Abstract:
A network element has at least one input, to which an optical signal can be fed, and at least one output, which is equipped to emit an optical signal; a first coupler having an input linked to the network element input and a first and a second output; an optical receiver having at least one input coupled to the second output of the first coupler and at least one output; an optical sender having at least one input of which is linked to the output of the optical receiver; a signal processing device being arranged in the signal path; a second coupler having a first input linked to the first output of the first coupler, a second input linked to the output of the optical sender, and an output which is linked to the first output of the network element.

Description:
BACKGROUND 
     The invention relates to a network element with at least one first input, to which an optical signal may be supplied, and at least one first output, which is designed to emit an optical signal. 
     US 2009/0175620 A1 discloses a network element of the type stated in the introduction. This known network element has a first input, to which an optical signal may be supplied. The optical signal comprises a plurality of different channels with in each case one dedicated frequency or wavelength. Furthermore, the known network element comprises an optical switch, which applies individual channels of the optical input signal to predeterminable outputs by means of optical filters. In addition, a further input may be provided, being adapted to add an additional signal to the output signal. The network element may therefore be used for converting predeterminable channels of the optical input signal, unchanged, into the output signal and extracting individual channels of the optical signal and providing said individual channels at a further electrical or optical output. Finally, the network element may receive an input signal via a further input which is likewise supplied to the output signal. 
     However, this known device has the disadvantage that, owing to the use of the optical switch, high insertion losses occur. In some embodiments, the filters used in the optical switch may cause phase ripple, which has a disadvantageous influence on the signal quality and restricts the possibility of cascading a plurality of said network elements. If the capacity of an optical network equipped with the known network element is intended to be increased by virtue of a greater number of optical channels with in each case a dedicated, associated wavelength, at least some of the optical switches need to be replaced. Therefore, the increase in the capacity of the optical network involves considerable complexity which means longer downtimes of the optical network. Furthermore, the known network elements cannot be scaled during operation, i.e. it is not possible to increase or lower the number of channels after installation. 
     It is therefore an object of the present invention to provide a network element having fewer insertion losses, having less influence on the signal quality and allowing easy scalability of the optical network equipped with the network element. 
     SUMMARY 
     The disclosed network element has a first input, to which an optical input signal may be supplied. The optical signal comprises a plurality of different channels with in each case one associated frequency or wavelength. The input signal is converted into an output signal, the user data of at least one channel being at least partially manipulated. 
     In some embodiments of the invention, the proposed network element may comprise a reconfigurable, optical add-drop multiplexer, which removes a data stream from the input signal and/or adds a data stream and/or changes a data stream. In other embodiments of the invention, the network element may regenerate an optical signal in order to compensate for the decreasing signal quality owing to the dispersion of the fibers carrying the signal. In this case, the regeneration may include amplification and/or pulse shaping and/or timing correction. 
     Instead of the optical switch, the invention proposes the use of an optical coupler, which couples out a predeterminable component of the input signal. This component is converted into an electrical signal by means of an optical receiver. The electrical signal may then be processed by means of a signal-processing device. The output signal of the signal-processing device may be converted into an optical signal by means of an optical transmitter. Then, the modified optical signal may be supplied back to the output signal by means of a second coupler. 
     In contrast to the known network element, the selectivity in respect of a predeterminable optical wavelength and therefore the selectivity in respect of a predeterminable channel of an optical signal comprising a plurality of channels is not achieved by an optical filter, but by the predeterminable mid-frequency and the finite bandwidth of an optical receiver, which converts the optical signal into an electrical signal. In this way, the mid-frequency and therefore the channel to be selected may be selected in a simple manner using the optical receiver. 
     The coupling of the individual assemblies of the network element may be performed directly, i.e. without any further, interposed component, or indirectly, i.e. via at least one interposed component or via an assembly comprising a plurality of components. 
     In some embodiments of the invention, the optical receiver may be designed to only convert light of an individual, predeterminable channel or wavelength range into an electrical signal. In this way, the network element according to the invention allows coupling-out or regeneration of an individual, predeterminable partial data stream from an input signal, which transports a plurality of independent data streams in independent channels. 
     In some embodiments of the invention, provision may be made for the signal-processing device to provide a selected partial data stream as electrical data stream by means of a further output. The partial data stream may comprise the data stream of one channel or at least part of the data stream of one channel. Furthermore, the signal-processing device may have a further input, by means of which a data stream may be modulated onto the optical input signal. In this way, the data stream of one channel of the input signal may be branched off, for example in order to conduct said data stream to another destination on another optical fiber. The now redundant transmission capacity may then be used by a further data stream, which is conducted from another feed-in point to the network element according to the invention. 
     A particularly reliable selection of a channel or wavelength range from the optical input signal may be achieved by means of a local oscillator, which provides an optical signal of a predeterminable wavelength, which approximately corresponds to the wavelength of the subcarrier of the respective channel. For this purpose, the local oscillator may comprise, for example, a light-emitting diode, a semiconductor laser or another optoelectronic semiconductor component. If the local oscillator is adjustable, the signal coupled out by the optical receiver may be varied in a particularly simple manner by changing the wavelength emitted by the local oscillator. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be explained in more detail below with reference to exemplary embodiments and figures, without the general concept of the invention being restricted in any way. In the figures: 
         FIG. 1  shows the block diagram of a network element proposed according to the invention. 
         FIG. 2  shows the block diagram of an embodiment of the network element according to the invention with analog signal processing. 
         FIG. 3  shows the block diagram of an embodiment of the network element according to the invention with digital signal processing. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  shows an embodiment of the network element  1  proposed according to the invention. The network element  1  has a first input  11 , to which an optical input signal may be supplied. The input signal received via the first input  11  may be supplied in the free optical path. Usually, however, the input signal is supplied by means of an optical waveguide known per se or an optical fiber. The optical input signal may comprise a plurality of useful data streams, which are transported in independent channels independently of one another. In this case, each channel may have a unique wavelength or carrier frequency which differs from the carrier frequency of an adjacent channel. In some embodiments of the invention, the difference between two adjacent carrier frequencies may be approximately 100 GHz. In other embodiments of the invention, the difference may be approximately 50 GHz or approximately 25 GHz or 1 GHz or less than 1 GHz. 
     Downstream of the input  11  of the network element  1 , the input signal arrives at a first coupler  100 . The first coupler  100  has at least one input  110  and at least two outputs  121  and  122 . The coupler  100  may be in the form of a fused coupler, in which a coupling-out optical waveguide is arranged so as to bear against the input optical waveguide, with the result that a predeterminable component of the signal is transmitted into the coupling-out optical waveguide by means of crosstalk. In other embodiments of the invention, the coupler  100  may be an interference coupler. In some embodiments of the invention, the coupler  100  may have more than the two output waveguides  121  and  122  illustrated in  FIG. 1 . In some embodiments, a plurality of couplers  100  may be cascaded in order to provide a plurality of output waveguides. For example, the number of output waveguides may be between two and ten. Correspondingly, the devices  400 ,  800  and  500  are then also provided multiply. For the understanding of the invention, however, only a single coupler  100  with two output waveguides  121  and  122  is described in the description below. 
     In the second output waveguide  122  of the coupler  100 , a predeterminable component of the optical signal is coupled out of the input signal at the input  11 . The component may be approximately 5% to approximately 25%. In some embodiments of the invention, the component is between 10 and 15%. The remaining signal leaves the coupler  100  via the output  121 . 
     The signal on the optical fiber  122  is supplied to the first input  410  of an optical receiver  400 . The optical receiver  400  converts the optical input signal at the input  410  into an electrical signal, which is provided at the output  420 . The input signal supplied to the optical receiver  400  still comprises all of the channels comprised in the input signal. In order to select a predeterminable channel from the input signal, a local oscillator  700  is used in the embodiment illustrated in  FIG. 1 . The local oscillator  700  has outputs  720 , at which an optical signal of the local oscillator is provided. The local oscillator  700  may comprise an optoelectronic semiconductor component, for example a semiconductor laser. In this case, the local oscillator  700  provides an optical signal with a small bandwidth, said signal having approximately the same wavelength as the optical carrier of the channel to be extracted from the input signal. The optical signal of the local oscillator is supplied to the optical receiver  400  via a second input  412 . 
     In the optical receiver  400 , the superimposition of all of the wavelengths provided in the input signal with the output signal of the local oscillator  700  results in different mixed products. In this case, the bandwidth of the optical receiver  400  is set in such a way that all of the mixed products apart from one are outside the bandwidth of the optical receiver. In this way, the desired useful data stream of the selected channel is provided at the output  420  of the optical receiver  400 . 
     The useful data stream is supplied as electrical signal to the input  810  of the signal-processing device  800 . The signal-processing device  800  may comprise an analog and/or a digital circuit. The signal-processing device  800  may be provided for manipulating the data stream in a predeterminable manner. In some embodiments, the signal-processing device  800  may perform regeneration of the signal. The regeneration may include improvement of the timing, amplification and/or pulse shaping. In other embodiments of the invention, the signal-processing device  800  may perform inversion of the signal. In some embodiments, the signal-processing device  800  may perform format conversion of the input signal. In some embodiments of the invention, the signal-processing device  800  may provide the useful data stream via a second output  822 . The useful data stream may then leave the network element  1  via the second output  14  as an optical or electrical signal. 
     In some embodiments of the invention, the signal-processing device  800  may receive a new useful data stream via the second input  812 . This new useful data, stream may be supplied to the network element  1  via the input  13  as an optical or electrical signal. 
     The useful signal processed by the signal-processing device  800  leaves the signal-processing device  800  via the output  820 . 
     The output signal of the signal-processing device  800  is supplied to an optical transmitter  500  via the input  510  thereof. The optical transmitter may comprise an optoelectronic semiconductor component, which, in a manner known per se, generates an optical, modulated carrier signal. In this way, a signal is provided at the output  520  of the optical transmitter  500  which comprises at least the information of the data stream which was provided at the output  820  of the signal-processing device  800 . 
     In some embodiments of the invention, the optical transmitter  500  may comprise a modulator, which modulates an optical carrier signal, which is provided by the local oscillator  700  and is supplied to the optical transmitter via the input  512  thereof. In this way, the carrier frequency of the signal provided at the output  520  may be monitored with considerable accuracy. 
     The optical data signal provided at the output  520  of the optical transmitter  500  is supplied to a second optical coupler  200  via an input  212 . The optical coupler  200  may be a fused coupler or an interference coupler, in the same way as the first coupler  100 . Furthermore, the input signal which has left the first coupler  100  via the output  121  is still supplied to the coupler  200  via a first input  211 . 
     In the second coupler  200 , the input signal interferes with the signal provided at the output  520 . In order to be able to adjust the phase difference between the two signals provided for the interference to a predeterminable value, in some embodiments of the invention a delay element  600  with an input  610  and an output  620  may be provided. In some embodiments of the invention, the delay element  600  may be an optical waveguide with a predeterminable length, with the delay corresponding to the propagation time of the signal on this optical waveguide. 
     The signal provided at the output  820  of the signal-processing device  800  may, in some embodiments of the invention, be formed in such a way that it comprises a signal component which cancels the signal originally transported on this channel in the event of interference in the second coupler  200 . In this way, the channel in question at the output  220  of the second coupler  200  represents the data signal supplied via the input  13  and/or the data signal regenerated or changed in the signal-processing device  800 . The optical signal emerging from the second coupler  200  leaves the network element  1  via the output  12  thereof. In turn, the output  12  may comprise a free optical path or an optical waveguide. 
     In some embodiments of the invention, the network element  1 , as optoelectronic semiconductor component, may be integrated monolithically on a single substrate. In other embodiments of the invention, the network element  1  may have a partially integrated configuration. In this case, only some of the elements illustrated in  FIG. 1  of the network element  1  are integrated monolithically on a substrate. Other elements may then be provided on one or more further substrates. A plurality of semiconductor substrates may be combined in a single housing. In some embodiments of the invention, the individual components of the network element  1  may be arranged in different housings, which are combined on a circuit carrier, for example a printed circuit board. 
     In some embodiments of the invention, the signal-processing device  800  may comprise a programmable logic circuit or a use-specific semiconductor chip. In some embodiments of the invention, the signal-processing device  800  may comprise a microprocessor or a microcontroller, on which software is run such that the signal-processing device  800  implements the function for which it is intended. 
       FIG. 2  shows an embodiment of a network element  1  according to the invention which will, be used to explain the design of the optical receiver  400 , the signal-processing device  800  and the design of the optical transmitter  500  in more detail. 
     The embodiment of the invention shown in  FIG. 2  comprises first and second couplers  100  and  200  and a delay element  600 , as explained in connection with  FIG. 1 . Furthermore, the network element shown in  FIG. 2  also comprises a local oscillator  700 , which provides an optical signal of a predeterminable wavelength. 
     The optical receiver  400  shown in  FIG. 2  comprises a third coupler  300  with a first input  311 , a second input  312 , a first output  321  and a second output  322 . In some embodiments, the third coupler  300  may be a 2×2 multimode interference coupler. In this case, in the third coupler  300  the input signal supplied via the input  410  and the signal of the local oscillator  700  are brought to interference. The interference signal is provided at the two outputs  321  and  322  of the third coupler  300  and is supplied to a difference signal detector  440  via the inputs  441 . 
     In some embodiments of the invention, provision may be made for a device  470  for polarization regulation to be arranged in the signal path between the input  410  of the optical receiver  400  and the first input  311  of the third coupler  300  and in the signal path between the second input  412  of the optical receiver  400  and the second input  312  of the third coupler  300 . In this way, the polarization of the signal supplied to the third coupler  300  may be tracked, with the result that the polarization always has the same value or the actual value of the polarization only deviates from a setpoint value by a predeterminable difference. In this way, reliable superimposition of the signals in the third coupler  300  is ensured, with the result that the third coupler  300  may provide an optical signal with a high quality factor at the input  441  of the difference signal detector  440 . 
     In other embodiments of the invention, the device  470  for polarization regulation may also be replaced by a steady-state polarization filter. In such an embodiment of the invention, provision may be made for a twofold configuration of the optical receiver  400 , with the result that an associated optical receiver  400  is provided for each polarization direction. 
     The difference signal detector  440  provides an electrical signal at its output  443  which is a function of the difference in the intensities of the optical signals present at the input  441 . The difference signal detector  440  may have a finite bandwidth, which is selected such that all of the mix products of the input signals at the input  410  with the signal of the local oscillator  700  are outside this bandwidth, with the exception of a predeterminable mix product, which is formed from the channel to be selected of the input signal. The electrical signal of the difference signal detector  440  then leaves the optical receiver  400  and is supplied to the signal-processing device  800  via the first input  810  thereof. 
     In the exemplary embodiment illustrated, the signal-processing device  800  has analog electronic signal processing. The signal present at the input  810  may be filtered by an optional low-pass filter  830 . In this case, the low-pass filter  830  ensures that the subsequent signal processing only includes the useful signals of the selected optical transmission channel. If the bandwidth limitation of the difference signal detector  440  is selected correspondingly, the low-pass filter  830  may also be dispensed with in some embodiments of the invention. 
     In some embodiments of the invention, the data signal extracted from the selected optical transmission channel may leave the signal-processing device  800  via the second output  822 . The signal may then be provided at the output  14  of the network element  1 . 
     In some embodiments of the invention, the extracted data signal may be supplied to an inverter  840 . In this way, an inverted data signal is provided at the first output  820  of the signal-processing device  800 , said inverted data signal modulating the optical signal leaving the optical transmitter  500  via the output  520  in such a way that said optical signal interferes with the original optical input signal in the second coupler  200  in such a way that the selected channel in the output signal is canceled or is only present in unmodulated form. As a result, the carrier signal is available for a further useful data stream. 
     This further useful data stream may be supplied to the network element  1  via the input  13 . In this case, the input  13  is coupled to the input  812  of the signal-processing device  800 , with the result that the data signal may leave the signal-processing device  800  likewise via the output  820  thereof. The electrical signal at the output  820  therefore comprises the inverted original signal and the data stream to be modulated instead of the original data signal. The data stream supplied via the connection  13  may comprise the identical useful data which were previously provided via the output  14 . The useful data may then be subjected to regeneration between the output  14  and the input  13 . In other embodiments of the invention, such regeneration may also be performed within the signal-processing device  800 . In yet another embodiment of the invention, the data stream leaving the output  14  may comprise different useful data than the data stream supplied to the input  13 . 
     The electrical signal produced in the signal-processing device  800  is supplied to the optical transmitter  500 . In the embodiment illustrated, the optical transmitter  500  comprises a modulator  530 . An optical carrier is supplied from the local oscillator  700  to the modulator, said optical carrier being modulated with the data stream of the signal-processing device  800 . The modulation may comprise, for example, amplitude modulation, phase modulation or another form of modulation known per se. 
     Furthermore, the optical transmitter may comprise additional components, for example an amplifier  540 , which may implement impedance conversion and/or match the current intensity and/or the voltage of the electrical signal. In some embodiments of the invention, the amplifier  540  may implement filtering of the data stream. 
     In other embodiments of the invention, the optical transmitter  500  may also have a different design and comprise a directly modulatable semiconductor laser instead of the modulator  530 , for example. 
     The signal emitted by the optical transmitter  500  via the output  520  is then brought to interference with the original signal in the second coupler  200 . In the process, the data signal inverted in the inverter  840  cancels the original data signal, with the result that the selected channel now only comprises the useful data supplied via the input  13  at the output of the second coupler  200 . 
       FIG. 3  shows an embodiment of the invention which comprises digital data processing. The embodiment shown in  FIG. 3  also comprises a first coupler  100 , a second coupler  200  and a local oscillator  700 , as explained in connection with  FIGS. 1 and 2 . Part of the optical input signal which has been coupled out via the first coupler  100  is supplied to the optical receiver  400 . Furthermore, an optical signal of a local oscillator  700  is supplied to the optical receiver  400 , as described above. In some embodiments of the invention, both optical signals may be supplied to a device  470  for polarization regulation, as described above in connection with  FIG. 2 . In other embodiments of the invention, the device  470  for polarization regulation may also be dispensed with or may be replaced by a simple polarization filter. 
     Then, the optical signals arrive at a device  450 , with which the intensity and/or the phase of the input signal may be extracted. In some embodiments of the invention, the device  450  may comprise a multimode interference coupler. In some embodiments of the invention, the multimode interference coupler may be a 2×4/90° hybrid or comprise such a hybrid. In this case, the device  450  has two inputs  451  and  452  and four outputs  453 . The outputs  453  may be coupled in pairs to associated difference signal detectors  440  and  445 . 
     The difference signal detectors  440  and  445  have a finite bandwidth, with the result that, in interaction with the local oscillator  700 , they selectively extract the data signal of a predeterminable channel of the input signal. 
     The embodiment of the invention illustrated in  FIG. 3  shows the most general case, in which both the intensity and the phase of the optical carrier signal are determined by means of two difference signal detectors  440  and  445 . In this way, an optical data stream may be decoded which codes a plurality of data bits in a symbol duration by means of quadrature amplitude modulation. If the optical data signal has only amplitude modulation or only phase modulation, one of the difference signal detectors  440  and  445  may also be dispensed with in some embodiments of the invention. 
     Analog-to-digital converters are arranged at the output  443  of the difference signal detectors in order to generate a digital data stream from the electrical output signal of the difference signal detectors  440  and  445 . The digital data stream then leaves the optical receiver  400  via the outputs  420  thereof and is supplied to the signal-processing device  800 . 
     Even when only in each case one individual line connection is illustrated at the output of the analog-to-digital converters  460  and  461 , this may of course include a plurality of physical conductors, for example for transmitting a plurality of digital data bits in parallel and/or as a connection to ground. The individual line illustrated in  FIG. 3  therefore describes a logic connection between the optical receiver  400  and the signal processing device  800  and not an individual physical line. 
     As already explained in connection with  FIG. 2 , the signal-processing device  800  may have low-pass filters  431  and  432 . In other embodiments of the invention, these low-pass filters may also be dispensed with or may be part of the optical receiver  400 . Likewise, the analog-to-digital converters  460  and  461  may in some embodiments also be part of the signal-processing device  800  and not be arranged in the optical receiver  400 . 
     Then, the signal arrives at a logic circuit  850  with at least one input  851  and at least one output  852 . The logic circuit  850  may implement inversion of the data signal in some embodiments of the invention, as described in connection with  FIG. 2 . In this way, the output signal of the signal-processing device  800  comprises a signal component which cancels the data signal originally transported on the selected channel by means of interference in the second coupler  200 . 
     Furthermore, the logic circuit  850  may implement further modifications of the supplied data signal. For example, regeneration of the signal may be performed in the logic circuit  850 . In other embodiments of the invention, the logic circuit  850  may perform format conversion of the signal. In yet another embodiment of the invention, the logic circuit  850  may provide the data stream or a part thereof via the output  14  of the network element  1  and/or receive a data stream via the connection  13  of the network element  1 . For this purpose, the logic circuit  850  may comprise a digital signal processor, a programmable logic circuit, a microprocessor or a microcontroller, which implements the respectively desired modifications of the data stream supplied via the connection  851 . 
     The data stream modified in the logic circuit  850  leaves the logic circuit via the output  852 . The digital data signal provided via the output  852  leaves the device  800  via the output  820  thereof. The data signal comprises a digital data stream, which represents the desired waveform with which the optical carrier is intended to be modulated in the downstream optical transmission device  500 . 
     For the modulation of the optical carrier, an I/Q modulator  550  is used as shown in  FIG. 3 . The unmodulated optical carrier is supplied from the local oscillator  700  to the I/Q modulator  550 . Furthermore, at least one electrical analog signal is supplied to the modulator  550 , the waveform of said analog signal intending to be modulated onto the optical carrier. In order to produce this analog electrical signal, at least one digital-to-analog converter  560  and  561  is provided in the optical transmitter  800 . In other embodiments of the invention, the digital-to-analog converters may also be part of the signal-processing device  800 . The digital-to-analog converters  560  and  561  receive the data stream of the device  800  and provide the input signal for the modulator  550  at the outputs of said converters. 
     In some embodiments of the invention, the optical transmitter  500  may also comprise optional amplifiers  541 , by means of which impedance matching, amplification or filtering of the analog signals may be performed. If the output signal of the optical transmitter  500  has only simple amplitude or phase modulation, in some embodiments of the invention a single modulator  550  may also be used, which only receives one of the analog signals illustrated. 
     The optical signal emitted by the optical transmitter  500  is in turn brought to interference in the second coupler  200 , as has already been described in connection with  FIGS. 1 and 2 . 
     The invention is not, of course, limited to the embodiments represented in the figure and the illustrative embodiments. The above description should therefore not be regarded as limiting, but as illustrative. The following claims should be understood in such a way that a mentioned feature is provided in at least one embodiment of the invention. This does not exclude the possibility of the presence of further features. Insofar as the claims and the above description define “first” and “second” features, then this notation serves to differentiate between two similar features without stipulating an order of precedence.