Coherent light generators – Particular beam control device – Modulation
Reexamination Certificate
1998-12-01
2002-01-08
Leung, Quyen (Department: 2881)
Coherent light generators
Particular beam control device
Modulation
C372S096000, C372S050121, C372S038020
Reexamination Certificate
active
06337868
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a distributed feedback (DFB) semiconductor laser which can change its polarization mode of output light according to its stimulated condition and can be used as a signal light source for optical transmission and the like, and to a driving method of the DFB semiconductor laser.
2. Related Background Art
In a conventional oscillation polarization mode selective DFB semiconductor laser as disclosed in Japanese Patent Application Laid-Open No. 7(1995)-162088, for example, the relation between the wavelength dependency of gains created in its active layer and Bragg wavelengths determined by the pitch and the like of its diffraction grating is controlled, and a multi-electrode structure is adopted for that purpose.
As an oscillation polarization mode selective dynamic single mode semiconductor laser, the following device has been developed and proposed. The oscillation polarization mode selective device has a structure that can be modulated by a digital signal which is produced by superposing a minute-amplitude digital signal on a bias injection current. The device is a DFB semiconductor laser in which a distributed reflector of a grating is introduced into a semiconductor laser resonator or cavity and wavelength selectivity of the grating is utilized. In the device, strain is introduced into an active layer of a quantum well structure, or its Bragg wavelength is located at a position shorter than a peak wavelength of a gain spectrum, so that gains for transverse electric (TE) mode and transverse magnetic (TM) mode are approximately equal to each other for light at wavelengths close to an oscillation wavelength, under a current injection condition near an oscillation threshold. Further, a plurality of electrodes are arranged and currents are unevenly injected through those electrodes. An equivalent refractive index of the cavity is unevenly distributed by the uneven current injection, and oscillation occurs in one of the TE mode and the TM mode and at a wavelength which satisfies a phase matching condition and takes a minimum threshold gain. When the balance of the uneven current injection is slightly changed to vary a competitive relation of the phase condition between the TE mode and the TM mode, the oscillation polarization mode and wavelength of the device can be switched.
In that semiconductor device, an anti reflection coating is provided on one end facet to asymmetrically employ effects of the uneven current injection into its output-side portion and its modulation-electrode portion. Alternatively, lengths of the electrodes are made different from each other to introduce a structural asymmetry.
Further, Japanese Patent Application Laid-Open No. 2(1990)-117190 discloses a semiconductor laser apparatus which has two serially- or parallel-arranged semiconductor devices that primarily generate or amplify light waves in a predetermined polarization mode and another polarization mode, respectively.
However, the above conventional oscillation polarization mode selective DFB semiconductor laser, which selects the lapsing polarization mode depending on the phase condition, is sensitive to the phase at the end facet. As a result, the lapsing wavelength and polarization mode of the device depend on the current injection condition in a complicated fashion, and fluctuation in characteristics of the lapsing polarization mode appears among individual devices. In particular, regarding the fluctuation among devices, it is difficult to selectively impart a gain to one of the competing polarization modes when a current injected into one of two waveguide portions is increased, so that a lapsing polarization switching point varies among the devices.
Further, in the conventional laser, the phase relations of resonant light in the cavity for respective polarization modes are controlled by the balance of currents injected into a uniform structure extending in the cavity direction (a uniform active layer and a uniform diffraction grating), and the lapsing polarization mode is thus switched by changing the polarization mode whose oscillation threshold gain is the smallest. Therefore, the conventional laser suffer the following disadvantages.
(1) Since the light phase is controlled, the lapsing polarization mode is again returned to the TE mode, for example, when the switching from the TE mode to the TM mode is conducted by increasing a current injected into a certain region and thereafter this current is further increased.
(2) Due to the multi-electrode structure, a common polarization mode is not selected among plural regions when currents injected into those regions increase. The respective regions are thus brought into independent lapsing conditions, and those conditions influence each other to cause a plurality of longitudinal modes.
On the other hand, in the above another conventional DFB semiconductor laser, the light wave in the predetermined polarization mode is generated or amplified by the choice of its geometrical shape, so that its yield varies due to processing fluctuations of etching depth and edge width during its ridge fabrication process.
An object of the present invention is to provide a distributed feedback semiconductor laser which can simplify its stimulated condition (typically, its current injection condition) for causing the switching of its oscillation or lapsing polarization mode, a method for driving the semiconductor laser, a light source apparatus which can perform the modulation with a large extinction ratio using the semiconductor laser, an optical transmission method using the semiconductor laser, an opto-electric converting apparatus suitably usable in the optical transmission method, an optical transmission system using the opto-electric converting apparatus, and so forth.
SUMMARY OF THE INVENTION
A distributed feedback semiconductor laser of the present invention includes a cavity extending in a cavity-axial direction and including a plurality of regions, and a plurality of waveguides with a diffraction grating and an active layer extending along the cavity-axial direction. The waveguides are formed in the regions, respectively, coupled to each other along the cavity-axial direction, and define different first and second polarization modes (typically, TE mode and TM mode). The semiconductor laser further includes a light intensity distribution control portion formed in the cavity and having a function to relatively and locally strengthen light intensity distributions for the first and second polarization modes in the cavity-axial direction with a polarization dependency, and a control unit (typically, a current injection unit) for independently stimulating at least two of the regions.
The following specific structures are possible based on the above fundamental structure.
The light intensity distribution control portion imparts a larger action, which relatively strengthens a light intensity distribution at a place of the light intensity distribution control portion in the cavity, to one of the first and second polarization modes, as compared to the other of the first and second polarization modes. More specifically, the light intensity distribution control portion imparts substantially no action to the other of the first and second polarization modes.
The light intensity distribution control portion may comprise a phase shift section for shifting a phase of a periodical change in a refractive index for one of the first and second polarization modes due to the diffraction grating while not shifting a phase of a periodical change in a refractive index for the other of the first and second polarization modes due to the diffraction grating. Further, the phase shift section may shift the phase of the periodical change in the refractive index for one of the first and second polarization modes by 180 degrees. The phase shift portion shifts the phase for one polarization mode such that its light intensity is strengthened at a place of the shift section, while shifting the phase for the ot
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Leung Quyen
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