Abstract:
High speed optical modulators can be made of k modulators connected in series disposed on one of a variety of semiconductor substrates. An electrical signal propagating in a microwave transmission line is tapped off of the transmission line at regular intervals and is amplified by k distributed amplifiers. Each of the outputs of the k distributed amplifiers is connected to a respective one of the k modulators. Distributed amplifier modulators can have much higher modulating speeds than a comparable lumped element modulator, due to the lower capacitance of each of the k modulators. Distributed amplifier modulators can have much higher modulating speeds than a comparable traveling wave modulator, due to the impedance matching provided by the distributed amplifiers.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority from U.S. Provisional applications No. 60/495,402, No. 60/495,403 and No. 60/495,404 filed Aug. 15, 2003. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to optical modulators for use in optoelectronic integrated circuits.  
       BACKGROUND OF THE INVENTION  
       [0003]     Optical fibers have been widely used for the propagation of optical signals, especially to provide high speed communications links. Optical links using fiber optics have many advantages compared to electrical links: large bandwidth, high noise immunity, reduced power dissipation and minimal crosstalk. Fiber optic communications links can operate with carrier frequencies in the THz range. In communications systems where optical fibers are used to transport optical communications signals, various optoelectronic devices are used to control, modify and process the optical signals.  
         [0004]     An integrated optical modulator is a key component of an optical communications system. An optical modulator uses an electrical signal to modulate some property of an optical wave, like 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.  
         [0005]     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, such as: high yields, low fabrication costs and continuous process improvements.  
         [0006]     Previously fabricated silicon-based PIN diode optical modulators have been designed for integrated silicon waveguides with large cross sectional dimensions on the order of several microns. These large modulators are relatively low speed devices capable of modulation at rates in the tens of megahertz, and such low speed devices are not suitable for use in high speed GHz rate systems.  
       SUMMARY OF THE INVENTION  
       [0007]     High speed optical modulators can be made of 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 is tapped off of the transmission line at regular intervals and is amplified by k distributed amplifiers. Each of the outputs of the k distributed amplifiers is connected to a respective one of the k modulators. Distributed amplifier modulators can have much higher modulating speeds than a comparable lumped element modulator, due to the lower capacitance of each of the k modulators. Distributed amplifier modulators can have much higher modulating speeds than a comparable traveling wave modulator, due to the impedance matching provided by the distributed amplifiers. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  is a block diagram of a prior art traveling wave integrated optical modulator.  
         [0009]      FIG. 2  is a block diagram of an integrated distributed amplifier optical modulator, according to one embodiment of the present invention.  
         [0010]      FIG. 3  is a block diagram of an integrated distributed amplifier optical modulator, according to an alternate embodiment of the present invention.  
         [0011]      FIG. 4  is a block diagram of an integrated distributed amplifier optical modulator, according to another embodiment of the present invention.  
         [0012]      FIG. 5  is an overall block diagram of a Mach-Zender Interferometer, incorporating one of the optical modulators of the present invention.  
         [0013]      FIG. 6  is an overall block diagram of a Mach-Zender Interferometer, incorporating two of the optical modulators of the present invention.  
         [0014]      FIG. 7  is an overall block diagram of a Mach-Zender Interferometer, incorporating one of the optical modulators of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0015]      FIG. 1  is a block diagram of a prior art traveling wave (TW) integrated optical modulator. Traveling wave modulator  100  is made of transmission line (TL)  110  and series connected k modulating elements  140 - 1  to  140 -k (M- 1  to M-k). TL  110  receives modulating signal  105  at its input port and has k output ports  115 - 1  to  115 -k. The output ports  115 - 1  to  115 -k can be equally spaced apart along the length of TL  110 , which would correspond to equal amounts of delay between the output ports. The distribution of ports along the length of TL  110  can follow some other distribution. Each of the outputs  115 -j is delayed relative to the previous output  115 -(j−1), depending on the length of the transmission line between any two adjacent output ports. Each of the k outputs is connected to the modulating input terminals of the respective k optical modulating elements  140 - 1  to  140 -k connected in series.  
         [0016]     Optical carrier  130  is connected to the carrier input of the first modulator  140 - 1 . The modulated output  150  of the series of modulating elements is at the end of the chain of modulators, at the output of modulating element  140 -k. Optical wave  130  is first modulated by electrical signal  115 - 1  from transmission line  110  in modulating element M- 1  ( 140 - 1 ). Output  145 - 1  of modulating element  140 - 1  is connected to the input of modulating element M- 2  ( 140 - 2 ), where the optical wave is further modulated by electrical signal  115 - 2  from transmission line  110 . Each successive modulating element in the series can provide additional modulation. This process continues through the k stages of modulation, until the fully modulated output  150  of the last modulating element M-k ( 140 -k) is generated.  
         [0017]     The velocity of the electrical wave propagating in the transmission line  110  is typically faster than the optical wave propagating in the series of modulating elements  140 . In order to match the overall electrical velocity in the TL  110  to the average optical velocity in the series of modulators, TL  110  is 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.  
         [0018]     A TW modulator can be equivalent to a lumped element modulator, where the total capacitance of the series connected modulating elements is equal to the capacitance of the lumped element modulator. A traveling wave modulator can be capable of faster operation as compared to an equivalent lumped element modulator, because the capacitance of each of the individual k modulating elements is 1/k of the capacitance of a lumped element modulator.  
         [0019]     Among the disadvantages of a TW modulator is the poor impedance match between the outputs of the transmission line (TL) and the electrical inputs of the optical modulators. Another disadvantage is that the amplitude of the electrical signal in the TL tends to be attenuated as it travels through the TL. As a result, the amplitude of the electrical output signals towards the end of the TL are also attenuated and this means that the optical wave propagating through the modulators at the end of the series of modulators, receives less modulation than it did at the start of the series of modulators.  
         [0020]      FIG. 2  is a block diagram of an integrated distributed amplifier (DA) optical modulator  200 , according to one embodiment of the present invention. DA modulator  200  is made of distributed amplifiers  220 - 1  to  220 -k (A- 1  to A-k) and series connected k modulating elements  240 - 1  to  240 -k (M- 1  to M-k). The modulating elements can be PN or other types of modulators. Each of the distributed amplifiers  220 - 1  to  220 -k receives modulating signal  205  on its input port and has k output ports  225 - 1  to  225 -k. The outputs  225 - 1  to  225 -k of amplifiers  220 - 1  to  220 -k are connected in parallel to the modulating input terminal of the respective k optical modulating elements  240 - 1  to  240 -k connected in series.  
         [0021]     Optical carrier  230  is connected to the carrier input of the first modulating element  240 - 1 . The modulated output  250  of the series of modulating elements is at the end of the chain of modulators. Optical wave  230  is modulated by electrical signal  225  in each of the modulating elements  240 - 1  to  240 -k (M- 1  to M-k). Each successive modulating element in the series can provide additional modulation. This process continues through the k stages of modulation, until the fully modulated output  250  of the last modulating element  240 -k (M-k) is generated.  
         [0022]     Distributed amplifiers  220 - 1  to  220 -k provide many advantages compared to the prior art TW modulator shown in  FIG. 1 , such as impedance matching between the outputs of the distributed amplifiers  220 - 1  to  220 -k and the k modulating elements, adjustable gain control and automatic gain control. The output impedance of any output port  225 -j of distributed amplifier  220 -j can match the input impedance of the corresponding modulating signal input port of modulating element  240 -j.  
         [0023]     Electrical signal  225 , amplified from modulating input signal  205 , is used to modulate the optical wave propagating through the series of modulating elements  240 - 1  to  240 -k. For signal  225  to effectively modulate optical wave  230  in modulator  200 , the time delay between the first  240 - 1  and last  240 -k modulating elements should be as short as possible.  
         [0024]      FIG. 3  is a block diagram of an integrated distributed amplifier (DA) optical modulator  300 , according to one embodiment of the present invention. DA modulator  300  is made of transmission line (TL)  310 , distributed amplifiers  220 - 1  to  220 -k (A- 1  to A-k) and series connected k modulating elements  240 - 1  to  240 -k (M- 1  to M-k). TW  310  receives modulating signal  205  on its input port and has k output ports  315 - 1  to  315 -k. Each of the outputs  315 -j is delayed relative to the previous output  315 -(j−1), depending on the length of the transmission line between any two adjacent output ports. Each of the k outputs of the TL is connected to the respective inputs of amplifiers  220 - 1  to  220 -k. The outputs  225 - 1  to  225 -k of amplifiers  220 - 1  to  220 -k are connected to the modulating input terminal of the respective k optical modulating elements  240 - 1  to  240 -k connected in series.  
         [0025]     Transmission line  310  can also be made of several shorter transmission lines connected in series, and the output of each shorter transmission line can be connected to a buffer. The output of each buffer can be connected to the input of the next transmission line and to the input of the respective distributed amplifier. The buffers can stabilize the amplitude of the signal in the transmission line and the amplitude of the signals connected to the distributed amplifiers.  
         [0026]     Optical carrier  230  is connected to the carrier input of the first modulating element  240 - 1 . The modulated output  350  is generated at the end of the series of modulating elements, at the output of modulating element  240 -k. Optical wave  230  is first modulated by electrical signal  315 - 1  from transmission line  310  in modulating element ( 240 - 1 . Output  245 - 1  of modulating element  240 - 1  is connected to the input of modulating element  240 - 2 , where the optical wave is further modulated by electrical signal  315 - 2  from TL  310 . Each successive modulating element in the series can provide additional modulation. This process continues through the k stages of modulation, until the fully modulated output  350  of the last modulating element  240 -k is generated.  
         [0027]     Distributed amplifiers  220 - 1  to  220 -k provide many advantages compared to the prior art TW modulator shown in  FIG. 1 , such as impedance matching between the outputs of the TL and the k modulating elements, adjustable gain control and automatic gain control. The input impedance of any input port of distributed amplifier  220 -j can be designed to be equal or higher than the output impedance of the corresponding output port  315 -j of TL  310 . The output impedance of any output port  225 -j of distributed amplifier  220 -j can match the input impedance of the corresponding modulating input port of modulating element  240 -j.  
         [0028]     Since an electrical signal propagating in TL  310  will be attenuated as it travels through TL  310 , each of the amplifiers  220 - 1  to  220 -k can have an adjustable gain control to compensate for the attenuation and thus provide the same amplitude signal to each of the modulating elements  240 - 1  to  240 -k. Another way to compensate for attenuation in TL  310 , is to provide each of the amplifiers  220 - 1  to  220 -k with an automatic gain control (AGC) to stabilize the output amplitude of the amplifiers.  
         [0029]      FIG. 4  is a block diagram of an integrated distributed amplifier optical modulator, according to an alternate embodiment of the present invention. The DA modulator  400  of  FIG. 4  is very similar to the DA modulator  200  of  FIG. 3 , except that multi-tap delay line  460  is used instead of transmission line  310 . TL  310  of  FIG. 3  can be understood as a type of multi-tap delay line. Delay line  460  is made of series connected delay elements  460 - 1  to  460 -(k−1) [D- 1  to D-(k−1)]. The output  465 -j of any delay  460 -j is connected to the next delay  460 -(j+1) and to the input of amplifier  240 -(j+1). The other similarly numbered elements of  FIG. 4  provide the same functions as previously discussed with respect to  FIG. 3 .  
         [0030]     The output of each delay element  460 -j can be stabilized by connecting a suitable buffer to the output and then connecting the output of the buffer to the next delay element  460 -(j+1) and respective amplifier  220 -(j+1). The delay elements can be made of any of a variety of delay elements, such as transmission lines and transistor based devices. The delay lines can also include electronically controlled variable delay lines.  
         [0031]     The DA modulators of  FIGS. 2, 3  and  4  can be any of a variety of electroabsorptive modulators, such as phase modulators, forward and reverse biased PN modulators and MOS capacitor modulators.  
         [0032]     More information about PN modulators can be found in the following U.S. patent applications, which are incorporated herein by reference: “PN Diode Optical Modulators Fabricated In Rib Waveguides,” “PN Diode Optical Modulators Fabricated In Strip Loaded Waveguides,” “PN Diode Optical Modulators With Variegated PN Junctions” and “Doping Profiles In PN Diode Optical Modulators,” filed on Aug. 11, 2004.  
         [0033]     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), 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.  
         [0034]     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.  
         [0035]      FIG. 5  is an overall block diagram of a Mach-Zender Interferometer, incorporating one of the DA modulators of the present invention.  FIG. 5  is an overall block diagram of a Mach-Zender Interferometer (MZI)  500 , incorporating any one of the DA modulators of the present invention. Optical wave  501  of fixed frequency and amplitude is input to splitter  502 , which divides optical wave  501  into two optical waves  503  and  504  of equal amplitude propagating through the two arms of MZI  500 . Optical wave  503  is input to DA modulator  505 , which can cause a phase shift in optical wave  503  and produce optical wave  507  as a result of applied electrical voltage  506 . Modulated wave  507  and unmodulated wave  504  are summed in combiner  509  to generate output  511 . Depending on the phase relationship between the two waves  507  and  504 , combining the two waves can cause constructive or destructive interference, which can result in intensity modulated wave  511 . Modulation of optical wave  501  is produced by an electrically controlled phase shift in DA modulator  505 .  
         [0036]     MZI  500  is one of many well known devices or systems which can be used to modulate an optical wave. Other types of optical modulating systems, which can use any one of the DA modulators of the present invention, include but are not limited to: an MZI modulator with a DA modulator in both arms of the MZI as shown in  FIG. 6 , a ring modulator consisting of a waveguide coupled to a ring resonator, where the ring resonator contains a DA modulator, a Fabry-Perot (FP) cavity where the DA phase modulator is part of the FP cavity, and an MZI modulator where either one or each of its arms contains one or more of the above ring modulators or FP modulators having a DA modulator.  
         [0037]      FIG. 6  is an overall block diagram of a Mach-Zender Interferometer, incorporating two of the optical modulators of the present invention.  FIG. 6  is an overall block diagram of a Mach-Zender Interferometer (MZI)  500 , incorporating any two identical DA modulators of the present invention. Optical wave  501  of fixed frequency and amplitude is input to splitter  502 , which divides optical wave  501  into two optical waves  503  and  504  of equal amplitude propagating through the two arms of MZI  500 .  
         [0038]     Optical wave  503  is input to DA modulator  505 , which can cause a phase shift in optical wave  503  and produce optical wave  507  as a result of applied electrical signal  506 . Optical wave  504  is input to DA modulator  505 A, which can cause an opposite phase shift in optical wave  504  and produce optical wave  508  as a result of applied electrical voltage  506 A. Applied signal  506 A is the inverse of modulating signal  506 . MZI modulator  600  uses signals  506  and  506 A as a differential modulating signal, which can result in the cancellation of noise, which may be present in the modulating signal  506 .  
         [0039]     Modulated wave  507  and modulated wave  508  are summed in combiner  509  to generate output  511 . Depending on the phase relationship between the two waves  507  and  508 , combining the two waves can cause constructive or destructive interference, which can result in intensity modulated wave  511 . Modulation of optical wave  501  is produced by the electrically controlled phase shifts in DA modulators  505  and  505 A.  
         [0040]      FIG. 7  is an overall block diagram of a Mach-Zender Interferometer, incorporating one of the distributed amplifier optical modulators of the present invention. MZI modulator  700  is made of optical splitter  702 , transmission line (TL)  710 , distributed amplifiers  720 - 1  to  720 -k (A- 1  to A-k), series connected k modulating elements  740 - 1  to  740 -k (M- 1  to M-k) and optical combiner  709 .  
         [0041]     Optical wave  730  of fixed frequency and amplitude is input to splitter  702 , which divides optical wave  730  into two optical waves  703  and  704  of equal amplitude propagating through the two arms of MZI  700 . Optical wave  703  is input to the half of the modulating elements that are in the upper arm of MZI modulator  700 , which can cause a phase shift in optical wave  703  and produce optical wave  745 -(k−1) as a result of the applied electrical signals. Optical wave  704  is input to the half of the modulating elements that are in the lower arm of MZI modulator  700 , which can cause a phase shift in optical wave  704  and produce optical wave  745 -k as a result of the applied electrical signals.  
         [0042]     Modulated wave  745 -(k-1) and modulated wave  745 -k are summed in combiner  709  to generate output  750 . Depending on the phase relationship between the two waves  745 -(k-1) and  745 -k, combining the two waves can cause constructive or destructive interference, which can result in intensity modulated wave  750 . Modulation of optical wave  730  is produced by the electrically controlled phase shifts in MZI modulator  700 .  
         [0043]     TW  710  receives modulating signal  705  on its input port and has k output ports  715 - 1  to  715 -k. Each of the outputs  715 -j is delayed relative to the previous output  715 -(j−1), depending on the length of the transmission line between any two adjacent output ports. Each of the k outputs of the TL is connected to the respective inputs of amplifiers  720 - 1  to  720 -k.  
         [0044]     The odd numbered outputs  725 - 1  to  725 -(k−1) of amplifiers  720 - 1  to  720 -(k−1) are connected to the modulating input terminal of the respective k/2 optical modulating elements  740 - 1  to  740 -(k−1) connected in series in the upper arm of MZI modulator  700 .  
         [0045]     The even numbered outputs  725 - 2  to  725 -k of amplifiers  720 - 2  to  720 -k are connected to the respective inputs of signal inverters  726 - 2  to  726 -k. The outputs  727 - 2  to  727 -k of respective signal inverters  726 - 2  to  726 -k are connected to the respective inputs of modulating elements  740 - 2  to  740 -k in the lower arm of MZI modulator  700 .  
         [0046]     MZI modulator  700  uses the odd numbered ports and the even numbered ports of TL  710  as a differential modulating signal, which can result in the cancellation of noise, which may be present in the modulating signal  705 . To provide for the same amount of modulation in the upper and lower arms of MZI  700 , the number of modulating elements in the upper and lower arms should be equal, so that k, the total number of modulating elements, should be an even number.  
         [0047]     Distributed amplifiers  720 - 1  to  720 -k provide many advantages such as impedance matching between the outputs of the TL and the k modulating elements, adjustable gain control and automatic gain control.  
         [0048]     Although the present invention has been described in detail, it should be understood that various changes, substitutions and alterations can be made without departing from the spirit and scope of the invention.