PATENT DOCUMENT

Publication Number: US-11506535-B1
Application Number: US-202017014902-A
Country: US
Kind Code: B1

Title: Diffraction grating design

Abstract:
Configurations for a diffraction grating design and methods thereof are disclosed. The diffraction grating system can include an input waveguide located at a first location on or near a Rowland circle and multiple output waveguides located at a second and third location on or near the Rowland circle. The input waveguide may be located between the output waveguides and this configuration of input and output waveguides can reduce the footprint size of the device. In some examples, the optical component can function as a de-multiplexer. Additionally, the optical component may separate the input wavelength band into two output wavelength bands which are separated from one another by approximately 0.1 μm.

Claims:
What is claimed is: 
     
       1. An optical device, comprising:
 a planar waveguide defining an input light path; 
 an input waveguide configured to emit light along the input light path; 
 a set of grating facets configured to:
 receive the emitted light from the input waveguide; and 
 reflect the emitted light as a first reflected light and a second reflected light; 
 
 a first output waveguide defining a first reflected light path and configured to receive the first reflected light along the first reflected light path; and 
 a second output waveguide defining a second reflected light path and configured to receive the second reflected light along the second reflected light path, wherein:
 the first reflected light is a first wavelength of light reflected at a first angle and to the first output waveguide; 
 the second reflected light is a second wavelength of light reflected at a second angle and to the second output waveguide; and 
 a power distribution of the first wavelength range of light received by the first output waveguide and the second wavelength range of light received by the second output waveguide is based at least in part on a blaze angle of the set of grating facets. 
 
 
     
     
       2. The optical device of  claim 1 , wherein the input waveguide is located between the first output waveguide and the second output waveguide. 
     
     
       3. The optical device of  claim 1 , wherein the emitted light is in a broadband wavelength range of at least one μm. 
     
     
       4. The optical device of  claim 1 , wherein:
 the first reflected light is in a first wavelength range; and 
 the second reflected light is in a second wavelength range spaced apart from the first wavelength range by at least 0.1 μm. 
 
     
     
       5. The optical device of  claim 1 , wherein:
 the input waveguide is located at a first position on a Rowland circle; 
 the first output waveguide is located at a second position on the Rowland circle; and 
 the second output waveguide is located at a third position on the Rowland circle. 
 
     
     
       6. The optical device of  claim 1 , wherein the first angle is equal to the second angle. 
     
     
       7. The optical device of  claim 1 , wherein:
 the first angle is between the input light path and at least one of first or second output light paths; and 
 the first angle is selected such that an optical loss associated with the first wavelength range of light is equal to an optical loss associated with the second wavelength range of light. 
 
     
     
       8. An optical device, comprising:
 a first light emitting element configured to emit a first input light along a first input light path and positioned at a first location on a Rowland circle; 
 a second light emitting element configured to emit second input light along a second input light path; 
 a light receiving element configured to receive reflected light along a reflected light path and positioned at a second location on the Rowland circle and between the first light emitting element and the second light emitting element; and 
 a diffraction grating configured to:
 receive light along the input light path of the light emitting element; and 
 reflect light along the reflected light path to the light receiving element. 
 
 
     
     
       9. The optical device of  claim 8 , wherein:
 the reflected light path is a first reflected light path; 
 the light receiving element is a first light receiving element; 
 the optical device further comprises a second light receiving element configured to receive reflected light along a second reflected light path; and 
 at least a subset of a set of grating mirrors is configured to receive light in a first wavelength band of light, the first wavelength band of light comprising:
 a second wavelength band of light reflected along the first reflected light path to the first light receiving element; and 
 a third wavelength band of light separated from the second wavelength band of light, the third wavelength band of light reflected along a third reflected light path to the second light receiving element; and 
 
 the input light path is located between the reflected light path and the second reflected light path. 
 
     
     
       10. The optical device of  claim 9 , wherein the second wavelength band of light and the third wavelength band of light have approximately equal average transmission powers. 
     
     
       11. The optical device of  claim 9 , wherein the second wavelength band of light is spaced apart from the third wavelength band of light by at least 0.1 μm. 
     
     
       12. The optical device of  claim 8 , wherein the diffraction grating is an Echelle grating. 
     
     
       13. The optical device of  claim 8 , wherein the second wavelength band of light and the third wavelength band of light are separated by a separation wavelength band. 
     
     
       14. The optical device of  claim 8 ; wherein:
 the light receiving element is a first light receiving element; 
 the reflected light path is a first reflected light path; 
 the reflected light is a first reflected light; and 
 the optical device further comprises a second light receiving element configured to receive second reflected light on a second reflected light path. 
 
     
     
       15. The optical device of  claim 8 , further comprising:
 the light receiving element is a first light receiving element; 
 the reflected light path is a first reflected light path; 
 the reflected light is a first reflected light; and 
 the optical device further comprises:
 a second light receiving element configured to receive second reflected light on a second reflected light path, wherein the first light receiving element and the second light receiving element are both located on a first side of the light emitting element. 
 
 
     
     
       16. The optical device of  claim 8 , further comprising a doped material positioned between the light emitting element and the light receiving element. 
     
     
       17. A method for splitting light, comprising:
 emitting light in a broadband wavelength range from a first position on a Rowland circle; 
 reflecting the light from an Echelle grating in the broadband wavelength range; 
 receiving a first reflected light in a first wavelength band of the broadband wavelength range and at a second position on the Rowland circle; and 
 receiving a second reflected light in a second wavelength band of the broadband wavelength range and at a third position on the Rowland circle, the second wavelength band separated from the first wavelength band by at least 0.1 μm, wherein the first position on the Rowland circle is between the second position and the third position. 
 
     
     
       18. The method of  claim 17 , wherein emitting light comprises:
 emitting light on an optical path between a first reflected optical path and a second reflected optical path.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a nonprovisional of and claims the benefit under 35 U.S.C. § 119(e) of U.S. Provisional Patent Application No. 62/897,553, filed Sep. 9, 2019, and entitled “Echelle Grating Design,” the contents of which are incorporated herein by reference as if fully disclosed herein. 
    
    
     FIELD 
     This disclosure relates generally to an optical system with emitting and receiving waveguides and an Echelle grating. More particularly, embodiments herein relate to an optical system for multiplexing or de-multiplexing via emitting and receiving waveguides and an Echelle grating. 
     BACKGROUND 
     Diffraction gratings may be used in various optical instruments such as monochromators, lasers, or for holographic memory. An Echelle grating is one type of diffraction grating where the input light travels through a medium and multi-path interference of light can cause the wavelengths of the reflected light to combine or separate. The different optical paths traveled by the light can lead to phase errors. 
     The diffraction grating design can affect the design and performance of an optical component, such as the spacing between the input and output channels of the optical component, which can lead to unwanted channel crosstalk. Additionally, small variations in the reflective facets of the diffraction grating caused by imperfections in the grating material may generate optical losses and deviations in the reflected light. 
     SUMMARY 
     Embodiments of the systems, devices, methods, and apparatuses described in the present disclosure are directed to a photonics device for de-multiplexing light. Also described are systems, devices, methods, and apparatuses directed to receiving light in a wavelength range and outputting light in a first wavelength band and a second wavelength band, which may be separated by at least 0.1 μm. An input waveguide may provide the light through a planar waveguide and reflect off of a diffraction grating, as a first output light received by a first output waveguide and a second output light received by a second output waveguide. The input waveguide may be positioned between the first and second output waveguides. 
     In some examples, the present disclosure describes an optical device. The optical device may include a planar waveguide that defines an input light path, an input waveguide, and a set of grating facets. The input waveguide may emit light along the input light path and the set of grating facets may receive the emitted light from the input waveguide and may reflect the emitted light as a first reflected light and a second reflected light. The optical device may include a first output waveguide defining a first reflected light path and configured to receive the first reflected light along the first reflected light path and a second output waveguide defining a second reflected light path and configured to receive the second reflected light along the second reflected light path. In some examples, the input waveguide may be located between the first output waveguide and the second output waveguide and the emitted light may be in a broadband wavelength range. In some examples the first reflected light may be in a first wavelength range and the second reflected light may be in a second wavelength range spaced apart from the first wavelength range by at least 0.1 μm. 
     In some examples, the input waveguide may be located at a first position on a Rowland circle, the first output waveguide may be located at a second position on the Rowland circle, and the second output waveguide may be located at a third position on the Rowland circle. In some examples, the set of grating facets may reflect a first wavelength range of light at a first angle and to the first output waveguide and may reflect a second wavelength range of light at a second angle and to the second output waveguide, where the first angle is equal to the second angle. In some examples, the first angle may be between the input light path and at least one of first or second output light paths, and the first angle may be selected such that an optical loss associated with the first wavelength range of light is equal to an optical loss associated with the second wavelength range of light. In still further examples, a power distribution of the first wavelength range of light may be received by the first output waveguide, and the second wavelength range of light received by the second output waveguide is based at least in part on a blaze angle of the set of grating facets. 
     In some examples, the present disclosure describes an optical device. The optical device may include a light emitting element, a light receiving element and a diffraction grating. The light emitting element may emit input light along an input light path and positioned at a first location on a Rowland circle. The light receiving element may receive reflected light along a reflected light path and positioned at a second location on the Rowland circle. The diffraction grating may receive light from the light emitting element, traveling along its input light path, and may reflect light along the reflected light path to the light receiving element. In some examples, the reflected light path is a first reflected light path, the light receiving element is a first light receiving element, and the optical device may include a second light receiving element configured to receive reflected light along a second reflected light path. Continuing the example, at least a subset of a set of grating mirrors may be configured to receive light in a first wavelength band of light, and the first wavelength band of light may include a second wavelength band of light reflected along the first reflected light path to the first light receiving element and a third wavelength band of light separated from the second wavelength band of light, the third wavelength band of light reflected along a third reflected light path to the second light receiving element. Additionally, the input light path is located between the reflected light path and the second reflected light path. In some examples, the second wavelength band of light and the third wavelength band of light may have approximately equal average transmission powers. In some examples, the second wavelength band of light may be spaced apart from the third wavelength band of light by a 0.1 μm separation wavelength band. In some examples, the diffraction grating is an Echelle grating. 
     In some examples, the light emitting element may be a first light emitting element, the input light may be a first input light, the input light path may be a first input light path, the optical device may include a second light emitting element that may emit second input light along a second input light path, and the light receiving element may be located between the first light emitting element and the second light emitting element. In still further examples, the light receiving element may be a first light receiving element, the reflected light path may be a first reflected light path, and the optical device may include a second light receiving element that may receive second reflected light on a second reflected light path. In still further examples, the light receiving element may be a first light receiving element, the reflected light path may be a first reflected light path, the reflected light is a first reflected light, and the optical device may include a second light receiving element that may receive second reflected light on a second reflected light path, where the first light receiving element and the second light receiving element are both located on a first side of the light emitting element. In some examples, the optical device may include a doped material positioned between the light emitting element and the light receiving element. 
     In some examples, the present disclosure describes a method for splitting light. The method may include emitting light in a broadband wavelength range, reflecting the light from an Echelle grating in the broadband wavelength range, receiving a first reflected light in a first wavelength band, and receiving a second reflected light in a second wavelength band, the second wavelength band separated from the first wavelength band by at least 0.1 μm. In some examples, the method may include emitting light on an optical path between a first reflected optical path and a second reflected optical path. In some examples, the method may include emitting light comprises emitting light from a first position on a Rowland circle, receiving the first reflected light comprises receiving the first reflected light at a second position on the Rowland circle, and receiving the second reflected light comprises receiving the second reflected light at a third position on the Rowland circle. In some examples, the first position on the Rowland circle may be between the second position and the third position. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a block diagram of an optical component. 
         FIG. 2  illustrates a simplified optical component with a diffraction grating. 
         FIG. 3A  illustrates an optical component with a diffraction grating. 
         FIG. 3B  illustrates an optical component with a diffraction grating. 
         FIGS. 4A-4C  illustrate reflective facets of a diffraction grating. 
         FIGS. 5A and 5B  illustrate spectrum plots at different blaze angles. 
         FIG. 6  illustrates a process flow for operating an optical component. 
     
    
    
     It should be understood that the proportions and dimensions (either relative or absolute) of the various features and elements (and collections and groupings thereof) and the boundaries, separations, and positional relationships presented between them, are provided in the accompanying figures merely to facilitate an understanding of the various embodiments described herein and, accordingly, may not necessarily be presented or illustrated to scale, and are not intended to indicate any preference or requirement for an illustrated embodiment to the exclusion of embodiments described with reference thereto. 
     DETAILED DESCRIPTION 
     Reference will now be made in detail to representative embodiments illustrated in the accompanying drawings. It should be understood that the following description is not intended to limit the embodiments to one preferred embodiment. To the contrary, it is intended to cover alternatives, modifications, and equivalents as can be included within the spirit and scope of the described embodiments as defined by the appended claims. 
     Generally, diffraction gratings may be designed to reduce channel crosstalk, phase errors, and optical losses to the input and output light. Different factors that may be considered when designing an optical component with a diffraction grating may include, but are not limited to, the positioning of the input waveguide(s) and the output waveguide(s) relative to one another, the radius of curvature of the diffraction grating, the input and output wavelength(s), the angle between the input waveguide(s) and the output waveguide(s), and so forth. In some examples, the input waveguide(s) and the output waveguide(s) may provide a reduced footprint size of the diffraction grating. 
     Additionally, the performance of the diffraction grating may be sensitive to the quality of the reflector facets, also known as the facets, or the grating teeth. The terms “reflector facets,” “teeth,” “grating teeth,” and “facets,” may be used interchangeably herein. The fabrication of the diffraction grating may affect the size of the reflective facets as defining the facets in a planar substrate may be difficult, especially when the reflective teeth have a small width and/or small height. The fabrication process may produce corner effects, such as rounded corners, that can cause undirected scattering of light. 
     Disclosed herein are optical components including a diffraction grating. The optical component may include a diffraction grating, an input waveguide(s) and an output waveguide(s). The input waveguide may be located between the output waveguides and all of the waveguides may be located on or near a Rowland circle. The placement of the first location(s) between the second location(s) (or vice versa) can reduce the footprint size of the device. 
     In some examples, the diffraction grating can be a de-multiplexer that separates the input wavelength band of light into at least two output wavelength bands of light that are separated from one another. In some examples, the optical component may have angles between the input and output waveguides that are similar or are the same. Similar angles between the input and output waveguides may produce similar optical losses of the different wavelength bands. The angle selection can reduce the differences in average transmissions between the different wavelength bands and can increase the diffraction efficiency. 
     In some examples, the width, height, and blaze angle of the reflective facets can be tuned to lead to easier fabrication, to reduce the amount of fluctuations in optical losses, to reduce the size of the device, and so forth. 
     These and other embodiments are discussed below with reference to  FIGS. 1-6 . However, those skilled in the art will readily appreciate that the detailed description given herein with respect to these Figures is for explanatory purposes only and should not be construed as limiting. 
     Representative applications of methods and apparatuses according to the present disclosure are described in this section. These examples are being provided solely to add context and aid in the understanding of the described examples. It will thus be apparent to one skilled in the art that the described examples may be practiced without some or all of the specific details. Other applications are possible, such that the following examples should not be taken as limiting. 
       FIG. 1  illustrates a block diagram of an optical component. In  FIG. 1 , the optical component  100  may be a de-multiplexer that may receive input light  110  from a light source  135  and route the received light to output light  120 , which may be received by a detector  130 . The input light  110  may be a single input and the output light  120  may include two or more outputs such as output light  120 A and output light  120 B. Where discussed herein, the output light may be labeled generally as the output light  120 , and individual instances or components with a separate element number such as output light  120 A and output light  120 B, and so forth. Although the light source  135  is depicted as emitting light directly into the de-multiplexer, there may be additional optical components between the light source  135  and the de-multiplexer, and there may not be direct optical coupling between the de-multiplexer and the detector  130 . 
     The optical component  100  may separate the output light  120  and the light separation may be wavelength dependent. The light may be separated and output as individual wavelengths or wavelength bands. In some examples, the input light  110  may be in a wavelength range of approximately 1.3 μm-2.5 μm and the output light  120 A and  120 B may be in two separate wavelength bands, both of which may be within the input light wavelength range. The separation of the input light will be discussed in further detail with reference to  FIGS. 2-5B . 
     The input light  110  may be provided by an input waveguide  115  and the output light  120  may be received by output waveguides  125 A and  125 B. As discussed herein, the output waveguides may be referred to as output waveguides  125  or individually with a separate element number for each output waveguide such as output waveguide  125 A and  125 B. In some examples, the input and output waveguides may be strip waveguides. Although the input light  110  may be discussed as emitting light into the optical component  100 , it may be understood that a light source not illustrated in  FIG. 1  may provide light to the input waveguide  115 . Similarly, the output waveguides  125  may be discussed as receiving output light  120 , but the output waveguides  125  may provide the output light  120  to one or more light detectors that are not illustrated in  FIG. 1 . The detector(s) can include any type of diode that can respond to or measure photons impinging on its active area. The detector(s) can generate one or more detector signals indicative of the output light. 
     Optical Component with a Diffraction Grating 
       FIG. 2  illustrates a simplified optical component with a diffraction grating. The optical component  200  of  FIG. 2  may include an input waveguide  215 , multiple output waveguides  225 , and a diffraction grating  230 . As illustrated in  FIG. 2 , the diffraction grating  230  may have a radius of curvature that is associated with an imaginary diffraction grating circle  235 . The radius of curvature of the diffraction grating  230  may also be associated with another imaginary circle referred to herein as the Rowland circle  240 . The Rowland circle  240  has a diameter that is approximately equal to the radius of the diffraction grating circle  235 . In some examples, the input waveguide  215  and the output waveguides  225 A,  225 B, and  225 C may be on the Rowland circle  240 . When the input waveguide  215  emits input light  210  from somewhere on the Rowland circle  240  and toward the diffraction grating  230 , then a beam reflecting off of the diffraction grating  230  may be split into reflected beams that may come into focus at other points on the Rowland circle  240 . The single input waveguide and the three output waveguides are used for explanatory purposes only, as the optical component  200  may include one or more input waveguides and one or more output waveguides as appropriate. Additionally, the positioning of the input and output waveguides with respect to each other will be discussed in further detail with respect to  FIGS. 3A-4C . 
     In  FIG. 2 , for explanatory purposes only, the input waveguide  215  is located at a first position and the output waveguides  225  are located all to one side of the input waveguide  215 ; however, in other examples, the input and output waveguides may be arranged in different configurations. In some examples and as discussed with reference to  FIGS. 3A-4C , the output waveguides  225  may be located on both sides of the input waveguide  215 , there may be two input waveguides  215  that may be located on both sides of an output waveguide  225 , and so forth. 
     The input waveguide  215  may emit input light  210  into a planar waveguide or slab waveguide  260 , where the slab waveguide is represented by the shaded area. In  FIG. 2 , the slab waveguide  260  is defined by the optical path of the input light optical path and the optical path of the output light. Although the shaded area in  FIG. 2  depicts the slab waveguide  260 , in some examples, the slab waveguide  260  may extend outside of the shaded area of  FIG. 2 . In some examples, the area outside of the slab waveguide  260  may be a doped material to prevent the attenuation of input light and output light. The terms “planar waveguide” and “slab waveguide” may be used interchangeably herein. 
     In some examples, the input light  210  may be received by the input waveguide  215  from one or more light emitters (not shown in  FIG. 2 ), and the input light  210  may propagate from the input waveguide  215  into the slab waveguide  260  toward the diffraction grating  230 . The slab waveguide  260  may be optically coupled to the diffraction grating  230 , so that the slab waveguide  260  may emit light that will reflect off of the diffraction grating  230 . The input light  210  may then reflect off of the diffraction grating  230  and output light  220 A,  220 B, and  220 C, and after being reflected may propagate back through the slab waveguide  260  toward the output waveguides  225 A,  225 B, and  225 C, respectively. As previously discussed, the input waveguide  215  and the output waveguides  225  may be strip waveguides. The input waveguide  215  and the output waveguides  225  may be optically coupled to the slab waveguide  260  to reduce the loss of light at the interface of the waveguides. 
     Also shown in the expanded section of  FIG. 2  are the reflective facets  250  of the diffraction grating  230 . The expanded section of  FIG. 2  is for illustrative and explanatory purposes and is not to scale. Additionally, although four reflective facets  250  are illustrated, the diffraction grating  230  may include any appropriate number of reflective facets  250 . Each of the reflective facets  250  may be approximately equidistant from one another by a distance d. The reflective facets  250  of the diffraction grating  230  will be discussed with reference to  FIGS. 4A-4C . Additionally, the optical component  200  may include an input waveguide  215  for emitting light  210  toward the diffractive grating  230  and the output light  220  may be received at output waveguides  225 . As used herein, the term “reflective facets” may be used interchangeably with “grating mirrors,” “grating facets,” and “teeth.” 
     The locations of the input waveguide  215  and the output waveguides  225  may depend at least partially on the radius of curvature of the diffraction grating  230 . In some examples, the input waveguide  215  and the output waveguides  225  are located adjacent to the Rowland circle  240 , and the Rowland circle  240  depends on the radius of curvature of the diffraction grating  230 . Additionally, the reflective facets  250  of the diffraction grating  230  may determine the angle at which the input light reflects off of the diffraction grating  230  in conjunction with the radius of curvature of the diffraction grating  230 . In turn, the angle at which the light reflects determines the location of the output waveguides  225  on the Rowland circle  240 . Although the input and output waveguides are discussed as being located on the Rowland circle  240 , in some examples, the input and output waveguides may not be located on the Rowland circle  240 . 
     In  FIG. 2 , the input light  210  propagates in the slab waveguide  260 . As the light propagates, this may be discussed herein as the light being emitted on an input light path or an optical path, where the terms “light path” and “optical path” may be used interchangeably. Similar to the input light, when the output light  220  propagates from the diffraction grating  230  through the slab waveguide  260 , this may be described as the output light being received on an output light path. 
       FIG. 3A  illustrates an optical component with a diffraction grating.  FIG. 3A  illustrates an optical component  300  with a diffraction grating  330  that receives input light  310  from an input waveguide  315 . The input light  310  may propagate on an input light path to the diffraction grating  330  and reflect off of the diffraction grating  330  as output light  320 . As shown in  FIG. 3A , the output light  320  may propagate on two different output light paths and the output light  320 A may be received by a first output waveguide  325 A and the output light  320 B may be received by a second output waveguide  325 B. The two output waveguides  325  may be located on both sides of the input waveguide  315 . Although a single input waveguide and two output waveguides are illustrated in  FIG. 3A , any appropriate number of input and output waveguides may be used so long as the number of input waveguides is fewer than the number of output waveguides. 
     Angle  370 A may be between the input light path  310  and the output light path  320 A. Angle  370 B may be between the input light path  310  and the output light path  320 B. The angles  370 A and  370 B can depend on various factors including, but not limited to, the properties of the diffraction grating  330  (e.g., spacing of the reflective facets, radius of curvature of the diffraction grating, and so forth) and the wavelength of the input light  310 . In the example of  FIG. 3A , the optical component  300  functions as a de-multiplexer; the diffraction grating  330  may de-multiplex the input light  310  received from the input waveguide  315  by reflecting and separating the input light  310  into output light  320 A and  320 B. The diffraction grating  330  may reflect incoming light at different angles depending on the wavelength of light. The diffraction grating  330  may be used to separate light and direct the reflected light along the output paths  320 A and  320 B, where the reflected light on the output paths  320 A and  320 B may have different wavelength ranges that do not overlap. As previously discussed, the wavelengths of the input light  310  may be in a broadband wavelength range, the output light  320 A may be in a first wavelength range, and the output light  320 B may be in a second wavelength range, which may be separated from the first wavelength range by a separation wavelength band of at least 0.1 μm. In some examples, the separation wavelength band may be more or less than 0.1 μm. Although the diffraction grating  330  is discussed herein as reflecting light, it may be understood that the reflective facets of the diffraction grating reflect the light. 
     In some example embodiments, a “broadband wavelength range” may be generally a set of emitted broadband wavelengths and/or detected broadband wavelengths over the approximate range of 1 μm. In some examples, the 1 μm emitted and/or detected broadband wavelengths may be in the “broadband” range of approximately 1.0 μm and 3.0 μm. Accordingly, embodiments described herein may operate over, or employ, an operating range that may correspond to, or be encompassed in, a broadband wavelength range. Examples of such operating ranges include 1.0 μm-2.0 μm, 1.3 μm-2.3 μm, 1.4 μm-2.4 μm, 1.5 μm-2.5 μm, and so forth. Although specific wavelength ranges may be discussed, any appropriate wavelength or wavelength range may be emitted and/or detected by the photonics elements described herein, depending on the use and construction of those elements. 
     In some examples, the diffraction grating  330  may be an Echelle grating. The Echelle grating may be designed to have a reduced footprint size by leveraging the wavelength separation between the first wavelength band and the second wavelength band. With the separation between wavelength bands, the optical component  300  may include separate waveguides which may be coupled to separate detectors for detecting the different wavelength bands. For example, as shown in  FIG. 3A , a first detector can be coupled to the first output waveguide  325 A for detecting output light  320 A and a second detector can be coupled to the second output waveguide  325 B for detecting output light  320 B. Light path  320 A may include the first wavelength band, and light path  320 B may include the second wavelength band. 
     An Echelle grating may be multi-functional and used for either one or both of diffraction and refocusing of the input light. The multi-functional diffraction grating can lead to a reduction in the grating size and the overall optical device size, but in some examples this may lead to the possible locations of the input and output waveguides being reduced. By separating the output wavelength bands using the multi-functional diffraction grating, the location options of the input waveguide  315  and the output waveguides  325  can increase. 
     In  FIG. 3A , the input waveguide  315  is positioned at a first location on or near the Rowland circle, and the first output waveguide  325 A is positioned at a second locations on or near the Rowland circle. In the example of  FIG. 3A , the first location of the input waveguide  315  is between the second and third locations of the second and third waveguides  325 A and  325 B, respectively, on or near the Rowland circle  335 . 
     The radius of curvature of the diffraction grating  330  and thus, the radius of the Rowland circle  335  can be selected by considering one or more of: the angles  370 , the location of the input waveguide  315 , and the location of the output waveguides  325 . By placing the input waveguide  315  between the output waveguides  325  (or vice versa in the example of  FIG. 3B ), the radius of the Rowland circle  335  can be reduced, which may allow for a smaller-sized optical component  300 . 
     In addition to reducing the footprint size, the diffraction grating design may be configured to improve optical performance. In some examples, the optical performance may be improved by reducing the optical loss of the optical component  300 . In  FIG. 3A , by locating the input waveguide  315  between the output waveguides  325 , the angles  370  may be reduced, which may result in a reduction of optical losses. In some examples, the angle  370 A is substantially similar (e.g., equal or within a five percent deviation) to the angle  370 B, where the angle  370 A is associated with the optical losses and phase errors associated with the first wavelength band and the angle  370 B is associated with the optical losses and phase errors associated with the second wavelength band. Thus, the optical losses can be balanced and the likelihood of the longer wavelength band experiencing a higher optical loss relative to the shorter wavelength band may be reduced. In addition to balancing the optical losses between the two different wavelength bands, reducing the angles  370  can lead to less undirected, scattered light. A smaller angle  370  can allow the input light  310  to be incident on the reflective facets of the diffraction grating  330  at higher angles. This will be discussed in further detail with reference to  FIGS. 4A-4C . 
       FIG. 3B  illustrates an optical component with an Echelle grating. The optical component  301  of  FIG. 3B  is similar to the optical component  300  of  FIG. 3A , except for the positioning of the input waveguides  315 A and  315 B with respect to the output waveguide  325 . In  FIG. 3B , the output waveguide  325  is positioned between the input waveguides  315 A and  315 B. For the purposes of discussion, similarly numbered elements may have similar characteristics and functionality. In  FIG. 3B , the optical component  301  is a multiplexer and the diffraction grating  330  can combine light  310 A and  310 B from input waveguides  315 A and  315 B to provide output light  320  to the output waveguide  325 . In some examples, the diffraction grating  330  can be used to multiplex (e.g., combine) light having multiple wavelengths or wavelength ranges. Although  FIG. 3B  illustrates two input waveguides and a single output waveguide, in other multiplexer examples, any appropriate number of input waveguides and output waveguides may be used so long as there are fewer output waveguides than input waveguides. 
     In some examples, the diffraction grating  330  of  FIG. 3B  may be an Echelle grating. An Echelle grating may be particularly suitable for generating higher diffractive orders of light due to the nature of the periodic structure of the reflective facets (the reflective facets are not illustrated in  FIG. 3B ). The reflective facets will be discussed in further detail with reference to  FIGS. 4A-4C . In some examples, Echelle gratings may provide high dispersion having a small footprint size. 
       FIGS. 4A-4C  illustrate reflective facets of a diffraction grating. In  FIG. 4A , the reflective facets  410 A of the diffraction grating  400 A may include different features which may be varied depending on the desired optical performance. The varied features may include the periodicity of the reflective facets, the height of the reflective facets, the blaze angle, the length of both sides of the reflective facets, and so forth. The blaze angle  405 A may be the angle of the reflective facet of the diffractive grating measured relative to the plane of incidence. In some examples, the reflective facets  410 A of a diffraction grating  400 A can have a low period  420 A where the adjacent reflective facets  410 A can be widely spaced from one another. In some examples, the diffraction grating  400 A may have reflective facets  410 A with a high-blaze angle  405 A. 
     As shown in  FIG. 4B , reflective facet angle  405 B is lower than reflective facet angle  405 A of  FIG. 4A . In  FIG. 4B , light with higher angles of incidence can allow for a lower facet angle without compromising optical performance such as diffraction efficiency. Additionally, the reflective facet width  410 B can be greater than the reflective facet width  410 A. The reflective facet width  410  may also be generally referred to as the period or periodicity of the grating  400 . A wider reflective facet period  410 B can lead to easier fabrication of the diffractive grating  400 B. As one example, the reflective facet period  410 B can be approximately 3.18 μm for the grating  400 B, whereas the reflective facet period  410 A can be approximately 0.57 μm for the diffractive grating  400 A. 
     In some examples, a wider reflective facet width  410 B can result in less rounding of the corners of the grating teeth during fabrication processes. Reducing the amount of rounding of the corners of the grating teeth can result in reducing the amount of undirected, scattered light. In some examples, the rounded corners may cause unwanted, large fluctuations in the optical losses. In some examples, the diffraction grating  400 B may be able to tolerate rounded corners when the reflective facet width  410 B is larger. Further, in some examples, the reflective facet width  410 B may be related to the targeted reflected wavelength or ranges of wavelengths. For example, the reflective facet width  410 B may be increased to accommodate an increased target wavelength or range of wavelengths. 
     In some examples, the size of the optical component may be balanced with the optical performance. For example, the diffractive grating design may not be based solely on the lowest possible reflective facet angle and the allowable size of the diffraction grating  400 B. Other factors may be considered such as the complexity of the fabrication, as discussed herein. 
     In some examples, the diffraction grating  400 B may have a selected blaze angle  405 B and the dispersion of light may be related to the blaze angle. In some instances, when the angle between the input light and a ray of normal incidence to the reflective facet is approximately equal to the blaze angle, light at a certain grating order may have improved diffraction efficiency. 
     In  FIG. 4C , the height  415 C of the reflective facets  410 C of the diffraction grating  400 C may be greater than the reflective facet height  415 B of the diffraction grating  400 B of  FIG. 4B . In some instances, tuning the blaze angle and the grating order of the output light can allow for taller reflective facets. 
     In some examples, the larger height  415 C can reduce the complexity of the fabrication of the diffraction grating  400 C. As one example, the height  415 C can be approximately 0.547 μm for the diffraction grating  400 C, whereas the height  415 B can be approximately 0.275 μm for the diffraction grating  400 B. Because the blaze angle is related to the angle between the input light and the output light, the angle between the input and output light may be used to select the height  415 C. 
     In some examples, the angle between the input and output light may be tuned such that the optical loss associated with the first wavelength band (e.g., of a first input light path) can be similar to the optical loss associated with the second wavelength band (e.g., of the second input light path). In some examples, the diffracted output light may overlap with the input light, which may help reduce imaging problems specific to optical components that use broadband wavelength ranges of light. 
       FIGS. 5A and 5B  illustrate sample spectrum plots at different blaze angles. The spectrum plot of  FIG. 5A  is representative of a blaze angle of approximately 3.5° and the spectrum plot of  FIG. 5B  is representative of a blaze angle of approximately 4.7°. In some examples, the blaze angle may be tuned such that the power of the input light can be balanced (e.g., have a small dynamic range) across multiple wavelengths of both the first wavelength band and the second wavelength band, which may be spaced apart from the first wavelength band by at least 0.1 μm and generally in the broadband wavelength range as described herein. It should be understood that the spectrum plots of  FIGS. 5A and 5B  are illustrative rather than intended to show particular or limiting information. 
     The power of the light for the first wavelength band relative to the second wavelength band may differ more with a smaller blaze angle than with a larger blaze angle, so the larger angle may be selected such that the difference in the output light transmissions between the first wavelength band (of the first input light) and the second wavelength band (of the second input light) is reduced. In some examples, the average transmission of the first wavelength band may be equal to the average transmission of the second wavelength band. 
     In some examples, the diffraction grating may have blaze angles to reflect light such that an optical separation is maintained between an input waveguide (e.g., to a laser) and an output waveguide (e.g., to a detector). Without the optical separation, the waveguides may be subject to optical coupling that can lead to unwanted effects such as the self-mode of light in the input waveguide competing with a lasing mode. The self-mode of light in the input waveguide may be due to light reflecting from the diffraction grating that may return to the input waveguide and can cause self-mode lasing. 
     In some examples, the wavelengths received at the output waveguide(s) can be associated with a lower level of granularity such that the wavelength band between 2.0 μm-2.1 μm may be extracted. A lower level of granularity can refer to a large spacing between wavelengths so that a signal may be detected at the output waveguide(s). In some examples, the angles between the input waveguide(s) and the output waveguide(s) may be related to the extracted wavelength band. 
     The optical performance of the diffraction grating may be sensitive to the quality of the fabrication and the grating material and the fabrication of the diffraction grating may present challenges, especially when the reflective facet width and/or height of the teeth are small. For example, variations and imperfections of the reflect facet of the diffraction grating can lead to phase errors, optical crosstalk, variations in the path lengths of the returned light, variations in the scattering, or the like. In addition to or instead of configuring the diffraction grating design in consideration of the size of the optical component and the optical performance, other considerations may include fabrication costs, yield, and complexity. 
       FIG. 6  illustrates a process flow for operating a diffraction grating. At operation  605  of process  600 , the light source may emit light towards the diffraction grating. The wavelength range of the input light may be a broadband wavelength range as described herein. At operation  610 , the light can propagate in a planar waveguide to the diffraction grating. In some examples, the input light may be emitted into the planar waveguide using a strip waveguide which may be located on or near the Rowland circle. At operation  615 , the diffraction grating may reflect the light to a first and second waveguide, which may be near or on the Rowland circle. In some examples, the reflected light may be separated into two wavelength bands that are separated by approximately 0.1 μm. 
     Although the disclosed examples have been fully described with reference to the accompanying drawings, it is to be noted that various changes and modifications will become apparent to those skilled in the art. Such changes and modifications are to be understood as being included within the scope of the disclosed examples as defined by the appended claims.

Metadata:
Filing Date: 20200908
Publication Date: 20221122
Grant Date: 20221122
Priority Date: 20190909
Inventors: TU, YONGMING
BISMUTO, ALFREDO
Assignee: APPLE INC
CPC Classifications: [{"code": "G02B27/1006", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02B5/1861", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B27/1086", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01J3/28", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B5/1861", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01J3/1809", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02B6/34", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01J3/20", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B27/4233", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01J3/20", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B6/34", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B5/1861", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01J3/28", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01J3/1809", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02B27/4233", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01J3/20", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01J3/1809", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01J3/0218", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B6/2931", "inventive": true, "first": false, "tree": "[]"}]
Family ID: 84104864