Position detecting apparatus of optical interferometry

A position detecting apparatus utilizing optical interferometry changes the wavelength of a light beam from the light source. The variation in the wavelength causes an increment and decrement Cx, Co in the number of waves in the measurement and reference lengths Lx, Lo. The position data calculating section calculates the measurement length Lx on the basis of the detected increment and decrement Cx, Co and the reference length Lo according to an equation, Lx=Lo(Cx/Co). The position detecting apparatus can easily detect an absolute position of an object to be detected.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a position detecting apparatus used for 
detecting a displacement position or an angle of the rotation axis of a 
working stage in a precision industrial machine or a machining tool, and 
more particularly, to a position detecting apparatus utilizing optical 
interferometry, in which a reference light from a reference plane and a 
measurement light from a measurement plane are brought together so as to 
form interference light, so that a difference between the optical path 
lengths of the reference light and the measurement light is measured on 
the basis of the optical intensity of the interference light, whereby 
position data of an object is obtained based on the difference. 
2. Description of the Prior Art 
FIG. 13 illustrates a laser length measuring instrument, which is an 
example of a position detecting apparatus of optical interferometry 
employing an optical heterodyne method. The laser length measuring 
instrument forms interference light ML by bringing together a laser beam 
reflected from a movable mirror 10 on an object and a laser beam reflected 
from a stationary mirror 11. The variation in the optical intensity of the 
interference light ML is detected in response to the movement of the 
movable mirror 10, to thereby obtain the displacement X of the object. 
Laser source 12 as a light source or emits a light beam towards a 
separation plane P1. The laser source 12 comprises a He--Ne laser which 
outputs two laser beams (light beams OL) on the respective planes of 
polarization which are orthogonal to each other. The laser beams have 
different frequencies f1 and f2, respectively. A polarization beam 
splitter 13 receives laser beams so as to split the beams into measurement 
light L1 of frequency f1 and reference light L2 of frequency f2 at the 
separation plane P1. 
The measurement light L1 is irradiated toward a measurement plane which is 
defined on the movable mirror 10 fixedly mounted on an object. The 
measurement light L1 reflected from the measurement plane is subjected to 
Doppler modulation of frequency .DELTA.f in proportion to the velocity of 
the movable mirror 10 in the direction X, and then returns to the 
polarization beam splitter 13. The reference light L2 is reflected at a 
reference plane on the stationary mirror 11, and then returns to the 
polarization beam splitter 13. The polarization beam splitter 13 brings 
together the measurement light L1 and the reference light L2 so as to form 
interference light ML for measurement. The movement of the object causes 
an increase or a decrease in the optical length on which the measurement 
light L1 travels from the separation plane P1 to a point P2 on the 
polarization beam splitter 13. The increase or decrease in the length 
serves to shift the phase of the measurement light L1 with respect to that 
of the reference light L2 at the point P2. The shift in the phase causes 
the optical intensity of the measurement interference light ML to be 
varied. 
A beam receiver 14 electrically detects the optical intensity in the 
interference of the measurement interference light ML. In other words, the 
beam receiver 14 photoelectrically converts the measurement interference 
light ML into a measurement electrical signal Fp (beat signal) which has a 
differential frequency obtained from f1.+-..DELTA.f and f2. 
The light beam OL is split at a beam splitter 15, so that the split light 
beam is supplied to a photoelectric device 16, which photoelectrically 
converts the above mentioned two laser beams with different frequencies 
into a reference electrical signal Fr (beat signal) of a differential 
frequency obtained from f1 and f2. 
The phase shift between the measurement and reference electrical signals Fp 
and Fr indicates the difference in the optical length between the 
measurement and reference lights L1 and L2. The phase shift of the 
measurement electrical signal Fp with respect to the reference electrical 
signal Fr represents the relative displacement X of the object or the 
movable mirror 10 since the phase of the reference electrical signal Fr is 
fixed because of the fixed optical length of the reference light L2, which 
is represented by the reference electrical signal Fr. This principle is 
the basis for the calculation of the phase shift between the measurement 
and reference electrical signals Fp and Fr by the calculation circuit 17 
while the object is moving, so that the relative displacement X is 
measured as position data of the object on the basis of the phase shift. 
Assuming that the wavelength of a laser beam is .lambda., when the object 
is displaced by an amount X, the phase shift between the signals Fp and Fr 
is designated as 4.pi.(X/.lambda.). The phase of the measurement 
electrical signal Fp accordingly coincides with that of the reference 
electrical signal Fr for every cycle of the phase, that is, every time the 
phase shift is changed by .lambda./2. The calculation circuit 17 thus 
includes a measurement device for determining a measurement value .DELTA.x 
within a range between 0 and .lambda./2, and a counter for counting the 
number Xu of cycles of the phase shift based on the determined measurement 
value .DELTA.x. The position data X is output based on the equation 
X=(.lambda./2)Xu+.DELTA.x. 
The calculation circuit 17 measures a phase shift between the signals Fp 
and Fr in accordance with a predetermined sampling time. The sampling time 
is set such that the amount of variation between the last measurement 
value .DELTA.x(last) and the current measurement value .DELTA.x(curr) 
remains within a range of .+-..lambda./4. This setting of sampling time 
enables a simple and reliable counting of the number Xu of cycles by 
comparison between the measurement values .DELTA.x(last) and 
.DELTA.x(curr). 
When the inequality .DELTA.x(curr)-.DELTA.x(last).ltoreq.-.lambda./4 is 
established, that is, the current measurement value is smaller than the 
last measurement value by the amount equal to or more than .lambda./4, the 
measurement device determines that an additional cycle is achieved for the 
number Xu of completed cycles, so that an up pulse is output from the 
measurement device so as to increase the value of the counter by one. For 
instance, assume that the last measurement value or 
.DELTA.x(curr)=.lambda./4 and the current measurement value or 
.DELTA.x(last)=(3.lambda.)/4. Since 
.DELTA.x(curr)-.DELTA.x(last)=-.lambda./2 is established, the value of the 
counter is increased by one. The measurement value can possibly become 
.lambda./4 in two ways. One is in a case where the measurement value is 
increased from (3.lambda.)/4 after the transition to the next cycle. The 
other is in a case where the measurement value is simply decreased from 
(3.lambda.)/4. However, since the sampling time has been set such that the 
amount of variation between values of two successive measurements would 
stay within the range of .+-..lambda./4, the variation to the 
.DELTA.x(curr) can be determined as the result of an increase after the 
transition to the subsequent cycle. Likewise, when the inequality 
.DELTA.x(curr)-.DELTA.x(last).gtoreq..lambda./4 is established, in other 
words, the current measurement value is larger than the last measurement 
value by the amount equal to or more than .lambda./4, the measurement 
device determines that the number Xu of cycles should be decreased by one, 
and thus outputs a down pulse so as to decrease the value of the counter 
by one. 
A part of the light beam OL split at the beam splitter 15 is also supplied 
to a photoelectric device 18.. The electrical signal is obtained from the 
photoelectric device 18 and is then supplied to a laser tuning circuit 19. 
The laser tuning circuit 19 is designed to stabilize the laser source 12. 
The above described position detecting apparatus utilizing optical 
interferometry, however, requires movement of an object because the 
apparatus measures the variation in the optical intensity of interference 
light, which variation is caused by an increase or a decrease of the 
optical length of the measurement light. The apparatus is accordingly 
required to set a reference point at the beginning of the position 
detection, so that the relative displacement of the object from the 
reference point is incrementally measured. This principle causes the 
following disadvantage: (1) the erroneous counting of the number Xu of 
cycles will be accumulated in the measurement; (2) the interruption of the 
optical path of measurement light will lead to the loss of the current 
position, so that the object is required to return to the home position so 
as to again establish the reference point; and (3) the switching-off of 
the apparatus will cause the loss of the current position, so that the 
reference point must be established every time the apparatus is switched 
on. 
In addition, the above described apparatus requires a large and expensive 
light source, such as He--Ne laser sources for two frequencies. Some may 
propose the utilization of a small semiconductor laser instead, which 
leads to another disadvantage. The wavelength of a semiconductor laser 
cannot be stabilized sufficiently and hence, the variation and the error 
in the wavelength undesirably affect the measurement accuracy, in 
particular, when employed in a Michelson interferometer. 
Further, the ambient environments of measurement and the movement of the 
object may incur fluctuation in the air around the optical path, which 
will lead to unreliable measurement data. 
SUMMARY OF THE INVENTION 
The present invention therefore aims to provide a position detecting 
apparatus utilizing optical interferometry capable of easily detecting an 
absolute position of an object to be detected. 
The present invention further aims to provide a small and inexpensive 
position detecting apparatus utilizing optical interferometry wherein the 
variation of the wavelength of a semiconductor laser does not affect the 
measurement accuracy when a semiconductor laser is employed as a light 
source. 
The present invention still further aims to provide a position detecting 
apparatus utilizing optical interferometry resistive to the changes of the 
ambient environment of the measurement. 
According to the first aspect of the present invention, there is provided a 
position detecting apparatus of optical interferometry, comprising: a 
light source capable of emitting a light beam having an interference 
capability; a wavelength control section capable of varying a wavelength 
of the light beam emitted from the light source; a relative displacement 
interference section capable of bringing reference and measurement light 
together so as to output interference light for relative displacement, the 
reference light having been separated from the light beam at a separation 
plane and reflected from a reference plane, the measurement light having 
been separated from the light beam and reflected from a measurement plane 
defined on an object; a reference length interference section capable of 
bringing rays of light together so as to output interference light for 
reference length on the basis of a reference length light beam separated 
from the light beam, the rays of light having a difference in optical 
length corresponding to a predetermined reference length; and a position 
data determining section capable of determining position data of the 
object on the basis of the predetermined reference length and variation in 
optical intensities of the interference lights for relative displacement 
and reference length when the wavelength control section varies the 
wavelength of the light beam. 
With the above arrangement, position data of an object can be detected on 
the basis of a predetermined reference length and variations in the 
optical intensities for the interference light for relative displacement 
and reference length when the wavelength control section varies the 
wavelength of the light beam, so that an absolute position of the object 
can easily and reliably be detected. The current position can be detected 
as an absolute position irrespective of any erroneous counting to be 
accumulated, any interruption of the optical length, or the turning off of 
the apparatus. It is accordingly possible to omit returning of the object 
to the home position, thereby leading to a reduction in the sequences and 
a reliable detection. 
In addition, the accuracy in varying the wavelength can be rough enough to 
measuring the increment and decrement in the number of waves, so that an 
accurate detection can be achieved even when using as a light source a 
semiconductor laser which has an unstable oscillation frequency. 
Therefore, it is possible to provide a compact and inexpensive position 
detecting apparatus with a highly accurate detection. 
Furthermore, variations in the ambient environment, such as air 
fluctuation, do not hinder a stable measurement, so that highly accurate 
measurement can be achieved under any ambient environment. 
According to the second aspect of the invention, the reference length 
interference section of the first aspect may comprise a reference plane 
capable of reflecting the rays separated from the reference length light 
beam at a separation plane; and a measurement plane capable of reflecting 
the rays separated from the reference length light beam from the 
separation plane; the interference light being formed by bringing together 
the rays from the reference and measurement planes, the rays from the 
measurement plane having a difference in optical length corresponding to 
the predetermined reference length with respect to the rays from the 
reference plane. The reference length interference section can be realized 
with a simple structure. 
According to the third aspect of the present invention, in addition to the 
second aspect, the position data determining section may comprise a first 
counting circuit capable of counting increment and decrement Co of waves 
in the interference light for reference length when the wavelength control 
section varies the wavelength of the light beam; a second counting circuit 
capable of counting increment and decrement Cx of waves in the 
interference light for relative displacement when the wavelength control 
section varies the wavelength of the light beam; and an absolute position 
data calculation circuit capable of calculating a position Lx of the 
object with respect to the separation plane in the relative displacement 
interference section on the basis of an equation, Lx=Lo(Cx/Co), wherein Lo 
is the reference length defined between the separation and measurement 
planes in the reference length interference section. The position data 
calculating section can be realized with a simple structure. 
According to the fourth aspect of the present invention, in addition to the 
first aspect, the position data determining section may comprise an 
absolute position data calculation section capable of calculating absolute 
position data of the object on the basis of the reference length and the 
variation in optical intensities for the interference light for relative 
displacement and reference length when the wavelength control section 
varies the wavelength of the light beam; a relative position data 
calculation section capable of calculating relative position data of the 
object with respect to an absolute position of the object on the basis of 
the variation in optical intensity of the interference light for relative 
displacement in response to movement of the object; and a composite 
calculation section for combining the absolute and relative position data 
so as to determine a position of the object. 
With the above arrangement, the detection of position data by varying the 
wavelength, which requires a longer response time, can be carried out in 
an initial sequence after turning on the switch, so that only relative 
position data needs to be obtained thereafter in the subsequent sequences. 
The response for position detection can be improved in the subsequent 
sequences after the initial sequence by omitting the time-consuming 
detection by varying the wavelength. Further, unlike sole incremental 
detection, an absolute position of the object can easily be detected by 
the absolute position data calculation section irrespective of any 
erroneous counting during measurement or interruption in the optical path, 
which would normally cause the loss of the current position, or the 
turning off of the switch during measurement. It is not required to return 
the object to the home position. 
According to the fifth aspect of the present invention, the position data 
determining section further comprises a wavelength variation detecting 
section for detecting variation in the wavelength of the light beam from 
the light source on the basis of the interference light for relative 
displacement from the relative displacement interference section; and a 
wavelength correcting section for correcting the relative position data on 
the basis of the variation detected by the wave length variation detecting 
section. 
With the above arrangement, when the wavelength of the light beam from the 
light source varies as time elapses, so as to exhibit a tendency in which 
the value of the relative position data follows non-linearity compared to 
the beginning of the detection, such a tendency can be balanced by 
correcting the variation of the wavelength. It is possible to detect a 
highly accurate absolute position irrespective of the variation in the 
wavelength. 
According to the sixth aspect of the present invention, the light source 
comprises a semiconductor laser, and the wavelength control section varies 
the wavelength of the light beam by changing the temperature of the 
semiconductor laser. According to the seventh aspect of the present 
invention, the wavelength control section varies the wavelength of the 
light beam by causing stress in a medium, through which the light beam is 
transmitted, so as change the refractive index of the medium. According to 
the eighth aspect of the present invention, the wavelength control section 
varies the wavelength of the light beam by applying an electrical and/or a 
magnetic field to a medium, through which the light beam is transmitted, 
so as to change the refractive index of the medium. According to the ninth 
aspect of the invention, the wavelength control section varies the 
wavelength of the light beam using a Doppler effect caused by rotation of 
a rotatable plate of a predetermined refractive index. With any of these 
features, the wavelength of the light beam from the light source can be 
varied with a simple structure.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 illustrates a schematic representation of a position detecting 
apparatus 20 utilizing optical interferometry according to a first 
embodiment of the present invention. The position detecting apparatus 20 
comprises a light source 21 for emitting a light beam OL. The wavelength 
of the light beam OL can optionally be varied by a wavelength control 
section 22. A relative displacement interference section 23 brings 
together reference light and measurement light out of the light beam OL so 
as to output interference light L3 for relative displacement. The 
reference and measurement light have a difference in optical length 
corresponding to a distance to an object. A reference length interference 
section 24 brings together two rays of a light beam out of a reference 
length light beam SOL so as to output interference light L4 for reference 
length. The two light rays have a difference in optical length 
corresponding to a predetermined reference length. A position data 
calculating section 25 calculates position data of an object on the basis 
of the reference length and the variations in the optical intensity of the 
interference light L3, L4 for relative displacement and reference length. 
The position detecting apparatus 20 causes variation in optical intensity 
of the interference light L3, L4 for relative displacement and reference 
length, without moving an object, by changing the wavelength of the light 
beam OL from the light source 21. 
The light source 21 includes, as shown in FIG. 2, a semiconductor laser 30 
of GaAs type or the like. The semiconductor laser 30 is driven by a 
semiconductor laser driver circuit 31 and outputs a laser light beam OL 
which has a desired wavelength. The output laser light beam OL is 
directed, via two beam splitters 32 and 33, to a separation plane P3 of 
the relative displacement interference section 23. 
The relative displacement interference section 23 includes an interference 
unit 34 for splitting the light beam OL into two light beams, namely a 
reference and a measurement light L5, L6 in the relative displacement 
interference section 23. The light L5, L6 have planes of polarization 
orthogonal to each other. The reference light L5 is separated from the 
light beam OL at the separation plane P3 through reflection at a 
stationary polarization mirror 35, that is, a reference plane. The 
measurement light L6 is separated from the light beam OL at the separation 
plane P3 by transmission through the polarization mirror 35. The 
measurement light L6 is then reflected at a measurement plane or a 
reflection mirror 36 which is fixed to an object to be detected. The 
reference light L5 from the polarization mirror 35 and the measurement 
light L6 from the reflection mirror 36 are brought together at the 
separation plane P3 for forming interference light L3 for relative 
displacement. The difference in optical length between the reference and 
measurement lights L5 and L6 is equal to twice the length Lx defined 
between the polarization mirror 35 and the object namely the reflection 
mirror 36. The optical waves existing in a path corresponding to the 
difference 2Lx of optical length cause, as shown in FIG. 3, a phase shift 
.theta. between the reference and measurement light L5, L6, which phase 
shift .theta. is represented by the number of waves. The phase shift 
.theta. in turn causes interference fringes of the interference light L3 
for relative displacement. 
The reference length interference section 24 includes an interference unit 
37 for splitting the light beam SOL into two light beams, namely, 
reference and measurement light L7, L8 in the relative displacement 
interference section 24. The two light beams have planes of polarization 
orthogonal to each other. The reference light L7 is separated from the 
reference length light beam SOL at a separation plane P4 through 
reflection at a reference plane, namely, a stationary polarization mirror 
38. The measurement light L8 is separated from the light beam SOL by 
transmission through the polarization mirror 38 and is then reflected at a 
measurement plane or a stationary reflection mirror 39. The reference 
light L7 from the polarization mirror 38 and the measurement light L8 from 
the reflection mirror 39 are brought together at the separation plane P4 
for forming interference light L4 for reference length. The polarization 
and reflection mirrors 38, 39 are positioned at a distance of the 
reference length Lo, so that the difference in optical length between the 
reference and measurement light L7, L8 is equal to twice the reference 
length. The optical waves existing in a path corresponding to the 
difference 2Lo of the optical length cause a phase shift .theta. between 
the measurement and reference lights L7, L8, which phase shift .theta. is 
represented by the number of waves. The phase shift .theta. in turn causes 
interference fringes of the interference light L4 for reference length. 
It should be noted that the polarization mirrors 35, 38 in the relative 
displacement and reference length interference sections 23, 24 may 
respectively comprise a beam splitter 13 of FIG. 13 which has an angular 
plane of 45 degree with respect to the light beam. In this case, a light 
beam irradiated to the beam splitter is split into two light beams: the 
first one goes straight and the second one goes in a direction 
perpendicular to the first one. The two split light beams are respectively 
reflected at the reference and measurement planes and are then brought 
together at the beam splitter for an output of an interference light L3, 
L4. 
The position data calculating section 25 comprises a first counting circuit 
40 for counting increment and decrement Cx in the number of waves of the 
interference light L3 for relative displacement, and a second counting 
circuit 41 for counting increment and decrement Co in the number of waves 
of the interference light L4 for reference length. The counted increment 
and decrement Cx, Co are supplied to an absolute position data calculation 
circuit 42 where a position Lx of the object from the separation plane P3 
in the relative displacement interference section 25 is calculated from 
the equation Lx=Lo(Cx/Co) on the basis of a principle which will be 
described later, wherein Lo represents the reference length from the 
separation plane P4 to the measurement plane 39 in the reference length 
interference section 24. The value Lx is output as position data Pout of 
the object. 
The interference light L3 for relative displacement, which is irradiated to 
the position data calculating section 25, is photoelectrically converted 
into two signals for optical intensity at a beam receiver 43. The two 
signals have different phases from each other. The electrical signals are 
then voltage amplified in an amplification circuit 44. The amplified 
electrical signals are supplied to the counting circuit 40, where the 
increment and decrement Cx of the number of waves are counted on the basis 
of the two signals for optical intensity. 
The interference light L3 for relative displacement, which is irradiated to 
the beam receiver 43, is converted to circular polarized light by a 
.lambda./4 retardation sheet 45, as is apparent from FIG. 4, and is then 
split into two light beams by a beam splitter 46. The split light beams 
are respectively transmitted through first and second polarization plates 
47, 48, which have polarizing angles shifted by .pi./4 from each other. 
The two transmitted light beams are converted into electrical signals S1, 
S2 by first and second photoelectric devices 49, 50, respectively. The 
electrical signals S1, S2 have sinusoidal waveforms with a .pi./2 phase 
shift. The cycle of the waveforms corresponds to the displacement 
X=.lambda./2 of the object. This type of method my generally be used in 
the field of an interferometry for making signals of different phases by 
employing a .lambda./4 retardation sheet and two polarizing plates. 
Alternatively, two polarization plates my be employed to have a .pi./2 or 
(3.pi.)/4 shift in polarizing angles so as to provide electrical signals 
having .pi. or (3.pi.)/2 phase shift. 
The amplification circuit 44 serves to convert the electrical signals S1, 
S2 from an electrical current form to a voltage form, by using a 
current/voltage circuit. The converted signal is then amplified to a 
sufficient voltage level by an amplifier. 
FIG. 6 illustrates the counting circuit 40 including a comparator 51 for 
converting the voltage signals S1, S2 into pulse signals, and an UP/DOWN 
counter 52 for counting the number N of cycles of the signals S1, S2 on 
the basis of the pulse signal from the comparator 51. The comparator 51 
compares the levels of the signals S1, S2, as shown in FIG. 5, with a 
predetermined reference level, such as the median of the amplitude of the 
sine wave. When the levels of the electric signals S1, S2 are detected to 
be larger than the reference level, an H signal is output. This comparator 
51 is designed to determine first to fourth quarter sections for a cycle 
of a signal for optical intensity on the basis of the pulse signals for 
the signals S1, S2. The UP/DOWN counter 52 counts the number N of cycles 
and detects the direction of a variation in phase shift, both on the basis 
of the changes of the pulse signals in the respective first to fourth 
quarter sections. 
The counting circuit 40 further includes an interpolation circuit for 
determining the relative locations of the electrical signals S1, S2 within 
the cycle. The electrical signals S1 and S2 are expressed by: 
EQU S1=A1cos (.theta.)+B1 
EQU S2=A2sin (.theta.)+B2 (1) 
The constants A1, A2, B1 and B2 can be previously measured and eliminated 
from the equations as follows: 
EQU S1=cos (.theta.) 
EQU S2=sin (.theta.) (2) 
The electrical angle .theta. for the electrical signals S1, S2 can be 
expressed by: 
##EQU1## 
thereby calculating the electrical angle .theta. corresponding to the 
phase shift within one cycle or .lambda./2 of the wave. 
The counting circuit 40 further includes a combining circuit 54 for 
calculating increment and decrement Cx of waves on the basis of 
##EQU2## 
wherein N represents the number of cycles; and .theta. represents the 
electrical angle. A method for generating countable waves will be 
described later. 
The interference light L4 for reference length, which is irradiated via two 
reflection mirrors 55, 56 to the position data calculating section 25, is 
converted into an electrical signal in the beam receiver 57, and is then 
subjected to voltage amplification in an amplification circuit 58. The 
amplified electrical signal is supplied to a counting circuit 41. The 
functions of the beam receiver 57, the amplification circuit 58 and the 
counting circuit 41 are the same as those of the beam receiver 43, the 
amplification circuit 44 and the counting circuit 40, respectively, so the 
detailed description thereof is omitted here. 
The absolute position data calculation circuit 42 calculates a distance Lx 
between the separation plane P3 and an object on the basis of the 
reference length Lo, and increment and decrement Cx, Co in the number of 
waves of the interference light L3, L4 for relative displacement and 
reference length. The increment and decrement Cx, Co is caused by a 
continuous variation, in a digital or an analog manner, in the wavelength 
of the light beam OL and the reference length light beam SOL from the 
light source 21. 
Assume that the wavelength control section 22 continuously reduces from 
.lambda.1 to .lambda.2 the wavelength of the light beam OL emitted from 
the light source 21, in stead of the displacement X which an object moves 
along. The reduction in the wavelength, in the optical length 2Lx of FIG. 
7 on which the measurement light L6 reciprocates from the separation plane 
P3 in the relative displacement interference section 23, causes the number 
of waves to be increased from n1 to n2. Every .lambda./2 increment in the 
number of waves causes the counting of brightness and shadow in an 
interference fringe, in a similar manner where the object is displaced by 
the amount X=.lambda./2. FIG. 3 illustrates no phase shift between the 
reference and measurement lights L5, L6 upstream of the separation plane 
P3. After being separated at the separation plane P3, the measurement 
light L6 of frequency f1 additionally travels along the optical length 2Lx 
compared with the reference light L5. When the measurement light L6 
returns to the separation plane P3, the number of waves in the optical 
length 2Lx causes a phase shift .theta.1 to the reference light L5. This 
phase shift .theta.1 is represented by an optical intensity of the 
interference light L3 for relative displacement. When the wavelength is 
reduced, the number n of optical waves is gradually increased, as shown in 
FIG. 7. As the number n of waves increases, the phase of the measurement 
light L6 gradually varies at the separation plane P3 from .theta.1 via 
.theta.12 to .theta.2 with the phase of the reference light L5 being 
maintained at .theta.1. The variation in the optical intensity due to the 
variation in the wavelength is photoelectrically converted in the beam 
receiver 43, so that electrical signals S1 and S2 are obtained. According 
to this principle, when the wavelength .lambda. of the light beam OL is 
varied with an object maintained at position X such that the number n of 
waves is varied by .+-.1/2, the sinusoidal waveform of the electrical 
signals S1, S2 varies by one cycle. 
The wave number data Cx, which has been calculated in the counting circuit 
40, corresponds to the increment in the number of waves in the measurement 
length Lx before and after the change of the wavelength, that is, 
.DELTA.Cx=n2-n1. The measurement length Lx before the change of the 
wavelength is expressed as follows: 
##EQU3## 
wherein C represents an optical velocity and C/f1 represents the 
wavelength of an optical wave. Likewise, the measurement length Lx after 
the change of the wavelength is expressed as follows: 
##EQU4## 
Accordingly, 
##EQU5## 
is obtained. It is understood that the counted variation .DELTA.Cx in the 
number of waves is proportional to the measurement length Lx. 
Likewise, the variation Co, which is counted in the counting circuit 41, 
corresponds to the increment in the number of waves in the reference 
length Lo before and after the change of the wavelength, that is, 
.DELTA.Cx=n4-n3, wherein n3 represents the number of waves existing in the 
reference length Lo before the change of the wavelength, and n4 represents 
the number of waves existing in the reference length Lo after the change 
of the wavelength. It is also understood from 
##EQU6## 
that the increment .DELTA.Co is proportional to the reference length Lo. 
Accordingly, 
##EQU7## 
is established. The absolute position data calculation circuit 42 
calculates the measurement length Lx from the above equation, and outputs 
position data Pout of the object. 
The position detection apparatus 20 utilizing optical interferometry 
according to the present invention can detect a position of an object, 
namely the measurement length Lx, in the form of an absolute position from 
the measurement point (separation plane) P3. It is unnecessary to first 
locate an object at a reference position for setting a home position, 
thereby achieving prompt position detection. Further, it is not necessary 
for the wavelength of the light beams OL, SOL to be well stabilized as 
long as the number n of optical waves can be varied in the measurement and 
reference length Lx, Lo before and after the change of the wavelength so 
as to ensure the calculation of the increment and decrement Cx, Co. The 
displacement detection can be performed stably without being effected by 
the variation in the wavelength of the light beam. Furthermore, when the 
measurement and reference length Lx, Lo are established under a common 
ambient environment, wave number data Cx, Co can be affected by a common 
fluctuation, so that the effects on both data Cx, Co can be balanced with 
each other during the calculation of the measurement length Lx. A 
measurement which is resistant to an ambient environment can be obtained. 
The above described first embodiment employs two light beams split by 
polarization for outputting a plurality of signals with different phases 
from one another. However, the apparatus may employ a standard Fizeau or 
Michelson interferometer without utilizing polarization. In this case, a 
polarization mirror may be a partially transmissible plane, and a beam 
receiver receives two beams of interference light. The mirror may also 
well be a locally transmissible mirror. A counting circuit may count the 
fringes of the interference light. In this event, if the reference plane 
may comprise planes with a step of .lambda./8, from each of which 
interference light is received, the apparatus can detect the direction of 
movement of an object and the position of an object within .lambda./2. 
The position detection apparatus according to the second embodiment of the 
present invention is characterized by the structure of a position data 
calculating section 60, as shown in FIG. 8. The other components have the 
same structure as that of the position detection apparatus 20 of the first 
embodiment, so the detailed description can be omitted here. 
The position data calculating section 60 includes an absolute position data 
calculation section 61 and a relative position data calculation section 
62. The absolute position data calculation section 61 calculates absolute 
position data Pabs of an object on the basis of the reference length Lo 
and the increment and decrement Cx, Co in the number of waves for the 
interference lights L3, L4 for relative displacement and reference length 
when the wavelength control section 22 varies the wavelength of the light 
beam OL. The relative position data calculation section 62 calculates a 
displacement of the object from its absolute position, that is, relative 
position data Pinc with respect to its absolute position, on the basis of 
the increment and decrement Cx in the number of waves in the interference 
light L6 for relative displacement L6 when the object moves. The absolute 
position data Pabs and relative position data Pinc of the object are 
combined in a composite calculation section 63 for an output as position 
data Pout which indicates the position of the object. 
The absolute position data calculation section 61 employs the above 
described principle in the same manner as the absolute position data 
calculation circuit 42 in the first embodiment. For instance, the initial 
position Lini of an object is detected by changing the wavelength of a 
light beam OL or the like in the initial sequence after the apparatus is 
turned on 
The relative position data calculation section 62 calculates a relative 
displacement Pinc from the initial position Lini on the basis of the 
following equation: 
##EQU8## 
The position detecting apparatus according to the second embodiment is 
capable of detecting position data with the wavelength of a light beam 
being varied, which requires a relatively long response time, only during 
the initial sequence after the apparatus is turned on. The subsequent 
sequence can omit the absolute position data being detected through the 
change of the wavelength, thereby reducing the response time in the 
subsequent sequences after the initial sequence. Further, unlike an 
apparatus employing the sole detection of incremental position data, the 
position detecting apparatus in the second embodiment has an advantage, in 
which an absolute position of an object can easily be obtained in the 
absolute position data calculation section 61 even if an erroneous 
counting has been conducted during the measurement, the last position has 
become untraceable due to the interruption of the optical path, or the 
apparatus has been turned on and off during the measurement. It is 
unnecessary to bring an object to a home position after such an accident. 
The position detection apparatus according to the second embodiment has a 
tendency for the values of relative displacement Pinc come to exhibit 
non-linear variation compared with the beginning of the detection as the 
oscillation frequency of a laser beam is varied. The position detection 
apparatus according to the third embodiment of the present invention can 
overcome this problem by finding and correcting the variation in the 
oscillation frequency f, that is, the variation in the wavelength 
.lambda.. The other components have the same structure as that in the 
second embodiment. 
FIG. 9 illustrates a position data calculating section 70 according to the 
third embodiment. The position data calculating section 70 includes a 
wavelength variation detecting section 71 for detecting variation in the 
wavelength of the light beam OL from the light source 21 on the basis of 
the interference light L4 for reference length from the reference length 
interference section 24, and a wavelength correcting section 72 for 
correcting relative position data Pinc on the basis of the detected 
variation of the wavelength. 
The wavelength variation detecting section 71 first latches and stores as 
initial data CoL the increment and decrement Co in the number of waves 
from the counting circuit 41 at the start of generating a relative 
position data Pinc. The section 71 then calculates a variation 
.DELTA..lambda. in the wavelength after a lapse of a predetermined time 
based on the following equation by using data Co then measured: 
##EQU9## 
The wavelength correcting section 72 then calculates wavelength .lambda.c 
using the calculated variation .DELTA..lambda. on the basis of the 
following equation: 
EQU .lambda.c=.lambda.+.DELTA..lambda. (12) 
Equation 10 can be calculated using the wavelength .lambda.c so as to 
provide Pinc which is corrected by an amount of variation .DELTA..lambda. 
in the wavelength. As a result, position data Pout can be obtained having 
sufficient linearity. 
In a case where the position detecting apparatus according to the present 
invention is used as a relative position detecting apparatus, the 
apparatus may output Pinc corrected by an amount of variation 
.DELTA..lambda. in the wavelength. In this case, an absolute position data 
calculation section is unnecessary. 
It should be noted that the following method can be applied in order to 
obtain an absolute wavelength of a laser beam. 
Assume that a semiconductor laser with .lambda.=780 nm is used. Define the 
optical length for the polarization mirror 38 as y, while the optical 
length for the reflection mirror 39 is defined as y+.beta., with the 
difference in optical length determined as .beta. in the reference length 
interference section 24. The increment and decrement Co in the number of 
waves with an ideal wavelength .lambda. is designated as Coi, whereas the 
increment and decrement Co in the number of waves by an actual 
semiconductor laser is designated as Cor. When a light beam having an 
ideal wavelength .lambda. is introduced, a difference .DELTA.n in the 
number of waves between the optical paths for the polarization and 
reflection mirrors 38, 39 can be expressed by: 
##EQU10## 
Here, it is possible to identify the ideal wavelength .lambda. of the 
semiconductor laser, the value Coi of data Co for the ideal wavelength 
.lambda., and the difference .beta. in optical length. 
The actual incident light beam from a semiconductor laser with a wavelength 
of (.lambda.+.alpha.)generates a difference .DELTA.n' in number of waves 
between the optical lengths for the polarization and reflection mirrors 
38, 39 as designated below: 
##EQU11## 
Equation (14) represents the counting of the number of waves when the 
wavelength is changed from .lambda. to (.lambda.+.alpha.), so that the 
following equation can be obtained: 
##EQU12## 
Thus, the wavelength .lambda.' of the semiconductor laser which is 
actually used is expressed by: 
EQU .lambda.'=.lambda.+.alpha. (16) 
The wavelength .lambda.' an semiconductor laser in use can be measured by 
setting a known difference .beta. in optical length, measuring increment 
and decrement Co in the number of waves for a stabilized laser having a 
known reference wavelength .lambda., and measuring increment and decrement 
Co in the number of waves for an actual semiconductor laser for 
comparison. It is possible to calculate position data with high accuracy 
without an error caused by the variation in wavelength. 
An example of the wavelength control section 22 will next be described in 
detail. FIG. 10 illustrates the wavelength control section 22 comprising a 
temperature sensor 73 for sensing the temperature of the semiconductor 
laser 30, and a temperature adjuster 74 for adjusting the temperature of 
the semiconductor laser 30 based on the sensed temperature. The wavelength 
control section 22 varies the wavelength of the light beam OL by varying 
the temperature of the semiconductor laser 30. 
The temperature adjuster 74 has a heater 75 attached to the semiconductor 
laser 30, and a temperature control unit 76 for outputting a heating or 
cooling command to the heater 75. Upon receipt of a request signal RQabs 
for measuring a position from the position data calculating section 25, 
the temperature control unit 76 outputs a heating or cooling command to 
the heater 75 until the temperature of the semiconductor laser 30 becomes 
a predetermined target temperature. When the temperature sensor 73 
determines that the temperature of the semiconductor laser 30 has reached 
the target temperature, the temperature control unit 76 outputs a 
detection command Oabs to the position data calculating section 25. The 
temperature control unit 76 may output a heating or cooling command after 
an output of the detection command Oabs so as to normalize the wavelength 
of the semiconductor laser 30. The wavelength of a semiconductor laser 30 
depends upon temperature, such as 2.4 nm/10.degree. C. for a wavelength 
.lambda.=780 nm, although it sometimes depends on a type of product or its 
basic wavelength. It should be noted that the present invention does not 
require an accuracy in variation of the wavelength .lambda., so that the 
wavelength can be roughly changed. The temperature of the semiconductor 
laser 30 is accordingly not required to be controlled accurately. It is 
not necessary to conduct feed-back control utilizing the temperature 
sensor 73. 
The wavelength control section 22 may vary the oscillation wavelength of an 
output laser beam by varying the temperature of the connection in a 
semiconductor through excited current in the semiconductor laser 30. The 
semiconductor laser driver circuit 31 controls the excited current. The 
oscillation wavelength is varied toward a longer wavelength as the excited 
current increases. 
Upon receipt of a request signal RQabc for measuring a position, the 
wavelength control section 22 outputs a drive current controlling signal 
to the semiconductor laser driver circuit 31. The driver circuit 31 varies 
the drive current for the semiconductor laser 41 to vary an optical 
output. When the drive current reaches a desired level so as to obtain a 
desired temperature level, a detection command Oabs is supplied to the 
position date detecting section 25. After outputting the detection command 
Oabs, the wavelength control section 22 may supply the original drive 
current so as to bring the wavelength of the semiconductor laser 31 back 
to the normal condition. The wavelength of the semiconductor laser 30 
depends upon an electric product such as 4 nm/.DELTA.7 mW for a wavelength 
of 780 nm, although it may sometimes depend on a type of product or its 
basic wavelength. 
Further, the wavelength control section 22 may cause stress in a medium 
through which the light beam OL passes, to thereby vary a reflective index 
of the medium, which leads to a variation in the wavelength of the light 
beam OL. 
Still further, the wavelength control section 22 may apply an electric 
field to the light beam OL by using electrooptical modulation, to thereby 
vary a reflective index of the light beam OL, which leads to a variation 
in the wavelength of the light beam OL. This phenomena is called the 
Pockels effect. Preferably, the optical path may comprise a material which 
is significantly affected by this phenomena. 
Furthermore, the wavelength control section 22 may apply a magnetic field 
to the light beam OL by using magneto-optic modulation, to thereby vary a 
reflective index of the light beam OL, which leads to a variation in the 
wavelength of the light beam OL. This phenomena is called the Faraday 
effect. Preferably, the optical path may comprise a material which is 
significantly affected by this phenomena. 
Still further, the wavelength control section 22 may rotate a rotatable 
plane with a predetermined reflective index at a high velocity, so that 
the wavelength of the light beam OL is varied through a Doppler effect of 
the light beam OL which passes through the rotatable plane. 
Alternatives for determining the reference length Lo in the reference 
length interference section 24 will next be described in detail. 
FIG. 11A illustrates an interference unit comprising a glass plate 77. The 
glass plate 77 has a polarization mirror 78 as an incident plane, and a 
reflection mirror 79 positioned at the reference length Lo from the 
polarization mirror 78. The reference light L7 out of the light beams SOL 
having orthogonal polarization planes is reflected at the polarization 
mirror 78 in the glass plate 77. The measurement light L8 is transmitted 
through the polarization mirror 78 and is then reflected at the reflection 
mirror 79 to exit from the glass plate 77. The reflected reference and 
measurement light L7, L8 are brought together so as to form interference 
light L4 for reference length directed toward the beam receiver 57. It is 
accordingly possible to simplify the structure by incorporation of the 
reference and measurement planes into the interference unit 37. The 
polarization mirror 77 may comprise a partially or locally transmissible 
plane for a measurement without polarization. Alternatively, the mirror 
planes 77 and 78 may be substituted by a general glass plane so as to 
utilize its reflection or transmission characteristics. The reflection 
mirror 79 may be formed as a cubic. 
FIG. 11B illustrates an interference unit made of an optical fiber 80. The 
optical fiber 80 has a polarization mirror 81 as an incident plane at one 
end. The other end is subjected to a reflection treatment so as to have a 
mirror 82. The reference light L7 out of the light beams SOL having 
orthogonal polarization planes is reflected at the polarization mirror 81. 
The measurement light L8 is transmitted through the polarization mirror 81 
and then advances through the optical fiber 80. The mirror 82 reflects the 
measurement light L8 back along the optical fiber 80. The light L8 finally 
exits from the incident plane. The reflected reference and measurement 
lights L7, L8 are brought together so as to form interference light L4 for 
reference length, directed toward the beam receiver 57. Although it is 
difficult to take a longer reference length Lo in the interference unit 37 
shown in FIG. 2, the utilization of an optical fiber enables an 
interference unit of a longer reference length with a simple structure and 
a reduced space. The polarization mirror 81 may of course comprise a 
partially or locally transmissible plane, and the mirror planes 81 and 82 
may utilize reflection or transmission of a general optical fiber. 
The interference unit shown in FIG. 11C includes a polarization mirror 84 
attached to a housing 83 as an incident plane, and a reflection mirror 85 
for subjecting an introduced light beam within the housing 83 to 
continuous reflection until it finally exits from the incident plane. The 
reference light L7 out of the light beams SOL having orthogonal 
polarization planes is reflected at a polarization mirror 84 in this 
interference unit. The measurement light L8 is transmitted through the 
polarization mirror 84 and is then subjected to continuous reflection at 
the reflection mirror 85 within the housing 83 until it finally exits from 
the incident plane. The reflected reference and measurement light L7, L8 
are brought together so as to form an interference light L4 for reference 
length, directed toward the beam receiver 57. This interference unit 
enables the reference length of a longer path with a reduced space, 
similar to the case in FIG. 11b. 
Although light beams reflected from the polarization mirrors 78, 81, 84 and 
the reflection mirrors 79, 82, 85 are brought together for an output in 
the foregoing FIGS. 11A to 11C, light beams can be supplied as 
transmission light. In this case, the polarization mirrors 78, 81, 84 and 
the reflection mirrors 79, 82, 85 may comprise a partially transmissible 
plane (a partially reflecting plane). Assume that this structure is 
employed in FIG. 11A. The reference length light beam SOL from the light 
source 21 is partially transmitted through the polarization mirror 78 and 
advances toward the reflection mirror 79 which is a partially 
transmissible plane. The reflection mirror 79 allows the light beam SOL to 
be partially transmitted to the outside. The polarization mirror 78 allows 
the reflected light beam SOL to be partially reflected at the reflection 
mirror 79. The light beam SOL is thereafter transmitted through to the 
reflection mirror 79. The last transmitted light beam and the initial 
transmitted light beam both through the reflection mirror 79 are brought 
together so as to form interference light, directed toward the beam 
receiver 57. The interference of the transmission light beams, contrary to 
the interference of the reflected light beams, enables the beam splitter 
55 to be omitted so as to simplify the structure, since the light beams do 
not travel toward the light source 21. Further, it is possible to 
integrate the beam receiver 57 and the interference unit 37 together, 
thereby leading to a compact apparatus. 
FIG. 12 illustrates a position detecting apparatus utilizing optical 
interferometry according to a fourth embodiment of the present invention. 
The fourth embodiment employs the optical heterodyne method using a light 
source with light beams of different wavelengths. Since the optical 
heterodyne method of this type has been described in detail referring to 
FIG. 13, the detailed explanation thereof is not presented here. The 
structures similar to those in the foregoing first embodiment are given 
the same reference numerals, so the detailed description thereof is 
omitted. 
The position detecting apparatus according to the fourth embodiment 
comprises the light source 21 which includes an acousto-optic modulating 
device 100 for modulating the frequency of a light beam OL emitted from 
the semiconductor laser 30. The acousto-optic modulating device 100 
modulates the light beam OL to provide light beams OL, for an output, of 
frequencies f1, f2 on orthogonal polarization planes. The frequencies f1, 
f2 have a frequency difference f0. 
The output light beam OL is supplied to the relative displacement 
interference section 23 and to a photoelectric device 102 of the position 
data calculating section 25 via a beam splitter 101. The light beam OL 
supplied to the relative displacement interference section 23 is split 
into measurement light L6 of frequency f1 and reference light L5 of 
frequency f2. The reference and measurement lights L5, L6 are respectively 
reflected at the mirrors 35, 36 and are brought together. When the 
reflection mirror 36 is relatively displaced, the measurement light L6 is 
subjected to Doppler modulation, whereby the frequency thereof is varied 
to be f1.+-..DELTA.f. The interference light L3 for relative displacement, 
which has been formed by bringing the light beams together, is 
photoelectrically converted in the beam receiver 43 in the position data 
calculating section 25 so as to provide an electrical signal Fp1 having a 
beat frequency f2-(f1.+-..DELTA.f). As described above, a phase shift is 
measured on the basis of the electrical signal Fp1 and the reference 
signal Fr of a beat frequency f2-f1 from the photoelectric device 102. An 
amount of displacement can be calculated on the basis of the measured 
phase shift. When the wavelength control section 22 varies the wavelength 
of the semiconductor laser 30 by .DELTA..lambda., the number of the waves 
is varied in the optical lengths between the polarization mirror 35 (a 
split plane P3) and the reflection mirror 36. This variation apparently 
presents a displacement X, so that increment and decrement Cx in the 
number of waves can be obtained as position data of the object. 
The reference length light beam SOL separated from the light beam OL is 
supplied to the reference length interference section 24 so as to form 
interference light L4 for reference length in a similar way to the 
relative displacement interference section 23. The interference light L4 
has a phase shift corresponding to the reference length Lo. The 
interference light L4 for reference length is photoelectrically converted 
in the beam receiver 47 of the position data detection section 25, so that 
an electrical signal Fp2 of a beat frequency f2-f1 is output. As described 
above, a phase shift is measured on the basis of the electrical signal Fp2 
and the reference signal Fr of a beat frequency f2-f1 from the 
photoelectric device 102. An amount of displacement is accordingly 
calculated on the basis of the measured phase shift. The change of 
frequency of the semiconductor laser 30 enables increment and decrement Co 
in the number of waves to be obtained. 
The counted increment and decrement Co, Cx in the number of waves are 
proportional to the reference length Lo and the measurement length Lx, 
respectively, so that the measurement length Lx can be calculated based on 
the equation (9). The calculated measurement length Lx is output as 
absolute position data Pout. 
The calculation of the increment and decrement Co, Cx in the number of 
waves can be conducted, unlike the foregoing first embodiment, using AC 
signals Fr, Fp1 and Fp2, so that the disadvantage due to the variation in 
DC level can be prevented. Further, any unnecessary frequencies due to 
noise can be cut off, leading to stable detection. 
It should be noted that the oscillation frequency may be controlled, as 
shown in FIG. 13, by introducing the light beam OL into a laser tuning 
circuit. The light source 21 may comprise a He--Ne laser, not shown, in 
place of a semiconductor laser 30. In this case, the Zeeman effect may be 
utilized to obtain two types of light beam. This effect is known to 
utilize the fact that a spectrum is split due to a strong magnetic field. 
The Zeeman effect can be realized by locating a laser tube within a 
magnetic field formed by a coil supplied with a voltage. Alternatively, 
the generation of two types of light beam can be achieved by any other 
method.