Abstract:
Various embodiments described herein comprises an optoelectronic device comprising a waveguide structure including a plurality of optical modulator elements each having an optical property that is adjustable upon application of an electrical signal so as to modulate light guided in the waveguide structure. The optoelectronic device also comprises a plurality of amplifiers in distributed fashion. Each amplifier is electrically coupled to one of the optical modulators to apply electrical signals to the optical modulator.

Description:
PRIORITY APPLICATIONS 
       [0001]    This application is a divisional of U.S. application Ser. No. 11/540,172, filed on Sep. 29, 2006 and entitled “DISTRIBUTED AMPLIFIER OPTICAL MODULATOR” which is incorporated herein by reference in its entirety, and which is a continuation-in-part of U.S. application Ser. No. 11/363,512, filed on Feb. 27, 2006, which is in turn a continuation of U.S. application Ser. No. 10/917,927, filed on Aug. 13, 2004 and which is incorporated by reference in its entirety, which claims priority under 35 U.S.C. § 119(e) to U.S. Provisional Application Ser. No. 60/495,402, No. 60/495,403, and No. 60/495,404 filed on Aug. 15, 2003. 
     
    
     BACKGROUND 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates generally to the field of optical modulators and more particularly to a distributed amplifier optical modulator for use in optoelectronic integrated circuits. 
         [0004]    2. Description of the Related Art 
         [0005]    Optical fibers have been widely used for the propagation of optical signals, especially to provide high speed communication links. Optical links using fiber optics have many advantages compared to electrical links. These advantages include large bandwidth, high noise immunity, reduced power dissipation, and reduced crosstalk. Fiber optic communication links can operate with carrier frequencies in the THz range. In communication systems where optical fibers are used to transport optical communication signals, various optoelectronic devices are used to control, modify, and process the optical signals. 
         [0006]    An integrated optical modulator is one such component of an optical communication system. Certain optical modulators use an electrical signal to modulate some property of an optical wave, such as the phase or the amplitude. A modulated optical wave can be sent on a fiber optic link or processed by other optical or optoelectronic devices. 
         [0007]    Integrated optoelectronic devices made of silicon are highly desirable since they can be fabricated in the same foundries used to make VLSI integrated circuits. Optoelectronic devices integrated with their associated electronic circuits can eliminate the need for more expensive hybrid optoelectronic circuits. Optoelectronic devices built using a standard CMOS process have many advantages, including high yields, low fabrication costs, and continuous process improvements. 
         [0008]    Distributed amplifier optical modulators have been suggested that use a plurality, e.g., k modulators connected in series disposed on one of a variety of semiconductor substrates or wafers. An electrical signal propagating in a microwave transmission line in such distributed amplifier modulators is tapped off of the transmission line at regular intervals and is amplified by k distributed amplifiers which output the electrical signal to a respective one of the k modulators. These distributed amplifier modulators can have much higher modulating speeds than a comparable single lumped element modulator, as the distributed arrangement allow the input capacitance of the distributed amplifier blocks to be absorbed into the characteristic impedance of the input transmission line. The distributed amplifier modulators driving a plurality of lumped modulator elements can also have lower power consumption than comparable traveling wave modulators having a terminated output transmission line because no power is dissipated in the termination resistance of the output transmission line. However, the speed, performance reliability, and the size of the distributed amplifier modulators can be improved. 
       SUMMARY 
       [0009]    One embodiment of the invention comprises an optoelectronic device. The optoelectronic device comprises a waveguide structure including a plurality of optical modulator elements each having an optical property that is adjustable upon application of an electrical signal so as to modulate light guided in said waveguide structure, and a plurality of amplifiers, each amplifier electrically coupled to one of said optical modulators to apply electrical signals thereto. At least one of the plurality of amplifiers may comprise one or more first type transistors electrically connected between a first reference potential and an output node and one or more second type transistors electrically connected between said output node and a second reference potential, said first and second type transistors being complementary. 
         [0010]    Another embodiment of the invention comprises an optoelectronic device. The optoelectronic device comprises a waveguide structure including a plurality of optical modulator elements each having an optical property that is adjustable upon application of an electrical signal so as to modulate light guided in the waveguide structure. The optoelectronic device also comprises a plurality of amplifiers wherein each amplifier is electrically coupled to one of the optical modulators to apply electrical signals to the optical modulator. At least one of the plurality of amplifiers comprises a single pair of complementary transistors. 
         [0011]    Another embodiment of the invention comprises an optoelectronic device. The optoelectronic device comprises a waveguide structure including a plurality of optical modulator elements each having an optical property that is adjustable upon application of an electrical signal so as to modulate light guided in the waveguide structure. The optoelectronic device also comprises a plurality of amplifiers wherein each amplifier is electrically coupled to one of the optical modulators to apply electrical signals to the optical modulator. At least one of the plurality of amplifiers comprises bandwidth extension circuitry. 
         [0012]    Another embodiment of the invention comprises an optoelectronic device. The optoelectronic device comprises a waveguide structure including a plurality of optical modulator elements each having an optical property that is adjustable upon application of an electrical signal so as to modulate light guided in the waveguide structure. The optoelectronic device further comprises a capacitive degeneration bypass, a negative resistance element, or a transformer to increase bandwidth. The optoelectronic device also comprises plurality of amplifiers wherein each amplifier is electrically coupled to one of the optical modulators to apply electrical signals to the optical modulator and at least one of the plurality of amplifiers comprises a push-pull driver circuit. 
         [0013]    Another embodiment of the invention comprises an optoelectronic device. The optoelectronic device comprises a waveguide structure including a plurality of optical modulator elements each having an optical property that is adjustable upon application of an electrical signal so as to modulate light guided in the waveguide structure. The optoelectronic device also comprises a plurality of amplifiers wherein each amplifier is electrically coupled to one of the optical modulators to apply electrical signals to the optical modulator. At least one of the plurality of amplifiers comprises a push-pull driver circuit. The optoelectronic device further comprises at least one additional amplifier stage electrostatically or magnetically coupled to one of said amplifiers to increase bandwidth. 
         [0014]    Another embodiment of the invention comprises an optoelectronic device. The optoelectronic device comprises a waveguide structure including a plurality of optical modulator elements each having an optical property that is adjustable upon application of an electrical signal so as to modulate light guided in the waveguide structure. The optoelectronic device also comprises a plurality of amplifiers wherein each amplifier is electrically coupled to one of the optical modulators to apply electrical signals to the optical modulator. The optoelectronic device also comprises a plurality of waveform shaping circuits each having a signal input port and a signal output port wherein each of the signal output ports is electrically connected to a respective one of the plurality of amplifiers and each of the plurality of waveform shaping circuits include a control input for establishing rise time, overshoot, fall time, undershoot, or duty cycle of an electrical signal applied to the optical modulator element. 
         [0015]    Another embodiment of the invention comprises an optoelectronic device. The optoelectronic device comprises a waveguide structure including a plurality of optical modulator elements each having an optical property that is adjustable upon application of an electrical signal so as to modulate light guided in the waveguide structure. The optoelectronic device also comprises a plurality of amplifiers, each amplifier electrically coupled to one of the optical modulators to apply electrical signals to the optical modulator. The optoelectronic device also comprises a plurality of lumped element LC circuits connected in series such that an electrical signal can pass through the lumped element LC circuits, and each of the lumped element LC circuits is electrically connected to respective inputs of the plurality of amplifiers. 
         [0016]    Another embodiment of the invention comprises an optoelectronic device. The optoelectronic device comprises a waveguide structure including a plurality of optical modulator elements each having an optical property that is adjustable upon application of an electrical signal so as to modulate light guided in the waveguide structure. The optoelectronic device also comprises a plurality of amplifiers wherein each amplifier is electrically coupled to one of the optical modulators to apply electrical signals to the optical modulator, and wherein the waveguide structure comprises a path that undulates laterally or is folded such that the distance light propagates within the waveguide structure is substantially larger than the length of the waveguide structure. 
         [0017]    Another embodiment of the invention comprises an optoelectronic device. The optoelectronic device comprises a waveguide structure including a plurality of optical modulator elements each having an optical property that is adjustable upon application of an electrical signal so as to modulate light guided in the waveguide structure. The optoelectronic device also comprises a plurality of amplifiers wherein each amplifier is electrically coupled to one of the optical modulators to apply electrical signals to the optical modulator. At least one of the plurality of amplifiers comprises a push-pull driver circuit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    For a more complete understanding of the present invention and for further features and advantages, reference is now made to the following description taken in conjunction with the accompanying drawings. 
           [0019]      FIG. 1  is a block diagram of an integrated distributed amplifier optical modulator. 
           [0020]      FIG. 2  is a block diagram of a single distributed amplifier stage connected to a respective modulating element. 
           [0021]      FIG. 3  is a block diagram of electrical delay line portions in connection with a single distributed amplifier stage connected to a respective modulating element. 
           [0022]      FIG. 3A  is a block diagram of electrical delay line portions in connection with multiple capacitors and multiple distributed amplifier stages connected to respective modulating elements. 
           [0023]      FIG. 4  is a block diagram of an alternative embodiment of a distributed amplifier connected to a respective modulating element that employs mutual coupling for increasing transition speed. 
           [0024]      FIG. 5  is a diagram of bandwidth extension techniques that can be used in combination with the amplifiers discussed herein. 
           [0025]      FIGS. 6A and 6B  illustrate inductive coupling for a differential and a single ended amplifier, respectively. 
           [0026]      FIGS. 7A and 7B  illustrate capacitive degeneration bypass for a differential and a single ended amplifier. 
           [0027]      FIG. 8  illustrates a negative resistance cell for a differential amplifier. 
           [0028]      FIG. 9A  is a block diagram illustrating a method of incorporating waveform shaping circuitry with a distributed amplifier connected to a respective modulating element. 
           [0029]      FIGS. 9B and 9C  are block diagrams illustrating alternative methods of incorporating waveform shaping circuitry with a distributed amplifier connected to a respective modulating element. 
           [0030]      FIG. 10  is a block diagram of a Mach-Zender Interferometer. 
           [0031]      FIG. 11  is a block diagram illustrating a Mach-Zender Interferometer including a plurality of phase modulators driven by amplifiers connected to an artificial transmission line. 
           [0032]      FIG. 12  is a block diagram illustrating a Mach-Zender Interferometer including a plurality of modulators driven by amplifiers connected to an active delay line. 
           [0033]      FIGS. 12A-12B  are block diagrams illustrating a Mach-Zender Interferometer with folded modulators. 
           [0034]      FIG. 13  is a block diagram illustrating the use of a reference clock signal in a delay locked loop for stabilizing variable delay elements. 
           [0035]      FIG. 14  is a block diagram illustrating the use of a reference delay block in a delay locked loop for stabilizing variable delay elements. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0036]      FIG. 1  is a block diagram of an integrated distributed amplifier (DA) optical modulator. DA modulator  100  includes a transmission line (TL)  120 , k distributed amplifiers  140 - 1  to  140 - k  and series connected k modulating elements  170 - 1  to  170 - k . TL  120  receives a modulating signal  110  on its input port and has k output signals  130 - 1  to  130 - k . Each of the output signals  130 - i  is delayed relative to the previous output signal  130 -( i −1). The amount of delay depends on the length of the transmission line between any two adjacent output ports. Each of the k output signals of the TL  120  is connected to the respective inputs of the amplifiers  140 - 1  to  140 - k . Each of the output signals  150 - 1  to  150 - k  of amplifiers  140 - 1  to  140 - k  is connected to the modulating input terminal of a respective k optical modulating element  170 - 1  to  170 - k.    
         [0037]    Optical wave  160  is connected to the optical input of the first modulating element  170 - 1 . The modulated optical output  190  is generated at the end of the series of modulating elements, at the output of modulating element  170 - k . Optical wave  160  is first modulated by modulating element  170 - 1  driven by the electrical signal  150 - 1  from the TL  120 . Output  180 - 1  of the modulating element  170 - 1  is connected to the input of the modulating element  170 - 2 , where the optical wave is further modulated by electrical signal  150 - 2  from the TL  120 . Each successive modulating element in the series can provide additional modulation. This process continues through the k stages of the modulator, until the fully modulated optical output  190  of the last modulating element  170 - k  is generated. 
         [0038]      FIG. 2  is a block diagram of one of the k distributed amplifiers connected to a respective one of the k modulating elements, in accordance with one embodiment of the present invention. The TL  120  receives the modulating signal  110  on its input port and outputs a signal  130 - i  to the amplifier  140 - i . In accordance with certain preferred embodiments of the present invention, the amplifier  140 - i  is a push-pull driver circuit comprising of a stack of two p-type devices  260  and two n-type devices  270 . Using a stack of two or more transistors is advantageous, because transistor stacking prevents high voltage breakdown. The voltage required to drive an optical modulating element from the ground potential to the VDD potential may be too high for transistors having low voltage breakdowns. Therefore, if any of these transistors are exposed to the high voltage needed for driving an optical modulating element, the reliability of the transistors is reduced over time, and they could become permanently damaged. Therefore, according to the preferred embodiments of the present invention, a stack of two or more transistors is used so that the voltage potential is split equally between the two devices. This reduces the probability that any one of the transistors would break down as a result of excessively high voltage. 
         [0039]    Additionally, the use of complementary transistors in the amplifier  140 - i  is also advantageous because complementarily configurations result in lower power consumption. Furthermore, certain embodiments of the DA optical modulator can therefore achieve a high voltage swing across the terminals of the modulating elements. In order to achieve the highest possible voltage swing, the voltage swing should be substantially equal to the VDD potential. That can be achieved by using a complementary configuration. Thus, in accordance with various preferred embodiments of the present invention, an amplifier  140 - i  includes a stack of two p-type transistors and two n-type transistors. 
         [0040]    However, even though a stack of two complementary transistors is shown in  FIG. 2 , the push-pull amplifier could include only a single pair of complementary transistors. Additionally, the push-pull amplifier could also include a pair of more than two complementary transistors. Furthermore, the n-type devices that are used could be either NFET or npn transistors and the p-type devices that are used could be either PFET or pnp transistors. Other types of transistors, other numbers of transistors, and other arrangements of transistors are also possible. 
         [0041]    Referring back to  FIG. 2 , amplifier  240  may be configured for active breakdown protection to assure that there is a generally equal voltage drop across the terminals of both transistors in the upper stack, which may comprise of two n-type transistors, and that neither of the transistors in the upper stack are subject to very high voltage. Similarly, amplifier  250  may be configured for active breakdown protection to assure that there is a generally equal voltage drop across the terminals of both transistors of the lower stack, which may comprise of two p-type transistors, and that neither of the transistors in the lower stack are subject to very high voltage. The amplifiers  240  and  250  may, for example, monitor the mid-point voltage between the two transistors in the stack and adjust the gate voltage on the bottom transistor according to the value of the mid-point voltage. This adjusting of the gate voltage can assure that equal voltage is dropped across the terminals of each transistor in the stack. Thus, the amplifiers  240  and  250  can actively regulate the current signal that goes through the transistors and prevent premature failure of the transistors. 
         [0042]    The output signal  150 - i  of the amplifier  140 - i  is connected to an input port of the modulating element  170 - i . The output signal  150 - i  modulates the optical wave  160  to produce a modulated optical wave  180 - i . The modulated optical wave  180 - i  may be input to a next successive modulating element for further modulation. 
         [0043]    A capacitor  220  is connected between the amplifiers  240  and  250 . The capacitor is connected such that it connects the source of a first NFET transistor and the drain of a second NFET transistor to the source of a first PFET transistor and the drain of a second PFET transistor. The addition of this capacitor is advantageous in that it can result in capacitive coupling between the transistors. This coupling stabilizes the voltage difference between the midpoint node of the NFET transistors and the midpoint node of the PFET transistors during transitions and helps enhance high frequency performance of the active breakdown protection circuits by fixing their relative voltage. 
         [0044]    The velocity of the electrical signal propagating in the TL  120  is typically faster than the velocity of the optical wave propagating in the series of modulating elements  140 - 1  to  140 - k . In order to match the overall velocity of the electrical signal in the TL  120  to the average optical velocity in the series of modulators, the TL  120  may be designed to have sufficient delay between the output ports to slow down the overall electrical signal to match the speed of the optical signal in the series of modulators. Accordingly, as illustrated in  FIG. 3 , the TL  120  can be designed to have a series of electrical delay lines  320 . The TL  120  could be designed so that the input of each amplifier  140 - i  is connected to a respective one of the electrical delay lines  320 . Additionally, the electrical delay lines  320  may be active electrical delay lines comprising of devices which can provide signal amplification such as transistors. The electrical delay lines  320  can also be passive delay lines comprising, for example, of circuit elements such as resistors, capacitors, inductors, transformers, and the like. 
         [0045]    Generally, the characteristic impedance of a transmission line is determined by the intrinsic inductance and capacitance of metal lines associated with the transmission line. However, when circuit elements such as amplifiers having lumped input capacitances are connected to the transmission line, the characteristic impedance of the transmission line is affected by the input capacitance of such circuit elements. Thus as one or more amplifiers are connected to various points of a transmission line, such as the TL  120 , characteristic impedance may change along the length of the TL  120 . This may result in impedance discontinuities along the length of the TL  120 . 
         [0046]    To reduce the effects of impedance discontinuities along the TL  120 , one or more capacitors may be connected to the TL  120 , as illustrated in  FIG. 3A . Connecting these capacitors ensures a more uniform distribution of capacitance along the length of the TL  120  and thus results in a reduction of impedance discontinuities along the length of the TL  120 . Transmission lines having a large number of lumped capacitive loads generally have lower characteristic impedances. Additionally, due to more frequent distribution of lumped capacitors, their performance is closer to ideal transmission lines. 
         [0047]    As illustrated, capacitors C D1 -C D3  can be connected to the TL  120  between any two inputs nodes of the distributed amplifiers  140 - 1  to  140 - 3 . In this embodiment, the capacitors are connected to the TL  120  at uniform intervals such that they are located at a same distance from one other. The capacitors are generally connected between various electrical delay lines  320 . Thus, one or more capacitors can be disposed between any two amplifier stages. Other configurations are also possible. For example, in one embodiment, the capacitors are connected to the TL  120  at non-uniform intervals such that not all of the capacitors are located at same distances from one another. 
         [0048]    In certain embodiments, the capacitors C D1 -C D3  are selected such that their capacitance is equivalent to the input capacitance of the amplifiers. In other embodiments where the amplifier stage includes a smaller input stage and a larger output stage, the input stage itself may be used as a capacitive load to achieve better capacitance matching. However, before being used as a capacitive load, generally the input stage is powered-down so that it does not consume a large amount of power. The capacitors may be a metal-oxide-semiconductor capacitor, a metal-insulator-metal capacitor, a fringe capacitor, a transistor, or other types of capacitors known in the art. 
         [0049]      FIG. 4  illustrates a method of increasing transition speed in a DA optical modulator by mutually coupling two peaking networks. The peaking network  410  of  FIG. 4  is connected to a predriver  420  which outputs an electrical signal  425  to the amplifier  140 - i . On the output side of the amplifier  140 - i , the peaking network  430  is disposed between the output port of the amplifier  140 - i  and the electrical input port of the modulating element  170 - i . This arrangement also results in mutual coupling between the predriver  420  and the peaking network  430  of the output stage. The advantages of mutual coupling are discussed more fully below. The peaking networks  410  and  430  may comprise of a single circuit element, such as a transformer or a group of individual inductors. In various embodiments, the inductors included in the peaking networks  410  and  430  can be placed in a concentric or overlapping pattern. Such arrangements create mutual magnetic flux coupling between the inductors. The polarity of this mutual coupling can be chosen such that high-frequency magnetic energy from the predriver  420  can be coupled into the amplifier  140 - i  to achieve bandwidth extension. 
         [0050]    In various preferred embodiments of the present invention, the predriver  420  is comprised of a differential amplifier followed by a differential to single ended converter. The converter converts the output of the differential amplifier to a single signal that can be input to the amplifier  140 - i . Because there is only one signal that is input to the amplifier  140 - i , only a portion of the differential amplifier used in the predriver  420  is needed for outputting the signal  425 . Therefore, a portion of the differential amplifier including the branch that is not connected to the output stage is not used. The energy from this unused portion of the differential amplifier is coupled into the peaking network  430  to enhance transition speed. This can be achieved because, even though the branch may be unused, due to the balanced operation of the differential amplifier, a high frequency AC current may still flow through it. This current can be converted to magnetic energy by coupling an inductor (primary coil) of the peaking network  410  to an inductor (secondary coil) of the peaking network  430 . The magnetic energy generated can be converted back to a current of the same polarity as the AC current in the peaking network  430 . 
         [0051]    The mutual inductive coupling is achieved in two ways. According to a small signal approach, the inductor resonates with the parasitic capacitance of the output node and to a certain extent cancels the parasitic capacitance. Parasitic capacitance is undesirable because it could create a pole resulting in a frequency roll-off in an amplifier driving a modulating element. Therefore, any effort to eliminate or neutralize the parasitic capacitance is desirable as it provides for operating at higher frequencies. According to a large signal approach, the inductor inhibits current changes at the output node and produces overshoot and undershoot. These effects are sometimes desirable because they assist in charging and discharging the parasitic capacitance of the modulating element. During signal switching, mutual coupling between stages helps to enhance the effects of the peaking inductor. This also translates to shorter rise time and fall time. 
         [0052]    Although the preferred embodiments of the present invention involve inductive coupling, it is well known in the art that various different circuit elements and different configurations can be used to create the peaking networks  410  and  430  and the predriver  420 . For example, capacitors can be used in the peaking networks  410  and  430 , in which case instead of inductive coupling, capacitive coupling can be used. Combination of inductive and capacitive coupling may also be possible. Other variations may be used as well. 
         [0053]      FIG. 5  illustrates some additional bandwidth extension techniques that when used in combination with any of the k amplifiers in a DA optical modulator can result in increased bandwidth extension in the DA optical modulator. These techniques include the use of inductive peaking described above and in more detail below with respect to  FIGS. 6A and 6B , the use of capacitive bypass of resistive degeneration described in more detail below with respect to  FIGS. 7A and 7B , and the use of a negative resistance cell, described in more detail below with respect to  FIG. 8 . 
         [0054]      FIGS. 6A and 6B  are diagrams of two example circuits achieving bandwidth extension through inductive peaking.  FIG. 6B  shows how inductive peaking can be achieved with a single-ended amplifier by connecting an inductor  640  in series with a resistor  660  in an amplifier circuit  650 . This technique when used in combination with any amplifier  140  of the DA optical modular can result in increased bandwidth extension and reduced circuit area. 
         [0055]      FIG. 6A  shows inductors  610  and  620  both having the same value of inductance, coupled to a differential amplifier  630 . The two inductors create mutual inductive coupling and inductive peaking. Inductive peaking takes advantage of the fact that inductor impedance increases with frequency. Thus, a high frequency roll-off of an amplifier gain can be compensated by including inductors in series with resistors. In differential amplifiers, mutual inductive coupling can be used to increase bandwidth extension or reduce inductor area. 
         [0056]      FIGS. 7A and 7B  are diagrams of two example circuits that achieve bandwidth extension through capacitive bypass of resistive degeneration. As shown in  FIG. 7A , a capacitor  750  is used to bypass the resistance of resistors  760  and  780 . Generally, the resistors  760  and  780  degenerate the circuit gain. This is because generally degeneration resistors reduce amplifier gain. However, when, a capacitor is placed in parallel with the resistors, the collective impedance of the RC combination can decrease at high frequencies. When the reduction in gain degeneration at high frequencies coincides with the gain roll-off that is due to circuit poles, the circuit achieves a relatively constant gain. The reduction in gain degeneration increases gain and the gain roll-off reduces gain. These two effects offset each other and result in a relatively constant bandwidth extension for the circuit. When the capacitor  750  is added to the circuit shown in  FIG. 7A , the resistors  760  and  780  are bypassed at high frequencies. Therefore, at high frequencies the gain degeneration is bypassed and gain is restored to its original value. This process results in bandwidth extension at high frequencies.  FIG. 7B  shows how capacitive bypass of resistive degeneration can be achieved with a single-ended amplifier by connecting a capacitor C in parallel with a resistor R 2 . This technique of capacitive bypass of resistive degeneration can be used in combination with any amplifier  140  (see  FIG. 1 ) of the DA optical modular to create increased bandwidth extension. 
         [0057]      FIG. 8  is a diagram of an example circuit that achieves bandwidth extension by using a negative resistance cell. A negative resistance cell is a combination of circuit elements for which the current and voltages produced across its terminals represent a negative resistance. The use of a negative resistance cell could result in increased speeds for signal transitions at high frequencies and in turn in bandwidth extension. A negative resistance cell when used in combination with any amplifier  140  of  FIG. 1  of the DA optical modular could result in increased bandwidth extension. 
         [0058]    The distributed amplifiers of the DA optical modulator  100  can also be equipped with waveform shaping circuitry for the purpose of improving circuit performance and compensating for waveform non-idealities in the optical modulating elements.  FIG. 9A  shows a block diagram of a circuit incorporating such waveform shaping circuitry. The modulating signal  130 - i  of  FIG. 9A  is input to a predriver  905  which in turn outputs a signal  910 . The signal  910  is input to both the amplifier  140  and a waveform shaping control  930 . The waveform shaping control  930  also receives a waveform shaping adjustment signal  920 . The waveform shaping adjustment signal  920  maybe a binary waveform that is created by a user&#39;s manual adjustment of a waveform control interface  950 . For example, the waveform control interface  950  may allow the user to adjust various waveform characteristics of the modulating signal  130 - i , such as the rise time, fall time, overshoot, undershoot, or duty cycle. In certain embodiments each of the adjustable waveform characteristics can be adjusted independent of the others. In other embodiments, adjusting one characteristic may result in adjustment of other characteristics. For example, adjusting the rise time may result in adjustment of overshoot and vice versa, and adjusting the fall time may result in adjustment of undershoot and vice versa. Various other arrangements are also possible. 
         [0059]    The waveform shaping control  930  may include a digital decoder capable of decoding the binary adjustment signal  920 . After decoding the adjustment signal  920 , waveform shaping control  930  may use the decoded signal to adjust specific waveform characteristics of the modulating signal  130 - i . An adjusted signal  980  may then be outputted from the waveform shaping control  930  to a summing unit  940 . The summing unit  940  can sum the adjusted signal  980  with the output of the amplifier  140 - i . The final modified modulating signal  150 - i  may then be input to the modulating element  170 - i  to modulate the optical wave  160 . The amplifier  140  of  FIG. 9  can be any type of the variety of amplifiers known in the art and is not limited to a push-pull amplifier. Other variations are also possible. 
         [0060]    In an alternative embodiment, a waveform monitoring circuit (eye monitor) can be used to monitor the characteristics of the output signal  150 - i , as illustrated in  FIG. 9B . The eye monitor  960  can monitor the output signal  150 - i  and extract characteristic information, e.g., rise time, fall time, undershoot, overshoot, and duty cycle from the signal  150 - i . This information is then provided to the waveform shaping control  930 . The waveform shaping control  930  then compares the characteristic information of the signal  150 - i  to preprogrammed desired waveform settings. Waveform shaping circuits within the waveform shaping control  930  are then adjusted to reduce the differences between the measured characteristics and the desired characteristics. The preprogrammed desired waveform settings can be defined in hardware or software or may be adjustable by a user. The latter approach is advantageous as it affords more flexibility. The use of feedback has an advantage of reducing sensitivity to variations in electrical and optical process parameters. In an alternative embodiment, the eye monitor  960  monitors the modulated optical signal  180 - i  and the waveform shaping control  930  adjusts the waveform shaping circuits so that desired waveform characteristics are achieved in the optical signal  180 - i . Other variations are also possible. 
         [0061]      FIG. 9C  illustrates an alternative embodiment for shaping the waveform characteristics of the modulating signal  130 - i . The output  980  of the waveform shaping control  930  is input to two voltage controlled current sources  910 . The current sources  910  can provide time-varying, data dependent currents to change the rate at which the modulating element  170 - i  is charged or discharged. Consequently, transition times can be decreased or increased, and overshoot or undershoot can be introduced by turning on one or both of the sources. Additional alternative embodiments include using only one voltage controlled current source or using one or more current controlled voltage sources, voltage controlled voltage sources, or current controlled current sources. 
         [0062]      FIG. 10  is a block diagram of a Mach-Zender Interferometer (MZI)  1000 , incorporating two DA optical modulators. Optical wave  1010  having a specific frequency and amplitude is input to splitter  1020 , which divides optical wave  1010  into two optical waves  160 - 1  and  160 - 2  propagating through a first and a second arm of the MZI  1000 . 
         [0063]    Optical wave  160 - 1  is input to DA modulator  1050 , which can cause a phase shift in optical wave  160 - 1  and produce optical wave  190 - 1  as a result of applied electrical signal  110 - 1 . Optical wave  160 - 2  is input to the DA modulator  1060 , which can cause for example an opposite phase shift in optical wave  160 - 2  and produce optical wave  190 - 2  as a result of applied electrical voltage  110 - 2 . Applied signal  110 - 2  may be the inverse of the modulating signal  110 - 1  which would result in a destructive interference. In some embodiments, the MZI modulator  1000  uses signals  110 - 1  and  110 - 2  as differential modulating signals, which can result in the cancellation of noise present in the modulating signal  110 . 
         [0064]    Modulated wave  190 - 1  and modulated wave  190 - 2  are summed in combiner  1070  to generate output  1090 . Depending on the phase relationship between the two waves  190 - 1  and  190 - 2 , combining the two waves can cause constructive or destructive interference, which can result in intensity modulated wave  1090 . Amplitude modulation of optical wave  1010  is produced by the electrically controlled phase shifts in DA modulators  1050  and  1060 . 
         [0065]      FIG. 11  is a block diagram of an MZI modulator, according to one embodiment of the present invention. The MZI modulator  1100  of  FIG. 11  includes an artificial transmission line (ATL) instead of the electrical transmission line  120  shown in  FIG. 1 . The velocity of the electrical wave propagating in an electrical transmission line is typically not equal to the velocity of the optical wave propagating in the series of modulating elements  140 . Therefore, an ATL can be used to match the propagation velocity of the electrical signal to that of the optical wave. Additionally, an ATL can offer a more compact footprint and can provide increased placement flexibility. The ATL of  FIG. 11  is comprised of lumped LC delay elements  1130 . The LC delay elements  1130  are generally comprised of two inductors  1120 -( i −1) and  1120 - i  connected on one side to a capacitor C. The combination of the two inductors and the capacitor C tend to behave like a transmission line. The characteristic impedance Z 0  of the ATL that results from the combination of such LC delay elements is equal to the square root of L/C (Z 0 =√{square root over (L/C)}) and the delay of each LC element is equal square root of LC (T=√{square root over (LC)}). 
         [0066]    In various preferred embodiments of the present invention, the ATL is a differential ATL, as illustrated in  FIG. 11 . A differential ATL can take advantage of mutual magnetic coupling between inductors and also provides for differential capacitor placement. Mutual coupling between the inductors results in achieving the same effective inductance in a smaller area. Therefore, mutual coupling offers the ability to reduce the physical dimensions of the required inductors and capacitors. Although the LC delay elements shown in  FIG. 11  are comprised of two inductors and a capacitor, it must be noted that a variety of different circuit elements and configurations can be used to create the LC delay elements that make up an ATL. For example, instead of the two inductors, two capacitors may be used to create capacitive coupling as opposed to inductive coupling. Various other arrangements are also possible. 
         [0067]    The modulating signal  110  is run through an ATL driver  1105  and the output from the driver  1105  is input into the amplifiers  1110 - 1  to  1110 - k . The amplifiers  1110 - 1  to  1110 - k  are differential amplifiers generally comprising of two push-pull amplifiers. The outputs from the two different arms of the amplifiers  1110 - 1  to  1110 - k  are input to corresponding modulating elements in the two arms of the MZI  1100 . To prevent reflections and reduce their undesirable interference with the transmitted signal, a matching termination resistor  1140  is placed at the far end of the ATL. The resistor  1140  may absorb all of the electrical energy and not reflect any of the energy back. It is advantageous to design the MZI  1100  in such a way that the output impedance of the ATL driver  1105  matches the characteristic impedance of the ATL. 
         [0068]      FIG. 12  shows a block diagram of an MZI  1200  that uses an active delay line, instead of an actual electrical transmission line or an ATL. The use of an active delay line is advantageous because it can perform the same functionalities as an electrical transmission line, but requires a smaller area. The active delay line of various preferred embodiments of the present invention is comprised of multiple active delay elements  1220 - 1  to  1220 - k . The delay of each active delay element is adjusted by a delay locked loop (DLL)  1210 , the operation of which is described in more details with respect to  FIGS. 13 and 14 . The delay of the active delay elements is adjusted so that the velocity of the modulating signal traveling through the active delay line matches the velocity of the optical wave traveling through the modulating elements of the MZI  1200 . The DLL  1210  uses either an internal or an external reference signal for the purpose of stabilizing delays of the active delay elements. The reference signal could be a periodic binary signal. 
         [0069]    The modulating signal  110  is run through a driver  1205  and the output from the driver  1205  is input into an array of amplifier stages. As illustrated, the MZI  1200  has a first end and a second end, such that both the array of amplifier stages and the modulating elements in each of the first and the second arms of the MZI  1200  progress from the first end to the second end. The modulating signal that is input into the first amplifier stage  1230 - 2  is delayed by the delay element  1220 - 1  before being input into the next amplifier in the array of amplifier stages in the MZI  1200 . As the modulating signal moves from the first end of the MZI  1200  to the second end, it is input sequentially into each one of the delay elements  1220 - 1  to  1220 - k  and it may be further delayed by each one of these delay elements. 
         [0070]    The amplifiers  1230 - 2  to  1230 - k  are differential amplifiers and generally comprise two push-pull amplifiers. The outputs from the two different arms of the amplifiers  1230 - 2  to  1230 - k  are input to corresponding modulating elements in the two arms of the MZI  1200 , such that the first amplifier stage drives the first upper and the first lower modulating elements  170 -U 1  and  170 -L 1 , and the second amplifier stage drives the second two modulating elements in each of the first and the second arms of the MZI  1200 . This process is repeated from the first end to the second end such that as the modulating signal moves from the first end to the second, each of the amplifiers in the array of amplifier stages drives a corresponding upper and lower modulating element from the first end to the second end. 
         [0071]    To take advantage of the smaller area needed by the active delay line as compared to an electrical transmission line, the optical wave and associated optical modulating elements can be nonlinear, meandered, or folded, as illustrated in  FIG. 12 . Each of the separated curved shaded areas in  FIG. 12  represents one of the modulating elements  170 -U 1  to  170 -Uk or  170 -L 1  to  170 -Lk. In this embodiment, the arms of the MZI  1200  follow a sinusoidal path. The arms, however, may be different and can have different shapes. For example, the arms may follow a path having turns that are less sharp than the sinusoidal path shown in  FIG. 12 . Other shapes may also be used. These shapes may be curved or comprise linear portions. In various embodiments, the arms undulate many times, e.g., 3, 4, 5, 6, 10, 20, or more times. Accordingly, the path that the arms follow shifts laterally many times along a longitudinal direction to increase the optical path length for a given length of the MZI  1200 . This arrangement can reduce the required size of the MZI  1200  and provide increased placement flexibility for placing the MZI. 
         [0072]    In other embodiments, the traveling path of both the optical wave and the modulating signal may be folded back, as illustrated in  FIG. 12A . With this arrangement the velocity of the modulating signal traveling through the delay line can substantially match the velocity of the optical wave traveling through the modulating elements of the MZI  1260 . In  FIG. 12A , each of the separated rectangular shaded areas represents one of the modulating elements  170 -U 1  to  170 -Uk or  170 -L 1  to  170 -Lk. As illustrated, the MZI  1260  of  FIG. 12A  has a first end and a second end, such that the array of amplifier stages progress from the first end to the second end. It should be noted that line  190  is an optical path and not an electrical line and it is not connected to the amplifiers  1230 - 2  to  1230 - k . Line  190  is generally placed on a layer either above or below the layer the amplifiers  1230 - 2  to  1230 - k  are placed on. 
         [0073]    The optical modulating elements in the first arm of the MZI  1260  are optically connected and follow an optical path from the first end to the second end in a first row of the first arm and then back from the second end to the first end in a second row of the first arm. The first row of the first arm is the row of modulating elements containing modulating elements  170 -U 1  to  170 -U 4  and the second row of the first arm is to the row containing the modulating elements  170 -U 5  to  170 -Uk. The optical modulating elements in the second arm of the MZI  1260  are optically connected and follow an optical path from the first end to the second end in a second row of the second arm and then back from the second end to the first end in a first row of the second arm. The first row of the second arm is the row of modulating elements containing modulating elements  170 -L 1  to  170 -L 4  and the second row of the second arm is to the row containing the modulating element  170 -L 5  to  170 -Lk. Thus, as illustrated in  FIG. 12A , each of the optical waves  160 - 1  and  160 - 2  traveling through the first and the second arms of the MZI  1260  follow a path from the first end to the second and then back from the second end to the first end. Accordingly, the optical waves travel through multiple rows that are substantially parallel. In this embodiment, each of the first and the second arms of the MZI  1260  are folded back and undulate once. In certain configurations, however, the arms may undulate two or more times and the rows need not be linear or parallel. 
         [0074]    As illustrated, as the amplifiers progress from the first end to the second end, adjacent amplifiers do not drive consecutive optical modulators in the two arms of the MZI  1260 . Adjacent amplifiers, generally, drive modulating elements that are in two different rows of each of the arms of the MZI  1260 . For example, the first amplifier stage  1230 - 2  drives the first upper and lower modulating elements  170 -U 1  and  170 -L 1 , but the second amplifier stage  1230 - 3  does not drive the second upper and lower modulating elements. Instead, the second amplifier stage  1230 - 3  drives the last upper and lower modulating elements  170 -Uk and  170 -Lk. Thus, the first two amplifier stages from the left of the MZI  1260  drive the first modulating elements from the left in both of the first and the second rows of both the first and the second arms of the MZI  1260 . The second two amplifier stages from the left drive the second modulating elements from the left in both of the first and the second rows of both the first and the second arms of the MZI  1260 . This process continues such that the next two amplifier stages from the left drive the next corresponding modulating elements in both the first and the second rows of both the first and the second arms of the MZI  1260 . Thus, the first amplifier stage drives modulating elements that are located earlier, upstream or rearward in the optical path and the second amplifier stage drives modulating elements that are located later, downstream or forward in the optical path. 
         [0075]    Because spatially consecutive amplifiers do not drive spatially consecutive modulating elements, the modulating signal is delayed by a different amount at each one of the different amplifier stages. For example, because the first amplifier stage drives earlier upstream modulating elements, the second amplifier stage drives later downstream modulating elements, and the third amplifier stage drives modulating elements that are in between the earlier upstream and the later downstream modulating elements, the modulating signal is delayed more by the second amplifier stage than by the third amplifier stage. Thus, as the modulating signal progresses from the first end to the second end, the amount of delay applied to the signal generally increases and decreases at consecutive amplifier stages. This applied delay is generally from the folded back transmission line. 
         [0076]    As  FIG. 12A  illustrates, the MZI  1260  is folded in half. Various other configurations are possible. For example, in some embodiments, the MZI may be folded three or more times to further reduce the size of the MZI  1260 . Other embodiments are also possible. For example,  FIG. 12B  illustrates an alternative embodiment of the MZI  1260 . The difference between the MZI  1270  of  FIG. 12B  and the MZI  1260  of  FIG. 12A  is the direction of travel of the optical waves. In the MZI  1260  of  FIG. 12A , the optical wave  160 - 1  first travels from the first end to the second end in the first row of the first arm and then it travels back from the second end to the first end in the second row of the first arm, and the optical wave  160 - 2  first travels from the first end to the second end in the second row of the second arm and then it travels back from the second end to the first end in the first row of the second arm. However, in the MZI  1270  of  FIG. 12B , the optical wave  160 - 1  first travels from the second end to the first end in the second row of the first arm and then it travels back from the first end to the second end in the first row of the first arm, and the optical wave  160 - 2  first travels from the second end to the first end in the first row of the second arm and then it travels back from the first end to the second end in the second row of the second arm. Thus, as illustrated, the optical wave  160  of the MZI  1260  follows an optical path that is split at the first end and recombined again at the first end, while the optical wave  160  of the MZI  1270  follows an optical path that is split at the first end and recombined at the second end. It should be noted that line  160  is an optical path and not an electrical line and it is not connected to the amplifiers  1230 - 2  to  1230 - k . Line  160  of the MZI  1270  is generally placed on a layer either above or below the layer the amplifiers  1230 - 2  to  1230 - k  are placed on. 
         [0077]      FIG. 13  illustrates the use of a reference clock signal in a DLL for stabilizing variable delay elements. The reference signal used in  FIG. 13  is a reference clock signal  1310 . The reference clock signal  1310  is input to a variable delay cell  1370  which also receives a delay adjustment signal  1380 . The output of the variable delay cell  1370  is compared to the clock&#39;s inverted waveform in the phase detector  1340 . The phase detector  1340  outputs an error signal corresponding to the phase difference between the output of the variable delay cell  1370  and the clock&#39;s inverted waveform. The error signal of the phase detector is run through a charge pump  1350  where it is converted into current pulses. The current pulses are input to a loop filter  1360 . The loop filter  1360  is generally a low pass filter used to assure that the phase adjustments are sufficiently slow. This process achieves increased gain for the phase control loop and generally reduces the steady state error. The loop filter  1360  outputs a delay adjustment signal  1380 . This delay adjustment signal is sent to the active delay elements  1220 - 1  to  1220 - k  of  FIG. 12  and is also fed back to the variable delay element  1370 . 
         [0078]      FIG. 14  illustrates the use of a reference delay block in a DLL for stabilizing variable delay elements. The reference delay block  1420  of  FIG. 14  is generally comprised of a section of the transmission line known to have delay. This is because passive circuit components tend to exhibit less process and temperature variations compared to transistors and therefore, their use as a delay reference will improve stability of the active delay line. The signal  1410  which is input to both the reference delay block  1420  and the variable delay cell  1460  does not have to be periodic and it could be arbitrary. This is because in a DLL using a reference delay block only transitions are of interest. 
         [0079]    The DA modulators of the present invention can be any of a variety of electroabsorptive modulators, such as phase modulators, forward and reverse biased PN modulators, and MOS capacitor modulators. Any of the DA modulators of the present invention can be fabricated on a variety of substrates or wafers, such as: a layer of monocrystalline silicon, silicon on insulator (SOI) substrate, a layer of sapphire, an air filled cavity and a five layer substrate of three layers of monocrystalline silicon with two layers of dielectric between them. It is also possible to use gallium arsenide or indium phosphide substrates or wafers to construct DA modulators of the present invention. Other substrates and platforms may also be used. 
         [0080]    One advantage of fabricating distributed amplifier modulators of the present invention on a silicon or SOI substrate, is the ability to use low cost and well developed CMOS processes for the fabrication of the optical, optoelectronic and electronic devices on the same substrate or wafer. If a distributed amplifier modulator is fabricated on a silicon or SOI substrate, then silicon optoelectronic elements such as the modulating elements can be formed at the same time and of the same silicon used to form the silicon body of a transistor, such as a CMOS transistor. 
         [0081]    A wide variety of variations are possible. Components may be added, removed, or reordered. Different components may be substituted out. The arrangement and configuration may be different. Similarly, processing steps may be added or removed, or reordered. 
         [0082]    Those skilled in the art will appreciate that the methods and designs described above have additional applications and that the relevant applications are not limited to those specifically recited above. Also, the present invention may be embodied in other specific forms without departing from the essential characteristics as described herein. The embodiments described above are to be considered in all respects as illustrative only and not restrictive in any manner.