Abstract:
Optical modulators with reduced temperature dependence on bias control are described. A set of bias electrodes is arranged relative to a set of RF electrodes in a manner which results in the operating point of the device remaining relatively constant as a function of temperature. The arrangement of the bias electrodes relative to the RF electrodes includes a physical offset of one set of electrodes relative to the other, with or without a reversal of polarity of one set of electrodes relative to the other. Arrangements according to the present invention create a symmetrical electrode arrangement from a temperature-induced stress point of view so that the operating point of the device remains relatively constant as a function of temperature.

Description:
BACKGROUND  
         [0001]    The present invention relates generally to optical modulators and, more particularly, to optical modulators having electrodes proximate waveguides for biasing optical outputs.  
           [0002]    Technologies associated with the communication of information have evolved rapidly over the last several decades. Optical information communication technologies have evolved as the technology of choice for backbone information communication systems due to, among other things, their ability to provide large bandwidth, fast transmission speeds and high channel quality. Optical modulators are used to impress data onto optical carriers for transmission through optical fiber.  
           [0003]    Compared to their bulk counterparts, guided-wave electro-optic modulators offer a drastic reduction in the required driving power combined with a bandwidth extending well into the 40 GHz band. In an electro-optic modulator, an electric signal is applied to electrodes embedded in a substrate and proximate to the waveguide(s), to induce an electric field which in turn causes, via the electro-optic effect, phase modulation. The latter is typically converted into amplitude modulation by redistributing light among output channels, of which some may correspond to guided modes and others to radiation modes.  
           [0004]    One example of a guided-wave, electro-optic modulator is the Mach-Zehnder interferometer (MZI). As shown in FIGS.  1 ( a ) and  1 ( b ), the waveguide  8  of an MZI typically includes an input section  10 , a symmetric input Y-junction  12 , two interferometer arms  14  and  16 , a symmetric output Y-junction  18 , and an output section  19 . The input Y-junction  12  splits light launched into the input section  10  into two waves propagating through the interferometer arms  14  and  16 . The waves are combined and interfere, with a certain phase difference between them, in the output Y-junction  18 . The differential phase determines the split of optical power between the fundamental (even, FIG. 1( a )) and first-order (odd, FIG. 1( b )) mode. The output single-mode section  19  supports the fundamental mode but rejects the first-order mode, which is below cut-off and diffracts into the substrate as it propagates towards the output endface of the substrate. If the differential phase equals an even integer of pi, only the fundamental mode is excited in the output Y-junction  18  and propagates, with very little loss, through the output section  19 . However, when the differential phase is an odd integer of pi, only the first-order mode is excited in the output Y-junction  18  and subsequently rejected by the output section  19 . The differential phase is affected by asymmetries that the interferometer arms  14  and  16  may have, referred to herein as intrinsic bias, as well as by a voltage applied to electrodes (not shown in FIGS.  1 ( a ) and  1 ( b )) that are disposed proximate the interferometer arms. As discussed in more detail below, with respect to FIG. 2, the voltage applied to these electrodes is used to control the differential phase and achieve amplitude modulation of the optical carrier.  
           [0005]    The intrinsic bias of the interferometer is affected by many factors, such as asymmetries (intentional or caused by fabrication tolerances), temperature, surface condition, etc. Of these factors, temperature should be considered in most applications, since typically no temperature stabilization is permitted in commercial, packaged devices. It is known that the temperature dependence of bias is caused by at least two components, one being a surface-state-related bias drift and the other an electrode-introduced stress. The drift component accounts for non-reversible changes in bias that are temperature dependent but do not directly follow changes in temperature. For example, when the modulator goes through the up-ramp of a temperature cycle and subsequently through a symmetric down-ramp, the bias after the cycle does not return to the pre-cycle value and is in fact determined by the whole history of previous temperature changes. On the other hand, the electrode-introduced stress produces, via the elasto-optic effect, a bias component that is uniquely determined by temperature. If, in the above exemplary modulator of FIGS.  1 ( a ) and  1 ( b ), only the stress component of bias would be present, such a modulator could undergo multiple symmetric temperature cycles with its operating point following the same curve during a cycle and returning to the starting point at the end of the cycle.  
           [0006]    Accordingly, Applicant would like to provide modulators and methods of making and operating modulators which substantially reduce or eliminate the stress-induced temperature dependent component of bias in such devices.  
         SUMMARY  
         [0007]    Optical modulators with reduced temperature dependence of intrinsic bias are described. A set of bias electrodes is arranged relative to a set of RF electrodes in a manner which results in the operating point of the device remaining relatively constant as a function of temperature. The arrangement of the bias electrodes relative to the RF electrodes includes a physical offset of one set of electrodes relative to the other, with or without a reversal of polarity of one set of electrodes relative to the other.  
           [0008]    According to one exemplary embodiment of the present invention, an optical modulator includes a substrate; an optical waveguide formed in the substrate; a first set of electrodes provided on the substrate proximate the optical waveguide for establishing an electric field therebetween to modulate the optical signal, the first set of electrodes including two ground electrodes and one RF potential electrode; and a second set of electrodes provided on the substrate proximate the optical waveguide for establishing an electric field therebetween to bias the optical modulator, the second set of electrodes including one ground electrode and two bias potential electrodes.  
           [0009]    According to another exemplary embodiment of the present invention, an optical modulator includes a substrate, an optical waveguide formed in the substrate; a first set of electrodes provided on the substrate for establishing an electric field therebetween to modulate the optical signal, the first set of electrodes including two ground electrodes and one RF potential electrode; and a second set of electrodes provided on the substrate for establishing an electric field therebetween to bias the optical modulator, the second set of electrodes including two ground electrodes and one bias potential electrode; wherein a center of the one RF potential electrode is offset from a center of the one bias potential electrode by a predetermined distance relative to an axis running through the optical waveguide.  
           [0010]    According to yet another exemplary embodiment of the present invention, waveguide means for guiding an optical signal through the optical modulator; electrical modulation means for modulating the optical signal, and bias means for biasing the optical modulator, wherein the electrical modulation means and the bias means are arranged in a manner which reduces a temperature dependence of the bias means.  
           [0011]    According to a still further embodiment of the present invention, a method for making optical modulators includes the steps of providing a substrate; forming a waveguide in the substrate for guiding an optical signal through the optical modulator; disposing a first set of electrodes on the substrate for modulating the optical signal; disposing a second set of electrodes on the substrate for biasing the optical modulator; and arranging the first set of electrodes and the second set of electrodes to reduce a temperature dependence of the biasing of the optical modulator. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    The accompanying drawings illustrate exemplary embodiments of the present invention, wherein:  
         [0013]    [0013]FIG. 1( a ) depicts a conventional waveguide MZI with fundamental mode excitation;  
         [0014]    [0014]FIG. 1( b ) depicts a conventional waveguide MZI with first-order mode excitation;  
         [0015]    FIGS.  2 ( a ) and  2 ( b ) show the arrangement of electrodes and resulting electric field lines for conventional optical modulators having an x-cut and a z-cut substrate, respectively;  
         [0016]    [0016]FIG. 3 shows the arrangement of an RF set of electrodes and a bias set of electrodes relative to waveguide arms in a conventional z-cut optical modulator;  
         [0017]    [0017]FIG. 4 shows the arrangement of an RF set of electrodes and a bias set of electrodes relative to waveguide arms in an optical modulator according to an exemplary embodiment of the present invention;  
         [0018]    [0018]FIG. 5( a ) is a cross-section of the device of FIG. 4 taken through the RF set of electrodes which shows exemplary electric field lines therebetween;  
         [0019]    [0019]FIG. 5( b ) is a cross-section of the device of FIG. 4 taken through the bias set of electrodes which shows exemplary electric field lines therebetween;  
         [0020]    [0020]FIG. 6 is a graph depicting bias voltage as a function of temperature of conventional (dotted lines) and inventive (solid lines) optical modulators for a first test run;  
         [0021]    [0021]FIG. 7 is a graph depicting bias voltage as a function of temperature of conventional (dotted lines) and inventive (solid lines) optical modulators for a second test run;  
         [0022]    [0022]FIG. 8 depicts a conventional set of bias electrodes from a packaging perspective;  
         [0023]    [0023]FIG. 9 depicts a set of bias electrodes according to an exemplary embodiment of the present invention from a packaging perspective;  
         [0024]    [0024]FIG. 10 depicts an RF electrode and bias electrode layout for optical modulators according to another exemplary embodiment of the present invention; and  
         [0025]    [0025]FIG. 11 depicts an RF electrode and bias electrode layout for optical modulators according to a still further exemplary embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0026]    The following detailed description of the invention refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. Also, the following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims.  
         [0027]    Devices and methods according to exemplary embodiments of the present invention provide waveguide optical modulators having biasing circuits with reduced temperature dependence as compared with conventional modulators. The purely exemplary embodiments discussed herein involve modulators employing MZIs, however those skilled in the art will appreciate that the present invention is equally applicable to any waveguide optical modulators where electrodes are positioned proximate waveguides.  
         [0028]    The positioning of the electrodes for both the RF electrode set and the bias electrode set relative to the interferometer arms is driven by the choice of substrate material and the manner in which the substrate material is diced. Of the various uniaxial materials used to fabricate optical modulators, lithium niobate (LiNbO 3 ) or lithium tantalate (LiTaO 3 ) are popular substrate choices. LiNbO 3  is widely used due to its combination of low loss characteristics, large electro-optic coefficients and high optical transparency in the near infrared wavelengths used for telecommunications. LiNbO 3  wafers are available in three different crystal cuts (x-, y-, and z-cut). FIGS. 2 a  and  2   b  illustrate cross-sections of x-cut and z-cut, respectively, LiNbO 3  substrates  31 . For the most pronounced electro-optic effect, the strongest component of the applied electric field is aligned with the z-axis of the crystal (because the z-axis has the largest electro-optic coefficient) to take advantage of what is referred to in the art as the “r 33  coefficient”. On z-cut LiNbO 3  substrates, vertical electrical fields are used with a transverse magnetic (TM) optical mode to take advantage of the r 33  coefficient, whereas in x-cut LiNbO 3  substrates, horizontal electrical fields and a transverse electric (TE) optical mode utilize the r 33  coefficient. This can be seen in FIG. 2( a ), where MZI arm  16  is placed between electrodes  22  and  26 , and arm  14  is placed between electrodes  26  and  24  such that the electric field lines passing through the waveguide arms are primarily horizontal. In FIG. 2( b ), for a z-cut crystal, the RF  26  and ground electrode  24  are placed directly over waveguide arms  16  and  14  such that the electric field lines passing through the waveguide arms are substantially vertical.  
         [0029]    [0029]FIG. 3 shows an exemplary z-cut optical modulator using an MZI having coplanar waveguide electrodes  22 - 26  formed over the optical waveguide  8 . Electrodes  22  and  24  are grounded, while electrode  26  is supplied with an RF signal. In operation, when a carrier wave from a light source, for example a DFB laser, enters at optical waveguide input  10 , the carrier power is evenly split at the first Y junction  12  into the two light channels of the MZI arms  14  and  16 . By applying a voltage between the electrode  26  and ground electrodes  22  and  24 , oppositely oriented electric field vectors exist in the crystal, one in each MZI arm  14  and  16 . Consequently, the carrier light wave within each of the arms is complementarily phase shifted relative to one another in push-pull fashion. Light from each arm is then combined at Y junction  18  where constructive or destructive interference resulting from combining phase shifted carrier waves causes signal intensity modulation. When the total phase shift Θ between the carrier waves in arms  14  and  16  is such that Θ=π, light entering the device at input  10  radiates into the substrate and results in zero channel output at output  19 .  
         [0030]    Another set of electrodes  27 - 29  is also shown in FIG. 3. These electrodes provide bias control for the modulator to offset changes in the operating point of the device that cause the phase shift of the modulated carrier waves to deviate from the intended phase shift. Among other things, the operational point of the device may change due to changes in wavelength of the input optical signal. A feedback loop (not shown) samples a portion of the light from output  19 , determines the actual operating point and compares that with the intended operating point to determine the amount of corrective bias voltage to apply via electrodes  27 - 29 . As with the RF electrodes  22 - 26 , note that the bias electrodes  27 - 29  in the conventional modulator of FIG. 3 are disposed in the same manner relative to the MZI arms  14  and  16 , i.e., a ground electrode  29  is positioned over MZI arm  14  and an electrode driven to the bias voltage potential V b  is positioned over MZI arm  16 . More specifically note that the RF and bias sets of electrodes are aligned and that the centers of the potential electrodes  26  and  28  are positioned on the same side of an axis  30  running through the waveguide of the conventional z-cut optical modulator.  
         [0031]    Although described herein as the “RF set of electrodes” and the “bias set of electrodes”, those skilled in the art will appreciate that biasing of the optical modulators described herein can occur through both the RF set of electrodes and the bias set of electrodes, as well as solely via the bias set of electrodes. The former arrangement may provide an advantage in that the voltage applied via the RF and bias set of electrodes will be lower compared to using only the bias electrode set for biasing and, consequently, the optical modulator will have a reduction in voltage-dependent drift.  
         [0032]    In the conventional Z-cut modulator of FIGS.  2 ( b ) and  3 , both the RF and bias section have the same structural asymmetry with respect to the interferometer in the sense that the hot (center) electrodes of both sections are positioned over the same interferometer arm (lower arm  16  in FIG. 3). As a result, the stress contributions of the RF and bias electrodes add to each other “in phase”. When temperature varies, the total mechanical stress applied to the interferometer changes as the sum of the changes in stress introduced by each electrode section. As a result, the intrinsic phase angle of the interferometer can vary quite significantly—by as much as 120° or even more for a 60° C. temperature change. The latter is comparable to what is typically used in environmental temperature cycle testing. To compensate for a given change in the intrinsic phase angle, the bias voltage V b  would have to be adjusted accordingly in order to maintain the desired operating point of the modulator, such as quadrature, maximum, minimum, or any other. For example, a 120° change in the intrinsic phase angle would require an adjustment of the bias voltage equal to ⅔ Vpi, which is typically several voltages. Such a slowly varying voltage can contribute to the voltage-dependent component of bias drift and eventually deteriorate the long-term stability of the modulator. Therefore, it is highly desirable to eliminate, or at least significantly reduce, the temperature dependence of bias.  
         [0033]    The reversible change in bias, caused as described above by a temperature-dependent electrode stress, can be significantly reduced by the present invention, one exemplary embodiment of which is shown in FIG. 4. As seen, the set of bias electrodes  42 - 46  is offset in such a way that the center electrode  46  of the bias section is positioned over the upper arm  14  while the center electrode  26  of the set of RF electrodes  22 - 26  is positioned over the lower arm  16 . Also note that in the set of bias electrodes  42 - 46 , the center electrode is grounded while the two outer electrodes  42  and  44  are held at the bias voltage V b . Compare this exemplary embodiment of the present invention with the conventional bias electrode arrangement in FIGS.  2 ( b ) and  3 . In the conventional arrangement, the center electrode  28  of the bias set of electrodes  27 - 29  is positioned over the same interferometer arm as the center electrode  26  of the RF set of electrodes  22 - 26 . Moreover, the conventional arrangement uses the same polarity pattern for both the RF set of electrodes as the bias set of electrodes, i.e., the “hot” electrode is centered between two ground electrodes.  
         [0034]    The changes illustrated in FIG. 4, offset and reversed polarity of the bias section, ensure that the RF and bias sections contribute “out of phase” mechanically but “in phase” electro-optically. Since the bias set of electrodes  42 - 44  is offset with respect to the positioning of the RF set of electrodes  22 - 26  relative to the waveguide arm  14  and  16 , the direction of the stress-induced bias change caused by the bias section will be opposite to that caused by the RF section. Thus, the total temperature-induced change in operating point will be reduced, or even completely eliminated if the impact of the bias section completely compensates that of the RF section. At the same time, the electro-optic modulation of the bias section adds to that induced by the RF electrode. Therefore, both electrodes can be used for biasing, just as in the standard scheme.  
         [0035]    FIGS.  5 ( a ) and  5 ( b ) illustrate the distribution of electric field lines for the RF and bias sections of the exemplary embodiment of FIG. 4, respectively. It can be noted from these figures that the RF set of electrodes  22 - 26  imparts electro-optic modulation primarily by decreasing the index of the lower waveguide arm  16 , while the bias set of electrodes  42 - 46  mostly by increasing the index of the upper waveguide arm  14 . In both cases, the differential phase, i.e. the phase difference between the upper and lower arm, increases with applied voltage. Thus, the electro-optic modulation of the RF set of electrodes  22 - 26  adds in phase to that of the bias set of electrodes  42 - 46 . Also seen in FIGS.  5 ( a ) and  5 ( b ) is an insulation buffer film  50 , such as SiO 2  or Al 2 O 3 , which may be used as a buffer to minimize z-cut LiNbO 3  optical losses that occur through TM mode absorption in the electrode metal. Buffer films may also be beneficial in x-cut LiNbO 3  devices operating at high frequency.  
         [0036]    Applicant has tested exemplary embodiments of the present invention to determine their efficacy in reducing or eliminating the temperature dependence of bias control in optical modulators. For this test, an array of Mach-Zehnder interferometers was formed in Z-cut LiNbO 3  with each modulator having an identical RF electrode and either a conventional or a bias section modified in accordance with the present invention. The modulators were temperature cycled on a hot plate between room temperature (RT) and about 85° C. During temperature cycling, the operating point of the device under test was maintained at the maximum of the transfer curve by using a feedback loop including a fiber-optic coupler and bias controller. The voltage generated by the bias controller and applied to both the RF and bias sections was recorded versus temperature. The half-wave voltage V π  of the combined (RF+bias) electrode was determined in a separate measurement. V π  of both the conventional and the modified optical modulators was the same, confirming that the RF and bias electrodes contributed in-phase electro-optically. The temperature-dependent bias variation was calculated in degrees as Δφ(T)=(V(T)−V RT )/V π *180°, where V(T) and V RT  are the bias voltages at temperature T and room temperature, respectively. FIG. 6 shows the measured dependence of bias versus temperature, wherein the dotted lines represent the results for conventional devices and the solid lines represent results for optical modulators having a bias set of electrodes arranged according to the present invention. Therein it can be seen that while the bias voltage changed significantly as a function of temperature for the conventional optical modulators, it remained relatively stable as a function of temperature for modulators according to the present invention.  
         [0037]    Applicant recognizes, however, that the ability of modulators according to the present invention to reduce the temperature-dependent nature of bias control will depend on how well the temperature-induced stresses in the bias set of electrodes match the temperature-induced stresses in the RF set of electrodes. Thus, for modulators wherein the asymmetries are such that a good match does not occur it is expected that the bias voltage will vary somewhat more as a function of temperature than the test recorded in FIG. 6 depicts. Nonetheless the temperature performance of modulators constructed in accordance with the present invention is still expected to greatly exceed that of modulators fabricated in accordance with the conventional techniques of FIGS.  2 ( b ) and  3 . Such an example is seen in FIG. 7 for another set of conventional modulators and modulators designed in accordance with the present invention. These devices were tested in a similar manner to that described above with respect to the graph of FIG. 6 and, again, the dotted lines represent the results for conventional devices and the solid lines represent results for optical modulators having a bias set of electrodes arranged according to the present invention. In the example of FIG. 7, it can be seen that the bias voltage for the modulators fabricated in accordance with the present invention does vary as a function of temperature, however the variance is still much less than that of the conventional devices.  
         [0038]    Optical modulators are typically sold in commercial packages with ground and voltage contacts. FIG. 8 illustrates a manner in which conventional bias electrodes are laid out in a package to provide for a ground contact and a voltage contact on the same side of the package. Ideally, although not required for the present invention, modulators fabricated in accordance with the present invention could be provided with contact pads disposed at substantially the same package position as their conventional counterparts. An example of an electrode layout which supports this concept is shown in FIG. 9, wherein a ground contact and a voltage contact are provided on the same side of the package as the contacts shown in FIG. 8, while also providing for the offset electrical and physical connections for bias electrode arrangements according to the present invention.  
         [0039]    [0039]FIG. 10 depicts another exemplary embodiment of the present invention wherein an x-cut substrate is employed. Therein, the interferometer arms are positioned substantially in the electrode gaps and TE-polarized light is provided to the modulator in order to utilize the r 33  coefficient. The RF electrode set  106 - 110  is offset with respect to interferometer arms  14  and  16  in order to cause chirp, i.e., both phase and amplitude modulation, in the optical output. Chirp is typically used to offset the impact of dispersion in fiber-optic transmission links. The offset of the RF electrode set creates an unbalanced mechanical strain of arms  14  and  16 . If not compensated, this unbalanced strain would result in a strong temperature dependence of bias. In the exemplary embodiment of FIG. 10, compensation is provided by offsetting the bias set of electrodes  100 - 104  in the direction opposite from the offset of the RF electrode set. More specifically, it can be seen that the center of the RF potential electrode  106  is offset to one side of axis  30  by a spacing which is substantially less than a spacing between the two waveguide arms  14  and  16 , while the center of the bias potential electrode  100  is offset to the other side of axis  30  by a similar spacing. Unlike the embodiment of FIG. 4, in this exemplary embodiment the bias set of electrodes  102 - 104  and the RF set of electrodes  106 - 110  share the same electrical configuration, i.e., the center electrodes  100  and  106  are the potential electrodes and the outer electrodes  102 ,  104 ,  108  and  110  are the ground electrodes.  
         [0040]    [0040]FIG. 11 illustrates another z-cut exemplary embodiment of the present invention. In this embodiment, as in the embodiment of FIG. 10, the bias and RF set of electrodes share the same electrical configuration, i.e., the center electrodes  26  and  126  are the potential electrodes and the outer electrodes  22 ,  24 ,  122  and  124  are the ground electrodes. Here, the centers of the potential electrodes  26  and  126  are again disposed on opposite sides of axis  30  and offset from one another by a spacing which is substantially similar to the spacing between the waveguide arms  14  and  16 . Since the polarity of the electrodes is the same in both sets of electrodes, the optical axis in the area  130  of the substrate in which the bias set of electrodes is disposed should be reversed relative to the optical axis in the area of the substrate in which the RF set of electrodes is disposed. Various techniques can be applied to flip the optical polarity of a section of the substrate, e.g., electric field poling or proton exchange.  
         [0041]    The above-described exemplary embodiments are intended to be illustrative in all respects, rather than restrictive, of the present invention. Thus the present invention is capable of many variations in detailed implementation that can be derived from the description contained herein by a person skilled in the art. All such variations and modifications are considered to be within the scope and spirit of the present invention as defined by the following claims. No element, act, or instruction used in the description of the present application should be construed as critical or essential to the invention unless explicitly described as such. Also, as used herein, the article “a” is intended to include one or more items.