Coherent light generators – Particular component circuitry – Optical pumping
Reexamination Certificate
1999-06-28
2003-09-09
Ip, Paul (Department: 2828)
Coherent light generators
Particular component circuitry
Optical pumping
Reexamination Certificate
active
06618406
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to an optical semiconductor diode driver circuit and optical transceiver modules. In more detail, the invention relates to an optical transmission device used for optical communications or optical data links, and more particularly, to an optical semiconductor diode driver circuit and optical transceiver modules fully operative with a low voltage power supply, under a severe condition that the difference between a forward bias voltage of the diode and a power supply voltage for the driver circuit, required for driving the optical semiconductor diode, is fairly small.
The currently rapid growth multimedia is supported by various network backbones from WAN (wide area network) to LAN (local area network). Among them, optical communication and FDDI (fiber distributed data interface) are typically used, for which high-speed and high-performance optical transmission devices are indispensable. Recently, in particular, not only for high-speed telecommunication but also for the application to data communication between computers, the need for reliable and high-throughput optical interconnection technologies is progressively increasing.
To cope with the requirement, fiber channels and IEEE 1394 have been standardized and several data links with use of optical fibers are going to be developed. As downsizing of computers is also progressing rapidly, PC (personal computers) and/or EWS (engineering workstation) become a standard data processing tool. So, regarding specifications required for interconnection, much importance has been attached to applicability from a practical viewpoint in addition to a high performance in throughput. That is, from the viewpoint of practical application to a system, there is a demand for devices operative with a low power consumption for abbreviation of a cooling system, with the same electrical input/output interface as that of ICs used in a computer system, and preferably with the same power supply voltage. From the viewpoint of the cost effectiveness of the system, there is a strong demand for realization of low-cost optical transceiver modules.
Especially in the IEEE 1394 standard just fixed, for purposes of reducing the link cost by optical fibers and optical transceiver modules, it is presumed that the combination of plastic fibers and a light source of the red wavelength band matched to a low transmission loss through fibers will be rapidly grown. A semiconductor laser diode (LD) oscillating with a wavelength of about 650 nm is used as a light source of the red wavelength band.
However, through investigations by the Inventor, employment of such a red LD has revealed to cause a new problem different from those of optical transmitters in conventional optical telecommunication using infrared LDs with wavelength bands longer than 1.3 um. This problem is as follows.
In case of an optical communication device using the infrared LD, since the photon energy emitted from the LD is 1 eV or less, the applying voltage to the LD under operation is from 1.2 V to 1.5 V at most. However, since the photon energy is inversely proportional to the wavelength and the band gap energy of the semiconductor is proportional to the photon energy, the forward bias voltage of the diode required for operation of the optical semiconductor diode with a red region becomes high compared with the infrared LD. For example, in case of LD oscillating at a wavelength of 650 nm or less, the forward voltage necessary for the LD drive is at least 2 V. Considering it into account of the voltage drop due to an internal resistance increase and/or the need for a large driving current, it must be expected that the forward voltage increases to about 2.5 V maximum.
On the other hand, a usual power supply voltage for operating recent signal processor ICs or memory ICs is being reduced from 5 V to 3.3 V, and also the voltage of 3.3 V or less is expected to be the standard in a near future. Additionally, the finer micro-processes for decreasing the power consumption of signal processor ICs and for getting their processing speed have been developed one after another, and the shift of decreasing the internal operation voltage inside ICs to the order of 2 V or less is also rapid. However, in order to avoid complexities and cost-increasing disadvantages on building the system with mixing interfaces of different voltage levels, it is a general trend that input/output interfaces of ICs are brought into matching with the common logic level of the 3.3 V system, even when the IC internal operation voltage is lower than 3.3 V.
Therefore, it is important to realize a LD driver circuit that can be designed to guarantee a fully functional operation on a single 3.3 V power supply. In other words, it is indispensable to realize a diver IC capable of driving the LD with the forward bias voltage of 2.5 V, maximum, on a power source of 3.3 V. Even if we consider just 5% fluctuation in power supply voltage that is the minimum requirement for any system, the difference between the source voltage and the forward operation voltage of LD is 0.635 V only. If we suppose that a maximum power supply change of 10% is allowed as in popular cases and a voltage drop by wiring resistance inside an IC package is assumed to be small of 47 mV, the voltage difference may decrease by 0.4 V.
FIG. 27
shows a schematic circuit diagram of a conventional LD driver circuit. Since the conventional driver circuits have been usually designed to operate with a source voltage of 5 V or a higher voltage, a sufficient output voltage for a LD operation was ensured, which is much greater than the forward voltage of LD. Then it was easy to share a voltage necessary for a current switching transistor and a control transistor of a constant current source in the LD driver circuit respectively. Therefore, as shown in
FIG. 27
, the stacked configuration of the two sub-circuits, a differential transistor switch circuit and a high-accuracy constant current generating circuit, has been widely employed to ensure high-speed and relatively large switched current, and precise output amplitude in the driver IC.
Here the LD driver circuit shown in
FIG. 27
is illustrated briefly but more concretely. This circuit includes a pre-driver circuit at its input portion and a large-current switch output circuit at its output stage. The pre-driver works as a pulse shaper, which is made up of a differential limiting amplifier and emitter followers. The output circuit is made up of differential transistors for switching on and off the current of the constant current source according to the high and low level of the logic input respectively. And also a constant current source is connected to the collector of the final output transistor for applying a DC bias current to the LD. In order to achieve a properly functional operation of the output stage on this circuit configuration, it is important to keep the total voltage margins of the collector-emitter bias sufficient for non-saturated operation of the output differential switching transistors, and necessary for operating the constant current source connected to their emitters.
FIG. 28
shows voltage-current characteristics of a typical high speed npn transistor. When the base-fixed constant current generating circuit in
FIG. 27
is designed on trial considering such npn transistor characteristics as shown in
FIG. 28
, it has been confirmed that the total voltage necessary for the reference resistance bias and the collector-emitter voltage of the control transistor for the constant current source is more than 0.5 V, even if paying careful attention to minimizing the operation biases. However, to design a 3.3V driver IC with use of the above configuration, even when assuming the aforementioned voltage difference 0.635 V, if it is pile up thereon, the voltage that can be applied to the switch transistor is about 0.1 V only. Then it is impossible to achieve the normal operation of the switch transistors in the linear region.
This problem might be solved by supplement an insuffici
Hogan & Hartson LLP
Ip Paul
Kabushiki Kaisha Toshiba
Zahn Jeffrey N
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