Coherent light generators – Particular temperature control – Heat sink
Reexamination Certificate
2000-03-14
2002-09-24
Davie, James (Department: 2881)
Coherent light generators
Particular temperature control
Heat sink
Reexamination Certificate
active
06456635
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a semiconductor laser device suitable for constructing an optical pickup reading information from and writing information to different types of optical discs.
BACKGROUND ART
Optical discs are used not only for video/audio recording but also as computer external storage media because they have a large recording capacity and a random access characteristic that the access time is not varied irrespective of an information-recording position. However, different types of optical discs are used according to purpose. Thus, characteristics demanded of the optical pickups differ depending on the type of optical discs for which the optical pickups are used.
In optical pickups reading compact discs (CD) that are used to record audio information and CD-ROM media formed by formatting the CD as computer external storage media, an optical spot having a large diameter can be used. Thus, an infrared semiconductor laser device emitting a light beam having a wavelength in the vicinity of 780 nm is used as a light source, and a condenser lens having a numerical aperture (NA) of about 0.45 is used.
In optical pickups which read information from a digital versatile disc (DVD) having a large memory capacity and used to record video information or the like, it is necessary to make the optical spot smaller. Thus, as the light source of the DVD optical pickup, a red semiconductor laser device emitting a light beam having a wavelength of 630-680 nm is used, and a condenser lens having a NA of about 0.6 is used. The optical pickup for DVD can also read information from the CD and the CD-ROM by adjusting the NA of the condenser lens by, for example, using a diaphragm.
Optical discs called “CD-R” which have an information recording format common to the CD and to which information can be written only once are spreading widely. This is because the optical discs CD-R are not only readable by an ordinary optical pickup for the CD, but also inexpensive. As the material of the CD-R, in consideration of compatibility with the CD, a recording film made of an organic material is used which is designed such that read and write of information can be preferably accomplished for light having a wavelength in the neighborhood of 780 nm. Thus, the CD-R disc is not readable unless an optical pickup using an infrared laser as its light source is employed.
In order for one optical pickup to read information of different types of optical discs such as CD, CD-ROM, CD-R, and DVD, a red semiconductor laser and an infrared semiconductor laser are required as its light source.
Changing the optical pickup for each type of the optical discs causes inconvenience and the device size will become large. Thus, there is a growing demand for the development of an optical pickup that is capable of reading information from and writing information to different types of optical discs, has a size not different from that of the conventional optical pickup for CD, and can be produced with a technique equivalent to that employed to manufacture the conventional optical pickup for CD.
To allow the optical pickup to read information from and write information to different types of optical discs, two kinds of laser light sources, namely, the red semiconductor laser and the infrared semiconductor laser are required, as described above. The sizes of the laser light sources define the size of the optical pickup. That is, a conventional semiconductor laser device has an internal construction shown, for example, in FIG.
6
.
In
FIG. 6
, a semiconductor laser element
3
is fixed to a heat sink block
2
formed integrally with a mount
1
made of a metal disc, using an electrically conductive adhesive material or a soldering material (not shown). The semiconductor laser element is typically about 200 &mgr;m in width, about 250 &mgr;m in length, and about 100 &mgr;m in thickness. Because flat planes are formed on the heat sink block
2
by molding, the length of one side of the heat sink block
2
is set to 2 mm or more.
A monitoring photodiode (PD)
6
is provided in a recess
1
a
of the mount
1
for monitoring the intensity of light emitted from a rear end face of the semiconductor laser element
3
. The monitoring PD
6
is fixed by an electrically conductive adhesive material or a metallic soldering material (not shown) applied in advance to a bottom surface of the recess la and to a lower surface of the monitoring PD
6
.
To accommodate the semiconductor laser element
3
, the heat sink block
2
, and the monitoring PD
6
in one package, after they are covered with a cap (not shown) having a laser beam-emitting window, the cap is welded to the mount
1
. The welding of the cap is performed in an atmosphere of an inert gas such as nitrogen, argon or the like or dry air so that moisture does not remain in the package. The cap is welded to the mount
1
with no gap therebetween to keep the inside of the package airtight and prevent moisture from penetrating into the package from outside so that semiconductor devices such as the semiconductor laser element
3
and the monitoring PD
6
are prevented from deteriorating over a long period of time.
To electrically connect the semiconductor devices with the outside, there are provided a plurality of lead pins penetrating through the mount
1
in a manner insulated from the mount
1
. More specifically, an upper electrode
3
a
of the semiconductor laser element
3
and a flat portion
51
a
of a lead pin
51
having a diameter of about 0.2 mm are electrically connected to each other with a gold wire
501
. A surface electrode
6
a
of the monitoring PD
6
and a front end of a lead pin
52
are connected to each other with a gold wire
502
. For insulation, the mount
1
and the lead pins
51
,
52
are spaced by a gap of 0.1 mm or more and fixed to each other with an insulating material.
The inner diameter of the cap is such that the cap does not contact the lead pins. To keep the inside of the cap airtight, a part of the cap that contacts the mount
1
has a flat surface having a width of about 0.5 mm. Therefore, it is necessary that the mount
1
should have a diameter of more than 3.8 mm.
An optical axis C of the semiconductor laser element
3
passes through an emission point
301
and is perpendicular to a front end surface of the semiconductor laser element
3
. If the distance between the optical axis of the semiconductor laser that is used as a light source and the axis of a condenser lens is sufficiently short, preferably less than 80 &mgr;m, it is possible to read information from an optical disc. That is, as the distance between the optical axis of the semiconductor laser element and the axis of the condenser lens becomes longer, spherical aberration increases quadrically. As a result, the spot diameter of the condensed light becomes large and thus information cannot be read. For example, in an optical disc system, a tolerable spherical aberration limit is Marechal limit (0.07&lgr;, where &lgr; is a wavelength of a laser beam). In the case of the condenser lens used in an ordinary optical pickup, if the distance between the axis of the condenser lens and the optical axis of the semiconductor laser element is about 80 &mgr;m, then the spherical aberration exceeds Marechal limit.
If the infrared semiconductor laser element and the red semiconductor laser element are arranged simply side by side to be used as the light sources, the distance L between the optical axis of an infrared laser beam and that of a red laser beam will be more than 3.8 mm because the diameter of the mount
1
is more than 3.8 mm or more, as described above. In this case, the distance between the center axis of the condenser lens and the optical axis of each laser beam exceeds 80 &mgr;m without failure. Accordingly, without shifting the condenser lens, it is impossible to read information from and write information to different types of optical discs.
To solve this problem, an optical pickup as shown in
FIG. 7
is described in Nikkei Electronics No. 687, page 138, published on
Kohashi Ikuo
Shiomoto Takehiro
Alexander John B.
Conlin David G.
Davie James
Edwards & Angell LLP
Sharp Kabushiki Kaishiki
LandOfFree
Semiconductor laser device does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Semiconductor laser device, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Semiconductor laser device will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2888331