Abstract:
A semiconductor based electro-optic modulator comprising a capacitively coupled ground allowing for DC biasing of the modulator and a pre-distortion circuit for alleviating RF skin effect and for increasing bandwidth of modulator. Electrical components and functions of modulator partly located on an alumina pane. Reduction of thermally-induced stresses by connecting modulator ground to package ground via alumina pane is also disclosed.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates generally to electro-optic modulators. More particularly, the present invention relates to electro-optic modulators having extended bandwidths.  
       BACKGROUND OF THE INVENTION  
       [0002]     Electro-optic modulators are used in optical communications systems to rapidly modulate and optical signal in accordance with an electrical signal. In an electro-optic mode converter, a type of electro-optic modulator, an input beam of light having an input state of polarization (SOP) impinges on and traverses through an electro-optic waveguide that is subjected to an applied electric field. The applied electric field modifies the modal properties of the waveguide via the electro-optic effect. When the input SOP is not aligned with a principal axis of the waveguide, and the propagation speed of the light through the two principle axes differs, the beam of light at the output of the waveguide will generally have an output SOP different from the input one. With proper choice of input SOP, waveguide geometry and applied electrical field, it is possible to have the output SOP orthogonal to the input SOP. This allows the use of the electric field to control the mode conversion so that the output optical signal is modulated in accordance with a signal used to generate the electric field.  
         [0003]     Structurally, an electro-optic mode converter or a modulator will usually include an electro-optic guiding layer such as a III-V semiconductor or LiNbO 3 -type material, an optical waveguide defined in the optical guiding layer and an electrode structure disposed in the vicinity of the optical waveguide. As a voltage is applied to the electrodes, an electric field is formed across the optical waveguide and modifies the modal properties of the waveguide such as the orientation of its principal axes and/or its birefringence thus allowing for a modification of the SOP of a light beam traversing the optical waveguide. In the ideal, the principle axes are rotated to 45° from an X-Y orientation, and the birefringence of the axes is then modulated to control the phase relationship of the two fundamental hybrid optical modes. The phase relationship in turn determines the output SOP.  
         [0004]     U.S. Pat. No. 5,566,257, hereinafter referred to as &#39;257, issued Oct. 15, 1996 to Jaeger et al., which is incorporated herein by reference, discloses an electro-optic modulator having an electrode structure with two spaced apart conductive strips or electrodes disposed on either side of a single semiconductor optical waveguide. Each electrode includes projections, or fins, projecting towards the other conductive strip and disposed so as to affect the optical permittivity tensor of the waveguide material upon a voltage being applied to the electrodes. At the end of the projections, adjacent the waveguide, are pads, the geometry of which having an impact on the electrode structure capacitance.  
         [0005]     The electrode structure of &#39;257 is referred to as a “slow wave” electrode structure because it matches a microwave index to the optical index of the waveguide, so that a signal applied to the electrodes propagates through the signal path at the same velocity that the optical signal propagates through the waveguide. As a result, the optical signal can be cleanly modulated in accordance with the changing electric field generated by the application of a signal to the electrodes.  
         [0006]     The teachings of &#39;257 provide an electro-optic modulator requiring less electrical and optical power and capable of running at higher frequency than Mach-Zehnder type slow wave modulators such as described in U.S. Pat. No. 5,150,436, hereinafter referred to as &#39;436, issued Sep. 22, 1992 to Jaeger et al., which is incorporated herein by reference.  
         [0007]     For efficient operation, electro-optic mode modulators such as the one disclosed in &#39;257 usually require a bias voltage to adjust the operating point of the modulator. The bias voltage may be applied to the signal electrode with the ground electrode connected to the ground of the package housing the mode converter in order to achieve current return. This method of biasing requires that a DC blocking circuit be disposed at the input of the electrode structure in order to prevent excessive voltage due to the biasing voltage from appearing in the modulation driving circuit. Furthermore, the DC blocking circuit must be such that it does not affect the modulation signal across the operational bandwidth of the modulator.  
         [0008]     It would thus be desirable to have a mechanism for applying a biasing voltage to the mode converter that does not require a DC blocking circuit and that does not impair the operational bandwidth of the mode converter.  
         [0009]     Additionally, electro-optic modulators as disclosed in &#39;257 tend to have their electrodes exhibit a resistive loss of the electrical signal that increases as the frequency of the electrical signal is increased. This is due to the skin effect and leads to a substantial reduction of the electro-optic bandwidth of the modulator.  
         [0010]     Consequently, it would be desirable to have a mechanism for alleviating the frequency dependent skin effect.  
         [0011]     Another concern for electro-optic mode converters or modulators such as disclosed in &#39;257 relates to their grounding. In order for the mode converter or modulator to exhibit proper radio frequency (RF) behaviour, the ground electrode of the modulator should be in electrical contact with the ground of the modulator package and, the connection length between the modulator and package grounds should be as short as possible. Conventional methods of accomplishing this connection usually lead to the appearance of mechanical stress in the mode converter as the temperature of the package varies. Such stresses adversely affect the performance of the mode converter, and can induce an unwanted strain-optic effect in the waveguide that changes its known parameters.  
         [0012]     It would thus be desirable to have a grounding mechanism that provides proper RF behaviour and does not affect the mode converter performance as the temperature of the package housing the mode converter is varied.  
       SUMMARY OF THE INVENTION  
       [0013]     It is an object of the present invention to obviate or mitigate at least one disadvantage of previous electro-optic mode converters.  
         [0014]     In a first aspect of the present invention, there is provided an electro-optic modulator. The modulator comprises and AC coupled ground electrode, a signal path electrode and a waveguide. The signal path electrode carries an electrical signal, and is disposed substantially parallel to the AC coupled ground electrode. The waveguide is disposed between the signal path and AC coupled ground electrodes. The waveguide transmits an optical signal and modulates the polarization of the optical signal in accordance with an electric field generated between the signal path and AC coupled ground electrodes in response to the transmission of the electrical signal through the signal path electrode.  
         [0015]     In an embodiment of the first aspect of the present invention, the AC coupled ground electrode and the signal path electrode each include a series of projections, extending from an edge towards the waveguide, and each of the series of projections imparts a capacitance to the signal path. In another embodiment, the AC coupled ground electrode includes a plate capacitively coupled to ground by at least one capacitor, the capacitor optionally being formed by having the AC coupled ground electrode is connected to a plurality of ground plates by a dielectric, the connection through the dielectric forming a capacitor to capacitive couple the AC ground electrode to ground. In a further embodiment, the electro-optic modulator includes a biasing means coupled to the AC coupled ground electrode. The biasing means biases the AC coupled ground electrode, the bias level can be used to set an operating point for the modulator.  
         [0016]     In another embodiment, the signal path electrode includes a signal path input for receiving the voltage signal and a signal path terminal end having resistive termination to attenuate back reflections, where optionally the signal path electrode is disposed on an optical semiconductor chip and the signal path input and the signal path terminal end, are disposed on a separate chip and are connected to the signal path electrode by bond wires. In another embodiment of the present invention the waveguide is disposed on a semiconductor material, which is optionally Al x Ga 1-x As, x being selected from 0 and 1. In another embodiment, the electro-optic modulator includes a predistortion network connected to an input to the signal path electrode, the predistortion network for distorting signals transmitted across the signal path electrode to compensate for signal distortion in the signal path electrode, where optionally the predistortion network includes interconnected resistors and capacitors to distort the signals to compensate for skin effect resistive loss in the signal path electrode.  
         [0017]     In other embodiments, the AC coupled ground electrode is mounted on the surface of a first chip, and is capacitively coupled to a ground plate on the surface of a second chip, which is connected to a ground plane on the base of the second chip by at least one of vias and edge wrap around connections. In one embodiment, the AC coupled ground electrode is indirectly capacitively coupled to the ground plate.  
         [0018]     Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]     Embodiments of the present invention will now be described, by way of example only, with reference to the attached Figures, wherein:  
         [0020]      FIG. 1  is a perspective view of a preferred embodiment of the electro-optic modulator;  
         [0021]      FIG. 2  is a cross-sectional view of the electro-optic modulator of  FIG. 1 ;  
         [0022]      FIG. 3  is a cross-sectional view of the electro-optic modulator of  FIG. 1  depicting an insulator disposed between the chips of the modulator;  
         [0023]      FIG. 4  is a graph showing the improved performance of the electro-optic modulator of the present invention over a prior art modulator; and  
         [0024]      FIG. 5  is a cross-sectional view of the elector-optical modulator of  FIG. 1  depicting means for grounding the modulator. 
     
    
     DETAILED DESCRIPTION  
       [0025]     Generally, the present invention provides a system for extending the operational bandwidth of an opto-electronic polarization modulator.  
         [0026]     In the system of the present invention, an electrical signal path is used as an electrode, as is an AC coupled ground. As an electrical signal is applied to the electrical signal path, an electric field is generated between the electrodes. This electric field is used to modulate the polarization state of light input to the modulator. The AC coupled ground allows for the application of a DC biasing voltage, which allows easy selection of an operating point of the circuit. The AC coupling is achieved by capacitively coupling a DC biased plate to a ground plate. The physical connection can be a series of ground plates, on the same chip, or to a single ground plate, with a dielectric disposed between the two. This creates a distributed capacitance between the two plates, which appears substantially as a short circuit over the operational bandwidth of the modulator, and an open circuit to the applied DC voltage. As a result, a biasing voltage can be applied, without the difficulty experienced in the prior art. The use of a series of ground plates allows for a discretized distributed capacitance, so that in the event that one of the segments has the dielectric misapplied, the capacitive coupling is still maintained. Further enhancements, including the use of discrete large capacitors connected to the AC coupled ground allow the ground to have the desired characteristics while maintaining a wide frequency response band.  
         [0027]     A perspective view of a preferred embodiment of the present invention is shown in  FIG. 1 . There, electro-optic modulator  1  is shown as having a first chip  2 , which includes a semiconductor material and an electro-optic semiconductor-based waveguide  3 , disposed between a first electrode  4  and a second electrode  5 . The semiconductor-based waveguide  3  is preferably a ridge waveguide including Al x Ga 1-x As, x being between 0 and 1.  
         [0028]     As seen in  FIG. 2 , which is a cross-sectional view taken along line AB of  FIG. 1 , first electrode  4  is disposed atop an insulating layer  10 , which is overlapping grounding pads  11 . Grounding pads  11  may be disposed on an insulating buffer layer (not shown).  
         [0029]     Also depicted in  FIG. 1 , are first end  12  and second end  13  of the signal path electrode  5 . Additionally, first electrode  4  and second electrode  5  are shown as being substantially parallel to each other and to waveguide  3 . However, such parallelism is not necessary to practice the present invention.  
         [0030]     Continuing with  FIG. 1 , first electrode  4  and second electrode  5  are provided with a plurality of projections  14  extending from a side of the electrodes towards waveguide  3 . Projections  14  are for imparting a capacitance to modulator  1 . Some possible designs of projections  14  have been previously disclosed in &#39;257 and &#39;436 where the appellations “fins” and “fins” with “pads” are used instead of “projections”.  
         [0031]     Another chip, chip  15 , is shown disposed on the left hand side of chip  2  and is preferably made of an insulator material containing, for example, alumina i.e. Al 2 O 3 . Chip  15  includes conductive input segments  20   a  and  20   b,  the latter being in electrical contact with first end  12  of electrode  5 , the electrical contact being provided by one of conductive wires  21 , which are preferably gold wires. Disposed between conductive input segments  20   a  and  20   b,  and electrically coupled to conductive input segments  20   a  and  20   b,  is a pre-distortion circuit  22 , also referred to as a predistorer, which will typically include, but is not limited to, a resistor and a capacitor. Predistortion circuit  22 , as illustrated, is part of the presently preferred embodiment, but should not be viewed as essential.  
         [0032]     Chip  15  is also depicted as including conductive terminal segment  23  being in electrical contact with second end  13  of signal path electrode  5 , the electrical contact being provided by a bond wire. Connected between conductive terminal segment  23  and ground  24  is resistive termination  25 . One skilled in the art will appreciate that the signal path is resistively terminated to ensure that constant impedance is found on all parts of the signal path. Chip  15  includes ground electrode  30  which is in electrical contact with grounding pads  11  using a set of conductive wires  21 . Additionally, capacitors  31  are disposed atop ground electrode  30 , and can be formed by sandwiching insulating layer  32  between a top plate and the ground electrode  30  as shown in  FIG. 2 . Moreover, conductive bias pad  33  is disposed on chip  15  and is electrically coupled to first electrode  4  via one of conductive wires  21 .  
         [0033]     An additional feature of the present invention is depicted in  FIG. 3  where a space separating chips  2  and  15  includes a dielectric material  34  such as, for example, a benzocyclobutene-based polymer, for reducing impedance of conductive wires  21 . The application of the dielectric material  34  is preferably done in a manner that minimizes the stress imparted to chip  2 .  
         [0034]     Fabrication and micro-fabrication techniques for depositing patterns of conductive materials on insulator and semiconductor materials as well as techniques for depositing insulating layers on insulators, conductors and semiconductors together with other fabrication techniques used in the practice of the present invention are well known in the art of device fabrication and will not be described here.  
         [0035]     Further structural and operational features of electro-optic modulator I will now be described in relation to the operation of the modulator.  
         [0036]     A light beam  35 , having an input state of polarization (SOP) is provided to waveguide  3  at input port  40  and propagates through waveguide  3 . An electrical signal is applied to conductive input segment  20   a,  and is then carried to the signal path electrode  5  over a bond wire. Prior to passing to signal path electrode  5 , the input electrical signal is modified by predistorer  22  to compensate for anticipated distortion related to the skin effect resistive loss in the signal path. As a signal is carried along signal path electrode  5 , an electric field is generated between electrodes  4  and  5 , which resulting in a field across waveguide  3 . The electric field across waveguide  3  modifies the optical properties of waveguide  3  and affects one or both of the birefringence and the orientation of principal axes of the waveguide. Light beam  35  will thus generally have its initial SOP modulated in accordance with the input electrical signal as it propagates through waveguide  3 , exiting waveguide  3  at port  50 . Modulation of the voltage signal leads to a modulation of the electric field, which modulates the optical anisotropy of waveguide  3  thereby modulating the SOP of light beam  35 .  
         [0037]     The electrical signal applied to signal path electrode  5  may include a high frequency AC modulation extending up to and beyond 40 GHz. At such high frequencies, the electrodes are susceptible to the skin effect. As a result, high frequency AC current has a tendency to reside near the surface of signal path electrode  5 , which results in an effective augmentation of the resistive losses of modulator  1 . This decreases the effective bandwidth of the system. Pre-distortion circuit  22 , as previously described, can be used to intentionally distort the signal path to pre-emptively counter the skin effect. The performances of electro-optic modulator  1 , using a predistortion network  22 , and that of an electro-optic modulator not having a pre-distortion circuit are shown in  FIG. 4 .  
         [0038]      FIG. 4  presents a graph  51  plotting the electro optic frequency response, or electro-optic S 2 , as a function of frequency. Electro-optic S 21  is calculated as 20*log10(Vo/Vi) where Vo is the amplitude of the optical modulation as detected by an infinite bandwidth optical power detector and Vi is the amplitude of the electrical signal applied to conductive input segment  20   a.  The 3 dB bandwidth of modulators such as modulator  1  is determined by using their low frequency response as the 0 dB reference and then determining the frequency at which the response decreases by 3 dB.  FIG. 4  shows trace  52  measured for a modulator not having pre-distortion circuit  22 . In this case, the low frequency response is −0.5 dB and, therefore, the frequency at which the response has dropped by 3 dB is approximately 35 GHz. In the case of the response of modulator  1 , i.e. a modulator including pre-distortion circuit  22 , trace  53  shows a low frequency response of −1.5 dB. Therefore, the frequency at which the response has dropped by 3 dB is approximately 41 GHz. Thus, in the example of graph  51 , the presence of pre-distortion circuit  22  has improved the modulator bandwidth by approximately 6 GHz.  
         [0039]     The geometry of ground electrode  4  and signal path electrode  5  is central to the performance of modulator  1 . The geometry of electrodes  4  and  5  is preferably designed to match the phase velocity of the electrical signal with the group velocity of light beam  35  as it travels through waveguide  3 .  
         [0040]     Additionally, the voltage signal driving circuit (not shown) will have a nominal impedance, which is typically  50 n. The geometry of electrodes  4  and  5  is preferably designed to have a characteristic impedance matched to the voltage signal driving circuit. In order to avoid electrical back reflections from modulator I to the voltage driving circuit, resistive termination  25  is impedance-matched to the nominal impedance of the voltage signal driving circuit and electrodes  4  and  5 , and connected between conductive terminal segment  23  and ground  24 .  
         [0041]     In order to operate efficiently, electro-optic modulator  1  is usually required to function within an operating range attained through a DC bias voltage. The AC coupled ground electrode  4  can easily be DC biased to select an operating point for the modulator. In previous designs such as, for example, those disclosed in &#39;257, it is also possible to apply a DC bias to the signal electrode. However, as mentioned above, this requires that a DC blocking circuit be disposed at the electrical input of the modulator in order to prevent excessive voltage, resulting from the DC biasing voltage, from appearing in the modulation driving circuit. Furthermore, the DC blocking circuit must be designed so that it does not affect the modulation signal across the very wide operational bandwidth of the modulator.  
         [0042]     The present invention allows for the DC bias voltage to be applied to AC coupled ground electrode  4  by connecting a DC bias voltage source (not shown) to conductive bias pad  33 , which is in electrical contact with AC coupled ground electrode  4  via a bond wire. AC coupled ground electrode  4  is capacitively coupled to grounding pads  11  and is in electrical contact with capacitors  31  via conductive wires  21 . Grounding pads  11  are in turn in electrical contact with ground electrode  30  via conductive wires  21 . Furthermore, capacitors  31  capacitively couple the AC coupled ground electrode  4  to ground electrode  30 , which is ultimately connected to ground  24 . The use of discrete high capacitance capacitors  31  extend the operation bandwidth of the system to include lower frequencies.  
         [0043]     This manner of applying the DC bias voltage to modulator  1  provides a capacitively coupled ground, also referred to as an AC coupled ground, which alleviates the need for a DC blocking circuit at the electrical input of modulator  1 . Furthermore, the capacitors formed by AC coupled ground electrode  4  and grounding pads  11  can be made to have sufficient capacitance to provide an effective low impedance ground path at low frequencies and yet, provide low inductance current paths for currents flowing into and out of AC coupled ground  4 , the low inductance being important in order to maintain a low impedance at the high frequencies. Additionally, the disposition of the capacitors formed by AC coupled ground electrode  4  and grounding pads  11  along the transmission axis of the waveguide, i.e. along the line joining ports  40  and  50 , allow for a substantially constant impedance along waveguide  3 .  
         [0044]     Capacitors  31  formed between ground plates and ground electrode  30  modify behaviour of modulator  1  at low frequencies and will usually have higher capacitance values than those of the capacitors formed between ground electrode  4  and grounding pads  11 . This presently preferred feature provides a simple mechanism to extend the lower bandwidth of the circuit by providing a current path that appears as a low impedance at very low frequencies.  
         [0045]     In order to prevent unwanted electrical modes of propagation along AC coupled ground electrode  4  and signal path electrode  5  upon the modulator being packaged, ground electrode  30  can be electrically connected to a package ground through ground  24 . An illustration of such an embodiment is provided in  FIG. 5 , which illustrates a partial cross-sectional view of modulator  1 . Conductive through connection  60  is disposed in a bore through ground electrode  30  and the second chip  15 . Conductive through connection  60 , also referred to as a via, which is in physical contact with ground electrode  30  and lower ground electrode  30   b,  is fastened to conductive package  62  by a conductive attach material  64 , for example solder or a conductive adhesive, which is connected to ground  24 . A plurality of through connections  60  are disposed in a similar manner at a plurality of locations on ground electrode  30 . Alternatively, or in addition to conductive through connections  60 , ground electrode  30  may include an edge wrap around connection, such as metalized wall  63 , allowing electrical contact between ground electrode  30  and lower ground electrode  30   b.  Lower ground electrode  30   b,  also referred to as a ground plane, makes contact to conductive package  62  by a conductive attach material  64 .  
         [0046]     In addition to preventing unwanted electrical modes of propagation along first electrode  4  and second electrode  5 , the electrical connection mechanisms of ground electrode  30  to the ground  24  allow for a substantial reduction of temperature related effects on the performance of modulator  1  by reducing the need for the optical transmission chip  2  to be secured to the packaging in a manner that would apply stress to the chip under temperature changes. Void A prevents second attach material  64   b,  which may be conductive or non-conductive and used to secure chip  2  to conductive package  62 , from wicking upwards into void B during assembly, thereby reducing mechanical interaction (which may result from temperature changes) of chip  2  and second chip  15 . Reduced mechanical interaction-of chip  2  and second chip  15  facilitates maintaining a stable position of chip  2  relative to conductive package  62 , allowing position of waveguide  3  to be stable relative to conductive package  62 . Positional stability of waveguide  3  relative to conductive package  64  facilitates stable optical coupling into waveguide  3  at input port  40  by an input coupling system (not shown) and out of waveguide  3  at output port  50  by an output coupling system (not shown), both input and output coupling systems being in stable position relative to conductive package  62 .  
         [0047]     The structures and functions described with relation to chip  15  could be implemented in chip  2 . However, some of these structures and functions are easier and more economical to implement on an insulator material such as the one of chip  15  than it is on a semiconductor material described in chip  2 . As an example, implementing pre-distortion circuit  22  on chip  15  is more economical than it would be to implement it on chip  2 .  
         [0048]     The above-described embodiments of the present invention are intended to be examples only. Alterations, modifications and variations may be effected to the particular embodiments by those of skill in the art without departing from the scope of the invention, which is defined solely by the claims appended hereto.