U.S. patent application number 10/071102 was filed with the patent office on 2002-07-04 for optical transmitter and optical transmitting apparatus using the same.
This patent application is currently assigned to Hitachi, Ltd.. Invention is credited to Hamagishi, Takahiro, Hasegawa, Atsushi, Tokita, Shigeru.
Application Number | 20020085258 10/071102 |
Document ID | / |
Family ID | 12530910 |
Filed Date | 2002-07-04 |
United States Patent
Application |
20020085258 |
Kind Code |
A1 |
Tokita, Shigeru ; et
al. |
July 4, 2002 |
Optical transmitter and optical transmitting apparatus using the
same
Abstract
An optical transmitter for use in an optical transmitting system
based on PDS (Passive Double Star) technology, does not erroneously
output an optical signal when the optical transmitter is powered
on/off. The optical transmitter has a current source 1 for
outputting a drive current having a magnitude corresponding to an
input control signal, a Laser diode (LD) for generating an optical
output signal based on the received drive current, a modulator 9
for controlling the supply and cutoff of the drive current to the
Laser diode (LD), a source voltage detector 3 for monitoring a
source voltage to detect whether the source voltage is lower than a
predetermined voltage, and a switch circuit 4 for outputting a
control signal to the current source 1 to stop the supply of the
drive current when the source voltage is determined to be lower
than the predetermined voltage.
Inventors: |
Tokita, Shigeru;
(Yokohama-shi, JP) ; Hasegawa, Atsushi;
(Yokohama-shi, JP) ; Hamagishi, Takahiro;
(Yokohama-shi, JP) |
Correspondence
Address: |
MATTINGLY, STANGER & MALUR, P.C.
1800 DIAGONAL ROAD
SUITE 370
ALEXANDRIA
VA
22314
US
|
Assignee: |
Hitachi, Ltd.
|
Family ID: |
12530910 |
Appl. No.: |
10/071102 |
Filed: |
February 11, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10071102 |
Feb 11, 2002 |
|
|
|
09247809 |
Feb 11, 1999 |
|
|
|
Current U.S.
Class: |
398/192 ;
398/182 |
Current CPC
Class: |
H04B 10/564 20130101;
H04B 10/504 20130101 |
Class at
Publication: |
359/187 ;
359/180 |
International
Class: |
H04B 010/04 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 20, 1998 |
JP |
P10-038642 |
Claims
1. An optical transmitter, for coupling to communication devices
though an optical divider/coupler, having a source outputting a
drive current, a light-emitting element, for outputting an optical
signal to an optical fiber coupled to at least one of the
communication devices, that is driven by the drive current for
generating an optical output signal and, a modulator controlling
the supply and cutoff of the drive current to the light-emitting
element, comprising: a detecting circuit that detects a source
voltage; and a control circuit that stops, if the detected source
voltage is lower than the predetermined voltage, the supply of the
drive current to the light-emitting element.
2. The optical transmitter according to claim 1, further comprising
a logic circuit that stops the supply of the drive current to the
light-emitting element in response to an externally supplied
shut-down signal.
3. The optical transmitter according to claim 1, further comprising
a temperature detector that measures a temperature of the
light-emitting element and a pulse width correction circuit for
varying, according to a measured temperature, a pulse width of a
light-on/off signal to be supplied to the modulator.
4. The optical transmitter according to claim 1, wherein the
light-emitting element is a laser diode, and wherein the control
circuit has a switch circuit that cuts off the drive current from
the current source in the state of the voltage of the source being
lower than a predetermined voltage.
5. The optical transmitter according to claim 1, further
comprising: a photodiode that converts part of an optical output
signal of the light-emitting element into an electrical signal; an
automatic power control circuit that outputs, in response to the
electrical signal from the photodiode, a control signal for making
an optical power of the optical output signal constant; and a
switch circuit that transmits the control signal outputted from the
automatic power control circuit to the current source if the
detected source voltage is over the predetermined voltage.
6. The optical transmitter according to claim 5, wherein the
automatic power control circuit has a buffer circuit that performs
level conversion of the light-on/off signal, a first peak hold
circuit that holds a maximum output level of the buffer circuit, a
second peak hold circuit that holds a maximum output level of the
photodiode, and a comparator that makes a comparison between output
levels of the first peak hold circuit and the second peak hold
circuit.
7. An optical transmitter, for coupling to communication devices
though an optical divider/coupler, having a source outputting a
drive current, a light-emitting element, for outputting optical
signal to an optical fiber coupled to at least one of the
communication devices, that is driven by the drive current for
generating an optical output signal, and a modulator controlling
the supply and cutoff of the drive current to the light-emitting
element in response to an externally supplied light-on/off signal,
the optical transmitter comprising: a source voltage detector that
monitors a source voltage; and a light-emission cutoff circuit,
connected to the modulator, that controls a level of the
light-on/off signal to be inputted to the modulator in response to
the monitored source voltage, wherein the modulator cuts off supply
of the drive current to the light-emitting element when the
monitored source voltage is lower than the predetermined
voltage.
8. The optical transmitter according to claim 7, wherein the
optical transmitter further comprises a switch circuit that stops,
if the monitored source voltage is lower than the predetermined
voltage, the supply of the drive current to the light-emitting
element.
9. An optical transmitter, for coupling to communication devices
though an optical divider/coupler, having a source outputting a
drive current, a light-emitting element, for outputting an optical
signal to an optical fiber coupled to at least one of the
communication devices, that is driven by the drive current for
generating an optical output signal, a flip-flop circuit generating
a light-on/off signal based on an externally supplied data signal
and an externally supplied clock signal, and a modulator
controlling the supply and cutoff of the drive current to the
light-emitting element in response to the light-on/off signal,
wherein the flip-flop circuit changes the level of the light-on/off
signal to be outputted to cause the modulator to cut off supply of
the drive current when a source voltage is lower than a
predetermined voltage and maintains a state in which the drive
current is cut off until the data signal and the clock signal for
directing light emission are supplied even after the source voltage
has reached the predetermined voltage.
10. The optical transmitter according to claim 9, wherein the
flip-flop circuit has a source voltage detector that detects
whether the source voltage is found to be lower than the
predetermined voltage, a D-type flip-flop circuit composed of a
first gate circuit that samples the data signal in synchronization
with the clock signal, a first logic state hold circuit that holds
an output of the first gate circuit, a second gate circuit that
samples the output held in the first logic state hold circuit in
synchronization with the clock signal, and a second logic state
hold circuit that holds an output of the second gate circuit, and a
first logic state modify circuit and a second logic state modify
circuit that puts the hold states of the first logic state hold
circuit and the second logic state hold circuit respectively into a
low-level state when the source voltage is found to be lower than
the predetermined voltage.
11. An optical transmitter, for coupling to communication devices
though an optical divider/coupler, having a source outputting a
drive current, a light-emitting element, for outputting optical
signal to an optical fiber coupled to at least one of the
communication devices, that is driven by the drive current for
generating an optical output signal, and a modulator controlling a
supply and cut off of the drive current to the light-emitting
element, the optical transmitter comprising: detecting means for
detecting a source voltage; and control means for stopping the
supply of a drive current to the light-emitting element, if the
source voltage is found to be lower than the predetermined voltage,
the predetermined voltage being defined by the level causing
erroneous operation of the light-emitting element.
12. The optical transmitter according to claim 11, wherein the
control means stops the supply of the drive current from the source
to the light-emitting element when the detected source voltage is
lower than the predetermined voltage.
13. The optical transmitter according to claim 11, wherein the
modulator controls the supply and cutoff of the drive current to
the light-emitting element in response to an externally supplied
light-on/off signal, and wherein the control means sets a level of
the light-on/off signal to be inputted to the modulator, wherein
the modulator cuts off supply of the drive current to the
light-emitting element when the detected source voltage is lower
than the predetermined voltage.
14. The optical transmitter according to claim 11, further
comprising temperature detector means for measuring a temperature
of the light-emitting element and a pulse width correction circuit
for varying, according to a measured temperature, a pulse width of
a light-on/off signal to be supplied to the modulator.
15. The optical transmitter according to claim 11, further
comprising: a photodiode for converting part of an optical output
signal of the light-emitting element into an electrical signal;
automatic power control means for outputting, in response to the
electrical signal from the photodiode, a control signal for making
an optical power of the optical output signal constant; and switch
means for transmitting the control signal outputted from the
automatic power control circuit to the current source if the source
voltage is over the predetermined voltage.
16. A drive current controlling method for an optical transmitter,
for coupling to communication devices though a optical
divider/coupler, having a source outputting a drive current, a
light-emitting element, for outputting an optical signal to an
optical fiber coupled to at least one of the communication devices,
that is driven by the drive current for generating an optical
output signal, and a modulator controlling a supply and cutoff of
the drive current to the light-emitting element, the method
comprising: detecting a source voltage of the source, and stopping
the supply of a drive current to the light-emitting element, if the
source voltage is found to be lower than the predetermined voltage,
the predetermined voltage being defined by the level causing
erroneous operation of the light-emitting element and being greater
than zero.
17. The method according to claim 16, wherein the stopping stops
the supply of the drive current from the source to the
light-emitting element.
18. The method according to claim 16, further comprising:
controlling the supply and cutoff of the drive current to the
light-emitting element in response to an externally supplied
light-on/off signal, and wherein the controlling sets a level of
the light-on/off signal to be inputted to the modulator and wherein
the modulator cuts off supply of the drive current to the
light-emitting element when the detected source voltage is lower
than the predetermined voltage.
19. A drive current controlling method for an optical transmitter,
for coupling to communication devices though an optical
divider/coupler, having a source outputting a drive current, a
light-emitting element, for outputting optical signals to an
optical fiber coupled to at least one of the communication devices,
that is driven by the drive current for generating an optical
output signal, a flip-flop circuit generating a light-on/off signal
based on an externally supplied data signal and an externally
supplied clock signal, and a modulator controlling the supply and
cutoff of the drive current to the light-emitting element in
response to the light-on/off signal, the method comprising:
changing the level of the light-on/off signal to be outputted to
cause the modulator to cut off supply of the drive current when a
source voltage is lower than a predetermined voltage which is
greater than zero; and maintaining a state in which the drive
current is cut off until the data signal and the clock signal for
directing light emission are supplied even after the source voltage
has reached the predetermined voltage.
20. The method according to claim 19, wherein the maintaining
further includes: sampling the data signal in synchronization with
the clock signal by a first gate circuit; holding an output of the
first gate circuit by a first logic state hold circuit; sampling
the output held in the first logic state hold circuit by a second
gate circuit in synchronization with the clock signal; holding an
output of the second gate circuit by a second logic state; and
putting the hold states of the first logic state hold circuit and
the second logic hold circuit respectively into a low-level state
by a first logic state modify circuit and a second logic state
modify circuit when the source voltage is lower than the
predetermined voltage; wherein the first gate circuit, the first
logic state hold circuit, the second gate circuit and the second
logic state hold circuit compose a D-type flip-flop circuit.1
Description
FIELD OF THE INVENTION
[0001] 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
[0002] 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.
[0003] 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 Q1 and
a transistor Q2 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 Q2 is turned on, the laser diode
(LD) is supplied with a drive current from the current source 101
to generate an optical pulse signal.
[0004] 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.
[0005] 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.
[0006] 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
[0007] 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.
[0008] 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.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] These and other objects of the invention will be seen by
reference to the description, taken in connection with the
accompanying drawing, in which:
[0010] FIG. 1 is a block diagram illustrating a basic configuration
of an optical transmitter practiced as a first preferred embodiment
of the invention;
[0011] FIG. 2 is a circuit diagram illustrating a circuit
configuration of the first preferred embodiment;
[0012] FIG. 3 is a diagram illustrating a constitution of an
automatic power control circuit;
[0013] FIG. 4 is a diagram illustrating a relationship between the
variation in source voltage Vcc and an abnormal operation;
[0014] FIG. 5 is a diagram illustrating another circuit
configuration of the first preferred embodiment;
[0015] FIG. 6 is a block diagram illustrating a basic configuration
of an optical transmitter practiced as a second preferred
embodiment of the invention;
[0016] FIG. 7 is a circuit diagram illustrating a circuit
configuration of the second preferred embodiment;
[0017] FIG. 8 is a block diagram illustrating a basic configuration
of an optical transmitter practiced as a third preferred embodiment
of the invention;
[0018] FIG. 9 is a circuit diagram illustrating a circuit
configuration of the third preferred embodiment;
[0019] FIG. 10 is a circuit diagram illustrating a circuit
configuration of a flip-flop circuit 6 shown in FIG. 9;
[0020] FIG. 11 is a diagram illustrating a configuration of an
optical transmitter having a temperature-compensating capability;
and
[0021] FIG. 12 is a block diagram illustrating a configuration of a
related-art type of optical transmitter.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0022] The invention will be described in further detail by way of
example with reference to the accompanying drawings.
[0023] Now, referring to FIG. 1, there is shown a basic
configuration of an optical transmitter practiced as a first
preferred embodiment of the present invention. As shown, the
optical transmitter comprises a current source 1 for outputting a
drive current, a Laser diode (LD) for generating an optical output
signal based on the drive current, a modulator 9 for controlling
the supply and cutoff of the drive current to the Laser diode (LD)
according to a light-on/off signal (SG), a photodiode (PD) for
converting part of the optical output signal into an electrical
signal, an automatic power control circuit 2 for maintaining the
optical power of the optical output signal at a constant level, a
source voltage detector 3 for detecting that the source voltage is
lower than a predetermined reference level, and a switch circuit 4
for controlling the current output of the current source 1. The
value of the predetermined reference level depends on the type of
LD. The first preferred embodiment includes a source voltage
detector 3 and a switch circuit 4 that stops the current output of
the current source 1 when the source voltage determines that the
source voltage is lower than the reference level. Also, when the
source voltage exceeds the reference level, the operation of the
automatic power control circuit 2 maintains the optical output
power of the Laser diode (LD) at a constant level. In this
invention, the source voltage detector is supplied with a power
supply potential 11 (Vcc) in common with LD. Therefore, the source
voltage detector precisely detects whether the source voltage is
lower than the predetermined reference level without being
influenced by the operation of Ld. FIG. 2 shows a circuit
configuration of the optical transmitter. It should be noted that
the circuit to be described below assumes that a power supply
potential 11 (Vcc) relative to ground potential 12 (GND) is 3.3 V
and the level difference between the positive-phase and
negative-phase light-on/off signals (SG) is 300 mV.
[0024] As shown, the modulator 9 is composed of transistors Q1 and
Q2 with the emitters thereof connected is common. The transistors
Q2 and Q1 are applied at the bases thereof with the positive-phase
and negative-phase light-on/off signals (SG), respectively. When
the positive-phase light-on/off signal is at a high level (for
example, 2.5 V), the negative-phase light-on/off signal is at a low
level (for example, 2.2 V) and vice versa. The transistor Q2 is
connected at the collector thereof to the Laser diode (LD) and the
emitter thereof to the current source 1, supplying a drive current
to the Laser diode (LD) when the inputted light-on/off signal (SG)
is high.
[0025] The current source 1 is constituted by an N-channel
field-effect transistor MN3. This field-effect transistor MN3
adjusts the magnitude of the drive current to be supplied to the
Laser diode (LD) according to the level of the signal inputted in
the gate of this transistor.
[0026] The automatic power control circuit 2 is composed of two
buffer circuits 21 and 25, two peak-hold circuits 22 and 23, and a
comparator 24 as shown in FIG. 3, which outputs an optical output
control signal.
[0027] The source voltage detector 3 is composed of a first series
circuit consisting of a transistor Q9 and a resistor R1, a second
series circuit consisting of transistors Q6 and Q7 and a resistor
R2, and a comparator consisting of N-channel field-effect
transistors MN1 and MN2, P-channel field-effect transistors MP1 and
MP2, and a current source 1. It should be noted that use of the
field-effect transistors MP1 and MP2 is advantageous in enhancing
the accuracy and speed of voltage decision in the comparator over a
conventional general arrangement using a resistor instead.
[0028] The switch circuit 4 is composed of two switch elements SN
and SP. The switch elements SN and SP are constituted by an
N-channel field-effect transistor and a P-channel field-effect
transistor, respectively.
[0029] The following describes the operation of the optical
transmitter shown in FIG. 3.
[0030] In the automatic power control circuit 2, the positive-phase
and negative phase light-on/off signals (SG) are inputted in the
buffer 21 for input differential amplification. The amplified
output is inputted in the peak-hold circuit 22 in which the maximum
level is maintained. On the other hand, the electrical signal
outputted from the photodiode (PD) is amplified by the buffer
circuit 25 and the maximum level of the amplified signal is
maintained in the peak-hold circuit 23. The amplified output from
the buffer circuit 21 has a certain offset, which is adjusted
before-hand so that the electrical signal outputted from the buffer
21 becomes the same level as that of the electrical signal
outputted from the buffer 25 when the positive-phase light-on/off
signal (SG) is at the low level. The comparator 24 varies the level
of the optical output control signal to be outputted so that the
hold outputs of the peak-hold circuits 22 and 23 become equal to
each other.
[0031] The automatic power control circuit 2 for executing the
above-mentioned operations can maintain the optical output power of
the light-emitting diode (LD) at a constant level without being
affected by the mark ratio (the ratio between high and low levels)
of the light-on/off signals (SG).
[0032] In the source voltage detector 3, the voltage Vc at point
"c" in the first series circuit is represented in relation (1)
below. In this relation, Vbe is indicative of the potential between
the base and emitter of the transistor, which is about 0.8 V
(constant). Like Vbe, the potential that becomes constant relative
to ground potential GND regardless of source voltage Vcc is
hereafter referred to as a GND reference potential.
Vc=Vbe (1)
[0033] The potential Vd at point "d" in the second series circuit
is represented in relation (2) below. Like potential Vd, a
potential that decreases as the supply potential Vcc decreases is
hereafter referred to as a Vcc reference potential.
Vd=Vcc-2Vbe (2)
[0034] The gates of the field-effect transistors MN1 and MN2 of the
comparator are connected to point "c" and point "d" respectively,
so that the field-effect transistor MN1 is OFF when Vd>Vc and ON
otherwise. Hence, the voltage decision signal outputted from the
comparator becomes the high level (approximately equal to Vcc) when
Vd>Vc (namely, when the source voltage Vcc is greater than
3Vbe); otherwise, this signal becomes the low level (approximately
equal to GND). 3Vbe is indicative of a predetermined reference
potential, for example about 2.4 V (constant). As shown in FIG. 4,
3Vbe becomes higher than the level at which the circuit for
generating the light-on/off signals (SG) possibly functions
erroneously.
[0035] In the switch circuit 4, when the high-level voltage
decision signal is inputted from the comparator of the source
voltage detector 3, the switch element SN goes on and the switch
element SP goes off. Consequently, the optical output control
signal is transmitted from the automatic power control circuit 2 to
the gate of MN3 of the current source 1 through the switch element
SN. This causes the current source 1 to output a current according
to the optical output control signal, making the light-emitting
diode (LD) emit a laser beam when the transistor Q2 is on.
[0036] On the other hand, when the low-level voltage decision
signal is inputted in the switch circuit 4 from the comparator of
the source voltage detector 3, the switch element SN turns off and
the switch element PS turns on. Consequently, supplying of the
output signal of the automatic power control circuit 2 to the gate
of MN3 of the current source 1 is cut off, setting the potential of
the base to approximately the GND level. This causes the current
source 1 to stop outputting the current, keeping the Laser diode
(LD) in the light-off state also when the transistor Q2 is on.
[0037] An AND circuit 10 is normally inputted with a shut-down
signal SD of high level (Vcc) to transmit the optical output
control signal to the switch circuit 4 without change. When the
shut-down signal SD becomes a low level, the AND circuit 10 causes
the current source 1 to stop outputting the current by setting the
signal to the switch circuit 4 to a low level regardless of the
level of the source voltage Vcc. Consequently, the LD of the
transmitter might be controlled by the shut-down signal SD
notwithstanding the level of the source voltage Vcc.
[0038] Thus, the optical transmitter shown in FIG. 2 operates so
that the same stops supplying the current to the Laser diode (LD)
when the source voltage is lower than 3Vbe regardless of the level
of the light-on/off signal (SG). Consequently, no optical signal is
outputted due to erroneous light emission at a power on/off
operation. In addition, if the source voltage is normal, an optical
signal having a constant optical output power can be outputted
according to the light-on/off signal (SG).
[0039] FIG. 5 is a circuit diagram of another example of the
optical transmitter shown in FIG. 1.
[0040] This optical transmitter has a source voltage cutoff circuit
7 in which the capabilities of the source voltage detector 3 shown
in FIG. 2 are incorporated. An automatic power control circuit 2, a
modulator 9, and a Laser diode (LD) are generally the same in
capability as those shown in FIG. 2. For a current source 1, a
transistor Q3 is used, the Laser diode (LD) being driven by the
collector current of the transistor Q3.
[0041] In the source voltage cutoff circuit 7, a transistor Q8 is
controlled in the corrector current thereof by an optical output
control signal generated by the automatic power control circuit 2.
Transistors Q4 through Q7 constitute a current-mirror circuit. When
the source voltage Vcc is normal, the collector current of the
transistor Q8 is transmitted to the collector of the transistor Q4.
According to this collector current, the potential of the base of
the transistor Q4 varies, thereby controlling the magnitude of the
output current of the current source 1.
[0042] Potential Va at point "a" of a series circuit composed of
transistors Q9 and Q10 and a resistor R1 becomes a potential
relative to GND represented in relation (3) below.
Va=2Vbe (3)
[0043] On the other hand, potential Vb at point "b" of a series
circuit composed of the transistors Q6 and Q7 becomes a potential
relative to Vcc represented in relation (4) below.
Vb=Vcc-2Vbe (4)
[0044] A transistor Q11 is connected at the base thereof to point
"a" and at the emitter thereof to point "b". Hence, a condition in
which the transistor Q11 is turned on is represented in relation
(5) below.
Va-Vb>Vbe (5)
[0045] If the source voltage Vcc is lower than 3Vbe, the condition
of relation (5) is satisfied, so that conduction is provided
between the collector and emitter of the transistor Q11, allowing
the collector current of the transistor Q8 to flow through the
transistor Q11. This allows little current to flow to the
transistor Q6, thereby preventing the collector current of the
transistor Q8 from being transmitted to the transistor Q4. This
consequently stops the current output of the current source 1.
[0046] As described, the optical transmitter shown in FIG. 5
operates such that the current supply to the Laser diode (LD) is
stopped if the source voltage is lower than 3Vbe regardless of the
level of the light-on/off signal (SG). Therefore, no erroneous
optical signal is outputted at a power on/off operation. If the
source voltage is normal, the optical transmitter shown in FIG. 5
can output a constant optical output signal according to the
light-on/off signal (SG). Further, as compared with the circuit
shown in FIG. 2, the transistors used in the circuit configuration
of this optical transmitter can all be of a bipolar type, thereby
facilitating integration of a this optical transmitter into one
chip.
[0047] FIG. 6 is a block diagram illustrating the basic
configuration of the optical transmitter practiced as an
alternative embodiment. As shown, this embodiment comprises a
current source 1, a modulator 9, a Laser diode (LD), a photodiode
(PD), an automatic power control circuit 2, a source voltage
detector 3, and a light emission cutoff circuit 5. The components
other than the light emission cutoff circuit 5 have generally the
same capabilities as those of the components described with
reference to FIG. 1.
[0048] The light emission cutoff circuit 5 has a capability of
controlling the level of the light-on/off signal inputted in the
modulator 9. When the source voltage is lower than the reference
level, the light emission cutoff circuit 5 sets the level of the
light-on/off signal to the level at which the modulator 9 cuts off
the drive current.
[0049] This embodiment may be provided with the capability of
controlling the current output of the current source 1 by the
switch circuit 4 described with reference to FIG. 1. If this
capability is provided, the erroneous light emission of the Laser
diode (LD) can be prevented more reliably.
[0050] FIG. 7 shows a circuit configuration added with the
capability of controlling the current output of the current source
1.
[0051] The light emission cutoff circuit 5 is composed of a
2input/2-output buffer circuit 50 and base resisters RB1 and RB2
connected respectively to the outputs of the buffer circuit 50.
Positive-phase and negative-phase light-on/off signals are inputted
respectively in the inputs of the buffer circuit 50. The outputs of
the light emission cutoff circuit 5 are connected respectively to
the bases of the transistors Q2 and Q1 constituting the modulator
9.
[0052] A switch circuit 7 has generally the same configuration as
that of the switch circuit shown in FIG. 5. However, a difference
lies in that, while the source voltage Vcc is supplied to the
collector of the transistor Q11 in the switch circuit shown in FIG.
5, the positive-phase output signal line of the light emission
cutoff circuit 5 is connected to the collector of the transistor
Q11 in the switch circuit 7.
[0053] When the source voltage Vcc is normal, the switch circuit 7
performs generally the same operation as that of the switch circuit
shown in FIG. 5, transmitting the optical output control signal to
the current source 1 to output therefrom the drive current having a
magnitude according to the optical output control signal.
[0054] When the source voltage Vcc is lower than 3Vbe, the
transistor Q11 is on, so that the collector current of the
transistor Q11 flows from the buffer circuit 50 through the base
resistor RB2. For example, if the base resistor RB2 is set to 200
ohms and the collector current of the transistor Q11 to 3 mA, then,
when the transistor Q11 is on, the base voltage of the transistor
Q2 drops by 600 mV, surely going lower than the base voltage of the
transistor Q1. This turns off the transistor Q2, stopping the
current supply to the Laser diode (LD).
[0055] As described, the optical transmitter shown in FIG. 7
operates such that, when the source voltage becomes lower than
3Vbe, the current output of the current source 1 is stopped and, at
the same time, the light-on/off signals are set to a level at which
the modulator 9 cuts off the drive current. This prevents with
reliability the erroneous light emission of the Laser diode (LD)
from occurring.
[0056] FIG. 8 is a block diagram illustrating the basic arrangement
of an optical transmitter practiced as a third preferred embodiment
of the invention, the optical transmitter using a flip-flop circuit
6 associated with the invention. As shown, the third preferred
embodiment comprises a current source 1, a modulator 9, a Laser
diode (LD), a photodiode (PD), an automatic power control circuit
2, and the flip-flop circuit 6. The components of the third
preferred embodiment other than the flip-flop circuit 6 have
generally the same capabilities as those of the first or second
preferred embodiment.
[0057] The flip-flop circuit 6 generates a light-on/off signal (SG)
based on a data signal DT and a clock signal CL that are supplied
externally and outputs the generated light-on/off signal (SG) to
the modulator 9. If the source voltage is lower than a
predetermined reference voltage, the level of the light-on/off
signal (SG) is set to a level at which the modulator 9 cuts off the
drive current. Further, the state in which the drive current is cut
off is kept until, after the source voltage goes over the reference
voltage, the data signal DT and the clock signal CL for directing
light emission are supplied.
[0058] The flip-flop circuit 6 comprises a first gate circuit 31
for sampling the data signal DT in synchronization with the clock
signal CL, a first logic state hold circuit 32 for holding an
output of the first gate circuit 31, a second gate circuit 33 for
sampling the output of the first logic state hold circuit 32 in
synchronization with the clock signal CL, a second logic state hold
circuit 34 for holding an output of the second gate circuit 33, a
source power detector 3 for detecting that the source voltage is
lower than the predetermined reference voltage, and first and
second logic state modify circuits 35 and 36 for putting the hold
states of the first and second logic state hold circuits 32 and 34
respectively into the low level states when the source voltage is
lower than the reference voltage.
[0059] The gate circuit 31, the logic state hold circuit 32, the
gate circuit 33, and the logic state hold circuit 34 constitute a
D-type flip-flop of master-slave type.
[0060] FIGS. 9 and 10 show a circuit configuration of the optical
transmitter using the flip-flop circuit 6.
[0061] Referring to FIG. 9, the base of the transistor Q3 for use
as the current source 1 is supplied with the optical output control
signal of the automatic power control circuit 2. Output signals Vo1
and Vo2 of the flip-flop circuit 6 are supplied to the bases of the
transistors Q1 and Q2 respectively of the modulator 9. Here, the
flip-flop circuit 6 is supplied with Vcc-reference data signal DT
and clock signal CL.
[0062] FIG. 10 shows a circuit configuration of the flip-flop
circuit 6.
[0063] In the figure, the first gate circuit 31 is composed of
transistors T1, T2, T9, and T10. The first logic state hold circuit
32 is composed of registers R11 and R12 and transistors T3 and T4.
The second gate circuit 33 is composed of transistors T5, T6, T11,
and T12. The second logic state hold circuit 34 is composed of
resistors R13 and R14 and transistors T7 and T8.
[0064] The source voltage detector 3 and the first and second logic
state modify circuits 35 and 36 are integrated into a logic state
modify circuit 39. This logic state modify circuit 39 is composed
of transistors Qf1, Qf2, Qf3, and Qf4, and a resistor Rf1.
[0065] The following describes the operation of the optical
transmitter shown in FIGS. 9 and 10.
[0066] The positive-phase and negative-phase clock signals CL are
inputted in the transistors T10 and T11 and the transistors T9 and
T12, respectively, of the gate circuits 31 and 33. At this moment,
the potentials at point "a1" and point "a2" become Vcc reference
voltage Va. Point "b" of the series circuit constituted by the
transistors Qf1 and Qf2 and the resistor Rf1 presents GND reference
voltage Vb.
[0067] The transistors Qf3 and Qf4 are connected at the bases
thereof commonly to point "b", at the emitters thereof to point
"a1" and point "a2", respectively, and the collectors thereof to
one of the differential output pair of the logic state hold circuit
32 and one of the differential output pair of the logic state hold
circuit 34, respectively.
[0068] Consequently, when Vb-Va>Vbe, where the source voltage
Vcc goes lower than the normal operating level of the flip-flop
circuit, the transistors Qf3 and Qf4 are turned on, providing
conductance between the collector and emitter of each of these
transistors. This causes the currents of current sources If1 and
If2 to flow through load resistors R11 and R14 through the
transistors Qf3 and Qf4 respectively, thereby fixing the outputs of
the logic state hold circuits 32 and 33 on the sides connected to
the resistors R11 and R14 respectively to the low level. Further,
the levels of the other outputs of the logic state hold circuits 32
and 34 are also fixed, thereby setting the potential of the output
of the flip-flop circuit 6 to the state of Vo1>Vo2. This causes
the modulator 9 to cut off the drive current of the Laser diode
(LD).
[0069] Thus, in the optical transmitter shown in FIG. 9, the
modulator 9 performs the operation of cutting off the drive current
when Vb-Va>Vbe in which the source voltage Vcc goes lower than
the normal operating level, so that no optical signal is
erroneously outputted at a power on/off operation.
[0070] In addition, in this flip-flop circuit 6, the logic state
hold circuits 32 and 34 hold the output state established when
Vb-Va>Vbe until inputting of the data signal DT and the clock
signal CL starts after the source voltage goes over the normal
operating level (Vb-Va<Vbe). Therefore, the output of the
flip-flop circuit 6 is kept in the state of Vo1>Vo2, thereby
continuing the state in which the drive current of the Laser diode
(LD) is cut off.
[0071] In related-art flip-flop circuits, whether the output when
the source voltage has gone up to the normal operating level is
high or low is uncertain. This possibly causes erroneous light
emission even after the source voltage has gone up to the normal
operating level.
[0072] On the contrary, in the optical transmitter shown in FIG. 9,
even when the source voltage exceeds the normal operating level,
the cutoff of the drive current of the Laser diode (LD) is
continued until inputting of the data signal DT and the clock
signal CL starts, thereby preventing the erroneous light emission
at a power on/off operation.
[0073] FIG. 11 is a block diagram illustrating a configuration of
an optical transmitting apparatus having a temperature compensating
capability. The Laser diode (LD) has a characteristic that the
pulse width of the output optical signal varies with temperature.
To compensate for this variation, the optical transmitter shown in
FIG. 11 has a pulse width correction circuit 51, a temperature
detector 52, a controller 53, an optical transmitter 54, and an
optical fiber 55. For the optical transmitter 54, any of the
above-mentioned embodiments is available. The optical output signal
of the Laser diode (LD) in the optical transmitter 54 is
transmitted through the optical fiber 55.
[0074] The pulse width correction circuit 51 generates light-on/off
signals (SG) according to a data signal and a clock signal CL. The
pulse width correction circuit 51 also varies, according to a
control signal generated by the controller 53, the period in which
the high level of the light-on/off signal (SG) to be outputted is
maintained. The temperature detector 52 measures the temperature of
the Laser diode (LD) in the optical transmitter. Based on the
result of this measurement, the controller 53 outputs a control
signal to the pulse width correction circuit 51 so that the optical
output signal of the Laser diode (LD) is corrected to a proper
width. For example, if the pulse width of the optical output signal
is smaller than the specified value due to temperature variation,
the controller 53 controls the period such that the period in which
the light-on/off signal (SG) is at the high level is continued
longer.
[0075] Each of the optical transmitters practiced as the preferred
embodiments of the invention is combined with an optical receiver,
not shown, into an optical communication device. This optical
communication device is especially effective for use in an optical
transmission system in which, like the PDS technology, plural
optical communication devices are interconnected in a star by a
passive optical divider/coupler such as a star coupler. In other
words, by using each of the above-mentioned optical transmitters, a
system is realized in which communication between optical
communication devices is not interfered with by a power on/off
operation performed on each optical communication device.
[0076] As described and according to the invention, there is
provided an optical transmitter that does not erroneously output an
optical signal when the optical transmitter is powered on/off.
[0077] While the preferred embodiments of the present invention
have been described using specific terms, such description is for
illustrative purposes only, and it is to be understood that changes
and variations may be made without departing from the spirit or
scope of the appended claims.
* * * * *