U.S. patent application number 09/239667 was filed with the patent office on 2001-08-30 for method and apparatus for optical sectioning and imaging using time- gated parametric image amplification.
Invention is credited to HARIHARAN, ANAND, HARTER, DONALD J., SQUIER, JEFF, SUCHA, GREGG.
Application Number | 20010017727 09/239667 |
Document ID | / |
Family ID | 22903193 |
Filed Date | 2001-08-30 |
United States Patent
Application |
20010017727 |
Kind Code |
A1 |
SUCHA, GREGG ; et
al. |
August 30, 2001 |
METHOD AND APPARATUS FOR OPTICAL SECTIONING AND IMAGING USING TIME-
GATED PARAMETRIC IMAGE AMPLIFICATION
Abstract
An optical parametric amplifier pumped by ultrashort optical
pulses provides time-gated image amplification or time-gated image
frequency conversion, resulting in optical sectioning of an object
under test and/or background rejection of improperly timed light.
An ultrashort laser pulse of one frequency is used to illuminate an
object under test. An ultrashort pulse light beam at a signal
frequency transmitted through or scattered from the object is
optically mixed with an ultrashort laser pulse at a pump frequency
in a nonlinear optical medium. This mixing produces an amplified
image of a particular optical section of the object at the signal
frequency in addition to producing a frequency converted image of
the same optical section at an idler frequency. This time-gated
amplification can be used in conjunction with a confocal imaging
system, or a conventional imaging system. The resolution of optical
sectioning is determined by the temporal widths of the signal and
pump pulses and by the group velocity walkoff in the nonlinear
medium. By illuminating the target with a train of closely spaced
ultrashort pulses, an image of multiple sections can be amplified
and downconverted within a single laser shot, giving a contour
image of the target. The signal light can also be fluorescence from
the object, excited by a short laser pulse, either through
single-photon or multi-photon absorption. In this case, the signal
light is incoherent with respect to the pump light. By using
quasi-phase-matched nonlinear optical crystals as the amplifying
medium, advantages such as an increased acceptance angle and lower
pump thresholds are obtained.
Inventors: |
SUCHA, GREGG; (MANCHESTER,
MI) ; HARIHARAN, ANAND; (ANN ARBOR, MI) ;
HARTER, DONALD J.; (ANN ARBOR, MI) ; SQUIER,
JEFF; (SAN DIEGO, CA) |
Correspondence
Address: |
SUGHRUE MION ZINN MACPEAK & SEAS
2100 PENNSYLVANIA AVENUE NW
WASHINGTON
DC
200373213
|
Family ID: |
22903193 |
Appl. No.: |
09/239667 |
Filed: |
January 29, 1999 |
Current U.S.
Class: |
359/326 ;
359/330; 359/333 |
Current CPC
Class: |
G01N 21/4795 20130101;
G01S 17/89 20130101; G02F 1/39 20130101 |
Class at
Publication: |
359/326 ;
359/330; 359/333 |
International
Class: |
G02F 001/35 |
Claims
What is claimed is:
1. An ultrafast time-gated optical parametric amplification system
using time-gated optical parametric amplification of light received
from an object, comprising: an ultrashort pulse laser system
configured to generate an optical illumination beam at an
illumination frequency and an ultrashort optical pump pulse at a
pump frequency, said ultrashort pulse laser system directing the
optical illumination beam toward the object; a nonlinear optical
medium arranged to receive light from the object at a signal
frequency in response to interaction of the optical illumination
beam with the object, said ultrashort pulse laser system pumping
said nonlinear optical medium with the pump pulse, such that the
pump pulse spatially and temporally overlaps a portion of the light
from the object within the nonlinear optical medium, resulting in
time-gating and optical parametric amplification of the portion of
the light from the object from interaction with the pump pulse
within said nonlinear optical medium; an optical detector
responsive to output light emitted by the nonlinear optical medium
to produce detection signals.
2. The system according to claim 1, wherein said optical parametric
amplification system operates as a LIDAR.
3. The system according to claim 1, wherein said optical parametric
amplification system images an object, and said system further
comprises an imaging device adapted to form am image of the object
in accordance with said detection signals, said detection signals
comprising object image signals.
4. The system according to claim 1 or 3, wherein said ultrashort
pulse laser system comprises a synchronizer for controlling a
relative timing of the optical illumination beam and the pump pulse
in order to control which portion of the light from the object is
parametrically amplified and time-gated by the pump pulse within
said nonlinear optical medium, thereby determining which portion of
the object is detected by the optical illumination beam.
5. The system according to claim 4, wherein said synchronizer
includes a variable optical delay device adapted to adjust the
length of an optical path of the pump pulse to control a relative
timing of the arrival of the pump pulse and the arrival of the
imaging light at said nonlinear optical medium
6. The system according to claim 1 or 3, wherein said ultrashort
pulse laser system comprises: an ultrashort pulse laser configured
to generate the optical illumination beam and the pump pulse from a
common pulse; a frequency converter adapted to convert a frequency
of the optical illumination beam to the illumination frequency; and
a variable optical delay device adapted to adjust the length of an
optical path of the pump pulse to control a relative timing of the
arrival of the pump pulse and the arrival of the light from the
object at said nonlinear optical medium.
7. The system according to claim 6, wherein: said ultrashort pulse
laser comprises a laser source adapted to generate a primary
ultrashort optical pulse at a first frequency; and said frequency
converter comprises: a frequency doubler responsive to the primary
ultrashort optical pulse to generate the pump pulse; and an optical
parametric generator responsive to the pump pulse to generate the
optical illumination beam.
8. The system according to claim 6, wherein said frequency
converter comprises at least one of: a second harmonic generator;
an optical parametric generator; and a frequency mixer.
9. The system according to claim 6, wherein said variable optical
delay device comprises an adjustable optical delay line.
10. The system according to claim 1 or 3, wherein said ultrashort
pulse laser system comprises: a first ultrashort pulse laser
adapted to generate the optical illumination beam; a second
ultrashort pulse laser adapted to generate the pump pulse; a
synchronizer coupled to the first and second ultrashort pulse
lasers, for controlling a relative timing of the arrival of the
pump pulse and the arrival of the light from the object at said
nonlinear optical medium; and a frequency converter adapted to
convert a frequency of the optical illumination beam to the
illumination frequency.
11. The system according to claim 10, wherein said synchronizer is
an electronic synchronization unit.
12. The system according to claim 1 or 3, wherein the output light
detected by said optical detector includes amplified light at the
signal frequency generated from the interaction of the light from
the object with the pump pulse within said nonlinear optical
medium.
13. The system according to claim 1 or 3, wherein the output light
detected by said optical detector includes light at an idler
frequency generated from the interaction of the light from the
object with the pump pulse within said nonlinear optical
medium.
14. The system according to claim 13, wherein said pump pulse
causes nondegenerate optical parametric amplification of the light
from the object within said nonlinear optical medium, such that a
frequency-converted signal is generated at the idler frequency
which is different from the signal frequency, the system further
comprising: an optical filter disposed in an optical path between
said nonlinear optical medium and said optical detector, said
optical filter blocking light emitted from the nonlinear optical
medium at the signal frequency.
15. The system according to claim 13, wherein: said pump pulse
causes degenerate optical parametric amplification of the light
from the object within said nonlinear optical medium, such that the
idler frequency is the same as the signal frequency: and the output
light includes amplified light at the signal frequency generated
from the interaction of the light from the object with the pump
pulse within the nonlinear optical medium and unamplified light at
the signal frequency.
16. The system according to claim 1 or 3, further comprising: an
optical filter disposed in an optical path between said nonlinear
optical medium and said optical detector, said optical filter
blocking light emitted from the nonlinear optical medium at the
pump frequency.
17. The system according to claim 1 or 3, wherein said nonlinear
optical medium is disposed at a real image plane within the
system.
18. The system according to claim 1 or 3, wherein said nonlinear
optical medium is disposed at a Fourier plane within the
system.
19. The system according to claim 1 or 3, wherein said nonlinear
optical medium is disposed at a plane other than a real image plane
and a Fourier plane within the system.
20. The system according to claim 1 or 3, wherein said nonlinear
optical medium is a quasi-phase-matched nonlinear optical
crystal.
21. The system according to claim 20, wherein the
quasi-phase-matched nonlinear optical crystal is a
periodically-poled lithium niobate crystal.
22. The system according to claim 1 or 3, wherein said nonlinear
optical medium is an optical waveguide structure.
23. The system according to claim 1 or 3, wherein said nonlinear
optical medium is a periodically poled ferroelectric optical
material comprising one of a lithium niobate crystal, a lithium
tantalate crystal, an MgO:LiNbO.sub.3 crystal, and a KTP or KTP
isomorph family crystal.
24. The system according to claim 3, wherein said imaging device
forms a surface contour image of the object.
25. The system according to claim 3, wherein said imaging device
forms a cross-sectional image of the object.
26. The system according to claim 1 or 3, wherein said ultrashort
pulse laser system generates the optical illumination beam as a
single ultrashort optical pulse at the illumination frequency.
27. The system according to claim 1 or 3, wherein: said ultrashort
pulse laser system comprises a pulse shaper adapted to produce an
optical illumination beam that comprises a sequence of ultrashort
optical pulses; a plurality of pulses in the sequence of pulses
interacts with a single pump pulse within said nonlinear optical
medium, such that said single pump pulse time gates and
parametrically amplifies each of the plurality of pulses; and said
optical detector detects output light corresponding to each of said
plurality of pulses.
28. The system according to claim 27, wherein said pulse shaper
controls temporal spacings and relative pulse intensities of pulses
in the sequence of ultrashort optical pulses
29. The system according to claim 27, wherein said imaging device
forms a three-dimensional image of a surface of the object or a
three-dimensional image of a section of the object from the image
detection signals corresponding to said single pump pulse.
30. The system according to claim 1 or 3, wherein: said ultrashort
pulse laser system generates a second optical illumination beam and
a second optical pump pulse, said ultrashort pulse laser system
directing the second optical illumination beam toward the object;
and said nonlinear optical medium receives second light from the
object in response to interaction of the second optical
illumination beam with the object, said ultrashort pulse laser
system pumping said nonlinear optical medium with the second pump
pulse, so that the second pump pulse spatially and temporally
overlaps a portion of the second imaging light within the nonlinear
optical medium, whereby a timing of the second pump pulse relative
to the second light is different from a timing of the pump pulse
relative to the first light from the object, such that the portion
of the second light amplified by the second pump pulse corresponds
to a different portion of the object than the amplified portion of
the first light from the object.
31. The system according to claim 3, wherein: said ultrashort pulse
laser system generates a plurality of optical illumination beams
and a plurality of corresponding ultrafast optical pulses that are
directed toward the object; said nonlinear optical medium receives
light from the object in response to interaction of each of the
optical illumination beams with the object; and said ultrashort
pulse laser system pumps said nonlinear optical medium with said
plurality of pump pulses in synchronization with the arrival of
light from respective optical illumination beams, such that the
relative timing of corresponding optical illumination beams and
pump pulses is varied so that a portion of the light from the
object amplified for different optical illumination beams
corresponds to different portions of the object.
32. The system according to claim 31, wherein said imaging device
forms a three-dimensional image of a surface of the object or a
three-dimensional image of a section of the object from a plurality
of light signals emitted from the nonlinear optical medium
corresponding to the plurality of optical illumination beams.
33. The system according to claim 1 or 3, wherein said ultrashort
pulse laser system illuminates the object with the optical
illumination beam collinearly with an optical axis of said optical
detector.
34. The system according to claim 3, wherein the imaging light is
light from the illumination beam scattered from a surface of the
object, the illumination frequency is the signal frequency, and
said imaging device generates a topographic image of the
object.
35. The system according to claim 3, wherein the illumination beam
excites a fluorescent medium introduced into the object, causing
the fluorescent medium to emit imaging light at the signal
wavelength which is different from the illumination wavelength, and
said imaging device generates a cross-sectional image of the
object.
36. The system according claim 1 or 3, wherein the signal frequency
is a third harmonic of the illumination frequency, generated at a
surface of the object.
37. The system according to claim 3, wherein the system is a
confocal imaging system.
38. The system according to claim 15, wherein the imaging system is
operated in a degenerate mode, giving interferometrically sensitive
gain.
39. The system according to claim 38, wherein the system is an
optical coherence tomography system.
40. The system according to claim 38, wherein a time delay between
the pump pulse and the optical illumination beam is adjusted for
successive pump pulses in a sequence of pump pulses to effect
scanning of the object.
41. The system according to claim 3, wherein said imaging device
uses the image detection signals generated by said optical detector
to make fluorescence lifetime imaging measurements.
42. A method of imaging an object using time-gated optical
parametric amplification of light received from the object, the
method comprising the steps of: transmitting an optical
illumination beam, at an illumination frequency, toward the object;
receiving imaging light, at a signal frequency, from the object in
response to interaction of the illumination beam with the object,
and directing the imaging light into a nonlinear optical medium;
pumping the nonlinear optical medium with an ultrashort optical
pump pulse, at a pump frequency, that spatially and temporally
overlaps a portion of the imaging light within the nonlinear
optical medium, resulting in time-gating and optical parametric
amplification of the portion of the imaging light from interaction
with the pump pulse within the nonlinear optical medium; and
forming an image of the object in response to output light emitted
from the nonlinear optical medium.
43. The method according to claim 42, wherein the output light used
to form the image of the object includes amplified light at the
signal frequency generated from the interaction of the imaging
light with the pump pulse within the nonlinear optical medium.
44. The method according to claim 42, wherein the output light used
to form the image of the object includes light at an idler
frequency generated from the interaction of the imaging light with
the pump pulse within the nonlinear optical medium.
45. The method according to claim 44, wherein nondegenerate optical
parametric amplification of the imaging light results from
interaction of the imaging light with the pump pulse, such that a
frequency-converted image signal is generated at the idler
frequency which is different from the signal frequency, the method
further comprising the step of: filtering out light emitted from
the nonlinear optical medium at the signal frequency, such that the
output light used to form the image of the object does not include
light at the signal frequency.
46. The method according to claim 44, wherein: degenerate optical
parametric amplification of the imaging light results from
interaction of the imaging light with the pump pulse, such that the
idler frequency is the same as the signal frequency; and the output
light used to form the image of the object includes amplified light
at the signal frequency generated from the interaction of the
imaging light with the pump pulse within the nonlinear optical
medium and unamplified light at the signal frequency.
47. The method according to claim 42, further comprising the step
of: filtering out light emitted from the nonlinear optical medium
at the pump frequency, such that the output light used to form the
image of the object does not include light at the pump
frequency.
48. The method according to claim 42, further comprising the step
of positioning the nonlinear optical medium at a real image plane
within an imaging system.
49. The method according to claim 42, further comprising the step
of positioning the nonlinear optical medium at a Fourier plane
within an imaging system.
50. The method according to claim 42, further comprising the step
of positioning the nonlinear optical medium at a plane other than a
real image plane and a Fourier plane within an imaging system.
51. The method according to claim 42, wherein the pumping step
includes pumping the nonlinear optical medium that is a
quasi-phase-matched nonlinear optical crystal.
52. The method according to claim 51, wherein the
quasi-phase-matched nonlinear optical crystal is a
periodically-poled lithium niobate crystal.
53. The method according to claim 42, wherein the pumping step
includes pumping the nonlinear optical medium that is a nonlinear
optical waveguide structure.
54. The system according to claim 1 or 3, wherein said nonlinear
optical medium is a periodically poled ferroelectric optical
material comprising one of a lithium niobate crystal, a lithium
tantalate crystal, an MgO:LiNbO.sub.3 crystal, and a KTP or KTP
isomorph family crystal.
55. The method according to claim 42, wherein the image formed of
the object is a surface contour of the object.
56. The method according to claim 42, wherein the image formed of
the object is a cross-section of the object.
57. The method according to claim 42, wherein the optical
illumination beam corresponding to a single pump pulse comprises a
single ultrashort optical pulse at the illumination frequency.
58. The method according to claim 42, wherein the optical
illumination beam comprises a sequence of ultrashort optical pulses
that are time gated and parametrically amplified within the
nonlinear optical medium by a single pump pulse, and wherein the
image of the object comprises a plurality of images of different
portions of the object respectively corresponding to the sequence
of ultrashort optical pulses.
59. The method according to claim 58, further comprising the step
of controlling temporal spacings and relative pulse intensities of
pulses in the sequence of ultrashort optical pulses.
60. The method according to claim 58, wherein the plurality of
images form a three-dimensional image of a surface of the object or
a three-dimensional image of a section of the object.
61. The method according to claim 42, wherein the optical
illumination beam and the pump pulse are derived from a common
pulse.
62. The method according to claim 42, wherein the optical
illumination beam is frequency converted relative to the pump
pulse, such that the illumination frequency of the optical
illumination beam is less than the pump frequency of the pump
pulse.
63. The method according to claim 42, further comprising the steps
of: transmitting a second optical illumination beam toward the
object; receiving second imaging light from the object in response
to interaction of the second illumination beam with the object, and
directing the second imaging light into the nonlinear optical
medium; and pumping the nonlinear optical medium with a second
ultrashort optical pump pulse that spatially and temporally
overlaps a portion of the second imaging light within the nonlinear
optical medium, wherein a timing of the second pump pulse relative
to the second imaging light is different from a timing of the pump
pulse relative to the imaging light, such that the portion of the
second imaging light amplified by the second pump pulse images a
different portion of the object than the amplified portion of the
imaging light.
64. The method according to claim 42, wherein: the transmitting
step includes transmitting a plurality of optical illumination
beams; the pumping step includes pumping the nonlinear optical
medium with a plurality of ultrafast optical pump pulses in
synchronization with the plurality of optical illumination beams;
and the relative timing of corresponding illumination beams and
pump pulses is varied such that the portion of the imaging light
amplified for different optical illumination beams corresponds to
different portions of the object.
65. The method according to claim 64, wherein the forming step
includes forming a three-dimensional image of the object from a
plurality of light signals emitted from the nonlinear optical
medium corresponding to the plurality of illumination beams.
66. The method according to claim 42, further comprising the step
of controlling a relative timing of the arrival of the pump pulse
and the arrival of the imaging light at the nonlinear optical
medium to control which portion of the imaging light is
parametrically amplified and time-gated by the pump pulse, thereby
controlling which portion of the object is imaged.
67. The method according to claim 42, wherein the object is
illuminated by the illumination beam collinearly with an optical
axis of the nonlinear optical medium.
68. The method according to claim 42, wherein the imaging light is
light from the illumination beam scattered from a surface of the
object, the illumination frequency is the signal frequency, and the
image of the object is a topographic image.
69. The method according to claim 42, wherein the illumination beam
excites a fluorescent medium introduced into the object, causing
the fluorescent medium to emit the imaging light at the signal
wavelength which is different from the illumination wavelength, and
wherein the image of the object is a cross-sectional image.
70. The method according claim 42, wherein the signal frequency is
a harmonic of the illumination frequency.
71. The method according to claim 42, wherein the image of the
object is used to make fluorescence lifetime imaging
measurements.
72. The system according to claims 1 or 3, wherein said ultrashort
pulse laser system generates laser pulses having a pulsewidth of
less than 2 ns.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to laser-based imaging systems
which are used for time-gated imaging, imaging through turbid
media, optical sectioning, metrology, image amplification,
frequency conversion of images, and confocal microscopy.
[0003] 2. Description of the Related Art
[0004] Research relating to optical parametric image amplification
has concerned the upconversion of weak infrared images, and the
selective amplification of certain spatial frequencies without
regard to time resolution. As reported by J. Watson et al. in
"Imaging in diffuse media with ultrafast degenerate optical
parametric amplification," Optics Letters, Vol. 20, p. 231 (1995),
time-resolved, degenerate optical parametric image amplification
has been used for transillumination imaging through turbid media by
providing a sub-picosecond time gate to temporally discriminate
against scattered photons. This method, however, does not provide
optical sectioning of the object or surface contour information.
These optical parametric amplification (OPA) imaging techniques
have employed the OPA either at an image plane or in the Fourier
plane of the optical system.
[0005] Ultrafast time-gated imaging has also been employed to
observe fast processes, such as the propagation of light pulses
through various media. Time-gating has been performed using
techniques other than optical parametric amplification. These
techniques include: LIF holography, as disclosed by J. A. Valdmanis
et al. in "Three-dimensional imaging with femtosecond optical
pulses," Optical Society of America, Conference on Lasers and
Electro-Optics, Vol. 7, paper CTUA1, (1990); picosecond Kerr
shutters, as disclosed by M. A. Duguay et al. in "Ultrahigh speed
photography of picosecond light pulses and echoes," Appl. Opt.,
Vol. 10, pp. 2162-2170 (1970) and by L. Wang, et al. in Science,
Vol. 253, p. 769 (1991); and sum-frequency cross-correlation, as
disclosed by K. M. Yoo et al. in Optics Letters, Vol. 16, p. 1019
(1991). Time-gated upconversion using pulses as short as 65 fsec
has been used to measure biological specimens such as the corneal
structure of rabbit eyes, as disclosed by Fujimoto et al. in
"Femtosecond optical ranging in biological systems," Optics
Letters, Vol. 11, p. 150 (1986). In this method, the ranging was
performed one point at a time, and required raster scanning of the
beam over the specimen.
[0006] Subsequently, an optical coherence tomography (OCT)
technique was disclosed by E. A. Swanson et al. in "High-speed
optical coherence domain reflectometry," Optics Letters, Vol. 17,
p. 151 (1992), which employs only linear interferometry without any
nonlinear optical interaction. Time-gated imaging by ultrashort
pulses using second harmonic generation (SHG) was first disclosed
by Diels et al. in "Imaging with femtosecond pulses," Appl. Opt.,
Vol. 31, p. 6869 (1992) and in "Ultrafast diagnostics," Revue Phys.
Appl., Vol. 22, p. 1605 (1987). In this method, a gating pulse was
used to time-gate and upconvert entire images of objects which were
illuminated by an ultrashort pulse. However, this method does not
provide any amplification of the image, and provides only a single
contour or surface section.
[0007] Surface metrology measurement using ultrafast lasers in
conjunction with sum-frequency mixing is disclosed in U.S. Pat. No.
5,585,893 to Hariharan, et. al., entitled "Ultrashort pulsewidth
laser ranging system employing a time gate producing an
autocorrelation and method therefor." In this method, a focused
laser beam is scanned over the surface of the target in order to
map out the surface topography. Again, in this method, there is no
light amplification, and raster scanning is required to build up an
image of a surface.
SUMMARY OF THE INVENTION
[0008] It is an object of the present invention to employ optical
parametric amplification (OPA) in conjunction with conventional,
Fourier or confocal imaging systems to achieve high gain and low
noise amplification of signal light reflected from or transmitted
through an object in order to produce an amplified image of the
object.
[0009] It is a further object of the present invention to improve
image resolution in confocal microscopy using optical parametric
amplification.
[0010] It is another object of the present invention to use the
time gating capability of optical parametric amplification to
discriminate against scattered light.
[0011] A further object of the present invention is to use the
time-gating capability of optical parametric amplification to
provide optical sectioning of an object under test, similar to that
obtained with optical coherence tomography (OCT).
[0012] A still further object of the present invention is to use
the time gating capability of optical parametric amplification to
produce a new method of fluorescence lifetime imaging.
[0013] Another object of the present invention is to use
quasi-phase-matched nonlinear optic materials as the amplifying
medium in an imaging system, thereby providing large angular
acceptance and low pump thresholds.
[0014] Yet another object of the present invention is to lower the
required excitation power of an illuminating beam in an imaging
system, thereby allowing increased observation time, reducing
photobleaching and enhancing the viability of cells being
imaged.
[0015] The aforesaid objects are achieved individually and in
combination, and it is not intended that the present invention be
construed as requiring two or more of the objects to be combined
unless expressly required by the claims attached hereto.
[0016] The present invention employs optical parametric
amplification (OPA) in a nonlinear optical medium pumped by an
ultrashort (less than 2 ns) pulse laser at frequency .omega..sub.p,
to amplify and time-gate the scattered light from a target object
illuminated by an ultrashort laser pulse at a signal frequency
.omega..sub.s. In the process, another amplified signal is
generated at the idler frequency, .omega..sub.i. This amplified
light is recorded using a CCD camera or other imaging device. This
technique can be used in conjunction with confocal imaging methods.
By amplifying and time-gating the scattered light, optical
sectioning of the object is achieved, enabling an image of an
isometric contour of the object surface or interior to be produced.
In the case of nondegenerate OPA, detection of the idler frequency
instead of the signal frequency also achieves frequency conversion
of the image. Standard gated image intensifiers (e.g., microchannel
plates) have a time-gate window of approximately 100 ps which can
resolve depth features with only approximately 1 cm resolution. By
using ultrashort pulses (e.g., 100 fs) it is possible to resolve
surface features with a resolution of approximately 10 microns. By
using still shorter pulses, the longitudinal resolution improves
further (e.g., down to 2 microns using 20 fs pulses).
[0017] Sum-frequency gating and Kerr gating yield comparable
resolution when pumping with ultrashort pulses, but do not provide
amplification, and, in fact, usually are most inefficient. Photon
efficiencies typically do not exceed 10% with these systems. In
contrast, by using ultrafast, time-gated, optical parametric image
amplification (UTOPIA) it is possible to obtain both image
amplification and time-gating simultaneously. The parametric
amplification method of the present invention can be performed in
collinear or noncollinear geometries, can be either degenerate or
nondegenerate, and can employ type-I or type-II phase matching or
quasi-phase matching.
[0018] Further, the technique of the present invention can be used
in conjunction with either a confocal imaging system or a
conventional or Fourier imaging system. If collinear, degenerate
OPA is used, then the amplified contour image is superimposed on
the unamplified image at the same frequency (since the idler
frequency .omega..sub.1 is the same as the signal frequency
.omega..sub.s). This provides a convenient method of registration
between the contour image and the visual image of the object. With
degenerate OPA, the image amplification factor is sensitive to the
relative optical phase between the pump and signal pulses. In some
cases it may be desirable to obtain only the contour image with
maximum discrimination against any background light. In these
cases, it is advantageous to use nondegenerate UTOPIA which gives
simultaneous image amplification, time-gating, and frequency
conversion to the idler frequency. Illumination of the target with
pulses at a wavelength near 1550 nm is particularly advantageous in
many cases because this wavelength is considered to be eyesafe.
[0019] By illuminating the target with a single pulse, an isometric
contour (or contours) corresponding to a particular depth level of
the target surface (i.e., an optical section) is obtained. Then, by
adjusting the optical path length (time delay) traversed by either
the pump or signal pulses, a number of different contours can be
obtained, whose spacings correspond to the adjustments in optical
path difference. Thus, a multiple contour image can be built up
from a number of single-contour images. If, instead, the target is
illuminated by a sequence of N closely-spaced ultrashort pulses
during the pump pulse period, then a multiple contour image with
the contours corresponding to N different depth levels of the
target surface is obtained with a single pump pulse. If the pump
laser pulse is sufficiently powerful, then this multiple-contour
image can be acquired using a single laser shot, making it possible
to obtain topographic images of objects which are moving very
rapidly, e.g., even at hypersonic velocities. While
multiple-contour images have been obtained using interferometric
methods, the contours so obtained are very closely spaced (e.g., at
a fixed spacing of one wavelength of the light) which gives very
high resolution, and which limits the total depth which can be
probed with a CCD imaging system due to the finite number of pixels
which comprise the CCD array. The UTOPIA system of the present
invention can cover a large dynamic range in feature depth by
adjusting the spacings between the optical pulses in the sequence.
With resolution of 10 microns, it is still possible to map out a
depth range of over 100 mm with no ambiguity.
[0020] The choice of the nonlinear optical medium for performing
optical parametric amplification is an important aspect of the
present invention. The advantages of using a noncritical phase
matching geometry have been demonstrated in type I nonlinear
crystals. Quasi-phase-matched crystals have significant advantages
over type I and type II phase-matched crystals, as described by M.
Yamada et al. in Appl. Phys. Lett., Vol. 62, p. 436 (1993). In
particular, periodically-poled lithium niobate (PPLN) has a large
nonlinear coefficient and can be tailored to the desired phase
matching conditions, such as frequency and acceptance angle. PPLN
enables noncritical phase matching, thus increasing the acceptance
angle of the UTOPIA system. Thus, according to the invention, the
nonlinear optical medium is preferrably a periodically poled
ferroelectric optical material, including but not necessarily
limited to lithium niobate, lithium tantalate, MgO:LiNbO.sub.3, KTP
and crystals of the KTP isomorph family.
[0021] The above and still further objects, features and advantages
of the present invention will become apparent upon consideration of
the following detailed description of specific embodiments thereof,
particularly when taken in conjunction with the accompanying
drawings wherein like reference numerals in the various figures are
utilized to designate like components.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIG. 1 is a schematic diagram of an optical parametric
imaging system according to an exemplary embodiment of the present
invention.
[0023] FIG. 2 is a schematic diagram of an apparatus used to
experimentally demonstrate the fundamental operation of an imaging
system employing parametric image amplification with ultrashort
pulses.
[0024] FIG. 3 is a weakly amplified, magnified image of the letter
"a" created by a mask in the object plane of the imaging system
illustrated in FIG. 2.
[0025] FIG. 4 is a strongly amplified, magnified image of the
letter "a" using the imaging system illustrated in FIG. 2.
[0026] FIGS. 5(a)-5(c) are schematic diagram illustrating that the
nonlinear optical medium of the imaging system of the present
invention can be positioned in an image plane, the Fourier plane,
or some other arbitrary plane of the imaging system.
[0027] FIG. 6 is a schematic diagram of an imaging system according
to another embodiment of the present invention in which the target
is illuminated by a sequence of pulses to produce a
multiple-contour image from a single laser shot.
[0028] FIG. 7 is a schematic diagram of an imaging system according
to another embodiment of the present invention in which the pump
pulse and the signal pulse originate from separate laser systems
which are synchronized by an electronic synchronization
circuit.
[0029] FIG. 8(a) illustrates the basic arrangement of a
conventional confocal system, and FIG. 8(b) illustrates a confocal
system which employs optical parametric amplification in accordance
with the present invention.
[0030] FIG. 9 is a schematic diagram of a parametrically amplified
confocal imaging system using a waveguide nonlinear optical medium
as the amplifying medium and as the limiting aperture.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0031] According to the present invention, ultrafast time-gated
optical parametric image amplification (UTOPIA) is employed in
conjunction with a variety of imaging systems such as conventional,
Fourier, and confocal imaging systems. Advantageously, the present
invention amplifies signal light via optical parametric
amplification (OPA) which provides both high gain and low noise,
e.g., gains of up to 80 dB are commonly obtained with OPA.
Consequently, the quantum efficiency achieved by the present
invention in the described imaging systems is much greater than
with other techniques. As reported by M. I. Kolobov et al. in
"Sub-shot-noise microscopy: imaging of faint phase objects with
squeezed light," Optics Letters, Vol. 18, p. 849 (1993), parametric
amplification in combination with imaging also holds the
possibility of enabling sub-shot-noise imaging. In contrast to
previous OPA imaging systems using ultrashort pulses, the UTOPIA
system of the present invention can be used in reflection (in
addition to transillumination), thus giving optical sectioning with
depth resolution determined primarily by the pulsewidth. Also, the
use of quasi-phase-matched (QPM) nonlinear media provides both high
gain, noncritical phase matching, and large angular acceptance,
thus increasing the field of view. Quasi phase matching has
additional advantages over type-I and type-II phase matching in
terms of background-free amplification and superior time (range)
resolution. Nonlinear optical media which are preferred according
to the invention include periodically poled ferroelectric optical
materials, including lithium niobate, lithium tantalate,
MgO:LiNbO.sub.3, KTP and crystals of the KTP isomorph family.
[0032] An ultrafast tomographic optical parametric image
amplification system 10 for generating an image of a
three-dimensional object under test according to an exemplary
embodiment of the present invention is shown in FIG. 1. Imaging
system 10 includes an ultrashort pulse laser (USPL) 12 which
generates an ultrashort laser pulse at a pump frequency
.omega..sub.p. The ultrashort laser pulse from laser 12 is split
into two beams, a pump beam and an illumination beam, by a beam
splitter 14 or other conventional beam-splitting or beam-separating
mechanism. The ultrashort pulse of the illumination beam is
directed to a frequency conversion device 16, such as an optical
parametric generator, which frequency converts the pulse to a
signal frequency .omega..sub.s that is lower than the pump
frequency .omega..sub.p. The frequency-converted pulse at the
lower, signal frequency .omega..sub.s is used as the signal pulse
to illuminate a target object.
[0033] Imaging system 10 includes an image acquisition device 20,
such as a charge-coupled device (CCD) camera, a focal plane array,
or a vidicon used to form an image of the target object. For
convenience, the image acquisition device 20 will hereinafter be
referred to as a "camera" or "detector"; however, it will be
understood that the image acquisition device can be any appropriate
device for detecting or registering object image signals, including
the aforementioned devices. The imaging system may have only one
real image plane at the camera focal plane, or it may have one or
more intermediate real image planes between the object and the
camera. An imaging device 18 receives image signals acquired by
camera 20 and generates an image from the image signals and/or
records the image signals for later image generation. The imaging
device 18 can comprise any conventional image generation and/or
image storage device, including, but not limited to: a visual
display (e.g., a cathode ray tube or a light emitting diode array),
a printer, a photographic image generating device, and an image
signal recording or storage device (e.g., video tape, RAM, etc.).
It will be appreciated that, in addition to the imaging
applications descried above and hereafter, the invention, wihtout
necessarily imaging an object, finds application as a LIDAR
system.
[0034] As shown in FIG. 1, the target object can be illuminated
collinearly with the optical axis of the imaging system by
projecting the signal pulse toward the object along the optical
axis of the camera 20. Specifically, a beam splitter 21 lying along
the optical axis of camera 20 directs the signal pulse through a
lens system 23 toward the object. A nonlinear optical medium 22,
such as periodically-poled lithium niobate (PPLN), is positioned at
some arbitrary intermediate image plane in the imaging system along
the optical axis of camera 20, and serves as an optical parametric
amplifier. The signal pulse light scattered from the target is
collected by lens system 23, passes through beamsplitter 21 and a
dichroic mirror 25, and is imaged onto the nonlinear optical medium
22.
[0035] The pulse of the pump beam at the higher, pump frequency,
.omega..sub.p, is used as the pump pulse for the nonlinear optical
medium 22. In order to controllably synchronize the arrival of the
pump pulse and the reflected light of the signal pulse at the
nonlinear optical medium 22, the pump pulse is time delayed by an
adjustable or variable optical delay line 24. The pump pulse is
then directed onto the nonlinear optical medium 22 by dichroic
mirror 25. The pump pulse interacts with the signal radiation in
the nonlinear optical medium 22 in such a manner that the nonlinear
optical medium 22 amplifies portions of the image which arrive
synchronously with the pump pulse at the nonlinear optical medium
22. Simultaneously, a frequency-converted image is generated at the
idler frequency, .omega..sub.i, which is different from the signal
frequency .omega..sub.s in the case of nondegenerate optical
parametric amplification (OPA).
[0036] A frequency-selective filter 26 is used to block the light
at the pump and signal frequencies, allowing only the image at the
idler frequency .omega..sub.i to pass through lens system 27 to the
camera 20. The resulting image is a single contour or set of
contours which all correspond to surface features which are
equidistant (in terms of optical path length) from the reference
plane, which can be defined as the input surface of the nonlinear
optical medium 22. This contour is an amplified image of an optical
section of the object under test. Because the detection is
performed at the idler frequency, the detection is background
free.
[0037] By changing the optical delay experienced by the pump pulse
(i.e., by adjusting the adjustable optical delay line 24), a
similar contour is obtained which corresponds to a different
optical section of the object, displaced in depth from the previous
section by an amount equal to the change in optical path length of
the pump pulse. By repeatedly changing the optical delay of the
pump pulse and acquiring contour images, a complete
three-dimensional topographic image of the object surface or
tomographic image of the object interior can be built up from a
series of contour images.
[0038] The depth resolution is determined by the pulsewidths of the
pump and signal pulses, and by the group velocity walkoff between
the pump and signal pulses in the nonlinear optical medium 22. In
the case where the illumination is not collinear, the image
contours recorded by camera 20 do not correspond to sections which
are equidistant from the optical axis, but which are skewed with
respect to the optical axis; accordingly the detected image signals
must be mathematically interpreted to account for the relative
propagation angle.
[0039] While a blocking filter that passes the idler frequency and
blocks the signal and pump frequencies advantageously eliminates
background noise, the blocking filter can be configured to block
the pump radiation at frequency .omega..sub.p and the idler
radiation at frequency .omega..sub.1 while passing the signal
radiation at frequency .omega..sub.s. In this arrangement, the
image captured by the camera will consist of the amplified contour
image (corresponding to the timing of the pump pulse) superimposed
on the unamplified image of the whole object surface which is not
time gated. This combination of the amplified image section and the
unamplified surface image provides a convenient means of
registration between the visual image and the optical sections of
the surface.
[0040] In the degenerate case, where the signal and idler
frequencies are equal, the blocking filter blocks only the pump
radiation at frequency .omega..sub.p and passes the amplified image
at frequency .omega..sub.s=.omega..sub.i. In this case, the gain of
the OPA is dependent on the relative optical phase between the pump
and signal pulses when they are incident on the nonlinear optical
medium 22.
[0041] Referring to FIG. 2, a schematic diagram of an apparatus 30
used to experimentally demonstrate the operational principles of
parametric image amplification with ultrashort pulses is shown.
Pulses from a fiber chirped pulse amplification (CPA) system are
produced at a wavelength of 1550 nm, giving 70 mW average power, at
a repetition frequency of 20 kHz, and a pulse energy of 3.5 .mu.J.
These pulses are incident at the input of the apparatus 30, where
they are frequency doubled by a frequency-doubling crystal 32 to
produce 30 mW of 780 nm light. This light powers an optical
parametric generation (OPG) crystal 34, which is tuned to produce
1300 nm wavelength radiation. The residual 780 nm pump light is
separated from the 1300 nm light through a dichroic 36. The pulses
widths are approximately 700 fs in duration. The 1300 nm light is
used to illuminate the object, while the 780 nm light is used to
pump a periodically poled lithium niobate (PPLN) crystal 38, which
is the nonlinear optical material used to provide the image
amplification. More specifically, a beam splitter 40 directs the
reflected illumination light toward a dichroic 42 which combines
the reflected illumination light with the pump pulse and directs
them toward the PPLN crystal 38. The 1300 nm light reflected from
the object is passed through the PPLN crystal 38 so as to be
co-propagating with the pump beam. The delay between the 1300 nm
light reflected from the object and the 780 nm pump light is
adjusted by a variable delay line 44 to provide optimal temporal
overlap (and thus optimal gain) in the PPLN crystal. The output of
the PPLN crystal is then imaged onto a camera 46, and the image is
detected at 1300 nm (in this experimental configuration, a
frequency filter is placed between the PPLN crystal 38 and the
camera 46).
[0042] Using the experimental configuration shown in FIG. 2, the
amplification was first tested simply by using a mirror as an
object, resulting in the amplification of the 1300 nm pulse by a
factor of 300-350 when a pump pulse of 71 nJ at 780 nm was used.
The approximate beam area over which amplification occurred was 76
.mu.m.
[0043] The time gating aspect of the present invention was
illustrated by tuning the OPG crystal 34 to create many satellite
pulses, up to sixteen, separated in time. With the pump arm blocked
(i.e., the pump pulse was prevented from reaching the PPLN crystal
38), only a low background was detected by the camera 46. With the
pump arm unblocked, an image of the illumination beam appeared.
This image could be made to appear and disappear depending on the
timing of the pump beam pulse, indicating that selective
amplification of the individual satellite pulses was taking place.
This amplification was verified independently by cross correlating
the output.
[0044] Next, a small, thin mask of the letter "a" was placed at the
object plane. The "a" itself was black and of low reflectivity. The
mask was placed directly on the mirror, consequently producing a
reverse contrast image. The "a" is formed at the camera by the
absence of light, i.e., the image appears where there is no light.
The 1300 nm illumination beam was just large enough to slightly
overfill the mask. The image resulting from weakly amplified 1300
nm pulses is shown in FIG. 3. Note that the image contrast is
degraded due to light spilling over into areas of the "a" but
nonetheless it is visible and distinguishable. This degraded image
is attributable to spatially nonuniform gain due to the beam
profile of the pump beam; i.e., the highest gain is at the center
of the beam.
[0045] When the timing delay between the reflected and pump pulses
is changed by more than a pulsewidth, the image disappears,
demonstrating the time-gating aspect of the system. It should also
be noted that the image is magnified by approximately a factor of
five, so the system forms a low-power, wide-field microscope.
[0046] In FIG. 4, a strongly amplified image is shown. The image
areas of greatest intensity show the highest gains. That there is
more gain at the center of the image than at the edges is due to
the fact that a Gaussian beam mode is used in both the illumination
and pump beams. Thus, the field is not flat in amplitude. Contrast
in the image is reduced, but this is believed to be due to a
combination of the low initial image contrast, and lack of dynamic
range of the camera. The stronger amplification in the case of FIG.
4 is obtained by timing the reflected and pump pulses so that they
are more closely coincident within the nonlinear optical
medium.
[0047] Thus, the parametric amplification of a magnified image has
been demonstrated using the experimental apparatus shown in FIG. 2.
The measured gains, and pump fluences demonstrate that this
technique can be readily scaled to a confocal microscope system
using presently available lasers, microscopes, nonlinear media, and
detectors.
[0048] As shown in FIG. 5(a), the nonlinear optical medium can be
positioned at or near a real image plane of the object in the
imaging system. As shown in FIG. 5(b), the nonlinear optical medium
can also be positioned at or near a Fourier plane of the object in
the imaging system, resulting in a reduction in the degradation of
resolution of the target radiation which normally accompanies
optical parametric nonlinear interactions in a medium of finite
thickness, as pointed out in U.S. Pat. No. 3,629,602, to Firester.
The amplified radiation at either .omega..sub.s or .omega..sub.1
must then, in turn, be optically Fourier transformed back to a real
image of the object. This can be accomplished by a single lens in
the simplest case. The imaging system may not have any real image
plane within the system between the object and the camera. In this
case, the nonlinear optical medium is inserted at any arbitrary
plane between the object and the camera, as shown in FIG. 5(c).
[0049] FIG. 6 is a schematic diagram illustrating an imaging system
60 according to another embodiment of the present invention in
which the target is illuminated by a sequence of N pulses at the
signal frequency .omega..sub.s. The pulse sequence is obtained by
passing the signal pulse through a pulse shaper 62. The scattered
target radiation is then selectively, parametrically amplified in
the nonlinear optical medium by interaction with the pump pulse.
Only those portions of the radiation are amplified which correspond
to particular sections of surface, resulting in a multiple contour
image of the object surface. The longitudinal separation between
sections of surface corresponds to the spacing between signal
pulses emanating from the pulse shaper 62 and illuminating the
target. It is thus possible to obtain a multiple contour image of
the target surface using a single laser shot without recourse to
adjusting the optical path length traversed by the pump pulse.
[0050] FIG. 7 illustrates an imaging system 70 in accordance with
another embodiment of the present invention, wherein two separate
ultrashort pulse laser sources 72 and 74 are used to generate the
pump pulses and the signal pulses, respectively. Laser sources 72
and 74 are electronically synchronized via a synchronization unit
76. Using this arrangement, it is no longer necessary to insure
that the optical path lengths traveled by the pump pulse and signal
pulse are substantially equal to insure coincidence inside the
nonlinear optical medium. Near coincidence can be insured by
electronically adjusting the timing delay between the two laser
sources via synchronization unit 76. (See, U.S. Pat. No. 5,778,016
to Sucha et al.) This synchronization enables topographic sections
of surfaces to be obtained by the previously described UTOPIA
techniques even for remote objects (e.g., at a range of 100 meters)
for which the equivalent optical delay would be impractically
large.
[0051] In accordance with another embodiment, the optical
parametric image amplification technique of the present invention
can be applied to confocal microscopy, e.g., imaging in a confocal
single-photon excitation, laser fluorescence, microscope system.
The intensity point spread function for such a system produces a
quadratic dependence in the detected intensity, resulting in the
optical sectioning capability of the system, as reported by M. Gu
et al. in "Three-dimensional image formation in confocal microscopy
under ultra-short-laser-pulse illumination," Journal of Modern
Optics, Vol. 42, No. 4, pp. 747-762 (1995).
[0052] An example of a single-photon fluorescence microscope 80
having a conventional confocal geometry for imaging in the
back-scattered or reflective direction is illustrated in FIG. 8(a).
Light having a first wavelength is transmitted from light source 82
through dichroic mirror 84 and imaging lens 86 into the specimen 88
along an image plane. The incident light excites a fluorescent
medium which has been introduced into the specimen 88, and causes
the fluorescent medium to emit light of a different wavelength. The
back-scattered light emitted by the fluorescent medium in the
specimen 88 is reflected by the dichroic mirror 84 towards a
detector 90. The same technique can be applied in a transmissive
geometry as well. For the single photon case, in a normal confocal
geometry, there will be appreciable fluorescence throughout the
focal volume. Hence, a pinhole 92 at the conjugate image plane must
be used to block the out-of-focal-plane fluorescence, in order to
generate an image.
[0053] Because of the intensity-squared dependence, a confocal
system is able to produce two-dimensional, cross sectional images
of microscopic objects. A series of these two dimensional
cross-sections taken at different axial levels within the specimen
can be recombined to produce a full high-resolution,
three-dimensional image of the specimen. The resolution of this
system in part is determined by the pinhole detector size. An
infinitely small pinhole produces the highest resolution, but
because no photons are detected, the signal-to-noise ratio is zero.
Thus, the pinhole size is chosen to yield the most optimal
compromise between resolution, and signal-to-noise ratio.
[0054] The resolution of a confocal microscopy system can be
enhanced by the time gating feature of optical parametric
amplification. In accordance with one novel aspect of the present
invention, parametric image amplification is used for the first
time with confocal imaging systems. The parametric image
amplification is to be contrasted with previous parametric image
amplification schemes, such as in aforementioned U.S. Pat. No.
3,629,602 to Firester, in that these conventional systems to date
have tried, with limited success, to amplify images with high
(nonzero) spatial frequency content. By scanning the object with a
confocal system, the DC or zero spatial frequency is amplified at
all times. The higher spatial frequencies are purposely
discriminated against by the confocal system. Thus, while
traditional parametric imaging systems are required to account for
the effect of the amplification process on the spatial frequency
spectral amplitude and phase content of the image, the technique of
the present invention need consider only the effect on the
amplitude of the DC component. (Spatial frequencies are assigned to
the object and the confocal imaging system. The confocal system has
a nonzero spatial frequency optical transfer function. This
transfer function enables both the illumination beam and the
detected fluorescence to be focused to a point. The spatial
frequencies described herein refer to those found in the object,
not the microscope.) In this manner, amplified images of higher
resolution are attainable. Further, by performing confocal
parametric image amplification, amplified, high-resolution, three
dimensional images are possible for the first time.
[0055] In relation to confocal microscopy, the time-gating aspect
of the present invention has advantages related to reducing the
effects of scattered light. Many multi-photon confocal systems
benefit from not having a pinhole at the detector, e.g., the
fluorescence or reflected signal does not need to be descanned and
the detector can be a two dimensional array such as a CCD, which
results in a considerable simplification of the system. In
addition, the frame rate capability of this type of confocal system
tends to be quite high, readily achieving real-time, video rates
for example. The difficulty of many of these systems is that they
are unable to discriminate against the scattered light. In
accordance with the present invention, parametric image
amplification of the signal in these systems is used to time gate
against the scattered light, improving the signal-to-background
ratio in these systems. Such time gating can also be employed to
aid in confocal systems which have pinhole detection.
[0056] Referring to FIG. 8(b), a confocal microscope 100 in
accordance with the present invention includes a nonlinear optical
medium 102 placed at the detector location to parametrically
amplify the detected signal. No pinhole is provided in this system.
An illumination pulse having a signal frequency .omega..sub.s is
transmitted through dichroic mirror 104 and imaging lens 106 into a
specimen 108 along an image plane. The incident light pulse excites
a fluorescent medium which has been introduced into the specimen
108, and causes the fluorescent medium to emit light of a different
wavelength. The back-scattered light emitted by the fluorescent
medium in the specimen 108 is reflected by the dichroic mirror 104
towards a detector 110. A pump pulse having a pump frequency
.omega..sub.s is transmitted from a laser source 112 through
dichroic mirror 104 and is incident on nonlinear optical medium
102, such that the pump pulse is spatially and temporally
overlapping with the back-scattered light emitted by the
fluorescent medium in the specimen 108. The same technique can be
applied in a transmissive geometry as well. For purposes of
illustration, the pump rays and reflected imaging light rays
converging toward the nonlinear optical medium 102 in FIG. 8(b) are
separated spatially; specifically, the imaging light is shown
converging at a greater angle than the pump light. It will be
understood that, in the actual system, the pump light and reflected
imaging light are made to overlap, i.e., they converge at
substantially the same angle.
[0057] Note that due to the confocality condition, the nonlinear
optical medium 102 is not required to be pumped with a planar beam,
but can use a focused beam. This is in contrast to traditional
parametric image amplification techniques which require an
essentially planar pump beam to preserve image fidelity. The
amplified signal produced in this manner, results in improvements
in signal-to-noise ratio and resolution. Additionally, it can be
used to lower the excitation power of the illuminating beam, which
results in increased observation time (bleaching rates are lowered)
and enhanced cell viability. In general, cells absorb the short
wavelength excitation light, resulting in aberrant behavior or cell
death. By lowering the excitation powers necessary for imaging, the
cells absorb less energy, and remain viable for longer periods.
These improvements are due to the gain provided by parametric
amplification.
[0058] Assuming square pulses (in time) these gains can be roughly
estimated. Time gating with a 100-fs pulse of the detected
fluorescence results in a net reduction of photons by (100-fs
gate)/(1-10-ns fluorescence)=10.sup.4 to 10.sup.5 when detected at
the background free (idler) frequency. (Clearly, this method also
benefits by using a gating pulse of 1 to 10 ns in duration--roughly
equivalent to the fluorescent lifetime of the reporter molecule.
Alternatively, a "burst" of gating pulses can be used, the burst
lasting for the fluorescent lifetime. The number of pulses in the
burst can be as many as can be conveniently produced.) However,
assuming a conservative gain of 10.sup.6 amplification, there still
is a net gain of 10-100. This conservative gain number is used,
since the parametric amplification process also creates noise
photons. At these gains, the parametric noise is minimal and there
is a net increase in overall image signal-to-noise ratio. Thus, the
pinhole can be reduced in size proportionately without sacrificing
signal-to-noise ratio. By detecting at the signal frequency there
is no reduction of the net photons, simply the net gain of 10.sup.6
in signal over the gating pulse period. Note that, in this simple
example, the fact that an incoherent source is being amplified has
been ignored. Only the dipoles aligned with the correct
polarization to be phase-matched in the parametric amplifier will
be amplified.
[0059] The optical parametric image amplification technique of the
present invention can also be applied in a two-photon excitation
confocal microscopy system. In two-photon confocal microscopy, a
pinhole is not necessary at the detection plane, as the optical
sectioning is inherent to the two-photon absorption process that
scales as the square of the excitation intensity. See, M. Gu et
al., "Effects of a finite-sized pinhole on 3D image formation in
confocal two-photon fluorescence microscopy," Journal of Modern
Optics, Vol. 40, No. 10, pp. 2009-2024 (1993). However, the
combination of two-photon absorption with a pinhole detector does
result in a sharper point spread function in the paraxial
diffraction theory limit when compared to the single photon case.
Thus, parametric image amplification can be employed as in the
single photon excitation case, in identical geometries, with
comparable or potentially superior gains in resolution.
[0060] The optical parametric image amplification technique of the
present invention is also applicable when scanning confocal
microscopy using pulsed illumination is employed. See, S. Hell et
al., "Pulsed and cw confocal microscopy; a comparison of resolution
and contrast," Optics Communications, Vol. 113, pp. 144-152 (1994).
In this instance, the detected frequency is the excitation
frequency as in a standard microscope. The parametric amplified
signal in this case is then just the excitation light that is
reflected (or transmitted depending on the geometry of the
microscope) back from the specimen.
[0061] The optical parametric image amplification technique of the
present invention is also applicable when harmonic confocal
microscopy is used. In this case, a harmonic of the excitation
frequency is detected and used for image formation. The harmonic
can be generated from the specimen itself, as in third harmonic
interface imaging, as reported by M. Muller et al. in
"3D-microscopy of transparent objects using third-harmonic
generation," Journal of Microscopy, Vol. 191, No. 3, pages 256-274,
1998, or can be the result of reporter molecules designed to
produce a harmonic of the excitation frequency.
[0062] In each of the aforementioned techniques, the optical
parametric amplification process has been described as a method of
improving the resolution of confocal microscopy instruments. It is
important to note, however, that the same embodiment can also be
used simply to amplify a weak image. The parametric amplification
technique can then be used as previously described, but no
additional reduction in pinhole size is necessary. This results in
an increase in image intensity, but no increase in image
resolution. Conversely, the excitation power can be lowered to
reduce damage to the specimen, and the parametric amplification
used to compensate for the lower power. In this manner, the image
remains comparable in resolution and signal-to-noise ratio, but the
specimen is exposed to less damaging radiation, thus extending
specimen viability.
[0063] The application of time gating with optical parametric
amplification to discriminate against scattered signals can achieve
the same effect as optical coherence tomography (OCT). To date,
most OCT systems for imaging through scattering media accomplish
this discrimination through interferometry, though recently
attempts have been made at imaging in diffuse media using
degenerate optical parametric amplification, as reported in the
aforementioned article by J. Watson et al. Improvements in the
sectioning discrimination and the penetration depth have been
obtained by combining OCT with confocal microscopy, as described by
Izatt et al. in "Optical coherence microscopy in scattering media,"
Optics Letters, Vol. 19, p. 590 (1994). The advantage of time
gating in the non-degenerate case is that interferometric
sensitivity is not required, and background free operation is
possible. In the case of degenerate OPA, the system is essentially
a form of OCT, but provides amplification without any additional
background noise from a "local oscillator," as occurs with the
heterodyne gain obtained with conventional OCT systems. Further,
phase information from the illuminating beam may be more readily
extracted in the time gating geometry as opposed to the
interferometric, using for example, any of a number of the recently
developed frequency-resolved optical gating schemes. See, e.g., R.
Trebino et al., "Measuring ultrashort laser pulses in the
time-frequency domain using frequency-resolved optical gating,"
Rev. Sci. Instrum. 68 (9), September 1997.
[0064] By operating the confocal UTOPIA system in the degenerate
mode (i.e., .omega..sub.s=.omega..sub.1), the gain becomes
interferometrically sensitive to the relative optical phase between
the pump pulse and the signal pulse scattered from the object. In
this case, the system is a particular form of OCT in which the
signal gain is provided by parametric amplification rather than by
the heterodyne gain mechanism present in conventional OCT. This
circumvents certain disadvantages of heterodyne gain, the chief one
being the increase in noise background induced by the strong "local
oscillator." Since parametric amplification is inherently quiet,
this system provides the advantages of OCT systems with a quieter
mechanism for signal amplification. This system can be implemented
using the configuration shown in FIG. 8(b). The time delay between
the pump and signal pulses is rapidly scanned, as in an OCT
system.
[0065] The system shown in FIG. 8(b) is essentially a bulk optic
system. Referring to FIG. 9, an imaging system 120 in accordance
with another embodiment of the present invention includes a
nonlinear optical medium formed in a PPLN (or other QPM) waveguide
structure 122. In this case, no additional aperture (e.g., a
pinhole or the like) is required near the detector because the
single-mode nature of the waveguide provides an effectively very
small aperture. The advantage of the nonlinear optical medium
waveguide structure 122 is that it has a greatly reduced pump power
requirement, allowing the use of a simple modelocked laser
oscillator (unamplified) as the pump source. This feature provides
a great reduction in system size, complexity and cost.
[0066] In accordance with another embodiment of the present
invention, the combination of optical parametric amplification and
confocal microscopy can be used to make fluorescent lifetime
imaging measurements. In this case, the actual geometry of the
imaging system remains the same as in FIG. 8(b). However, two
gating pulses are used which are separated by a variable time
delay. Two separate images are formed: one with the first gating
pulse, and one with the second gating pulse. The first image is
then divided by the second. The value of this ratio is related to
the local lifetime of the reporter molecule. For instance, if there
has been little lifetime decay the ratio is approximately unity.
For substantial decay, the ratio approaches zero. A previously
determined look-up-table (LUT) assigns a value to the fluorescent
lifetime on a pixel-to-pixel basis within the image. A series of
these images taken with various time delays between the two gating
pulses increases the accuracy of the technique.
[0067] This method has the advantage over other lifetime imaging
techniques in that very slow detectors can be used, and saturation
of the molecule (which results in a loss of image resolution) is
not required as in double pulse fluorescent lifetime imaging. See,
A. Buist et al. "Double pulse fluorescent lifetime measurements,"
Journal of Microscopy, 186 (3) 212 (1997). Further, this technique
works with either single photon or two photon excitation, in
contrast to those techniques that require saturation of the
fluorophore.
[0068] Having described preferred embodiments of a new and improved
method and apparatus for optical sectioning and imaging using
time-gated parametric image amplification, it is believed that
other modifications, variations and changes will be suggested to
those skilled in the art in view of the teachings set forth herein.
It is therefore to be understood that all such variations,
modifications and changes are believed to fall within the scope of
the present invention as defined by the appended claims.
[0069] The disclosures of all of the aforementioned articles and
patents are incorporated herein by reference in their
entireties.
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