High performance telescopic etalon filters

Coherent light generators – Particular resonant cavity – Specified cavity component

Reexamination Certificate

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Details

C372S019000

Reexamination Certificate

active

06181726

ABSTRACT:

TECHNICAL FIELD
The present invention relates to the use and construction of high quality etalon filters. In particular, etalons used as telescopic filters.
BACKGROUND OF THE INVENTION
In telescopic astronomy it is frequently useful to observe objects using optical filters. Filters are particularly useful for observing objects at specific wavelength bandwidths. Additionally, filters are useful for reducing the overall level of light observed making them advantageous for observation of very bright objects such as the sun.
When attempting to observe objects at specific wavelengths, filters having highly precise bandwidths and passbands are required. Unfortunately, it has proven difficult to mass produce etalon filters having the requisite optical specifications. Furthermore, etalon filters typically suffer from thermally induced variations from the desired performance specifications.
One type of useful filter is the Fabry-Perot etalon filter. Although an excellent filter, it has not yet been possible to mass produce etalons having sufficient quality. Until now, the construction of such etalons has thus far been a highly sensitive “craft” able to manufacture only a few units at a time. As a result, etalons of this type are extremely expensive.
The characteristics of Fabry-Perot etalons are well known in the art and are discussed in a number of classic texts. For example, M. Born and E. Wolf, “Principles of Optics” Pergamon Press (1980) incorporated herein by reference. In general, a Fabry-Perot etalon consists of two parallel optically flat surfaces separated by a gap. The two surfaces may have an optical coating applied to their surfaces or may be uncoated. The surfaces can be the opposing faces of two separate plates separated by a gap, the gap being filled with air or a vacuum. Such an etalon is referred to as an “air-spaced etalon”. An etalon may also be constructed using two parallel surfaces on opposite sides of a single solid plate. This is referred to as a “solid etalon”. Both types are used extensively in spectral analysis, laser-line narrowing, mode selection, and as integral components in the construction of ultra-narrow band optical filters, as well as many other instances where spectral selection and filtering is desired.
An air-spaced etalon can be made extremely thermally stable, whereas a solid etalon is subject to changes in its optical thickness depending on changes in ambient temperature, thereby causing the passband of the etalon to change with changing temperature. Because it is desirable to have a stable passband, such solid etalons are not desirable.
Historically, air-spaced etalons have been constructed using two different designs.
FIG. 1
shows an etalon
1
having two parallel optically flat surfaces (also called plates or etalon plates)
10
,
11
separated by spacers
14
,
15
, which define a gap
17
equal to the thickness of the spacers
14
,
15
. As the ambient temperature changes, the spacers
14
,
15
expand and contract leading to an expansion and contraction of the gap
17
which changes the passband of the etalon
1
.
FIG. 2
illustrates an alternative design known as a “re-entrant” etalon
2
. Such etalons feature a third plate known as a “riser”
19
. Typically, re-entrant etalons
2
are used when a gap
17
of less than about 0.5 millimeters (mm) is desired. The gap
17
in such structures is defined by the difference in length between the spacers
14
and
15
and the thickness of the riser
19
. The optical quality of the etalon
2
and consequently its efficiency is governed by two factors, the flatness of the plates
10
,
11
and the parallelism of the gap
17
. In any etalon, the flatness of the plates is a limiting parameter. In the air-spaced etalon, the parallelism of the gap is controlled by the ability to form spacers demonstrating adequate parallelism and, in the case of the re-entrant design (FIG.
2
), the ability to form an adequately parallel riser
19
.
The chief difficulty in manufacturing telescopic etalon filters is that to meet the necessary optical tolerances, extremely precise, time consuming, and expensive manufacturing techniques must be used. Although it may be possible to hand-manufacture small numbers of air-spaced etalons of the type described above, such techniques are so specialized that only a few persons in the world can make such filters. These techniques are more in the nature of an art and not at all suited to mass production. In fact, there is no presently known method for constructing such devices in large quantities. What is needed is an etalon having a high degree of optical precision and a high degree of thermal stability as well as a method of mass producing such etalons.
SUMMARY OF THE INVENTION
Accordingly, the principles of the present invention contemplate an air-spaced etalon filter having a high degree of optical quality, a high degree of thermal stability, and a method for mass producing such etalons.
The etalon comprises an etalon plate having a first surface and a second etalon plate having a second surface. The first and second surfaces being separated by at least one spacer. Said separation defining a gap. The precision of the gap being enhanced by optically contacting the etalon plates to the spacers. Said spacers being formed to a high degree of uniformity such that thickness of the gap is highly uniform across the entire surface of the etalon plates.
The principles of the present invention may be used to construct etalon filters having excellent flatness and parallelism across the entire etalon surface. One such embodiment contemplates using a centrally located spacer. This embodiment may also incorporate peripherally mounted spacers placed around the etalon edges. Additionally, another embodiment improves plate flatness and parallelism without obstructing a central aperture by using peripherally mounted spacers of relatively large size. By optically contacting large spacers to etalon plates, the spacers contact large surface areas of the plates, thereby bending the plates into conformity with the spacers leading to excellent etalon plate flatness and parallelism.
Additionally, a third optical element having a third surface may be optically contacted to the surface of said second etalon plate. The separation between the third surface and the first surface defines a gap.
Also, in accordance with the principles of the present invention a method of mass producing the etalon filters of the present invention is disclosed. The etalon plates being formed using standard precision optical manufacturing techniques. The plates having appropriate thicknesses and having the appropriate coatings. A spacer substrate is then provided. The spacer substrate, having an appropriate thickness, is also formed using standard precision optical manufacturing techniques. Individual spacers are cut from the spacer substrate. Spacers cut from the same local area of the spacer substrate are optically contacted with the etalon plates to form an assembled etalon. The same principles may be used to construct re-entrant type etalons.
Other features of the present invention are disclosed or apparent in the section entitled “DETAILED DESCRIPTION OF THE INVENTION”.


REFERENCES:
patent: 5028879 (1999-02-01), Ershov
patent: 5856991 (1999-01-01), Ershov
patent: 5889590 (1999-03-01), Duggal et al.

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