Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1999-02-11
2003-05-06
Chan, Jason (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200, C359S199200, C359S199200, C359S199200
Reexamination Certificate
active
06559997
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an optical transmitter, and in particular, to an optical transmitter suitable for use in an optical transmitting system such as one based on PDS (Passive Double Star) technology.
BACKGROUND OF THE INVENTION
FIG. 12
shows a related-art type of optical transmitter. As shown, the optical transmitter is composed of a flip-flop circuit
106
, a modulator
109
, a Laser diode (hereinafter LD), a current source
101
, a photodiode (PD), and an automatic power control circuit
102
.
The flip-flop circuit
106
captures a data signal DT in synchronization with a clock signal CL to output light-on/off signals (of positive phase and negative phase). A transistor Q
1
and a transistor Q
2
of the modulator
109
are supplied at the bases thereof with the positive-phase light-on/off signal and the negative-phase light-on/off signal to perform a differential operation. When the transistor Q
2
is turned on, the laser diode (LD) is supplied with a drive current from the current source
101
to generate an optical pulse signal.
The photodiode (PD) converts part of the optical signal outputted from the Laser diode (LD) into an electrical signal. In order to set the amplitude of this electrical signal to a predetermined level, the automatic power control circuit
102
adjusts the magnitude of the current coming from the current source
101
. Consequently, the optical output power of the Laser diode (LD) is maintained at a constant level.
Details of the above-mentioned optical transmitter are disclosed in Japanese Laid-open Patent No. Hei 6-97889, for example. In an optical transmission system for use in a public communication network, a station-side communication device is connected to plural subscriber-side communication devices by fiber optics. Some such optical transmission systems are based on PDS technology in which the optical fiber of the station-side communication device is coupled with each of the optical fibers of the plural subscriber-side communication devices through a passive optical divider/coupler such as a star coupler. Also, in the above-mentioned optical transmission system, while communication is being performed between the station-side communication device and one of the subscriber-side communication devices, another subscriber-side device may be powered on/off.
However, in the communication device having the above-mentioned related-art optical transmitter, the circuit operation in the communication device may be made unstable by the power on/off operation, causing the Laser diode (LD) to emit the light erroneously. This erroneous light emission is caused when the source voltage drops more than the threshold, thereby causing the flip-flop circuit
106
to output an error light-on signal to the modulator
109
by way of example. Consequently, if the communication device having the above-mentioned related-art type of optical transmitter is used in an optical transmission system based on PDS technology, the optical signal caused by erroneous light emission at a power on/off operation, affects the optical fibers of other communication devices through the optical divider/coupler, thereby interfering with the communication of these devices.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an optical transmitter that does not output an optical signal erroneously at a power on/off operation and an optical transmitting apparatus using such an optical transmitter.
In carrying out the invention and according to one aspect thereof, there is provided an optical transmitter having a current source for outputting a drive current having a magnitude corresponding to a control signal to be inputted, a Laser diode that is driven by the drive current for generating an optical output signal, and a modulator for controlling the supply and cutoff of the drive current to the Laser diode, the optical transmitter comprising: a detecting circuit for monitoring a source voltage to detect whether the source voltage is lower than a predetermined voltage and supplied a power supply potential
11
(Vcc) in common with LD; and a control circuit for stopping, if the source voltage is found lower than the predetermined voltage, the supply of the drive current to the Laser diode.
REFERENCES:
patent: 4757193 (1988-07-01), Kollanyi
patent: 5379145 (1995-01-01), Nodine
patent: 5548435 (1996-08-01), Tahara et al.
patent: 5706112 (1998-01-01), Morita et al.
patent: 6040931 (2000-03-01), Miyazaki et al.
patent: 06097889 (1994-08-01), None
Hamagishi Takahiro
Hasegawa Atsushi
Tokita Shigeru
Chan Jason
Hitachi , Ltd.
Mattingly Stanger & Malur, P.C.
Tran Dzung
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