Optics: measuring and testing – By light interference
Reexamination Certificate
1999-10-18
2003-04-29
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By light interference
Reexamination Certificate
active
06556304
ABSTRACT:
RELATED APPLICATIONS
This application claims the priority of Japanese Patent Application No. 11-055113 filed on Mar. 3, 1999, which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of supporting a reference plate for a lightwave interferometer; and, in particular, to a method of supporting a reference plate for a lightwave interferometer when the reference plate is attached to the main body of the interferometer.
2. Description of the Prior Art
Since the emergence of lasers which are light sources having a favorable coherence, lightwave interferometers which can measure surface forms of optical parts with a high accuracy have come into widespread use. A lightwave interferometer divides a luminous flux from a light source into two, employs one of the resulting two luminous fluxes as an object lightwave irradiating a sample to be inspected and thereby carrying the state of phase of the sample, employs the other as a reference lightwave irradiating a reference plate and thereby carrying the state of phase of the reference plate, and recombines these two lightwaves together, so as to obtain interference fringes corresponding to the phase difference between the lightwaves, thus only comparing the object lightwave and the reference lightwave with each other for the measurement. The detectable difference between the two lightwaves in the lightwave interferometer is about ½ of the wavelength of the employed lightwaves (corresponding to one line of the interference fringes), whereby very fine comparisons can be carried out.
In such a very fine comparison, it is important how to make the reference plate with a high precision and how to support it with the main body of the interferometer while keeping its precision.
FIG. 10
shows an example of a conventional circular reference plate for measuring a plane. The reference plate
101
is made of a material having a low coefficient of thermal expansion, e.g., quartz or ceramics. In optical systems in which a luminous flux is transmitted through the reference plate
101
, in particular, uniformity is required in the refractive index distribution within the material.
Though depending on the accuracy required in measurement, the reference surface
102
is polished with such a high precision that the amount of deviation from a perfect plane is {fraction (1/20)} or less of the employed wavelength of light. Though no such a high precision as that of the reference surface
102
is required, the rear surface
103
opposite the reference surface
102
is polished with such a precision that the abovementioned amount is about ⅕ to {fraction (1/10)} of the employed wavelength of light, and is provided with an antireflective coating for the employed wavelength of light in order to reduce reflection noise. Further, the outer peripheral face
104
of the reference surface
102
is polished or lapped with a roughness on the order of #
400
to #
800
in general.
There have been some attempts to attach the reference plate
101
thus finished with a high precision to the main body of the interferometer while keeping the precision.
The example shown in
FIG. 11
shows, in a barrel
110
for accommodating the reference plate
101
and mounting it to the main body portion of the interferometer, the reference plate
101
being inserted while a shoulder
111
of the barrel
110
is used as an abutment. A pressure ring
112
is used for preventing the reference plate
101
from rattling in excess, and is fixed with a slight gap from the reference plate
101
so as not to affect the precision of the reference plate
101
. A screw
113
is used for attaching the reference plate unit to the main body of the interferometer and keeping a stable relative positional relationship between the main body portion of the interferometer and the reference plate
101
.
However, such a method has the following two problems in terms of the accuracy in measurement.
(1) The barrel
110
made of a metal such as aluminum and the reference plate
101
made of glass, ceramics, or the like come into contact with each other at the shoulder
111
of the barrel
110
, whereby the reference plate
101
presses this contact portion with its own weight. Since the shoulder
111
of the barrel
110
processed by cutting or the like and the polished surface of the reference plate
101
have different degrees of processing precision, it is hard to attain a uniform contact state between these members
101
,
111
, whereby they come into contact with each other by point contact of at least three points as shown in FIG.
12
. Letting A, B, and C be the three points of contact, the part of the reference plate
101
outside the triangle ABC is flexed by the own weight of the reference plate
101
, so as to be changed into a form schematically indicated by contour lines
120
in FIG.
12
. Thus, the measurement using this reference plate
101
may yield an error in measurement corresponding to the change in form of the reference plate
101
such as that indicated by the contour lines
120
.
(2) As shown in
FIG. 11
, the reference plate
101
is unfixed though it does not drop out of the barrel
110
, thus being able to rotate freely about the optical axis at its mounted position. This feature does not cause any problem as long as the reference plate
101
is a perfect plane so that the form of the sample to be compared can be measured however it is rotated. In practice, however, it is hard to form a perfect plane, and the reference plate
101
has different plane forms, though slightly, among its portions. Since the interference measurement is based on the comparison as mentioned above, comparative data, i.e., measured values, of the sample will become unstable if the reference plate
101
to be compared rotates freely. Also, local differences in the form of the reference plate
101
may change the contact positions between the sample and the shoulder
111
of the barrel
110
, whereby the supporting of the reference plate
101
according to the above-mentioned method may affect the amount of flexure of the reference plate
101
as well.
FIG. 13
shows a method of supporting a reference plate which can overcome the shortcomings of the conventional example shown in FIG.
11
.
In this method, a cushioning material
121
made of thick paper or the like is held between the reference plate
101
and the shoulder
111
of the barrel
110
, which have been in contact with each other due to the own weight of the former in the conventional example shown in FIG.
11
. Thus, the error in precision of the shoulder
111
of the barrel
110
due to machining is absorbed by the elasticity of the cushioning material
121
, so that the reference plate
101
comes into contact therewith by the whole outer peripheral face of the reference surface. As a consequence of this improvement, the reference plate
101
is kept from changing its form due to its contact points as shown in
FIG. 12
, and only yields changes in form due to its own weight flexure caused as being supported by the whole periphery as shown in FIG.
14
.
In this case, however, the cushioning member
121
held between the reference plate
101
and the shoulder
111
of the barrel
110
is kept being pressed with the weight of the reference plate
101
, whereby it may lose its elasticity over time and may reduce cushioning effects. Namely, as the time passes, the form of the reference plate
101
may change from the state shown in
FIG. 14
to that shown in FIG.
12
.
FIG. 15
shows a method of supporting the reference plate
101
which overcomes such a problem. In this method, the reference plate
101
is not mounted on the shoulder
111
of the barrel
110
. Instead, an adhesive
132
is injected by a syringe or the like into small holes
131
equidistantly formed in the circumference of the barrel
110
, so as to bond and secure the reference plate in a suspended state. According to this method, nothing comes into contact with the reference surface
10
Font Frank G.
Fuji Photo Optical Co., Ltd.
Lee Andrew H.
Snider Ronald R.
Snider & Associates
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