Patent Publication Number: US-2023143150-A1

Title: Optical device having a light-emitting structure and a waveguide integrated capacitor to monitor light

Description:
STATEMENT OF GOVERNMENT RIGHTS 
     This invention was made with Government support under Award No. DE-AR0001039 awarded by DOE, Office of ARPA-E. The Government has certain rights in this invention. 
    
    
     BACKGROUND 
     Optical systems include optical devices that can generate, process, and/or carry optical signals from one point to another point. In certain implementations, optical systems such as optical communication systems may facilitate data communication over longer distances with higher bandwidth using smaller cable width (or diameter) in comparison to communication systems using electrical wires. In an optical communication system, a light may be generated by a light source such as a laser. In some optical systems, external light monitoring devices such as photodiodes are used to monitor the light generated by the light source. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various examples will be described below with references to the following figures. 
         FIG.  1    depicts an example optical device having a light-emitting structure and a waveguide integrated capacitor for monitoring light. 
         FIG.  2    depicts a cross-sectional view of an example optical device. 
         FIG.  3    depicts a top view of an example optical device. 
         FIG.  4    depicts a top view of another example optical device. 
         FIG.  5    depicts a cross-sectional view of an example optical device. 
         FIG.  6    depicts a cross-sectional view of another example optical device. 
         FIG.  7    depicts a block diagram of an example optical system having an example optical device. 
         FIG.  8    depicts a block diagram of an example multi-chip module having an electronic chip and a photonic chip having an example optical device. 
         FIG.  9    depicts a flow diagram of an example method of fabricating an example optical device. 
         FIG.  10    depicts a flow diagram of another example method of fabricating an example optical device. 
     
    
    
     It is emphasized that, in the drawings, various features are not drawn to scale. In fact, in the drawings, the dimensions of the various features have been arbitrarily increased or reduced for clarity of discussion. 
     DETAILED DESCRIPTION 
     The following detailed description refers to the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the following description to refer to the same or similar parts. It is to be expressly understood that the drawings are for the purpose of illustration and description only. While several examples are described in this document, modifications, adaptations, and other implementations are possible. Accordingly, the following detailed description does not limit disclosed examples. Instead, the proper scope of the disclosed examples may be defined by the appended claims. 
     Optical systems may include various optical devices (e.g., components) such as, but not limited to, light sources (e.g., lasers), optical modulators, optical filters, optical amplifiers, optical couplers, waveguides, optical combiners, optical multiplexers, optical demultiplexers, optical resonators, or photodetectors (e.g., photodiodes). Some optical systems may include light monitoring circuits that monitor optical signals contained within one or more such optical components. Such monitoring of the light may be useful in correcting certain operational parameters, for example, biasing conditions in the presence of changing environmental conditions or aging of the optical devices. 
     A common technique used in some light monitoring circuits entails extracting a small portion of the light from an optical component using one or more optical splitters. The extracted light may be routed to one or more separate photodetectors (e.g., photodiodes) that convert the extracted light into an electrical signal (e.g., electrical current). Such extraction of the light and routing of the extracted light to the separate photodetectors may result in loss of useful optical power. Further, the photodetectors used to convert the light into electrical signals may not have been properly calibrated, resulting in an inaccurate measurement of the light. Moreover, in an optical system having several optical components (e.g., light source, ring resonators, etc. cascaded in a long chain), the use of the abovementioned light monitoring technique may result in increased optical losses. Additionally, the above-described light monitoring technique may suffer from uncertainties in the splitting ratio among the various splitters used, even if the designs of the splitters are similar. Furthermore, the light from the splitters may be routed across a chip to the photodetector which may lead to further uncertainties in the power level or unavoidable waveguide crossings. In some implementations, the use of additional structural elements such as the separate photodetectors may require additional space leading to an increase in the overall footprint of the optical system and/or require compromise on internal structure and/or efficiency of the ring resonator. 
     In accordance with one or more examples presented herein, an optical device such as an optical light source is provided that includes on-chip monitoring of photon density inside a cavity of the optical device without extracting a portion of the light into a separate detector thereby reducing losses and negative impacts from light reflections. The footprint of the example optical device is small and is compatible with the heterogeneous III-V on Silicon. 
     The example optical device includes a light-emitting structure to emit light upon application of electricity to the optical device. Further, the optical device includes a waveguide integrated capacitor that is formed integral to the structure of the optical device. In particular, the waveguide integrated capacitor may be a metal-oxide-semiconductor (MOS) capacitor formed under the light-emitting structure to monitor the light emitted by the light-emitting structure without extracting light out of the optical device (e.g., to a separate photodetector downstream of the light-emitting structure). In some examples, the waveguide integrated capacitor includes a waveguide region carrying at least a portion of the light emitted by the light-emitting structure. The waveguide region includes one or more photon absorption sites causing the generation of free charge carriers relative to an intensity of the light confined in the waveguide region resulting in a change in the conductance of the waveguide region. 
     In some examples, a monitoring circuit may be electrically coupled to the optical device to monitor light confined inside the optical device. In particular, the monitoring circuit may be electrically coupled to the waveguide integrated capacitor(s) at one or more monitoring sites within the optical device to cause the generation of electrical signals representative of intensities of light contained in the optical devices at the respective monitoring sites. Using the electrical signals generated via the waveguide integrated capacitor(s), the monitoring circuit may be configured to determine an optical parameter, such as but not limited to, an efficiency of the optical device. The use of the waveguide integrated capacitor may obviate the need for separate photodiodes to monitor the light, resulting in a compact footprint and reduced complexity of an optical system employing the proposed optical device. 
     Referring now to the drawings, in  FIG.  1   , an example optical device  100  is presented. The optical device  100  may be a light source such as a laser that may be disposed in an optical system (not shown) for generating light and providing light to other optical devices in the optical system. The optical device  100  of  FIG.  1    may include electrical contacts  102 A,  102 B,  102 C, a light-emitting structure  104 , and a waveguide integrated capacitor  106 . In the example depicted in  FIG.  1   , three electrical contacts  102 A- 102 C are shown for illustration purposes. In some other examples, the optical device  100  may include a fewer or greater number of electrical contacts. The optical device  100  may receive electrical power and/or reference monitoring voltages through one or more of the electrical contacts  102 A- 102 C. 
     Upon application of electrical power to the optical device  100  through one or more of the electrical contacts  102 A- 102 C, the light-emitting structure  104  may emit light. The light-emitting structure  104  may be a region of semiconductor material(s) that generates light based on the excitation of charge carriers (e.g., electrons) due to an electric field caused across the light-emitting structure by the applied electrical power. For example, the light-emitting structure  104  may be a diode such as a light-emitting diode. In some other examples, the light-emitting structure  104  may include a heterogeneous quantum well structure or a quantum dot structure to generate the light. Additional details of the light-emitting structure  104  are described in conjunction with  FIGS.  5 - 6   . 
     The waveguide integrated capacitor  106  may be a MOS capacitor formed within the device structure of the optical device  100  as opposed to photodiodes used with traditional optical devices that are built outside of the traditional optical devices and receiving light through a drop port. The waveguide integrated capacitor  106  may aid in the detection of light contained within the optical device  100  without diverting any portion of the light outside of the optical device  100 . In one example, the waveguide integrated capacitor  106  may include photon absorption sites (see  FIG.  2   ) that may cause the generation of free charge carriers relative to the intensity of the optical signal inside the optical device  100 . As will be understood, the generation of free charge carriers may result in a change (e.g., increase) in the conductance of a given region (e.g., a waveguide region) of the optical device. The changes in the conductance of the given region may cause variations in the current passing through the given region which may be monitored by the monitoring circuit (not shown in  FIG.  1   , see  FIG.  4   ). Additional details regarding an example waveguide integrated capacitor, such as the waveguide integrated capacitor  106 , are described with respect to  FIG.  2   . 
     Referring now to  FIG.  2   , a cross-sectional view  200  of an example optical device  202  is presented. The optical device  202  may be an example representative of the optical device  100  and include a light-emitting structure  204  and a waveguide integrated capacitor  206 . In some examples, the optical device  202  may include a waveguide region  207 , a first buffer semiconductor region  208 , and an insulating layer  210 . The waveguide region  207 , the first buffer semiconductor region  208 , and the insulating layer  210  together define a metal-oxide-semiconductor (MOS) capacitor, also referred herein as, the waveguide integrated capacitor  206 . In particular, the waveguide region  207  for a part of the waveguide integrated capacitor  206 . The insulating layer  210  may be formed between the waveguide region  207  and the first buffer semiconductor region  208  such that the insulating layer  210  may act as an electric insulator between two electrically conductive regions, for example, the waveguide region  207  and the first buffer semiconductor region  208 . 
     The optical device  202  may be formed using a substrate  212 . In some examples, the substrate  212  may be a silicon on insulator (SOI) substrate that may include a base substrate layer  216 , a base oxide layer  214 , and device layer  218 . The base substrate layer  216  may be made of semiconductor material, for example, silicon (Si). Other examples of materials that may be used to form the base substrate layer  216  may include III-V semiconductors, such as indium phosphide (InP), germanium (Ge), gallium arsenide (GaAs), aluminum gallium arsenide (AlGaAs), indium gallium arsenide (InGaAs), indium arsenide (InAs), or combinations thereof. Further, as depicted in  FIG.  2   , the substrate  212  may include a base oxide layer  214  disposed on an underlying base substrate layer  216 . For example, the base oxide layer  214  may be formed by oxidizing the substrate  212 . In the implementation of  FIG.  2   , for the base substrate layer  216  made of silicon, the base oxide layer  214  may comprise silicon dioxide (SiO 2 ), which may be formed in the presence of oxygen at a temperature in the range from 900° C. to 1380° C. In some examples, the base oxide layer  214  may be a buried oxide (BOX) layer (e.g., the SiO 2  may be buried in the base substrate layer  216 ). In some examples, a layer of the SiO 2  may be buried in the base substrate layer  216  at a depth ranging from less than 100 nm to several micrometers from the wafer surface depending on the application. Other examples of the base oxide layer  214  may include, but are not limited to, Silicon Nitride (Si 3 N 4 ), Aluminum oxide (Al 2 O 3 ), Hafnium Dioxide (HfO 2 ), diamond, silicon carbide (SiC), or combinations thereof. 
     Further, the substrate  212  may include a device layer  218  disposed on top of the base oxide layer  214 . In the example implementation of  FIG.  2   , the device layer  218  is composed of silicon. The device layer  218  may be suitably shaped (e.g., via techniques such as photolithography and etching) to form one or more regions, such as, the waveguide region  207  and a non-waveguide region  209  isolated via an air-trench  219 . The waveguide region  207  carries an optical signal during the operation of the optical device  202 . In some examples, the waveguide region  207  may include a first-type doping (e.g., p-type doping) or compensation doped to generate net doping of the first-type. In one example, the waveguide region  207  may represent a cross-section of an annular or ring-shaped optical waveguide (see  FIG.  4   , for example). In another example, the waveguide region  207  may represent a cross-section of a linear optical waveguide (see  FIG.  3   , for example). The waveguide region  207  may be undoped, resulting in improved sensitivity to a reference variable voltage applied by the monitoring circuit, for example, the monitoring circuit  108  depicted in  FIG.  1   . 
     As depicted in an enlarged view  222  of a portion  224  of the waveguide region  207 , the waveguide region  207  may have one or more photon absorption sites  226 . The term “photon absorption sites” as used herein may refer to crystal imperfections or defects in the bulk of the material of the waveguide region  207 , surface imperfections at the boundaries of the waveguide region  207 , or both. In some examples, the photon absorption sites may have resulted from imperfections in the manufacturing process. In some examples, some photon absorption sites may be intentionally created. The photon absorption sites  226  may absorb photons and cause the generation of free charge carriers relative to the intensity of the optical signal impinging thereon inside the waveguide region  207 . The conductance of the waveguide region  207  depends on the amount of the free charge carriers, such that an increase in the optical signal causes an increase in the conductance of the waveguide region  207 . 
     The insulating layer  210  is disposed over the waveguide region  207  and/or the non-waveguide region  209 . In particular, the insulating layer  210  is formed such that the insulating layer  210  is sandwiched between the waveguide region  207  and the first buffer semiconductor region  208 . The insulating layer  210  may be formed of one or more dielectric materials, including but not limited to, native oxides of the materials of the waveguide region  207  or the first buffer semiconductor region  208 , or both, or external dielectric materials such as high-k dielectrics or polymers which can be formed by deposition, oxidation, wafer bonding or other dielectric coating methods. Other non-limiting examples of the dielectric materials that can be used to form the insulating layer  210  may include, SiO 2 , Si 3 N 4 , Al 2 O 3 , HfO 2 , polyimide, benzocyclobutene (BCB), or combinations thereof. 
     Further, the first buffer semiconductor region  208  may be made of semiconductor material, such as a III-V semiconductor material. Examples of the III-V semiconductor materials that may be used to form the first buffer semiconductor region  208  may include, but are not limited to, GaAs, Gallium nitride (GaN), or Indium nitride (InN). The first buffer semiconductor region  208  may be formed over the insulating layer  210  using techniques such as, but not limited to, deposition, wafer bonding, monolithic growth, or other fabrication techniques. In some examples, the first buffer semiconductor region  208  may include a second-type doping (e.g., n-type doping) different from the first-type doping. 
     The light-emitting structure  204  may be representative of an example of the light-emitting structure  104  and is capable of generating light based on the excitation of charge carriers (e.g., electrons) due to an electric field caused across the light-emitting structure  204  by the electrical power applied via metal contacts (described later). For example, the light-emitting structure  204  may be a diode such as a light-emitting diode. In some other examples, the light-emitting structure  204  may include a heterogeneous quantum well structure (see  FIG.  5   ) or a quantum dot structure (see  FIG.  6   ) to generate the light. 
     The light-emitting structure  204  may be formed over at least a portion of the first buffer semiconductor region  208 . In particular, the light-emitting structure  204  may be formed on a surface of the first buffer semiconductor region  208  above the waveguide region  207 . The light-emitting structure  204  may include an optical gain region  236  and a second buffer semiconductor region  238 . The optical gain region  236  may be formed over the waveguide integrated capacitor  206 , more particularly, on the surface of the first buffer semiconductor region  208  above the waveguide region  207 . The second buffer semiconductor region  238  may be formed over the optical gain region  236 . The second buffer semiconductor region  238  may be made of semiconductor material, such as III-V semiconductor materials, for example, GaAs, GaN, or InN. In some examples, the second buffer semiconductor region  238  may have a different type of doping as compared to the first buffer semiconductor region  208 . In particular, if the first buffer semiconductor region includes the second-type (e.g., n-type) doping, the second buffer semiconductor region  238  may include the first-type (e.g., p-type) doping. Forming the buffer semiconductor regions  208  and  238  to have such different types of doping may lower the optical propagation loss inside the optical device  202 . 
     Furthermore, in some examples, the optical device  202  may include a first contact region  228 , a second contact region  230 , and a third contact region  231 . For illustration purposes, in  FIG.  2   , the contact regions  228 ,  230 , and  231  are shown as made of silicon. In some other examples, the contact regions  228  and  230  may be made of other semiconductor materials including, but not limited to, InP, Ge, GaAs, AlGaAs, InGaAs, or combinations thereof. The first contact region  228  may include the first-type doping and is disposed in contact with the waveguide region  207 . Further, the second contact region  230  may include the second-type doping and is disposed in contact with the first buffer semiconductor region  208 . The third contact region  231  may include the first-type doping and is disposed in contact with the second buffer semiconductor region  238 . In particular, in some examples, the third contact region  231  may be formed over the second buffer semiconductor region  238 . 
     Moreover, in some examples, the optical device  202  may include metal contacts, such as, a first metal contact  232 , a second metal contact  234 , and a third metal contact  235  (hereinafter collectively referred to as metal contacts  232 - 235 ). As depicted in  FIG.  2   , the first metal contact  232  and the second metal contact  234  are respectively disposed in electrical contact (e.g., in direct physical contact or via any intermediate electrically conductive material) with the first contact region  228  and the second contact region  230 . The third metal contact  235  is disposed in electrical contact with the third contact region  231 . In some examples, the metal contacts  232 ,  234 , and  235  may be formed on top of (i.e., vertically over) the first contact region  228 , the second contact region  230 , and the third metal contact  235 , respectively. Examples of materials used to form the metal contacts  232  and  234  may include, but are not limited to, copper (Cu), gold (Au), Al, and/or platinum (Pt). In an example, in an optical system (see  FIG.  7   , for example), a monitoring circuit may be electrically connected to the contact regions  228  and  230  via respective metal contacts  232  and  234 . Further, for the optical device  202  to generate light, operating electric power may be applied to the optical device  202  across the metal contacts  234  and  235 . 
     For example, during operation, electrical power (e.g., operating voltage) may be applied to the optical device  202  across the metal contacts  234  and  235 . The application of the operating voltage may cause the generation of light through the optical gain region  236 . At least a portion of the generated light may be confined in the waveguide region  207 , the first buffer semiconductor region  208 , and the optical gain region  236 . Such confinement of light in the waveguide region  207  (also referred to as a modal overlap) allows for efficient coupling into passive regions where the first buffer semiconductor region  208  is etched off to create laser mirrors (not shown) or into other devices in a photonic integrated circuit that are composed entirely of silicon, such as waveguides, modulators, detectors, multiplexers, de-multiplexers, etc. 
     The waveguide integrated capacitor  206  aids in monitoring the light confined into the waveguide region  207  without using any external photodiodes or other devices such as splitters. To monitor the light emitted by the light-emitting structure  204 , a reference voltage is applied to the second contact region  230 , and a current flowing through the first contact region is measured. As previously noted, the photon absorption sites  226  may absorb photons and cause the generation of free charge carriers relative to the intensity of the optical signal impinging thereon inside the waveguide region  207 . The conductance of the waveguide region  207  depends on the amount of the free charge carriers, such that an increase in the optical signal causes an increase in the conductance of the waveguide region  207 . Consequently, the current flowing through the waveguide integrated capacitor  206  may vary. The variation in the current is proportional to the change in the conductance of the waveguide region indicative of the light emitted by the light-emitting structure. As will be appreciated, the use of a waveguide integrated capacitor such as the waveguide integrated capacitor  206  may aid in the detection of light within the optical device without the need of diverting any portion of the light out of the waveguide region  207 . Further, in some examples, the use of the waveguide integrated capacitor may obviate the need for separate photodiodes to monitor the light, resulting in a compact footprint and reduced complexity of the proposed optical system. Moreover, in some examples, by using the waveguide integrated capacitors in the optical components and a common monitoring circuit, tasks such as operation monitoring and debugging can be easily performed in the proposed optical system. 
     Turning to  FIG.  3   , a top view  300  of an example optical device  302  is depicted. The optical device  302  is a linear light source (e.g., laser). The optical device  302  may be an example representative of the optical device  202  and may include one or more material regions that are similar to those described in  FIG.  2    for the optical device  202  although not all such regions are depicted in  FIG.  3    for simplicity of illustration. In some examples, instead of having a continuous contact region such as the contact region  228  along the circumference of the waveguide region  207  of  FIG.  2   , the contact region may be split into a plurality of sections to allow monitoring of light intensities at respective locations inside the optical device  302 . For example, the optical device  302  may include contact regions such as the contact regions  304 ,  306 , and contact region sections  308 A,  308 B, and  308 C. For illustration purposes, in the top-view  300  of  FIG.  3   , the contact regions  304 ,  306 , a waveguide region  307 , and the contact region sections  308 A,  308 B, and  308 C are depicted. Although the optical device  302  is shown to include three contact region sections  308 A- 308 C, the use of a fewer or greater number of contact region sections is envisioned within the purview of the present disclosure. The optical device  302  may have the same cross-section as depicted in  FIG.  2    at several locations along the length “L” of the optical device  302 , more particularly, when taken at the locations of the contact region sections  308 A,  308 B, and  308 C. For example, a cross-section of the optical device  302  taken at the example location  3 - 3  along the contact region section  308 B may look similar to the cross-sectional view  200  depicted in  FIG.  2   . Also, the optical device  302  may include metal contacts (not shown) disposed in contact with each of the contact regions  304 ,  306 , and the contact region sections  308 A- 308 C. 
     The contact regions  304  and  306  are examples representative of the contact regions  230  and  231  of  FIG.  2    and are disposed similarly as the contact regions  228  and  231 . In particular, the contact region  304  is disposed in contact with the first buffer semiconductor region (not shown) of a waveguide integrated capacitor of the optical device  302  along the length of the optical device  302 . The contact region  306  is disposed over a second buffer semiconductor region (similar to the second buffer semiconductor region  238 , not shown) of a light-emitting structure of the optical device  302 . Further, the contact region sections  308 A- 308 C are representative of the contact region  228  of  FIG.  2   . In particular, the contact region sections  308 A- 308 C are physically and/or electrically isolated from each other and are disposed over the waveguide region  307 . 
     In one example, to measure the light inside the optical device  302 , a reference sinusoidal voltage may be applied to the contact region  304 , and electrical current flowing through one or more of the contact region sections  308 A- 308 C may be monitored via a monitoring circuit (not shown). As previously noted, in a similar fashion as described with reference to the waveguide region  207 , changes in the light intensities inside the waveguide region  307  may also cause changes in the conductance of the waveguide region  307 . Consequently, the current flowing through the waveguide integrated capacitor of the optical device  302  may vary. The variation in the current is proportional to the change in the conductance of the waveguide region  307  indicative of the light emitted by the light-emitting structure. Measurement of electrical current through the contact region sections  308 A- 308 C may be indicative of light intensities contained inside the waveguide region at the location of the respective contact region sections. 
     Referring now to  FIG.  4   , a top view  400  of an example optical device  402  is presented. The optical device  402  may be an annular light source (e.g., a ring laser). The optical device  402  may be an example representative of the optical device  202  and may include one or more material regions that are similar to those described in  FIG.  2    for the optical device  202  although not all such regions are depicted in  FIG.  4    for simplicity of illustration. In the example implementation shown in  FIG.  4   , the optical device  402  is shown to include a coupling waveguide  401  and a ring laser waveguide  403 . The coupling waveguide  401  may be disposed adjacent to and is evanescently coupled to the ring laser waveguide  403 . The coupling waveguide  401  may include output ports  405 A and  405 B. The light generated in the ring laser waveguide  403  may be coupled into the coupling waveguide  401  and can be supplied to other optical devices (not shown) via one or both of the output ports  405 A and  405 B. In some examples, the ring laser waveguide  403  may include a waveguide integrated capacitor such as the waveguide integrated capacitor  206  shown in  FIG.  2    to detect the light intensity inside the ring laser waveguide  403 . In some examples, both the coupling waveguide  401  and the ring laser waveguide  403  may include waveguide integrated capacitors. 
     In a similar fashion as described in  FIG.  3   , in the optical device  402 , one or more of the contact regions may be split into a plurality of sections to allow monitoring of light intensities at respective locations inside the optical device  402 . For example, the optical device  402  may include contact regions such as the contact regions  404 ,  406 , and contact region sections  408 A,  408 B, and  408 C. Instead of having a single contact region such as the contact region  228  along the circumference of the waveguide region  207  of  FIG.  2   , the optical device  402  includes contact region sections  408 A,  408 B, and  408 C. For illustration purposes, in the top view  400  of  FIG.  4   , the contact regions  404 ,  406 , a waveguide region  407 , and the contact region sections  408 A,  408 B, and  408 C are depicted. Although the optical device  302  is shown to include three contact region sections  408 A- 408 C, the use of a fewer or greater number of contact region sections is envisioned within the purview of the present disclosure. The optical device  402  may also have the same cross-section as depicted in  FIG.  2    at several locations along the annulus of the ring laser waveguide  403 , more particularly, when taken at the locations of the contact region sections  408 A,  408 B, and  408 C. For example, a cross-section of the optical device  402  taken at the example location  4 - 4  along the contact region section  408 B may look similar to the cross-sectional view  200  depicted in  FIG.  2   . Also, the optical device  402  may include metal contacts (not shown) disposed in contact with each of the contact regions  404 ,  406 , and the contact region sections  408 A- 408 C. 
     The contact regions  404  and  406  are example representatives of the contact regions  230  and  231  of  FIG.  2    and are disposed in a similar fashion as the contact regions  230  and  231 . In particular, the contact region  404  is disposed in contact with the first buffer semiconductor region (not shown) of a waveguide integrated capacitor of the optical device  402  along the annulus of the optical device  402 . Further, the contact region  406  is disposed over a second buffer semiconductor region (similar to the buffer semiconductor region  238 , not shown) of a light-emitting structure of the optical device  402 . Further, the contact region sections  408 A- 408 C are representatives of the contact region  228  of  FIG.  2   . In particular, the contact region sections  408 A- 408 C are physically isolated from each other and are disposed over the waveguide region  407  along the circumference of the waveguide region  407 . 
     In one example, to measure the light inside the optical device  402 , a reference sinusoidal voltage may be applied to the contact region  404 , and electrical current through one or more of the contact region sections  408 A- 408 C may be monitored via a monitoring circuit (not shown). As previously noted, in a similar fashion as described with reference to the waveguide region  207 , changes in the light intensities inside the waveguide region  407  may also cause changes in the conductance of the waveguide region  407 . Consequently, the current flowing through the waveguide integrated capacitor of the optical device  402  may vary. The variation in the current is proportional to the change in the conductance of the waveguide region indicative of the light emitted by the light-emitting structure. Measurement of electrical current through the contact region sections  408 A- 408 C may indicate light intensities contained inside the waveguide region at the location of the respective contact region sections. 
       FIG.  5    depicts a cross-sectional view  500  of an optical device  502 , in accordance with an example. The optical device  502  of  FIG.  5    may be representative of one example of the optical device  202  of  FIG.  2    and may include one or more structural elements similar, in one or more aspects, to those described in  FIG.  2   —description of which is not repeated herein for the sake of brevity. For example, in  FIG.  5   , the optical device  502  is shown to include a light-emitting structure  504 , a waveguide integrated capacitor  506 , a waveguide region  507 , a first buffer semiconductor region  508 , an insulating layer  510 , a substrate  512  having a base substrate layer  516 , a base oxide layer  514 , and a device layer  518 , non-waveguide region  509 , contact regions  528 ,  530 , and metal contacts  532 ,  534 . 
     Further, the light-emitting structure  504  may include optical gain region  536 , a second buffer semiconductor region  538 , a contact region  531 , and a metal contact  535 . The waveguide region  507  and the non-waveguide region  509  are formed in the device layer  518  of the substrate  512 . The waveguide region  507  includes photon absorption sites (not shown) similar to the photon absorption sites  226  shown in  FIG.  2   . The insulating layer  510  is disposed over the waveguide region  507  and/or the non-waveguide region  509 . Further, the first buffer semiconductor region  508  is formed over the insulating layer  510 . The light-emitting structure  504  may be formed over at least a portion of the first buffer semiconductor region  508 . In particular, the light-emitting structure  504  may be formed on a surface of the first buffer semiconductor region  508  above the waveguide region  507 . 
     In the example of  FIG.  5   , the optical gain region  536  in the light-emitting structure  504  includes a quantum well structure. In  FIG.  5   , the optical gain region  536  is shown to include a quantum well region  540 . In particular, the quantum well region  540  is made of several III-V layers of different composition and doping to provide lateral carrier (electrons and holes) confinement and are also referred to as separate confinement heterostructures (SCH) and quantum wells (QW) or as an active region. In the quantum well region  540 , carriers are confined in one dimension but can move in the other two dimensions. The carrier distribution in the quantum well region  540  caused by a current flow through optical gain region  536 , facilitates light generation or optical gain at a wavelength close to the bandgap of the material making up the quantum well region  540 . 
       FIG.  6    depicts a cross-sectional view  600  of an example optical device  602 . The optical device  602  of  FIG.  6    may be representative of one example of the optical device  202  of  FIG.  2    and may include one or more structural elements similar, in one or more aspects, to those described in  FIG.  2   —description of which is not repeated herein for the sake of brevity. For example, in  FIG.  6   , the optical device  602  is shown to include a light-emitting structure  604 , a waveguide integrated capacitor  606 , a waveguide region  607 , a first buffer semiconductor region  608 , an insulating layer  610 , a substrate  612  having a base substrate layer  616 , a base oxide layer  614 , and a device layer  618 , non-waveguide region  609 , contact regions  628 ,  630 , and metal contacts  632 ,  634 . The light-emitting structure  604  may include optical gain region  636 , a second buffer semiconductor region  638 , a contact region  631 , and a metal contact  635 . The waveguide region  607  and the non-waveguide region  609  are formed in the device layer  618  of the substrate  612 . The waveguide region  607  may include photon absorption sites (not shown) similar to the photon absorption sites shown in  FIG.  2   . The insulating layer  610  is disposed over the waveguide region  607  and/or the non-waveguide region  609 . Further, the first buffer semiconductor region  608  is formed over the insulating layer  610 . The light-emitting structure  604  may be formed over at least a portion of the first buffer semiconductor region  608 . In particular, the light-emitting structure  604  may be formed on a surface of the first buffer semiconductor region  608  above the waveguide region  607 . 
     In the example of  FIG.  6   , the optical gain region  636  in the light-emitting structure  604  includes a quantum dot structure. In  FIG.  3   , the optical gain region  636  is shown to include a quantum dot region  640 . The quantum dot region  640  is made of several III-V layers of different compositions and doping to provide lateral carrier (electrons and holes) confinement and are also referred to as SCH, quantum dots, or an active region. In particular, the quantum dot region  640  is confined in three dimensions and has a discrete energy spectrum, like an atom. The carrier distribution in the quantum dot region  640  caused by a current flow through optical gain region  636 , facilitates light generation or optical gain at a wavelength close to the bandgap of the material making up the quantum dot region  640 . 
       FIG.  7    presents an example optical system  700 . The optical system  700  may include an optical device such as the optical device  202  capable of generating light and a monitoring circuit  702  to monitor the light inside the optical device  100 . In some examples, the optical system  700  may include more than one optical device, without limiting the scope of the present disclosure. 
     As previously noted, the optical device  202  includes the light-emitting structure  204  that generates light, and the waveguide integrated capacitor  206  that aids in detecting the light intensity inside the optical device  202  without extracting the light outside of the waveguide region  207 . Upon application of a voltage across the waveguide integrated capacitor  206 , the photon absorption sites  226  may cause the generation of free charge carriers relative to the intensity of the optical signal inside the optical device. As will be understood, the generation of free charge carriers may result in a change (e.g., increase) in the conductance of a given region (e.g., a waveguide region  207 ) within the respective optical components. The changes in the conductance of the given region may cause variations in the current passing through the given region which may be monitored by the monitoring circuit  702 . 
     The monitoring circuit  702  may be electrically coupled to the waveguide integrated capacitor  206  at one or more monitoring sites. In some examples, the monitoring circuit  702  may cause the waveguide integrated capacitor  206  to generate electrical signals indicative of the light intensities at the monitoring site. To effect the generation of the electrical signals, in some examples, the monitoring circuit  702  may include a lock-in amplifier  708  and a preamplifier  710 . In some examples, the lock-in amplifier  708  may generate a reference variable voltage signal, for example, a sinusoidal signal. For a given waveguide integrated capacitor, the lock-in amplifier  708  may determine a frequency of the reference variable voltage signal based on the conductance of a waveguide region in the given waveguide integrated capacitor and a capacitance of the given waveguide integrated capacitor. In one example, the lock-in amplifier  708  may determine the frequency (F 0 ) of the reference variable voltage signal based on an example relationship of equation (1). 
     
       
         
           
             
               
                 
                   
                     F 
                     0 
                   
                   = 
                   
                     
                       G 
                       WG 
                     
                     
                       2 
                       ⁢ 
                       π 
                         
                       * 
                       C 
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                       
                   
                     ( 
                     1 
                     ) 
                   
                 
               
             
           
         
       
     
     where, G WG  represents the conductance of the waveguide region  207  in the given waveguide integrated capacitor and C represents the capacitance of the given waveguide integrated capacitor  206 . In certain other examples, the frequency (F 0 ) of the reference variable voltage signal may be set to any value greater than 
     
       
         
           
             
               
                 G 
                 WG 
               
               
                 2 
                 ⁢ 
                 π 
                   
                 * 
                 C 
               
             
             . 
           
         
       
     
     The monitoring circuit  702  may apply a reference variable voltage signal having the frequency F 0  to the waveguide integrated capacitor  206 . As previously noted, the conductance of the waveguide region  207  may change depending on the intensity of the optical signal therein. Consequently, the current flowing through the waveguide region  207  may also vary. In particular, the magnitude of electrical current generated by the waveguide integrated capacitor  206  may be influenced by the light intensity inside the waveguide region  207 , because the conductance of the waveguide region at the monitoring site changes due to the presence of free carriers created by the absorption of photons at the photon absorption sites  226 . 
     The monitoring circuit  702  may measure the electrical signals (e.g., electrical currents) through the waveguide integrated capacitor  206  that is in turn representative of the intensity of the light contained inside waveguide region  207 . In some examples, the electrical current received by the monitoring circuit  702  from the waveguide integrated capacitor  206  may be weak in strength. The preamplifier  710  may amplify the electrical currents for further processing by the lock-in amplifier  708 . 
       FIG.  8    depicts a block diagram of an example multi-chip module  800 . In some examples, the multi-chip module  800  is implemented as a sub-system within an electronic system such as, but not limited to, computers (stationary or portable), servers, storage systems, wireless access points, network switches, routers, docking stations, printers, or scanners. Such electronic systems may be offered as stand-alone products, packaged solutions, and can be utilized on one-time full product/solution purchases or pay-per-use or consumption basis. In an example implementation, the multi-chip module may include at least one electronic chip such as an electronic chip  802  and at least one photonic chip such as a photonic chip  804  mounted on a circuit board  806 . The circuit board  806  may be a printed circuit board (PCB) that includes several electrically conductive traces (not shown) to interconnect the electronic chip  802  and the photonic chip  804  with each other and with other components disposed on or outside of the circuit board  806 . Non-limiting examples of the electronic chip  802  may include IC chips such as, but not limited to, an application-specific integrated circuit (ASIC), a field-programmable gate array (FPGA) chip, a processor chip (e.g., central processing unit and/or graphics processing unit), a memory chip, a wireless communication module chip, power supply chips or modules, electronic devices such as capacitors, inductors, resistors, or the like. In one example, the electronic chip  802  may be configured to operate as a photonics controller. During the operation of the multi-chip module  800 , the electronic chip  802  may be configured to send and receive data and/or control signals to the photonic chip  804 . 
     The photonic chip  804  may include one or more optical devices such as but not limited to, or optical detectors, optical filters, optical cables, waveguides, optical modulators, light sources (e.g., lasers), and the like. The photonic chip  804  may function as an optical receiver, optical transmitter, optical transceiver, optical communication and/or processing medium for the data and control signals received from the electronic chip. In some examples, the photonic chip  804  may include an optical device  202  as depicted in  FIG.  2   , for example, the description of which is not repeated herein for the sake of brevity. Use of the other optical devices such as the optical devices  102 ,  302 ,  402 ,  502 , or  602  in the photonic chip  804  is also envisioned within the purview of the present disclosure. Further, in some examples, the electronic chip  802  may also function as a monitoring circuit, such as, the monitoring circuit  702  described in  FIG.  7   , to monitor light inside the optical device  202  of the photonic chip  804 . In certain other examples, the multi-chip module  800  may include an additional electronic chip or circuit that functions as the monitoring circuit  702 . 
       FIG.  9    depicts an example method  900  of forming an optical device such as the optical device  100  of  FIG.  1   . For illustration purposes the method  900  is described in conjunction with  FIG.  1   , however, the method steps described herein may also apply to other example optical devices described hereinabove. 
     At block  902 , a substrate is provided. The substrate may be SOI substrate in one example. Further at block  904 , a waveguide integrated capacitor such as the waveguide integrated capacitor  106  may be formed using the substrate wherein the waveguide integrated capacitor may include a waveguide region comprising one or more photon absorption sites. The photon absorption sites may be imperfections in the bulk of the material of the waveguide region, surface imperfections at the boundaries of the waveguide region, or both. In some examples, photon absorption sites may have resulted from imperfections in the manufacturing process. In some examples, photon absorption sites may be intentionally created. Further, at block  906 , a light-emitting structure such as the light-emitting structure  104  may be formed over the waveguide integrated capacitor, wherein the light-emitting structure emits light upon application of electricity to the optical device. Additional details of forming the waveguide integrated capacitor and the light-emitting structure are described in conjunction with  FIG.  10   . The waveguide region contains at least a portion of the light generated by the light-emitting structure and the photon absorption sites cause the generation of free charge carriers relative to an intensity of the light confined in the waveguide region resulting in a change in the conductance of the waveguide region. 
     Referring now to  FIG.  10   , an example method  1000  of forming an optical device such as the optical device  202  of  FIG.  2   . For illustration purposes the method  1000  is described in conjunction with  FIG.  2   , however, the method steps described herein may also apply to other example optical devices described hereinabove. 
     At block  1002 , a substrate, such as, the substrate  212  may be provided. The substrate  212  may be an SOI substrate having the base substrate layer  216 , the base oxide layer  214 , and the device layer  218 . Further, at block  1004 , a waveguide integrated capacitor such as the waveguide integrated capacitor  206  may be formed using the substrate  212 . In one example, forming the waveguide integrated capacitor  206  may include forming, at block  1006 , the waveguide region  207  and the non-waveguide region  209  into the substrate  212 . As previously noted, in some examples, the imperfections are intentionally formed into the waveguide region  207  to have photon absorption sites such as the photon absorption sites  226 . In some cases, the photon absorption sites  226  are caused by imperfections in the manufacturing processes. Further, in some examples, the waveguide region  207  may be lightly doped to achieve a first-type doping. In particular, the waveguide region  207  and the non-waveguide region  209  may be formed by photolithographically defining and masking the areas for waveguide region  207  and the non-waveguide region  209  and then chemically and/or mechanically etching the unmasked areas. Further, forming the waveguide integrated capacitor  206  may include forming, at block  1008 , the insulating layer  210  over the waveguide region  207  and the non-waveguide region  209 . The insulating layer  210  may be formed using thermal growth techniques and/or using deposition techniques, such as, chemical vapor deposition (CVD), for example. Furthermore, forming the waveguide integrated capacitor  206  may include forming, at block  1010 , a first buffer semiconductor region  208  over the insulating layer  210  using thermal growth techniques and/or using deposition techniques, such as, CVD, for example, or wafer bonding. 
     Moreover, in some examples, the method  1000  may include forming, at block  1012 , the light-emitting structure such as the light-emitting structure  204  over the waveguide integrated capacitor  206 . The light-emitting structure  204  may be formed by forming an optical gain region such as the optical gain region  236  and a second buffer semiconductor region such as the buffer semiconductor region  238 . For example, at block  1014 , the optical gain region  236  may be formed over the first buffer semiconductor region  208 . As described earlier, the optical gain region  236  may include a quantum well structure (see  FIG.  5   ) or a quantum dot structure (see  FIG.  6   ). In particular, in some examples, the optical gain region  236  may be formed on the top surface of the first buffer semiconductor region  208  above the waveguide region  207  using techniques such as, but not limited to, thermal growth or CVD, wafer bonding, molecular beam epitaxy (MBE) for example. Further, at block  1016 , the second buffer semiconductor region  238  may be formed over the optical gain region  236  using similar techniques used to form the first buffer semiconductor region  208 . 
     Moreover, in some examples, at block  1018 , one or more contact regions such as the contact regions  228 ,  230 , and  231  may be formed. The contact regions  228 ,  230 , and  231  may be formed using techniques such as, but not limited to, thermal growth and/or CVD, wafer bonding, MBE and performing doping with respective impurities. For example, the contact regions  228  and  231  are doped to include first-type doping, and the contact region  230  is doped to include the second-type doping. Further, in some examples, at block  1020 , metal contacts such as the metal contact  232 ,  234 , and  235  are formed over the contact regions  228 ,  230 , and  231 , respectively. 
     The terminology used herein is for the purpose of describing particular examples and is not intended to be limiting. As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. The term “another,” as used herein, is defined as at least a second or more. The term “coupled to” as used herein, is defined as connected, whether directly without any intervening elements or indirectly with at least one intervening element, unless indicated otherwise. For example, two elements may be coupled to each other mechanically, electrically, optically, or communicatively linked through a communication channel, pathway, network, or system. Further, the term “and/or” as used herein refers to and encompasses any and all possible combinations of the associated listed items. It will also be understood that, although the terms first, second, third, etc. may be used herein to describe various elements, these elements should not be limited by these terms, as these terms are only used to distinguish one element from another unless stated otherwise or the context indicates otherwise. As used herein, the term “includes” means includes but not limited to, the term “including” means including but not limited to. The term “based on” means based at least in part on. 
     While certain implementations have been shown and described above, various changes in form and details may be made. For example, some features and/or functions that have been described in relation to one implementation and/or process may be related to other implementations. In other words, processes, features, components, and/or properties described in relation to one implementation may be useful in other implementations. Furthermore, it should be appreciated that the systems and methods described herein may include various combinations and/or sub-combinations of the components and/or features of the different implementations described. Moreover, method blocks described in various methods may be performed in series, parallel, or a combination thereof. Further, the method blocks may as well be performed in a different order than depicted in flow diagrams. 
     Further, in the foregoing description, numerous details are set forth to provide an understanding of the subject matter disclosed herein. However, an implementation may be practiced without some or all of these details. Other implementations may include modifications, combinations, and variations from the details discussed above. It is intended that the following claims cover such modifications and variations.