Abstract:
An optical selector for an asynchronous transfer mode (ATM) optical network, the said selector being capable of selecting ATM cells of bits having a frequency of arrival f less than or equal to a predetermined frequency (PCR), the said optical selector comprising: 
     an input having a first device capable of blocking the entry of bits; 
     an output having a second device capable of permitting the outflow of bits; 
     an ATM cell recognition unit constructed in such a way that it recognizes the headers of ATM cells and connected to the said input and to the said output for the transit of bits from the said input to the said output; and 
     an optical decision unit connected operationally to the said first device, to the said second device and the said ATM cell recognition unit.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on European Patent Application No. 98203132.0 filed on Sep. 17, 1998 and U.S. Provisional Application No. 60/101,098 filed on Sep. 18, 1998, the content of which is incorporated hereinto by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical selector and a method for selecting ATM cells having a frequency of arrival, f, less than or equal to a predetermined frequency. 
     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 offered by optical fibres, while the operations of processing the optical signals, such as multiplication 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. 
     In the switching nodes of an optical network, where multiple communication channels converge and originate, there is a widely felt need for devices capable of carrying out wholly optical operations on sequences of bits (cells). 
     For example, an important operation in known asynchronous transfer mode (ATM) networks, which use a method of statistical multiplexing of data packets originating from separate sources, is that of checking the frequency of the cells (peak cell rate, PCR) arriving at the input of a network. 
     ATM is a transmission method which consists in grouping the data transmitted from various sources into packets of digital data (cells), each of which consists of a payload of 43 bytes and a header of 5 bytes. The header comprises various fields containing various data used by the nodes of the ATM network to control the switching of the cells. This transmission method also provides 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 has to 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. 
     However, in the case to which the present description and claims relate, the expression “ATM network” is not used in the limiting sense but also comprises all the systems which, like the conventional ATM, are based on a negotiation of the cell frequency before the opening of a predetermined connection. 
     The “virtual scheduling algorithm” (VSA) is one of the standard algorithms included in ITU-T Recommendation I.371, for PCR checking in ATM networks. This algorithm requires that, at the moment at which a new cell relating to a given connection between two ATM nodes arrives at one of the said access or interconnection points, a device (“policer”) capable of controlling the traffic in this network calculates the time elapsed since the transit of the last cell and that, if this time is less than a time T PCR  equal to the inverse of the PCR (T PCR =1/PCR), the new cell must be rejected. 
     There are known electronic devices for implementing the VSA. 
     U.S. Pat. No. 5,541,913 describes a PCR checking device comprising a plurality of processors in parallel, each of which implements a modified virtual scheduling algorithm and thus generates conformity signals for a cell for which an activation circuit has been activated. From these conformity signals, a plurality of which may be provided for each cell of the incoming ATM flow, an arbitration module generates a discharge signal, which indicates whether this cell must or must not be included in an output flow from a discharge circuit, and feedback signals, which indicate to a corresponding processor whether or not to carry out an update, already provided, of the specified arrival time used in the aforesaid algorithm. The architecture of the device permits the implementation of any check method by means of an adaptation of the arbitration module and of the activation circuit. 
     Man-Yeong Jeon et al. [“Implementation of a Peak Cell Rate Policer using the Virtual Scheduling Algorithm”, Proceedings of IEEE ICC &#39;96, Dallas, Tex., USA, p. 762-766 (1996)] describe the architecture of a checking device which implements the VSA to check the PCR and discuss some problems relating to its implementation. They also illustrate some experimental results obtained for various values of PCR, to demonstrate that the device carries out its checking function in a satisfactory way. 
     However, there are no known optical devices for implementing a policer for checking the PCR in ATM networks. 
     SUMMARY OF THE INVENTION 
     A first aspect of the present invention is therefore an optical selector for an ATM optical network, the said selector being capable of selecting ATM cells of bits having a frequency of arrival, f, less than or equal to a predetermined frequency (PCR), the said optical selector comprising: 
     an input having a first device capable of blocking the entry of bits; 
     an output having a second device capable of permitting the outflow of bits; 
     an ATM cell recognition unit constructed in such a way that it recognizes the headers of ATM cells and connected to the said input and to the said output for the transit of bits from the said input to the said output; and 
     an optical decision unit connected operationally to the said first device, to the said second device and to the said ATM cell recognition unit. 
     In the course of the present description and of the attached claims, the expression “optical decision unit” is used to indicate a device which, having received an optical impulse at its input., is capable of supplying two optical impulses at the output, for example two optical control impulses, in which each output optical impulse has a predetermined temporal phase shift with respect to an input optical impulse and also having a predetermined duration. 
     Preferably, the said first device consists of a first optical gate. More preferably, the said first optical gate is in the open state when it is in normal operating conditions. 
     Preferably, the said second device consists of a second optical gate. More preferably, the said second optical gate is in the closed state when it is in normal operating conditions. 
     Advantageously, the said ATM cell recognition unit is an optical device capable of sending an optical impulse to the said optical decision unit when it recognizes the header of an ATM cell. 
     Preferably, the said optical decision unit processes the said optical impulse to supply a first optical control signal for the said first device, the said first optical control signal being capable of changing the state of the said first optical gate from open to closed for a time substantially equal to T, starting from when a time substantially corresponding to the duration T C  of an ATM cell has elapsed since the recognition of the header of an ATM cell. Additionally, the said optical decision unit processes the said optical impulse to supply a second optical control signal for the said second device, the said second optical control signal being capable of changing the state of the said second optical gate from closed to open for a time substantially equal to the duration T C  of an ATM cell, starting from the time of recognition of the header of an ATM cell by the said ATM cell recognition unit. 
     Preferably, the said optical decision unit also comprises a first output for the said first optical control signal connected to the said first device. More preferably, the said optical decision unit also comprises a second output for the said second optical control signal connected to the said second device. 
     A second aspect of the present invention is therefore a method for selecting ATM cells of bits having an arrival frequency, f, less than or equal to a predetermined frequency (PCR), the said method comprising: 
     a) the input of bits through suitable open input means, 
     b) the passage of the said bits through an ATM cell recognition unit capable of recognizing the headers of ATM cells, towards suitable closed output means; 
     c) the sending of a command, when the header of an ATM cell is recognized by the said ATM cell recognition unit, 
     i) for the opening of the said output means for a time substantially equal to the duration (T C ) of an ATM cell and, at the end of this time T C , 
     ii) for the closing of the said input means for a time 
     
       
           T =1 /PCR−T   C   
       
     
      d) the maintenance of the initial state, if the header of an ATM cell is not recognized by the said ATM cell recognition unit, in such a way that the said input means remain open for the input of an ATM cell and the said output means remain closed, thus eliminating the bits which have entered and whose header has not been recognized. 
     Preferably, the step c) also comprises providing a first optical control impulse for the said output means and a second optical control impulse for the said input means. 
     Advantageously, the said first optical control impulse has a duration substantially equal to T C , starting from the moment of recognition of the header of an ATM cell. 
     More advantageously, the said second optical control impulse has a duration substantially equal to T, starting from the moment when a time substantially equal to T C  has elapsed since the recognition of the header of an ATM cell. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Characteristics and advantages of the invention will now be illustrated with reference to embodiments represented by way of example and without restriction in the attached figures, in which: 
     FIG. 1 is a schematic representation of an optical selector according to the invention; 
     FIG. 2 is a schematic representation of an optical decision unit applicable in the embodiment of the optical/selector shown in FIG. 1; 
     FIG. 3 is a schematic representation of an 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 decision unit 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 decision unit shown in FIG. 2; 
     FIG. 6 is a schematic representation of a third embodiment of a variable delay unit of the optical decision/unit shown in FIG. 2; 
     FIG. 7 is a schematic representation of a first embodiment of an optical flip-flop of the optical decision unit shown in FIG. 2; 
     FIG. 8 is a schematic representation of an optical switch included in the variable delay unit shown in FIG. 4; 
     FIG. 9 is a schematic representation of the operation of the optical selector shown in FIG. 1; 
     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 decision unit shown in FIG. 2; 
     FIG. 11 is a schematic representation of a third embodiment of an optical flip-flop of the optical decision unit shown in FIG.  2 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     According to the embodiment shown in FIG. 1, the optical selector  8000  comprises an optical input gate  10 , an optical output gate  20 , a cell recognition unit  30  and an optical decision unit  40  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. 
     Typically, an optical gate is a device having an input and an output for an optical signal in transit and at least one input for at least one control signal capable of changing 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. 
     The optical input gates  10  and output gates  20  consist, for example, of conventional interferometers of the Mach-Zehnder type. As illustrated in FIG. 3, each gate comprises a 1×2 input coupler  21 , a 2×1 output coupler  22 , a 2×1 coupler  27  for an optical control signal  100 , 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 fibre type doped with rare earths or of the semiconductor type. In the illustrated embodiment, they are of the semiconductor type. Preferably, the said couplers  21 ,  22  and  27  are conventional 50/50 (3 dB) directional couplers made from optical fibres or waveguides. Alternatively, a conventional optical circulator (not shown) may be used in place of the said coupler  27  for the optical control signal  100 . The input coupler  21  divides an input optical signal into two signals having substantially the same intensity, which pass 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 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 . 
     The amplifiers  23  and  26  are conventional optical semiconductor amplifiers, each consisting, for example, of an active InGaAsP waveguide 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. 
     The supply current of the optical semiconductor amplifiers of the optical gate  10  is regulated in such a way as to impart a phase delay of 0° or 360° (2π) to the optical signals propagated in the optical paths  24  and  25 . In this way they interfere constructively in the output coupler  22  and the optical gate  10  is open (allowing the passage of the optical signal). On the other hand, the supply current of the optical semiconductor amplifiers of the optical gate  20  is regulated in such a way as to impart a phase delay of 180° (π) to the optical signals propagated in the optical paths  24  and  25 . In this way, they interfere destructively in the output coupler  22 , and the optical gate  20  is closed (blocking the passage of the optical signals). 
     Consequently, 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. On the other hand, in the presence of a control signal  100  (supplied to the amplifier  26  by the said third coupler  27 ) having a suitable intensity and wavelength, the optical signals in the two optical paths  24  and  25  are phase shifted again and the state of the optical gate  10  thus changes from open to closed and that of the optical gate  20  changes from closed to open. 
     Additionally, the gain of the said optical semiconductor amplifiers  23  and  26  is regulated in such a way that, at the input of the said output coupler  22 , the optical signals originating from the two guided optical paths  24  and  25  have the same intensity. 
     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 along the optical path of the input signal. According to this embodiment, the gate is closed by means of an optical control signal having a wavelength and intensity such that the optical amplifier is saturated, thus preventing a further optical signal at the input of the amplifier from being transmitted to its output. 
     According to a preferred embodiment, the cell recognition unit  30  is a wholly optical device capable of recognizing the header of an ATM cell at its input. The said device allows the ATM cells of bits reaching its input from the optical fibre connection  1  to pass 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  along the optical fibre connection  2  for the optical decision unit  40 . For example, European Patent Application No. 97201988.9 in the name of the applicant describes a wholly optical device (not shown) which generates an output optical impulse 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 of N bits (a cell) 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 with M bits; 
     means for carrying out a logical OR operation between the bits of the said second parallel pattern with M bits, and for generating in this way the said output optical impulse. 
     According to the embodiment shown in FIG. 2, the optical decision unit  40  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 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 . 
     Each of the connections  3 ,  4 ,  6 ,  7 ,  8 ,  9  is preferably made from a conventional optical fibre or waveguide. 
     The couplers  41 - 43  are conventional couplers. Preferably, they are 50/50 (3 dB) directional couplers. 
     The delay units  44  and  45  are, typically, constant delay units, in which there is a fixed and non-modifiable time interval between the moment at which the signal appears at their input and that at which the same signal begins to be available at the output, while the unit  46  is preferably of the variable delay type, in other words one in which the said time interval is modifiable. 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 which is 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 a 1×2 input switch  461 , a 2×1 output 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 predetermined delay line and a section of fibre of negligible length respectively, and one output. 
     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 having 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, while in the presence of the control signal the signal is switched 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 , 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 and, for the amplifier  4660 , an electrical control signal  4600 . 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. The couplers  4610  and  4620  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, each consisting of an active InGaAsP waveguide 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 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 electrical control signal  4600  is 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 electrical control signal  4600  (in normal operating conditions), an optical signal at the input of the switch is switched to the output  11  while, in the presence of the electrical control signal  4600 , it is switched to the output  12 . Additionally, the gain of the said optical semiconductor amplifiers  4650  and  4660  is regulated in such a way that at the input of the said output coupler  4620  the optical signals originating from the two guided optical paths  4630  and  4640  have the same intensity. 
     Alternatively, as seen previously in the case of the optical gates  10  and  20 , the control signal  4600  may be optical. 
     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 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). The said optical gates  4602  consist, for example of conventional interferometers of the Mach-Zehnder type as illustrated in FIG.  3 . 
     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  4606  in FIGS. 5 c  and  5   d ) which from time to time direct the input signal, 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 . 
     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 a loop. Preferably, the said loop consists of an optical fibre 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 another until a signal causing the transition from one state to the other is applied to them. For example, there are 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 a reset optical 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, of 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 intensity 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 optical set impulse is coupled to the structure  474  by means of the coupler  471  and the optical reset impulse is coupled by means of the control input  476  of the optical gate  473 . 
     The coupler  471  divides the optical set impulse into two impulses having substantially the same intensity 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 intensity 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 intensity. 
     In the presence of the optical reset impulse, the optical gate  473  is closed 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 intensity 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 similar, 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  260  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 intensity. 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 intensity, 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  999  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 with a negligible delay with respect to the interval of the said optical control impulse  222  at the input of the optical decision unit  40  the optical set impulse  888  causes the emission of an optical signal, at a substantially constant intensity, from the output of the flip-flop  47 . On the other hand, 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, 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 . 
     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 throughout the duration (T) of the said optical impulse. 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 throughout the duration (T C ) of the said optical impulse. 
     FIGS.  9 ( a )- 9 ( d ) represent schematically the operation of the optical selector  8000  according to the invention. An ATM cell  1000  at the input of the selector  8000 , 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 decision unit  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  from 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 decision unit  40  p 2  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 incoming cells from entering the optical selector  8000  after all the bits of the cell  1000  have passed through the optical gate  10  [FIG.  9 ( c )]; and 
     3) with a further delay T [T=(1/PCR)−T C ] with respect to T C , the optical decision unit  40  again causes the opening of the input gate  10  (interrupts the emission of the said second optical signal). Therefore, if a second cell  2000  arrives in the optical selector  8000  before a time interval T PCR =1/PCR=T C +T has elapsed after the arrival of the first cell  1000  (has an arrival frequency f&gt;PCR), 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, some of the bits of the second cell  2000  are still at the input of the gate  10 , the said bits are not recognized by the cell recognition device  30  as the header of an ATM cell and, therefore, the optical impulse  222  is not transmitted to the optical decision unit  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, even if incomplete cells of bits enter the optical selector  8000 , they cannot then emerge from it. 
     To summarize, 
     incoming bits can, in normal conditions, enter the optical selector  8000  through the gate  10  (which is in the open state) and reach the gate  20 , passing through the cell recognition device  30 ; 
     until the cell recognition device  30  recognizes the header of an ATM cell, the initial state of the selector  8000  is maintained (gate  10  open and gate  20  closed), and the incoming bits are thus blocked at the output (destructive interference in the output gate  20 ); 
     on the other hand, when the cell recognition unit  30  recognizes the header of an ATM cell, 
     i. the cell recognition unit  30  transmits the optical impulse  222  along the connection  2 ; and the optical decision unit  40   
     ii. causes the output gate  20  to open immediately after the arrival of the optical impulse  222  (to permit the output of the recognized ATM cell) and keeps it open for a time interval equal to T C  (to enable all the bits of the ATM cell to pass through the gate  20 ); 
     iii. causes the input gate  10  to close after a time T C  (to enable all the bits of the cell  1000  to pass through the gate  10 ) and keeps it closed for a time interval equal to T (to prevent new incoming cells at the input of the optical selector  8000 , with f&gt;PCR, from entering the optical selector). 
     For this reason, with respect to the arrival of the optical impulse  222  at the input  50  of the optical decision unit  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 T PCR , and by a delay equal to T with respect to the set impulse  666 . 
     Since T [T=(1/PCR−T C )], as seen previously, depends on the PCR negotiated between the source and the ATM network, the value of this delay is preferably modifiable by means of the variable delay unit  46 . 
     On the other hand, since the duration T C  of an ATM cell is fixed, the delays T C  are typically obtained by means of the constant delay units  44  and  45 . 
     The optical selector  8000  according to the invention can therefore be used to permit the passage to the output of only those ATM cells which arrive at its input with a time interval, with respect to each other, greater than or equal to T PCR  (they arrive with a frequency f≦PCR) and thus to provide a wholly optical checker for ATM networks. 
     The optical selector  8000  is also capable of eliminating, in the output gate  20 , the bits which enter from the input gate  10  but which are not subsequently recognized by the ATM cell recognition unit  30 .