 |
Description  |
|
|
The invention relates generally to a method and apparatus for the optical
detection of transient motion in an object, and more specifically to the
noncontact detection of ultrasound during nondestructive testing.
BACKGROUND OF THE INVENTION
The generation and detection of ultrasonic acoustic waves is widely used
for imaging the interior of solid objects in nondestructive testing and
evaluation (NDE), as well as for dimension gauging and for measuring
material properties such as elastic constants. The most common devices
used for generation and detection of ultrasound are piezoelectric
transducers, which typically have very high sensitivity. These transducers
require direct contact with the object being tested or indirect contact
through a liquid column or a solid wedge. However, in many applications
completely noncontact ultrasonic inspection, where there is no physical
contact, either directly or indirectly, with the object being tested, is
desirable. Examples include inspection of objects which are at high
temperature, objects with curved surfaces, objects sensitive to
contamination, or where conditions require that fast scans be preformed.
Ultrasonic inspection can be performed without direct or indirect physical
contact using a pulsed laser that generates ultrasound at the surface of
the object to be inspected, which then propagates to the object's
interior, in combination with optical interferometric apparatus and
methods that detect the very small undulations created by reflected waves
reaching the surface of the object. This technique is generally called
laser-ultrasonics (LUT) or laser-based-ultrasound (LBU). A description of
several applications of LUT can be found in "Nondestructive evaluation
with laser ultrasound," Mechanical Engineering, Vol. 116, p.63, 1994.
However, because of the small amplitude of typical ultrasonic
displacements compared to the light wavelength, optical detection remains
a challenge. Thus, various techniques have been investigated and developed
for optical detection of ultrasound, as described, for example, by J. P.
Monchalin, "Optical detection of ultrasound," IEEE Transactions on
Ultrasonics, Ferroelectrics, and Frequency Control, Vol. UFFC-33, p. 485,
1986, and by J. W. Wagner, "Breaking the sensitivity barrier: the
challenge for laser ultrasonics," IEEE Ultrasonics Symposium,
1051-0117/92, p. 791, 1992.
Laser-ultrasonics instrumentation for use in the field requires an
interferometric system that is lightweight, compact, and environmentally
rugged. Conventional interferometers do not possess these characteristics
because they require the exact alignment of optical components on a rigid
and heavy optical bench. Assembling the interferometer with optical fibers
eliminates optical alignment problems and the need of an optical bench.
However, perturbations such as temperature changes and vibrations cause a
substantial change of the fiber delay and consequently most fiber-optic
interferometers are adversely susceptible to perturbations of this type.
The J. E. Bowers and G. S. Kino, U.S. Pat. No. 4,572,949, Feb. 25, 1986,
discloses prior art of a fiber-optic ultrasonic detector that avoids the
problems caused by temperature changes and vibrations by using a
configuration known in the art as a Sagnac interferometer. A detailed
explanation of its working principle is also found in the corresponding
patent document. FIG. 6, adapted from the referred patent, shows this
prior art. Unfortunately, in the Sagnac interferometer, one-half of the
light reaching the detector does not convey a signal useful for detecting
desired information, and therefore, that light increases the background
noise and limits the instrument sensitivity. Moreover, the response
obtained by the Sagnac interferometer is proportional to the square of the
ultrasonic signal amplitude, which is very small, unless a phase modulator
is inserted inside the loop. Inserting the phase modulator creates
harmonic distortion and consumes electrical power, and in addition the
modulator is expensive. An alternative embodiment disclosed in the
referred patent is to use a polarization controller inside the Sagnac loop
and take advantage of the fiber residual birefringence, but with this
approach the advantages of environmental insensitivity are lost because
the interferometer is sensitive to perturbations to the fiber
birefringence.
SUMMARY OF THE INVENTION
It is a purpose of this invention to disclose a device for laser detection
of ultrasound using a fiber-optic interferometer that is environmentally
rugged and particularly advantageous for use in the field. Elements of
this invention were disclosed at two conferences: J. Alcoz, "Stable
fiber-optic interferometer for ultrasonic detection using passive
components," 1996 Optical Society of America Annual Meeting (Paper
TuQQ52), Oct. 22, 1996, and in J. Alcoz, C. Duffer, and S. Nair,
"Noncontact detection of ultrasound with rugged fiber-optic
interferometer," IEES 1996 Ultrasonic Symposium (Paper B-3), Nov. 4, 1996.
The prior art in optical detection of ultrasound using fiber-optic
interferometers requires the use of one or more of the following
components as described, for example, in the cited review paper by
Monclialin: (1) temperature-stabilized high-coherence lasers (e.g.,
long-path-difference interferometers), (2) electronic feedback with fiber
stretchers or other optical-path correction (e.g., stabilized Michelson
and Mach-Zendlier interferometers), and (3) optical frequency or phase
modulation (e.g., heterodyne interferometers or the previously mentioned
Bowers' patent). In contrast, the present invention does not require any
of these components or methods. In particular, the fiber-optic
interferometer in the present invention remains passively locked at its
more sensitive operating point under wide temperature variations inside
the interferometer enclosure.
The present invention consists of a path-matched interferometer of the
Sagnac class. Light from a laser diode, or another monochromatic source of
arbitrary coherence, simultaneously travels both clockwise and
counterclockwise along an optical loop which includes the path of the
light illuminating the object being inspected, reflecting from that object
and then collected and coupled into the fiber. Means are provided to
assure that all the light reaching the detectors has completed the
closed-circuit Sagnac loop. Means are also provided to passively control
the polarization and phase of the light in the interferometer. The output
light is detected by two photodetectors working in a differential mode.
Ultrasonic vibrations from the object being inspected modulate the
amplitude of the detected signal.
The invention should not be viewed as restricted to sensing ultrasonic
vibrations on a surface without contacting the surface. It should be
obvious that the invention can be readily used on contact with the surface
of the object being inspected. It can also be readily used by embedding
the illuminating fiber in the object being inspected.
For the purposes of this disclosure, the phrase "illuminating the object"
refers to any means of directing light to the object where transient
motion is sensed. In the same manner, "reflected light" refers to any
light that propagates backwards from the object being inspected towards
the sensing apparatus, regardless of the physical origin of backscatter.
In its preferred embodiment, light from a laser diode is coupled with a
fiber directional coupler into two polarization-maintaining (PM) optical
fibers. Polarized light propagating along the slow mode in one fiber is
coupled by a polarizing beam-splitter into the slow mode of a third PM
fiber. A fiber polarization controller converts this mode into right
circular polarization that is focused with optical lenses on the monitored
area of the object being investigated. Part of the reflected or scattered
light is collected by the lens and coupled to the fiber, and the
polarization controller converts this light into the fast polarization
mode. The polarizing beam-splitter directs this light to the second PM
fiber, and the light returns to the first directional coupler after
completing the optical loop in the clockwise direction. Likewise, light
originally coupled to the fast mode of the second fiber travels
counterclockwise along the identical optical path.
A polarizing beamsplitter with its axes at 45 degrees with respect to the
axes of the loop fibers is used to combine the clockwise and
counterclockwise beams of light and to obtain two interference signals.
Because one fiber arm of the loop is much longer than the second fiber
arm, the area of the object being inspected is at an asymmetrical location
with respect to the loop center and ultrasonic displacements will generate
transient phase differences between the counter-propagating beams. The
static phase difference between these light beams is fixed at 90 degrees
by the phase-retardation produced by a polarization controller at the
input or output of the Sagnac loop. The two interference signals are
amplitude modulated by the ultrasound but are 180 degrees out of phase.
They are subtracted electronically to double the desired ultrasonic signal
while most noise sources, such as intensity noise of the laser, are
canceled.
Perturbations to the optical path, due for example to temperature changes
or mechanical vibrations, equally affect light propagating in both
directions in the path, as long as the perturbations are slow compared
with the transit time of the loop. Thus, they do not produce an output
signal, nor do they change the operating point of the interferometer. This
is a consequence of the perfect optical-path-match between the clockwise
and counterclockwise propagating light. It is pointed out that, although
the beams change their polarization state several times while traveling
along the optical path, at each specific point in this closed-circuit path
the polarization states of the counter-propagating modes coincide.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be described in further detail with reference to the
accompanying drawings, in which:
FIG. 1 is a schematic diagram of a preferred embodiment of the ultrasonic
detector utilizing PM optical fibers.
FIG. 2 is a schematic diagram of an embodiment of the ultrasonic detector
utilizing discrete optical components.
FIG. 3 is a schematic diagram of an alternative embodiment of the
ultrasonic detector using a combination of discrete optical components and
PM optical fibers.
FIG. 4 is a schematic diagram of an alternative embodiment of the
ultrasonic detector that does not use polarizing beam-splitters.
FIG. 5 is a schematic diagram of an alternative embodiment of the
ultrasonic detector having increased efficiency in its use of light.
FIG. 6 is a prior art schematic diagram of an ultrasonic detector using a
fiber-optic Sagnac interferometer.
DETAILED DESCRIPTION OF THE INVENTION
The principle of ultrasonic detection utilizing an optical bench is first
described in reference to FIG. 2. This embodiment, operating at ultrasonic
frequencies higher than those normally used in industrial NDE, is used to
clarify the explanation of its operation. The reason it is not practical
for use in industrial NDE is that a very long optical path (greater than
10 meters) is required at these ultrasonic frequencies. The operational
details of using optical fiber are described later in a preferred
embodiment.
The light from a linearly polarized laser 1 is converted to circular
polarization by a quarter-waveplate 2. The light components polarized
vertically and horizontally are 90 degrees out of phase. A 50/50
non-polarizing beam-splitter 3 splits the light into two equal-power
beams, the clockwise (c.w.) beam traveling towards the mirror 5 and the
counterclockwise (c.c.w.) beam traveling towards the polarizing
beam-splitter 4. The c.w. beam is reflected by mirrors 5 and 6, which
serve as an optical delay, and reaches the polarizing beam-splitter 4. The
horizontally polarized component of the c.w. beam is transmitted towards a
light dump 15 while the vertically polarized component is reflected
towards a second quarter-waveplate 7. The quarter-waveplate 7 has its axes
rotated 45 degrees with respect to the c.w. beam and converts this
vertically polarized light into right circular polarized light. This light
beam is focused by a lens 8 on the ultrasonically vibrating surface 9.
The reflected or scattered beam from surface 9 is left circular polarized
and part of it is collected by lens 8. This collected light is converted
by the quarter-waveplate 7 to horizontally polarized light which is
transmitted by the polarizing beam-splitter 4, without any light being
reflected towards mirror 6, completing the clockwise path of the loop.
In like maimer, the vertical component of the c.c.w. beam propagating from
the non-polarizing beam-splitter 3 towards the polarizing beam-splitter 4
is reflected towards the light dump 15 by the polarizing beam-splitter 4,
and the horizontally polarized beam is transmitted towards the
quarter-waveplate 7 which converts the polarization of this light to left
circular polarized light. This beam is focused by lens 8 on the
ultrasonically vibrating surface; it is reflected by the surface as right
circular polarized light; and finally it is converted to vertical
polarized light by waveplate 7. All of this beam is reflected by the
polarizing beam-splitter 4 and completes the c.c.w. path 6-5-3. It should
be noted that the path between beam-splitter 4 and the surface 9 is part
of the Sagnac loop, even though it is a linear path. This is so because
light travels towards the surface with polarization which is orthogonal to
the polarization it has upon returning from the surface, and the c.c.w.
light changes polarization modes in reverse order to that of the c.w.
light.
One-half of the power of the vertically polarized c.w. and the horizontally
polarized c.c.w. beams are sent towards a compensating waveplate 10 by the
non-polarizing beam-splitter 3. The purpose of waveplate 10 is described
in the following paragraphs. A second polarizing beam-splitter 11 at 45
degrees with respect to the c.w. and c.c.w. polarization axes combines
these two vertically and horizontally polarized beams producing two new
beams traveling towards photodetector devices 12 and 13. These beams are
the result of the interference of the counter-propagating modes, and thus
their amplitudes are modulated by the phase difference between the
interfering modes. This is well known in the art of interferometry. In the
absence of an ultrasonic signal, the c.w. and c.c.w. beams travel exactly
the same optical path inside the loop, without any relative phase shift
being introduced between them. However, the first quarter-waveplate 2
introduces outside the loop a 90 degrees static bias between the two beams
which is present until they interfere at the polarizing beam-splitter 11.
The output of the interferometer is a (vertically shifted) cosine function
of this phase difference, as in every single-path interferometer. Thus,
the 90 degrees static bias assures that the operating point of the
interferometer is fixed at the linear part of the response curve. In the
presence of ultrasonic vibrations, the surface 9 of the object being
inspected will move in the time increment between which it is reached by
the c.c.w. beam traveling first through the short arm 3-4-7-8-9 and the
corresponding portion of the c.w. beam traveling first through the long
arm 3-5-6-4-7-8-9. This will produce a dynamic phase shift between the two
interfering beams. When the time delay difference between these beams
traveling in the short and long arms is equal to half the period of the
ultrasonic center frequency, or an odd multiple of it, one beam will see a
peak of the wave when the corresponding portion of the other beam sees a
valley, producing the largest relative phase modulation.
In addition to the 90 degrees static phase-shift provided by the first
waveplate 2, the only other possible sources of static phase difference
are spurious birefringence retardation at the input and output glass of
beam-splitter 3 or the input glass of beam-splitter 11. These non-desired
shifts are compensated by a phase-retardation waveplate 10. Alternatively,
the 2 and 10 waveplates can be interchanged in their positions or be
placed together either at the output or at the input of the loop.
The output signal can be doubled and, at the same time, the contribution of
noise can be greatly diminished, by using a balanced detector. This takes
advantage of the fact that the outputs detected by the photodetectors are
complementary, i.e., when the power reaching the first detector increases
due to a change in phase difference, the power reclining the second
detector decreases. Balanced detectors are widely used in sensors based on
the interference of orthogonal polarizations. In this embodiment the
outputs of the photodetectors are subtracted electronically at 14 to
obtain the desired signal.
The preferred embodiment of the invention is shown in FIG. 1. The
embodiment disclosed in FIG. 1 and discussed in detail presents an
advantage with respect to the embodiment of FIG. 2.
In the embodiment shown in FIG. 1 light is guided by optical fibers and no
alignment of discrete components on an optical bench is necessary.
Furthermore, the long optical delay needed in one arm of the loop is
achieved by a compact coil of fiber.
The light from a laser diode 1 is coupled to a single mode fiber. A
fiber-polarization-controller (FPC) 2 is used to adjust the polarization
of the light injected at point 3 to the input polarization-maintaining
(PM) fiber of the interferometer. PM fiber has two possible modes of
propagation with polarized light aligned along orthogonal axes. These
modes are a slow and a fast propagating mode. This is a result of
birefringence introduced during manufacture. A 50/50 PM fiber
directional-coupler 4 splits the light from the laser diode 1 into two
equal power c.w. and c.c.w. beams. The c.w. light travels through a long
length of PM fiber 5, which results in a time delay, before reaching the
fiber polarizing-beam-splitter (PBS) 6. The c.w. light propagating along
the fast mode of the PM fiber 5 is coupled towards a light dump 8, while
the c.w. light polarized along the slow mode is transmitted towards a
second FPC 7. The c.w. light transmitted towards FPC 7 is focused by a
lens assembly 9 on the surface 10 of the object being inspected where
ultrasonic waves are present. Part of the reflected or backscattered light
is collected by the lens assembly 9 and coupled back into the fiber. The
FPC 7 is adjusted so that it works as a quarter-waveplate, thus converting
the c.w. linearly-polarized light (slow-mode) into right-circular
polarized light at its output and the collected left-circular polarized
light into light polarized along the PM fiber fast-mode. The PBS 6 directs
all of this light to the short arm of the loop, and it reaches the
directional coupler 4, completing the remaining portion of the c.w. path.
In like manner, light coupled by fiber-directional-coupler 4 to the short
arm of the loop is split at PBS 6 into the slow mode which is directed
towards the light dump 8, and the fast mode that travels towards the FPC
7, which converts it to left-circular polarized light, and is focused by
the lens 9 onto the surface 10 of the object being inspected. Part of the
reflected or backscattered light is collected by lens 9 and coupled back
into the fiber. The FPC 7 converts the collected c.c.w. light, which is
right-circular polarized, into light linearly polarized along the
slow-mode of the PM fiber. The PBS 6 couples all of the c.c.w. light
towards the long arm of the loop. The c.c.w. light is delayed by the long
PM fiber arm 5 and reaches the directional-coupler 4, completing the
remaining portion of the c.c.w path of the loop.
One-half of the c.w. and c.c.w propagating light is coupled by the
directional coupler 4 into the output PM fiber 4-11. This fiber is
connected at splice point 11 to the input PM fiber of a second fiber
polarizing-beam-splitter PBS 12. The axes at splice point 11 of the two PM
fibers are rotated 45 degrees with respect to each other. Thus, the c.w.
and c.c.w. modes interfere at this point producing two orthogonally
polarized modes that are separated by PBS 12 and detected by
photodetectors 13 and 14. The resulting interference signals are
complementary and are electronically subtracted at 15 to obtain a signal
modulated by the ultrasound present at the surface of the object being
inspected.
The FPC 2 is adjusted so that equal power is coupled at input point 3 to
the slow and fast modes and so that the static phase difference between
the modes when they interfere at splice point 11 is 90 degrees. In
general, when this condition is obtained, the light coupled at point 3
will not be circularly polarized, i.e. the fast and slow modes generated
will not have a phase difference of 90 degrees. The input and output to
the Sagnac loop introduce phase-shifts, because the PM fiber lengths to
the left of 4 (i.e., 3-4 and 4-11) are not part of the path-matched Sagnac
loop.
This is a disadvantage with respect to the embodiment of FIG. 1 because the
fiber lengths outside the proper Sagnac loop are sensitive to
environmental perturbations and to frequency variations of the laser. The
environmental perturbations are minimized by making the length of these
fibers as short as possible. The effect produced by frequency variation is
minimized if the two fiber lengths are of the same type of fiber and of
the same length. The reason is that light propagating in the slow mode at
the input fiber will propagate in the fast mode at the output fiber and
vice versa. Thus, variations in the beat-length between the modes, due to
changes in laser frequency, are canceled if the fibers are of identical
length.
FIG. 3 shows an alternative embodiment of the invention. This embodiment
combines discrete optical components with PM optical fiber parts,
therefore being a hybrid between the embodiments disclosed in FIG. 1 and
FIG. 2. The operating principle is similar to the one described in
relation to FIG. 1 since the components of this embodiment are: laser
diode 1, quarter-waveplate 2, 50/50 beam-splitter 3, PBS 4, PM fiber 5, PM
fiber 6, quarter-waveplate 7, focusing lens 8, probe case 9, fiber holder
10, PBS 11, photodetectors 12-13, compensating waveplate 14, and
electronic subtraction circuit 15. The waveplate 2 converts the light from
the laser 1 into circular polarized light, which is split by beam-splitter
3 and directed into the c.w. and c.c.w. paths. The PBS 4 sends one
polarization mode of each path along the PM fiber 6 towards the probe 9.
The waveplate 7 in the probe converts each mode to circular polarized
light and light reflected from the surface of the object being inspected
is converted by waveplate 7 back to the orthogonal mode. The output PBS
11, with its axes at 45 degrees with respect to the fiber, produces the
two interference beams which are detected by the photodetectors 12 and 13,
and thereafter are subtracted electronically at 15. Waveplate 14
compensates for spurious birefringence as described in relation to FIG. 2
and can also be used to rotate the c.w. and c.c.w. polarization axes 45
degrees without physically rotating PBS 11.
FIG. 4 shows another embodiment of this invention. This embodiment is
similar to the embodiment disclosed in FIG. 1, except that the polarizing
beam-splitter in FIG. 1 is replaced by a PM directional coupler 6 as shown
in FIG. 4, and two lengths of polarizing fiber 15 and 16 are inserted in
the Sagnac loop. The rest of the components are identical to those
disclosed in FIG. 1. Polarizing (PZ) fiber only supports the propagation
of a single polarization mode, contrary to the effect of PM fiber,
consequently it polarizes light coupled at its input, as described by M.
J. Messerly, R. C. Mikkelson, and J. R.. Onstott, "A broadband single
polarization optical fiber," Journal of Lightwave Technology, Vol. 9,
1991. PZ fiber is employed in this embodiment to eliminate light that
propagates backwards in the PZ fiber without having traversed the complete
Sagnac loop. The light from a laser diode 1 is coupled to FPC 2 which is
used to adjust the polarization of the light injected at point 3 to the
input PM fiber of the interferometer, as explained in relation to the
embodiment of FIG. 1. A 50/50 PM fiber directional-coupler 4 splits the
light into two equal power c.w. and c.c.w. beams. The c.w. propagating
light travels through a long length of PM fiber 5, which results in a time
delay, and through a length of PZ fiber 15. This eliminates the linearly
polarized fast mode. One-half of the light propagating along the slow mode
of the fiber is coupled towards a light dump 8, while the other half
travels towards FPC 7 adjusted as a quarter-waveplate. The c.w. output
light from FPC 7, which is right-circularly polarized light, is focused by
a lens assembly 9 on the surface of the object being inspected where
ultrasonic waves are present. Part of the reflected or backscattered light
is collected by the lens assembly 9 and is coupled back to the fast mode
of the PM fiber. The 50/50 PM directional-coupler 6 directs one-half of
this light to the short arm of the loop. The other half is coupled into
the long arm and eliminated by the PZ fiber 15, because the propagation
axis of the PZ fiber is aligned with the slow mode. However, the PZ fiber
16 has its propagation axis aligned with the fast mode of the short arm,
and the light can complete the c.w. path. In the same manner, c.c.w.
propagating light coupled by directional coupler 4 to the short arm
becomes polarized by the PZ fiber 16 along the fast mode and one-half of
the light is coupled at directional coupler 6 into the fast mode of the
third arm. This c.c.w. light travels towards the surface of the object
being inspected and part of the reflected or backscattered light collected
by lens 9 is completed into the fiber and traverses the complete c.c.w.
path polarized along the slow mode of the long arm. The fiber carrying the
light from directional coupler 4 is spliced at point 11 to a PZ fiber 12
with its axes rotated 45 degrees, so as to produce an interference signal
that is detected by a photodetector 13. This embodiment has the advantage
of not requiring a polarizing beam-splitter, but is not very efficient in
its use of light power.
FIG. 5 shows yet another embodiment of this invention. This embodiment is
similar to that disclosed in FIG. 1, except that the input 50/50
directional-coupler 4 is replaced by a polarizing beam-splitter 4 and an
optical circulator 17 is added. This embodiment has the advantage of
increased efficiency because no light is lost as a result of dumping to
unused ports. The light from a laser diode 1 is coupled to FPC 2 which is
used to adjust the polarization of the light injected at point 3 to the
input PM fiber of the interferometer. An optical circulator 17 built with
PM fiber transmits all the light towards PBS 4. Fiber optical circulators
are described, for example, by Jay Van Delden, "A new approach to fiber
coupling" Photonics Spectra, January 1992. PBS 4 splits the light into a
component polarized along the slow mode and traveling c.w., and a
component polarized along the fast mode and traveling c.c.w. The c.w.
light travels through a long length of PM fiber 5, which results in a time
delay, and reaches the fiber-polarization-splitter 6. All the c.w. light
is transmitted towards FPC 7, adjusted as a quarter-waveplate. This light
from FPC 7 is focused by a lens assembly 9 on the surface where ultrasonic
waves are present. Part of the reflected or backscattered light is
collected by lens 9 and is coupled back into the fiber. PBS 6 directs all
of this light to the short arm of the loop, polarized along the fast mode.
In like manner, all of the light coupled by PBS 4 to the short arm travels
towards the FPC 7 and the light from FPC 7 is focused by lens assembly 9
on the surface of the object being inspected. Part of the reflected or
backscattered light is collected by lens 9 and is coupled to the long arm,
polarized along the slow mode. The c.w. and c.c.w. components are both
completely transmitted by PBS 4 towards the optical circulator 17 which
sends these beams traveling towards port 11 to interfere and be detected
by differential detector 13-14-15 as explained in the discussion of the
operation of the embodiment shown in FIG. 1.
It should be further realized that numerous other embodiments may be
considered without departing from the scope of the invention.
* * * * *
|
|
 |
|
 |
Description |
|
