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This invention relates to interferometers for precise determination of
lengths and more particularly, to a highly sensitive interferometer which
can produce interference fringes one fringe separation of which
corresponds to .lambda./2N (.lambda. is a wavelength of an incident light
and N is an integer which is 2, 3, 4 . . . ).
In order to detect a very small linear displacement of an object to be
measured on the order of Angstrom by means of an interferometer in a
highly precise manner, the interferometer itself must be stable in
position and an inclination of the interferometer as a whole must not have
an influence upon the precise determination of lengths.
As one of means of stabilizing the interferometer per se, heretofore, it
has been proposed to utilize the principle of so-called common path
interference. This consists in that two light beams to be interfered with
each other are caused to travel a common light path as far as possible,
and in that the path difference is not affected even when the alignment of
the interferometers changes due to, for example, thermal or mechanical
causes.
In such a construction as the Michelson interferometer which has heretofore
been used as an interferometer for precise determination of lengths in
general, there is a risk of a part or whole of the interferometer being
inclined by various causes. This inclination results in a change in length
of the light path of either a reference light beam or a measuring light
beam, and as a result, the measuring light beam is subjected to a change
other than the change of the light beam per se, thereby making it
difficult to obtain a stabilized interference fringe.
Heretofore, it has been proposed to provide an interferometer for precise
determination of lengths, which makes use of an autocollimator or a
special trilateral reflecting mirror for the purpose of obviating the
Abbe's error.
The use of the autocollimator, however, has the disadvantages that the
Abbe's error must be corrected in an electrically complex manner, and that
the autocollimator becomes worse in precision due to floating of the air
and the like.
The use of the trilateral reflecting prism has the disadvantage that the
trilateral reflecting prism must be arranged in a three dimensional manner
so that its adjustment in position becomes complex, and that the light
beams for producing the interference fringe are not located in the same
plane on which an object being measured is positioned so that the
interferometer becomes complex in construction.
In addition, both these conventional interferometers produce interference
fringes one fringe separation of which corresponds to .lambda./4, and as a
result, their sensitivity is low.
Heretofore, it has been proposed to insert an optical system for folding a
reference optical path and a measuring optical path for the purpose of
increasing the sensitivity of the interferometer. Such kind of an
interferometer has the disadvantage that the optical system inserted for
the purpose of folding the optical path must have a highly precise surface
and must be made of a material having an extremely homogeneous property.
As a result, the interferometer becomes excessively expensive.
In general, the sensitivity of an interferometer can be improved through
the frequent folding of the light path by means of a suitable means. In
the means heretofore proposed, an optical system consisting of a
combination of reflecting optical systems such as a plane mirror, corner
cube mirror and the like is arranged in the reference light path and in
the measuring light path separately so as to fold the light path.
Such conventional means, however, has the disadvantage that in order to
obtain uniform interference fringes the surface precision of the optical
system for folding the light path must be made extremely high, and that
the adjustment must be effected by a highly skilled operator. If use is
made of a light path folding optical system common to both the reference
and the measuring light paths, it is possible to lower the surface
precision of the optical system for folding the light path and make the
adjustment easy.
In case of precisely determining lengths by means of the interferometer,
reflecting optical systems are relatively moved in the direction of
optical axis. In this case, if one of the reflecting optical systems is
rotated, the light beam is transversely displaced. As a result, if a
coherent light incident on the interferometer has a small cross section,
the transverse displacement of the light beam results in a reduction of
the area on which interference fringes are produced. As a result, in case
of photoelectrically detecting the interference fringes, the modulated
output from a photoelectric detector is decreased to reduce the contrast
of the apparent interference fringes.
This is particularly important when the interferometer is provided for a
carriage of a large machine tool. If the carriage is moved, for example,
by 10 meters and during its movement if it is inclined from the horizontal
direction by an angle of 40", the amount of transverse displacement of the
light beam becomes equal to 2.times.10m.times.(2.times.10.sup.-.sup.4 rad)
= 4 mm. As a result, even though the diameter of the light beam is on the
order of 10 millimeters, the distinctness of the apparent interference
fringes, due to the transverse displacement of the light beam, becomes
considerably decreased.
An object of the invention is to provide an interferometer for precise
determination of lengths wherein two light beams to be interfered with
each other are caused to travel a common light path as far as possible and
even when the interferometer is slightly misaligned due to various causes,
these light paths are prevented from being changed in the path difference,
thereby obtaining stabilized interference fringes.
Another object of the invention is to provide an interferometer which can
make one fringe separation .lambda./4, obtain a contrast of the
interference fringes independently of the alignment of moving mirrors and
the like constituting the interferometer.
A further object of the invention is to provide an interferometer for
precise determination of lengths which can obviate the Abbe's error in a
less expensive manner without utilizing the autocollimator or trilateral
reflecting mirror.
A still further object of the invention is to provide an interferometer
which does not make use of a highly precise optical system for the purpose
of folding the light path and which is highly sensitive and less expensive
.
Another object of the invention is to provide an interferometer which makes
use of a laser as a light source and which can obviate the influence of
the reflected light emerging from the interferometer upon the laser, i.e.,
can obviate so-called back talk.
A further object of the invention is to provide a highly sensitive
interferometer which makes use of a light path folding optical system
common to both the reference and the measuring light paths.
Another object of the invention is to provide a highly sensitive
interferometer which can utilize a laser as a light source without
involving a back talk phenomenon in which the reflected light from the
interferometer has an effect upon the laser.
Another object of the invention is to provide a highly sensitive
interferometer completely symmetrical as a whole and having an optical
system substantially common to both the reference and measuring light
paths and capable of changing the path difference only by a relative
displacement between the reference reflector and the measuring reflector
without changing the path difference by inclination of a beam splitter or
the reference and measuring reflectors.
Another object of the invention is to provide an interferometer which can
measure not only minute displacement but also the pitch or drunkenness
error of screw threads or the pitch of a rack.
A further object of the invention is to provide a highly sensitive
interferometer which can produce interference fringes one fringe
separation of which corresponds to .lambda./2N (.lambda. is a wavelength
of an incident light and N is an integer which is 2, 3, 4 . . . ).
The invention will now be described in detail in connection with the
attached drawings, wherein:
FIG. 1 is a perspective view showing a preferred exemplary embodiment of
the interferometer for precise determination of lengths according to the
invention;
FIG. 2 is a perspective view showing a modified embodiment of
retroreflecting optical means shown in FIG. 1;
FIG. 3 is a perspective view showing a modified embodiment of beam
splitting means shown in FIG. 1;
FIG. 4 is a diagrammatic sectional view showing a modified embodiment of
bilateral reflecting optical means shown in FIGS. 1 to 3;
FIG. 5 is a block diagram showing a device for photoelectrically detecting
interference fringes;
FIG. 6 is a perspective view showing another modified retroreflecting
optical means according to the invention;
FIG. 7 is a plan view showing the inventive interferometer applied to
measure the pitch or drunkeness error of screw threads;
FIGS. 8 to 10 are front elevations showing optical elements and light paths
shown in FIG. 7;
FIG. 11 is a plan view showing a modified embodiment of the interferometer
shown in FIG. 7;
FIG. 12 is a plan view showing another modified embodiment of the
interferometer shown in FIG. 7;
FIG. 13 is a perspective view showing another embodiment of the
interferometer according to the invention;
FIG. 14 is a perspective view showing a modified embodiment of the
interferometer shown in FIG. 13;
FIG. 15a is a plan view of another modified embodiment of the
interferometer shown in FIG. 13;
FIG. 15b is the front elevation of FIG. 15a;
FIG. 16 is a perspective view showing a still further embodiment of the
interferometer shown in FIG. 13;
FIG. 17 is a perspective view showing another embodiment of the
interferometer according to the invention;
FIG. 18 is a plan view showing a device for photoelectrically detecting
interference fringes;
FIG. 19 is a perspective view showing modified bilateral reflecting optical
means of the interferometer shown in FIG. 17;
FIG. 20 is a front elevation showing the construction and light paths of
another embodiment of the interferometer according to the invention, seen
from the incident light side;
FIG. 21 is a front elevation showing the construction and light paths of a
modified embodiment of the interferometer shown in FIG. 20;
FIG. 22 is the plan view showing a relative arrangement between the beam
splitting means shown in FIG. 17 and the rotator and a cat's eye;
FIG. 23 is the plan view showing a relative arrangement between the beam
splitting means shown in FIG. 17, the rotator and a corner cube prism;
FIG. 24 is a perspective view showing the construction and light paths of a
still further embodiment of the interferometer shown in FIG. 17; and
FIG. 25 is a perspective view showing the construction and light paths of
another embodiment of the interferometer according to the invention.
In FIG. 1 is shown an exemplary embodiment of the interferometer for
precise determination of lengths according to the invention. Reference
numeral 1 designates a thin coherent incident light, for example, a laser
light incident on a beam splitting plate 2, i.e. a double refraction
crystal plate made, for example, of calcite whose optical axis is inclined
from the direction of the incident light. The light incident on the double
refractive crystal plate 2 is separated into an ordinary light beam that
oscillates in a plane perpendicular to a plane including the optical axis
of the plate 2 and an extraordinary light beam that oscillates in the
plane including the optic axis of the plate 2. These ordinary and
extraordinary light beams travel unequal optical paths 11, 21,
respectively.
The ordinary light beam travelling the optical path 11 shown by a full line
after passing through the crystal plate 2 is reflected by a
retroreflecting triangular prism 3, constituting appropriate optical
means, and travels an optical path 12 through the crystal plate 2. The
extraordinary light beam travelling the optical path 21 shown by dot and
dash lines after passing through the crystal plate 2 is reflected by
another retroreflecting triangular prism 4 and travels an optical path 22
through the crystal plate 2.
The ordinary and extraordinary light beams reincident on the crystal plate
2 are reunited, travel a common optical path 13 and are incident on a
bilateral reflecting triangular prism 5 whose edge line of the bilateral
reflecting surfaces is substantially perpendicular to those of the
retroreflecting prisms 3, 4.
The light beam incident on the bilateral reflecting prism 5 is reflected by
it in the same direction from which it came. The light beam emerging from
the bilateral reflecting prism 5 passes through a rotator 6 which can
rotate the polarizing plane of the light traversing it by 90.degree. and
is incident again on the crystal plate 2.
The light beam incident again on the crystal plate 2 is separated into two
light beams that travel unequal optical paths 14, 24 and emerge from the
crystal plate 2. The light beams travelling along the light paths 14, 24
are incident on the bilateral reflecting prism 4 and after reflected
incident again on the crystal plate 2 and travel light paths 15, 25 within
the crystal plate 2.
These light beams along the light paths 15, 25 after passing through the
crystal plate 2 are reunited and are allowed to emerge therefrom. If the
light beam emerged from the crystal plate 2 is incident on a polarizing
element such as a polarizing plate 7 whose polarizing axes are inclined
from the polarizing direction of the light traversing it by 45.degree., it
is capable of detecting interference fringes corresponding to the path
difference between the two polarized light beams.
As seen from the above, there are two light paths, that is, a light path
(I): 1 .fwdarw. 11 .fwdarw. 12 .fwdarw. 13 .fwdarw. 23 .fwdarw. 14
.fwdarw. 15 .fwdarw. 16 and a light path (II): 1 .fwdarw. 21 .fwdarw. 22
.fwdarw. 13 .fwdarw. 23 .fwdarw. 24 .fwdarw. 25 .fwdarw. 16. The light
beam travelling the optical path (I) is reflected once by the prisms 3, 4,
respectively, while the light beam travelling the optical path (II) is
reflected twice by the prism 4.
Since the light beam travelling the light path (I) is located outside the
light beam travelling the light path (II) with respect to the prisms 3, 4
and these two light paths I and II pass through the same crystal plate 2,
the distance between the light paths 11 and 21 is the same as that between
the light paths 14 and 24. As a result, even when that part of the
interferometer which is surrounded by two dotted chain lines 8 as a whole
is inclined or displaced with respect to the prisms 3, 4, the path
difference between these two light paths I and II is not changed at all.
That is, a relative displacement or rotation between the prisms 3, 4 and
the interferometer portion 8 has no effect upon the interference fringes,
so that it is possible to obtain extremely stable interference fringes.
Either of the retroreflecting triangular prisms 3, 4 may be used as a
movable reflecting mirror of the interferometer when it is used for the
precise determination of lengths. In this case, one of the retroreflecting
triangular prisms 3, 4 may be made stationary, while the other may be made
movable. If the wavelength of the incident light beam is .lambda., the
intensity of the interference fringes periodically changes every time the
relative displacement between the rectangular prisms 3 and 4 is effected
by .lambda./2.
In FIG. 2 is shown another embodiment of the interferometer according to
the invention. The retroreflecting triangular prism 4 shown in FIG. 1 is
divided here into two prisms 3' and 4 and the prism 3' is secured to a
supporting base to which is secured the prism 3. The other constructional
elements are the same as those shown in FIG. 1.
In the present embodiment, the light beam travelling the ordinary light
beam path 11 after passing through the crystal plate 2 is reflected by the
rectangular prism 3 and travels through the crystal plate 2, the bilateral
reflecting prism 5 and the crystal plate 2 in succession and is incident
on the retroreflecting triangular prism 3' and reincident on the crystal
plate 2.
On the other hand, the light beam travelling the extraordinary light beam
path 21 travels the light path (II) and is reflected two times by the
prism 4 as in the case of FIG. 1. In the present embodiment, the distance
between the light paths 11 and 21 is also the same as that between the
light paths 14 and 24. As a result, the relative displacement and rotation
between the interferometer portion surrounded by the two dotted chain
lines 8 and the rectangular prism 3, 3', 4 have no effect upon the path
difference between these two light paths I and II. In addition, a rotation
of either the interferometer portion 8 or prisms 3, 3' or the prism 4
about points on a center axis of the prisms 3, 3' has no effect upon the
interference fringes.
As a result, in the present interferometer only a change in direction of
the optical axes between the prisms 3, 3' and the prism 4 results in a
change in the path difference. In addition, the two wave fronts of the
light beams to be interfered with each other are not inclined from the
above mentioned inclinations so that stabilized interference fringes are
obtained. In the present embodiment, either the retroreflecting triangular
prisms 3, 3' or the prism 4 may be used as a movable reflecting mirror.
In FIG. 3 is shown a modified embodiment of the interferometer portion 8
shown in FIG. 1. In this embodiment, the beam splitting plate 2 is
replaced by a prism 9. The prism 9 is composed of a triangular prism 91
and a parallelogram prism 92, the latter being separated from the former
by a cemented surface which is provided with a polarizing film 93
deposited by vacuum evaporation, etc. As a result 100% of that part of the
incident light which is a linear light polarized in the incident plane
(hereinafter this is called P polarized light) passes through the
polarizing film 93, while 100% of the linear light polarized in a plane
normal to the incident plane (hereinafter this is called S polarized
light) is reflected by the polarizing film 93.
If the light beam travelling the optical path I be the P polarized light,
the incident light 1 after having passed through the polarizing film 93
travels the light path 11, is reflected by the prism 3 then travels the
light paths 12, 13 and passes through the bilateral reflecting triangular
prism 5.
The light beam emerging from the bilateral reflecting triangular prism 5 is
incident on the rotator 6 for rotating the polarizing plane of the light
traversing it by 90.degree., changed into the S polarized light and
reflected by the polarizing film 93. The light beam reflected by the
polarizing film 93 travels the light path 14 and is reflected by a
reflecting part 94 of the parallelogram prism 92 which is opposed to the
polarizing film 93 and incident on the retroreflecting triangular prism 4.
The light beam incident on the retroreflecting triangular prism 4 is
reflected by it, travels the light path 15 and is reflected by the
polarizing film 93, travels the light path 16 and arrives at the
polarizing plate 7.
If the light beam travelling the optical path II is the S polarized light,
the incident light is reflected by the polarizing film 93 and the
reflecting part 94, travels the light path 21 and is incident on the
retroreflecting triangular prism 4. The light beam incident on the prism 4
is reflected by it, travels the light path 22, is reflected by the
reflecting part 94 and the polarizing film 93, travels the light path 13
and arrives at the bilateral reflecting triangular prism 5.
The light beam passing through the prism 5 is incident on the 90.degree.
rotator 6 is changed into the P polarized light and passes through the
polarizing film 93. The light beam emerging from the polarizing film 93
travels the light path 24, the prism 4, the light path 25, the polarizing
film 93 and the light path 16 and arrives at the polarizing plate 7.
The light beams that travel the optical paths I and II when reunited
interfere with each other to produce the interference fringes.
The light path I is located outside the light path II with respect to the
prisms 3, 4, and the distance between the light paths I and II with
respect to the right side of the interferometer is equal to that between
the same paths with respect to the left side.
As a result, a change in the relative displacement and relative rotation
between the interferometer part and the retroreflecting triangular prisms
3, 4 has no effect upon the path difference between the two light paths I
and II so that significantly stable interference fringes can be obtained.
In FIG. 4 is shown a modified embodiment of the bilateral reflecting
triangular prism 5 shown in FIGS. 1 to 3. Here the bilateral reflecting
triangular prism 5 is replaced by two mutually perpendicular reflecting
mirrors 51, 52 between which is arranged the rotator which may be arranged
at any position on the light path 13 between the reflecting mirrors 51,
52.
Alternatively, in the embodiments shown in FIGS. 1 to 3, the rotator 6 may
be arranged on the light path 13 in front of or in the rear of the
bilateral reflecting prism 5.
Each of the rectangular prisms 3, 3' and 4 shown in FIGS. 1 to 3 may also
be replaced by two mutually perpendicular reflecting mirrors.
In addition, the retroreflecting triangular prisms 3, 4 may be interchanged
such that the path difference between the light paths I and II is produced
by the relative change in position between the two prisms 3 and 4.
For example, the prisms 3, 4 shown in FIG. 1 may be rotated by 180.degree.
so as to locate the prism 3 at the left side and the prism 4 at the right
side of the interferometer.
In FIG. 5 is shown a detector system for photoelectrically detecting the
interference fringes obtained by the interferometer according to the
invention. The light beam 16 emerging from the interferometer is separated
into two light beams by means of a semitransparent mirror 30. The light
beam passing through the mirror 30 goes through a quarter-wavelength plate
31 whose polarizing axis is parallel with or perpendicular to the drawing.
Reference numerals 32, 33 designate rotators for rotating the polarizing
plane of the traversing light by 45.degree., respectively.
The light beams separated by the semitransparent mirror 30 being rotated by
the rotators 32, 33 in their polarizing planes by 45.degree., are
separated by means of polarizing prisms 34, 35 and arrive at light
detectors 36, 37 and 38, 39, respectively. The interference fringes
produced by the light detectors 36 and 37 are different in phase from each
other by 180.degree..
The outputs from these two light detectors 36, 37 are amplified by a
differential amplifier 40 to obtain a signal exclusive of a direct-current
component in response to the interference fringes. Similarly, the outputs
from the two light detectors 38, 39 are amplified by a differential
amplifier 41 to obtain a signal exclusive of a direct-current component in
response to the interference fringes. The output signal from the
differential amplifier 41 is displaced 90.degree. in phase by means of the
1/4 wavelength plate 31 from the output signal from the differential
amplifier 40.
These two signals are supplied to a reversible counter 42 which can count
the interference fringes produced due to an increase or decrease of the
path difference. A part surrounded by two dotted chain lines 43 shows a
polarizing element corresponding to the polarizing plate 7 shown in FIGS.
1 to 3.
In front of each of the light detectors 36, 37, 38, 39 may be arranged a
polarizing plate, and its polarizing plane may be rotated to adjust the
intensity of light incident on these light detectors. As a result, it is
possible to count the interference fringes without being influenced by a
change in light intensity from a light source.
In the embodiments shown in FIGS. 1 to 3, the light path I is located
outside the light path II with respect to the prisms 3, 3', 4 and these
two light paths I, II are equally spaced apart from each other, so that
the relative displacement and relative rotation between the interferometer
part 8 and the retroreflecting triangular prisms 3, 3', 4 have no
influence upon the path difference between the two light paths I, II,
thereby obtaining extremely stable interference fringes.
In FIG. 6 is shown another embodiment of the interferometer according to
the invention. In this embodiment, the incident light 1 which is a linear
polarized light having a polarizing plane inclined by 45.degree., or a
circular or elliptical polarized light, is incident on the beam splitter 9
composed of the triangular prism 91 and the parallelogram prism 92, the
latter being separated from the former by the cemented surface which is
provided with the polarizing film 93 as in the embodiment shown in FIG. 3.
In the present embodiment, substantially 100% of that component of the
incident light 1, which is the linearly polarized light oscillating in the
incident plane of the film 93, passes through the polarizing film 93,
while substantially 100% of that component of the incident light 11, which
is the linearly polarized light oscillating in a plane normal to the
incident plane of the film 93, is reflected by that film.
The light beam 11 passing through the polarizing film 93 is reflected by
the retroreflecting triangular prism 3 and incident again as the light
beam 12 on the beam splitter 9. The light beam 12 passing through the film
93 is reflected by the bilateral reflecting triangular prism 5 the edge
line of which is perpendicular to that of the retroreflecting triangular
prism 3. The light beam reflected by the bilateral reflecting triangular
prism 5 passes through the rotator 6 for rotating by 90.degree. the
polarizing plane of the light to obtain the linearly polarized light beam
the polarizing plane of which is perpendicular to the incident plane of
the polarizing film 93.
This light beam is reflected by the film 93 and then reflected by the total
reflecting plane 94 of the parallelogram prism 92 to obtain the light beam
14.
The light beam 14 is reflected by the retroreflecting triangular prism 3'
to obtain the light beam 15. This beam is reflected by the total
reflecting face 94 and the polarizing film 93 passed through the beam
splitter 9 and emerges as the light beam 16.
That part of the incident light 1 which is reflected by the film 93 and the
plane 94 and passing through the beam splitter 9 is reflected by a cat's
eye composed of a lens 17 and a reflecting mirror 18 located at the focal
point of the lens 17, and it travels as a light beam 22 in the reverse
direction. This light beam 22 after passing through a rotator 19 for
rotating by 90.degree. the polarizing plane of the traversing light
becomes the linearly polarized light vibrating on a single plane parallel
to the incident plane of the polarizing film 93. This light beam is
incident on the beam splitter 9 and passes through the film 93.
The light beam passed through the film 93 and the rotator 6 is reflected by
the bilateral reflecting triangular prism 5 and is incident on the beam
splitter 9. The polarized plane of the light beam 13 is rotated by
90.degree. with respect to the light beam 22 so that the light beam 13 is
reflected by the film 93 and the surface 94 and emerges as a light beam 24
from the beam splitter 19. This light beam 24 is reflected by the cat's
eye 17, 18 and after having passed through a rotator 20 to produce a
linear polarized light 25 vibrating on a single plane parallel with the
incident plane of the polarizing film 93. This light beam 25 is incident
on the beam splitter 9, passes through the polarizing film 93 and emerges
as the light beam 16 from the beam splitter 9.
The combined light beam 16 travels the same light path and passes through a
polarizing element 7 the polarizing plane of which is inclined by
45.degree. from the polarizing planes of the combined light beams 16, and
as a result, interference fringes can be observed. These interference
fringes are detected by photoelectric detectors as shown in FIG. 5, and
the amount of movement of the cat's eye 17, 18 can be measured by well
known methods.
As seen from the above, the relative displacement and rotation between the
interferometer part composed of beam splitter 9, bilateral reflecting
triangular prism 5 and rotator 6 on the one hand, and retroreflecting
triangular prisms 3, 3' and cat's eye 17, 18 on the other hand, have no
effect upon the path difference between the two optical paths whereby
stabilized interference fringes can be obtained.
In addition, the interferometer according to the invention is so
constructed that the distance between successive fringes corresponds to
.lambda./4 and the light beam is prevented from being transversely
displaced. As a result, distinctiveness of the interference fringes is
obtained independently of the alignment of the movable mirror and the like
constitutional elements of the interferometer.
In the present embodiment, use may be made of the beam splitting plate or
double refraction crystal plate shown in FIGS. 1 and 2 instead of the beam
splitter 9 having the polarizing film 93 to obtain the same effect as
described above.
In addition, the cat's eye 17, 18 used as the movable mirror may be
replaced by a corner cube prism which constitutes the same retroreflecting
mirror as the cat's eye. In this case, it is preferable to make one of the
edge lines of the reflecting surfaces of the corner cube prism horizontal
or vertical. In general, the light beam reflected by the corner cube prism
is an elliptical polarized light so that it is desirous to dispose a
desired polarizing element at the emergence side of the rotators 19, 20 or
to use a wavelength plate for changing the elliptical polarized light into
the linearly polarized light instead of the rotators 19, 20.
The rotator 6 is intended to rotate the linearly polarized light by
90.degree. so that it is not always necessary to locate the rotator 6 at
the position shown in FIG. 6. The rotator 6 may be disposed at any desired
position on the light path 13 between the beam splitter 9 and the
bilateral reflecting triangular prism 5. In addition, the rotators 19, 20
may be disposed in the light paths 21, 24 incident on the cat's eye 17,
18. The same may be applied to the above described wavelength plate. Even
when the rotators 19, 20 are disposed in the incident light paths 21, 24
of the cat's eye, the polarizing elements must be arranged at the
emergence side of the cat's eye 17, 18. Various modifications in the
arrangement of the rotator, the wavelength plate or the polarizing element
are possible by the knowledge of polarization analysis.
In case of using the beam splitter 9 having the polarizing film 93, if the
vibration plane of the incident light is not accurately coincident with
the inclination of the polarizing film 93, two light beams emerge from
that surface of the beam splitter 9 which is opposed to the total
reflecting surface 94 to produce surplus interference fringes having a bad
effect upon observation. In order to prevent the production of such
surplus interference fringes, the retroreflecting triangular prism 3, 3'
and the cat's eye 17, 18 are not arranged in phase, but the former is
relatively displaced with respect to the latter such that both light paths
passing through the retroreflecting triangular prisms 3, 3', and the light
paths passing through the cat's eye 17, 18 do not pass through the same
light path in the beam splitter 9. If the inclination of the vibrating
plane of the light is precisely aligned with the polarizing film, the
relative displacement between the retroreflecting triangular prisms 3, 3'
and the cat's eye 17, 18 has no effect upon the interference fringes being
observed.
As a light source, a laser is particularly beneficial for precise
determination of large lengths, but the light source itself is outside the
scope of the inventive. Use may be made of a two-wavelength laser as the
laser light source. A representative two-wavelength laser consists in that
a magnetic field is applied to an optical axis direction so as to produce
the Zeeman effect whereby the wavelengths between the mutually independent
circular polarized lights are different from each other.
The use of a 1/4 wavelength plate having an axial direction inclined from
the horizontal surface by 45.degree. makes it possible to convert two
circular polarized lights emitted from the two-wavelength laser into two
linearly polarized lights vibrating in horizontal and vertical directions,
respectively. If such light beams are incident on the interferometer
according to the invention, the light beams travelling the light paths I
and II become different in wavelength. If the light beam emerging from the
interferometer and passed through the polarizing element is detected by
the light detector, there is produced a beat whose frequency is equal to
the difference between the frequencies of the two light beams.
In addition, a part of the light incident on the interferometer is taken
out by means of a semitransparent mirror, etc. and this taken out light is
allowed to pass through the polarizing element and is received by the
light detector, thereby detecting a beat between the two polarized lights.
The number of beats obtained from these light detectors is counted by
means of a counter and then it is possible to obtain from the differences
between these two beats counts proportional to the amount of movement of a
carriage being measured.
As stated hereinbefore, the invention is capable of obtaining stabilized
interference fringes one fringe separation of which corresponds to
.lambda./4 and making the contrast of the interference fringes independent
of the alignment of the movable mirror and the like constitutional
elements of the interferometer, thereby reliably and simply effecting a
precise determination of lengths.
In FIG. 7 is shown an inventive interferometer embodiment which can measure
the pitch or the drunkenness error of screw threads. Referring to FIG. 7,
A designates a light beam which is directly incident on a beam splitter 61
having a polarizing film. As a result, the light beam A is separated into
a linearly polarized light a.sub.1 vibrating on the plane of the drawing
in FIG. 7 and a linearly polarized light b.sub.1 vibrating on a plane
normal to the plane of the drawing in FIG. 7. The linearly polarized light
a.sub.1 is reflected by a retroreflecting triangular prism 62 to produce a
reflected light a.sub.2. This light after passing again through the beam
splitter 61 is incident on a rotator 63 to produce a linearly polarized
light a.sub.3 whose polarizing plane is rotated 90.degree. with respect to
the light a.sub.2. This is incident on a bilateral reflecting triangular
prism 64. The above mentioned light path, seen from the front, is shown in
FIG. 8.
The linearly polarized light a.sub.3 incident on the bilateral reflecting
triangular prism 64 is reflected by it to produce a reflected light
a.sub.4 which is then incident on another beam splitter 65 having a
polarizing film. The polarizing plane of the linearly polarized light
a.sub.2 passed through the beam splitter 61 is rotated by 90.degree. by
means of the rotator 63 so that the light a.sub.4 is reflected by the
polarizing film of a beam splitter 65 which is similar in construction to
the beam splitter 61, and it is then reflected by a triangular prism 66 to
produce a reflected light a.sub.5 | | |
