Patent Publication Number: US-2022229162-A1

Title: Scanning lidar with optical switching

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. provisional application Ser. No. 62/850,573 filed May 21, 2019, the disclosure of which is hereby incorporated by reference in its entirety herein. 
    
    
     TECHNICAL FIELD 
     This disclosure relates to scanning lidar and a method for improving resolution of a scanning lidar system. 
     BACKGROUND 
     Lidar is an active remote sensing technology that uses light from a transmitter reflected by objects within a field of view (FOV) to determine the range or distance to the objects. This information can be processed to generate an image or otherwise used for mapping, object identification, object avoidance, navigation, etc. in various types of vehicles, such as automotive vehicles or drones, for example. While a number of lidar solutions have been proposed and may be acceptable for particular applications, various strategies have associated disadvantages that may make them unsuitable in other applications. For example, various lidar systems have limited power and associated limited detection range to maintain eye safety, use moving parts to mechanically scan the FOV, have limited frame rates, and are limited in adverse environmental conditions, such as fog, haze, rain, snow, etc. 
     SUMMARY 
     A real-time scanning lidar system and method include various embodiments that incorporate all-optical switching so that no moving parts are required. Various embodiments facilitate scanning of a larger field of view (FOV) in three-dimensional space to provide multi-dimensional real-time data, such as location, range, polarization, velocity, etc., with respect to objects within the FOV. 
     In one or more embodiments, a scanning lidar system includes a transmitter having a laser and a first optical switch configured to receive laser pulses from the laser, a first plurality of fibers each coupled to a different one of a plurality of outputs of the first optical switch, a first at least one optical element configured to receive the laser pulses from at least one of the first plurality of fibers and to redirect the laser pulses to illuminate at least a portion of a field of view, a receiver including a second optical switch and at least one detector, a second plurality of fibers each coupled to a different input of the second optical switch, an output of the second optical switch coupled to the at least one detector, a second at least one optical element configured to receive the laser pulses reflected from the field of view and to redirect received reflected pulses to at least one of the second plurality of fibers, and at least one controller configured to control the first optical switch to direct the laser pulses from an input of the first optical switch to each of the plurality of outputs in turn, to control the second optical switch to direct light from each of the second plurality of fibers in turn to the output of the second optical switch, and to process signals from the at least one detector to generate data representing the field of view. The first optical switch and the second optical switch may be all-optical switches that have no moving parts associated with switching light from the input to one of the plurality of outputs, or from the plurality of outputs to the input, respectively. The first and second optical switches may include a Faraday rotator. In at least one embodiment, the first and second optical switches comprise a magneto-optic switch. 
     In various embodiments, the system may include a first microprocessor-based controller configured to control the first optical switch and a second microprocessor-based controller in communication with the first controller and configured to control the second optical switch. 
     In one or more embodiments, the laser comprises a fiber laser or any pulsed laser source connect to a fiber configured to generate pulses having an SWIR wavelength between 900 nanometers (nm) and 1700 nanometers (nm). In at least one embodiment, the laser is configured to generate pulses having a nominal wavelength of 1550 nanometers (nm). 
     In one or more embodiments, the at least one first optical element comprises an aspherical lens configured to form an output beam having an elliptical cross section. The optical element may be shaped to form an output beam having an angular divergence along a first axis of at least 20 times the angular divergence along a second axis perpendicular to the first axis. In at least one embodiment, the output beam has an angular divergence of 40 degrees in a horizontal direction and one degree in a vertical direction. 
     In one or more embodiments the system includes a plurality of optical elements, each associated with one of the first plurality of fibers to provide the output beam that illuminates the FOV. 
     In one or more embodiments, the at least one second optical element comprises collection optics positioned upstream of beam shaping optics, the beam shaping optics configured to form beams having an elliptical cross section. The beam shaping optics may provide a beam having an elliptical cross section with an angular divergence along a second axis at least 20 times the angular divergence along a first axis perpendicular to the second axis. In one embodiment, the receiver includes collection optics and beam shaping optics to provide received light beams having a horizontal divergence of one degree and a vertical divergence of 20 degrees. 
     In one or more embodiments, the first plurality of fibers is arranged in a first linear array and inputs of the second plurality of fibers are positioned in a second linear array orthogonal to the first linear array. 
     In various embodiments, the at least one detector comprises an avalanche photodiode. In at least one embodiment, the at least one detector comprises a plurality of detectors configured to operate in parallel. 
     In one or more embodiments, the laser is configured to transmit polarized pulses and the detector comprises a plurality of detectors each configured to detect received light having a different angle of polarization. The at least one controller may be configured to process signals from the plurality of detectors to generate degree of polarization or angle of polarization data representing the field of view. In at least one embodiment, the first at least one optical element and the second at least one optical element each comprises a polarization rotator controlled by the at least one controller and a polarization phase mask. In at least one embodiment, the first at least one optical element and the second at least one optical element each comprise a first linear polarizer, a Faraday rotator, a half-wave retarder phase mask, and a second linear polarizer. The at least one controller may be configured to operate the laser and the first and second optical switches to refresh the data at 100 Hz to provide 100 frames per second. 
     In various embodiments, a modular system is provided having a control unit coupled by a fiber bundle to an optical head. In one embodiment, the system includes a housing containing the transmitter, the receiver, and the at least one controller and at least one optical head located outside of the housing and containing the first at least one optical element and the second at least one optical element, wherein the first plurality of fibers and the second plurality of fibers extends between the first housing and optical head. The housing of the control unit may contain a transmitter and a receiver associated with each of a plurality of remotely located optical heads each coupled by an associated fiber bundle. 
     In at least one embodiment, a vehicle includes a lidar system as described herein with a controller unit connected to one or more head units. The vehicle may include a lidar system having a stationary controller unit optically coupled to a head unit mounted on an actuator configured to rotate the mounted head unit. 
     Embodiments may also include a method for lidar scanning that includes generating laser pulses, optically switching the laser pulses to each of a first plurality of fibers arranged in a first linear array to illuminate a field of view, directing light reflected from an object illuminated by the laser pulses within the field of view to a second plurality of fibers arranged in a second linear array, optically switching light from the second plurality of fibers to direct the light to at least one detector, and processing signals from the detector to generate data representing the field of view. Generating the laser pulses may include generating laser pulses using a fiber laser having an output wavelength of between 900 nm and 1700 nm. In one embodiment, the nominal output wavelength is 1550 nm to provide increased power for remote sensing while meeting eye safety requirements. 
     In one or more embodiments, the method includes optically switching the laser pulses and optically switching the light from the second plurality of fibers by controlling an all-optic solid state switch having a Faraday rotator without any moving parts, which may be implemented by a magneto-optic switch, for example. 
     In one or more embodiments, the method may include shaping the laser pulses output from the first plurality of fibers to form pulsed beams having elliptical cross-sections with an angular divergence along a first axis at least 20 times greater than an angular divergence along a second axis perpendicular to the first axis. In one embodiment, the angular divergence along the first axis is 40 degrees, while the angular divergence along the second axis is one degree. 
     Various embodiments may include scanning the FOV by directing laser pulses sequentially to a first linear array of fibers positioned orthogonally relative to the second linear array of fibers that receive the reflected light from objects within the FOV. 
     In at least one embodiment, the method includes optically switching the light from the second plurality of fibers to a single fiber coupled to the detector. In some embodiments, a plurality of detectors is provided and optically switching the light from the second plurality of fibers comprises optically switching light from different groups of fibers to different detectors operating in parallel. 
     In one or more embodiments, the method includes polarizing the laser pulses that illuminate the field of view, detecting polarization of the light reflected from the object illuminated by the laser pulses, and generating polarization data representing the field of view based on detecting the polarization of the light reflected from the object. The method may also include varying polarization of the laser pulses to increase light intensity detected from a selected one of the second plurality of fibers relative to light intensity from fibers adjacent to the selected fiber. 
     Embodiments also include a method for improving scanning resolution. The method includes generating a continuous wave (cw) or pulsed laser beam, scanning the laser beam along a first direction to generate a transmitted laser beam to illuminate a field of view, directing light reflected from an object illuminated by the transmitted laser beam within the field of view along a second direction orthogonal to the first direction to form a received beam provided to at least one detector, processing signals from the at least one detector to generate a two-dimensional array of pixels, and varying an intensity profile within a selected pixel to move detected peak intensity in a continuous manner within the selected pixel by synchronously varying polarization of the transmitted laser beam and the received beam provided to the at least one detector. 
     In one or more embodiments, the method includes varying the intensity profile of the detected received beam within a pixel by directing the generated laser beam or the transmitted laser beam through a Faraday rotator and a phase mask, and controlling the Faraday rotator to vary the polarization of the transmitted laser beam. The method may also include directing the received beam through a second Faraday rotator and a second phase mask, wherein the second Faraday rotator is controlled synchronously with the first Faraday rotator. In one embodiment, the phase mask comprises a half-wave retarder phase mask. In one embodiment, the phase mask comprises a vortex retarder. 
     In various embodiments, the method includes optically switching the laser beam to direct light to sequentially illuminate a plurality of transmitter fibers arranged in a first linear array and directing light reflected from the object to a plurality of receiver fibers arranged in a second linear array orthogonally positioned relative to the first linear array. 
     Embodiments may also include a system having a laser configured to generate a laser beam, a first Faraday rotator positioned upstream of a first half-wave retarder phase mask, the first Faraday rotator and the first phase mask selectively varying polarization of an output beam in response to a polarization control signal, a first magneto-optical switch configured to receive the output beam and selectively redirect the output beam to one of a plurality of outputs in response to a transmit scanning control signal to scan the output beam along a first direction to illuminate a field of view, at least one optical element configured to collect light reflected from the field of view along a second direction orthogonal to the first direction, a second magneto-optical switch configured to selectively redirect light from each of a plurality of inputs to an output in response to a receive scanning control signal to scan the light reflected from the field of view along the second direction, a second Faraday rotator positioned to receive light from the output of the second magneto-optical switch and positioned upstream of a second half-wave retarder phase mask, the second Faraday rotator and the second phase mask selectively varying polarization of the received light in response to the polarization control signal, at least one detector positioned to receive light from the second phase mask, and at least one controller configured to generated the transmit and receive scanning signals to control the first and second magneto-optical switches, to process signals from the at least one detector to generate a two-dimensional array of pixels, and to vary an intensity profile within a selected pixel to move peak intensity within the selected pixel by generating the polarization control signal to synchronously control the first and second Faraday rotators to vary polarization of the transmitted laser beam and the received light provided to the at least one detector. 
     In at least one embodiment, a scanning lidar system includes a central module including a laser and an optical splitter configured to divide output from the laser among a plurality of splitter outputs, a first plurality of connecting fibers each coupled to a different one of the plurality of splitter outputs, and a plurality of optical heads each coupled to the central module by one of the first plurality of connecting fibers. Each optical head includes a first optical switch having an input coupled to an associated one of the first plurality of connecting fibers, a plurality of transmission fibers coupled to an associated output of the first optical switch and having outputs arranged in a linear array along a first direction, at least one transmitter optical element configured to receive laser pulses from at least one of the plurality of transmission fibers and to redirect the laser pulses to illuminate at least a portion of a field of view, a second optical switch, a plurality of receiver fibers each coupled to a different input of the second optical switch, an output of the second optical switch coupled to at least one detector, and at least one receiver optical element configured to receive the laser pulses reflected from the field of view and to redirect received reflected pulses to at least one of the receiver fibers. The system also includes at least one controller configured to control the first optical switch and the second optical switch and to process signals from the at least one detector to generate data representing the field of view, wherein the at least one controller and the at least one detector are disposed within either the central module or the optical head. 
     In one or more embodiments, a method includes generating laser pulses in a central module, splitting the laser pulses in the central module to deliver the laser pulses to each of a plurality of remotely located optical heads. In each of the plurality of remotely located optical heads, the method includes optically switching the laser pulses to each of a first plurality of fibers arranged in a first linear array to illuminate a field of view, directing light reflected from an object illuminated by the laser pulses within the field of view to a second plurality of fibers arranged in a second linear array, and optically switching light from the second plurality of fibers to direct the light to at least one detector. The method also includes processing signals from the detector to generate data representing the field of view. 
     In at least one embodiment, a scanning lidar system includes a central module including a pulsed laser, a first magneto-optical optical switch, and a plurality of optical splitters, the magneto-optical switch configured to switch laser output to one of the plurality of optical splitters, each of the plurality of optical splitters configured to divide pulses from the laser among a plurality of splitter outputs, and a plurality of connecting fibers coupled to each output of each of the plurality of optical splitters. A plurality of optical heads remotely located relative to the central module and each coupled to the central module by one of the plurality of connecting fibers associated with each of the plurality of splitter outputs each include a plurality of transmission fibers coupled to each of the plurality of fiber splitters and having outputs arranged in a linear array along a first direction, at least one transmitter optical element configured to receive laser pulses from at least one of the plurality of transmission fibers and to redirect the laser pulses to illuminate at least a portion of a field of view, a second magneto-optical switch, a plurality of receiver fibers each coupled to a different input of the second magneto-optical switch, an output of the second magneto-optical switch coupled to at least one detector, and at least one receiver optical element configured to receive the laser pulses reflected from the field of view and to redirect received reflected pulses to at least one of the receiver fibers. the system also includes at least one controller configured to control the first magneto-optical switch and the second magneto-optical switch and to process signals from the at least one detector to generate data representing the field of view, wherein the at least one controller and the at least one detector are disposed within either the central module or the optical head. 
     In one or more embodiments, a method includes generating laser pulses in a central module, optically switching the laser pulses to each of a plurality of fiber splitters in the central module, and splitting the laser pulses by each fiber splitter to deliver each laser pulse to each of a plurality of optical heads remotely located relative to the central module. The method includes, in each of the plurality of optical heads, directing the laser pulses from the plurality of fiber splitters to each of a plurality of transmission fibers arranged in a first linear array to illuminate a field of view, directing light reflected from an object illuminated by the laser pulses within the field of view to a plurality of receiver fibers arranged in a second linear array arranged orthogonally to the first linear array, and optically switching light from the receiver fibers to direct the light to at least one detector. The method also includes processing signals from the detector to generate data representing the field of view. 
     One or more embodiments may provide advantages for various applications. For example, various embodiments according to the disclosure provide a scanning lidar operating at an eye-safe wavelength that does not require any moving parts by using magneto-optic switches orthogonally oriented fiber arrays to scan a field of view and receive reflected light to create an image or multi-dimensional data representing the FOV. One or more embodiments provide improved range, frame rate, reliability, scalability, and robust operation under adverse weather conditions making them suitable for use in autonomous vehicles and drones, for example. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram illustrating operation of a representative embodiment of a system or method for scanning lidar. 
         FIGS. 2 and 3  are diagrams illustrating beam cross-sections from a linear array of fibers for a transmission beam or received reflected light directed into an orthogonally positioned linear array of fibers in a representative embodiment. 
         FIG. 4  illustrates multi-dimensional data generated from detector signals associated with overlapping points or pixels corresponding to a combination of the optically switched or scanned transmitted and received light pulses. 
         FIG. 5  illustrates a modular configuration or architecture for a scanning lidar sensor having a controller or central unit and at least one optical head of a representative embodiment. 
         FIG. 6  illustrates a modular configuration or architecture for a scanning lidar sensor having a central unit with an optical splitter upstream of a transmitter optical switch coupled to multiple optical heads in another representative embodiment. 
         FIG. 7  illustrates a modular configuration or architecture for a scanning lidar sensor having a central unit with an optical splitter downstream of a transmitter optical switch coupled to multiple optical heads in another representative embodiment. 
         FIG. 8  illustrates a representative embodiment of a vehicle having a scanning lidar sensor with a rotating optical head and/or one or more stationary optical heads. 
         FIG. 9  is a block diagram illustrated a scanning lidar with optical switches and multiple detectors to detect polarization of reflected light according to various embodiments. 
         FIG. 10  illustrates a representative embodiment of a system or method for improving resolution within a pixel of a laser scanned FOV by manipulating or scanning polarization of the transmitted and received light using a Faraday rotator and phase mask. 
         FIG. 11  illustrates increasing intensity within a portion of a pixel of a laser scanned FOV by manipulating polarization of the transmitted and received light according to various embodiments. 
         FIG. 12  is a flowchart illustrating operation of a system or method for lidar scanning using optical switching according to one or more embodiments. 
         FIG. 13  is a flowchart illustrating operating of a system or method for increasing resolution of a laser scanned FOV using polarization manipulation according to one or more embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. The figures are not necessarily to scale; some features may be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention. 
     As used in this description, an image or related terminology is not limited to a visual representation and refers more generally to a data representation of a field of view (FOV). Different types of data, such as location/position, distance/range, intensity, polarization, velocity, etc., may be collected for each measured point or pixel within the FOV to provide a multi-dimensional data array that may be processed by a controller without generating a visual representation of the data. Similarly, references to a pixel do not imply or require a visual representation or display of associated data, or an area on a display screen, but refer more generally to a discrete measurement point or observation point within the FOV, with underlying discrete measurements for a particular pixel location referred to as sub-pixels that may be used to improve or enhance the resolution within the pixel. For example, sub-pixels corresponding to measurements generated for a particular (x, y) pixel location from different laser pulses or different characteristics/properties of the laser pulse provide additional data that may be used to detect or identify time domain or spatial domain changes within the pixel to enhance resolution. 
     A vehicle is used in its most general sense as something used to carry, transport, or convey something or self-propelled mechanized equipment. 
     An optical switch refers to an all-optical switch that maintains the signal as light from input to output in contrast to an electro-optic switch that converts the optical signal to an electric signal and back to an optical signal to route light signals or pulses from one channel to another, i.e. from an input to one of a plurality of outputs, or from one of a plurality of inputs to an output. The all-optical switch may be controlled by an electric signal or electronic controller to provide spatial domain switching of optical signals or pulses. An optical switch with no moving parts refers to a device that does not have any moving mechanical components to perform the switching operation, i.e. excludes movable mirrors such as those provided in MEMS based photonic switches. 
     An optical element refers to any element or component that acts upon light including discrete elements such as mirrors, lenses (including graded index or gradient index lenses), prisms, gratings, etc. as well as integrated optics and holographic optical elements that may also act on incident light to redirect the light and/or modify one or more properties of the light. 
       FIG. 1  is a block diagram illustrating operation of a representative embodiment of a system or method for scanning lidar. System  100  includes a transmitter  102  configured to illuminate a field of view (FOV)  104  having at least one object  106  with reflected light from object  106  being detected by a receiver  108 . Transmitter  102  includes a laser  110  and a first optical switch  112  configured to receive laser pulses from the laser  110 . In various embodiments, laser  110  is a fiber laser operating in a pulsed mode in the SWIR range with an output nominal wavelength between 900 nm and 1700 nm. In at least one embodiment laser  110  operates at a nominal output wavelength of 1550 nm in the eye-safe region so that transmitter  102  may operate with higher power to provide longer range and improved imaging/sensing performance Laser  110  may be operated to provide a data frame rate of between 100-500 Hz, for example, with laser pulse rates between 100-500 KHz, for example. Of course, the data frame rate and laser pulse repetition rate will vary based on the particular application and implementation. 
     In the representative embodiment illustrated in  FIG. 1 , first optical switch  112  is an electronically controlled all-optical 1×N switch to transfer optical pulses output by fiber laser  110  from the input of switch  112  to one of the N outputs as controlled by an associated controller, such as controller(s)  120 . In one embodiment, optical switch  112  is implemented by a 1×32 magneto-optical switch similar to commercially available switches offered by Agiltron, Inc. of Woburn, Mass., USA or Primanex, Inc. of Qingdao, Shandong, China. A magneto-optical switch includes a Faraday rotator to switch the optical pulses so that the switch includes no moving parts to perform the switching operation. 
     Each of the outputs of switch  112  is coupled to an associated fiber positioned in a linear array along a first axis or direction. In one embodiment, the linear array of transmitter fibers  114  is oriented vertically. The outputs of fibers  114  are positioned at the focal plane of transmission optics  116 , which may be implemented by at least one optical element configured to receive the laser pulses from fibers  114  and to redirect the laser pulses such that the light  118  is directed into a different angle to illuminate a corresponding portion of the FOV  104  containing one or more objects  106 . The at least one optical element may include a diverging lens or one or more asymmetric, aspherical, and/or cylindrical optical elements to provide an oval or elliptical output beam at a specified angle based on the desired coverage portion of FOV  104  (best illustrated in  FIG. 2 ). The at least one optical element may include one or more lenses with each lens associated with a single one of fibers  114 , a group of fibers  114 , or all fibers  114 . 
     Receiver  108  receives central light rays  130  and off-axis light rays  132 ,  136  within reflected light  138  from object  106  within the FOV  104 . Receiver optics  140  includes at least one optical element configured to collect and receive the laser pulses reflected from the FOV  104  and to redirect received reflected pulses to at least one of a second plurality of fibers  150 . Fibers  150  are arranged in a linear array oriented orthogonally relative to the linear array of the transmitter fibers  114 . In one embodiment, fibers  150  are oriented horizontally (best shown in  FIG. 3 ). Of course, the orientation of the linear fiber arrays of the transmitter and receiver may be reversed with the transmitter fibers oriented horizontally and the receiver fibers oriented vertically. Other orthogonal orientations are also possible depending on the particular application and implementation. 
     In the representative embodiment of  FIG. 1 , receiver optics  140  are configured to collect and redirect off-axis reflected light rays  132  to an associated fiber  152 , while representative central rays  130  are collected and redirected to a fiber  154  and representative off-axis rays  136  are collected and redirected to a fiber  156  within fiber array  150 . Receiver optics  140  includes at least one optical element, which may include collection optics upstream of beam shaping optics configured to form beams having an elliptical cross section. The at least one optical element may include a converging lens or one or more asymmetric, aspherical, and/or cylindrical optical elements to provide an oval or elliptical output beam. The at least one optical element may include one or more lenses with each lens associated with a single one of fiber array  150 , a group of fibers within fiber array  150 , or all fibers of fiber array  150 . 
     Each fiber of fiber array  150  is coupled to a different input of a second optical switch  160 . Optical switch  160  may be implemented by an all optical switch with no moving parts similar to first optical switch  112 . In one embodiment optical switch  160  is a M×1 magneto-optical switch that sequentially connects one of the M inputs coupled to an associated fiber within fiber array  150  to an output connected to detector(s)  164  as controlled by associated controller(s)  162 . In one embodiment optical switch  160  is a 32×1 magneto-optical switch such that system  100  provides a 32×32 array of pixels or data for a 40° horizontal by 20° vertical FOV as described in greater detail herein. Of course, the vertical and horizontal extent, number of pixels, and FOV covered need not be symmetric or proportional to the representative embodiments and will vary by application and implementation. 
     In one embodiment, at least one controller  120 ,  160  is a microprocessor-based controller having associated non-transient memory or computer readable media for storing data representing instructions executable by the controller(s) to perform one or more control functions or algorithms and is thereby configured to control the first optical switch  112  to direct the laser pulses from fiber laser  110  from an input of the first optical switch  112  to each of the plurality of outputs coupled to associated fibers  114  in turn. The at least one controller  120 ,  160  controls the second optical switch  160  to direct light from each of the second plurality of fibers  150  in turn to the output of the second optical switch  160 , and to process signals from the at least one detector  164  to generate data representing the field of view. Each of detector(s)  164  may be implemented by a photodiode such as an avalanche photodiode (APD), a PIN diode, a Schottky barrier photodiode, or any other optical detector with similar sensitivity that provides the desired signal-to-noise ratio (SNR) for a particular application. Where more than one controller is provided, the controllers may communicate to exchange data and/or coordinate or cooperate to perform a particular task, function, algorithm, etc. 
     In general, the processes, methods, or algorithms disclosed herein can be performed by a processing device, controller, or computer, which can include any existing programmable electronic control unit or dedicated electronic control unit or controller. Similarly, the processes, methods, or algorithms can be stored as data and instructions executable by a controller or computer in many forms including, but not limited to, information permanently stored on non-writable storage media such as ROM devices and information alterably stored on writeable storage media including electronic, magnetic, and/or optical storage devices. Certain processes, methods, or algorithms may also be implemented in a software executable object. Alternatively, the processes, methods, or algorithms can be embodied in whole or in part using suitable dedicated or custom hardware components, such as Application Specific Integrated Circuits (ASICs), Field-Programmable Gate Arrays (FPGAs), state machines, controllers, or any other hardware components or devices, or a combination of hardware, software and firmware components. Similarly, illustration or description of a process, algorithm or function in a particular sequence or order may not be required to perform the described operation or outcome. Some processes, functions, algorithms, or portions thereof may be repeatedly performed, performed in a different sequence, or omitted for particular applications. 
       FIG. 2  is a diagram illustrating a transmission beam cross-section or front view for a linear array of fibers in a representative embodiment. As illustrated in  FIG. 2 , the optics in the transmitter may not be symmetric but rather asymmetric, aspherical, or cylindrical in nature, so that each beam of the output light  118  is shaped by the non-spherical optics to have an oval or elliptical beam cross section or front view. Each beam has a very wide angular divergence in the horizontal plane or direction  220  and narrow angular divergence in the vertical plane or direction  230 , or vice versa. In one embodiment, the horizontal angular divergence is 40 degrees with a vertical angular divergence of 1 degree. This beam shape can be achieved, for example, by placing a small aspherical/cylindrical lens in front of every fiber  114  within linear array  210  or a larger lens common to groups or all fibers as previously described. As such, the transmitter directs each laser pulse across a wide horizontal coverage and narrow vertical coverage or vice versa. Additional vertical coverage for the FOV is provided by scanning the laser pulses using the optical switch across adjacent fibers to cover the desired FOV. Each pulse may be separated in time to reduce or eliminate overlap. 
       FIG. 3  is a diagram illustrating a cross-section of received reflected light directed into a linear array  310  of fibers  150  arranged orthogonally relative to the linear array  210  of transmission fibers  114  ( FIG. 2 ) in a representative embodiment. As previously described, the receiver optics is configured to receive the laser pulses reflected from the FOV and to redirect received reflected light to each of the fibers  150 . The fibers are scanned or switched by the associated optical switch to direct the received light to the detector(s). As described with respect to  FIG. 1 , the received light is directed or coupled to one of the plurality of fibers  150  in the linear array  310 . The receiving “beams” in space look very similar to the transmission beams only rotated by 90 degrees, as generally illustrated in  FIGS. 2 and 3 . The shaping of the receiving beams to the oval or elliptical cross section is performed using similar techniques and components as described above with respect to shaping the transmission beam. In one embodiment, the received light “beams” each cover an angle in the horizontal direction  220  of 1° and an angle in the vertical direction  230  of 20°. 
     Although the orthogonally positioned linear arrays of fibers are illustrated as a vertical transmission array and a horizontal receiver array, any other orthogonal orientation is possible. Similarly, although the representative embodiments include examples having a transmission fiber array with 32 fibers and a receiver fiber array of 32 fibers, the transmission and receiver arrays may contain different numbers of fibers depending on the particular application. Likewise, although transmission optics generate beams having angular divergence of 40×1 degrees and receiver optics to generate light having angular divergence of 1×20 degrees, the transmitter and receiver optics may be selected to provide different angular divergences depending on the particular application. 
       FIG. 4  illustrates multi-dimensional data generated from detector signals associated with overlapping points or pixels corresponding to a combination of the optically switched or scanned transmitted and received light pulses. The transmitted elliptical pulses  118  are scanned using the transmitter optical switch to direct pulses into each fiber sequentially or in turn as indicated at  400 . The received light or “beams”  180  are scanned using the receiver optical switch to direct light from each receiver fiber to the detector as represented at  410 . The intersection of the transmitted and received beam provides an array of pixels  420  representing the data generated by the detector signals for a particular position/location. As such, if the transmitter optical switch is configured to direct one or more laser pulses to fiber indexed i and the receiving optical switch is configured to direct received light from fiber number j to the detector(s), then the light signal that will be received at the detector will come from the angle defined by the (i, j) th  element of the 2D pixel array of pixels  420  defined by the combination of the transmitting and receiving beams. As defined in greater detail below, multiple laser pulses may be provided to one transmitter/receiver fiber pair before operating the switches to scan to the next adjacent transmitter/fiber pair to provide multiple measurements for each pixel  420  to improve SNR as described below. Similarly, switching may occur after each laser pulse with multiple data associated with each pixel combined or otherwise processed to provide desired performance. 
     In one representative embodiment, each laser pulse associated with each of 32 transmission fibers has an instantaneous FOV of 40°×1°. Each receiver instantaneous FOV is 1°×20° and the laser is scanned from top down and the receiver from left to right, creating a matrix of pixels  420  that include 32×32 (or more generally N×M) pixels defined by the intersection of the transmitted and received light. The scanning is based on transmitter/receiver optical switches implemented by 1×32 magneto-optic switches with a switching time of less than 10 μsec. The fiber laser characteristics are assumed to provide PRF*32*32 pulses/second or approximately 100 k pulses/second to provide the desired SNR with an average laser power of 100 k*30 microjoules=3 W. Representative signal calculations are shown below. 
     
       
         
           
               
               
               
               
             
               
                   
                   
               
               
                   
                 Parameter 
                 Value 
                 Description 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 E p   
                 30 
                 μJ 
                 Pulse energy 
               
               
                   
                 τ p   
                 10 
                 ns 
                 Pulse width 
               
               
                   
                 IL tx   
                 5 
                 db 
                 Insertion loss of TX switch 
               
               
                   
                 IL rx   
                 5 
                 db 
                 Insertion loss of RX switch 
               
            
           
           
               
               
               
               
            
               
                   
                 A r   
                 πr 2   
                 Area of Rx optics 
               
            
           
           
               
               
               
               
               
            
               
                   
                 R 
                 2.5 
                 cm 
                 Radius of receiver optics 
               
               
                   
                 R 
                 200 
                 m 
                 Test range to target 
               
            
           
           
               
               
               
               
            
               
                   
                 Σ 
                   0.06 
                 Reflectance of target 
               
               
                   
                 S t   
                 0.1° × 0.1° 
                 Reflecting surface size at R 
               
               
                   
                 TX FOV   az   
                 40° 
                 Transmitter azimuthal FOV 
               
               
                   
                 TX FOV   el   
                  1° 
                 Transmitter elevation FOV 
               
            
           
           
               
               
               
               
               
            
               
                   
                 PRF 
                 100 
                 Hz 
                 Frame rate 
               
               
                   
                   
               
            
           
         
       
     
     The instantaneous peak power that falls on the target is provided by: 
     
       
         
           
             
               P 
               target 
             
             = 
             
               
                 
                   E 
                   p 
                 
                 
                   τ 
                   p 
                 
               
               ⁢ 
               I 
               ⁢ 
               
                   
               
               ⁢ 
               
                 L 
                 tx 
               
             
           
         
       
     
     Assuming a target size as small as the minimal resolution for the system to detect, S t , the reflected signal from this target object is given by: 
     
       
         
           
             
               P 
               reflected 
             
             = 
             
               
                 
                   
                     P 
                     target 
                   
                   ⁢ 
                   
                     S 
                     t 
                   
                 
                 
                   T 
                   ⁢ 
                   
                     X 
                     
                       F 
                       ⁢ 
                       O 
                       ⁢ 
                       V 
                     
                     
                       e 
                       ⁢ 
                       l 
                     
                   
                   ⁢ 
                   T 
                   ⁢ 
                   
                     X 
                     
                       F 
                       ⁢ 
                       O 
                       ⁢ 
                       V 
                     
                     
                       A 
                       ⁢ 
                       Z 
                     
                   
                 
               
               ⁢ 
               σ 
             
           
         
       
     
     In a worst case scenario, the reflected signal from the target object is distributed evenly over half a dome toward the transmitter unit, and therefore the collecting optics in the receiver collects the following amount of power from the target object: 
     
       
         
           
             
               P 
               received 
             
             = 
             
               
                 
                   
                     P 
                     reflected 
                   
                   ⁢ 
                   
                     A 
                     r 
                   
                 
                 
                   R 
                   2 
                 
               
               ⁢ 
               I 
               ⁢ 
               
                   
               
               ⁢ 
               
                 L 
                 
                   r 
                   ⁢ 
                   x 
                 
               
             
           
         
       
     
     Inserting the values for each one of the parameters above gives: 
       P received ≅2*10 −10 W
 
     While it may be difficult to detect such a signal from a single pulse, repeating the pulses to generate sub-pixel measurements for each pixel with a frame rate of 100 Hz, for example, can provide an improvement factor of 10 for the SNR, so the effective received signal will be: 
       P received √{square root over (PRF)}≅2*10 −9 W
 
     This signal level can clearly be detected by a sensitive photodiode such as an avalanche photodiode or similar optical detector. 
     The laser pulse frequency can be increased with an upper system constraint established by a rate at which transmitted pulses may overlap or intersect at the receiver if the pulses are close enough in time, i.e. a pulse is transmitted before the previously transmitted pulse has been reflected by an object and detected by the receiver. Assuming a buffer time of 2 microseconds (600 m) between pulses to prevent the pulses from intermixing, the maximum laser frequency for a typical application would be 500 KHz. This repetition rate may provide a corresponding rate of 500K sub-pixels/sec and an average laser power of 15 W. As 15 W is a rather large laser average power, multiple detectors may be provided to operate in parallel to cover the FOV of the system. For example, a system having four avalanche photodiodes in parallel instead of a single detector as described above facilitates a 125 KHz laser (3.8 W average laser power) while providing 500K sub-pixels/sec data rate. Similarly, a system having eight detectors with a 125 KHz laser frequency can generate 1M sub-pixels/sec. A representative system having multiple detectors operating in parallel is illustrated and described with respect to  FIG. 9 . Of course, the polarization filters would be omitted so that all parallel detectors are detecting the same characteristic of the received light. 
     Alternatively, or in combination, search resources may be allocated based on detection of objects in previous frames. This strategy will effectively increase the resolution of detection without the need to increase the number of overall pixels in the system by allocating additional search sub-pixels directed only to areas where objects were detected in previous frames. 
       FIG. 5  illustrates a modular configuration or architecture for a scanning lidar sensor having a controller or central unit (CU) and at least one optical head (OH) of a representative embodiment. The various modular configurations or architectures illustrate in  FIGS. 5-7  are understood to have components similar to those described with respect to the embodiment of  FIG. 1  unless otherwise stated, and are not described again in detail. However, various components may be arranged differently within either the CU or OH depending on the particular embodiment, and additional components may be used to facilitate specific embodiments. 
     System  500  includes a CU module  510  with a first housing and OH module  560  in a second housing coupled by a fiber bundle  532 . In the representative embodiment of  FIG. 5 , CU module  510  includes fiber laser  512 , first optical switch  514 , controller(s)  516 , second optical switch  518  and detector(s)  520 . Fiber laser  512  is coupled to first optical switch  514  via fiber  522 , or may be directly coupled. Transmitter fibers  532  are coupled to the output of first optical switch  514 . Similarly, receiver fibers  534  are coupled to second optical switch  518 . Fiber bundle  530  includes transmitter fibers  532  and receiver fibers  534 . OH  560  includes transmitter optics  540  and receiver optics  550 . As such, OH module  560  contains only optical fibers and optical components, and is connected to the CU  510  through optical fiber bundle  530 . As illustrated and described in greater detail with respect to  FIGS. 6-7 , multiple OH modules can be connected to a single CU module to expand the covered FOV of the system. In this embodiment, the CU module  510  contains the laser, optical switches, detection electronics, and processor. The OH module  560  is relatively small in dimensions and can be remotely located relative to the CU module  510 . As such, the OH module  560  may be placed in various locations of a vehicle, drone, robot, etc. 
       FIG. 6  illustrates a modular configuration or architecture for a scanning lidar sensor having a central unit with an optical splitter upstream of a transmitter optical switch coupled to multiple optical heads in another representative embodiment. System  600  includes a plurality of OH modules  602  coupled to a remotely located CU module  610 . CU module  610  includes laser  612  and a fiber splitter  628 , implemented by a 1×4 fiber splitter in this representative embodiment. Controller(s)  616  and detector(s)  620  may be located either within CU module  610  or one or more OH modules  602 . Plurality of OH modules  602  include four OH modules  660 ,  662 ,  664 , and  666  in this representative embodiment. Each of the OH modules includes a transmitter optical switch  614 , a receiver optical switch  618 , transmitter linear array and associated optics  640 , and a receiver linear array and associated optics  650 . As such, system  600  includes one CU  610  and N independent OH modules  602  ( 4  in this example), each covering a specific part of the overall FOV of the system. This architecture maintains independency for each OH  602  to have its own associated scan pattern, but the overall solution may be more expensive and have larger size and complexity. 
       FIG. 7  illustrates a modular configuration or architecture for a scanning lidar sensor having a central unit with an optical splitter downstream of a transmitter optical switch coupled to multiple optical heads in another representative embodiment. System  700  includes a plurality of OH modules  702  and a single CU module  710 . The CU module  710  includes a laser  712 , a transmit switch  714  with output fibers  732  connected each connected to one of a plurality of fiber splitters  728 . In the representative embodiment illustrated, (32)—1×4 fiber splitters  728  are provided so that each of the fiber splitters  728  is coupled to all of the OH modules  702  by associated transmission fibers  730 . Controller(s)  716  and Detector(s)  720  may also be provided either within CU module  710  or one or more OH modules  702 . 
     In the embodiment of  FIG. 7 , OH modules  702  include a first OH module  760 , a second OH module  762 , a third OH module  764 , and a fourth OH module  766  remotely located from CU module  710  and coupled by one or more fiber bundles  730 . Each OH module  702  includes a receiver optical switch  718 , transmitter linear array and transmitter optics  740 , and receiver linear array and receiver optics  750 . In this embodiment, system  700  includes a single CU module  710  and N non-independent OH modules  702 , where each scan step is done in parallel in each OH module  760 ,  762 ,  764 , and  766 . This facilitates smaller OH modules  702  that are generally lower cost, but the scan pattern of the overall field of view is fixed and is a replica of the scan pattern of one of the OH modules  702 . 
       FIG. 8  illustrates a representative embodiment of a vehicle having a scanning lidar sensor with a rotating optical head and/or one or more stationary optical heads. Vehicle  800  includes a plurality of OH modules  802  coupled to a remotely located CU module  810 . OH modules may be placed at various positions around the vehicle  800  such as OH modules  812 ,  814  positioned on respective side-view mirrors, OH modules  816 ,  818  placed on a front grille or bumper. Fixed OH modules may also or alternatively placed at any number of positions including different faces/corners of the vehicle  800 , inside headlights, frames, mirrors, etc. One or more embodiments may utilize the headlight integrated optics to collect light for the OH units. 
     Alternatively, or in combination, vehicle  800  may include an OH module  820  connected to a motor/actuator  830  that rotates to scan 360° as represented at  840 . One or more rotating units may integrate an OH module with a rotating mirror, or deliver the light from an OH module through a rotating mirror on the roof or side of a vehicle. The CU module may deliver the laser light to/from the rotating OH module through one or more free space optical elements with no physical connection between the rotating OH module and the fixed CU module. 
       FIG. 9  is a block diagram illustrated a scanning lidar with optical switches and multiple detectors to detect polarization of reflected light according to various embodiments. System  900  includes a laser  910  configured to transmit polarized pulses. In the illustrated embodiment, system  910  includes a polarizer  912  between laser  910  and transmitter optical switch  914 . Polarizer  912  may be omitted if laser  910  generates polarized light suitable for the application. Optical switch  914 , transmitter fibers  916  and transmitter linear array and optics  918  are configured to preserve the polarization of the polarized pulses to deliver associated polarized output beams  920  similar to the previously described embodiments. Reflected light  930  from objects within the FOV passes through receiver collection optics and beam shaping optics and into the receiver linear array  932  of fibers  934 . Switch  936  scans fibers  934  to sequentially couple each of fibers  934  to optical splitter  938 , which splits or redirects the light to outputs  940  connected to detectors  970 ,  972 ,  974 , and  976  operating in parallel. Each detector  970 ,  972 ,  974 , and  976  may have an associated polarization filter  950 ,  952 ,  954 , and  956  to detect light having corresponding polarizations. 
     As illustrated in  FIG. 9 , the laser pulses transmitted through each one of the fibers  916  is polarized, with the collection optics of the system, including the free space optical elements  918 ,  932  and the receiving optical fibers  934  selected to maintain the polarization of the reflected signal from the target object. As such, system  900  can collect information not just on the reflectance of the target object or its shape, but also on its Degree Of Polarization (DOP) and Angle Of Polarization (AOP). Because each detector  970 ,  972 ,  974 , and  976  has a different linear polarizer  950 ,  952 ,  954  aligned at angles of 0, 45, and 90 degrees or circular polarizer  956  in front of its front surface, the different polarization components of each reflected signal may be used to calculate the DOP and AOP of natural and/or man-made targets very accurately. DOP/AOP information may be used to advantage in driver assistance systems and autonomous vehicles. 
     AOP and/or DOP images are very robust and survive strong interference by the atmosphere such as scattering through fog, haze or rain. In such atmospheric conditions, the reflected signal intensity from a target tends to diminish and be masked by the light reflected and scattered by the particles or water droplets in its optical path. However, DOP/AOP information from the target is maintained and therefore can be used to sense or see through fog and haze to much longer ranges relative to simple intensity information. Polarization information may also help detect and classify small objects on smooth surfaces (such as a flat tire on the road, or a pit or crack in a road) that are almost undetectable by using just the intensity image of the reflected signal. Polarization information may also help detect and classify surface characteristics to alert a driver, such as a road surface containing water, oil, ice, black ice etc. 
       FIG. 10  illustrates a representative embodiment of a system or method for improving resolution within a pixel of a laser scanned FOV by manipulating or scanning polarization of the transmitted and received light using a Faraday rotator and phase mask to generate sub-pixel data. This strategy may be employed in one or more of the previously described embodiments, or other laser scanning systems to increase the resolution of scan of the system. The system or method involve inserting a polarization phase mask and polarization manipulation component into the optical train of both the transmitted and received light. The polarization manipulation may be performed electronically such that there are no moving parts in the scanning process as previously described with respect to one or more embodiments. There are various ways to provide polarization manipulation within the optical path of the transmitted and received beams other than illustrated in  FIG. 10  that are within the scope of the claimed subject matter and will be recognized by those of ordinary skill in the art. 
     System  1000  includes an optical path  1020  for the transmitted and received beams that includes a linear polarizer  1022  positioned upstream of a Faraday rotator (FR)  1024 , which, in turn is positioned upstream of a phase mask (PM)  1028  and a second linear polarizer  1050 . System  1000  may be placed in front of each transmission fiber in any of the previously described embodiments, for example. 
     Phase mask  1028  may comprise a liquid crystal polymer (LCP) retarder, which is half-wave retarder designed to affect the radial and azimuthal polarization of optical fields. A commercially available vortex retarder, for example, has a constant retardance across the clear aperture but its fast axis rotates continuously over the area of the optic. There is no practical limit to the flexibility in designing the fast axis distribution on the phase mask to produce a desired distribution of polarized light immediately downstream of the phase mask. In the representative embodiment of  FIG. 10 , phase mask  1028  is a half-wave retarder phase mask that includes three columns  1030 ,  1032 , and  1034  each having a retardation axis rotated at a different angle. This design allows the Faraday rotator (FR)  1024  to turn or rotate the input linear polarization produced by the linear polarizer  1022  to be parallel to one of the retardation axes  1030 ,  1032 ,  1034  on the phase mask  1028 . The FR  1024  is controlled electronically by supplying variable voltage. If the selected polarization direction is parallel to the retardation axis in the first column  1030  of the PM, the intensity of the light that follows the second polarizer  1050  will have peak intensity along the line of column  1030 , with reducing intensity toward column  1032  and  1034  that have axes rotated relative to the input polarizer  1022 . The FR  1024  can then select a second angle of rotation that will align the polarization to the second column  1032  of the PM  1028 . In that case the peak intensity following the second polarizer  1050  will move to the second column  1032  and the other columns  1030 ,  1034  will have lower intensity and look much darker. In a similar manner, the FR  1024  can select any column  1030 ,  1032 ,  1034  to be enhanced in intensity after the second polarizer  1050  and therefore “scan” the peak intensity across the horizontal direction with three sub-pixels Similar optics may be placed in front of the receive channel of the system with the same scan mechanism, but with the receiver PM rotated 90 degrees relative to the transmission PM. This will allow the transmitter/receiver pair to enhance a different square in the PM and therefore a different portion of the selected pixel based on the nine possible sub-pixels of the field of view of each of the fibers. This strategy may be applied to a single pixel location to provide sub-pixel resolution by scanning the polarization using the FR across the PM, which results in scanning the peak intensity of the received pulses across a pixel rather than a portion or region of the FOV represented by a group of pixels. 
     The combination of the transmitter and receiver polarization scan patterns provides the ability to emphasize the power of a specific pixel within a group of pixels, or a particular region within a single pixel as generally represented in  FIG. 11 . 
     With reference to  FIGS. 10 and 11 , pattern  1110  represents peak intensity distribution of a transmitted beam having system  1000  in the optical path after the light exits the second polarizer  1050  with the FR  1024  controlled to align with the first column  1030  of PM  1028 . This results in column  1120  having higher peak intensity relative to column  1122  and column  1124 . Pattern  1130  represents peak intensity distribution of received light having a system  1000  in the optical path, but with PM  1028  rotated by 90 degrees and the FR controlled in a coordinated manner as the FR in the transmission path, after the received light exits polarizer  1050 . This results in the top row  1132  having higher peak intensity than the middle row  1134  and bottom row  1136 . The combined transmitter/receiver scan  1150  illustrates the resulting distribution of power or intensity with pixel or sub-pixel  1152  having a higher peak intensity than surrounding sub-pixels/pixels. By switching or scanning the polarization state, the position of the brightest pixel or sub-pixel can be quickly shifted to various positions to scan an area of interest with variable intensity to improve resolution. 
       FIG. 12  is a flowchart illustrating operation of a system or method for lidar scanning using optical switching according to one or more embodiments. The system or method  1200  include generating laser pulses at  1210 , which may include generating laser pulses using a fiber laser having an output nominal wavelength of between 900 nm and 1700 nm. The system or method include optically switching the laser pulses to each of a first plurality of fibers arranged in a first linear array to illuminate a field of view at  1220 . This may include shaping the laser pulses to form an elliptical beam with at least 20 times greater angular divergence along a first axis relative to angular divergence along a perpendicular second axis as represented at  1222 . 
     Block  1230  represents directing light reflected from an object illuminated by the laser pulses within the field of view to a second plurality of fibers arranged in a second linear array. This may include collecting and shaping light to form elliptical beams with an angular divergence along a second axis at least 20 times greater than the angular divergence along a perpendicular first axis at  1232 . The system or method may also include optically switching light from the second plurality of fibers to direct the light to at least one detector as represented at  1240 , which may include optically splitting the received light to direct a portion of the received light to each of a plurality of detectors arranged in parallel as indicated at  1242 . The system or method may further include detecting the angle and/or degree of polarization of the received light as represented at  1244 . The detector signals are then processed to generate data representing the field of view as represented by block  1250 . 
       FIG. 13  is a flowchart illustrating operating of a system or method for increasing resolution of a laser scanned FOV using polarization manipulation according to one or more embodiments. System or method  1300  includes generating a laser beam at  1310 , which may be a pulsed or continuous wave (cw) beam. The laser beam is scanned along a first direction to generate a transmitted beam to illuminate a field of view is indicated at  1320 . This may include optically switching the laser beam to scan a first linear array of fibers as indicated at  1322 . 
     The system or method  1300  may include directing light reflected from an object illuminated by the transmitted laser beam within the field of view along a second direction orthogonal to the first direction to form a received beam provided to at least one detector as represented at  1330 . This may include optically switching received light from a linear array of fibers positioned orthogonally relative to the first linear array of fibers as indicated at  1332 . Block  1340  represents processing signals from the at least one detector to generate a two-dimensional array of pixels. Block  1350  represents varying an intensity profile within a selected pixel or group of pixels to move peak intensity in a continuous manner within the selected pixel or across the group of pixels by synchronously varying polarization of the transmitted laser beam and the received beam provided to the at least one detector. This may include directing the transmitted beam and the received light through associated polarizers, Faraday rotators, and phase masks as represented at  1352 , and synchronously controlling the Faraday rotators to manipulate or vary the polarization as represented at  1354 . 
     While representative embodiments are described above, it is not intended that these embodiments describe all possible forms of the claimed subject matter. The words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the disclosure and claimed subject matter. Additionally, the features of various implementing embodiments may be combined to form further embodiments not explicitly described or illustrated, but within the scope of the disclosure and claimed subject matter and recognizable to one of ordinary skill in the art. 
     While various embodiments may have been described as providing advantages or being preferred over other embodiments or prior art implementations with respect to one or more desired characteristics, as one of ordinary skill in the art is aware, one or more features or characteristics may be compromised to achieve desired overall system attributes, which depend on the specific application and implementation. These attributes include, but are not limited to: cost, strength, durability, life cycle cost, marketability, appearance, packaging, size, serviceability, weight, manufacturability, ease of assembly, etc. Embodiments described as less desirable than other embodiments or prior art implementations with respect to one or more characteristics are not necessarily outside the scope of the disclosure and may be desirable for particular applications.