OPTICAL PULSE STRETCHER

Abstract: A method of producing a fast-rising, fast-falling, relatively long pulse of laser light from at least one fast-rising, fast-falling, short input pulse of laser light, comprising: providing a plurality of short laser pulses of similar amplitude and pulse width, with a substantially constant time shift between pulses that is much smaller than the pulse width; and summing the time-shifted laser pulses into a single long substantially flat pulse, having fast rise and fall times.

Description: OPTICAL PULSE STRETCHER FIELD OF INVENTION The present invention relates generally to systems of three-dimensional imaging and in particular, to the stretching of optical pulses for use in such systems. BACKGROUND OF THE INVENTION Three dimensional optical imaging systems, hereinafter referred to as "3D cameras", that are capable of providing distance measurements to objects and points on objects that they image, are used for many different applications. Among these applications are profile inspection of manufactured goods, CAD verification, robot vision, geographic surveying and imaging objects selectively as a function of distance. Some 3D cameras provide simultaneous measurements to substantially all points of objects in a scene they image. Generally, these 3D cameras comprise a light source, such as a laser, which is pulsed or shuttered so that it provides pulses of light for illuminating a scene being imaged and a gated imaging system for imaging light from the light pulses that is reflected from objects in the scene. The gated imaging system comprises a camera having a photosensitive surface, hereinafter referred to as a "photosurface", such as a CCD camera, and a gating means for gating the camera open and closed, such as an optical shutter or a gated image intensifier. The reflected light is registered on pixels of the photosurface of the camera only if it reaches the camera when the camera is gated open. To image a scene and determine distances from the camera to objects in the scene, the scene is generally illuminated with a train of light pulses radiated from the light source. For each radiated light pulse in the train, following an accurately determined delay from the time that the light pulse is radiated, the camera is gated open for a period of time hereinafter referred to as a "gate". Light from the light pulse that is reflected from an object in the scene is imaged on the photosurface of the camera if it reaches the camera during the gate. Since the time elapsed between radiating a light pulse and the gate that follows it is known, the time it took imaged light to travel from the light source to the reflecting object in the scene and back to the camera is known. The time elapsed is used to determine the distance to the object. To correct for differences in reflectivity of surfaces on the scene, a normalization procedure is carried out. A non-gated measurement (or a measurement wherein the gate is very long) is taken. The different amounts of light that fall on pixels in the non-gated measurement are indicative of the differences in reflectivity of surfaces on the scene. These values are used to correct gated measurements to provide true depth determination. In some of these 3D cameras, the amount of light registered by the pixel during a time that the camera is gated open is also used to determine the distance. The accuracy of measurements made with these 3D cameras is a function of the rise and fall times of the light pulses, how fast the gating means can gate the camera open and closed, pulse widths and gate duration. A 3D camera using a pulsed source of illumination and a gated imaging system are described in "Design and Development of a Multi-detecting two Dimensional Ranging Sensor", Measurement Science and Technology 6 (September 1995), pages 1301-1308, by S. Christie, et al., and in "Range-gated Imaging for Near Field Target Identification", Yates et al, SPIE Vol. 2869, p374 - 385 which are herein incorporated by reference. Other "gated" 3D cameras and examples of their uses are found in PCT Publications WO97/01111, WO97/01112, and WO97/01113 which are incorporated herein by reference. An optical shutter suitable for use in 3D cameras is described in PCT patent application PCT/IL98/00060, the disclosure of which is incorporated herein by reference. Ideally, a long, square laser pulse is desired, with the next best thing being a fast-rising, fast- falling, long, flat laser pulse. Yet, among available laser systems, some, such as the quasi- continuous lasers, generate slow-rising long laser pulses. Others, such as Q-switch lasers, produce very short pulses that are fast-rising and fast falling. However, the desired fast-rising, fast- falling, long, flat laser pulse cannot be produced by either system. Therefore, some pulse- shaping techniques become necessary. It should be understood that for these measurement systems the flatness of the pulse is very important in determining the accuracy of the distance measurement. Prior art review reveals that US patent 4,671,605, US patent 4,128,759, US patent 4,053,763, US patent 4,976,518 and the article , "A Fiber-Optics Matched Delay Filter for RF Direction Finding," by Pappert, S. A., et al., Journal of Lightwave Technology, Vol. LT-3, No. 2., Apr., 1985 all deal with the use of a plurality of optical delay lines for filtering and pulse shaping applications. Yet none describes a method of application suitable for producing a fast-rising, fast-falling, relatively long, relatively flat laser pulse. It would be advantageous to have a system that produces such a pulse. SUMMARY OF THE INVENTION It is an object of some aspects of the present invention to provide a fast-rising, fast- falling, relatively long, flat laser pulse. In preferred embodiments of the invention, the pulse is used in gated, three-dimensional imaging systems. In one aspect of some preferred embodiments of the present invention, an optical pulse stretcher system is constructed, in which a sequential series of fast-rising, fast-falling, short laser pulses is added together to form a fast-rising, fast-falling, relatively long, substantially flat laser pulse. In some preferred embodiments of the present invention, the optical pulse stretcher comprises a laser which has a fast-rising and fast-falling pulse (such as a Q-switch laser) and a series of delay lines. Preferably the series of delay lines comprises a fiber bundle comprising optical fibers, wherein each optical fiber constitutes a specific delay line. In these preferred embodiments of the present invention, a fast-rising, fast-falling, short, input pulse of laser light is transmitted via the fiber bundle. In some preferred embodiments of the present invention, the optical fibers are of identical material and index of refraction. Preferably, the optical fibers have successively greater lengths which increase in equal increments. Therefore, the travel time through each delay line is successively greater, by an equal amount. As a result, the fast-rising, fast-falling, short input laser pulse exits the fiber bundle as a sequential series of fast-rising, fast-falling, short pulses, with equal incremental delays between them. Preferably, the incremental delays are in the order of fractions of nanosecond. Preferably, in forming the optical bundle, the optical fibers, having proximal ends and distal ends with respect to the laser, are aligned at their proximal ends to form an input surface. Preferably, an appropriate binder, as known in the art, is used to bind the proximal ends together. Preferably, the input surface is then highly polished. Preferably, the input surface is coated with an anti-reflection coating, as known in the art. Preferably, the optical fibers, are aligned at their distal ends to form a first output surface. Preferably, an appropriate binder is used to bind the distal ends together. Preferably, the first output surface is then highly polished. Preferably, the first output surface is coated with an anti-reflection coating as known in the art. In some preferred embodiments of the present invention, a relatively thick optical fiber is coupled to the first output surface. The thick optical fiber comprises a proximal end and a distal end. Preferably, the surfaces at the proximal end and at the distal end are highly polished and coated with anti-reflection coating. The thick optical fiber has a core, a cladding and a diameter that is substantially the same as the overall diameter of the fiber bundle. The preferably polished and coated surface at the distal end forms a second output surface. The purpose of the thick optical fiber is to mix the short pulses emanating from the first output surface, in order to minimize the effects of spatial offset of the fibers. Alternatively, a light guide for reducing a beam diameter, as known in the art, having a proximal end and a distal end, is coupled to the first output surface at its proximal end. The light guide comprises an output surface at its distal end which forms a second output surface. Alternatively, the individual cores of the fibers are fused together, at the distal end, to form a single fiber, whose diameter is progressively reduced. This is effective to mix the light from the various fibers. Alternatively, an optical system adjacent to the first output surface focuses the sequential series of short pulses emanating from the first output surface onto a single optical fiber. The single optical fiber comprises a proximal end and a distal end. Preferably, the surfaces at the proximal end and at the distal end are highly polished and coated with an anti-reflection coating. The preferably polished and coated surface at the distal end forms a second output surface. Preferably, a collimator (expander) near the second output surface substantially lines up the sequential series of short pulses emanating from the second output surface parallel with the collimator axis. In effect, the sequential series of short pulses forms a long pulse. Since the short pulses are fast-rising and fast-falling, the long pulse is likewise fast-rising and fast- falling. The rise-time slope for the short input pulse issued from the laser and for the long pulse emanating from the collimator are substantially the same, excluding time variance because of fiber dispersion. It should be noted that because the optical fibers are relatively short, pulse dispersion in the optical fibers is negligible. It should be understood that, in preferred embodiments of the invention, constructions similar to those at the output are also provided at the input. The reason for providing such input "couplers" is to mix the light so that the fibers are equally illuminated. It should be noted that because of the incremental delays between short pulses at the distal end of the fiber bundle, the long pulse is time incoherent. However, this is immaterial for use of the long pulse in gated, three-dimensional imaging of the type described in the background of the invention. While the aforementioned description relates to the generation of a single long pulse from a single fast-rising, fast-falling, short input pulse, it should be understood that as the laser issues a pulse train of fast-rising, fast-falling short input pulses, a train of fast-rising, fast falling, long pulses are generated. Preferably, the object to be illuminated for gated, three-dimensional imaging is positioned beyond the collimator with respect to the laser. In some preferred embodiments of the present invention, an expander is also positioned between the laser and the input surface. This may be desirable in cases where the short input pulse has a beam diameter that is smaller than the overall diameter of the fiber bundle. In some preferred embodiments of the present invention, the optical fibers in the fiber bundle are of equal length, while incremental variations in their indices of refraction produce the desired equal incremental delays. This effect is preferably provided in place of the incremental increases in length between optical fibers. Alternatively, the desired equal incremental delays are achieved by a combination of incremental length increases and incremental variations in the indices of refraction of the optical fibers. In some preferred embodiments of the present invention, each delay line is made up of a plurality of optical fibers, in order to even out effects of spatial offset and (or) non-uniformity of the input beam. Preferably, optical fibers from each delay line are distributed about the input surface so as to even out effect of non-uniformity of the beam. Preferably, optical fibers from each delay line are distributed about the first output surface so as to even out the spatial offset of the various delayed beams. The short input pulse has a cross-sectional intensity profile associated with it. Preferably, the plurality of optical fibers in each delay line is used also to correct for the intensity profile. Preferably, the input surface is divided into a central circle and one or more peripheral rings. Optical fibers from each delay line with a given time delay are distributed equally within the central circle and within each peripheral ring. Preferably, in a similar way, the first output surface is divided into a central circle and one or more peripheral rings. Optical fibers from each delay line are distributed equally within the central circle and within each peripheral ring. In some preferred embodiments of the present invention, the optical pulse stretcher comprises a plurality of fast-rising, fast falling, short-pulse lasers (such as diode lasers or active Q-switch lasers), preferably, positioned side by side. Preferably, the lasers are of very similar characteristics, since this affects the flatness of the pulse. Preferably, triggering circuitry triggers the lasers in a sequential manner such that each laser emits pulses having substantially the same amplitude and pulse width, wherein the elapsed time between the triggering of one laser and the triggering of the next laser in the sequence is substantially less than the pulse width. Preferably, the lasers are coupled to a single optical fiber, having a proximal end and a distal end. Preferably, the surfaces at the proximal end and at the distal end are highly polished and coated with an anti-reflection coating, wherein the proximal surface forms an input surface and wherein the distal surface forms an output surface. Preferably, the coupling of the lasers to the optical fiber is achieved by a focusing lens which focuses the short laser pulses onto the input surface. Preferably, a collimator (expander) between the output surface and an object to be illuminated lines up the laser light at the distal end, thus forming one long pulse of laser light. There is thus provided, in accordance with a preferred embodiment of the invention, a method of producing a fast-rising, fast-falling, relatively long pulse of laser light from a least one fast-rising, fast- falling, short input pulse of laser light, comprising: providing a plurality of short laser pulses of similar amplitude and pulse width, with a substantially constant time shift between pulses that is much smaller than the pulse width; and summing the time-shifted laser pulses into a single long substantially flat pulse, having fast rise and fall times. In a preferred embodiment of the invention, providing comprises: providing a single short pulse; producing a plurality of time coincident, substantially equal amplitude pulses from said short pulse; and delaying substantially equal portions of the short input pulse by equal increments of time. Preferably, delaying comprises passing said equal portions through delay lines comprising optical fibers. In a preferred embodiment of the invention, the method includes constructing the delay lines with equal increments of delay between them. Preferably, the method includes constructing each delay line from a single optical fiber. In a preferred embodiment of the invention, in which the optical fibers each have a proximal end that is illuminated by the short input pulse and a distal end the method includes: aligning the optical fibers at their proximal ends to form an input surface; and aligning the optical fibers at their distal ends to form a first output surface. In a preferred embodiment thereof, the method includes constructing each delay line from a group of identical optical fibers. Preferably, the optical fibers each have a proximal end that is illuminated by the short input pulse and a distal end and the method includes: aligning the optical fibers at their proximal ends to form an input surface; and aligning the optical fibers at their distal ends to form a first output surface. Preferably, aligning the fibers at their proximal ends to form said input surface comprises distributing the optical fibers of each delay line substantially evenly about the input surface. Preferably, distributing the optical fibers substantially evenly about the input surface comprises: dividing the input surface into a central circle and one or more peripheral rings; and distributing the optical fibers from each delay line substantially equally within the central circle and within each peripheral ring. Preferably, aligning the fibers at their distal ends to form said first output surface comprises distributing the optical fibers of each delay line substantially evenly about the first output surface. Preferably, distributing the optical fibers substantially evenly about the first output surface comprises: dividing the first output surface into a central circle and one or more peripheral rings; and distributing the optical fibers from each delay line substantially equally within the central circle and within each peripheral ring. In a preferred embodiment thereof, the method includes bonding the optical fibers together at their proximal ends; bonding the optical fibers together at their distal ends; and polishing the input and first output surfaces. In a preferred embodiment of the invention, bonding includes fusing the optical fibers at at least one of the distal and proximal ends to form a single fiber. Preferably, it also includes reducing the diameter of the fused fiber. Preferably, the method includes coating the input and first output surfaces with an anti- reflection coating. In a preferred embodiment of the invention, wherein the laser generates a beam having a diameter the method includes: expanding the beam diameter of the laser such that it is substantially equal to that of the input surface; and coupling the expanded laser beam to the optical fibers at the input surface. Preferably, combining comprises mixing the laser light between a first and a second output surfaces so that the laser light emanates from the second output surface as a relatively uniform beam. Preferably, combining also comprises forming a collimated beam from the light emanating from an output surface. In accordance with a preferred embodiment of the invention, producing comprises: providing a plurality of fast-rising, fast-falling, short-pulse lasers; sequentially triggering the lasers to produce said plurality of short pulses. Preferably, triggering comprises sequentially applying a driving voltage to the plurality of lasers, with an elapsed time between the triggering of one laser and the triggering of another that is shorter than the time extent of the pulses. Preferably, combining comprises focusing all input laser pulses onto the input of an optical transmission line. Preferably, the method includes eollimating laser light exiting the optical transmission line. In a preferred embodiment of the invention, wherein the optical transmission line is an optical fiber line having a proximal, input, end and a distal, output, end, the method includes: polishing proximal and distal end surfaces of the optical fiber; and coating the proximal and distal surfaces of the optical fiber with an anti-reflection coating. In preferred embodiments of the invention, the plurality of pulses are geometrically spaced apart. There is further provided, in accordance with a preferred embodiment of the invention apparatus for producing substantially flat laser pulses having fast rise and fall times, comprising: a source of short laser pulses of similar amplitude and pulse width, with a time shift between pulses that is much smaller than the pulse width; and a light summer that receives the time-shifted laser pulses and sums them into a single long pulse. Preferably, the source comprises: a laser that produces a fast-rising, fast-falling, short input pulse of laser light; and a plurality of delay lines comprising optical fibers, each having a proximal end illuminated by the laser light and a distal end, said delay lines delaying substantially equal portions of the laser pulse by different delay times, said delay times being different by equal increments. In a preferred embodiment of the invention, each delay line is constructed from a single optical fiber. In a preferred embodiment of the invention, the optical fibers are aligned at their proximal ends to form an input surface; and the optical fibers are aligned at their distal ends to form a first output surface. In a preferred embodiment of the invention, each delay line is constructed from a group of identical optical fibers. In a preferred embodiment of the invention: the optical fibers are aligned at their proximal ends to form an input surface; and the optical fibers are aligned at their distal ends to form a first output surface. Preferably, the optical fibers are aligned at their proximal ends so that optical fibers from each delay line are distributed substantially evenly about the input surface. Preferably, the input surface is divided into a central circle and one or more peripheral rings; and optical fibers from each delay line are distributed substantially equally within the central circle and within the peripheral rings. Preferably, the optical fibers are aligned at their distal ends so that optical fibers from each delay line are distributed substantially evenly about the first output surface. Preferably, the first output surface is divided into a central circle and one or more peripheral rings; and optical fibers from each delay line are distributed substantially equally within the central circle and within the peripheral rings. Preferably, the proximal ends of the optical fibers are bonded together; the distal ends of the optical fibers are bonded together; and the input and first output surfaces are polished. Preferably, the fibers are fused together at at least one of the distal and proximal ends to form a single fiber. Preferably, the diameter of the single fiber is reduced away from the single fiber fusing. Preferably, the input and first output surfaces are coated with an anti- reflection coating. Preferably, a collimator is positioned between the laser and the input surface. In a preferred embodiment of the invention a light guide for reducing a beam diameter, as known in the art, is coupled to the first output surface, wherein the light guide comprises a distal surface which forms a second output surface. Preferably, a relatively thick optical fiber, having a diameter that is substantially equal to the diameter of the first output surface and comprising a proximal surface and a distal surface, is coupled to the first output surface, wherein its distal surface forms a second output surface. Preferably, a lens system, coupled to the first output surface, focuses the light exiting from the first output surface onto a single optical fiber having a proximal surface and a distal surface, wherein the distal surface forms a second output surface. Preferably, a collimator (expander) lens system is positioned between the second output surface and an object to be illuminated. Preferably, the delay lines have different lengths. Alternatively or additionally, the delay lines have different light velocities. In a preferred embodiment of the invention, source comprises: a plurality of fast-rising, fast-falling short-pulse lasers, having similar characteristics; and triggering circuitry that sequentially triggers the lasers such that each of them emits pulses having substantially the same pulse width, wherein the elapsed time between the triggering of one laser and the triggering of the next laser in the sequence is substantially less than the pulse width. Preferably, the summer comprises an optical fiber to which the lasers pulses are coupled at a proximal, input, end thereof and a distal end. Preferably, optical fiber surfaces at the distal and proximal ends are highly polished and coated with an anti-reflection coating. In a preferred embodiment of the invention, the apparatus includes a focusing lens, positioned between the lasers and summer, which receives the plurality of laser pulses and focuses them onto an input of the summer. Preferably, the apparatus includes a collimator positioned between an output of the summer and an object to be illuminated by the laser light. In a preferred embodiment of the invention, the plurality of pulses are geometrically spaced from each other. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be more clearly understood from the following non-limiting detailed description of preferred embodiments thereof, taken together with the appended drawings, in which: Fig. 1 is a schematic representation of an optical pulse stretcher, in accordance with a preferred embodiment of the present invention. Fig. 2 is a graphic representation of a fast rising, long, flat laser pulse generated by an optical pulse stretcher in accordance with a preferred embodiment of the present invention, wherein the fiber bundle comprises six delay lines of one optical fiber each. Fig. 3 is a graphic representation of a fast-rising, fast-falling long, flat laser pulse generated by an optical pulse stretcher in accordance with a preferred embodiment of the present invention, wherein the fiber bundle comprises 41 delay lines of one optical fiber each. Fig. 4 is a schematic representation of an optical pulse stretcher, in accordance with another prefeπed embodiment of the present invention. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Reference is now made to Fig. 1, which schematically illustrates an optical pulse stretcher 10, in accordance with some preferred embodiments of the present invention. In these preferred embodiments, optical pulse stretcher 10 comprises a laser 20, preferably a Q-switch laser, a fiber bundle 30 of optical fibers 35, a light guide 40 and a collimator 60. In accordance with these preferred embodiments, a fast-rising, fast-falling, short input pulse of laser light is emitted from laser 20 and transmitted via optical fibers 35 of fiber bundle 30. In a preferred embodiment of the present invention, optical fibers 35 are of identical material and index of refraction. Preferably, optical fibers 35 are successively different in lengths by equal incremental steps. Therefore, the travel time through each of optical fibers 35 is successively greater by an equal amount. As a result, the fast-rising, fast- falling, short input pulse exits fiber bundle 30 as a sequential series of fast-rising, fast-falling, short pulses 50, with equal incremental delays between them. Preferably, the incremental delays are in the order of fractions of nanosecond. Preferably, in forming fiber bundle 30, optical fibers 35, having proximal ends 32 with respect to the laser, are aligned at proximal ends 32 along an input surface 34 which is perpendicular to the input pulse. Preferably, a binder 37 of some appropriate material, as known in the art, is used to bind proximal ends 32 together. Preferably, input surface 34 is highly polished and coated with an anti-reflection coating, as known in the art. Preferably, in a similar manner, distal ends 36 of optical fibers 35 are aligned at a first output surface 38. Preferably, binder 37 is used to bind distal ends 36 together. Preferably, first output surface 38 is highly polished and coated with an anti -reflection coating. Preferably, the fibers are closely packed (and not as shown on the drawing in which the spacing has been expanded to clarity of presentation). Preferably, a light guide 40 for reducing a beam diameter, as known in the art, having a proximal end 42, is coupled to first output surface 38 of fiber bundle 30. Preferably, light guide 40 comprises a distal end 44 and an output surface which forms second output surface 46. The purpose of light guide 40 is to mix sequential series of short pulses 50 emanating from first output surface 38 and minimize spatial offset. This mixer may be formed by coupling a conical light guide as shown in Fig. 1, or by simply fusing the cores of the fibers and preferably, progressively reducing the diameter of the resulting single fiber. In a preferred embodiment of the invention, a structure similar to that provided at the output is provided at the input. This helps to even out the light distribution on the fibers, considering that the input beam may have a cross-sectional variation in intensity dependent on the mode of the laser. Preferably, a collimator 60 is positioned near second output surface 46, parallel to second output surface 46. Sequential series of short pulses 50, passing through collimator 60, exit as a collimated long pulse 70. Since long pulse 70 is made up of fast-rising, fast-falling, short pulses, it is also fast-rising and fast-falling. In general, the rise-time slopes for the short input pulse that is issued from the laser, for each short pulse of sequential series 50, emanating from second output surface 46 and for long pulse 70, emanating from the collimator are substantially the same. It should be noted that there is a close relation between pulse width of the long pulse and the incremental delays between short pulses. It should also be noted that because the incremental delays between short pulses in sequential series 50, long pulse 70 is time incoherent. However, this is immaterial for use of the long pulse in gated, three-dimensional imaging systems referred to above. While the aforementioned description relates to the occurrences of one fast-rising, fast- falling short input pulse, it should be understood that as laser 20 issues a train of fast-rising, fast-falling, short pulses with time, the preferred embodiments described herein will issues a train of fast-rising, fast- falling, long pulses with time. Preferably, an object to be illuminated for gated, three-dimensional imaging is positioned at point 80, some distance from collimator 60. Reference is now made to Fig 2 which is a theoretical graphic representation of fast- rising, fast-falling, long, flat pulse 70 in accordance with a preferred embodiment of the present invention. The figure illustrates a preferred embodiment of six delay lines, with equal incremental length steps of 299.7 mm between them. Thus, sequential series of short pulses 50 is made up of six short pulses. As the figure illustrates, it is desired that the short pulses be spaced at equal intervals for long pulse 70 to have a flat plateau. Note that the rise-time slope of long pulse 70 is substantially the same as the rise-time slope of short pulses of sequential series 50. In the preferred embodiment illustrated in Fig. 2, the full width at maximum height (FWMH) value of the short input pulse is 1 nanosecond. The incremental delay between delay lines is 0.666 nsec The resultant rise time of long pulse 70 is 0.966. The resultant FWMH of long pulse 70 is 5.533 nsec. The ripple value is 7.784 x 10 "5 , a very small, if theoretical value. As the figure illustrates, it is possible to achieve a relatively flat top with a small ripple value using only 6 fibers. The same physical construction of 6 delay lines can be used for generating a long pulse with a smaller FWMH, but with slightly faster rise and fall times. Conversely, the same physical construction of 6 delay lines can be used for generating a long pulse with a larger FWMH, but with slower rise and fall times. Reference is now made to Fig 3 which is a theoretical graphic representation of fast- rising, fast-falling, long, flat pulse 70 in accordance with a preferred embodiment of the present invention. The figure illustrates a preferred embodiment of 41 delay lines, each of one optical fiber, with equal incremental length steps of 58.5 mm between them. Thus, sequential series of short pulse 50 is made up of 41 short pulses. In the preferred embodiment illustrated in Fig. 3, the FWMH value of the short input pulse is 0.2 nanosecond. The incremental delay between delay lines is 0.13 nsec. The resultant rise time of long pulse 70 is 0.195. The resultant FWMH of long pulse 70 is 5.33 nsec. The ripple value is 8.931xl0 "s . Again, the same physical construction can be used for a long pulse of longer or shorter FWMH; however, again, this will affect other relevant parameters. It should be understood from the above examples that the present invention can be used to produce relatively long pulses having fast rise and fall times to meet a wide range of pulse widths and other characteristics by varying the number of delay lines, their incremental delays and the pulse width of the input pulse. In comparing Fig. 2 and Fig. 3, one notes that with a larger number of delay lines, smaller incremental delay between them and smaller values of FWHM for the short input pulse, the resultant long pulse will have faster rise and fall times. Reference is now made to Fig. 4, which schematically illustrates an optical pulse stretcher 100, in accordance with some other preferred embodiments of the present invention. In these preferred embodiments, optical pulse stretcher 100 comprises a plurality of fast-rising, fast- falling, short-pulse lasers 110 preferably, of very similar characteristics, that are positioned side by side. Preferably, triggering circuitry 180 triggers the lasers in a sequential manner such that each laser emits pulses having substantially the same amplitude and pulse width, wherein the elapsed time between the triggering of one laser and the triggering of the next laser in the sequence is substantially less than the pulse width. Preferably, lasers 110 are coupled to a single optical fiber 120, having a proximal end 130 and a distal end 140. Preferably, the surface at proximal end 130 is highly polished and coated with an anti-reflection coating, wherein the proximal surface forms an input surface 135. Preferably, the surface at the distal end is highly polished and coated with an anti-reflection coating, wherein the distal surface forms an output surface 145. Preferably, the coupling of lasers 110 to optical fiber 120 is achieved by means of a focusing lens 115 which focuses the short laser pulses onto input surface 135. Preferably, a collimator 150 between output surface 145 and an object to be illuminated 160, lines up the laser light emanating from output surface 145, thus forming long pulse 170 of laser light. The present invention has been described using non-limiting detailed descriptions of preferred embodiments thereof that are provided by way of example and are not intended to limit the scope of the invention. Variations of embodiments described will occur to persons of the art. Furthermore, the terms "comprising," "comprise," include/' and "including" or the like, shall mean, when used in the claims, "including but not necessarily limited to." The scope of the invention is limited only by the following claims:

Claims: CLAIMS 1. A method of producing a fast-rising, fast- falling, relatively long pulse of laser light from a least one fast-rising, fast- falling, short input pulse of laser light, comprising: providing a plurality of short laser pulses of similar amplitude and pulse width, with a substantially constant time shift between pulses that is much smaller than the pulse width; and summing the time-shifted laser pulses into a single long substantially flat pulse, having fast rise and fall times. 2. A method according to claim 1 wherein providing comprises: providing a single short pulse; producing a plurality of time coincident, substantially equal amplitude pulses from said short pulse; and delaying substantially equal portions of the short input pulse by equal increments of time. 3. A method according to claim 2, wherein delaying comprises passing said equal portions through delay lines comprising optical fibers. 4. A method according to claim 3 and including constructing the delay lines with equal increments of delay between them. 5. A method according to claim 4 and including constructing each delay line from a single optical fiber. 6. A method according to claim 5, wherein said optical fibers each have a proximal end that is illuminated by the short input pulse and a distal end and including: aligning the optical fibers at their proximal ends to form an input surface; and aligning the optical fibers at their distal ends to form a first output surface. 7. A method according to claim 4 and including constructing each delay line from a group of identical optical fibers. 8. A method according to claim 7, wherein said optical fibers each have a proximal end that is illuminated by the short input pulse and a distal end and including: aligning the optical fibers at their proximal ends to form an input surface; and aligning the optical fibers at their distal ends to form a first output surface. 9. A method according to claim 8, wherein aligning the fibers at their proximal ends to form said input surface comprises distributing the optical fibers of each delay line substantially evenly about the input surface. 10. A method according to claim 9, wherein distributing the optical fibers substantially evenly about the input surface comprises: dividing the input surface into a central circle and one or more peripheral rings; and distributing the optical fibers from each delay line substantially equally within the central circle and within each peripheral ring. 11. A method according to any of claims 8-10, wherein aligning the fibers at their distal ends to form said first output surface comprises distributing the optical fibers of each delay line substantially evenly about the first output surface. 12. A method according to claim 11, wherein distributing the optical fibers substantially evenly about the first output surface comprises: dividing the first output surface into a central circle and one or more peripheral rings; and distributing the optical fibers from each delay line substantially equally within the central circle and within each peripheral ring. 13. A method according to any of claims 6 or 8-12 and including: bonding the optical fibers together at their proximal ends; bonding the optical fibers together at their distal ends; and polishing the input and first output surfaces. 14. A method according to claim 13 wherein bonding includes fusing the optical fibers at at least one of the distal and proximal ends to form a single fiber. 15. A method according claim 14 and including reducing the diameter of the fused fiber. 16. A method according to any of claims 6 or 8-15 and including coating the input and first output surfaces with an anti-reflection coating. 17. A method according to any of claims 6 or 8-16, wherein the laser generates a beam having a diameter and comprising: expanding the beam diameter of the laser such that it is substantially equal to that of the input surface; and coupling the expanded laser beam to the optical fibers at the input surface. 18. A method according to any of claims 6 or 8-17, wherein combining comprises mixing the laser light between a first and a second output surfaces so that the laser light emanates from the second output surface as a relatively uniform beam. 19. A method according to any of claims 6 or 8-18, wherein combining also comprises forming a collimated beam from the light emanating from an output surface. 20. A method according to claim 1, wherein producing comprises: providing a plurality of fast-rising, fast-falling, short-pulse lasers; sequentially triggering the lasers to produce said plurality of short pulses. 21. A method according to claim 20, wherein triggering comprises sequentially applying a driving voltage to the plurality of lasers, with an elapsed time between the triggering of one laser and the triggering of another that is shorter than the time extent of the pulses. 22. A method according to claim 20 or claim 21, wherein combining comprises: focusing all input laser pulses onto the input of an optical transmission line. 23. A method according to claim 22 and including: collimating laser light exiting the optical transmission line. 24. A method according to claim 23, wherein the optical transmission line is an optical fiber line having a proximal, input, end and a distal, output, end, and including: polishing proximal and distal end surfaces of the optical fiber; and coating the proximal and distal surfaces of the optical fiber with an anti-reflection coating. 25. A method according to any of the preceding claims wherein the plurality of pulses are geometrically spaced apart. 26. Apparatus for producing substantially flat laser pulses having fast rise and fall times, comprising: a source of short laser pulses of similar amplitude and pulse width, with a time shift between pulses that is much smaller than the pulse width; and a light summer that receives the time-shifted laser pulses and sums them into a single long pulse. 27. Apparatus according to claim 26 wherein the source comprises: a laser that produces a fast-rising, fast-falling, short input pulse of laser light; and a plurality of delay lines comprising optical fibers, each having a proximal end illuminated by the laser light and a distal end, said delay lines delaying substantially equal portions of the laser pulse by different delay times, said delay times being different by equal increments. 28. Apparatus according to claim 27, wherein each delay line is constructed from a single optical fiber. 29. Apparatus according to claim 28, wherein: the optical fibers are aligned at their proximal ends to form an input surface; and the optical fibers are aligned at their distal ends to form a first output surface. 30. Apparatus according to claim 27, wherein each delay line is constructed from a group of identical optical fibers. 31. Apparatus according to claim 30, wherein: the optical fibers are aligned at their proximal ends to form an input surface; and the optical fibers are aligned at their distal ends to form a first output surface. 32. Apparatus according to claim 31, wherein the optical fibers are aligned at their proximal ends so that optical fibers from each delay line are distributed substantially evenly about the input surface. 33. Apparatus according to claim 32, wherein: the input surface is divided into a central circle and one or more peripheral rings; and optical fibers from each delay line are distributed substantially equally within the central circle and within the peripheral rings. 34. Apparatus according to any of claims 31-33, wherein the optical fibers are aligned at their distal ends so that optical fibers from each delay line are distributed substantially evenly about the first output surface. 35. Apparatus according to claim 34, wherein: the first output surface is divided into a central circle and one or more peripheral rings; and optical fibers from each delay line are distributed substantially equally within the central circle and within the peripheral rings. 36. Apparatus according to any of claims 29 or 31-35, wherein: the proximal ends of the optical fibers are bonded together; the distal ends of the optical fibers are bonded together; and the input and first output surfaces are polished. 37. Apparatus according to claim 36 wherein the optical fibers are fused at at least one of the distal and proximal ends to form a single fiber. 38. Apparatus according claim 37 and wherein the diameter of the fused fiber is progressively reduced away from the individual fibers. 39. Apparatus according to any of claims 29 or 31-38, wherein the input and first output surfaces are coated with an anti-reflection coating. 40. Apparatus according to any of claims 29 or 31-39, wherein a collimator is positioned between the laser and the input surface. 41. Apparatus according to any of claims 29 or 31-40 wherein a light guide for reducing a beam diameter, as known in the art, is coupled to the first output surface, wherein the light guide comprises a distal surface which forms a second output surface. 42. Apparatus according to any of claims 29 or 31-41 wherein a relatively thick optical fiber, having a diameter that is substantially equal to the diameter of the first output surface and comprising a proximal surface and a distal surface, is coupled to the first output surface, wherein its distal surface forms a second output surface. 43. Apparatus according to any of claims 29 or 31-41 wherein a lens system, coupled to the first output surface, focuses the light exiting from the first output surface onto a single optical fiber having a proximal surface and a distal surface, wherein the distal surface forms a second output surface. 44. Apparatus according to any of claims 36-43, wherein a collimator (expander) lens system is positioned between an output surface and an object to be illuminated. 45. Apparatus according to any of claims 27-44, wherein the delay lines have different lengths. 46. Apparatus according to any of claims 27-45, wherein the delay lines have different light velocities. 47. Apparatus to claim 26 wherein the source comprises: a plurality of fast-rising, fast-falling short-pulse lasers, having similar characteristics; and triggering circuitry that sequentially triggers the lasers such that each of them emits pulses having substantially the same pulse width, wherein the elapsed time between the triggering of one laser and the triggering of the next laser in the sequence is substantially less than the pulse width. 48. Apparatus according to claim 47, wherein the summer comprises an optical fiber to which the lasers pulses are coupled at a proximal, input, end thereof and a distal end. 49. Apparatus according to claim 48, wherein optical fiber surfaces at the distal and proximal ends are highly polished and coated with an anti -reflection coating. 50. Apparatus according to any of claims 47-49, and including a focusing lens, positioned between the lasers and summer, which receives the plurality of laser pulses and focuses them onto an input of the summer. 51. Apparatus according to any of claims 47-50 and including a collimator positioned between an output of the summer and an object to be illuminated by the laser light. 52. Apparatus according to any of claims 26-51 wherein the plurality of pulses are geometrically spaced from each other.

Inventors: SCHUSTER ISRAEL, YAHAV GIORA, SCHUSTER, ISRAEL, YAHAV, GIORA
Applicants: 3DV SYSTEMS LTD, SCHUSTER ISRAEL, YAHAV GIORA, 3DV SYSTEMS, LTD., SCHUSTER, ISRAEL, YAHAV, GIORA
Application year: 1998
Main classification: H01S3
IPC codes: G02B6, H01S3