Phase modulation of grazing incidence interferometer

Phase is modulated in a grazing incidence interferometer by modulating the incidence angle and spacing the test surface far enough from the reference surface so that the incidence angle modulation changes the phase relationship between the interferring beam reflected from the test surface and the reference beam reflected from the reference surface substantially more than it changes the sensitivity of the interferometer to surface variations between the reference and test surfaces. The preferred way of accomplishing this is with tiltable mirror 15 modulating the incidence angle slightly relative to prism 11 with its reference surface 12 and test surface 13 spaced from reference surface 12 in interferometer 10. An electromagnetic element 16 such as a galvanometer is preferred for tilting mirror 15.

BACKGROUND 
Grazing incidence interferometers are generally known for determining the 
topography of a test surface, and among other advantages, they are easily 
adjustable in sensitivity to provide a zoom effect allowing a test surface 
to be measured with varying precision. They change sensitivity by 
adjusting the incidence angle; and they use a prism surface as a reference 
surface and place the test surface in contact with the reference surface. 
Another development in interferometry separate from grazing incidence 
interferometers is phase modulation to move fringes so as to distinguish 
between hills and valleys on the test surface. This can be done manually 
with a finger tip touch by an observer who notices the fringe movement and 
determines the direction of test surface slopes. Phase modulation is also 
used in detection systems involving computer analysis to make the same 
distinctions between hills and valleys and use the computer to process and 
present information about the test surface. Phase modulation in computer 
controlled systems has been accomplished by piezoelectric transducers 
arranged to move the reference mirror. 
To apply phase modulation as developed in the interferometry art to a 
grazing incidence interferometer would require relative movement between 
the test surface and reference surface that are normally placed in 
contact. Moving either the test surface or the reference surface relative 
to the other is so difficult and troublesome that phase modulation has not 
been applied to grazing incidence interferometers. 
My invention recognizes a simple and effective way to accomplish phase 
modulation in a grazing incidence interferometer. It avoids the problems 
of moving the test or reference surfaces relative to each other and 
achieves excellent results in a simple and inexpensive way. 
SUMMARY OF THE INVENTION 
To modulate phase in a grazing incidence interferometer, I modulate the 
incidence angle of the interferometer, and to avoid changing the 
instrument's sensitivity as the incidence angle changes, I space the test 
surface far enough from the reference surface so that the incidence angle 
modulation changes phase substantially more than sensitivity. Although the 
instrument's sensitivity unavoidably changes somewhat as incidence angle 
varies near the critical angle for the instrument, I have found that 
spacing the test surface from the reference surface scales up or enlarges 
the different distances travelled by the interfering beams so that phase 
is affected much more than sensitivity. Sensitivity changes can then be 
either ignored as minor inaccuracies, or can be compensated for by a 
computer in the detection system. Phase modulation is then easily achieved 
by tilting a mirror in the incident beam path.

DETAILED DESCRIPTION 
Grazing incidence interferometer 10 as schematically shown in the drawings 
uses a prism 11 providing a reference surface 12; and test surface 13, 
instead of being in contact with reference surface 12, is spaced a small 
and fixed distance away from surface 12 according to my invention. This 
spacing should be at least 0.015 millimeters and is preferably about 
0.075 millimeters with a larger maximum spacing possible but limited by 
other practicalities. 
As exaggerated in FIG. 2, incident rays I.sub.1 and I.sub.2 refract through 
prism 11 with I.sub.1 reflecting off test surface 13 and I.sub.2 
reflecting off reference surface 12 to interfere in reflected beam 
R.sub.1,2. During the time that ray I.sub.1 passes through points A, B and 
C in reflecting off test surface 13, ray I.sub.2 passes only the distance 
between points D and C, establishing different path lengths for 
interference purposes, depending on the contour of test surface 13. 
Spacing test surface 13 at a distance from reference surface 12 
proportionally enlarges the distance between points A, B and C relative to 
distance D, C. Although a change in incidence angle changes the distances 
travelled by both beams, the longer distance A, B, C maintains the same 
relatively large proportion to the shorter distance D, C and changes by a 
larger magnitude. From this relationship, slight changes in the incidence 
angle that barely affect sensitivity are adequate to change phase enough 
to move fringes for phase modulation purposes. In other words, by spacing 
test surface 13 away from reference surface 12, a slight change in 
incidence angle affects the phase relationship between rays R.sub.1 and 
R.sub.2 to a much greater extent than the same change in incidence angle 
affects the sensitivity of interferometer 10. This can be seen intuitively 
from the drawings, and can be proved mathematically, but the proof is 
complex and is omitted for simplicity. 
Changing the incidence angle of interferometer 10 can be done in several 
ways, and the preferred way as schematically shown in FIG. 1 is with a 
folding mirror 15 that is tiltable, preferably by an electromagnetic drive 
such as galvanometer 16 turning shaft 17 supporting mirror 15. 
Piezoelectric elements, other electromagnetic drives, and even mechanical 
drives are possible for tilting mirror 15 slightly in a movement that is 
preferably uniform and regular to modulate the incidence angle slightly 
and thereby modulate phase and move the interference fringes for the 
benefit of the detection system. 
The overall sensitivity of interferometer 10 is adjusted in a generally 
known way by changing the orientation of mirror 20 to adjust the incidence 
angle. This establishes any zoom effect and adjusts the number of fringes 
desired for the surface 13 being measured. Instrument sensitivity is 
established by the angle of mirror 20 and generally remains constant while 
mirror 15 tilts to modulate the incidence angle slightly and primarily 
affect phase relationship. 
Interferometer 10 also benefits from a "coherence buster" formed as a 
ground glass disk 31 rotated in the path of the incident beam by a motor 
32. A rotating ground glass has been used in other interferometers to 
reduce the coherence of the incident beam for other purposes, but in 
interferometer 10 this is especially advantageous in reducing the effect 
of multiple reflections between reference surface 12 and test surface 13, 
which are both polished surfaces. Such multiple reflections produce high 
finesse fringe lines that cause problems for the detection system, which 
is arranged to process cosine fringes produced by single reflections from 
test surface 13 and reference surface 12. 
Interferometer 10 can also have many other refinements that are generally 
known and used in grazing incidence interferometers, but are omitted from 
the drawings for simplicity. Those skilled in the art of grazing incidence 
interferometers will be able to apply this invention in many ways, once 
they understand the basic relationship involved.