Note: Descriptions are shown in the official language in which they were submitted.
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"OPTICAL DEVICE FOR PROCESSING AN OPTICAL IMPULSE"
The present invention relates to an optical device
for generating, from an input optical impulse, a
first and a second output optical impulse, each
having a preselected temporal phase shift with
respect to the input optical impulse and a
preselected duration.
At the present time, in telecommunications
networks, optical technology is predominantly used
for the transmission of optical signals, in which use
is made of the known wideband properties of optical
fibres, while the operations of processing the
optical signals, such as multiplexing and switching,
are carried out by means of opto-electronic devices.
However, opto-electronic devices have the known
disadvantages of electronic circuits which are
becoming bottlenecks in optical communications
systems and in optical networks. This is because
electronic devices have a narrow bandwidth compared
with the optical band available in optical
communications systems, and are generally based on a
relatively slow serial processing of the signals.
Research is therefore being increasingly directed
towards the possibility of using optics not only for
the transmission of signals but also for their
processing. This is due to the fact that,
potentially, wholly optical devices have a wide
bandwidth and are transparent to the bit rate, the
format and the code of the transmission.
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The inventors of the present invention have noted
the necessity of an optical device which, on
receiving an optical impulse at its input, is capable
of supplying two optical impulses at the output, for
example two optical control impulses, each having a
preselected temporal phase shift with respect to the
input optical impulse and also having a preselected
duration.
A first aspect of the present invention is
therefore an optical device comprising:
- an input for an input optical impulse;
- an optical beam splitting element for supplying,
from the said input optical impulse, a first pair
of optical impulses and a second pair of optical
impulses;
- at least a first optical delay element to delay, by
a predetermined time, at least one of the said
optical impulses of the said first pair;
- at least a second optical delay element to delay,
by a predetermined time, at least one of the said
optical impulses of the said second pair;
- a first optical processing element capable of
supplying a first output optical signal when
commanded by the said first pair of optical
impulses;
- a second optical processing element capable of
supplying a second output optical signal when
commanded by the said second pair of optical
impulses;
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- a first output connected to the said first optical
processing element for the said first output
optical signal;
- a second output connected to the said second
optical processing element for the said second
output optical signal.
Typically, the said first output optical signal has
a duration which is modified with respect to at least
one of the said optical impulses.
Generally, the said second output optical signal
has a duration which is modified with respect to at
least one of the said optical impulses.
Preferably, the said optical beam splitting element
comprises a first optical beam splitter, to form a
first and a second optical impulse from the said
input optical impulse, and a second and a third
optical beam splitter to form the said first pair of
optical impulses and the said second pair of optical
impulses from the said first optical impulse and from
the said second optical impulse, respectively.
More preferably, the said optical device also
comprises at least a third optical delay element to
delay, by a predetermined time, at least one of the
said first and second optical impulses.
Typically, the said first optical processing
element consists of an optical flip-flop. Preferably,
the said second optical processing element also
consists of an optical flip-flop.
A second aspect of the present invention is
therefore a method for supplying, from an input
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optical impulse, a first and a second output optical
impulse, each having a preselected time delay with
respect to the input optical impulse, and also having
a preselected duration, the said method comprising
the steps of
a) supplying the said input optical impulse;
b) supplying a first pair of optical impulses and a
second pair of optical impulses from the said
input optical impulse;
c) imparting a predetermined delay to at least one of
the said optical impulses of the said first pair;
d) imparting a predetermined delay to at least one of
the said optical impulses of the said second pair;
e) supplying the said first output optical impulse as
a function of the said optical impulses of the
said first pair;
f) supplying the said second output optical impulse
as a function of the said optical impulses of the
said second pair.
Preferably, one of the said optical impulses of the
said first pair is delayed by a predetermined time
with respect to the other.
Advantageously, one of the said optical impulses of
the said second pair is delayed by a predetermined
time with respect to the other.
According to one embodiment, step e) consists in
supplying an output optical signal at the command of
one of the two optical impulses of the said first
pair and interrupting the transmission of the said
output optical signal at the command of the other of
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the two optical impulses of the said first pair, in
such a way as to supply the said first output optical
impulse.
According to another embodiment, step f) consists
in supplying an output optical signal at the command
of one of the two optical impulses of the said second
pair and interrupting the transmission of the said
output optical signal at the command of the other of
the two optical impulses of the said second pair, in
such a way as to supply the said second output
optical impulse.
Preferably, the said first output optical impulse
has a duration which is modified with respect to the
said input optical impulse.
Even more preferably, the said first output optical
impulse is time-delayed with respect to the said
input optical impulse.
Preferably, the said second output optical impulse
has a duration which is modified with respect to the
said input optical impulse.
Even more preferably, the said second output
optical impulse is time-delayed with respect to the
said input optical impulse.
Characteristics and advantages of the invention
will now be explained with reference to embodiments
shown by way of example and without restriction in
the attached figures, in which:
- Fig. 1 is a schematic representation of a form of
application of the optical device according to the
invention in an optical selector;
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- Fig. 2 is a schematic representation of an optical
device according to the invention;
- Fig. 3 is a schematic representation of one
embodiment of an optical gate of the optical
selector shown in Fig. 1;
- Fig. 4 is a schematic representation of a first
embodiment of a variable delay unit of the optical
device shown in Fig. 2;
- Fig. 5 is a schematic representation of four
variants (Figs. 5a-5d) of a second embodiment of a
variable delay unit of the optical device shown in
Fig. 2;
- Fig. 6 is a schematic representation of a third
embodiment of a variable delay unit of the optical
device shown in Fig. 2;
- Fig. 7 is a schematic representation of a first
embodiment of an optical flip-flop of the optical
device shown in Fig. 2;
- Fig. 8 is a schematic representation of an optical
switch included in the variable delay unit of Fig.
4;
- Fig. 9 is a schematic representation of the
operation of the optical selector shown in Fig. 1;
- Fig. 10 is a schematic representation of a second
embodiment of an optical flip-flop of the optical
device shown in Fig. 2;
- Fig. 11 is a schematic representation of a third
embodiment of an optical flip-flop of the optical
device shown in Fig. 2.
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According to the embodiment shown in Fig. 2, the
optical device 40 according to the invention
comprises a first 1x2 coupler 41, a second 1x2
coupler 42 and a third 1x2 coupler 43, a first delay
unit 44, a second delay unit 45 and a third delay
unit 46, a first flip-flop 47 and a second flip-flop
48, and suitable optical connections 3, 4, 6, 7, 8,
9. The input of the first coupler 41 forms the input
50 of the optical device 40, the output of the flip-
flop 48 forms its first output 51 and the output of
the flip-flop 47 forms its second output 52. The
connections 3, 4, 6, 7, 8, 9 preferably consist of
optical fibres or waveguides.
The couplers 41-43 are conventional couplers.
Preferably, they are 1x2 directional 50/50 (3 dB)
couplers.
The delay units 44, 45 and 46 may either be of the
constant delay type (the time interval between the
moment at which the signal appears at the input and
that at which the same signal begins to be available
at the output is substantially fixed and not
modifiable) or of the variable delay type (the time
interval between the moment at which the signal
appears at the input and that at which the same
signal begins to be available at the output is
modifiable) .
For example, in the illustrated embodiment, the
delay units 44 and 45 are of the constant delay type,
while the unit 46 is of the variable delay type.
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The constant delay units 44 and 45 are made, for
example, from a section of optical fibre having a
length L = v*T, where T is the delay to be obtained
and v (v = c/n) is the velocity c of light inside an
optical fibre having a refractive index n.
According to the embodiment shown in Fig. 4, the
variable delay unit 46 consists of an input 1x2
switch 461, an output 2x1 switch 462 and a certain
number of 2x2 switches, disposed in series between
the two input and output switches 461 and 462. The
input switch 461 has one input and two outputs
connected, respectively, to a predetermined optical
fibre delay line and to a section of optical fibre of
negligible length (which introduces a negligible
delay in an optical signal passing through it). The
i-th switch has two inputs, one for a predetermined
optical fibre delay line and the other for a section
of fibre of negligible length, and two outputs
connected to a further predetermined delay line and
to a further section of fibre of negligible length.
Finally, the output switch 462 has two inputs, for a
predetermine delay line and a section of fibre of
negligible length respectively, and one output.
In order to enable M different delays to be
provided, the unit 46 preferably consists of a
number, equal to log2M, of switches connected in
series, including the input switch 461, in addition
to the output switch 462. Additionally, the
predetermined optical fibre delay line connected to
the output of the i-th switch preferably has a length
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such that it causes a delay equal to T/2i (1 < i <_
log2M) in the signal in transit.
The switches in series are controlled by suitable
control signals cl-Clog2M which determine the path of
the signal in transit along the delay lines and/or
along the sections of fibre of negligible length and,
consequently, the delay applied to this signal in
transit. This is because the signal arriving at one
of the two inputs of the switch is sent to one output
rather than to another, according to the presence or
absence of the control signal.
Typically, a switch is a device provided with at
least one input, at least two outputs for a signal in
transit and at least one input for at least one
control signal. In the absence of the control signal,
the input signal leaves the device through one of the
said outputs, whereas in the presence of the control
signal the signal is diverted to another of the said
outputs.
The said switch may consist of a device having a
conventional interferometric structure of the Mach-
Zehnder type. Fig. 8 shows, for example, the 1x2
input switch 461. This comprises an input coupler
4610, an output coupler 4620, two guided optical
paths 4630 and 4640, a coupler 4670 (not shown) for
an optical control signal 4600 and two outputs 11 and
12. In turn, each of the two guided optical paths
4630 and 4640, preferably consisting of optical
fibres or waveguides, comprises a conventional
optical amplifier 4650 and 4660, respectively. The
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said optical amplifiers 4650 and 4660 are, for
example, of the type consisting of optical fibres
doped with rare earths or of the semiconductor type,
and their gain is regulated in such a way that the
optical signals arriving from the two guided optical
paths 4630 and 4640 have the same strength at the
input of the said output coupler 4620.
The couplers 4610, 4620 and 4670 are conventional
couplers. Preferably, they are directional 50/50
couplers consisting of optical fibres or waveguides
(3 dB) .
According to one embodiment, the amplifiers 4650
and 4660 are conventional optical semiconductor
amplifiers consisting of active InGaAsP waveguides
surrounded by a shell of InP. By regulating the
supply current of the said optical semiconductor
amplifiers or by illuminating the active waveguide
with a predetermined optical control signal, it is
possible to vary the density of the charge carriers
inside the said active waveguide and, consequently,
its refractive index. For the construction of the
switch 461, the supply current of the said optical
amplifiers 4650 and 4660 is regulated in such a way
as to impart a predetermined phase shift between the
optical signals which are propagated in the optical
paths 4630, 4640 and which are then added in a
coherent way in the output coupler 4620. For example,
the said supply current is regulated in such a way
that the signals propagated in the two optical paths
4630 and 4640 interfere constructively in the output
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11 and destructively in the output 12. Conversely,
the strength and the wavelength of the optical
control signal 4600 are selected in such a way as to
make the signals propagated in the two optical paths
4630 and 4640 interfere constructively in the output
12 and destructively in the output 11. In this way,
in the absence of an optical control signal 4600
(normal operating conditions), an optical signal at
the input of the switch is diverted towards the
output 11 while, in the presence of the optical
control signal 4600, it is diverted towards the
output 12.
Alternatively, the control signal 4600 may be
electrical.
According to a second embodiment, the variable
delay unit 46 consists of a conventional tree
structure such as that shown in Figs. 5a-5d. This
structure comprises an input 4607, a plurality of
sections of optical fibre (indicated as a whole by
the number 4603 in Fig. 5), each having a
predetermined length, and an output 4608. In Figs. 5a
and 5b, conventional beam splatters (for example, 1x2
directional couplers in series, indicated as a whole
by the number 4604 in Figs. 5a and 5b) repeatedly
divide an input optical signal and transmit it in the
various sections of optical fibre 4603. The outputs
of these latter sections of optical fibre 4603 are
then coupled by suitable couplers (for example, 2x1
couplers in series, indicated as a whole by the
number 4605 in Fig. 5) to return to a single optical
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fibre at the output 4608. Optical gates (indicated as
a whole by the number 4602 in Figs. 5a and 5b) are
present at the outputs of the various sections of
optical fibre 4603, only the gate corresponding to
the signal delayed by the desired quantity being open
(allowing the signal to pass), while the others are
closed (blocking the passage of the signal).
Alternatively, as shown in Figs. 5c and 5d, in place
of the said beam splatters 4604 it is possible to
connect 1x2 switches (indicated as a whole by the
number 4606 in Figs. 5c and 5d) which direct the
input signal as required, according to the delay
which is to be imparted to it, into different
sections of optical fibre 4603. The said switches
4606 may be, for example, of the type described
previously and shown in Fig. 8.
In turn, the said optical gates 4602 consist, for
example, of a conventional interferometer of the
Mach-Zender type as shown in Fig. 3.
Typically, an optical gate is a device provided
with an input and an output for an optical signal in
transit and with at least one input for at least one
control signal capable of varying the normal state of
the said gate. More particularly, the said at least
one control signal closes a gate which is normally
open, or vice versa.
According to the embodiment shown in Fig. 3, each
gate comprises a 1x2 input coupler 21, a 2x1 output
coupler 22, a first optical propagation path 24 and a
second optical propagation path 25 and a first
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optical amplifier 23 and a second optical amplifier
26. The said optical amplifiers 23 and 26 are
conventional optical amplifiers, for example of the
optical fibre type doped with rare earths or of the
semiconductor type. In the embodiment shown, they are
of the semi-conductor type. Typically, the said
couplers 21 and 22 are conventional 50/50 directional
couplers (3 dB) consisting of optical fibres or
waveguides. The input coupler 21 divides an input
optical signal into two signals of approximately the
same strength which travel along the two paths 24 and
25 of the interferometer. Each of the optical paths
24 and 25 preferably consists of an optical fibre or
of a waveguide. The first optical semiconductor
amplifier 23 is connected in the first path 24, while
the second optical semiconductor amplifier 26 is
connected in the second path 25. Each of the
semiconductor amplifiers 23 and 26 consists, for
example, of an active InGaAsP waveguide surrounded by
a shell of InP, and their supply current is
regulated, as stated previously, in such a way as to
impart a predetermined phase shift between the
optical signals propagated in the optical paths 24
and 25. When the said signals are phase shifted with
respect to each other by 180° (~), they interfere
destructively in the output coupler 22 and the gates
are closed (blocking the passage of the optical
signal), whereas when they are shifted by 0° or 360°
(2~), they interfere constructively and the gates are
open (allowing the optical signal to pass).
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Additionally, by means of an optical control signal
100 having suitable strength and wavelength it is
possible to obtain, in a way similar to that
demonstrated for the switches, a further phase shift
between the optical signals in the two optical paths
24 and 25 and thus to change the state of the gates
from closed to open or from open to closed. The
optical control signal 100 is supplied to one of the
two amplifiers (to the amplifier 26 in the case of
Fig. 3) by means of a third 2x1 conventional coupler
27 consisting of optical fibre or a waveguide.
Alternatively, the optical control signal 100 is
supplied by means of a conventional optical
circulator. Additionally, the gain of the said
optical semiconductor amplifiers 23 and 26 is
regulated in such a way that the optical signals from
the two guided optical paths 24 and 25 have the same
strength at the input of the said output coupler 22.
According to one embodiment, an optical filter (not
shown) is connected at the output of the coupler 22
to filter the ASE (Amplified Spontaneous Emission)
generated by the optical amplifiers 23 and 26 and to
reduce the quantity of noise transmitted to the
following stages. The said optical filter may be of
the interference type such as a conventional
reference filter, a Mach-Zehnder filter, an
interference grating or a diffraction grating.
Alternatively, the said gates may be made by
connecting a single optical semiconductor amplifier
directly into the optical path of the input signal.
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Given the connection losses due both to the
reflections of the signal at the input and output of
the amplifier and to the material of which the
amplifier consists, a gate will be open when the gain
of the amplifier is greater than the connection
losses, in other words when the supply current is
greater than a predetermined threshold value.
Conversely, the gate will be closed when the gain of
the amplifier is less than the connection losses, in
other words when the supply current is below the said
threshold value. According to another variant, the
gate may be closed by means of an optical control
signal having a wavelength and strength such that the
optical amplifier is saturated and thus preventing a
further optical signal at the input of the amplifier
from being transmitted to its output.
According to a third embodiment, the variable delay
unit 46 has a structure of the loop type, as shown in
Fig. 6. In this type of structure, the input optical
signal is delayed by making it circulate for a
predetermined number of times in an optical fibre
loop having a predetermined length according to the
delay which is to be obtained. The signal is then
collected at the output by means of a 2x2 switch 4601
(of the type shown in Fig. 8, for example) which has
the function of inserting the signal into the said
optical fibre loop and of extracting it at the output
of the loop.
Typically, the optical flip-flops 47 and 48 are
two-state devices which remain in one state or the
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other until a signal causing the transition from one
state to the other is applied to them. For example,
they may be conventional optical devices of the Set-
Reset (SR) type with two inputs and one output, in
which an optical set impulse at one of the two inputs
sets the state of the output to 1 (emission of an
output optical signal) and an optical reset impulse
at the other input sets the state of the output to 0
(absence of an output optical signal). The output of
the device remains in the state 1 until an optical
reset impulse causing the transition from the state 1
to the state 0 is applied to one of the two inputs.
In turn, the output remains in the state 0 until an
optical set impulse causing the transition from the
state 0 to the state 1 is applied to the other of the
two inputs.
For example, according to the embodiment shown in
Fig. 7, each of the optical flip-flops 47 and 48
consists of a loop-type optical fibre structure 474
in which a conventional 2x2 coupler 471 and an
optical gate 473 are connected. The coupler 471 is
preferably of the 50/50 directional type and the
optical gate 473 is, for example, one of the types
described previously. Additionally, each flip-flop
has a first set input 475 for an input optical
impulse (set), a second reset input 476 for an
optical control impulse (reset) and an output 477.
Following an optical set impulse at the input 475 and
in the absence of the optical reset impulse, an
output optical signal having substantially constant
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_ _ lg _
strength is transmitted to the output 477 of the
flip-flop. Conversely, in the presence of the optical
reset impulse, no optical signal is transmitted to
the output 477 of the flip-flop.
The coupler 471 subdivides the optical set impulse
into two impulses having substantially the same
strength and transmits one of them into the loop
structure 474 and the other to the output 477. In the
absence of the optical reset impulse, the optical
gate 473 is open and allows the optical set impulse
to pass in the loop structure 474 to the coupler 471.
Every time the optical set impulse transmitted into
the loop structure 474 returns to the coupler 471,
half of its strength is again transmitted to the
output and the other half into the loop structure
474. Preferably, the total length of the optical
fibre loop structure 474 is selected in such a way
that the optical set impulse which is made to
circulate in it is subsequently transmitted to the
output, at the tail of the preceding optical impulse
transmitted to the output. In other words, the
propagation time in the loop structure 474 is
preferably equal to the duration of the optical set
impulse. In this way, an optical signal having a
duration equal to a multiple of the duration of the
set impulse is transmitted to the output 477 of the
flip-flop. Additionally, the gain of the optical
semiconductor amplifiers of the gate 473 is
preferably selected in such a way as to compensate
for the losses undergone by the optical set impulse
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during its propagation in the loop structure 474 and
thus to ensure that the said output optical signal
has a constant strength.
In the presence of the optical reset impulse the
optical gate 473 closes and thus interrupts the
propagation of the said optical set impulse in the
loop structure 474. Preferably, the optical reset
impulse has a duration equal to that of the optical
set impulse. In this way, the optical gate 473
remains closed for a sufficiently long time to
extinguish the optical set impulse in the loop
structure 474. Consequently, while the optical reset
impulse is absent, the optical set impulse is
continually retransmitted to the output, in such a
way that an optical signal of virtually constant
strength is present at the output of the flip-flop.
Following the arrival of the optical reset impulse,
however, the transmission of the said optical signal
to the output 477 of the flip-flop is blocked. At the
output of the flip-flop, therefore, there is an
output optical impulse which is temporally aligned
with the arrival of the optical set impulse and has a
duration equal to the difference between the time of
arrival of the optical reset impulse and the
preceding time of arrival of the optical set impulse.
According to one embodiment, an optical filter (not
shown) is connected in the loop structure 474 to
filter the ASE (Amplified Spontaneous Emission)
generated by the optical amplifiers present in the
optical gate 473 and to reduce the quantity of noise
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accumulated along the said loop structure 474. The
said optical filter may be of the interference type
such as a conventional reference filter, a Mach-
Zehnder filter, an interference grating or a
diffraction grating.
According to another embodiment, the optical flip-
flops 47 and 48 may be of the type described in
patent application EP 97122771 in the name of the
present applicant. For example, as shown in Fig. 10,
they may be formed in free space where the light
beams are propagated in a vacuum or in the atmosphere
between optical elements such as filters, prisms and
mirrors.
According to the embodiment in Fig. 10, a flip-flop
comprises a first optical beam splatter 200 having a
first input for an optical set impulse 210, a second
input and an output, a second optical beam splatter
220 having an input coupled optically to the output
of the first optical beam splatter 200 and two
outputs, a third optical beam splatter 280 having a
first input coupled optically to an output of the
second optical beam splatter 220 and a first output
coupled optically to the second input of the first
optical beam splatter 200 in such a way as to form a
loop. The said optical beam splatter 280 also
comprises a second input for an optical reset impulse
230 and a second output coupled optically to an
optical amplifier 260 included in the said loop. The
said optical beam splatters are, for example,
conventional partially reflecting mirrors or
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conventional prisms. A reflecting element 240, such
as a prism, a mirror or the like, optically couples
the optical amplifier 260 to an output of the optical
beam splatter 220. The said optical amplifier 260 is
a conventional optical amplifier, for example of the
fibre type doped with rare earths or of the
semiconductor type.
Preferably, an optical filter 300 is connected in
the loop 22 to filter the ASE (Amplified Spontaneous
Emission) generated by the optical amplifier 260 and
to reduce the quantity of noise accumulated along the
loop. The said optical filter 300, as stated
previously, may be of the interference type such as a
conventional reference filter, a Mach-Zehnder filter,
an interference grating or a diffraction grating.
The operation of this embodiment of the flip-flop
is entirely analogous to that of the preceding one.
An optical set impulse 210 input into the first
optical beam splatter 200 enters the loop. The second
beam splatter 220 divides the said optical set
impulse into an optical signal 140 which leaves the
loop and into an optical feedback signal which is
transmitted to the optical amplifier 260. The optical
feedback signal is amplified by the optical amplifier
260 and then retransmitted to the optical beam
splatter 220 after two reflections by the optical
beam splatters 280 and 200. The optical beam splatter
220 divides the optical feedback signal into a first
portion, which maintains the output optical signal
140 even after the termination of the optical set
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impulse 210, and a second portion which is again
transmitted along the loop. The output optical signal
140 is then interrupted by an optical reset impulse
230 which saturates the optical amplifier 260 and
thus blocks the propagation of the optical feedback
signal along the loop.
Preferably, in this case also the gain of the
optical amplifier is such that it compensates for the
losses undergone by the optical feedback signal in
the loop and the propagation time along the loop is
equal to the duration of the set impulse 210.
In one embodiment, the wavelength of the optical
reset impulse 230 is different from that of the
optical set impulse 210. The filter 300 can thus be
selected in such a way as to allow the wavelength of
the optical set impulse 210 to pass and to stop that
of the optical reset impulse 230, preventing the
optical reset impulse 230 from being transmitted to
the output.
According to a further embodiment shown in Fig. 11,
the loop shown in Fig. 10 also comprises an optical
gate 360, an optical beam splitter 340 for supplying
the optical reset impulse 230 to the optical gate 360
and a reflecting element 320 in place of the optical
beam splitter 280. The optical gate is, for example,
of the same type as those described previously. The
operating principle of this embodiment is entirely
analogous to that of the preceding embodiment.
With reference to an example of operation of the
optical device 40, an optical impulse 222 at the
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input of the coupler 41 is divided into two optical
impulses 444 and 333 of approximately equal strength.
The optical impulse 333 is transmitted along the
connection 3 towards the constant delay unit 45 which
delays it by a length of time equal to a
predetermined time interval T~. The optical impulse
333, delayed by the unit 45, is then divided into two
further optical impulses 666 and 777 by the coupler
42. The first impulse 666 is transmitted to the set
input of the flip-flop 48 through the connection 6,
while the second impulse 777 is transmitted to the
variable delay (T) unit 46 and then, through the
connection 7, to the reset input of the said flip-
flop 48. In this way, with a delay T~ in the arrival
of the control impulse 222 at the input of the
optical device 40, the optical impulse 666 (the set
impulse) causes the emission of an optical signal, at
substantially constant strength, from the output of
the flip-flop 48. Then, after a delay T with respect
to T~, the optical impulse 777 (the reset impulse)
blocks the transmission of the said optical signal to
the output of the flip-flop 48.
In turn, the optical impulse 444 is transmitted
along the connection 4 to the coupler 43, which
produces an optical impulse 888 which, through the
connection 8, arrives at the set input of the flip-
flop 47, and an optical impulse 99 which, through the
connection 9, arrives at the reset input of the said
flip-flop 47. The said optical impulses 888 and 999
are temporally phase-shifted with respect to each
CA 02282251 1999-09-15
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other by a time T~ by the constant delay unit 44. In
this way the optical impulse 888 (the set impulse),
with a negligible delay with respect to the arrival
of the said optical control impulse 222 at the input
of the optical device 40, causes the emission of an
optical signal, of a substantially constant strength,
from the output of the flip-flop 47. Additionally,
the optical impulse 999 (the reset impulse), with a
delay T~ after the arrival of the said control
impulse 222, blocks the transmission of the said
optical signal to the output of the flip-flop 47.
Consequently, according to the illustrated
embodiment, the optical device 40 generates, as a
result of the optical control impulse 222 at the
input, two optical impulses, of which
- the one at the output 52 has a duration of T~ and
is aligned temporally with the arrival of the
optical control impulse 222 at the input; and
- the one at the output 51 has a duration of T and is
delayed, with respect to the arrival of the optical
control impulse 222 at the input, by a time equal
t o T~ .
Therefore, if the delay units are selected in a
suitable way, the optical device 40 according to the
invention, when it has received an optical impulse at
its input, generates two optical impulses at the
output, each having a desired delay with respect to
the arrival of the input optical impulse, and also
having a desired duration. For example, in the
absence of the delay unit 45, both of the output
CA 02282251 1999-09-15
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optical impulses can be aligned temporally with
respect to the arrival of the input optical impulse
and can have, depending on the way in which the delay
units 44 and 46 are selected, durations which are
equal to or different from each other and equal to or
different from the input optical impulse. Conversely,
in the presence of a delay unit for both of the
optical impulses 333 and 444, both of the output
optical impulses can be temporally phase-shifted with
respect to the arrival of the input optical impulse,
and can have, depending on the way in which the delay
units 44 and 46 are selected, durations which are
equal to or different from each other and equal to or
different from the input optical impulse.
The optical device 40 according to the invention
may be used, for example, to control the opening and
closing times of the input and output of an optical
selector to control the frequency of arrival (f) of
cells in optical networks using asynchronous transfer
mode (ATM).
ATM is a transmission method consisting in grouping
the data transmitted from various sources in packets
of digital data (cells) which consist of a payload of
43 bytes and a header of 5 bytes. The header
comprises various fields containing data used by the
nodes of the ATM network to control the switching of
the ATM cells. This transmission method also includes
a negotiation of the cell transmission frequency
(PCR) before the opening of a connection between a
CA 02282251 1999-09-15
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source and an ATM network or between two different
ATM networks.
The PCR check is a very important operation in
optical ATM networks. It must be carried out both at
the point at which the traffic sources have access to
an ATM network and at points of interconnection
between different ATM networks, and consists in
checking that each source or network does not
generate cells at a frequency (f) greater than that
negotiated, in other words greater than the frequency
(PCR) which has been assigned to it by the network to
which it is connected.
Fig. 1 shows an optical selector 8000 which acts as
a PCR checker in ATM networks, using an optical
device according to the present invention. The said
optical selector 8000 can permit the passage of only
those ATM cells which appear at its input with a
frequency f (cells per second) less than or equal to
a predetermined frequency PCR (f 5 PCR).
As shown in Fig. l, the optical selector 8000
comprises an input optical gate 10, an output optical
gate 20, an ATM cell recognition device 30 and an
optical device 40, according to the invention,
connected in a loop structure consisting of 5 optical
connections 1, 2, 5, 15 and 16.
Preferably the said optical connections 1, 2, 5, 15
and 16 consist of conventional optical fibres or
waveguides.
The input optical gate 10 and the output optical
gate 20 are, for example, of the type shown in Fig.
CA 02282251 1999-09-15
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3. In normal operating conditions (in the absence of
a control signal), the input gate 10 is in an open
state, while the output gate 20 is in a closed state.
The optical impulse from the output 51 of the optical
device 40 forms the control signal for the input
optical gate 10, while the optical impulse from the
output 52 of the optical device 40 forms the control
signal for the output optical gate 20. Consequently,
in the presence of an optical impulse arriving,
through the optical fibre connection 16, from the
flip-flop 48, the input optical gate 10 changes its
state from open to closed and maintains it until the
said optical impulse is present (for a time equal to
T). In turn, in the presence of an optical impulse
arriving, through the optical fibre connection 15,
from the flip-flop 47, the output optical gate 20
changes its state from closed to open and maintains
it while the said optical impulse is present (for a
time equal to T~) .
According to a preferred embodiment, the cell
recognition device 30 is a wholly optical device
capable of recognizing the header of an ATM cell at
its input. The said device permits the passage of the
cells of bits arriving at its input from the optical
fibre connection 1 to the optical fibre connection 5
and, when it recognizes that the header of an ATM
cell is present at its input, sends an optical
control impulse 222 for the optical device 40 along
the optical fibre connection 2. For example, European
patent application No. 97201988.9 filed by the
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present applicant describes a wholly optical device
(not shown) which generates an optical impulse at the
output when it recognizes a predetermined sequence of
bits (the header of an ATM cell) at its input. The
said device comprises
- a series/parallel converter for converting a series
(cell) of N bits into a corresponding spatial
pattern of N bits carrying the same information;
- optical means for generating, from the said spatial
pattern of N bits, a first two-dimensional image
consisting of N rows and M columns;
- optical means for carrying out a logical AND
operation between the elements of the said first
two-dimensional image and those of a predetermined
second two-dimensional image having N rows and M
columns, and for generating in this way a third
two-dimensional image having N rows and M columns;
- means for carrying out a logical XOR operation
between the elements of each column of the said
third two-dimensional image, and for generating in
this way a second parallel pattern of M bits;
- means for carrying out a logical OR operation
between the bits of the said second parallel
pattern of M bits, and for generating in this way
the said output optical impulse.
Figures 9(a)-9(d) represent schematically the
operation of the optical selector 8000 shown in Fig.
1. An ATM cell 1000, characterized by a predetermined
frequency f of arrival (cells per second), is
transmitted from the input gate 10, which is normally
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in the open state, to the cell recognition device 30,
through the connection 1. The cell recognition device
30 permits the passage of the cell 1000 along the
connection 5 towards the output gate 20 and, when it
recognizes that the header of an ATM cell is present
at its input, generates a first optical impulse 222,
temporally aligned with the header of the cell 1000,
and transmits it to the optical device 40 through the
connection 2 [Fig. 9(a)]. At this point,
1) with a negligible delay with respect to the
generation of the optical impulse 222,
- the optical device 40 generates a first optical
signal at the output 52 which, through the
connection 15, arrives at the output gate 20.
The said first optical signal changes the state
of the output gate 20 from closed to open and
thus permits the passage of the cell 1000 out of
the output of the optical selector 8000 [Fig.
9(b)];
2) with a delay T~, equal to the duration of the cell
1000, with respect to the generation of the
optical impulse 222, the optical device 40
- generates a second optical signal at the output
51 which, through the connection 16, arrives at
the input gate 10. The said second optical
signal changes the state of the input gate 10
from open to closed and thus prevents other
arriving cells from entering the optical
selector 8000 [Fig. 8(c)]; and
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- again causes the output gate 20 to close
(interrupts the emission of the said first
optical signal) [Fig. 9(d)] to prevent other
bits which may have entered the selector 8000
and which do not belong to the cell 1000 from
leaving it;
3) with a further delay T [T=(1/PCR)-T~] with respect
to T~, the optical device 40 again causes the
input gate 10 to open (interrupts the emission of
the said second optical signal) to permit the
passage of other ATM cells which arrive in the
selector 8000 with a delay greater than or equal
to TP~R with respect to the cell 1000.
In this way, if a second cell 2000 arrives in the
optical selector 8000 before a time interval TP~R =
T~+T = 1/PCR has elapsed after the arrival of the
first cell 1000, the input gate 10, being still in
the closed state, prevents it from entering the
optical selector 8000 [Fig. 9(d)]. Additionally, even
if, when the input gate 10 changes its state from
closed to open, part of the second cell 2000 is still
at the input of the gate 10, the said remaining part
of the cell 2000 is not recognized by the cell
recognition device 30 and, therefore, the optical
impulse 222 is not transmitted to the optical device
40. Consequently the latter does not cause the output
gate 20 to open and does not permit the said
remaining part of the cell 2000 to pass to the output
of the optical selector 8000. In this way, incomplete
CA 02282251 1999-09-15
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cells of bits at the input of the optical selector
8000 cannot pass to its output.
To summarize,
- a first arriving cell 1000 can, in normal
conditions, enter the optical selector 8000
through the gate 10 and reach the cell
recognition device 30;
- the cell recognition device 30 transmits the
optical impulse 222 along the connection 2 when
it recognizes the header of the cell 1000;
- the state of the output gate 20 changes from
closed to open immediately after the arrival of
the optical impulse 222;
- the state of the input gate 10 changes from open
to closed after a time T~ (after the whole of the
cell 1000 has passed through the gate 10);
- the output gate 20 remains open for a time
interval equal to T~ (after the whole of the cell
1000 has passed through the gate 20);
- the input gate 10 remains closed for a time
interval equal to T (to prevent new cells
arriving at the input of the optical selector
8000 with f > PCR from entering the device).
This is because, with respect to the arrival of the
optical impulse 222 at the input 50 of the optical
device 40:
- the set impulse 888 is not delayed;
- the reset impulse 999 is delayed by a fixed delay
equal to the duration T~ of the cell 1000;
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- the set impulse 666 is delayed by a fixed delay
equal to the duration T~ of the cell 1000;
- the reset impulse 777 is delayed by a total delay
equal to 1/PCR, and, with respect to the set
impulse 666, by a delay equal to T.
The variable delay unit 46 enables the value of T
to be changed according to the PCR negotiated between
a source and an ATM network [T = (1/PCR) - T~].
The device 40 according to the invention, by
supplying two output optical impulses, temporally
phase-shifted with respect to each other by a time
equal to T~ and having durations of T~ and T,
therefore enables the open and closed times of the
input optical gate 10 and the output optical gate 20
of the optical selector 8000 to be controlled and
thus enables a wholly optical controller to be
provided for ATM networks.
