Contact image sensor using switchable Bragg gratings
A contact image sensor comprises: a light source providing a collimated beam; a detector and a switchable grating array comprising first and second transparent substrates sandwiching an array of switchable grating elements with transparent electrodes applied to said substrates, said substrates together providing a total internal reflection light guide. A first transmission grating layer overlays said first substrate. A second transmission grating layer overlays said second substrate. A quarter wavelength retarder layer overlays said second transmission grating layer. A platen overlays said quarter wavelength retarder layer; a polarization-rotating reflecting layer overlaying said first transmission grating layer. An input coupler for directing light from said light source into said light guide and an output coupler for extracting light out of said light guide towards said detector are also provided.
1. A contact image sensor comprising:
a light source providing a collimated beam;
a platen;
a first array of switchable grating elements disposed in a layer for coupling light from said source towards said platen;
a plurality of detector elements spatially distributed in two dimensions;
a plurality of waveguiding cores disposed in a layer; and
a second array of switchable grating elements disposed in a layer for coupling light reflected from said platen into a TIR path to said detector elements via said plurality of waveguiding cores,
wherein each waveguiding core transmits light to a unique detector element;
wherein the contact image sensor is configured to perform a sequence of switching steps, wherein in each step one grating element of said first array of switchable grating elements and one element of said second array of switchable grating elements are switched into their diffracting states with all other grating elements remaining in their non-diffracting states.
2. The apparatus of claim 1 , wherein each said grating element is switchable between a diffracting state and a non-diffracting state.
3. The apparatus of claim 1 , wherein said plurality of waveguiding cores comprises a multiplicity of parallel strip cores separated by low refractive index cladding material.
4. The apparatus of claim 1 , wherein said plurality of waveguiding cores is formed within said layer containing said second array of switchable grating elements.
5. The apparatus of claim 4 , wherein said waveguide cores are formed of regions of high refractive index HPDLC material separated by regions of low refractive index HPDLC material.
6. The apparatus of claim 1 , wherein said plurality of waveguiding cores is formed within a layer overlapping said second array of switchable grating elements.
7. The apparatus of claim 1 , wherein said grating elements of said second array of switchable grating elements have refractive index modulation varying along the length of the array.
8. The apparatus of claim 1 wherein light in each of the plurality of waveguide cores is coupled to one of the plurality of detector elements by either a grating or a prism.
9. The apparatus of claim 1 , wherein said switchable grating elements are switched in their array order.
10. The apparatus of claim 4 , wherein the output signal from said plurality of detector elements is read out in synchronism with the switching of said switchable grating elements.
11. The apparatus of claim 2 , wherein said diffracting state exists when an electric field is applied across said switchable grating elements and said non-diffracting state exists when no electric field is applied.
12. The apparatus of claim 2 , wherein said diffracting state exists when no electric field is applied across said switchable grating elements and said non-diffracting state exists when an electric field is applied.
13. The apparatus of claim 1 , further comprising at least one element selected from the group of an air space, a polarization modifying layer, a diffractive optical element and a homogeneous refractive index material layer, and a microlens array and a low index material layer.
14. The apparatus of claim 1 , wherein said source comprises a laser and a collimator lens.
15. The apparatus of claim 1 , wherein said light is coupled into each of said first or second array via a grating or a prismatic element.
16. The apparatus of claim 1 , wherein each said array of switchable grating elements is formed in a layer sandwiched by transparent substrates, said substrates supporting transparent electrodes, and said substrates together providing a total internal reflection light guide.
17. The apparatus of claim 1 , wherein each of said switchable grating elements are selected from the group of switchable Bragg gratings (SBGs), gratings recorded in liquid crystal and polymer material system, gratings recorded in a holographic polymer dispersed liquid crystal material system or gratings recorded in a reverse mode holographic polymer dispersed liquid crystal material system.
18. The apparatus of claim 1 , wherein said waveguiding cores include at least one of curved core segments in proximity to said detector elements and core segments disposed between adjacent detector elements.
19. The apparatus of claim 16 , wherein said switchable grating elements are elongate with a longer dimension perpendicular to a principal beam propagation direction of said light guide.
20. A method of making a contact image measurement, comprising the steps of:
a) providing an apparatus comprising: a light source providing a collimated beam; a platen; a plurality of detector elements spatially distributed over two dimensions; a plurality of waveguiding cores disposed in a layer; a first array of switchable grating elements for coupling light from said source towards said platen; and a second array of switchable grating elements for coupling light reflected from said platen into a TIR path to said detector elements; each said waveguide core overlaying a unique detector element;
b) coupling light from said source into a total internal reflection path in said first array of switchable grating elements;
c) switching one grating element in each of said first and second arrays of switchable grating elements into their diffracting state with all other elements remaining in their non-diffracting states;
d) diffracting said light towards said platen via said first array of switchable grating elements;
e) transmitting light incident on a region of an external surface of said platen in contact with an external material and reflecting light not incident on said region from said external surface;
f) coupling light reflected from said platen into said plurality of waveguiding cores via at least one element of said second array of switchable grating elements in said diffracting state;
g) coupling said light out of said plurality of waveguide cores onto said detector elements;
h) repeating steps c) to g) with different pairs of switchable grating elements being switched into their diffracting states.