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Description  |
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FIELD OF INVENTION
This invention relates generally to gauges and sensors for measuring
physical distances, and particularly to optical sensors for measurement of
distances from 0.1 to 10 inches for manufacturing applications.
BACKGROUND
In the manufacturing environment, the traditional method of measuring
dimensions is with mechanical gauges. Linear distances are measured with
rulers, calipers, metal tape or precision mechanical stages. These methods
involve mechanical contact with the surfaces, and can be very slow because
of the manual adjustments required. Further, many types of objects such as
rotating shafts, spindles and wheels cannot be conveniently measured
mechanically while they are in motion.
Electronic alternatives to mechanical gauges are usually based on
capacitance or eddy currents. These devices have the advantage of a direct
measurement of a dimension without manual comparison with a standard. This
direct measurement capability increases the speed of measurement and
reduces errors due to distortion when a reference piece is contacted with
the test piece. However, electronic gauges can be material dependent, and
usually do not work at all with non-metallic materials such as fiberglass,
plastics and some kinds of composite materials. Also, their operational
range may be very limited.
Because of the recognized limitations of mechanical gauges, alternative
optical methods involving time-off flight laser radar are of interest. The
following articles discuss various examples of related prior art in laser
techniques for distance measurement.
In an article entitled "Laser Radar Range Imaging Sensor For Commercial
Applications" by K. G. Wesolowicz and Robert E. Sampson, SPIE Proc. Vol.
783, p. 152 (1987), there is described an imaging laser radar system
employing a single frequency (0.72 GHz) intensity modulation of a GaAlAs
laser diode operating at 0.82 .mu.m. The target range x is obtained from
the phase delay of the modulation. Since the phase has an implicit 2.pi.
ambiguity, the range measurement has a corresponding ambiguity interval.
For a modulation frequency v of 0.72 GHz, this interval is 8.2 inches. The
article claims a resolution for this radar of between 0.032 and 0.004
inches at 3 feet, depending on the type of target, the measurement time,
and the application. It is not clear that the absolute accuracy of the
instrument is also 0.004 inch, since this would appear to require 40 ppm
linearity in the phase measurement.
In an article entitled "Laser-diode Distance Meter in a KERN DKM 3A
Theodolite" by A. Greve and W. Harth, Applied Optics, Vol. 23, No. 17, p.
2982 (1984), there is described an intensity-modulated laser radar that
uses a phase locking technique to measure the relative phase .phi.. The
modulation frequency is in the 1.745-1.8 GHz range. The ambiguity in the
range measurement, at least in principle, can be removed by varying the
modulation frequency. In Table II of the article, a distance measurement
variation of 85 .mu.m at 2.9 m is claimed. FIG. 3 of the article shows
large distortions in the measurement curves that imply a much lower
absolute accuracy.
In an article entitled "High-Precision Fiber-Optic Position Sensing Using
Diode Laser Radar Techniques" by G. Abbas, W. R. Babbitt, M. de la
Chapelle, M. Fleshner, J. D. McClure, and E. Vertatschitsch, there is
described a linear position sensor with fiber-optic signal distribution.
The sensor uses a frequency-chirped, intensity-modulated laser diode. The
intensity-modulation bandwidth is 6 GHz. Absolute distance is obtained by
determining the beat frequency between the laser modulation and the
delayed modulation of the return signal. The beat frequency is found by
high-speed digital Fourier transforming of the beat signal. This approach
has the important advantage that several sensor heads may be connected by
fiber optics to the same source and detection module, provided that the
possible variations in range to each of the heads do not overlap. The
experimental system described in the article achieves 58 .mu.m RMS range
error over 100 cm using a 1 ms chirp duration and a signal processing time
of 50 .mu.sec. A resolution of 10 .mu.m is projected for an improved
version of this system. Although the achievements and specifications of
this instrument are consistent with some of the objectives of the present
invention, the system uses a highly cooperative target (a retroreflector)
and expensive radio-frequency hardware. Modification of this system for
non-cooperative surfaces in manufacturing may not be practical or cost
effective.
In an article entitled "Utilizing GaAlAs Laser Diodes as a Source (sic) for
Frequency Modulated Continuous Wave (FMCW) Coherent Laser Radars" by A.
Slotwinski, F. Goodwin and D. Simonson, SPIE Vol. 1043, p. 245 (1989),
there is described an instrument that uses optical interferometry to
generate beat signals between local and time-delayed optical frequencies.
The frequency modulation is achieved by thermal tuning of a laser diode
cavity length. The thermal tuning is easily effected by precisely
controlled variation of the laser excitation current and is thus much
easier to obtain over large bandwidths (>5 GHz) than an
intensity-modulation chirp. The article claims a resolution of 1 mil (25
.mu.m) by using a reference length for continuous calibration. However,
high accuracy and reliability can only be obtained with carefully
characterized and monitored single-mode laser diodes. The commercial
system is also very expensive and may be sensitive to vibration.
Laser interferometers are widely employed for high-precision displacement
measurement and very-high resolution surface profilometry. An example of a
commercial instrument in use is the well-known Hewlett Packard Laser
Gauge. However, a problem with these instruments relates to the
interference phase ambiguity. Interferometry with a single, constant
wavelength cannot be used to measure a distance without ambiguity of
one-half of one wavelength. Thus, the beam cannot be broken and only
highly-reflective, "cooperative" targets such as mirrors and
retroreflectors can be used. This seriously limits the applicability of
interferometry for measurement tasks in manufacturing.
One known method to extend the range of metrology applications for
interferometry is to measure the interferometric phase at two or more
distinct wavelengths. This is the method that most closely relates to the
present invention. The difference between the interferometric phase
measurements at two vacuum wavelengths .lambda..sub.1 and .lambda..sub.2
corresponds to a synthetic wavelength .LAMBDA. given by
1/.LAMBDA.=1/.lambda..sub.1 -1/.lambda..sub.2 ( 1)
If the measured interferometric phase is .phi..sub.1 and .phi..sub.2 for
the two wavelengths, then the distance x can be measured to within an
interval .LAMBDA. by using
x=.LAMBDA.(.phi..sub.1 -.phi..sub.2)/4.pi. (2)
In that the synthetic wavelength may be large compared to visible-light
wavelengths, it is possible to measure larger distances before phase
ambiguities contribute to measurement errors. If several synthetic
wavelengths of different size are used, it is possible to make
measurements of successively higher precision to "ladder down" to high
accuracy without the inconvenience of phase ambiguities. One of the
advantages of this approach over the intensity-modulation techniques
described above is that the synthetic wavelength can be made very much
smaller and the precision proportionally better than is practical with
direct-detection techniques.
An article entitled "Absolute distance interferometry, "by N. A. Massie and
H. John Caulfield, SPIE Proceedings, Vol. 816, pp. 149-157 (1987)
summarizes the prior art for this type of multiple-wavelength laser
ranging technology. The basic principles are described, and several
implementations described in other journal articles are presented. Most of
these examples involve complex and expensive gas or tunable dye lasers for
generating multiple wavelengths. In most cases, no more than two
wavelengths are obtainable at any one time from the source, thus
increasing the complexity by requiring time-multiplexing and automated
laser tuning.
In an article entitled "Absolute optical ranging with 200-nm resolution" by
C. Williams and H. Wickramasinge, Optics Letters, Vol. 14, pp. 542-544
(1989), there is described a two-wavelength interferometer requiring the
use of two independently-controlled and aligned GaAlAs single-mode laser
diodes. This system has the advantage that it uses relatively inexpensive
and compact laser diodes. However, the data shows a very small
demonstrated operational range (less than 1 mm) and a complex and
expensive system of acousto-optic modulators for time-multiplexing the
signals for the two wavelengths was used. Changing the synthetic
wavelength required modifying the operating conditions (changing the
temperature and excitation levels) of the lasers.
In an article entitled "Two-wavelength scanning spot interferometer using
single-frequency diode laser" by A. J. den Boef, Appl. Opt., Vol. 27, pp.
306-311 (1988), there is described the use of two single-frequency laser
diodes operating simultaneously, with the wavelength separation achieved
by polarization. Only one synthetic wavelength is available at a given
time.
In an article entitled "Laser diode technologies for in-process metrology,"
by P. de Groot, SPIE Proc., Vol. 1333, Paper 21 (San Diego, July, 1990),
there is described a two-wavelength interferometer using a single,
multiple-wavelength laser diode. The laser used was a conventional
two-wall Fabry-Perot device exhibiting multiple longitudinal oscillation
modes over a spectral width of about 1.2 nm. The interferometric phase for
each of the wavelengths was detected with a wavelength-selective detection
system involving a diffraction grating. Only one synthetic wavelength (700
.mu.m) was used because of the limited spectral width of the source.
Further, the multimode diode had poor temporal coherence and could not be
used for interferometry for optical path lengths exceeding 1 mm.
In an article entitled, "Interferometric displacement sensing by visibility
modulation," by T. A. Berkoff and A. D. Kersey, OFS '89, 78-82 (Springer
Verlag, 1989), there is described a fiber interferometer in which an
integrated-optic intensity modulator is used to generate a
carrier-suppressed, synthesized two-wavelength source with frequency
separation between 10 and 100 MHz. Two kinds of measurements are
described. One is the determination of distance by measuring the
frequencies for which the fringe visibility is nulled. The other is
displacement sensing by locking onto one of these fringe-visibility
minima. Since the distance measurement method described in this article
requires a variation frequency separation (90 MHz) that is comparable to
the largest attainable separation (100 MHz), there is little advantage of
this approach over the FMCW method described by Slotwinski et al. cited
herein. Also, because the 100 MHz frequency separation of the synthesized
modes is relatively small, the resulting resolution is only .+-.2 mm.
The above cited prior art does not adequately meet the simultaneous
requirements of accuracy, operational range, cost and low power for the
measurement applications of interest to this disclosure.
SUMMARY OF THE INVENTION
The foregoing and other problems are overcome by method and apparatus for
optical measurement of distance using optical interferometry with a
continuously tuned two-wavelength laser source. The detection method is
based on analysis of a fringe-contrast curve, but does not require that
the frequency separation to be variable over a range comparable to the
largest possible separation. Since it is generally much easier technically
to have two simultaneous emissions that are widely separated in optical
frequency than it is to vary an emission frequency continuously over a
wide range, the invention does not require any costly advance in optical
component technology in order to achieve very high absolute accuracy, even
with very weakly reflecting targets.
The technique employed by the instant invention will be referred to as
Chirped Synthetic Wavelength (CSW) laser radar. An embodiment of the
present invention consists of a coherent optical source comprised of two
single-frequency lasers with a tunable frequency separation, a compact
probe or sensor head, a compensating reference interferometer for
continuously measuring the relative frequency of the two lasers, and
detection and signal processing means for extracting distance information.
In accordance with a method of the present invention, and an apparatus for
accomplishing same, a first step transmits the combined emissions from the
two lasers to the probe through an optical fiber, which serves to direct
the light onto a target. A second step mixes the reflected light from the
surfaces of the target and the reflections from the fiber endface and
returns the light through the fiber back to the source. The relative phase
of the resultant interference pattern is modulated by opto-mechanical
means incorporated into the probe, resulting in a periodic signal. A
further step rectifies and filters the interference signal, resulting in a
voltage proportional to the contrast of the interference fringes that
produce the interference signal. During data acquisition, the frequency
separation of the lasers is linearly varied or "chirped," resulting in a
variation in the fringe contrast. In a final step, a computer analyzes the
fringe contrast variation as a function of time and obtains the distance
from the fiber endface to the target.
It is another object of the invention to measure distances to an accuracy
of 0.001 inch, for a variety of materials including those that are not
measurable by electronic means, over a range of 0.1 to 10 inches, without
mechanically contacting the surfaces involved. For reasons of eye safety,
the optical power should be limited to 100 .mu.W at the measurement probe.
It is further an object of this invention that the technology used not be
intrinsically expensive to implement.
It is another object of the invention to provide a method and apparatus
with the following advantageous characteristics: high accuracy (<1 mil)
due to the dual wavelength source, no ambiguity interval, reduced
vibration sensitivity compared to conventional interferometers,
accommodates moving targets such as rotating shafts and wheels, no
component of the preferred embodiment is intrinsically expensive,
cooperative targets such as retroreflectors are not required, and works
over 10 inch range with narrow (0.1 inch), low power (0.1 mW) beam.
BRIEF DESCRIPTION OF THE DRAWINGS
The features set forth above and other features of the invention are made
more apparent in the ensuing Detailed Description of the Invention when
read in conjunction with the attached Drawings, wherein:
FIG. 1 is a schematic diagram showing a preferred embodiment of the
invention;
FIG. 2 is a block diagram of the electronics used in to drive the laser
diodes in the preferred embodiment of the invention;
FIG. 3 is a block diagram of the electronics used to analyze the
interference patterns in the preferred embodiment of the invention;
FIG. 4 is a graph depicting the theoretical fringe contrast x=2,098 inch
(53.29 mm);
FIG. 5 is a graph depicting the experimental fringe contrast, same
parameters as FIG. 4;
FIG. 6 is a graph depicting the absolute ranging error as a function of the
position of a motorized stage equipped with an optical encoder; and
FIG. 7 is a graph depicting the measured profile of the Lincoln memorial
relief found on the back of a penny.
DETAILED DESCRIPTION OF THE INVENTION
Referring to FIG. 1, a measurement system constructed and operated in
accordance with a preferred embodiment of the present invention is shown.
The system is composed of a two-frequency laser source, a reference
interferometer and a fiber-coupled target interferometer. It is designed
for an operating range of 0.1 to 10 inches (2.5 to 250 mm), with a typical
range resolution of 1 mil (25 .mu.m) RMS for unpolished metallic targets.
As shown in FIG.1, the beams from lasers LD1 and LD2 are collimated by
optical elements 100, 102, combined by a prism 104 and sent through an
isolator 106 to protect them from optical feedback. The combined beam
passes through beam combiner 108, which may be a prism, to fiber optic
coupler 110 where the beam enters a single-mode fiber 112 and is sent to
the target 114 in the form of a 0.01-inch (250 .mu.m) diameter collimated
beam. The small beam size is obtained with a 0.18 pitch graded-index
(GRIN) lens 113. The fold mirror 115 delivers the combined beam onto
target 114. The interference signal is obtained by combining the light
scattered from the target surface with the natural 4% Fresnel reflection
from the end of an optical fiber. The light returning through the fiber is
directed onto the detector PD1 by beam combiner 108.
The unwanted reflection at the source end of the fiber is deflected by
cleaving the fiber at an angle. Approximately 100 .mu.W of laser light is
directed towards the target. The target detector PD1 receives 0.2 .mu.W of
light reflected from the fiber endface and less than 1 nW of the light
scattered from the target.
The phase modulation carrier signal for modulating the relative phase of
the interference pattern is generated by a piezo-electric transducer
(PZT1), which oscillates fold mirror 115 at 1 kHz with an amplitude of
0.05 mils (1.25 .mu.m). This allows the interference signal to pass
through two cycles of constructive and destructive interference. Thus, a
signal at the detector PD1 is generated with an AC amplitude proportional
to fringe visibility.
An additional branch may be provided in the configuration in FIG. 1 for
employing a compensating reference interferometer. Laser diodes are
extremely sensitive to small variations in temperature (30 GHz/.degree.C.
typical) and the thermoelectric coolers in the diode mounts are incapable
of maintaining the required temperature stability. The reference Michelson
interferometer 118, shown in FIG. 1, is included to monitor the drift in
synthetic wavelength over time. The paths of the interferometer are fixed.
Beam combiner 108 can serve to supply a portion of the combined beam to
the reference interferometer. The interference pattern in the compensating
reference interferometer is phase modulated by transducer PZT2, in the
same manner as the target interference pattern is phase modulated by PZT1.
The resulting interference pattern output by 118 is sensed by reference
detector PD2.
The linear translation stage 116 is used to calibrate the system and
provides the exact distance for the measured distance to be compared
against in evaluating the system. In actual operation, this stage would
not be present.
The following theoretical description of signal generation and analysis for
CSW laser radar will be useful in understanding this embodiment.
The round-trip optical path length z=2x is defined relative to an arbitrary
point of origin in the target beam for which the interferometric phase is
zero. This path length z is equal to twice the physical distance to the
object times the phase-velocity refractive index of the medium. In an
ideal, intensity-balanced interferometer having a two-color source with
vacuum emission wavenumbers k.sub.1 =2.pi./.lambda..sub.1 and k.sub.2
=2.pi./.lambda..sub.2 there are two simultaneous interference patterns
I.sub.1 =j.sub.1 (1+cos (k.sub.1 z)) (3a)
I.sub.2 =j.sub.2 (1+cos(k.sub.2 z)) (3b)
These patterns add together by incoherent superposition, resulting in a new
pattern
I=1+V(z,.beta.).multidot.cos (kz) (4)
where k is the average wavenumber and the coefficient
V(Z,.beta.)=cos (.beta.Z) (5)
is characterized by the synthetic wavenumber .beta.=2.pi./.LAMBDA.. The
distance Z is twice the physical distance to the target times the
group-velocity refractive index. For relative distances measured in air,
the difference between Z and z is very small and will be neglected.
A mechanical transducer modulates the distance slightly to generate a
signal at the detector with an AC amplitude proportional to the fringe
visibility .vertline.V(z,.beta.).vertline.. The synthetic-wavelength chirp
is achieved by changing .beta. an amount .gamma. over a period T. The
tuning is done linearly with respect to time t, so that
.beta.(t)=(t/T).gamma.+.beta.(0) (6)
The phase of the visibility curve can now be expressed as a time-dependent
function having a "chirp beat frequency," in cycles per period T, of
f=z.gamma./2.pi. (7)
and a synthetic phase offset
.PHI.(0)=z.beta.(0) (8)
These frequency and phase parameters are extracted from the visibility
curve by Fourier Transform analysis, counting fringes as a function of
time, or some equivalent method of frequency and phase detection. The
phase .PHI.(0) represents the synthetic phase for the synthetic wavelength
.LAMBDA.(0) corresponding to .beta.(0).
As is the case with all two-wavelength interferometers, the synthetic phase
offset .PHI.(0) has a 2.pi. ambiguity associated with it. This difficulty
is made explicit by
.PHI.(0)= +2.pi.m.sub.0 (9)
where 0< <2.pi. is the relative phase obtained directly from the
fringe-visibility data, and m.sub.o is an integer expressing the 2.pi.
phase ambiguity. This problem is solved in chirped synthetic wavelength
interferometry by making use of frequency f. Solving Eq.(7) for z and
substituting the result into Eq.(8), we obtain the estimate
.PHI.'(0)-(2.pi.f/.gamma.).beta.(0) (10)
of the synthetic phase. Remembering that m.sub.0 is by definition an
integer
m.sub.0 =Int{1/2.pi.(.PHI.'(0)' ){ (11)
where the function Int{} is equal to the nearest integer to its argument.
The phase .PHI.(0) no longer has a 2.pi. ambiguity and can be used to
calculate the absolute optical path difference:
z=.LAMBDA.(0).PHI.(0)/2.pi. (12)
The ability to determine the synthetic phase without ambiguity, using a
two-wavelength source with limited tuning bandwidth, is a significant
advantage of the invention.
Continuous calibration may be provided by a separate compensating reference
interferometer having a fixed optical path difference z.sub.ref. The
corresponding phase .PHI.(0).sub.rcf is used in an inverted form of
Eq.(12):
.LAMBDA.(0)=2.pi.z.sub.ref /.PHI.(0).sub.ref (13)
This calculation assumes that .PHI.(0).sub.ref does not vary by more than
.+-..pi. from some initial estimate that is made when the instrument is
first turned on. A similar calculation for the synthetic wavenumber chirp
excursion .gamma. for use in Eq.(10) can also be made from a value
f.sub.ref for the chirp beat frequency in the reference interferometer and
an inverted form of Eq.(7):
.gamma.=2.pi.f.sub.ref /Z.sub.ref (14)
These calibrations make the measurement independent of the frequency drifts
that are characteristic of laser diode sources.
The two-color source in FIG. 1 may be, for example, a pair of Sharp LTO80
wavelength-stabilized laser diodes, operating at 26.degree. C. and DC
biased at 45 mA for an average power output of 2.5 ms. The average
wavelength is 780 nm and the frequency separation at t=0 is approximately
190 GHz, corresponding to an equivalent or synthetic wavelength of 60 mils
(1.5 mm). A chirp bandwidth of 32 GHz is obtained by simultaneously
current tuning both lasers over a period T=190 ms in opposite directions.
Trigger 208 ensures the simultaneous operation of the chirp generators
204, 206. A 3 mA current modulation, supplied by the chirp generators 204,
206, is superimposed on the DC bias, supplied by the DC power sources 200,
202, as shown in FIG. 2. The current modulation introduces a 2 ms
variation in power output of each of the diodes; however, the power
variations are complementary, so that the total power output is constant.
The coherence length of the source is approximately 30 inches (about 1
meter). Noise that would otherwise be generated by interference of the
return signal with accidental reflections in the apparatus is suppressed
by using an optical fiber 112 that is long (>1 meter) compared to the
coherence length of the lasers.
Referring to the block diagram in FIG. 3, the interferometric signal
received by PD1 is transmitted through high-pass filter 302 with a cutoff
frequency of 800 Hz, then rectified and transmitted through low-pass
filter 306 with a cutoff frequency of 500 Hz to yield the fringe-contrast
curve. Although the interferometer used for the target signal is in
principle a multiple-beam Fabry-Perot interferometer, its finesse is so
low that the two-beam analysis for the fringe contrast function may still
be used. This is shown in FIG. 4 and FIG. 5, where the theoretical and
experimental visibility curves are compared for a distance x of 2.10
inches (51.3 mm). The effect of the varying relative power output of the
diodes manifests itself as a change in modulation depth of the visibility
curve as a function of time, as is evident from the figures. To the first
order, this relative power variation does not effect the phase or
frequency of the modulation.
The system is controlled by a computer 308, shown in FIG. 3, that
simultaneously acquires 2048 points of fringe-contrast data for both the
target and reference interferometers during the T=190 ms chirp period. The
values for .PHI.(0) and f in the experimental apparatus are obtained from
the fringe contrast data by a least-squares linear fit to a plot having as
an ordinate, the phase .PHI. in units of 2.pi., and as an abscissa, the
positions of the minima in the fringe contrast curve in units of time t
normalized to the chirp period T. Alternative signal processing means rely
on the well-known Digital Fourier Transform algorithm to extract phase and
frequency. The system is simultaneously calibrated using the reference
length z.sub.ref, Eqs. (13),(14) and the corresponding phase and frequency
information .PHI.(0).sub.ref, f.sub.ref. Eqs.(10) through (12) are then
used to calculate the target distance z.
The accuracy of the instrument can be determined by comparing CSW range
measurements to the optical encoder readings on a precision translation
stage. The results in FIG. 6 for a data acquisition rate of 5 Hz show an
RMS absolute error of 5 .mu.m over a 150 mm displacement. It is also
possible to profile objects with non-specular surfaces. The curve in FIG.7
is a scan of the Lincoln memorial that appears on the back side of a
penny, obtained at a distance of 2.5 inches from the probe. It is
noteworthy that this profile is obtained without focusing the probe beam.
Like all optical heterodyne laser radars, the present system has the
advantage of high sensitivity with low light levels and common detectors.
However, since the CSW laser radar is a coherent detection device, the
signal strength depends greatly on the particular characteristics of the
target surface and the resultant speckle pattern produced when the sur | | |
