Variable wavelength semiconductor laser and optical module

Details Variable wavelength semiconductor laser and optical module Variable wavelength semiconductor laser and optical module

2002-05-29

2004-02-10

Ip, Paul (Department: 2828)

Coherent light generators

Particular beam control device

Tuning

C372S043010, C372S044010, C372S050121, C372S099000, C372S101000, C372S102000

Reexamination Certificate

active

06690688

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a broad-band wavelength-tunable semiconductor laser needed in an optical fiber communication technique used for a telephone exchange network of a trunk line or the like, particularly a wavelength division multiplexing system that simultaneously uses laser beams having different wavelengths for signal transmission, and, more particularly, to a wavelength-tunable semiconductor laser having an optical waveguide with a front light reflection area comprising a sampled grating mirror and a rear light reflection area comprising a super-structure-grating mirror that are located before and after an active region, respectively, and an optical module using the wavelength-tunable semiconductor laser as a light source.
2. Description of the Related Art
An SSG-DBR wavelength-tunable semiconductor laser using a so-called SSG (Super-Structure-Grating) DBR (Distributed Bragg Reflectors) as a diffraction grating is known as one of the wavelength-tunable semiconductor lasers having a conventional multiple-electrode DBR structure.
FIG. 11
is a schematic diagram showing the structure of a conventional SSG-DBR wavelength-tunable semiconductor laser reported by H. Ishii et. al. in IEEE Journal of Quantum Electronics, vol. 32, No. 3, 1996, pp 433-441, and, more particularly,
FIG. 11
shows a cross-sectional view of a wavelength-tunable semiconductor laser in the direction parallel to the optical axis.
In
FIG. 11
, reference numeral
1
represents an active region;
2
, a front light reflection area;
3
, a rear light reflection area;
4
, a phase control area;
5
, an optical waveguide formed of InGaAsP;
6
, an n-type InP substrate;
7
, an n-type InP cladding layer;
8
, a p-type InP cladding layer;
9
, a p-type InGaAsP contact layer;
13
, an n-type electrode;
14
a
,
14
b
,
14
c
,
14
d
, p-type electrodes;
15
, a laser beam emitted from the front facet of an optical resonator; and
21
,
22
, the pitch variations of the diffraction grating (i.e., one period of modulation) of each of a front SSG-DBR mirror and a rear SSG-DBR mirror, respectively.
Here, the SSG-DBR mirror has the following periodic structure. That is, when in a structure having both the ends spaced at a predetermined distance, the pitch of the diffraction grating in the structure is linearly and continuously varied (linearly chirped) from &Lgr;
a
to &Lgr;
b
in the direction from one end to the other end, and the structure is set as one period &Lgr;
s
, the above structure of one period &Lgr;
s
is repeated by plural periods (&Lgr;
s
) to thereby achieve the periodic structure. The reflection peak spectrum of the SSG-DBR mirror has plural reflection peaks at a wavelength interval of &dgr;&lgr;=&lgr;
0
2
/(2n
eq
X&Lgr;
s
) over the wavelength range from &lgr;
a
=2n
eq
X&Lgr;
a
to &lgr;
b
=2n
eq
X&Lgr;
b
. Here, n
eq
represents the equivalent refractive index of the optical waveguide, and &lgr;
0
represents the center wavelength.
In the front SSG-DBR mirror constituting the front light reflection area
2
and the rear SSG-DBR mirror constituting the rear light reflection area
3
of the conventional wavelength-tunable semiconductor laser, one period
21
of the front SSG-DBR mirror and one period
22
of the rear SSG-DBR mirror are respectively repeated by plural periods in
FIG. 11
(the repetitive arrangement is not shown in FIG.
11
). The SSG-DBR wavelength-tunable semiconductor laser is designed so that the wavelength intervals of the reflection peaks of the front and rear SSG-DBR mirrors are slightly different from each other by using a method of varying the distance of the one period
22
with respect to the distance of the one period
21
above-mentioned.
Next, the operation of the conventional SSG-DBR wavelength-tunable semiconductor laser shown in
FIG. 11
will be described.
As shown in
FIG. 11
, the active region
1
, the front light reflection area
2
, the rear light reflection area
3
, and the phase control area
4
are integrated into the optical waveguide
5
. P-type electrodes
14
a
,
14
b
,
14
c
,
14
d
, which are electrically separated from one another, are located in the respective areas. Active-layer current is injected into the active region
1
by applying a forward bias voltage across the p-type electrode
14
b
opposite the active region
1
and the n-type electrode
13
located at the rear surface side of the semiconductor substrate, whereby spontaneous emission light having a broad wavelength range is emitted.
The spontaneous emission light is repetitively reflected and amplified by the front SSG-DBR mirror located in the front light reflection area
2
and the rear SSG-DBR mirror located in the rear light reflection region
3
while it propagates in the optical waveguide
5
located in the optical resonator, and laser beams having one wavelength are controlled to be finally selected by controlling refractive index of each area due to current injection to the front light reflection area
2
and/or the rear light reflection area
3
and the phase control area
4
, whereby laser oscillation at a single wavelength is achieved with a threshold current.
The laser oscillation wavelength control of the conventional wavelength-tunable semiconductor laser will be described in more detail.
FIG. 12A
shows reflection peak spectra of the front SSG-DBR mirror and the rear SSG-DBR mirror formed in the front light reflection area
2
and the rear light reflection area
3
when no current is injected into these areas
2
and
3
.
FIG. 12B
shows the comparison between the reflection peak spectrum of the rear SSG-DBR mirror when current is injected into the rear light reflection area
3
and the reflection peak spectrum of the front SSG-DBR mirror when no current is injected into the front light reflection area
2
. In
FIGS. 12A and 12B
, the abscissa represents the wavelength and the ordinate represents the reflectivity. Further, &lgr;
1
represents the wavelength at which the reflection peaks of the front and rear SSG-DBR mirrors are coincident when no current is injected into each of the front light reflection area
2
and the rear light reflection area
3
, and &lgr;
2
represents the wavelength at which the reflection peaks of the front and rear SSG-DBR mirrors are coincident when current is injected into the rear light reflection area
3
. These reflection peak spectra comprise plural reflection peaks that are different in intensity from each other and have extremely narrow line widths, which is the general characteristic of the SSG-DBR wavelength-tunable semiconductor laser.
As described above, in the initial state when both the front SSG-DBR mirror control current and the rear SSG-DBR mirror control current are equal to zero, the wavelength at which the reflection peaks of the front and rear SSG-DBR mirrors in the front light reflection area
2
and the rear light reflection area
3
respectively are coincident with each other is equal to &lgr;
1
. As a result, the light having the wavelength &lgr;
1
is strongly reflected from the front and rear SSG-DBR mirrors, so that the loss of light beams at the wavelength &lgr;
1
is much smaller than light beams having the other wavelengths. That is, the gain of light at the wavelength &lgr;
1
is relatively increased as compared with the gain of light at the other wavelengths, so that the wavelength-tunable semiconductor laser starts laser oscillation at the wavelength &lgr;
1
. The reason why the reflection peaks of the front and rear SSG-DBR mirrors are coincident with each other at only the wavelength &lgr;
1
and the other reflection peaks at the other wavelengths are not coincident with each other resides in that a minute displacement occurs in the wavelength interval of the reflection peak spectrum between the front and rear mirrors due to the difference in pitch of the diffraction grating which is caused by the difference in distance between the respective periods
21
and
22
and thus the reflection peaks are coincident with each other at

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