Semiconductor laser device capable of suppressing leakage...

Details Semiconductor laser device capable of suppressing leakage... Semiconductor laser device capable of suppressing leakage...

2001-11-16

2004-11-30

Wong, Don (Department: 2821)

Coherent light generators

Particular active media

Semiconductor

C372S045013

Reexamination Certificate

active

06826218

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor laser device having a protective coating with a specified reflectance formed on light emitting end surface, and to a method for manufacturing the same.
As shown in
FIG. 5
, most semiconductor laser devices are composed of, for example, protective coatings
2
a
and
2
b
, each having an identical reflectance, formed on light emitting end surfaces
1
a
and
1
b
of a GaAs laser chip
1
. Reference numeral
3
denotes an active layer of the laser chip
1
. In the case where the protective coatings
2
a
and
2
b
are composed of Al
2
O
3
in
FIG. 5
, if a refractive index of the Al
2
O
3
film is set to 1.60 while a refractive index of the laser chip
1
is set to 3.50, a reflectance of the protective coatings
2
a
and
2
b
corresponding to a coating thickness d varies as shown in
FIG. 6
(provided that a laser emission wavelength &lgr;=7800 Å).
FIG. 6
indicates that regardless of the coating thickness d of the protective coatings
2
a
and
2
b
, the reflectance thereof is smaller than that of the case without the protective coatings
2
a
and
2
b
(i.e. the reflectance of the light emitting end surfaces
1
a
and
1
b
). The reflectance becomes smallest when an optical coating thickness (refractive index n×coating thickness d) is an odd multiple of &lgr;/4, while the reflectance becomes approximately equal to that in the case without the protective coatings
2
a
and
2
b
when the optical coating thickness is an integral multiple of &lgr;/2. This is because the refractive index (1.60) of the protective coatings
2
a
and
2
b
is smaller than the refractive index (3.50) of the GaAs laser chip
1
.
Contrary to this, in the case where the refractive index of the protective coatings
2
a
and
2
b
is larger than the refractive index of the GaAs laser chip
1
(for example, if such material as Si is used as the protective coatings
2
a
and
2
b
, the reflectance thereof is larger than that in the case without the protective coatings
2
a
and
2
b
, regardless of the coating thickness), the reflectance becomes largest when the optical coating thickness is an odd multiple of &lgr;/4, while the reflectance becomes approximately equal to that in the case without the protective coatings
2
a
and
2
b
when the optical coating thickness is an integral multiple of &lgr;/2.
In the case of high output semiconductor laser devices with optical output as high as 20 mW or more, as shown in
FIG. 7
, for increasing optical output Pf from the side of a main emitting end surface (front end surface), the reflectance of the protective coating
12
a
on the side of the main emitting end surface
11
a
is generally set lower than that in the case without the protective coating
12
a
, while the reflectance of the protective coating
12
b
on the side of a rear emitting end surface
11
b
is set higher than that in the case without the protective coating
12
b
. For example, the reflectance of the protective coating (Al
2
O
3
)
12
a
is set to approx. 15% or less. This reflectance is obtained with the coating thickness of approx. 700 Å to 1600 Å.
The protective coating
12
b
on the rear emitting end surface
11
b
, if composed with use of a film having a refractive index larger than that of the laser chip
11
, is not capable of providing a sufficiently high reflectance as a single layer. Accordingly, there are laminated an Al
2
O
3
film with a thickness of &lgr;/4 as a first layer
14
and a third layer
16
, and an amorphous Si with a thickness of &lgr;/4 as a second layer
15
and a fourth layer
17
. Then finally, there is laminated an Al
2
O
3
film with a thickness of &lgr;/2 as a fifth layer
18
. This makes it possible to form a protective coating
12
b
having a reflectance as high as approx. 85% or more. It is noted that reference numeral
13
denotes an active layer.
Description will now be given of a method for forming protective coatings
2
a
and
2
b
having the above-described reflectance on light emitting end surfaces
1
a
and
1
b
of a semiconductor laser chip
1
.
First, as shown in
FIG. 8
, there is formed by scribing a cleavage line
25
extensively disposed between an electrode
22
of an arbitrary element in a semiconductor laser wafer
21
and an electrode
23
of an adjacent element in direction orthogonal to an emitting section (channel)
24
. Then, as shown in
FIG. 9
, the semiconductor laser wafer
21
is cleaved and divided into a plurality of laser bars (bar-shaped laser chips)
26
.
Next, as shown in
FIG. 10
, a plurality of the divided laser bars
26
are set in a laser bar fixing device
27
such that the electrodes
22
are laid on top of each other. All the laser bars
26
should be set so that an emitting end surface
28
a
and an emitting end surface
28
b
face the same side. Next, on the emitting end surfaces
28
a
and
28
b
of a laser bar
26
fixed to the laser bar fixing device
27
, there is formed a protective coating having a specified reflectance, generally with use of a vacuum evaporator
29
exemplarily shown in FIG.
11
. The vacuum evaporator
29
is provided with a vapor source
31
, a holder
32
for holding a plurality of the laser bar fixing devices
27
, and a crystal oscillator
33
for monitoring the thickness of evaporated films, all in a chamber
30
.
Following description discusses procedures of forming the protective coating. First, in the case for evaporating a protective coating onto the emitting end surface
28
a
, the holder
32
is disposed such that the emitting end surface
28
a
of a laser bar
26
faces the vapor source
31
side as shown in FIG.
11
. Then, the chamber
30
is evacuated through a duct
34
. After a specified degree of vacuum is obtained, an evaporation material
35
put in the vapor source
31
is heated and evaporated by electron beams and the like so that a protective coating is evaporated onto the emitting end surface
28
a
of the laser. After evaporation is completed, the holder
32
is then rotated 180° for evaporating a protective coating onto the emitting end surface
28
b
based on the same procedures.
Here, a forming speed (evaporation rate) for forming a protective coating on the both light emitting end surfaces
28
a
and
28
b
is controlled to be approximately constant till completion of evaporation. The evaporation rate is in this case controlled with heating temperature. In the case of electron beam evaporation, therefore, the evaporation rate may be controlled with intensity of electron beams. It is well known that in the case of resistance heating, the evaporation rate is controlled with an amount of electric current passed through a resistance. The evaporation rate is generally set to the range between several Å/sec to 30 Å/sec with the evaporation material of Al
2
O
3
. Evaporation is conducted while coating thickness is monitored with use of the crystal oscillator
33
. Evaporation is terminated when a specified coating thickness is obtained.
In the case of a high output type semiconductor laser device shown in
FIG. 7
, there is formed a low reflecting protective coating
12
a
(having a reflectance of approx. 15% or less) on the side of the main emitting end surface
11
a
, and then there is formed in succession a multilayered high reflecting protective coating
12
b
on the side of the rear emitting end surface
11
b
. The multilayered high reflecting protective coating
12
b
is composed of a laminated structure made up of: a first layer
14
and a third layer
16
each consisting of an Al
2
O
3
film with a thickness equal to &lgr;/4; a second layer
15
and a fourth layer
17
each consisting of an Si film with a thickness equal to &lgr;/4; and a fifth layer
18
consisting of an Al
2
O
3
film with a thickness equal to &lgr;/2. For evaporation of this film, Al
2
O
3
and Si are mounted on the vapor source
31
as evaporation materials
35
. Then the first layer
14
, the third layer
16
, and the fifth layer
18
consisting of an Al
2
O
3
film a

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