U.S. patent application number 10/600974 was filed with the patent office on 2004-01-08 for semiconductor laser device, optical pickup and fabrication method of semiconductor laser device.
Invention is credited to Honda, Masayuki, Shiomoto, Takehiro.
Application Number | 20040004983 10/600974 |
Document ID | / |
Family ID | 26601674 |
Filed Date | 2004-01-08 |
United States Patent
Application |
20040004983 |
Kind Code |
A1 |
Shiomoto, Takehiro ; et
al. |
January 8, 2004 |
Semiconductor laser device, optical pickup and fabrication method
of semiconductor laser device
Abstract
A semiconductor laser device and optical pickup in which the
reflectance of the side beam at a header portion will not adversely
affect the characteristics of the optical pickup, and superior in
productivity, and a method of fabricating such a semiconductor
laser device are obtained. A reflector is attached on a side beam
incident region of a leading end plane of a header mounted with a
laser chip that emits a laser beam. Said side beam is one of the
two side beams generated by the reflected .+-. first order beams
and fed back through the optical system returning towards the
header portion to strike the side beam incident region. The
reflector reflects side beam outside the optical system.
Inventors: |
Shiomoto, Takehiro;
(Kashihara-shi, JP) ; Honda, Masayuki;
(Sakurai-shi, JP) |
Correspondence
Address: |
EDWARDS & ANGELL, LLP
PO BOX 9169
Boston
MA
02209
US
|
Family ID: |
26601674 |
Appl. No.: |
10/600974 |
Filed: |
June 20, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10600974 |
Jun 20, 2003 |
|
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|
09971207 |
Oct 4, 2001 |
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Current U.S.
Class: |
372/43.01 ;
372/36 |
Current CPC
Class: |
H01S 5/0236 20210101;
H01L 2224/48247 20130101; H01S 5/023 20210101; H01S 5/005 20130101;
H01S 5/02326 20210101; H01S 5/02212 20130101; H01S 5/0233 20210101;
H01L 2224/48091 20130101; H01S 5/0235 20210101; H01L 2224/48091
20130101; H01L 2924/00014 20130101 |
Class at
Publication: |
372/43 ;
372/36 |
International
Class: |
H01S 003/04; H01S
005/00 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 6, 2000 |
JP |
2000-307462(P) |
Apr 11, 2001 |
JP |
2001-112770(P) |
Claims
What is claimed is:
1. A semiconductor laser device employed in an optical pickup of a
3-beam method that divides one laser beam into three beams by an
optical system, said three beams being a 0th-order beam and .+-.
first order beams, and directs the three beams towards an optical
recording medium to detect information recorded on said recording
medium and detecting tracking error information during said
detection by the 0th-order beam and .+-. first order beams
reflected from said recording medium, wherein a reflector is
attached on a side beam incident region of a leading end plane of a
header portion mounted with a laser chip emitting said laser beam,
said side beam being one of two side beams generated by said
reflected .+-. first order beam and fed back through said optical
system returning towards said header portion to strike said side
beam incident region, said reflector reflecting said side beam
outside said optical system.
2. The semiconductor laser device according to claim 1, wherein
said reflector is attached at the leading end plane of the header
portion so that a distance between a reflecting plane of said
reflector and a light emitting point at an outgoing end plane of
said laser chip is at least 50 .mu.m and not more than 150
.mu.m.
3. The semiconductor laser device according to claim 1, wherein
said reflecting plane of said reflector is tilted having an angle
of at least 10 degrees with respect to a plane perpendicular to a
main beam generated by said 0th-order beam and fed back through
said optical system.
4. The semiconductor laser device according to claim 1, wherein
said reflector has a cross section of a saw-toothed configuration,
and includes an inclination plane of a plurality of steps.
5. The semiconductor laser device according to claim 1, wherein
said reflector is formed of any one of a synthetic resin and
metal.
6. The semiconductor laser device according to claim 5, wherein
said synthetic resin includes a thermosetting resin.
7. The semiconductor laser device according to claim 5, wherein
said metal includes a metal of a hardness lower than the hardness
of the metal forming the header portion.
8. A method of fabricating the semiconductor laser device recited
in claim 1, comprising the steps of attaching at said side beam
incident region at a leading end plane of said header portion a
base material of a reflector formed of a metal that is softer than
the metal forming said header portion or a synthetic resin prior to
curing, and then shaping said base material into a reflector of a
predetermined configuration.
9. A semiconductor laser device having a stem with a semiconductor
laser chip mounted, wherein said stem includes a mount plane where
said semiconductor laser chip is mounted, and a cross plane
crossing said mount plane and facing a laser irradiated body on
which a laser beam emitted from said semiconductor laser chip
strikes, wherein said cross plane is covered with a
reflectance-reducing material reducing the reflectance to said
laser beam lower than the reflectance of said cross plane so that
an amount of light of said laser beam reflected at said cross plane
to be directed towards said laser irradiated body is reduced.
10. The semiconductor laser device according to claim 9, wherein
said reflectance-reducing material scatters and/or absorbs said
laser beam directed towards said cross plane.
11. The semiconductor laser device according to claim 9, wherein
said reflectance-reducing material is applied continuously to said
mount plane, and a portion of said reflectance-reducing material
applied on said mount plane is used as a bonding material to
die-bond said semiconductor laser chip to said stem.
12. The semiconductor laser device according to claim 9, wherein a
crossing portion of said mount plane and said cross plane is
subjected to an R configuration, and said material is applied at a
region adjacent to the region of the R configuration.
13. The semiconductor laser device according to claim 9, wherein
said reflectance-reducing material includes a conductive die bond
paste.
14. The semiconductor laser device according to claim 13, wherein
said conductive die bond paste includes an epoxy resin and
silver.
15. The semiconductor laser device according to claim 9, wherein
said reflectance-reducing material includes at least one type of an
epoxy resin and an UV resin, and at least one type of silica and
carbon powder.
16. An optical pickup comprising: a semiconductor laser device
mounted with a semiconductor laser chip; a diffraction grating
diffracting a laser beam emitted from said semiconductor laser
chip; a beam splitter partially splitting said diffracted laser
beam; and a photodetector detecting an intensity of the laser beam
split by said beam splitter, wherein the semiconductor laser device
recited in claim 9 is employed as said semiconductor laser
device.
17. The optical pickup according to claim 16, wherein the laser
beam emitted from said semiconductor laser chip is divided into
three major beams by said diffraction grating, said three major
beams including a main beam and two side beams, said three beams
being directed to an optical disk and reflected from the optical
disk, at least two of said three beams being split partially by
said beam splitter, and a split beam detection output is obtained
from said photodetector for said two split beams, whereby a
tracking error signal corresponding to a tracking status of said
main beam directed to said optical disk is obtained.
18. A method of fabricating a semiconductor laser device including
a stem mounted with a semiconductor laser chip, said method
comprising the steps of: preparing a stem including a mount plane
mounting said semiconductor laser chip, and a cross plane crossing
said mount plane and facing a laser irradiated body on which a
laser beam emitted from said semiconductor laser chip strikes,
covering said cross plane with a material reducing reflectance to
the laser beam lower than the reflectance of said cross plane so
that an amount of light of said laser beam reflected at said cross
plane to be directed to a laser irradiated body is reduced, and
mounting said semiconductor laser chip on said mount plane.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a semiconductor laser
device, an optical pickup and a fabrication method of a
semiconductor laser device. More particularly, the present
invention relates to a semiconductor laser device and optical
pickup used in an optical disk system that carries out tracking
control by the 3-beam method and a fabrication method of a
semiconductor laser device.
[0003] 2. Description of the Background Art
[0004] A conventional optical pickup carrying out tracking control
by the 3-beam method is shown in FIG. 8A. Referring to FIG. 8A, a
semiconductor laser chip 104 is mounted on a mount plane 102a of,
for example, a stem 102. In stem 102 is mounted a photodetector 112
on an orthogonal plane 102b substantially orthogonal to mount plane
102a. The laser beam emitted from semiconductor laser chip 104 is
divided into the 0th-order beam i.e., a main beam B0, and side
beams B1 and B2 which are the .+-. first order beams. These three
beams are collected on an information recording plane 116a of an
optical disk 116. The three collected spots are disposed along a
direction tilted several degrees (.psi.) to the information track
direction where the signal pits are aligned, as shown in FIG.
8B.
[0005] In the case where main beam B0 is located at the center of
the information track which is a sequence of signal pits recording
signals, the two side beams B1 and B2 are located at opposite
directions about the center of the information track by equal
distance. In other words, the two side beams cover the same area of
signal pits. In contrast, when the main beam B0 is deviated from
the center of the information track, the area of signal pits
covered by the two side beams B1 and B2 will differ from each
other.
[0006] The reflected light from optical disk 116 enters
photodetector 112 through an optical element that has beam split
capability such as a hologram 114b. Hologram 114b and a diffraction
grating 114a are formed on a transparent substrate 114. This
transparent substrate 114 is generally formed integrally with stem
102. Photodetector 112 is divided into a plurality of detector
elements so that side beam B1 is detected by a photodetector
element 112d whereas side beam B2 is detected by a photodetector
element 112e, for example.
[0007] The position relationship between the information track and
the main beam is detected as set forth below by photodetector 112
that detects the side beams. When the main beam is located at the
center of the information track, the signals from the photodetector
detecting the two side beams are equal in level. More specifically,
the signal intensity Sd from photodetector element 112d is equal to
the signal intensity Se from photodetector element 112e. The
relationship of Sd=Se is established. When the main beam is offset
from the center of the information beam, the signal of one side
beam will become greater than the signal of the other side beam
according to the offset direction. By detecting the difference
(Sd-Se) of the signal levels of the side beams and adjusting the
position or the like of an objective lens 115 having a
beam-condensing function so that the difference becomes zero, the
main beam can be maintained at the center of the information track.
This is the mechanism of the tracking error control by the general
3-beam method.
[0008] Although the optical pickup employing the 3-beam method
relies on the characteristic of the beam splitter, not all the
light reflected from the optical disk enters the photodetector. The
reflected light partially passes straight through the hologram to
return to the proximity of the light emitting point X of
semiconductor laser chip 104 as a main beam R0 and side beams R1
and R2, i.e. three return beams as shown in FIG. 8A and FIG. 9.
These three beams are spaced apart from each other by the distance
d of approximately 70-120 .mu.m in the proximity at the side plane
of semiconductor laser chip 104. The return side beam R1 incident
on plane 102b orthogonal to semiconductor laser chip mount plane
102a of stem 102 is reflected at orthogonal plane 102b to return
towards optical disk 116. This return side beam is diffracted at
hologram 114b to directly enter photodetector elements 112d and
112e, or reflected at each plane of objective lens 115, the surface
of optical disk 116, information recording plane 116a and the like.
This reflected side beam is further diffracted by hologram 114b and
directly enters photodetector element 112d or 112e to disturb the
tracking control signal.
[0009] To eliminate such a problem, several conventional measures
were taken. For example, with respect to R2: (a) the thickness of
semiconductor laser chip 104 was adjusted so as to pass above the
position of light emitting point X of semiconductor laser chip 104;
and (b) the reflectance at the semiconductor laser chip end plane
is reduced to 10% and below to prevent much of the light quantity
from returning to the optical disk. Furthermore, the other side
beam R1 was caused: to (a) be incident upon stem 102 to be
scattered; or (b) to be incident upon a submount (not shown) having
a low reflecting film.
[0010] FIG. 9 shows an example of the above structure disclosed in
Japanese Patent Laying-Open No. 62-52737 by the present applicant.
The light emitting point X of semiconductor laser chip 104 is set
to be in the proximity of the middle of the chip's height, i.e.
approximately 50 .mu.m from mount plane 102a of the stem. Since
return side beam R2 is apart from return main beam R0 by a distance
of approximately d=70-120 .mu.m, side beam R2 will pass over
semiconductor laser chip 104 and not return to the optical disk. In
contrast, return side beam R1 will be incident on stem surface
102b. Therefore, the surface 102b of the stem is roughened for
scattering. By this roughening process, the quantity of light that
is reflected to return to the optical disk among the return side
beam is significantly reduced. In practice, a tracking error signal
of high reliability can be obtained by configuring an optical
pickup using such a semiconductor laser. Stable tracking control
can be achieved.
[0011] However, it has been identified recently that there is a
case where the reflecting effect of return side beam R1 at
orthogonal plane 102b of the stem cannot be prevented sufficiently.
Possible causes of such an event are set forth below.
[0012] (a) Various types of optical disks have been developed.
[0013] (b) A control method called differential push-pull (DPP)
using side beams similar to the 3-beam method has been employed as
the tracking control method. In the DPP method, the position
relationship of the three beams, the relationship between the beam
direction and the optical disk track, and the like differ from
those of the 3-beam method.
[0014] (c) A high power laser of more than 50 mW at the end plane
of the semiconductor laser chip is employed for the optical pickup
of the information rewritable optical disk.
[0015] It is now necessary to further reduce the reflectance at
orthogonal plane 102b of the stem to deal with the above-described
causes. The following methods are known to reduce the reflectance
at stem orthogonal plane 102b in addition to the plane roughening
process.
[0016] (a) As shown in FIG. 10, an inclination portion 121 is
provided at the stem orthogonal plane 102b where return side beam
R1 strikes (Japanese Patent Laying-Open No. 61-250844).
[0017] (b) The light emitting point of the semiconductor laser is
set upwards remote from mount plane 102a, and apply a low
reflecting material at the end plane of the semiconductor laser
chip at the side closer to the mount plane (Japanese Patent
Laying-Open No. 62-18080).
[0018] (c) A nonreflective coating is applied on the stem (header
portion) where side beam R1 strikes (Japanese Patent Laying-Open
No. 61-250845).
[0019] These above methods allow the reflectance of the return side
beam to be reduced significantly. For example, according to the
structure of the above (a) shown in FIG. 10, return light R1 is
reflected at inclination plane 121 provided at surface 113b of
header 113 to be radiated as reflected light Rir out from the
optical system. Therefore, light interference and the like will not
occur at the optical system of the optical pickup. However, each of
the above methods has an incidental problem. For example, the
method of providing an inclination portion 121 at orthogonal plane
102b at the stem shown in FIG. 10 (Japanese Patent Laying-Open No.
61-250844) has difficulty in mass production. There was a fatal
problem in productivity. This problem will be described in detail
with reference to FIGS. 11A and 11B.
[0020] In general, header portion 113 is formed on stem 102 by
press-molding. Specifically, header portion 113 is formed by
pressing a warp-like (thin elongated sheet) iron material 120 with
a die 151. In order to form a projection corresponding to head
portion 113 from iron material 120 which is a flat sheet, an
extremely strong stress must be applied to die 151. It is to be
particularly noted that the portion 151b facing the leading end
plane 113b (102b) of header portion 113 receives the greatest force
in order to maintain leading end plane 113b planar. If the
aforementioned inclination plane 121 is to be provided at the
leading end plane 113b (102b) of header portion 113, die 151 must
be formed with an inclination plane formation portion of a
projection or recess corresponding to inclination plane 121.
However, since this inclination plane formation portion corresponds
to the region where the greatest force is applied, this inclination
plane formation portion is easily susceptible to damage by the
stress. Thus, the productivity thereof was extremely poor. In the
worst case, this will induce damage of die 151 per se. In practice,
it was impossible to provide stem 102 with header portion 113
having inclination plane 121 as described above directly formed by
mass production.
[0021] Also, the method of applying a low reflective material at
the end plane of the semiconductor laser chip at the closer side to
the mount plane (Japanese Patent Laying-Open No. 62-18080) as well
as the method of applying a nonreflective coating on the header
portion (Japanese Patent Laying-Open No. 61-250845) causes the
fabrication process of such a semiconductor laser device to become
complicated, resulting in increase in the fabrication cost.
SUMMARY OF THE INVENTION
[0022] The object of the present invention is to provide a
semiconductor laser device that can be easily fabricated and having
reflectance of the returning side beam reduced to an acceptable
level, an optical pickup, and a method of fabricating such a
semiconductor laser device.
[0023] According to an aspect of the present invention, a
semiconductor laser device is employed in an optical pickup of the
3-beam method that divides one laser beam into three beams which
are the 0th-order beam and .+-. first order beams using an optical
system for radiation to an optical recording medium and that
detects information recorded on the recording medium as well as
detect tracking error information in the detection mode by the
reflected 0th-order beam and reflected .+-. first order beams from
the recording medium. The semiconductor laser device is
characterized in that a reflector is attached on a side beam
incident region of a leading end plane of a header portion mounted
with a laser chip emitting the laser beam. The side beam is one of
the two side beams generated by the reflected .+-. first order
beams and fed back through the optical system returning towards the
header portion to strike the side beam incident region. The
reflector reflects the side beam outside the optical system.
[0024] According to the present invention, the stem and header
portion are formed by press-working using a die, and then a
reflector which is independently formed from the stem is attached
at the leading end plane of the header portion. Therefore, the
disadvantage of breakage of the inclination plane formation portion
or the die per se encountered using the conventional die does not
occur. Mass production is possible. Furthermore, the properties of
the optical pickup will not be degraded since the side beam is
reflected outside the optical system by the reflector.
[0025] The reflector is preferably attached to the leading end
plane of the header portion so that the distance between the
reflecting plane and the light emitting point at the emitting end
of the laser chip is at least 50 .mu.m and not more than 150
.mu.m.
[0026] By setting the position of attaching the reflector within
this range, the side beam can be reflected outside the optical
system without being influenced by the property of the optical
pickup in which the semiconductor laser device is employed.
[0027] The reflector preferably has the reflecting plane set
inclined at least 10 degrees to the plane orthogonal to the main
beam that is generated by the 0th-order beam and fed back through
the optical system. Accordingly, reentrance of the side beam to the
optical system can be prevented effectively.
[0028] Preferably, the reflector has an inclination plane of a
plurality of stages whose cross section is a saw-toothed. By taking
such a configuration, the protruding height of the reflector from
the leading end plane of the header portion can be suppressed to
avoid being an obstruction in the subsequent process.
[0029] The reflector may be formed of a material of synthetic resin
or metal, further preferably a material that can easily be attached
to the header portion by an adhesive.
[0030] Thermosetting resin can be employed as the synthetic resin
for the reflector. In this case, the reflector will not peel off
from the header portion even if a heating process is applied in the
assemble process after the reflector is attached or in the
soldering process for connection with external circuitry by virtue
of the nature of the thermosetting resin.
[0031] A metal with hardness lower than that of the metal forming
the header portion can be used as the metal for the reflector.
Indium, gold, aluminium, silver and the like meet the requirement
of such a metal when the header portion is mainly formed of iron.
By using such metals, the reflector can be attached to the header
portion without using an adhesive.
[0032] A method of fabricating a semiconductor laser device of the
present aspect includes the steps of attaching a base material of a
reflector formed of metal that is softer than the metal forming the
header portion or of a synthetic resin prior to being cured at a
reflected side beam incident region at the leading end plane of the
header portion, and then shaping the base material into a reflector
having a predetermined configuration.
[0033] According to this method, the amount of material used for
the reflector can be minimized, in addition to the advantage that
mass production is facilitated. Thus, the cost can be reduced.
[0034] According to another aspect of the present invention, a
semiconductor laser device includes a stem where a semiconductor
laser chip is mounted. The stem includes a mount plane where the
semiconductor laser chip is mounted, as well as a cross plane
crossing the mount plane and facing a laser beam irradiated body on
which the laser beam emitted from the semiconductor laser strikes.
The cross plane is covered with a reflectance-reducing material
that reduces the reflectance to the laser beam lower than the
reflectance of the cross plane so that the amount of light
reflected at the cross plane to be directed towards the laser
irradiated body is reduced.
[0035] By the above structure, the reflectance of beam incident on
the stem among the return side beam can be reduced. Therefore, a
tracking error signal of high reliability can be obtained even in
the case where a high output power semiconductor laser chip is
employed as the light source of an optical disk system of the
information rewritable type or new type and a tracking error
control system of the 3-beam method or a similar method thereof
such as DPP is employed. The above laser light irradiated body
refers to an optical disk or the like.
[0036] In the semiconductor laser device of the present aspect, the
reflectance-reducing material scatters and/or absorbs the laser
beam directed towards the cross plane.
[0037] By scattering and/or absorbing the laser beam, the amount of
light reflected at the cross plane to return towards the optical
disk can be reduced. Therefore, the tracking error signal will not
be disturbed substantially even if tracking control by the 3-beam
method or similar DPP method is carried out in an optical disk
system of the rewritable or new type using a high output power
semiconductor laser chip.
[0038] In the semiconductor laser device of the present aspect, the
reflectance-reducing material is preferably applied continuously to
the mount plane. The portion of the reflectance-reducing material
applied on the mount plane is used as the bonding material to
die-bond the semiconductor laser chip to the stem.
[0039] In the case where the above-described material of reducing
the reflectance is an adhesive such as a die bond material, the
material can be applied continuously from the mount plane where the
semiconductor laser chip is die-bonded through the cross portion to
the cross plane. For example, when the silver paste is the material
to reduce reflectance, die-bonding can be effected using the silver
paste. Here, by applying the silver paste to spread from the die
bond to the cross plane, the silver paste applied at the cross
plane of the stem to reduce the reflectance is continuous. This
continuation of the silver paste allows the lower portion of the
end plane of the semiconductor laser chip to be covered with the
silver paste. The scattering effect is not lost even if the
striking position of the return side beam varies slightly.
Therefore, the reflectance will not increase. In this case, there
is little adverse effect on the electric characteristics even if
the silver paste is applied on the mount plane since the silver
paste per se has conductivity. There is a great advantage in using
silver paste for the die bond of the semiconductor laser chip.
[0040] In the semiconductor laser device of the present aspect, the
crossing portion between the mount plane and the cross plane has,
for example, an R configuration. The reflectance-reducing material
is applied at the region adjacent to the region of the R
configuration.
[0041] Since the return laser beam striking the portion of the R
configuration does not return to the laser irradiated body side and
reflected in a direction greatly deviated from that direction, the
above material does not have to be applied on the R configuration
portion. By minimizing the area where the material is applied, the
risk of applying the material over the light emitting point at the
end plane of the semiconductor laser chip can be reduced.
[0042] In the semiconductor laser device of the present aspect, the
reflectance-reducing material is, for example, a conductive die
bond paste.
[0043] Therefore, there is no problem even if the material is
attached to the mounting plane of the semiconductor laser chip
since the applied material is a conductive die bond paste that
die-bonds the semiconductor laser chip to the stem. Thus,
fabrication of a semiconductor laser device is facilitated.
[0044] In the semiconductor laser device of the present aspect, the
conductive die bond paste can include, for example, an epoxy resin
and silver.
[0045] The conductive die bond paste of reducing the reflectance is
a silver paste having silver added to the epoxy resin. The silver
has at least one type of shape of a needle crystal or flake crystal
to promote scattering and suppress reflectance of the beam towards
the laser irradiated body. Thus, the reflectance can be set low
enough.
[0046] In the semiconductor laser device of the present aspect, the
reflectance-reducing material can include at least one type of
epoxy resin and UV resin, for example, and at least one type of
silica and carbon powder.
[0047] By using the above material, absorption with respect to the
laser beam can be increased. Furthermore, the above material has
high adherence with the stem even when subjected to high
temperature during the fabrication and usage of the semiconductor
laser device. Therefore, the reflectance will not be increased. The
UV resin refers to resin that is cured when exposed to ultraviolet
ray.
[0048] The optical pickup of the present invention includes a
semiconductor laser device mounted with a semiconductor laser chip,
a diffraction grating diffracting the laser beam emitted from the
semiconductor laser chip, a beam splitter partially splitting the
diffracted laser beam, and a photodetector detecting the intensity
of the laser beam split by the beam splitter. The semiconductor
laser employed in the optical pickup is any one of the
above-described semiconductor laser device.
[0049] By using any of the above-described semiconductor laser
device in the optical pickup, the reflectance of light towards the
cross plane of the stem can be reduced. Therefore, a tracking error
signal of high reliability can be obtained even if a high output
power semiconductor laser chip is employed.
[0050] In the optical pickup of the present invention, the laser
beam emitted from the semiconductor laser chip is divided into the
three major beams, i.e., one main beam and two side beams, by the
diffraction grating. These three beams are directed to an optical
disk and reflected therefrom. At least two beams of the three beams
are partially split by the beam splitter. A split beam detection
output is obtained for the two split beams by the photodetector. A
tracking error signal corresponding to the tracking status of the
main beam directed onto the optical disk is obtained.
[0051] According to the above structure, a tracking error signal of
high reliability can be obtained even using a high output power
semiconductor laser chip in the optical pickup employing a tracking
error control system of the 3-beam method or a similar method using
the side beams such as the DPP. In other words, a tracking error
signal of high reliability can be obtained even if a high output
power semiconductor laser chip is used in an optical disk system,
regardless of whether the optical disk is of the information
rewritable type or new type.
[0052] A method of fabricating the semiconductor laser device of
the present aspect is directed to fabricating a semiconductor laser
device including a stem where a semiconductor laser chip is to be
mounted. The fabrication method includes the steps of preparing a
stem including a mount plane where the semiconductor laser chip is
to be mounted and a cross plane crossing said mount plane and
facing a laser irradiated body on which a laser beam emitted from
said semiconductor laser chip strikes covering the cross plane with
a material reducing the reflectance to the laser beam lower than
the reflectance of the cross plane so that the amount of light of
the laser beam reflected thereat and directed to the laser
irradiated body side is reduced, and mounting the semiconductor
laser chip at the mount plane.
[0053] By the above fabrication method, a semiconductor laser
device that provides a tracking error signal of high reliability
can be obtained even if a high output power semiconductor laser
chip is employed in the tracking control by the 3-beam or DPP
method in an optical disk system of the rewritable type or new
type. The semiconductor laser device is incorporated in the above
structure to allow stable tracking control. The order of the step
of applying the material reducing reflectance and the step of
mounting the semiconductor laser chip at the mount plane is of no
concern. In other words, either step can be carried out first.
[0054] The foregoing and other objects, features, aspects and
advantages of the present invention will become more apparent from
the following detailed description of the present invention when
taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0055] FIG. 1 is a perspective view of a semiconductor laser device
according to a first embodiment of the present invention with the
cap removed.
[0056] FIGS. 2, 3 and 4 are partial enlarged schematic cross
sectional views of the header unit according to first, second and
third embodiments, respectively, of the present invention.
[0057] FIG. 5 shows the optical pickup according to a fourth
embodiment of the present invention.
[0058] FIGS. 6A and 6B show the main part of a semiconductor laser
device of the present invention employed in the optical pickup of
FIG. 5, wherein FIG. 6A corresponds to the case where a silver
paste is applied at the cross plane and FIG. 6B corresponds to the
case where a silver paste is applied at both the cross plane and
the die bond of the semiconductor laser chip in a continuous
manner.
[0059] FIG. 7 shows the results of a preliminary test indicating
that the reflectance is reduced lower than the reflectance at the
laser chip end plane when the silver paste is applied.
[0060] FIG. 8A shows a conventional optical pickup.
[0061] FIG. 8B shows the arrangement of the information track and
the spots of three beams.
[0062] FIG. 9 shows the position of the three return beams in the
conventional optical pickup.
[0063] FIG. 10 is a partial enlarged schematic sectional view of
the header portion of a conventional semiconductor laser
device.
[0064] FIGS. 11A and 11B schematically show the formation method of
the header portion of a conventional semiconductor laser device,
corresponding to the status before and during shaping,
respectively.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0065] Embodiments of the present invention will be described
hereinafter with reference to the drawings.
[0066] First Embodiment
[0067] The first embodiment will be described with reference to
FIGS. 1 and 2. FIG. 1 is a perspective view of the semiconductor
laser device of the present invention with the cap removed. FIG. 2
is a partial enlarged schematic cross sectional view of the header
portion.
[0068] A semiconductor laser device 1 includes a disk-like stem 2,
a header portion 3 protruding substantially at the center area of
stem 2, and a laser chip 4 mounted on an upper plane 31 of header
portion 3.
[0069] By press working an iron material using a die, header
portion 3 is formed integral with stem 2. The upper plane 31 of
head portion 3 (laser chip mount plane) 31 and leading end plane
(cross plane) 32 are formed as planar planes at right angles to
each other, as shown in FIG. 2. In the illustrated example, both
side planes 33 and a bottom plane 34 are formed as inclination
planes and a curved plane, respectively. However, the configuration
of these planes are not limited thereto.
[0070] A conventional laser chip is employed here for laser chip 4.
Preferably, a laser chip having an outgoing end plane 41 of low
reflectance or applied with a nonreflective coating is employed for
the purpose of suppressing reflection of main beam R0. Laser chip 4
is fixed by a die bond material such as solder or silver paste at
upper plane 31 of header portion 3 so that outgoing end plane 41 is
flush with leading end plane 32 of header portion 3. Laser chip 4
is connected through a gold wire 6 to lead 51 out of one pair of
leads 51 and 52 projecting from stem 2 at both sides of header
portion 3.
[0071] A reflector 7 is attached to leading end plane 32 of header
portion 3. Referring to FIG. 2, reflector 7 serves to reflect in a
direction L.sub.r outside the optical system one of the two side
beams R1 and R2 generated by the reflected .+-. first order beams
fed back through the optical system and returned towards header
portion 3, i.e., side beam R1 here. For this purpose, reflector 7
is attached to a side beam incident region 35 where side beam R2
strikes at leading end plane 32 of head portion 3.
[0072] In the present embodiment, reflector 7 is formed in
triangular prism configuration, and attached on the above-described
position of header portion 3 using an adhesive. The material of
reflector 7 may be any metal or synthetic resin, provided that it
can be attached to header unit 3 with an adhesive.
[0073] Reflector 7 is dimensioned so that the upper edge 71u of its
reflecting plane 71 is located 50 .mu.m (indicated by a in FIG. 2)
from a light emitting point 72 at outgoing plane 41 of laser chip
4, and so that the lower edge 71d of reflector plane 71 is located
150 .mu.m (indicated by b in FIG. 2) from light emitting point 42.
In order words, reflector 7 is attached to leading end plane 32 of
header portion 3 so that the distance between reflecting plane 71
of reflector 7 and light emitting point 42 of laser chip 4 is at
least 50 .mu.m and not more than 150 .mu.m.
[0074] Furthermore, the inclination angle of reflecting plane 71 of
reflector 7, i.e. the angle (indicated by .theta. in FIG. 2) with
the perpendicular plane to main beam L.sub.m generated by the
0th-order beam and fed back through the optical system, is at least
10 degrees. This is effective in preventing side beam L.sub.s2 from
reentering the optical system.
[0075] Header portion 3 can be attached to reflector 7 before or
after laser chip 4 is attached to header portion 3.
[0076] Semiconductor laser device 1 has a cap not shown attached
and sealed hermetically to be employed for the optical pickup.
[0077] The synthetic resin used for reflector 7 may be
thermosetting resin. In this case, the thermosetting resin is
potted at leading end plane 32 of header portion 3 to be cured in a
heaped up manner. The formed inclination plane is reflecting plane
71. The usage of thermosetting resin for reflector 7 provides the
advantage that reflector 7 will not peel off from header portion 3
even when subjected to a heating process during the assembly
process after reflector 7 is attached or during the soldering
process for connection with external circuitry by virtue of the
thermosetting nature.
[0078] Second Embodiment
[0079] A semiconductor laser device according to a second
embodiment of the present invention and a method of fabricating
this semiconductor laser device will be described with reference to
FIG. 3 showing a partial enlarged schematic cross section of the
header portion. Components corresponding to those of the first
embodiment have the same reference characters allotted, and
description thereof will not be repeated.
[0080] In the second embodiment, reflector 7 is formed of a metal
lower in hardness than the metal forming header portion 3. For
example, in the case where header portion 3 is formed mainly by the
metal of iron, indium is employed as the material for reflector 7.
The metal material of reflector 7 is not limited to indium, and may
be, for example, gold, silver, aluminium, or the like.
Alternatively, a synthetic resin prior to curing can be used
instead of such metal.
[0081] In the case where the above material is employed for
reflector 7, the base material of the reflector such as indium is
fused at leading end plane 32 of header portion 3. The base
material is molded to a predetermined configuration with a die to
form reflector 7. In the case this method is employed, the adhesive
for the attachment of reflector 7 is not required, as compared to
the first embodiment. The amount of material used for reflector 7
can be minimized.
[0082] In shaping the base material into reflector 7, the
configuration of reflector 7 is not limited to the exemplified
triangular prism of the first embodiment. As shown in FIG. 3, the
reflector can have a saw-toothed cross section, including a
plurality of stages of inclination planes 71a, 71b and 71c. Such a
configuration of the reflector allows the protruding height H of
the reflector from leading end plane 32 of header 3 to be
suppressed. The lower height will prevent reflector 7 from being an
obstruction in the subsequent process.
[0083] Third Embodiment
[0084] A semiconductor laser device according to a third embodiment
of the present invention will be described with reference to FIG. 4
which is a partial enlargement schematic cross section of the
header portion. Components corresponding to those of the first
embodiment have the same reference characters allotted, and
description thereof will not be repeated.
[0085] In the third embodiment, a gold ball 7A is employed as
reflector 7. Specifically, a gold wire is ball-bonded to side beam
incident region 35 at leading end plane 32 of header portion 3 by a
wire bonding device. The gold wire is cut to form reflector 7. It
is to be noted that a gold wire is generally thin having a diameter
of approximately 50 .mu.m, and that the ball bond position can be
determined at the accuracy of the ball bonder. Therefore, ball 7A
can be formed in high precision. Since the time required to form
the reflector is substantially equal to the period of time required
for the wire bonding process on the laser chip, the attaching
process of reflector 7 to header portion 3 can be improved in
speed.
[0086] Fourth Embodiment
[0087] FIG. 5 shows an optical pickup 30 according to a fourth
embodiment of the present invention. The semiconductor laser device
includes a stem 2 integral with the header portion. A semiconductor
laser chip 4 is mounted on stem 2. Stem 2 further includes a
photodetector 12. In the description of the fourth embodiment and
et seq., distinction between the stem and header portion is not
particularly rendered. Above stem 2 is provided a transparent
substrate 14 including a diffraction grating 14a and a hologram
14b. An optical disk 16 is positioned further above.
[0088] The laser beam emitted from semiconductor laser chip 4 is
split into a main beam B0 and two side beams B1 and B2 by
diffraction grating 14a provided at transparent substrate 14. These
three beams are collected by an objective lens 15 on an information
recording plane 16a of optical disk 16. The collected three spots
are arranged along a direction tilted several degrees (.psi.) with
respect to the information track where the signal pits are aligned.
Therefore, when main beam B0 is located at the center of the
information track where signals are recorded, the two side beams B1
and B2 are located at opposite directions from the center of the
information track by equal distance. If main beam B0 is deviated
from the center of the information track, the area of the two side
beams B1 and B2 covering the information track will differ from
each other.
[0089] The reflected light from optical disk 16 is caused to be
incident on photodetector 12 by an optical element with the beam
split capability such as hologram 14b. Diffraction grating 14a and
hologram 14b are provided at transparent substrate 14. Transparent
substrate 14 is generally integrated with stem 2. Photodetector 12
is divided into a plurality of detector elements. For example, side
beam B1 is detected by a photodetector element 12d whereas side
beam. B2 is detected by a photodetector element 12e. Tracking error
control by the general 3-beam method is effected using these
optical elements.
[0090] The three laser beams partially pass through hologram 14b
which is a beam splitter towards a cross plane 2b of the stem. The
present invention is characterized in that a silver paste 17 formed
of a material that reduces reflectance is applied on cross plane 2b
of the stem. Cross plane 2b is the plane corresponding to leading
end plane 32 of the header portion in the case where distinction
between the stem and the header portion is made.
[0091] FIG. 6A shows a main part of the semiconductor laser
employed in the optical pickup of the4 present embodiment. The
crossing portion between mount plane 2a of stem 2 and cross plane
2b of the semiconductor laser device is subjected to an R
configuration as shown in FIG. 6A. Mount plane 2a is the plane
corresponding to upper plane 31 of the header portion in the case
where distinction between the stem and header portion is made. In
fabrication of this semiconductor laser device, a silver paste 17
formed of a reflectance-reducing material is applied at the region
of cross plane 2b of the stem adjacent to the region of the R
configuration. Then, the temperature is raised to cure silver paste
17. Next, semiconductor laser chip 4 is mounted on mount plane 2a
of stem 2. In order to avoid the stress caused by heat during
die-bonding, a soft metal 8 such as indium can be used as the
bonding material of stem 2 and semiconductor laser chip 4. Since
indium has favorable heat conductivity, a semiconductor laser chip
of higher output power can be employed.
[0092] Semiconductor laser chip 4 can be die-bonded using silver
paste. In this case, silver paste 18 for die-bonding is to be
applied so as to spread towards cross plane 2b to be continuous
with silver paste 7 applied on cross plane 2b of stem 2 to reduce
the reflectance, as shown in FIG. 6B. In other words, the silver
paste is applied continuously at cross plane 2b and mount plane 2a
including the region of the R configuration. The continuity of the
silver paste allows the lower portion of the end plane of the
semiconductor laser chip to be covered with silver paste. The
scattering effect will not be degraded even if the striking
position of return side beam R1 slightly varies. Therefore, the
reflectance will not be increased. The silver paste located at
mount plane 2a will not adversely affect the electrical
characteristics since silver paste per se is conductive. This is a
great advantage in using silver paste for the die bond of
semiconductor laser chip 4.
[0093] Since stem 2 is made of metal, hard soldering material such
as an alloy of gold and tin that is thermally and mechanically
stable cannot be used in the case where semiconductor laser chip 4
is directly die-bonded to the stem. In the case where a submount
such as of silicon, silicon carbide (SiC) or the like is used, the
aforementioned hard soldering material can be used in die-bonding
semiconductor laser chip 4 on the submount. In this case, silver
paste 7 that increases the scattering property and reduces
reflectance is to be applied to the submount.
[0094] As an alternative method of fabricating the semiconductor
laser device, first semiconductor laser chip 4 is mounted, and then
silver paste 17 that improves the scattering effect is applied.
Since semiconductor laser chip 4 is first mounted according to this
fabrication method, caution must be exercised so that silver paste
7 does not cover light emitting point X of semiconductor laser chip
4. There is no problem even if silver paste 7 adheres to the end
plane of semiconductor laser chip 4 as long as light emitting point
X is not covered. In other words, even if the silver paste adheres
to the end plane of the semiconductor laser chip, the problem of
electrically short-circuiting between the p type impurity layer and
the n type impurity layer of semiconductor chip 4 will not occur
since the end plane of semiconductor chip 4 is coated with an
insulation film.
[0095] By setting light emitting point X sufficiently remote from
mount plane 2a, it is not so difficult to prevent the silver paste
from covering light emitting point X. For example, light emitting
point X is to be set at least 30 .mu.m higher than mount plane 2a,
similar to the conventional semiconductor laser device disclosed in
Japanese Patent Laying-Open No. 62-52737.
[0096] By employing the above-described semiconductor laser device
or optical pickup, the amount of light reflected at the stem cross
plane towards the optical disk can be reduced even if a high output
power semiconductor laser chip is used. Tracking error control by
the 3-beam method can be effected stably. Such a stable tracking
error control can be maintained irrespective of the type of the
optical disk such as the rewritable type or new type as long as the
tracking error control is effected by the 3-beam or similar DPP
method.
[0097] The experiments for preliminary evaluation to ascertain the
feature of the fourth embodiment of the present invention will be
described hereinafter. The inventors of the present invention
studied where the side beams return. Side beam R2 of the two side
beams that return passes over semiconductor laser chip 4, as in the
conventional case, and will not return to the optical disk. The
other R1 of the side beams strikes against cross plane 2b of the
stem. The region around the crossing line between mount plane 2a
and cross plane 2b of the stem has an R configuration with a smooth
surface. When side beam R1 strikes against the region of the R
configuration around this cross plane, side beam R1 is partially
reflected upwards and will not return to the optical disk. It was
found that the reflectance is reduced to a level substantially
acceptable.
[0098] Main beam R0 of the return beam comes back to light emitting
point X of the semiconductor chip. The distance (height) of light
emitting point X from mount plane 2a has an error of approximately
10 .mu.m about 55 .mu.m. The radius of curvature of the R around
the crossing between mount plane 2a and cross plane 2b is
approximately 30-60 .mu.m, approximately 45 .mu.m in average. Since
the distance between main beam R0 and side beam R1 is approximately
60-120 .mu.m, it was identified that the possibility of side beam
R1 striking the region of the R configuration is high.
[0099] The reflectance of the laser beam on cross plane 2b of the
stem was actually measured to obtain the results shown in FIG. 7 as
the comparative example. It was appreciated from the results that
the reflectance was 20-45 %, approximately 35% in average. In other
words, the reflectance was equal to or greater than the reflectance
of 32% when the end plane of the semiconductor laser chip is not
particularly coated.
[0100] As a way of reducing the reflectance at cross plane 2b of
the stem, the approach of further scattering the light was
considered. As a result, it was found that, as the material of
scattering light, a conductive die bond paste having silver filler
added to epoxy resin, i.e. silver paste, exhibited favorable
characteristics. When silver paste is applied as shown in FIG. 7,
the reflectance falls in the range of 7-20%, and the center of the
distribution was substantially 14%. A reflectance lower than that
at the end plane of a semiconductor laser chip not subjected to
coating is obtained.
[0101] The reason why the reflectance is low of the conductive
paste that employs silver assumed to have high reflectance as a
filler is that the silver filler of the silver paste is of the
needle crystal or flake crystal form. The scattering effect with
respect to the laser beam is great since the direction of the
needle or flake crystals is random. Thus, the reflectance is
reduced.
[0102] The present invention was accomplished based on the above
results of preliminary study.
[0103] Fifth Embodiment
[0104] In the fifth embodiment of the present invention, the method
of absorbing the laser beam is employed as the method of reducing
the reflectance at cross plane 2b of the stem. A resin blackened
with filler is applied instead of the silver paste used in the
fourth embodiment. Here, the reflectance of cross plane 2b of the
stem was lower than the value of the end plane of the semiconductor
chip not subjected to coating.
[0105] The preferably resin is a thermosetting epoxy resin that has
favorable adherence with the stem or an UV resin that is cured by
exposure to ultraviolet ray. These resins are advantageous in that
they do not easily peel off even when subjected to a thermal cycle.
It is to be noted that they is a heating process of 200-300.degree.
C., when the semiconductor laser chip is mounted at the stem and a
further heating process for testing after the mounting step.
Therefore, peel off will not easily occur by using the foregoing
resin.
[0106] As the filler to be added to the resin, silica or carbon is
used. The reflectance can be reduced as a function of increasing
the ratio of the added filler of these materials. However, if the
added ratio is too high, the adherence will be degraded so that the
resin will easily peel off. It is desirable to set the added amount
as high as possible within the range that avoids peel off.
[0107] In the case where epoxy resin or UV resin added with filler
as the reflectance-reducing material is employed as described
above, it is desirable to apply the resin only at cross plane 2b as
shown in FIG. 6A. Although the resin may be spread continuously to
mount plane 2a including the cross portion of the R configuration
as shown in FIG. 6B, a result as favorable as that for silver paste
cannot be achieved since the adherence and the heat conductivity of
the semiconductor laser chip are not favorable as by the silver
paste. However, it is desirable to dispose the resin, even when of
a material that reduces reflectance mainly by absorption,
continuously at the mount plane including the crossing portion and
the cross plane when the material is superior in adherence and heat
conductivity. Specifically, it is desirable to use such material as
a reflectance-reducing material at cross plane 2b and the
neighborhood of the crossing portion, and as a bonding material to
die-bond the semiconductor laser chip at mount plane 2a.
[0108] The present invention is not limited to that described in
the foregoing. For example, any device employed in the optical
pickup of the 3-beam method (or a similar tracking method using
side beams), even if absent of the hologram or diffracting grating,
is included in the semiconductor laser device of the present
invention.
[0109] Although the present invention has been described and
illustrated in detail, it is clearly understood that the same is by
way of illustration and example only and is not to be taken by way
of limitation, the spirit and scope of the present invention being
limited only by the terms of the appended claims.
* * * * *