U.S. patent application number 12/772585 was filed with the patent office on 2010-08-19 for optical receiver.
This patent application is currently assigned to Sumitomo Electric Industries, Ltd.. Invention is credited to Keiji Tanaka, Katsumi Uesaka.
Application Number | 20100209100 12/772585 |
Document ID | / |
Family ID | 37448400 |
Filed Date | 2010-08-19 |
United States Patent
Application |
20100209100 |
Kind Code |
A1 |
Uesaka; Katsumi ; et
al. |
August 19, 2010 |
OPTICAL RECEIVER
Abstract
The present invention provides an optical receiver able to
monitor the level of the optical input signal in accurate even when
the level is quite small. The optical receiver comprises a
photodiode to generate a photocurrent Ipd, a current mirror circuit
to reflect the photocurrent into a mirrored current Imon, a
current-to-voltage converter to convert the mirrored current Imon
to a voltage signal, switch to connect/cut the current mirror
circuit with the current-to-voltage converter, and a correction
unit for subtracting a signal when the switch is connected from a
signal when the switch is cut.
Inventors: |
Uesaka; Katsumi;
(Yokohama-shi, JP) ; Tanaka; Keiji; (Yokohama-shi,
JP) |
Correspondence
Address: |
SMITH, GAMBRELL & RUSSELL
1130 CONNECTICUT AVENUE, N.W., SUITE 1130
WASHINGTON
DC
20036
US
|
Assignee: |
Sumitomo Electric Industries,
Ltd.
Osaka
JP
|
Family ID: |
37448400 |
Appl. No.: |
12/772585 |
Filed: |
May 3, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11416389 |
May 3, 2006 |
|
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12772585 |
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Current U.S.
Class: |
398/9 ;
398/202 |
Current CPC
Class: |
H04B 10/66 20130101;
H04B 10/0799 20130101 |
Class at
Publication: |
398/9 ;
398/202 |
International
Class: |
H04B 10/08 20060101
H04B010/08; H04B 10/06 20060101 H04B010/06 |
Foreign Application Data
Date |
Code |
Application Number |
May 10, 2005 |
JP |
2005-137421 |
Jul 28, 2005 |
JP |
2005-219411 |
Claims
1-16. (canceled)
17. An optical receiver including a power monitoring circuit for
outputting a monitoring signal corresponding to a photocurrent
generated by a photodiode by receiving an optical signal with an
optical level, the optical receiver comprising: a current mirror
circuit connected to the photodiode, the current mirror circuit
generating a mirrored current by receiving the photocurrent; one
current-to-voltage converter provided in the power monitoring
circuit for converting the mirrored current into a first voltage
signal with a substantial offset when the variable conversion gain
of the current-to-voltage converter is set to a first gain, and for
converting the mirrored current into a second voltage signal with
the substantial offset when the variable conversion gain of the
current-to-voltage converter is set to a second gain different from
the first gain; and a correction unit for subtracting the first
voltage signal from the second voltage signal to cancel the
substantial offset of the current-to-voltage converter.
18. The optical receiver according to claim 17, wherein the
current-to-voltage converter is a trans-impedance amplifier
including an inverting amplifier and a resistor connected between
input and output terminals of the inverting amplifier, the
inverting amplifier having the substantial offset, and the first
gain of the current-to-voltage converter is obtained by setting the
resistor to a first resistance and the second gain of the
current-to-voltage converter is obtained by setting the resistor to
a second resistance.
19. The optical receiver according to claim 17, wherein the
current-to-voltage converter includes a resistor and a voltage
follower, the mirrored current flowing in the resistor and the
voltage follower detecting a voltage drop induced in the resistor
due to flowing of the mirrored current, and the conversion gain of
the current-to-voltage convertor is set by changing the resistance
of the resistor.
20. A method for monitoring an input optical level of an optical
signal received by an optical receiver, the method comprising the
steps of: converting the optical signal into a photocurrent by a
photodiode; reflecting the photocurrent into a mirrored current by
a current mirror circuit; converting the mirrored current into a
first voltage signal with a first conversion gain by one
current-to-voltage converter with a substantial offset; converting
the mirrored current into a second voltage signal with a second
conversion gain by the one current-to-voltage converter; and
subtracting the second voltage signal from the first voltage signal
to cancel the substantial offset of the one current-to-voltage
converter.
21. The method of claim 20, wherein the one current-to-voltage
converter includes an inverting amplifier with the substantial
offset and a feedback resistor connected between an input terminal
and an output terminal of the inverting amplifier, and the first
and second conversion gains of the one current-to-voltage converter
are set by varying the resistance of the feedback resistor.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to an optical receiver for
receiving an optical signal in the optical communication
system.
[0003] 2. Related Art
[0004] Japanese Patent published as JP-2003-198279A has disclosed
one type of optical receiver with a circuit for monitoring an
optical input level, in which the output of the differential
amplifier of the voltage follower is positively shifted to reduce
an error due to the non-linearity of the output thereof by
inserting a diode between the output of the differential amplifier
of the voltage follower and the output of the feedback loop.
[0005] Conventional monitoring unit such as those disclosed in the
Japanese Patent mentioned above includes a voltage follower
configured by a differential amplifier whose non-inverting input is
connected to a resister that converts a photocurrent generated by a
photodiode by receiving the optical input signal into a voltage
signal with a reference to the ground. When the optical input level
is quite small for the photodiode to generate only a faint
photocurrent, a voltage drop generated in the resistor by this
photocurrent becomes quite small and the input level of the voltage
follower is very close to the ground, to cause an output voltage
error because of the nonlinear characteristic of the differential
amplifier when its output is very small.
[0006] In the optical receiver disclosed in the Japanese Patent
mentioned above, by inserting a diode in the output of the
differential amplifier, the output thereof may be within a range
securing the linearity even when the output of the feedback loop
closes the ground level because the output of the differential
amplifier is positively shifted by a forward voltage of the diode.
However, the input of the differential amplifier is left in a very
small voltage close to an input offset voltage of the differential
amplifier when the optical input level is small. Thus, it is quite
hard to monitor the optical input level in accurate when the level
thereof is quite small.
[0007] Accordingly, the present invention, based on subjects
mentioned above, is to provide an optical receiver with a function
to accurately monitor the optical signal with a quite wide dynamic
range.
SUMMARY OF THE INVENTION
[0008] First aspect of the present invention relates to an optical
receiver, in particular, relates to a configuration of a circuit
for monitoring an optical input level and for outputting a
monitoring signal. First configuration of the optical receiver
includes a photodiode, a current-to-voltage converter with a
variable conversion gain, and a correction unit. The photodiode
generates a current by receiving an optical input signal with a
level. The current-to-voltage converter is configured to convert
the current into first and second voltage signals with the first
and second conversion gains, respectively. The correction unit
subtracts the second voltage signal from the first voltage
signal.
[0009] Second configuration of the optical receiver includes a
photodiode, a current mirror circuit, a current-to-voltage
converter, a switch, and a correction unit. The current mirror
circuit, arranged between the photodiode and the switch, generates
a current reflecting the photocurrent generated in the photodiode.
The current-to-voltage converter of this configuration converts the
output current of the current mirror circuit into a corresponding
voltage signal. The switch, arranged between the current mirror
circuit and the current-to-voltage converter, cuts or connects a
current path between the current mirror circuit and the
current-to-voltage converter. The correction unit subtracts a first
signal, which is obtained by the current-to-voltage converter when
the switch is cut, from a second signal obtained by the
current-to-voltage converter when the switch is connected.
[0010] Third configuration of the optical receiver includes a
photodiode, a trans-impedance amplifier including a differential
amplifier with an inverting input terminal, a non-inverting input
terminal and an output terminal, and a resistor connected between
the inverting input and output terminals, and a correction unit.
The trans-impedance amplifier converts the photocurrent generated
in the photodiode into a voltage signal. The correction unit
subtracts an input signal appeared in the inverting input terminal
from the output voltage.
[0011] According to configurations of the optical receiver
described above, an offset voltage inherently attributed to the
current-to-voltage converter may be eliminated from the monitored
output of the optical receiver. Therefore, even when the optical
input level becomes quite small, a monitoring error caused by the
offset voltage of the current-to-voltage converter can be
maintained.
[0012] The second aspect of the present invention relates to a
method for monitoring an optical input level of an optical input
signal and for outputting a signal indicating the optical input
level. The first method comprises steps of; (1) converting the
optical input signal into a photocurrent by a photodiode, (2)
converting the photocurrent into a first voltage signal with a
first conversion gain, (3) converting the photocurrent into a
second voltage signal with a second conversion gain, and (4)
subtracting the second signal from the first signal.
[0013] The second method comprises steps of: (1) generating a
mirrored current, which reflects a photocurrent, by a current
mirror circuit that receives the photocurrent generated by a
photodiode by receiving an optical input signal with a level, (2)
generating a first signal by connecting the current path between
the current mirror circuit and the current-to-voltage converter,
(3) generating a second signal by cutting the current path between
the current mirror circuit and the current-to-voltage converter,
and (4) subtracting the second signal from the first signal.
[0014] The third method comprises steps of: (1) generating a
photocurrent by a photodiode by receiving an optical input signal,
(2) converting the photocurrent into a voltage signal by a
current-to-voltage converter with a type of trans-impedance
amplifier including a differential amplifier with output, inverting
input, and non-inverting input terminals and a resistor connected
between the inverting input and output terminals of the
differential amplifier, and (3) subtracting a voltage signal
induced in the inverting input terminal from the voltage signal
appeared in the output terminal of the differential amplifier.
[0015] Since these methods may eliminate an effect of an offset
inherently attributed to the current-to-voltage converter, or to
the differential amplifier, a photocurrent generated in the
photodiode of the receiver can be monitored in accurate even when
the optical input level becomes small.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 is a block diagram of the optical receiver according
to first embodiment of the present invention;
[0017] FIG. 2 schematically shows the current-to-voltage converter
and a filter circuit each configured in the optical receiver shown
in FIG. 1;
[0018] FIG. 3 is a flow chart for calculating the optical level by
the power monitoring unit shown in FIG. 1;
[0019] FIG. 4 shows simulation results of the monitored output from
the correction unit against the optical input level for various
optical receivers including that of the present invention;
[0020] FIG. 5 is a block diagram of an optical receiver with a
current mirror circuit according to second embodiment of the
invention;
[0021] FIG. 6 is a block diagram of an optical receiver according
to third embodiment of the present invention;
[0022] FIG. 7 is a block diagram of an optical receiver according
to fourth embodiment of the present invention, which provides a
switch between the current mirror circuit and the
current-to-voltage converter;
[0023] FIG. 8 is a block diagram of an optical receiver according
to fifth embodiment of the invention, which provides an offset
current source to the current-to-voltage converter;
[0024] FIG. 9 is a block diagram of an optical receiver according
to sixth embodiment of the present invention, which compare the
input voltage with the output voltage of the current-to-voltage
converter; and
[0025] FIG. 10 is a block diagram of an optical receiver according
to seventh embodiment of the present invention, which provides a
package for enclosing the current mirror circuit, the filter
circuit, the photodiode, the trans-impedance amplifier and the gain
controller.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0026] Next, preferred embodiments of the present invention will be
described as referring to accompanying drawings. In the drawings
and their explanations, the same numerals or symbols will refer to
the same elements without overlapping explanations.
[0027] FIG. 1 is a block diagram of an optical receiver according
to one embodiment of the present invention. The optical receiver 1,
which is one of optical modules to output an electrical signal Sout
corresponding to an input optical signal Oin to the outside,
comprises a photodiode (hereinafter denoted as PD) 2, a monitoring
unit 3, a pre-amplifier 4, and a main amplifier 5. The PD generates
a photocurrent Ipd that reflects the input optical signal Oin. The
monitoring unit 3, connected to a cathode of the PD 2, monitors the
level of the input signal Oin. The pre-amplifier 4, connected to an
anode of the PD 2, converts the photocurrent into a voltage signal.
The main amplifier 5, connected to the pre-amplifier, amplifies the
voltage signal and outputs thus amplified signal as an output
signal Sout to the outside of the optical receiver 1.
[0028] The monitoring unit 3 includes a current-to-voltage
converter (hereinafter denoted as I/V-C) 6, an analog-to-digital
converter (hereinafter denoted as A/D-C) 7, a selector 8, a
register 9, and an arithmetic logic unit (hereinafter denoted as
ALU) 10. The I/V-C 6 is configured to have a variable conversion
gain. The A/D-C 7 converts a voltage signal, which is output from
the I/V-C 6, into a digital form. The selector 8 sends the digital
signal converted by the A/D-C 7 into the register 9 that stores
thus sent digital signal. The I/V-C 6, the selector 8 and the ALU
10 are controlled by a controller 11. These A/D-C 7, the selector
8, the register 9, and the ALU 10 constitute a correction unit 18
to generator a monitoring signal Vmon corresponding to the input
signal Oin.
[0029] The I/V-C 6, connected to the cathode of the PD 2, converts
the photocurrent Ipd into a voltage signal. The I/V-C 6 is
configured to vary a conversion ratio from the current Ipd to the
voltage signal based on a control signal supplied from the
controller 11.
[0030] FIG. 2 shows the I/V-C 6 in detail. As shown in FIG. 2, the
I/V-C 6 is one type of trans-impedance amplifier including a
differential amplifier 12 with inverting input, non-inverting input
and output terminals and a variable resistor 13 connecter between
the inverting input and output terminals of the differential
amplifier. The resistor 13 is configured as a variable
trans-impedance. Based on the control signal supplied from the
controller 11 to the variable resistor 13, the resistance of this
variable resistor 13 may be changed, which varies the conversion
gain.
[0031] The non-inverting input terminal of the differential
amplifier 12 receives a reference voltage 14, which defines the
output level of the differential amplifier 12. On the other hand,
the inverting input terminal of the differential circuit 12
connects the cathode of the PD 2 via a filter circuit 15 to remove
high frequency components contained in the photocurrent Ipd. Thus,
an average of the photocurrent Ipd may be supplied to the inverting
input terminal of the differential amplifier 12 and the variable
resistor 13. Specifically, the filter circuit 15 includes a
resistor 16 connected between the inverting input terminal of the
differential amplifier 15 and the cathode of the PD 2, and a
capacitor 17 connected between the cathode of the PD 2 and the
ground. The output of the I/V-C 6 is led to the A/D-C 7 to convert
the voltage signal into a digital form.
[0032] The output terminal of the A/D-C 7 connects the selector 8.
This selector 8 is configured to receive the digital signal from
the A/D-C 7 and to output this digital signal to one of registers,
namely, the first register 9a or the second register 9b, commanded
by the control signal supplied from the controller 11. Registers,
9a and 9b, store the digital signals sent from the selector 8.
[0033] The register unit 9 connects the ALU 10 configured to read
the digital signal from one of the registers, 9a and 9b, commanded
by the control signal supplied from the controller 11, to calculate
the difference between two digital signals. The ALU 10 outputs this
difference to the outside of the receiver 1 as a monitoring signal
Vmon of the optical input signal Oin.
[0034] The controller 11 is configured to output a signal to the
I/V-C 6 so as to vary the resistance of the variable resistor 13,
and to output a signal to the selector 8 so as to store the first
digital signal output from the A/D-C 7 into the first register 9a.
The controller 11 is configured, subsequent to the outputting of
the control signal to the selector 8 to store the digital signal
into the first register 9a, to output a signal to the I/V-C 7 so as
to vary the resistance of the variable resistor 13 to a value
different from the current value, and to output a control signal to
the selector 8 to store another digital signal currently output
from the A/D-C 7 into the second register 9b. The controller 11 is
further configured to output a control signal to the ALU 10 such
that, after storing two digital signals into respective registers,
9a and 9b, the ALU 10 reads out these digital signals from the
registers, 9a and 9b, and starts the subtraction of the second data
from the first data to obtain the monitoring signal Vmon.
[0035] Next, a process for calculating the monitoring signal Vmon
will be described as referring to a flow chart shown in FIG. 3.
[0036] First, subsequent to the reception of the optical signal Oin
by the PD 2 and receiving a command to start the monitoring of the
optical input level from the outside of the receiver 1, the
controller 11 sends the control signal to the I/V-C 6 to set the
resistance of the variable resistor at step S1. In this step,
assuming that the photocurrent Ipd from the PD 2, an input offset
voltage of the differential amplifier are Ipd and Vofs,
respectively, the output signal V1 output from the differential
amplifier 12 becomes;
V1=VREF+Ipd.times.R+Vofs, (1)
where VREF is the input voltage to the non-inverting input terminal
of the differential amplifier 12.
[0037] Next, after a processing period in the A/D-C 7, the
controller 11 sends the command to the selector 8 to store the
digital signal D1 output from the A/D-C 7 into the first register
9a at step S2. Here, assuming that bit width of the A/D-C 7 and the
reference voltage operable within the A/D-C 7 are N and Vadc
(>0), respectively, the digital signal D1 becomes;
D1=V1.times.(2.sup.N-1)/Vadc=(VREF+Ipd.times.R+Vofs).times.(2.sup.N-1)/V-
adc. (2)
[0038] Subsequently, the controller 11 commands the I/V-C 6 to
reduce the resistance of the variable resistor 13, for example, by
half at step S3. In this step, the voltage signal V2 output from
the I/V-C 6 becomes;
V2=VREF+Ipd.times.R/2+Vofs. (3)
[0039] Next, the controller 11 commands, after the processing
period in the A/D-C 7, to the selector 8 to store the digital
signal D2 output from the A/D-C 7 corresponding to the voltage
signal V2 into the second register 9b at step S4. The digital
signal D2 may be calculated in the A/D-C 7 as follows;
D2=V2.times.(2.sup.N-1)/Vadc=(Ipd.times.R/2+Vofs).times.(2.sup.N-1)/Vadc-
. (4)
[0040] Subsequently, the controller 11 sends the command to the ALU
10 to subtract the second signal D2 from the first signal D1 to get
the monitoring signal Vmon at step S5. The ALU 10 outputs this
monitoring signal Vmon to the outside of the receiver 1. The
subtraction in the ALU 10 is carried out based on the following
equation;
Vmon=2.times.(D1-D2). (5)
When the ratio of the conversion gain of the I/V-C 6 at step S3 to
the conversion gain at step S1 is m, Vmon becomes;
Vmon=(D1-D2)/(1-m). (6)
[0041] FIG. 4 shows a simulation result under a condition that the
variable resistor R, the bit width N of the A/D-C 7, the reference
voltage Vadc, and the input offset voltage Vofs of the differential
amplifier 12 are R=12.5 k.OMEGA., N=15, VREF=2.5 V, and Vofs=.+-.3
mV, respectively. In a conventional monitoring unit, the
discrepancy from a theoretical lien becomes very large in a region
where the optical input level is quite small. Assuming the offset
voltage of the differential amplifier 12 is Vofs, the monitoring
signal Vmon by the variable resistor R due to the photocurrent Ipd
generated by the PD 2 becomes Vmon=Ipd*R+Vofs. Therefore, a
quantizing error ERROR involved in the monitoring signal Vmon is
denoted as:
ERROR=10.times.log.sub.10(1+Vofs/Ipd/R) [dB]. (7)
[0042] Thus, the input offset voltage Vofs influences on the
quantizing error ERROR as the photocurrent Ipd becomes smaller. To
obtain a precise result for monitoring the optical input level, it
would be effective to make the resistance of the variable resistor
13 large. However, such large resistance would saturate the
monitoring unit 3 at medium or large input levels. Therefore, to
increase the resistance is a restricted means.
[0043] According to the optical receiver 1 described above, the
photocurrent Ipd generated by the PD 2 based on the optical input
signal Oin is converted into a voltage signal by the I/V-C 6. In
this conversion process, at least two sets of the conversion gain
may be set to generate a pair of voltage signals, V1 and V2. The
monitoring signal Vmon is derived from the difference between these
two voltage signals, V1 and V2. Accordingly, the offset voltage
Vofs involved within both signals, V1 and V2, may be cancelled.
Thus, even when the optical signal has a small level, the accuracy
of the monitoring signal is maintained.
Second Embodiment
[0044] FIG. 5 is a block diagram showing an optical receiver 1b
according to the second embodiment of the present invention. The
optical receiver 1a provides, in addition to the embodiment shown
in FIG. 2, a current mirror circuit 22 between the resistor 16 in
the filter circuit 15 and the inverting input terminal of the
differential amplifier 12. The current mirror circuit 22 has two
current paths each including transistor, Tr1 or Tr2, whose bases
and emitters are commonly connected. In the current mirror circuit
22, the photocurrent Ipd flowing out from the first transistor Tr1,
namely, the first current path, shows a constant relation to the
current Imon flowing out from the second transistor Tr2, namely,
the second current path. The common emitters of transistors, Tr1
and Tr2, are connected to the power supply Vcc. The first current
path is connected to the PD 2 via the filter circuit 15, while, the
second current path is connected to the I/V-C 6. In the current
mirror circuit 22, the current Imon flowing in the I/V-C 6 may be
isolated from the photocurrent Ipd with the constant ratio.
Accordingly, the former current Imon flowing in the I/V-C 6 may be
independently adjusted and generally called as the mirrored
current.
Third Embodiment
[0045] FIG. 6 shows another embodiment of the optical receiver that
includes a modified current-to-voltage converter 6a. The optical
receiver 1b includes a variable resistor 13a and a voltage follower
12a, where they constitute the I/V-C 6a. The variable resistor 13a
is connected, in one terminal thereof, to the second current path
of the current mirror circuit 22, while, the other terminal is
grounded. Thus, the variable resistor 13a generates a voltage drop
Vin depending on the mirrored current Imon flowing out from the
current mirror circuit 22. The resistance of the variable resistor
13a may be controlled by the control signal supplied from the
controller 11. The voltage drop Vin appeared in this variable
resistor 13a is led to the A/D-C 7 via the voltage follower
12a.
Fourth Embodiment
[0046] FIG. 7 is a block diagram showing still another embodiment
of optical receiver according to the present invention. The optical
receiver 1c shown in FIG. 7 provides a current mirror circuit 22
and a modified monitoring unit 3b. The optical receiver 1c
generates two signals each based on the photocurrent Ipd. The first
transistor Tr1 provides the current path for the photocurrent Ipd.
The second transistor Tr2 connected to the resistor 19 via the
switch SW2 and to the inverting input terminal of the differential
amplifier 12 via the switch SW1. These switches, SW1 and SW2, are
complementarily operated to distribute the mirrored current Imon
from the current mirror circuit 22 to one of the resistor 19 or the
differential amplifier 12. The sequence of these switches, SW1 and
SW2, are controlled by the signal sent from the controller 11.
[0047] When the first switch SW1 is put off, while, the second
switch SW2 is put on by the controller 11, which connects the
resistor 19 with the current mirror circuit 22, no current from the
second transistor Tr2 of the current mirror circuit 22 flows into
the I/V-C 6. Thus, the output of the I/V-C 6 reflects only the
input offset of the differential amplifier 12.
[0048] Subsequent to the process above, the first switch SW1 is put
on and the second switch SW2 is put off by the controller 11. Then,
the current Imon output from the second transistor Tr2 flows into
the I/V-C 6, and the I/V-C 6 converts this current into a voltage
signal. In this step, the output of the I/V-C 6 includes both the
voltage signal derived from the current Imon and the input offset
of the differential amplifier 12. The correction unit processes
this output voltage by the same procedure already mentioned. By
subtracting the output voltage obtained in the former process from
the output voltage by the present process, only voltage signal
generated by the mirrored current Imon flowing in the second
transistor TR2 that reflects the photocurrent Ipd can be extracted.
Thus, the influence from the offset voltage of the differential
amplifier 12 in the I/V-C 6 may be compensated.
Fifth Embodiment
[0049] When a negative offset appears in the output of the
differential amplifier 12, which the A/D-C 7 cannot convert into a
digital value, a positive offset is intentionally added by adding
an offset current Iofs to the inverting input terminal of the
differential amplifier 12 as shown in FIG. 8 such that the offset
appears in the output of the differential amplifier 12 always
becomes positive.
[0050] The magnitude of the currents flowing in respective paths of
the current mirror circuit 22 may be adjusted by varying the
collector size of each transistor, Tr1 or Tr2. Further, even when
the collector size of transistors is substantially equal to each
other, to insert a resistor between each emitter and the power
supply Vcc, which is not shown in FIG. 7, and to adjust the
resistance thereof determines the ratio a of the currents flowing
in respective paths.
[0051] According to the optical receiver id of the present
embodiment, the I/V-C 6 sequentially and alternately generates two
voltage signals, one of which corresponds to the mirrored current
Imon added by the input offset voltage Vofs of the differential
amplifier 12, and the other of which corresponds only to the input
offset voltage Vofs. The corrected monitoring signal Vmon may be
derived from the subtraction of these two signals. Accordingly, the
offset voltage inherently attributed to the differential amplifier
12 may be eliminated, the output Vmon of the receiver 1d as the
monitored optical level maybe corrected even when the input optical
level is quite small. Moreover, the offset voltage Vofs is affected
by the operating temperature. Therefore, to eliminate the influence
of the offset voltage may enhance the stability and the accuracy of
the monitoring of the optical input level against the
temperature.
Sixth Embodiment
[0052] FIG. 9 shows an optical receiver 1e according to sixth
embodiment of the invention, which provides a modified monitoring
unit 3d. This monitoring unit 3d includes a selector 24 arranged
between the I/V-C 6 and the A/D-C 7.
[0053] The selector 24 selects one of the inverting input and the
output of the differential amplifier 12 and provides the selected
signal to the A/D-C 7. The controller sends a control signal to the
selectors, 24 and 8, to choose the output of the differential
amplifier 12.
[0054] Next will describe an algorithm to evaluate the monitoring
output Vmon according to the present embodiment.
[0055] When the input voltage of the inverting input terminal of
the differential amplifier 12 is set to be VREF, the voltage
appears in the inverting input of the differential amplifier 12
becomes;
Vin=VREF.+-.Vofs, (8)
where Vofs is an input offset of the differential amplifier 12.
Assuminig the mirrored current flows into the I/V-C 6 is Imon, the
output of the differential amplifier 12 is given by;
Vout=VREF.+-.Vofs-Imon.times.R, (9)
By using the current ratio a of Imon to Ipd, the equation (9) can
be rewritten as:
Vout=VREF.+-.Vofs-Ipd/.alpha..times.R. (10)
Therefore, by subtracting Vin from Vout, the input offset voltage
Vofs of the differential amplifier can be cancelled.
[0056] In the present embodiment, even when the selector 24 selects
the input Vin of the differential amplifier 12, the resistor 13 is
not cut off and has a substantial resistance, which maintains the
closed loop for the differential amplifier 12. The A/D-C 7
sequentially converts above signals into corresponding digital
forms. The output of the A/D-C 7 is led to the selector 8 in the
correction unit 18 and the same procedures with those already
mentioned are carried out to generate the corrected monitoring
signal Vmon.
[0057] Thus, even in the monitoring unit 3d, the input offset
voltage of the differential amplifier 12 may be corrected to
enhance the accuracy of the monitoring signal Vmon even when the
input optical level is quite small.
Seventh Embodiment
[0058] FIG. 10 is a block diagram of the optical receiver according
to sixth embodiment of the present invention. The optical receiver
if shown in FIG. 10, in addition to the monitoring unit 3d,
provides a package 30 that installs the PD 2, the filter circuit
15, the current mirror circuit 22, the trans-impedance-amplifier
4a, and the second controller 27. The package 30 may be a CAN type
package with a co-axial form. The trans-impedance amplifier 4a has
a configuration of a variable performance in the current-to-voltage
conversion gain against the frequency bandwidth. The second
controller 27 generates a control signal Samp to adjust the
performance of the trans-impedance amplifier 4a.
[0059] Features of this optical receiver if are, (1) the current
mirror circuit 22 is installed within the package 30 with the PD 2,
and (2) the conversion gain of the trans-impedance amplifier 4a is
variable and the second controller 27 may adjust this conversion
gain. Moreover, in addition to above two features, (3) the
reference level VREF supplied to the differential amplifier 12 may
be adjusted outside of the monitoring unit 3d.
[0060] Specifically, one output terminal of the current mirror
circuit 22 is connected to the inverting input terminal of the
differential amplifier 12 via one lead terminal of the package 30.
Moreover, this lead terminal is also connected to the second
controller 27. Here, the input impedance of the second controller
27 is set quite high so as to ignore the inflow of the mirrored
current Imon thereinto.
[0061] The non-inverting input terminal of the differential
amplifier 12 is exposed to the outside of the monitoring unit 3d to
receive the reference level VREF. The input voltage Vin of the
inverting input terminal may be determined based on this reference
level VREF. Accordingly, by varying this reference level VREF, the
input voltage Vin of the inverting input terminal may be
varied.
[0062] On the other hand, the input voltage Vin is also brought to
the second controller 27 within the package 30. The second
controller 27 may generate the control signal Samp based on this
input voltage Vin. That is, in the optical receiver if, the
performance of the conversion gain against the frequency bandwidth
of the trans-impedance amplifier 4a can be varied by adjusting the
reference level VREF input to the terminal 14. The trans-impedance
amplifier 4a may provide the same configuration with those of the
differential amplifier 12, namely, an inverting amplifier with a
feedback resistor connected between the input and output terminals
thereof. In such configuration, when the feedback resistor may be
variable in the resistance thereof by the control signal Samp, a
trans-impedance amplifier with a variable performance of the
conversion gain against the frequency bandwidth may be simply
realized. Thus, the lead terminal of the package 30 for outputting
the mirrored current Imon and another lead terminal to supply the
control signal to the second controller 27 may be common to reduce
a count of lead terminals of the package 30.
[0063] In the present embodiment, the reference level VREF3 varies
its level to adjust the performance of the trans-impedance
amplifier 4a. Two inputs of the selector 24, which is the input
voltage Vin and the output Vout of the differential amplifier, are
determined based on this external reference VREF. Accordingly, the
input and output voltages, Vin and Vout, may not be always in a
convertible range for the A/D-C 7. In this case, a level shifter
may be added between selector 24 and the A/D-C 7 to bring the level
of two signals, Vin and Vout, within an appropriate range for the
A/D-C 7.
[0064] The optical receiver 1e of the present embodiment provides
the trans-impedance amplifier 4a with the variable performance in
the conversion gain against the frequency bandwidth. When the
frequency, namely, the transmission speed of the optical input
signal has relatively high, the conversion gain of the
trans-impedance amplifier 4a may be reduced to widen the frequency
bandwidth thereof. On the other hand, when the signal frequency is
relatively low, the bandwidth of the trans-impedance amplifier 4a
may be narrowed to enhance the conversion gain thereof.
[0065] The present invention is not restricted to those embodiments
described in the specification and accompanying drawings. For
example, the selector 8, the registers, 9a and 9b, and the ALU 10
may be configured independently, or, may be realized by the
software on single digital processor. Moreover, the photodiode 2
for a light-receiving device may be replaced by an avalanche
photodiode (APD). Thus, it will be apparent to those skilled in the
art that numerous modifications and variations can be made without
departing from the scope of the invention as defined in the
appended claims.
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