1. Field of the Invention
This invention pertains to the general field of optical filters and, in particular, to temperature-stable and tunable high-performance etalon filters.
2. Description of the Prior Art
Etalons are well known optical devices that consist of two reflective surfaces parallel to one another and spaced apart by solid spacers to produce a predetermined optical length (the “cavity length”). They may consists simply of a solid parallel plate with reflective surfaces (so called “solid etalons”) or of two plates with an air gap between them that defines the cavity (so called “air-spaced etalons”), as illustrated in FIG. 1. A hybrid form of etalon (so called “re-entrant etalon”) utilizes an additional solid structure with a reflective surface (a “riser”) between the two plates in order to achieve narrower cavity lengths than practically obtainable with the use of spacers.
When illuminated with a broadband collimated light, etalons produce a transmission beam and a reflection beam with periodic spectra characterized by very narrowband spikes centered at wavelength determined by the physical properties and dimensions of the etalon. A typical etalon transmission spectrum is illustrated in FIG. 2. With reference to air-spaced etalons, in particular, the specific center wavelength λ′ of the passband (the spectral spike) and the period between spectral spikes (commonly referred to in the art as channel spacing or free spectral range—FSR—of the device) are a function of the optical length of the etalon's cavity. This disclosure is limited to a discussion of transmission operation because those skilled in the art would readily understand that it is similarly applicable to reflection operation.
In particular, referring for example to the etalon 10 and the intensity spectrum 12 of FIGS. 1 and 2, respectively, minor changes in the optical length L (corresponding to the geometric length L′ shown in the figures) of the cavity 14 will cause a shift of the periodic spectrum along the wavelength axis, as indicated by arrows 16. As is well understood by those skilled in the art, varying the optical length L of the cavity also produces a change in the width of the spectral spike and in the free spectral range of the etalon.
These properties of etalons are very advantageous for many optical applications. In particular, etalons are used as high-performance filters to isolate light of a very a precise frequency, as may be needed for a particular application. In telescopic astronomy, for instance, such filters are particularly useful for observing objects at specific wavelengths. Since the exact wavelength of each peak is a function of the exact optical length L of the cavity, it has been most important in the art to build etalon filters with precise and uniform spacing between the two plates (18,20) constituting the etalon (FIG. 1). To that end, very precisely machined spacers 22,24 of equal thickness L′ are used, typically uniformly distributed around the annular periphery of the plates in a sufficient number to separate the plates and produce a cavity of uniform optical length L. Moreover, these spacers are typically made of materials having a low coefficient of thermal expansion. (It is noted that L′ is the physical cavity length corresponding to the desired optical path length L, the two quantities being related by the equation L=nL′, where n is the index of refraction of the medium in the cavity.).
In practice it has been difficult and expensive to achieve the desired degree of perfection because of the very narrow tolerances (in the order of nanometers) required for the level of performance associated with extremely narrowband applications. U.S. Pat. Nos. 6,181,726 and 6,215,802 disclosed several advances over the prior art whereby the uniformity of the etalon's optical length was improved. According to one approach described in the patents, all the spacers used to form the etalon are selected from a common local area of a spacer substrate produced by standard-precision optical manufacturing techniques. It was discovered that, as a result of this selection, the spacers tend to have substantially more uniform thickness and, therefore, they produce a more uniform etalon cavity. According to another, complementary approach, an additional spacer from the same local substrate area is used at the center of the etalon, thereby providing a correction to plane deformations produced by the optical contact of the peripheral spacers with the etalon plates.
While the techniques described in these patents provide a significant improvement over the etalons previously known in the art, they are very labor-intensive and therefore expensive to practice. In addition, the resulting etalons, while more uniform in the optical length of the cavity, are not necessarily tuned to the precise desired wavelength. Copending U.S. Ser. No. 10/795,167 discloses a solution to this problem based on the use of counterbalanced forces applied to the etalon elements. This approach constitutes another significant advance in the art, but it does not address the problem of changes in performance (in terms of center wavelength and FSR) produced by thermal variations. Therefore, there is still a need for an etalon structure that is relatively insensitive to thermal effects and that produces extremely accurate tuning of the optical length of the etalon cavity. The present invention provides a solution to this remaining challenge that also produces a greater range of angular acceptance and a mechanism for thermally tuning the etalon to a precise level of performance.