and satisfying d 0 =d n /d d ;
selecting said predetermined deviation to be in a range given by 0.01≦|Δd|/d d ≦0.05.
3. The method of claim 1 , wherein said reference optical resonator design is a three mirror design.
4. The method of claim 1 , further comprising applying one or more perturbations to said optical resonator during assembly to minimize measured excitation of said higher order modes during assembly test.
5. The method of claim 4 , wherein said one or more perturbations are selected from the group consisting of: cavity mirror rotation, cavity mirror translation, changing cavity length, changing a stress applied to a cavity optical element, and changing cavity axis alignment.