Monolithic multi-wavelength semiconductor laser unit

Details Monolithic multi-wavelength semiconductor laser unit Monolithic multi-wavelength semiconductor laser unit

2000-08-15

2003-09-09

Leung, Quyen (Department: 2828)

Coherent light generators

Particular active media

Semiconductor

C372S045013

Reexamination Certificate

active

06618420

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention pertains to optical communication, optical interconnects, and optical signal processing employing semiconductor lasers. More particularly, the present invention relates to a monolithic multi-wavelength semiconductor laser unit for outputting laser lights having different wavelengths from a single monolithically integrated semiconductor chip, which can be used in optical memory disk applications.
2. Description of the Related Art
An optical memory disk system is widely put to practical use since it is small-sized and it can record a large capacity of information. Especially, in a digital versatile disk (DVD) system, its practical use has been rapidly promoted as a main system such as a movie, a multi-media application for a next generation and the like. On the other hand, a compact disk (CD) system or a compact disk-recordable (CD-R) system has been conventionally widespread as an optical memory disk. It is desirable that the DVD system has compatibility with the CD system. That is, it is necessary that the DVD system can read and write data from and to a disk of CD or CD-R. Those optical memory disk systems employ an optical pickup using a semiconductor laser, in order to read out the information recorded on the disk, and/or write the information on the disk.
FIG. 1
is an explanation view showing a typical configuration proposed as an optical pickup of a conventional DVD system. That is, the optical pickup in
FIG. 1
has compatibility with the disks of CD and DVD, and has an optical integration unit
1
for DVD and an optical integration unit
2
for CD and CD-R. A laser light having a wavelength of 650 nm emitted by the optical integration unit
1
for DVD passes through a dichroic prism
3
, and also passes through a collective lens
4
, a beam-rising mirror
5
, a wavelength selection diaphragm
6
and an objective lens
7
, and then reaches an optical memory disk
9
. On the other hand, the prism
3
reflects a laser light having a wavelength of 780 nm, emitted from the optical integration unit
2
to the recording surface of CD. Then, it passes through an optical path substantially equal to that of the laser light emitting the radiation with the wavelength of 650 nm for DVD, and reaches a CD or CD-R disk
8
. And, the return lights from the disk pass through optical paths opposite to the above-mentioned paths, and reach the optical integration units
1
,
2
for DVD or CD, respectively.
However, the conventional optical pickup uses the two different optical integration units
1
,
2
in order to obtain the laser light with the wavelength of 650 nm and that with the wavelength of 780 nm. Thus, its configuration becomes complex which results in a problem that it is difficult to make the optical pickup smaller and lighter. Also, the finely positional adjustments for respective light sources must be done to thereby require a very long time for assembling the optical system.
In order to solve such problems, a multi-wavelength semiconductor laser unit is proposed which can independently output laser lights having two different wavelengths of 650 nm and 780 nm from .one chip, as shown in
FIG. 2A
, which is proposed by Uchizaki et al. in Japanese Published Unexamined Patent Application No.P2000-11417A (hereinafter referred as “Uchizaki et al.”). The semiconductor laser in
FIG. 2A
radiates the laser lights having the two different wavelengths. So, two active regions are arranged in parallel to each other in the optical axis directions. In
FIG. 2A
, the laser structure referred to as “a selectively buried ridge (SBR) structure” is shown. That is, p-type InGaAlP cladding layers
109
,
119
are formed in ridge geometries. Then, both sides of the ridge are sandwiched by n-type GaAs layers
123
having column V elements Arsenic (As) different from the column V elements Phosphorus (P) in the cladding layers
109
,
119
. The n-type GaAs layers serves as current-blocking regions to thereby channeling current into active layers. At the same time, the GaAs layer in which band gap is narrower than that of the active layer absorbs a light transmitted through the active layers in the lower portions, at both the sides of the ridge
109
,
119
.
In detail, even in any of laser portions
100
,
101
, n-type buffer layers
102
,
112
, n-type InGaAlP cladding layers
103
,
113
, InGaAlP waveguide layers
104
,
114
, multi-quantum well (MQW) active layers
105
,
115
, InGaAlP waveguide layers
106
,
116
, first p-type InGaAlP cladding layers
107
,
117
, p-type InGaP etching stop layers
108
,
118
, second p-type InGaAlP cladding layers
109
,
119
, p-type InGaP conduction layers
110
,
120
, n-type current-blocking layers
123
and p-type GaAs contact layers
122
are laminated on substrates
124
in this order.
Here, in the first laser portion
100
emitting the radiation with the wavelength of 780 nm, the active layer
105
has the MQW structure composed of Ga
0.9
Al
0.1
As quantum well layer and Ga
0.65
Al
0.35
As barrier layer. In the second laser portion
101
emitting the radiation with the wavelength of 650 nm, the active layer
115
has the MQW structure composed of In
0.5
Ga
0.5
As quantum well layer and In
0.5
(Ga
0.5
Al
0.5
)
0.5
P barrier layer.
That is, the active layer
105
emitting the radiation with the wavelength of 780 nm and the cladding layers
103
,
107
and
109
have column V elements different from each other, namely, P and As. Also, the active layer
41
emitting the radiation with the wavelength of 650 nm and the cladding layers
22
,
26
and
28
have column V elements P common to each other. This structure enables the compositions and the film thicknesses of the cladding layers
103
,
107
and
109
, and
113
,
117
and
119
in the devices
100
,
101
, the compositions and the film thicknesses of the high conductivity films
110
,
120
, the compositions and the film thicknesses of the current-blocking layers
123
and the compositions and the film thicknesses of the contact layers
122
to have the commonalities to each other, and also enables the manufacturing processes to be very easy and further enables the control accuracies to be very high.
However, this also brings about a problem.
FIG. 2B
describes this problem. In
FIG. 2B
, the compositions, the carrier concentrations and the film thicknesses of the respective semiconductor layers are plotted, from the n-type cladding layer
103
to the p-type third cladding layer
109
, along the lamination direction when a forward bias of 2.5 V is applied across the electrodes. The film thicknesses of the respective semiconductor layers are considered to accordingly simulate the energy band diagram, the Fermi-level diagram and the distribution of electron current densities. In the n-type cladding layer
103
, the Al mole fraction is 0.7 as In
0.5
(Ga
0.3
Al
0.7
)
0.5
P, the carrier concentration is 2×10
17
cm
−3
and the layer thickness is about 1 &mgr;m. The compositions of the waveguide layer
104
and the buffer layer
102
are defined as undoped In
0.5
(Ga
0.5
Al
0.5
)
0.5
P. The active layer
105
is a double quantum well (DQW) composed of two quantum well layers, namely, a first quantum well layer in contact with the n-type cladding layer
103
and a second quantum well layer adjacent to a right side of the first quantum well layer. Each of them is assumed to have the film thickness of 10 nm and the composition of undoped Al
0.1
Ga
0.9
As. There are the two p-type cladding layers
107
,
109
sandwiching the etching stop layer
108
between them. However, they are designed such that all of the compositions are equal to that of the n-type cladding layer
103
, the carrier concentrations are 1×10
17
cm
−3
and the total thickness is equal to that of the n-type cladding layer. A longitudinal length of a resonator is defined as 600 &mgr;m.
In a conduction band edge in
FIG. 2B
, there is a large peak reaching 45% with respect to a depth of the first quantum well layer, at a boundary between the firs

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