Abstract:
Method and apparatus are contemplated for electrical-to-optical conversion coupled to an electrical switching fabric, wherein the number of lasers supplying optical carrier beams in electrical-to-optical conversion is less than the number of optical modulators. Cost savings for a reduced number of lasers may be considerable. Further, the shared laser bank supplying optical carrier beams may comprise shared control and monitoring electronics, resulting in a further cost savings. The shared laser bank may comprise at least one redundant laser. Optical modulators may be grouped into modules for ease of replacement and further cost savings. Optical signal conditioning may be applied to the lasers, and the conditioned beams may be shared among the modulators. Soliton pulses may be generated at a desired data rate, distributed to form a plurality of distributed pulse trains, and each pulse stream may be modulated with information from the same or different information channels.

Description:
FIELD  
         [0001]    The field of interest is optical networks, and more specifically, optical switching fabrics.  
         BACKGROUND  
         [0002]    An optical switching cross-connect comprises equipment that switches or routes information received from one or more fiber optic media input lines, and transmits the information out through one or more fiber optic media output lines. The connecting of input lines to output lines through the optical cross-connect can occur in any combination or permutation.  
           [0003]    [0003]FIG. 1 shows a general optical cross-connect  100  with an electrical switching fabric. In this prior art embodiment, each incoming line  102  is fed into the demultiplexer section  104  wherein a demultiplexer  106  separates the multiple wavelengths on each incoming line. In a central portion  108  optical-to-electrical translation of incoming optical signals is accomplished, followed by the switching, which is accomplished electrically. The electrical switching fabric output is then converted via electrical-to-optical translation to optical signals. Finally, each multiplexer  1   12  in the multiplexer section  10  places several wavelengths onto an output optical transmission line  114 .  
           [0004]    [0004]FIG. 2 (prior art) shows a conventional electrical-to-optical (EO) conversion  200 . Each input line  202 , typically carrying an optical signal comprising information on a single wavelength carrier, is connected to an optical receiver  204 , which translates the optical signals received on the input line into electrical signals. An electrical switching fabric  206  routes each electrical signal to its intended output line  208 . The electrical signal output from the electrical switching fabric  206  is then fed to an optical transmitter  210 , where the electrical signal modulates an optical laser carrier beam generated by a laser within the optical transmitter. The output of the optical transmitter  210  is fed into an optical transmission line  212 , which is typically a fiber optic cable.  
           [0005]    [0005]FIG. 3 (prior art) shows a typical optical laser transmitter module  300 , the module comprising a Continuous Wave (CW) fixed International Telecommunications Union (ITU) grid wavelength laser  302 , and an external modulator  304  that modulates the laser carrier beam with information from an Electrical Data Input  306 , which data has come from the electrical switching fabric  206  (see FIG. 2). A tap  312  diverts some of the light energy emitted from the laser to a wavelength locker  314 , which provides feedback to control circuitry  316  that serves to maintain a specific wavelength of the ITU grid wavelength laser  302 . Monitoring circuitry  318  monitors the wavelength and power of the ITU grid wavelength laser  302 .  
           [0006]    As seen in FIG. 2, prior art EO conversion employs one optical transmitter  210  for each output line coming from the electrical switching fabric  206 . Prior art further depicted in FIG. 3 shows that each optical transmitter contains at least one laser that supplies the optical carrier to be modulated by the external modulator  304 , which is then output to an Optical Data Output  310 .  
           [0007]    Optical cross-connect architecture comprises both optical and electrical switching fabrics. Electrical switching fabrics require optical-to-electrical (OE) conversion circuitry and electrical-to-optical (EO) conversion circuitry. In designing electrical switching fabrics, EO conversion circuitry is the predominant cost factor. Reducing costs of EO conversion circuitry would have a major impact on overall cost of an electrical switching fabric-based optical cross-connect installation. The set of lasers providing output carrier beams to the output modulators is a major expenditure in EO conversion. A reduction in the total number of lasers needed to produce all output channels would result in a significant cost saving.  
         SUMMARY  
         [0008]    Method and apparatus is provided for supplying output carrier optical signals to output modulators through the use of a reduced number of lasers that comprise a shared laser bank. The total number of lasers employed is less than the total number of optical modulators being supplied with optical carriers.  
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0009]    [0009]FIG. 1 shows a general optical cross-connect with an electrical switching fabric, as in the prior art.  
         [0010]    [0010]FIG. 2 shows a conventional OE and EO conversion apparatus for an electrical switching fabric, as in the prior art.  
         [0011]    [0011]FIG. 3 shows a diagram of a laser transmitter module, also called an optical transmitter, as in the prior art.  
         [0012]    [0012]FIG. 4 shows an EO conversion apparatus for an electrical switching fabric, according to an embodiment of the present invention.  
         [0013]    [0013]FIG. 5 shows a shared DWDM laser bank, according to an embodiment of the present invention.  
         [0014]    [0014]FIG. 6 shows a shared DWDM laser bank with multiple tunable CW lasers, according to an embodiment of the present invention.  
         [0015]    [0015]FIG. 7 shows a shared DWDM laser bank, with common control and monitoring for subgroups of lasers, power splitters grouped into subgroups, and modulators grouped into modular sub-units, according to an embodiment of the present invention.  
         [0016]    [0016]FIG. 8 shows a shared DWDM laser bank with optical signal conditioners, according to an embodiment of the present invention.  
         [0017]    [0017]FIG. 9 shows a shared DWDM laser bank with soliton generators, according to an embodiment of the present invention.  
         [0018]    [0018]FIG. 10 depicts modulating data which is input to an optical modulator that modulates soliton pulses, and the soliton pulse output, which includes a parity bit, according to one embodiment of the present invention.  
         [0019]    [0019]FIG. 11 shows a plurality of optical modulators grouped into a modular unit, according to one embodiment of the present invention.  
         [0020]    [0020]FIG. 12 shows a shared laser bank whose wavelengths are multiplexed, split into a set of multiplexed beams, and subsequently de-multiplexed and modulated, according to an embodiment of the invention.  
     
    
     DETAILED DESCRIPTION  
       [0021]    An embodiment  400  of the present invention, shown in FIG. 4, has a set of M input optical transmission lines  401 , which are typically fiber optic cables. An optical carrier beam comprising N modulated wavelengths (in FIG. 4, N=4) is demultiplexed by a demultiplexer  402  in the demultiplexer section  403 , and each of the signals, whose carrier wavelengths are respectfully λ 1 , λ 2 , λ 3 , λ 4 , is fed to an optical receiver  404 , which converts the modulated wavelength into a electrical signal. For M transmission lines, each of which carries N modulated wavelengths, a total of M*N signals is fed into an electrical switching fabric  408 . If there is a different number of modulated wavelengths Ni for each input optical carrier beam  401  (carrier beams are indexed i=1,2,3, . . . ), then the total number of signals fed into the electrical switching fabric  408  is Σ N i , i=1,M. The electrical signals are fed via input lines  406  into the electrical switching fabric  408 , which routes the electrical signals to electrical output lines  410 , the routing being determined by the configuration of the electrical switching fabric  408 . Each output electrical signal is then fed, via an electrical output line  410 , into an optical modulator  416 . Each optical modulator  416  modulates a laser carrier beam carried on an optical transmission line  414 , with an electrical signal supplied by output line  410 ; alternatively another information channel source (not shown) may be used to modulate an optical modulator. Some of the information channels feeding optical modulators may be redundant, i.e., there may be a plurality of identical output electrical signals that produce identically modulated output optical signals, which may serve to increase the probability of correctly transmitting the data to its final destination and/or be used for a broadcast application wherein identical information is sent to many destinations.  
         [0022]    A shared laser bank  412  comprises a set of lasers of wavelengths λ A , λ B , λ C , λ D , the laser outputs of which are a set of carrier beams that are routed via optical transmission lines  414 , comprising e.g., fiber optic cables, to the optical modulators  416 . There are more optical transmission lines  414  than lasers in the shared laser bank  412 ; hence a small number of lasers supplies carrier beams to a larger number of optical modulators  416 . The output of each optical modulator  416  is a modulated optical beam. The modulated optical beams are then fed into output optical transmission lines  418 , typically comprising fiber optic transmission cables, and then to the multiplexer section  420 . A sub-group of modulators  424  feeds modulated signals to a multiplexer  426 , each modulated signal having a different carrier wavelength λ i , where i=A, B, . . . . The carrier wavelengths λ A , λ B , . . . of the output signals to the multiplexer may be the same as or different than the wavelengths of the input signals λ 1 , λ 2 , . . . carried by input lines  401 . The multiplexed signal is then transmitted out through an output line  428 , typically comprising fiber optic cable.  
         [0023]    An embodiment of a shared Dense Wavelength Divison Multiplex (DWDM) laser bank  500  is shown in FIG. 5. A set of CW lasers  502 , each with its own respective wavelength λ A , λ B , λ C , λ D , each with a tap  504  and a locker  506  that effects frequency stability through a feedback loop, provides input to a set of power splitters  508 , each of which splits its input beam into several output beams. As each laser provides carrier beams for a plurality of output lines, the total number of output lines is greater than the number of lasers in the laser bank. A tunable CW laser  510  that has a tap  504  and a locker  506 , feeds a 1×4 switch  512 , serves as a redundant laser, providing an alternate input to the power splitters  508  in the event of a fixed CW laser failure. The tunable CW laser  510  can be tuned to the wavelength output of the failed laser, and by choosing the appropriate route through the use of the 1×4 switch  512 , the tunable CW laser  510  provides an alternate laser carrier beam to the respective modulator.  
         [0024]    Outputs  514  provide carrier beams to, e.g., the optical modulators  416  of FIG. 4. Control and monitoring circuitry  516  is common to all lasers within the shared laser bank  500 . In the embodiment illustrated in FIG. 5, a total of 5 lasers (four fixed wavelength CW lasers  502 , and one tunable CW laser  510 ) provides carrier beams to 24 outputs.  
         [0025]    [0025]FIG. 6 shows another embodiment  600  of a DWDM laser bank. Each of the lasers  602  is a tunable CW laser, which allows for flexibility in the wavelengths of the output carrier beams directed to outputs  608 .  
         [0026]    [0026]FIG. 7 shows a Shared DWDM Laser Bank  700  wherein control and monitoring circuitry  704  is common to a subset  702 , also called module, of the shared laser bank. Several modules of control and/or monitoring circuitry  704  can then control and/or monitor various groups of lasers, producing different inputs for different subgroups of splitters  706 , and hence different outputs for each of the subsets of outputs leaving the splitters  706 . External modulators  710  can be grouped as a module  712  within the same physical structure, allowing for more efficient manufacture and ease of replacement. Each modulator receives at least one electrical signal  714 , and at least one optical carrier  708 . Modular grouping  1100  of modulators allowing for more efficient manufacture and ease of replacement, is shown in detail in FIG. 11.  
         [0027]    [0027]FIG. 8 shows another embodiment  800  of a shared DWDM laser bank, this embodiment featuring optical signal conditioning. A CW laser  802 , with a tap  804  and a locker  806 , feeds an optical signal into a signal conditioner  808 . The signal conditioner  808  shapes the optical signal in a predetermined fashion, controllable through control circuitry (not shown) that may be common to a plurality of signal conditioners  808 , and monitored by monitoring circuitry (not shown) that may be common to a plurality of signal conditioners  808 ; alternatively each signal conditioner may have its own control and monitoring circuitry. The output of each signal conditioner is fed into a power splitter  810 , and outputs  814  provide carrier signals for, e.g., optical modulators such as  416  in FIG. 4. In one implementation of this embodiment, each of the signal conditioners  808  may condition its input signal differently, and so provide carrier signals that are unique to the optical modulators which they respectively feed.  
         [0028]    Use of solitons in optical networks reduces or eliminates the need for chromatic dispersion compensators, and thus enables interfacing directly into an ultra-long-haul network. FIG. 9. shows yet another embodiment  900  of a shared DWDM laser bank. In this embodiment, carrier signals are generated by a set of soliton pulse generators  902 , each with a different characteristic wavelength of light λ A , — B , λ C , . . . λ i . Soliton pulses are usually produced at a fixed rate, the rate being typically  10  Gigabits per second (Gbps). The output of a soliton pulse generator  902  is fed into a splitter  904 , usually containing an amplifier. Each of the outputs of the splitter  904  is typically fed into an external optical modulator  908 , and a portion of the signal via a tap  910  is fed to a synchronizer  912 , which synchronizes the rate of data stream  914  coming from the switching fabric, typically synchronized to the soliton pulse rate of the soliton pulses entering the external optical modulator  908 . The modulator  908  either passes or blocks each pulse according to the data stream  914  supplied by the synchronizer  912 , thus forming the desired information bit stream that is sent onto the output optical transmission line (not shown).  
         [0029]    The data rate may be synchronized to exactly match that of the soliton pulse rate. Alternatively, the data rate may be synchronized according to a scheme  1000  such as shown in FIG. 10, wherein every 10 th  soliton signal  1002  is modulated by a sum bit of the previous nine data bits, thus providing a check sum as an error correction mechanism. In similar fashion, data encoding of soliton pulses may be set forth according to any scheme, e.g., one-to-one correspondence with data rate, offset by one check-sum bit, offset by several error correction bits, aperiodic, etc.  
         [0030]    In all of the embodiments described thus far, distributing of optical carrier beams is accomplished through splitting of an optical beam. It is appreciated by those of ordinary skill that other techniques may be employed to distribute an optical beam, e.g. in the case of an optical beam comprising a plurality of optical beams which may be of differing wavelengths that may be multiplexed onto a trunk line, the distribution may be accomplished using, e.g., one or more distribution devices including, but not limited to optical add/drop elements, add/drop multiplexers, wavelength routers, wavelength filters, circulators and combinations thereof. An illustration is shown in FIG. 12. Here a laser  1202  produces a carrier beam of wavelength XA, which then passes through a 2×1 switch  1206 , enabling redundancy provided by a tunable CW laser  1204  and a 1×4 switch  1208 . A laser carrier beam  1210  of wavelength λ A  then feeds into an optical Wavelength Division Multiplexing (WDM) multiplexer  1212 , where it is multiplexed with other laser carrier beams, here λ B , λ C , λ D . The multiplexed beam is then fed to a beam splitter  1214 , splitting the multiplexed beam into a plurality of daughter multiplexed beams, each containing wavelengths λ A , λ B , λ C , and λ D . A daughter multiplexed beam  1216  feeds into a WDM de-multiplexer, where the multiplexed beam  1216  is distributed to form a plurality of laser carrier beams of single wavelength λ A , λ B , λ C , and λ D , respectively. A laser carrier beam  1220  of wavelength λ A  then feeds into an optical modulator  1224 , where it is modulated by information on an information channel  1222 . The modulated laser beam  1226  is then output for transmission.  
         [0031]    Having illustrated and described the principles of the invention in the above-described embodiments, it should be apparent to those skilled in the art that the embodiments can be modified in arrangement and detail without departing from such principles. In view of the many possible embodiments to which the presented may be applied, it should be recognized that the illustrated embodiments are only examples of the invention and should not be taken as a limitation on the scope of the invention. Rather, the invention is defined by the following claims. I therefore claim as my invention all such embodiments that come within the scope and spirit of these claims.