Method of driving a semiconductor optical functional device

Abstract: A semiconductor optical functional device is divided into two regions of a first region 1 and a second region 2 adjacent to each other in a longitudinal direction of a semiconductor optical waveguide. The first region 1 is provided with a region including an MQW structure in which a compressive strain is introduced, and the second region 2 is provided with a region including an MQW structure in which a tensile strain is introduced. Electrodes 3 and 4 formed separately and independently from each other are respectively disposed on the first region 1 and the second region 2, and bias voltages applied to the electrodes 3 and 4 are adjusted so that transmissivities for light having a TE mode component and light having a TM mode component are independently controlled.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor optical functional device, and particularly to an optical functional device which is used for an optical fiber communication system and is capable of performing mode filtering with respect to a TE mode and a TM mode which are polarization modes in which light orthogonally intersects. 2. Description of the Related Art Up to now, a semiconductor laser has been often used as a light source in a ultra high speed and long distance optical fiber communication system. However, in a conventional semiconductor laser, there is a fear that a noise occurs in the inside of the semiconductor laser due to minute reflected returning light from an optical fiber so that a transmission code error occurs. Thus, in an optical fiber communication system using a semiconductor laser, an optical isolator is generally employed to remove the light returning to the semiconductor laser. In the above described conventional communication system, the optical isolator is used to remove the returning light to the semiconductor laser. However, the optical isolator requires a number of steps for processing and assembling materials, and is not only expensive but also poor in reliability and stability. Thus, when the optical isolator is used for an optical fiber communication system, there arises a problem that the reliability and stability of the whole system are damaged. In EP469681 a semiconductor optical device is disclosed having a first region and a second region adjacent to the first region in a longitudinal direction of a semiconductor optical waveguide, wherein the first region includes an MQW region in which a compressive strain is introduced into the well and the second region includes a MQW in which a tensile strain is introduced into a well. SUMMARY OF THE INVENTION The present invention is as presented in claims 1 and 2, and has been made in view of the above described problem possessed by the conventional technique, and an object thereof is therefore to provide a semiconductor optical functional device which is capable of removing returning light to a semiconductor laser without using an optical isolator. In order to achieve the above object, according to the present invention, a semiconductor optical functional device is constituted by two regions of a first region and a second region adjacent to each other in a longitudinal direction of a semiconductor optical waveguide, and is characterized in that the first region has an MQW (Multiple Quantum Well) structure in which a compressive strain is introduced to a well, and the second region has an MQW structure in which a tensile strain is introduced to a well. Also, according to the present invention, in the method of driving a semiconductor optical functional device of the invention, the first region and the second region are provided with electrodes formed separately and independently from each other, and the first region and the second region perform an amplifying operation when a forward bias voltage is applied to the electrodes, and perform an absorbing operation when a reverse bias voltage is applied to the electrodes. Also, according to the present invention, a method of driving the semiconductor optical functional device of the second aspect is characterized in that a reverse bias voltage is applied to the electrode disposed on the first region; and a forward bias voltage is applied to the electrode disposed on the second region, the forward bias voltage being so small that laser oscillation does not occur in the second region, whereby when light having a TE mode component and a TM mode component is inputted into the first region, the semiconductor optical functional device is driven as a semiconductor integrated polarization mode filter which makes only the light having the TM mode component pass through and outputs the passed light. Also, according to the present invention, a method of driving the semiconductor optical functional device of the second aspect is characterized in that a forward bias voltage is applied to the electrode disposed on the first region, the forward bias voltage being so small that laser oscillation does not occur in the first region; and a reverse bias voltage is applied to the electrode disposed on the second region, whereby when light having a TE mode component and a TM mode component is inputted into the first region, the semiconductor optical functional device is driven as a semiconductor integrated polarization mode filter which makes only the light having the TE mode component pass through and outputs the passed light. BRIEF DESCRIPTION OF THE DRAWINGS The above and other objects, advantages and features of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawing in which: Figs. 1A and 1B are views for explaining an operation principle in the case where a semiconductor optical functional device of the present invention is activated as a semiconductor integrated polarization mode filter, Fig. 1A being a sectional view showing the structure thereof, and Fig. 1B being a view showing the distribution of electric field in the respective TE mode and TM mode. Figs. 2A and 2B are views showing an embodiment in which a semiconductor optical functional device of the present invention is used as an integrated polarization mode filter, Fig. 2A being a view showing an example in which when light of a TE mode component and a TM mode component is inputted, only light of the TM mode component is emitted, and Fig. 2B being a view showing an example in which when light of a TE mode component and a TM mode component is inputted, only light of the TE mode component is emitted. DETAILED DESCRIPTION OF THE INVENTION Operation principles of a semiconductor optical functional device according to the present invention will be hereinafter described. As shown in Fig. 1A, the structure of the semiconductor integrated polarization mode FILTER is the same as that of a semiconductor laser including a multiple oscillator structure. That is, an oscillator using a normal laser is divided into two regions of a first region and a second region adjacent to each other in a longitudinal direction of a semiconductor optical waveguide. The first region 1 is provided with a region having an MQW structure in which a compressive strain is introduced into a well, and the second region 2 is provided with a region having an MQW structure in which a tensile strain is introduced into a well. Further, electrodes 3 and 4 formed separately and independently from each other are disposed on the first region 1 and the second region 2, respectively. If the compressive strain or the tensile strain is introduced into a quantum well, the band structure of the well is changed. Here, with respect to the compressive strain MQW, since the first level of a valence band thereof is an HH band, the region performs an amplifying or absorbing operation with respect to light of the TE mode. On the other hand, in the tensile strain MQW, since the first level of a valence band thereof is an LH band, the region performs an amplifying or absorbing operation with respect to light of the TM mode. Whether the amplifying operation is performed or absorbing operation is performed, is controlled by a voltage applied to the electrodes provided on the respective regions. When the voltage applied to the electrode is a forward bias voltage, the amplifying operation is performed, and when the applied voltage is a reverse bias voltage, the absorbing operation is performed. Next, an operation principle as a semiconductor integrated polarization mode filter will be described. Figs. 1A and 1B are views for explaining the operation principle in the case where a semiconductor optical functional device of the present invention is activated as the semiconductor integrated polarization mode filter, Fig. 1A being a sectional view showing the structure thereof, and Fig. 1B being a view showing the distribution of electric field in the respective TE mode and TM mode. For example, as shown in Fig. 1A, in the case where a reverse bias voltage is applied to the first region 1 having the compressive strain MQW structure, a forward bias voltage, which is so small that laser oscillation is not caused, is applied to the second region 2 having the tensile strain MQW structure, and light having two mode components of TE and TM is inputted into the first region 1, as shown in Fig. 1B, in the TE mode component and the TM mode component of the incident light, the TE mode component is absorbed in the region having the compressive strain MQW structure in the first region 1, and only light having the TM mode component is emitted from the region having the tensile strain MQW structure in the second region 2. Also, in the case where a forward bias voltage, which is so small that laser oscillation is not caused, is applied to the first region 1 having the compressive strain MQW structure, a reverse bias voltage is applied to the second region 2 having the tensile strain MQW structure, and light having two mode components of TE and TM is inputted into the first region 1, in the TE mode component and the TM mode component of the incident Figs. 2A and 2B are views showing an embodiment in which a semiconductor optical functional device of the present invention 2 is used as an integrated polarization mode filter. Fig. 2A is a view showing an example in which when light of the TE mode component and TM mode component is inputted, only light of the TM mode component is emitted, and Fig. 2B is a view showing an example in which when light of the TE mode component and TM mode component is inputted, only light of the TE mode component is emitted. In order to activate the semiconductor optical functional device as the semiconductor integrated polarization mode filter, as shown in Figs. 2A and 2B, AR films 23 are deposited to the structure shown in Figs. 2A and 2B, and the value of forward bias voltage applied to the first region 11 or the second region 12 is made so small that laser oscillation does not occur in the region where the forward bias voltage is applied. According to this, in the case where incident light has two modes of TE and TM, by selecting the way of applying bias voltages shown in Fig. 2A or 2B, the TE mode component or TM mode component of the incident light is selectively cut off, and only light of the remaining mode is amplified and is made to pass through. Accordingly, the semiconductor device functions as the semiconductor integrated polarization mode filter. According to the present invention, bias voltages different from each other are applied to the electrodes disposed on the first region and the second region, and in the application of the forward bias voltage, the bias voltage is made so small that laser oscillation does not occur in the region to which the bias voltage is applied, so that when light having the TE mode component and TM mode component is inputted, the semiconductor optical functional device can be driven as the semiconductor integrated polarization mode filter in which only light of either one of the components is passed through and is outputted. As a result, when the semiconductor optical functional device is used for an optical fiber communication system, minute returning light from an optical fiber is absorbed, so that a fear of causing a transmission code error can be eliminated. While the present invention has been described in connection with certain preferred embodiments, it is to be understood that the subject matter encompassed by way of the present invention is not to be limited to those specific embodiments. On the contrary it is intended for the subject mattter of the invention to include all alternatives, modifications and equivalents as can be included within the scope of the following claims.

Claims: A method of driving a semiconductor optical functional device having a first region (1) including an MQW structure in which a compressive strain is introduced into a well thereof, and a second region (2) adjacent to the first region in a longitudinal direction of a semiconductor optical waveguide and including an MQW structure in which a tensile strain is introduced into a well thereof, said first region and second region including electrodes formed separately and independently from each other, respectively, said first region and second region perform an amplifying operation when a forward bias voltage is applied to the electrode, and perform an absorbing operation when a reverse bias voltage is applied to the electrode, comprising: applying a reverse bias voltage to the electrode disposed on the first region; and applying a forward bias voltage to the electrode disposed on the second region, the forward bias voltage being less than a value required to support lasing in the second region, whereby when light having a TE mode component and a TM mode component is inputted into the first region, the semiconductor optical functional device is driven as a semiconductor integrated polarization mode filter in which light having the TM mode component is passed through and is outputted. A method of driving a semiconductor optical functional device having a first region including an MQW structure in which a compressive strain is introduced into a well thereof, and a second region adjacent to the first region in a longitudinal direction of a semiconductor optical waveguide and including an MQW structure in which a tensile strain is introduced into a well thereof, said first region and second region including electrodes formed separately and independently from each other, respectively, said first region and second region perform an amplifying operation when a forward bias voltage is applied to the electrode, and perform an absorbing operation when a reverse bias voltage is applied to the electrode, comprising: applying a forward bias voltage to the electrode disposed on the first region, the forward bias voltage being less than a value required to support lasing in the first region; and applying a reverse bias voltage to the electrode disposed on the second region, whereby when light having a TE mode component and a TM mode component is inputted into the first region, the semiconductor optical functional device is driven as a semiconductor integrated polarization mode filter in which light having the TE mode component is passed through and is outputted.

Inventors: HUANG YIDONG, HUANG, YIDONG
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1997
Main classification: H01S5
IPC codes: H01S3, G02B6, H01S5, G02F1