Abstract:
An optical power limiter is designed for use with high gain optical systems by providing intensity reduction by the use of a non-linear fluid. As the intensity of light increases in the fluid, the index of refraction becomes non-linear very rapidly. This results in a plasma being formed within the fluid. Picosecond reaction time is provided. Recovery time of less than a microsecond is required for the system to return to its starting point.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention is for high gain optical systems designed for use with low level light sources. In particular, the system is designed for broadband application with picosecond reaction time. This rapid reaction is due to non-linear optical processes dependent on the electric field intensity. 
   2. Description of the Prior Art 
   The requirement to limit light intensity for a given application, such as photography, is well known. In general, mechanical or electro-optical shutters and similar light limiting devices are used to prevent optical burn-out of detectors through overexposure of light intensity. The fastest known reaction times are in the order of nanosecond reaction times. Typical modern reaction systems can be destroyed in picoseconds with rapid surges in optical intensity. Such reaction time is of major significance when lasers now provide means for intensity bursts which may themselves last only a few picoseconds. 
   SUMMARY OF THE INVENTION 
   The device provides a reusable limiter capable of limiting optical light intensity within picoseconds of the intensity&#39;s initiation. Unusually high bursts of intensity are thus prevented from causing damage to sensitive detectors. Sensitive detectors include the human eye. Protection is provided by the use of an optical cell containing a non-linear fluid. Incoming light is focused by means of a lens or other optical system so that a focal waist is formed within the fluid. 
   At low intensities, the index of refraction is not altered appreciably by the incoming radiation, which passes through the cell unattenuated. For sufficiently high intensities, the index of refraction is altered in a non-linear fashion that causes the radiation to be self-focused to a still smaller waist. This further increases the intensity, which in turn increases the self-focusing, etc. This unstable process continues until the intensity is sufficient to cause optical breakdown of the fluid. At this point, a plasma is formed which effectively attenuates the radiation. Upon reduction of the intensity, the index of refraction decays rapidly to its original value. The transmission of the cell returns to normal with a recovery time determined by the hydrodynamic properties of the fluid, typically less than a microsecond. This contrasts to the rise time of the plasma, which can occur in a few picoseconds. 
   Light whose intensity has been limited in this manner is then passed through further desired optical systems. These systems may include a recollimating lens or a bandpass filter with a specific spectral line being observed. Light passing through such a lens or bandpass filter is then incident on an optical detector such as a photodiode, TV camera, human eye, or other device. 
   Accordingly, it is an object of the present invention to provide a high gain optical power limiter with picosecond reaction time to protect sensitive optical detection devices. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a ray diagram of the present invention including block illustrations of the stages of the present invention; and 
       FIG. 2  is a graph of output power versus input power for the present invention&#39;s non-linear fluid cell. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   In  FIG. 1  the present invention takes an input light beam  10  which can be either monochromatic light, such as a laser system, or broadband background light typically observed in astronomical uses. The light passes through a first lens  12  where it is focused to a focal waist  14  within a liquid cell  16 . Within liquid cell  16 , a non-linear fluid  18  is laced. Non-linear fluid  18  may be any fluid whose index of refraction is dependent in a non-linear fashion on the electric field intensity within the fluid. 
   Acceptable working non-linear fluids have been found to be pure carbon disulfide, CS 2 , or a mixture of carbon disulfide and ethanol. These fluids follow the general equation
 
 N=N+N   0   +N   2   E   2  
 
where
         N=the index of refraction at a given location and point in time within liquid cell  16 ,   N 0  represents the index of refraction of the non-linear fluid without an electric field present,   N 2  represents a minimum constant with any electric field present, and   E 2  represents the intensity of the electric field within liquid cell  16 .
 
This equation shows that for the focal waist  14  of light beam  10  in liquid cell  16 , as the intensity increases, the index of refraction becomes quadratically greater. For sufficiently high intensities, catastrophic self-focusing occurs. At this point, a plasma is formed in focal waist region  14  of liquid cell  16  such that most of the light intensity is absorbed by the plasma. Only a small amount of the present intensity passes through focal waist region  14  and exits from liquid cell  16 .
       

   The exiting light can then be refocused by lens  20  which serves to direct the light along a desired optical path. This light can then be observed by a photodetector  22 , such as a silicon photodiode and measured. In  FIG. 1 , detector  22  is shown used in combination with a pinhole screen  24  and a bandpass filter  26 . Both pinhole  24  and bandpass filter  26 , such as a 1.06 μm filter, merely represent improvements in detector performance for a specific application. For example, in laser rangefinder systems, a specific laser wavelength will be monitored. Information about the target is provided by the arrival time of light received. This light can be either reflected light or emitted light. For emitted light, there is a definite destructive threshold if the laser beam looks directly down the system. Detector burn-out would rapidly occur. In the present invention, such burn-out can be avoided despite bandpass filter  26  allowing the desired laser wavelength to reach detector  22 . A well-focused image via pinhole  24  can be monitored on detector  22  without fear of detector burn-out. 
   The transmission of energy through liquid cell  16  is approximately linear at low levels of intensity. It is only as the intensity increases that the quadratic effect causes significant departure from linear performance. Because of the instantaneous reaction between light and focal waist region  14  of non-linear fluid  18 , picosecond reaction time is available. Laser pulses of less than 40 picoseconds have been limited effectively. Recovery time of non-linear fluid  18  for carbon disulfide is less than one microsecond. Without band-pass filter  26 , the limiter reacts over a broadband because it responds to the electric field, E, rather than wavelength or frequency. 
   The reaction time is faster than has ever been achieved before and thus permits the limiter to function against intensity pulses which may themselves last only a few pico-seconds. The recovery of non-linear fluid  18  permits the limiter to be reusable. 
   A specific cut-off level for intensity reaction can be determined by varied concentration of carbon disulfide, CS 2 , and ethanol. Different combinations will permit different cells to provide different response curves. The limiter is of relatively small size and thus can be retrofitted into existing optical systems while providing the cost efficiencies not possible in electric automatic gain circuits. 
     FIG. 2  is a graph of output power from liquid cell  16  versus input power to liquid cell  16  for a liquid cell with only CS 2  as the working fluid. Arrow  30  represents the critical power threshold at which a plasma forms within the working fluid. For all input power levels greater than the one represented by arrow  30 , a plasma will occur in focal waist region  14 . Line  32  represents the power low intensity limit for linear behavior. As shown by line  32 , without a change in the index of refraction, the output power can otherwise undergo rapid growth and overwhelm the system. Dashed line  34  represents the high intensity limit cut-off of the present invention. As shown by plotted points  36 , as input power increases, the matching output power deviates from line  32  and follows cut-off limit line  34 . For most applications, pure Cs 2  provides the lowest cut-off power. The cut-off power may be increased to any value desired by admixing various amounts of ethanol. 
   It is obvious to those skilled in the art that numerous modifications on the above invention may be made.