Abstract:
A system and method for producing an 8-QAM-modulated signal are disclosed. The methodology, in an exemplary expedient, generally comprises splitting light from a CW laser into two parts; modulating the first part with a first signal and modulating the second part with a second signal; phase shifting the modulated second part by about π/4; combining the modulated first part with the phase shifted and modulated second part to produce a four-level modulated signal; and phase modulating the four-level modulated signal with a third signal with a phase modulation of (0, π/2). Several variations of this method are described herein.

Description:
FIELD OF THE DISCLOSURE 
     The present disclosure relates generally to communication networks, and more particularly, to methods and apparatus for directly converting a plurality of binary electrical signals into a single eight-level quadrature-amplitude-modulated (8-QAM) optical signal. 
     BACKGROUND 
     Wave division multiplexing (WDM) optical networks are well known. A WDM channel is typically transmitted by a single mode semiconductor laser, where information to be communicated is imposed on the light by modulating the laser current or by externally modulating the light by applying a voltage to a modulator coupled to the laser source. A receiver subsequently photo-detects and converts the light into electric current either by direct or coherent detection. 
     Due to the rapid growth of optical networks and the need for greater capacity, significant research has focused on finding efficient multi-level optical modulation formats. Any digital modulation scheme uses a finite number of distinct signals to represent digital data. Phase-shift-keying (PSK) uses a finite number of phases, each assigned a unique pattern of binary bits. Usually, each phase encodes an equal number of bits, and each pattern of bits forms the symbol that is represented by the particular phase. The demodulator, which is designed specifically for the symbol-set used by the modulator, determines the phase of the received signal and maps it back to the symbol it represents, thereby recovering the original data. The receiver compares the phase of the received signal to a reference signal. This expedient utilizes coherent detection and is referred to as CPSK. 
     Alternatively, in lieu of using the bit patterns to establish the phase of the wave, CPSK employs differential phase changes. The demodulator then determines these phase changes in lieu of the actual phase of the signal. This scheme is referred to as differential phase-shift keying (DPSK), and is easier to implement than PSK as there is no need for the demodulator to maintain the reference signal to determine the exact phase of the received signal. 
     BPSK (also sometimes called PRK, Phase Reversal Keying) is the simplest form of PSK. It utilizes a pair of phases separated by 180° and is known as 2-PSK. 
     Quaternary or quadriphase PSK, 4-PSK, or 4-QAM (QPSK) uses four points on a constellation diagram as is known in the art. The four-phase QPSK can encode two bits per symbol—twice the rate of BPSK—and experimentation has demonstrated that this may double the data rate compared to a BPSK system while maintaining the bandwidth of the signal. Alternatively, QPSK can maintain the data-rate of BPSK at half the requisite bandwidth. 
     Optical modulations based on four-level quadrature-phase-shift-key (QPSK) have been effectively demonstrated for both 40 Gb/s and 100 Gb/s optical transmission. In the quest for even higher spectral efficiency, eight-level 8-PSK modulation has been proposed and demonstrated experimentally. 
     8-QAM is another eight-level modulation format. In comparison to 8-PSK, 8-QAM is tolerant of greater noise (on the order of 1.6 dB), with identical spectral utilization. 
       FIG. 1  is a schematic of an 8-PSK modulator  100 , which comprises an optical splitter  104  that splits the incoming light from a CW laser source  102  into two components—a first part  106  and a second part  108 . The first and second parts  106 ,  108  are modulated by Mach-Zehnder Modulators MZM 1   110  and MZM 2   112 , which are driven by binary signals DATA 1  and DATA 2 , respectively, and biased at the null point with a driving swing of 2 Vπ. The modulated lower part from MZM 2   112  is applied to a phase shifter  114  to impose a phase shift of π/2. The modulated first part  106  and modulated and phase-shifted lower part are combined by a 1:1 optical combiner  118  and the output thereof subsequently phase-modulated by (0, π/4) with binary signal DATA 3  at phase-modulator  120  to produce the 8-PSK signal. 
     8-QAM encodes the signal in both amplitude and phase, thus making 8-QAM more difficult to practically implement than 8-PSK. 
     SUMMARY 
     In accordance with a first aspect, an apparatus for producing an 8-QAM-modulated signal is disclosed herein. The apparatus comprises a first coupler that receives light from a CW laser and splits the light into two parts; a first modulator that receives and modulates the first part with a first signal and a second modulator that receives and modulates the second part with a second signal; a phase shifter coupled to the second modulator that shifts a phase of the modulated second part by approximately π/4; a second coupler that combines the modulated first part with the phase-shifted and modulated second part to produce a four-level modulated signal; and a phase modulator that receives the four-level modulated signal and further modulates the four-level modulated signal with a third signal with a phase modulation of (0, π/2). The first and second modulators are preferably phase-asymmetric Mach-Zehnder (MZ) modulators. 
     In accordance with another aspect, an apparatus for producing an 8-QAM-modulated signal comprises: a phase modulator that receives light from a CW laser and modulates the light with a first signal with a phase modulation of (0, π/2); a first coupler that receives the modulated light and splits the modulated light into two parts; a first modulator that modulates the first part with a second signal and a second modulator that modulates the second part with a third signal; a phase shifter that shifts a phase of the modulated second part by about π/4 (it will be appreciated by those skilled in the art that π/4 is an optimal setting for a MZM modulator with a very high extinction ratio, but for a practical MZM modulator with a moderate extinction ratio, the optimal phase shift may vary slightly); and a second coupler that combines the modulated first part with the phase shifted and modulated second part to produce an 8-QAM-modulated signal. 
     In accordance with yet another aspect, a method for producing an 8-QAM-modulated signal, comprises: splitting light into two parts; modulating the first part with a first signal and modulating the second part with a second signal; phase shifting the modulated second part by approximately π/4; combining the modulated first part with the phase shifted and modulated second part to produce a four-level modulated signal; and phase modulating the four-level modulated signal with a third signal with a phase modulation of (0, π/2). 
     In accordance with still another aspect, a method for producing an 8-QAM-modulated signal, comprises: a phase modulator that receives light and modulates the light with a first signal; phase modulating light with a first signal with a phase modulation of (0, π/2); splitting the phase modulated light into two parts; modulating the first part with a first signal and modulating the second part with a second signal; phase shifting the modulated second part by about π/4 and combining the modulated first part with the phase shifted and modulated second part. 
     These aspects of the invention and further aspects and advantages thereof will become apparent to those skilled in the art as the present invention is described with particular reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic of a device for producing an 8-PSK-modulated signal; 
         FIG. 2  is a schematic of an apparatus for producing an 8-QAM-modulated signal in accordance with an exemplary embodiment of the disclosure; 
         FIG. 3   a  is a simulated I-Q constellation diagram of the signal after amplitude and phase-asymmetric modulation; 
         FIG. 3   b  is a simulated I-Q constellation diagram of the signal after π/2 phase modulation; 
         FIG. 4  is a schematic of an apparatus for producing an 8-QAM-modulated signal in accordance with an exemplary embodiment of the disclosure; 
         FIG. 5  is a schematic of an apparatus for producing an 8-QAM-modulated signal in accordance with an exemplary embodiment of the disclosure; 
         FIG. 6  is a schematic of an apparatus for producing an 8-QAM-modulated signal in accordance with another exemplary embodiment of the disclosure; and 
         FIG. 7  is a schematic of an apparatus for producing an 8-QAM-modulated signal in accordance with yet another exemplary embodiment of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments will be described with reference to the accompanying drawing figures wherein like numbers represent like elements throughout to the extent possible. Before embodiments are explained in detail, it is to be understood that the disclosure is not limited in its application to the details of the examples set forth in the following description or illustrated in the figures. The disclosure suggests other embodiments and of being practiced or carried out in a variety of applications and in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein are meant to encompass the items listed thereafter and equivalents thereof as well as additional items. 
       FIG. 2  is a schematic of a first exemplary embodiment of an apparatus  200  for converting three binary electrical signals into a single 8-QAM-modulated optical signal. In this regard, light generated by a constant wave (CW) laser source  202  is received by a first 3 dB 1:1 optical coupler  204  that equally splits the laser light into a first (upper) part  206  and a second (lower) part  208 . The upper part  206  is applied to a first modulator  210 , which is biased at the null point and driven by a first binary electrical signal (DATA 1 ) with a driving swing voltage of 2 Vπ. Similarly, the lower part  208  is applied to a second modulator  212 , which is biased at the null point and driven by the second binary electrical signal (DATA 2 ) with a driving swing voltage of 2 Vπ. 
     Preferably, first and second modulators  210 ,  212  are phase-asymmetric Mach-Zehnder (MZ) modulators (hereinafter referred to as “MZM 1   210 ” and “MZM 2   212 ”), each of which utilizes an optical interferometer fabricated from a nonlinear material in which the velocity of light is varied by an applied electrical field to selectively block or transmit incident laser light as a function of the externally applied driving voltage. The driving swing voltage is either applied to a single input or applied differentially at a pair of inputs. The MZ modulators operate at the full serial data rate of the optical channel and the output of the modulator driver typically has the largest amplitude and highest bandwidth of any electrical signal in the system. As will be appreciated by those skilled in the art, MZ modulators are typically designed to minimize variations or imperfections in the amplified electrical waveform directly to the optical signal to effectively reduce distortion that can affect bit-error rate (BER) and consequent operating distance of the fiber link. The illustrative embodiments herein utilize dual-parallel MZMs; however the invention is not limited to these expedients. 
     MZM 1   210  and MZM 2   212  are effectively operable as zero-chirp 0/π phase modulators. As illustrated in  FIG. 2 , the lower part  208  is phase modulated (0, π) by MZM 2   212 , then applied to a phase shifter  214  to impose a phase shift of π/4, and thereafter passed to a 5.7 dB power attenuator  216 . The modulated, phase-shifted and attenuated lower part is subsequently input to a second 3 dB optical coupler  218  that combines the lower part  208  with the modulated upper part  206  from MZM 1   210 . The combined signal from optical coupler  218  is a four-level QAM signal  220 , which is represented in  FIG. 3(   a ) as a simulated I-Q constellation diagram using 19 Gb/s electrical binary signals and an 18 GHz 3-dB optical modulation bandwidth. The four-level modulated optical signal  220  is subsequently modulated by a phase modulator  222  driven by a third binary electrical signal (DATA 3 ) with a phase modulation of (0, π/2). Advantageously, the output of phase modulator  222  is an optimal circular eight-level 8-QAM-modulated optical signal—represented in the I-Q constellation diagram of  FIG. 3  ( b ). 
       FIG. 4  is a schematic of a second exemplary embodiment  400  for achieving 5.7 dB power attenuation by reducing the driving swing of MZM 2  from 2 Vπ to 0.7 Vπ. In this embodiment, all components of the apparatus are similar to the expedient described above and illustrated in  FIG. 4 . As in the first exemplary embodiment of  FIG. 4 , light generated by a CW laser source  402  is received by a first 3 dB 1:1 optical coupler  404  that splits the light into an upper part  406  and a lower part  408 . The upper part  406  is applied to MZM 1   410 , which is biased at the null point and driven by a first binary electrical signal (DATA 1 ) with a driving swing voltage of 2 Vπ. Similarly, the lower part  408  is applied to MZM 2   212 , which is biased at the null point and driven by the second binary electrical signal (DATA 2 ) with, in this embodiment, a driving swing voltage of 0.7 Vπ. The lower part  408  is thereafter phase modulated (0, π) by MZM 2   412 , applied to a π/4 phase shifter  414 , and then input to a second 3 dB optical coupler  418  that combines the lower part  408  with the modulated upper part  406  from MZM 1   410 . The four-level modulated optical signal  420  from coupler  418  is thereafter modulated by a phase modulator  422  driven by a third binary electrical signal (DATA 3 ) with a phase modulation of (0, π/2) to produce the 8-QAM-modulated optical signal. 
       FIG. 5  is a schematic of a third exemplary embodiment  500  that advantageously achieves the desired power attenuation by substituting a 4:1 optical coupler  504  in lieu of the 3 dB optical coupler  204 / 404  shown in  FIGS. 2 and 4 . Thus, all components are similar to the expedients described above and illustrated in  FIGS. 2 and 4 . Light from a CW laser source  502  is received by a 4:1 optical coupler  504  that splits the light into an upper part  506  and a lower part  508 . The upper part  506  is applied to MZM 1   510 , which is biased at the null point and driven by a first binary electrical signal (DATA 1 ) with a driving swing voltage of 2 Vπ. Similarly, the lower part  508  is applied to MZM 2   512 , which is biased at the null point and driven by the second binary electrical signal (DATA 2 ) with a driving swing voltage of 2 Vπ. The lower part  508  is phase modulated (0, π) by MZM 2   512 , applied to a π/4 phase shifter  514 , and thereafter input to a second 3 dB optical coupler  518  that combines the lower part  508  with the modulated upper part  506  from MZM 1   510 . The four-level modulated optical signal  520  from coupler  518  is thereafter modulated by a phase modulator  522  driven by a third binary electrical signal (DATA 3 ) with a phase modulation of (0, π/2) to produce the 8-QAM-modulated optical signal. 
       FIG. 6  is a schematic of a fourth illustrative embodiment  600  that introduces the phase modulation (0, π/2) prior to dividing the light into constituent upper and lower parts. In this regard, the apparatus  600  comprises a phase modulator  620  that receives light from CW laser  602  and modulates the light with a first binary electrical signal (DATA  1 ) with a phase modulation (0, π/2). The phase-modulated light is received by a first coupler  604  that splits the modulated light into an upper part  606  and lower part  608 . The upper part  606  is applied to a first modulator  610 , which is biased at the null point and driven by a first binary electrical signal (DATA 2 ) with a driving swing voltage of 2 Vπ. Similarly, the lower part  608  is applied to a second modulator  612 , which is biased at the null point and driven by the second binary electrical signal (DATA 2 ) with a driving swing voltage of 2 Vπ. As in the exemplary embodiment of  FIG. 2 , the lower part  608  is phase modulated (0, π) by MZM 2   612 , applied to a π/4 phase shifter  614 , and thereafter passed to a 5.7 dB power attenuator  616  (not shown in the  FIG. 6 , I have attached a document showing the corrected  FIG. 6 ). The modulated, phase-shifted and power-attenuated lower part is then input to a second 3 dB optical coupler  618  that combines the lower part  608  with the modulated upper part  606  from MZM 1   610 . It will be appreciated by those skilled in the art that this modification may be incorporated in the embodiments of  FIGS. 4 and 5 . 
     In the above we have assumed that the (0,π/2) phase modulation is achieved by using the common phase modulator, where the phase modulation is linearly proportional to the applying driving electrical voltage. However, such a phase modulator will linearly transfer the amplitude jitter of the driving electrical signal into the phase jitter of the modulated optical signal and therefore may degrade the performance of the generated optical signal. In  FIG. 7  we disclose a new MZM-based (0,π/2) phase modulator, where the (0, π/2) phase modulation is achieved by interfering one (0,π) MZM-modulated light with a π/2 phase-shifted CW light. Due to the nonlinear electrical-to-optical response of MZM modulator, the proposed new (0,π/2) phase modulator can effectively suppress electrical-to-optical jitter transfer. 
       FIG. 7  is a schematic of another exemplary apparatus that is operable as a chirp-less (0, π/2) phase modulator  700 . In this expedient, the light from a CW laser is split by a first coupler  704  into an upper part  706  and lower part  708 . The upper part  706  is applied to a modulator  710 , which is biased at the null point and driven by a binary electrical signal (DATA) with a driving swing voltage of 2 Vπ. The lower part  708  is applied to a π/2 phase shifter  714 , and thereafter input to a second 3 dB optical coupler  718  that combines the lower part  708  with the modulated upper part  706  from MZM 1   710  to produce the 8-QAM-modulated optical signal. 
     The foregoing detailed description is to be understood as being in every respect illustrative and exemplary, but not restrictive, and the scope of the invention disclosed herein is not to be determined from the description of the invention, but rather from the claims as interpreted according to the full breadth permitted by the patent laws. It is to be understood that the embodiments shown and described herein are only illustrative of the principles of the present invention and that various modifications may be implemented by those skilled in the art without departing from the scope and spirit of the invention.