Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
2000-03-03
2003-07-22
Chan, Jason (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C372S029011
Reexamination Certificate
active
06597485
ABSTRACT:
CROSS REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims priority of Japanese Patent Application No. 11-75025, filed Mar. 19, 1999, the contents being incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical transmitter circuit which enables reliable monitoring of optical signals generated by a light emitting element, such as a semiconductor laser, with a comparatively inexpensive light receiving element, such as a photodiode. More particularly, the present invention relates to an optical transmitter circuit having a circuit to detect continuous “1” bits in the input data in order to perform update timing of a current control signal or deterioration evaluation of the light emitting element.
2. Description of the Related Art
Optical transmitter circuits are known.
FIG. 13
illustrates an example of a conventional optical transmitter circuit, including a light emitting element
101
such as a semiconductor laser, a light receiving element
102
such as a photodiode to monitor light, a current-voltage converter (I/V)
103
, an auto power control (APC) amplifier
104
, a drive circuit
105
, a sample hold circuit
106
, and an analog switching circuit
107
.
In operation of the conventional optical transmitter circuit shown in
FIG. 13
, the drive circuit
105
supplies drive current to the light emitting element
101
in accordance with input data (DATA). An optical signal is generated by the light emitting element
101
in accordance with the input data (DATA). The optical signal is detected by the monitoring light receiving element
102
, converted into voltage by the current-voltage converting circuit
103
, and input into the APC amplifier
104
. The APC amplifier
104
compares the output signal of the current-voltage converting circuit
103
and a reference value, and inputs a signal corresponding to the comparative differential thereof into the sample hold circuit
106
via the analog switching circuit
107
.
The analog switching circuit
107
receives the output signal of the APC amplifier
104
when the data (DATA) converted to optical signals is transmitted at level “1”, which output signal is input and held in the sample hold circuit
106
. The held signal is input into the drive circuit
105
as a current control signal, and the drive current of the light emitting element
101
is controlled so that the optical output remains constant.
FIGS. 14A-14D
are diagrams explaining the operation of the conventional example of the optical transmitter circuit shown in FIG.
13
. More specifically,
FIG. 14A
illustrates the data (DATA) input to the drive circuit
105
;
FIG. 14B
illustrates the optical output signal of the light emitting element
101
;
FIG. 14C
illustrates the output signal of the current-voltage converting circuit
103
; and
FIG. 14D
illustrates the drive current supplied to the light emitting element
101
.
In the example shown in
FIGS. 14A-14D
, when the optical output signal of the light emitting element
101
corresponding to the data (DATA) that is input at time t
1
decreases, as indicated by the OUTPUT DETERIORATION in
FIG. 14B
, the output signal of the current-voltage converting circuit
103
also decreases, as shown in FIG.
14
C. The decreased output signal of the current voltage converting circuit
103
is held by the sample hold circuit
106
via the analog switching circuit
107
, and a drive current is supplied to the light emitting element
101
from the drive circuit
105
as a current control signal corresponding to the data (DATA) that is input at the following time t
2
. More particularly, as shown in
FIG. 14D
, since the drive current corresponding to the data (DATA) that is input at time t
2
is increased more than the drive current corresponding to the data (DATA) input at time t
1
, the optical output signal is controlled to a specified level, as indicated by the OUTPUT RECOVERY of FIG.
14
B.
Furthermore, a circuit is known in which stabilization of the optical output is achieved by converting the output current of the light receiving element, which converts the optical output of the light emitting element, into voltage with a current-voltage converting circuit. When the converted optical output of the light emitting element reaches a specified level or above, the circuit determines the output to be a significant detection signal, holds it as a sample, and controls the drive current of the light emitting element in accordance with the held value. An example of this type of circuit is disclosed in Japanese Unexamined Laid-Open Patent Application Publication JP9-18054, wherein the input data is delayed and is considered to be a significant detection signal, and the output signal of the current-voltage converting circuit at this time is held as a sample.
When data (DATA) that is input is increased in speed, e.g., from a low speed of 50 Mbps to 150 Mbps, the light emitting element
101
has sufficient response speed since the light emitting element
101
is generally a semiconductor laser. However, a photodiode is generally used as the light receiving element
102
. Because the photodiode has an increased surface area in order to increase the light receiving sensitivity, the capacitance C
PD
of the photodiode is generally, for example, 20 pF or above. Accordingly, when the current-voltage converting circuit
103
includes a resistance R, the band region f
O
is determined by the equation f
O
=½&pgr;RC
PD
, so it is extremely difficult to broaden the band. In other words, when a comparatively inexpensive photodiode is used as a light receiving element
102
to monitor the light emitting element
101
, the response characteristics are not sufficient to detect an optical signal having a high speed of 150 Mbps or more.
FIGS. 15A-15D
are diagrams illustrating problems occurring with the APC amplifier
104
operation in accordance with the conventional optical transmitter circuit. More specifically,
FIG. 15A
illustrates the data (DATA) input to the drive circuit
105
;
FIG. 15B
illustrates the optical output signal of the light emitting element
101
;
FIG. 15C
illustrates the output signal of the current-voltage converting circuit
103
; and
FIG. 15D
illustrates the drive current supplied to the light emitting element
101
. When the input shown in
FIG. 15A
occurs, the sample hold circuit
106
samples and holds the output signal of the APC amplifier
104
when the data (DATA) has been input. At this time, since the response speed of the light receiving element
102
is lower than the speed of the input data (DATA), the output signal of the current-voltage converting circuit
103
changes, as shown in FIG.
15
C.
Moreover, the current control signal corresponding to the value that has been sampled and held is updated at the times t
1
, t
2
, t
4
, and t
6
, as indicated by the arrows showing the CURRENT UPDATE VALUE in
FIG. 15
, and the drive current supplied to the light emitting element
101
from the drive circuit
105
is updated. Accordingly, the output signal of the current-voltage converting circuit
103
resulting from the monitoring of the optical output produced by the light receiving element
102
in response to an initial input value of “1” becomes equal to or less than the set value indicated by the broken line at the time t
1
in FIG.
15
C. Because this output signal of the current-voltage converting circuit
103
is sampled and held, it is judged to be a lower optical output than a specified level, and the drive current supplied to the light emitting element
101
for the next continued data value of “1” increases, as shown in FIG.
15
D. Accordingly, the optical output signal of the light emitting element
101
at this time becomes greater than the initial optical output, as shown in FIG.
15
B.
In this state, at the following time t
3
, the drive current corresponding to the input data (DATA) “1” is supplied to the light emitting element
101
. At this time, even if the optical output of the l
Ikeuchi Tadashi
Inoue Tadao
Matsuyama Toru
Miki Makoto
Ueno Norio
Chan Jason
Fahmy Sherif R.
Fujitsu Limited
Staas & Halsey , LLP
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