Method of and apparatus for optically measuring displacement

The magnitude of displacement of an object to-be-measured is measured by projecting light beams on the object to-be-measured and a reference plane and utilizing the interference of lights reflected therefrom. The reflected lights from the object to-be-measured and the reference plane have their phase difference changed cyclically and forcibly. The discontinuous change of the magnitude of phase shift at the moment at which the phase difference of the two reflected lights caused by the displacement of the object to-be-measured has been compensated, and the magnitude of displacement of the object to-be-measured is found from the counted result of the discontinuous changes and the magnitude of phase shift at the moment at which the phase difference has been compensated.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method of and apparatus for optically 
measuring a displacement for use in the displacement measurement etc. of a 
machine which requires accurate positioning, for example, a semiconductor 
production plant or a coordinate measuring instrument. 
2. Description of the Prior Art 
A method of optically measuring the displacement of an object 
to-be-measured is, for example, an interferometric method disclosed in the 
specification of U.S. Pat. No. 4,298,283. This method is such that light 
is projected on a reference plane and a plane to-be-measured, that the 
phases of resulting reflected lights are cyclically and forcibly changed 
by a phase shifter, and that the magnitude of the displacement of the 
object to-be-measured is evaluated from the magnitude of phase shift at 
the moment at which the phase change generated by the displacement of the 
object has been compensated by the phase shifter. 
This method, however, cannot be used for a positioning displacement sensor 
because the measurement range thereof is as narrow as approximately a half 
of the wavelength of a light source employed. Moreover, even when the use 
is restricted to the measurements of minute displacements, the limited 
measurement range makes it necessary to adjust the phases of reflected 
lights with a Babinet-Soleil compensator before the measurement and to 
start the measurement from the middle of the measurement range. This 
initialization requires a long time. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a method of and apparatus 
for optically measuring a displacement which have a wide measurement range 
and which dispense with initialization. 
When an object to-be-measured moves beyond a measurement range in a plus 
direction by way of example, a magnitude of phase shift at the moment of 
the compensation of the phase difference between reflected lights, in 
other words, a measurement value, which has been initialized to a middle 
position within the measurement range at first, increases therefrom 
gradually; and upon reaching its maximum value, it changes discontinuously 
from the maximum value to the minimum value and returns to the middle 
position again; this cycle being repeated. Now, letting .lambda. denote 
the wavelength of a light source, one cycle of the above situation 
corresponds to a fixed magnitude of displacement (.lambda./2). Therefore, 
the measurement range can be widened by counting such discontinuous 
changes. 
The present invention is characterized in that the phase difference between 
lights reflected from an object to-be-measured and a reference plane is 
forced to fluctuate by means of a phase shifter, that the discontinuous 
change of a magnitude of phase shift is counted at the moment at which a 
phase difference developed by the displacement of the object 
to-be-measured is compensated by the phase shifter, and that the 
displacement magnitude of the object to-be-measured is obtained from the 
counted result and the magnitude of phase shift at the moment of the 
compensation of the phase difference.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Now, an embodiment of the present invention will be described with 
reference to FIGS. 1-13. In FIG. 1, numeral 1 designates a light source 
emitting coherent linear polarization, for example, a laser oscillator, 
and the oscillation plane of the linear polarization is in a direction 
defining an angle of 45 degrees to the sheet of the drawing. A polarizing 
beam splitter 2 is arranged on the optical path of the light source 1, and 
it reflects light B.sub.1 of a component having an oscillation plane 
perpendicular to the sheet of the drawing and transmits light B.sub.2 of a 
component having an oscillation plane parallel thereto. A quarter-wave 
plate 3 and an object to-be-measured 6 are arranged on the optical path of 
the beam B.sub.1 reflected by the polarizing beam splitter 2. On the other 
hand, a quarter-wave plate 4 and a reference plane 5 are arranged on the 
optical path of the beam B.sub.2 transmitted through the polarizing beam 
splitter 2. When the aforementioned linear polarization is reciprocated 
once through the quarter-wave plate 3 or 4, the oscillation plane thereof 
is rotated 90 degrees. A phase shifter 7, a polarizer 8 and a 
photodetector 9 are arranged on the optical path of the beams B.sub.1 and 
B.sub.2 having passed through the polarizing beam splitter 2. The phase 
shifter 7 is, for example, an electrooptic crystal having the Pockels 
effect. By applying a voltage to this electrooptic crystal, the refractive 
index thereof for the beam having the oscillation plane perpendicular to 
the sheet of the drawing and that for the beam having the oscillation 
plane parallel thereto are changed to control the phase difference between 
the two beams. The polarizer 8 transmits only an oscillating component 
defining an angle of 45 degrees to the sheet of the drawing. The 
photodetector 9 converts into an electric signal the intensity of the 
transmitted light of the polarizer 8 changing in accordance with the phase 
difference of the beams B.sub.1 and B.sub.2, and applies the electric 
signal to a sampling circuit 10. This sampling circuit 10 consists of a 
differentiation circuit 11, a comparator 12, edge detection means, e.g., a 
monostable multivibrator 13, a gate circuit 14, an AND circuit 15, and a 
sample-and-hold circuit 16. Here, the monostable multivibrator 13 is 
triggered by the falling edge of the output pulse of the comparator 12. 
The phase shifter 7, gate circuit 14 and sample-and-hold circuit 16 are 
supplied with the output voltage of an oscillator 18 through an A.C. 
amplifier 17. Thus, the phase shifter 7 forces the phase difference of the 
beams B.sub.1 and B.sub.2 to fluctuate, and the gate circuit 14 switches 
its output to a high level or a low level. A counter circuit 21 is 
connected to the output side of the sampling circuit 10. This counter 
circuit 21 is constructed of an up/down pulse generator circuit 49 and an 
up/down counter 30. The up/down pulse generator circuit 49 consists of 
comparators 22 and 23, edge detection means, e.g., monostable 
multivibrators 24, 25, 26 and 27, and AND circuits 28 and 29. Here, the 
monostable multivibrators 24 and 25 are respectively triggered by the 
rising edges of the output pulses of the comparators 22 and 23, and their 
outputs are input to the AND circuit 28. In addition, the monostable 
multivibrators 26 and 27 are respectively triggered by the falling edges 
of the output pulses of the comparators 23 and 22, and their outputs are 
input to the AND circuit 29. The output of the AND circuit 28 is connected 
to the UP terminal of the up/down counter 30, and that of the AND circuit 
29 to the DOWN terminal thereof. 
FIGS. 2A and 2B thru FIGS. 10A and 10B are diagrams for explaining the 
operation of the counter circuit 21, and a case of displacing the object 
to-be-measured 6 at a fixed speed is taken as an example. FIGS. 2A and 2B 
show the outputs of the sample-and-hold circuit 16, FIGS. 3A and 3B show 
the outputs of the comparator 22, FIGS. 4A and 4B show the outputs of the 
monostable multivibrator 24, FIGS. 5A and 5B show the outputs of the 
monostable multivibrator 27, FIGS. 6A and 6B show the outputs of the 
comparator 23, FIGS. 7A and 7B show the outputs of the monostable 
multivibrator 25, FIGS. 8A and 8B show the outputs of the monostable 
multivibrator 26, FIGS. 9A and 9B show the outputs of the AND circuit 28, 
and FIGS. 10A and 10B show the outputs of the AND circuit 29. FIGS. 2A, 
3A, . . . and 10A correspond to a case where the object to-be-measured 6 
is displaced in a plus direction (in a direction in which the magnitude of 
the displacement increases) from the position of an origin (the position 
at which the object to-be-measured 6 starts the displacement), while FIGS. 
2B, 3B, . . . and 10B correspond to a case where the object to-be-measured 
6 is displaced in a minus direction (a direction in which the displacement 
magnitude decreases). Besides, the reference voltage V.sub.1 of the 
comparator 22 is set between 0 (zero) and V.sub..pi., while the reference 
voltage V.sub.2 of the comparator 23 is set between -V.sub..pi. and 0, 
and the pulse widths of the output pulses of the monostable multivibrators 
24, 25, 26 and 27 are made sufficiently small beforehand (refer to FIGS. 
3A and 3B thru FIGS. 8A and 8B). 
In the case of displacing the object to-be-measured 6 in the plus direction 
under these conditions, when the output of the sample-and-hold circuit 16 
(refer to FIG. 2A) changes discontinuously from -V.sub..pi. to 
V.sub..pi., the pulses generated by the monostable multivibrators 24 and 
25 (refer to FIGS. 4A and 7A) overlap in time, and hence, the AND circuit 
28 outputs the up pulse (refer to FIG. 9A). In the case of displacing the 
object to-be-measured 6 in the minus direction, when the output of the 
sample-and-hold circuit 16 (refer to FIG. 2B) changes discontinuously from 
V.sub..pi. to -V.sub..pi., the pulses generated by the monostable 
multivibrators 26 and 27 (refer to FIGS. 8B and 5B) overlap, and the AND 
circuit 29 outputs the down pulse (refer to FIG. 10B). 
Although the above description concerns the case where the displacement 
speed is constant, the same applies to a case where the displacement speed 
changes. 
Next, the operation of the embodiment of the present invention will be 
described. In the linear polarization emitted from the light source 1, the 
light of the component having the oscillation plane perpendicular to the 
sheet of the drawing is denoted by B.sub.1, and the light of the component 
having the oscillation plane parallel thereto is denoted by B.sub.2. Then, 
the beam B.sub.1 is reflected by the polarizing beam splitter 2, 
transmitted through the quarter-wave plate 3 and reflected by the object 
to-be-measured 6, and it arrives at the polarizing beam splitter 2 via the 
quarter-wave plate 3 again. Since the beam B.sub.1 has reciprocated 
through the quarter-wave plate 3, the oscillation plane thereof at this 
time is parallel to the sheet of the drawing owing to the rotation of 90 
degrees, and this beam is transmitted through the polarizing beam splitter 
2. On the other hand, the beam B.sub.2 is transmitted through the 
polarizing beam splitter 2 and enters this polarizing beam splitter 2 via 
the path of the quarter-wave plate.fwdarw.the reference plane 5.fwdarw.the 
quarter-wave plate 4. Since the oscillation plane of the beam B.sub.2 is 
perpendicular to the sheet of the drawing owing to the rotation of 90 
degrees, this beam is reflected by the polarizing beam splitter 2. The 
beams B.sub.1 and B.sub.2 emergent from the polarizing beam splitter 2 
reach the polarizer 8 via the phase shifter 7. Here, in the beams B.sub.1 
and B.sub.2, only the light components having the oscillation planes of 
45.degree. to the sheet of the drawing pass through the polarizer 8, and 
they give rise to interference, so that the intensity of the transmitted 
light through the polarizer 8 changes in accordance with the phase 
difference between the beams B.sub.1 and B.sub.2. This intensity is 
converted into the electric signal by the photodetector 9, and the 
electric signal is input to the sampling circuit 10. Meanwhile, the output 
voltage of the oscillator 18 is amplified by the A.C. amplifier 17 and 
then applied to the phase shifter 7, whereby the phase difference of the 
beams B.sub.1 and B.sub.2 is forced to fluctuate by .phi.f in the 
following equation (1): 
EQU .phi.f=A sin wt (1) 
Here, an amplitude A is set somewhat larger than .pi.. In addition, a phase 
difference .phi. which is based on the difference between an optical path 
length l from the object to-be-measured 6 to the center of the polarizing 
beam splitter 2 and an optical path length l.sub.o from the reference 
plane 5 to the center of the polarizing beam splitter 2 is given by Eq. 
(2): 
##EQU1## 
Accordingly, the phase difference .phi..sub.t of the beams B.sub.1 and 
B.sub.2 becomes as indicated by Eq. (3): 
##EQU2## 
Letting I.sub.1 and I.sub.2 denote the intensities of the respective beams 
B.sub.1 and B.sub.2 exhibited when they have passed through the polarizer 
8, the intensity I of the interference light is expressed by Eq. (4): 
##EQU3## 
This intensity of the interference light is converted by the photodetector 
9 into the electric signal, which is passed through the differentiation 
circuit 11 and the comparator 12, whereupon the monostable multivibrator 
13 generates a pulse at a time at which the interference signal indicated 
by Eq. (4) becomes the maximum. The phase difference .phi..sub.t of the 
beams B.sub.1 and B.sub.2 at this time is given by Eq. (5): 
EQU .phi..sub.t =2n.pi.(n=0,.+-.1,.+-.2, . . . ) (5) 
While .phi.f indicated by Eq. (1) is caused to fluctuate in the range of 
-A.ltoreq..phi.f.ltoreq.A, only the pulse generated for 
-.pi..ltoreq..phi.f.ltoreq..pi. is selected by the gate circuit 14 as well 
as the AND circuit 15, and the input voltage of the phase shifter 7 at 
that time t is sampled by the sample-and-hold circuit 16 so as to find the 
magnitude of phase shift .phi.f at that time. 
With this measure, valid pulses are generated when .phi.f assumes a value 
satisfying Eq. (6) indicated below, save when the phase difference .phi. 
expressed by Eq. (2) and arising in correspondence with the displacement 
of the object to-be-measured 6 becomes (2 m+1).pi. (m: integer): 
EQU .phi.'+.phi.f=0 (6) 
Here, .phi.' is defined by Eq. (7): 
##EQU4## 
where m denotes an integer. 
In addition, when .phi. becomes (2 m+1).pi., pulses generated at times 
satisfying Eq. (8) becomes valid: 
EQU .phi.f=.+-..pi. (8) 
Accordingly, FIG. 11 is obtained in which the phase difference .phi. is 
taken on the axis of abscissas and the magnitude of phase shift .phi.f and 
the applied voltage V.sub.s of the phase shifter 7 at the generation of 
the valid pulse are taken on the axes of ordinates. 
.phi..sub.0 is let denote the phase difference of the beams B.sub.1 and 
B.sub.2 respectively reflected by the object to-be-measured 6 and the 
reference plane 5 when the object to-be-measured 6 lies at the position of 
the origin at first, and .phi..sub.0 ' is let denote a remainder obtained 
by subtracting integral times of 2.pi. from .phi..sub.0. Besides, V.sub.s0 
is let denote the output voltage of the sample-and-hold circuit 16 
produced when the object to-be-measured 6 lies at the position of the 
origin. In terms of the magnitude of phase shift, V.sub.s0 is multiplied 
by .pi./V.sub..pi. to become -.phi..sub.0 '. It is assumed that the 
object to-be-measured 6 be displaced in the plus direction from the 
position of the origin, to increase the phase difference .phi. from 
.phi..sub.0 to .phi..sub.1. V.sub.s decreases with increase in .phi., but 
when .phi. has exceeded (2 n-1).pi., V.sub.s changes from -V.sub..pi. to 
V.sub. .pi. discontinuously at that time. Here, V.sub..pi. indicates a 
voltage required for giving rise to a phase difference .pi. between the 
reflected beams B.sub.1 and B.sub.2. Upon a further displacement, V.sub.s 
decreases gradually, and it changes from -V.sub..pi. to V.sub..pi. again 
when .phi. has exceeded (2 n+1).pi.. V.sub.s1 is let denote the output 
voltage of the sample-and-hold circuit 16 produced when .phi. has 
increased to .phi..sub.1. In terms of the magnitude of phase shift, 
V.sub.s1 is multiplied by .pi./V.sub..pi. to become -.phi..sub.1 '. Here, 
.phi..sub.1 ' indicates a remainder obtained by subtracting integral times 
of 2.pi. from .phi..sub.1. Accordingly, the magnitude of displacement at 
this time can be found in a way described below. 
The discontinuous change of the output voltage of the sample-and-hold 
circuit 16 from -V.sub..pi. to V.sub..pi. is counted in + by the counter 
circuit 21. The counted result is multiplied by .lambda./2, and the 
product has 
##EQU5## 
added thereto. 
In a case where the object to-be-measured 6 is displaced in the minus 
direction, the output voltage of the sample-and-hold circuit 16 changes 
from V.sub..pi. to -V.sub.90 discontinuously when the phase difference 
.phi. exceeds odd-number times of .pi., i.e., (2 n+1).pi., (2 n+3).pi., . 
. . etc. in the decreasing process thereof. Therefore, the discontinuous 
changes are counted in minus by the counter circuit 21, and the others are 
as in the above case of finding the displacement in the plus direction, 
whereby the displacement can be found. 
FIG. 12 shows an example of an output circuit for executing the calculation 
processing described above. An A/D converter 31 converts the output 
voltage of the sample-and-hold circuit 16 into a digital value expressive 
of the fraction .phi.' of the phase difference, and applies the digital 
value to a subtraction circuit 33 to be described below. An origin 
position memory circuit 32 stores the fraction .phi..sub.0 ' of the phase 
difference exhibited when the object to-be-measured 6 lies at the position 
of the origin, and applies it to the subtraction circuit 33. This 
subtraction circuit 33 subtracts the content of the origin position memory 
circuit 32 from the phase difference .phi..sub.1 ' at the final position. 
An adder 34 adds the content of the subtraction circuit 33 as multiplied 
by .lambda./4.pi. and the content of the counter circuit 30 as multiplied 
by .lambda./2. When a reset signal is applied to a terminal A, the content 
of the counter 30 is cleared, and simultaneously, the fraction .phi..sub.0 
' of the phase difference at the position of the origin is stored in the 
origin position memory circuit 32. 
FIG. 13 shows an example of the gate circuit 14 shown in FIG. 1. The gate 
circuit 14 consists of comparators 37 and 38 and an OR circuit 39. The 
reference voltage of the comparator 37 is set at V.sub..pi., and that of 
the comparator 38 at -V.sub..pi.. The outputs of the comparators 27 and 38 
are input to the OR circuit 39. According to this circuit, the output 
becomes `high` only when the input voltage lies in the range of V.sub..pi. 
--V.sub..pi.. When the AND between this high output and the output of the 
monostable multivibrator 13 is taken by the AND circuit 15, only pulses 
generated at this time can be selected. 
FIG. 14 is a block diagram showing another embodiment of the present 
invention, with a counter circuit omitted therefrom. In FIG. 14, 
components assigned the same numerals as in FIG. 1 are identical portions. 
A polarizing beam splitter 40 is arranged on the optical path of the beams 
having passed through the phase shifter 7. Polarizers 41 and 42 are 
respectively arranged on the reflected beam path and transmitted beam path 
of the polarizing beam splitter 40. The transmitting axis of the polarizer 
41 defines an angle of 45.degree. to the sheet of the drawing, while that 
of the polarizer 42 defines an angle of -45.degree.. Numerals 43 and 44 
indicate photodetectors, and numeral 45 indicates a differential 
amplifier. A monostable multivibrator 13' is triggered by both the rising 
edge and falling edge of the output pulse of the comparator 12. A gate 
circuit 14' provides a `high` output when the applied voltage of the phase 
shifter 7 lies in a range of -V(.pi./2) to V(.pi./2). Here, V(.pi./2) 
denotes a voltage which is required for causing the phase shifter 7 to 
generate a phase difference of .pi./2. 
Next, the operation of this embodiment will be described. The beam B.sub.1 
(having the oscillation plane parallel to the sheet of the drawing) 
reflected from the object to-be-measured 6 and the beam B.sub.2 (having 
the oscillation plane perpendicular to the sheet of the drawing) reflected 
from the reference plane 5 via the same paths as in the embodiment of FIG. 
1 reach the polarizing beam splitter 40 via the phase shifter 7, and are 
split into reflected beams and transmitted beams here. The reflected beams 
are denoted by B.sub.1 ' and B.sub.2 ', and the transmitted beams by 
B.sub.1 " and B.sub.2 ". The reflected beams B.sub.1 ' and B.sub.2 ' are 
transmitted through the polarizer 41 to give rise to interference. The 
intensity of this interference light is indicated by I.sub.1. In addition, 
the transmitted beams B.sub.1 " and B.sub.2 " are transmitted through the 
polarizer 42 to give rise to interference. 
The intensity of this interference light is indicated by I.sub.2. Here, the 
transmitting axes of the polarizers 41 and 42 have different directions. 
Therefore, though the mean values of the intensities I.sub.1 and I.sub.2 
are equal, the phases of intensity change verses the phase difference of 
the reflected beams shift by 180.degree.. FIG. 15 and FIG. 16 show this 
situation, and the phase difference .phi. is taken on the axes of 
abscissas, while the intensities I.sub.1 and I.sub.2 of the lights are 
respectively taken on the axes of ordinates. Accordingly, when the 
interference light intensities are converted into electric signals by the 
photodetectors 43 and 44 and the difference of the signals is taken by the 
differential amplifier 45, the output e.sub.s of the amplifier changes as 
shown in FIG. 17. Accordingly, when .phi..sub.t of Eq. (3) mentioned 
before meets Eq. (9), the output of the differential amplifier 45 becomes 
null. 
EQU .phi..sub.t =.phi.+.phi.f=n.pi.+(.pi./2) (9) 
Here, n is an integer. 
That is, the following equation (10) holds on the basis of Eq. (3) and Eq. 
(9): 
##EQU6## 
Here, the amplitude A of the phase .phi.f which is caused to fluctuate by 
the phase shifter 7 is set to at least .pi./2. On this occasion, .phi.f 
fluctuates within a range of -A.ltoreq..phi.f.ltoreq.A. When Eq. (9) is 
met, the output of the differential amplifier 45 crosses zero, so that the 
output of the comparator 12 changes from `high` to `low` or from `low` to 
`high` to trigger the monostable multivibrator 13' and to generate a 
pulse. Only such pulses generated when the phase shift magnitude .phi.f of 
the phase shifter 7 lies in a range of -.pi./2 to .pi./2 are selected by 
the gate circuit 14' as well as the AND circuit 15, applied voltages to 
the phase shifter 17 at those times are sampled by the sample-and-hold 
circuit 16, and the magnitudes of phase shifts .phi..sub.s in those cases 
are found. Herein, the relationship between the phase difference .phi. of 
the reflected beams B.sub.1 and B.sub.2 and the output voltages of the 
sample-and-hold circuit 16, namely, the applied voltages V.sub.s to the 
phase shifter 7 become as shown in FIG. 18. Accordingly, the magnitude of 
displacement can be found in a way as stated below. 
Each time the object to-be-measured 6 is displaced .lambda./4, the 
generated discontinuous change of the output voltage of the 
sample-and-hold circuit 16 is counted by the counter circuit (not shown). 
The counted result is multiplied by .lambda./4, and the product has 
##EQU7## 
added thereto. Here, V.sub.s0 and V.sub.s1 denote the output voltages of 
the sample-and-hold circuit 16 respectively provided when the object 
to-be-measured 6 lies at the origin position and the final position. 
FIG. 19 is a block diagram showing still another embodiment of the present 
invention, in which the phases of reflected beams are caused to fluctuate 
by the use of an actuator such as electrostricitve element. Also in FIG. 
19, a counter circuit is omitted from illustration. In FIG. 19, the same 
numerals as in FIG. 1 denote identical portions. A polarizing beam 
splitter 46 is arranged on the optical path of the polarization from the 
light source 1, and a mirror 47 is arranged on the optical path of 
transmitted light through the polarizing beam splitter 46. The front 
surface of the mirror 47 serves as a reference plane. The mirror 47 is 
attached to an electrostrictive element 48. 
The light emitted from the laser oscillator 1 reaches the polarizing beam 
splitter 46, to be split into the reflected beam B.sub.1 and the 
transmitted beam B.sub.2. The reflected beam B.sub.1 is reflected from the 
object to-be-measured 6, and reaches the photodetector 9 via the 
polarizing beam splitter 46. The transmitted beam B.sub.2 is reflected 
from the front surface of the mirror 47 and reaches the polarizing beam 
splitter 46, in which it is reflected to cause interference with the 
reflected beam B.sub.1. The intensity change of the interference light is 
converted into an electric signal by the photodetector 9. Meanwhile, the 
electrostrictive element 48 has the output voltage of the A.C. amplifier 
17 applied thereto so as to be vibrated with a constant amplitude at all 
times. Thus, the phase shifter for causing the phases of the beams B.sub.1 
and B.sub.2 to fluctuate can be constructed by vibrating the mirror 47 
mounted on the electrostrictive element 48 in a direction perpendicular to 
the optic axis thereof. The operations of the other portions are the same 
as in the first embodiment. 
Also FIG. 20 shows still another embodiment of the present invention, which 
consists in a method of measuring displacements in a plurality of 
directions by the use of a single phase shifter of the electrooptic 
crystal type. FIG. 20 exemplifies a case of measuring displacements in two 
directions X and Y. In FIG. 20, the same numerals as in FIG. 1 denote 
identical portions. Besides, dashed numerals denote the same portions as 
indicated by the same numerals without dashes. 
The light source 1 for emitting coherent linear polarization gives forth 
the linear polarization which has a polarization plane in a direction of 
45.degree. with respect to the sheet of the drawing, and which enters the 
electrooptic crystal 7. The electrooptic crystal 7 has a voltage applied 
thereto forcibly and cyclically beforehand, to give phase changes to 
respective lights having polarization planes vertical and horizontal to 
the sheet of the drawing. Light emergent from the electrooptic crystal 7 
is split into two, a transmitted beam and a reflected beam by a polarizing 
beam splitter 101. Using the transmitted beam, the displacement of the 
object to-be-measured 6 in the X direction is measured. In addition, the 
reflected beam is changed by mirrors 102 and 103 into a beam rectilinearly 
propagating in the Y direction, with which the displacement of the object 
to-be-measured 6' in the Y direction is measured. The way of the 
measurement is the same as in the method illustrated in FIG. 1. 
According to the present measurement method, the displacements in the 
plurality of directions can be measured with the single phase shifter. 
Further, when the laser oscillator is employed as the light source 1, the 
laser beam spreads comparatively little owing to an excellent directivity. 
Nevertheless, in a case where a distance from the light source 1 to the 
phase shifter such as electrooptic crystal 7 has become long in the method 
illustrated in FIG. 1, the diameter of the beam reaching the phase shifter 
7 enlarges, and the entrance plane of the phase shifter 7 needs to be 
large in area. As a result, the magnitude of phasic change relative to the 
applied voltage decreases to the disadvantage. According to the present 
measuring method, this problem can also be solved. 
FIGS. 21 thru 23 show circuit arrangement diagrams of further edge 
detection means. FIG. 21 shows the edge detection means to detect a rising 
edge, which consists of a delay element 107, an inverter 110, and an AND 
circuit 105 for taking the AND between an input signal and a signal 
produced via the delay element 107 and the inverter 110. In addition, FIG. 
22 shows the edge detection means to detect a falling edge, which consists 
of a delay element 108, an inverter 111, and an AND circuit for taking the 
AND between a signal from the delay element 108 and a signal from the 
inverter 111. Besides, FIG. 23 shows the edge detection means to detect 
rising and falling edges, which consists of a delay element 109, and an 
exclusive OR circuit 112 for taking the exclusive OR between an input 
signal and a signal from the delay element 109. 
Even when such edge detection means are employed, operations and effects 
similar to those of the monostable multivibrators as the edge detection 
means in the respective embodiments can be attained. 
As described above, according to the present invention, displacements in a 
wide range can be measured irrespective of the wavelength of a light 
source used.