Note: Descriptions are shown in the official language in which they were submitted.
CA 02552465 2006-07-18
OPTICAL APPARATUS AND METHOD FOR DISTANCE MEASURING
FIELD OF THE INVENTION
[0001] The invention relates to optical sensors for
determining the distance of target. In particular, the
invention relates to optical fiber sensors based on low-
coherence interferometry for determining the distance between
a target and a probe head.
BACKGROUND OF THE ART
[0002] Several techniques exist to optically measure the
distance of a target relative to a reference point. One of
these techniques is laser triangulation. When sufficiently
high numerical apertures can be used, laser triangulation
techniques offer resolution at the micrometer scale. However,
in the case where it is not possible to use large numerical
apertures, laser triangulation techniques are less precise.
[0003] Time-of-flight techniques can also be used to
determine the distance to a target. However, these techniques
are based on using pulsed light sources and are generally used
to measure long distances (in the kilometer range) with
typical resolution at the meter range. Resolution of time-of-
flight techniques can be improved when shorter light pulses
and faster detection schemes are used, but a costly and
complex measurement system is then required.
[0004] The Fourier transform analog to the time-of-flight
technique is called Phase modulation telemetry. It proceeds by
measuring the relative phase of the light modulated at
frequency f, and coming back from a reflection on the target
object. The phase difference can be related to the distance of
the object. This technique has allowed demonstration of
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resolution in the range of 1 mm, but with an larger absolute
distance value of the order of 5 mm due to the severe
requirements on the phase value measurement at high modulation
frequency.
[0005] Another way to measure a distance to a target is to
use an interferometer with coherent light. Typical
interferometers offer very good resolution (at the sub-
wavelength range). However, the distance range that they
measure is limited due to the phase ambiguity problem. The
phase ambiguity can be removed by the use of dual wavelength
sources as suggested by many authors. However, the requirement
on the wavelength stability and the phase locking of both
sources make the systems costly and unreliable.
[0006] One way to measure distances in the range of 0 to
20 mm is to use an optical sensor based on low-coherence
interferometry. Low-coherence interferometers use a light
source having a broadband intensity spectrum (in comparison to
highly coherent sources such as lasers which have very narrow
intensity spectrum) to determine the position of a target. In
one of such existing systems, the spectrum intensity is
coupled to a scanning Michelson interferometer. The scanning
Michelson interferometer has a reference arm and a measuring
arm, the measuring arm being the arm used for measuring the
distance to the target. In such existing systems, the length
of the reference arm needs to be scanned by a mechanical means
during the measuring process.
[0007] There is a need for an optical sensor for distance
measuring of a target that could be used in narrow conduits,
that could measure distances in the range of a few micrometers
to a few millimeters with good resolution, and that does not
require moving mechanical parts.
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SUMMARY
[0008] Therefore, it is an aim of the present invention to
provide an optical sensor for distance measuring of a target
which addresses issues associated with the prior art.
[0009] The invention provides an optical sensor for
distance measuring of a target that can be used in narrow
conduits, that can measure distances in the range of a few
micrometers to a few millimeters with good resolution, and
that does not require moving mechanical parts.
[0010] Therefore, in accordance with the present invention,
there is provided an optical sensor for determining a distance
between a target and a probe head. The optical sensor
comprises a light source delivering a broadband spectrum; a
fiber-optic Michelson interferometer (FOMI) for creating an
interferogram; and an analyzer unit for receiving the
interferogram from an output of the FOMI and for determining
the distance of the target by finding a position corresponding
to a maximum in a Fourier transform of the interferogram. The
FOMI comprises a probing optical fiber terminated by the probe
head and a reference optical fiber, the reference optical
fiber having an optical path length of fixed length.
[0011] The invention also provides an optical sensor for
determining a distance between a target and a probe head. The
optical sensor comprises: a light source delivering a
broadband spectrum; a fiber-optic Michelson interferometer
(FOMI) for creating an interferogram; and an analyzer unit for
receiving the interferogram from an output of the FOMI and for
determining the distance of the target by finding a position
corresponding to a maximum in a Fourier transform of the
interferogram, the FOMI comprising: a probing optical fiber
coupled at one end to the light source and having at the other
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end the probe head, the probing optical fiber for propagating
a first amplitude portion of the broadband spectrum from the
coupled end towards the probe head and for illuminating the
target with the first amplitude portion of the broadband
spectrum exiting the probe head and for propagating back
toward the output of the FOMI a reflected spectrum from the
target, whereby a measuring signal is provided at the output;
and a reference optical fiber coupled at one end to the light
source, the reference optical fiber for propagating a second
portion of amplitude of the broadband source along a reference
optical path of fixed length, whereby a reference signal is
provided at the output, the addition of the measuring and
reference signals forming the interferogram.
[0012] The invention also provides the reference optical
fiber with a retro-reflector. The invention also provides the
probe head with a GRadient INdex (GRIN) lens. The invention
also provides the analyzer unit with a spectrometer for
dispersing the interferogram as a function of wavelength along
a detection axis of a linear detector array and for detecting
an intensity of the dispersed interferogram as a function of a
position along the detection axis.
[0013] The invention also provides an optical probe for use
in distance measuring of a target. The optical probe
comprises: a light source delivering a broadband spectrum; and
a fiber-optic Michelson interferometer (FOMI) for creating an
interferogram at an output of the FOMI, the FOMI comprising: a
probing optical fiber coupled at one end to the light source
and having at the other end a probe head, the probing fiber
for propagating a first amplitude portion of the broadband
spectrum from the coupled end towards the probe head and for
illuminating the target with the first amplitude portion of
the broadband spectrum exiting the probe head and for
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propagating back toward the output of the FOMI a reflected
spectrum from the target, whereby a measuring signal is
provided at the output; and a reference optical fiber coupled
at one end to the light source, the reference optical fiber
for propagating a second portion of amplitude of the broadband
source along a reference optical path of fixed length, whereby
a reference signal is provided at the output, the addition of
the measuring and reference signals forming the interferogram.
[0014] The invention also provides the reference optical
fiber with a retro-reflector. The invention also provides the
probe head with a GRIN lens.
[0015] The invention also provides a method for determining
a distance separating a target from a probe head of a probing
optical fiber. The method comprises: creating an interferogram
by combining a measuring signal with a reference signal;
measuring the intensity of the interferogram as a function of
wavelength; and determining the distance of the target by
finding a position corresponding to a maximum in a Fourier
transform of the detected intensity, the measuring signal
being obtained by propagating a first amplitude portion of a
broadband spectrum into the probing optical fiber towards the
probe head, illuminating the target with the first amplitude
portion of the broadband spectrum exiting the probe head and
propagating back, into the probing optical fiber and towards
an output, a reflected spectrum from the target whereby the
measuring signal is provided at the output; the reference
signal being obtained by propagating a second amplitude
portion of the broadband spectrum along a reference optical
path of fixed length, whereby the reference signal is provided
at the output.
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[0016] The invention also provides, at the step of
measuring, dispersing the interferogram as a function of
wavelength along a detection axis and detecting the intensity
as a function of a position along the detection axis. The
invention also provides determining the distance separating an
object to the probe head.
According to one aspect, there is provided an optical sensor
for determining a distance between a target and a probe head.
The sensor comprises: a low-coherence light source delivering
a broadband spectrum; a fiber-optic Michelson interferometer
(FOMI) for creating an interferogram; and an analyzer unit for
determining the distance to the target and comprising a
Fourier transform unit for calculating a Fourier transform of
the spectral distribution of the interferogram, the distance
being determined by finding a value of the distance
corresponding to a maximum in the Fourier transform. The FOMI
comprises : a probing optical fiber having at a first end the
probe head and coupled at a second end to the light source
such that a first amplitude portion of the broadband spectrum
is to propagate from the second end towards the probe head and
such that a measuring signal resulting from a reflection of
the first amplitude portion on the target is to propagate back
in the probing optical fiber from the first end toward an
output of the FOMI, the probe head being an integral element
having a first end surface and a second end surface, the first
end surface being in contact with the first end of the probing
optical fiber to receive the first amplitude portion, and the
first amplitude portion exiting the probe head from the second
end surface to directly illuminate the target; and a reference
optical fiber coupled at one end to the light source such that
a second portion of amplitude of the broadband source is to
propagate along a reference optical path of fixed length to
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provide a reference signal at the output, the probing optical
fiber and the reference optical fiber being coupled such that
the measuring signal and the reference signal are to be
combined to form the interferogram.
According to another aspect, there is provided an optical
probe for use in distance measuring of a target. The probe
comprises: a low-coherence light source delivering a broadband
spectrum; and a fiber-optic Michelson interferometer (FOMI)
for creating an interferogram at an output of the FOMI. The
FOMI comprises: a probing optical fiber having at a first
end a probe head and coupled at a second end to the light
source such that a first amplitude portion of the broadband
spectrum is to propagate from the second end towards the probe
head and such that a measuring signal resulting from a
reflection of the first amplitude portion on the target is to
propagate back in the probing optical fiber from the first end
toward the output of the FOMI, the probe head being an
integral element having a first end surface and a second end
surface, the first end surface being in contact with the first
end of the probing optical fiber to receive the first
amplitude portion, and the first amplitude portion exiting the
probe head from the second end surface to directly illuminate
the target; and a reference optical fiber coupled at one end
to the light source such that a second portion of amplitude of
the broadband source is to propagate along a reference optical
path of fixed length to provide a reference signal at the
output, the probing optical fiber and the reference optical
fiber being coupled such that the measuring signal and the
reference signal are to be combined to form the interferogram.
According to still another aspect, there is provided a method
for determining a distance separating a target from a probe
head of a probing optical fiber. The method comprises:
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propagating a first amplitude portion of a low-coherence
broadband spectrum into the probing optical fiber towards the
probe head; propagating the first amplitude portion from the
probing optical fiber to a first end surface of the probe head
in contact with the probing optical fiber and to a second end
surface of the probe head; directly illuminating the target
with the first amplitude portion exiting the probe head from
the second end surface; propagating back, into the probing
optical fiber, a measuring signal resulting from a reflection
of the first amplitude portion on the target; propagating a
second amplitude portion of the broadband spectrum along a.
reference optical path of fixed length to obtain a reference
signal; creating an interferogram by combining the measuring
signal with the reference signal; measuring an intensity of
the interterogram as a function of wavelength; and determining
the distance of the target by finding a value of the distance
corresponding to a maximum in a Fourier transform of the
spectral distribution of the detected intensity.
DESCRIPTION OF THE DRAWINGS
[0017] In order for the invention to be readily understood,
embodiments of the invention are illustrated by way of example
in the accompanying drawings.
[0019] Figure 1 is a schematic view ofan optical sensor for
distance measuring of a target, in accordance with an
embodiment of the present invention;
[0019] Figure 2 is a schematic view of a particular embodiment
of the optical sensor of Figure 1;
[0020] Figure 3 is a graphic representation of the Fourier
transform of the output signal of the optical sensor of Fig.
2, as a function of the target distance; and
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[0021) Figure 4 is a flow chart for a. method of distance
measuring, in accordance with another embodiment of the
present invention.
DETAILED DESCRIPTION
[0022] In the following description of the embodiments,
references to the accompanying drawings are by way of
illustration of an example by which the invention may be
practiced.
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[0023] Figure 1 is a schematic view of the present
invention which provides an optical sensor 10 that can be used
to determine the distance 13 of a target 11 relative to a
probe head 43 of the sensor 10 . The sensor 10 comprises a
broadband light source 21 coupled to the input 31 of a fiber-
optic Michelson interferometer 30, and an analyzer unit 50
coupled to the output 33 of the interferometer 30. In
accordance with an embodiment of the present invention, the
interferometer 30 comprises a bi-directional coupler 35, the
coupler 35 coupling the light source 21, the reference arm 37,
the measuring arm 39 and the output 33 of the interferometer.
The reference arm 37 is an optical fiber terminated by a
retro-reflector 38, so as to provide a reference optical path
length having a fixed length. The measuring arm 39 is formed
by an optical fiber, referred to as the probing optical fiber
41, and by the target 11 located at a distance d 13 from the
probe head 43 of the probing fiber 41. Thus, the optical path
length of the measuring arm 39 varies with the distance 13 to
the target. In one embodiment, the probe head 43 comprises a
GRIN lens.
[0024] The optical probe 20, which comprises the source 21
coupled to the Michelson interferometer 30, is used to create
an interferogram containing information about the distance 13
to the target 11. Once the interferogram is formed, the
analysis unit 50 interprets the interferogram in order to
determine the distance 13 to the target 11.
[0025] The present invention takes advantage of low-
coherence interferometry for measuring the distance 13 to the
target 11. Light source 21 is a broadband source such as, for
example, a light emitting diode (LED). In an embodiment of the
present invention, light source 21 is emitting a broadband
spectrum having a width of about 20 nm and being centered
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around 820 nm, providing the possibility of measuring a range
of distances of more than 10 mm with a precision of tens of
microns. Obviously, other types of sources can be used.
Furthermore, the light source and detector parameters may be
different from those described above so as to provide other
ranges of distance measuring and precision.
[0026] Light source 21 emits a wave, which travels towards
the coupler 35. The coupler 35 divides the wave in two, one
segment is directed into the reference arm 37 and is
propagated back along the reference arm 37 and towards the
coupler 35 after having been reflected by the retro-reflector
38. Another segment is directed into the probing optical fiber
41, propagates towards the probe head 43, and exits the probe
head 43 so as to illuminate the target 11. Target 11 reflects
back some of the light and some of this reflected spectrum is
collected by the probe head 43 and travels back through the
probing optical fiber 41 towards the coupler 35. Coupler 35
combines those reflected signals (one from the reference arm
37 and one from the measuring arm 39) and directs them toward
the interferometer output 33. Coupler 35 is, for example, a
50/50 fused-fiber coupler. At the output 33 of the
interferometer 30, an interferogram is thus created in the
spectral domain. This interferogram is then directed to the
analyzer unit 50 which determines the distance 13.
[0027] Figure 2 illustrates, in more detail, the components
of an embodiment of the analyzer unit 50. The unit 50
comprises a spectrometer 51, a Fourier transform unit 60 and a
display 70. The spectrometer 51 serves to disperse the
incoming interferogram as a function of the wavelength along a
detection axis 58 of a linear detector array 59 and to detect
the interferogram via the linear detector array 59. One
possible way to realize the above is to use a grating 55 in
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combination with lens 53 and 57 in a configuration such as
illustrated in Figure 2. Obviously, other ways that are known
to the person skilled in the art exist to disperse the
interferogram such as, for example, prisms, could be used as
well. Once dispersed on the linear detector array 59, the
intensity of the interferogram as a function of wavelength can
be obtained. The intensity of the interferogram as a function
of wavelength may be expressed as:
dl = f(2)[1+ v*cos(coz-)jd2 , (1)
w=27r¨c
with and
A c
where 2k., 2*L, c, v and f(2)are respectively the
wavelength, the difference between the optical length of the
reference arm and the optical path length of the measuring
arm, the speed of light, the contrast of the fringes of the
interferogram, and the intensity spectrum of the light source
21. By assuming that the light source emits a uniform
intensity as a function of wavelength, this intensity will
produce upon detection by the linear detection array 59 an
electrical signal V(A) :
W)=q1+v*cos(471L)I
(2)
A
where Vo is a constant.
[0028] By assuming a linear variation of the wavelength
along the detection axis 58 of the linear detector array 59:
2=21+ax , (2) may be written as follows:
[
4g _
L ,
V(x)=V0)
1+v*cos( ( 3)
Al + ax
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where X1 is the shortest wavelength of the detected signal
and a is a proportionality constant. The bandwidth AX of the
intensity spectrum of the light source being quite smaller
than Xi (typically tens of nm), we can use the following
1
approximation: ____ 1 (1-) and equation (3) becomes:
Al +ax Ai Ai
Wx)=V0{ 1+v*co:{411L(1
A i axl
Al (4)
[0029]
Expressed in terms of the pixels of the linear
detector array 59, normalized V(x) becomes:
_
aq,
V =l+v*co!3[4ItILO 1) (5)
q
Al Ai _
where q = 0, 1, 2, ....................................................... ,
2047 for a linear detector array
having 2048 pixels.
[0030] In
one embodiment of the present invention, linear
detection array 59 has 2048 pixels and the detected intensity
of the interferogram is thus a vector of 2048 Vq values, each
Vq value corresponding to a wavelength.
[0031] To
determine the difference between the optical path
length of the reference arm and the optical path length of the
measuring arm (2L), a discrete Fourier transform of the vector
of 2048 Vc, values is performed :
1j271" P
---q
Cn --1- n
E vae ( 6 )
' Vn q '
where p = 0, 1, 2, ......, 1023 et n = 2048. Graph 61,
illustrated in Figure 3, displays the imaginary portion of the
Fourier transform of the detected dispersed intensity as a
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function of the target distance. The x-axis 65 corresponds to
the target distance, whereas the y-axis 63 corresponds to the
coefficients Cp of the imaginary portion of the Fourier
transform. The maximum 67 of the graph 61 is located at a
position corresponding to the distance d 13 of the target 11.
(0032]
Turning now to Figure 4, a method 80 for determining
a distance of a target relative to the probe head 43 of the
optical sensor 10 will be described. The method 80 comprises
obtaining a measuring signal (step 71) and obtaining a
reference signal (step 73). Step 71 is provided by propagating
a first amplitude portion of a broadband spectrum into a
probing optical fiber 41 towards the probe head 43,
illuminating the target 11 with the first amplitude portion of
the broadband spectrum exiting the probe head 43 and
propagating back into the probing optical fiber 41 and towards
an output 33, a reflected spectrum from the target 11, whereby
a measuring signal is provided at the output 33.
[0033] Step 73 is provided by propagating a second
amplitude portion of the broadband spectrum into a reference
optical fiber 37 having a predetermined length and propagating
back the second amplitude portion into the reference fiber 37
towards the output, after reflection at one end 38 of the
reference fiber 37, whereby the reference signal is provided
at the output. Step 75 of obtaining an interferogram using the
measuring and reference signals is provided by combining the
measuring and reference signals at the coupler whereby an
interferogram is created.
[0034] The interferogram is then dispersed along a
detection axis as a function of the wavelength and detected as
a function of the wavelength (step 77). The detected intensity
is Fourier transformed at step 78. At step 79, the distance 13
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of the target 11 is determined by finding a position
corresponding to a maximum of the Fourier Transform.
[0035] Optical sensor 10 can be realized with different
types of light source 21 and different types of spectrometer
51. For example, in one embodiment, light source 21 is a
superluminescent diode from Superlum Diodes LTD, emitting
mW at 820 nm and having a bandwidth of 20 nm; the linear
detector array 59 is a CCD of 2048 pixels from ALPHAS that has
a spectral range between 320 and 1100 nm; and the spectrometer
51 comprises a diffraction grating 55 from Edmund that has
1200 1/mm.
[0036] In an alternative embodiment, light source 21 is a
superluminescent diodes from DenseLight, emitting 10 mW at
1310 nm and having a bandwidth of 30 nm; the linear detector
array 59 is a CCD of 512 pixels from Sensor Unlimited that has
a spectral range between 900 and 1700 nm; and the diffraction
grating 55 is chosen appropriately so has to dispersed the
light source spectrum on the CCD array.
[0037] In a yet alternative embodiment, light source 21 is
a ELED (edge-emitting) diodes from PL-LD, directly coupled to
a single mode fiber, emitting 5 AW at 1310 nm; the linear
detector array 59 is an array of Silicon photodetectors
comprising 2048 pixels from Perkin Elmers; and the diffraction
grating 51 is chosen appropriately so has to dispersed the
light source spectrum on the CCD array.
[0038] As will be obvious for someone skilled in the art,
the parameters of sensor 10 may be adjusted so as to provide a
desired distance measurement range with a desired resolution.
For example, it is possible to select a light source 21 having
a desired spectrum width and adjust the resolution of the
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dispersive device 51 in order to provide a distance measuring
having determined measurement range and resolution.
0039] The scope of the invention is intended to be limited
solely by the scope of the appended claims.
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