Patent Publication Number: US-2020280171-A1

Title: Single-facet, variable-confinement optical waveguide amplifier

Description:
TECHNICAL FIELD 
     Embodiments presented in this disclosure generally relate to integrating a variable-confinement optical waveguide amplifier with a semiconductor-based photonic chip. 
     BACKGROUND 
     Several challenges are encountered when integrating laser sources or other optically active components with a semiconductor-based photonic chip. For example, an efficient coupling of light between the laser source and the photonic chip can require a complex and costly optical alignment process. To support higher data rates (e.g., through faster modulation and/or more optical channels), the laser source may be scaled to higher power levels. In some cases, additional optical components such as lenses and isolators may be needed to protect against optical feedback. In some cases, it may be necessary to attach a laser source to a submount before integrating with the photonic chip, which increases fabrication costs and reduces overall fabrication yields. 
     Semiconductor optical amplifiers (SOAs) are another possibility for achieving the higher optical power levels needed for higher data rates. Some applications of SOAs include fiber-to-fiber or waveguide-to-waveguide amplification, and input and output ports are typically located on opposing facets of a substrate. However, aligning the opposing facets with waveguides of a photonic chip is a complex process, often requiring alignment along six axes. Alignment is made even more difficult by the small inherent mode size of conventional SOAs, requiring a sub-micron alignment. Further, output power is limited by low saturation power of the SOA structures. Reflective SOAs (RSOAs) may also be used as a gain medium for an external cavity laser that interfaces at a single facet with a photonic chip. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the above-recited features of the present disclosure can be understood in detail, a more particular description of the disclosure, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this disclosure and are therefore not to be considered limiting of its scope, for the disclosure may admit to other equally effective embodiments. 
         FIG. 1  is a top view of an optical system comprising a variable-confinement optical waveguide emitter, according to one or more embodiments. 
         FIGS. 2 and 3  are cross-sectional views of a variable-confinement slab-coupled optical waveguide (SCOW) emitter, according to one or more embodiments. 
         FIG. 4  is a side view of a variable-confinement SCOW emitter, according to one or more embodiments. 
         FIG. 5  is a side view of a plurality of layers providing a tapering in a first dimension, according to one or more embodiments. 
         FIG. 6  is a top view of one or more layers providing a tapering in a second dimension, according to one or more embodiments. 
         FIG. 7  is a top view of a variable-confinement optical waveguide emitter having an extended optical confinement region, according to one or more embodiments. 
         FIG. 8  is a top view of a variable-confinement optical waveguide emitter having a partially passive turning waveguide section, according to one or more embodiments. 
         FIG. 9  is a top view of a variable-confinement optical waveguide emitter having a partially passive input waveguide section and output waveguide section, according to one or more embodiments. 
         FIG. 10  is a top view of a variable-confinement optical waveguide emitter with a transition waveguide section providing multiple optical confinement transitions, according to one or more embodiments. 
         FIG. 11  is a top view of a variable-confinement optical waveguide emitter with turning mirrors in the turning waveguide section, according to one or more embodiments. 
         FIG. 12  is a method for use with an optical waveguide emitter disposed on a semiconductor substrate, according to one or more embodiments. 
     
    
    
     To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements disclosed in one embodiment may be beneficially utilized on other embodiments without specific recitation. 
     DESCRIPTION OF EXAMPLE EMBODIMENTS 
     Overview 
     One embodiment presented in this disclosure is an optical apparatus comprising a semiconductor substrate and an optical waveguide emitter. The optical waveguide emitter comprises an input waveguide section extending from a facet of the semiconductor substrate, a turning waveguide section optically coupled with the input waveguide section, and an output waveguide section extending to the same facet and optically coupled with the turning waveguide section. One or more of the input waveguide section, the turning waveguide section, and the output waveguide section comprises an optically active region. 
     Another embodiment presented in this disclosure is an optical system comprising a photonic chip comprising a first waveguide and a second waveguide, a semiconductor substrate comprising a facet, and an optical waveguide emitter disposed on the semiconductor substrate. The optical waveguide emitter comprises an input waveguide section extending from the facet and optically coupled with the first waveguide, a turning waveguide section optically coupled with the input waveguide section; and an output waveguide section extending to the facet and optically coupled with the turning waveguide section. One or more of the input waveguide section, the turning waveguide section, and the output waveguide section comprises an optically active region. 
     Another embodiment presented in this disclosure is a method for use with an optical waveguide emitter disposed on a semiconductor substrate. The method comprises receiving an optical signal at an input waveguide section extending from a facet of the semiconductor substrate, propagating the optical signal through a turning waveguide section, and emitting an amplified optical signal from an output waveguide section that extends to the same facet and is optically coupled with the turning waveguide section. One or more of the input waveguide section, the turning waveguide section, and the output waveguide section provide an optical gain to the optical signal. 
     Example Embodiments 
     Embodiments described herein include an optical apparatus having an optical waveguide emitter. In some embodiments, the optical apparatus is implemented as a SOA. The optical waveguide emitter includes an input waveguide section that is optically coupled with an output waveguide section through a turning waveguide section. The direction of light propagation through the optical waveguide emitter may be controlled using the turning waveguide section, such that the input waveguide section and the output waveguide section need not extend to opposing facets of a semiconductor substrate. For example, in some embodiments, the input waveguide section and the output waveguide section extend to a same facet of the semiconductor substrate. Beneficially, performing optical alignment of the optical waveguide emitter with another optical device (e.g., a photonic chip) may be less complex than when the input waveguide section and the output waveguide section extend to the opposing facets. 
     In some embodiments, the optical waveguide emitter is a variable-confinement optical waveguide emitter, and comprises one or more transition waveguide sections extending between waveguide sections having different optical confinements. For example, the turning waveguide section may have a greater optical confinement (e.g., a smaller optical mode size) than the input waveguide section and/or the output waveguide section. Beneficially, a turning waveguide section having a greater optical confinement may be implemented with a smaller bend radius, providing a lower optical loss through the turning waveguide section and supporting a reduced overall size of the optical waveguide emitter. In some embodiments, one or both of the input waveguide section and the output waveguide section comprises a SCOW. Beneficially, the larger optical mode size provided by the SCOW provides an improved optical coupling and misalignment tolerance of the optical waveguide emitter with, e.g., the photonic chip. 
     In some embodiments, one or more of the input waveguide section, the turning waveguide section, the transition waveguide section(s), and the output waveguide section comprises an optically active region. Any suitable optical gain material(s) may be used in the optically active region(s), such as quantum wells (QWs), quantum dots (QDs), quantum wires, etc., which may be electrically pumped and/or optically pumped. The arrangement and/or optical gain material(s) of the optically active region(s) may be selected to provide desired optical characteristics, such as one or more high power stages and/or one or more high gain stages of the optical waveguide emitter. 
       FIG. 1  is a top view of an optical system  100  comprising a variable-confinement optical waveguide emitter  120  (also referred to as optical waveguide emitter  120 ), according to one or more embodiments. In some embodiments, the optical waveguide emitter  120  comprises a SOA, although other implementations of the optical waveguide emitter  120  are also possible. 
     The optical system  100  comprises an optical apparatus  105  that is optically coupled with a photonic chip  110 . The optical apparatus  105  comprises a semiconductor substrate  115 , from which various optical and electrical components may be grown, patterned, etched, deposited, or eutectically bonded. In some embodiments, the semiconductor substrate  115  comprises a bulk silicon (Si) substrate, although other semiconductor materials are also contemplated. In some embodiments, the thickness of the semiconductor substrate  115  is between about 0.3 millimeters (mm) and about 1 mm. However, dimensions of the semiconductor substrate  115  may differ to account for new diameters and/or thicknesses desired in Si (or other semiconductor material) fabrication industries. 
     One or more features and/or materials of the optical waveguide emitter  120  are pre-processed in the semiconductor substrate  115 . The optical waveguide emitter  120  comprises an input waveguide section  135 , a first transition waveguide section  140 , a turning waveguide section  145 , a second transition waveguide section  150 , and an output waveguide section  155 . 
     The input waveguide section  135  extends from a facet  125  of the semiconductor substrate  115 . The input waveguide section  135  receives an optical signal at an input port  136  arranged at the facet  125 , and propagates the optical signal along a length of the input waveguide section  135 . The first transition waveguide section  140  extends between the input waveguide section  135  and the turning waveguide section  145 . 
     In some embodiments, the turning waveguide section  145  has a greater optical confinement than the input waveguide section  135 . Beneficially, the turning waveguide section  145  having a greater optical confinement may be implemented with a smaller bend radius, providing a lower optical loss through the turning waveguide section  145  and supporting a reduced overall size of the optical waveguide emitter  120 . In one embodiment, the turning waveguide section  145  has a greater optical confinement along one dimension. In another embodiment, the turning waveguide section  145  has a greater optical confinement along two dimensions. The turning waveguide section  145  is depicted as U-shaped, although other arrangements are also contemplated. In some embodiments, the turning waveguide section  145  has a bend radius between 25 microns and 1000 microns. In some embodiments, the turning waveguide section  145  has a bend radius between about 50 microns and 500 microns. 
     The first transition waveguide section  140  extends between the input waveguide section  135  and the turning waveguide section  145 . The first transition waveguide section  140  gradually changes the optical confinement of a propagating optical signal (e.g., a size of the optical mode) along one or more dimensions, through a gradual increase or decrease of one or more material layers proximate to the optical waveguide. In some embodiments, the one or more material layers provide a greater average refractive index than that of the optical waveguide, such that the optical mode is more confined for an increase of the one or more material layers, and the optical mode is less confined for a decrease of the one or more material layers. 
     In one example, the increase of the one or more material layers of the first transition waveguide section  140  comprises an increased number of material layers, and changing the optical confinement is accomplished through gradually decreasing the number of material layers. In another example, the increase of the one or more material layers comprises an increased dimensioning of the one or more material layers, and changing the optical confinement is accomplished through decreasing the dimensioning (e.g., tapering the one or more material layers) in one or more dimensions. Thus, in the optical waveguide emitter  120 , the one or more material layers of the first transition waveguide section  140  gradually increase along a direction of propagation of the optical signal (from the input waveguide section  135  to the turning waveguide section  145 ) to increase the optical confinement. 
     The second transition waveguide section  150  extends between the turning waveguide section  145  and the output waveguide section  155 . The second transition waveguide section  150  gradually changes the optical confinement of a propagating optical signal (e.g., a size of the optical mode) along one or more dimensions, through a gradual increase or decrease of one or more material layers proximate to the optical waveguide. In some embodiments, the one or more material layers provide a greater average refractive index than that of the optical waveguide, such that the optical mode is more confined for an increase of the one or more material layers, and the optical mode is less confined for a decrease of the one or more material layers. In the optical waveguide emitter  120 , the one or more material layers of the second transition waveguide section  150  gradually decrease along a direction of propagation of the optical signal (from the turning waveguide section  145  to the output waveguide section  155 ) to decrease the optical confinement. 
     The output waveguide section  155  extends to the same facet  125  of the semiconductor substrate  115 . The optical signal when propagated through the output waveguide section  155  exits through an output port  156  arranged at the facet  125 . In some embodiments, one or both of the input waveguide section  135  and the output waveguide section  155  comprises a SCOW. Stated another way, the input waveguide section  135  and/or the output waveguide section  155  have a lesser optical confinement than the turning waveguide section  145 , such that the optical waveguide emitter  120  receives and/or provides a large size optical mode. In an alternate embodiment, the input waveguide section  135  and the output waveguide section  155  extend to different facets arranged along a same side of the semiconductor substrate  115 . 
     One or more of the input waveguide section  135 , the first transition waveguide section  140 , the turning waveguide section  145 , the second transition waveguide section  150 , and the output waveguide section  155  comprises an optically active region. Thus, the optical waveguide emitter  120  comprises one or more optically active regions to provide an amplified optical signal at the output port  156 . In cases where the optical waveguide emitter  120  comprises a plurality of optically active regions, the plurality of optically active regions of may be of a same type or of different types (e.g., providing different optical power levels or optical gain levels). In some embodiments, each of the one or more optically active regions comprises one or more of quantum wells, quantum dots, and quantum wires. However, other types of optically active materials are also contemplated. Further, the one or more optically active regions may be electrically pumped and/or optically pumped. 
     The photonic chip  110  and/or the optical waveguide emitter  120  may have a suitable configuration to mitigate back reflections when transmitting optical signals between the photonic chip  110  and the optical waveguide emitter  120 . In some embodiments, the photonic chip  110  has an edge-coupled structure with an output port and an input port that are arranged to align with the input port  136  and the output port  156  of the optical waveguide emitter  120 . In some embodiments, one or both of the input port  136  and the output port  156  are angled relative to a long axis of the input waveguide section  135  and/or the output waveguide section  155 . For example, the long axis of the input waveguide section  135  and/or the output waveguide section  155  may be substantially orthogonal to the facet  125 , and the input port  136  and/or the output port  156  may be non-orthogonal to the facet  125 . In some embodiments, the angle of the input port  136  and the output port  156  have a same angling relative to the facet  125 . In some embodiments, an antireflective coating  130  may be applied to the facet  125  to mitigate back reflections. Some non-limiting examples of the antireflective coating  130  include one or more layers of aluminum oxide (Al 2 O 3 ), titanium dioxide (TiO 2 ), silicon dioxide (SiO 2 ), silicon nitride (SiN), magnesium fluoride (MgF 2 ), tantalum pentoxide (Ta 2 O 5 ), etc. deposited at the facet  125 . 
     The photonic chip  110  comprises a first waveguide  160  (e.g. a sub-micron waveguide), a first spot size converter  161 , a second waveguide  165  (e.g. a sub-micron waveguide), and a second spot size converter  166 . Although not shown, the first waveguide  160  may be optically coupled with an optical source, such as an integrable tunable laser assembly (ITLA) that transmits a continuous wave (CW) optical signal (e.g., an unmodulated optical signal) through the first waveguide  160 . Other types of optical sources and/or optical signals are also contemplated. In some embodiments, the output power provided by the optical source is insufficient for performing coherent modulation, and the optical waveguide emitter  120  provides an amplified optical signal from the output port  156  that has an output power sufficient for performing coherent modulation. 
     In some embodiments, the first waveguide  160  (e.g., a sub-micron waveguide) of the photonic chip  110  routes the CW optical signal to the first spot size converter  161 . Because the optical mode of the CW optical signal in the first waveguide  160  may be much smaller than the mode size of the waveguide in the input waveguide section  135  (e.g., a SCOW), the first spot size converter  161  increases the mode size to better match the mode of the waveguide in the input waveguide section  135 . As such, the optical coupling efficiency between the photonic chip  110  and the optical waveguide emitter  120  is improved. In one alternate embodiment, the optical mode of the CW optical signal in the first waveguide  160  and/or the second waveguide  165  is similar to the mode size of the waveguide in the input waveguide section  135  and/or the output waveguide section  155 , such that the photonic chip  110  need not include the first spot size converter  161  and/or the second spot size converter  166 . In another alternate embodiment, the optical mode of the CW optical signal in the first waveguide  160  and/or the second waveguide  165  is greater than the mode size of the waveguide in the input waveguide section  135  and/or the output waveguide section  155 , such the first spot size converter  161  may decrease the mode size and/or the second spot size converter  166  may increase the mode size. 
     Thus, during operation of the optical system  100 , after the CW optical signal propagates through the first spot size converter  161 , the CW optical signal exits the photonic chip  110  and is received by the optical waveguide emitter  120  at the input port  136 . The CW optical signal propagates through the input waveguide section  135  and through the first transition waveguide section  140 . After the mode size of the CW optical signal is reduced by the first transition waveguide section  140 , the CW optical signal is reoriented as it propagates through the turning waveguide section  145 . The mode size of the CW optical signal is increased by the second transition waveguide section  150 , and the CW optical signal is propagated along the output waveguide section  155 . The (now amplified) CW optical signal exits the optical waveguide emitter  120  at the output port  156 , and is received by the photonic chip  110  at the second spot size converter  166 . The second spot size converter  166  reduces the mode size of the CW optical signal, which is provided to the second waveguide  165 . 
     Thus, in the optical system  100 , the optical waveguide emitter  120  provides an optical amplification for optical signals carried on the photonic chip  110  with a relatively simple optical alignment process. The large optical mode supported by the input waveguide section  135  and/or the output waveguide section  155  improves a coupling efficiency and a misalignment tolerance. The integrated mode transition (or conversion) provided by first transition waveguide section  140  and/or the second transition waveguide section  150  allows use of a tightly-confined mode for sharp turns and a large SCOW mode for external coupling. The optical system  100  also supports multi-stage amplification with a high-gain, low-power SOA (in the tightly-confined regions) on the same chip as a low-gain, high-power SOA (in the SCOW regions). 
       FIGS. 2 and 3  are cross-sectional views  200 ,  300  of a variable-confinement SCOW emitter, according to one or more embodiments. The features depicted in the views  200 ,  300  may be used in conjunction with other embodiments described herein. For example, the SCOW emitter depicted in the views  200 ,  300  may be included in the variable-confinement optical waveguide emitter  120  of  FIG. 1 . The view  200  may represent a high optical confinement region, such as the turning waveguide section  145  of the optical waveguide emitter  120 . The view  300  may represent a low optical confinement region, such as the input waveguide section  135  and/or the output waveguide section  155  of the optical waveguide emitter  120 . 
     In the view  300 , the SCOW emitter comprises a first cladding layer  205 , a waveguide layer  210  arranged over the first cladding layer  205 , an optically active region  215  arranged over the waveguide layer  210 , and a second cladding layer  225  arranged over the optically active region  215 . Any suitable optical gain material(s) may be used in the optically active region  215 , such as QWs, QDs, quantum wires, etc., which may be electrically pumped and/or optically pumped. 
     In some embodiments, the SCOW emitter is formed over a submount, such as a semiconductor wafer (e.g., a silicon substrate). For example, the first cladding layer  205  may contact the submount. In some embodiments, the waveguide layer  210  is doped a first conductivity type, and the second cladding layer  225  is doped a different, second conductivity type. In some embodiments, the first cladding layer  205  is doped the first conductivity type. Although not shown, a second waveguide layer may be arranged over the optically active region  215 , and the second waveguide layer is doped the second conductivity type. 
     The SCOW emitter is formed as a ridge  230  extending from a slab  235 . The slab  235  is formed in the waveguide layer  210 . The ridge  230  is formed partly in the second cladding layer  225 , partly in the mode conversion layer  220 , partly in the optically active region  215 , and partly in the waveguide layer  210 . Thus, the ridge  230  forms part of a ridge waveguide which generally confines the optical signal within a portion of the waveguide layer  210  (represented in the view  300  as the optical mode  305 ). Stated another way, the majority of the power of the optical signal is confined within the region defined by the optical mode  305 . 
     Unlike other semiconductor optical amplifiers (SOAs) that include an active region at or near the middle of the optical mode  305 , in the SCOW emitter the optically active region  215  is located near a border of the optical mode  305  (e.g., the top portion of the optical mode  305 ). Thus, most of the optical signal propagates in the slab  235 , apart from the optically active region  215  and the ridge  230 . 
     In some embodiments, the waveguide layer  210  has a thickness between 3-5 microns and is formed from a III-V semiconductor material or alloy. In some embodiments, the width of the ridge  230  (as shown, in the left-right direction) is between 3-5 microns. With such dimensioning, the diameter of the optical mode  305  may be 4-5 microns, which is much larger than most SOAs that support single mode amplification. As the mode size increases, the optical signal typically has multiple modes. However, the SCOW emitter can have a large mode size and still support single mode amplification because of slab regions  245 A,  245 B of the slab  235 . As an optical signal propagating in the SCOW emitter generates additional modes, these modes are transmitted into, and filtered out, by the slab regions  245 A and  245 B. In this manner, the SCOW emitter supports single mode operation at larger mode sizes supported by other SOAs. In one embodiment, the SCOW emitter is a single-mode amplifier with a mode size of the fundamental mode greater than 2.5 microns 1/e 2  diameter; other modes supported by the waveguide experience a net loss because of the coupling to the slab. The relationship 1/e 2  is a typical metric for describing the size of a Gaussian beam. 
     As mentioned above, the relatively large size of the optical mode  305  relaxes the alignment tolerances for aligning the SCOW emitter to the spot size converters of the photonic chip. Further, the amplification generated by the SCOW emitter can compensate for the higher losses suffered when data rates are increased. For example, the SCOW emitter can be used in a transmitter that has an optical signal greater than 50 GHz and supporting data rates between 100 Gbps and 1 Tbps. 
     In the high optical confinement region illustrated in the view  200 , the SCOW emitter further comprises a mode conversion layer  220  that is arranged over the optically active region  215 , and the second cladding layer  225  is arranged over the mode conversion layer  220 . The mode conversion layer  220  comprises one or more layers of material(s) having a higher refractive index than the waveguide layer  210 . In this way, the mode conversion layer  220  provides a greater optical confinement of a propagating optical signal, illustrated as a confined optical mode  240 . As mentioned above, the greater optical confinement available through the mode conversion layer  220  supports lower optical loss and/or smaller dimensioning of the SCOW emitter. 
     Although illustrated as being arranged over the optically active region  215 , the mode conversion layer  220  may have alternate arrangement within the SCOW emitter. In one alternate embodiment, the optically active region  215  is arranged over the mode conversion layer  220 . In another alternate embodiment, the mode conversion  220  replaces the optically active region  215  in a passive region of the SCOW emitter. 
       FIG. 4  is a side view  400  of a variable-confinement SCOW emitter, according to one or more embodiments. The features depicted in the view  400  may be used in conjunction with other embodiments described herein. For example, the SCOW emitter depicted in the view  400  may correspond to the cross-sectional views  200 ,  300  of  FIGS. 2 and 3 . More specifically, in some examples, the transition waveguide section in the view  400  may correspond to the first transition waveguide section  140  and/or the second transition waveguide section  150  of  FIG. 1 , the region of high optical confinement may correspond to the turning waveguide section  145 , and the region of low optical confinement may correspond to the input waveguide section  135  and/or the output waveguide section  155 . However, the transition waveguide section, the region of high optical confinement, and/or the region of low optical confinement may correspond to different portions of the optical waveguide emitter  120 . 
     In the view  400 , the mode conversion layer  220  comprises a first region  405  having a substantially constant height, and a second region  410  having a gradually increasing (or decreasing) height. The first region  405  corresponds to the region of high optical confinement (e.g., the confined optical mode  240 ), and the second region  410  corresponds to a region of transitioning optical confinement (e.g., a transitioning optical mode  420  between a low optical confinement and a high optical confinement). In some embodiments, within the second region  410 , the one or more layers of the mode conversion layer  220  provide a tapering in one or more dimensions to provide an increased optical confinement. In a third region  415 , the height of the mode conversion layer  220  may be zero, providing a region of low optical confinement (e.g., the optical mode  305 ). 
     The mode conversion layer  220  may have any suitable shape in the second region  410  to provide a desired transition of the optical confinement. For example, the mode conversion layer  220  may change continuously or non-continuously in the second region  410 . In some embodiments, the mode conversion layer  220  changes linearly in the second region  410 . In other embodiments, the mode conversion layer  220  changes non-linearly in the second region  410 . 
     In one embodiment, the mode conversion layer  220  comprises a single material layer, and the tapering of the mode conversion layer  220  is achieved through variable-rate selective area growth of the single material layer. In another embodiment, the mode conversion layer  220  comprises a plurality of material layers, and the tapering of the mode conversion layer  220  is achieved through decreasing the number of material layers. Such an embodiment is illustrated in a side view  500  of  FIG. 5 , where a plurality of material layers  510  provide a stepped tapering of the mode conversion layer  220  in the second region  410 . 
     The side views  400 ,  500  illustrate a transitioning of the optical confinement along one dimension (i.e., the vertical dimension). Additionally or alternately, the mode conversion layer  220  may provide a transitioning of the optical confinement along one or more other dimensions. For example,  FIG. 6  is a top view  600  of one or more material layers  610  that provide a tapering of the mode conversion layer  220  in a second dimension. Referring also to the side views  400 ,  500 , the tapering illustrated in the view  600  provides a transitioning of the optical confinement in a dimension that extends into and out of the page. 
     The example implementation of the optical waveguide emitter  120  in  FIG. 1  is depicted as having the input port  136  angled similarly to the output port  156 , having the input waveguide section  135  and the output waveguide section  155  with similar lengths, having the first transition waveguide section  140  and the second transition waveguide section  150  with similar lengths, and having the turning waveguide section  145  that is symmetrical. Additionally, each of the input waveguide section  135 , the first transition waveguide section  140 , the turning waveguide section  145 , the second transition waveguide section  150 , and the output waveguide section  155  are depicted as having an optically active region  215 . 
     However, other implementations of the optical waveguide emitter  120  are also contemplated. In some embodiments, as illustrated in a top view  700  in  FIG. 7 , the optical waveguide emitter  120  includes an extended optical confinement region  705 , and the output waveguide section  155  is longer than the input waveguide section  135 . Due to the extended optical confinement region  705  being the input side (i.e., the side including the input waveguide section  135 ) of the turning waveguide section  145 , the turning waveguide section  145  has an asymmetrical appearance. In an alternate embodiment, the extended optical confinement region  705  is arranged on the output side (i.e., the side including the output waveguide section  155 ) of the turning waveguide section  145 . In another alternate embodiment, the input side and the output side of the turning waveguide section  145  each include an extended optical confinement region  705 . 
     In some embodiments, the optical waveguide emitter  120  comprises one or more optically passive sections. Each active section generally includes an optically active region  215  having suitable optical gain material(s). In some embodiments, a passive section does not include the optical gain material(s). In other embodiments, the passive section includes the optical gain material(s), but optical gain provided by the optical gain material(s) is mitigated through techniques such as QW intermixing. 
     Further, as mentioned above, the optical waveguide emitter  120  comprises one or more optically active regions. In cases where the optical waveguide emitter  120  comprises a plurality of optically active regions, the plurality of optically active regions of may be of a same type or of different types (e.g., providing different optical power levels or optical gain levels). 
     As illustrated in a top view  800  in  FIG. 8 , the optical waveguide emitter  120  includes a partially passive turning waveguide section  145 . More specifically, the turning waveguide section  145  comprises a first active section  805  comprising an optically active region, a second passive section  810 , and a third active section  815 . In other embodiments, the turning waveguide section  145  includes a different number of active sections. For example, the turning waveguide section  145  may be entirely passive. 
     As illustrated in a top view  900  in  FIG. 9 , the optical waveguide emitter  120  includes a partially passive input waveguide section  135  and a partially passive output waveguide section  155 . More specifically, the input waveguide section  135  comprises a first passive section  905  and a second active section  910 , and the output waveguide section  155  comprises a third active section  915  and a fourth passive section  920 . As shown, the first passive section  905  and the fourth passive section  920  are arranged at the input port  136  and the output port  156 , respectively. In other embodiments, the first passive section  905  and/or the fourth passive section  920  are arranged away from the input port  136  and/or the output port  156 . In other embodiments, the input waveguide section  135  and/or the output waveguide section  155  includes a different number of passive sections. For example, the input waveguide section  135  and/or the output waveguide section  155  may be entirely passive. 
     As illustrated in a top view  1000  in  FIG. 10 , the optical waveguide emitter  120  includes a transition waveguide section providing multiple optical confinement transitions. More specifically, the second transition waveguide section  150  comprises a first waveguide subsection  1005  extending to the turning waveguide section  145  and providing a first optical confinement transition, a second waveguide subsection  1015  extending to the output waveguide section  155  and providing a second optical confinement transition, and an intermediate waveguide subsection  1010  extending between the first waveguide subsection  1005  and the second waveguide subsection  1015 . The intermediate waveguide subsection  1010  may be passive, or may include an optically active region. 
     In other embodiments, the first transition waveguide section  140  comprises multiple optical confinement transitions in addition to, or alternate to, the second transition waveguide section  150 . In other embodiments, the first transition waveguide section  140  and/or the second transition waveguide section  150  include a different number of waveguide subsections providing optical confinement transitions and/or a different number of intermediate waveguide subsections. 
     As illustrated in a top view  1100  in  FIG. 11 , the optical waveguide emitter  120  includes multiple turning mirrors in the turning waveguide section  145 . More specifically, the turning waveguide section  145  comprises a first waveguide subsection  1105  extending to the first transition waveguide section  140 , a second waveguide subsection  1110  extending from the first transition waveguide  140  to a third waveguide subsection  1115 , which extends to the second transition waveguide section  150 . A first turning mirror  1125 - 1  is disposed at the intersection of the first waveguide subsection  1105  and the second waveguide subsection  1110 , and a second turning mirror  1125 - 2  is disposed at the intersection of the second waveguide subsection  1110  and the third waveguide subsection  1115 . As shown, the first waveguide subsection  1105  is inline with the first transition waveguide section  140 , the second waveguide subsection  1110  is orthogonal to the first waveguide subsection  1105 , and the first turning mirror  1125 - 1  is oriented at 45 degrees relative to the first waveguide subsection  1105 . The third waveguide subsection  1115  is orthogonal to the second waveguide subsection  1110 , and the second turning mirror  1125 - 2  is oriented at 45 degrees relative to the second waveguide subsection  1110 . The third waveguide subsection  1115  is inline with the second transition waveguide section  150 . In this way, an optical signal exiting the first transition waveguide section  140  propagates along the first waveguide subsection  1105  and is reflected by the first turning mirror  1125 - 1  onto the second waveguide subsection  1110 . As shown in the inset  1120 , an optical signal  1135  propagating along the second waveguide subsection  1110  is incident on a surface  1130  of the turning mirror  1125 - 2 , and is reflected by the turning mirror  1125 - 2  onto the third waveguide subsection  1115 . The optical signal  1135  then enters the second transition waveguide section  150 . Although the first turning mirror  1125 - 1  and the second turning mirror  1125 - 2  are described as being oriented at 45 degrees for orthogonal waveguide subsections, the different waveguide subsections and the turning mirrors of the turning waveguide section  145  may have any suitable alternate arrangement. 
     The turning mirrors  1125 - 1 ,  1125 - 2  may have any suitable configuration. In some embodiments, the turning mirrors  1125 - 1 ,  1125 - 2  are sized such that most or all of the optical mode of the optical signal  1135  is incident on the turning mirrors  1125 - 1 ,  1125 - 2 . In some embodiments, each of the turning mirrors  1125 - 1 ,  1125 - 2  extends from an etched pocket formed into the semiconductor substrate  115 . For example, the etched pocket may be formed using a dry etching process, such as reactive-ion etching (RIE), inductively coupled plasma (ICP)-RIE, focused ion beam (FIB), chemically-assisted ion beam etching (CAIBE), and so forth. In a total internal reflection (TIR) implementation of the turning mirrors  1125 - 1 ,  1125 - 2 , the etched pocket may be coated or filled with a dielectric material (e.g., SiO 2  or SiN) for surface protection and/or passivation. In a non-TIR implementation of the turning mirrors  1125 - 1 ,  1125 - 2 , the etched pocket may be coated or filled with a metallic material (e.g., silver or gold). 
       FIG. 12  is a method  1200  for use with an optical waveguide emitter disposed on a semiconductor substrate, according to one or more embodiments. The method may be used in conjunction with other embodiments, such as during operation of the optical waveguide emitter  120 . The method  1200  begins at block  1205 , where an optical signal is received in an input waveguide section extending from a facet. In some embodiments, the facet is formed in a semiconductor substrate. At block  1215 , the mode size of the optical signal is reduced by propagating the optical signal through a first transition waveguide section. At block  1225 , the optical signal with the reduced mode size is propagated through a turning waveguide section. At block  1235 , the mode size of the optical signal is increased by propagating the optical signal through a second transition waveguide section. At block  1245 , an amplified optical signal is emitted from an output waveguide section extending to the same facet. The method  1200  ends following completion of block  1245 . 
     In the preceding, reference is made to embodiments presented in this disclosure. However, the scope of the present disclosure is not limited to specific described embodiments. Instead, any combination of the described features and elements, whether related to different embodiments or not, is contemplated to implement and practice contemplated embodiments. Furthermore, although embodiments disclosed herein may achieve advantages over other possible solutions or over the prior art, whether or not a particular advantage is achieved by a given embodiment is not limiting of the scope of the present disclosure. Thus, the preceding aspects, features, embodiments and advantages are merely illustrative and are not considered elements or limitations of the appended claims except where explicitly recited in a claim(s). 
     Aspects of the present disclosure are described with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems) and computer program products according to embodiments presented in this disclosure. It will be understood that each block of the flowchart illustrations and/or block diagrams, and combinations of blocks in the flowchart illustrations and/or block diagrams, can be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general purpose computer, special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks. 
     These computer program instructions may also be stored in a computer readable medium that can direct a computer, other programmable data processing apparatus, or other devices to function in a particular manner, such that the instructions stored in the computer readable medium produce an article of manufacture including instructions which implement the function/act specified in the flowchart and/or block diagram block or blocks. 
     The computer program instructions may also be loaded onto a computer, other programmable data processing apparatus, or other devices to cause a series of operational steps to be performed on the computer, other programmable apparatus or other devices to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide processes for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks. 
     The flowchart and block diagrams in the Figures illustrate the architecture, functionality and operation of possible implementations of systems, methods and computer program products according to various embodiments. In this regard, each block in the flowchart or block diagrams may represent a module, segment or portion of code, which comprises one or more executable instructions for implementing the specified logical function(s). It should also be noted that, in some alternative implementations, the functions noted in the block may occur out of the order noted in the figures. For example, two blocks shown in succession may, in fact, be executed substantially concurrently, or the blocks may sometimes be executed in the reverse order, depending upon the functionality involved. It will also be noted that each block of the block diagrams and/or flowchart illustration, and combinations of blocks in the block diagrams and/or flowchart illustration, can be implemented by special purpose hardware-based systems that perform the specified functions or acts, or combinations of special purpose hardware and computer instructions. 
     In view of the foregoing, the scope of the present disclosure is determined by the claims that follow.