BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described in more detail by way of example with reference to the accompanying drawings, in which:
FIG. 1 shows tracks on an optical disk and the path of a scanning spot, in accordance with the present invention;
FIGS. 2a and 2b show a semiconductor laser in a front view and a top view, respectively;
FIG. 3a and FIG. 3b show a wavelength tuneable semiconductor laser in a side view and in a top view, respectively;
FIG. 4 shows a pair of gratings to be used with a wavelength tuneable laser according to FIG. 3;
FIG. 5 shows a top view of a wavelength tuneable semiconductor laser;
FIG. 6 shows a semiconductor laser with additional contacts for applying a voltage;
FIG. 7 shows a further example of a path of a light spot on an optical disk;
FIG. 8 shows a flow chart of a method according to the present invention;
FIG. 9 shows a setup for generating several beams to be projected onto an optical disk according to prior art;
FIG. 10 shows tracks of an optical disk and multiple light spots projected onto the optical disk, according to prior art; and
FIG. 11 shows segments of a photo-detector according to prior art.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows tracks on an optical disk and the path of a scanning spot, in accordance with the present invention. A central track 16 and two neighbouring tracks 24, 26 are shown that are provided on an optical disk 10. The path of the scanning spots 14 in a conventional optical disk drive is shown with a dashed line 28. The path of a scanning spot according to the present invention is shown with a solid line 30. The periodical dots on line 30 represent the sampling points. The amplitude of the wiggle is A, the period is T. In this particular case, the wiggle amplitude A is exactly one track, however, this is not a requirement for the invention. In this example, the main track 16 is sampled every T/2, whereas the two adjacent tracks 24, 26 are sampled every T. In case T is smaller than X/4NA, the full signal at the adjacent tracks may be recovered. As discussed, sampling at such a high rate may not be necessary for the present invention.
FIGS. 2a and 2b show a semiconductor laser in a front view and a top view, respectively. By a layered structure a semiconductor laser 32 is provided. The semiconductor laser 32 comprises a thin active layer 34 of a typical thickness of 150 mn. Only a stripe 36 of the active layer 34 is conducting. Thus, when a current is injected via contact 38, this current only flows through the narrow stripe 36 which is typically 5 mm wide. The front face 40 and the rear face 42 of the structures are reflecting so that they define a laser cavity. A beam of laser light 12 is emitted by the semiconductor laser 32.
FIG. 3a and FIG. 3b show a wavelength tuneable semiconductor laser in a side view and in a top view, respectively. A current is injected into the contact 38 of the wave- length tuneable laser diode 18. The current is injected into the active layer 34 through the stripe 36. The light generated in the cavity passes through a gap 44 into an additional structure with a Bragg-reflector/grating 46. The pitch and the effective refractive index of this structure control the wavelength of the light 12 that is emitted at the opposite side of the laser 18. The effective refractive index may be controlled by a secondary current that is applied over a further contact 48. By this signal applied over the contact 48 the wavelength of the laser 18 can be tuned. On the basis of such a tuned wavelength, a local displacement may be achieved as will be described below.
FIG. 4 shows a pair of gratings to be used with a wavelength tuneable laser ac- cording to FIG. 3. The light beam 12 from a wavelength tuneable laser, as for example shown in FIG. 3, is projected onto a first grating 50 and from there on to a second grating 52. The gratings 50, 52 have a pitch p. When the wavelength changes from X to X+AX the diffraction angle changes with Ap =AX/p. This results in a lateral shift of Ax =xAo =(x/p) AX. When Ax =21im and AX =5 nm, we must have x/p =400. Thus, for example x =500 ,m and p =1.25 ,um will do, which are reasonable numbers. The grating pair may also be etched on the surfaces of a prism.
FIG. 5 shows a top view of a wavelength tuneable semiconductor laser. In this embodiment a Bragg-reflector 54 with a variable pitch is used. The pitch of the Bragg- reflector 54 changes from s on side to s+As on the other side. The change in pitch As is very much exaggerated in the drawing. When a tuning signal is changed, the lateral position where the light leaves the cavity will change as well.
FIG. 6 shows a semiconductor laser with additional contacts for applying a voltage. The contact 38 for current injection is separated from two further contacts 56, 58 by two insulating layers 60, 62. Voltages V56 and V58 applied to these contacts 56, 58 make the electric field distribution in the device left-right asymmetric (provided that the two voltages are unequal). This will induce an asymmetry in the refractive index of the active layer, which makes the transversal mode of the laser cavity asymmetric. The light distribution at the exit surface of the device will then be asymmetric as well, so that the source point is effectively displaced from left to right (or vice versa).
The embodiment described in relation to FIG. 6 can also be varied in that currents are injected over the two additional contacts 56, 58. If these currents are injected with dif- ferent amounts, the desired asymmetry is achieved.
FIG. 7 shows a further example of a path of a light spot on an optical disk. A tra- jectory 64 of the scanning spot with intermittent wiggles is shown. Such an intermittent wobbling is sufficient for 3SPP. For XTC a more continuous sampling of the neighbouring tracks is required.
It is noted that the embodiments of the present invention can be different from the examples shown in the drawings and described above. For example, it is not required to use only one scanning spot that oscillates relative to the tracks. Rather, the prior art technique of using several scanning spots can be combined with the wobbling of the scanning spot ac- cording to the present invention.
Equivalents and modifications not described above may also be employed without departing from the scope of the invention, which is defined in the accompanying claims.
Any reference sign in the following claims should not be construed as limiting the claim. It will be obvious that the use of the verb “to comprise” and its conjugations does not exclude the presence of any other elements besides those defined in any claim. The word “a” or “an” preceding an element does not exclude the presence of a plurality of such elements.
Patent Application
20080094946
