Optical transmitter

Details Optical transmitter Optical transmitter

1999-04-27

2002-11-12

Pascal, Leslie (Department: 2633)

Optical: systems and elements

Deflection using a moving element

Using a periodically moving element

C359S199200

Reexamination Certificate

active

06480314

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical transmitter having a laser diode and capable of controlling its driving current by an automatic power control (automatic power control: APC hererinafter) circuit, and more particularly to an optical transmitter capable of controlling the optical output in transient response at the time of optical output cut-off or cancellation of optical output cut-off.
2. Prior Art
An optical transmitter includes a laser diode (light amplification by stimulated emission of radiation diode: LD hereinafter), and by controlling its driving current, a signal corresponding to transmission information is set to an optical receiver. However, since the LD changes in its characteristics depending on temperature changes or aging effects, if the same driving current is supplied, the output optical power from the LD fluctuates. As a result, the transmission output power may decrease to deteriorate the S/N (signal to noise) ratio, or the LD or the optical receiving element at the transmission destination may be broken by an excessive driving current. In the conventional optical transmitter, accordingly, to enhance its reliability, by using APC circuit and driving current control circuit, it is attempted to compensate for fluctuations of characteristics due to temperature changes or aging effects of LD and limit excessive driving current.
The APC circuit receives part of the LD output or back light by a photo diode, feeds back its reception power to control the driving current of the LD, so that a desired optical power is constantly delivered from the LD. The driving current control circuit controls the driving current supplied to the LD so as not to exceeds a limit value predetermined according to the LD standard.
FIG. 1
shows an outline of such conventionally proposed constitution of an optical transmitter. This optical transmitter comprises an LD
11
for delivering a signal light
10
depending on a driving current, and a photo diode
13
for receiving part of output light of the LD
11
or back light
12
. The optical transmitter further comprises a current-voltage converting circuit
14
for converting a received current generated by receiving part of the output light from the LD
11
or back light
12
by the photo diode
13
into a voltage value, an average detecting circuit
15
for detecting the average of voltage values converted by the current-voltage converting circuit
14
, and a reference current control circuit
16
for detecting the mark rate which is the appearance ratio of “1” state and “0” state of transmission data, and issuing a corresponding voltage value, in which an average voltage V
1
detected by the average detecting circuit
15
and a reference voltage V
2
detected by the reference voltage control circuit
16
depending on the mark rate of transmission rate are entered into an optical output control circuit
17
.
The optical output control circuit
17
issues a control signal so that the entered average voltage V
1
and reference voltage V
2
may be equal to each other, and its output is entered into a driving current control circuit
18
. The driving current control circuit
18
can control a laser driving circuit
19
for generating a driving current of the LD
11
as disclosed in, for example, Japanese Laid-open Patent No. 7-193540, and by controlling so that the laser driving circuit
19
may not pass any current exceeding the preset value of the LD
11
, the LD
11
is prevented from generating an excessive optical output. The output of the driving current control circuit
18
which is controlled thus by the optical output control circuit
17
is entered in the laser driving circuit
19
, and the driving current of the LD
11
can be controlled, so that the average voltage value detected by the average detecting circuit
15
by changing the optical power of the optical signal can be controlled.
That is, the optical output control circuit
17
can raise the average voltage by controlling the driving current control circuit
18
and laser driving circuit
19
so that the power of the output light from the LD
11
may be larger when the entered average voltage V
1
is smaller than the reference voltage V
2
, and thereby the difference between the average voltage V
1
and reference voltage V
2
can be decreased. On the other hand, when the entered average voltage V
1
is larger than the reference voltage V
2
, by controlling the driving current control circuit
18
and the laser driving circuit
19
so that the power of the output light from the LD
11
may be smaller, the average voltage is lowered, and the difference between the average voltage V
1
and reference voltage V
2
can be decreased. In this way, the optical output control circuit
17
issues a control signal corresponding to the difference between the average voltage V
1
and reference voltage V
2
to the driving current control circuit
18
in order to obtain a desired output light from the LD
11
. As a result, the optical output power of the LD
11
is maintained at a constant optical output depending on the reference voltage V
2
of the reference voltage control circuit
16
.
In such optical transmitter, it is possible to change the driving current by the laser driving circuit
19
so as to compensate for threshold current due to, for example, temperature changes of the LD
11
or fluctuations of the output power of the LD
11
due to variation of slope efficiency. Or, against characteristic changes due to aging effects of the LD
11
, the driving current can be changed so as to compensate for the variation, so that the output power of the LD
11
may be maintained constant.
By the way, the optical transmitter includes an optical output cut-off processing circuit
22
for controlling the laser driving circuit
19
for cutting off the optical output by an optical output forced cut-off input
20
from outside and a detection signal
21
of the preset condition of optical output cut-off. Accordingly, the optical output during optical output constant control can be cut off. Depending on the preset condition of optical output cut-off, herein, the optical output cut-off may be detected when the aforesaid mark rate is lower than the specified value, for example, due to cut-off the transmission data or clock. Thus, when the optical output cut-off is recognized by the optical output forced cut-off input
20
or optical output cut-off condition detection signal
21
, the optical output cut-off processing circuit
22
controls the laser driving circuit
19
, and cuts off the optical output from the LD
11
. On the other hand, when the optical output forced cut-off input
20
or optical output cut-off condition detection signal
21
is canceled, the optical output cut-off control of the laser driving circuit
19
is canceled, so that an optical signal controlled at a constant optical output can be issued.
The technology relating to such optical transmitter is disclosed, for example, in Japanese Laid-open Patent No. 7-193540 and Japanese Laid-open Patent No. 5-075547.
Japanese Laid-open Patent No. 7-193540 discloses an optical transmitter for communication for transmitting video by controlling the optical output constant by an optical automatic output control (auto power control) circuit, and using a semiconductor laser diode controlled so that the driving current may not exceed a specific value by using a limiter circuit, in which for the safety of the operator in case of emergency, the circuit for output shut-down of the semiconductor laser diode is realized simply by using a limiter circuit.
Japanese Laid-open Patent No. 5-075547 discloses an optical transmitter comprising a semiconductor laser, a semiconductor laser driving circuit for feeding a semiconductor laser driving current composed of a light modulation signal current for modulating the intensity of exit light of the semiconductor laser by an input signal and a direct-current bias signal into the semiconductor laser, a mark rate detecting circuit for detecting t

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