Abstract:
A method for enabling bidirectional data communication using a single optical carrier and a single laser source with the aid of an integrated, colorless demodulator and detector for frequency modulated signals, and a reflective modulator. A receiving optical system holds a technique for demodulation and detection of optical frequency modulated signals, enabling remodulation of the incoming signal to establish bidirectional communication with the transmitting optical system, without introducing a high penalty. A colorless demodulator and detector, which provides the functionality of a periodic filtering device for demodulation of the downstream, and also detection capability. The principle of operation of the CDD relies on the introduction of a comb transfer function with the help of a Semiconductor Optical Amplifier, by providing a reflected feedback signal to the CDD&#39;s active element. This periodic transfer function is obtained by an optical cavity and allows for wavelength-independent operation on a given wavelength grid.

Description:
TECHNICAL FIELD 
     The present invention relates to a receiving optical apparatus of a bidirectional optical link that uses a single wavelength with demodulation and detection of frequency modulated signal in a colorless and integrated way, next to re-modulating this signal with the aid of reflective modulation. 
     The present invention also refers to a method for enabling bidirectional data communication using a single optical carrier modulated in frequency and an apparatus as the cited one, at a user premises. 
     BACKGROUND ART 
     Optical fiber communications is one of the drivers to enable broadband services to be delivered by an operator to the customers that can be spread over larger geographic areas. Optical fiber is used as transmission medium because it offers several advantages compared to the copper wires, such as the traditional twisted pair. Fiber-to-the-X (FTTx) technology (X can stand for Curb, Node, Building, Home or other) has been extensively studied worldwide, for delivering high bandwidth to users and for converging wireless and wireline. 
     An important point for FTTx is the capability for building future-proof broadband networks with low installation and operating expenditures. While active optical networks (AONs) exist, taking advantage of repeaters and switches for reach extension and routing, passive optical networks (PONs) are also gaining attention due to the fact that no active components are deployed in the distribution plant between the operator and the customers. In this way, cost deriving from maintenance of active devices can be kept low as they are situated either at the central office of the provider or located at the customer premises. 
     The capacity and number of served users can be expanded by taking multiplexing technologies into account in the architecture of the access network (AN), regardless if it is of active or passive nature. As the optical fiber is suitable to transmit on multiple optical frequencies, wavelength division multiplexing (WDM) can lead to a significant improvement in cost and capacity, as fiber infrastructure can be shared between the customers while more data signals can be transmitted on different wavelengths. This kind of AN has an optical multiplexer situated between the operator and its customers, and is herein referred to as a WDM-AN. 
     Furthermore, each wavelength can, for the case of a so-called hybrid AN, be divided into time slots by means of time division multiplexing (TDM) for splitting the signal in its power to a bunch of customers instead of only one. Although this procedure leads to a reduction in the data rate per user, the naturally high data rates that can be achieved for each wavelength, thanks to the maturity of optical transmitters, ensures that the net data rates for the single customers still stay high. This kind of AN has, in addition to the multiplexer of the WDM-AN, a power splitter located at each output of the multiplexer, while the customers are connected to the outputs of the power splitter. Such an AN is herein referred to as a WDM/TDM-AN. 
     Expanding the AN by multiplexing means that the cost can be reduced due to a shared infrastructure at the fiber distribution plant and also at the central office of the network operator, referred to as the optical line terminal (OLT) herein, where several light sources and expensive equipment such as modulators and devices for signal conditioning are located. One requirement for introducing multiplexing into the AN is to keep the customer premises equipment, referred to as the optical network unit (ONU) herein, identical and therefore agnostic to these multiplexing techniques. A reflective design without active optical source or a design with a tunable optical source allows to have one single ONU, which can be used at any position inside the AN (i.e. the ONU is operable with different wavelengths and at different ports or a power splitter). Such a design that is suitable for mass deployment of ONU ensures cost effectiveness as the ONU will determine the expenditures for an AN with a high number of users. 
     The use of reflective modulators integrated together with optical amplifiers at the customer premises is a promising solution for the ONU. In this way, the loss over the network can be overcome while imprinting upstream transmission data on the incoming signal. An efficient AN uses a single wavelength as optical input signal for an ONU, which carries the data transmission from the OLT, referred to as the downstream, and also the data transmission from the ONU back towards the OLT, referred to as the upstream. These two data streams are present at the same time due to the bidirectional nature of communication. 
     Realistic deployment of the access networks discussed above, require ONUs that are not wavelength-dependent (color-agnostic or colorless) and are capable of re-using the same downstream signal wavelength for modulating the upstream data. Re-modulation of downstream can be efficiently done by using orthogonal modulation formats, as they avoid crosstalk between down- and upstream. However, the design of the ONU becomes more complicated as more complex modulation formats have to be used (compared to the simplest intensity modulation format), which may prevent a cost-effective deployment of customer premises equipment. This disadvantage is mainly motivated by the inability of photo-detectors to acquire information from the optical signal such as phase or frequency in addition to its intensity. Therefore, modulation formats that imprint data in the phase or frequency of the optical signal require in principle additional components such as filters or other information-converting structures [Prat05] [Martinez08]. 
     Promising candidates for a reflective modulator, which can take the advantage of just having to modulate the intensity of the constant-envelope downstream signal, are the reflective semiconductor optical amplifier (RSOA), the reflective electro-absorption modulator or integrated versions of semiconductor optical amplifier (SOA) and reflective electro-absorption modulator (REAM), where the SOA acts as amplifier to overcome also the losses of the REAM or any reflective active optical component capable of intensity modulating the upstream data signal. 
     REFERENCES 
     
         
         [Prat05] J. Prat et al., “Full-Duplex Single Fiber Transmission Using FSK Downstream and IM Remote Upstream Modulations for Fiber-to-the-Home,” Phot. Tech. Lett., vol. 17, pp. 702-704 (2005). 
         [Martinez08] J. J. Martinez et al., “Novel WDM-PON Architecture Based on a Spectrally Efficient IM-FSK Scheme Using DMLs and RSOAs,” J. Lightwave Tech., vol. 26, pp. 350-356 (2008). 
       
    
     SUMMARY OF THE INVENTION 
     The present invention provides a method for enabling bidirectional data communication using a single optical carrier and a single laser source with the aid of an integrated, colorless demodulator and detector for frequency modulated signals, and a reflective modulator. 
     The invention also provides a receiving optical apparatus of a bidirectional optical link that uses a single wavelength with demodulation and detection of frequency modulated signal in a colorless and integrated way, that be following disclosed. 
     According to the present invention, an optical transmission network from type of either AN, WDM-AN or WDM/TDM-AN is provided, comprising:
         an OLT containing optical sources for transmitting frequency modulated downstream data signals towards the ONUs and also receivers for the reception of the upstream data from several ONUs, transmitted in intensity modulation;   a fiber distribution plant, referred to as the optical distribution network (ODN) herein, comprising fiber to connect several units at the border of the ODN. It can also include one or more interconnecting points, referred to as the remote nodes (RN), for relaying the data traffic from the OLT to the ONUs and backwards. These conjunction points between different segments of the ODN are not further specified;   ONUs that comprise receivers for the downstream after being optically demodulated into an intensity modulated format, and reflective modulators that use the downstream signals for the upstream transmission.       

     The ONU holds a technique for demodulation and detection of optical frequency modulated downstream signals, enabling remodulation of the downstream signal with upstream data without introducing a high penalty. This is achieved by means of a colorless demodulator and detector (CDD), which provides the functionality of a periodic filtering device for demodulation of the downstream, and also a detection capability. It is used after the splitter that extracts part of the incoming signal into the ONU for downstream detection. The CDD is adjusted to the frequency deviation of the downstream signal, but not further specified at the moment (details can be seen in the next section). The principle of operation of the CDD relies on the introduction of a comb transfer function with the help of a SOA, by providing a reflected feedback signal to this active element used in the CDD. This periodic transfer function is obtained by an optical cavity and allows for wavelength-independent operation on a given wavelength grid, such as the ITU WDM wavelength grid. 
     The upstream can be then intensity modulated onto the incident downstream signal, due to the fact that the incoming frequency modulated signal has in principle a constant envelope. The upstream modulator is preferably from reflective nature, but does not necessarily have to be. The chosen modulation scheme allows reusing the wavelength also for upstream transmission without passing parts of the downstream back to the upstream receiver, which would introduce a significant reception penalty for the upstream. 
     A control loop may be used together with a tapped electrical downstream signal to provide a feedback to the CDD for stabilization of its operation point. 
     Two preferred embodiments for the ONU are considered, which depend on the capability of concurrent upstream transmission during the downstream detection. For the first preferred embodiment, the ONU is extended with an optical transmitter in addition to the CDD used for detection, to provide full-duplex transmission. In this case, the downstream is remodulated with the said optical transmitter. For the second preferred embodiment, the CDD inside the ONU is not only used for detection of the downstream but also for transmission of the upstream data. In this latter case, a simpler realization of the ONU is provided at the cost of half-duplex transmission. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more detailed description of the present invention will be given by taking reference to the accompanying drawings which are showing examples for realizing the present invention, allowing providing an illustrative description for the preferred embodiments. 
       It shall be stressed that the drawings provided contain all essential information that is believed to be most relevant for an understanding of the principles of the present invention. Furthermore, no attempt is made to show unnecessarily detailed structures of the present invention, for ease of understanding. In this way, non-essential elements were skipped from some of the drawings. For the cases where no example is given, it is left to those who are skilled in art, how the fundamental structures depicted in the drawings may be realized in practice. 
         FIG. 1  depicts the AN with downstream data streams modulated in their frequency and upstream data sent modulated in its intensity, in which context the ONU enables the colorless demodulation and detection of the downstream signals at the customer premises, according to the present invention. 
         FIG. 2  depicts an ONU for full-duplex transmission in ANs with orthogonal modulation formats for both bit stream directions, including a CDD and a control mechanism for adjusting the CDD, according to the present invention. 
         FIG. 3  depicts an ONU for half-duplex transmission in ANs with orthogonal modulation formats for both bit stream directions, including a CDD and a control mechanism for adjusting the CDD, according to the present invention. 
         FIG. 4  depicts the CDD with its physical properties, as it is included in the ONUs of the AN, according to the present invention. 
         FIG. 5  depicts a preferred embodiment of the CDD, according to the present invention. 
         FIG. 6  depicts another preferred embodiment of the CDD, according to the present invention. 
         FIG. 7  depicts the functionality for spectral sub-channel selection via the CDD, according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Before an explanation of the present invention is given, it has to be noted that the present invention is not limited in its applicability to the details set in the following discussion or the examples provided. The present invention can be carried out in various embodiments. It shall be understood that certain features that are described in the context of different embodiments, may also be provided for a certain specific embodiment. In turn, certain features that are described in context of a single embodiment may also be provided for any other suitable embodiment included in the description of the present invention. 
     The terms “comprises”, “comprising”, “includes”, “including”, and “having” are supposed to be understood as “including but not limited to”, while the term “consisting of” has the same meaning as “including and limited to”. The term “consisting essentially of” means that the structure may include additional parts, but only if these additional parts do not perturb or alter the basic and novel characteristics of the claimed structure. Furthermore, the singular form “a”, “an”, and “the” include also plural references unless it is otherwise dictated by the context. 
     Detailed Description of the Method and Apparatus of this Invention 
     A novel ONU is introduced based on a solution with integrated semiconductor devices for colorless demodulation and detection of the downstream signal while remodulating the downstream with upstream data via a simple intensity modulator. This method enables significant performance improvement in terms of sensitivity for the upstream reception at the OLT due to the orthogonal modulation formats used. 
     Considering the preferred embodiment of the present invention, modifications are required only at the ONU and do not perturb the fiber distribution plant nor the OLT (besides using frequency modulation instead of intensity modulation for the downstream signal), nor do other preferred embodiments of the present invention. 
     In discussion of several figures herein, similar numbers refer to similar parts. The drawings further do not have to be scaled. 
     1. Operation of an Apparatus Implementing the Proposed Method 
     Reference is now made to  FIG. 1  which illustrates the AN architecture  100 , in which context the present invention is embedded. 
     The AN  100  comprises an OLT  110 , an ONU  130  that are connected via the ODN  120  which includes a bidirectional link  140 . Signals originating from the OLT  110  towards the ONU  130  are referred to as downstream signals, while signals originating from the ONU  130  towards the OLT  110  are referred to as upstream signals. 
     In its simplest version of the AN  100 , the ODN  120  comprising a bidirectional link  140 , which can be established by a single optical fiber. In an extended version, the ODN  120  would incorporate techniques of WDM and TDM as described before. 
     The OLT  110  includes the optical source of the AN  100  and an optical frequency modulator, used for imprinting the optical carrier with downstream data. The frequency deviation of the frequency shift keyed downstream modulation is adjusted to the parameters used in the design of the CDD in the ONU  130 . The bidirectional data transmission along the fiber link  140  has to be splitted/merged to the two unidirectional paths of the optical transmitter and receiver of the OLT  110 , e.g. with the help of an optical circulator. The optical receiver of the OLT  110  includes further photo detectors such as PIN diodes or avalanche photo diodes. 
     Depending on the requirements inside the AN  100 , the OLT  110  may further have other components such as optical amplifiers, means of dispersion compensation, optical WDM multiplexer/demultiplexer and electronic signal conditioning. The OLT  110  is capable to perform higher layer functionalities and is interconnected to an operator interface according to modern AN standards. 
     The OLT  110  and the fiber infrastructure of the AN  100  can be shared between multiple operators, which are delivering different types of services towards the customers. The ONU  130  then has to be capable to switch between different operators in a proper way, to enable this feature of multi-operability in the AN  100 . 
     The ONU  130 , hosted at the customer premises and responsible for downstream reception and upstream data transmission, is now discussed in more detail in its preferred embodiments of the present invention. 
     Reference is now made to  FIG. 2  illustrating an ONU  200  with a CDD for full-duplex data transmission, in accordance with the first preferred embodiment of the present invention. 
     The ONU  200  is connected by line  201  towards the ODN  120  of the AN  100 . An optical coupler  210  splits off a part of the downstream  202  and relays this part  211  to the CDD  220 , which is used for demodulation and detection of the downstream. The detected electrical signal  224  is fed to the electrical receiver  270 . This in turn contains methods for electrical signal conditioning and higher layer functionalities. The interconnection to the user interface that is compliant with an AN standard is made via line  271 . 
     As the CDD  220  may cause reflections, indicated by the signal  213 , an isolator  240  may be placed in between the optical coupler  210  and the CDD  220 . For the case that there is no isolator  240 , the signal  214  will overlap with the upstream  216  at the launched output signal  203  of the ONU  200 . 
     The key part, the CDD  220  contains a reflective interface (RINT)  221 , an amplifier (DAMP)  222  and a detector (DET)  223 . The reflective facet that is attached to the DET  223  provides feedback towards the RINT  221  at the entrance of the CDD  220 . Together with the bidirectional DAMP  222 , a periodic transfer function for the CDD  220  is established via the mirror reflections of the RINT  221  and the reflective facet attached to the DET  223 . While the RINT  221  may be constructed passively, the DAMP  222  can be a SOA or any other suitable amplifier, and the DET  223  may be an REAM or any other suitable reflective detector. The optical length of the CDD  220 , determined by the geometrical lengths and refractive indices of the DAMP  222  and the DET  223 , as well as the geometry of the RINT  221  and its refractive index, determines the free spectral range (FSR) of the generated comb that is used for demodulation of the frequency modulated downstream signal. 
     To align the comb of the detection function of the CDD  220  to the spectrum of the incident downstream signal, the CDD  220  is connected with a bias controller (BCTL)  250 . This controller provides the electrical bias signals  251 ,  252  and  253  for the RINT  221 , the DAMP  222  and the DET  223 , respectively. The RINT  221  may not require a bias signal  251 . The alignment of the comb is obtained by small variations of the bias signals  251 ,  252  and  253  around their usual bias points. Alternatively, also the temperature of the semiconductor-based CDD  220  could be varied. On which of the bias signals these variations are applied, depends on the chosen realization for the RINT  221 , DAMP  222  and DET  223 . The feedback signal for the BCTL  250  is obtained from the detected downstream signal  224 . Alternatively, the steering signal  225  for the BCTL  250  may stem from the receiver (RX)  270  which can be also provided in addition to the detected signal  224 . 
     The BCTL  250  includes an electronic control circuit and an electrical driver circuit that is capable of driving the RINT  221 , the DAMP  222  and the DET  223  with appropriate electrical signals, depending on the physical realization of the CDD  220 . 
     The other part of the downstream signal is relayed as signal  215  to pass to the optical transmitter (OTX)  230  which performs the function of remodulating the constant envelope downstream signal  215  with upstream data. For this reason, an intensity remodulator (REM)  232  is used, which might be reflective as shown in  FIG. 2 . The modulator can be from for example a REAM, a RSOA or other suitable intensity modulators. For the case that REM  232  introduces optical losses, an amplifier (RAMP)  231  may be placed in the OTX  230 . This amplifier can be a SOA that might be also integrated together with the REM  232 . 
     The transmitter (TX)  260  contains high layer functionalities as well as adaptation of the electrical driving signal  261 . Line  262  connects the TX  260  to the user interface according to an AN standard. The TX  260  is capable of driving the REM  232  in a proper way, including adaptation of the base-band frequency data signal and adaptation of the bias point in terms of adjustment of the bias current and/or voltage for the REM  232 . A direct interconnection  272  between RX  270  and TX  260  may be present for higher layer functionality such as signaling inside the AN. 
     Although a reflective OTX  230  is preferred for wavelength reuse, it may comprise a direct or external modulated laser diode that may also be tuned in its wavelength. 
     Reference is now made to  FIG. 3  illustrating an ONU  300  with a CDD for half-duplex data transmission, in accordance with the second preferred embodiment of the present invention. 
     The ONU  300  is connected by line  301  towards the ODN  120  of the AN  100 . The downstream  302  enters the CDD  320 , which is used for demodulation and detection of the downstream. The detected electrical signal  324  is fed to the electrical receiver  370 . This in turn contains methods for electrical signal conditioning and higher layer functionalities. The interconnection to the user interface that is compliant with an AN standard is made via line  371 . 
     The CDD  320  contains a reflective interface RINT  321 , an amplifier DAMP  322  and a reflective detector and remodulator (RDR)  323 . The reflective nature of the RDR  323  provides feedback towards the RINT  321  at the entrance of the CDD  320 . Together with the bidirectional DAMP  322 , a periodic transfer function for the CDD  320  is established via the gain ripple of the DAMP  322 . While the RINT  321  may be constructed passively, the DAMP  322  can be a SOA or any other suitable amplifier, and the RDR  323  may be an REAM or any other suitable reflective device. The optical length of the CDD  320 , determined by the lengths and refractive indices of the DAMP  322  and the RDR  323 , as well as the geometry of the RINT  321  and its refractive index, determines the FSR of the generated comb that is used for demodulation of the frequency modulated downstream signal. 
     The alignment of the comb of the detection function of the CDD  320  to the spectrum of the incident downstream signal is made in a similar way as for the ONU  200 . Therefore, the CDD  320  is connected with a bias controller BCTL  350 . This controller provides the electrical bias signals  351 ,  352  and  353  for the RINT  321 , the DAMP  322  and the RDR  323 , respectively. The RINT  321  may not require a bias signal  351 . The alignment of the comb is obtained by small variations of the bias signals  351 ,  352  and  353  around their usual bias points. On which of the bias signals these variations are applied, depends on the chosen realization for the RINT  321 , DAMP  322  and RDR  323 . The feedback signal for the BCTL  350  is obtained from the detected downstream signal  324 . Alternatively, the steering signal  325  for the BCTL  350  may stem from the RX  370  which can be also provided in addition to the detected signal  324 . The BCTL  350  includes an electronic control circuit and an electrical driver circuit that is capable of driving the RINT  321 , the DAMP  322  and the DET  323  with appropriate electrical signals, depending on the physical realization of the CDD  320 . 
     The CDD  320  is also used for upstream data transmission. For this reason, the ONU  300  is operated in half-duplex, meaning that at the time during the downstream detection there is no upstream transmission and vice versa. The optical carrier is then remodulated by the CDD  320  and carries the upstream data at the output signal  303 . 
     For modulation, the RDR  323  may be biased differently than for the detection, so that the BCTL  350  will adjust the bias signal  353  during upstream transmission. For this reason, a signal  326  is provided from the TX  360  towards the BCTL  350 . The upstream data, provided by the TX  360 , is fed via the data interface  324  to the RDR  323 . 
     The transmitter TX  360  contains high layer functionalities as well as adaptation of the electrical driving signal  324 . Line  362  connects the TX  360  to the user interface according to an AN standard. The TX  360  is capable of driving the RDR  323  in a proper way, including adaptation of the high frequency data signal. A direct interconnection  372  between RX  370  and TX  360  may be present for higher layer functionality such as signaling inside the AN. 
     In principle, downstream detection could take place at the same time as upstream modulation, since the downstream carries a constant envelope—although a high penalty may derive from the upstream transmission. As there will be crosstalk from the upstream data into the detected downstream signal due to the shared interconnection  324  towards the RDR  323 , the data provided between the interface  372  between TX  360  and RX  370  can be used to inform the RX  370  about the transmitted upstream data, which can then be used by the RX  370  to counteract distortions in the detected downstream. 
     This can be applied not only to base-band digital signals but also to radio or pass-band signals, by adjusting the frequency deviation and center. 
     2. Device for Colorless Demodulation and Detection 
     Reference is now made to  FIG. 4  illustrating the principal properties and requirements for the CDD of the present invention. 
     The CDD  400  is a structure with an optical input  401 , and one or more electrical ports  402 , which can be used for biasing the device while at least one port is used to obtain the detected high frequency input signal, which is derived from the signal that enters the optical input  401 . 
     The CDD  400  converts the frequency modulation of the incident optical signal at the input port  401  into intensity modulation, which is present on the electrical output signal at the port(s)  402 . 
     The transfer function  410  for the detected signal has a periodicity in its frequency, which is defined by the free spectral range (FSR),
 
 T ( f +FSR)= T ( f )
 
     The formed comb-like function is further defined by the full-width half-maximum (FWHM) bandwidth Δf of its transmission peaks. 
     According to the present invention, the following conditions have to be fulfilled by the CDD.
         the periodic transmission function is given over the whole optical wavelength range in which data transmission is considered   the FSR is adjusted to the bit rate of the incoming signal that enters via the optical port  401 , so that colorless operation can be guaranteed   the bandwidth Δf is fixed in accordance with the frequency deviation used for the frequency modulation of the incident optical signal   the center frequency of one of the transmission peaks can be adjusted by proper means via the electrical port(s)  402  to the incident optical signal, to ensure optimal operation of the CDD. The frequency shift δf that can be applied to the transfer function T  410 , to relocate the spectral comb for aligning it towards a new, shifted transfer function T*  420 , shall be in the range of up to +/−FSR/2, in words: the half FSR in positive and negative direction.       

     The alignment of the incoming frequency shift keyed data signal with the CDD  400  is made so that
         the frequency f 1    421  that corresponds to the logical 1-bits (i.e. marks in the bit stream) corresponds with one of the peaks of the transfer function T*  420  and experience in turn maximum transmission   the frequency f o    422  that corresponds to the logical 0-bits (i.e. spaces in the bit stream) corresponds with one of the valleys of the transfer function T*  420  and experience in turn minimum transmission       

     Different preferred embodiments are possible for the CDD  400 , which are now explained in more detail. 
     Reference is now made to  FIG. 5  illustrating a preferred embodiment of the CDD for the present invention. 
     The CDD  500  comprises an incoming waveguide (IWG)  510 , from which the downstream is entering, a DAMP  530  for amplification, and detector/reflective detector and remodulator (DET/RDR)  540 . In addition, a cavity is formed by the RINT  550  and the high reflective coating (HRC)  560 . The RINT  550  may be separated from the DAMP  530  by a piece of passive waveguide (PWG)  520 , which will determine together with the optical lengths of the DAMP  530  and the DET/RDR  540  the FSR of the cavity. The PWG  520  may be discarded, so that the RINT  550  faces directly the DAMP  530 . 
     The IWG  510  can be either a passive waveguide if there is a photonic integrated solution for the ONU, or a fiber-optic waveguide such as a single-mode fiber, which includes then also means of fiber-to-waveguide coupling. 
     The DAMP  530  may be a suitable optical amplifier, such as a SOA. The DET/RDR  540  may be a suitable optical detector with eventual remodulation capabilities, such as a REAM. 
     Electrical interfaces exist for the DAMP  530  and the DET/RDR  540 . The pump for the DAMP  530  is provided via the electrical interface composed by the connections  532 ( a, b ), while the bias for the DET/RDR  540  is provided by the connections  542 ( a, b ), from which also the detected high frequency data signal of the downstream may be obtained from. 
     Electrodes  531 ( a, b ) attached to the DAMP  530  allow to inject a bias into the DAMP  530 , while electrodes  541 ( a, b ) are attached for the same reason at the DET/RDR  540 . Electrodes  531   b  and  541   b  may be joined and may be fed via a common electrical interface  532   b . In this case, the electrical interface  542   b  can be discarded. 
     While the HRC  560  provides a high reflectivity, derived from e.g. a dielectric multilayer coating, the RINT  550 , composed out of another dielectric multilayer coating or other photonic integrateable structures such as photonic crystal mirrors with partial reflectivity, is optimized in its reflectivity towards the DAMP  530  to obtain the best detection performance for the CDD  500 . 
     The tuning of the spectral comb function of the CDD  500  in its shift of spectral maxima and minima is obtained by an adjustment of the optical length of the cavity that is formed between the RINT  550  and the HRC  560 . 
     For the CDD  500 , this adjustment is made either via the DAMP  530  or the DET/RDR  540 , by adjusting the bias points of either one or both elements, without perturbing the detection performance. In this way the comb is aligned to the incoming downstream signal, and the frequency modulated data will be available as intensity modulated electrical signal at the electrical interface  542 ( a, b ) of the DET/RDR  540 . 
     Reference is now made to  FIG. 6  illustrating another preferred embodiment of the CDD for the present invention. 
     The CDD  600  originates from the CDD  500  and comprises an IWG  610 , a PWG  620 , a DAMP  630 , a DET/RDR  640 , which are placed inside a cavity that is formed by the RINT  650  and the FRC  660 . The PWG  620  may be discarded, so that the RINT  650  faces directly the DAMP  630 . The IWG  610  can be either a passive waveguide if there is a photonic integrated solution for the ONU, or a fiber-optic waveguide, which includes then also means of fiber-to-waveguide coupling. 
     Electrical interfaces  632 ( a, b ) and  642 ( a, b ) connect to electrodes  631 ( a, b ) and  641 ( a, b ) to provide the drive and bias for the DAMP  630  and the DET/RDR  640 . The detected high frequency data signal of the downstream may be obtained from the electrical interface  642 ( a, b ). 
     In addition, a phase-shifting section (PHS)  670  is placed inside the cavity to tune the spectral comb function of the CDD  600  in its shift of spectral maxima and minima. This PHS  670  can be from any suitable type and introduces e.g. a thermo-optical or electro-optical phase shift, and will be in principle from smaller dimensions than the DAMP  630 . According to its physical realization, the electrodes  671 ( a, b ) may also be heaters in the case of a thermo-optical device. An electrical interface  672 ( a, b ) exists for the PHS  670 , which is fed from the BCTL that is present in the ONU. 
     Besides demodulation and detection of frequency modulated downstream signals, the CDD can also be used as a tunable interleaver. This allows to select sub-channels in the actual wavelength channels, which can be an additional aspect of functionality, e.g. in multi-operated ANs where different operators share the fiber plant and transmit their data in different sub-channels. 
     Reference is now made to  FIG. 7  illustrating the spectral allocation  700  for the functionality for spectral sub-channel selection via the CDD, according to the present invention. 
     For the case that the CDD has a high finesse, the latter defined as the ratio between the FSR  720  and the FWHM bandwidth  721 , additional downstream signals can be inserted between two maxima of the transfer function of the CDD. These additional signals will be suppressed for detection since they are rejected by the CDD. Depending on the width of the FSR  720 , which corresponds to the channel spacing in the case of an WDM-based WDM-AN or WDM/TDM-AN, a specific number of sub-channels can be allocated. These sub-channels are in turn filled with the said additional downstream signals. 
     The overall wavelength channel, defined by the lower wavelength f TL    701  and the upper wavelength f TH    702 , in which the transmission function  730  of the CDD can be tuned according to the requirements of the CDD, is now divided into independent sub-channels, each of them having a bandwidth of Δf  713 , where in general Δf  713  is much smaller than FSR  720  for a high finesse. For the example given in  FIG. 7 , four sub-channels are available, carrying the downstream signals  712 ( a, b, c, d ). These signals may for example stem from different operators of an AN. 
     For selecting one of the sub-channels, the transmission function  730  is now tuned by the shift δf  731  that can be obtained for the CDD, so that the comb function  732  settles for the detection at the desired sub-channel, which is for the case of  FIG. 7  the one that holds the downstream signal  712   c , which has the frequencies f 1C  and f 0C  mapped to its 1- and 0-bits. 
     While preferred embodiments of the invention have been shown and described herein, it will be understood that such embodiments are provided by way of example only. Numerous variations, changes and substitutions will occur to those skilled in the art without departing from the spirit of the invention. Accordingly, it is intended that the appended claims cover all such variations as fall within the spirit and scope of the invention.