Optical communications – Transmitter – Including compensation
Reexamination Certificate
2000-03-07
2004-04-27
Chan, Jason (Department: 2733)
Optical communications
Transmitter
Including compensation
C398S023000
Reexamination Certificate
active
06728495
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a light output control circuit applicable for optical communication etc. having a driving circuit of a light emission element such as a semi-conductor laser (LD: laser diode) or a light emission diode (LED) and more particularly a light output control circuit for controlling constant light output level provided with a warning function of deteriorated light output.
BACKGROUND OF THE INVENTION
In equipment such as optical transmission equipment which uses a light emission element, it is generally required to control light output to maintain at a predetermined value.
The efficiency of a light emission element such as an LD largely depends on temperature and also has a property of aged deterioration. It is therefore necessary to control to feed appropriate driving current to the light emission element in order to maintain light output level constant in any operating condition.
The APC (automatic power control) using negative feedback control has been introduced in a light output control circuit to control constant light output emitted by a light emission element.
In a light output control circuit to drive an emission element, a function is usually provided to monitor output power. A warning is issued when the output power decreases below a predetermined level. This facilitates to know appropriate time to replace the emission element caused by the breakdown or aged deterioration.
In
FIG. 31
, there is shown the first configuration example of a conventional light output control circuit to drive a light emission element. In this
FIG. 31
, pulse transmission data (DATA) is inputted with transmission input clock (CLK) to a D-type flip-flop (hereafter referred to as ‘DFF’) circuit
100
, to be outputted to an LD driving circuit
101
after the pulse width of the input data (DATA) is corrected.
LD driving circuit
101
, after receiving an output signal of DFF circuit
100
, feeds a driving current to a laser diode (LD) to produce the LD emit light corresponding to the input data (DATA).
By using a light output control signal outputted from a comparator
102
, a negative feedback function is carried out to produce stronger light emission when an LD light output is small, or to produce weaker light emission when the LD light output is large.
A photodiode (PD) receives a portion of the light outputted from the laser diode (LD) and converts the received light to the corresponding current to output. A current/voltage converter
103
converts the received current signal from the PD into a voltage signal.
A reference voltage generator
104
generates a reference signal from an input data (DATA) to output to comparator
102
. It may also be possible that reference voltage generator
104
produces the reference voltage from a constant voltage generator.
A peak detection circuit
105
receives an output signal from current/voltage converter
103
to detect a peak value of the signal to forward to comparator
102
. A capacitor is usually used in peak detection circuit
105
to detect the peak value.
Comparator
102
produces a differential value between the values of the reference signal from reference voltage generator
104
and the peak signal from peak detection circuit
105
, then to feed to LD driving circuit
101
as a light output control signal.
Accordingly, the circuit shown in
FIG. 31
performs the negative feedback control: a light output control signal from comparator
102
acts to increase an LD light output when the light output is small, and to the contrary to decrease the LD light output when the light output is large. Thus the LD light output is controlled to maintain a constant value.
In the above-mentioned first configuration of a conventional light output control circuit, there is shown in
FIG. 32
an example of a circuit which outputs a warning signal of a deteriorated light output. In
FIG. 32
, the loop to control driving current illustrated in
FIG. 31
is not shown.
Also in
FIG. 32
, current/voltage converter
103
and peak detection circuit
105
shown in
FIG. 31
are integrated into one as a monitoring portion
106
. A level comparator
107
compares an output of monitoring portion
106
to a threshold value which corresponds to the warning generation level led from constant voltage generator
108
. When an output of monitoring portion
106
decreases below the threshold value from constant voltage generator
108
, level comparator
107
outputs a warning signal of a deteriorated light output.
FIG. 33
shows the second configuration example of a conventional light output control circuit. Compared to the first configuration shown in
FIG. 31
, digital control is introduced in the driving current control circuit in
FIG. 33
so that the circuit can easily be fabricated into an LSI.
In the configuration shown in
FIG. 33
, LD driving circuit
101
modulates driving current according to transmission data supplied by DFF circuit
100
to feed to the light emission element (LD). A peak value of the driving current (hereafter simply referred to as ‘driving current value’) is controlled so that a driving current value is proportional to a digital value inputted to a digital-to-analog converter
110
.
The above digital value is fed from a pre-stage counter
109
, and therefore the produced driving current value is proportional to a value of counter
109
. A photo diode (PD) for monitoring produces a monitoring current proportional to the light amount emitted from an emission element (LD). Then, the monitoring current value is converted to a voltage value in monitoring portion
106
, to be compared in comparator
102
to a reference voltage
104
(i.e. a target value).
A counter value of counter
109
is changed using the result of comparison performed by comparator
102
in which a differential amplifier is used. Namely, when an output of monitoring portion
106
is smaller than the reference value
104
, the counter value in counter
109
is increased by 1 to increase a driving current value. Also, when an output of monitoring portion
106
is greater than the reference value, the value in counter
109
is decreased by 1 to reduce the driving current. Through the operation of a negative feedback amplification described above, a light output is controlled to fix to the constant reference value.
Here, in the light output control circuit shown in
FIG. 33
, the driving current is controlled at the precision determined by the resolution which is fixed by the least significant bit (LSB) in digital-to-analog converter
110
. For example, when digital-to-analog converter
110
is composed of 10 bits, the obtained resolution becomes 2
10
=1024. When an output current to be controlled by this circuit ranges from 10 mA to 100 mA, a current source in LD driving circuit
101
is designed so that the LSB corresponds to 0.1 mA.
Also, the variation of digital values ranging from 100 to 1000 is so designed as to correspond to the variation of a driving current from 10 mA to 100 mA. Then, since the current 0.1 mA of the LSB corresponds 1% of the least value (10 mA) of the driving current, an accurate control having approximately 1% accuracy of driving current (also proportional to light output) can be achieved.
In the aforementioned second configuration of the conventional light output control circuit, a circuit to issue a light output deterioration warning may be similar to the deterioration warning circuit shown in
FIG. 32
in the first configuration of the conventional light output control circuit shown in FIG.
31
.
In an access system for optical communication system which has been in practical use in recent years, a burst transmission system is known as a required transmission system between subscribers and a switching office, in which data partitioned into cells are intermittently transmitted.
FIG. 34
illustrates an operational problem which may occur when the conventional light output control circuit shown in
FIG. 31
or
FIG. 33
is applied for the burst transmission system.
In
FIG. 34
, the horizontal axis and the
Ikeuchi Tadashi
Inoue Tadao
Matsuyama Toru
Miki Makoto
Murakami Norio
Chan Jason
Fujitsu Limited
Li Shi K.
Staas & Halsey , LLP
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