Coherent light generators – Particular component circuitry – Having feedback circuitry
Reexamination Certificate
1999-01-15
2001-02-20
Font, Frank G. (Department: 2877)
Coherent light generators
Particular component circuitry
Having feedback circuitry
C372S038020, C372S038070, C372S029011, C372S026000, C359S199200, C359S199200
Reexamination Certificate
active
06192060
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an optical transmitter in which a laser is used.
2. Description of the Related Art
Conventionally, an optical transmitter using a laser employs a compensator which compensates for variations of threshold current and differential quantum efficiency due to temperature variation or deterioration with the passage of time in an output of the laser.
For example, an automatic power control (hereinafter referred to simply as APC) is disclosed in Japanese Patent Laid-Open No. 90673/1993 that a control loop which receives and converts back light of a laser into a current by means of a photodiode, converts the current into a voltage, determines an average value of the voltage and controls a bias current with the average value and another control loop which detects a peak value of the voltage converted from the output current of the photo-diode and controls a modulation current for driving the laser with the peak value.
FIG. 1
is a block diagram showing an example of a conventional optical transmitter circuit having a function of compensating for variations of threshold current and differential quantum efficiency caused by temperature variation and deterioration with the passage of time of a laser.
Referring to
FIG. 1
, input signal
1
from an input terminal is applied to driver
43
, and laser
4
is driven by driver
43
and bias circuit
42
. Part of an output (back light) of laser
4
is monitored and converted into a current by photodiode
5
, and the current is converted into a voltage by current/voltage converter
45
.
An average value of the output voltage of current/voltage converter
45
obtained by first average value detector
47
and an average value of input signal
1
obtained by second average value detector
48
are compared with each other and the difference between them is amplified by amplifier
49
, and bias circuit
42
is controlled with the output of amplifier
49
.
Meanwhile, a peak value of the output from current/voltage converter
45
is detected by peak value detector
46
, and driver
43
drives laser
4
with the peak value so that laser
4
outputs light.
With the optical transmitter described above, however, in order to compensate for a characteristic variation by temperature variation or deterioration with the passage of time of the laser output, it is required to convert the current for monitoring back light of the laser into a voltage and detect the peak value of the voltage, and this restricts the modulation rate of the optical transmitter.
The reason is that, where the modulation rate is higher than a certain level, the peak value cannot be detected by the peak value detector due to a limitation in frequency characteristic of the photodiode which monitors the back light of the laser.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an optical transmitter circuit which compensates for temperature variation and deterioration with the passage of time of threshold current and differential quantum efficiency of a laser even when the laser is driven at a high modulation speed and allows production of an optical output having a fixed optical power and free from a pulse width distortion by converting monitor current of the laser into a voltage and detecting an average value of the voltage.
In order to attain the object described above, in an optical transmitter of the present invention, back light of a laser driven by a pulse current driver which receives an input signal is monitored by a photodiode, and the monitor current is converted into a voltage by a current/voltage converter. The voltage is averaged by an average value detector, and the average value is inputted to a bias controller and an operation circuit. Meanwhile, the input signal is also receive into a mark-space ratio detector, by which a mark-space ratio of the input signal is detected, and the mark-space ratio is supplied to both of the bias control circuit and the operation circuit similarly.
The operation circuit makes the main point of the present invention, and divides the average value inputted thereto by the mark-space ratio and outputs a resulting quotient value to a pulse current controller. The output of the operation circuit maintains a fixed value even if the mark-space ratio of the input signal varies, and the pulse current driver is driven with the output of the operation circuit. Further, the bias controller controls the output of the bias circuit so that the bias current of the laser may be substantially equal to threshold current. The laser is driven with the outputs of the pulse current driver and the bias circuit and outputs a required optical signal.
Since the optical transmitter of the present invention employs the operation circuit in place of the conventional peak value detector as described above, an optical output having a fixed optical power and free from a pulse width distortion can be obtained also upon high rate modulation of the laser.
REFERENCES:
patent: 4663760 (1987-05-01), Shimada et al.
patent: 5127015 (1992-06-01), Chikugwa et al.
patent: 5177755 (1993-01-01), Johnson
patent: 5448629 (1995-09-01), Bosch et al.
patent: 5488621 (1996-01-01), Slawson et al.
patent: 5526164 (1996-06-01), Link et al.
patent: 5548435 (1996-08-01), Tahara et al.
patent: 5966395 (1999-10-01), Ikeda
patent: 05063647 (1991-08-01), None
patent: 5-90673 (1993-04-01), None
Font Frank G.
McGinn & Gibb PLLC
NEC Corporation
Rodriguez Armando
LandOfFree
Optical transmitter does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Optical transmitter, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Optical transmitter will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2569763