Abstract:
Duobinary and NRZ modulation of an X-Gb/s optical signal is achieved with a lumped element InP Mach-Zehnder device configured to operate at X/k-Gb/s where k&gt;1 and arranged in a push-pull configuration.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates generally to the field of optical communications and in particular to the generation of duobinary and Non-Return-To-Zero (NRZ) optical signals with a non-linear, lumped-element InP Mach-Zehnder structure. 
       BACKGROUND OF THE INVENTION 
       [0002]    One impediment to the widespread adoption of 40-Gb/s optical transmission systems is the absence of a transponder exhibiting cost, footprint and power characteristics superior to those exhibited by 4×10-Gb/s systems. 
       SUMMARY OF THE INVENTION 
       [0003]    In accordance with the principles of the invention, we describe a compact, lumped-element electrode InP Mach-Zehnder device capable of sustained 42.7-Gb/s operation constructed from a transmitter intended for 10 Gb/s operation. Both optical duobinary (DB) and optically-equalized non-return-to-zero (NRZ) formats yield excellent results. 
         [0004]    Accordingly, one aspect of the present invention is applicable to a method for generating a duobinary optical signal at X-Gb/sec through the use of a non-linear lumped element Mach-Zehnder device designed to be operated at X/k-Gb/sec where k&gt;1. In this inventive manner, relatively high-yield, low cost devices may be used to generate high-quality signals having formats and rates of substantial interest. Subsequent filtering of these signals further improves their desirable characteristics. 
         [0005]    Viewed from another aspect, the present invention is applicable to a method for generating non-return-to-zero (NRZ) signals. 
         [0006]    Viewed from yet another aspect, the present invention is applicable to a method of filtering duobinary signals. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0007]    A more complete understanding of the present invention may be realized by reference to the accompanying drawing in which: 
           [0008]      FIG. 1  is a schematic block diagram of a lumped-element InP Mach-Zehnder modulator according to the present invention; 
           [0009]      FIG. 2  is a graph showing the electro-optic transfer function of the InP structure according to the present invention along with a comparison to a traveling-wave design as well as an ideal 5 th  order Bessel transfer function; 
           [0010]      FIG. 3  is a series of graphs showing both (a) unfiltered and (b) filtered 42.7-Gb/s optical duobinary eyes and (c) spectra before (dashed) and after (solid) receive-side filtering by a 100/50-GHz Wavelength Division Multiplexed interleaver; 
           [0011]      FIG. 4  is a block diagram showing (a) general optical duobinary filtering and (b) modulation according to the present invention; 
           [0012]      FIG. 5  is a graph showing BER vs. OSNR for optically unfiltered (squares) and filtered (circles) duobinary as well as optically equalized NRZ (triangles), all at 42.7 Gb/s.; 
           [0013]      FIG. 6  is a series of graphs showing 42.7-Gb/s NRZ eye diagrams and specta before [(a), (c) dashed] and after [(b), (c) solid] optical equalization; 
           [0014]      FIG. 7  shows schematically  FIG. 7(   a )- FIG. 7(   c ) the equalization process,  FIG. 7(   d ) the optical equalizer employed, and  FIG. 7(   e ) the operations and stages of the equalizer; and 
           [0015]      FIG. 8  is a graph showing chromatic dispersion tolerance of optically filtered duobinary (circles) and optically equalized NRZ with fixed (solid triangles) and with dispersion-optimized OEQ (open triangles). 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    The following merely illustrates the principles of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the invention and are included within its spirit and scope. 
         [0017]    Furthermore, all examples and conditional language recited herein are principally intended expressly to be only for pedagogical purposes to aid the reader in understanding the principles of the invention and the concepts contributed by the inventor(s) to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. 
         [0018]    Moreover, all statements herein reciting principles, aspects, and embodiments of the invention, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents as well as equivalents developed in the future, i.e., any elements developed that perform the same function, regardless of structure. 
         [0019]    Thus, for example, it will be appreciated by those skilled in the art that the diagrams herein represent conceptual views of illustrative structures embodying the principles of the invention. 
         [0020]    According to one aspect of the invention, 42.7-Gb/s operation of a lumped-element electrode InP Mach-Zehnder modulator (MZM) originally intended for 10.7-Gb/s is described. This MZM exhibits a steeper frequency roll-off beyond 15 GHz than an equivalent travelling-wave design. 
         [0021]    Turning now to  FIG. 1 , there is shown a schematic block diagram of an exemplary, lumped element, InP Mach-Zehnder modulator according to the present invention. More particularly—and for exemplary purposes—a number of 10.664 Gb/s signals  110  are applied to a 4:1 multiplexer  120  which outputs the multiplexed 10.664 signals  110  as 42.7-Gb/s signals  133  and  135  which represent DATA, and  DATA  respectively and which are applied to a respective electrodes  143 ,  145  each disposed on a respective arm of a Mach-Zehnder modulator  140  (MZM). 
         [0022]    As shown in this FIG.  1 —and according to a preferred embodiment—the Mach-Zehnder modulator MZM  140  has lumped-element electrodes,  143 ,  145  which those skilled in the art will readily appreciate enable small size, low cost and high yield manufacture, yet exhibit a steeper frequency roll-off beyond ˜15 GHz than the more involved travelling-wave electrode approaches designed to operate at substantially 40 Gb/s. 
         [0023]      FIG. 2  shows the electro-optic transfer characteristics (input-to-output scattering parameter S 21 ) of the two MZM arms, and contrasts them to a travelling-wave LiNbO 3  device with similar 3-dB bandwidth, rated for 10.7-Gb/s operation. The dc switching voltage of the device was V π =2.3 V, and the dc extinction ratio was 16 dB, since—in the exemplary embodiment—the MZM splitting ratios were tailored to opti-mize negative chirp performance at 10 Gb/s. The cw output power of the DFB-MZM module  150  was +7 dBm. 
         [0024]    Optical duobinary modulation at 42.7-Gb/s is achieved by biasing the modulator at maximum extinction and directly applying a 42.7-Gb/s bit sequence, we simultaneously used the MZM as a duobinary low-pass filter and as a modulator. As is evident from  FIG. 2 , the steeper roll-off of the InP MZM compared to a LiNbO 3  travelling-wave design better matches a 5 th -order Bessel low-pass characteristics of ˜12 GHz 3-dB bandwidth, known to be well suited for 42.7-Gb/s duobinary filtering. The 42.7-Gb/s bit sequence was generated by multiplexing four properly delayed copies of a 10.7-Gb/s 2 31 −1 pseudo-random bit sequence. The modulator was driven in push-pull operation with a differential drive voltage of 7 V pp  (3.5 V pp  on each MZM arm). 
         [0025]      FIG. 3  shows the resulting eye diagrams in which  FIG. 3(   a ) is straight out of the modulator and  FIG. 3(   b ) is after optical filtering by a 100/50-GHz wavelength-division multiplexing (WDM) interleaver having a 43-GHz 3-dB bandwidth.  FIG. 3(   c ) shows the optical spectra before and after optical filtering. As can be readily appreciated, the interleaver suppresses higher-frequency spectral components, which are sub-optimally filtered by the MZM&#39;s non-ideal low-pass characteristics. 
         [0026]    An overview of this operation may be quickly understood with reference to  FIG. 4  which shows in block diagram form this optical duobinary filtering and modulation according to the present invention. In particular, in  FIG. 4(   a ), a 40 G NRZ input data signal  410  is applied to a R/4 filter  420  which results in a filtered 40 G signal  430 . This filtered 40 G signal is applied to the arms of the 10-Gb/s MZM  440  thereby resulting in an optical intensity as depicted in diagram  450 . In this inventive manner, the 10-Gb/s MZM acts as a duobinary filter for 40-Gb/s NRZ signals  470   FIG. 4(   b ). 
         [0027]    Turning now to  FIG. 5  there it shows the bit error ratio (BER) as a function of the optical signal-to-noise ratio (OSNR, 0.1-nm reference bandwidth, both noise polarizations), as measured with a beat-noise limited receiver of 40-GHz electrical bandwidth. The squares and circles represent the optically unfiltered (1.2-nm optical receiver bandwidth) and the interleaver-filtered DB signal, respectively. As shown, optical filtering not only eliminates an error floor at BER˜10 −8 , but also lowers the required OSNR at BER=10 −3  from 21 dB to 17.7 dB; both values demonstrate excellent performance 6 . 
         [0028]    Biasing the modulator at quadrature, and driving it with the same 42.7-Gb/s bit sequence as described above, but at a reduced differential drive voltage swing of 3.5 V pp  (1.75 V pp  on each MZM arm), a heavily low-pass filtered NRZ signal exhibiting an almost closed eye as shown in  FIG. 6(   a ) was generated. Using a two-tap optical feed-forward equalizer (OEQ), the NRZ signal was restored to that shown in  FIG. 6(   b ).  FIG. 6(   c ) shows the spectra before (dashed) and after (solid) the OEQ, evidencing the restoration of high-frequency spectral components. 
         [0029]    For our demonstrative purposes, the OEQ employed was fabricated in 0.80% index contrast Si:SiO 2  planar lightwave circuit technology. A tap delay of 20 ps, shorter than the bit duration of 23.4 ps, enhances OEQ performance and allows for simultaneous equalization of multiple 50-GHz (or 100-GHz) spaced WDM channels. The device has a 4-dB fiber-to-fiber insertion loss. Being fully passive, pre-emphasis of high-frequency signal components comes at the expense of attenuating the carrier, which leads to a 9-dB reduction in average signal power, compensated by transmit amplification. 
         [0030]      FIG. 7  shows schematically the operation of the equalization filter. In particular  FIG. 7(   e ) shows a distorted waveform applied to an input of the equalization filter is “cleaned up” by the successive stages of the filter to produce the equalized waveform output therefrom. Operationally, the equalization has the effect of removing energy from the ‘0’-bits of the waveform and relocating that energy to the ‘1’-bits. This operation is shown schematically in  FIG. 7(   a ),  FIG. 7(   b ) and  FIG. 7(   c ) which correspond to the waveform upon input, after the first stage and after the second stage of the equalization filter, respectively.  FIG. 7(   d ) is a schematic of the Si:SiO 2  planar lightwave circuit described above wherein (a), (b), and (c) denote the approximate locations of waveforms  FIG. 7(   a ),  FIG. 7(   b ) and  FIG. 7(   c ) within its structure. 
         [0031]    Turning our attention now back to  FIG. 5 , the BER performance of the optically equalized NRZ signal is shown. A back-to-back required OSNR of 17 dB at BER=10 −3  was achieved, which is only 0.5 dB worse than the performance of an optimised 42.7-Gb/s LiNbO 3 -based NRZ transmitter in the same receiver setup. 
         [0032]    The chromatic dispersion tolerance of the present invention may be understood with reference to  FIG. 8 . In particular,  FIG. 8  shows the dispersion tolerance (required OSNR for BER=10 −3 ) of duobinary modulation filtered by the 100/50-GHz interleaver (circles) as well as of the optically equalized NRZ signal with fixed (back-to-back optimised) OEQ settings (open triangles) and with the OEQ optimised separately for each dispersion point (solid triangles). The modulator chirp due to the finite extinction ratio of the MZM is evident from the asymmetry of the penalty curves. The full width of the dispersion window at 2 dB OSNR penalty amounts to 110 ps/nm for NRZ with fixed OEQ, 330 ps/nm for NRZ with dispersion-optimized OEQ, and 255 ps/nm for DB. The dispersion tolerance for NRZ matches or exceeds reported data using 42.7-Gb/s rated LiNbO 3  modulators. The tolerance for optically filtered DB is slightly lower than the expected ˜300-ps/nm window, which we mainly attribute to residual modulator chirp. 
         [0033]    At this point, those skilled in the art will appreciate the significance of our operation of a 10.7-Gb/s highly integrated lumped-element electrode InP MZM at 42.7 Gb/s using duobinary and optically equalized NRZ signals. In addition, duobinary features lower optical complexity, little modulation insertion loss, and a large dispersion tolerance, at the modest expense of slightly higher drive voltage requirements, although lower drive voltages can advantageously be used a the expense of increased insertion loss. Optically equalized NRZ modulation needs lower drive voltages at the expense of an additional optical component; the OEQ lends itself to integration and can be shared among WDM channels. The back-to-back sensitivities as well as the dispersion tolerances are comparable to those obtained from 42.7-Gb/s rated LiNbO 3  modulators. 
         [0034]    At this point, while we have discussed and described our invention using some specific examples, those skilled in the art will recognize that our teachings are not so limited. In particular, while we have discussed an operation of the invention at particular rates (i.e., 10.7 Gb/s devices having 42.7 Gb/s bit sequences applied thereto, our invention may advantageously operate with other variations of rated operation rate of the Mach-Zehnder and applied bit rates. Accordingly, our invention should be only limited by the scope of the claims attached hereto.