Abstract:
A method and apparatus for propagating a laser beam. The laser beam pulse is passed through a first lens which focuses the laser beam pulse at a focal point of the first lens. An electronegative gas at substantially atmospheric pressure is configured to surround the focal point in order to suppress an ionization effect by the laser beam pulse at the focal point.

Description:
BACKGROUND 
     The present disclosure relates generally to laser beam propagation and, more particularly, to methods and apparatus for preventing air ionization at regions of high laser beam intensity. 
     Laser beam pulses may be used in various industrial and military applications. For example, laser beam pulses can be reflected off of a selected target in order to determine a distance to the target. Devices for generating and recording such laser beam pulses generally include a lens system that changes the width of the beam. The laser beam pulse generally passes through a focal point of the lens system. As the laser beam pulse passes through the focal point, the energy density of the laser beam pulse can become high enough to ionize air at the focal point, which can impair the laser beam for its intended use. Therefore, there is a need for preventing air ionization resulting from high energy densities in laser beam pulse propagation devices. 
     SUMMARY 
     According to one embodiment, an apparatus for propagating a laser beam includes: a first lens for converging the laser beam to a focal point of the first lens; and an electronegative gas located at the focal point to suppress a photoionization effect at the focal point, wherein the pressure of the electronegative gas is substantially one atmosphere. 
     According to another embodiment, a method of propagating a laser beam includes: passing the laser beam through a first lens to focus the laser beam at a focal point of the first lens; and surrounding the focal point with an electronegative gas at a substantially atmospheric pressure to suppress an ionization effect by the laser beam at the focal point. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of this disclosure, reference is now made to the following brief description, taken in connection with the accompanying drawings and detailed description, wherein like reference numerals represent like parts: 
         FIG. 1  illustrates as exemplary embodiment of an apparatus for propagating a laser beam pulse; 
         FIG. 2  illustrates an alternate embodiment of an apparatus for propagating a laser beam pulse; and 
         FIG. 3  shows an exemplary optical cell that may be used with the exemplary laser beam propagation device of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION 
     A detailed description of one or more embodiments of the disclosed apparatus and method is presented herein by way of exemplification and not limitation with reference to the Figures. 
       FIG. 1  shows an exemplary embodiment of a laser beam propagation device  100  of the present disclosure. The exemplary device  100  includes a laser  120 , various optical elements, such as lenses  102 ,  104  and  108  and beam splitter  106 , and an optical cell containing an electronegative gas to inhibit air ionization by the laser beam. The exemplary laser may be for example a Nd:YAG laser or any other suitable laser for providing a laser beam pulse. The laser  120  generates a laser beam pulse  122  that propagates towards beam splitter  106 . The beam splitter  106  directs a portion of the laser beam  122  through a lens system that includes a first lens  102  and a second lens  104 . A portion of the laser beam  122  may also be directed through lens  108  to be recorded at a recording device  110  for recording a parameter of the laser beam. The recording device  110  may obtain an image of the target or record a time-of-arrival of the laser beam pulse. In various embodiments, the recording device may be a camera, a monitor or television display for displaying an optical image, or an infrared imaging device such as a near infrared (NIR) imaging device, a shortwave infrared (SWIR) imaging device, a mid-wave infrared (MWIR) imaging device, a long wave infrared (LWIR) imaging device, or a far infrared (FIR) imaging device, among others. 
     The first lens  102  and the second lens  104  may be used to change a width of the laser beam pulse. The laser beam pulse  122  propagates through a focal point  114  of the first lens  102 . As the laser beam pulse  122  converges at the focal point  114 , the energy density per unit volume of the laser beam pulse  122  increases. When air is located at the focal point  114 , the energy density of the laser beam pulse at the focal point  114  may exceed an ionization potential of the air. Air ionization impairs the laser beam pulse  122  for its intended use. Therefore, the device  100  includes an optical cell  116  centered at the focal point  114  that includes an electronegative gas  118  at approximately one atmosphere. The electronegative gas  118  in general has a higher ionization potential than air. Thus, the electronegative gas  118  is resistant to ionization by the laser beam pulse at the focal point  114 . 
     In the exemplary laser beam propagation device  100 , the optical cell  116  contains electronegative gas  118  that is held at substantially atmospheric pressure, i.e., one atmosphere. Exemplary electronegative gases may include, for example, sulfur hexafluoride. Sulfur hexafluoride is an inert, non-toxic, non-flammable gas that provides no light absorption within the range of visible light through the medium wavelength infrared. Sulfur hexafluoride also has a low leak rate due to its high molecular weight. Alternate electronegative gases that may be used include chlorfluorocarbons (i.e., Freon), such as such as Dichlorodifluoromethane (R-12), 1,2-Dichlorotetrafluoroethane (R-114), 1,1,1,2-Tetrafluoroethane (R-134a), Octafluorocyclobutane (R-C318), and Perfluorobutane (R-3-1-10), for example. Since the pressure of the electronegative gas is about  1  atmosphere, the optical cell may be made of thin material and/or flexible material without concern for withstanding pressure differentials across the walls of the optical cell. In contrast, prior art devices include gases held at high pressures or a vacuum, thereby requiring thick optical cell walls to withstand high pressure differentials as well as additional equipment such as pressure pumps and various electronic circuits. The exemplary optical cell  116  may further include a flexible wall or a bellows that may expand and/or contract in order to maintain the internal pressure in the optical cell at substantially one atmosphere. In an exemplary use of exemplary laser beam propagation device  100 , the laser beam that exits the second lens  104  is reflected off of a selected target. The reflected beam returns via the second lens  104 , passes through the optical cell  116  and focal point  114  and through first lens  102 . A portion of the returning beam the passes through the beam splitter  106  and third lens  108  to be recorded at the recording device. The recording device may record a time-of-arrival of the original laser beam pulse and the reflected laser beam pulse and use time difference in the times-of-arrival to determine a distance to the selected target. 
       FIG. 2  shows an alternate embodiment of a laser beam propagation device  200  of the present disclosure. The alternate device  200  includes an exemplary laser  120  that generates a laser beam pulse  122  that is directed towards beam splitter  106 . The beam splitter  106  directs a portion the laser beam pulse through a lens system that includes a first lens  102  and a second lens  104 . A portion of the beam may also be directed through lens  108  which focuses the portion of the beam at the exemplary recording device  110 . The first lens  102  and second lens  104  are contained within exemplary optical cell  216   a  and may be held in place within the optical cell  216   a  using various holding devices (not shown). In another embodiment, alternate device  200  includes an optical cell  216   b  that contains first lens  102 , second lens  104 , beam splitter  106  and third lens  108 . In additional alternate embodiments, any number of optical elements may be included in the optical cell, including any or the first lens  102 , second lens  104 , beam splitter  106 , third lens  108 , the laser  120  and recording device  110 , among others. 
       FIG. 3  shows an exemplary optical cell  216  that may be used with the exemplary laser beam propagation device of  FIG. 2 . The exemplary optical cell  216  may include a cover  302  such as a hinged lid, sliding cover or other suitable element opening the optical cell and closing and sealing the optical cell. The exemplary optical cell  216   a  may be opened to allow replacing, cleaning and/or adjusting the first lens  102  and the second lens  104 , for example. The optical cell  216  may then be closed and sealed to prevent gas transfer with the outside environment. Once sealed, electronegative gas may be introduced into the optical cell  216 . The exemplary optical cell  216  may provide a port for a gas source  304  to provide the electronegative gas. The gas source  304  is coupled to the optical cell  216  and fills the optical cell  216  with the electronegative gas once the optical cell  216  is sealed. The electronegative gas may be used to prevent air ionization or to purge the optical cell, in various embodiments. The gas source may  304  may be a detachable source or container such as a gas bottle. In one embodiment, a flow element  306  may control the flow of the electronegative gas from the gas source into the optical cell. The exemplary optical cell  216  may further include a flexible wall or a bellows  308  that may expand and/or contract in order to control or to equalize a pressure of the gas within the optical cell as the gas is introduced, thereby maintaining a low pressure differential across the optical cell walls. Since a low pressure differential is across the optical cells walls, the optical cell walls can be thin, thereby providing little optical interference with the laser beam pulse at the cell wall. In addition, pressure pumps are not needed, thereby cutting down on the weight of the devices discussed herein. Optical cells enable construction of lightweight, portable devices that are easy to repair and/or replace. 
     Elements of the embodiments have been introduced with either the articles “a” or “an.” The articles are intended to mean that there are one or more of the elements. The terms “including” and “having” are intended to be inclusive such that there may be additional elements other than the elements listed. The conjunction “or” when used with a list or string of at least two terms is intended to mean any term or combination of terms. The term “secure” relates to one component being coupled either directly to another component or indirectly to the another component via one or more intermediate components. 
     While the disclosure has been described with reference to a preferred embodiment or embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the disclosure. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the disclosure without departing from the essential scope thereof Therefore, it is intended that the disclosure not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this disclosure, but that the disclosure will include all embodiments falling within the scope of the appended claims.