Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1999-03-01
2002-03-26
Chan, Jason (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200, C372S034000
Reexamination Certificate
active
06362910
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an optical transmitter for transducing an electrical signal into an optical signal for transmission, and more particularly to an optical transmitter for stabilizing transition times (rising time and falling time) of an optical signal against a varying operating temperature. The present invention is also related to an optical transmission system using this optical transmitter.
An example of conventional optical transmitters is shown, for example, in JP-A-2-215239 (hereinafter called the “prior art (1)”).
FIG. 1
illustrates the configuration of this optical transmitter.
The illustrated optical transmitter is composed of an amplifier
4
, a modulator
2
, a current source
3
, and a light emitting element
1
. Data signals of positive phase and negative phase are inputted to terminals N
1
, N
2
, respectively. The data signals are amplified by the amplifier
4
which is composed of transistors Q
3
-Q
8
, a current source I
1
, and resistors R
1
-R
4
, and inputted to bases of a pair of differential transistors Q
1
, Q
2
in the modulator
2
. The modulator
2
controls to conduct and break a driving current generated by the current source
3
in response to the signals inputted to bases of the transistors Q
1
, Q
2
. As a result, a modulated current signal is outputted to the light emitting device
1
connected to a collector of the transistor Q
1
, causing the light emitting device
1
to generate an optical signal.
Another example of conventional optical transmitters is shown, for example, in JP-A-10-229232 (hereinafter called the “prior art (2)”).
FIG. 2
illustrates the configuration of this optical transmitter. The illustrated optical transmitter is composed of a pair of differential transistors
101
for supplying a semiconductor laser diode
113
with a modulated driving current; a transistor
104
for supplying the semiconductor laser diode or laser diode
113
with a bias current; and emitter follower transistors
102
,
103
for driving the differential transistor pair
101
, wherein a pulsed driving current for alternately driving the semiconductor laser diode
113
is controlled by an automatic power control (APC) voltage. More specifically, the APC voltage is used to control a current flowing through the emitter follower transistors
102
,
103
for driving the differential transistor pair
101
, so that even if the pulsed driving signal to the differential transistor pair
101
varies to cause fluctuations in the speed of the differential transistor pair
101
, the speed of the emitter follower transistors
102
,
103
can be changed to cancel a fluctuating portion of the speed of the differential transistor pair
101
.
In addition, it has been known that as a base-to-collector voltage of a transistor forming part of a modulator in an optical transmitter changes due to a varying temperature, a parasitic capacitance between the base and the collector of the transistor also changes, thereby resulting in deformation of the optical signal waveform. To solve this problem, JP-A-9-83456 (hereinafter called the “prior art (3)”) describes a technique for controlling a base voltage of the transistor in accordance with the temperature to compensate for a temperature dependency of the base-to-collector voltage of the transistor.
SUMMARY OF THE INVENTION
The optical transmitter according to the prior art (1), however, implies a problem that the optical signal rises and falls at different times depending upon the operating temperature. This problem results from the temperature characteristic of a bipolar transistor or a field effect transistor which forms part of the differential transistor pair of the modulator.
The following equations (1), (2) expresses the input/output characteristics (modulated current versus differential input voltage characteristics) of a modulator composed of bipolar transistors and a modulator composed of field effect transistors:
Im
=
Is
1
+
exp
(
q
·
Δ
V
K
B
·
T
)
(
1
)
Im
=
1
2
(
Is
+
q
·
D
k
B
·
T
·
W
L
·
Co
·
Δ
V
·
(
4
Is
q
·
D
k
B
·
T
·
W
L
·
Co
)
-
Δ
V
2
)
where
(
2
)
Δ
V
<
Is
q
·
D
k
B
·
T
·
W
L
·
Co
(
3
)
Im is a modulated current; &Dgr;V, a differential input voltage; Is, a current source current; q, a charge; k
B
, the Boltzmann's factor; T, an absolute temperature; W, a gate width; L, a gate length; Co, a gate capacitance per unit area; and D, a diffusion constant.
The equations (1), (2) respectively include a term “&Dgr;V/T.” It can be seen that the input/output characteristic of the modulator varies depending on the operating temperature.
FIGS. 3A-3D
are diagrams for explaining the temperature dependency of the waveform of a modulated current signal which is generated using a modulation control signal in a conventional optical transmitter. A modulator illustrated in
FIG. 3A
exhibits a reduced slope (a changing rate of the modulated current with respect to a change in a differential input voltage) of the input/output characteristic (a modulated current Im with respect to a differential input voltage: &Dgr;V=V
1
−V
2
; where V
1
, V
2
are base voltages of transistors Q
1
, Q
2
) when the operating temperature rises from T
1
K to T
2
K, as illustrated in FIG.
3
C. The reduced slope of the input/output characteristic in turn results in an extended linear input range for the differential input voltage.
The optical transmitter according to the prior art (1) supplies the modulator having the temperature characteristic as mentioned with an input signal of a constant voltage amplitude as illustrated in
FIG. 3B
irrespective of the operating temperature. On the other hand, rising/falling times of the modulated current signal Im is determined by a transition time of the modulation control signal in a linear input voltage range. Due to the temperature dependency of the modulated current signal Im, if the operating temperature changes from T
1
to T
2
to cause a change in the slope of the input/output characteristic, the rising/falling times of the modulated current signal change as illustrated in
FIG. 3D
, where a solid line and a dotted line represent the modulated current signal at temperatures T
1
and T
2
, respectively. Therefore, an optical signal generated from the modulated current signal will have a temperature dependency in rising/falling times.
Further, a laser diode and a light emitting diode (LED), which are light emitting elements, have their input/output characteristics changing depending on the temperature.
FIG. 4A
is a graph illustrating the input/output characteristic of a laser diode. As illustrated in
FIG. 4A
, as an operating temperature increases from T
1
to T
2
and further to T
3
(T
1
<T
2
<T
3
) in the laser diode, a threshold value for a driving current I
LD
for the laser diode to start emitting light also increases from Ith
1
to Ith
2
and Ith
3
. Also, as the operating temperature increases from T
1
to T
2
and further to T
3
, a slope efficiency (&Dgr;Po/&Dgr;I
LD
), which is the slope of the input/output characteristic (driving current I
LD
versus output light amount Po), decreases as indicated by a solid line, a dotted line and a one-dot chain line.
Therefore, assuming that a collector current IQ
1
of a transistor Q
1
, which serves as a driving current for a laser diode as illustrated in
FIG. 4B
, is constant irrespective of the operating temperature, the output light amount Po of the laser diode will decreases as the temperature rises, as illustrated in FIG.
4
C. In
FIG. 4C
, a waveform drawn by a solid line indicates the output light amount Po when the operating temperature is at T
1
, and a waveform drawn by a dotted line indicates the output amount Po when the operating temperature is at T
2
.
Similarly, when a light emitt
Chan Jason
Hitachi , Ltd.
Huynh Tuanson
Kenyon & Kenyon
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