Semiconductor laser device

Details Semiconductor laser device Semiconductor laser device

2002-02-14

2003-11-04

Davie, James (Department: 2828)

Coherent light generators

Particular active media

Semiconductor

C372S096000

Reexamination Certificate

active

06643309

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a variable wavelength semiconductor laser which is required mainly for wavelength multiplex optical fiber communication, particularly to a variable wavelength semiconductor laser of multiple-electrode DBR structure wherein distributed Bragg reflectors (DBR) made of a semiconductor are disposed before and after an active region and a phase control region, and a control electrode is provided in each region.
BACKGROUND ART
An example of a variable wavelength semiconductor laser having a multiple-electrode DBR structure, specifically a sampled grating DBR laser which uses a sampled grating for the diffraction grating portion, will be described below.
FIG. 11
is a schematic sectional view of a device having the constitution of sampled grating DBR
20
laser of the prior art described by V. Jayaraman et al. in IEEE J. Quantum Electronics, vol. 29, No. 6, 1993, pp 1824-1834.
In
FIG. 11
, reference numeral
1
denotes a forward light reflection region (also referred as forward mirror),
2
denotes a backward light reflection region (also referred as backward mirror),
3
denotes an active region,
4
denotes a phase control region,
5
denotes an n-type electrode,
6
denotes an n-type InP lower cladding layer.
7
denotes a p-type InP upper cladding layer,
8
denotes a p-type InGaAsP contact layer,
9
and
10
denote pitch modulation periods of the forward mirror and the backward mirror, respectively,
11
denotes a laser beam emitted from the forward end face of a laser resonator,
12
denotes an InGaAsP optical waveguide layer,
13
denotes a diffraction grating portion,
14
denotes discontinuity in the diffraction grating, and
15
denotes a p-type electrode.
The forward light reflection region
1
and the backward light reflection region
2
have sampled grating DBR mirrors formed on the InGaAsP optical waveguide layers on the forward end face side and the backward end face side of the laser resonator, respectively. Combined length of one diffraction grating portion
13
and one non-diffracting portion
14
(portion without diffraction grating formed therein) is called the pitch modulation period. In
FIG. 11
, the pitch modulation period of the forward mirror is indicated by reference numeral
9
and the pitch modulation period of the backward mirror is indicated by reference numeral
10
. Each of the forward mirror and the backward mirror has a plurality of pitch modulation periods, while the pitch modulation period of the forward mirror and the pitch modulation period of the backward mirror are set to different values in general.
An optical waveguide of a laser oscillator has an InGaAsP layer sandwiched by the lower cladding layer
6
and the upper cladding layer
7
which have a forbidden band gap greater than that of the InGaAsP layer, while the backward light reflection region
2
, the active region
3
, a phase control region
4
, and the forward light reflection region
1
are formed on the InGaAsP layer.
The active region
3
is constituted from an n-type InGaAsP strained quantum well which has a forbidden band gap smaller, on average, than the forbidden band gap of the InGaAsP optical waveguide layer
12
that constitutes the forward light reflection region
1
and the backward light reflection region
2
, and the phase control region
4
comprises an InGaAsP optical waveguide layer having the same composition as that of the forward light reflection region
1
and the backward light reflection region
2
.
Now the operation of the sampled grating DBR (SSG-DBR) laser of the prior art shown in
FIG. 11
will be described below.
As shown in
FIG. 11
, the p-type electrodes
15
are formed separately on the active region
3
, the forward light reflection region
1
, the backward light reflection region
2
and the phase control region
4
. When a forward bias voltage is applied between the p-type electrodes
15
which are formed separately on the active region
3
and the n-type electrode
5
, current flows into the active region
3
so that spontaneous emission of light ranging over a broad band of wavelengths takes place in the active region
3
. The emitted light propagates through the optical waveguide formed in the optical resonator, and light of a particular wavelength is reflected on a forward sampled grating DBR mirror formed in the forward light reflection region
1
and a backward sampled grating DBR mirror formed in the backward light reflection region
2
repetitively and is amplified in the active region, thus achieving laser oscillation.
In the sampled grating DBR laser shown in
FIG. 11
, supplying a current to the forward light reflection region
1
or the backward light reflection region
2
and the phase control region
4
results in a wavelength to be selected so that, at the single wavelength selected according to the current supplied, laser oscillation occurs.
A process of controlling the oscillation wavelength of the laser shown in
FIG. 11
will now be described in detail.
FIG. 13
shows a reflectivity spectrum
16
of the forward mirror
1
and a reflectivity spectrum
17
of the backward mirror
2
in case current is not supplied in the forward and backward light reflection regions
1
,
2
. FIG.
14
shows a reflectivity spectrum
18
of the backward mirror
2
in case current is supplied only to the backward light reflection region
2
and the reflectivity spectrum
17
of the forward mirror
1
without current supply in comparison.
In FIG.
13
and
FIG. 14
, wavelength is plotted along the abscissa and power reflectivity is plotted along the ordinate. &lgr;
1
is a wavelength at which the peaks of reflectivity of the forward and backward mirrors coincide in case current is not supplied to the forward and backward light reflecting regions
1
,
2
, and &lgr;
2
is a wavelength at which the peaks of reflectivity of the forward and backward mirrors coincide in case current is supplied only to the backward light reflecting region
2
.
The reflection spectrum of the sampled grating DBR consists of a plurality of sharp peaks of reflectivity which are different in height, although all peaks of reflectivity are shown in the diagram to have equal height since it is the sole object to show the wavelengths in FIG.
13
and FIG.
14
.
As described previously,
FIG. 13
shows the reflectivity spectrum in the initial state when the forward mirror control current and the backward mirror control current are both zero. In this case, the reflectivity spectra of the forward and the backward mirrors coincide at wavelength &lgr;
1
. As a result, power loss of light becomes extremely smaller at the wavelength &lgr;
1
compared to the other wavelengths, which means relatively high gain of light at the wavelength &lgr;
1
and therefore the laser oscillates at the wavelength &lgr;
1
.
The oscillation wavelength of the laser can be changed by applying a forward bias voltage to one or both of the forward light reflection region
1
and the backward light reflection region
2
to supply current to the region, equivalently changing the refraction indices of the light reflection regions
1
and
2
by way of free carrier plasma effect.
As shown in
FIG. 14
, for example, in case current is supplied only to the backward light reflection region
2
, the reflectivity spectrum
18
of the backward mirror shifts to a shorter wavelength due to a decrease in the refraction index of the backward mirror, and the wavelength at which the reflectivity spectra of the forward and backward mirrors coincide changes to &lgr;
2
. As a result, light of wavelength &lgr;
2
propagates in the laser resonator and is amplified so that laser oscillation eventually occurs. The oscillation wavelength can be changed similarly in case current is supplied only to the forward light reflection region
1
and in case current is supplied to both the forward light reflection region
1
and the backward light reflection region
2
. Thus oscillation wavelength of the laser can be changed freely by supplying current to the light reflection regions where the

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