1. Field of the Invention
The present invention relates to a semiconductor optical device having an active layer of a quantum well structure, such as an oscillation polarization mode selective semiconductor laser whose oscillation polarization mode can be changed by a direct modulation method with its dynamic wavelength fluctuation being oppressed even at the time of high-speed modulation and an polarization-insensitive optical amplifier which can amplify light substantially irrespective of its polarized condition, a use method of the semiconductor optical device and a method for driving the semiconductor optical device. The present invention also relates to a light source apparatus, an optical communication method and an optical communication system using the optical semiconductor device with an active layer of a quantum well structure.
2. Related Background Art
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 small-amplitude digital signal on a bias injection current (see, for example, Japanese Patent Laid-Open Application No. 7(1995)-162088). The device is a distributed feedback (DFB) laser in which a distributed reflector of a grating is provided in a semiconductor laser resonator or cavity and wavelength selectivity of the grating is utilized. In the device, a bulk active layer is used, or strain is introduced into an active layer of a quantum well structure and 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 its oscillation wavelength, under a current injection condition near its 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 its 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 matching condition between the TE mode and the TM mode (i.e., in which mode is the threshold lower than the other's threshold in a state which satisfies the phase matching condition), the oscillation polarization mode and wavelength of the device can be switched.
In that semiconductor device, an antireflection coating is provided on one end facet to asymmetrically employ effects of the uneven current injection into its output-side portion (a modulation current is not injected into the output-side portion with one electrode so as not to fluctuate an output power) and its modulation-electrode portion (a portion where the other electrode is arranged). Alternatively, lengths of those electrodes are made different from each other to obtain a structural asymmetry.
Further, Japanese Patent Laid-Open Application No. 2-117190 discloses a semiconductor laser apparatus in which two semiconductor structures are arranged serially or in parallel. One of the semiconductor structures principally oscillates or amplifies a light wave in a predetermined polarization mode, and the other one chiefly oscillates or amplifies a light wave in another polarization mode. Those semiconductor structures are formed on a common layer plane or in parallel layer planes.
In the polarization selective DFB laser in which the oscillation polarization mode is selected depending on its above-discussed phase condition (i.e., the oscillation in a polarization mode having a lower threshold is selected at a wavelength which satisfies its phase condition), it is important to approximately equalize gains for the TE mode and the TM mode with each other in its cavity. As an active layer for such a purpose, there have been proposed a method of forming a bulk active layer with about the same TE-mode and TM-mode gains, a method of arranging an active layer with a dominant TM-mode gain obtained by the introduction of strain or the like thereinto in sequence with an active layer for generating a TE-mode gain (see Japanese Patent Laid-Open Application No. 2(1990)-117190).
Those structures, however, have the following drawbacks. In the bulk active layer, though the TE gain can be made equal to the TM gain, its threshold current tends to increase. Therefore, its threshold current needs to be reduced by its quantization or the like.
On the other hand, in the method of Japanese Patent Laid-Open Application No. 2-117190 in which active layers with dominant TE and TM gains are serially or in parallel are arranged, the number of its growth steps is increased and its fabrication process is complicated since active layers with different strains must be formed.
Furthermore, in those active layers, a wavelength band width, over which gains are substantially constant or uniform under a near-threshold condition, is narrow. For example, in a simple quantum well structure, a wavelength range, over which the gain is within a three-dB-down value from its peak and which includes a peak wavelength, is approximately 20 nm. Therefore, considering into account difficulties of the wavelength control near the gain peak wavelength of the active layer and the control of a grating pitch, it is hard to fabricate a low-threshold current laser with a good reproducibility and a stable oscillation wavelength by precisely controlling those gain peak wavelength and grating pitch.
In addition, when considering it necessary that respective wavelength ranges, over which TE-mode and TM-mode gains are substantially uniform and balance with each other, overlap on each other, it is desirable that a wavelength range, over which a gain for each polarization mode is approximately constant, is sufficiently wide. Furthermore, when an array laser consisting of a plurality of lasers with different oscillation wavelengths are to be achieved by using a common active layer, it is necessary that a wavelength range, over which each gain is approximately uniform, be expanded.