Abstract:
A method for achieving optimum coaxial lidar configuration using optical fibers. Optical fibers are used with mirrors or lenses to create light paths that can achieve an optimum lidar configuration while employing fewer components than that of prior art for a more simplified, lightweight, and less expensive system to produce. The lidar components, including the laser source and the light detector unit, may be placed in a separate housing. A separate housing eliminates unnecessary weight in the optical telescope assembly, makes scanning of the system easier, and enables a better omni-directional cloud height indicator.

Description:
This application claims the benefit of U.S. Provisional Application No. 61/068,927, filed Mar. 11, 2008, which is hereby incorporated by reference in its entirety. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to lidar. 
     BACKGROUND OF THE INVENTION 
     The very earliest cloud height measurements utilized ceiling balloons of various weights and lifts, inflated with helium. The time interval between the release of the balloon and its entry into the base of the clouds was recorded. The point of entry for layers aloft was considered as midway between the point at which the balloon began to fade and that when it completely disappeared. With surface based clouds, the time interval ended when the balloon completely disappeared. During the day, red balloons were used with thin clouds and black balloons were used with thicker clouds and at night, a battery-powered light was attached. Naturally, accuracy using this method depended somewhat on the reactions and eyesight of the observer and could be complicated by such issues as wind and local topography. 
     In the 1930s, a methodology originated whereby a beam from a ceiling light was projected at a 45-degree elevation into the sky. The projector was rotated about the vertical axis until the light beam hit the lowest cloud. An observer paced off the distance from the projector to a point directly below the illumination spot. With the geometry of this scheme, the paced distance equaled the height of the cloud. This technique was quickly abandoned in favor of a vertically shining light with a clinometer at a previously measured baseline. Knowing the baseline length and elevation angle in this right triangle situation made it easy to determine the height with a lookup table. 
     Much of the human introduced subjectivity was later removed by automation using a photocell that scanned the vertical path until the spot of light on the cloud was detected. The projector light was modulated so the photocell received less interference from ambient light during daytime use. The angle of inclination was displayed automatically at the observer&#39;s console. 
     The next version of cloud height indicators (CHI) was the rotating beam ceilometer. As the name implies, the beam of light rotated, and the vertically looking detector measured any cloud hits directly overhead. The angle of the cloud hits was displayed either on a scope or on a recorder chart. Height measurements were limited to heights no greater than ten times the base line. Above this ratio, the value of the tangent function increased too quickly to ensure the accuracy of a measurement. 
     Light detection and ranging (LIDAR) is a method that can be used to characterize the atmosphere, and many methods have been developed to produce LIDAR systems for specific applications. A LIDAR system usually includes a light transmitter, a receiver—including optics and an electronic light detection device, and some type of timing circuitry. LIDARs for cloud ceiling measurement began service with the National Weather Service (NWS) CHI service in 1985. The sensor sends laser pulses vertically into the atmosphere. The pulse rate varies with the temperature to maintain a constant power output. The time interval between the pulse transmission and the reflected reception is used to determine the cloud height. The reporting limit of this instrument for the NWS is 3800 m (12,000 ft). 
     Different configurations for the transmitting and receiving optics have been developed over the years including the bi-axial or side-by-side configuration, where the transmitter and receiver optics are separate but adjacent. This produces problems in overlap of the transmitting beam and the receiving field of view for certain ranges. The advantage, however, is that the full power of the laser is transmitted, and the full return power from atmospheric backscattering is received by the receiving optics. In a coaxial configuration, the transmitting and receiving optical pathways share the same centroid. For full overlap of the transmitting and receiving optical ray traces, some type of beam splitter is usually employed resulting in up to a 75% loss in optical power due to the two passes through a 50/50 beam splitter, for example. Additionally the receiving field of view includes the optical surfaces where the transmitter light exits. Reflections from these surfaces can result in excessively high power at the receiver, disrupting measurements. 
     Other configurations have been introduced using a beam-splitting mirror with a hole in it, creating a coaxial configuration, where the transmitting beam suffers no loss on the outgoing path, and the receiving beam is less contaminated with crosstalk from the transmitting beam reflecting off of the outgoing optics. However, while crosstalk is reduced, it is not eliminated. Multiple reflections within the lens can bring light back from the transmitter to the receiver. 
     Needs exist for improved methods of measuring cloud heights. 
     SUMMARY OF THE INVENTION 
     The present invention is methods for achieving optimum coaxial lidar configuration using optical fibers. Optical fibers are used with mirrors or lenses to create light paths that can achieve an optimum lidar configuration while employing fewer components than that of prior art for a more simplified, lightweight, and less expensive system to produce. The lidar components, including the laser source and the light detector unit, may be placed in a separate housing. A separate housing eliminates unnecessary weight in the optical telescope assembly, makes scanning of the system easier, and enables a better omni-directional cloud height indicator. Optical fibers come in a variety of core diameters and Numerical Apertures (NA). The NA determines the divergence angle of the light coming out of the fiber, as well as the acceptance angle for incoming light. 
     The technology has the following characteristics: 
     1. Coaxial Transmit and Receive Paths: transmitted and received optical power is along the same axis. This allows for better overlap of fields of view over a longer range 
     2. Optical Fiber Based: optical fibers are used in key areas to create coaxial transmit and receive beam paths in a compact and lightweight configuration. 
     3. Low Cross-Talk: the receiver field of view is isolated from receiving light that would reflect off of internal components of the lidar system, eliminating large spikes in return power that would complicate atmospheric lidar measurements. 
     4. Lightweight: reduction of the weight of the optical assembly allows fast scanning of the optical assembly using low-power motor assemblies. 
     5. Increased Optical Efficiency: by achieving a coaxial configuration without the use of beam splitters, and optimizing the transmit and return cross-sectional areas, a more sensitive and optically efficient system is produced. This optimal ratio of transmitting area to receiving area is approximately 1:1. 
     These and further and other objects and features of the invention are apparent in the disclosure, which includes the above and ongoing written specification, with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a prior art system where the transmitted beam and the received light share the same aperture. 
         FIG. 2  shows another prior art coaxial system where the transmitted light exits in the center sub-aperture of the lens, and a portion of the return light is reflected off of a mirror with a hole in it into the detector. 
         FIG. 3  shows one embodiment that utilizes optical fibers to guide transmitted and received light through lenses. 
         FIG. 4  is an embodiment where the receiver optical fiber is inverted and the receiver optical configuration utilizes a parabolic or spherical mirror to reflect the received light into the receiver optical fiber which guides light to the detector. 
         FIG. 5  shows an embodiment where the optical fibers are both inverted, employing independent parabolic or spherical mirrors to achieve the desired optical paths. 
         FIG. 6  is a configuration where the brightest part of the beam is then on the outer radius of the transmitted beam and unblocked by the transmitter fiber following reflection off of the parabolic (or spherical) mirror. 
         FIG. 7  is a configuration where an axicon is employed to offset the brightest portion of the beam axially outward. 
         FIG. 8  is another embodiment where the receiver fiber is upright and receives light from a secondary mirror located behind the transmitter fiber. 
         FIG. 9  is an embodiment where the transmitting optical fiber is inverted off angle to a parabolic (or spherical) mirror. 
         FIG. 10  is an embodiment where the transmitting fiber is directed downward toward a mirror. 
         FIG. 11  shows the system of  FIG. 3  from a perspective view, where the transmitting optical fiber is connected to a collimating optical subsystem that is supported by the receiving lens. 
         FIG. 12  shows the system of  FIG. 8  where the transmitting collimator assembly includes a mirror on its back surface which serves as the receiver secondary mirror. 
         FIG. 13  is an embodiment where the transmitter optical fiber or the transmitter itself, and the receiver optical fiber or the receiver itself are located symmetrically on opposite sides. 
         FIG. 14  is an embodiment where optically flat mirrors are used in combination with a collimating lens. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows a prior art system  13 . In  FIG. 1 , a transmitted beam  15  is sent from a transmitter  17 . Both the transmitted beam  15  and a received light  21  share an aperture. A beam-splitter  19  allows a portion  21  of the transmitted beam  15  from the laser transmitter  17  through to the lens  23  and reflects part of the return light  25  into a detector  27 . 
       FIG. 2  shows another coaxial system  29  where transmitted light  31  from a transmitter  32  exits in a center  33  sub-aperture of a lens  35 , and a portion of return light  37  is reflected off of a mirror  39  with a hole  41  in it into a detector  43 . The configuration of  FIG. 2  reduces reflections of the transmitter off of the lens  35  into the detector  43 , which can saturate the detector electronics. 
     The current invention utilizes a coaxial configuration, but achieves it through different configurations that offer distinct advantages. 
       FIG. 3  shows one embodiment  51 , which utilizes optical fibers to guide transmitted and received light through lenses. The transmitter optical fiber  53  comes from a laser source  55 . Light  57  is emitted from a fiber end  59  and is collimated via a lens  61 . Received light  63  is focused by a ring lens  65  into a receiver optical fiber end  67 , through an optical fiber  69  and to a detector  71 . This configuration more completely eliminates the possibility of back reflections from the transmitted beam  57  from entering the receiver  67  as the transmitting and receiving lenses  61 ,  65  are separated and insulated from each other. This configuration is also more compact than prior art and allows separation of the optical assembly from the electronic assembly for ease of manufacturing and maintenance. To create the system of  FIG. 3 , two collimating lenses are fabricated such that one lens  61  is small and disposed in the center of a larger lens  65  with a cutout in the center. 
       FIG. 4  shows an embodiment  73  where a receiver optical fiber  75  is inverted and a receiver optical configuration utilizes a parabolic or spherical mirror  77  to reflect received light  79  into a receiver optical fiber end  81  which guides light through the optical fiber  75  and to a detector  83 . Transmitted light  85  travels through a transmitter optical fiber  87  and comes from a laser source  89 . The transmitted light  85  passes through a collimating lens  91  in the center of the apparatus  73 . This configuration provides a more compact size than the system of  FIG. 3 , while maintaining the advantage of better isolation of transmit and receive optical paths. 
       FIG. 5  shows an embodiment  93  where both a transmitter optical fiber  95  from a laser  97  and receiving optical fiber  99  to a detector  101  are inverted. The embodiment  93  of  FIG. 5  also employs independent parabolic or spherical mirrors  103 ,  105  to achieve desired optical paths. Transmitted light  107  exits the laser-coupled optical fiber  95  from an optical fiber end  109  and reflects off of the collimating parabolic or spherical mirror  103  to exit the center of the system  93 . Return light  111  reflects off of the ring parabolic (or spherical) receiver mirror  105  and is focused into the detector-coupled receiver optical fiber  99  through the optical fiber end  113 . 
     To optimize the transmitted energy, the exit beam profile of the transmitter optical fiber is a Gaussian profile  127  that has been turned inside out  115  through use of an axicon  117  or waxicon  119  as shown in  FIG. 6  and  FIG. 7 . In the case of the configuration  121  in  FIG. 6 , the brightest part of a beam  123  is then on the outer radius  125  of the transmitted beam and unblocked by the transmitter fiber  95  following reflection off of the parabolic (or spherical) mirror  105 . The incoming fiber  95  and outgoing fiber enter an optical circulator  129  and then a collimating lens  131  and the waxicon  119 . In the configuration in  FIG. 7 , an axicon  117  is employed to offset the brightest portion of the beam axially outward  133  so that the brightest part of a beam  135  is at edges  137  of the transmitted beam and unblocked by the transmitter fiber  95  following reflection off of the parabolic (or spherical) mirror  105 . The incoming fiber  95  pass light through a collimating lens  139 , through the axicon  117 , through a focusing lens  141  and into an outgoing fiber  99 . 
       FIG. 8  shows an embodiment  143  where a receiver fiber  145  is upright and receives light  147  from a secondary mirror  153  located behind a transmitter fiber  151  and receives light at fiber end  146  before a detector  148 . The secondary mirror  153  receives light from the parabolic (or spherical) primary mirror  149  below the receiver optical fiber  145 . Transmitted light  155  leaves a laser  157 , travels through a transmitter fiber  151  and through a fiber end  159 . The transmitted light  155  passes through a collimating lens  161  that is small and goes in the center of the apparatus  143 . The configuration in  FIG. 8  provides a compact co-axial configuration. 
       FIG. 9  shows an embodiment  163  where a transmitting optical fiber  165  is inverted off angle to a parabolic (or spherical) mirror  167 . Transmitted light  169  is generated by a laser  171 , travels through the transmitting optical fiber  165  and exits a fiber end  173  and then reflects off of the mirror  167  and is collimated by the collimating lens  175 . Received light  177  is reflected off a second mirror  179  and up to an underside  181  of the first mirror  167 . The underside  181  of the first mirror  167  then reflects the received light  177  into a received fiber end  183 , through a receiver optical fiber  185  and into a detector  187 . 
       FIG. 10  shows an embodiment  189  where a transmitting fiber  191  is directed downward toward a mirror  193 . A beam  195  travels from a laser  199 , through the transmitting fiber  191 , through a fiber end  201 , is reflected off the mirror  193  and is reshaped using the system of  FIG. 6  or  FIG. 7  such that the highest power section of the beam  195  is not blocked before it is collimated by a collimating lens  197 . Received light  203  is reflected off a second mirror  205  and up to an underside  207  of the first mirror  193 . The underside  207  of the first mirror  193  then reflects the received light  203  into a received fiber end  209 , through a receiver optical fiber  211  and into a detector  213 . 
       FIG. 11  shows the system  51  of  FIG. 3  from a perspective view, where the transmitting optical fiber  53  is connected to a collimating optical subsystem  215  that is supported by the receiving lens  61 . 
       FIG. 12  shows the system  143  of  FIG. 8  where a transmitting collimator assembly  217  includes a mirror  153  on its back surface which serves as the receiver secondary mirror. Received light  147  is reflected one or more times from the mirror  149  to an underside  219  of the first mirror  153  before entering a receiver optical fiber end  146  in an opening  221  in the mirror  149 . 
       FIG. 13  shows an embodiment  223  where a transmitter optical fiber head  233 , transmitter optical fiber  225  or a transmitter  227  itself, and a receiver optical fiber head  235 , receiver optical fiber  229 , or a receiver  231  itself are located symmetrically on opposite sides of the device  223 . A parabolic or spherical transmitter mirror  237  in the center of the device  223  collimates transmitted light  239  from the transmitter optical fiber  225 . A parabolic (or spherical) ring mirror  241  focuses return light  243  into the receiver optical fiber  229 . The system of  FIG. 13  is more compact than previous devices. 
       FIG. 14  shows an embodiment  245  where optically flat mirrors  247  are used in combination with a collimating lens  249 . This embodiment maintains the symmetry of the system of  FIG. 13 . A transmitter  251 , transmitter optical fiber  253  and transmitter optical fiber head  255  are located on an opposite side of the device  245  than the receiver optical fiber head  257 , receiver optical fiber  259  and detector  261 . A compact mirror assembly in the middle consists of the center mirror  247  for reflecting a transmitter beam  263  to the collimating lens  249 . A ring mirror  265  reflects focused return light  267  from the outer section of the lens  249  into the receiver optical fiber  259 . 
     While the invention has been described with reference to specific embodiments, modifications and variations of the invention may be constructed without departing from the scope of the invention.