Abstract:
An optical pulse generator comprising a phase modulator, wherein the phase modulator is configured to phase modulate and intensity modulate a continuous wave (CW) light signal in accordance with a received electrical signal, and an all-pass filter, the all-pass filter being coupled to the phase modulator to generate a pulsed light signal by filtering the modulated light signal from the phase modulator.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to the field of optical signal processing, and more specifically to apparatus and methods related to optical signal modulation. 
       BACKGROUND INFORMATION 
       [0002]    Silicon photonics has the potential to change many aspects of data transport, but has not yet substantially impacted long haul transport. Part of the reason for this is that modulators in this platform are “chirped” in the sense that they generate signals that exhibit chirp. As is well known, chirp can degrade spectral efficiency and long haul transport characteristics. 
         [0003]    In silicon and indium phosphide (InP) photonic integrated circuits, electro-optic modulators can create optical phase shifting by using the Kramers-Kronig effect, where a change in optical loss induces a change in optical path length within an optical waveguide. This approach is effective in creating an electro-optic response, often referred to as the plasma-dispersion effect, in materials that do not have an intrinsic electro-optic response. When used for optical modulation within a push-pull Mach-Zehnder modulator (MZM), however, there is a resulting change in modulator optical path length during signal modulation that results in a chirped transmission signal. As mentioned, modulator chirp can be detrimental to system spectral efficiency, system scalability, and transmission performance. 
       SUMMARY OF THE INVENTION 
       [0004]    An exemplary embodiment of a substantially chirp-free pulse generator in accordance with the present invention combines a plasma dispersion phase modulator (PDPM) and an all-pass filter (APF). The substantially chirp-free pulse train can be used with a chirped intensity modulator to generate an effectively chirp-free return-to-zero (RZ) transmission. The RZ transmission can be effectively chirp-free because the optical frequency shifts that result from the modulator chirp can be confined to the regions between the pulses in the pulse train. 
         [0005]    Furthermore, various embodiments for generating a substantially chirp-free pulse train can be extremely efficient because the resulting pulse train peak power can be larger than the continuous wave (CW) power input into the pulse generator. 
         [0006]    Various embodiments make it possible to generate an effectively chirp-free RZ transmission from an integrated optic platform that typically provides chirped modulation. Effectively chirp-free RZ transmission can be achieved when it previously was typically not possible in a silicon photonic based platform. By creating a chirp free transmission capability for the silicon photonics platform, new applications may be opened up for this technology. 
         [0007]    Furthermore, electro-absorption modulator (EAM) designs in accordance with various embodiments can also be used in a platform such as indium phosphide (InP) to generate effectively chirp-free transmissions, opening up new opportunities to optimise power consumption and use of space with this technology platform. 
         [0008]    The aforementioned and other features and aspects of various embodiments are described in greater detail below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a schematic representation of an exemplary embodiment of an effectively chirp-free data modulator system in accordance with the present invention. 
           [0010]      FIGS. 2A and 2B  show the amplitude and phase, respectively, of a pulse train generated by an exemplary embodiment of a substantially chirp-free pulse generator in accordance with the present invention. 
           [0011]      FIGS. 3A through 3D  show the change in transmissivity and phase of an all-pass filter (APF) as a function of frequency and the output of a plasma dispersion phase modulator (PDPM) in the system of  FIG. 1 . 
           [0012]      FIGS. 4A and 4B  show the amplitude and phase, respectively, of the output of the PDPM, and  FIGS. 4C through 4E  show the intensity and phase of a signal at the output of the APF. 
           [0013]      FIGS. 5A through 5C  illustrate the minimal extinction ratio variation and chirp arising from a given variation in the spectral response of the APF in the case of a pulse train with an extinction ratio of approximately 10 dB. 
           [0014]      FIGS. 6A through 6C  illustrate the tunable change in chirp arising from a variation in the spectral response of the APF in the case of a pulse train with an extinction ratio of approximately 20 dB. 
           [0015]      FIGS. 7A through 7C  illustrate the ability to generate an opposite chirp from a variation in the spectral response of the APF in the case of a pulse train with an extinction ratio of approximately 20 dB. 
           [0016]      FIGS. 8A and 8B  illustrate the occurrence of chirp in a chirped non-return-to-zero (NRZ) data-modulated signal generated by a chirped data modulator. 
           [0017]      FIGS. 9A and 9B  illustrate the occurrence of chirp in a minimal-chirp return-to-zero (RZ) signal generated by the data modulator system of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0018]      FIG. 1  is a block diagram of an exemplary embodiment of a data modulator system  100  in accordance with the present invention. The system  100  comprises an exemplary embodiment of an optical pulse generator  110  coupled to an optical modulator  120 . The pulse generator  110  comprises a high-speed plasma-dispersion phase modulator (PDPM)  111  coupled to an all-pass filter (APF)  112 . The APF  112  comprises a thermo-optic phase shifter (TOPS)  113  and a thermo-optic tunable coupler (TOTC)  114  arranged within a waveguide ring resonator  115 . A substantially monochromatic continuous wave (CW) light signal is applied via optical waveguide to the PDPM  111  and is predominantly phase modulated thereby. As shown below, the PDPM  111  also imparts some intensity modulation to the CW light signal. 
         [0019]    As described in greater detail below, the pulse generator  110  can generate a pulse train that is substantially chirp-free (e.g., having a chirp factor ≦±0.2), even though it is implemented using fabrication technologies, such as silicon, germanium, or indium phosphide (InP) based technologies, that are susceptible to generating chirped signals. Moreover, the pulse generator  110  can be used in conjunction with a modulator which is also implemented using such technology to generate a data-modulated output that is effectively chirp-free. In addition, for applications in which some chirp is desired, the pulse train generator  110  can be tuned to provide variable degrees of chirp. These features are described in greater detail below. 
         [0020]    The PDPM  111  creates the required spectral content for a high quality pulse stream. The APF  112  modifies the relative phases of the output tones from the modulator  111  to turn the predominantly phase-modulated light into a substantially chirp-free pulse stream. It should be noted that the peak output pulse power can be larger than the CW power in the waveguide prior to the PDPM  111 , even after accounting for the optical loss in the PDPM  111  due to carrier absorption. 
         [0021]    The spectral response of the APF  112  can be tuned using the TOPS  113 ; i.e., by adjusting the phase shift introduced by the TOPS by controlling the temperature of the TOPS in a conventional way. Referring to  FIG. 3B , the TOPS  113  controls the spectral positions of the dips (or resonances) in the APF transmission characteristic. The depths or heights of the transmission dips (or resonances) can be controlled using the TOTC  114 ; i.e., by adjusting the coupling provided by the TOTC by controlling the temperature of the TOTC in a conventional way. As described below, changing the spectral response of the APF  112  can provide variable degrees of positive as well as negative chirp, thereby allowing control over the compression or expansion of pulses as they propagate over fiber. 
         [0022]    The frequency of the pulse train generated by the pulse generator  110  is determined by the drive signal applied to the PDPM  111 . In an exemplary embodiment, the PDPM  111  is driven by a 10 GHz RF drive signal and the APF  112  has a free spectral range (FSR) of 20 GHz.  FIGS. 2-7  described below pertain to a 10 GHz embodiment. Note that while a 10 GHz implementation is described, the present invention is not limited to any particular pulse frequency and may be implemented for a wide range of pulse frequencies well below 10 GHz and extending well beyond 100 GHz. 
         [0023]    The modulator  120  encodes data onto the pulse stream generated by the substantially chirp-free pulse generator  110  to generate an effectively chirp-free return-to-zero (RZ) data-modulated output. The RZ output can be effectively chirp-free because the optical frequency shifts that result from the modulator chirp can be confined to the regions between the pulses in the pulse train. Such effectively chirp-free operation can be achieved with a variety of are chirped intensity modulators including, for example, Mach-Zehnder modulators (MZM), electro-absorption modulators (EAM), and optical modulators implemented in silicon photonics structures or indium phosphide (InP). Such modulators are referred to herein as chirped intensity modulators in that they introduce intensity modulation and phase shifting of the modulated signal. 
         [0024]    In the embodiment shown in  FIG. 1 , the modulator  120  comprises a 50/50 coupler  121 , with a first output coupled to a TOPS  122  in series with a PDPM  124 , and a second output coupled to a PDPM  123 . (Note, however, that the coupling ratio of the coupler  121  may be something other than 50/50, so that the modulator can produce a large modulation extinction ratio even though there may be a difference in optical propagation loss between the two arms of the modulator.) The PDPMs  123  and  124  are driven by complementary data signals. The outputs of the PDPMs  123  and  124  are coupled to a further coupler  125 , an output of which generates the aforementioned effectively chirp-free RZ data-modulated output. 
         [0025]      FIGS. 2A and 2B  show the amplitude and phase, respectively, of a substantially chirp-free pulse train generated by the generator  110 . The pulse train shown has an extinction ratio of approximately 13 dB. 
         [0026]      FIGS. 3A and 3B  show the spectral response of the APF  112 , with  FIG. 3B  having an expanded vertical scale. The output spectrum of the PDPM  111  is also shown in  FIG. 3A . The output of the PDPM  111  shown in  FIG. 3A  can be described as including the carrier wavelength—represented by the tallest vertical line  301  in the center of FIG.  3 A—and the modulation sidebands due to the modulation imparted by the PDPM  111 , represented by the shorter vertical lines  302   a,b  and  303   a,b  on either side of line  301 .  FIG. 3C  shows the variation of the phase of the APF  112  with frequency with the PDPM output superimposed. The linear component of the change in the APF  112  phase response with frequency is due to a constant group delay component from the APF  112 .  FIG. 3D  shows the frequency dependent change in optical phase caused by the APF  112  with the linear component of the phase change removed for clarity, which corresponds to removing the constant group delay component. As shown in  FIG. 3D , the APF  112  subjects the modulation sideband tones  302   a,b  to a phase shift of approximately 8 radians (or ˜1.5 radians) relative to the carrier tone  301 . 
         [0027]      FIGS. 4A and 4B  show the amplitude and phase, respectively, of the output of the PDPM  111 . As shown, the CW light is predominantly phase modulated with some amplitude modulation (˜0.35 dB). The peak intensity after the modulator  111  is approximately −0.9 dB.  FIG. 4C  shows the signal at the output of the APF  112 , which has a peak intensity of approximately +1 dB. The phase of the APF output signal is shown in  FIG. 4D  and on an expanded scale in  FIG. 4E . Note that the phase variation is less than approximately 0.03 radians. 
         [0028]      FIG. 5A  shows a plot of the frequency-dependent response of the APF  112  shifted over a given frequency range, in this illustration 0.4π radians (4 GHz), or 20% of the FSR (20 GHz) of the APF. The spectrum of the output of the PDPM  111  is also shown thereon as represented by spectral lines  501  and  502   a,b.  As shown in  FIG. 5A  the APF  112  is tuned so that the carrier wavelength  501  is located near, but off of a resonance of the APF response. Similarly, the sidebands  502   a,b  are also set to an off-resonance condition and so like the carrier  501  experience minimal loss through the APF  112 . As shown in  FIG. 5A , the maximum on-resonance loss of the APF is approximately 1 dB. As such, the primary filtering introduced by the APF  112  entails the frequency-dependent optical phase shifting of the phase modulated output of the PDPM  111  rather than any frequency-dependent optical loss. Note in  FIG. 5A  that the resonances in the spectral response of the APF  112  remain between the spectral components  501  and  502   a,b  as the spectral response of the APF  112  is shifted over the 0.4π radian range shown. 
         [0029]      FIGS. 5B and 5C  show the corresponding intensity and phase of the APF output signal over time. As shown in  FIG. 5B , the pulse extinction ratio varies from approximately 9.9 dB to 10.8 dB when the spectral response of the APF is moved ˜0.4π radians, or 20% of the filter FSR, demonstrating that the pulse extinction ratio is substantially insensitive to the spectral locations of the APF resonances. Also, as shown in  FIG. 5C , when the APF spectral response is shifted 0.4π radians, the phase variation across each pulse remains less than approximately 0.2 radians (or approximately 12 degrees), which translates into a chirp of approximately ±1 GHz. This corresponds to a Mach-Zehnder Modulator (MZM) chirp parameter of approximately 0.2, which is a relatively small phase variation compared to the approximately 40 degree phase variation (±3.4 GHz) seen from a typical 0.7 chirp parameter LiNbO 3  MZM with a 10 dB pulse extinction ratio. 
         [0030]      FIG. 6A  shows a plot of the frequency response of the APF  112  shifted over a given range (˜0.2π radians, or 10% of the filter FSR) in the case of a pulse extinction ratio of approximately 20 dB. The spectrum of the output of the PDPM  111  is also shown thereon.  FIGS. 6B and 6C  show the corresponding intensity and phase spectra of the APF output signal. As shown, the pulse extinction ratio remains greater than 20 dB while the effective chirp factor changes from being negligible to a value of approximately 0.7 (similar to that from a chirped LiNbO 3  MZM) by changing only the spectral location of the APF spectral response. 
         [0031]      FIGS. 7A through 7C  illustrate the same case as in  FIGS. 6A-6C , except that the opposite chirp on pulses is demonstrated. Compare the relative locations of the APF resonances and the phase modulator output spectrum with those shown in  FIG. 6A . The generator of the present invention can thus generate pulses with positive or negative chirp by tuning the APF  112  to shift the spectral response of the APF over a frequency range. 
         [0032]    Because off-resonance filtering is used (i.e., the APF resonance frequencies or the frequencies at the bottoms of the dips in the APF spectral response are different from the frequencies of the spectral components of the PDPM output), the pulse generator extinction ratio is insensitive to the spectral position of the APF resonance: e.g., the resonance can be moved 20% of the FSR with little change in the pulse extinction ratio. Thus for a 10 dB pulse extinction ratio, the chirp remains small when the APF resonance is moved 20% of the FSR. For a 20 dB pulse extinction ratio, the chirp can be tuned from a negligible amount to an effective chirp factor of ˜0.7, by changing only the spectral location of the APF resonance. 
         [0033]    An exemplary embodiment of a 10 GHz pulse generator in accordance with the present invention was simulated with favorable results. In the simulation, the PDPM  111  had a length of approximately 3.3 mm and doping of approximately 1e17. The APF  112  had an FSR of 20 GHz, and a round trip loss of ˜0.22 dB, with an assumed passive waveguide loss of 1.5 dB. 
         [0034]    The RF power required to create a pulse train with a given extinction ratio was comparable to an MZM. For instance, for a 10 dB extinction ratio, the phase modulator drive voltage required is ˜31% of Vπ as opposed to ˜32% for a push-pull MZM. For a 12 dB extinction ratio, the required voltage is ˜35% of Vπ compared to ˜36% for a push-pull MZM, and for a 15 dB extinction ratio, ˜39% of Vπ, as opposed to ˜40.5% for a push-pull MZM. 
         [0035]      FIGS. 8A and 8B  illustrate the occurrence of chirp in a chirped non-return-to-zero (NRZ) data-modulated signal generated by a chirped data modulator, such as the modulator  120  shown in  FIG. 1 . (Note that in this case, the signal shown is generated by the modulator  120  operating alone, without receiving pulses from the pulse generator  110 .) The chirp is manifested as a change in color at the leading and trailing edges of each NRZ pulse. As shown in  FIGS. 8A and 8B , maximum chirp occurs at approximately 60% of peak power of the NRZ pulse stream, as represented by point  810 . 
         [0036]      FIGS. 9A and 9B  illustrate the occurrence of chirp in a minimal-chirp return-to-zero (RZ) signal generated by an exemplary embodiment of a data modulator system in accordance with the present invention, such as the system  100  of  FIG. 1 . As shown in  FIGS. 9A and 9B , maximum chirp occurs at approximately 0% of peak power of the RZ pulse stream, as represented by point  910 . Since the signal power is at or near zero when the chirp is maximum, the manifestation of the chirp in the RZ signal will be minimal, thus yielding an effectively chirp-free signal. 
         [0037]    It is understood that the above-described embodiments are illustrative of only a few of the possible specific embodiments which can represent applications of the invention. Numerous and varied other arrangements can be made by those skilled in the art without departing from the spirit and scope of the invention.