Abstract:
A method and apparatus for optical return-to-zero (RZ) modulation based on a single Mach-Zehnder modulator driven by non-return-to-zero (NRZ) electrical signals. The method and apparatus allow for continuously electrically tunable duty cycles and lead to chirped-RZ formats. A “push-pull” embodiment involves driving one control arm of the Mach-Zehnder with a differentially encoded version of an NRZ data stream and driving the other control arm with an inverted and time-delayed copy of the same differentially encoded data stream. A “push-push” embodiment involves driving one control arm of the Mach-Zehnder with a differentially encoded version of an NRZ data stream and driving the other control arm with a time-delayed but non-inverted copy of the same differentially encoded data stream. In one or more embodiments, the duty cycle of the RZ modulation is controlled via the selection of the time delay between the electrical signals that drive the two arms of the Mach-Zehnder.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit of the filing date of U.S. provisional application No. 60/448,735, filed on Feb. 20, 2003. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    This invention relates generally to the field of optical telecommunications, and in particular, to return-to-zero (Rz) modulators in optical transmitters.  
           [0004]    2. Description of the Related Art  
           [0005]    In the field of optical communications, RZ formats are often preferred over non-return-to-zero (NRZ) formats due to their increased robustness to a variety of distortions that are typically encountered in optical fiber propagation and in filtering and reception.  
           [0006]    The most commonly employed RZ transmitter structures make use of an NRZ data modulator either in combination with a sinusoidally driven intensity modulator acting as a pulse carver, or in combination with an actively mode-locked laser. Efforts to reduce RZ transmitter complexity have led to designs that (i) use a single electro-optic modulator fed by an electrical RZ signal, (ii) employ an NRZ-driven phase modulator followed by a passive optical delay interferometer, or (iii) drive a Mach-Zehnder intensity modulator between its transmission minima with an NRZ signal to generate RZ pulses upon level changes in the NRZ drive signal.  
           [0007]    More information on designs (i), (ii), and (iii) can be found in: N. M. Froberg et al., “Generation of 12.5 Gbit/s soliton data stream with an integrated laser-modulator transmitter,” Electron. Lett., vol. 30, 1880-1881 (1994); P. J. Winzer and J. Leuthold, “Return-to-Zero Modulator Using a Single NRZ Drive Signal and an Optical Delay Interferometer,” Photon. Technol. Lett., vol. 13, 1298-1300 (2001) (herein “Winzer &#39;01”); and J. J. Veselka et al., “A soliton transmitter using a cw laser and an NRZ driven Mach-Zehnder modulator,” Photon. Technol. Lett., vol. 8, 950-952 (1996), respectively, each incorporated herein by reference in its entirety.  
           [0008]    As the demand for more bandwidth grows, the market pressure to reduce the cost, size, and complexity of RZ transmitters increases.  
         SUMMARY OF THE INVENTION  
         [0009]    Problems in the prior art are addressed in accordance with principles of the present invention by a method and apparatus for optical return-to-zero (RZ) modulation that are based on a single Mach-Zehnder modulator driven by non-return-to-zero (NRZ) electrical control signals. The method and apparatus allow for continuously electrically tunable duty cycles and lead to chirped-RZ formats. One embodiment, a “push-pull” operation, involves driving one control arm of the Mach-Zehnder with a differentially encoded version of an NRZ data stream and driving the other control arm with an inverted and time-delayed copy of the same differentially encoded data stream. Another embodiment, a “push-push” operation, involves driving one control arm of the Mach-Zehnder with a differentially encoded version of an NRZ data stream and driving the other control arm with a time-delayed but non-inverted copy of the same differentially encoded data stream. In one or more embodiments, the duty cycle of the RZ modulation is controlled via the selection of the time delay between the electrical signals that drive the two arms of the Mach-Zehnder.  
           [0010]    In one embodiment, the present invention is an apparatus for generating a modulated optical signal. The apparatus includes a signal splitter adapted to receive and split an input data signal into first and second copies, a delay element adapted to receive and delay the first copy relative to the second copy, and an optical signal modulator adapted to modulate light fed to the modulator in accordance with first and second control signals based on the delayed first copy and the second copy, respectively, to generate the modulated optical signal.  
           [0011]    In another embodiment, the present invention is a method for generating a modulated optical signal. The method involves splitting an input data signal into first and second copies, delaying the first copy relative to the second copy, and modulating light based on the delayed first copy and the second copy to generate the modulated optical signal. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    Other aspects, features, and advantages of the present invention will become more fully apparent from the following detailed description, the appended claims, and the accompanying drawings in which:  
         [0013]    [0013]FIG. 1 depicts two different embodiments of a chirped-RZ transmitter according to the present invention.  
         [0014]    [0014]FIG. 2 depicts exemplary waveforms for the intensity and phase of the signal out of the Mach-Zehnder modulator for the push-pull configuration (FIG. 2( a )) and the push-push configuration (FIG. 2( b )). 
     
    
     DETAILED DESCRIPTION  
       [0015]    Reference herein to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments mutually exclusive of other embodiments.  
         [0016]    The Transmitters  
         [0017]    [0017]FIG. 1 depicts two different embodiments of a chirped-RZ transmitter according to the present invention. These embodiments represent modifications of duobinary and alternate-mark-inversion NRZ transmitters. For duobinary signaling, a phase change occurs whenever there is an odd number of ‘0’s between two successive ‘1’s, whereas for AMI the phase changes for each ‘1’ (even for adjacent ‘1’s), independent of the number of ‘0’s in between. More information on such transmitters can be found in T. Franck et al., “Duobinary transmitter with low intersymbol interference,” Photon. Technol. Lett., vol. 10, 597-599 (1998) (herein “Franck &#39;98”), incorporated herein by reference in its entirety.. As discussed in the following, each transmitter results in a modulated optical output signal that exhibits a unique set of characteristics.  
         [0018]    Push-Pull  
         [0019]    [0019]FIG. 1( a ) depicts a “push-pull” embodiment of chirped-RZ (CRZ) transmitter  100 , as well as associated electrical and optical waveforms  102  and  104 , respectively, according to one embodiment of the present invention.  
         [0020]    CRZ transmitter  100  includes (optional) differential encoder  106 , continuous-wave (CW) laser  108 , dual-drive Mach-Zehnder modulator (MZM)  110 , non-inverting driver amplifier  112 , inverting driver amplifier  114 , and variable delay element  116  of delay τ.  
         [0021]    Operationally, CW laser  108  feeds MZM  110  with an optical signal that is modulated by the MZM with a differentially encoded representation of an electrical NRZ data signal that has a bit period of T seconds. In particular, differential encoder  106  receives the electrical NRZ data signal and translates it to a differentially encoded signal that is split into two paths. One path is fed to non-inverting driver amplifier  112 , which drives one electrical control arm of MZM  110 . The second path feeds delay element  116  where the signal is delayed by τ seconds, where τ≦T. The delayed signal out of the delay element is then fed to inverting driver amplifier  114 , which drives the other control arm of MZM  110 . Electrical waveforms  102  correspond to the differentially encoded signal from driver amplifier  112  and the inverted, delayed, differentially encoded signal from driver amplifier  114 .  
         [0022]    The differential encoder operates by translating each occurrence of a logical “1” in the electrical NRZ data signal into a level change on the encoder&#39;s output. For example, an NRZ data signal representing the bit pattern  1000111  would be encoded as SNNNSSS, where S denotes a level shift and N denotes no level shift. Such a differential encoding scheme is discussed in more detail in Franck &#39;98. Note that it is not strictly necessary to precode the signals at the transmitter. In an alternative implementation, the differential encoder can be omitted, the MZM can be modulated with the uncoded NRZ data signal, and appropriate decoding can be done at the receiver, as would be understood by one skilled in the art. However, in practice, precoding at the transmitter leads to a more noise-tolerant system than performing decoding at the receiver.  
         [0023]    Note that MZM  110  is biased for destructive interference in the absence of drive level changes between its control arms. Thus, the output power of the MZM in the absence of level transitions is essentially zero. However, as a result of changes in the control arm drive voltages (see, for example, waveforms  102 ) that result from logical ones in the electrical NRZ data signal, pulses are produced (e.g., pulses  118  of waveform  104 ) in the output of the MZM corresponding to where the interference properties of the MZM are altered by the two modulating electrical NRZ waveforms  102 . The duration of each pulse (i.e., its pulsewidth) is determined by the electrical delay τ and the rise/fall times of the MZM control arm drive signals.  
         [0024]    Push-Push  
         [0025]    [0025]FIG. 1( b ) depicts a “push-push” embodiment of chirped-RZ transmitter  120 , as well as associated electrical and optical waveforms  122  and  124 , respectively, according to another embodiment of the present invention.  
         [0026]    CRZ transmitter  120  includes (optional) differential encoder  126 , continuous-wave laser  128 , dual-drive Mach-Zehnder modulator  130 , non-inverting driver amplifiers  132  and  134 , and delay element  136  of delay τ.  
         [0027]    Operationally, it should be noted that corresponding elements of CRZ transmitter  120  behave similarly to those of CRZ transmitter  100 . Namely, CW laser  128  feeds MZM  130  with an optical signal that is modulated by the MZM with a differentially encoded representation of an electrical NRZ data signal that has a bit period of T seconds. In particular, differential encoder  126  receives the electrical NRZ data signal and translates it to a differential signal, still in NRZ format. The resulting differentially encoded signal is split into two paths. One path is fed to non-inverting driver amplifier  132 , which drives one electrical control arm of MZM  130 . The second path feeds delay element  136  where the signal is delayed by τ seconds, where τ≦T. The delayed signal out of the delay element is then fed to non-inverting driver amplifier  134 , which drives the other control arm of MZM  130 . Electrical waveforms  122  correspond to the differentially encoded signal from driver amplifier  132  and the delayed, differentially encoded signal from driver amplifier  134 .  
         [0028]    MZM  130  is biased for destructive interference in the absence of drive level changes between its control arms. Thus, the output power of the MZM in the absence of level transitions is essentially zero. However, as a result of changes in the control arm drive voltages (see, for example, waveforms  122 ) that result from logical ones in the electrical NRZ data signal, pulses are produced (e.g., pulses  138  of waveform  124 ) in the output of the MZM corresponding to where the interference properties of the MZM are altered. The duration of each pulse (i.e., its pulsewidth) is determined by the electrical delay τ and the rise/fall times of the MZM control arm drive signals.  
         [0029]    Pulsewidth and Waveform Characteristics  
         [0030]    [0030]FIG. 2 depicts exemplary waveforms for the intensity and phase of the signal out of the MZM for the push-pull configuration (e.g., FIG. 2( a )) and the push-push configuration (e.g., FIG. 2( b )) for electrical delays of τ equal to T, 0.5·T, and 0.1·T. In each case, the electrical MZM control signal is assumed to have a 10%-90% rise time of 0.4·T, corresponding to a moderate drive bandwidth of 0.9/T. The drive levels of the control signals are chosen equal to the MZM&#39;s switching voltage V π . This results in destructive interference at the output of the MZM under nominal circumstances (e.g., NRZ data=0). As shown in FIG. 2, relatively short RZ pulses can be generated without the need for exceedingly high electrical-drive bandwidths.  
         [0031]    One difference between the push-pull embodiment and the push-push embodiment is that the push-pull embodiment yields a substantially constant peak pulse power, independent of τ, while the peak pulse power decreases with τ in the push-push implementation. This is because, for push-pull operation, the drive-voltage difference Δμ(t)=Δ 1 (t)−μ 2 (t) between the two MZM control arms, which is responsible for the optical power transmission of the MZM, always passes a transmission maximum at Δμ(t)=0 when switching between the transmission minima that are present at Δμ(t)=V π −0 and Δμ(t)=0−V π  (i.e., the voltage differences associated with no control arm drive level changes).  
         [0032]    For push-push operation, on the other hand, constructive interference in the MZM, leading to RZ-pulse peaks, is found at times of maximum drive voltage difference Δu . As can be seen from FIG. 1( b ), this difference is reduced once τ falls short of the modulation rise time. To avoid the excess modulation insertion loss introduced by this effect, the drive voltage can be increased. Conversely, in circumstances when a higher modulator insertion loss is acceptable, the push-push embodiment may be used for control arm drive voltages smaller than V π , while the push-pull implementation involves drive levels substantially equal to V π  on both arms of the MZM, or degradations in extinction ratio will be encountered.  
         [0033]    Regarding the phase of the optical pulses, FIG. 2( a ) reveals that the push-pull implementation yields alternate-chirp RZ signals, with lower phase excursions at reduced duty cycles. Signals of this kind can offer potential advantages for non-linear fiber propagation as discussed in R. Ohhira, D. Ogasahara, and T. Ono, “Novel RZ signal format with alternate-chirp for suppression of nonlinear degradation in 40 Gb/s based WDM,” Proc. OFC&#39;01, paper WM2 (2001), incorporated herein by reference in its entirety.  
         [0034]    The push-push implementation, on the other hand, in addition to a π-phase jump for every RZ pulse (see FIG. 2( b )), typically generates linear phase transitions of alternating sign over the pulse duration. In other words, it lets adjacent pulses experience opposite frequency shifts, as discussed in Winzer &#39;01. In the limit as τ→T and as the rise and fall times of the control arm drive signals approach zero, both embodiments of the present invention can produce unchirped, alternate-mark-inversion, NRZ signals out of the MZM.  
         [0035]    Note that various alternative implementations may be substituted for the exemplary implementations illustrated in FIGS.  1 ( a ) and  1 ( b ). For example, a push-push implementation that replaces each non-inverting driver amplifier ( 132  and  134 ) in the embodiment of FIG. 1( b ) with an inverting driver amplifier, two inverting driver amplifiers, or no driver amplifiers at all (assuming drive levels from the differential encoder were sufficient) would be within the spirit and scope of the present invention. Similarly, in the push-pull implementation of FIG. 1( a ), equivalent arrangements of driver amplifiers including swapping the location of inverting and non-inverting driver amplifiers ( 114  and  112 ), while making appropriate voltage offset adjustments, using no driver amplifier in place of non-inverting driver amplifier  112  while using inverting driver amplifier  114 , and other equivalent arrangements as would be understood by one skilled in the art, would remain within the scope and spirit of the present invention.  
         [0036]    Also, a splitter, as described herein, should be understood to include any active or passive electronic device that produces two substantially identical or logically inverted copies of one data stream as would be understood to one skilled in the art. Similarly, the process of splitting should be understood to include any active or passive process that produces two substantially identical or logically inverted copies of one data stream.  
         [0037]    Additionally, it should be noted that, in the push-pull embodiment of the present invention depicted in FIG. 1( a ), the order of delay component  116  and inverting driver amplifier  114  maybe reversed (i.e., the signal out of differential encoder  106  maybe split and amplified, inverted, and then delayed before being fed to MZM  110 ) while remaining within the scope of the present invention. Alternatively, driver amplifier  112  and inverting driver amplifier  114  can be deleted and a single dual-output (one output invert) driver amplifier can be inserted after differential encoder  106 . In this alternative arrangement, one output of the dual-output driver amplifier is fed to delay element  116 , which in turn feeds MZM  110  and the other output is fed to MZM  110  directly.  
         [0038]    In a similar manner, in the push-push embodiment of the present invention depicted in FIG. 1( b ), the order of delay component  136  and driver amplifier  134  may be reversed. Alternatively, driver amplifiers  132  and  134  can be deleted and a single dual-output driver amplifier can be inserted after differential encoder  126 . In this alternative arrangement, one output of the dual-output driver amplifier is fed to delay element  136 , which in turn feeds MZM  130  and the other output is fed MZM  130  directly. Alternatively, in the latter arrangement, the driver amplifier may be of the single output variety and a single output lead from the driver amplifier can be directly split or fed to a printed-circuit board trace that is then split between the delay element and the direct feed to the MZM. Other equivalent arrangements are within the scope and spirit of the present invention as would be understood to one skilled in the art.  
         [0039]    Note that the elements of the present invention may be implemented by various techniques and in various technologies while remaining within the spirit and scope of the invention. These techniques and technologies include, but are not limited to, integrated optics (including silica on silicon substrate or Si:SiO 2 ), fiber optics, free space optics, thin film, InGaAs, InP, and LiNbO 3  subsystems.  
         [0040]    Note that in one or more embodiments of the present invention, variable delay elements  116  and  136  of FIGS.  1  ( a ) and  1  ( b ), respectively, can be dynamically configured by an integrated or external controller (not illustrated).  
         [0041]    While this invention has been described with reference to illustrative embodiments, this description should not be construed in a limiting sense. Various modifications of the described embodiments, as well as other embodiments of the invention, which are apparent to persons skilled in the art to which the invention pertains are deemed to lie within the principle and scope of the invention as expressed in the following claims.