Up-conversion of electromagnetic radiation within a wavelength range
Prior art techniques for converting the wavelength of electromagnetic radiation by non-linear interactions such as sum frequency generation SFG produces blurred images when applied to polychromatic images. The invention provides an improved arrangement ( 15 ) for up-converting incoming electromagnetic radiation with dissimilar first wavelengths (λ,) distributed in a first wavelength interval (Δλ,) into up-converted electromagnetic radiation comprising corresponding dissimilar up-converted wavelengths (λ 3 ), smaller than the first wavelengths (Δ 1 ). The arrangement applies a nonlinear crystal ( 5 ) and a laser ( 16 ) for providing a laser beam ( 10 ) of second wavelength, different from the first wavelengths (λ 1 ) inside the nonlinear crystal, and a Fresnel zone plate ( 17 ) for focusing or converging the incoming radiation inside the nonlinear crystal in spatial overlap with the laser beam. The up-converted radiation is produced inside the non-linear crystal by sum frequency generation between the incoming radiation and the laser beam. The arrangement ( 15 ) can comprise an image forming lens arrangement ( 20 ) for forming an object image ( 2 ), a second zone plate ( 18 ) for forming an up-converted image ( 3 ) and an image sensor ( 19 ) on which the up-converted image ( 3 ) is formed.
1. An arrangement for up-converting incoming polychromatic electromagnetic radiation (EMR) in wavelength interval Δλ 1 into up-converted EMR in corresponding wavelength interval Δλ 3 of smaller wavelengths than the wavelength interval Δλ 1 , the arrangement comprising:
a nonlinear crystal;
a laser for providing a laser beam having a second wavelength, λ 2 , different from the first wavelengths; and
a first zone plate for converging the incoming EMR inside the nonlinear crystal;
wherein the nonlinear crystal is selected for producing up-converted EMR by sum frequency generation between the incoming EMR and the laser beam; and
wherein the nonlinear crystal, the laser, and the first zone plate are arranged so that:
the laser beam will be provided in the nonlinear crystal;
the incoming EMR will be converged in the nonlinear crystal by the first zone plate and in spatial overlap with the laser beam; and
the laser beam will not be converged in the nonlinear crystal by the first zone plate.
2. The arrangement according to claim 1 , wherein the arrangement comprises an optical cavity for providing an increased intra-cavity power of the laser beam, and wherein the non-linear crystal is arranged inside the optical cavity.
3. The arrangement according to claim 1 , wherein the first zone plate has a phase distribution function equivalent to a spherical convex lens.
4. The arrangement according to claim 1 , wherein the first zone plate has at least 8 zones.
5. The arrangement according to claim 1 , further comprising a second zone plate for receiving and collimating up-converted EMR from the non-linear crystal.
6. The arrangement according to claim 1 , wherein the arrangement comprises imaging optics arranged before the first zone plate for producing a two-dimensional image from the incoming EMR.
7. A method of using the arrangement according to claim 1 to shift the effective sensitivity range of an image sensor towards longer wavelengths comprising:
positioning the arrangement of claim 1 in front of an image sensor; and
providing up-converted EMR to the image sensor so as to shift the effective sensitivity range of the image sensor towards longer wavelengths.
8. A method for up-converting incoming polychromatic electromagnetic radiation (EMR) into up-converted EMR with wavelength smaller than the incoming EMR, the method comprising:
converging incoming polychromatic EMR in a nonlinear crystal by means of a first zone plate;
providing a laser beam of a second wavelength, λ 2 , in the nonlinear crystal, where the first zone plate is arranged so that the laser beam is not converged in the nonlinear crystal by the first zone plate; and
producing up-converted EMR by sum frequency generation between the incoming EMR and the laser beam in a spatial overlap between the incoming EMR and the laser beam inside the non-linear crystal.
9. The method according to claim 8 , wherein the first zone plate has a phase distribution function equivalent to a spherical convex lens.
10. The method according to claim 9 , further comprising producing a two-dimensional image from the incoming EMR on the first zone plate.
11. The method according to claim 9 , further comprising collimating the produced up-converted EMR by means of a second zone plate.
12. The method according to claim 8 , wherein at least some of the incoming polychromatic EMR has a wavelength longer than 750 nm.
13. The method according to claim 8 , wherein the incoming polychromatic EMR comprises incoherent radiation.
14. The method according to claim 8 , wherein the incoming polychromatic EMR contains image information.
15. The method of claim 8 , wherein the incoming polychromatic EMR comprises coherent radiation or a combination of coherent and incoherent radiation.