1. Field of the Invention
The present invention relates to an optical transmitter, in particular, an optical transmitter to driver a semiconductor optical device integrating a semiconductor laser diode (hereafter denoted as LD) with a semiconductor optical modulator of a type of the electro-absorption (hereafter denoted as EA) modulator.
2. Related Background Art
Recent optical communication system often applies a semiconductor optical device integrating an LD with an EA modulator to modulate light generated in the LD. An optical transmitter applied in such a system inevitably involves a semiconductor optical device with the LD and the EA modulator, and a driver circuit for driving both the LD and the EA modulator.
Various drivers for the EA modulator have been known. For instance,
The driver circuit disclosed in the U.S. Pat. No. 6,882,667 gives a solution to solve the subject above described, that is, the LD 11 and the EA modulator 12 in the cathode thereof are commonly grounded, the anode of the LD 11 receives the bias current from the positive power supply 6 through the current source 35, and the anode of the EA modulator 12 is negatively biased by the bias circuit 36 through the inductor 33. Further, the driver 20, which has two outputs showing the differential mode and each pulled up to the power supply 7 for the driver 20, drives the EA modulator 12 by the AC mode through the capacitor 42.
However, the arrangement shown in
An aspect of the present invention relates to an optical transmitter comprising a semiconductor optical device and a driver to drive the semiconductor optical device. The semiconductor optical device integrates the LD with the EA modulator on a single semiconductor substrate by an arrangement where the back surface of the substrate provides a cathode electrode common to the LD and the
EA modulator. The driver has the differential arrangement with two outputs complementary to each other to drive the EA modulator in the differential mode. One of the outputs is coupled with the anode of the EA modulator though a capacitor, while, the other outputs is coupled with the common cathode of the EA modulator through another capacitor.
In the arrangement of the optical transmitter, the anode of the LD may be positively biased, while, the anode of the EA modulator may be negatively biased each through an inductor. Because the EA modulator may be driven in the differential mode, the amplitude of the modulation signal output from the outputs of the driver may be half of the conventional arrangement of the single phase driving. Moreover, because the LD and the EA modulator may be oppositely biased with respect to the common cathode, the power supply for the LD and that for the driver may lower the voltage thereof, which may make the conventional active device, namely, the silicon based device such as Si-CMOS, SiGe HBT, and so on, may be usable in the driver instead of the compound semiconductor based active device, which may reduce the cost of the optical transmitter.
The foregoing and other purposes, aspects and advantages will be better understood from the following detailed description of a preferred embodiment of the invention with reference to the drawings, in which:
Next, some preferred embodiments according to the present invention will be described as referring to accompany drawings. In the description of the drawings, the same numerals or symbols will refer to the same elements without overlapping explanations.
The semiconductor optical device 10 may integrate the LD 11 with the EA modulator 12 on the single semiconductor substrate. The bottom surface of the semiconductor substrate provides a cathode common to the LD 11 and the EA modulator 12, and this common cathode is grounded through the inductor 31. The LD 11 and the EA modulator 12 are oppositely biased with respect to the ground 5. That is, the anode of the LD 11 is coupled with the positive power supply 6 through a series circuit of the current source 35 and the inductor 32 to receive the bias current. While, the anode of the EA modulator 12 is negatively biased by the power supply 36 through the inductor 33. A termination resistor 13 for the driver 20 is connected between the anode and cathode of the EA modulator 12 in the optical device 10.
The driver 20 may drive the EA modulator 12. The driver 20 may have an arrangement of the differential circuit to drive the EA modulator 12 in the differential mode. Specifically, one of outputs of the driver 20 is coupled with the anode of the EA modulator 12 through the capacitor 41 as the first capacitor, while, the other output of the driver 20 is coupled with the cathode of the EA modulator 12 through the other capacitor 42 as the second capacitor. Thus, the EA modulator 12 may be driven in the differential mode.
The optical transmitter 1 shown in
EA modulator 12 is driven in the AC mode. Moreover, the power supply 7 for the driver 20 may be lowered to 3.3V because the driver 20 couples with the EA modulator 12 in the AC mode . Thus, the DC-DC converter, which is not explicitly shown in
Moreover, the driver 20 shown in
In the conventional optical transmitter, active devices made of GaAs, InP, and so on are necessary to obtain such large signals at high frequencies. However, active devices made of such compound semiconductor materials generally show cost ineffective. The arrangement of the differential driving according to the present embodiment makes the implementation of silicon-based devices, such as Si, SiGe and so on, possible to reduce the cost of the optical transmitter.
The differential arrangement of the driver 20 may also omit the pull-up elements, 43 and 44, shown in
The optical device 10A further provides a capacitor 14 as the third capacitor between the anode and cathode of the LD 11 in addition to the arrangement of the optical device 10. The optical transmitter 1A of the second embodiment has advantages substantially same with those of the first embodiment. Moreover, the optical transmitter 1A of the present embodiment may bypass high frequency components leaked from the modulation signal applied to the EA modulator 12 from the driver 20, which may suppress the LD 11 from being modulated by the modulation signal leaked through the cathode common to the LD 11 and the EA modulator 12. Thus, the degradation of the transmission quality of the optical transmitter 1A may be prevented.
The mechanism to suppress the degradation of the transmission quality described above is further explained as referring to
Referring to
On the other hand, the capacitor 14 connected between the anode and the cathode of the LD 11 may operate as a bypassing capacitor for the LD 11 which may suppress the LD 11 from being modulated by the modulation signal. In this arrangement, the amplitude of the modulation signal applied to the EA modulator 12 becomes half of those of the conventional arrangement, namely, about 1 Vp-p.
The common cathode of the LD 11 and the EA modulator is coupled with the anode of the LD 11 through a capacitor 37 as the fourth capacitor with large capacitance and grounded through the inductor 31. That is, the capacitor 37 may be regarded as a short circuit for high frequencies and one of the output signals of the driver 20 is provided to the common cathode of the LD 11 and the EA modulator 12 through the capacitor 42 and the other capacitor 37. Other arrangements are substantially same with those of the optical transmitted 1A of the former embodiment.
The optical transmitter 1B drives the EA modulator 12 in the differential mode by the driver 20, which may make the amplitude of the modulation signal in half of those provided in the conventional arrangement where the EA modulator 12 is driven by the single phase signal. The optical transmitter 1B may lower the positive power supply for the driver 20, which may not only reduce the power consumption of the driver 20 but also widen a type of active devices for the driver 20 to Si, SiGe and so on.
The optical device 10B also provides the capacitor 14 connected in parallel to the LD 11 within the optical device 10A to bypass the modulation signal, which may suppress the degradation of the transmission quality of the optical transmitter 1B.
Because the optical transmitter 1B applies one of the modulation signal directly to the anode of the LD 11, which may possibly drive the LD 11 in the AC mode. However, the capacitor 37 with the large capacitance may bypass the modulation signal and operate the LD 11 in substantially DC mode.
In the foregoing detailed description, the method and apparatus of the present invention have been described with reference to specific exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the present invention. The present specification and figures are accordingly to be regarded as illustrative rather than restrictive.
Number | Date | Country | Kind |
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2011-008147 | Jan 2011 | JP | national |