Optical communications – Transmitter
Reexamination Certificate
2001-05-15
2004-04-27
Ngo, Hung N. (Department: 2633)
Optical communications
Transmitter
C315S2090SC, C330S288000, C327S109000
Reexamination Certificate
active
06728494
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to light-emitting device drive circuits and optical transmission systems using the circuits and, more specifically, to the circuits for driving a light-emitting device used for an optical transmission circuit and other circuits in an optical communication apparatus, and an optical transmission system using the light-emitting device drive circuit.
2. Description of the Background Art
As being well known, with recent advancement of technology, optical fibers have been able to achieve wide-band, low-loss transmission. Therefore, the optical fibers have come to be applied more to a backbone system for high-speed, large-capacity transmission carried out typically over the Internet. In the future, the optical fibers are expected to be applied further to a backbone-to-home access system, home network, and other networks.
To achieve such next-generation digital home networks, an interface, which is able to transmit a large amount of digital signals at high speed for a long distance with high quality and at low cost, is needed. Among potential protocols of such interface is IEEE 1394, which standardizes digital signals of 100 Mbps, 200 Mbps, 400 Mbps, and other transmission rates. Under IEEE 1394, however, a transmission medium for use is implemented by an electrical cable, which enables transmission only for a short distance of 4.5 m. To make the distance far longer, the transmission should be optically achieved by using an optical fiber, which is not affected by disturbance due to electromagnetic waves, instead of using the electrical cable.
The optical fibers are exemplarily classified into glass optical fibers (hereinafter, GOFs), polymer-clad fibers (hereinafter, PCFs), and plastic optical fibers (hereinafter, POFs), according to the difference of materials. The GOFs are suitable for long-distance transmission over the backbone system, for example, because of their extremely small transmission loss. However, the GOF's core through which an optical wave passes is so small in diameter (10 to 50 &mgr;m) that connectors and other components used in the system have to be made with high accuracy, thereby increasing their manufacturing cost. Moreover, the GOF's core is made of glass, which is inflexible and easy to be broken, and therefore extreme caution is required in handling the GOFs. The PCF's core is no less than 200 &mgr;m in diameter, which is larger than that of the GOFs, but also made of glass as the GOF's core. Therefore, extreme caution is required also in handling the PCFs. On the other hand, the POF's core is approximately 1 mm in diameter, which is extremely larger than those of the other two, and therefore connectors and other components used in the system can be made without requiring high accuracy, thereby reducing their manufacturing cost. Moreover, the POFs are entirely made of plastic material, and therefore they are easy to handle and pose no danger for use at home. Therefore, a POF optical transmission technique based on IEEE 1394 is coming to more attention for realizing an interface of the next-generation digital home network.
The POF's core is generally made of polymethyl methacrylate (hereinafter referred to as PMMA) type material.
FIG. 4
shows transmission loss characteristics of a PMMA-type POF with respect to a wavelength &lgr;. As shown in
FIG. 4
, low transmission loss is observed in optical waves with their wavelength bands ranging from 450 to 540 nm, from 560 to 580 nm, and from 640 to 660 nm. Therefore, for high-speed and long-distance signal transmission, a light source suitable for one of those wavelength bands should be selected. For example, a light source for a wavelength band of 640 to 660 nm is selected. Furthermore, in consideration of cost and eye safety when a user directly views light, the light source for use at home or other purposes is preferably a light-emitting diode (hereinafter, LED) rather than a semiconductor laser diode (LD). For this reason, one potential interface is realized by an optical transmission system using the POF and the LED for the wavelength band within 640 to 660 nm.
However, if the LED is selected as the light source, what is concerned is how fast the response is. More specifically, the LED for 640 to 660 nm has a frequency bandwidth of approximately 100 MHz, and therefore digital signals of 200 or 400 Mbps under IEEE 1394 cannot be transmitted through this LED. Therefore, a method of compensating the LED's bandwidth has been suggested using an electrical circuit.
One example of a conventional light-emitting device drive circuit used in an optical transmission circuit is disclosed in Japanese Patent Laid-Open Publication No. 9-83442 (1997-83442).
FIG. 5
is a schematic diagram showing the structure of the conventional light-emitting device shown in this publication.
FIG. 6
shows an example of a signal waveform at each component of the conventional light-emitting device drive circuit of FIG.
5
. In
FIG. 5
, the conventional light-emitting device drive circuit includes a signal current source
41
for outputting a signal current i
1
corresponding to a transmission signal, a differential current source
42
for outputting a differential current i
2
corresponding thereto, a signal adder
43
, and a light-emitting device
44
.
In general, if the light-emitting device
44
whose bandwidth is insufficient for the transmission signal is driven only by the signal current i
1
having a rectangular waveform shown in (a) of
FIG. 6
, a light output Pout outputted from the light-emitting device
44
has such a waveform as that of the transmission signal with blunt rising and falling edges ((e) of FIG.
6
). Such waveform is caused due to the capacity and internal resistance of the light-emitting device
44
itself. With such structure, transmitting an optical signal at high speed can not be achieved. On the other hand, the differential current i
2
has a differential waveform with its steep peaks appearing at the rising and falling edges of the transmission signal ((b) of FIG.
6
). Therefore, the signal current i
1
having a rectangular waveform and the differential current i
2
having the differential waveform are added together by the signal adder
43
, and an output therefrom is an injection current Iin having a waveform with its steep peaks appearing at the rising and falling edges of the transmission signal. This injection current Iin drives the light-emitting device
44
, and an output therefrom is the light output Pout having a desired band-compensated waveform (rectangular waveform) ((d) of FIG.
6
).
As being evident from (d) of
FIG. 6
showing the waveform of the light output Pout, the light-emitting device
44
constantly emits light even though the digital signal is at the low level (L) (refer to a slanted part in the drawing). Such light emission is unavoidable because predetermined direct current components have to be included in the injection current Iin for preventing waveform distortion that occurs when the peak value at the falling edge of the injection current Iin becomes below zero. If not prevented, this distortion leads to distortion in waveform of the light outputted from the light-emitting device
44
.
Such light emission, however, acts as noise, affecting the transmission characteristics to deteriorate a signal-to-noise (S/N) ratio of the digital signal after transmission.
SUMMARY OF THE INVENTION
Therefore, an object of the present invention is to provide a light-emitting device drive circuit and an optical transmission system using the circuit that ensure good transmission capabilities by compensating the bandwidth and also bringing the low level of the light output waveform down to zero (or approximately zero) to improve the S/N ratio of the digital signal.
The present invention has the following features to attain the object above.
A first aspect of the present invention is directed to a circuit that drives a light-emitting device based on an inputted digital signal, the circuit
Furusawa Satoshi
Morikura Susumu
Numata Kazunori
Matsushita Electric - Industrial Co., Ltd.
Ngo Hung N.
Wenderoth , Lind & Ponack, L.L.P.
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