Patent Publication Number: US-2013230267-A1

Title: High speed optical transmitter producing modulated light signals

Description:
FIELD 
     The present invention relates to optical devices and particularly, to optical transmitters. 
     BACKGROUND 
     Optical systems are increasingly being used for a variety of applications such as communications and communications between electrical devices such as servers. These networks make use of transmitters that generate the light signals at one of the electrical devices. In some instances, these transmitters modulate the light signals at high speeds on the order of 25 GHz. As the use of these transmitters has increased, it has become desirable to increase the number of light signals produced by a single device. Increasing the number of light signals produced by a single transmitter can increase the distance between different features of the transmitter. This increased distance can slow down the possible modulation speed of the light signals and increase the size of the device. As a result, there is a need for a compact transmitter that can generate multiple light signals that are each modulated at high speed. 
     SUMMARY 
     An optical system includes a transmitter having waveguides defined in a layer of a light-transmitting medium positioned on a base. A portion of the waveguides are transition waveguides that each guides a different transition light signal. The transmitter also includes modulators positioned on the base. Each modulator includes a modulator waveguide that receives one of the transition light signals and guides the received transition light signal through the modulator. The system also includes drive electronics in electrical communication with the modulators. The drive electronics apply electrical energy to each of the modulators such that an electrical field is generated within the modulator waveguide. Each electrical field is generated so as to modulate one of the transition light signals into a modulated signal. The system includes multiple drive paths. A drive path length is the length of an electrical path from a contact pad on the drive electronics to a location where the electrical field is formed in one of the modulator waveguides. The modulators are constructed and arranged on the transmitter such that the drive path length for each of the modulators is less than 0.5 mm. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1A  is a schematic of a system that includes an optical transmitter and a receiver. 
         FIG. 1B  is a schematic of another embodiment of a system that includes an optical transmitter and a receiver. 
         FIG. 2A  through  FIG. 2C  illustrate a transmitter that is suitable for use as a transmitter in a system such as the system of  FIG. 1A .  FIG. 2A  is a topview of the transmitter. 
         FIG. 2B  is a sideview of the transmitter taken looking in the direction of the arrow labeled B in  FIG. 2A . 
         FIG. 2C  is a cross-section of the transmitter taken along the line labeled C in  FIG. 2A . 
         FIG. 3A  through  FIG. 3C  are each a cross section of a modulator that is suitable for use as the transmitters of  FIG. 1A  through  FIG. 2C . 
     
    
    
     DESCRIPTION 
     A transmitter includes a multiple modulators positioned along an edge of the transmitter. Each of the modulators is configured to guide a different light signal through a modulator waveguide. The transmitter is used in conjunction with off board modulator driver electronics that are configured to generate an electrical field in each of the modulator waveguides. The modulator driver electronics generate the electrical fields such that the different light signals are each modulated at a rate of at least 25 GHz. In order to efficiently operate modulators at these speeds, the electrical path from contact pads on the modulator driver electronics to the location where the electrical field in the modulator waveguides must have a length of less than 0.5 mm. When prior modulator structures are combined with large numbers of modulators and off board modulator driver electronics, these drive path lengths become very difficult to achieve. The structure of the modulators in the disclosed transmitter combined with the arrangement of the modulators on the transmitter allow these drive path lengths to be achieved even when a large number of modulators are present on a single transmitter. 
       FIG. 1A  is a schematic of a system that includes an optical transmitter and a receiver. The transmitter includes multiple waveguides, one or more lasers  10 , a splitter  12 , and modulators positioned on a common platform  14 . The waveguides include one or more source waveguides  16 , transition waveguides  18 , and output waveguides  20 . 
     The system also includes laser driver electronics  22 . Although not shown in  FIG. 1A , the laser driver electronics  22  are in electrical communication with the one or more lasers  10  and are configured to operate the one or more lasers  10  such that each of the lasers  10  generates and outputs a light signal. The system also includes modulator driver electronics  24 . The modulator driver electronics  24  are in electrical communication with each of the modulators and are configured to operate the modulators such that the modulators modulate the intensity of a light signal being guided through the modulator. Although  FIG. 1A  shows the laser driver electronics  22  and modulator driver electronics  24  as being separate components, the laser driver electronics  22  and modulator driver electronics  24  can be included on the same component. 
     During operation of the system, the laser driver electronics  22  operate the one or more lasers  10  such that they each generate a light signal. The generated light signal(s) are each received at one of the source waveguides  16 . The source waveguides  16  carry the light signal(s) to the splitter  12 . The splitter  12  splits the received light signal(s) into multiple transition light signals. Each of the transition light signals is received on a different one of the transition waveguides  18 . Each of the transition waveguide  18  guides the received transition light signal to a different one of the modulators. The modulators each include a modulator waveguide  25  that guides the received transition light signal through the modulator. The modulator driver electronics  24  are configured to operate each of the modulator such that the transition light signal being guided through the modulator waveguide  25  is modulated into a modulated light signal. The output waveguides  20  each receives one of the modulated light signals and guides the received modulated light signals to a facet  28 . 
     The system also includes multiple optical fibers  26  and a receiver. Each of the optical fibers  26  is aligned with a facet  28  on the transmitter so as to receive a modulated light signal from the transmitter. Each of the optical fibers  26  guides the received modulated light signal to the receiver. 
     The receiver includes sensor waveguides  30  positioned on a common platform  32 . The sensor waveguides  30  are each aligned with one of the optical fibers  26  such that each of the sensor waveguides receives one of the modulated light signals from one of the optical fibers  26 . The receiver also includes light sensors  34  positioned on the common platform  32 . Each of the sensor waveguides  30  guides the received modulated light signal to one of the light sensors  34 . The light sensors  34  are configured to convert the received modulated light signal to an electrical signal that is further processed by electronics (not shown) in electrical communication with the receiver. 
     Although  FIG. 1A  shows each of the optical fibers  26  routing a modulated light signal to a common receiver, the optical fibers  26  can route different modulated light signals to different receivers in different locations. As a result, the transmitter can be used to transmit modulated light signal and/or data to different locations. 
     The transmitter can include more than one laser  10 . For instance,  FIG. 1B  is a schematic of a system that includes an optical transmitter having multiple lasers  10 . The laser driver electronics  22  operate the one or more lasers  10  such that they each generate a light signal. Different light signals can be at the same wavelength or different light signals. The generated light signal(s) are each received at a transition waveguide  18 . Each of the transition waveguides  18  guides the received transition light signal to a different one of the modulators. The modulators each include a modulator waveguide  25  that guides the received transition light signal through the modulator. The modulator driver electronics  24  are configured to operate each of the modulator such that the transition light signal being guided through the modulator waveguide  25  is modulated into a modulated light signal. The output waveguides  20  each receives one of the modulated light signals and guides the received modulated light signals to a facet  28 . 
     The system also includes multiple optical fibers  26  and a receiver. Each of the optical fibers  26  is aligned with a facet  28  on the transmitter so as to receive a modulated light signal from the transmitter. Each of the optical fibers  26  guides the received modulated light signal to the receiver. 
     The receiver includes sensor waveguides  30  positioned on a common platform  32 . The sensor waveguides  30  are each aligned with one of the optical fibers  26  such that each of the sensor waveguides receives one of the modulated light signals from one of the optical fibers  26 . The receiver also includes light sensors  34  positioned on the common platform  32 . Each of the sensor waveguides  30  guides the received modulated light signal to one of the light sensors  34 . The light sensors  34  are configured to convert the received modulated light signal to an electrical signal that is further processed by electronics (not shown) in electrical communication with the receiver. 
     Although  FIG. 1B  shows each of the optical fibers  26  routing a modulated light signal to a common receiver, the optical fibers  26  can route different modulated light signals to different receivers in different locations. As a result, the transmitter can be used to transmit modulated light signal and/or data to different locations. 
       FIG. 2A  through  FIG. 2C  illustrate a transmitter that is suitable for use as a transmitter in a system such as the system of  FIG. 1A .  FIG. 2A  is a topview of the transmitter.  FIG. 2B  is a sideview of the transmitter taken looking in the direction of the arrow labeled B in  FIG. 2A .  FIG. 2C  is a cross-section of the transmitter taken along the line labeled C in  FIG. 2A . The transmitter is within the class of optical devices known as planar optical devices. These devices typically include one or more waveguides immobilized relative to a substrate or a base. The direction of propagation of light signals along the waveguides is generally parallel to a plane of the device. Examples of the plane of the device include the top side of the base, the bottom side of the base, the top side of a substrate included in the base, and/or the bottom side of the substrate. 
     A suitable platform for building a transmitter according to  FIG. 2A  through  FIG. 2C  includes a light-transmitting medium  40  positioned on a base  42 . The waveguides guide the different light signals through the light-transmitting medium  40 .  FIG. 2C  is a cross-section of a source waveguide  16 ; however, the transition waveguides  18  and output waveguides  20  can also be constructed as shown in  FIG. 2C . The light-transmitting medium  40  includes a ridge  44  defined by trenches  46  that extend into the light-transmitting medium  40  on opposing sides of the ridge  44 . In  FIG. 2A , only the portion of the trench  46  adjacent to the laser  10  is shown for the purposes of simplifying the illustration. The ridge  44  defines an upper portion of the waveguide. Accordingly, the waveguides include a ridge  44  of the light-transmitting medium  40  extending upward from slab regions of the light-transmitting medium  40  located on opposing sides of the ridge  44 . 
     The portion of the base  42  adjacent to the light-transmitting medium  40  is configured to reflect light signals being guided in the ridge  44  back into the ridge  44  in order to constrain light signals in the waveguide. For instance, the portion of the base  42  adjacent to the first light-transmitting medium  40  can be an optical insulator  48  with a lower index of refraction than the light-transmitting medium  40 . The drop in the index of refraction can cause reflection of a light signal from the light-transmitting medium  40  back into the light-transmitting medium  40 . The base  42  can include the optical insulator  48  positioned on a substrate  50 . 
     In one example, the platform is a silicon-on-insulator wafer. A silicon-on-insulator wafer includes a silicon layer positioned on a base  42 . The layer of silicon serves as the light-transmitting medium  40 . The base  42  of the silicon-on-insulator wafer also includes a layer of silica positioned on a silicon substrate. The layer of silica serves as the optical insulator  48  while the silicon substrate serve as a substrate  50  for the base  42 . 
     The transmitter includes a laser chip  52 . The illustrated laser chip  52  includes a single laser  10  although it is possible to build the transmitter that makes use of multiple lasers. Suitable lasers  10  include Fabry-Perot lasers. The laser  10  includes a ridge  54  that extends upwards from a platform and at least partially defines a laser waveguide on the laser chip  52 . The laser chip  52  is positioned in a recess  56  that extends into at least the light-transmitting medium  40 . In some instances, the recess  56  extends into the base  42 . The laser chip  52  is inverted in that the ridge  54  defining the laser waveguide is positioned between the platform of the laser chip  52  and the base  42  of the transmitter. Accordingly, the location of the ridge  54  is shown by dashed lines in  FIG. 2A . Laser driver electronics  22  (not show) are in electrical communication with the laser  10  and are configured to operate the laser  10  such that the laser  10  generates and outputs a light signal. 
     The laser chip  52  is placed in the recess  56  such that the laser  10  is aligned with the source waveguide  16 . As a result, during operation of the transmitter, the source waveguide  16  receives the light signal output by the laser  10 . Suitable methods, structures, and configurations for mounting a laser chip  52  on a silicon-on-insulator  48  wafer with the proper alignment are disclosed in U.S. patent application Ser. No. 08/853,104, filed on May 8, 1997, entitled “Assembly of an Optical Component and an Optical Waveguide, now issued as U.S. Pat. No. 5,881,190, and also in U.S. patent application Ser. No. 12/215,693, filed on Jun. 28, 2008, entitled “Interface Between Light Source and Optical Component,” each of which is incorporated herein in its entirety. The method of fabrication, operation, and mounting disclosed in U.S. patent application Ser. No. 08/853,104 and/or Ser. No. 12/215,693 can be use din conjunction with the transmitter of  FIG. 1A  through  FIG. 2C . 
     The splitter  12  need not be a wavelength dependent splitter  12 . For instance,  FIG. 2A  shows the source waveguide  16  guiding the light signal to a series of y-junctions that serve as the splitter  12 . Y-junctions are example of splitters  12  that do not split up an incoming light signal into transition light signals that each has a different selection of wavelengths. Other examples of suitable wavelength independent splitters  12  include, but are not limited to, Multimode Interference couplers (MMIs), and directional couplers. In some instances, the splitter  12  is a wavelength dependent splitter  12  that splits the incoming source signal into transition light signals that each has a different selection of wavelengths. Examples of suitable wavelength dependent splitters  12  include, but are not limited to, arrayed waveguide gratings, echelle gratings, and bragg gratings. 
     The transmitter includes modulators positioned along an edge of the transmitter. The transition waveguides  18  each guides one of the transition light signals to a different one of the modulators. In order to simplify  FIG. 2A , the details of the modulator construction are not shown in  FIG. 2A . However, the modulator construction is evident from other illustrations such as  FIG. 3A  through  FIG. 3C . The modulators each include a modulator waveguide  25  configured to guide the received transition light signal through an electro-absorption medium  61 . For instance, a ridge  60  of the electro-absorption medium  61  can extend upward from slab regions  62  of the electro-absorption medium  61 . Accordingly, the modulator waveguides  25  are each partially defined by the top and lateral sides of the ridge  60  of the electro-absorption medium  61 . 
     The modulators each include a first contact pad  63  and a second contact pad  64  for providing electrical communication between the modulator and the modulator driver electronics  24 . The modulator driver electronics  24  can be “off board” as shown in  FIG. 2A . For instance, the modulator driver electronics  24  can be included on a component that is in addition to and/or separate from the transmitter and the additional component can be positioned adjacent to the transmitter as shown in  FIG. 2A . This “off board” arrangement is in contrast to the “on board” arrangements where the modulator driver electronics  24  are integrated directly onto the transmitter or are included on a flip chip bonded on the top of the transmitter. 
     The modulator driver electronics  24  can also include drive pads  66 . Suitable drive pads  66  include contact pads. An electrical conductor such as a wire  68  can provide electrical communication between the first contact pad  63  and the second contact pad  64  of a modulator and the drive pads  66 . An electrical conductor such as a wire  68  can be connected to the first contact pad  63  and the second contact pad  64  of a modulator and the drive pads  66  using technologies such as wire bonding. 
     The modulator driver electronics  24  are configured to apply electrical energy to the drive pads  66  such that an electrical field is formed in the modulator waveguide  25 . The modulator driver electronics  24  vary the electrical field so as to modulate the transition light signal traveling through the modulator waveguide  25 . This modulation of the transition light signal results in the generation of a modulated light signal that exits from the modulator. The modulator driver electronics  24  can modulate each of the transition light signals such that different modulated light signal are the same or different. For instance, different modulated light signals can be modulated at different frequencies or at the same frequency. Further, different modulated light signals can be modulated to include different data or the same data. 
     The modulated light signals are each received by one of the output waveguides  20 . The output waveguides  20  each guides one of the modulated light signals to a facet  28  through which the modulated light signal can exit the transmitter. 
     The length of the electrical path from the drive pads  66  to the location where the electrical field is formed in the modulator waveguide  25  (the drive path length) affects the speed at which the modulators are able to modulate the modulated light signal. For instance, increasing the drive path length for the first contact pad  63  and/or the second contact pad  64  associated with a single modulator reduces the modulation speeds that are possible for a given power level. It is generally desirable to modulate the modulated light signal in the RF range (frequency in a range of 3 kHz to 400 GHz). For communications applications, it is generally desirable to modulate the modulated light signal in a range of 100 MHz to 400 GHz. However, the Applicant has found that in order to effectively modulate a modulated light signal at a rate of 25 GHz, the drive path length for the first contact pad  63  and the second contact pad  64  both need to be less than 1 mm. 
     It becomes more difficult to achieve the require drive path lengths as the number of modulators on the transmitter increases. For instance, increasing the number of modulators can be achieved by staggering the locations of the modulators on the transmitter. Staggering the locations of the modulators means the drive path lengths will be different for different modulators and that the drive path length can become undesirably large as the number of modulators increases. The arrangement of  FIG. 2B  overcomes these challenges. The modulators are positioned along an edge of the transmitter in order to reduce the distance between the modulators and the modulator driver electronics  24 . This reduced distance shortens the drive path lengths. In some instances, the modulators are arranged such that the distance between the edge of the transmitter and the furthest point of each modulator waveguide  25  (labeled D in  FIG. 2A ) is less than 1 mm, 0.5 mm, or 0.25 mm. In cases where the modulator waveguides  25  are partially defined by a ridge  60  extending upward from slab regions  62 , the furthest point of each modulator waveguide  25  is the furthest portion of the ridge  60  from the edge of the transmitter. 
     The modulators are lined up along the edge of the transmitter. For instance, the modulator waveguides  25  are arranged so the direction of light signal propagation through each modulator waveguide  25  is parallel to a common line (labeled C in  FIG. 2A ). Accordingly, the length of each modulator waveguide  25  is also parallel to the common line. Additionally, the modulator waveguides  25  are arranged so that common line can concurrently pass through each of the modulator waveguides  25 . Further, the modulator waveguides  25  are arranged so the common line can concurrently pass through the same location in each of the modulator waveguides  25 . For instance, the common line can concurrently pass through the left side of the ridge  60  that defines each of the modulator waveguides  25  or the common line can concurrently pass through the right side of the ridge  60  that defines each of the modulator waveguides  25 . Alternately, the common line can concurrently pass through the center of the ridge  60  that defines each of the modulator waveguides  25 . In this arrangement, the modulator waveguides  25  are optically aligned. For instance, the modulated light signal that exited from the lowest modulator shown in  FIG. 2A  would pass through each of the modulator waveguides  25  in the line of modulators if there were not other components (other waveguides) between those modulators that interfered with the path of the modulated light signal. 
     When the modulators are lined up along the edge, the distance between the first contact pad  63  and the edge of the transmitter remains the same for each of the different modulators and the distance between the second contact pad  64  and the edge of the transmitter remains the same for each of the different modulators as is evident from  FIG. 2A . As a result, the drive path lengths for the different modulators can be substantially constant. For instance, the drive pads  66  on the modulator driver electronics  24  can be arranged such that each drive pads  66  is about the same distance from an edge of the modulator driver electronics  24  as shown in  FIG. 2A . As a result, the drive path length for each of the first contact pads  63  is about the same and the drive path length for each of the first contact pads  63  is about the same. 
     The distance between the drive pads  66  and the edge of the modulator driver electronics  24  can also affect the drive path length as is evident from  FIG. 2A . The distance between the drive pads  66  and the edge of the modulator driver electronics  24  (labeled E in  FIG. 2A ) is typically less than 300 μm, or 100 μm, or 10 μm. Although the modulator driver electronics  24  are “off board” and are a separate component from the transmitter, the edge of the modulator driver electronics  24  can be substantially parallel to the edge of the transmitter. In some instances, the edge of the modulator driver electronics  24  contacts the edge of the transmitter and is parallel to the edge of the transmitter as is shown in  FIG. 2A . The modulator driver electronics  24  can optionally be immobilized relative to the transmitter. For instance, the modulator driver electronics  24  can optionally be epoxied to the transmitter 
     In  FIG. 2A , a portion of the modulators include transition waveguides  18  located between the modulator and the edge of the transmitter that is closest to the modulators. These transition waveguides  18  can alternately be positioned on the opposing side of the modulators; however, these transition waveguides  18  would then cross one or more output waveguides  20  before being connected to the desired modulator. Since waveguide intersections are a source of optical loss, the arrangement of  FIG. 2A  may be more desirable. 
     Although the transmitter of  FIG. 2A  is shows with four modulators, the transmitter can include other numbers of modulators and the associated waveguides. In some instances, the transmitter includes more than three modulators arranged as shown in  FIG. 2A  or more than more than five modulators arranged as shown in  FIG. 2A . 
     A schematic of the transmitter of  FIG. 2A  is in accordance with  FIG. 1A ; however, the transmitter of  FIG. 2A  can be modified to have a schematic in accordance with  FIG. 1B  by replacing the single laser  10 , source waveguide  16 , and splitter  12  of  FIG. 2A  with multiple different lasers. 
     Suitable modulators for satisfying the above size limitations are Franz-Keldysh modulators. Accordingly, in some instances the modulators shown in  FIG. 2A  are each a Franz-Keldysh modulator.  FIG. 3A  is a cross section of a Franz-Keldysh modulator that can serve as the modulators of  FIG. 2A  through  FIG. 2C . As will become evident from the following discussion, the modulator includes multiple doped regions  72 . In the cross section of  FIG. 3A , the perimeter of portions of the doped regions  72  are illustrated with dashed lines to prevent them from being confused with interfaces between different materials. The interfaces between different materials are illustrated with solid lines. The modulator is configured to apply an electric field to the electro-absorption medium  61  in order to intensity modulate the transition light signals received by the modulator. 
     A ridge  60  of electro-absorption medium  61  extends upward from a slab region  62  of the electro-absorption medium  61 . Accordingly, the modulator waveguide  25  is partially defined by the top and lateral sides of the ridge  60  of electro-absorption medium  61 . The slab regions  62  of the electro-absorption medium  61  and the ridge  60  of the electro-absorption medium  61  are both positioned on a seed portion  70  of the light-transmitting medium  40 . As a result, the seed portion  70  of the light-transmitting medium  40  is between the electro-absorption medium  61  and the base  42 . In some instances, the seed portion  70  of the light-transmitting medium  40  is continuous with the portion of the light-transmitting medium  40  included in the transition waveguide  18  from which the modulator receives the transition light signals. In these instances, when a transition light signal travels from a transition waveguide  18  into the electro-absorption medium  61 , a portion of the transition light signal enters the seed portion  70  of the light-transmitting medium  40  and another portion of the transition light signal enters the electro-absorption medium  61 . Accordingly, the seed portion  70  of the light-transmitting medium  40  is included in the modulator waveguide  25  in the sense that the modulator waveguide  25  extends from the base  42  to the top of the ridge  60  of the electro-absorption medium  61 . During fabrication of the modulator, the electro-absorption medium  61  can be grown on the seed portion  70  of the light-transmitting medium  40 . 
     Doped regions  72  are both in the slab regions  62  of the electro-absorption medium  61  and also in the ridge  60  of the electro-absorption medium  61 . For instance, doped regions  72  of the electro-absorption medium  61  are positioned on the lateral sides of the ridge  60  of the electro-absorption medium  61 . In some instances, each of the doped regions  72  extends up to the top side of the electro-absorption medium  61  as shown in  FIG. 3A . Additionally, the doped regions  72  extend away from the ridge  60  into the slab region  62  of the electro-absorption medium  61 . The transition of a doped region  72  from the ridge  60  of the electro-absorption medium  61  into the slab region  62  of the electro-absorption medium  61  can be continuous and unbroken as shown in  FIG. 3A . 
     Each of the doped regions  72  can be an N-type doped region  72  or a P-type doped region  72 . For instance, each of the N-type doped regions  72  can include an N-type dopant and each of the P-type doped regions  72  can include a P-type dopant. In some instances, the electro-absorption medium  61  includes a doped region  72  that is an N-type doped region  72  and a doped region  72  that is a P-type doped region  72 . The separation between the doped regions  72  in the electro-absorption medium  61  results in the formation of PIN (p-type region-insulator  48 - n -type region) junction in the modulator. 
     In the electro-absorption medium  61 , suitable dopants for N-type regions include, but are not limited to, phosphorus and/or arsenic. Suitable dopants for P-type regions include, but are not limited to, boron. The doped regions  72  are doped so as to be electrically conducting. A suitable concentration for the P-type dopant in a P-type doped region  72  includes, but is not limited to, concentrations greater than 1×10 15  cm −3 , 1×10 17  cm −3 , or 1×10 19  cm −3 , and/or less than 1×10 17  cm −3 , 1×10 19  cm −3 , or 1×10 21  cm −3 . A suitable concentration for the N-type dopant in an N-type doped region  72  includes, but is not limited to, concentrations greater than 1×10 15  cm −3 , 1×10 17  cm −3 , or 1×10 19  cm −3 , and/or less than 1×10 17  cm −3 , 1×10 19  cm −3 , or 1×10 21  cm −3 . 
     The first contact pad  63  and the second contact pad  64  are each positioned on the slab region  62  of the electro-absorption medium  61 . In particular, the first contact pad  63  and the second contact pad  64  each contact a portion of a doped region  72  that is in the slab region  62  of the electro-absorption medium  61 . Accordingly, the each of the doped regions  72  is doped at a concentration that allows it to provide electrical communication between an electrical conductor and one of the doped regions  72  in the electro-absorption medium  61 . As a result, the modulator driver electronics  24  can apply electrical energy to the first contact pad  63  and the second contact pad  64  in order to apply the electric field to the electro-absorption medium  61 . 
     During operation of the modulators of  FIG. 3A , the modulator driver electronics  24  apply electrical energy to the first contact pad  63  and the second contact pad  64  so as to form an electrical field in the electro-absorption medium  61 . For instance, the electronics can form a voltage differential between the doped regions  72 . The electrical field can be formed without generating a significant electrical current through the electro-absorption medium  61 . The electro-absorption medium  61  can be a medium in which the Franz-Keldysh effect occurs in response to the application of the electrical field. The Franz-Keldysh effect is a change in optical absorption and optical phase by an electro-absorption medium  61 . For instance, the Franz-Keldysh effect allows an electron in a valence band to be excited into a conduction band by absorbing a photon even though the energy of the photon is below the band gap. To utilize the Franz-Keldysh effect the active region can have a slightly larger bandgap energy than the photon energy of the light to be modulated. The application of the field lowers the absorption edge via the Franz-Keldysh effect and makes absorption possible. The hole and electron carrier wavefunctions overlap once the field is applied and thus generation of an electron-hole pair is made possible. As a result, the electro-absorption medium  61  can absorb light signals received by the electro-absorption medium  61  and increasing the electrical field increases the amount of light absorbed by the electro-absorption medium  61 . Accordingly, the electronics can tune the electrical field so as to tune the amount of light absorbed by the electro-absorption medium  61 . As a result, the electronics can intensity modulate the electrical field in order to modulate the light signal. Additionally, the electrical field needed to take advantage of the Franz-Keldysh effect generally does not involve generation of free carriers by the electric field. 
     Suitable electro-absorption media  61  include semiconductors. However, the light absorption characteristics of different semiconductors are different. A suitable semiconductor for use with modulators employed in communications applications includes Ge 1-x Si x  (germanium-silicon) where x is greater than or equal to zero. In some instances, x is less than 0.05, or 0.01. Changing the variable x can shift the range of wavelengths at which modulation is most efficient. For instance, when x is zero, the modulator is suitable for a range of 1610-1640 nm. Increasing the value of x can shift the range of wavelengths to lower values. For instance, an x of about 0.005 to 0.01 is suitable for modulating in the c-band (1530-1565 nm). 
     Additional details about the fabrication, structure, incorporation into an optical device such as the transmitter, and operation of a modulator having a cross section according to  FIG. 3A  can be found in U.S. patent application Ser. No. 12/653,547, filed on Dec. 15, 2009, entitled “Optical Device Having Modulator Employing Horizontal Electrical Field,” and incorporated herein in its entirety. 
     The modulator of  FIG. 3A  can be modified as shown in  FIG. 3B .  FIG. 3B  is a cross section of another embodiment of a suitable Franz-Keldysh modulator. The perimeter of portions of doped regions  72  shown in  FIG. 3A  are illustrated with dashed lines to prevent them from being confused with interfaces between different materials. The interfaces between different materials are illustrated with solid lines. A first doped zone  80  and a second doped zone  82  combine to form each of the doped regions  72 . In some instance, the first doped zone  80  is located in the light-transmitting medium  40  but not in the electro-absorption medium  61  and the second doped zone  82  is located in the electro-absorption medium  61 . The first doped zone  80  can contact the second doped zone  82  or can overlap with the second doped zone  82 . In some instances, the first doped zone  80  and the second doped zone  82  overlap and at least a portion of the overlap is located in the light-transmitting medium  40 . In other instances, the first doped zone  80  and the second doped zone  82  overlap without any overlap being present in the electro-absorption medium  61 . 
     The first doped zone  80  and the second doped zone  82  included in the same doped region  72  each includes the same type of dopant. For instance, the first doped zone  80  and the second doped zone  82  in an n-type doped region  72  each includes an n-type dopant. The first doped zone  80  and the second doped zone  82  included in the same doped region  72  can have the same dopant concentration or different concentrations. 
     Although  FIG. 3A  and  FIG. 3B  illustrates the slab regions  62  of the electro-absorption medium  61 , the slab regions  62  of the electro-absorption medium  61  may not be present. For instance, the etch that forms the slab regions  62  of the electro-absorption medium  61  may etch all the way through the slab regions  62 . In these instances, the first doped zone  80  and the second doped zone  82  are both formed in the light-transmitting medium  40 . 
     Although  FIG. 3B  shows the first doped zone  80  not extending down to the optical insulator  48 , the first doped zone  80  can extend down to the optical insulator  48  or into the optical insulator  48 . 
     The modulator of  FIG. 3A  can be modified as shown in  FIG. 3C .  FIG. 3C  presents another embodiment of a suitable Franz-Keldysh modulator. The perimeter of portions of doped regions  72  shown in  FIG. 3C  are illustrated with dashed lines to prevent them from being confused with interfaces between different materials. The interfaces between different materials are illustrated with solid lines. 
     The doped regions  72  each includes a portion that extends into the ridge  60  of electro-absorption medium  61  and another portion that extends into the slab region  62  of the electro-absorption medium  61 . The doped region  72  extends further into the slab region  62  of the electro-absorption medium  61  than the doped region  72  extends into the ridge  60  of the electro-absorption medium  61 . For instance, the portion of each doped region  72  in the slab region  62  of the electro-absorption medium  61  is thicker than the portion in the ridge  60 . Reducing the extension of the doped region  72  into the ridge  60  reduces the interaction between the doped region  72  and a light signal being guided through the ridge  60 . As a result, a reduced extension of the doped region  72  into the ridge  60  reduces optical loss. Extending the doped region  72  further into the slab regions  62  allows the electrical field formed between the doped regions  72  to move closer to the base  42 . As a result, the extension of the doped regions  72  further into the slab increases the portion of the light signal that interacts with the electrical field. Accordingly, problems associated with increasing the thickness of the slab regions  62  do not arise because they can be addressed by extending the doped regions  72  further into the slab regions  62 . 
     A suitable thickness for the portion of the doped region  72  in the ridge  60  (labeled T R  in  FIG. 3C ) includes a thickness greater than 0.01, 0.075, 0.1, or 0.125 μm and/or less than 0.175, 0.2, or 0.5 μm. A suitable thickness for the portion of the doped region  72   40  in the slab region  62  of the electro-absorption medium  61   27  (labeled T S  in  FIG. 3C ) includes a thickness greater than 0.175, 0.2, or 0.225 μm and/or less than 0.275, 0.3, 0.325, or 0.8 μm. A suitable thickness ratio (ratio of thickness of portion of doped region  72  in the slab region  62 : thickness of portion of doped region  72  in the ridge  60 ) includes ratios greater than 1, 1.25, or 1.5 and/or less than 2.0, 2.5, and 3. 
     The doped regions  72  can each be a result of combining a first doped zone  80  (not shown in  FIG. 3C ) and a second doped zone  82  (not shown in  FIG. 3C ). The first doped zone  80  can be located in the slab region  62  of the electro-absorption medium  61  and the second doped zone  82  can be located both in the ridge  60  and in the slab region  62  of the electro-absorption medium  61 . The first doped zone  80  and the second doped zone  82  included in the same doped region  72  each includes the same type of dopant. For instance, the first doped zone  80  and the second doped zone  82  in an n-type doped region  72  each includes an n-type dopant. The first doped zone  80  and the second doped zone  82  included in the same doped region  72  can have the same dopant concentration or different concentrations. Additionally, the first doped zone  80  can contact the second doped zone  82  so as to form the doped region  72  or can overlap with the second doped zone  82  so as to form the doped region  72 . In some instances, the first doped zone  80  and the second doped zone  82  overlap and at least a portion of the overlap is located in slab region  62  of the electro-absorption medium  61 . 
     Although  FIG. 3C  shows the doped region  72  not extending down to the optical insulator  48 , the doped region  72  can extend down to the optical insulator  48  or into the optical insulator  48 . 
     The modulator driver electronics  24  can operate the Franz-Keldysh modulators of  FIG. 3B  and  FIG. 3C  in a manner that is analogous to the operation of the Franz-Keldysh modulators of  FIG. 3A . Additional details about the fabrication, structure, incorporation into an optical device such as the transmitter, and operation of a modulator having a cross section according to  FIG. 3A  through  FIG. 3C  can be found in U.S. patent application Ser. No. 13/385,099, filed on Feb. 1, 2012, entitled “Optical Component Having Reduced Dependency on Etch Depth,” and incorporated herein in its entirety. 
     The modulators of  FIG. 3A  through  FIG. 3C  are suitable for use as the modulators of  FIG. 2A  because of their compact size. For instance, the width of the slab regions  62  in each of  FIG. 3A  through  FIG. 3C  (labeled W, slab region) can be less than 40 μm, 30 μm, or 20 μm. Additionally or alternately, the distance between the outermost end of the contact pad and the ridge  60  of the electro-absorption medium  61  (labeled d in  FIG. 3A ) can be less than 40 μm, 30 μm, or 10 μm. These small dimensions allow these modulators to have a very close proximity to other features on the transmitter. For instance, these dimension allow the modulators to be positioned near the edge of the transmitter. The ability to position these modulators near the edge of the transmitter makes it possible to achieve the small drive path lengths disclosed above. Another example of transmitter features that can be positioned close to these modulators are transition waveguides  18 . As discussed above, a portion of the modulators include transition waveguides  18  located between the modulator and the edge of the transmitter that is closest to the modulators. The ability to place the modulators close to these transition waveguides  18  increases the number of transition waveguides  18  that can be positioned between the edge and the modulators without affecting the modulation speed. 
     Modulator types other than Franz-Keldysh modulators can be employed in the above transmitter. However, where Franz-Keldysh modulators directly modulate intensity, many other modulator types modulate phase and are accordingly incorporated into a Mach-Zehnder interferometer in order to modulate intensity. Modulators that manipulate a depletion region in a waveguide are an example of a phase modulator that is typically incorporated into a Mach-Zehnder interferometer. Since the waveguides associated with a Mach-Zehnder interferometer require more space on the transmitter than a Franz-Keldysh modulator, the Franz-Keldysh modulator may provide a more compact transmitter. For instance, the length of the Franz-Keldysh modulators can be selected such that a distance from a location on one of the modulators to the same location on the next modulator (labeled P) in  FIG. 2A  is less than 2 mm, 1 mm, or 0.5 mm. 
     In the modulators of  FIG. 3A  through  FIG. 3C , the region of the light-transmitting medium  40  or electro-absorption medium  61  between the doped regions  72  can optionally be undoped or lightly doped as long as the doping is insufficient for the doped material to act as an electrical conductor that electrically shorts the modulator. 
     In the modulators of  FIG. 3A  through  FIG. 3C , the electrical field is essentially formed between the portions of the doped region  72  located in the ridge  60  of the electro-absorption medium  61 . As a result, the drive path length extends from the drive pads  66  to the location where the doped region  72  of the active ends under the ridge  60 . For instance, in the modulator of  FIG. 3A , one of the drive path lengths extends from one of the drive pads  66  to the location labeled DPL. In the case shown in  FIG. 3A , the drive path length consists of the length of the wire  68  from the drive pad  66  to the contact pad and the distance between the location where the wire  68  is bonded to the contact pad to the location labeled DPL. 
     Although  FIG. 2A  through  FIG. 3C  illustrate the first contact pad  63  being positioned on an opposite side of the ridge  60  of the electro-absorption medium  61  from the second contact pad  64 , the first contact pad  63  and the second contact pad  64  can be positioned on the same side of the ridge  60  of the electro-absorption medium  61 . In these instances, the transmitter can include a metal trace that extends from one of the contact pads, across the ridge  60  of the electro-absorption medium  61  and/or across the ridge  44  of light-transmitting medium  40 , and into contact with the doped region  72  on that side of the ridge  60  of the electro-absorption medium  61 . Accordingly, the metal trace provides electrical communication between a contact pad on one side of the modulator waveguide  25  and a doped region  72  on the other side of the modulator waveguide  25 . In these instances, the metal trace is part of the drive path length. As a result, the length of the metal trace can affect the modulation speed. 
     In one example of the transmitter, the waveguides (source waveguides  16 , transition waveguides  18  and output waveguides  20 ) are single mode waveguides. The single mode waveguides can also be large core single mode waveguides. For instance, each of the source waveguides  16 , transition waveguides  18  and output waveguides  20  can have a cross section according to  FIG. 2C  and can have a ridge  44  width (labeled w in  FIG. 2C ) greater than 1 μm or 2 μm and/or less than 4 μm or 5 μm, a ridge  44  height (labeled h in  FIG. 2C ) greater than 0.5 μm or 1 μm and/or less than 2 μm or 2.5 μm, and a thickness (labeled T in  FIG. 2C ) greater than 1 μm or 2 μm and/or less than 4 μm or 5 μm. In this same example of the transmitter, the modulators can each be constructed according to  FIG. 3A ,  FIG. 3B , or  FIG. 3C  with a modulator waveguide  25  having a ridge  60  width (labeled wr in  FIG. 3A ) greater than 0.2 μm or 0.4 μm and/or less than 0.8 μm or 1 μm, a ridge  60  height (labeled hr in  FIG. 3A ) greater than 1 μm or 2 μm and/or less than 2.8 μm or 3 μm, and a waveguide thickness (labeled Tr in  FIG. 2C ) greater than 1 μm or 2 μm and/or less than 4 μm or 5 μm. 
     Using the above transmitter construction and the above modulators, the transmitter and modulator driver electronics  24  can be constructed so as to provide drive path lengths less than 1 mm, 0.5 mm, or 0.25 mm. 
     The transmitter can include components in addition to the components shown in  FIG. 1A  through  FIG. 2C . For instance, the transmitter can include a tap or tap waveguide configured to tap off a portion of one of the light signals (transition signal, source signal, modulated signal). The transmitter can also include a light sensor or monitor that receives the tapped portion of the light signal. The light sensor or monitor can convert the received light signal to an electrical signal. Electronics can use the electrical signal to adjust the intensity of light being generated by the one or more lasers  10  in a feedback loop. In one example, the transmitter is constructed according to  FIG. 2A  and a tap waveguide taps off a portion of the light signal from the source waveguide  16 . The tap waveguide guides the tapped portion of the light signal to a light sensor and electronics adjust the output from the laser  10  in response to output from the light sensor. Additionally or alternately, the transmitter can include a combiner or multiplexer. For instance, the transmitter can include a combiner or multiplexer that combines two or more of the modulated signals into a single output signal that is processed further by the transmitter and/or is received by an optical fiber  26 . In one example, the transmitter of  FIG. 2B  includes a multiplexer that combines multiple modulated signals into an output signal that is an optical fiber  26 . In this instance, the lasers  10  that are the source of the combined modulated signals can each have a different wavelength so the output signal can be demuliplexed later. 
     Other embodiments, combinations and modifications of this invention will occur readily to those of ordinary skill in the art in view of these teachings. Therefore, this invention is to be limited only by the following claims, which include all such embodiments and modifications when viewed in conjunction with the above specification and accompanying drawings.