GB2322455A

GB2322455A - Cylinder lens and plates for beam shaping for an optical pickup

Info

Publication number: GB2322455A
Application number: GB9707538A
Authority: GB
Inventors: Chul-Woo Lee; Pyong-Yong Seong; Tae-Kyung Kim
Original assignee: Samsung Electronics Co Ltd
Current assignee: Samsung Electronics Co Ltd
Priority date: 1997-01-28
Filing date: 1997-04-14
Publication date: 1998-08-26
Anticipated expiration: 2017-04-14
Also published as: GB2322455B; NL1008116C2; NL1008116A1; RU2176097C2; CN1189669A; MY118337A; CN1156722C; DE19802389B4; GB9707538D0; DE19802389A1; KR19980066783A; KR100288967B1; HK1009876A1; JPH10208291A

Abstract

A beam from a light source 51 is shaped by a cylinder lens 53 and plates 55,57. The beam may be shaped from an elliptical cross-section to a circular cross-section, by reducing the long-axis of the ellipse, or vice versa. The cylinder lens 53 may have a surface with a negative power 533 and a surface with a positive power 535 of greater magnitude. An optical pickup employs such a beam-shaper and further includes an objective lens 60 for focusing light from the plates 55,57. The pickup may also include a collimating lens 59. One surface 555 of the first plate 55 may act as a beam-splitter and reflect light transmitted from the second plate 57 to a photo detector 63. The photo detector 63 may detect light according to an astigmatic method.

Description

2322455 AN OPTICAL SYSTEM FOR BEAM SHAPING AND AN OPTICAL PICKUP EMPLOYING

THE SAME The present invention relates to an optical system for shaping beams and an optical pickup employing the same, and more particularly, to an optical system for shaping light beam output from a light source into a desired form and an optical pickup employing the same.

An optical pickup for an optical storage medium such as a compact disk (CD) and a digital versatile disk (DVD), uses a laser source for outputting a light beam having an elliptical cross-section. The laser source outputs light generated by an active layer of a laser diode in the form of a divergent beam. Referring to Figure 1, generation of the laser light is briefly explained.

Figure 1 shows an elliptical light beam output from a laser diode. In Figure 1, a direction of junction surface in the laser diode, that is, a direction parallel with an active layer is expressed as "ill' and a direction perpendicular to the junction surface is expressed as 11111.

Here, the direction J_ coincides with the direction of current flowing through the active layer in the laser diode. In the case of a laser diode (Model No. PS010-00 manufactured in Blue Sky Research), an active layer region has a size of igm (direction 1) X 3lim (direction 11) centered at a point B shown in Figure 1. The laser light is generated from the active layer region. Since the light output through the active layer region starts at two different points A and B, the output light has astigmatic distance AZ representing a distance between the points A and B. A divergence angle of the laser light is generally 20400 in case of 0, and 8200 in case of 01, and thus ' the output light beam has an elliptical cross-section with respect to the optical axis. Particularly, the long axis having a large beam diameter coincides with the direction -L and the short axis having a small beam diameter coincides with the direction 11 parallel with the junction 5 surface.

However, since an objective lens for a light storage medium is circular, a light beam having a circular crosssection is needed to enhance a light utilization efficiency. The conventional beam shaping methods proposed by this need are described below with reference to Figures 2A through 4.

The optical system shown in Figures 2A and 2B includes two cylindrical lenses 11 and 12. Figures 2A shows the lenses 11 and 12 viewed from the plane coincident with the direction 11 and Figure 2B shows the lenses 11 and 12 viewed from the plane parallel with the direction 1. The lenses 11 and 12 have a different focal length, respectively. The diverging light beam output from the laser source as referred in Figure 1, is collimated by a collimating lens (not shown) and then is incident to the lens 11. The lens 11 has a plano- concave shape at the direction coincident with the direction 11, to accordingly diverge the light incident thereto in parallel with the direction 11. The lens 11 transmits the light incident in parallel with the direction 1 without refraction. The light output from the lens 11 is incident to lens 12. The lens 12 outputs the light incident from the plano-concave lens 11 in parallel with the direction 11 in the form of substantially parallel light. The light incident at the direction parallel with the direction 1 is transmitted without refraction via the lens 12, to accordingly keep it substantially parallel light. Thus, at the direction parallel with the direction 11 shown in Figure 2A, the incident light having a beam diameter Wi is changed into that having a larger beam diameter Wo. As a result, the elliptical light beam output from the laser source is shaped into the light beam having a 5 substantially circular cross-section.

Figure 3 shows a conventional beam shaping prism. A light beam incident to the prism of Figure 3 is a light beam elliptically output from a laser source and then collimated by a collimating lens, as in Figure 2A and 2B. The collimated light beam is incident to a surface 23 of a prism 21. At the incident plane shown in Figure 3, the light beam of the small diameter direction having an incident angle Oi is refracted by a refractive angle 0, by the prism 21 having a refractive index n and then is output from the surface 25. The prism 21 changes the diameter wi of the light beam incident to the incident plane of Figure 3 into a larger diameter Wo. However, the prism 21 does not nearly change the beam diameter of the light incident to another incident plane perpendicular to the incident plane. Thus, the light beam output from the surface 25 becomes substantially circular.

Figure 4 shows a conventional optical system for shaping a beam using a micro-lens. The light output from an active layer 41 has an elliptical cross-section as described above referring to Figure 1. The light is incident to the micro-lens 42 distant by several micrometers from the active layer 41. The micro-lens 42 has an optical f eature in which the light incident with respect to the small diameter direction as shown in Figure 4 as dotted lines is transmitted without being substantially refracted. However, with respect to the large diameter direction shown as a solid line, the micro- lens 42 refracts the incident light via a convex surface 421 to become substantially parallel light, and diverges the light via a surface 423 to be substantially coincident with the beam diameter to the direction of the small beam diameter.

Since it is difficult to manufacture the abovedescribed cylindrical lenses having excellent wavefront aberration and to adjust an optical axis, the method using the cylindrical lenses is rarely used.

In the case of the prism, since a desired beam shaping operation can be done only when substantially parallel light is incident, a separate collimating lens is needed to collimate the diverging light beam output from the laser source, which causes a long light path distance and makes it difficult to manufacture a compact optical pickup.

In the case of the method using the micro-lens, the micro-lens should be assembled in the output window of the laser diode, which makes it difficult to assemble the micro-lens with the laser diode without being a laser diode manufacturer and raises a manufacturing cost. It is also difficult to manufacture a micro-lens having an excellent performance.

With a view to solving or reducing the above problems, it is an aim of embodiments of the present invention to provide a beam-shaping optical system for maximizing a light utilization efficiency and wavefront aberration.

It is another aim of embodiments of the present invention to provide an optical pickup employing the above beam-shaping optical system.

- 5 According to a f irst aspect of the present invention, there is provided a beam-shaping optical system comprising: a light source; a plurality of plates; and a cylindrical lens disposed between the light source and the plurality of the plates, wherein the plurality of the plates and the cylindrical lens have an optical feature in which the light output from the light source is beamshaped to have a desired shape in the cross- section of the light beam, respectively.

Preferably, said plurality of plates and said cylindrical lens shape a beam of an output light so that the beam diameter of the light output from said light source is the same as one of the long-axis diameter and is the short-axis diameter of the beam-shaped light, when light having a substantially circular beam cross-section is output from said light source.

Said plurality of plates and said cylindrical lens may shape the beam of the light output from said light source so that the beam diameter of the beam-shaped light having a substantially circular beam cross-section is the same as one of the long-axis diameter and the short-axis diameter of the light output from said light source, when light having a substantially elliptical beam cross-section is output from said light source.

Said plurality of plates may have a divergency of the light beam on a first reference plane smaller than that on a second reference plane, in which said first reference plane is in parallel with a large beam diameter of the light output from said light source and said second reference plane is in parallel with a small beam diameter of the light output from said light source.

Each of said plurality of plates is preferably a plane-parallel plate having an optical feature refracting the incident light.

Preferably, said plurality of plates comprise two plates which have a plane symmetrical relationship with respect to the plane perpendicular to the optical axis of said light source and transmit the incident light.

Preferably, said cylindrical lens has a cylindrical surface whose longaxis is an axis parallel with said second reference plane and facing said plurality of plates, and said cylindrical surface has a positive optical power.

Preferably, said cylindrical lens has a cylindrical surface whose longaxis is an axis parallel with said second reference plane and facing said light source, and said cylindrical surface has a negative optical power.

Said negative power preferably has a relatively minute magnitude compared with said positive optical power.

There is also provided according to a second aspect of the present invention, an optical pickup for an optical storage medium comprising: a laser source for outputting light having a substantially elliptical crosssection in the form of a divergent beam; a plurality of plates; a cylindrical lens disposed between the laser source and the plurality of the plates; and an objective lens for focusing the light incident from the plurality of plates in the optical storage medium, wherein the plurality of the plates and the cylindrical lens have an optical feature in which the light output from the light source is beam-shaped to have a desired shape in the cross-section of the light beam, respectively.

Preferably, said plurality of plates and said cylindrical lens shape the bean of the light having a substantially elliptical cross-section to have a beam diameter which is substantially same as the short-axis diameter of the substantially elliptical light beam.

Preferably, said plurality of plates have the divergency of the light beam on a first reference plane smaller than that on a second reference plane, in which said first reference plane is in parallel with a large beam diameter of the light output from said light source is and said second reference plane is in parallel with a small beam diameter of the light output from said light source.

preferably, said plurality of plates have a plane- parallel plate having an optical feature refracting the incident light, respectively.

Said pickup may further comprise a photo detector, wherein a first plate located closely to said photo detector reflects the light incident from a second plate located away from said photo detector toward said photo detector.

Said photo detector preferably has a structure adapted f or detecting light according to an astigmatic method.

- 8 The plate surface of said f irst plate located closely to said photo detector preferably reflects the light incident from said second plate.

Preferably, the optical axis of said laser source is not parallel with that of said objective lens and, the optical pickup further comprising a reflection mirror reflecting the light incident from said second plate toward said objective lens.

Preferably, the optical axis of said laser source is disposed perpendicularly to that of said objective lens.

Said cylindrical lens preferably has a cylindrical is surface whose long-axis is an axis parallel with said second reference plane and facing said plurality of plates, and said cylindrical surface has a positive optical power.

Preferably, said cylindrical lens has a cylindrical surface whose long-axis is an axis parallel with said second reference plane and facing said light source, and said cylindrical surface has a negative optical power.

said negative optical power preferably has a relatively minute magnitude compared with said positive optical power.

Preferably, a collimating lens is provided for collimating the light incident via said plurality of plates and transmitting the light to said objective lens.

According to a third aspect there is provided a beamshaping optical system for beam-shaping a light beam which is output from a light source and has an elliptical beam cross-section, into a circular cross-sectional light beam, the optical system comprising: a cylindrical lens for receiving a light beam having an elliptical cross-section and outputting the light beam as in the form that beam diameter in a long axis direction is reduced and beam diameter in a short axis direction is in due form; and a plurality of plates for correcting astigmatism of the light beam which is output from said cylindrical lens and have a substantially circular beam cross-section.

Said cylindrical lens preferably has a cylindrical surface whose long axis is an axis parallel with a short axis direction of said elliptical light beam and facing said plurality of plates, wherein said cylindrical surface has a positive optical power.

Preferably, said cylindrical lens has a cylindrical surface whose long axis is an axis parallel with a short axis direction of said elliptical light beam facing said light source, wherein said cylindrical surface has a negative optical power.

Preferably, said negative optical power has a relatively minute magnitude compared with said positive optical power.

Said plurality of plates preferably have the divergency of the light beam on a plane which parallels a large beam diameter of the light output from said light source smaller than that of the light beam on a plane which parallels a small beam diameter of the light output from said light source.

Preferably, each of said plurality of plates is a plane-parallel plate of an optical feature refracting the incident light.

is Said plurality of plates preferably comprise two plates which have a plane symmetrical relationship with respect to the plane perpendicular to the optical axis of said light source and transmit the incident light.

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:

Figure 1 is a view for explaining a laser source for outputting an elliptical light beam; Figures 2A and 2B are views for explaining a conventional optical system for shaping a beam using a cylindrical lens; Figure 3 shows a conventional beam shaping prism; Figure 4 shows a conventional optical system for shaping a beam using a micro-lens; Figures 5A and 5B show a beam-shaping optical system according to an embodiment of the present inventiony in which Figure 5A shows the optical system viewed according to the direction of the large beam diameter of the light output from a laser source, and Figure 5B shows the optical system viewed according to the direction of the small beam diameter of the light output from the laser source; and Figure 6 is a view showing an optical system according to an embodiment of the present invention.

An optical system and an optical pickup employing the same according to the present invention will be described in detail with reference to the accompanying drawings.

Figures 5A and 5B are different views showing an optical system according to an embodiment of the present invention Figure 5A shows the optical system viewed according to the direction of the large beam diameter of the light output from a laser source, and Figure 5B shows the optical system viewed according to the direction of the small beam diameter of the light output from the laser source. The direction of the large beam diameter of the light output from the laser source coincides with the direction -L which has been described with reference to Figure 1, while the direction of the small beam diameter of the light output from the laser source coincides with the direction 11 thereof. in optical systems in accordance with the principles of the present invention, the elliptical light beam output from the laser - source is f inally beam-shaped into a substantially circular beam. Therefore, the directions -L and 11 are used for representation of the directions of the large beam diameter and the small beam diameter of the light output from the laser source.

The optical system includes a cylindrical lens 53, two plates 55 and 57 and a collimating lens 59. A laser diode 51 outputs laser light having a substantial elliptical cross-section in the form of a divergent beam.

The laser light is incident to the cylindrical lens 53.

The cylindrical lens 53 includes surfaces 533 and 535, whose axis is parallel with the direction 11. The cylindrical lens 53 is manufactured to have the surface 533 having a negative optical power with respect to the direction -Land the surface 535 having a positive optical power with respect thereto. Then, the optical power of the surface 533 has a relatively minute magnitude compared with the optical power of the surface 535. Since the cylindrical lens 53 does not limit the present invention, it is possible to modify the cylindrical lens 53 to have an axis parallel with the direction 11 and surfaces respectively having a bi-convex shape with respect to the direction 1.

When a light is incident from the laser diode 51, the cylindrical lens 53 having the above configuration refracts the incident light to reduce the light beam divergency as shown in Figure 5A with respect to the direction L. However, the cylindrical lens 53 outputs the incident light as it is as shown in Figure 5B with respect to the direction 11. Therefore, the divergency with respect to the direction -L is slightly reduced in the light output from the cylindrical lens 53 and the divergency with respect to the direction 11 is maintained in substantially the same f orm as that output f rom the laser diode 51.

A first plate 55 which receives the light from the cylindrical lens 53 is a plane-parallel plate whose surfaces 553 and 555 are parallel with each other. The first plate 55 is disposed to have an inclination angle 0 on the basis of the line parallel with an optical axis of the laser diode 51. In a case that the refraction of the light by the cylindrical lens 53 is fixed, the beam diameter with respect to the direction L and the beam diameter with respect to the direction 11 can be made to be the same by controlling the inclination angle 0 of the first plate 55. However, as is well known, when the f irst plate 55 is slantingly inserted onto a path of the divergent or convergent light, coma or astigmatism occurs.

The coma can be corrected using plates which are disposed in the crossing form at different positions along the light path. Thus, a second plate 57 including a surface 573 facing the surface 555 of the first plate 55 is used. The second plate 57 is also a plane-parallel plate in which the surface 573 is parallel with the surface 575. The plates 55 and 57 are disposed to have a surface symmetrical relationship with respect to the surface perpendicular to the optical axis. The surface symmetrical relationship is shown in Figure 5A. Thus, when the f irst plate 55 is tilted by the angle 0 on the basis of the line parallel with the optical axis of the laser diode 51, the second plate 57 is tilted by an angle of -0. The plates 55 and 57 are manufactured using an optical device having the same refractive index, such as for example glass. Meanwhile, the astigmatism generated by insertion of the plates 55 and 57 is compensated by the cylindrical lens 53 used in the optical system. The cylindrical lens 53 also compensates an astigmatic difference described with reference to Figure 1, that is, an astigmatic difference occurring due to the different starting points at the active layer region of the light output from the laser diode.

The first plate 55 refracts the light incident from the cylindrical lens 53 via the plate surface 553 toward the direction -L of Figure 5A, and transmits the light without being substantially refracted toward the direction 11 of Figure 5B. The second plate 57 also performs the same function as that of the plate 55. That is, the second plate 57 refracts the light incident from the plate surface 555 of the first plate 55 via the plate surface 573 toward the direction 1 of Figure 5A, and transmits the light without being substantially refracted toward the direction 11 of Figure 5B. Therefore, the light transmitting the f irst and second plates 55 and 57 becomes a circular light beam in which the beam cross-section of the direction _L is the substantially same as that of the direction 11, as can be seen from Figures SA and 5B. Also, since the beam shaping is accomplished to be same as the small beam diameter not a large beam diameter, the size of an optical spot formed on a signal recording surface of an optical recording medium by the beam-shaped light can be reduced. A collimating lens 59 facing the plate surface 575 of the second plate 57 collimates the divergent light output from the second plate 57. Therefore, if an objective lens is installed in the rear end of the collimating lens 59, a complete optical system which can be used in an optical pickup is manufactured.

The above-described embodiments referring to Figures 5A and 5B have been described with respect to a laser source outputting a light beam having a substantially elliptical cross-section. However, there are various light beams requiring beam shaping including white light or natural light. Accordingly, it is apparent to a person skilled in the art that various modifications for shaping such light beams are possible within the scope of the present invention.

As a modification, the above-described laser diode 51 is replaced by a light source outputting a light beam having a circular cross-section. In this case, the light beam having a circular cross-section is shaped into a light beam having an elliptical cross-section which is the same as one of the long-axis beam diameter and the shortaxis beam diameter of the elliptical beam cross-section.

As an alternative modification, a light source outputting a light beam having an elliptical cross-section is located in the position of the collimating lens 59. In this case, the light beam having an elliptical cross section is shaped into a light beam having a circular cross-section which is the same as one of the long-axis beam diameter and the short-axis beam diameter of the elliptical cross-section.

Since these modifications are apparent to one having an ordinary skill in the art, further explanation of these modifications is omitted.

Figure 6 is a view showing an optical system employing the above-described embodiments referring to Figures 5A and 5B. In f igure 6, a laser diode 51, a cylindrical lens 53 and a second plate 57 are disposed in the same manner as those of Figure 5A and perf orm. the same functions as those thereof. However, the plate surface 555 of the first plater 55 transmits the light output from the plate surface 535 and reflects the light output from the second plate surface 57, differently from the description referring to Figure 5A. That is, the plate surface 555 has a well-known optical feature of a beam splitter. Therefore, the light incident from the second plate surface 57 is reflected by the plate surface 555.

The optical pickup shown in Figure 6 further includes a reflection mirror 58, an objective lens 60 and a photo detector 63 in addition to the above-described optical devices. The optical pickup of Figure 6 is disposed so that the optical axis of the laser diode 51 is not parallel with that of the objective lens 60 for focusing the incident light on the signal recording surface of an optical recording medium. Also, the reflection mirror 58 is disposed so that the light incident from the second plate 57 is reflected toward the collimating lens 59. Thus, the optical axis of the laser diode 51 is perpendicular to that of the objective lens 60, by which the optical pickup can be compactly manufactured.

The light reflected from the signal recording surface of the optical recording medium 61 transmits the objective lens 60 and the collimating lens 59 and then is incident to the reflection mirror 58. The light incident to the reflection mirror 58 has a convergent form converged by the collimating lens 59. Accordingly, the light reflected from the reflection mirror 58 and then refracted from the second plate 57 has a convergent form. Thus, the light reflected from the plate surface 555 travels in the convergent form as well as has astigmatism. This is because the convergent light reflected from the plate surface 555 is light transmitting only one plate 57 differently from the light which is focused on-the signal recording surface of the optical recording medium 61. To use the astigmatism possessed by the light reflected from the plate surface 555 in a focusing servo, the embodiment uses a photo detector 63 having a structure adapted for employing a well-known astigmatic method.

As described above, optical systems according to 30 embodiments of the present invention uses a cylindrical lens and the plates for shaping a beam, to thereby beamshape the elliptical or circular light output from the light source at low cost. In addition, the beam shaping with respect to the direction of the large beam diameter is performed to be coincided with that of the small beam diameter with respect to the elliptical light output from the light source, thereby maximizing a utilization efficiency and a wavefront aberration of the laser light.

Also, since the light output from the beam-shaping optical system is diverged and the optical system uses a plurality of plates, the light reflected from the optical recording medium is converged and has astigmatism as it is. As a result, a focusing servo can be done using the astigmatic method and does not need to use a separate light-receiving lens to converge the light to the photo detector.

Further, since an optical axis of the laser source is perpendicular to the optical axis of the objective lens, a compact optical pickup can be accomplished.

While only certain embodiments of the invention have been specifically described herein, it will be apparent that numerous modifications may be made thereto without departing from the scope of the invention.

The readerOs attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

Each feature disclosed in this specification (including any accompanying claims, abstract and drawings), may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

The invention is not restricted to the details of the foregoing embodiment (s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

Claims

A beam-shaping optical system comprising:

a light source; a plurality of plates; and a cylindrical lens disposed between said light source and said plurality of plates, wherein said plurality of plates and said cylindrical lens have an optical feature in which the light output from the light source is beam- shaped to have a desired shape in the cross-section of the light beam, respectively.
2. The beam-shaping optical system according to claim 1, wherein said plurality of plates and said cylindrical lens shape a beam of an output light so that the beam diameter of the light output from said light source is the same as one of the long-axis diameter and the short-axis diameter of the beam-shaped light, when light having a substantially circular beam cross-section is output from said light source.
3. The beam-shaping optical system according to claim 1, wherein said plurality of plates and said cylindrical lens shape the beam of the light output from said light source so that the beam diameter of the beam-shaped light having a substantially circular beam cross-section is the same as one of the long-axis diameter and the short-axis diameter of the light output from said light source, when light having a substantially elliptical beam cross-section is output from said light source.
4. The beam-shaping optical system according to claim 3, wherein said plurality of plates have a divergency of the light beam on a first reference plane smaller than that on a second reference plane, in which said first reference plane is in parallel with a large beam diameter of the light output from said light source and said second reference plane is in parallel with a small beam diameter of the light output from said light source.
5. The beam-shaping optical system according to claim 4, wherein each of said plurality of plates is a planeparallel plate having an optical feature refracting the incident light.
6. The beam-shaping optical system according to claim 5, wherein said plurality of plates comprise two plates which have a plane symmetrical relationship with respect to the plane perpendicular to the optical axis of said light source and transmit the incident light.
7. The beam-shaping optical system according to claim 4, wherein said cylindrical lens has a cylindrical surface whose long-axis is an axis parallel with said second reference plane and facing said plurality of plates, and said cylindrical surface has a positive optical power.
8. The beam-shaping optical system according to claim 4, wherein said cylindrical lens has a cylindrical surface whose long-axis is an axis parallel with said second reference plane and facing said light source, and said cylindrical surface has a negative optical power.
9. The beam-shaping optical system according to claim 8, wherein said negative power has a relatively minute magnitude compared with said positive optical power.
10. An optical pickup for an optical storage medium comprising:

a laser light source f or outputting light having a substantially elliptical cross-section in the form of a divergent beam; a plurality of plates; a cylindrical lens disposed between the laser source and the plurality of plates; and an objective lens for focusing the light incident from said plurality of plates in the optical storage medium, wherein said plurality of plates and said cylindrical lens have an optical feature in which the light output from said light source is beam- shaped to have a substantially circular shape in the cross-section of the light beam, respectively.
11. The optical pickup according to claim 10, wherein said plurality of plates and said cylindrical lens shape the beam of the light having a substantially elliptical cross-section to have a beam diameter which is substantially same as the short-axis diameter of the substantially elliptical light beam.
12. The optical pickup according to claim 11, wherein said plurality of plates have the divergency of the light beam on a first reference plane smaller than that on a second reference plane, in which said first reference plane is in parallel with a large beam diameter of the light output from said light source and said second reference plane is in parallel with a small beam diameter of the light output from said light source.
13. The optical pick-up according to claim 12, wherein said plurality of plates have a plane-parallel plate having an optical feature refracting the incident light, respectively.
14. The optical pickup according to claim 13, wherein said plurality of plates comprise two plates which have a plane symmetrical relationship with respect to the plane perpendicular to the optical axis of said light source and transmit the incident light.
15. The optical pickup according to claim 14, further comprising a photo detector, wherein a first plate located closely to said photo detector reflects the light incident from a second plate located away from said photo detector toward said photo detector.
16. The optical pickup according to claim 15, wherein said photo detector has a structure adapted for detecting light according to an astigmatic method.
17. The optical pickup according to claim 16, wherein the plate surface of said first plate located closely to said photo detector reflects the light incident from said second plate.
18. The optical pickup according to claim 17, wherein the optical axis of said laser source is not parallel with that of said objective lens and, the optical pickup further comprising a reflection mirror reflecting the light incident from said second plate toward said objective lens.
19. The optical pickup according to claim 18, wherein the optical axis of said laser source is disposed perpendicularly to that of said objective lens.
20. The optical pickup according to claim 12, wherein said cylindrical lens has a cylindrical surface whose long-axis is an axis parallel with said second reference plane and facing said plurality of plates, and said cylindrical surface has a positive optical power.
21. The optical pickup according to claim 20, wherein said cylindrical lens has a cylindrical surface whose long-axis is an axis parallel with said second reference plane and facing said light source, and said cylindrical surface has a negative optical power.
22. The optical pickup according to claim 21, wherein said negative optical power has a relatively minute magnitude compared with said positive optical power.
23. The optical pickup according to claim 10, further comprising a collimating lens for collimating the light incident via said plurality of plates and transmitting the light to said objective lens.
24. A beam-shaping optical system for beam-shaping a light beam which is output from a light source and has an elliptical beam cross-section, into a circular crosssectional light beam, the optical system comprising:

a cylindrical lens for receiving a light beam having an elliptical crosssection and outputting the light beam as in the form that beam diameter in a long axis direction is reduced and beam diameter in a short axis direction is in due form; and a plurality of plates for correcting astigmatism of the light beam which is output from said cylindrical lens and have a substantially circular beam cross-section.
25. The beam-shaping optical system according to claim 24, wherein said cylindrical lens has a cylindrical surface whose long axis is an axis parallel with a short axis direction of said elliptical light beam and facing said plurality of plates, wherein said cylindrical surface has a positive optical power.
26. The beam-shaping optical system according to claim 25, wherein said cylindrical lens has a cylindrical surface whose long axis is an axis parallel with a short axis direction of said elliptical light beam facing said light source, wherein said cylindrical surface has a negative optical power.
27. The beam-shaping optical system according to claim 26, wherein said negative optical power has a relatively minute magnitude compared with said positive optical power.
28. The beam-shaping optical system according to claim 24, wherein said plurality of plates have the divergency of the light beam on a plane which parallels a large beam diameter of the light output from said light source smaller than that of the light beam on a plane which parallels a small beam diameter of the light output from said light source.
29. The beam-shaping optical system according to claim 28, wherein each of said plurality of plates is a planeparallel plate of an optical feature refracting the incident light.
30. The beam-shaping optical system according to claim 10 29, wherein said plurality of plates comprise two plates which have a plane symmetrical relationship with respect to the plane perpendicular to the optical axis of said light source and transmit the incident light.

is
31. A beam shaping optical system substantially as herein described with reference to Figures SA to 6 of the accompanying drawings.

30. The beam-shaping optical system according to claim 29, wherein said plurality of plates comprise two plates which have a plane symmetrical relationship with respect to the plane perpendicular to the optical axis of said light source and transmit the incident light.

31. A beam shaping optical system substantially as herein described with reference to Figures SA to 6 of the accompanying drawings.

-7.6 - CLAIMS A beam-shaping optical system comprising:

a light source; a plurality of plates; and a cylindrical lens disposed between said light source and said plurality of plates, wherein said system has an optical feature in which the light output from the light source is beam-shaped to have a desired shape in the cross- section of the light beam.

2. The beam-shaping optical system according to claim 1, wherein said plurality of plates and said cylindrical lens shape a beam of an output light so that one of a long-axis diameter and a short-axis diameter of the beam-shaped light having a substantially elliptical cross-section is the same as the beam diameter of the light output from said light source, when light having a substantially circular beam cross-section is output from said light source.

3. The beam-shaping optical system according to claim 1, wherein said plurality of plates and said cylindrical lens shape the beam of the light output from said light source so that the beam diameter of the beam-shaped light having a substantially circular beam cross-section is the same as one of the long-axis diameter and the short-axis diameter of the light output from said light source, when light having a substantially elliptical beam cross-section is output from said light source.

4. The beam-shaping optical system according to claim 3, wherein said plurality of plates have a divergency of the light beam on a first reference plane smaller than that on a second reference plane, in which said first reference plane is in parallel with a large beam diameter of the light output from said light source and said second reference plane is in parallel with a small beam diameter of the light output from said light source.

5. The beam-shaping optical system according to any preceding claim, wherein each of said plurality of plates is a plane-parallel plate having an optical feature refracting the incident light.

is 6. The beam-shaping optical system according to any preceding claim, wherein said plurality of plates comprise two plates which have a plane symmetrical relationship with respect to the plane perpendicular to the optical axis of said light source and transmit the incident light.

7. The beam-shaping optical system according to claim 4, wherein said cylindrical lens has a cylindrical surface whose long-axis is an axis parallel with said second reference plane and facing said plurality of plates, and said cylindrical surface has a positive optical power.

8. The beam-shaping optical system according to claim 4 or 7, wherein said cylindrical lens has a cylindrical surface whose long-axis is an axis parallel with said second reference plane and facing said light source, and said cylindrical surface has a negative optical power.

9. The beam-shaping optical system according to claim 8 when dependent on claim 7, wherein said negative power has a relatively minute magnitude compared with said positive optical power.

- N - 10. An optical pickup for an optical storage medium comprising:

a laser light source for outputting light having a substantially elliptical cross-section in the form of a divergent beam; a plurality of plates; a cylindrical lens disposed between the laser source and the plurality of plates; and an objective lens for focusing the light incident from said plurality of plates in the optical storage is medium, wherein said system has an optical feature in which the light output from said light source is beam-shaped to have a substantially circular shape in the cross-section of the light beam.

11. The optical pickup according to claim 10, wherein said plurality of plates and said cylindrical lens shape the beam of the light having a substantially elliptical cross-section to have a beam diameter which is substantially same -as the short-axis diameter of the substantially elliptical light beam.

12. The optical pickup according to claim 10 or 11, wherein said plurality of plates have the divergency of the light beam on a first reference plane smaller than that on a second reference plane, in which said first reference plane is in parallel with a large beam diameter of the light output from said light source and said second reference plane is in parallel with a small beam diameter of the light output from said light source.

13. The optical pick-up according to any preceding claim wherein said plurality of plates have a plane-parallel plate having an optical feature refracting the incident light.

14. The optical pickup according to any preceding claim, wherein said plurality of plates comprise two plates which have a plane symmetrical relationship with respect to the plane perpendicular to the optical axis of said light 10 source and transmit the incident light.

15. The optical pickup according to claim 14, further comprising a photo detector, is wherein a first plate located closely to said photo detector reflects the light incident from a second plate located away from said photo detector toward said photo detector.

16. The optical pickup according to claim 15, wherein said photo detector has a structure adapted for detecting light according to an astigmatic method.

17. The optical pickup according to claim 15 or 16, wherein the light is reflected by the plate surface of said first plate.

18. The optical pickup according to claim 14 or any succeeding claim, wherein the optical axis of said laser source is not parallel with that of said objective lens and, the optical pickup further comprising a reflection mirror reflecting the light incident from said second plate toward said objective lens.

19. The optical pickup according to claim 18, wherein the optical axis of said laser source is disposed perpendicularly to that of said objective lens.

20. The optical pickup according to claim 12, wherein said cylindrical lens has a cylindrical surface whose long-axis is an axis parallel with said second reference plane and facing said plurality of plates, and said cylindrical surface has a positive optical power.

21. The optical pickup according to claim 20, wherein said cylindrical lens has a cylindrical surface whose long-axis is an axis parallel with said second reference plane and facing said light source, and said cylindrical surface has a negative optical power.

22. The optical pickup according to -claim 21, wherein said negative optical power has a relatively minute magnitude compared with said positive optical power.

23. The optical pickup according to claim 10, further comprising a collimating lens for collimating the light incident via said plurality of plates and transmitting the light to said objective lens.

24. A beam-shaping optical system for beam-shaping a light beam which is output from a light source and has an elliptical beam cross-section, into a circular crosssectional light beam, the optical system comprising:

a cylindrical lens for receiving a light beam having an elliptical crosssection and outputting the light beam as in the form that beam diameter in a long axis direction is reduced and beam diameter in a short axis direction is substantially unchanged; and a plurality of plates for correcting astigmatism of the light beam which is output from said cylindrical lens and have a substantially circular beam cross-section.

25. The beam-shaping optical system according to claim 24, wherein said cylindrical lens has a cylindrical surface whose long axis is an axis parallel with a short axis direction of said elliptical light beam and facing said plurality of plates, wherein said cylindrical surface has a positive optical power.

26. The beam-shaping optical system according to claim 25, wherein said cylindrical lens has a cylindrical surface whose long axis is an axis parallel with a short axis direction of said elliptical light beam facing said light source, wherein said cylindrical surface has a negative optical power.

27. The beam-shaping optical system according to claim 26, wherein said negative optical power has a relatively minute magnitude compared with said positive optical power.

28. The beam-shaping optical system according to claim 24, wherein said plurality of plates have the divergency of the light beam on a plane which parallels a large beam diameter of the light output from said light source smaller than that of the light beam on a plane which parallels a small beam diameter of the light output from said 1-ight source.

29. The beam-shaping optical system according to claim 28, wherein each of said plurality of plates is a plane parallel plate of an optical feature refracting the incident light.