Optics: measuring and testing – Dimension – Thickness
Reexamination Certificate
2002-05-03
2004-11-16
Rosenberger, Richard A. (Department: 2877)
Optics: measuring and testing
Dimension
Thickness
C356S485000
Reexamination Certificate
active
06819438
ABSTRACT:
TECHNICAL FIELD
The present invention relates to optical devices and more particularly to the manufacture of high quality micro-optical devices.
BACKGROUND ART
FIG. 1
depicts section of an exemplary conventional micro-optical element
100
. The element
100
is in the form of a substantially flat glass plate
110
. The glass plate
110
has a primary upper surface
110
A and primary lower surface
110
B. The glass plate
110
could be in the shape of a square, rectangle, circle or have some other shape. The glass plate could, for example, be for use in one or more etalons or other optical devices.
The primary upper surface
110
A and primary lower surface
110
B of the glass plate
110
are separated by a distance “d”. The transmission characteristics of the glass plate
110
between the primary upper surface
110
A and primary lower surface
110
B, and more particularly what is commonly referred to as the optical thickness or optical path distance (OPD), will vary with variations in the distance d. In many applications, it is important that the OPD throughout the glass plate
110
be uniform.
A single optical element, such as the plate
110
, may form what is commonly characterized as a flat, from which multiple smaller optical sub-elements will be formed. More particularly, a flat is sliced to separate the multiple optical sub-elements. Each separate sub-element may, for example, then be used individually in the manufacture an optical device. Accordingly, it is desirable for each of the multiple sub-elements within the flat, e.g. glass plate
110
, to have uniform transmission characteristics, which in turn requires that that the OPD throughout the flat be uniform or uniform within an acceptable tolerance.
To establish a uniform OPD throughout the sub-elements, the conventional practice is to first physically measure the thickness of the optical element. Polishing is performed to remove material from one or more surface of the element based on the physical measurement. The uniformity of the OPD for the element is then optically measured. If required, further polishing and optical measurement of element's OPD uniformity is performed. Once sufficient uniformity has been achieved, the element is sliced in order to separate the individual optical sub-elements.
For example, referring again to
FIG. 1
, in the case of the glass plate
110
the distance between the primary upper and lower surfaces
110
A and
110
B at various locations on the plate is measured, using a micrometer, to determine the distance d at each of the measured locations. These determined distances are then used to determine, on a relatively coarse basis, the uniformity of the apparent OPD, which can be computed using the determined distances as is well understood in the art.
Based on the measured distances, a determination is also made as to how much material must be removed from particular areas of one or both of the primary surfaces
110
A and
110
B to form a plate having a sufficiently uniform thickness d, and therefore a sufficiently uniform apparent OPD, for the particular application. The material is removed by polishing the optical element.
The goal of this initial polishing is to make the surfaces
110
A and
110
B of the plate perfectly parallel, and hence the apparent OPD perfectly uniform. However, as is recognized by those skilled in the art, the true OPD and the apparent OPD will often vary for at least two reasons. First, the true distance d may differ from the physically measured distance. This is particularly true in the fabrication of micro-optical elements. Additionally, there may be variations in the refractive index of the glass forming the optical element, e.g. the plate
110
. Hence, even if the plate
110
has the exact same thickness, and therefore the exact same apparent OPD, at two locations, the true OPD at these locations may vary due to differences in the refractive indexes of the plate material at these locations. Accordingly, to obtain uniformity of the true OPD, it may be necessary for the thickness of the element, e.g. the distance d, to vary at different locations. Therefore, even if the initial polishing results in the surfaces
110
A and
110
B of the plate being perfectly parallel, this may not result in sufficient uniformity of the true OPD.
Therefore, after the initial polishing, an interferometer (not shown) is typically used to perform an optical measurement by directing a broad beam of light
120
over the entire surface
110
A of the plate
110
. As is well understood in the art, by visually examining the shading of the light
130
passing through the plate
110
, a more accurate determination of the uniformity of the true OPD can be determined.
If the plate surfaces
110
A and
110
B are perfectly parallel, making the distance d perfectly uniform, and the plate material has a constant refractive index throughout the plate, the OPD will also be perfectly uniform. In such a case, the interferometer light
130
which passes through the plate will appear either light or dark, but in any event of a constant shade of gray or of a constant color when viewed with the naked eye. It will be recognized that the fact that the passing light
130
is light or dark is unimportant. Rather, what is important is that the passing light
130
appears to have a uniform shade or color.
However, if the passing light
130
appears to have a non-uniform shade or color, further polishing is performed on those areas of the appropriate surface(s)
110
A and
110
B corresponding to the areas of non-uniform shading. This further polishing will be performed whether the non-uniform shading or color, reflecting a non-uniformity of the true OPD, is caused by the plate surfaces
110
A and
110
B not being perfectly parallel or by the plate material having a varying refractive index or both. It will be understood, if the refractive index varies, uniform shading, and hence a uniform true OPD, can only be achieved if the distance d actually varies slightly in different areas of the optical element, to offset the index variations. After this further polishing, the plate
110
is re-checked, using the interferometer. After inspection indicates a sufficiently uniform true OPD, the plate
110
is sliced into multiple optical sub-elements.
Hence, if the fabricator perceives, through a visual inspection, that a variation in shading or color exist in the middle of the optical element, a small portion of the material in the middle of the element is removed by polishing. If visual inspection of the passing light indicates that the shading is sufficiently uniform, no further polishing is performed and the optical element can be sliced into respective sub-elements or used in a further flat assembly as will be described below with reference to FIG.
2
.
FIG. 2
shows an exemplary portion of a conventional flat of micro-optical devices, commonly characterized as etalons. The depicted etalon is formed of a portion of top plate
210
A and bottom plate
210
B, which are both formed of glass. Plates
210
A and
210
B are substantially identical to plate
110
of FIG.
1
. The glass plates
210
A and
210
B are separated by spacers
260
A and
260
B to create a cavity
270
. It will be understood that additional spacers (not shown) are used to separate the plates
210
A and
210
B to form the other etalons which will be sliced from the flat.
In the
FIG. 2
example, since air fills the cavity
270
, the depicted etalon is of the type commonly referred to as an air space etalon. As will be recognized by those skilled in the art, if glass filled the cavity
270
, the etalon would be of a type commonly referred to as a solid etalon. If water, or some other liquid, filled the cavity
270
, the etalon would be of a type commonly referred to as a liquid etalon.
Also disposed within the cavity is a top coating layer
230
A formed on lower main surface of the top glass plate
210
A, and a bottom coating layer
230
B formed on the upper main surface of the bottom glass plate
210
B. The top and bottom coatings are separate
McCreath William
Neily Richard A.
Parker David
Barth Vincent P.
GSI Lumonics Corporation
Maine & Asmus
Rosenberger Richard A.
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