Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2001-07-18
2003-06-17
Ip, Paul (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S045013, C372S046012, C372S096000, C372S020000
Reexamination Certificate
active
06580740
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to semiconductor laser devices, and more particularly to a semiconductor laser device that selectively absorbs a portion of the light radiated from an active layer of the semiconductor laser device.
2. Discussion of the Background
With the recent demand for increased bandwidth for data communications, optical networks and the components essential for their operation are being closely studied. To provide a light source for such optical networks, semiconductor laser devices such as the distributed feedback (DFB) semiconductor laser device, the distributed Bragg reflector (DBR) semiconductor laser device, and the fiber Bragg grating (FBG) semiconductor laser module, or the like have been used. Each of these devices include a wavelength selecting structure capable of selecting a lasing wavelength independent of the optical gain distribution of the active layer and emitting the selected wavelength from the laser device.
For example,
FIG. 13
shows a cross section of an exemplary distributed feedback semiconductor laser
1300
(hereinafter, referred to as a DFB laser). As seen in this figure, the DFB laser has an active layer
1301
wherein radiative recombination takes place, and a diffraction grating
1303
for changing the real part and/or the imaginary part of the refractive index (complex refractive index) periodically, so that only the light having a specific wavelength is fed back for wavelength selectivity. The diffraction grating
1303
is comprised of a group of periodically spaced parallel rows of grating material
1305
surrounded by a cladding material
1307
(typically made of InP material) to form a compound semiconductor layer that periodically differs in refractive index from the surroundings. In a DFB laser having such a diffraction grating
1303
in the vicinity of its active layer
1301
, the lasing wavelength &lgr;
DFB
which is emitted from the DFB laser is determined by the relation:
&lgr;
DFB
=2
n
eff
&Lgr;,
where &Lgr; is the period of the diffraction grating as shown in
FIG. 13
, and n
eff
is the effective refractive index of the waveguide. Thus, the period &Lgr; of the diffraction grating and the effective refractive index n
eff
of the waveguide can be adjusted to set the lasing wavelength &lgr;
DFB
independent of the peak wavelength of the optical gain of the active layer.
This setting of the lasing wavelength &lgr;
DFB
independent of the peak wavelength of the optical gain of the active layer allows for essential detuning of the DFB laser device. Detuning is the process of setting the emitted lasing wavelength of a laser to a different value than the peak wavelength of the optical gain of the active layer to provide more stable laser operation over temperature changes. As is known in the art, a larger detuning value (that is, a large wavelength difference between the emitted lasing wavelength and the peak wavelength of the optical gain of the active layer) can improve high speed modulation or wide temperature laser performance.
In addition to detuning, the lasing wavelength &lgr;
DFB
may also be set independent of the peak wavelength of the optical gain of the active layer in order to the obtain different characteristics of the semiconductor laser device. For example, when the lasing wavelength of the DFB laser is set at wavelengths shorter than the peak wavelength of the optical gain distribution, the differential gain increases to improve the DFB laser in high-speed modulation characteristics and the like. Where the lasing wavelength of the DFB laser is set approximately equal to the peak wavelength of the optical gain distribution of the active layer, the threshold current of the laser device decreases at room temperature. Still alternatively, setting &lgr;
DFB
at wavelengths longer than the peak wavelength improves operational characteristics of the DFB laser, such as light intensity output and current injection characteristics, at higher temperatures or at a high driving current operation. It is noted, however, that where the lasing wavelength &lgr;
DFB
is set shorter or longer than the peak wavelength of the optical gain distribution of the active layer, undesirable lasing may actually occur at the peak wavelength of the optical gain distribution of the active layer. Thus, the peak wavelength is generally suppressed in order to optimize the emitted light at the lasing wavelength.
The conventional DFB laser such as that disclosed in
FIG. 13
can be broadly divided into a refractive index coupled type laser and a gain coupled type laser. In the refractive index coupled DFB laser, the compound semiconductor layer constituting the diffraction grating has a bandgap energy considerably higher than the bandgap energy of the active layer and the bandgap energy of the lasing wavelength. Thus, bandgap wavelength (which is a wavelength conversion of the bandgap energy) of the diffraction grating is typically at least 100 nm shorter than the lasing wavelength and is usually within the range of 1200 nm-1300 nm if the &lgr;
DFB
is approximately 1550 nm. In the gain coupled DFB laser, the bandgap wavelength of the compound semiconductor layer constituting the diffraction grating is longer than the lasing wavelength and is typically about 1650 nm if the &lgr;
DFB
is approximately 1550 nm.
FIGS. 14
a
and
14
b
show the operational characteristics of an exemplary refractive index coupled laser and gain coupled laser respectively. Each of these figures includes &lgr;e, &lgr;g, &lgr;max, and &lgr;InP shown plotted on an abscissa which shows wavelength increasing from left to right in the figures. In this regard, &lgr;e is the selected lasing wavelength of the DFB laser 1300, &lgr;max is the peak wavelength of the optical gain distribution of the active layer
1301
, &lgr;g is the bandgap wavelength of the diffraction grating material
1305
, and &lgr;InP is the bandgap wavelength of the surrounding InP material
1307
. As seen in
FIGS. 14
a
and
14
b
, the bandgap wavelength &lgr;InP is typically 920 nm and the bandgap wavelength &lgr;g is closely related to the absorption loss of the diffraction grating which is shown by the broken curves
1403
and
1403
′. Moreover, the refractive index of a material increases as the bandgap wavelength of the material increases as shown by the arrows
1405
. Thus, as seen in the figures, the refractive index of the diffraction grating having the bandgap wavelength &lgr;g is generally higher than the refractive index of the surrounding InP layer having the bandgap wavelength &lgr;InP.
FIG. 14
a
shows an exemplary refractive index coupled DFB laser wherein the lasing wavelength &lgr;e which is greater than the bandgap wavelength &lgr;g of the diffraction grating layer. Specifically, the DFB laser has a lasing wavelength &lgr;e of 1550 nm, has a bandgap wavelength &lgr;g of 1250 nm, and satisfies the relationship:
&lgr;g<&lgr;max <&lgr;e.
Thus, the DFB laser of FIG.
14
(
a
) reflects &lgr;e−&lgr;g=300 nm. With the refractive index coupled DFB laser, the absorption loss curve
1403
does not cross the lasing wavelength &lgr;e and therefore absorption loss at &lgr;e is very small. Accordingly, the DFB laser of
FIG. 14
a
, has the advantage of a low threshold current and favorable optical output-injection current characteristics. However, as also shown in
FIG. 14
a
, in a refractive index coupled DFB laser, the absorption loss curve
1403
also does not cross the peak wavelength of the optical gain distribution of the active layer &lgr;max. Therefore, assuming that the absorption coefficient with respect to the lasing wavelength &lgr;e of the DFB laser is &lgr;e and the absorption coefficient with respect to the bandgap wavelength of the active layer, or the peak wavelength &lgr;max of the optical gain distribution of the active layer, is &agr;max, then &agr;e is approximately equal to &agr;max which is approximately equal to zero. This means that the absorption curve
1403
affects nei
Funabashi Masaki
Kasukawa Akihiko
Yatsu Ryosuke
Ip Paul
Nguyen Tuan
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
The Furukawa Electric Co. Ltd.
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