Optical pick-up unit
View Patent ↗An optical pick-up including a fold mirror having an achromatic quarter-wave plate coating is provided. To allow the thin film coating to efficiently provide the quarter-wave retardation, the optical pick-up is configured such that the polarization of light incident on the fold mirror is at a predetermined angle. According to one embodiment, the predetermined angle is provided by rotating the light sources. According to another embodiment, the predetermined angle is provided by rotating a plane containing the light sources and the beam combiners. In each embodiment, the beam combiners and the fold mirror are arranged along the same axis to further improve efficiency.
1. An optical pick-up apparatus comprising:
a plurality of light sources including a first light source for emitting light at a first wavelength and a second light source for emitting light at a second other wavelength;
at least one beam combiner for transmitting the light emitted from each light source in the plurality of light sources along a common optical path in a first direction parallel to a first axis;
a reflector for redirecting the light transmitted in the first direction in a second direction, the second direction parallel to a second axis substantially perpendicular to the first axis, the reflector including a coating for providing substantially quarter-wave retardation at the first and second wavelengths at a predetermined angle of incidence;
an objective lens for focusing the light redirected by the reflector onto an optical disc; and
at least one photo-detector for detecting light reflected from the optical disc,
wherein the at least one beam combiner and the reflector are disposed along the first axis,
wherein the light transmitted in the first direction from the at least one beam combiner includes linearly polarized light having its electric field vector substantially perpendicular to the first axis and at an oblique angle to the second axis, and
wherein the coating includes a first plurality of alternating layers of isotropic materials having contrasting refractive indices, a thickness and refractive index of each layer in the first plurality selected such that the coating functions as a C-plate at the first and second wavelengths.
2. An optical pick-up apparatus according to claim 1 , wherein the plurality of light sources includes a third light source for emitting a third beam of light at a third wavelength, each of the first, second, and third wavelengths being different.
3. An optical pick-up apparatus according to claim 2 , wherein the at least one beam combiner includes a first beam combiner for receiving the light emitted from the first light source, a second beam combiner for receiving the light emitted from the second light source, and a third beam combiner for receiving the light emitted from the third light source, each of the first, second, and third beam combiners disposed along the first axis.
4. An optical pick-up apparatus according to claim 3 , wherein the first, second, and third beam combiners are wavelength dependent polarization beam combiners for, in a first pass, directing light from the first, second, and third light sources towards the reflector, and in a second pass, directing light at the first, second, and third wavelengths from the reflector towards the at least one photodetector in dependence upon a substantially 90 degree phase shift provided by the reflector.
5. An optical pick-up apparatus according to claim 3 ,
wherein the first beam combiner is for reflecting light at the first wavelength having a first polarization from the first light source to the second beam combiner, and for passing light at the first, second, and third wavelengths having a second orthogonal polarization from the second beam combiner to the at least one photodetector,
wherein the second beam combiner is for reflecting light at the second wavelength having the first polarization from the second light source to the third beam combiner, for passing the light at the first wavelength having the first polarization from the first beam combiner to the third beam combiner, and for passing light at the first, second, and third wavelengths having the second orthogonal polarization from the third beam combiner to the first beam combiner, and
wherein the third beam combiner is for reflecting light at the third wavelength having the first polarization from the third light source to the reflector, for passing the light at the first and second wavelengths having the first polarization from the second beam combiner to the reflector, and for passing light at the first, second, and third wavelengths having the second orthogonal polarization from the reflector to the second beam combiner.
6. An optical pick-up apparatus according to claim 3 , wherein each of the first, second, and third beam combiners are oriented to provide the linearly polarized light having its electric field vector substantially perpendicular to the first axis and at an oblique angle to the second axis.
7. An optical pick-up apparatus according to claim 3 , wherein each light source in the plurality of light sources and the at least one beam combiner are disposed in a same plane, the same plane rotated about the first axis relative to the second axis at an angle substantially equal to the oblique angle.
8. An optical pick-up apparatus according to claim 3 , wherein each of the first, second, and third beam combiners includes a polarization beam splitting cube.
9. An optical pick-up apparatus according to claim 3 , wherein each of the first, second, and third beam combiners includes a polarization beam splitting plate, each polarization beam splitting plate disposed at angle to the first axis substantially equal to ±45 degrees.
10. An optical pick-up apparatus according to claim 9 , including a plate for reducing astigmatism, the plate disposed along the first axis with the polarization beam splitting plates such that each successive plate is tilted about an orthogonal axis.
11. An optical pick-up apparatus according to claim 2 , wherein the at least one photo-detector includes a first photo-diode co-packaged with the first light source, a second photo-diode co-packaged with the second light source, and a third photo-diode co-packaged with the third light source.
12. An optical pick-up apparatus according to claim 11 , wherein the at least one beam combiner includes a first dichroic beam splitting cube disposed along the first axis, the first dichroic beam splitting cube including an interface for passing light at the first wavelength and reflecting light at the second wavelength.
13. An optical pick-up apparatus according to claim 12 , wherein the at least one beam combiner includes a second dichroic beam splitting cube disposed along the first axis, the second dichroic beam splitting cube including an interface for passing light at the first and second wavelengths and for reflecting light at the third wavelength.
14. An optical pick-up apparatus according to claim 11 , including a polarizing hologram for diffracting light at the first, second, and third wavelengths in dependence upon a substantially 90 degree phase shift provided by the reflector, the polarizing hologram selected for directing the diffracted light at the first wavelength towards the first photo-diode, the diffracted light at the second wavelength towards the second photo-diode, and the diffracted light at the third wavelength towards the third photo-diode.
15. An optical pick-up apparatus according to claim 11 , wherein each of the light sources in the plurality of light sources and the at least one beam combiner are disposed in a plane substantially perpendicular to the second axis.
16. An optical pick-up apparatus according to claim 11 , wherein each of the light sources in the plurality of light sources are rotated such that the linearly polarized light transmitted in the first direction by the at least one beam combiner has its electric field vector substantially perpendicular to the first axis and at an oblique angle to the second axis.
17. An optical pick-up apparatus according to claim 1 , comprising at least one tilted edge filter for protecting the first and second light sources from light reflected from the at least one beam combiner.
18. An optical pick-up apparatus according to claim 1 , wherein the oblique angle is substantially equal to 45 degrees.
19. An optical pick-up apparatus according to claim 1 , wherein the predetermined angle is substantially equal to 45 degrees.
20. An optical pick-up apparatus according to claim 1 , wherein the coating is designed to allow a portion of the light transmitted in the first direction and incident on the reflector to be tapped.
21. An optical pick-up apparatus according to claim 1 , wherein the coating includes a second plurality of alternating layers of isotropic materials having contrasting refractive indices, a thickness and refractive index of each layer in the second plurality selected such the coating functions as a high reflector at the first and second wavelengths.
22. An optical pick-up apparatus according to claim 21 , wherein the first and second plurality of alternating layers are selected such that the C-plate functionality and the high reflector functionality is integrated.