Raman despeckling with spectral control
An apparatus and method for controlling the spectrum of stimulated Raman scattering that is used for despeckling of digitally projected images. The stimulated Raman scattering is utilized to add wavelength diversity for reduced speckle and to change the color of the light to a more desirable combination of wavelengths. Digital projection with color-sequential projectors may be enabled by alternately switching the Raman spectrum between green and red. Improved projector transmission may be achieved by minimizing the amount of yellow light generated in the Raman spectrum.
1. An optical apparatus comprising:
a pulsed green laser;
a Raman-shifting device; and
a digital projector;
wherein the pulsed green laser generates a green light with a light pulse that illuminates the Raman-shifting device; the light pulse is shaped in a time domain to generate a desired spectrum from the Raman-shifting device, and the desired spectrum illuminates the digital projector.
2. The apparatus of claim 1 wherein the desired spectrum has a lower speckle characteristic than the green light.
3. The apparatus of claim 1 wherein the desired spectrum has a higher luminous efficacy than the green light.
4. The apparatus of claim 1 wherein the light pulse is shaped in the time domain to be a square shape.
5. The apparatus of claim 4 wherein the desired spectrum is red.
6. The apparatus of claim 5 wherein the light pulse is shaped in the time domain to alternate between a square shape of a first amplitude and a square shape of a second amplitude; the first amplitude is different than the second amplitude; and the desired spectrum alternates between green light and red light.
7. The apparatus of claim 6 wherein the digital projector comprises a single light valve; and the single light valve forms a full color digital image by synchronizing the single light valve with a source of blue light and with the desired spectrum that alternates between green light and red light.
8. The apparatus of claim 1 wherein the desired spectrum comprises a green band and a red band; and the desired spectrum has a gap between the green band and the red band.
9. The apparatus of claim 8 wherein the gap matches a low transmission band of the digital projector.
10. A method of despeckling comprising:
generating a pulsed green laser light;
Raman-shifting the pulsed green laser light to generate a Raman-shifted laser light;
using the Raman-shifted light to illuminate a digital projector; and
projecting a digital image with the digital projector;
wherein the pulsed green laser light has a light pulse that is shaped in a time domain to Raman-shift the laser light to a desired spectrum.
11. The method of claim 10 wherein the desired spectrum has a lower speckle characteristic than the pulsed green laser light.
12. The method of claim 10 wherein the desired spectrum has a higher luminous efficacy than the pulsed green laser light.
13. The method of claim 10 wherein the light pulse is shaped in the time domain to be a square shape.
14. The method of claim 13 wherein the desired spectrum is red.
15. The method of claim 14 wherein the light pulse is shaped in the time domain to alternate between a square shape of a first amplitude and a square shape of a second amplitude; the first amplitude is different than the second amplitude; and the desired spectrum alternates between green light and red light.
16. The method of claim 15 wherein the digital projector comprises a single light valve; and the single light valve forms a full color digital image by synchronizing the single light valve with a source of blue light and with the desired spectrum that alternates between green light and red light.
17. The method of claim 10 wherein the desired spectrum comprises a green band and a red band; and the desired spectrum has a gap between the green band and the red band.
18. The method of claim 17 wherein the gap matches a low transmission band of the digital projector.