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 detector;
an array of switchable grating elements sandwiched by first and second transparent substrates supporting transparent electrodes, said substrates together providing a total internal reflection light guide;
a first transmission grating layer overlaying said first substrate;
a second transmission grating layer overlaying said second substrate;
a quarter wavelength retarder layer overlaying said second transmission grating layer;
a platen overlaying 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; and
wherein the contact image sensor is configured to perform a sequence of switching steps, wherein in each step two 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 grating element diffracts only one polarization of said light.
4. The apparatus of claim 1 , wherein said switched grating elements are separated by a fixed number of elements.
5. The apparatus of claim 1 , wherein said grating elements are switched in their array order.
6. The apparatus of claim 1 , wherein the output signal from said detector is read out in synchronism with the switching of said grating elements.
7. The apparatus of claim 2 , wherein said diffracting state exists when an electric field is applied across said switchable grating element and said non-diffracting states exists when no electric field is applied.
8. The apparatus of claim 2 , wherein said diffracting state exists when no electric field is applied across said switchable element and said non-diffracting states exists when an electric field is applied.
9. The apparatus of claim 1 , further comprising at least one air space.
10. The apparatus of claim 1 , wherein said source comprises a laser and a collimator lens.
11. The apparatus of claim 1 , wherein said light is coupled into or out of said light guide via a grating or a prismatic element.
12. The apparatus of claim 1 , wherein said transparent substrates are fabricated from plastic.
13. The apparatus of claim 1 , wherein said detector comprises at least one photosensitive element.
14. The apparatus of claim 1 , wherein said detector is one of a linear array or a two-dimensional array.
15. The apparatus of claim 1 , wherein said gratings are 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.
16. The apparatus of claim 1 , wherein said grating elements are elongate with longer dimension perpendicular to a principal beam propagation direction of said light guide.
17. A method of making a contact image measurement, comprising the steps of:
providing an apparatus comprising: a light source providing a collimated beam; an array of switchable grating elements sandwiched by first and second transparent substrates each supporting transparent electrodes, said substrates together providing a total internal reflection light guide; a first transmission grating layer overlaying said first substrate; a second transmission grating layer overlaying said second substrate; a quarter wavelength retarder layer overlaying said second transmission grating layer; a platen overlaying 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 a detector;
coupling light from said source into a total internal reflection path in said light guide;
switching first and second grating elements of said SBG array into their diffracting state with all other elements remaining in their non-diffracting states;
diffracting said light towards said platen via said first grating element;
deflecting the path of said light using said second transmission grating;
converting said light from P polarized to circularly polarized using said quarter wavelength retarder layer;
transmitting light incident on a region of the external surface of said platen in contact with an external material and reflecting light not incident on said region from said external surface;
converting said reflected light from circularly polarized light to S-polarized using said quarter wavelength retarder layer;
transmitting said S-polarized light through first transmission grating without deviation;
transmitting said light through said SBG array without deviation;
reflecting said light and converting it from S to P polarization using said polarizing reflector;
deflecting the path of said P-polarized light using said first transmission grating;
coupling said P-polarized light into a total internal reflection path in said light guide via second active SBG element;
coupling said light out of said light guide via said output coupler;
receiving said light at said detector;
repeating steps c) to o) with different pairs of SBG elements being switched into their diffracting states.
18. The method of claim 17 , wherein said switched SBG elements are separated by a fixed number of elements.
19. The method of claim 17 , wherein said SBG elements are switched in their array order.