Abstract:
A method and a device for adjusting a laser in an optical network. At least one alive message is transmitted from a first optical component towards a second optical component. A confirmation message is transmitted from the second optical component to the first optical component determining the wavelength of the laser to be used based on the alive message received by the second optical component. Furthermore, an optical communication system is provided with an optical element.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to a method and to a device for adjusting a (tunable) laser in an optical network. 
     A passive optical network (PON) is a promising approach regarding fiber-to-the-home (FTTH), fiber-to-the-business (FTTB) and fiber-to-the-curb (FTTC) scenarios, in particular as it overcomes the economic limitations of traditional point-to-point solutions. 
     Several PON types have been standardized and are currently being deployed by network service providers worldwide. Conventional PONS distribute downstream traffic from the optical line terminal (OLT) to optical network units (ONUs) in a broadcast manner while the ONUs send upstream data packets multiplexed in time to the OLT. Hence, communication among the ONUs needs to be conveyed through the OLT involving electronic processing such as buffering and/or scheduling, which results in latency and degrades the throughput of the network. 
     In fiber-optic communications, wavelength-division multiplexing (WDM) is a technology which multiplexes multiple optical carrier signals on a single optical fiber by using different wavelengths (colors) of laser light to carry different signals. This allows for a multiplication in capacity, in addition to enabling bidirectional communications over one strand of fiber. 
     WDM systems are divided into different wavelength patterns, conventional or coarse and dense WDM. WDM systems provide, e.g., up to 16 channels in the 3rd transmission window (C-band) of silica fibers of around 1550 nm. Dense WDM uses the same transmission window but with denser channel spacing. Channel plans vary, but a typical system may use 40 channels at 100 GHz spacing or 80 channels with 50 GHz spacing. Some technologies are capable of 25 GHz spacing. Amplification options enable the extension of the usable wavelengths to the L-band, more or less doubling these numbers. 
     Current WDM PON proposals use arrayed waveguide gratings (AWGs) or some other wavelength selective element to distribute only a single wavelength to a single customer or a group of customers. 
     In a WDM PON concept, tunable lasers are used as upstream source for the ONUs, wherein the upstream wavelength has to be determined in order to pass the AWG. 
     At present, tunable lasers have a pre-determined wavelength selection, i.e. when being produced, the laser is characterized and an individual table for each single laser contains parameters to set the laser to a given wavelength. This characterization process results in a significant portion of tunable laser costs. Also, in order to provide a reference signal for this calibration, wavelength selective elements, such as etalon filters with associated monitor diodes, have to be added to the laser platform. This further increases the costs of the laser as additional components and assembly steps are required. 
     The problem to be solved is to overcome the disadvantages stated above and allow for an efficient utilization of a tunable laser. 
     BRIEF SUMMARY OF THE INVENTION 
     This problem is solved according to the features of the independent claims. Further embodiments result from the depending claims. 
     In order to overcome this problem, a method for adjusting a laser in an optical network, in particular in a component or element of the optical network, is suggested
         (a) wherein at least one alive message is transmitted from a first optical component towards a second optical component;   (b) wherein a confirmation message is transmitted from the second optical component to the first optical component determining the wavelength of the laser to be used based on the alive message received by the second optical component.       

     The laser is a tunable laser providing a wavelength that can be adjusted at or by the first optical component. The laser may be deployed at or with the first optical component. 
     It is noted that the first optical component may be an optical network unit (ONU) or an optical line terminal (OLT). For example, if the first optical component is an ONU, the second optical component is an OLT and if the first optical component is an OLT, the second optical component is an ONU. 
     Advantageously, the first optical component&#39;s laser can be adjusted dynamically to the conditions of the optical network and does not have to be produced in a way that it precisely provides a particular wavelength. Instead, the laser is dynamically tuned to the wavelength that allows for data transmission. In case the laser (over time) changes its wavelength, e.g., due to temperature influences, it can be readjusted. 
     The alive message may be a message sent continuously and/or iteratively that indicates to the second optical component that the first optical component is active. In case the second optical component receives the first optical component&#39;s alive message, it sends the confirmation message that the current wavelength utilized by the laser of the first optical component can be used for data transmission purposes. 
     It is noted that the first optical component conveys alive messages at varying wavelengths, i.e. alive messages are sent when the laser is adjusted to different wavelengths. In other words, the range of wavelengths is being scanned by producing alive messages at different wavelengths. 
     Hence, the first optical component after having received the control signal may emit alive messages at different wavelengths. Several such alive messages may be conveyed via different wavelengths towards the second optical component until a suitable wavelength is found. It is noted that albeit being conveyed towards the second optical component, a portion of the alive messages may not pass an intermediate optical component, e.g., a wavelength filter between the first optical component and the second optical component. A suitable wavelength for the laser is found when the second optical component receives an alive message and indicates via said confirmation message to the first optical component that this current wavelength can be used. 
     According to an embodiment, the laser is adjusted to different wavelengths and several alive messages are sent from the first optical component to the second optical component at such different wavelengths. 
     Hence, the first optical component “scans” the second optical component by emitting alive messages at different wavelengths. 
     Such scanning is advantageously provided at the sender (here the first optical component). The sender receives the confirmation message in case the receiver was able to detect the alive message. The receiver may have in particular detected at least one alive message from the sender or an alive message with a power level that reaches and/or exceeds a predetermined threshold. Hence, the confirmation message from the second optical component may be emitted in case the alive message detected shows a sufficient degree of power (or signal-to-noise ratio or the like). 
     Pursuant to an embodiment, prior to step (a) a control signal is conveyed towards the first optical component. 
     Hence, the control signal may be a trigger for the first optical component to emit the alive messages and thus the laser of the first optical component (e.g., the ONU) may (successively) be set to a wavelength that is successfully received by the second optical component (e.g., the OLT). Then, upstream traffic from the first optical component to the second optical component is feasible. 
     In a next embodiment, the method comprises the following steps:
         several alive messages are conveyed towards the first optical component;   the first optical component determines an acceptance interval based on such alive messages;   a wavelength is chosen by the first optical component within this acceptance interval.       

     In this scenario, the second optical (e.g., the OLT) component conveys alive messages towards the first optical component (e.g., the ONU). 
     The alive message may be a message sent continuously and/or iteratively that indicates to the first optical component that the second optical component is active. 
     The wavelength chosen may be a wavelength in or around the middle of said acceptance interval. 
     Said acceptance interval can be determined by the first optical component by adjusting its laser to different wavelengths and scanning for the alive messages. The acceptance interval allows reception of the alive message sent. If the reception is no longer possible due to the laser&#39;s wavelength selected, the limit of the acceptance interval may be reached. It is noted that at the border of the acceptance interval, a quality or level of reception can be determined to define whether or not a particular wavelength still falls within the acceptance interval. 
     The acceptance interval can be determined by scanning (i.e. varying the wavelength of the laser) from a low to a high wavelength or the other way round. 
     By setting the wavelength in or around the center of the acceptance interval, the wavelength of the laser may drift in both directions until reaching the border of the acceptance interval. This provides an efficient implementation of a safety margin. 
     Pursuant to another embodiment, the control signal is conveyed in downstream direction from an optical access point, in particular an optical line terminal. 
     In another embodiment, a reception power level is monitored and a control message is sent towards the first optical component in case the power level reaches and/or exceeds a predetermined threshold. 
     Such monitoring and sending of the control message can be conducted by the second optical component. 
     In a further embodiment, the first optical component tunes its laser pursuant to the control message received. 
     Hence, an efficient mechanism to compensate any drift, e.g., due to aging or temperature change, is provided. Preferably, the OLT may convey such control message to an ONU once it has determined that the power level of the signal received deteriorates. 
     It is also an embodiment that the control signal is received by a receiver susceptible to a broad spectrum. 
     Such receiver can be a photo diode at the first optical component. 
     According to an embodiment, the first optical component is an optical network unit (ONU). 
     According to another embodiment, the first optical component tunes its laser to an extreme value of an acceptance range. 
     Hence, the first optical component may adjust its laser towards a minimum or a maximum wavelength of the wavelengths that can be utilized for conveying traffic towards the OLT. This extreme value can be utilized as a starting point for scanning a range of wavelengths, e.g., varying the laser&#39;s wavelength towards the respective other extreme value. 
     The problem stated above is also solved by an optical element comprising
         a tunable laser,   a processing unit that is arranged
           for transmitting alive messages towards a second optical component;   for receiving a confirmation message from the second optical component;   for using the wavelength of the tunable laser that corresponds to the confirmation message received.   
               

     It is noted that the processing unit may be or comprise optical and/or electrical means. It may in particular comprise a processor, optical elements, hardwired circuits or the like. 
     The further embodiments described above apply to the optical elements accordingly. 
     In yet another embodiment the processing unit of the optical element is arranged for receiving a control signal prior to transmitting said alive messages towards a second optical component. 
     According to a next embodiment, the processing unit can be arranged
         for determining an acceptance interval based on alive messages received;   for determining a wavelength to be used for the tunable laser within said acceptance interval.       

     Hence, the optical element may utilize alive messages received from the second optical component to determine the acceptance interval. 
     The problem stated above is also solved by an optical element that comprises a processing unit that is arranged such that the steps described above are executable thereon. 
     Pursuant to yet an embodiment, the optical element is or is associated with an OLT or an ONU. 
     The problem stated supra is further solved by an optical communication system comprising at least one optical element as described herein. 
     Embodiments of the invention are shown and illustrated in the following figures: 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         FIG. 1  shows a schematic scenario with an OLT connected via a wavelength filter towards several ONUs. 
         FIG. 2  shows a schematic message sequence chart between an OLT and an ONU, wherein the OLT sends a control signal downstream towards an ONU thereby notifying the ONU to tune is laser to the correct wavelength; 
         FIG. 3  shows a schematic message sequence chart between an OLT and an ONU that visualizes an option, according to which an additional centering step within an acceptance range of wavelengths can be performed. 
     
    
    
     DESCRIPTION OF THE INVENTION 
     This approach suggested uses tunable lasers without any need for their prior characterization. In other words, the absolute wavelength of the laser may be unknown, albeit the laser can be utilized in an efficient manner. 
       FIG. 1  shows a schematic scenario with an OLT  101  connected via a filter  102  (e.g., a wavelength filter or an AWG) towards several ONUs  103 ,  104 . A direction from the OLT  101  towards the ONU  103 ,  104  is referred to as a downlink or down-stream direction, whereas the opposite direction from the ONU  103 ,  104  towards the OLT  101  is referred to as uplink or upstream direction. 
     The solution provided in particular suggests using free-running tunable lasers for an upstream direction, wherein wavelength calibration of the laser is provided or conducted by the OLT. 
       FIG. 2  shows a schematic message sequence chart between an OLT  201  and an ONU  202 . The OLT  201  sends a control signal  203  downstream towards an ONU thereby notifying said attached ONU  202  to tune its laser to the correct wavelength. This control signal  203  can be received by the ONU  202  independently of a particular wavelength, since typical photodiode receivers have a broad sensitivity range, usually more than 200 nm. 
     In upstream direction, the OLT  201  may receive no particular response from the ONU  202  upon startup, as the tunable laser of the ONU  202  may most likely emit signals on the wrong wavelength so that the wavelength selective splitter blocks this upstream transmission (over the wrong wavelength). 
     When the ONU  202  receives the control signal  203  indicating that the ONU needs to tune itself, the ONU  202  may tune its laser to one of the extreme values available throughout the tuning range, i.e. a minimum wavelength or a maximum wavelength. Then, the ONU  202  may initiate a wavelength scan (preferably conducted with a slow scanning speed) thereby (e.g., continuously and repeatedly) sending a message  204  indicating to the OLT  201  that the ONU “is alive”. Such message  204  is also referred to as “I am alive”-message. 
     When the wavelength lies within an acceptance range of the respective port of the wavelength selective filter at the splitter site, upstream communication passes the selective filter and is conveyed towards the OLT  201 . The OLT  201  responds with a “STOP SCANNING” message  205  indicating that the ONU  202  is now tuned to the correct wavelength and both OLT and ONU may negotiate the remaining connection parameters  206 . 
     As an alternative, the approach suggested also works without the control signal  203  first sent from the OLT  201  to the ONU  202 . In this scenario, the ONU  202  may be switched on and a said message  204  is sent from the ONU  202  towards the OLT  201 . The message  204  is sent by the ONU  202  at different wavelengths until said message  205  is received. Then, the ONU  202  stops scanning and uses the current wavelength for further communication with the OLT  201 . 
     It is noted that in a TDM environment, the control signal  203  may be or comprise a broadcast signal conveying resources (e.g., time slots) towards new ONUs. In the example shown in  FIG. 2 , the ONU  202  may become aware via said control signal  203  of resources to be utilized in the time division domain. 
     As an addition, as an option, the OLT  201  may continuously monitor the received power level from the respective ONU  202  as indicated in a step  207 . If that power level decreases below a predefined threshold, the OLT  201  will send a control message  208  to the ONU  202  indicating that it is reaching the borders of the wavelength selective splitter and that the ONU  202  has to re-adjust its laser. The ONU  202  may then process such re-adjustment of its laser (not shown in  FIG. 2 ). 
       FIG. 3  shows a schematic message sequence chart between an OLT  301  and an ONU  302  that visualizes an option, according to which an additional centering step can be performed. In this case, the OLT  301  may repeatedly send an “I hear you” message  303  towards the ONU  302  and the ONU  302  may scan towards shorter wavelengths, then towards longer wavelengths (or the other way round: first towards longer wavelengths, then towards shorter wavelengths) until it does no longer receive the OLT&#39;s “I hear you” message  303  (see step  304 ). 
     Then, the ONU may tune to an average value of these two extreme values of wavelengths (shortest wavelength and longest wavelengths of a frequency range that allows reception of the OLT&#39;s “I hear you” message  303 —this range is also referred to as “acceptance range”). Such average value of the extreme values may be a wavelength in or around the middle of the acceptance range of the wavelength selective device (see step  305 ). 
     It is an advantage that an expensive step in the production of a tunable laser, e.g. stabilization, can be omitted thus reducing the overall costs of the system. 
     List Of Abbreviations: 
     
         
         AWG Arrayed Waveguide Grating 
         OLT Optical Line Terminal 
         ONU Optical Network Unit 
         PON Passive Optical Network 
         UDWDM Ultra Dense WDM 
         WDM Wavelength Division Multiplex