This invention relates to a method of manufacturing a beam-shaper photodiode unit, a method of manufacturing a wafer of photodiode beam-shaper units, a beam-shaper photodiode unit, a wafer of photodiode beam-shaper units, a beam-shaper and to a method of shaping a beam in an optical device.

A convenient method for the miniaturisation of a light path for an optical recording unit is to glue the optical components, including a laser, onto a photodiode detector chip. Such a system is described below, with reference to Figures 5 and 6.

A laser detector grating unit (LDGU) with low building height and a slimmer light path is constituted as follows.

Coupling of the light beam to one side of an LDGU results in a large reduction of the building height and results in a simpler assembling of the laser.

Fig. 5 shows the concept of current LDGU (source: Philips). The position of photodiodes 70 with respect to a laser 72 and the wiring results in the diameter of this device determining the building height. Also notice that the laser 72 is perpendicular to the base plate, which results in a complicated assembly.

Fig. 6 shows embodiments in which the outgoing light beam is perpendicular with the assembly base-plate. In the Figures 5 and 6 the laser 72, with or without a sub- mount, is placed perpendicular with the base-plate on a photodiode chip 74. The photodiode chip 74 in its turn is placed on the base-plate (the housing). The beam-splitter grating 76 is positioned on, over or beside the photodiode/laser sub-assembly.

In the embodiment of fig. 6 a prism 78 or mirror is attached onto the photodiode. The laser chip is mounted on the rim (edge) of the photodiode, so no sub-mount is needed.

One example of an existing beam-splitter is a semi-transparent flat mirror (at an angle of 45 degrees w. r. t the beam) in which the laser light on its way to the disc is partly reflected and the light reflected by the disc is partly transmitted by the semi-mirror and passed on towards the photodiodes. A second example of a beam-splitter is a semi-reflecting beam-splitter cube.

A problem arises in the gluing of optical components to a photodiode detector chip in that components must be placed in an exact position within very small tolerances. In view of this, it is advisable to reduce the number of components to a minimum that have to be placed individually. For this reason as many components as possible have to be positioned and glued onto the photodiode detector chip whilst the chips are still held together in the form of a wafer during manufacture. After production, the wafer is divided into individual light path sections.

The particular components that have to be positioned individually on the detector chips are the laser and collimator lens. Also, a beam-shaper having toroidal surfaces has to be positioned individually.

It is an object of the present invention to seek to reduce the number of components that must be glued individually on the photodiode detector chips discussed above and to reduce a light path in a beam-shaper photodiode unit.

According to a first aspect of the present invention a method of manufacturing a beam-shaper photodiode unit comprises: a) securing a laser unit and a collimator lens to each of a plurality of photodiode chips, which photodiode chips form part of a photodiode wafer; b) securing at least one beam-shaper element across a plurality of said photodiode chips; and c) separating resulting beam-shaper photodiode units from each other by separating the photodiode wafer into individual photodiode units, each photodiode unit carrying a portion of said at least one beam-shaper element.

Preferably, two beam-shaper elements are secured across the plurality of said photodiode chips. The beam-shaper elements are preferably tetrahedrons in cross-section, which may have a truncated wedge and/or a truncated prism shape. The two beam-shaper elements are preferably secured side by side and may have an air gap between them, or between faces thereof which form a light path.

The or each beam-shaper element is preferably an elongate strip.

The or each beam-shaper element may be aligned with respect to edges of the photodiode wafer. The or each beam-shaper element preferably extends across the width of the photodiode chip.

Side faces of the or each beam-shaper element are preferably left substantially unfinished after separation of the photodiode wafer. Thus, manufacturing time and cost is advantageously reduced.

The or each beam-shaper element may have a reflective face, which is preferably a lower face of a beam-shaper element having a prismatic/truncated-prismatic cross-section. Reflection from the lower face advantageously allows the folding of the light path through the beam-shaper, which allows beneficial building height reduction.

Preferably, the beam-shaper element causes a thickness of an incoming beam to be reduced after passage therethrough. An advantageous reduction in building height is thus achieved.

Preferably at least one beam-shaper element has a reflective surface/coating applied thereto.

According to a second aspect, the invention extends to a method of manufacturing a wafer of beam-shaper photodiode units comprising steps a) and b) of the first aspect.

According to a third aspect of the invention a beam-shaper photodiode unit is manufactured according to the first aspect of the invention.

According to a fourth aspect of the invention a wafer of beam-shaper photodiode units is manufactured according to the second aspect of the invention.

According to a fifth aspect of the invention a beam-shaper has at least one internally reflective surface adapted to internally reflect a light beam passing through the beam-shaper.

Preferably the at least one internally reflective surface is a lower surface.

Preferably the beam-shaper comprises at least one beam-shaper element that is a prism having the internally reflecting surface.

Preferably the beam-shaper causes a reduction in light beam thickness after passage through the beam-shaper and reflection from the internally reflective surface.

According to a sixth aspect of the invention a beam-shaper comprises at least one polyhedron arranged to extend across a width of a photodiode base to which it is to be secured.

The at least one polyhedron is preferably elongate, preferably being at least two times longer than the width of a collimating lens secured to the photodiode base to which the beam-shaper is to be secured. Thus, since the polyhedron extends across the width of the photodiode base, it can advantageously be formed in strips, resulting in significantly more accurate manufacture and also lower manufacture costs, because individual placing of beam- shapers is avoided.

According to a seventh aspect of the invention a beam-shaper photodiode unit comprises a photodiode base having a beam-shaper secured thereto, wherein the beam-shaper extends substantially across the width of the photodiode base.

According to an eighth aspect of the present invention, a method of shaping a beam in an optical device comprises directing the beam to a prism-shaped beam-shaper element at an angle to cause the beam to enter the beam-shaper element and to be reflected internally from at least one internal surface of the element before leaving the element.

Preferably, the beam is passed through a wedge-shape element before passing through the prism-shaped element.

All of the features described herein may be combined with any of the above aspects in any combination.

For a better understanding of the invention and to show how the same may be brought into effect, specific embodiments will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a schematic side view showing the path of a light beam through a first embodiment of beam-shaper ; Fig. 2 is a schematic side view showing the path of a light beam through a second embodiment of beam-shaper; Fig. 3 shows schematic side and top views of a device incorporating the second embodiment of beam-shaper; Fig. 4 shows a schematic view from above of a wafer incorporating a plurality of the devices shown in Figure 3; Fig. 5 shows an exploded view of a prior art laser detector grating unit (LDGU); and Fig. 6 shows another prior art LDGU arrangement in schematic detail.

In Figure 3 a section of an optical recording unit is shown. A laser 10 directs a beam at a collimator lens 12, through a beam-shaper 16 which directs the beam onto an objective lens (not shown) and an optical disk (not shown). The light is then reflected from the optical disk back through the beam-shaper 16 and the collimator lens to be deflected by a beam-splitter (not shown for clarity) located between the collimator lens 12 and the laser 10.

The beam-splitter directs the reflected light onto a photodiode chip 14 having detector regions 18a and 18b.

It has been realised by the applicant that if it is possible to combine a particular component into a strip which can be secured to a wafer as shown in Figure 4 comprising a plurality of optical recording units, then positioning tolerances of the strip will be dramatically reduced compared to those for locating the individual components separately.

Afterwards, the wafer can be divided into the separate units forming individual light paths.

A beam-shaper with toroidal surfaces does not lend itself to being produced in a strip. Hence, the applicant proposes that an alternative type of beam-shaper 16 as shown in Figure 1. This beam-shaper comprises a first wedge 20 having a generally vertical first surface 22 which receives an incoming light beam from the collimator lens 12. The light beam passes through the first wedge 20 to be refracted upwards from a second surface 24 which is inclined downwards with respect to the first surface 22.

The upwardly refracted light beam then enters a second wedge 26 having a first surface 28 inclined downwardly, such that the upwardly refracted light beam enters the second wedge 26 perpendicularly to the first surface 28 thereof. The light beam then passes towards a second surface 30 of the second wedge 26, which second surface 30 is inclined upwardly from the horizontal at such an angle to cause the light beam to be refracted out of the second wedge 26 to resume a horizontal track before passing onto the objective lens mentioned above.

The purpose of a beam-shaper is to decrease the diameter of the collimated light beam in one direction (in this case in the direction perpendicular to the plane of the drawing) and not in the other direction, thus increasing the total amount of light captured by the circular pupil of the objective lens.

The diffraction of the light beam emitted by the laser is larger in a direction in the plane of fig. 4 than in a direction perpendicular to it. Typical values for the full width at half maximum intensity (FWHM) for this Gaussian shaped intensity distribution are 22 degrees and 10 degrees. The rim intensity of the light beam entering the pupil of the objective lens must be larger than a certain value in order to obtain a suitably small spot to perform correct read-out of the information on the disc. Therefore the smallest diverging angle of the laser determines how much light can be captured by the objective lens. In order to increase this total amount of light captured by the objective lens, necessary for writing data at high intensity, the pupil diameter of the collimating lens is chosen larger than the diameter of the objective lens. This results in a collimated beam diameter, which is appropriate in one

direction but over-filled in the other. The purpose of the beam-shaper is to decrease the beam diameter in the over-filled direction and thus increase the total amount of captured light.) The beam-shaper 16 shown in Figure 1 can be manufactured in strips spanning a number of photodiode units arranged on a wafer (see Figure 4). The separate units being divided after laying down and gluing the first and second wedges 20 and 26 in the configuration shown in Figure 4 and described below. The side faces that would be revealed after separation of the separate elements do not require specific finishing, because those faces are not used for transmission of light, given that only the first and second surfaces of each of the wedges 20 and 26 are used, rather than the new side faces that would be produced during separation of adjacent units.

By producing the beam-shaper in strips 48 and 50 to be laid across a wafer, considerably better tolerances are achieved when compared with locating each beam-shaper separately.

The elements of the beam-shaper 16 are made of glass using known methods for producing a wedge-shaped strip. The wedge strips 48 and 50 are then glued across a wafer in the correct position using an adhesive. As shown in Figure 4 multiple strips 48,50 can be placed on a wafer to produce an array of optical units, each having a beam-shaper 16.

After the wedge-shaped strips 48, 50 have been secured in position on the wafer, the individual optical units can be separated.

In the configuration shown in Figure 1 the output beam is shifted through a distance larger than would typically be required, which makes the light path (the distance between the lowest edge of the light beam and the upper edge to its whole track) rather thick.

A second embodiment of beam-shaper is shown in Figure 2. In that configuration a wedge 34 has a vertical first surface 36 through which the light beam enters.

The light beam passes through the wedge 34 and is refracted downwards by an upwardly inclined second surface 38. The light beam then passes towards a prism 40 with the light beam entering at a perpendicular angle to a first surface 42 of the prism 40. The light beam continues its downward movement through the prism 40 to be reflected internally from a lower face 44. The lower face 44 has an exterior reflective coating to enable the internal reflection. The reflected beam then passes to a second surface 46 of the prism 40, which second surface is inclined upwardly at an angle chosen to cause the light beam to be refracted to resume a horizontal path towards the objective lens.

In the situation described above the prism 40 is glued directly onto the photodiode chip, which requires the provision of the reflective coating, to prevent light passing out of the lower surface and onto the photodiode chip 14 in an unintended location.

The second embodiment is manufactured in the same way as the first embodiment, with the first wedge 34 and prism 40 being manufactured in strips and glued in position on a wafer of photodiode units. Thus, the beam-shaper comprises at least one polyhedron (the prism 40) arranged to extend across a width of a photodiode base 14 to which it is secured. The at least one polyhedron is elongate, and in the example shown is at least two times longer than the width of the collimating lens 12 secured to the photodiode base 14 to which the beam-shaper is secured.

The beam-shapers 16 described herein are suitable for on chip integration with chips having a low building height. Significant production time and production cost advantages result from securing strips onto a photodiode wafer before separation of the wafer in separate photodiode units.

Also, the folding of the beam in the second embodiment by internal reflection in the prism-shaped element is particularly advantageous for achieving low building height.

A simple reflective coating allows the internal reflection.