Semiconductor laser device

Details Semiconductor laser device Semiconductor laser device

2001-12-20

2004-03-16

Davie, James (Department: 2828)

Coherent light generators

Particular active media

Semiconductor

C372S046012

Reexamination Certificate

active

06707836

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor laser device that is used suitably as a light source for optical information processing, optical measurement or the like.
2. Description of Related Art
In recent years, because of their advantage of being compact and inexpensive, semiconductor laser devices have been used widely as a light source for optical information processing, optical measurement and the like.
Among those semiconductor laser devices, Fabry-Pèrot (referred to as “FP” in the following)-type semiconductor laser devices using a FP resonator in which the semiconductor's cleavage planes serve as mirrors are easy to manufacture and thus mass-produced as a light source for a pickup of optical disks. However, they have disadvantages in that their emission wavelength changes depending on temperature and that a laser oscillates at a plurality of longitudinal modes during a high-speed modulation.
In order to solve the above problems without compromising the compactness of the devices, a known semiconductor laser device adopted a technique in which a diffraction grating having a sharp wavelength selectivity is provided in the semiconductor resonator so as to perform light feedback. One of the semiconductor laser devices of this type is a distributed Bragg reflector (referred to as “DBR” in the following) type semiconductor laser device. The DBR type semiconductor laser device has a structure in which an active region for amplifying light and a DBR region provided with a diffraction grating are connected optically so that the light feedback is performed by utilizing a Bragg reflection in the DBR region.
The DBR type semiconductor laser device has the following advantages: (i) by applying electric current to the DBR region, a Bragg wavelength serving as the emission wavelength can be changed easily; (ii) since the active region and the DBR region are provided independently, the degree of design/production flexibility is high.
The following is a description of a conventional DBR type semiconductor laser device as a typical example. The structure of this DBR type semiconductor laser device described here is disclosed in IEEE JOURNAL OF QUANTUM ELECTRONICS VOL. 27, p. 1609.
FIG. 5
is a partially sectional perspective view showing a DBR type semiconductor laser device with a conventional structure. The DBR type semiconductor laser device is divided into three regions along its optical resonance direction. Numeral
201
denotes an active region, numeral
202
denotes a phase control region, and numeral
203
denotes a DBR region. Next, its layered structure will be described. On an n-type GaAs substrate (n-type substrate)
204
, an n-type Al
0.6
Ga
0.4
As first cladding layer
205
and an active layer
206
are formed. The active layer
206
includes an undoped GaAs single quantum well and undoped Al
x
Ga
1−x
As (x=0.3 to 0.6) distributed refractive index (GRIN) layers that are arranged so as to sandwich the GaAs single quantum well from both external sides. An active layer
206
a
in the phase control region
202
and the DBR region
203
is disordered by Si ion implantation, thus providing a low-loss treatment for Bragg wavelength light. A rib-shaped n-type Al
0.3
Ga
0.7
As optical guiding layer
208
is formed on the active layer
206
. The optical guiding layer
208
in the DBR region
203
is provided with a diffraction grating
208
a.
On the optical guiding layer
208
, an n-type Al
0.3
Ga
0.7
As second cladding layer
211
and a p-type GaAs contact layer
212
are formed.
The contact layer
212
is arranged separately in the active region
201
, the phase control region
202
and the DBR region
203
so that current can be applied independently to each region, and a p-electrode
213
further is provided thereon. The p-electrode
213
is formed immediately above the contact layer
212
in a rib region. In order to narrow the current applied from the p-electrode
213
to the rib region, an insulating layer
215
is provided immediately below the p-electrode
213
in a region other than the rib region. An n-electrode
214
is provided below the n-type substrate
204
.
The following is a description of an operation of the conventional DBR type semiconductor laser device structured as above.
First, the current applied from the p-electrode
213
of the active region
201
is narrowed into the rib region by the insulating layer
215
and reaches the active layer
206
, so that the active layer
206
in the rib region emits light. The rib region serves as a waveguide channel so as to propagate the emitted light.
When the semiconductor laser device is used as a light source for optical information processing or optical measurement, a single transverse mode is required. To meet this requirement, it is necessary to confine the guided light in the transverse direction effectively. In this DBR type semiconductor laser device with the conventional structure, since the rib-shaped optical guiding layer
208
is provided in the waveguide channel region, the effective refractive index in the waveguide channel region is lower than that in its outer regions, so that the guided light is confined in the transverse direction.
The waveguide structure of the semiconductor laser device that has been described here is a rib waveguide type. However, the operation is essentially the same in other refractive index waveguide structures, for example, a ridge waveguide type.
In this DBR type semiconductor laser device, an end face near the active region
201
and a DBR formed of the diffraction grating
208
a
in the DBR region
203
serve as two reflecting mirrors so as to form a resonator, so that the guided light is amplified in the active region
201
and emitted as a laser beam.
A plurality of longitudinal modes that satisfy a phase condition of the laser oscillation are present in the DBR type semiconductor laser device like in the FP type semiconductor laser device. Among these longitudinal modes, only the longitudinal mode having a wavelength closest to a Bragg wavelength of the DBR is Bragg-reflected mainly and satisfies an amplitude condition of the laser oscillation. Thus, the single longitudinal mode can be achieved. In this case, the Bragg wavelength &lgr;
b
is determined by an equation below.
&lgr;
b
=2
N
eq
&Lgr;/q
  (1)
where N
eq
represents an equivalent refractive index of the DBR region
203
, &Lgr; represents a period of the diffraction grating, and q represents an order of the Bragg reflection. For instance, the Bragg wavelength is 850 nm in the DBR type semiconductor laser device of this conventional example.
The emission wavelength is controlled by current applied to the DBR region
203
. When the current is applied, this changes the refractive index, that is, N
eq
in the DBR region. Therefore, the Bragg wavelength can be controlled according to Equation (1).
In this case, however, the emission wavelength only can change discontinuously at an interval of the longitudinal mode. This is because, among the longitudinal modes satisfying the phase condition as above, the one having a wavelength closest to the Bragg wavelength oscillates. In order to allow the emission wavelength to change continuously, it is necessary to control the phase of the guided light, thereby changing the wavelength of the longitudinal mode. For this purpose, the phase control region
202
for changing the phase of the guided light is provided inside the resonator. By applying current to the phase control region
202
so as to change the equivalent refractive index in this region, the phase of the guided light is controlled.
Thus, by setting the current applied to the phase control region
202
and the DBR region
203
appropriately, it is possible to allow the emission wavelength to change continuously.
The change in a refractive index N of a semiconductor by current application mainly is attributable to a plasma effect and a heat effect.
The plasma effect is caused by an applied carrier. The refractive index change

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