EP1004954B9 - Dispositif optique de traítement de signaux optiques numériques - Google Patents

Dispositif optique de traítement de signaux optiques numériques Download PDF

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EP1004954B9
EP1004954B9 EP19980203978 EP98203978A EP1004954B9 EP 1004954 B9 EP1004954 B9 EP 1004954B9 EP 19980203978 EP19980203978 EP 19980203978 EP 98203978 A EP98203978 A EP 98203978A EP 1004954 B9 EP1004954 B9 EP 1004954B9
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Prior art keywords
bits
optical
pattern
sequence
phase
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EP1004954B1 (fr
EP1004954A1 (fr
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Pierpaolo Boffi
Davide Piccinin
Mario Martinelli
Damiano Rossetti
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Cisco Systems International BV
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Cisco Systems International BV
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    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06E—OPTICAL COMPUTING DEVICES; COMPUTING DEVICES USING OTHER RADIATIONS WITH SIMILAR PROPERTIES
    • G06E1/00—Devices for processing exclusively digital data

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  • the present invention relates to a device for processing digital optical signals. More particularly, the present invention relates to an optical device for comparing at least one sequence of N bits with at least one predetermined sequence of N reference bits, an optical communication system in which this comparison is made, and a method for making this comparison.
  • the pattern recognition is carried out by means of a conventional operation of correlation between a reference pattern and a test pattern.
  • reference pattern denotes a predetermined pattern which is to be recognized.
  • test pattern denotes any other pattern which is to be compared with the reference pattern.
  • c(x,y) ⁇ s *( ⁇ , ⁇ ) f ( ⁇ + x , ⁇ + y ) d ⁇ d ⁇
  • s(x,y) and f(x,y) are two-dimensional patterns
  • (x, y) are spatial coordinates of the said two-dimensional patterns
  • ( ⁇ , ⁇ ) are conventional integration variables and the asterisk indicates the complex conjugation.
  • Figure 1 shows a conventional optical correlator according to Vander Lugt (A. Vander Lugt, "Signal detection by complex spatial filtering", IEEE trans. Inform. Theory, vol. 10, p. 139, 1964).
  • This device comprises a first lens 21 having a focal length f 1 , an optical filter 22 and a second lens 23 having a focal length f 2 .
  • the two lenses 21 and 23 are at a distance of f 1 +f 2 from each other, and the optical filter 22 is located in the rear focal plane of the first lens 21, which corresponds to the front focal plane of the lens 23.
  • the input plane 11 and the output plane 14 of the device shown in Figure 1 are intended to indicate, respectively, the front focal plane of the lens 21 and the rear focal plane of the lens 23.
  • the optical filter 22 may be, for example, a matched filter (MF) or a phase only filter (POF).
  • MF matched filter
  • POF phase only filter
  • the matched filter and the phase only filter process the patterns in different ways from each other.
  • the convolution f(x,y) ⁇ h(x,y) corresponds to the correlation c(x,y) between the test pattern and the reference pattern as defined by equation (2).
  • Vander Lugt correlator with a phase only filter is not a true correlation operation as conventionally defined in mathematics, for the purposes of the present invention the functions ac'(x,y) and cc'(x,y) are considered to be an autocorrelation and a cross-correlation respectively.
  • a parameter D' indicating the ratio between the intensity (
  • a parameter D indicating the ratio between the intensity (
  • the parameters D and D' are therefore indicators of the discrimination capability CD of a correlator device.
  • phase only filter has an overall performance which is better than that of the matched filter when it is connected in a Vander Lugt correlator for pattern recognition
  • K.C. Macukow et al. "Phase only filter as matched spatial filter with enhanced discrimination capability", Optics communications, vol. 64, p. 224, 1987; L.P. Yaroslavsky, "Is the phase only filter and its modifications optimal in terms of discrimination capability in pattern recognition?", Applied Optics, vol. 31, p. 1677, 1992; L.A. Romero et al., “Comparison between the peak-to-sidelobe ration of the matched and the phase only filters", Optics Letters, vol. 16, p. 253, 1991; B.V. Kumar et al., “Phase only filter with improved signal to noise ratio", Applied Optics, vol. 28, p. 250, 1989].
  • US Patent 5 214 534 also describes a method for carrying out a correlation of a pattern in a Fourier transform correlator.
  • This method consists in encoding an input pattern as a phase only object having a standardized amplitude and a phase which is a function of the intensity of the said input pattern.
  • the said method also consists in obtaining the Fourier transform of this object, in filtering the Fourier transform of the said object with a two-dimensional phase only filter in which a reference pattern has been recorded, and, finally, in obtaining the inverse Fourier transform of the said object which has been filtered in this way.
  • US Patent 5 523 881 describes an optical signal processor which uses a coherent light source, a first and a second spatial light modulator and a beam splitter.
  • the light from the said source is reflected by the beam splitter and sent to the first spatial light modulator where it is modulated by multi-phase modulation in accordance with an input pattern.
  • the said coherent light, modulated in this way is then subjected to the Fourier transform, sent to the said second spatial light modulator in which it is modulated by multi-phase modulation in accordance with a reference pattern, and then subjected to the inverse Fourier transform.
  • a signal dependent on the correlation operation between the said input pattern and the said reference pattern is thus obtained at the output.
  • Figures 14 and 15 show the values of the parameter D' found in this way for the reference byte 01000111 (71) and 01001110 (78) respectively.
  • all the test bytes, which were different from the reference byte were discriminated (gave a value of D' less than 1) from the reference byte, and the worst case (lowest CD') was found with the test byte 01001111 (79), in other words with one of the bytes which differed from the reference byte by one bit only.
  • the worst value of CD' was found to be less than 1% in the case of the reference byte (71), and approximately 13% in the case of the reference byte (78).
  • a device which is capable of analysing the value of the intensity of the signal provided by the operation of comparison between the two sequences, and to determine whether or not this value is equal to the maximum value which this intensity has when the test sequence is identical to the reference sequence.
  • the said device has to be more sensitive to be able to distinguish the reference sequence from all the possible input test sequences. For example, with a value of CD' (or CD) equal to 1%, the said device has to be capable of detection variations of intensity of 1%, provided that there is no background noise.
  • Figures 4 and 5 show the values of the parameter D' which were obtained in the case of the reference bytes 01000111 (71) and 01001110 (78) respectively for the 256 possible input test bytes.
  • the inventors therefore set up various other working hypotheses concerning the question of how to distinguish a sequence of N reference bits from its complementary without adversely affecting the capability of discriminating this reference sequence of N bits from the other test sequences of N bits. In the course of this research, they unexpectedly found that this could be achieved by using a suitable 0/ ⁇ phase modulation, with 0 ⁇ 1, for the input bits.
  • the present invention therefore relates to an optical device comprising
  • the device according to the present invention is not only able to discriminate a predetermined sequence of N reference bits from its complementary sequence, but can also improve the mean capability of discrimination of the reference sequence of N bits from all the other test sequences obtainable with the aforesaid N bits (that is, it decreases the mean value of the parameter D or D'). In other words, it enables the number of sequences of bits which can be recognized to be increased above the level for known devices.
  • the device according to the invention also has the advantage of carrying out a discrimination operation in real time, in other words in a period equal to the duration of the propagation of the bits within the device. It therefore introduces no limitations of bit rate when it is connected in an optical switching network or in an optical communication system.
  • the said at least one sequence of N bits is also the complementary sequence of the said predetermined spatial reference pattern of N bits.
  • the value of the parameter ⁇ is selected in such a way as to optimize the discrimination of the said predetermined spatial reference pattern of N bits from the said complementary sequence. More advantageously, the value of the parameter ⁇ is selected in such a way as to optimize the discrimination of the said predetermined spatial reference pattern of N bits from all the possible sequences of N bits.
  • the said value of the parameter ⁇ is from 0.3 to 0.95.
  • the said value of the parameter ⁇ is from 0.7 to 0.9.
  • the said first element comprises a laser source and a phase modulator capable of carrying out the said 0/ ⁇ phase modulation.
  • the said first element also comprises an opto-electronic device capable of converting an input digital optical signal, having a modulation different from 0/ ⁇ , into an electrical control signal for the said 0/ ⁇ phase modulator.
  • the said series-parallel converter comprises a 1xN splitter for cloning the said digital optical signal into N digital optical signals, and N delay lines for delaying the said N digital optical signals by a predetermined delay for each signal, in such a way as to supply the said spatial pattern of N bits at the output of the said N delay lines.
  • the said N delay lines comprise heater devices.
  • the said second element carries out an operation of convolution of the said spatial pattern of N bits and the said predetermined spatial reference pattern of N bits.
  • the said second element capable of carrying out a comparison operation is an optical element capable of supplying at its output an optical signal having an intensity whose value depends on the result of the said comparison operation.
  • the said second element is a Vander Lugt correlator.
  • the said Vander Lugt correlator comprises a first optical lens, a second optical lens and a phase only filter.
  • the said optical device also comprises a detector element for detecting the said intensity of the said signal at the output of the said second element, and a comparator element capable of comparing the said intensity with a threshold of intensity having a predetermined value, to determine whether or not the said spatial pattern of N bits is identical to the said predetermined spatial reference pattern of N bits.
  • the said detector element is typically a photodetector for converting the said optical signal into a corresponding electrical output signal having a predetermined voltage.
  • the said comparator is typically an electronic threshold circuit capable of comparing the said voltage with a voltage threshold having a predetermined value, to determine whether or not the said spatial pattern of N bits is identical to the said predetermined spatial reference pattern of N bits.
  • the said optical device is operationally connected to a processor capable of determining, for each predetermined spatial reference pattern, the value of the said parameter ⁇ which optimizes the discrimination of the said predetermined spatial reference pattern of N bits from its complementary sequence, and of causing the said first element to carry out the said 0/ ⁇ phase modulation and causing the said second element to optimize the said comparison operation.
  • the said processor determines, for each predetermined spatial reference pattern, the value of the said parameter ⁇ which optimizes the discrimination of the said predetermined spatial reference pattern of N bits from all the possible sequences of N bits.
  • the present invention relates to an optical communication system comprising:
  • the said light source is a laser source.
  • the said optical transmission line comprises an optical fibre. More advantageously, it comprises an optical cable.
  • the said second apparatus comprises:
  • the said at least one sequence of N bits is also the complementary sequence of one of the said M predetermined spatial reference patterns of N bits.
  • the said parameter ⁇ is selected in such a way as to optimize the discrimination of the said M predetermined spatial reference patterns from the said complementary sequence. More preferably, the said parameter ⁇ is selected in such a way as to optimize the discrimination of the said M predetermined spatial reference patterns from all the possible sequences of N bits.
  • the present invention relates to a method for comparing an optical spatial pattern of N bits with a predetermined spatial reference pattern of N bits comprising the phases of:
  • the phase b) supplies an optical signal having an intensity whose value depends on the result of the said convolution operation.
  • phase c) consists in converting the said optical signal into a corresponding electrical signal having a predetermined voltage
  • phase d) consists in comparing the value of the said voltage with a voltage threshold having a predetermined value for determining whether or not the said optical spatial pattern of N bits is identical to the said predetermined spatial reference pattern of N bits.
  • the embodiment of the optical device 500 comprises a first element 100 for supplying a digital optical signal comprising at least one serial optical sequence 1000 of N binary bits, suitably phase modulated, a series-parallel converter 6 and a second element 9 for carrying out an operation of convolution in free space and in parallel of a predetermined reference sequence of N binary bits and the said serial optical test sequence 1000 ( Figure 6).
  • the said first element 100 for supplying the said optical sequence 1000 of N bits comprises, for example, a laser source 120 and a phase modulator 130.
  • the said laser source 120 is, for example, a laser diode, emitting at the wavelengths of an optical signal for telecommunications, for example in the range from approximately 1300 to 1600 nm, or, preferably, in the range from approximately 1500 to 1600 nm.
  • the phase modulator 130 is a conventional optical modulator, consisting, for example, of a waveguide on an LiNbO 3 substrate associated with two electrodes.
  • the said modulator 130 carries out a binary phase modulation of the optical signal emitted by the laser source 120 according to a digital electrical pilot signal 110 which carries the digital information to be transmitted at a predetermined bit rate.
  • the said phase modulator 130 associates with the optical signal emitted by the laser source 120 a phase of
  • the said optical sequence 1000 of N bits is phase modulated by the 0- ⁇ modulation.
  • the series-parallel converter 6 can convert the said serial sequence 1000 of N bits, formed in the above way, into a spatial pattern 3000 of N bits carrying the same information as the serial sequence 1000.
  • T b is the duration of the bit, in other words the inverse of the bit rate
  • the splitter 61 is, for example, a single 1xN fused-fibre coupler, or is formed from an equivalent number of 1x2 fused-fibre splitters connected in cascade to form a 1xN splitter.
  • the splitter 61 may also be produced by other technologies such as that of integrated optics or holographic diffraction.
  • optical delay lines 62 are, for example, sections of optical fibre or waveguides of suitable length.
  • thermo-optical phase controllers are located at the output of the optical delay lines 62 or, alternatively, along them, and precisely regulate the phase lag of each of the N signals in such a way that the phase relation between the N bits of the said spatial pattern 3000 is the same as that between the N bits of the optical sequence 1000 at the input of the series-parallel converter 6.
  • the said heaters suitably regulate the temperature of the said delay lines 612 to adjust the lengths of the said lines 612 and consequently the phases of the N bits of the said spatial pattern 3000.
  • optical delay lines 62 consist of sections of optical fibre
  • conventional piezoelectric devices capable of regulating the lengths of the said sections of optical fibre.
  • the said second element 9 for carrying out an operation of convolution in free space and in parallel of the said reference sequence of N bits and the said serial optical test sequence 1000 consists of a conventional Vander Lugt correlator of the type described previously with reference to Figure 1, comprising a first convex lens 21, a phase only filter 22 and a second convex lens 23.
  • the said second element 9 supplies at its output an optical signal 2000 having an intensity whose value depends on the result of the operation of comparing the said reference sequence of N bits with the said serial optical test sequence 1000.
  • the phase only filter 22 has a transfer function with a phase ⁇ ( ⁇ , ⁇ ) substantially equal to the conjugate phase of the optical field which is incident on the said filter when the test sequence of N bits is equal to the reference sequence. More particularly, this phase only filter 22 has a transfer function with a phase ⁇ ( ⁇ , ⁇ ) substantially equal to the conjugate phase of the Fourier transform of the reference sequence of N bits [equation (4)].
  • SLM spatial light modulator
  • This device consists of an array of N liquid crystal cells which impart a phase lag to the incident optical field according to the conjugate of the phase information contained in the Fourier transform of the reference sequence of N bits.
  • the said phase lag is obtained by controlling the electrical potential difference applied to the said liquid crystal cells by an electrical control system. This is achieved because, owing to the birefringent properties of the liquid crystals, it is possible to obtain a rotation of the polarization plane of the light incident on the cells, in other words a change of phase of the incident light, by applying a predetermined potential difference to the said cells.
  • phase only filter 22 may also consist of a conventional phase mask made by known holographic or diffractive lithographic methods.
  • Vander Lugt correlator comprising a first and a second convex optical lens and a conventional matched filter.
  • the said second element 9 may also consist of other types of devices capable of carrying out an operation of convolution of two sequences of bits, such as a conventional joint transform correlator (JTC), a correlator of the type described in Patent Application No. 982002411.9 filed by the present applicant, or suitable conventional electronic devices.
  • JTC joint transform correlator
  • a correlator of the type described in Patent Application No. 982002411.9 filed by the present applicant or suitable conventional electronic devices.
  • Figure 2 shows an embodiment of an optical transmission system according to another aspect of the present invention.
  • the optical transmission system in Figure 2 comprises a transmitter A, an optical transmission line 4 and a receiver B.
  • the transmitter A comprises a laser source 2 connected optically to one input of a phase modulator 3.
  • the output of the phase modulator 3 is connected to the optical transmission line 4 which, in turn, is connected optically to the input of the receiver B.
  • the laser source 2 is, for example, a laser diode, emitting at the wavelengths of an optical signal for telecommunications, for example in the range from approximately 1300 to 1600 nm, or, preferably, in the range from approximately 1500 to 1600 nm.
  • the phase modulator 3 is a conventional optical modulator, consisting, for example, of a waveguide on an LiNbO 3 substrate associated with two electrodes.
  • the said modulator 3 carries out a binary phase modulation of the optical signal emitted by the laser source 2 according to a digital electrical pilot signal 110 which carries the digital information to be transmitted at a predetermined bit rate.
  • the said phase modulator 3 associates with the optical signal emitted by the laser source 2 a phase of
  • the optical transmission line 4 typically comprises an optical fibre. Preferably, it comprises an optical cable.
  • the optical transmission line 4 comprises at least one conventional optical amplifier, for example one of the erbium-doped fibre type.
  • the receiver B comprises a 1xM splitter 5 for separating the input signal into M outputs.
  • Each of the M outputs of the splitter 5 is connected to a series-parallel converter 6, each comprising N outputs made, for example, from optical fibre.
  • the N outputs of each series-parallel converter 6 are optically connected to one of M elements 9.1-9.M, of the type described previously, for carrying out an operation of convolution in free space and in parallel of a spatial reference pattern of N bits and a spatial test pattern of N bits ( Figure 6).
  • the output of each element 9.1-9.M is connected to a different photodetector 7 which in turn is connected to a threshold circuit 8.
  • the splitter 5 is, for example, a single 1xM fused-fibre coupler, or consists of a plurality of fused-fibre couplers (of the 1x2 type for example) connected in cascade to form a 1xM splitter.
  • the splitter 5 may also be produced by other technologies such as those of integrated optics or holographic diffraction.
  • the series-spatial converters 6 are, for example, of the type described previously in relation to Figure 3.
  • the devices 9.1-9.M may be, as stated previously, Vander Lugt correlators ( Figure 1), each comprising a first convex lens 21, a phase only filter 22 and a second convex lens 23.
  • the devices 9.1-9.M may consist of other types of conventional correlator, such as a conventional joint transform correlator (JTC), a correlator of the type described in Patent Application No. 98202411.9 filed by the present applicant, or conventional electronic devices capable of carrying out an operation of convolution of a reference byte and a test byte.
  • JTC joint transform correlator
  • Each of the devices 9.1-9.M is constructed in such a way that it recognizes a predetermined binary reference sequence of N bits among all the possible sequences (2 N ) arriving from the optical transmission line 4.
  • the receiver B is thus capable of discriminating, from all the 2 N possible sequences arriving at its input, those which are identical to at least one of M reference sequences (where M ⁇ 2 N ).
  • the receiver B will comprise only one series-parallel converter 6, a single element 9, a single photodiode 7 and a single threshold circuit 8.
  • These reference sequences may be, for example, an address of a cell for a transmission of the asynchronous type (asynchronous transfer mode, ATM) or a CDMA (code division multiple access) transmission code.
  • asynchronous type asynchronous transfer mode, ATM
  • CDMA code division multiple access
  • the photodetector 7 is, for example, a PIN photodiode made from InGaAs, such as the ETX75 FJ SLR model, marketed by Epitaxx Optoelectronics Devices, 7 Graphics Drive, West Trenton, NJ, USA.
  • the threshold circuit 8 is, for example, a conventional electronic circuit.
  • the photodetector 7 detects the intensity of the optical signal 2000 at the output of the corresponding element 9 and converts it into a corresponding value of voltage V.
  • the threshold circuit 8 compares this voltage value V with a threshold voltage value which is selected in a conventional way to determine whether or not the sequences of N bits arriving from the optical transmission line 4 are identical to the predetermined reference sequence.
  • Figure 16 in which the same numerical references are used to indicate components of the same type as those described previously, shows a second embodiment of the device 500 according to the present invention.
  • the device 500 in Fig. 6 also comprises an opto-electronic circuit 43, a photodetector 7, a threshold circuit 8 and a processor 44.
  • the opto-electronic circuit 43 comprises, typically, a photodiode, a threshold circuit and an electronic amplifier, all of conventional types (not shown).
  • the opto-electronic circuit 43 converts a digital optical signal, having a modulation different from 0/ ⁇ and arriving from a transmission line (of the optical fibre type for example) 41, into a corresponding electrical signal 110.
  • This electrical signal 110 is used as the pilot signal of the phase modulator 130 of the device 500 which modulates the optical signal generated by the laser source 120 by an 0/ ⁇ modulation.
  • the sequence of N bits 1000 modulated in this way by the phase modulator 130 is sent to the series-parallel converter 6 and to the second element 9 in Figure 6.
  • the output optical signal 2000 of the second element 9 is then sent to the photodetector 7 and then to the threshold circuit 8.
  • the device in Figure 16 may be used, for example, in the receiver of a conventional optical transmission system in which at least one digital optical signal comprising sequences of N bits, modulated by a conventional modulation such as an NRZ (non return to zero) or RZ (return to zero) amplitude modulation or a 0/ ⁇ phase modulation, is transmitted.
  • a conventional modulation such as an NRZ (non return to zero) or RZ (return to zero) amplitude modulation or a 0/ ⁇ phase modulation
  • the opto-electronic circuit 43 is preferably associated with a conventional device capable of carrying out a detection of the coherent type.
  • the said at least one digital optical signal arrives along the transmission line 41 at the input of the device in Figure 16.
  • the circuit 43 carries out the optical-to-electrical conversion of the said digital optical signal comprising the sequences of N bits, and thus supplies the electrical pilot signal 110 to the phase modulator 130.
  • the processor 44 determines the parameter ⁇ , as described previously, and operates
  • the optical signal 2000 supplied by the second element 9 is then converted by the photodiode 7 into an electrical signal whose voltage is compared by the circuit 8 with a threshold voltage which is selected in a conventional way to determine whether or not the incoming test sequences are identical to the predetermined reference sequence.
  • the device in Figure 16 can recognize more than one predetermined reference sequence of N bits among those arriving at the receiver.
  • the inventors have developed a computer program capable of simulating the behaviour of a device according to the invention.
  • curve A shows the values of the parameter D' obtained in this way, using as the test sequence the complementary byte of the reference byte 78.
  • curve B shows for each value of a the highest value (worst case) of the parameter D' which was obtained with all the 256 test bytes with the exception of those identical to the reference byte 78 and its complementary.
  • the curve B therefore represents, for each value of the parameter ⁇ considered, the worst case of the discrimination capability CD of a device according to the invention for all the test bytes with the exception of those identical to the reference byte 78 and its complementary.
  • the optimal modulation level ⁇ o which optimized the discrimination of the reference sequence of bits 78 from all the other test sequences, including the complementary, was found.
  • the optimal modulation level ⁇ o (corresponding in this case to the minimum point of the curve B) was found to be equal to 0.72 ⁇ ; in other words, the optimal value ⁇ o of the parameter ⁇ was found to be equal to 0.72.
  • the parameter D' for the complementary sequence was found to be equal to 0.4537 while, for the sequence with the worst discrimination, D' was found to be equal to 0.6635 (see curves A and B).
  • Figure 8 shows the results of a further simulation carried out to determine the variation of the parameter D', using:
  • Figure 9 shows the values of the parameter D' which were obtained with the test bytes (206, 14, 110, 94, 70, 74, 76, 79) which differ from the reference byte 01001110 (78) by only one byte, and with its complementary byte (177).
  • the curve E shows the values of the parameter D' obtained with level of modulation ⁇ equal to ⁇ , in other words with a conventional modulation, while the curve F shows the values of the parameter D' obtained, according to the invention, with the optimal modulation level ⁇ o equal to 0.72 ⁇ .
  • the device according to the invention therefore made it possible to overcome the problem of the inability to distinguish a predetermined reference sequence from its complementary and, on average, to increase the ability to distinguish it from all the other test sequences.
  • Figure 10 shows the optimal values ⁇ (on the vertical axis) found in this way for each reference byte (indicated in decimal values on the horizontal axis). It will be noted from Figure 10 that the optimal modulation levels ⁇ o belong, for virtually all the bytes, to a limited set of values ranging from 0.7 ⁇ to 0.9 ⁇ , corresponding to 0.7 ⁇ ⁇ o ⁇ 0.9.
  • Figure 11 shows the maximum value of the parameter D' which was obtained for each of 256 reference bytes for the corresponding optimal modulation level ⁇ o found previously. From time to time, the test bytes did not include the one equal to the selected reference byte.
  • Figure 12 shows the maximum values of the parameter D' obtained in this way for each reference byte.
  • Figure 13 shows the results obtained in this way for the relative variation of the parameter D' (on the vertical axis). For each reference byte (shown on the horizontal axis in decimal notation), the increase of the value of the parameter D' due to the use of a modulation value of 0.83 ⁇ instead of the optimal value was found to be contained within 20%.
  • Figure 17 shows the worst value of the parameter D' which was obtained for each of 256 reference bytes with a conventional 0/ ⁇ modulation, considering all the 2 N test sequences with the exception of the complementary sequence and the sequence identical to the reference sequence.
  • the modulation according to the invention not only provided a good capability of discriminating the complementary sequence but also made it possible to obtain, on the average, much lower values of the parameter D' than those obtained with a conventional modulation.
  • Figure 18 which illustrates the difference between the values of D' obtained in Figure 17 and those obtained in Figure 12, shows how the values of D' obtained with the conventional modulation 0/ ⁇ were found to be, on average, higher than those obtained according to the invention.
  • the procedure of determining the parameter ⁇ and, consequently, the optimal modulation level is independent of the length (N) of the reference sequence of bits, and of the particular embodiment of the second element 9. For example, further simulations were carried out for different lengths of the numerical sequence, in other words for values N equal to 7 and 5. In this case also, an optimal value ⁇ o typically ranging from 0.7 to 0.9 was obtained.
  • the device 500 in Figure 6 also comprises a phase mask (not shown).
  • the said phase mask is located at the input of the second element 9.
  • it may be located on the input plane 11 of the Vander Lugt correlator shown in Figure 1.
  • phase masks suitable for the purposes of the invention are those produced by Lasiris, which uses laser scribing methods, or by RPC, which uses lithographic methods with ultraviolet radiation. These methods of scribing and the performance of the diffractive optical elements thus produced are described, for example, by A. Asselin et al. ("Diffractive optics at NOI", National Optics Institute, vol. 5, pp. 1-8, 1994).
  • the said phase mask may accentuate the existing differences between sequences of N bits which are very similar to each other (for example, in the case of sequences which differ from each other by one bit only) and is preferably carried out in such a way as to imprint a predetermined phase shift on the bits which in the input test sequence occupy the same position, in the plane x,y, as the bits set to 1 in the reference sequence of N bits.
  • the phase only filter 22 of the Vander Lugt correlator shown in Figure 1 is preferably operated in such a way that the phase ⁇ ( ⁇ , ⁇ ) of its transfer function [equation (4)] is equal to the sum of the conjugate phase of the Fourier transform of the reference sequence of N bits and the phase shift introduced by the mask.
  • the said optimal phase shift value of the phase mask was calculated by using, for the input sequence, a conventional phase modulation level ⁇ (equal to ⁇ ).
  • the said optimal phase shift value of the phase mask was then calculated by using, for the input sequence, a phase modulation level ⁇ equal to ⁇ according to the invention.
  • phase mask provided values of D' which were similar overall to those obtained in the absence of a phase mask and with an optimal modulation of 0.72 ⁇ of the bits of the input sequence ( Figure 8).

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Claims (10)

  1. Dispositif optique (500) comprenant
    un premier élément (100) apte à délivrer un signal optique numérique comprenant au moins une séquence (1000) de N bits modulée par une modulation de phase appropriée;
    un convertisseur série-parallèle (6) pour convertir ladite au moins une séquence (1000) de N bits en des profils spatiaux (3000) de N bits en parallèle, et véhiculant la même information que ladite séquence (1000) de N bits; et
    un second élément (9) apte à exécuter une opération de comparaison entre ledit profil spatial (3000) de N bits et un profil spatial de référence prédéterminé de N bits, et délivrer à la sortie un signal (2000) possédant une intensité, dont la valeur dépend du résultat de ladite opération de comparaison, caractérisé en ce que ladite modulation de phase appropriée 0/απ avec 0 < α < 1.
  2. Dispositif (500) selon la revendication 1, caractérisé en ce que ladite au moins une séquence de N bits est également la séquence complémentaire dudit profil spatial de référence prédéterminé de N bits.
  3. Dispositif (500) selon la revendication 2, caractérisé en ce que la valeur du paramètre α est choisie de manière à optimiser la discrimination dudit profil spatial prédéterminé de référence de N bits à partir de ladite séquence complémentaire.
  4. Dispositif (500) selon la revendication. 3, caractérisé en ce que la valeur du paramètre α est choisie de manière à optimiser la discrimination dudit profil spatial prédéterminé de référence de N bits à partir de l'ensemble des séquences possibles de N bits.
  5. Dispositif (500) selon l'une quelconque des revendications 1 à 4, caractérisé en ce que ledit premier élément (100) comprend une source laser (120) et un modulateur de phase (130) apte à exécuter ladite modulation de phase 0/απ.
  6. Dispositif (500) selon l'une quelconque des revendications 1 à 5, caractérisé en ce que ledit second élément (9) exécute une opération de convolution sur ledit profil spatial (3000) de N bits et sur ledit profil spatial prédéterminé de référence de N bits.
  7. Dispositif (500) selon la revendication 6, caractérisé en ce que ledit second élément (9) est un élément optique qui délivre à sa sortie un signal optique (2000) possédant une intensité dont la valeur dépend du résultat de ladite opération de comparaison.
  8. Dispositif (500) selon la revendication 7, caractérisé en ce que ledit second élément (9) est un corrélateur de Vander Lugt.
  9. Système de communication optique comprenant:
    au moins un premier dispositif (A) comprenant une source de lumière (2) et un modulateur de phase (3), pour délivrer un signal optique numérique comprenant au moins une séquence (1000) de N bits modulée par une modulation de phase O/απ appropriée dans laquelle on a 0 < α < 1;
    une ligne de transmission optique (4), connectée optiquement audit premier dispositif (A) pour véhiculer ledit signal optique numérique;
    un second dispositif (B) connecté optiquement à ladite ligne de transmission optique (4), ledit second dispositif optique (B) comprenant:
    i. un convertisseur série parallèle. (6) pour convertir ladite au moins une séquence (1000) de N bits en un profil spatial (3000) de N bits en parallèle véhiculant la même information que ladite séquence (1000) de N bits; et
    ii. un élément (9) apte à exécuter une opération de comparaison dudit profil spatial (3000) de N bits avec un profil spatial prédéterminé de référence de N bits et de délivrance, à la sortie, d'un signal (2000) possédant une intensité dont la valeur dépend du résultat de ladite opération de comparaison;
    iii. un élément détecteur (7) pour détecter ladite intensité dudit signal (2000) sur la sortie dudit élément (9); et
    iv. un élément comparateur (8) accordé audit élément détecteur (7) et apte à comparer ladite intensité à un seuil d'intensité possédant une valeur prédéterminée, pour déterminer si ledit profil spatial (3000) de N bits est identique ou non audit profil spatial prédéterminé de référence de N bits.
  10. Procédé pour comparer un profil spatial optique de N bits à un profil spatial prédéterminé de référence de N bits comprenant les étapes consistant à:
    a) moduler lesdits N bits dudit profil spatial optique au moyen d'une modulation de phase appropriée;
    b) exécuter une opération de convolution sur ledit profil spatial de N bits et sur ledit profil spatial prédéterminé de référence de N bits de manière à délivrer un signal ayant une intensité dont la valeur dépend du résultat de ladite coopération de convolution;
    c) détecter ladite intensité;
    d) comparer la valeur de ladite intensité à un seul d'intensité ayant une valeur prédéterminée, pour déterminer si ledit profil spatial optique de N bits est identique ou non audit profil spatial prédéterminé de référence de N bits,
    caractérisé en ce que ladite modulation de phase appropriée est 0/απ, avec 0 < α < 1.
EP19980203978 1998-11-25 1998-11-25 Dispositif optique de traítement de signaux optiques numériques Expired - Lifetime EP1004954B9 (fr)

Priority Applications (2)

Application Number Priority Date Filing Date Title
EP19980203978 EP1004954B9 (fr) 1998-11-25 1998-11-25 Dispositif optique de traítement de signaux optiques numériques
DE69823839T DE69823839T2 (de) 1998-11-25 1998-11-25 Optisches Gerät zur Verarbeitung von digitalen optischen Signalen

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Application Number Priority Date Filing Date Title
EP19980203978 EP1004954B9 (fr) 1998-11-25 1998-11-25 Dispositif optique de traítement de signaux optiques numériques

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GB2445588A (en) 2006-12-16 2008-07-16 Qinetiq Ltd Optical Correlation Apparatus with parallel optical signals
GB2560584B (en) 2017-03-17 2021-05-19 Optalysys Ltd Optical processing systems

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US4976520A (en) * 1988-09-09 1990-12-11 Grumman Aerospace Corporation Common path multichannel optical processor
US5214534A (en) * 1991-06-19 1993-05-25 The United States Of America As Represented By The Secretary Of The Air Force Coding intensity images as phase-only images for use in an optical correlator
US5327286A (en) * 1992-08-31 1994-07-05 Texas Instruments Incorporated Real time optical correlation system
DE69622406T4 (de) * 1995-04-28 2003-10-30 Forskningscenter Riso, Roskilde Phasenkontrastbilderzeugung

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EP1004954B1 (fr) 2004-05-12
EP1004954A1 (fr) 2000-05-31
DE69823839D1 (de) 2004-06-17

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