Abstract:
A method and system for performing OTDM. Laser wavelength tuning is used to create appropriate time differentials between bits in a combined optical output data stream.

Description:
FIELD OF THE INVENTION 
   The invention relates to optical time division multiplexing (OTDM), and in particular to an OTDM transmitter. 
   BACKGROUND 
   In a conventional OTDM transmitter, several optical signals modulated at bit rate B using the same carrier frequency are multiplexed optically to form a composite optical signal at a higher bit rate nB, where n is the number of multiplexed optical channels. 
   Specifically, multiplexing of these n constituent bit streams is achieved by launching them into an optical fiber with time delays. The bit stream in the j-th channel is delayed optically by an amount (j−1)/nB, where j=1, . . . . , n. The outputs of all channels are combined to form a composite signal as a return-to-zero (RZ) signal. The composite bit stream has a bit slot T=1/nB. Furthermore, in the composite bit stream, n consecutive bits in each interval of duration 1/B belong to n different channels, as required by the TDM scheme. 
   The optical delays above are typically implemented by using fiber segments of controlled lengths. As an example, a 1 mm fiber length introduces a delay of about 5 ps. Moreover, the relative delay in each channel must be precisely controlled to ensure the proper alignment of bits belonging to different channels. For a precision typically required for a 40 Gb/s OTDM signal, the delay length should be controlled to within 20 μm. 
   However, as link rate increases beyond 40 Gbs, conventional OTDM systems and methods begin to experience problems such as timing inaccuracy and smeared time differentials between any two bits of the output composite signal launched into the optical fiber. 

   
     BRIEF DESCRIPTION OF THE FIGURES 
     The accompanying drawings which are incorporated in and form a part of this specification illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
       FIG. 1  shows an OTDM transmitter in accordance with one embodiment of the invention. 
       FIG. 2  shows a flow chart outlining steps for performing OTDM in accordance with one embodiment of the invention. 
       FIG. 3  shows a WDM system incorporating OTDM channels in accordance with one embodiment of the invention. 
   

   DETAILED DESCRIPTION 
   Reference is now made in detail to embodiments of the invention. While the invention is described in conjunction with the embodiments, the invention is not intended to be limited by these embodiments. On the contrary, the invention is intended to cover alternatives, modifications and equivalents which may be included within the spirit and scope of the invention as defined by the appended claims. Furthermore, in the following detailed description of the invention, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, as is obvious to one ordinarily skilled in the art, the invention may be practiced without these specific details. In other instances, well-known methods, procedures, components, and circuits have not been described in detail so that aspects of the invention will not be obscured. 
   Referring now to  FIG. 1 , an OTDM transmitter  100  is shown in accordance with one embodiment of the invention. Transmitter  100  comprises four sources  151 – 154 , four modulators  171 – 174 , and four group velocity dispersive elements  181 – 184 . These sources ( 151 – 154 ), modulators ( 171 – 174 ) and group velocity dispersive elements ( 181 – 184 ) are arranged as four input channels  141 – 144  of transmitter  100 . Transmitter  100  also comprises a combiner  105  and a wavelength converter  110 . Furthermore, transmitter  100  is coupled an optical link  130 . 
   As shown within channel  141 , source  151  is coupled to modulator  171  that is in turn coupled to group velocity dispersive element  181 . Similarly, channels  142 – 144  are formed wherein sources  152 – 154  are coupled respectively to modulators  172 – 174  that are in turn coupled respectively to group velocity dispersive elements  182 – 184 . 
   Referring still to  FIG. 1 , sources  151 – 154  are continuous wave (CW) tunable lasers. In channel  141 , source  151  provides to modulator  131  a tunable CW laser beam  101  of wavelength λ 1 . Modulator  131  modulates laser beam  101  and generates therefrom an output RZ bit stream  111 . Bit stream  111  undergoes group velocity dispersion as it traverses the group velocity dispersive element  181  and emerges therefrom as a constituent RZ bit stream  191  of a composite bit stream  122 . 
   Similarly, in channel  142 , the constituent RZ bit stream  192  is generated from a CW tunable laser beam  102  of wavelength λ 2  that undergoes modulation (at modulator  172 ) and group velocity dispersion (at group velocity dispersive element  182 ). In channel  143 , the constituent RZ bit stream  193  is generated from a CW tunable laser beam  103  of wavelength λ 3  that undergoes modulation at (modulator  173 ) and group velocity dispersion (at group velocity dispersive element  182 ). In channel  144 , the constituent RZ bit stream  194  is generated from a CW tunable laser beam  104  of wavelength λ 4  that undergoes modulation (at modulator  174 ) and group velocity dispersion (at group velocity dispersive element  184 ). 
   The constituent bit streams  191 – 194  are adapted to be time-division-multiplexed into composite bit stream  122 . As such, they are constituents of composite bit stream  122 , which is also a RZ bit stream. These constituent bit streams ( 191 – 194 ) are first combined at combiner  105 , resulting in composite bit stream  122  as the output of combiner  105 . The composite bit stream  122  undergoes modification within wavelength converter  110  and emerges therefrom as a composite bit stream  132  having a wavelength λv adapted for transmission on optical link  130 . 
   Also as understood herein, wavelengths λ 1 , λ 2 , λ 3  and λ 4  need not be the same. As such, in an alternative embodiment where λ 1 , λ 2 , λ 3  and λ 4  are not be the same, a wavelength multiplexer can be used instead of a combiner such as combiner  105 . 
   For an OTDM transmitter in accordance with the present embodiment, each of the optical signals (constituent bit streams) is modulated at approximately bit rate B. These bit streams are optically time-division-multiplexed to form a composite optical signal at a higher bit rate nB, where n is the number of multiplexed optical channels. Specifically, time-division-multiplexing of these n constituent bit streams into composite bit stream  122  is achieved by launching them into an optical fiber with time delays. The modulated bit stream in the j-th channel is delayed optically by an amount (j−1)/nB, where j=1, . . . , n. The outputs of all channels are combined to form a composite signal as a pulsed signal such as a RZ signal. The multiplexed composite bit stream has a bit slot T=1/nB. Furthermore, in the composite bit stream, n consecutive bits in each interval of duration 1/B belong to n different channels, as required by the TDM scheme. 
   Specifically, continuing with  FIG. 1 , in the present embodiment, each of optical signals (bit streams  111 – 114 ) is modulated at approximately bit rate B (=10 Gb/s). Bit streams  111 – 114  are time-division-multiplexed optically to form composite bit stream  122  at a higher bit rate nB (=40 Gb/s), where n (=4) is the number of multiplexed optical channels  141 – 144 . Specifically, time-division-multiplexing of these n (=4) constituent bit streams  191 – 194  into composite bit stream  122  is achieved by launching them consecutively into optical link  132  with time delays. The modulated bit stream in the j-th channel is delayed optically by an amount (j−1)/nB (=j−1)/(40 Gb/s)), where j=1, 2, 3 and 4. The outputs of all channels are combined to form time-division-multiplexed composite signal  122  as a pulsed signal such as a RZ signal. The multiplexed composite bit stream has a bit slot T=1/nB (=1/(40 Gb/s)). Furthermore, in the composite bit stream, 4 consecutive bits in each interval of duration 1/B (=1/(10 Gb/s)) belong respectively to 4 different channels  141 – 144 , as required by the TDM scheme. As such, OTDM transmitter  100  outputs the composite RZ bit stream  132  as a 40 Gb/s RZ bit stream that is launched into optical link  130  for optical transmission. 
   In the present embodiment, through time delays produced by source wavelength tuning, a time separation of 1/(40 Gb/s) can be produced between each two consecutive bits on composite bit streams  122  and  132 . As such, to properly align constituent bit streams  191 – 194  as they are combined to form composite bit stream  122 , the optical delay amounts of 0/(40 Gb/s), 1/(40 Gb/s), 2/(40 Gb/s), and 3/(40 Gb/s) are introduced respectively into constituent bit streams  191 – 194 . As described next, these time delays are implemented by tuning wavelengths λ 1 , λ 2 , λ 3  and λ 4  (of sources  151 – 154  respectively). 
   Referring still to  FIG. 1 , each of sources  151 – 154 , being a CW tunable laser, produces a laser beam whose wavelength can be tuned. As such, λ 1  of source  151  and λ 2  of source  152  are tuned in order to produce the proper interleaving of bit stream  191  and bit stream  192  within the composite bit streams  122  and  132 . Specifically, λ 1  and λ 2  are tuned to produce respectively bit streams  111 – 112  so that a time differential of T=1/(40 Gb/s) separates the bits within bit stream  191  from those within bit stream  192  as they emerge respectively from dispersive elements  181 – 182 . Similarly, λ 2  and λ 3  are tuned to produce respectively bit streams  112 – 113  so that a time differential of T=1/(40 Gb/s) separates the bits within bit stream  192  from those within bit stream  193  as they emerge respectively from dispersive elements  182 – 183 . Similarly, λ 3  and λ 4  are tuned to produce respectively bit streams  113 – 114  so that a time differential of T=1/(40 Gb/s) separates the bits within bit stream  193  from those within bit stream  194  as they emerge respectively from dispersive elements  183 – 184 . Hence, by tuning λ 1 , λ 2 , λ 3  and λ 4 , constituent bit streams  191 – 194  can be combined by combiner  105  into composite bit stream  122  that has the proper OTDM time spacing between every consecutive bit. 
   In the present embodiment, wavelength converter  110  is implemented with a vertical lasing semiconductor optical amplifier (VLSOA) whose vertical laser has wavelength λv. Specifically, as wavelength converter  110 , VLSOA&#39;s vertical lasing generates composite bit stream  132  with wavelength kv. As such, as composite bit stream  132  emerges from wavelength converter  130 , its wavelength λv is independent of various wavelengths λ 1 , λ 2 , λ 3  and λ 4 ) contained within composite bit stream  122 . Moreover, composite bit stream  132  is amplified because of the amplifying function of wavelength converter  110  implemented as a VLSOA. 
   However, as understood herein, wavelength converter  110  need not be implemented with a VLSOA. For example, in one embodiment of the invention, wavelength converter  110  is implemented using four-wave mixing. In another embodiment, wavelength converter  110  is implemented with a semiconductor optical amplifier (SOA). In yet another embodiment of the invention, wavelength converter  110  is implemented with a Mach-Zehner-SOA (MZ-SOA). 
   Also, as understood herein, channels  141 – 144  need not be 10 Gb/s per channel. For example, in another embodiment, each of bit streams ( 111 – 114 ) is modulated at approximately 40 Gb/s, thereby resulting in composite bit stream  132  that is approximately 160 Gb/s. Moreover, as understood herein, the present embodiment need not be implemented with n (=4) input channels such as channels  141 – 144 . For example, in another embodiment, n (not=4) channels are time-division-multiplexed. 
   Furthermore, as understood herein, the constituent bit streams need not be generated from CW tunable lasers with modulators. For example, rather than the tunable laser sources being modulated by modulators to generate the constituent bit streams, tunable laser sources that are directly modulated lasers can be used to generate the constituent bit streams. Accordingly, in another embodiment of the invention, the constituent bit streams are generated from directly modulated laser sources. 
   Referring now to  FIG. 2 , a flow chart  200  is shown outlining steps in performing OTDM transmission in accordance with one embodiment of the invention. 
   In step  205 , n bit streams of approximately bit rate B are generated by modulating each of n output laser beams respectively from n tunable CW laser sources. However, as understood herein, these n bit streams need not be generated as such. For example, in another embodiment of the invention, these n bit streams can be generated from n tunable laser sources that are directly modulated. 
   In step  210 , n group velocity dispersed bit streams are generated by introducing group velocity dispersion into each of the n bit streams by passing each modulated bit streams through a group velocity dispersive element. 
   In step  215 , a multi-wavelength composite bit stream of approximately bit rate nB is formed by combining the n group velocity dispersed bit streams. The multi-wavelength composite bit stream contains wavelengths from the n group velocity dispersed bit streams. As understood herein, these n group velocity dispersed bit streams can be combined with a optical combiner or a wavelength multiplexer. 
   In step  220 , a single-wavelength composite bit stream of approximately bit rate nB is generated by passing the multi-wavelength composite bit stream through a wavelength converter. The single-wavelength composite bit stream has a wavelength that is selected for optical transmission by a optical link. In the present embodiment, the wavelength converter is implemented with a VLSOA. However, as understood herein, in one embodiment of the invention, the wavelength converter is implemented with a SOA. In another embodiment of the invention, the wavelength converter is implemented with MZ-SOA. In yet another embodiment of the invention, the wavelength converter is implemented with four-wave-mixing. 
   In query step  225 , either the multi-wavelength composite bit stream or the single-wavelength composite bit stream is examined to see if proper OTDM time differential occurs between every two consecutive bits. If yes, then step  235  is implemented. If no, then step  230  is performed. 
   In step  230 , time positions of bits within the single-wavelength composite bit stream are adjusted to create proper OTDM time differential between every two consecutive bits by tuning some or all of n tunable laser sources. Query step  225  is then performed again. 
   In step  235 , the single-wavelength composite bit stream is launched to an optical link for optical transmission. 
   Referring now to  FIG. 3 , a wave-division-multiplexing (WDM) system  300  incorporating OTDM channels  341 – 342  is shown in accordance with one embodiment of the invention. WDM system  300  comprises input OTDM channels  341 – 342 , a WDM multiplexer  305  and an optical link  330 . Channel  341  contains a composite 4B Gb/s bit stream  132  (λv) resulting from performing OTDM on four B Gb/s bit streams  191 – 194  (respectively λ 1 , λ 2 , λ 3  and λ 4 ) in accordance with one embodiment of the invention. Channel  342  contains a composite 4B Gb/s bit stream  432  (λv′) resulting from performing OTDM on four B Gb/s bit streams  491 – 494  (respectively λ 1 ′, λ 2 ′, λ 3 ′ and λ 4 ′) in accordance with one embodiment of the invention. 
   As understood herein, wavelengths λ 1 , λ 2 , λ 3  and λ 4  need not be the same. Similarly, wavelengths λ 1 ′, λ 2 ′, λ 3 ′ and λ 4 ′ need not be the same. As such, in an alternative embodiment wherein λ 1 , λ 2 , λ 3  and λ 4  are not the same, and wherein λ 1 ′, λ 2 ′, λ 3 ′ and λ 4 ′ are not the same, wavelength multiplexers can be used instead of combiners. 
   Continuing with  FIG. 3 , channel  341  is coupled to WDM multiplexer  305 ; channel  342  is also coupled to WDM multiplexer  305 . WDM multiplexer  305  is in turn coupled to optical link  330 . Composite bit stream  132  of λv and composite bit stream  432  of λv′ are wavelength-division-multiplexed to form a composite bit stream  332  having a bandwidth of 8B Gb/s, and carrying wavelengths of λv and λv′. 
   Specifically, composite bit stream  132  as shown in  FIG. 3  is generated by OTDM performed in accordance with the embodiment described in  FIG. 1 . Similarly, composite bit stream  432  as shown in  FIG. 3  is generated by OTDM performed in accordance with the embodiment described in  FIG. 1 . 
   As understood herein, WDM system  300  need not be limited to two OTDM channels  341 – 342  as shown. For example, in one embodiment, a WDM system is implemented with m OTDM channels. As such, an optical signal having bandwidth of 4 mB Gb/s is transmitted by the WDM system to an optical link. Moreover, in another embodiment, a WDM system is implemented with m OTDM channels wherein each OTDM channel carries a composite bit stream formed by time-division-multiplexing n bit streams. As such, an optical signal having bandwidth of mnB Gb/s is transmitted by the WDM system to an optical link. 
   In the present embodiment, the wavelength converters  110  and  410  are implemented with VLSOAs. However, as understood herein, in one embodiment of the invention, the wavelength converters  110  and  410  are implemented with SOAs. In another embodiment of the invention, the wavelength converters  110  and  410  are implemented with MZ-SOAs. In yet another embodiment of the invention, the wavelength converters  110 – 410  are implemented using four-wave mixing. 
   Furthermore, as understood herein, the constituent bit streams need not be generated from CW tunable lasers with modulators. For example, rather than the tunable laser sources being modulated by modulators to generate the constituent bit streams, tunable laser sources that are directly modulated lasers can be used to generate the constituent bit streams. Accordingly, in another embodiment of the invention, the constituent bit streams are generated from directly modulated laser sources. 
   The foregoing descriptions of specific embodiments of the invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations are possible and consistent with the discussion above. The embodiments were chosen and described in order to explain the principles and the application of the invention, thereby enabling others skilled in the art to utilize the invention in its various embodiments and modifications according to the particular purpose contemplated. The scope of the invention is intended to be defined by the claims appended hereto and their equivalents.