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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a measuring and testing method and device
using light interference and, in particular, light interference by light
beams of different frequencies, i.e., heterodyne detection techniques.
2. Discussion of Related Art
Measurement and Testing Interferometry
It is well known that optical interferometry plays a vital and useful role
in scientific and industrial applications. Over the years, optical
interferometry has been used for a wide variety of applications that
include measurements of material thickness and changes in thickness,
biomedical uses such as real-time DNA detection, surface structure
characterization, gas flow and plasma temperature measurement, particle
velocity measurement, electric and magnetic field sensing, rotation and
stress measurements, magnetic force microscopes, optical scanning
microscopes for non-destructive testing of integrated electronic and
optical circuits, optical tweezers for micromanipulation, and a host of
other applications, such as disclosed in, e.g., P. Hariharan, Handbook of
Optics, Vol. II, 2nd Edition, M. Bass Editor in Chief, McGraw Hill, New
York, Ch. 21, Section 21.1 to 21.25, 1995, and Springer Series in Optical
Sciences, P. K. Rastogi, Editor, 68, pp. 1-6, Springer-Verlag, New York
(1994).
One basic phase measurement method is called the quasi-heterodyne
phase-step method where the local intensity of the interference pattern is
sampled at fixed phase steps, such as disclosed in J. Schwider, Progress
in Optics, North Holland, Amsterdam, 28, 273 (1980). This method allows
only modulo-2 .pi. interference measurements and, to get a complete phase
map, the continuity of the phase function must be assumed. Further, well
known phase interpolation techniques, such as disclosed in K. Creath,
Springer Series in Optical Sciences, 68, 5, P. K. Rastogi, Editor, pp.
109-148, Springer-Verlag, New York (1994), must be used. This method
typically offers as measurement accuracy one (1) percent of a fringe
interference phase measurement accuracy.
An interferometer that offers higher accuracy, i.e., better than 1/1000 of
a fringe interference phase measurement, and also avoids both the phase
interpolation problem (associated with the quasi-heterodyne methods) and
the sign ambiguity of classical interferometry, is the heterodyne
interferometer, such as discussed in J. Schwider, Progress in Optics,
North Holland, Amsterdam, 28, 273 (1980); J. H. Bruning et at., Applied
Optics, 13, 2693 (1974); and R. Dandliker, Progress in Optics, North
Holland, Amsterdam, 17, 1 (1980).
In this interferometer, a high speed photo-detector generates an electrical
signal via heterodyne detection of the interfering signal and reference
optical beams that have slightly different (e.g., by 1 MHz) optical
frequencies. The phase of this heterodyne detected electrical signal
relative to a stable, external electrical signal determines the measured
local optical phase of the test medium by an electronic phase meter which
is typically used to measure the phase difference between the two
electrical signals, such as disclosed in Mastner, V. Masek, Rev. Sci.
Instrum., Vol. 51, (1980) p. 926. Using this localized phase information
and mechanically scanning the test medium by the mechanical motion of a
detector, the overall phase distribution of the test medium can be
reconstructed for test and evaluation purposes.
One such use of the heterodyne interference method was implemented for
strain measurements via a two reference beam, holographic interferometric
set-up, as disclosed in R. Dandliker, B. Eliasson, Exp. Mech., 19, 93
(1979); and R. Thalmann and R. Dandliker, Applied Optics, 26, 1964 (1987).
Here, a frequency difference of 100 KHz between the two reference beams
during the reconstruction process was generated using two acousto-optic
modulators (AOMs) set for opposite Doppler shifts, with one AOM driven by
40 MHz and the other by 40.1 MHz. The phase differences between the two
100 KHz beat signals generated at the output photo-detector pair are
measured using a zero-crossing electronic phase-meter which interpolates
the phase angle to 0.1.degree. and also counts the multiples of
360.degree. .
One important conclusion of this heterodyne experiment was that the
heterodyne fringe interpolation technique did not restrict the phase
measurement accuracy. In fact, it was mainly the air turbulence and
mechanical hologram repositioning that limited instrument performance.
Another heterodyne interferometer measurement instrument was tested
recently using phase-locked PZT-tunable diode-pumped ND:YAG lasers and
acousto-optic (AO) devices, as disclosed in E. Gelmini, et al., Rev. Sci.
Instrum., 66, 8, 4073 (1995).
A common theme with nearly all heterodyne interferometers is the use of the
Doppler shifting property of AO devices to generate the color shifts in
the optical beams used in the interferometry. These AO device-based
interferometers are constructed using several mirrors, beam splitters,
beam combiners, and possibly a host of other optical and mechanical
components laid out over a large test area (e.g., 1 m.sup.2). Furthermore,
mechanical motion of mirrors is typically used for scanning the optical
beams used in the interference process for gathering phase data for a
given test area.
Because each component of an interferometer is a possible source of
unwanted phase noise (e.g., through mechanical vibration of a mirror), in
general, these conventional heterodyne optical interferometers have to be
built on costly air-isolation optical tables with special thermal and
mechanical vibration protection. Even in systems that do not appreciably
suffer from these problems, there is still a need for a high optical
power, high speed scanning interferometer system providing accurate
diagnostic measurements.
Signal Processing Interferometry
Two significant Bragg cell-based optical interferometers have been
developed mainly for such optical signal processing applications. These
are the Mach-Zehnder Acousto-Optic (AO) interferometer, and the in-line
Koester prism AO interferometer, such as disclosed in A. Vander Lugt,
"Interferometric Spectrum Analyzer," Applied Optics, Vol. 20, No. 16,
(1981) pp. 2770-2779, and M.D. Koontz, "Miniature Interferometric Spectrum
Analyzer," Optical Information Processing II, Proc. Soc. Photo-Opt.
Instrum. Eng. 639, (1986) pp. 126-130. Although the in-line Koester prism
design offers much improved mechanical and vibrational stability than the
Mach-Zehnder AO design, there still remains key sources of optical phase
instabilities due to the use of independent Koester prism components for
the optical beam splitting and beam combining operations. Depending on the
application requirements, one optical interferometer might be preferred
over the other but both suffer from mechanical instabilities which
influence phase measurements.
The Riza Interferometer for Signal Processing
Over the last several years, the present inventor has developed and
experimentally demonstrated a compact, heterodyne and baseband-type,
optical interferometer architecture for a host of photonic information
processing applications such as phased array antenna/radar control (N. A.
Riza, Ph.D Thesis, California Inst. of Tech., Pasadena, U.S.A., Oct.,
(1989); N. A. Riza, IEEE Photonics Tech. Lett., Vol. 4, No. 2, 177-179
(Feb. 1992); N. A. Riza, IEEE Photonics Tech. Lett., Vol. 4, No. 9,
1073-1075 (Sept. 1992); N. A. Riza, IEEE/OSA J. of Lightwave Tech., Vol.
10, No. 12, 1974-1984 (Dec. 1992); and N. A. Riza, Applied Optics, Vol.
33, No. 17, 3712-3724 (June 1994)) and radio frequency (rf) signal
correlation (N. A. Riza, Applied Optics, Vol. 33, No. 14, 3060-3069 (May
1994)), convolution (N. A. Riza, IEEE Photonics Tech. Lett., Vol. 7, No.
3, 339-341 (March 1995)), notch filtering (N. A. Riza, SPIE Proc. 2155,
413-419 (1994)), and spectrum analysis operations (N. A. Riza, Applied
Optics, Vol. 31, No. 17, 3194-3196 (June 1992)).
This basic interferometric architecture is shown in FIG. 1, and consists of
two Acousto-Optic (AO) devices such as Bragg cells 11 and 12 in an in-line
configuration, where the first Bragg cell 11 acts as an optical beam
splitter and the second Bragg cell 12 acts as an optical beam combiner.
Thus, using only four optical components (the two Bragg cells 11 and 12
and two spherical lenses 13.sub.1 and 13.sub.2) all in the path of the
interfering optical beams, a compact, low component count interferometer
10 is realized. This heterodyne/baseband interferometer 10 has an
important property that is desirable for all optical interferometers;
namely, excellent mechanical stability and tolerance to optical phase
instabilities via the almost common-path in-line design.
This interferometer 10 is collinear, except between the two Bragg cells
where the two interfering beams are physically separated, although still
in-line and in close proximity (e.g., within 1 cm). Thus, any thermal,
mechanical, or air turbulence affects impinging on this instrument have
almost the same affect on both interfering beams and therefore on an
output photo detector 15, such as a high speed photo diode, CCD baseband
sensor or other suitable form of photosensor. In fact, the heterodyne
detection operation via optical mixing at the photo detector 15 results in
the cancellation of this type of phase noise.
In operation, the interferometer 10 shown in FIG. 1 receives light from an
input laser (not shown), which is Bragg matched to the first Bragg cell
11. The first Bragg cell 11 is controlled by a radio-frequency (rf) signal
r(t) centered at a central frequency f.sub.c of the Bragg cell's operating
range. For the low diffraction efficiencies (e.g., <10%) needed for
optimal linear AO signal modulation, the first Bragg cell 11 produces a
strong undiffracted DC (i.e., unmodulated) beam and a weaker, deflected,
positive Doppler shifted, +1 order diffracted beam which has been
frequency shifted by the input rf signal r(t). Thus, the first Bragg cell
11 creates the two beams used in this heterodyne interferometer 10.
Non-magnifying (1:1) imaging optics consisting of two spherical lenses
13.sub.1 and 13.sub.2 or their equivalent are used to image the first
Bragg cell 11 onto an imaging plane of the second Bragg cell 12. The
imaging optics preserve the Bragg matching condition at the second Bragg
cell 12, the second Bragg cell 12 being fed by a rf signal s(t) centered
at a central operating frequency f.sub.c of the second Bragg cell 12,
which is the same as the first Bragg cell 11 in this example. The strong
DC beam from the first Bragg cell 11 generates, at the second Bragg cell
12, a weaker, deflected, negative Doppler shifted, -1 order diffracted
beam (or, optionally, positive -1 order diffracted beams, this option
being shown in FIG. 1 by a reverse oriented Bragg cell 12' in phantom).
After the second Bragg cell 12, the diffracted +1 and diffracted -1 (or
+1) order beams are collinear, meaning that the second Bragg cell 12 also
acts as a beam combiner for the interferometer 10.
A third spherical lens 13.sub.3 collimates the output beams of the second
Bragg cell 12 (or 12'). The strong DC beam from the second Bragg cell 12
(or 12') is not utilized in the signal processing and is blocked by a
spatial block 17, while the collinear +1 and -1 (or +1) order beams are
focused or imaged onto a high speed photo detector or detector array 15 by
a fourth spherical lens 13.sub.4.
Depending on the desired information processing application, the output
collinear +1 and -1 order beams (or +1 and +1 order beams) interfere with
one another and are heterodyne detected by an appropriately positioned
photo detector or detector array at a desired output plane and processed
by a variety of known means. Also, as shown in FIG. 1, the two-beam
interference can be optionally detected at either the Fourier plane of the
second Bragg cell 12 (or 12') by the photo detector 15', or the image
plane of the second Bragg cell 12 as shown in phantom by a photo detector
15". As an additional option, image inversion optics 19, such as a Dove
prism, can be inserted between the first and second spherical lenses
13.sub.1 and 13.sub.2 along the +1 order diffracted beam of the first
Bragg cell 11 for certain signal processing needs. As a further option, a
baseband sensor such as a CCD 15" can be used in place of the photo
detector 15 to detect a baseband signal.
The beat rf signal generated by the interference sensing photo detector 15
(or 15') is centered at a 2f.sub.c frequency carrier for the +1, -1 order
case, and is modulated by the required signal processing transform output
signal desired from the photonic processor. In the optional case using the
second Bragg cell 12' and the CCD 15", the +1, +1 orders interfere to
generate the desired baseband output signal.
Because a Bragg cell is an excellent device for introducing rf or wideband
(e.g., 50 Mhz to 1 GHz instantaneous bandwidth) electrical signals onto
the Bragg diffracted optical beam, it becomes possible to use this
interferometer to optically process a variety of electrical signals and,
in particular, the electromagnetic interference (EMI) sensitive microwave
or higher band electrical signals. Thus, the present inventor proposed and
experimentally demonstrated several versions of this interferometer shown
in FIG. 1 as various significant coherent signal processors as mentioned
above. As can be seen from a review of the articles cited above, the Riza
interferometer has been shown to be useful in a wide-variety of signal
processing applications, as opposed to measuring and testing applications.
It is important to note that the output of these Riza interferometers is
either the -1 or +1 order diffracted beams or the +1 and +1 (or -1 and -1)
diffracted order beam pairs, that interfere of the second Bragg cell's (12
or 12') Fourier or image plane. An important thing to note in the design
is that the DC beam is not detected and therefore not used in the various
signal processing applications.
As disclosed in R. G. Johnston and W. K. Grace, "Refractive index detector
using Zeeman interferometry," Applied Optics, Vol. 29, pp. 4720-4724,
1990, and U.S. Pat. No. 4,906,095 to R. G. Johnston entitled "Apparatus
and Method For Performing Two Frequency Interferometry", a heterodyne
interferometer can also be formed using the Zeeman effect laser that emits
two collinear laser lines with orthogonal polarizations. For example, the
Helium Neon Zeeman effect laser by Optra, Peabody, Mass., emits two laser
lines having a wavelength near .lambda.=632.8 nm, and differ only in
frequency by 250 KHz. Thus, the heterodyne detected signal generated by
this interferometer is at a 250 KHz electrical signal. The key point to
note about this heterodyne interferometer is that it is a non-scanning
interferometer, i.e., the test optical beam does not electronically scan
the sample material or test medium. Also, the heterodyne frequency is
fixed by the type of laser used, and is not tunable. The next paragraph
deals with scanning heterodyne interferometers.
Others have employed Bragg cells in such applications as the optical
scanning microscopes, such as disclosed in U.S. Pat. No. 4,627,730 to
Jungerman et at. In this microscope, coherent light at .lambda.=f.sub.O
impinges on a Bragg cell driven at a swept frequency f.sub.O (60-110 MHz).
A stationary reference beam (at .lambda.=f.sub.0) beam and a positive
doppler scanning beam (at .lambda.=f.sub.O +f.sub.b) impinge on a test
material and are reflected back through the Bragg cell. A negative doppler
frequency shifted diffracted portion of the returning reference beam (at
.lambda.=f.sub.O -f.sub.b) and the non-diffracted returning scanning
positive doppler beam (at .lambda.=f.sub.O +f.sub.b) are focused on a
detector, and circuitry selectively extracts phase and amplitude
information imparted by the test material to yield the height of its
surface features. A modification includes an internal optical reference in
the form of a second beam in a plane which is perpendicular to the scanned
output and impinges on a known flat surface. The Jungerman et at. patent
discloses an output light impinging on the photo detectors as a positive
doppler frequency (at .lambda.=f.sub.O +f.sub.b) scan beam and a negative
doppler frequency (at .lambda.=f.sub.O -f.sub.b) reference beam. An
important point to note is that when the Bragg cell frequency f.sub.b is
varied, the two output beams (i.e., .lambda.=f.sub.O +f.sub.b and
.lambda.=f.sub.O f.sub.b) also scan or move on the output photodiode
surface. Thus, when f.sub.b is changed, then the tiny output photodiode
must also be moved to track the scanned output pair beam. Thus, the
Jungerman device is not truly non-mechanical, as the output beams that
eventually heterodyne detect and generate the 2f.sub.b beat signal, are
physically moving at the output photodiode plane and causing optical loss
in the instrument.
Recently, M. S. Valera and A. N. Farley ("A High Performance Magnetic Force
Microscope," Measurement Sci. Tech., Vol. 7 (Jan. 1996), pp. 30-35) have
proposed a differential heterodyne optical interferometer for magnetic
force microscopy applications. Although the optical structure of this
heterodyne interferometer is based on a simple Bragg cell using a
reflective geometry, similar to Jungerman's patent, there is a key
difference between the output beams at the photodiode that generate the
heterodyne detected 2f.sub.B frequency signal. In the Valera and Farley
instrument, the output heterodyne signal at 2f.sub.B is generated by the
interference of the undiffracted zero doppler shifted light beam at a
f.sub.O light frequency, and the doubly diffracted twice positive doppler
shifted light beam at a f.sub.O +2f.sub.b light frequency. Valera and
Farley state that these two beams appear after double passage through the
Bragg cell, and appear on the optic axis. They call the f.sub.O frequency
beam the object beam and the diffracted f.sub.O +f.sub.B doppler shifted
and deflected beam the reference beam. Both beams are focussed and
incident on a reflective cantilever that vibrates at a f.sub.c frequency.
Valera and Farley state that "The spacing between these beams can be
adjusted by introducing additional lenses between the Bragg cell and the
objective lens." They also state that "Positioning and scanning of the
sample is undertaken by a monolithic flexure stage driven by piezoelectric
actuators" (in abstract of the paper). Thus, Valera and Farley use
mechanical methods to optically scan the sample, and do not suggest an
electronic, non-mechanical means for scanning optical beams on the test
material/target. In their instrument, the Bragg cell drive frequency
f.sub.B is fixed, and additional lenses and mechanical stage motion via
piezo-actuators is used to scan the test material for magnetic force
measurements. Thus, both optical beams on the cantilever are fixed and
stationary (i.e., non-scanning).
It would be extremely desirable to have an optical heterodyne
interferometer that has good phase/mechanical stability, plus has
non-mechanical optical beam scanning capability for rapid inspection and
evaluation of a test medium. Furthermore, it is desirable to have an
interferometer where the test beam rapidly scans the test medium, yet the
output light beams interfering at the output detector plane are fixed and
stationary to provide high heterodyne detection efficiency. A novel
optical heterodyne or baseband and intermediate frequency interferometer
is disclosed herein which results in the realization of a high speed
scanning optical interferometer with excellent mechanical stability and
phase noise suppression characteristics.
SUMMARY OF THE INVENTION
The present invention addresses slow mechanical beam scanning, output beam
motion, and vibration instability problems of conventional
interferometers, by introducing a new kind of heterodyne optical
interferometer that also has high speed, non-mechanical, inertialless beam
scanning capabilities, a stationary interfering output beam pair, plus has
a compact in-line design for minimizing air turbulence and other unstable
phase noise effects.
Specifically, according to a first embodiment, a reflective optical
interferometric scanner is provided including means for supplying coherent
light and means for splitting the coherent light into a first beam and a
second beam. The invention further includes a first acousto-optical device
having a first channel for selectively deflecting a first portion of the
first beam in accordance with a first frequency in a first direction and a
second channel for selectively deflecting a first portion of the second
beam in accordance with a second frequency in first direction, wherein the
difference between the first and second frequencies is fixed and a second
portion of the first and second beams are not deflected by the first
acousto-optical device. Additionally, the invention includes a second
acousto-optical device for deflecting a test beam, the test beam being
part of the first portion of the first beam, and a reference beam, the
reference beam being part of the second portion of the second beam, in a
second direction substantially perpendicular to the first direction. In
this embodiment, the test beam passes through an area of the test medium
in a two dimensional scanning pattern. A reflective element is positioned
to reflect the test beam and the reference beam back through the first and
the second acousto-optic devices. The invention further includes detector
means for detecting the test beam and for detecting reference beam, and
signal processing means for generating an intermediate frequency signal
from the test and reference beams, the intermediate frequency signal
bearing phase and amplitude information of the test medium.
In a second embodiment, a scanning spot heterodyne optical interferometer
includes means for providing a first coherent light beam and a second
coherent light beam having a frequency different from the first coherent
light beam. Light combining means combines the first and second light
beams into a collinear beam composed of two wavelengths. Further, first
means splits the collinear light beam into fixed beams unaffected by the
first means and test beams varying in spatial position in a first
direction in accordance with a signal input to the first means and
producing a frequency shift in the test beams relative to the fixed beams.
A first light deflector deflects the fixed and test beams in a second
direction perpendicular to the first direction. A test medium, onto which
the test beams impinge as it varies in spatial position in the first and
second directions perpendicular to an optical axis of the interferometer,
impartes a further frequency shift onto the test beams. A second means
recombines the fixed beams and the test beams from the first means and
providing the test beams with a further frequency shift relative to the
fixed beams, wherein the fixed beams and the test beams are collinear and
unmoving in at least the first direction. Means for detecting test medium
phase information from the test beam is also provided.
In yet another embodiment, a scanning heterodyne optical interferometer
includes means for providing a coherent light beam, and first means for
splitting the coherent light beam into a fixed beam unaffected by the
first means and a test beam varying in spatial position in accordance with
a signal input to the first means and producing a frequency shift in the
test beam relative to the fixed beam. The invention further includes a
test medium onto which a first part of the test beam impinges as it varies
in spatial position in a first direction perpendicular to an optical axis
of the interferometer, the test medium imparting a further frequency shift
onto the first part of the test beam but not a second part of the test
beam. The invention also includes a second means for recombining the fixed
beam from the first means and the test beam and providing the test beam
with a further frequency shift relative to the fixed beam, wherein the
fixed beam and the test beam are collinear and unmoving in at least the
first direction. Finally, means for detecting test medium phase
information from the first and second parts of the test beam is provided.
Yet another embodiment of the present invention is a scanning heterodyne
optical interferometer including means for providing a coherent light beam
and first means for splitting the coherent light beam into a fixed beam
unaffected by the first means and a first scanning beam shifting in
spatial position and frequency. This embodiment further includes second
means for splitting the first fixed beam into a second fixed beam
unaffected by either first or second means and a second scanning beam
shifting in spatial position and frequency, and for splitting the first
scanning beam into a third scanning beam shifting in spatial position and
a third fixed beam unaffected by the second means. The second and third
fixed beams are collinear and the second and third scanning beams are
collinear. Further, first and second polarizing beam splitters split
polarized light components of the collinear second and third fixed beams
and the second and third scanning beams. Components of each pass through a
test medium and other components of which act as a reference. Detecting
means for determining phase differences between respective components is
also provided.
Interferometers in accordance with the present invention can be used as a
variety of optical instruments such as holographic interferometers,
interferometric sensors, material characterization tools such as thin
film/surface characterization, diagnostic measurement systems such as
turbulence and flow/temperature assessment, holographic recording and
retrieval, shock wave measurements, material optical birefringence
measurements, free-space optical sensing such as wind tunnel, combustion,
and flame diagnostics and testing, fiber-optic remote sensing, | | |
