Abstract:
An electro-optic Mach-Zehnder modulator includes a first optical waveguide forming a first arm of the Mach-Zehnder modulator, and a second optical waveguide forming a second arm thereof. The first or second optical waveguide includes capacitive segments that are spaced apart from one another, each forming an electrical capacitor. A travelling wave electrode arrangement applies a voltage across the first or second optical waveguide. The travelling wave electrode arrangement includes waveguide electrodes arranged on the capacitive segments , an electrical line extending along a part of the first or second optical waveguide, the electrical line being arranged a distance from the waveguide electrodes, and connecting arrangements, each being assigned to one of the waveguide electrodes. Each connecting arrangement includes at least two connecting structures spaced apart from one another wherein the waveguide electrodes each are electrically connected to the electrical line via the assigned two connecting structures.

Description:
CROSS-REFERENCE TO A RELATED APPLICATION 
       [0001]    This application is a National Phase Patent Application of International Patent Application Number PCT/EP2012/061817, filed on Jun. 20, 2012, which claims priority of European Patent Application Number 11170538.0, filed on Jun. 20, 2011. 
       BACKGROUND 
       [0002]    The invention relates to an electro-optic Mach-Zehnder modulator and a method for fabricating a Mach-Zehnder modulator. 
         [0003]    It is known in the art to use Mach-Zehnder interferometers as electro-optic modulators in optical high-speed transmission systems, wherein, for example, travelling wave electrodes (TWE) are used to induce a phase shift in the interferometer arms. Examples of such TWE-Mach-Zehnder modulators are described in the publication “High-Speed III-V Semiconductor Intensity Modulators”, Robert G. Walker, IEEE Journal of Quantum Electronics Vol. 27, No. 3, March 1991. 
       SUMMARY 
       [0004]    It is an object of the invention to provide an electro-optical Mach-Zehnder modulator having an improved frequency response. It is another object of the invention to provide a method of fabricating such a Mach-Zehnder modulator. 
         [0005]    According to an exemplary embodiment of the invention, an electro-optical Mach-Zehnder modulator is provided, comprising
       a first optical waveguide forming a first arm of the Mach-Zehnder modulator;   a second optical waveguide forming a second arm of the Mach-Zehnder modulator, wherein   the first and/or the second optical waveguide comprises a plurality of capacitive segments spaced apart from one another, the capacitive segments each forming an electrical capacitor; and   a travelling wave electrode arrangement for applying a voltage across the first and/or the second optical waveguide, the travelling wave electrode arrangement comprising
           a plurality of waveguide electrodes arranged on the capacitive segments;   at least one electrical line (e.g. a microwave line) extending at least partially along a part of the first and/or the second optical waveguide, the electrical line being arranged in a distance from the waveguide electrodes; and   a plurality of connecting arrangements, wherein a connecting arrangement is assigned to each of the waveguide electrodes, the connecting arrangements each comprising at least two connecting structures spaced apart from one another or a single wide connecting structure having a width of at least 20% of the length of the assigned waveguide electrode, wherein the width of the wide connecting structure and the length of the waveguide electrode are measured along the first and/or the second optical waveguide, and wherein the waveguide electrodes each are electrically connected to the electrical line via the assigned two connecting structures or the assigned single wide connecting structure, respectively.   
               
 
         [0013]    The capacitive segments of the first and/or the second optical waveguide provide a capacitive load such that the travelling wave electrode arrangement is capacitively loaded. The waveguide electrodes and thus the capacitive segments of the first and the second optical waveguide are connected to the electrical line, wherein the connection between each waveguide electrode and the electrical line is established either via two connecting structures assigned to the respective waveguide electrode or via a single wide connecting structure assigned to the waveguide electrode. Conventional Mach-Zehnder modulators using capacitively loaded travelling wave electrodes comprise single small connecting structures (in the form of contact bridges) for connecting the waveguide electrodes to the electrical line. 
         [0014]    Using two connecting structures (e.g. in the form of contact bridges) and/or a wide connecting structure (e.g. a wide contact bridge) for connecting the waveguide electrodes to the electrical line modifies the electrical behaviour of a travelling wave electrode arrangement in that in addition to the capacitive load induced by the capacitive segments of the waveguides an inductive component is introduced. The additional inductive component can be for example used to shape the frequency behaviour of the electrical effective index of the travelling wave electrode arrangement in such a way that it either remains essentially constant over a pre-determined frequency range or decreases over a pre-determined frequency range. Further, the additional inductive component can be used to adapt the impedance of the travelling wave electrode arrangement as will be set forth further below. 
         [0015]    More particularly, using at least two connecting structures (e.g. a pair of connecting structures) or a single wide connecting structure per waveguide electrode, the effective microwave index of the travelling wave electrode arrangement can be adapted in such a way that it at least substantially equals the optical effective index of the first or the second optical waveguide, wherein the capacitive load of the electrodes may remain unchanged. For example, the pairs of connecting structures and/or the wide connecting structure are designed in such a way that the electrical effective index is 3.6 over a predetermined frequency range, e.g. from 20 to 40 GHz. 
         [0016]    Adapting the effective electrical index of the travelling wave electrode arrangement to the optical index of the waveguides has the effect that the velocity of the electrical field matches the velocity of the optical field such that an effective interaction between the electrical and the optical field can be achieved over a broad frequency range. The effective interaction between the electrical and the optical field in turn results in a high electro-optic bandwidth and may allow to use a lower driving voltage and/or shorter interferometer arms, i.e. shorter devices. 
         [0017]    As already mentioned above, the at least two connecting structures and/or the single wide connecting structure can be formed as contact bridges. For example, the electrical line is separated from the first and/or the second optical waveguide by an isolation groove, wherein the two connecting structures and/or the single wide connecting structure extend over the isolation groove. 
         [0018]    It is possible that some of the waveguide electrodes are connected to the electrical line via at least two connecting structures and other waveguide electrodes each are connected to the electrical line via a single wide connecting structure. However, it is also possible that identical connecting arrangements are assigned to the waveguide electrodes, i.e. each one of the waveguide electrodes is connected to the electrical line via two connecting structures or each one of the waveguide electrodes is connected to the electrical line via a single wide connecting structure. It is further noted that at least one of the connecting arrangements comprises exactly two connecting structures. However, it is also conceivable that at least one of the connecting arrangements comprises more than two connecting structures. 
         [0019]    If at least one of the waveguide electrodes is connected to the electrical line via a wide connecting structure, the inductive component introduced by the wide connecting structure is adapted by choosing the width—measured in a direction along the first and/or the second optical waveguide—of the wide connecting structure (or of the plurality of wide connecting structures if several or all connecting arrangements comprise a wide connecting structure) in such a way that the electrical effective index of the travelling wave electrode arrangement is substantially constant or decreases over at least a frequency range from 20 to 40 GHz. Similarly, if at least one waveguide electrode is connected via two connecting structures to the electrical line, the two connecting structures are configured and arranged (e.g. by adapting the width and/or the position of the connecting structures) in such a way that the electrical effective index of the travelling wave electrode arrangement is substantially constant or decreases over at least a frequency range from approximately 20 to 40 GHz. 
         [0020]    In an exemplary embodiment of the invention, the width of the wide connecting structure(s) (and/or the width and/or the location of the two connecting structures) is chosen in such a way or or the two connecting structures are configured and arranged in such a way that the electrical effective index of the travelling wave electrode arrangement is essentially constant or decreases over at least a frequency range from approximately 10 to 60 GHz. 
         [0021]    For example, the wide connecting structure—if present—has a width of at least one third or one half of the length—measured along the first and/or the second optical waveguide—of the assigned waveguide electrode. For example, the wide connecting structure has a width of at least 20 μm or at least 50 μm. In another embodiment, the width of wide connecting structure is at least 90% of the length—measured along the first and/or the second optical wave guide—of the assigned waveguide electrode or substantially equals the length of the assigned waveguide electrode. For example, the wide connecting structure has a width of at least 150 μm or at least 200 μm. If several or all connecting arrangements consist of a wide connecting structure, the wide connecting structures may have different widths. However, it is, of course, also possible that all of the provided wide connecting structures have the same width. 
         [0022]    Further, a plurality of connecting arrangements may comprise a wide connecting structure such that the Mach-Zehnder modulator comprises a plurality of wide connecting structures, wherein the wide connecting structures are arranged in a distance from one another (along the direction of the modulator, i.e. along the first and/or second optical waveguide) such that a gap is formed between them. 
         [0023]    However, it is also conceivable that the Mach-Zehnder modulator comprises a plurality of wide connecting structures, wherein the wide connecting structures are integrally connected to one another. For example, the plurality of wide connecting structures is formed by a continuous contact structure extending along the first and/or the second optical waveguide. In particular, a plurality of subsections (having the width of the assigned waveguide electrode) of the continuous contact structure form the plurality of wide connecting structures, wherein the subsections are integrally connected to one another via portions of the continuous contact structure. 
         [0024]    In other words, the plurality of wide connecting structures is formed by a single continuous structure that, for example, overlaps with the first or the second optical waveguide. For example, the continuous contact structure extends over a groove between the electrical line and the waveguide electrodes (i.e. between the electrical line and the capacitive segments of the waveguides) and/or over a space between neighbouring waveguide electrodes, wherein the groove and/or the space between neighbouring waveguide electrodes may be filled with an electrically isolating material such as a BCB (benzocyclobutene). 
         [0025]    It is further possible that both the first and the second optical waveguide comprise capacitive segments. Further, a first and a second electrical line can be provided, wherein the first electrical line is connected to the waveguide electrodes arranged on the capacitive segments of the first optical waveguide and the second electrical line is connected to the waveguide electrodes arranged on the capacitive segments of the second optical waveguide. For example, the electrical lines are coplanar lines arranged coplanar to the waveguide electrodes. 
         [0026]    However, it is also conceivable that either the first or the second optical waveguide does not comprise capacitive segments such that only one arm of the Mach-Zehnder modulator will be modulated, i.e. a voltage is applied to only one arm of the modulator. 
         [0027]    Further more, if at least one of the capacitive segments is connected to the electrical line via two connecting structures (a first and a second connecting structure) the first connecting structure can be connected to a first end of the waveguide electrode and the second connecting structure can be connected to a second end of the waveguide electrode. For example, a lateral side (extending perpendicular relative to the first and the second optical waveguide) of the first connecting structure is aligned with a face side of the waveguide electrode. 
         [0028]    However, it is also possible that the two connecting structures are arranged within a certain distance from the ends of the waveguide electrode. For example, the two connecting structures are placed in such a way that the distance between the respective end of the waveguide electrode and the connecting structures does not exceed a tenth of the length of the waveguide electrode. For example, the distance between the connecting structure and the respective waveguide electrode end is less than 20 μm. 
         [0029]    According to another example of the invention, the distance between the first and the second connecting structure is at least half of the length—measured along the first and/or the second optical waveguide—of the assigned waveguide electrode or at least two thirds of the length of the assigned waveguide electrode. 
         [0030]    The width and the location (e.g. the distance between the two connecting structures) can be used to adapt both the electrical effective index and the impedance of the travelling wave electrode arrangement. Similarly, the width of the wide connecting structure may have a considerable influence on both the electrical effective index and the impedance of the travelling wave electrode arrangement. 
         [0031]    In addition to the design of the two connecting structures and/or the width of the wide connecting structure(s), however, other means can be used for adapting the impedance of the travelling wave electrode arrangement. For example, the width—measured perpendicular to the first and/or the second optical waveguide—of the electrical line can be varied, wherein the width may be chosen in such a way that the impedance neither exceeds a pre-determined maximum nor drops below a pre-determined minimum, for example at least in the frequency range from 20 to 40 GHz. For example, the impedance lies within the range from 45 and 50 Ω or is substantially 50 Ω in this frequency range. 
         [0032]    The Mach-Zehnder modulator according to the invention has an input section and/or an output section that comprises for example, a Y-branch splitter or a multimode interference coupler (MMI coupler) connected to the first and the second optical waveguide. Further, the input section and/or the output section of the modulator may comprise a spot size converter for matching the input/output optical mode of the modulator to the mode of an optical fibre. An example of a possible principal optical and electrical layout of the Mach-Zehnder modulator according to the invention is described in the publication “45 GHz Bandwidth Travelling Wave Electrode Mach-Zehnder Modulator with Integrated Spot Size Converter”, D. Hoffmann, Proceedings International Conference on Indium Phosphide and Related Materials, p. 585, 2004, wherein the content of this article in respect to the optical and electrical design of the Mach-Zehnder modulator is incorporated by reference herewith. It is noted, however, that the invention is of course not restricted to a particular basic design of the Mach-Zehnder modulator. For example, the optical waveguides could also be formed as strongly guiding (“deep ridge”) waveguides as disclosed in U.S. patent application Ser. No. 12/638,372 (published as 2010/0150494 A1), which is also incorporated by reference herewith. 
         [0033]    The Mach-Zehnder modulator according to the invention is, for example, formed as a semiconductor device. In particular, the Mach-Zehnder modulator is fabricated using indium phosphide or gallium arsenide technology, i.e. the modulator is fabricated on an indium phosphide or a gallium arsenide substrate. 
         [0034]    For example, the capacitive segments of the modulator arms comprise a p-doped layer, wherein the p-doped layer can be removed between adjacent capacitive segments (the active sections of the waveguides) in order to reduce the optical loss in the waveguide sections between the capacitive segments (i.e. in the passive sections of the waveguides). It is possible to replace the removed p-doped sections between the capacitive segments by another material, e.g. by intrinsic or semi-insulating material, for example using the “selective area growth” technique as described in U.S. patent application Ser. No. 12/638,372 already mentioned above. 
         [0035]    The p-doped layer in the capacitive segments may form part of a p-i-n-diode, wherein the isolating region of the p-i-n-diode is formed by at least one isolating layer arranged between the p-doped layer and an n-doped layer or an n-doped substrate. It is, however, also possible that other kinds of diodes are realized by the capacitive segments such as n-i-n diodes and/or Schottky diodes. 
         [0036]    The invention also relates to a method of fabricating an electro-optic Mach-Zehnder modulator comprising the steps of:
       providing a first optical waveguide forming a first arm of the Mach-Zehnder modulator;   providing a second optical waveguide forming a second arm of the Mach-Zehnder modulator;   forming a plurality of capacitive segments of the first and/or the second optical waveguide, the capacitive segments being spaced apart from one another and each forming an electrical capacitor; and   providing a travelling wave electrode arrangement for applying a voltage across the first and/or the second optical waveguide, comprising   arranging a plurality of waveguide electrodes on the capacitive segments;   forming an electrical line extending along at least a part of the first and/or the second optical waveguide, the electrical line being arranged in a distance from the waveguide electrodes,   forming at least two connecting structures or a single wide connecting structure for each one of the waveguide electrodes in such a way that the waveguide electrodes are electrically connected to the electrical line via the at least two connecting structures or the single wide connecting structure, respectively, wherein the width of the wide connecting structure is chosen to be at least 20% of the length of the assigned waveguide electrode, the width of the wide connecting structure and the length of the waveguide electrode being measured along the first and/or the second optical waveguide.       
 
         [0044]    In an exemplary embodiment the method comprises pre-determining (pre-setting) a desired electrical effective index and/or a desired impedance of the travelling wave electrode arrangement, and choosing the width of the wide connecting structure dependent on the pre-determined (pre-set) electrical effective index (e.g. an electrical effective index range in a pre-determined frequency range) and/or desired impedance (e.g. an impedance range in a pre-determined frequency range) of the travelling wave electrode arrangement. 
         [0045]    In other words, the width of the wide connecting structure(s)—if present—can be used as a design parameter to meet requirements of the electrical design of the Mach-Zehnder modulator (as already explained above). For example, the width of the wide connecting structure(s) is varied (e.g. using a simulation, i.e. a model of the travelling wave electrode arrangement) until the desired electrical effective index range and/or the desired impedance range is met. This embodiment could also be used without specifying a particular minimal width (such as at least 20% of the length of the waveguides) of the wide connecting structure. 
         [0046]    Similarly, the design (width, number and/or position) of the at least two connecting structures can be chosen dependent on the pre-determined electrical effective index and/or impedance of the travelling wave electrode arrangement. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0047]    Embodiments of the invention will be described in more detail hereinafter with reference to the drawings. 
           [0048]      FIG. 1  illustrates a top view of an electro-optic Mach-Zehnder modulator according to an embodiment of the invention. 
           [0049]      FIG. 2  shows a detail of a conventional Mach-Zehnder modulator. 
           [0050]      FIG. 3  shows a detail of the Mach-Zehnder modulator illustrated in  FIG. 1 . 
           [0051]      FIG. 4  shows a detail of a Mach-Zehnder according to another embodiment of the invention. 
           [0052]      FIG. 5  depicts the frequency behaviour of an electrical effective index of the travelling wave electrode arrangement of different Mach-Zehnder modulators. 
           [0053]      FIG. 6  shows the frequency behaviour of the impedance of a travelling wave electrode arrangement of different Mach-Zehnder modulators. 
           [0054]      FIG. 7  shows the electro-optic frequency response of different Mach-Zehnder modulators. 
           [0055]      FIG. 8  shows a perspective view of a Mach-Zehnder modulator according to another embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0056]    The Mach-Zehnder modulator  1  according to an embodiment of the invention illustrated in  FIGS. 1 and 3  comprises a first and a second integrated optical waveguide  11 ,  12 , wherein the first and the second optical waveguides  11 ,  12  form the arms of an interferometer section of the Mach-Zehnder modulator. 
         [0057]    An input section of the Mach-Zehnder modulator  1  comprises an input waveguide  13  connected to a Y-coupler  14  whose branches are connected to the first and the second optical waveguide  11 ,  12 . Further, a spot size converter  15  is provided for reducing the coupling loss between an optical fibre (not shown) and the input waveguide  13 . An output section of the Mach-Zehnder modulator  1  comprises a multimode interference coupler (MMI)  16  connected to output ends of the first and second optical waveguide  11 ,  12 , wherein two output waveguides  17   a ,  17   b  are provided connected the MMI  16  and to spot size converters  18   a ,  18   b.    
         [0058]    Further, the Mach-Zehnder modulator  1  comprises a travelling wave electrode arrangement  2  having a plurality of waveguide electrodes  211 ,  212  arranged on top of capacitive segments  111 ,  121  (active waveguide sections) formed by the first and the second optical waveguides  11 ,  12 , respectively. The travelling wave electrode arrangement  2  further comprises a first (U-shaped) electrical line in the form of a first microwave line  31  and a second electrical line in the form of a second microwave line  32 , wherein a middle section of the first microwave line  31  and the whole second microwave line  32  extend parallel to the first and the second optical waveguides  11 ,  12  and substantially coplanar to the waveguide electrodes  211 ,  212 . It is noted that the U-shaped design of the first microwave line  31  is of course only exemplary. Other designs are also possible, for example, the first microwave line  31  could be a straight line or may have a first ending that crosses the optical waveguides  11 ,  12  and second ending that extends away from the optical waveguides  11 ,  12 . 
         [0059]    The microwave lines  31 ,  32  are disposed in a distance from the first and second optical waveguide  11 ,  12  and thus in a distance from the waveguide electrodes  211 ,  212 . Between the first and second optical waveguide  11 ,  12  and the microwave lines  31 ,  32  isolation grooves  41 ,  42  are arranged, the isolation grooves  41 ,  42  providing electrical isolation between the waveguide electrodes  211 ,  212  and the microwave lines  31 ,  32  (see  FIG. 3 ). 
         [0060]    A high frequency voltage is supplied to a first end (facing towards the input section of the modulator) of the microwave lines  31 ,  32 , whereas a second end of the microwave lines  31 ,  32  is terminated using for example a 50 Ω resistor. The basic optical and electrical layout of the Mach-Zehnder modulator  1  is described in the publication “45 GHz Bandwidth Travelling Wave Electrode Mach-Zehnder Modulator with Integrated Spot Size Converter”, D. Hoffmann, Proceedings International Conference on Indium Phosphide and Related Materials, p. 585, 2004, already mentioned above. However, as already set forth above, the invention is not restricted to a particular basic optical or electrical design of the Mach-Zehnder modulator. For example, the basic optical or electrical design of the modulator could also follow the modulator layout described in U.S. patent application Ser. No. 12/638,372. 
         [0061]    Each one of the waveguide electrodes  211  arranged on the first optical waveguide  11  is integrally connected to the first microwave line  31  via a connecting arrangement consisting of a pair of connecting structures in the form of contact bridges  311 ,  312 . Similarly, each one of the waveguide electrodes  212  arranged on the second optical waveguide  12  is integrally connected to the second microwave line  32  via a pair of contact bridges  321 ,  322  (see also  FIG. 3  showing in detail the pairs of contact bridges). The isolation grooves  41 ,  42  and/or the space between the waveguide electrodes  211 ,  212  could be filled with an insulating material such as BCB. 
         [0062]    The contact bridges  311 ,  312  and  321 ,  322 , respectively, are each positioned near endings  2111 ,  2112  and  2121 ,  2122  of the waveguide electrodes  211 ,  212  such that the distance between the contact bridges  311 ,  312  and  321 ,  322 , respectively, compares to the length of the waveguide electrodes  211 ,  212 ; e.g. the distance between the contact bridges is at least 80% or 90% of the length of the assigned waveguide electrode. It is noted that the waveguide electrodes  211 ,  212  do not necessarily extend over the complete length of the capacitive sections  111 ,  121 . Rather, it is possible that the capacitive sections  111 ,  121  are longer and extend beyond the ends of the assigned waveguide electrodes. 
         [0063]    The pairs of contact bridges  311 ,  312  and  321 ,  322 , respectively, introduce an inductive component (in addition to the capacitive load induced by the capacitive waveguide segments  111 ,  121 ), that can improve the frequency characteristics of the electrical effective index of the travelling wave electrode arrangement  2  and/or the frequency characteristics of the impedance of the travelling wave arrangement  2  (see  FIGS. 5 and 6 ). In order to optimize the frequency characteristics of the electrical effective index and/or the impedance of the travelling wave arrangement the distance between the contact bridges  311 ,  312  and  321 ,  322 , respectively, and the width (in the direction of the waveguides  11 ,  12 ) can be varied. 
         [0064]    For example, the width of the contact bridges  311 ,  312  and  321 ,  322  compares to the width of a contact bridge used in a conventional Mach-Zehnder modulator (comprising small single contact bridges for connecting the waveguide electrodes to the microwave lines as shown in  FIG. 2 ). For example, the width of the contact bridges  311 ,  312  and  321 ,  322  is in the range between 2 and 10 μm. 
         [0065]    According to another embodiment of the invention, instead of pairs of contact bridges  311 ,  312  and  321 ,  322  a single wide electrode could be used as illustrated in  FIG. 4 . According to  FIG. 4 , wide connecting structures in the form of continuous wide contact bridges  313 ,  323  are used to connect the first microwave line  31  to the waveguide electrodes  211  on the capacitive segment  111  of the first optical waveguide  11  and to connect the second microwave line  32  to the waveguide electrode  212  on top of the capacitive segment  121  of the second optical waveguide  12 , respectively. 
         [0066]    The width—measured along the longitudinal direction of the first and second optical waveguide  11 ,  12 —of the wide contact bridges  313 ,  323  is only slightly shorter than the length of the waveguide electrodes  211 ,  212 . It is also possible that the width of the contact bridges  313 ,  323  is identical to the length of the waveguide electrodes  211 ,  212 . 
         [0067]    The microwave line  31 , the wide contact bridge  313  and the waveguide electrode  211  may be integrally formed. Similarly, the other microwave line  32 , the wide contact bridge  323  and the waveguide electrode  212  may be integrally formed, wherein the microwave lines  31 ,  32 , the wide contact bridges  313 ,  323  and the waveguide electrodes  211 ,  212  may be generated by electroplating. 
         [0068]    The wide contact bridges  313 ,  323  similarly to the contact bridge pairs  311 ,  312  and  321 ,  322  allow to shape the effective microwave index (effective electrical index) of the travelling wave electrode arrangement  2  in a pre-determined frequency range. For example, the wide contact bridges  313 ,  323  are designed in such a way that the effective microwave index is substantially constant in a pre-determined frequency range (e.g. 20 to 40 GHz) or decreases in that frequency range. The frequency behavior of the microwave effective index of the travelling wave electrode arrangement comprising wide contact bridges as illustrated in  FIG. 4  and pairs of contact bridges as shown in  FIG. 3 , respectively, is shown in  FIG. 5 . 
         [0069]    Referring to  FIG. 5  (curve A), if each of the waveguide electrodes is connected to the respective microwave line via a pair of (e.g. conventionally sized) contact bridges as shown in  FIG. 1  (“two contact bridges design”), the microwave effective index at lower frequencies drops and is substantially constant at higher frequencies (e.g. in the range between 20 to 40 GHz). The index slightly raises again at higher frequencies (above 40 GHz). If all of the waveguide electrodes are connected via a single wide contact bridge as shown in  FIG. 4  (“wide contact bridge design”), the microwave effective index decreases over the range between 0 to 60 GHz (see curve B), wherein it shows a similar drop at lower frequencies (below 5 GHz) as curve A. 
         [0070]    It is noted that the frequency behavior of the microwave effective index will also be influenced by the design of the coplanar microwave lines  31 ,  32  (in particular by the width perpendicular to the optical waveguides  11 ,  12  of the microwave lines  11 ,  12 ). Therefore, the frequency behavior of the microwave effective index can be optimized by adapting the design of the microwave lines  31 ,  32 . 
         [0071]    Curve C also shown in  FIG. 5  relates to a conventionally designed travelling wave electrode arrangement (as shown in  FIG. 2 ) used in a conventional Mach-Zehnder modulator. The microwave effective index of such a travelling wave electrode arrangement increases over the frequency range between 10 and 60 GHz and thus especially at higher frequencies will not be adapted to the (substantially constant) optical index resulting in a reduced electro-optic bandwidth, c.f.  FIG. 7 . 
         [0072]      FIG. 6  illustrates that using the two contact bridges design or the wide contact bridge design according to the invention, may also flatten the frequency response of the impedance of the travelling wave electrode arrangement. For example, it is possible to obtain an impedance which in the range between approximately  10  and approximately 40 GHz lies between 45 and 50 Ω (curve D: two contact bridges design, curve E: wide contact bridge design). The impedance of a conventional electrode arrangement (single small contact bridges) represented by curve F decreases more strongly in the higher frequency range such that the conventional electrode arrangement can be adapted to a pre-determined impedance (e.g. the standard 50 Ω impedance) only within a small frequency range. For example, if the impedance is adapted within a lower frequency range, a reduction of the electrical power and reflections and thus frequency dependent power drops will occur in the higher frequency range due to the lower impedance in that frequency region. 
         [0073]      FIG. 7  shows the electro-optical behaviour of different Mach-Zehnder modulators having travelling wave electrode arrangement comprising the two contact bridges design (curve G), the wide contact bridge design (curve H) or the conventional design (curve I). The Mach-Zehnder modulators according to the invention, i.e. comprising the two contact bridges design (curve G) and the wide contact bridge design (curve H), respectively, show a significantly higher electro-optical bandwidth than a conventional modulator (curve I). In fact, the invention allows to nearly double the electro-optical bandwidth, although other properties of the modulator such as insertion loss and driving voltage may remain unchanged. 
         [0074]      FIG. 8  shows a perspective view of a Mach-Zehnder modulator  1  according to another embodiment of the invention. The capacitive waveguide segments  111 ,  121  (active waveguide segments) comprise a p-doped region  1111 ,  1211 , an isolating region  1112 ,  1212  and an n-doped region  1113 ,  1213  such that p-i-n diodes are formed that provide the capacitive load of the travelling wave electrode arrangement. The p-doped region is at least partially removed between adjacent capacitive waveguide segments such that passive waveguide segments  113  are formed. 
         [0075]    Further, the plurality of capacitive segments  111 ,  121  of the optical waveguides  11 ,  12  are connected to the first and the second microwave lines  31 ,  32  via a plurality of wide contact bridges  313 ,  323  that extend over the whole length of the capacitive segments  111 ,  121 , i.e. over the whole length of the waveguide electrodes  211 ,  212  arranged on top of the capacitive segments  111 ,  121 . 
         [0076]    The contact bridges  313 ,  323  connected to the waveguide electrodes  211 ,  212  are formed by continuous contact bridges  300 ,  300 ′ (contact structure) integrally formed with the waveguide electrodes  211 ,  212 , respectively, wherein the continuous contact bridges  300 ,  300 ′ extend along the first and the second optical wave guide  11 ,  12  and over both the capacitive segments  111 ,  121 , respectively, and the passive waveguide segments  113 . The gap between the passive waveguide segments  113  and the continuous contact bridge  300  could be filled with an isolating material such as BCB. The contact bridges  313 ,  323  are thus integrally connected to one another, wherein they are formed by subsections of the continuous contact bridges  300 ,  300 ′, respectively. These subsections are connected to one another by portions of the continuous contact bridges  300 ,  300 ′ between the contact bridges  313 ,  323 . 
         [0077]    It is noted that in another embodiment of the invention the passive waveguide segments  113  after removing the p-doped region can be re-grown using e.g. intrinsic (undoped) or semi-insulating material, e.g. by selective area growth as already mentioned above. An isolating material could be arranged between the re-grown passive waveguide segments and the continuous contact bridge  300 . Also, the capacitive waveguide segments  111 ,  121  do not necessarily form a p-i-n diode. Rather, other diode types such as n-i-n diodes could be formed. 
         [0078]    It is noted that features of the embodiments shown in  FIGS. 1 ,  3 ,  4  and  8  could also be used in combination. For example, some of the contact bridge pairs  311 ,  312  or  321 ,  322  in  FIG. 1  could be substituted by wide contact bridges  313 ,  323  as shown in  FIG. 4  or a continuous contact bridge  300  as depicted in  FIG. 8 . 
       Reference Signs 
       [0000]    
       
           1  Mach-Zehnder modulator 
           2  travelling wave electrode arrangement 
           11  first optical waveguide 
           12  second optical waveguide 
           13  input waveguide 
           14  Y-splitter 
           15 ,  18   a ,  18   b  spot size converter 
           16  MMI 
           17   a ,  17   b  output waveguides 
           31  first microwave line 
           32  second microwave line 
           41 ,  42  isolation groove 
           111 ,  121  capacitive segment 
           113  passive waveguide segment 
           211 ,  212  waveguide electrode 
           300 ,  300 ′ continuous contact bridge 
           311 ,  312 ,  321 ,  322  contact bridge 
           313 ,  323  wide contact bridge 
           1111 ,  1211  p-doped region 
           1112 ,  1212  isolating region 
           1113 ,  1213  n-doped region 
           2111 ,  2112  end of waveguide electrode 
           2121 ,  2122  end of waveguide electrode