WO2006051470A1

WO2006051470A1 - Laser diode with integrated quarter wave plate and grating

Info

Publication number: WO2006051470A1
Application number: PCT/IB2005/053645
Authority: WO
Inventors: Ming S. Chen
Original assignee: Arima Devices Corporation
Priority date: 2004-11-11
Filing date: 2005-11-07
Publication date: 2006-05-18
Also published as: TW200631006A

Abstract

In a laser diode (20), a quarter wave plate (25) and a grating (22) are integrated such that they form part of the cover glass window (22, 25, 25a-b) of the laser diode (20). The integration of the quarter wave plate and the grating into the laser diode reduces the cost and the production cycle of an optical pickup unit (OPU) for CD, DVD and BD (Blue-ray Disc) systems in which the inventive laser diode forms part, due to less components and assembly. Further, the overall performance of the OPU may be improved, and the OPU can be made more compact and the assembly becomes easier.

Description

Laser diode with integrated quarter wave plate and grating

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a laser diode for generating laser light of at least one specific wavelength, and an optical pickup unit for use in an optical system, such as a CD/DVD/BD system, comprising such a laser diode. ■

BACKGROUND OF THE INVENTION

An optical pickup unit (OPU), which is a key component for CD, DVD, and BD (Blue-ray Disc) systems, usually includes key components such as a semiconductor laser diode, a diffraction grating, a beam splitter, a collimating lens, a quarter wave plate, actuator with objective lens, and photo detectors. The laser diode generates a laser beam which is split up into the generally required three laser beams by the diffraction grating. The three laser beams, which are linearly polarized, are then reflected through the beam splitter, collimated in the collimating lens, converted into circularly polarized laser beams by the quarter wave plate, and focused by the objective lens on a rotating disc. The disc reflects the laser beams which are modulated by the information layer on the disc, and the beams are picked up by the objective lens, converted again into linearly polarized beams by the quarter wave plate, focused by the collimating lens, and, at least partly, deflected by the beam splitter to the photo detectors.

Concurrently with the consumer price for CD/DVD systems etc. being reduced and especially portable disc players becoming smaller and thinner, there is a demand for cheaper and smaller optical pickup units.

In Nemoto, K. and Miura, K.: A Laser Coupler for DVD/CD Playback - Using a Monolithic-integrated Two-wavelength Laser Diode, JSAP International, No.3, January 2001, there is disclosed a two-wavelength laser coupler with a laser diode, a photo detector IC chip, a micro-prism and a quarter wave plate being integrated in a small package. By integrating these components into the laser coupler package, a smaller number of parts is required in the OPU, whereby its size as well as its price may be reduced. However, by an increasing demand for cheaper and smaller CD/DVD/BD players, there is a need for further improvements to the optical pickup units. SUMMARY OF THE INVENTION

It is an object of the present invention to make norther reduction of the number of components of optical pickup units and thus costs possible. This and other objects are achieved by the provision of a laser diode according to claim 1 and an optical pickup unit for use in a CD/DVD/BD system comprising such a laser diode according to claim 6. Preferred embodiments of the present invention are defined in the dependent claims.

More specifically, a laser diode according to the present invention for generating laser light of at least one specific wavelength comprises a laser chip arranged to generate a laser beam, a casing, preferably meant for sealing of the laser chip from the environment, and a window, preferably forming part of the casing, which window is arranged such that the laser beam may pass through it. The inventive laser diode is characterized in that said window comprises a quarter wave plate for converting the generated laser beam being linearly polarized into a circularly polarized laser beam and a grating arranged to split up the laser beam into at least three laser beams.

The integration of the quarter wave plate and the grating into the laser diode reduces the cost and the production cycle of an OPU in which the inventive laser diode forms part, due to less components and assembly. Further, the overall performance of the OPU may be improved, and the OPU can be made more compact and the assembly becomes easier.

According to one embodiment of the invention, the quarter wave plate comprises a polymer as retarding material. Usually, the quarter wave plate in an OPU is located after the divergent laser beam is collimated, because the retardation of the quarter wave plate is generally sensitive to incident angle of the laser beam. That is basically because the quarter wave plate typically is made of materials like quartz crystal which has a high birefringence. The high birefringence results in that a true zero-order quarter wave plate made of quartz has to be very thin and is thereby very fragile. Therefore, quartz quarter wave plates are usually multiple-order retarders which are sensitive to incident angle of the laser beam. Polymer materials offer a lower birefringence than quartz and can therefore be made into true zero-order retarders of reasonable thickness, which can suitably be integrated into the laser diode casing.

According to another embodiment of the invention, a cover glass is located on each side of the quarter wave plate. The cover glasses protect and stabilize the quarter wave plate, which is usually in the form of a film. In another embodiment of the invention, at least the one cover glass surface which is nearest to the laser chip is coated with an anti-reflection coating. This prevents laser noise, Le. reflected light which feedbacks into the laser chip and causes instability of the laser power output, and reduces stray light. In yet another embodiment, the laser diode is separate from any photo detector. In a conventional laser diode grating unit (LDGU), which is similar to the two- wavelength laser coupler described above, a photo detector is integrated into the laser diode and a hologram element is arranged on top of the LDGU to deflect the light beam to the photo detector inside the laser diode. For an OPU including a LDGU, most key parameters of the OPU are determined by the LDGU which implies constraints for an OPU designer. If the OPU maker instead uses a discrete solution where the laser diode and the photo detector are separate, the OPU designer is able to change other parts of the OPU to get required specifications of the OPU. Further, conventional laser diodes, which unlike LDGU's are not combined with photo detectors, are still mainstream, much due to the higher cost and complexity of LDGU's.

Further features of, and advantages with, the present invention will become apparent when studying the appended claims and the following description.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will now be described in more detail with reference to the accompanying drawings, in which

Fig. 1 is a schematic view of a conventional optical pickup unit,

Fig. 2 is a schematic view of an embodiment of a laser diode according to the present invention, Fig. 3 is a schematic view of a part of the laser diode according to Fig. 2, illustrating the configuration of laser beams passing into and out from the laser diode, and Fig. 4 is a schematic view of an optical pickup unit including a laser diode according to Fig. 2.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In Fig. 1, a conventional optical pickup unit (OPU), which is a key component for CD, DVD, and BD (Blue-ray Disc) systems, is shown. This OPU includes a laser diode 10 with a semiconductor laser chip 11, a diffraction grating 12, a beam splitter 13, a collimating lens 14, a quarter wave plate 15, an actuator with an objective lens 16, and a photo detector 18. Generally, a servo lens (not shown) is also needed just in front of the photo detector 18 to generate a focus error signal. The laser diode 10 generates a laser beam which is split up into three laser beams by the diffraction grating 12. The three laser beams, which are linearly polarized, are then transmitted through the beam splitter 13, collimated in the collimating lens 14, converted into circularly polarized laser beams by the quarter wave plate 15, and focused by the objective lens 16 on a rotating optical disc 17. The optical disc 17 reflects the laser beams which are modulated by the information layer on the disc, and the beams are picked up by the objective lens 16, converted again into linearly polarized beams by the quarter wave plate 15, focused by the collimating lens 14, and, at least partly, deflected by the beam splitter 13 to the photo detector 18.

A disadvantage with this prior art OPU is the relatively large number of components in it, making a further reduction of the size and the prize of the OPU difficult. The present invention helps to solve this problem.

Fig. 2 shows one embodiment of a laser diode 20 which comprises a semiconductor laser chip 21 arranged on a submount inside a casing 29 of the laser diode.

The casing 29 is arranged to seal the laser chip 21 from the environment. Conventionally, the laser diode would include a cover glass window which is a part of the sealing of the laser chip and arranged so that a laser beam may pass through it. However, in accordance with the ^■ present invention, the ordinary cover glass window is replaced by a quarter wave plate (QWP) 25, cover glass plates 25a-b and a grating 22, which are all integrated into one unit. This combination of QWP 25, grating 22 and glass plates 25a-b functions as the ordinary cover glass window to seal the laser chip 21 and allow a laser beam to pass through it, but also exhibits the functions of the QWP and the grating which conventionally are located in ¹ the OPU outside the laser diode. The QWP 25 is arranged to convert a linearly polarized laser beam generated by the laser chip 21 into a circularly polarized laser beam, and the grating 22 is arranged to split up the laser beam into three laser beams for generating tracking error signals.

In a recordable and rewritable optical storage system, a high laser power is needed on the optical disc due to writing and recording information. If the coupling efficiency of the whole light path in the OPU is lower, then the power output from the laser diode must be higher. In the past, laser diodes used for DVD recording could not offer much high power output, hence the coupling efficiency of the OPU had to be higher. Therefore, polarized optical parts were needed in the OPU. In that case, the QWP has to be put after the polarized parts, like polarized beam splitter etc., and thus cannot be integrated into the laser diode. With recent progress of laser diode design and process, the power output level from a DVD laser diode is higher. With present technology, e.g. high power laser diodes and optical parts available, it is possible to design an OPU with unpolarized optical parts and a higher power DVD laser diode. Hence, the QWP can be integrated into the laser diode. Preferably, the retarding material of the QWP 25 is made of a polymer. By retarding material is meant the material which gives the QWP the function to resolve a light wave into two orthogonal linear polarization components and produce a phase shift, i.e. retardation, between them. As mentioned above, polymer materials offer a lower birefringence than quartz, and can therefore be made into true zero-order retarders of reasonable thickness. Zero-order retarders are much less sensitive to incident angle of a light beam than multiple-order retarders, and can therefore more suitably be integrated into the laser diode casing.

However, other materials may also be used as retarding material of the QWP 25. For example a liquid crystal may be used, since liquid crystal can be made as a true zero- order retarder. Or, maybe new technology will make it possible to use quartz as retarding material in the QWP integrated into the laser diode. Further, in future there might be other types of materials which are suitable for this application.

Referring again to Fig. 2, the cover glass plates 25a-b are located on each side of the QWP 25 in order to protect and stabilize the QWP which, usually, is in the form of a film. Preferably, at least the surface of the cover glass plate 25a which is nearest to the laser chip 21 is coated with an anti-reflection coating. Otherwise the reflection from the glass surface at the air boundary will affect laser noise, i.e. reflected light feedbacks into the laser chip and cause instability of the laser power output. In addition, the anti-reflection coating is useful to increase the light coupling efficiency of the whole light path if laser power output is an issue in the OPU, and to reduce stray light.

The grating 22 can be made of glass like quartz or organic materials. The grating may be a classical grating, e.g. a linear grating, or a phase shift grating, a hologram etc., depending on specific application.

In the embodiment shown in Fig. 2, the QWP 25 is located before the grating 22 in a forward direction of the laser beam, i.e. a direction out from the laser diode 20. If the QWP is made of a polymer as mentioned above, it does not matter whether the QWP or the grating is located first in the forward direction, since the retardation of the QWP is unsensitive to angle of incidence of the light beam. However, the distance from the laser chip 21 to the grating 22 is important, and determined by the overall light-path in the OPU. In Fig. 3, the combination of QWP 25, grating 22 and glass plates 25a-b from Fig. 2 is shown separate in an enlarged view. Light beams in a forward direction and a backward direction are shown with their polarizations before and after passing the combined QWP, grating and glass plates. In the forward direction, linearly polarized light from the laser chip 21 will become circularly polarized after passing through the QWP 25, or nearly circularly polarized due to certain compromise between different laser wavelenghts if it is a two- wavelength laser diode. Of course, the optical axis orientation and thickness of the QWP need to meet certain specifications. After further passing through the grating 22, the one beam will be splitted into three beams. The two "satellite" beams are used to generate tracking error signals to lock the desired track on an optical disc in between, while the central beam is used for reading out or recording the track. The light beams will continuously pass through other components inside the OPU, and arrive in the optical disc. The light modulated by an information layer on the disc will be reflected back, and certain amount of light will be feedbacked to the laser diode 20. After the light passes through the QWP 25 again, the polarization of the light will become linear and perpendicular to the direction of polarization of the light beam from the laser chip 21. Due to compromise in retardation of the QWP, disc birefringence etc. the polarization may not be exactly linear and perpendicular. Anyway, the feedbacked light will not interfere with light in the laser oscillation cavity of the laser chip 21, and the optical feedback noise in the laser diode 20 is thus minimized. _' Fig. 4 shows an OPU for use in e.g. a CD, DVD or BD system including a laser diode 20 according to Fig. 2. The OPU further includes a beam splitter 23, a collimating lens 24, an actuator with an objective lens 26, and a photo detector 28. As mentioned above in relation to the prior art OPU shown in Fig. 1, a servo lens (not shown) may also be included in front of the photo detector 28 to generate a focus error signal. The three circularly polarized light beams from the inventive laser diode 20 will propagate through the other parts of the OPU and arrive in the optical disc 27, where they are reflected back to the beam splitter 23. Part of the light will be transmitted by the beam splitter to the photo detector 28, and part will be feedbacked to the laser diode 21.

In the embodiment shown in Fig. 4, the laser diode 20 is separate from the photo detector 28. The advantages with the laser diode and the photo detector being separate are described above. Alternatively, however, the photo detector may be integrated into the laser diode, which could be preferred in certain application. It is to be understood that modifications of the above described laser diode and OPU can be made by people skilled in the art without departing from the spirit and scope of the invention.

Claims

CLAIMS:

1. A laser diode (20) for generating laser light of at least one specific wavelength, comprising a laser chip (21) arranged to generate a laser beam, a casing (29) for the laser chip (21), and a window (22, 25, 25a-b) for passing through the generated laser beam, wherein the window (22, 25, 25a-b) comprises a quarter wave plate (25) for converting the generated laser beam being linearly polarized into a circularly polarized laser beam and a grating (22) arranged to split up the generated laser beam into at least three laser beams.

2. A laser diode according to claim 1, wherein the quarter wave plate (25) comprises a polymer as retarding material.

3. A laser diode according to claim 1, wherein a cover glass (25a-b) is located on each side of the quarter wave plate (25).

4. A laser diode according to claim 3, wherein at least the one cover glass (25a) surface which is nearest to the laser chip (21) is coated with an anti-reflection coating.

5. A laser diode according to claim 1, wherein the laser diode (20) is separate from any photo detector.

6. An optical pickup unit for use in an optical system, such as a CD/DVD/BD system, comprising a laser diode (20) according to claim 1.

7. An optical pickup unit according to claim 6, wherein the laser diode (20) is separate from any photo detector.