1. Field of the Invention
The present invention relates to an optical transmitter with an external modulator, an optical transceiver including the optical transmitter with the external modulator, and methods of driving the same, and more particularly, to an optical transmitter with an electro-absorption optical modulator serving as an external modulator, an optical transceiver including the optical transmitter with the electro-absorption optical modulator, and methods of driving the same.
2. Description of the Related Art
In optical communications, an optical transceiver is a key device serving as an interface between an electric signal and an optical signal. The optical transceiver includes an optical transmitter performing electrical-optical conversion and an optical receiver performing optical-electrical conversion. In order to drive and control the optical transmitter and the optical receiver, the optical transceiver generally further includes a microcomputer, a driver circuit, a power supply circuit, and a temperature adjusting circuit.
In recent years, it has been requested that a transmission distance and a communication capacity are increased with the rapid expansion of fields to which optical communications are applied. Therefore, in order to satisfy the requirements of chromatic dispersion and spectral accuracy on long-haul transmission, dense wavelength division multiplexing (DWDM) transmission, or the like, an external modulating method is used for many structures of the optical transmitter installed in the optical transceiver. An electro-absorption optical modulator (EAM) which is easily integrally formed with a semiconductor laser has been known as one of means for realizing the external modulating method. An optical transmitter including the EAM is disclosed in Japanese Patent Application Laid-open No. Hei 09-179079 (patent family U.S. Pat. No. 5,917,637). A modulator integrated laser diode (MI-LD) serving as the optical transmitter includes a laser diode (LD) having an active layer emitting light and an EAM having an absorption layer absorbing the light to modulate the light. The LD and the EAM are integrally formed with each other. In the MI-LD, the active layer of the LD is directly connected with the absorption layer of the EAM, so the light emitted from the active layer of the LD is efficiently introduced into the absorption layer of the EAM. Thus, the MI-LD has an advantage in that a high-efficiency optical transmitter with an external modulator is obtained. The MI-LD can be formed using a single semiconductor chip, so the MI-LD has an advantage in that a reduction in size can be achieved by integration.
However, in order to drive the MI-LD described in Japanese Patent Application Laid-open No. Hei 09-179079, it is necessary to apply a forward bias voltage to LD side and to apply a backward bias voltage to the EAM side. That is, it is necessary to apply a positive applied voltage to an LD side electrode relative to a grounded common electrode for the LD and the EAM and to apply a negative applied voltage to an EAM side electrode relative thereto. Therefore, in order to drive the MI-LD, two different power supplies, namely, positive and negative power supplies are required. Furthermore, the use of the negative voltage power supply may cause generation of power supply noise and deterioration in characteristic of the optical transceiver which are caused by the negative voltage power supply. It is necessary to draw power supply wirings for the negative voltage power supply and to use parts for the negative voltage power supply. Thus, the MI-LD has a fear of reducing the general versatility in a case where the optical transmitter is installed in an apparatus or in use environments.