Coherent light generators – Particular resonant cavity – Distributed feedback
Reexamination Certificate
1999-04-15
2002-06-25
Ip, Paul (Department: 2828)
Coherent light generators
Particular resonant cavity
Distributed feedback
C372S097000
Reexamination Certificate
active
06411640
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor laser device with a phase shift region having a polarization dependency suitably usable as a light source for optical communications or the like, such as a distributed feedback (DFB) semiconductor laser capable of switching a polarization mode of its output light between two polarization modes (typically, transverse electric (TE) mode and transverse magnetic (TM) mode) depending on its driven condition. The present invention also relates to an apparatus or system using the laser device.
2. Related Background Art
In recent years, optical communication and optical information processing have been earnestly studied to cope with a rapid increase in information handling capacity due to development of multimedia and the like. A dynamic-single-mode device with a narrow spectrum has been needed to serve as a light source for those optical communications and for information processing. For such purposes, DFB semiconductor lasers and distributed Bragg reflector (DBR) semiconductor lasers have been developed and studied. Japanese Patent Application Laid-Open No. 2(1990)-159781 (JP '781), for example, discloses a polarization switchable laser which can emit an intensity-modulated signal with a high extinction ratio and can serve the purposes described above. In this device, a pumped condition in its portion is changed to perform the switching of its oscillation polarization mode.
FIG. 1
illustrates the polarization switchable device.
FIG. 1
is a cross-sectional view taken along a laser resonance (cavity-axial) direction of the device. The structure includes a light guide layer
1102
, an active layer
1103
, a clad layer
1104
, and a contact layer
1105
which are laid down over a substrate
1101
of InP. A uniform diffraction grating
1108
is formed at the interface between the light guide layer
1102
and the substrate
1101
. The contact layer
1105
is divided into three portions along the cavity-axial direction. Bias electrodes
1106
a
and
1106
b
and a control electrode
1107
are respectively deposited on the three portions of the contact layer
1105
. The control electrode
1107
is formed on a region for shifting the phase of an equivalent refractive index. A common electrode
1109
is formed on the bottom surface of the substrate
1101
. The control electrode
1107
and the bias electrodes
1106
a
and
1106
b
are electrically separated from each other, so a current can be independently injected through the control electrode
1107
. In the device of
FIG. 1
, the current injected into the phase shift region through the control electrode
1107
is changed or modulated under a condition under which appropriate bias currents are injected through the bias electrodes
1106
a
and
1106
b
. Thus, the equivalent refractive index is partly changed, and the shift amount of the equivalent refractive index is controlled for each of the two different polarization modes. Consequently, the relation between threshold gains for the two modes is changed and polarization switching is performed.
In the above proposal, the polarization modulation system itself is an advantageous one. However, in an ordinary DFB laser, oscillation in the TE mode is dominant over that in the TM mode, so polarization mode contention is difficult. JP '781 discloses no specific solution for this problem.
Further, in a DFB laser lacking built-in phase shift section, oscillation occurs at wavelengths at either or both ends of its stop band rather than at its Bragg wavelength in a single mode, due to adverse influences of fine unevenness in the diffraction grating and the phase at the end facet. In the above proposal, although the pumping condition is partially varied to introduce the phase shift, such phase shift due to the control of current injection is unstable and it is hence hard to achieve stable single-mode oscillation. Furthermore, the current for attaining the single-mode oscillation and the current for achieving the polarization switching vary among individual devices due to influences of the end-facet phase and so forth.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a semiconductor laser, such as a polarization switchable distributed feedback semiconductor laser, which includes a phase shift region with a polarization dependency such that light in a polarization mode influenced by a &lgr;/4 phase shift can be stably oscillated in a single longitudinal mode, an optical transmitter with the laser, and an optical transmission system or method using the laser.
It is a second object of the present invention to provide a semiconductor laser, such as a polarization switchable distributed feedback semiconductor laser, which can suppress unfavorable phenomenon, such as hole burning due to extreme concentration of light on a part and can be fabricated by a simple process without needing a complicated process required for the fabrication of a conventional &lgr;/4 phase shift diffraction grating.
It is a third object of the present invention to provide a semiconductor laser, such as a polarization switchable distributed feedback semiconductor laser, which can effect stable polarization-mode contention and effect a single-mode oscillation in each of the TE mode and the TM mode.
The objects of the present invention are achieved by the following lasers, transmitters and optical communication systems or methods.
A distributed feedback semiconductor laser of the present invention includes a waveguide with an active layer and a diffraction grating, which extends along a cavity-axial direction and is defined such that propagation of light in two different polarization modes is permitted in the waveguide; and a first phase shift region formed in the waveguide. The first phase shift region extends along the cavity-axial direction and has a polarization dependency that an effective refractive index for propagation light of the first phase shift region differs from an effective refractive index for propagation light of a region of the waveguide other than the first phase shift region such that a phase shift of a quarter wavelength of the propagation light is created for one of the two polarization modes and a phase shift of a half wavelength of the propagation light is created for the other of the two polarization modes in the first phase shift region.
The laser of the present invention can be typically constructed to act as a DFB semiconductor laser which can switch or modulate its oscillation polarization mode (in this specification, “switch”, “switchable” and the like are used in a broad sense including a modulation wherein the polarization mode is changed at a relatively high speed), but its structure is not limited thereto. For example, the laser of the present invention can also be constructed as a single-mode tunable semiconductor laser which can change its wavelength while its polarization mode remains unchanged, or a single-mode semiconductor laser which can stably oscillate in one polarization mode in a single mode.
Specifically, the following configurations of three types can be adopted based on the above fundamental structure.
In a first configuration, the laser can oscillate light in two different polarization modes of TE mode and TM mode, the active layer generates a larger gain for the TM mode than for the TE mode, and the first phase shift region creates a phase shift of a quarter wavelength for the propagation light in the TE mode and creates a phase shift of a half wavelength for the propagation light in the TM mode.
In this case, n
TE
, n
TM
and L are preferably determined such that &agr;=(4×L×n
TE
+2×&lgr;
TE
)/(4×L×n
TM
+&lgr;
TM
) is satisfied where n
TE
and n
TM
are effective refractive indices of the region other than the first phase shift region for the TE mode and the TM mode, respectively, &lgr;
TE
and &lgr;
TM
are wavelengths of the propagation light in the TE mode and the TM mode, respectively, n
1TE
an
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Ip Paul
Zahn Jeffrey N
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