Patent Publication Number: US-2007097350-A1

Title: Variable polarization attenuator

Description:
BACKGROUND OF THE INVENTION  
      Disclosed embodiments relate generally to laser based sensing devices. More particularly, disclosed embodiments relate to laser transmitting devices such as the type used in lidar systems. The discussion below is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.  
      Lidar systems are measuring systems that detect and locate objects using the same principles as radar, but using light from a laser. Lidar systems can be used on aircraft, for example, for a number of purposes. One example of a lidar system on an aircraft is an altimeter which uses laser range finding to identify a height of the aircraft above the ground. Another example of a lidar system on an aircraft could include a system which detects air turbulence. Other uses on aircraft are possible, for example including on-ground range finding for purposes of on-ground navigation of aircraft in proximity to airports, etc. Non-aircraft uses of lidar systems are also possible.  
      One potential problem with some lidar systems relates to the types of lasers used. Frequently, lidar systems use laser sources that are inexpensive and readily available. Many of these lasers emit light in the visible and near infrared wavelengths, which can be an eye hazard if the beam intensity is too high. While the aircraft is moving, the potential eye hazard is reduced, because it is unlikely that a person will have a prolonged exposure. However, while the aircraft is stationary on the ground, the ground crew could be exposed to this hazard. Some potential for eye hazards also exists in situations where an aircraft is flying at low airspeeds and a low altitude, though the risk is not likely to be high.  
     SUMMARY OF THE INVENTION  
      This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.  
      In accordance with an example embodiment, a laser transmitting system includes a laser source configured to transmit a laser beam at an original transmit intensity. The system also includes a laser intensity adjusting mechanism positioned in the path of the transmitted laser beam. The laser intensity adjusting mechanism is controllable to reduce the intensity of the laser beam from the original transmit intensity. The system also includes a controller coupled to the laser intensity adjusting mechanism. The controller is configured to automatically control the laser intensity adjusting mechanism based upon a safety criteria. Disclosed embodiments also include methods of controlling a laser transmitting system.  
      The laser transmitting system can be, in some embodiments, a lidar system such as a laser altimeter on board an aircraft. One example of a safety criteria is an altitude of the aircraft on which the laser transmitting system is positioned. Other examples of safety criteria can include identifying whether personnel are in the vicinity of the aircraft, a rate of speed of the aircraft, power level, etc. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a diagrammatic illustration of a laser transmitting system, in accordance with disclosed embodiments, positioned on an aircraft.  
       FIG. 2  is a block diagram illustrating an embodiment of a laser transmitting system.  
       FIGS. 3-1  and  3 - 2  are diagrammatic illustrations of a variable polarization attenuator used in embodiments of the laser transmitting system shown in  FIG. 2 .  
       FIG. 4  is a diagrammatic illustration and a plot illustrating transmission as a function of angle between the incident linear polarization vector and the polarization direction of the exit polarizer (analyzer) in an example variable polarization attenuator embodiment.  
       FIG. 5  is an illustration of an example variable polarization attenuator embodiment.  
       FIG. 6  is a flow diagram illustrating a disclosed method embodiment. 
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS  
       FIG. 1  is a diagrammatic illustration of an aircraft  100  with a laser transmitting system  110  in accordance with disclosed embodiments. Laser transmitting system  110  is, in exemplary embodiments, a lidar system, for example a laser altimeter system. This type of laser transmitting system  110  can be used to transmit a laser beam  112  toward a surface  120  (for example the ground, a structure or other types of surfaces). By detecting the reflected energy  114 , system  110  can detect the presence of surface  120 . Based upon the timing between transmission of laser beam  112  and detection of reflected energy  114 , a range or distance between system  110  (and thus aircraft  100 ) and surface  120  can be estimated using well known techniques.  
      As noted above, lidar systems frequently use laser sources that emit light in the visible and near infrared wavelengths, which can be an eye hazard if the beam intensity is too high. On the other hand, if the beam intensity is not high enough at high altitudes, the system performance may suffer. As a non-limiting example, a typical lidar system can range find to the ground when flying below altitudes of approximately 2500 feet. An eye hazard from the laser beam exists at much closer proximity, however, for example beginning at around 42 feet in one example. While the aircraft  100  is moving, the potential eye hazard is reduced because it is extremely unlikely that a person will have a prolonged exposure. However, while the aircraft is stationary on the ground, the ground crew could be exposed to this hazard. The potential eye hazard also exists at low altitudes and airspeeds, for example to an individual who would happen to be observing the aircraft through binoculars. In one example, the eye hazard range would be extended, when a person is viewing the aircraft through binoculars, to around 150 feet. These ranges are provided merely as examples, and those of skill in the art will recognize that the ranges depend on a number of factors, including the power or intensity of the transmitted beam. Disclosed embodiments include use of a power attenuating device or mechanism which adjusts the laser intensity under the control of a control. These embodiments attenuate the transmit beam from its original transmit intensity, reducing the power output to an eye safe level when eye hazards are most severe (low altitudes, on ground, etc).  
       FIG. 2  is a block diagram illustrating an embodiment of laser transmitting system  110  in greater detail. Laser transmitting system  110  includes a laser source  210  which is configured to transmit a laser beam  212  at an original transmit power or intensity. The laser beam  212  is typically a pulsed light, as opposed to a continuously transmitted beam. The original transmit intensity, combined with the transmission wavelength of the laser beam together can be hazardous to a human eye in some embodiments.  
      As shown in  FIG. 2 , laser transmitting system  110  also includes a laser intensity adjusting mechanism  220  positioned in the path of the transmitted laser beam  212  (transmitted at the original transmit intensity), and a controller  230 . Controller  230  is coupled to the laser intensity adjusting mechanism  220  and is configured to automatically control mechanism  220  based upon a safety criteria. A safety criteria analysis module or method implemented by controller  230  is illustrated at  235 . The laser intensity adjusting mechanism  220  is controlled by controller  230 , based upon a safety criteria, to reduce the intensity of the laser beam from the original transmit intensity in some situations. This attenuated laser beam is illustrated in  FIG. 2  at reference number  222 .  
      The safety criteria can be a number of different criteria determined in a variety of different manners. For example, the safety criteria can include an altitude of an aircraft  100  on which the laser transmitting system  110  is positioned. In one embodiment, if the laser source is transmitting while the aircraft is above some minimum altitude,  150  feet for example, laser intensity adjusting mechanism  220  is controlled such that there is no attenuation of the transmit intensity or power. In other words, laser beam  222  exiting mechanism  220  has substantially the same power or intensity as laser beam  212  entering mechanism  220 . However, when aircraft  100  drops below the minimum altitude, the safety criteria analysis performed by controller  230  identifies this fact and mechanism  220  is controlled accordingly to attenuate the power of the laser beam. This attenuation can be two state in nature (on or off), or it can be over some continuum (discrete or continuously changing), where the lower the aircraft altitude, the greater the attenuation of the transmitted beam. Other safety criteria can include, for example, the detection of personnel in the vicinity of the laser transmitting system, ground speed of the aircraft, airspeed of the aircraft, power level, etc.  
      In exemplary embodiments, the laser intensity adjusting mechanism  220  includes a variable polarizing attenuator (VPA). Variable polarizing attenuators are commonly available devices which attenuate light under the control of a control signal. Frequently, VPAs are voltage controlled devices. In accordance with disclosed embodiments, a VPA is used in a laser above ground level (LAGL) sensor system or other system under the control of a controller and based on a safety criteria to reduce the eye hazard by switching between high and low intensity states. As described above, for a LAGL sensor, the VPA will allow transmission at high intensity while it is above a defined hazard range, but will switch to one or more low intensity transmission states when it is below the defined hazard range, thereby protecting all ground personnel from the laser hazard.  
      A VPA implementation of laser intensity adjusting mechanism  220  is illustrated in  FIGS. 3-1  and  3 - 2 . The VPA includes an adjustable polarization component  310  (a variable polarization rotator) which would be coupled to a controller (e.g., controller  230 ) and an exit polarizer or analyzer  320 . The adjustable polarization component  310  is positioned relative to the source  210  (shown in  FIG. 2 ) and to the exit polarizer  320  such that the laser beam enters component  310  first, and after exiting component  310  enters into the exit polarizer  320 . The VPA uses adjustable polarization component  310  to rotate the incident beam  212  between two or more polarization states (two states shown, respectively, in  FIGS. 3-1  and  3 - 2 ) relative to a polarization direction (represented at reference number  322 ) of the exit polarizer  320 . The beam rotation before component  310  is illustrated at reference number  305 . The rotated beam between rotator  310  and exit polarizer  320  is represented at reference number  312 , and is shown to have an after rotation polarization as illustrated at reference number  315 . Note the different beam polarizations  315  shown in  FIGS. 3-1  and  3 - 2 . As can be seen in  FIGS. 3-1  and  3 - 2 ,  
      When the variable polarization rotator  310  is configured to rotate the beam polarization parallel to the exit polarizer direction  322  as shown in  FIG. 3-2 , the exit polarizer  320  will transmit the beam  222  at full intensity in this polarization direction (as represented at reference number  360 ). When the variable polarization rotator  310  is configured to rotate the beam polarization perpendicular to the exit polarizer direction  322  as shown in  FIG. 3-1 , it will attenuate the beam  222  to its lowest intensity. If the polarization is rotated to a state that is between parallel and perpendicular, the transmission percentage is proportional to the cosine squared of the polarization rotation angle. A plot of transmission (where  0  represents full attenuation and  1  represents full transmission) of an ideal polarizer response is shown in  FIG. 4  for the angle between the incident linear polarized vector  315  and the polarization exit polarizer (or analyzer) direction  322 . The VPA is most efficient with the use of a linear polarized laser, but can be used with a non-polarized laser if another polarizer is introduced before the variable polarization rotator. This typically reduces the transmission by an additional fifty percent for randomly polarized light.  
       FIG. 5  is an illustration of one embodiment of a variable polarization attenuator type of laser intensity adjusting mechanism  220 . As shown in  FIG. 5 , the VPA includes first and second polarization beam splitters (PBSs)  505  and  510 , though some embodiments would use only PBS  510 . PBS  505  is included in some embodiments to clean up the polarization of the laser beam  212 , as it would be oriented to receive beam  212  prior to the beam passing through other components of mechanism  220 . Positioned between PBS  505  and PBS  510  are a fixed half wave plate  515  and a variable wave plate  520 . Variable wave plate  520  provides the variable polarization rotator  310  function. In some embodiments, heaters  525  and  530  are included and are positioned on either side of the variable wave plate  520  for thermal stability purposes.  
      Referring now to  FIG. 6 , shown is a flow diagram  600  illustrating a method for controlling a laser transmitting system. At block  605  of  FIG. 6 , the method is shown to include the step of transmitting a laser beam at an original transmit intensity using a laser source. Then, at step  610 , the method is shown to include the step of identifying a safety criteria related to the transmitted laser beam at the original transmit intensity. Finally, at step  615 , the method is shown to include the step of controlling a laser intensity adjusting mechanism, positioned in the path of the transmitted laser beam, in order to satisfy the identified safety criteria by selectively reducing the intensity of the laser beam from the original transmit intensity. Implementation of these steps can be as described above with reference to the operation of system  110 .  
      Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are disclosed as example forms of implementing the claims.