Coherent light generators – Particular beam control device – Having particular beam control circuit component
Reexamination Certificate
1998-10-27
2001-09-18
Arroyo, Teresa M. (Department: 2881)
Coherent light generators
Particular beam control device
Having particular beam control circuit component
C372S038020
Reexamination Certificate
active
06292497
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a laser diode driving method and circuit for controlling an optical output and extinction ratio at a constant level in correspondence with deterioration of the laser diode with time.
An example of a laser diode driving circuit used in an optical transmission system or the like is disclosed in Japanese Patent Laid-Open No. 2-308584 in which the optical output from the laser diode is controlled at a constant level regardless of the ambient temperature.
FIG. 5
shows the arrangement of a conventional laser diode driving circuit for controlling the optical output from the laser diode.
A laser diode LD is driven by a current prepared by superposing a driving current Id and a bias current Is. The driving current Id is a pulse current based on transmission data, whereas the bias current Is is a base current for causing the LD to emit light by induced emission. A temperature sensor
110
generates a voltage corresponding to the ambient temperature and outputs the voltage as an analog temperature signal representing the ambient temperature to an A/D converter
120
. The A/D converter
120
converts the input temperature signal into a digital signal and outputs the digital signal to a memory
150
.
The memory
150
uses input digital signal as an address signal to read out digital data stored at the corresponding address from the memory
150
and output the data to a D/A converter
130
. The D/A converter
130
converts the input digital data into an analog signal and outputs the analog signal to a current controller
140
. The current controller
140
controls an emitter current Is common to transistors Q
1
and Q
2
in accordance with the analog signal from the D/A converter
130
.
The operation of the laser diode driving circuit will be described.
A pre-bias signal is applied to the base of the transistor Q
1
. Assume that the state in which the pre-bias signal voltage is higher than a reference voltage (−VR) is a disable state. In the disable state, the transistor Q
1
is turned on, the transistor Q
2
is turned off, and the laser diode LD is not driven. Assume that the state in which the pre-bias signal voltage is lower than the reference voltage (−VR) is an enable state. In the enable state, the transistor Q
1
is turned off, the transistor Q
2
is turned on, and the laser diode LD is driven by a current, i.e., a current which changes between Is and Is+Id, prepared by superposing the driving current Id and the bias current Is generated by the current controller
140
.
In the memory
150
, data about the bias current corresponding to the ambient temperature is stored. When data obtained by digitally converting a temperature signal representing the ambient temperature is input from the A/D converter
120
to the address line of the memory
150
, the memory
150
outputs data about the bias current corresponding to the ambient temperature to the data line.
The D/A converter
130
D/A-converts the bias current data output to the data line, and outputs the analog signal to the current controller
140
. The current controller
140
controls the emitter current of the transistors Q
1
and Q
2
in accordance with the analog signal output from the D/A converter
130
.
In the laser diode driving circuit, the emitter current Is is adjusted in correspondence with the ambient temperature. That is, when the ambient temperature changes, data on the address line changes, and data about a new bias current appears on the data line. The D/A converter
130
D/A-converts the data on the data line, and the current controller
140
converts the signal output from the D/A converter
130
into a current.
At this time, if the pre-bias signal changes to the enable state, the laser diode LD is driven by a current prepared by superposing the driving current Id on the new bias current Is adjusted in correspondence with the ambient temperature.
The laser diode driving circuit employs a feed forward controller. Since the circuit performs control for only optical output fluctuation conditions set in advance, it cannot control the optical output from the laser diode in correspondence with optical output fluctuation conditions other than ambient temperature fluctuations. For this reason, e.g., when the laser diode deteriorates with time to decrease the optical output, the optical output may be smaller than its lower limit defined in the optical transmission system.
In the laser diode driving circuit, light emission may delay, the extinction ratio may decrease, and the quality of the transmission system may degrade because no consideration is given to temperature fluctuations in differential quantum efficiency of the laser diode. That is, the optical output is controlled at a constant level by changing only the bias current without changing the driving current in correspondence with the ambient temperature.
FIGS. 1A and 1B
show the current vs. optical output characteristics of a general laser diode.
FIG. 1A
shows current vs. optical output characteristics when the bias current and the driving current are ideally distributed.
FIG. 1B
shows current vs. optical output characteristics when the driving current is kept constant.
In
FIGS. 1A and 1B
, t
1
, t
2
, and t
3
(t
1
<t
2
<t
3
) represent ambient temperatures; Isn, Idn, and Ithn (n=1, 2, 3), the bias current, the driving current, and the light emission threshold current of the laser diode; and Po, the optical output. The laser diode driving circuit controls the optical output Po at a constant level at the respective temperatures. The laser diode has such a characteristic that both the bias current Is and driving current Id required to obtain a constant optical output Po increase along with an increase in ambient temperature. As shown in
FIG. 1A
, it is ideal for efficiently driving the laser diode that the bias current Is is set about the light emission threshold current Ith of the laser diode, and the driving current Id is superposed on the bias current Is to keep the optical output constant.
In the laser diode driving circuit in
FIG. 5
for controlling the optical output at a constant level by changing the bias current Is while keeping the driving current Id constant, if the bias current Is
2
and the driving current Id
2
are optimum at an ambient temperature t
2
, but the ambient temperature decreases to t
1
, the set value Is
1
of the bias current becomes smaller than the light emission threshold current Ith
1
to delay light emission. If the ambient temperature increases from t
2
to t
3
, the set value Is
3
of the bias current exceeds the light emission threshold current Ith
3
to decrease the extinction ratio, failing to obtain a reliable extinction state.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a laser diode driving method and circuit capable of controlling an optical output and extinction ratio at a constant level against an optical output fluctuation factor such as deterioration of the laser diode with time that cannot be set in advance.
It is another object of the present invention to provide a laser diode driving method and circuit capable of controlling the optical output and extinction ratio of the laser diode at a constant level regardless of the ambient temperature.
To achieve the above objects, there is provided a laser diode driving method comprising the steps of adjusting, in accordance with an ambient temperature, a bias current set about a light emission threshold current of a laser diode, and a pulse current for causing the laser diode to emit light, and driving the laser diode by a current prepared by superposing the bias current and the pulse current, thereby controlling an optical output and extinction ratio of the laser diode at a constant level.
REFERENCES:
patent: 4797799 (1989-01-01), Inokuchi
patent: 5526164 (1996-06-01), Link
patent: 5812572 (1998-09-01), King
patent: 6055252 (2000-04-01), Zhang
patent: 59-112670 (1984-06-01), None
patent: 60-251731 (1985-12-01), None
patent: 2-308584 (1990-12-
Arroyo Teresa M.
Inzirillo Gioacchino
McGuireWoods LLP
NEC Corporation
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