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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical heterodyne measuring apparatus
for effecting measurement based on a phase difference and a frequency
difference between a reference and a measuring beat signals.
2. Discussion of the Prior Art
There is known an optical heterodyne measuring apparatus which is adapted
to measure physical quantities associated with a subject, depending upon a
phase difference or a frequency difference between a reference beat beam
which consists of two laser beams having mutually perpendicular
polarization planes and different frequencies, and a measuring beat beam
which consists of the reference beat beam which has been reflected by the
subject. Such a known optical heterodyne measuring apparatus usually uses
a single reference beat beam whose beat frequency is equal to a difference
between the frequencies of the two laser beams that constitute the
reference beat beam. Therefore, the beat frequency of the reference beat
beam is a fixed value which is determined by the frequencies of the two
laser beams produced by a laser device.
In such a known optical heterodyne measuring apparatus, the limitation of
the beat frequency of the reference beat beam causes insufficient accuracy
of measurement of the subject, or a relatively limited range of the
velocity of the subject to be measured. For example, where the beat
frequency is comparatively high, the apparatus may follow a comparatively
high speed or velocity of the subject, but suffers from comparatively low
resolution of measurement (i.e., cannot measure a quantity in sufficiently
small increments). Conversely, where the beat frequency is comparatively
low, the resolution of measurement is sufficient for a subject moving at a
relatively low speed, but the apparatus cannot sufficiently follow a
comparatively high speed or velocity of the subject, whereby the velocity
or position of the slowing moving subject cannot be accurately determined.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an optical
heterodyne measuring apparatus which permits sufficiently high resolution
of measurement, and sufficiently high ability of following a rapidly
moving subject.
The above object may be achieved according to the principle of the present
invention, which provides an optical heterodyne measuring apparatus for
effecting measurement of a subject, based on a phase difference and a
frequency difference between a reference beat beam consisting of two laser
beams having different frequencies, and a measuring beat beam which
consists of the reference beat beam reflected by the subject, the
apparatus including a light source device, a beat frequency detecting
device and a phase difference detecting device. The light source device
produces a first reference beat beam having a first beat frequency, and a
second reference beat beam having a second beat frequency which is lower
than the first beat frequency. The beat frequency detecting device detects
a beat frequency between the first beat frequency of the first reference
beat beam, and a beat frequency of a first measuring beat beam which
consists of the first reference beat beam reflected by the subject. The
phase difference detecting device detects a phase difference between the
second reference beat beam, and a second measuring beat beam which
consists of the second reference beat beam reflected by the subject.
In the optical heterodyne measuring apparatus of the present invention
constructed as described above, the beat frequency detecting device
detects the beat frequency between the comparatively high beat frequency
of the first reference beat beam, and a beat frequency of a first
measuring beat beam which consists of the first reference beat beam which
is reflected by the subject. Thus, an amount of displacement of the
subject can be measured in increments of a half of the wavelength of the
laser beams, or with a resolution corresponding to the half of the laser
beam wavelength. Independently of this measurement, the phase difference
detecting device detects the phase difference between the second reference
beat beam, and a second measuring beat beam which consists of the second
reference beat beam which is reflected by the subject. Thus, an amount of
displacement of the subject may be measured in increments of not larger
than the half of the laser beam wavelength, or with a resolution
corresponding to a value not larger than the half of the laser beam
wavelength. Therefore, the instant optical heterodyne measuring apparatus
assures sufficiently high resolution of measurement, and has sufficiently
high ability of following the subject moving at a relatively high speed.
In one form of the invention, the light source device comprises a laser
source for producing two linearly polarized laser beams having mutually
perpendicular polarization planes and different frequencies, a
non-polarizing beam splitter which reflects a component of each of the two
linearly polarized laser beams and transmits therethrough the other
component of each linearly polarized laser beam, and a frequency shifter
which receives the components of the linearly polarized laser beams
reflected by or transmitted through the non-polarizing beam splitter. The
frequency shifter adjusts frequencies of the received laser beams so as to
produce the first reference beat beam having the first beat frequency. The
other components of the two linearly polarized laser beams transmitted
through or reflected by the non-polarizing beam splitter provide the
second reference beat beam having the second beat frequency.
In another form of the invention, the light source device comprises a first
laser source for producing the first reference beat beam having the first
beat frequency, and a second laser source for producing the second
reference beat beam having the second beat frequency.
In a further form of the invention, the light source device comprises a
laser source for producing two linearly polarized laser beams having
mutually perpendicular polarization planes and different frequencies, a
non-polarizing beam splitter which reflects a component of each of the two
linearly polarized laser beams and transmits therethrough the other
component of each linearly polarized laser beam, a first frequency shifter
which receives the components of the two linearly polarized laser beams
reflected by or transmitted through the non-polarizing beam splitter and
which adjusts frequencies of the received laser beams, so as to produce
the first reference beat beam having the first beat frequency, and a
second frequency shifter which receives the other components of the laser
beams transmitted through or reflected by the beam splitter and which
adjusts the frequencies of the received laser beams, so as to produce the
second reference beat beam having the second beat frequency.
In a still further form of the invention, the measuring apparatus further
comprises a first reference-beam photosensor receiving the first reference
beat beam and producing a first reference beat signal, a second
reference-beam photosensor receiving the second reference beat beam and
producing a second reference beat signal, a first measuring-beam
photosensor receiving the first measuring beat beam and producing a first
measuring beat signal, and a second measuring-beam photosensor receiving
the second measuring beat beam and producing a second measuring beat
signal.
According to one arrangement of the above form of the invention, the
apparatus further comprises means for permitting the first and second
reference beat beams to be reflected by the subject, so as to produce the
first and second measuring beat beams, and directing the first and second
measuring beat beams to the first and second measuring-beam photosensors.
The beat frequency detecting device may comprise a first counter for
counting pulses of the first reference beat signal, a second counter for
counting pulses of the second reference beat signal, and means for
calculating a difference between counts of the first and second counters,
to thereby detect the beat frequency between the first beat frequency of
the first reference beat beam, and the beat frequency of the first
measuring beat beam. The phase difference detecting device may comprise a
pulse generator for generating a reference pulse signal having a
predetermined frequency, a gate which receives the reference pulse signal
and is open for a time duration corresponding to a phase difference
between the second reference beat signal and the second measuring beat
signal, and a phase counter for counting pulses of the reference pulse
signal for the time duration, to thereby detect the phase difference
between the second reference and measuring beat beams.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other objects, features and advantages of the present
invention will be better understood by reading the following description
of presently preferred embodiments of the invention, when considered in
connection with the accompanying drawings, in which:
FIG. 1 is a schematic view of one embodiment of an optical heterodyne
measuring apparatus of the present invention;
FIG. 2 is a view showing in detail a detecting circuit used in the
embodiment of FIG. 1; and
FIGS. 3 and 4 are fragmentary schematic views illustrating modified
embodiments of the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring first to FIG. 1, there is shown an optical heterodyne measuring
apparatus for measuring a velocity and an amount of displacement of a
corner cube 62, or a subject on which the corner cube 62 is fixed. The
apparatus uses a laser device in the form of a Zeeman laser source 10
which produces two linearly polarized laser beams which have mutually
perpendicular polarization planes and different frequencies. These two
laser beams are incident upon a non-polarizing beam splitter 12.
A component of each of the two linearly polarized laser beams produced by
the laser source 10 is reflected by the non-polarizing beam splitter 12,
while the other component of each incident laser beam is transmitted
through the beam splitter 12. The laser beams reflected by the beam
splitter 12 is transmitted through a polarizer 14 and a 1/4 waveplate 16,
whereby the two linearly polarized laser beams reflected by the beam
splitter 12 are converted into a circularly polarized laser beam. This
circularly polarized beam is reflected by a mirror 18 and is thereby
incident upon a frequency shifter 20.
The frequency shifter 20 includes, for example, a polarizing beam splitter
22 for splitting the circularly polarized laser beam from the mirror 18
into two linearly polarized laser beams, a pair of acoustooptical
modulators 24, 26 which are adapted to diffract the respective linearly
polarized laser beams from the beam splitter 22 and change the frequencies
of the received laser beams, and a polarizing beam splitter 28 which
combine the laser beams from the acoustooptical modulators 24, 26. Each
modulator 24, 26 is formed of a single crystal of tellurium dioxide or
molybdate, or formed of a glass material.
The acoustooptical modulators 24, 26 are provided with respective
piezoelectric elements 30, 32 which produce acoustic waves, so that the
incident linearly polarized laser beams are diffracted due to periodic
changes in the refractive index of the substrates of the modulators 24,
26, which occur depending upon the frequencies of the acoustic waves. The
frequencies of the diffracted laser beams are changed or shifted by
amounts equal to the frequencies of the surface acoustic waves, due to a
sort of the Doppler effect. As a result, the difference between the
frequencies of the two laser beams transmitted through the acoustooptical
modulators 24, 26 is increased as compared with that of the laser beams
incident upon the modulators, i.e., with that of the laser beams as
produced by the laser source 10. The thus frequency-shifted linearly
polarized laser beams are combined with each other by a polarizing beam
splitter 28, whereby there is produced a first reference beat beam B1
which has a comparatively high first reference beat frequency fHB.
In the frequency shifter 20 described above, the frequencies of drive
signals applied from a suitable driver circuit to the piezoelectric
elements 30, 32 are controlled so as to determine the first reference beat
frequency fHB suitable for measuring the subject (corner cube prism 62).
This beat frequency fHB does not exist as the light frequency of the first
reference beat beam B1 as emitted from the frequency shifter 20, but
should be recognized as a beat frequency of a first reference beat signal
BSl produced by a first reference-beam photosensor 40 (which will be
described), which receives the two different frequencies of the two
linearly polarized laser beams of the first reference beat beam B1. For
the sake of easy understanding, however, the first reference beat beam B1
will be described as having the frequency fHB. Similarly, a second
reference beat beam B2 will be described as having a second reference beat
frequency fLB.
The first reference beat beam B1 emitted from the polarizing beam splitter
28 of the frequency shifter 20 is reflected by a mirror 34 and is split by
a non-polarizing beam splitter 36, as indicated in solid lines in FIG. 1.
The first reference beat beam B1 transmitted through the non-polarizing
beam splitter 36 is received by the first reference-beam photosensor 40
through a polarizer 38. As a result, the first reference beat signal BSl
having the first reference beat frequency fHB is generated by the
photosensor 40, as indicated above, and is applied to a detecting circuit
64. This first reference beat frequency fHB is comparatively high.
The two linearly polarized laser beams transmitted through the
non-polarizing beam splitter 12 serve as the second reference beat beam B2
indicated in dashed lines in FIG. 1. The second reference beat beam B2
reflected by the beam splitter 36 is received by a second reference-beam
photosensor 44 through a polarizer 42. Since this second reference beat
beam B2 consists of the two linearly polarized laser beams having the
mutually perpendicular polariazation planes and different frequencies as
produced by the laser source 10, a second reference beat signal BS2
produced by the second reference-beam photosensor 44 has a second
reference beat frequency fLB which is lower than the first reference beat
frequency fHB of the first reference beat signal BSl. The second reference
beat signal BS2 is also applied to the detecting circuit 64.
Each of the polarizers 38, 42 is provided to adjust the ratio of the two
linearly polarized laser beams of the corresponding first or second
reference beat beam B1, B2 incident upon the first and second
reference-beam photosensors 40, 44.
In the present embodiment, the laser source 10, non-polarizing beam
splitter 12 and frequency shifter 20 constitute a major portion of a light
source device for producing the first reference beat beam B1 having the
comparatively high first reference beat frequency fHB, and the second
reference beat beam B2 having the comparatively low second reference beat
frequency fLB. The first reference beat frequency fHB is on the order of
MHz, while the second reference beat frequency fLB is on the order of a
few or several hundreds of kHz.
The first reference beat beam B1 reflected by the non-polarizing beam
splitter 36, and the second reference beat beam B2 transmitted through the
beam splitter 36 are split by a polarizing beam splitter 46 into reference
beams (S-type beams) and measuring beams (P-type beams). Described more
specifically, the S-type linearly polarized beam included in the first
reference beat beam B1, and the S-type linearly polarized beam included in
the second reference beat beam B2 are reflected by the polarizing beam
splitter 46, and are transmitted through a 1/4 waveplate 48, whereby the
S-type linearly polarized beams are converted into circularly polarized
beams. These circularly polarized bams are reflected by a corner cube
prism 50 and are again transmitted through the 1/4 waveplate, whereby the
circularly polarized beams are converted into two P-type linearly
polarized beams whose polarization planes are rotated by 90 degrees with
respect to the S-type beams. Thus, the P-type linearly polarized beams can
be transmitted through the polarizing beam splitter 46, and are incident
upon respective polarizers 52, 54.
On the other hand, the P-type linearly polarized beam included in the first
reference beat beam B1, and the P-type linearly polarized beam included in
the second reference beat beam B2 are transmitted through the beam
splitter 46, and a 1/4 waveplate 60, whereby the P-type beams are
converted into circularly polarized beams. The circularly polarized beams
are reflected by the corner cube prism 62 and are again transmitted
through the 1/4 waveplate 60, whereby the circularly polarized beams are
converted into S-type linearly polarized beams whose polarization planes
are rotated by 90 degrees with respect to the P-type beams. Accordingly,
the S-type linearly polarized beams are reflected by the beam splitter 46
and are incident upon the respective polarizers 52, 54.
Therefore, the polarizer 52 receives a first measuring beat beam D1
consisting of the reference and measuring beams derived from the first
reference beat beam B1. The first measuring beat beam D1 is received by a
first measuring-beam photosensor 56. Similarly, the polarizer 54 receives
a second measuring beat beam D2 consisting of the reference and measuring
beams derived from the second reference beat beam B2. The second measuring
beat beam D2 is received by a second measuring-beam photosensor 58. The
first measuring-beam photosensor 56 produces a first measuring beat signal
DS1 having a first measuring beat frequency fHD, while the second
measuring-beam photosensor 58 produces a second measuring beat signal DS2
having a second measuring beat frequency fLD.
The corner cube prism 62 is fixed on a desired subject, while the other
optical elements described above are fixedly accommodated in a suitable
housing. The subject carrying the corner cube prism 62 is adapted to be
moved in a Z-axis direction as indicated in FIG. 1. When the subject or
corner cube prism 62 is at rest, the first measuring beat frequency fHD of
the first measuring beat signal DS1 and the second measuring beat
frequency fLD of the second measuring beat signal DS2 are equal to the
first and second reference beat frequencies fHB and fLB of the first and
second reference beat signals BS1, BS2. When the corner cube prism 62 is
moved in the Z-axis direction, the measuring beams of the first and second
measuring beat beams D1, D2 are subject to a Doppler frequency shift,
whereby the first and second measuring beat frequencies fHD, fLD are
changed relative to the first and second reference beat frequencies fHB,
fLB. Therefore, an amount of the Doppler frequency shift, namely, the
velocity of the corner cube prism 62 (subject) is represented by a
difference between the first measuring and reference beat frequencies fHD
and fHB (or corresponding phase difference), and a difference between the
second measuring and reference beat frequencies fLD and fLB (or
corresponding phase difference).
Referring next to FIG. 2, there is shown in detail the arrangement of the
detecting circuit 64 which receives the first and second reference and
measuring beat signals BS1, BS2, DS1 and DS2.
The first measuring and reference beat signals DS1, BS1 having the
comparatively high first measuring and reference beat frequencies fHD, fHB
are converted into rectangular pulses by respective waveform shaping
elements 66, 68, which also serve to amplify the input signals. The
rectangular pulses from the elements 66, 68 are counted by respective
first and second counters 70, 72. Namely, the first counter 70 counts the
pulses of the first measuring beat signal DS1 having the beat frequency
fHD, while the second counter 72 counts the pulses of the first reference
beat signal BS1 having the beat frequency fHB. Counts CD and CB obtained
by the first and second counters 70, 72 after a predetermined counting
period are temporarily stored in respective first and second latches 74,
76. The content of the second latch 76 is applied to a sign-changing
circuit 78, wherein the sign of the input value is reversed. The output of
the sign-changing circuit 78 and the content of the first latch 74 are
added by an adder 80. In other words, the content of the second latch 76
is subtracted from the content of the first latch 74, and a difference
(CD-CB) obtained as an output of the adder 80 is applied to a third latch
82 and temporarily stored therein. Thus, the content (CD-CB) of the third
latch 82 represents the difference between the first measuring beat
frequency fHD and the first reference beat frequency fHB (phase difference
between the beat signals DS1 and BS1), which difference corresponds to an
amount of the frequency shift .DELTA.fp which is caused by the movement of
the corner cube prism 62. The difference (CD-CB) also represents the
velocity of the Z-axis movement of the corner cube prism 62 (or the
distance of movement per unit time), with a resolution or in increments of
a half of the wavelength of the laser beams produced by the laser source
10.
On the other hand, the second measuring and reference beat signals DS2, BS2
having the comparatively low beat frequencies fLD, fLB are converted into
rectangular pulses by respective waveform shaping elements 84, 86 similar
to the elements 66, 68. The pulses are applied to a gate 88. This gate 88
is open for a time period during which the beat signals DS2 and BS2 are
both present. A reference pulse generator 90 is provided to produce a
reference pulse signal KS having a predetermined clock frequency. The gate
88 also receives clock pulses of the pulse signal KS and applies the
received clock pulses to a phase counter 92, for the above-indicated time
period. Accordingly, the clock pulses are counted by the phase counter 92
for the time period during which the gate 88 is open. The clock frequency
of the reference pulse signal KS is 100 MHz, for example. A count C1
obtained by the phase counter 92 represents the phase difference between
the second measuring and reference beat signals DS2 and BS2, that is, the
movement velocity or movement distance per unit time of the corner cube
prism 62, in increments of not larger than the half of the laser beam
wavelength. The count C1 is stored in a fourth latch 94.
The contents of the third and fourth latches 82, 94 are processed by a CPU
(central processing unit) 96, to continuously calculate the movement
velocity or distance of the corner cube prism 62 and display the
calculated value on a display 102, according to a control program stored
in a ROM (read-only memory) 100 while utilizing temporary data storage
function of a RAM (random-access memory) 98.
Suppose an amount of displacement per unit time .DELTA.t of the corner cube
prism 62 in the Z-axis direction is represented by .DELTA.Zp, the amount
.DELTA.fp of the Doppler frequency shift is represented by the following
equation:
.DELTA.fp=(2/.lambda.).multidot.(.DELTA.Zp/.DELTA.t)
where, .lambda.: Wavelength of the laser beams
By integrating this equation, the movement distance Zp of the prism 62 in
the Z-axis direction is obtained according to the following equation:
Zp=(.lambda./2).intg..DELTA.fp dt
The above equation is converted into the following equation:
Zp=(.lambda./2).multidot.(CD-CB)
According to the above equation, the CPU 96 calculates the movement
distance Zp based on the content (CD-CB) of the third latch 82.
On the other hand, the CPU 96 calculates an amount of movement of the prism
62, in increments of not larger than the half of the wavelength of the
laser beams, based on the content C1 of the fourth latch 94. The output C1
of the fourth latch 94 is an integrated value of the count of the clock
pulses of the reference pulse signal KS having the frequency of 100 MHz,
for a time duration corresponding to the phase difference of the second
measuring and reference beat signals DS2 and BS2. Therefore, suppose the
second reference beat frequency fLB is 100 kHz, the movement distance of
the prism 62 may be measured with a resolution of .lambda./2000.
By adding the values in increments of .lambda./2 and the values in
increments of .lambda./2000 corresponding to the phase difference of the
beat signals DS2 and BS2, the amount or velocity of the movement of the
prism 62 is eventually calculated, and displayed on the display 102. Where
the phase counter 92 cannot follow a high velocity of the corner cube
prism 62 because of an extremely short period of time during which the
gate 88 is open, the movement amount or velocity of the prism 62 is
calculated based solely on the content of the third latch 82 which is
derived from the counts of the first and second latches 74, 76. Where the
first and second counters 70, 72 cannot count due to an extremely low
velocity (e.g., zero velocity) of the prism 62, the movement amount or
velocity of the prism 62 is calculated based solely on the content of the
fourth latch 94 which is derived from the phase counter 92.
It follows from the foregoing description that the first counter 70, second
counter 72, sign-changing circuit 78 and adder 80 constitute a major
portion of a beat frequency detecting device for obtaining the difference
.DELTA.fp between the comparatively high first reference beat frequency
fHB of the first reference beat signal BS1 and the comparatively high
first measuring beat frequency fHD of the first measuring beat signal DS1.
Thus, the amount or velocity of displacement of the corner cube prism 62
can be detected in increments of .lambda./2. On the other hand, the gate
88 and phase counter 92 constitute a major portion of a phase difference
detecting device for obtaining the phase difference between the second
reference beat signal BS2 having the comparatively low second reference
beat frequency fLB and the second measuring beat signal DS2. Thus, the
amount or velocity of displacement of the prism 62 can be detected in
increments of not larger than .lambda./2. As described above, the phase
difference detecting device is operable when the prism 62 is moved at an
extremely low speed, while the beat frequency counting device is operable
when the prism 62 is moved at an extremely high speed. Therefore, the
instant optical heterodyne measuring apparatus not only has sufficiently
high ability of following a high-speed movement of the subject (prism 62),
but also permits high-resolution measurement.
It is also noted that the first reference beat frequency fHB of the first
reference beat beam B1 or first reference beat signal BS1 can be suitably
adjusted by changing the frequencies of the drive signals applied to the
piezoelectric elements 30, 32 of the acoustooptical modulators 24, 26 of
the frequency shifter 20, depending upon the behavior of the relevant
subject (prism 62).
Referring next to FIGS. 3 and 4, modified embodiments of the invention will
be described. For easy understanding, the same reference numerals and
characters as used in FIG. 1 are used in FIGS. 3 and 4, to identify the
functionally equivalent or corresponding elements and beams.
The embodiment of FIG. 3 uses two separate Zeeman laser sources 106 and 108
which produce respectively the first reference beat beam B1 having the
comparatively high first reference beat frequency fHB, for example, about
5 MHz, and the second reference beat beam B2 having the comparatively low
second reference beat frequency fLB, for example, about 100 kHz. Each of
the first and second reference beat beams B1, B2 consists of two linearly
polarized laser beams which have mutually perpendicular polarization
planes and different frequencies. Thus, these two laser sources 106, 108
constitute a major portion of a light source device of the measuring
apparatus.
The embodiment of FIG. 4 uses a second frequency shifter 110 disposed
between the non-polarizing beam splitters 12 and 36, in addition to the
first frequency shifter 20 provided in the first embodiment of FIG. 1. The
second frequency shifter 110, which is constructed similarly to the
frequency shifter 20, is adapted to produce the second reference beat beam
B2 having the comparatively low second reference beat frequency fLB. That
is, the second reference beat frequency fLB can be suitably adjusted by
the second frequency shifter 110. In this embodiment, the laser source 10,
non-polarizing beam splitter 12, and frequency shifters 20, 110 constitute
a major portion of a light source device of the apparatus. In this case,
the light source device may be adapted to produce a single linearly
polarized laser beam, since the first and second frequency shifters 20,
110 may produce desired beat frequencies. However, the polarization plane
of the laser beam should be inclined 45 degrees with respect to the plane
of incidence of the polarizing meam splitter 46.
While the present invention has been described in its presently preferred
embodiments, it is to be understood that the present invention may be
embodied with various changes, modifications and improvements.
For instance, the principle of the invention is applicable to an optical
heterodyne measuring apparatus for measuring physical quantities, other
than the amount or velocity of a linear displacement of the subject (prism
62) as in the illustrated embodiments.
Further, a combination of the sign-changing circuit 78 and the adder 80
used in the illustrated embodiments may be replaced by a subtractor for
subtracting the content of the second latch 76 from the content of the
first latch 74.
It will be understood that other changes, modifications and improvements
may be made in the invention, without departing from the spirit and scope
of the invention defined in the following claims.
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