Reading/Writing Optical Device Having Temperature Compensation

Abstract

An optical data reading/writing device for reading/writing to an information layer, the device comprising at least a first radiation source (12) for generating a radiation beam and an optical system (10, 40, 44, 46, 48, 50, 52, 54, 56) for converging the radiation beam on the information layer and for converging the radiation beam reflected by the information layer onto a detector, wherein the optical system incorporates a wavelength sensitive structure which compensates for a temperature-induced defocusing of the optical system.

Description

For a better understanding of the invention, and to show how embodiments of the same may be carried into effect, reference will now be made, by way of example, to the accompanying diagrammatic drawings in which:

FIG. 1 is a schematic side view of a pre-collimator;

FIG. 2 is a schematic diagram of a prior art double optical writer;

FIG. 3 is a schematic side and front view of a beam-shaper/pre-collimator having a wavelength sensitive structure; and

FIG. 4 is a schematic side view of a blazed grating structure.

In order to address the problem of refractive index of an optical component (such as a pre-collimator, beam-shaper or sensor lens) varying with temperature it is proposed to integrate a wavelength sensitive (such as a grating structure or stepped phase structure) on the optical component, which structure compensates for the defocusing in temperature due to the change of the refractive index with temperature of the material. The wavelength sensitive structure forms part of an optical system of an optical read/write device.

For plastics materials the change of refraction index with temperature is larger than for glass materials. The invention is therefore most beneficial for plastics optical components.

FIG. 3 shows front and side views of a pre-collimator 28 (which could equally represent a servo-lens or beam-shaper) having a wavelength sensitive structure 30 on a front face thereof. A reader/writer incorporating the pre-collimator 28 shown in FIG. 3 is implemented, for example, in a structure like that shown in FIG. 2. Thus FIG. 2 would be changed only to introduce a pre-collimator or beam-shaper 28 (as in FIG. 3) for the pre-collimator 10 or beam-shaper 18 in FIG. 2.

In the case of a light path with two lasers (discrete lasers or separate laser-branches), such as that shown in FIG. 2, the pre-collimator 10 and beam-shaper 18 are behind one laser each, which means only one wavelength is of interest. It is also foreseen that three or more lasers may be used, for example to allow the use of three different formats, such as CD, DVD and Bluray Disc (BD).

One possibility to address the change of the laser wavelength with temperature is the use of a non-periodic phase structure as described in Appl. Opt. Vol. 40 no 35 6548-6560, the contents of which are incorporated herein by reference. The height h of the steps in the structure are in fixed steps with

$\begin{array}{cc}h=\frac{\lambda }{n-1}& \left(2\right)\end{array}$

Say that j is the number of steps. For that case the zone height is m_j.h with m_j an integer. If Δλ is the wavelength shift and Δn is the refracting index change (both due to temperature) the phase steps ΔΦ_j are:

$\begin{array}{cc}\Delta {\Phi }_j=-2\pi m_j·\left(\frac{\Delta \lambda }{\lambda }-\frac{\Delta n}{n-1}\right)& \left(3\right)\end{array}$

Another possibility is using other diffractive structures like a blazed grating or kinoform grating, which are also wavelength sensitive.

A blazed grating is a structure as shown in FIG. 4. When the angle of the gratings in the structure are such that the angle of refraction is the same as the angle of diffraction, then 100% of the laser power will go into one order (for instance the +1 order). This is the case for

$\begin{array}{cc}\Psi =\frac{\lambda }{p·\left(n-1\right)}& \left(4\right)\end{array}$

• • Ψ: the blaze angle
  • λ: the wavelength is the laser
  • p: grating pitch
  • n: refracting index of the grating material

The kinoform grating is like the blazed grating, but it has a more rounded shape.

For a lens having a wavelength sensitive diffractive grating structure, the power of the grating is part of the total power of the lens, which is the case of this invention.

The power of the grating structure is designed in such a way that the wavelength shift of the laser causes exactly the same focal length shift of the lens as by the change of the refracting index change with temperature of the body, however with opposite sign.

The servo or sensor lens 56 for the focus adjustment is also not in the common path: only in the detector path 26 and not in the laser path 14/22. The sensor lens 56 usually comprises a spherical and an astigmatic surface. It could also be that some chromatic aberration correction is implemented in this sensor lens 56.

The lens 56 will also cause defocus with temperature, because it is not in the common path. However this effect is not so strong, and is described by (approximation with planar convex or concave lens)

$\begin{array}{cc}\frac{z}{T}=\frac{{\left(1-\frac{{\mathrm{NA}}_{\mathrm{in}}}{{\mathrm{NA}}_{\mathrm{out}}}\right)}^2·f}{\left(2·m_c^2\right)·\left(n-1\right)·{\left(\frac{{\mathrm{NA}}_{\mathrm{in}}}{{\mathrm{NA}}_{\mathrm{out}}}\right)}^2}·\frac{n}{T}& \left(5\right)\end{array}$

with the parameters as described with formula (1)

When a single detector is applied the servo or sensor lens 56 in front of the detector must operate correctly at 2 wavelength ranges (although as mentioned above more than 2 wavelength ranges could be used). The temperature compensation on this lens is chosen for the most critical application, which is in general the shortest wavelength (for instance DVD λ=660 nm). Alternatively, a structure may be used to give a mixed effect on both wavelengths.

The above is also for the situation when a beam-shaper or pre-collimator is applied together with a dual wavelength laser (two laser-chips having different wavelength ranges in one package).

A similar setup can easily be devised in the case that the optical read/write device uses more than two wavelength ranges. For example, in the case of an optical reading/writing device for CD, DVD and BD applications there would be three branches as opposed to the two laser branches 14/22 in the embodiment described above.

Claims

1. An optical data reading/writing device for reading/writing to an information layer, the device comprising at least a first radiation source for generating a radiation beam and an optical system for converging the radiation beam on the information layer and for converging the radiation beam reflected by the information layer onto a detector, wherein the optical system incorporates a wavelength sensitive structure which compensates for a temperature-induced defocusing of the optical system.

2. An optical data reading/writing device as claimed in claim 1, in which the wavelength sensitive structure is a part of a refracting pre-collimator, a beam-shaper or a sensor lens of the optical system.

3. An optical data reading/writing device as claimed in claim 1, in which the wavelength sensitive structure is located out of a common path for the radiation beam.

4. An optical data reading/writing device as claimed in claim 3, in which the wavelength sensitive structure is located between the at least one radiation source and a pre-collimator/beam-shaper of the optical system.

5. An optical data reading/writing device as claimed in claim 3, in which the wavelength sensitive structure is located between a beam-splitter element and a detector element of the optical data reading/writing device.

6. An optical data reading/writing device as claimed in claim 1, in which the wavelength sensitive structure is a grating structure.

7. An optical data reading/writing device as claimed in claim 1, in which the wavelength sensitive structure is a stepped phase structure.

8. An optical data reading/writing device as claimed in claim 1, in which the wavelength sensitive structure is a non-periodic phase structure.

9. An optical data reading/writing device as claimed in claim 1, in which the wavelength sensitive structure is a diffractive structure.

10. An optical data reading/writing device as claimed in claim 1, which incorporates multiple radiation sources for reading/writing to different types or formats of information layer.

11. An optical data reading/writing device as claimed in claim 1, in which the wavelength sensitive structure faces its respective radiation source.

12. A method of compensating for temperature-induced defocusing of an optical system in an optical reading/writing device comprises including a wavelength sensitive structure in the optical system, which wavelength sensitive structure compensates for said defocusing.

13. The method as claimed in claim 12, in which the wavelength sensitive structure faces a radiation source of the reading/writing device.

14. The method of claim 12, which includes compensating for defocusing in at least two elements of the optical system.

15. The method of claim 14, in which each of said elements has an associated wavelength sensitive structure.

16. A refracting pre-collimator/beam-shaper/sensor lens for compensating for temperature defocusing incorporates a wavelength sensitive structure adapted to compensate for temperature defocusing.