Imaging system with confocally self-detecting laser

The invention relates to a confocal laser imaging system and method. The system includes a laser source, a beam splitter, focusing elements, and a photosensitive detector. The laser source projects a laser beam along a first optical path at an object to be imaged, and modulates the intensity of the projected laser beam in response to light reflected from the object. A beam splitter directs a portion of the projected laser beam onto a photodetector. The photodetector monitors the intensity of laser output. The laser source can be an electrically scannable array, with a lens or objective assembly for focusing light generated by the array onto the object of interest. As the array is energized, its laser beams scan over the object, and light reflected at each point is returned by the lens to the element of the array from which it originated. A single photosensitive detector element can generate an intensity-representative signal for all lasers of the array. The intensity-representative signal from the photosensitive detector can be processed to provide an image of the object of interest.

BACKGROUND OF THE INVENTIONS 
The invention generally relates to optical instruments and methods. More 
particularly, the invention relates to a confocal laser system for 
scanning a surface or other object with a laser beam and generating an 
image of the object. 
Optical scanning imaging techniques are employed in devices such as 
scanning laser microscopes (SLM), confocal scanning laser microscopes 
(CSLM), tandem scanning confocal microscopes (TSM), scanning laser 
ophthalmoscopes (SLO), and flying spot television (FSTV) devices. Confocal 
imaging systems can provide enhancements in contrast and in dynamic range. 
Certain of these imaging systems include moving optical elements for 
deflecting a laser beam, so that an illumination spot is swept across the 
object to be scanned. Other such systems employ mechanical elements to 
rotate an illuminated pinhole for the same purpose. In the TSM, a 
plurality of illumination spots is moved simultaneously, to provide source 
multiplexing, necessary because the source does not have the higher 
radiance (brightness) of a laser. 
A double scanning optical apparatus is disclosed in U.S. Pat. No. 4,764,005 
of Webb et al., the teachings of which are incorporated herein by 
reference. The apparatus utilizes multiple scanning elements, including a 
multifaceted rotating polygonal reflector scanner, to provide scanning of 
both incident and reflected light at television-rate frequencies. 
Additionally, certain flying spot imagers us a cathode ray tube (CRT) as a 
light source, with a single illuminated point scanned across the CRT face. 
The tube face is imaged onto the object to provide the illumination 
raster. 
A TSM is discussed in Petran et al., "Tandem-Scanning Reflected-Light 
Microscope," Journal of the Optical Society of America, Vol. 58, No. 5, 
pp. 661-664, May 1968. Petran et al. observe that reflected-light 
microscopy of semi-transparent material is usually unsatisfactory because 
of low contrast and light scattering. In the TSM, in which both the object 
plane and the image plane are scanned in tandem, only light reflected from 
the object plane is included in the image. In the Petran et al. system, 
the object is illuminated with light passing through holes in one sector 
or side of a rotating scanning disk, known as a Nipkow disc. The scanning 
disk is imaged by the objective at the object plane. Reflected-light 
images of these spots are directed to the diameterically opposite side of 
the same disk. Light can pass from the source to the object plane, and 
from the object plane to the image plane, only through optically congruent 
holes on diametrically opposite sides of the rotating disk. This 
configuration produces an image having enhanced contrast and sharpness 
relative to a conventional reflected-light microscope. 
Tandem scanning confocal arrangements and flying spot CRT configurations, 
however, are "light-starved" by the limited brightness of the illumination 
spot. In TSM configurations, this brightness limitation is partially 
compensated by the multiplex operation. TSM systems, however, are hampered 
by stray light scattered from the moving pinhole array. A general 
reference for microscopy is The Handbook of Biological Confocal 
Microscopy, Pawley, 2nd. ed., Plenum, 1991. 
A further advance in confocal scanning laser microscopy is disclosed in 
U.S. Pat. No. 5,028,802 (Webb et al. ) in which a laser source comprising 
a microlaser array scans the surface of an object. A beam splitter directs 
the light reflected from the object to a detector array. The detector 
array can be scanned in synchrony with scanning the microlaser array to 
detect light reflected from the object due to each microlaser in the laser 
source. Here, the "scanning" may be entirely an electronic switching 
operation, reducing the alignment, spatial filtering, optical aberration 
and mechanical distortion problems associated with most optical scanning 
arrangements. 
An alternative approach to scanning laser microscopy is disclosed in 
Juskaitis et al., "Fibre-Optic Based Confocal Scanning Microscopy with 
Semiconductor Laser Excitation and Detection," Electronics Letters, Vol. 
28, No. 11, pp. 986-988, May 1992; Juskaitis et al., "Fibre-Optic Based 
Confocal Microscopy Using Laser Detection," Optics Communications, Vol. 
99, No. 12, pp. 105-113, December 1992; and Juskaitis et al., "Spatial 
Filtering by Laser Detection In Confocal Microscopy." Optics Letters, Vol. 
18, No. 14, pp. 1135-1137, July 1993. As with previous systems, Juskaitis 
et al. image an object by projecting a laser beam at the object. However, 
instead of detecting reflected light from the object, Juskaitis et al. 
feedback the reflected light to the laser source. The laser source in 
turn, increases or decreases its power in response to the remitted light. 
Juskaitis et al. detect an image signal as a modulation on a power-monitor 
signal derived from a power monitor diode, located behind the laser 
source. Alternatively, in the case of a semiconductor laser, they measure 
the drive voltage to the laser. 
Further improvements over the above discussed systems are desirable. By way 
of example, the aforesaid Webb et al. patent solves light starvation 
problems and uses no moving parts, however, requires an array of detectors 
that is scanned in synchrony with a microlaser source array to achieve 
confocality. Arrays of detectors can be costly and can complicate the 
design of the microscope. By way of further example, Juskaitis et al. 
attempt an alternative approach to using the reflected light from the 
object to develop the imaging signal. However, Juskaitis et al. employ 
mirrors to scan the image of a laser source over the object being scanned. 
Accordingly, an object of the present invention is to provide an improved 
confocal laser imaging system that requires no moving parts. 
Another object of the present invention is to provide a compact and 
reliable laser imaging system that generates an image by detecting output 
intensity of a laser source. 
Other general and specific objects of the invention will in part be obvious 
and will in part appear hereinafter. 
SUMMARY OF THE INVENTION 
The invention relates to a confocal laser imaging system and method using 
self-detection. According to one preferred embodiment, the system includes 
a laser source which is preferably a laser array, a focusing element and a 
photosensitive detector. The laser source emits a laser beam which is 
focused along a first optical path at the object to be imaged. Light 
reflected back to the laser source from the object modulates the intensity 
of the laser beam. A beam splitter is located in the first optical path to 
deflect a portion of the laser beam from the first optical path along a 
second optical path onto the photosensitive detector. The photosensitive 
detector detects the intensity of the deflected portion of the laser beam, 
and generates an electrical signal indicative thereof. The generated 
electrical signal can be processed to yield an image of the object. The 
object can be for example, a liquid, a gas, or a solid. 
According to another embodiment, the invention includes a cube beam 
splitter. The cube beam splitter includes first and second opposing 
surfaces in the first optical path and a third surface in the second 
optical path, with a beam splitter for deflecting a portion of the laser 
beam from the first optical path along the second optical path. The laser 
source can be coupled to the first surface and the photosensitive detector 
can be coupled to the third surface. Additionally, a lens or other 
objective assembly can be mounted on the second surface for focusing the 
laser beam from the laser source to the object and for directing the light 
reflected from the object, due to the laser beam, back to the laser 
source. A lens can also be mounted on the surface for focusing the 
deflected portion of the laser beam onto the photosensitive detector. 
The laser source can be an electrically scannable microlaser array, 
including elements for generating a line or matrix array of discrete 
non-overlapping laser beams in response to electrical excitations 
addressed to the laser. Similarly, the photosensitive detector can be a 
single detector or a detector array. According to one preferred 
construction, the detector is a single avalanche photodiode detector, 
which receives output light from all or a plurality of the lasers of an 
array. According to another embodiment, the detector can be an array of 
avalanche photodiode detectors. As described in detail below, where a 
detector array is employed, the detectors of the array detect in unison. 
In the case where the laser source is a scannable microlaser array, the 
system includes addressing elements for generating time-varying electrical 
excitations to address the laser in an excitation scanning pattern. The 
beam splitter can also deflect a portion of each of the non-overlapping 
laser beams along the second optical path. Additionally, the 
photosensitive detector can be a single detector having sufficient 
detecting area such that each portion of the non-overlapping laser beams 
is incident on the single detector. For example, a microscope objective 
may image the entire lasing area onto a one millimeter diode. 
The invention includes a method for generating an image of an object. 
According to the invention, the method includes the following steps: 
projecting a laser beam along a first optical path at the object; 
directing the laser light reflected from the object back into the laser 
source to modulate the intensity of the laser beam; deflecting a portion 
of the laser beam from the first optical path along a second optical path 
to a photosensitive detector; detecting the intensity of the deflected 
portion and generating an electrical signal representative thereof; and 
generating an image of an object from the electrical signal. 
The invention will next be described in connection with certain illustrated 
embodiments. However, it should be clear to those skilled in the art that 
various modifications, additions and subtractions can be made without 
departing from the spirit or scope of the claims.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS 
The invention provides a confocal scannable laser imaging system that 
employs self-detection. According to the invention, the system illuminates 
a surface or other object with a laser beam, modulates the intensity of 
the laser beam in response to light reflected from the object, detects 
changes in the intensity of the of the laser beam reflecting this 
modulation, and generates an image of the object from the detected 
changes. 
FIG. 1 shows a block diagram of a confocal scannable laser imaging system 
in accordance with the invention. A laser source 10 illuminates an object 
18 with laser beams 14. The object 18 reflects portions of the beams 14 
back to the laser source 10. In response to the light reflected back from 
the object 18. Basically, the reflected light returned to the laser source 
10 from the object 18 reenters the laser cavity and affects the output 
intensity of the laser source 10. Thus, the source 10 experiences 
increases or decreases in intensity that correspond to the reflectance of 
the object at which its beam is directed. This property is referred to 
below as self-detection. 
A beam splitter or other suitable optical device (S, FIG. 3) directs a 
portion 15 of the light from laser beams 14 to a photodetector 20. The 
photodetector 20 provides an image signal via conductor 21 to an image 
output device 22. The photodetector 20 produces a time-varying image 
brightness signal on conductor 21 in response to changes in the intensity 
of laser light 15. Thus, the image signal is representative of the 
reflectance of the object 18. The image output device 22 processes the 
image signal on conductor 21 to generate an image of the object 18. 
According to the depicted embodiment, the laser source 10 is a N.times.M 
scannable microlaser array, where N and M are typically in the range of 
500 to 1000. In one prototype construction, the laser source was a 
10.times.10 matrix of microlasers, addressable in row--column fashion, 
each having a coherence length of approximately 70 mm. However, microlaser 
arrays of varying sizes and of varying coherence lengths can be employed 
with the invention. A laser scan drive 16 controls the time at which each 
microlaser in the array 10 is energized. The laser scan drive 16 can, for 
example, energize the microlasers in a sequence such that the array 10 is 
scanned in a conventional television raster fashion, at television scan 
rates. 
When energized, each microlaser in the array 10 illuminates a different 
portion of the object 18 with a laser beam 14. This is preferably achieved 
by an objective focusing assembly, such as a lens. During operation, the 
laser scan drive 16 sequentially scans the source 10, via drive line 23, 
to energize each microlaser. As reflected laser light from a particular 
microlaser source is reflected back to the microlaser source from the 
object 18, the particular microlaser source automatically, and essentially 
instantaneously, modulates its output intensity. The photodetector 20 
detects the output intensity from each microlaser by way of a deflected 
portion 15 of the laser light, and provides a correspondingly modulated 
output signal, while the source is on, along line 21 to image output 
device 22. The laser scan drive 16 controls the microlaser array 10 to 
provide scanned, e.g., raster illumination of the object 18. The drive 16 
also provides a SYNCH signal on conductor 24 to the image output device 
22. The SYNCH signal enables the image output device 22 to coordinate each 
image signal from detector 20, due to actuation of each microlaser source, 
synchronously with the electronic scanning actuation of source array 10. 
In this way, the image output device 22 provides a video image of the 
object 18. 
FIG. 2 depicts a scannable microlaser source 10 according to one embodiment 
of the invention. More particularly, FIG. 2 shows an array 10 of 
microlasers 12. Such microlasers can be, for example, low threshold 
electrically-pumped vertical-cavity surface-emitting diode lasers. A 
two-dimensional microlaser array of this embodiment is contained on an 
integrated circuit fabricated by AT&T Bell Laboratories, Murray Hill, N.J. 
Other microlaser arrays are available from Photonics Research, Inc., 
Broomfield, Colo. The lasers emit light perpendicularly to the surface of 
the chip. Each square centimeter of the chip contains approximately two 
million individual lasers. 
Current microlaser arrays consist of two interference mirrors formed by 
alternating layers of aluminum arsenide an gallium arsenide, around an 
active region of indium gallium arsenide, all grown on a gallium arsenide 
substrate. This structure is covered with a gold electrical contact and 
etched by chemically assisted ion beam lithography to form cylindrical 
lasers. 
A preferred practice of the invention utilizes a microlaser source array 10 
in which the individual lasers have diameters of between 1 .mu.m-5 .mu.m. 
In a typical embodiment, the microlasers are 2 .mu.m in diameter on 
centers 2 .mu.m apart, so that a 512.times.512 array, for example, is 
approximately 1 mm square. Those skilled in the art will appreciate that a 
512.times.512 array can be imaged to provide a level of detail compatible 
with current television, and that proposed high-definition television 
standards will require a 1024.times.1024 array. The microlaser array is 
preferably incorporated in conjunction with one or more lenses L. In a 
presently preferred embodiment, a single objective lens, such as a ten or 
twenty power microscope objective, may focus the entire laser array on the 
object, assuring that each laser receives back, and is modulated by, light 
reflected from the point which it illuminates. 
FIG. 3 depicts a microlaser microscope 100 with self-detection in 
accordance with the invention. A beam splitter S directs a portion of the 
light from scanned source array 10 onto the detector 20. Lens L focuses 
the light from source array 10 onto the object plane OB.sub.0, and also 
directs light reflected from the object back to source array 10. As used 
herein, the term lens refers to any optical assembly having focusing power 
for imaging, including both simple and compound lenses, curved mirrors, 
arrays of focusing elements or the like. The confocal configuration 
depicted in FIG. 3 employs a lens L' to direct a portion of the light 
emitted from source 10 onto corresponding regions of a detector 20, in a 
manner discussed below. Detector 20 can be, for example, a single 
avalanche photodiode detector (APD). Alternatively, detector 20 can be an 
array of detector elements in order to multiplex signal acquisition. Where 
detector 20 is an array, each detector in the array detects light from a 
portion of source 10 in unison. 
According to the invention, each laser 12 functions both as a confocal 
"moving" pinhole and as a "moving" spot laser. During operation of one 
embodiment, each microlaser 12 is energized for a period of time and then 
de-energized for a period of time, so that the array 10 appears to have a 
moving spot of light running along it. Lens L' focuses the whole array 10 
onto an APD 20 having a diameter of 0.7 mm. Accordingly, the APD 20 
detects light from the source 10, not light reflected from the object 
OB.sub.0. Light remitted from the object OB.sub.0 falls on its originating 
laser 12, so the small emitting face functions as a pinhole aperture or 
stop in the confocal condition, and causes that laser 12 to lase more 
strongly. One way to view this is that the laser 12 is acting 
simultaneously as a light amplifier, adding the amplified light to its 
output, which the APD 20 detects. The lasers 12 have a bandwidth of about 
10 nm, so the coherence length is approximately 70 mm. With this 
arrangement, bright parts of the object result in well defined differences 
in laser output intensity. 
In another realization of the invention, the laser drive voltage can be 
sensed. Although the drive voltage is in some ways more direct, it lacks 
the noise free gain of the APD 20 and is more complex to implement. 
The microlaser microscope differs optically from other confocal laser 
microscopes in that scanning takes place at an object plane OB.sub.0. All 
optical devices have two sets of distinct planes--those conjugate to the 
object and image and those conjugate to the pupil or aperture. Angles in 
one of these sets of planes translate to positions in the other set. All 
other scanning laser confocal microscopes use some device in an aperture 
plane to change the angle of the laser beam(s). That procedure then 
changes the position of the laser spot in the object planes. The confocal 
scanning laser microscope 100 is more like the disk scanning confocal 
microscopes in that its scanning takes place in an object plane. That 
difference allows fewer optical components, as shown in FIG. 3, because 
there need be no access to an aperture plane. 
One of the advantages of the microlaser microscope is that it can be 
multiplexed. That is, more than one laser 12 can be energized at once. It 
is not desirable to attain the level of multiplexing used by the disk 
scanners, since our VCSELs do better on a low duty cycle. Thus, according 
to one embodiment, the invention multiplexes, for example, a 
1000.times.1000 array of microlasers onto an array of one hundred APDs. 
According to other embodiments, different size laser source arrays can be 
multiplexed to different size detector arrays. In the case where 
multiplexing is employed, the source array 10 can be spatially divided 
into subarrays of microlasers. One photosensitive detector in the detector 
array 20 can be assigned to each subarray of microlasers. At any 
particular instant in time, one microlaser 12 in each subarray can be 
energized. However, all of the detectors in the array 20 detect in unison. 
Because the entire system can be implemented with one solid state device, a 
microscope constructed in accordance with the invention can be as small as 
a few millimeters in size. In particular, a confocal scanning laser 
microscope can be housed in a container approximately the size of a 35 mm 
film canister, or smaller. This compact size renders the invention 
especially useful for remote sensing applications, endoscopy, hand-held 
inspection microscopes, and many other applications requiring a small 
imaging system. 
FIG. 4 depicts a compact embodiment employing self-detection in accordance 
with the invention. The embodiment of FIG. 4 employs a cube beam splitter 
26. The microlaser source array 10 is mounted to a first surface 28 of the 
cube 26, and emits beams of light along a first optical path at the object 
18 through an objective lens or relay lens L. Lens L is mounted on a 
second surface 30 opposing surface 28. The lens L directs light from the 
source 10 to the object 18 and also directs light reflected from the 
object 18 back to its originating point on the surface of the laser source 
10. A photosensitive detector 20 mounts to a third surface 28 of the cube 
26. As discussed above, the detector 20 can be either a single detector of 
sufficient surface area or a scan addressable array of photosensitive 
detectors. The prism or cube 26 includes a beam splitter S' for directing 
a portion of the light from laser array 10 to the detector 20 by way of 
lens L'. As in the embodiments discussed previously, the detector monitors 
the output intensity of each of the microlasers 12 included in array 10 to 
generate an imaging signal, as shown in FIG. 1. The lens L' assures that 
all laser light reflected by the beam splitter reaches the photodiode. 
FIG. 5 depicts an alternate embodiment of the compact system of FIG. 4, the 
lenses L and L' can be replaced by a single lens L" coupled between the 
laser array 10 and the first surface 28. The lens L" can direct light from 
the source 10 to the object 18 and to the detector 20. The lens L" also 
directs light reflected from the object 18 back to its originating point 
on the surface of the laser source 10. 
A laser imaging device according to the present invention may also be 
implemented as shown in FIG. 6, requiring some special device fabrication 
rather than off-the-shelf components. In this embodiment, a laser array 
10' such as a VCSEL array is specially fabricated to emit a portion of its 
laser output from each end of the array. Thus, the row and column 
electrodes for matrix addressing, are desirably transparent, or partially 
reflective, defining lasing mirrors, and are fabricated on the front and 
back surfaces, respectively, with the addressing logic moved off chip, or 
off of the entire lasing surface region. In this embodiment, the lasers 
themselves act as beam splitters, with a portion of their beam passing to 
an APD or set of detectors 20 and the beams from the opposite ends of each 
laser of the array passing to an optical element 40 which directs it at 
the object 18 which is to be imaged. Optical element 40 may be implemented 
as a single objective lens assembly 42, or as a microlens array 44 with 
each microlens aligned with one or more lasers of the array 10'. Such a 
microlens array may be fabricated as a hologram by techniques known in the 
art, and after fabricating an initial master hologram they may be formed 
very cheaply or as a thin sheet manufactured by stamping or molding 
techniques, thus allowing the entire assembly to be implemented in a thin 
sheet, chip or board. 
It will thus be seen that the invention efficiently attains the objects set 
forth above, among those made apparent from the preceding description. It 
will be understood that changes may be made in the above construction and 
in the foregoing sequences of operation without departing form the scope 
of the invention. For example, while the invention has been described in 
connection with arrays of surface-emitting diode lasers, the invention can 
also be practiced with both other scannable and individual laser light 
sources. It is accordingly intended that all matter contained in the above 
description or shown in the accompanying drawings be interpreted as 
illustrative rather than in a limiting sense. 
It is also to be understood that the following claims are intended to cover 
all of the generic and specific features of the invention as described 
herein, and all statements of the scope of the invention which, as a 
matter of language, might be said to fall therebetween.