(12) INTERNATIONAL APPLICATION PUBLISHED UNDER THE PATENT COOPERATION TREATY (PCT)
`“.
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`(19) World Intellectual Property
`Organization
`International Bureau
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`(43) International Publication Date
`6 October 2016 (06.10.2016)
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`WIPO!IPCT
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`(10) International Publication Number
`WO 2016/156776 Al
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`(51)
`
`International Patent Classification:
`G02F 1/13 (2006.01)
`GO2B 5/18 (2006.01)
`G02F 1/29 (2006.01)
`G02B 27/42 (2006.01)
`G02B 26/08 (2006.01)
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`(21)
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`International Application Number:
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`PCT/GB20 16/000065
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`(22)
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`International Filing Date:
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`30 March 2016 (30.03.2016)
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`(25)
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`(26)
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`(30)
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`(72)
`(71)
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`Filing Language:
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`Publication Language:
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`English
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`English
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`Priority Data:
`62/178,041
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`31 March 2015 (31.03.2015)
`
`US
`
`Inventors; and
`Applicants : POPOVICH, Milan Momcilo [GB/GB]; 53
`Westfield Road, Leicester LE3 6HU (GB). WALDERN,
`Jonathan David [US/US];
`11481 Old Ranch Road, Los
`Altos Hills, CA 94024 (US). GRANT, Alastair
`John
`[US/US]; 1460 Maria Way, San Jose, CA 95117 (US).
`
`(81)
`
`Designated States (unless otherwise indicated, for every
`kind f national protection available): AE, AG, AL, AM,
`AO, AT, AU, AZ, BA, BB, BG, BH, BN, BR, BW, BY,
`
`BZ, CA, CH, CL, CN, CO, CR, CU, CZ, DE, DK, DM,
`DO, DZ, EC, EE, EG, ES, FI, GB, GD, GE, GH, GM, GT,
`HN, HR, HU,ID, IL, IN, IR, IS, JP, KE, KG, KN, KP, KR,
`KZ, LA, LC, LK, LR, LS, LU, LY, MA, MD, ME, MG,
`MK, MN, MW, MX, MY, MZ, NA, NG, NI, NO, NZ, OM,
`PA, PE, PG, PH, PL, PT, QA, RO, RS, RU, RW, SA, SC,
`SD, SE, SG, SK, SL, SM, ST, SV, SY, TH, TJ, TM, TN,
`TR, TT, TZ, UA, UG, US, UZ, VC, VN, ZA, ZM, ZW.
`
`(84)
`
`Designated States (unless otherwise indicated, for every
`kind ¢ regional protection available): ARIPO (BW, GH,
`GM, KE, LR, LS, MW, MZ, NA, RW, SD, SL, ST, SZ,
`TZ, UG, ZM, ZW), Eurasian (AM, AZ, BY, KG, KZ, RU,
`TJ, TM), European (AL, AT, BE, BG, CH, CY, CZ, DE,
`DK, EE, ES, FI, FR, GB, GR, HR, HU,IE,IS, IT, LT, LU,
`LV, MC, MK, MT, NL, NO, PL, PT, RO, RS, SE, SI, SK,
`SM, TR), OAPI (BF, BJ, CF, CG, CI, CM, GA, GN, GQ,
`GW, KM, ML, MR, NE, SN, TD, TG).
`Published:
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`with international search report (Art. 21(3))
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`the time limit for amending the
`before the expiration of
`claims and to be republished in the event g receipt ¢
`amendments (Rule 48.2(h})
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`(54) Titles METHOD AND APPARATUS FOR CONTACT IMAGE SENSING
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`FIG.9
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`WO2016/156776A.|IINTIAINNAININNIMITATIONTNATAAA
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`in
`(57) Abstract: A contact image sensor comprises: a waveguiding structure for propagating light in a first direction comprising,
`series, a first clad medium,a first core, a switchable grating clad, a second core, and a second clad medium sandwiched by transpar -
`ent substrates, patterned parallel electrode elements orthogonally traversing the waveguides, a light source, a platen and a detector.
`Switchable grating regions overlapped byafirst voltage-addressed electrode element diffract TIR light from the first core towards
`the platen. Switchable grating region overlapped by a second voltage-addressed electrode element diffract TIR light reflected from
`the platen into a TIR path within the sccond core.
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`

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`WO 2016/156776
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`PCT/GB2016/000065
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`METHOD AND APPARATUS FOR CONTACT IMAGE SENSING
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`PRIORITY CLAIM
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`This application claims priority from US Provisional Application Serial No.: 62/178,041
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`entitled METHOD AND APPARATUS FOR CONTACT IMAGE SENSINGfiled on 3 1 March
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`2015, which is hereby incorporated by reference in its entirety
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`CROSS REFERENCE TO RELATED APPLICATIONS
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`The following patent applications are hereby incorporated by reference herein in their entireties:
`
`PCT Application No.: PCT/GB20 12/000680 entitled IMPROVEMENTS TO HOLOGRAPHIC
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`POLYMERDISPERSED LIQUID CRYSTAL MATERIALS AND DEVICESwith filing date 7
`
`January 2013; PCT Application No.: PCT/GB2012/000729 entitled METHOD AND
`
`APPARATUS FOR SWITCHING ELECTRO OPTICAL ARRAYSwith filing date 6
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`September 2012; PCT Application No.: PCT/GB20 13/000005 entitled CONTACT IMAGE
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`SENSOR USING SWITCHABLE BRAGG GRATINGSwith filing date 7 January 2013; United
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`States Patent No.: 8,354,640 entitled OPTICALLY-BASED PLANAR SCANNER with issue
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`date 15 January 2013; PCT Application No.: PCT/GB20 14/000295 entitled METHOD AND
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`APPARATUS FOR CONTACT IMAGE SENSINGwith filing date 30 July 2014;
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`US Patent Application No. 13/506,389 entitled COMPACT EDGE ILLUMINATED
`
`DIFFRACTIVE DISPLAY,, United States Patent No. 8,233,204 entitled OPTICAL
`
`DISPLAYS, PCT Application No.: US2006/043938, entitled METHOD AND APPARATUS
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`20
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`FOR PROVIDING A TRANSPARENT DISPLAY, PCT Application No.: GB2012/000677
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`entitled WEARABLE DATA DISPLAY, United States Patent Application No.: 13/317,468
`
`entitled COMPACT EDGE ILLUMINATED EYEGLASS DISPLAY, United States Patent
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`Application No.: 13/869,866 entitled HOLOGRAPHIC WIDE ANGLE DISPLAY,and United
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`States Patent Application No.: 13/844,456 entitled TRANSPARENT WAVEGUIDEDISPLAY.
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`

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`WO 2016/156776
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`PCT/GB2016/000065
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`BACKGROUNDOF THE INVENTION
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`The present invention relates to an imaging sensor, and more particularly to a contact
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`image sensor using electrically switchable Bragg gratings.
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`A contact image sensor is an integrated module that comprises an illumination system, an
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`optical imaging system and a light-sensing system - all within a single compact component. The
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`object to be imaged is place in contact with a transparent outer surface (or platen) of the sensor.
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`Well known applications of contact image sensors include document scanners, touch sensors for
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`computer interfaces, bar code readers and optical identification technology. Another field of
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`application is in biometric sensors, where there is growing interest in automatic finger print
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`detection. Fingerprints are a unique marker for a person, even an identical twin, allowing trained
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`personel or software to detect differences bewtween individuals. Fingerprinting using the
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`traditional method of inking a finger and applying the inked finger to paper can be extremely
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`time-consuming. Digital technology has advancedthe art of fingerprinting by allowing images
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`to be scanned and the image digitized and recorded in a mannerthat can be searched by
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`computer. Problems can arise due to the quality of inked images. For example, applying too
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`muchortoo little ink may result in blurred or vague images. Further, the process of scanning an
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`inked image can be time- consuming. A better approach is to use "live scanning” in which the
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`fingerprint is scanned directly from the subject's finger. More specifically, live scans are those
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`procedures which capture fingerprint ridge detail in a manner which allows for the immediate
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`processing of the fingerprint image with a computer. Examples ofsuch fingerprinting systems
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`are disclosed in Fishbineet al. (U.S. Pat. Nos. 4,81 1,414 and 4,933,976); Becker (U.S. Pat. No.
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`3,482,498); McMahon(U.S. Pat. No. 3,975,711); and Schiller (U.S. Pat. Nos. 4,544,267 and
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`4,322,163).
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`A live scanner must be able to capture an image at a resolution of 500 dots per
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`inch (dpi) or greater and have generally uniform gray shading across a platen scanning area.
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`WO 2016/156776
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`PCT/GB2016/000065
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`There is relevant prior art in the field of optical data processing in which optical waveguides and
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`electro-optical switches are used to provide scanned iJlumination One prior art waveguide
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`illuminator is disclosed in U.S. Pat. No. 4,765,703. This device is an electro-optic beam deflector
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`for deflecting a light beam within a predetermined rangeof angle. It includes an array of channel
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`waveguides and plural pairs of surface electrodes formed on the surface of a planar substrate of
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`an electro-optic material such as single crystal LiNbO ,.
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`While the fingerprinting systems disclosed in the foregoing patents are capable of
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`providing optical or optical and mechanical fingerprint images, such systems are only suitable
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`for use at a central location such as a police station. Such a system is clearly not ideal for law
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`enforcement and security applications where there is the need to perform an immediate identity
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`and background check on an individual while in the field.
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`In general, current contact image
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`sensor technilogy tend to be bulky, low in resolution and unsuitable for operation in thefield.
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`Thusthere exists a need for a portable, high resolution, lightweight biometeric contact
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`image scanner.
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`WO 2016/156776
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`PCT/GB2016/000065
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`SUMMARYOF THE INVENTION
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`It is an object of the present invention to provide a portable, high resolution, lightweight
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`biometeric contact image scanner.
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`A contact image sensor according to the principles of the invention comprises: a
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`waveguiding structure for propagating light in a first direction comprising, in series disposed in a
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`layer sandwiched bytransparent substrates, a first clad medium,a first
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`cote, a switchable
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`grating clad, a second core, and a second clad medium; electrodes applied to opposing surfaces
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`of the substrates at least one patterned into a set of parallel elements orthogonally traversing the
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`cores; a light source optically coupled to the first and second cores; a platen in optical contact
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`with the waveguiding structure; and a detector optically coupled to the first and second core
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`regions. Switchable grating regions overlapped bya first voltage-addressed electrode element
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`are operative, in their diffracting state, to diffract TIR light from first core into a path leading to
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`the outer surface of the platen. Switchable gratings region overlapped by a second voltage-
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`addressed electrode element are operative, in their diffracting state, diffract TIR light reflected
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`from the platen into a TIR path to the detector along the second core.
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`In one embodiment the waveguiding structure comprises a multiplicity of the cores and
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`the clads cyclically arranged.
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`In one embodiment the voltages are applied sequentially, two electrodes at a time,to all
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`electrodes in the array.
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`In one embodiment the diffracting state exists when no electric field is applied across the
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`SBG elementand the non-diffracting state exists when an electric field is applied.
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`In one embodiment the diffracting state exists when an electric field is applied across the
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`SBG element and the non-diffracting state exists when noelectric field is applied.
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`WO 2016/156776
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`PCT/GB2016/000065
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`In one embodiment when contact is made with an external material at a region on the
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`platen a portion of the light incident at the region on the platen contacted by the external material
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`is transmitted out of the platen, wherein light incident on the outer surface of the platen in the
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`absence of the contact with an external material is reflected downwards.
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`In one embodiment when contact is made with an external material at a region on the
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`platen a portion of the light incident at the region on the platen contacted by the external material
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`is reflected downwards. Light incident on the outer surface of the platen in the absence ofthe
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`contact with an external material is transmitted out of the platen.
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`In one embodiment the output from the detector is read out in synchronism with the
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`switching of the electrode elements.
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`In one embodiment the light source is one of a laser or LED andthe light is coupled into
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`the waveguiding structure by one of a grating or a prism.
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`In one embodiment the switchable grating clad is a switchable Bragg grating recorded in
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`one of a HPDLC grating, uniform modulation grating or reverse mode HPDLC grating.
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`In one embodiment the switchable grating clad includes at least one of a fold grating or a
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`multiplexed grating or a rolled k-vector grating.
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`In one embodiment a method of making a contact image measurement comprising the
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`stepsof:
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`a) Providing a waveguiding structure for propagating light in a first direction comprising, in
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`series disposed in a layer sandwiched bytransparent substrates, a first clad medium,a first
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`core, a switchable grating clad, a second core, and a second clad medium; electrodes applied
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`to opposing surfaces of the substrates at least one patterned into a set of parallel elements
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`orthogonally traversing the cores; a light source optically coupled to the first and second
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`WO 2016/156776
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`PCT/GB2016/000065
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`cores; a platen in optical contact with the waveguiding structure; a detector optically coupled
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`to the first and second core regions;
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`b) coupling the light into a TIR path in the waveguiding structure;
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`c) an external material contacting a region on the external surface ofthe platen;
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`d) setting first and second electrode elements to a first voltage state and all other voltage-
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`addressed electrodes set to a second voltagestate;
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`e) switchable grating regions overlapped byafirst electrode element diffracting TIR light from
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`first core into a path to the platen outer surface;
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`f) switchable grating regions overlapped by a secondelectrode elementdiffracting light
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`reflected from one of the region or the platen external surface into a TIR path in the second
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`core; and
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`g) transmitting the reflected light to the detector.
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`In one embodimentthe first voltage state corresponds to a voltage being applied and the
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`second voltage state correspondsto no voltage being applied.
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`In one embodimentthe first voltage state corresponds to no voltage being applied and the
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`second voltage state correspondsto a voltage being applied.
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`In one embodimentat least a portion of the light incident at the region on the platen is
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`transmitted out of the platen, wherein at least a portion of the second optical path light not
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`incidentat the region is reflected.
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`In one embodimentat least a portion of the light incident at the region on the platen is
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`reflected, wherein at least a portion of the second optical path light not incident at the region
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`being transmitted out of the platen.
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`WO 2016/156776
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`PCT/GB2016/000065
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`In one embodiment the waveguiding structure comprises a multiplicity of the cores and
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`the clads cyclically arranged.
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`In one embodimentthe voltages are applied sequentially, two electrodesat a time,to all
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`electrodes in the array.
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`In one embodiment the output from detector is read out in synchronism with the
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`switching of the electrode elements.
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`In one embodimentthe contact image sensor is configured as a finger print scanner or a
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`touch sensor.
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`A more complete understanding of the invention can be obtained by considering the
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`following detailed description in conjunction with the accompanying drawings wherein like
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`index numerals indicate like parts. For purposesof clarity, details relating to technical material
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`that is knownin the technical fields related to the invention have not been described in detail.
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`WO 2016/156776
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`PCT/GB2016/000065
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`BRIEF DESCRIPTION OF THE DRAWINGS
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`FIG.IA is a schematic side elevation view of a contact image sensor using a tablet computer as a
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`light source in one embodiment.
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`FIG. IB is a detail of schematic side elevation view of a contact image sensor using a tablet
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`computer as a light source in one embodiment.
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`FIG.1C is schematic plan view of a contact image sensor using a tablet computer as a light
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`source in one embodiment.
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`FIG.2A is a plan view ofa transparent electrode array in one embodiment.
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`FIG.2B is a scan line displayed on a computer tablet used as an illumination source in one
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`embodiment.
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`FIG.3 is a plan view ofa fold grating in one embodiment
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`FIG.4 is schematic side elevation view of a contact image sensor using a bidirectional waveguide
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`in one embodiment.
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`FIG.5 is schematic side elevation view of a contact image sensor using a bidirectional waveguide
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`in one embodiment.
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`FIG.6 is schematic plan view of a waveguide containing a fold grating used in a contact image
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`sensor in one embodiment.
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`FIG.7A is a schematic drawing of a waveguide structure used in contact image sensor in one
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`embodiment.
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`FIG.7B is a detail of a waveguide structure used in contact image sensor using a unidirectional
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`illumination and detection waveguide in one embodiment.
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`FIG. 8A is a schematic side elevation view of a contact image sensor using a unidirectional
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`illumination and detection waveguide in one embodiment.
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`PCT/GB2016/000065
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`FIG.8B is a schematic plan view of a contact image sensor using a unidirectional illumination
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`and detection waveguide in one embodiment.
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`FIG.9 is a schematic plan view of a detail of contact image sensor using a unidirectional
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`illumination and detection waveguide in one embodiment.
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`FIG.10A is across section view showing ray propagation in a waveguide core in one
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`embodiment
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`FIG. 108 is three dimensional view showing ray propagation in a waveguide core in one
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`embodiment
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`FIG.1 1A is a schematic plan view of a contact image sensor using a unidirectional illumination
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`and detection waveguide in one embodiment.
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`FIG. 1IB is a schematic cross section view of a contact image sensor using a unidirectional
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`illumination and detection waveguide in one embodiment.
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`FIG.12A is a schematic cross section view showing a stage in the process of fabricating a
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`unidirectional illumination and detection waveguide in one embodiment.
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`FIG.12B is a schematic cross section view showing a stage in the process of fabricating a
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`unidirectional illumination and detection waveguide in one embodiment.
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`FIG.12C is a schematic cross section view showing a stage in the process of fabricating a
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`unidirectional illumination and detection waveguide in one embodiment.
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`FIG.12D is a schematic plan view of an electrode structure in one embodiment
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`FIG. 13 is a schematic plan view of a waveguide structure for a unidirectional illumination and
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`detection waveguide in one embodiment.
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`FIG. 14 is a schematic plan view of an electrode array coated substrate for a unidirectional
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`illumination and detection waveguide in one embodiment.
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`PCT/GB2016/000065
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`FIG. 15 is a schematic plan view of a waveguide structure for a unidirectional illumination and
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`detection waveguide in one embodiment.
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`FIG.16 is a flow chart for making a contact image measurementusing a unidirectional
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`illumination and detection waveguide in one embodiment.
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`FIG. 17 is a schematic cross section view of a contact image sensor using a bidirectional
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`waveguide and a multiplexed grating beam control layer in one embodiment.
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`FIG. 18 is a schematic plan view of a detail of the embodiment of FIG.17.
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`FIG. 19 is a detail of a multiplexed grating beam control layer used in the embodiment of FIG. 17.
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`FIG.20 is a grating characteristic of the embodiment of FIG .17.
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`WO 2016/156776
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`PCT/GB2016/000065
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`DETAILED DESCRIPTION OF THE INVENTION
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`It will be apparent to those skilled in the art that the present invention may bepracticed
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`with someorall of the present invention as disclosed in the following description. For the
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`purposes of explaining the invention well-known features of optical technology known to those
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`skilled in the art of optical design and visual displays have been omitted or simplified in order
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`not to obscure the basic principles of the invention. Unless otherwise stated the term "on-axis"
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`in relation to a ray or a beam direction refers to propagation parallel to an axis normal to the
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`surfaces of the optical components described in relation to the invention. In the following
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`description the terms light, ray, beam and direction may be used interchangeably and in
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`association with each other to indicate the direction of propagation of light energy along
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`rectilinear trajectories. Parts of the following description will be presented using terminology
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`commonly employed by those skilled in the art of optical design.
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`It should also be noted that in
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`the following description of the invention repeated usage of the phrase "in one embodiment"
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`does not necessarily refer to the same embodiment.
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`In the following description SBG (Switchable Bragg Grating) will refer to a Bragg
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`grating that can beelectrically switched between an active or diffracting state and an inactive or
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`non-diffractive state.
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`In the embodiments to be described below the preferred switchable grating
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`will be a Switchable Bragg Grating (SBG) recording in a Holographic Polymer Dispersed Liquid
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`Crystal (HPDLC) material. The principles of SBGswill be described in more detail below. For
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`the purposes of the invention an non switchable grating may be one based on any material or
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`process currently used for fabricating Bragg gratings. For example the grating may be recorded
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`in aholographic photopolymer material.
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`In some embodiments a non switchable grating may be
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`provided by a surfacerelief grating.
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`An (SBG)is formed by recording a volumephase grating, or hologram, in a polymer
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`dispersed liquid crystal (PDLC) mixture. Typically, SBG devices are fabricated by first placing a
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`thin film of a mixture of photopolymerizable monomers and liquid crystal material between
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`parallel glass plates. Techniques for making andfilling glass cells are well known in the liquid
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`crystal display industry. One or both glass plates support electrodes, typically transparent
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`indium tin oxide films, for applying an electric field across the PDLC layer. A volume phase
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`grating is then recorded by illuminating the liquid material with two mutually coherent laser
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`beams, which interfere to form the desired grating structure. During the recording process, the
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`monomers polymerize and the HPDLC mixture undergoes a phase separation, creating regions
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`densely populated by liquid crystal micro-droplets, interspersed with regions of clear polymer.
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`The alternating liquid crystal-rich and liquid crystal-depleted regions form the fringe planes of
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`the grating. The resulting volumephase grating can exhibit very high diffraction efficiency,
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`which may be controlled by the magnitude ofthe electric field applied across the PDLC layer.
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`When an electric field is applied to the hologram via transparent electrodes, the natural
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`orientation of the LC droplets is changed causing the refractive index modulation of the fringes
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`to reduce and the hologram diffraction efficiency to drop to very low levels resulting in for a
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`"non diffracting” state. Note that the diffraction efficiency of the device can be adjusted, by
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`means of the applied voltage, over a continuous range from near 100%efficiency with no
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`voltage applied to essentially zero efficiency with a sufficiently high voltage applied. U.S.
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`Patent 5,942, 157 and U.S. Patent 5,75 1,452 describe monomer and liquid crystal material
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`combinations suitable for fabricating SBG devices.
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`To simplify the description of the invention the electrodes and the circuits and drive
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`electronics required to perform switching of the SBG elements are not illustrated in the Figures.
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`Methods for fabricated patterned electrodes suitable for use in the present invention are disclosed
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`in PCT US2006/043938. Other methods for fabricating electrodes and schemes for switching
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`SBG devices are to be found in the literature. The present invention does not rely on any
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`particular method for fabricating transparent switching electrodes or any particular scheme for
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`switching arrays of SBGs. Although the description makes reference to SBG arrays the
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`invention may be applied using any type of switchable grating. To clarify certain geometrical of
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`aspects of the invention reference will be made to A Cartesian (XYZ )coordinate system where
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`appropriate.
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`In one embodiment illustrated in FIGS. 1-3 there is provided a contact image sensor in
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`whichline scanned illumination is provided by a computer tablet screen. The apparatus further
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`comprises a platen 102, a waveguide layer 101 and a lower substrate 103 im contact with the
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`screen ofthe table 104. The platen and lower substrate together provide a waveguide cell. Note
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`that in practice it will be advantageous for the waveguide to be fabricated in a separate cell
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`which is then laminated to the platen. As shown in the cross section detail of FIG. IB and the
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`plan view of FIG.LC. The waveguide layer comprises alternating of strips of switchable SBG
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`cladding 105 and polymer cores 106. Electrodes are applied to opposing faces of the platen and
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`lower substrate. As shown in FIG.2A at least one electrode 109 is patterned into column -shaped
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`elements 109 disposed in an orthogonal direction to the waveguide cores. The electrodes are
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`used to switch portions of the clad between non-diffracting states. A clad region in its diffracting
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`states couples light reflected from the platen surface into an adjacent core. All core regions
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`under an addressed electrode are switched simultaneously. FIG.1C shows one such grating
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`region indicated by 114 lying under the voltage addressed electrode 109. This grating region
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`diffracts the reflected light 1107 into the TIR light path 1108. As will be discussed later the
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`diffracting state may occur with or without an applied voltage across the grating according to the
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`type of holographic material system used. Note that the light has been illustrated as undergoing
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`TIR in the plane of the drawing. However, in practice, the grating orientation (as defined by the
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`grating k-vector) and reflected beam vector will result in a more complicated TIR path which
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`will typically result in rays undergoing a spiral TIR path down each core. The signals from the
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`waveguide cores are collected by a linear detector at the end of the waveguide. In the simplest
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`. embodiment each waveguide core abuts a pixel of the detector. However, other coupling
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`schemes should be apparent to those skilled in ther art. At any time the tablet 112 displays a
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`column 115 of width a few pixels against a black background 116 asillustrated in FIG. 2B. The
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`bright column is scrolled continuously across the tablet screen. The width of the column may be
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`just one pixel. In practice a width of several pixe!s may be required to achieve an adequate signal
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`level. The tablet based on LCoS or LED technology will normally emit light over a large cone
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`angle as indicated by ther rays 1100,1 101. A small portion of this light lying within a small solid
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`angle will be totally internally reflected at the outer surface of the platen as indicated by the rays
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`1103-1 104. The reflected light 1104 is then coupled into a waveguide by an active region of the
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`waveguide as discussed above.
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`In one embodiment wherever an external body such as.a finger
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`touchesthe platen, it "frustrates" the reflection process, causing light to leak out of the platen.
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`Thus, the parts of the skin that touch the platen surface reflect very little light, forming dark
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`pixels in the image. The imageis built up line byline into a finger print image. A key advantage
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`of this embodiment is that the tablet eliminates the need for a separate scanner layer allow
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`making the sensor thinner, cheaper and lower power consumption. However, the use of visible
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`20
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`light may preclude its application in many security applications.
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`The complex beam steering required to couple the light reflected from the platen in the
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`waveguide cores requires a grating structure referred to by the inventors as a fold grating. This
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`type of grating is normally used for changing beam direction and providing beam expansion
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`within a waveguide. Gratings designed for coupling light into or out of a waveguides are tilted
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`around an axis lying in the waveguide plane. Fold gratings have a more generalized tilt. In their
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`simplest tmplementation, as used in the present invention, they are tilted around an axis
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`perpendicular to the waveguide plane such they deflect beams in the waveguide plane. More
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`generally, they may have tilts defined by two rotation angles so that, for example, light can be
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`coupled into the waveguide and deflected into an orthogonal direction inside the waveguide, all
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`in one step.
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`Figure 3 is a plan view of a basic fold grating 118. When the set of rays 1110
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`encounter the grating, they diffract in a manner that changes the direction of propagation by 90°.
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`Note that when a ray encounters the grating, regardless of whether it intersects the grating from
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`above or below, a fraction of it changes direction and the remainder continues unimpeded. A
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`typical ray will interact many times with vertical ly (in the Y direction) while some light will be
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`moving laterally Gin the X direction). From a design perspective,
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`it is desirable to engineer the
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`amountof light 1111 emerging from the output edge of the grating to be uniformly distributed
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`laterally and the amount of light 1112 emerging fromthe side edge of the grating to be as small
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`as possible.
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`In one embodiment illustrated in FIG.4 an illumination layer sensor for providing line
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`scanned illumination and a detector layer fromreceiving reflected light from a platen and
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`transmitting it to an infrared detector are combined in a single SBG array waveguide layer. The
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`contact image sensor comprises
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`a waveguide grating layer 161 a transmission grating 162 a
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`platen 163, a light source 165 and a detector 166.
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`In one embodiment the transmission grating
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`10
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`15
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`20
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`and waveguide are air separated. In one embedment the transmission grating and waveguide are
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`separated by a think layer of low index nanoporous material. Light from the source is coupled
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`into a TIR path 1140 in the waveguide by a coupling grating 167. The waveguide contains an
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`array of switchable grating elements such as 164 which is shown in its diffracting state. The
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`graling elements switch in scrolling fashion, each element in its diffracung state diffracting light
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`output out of the waveguide toward the transmission grating. For example, the active element
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`164 diffracts the TIR light into the direcuion 1141, typically normal to the waveguide. The
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`transmission grating deflects the light into the direction ] 142 inside the platen meeting the platen
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`surface TIR conditions. Light reflected from the platen 1143 passes through the transmissive
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`grating without deviation since it is now off-Bragg (that is, it lies outside the diffraction
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`efficiency angular bandwidth ofthe grating as predicted by Kogelnik theory). The optical
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`surfaces of the waveguide are roughened to ensure that at least a small portion of the reflected
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`light from the platen enters the TIR path 1144 to the detector. Advantageously,
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`the source and
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`detector operate in the infrared. The detector is typically a linear detector.
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`In one embodiment illustrated in the cross sectional view of FIG.5 a contact image sensor
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`similar in concept to the one of FIG.4 comprises a waveguide 170, a first passive transmission
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`Bragg grating 171 a second passive transmission Bragg grating 174 , and a platen 175. The
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`waveguide contains a SBG grating array 176 comprising column shaped elements, an input
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`coupling grating 177 for coupling light from an infrared source 178 into the waveguide. The
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`apparatus further comprises a light output coupling grating 180 for directing light onto a linear
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`infrared detector array 181. The transmission Bragg gratings may be recorded in a holographic
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`photopolymer or in a HPDLC material, the latter providing an attractive passive medium owing
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`to its high index modulation capability. At any time two elements of the grating array, such as
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`the ones indicated by 176A,176B are in a diffracting state. We next consider the light path
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`20
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`through the contact image sensor. Light 1150 from the source is coupled into a TIR path 1151 in
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`the waveguide by the input grating. The active grating element 176B deflects the light out of the
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`waveguide into the direction 1152 which is typically normal to the waveguide. The second
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`passive grating 173 diffracts the light into the platen in the direction 1153. After reflection from
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`the platen outer surface the light 1154 is diffracted toward the waveguide by the first passive
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`grating 171 in the direction 1155 which is substantially normal to the waveguide. The light is
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`coupled into the TIR path 1156 by the active grating element 176A andis finally deflected out of
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`the waveguide by the output coupling grating onto the infrared detector. In one embodiment the
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`apparatus of FIG.5 further comprises polarization components such as half wave plates and
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`quarter wave plates for controllmg beam polarization to achieve more efficient bidirectional
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`transmission of light within the waveguide as disclosed in PCT Application No.:
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`PCT/GB20 14/000295 entitled METHOD AND APPARATUS FOR CONTACT IMAGE
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`SENSING
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`In one embodiment illust