Abstract:
This invention provides a means for high data multiple wavelength encoding with reduced bandwidth requirements by combining the outputs of a plurality of multiple wavelength processing modules. Each module produces a modulated optical pulse train with the pulses from each module being phase offset with respect to each other. The phase offsets are controlled by a feedback system, such that when combined, the plurality of multiple wavelength processing modules produce a high data rate sequence of substantially non-overlapping optical pulses at multiple wavelengths. The invention is compatible with highly integrated optical and electronic modules and because the wavelength processing modules are identical, additional spare modules can be included to provide redundancy.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates to the area of modulating optical sources to provide high data rate optical communications by means of encoding data on multiple wavelengths. This has application in such areas as the optical communications industry where Dense Wavelength Division Multiplexing (DWDM) achieves high data rate transmission by independently modulating data on to a multiplicity of optical beams, each with a different wavelength. The actual values of these wavelengths typically correspond to specific values defined by industry standards and often referred to as the ITU grid. These optical beams are typically independently modulated with data signals and then combined and propagated down a single optical fiber. Since the different wavelengths do not significantly interfere with each other the multiple wavelengths are effectively independent communications channels.  
           [0002]    Multiple wavelength sources are typically generated by having multiple laser diodes, each designed to emit at one of the required wavelengths. Each laser diode may be fabricated so that it emits at a particular wavelength as in the case of Distributed Feed Back (DFB) lasers where the emitting wavelength is determined by the physical spacing of a distributed Bragg grating that is part of the laser diode. These individual laser diodes are independently modulated either in the electronic or optical domain. As higher modulation rates are required the preferred approach is to modulate in the optical domain. The type of modulation has heretofore typically been of the conventional “non return to zero” type. As higher modulation rates are required (such as 40 Gbit/s), the preferred approach is “return to zero” type where data is encoded by the presence or absence of a short pulse. This approach imposes increasingly higher bandwidth requirements on all aspects of the modulation process, from the electronic to the optical domain. Furthermore, this approach of using multiple laser diodes, each fabricated to emit at a different wavelength present inventory problems because of the large number of different lasers. It is also difficult to design redundancy into such systems, again because they have a large number of different laser diodes. Alternative redundancy solutions, such as tunable lasers are more expensive solutions. In any event this approach of using multiple individual lasers is not an integrated solution.  
           [0003]    An alternative highly integrated (and therefore amenable to high volume, low cost fabrication) approach to generating multiple wavelengths for modulation than that of having multiple laser diodes, each radiating at different wavelengths has been described in two U.S. patent applications Express Mail Label No. EF246731316 US, Filing Date Dec. 8, 2000, Internal No. JH20003 &amp; Express Mail Label No. EF246731418 US, Filing Date Dec. 8, 2000, Internal No. JH20004. The disclosure of these U.S. patent applications is hereby incorporated herein by reference. The approach described in these applications involves such techniques as propagating radiation from a single high peak power gain switched laser diode through a non-linear medium to generate a set of wavelengths. The approach uses a combination of reflective fiber Bragg gratings designed to reflect at the desired wavelength values, a harmonically related optical pulse repetition rate and a resonant cavity in which the round trip time is also harmonically related to the pulse repetition rate and the frequency separation of the wavelength set. This approach enables generating repetitive pulsed radiation at a multiplicity of wavelengths and while it is a highly integrated approach, in order to achieve the high repetition rates suitable for the high data rate requirements involves technically difficult high bandwidth, high peak current issues. Furthermore, with this approach the energy per wavelength is also limited by high bandwidth, high peak current switching issues. Therefore there is an unmet need for a high data rate, multiple wavelength, integrated encoding method and apparatus that has built in redundancy and does not require high bandwidth modulation or high bandwidth, high peak current switching,.  
         SUMMARY OF THE INVENTION  
         [0004]    This invention provides a means for high data rate multiple wavelength encoding with reduced bandwidth requirements by means of first generating a plurality of sets of lower frequency pulsed multiple wavelengths, modulating each set and then combining the modulated wavelengths into a single fiber. The ratio of the pulse width to the repetition period is related to the number of the plurality of sets. The phase alignment of pulses from each set is such that when combined, the pulses form an evenly spaced, substantially non-overlapping pulse train that constitutes a data rate that is higher than the individual modulating rate of each set, but with a bandwidth compatible with the lower repetition rate of each set. The system can also have built in redundancy. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]    [0005]FIG. 1 is an illustration of the preferred embodiment of the invention taught herein.  
         [0006]    [0006]FIG. 2 is a more detailed illustration of the wavelength processing module.  
         [0007]    [0007]FIG. 3 is an illustration of a wavelength separator, modulator and combining module.  
         [0008]    [0008]FIG. 4 is an illustration of a typical optical and electronic pulse waveforms.  
         [0009]    [0009]FIG. 5 is an illustration of a wavelength processing module with reflective modulators.  
         [0010]    [0010]FIG. 6 is an illustration of a configuration with a redundant multiple wavelength generator.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0011]    A preferred embodiment of the invention for high data rate multiple wavelength encoding is illustrated in and described with reference to FIG. 1. A plurality (N) of Wavelength Processing Modules (WPM)  101 ,  102 ,  103  to  104  , which are described in more detail in FIG. 2, each produce a set of repetitive modulated optical pulses at a multiplicity of discrete wavelengths, which are each output on a single fiber connection  105 ,  106 ,  107  to  108 . The plurality of fibers are combined in a Wavelength Combining Module (WCM)  109 , such as a fiber coupling device. The Wavelength Combining Module  109  outputs the combined optical pulses as a high data rate modulated pulses train at a multiplicity of wavelengths on the output fiber  110 . A sample of the optical pulses are also routed by means of an other fiber  111 , to a Signal Analyzing Module (SAM)  112 , where the sample of the optical pulses is analyzed to produce relative phase information of the pulses from the plurality of Wavelength Processing Modules. This phase information  113  is sent to the Sync Generator Module (SGM)  114 . The Sync Generator Module  114  provides reference signals  115 ,  116 ,  117  to  118  which determine the relative phase of the optical pulses from each of the plurality of Wavelength Processing Modules. The Sync Generator Module  114  also sends control information to each of the Wavelength Processing Module, by means of the signal bus  119  to control the generation of the pulses that are used by the Signal Analyzing Module  112  to measure relative phase information.  
         [0012]    The Wavelength Processing Module (WPM) is described in more detail in FIG. 2. It consists of a Multiple Wavelength Generating Module which generates a set of repetitive pulsed radiation, or repetitive optical pulses, at a multiplicity of wavelengths. The phase of the repetitive optical pulses is determined by the reference signal  202 . The output of this module is routed, by means of a fiber  203  to a module  204 , described in more detail in FIG. 3, that separates the wavelengths, modulates individual wavelengths, re-combines the wavelengths and outputs the modulated optical pulses at a multiplicity of wavelengths on a single fiber  205 .  
         [0013]    The Wavelength Separator Modulator and Combiner (WSMC) module  204  is illustrated in more detail in FIG. 3, where a wavelength separator  301 , such as an arrayed waveguide grating, separates the wavelengths that arrive in a combined manner at the input  302  into “k” spatially separated individual wavelengths  303  to  304 . These individual wavelengths are routed through a set of k modulators  305  also labeled Ml to Mk. The modulated output pulses of this set of modulators are then re-combined in a wavelength combiner  306 , such as an arrayed wave guide grating and made available at the output of the module  307 .  
         [0014]    In FIG. 1 a plurality (N) of Wavelength Processing Modules  101 ,  102 ,  103  to  104 , are depicted. For descriptive purposes only, a value of N=4 which would indicate a plurality of 4 will be used in this description of the preferred embodiment. The outputs of the modules  101 ,  102 ,  103  and  104 , which are on the single fibers  105 ,  106 ,  107  and  108  are combined in the Wavelength Combiner Module  109 . The output  110  of this module consists of multiple wavelengths encoded at a high data rate. A small sample of this output is separated from the main output, for example by a fiber coupler, and is routed by means of another fiber  111  to a Signal Analyzer Module  112 . The output of this signal analyzer module  113  is made available to a Sync Generator Module  114 . This Sync Generator Module generates reference signals  115 ,  116 ,  117  and  118  which control the repetition rate and phase of the pulsed radiation of each of the multiple wavelength generators  101 ,  102 ,  103  and  104 . The Sync Generator module  114  can also override and control at least some of the modulators in each of the Wavelength Processing Modules  101 ,  102 ,  103  and  104 . By this means the Sync Generator Module  118  can isolate and control the phase of the pulses from the individual Wavelength Processing Modules. For example, the multiple wavelength generators  201  could generate a small number of wavelengths in addition to those that are required to be output and these wavelengths could be modulated (at a relatively low frequency) for phase alignment purposes. These wavelengths would then be the sample separated and output on  111  to be made available to the Signal Analyzer Module  112 . For example, the additional wavelengths could be selected from the main fiber by means of fibers coupled to the main fiber, with Bragg gratings that only allowed specific wavelengths to be coupled out. The ideal relative phase of the pulses from the different generators (at  112 ) consists of evenly spaced, substantially non-overlapping pulses. These pulse trains and typical examples of the modulated and combined versions are illustrated in FIG. 4. The pulse trains  401 ,  402 ,  403  and  404  show the 90 degree relative phase offset between the pulses from the four multiple wavelength generators. Pulse train  405  shows un-modulated versions of these four pulse trains after they have been combined to form an evenly spaced higher frequency pulse train. The signal  406  illustrates an un-modulated pulse train (such as  401 ) and  407  a typical modulating signal that would be applied to one modulator of the set of modulators  305 . The modulated output train is represented by  408 . In this example of the preferred embodiment, four signals at each of the k wavelengths, similar to  408 , but with the 90 degree phase offset would be combined in the wavelength combiner module  109  to produce a high frequency modulated pulse train, of which  409  is a typical example.  
         [0015]    At start up of the system, the Sync Generator  114  selectively and sequentially enables the individual pulse trains  401 ,  402 ,  403  and  404  of at least some of the wavelengths by means of the control data bus system  119  of FIG. 1. The signal analyzer module  112  detects the optical signals coupled from the Wavelength Combiner Module  109  and then the Sync Generator Module  114 , uses this information to phase align the pulse trains  401 ,  402 ,  403  and  404  by means of adjusting the phases of the reference signals  115 ,  116 ,  117  and  118 . After start up, the phase alignment is maintained by continuously monitoring the phase of the pulse trains of the additional wavelengths by means of the Signal Analyzer Module  112 .  
         [0016]    The modulating signal  407  clearly has a repetition rate that does not exceed the repetition of the signals  401 ,  402 ,  403  and  404 . Furthermore, the alignment between the modulating signal  407  and the un-modulated pulse train  406  is not critical. By this means, a high frequency modulated pulse train, such as illustrated in  409 , is generated using relatively low frequency modulators without critical alignment. Furthermore, the average power of the final high frequency pulse train, such as  409 , is increased by the number of independent pulse trains that are combined (which in this example is four), thus providing a means for generating a high power, high data rate, modulated pulse train without high bandwidth requirements.  
         [0017]    In an second preferred embodiment, the wavelength separator modulator and combiner modules,  204  in FIG. 2, consist of a single device that serves both as a wavelength separator and wavelength combiner. This is illustrated in FIG. 5, where the combined multiple wavelength is input by means of the input fiber  501  through an optical circulator  502  to a wavelength separator  503 , such as an arrayed waveguide grating, which separates the optical signal into k individual wavelengths  504  to  505 . These k wavelengths are applied to a set of k reflective modulators  506  (such as electro-absorption modulators) by which the pulsed radiation is modulated in a similar manner to that described above. One side  507  of the modulator set is highly reflective and reflects the radiation back through the modulator set, thus doubling the interaction length of the modulators, to be recombined by the same device  503  that separated the wavelengths and then to emerge through the optical circulator  502 . This circulator  502  routes the output signal through a second port  508 , to form the output of the module. A key advantage (in addition to longer interaction length and lower part count) of this reflective approach is that it enables very simple interconnection between the electronic signals driving the modulators, in that with this reflective configuration a set of electronic drivers can mate directly to the set of reflective modulators, with all drivers and modulators having exactly the same configuration, thus facilitating integration. Other aspects of this second preferred embodiment are similar to the first preferred embodiment.  
         [0018]    In a third preferred embodiment, the number of Wavelength Processing Modules ( 101 ,  102 ,  103  to  104  of FIG. 1) exceeds the number required to generate the required number of sets of multiple wavelength repetitive optical pulse trains. These Wavelength Processing Modules are all identical and therefore interchangeable, which means pulses from any one can Wavelength Processing Module can occupy the time slot of any other. Having more Wavelength Processing Modules than required and the fact that they are all identical, enables a redundant system, in which one or more spare Wavelength Processing Modules are available. For example, the system described in the first preferred embodiment could have five Wavelength Processing Modules, rather than four (as in the example). This would allow the Signal Analyzer Module  112  and the Sync Generator Module  114  in FIG. 1 to detect if one of the initial four Wavelength Processing Modules becomes defective, to disable it and then to enable the fifth (spare or redundant) Wavelength Processing Module phase aligned with (or occupying the same time slot of) the defective module. This enabled spare wavelength processing module is modulated with the same data channel with which the defective module had been modulated. Alternately, the Sync Generator Module  114  could continuously rotate the five Wavelength Processing Modules and in the event of one of them becoming defective, revert to only using the remaining four that are not defective. By such means, a high data rate multiple wavelength modulated pulse train can be generated by a system with built in redundancy. Another alternative redundant implementation, illustrated in FIG. 6, is to only have spare (or redundant) multiple wavelength generator modules (MWGM)  602  and no spare wavelength separator modulator and combiner modules (WSMC)  603 . The spare multiple wavelength generator module  601  can replace a defective generator by such means as a combination of thermo-optic switches that allow the output of the spare multiple wavelength generator to be routed so that it can replace a defective multiple wavelength generator. A thermo-optic switch typically consists of a temperature dependant waveguide or fiber splitter that allows routing of optical radiation to one of two routes by means of controlling heaters such as  604  in FIG. 6. The combination of heaters  604 , splitters  605  and combiners  606  allow the multiple wavelength output of the spare multiple wavelength generator module  601  to be routed to any of the wavelength separator modulator and combiner modules  603 ,  607 ,  608  or  609 . The output of the corresponding defective multiple wavelength generator, that is being replaced, would be disabled and ideally its defective status used to indicate that it should be replaced.  
         [0019]    It is understood that the above description is intended to be illustrative and not restrictive. Many of the features have functional equivalents that are intended to be included in the invention as being taught. For example, wavelength separators other than arrayed waveguide gratings, such as optical filters, could be used, or instead of modulator sets, fiber optical modulators could be used. Various other combinations of fiber elements, waveguide elements, modulator elements and wavelength combiner elements can be employed. Other examples will be apparent to persons skilled in the art.  
         [0020]    The scope of this invention should therefore not be determined with reference to the above description, but instead should be determined with reference to the appended claims, along with the fill scope of equivalents to which such claims are entitled.