Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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This invention relates generally to optical-
fiber transmission systems and more particularly to a
device for triggering an alarm system in the event of
insufficiency of the level of transmission on the line,
the device being intended for use in the receiver module
of a simplified transmission system ~or a single-fiber
optical communications link.
An.optical-fiber transmission system is constit-
uted by a transmitter module, a receiver module and a ~ib~r
extending between these modules. The transmitter module
comprises a light-emitting base constituted by a laser
diode, by a control photodiode and a laser/fiber optical
coupling system inserted in a connector plug for connect-
- ing said transmitter base to the optical-fiber cable, and
by an electronic control unit which is intended among
other design functions to re~ulate the optical trans-
mission power. The receiver module comprises a light-
detecting base constituted by a photodetector such as an
avalanche photodiode and by a photodiode/fiber optical
coupling system inserted in a connector plug for connect-
ing said base to the optical-fiber cable, and by an
electronic control unit which has the intended function,
among others, o. automatically regulating the ~ain of the
avalanche photodiodeO For a detailed description of the
receiver module, reference may usefully be made for
exampl.e to the article published in "The Bell System
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Technical Journal", volume 57, No 6, July - August 1978,
page 1837, entitled 1'Practical 45 - Mb/s Regenerator for
Lightwave Transmission".
In a transmission system which makes use of a
num~er of optical fibers, it is a known practice to
employ a microprocessor for the management of the receiver
module and in particular for triggeri~g an alarm system
in the event of insufficiency of the -transmission level.
However, the use of a microprocessor is not
lO,justified in the case of a transmission system which makes
use of a single optical fiber.
The invention is therefore directed to a device
for triggering an alarm system in the event of insuffi-
ciency of the transmission level for the receiver of a
transmission system which utilizes an optical fiber,
comprising :
~ a photodetector with automatic gain regulation ;
- a filter having a passband which is external to the
requency band employed for optical-fiber transmission,
~0 the input of said filter being connected to said photo-
detector,
- ~mplifying means connected to the output of said filter,
- rectifying means connected to the output of said amplify-
ing means,
- a voltage comparator circuit connected to the output of
said rectifying means, and
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- al~rm-triggering means connected to the output of said
voltage comparator circuit.
It will ~e noted that the electric circuit of
the alarm-triggerlng device described in the foregoing is
of particularly simple design.
In accordance with another distinctive feature
of the invention, the passband of the filter is external
to the frequency band employed for optical-fiber trans-
mission.
,J
Other features and advantages of the invention
will be more apparent upon consideration of the following
description and accompanying drawings, wherein :
- Fig. 1 is a diagram sho~m partly in the form of
blocks and representing the receiver module which com~rises
a first alternative embodiment of the alarm-triggering
device in accordance with the invention ;
- Fig. 2 is a diagram sho~ partly in the ~orm
of blocks and representing the receivex module which
comprises a secona alternative embodiment of the alarm-
triggering device in accordance with the invention ;
- Fig. 3 is a diagram sho~m partly in th~ form
of blocks and representing the receiver module which com-
prises a third alternative embodiment of the alarm-
triggering device in accordance with the invention.
In a first alternative embodiment o~ the -
invention which is illustrated in Fig. 1, the alarm-
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triggering device 1 in accordance with the invention is
connected at A to a photodetectox such as, for example,
an avalanche photodiode 2 provided within the receiver
module of a transmission system which makes use of a
single optical fiber. The input E of an amplifier A1 is
connected at A to the avalanche photodiode 2 and said
amplifier Al deli~ers at the output Sl pulses ha~ing a
peak-to-peak amplitude of 1 voltr for exampl~. A peak-to-
peak detector 3 i9 connected to the output Sl of the
'amplifier Al and ensures automatic gain regulation of the
a~alanche photodlode 2 ; the output S2 of said detector
is connected to a bias circuit 4 of the avalanche photo-
diode 2, the output S3 of said bias circuit being connected
to said avalanche photodiode. The avalanche photodiode 2
is connected to ground at A through a resistor R.
In the description which now follows, considera-
tion will be given to the structure of the alarm-triggering
device 1 in accordance with the first alternative embodi-
ment shown in Fig, 1.
Said alarm-triggering device 1 comprises a fiLter
5, the input E~ of which is connected to the avalanche
photodiode 2 through a resistor R' having a value which is
distinctly higher than that of the resistor ~. It will be
noted at this point that the filter 5 has a passband which
is external to the frequency band employed for trans-
mission by the pulse code. More specifically, the mean
frequency of the filter 5 can be either lower (the filter
5 in this case is a low-pass filter) or higher (the ilter
5 in this case is a high-pass filter) than the bottom or
top limits of the frequency band employed respectiuely for
transmission by the pulse code.
The output S4 of the filter 5 is connected to a
very-low-nolse ~mplifier 6/ the output S5 o w~ïch is
connected to an amplifier 7. At the output S~ of the
,
amplifier 7 is connected a rectifying circuit comprising
10 ,a diode D and a resistor Rl and, in series~ an integrating
circuit comprising a resistor R2 and a capacitor C. The
output of said integrating circuit is connected to one of
the input terminals E8 of a threshold comparator amplifier
8 whilst the other input terminal Eg of said amplifier 8
is connected to a source of variable reference voltage
generated at 9. A resistor R3 and an electroluminescent
diode D' for controlling an alarm system are connected in
series to the output S~ of the co~parator amplifier 8. It
will readily be apparent that the diode D' can be replaced
by any other suitable means without thereby departing from
the scope of the invention.
In Fig. 2, which illustrates a second alter-
native embodiment of the invention, the elements which
axe identical with those of Fig. 1 are designated by the
same references. The alarm-triggering device 1 is
connected to the output S1 of the amplifier Al ; more
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specifically, the input E~ of the filter 5 is connected
to the output Sl of the ampliier A1 and the output S4 of
said filter 5 is connected directly to the input of the
amplifier 7.
In Fig. 3, which illustrates a third embodiment
of the inven~ion, the elements which are identical with
those of ~ig. 2 are in turn designate* by the same reer-
ences. ~he alarm-triggexing device 1 described earlie~
is connected between the output Sl of the amplifier Al and
the bias circuit 4 of the avalanche photodiode 2. More
precisely, the output S8 of the comparator amplifier 8 is
connected to the electroluminescant diode D' and to the
input of the bias circuit 4. Thus the gain of the
avalanche photodiode 2 is automatically regulated by means
of the comparator amplifier 8.
The operation a~d the mode of utilization of-the
alarm-tri~gering device in accordance with the invention
may be deduced from the foregoing description and will
hereinafter be explained in greater detail.
In the first place and under normal operating
conditions, that is, with standardized incident optical
power, the coded electrical pulses produced by the
avalanche photodiode 2 are amplified by the amplifier A
and have a constant amplitude at the output by virtue of
a control loop circuit. The peak-to-peak ampli~ude of the
pulses is in fact measured by the peak-to-peak detector 3
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which consequently delivers a proportional direct-current
voltage. This voltage is applied to the bias circuit 4
of the avalanche photodiode 2 and therefore controls the
gain of this latter, said photodiode being also provided
with temperature drift. This control operation thus makes
it possible to maintain a standardized and constant ampli-
tude (a peak-to-peak amplitude of 1 volk, for example) of
the output signals in spite of any possible variations in
ambient temperature and variations in the incident optical
power.
Assuming now that there exists an insufficiency
of the line transmission level or in other words an in-
sufficiency of the incident optical power by reason of the
fact that the amplitude of the output signals remains
constant, the control system described in the foregoing
gives rise to o~erbiasing (with respect to normal biasing)
of the a~alanche photodioae 2 which therefore delivers at
its output a noise potential of the order of 1 volt, for
example.
The principle of alarm-system triggering lies in
the use of ~he energy within a frequency band which is not
employed for transmission of the pulse code i this is
obtained by means of the filter 5, the passband of which
is external to the frequency band employed by the pulse
code.
Thus, in accordance with the alternative
--8--
embodiment of Fig. 1, the noise is amplified by the first
low-noise ampliier 6 and by the second amplifier 7, then
rectified by the recti~ying circuit (D and Rl) and inte-
grated by the integrating circuit (R2 and C) in order to
produce a direct-current voltage which is proportional to
the e~fective value of the noiseO This voltage is then
compared with a reference vo~tage generated at 9 by the
amplifier 8. If this threshold voltage is attained, the
comparator amplifier 8 delivers a contxol signal to the
èlectroluminescent diode D' which triggers an alarm system
- of any suitable type.
It will be noted that the adjustment of said
reference voltage makes it possible to determine the
minimum optical alarm power ; for example, if the standard-
ized power is - 53 dBm, the alarm power is - 60 dBm.
The oregoing description remains applicable to
the alternative embodiments illustrated in Figs. 2 and 3.
It should be added that, in regard to the al-ternative
embodiment shown in Fig. 2, the noise is amplified solely
by the amplifier 7, ta~ing into account the sufficient
- level at the output o~ the amplifier A1. Furthermore, in
the alternative embodiment shown in Fig. 3, the alarm-
triggering device in accordance with the invention also
has the function o ensuring automatic gain regulation of
the avalanche photodiode.
