Patent Publication Number: US-2023161013-A1

Title: Methods and devices for reducing eye safety minimum distances in conjunction with illumination laser radiation

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application of PCT Application No. PCT/EP2021/070625, filed on 23 Jul. 2021, which claims the benefit and priority to German Patent Application No. 10 2020 115 703.7, filed on 24 Jul. 2020. The entire disclosures of the applications identified in this paragraph are incorporated herein by references. 
    
    
     FIELD 
     The present invention relates to methods and devices for reducing eye safety minimum distances during the active observation of objects and during fine tracking by means of illumination laser radiation. The eye safety minimum distance is also referred to as a non-ocular hazard distance, abbreviated NOHD. It represents the distance of a person from a laser radiation source at which the laser radiation impinging on the person falls below a predetermined exposure limit EGW. 
     SUMMARY 
     In order to illuminate an object in a defined environment, a minimum illumination intensity on the object must be achieved so that an optical sensor that detects the illumination radiation reflected by the object receives sufficiently many photons for an evaluation of the reflected illumination radiation. 
     In a first method known per se for operating a laser radiation device which has an illumination laser and an active laser, in a first operating mode of the laser radiation device in which the active laser radiates no active laser radiation the illumination laser is operated such that its illumination laser radiation has a first illumination laser radiant flux. 
     In a laser radiation device that has a first illumination laser beam path and a second illumination laser beam path, wherein laser radiation propagating in the first illumination laser beam path exits from a first aperture of the laser radiation device and laser radiation propagating in the second illumination laser beam path exits from a second aperture of the laser radiation device that is spatially separated from the first aperture, the illumination laser beam paths overlap. 
     In a second method known per se for adjusting an illumination laser radiant flux, a distance of a target to be illuminated by the illumination laser is measured with a distance measuring device different from the illumination laser. 
     A third method known per se is used to adjust parameters of an illumination laser, and has the following steps: Measuring a distance of a target to be illuminated by the illumination laser, determining parameters of the illumination laser as a function of the measured distance, emitting at least one laser radiation pulse, directed to the target, of the illumination laser operated with the determined parameters, checking whether laser radiation of the emitted laser radiation pulse is reflected to a detector of the illumination laser and then, if this is the case, determining the distance of the object that reflected the laser pulse as a function of the reflected and detected laser radiation. 
     For the illumination laser, it is desirable for many reasons (cross-sectional area, cleaning, signature, etc.) to have the smallest possible transmission aperture. This can result in high intensities and thus in undesirably large NOHD values, particularly in the vicinity of the aperture. 
     For the active observation of objects in space at different distances, it is desirable, particularly in urban use, to achieve low NOHD values for the use of the illumination laser in order to be able to avoid safety-related limitations during the illumination. In order to achieve low NOHD values in this target conflict, transmission apertures for the illumination laser radiation are currently used that are similar in size to transmission apertures of active lasers that emit significantly greater power levels. Another possibility is to limit the power radiated by an illumination laser to a minimum value that is just sufficient for the illumination function. It is also known to use illumination laser radiation with so-called “eye-safe wavelengths” that are concomitant with high exposure limit values (the exposure limit value increases with increasing wavelength). 
     The restriction of the illumination laser radiant flux is generally disadvantageous because it directly reduces the illumination quality and thus the object recognition quality and resolution. Illumination lasers that emit so-called eye-safe illumination laser radiation are very expensive and have low efficiency. 
     Against this background, the object of the invention is to specify methods for operating illumination lasers with very low NOHD values, which allow the use of illumination lasers in an urban environment while maintaining exposure limit values even in the case of illumination laser radiation wavelengths that are not generally considered to be “eye-safe.” Another object is to specify a device which allows the use of illumination lasers in an urban environment while maintaining exposure limit values. Such use should also be possible in particular with illumination laser radiation wavelengths which are not considered from the outset to be “eye-safe.” 
     This object is achieved in each case with the sum of the features of each independent claim. 
     According to the invention, the above-mentioned first method is characterized in that, in a second operating mode of the laser radiation device in which the active laser emits active laser radiation, the illumination laser is operated such that its illumination laser radiation has a second illumination laser radiant flux that is greater than the first illumination laser radiant flux. 
     According to the invention, the above-mentioned laser radiation device is characterized in that an overlap of the two illumination laser beam paths occurs only at a distance from the apertures which is greater than a predetermined minimum distance for each individual one of the two beam paths. 
     According to the invention, the second above-mentioned method is characterized in that an illumination laser radiant flux of the illumination laser is determined as a function of the measured distance and in that the target is subsequently illuminated by the illumination laser, wherein the illumination laser is operated such that it emits the determined illumination laser radiant flux. 
     According to the invention, the above-mentioned third method is characterized in that the distance of the object is compared with the distance of the target and in that, when the distance of the object is less than the distance of the target, parameters of the illumination laser are changed such that an exposure limit value at the location of the object is not exceeded by the laser radiation of the illumination laser impinging there. 
     A preferred embodiment of the first method is characterized in that the illumination laser is operated in the first operating mode and in the second operating mode such that it emits the illumination laser radiation in the form of illumination laser radiation pulses, wherein a repetition frequency with which the illumination laser radiation pulses are emitted is less in the first operating mode than in the second operating mode. 
     A preferred working method of the laser radiation device is characterized in that it is operated in such a way that the laser radiation propagating in the illumination laser beam paths is fed into the illumination laser beam paths in the form of illumination laser radiation pulses, wherein the illumination laser radiation pulses are fed alternately into the first illumination laser beam path and the second illumination laser beam path from one illumination laser radiation pulse to the next illumination laser radiation pulse. 
     A preferred embodiment of the second method according to the invention is characterized in that the illumination laser radiant flux is determined such that it also decreases as the distance decreases. 
     A preferred embodiment of the third method according to the invention is characterized in that the distance of the target is measured with a distance measuring device different from the illumination laser. 
     It is also preferred that the laser radiation impinging on the location of the object is modified by reducing an average intensity of the laser radiation of the illumination laser. 
     It is further preferred that the average intensity of the laser radiation is modified by reducing a repetition rate of the illumination laser. 
     Further advantages are described in the dependent claims, the description and the accompanying figures. 
     It is understood that the above features and those to be explained below can be used not only in the combination indicated in each case but also in other combinations or on their own, without departing from the scope of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention are shown in the drawings and explained in more detail in the following description. In this context, the same reference signs in different figures refer to the same elements or elements that are at least comparable in terms of their function. The figures show the following in a schematic form: 
         FIG.  1    shows a laser radiation device with which the methods according to the invention can be executed; 
         FIG.  2    shows an embodiment of a first method according to the invention with which the illumination laser is operated; 
         FIG.  3    shows an embodiment of a laser radiation device according to the invention; 
         FIG.  4    shows an embodiment of a second method according to the invention; and 
         FIG.  5    shows a flow chart as an embodiment of the third method according to the invention for adjusting parameters of an illumination laser. 
     
    
    
     DETAILED DESCRIPTION 
     Specifically,  FIG.  1    shows a laser radiation device  10 , which has a housing  12 . The housing  12  has a first illumination laser aperture  16 , through which illumination laser radiation  22  can exit the housing  12  in a first illumination laser beam path  52 . The housing  12  also has an active laser aperture  17 , through which an active laser beam can exit the housing  12 . 
     The laser radiation device  10  has an active laser  13 , a first illumination laser  14 , a deflecting mirror  36  that is reflective for the wavelength of the active laser radiation  20  and is transparent for the wavelength of the illumination laser radiation  22 , a tip-tilt mirror  24 , which is controllable with respect to its orientation in space, an active laser telescope  26 , a first illumination laser telescope  28 , an optical sensor  30 , a control unit  32 , and a distance measuring device  34 . 
     The illumination laser radiation  22  preferably has a different wavelength from the active laser radiation  20 . 
     The first illumination laser  14  is preferably configured to generate pulsed illumination laser radiation as illumination laser radiation  22 . Alternatively, the first illumination laser  14  is configured to generate the illumination laser radiation  22  as continuous-wave laser radiation. 
     The illumination laser radiation  22  exits from the housing  12  through the first illumination laser aperture  16  and, when the laser radiation device  10  is roughly aligned correctly, detects a target  38 . 
     A rough alignment is carried out, for example, by a mechanical alignment device which is configured to roughly align the housing  12 , and thus also the laser radiation  20 ,  22  emerging from the housing  12  in its azimuth and elevation, to the target  38 . The rough alignment is controlled, for example, by a radar system that, in one embodiment, also serves as a distance measuring device  34 . 
     The illumination laser radiation  42  reflected by the target  38  propagates through the active laser radiation telescope  26  and, via the tip-tilt mirror  24 , to the deflection mirror  36 , which is reflective for the wavelength of the active laser radiation  20  and transparent for the wavelength of the illumination laser radiation  22  and the reflected illumination laser radiation  42 . The reflected illumination laser radiation  42  passes through the deflection mirror  36  and is acquired by the optical sensor  30 . The signal of the optical sensor  30  generated in this manner is evaluated by evaluation software  44  of the control unit  32 , and the result of the evaluation is used by a drive unit  46  of the control unit  32  to drive the tip-tilt mirror  24 . The driving is such that the tip-tilt mirror  24  aligns any triggered active laser radiation  20  toward the target  38 . This alignment represents fine tracking. 
     The drive unit  46  also controls the first illumination laser  14  and the active laser  13  and processes signals of the distance measuring device  34 . 
     The control of the first illumination laser  14  takes place in such a way that the smallest possible NOHD value is set. For the calculation of the illumination laser parameters that lead to the smallest possible but still sufficiently large powers of the illumination laser radiation  22 , in the case of illumination lasers  14  operated in pulse operation, among other things two exposure limit values must be taken into account, namely a peak value EGW_peak (W/m 2 ) for each individual illumination laser radiation pulse and an average value EGW_average (W/m 2 ) for the average power of a sequence of illumination laser radiation pulses. In general, the peak value EGW_peak for an illumination laser radiation pulse is significantly greater than the average value EGW_average. This results in a low NOHD_peak value for low pulse repetition frequencies. 
     In the calculation of NOHD, further parameters are taken into account beyond the values EGW_peak and EGW_average, of which some are listed below, however this list is by no means exhaustive: 
     
       
         
           
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 Pulse peak power 
                 P_peak 
               
               
                   
                 Average power 
                 P_average = E_pulse * f_rep 
               
               
                   
                 Repetition rate 
                 f_rep (number of pulses per time unit) 
               
               
                   
                 Pulse energy 
                 E_pulse (J) 
               
               
                   
                 Divergence (beam  
                 (rad) 
               
               
                   
                 opening angle) 
                   
               
               
                   
                 Pulse intensity I_peak 
                 (W/m 2 ) 
               
               
                   
                 Pulse intensity I_average 
                 (W/m 2 ) 
               
               
                   
                 Wavelength 
                 Lambda (m) 
               
               
                   
                   
               
            
           
         
       
     
     For the active observation of objects and for fine tracking, comparatively large spatial regions are illuminated over a large area with illumination laser radiation  22 . Larger spatial regions are, for example, those which appear at a viewing angle of, for example, 2 mrad to 4 mrad as viewed from the aperture of the illumination laser. Illumination laser radiation  42  reflected by the target  38  or by another object is acquired by the active laser telescope  26  and converted by the optical sensor  30 , which is preferably a camera, into electrical signals that are then evaluated by the evaluation software  44  of the control unit  32 . Overall, the laser radiation device  10  is configured to carry out a method according to the invention or to control its sequence. The configuration is carried out in particular by corresponding programming of the control unit  32 . 
     In deviation from the illustration in  FIG.  1   , the illumination laser can also be arranged in a further housing separate from the housing  12  of the active laser  13 . The control unit  32  is then located in one of the two housings, for example. 
     In a further embodiment, the housing  12  accommodates in particular the active laser telescope, and the active laser  13  is arranged outside the housing  12  of the active laser telescope. The active laser radiation is then preferably guided into the housing  12  by means of at least one optical fiber. Alternatively or additionally, illumination laser radiation is guided into the housing  12  by means of one or more optical fibers. It is also preferred that the illumination laser radiation and the active laser radiation are combined with an optical coupler and both propagate through the active laser telescope and are emitted through the same aperture. 
       FIG.  2    shows an embodiment of the first method according to the invention, with which the illumination laser is operated. 
     In this embodiment, which relates to a laser radiation device  10  that has at least one illumination laser  14  and an active laser  13 , the illumination laser  14  is operated in two different operating modes. In a first step  100 , the illumination laser  14  is operated in a first operating mode of the laser radiation device  10 . The first operating mode is characterized in that the active laser  13  does not emit any active laser radiation. In this case, operation takes place with parameters of the illumination laser  14  at which the lowest possible NOHD value results while the power of the illumination laser radiation  22  is still sufficient for the illumination purposes. 
     In a second step  102 , it is checked whether active laser radiation  20  is to be emitted. If this is not the case, the method returns to the first step  100 . If, on the other hand, this is the case, in a third step the illumination laser  14  is operated in the second operating mode of the laser radiation device  10 . In the second operating mode, active laser radiation  20  is also emitted. The radiation power emitted with the active laser radiation  20  is generally so great that it significantly exceeds the exposure limit values. In this case, it is thus no longer necessary for the illumination laser  14  to comply with exposure limit values. Therefore, in the third step, and thus in the second operating mode, the illumination laser  14  is operated such that its illumination laser radiation  22  has a second illumination laser radiant flux that is greater than the first illumination laser radiant flux. With the larger illumination laser radiant flux, the signal-to-noise ratio of the illumination laser radiation  42  reflected by the object improves, which improves the fine tracking. 
     The illumination laser radiant fluxes of different magnitudes in the two operating modes are preferably realized in that, in the first operating mode and in the second operating mode, the illumination laser  14  emits illumination laser radiation  22  in the form of illumination laser radiation pulses, wherein a repetition frequency with which the illumination laser radiation pulses are emitted is less in the first operating mode than in the second operating mode. The power of the individual illumination laser radiation pulses remains constant here. Alternatively, the repetition frequency can also be maintained. In this case, the illumination laser is operated in the first operating mode with a lower individual pulse power. Furthermore, mixed forms are also possible, in which both the individual pulse power and the repetition frequency differ from operating mode to operating mode. From the third step  104 , the method always returns to step  102 , so that termination of the emission of active laser radiation triggers a return to the first operating mode. 
       FIG.  3    shows an embodiment of a further laser radiation device  50  according to the invention. The laser radiation device  50  differs from the laser radiation device  10  of  FIG.  1    on account of an additional, second illumination laser beam path  54 . Illumination laser radiation  22  propagating in the first illumination laser beam path  52  exits from a first illumination laser aperture  16  of the laser radiation device  50 , and illumination laser radiation  22  propagating in the second illumination laser beam path  54  exits from a second illumination laser aperture  56  of the laser radiation device  50  that is spatially separated from the first illumination laser aperture by a distance  58 . As a result, the illumination laser radiation  22  is divided. 
     The distance  58  of the illumination laser apertures  16 ,  56  from one another is selected such that the two illumination laser beam paths do not overlap within the eye-safe minimum distance NOHD defined by the exposure limit values to be maintained. The overlap of the two illumination laser beam paths occurs only at a distance from the apertures which is greater than a predetermined minimum distance NOHD for each individual one of the two illumination laser beam paths  52 ,  54 . The radiant fluxes propagating in the two illumination laser beam paths are summed only in the overlap region  60 . 
     The embodiment according to  FIG.  3   , in terms of its device aspects, deviates from the subject matter of  FIG.  1    on account of the additional second illumination laser  62  with its associated second illumination laser telescope  64 . In all other respects, the description of  FIG.  1    also applies to  FIG.  3   . 
     In a preferred embodiment, the laser radiation device  50  is operated such that the laser radiation propagating in the illumination laser beam paths is fed into the illumination laser beam paths in the form of illumination laser radiation pulses, wherein the illumination laser radiation pulses are fed alternately into the first illumination laser beam path  52  and the second illumination laser beam path  54  from one illumination laser radiation pulse to the next illumination laser radiation pulse. The average intensities of the individual illumination laser radiation of the two illumination laser beam paths are then summed on the illuminated object. 
       FIG.  4    shows an embodiment of a second method according to the invention. This method is used to adjust an illumination laser radiant flux. In a first step  200 , a distance of a target to be illuminated by the first illumination laser  14  and/or the second illumination laser  62  is measured with a distance measuring device  34  different from the illumination laser  14  and/or  62 . The distance measuring device  34  is preferably a radar device, as is also used for rough tracking. 
     In a second step  202 , an illumination laser radiant flux of the illumination laser  14  and/or  62  is determined as a function of the measured distance. Subsequently, in a third step  204  the target is illuminated by the illumination laser  14  and/or  62 , wherein the illumination laser  14  and/or  62  is operated such that it emits the determined illumination laser radiant flux. After this, the method returns to the first step  200 . The loop of these three steps is continuously run through repeatedly, so that the illumination laser radiant flux is continuously adapted to changing distances of the target  38 . Here, the illumination laser radiant flux is determined such that it also decreases as the distance decreases. The illumination laser radiant flux is in particular determined in each case in such a way that the smallest possible NOHD value results while the illumination laser radiation illuminating the target  38  still has sufficient intensity. 
       FIG.  5    shows a flow chart as an embodiment of the third method according to the invention for adjusting parameters of an illumination laser. In a first step  400 , it is checked whether a target  38  is to be observed. If no target is to be observed, the check is repeated from time to time without further steps of the methods described here being performed. If, on the other hand, a target is to be observed, then in the first step  400  a distance of a target  38  to be illuminated by the illumination laser  14  and/or  62  to the illumination laser  14  and/or  62  is measured. 
     The measurement is preferably carried out with a distance measuring device  34  different from the illumination laser  14  and  62 . This is, for example, a radar device. 
     In a second step  402 , parameters of the illumination laser  14  and/or  62  are determined as a function of the measured distance. 
     In a third step  404 , the illumination laser  14  and/or  62  operated with the specific parameters emits at least one illumination laser radiation pulse directed onto the target  38 . Based on the parameters of the illumination laser  14  and the first aperture  16 , the NOHD value for this illumination laser radiation pulse can be determined with pulse energy E_pulse, or E_0. 
     In a fourth step  406 , it is checked whether laser radiation of the emitted laser radiation pulse is reflected to the optical sensor  30  or to another detector of the illumination laser  14  and/or  62 . 
     If this is the case, then in a fifth step  408  the distance of the object that reflected the laser pulse is determined as a function of the reflected and detected illumination laser radiation  42 . Alternatively or additionally, the reflection is checked with an optical sensor  30 , which may be realized as a camera, as to whether it is imaging a human face. 
     In a sixth step  410 , the distance of the object from the illumination laser  14  and/or  62  is compared with the distance of the target  38  from the illumination laser  14  and/or  62 . If the distance of the object is less than the distance of the target, then in a seventh step  412  the parameters of the illumination laser  14  and/or  62  are modified such that an exposure limit value EGW at the location of the object is not exceeded by the laser radiation of the illumination laser  14  and/or  62  impinging there. The same applies to the case in which the reflection images a human face. The reflection with the smallest distance defines the permissible value for the NOHD for the average power P_average. 
     In one embodiment, the modification takes place in that an average intensity of the laser radiation of the illumination laser is modified. 
     This can be achieved, for example, by modifying the laser radiation impinging on the location of the object by reducing a repetition rate of the illumination laser. 
     For a given pulse energy, the maximum permissible repetition rate for the illumination laser can now be determined according to the “Technical Rules for the Occupational Health and Safety Ordinance on Artificial Optical Radiation (TROS laser radiation)” (P_average=E_pulse*f_rep. Subsequently, the method returns to the third step  404 . 
     In contrast, if it is determined in the sixth step  410  that no object is closer to the illumination laser  14  and/or  62  than the target  38 , the method returns to the third step  404  without modifying the illumination laser parameters. 
     If no reflection and thus no target is detected in the fourth step  406 , the method returns to the first step  400 , in which it is checked whether a target  38  is to be observed.