Abstract:
An arrangement for removing unwanted amplitude modulation from the output of an electro-optic phase modulator (formed within a silicon-on-insulator (SOI) system) includes resonant filters that are biased on the positive and negative slopes of the response signal. Therefore, as the amplitude response of one filter decreases, the amplitude response of the other filter increases, resulting in balancing the output and essentially eliminating amplitude modulation from the phase-modulated output signal. In one embodiment, ring resonators (formed in the SOI layer) are used to provide the filtering, where as the number of resonators is increased, the performance of the filtering arrangement is improved accordingly.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/652,608, filed Jan. 28, 2005. 
    
    
     TECHNICAL FIELD 
     The present, invention relates to a silicon-based electro-optic phase modulator and, more particularly, to a modulator formed within a silicon-on-insulator (SOI) structure and incorporating tunable ring filters to essentially “cancel” the (unwanted) amplitude modulation present on the modulated optical output signal. 
     BACKGROUND OF THE INVENTION 
     Significant advances in the ability to provide optical modulation in a silicon-based platform has been made, as disclosed in U.S. Pat. No. 6,845,198, issued to R. K. Montgomery et al. on Jan. 18, 2005 and assigned to the assignee of the present application. The Montgomery et al. modulator is based on forming a gate region of a first conductivity type to partially overlap a body region of a second conductivity type, with a relatively thin dielectric layer interposed between the contiguous portions of the gate and body regions. The doping in the gate and body regions is controlled to form lightly doped regions above and below the dielectric, thus defining the active region of the device. Advantageously, the optical electric field essentially coincides with the free carrier concentration area in the active device region. The application of a modulation signal thus causes the simultaneous accumulation, depletion or inversion of free carriers on both sides of the dielectric at the same time, resulting in operation at speeds in excess of 10 GHz. 
       FIG. 1  illustrates one exemplary arrangement of a silicon-based modulator device as disclosed in the Montgomery et al. reference. In this case, a “SISCAP” structure  1  in terms of a doped (i.e., “active”) silicon layer  2  (usually polysilicon) is disposed over a doped portion of a relatively thin (sub-micron) surface layer  3  of a silicon-on-insulator (SOI) wafer  4 , this thin surface layer  3  often being referred to in the art as the “SOI layer”. A thin dielectric layer  5  is located between the doped, active polysilicon layer  2  and the doped SOI layer  3 , with the layers disposed so that an overlap is formed, as shown in  FIG. 1 , to define an active region of the device. As mentioned above, free carriers will accumulate and deplete on either side of dielectric layer  5  as a function of voltages applied to SOI layer  3  (VREF 3 ) and/or polysilicon layer  2  (VREF 2 ). The modulation of the free carrier concentration results in changing the effective refractive index in the active region, thus introducing phase modulation of an optical signal propagating along a waveguide formed along the active region (the waveguide being in the direction perpendicular to the paper). 
     When constructing such a modulator as a pure frequency modulator (i.e., single sideband), a sawtooth ramp waveform, as shown in  FIG. 2 , is used to provide the modulating signal. In particular, an input signal is used to linearly change the phase from 0 to 2π, and then nearly instantaneously returning to 0 (and then repeating—modulo 2π). This linear phase shift results in a fixed frequency translation: ω 0 =δφ/δt. However, a problem arises with such modulators that are based on the free carrier effect to provide the desired modulation. That is, the optical absorption/attenuation characteristic of the modulator is a function of the total number of free carriers in the optical path. As a result, the application of a signal to modulate the phase of the optical signal will also affect the amplitude of the optical signal. This is problematic in that the unwanted amplitude modulation introduces error in the output signal.  FIG. 3  illustrates the presence of this amplitude modulation and the residual AM modulated signal components in the associated frequency spectrum. 
     Thus, a need remains in the art to remove, as much as possible, the AM modulation present within an SOI-based electro-optic phase modulator. 
     SUMMARY OF THE INVENTION 
     The need remaining in the prior art is addressed by the present invention, which relates to a silicon-based electro-optic phase modulator and, more particularly, to a modulator formed within a silicon-on-insulator (SOI) structure and incorporating an integrated filtering arrangement to essentially “cancel” the (unwanted) amplitude modulation present on the modulated optical output signal. 
     In accordance with the present invention, a filtering arrangement is formed that includes at least a pair of filters, a first filter biased at a near linear region of the amplitude response (positive or negative) and a second filter biased at the opposing near linear region (i.e., negative or positive, respectively). Presuming the first filter is biased along the negative slope region, the amplitude response curve shifts to exhibit more delay and the amplitude is reduced. The second filter, in this case, is biased along the positive slope region so as to increase the amplitude. By controlling the bias points for these two filters, therefore a “zero” amplitude response can be achieved, allowing for pure phase modulation to be provided. 
     In one embodiment a pair of tunable ring resonator filters may be utilized inasmuch as the processing involved to integrate such a device with a SOI-based electro-optic modulator is well-understood. In particular, a segment of each ring is doped and coupled to an electrode to provide for the desired tuning, where the application of the voltage to the doped area will modifying the effective index (and, therefore, filtered wavelength) for the ring. 
     In general, a plurality of such filtering elements may be used in combination, where the additional number of elements serves to improve the shape of both the phase and amplitude responses of the modulator by increasing the number of poles and zeroes in the filter response. 
     Various other embodiments and aspects of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Referring now to the drawings, 
         FIG. 1  illustrates, in a simplified cross-sectional view, an exemplary electro-optic phase modulator as formed in a silicon-on-insulator (SOI) structure; 
         FIG. 2  contains a plot of the ideal phase input and ideal frequency spectrum associated with the modulator of  FIG. 1 ; 
         FIG. 3  contains a similar plot of the phase, but also illustrating the amplitude modulated signal component that is present in the modulator of  FIG. 1 , the frequency spectrum including the residual AM components also being illustrated in  FIG. 3 ; 
         FIG. 4  illustrates, in a simplified view, a first embodiment of an amplitude-correcting arrangement formed in accordance with the present invention to counter the effects of amplitude modulation within an electro-optic phase modulator, where the amplitude and relative phase plots for this embodiment are also shown; 
         FIG. 5  illustrates an alternative embodiment of the present invention, using a pair of parallel waveguides to generate “amplitude compensation” in accordance with the present invention; and 
         FIG. 6  illustrates a variation of the embodiment of  FIG. 5 , where a plurality of filter elements are used to refine the shaping of both the phase and amplitude responses of an SOI-based electro-optic phase modulator. 
     
    
    
     DETAILED DESCRIPTION 
     An exemplary arrangement for substantially reducing the presence of amplitude modulation in a phase modulated output signal O from an electro-optic modulator is illustrated in  FIG. 4 . As shown, optical signal O is propagating along a waveguide  10 , where in most cases waveguide  10  will comprise a relatively thin (less than one micron) silicon surface layer of a silicon-on-insulator (SOI) structure. Moreover, the “active region” of waveguide  10  is best confined in the manner described above in association with  FIG. 1 , which illustrates an “active region”  5  having a relatively narrow width. Such an arrangement is important for single mode applications. 
     Referring again to  FIG. 4 , a first optical filtering element  12  (in this case, a ring waveguide) is disposed along waveguide  10  in a manner so as to out-couple a selected portion of the propagating signal. The Q of the ring (and as a result, the phase) defines the selectivity of the filter, where the higher the Q factor, the more selective the filter response. For the purposes of the present invention, a high Q factor is desired. First filtering element  12  (hereinafter referred to as “first ring filter  12 ” for the sake of discussion) is shown as including a tuning region  14 , where tuning region  14  comprises a doped portion of the SOI layer within which first ring filter  12  is formed. By the application of a voltage to tuning region  14 , the effective refractive index of that portion of first ring filter  12  is modified. The modification of the effective refractive index results in changing the wavelength of light that will out-couple from waveguide  10 . Thus, by adjusting the voltage applied to tuning region  14 , the filtered wavelength may be “tuned”. In accordance with the teachings of the present invention, first ring filter  12  is tuned so as to out-couple the signal at a predetermined wavelength “A”. Referring to the amplitude and phase plots associated with this portion of the arrangement, wavelength “A” is seen to be along the negative (downward) slope of the amplitude response. As the effective refractive index increases, the time delay of first ring filter  12  increases and the filter response curve shifts to the left. Thus, the output phase of the modulated optical output signal shifts to more delay and the amplitude is reduced. 
     A second ring filter  16  (which, in the most general case may comprise any suitable type of tunable optical filter) is illustrated as disposed along a separate section of waveguide  10 , where filter  16  includes a tuning region  18 . In accordance with the present invention, the voltage applied to tuning region  18  is controlled so that wavelength “B” filtered by second ring filter  16  will be along the positive (upward) slope of the amplitude response, as shown by the associated amplitude and phase response plots. Again, as the effective refractive index increases, the time delay of second ring filter  16  increases and the filter response curve shifts to the left. In this case, however, as the output phase of the modulated optical output signal shifts to more delay, the amplitude is increased. Therefore, in accordance with the present invention, the increase in amplitude associated with the second filtering element will offset the decrease in amplitude associated with the first filtering element and significantly reduce the residual amplitude modulation present in the output signal. 
     An alternative amplitude compensating arrangement of the present invention is shown in  FIG. 5 . In this case, an optical splitter formation is used, with each ring filter disposed along a separate one of the split paths. As shown, the optical phase modulated signal is applied as an input along a waveguide  20 , where as with the arrangement discussed above, waveguide  20  may be formed within the relatively thin (e.g., sub-micron) surface silicon layer of an SOI structure. Thereafter, waveguide  20  is split into two separate, parallel waveguides  22  and  24 . A first tunable filtering element  26  (in this case, a tunable ring filter) is disposed alongside waveguide  22  and functions to out-couple the signal propagating a predetermined wavelength from waveguide  22 . Tunable ring filter  26  includes a tuning region  28 , where the bias voltage applied to tuning region  28  will determine the specific wavelength that is out-coupled from waveguide  22 . As with the arrangement described above, tuning region  28  may comprise a heavily doped portion of the same sub-micron silicon layer used to form waveguide  22 , or a doped polysilicon material disposed over that portion of ring filter  26  or, alternatively, a silicide or other metal disposed over a predetermined portion of ring filter  26 . 
     In accordance with the present invention, and similar to the arrangement described above in association with  FIG. 4 , first ring filter  26  is tuned to filter the signal appearing at wavelength A, shown as along the negative slope of the amplitude response. The application of the appropriate voltage bias to tuning region  28  will allow for this desired wavelength to be selected. A second ring filter  30  is illustrated as disposed alongside waveguide  24 , where second ring filter  30  includes a tuning region  32 . In this case, the bias applied to tuning region  32  is adjusted until second ring filter  30  removes the wavelength at point “B”, associated with rising edge of the amplitude response. The combination of these two signals, therefore, will essentially remove any amplitude response from the modulator output. Specifically, the intensity of the output signal from the arrangement of  FIG. 5  can be expressed as follows: 
               I   out     =           I   A     +     I   B       2     +           I   A     ⁢     I   B         ⁢   cos   ⁢           ⁢     ϕ   .               
Thus, by maintaining cos φ at a constant value (near zero), the output intensity will exhibit little amplitude modulation, in accordance with the teachings of the present invention. For example, when a 2π phase shift is applied to a single phase modulator by the free carrier effect, the output intensity is reduced by 2.5 dB. For the inventive arrangement as shown in  FIG. 5 , a 2π phase shift results in the output I A  decreasing in intensity by 20%, while the intensity I B  increases by 20%. However, the total output intensity will change only slightly, since cos φ is maintained essentially constant. Thus, assuming a normalized output of unity, I OUT  will change to 0.99 (i.e. 1% change) when a 20% change in intensity is applied along waveguides  22  and  24 , providing a reduction in amplitude modulation by a factor of twenty. It is to be noted that the waveform applied to each arm is not linear, but the resulting phase shift is linear. Indeed, the change in phase is a direct result of a change in Q=CV, where ΔQ/Δt=constant from 0 to 2π.
 
     Inasmuch as the optical signal has been split to propagate along two separate signal paths, a degree of phase shift may occur, since the length of these two paths may not be perfectly equal. Therefore, it is preferred that a separate phase adjustment element  34  be disposed along waveguide  22  and/or waveguide  24  and utilized to compensate for any phase mismatch that may result. That is, the application of a bias voltage to phase adjustment element will introduce the proper time delay required to overcome any phase shift introduced by the arrangement. 
     As mentioned above, a plurality of separate filtering elements may be used in the arrangement of the present invention to better shape the phase modulation response and remove a larger portion of the unwanted (residual) amplitude modulation.  FIG. 6  illustrates a variation of the arrangement of  FIG. 5 , where in this case, a set of three separate tunable ring filters is stacked alongside each waveguide. That is, a plurality of three separate tunable ring filters  26 - 1 ,  26 - 2  and  26 - 3  are stacked along a portion of waveguide  22 , where each tunable ring filter includes a separate tunable region  28 - 1 ,  28 - 2  and  28 - 3 , with the possibility of applying a difference bias voltage to each region increasing the overall phase/frequency tuning range of the arrangement. A similar stacked arrangement of tunable ring filters  30 - 1 ,  30 - 2  and  30 - 3  is disposed alongside waveguide  24 . A set of tuning regions  32 - 1 ,  32 - 3  and  32 - 3  are associated with ring filters  30 - 1 ,  30 - 2  and  30 - 3  in a similar manner to provide an increased wavelength tuning range for the signal propagating along waveguide  24 . As mentioned above, increasing the number of rings (i.e., the number of resonances) increases the number of poles and zeroes in the filter response, providing more control over the tunability of the filter. 
     In the particular embodiment as shown in  FIG. 6 , there is an increased likelihood of a phase shift occurring between a first output signal propagating along a waveguide  42  disposed along ring filter  26 - 3  and a second output signal propagating along a waveguide  44  disposed along ring filter  30 - 3 . Therefore, a tunable phase shifter  46  is disposed along waveguide  42 , where by controlling the bias applied to tunable phase shifter  46  the optical path lengths of waveguides  42  and  44  can be equalized and phase shift eliminated. Preferably, a second tunable phase shifter  48  is disposed along waveguide  44  to provide for an additional degree of phase shift control. 
     While the subject matter of the present invention has been shown with various embodiments, it is to be understood that the scope of the invention is limited only by the claims appended hereto.