Optical device for processing an optical impulse

BACKGROUND OF THE INVENTION

1. Field of the Invention

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.

2. Technical Background

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.

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.

SUMMARY OF THE INVENTION

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;

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 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 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.

BRIEF DESCRIPTION OF THE DRAWINGS

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;

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. 5

a

-

5

d

) 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

;

FIGS.

9

(

a

) through

9

(

d

) are schematic representations 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

.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to the embodiment shown in

FIG. 2

, the optical device

40

according to the invention comprises a first 1×2 coupler

41

, a second 1×2 coupler

42

and a third 1×2 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 1×2 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.

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 1×2 switch

461

, an output 2×1 switch

462

and a certain number of 2×2 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 log

2

M, 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 such that it causes a delay equal to T/2

i

(1≦i≦log

2

M) in the signal in transit.

The switches in series are controlled by suitable control signals c

1

−c

log2M

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 1×2 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 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

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. 5

a

-

5

d.

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. 5

a

and

5

b,

conventional beam splitters (for example, 1×2 directional couplers in series, indicated as a whole by the number

4604

in

FIGS. 5

a

and

5

b

) 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, 2×1 couplers in series, indicated as a whole by the number

4605

in

FIG. 5

) to return to a single optical fibre at the output

4608

. Optical gates (indicated as a whole by the number

4602

in

FIGS. 5

a

and

5

b

) 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. 5

c

and

5

d,

in place of the said beam splitters

4604

it is possible to connect 1×2 switches (indicated as a whole by the number

460

G in

FIGS. 5

c

and

5

d

) 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 1×2 input coupler

21

, a 2×1 output coupler

22

, a first optical propagation path

24

and a second optical propagation path

25

and a first 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° (π), 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° (2π), they interfere constructively and the gates are open (allowing the optical signal to pass). 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 2×1 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. 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 2×2 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 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 2×2 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 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 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 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 splitter

200

having a first input for an optical set impulse

210

, a second input and an output, a second optical beam splitter

220

having an input coupled optically to the output of the first optical beam splitter

200

and two outputs, a third optical beam splitter

280

having a first input coupled optically to an output of the second optical beam splitter

220

and a first output coupled optically to the second input of the first optical beam splitter

200

in such a way as to form a loop. The said optical beam splitter

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 splitters are, for example, conventional partially reflecting mirrors or 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 splitter

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 splitter

200

enters the loop. The second beam splitter

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 splitter

220

after two reflections by the optical beam splitters

280

and

200

. The optical beam splitter

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 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 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

C

. 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

C

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

C

, 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 other by a time T

C

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

C

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

C

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 to T

C

.

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 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 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≦PCR).

As shown in

FIG. 1

, 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.

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

C

).

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 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.

FIGS.

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 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

C

, 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

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

C

] with respect to T

C

, 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 TPCR with respect to the cell

1000

.

In this way, if a second cell

2000

arrives in the optical selector

8000

before a time interval T

PCR

=T

C

+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 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

C

(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

C

(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

C

of the cell

1000

;

the set impulse

666

is delayed by a fixed delay equal to the duration T

C

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

C

].

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

C

and having durations of T

C

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.

Number	Name	Date	Kind
3838278	Duguay et al.	Sep 1974	A
4442550	Killat	Apr 1984	A
4809256	Smith et al.	Feb 1989	A
4856092	Wade	Aug 1989	A
5493434	Sasayama et al.	Feb 1996	A
5619360	Marshall et al.	Apr 1997	A
5930018	Effenberger	Jul 1999	A
6330089	Yamada	Dec 2001	B1

Optical device for processing an optical impulse

Information

Patent Number

Date Filed

Date Issued

Inventors

Original Assignees

Examiners

Agents

CPC

US Classifications

Field of Search

US

International Classifications

Abstract

Description

Claims

Priority Claims (1)

CROSS-REFERENCE TO RELATED APPLICATIONS

US Referenced Citations (8)

Foreign Referenced Citations (1)

Provisional Applications (1)