Patent Publication Number: US-2021181320-A1

Title: Performing speckle reduction using polarization

Description:
TECHNICAL FIELD 
     The present disclosure relates generally to a light detection and ranging (LIDAR) system that utilizes polarization for speckle reduction. 
     BACKGROUND 
     Traditional LIDAR systems operate by sending pulses toward a target and measuring the time the pulses take to reach the target and return. In such time-of-flight systems, the user learns information about the distance to the object, which when coupled with a scanner can provide a 3-D point cloud of the sensor&#39;s field-of-view. 
     SUMMARY 
     The present disclosure includes, without limitation, the following example implementations. 
     Some example implementations provide a method of operating a light detection and ranging (LIDAR) system including generating a beam of polarized light; and transforming a polarization state of the beam of polarized light at a rate faster than a rate of data collection at a plurality of detectors configured to detect light reflected from a target. In some embodiments, the method includes splitting light reflected from the target using a first polarizing beam splitter into a first output directed to a first detector and a second output directed to a second detector. In some embodiments, the method includes splitting the beam of polarized light into a local oscillator path and a target path using a first beam splitter; splitting the local oscillator path light into a first output and a second output using a second beam splitter; transmitting the target path light to a target and directing light reflected from the target to the first polarizing beam splitter using an optical path discriminator; mixing the first output of the second beam splitter and the first output of the first polarizing beam splitter using a first light mixer; mixing the second output of the second beam splitter and the second output of the first polarizing beam splitter using a second light mixer; receiving combined light from the first light mixer at the first detector; and receiving combined light from the second mixer at the second detector. In some embodiments, the first and second light mixers are configured to bias light to the first and second detectors in favor of light from the target path. In some embodiments, a variable polarization rotator is located before the first beam splitter and the second beam splitter is a second polarizing beam splitter. In some embodiments, transforming the polarization state of the beam of polarized light at a rate faster than the rate of data collection averages a spatial-mode coherence and mitigates signal-to-noise ratio fluctuation due to speckle effects. 
     Another example implementation provides a light detection and ranging (LIDAR) apparatus including an optical source configured to transmit polarized light to a target; a first polarizing beam splitter configured to split light reflected from the target into a first output directed to a first detector and a second output directed to a second detector; and a variable polarization rotator configured to transform a polarization state of the polarized light directed to the target at a rate faster than a rate of data collection at the first and second detectors. In some embodiments, the apparatus also includes a first beam splitter configured to split the polarized light into a local oscillator path and a target path; an optical path discriminator configured to transmit the target path light to a target and direct light reflected from the target to the first polarizing beam splitter; a second beam splitter configured to split the local oscillator path light into a first output and a second output; a first light mixer configured to mix light from a first output of the first polarizing beam splitter and the first output of the second beam splitter; and a second light mixer configured to mix light from a second output of the first polarizing beam splitter and the second output of the second beam splitter, wherein the first detector is configured to receive combined light from the first light mixer and the second detector is configured to receive combined light from the second light mixer. In some embodiments, the first and second light mixers are configured to bias light to the first and second detectors in favor of light from the target path. In some embodiments, the optical source includes a laser source and a second polarizing beam splitter. In some embodiments, the variable polarization rotator is located before the first beam splitter. In some embodiments, the second beam splitter is a third polarizing beam splitter. In some embodiments, the first mixer is configured to receive a reflected output from the first polarizing beam splitter and a transmitted output from the third polarizing beam splitter, and the second mixer is configured to receive a transmitted output from the first polarizing beam splitter and a reflected output from the third polarizing beam splitter. In some embodiments, the variable polarization rotator is located after the first beam splitter. 
     Another example implementation provides a light detection and ranging (LIDAR) apparatus including a wavelength division multiplexer (WDM) configured to combine light from a plurality of laser sources and transmit the combined light to a target; a first polarizing beam splitter configured to split light reflected from the target into a first output directed to a first detector and a second output directed to a second detector; and a variable polarization rotator configured to transform a polarization state of the combined light directed to the target at a rate faster than a rate of data collection at the first and second detectors. In some embodiments, the light from the plurality of laser sources has dissimilar wavelengths. In some embodiments, the apparatus also includes a first beam splitter configured to split the combined light from the WDM into a local oscillator path and a target path; an optical path discriminator configured to transmit the target path light to a target and direct light reflected from the target to the first polarizing beam splitter; a second beam splitter configured to split the local oscillator path light into a first output and a second output; a first light mixer configured to mix light from the first output of the first polarizing beam splitter and the first output of the second beam splitter; and a second light mixer configured to mix light from the second output of the first polarizing beam splitter and the second output of the second beam splitter, wherein the first detector is configured to receive mixed light from the first light mixer and the second detector is configured to receive mixed light from the second light mixer. In some embodiments, the first and second light mixers are configured to bias light to the first and second detectors in favor of light from the target path. In some embodiments, the apparatus also includes a second WDM between the first light mixer and the first detector, wherein the first detector is coupled to a first output of the second WDM; a third WDM between the second light mixer and the second detector, wherein the second detector is coupled to a first output of the third WDM; a third detector coupled to a second output of the second WDM; and a fourth detector coupled to a second output of the third WDM. In some embodiments, the first detector, second detector, third detector, and fourth detector are configured to detect light of different wavelengths. 
     These and other features, aspects, and advantages of the present disclosure will be apparent from a reading of the following detailed description together with the accompanying figures, which are briefly described below. The present disclosure includes any combination of two, three, four or more features or elements set forth in this disclosure, regardless of whether such features or elements are expressly combined or otherwise recited in a specific example implementation described herein. This disclosure is intended to be read holistically such that any separable features or elements of the disclosure, in any of its aspects and example implementations, should be viewed as combinable unless the context of the disclosure clearly dictates otherwise. 
     It will therefore be appreciated that this Brief Summary is provided merely for purposes of summarizing some example implementations so as to provide a basic understanding of some aspects of the disclosure. Accordingly, it will be appreciated that the above described example implementations are merely examples and should not be construed to narrow the scope or spirit of the disclosure in any way. Other example implementations, aspects, and advantages will become apparent from the following detailed description taken in conjunction with the accompanying figures which illustrate, by way of example, the principles of some described example implementations. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURE 
       Embodiments and implementations of the present disclosure will be understood more fully from the detailed description given below and from the accompanying drawings of various aspects and implementations of the disclosure, which, however, should not be taken to limit the disclosure to the specific embodiments or implementations, but are for explanation and understanding only. 
         FIG. 1  illustrates a LIDAR system according to example implementations of the present disclosure. 
         FIG. 2  illustrates a LIDAR system for performing material estimation according to example implementations of the present disclosure. 
         FIG. 3A  illustrates a triangle wave frequency modulation and echo signal, according to an example embodiment of the present disclosure. 
         FIG. 3B  illustrates a triangle wave frequency modulation and echo signal, as well as a counter-chirped signal, according to an example embodiment of the present disclosure. 
         FIG. 4  illustrates another LIDAR system for performing material estimation according to example implementations of the present disclosure. 
         FIG. 5  illustrates another LIDAR system for performing material estimation according to example implementations of the present disclosure. 
         FIG. 6  illustrates another LIDAR system for performing material estimation according to example implementations of the present disclosure. 
         FIG. 7  illustrates another LIDAR system for performing material estimation according to example implementations of the present disclosure. 
         FIG. 8  illustrates a LIDAR system for performing speckle reduction or material estimation according to example implementations of the present disclosure. 
         FIG. 9  illustrates another LIDAR system for performing speckle reduction or material estimation according to example implementations of the present disclosure. 
         FIG. 10  illustrates another LIDAR system for performing speckle reduction or material estimation according to example implementations of the present disclosure. 
         FIG. 11  illustrates a LIDAR system for performing speckle reduction and material estimation according to example implementations of the present disclosure. 
         FIG. 12  illustrates a LIDAR system for performing speckle reduction or material estimation according to example implementations of the present disclosure. 
         FIG. 13  depicts a flow diagram of a method for performing material estimation using polarized light in accordance with example implementations of the present disclosure. 
         FIG. 14  depicts a flow diagram of another method for performing material estimation using polarized light in accordance with example implementations of the present disclosure. 
         FIG. 15  depicts a flow diagram of a method for performing speckle reduction using polarized light in accordance with example implementations of the present disclosure. 
         FIG. 16  depicts a flow diagram of another method for performing speckle reduction using polarized light in accordance with example implementations of the present disclosure. 
         FIG. 17  is a block diagram of an example apparatus that may perform one or more of the operations described herein, in accordance with some embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     Example implementations of the present disclosure are directed to an improved scanning LIDAR system. Example implementations of the present disclosure are based on a type of LIDAR that uses polarization in order to gather additional data based on the different ways polarized light reflects off materials. Historically, LIDAR systems have not used light polarization as a way to gather additional data about a target or an environment; thus, such systems have not taken advantage of all the information that can potentially be gained from light received using a LIDAR system. 
     Traditional LIDAR systems operate by sending pulses toward a target and measuring the time the pulses take to reach the target and return. In such time-of-flight systems, the user learns information about the distance to the object, which when coupled with a scanner can provide a 3-D point cloud of the sensor&#39;s field-of-view. However, these traditional systems may have certain limitations, e.g., the inability to directly measure the velocity of the target, and the susceptibility to cross-talk from other like systems. An alternative LIDAR system can employ frequency-modulated continuous wave (FMCW) techniques to measure distance and velocity to yield a 4-D LIDAR system. 
     Example embodiments of the present disclosure additionally use polarization to enhance system performance by enabling material estimation and reducing signal-to-noise degradation due to speckle effects. Generally, the terms “speckle” and “speckle effects” are used to describe the phenomenon of light scattering off a diffuse surface. Such systems can provide depth and velocity information simultaneously for each location across a 2-D scan pattern. By using polarization optics, one can ascertain further information about a target&#39;s optical material properties or orientation. Furthermore, employing different configurations or polarization optics can mitigate the deleterious effects of speckle on the signal-to-noise ratio (SNR) of the system. 
     Example embodiments of the present disclosure involve using a co-propagating, cross-polarized beam of light as an outgoing signal. Due to polarization-based differences in the reflectivities of targets, these two signals can have different SNR measurements. One can then use this information to provide further insights into the surrounding environment including, but not limited to, determining material reflectivity or object orientation. 
     Employing similar system components and geometries, but operated with different polarization properties, one can mitigate the harmful effects of speckle on the SNR. Speckle can occur, for example, due to phase-front variations in turbulent air, or wavelength-scale reflections from uneven surfaces. These effects contribute to SNR fluctuations on the signal if the outgoing signal is fixed. By rapidly modulating the polarization state of the signal transmitted to a target, one can average the spatial-mode coherence, thereby reducing the negative effects of speckle. 
     Example implementations of the present disclosure can provide enhancements to any sensing market, such as, but not limited to, transportation, manufacturing, metrology, medical, and security. For example, in the automotive industry, such a device can assist with spatial awareness for automated driver assist systems or self-driving vehicles. 
       FIG. 1  illustrates a LIDAR system  100  according to example implementations of the present disclosure. The LIDAR system  100  includes one or more of each of a number of components, but may include fewer or additional components than shown in  FIG. 1 . As shown, the LIDAR system  100  includes optical circuits  101 . The optical circuits  101  may include a combination of active optical components and passive optical components. Active optical components may generate, amplify, or detect optical signals and the like. In some examples, the active optical circuit includes lasers at different wavelengths, one or more optical amplifiers, one or more optical detectors, or the like. 
     Passive optical circuits may include one or more optical fibers or waveguides to carry optical signals, and route and manipulate optical signals to appropriate input/output ports of the active optical circuit. The passive optical circuits may also include one or more fiber components such as taps, wavelength division multiplexers, splitters/combiners, polarization beam splitters, collimators, circulators, isolators, or the like. In some embodiments, as discussed further below, the passive optical circuits may include components to transform the polarization state and direct received polarized light to optical detectors using a polarizing beam splitter (PBS). 
     An optical scanner  102  includes one or more scanning mirrors that are swept along respective orthogonal axes to steer optical signals to scan an environment according to a scanning pattern. For instance, the scanning mirrors may be rotatable by one or more galvanometers. The optical scanner  102  also collects light incident upon any objects in the environment into a return laser beam that is returned to the passive optical circuit component of the optical circuits  101 . For example, the return laser beam may be directed to an optical detector by a polarization beam splitter. In addition to the mirrors and galvanometers, the optical scanning system may include components such as a quarter-wave plate, lens, anti-reflective coated window or the like. 
     To control and support the optical circuits  101  and optical scanner  102 , the LIDAR system  100  includes LIDAR control systems  110 . The LIDAR control systems  110  may include a processing device for the LIDAR system  100 . In embodiments, the processing device may be one or more general-purpose processing devices such as a microprocessor, central processing unit, or the like. More particularly, the processing device may be a complex instruction set computing (CISC) microprocessor, reduced instruction set computer (RISC) microprocessor, very long instruction word (VLIW) microprocessor, or processor implementing other instruction sets, or processors implementing a combination of instruction sets. The processing device may also be one or more special-purpose processing devices such as an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), a digital signal processor (DSP), network processor, or the like. 
     In some embodiments, the LIDAR control systems  110  may include a signal processing unit  112  such as a digital signal processor. The LIDAR control systems  110  are configured to output digital control signals to control optical drivers  103 . In some embodiments, the digital control signals may be converted to analog signals through signal conversion unit  106 . For example, the signal conversion unit  106  may include a digital-to-analog converter. The optical drivers  103  may then provide drive signals to active components of optical circuits  101  to drive optical sources such as lasers and amplifiers. In some embodiments, several optical drivers  103  and signal conversion units  106  may be provided to drive multiple optical sources. 
     The LIDAR control systems  110  are also configured to output digital control signals for the optical scanner  102 . A motion control system  105  may control the optical scanner  102  based on control signals received from the LIDAR control systems  110 . For example, a digital-to-analog converter may convert coordinate routing information from the LIDAR control systems  110  to signals interpretable by the galvanometers in the optical scanner  102 . In some embodiments, a motion control system  105  may also return information to the LIDAR control systems  110  about the position or operation of components of the optical scanner  102 . For example, an analog-to-digital converter may in turn convert information about the galvanometers&#39; position to a signal interpretable by the LIDAR control systems  110 . 
     The LIDAR control systems  110  are further configured to analyze incoming digital signals. In this regard, the LIDAR system  100  includes optical receivers  104  to measure one or more beams received by optical circuits  101 . For example, a reference beam receiver may measure the amplitude of a reference beam from the active optical circuit, and an analog-to-digital converter converts signals from the reference receiver to signals interpretable by the LIDAR control systems  110 . The optical receivers  104  may be in communication with a signal conditioning unit  107 , in some embodiments. Target receivers measure the optical signal that carries information about the range and velocity of a target in the form of a beat frequency, modulated optical signal. The reflected beam may be mixed with a second signal from a local oscillator. The optical receivers  104  may include a high-speed analog-to-digital converter to convert signals from the target receiver to signals interpretable by the LIDAR control systems  110 . 
     In some applications, the LIDAR system  100  may additionally include one or more imaging devices  108  configured to capture images of the environment, a global positioning system  109  configured to provide a geographic location of the system, or other sensor inputs. The LIDAR system  100  may also include an image processing system  114 . The image processing system  114  can be configured to receive the images and geographic location, and send the images and location or information related thereto to the LIDAR control systems  110  or other systems connected to the LIDAR system  100 . 
     In operation according to some examples, the LIDAR system  100  is configured to use nondegenerate laser sources to simultaneously measure range and velocity across two dimensions. This capability allows for real-time, long range measurements of range, velocity, azimuth, and elevation of the surrounding environment. In some example implementations, the system points multiple modulated laser beams to the same target. 
     In some examples, the scanning process begins with the optical drivers  103  and LIDAR control systems  110 . The LIDAR control systems  110  instruct the optical drivers  103  to independently modulate one or more lasers, and these modulated signals propagate through the passive optical circuit to the collimator. The collimator directs the light at the optical scanning system that scans the environment over a preprogrammed pattern defined by the motion control subsystem. The optical circuits may also include polarization wave plates to transform the polarization of the light as it leaves the optical circuits  101 . In embodiments, the polarization wave plate may be a quarter-wave plate and/or a half-wave plate. A portion of the polarized light may also be reflected back to the optical circuits  101 . For example, lensing or collimating systems may have natural reflective properties or a reflective coating to reflect a portion of the light back to the optical circuits  101 . 
     Optical signals reflected back from the environment pass through the optical circuits  101  to the receivers. Because the polarization of the light has been transformed, it may be reflected by a polarization beam splitter along with the portion of polarized light that was reflected back to the optical circuits  101 . Accordingly, rather than returning to the same fiber or waveguide as an optical source, the reflected light is reflected to separate optical receivers. These signals interfere with one another and generate a combined signal. Each beam signal that returns from the target produces a time-shifted waveform. The temporal phase difference between the two waveforms generates a beat frequency measured on the optical receivers (photodetectors). The combined signal can then be reflected to the optical receivers  104 . Configuration of optical circuits  101  for polarizing and directing beams to the optical receivers  104  are described further below. 
     The analog signals from the optical receivers  104  are converted to digital signals using an analog-to-digital converter (ADC). The digital signals are then sent to the LIDAR control systems  110 . A signal processing unit  112  may then receive the digital signals and interpret them. In some embodiments, the signal processing unit  112  also receives position data from the motion control system  105  and optical scanner  102 , as well as image data from the image processing system  114 . The signal processing unit  112  can then generate a 3D point cloud with information about range and velocity of points in the environment as the optical scanner  102  scans additional points. The signal processing unit  112  can also overlay a 3D point cloud data with the image data to determine velocity and distance of objects in the surrounding area. The system also processes the satellite-based navigation location data to provide a precise global location. 
       FIG. 2  illustrates a LIDAR system for performing material estimation according to example implementations of the present disclosure. The LIDAR system includes one or more of each of a number of components, but may include fewer or additional components than shown in  FIG. 2 . As shown, the LIDAR system includes a laser source  201 , a first beam splitter  203 , and a first polarizing beam splitter  205 . In this example embodiment, a single laser source  201  is split using the first beam splitter  203 , and then recombined using the first polarizing beam splitter  205  to generate a beam of co-propagating, cross-polarized light. The beam of cross-polarized light is then split into a target path and a local oscillator (LO) path using a second beam splitter  207 . The target path light can be amplified using an optical amplifier  209 , and directed to a target  217  through an optical path discriminator  211 . The target path light can be directed to the target  217  through a lens system  213  and a polarization wave plate  215 , in some embodiments. The light reflected from the target  217  can then be directed through the optical path discriminator  211  to a second polarizing beam splitter  219 . According to some embodiments, the optical path discriminator  211  can be a circulator or a beam splitter. In this example embodiment, the LO path light is transmitted from the second beam splitter  207  to a third polarizing beam splitter  229 . The second polarizing beam splitter  219  and the third polarizing beam splitter  229  are both configured to transmit light to the first mixer  221  and the second mixer  225 . In this example embodiment, the first mixer  221  and second mixer  225  are single-ended and each connects to the first detector  223  and the second detector  227 , respectively. However, the first and second mixers  221 ,  225  could have two or more outputs according to other embodiments. 
     According to the embodiment shown in  FIG. 2 , the reflected outputs from each of the second polarizing beam splitter  219  and the second polarizing beam splitter  229  are both directed to the first mixer  221 , while the transmitted outputs from the second and third polarizing beam splitters  219 ,  229  are both directed to the second mixer  225 . The use of the second polarizing beam splitter  219  and the second polarizing beam splitter  229  can differentiate the SNR measured on the first detector  223  and the second detector  227 . In some embodiments, the first and second mixers  221 ,  225  do not mix light equally from the LO path and the target path, but instead bias light coming from the target path. For example, the first and second mixers  221 ,  225  can bias light output to the first and second detectors  223 ,  227  at a ratio of 80/20, 90/10, or 99/1 in favor of the target path light, in some embodiments. 
     In this example embodiment, the system also includes a signal processing unit  250  in communication with the first detector  223  and the second detector  227  and configured to analyze signals from the detectors. The detectors measure the optical signal from the mixers and generate proportional electric signals. The signal processing unit  250  can compare the intensities of the signals from the first detector  223  and the second detector  227  and provide insights into the target material reflectivity, surface quality, or orientation. For example, an ice patch, puddle of water, or other reflective surface can largely reflect light only in one polarization parallel to the horizon, such that there is a large amount of light detected polarized parallel to the horizon but not much light detected polarized perpendicular to the horizon. If this is detected, the signal processing unit  250  can determine the reflectivity and orientation of the target material, and can potentially know that there is a puddle or ice patch ahead. A similar phenomenon can also occur with windows on a building that are highly reflective in one polarization. 
     The input ports of the first polarizing beam splitter  205  are co-polarized, in this example embodiment, but the splitter performs the polarization rotation upon reflection to create a beam of co-propagating, cross-polarized light. When used in reverse, as the second polarizing beam splitter  219  and third polarizing beam splitter  229  are arranged, a cross-polarized source will filter the two polarizations to their respective output ports. This behavior is not the same if free-space polarizing beam splitters are used, wherein a polarization rotator (such as a half wave-plate) is used on at least one of the ports in order to co-polarize the beams. 
       FIG. 3A  illustrates a triangle wave frequency modulation and echo signal, according to an example embodiment of the present disclosure. The FMCW LIDAR systems described in this disclosure modulate the frequency of the laser with a center frequency f c  over a sweep duration T s . The modulation can take any number of possible sweep patterns, such as a sawtooth, or a triangle wave, as shown in  FIG. 2B , which also shows a time-shifted “echo” (signal returned from a target) using the dashed line and delayed by Δt. The echo has a frequency shift due to the Doppler effect of a moving object. In this case, the Doppler shift Δf doppler , and the echo create different beat tones on the up-sweep and down-sweep of the triangle wave modulation. These beat frequencies are labeled Δf up  and Δf dn , respectively. The range and velocity can be calculated directly from these parameters using equations (1) and (2) below. 
     
       
         
           
             
               
                 
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     In equations (1) and (2) above, Bs is the sweep bandwidth for the modulated signal, and is the center wavelength of that sweep (defined by c/fc). 
       FIG. 3B  illustrates a triangle wave frequency modulation and echo signal, as well as a counter-chirped signal, according to an example embodiment of the present disclosure. According to some embodiments of the present disclosure, two laser sources can be used and one can modulate the two lasers (potentially at different wavelengths) with different patterns, e.g., the counter-chirp triangle modulation shown in  FIG. 3B . In this example, the transmitted lasers are shown as lines  301  and  303 , and their respective echoes are shown as lines  305  and  307 . This enables real-time range and velocity measurements since Δf up  and Δf dn  can be retrieved simultaneously, although reduced SNR in one polarization due to the various material properties of targets can affect measurements. Potential solutions to remedy this problem are discussed later in this disclosure. Although triangle waves have been used in the examples shown in  FIGS. 3A and 3B , this application is not limited to such waves, and the frequency modulation pattern for any laser can be different from the other lasers. 
       FIG. 4  illustrates another LIDAR system for performing material estimation according to example implementations of the present disclosure. The LIDAR system includes one or more of each of a number of components, but may include fewer or additional components than shown in  FIG. 4 . As shown, the LIDAR system includes two laser sources  401 ,  402  and a first polarizing beam splitter  405 . In this example embodiment, the first laser source  401  and the second laser source  402  are both connected to inputs of the first polarizing beam splitter  405  to generate a beam of co-propagating, cross-polarized light. While this example illustrates an embodiment with two laser sources, the application is not limited to two laser sources, and a larger number of light sources can be used in various embodiments. The beam of cross-polarized light is then split into a target path and a local oscillator (LO) path using a beam splitter  407 . The target path light can be amplified using an optical amplifier  409 , and directed to a target  417  through an optical path discriminator  411 . The target path light can be directed to the target  417  through a lens system  413  and a polarization wave plate  415 , in some embodiments. The light reflected from the target  417  can then be directed through the optical path discriminator  411  to a second polarizing beam splitter  419 . According to some embodiments, the optical path discriminator  411  can be a circulator or a beam splitter. In this example embodiment, the LO path light is transmitted from the beam splitter  407  to a third polarizing beam splitter  429 . The second polarizing beam splitter  419  and the third polarizing beam splitter  429  are both configured to transmit light to the first mixer  421  and the second mixer  425 . 
     In this example embodiment, two laser sources  401 ,  402  are used, and therefore the appropriate polarization should be routed to the final mixers  421 ,  425  in order to maximize the mixing efficiency. Because dissimilar lasers do not mix well, note that the beams that are mixed at the first mixer  421  and the second mixer  425  are from opposite ports of the second polarizing beam splitter  419  and the third polarizing beam splitter  429 . This is due to the s-polarization to p-polarization rotation process (and vice versa) that occurs when passing a linear polarization through the polarization wave plate  415 , off a target, and back through the polarization wave plate  415 . 
     In this example embodiment, the first mixer  421  and second mixer  425  are single-ended and each connects to the first detector  423  and the second detector  427 , respectively. However, the first and second mixers  421 ,  425  could have two or more outputs according to other embodiments. In some embodiments, the first and second mixers  421 ,  425  do not mix light equally from the LO path and the target path, but instead bias light coming from the target path. For example, the first and second mixers  421 ,  425  can bias light output to the first and second detectors  423 ,  427  at a ratio of 80/20, 90/10, or 99/1 in favor of the target path light, in some embodiments. This biasing of the light from the target path can increase the detection of the target path light, which may result in a more accurate detection of the target. 
       FIG. 5  illustrates another LIDAR system for performing material estimation according to example implementations of the present disclosure. The LIDAR system includes one or more of each of a number of components, but may include fewer or additional components than shown in  FIG. 5 . Similar to the embodiment shown above in  FIG. 4 , the LIDAR system includes two laser sources  501 ,  502  and a first polarizing beam splitter  505 . In this example embodiment, the first laser source  501  and the second laser source  502  are both connected to inputs of the first polarizing beam splitter  505  to generate a beam of co-propagating, cross-polarized light. In this example embodiment, a supplemental optical circuit is included that can be used to create a signal reference. This reference circuit includes a first beam splitter  506  that can split the cross-polarized beam of light and direct a portion of the light to a second splitter  531 , an optical delay line  535 , a third beam splitter  533 , a fourth polarizing beam splitter  537 , and two reference detectors  538 ,  539 . This reference signal can be tracked using the reference detectors  538 ,  539  and subsequently used to compensate for perturbations in the laser sources and to calibrate the system&#39;s performance. This reference circuit can be added to any of the other embodiments described in this disclosure. 
     The beam of cross-polarized light is also split into a target path and a local oscillator (LO) path using a fourth beam splitter  507 . The target path light can be amplified using an optical amplifier  509 , and directed to a target  517  through an optical path discriminator  511 . The target path light can be directed to the target  517  through a lens system  513  and a polarization wave plate  515 , in some embodiments. The light reflected from the target  517  can then be directed through the optical path discriminator  511  to a second polarizing beam splitter  519 . According to some embodiments, the optical path discriminator  511  can be a circulator or a beam splitter. In this example embodiment, the LO path light is transmitted from the fourth beam splitter  507  to a third polarizing beam splitter  529 . The second polarizing beam splitter  519  and the third polarizing beam splitter  529  are both configured to transmit light to the first mixer  521  and the second mixer  525 . 
     In this example embodiment, two laser sources  501 ,  502  are used, and therefore the appropriate polarization should be routed to the final mixers  521 ,  525  in order to maximize the mixing efficiency. Because dissimilar lasers do not mix well, the beams that are mixed at the first mixer  521  and the second mixer  525  are from opposite ports of the second polarizing beam splitter  519  and the third polarizing beam splitter  529 . This is due to the s-polarization to p-polarization rotation process (and vice versa) that occurs when passing a linear polarization through the polarization wave plate  515 , off a target  517 , and back through the polarization wave plate  515 . 
     In this example embodiment, the first mixer  521  and second mixer  525  are single-ended and each connects to the first detector  523  and the second detector  527 , respectively. However, the first and second mixers  521 ,  525  could have two or more outputs according to other embodiments. In some embodiments, the first and second mixers  521 ,  525  do not mix light equally from the LO path and the target path, but instead bias light coming from the target path. For example, the first and second mixers  521 ,  525  can bias light output to the first and second detectors  523 ,  527  at a ratio of 80/20, 90/10, or 99/1 in favor of the target path light, in some embodiments. This biasing of the light from the target path can increase the detection of the target path light, which may result in a more accurate detection of the target. 
       FIG. 6  illustrates another LIDAR system for performing material estimation according to example implementations of the present disclosure. The LIDAR system includes one or more of each of a number of components, but may include fewer or additional components than shown in  FIG. 6 . This example embodiment shows a two-laser design of a polarization-enhanced LIDAR system where two beams are sent to a target  617 . For simplicity, only two beams are shown in this example, but there are fundamentally no restrictions to the number of beams that can be sent to a target, and the system can be scaled up or down according to each embodiment. As shown, the LIDAR system includes two laser sources  601 ,  602  and a first polarizing beam splitter  605 . In this example embodiment, the first laser source  601  and the second laser source  602  are both connected to inputs of the first polarizing beam splitter  605  to generate a beam of co-propagating, cross-polarized light. Increasing the number of beams requires increasing the number of splitters used, or the number of outputs from each splitter that bookend the optical amplifier  609 . 
     In this example embodiment, the beam of cross-polarized light is split into two LO paths using a first splitter  606  and a second splitter  607  ahead of the optical amplifier  609 . In an alternative embodiment, the first and second splitters  606 ,  607  can be combined into a different type of splitter, such as a 1×3 splitter, or an active switch in some embodiments. This substitution or combination can simplify the system. The target path light is directed from the optical amplifier  609  to a third splitter  630 , which divides the light into two target paths. Each target path can be directed to the target  617  through a first optical path discriminator  611  or a second optical path discriminator  631 . The first target path light can be directed to the target  617  through a first lens system  613  and a first polarization wave plate  615 , while the second target path light is directed to through a second lens system  633  and a second polarization wave plate  635 . The first beam of light reflected from the target  617  can then be directed through the first optical path discriminator  611  to a second polarizing beam splitter  619 , while the second beam of light reflected from the target  617  can be directed through the second optical path discriminator  631  to a fourth polarizing beam splitter  639 . According to some embodiments, the optical path discriminators  611 ,  631  can be circulators or beam splitters. 
     In this example embodiment, the first LO path light is transmitted from the first beam splitter  606  to a fifth polarizing beam splitter  649 , while the second LO path light is transmitted from the second beam splitter  607  to a third polarizing beam splitter  629 . The second polarizing beam splitter  619  and the third polarizing beam splitter  629  are both configured to transmit light to the first mixer  621  and the second mixer  625 . Similarly, the fourth polarizing beam splitter  639  and the fifth polarizing beam splitter  649  are both configured to transmit light to the third mixer  645  and the fourth mixer  641 . 
     In this example embodiment, two laser sources  601 ,  602  are used, and therefore the appropriate polarization should be routed to the final mixers  621 ,  625 ,  641 ,  645  in order to maximize the mixing efficiency. Because dissimilar lasers do not mix well, note that the beams that are mixed at the first mixer  621  and the second mixer  625  are from opposite ports of the second polarizing beam splitter  619  and the third polarizing beam splitter  629 . Similarly, the beams mixed at the third mixer  641  and the fourth mixer  645  are from opposite ports of the fourth polarizing beam splitter  639  and the fifth polarizing beam splitter  649 . This is due to the s-polarization to p-polarization rotation process (and vice versa) that occurs when passing a linear polarization through the polarization wave plates  615 ,  635  off a target  617 , and back through the polarization wave plates  615 ,  635 . In this example embodiment, the first mixer  621 , second mixer  625 , third mixer  641 , and fourth mixer  645  are single-ended and each connects to the first detector  623 , second detector  627 , third detector  643 , and fourth detector  647 , respectively. 
       FIG. 7  illustrates another LIDAR system for performing material estimation according to example implementations of the present disclosure. Specifically,  FIG. 7  shows an example of a multiple laser, multi-beam embodiment of a polarization enhanced LIDAR system. The LIDAR system includes one or more of each of a number of components, but may include fewer or additional components than shown in  FIG. 7 . 
     As shown, the LIDAR system includes four laser sources  761 ,  763 ,  701 ,  702  and first and second wavelength division multiplexers (WDM)  765 ,  760 , which can separate the light by wavelength (i.e. color). The system uses two wavelengths for each polarization, with lasers  761  and  763  having p-polarization and lasers  701  and  702  having s-polarization. In this fashion, one can counter-chirp the two lasers to obtain real-time range and velocity measurements in addition to real-time material estimation information due to the variable response in polarization. One skilled in the art will realize that WDMs are not required to combine lasers into a single spatial mode if another optical degree of freedom is used for distinguishing between the lasers. Although two WDMs are shown in this example, the present disclosure is not limited to two colors. Any number of colors can be used in the various embodiments of this disclosure. 
     In this example embodiment the first and second WDMs are connected to the inputs of a first polarizing beam splitter  705  to generate the initial beam, and this beam is split into two LO paths using a first splitter  706  and a second splitter  707  ahead of the optical amplifier  709 . In an alternative embodiment, the first and second splitters  706 ,  707  can be combined into a different type of splitter, such as a 1×3 splitter, or an active switch in some embodiments. The target path light is directed from the optical amplifier  709  to a third splitter  730 , that divides the light into two target paths. Each target path can be directed to the target  717  through a first optical path discriminator  711  or a second optical path discriminator  731 . The first target path light can be directed to the target  717  through a first lens system  713  and a first polarization wave plate  715 , while the second target path light is directed through a second lens system  733  and a second polarization wave plate  735 . The first beam of light reflected from the target  717  can then be directed through the first optical path discriminator  711  to a second polarizing beam splitter  719 , while the second beam of light reflected from the target  717  can be directed through the second optical path discriminator  731  to a fourth polarizing beam splitter  739 . According to some embodiments, the optical path discriminators  711 ,  731  can be circulators or beam splitters. In an additional embodiment, a variable polarization rotator (VPR) can be introduced after the second beam splitter  707 . A single VPR (not shown) could be introduced before the third splitter  730 , in some embodiments, while in other embodiments two (or more) VPRs may be introduced after the third splitter  730 . 
     In this example embodiment, the first LO path light is transmitted from the first beam splitter  706  to a fifth polarizing beam splitter  749 , while the second LO path light is transmitted from the second beam splitter  707  to a third polarizing beam splitter  729 . The second polarizing beam splitter  719  and the third polarizing beam splitter  729  are both configured to transmit light to the first mixer  721  and the second mixer  725 . Similarly, the fourth polarizing beam splitter  739  and the fifth polarizing beam splitter  749  are both configured to transmit light to the third mixer  741  and the fourth mixer  745 . 
     In this example embodiment, because multiple laser sources are used the appropriate polarization should be routed to the final mixers  721 ,  725 ,  741 ,  745  in order to maximize the mixing efficiency. Because dissimilar lasers do not mix well, note that the beams that are mixed at the first mixer  721  and the second mixer  725  are from opposite ports of the second polarizing beam splitter  719  and the third polarizing beam splitter  729 . Similarly, the beams mixed at the third mixer  741  and the fourth mixer  745  are from opposite ports of the fourth polarizing beam splitter  739  and the fifth polarizing beam splitter  749 . This is due to the s-polarization to p-polarization rotation process (and vice versa) that occurs when passing a linear polarization through the polarization wave plates  715 ,  735  off a target  717 , and back through the polarization wave plates  715 ,  735 . In this example embodiment, the first mixer  721  directs mixed light to a third WDM  723 , which separates the light by wavelength and directs it to the first and second detectors  751 ,  752 . Likewise, the second mixer directs mixed light to a fourth WDM  727 , which separates the light by wavelength and directs it to the third and fourth detectors  753 ,  754 ; the third mixer  741  directs mixed light to a fifth WDM  743 , which separates the light by wavelength and directs it to the fifth and sixth detectors  757 ,  758 ; and the fourth mixer  745  directs mixed light to a sixth WDM  747 , which separates the light by wavelength and directs it to the seventh and eighth detectors  755 ,  756 . 
       FIG. 8  illustrates a LIDAR system for performing speckle reduction according to example implementations of the present disclosure. The LIDAR system includes one or more of each of a number of components, but may include fewer or additional components than shown in  FIG. 8 . This example embodiment shows an optical circuit that can remove the need for dual-axis components by using a single laser  801  and a polarizing beam splitter  805  at the source, and adding a variable polarization rotator (VPR)  812  to achieve speckle reduction. The beam of polarized light is then split into a target path and a local oscillator (LO) path using a first beam splitter  807 . The target path light can be amplified using an optical amplifier  809 , and directed to a target  817  through an optical path discriminator  811 . The target path light can be directed to the target  817  through the variable polarization rotator  812 , a lens system  813 , and a polarization wave plate  815 , in some embodiments. The light reflected from the target  817  can then be directed through the optical path discriminator  811  to a second polarizing beam splitter  819 . In this example embodiment, the LO path light is transmitted from the first beam splitter  807  to a second beam splitter  829 . The second polarizing beam splitter  819  and the second beam splitter  829  are both configured to transmit light to the first mixer  821  and the second mixer  825 . In this example embodiment, the first mixer  821  and second mixer  825  are single-ended and each connects to the first detector  823  and the second detector  827 , respectively. However, the first and second mixers  821 ,  825  could have two or more outputs according to other embodiments. 
     The VPR  812  can be controlled, in some embodiments, using a polarization rotating controller  850  that is configured to toggle the VPR  812  between two settings: one setting that performs no rotation, allowing p-polarization to stay p-polarized; and a second setting that performs, for example, a 90° rotation converting a p-polarized light to s-polarization. With this circuit design, the second splitter  829  can be a 2×2 splitter rather than a polarizing beam splitter, since all of the light propagating through it is co-polarized, and the second polarizing beam splitter  819  converts the cross-polarized light from the optical path discriminator  811  to co-polarized light on both waveguides leading to the mixers  821 ,  825  before the respective detectors  823 ,  827 . In exemplary embodiments, the polarization rotating controller  850  can achieve speckle reduction by controlling the operation of the VPR  812  to transform the polarization state of the target path light at a rate faster than the rate of sampling by the detectors  823 ,  827 . 
     According to various embodiments, the VPR  812  can be placed before or after the optical path discriminator  811 , although placement before requires a dual-axis optical path discriminator  811 . Similarly, it can be placed before the optical amplifier  809 , but again it requires a dual-axis amplifier which can lead to crosstalk and increased noise. In some embodiments, the first polarizing beam splitter  805  can be omitted if the laser source  801  generates a polarized beam. 
       FIG. 9  illustrates another LIDAR system for performing speckle reduction according to example implementations of the present disclosure. The LIDAR system includes one or more of each of a number of components, but may include fewer or additional components than shown in  FIG. 9 . This example embodiment shows an optical circuit where the VPR  912  is moved ahead of a first beam splitter  907  and reintroduces a third polarizing beam splitter  929  for the LO path. The beam of polarized light generated by the laser source  901  and the first polarizing beam splitter  905  is split into a target path and a LO path using a first beam splitter  907 . The target path light can be amplified using an optical amplifier  909 , and directed to a target  917  through an optical path discriminator  911 . The target path light can be directed to the target  917  through a lens system  913  and a polarization wave plate  915 , in some embodiments. The light reflected from the target  917  can then be directed through the optical path discriminator  911  to a second polarizing beam splitter  919 . The second polarizing beam splitter  919  and the third polarizing beam splitter  929  are both configured to transmit light to the first mixer  921  and the second mixer  925 . In this example embodiment, the first mixer  921  and second mixer  925  are single-ended and each connects to the first detector  923  and the second detector  927 , respectively. However, the first and second mixers  921 ,  925  could have two or more outputs according to other embodiments. 
     The VPR  912  can be controlled, in some embodiments, using a polarization rotating controller  950  that is configured to toggle the VPR  912  between two settings: one setting that performs no rotation, allowing p-polarization to stay p-polarized; and a second setting that performs, for example, a 90° rotation converting a p-polarized light to s-polarization. This design can potentially be very useful if one uses the VPR  912  to rapidly scramble (i.e. at a rate faster than the rate of sampling by the detectors  923 ,  927 ) between various orthogonal pairs of polarizations. The rapid “scrambling” of the polarization should mitigate SNR fluctuation due to speckle effects. Meanwhile, the use of orthogonal polarizations enables the material estimation. 
       FIG. 10  illustrates another LIDAR system for performing speckle reduction according to example implementations of the present disclosure. The LIDAR system includes one or more of each of a number of components, but may include fewer or additional components than shown in  FIG. 10 . This example embodiment shows an optical circuit that is a two-laser variant of the embodiment described above in reference to  FIG. 8 . Specifically, the system includes a first laser source  1001  and second laser source  1002  that are connected to inputs of a WDM  1005  to combine the two lasers if they have dissimilar wavelengths. If two similar wavelengths are used, one can use any modal combiner (such as a 2×2 splitter) to spatially couple the two lasers. The beam of light from the WDM  1005  is split into a target path and a LO path using a first beam splitter  1007 . The target path light can be amplified using an optical amplifier  1009 , and directed to a VPR  1012  ahead of an optical path discriminator  1011 . The target path light can be directed to the target  1017  through a lens system  1013  and a polarization wave plate  1015 , in some embodiments, and the light reflected from the target  1017  can then be directed through the optical path discriminator  1011  to a polarizing beam splitter  1019 . In this example embodiment, the LO path light is transmitted from the first beam splitter  1007  to a second beam splitter  1029 . The polarizing beam splitter  1019  and the second beam splitter  1029  are both configured to transmit light to the first mixer  1021  and the second mixer  1025 . In this example embodiment, the first mixer  1021  and second mixer  1025  are single-ended and each connects to the first detector  1023  and the second detector  1027 , respectively. In this particular embodiment, the VPR  1012  is placed before the optical path discriminator  1011 , and therefore a dual-axis optical path discriminator  1011  is used. As discussed above in reference to  FIG. 8 , the use of a WDM  1005  allows a counter-chirp to be used with the two lasers to obtain real-time range and velocity measurements in this particular embodiment. 
       FIG. 11  illustrates a LIDAR system for performing material estimation and speckle reduction according to example implementations of the present disclosure. Specifically,  FIG. 11  shows an example of a multi-beam VPR-based LIDAR system for material estimation and speckle reduction. The LIDAR system includes one or more of each of a number of components, but may include fewer or additional components than shown in  FIG. 11 . As shown, the LIDAR system includes two laser sources  1101  and  1102  that are combined using a WDM  1105 . The use of a WDM  1105  allows a counter-chirp to be used with the two lasers to obtain real-time range and velocity measurements in this particular embodiment. In an alternative embodiment, the LIDAR system illustrated in  FIG. 11  can be implemented with four laser sources and a first and second WDM, similar to the embodiment described above in reference to  FIG. 7 . 
     In this example embodiment, the beam from the WDM  1105  is split into two LO paths using a first splitter  1106  and a second splitter  1107  ahead of the optical amplifier  1109 . In an alternative embodiment, the first and second splitters  1106 ,  1107  can be combined into a different type of splitter, such as a 1×3 splitter, or an active switch in some embodiments. The target path light is directed from the optical amplifier  1109  to a third splitter  1130 , that divides the light into two target paths. Each target path can be directed to a respective VPR  1190 ,  1192 . After passing through the first VPR  1190  and the second VPR  1192 , the target path light can be directed to the target  1117  through a first optical path discriminator  1111  or a second optical path discriminator  1131 . The first target path light can be directed to the target  1117  through a first lens system  1113  and a first polarization wave plate  1115 , while the second target path light is directed through a second lens system  1133  and a second polarization wave plate  1135 . The first beam of light reflected from the target  1117  can then be directed through the first optical path discriminator  1111  to a first polarizing beam splitter  1119 , while the second beam of light reflected from the target  1117  can be directed through the second optical path discriminator  1131  to a second polarizing beam splitter  1139 . According to some embodiments, the optical path discriminators  1111 ,  1131  can be circulators or beam splitters. 
     In this example embodiment, the first LO path light is transmitted from the first beam splitter  1106  to a fifth beam splitter  1149 , while the second LO path light is transmitted from the second beam splitter  1107  to a fourth beam splitter  1129 . The first polarizing beam splitter  1119  and the fourth beam splitter  1129  are both configured to transmit light to the first mixer  1121  and the second mixer  1125 . Similarly, the second polarizing beam splitter  1139  and the fifth beam splitter  1149  are both configured to transmit light to the third mixer  1145  and the fourth mixer  1141 . 
     In this example embodiment, the first mixer  1121  directs mixed light to a second WDM  1123 , which separates the light by wavelength and directs it to the first and second detectors  1151 ,  1152 . Likewise, the second mixer  1125  directs mixed light to a third WDM  1127 , which separates the light by wavelength and directs it to the third and fourth detectors  1153 ,  1154 ; the third mixer  1141  directs mixed light to a fourth WDM  1143 , which separates the light by wavelength and directs it to the fifth and sixth detectors  1157 ,  1158 ; and the fourth mixer  1145  directs mixed light to a fifth WDM  1147 , which separates the light by wavelength and directs it to the seventh and eighth detectors  1155 ,  1156 . 
       FIG. 12  illustrates a LIDAR system according to example implementations of the present disclosure. The LIDAR system includes one or more of each of a number of components, but may include fewer or additional components than shown in  FIG. 12 . As shown, the LIDAR system includes a laser source  1201 , a first beam splitter  1203 , and a first polarizing beam splitter  1205 . In this example embodiment, a single laser source  1201  is split using the first beam splitter  1203 , and then recombined using the first polarizing beam splitter  1205  to generate a beam of co-propagating, cross-polarized light. The light can then be amplified using an optical amplifier  1209 , and directed to a target  1217  through an optical path discriminator  1211 . The target path light can be directed to the target  1217  through a lens system  1213  and a polarization wave plate  1215 . In some embodiments, the polarization wave plate  1215  (or some other element within the system) can reflect a portion of the light that is slightly offset, or ahead of, the light that exits the system, enters the target environment, and is reflected from the target  1217 . This internal reflection can be created by any waveguide-to-air interface (including a fiber-to-air interface), or a calibrated reflected optic in the path (including a lens, window, retroreflector, or partial mirror). Both the light reflected from the polarization wave plate  1215  and the light reflected from the target can be directed through the optical path discriminator  1211  to a second polarizing beam splitter  1219 . 
     According to some embodiments, the optical path discriminator  1211  can be a circulator or a beam splitter. The polarizing beam splitter  1219  then directs the light to detectors  1223  and  1227 , which are in communication with a signal processing unit  1250  configured to analyze signals from the detectors. In such an embodiment, a mixer, such as the mixers  221 ,  225  discussed above in reference to  FIG. 2 , is not required. The signal processing unit  1250  can compare the intensities of the signals from the first detector  1223  and the second detector  1227  and provide insights into the target material reflectivity, surface quality, or orientation. According to some embodiments, a variable polarization rotator can also be included within the system in order to transform the polarization state of the beam of polarized light, as discussed herein. 
       FIG. 13  illustrates flow chart of an example method  1300  for performing material estimation using polarized light in accordance with example implementations of the present disclosure. In embodiments, various portions of method  1300  may be performed by LIDAR systems of  FIGS. 1, 2, and 4-12 . With reference to  FIG. 13 , method  1300  illustrates example functions used by various embodiments. Although specific function blocks (“blocks”) are disclosed in method  1300 , such blocks are examples. That is, embodiments are well suited to performing various other blocks or variations of the blocks recited in method  1300 . It is appreciated that the blocks in method  1300  may be performed in an order different than presented, and that not all of the blocks in method  1300  may be performed. 
     At block  1301 , a beam of co-propagating, cross-polarized light is generated. In some embodiments, the co-propagating, cross-polarized light is generated using a polarizing beam splitter and a single laser source, while in other embodiments multiple laser sources can be used. In some embodiments, the multiple laser beams may have different wavelengths. The generated beam may be transmitted to a target, and reflected light from the target can be detected at a number of detectors. 
     At block  1303 , a material characteristic or orientation of the target is determined using the co-propagating, cross-polarized light. In one example embodiment, different polarizations of the cross-polarized light may be reflected from the target based on certain characteristics of the target. For example, an ice patch on a road may largely reflect light only in one polarization parallel to the horizon, such that there is a large amount of light detected polarized parallel to the horizon but not much light detected polarized perpendicular to the horizon. If this is the case, the reflectivity or orientation of the target i.e. the patch of ice, can be determined. A similar phenomenon can also occur with windows on a building that are highly reflective in one polarization. 
       FIG. 14  depicts a flow diagram of another method  1400  for performing material estimation using polarized light in accordance with example implementations of the present disclosure. In embodiments, various portions of method  1400  may be performed by LIDAR systems of  FIGS. 1, 2, and 4-12 . With reference to  FIG. 14 , method  1400  illustrates example functions used by various embodiments. Although specific function blocks (“blocks”) are disclosed in method  1400 , such blocks are examples. That is, embodiments are well suited to performing various other blocks or variations of the blocks recited in method  1400 . It is appreciated that the blocks in method  1400  may be performed in an order different than presented, and that not all of the blocks in method  1400  may be performed. 
     At block  1401 , a beam of co-propagating, cross-polarized light is generated. In some embodiments, the co-propagating, cross-polarized light is generated using a polarizing beam splitter and a single laser source, while in other embodiments multiple laser sources can be used. In some embodiments, the multiple laser beams may have different wavelengths. The generated beam may be transmitted to a target, and reflected light from the target can be detected at a number of detectors. In some embodiments, two or more laser sources can be used to generate a beam with different frequency patterns. In other embodiments, a WDM may be used to combine light with different wavelengths from two or more laser sources. 
     At block  1403 , the beam of co-propagating, cross-polarized light is split into a LO path and a target path. This can be done, for example, using a beam splitter. At block  1405 , the target path light is directed to the target using an optical path discriminator, and light reflected from the target is directed to a second polarizing beam splitter. At block  1407 , the second polarizing beam splitter splits light reflected from the target and directs it to at least two mixers. At block  1409 , the LO path light is split into two portions and also directed to the mixers. 
     At block  1411  the outputs from the second polarizing beam splitter and the third polarizing beam splitter are mixed using the mixers. In some embodiments, where a single laser source is used to generate the beam of co-propagating, cross-polarized light, the reflected outputs from the second and third polarizing beam splitters are combined using a first mixer, and the transmitted outputs from the second and third polarizing beam splitters are combined using a second mixer. In other embodiments, where two or more laser sources are used to generate the beam of co-propagating, cross-polarized light, the transmitted output from the second polarizing beam splitter is combined with the reflected output of the third polarizing beam splitter at the first mixer, while the reflected output from the second polarizing beam splitter is combined with the transmitted output of the third polarizing beam splitter at the second mixer. The first and second mixers are in communication with a first and second detectors, and combined light from the first mixer is received at the first detector and combined light from the second mixer is received at the second detector. 
     At block  1413 , a material characteristic or orientation of the target is determined based on a comparison of a SNR between signals from a first and second detector. As discussed above, in some cases the mixers can bias light to the detectors in favor of the target path. As discussed above, a comparison of the SNR between signals from the detectors can indicate the reflectivity or orientation of the target. 
       FIG. 15  depicts a flow diagram of a method for performing speckle reduction using polarized light in accordance with example implementations of the present disclosure. In embodiments, various portions of method  1500  may be performed by LIDAR systems of  FIGS. 1, and 8-12 . With reference to  FIG. 15 , method  1500  illustrates example functions used by various embodiments. Although specific function blocks (“blocks”) are disclosed in method  1500 , such blocks are examples. That is, embodiments are well suited to performing various other blocks or variations of the blocks recited in method  1500 . It is appreciated that the blocks in method  1500  may be performed in an order different than presented, and that not all of the blocks in method  1500  may be performed. 
     At block  1501 , a beam of polarized light is generated. In some embodiments, the co-propagating, cross-polarized light is generated using a polarizing beam splitter and a single laser source, while in other embodiments multiple laser sources can be used. In some embodiments, the multiple laser beams may have different wavelengths. The generated beam may be transmitted to a target, and reflected light from the target can be detected at a number of detectors. 
     At block  1503 , the polarization state of the polarized beam is transformed using a variable polarization rotator at a rate faster than the data collection rate from the detectors. As discussed above, rapidly modulating the polarization state of the signal transmitted to the target can average the spatial-mode coherence, thereby reducing the negative effects of speckle. 
       FIG. 16  depicts a flow diagram of another method for performing speckle reduction using polarized light in accordance with example implementations of the present disclosure. In embodiments, various portions of method  1600  may be performed by LIDAR systems of  FIGS. 1, and 8-12 . With reference to  FIG. 16 , method  1600  illustrates example functions used by various embodiments. Although specific function blocks (“blocks”) are disclosed in method  1600 , such blocks are examples. That is, embodiments are well suited to performing various other blocks or variations of the blocks recited in method  1600 . It is appreciated that the blocks in method  1600  may be performed in an order different than presented, and that not all of the blocks in method  1600  may be performed. 
     At block  1601 , a beam of polarized light is generated. In some embodiments, the co-propagating, cross-polarized light is generated using a polarizing beam splitter and a single laser source, while in other embodiments multiple laser sources can be used. In some embodiments, the multiple laser beams may have different wavelengths. The generated beam may be transmitted to a target, and reflected light from the target can be detected at a number of detectors. 
     At block  1603 , the beam of co-propagating, cross-polarized light is split into a LO path and a target path. This can be done, for example, using a beam splitter. At block  1605 , the target path light is directed to the target using an optical path discriminator, and light reflected from the target is directed to a first polarizing beam splitter. At block  1607 , the first polarizing beam splitter splits light reflected from the target and directs it to at least two mixers. At block  1609 , the LO path light is split into two portions and also directed to the mixers. In embodiments where the VPR is located ahead of the target path and LO path splitter, the LO path is split into two portions using a second polarizing beam splitter. Where the VPR is located after the LO path splitter, then a standard 2×2 splitter can be used to split the LO path and direct it to the mixers. 
     At block  1611 , the first and second light mixers mix light from the first polarizing beam splitter and the second polarizing beam splitter (or the second 2×2 splitter). The first and second mixers are in communication with first and second detectors, and combined light from the first mixer is received at the first detector and combined light from the second mixer is received at the second detector. As discussed above, in some cases the mixers can bias light to the detectors in favor of the target path. 
     At block  1613 , the polarization state of the polarized beam is transformed using a variable polarization rotator at a rate faster than the data collection rate from the detectors. By rapidly modulating the polarization state of the signal transmitted to the target, one can average the spatial-mode coherence and reduce the negative effects of speckle. 
       FIG. 17  illustrates a diagrammatic representation of a machine in the example form of a computer system  1700  within which a set of instructions, for causing the machine to perform any one or more of the methodologies discussed herein, may be executed. In alternative embodiments, the machine may be connected (e.g., networked) to other machines in a local area network (LAN), an intranet, an extranet, or the Internet. The machine may operate in the capacity of a server or a client machine in a client-server network environment, or as a peer machine in a peer-to-peer (or distributed) network environment. The machine may be a personal computer (PC), a tablet PC, a set-top box (STB), a Personal Digital Assistant (PDA), a cellular telephone, a web appliance, a server, a network router, a switch or bridge, a hub, an access point, a network access control device, or any machine capable of executing a set of instructions (sequential or otherwise) that specify actions to be taken by that machine. Further, while only a single machine is illustrated, the term “machine” shall also be taken to include any collection of machines that individually or jointly execute a set (or multiple sets) of instructions to perform any one or more of the methodologies discussed herein. 
     The exemplary computer system  1700  includes a processing device  1702 , a main memory  1704  (e.g., read-only memory (ROM), flash memory, dynamic random-access memory (DRAM), a static memory  1706  (e.g., flash memory, static random access memory (SRAM), etc.), and a data storage device  1718 , which communicate with each other via a bus  1730 . Any of the signals provided over various buses described herein may be time multiplexed with other signals and provided over one or more common buses. Additionally, the interconnection between circuit components or blocks may be shown as buses or as single signal lines. Each of the buses may alternatively be one or more single signal lines and each of the single signal lines may alternatively be buses. 
     Processing device  1702  represents one or more general-purpose processing devices such as a microprocessor, central processing unit, or the like. More particularly, the processing device may be complex instruction set computing (CISC) microprocessor, reduced instruction set computer (RISC) microprocessor, very long instruction word (VLIW) microprocessor, or processor implementing other instruction sets, or processors implementing a combination of instruction sets. Processing device  1702  may also be one or more special-purpose processing devices such as an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), a digital signal processor (DSP), network processor, or the like. The processing device  1702  is configured to execute processing logic  1726 , which may be one example of the signal processing unit  250  of  FIG. 2  or the polarization rotating controller  850  of  FIG. 8 , for performing the operations and steps discussed herein. 
     The data storage device  1718  may include a machine-readable storage medium  1728 , on which is stored one or more set of instructions  1722  (e.g., software) embodying any one or more of the methodologies of functions described herein, including instructions to cause the processing device  1702  to execute the signal processing unit  250  or the polarization rotating controller  850 . The instructions  1722  may also reside, completely or at least partially, within the main memory  1704  or within the processing device  1702  during execution thereof by the computer system  1700 ; the main memory  1704  and the processing device  1702  also constituting machine-readable storage media. The instructions  1722  may further be transmitted or received over a network  1720  via the network interface device  1708 . 
     The machine-readable storage medium  1728  may also be used to store instructions to perform the signal processing unit  250  of  FIG. 2  or the polarization rotating controller  850  of  FIG. 8 , as described herein. While the machine-readable storage medium  1728  is shown in an exemplary embodiment to be a single medium, the term “machine-readable storage medium” should be taken to include a single medium or multiple media (e.g., a centralized or distributed database, or associated caches and servers) that store the one or more sets of instructions. A machine-readable medium includes any mechanism for storing information in a form (e.g., software, processing application) readable by a machine (e.g., a computer). The machine-readable medium may include, but is not limited to, magnetic storage medium (e.g., floppy diskette); optical storage medium (e.g., CD-ROM); magneto-optical storage medium; read-only memory (ROM); random-access memory (RAM); erasable programmable memory (e.g., EPROM and EEPROM); flash memory; or another type of medium suitable for storing electronic instructions. 
     The preceding description sets forth numerous specific details such as examples of specific systems, components, methods, and so forth, in order to provide a good understanding of several embodiments of the present disclosure. It will be apparent to one skilled in the art, however, that at least some embodiments of the present disclosure may be practiced without these specific details. In other instances, well-known components or methods are not described in detail or are presented in simple block diagram format in order to avoid unnecessarily obscuring the present disclosure. Thus, the specific details set forth are merely exemplary. Particular embodiments may vary from these exemplary details and still be contemplated to be within the scope of the present disclosure. 
     Additionally, some embodiments may be practiced in distributed computing environments where the machine-readable medium is stored on and or executed by more than one computer system. In addition, the information transferred between computer systems may either be pulled or pushed across the communication medium connecting the computer systems. Embodiments of the claimed subject matter include, but are not limited to, various operations described herein. These operations may be performed by hardware components, software, firmware, or a combination thereof. 
     The preceding description sets forth numerous specific details such as examples of specific systems, components, methods, and so forth, in order to provide a good understanding of several embodiments of the present disclosure. It will be apparent to one skilled in the art, however, that at least some embodiments of the present disclosure may be practiced without these specific details. In other instances, well-known components or methods are not described in detail or are presented in simple block diagram format in order to avoid unnecessarily obscuring the present disclosure. Thus, the specific details set forth are merely exemplary. Particular embodiments may vary from these exemplary details and still be contemplated to be within the scope of the present disclosure. 
     Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiments included in at least one embodiment. Thus, the appearances of the phrase “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. In addition, the term “or” is intended to mean an inclusive “or” rather than an exclusive or. 
     Although the operations of the methods herein are shown and described in a particular order, the order of the operations of each method may be altered so that certain operations may be performed in an inverse order or so that certain operation may be performed, at least in part, concurrently with other operations. In another embodiment, instructions or sub-operations of distinct operations may be in an intermittent or alternating manner. 
     The above description of illustrated implementations of the invention, including what is described in the Abstract, is not intended to be exhaustive or to limit the invention to the precise forms disclosed. While specific implementations of, and examples for, the invention are described herein for illustrative purposes, various equivalent modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize. The words “example” or “exemplary” are used herein to mean serving as an example, instance, or illustration. Any aspect or design described herein as “example” or “exemplary” is not necessarily to be construed as preferred or advantageous over other aspects or designs. Rather, use of the words “example” or “exemplary” is intended to present concepts in a concrete fashion. As used in this application, the term “or” is intended to mean an inclusive “or” rather than an exclusive “or”. That is, unless specified otherwise, or clear from context, “X includes A or B” is intended to mean any of the natural inclusive permutations. That is, if X includes A; X includes B; or X includes both A and B, then “X includes A or B” is satisfied under any of the foregoing instances. In addition, the articles “a” and “an” as used in this application and the appended claims should generally be construed to mean “one or more” unless specified otherwise or clear from context to be directed to a singular form. Moreover, use of the term “an embodiment” or “one embodiment” or “an implementation” or “one implementation” throughout is not intended to mean the same embodiment or implementation unless described as such. Furthermore, the terms “first,” “second,” “third,” “fourth,” etc. as used herein are meant as labels to distinguish among different elements and may not necessarily have an ordinal meaning according to their numerical designation.