Optical: systems and elements – Optical modulator
Reexamination Certificate
2002-07-11
2004-05-25
Sugarman, Scott J. (Department: 2873)
Optical: systems and elements
Optical modulator
C359S238000, C372S029010
Reexamination Certificate
active
06741375
ABSTRACT:
BACKGROUND OF THE INVENTION
This application claims benefit of Japanese Patent Application No. 2001-211787 filed on Jul. 12, 2001, the contents of which are incorporated by the reference.
The present invention relates to optical modulator drive circuits and, more particularly, to optical modulator drive circuits for driving optical modulator such as laser diodes.
A laser beam emitted from a laser diode is used for such purposes as modulation of optical signal in optical fiber communication and reading of signal from a CD (Compact Disc) and DVD (Digital Versatile Disc). A laser diode drive circuit is used for generating a desired fixed laser beam for such a laser diode.
Prior art examples of such laser diode drive circuit is disclosed in, for instance, a Japanese Patent Laid-Open No. 3-73917 entitled “Optical Modulator Drive Circuit”.
FIG. 4
shows a typical laser diode drive circuit or modulation circuit. Referring to
FIG. 4
, a laser diode
110
is serially connected between an output terminal
106
of a semiconductor integrated circuit (i.e., drive IC)
100
and a power supply
121
. The drive IC
100
comprises a pair of differentially connected transistors
101
and
102
and a current source
103
connected to the common connection emitter of these transistors
101
and
102
.
A modulation signal or like drive input signal is inputted (i.e., applied) between input terminal
104
and
105
connected to the bases of the differential transistors
101
and
102
in the drive IC
100
. The collector of the transistor
101
is connected to a power source
122
, and the collector of the transistor
102
is connected via the output terminal
106
and the laser diode
110
to the power source
121
. The current source
103
has its other terminal connected to a power source
123
.
In the modulation circuit or laser diode drive circuit as shown in
FIG. 4
, by a modulation signal or a drive signal inputted between the input terminals
104
and
106
of the drive IC, the current in the current source
103
is distributed to the collectors of the transistors
101
to
102
for the control of the drive current in the laser diode
110
, which is connected to the output terminal
106
as the collector of the transistor
102
. Based on the drive current, the laser diode
110
outputs a modulated optical signal.
FIG. 5
shows an optical modulator drive circuit as another prior art example of the laser diode drive circuit. In the optical modulator drive circuit, a load
230
which is constituted by a resistor or an inductor, is connected between the output terminal
206
of the drive IC
200
including a MOS transistor
201
and a power source
221
. An optical modulator
210
including a laser diode is connected via an AC coupled capacitor
231
between the output terminal
206
and the power source
222
. An input terminal
204
is connected to the gate of transistor
201
, which has its source connected to a power source
223
and the drain connected to the output terminal
206
noted above.
The drain current is changed according to a modulation signal inputted to the gate input terminal
204
of the transistor
201
constituting the drive IC
200
. The modulation signal which is based on change in the output voltage from the output terminal
206
flows through the AC-coupling capacitor
231
in the optical modulator
210
.
In the semiconductor integrated circuit, however, the breakdown voltage is reduced with increasing operation speed. On the other hand, a high voltage is necessary for driving the laser diode. Therefore, the output voltage of the drive IC of such optical modulator may exceed the breakdown voltage of the drive IC and cause rupture of the IC.
SUMMARY OF THE INVENTION
The present invention has been made in view of the above prior art, and its object is to provide an optical modulator drive circuit with a low voltage as the output voltage of the driver IC.
According to an aspect of the present invention, there is provided an optical modulator drive circuit for driving an optical modulator by applying a drive signal from a drive output of the optical modulator drive circuit to the optical modulator, wherein a voltage clamp circuit is provided between the drive output and the optical modulator to clamp the voltage at the drive output to be lower than the breakdown voltage of the drive circuit.
According to another aspect of the present invention, there is provided an optical modulator drive circuit for driving an optical modulator by applying a drive signal from a drive output of the optical modulator drive circuit to the optical modulator, wherein a voltage clamp circuit is provided between the drive output and a load connected to a power supply to clamp the voltage at the drive output to be lower than the breakdown voltage of the drive circuit and the optical modulator is connected through a coupling capacitor to the drive circuit.
The voltage clamp circuit includes a transistor with a main current path thereof connected to the output terminal of the drive IC, a constant voltage being applied to a control terminal of the transistor. The voltage clamp circuit includes a uni-polar transistor with a main current path thereof connected to the output terminal of the drive IC, a constant voltage being applied to a control terminal of the transistor. The drive IC includes a pair of differentially connected transistors and a current source connected to the pair transistors. The drive IC is constituted by a single MOS transistor. The drive IC includes a MOS transistor and a current source connected in parallel with the transistor. The load is a resistor or an inductor. The drive circuit is semiconductor integrated circuit.
Other objects and features will be clarified from the following description with reference to attached drawings.
REFERENCES:
patent: 5377213 (1994-12-01), Honda
patent: 6278539 (2001-08-01), Ooi et al.
patent: 2002/0061035 (2002-05-01), Tokita et al.
patent: 3-73917 (1991-03-01), None
Hasan M.
McGinn & Gibb PLLC
NEC Communication Systems Ltd.
Sugarman Scott J.
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