Patent Application: US-97111101-A

Abstract:
an optical filter includes an input optical fiber bundle and an output fiber bundle . each of the bundles has one end having the fiber ends substantially two - dimensionally arrayed and another end substantially linearly arrayed . each input fiber is configured to receive a portion of a two - dimensional input image at the two - dimensional end and transmit the image portion to the one - dimensional end . a spectrally dispersive element receives the image portions from the input fiber bundle and outputs a predetermined spectral component to the output optical fiber bundle at the one - dimensional end , transmitting the image portion to the two - dimensional end . the output fiber bundle two - dimensional ends are arrayed in corresponding fashion to the first ends of the input fibers for spatially reconstructing the input image .

Description:
a description of one embodiment of the present invention will now be presented with reference to fig1 - 13b . the optical filter uses optical fiber bundles to maintain image integrity while accurately redirecting the spectral components after separation by a diffraction grating . with a suitably arranged fiber array , the image can be angularly and spatially separated without overlap in order to output a two - dimensional image at a desired wavelength . a block diagram of the system 10 of the present invention is given in fig1 with a schematic in fig2 . a two - dimensional input image 12 impinges on an input optical fiber array 14 . prior to entering the array 14 there may be placed , if desired , an optical element 16 to achieve magnification ( for higher - spatial - resolution imaging ) or reduction ( for greater field - of - view imaging ). the input array , or input fiber bundle , 14 comprises a plurality of optical fibers 140 , each having a first end 141 and a second end 142 . the arrangement of the fibers 140 depends upon the desired application , with a close - packed orientation being preferred for highest resolution and a more scattered , or loose - packed , orientation preferred for site - specific multipoint imaging . thus the invention is not intended to be limited to a particular packing embodiment . preferably the fibers &# 39 ; first ends 141 should be substantially coplanar for receiving the input image . the input array 14 digitizes the input image , with the spatial resolution thereof depending upon the resolution of the digitization process , which is determined by such characteristics as fiber diameter and packing density . as an example , 10 - μm single - mode fibers with interfiber spacings & lt ; 20 μm provides high spatial resolution . further , if the image is magnified prior to entering the input array 14 , a spatial resolution of & lt ; 1 μm is achievable . the second ends 142 of the input array 14 are oriented into a linear array to provide a linear image for subsequent spectral filtering ( see fig3 ). in a preferred embodiment of the invention , adjacent fibers 140 in the linear array are connected at the two - dimensional first ends 141 to spatially contiguous regions . this sequencing of fibers 140 reduces errors in spatial fidelity in the output spectral image . the optical image of the input array 14 at the one - dimensional second end 142 next becomes the object for a spectrally dispersive element 18 , such as , but not intended to be limited to , a monochromator , for dispersing the constituent spectral components . a monochromator may be used as a dispersing element when the slit width is wider than the fiber diameter and as a spectral filter when the slit width is narrower than the fiber diameter . preferably the one - dimensional input image is imaged at a 1 : 1 ratio at the output of the filter 18 . the narrow width of the second end 142 of the input array 14 , which constitutes , in effect , the width of the input slit of the filter 18 , coupled with the achievable spectral dispersion of a typical triple - grating monochromator , permits angstrom - level resolution of the output spectral image . an output optical fiber array 20 is configured substantially identically to the input array 14 . now the one - dimensional fiber array comprises the first ends 201 of the component fibers 200 and constitutes the output slit of the filter 18 . by adjusting the grating 180 of the filter 18 , a desired spectral component of the linear image can be obtained on the output array 20 . the tunable spectral dispersion analyzes the input light into its spectral components and permits a selection of a desired individual frequency component of the initial multicolor image while maintaining the spatial integrity of the linear input image . imaging the output of the spectrally dispersive element 18 onto a substantially identical one - dimensional fiber array 20 provides a one - to - one correlation between the spatial information contained in the two linear arrays 14 , 20 . light from each fiber 140 in the input array 14 to the filter 18 is filtered , with the results of the filtration obtained in the corresponding spatial counterpart fiber 200 of the output array 20 . since light at the spatial location of each fiber 200 of the output array 20 can be traced back to a corresponding origin 140 in the input array 14 prior to filtering , the second ends 202 of the fibers 200 of the output array 20 can be reconstituted into the same configuration as the original two - dimensional array to obtain a spectrally filtered , two - dimensional image of the original object at a desired wavelength . the final image can be detected using , for example , a ccd camera 22 , and then displayed , for example , in real time on a video monitor 24 . exemplary modes of detection include : 1 . if the original object is a dynamically evolving entity , such as fluorescence from an expanding laser - ablated plume , a fast , time - gated camera , such as an intensified charge - coupled device ( iccd ) camera , can be used to capture transient two - dimensional spectral images . at present , commercially available gate widths for such cameras permit attainable temporal resolutions for such dynamic images in the 10 - ns range . 2 . another mode of detection comprises time - integrated detection of images for which dynamic characteristics are not required . fast gating is not required for this mode , which is suitable , for example , for objects having extremely low light intensities . the exemplary system schematic of fig2 illustrates a two - dimensional object image 12 incident on the input array 14 and thereby digitized into a plurality of pixels . the light emerging from the second ends 142 is collected by a first concave mirror 25 and sent to a grating 180 as a collimated parallel beam . the first - order diffraction from the grating 180 is incident on a second concave mirror 26 . the image of the input one - dimensional array 14 is formed in the focal plane at a predetermined distance , here 12 in ., away from the second mirror 26 and comprises a ribbon of color comprising a series of horizontal colored lines . in each horizontal line , representing the image of a single pixel , the color varies from blue to red from one end to the other for an incident white light object image . each color line in the ribbon is therefore the image of a corresponding fiber 140 in the one - dimensional end 141 of the input array 14 . the one - dimensional end 201 of the output array 20 is vertically aligned on a desired spectral region of the band , for collecting light from the corresponding fibers 140 in the input array 14 . the image is then restored to its two - dimensional form at the two - dimensional end 202 of the output array 20 . in the exemplary embodiment shown in fig2 the filter 18 comprises a monochromator comprising a single square , plane reflection grating 1 × 1 in ., having 1200 grooves / mm . the grating 180 of filter 18 is mounted on a stage 181 to achieve vertical positioning of the grooves and permit three degrees of freedom of movement , including horizontal and vertical adjustments and vertical rotation . the zeroth order of diffraction from the grating 180 , while having the greatest intensity , is not usable , as there is no angular separation between the spectral components of the image . in this embodiment only the first - order diffraction was collected by the concave mirror 26 . both mirrors 25 , 26 comprise front - reflection concave mirrors having a 6 - in . diameter and a 12 - in . focal length . mirrors are believed preferable over a lens - based system in their elimination of chromatic aberration . the first mirror 25 is positioned 12 in . away from the one - dimensional end of the input array 14 ; the second mirror 26 , 12 in . away from the one - dimensional end of the output array 20 . thus the two mirrors 25 , 26 form a telescope of unit magnification in effect . this arrangement ensures that the beams incident on the grating are parallel beams and allows accurate one - to - one imaging between the one - dimensional array ends . the spectrum from the grating 180 avoids interfiber spatial and spectral overlap , thereby ensuring the smallest bandwidth and lowest cross - talk for the system 10 . the image is then collected by the detector 22 , which may comprise a gated ccd camera or optical multichannel analyzer ( oma ) system as desired or appropriate . in a preferred embodiment the input 14 and output 20 fiber arrays should be substantially identical . the two - dimensional ends comprise a 10 × 10 fiber array ; the one - dimensional ends 142 , 201 , a 100 × 1 array ( fig3 ). in one embodiment , each fiber 140 , 200 comprises silica and has a 200 - μm core diameter and a 240 - μm total diameter including the cladding . the arrays 14 , 20 each have a length of 24 mm at the one - dimensional end , and each fiber 140 , 200 has a length of 40 cm . the transmission range of the fibers spans the visible and infrared region , 400 - 1200 nm . the fiber ends are polished to a surface roughness better than 1 μm . the imaging arrangement necessitates that the one - dimensional structures are the same spatially , including the same microarrangement and spacing of each individual fiber end . otherwise , a fiber in the output one - dimensional end 201 would not be able to collect the light from its corresponding counterpart in the input one - dimensional end 142 of the fiber array or not be able to collect light at a desired wavelength . the two fiber arrays 14 , 20 in this embodiment are formed in this exemplary embodiment using 100 fibers , each 80 cm in length . using the device shown in fig4 two fibers 31 , 32 are glued in parallel fashion onto a glass slide 30 . two pieces of glass 33 , 34 are glued atop the fibers 31 , 32 , creating a slot 35 . the desired 100 fibers are then inserted into the slot 35 next to each other , and the fibers and glass slide 30 are glued together , for example , with an epoxy - type glue . after the glue is dry , the assembly is cut apart along the dotted line 36 with a diamond saw . both cut ends of each bundle are finally mechanically polished . the two - dimensional ends 141 , 202 of the arrays 14 , 20 are formed using specially designed fiber holders . two plastic disks , having diameters of 24 mm and thicknesses of 0 . 25 in ., each have 100 holes drilled therein in a 10 × 10 square array , each hole having a 508 - μm diameter , with a center - to - center distance of 2 mm between consecutive holes . a 1 . 375 - in .- long hypodermic needle having an inner diameter of 250 μm is inserted and glued into each of the 200 holes to provide support for the fibers . the disks are fitted and glued inside glass tubes . the fibers are inserted into the needles by pushing on all the fiber ends with , for example , a flat microslide to ensure coplanarity of the ends . then mounting wax is used to fix the fibers in position to form the two - dimensional arrays . it may be appreciated by one skilled in the art that additional methods may be contemplated , including different techniques and materials to achieve the same fiber arrays described herein . testing of the system 10 was performed using two kinds of light sources . a he — ne laser had monochromaticity and directionality to permit ease of alignment and calibration . in order to verify a one - to - one correspondence between the arrays 14 , 20 , the two - dimensional ends 141 of the input array 14 were illuminated by a lens - expanded beam of laser light at 632 . 28 nm , with the output image from the output array 20 monitored with a ccd camera ( fig5 ). the tilt of the illustrated image is because of a tilt of the camera . variations in the spatial intensity profile are due to variations in laser beam intensity at the input end . it was found that misalignments at fiber numbers 57 , 68 , and 77 were caused by mispositioning of the fibers during fabrication , and that fiber number 100 ( lower right - hand corner ) was broken . however , all fibers were illuminated on tuning the grating to the proper laser wavelength . even a slight detuning extinguished the illumination substantially completely . the spatial integrity of the two - dimensional image after transmission through the filter is an important issue . for example , if the image at each input one - dimensional end is larger than the actual size of the output one - dimensional array end ( fig6 ), it is possible for the light from one input fiber 140 to “ leak ” into other adjacent fibers , which is referred to as spatial cross - talk . this could manifest itself in spatial as well as spectral diffusion and resultant distortion of the transmitted image . the structure of the fiber array ( fig3 ) places fibers number 1 - 10 are in order horizontally , as are fibers 11 - 20 , etc . an exemplary check of cross - talk is shown in fig7 with the column 149 of fig3 comprising fibers 2 , 12 , 22 , . . . , 92 , illuminated exclusively . it can be seen that no illumination of adjacent columns was detected , which was also the result for all other columns and all rows . this is particularly significant for the close - packed fiber arrays 14 , 20 of the present system 10 . restoration of an input image was checked to ensure that the arrays properly rearrange the pixels following transformation into a one - dimensional array . two shapes , a triangle and a circle , positioned in two locations , were input , with the results shown in fig8 a - 8d . in this test , the fibers were more sparsely arranged , with a 2 - mm center - to - center distance , determining a lower resolution of the output image . the spectral response of the system 10 was also tested using broadband illumination from a xenon arc lamp , emitting in the 50 - 1500 - nm range . the lamp illuminated the input array 14 , and an oma detector was placed at the output array 20 , with angle tuning of the grating 180 allowing output frequency selectivity of the two - dimensional image . two examples at 430 and 807 nm ( fig9 a and 9b ) show continuous tunability , with the system range limited only by the wavelength the fiber can transmit , here 400 - 1200 nm . when the system 10 is tuned to a specific wavelength , other wavelengths should be efficiently rejected in the output , and this lack of “ spectral leakage ” is demonstrated in fig1 . tuning the system 10 to a wavelength in the green region ( λ = 550 nm ), for example , and scanning from 400 to 700 nm , covering most of the visible spectrum , leads to no evidence of emission at other wavelengths . this was verified at other transmission wavelengths as well . the transmission bandwidth can be obtained by high - resolution measurements of a spectral line detected by the oma . fig1 a and 11b show the bandwidth of the spectra depicted in fig9 a and 9b , respectively . the bandwidths ( fwhm ) for both short - and long - wavelength ranges are approximately 0 . 6 nm , indicating a very uniform bandwidth throughout the tunable range . the theoretical bandwidth , on the basis of the resolution of the diffraction grating and the diameter and position of the fibers used in the one - dimensional output array 20 , is calculated to be 0 . 66 nm , which is consistent with the experimentally observed values . this calculated value for bandwidth is obtained by considering the ratio of the spatial spread of the wavelength range in the plane of the band to the core diameter of the output one - dimensional fiber . if the position of a fiber in any of the one - dimensional arrays is off slightly relative to other fibers , the wavelength selected will be different from that expected . the oma was used to test fibers in column 5 for spectral uniformity along the fiber array . by choosing fibers in a column , it is ensured that fibers are sampled that are evenly spaced in the one - dimensional array ( see fig3 ). further , sampling any entire column affords data over a large range of the one - dimensional array and is thus truly representative of the maximum extent of the spectral nonuniformity . the wavelength of the peak value from each fiber was recorded fiber by fiber and are plotted in fig1 a and 12b , where a curvature is evident . the largest deviations in wavelength throughout the entire range are , respectively , 0 . 6 and 0 . 54 nm . this maximum variation is , however , within the 0 . 6 - nm bandwidth discussed previously . similar results have been obtained for fibers in other columns . the observed curvature is due to off - axis reflections from the concave mirror . if the system 10 filters a single - frequency light source , such as the output of a he — ne laser , the images of the first one - dimensional array 14 have a slight spatial curvature introduced by the off - axis fibers . as a result , the second one - dimensional array 20 will pick up slightly disparate wavelengths in a multiwavelength spectrum . interestingly , from fig1 a and 12b , the positions of the points are not random ; rather , they tend to form a smooth curve . this provides a remedy for alleviating this slight problem of spectral nonuniformity . the solution is to match the spatial arrangement of the second one - dimensional array according to the curvature of the imaging mirror , so that all the fibers can image the exact wavelength selected , thereby resulting in better spectral uniformity . an important function of the filter system 10 is to allow spatial patterns of a selected wavelength to pass through . in order to test this function , two object images were projected onto the input of the system simultaneously . one of these objects was a triangle at a wavelength of 450 nm , while the other was rectangular at a wavelength of 458 nm . the wavelengths were generated by an ar ion laser ( 458 nm ) and a xe arc lamp with a commercial filter of 450 nm . by appropriately tuning the angle of the grating 18 , only the desired pattern can be selected at the output of the system 10 and recorded by a ccd camera ( fig1 a and 13 b ). there is no observable interference between the images at these two different but closely spaced wavelengths . in both cases the output of the system 10 was identical to the shape and size of the corresponding spectral component in the dual - wavelength input object . these performance tests confirm the capability of a system as described herein to analyze a two - dimensional multiwavelength object into its spectral constituents , with a potential wavelength resolution on the order of angstroms and micrometer or better spatial resolution . gated viewing can provide the derivation of constituent monochromatic , two - dimensional images with temporal resolution of dynamic events on the 10 - ns time scale . the two - dimensional spectral imager is continuously tunable , and any wavelength that can be transmitted by the optical fibers and dispersed by the monochromator is available for viewing . numerous applications exist in the viewing of two - dimensional objects , in which the ability to view different spectral components while maintaining spatial integrity is important . for example , ccd imaging has been used to image laser - ablated plume dynamics by viewing the plume fluorescence [ 53 - 56 ]. the filter of the present invention is particularly suitable for studying plume emission during the laser ablation process for the growth of thin films . it is also possible to provide species - resolved , dynamic information about the spatial expansion and propagation of individual atomic and molecular species in a multicomponent laser - ablated plume . this is believed possible , as the demonstrated resolution and tunability of the spectral imager allows the isolation of emission from any atomic or molecular species and two - dimensional display in real time . other applications include , but are not intended to be limited to , plasma diagnostics , combustion diagnostics ( such as rocket plume imaging ), real - time process monitoring for fluorescent processes , medical imaging ( for example , the fluorescent detection of cancer cells ), and sensing of spatial temperature variations ( via blackbody radiation detection using a suitable fiber material ). it may be appreciated by one skilled in the art that additional embodiments may be contemplated , including different components and materials to achieve the same functionalities described herein . in the foregoing description , certain terms have been used for brevity , clarity , and understanding , but no unnecessary limitations are to be implied therefrom beyond the requirements of the prior art , because such words are used for description purposes herein and are intended to be broadly construed . moreover , the embodiments of the apparatus illustrated and described herein are by way of example , and the scope of the invention is not limited to the exact details of construction . g . kopp and m . derks , “ stay tuned : photonic filters color your world ,” photon . spect . 31 , 125 ( 1997 ). n . d . finkelstein , w . r . lempert and r . b . miles , “ rotational raman scattering measurements with a single mode laser source and narrow passband filter ,” opt . lett . 22 : ( 8 ), 537 ( 1997 ). j . sabbaghzadeh , w . buell , j . holder and m . fink , “ a very narrow high throughput rayleigh filter for raman spectroscopy ,” appl . phys . b , 60 : ( 2 - 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