Abstract:
A device and method for monitoring a light beam (e.g., laser beam) without intruding upon the light beam is disclosed. The device comprises a focusing lens which receives scattered light from a reflective or transmitting optical element and directs the scattered light to a photo detector. The photo detector generates one or more electrical signals which are proportional to an intensity distribution of the light beam for analysis by a processor. The analyzed electrical signals are then used to adjust a light beam source accordingly. The device may further comprise a display and printer for visually depicting the light beam characteristics.

Description:
FIELD OF INVENTION 
   The embodiments of the present invention relate to a non-intrusive device and method to subjectively and quantifiably measure and monitor the characteristics and parameters of a laser beam or beam of light wherein the parameters include the radiant energy power in the beam and/or the spatial energy power intensity distribution, which is typically referred to as the beam profile or beam intensity profile. The device and method may also incorporate a self-adjusting feature to automatically increase or decrease beam energy power and/or change or modify the beam intensity profile through closed loop electronic means. 
   BACKGROUND 
   Devices that monitor or measure energy, power or the spatial intensity profile of a light beam, such as a laser beam, have in the past been intrusive such that they intrude upon, and interrupt, the beam path in some fashion thereby extracting a portion of the energy power from the beam. In many cases the entire beam must be directed into a beam energy power meter or spatial energy intensity profile camera. 
   In high volume production manufacturing facilities using high power laser beams for materials processing applications, including laser welding, laser cutting, laser heat treating and laser free forming, laser power is routinely and continuously monitored. However, by intrusively dissecting a main primary laser beam and directing the dissected portion into a power meter, the routine and continuous monitoring results in a portion of the usable laser power being sacrificed. In these facilities the beam spatial intensity profile is only monitored or checked infrequently, and not on a continuous basis, because this procedure requires a costly interruption or stoppage of production of the company&#39;s products. 
   Because the quality of laser processed products is highly dependent on laser beam power and precise beam profile control it is desirable to non-intrusively and continuously monitor laser beam parameters while maintaining an ability to immediately make changes or compensations when the parameters unacceptably fluctuate or change over time. The embodiments of the present invention solve the aforementioned problems by non-intrusively monitoring only the unused small portion of scattered light or laser light that is conventionally lost off the surface of an optical lens or mirror that is typically required to reside in the beam path to accomplish the intended laser process application. 
   As described below, the patent literature includes numerous examples of light monitoring devices. 
   U.S. Pat. No. 4,842,404 discloses a system for monitoring the energy power in a light beam resulting in the least amount of intrusion into the light beam as possible. However, even with the system disclosed, a “beam splitter” optic is placed into the primary beam path to reflect a substantial portion of the beam into an orthogonal axis for directing it into an energy power meter while a majority of the primary beam is allowed to pass through. 
   U.S. Pat. No. 5,523,837 describes an “essentially non-intrusive laser power meter.” However, the invention utilizes a “beam splitter” that is intrusively placed into the beam path splitting off 1% to 5% of the beam which is then directed into a silicon photodiode power meter. 
   U.S. Pat. No. 4,792,690 discloses a laser beam monitoring device utilizing a transmitting crystal optic intrusively placed into the beam path that produces a color pattern that corresponds to the laser beam intensity distribution profile. U.S. Pat. No. 6,396,062 discloses a laser beam monitor that requires placement of a fluorescent screen into the primary laser beam path such that the screen is monitored by a camera wherein the images captured by the camera may then be displayed on a computer screen. 
   U.S. Pat. No. 4,745,280 discloses a laser beam profiling apparatus that requires intrusive attenuation of the beam accompanied by a device that scans the attenuated beam across a photo sensor which produces an electric signal proportional to the beam intensity so as to permit adjustment of the oscillation mode of the laser beam to achieve an efficient beam. U.S. Pat. No. 4,764,655 discloses a device to monitor the size of the focused laser beam and to perform a closed loop adjustment through a robot when necessary. 
   Each of the above-described devices is intended to monitor or measure the power level or beam mode quality of the light beam but each device is intrusive. A possible exception is an expensive custom made diffraction grating mirror, but such mirrors typically extract noticeably usable and excessive quantities of power from the primary beam. The embodiments of the present invention comprise non-intrusive devices which do not extract more energy power from the primary beam than would typically be lost to scatter. 
   SUMMARY 
   A first embodiment of the present invention comprises a light beam energy power measurement device including a focusing lens or mirror that collects and directs stray or scattered light from a primary light source to a photon detector, photodiode, photocell, photovoltaic detector device or any other device that converts or senses photon radiation and produces a proportional output electrical signal. 
   The produced proportional output electrical signal generates a quantitative display of relative radiant energy power associated with the light beam. This electrical signal may also be used as a benchmark reference to determine if the power level of the beam is remaining constant and, through a closed loop electronic circuit, may automatically drive a device that ramps up or ramps down the light or laser beam power source as required. 
   In a second embodiment of the present invention, the stray or scattered light is focused and directed to and passes through a pin-hole spatial filter and is then collected by a second lens which focuses and directs the light to one or more photo detector elements, such as a focal plane array, digital camera, or CCD camera, where said individual detector elements are also referred to as pixels. Each pixel produces an electrical signal proportionate to the power or energy in the radiant light source that strikes it. The light source is spread over many pixels, therefore, spatial intensity variations or spatial non-uniformities in the light intensity per unit area striking the array of pixels produces different but proportionate voltages from each pixel. The voltages generate a processed signal which is analyzed by a computer processor and displayed in a two or three dimensional intensity format. The processed signal may also be used in a closed loop fashion to automatically adjust the laser or light source for compensation of spatial intensity uniformity anomalies. 
   Other features, variations and embodiments will become evident from the detailed description, drawings and claims set forth below. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates a block diagram representation of optical and electrical relationships; 
       FIGS. 2(A)-2(C)  illustrate typical two-dimensional spatial intensity profiles for various laser beams; 
       FIG. 3  illustrates a photon optical and hardware arrangement along with a primary laser light beam as it propagates to and from a mirror; 
       FIG. 4  illustrates stray scattered light propagating to a focusing lens and a photo detector; 
       FIG. 5  illustrates the relationship between the primary laser beam and the stray scattering light profile propagation to and through the focusing lens; 
       FIG. 6  illustrates a donut-mode scattered light beam profile propagation to the photo detector; and 
       FIGS. 7A and 7B  illustrate a two-dimensional (A) and three-dimensional (B) image produced by the embodiments of the present invention. 
   

   DESCRIPTION 
   Reference is now made to the figures wherein like parts are referred to by like numerals throughout.  FIG. 1  shows a block diagram of a first embodiment of a laser beam monitor generally referred to by reference numeral  100 . A laser beam is generated by laser source  10 . In a typical processing application, the laser beam is directed toward a beam bending mirror  12  that directs the beam toward a final position where the laser beam interacts with a certain material. At a process station  16  the beam may heat up or melt the material during a heat treating, welding or similar application. Because no mirror is absolutely perfect, a small portion of the generated light beam is scattered off the surface of the mirror  12  and captured by a focusing lens  18  and then directed and focused onto a photo detector  20  where the photon light energy is converted to an electrical signal sent to a computer  22 . The computer  22  processes the electric signal and produces a visual image on monitor  24 . The computer  22  can also send a signal to a printer  26  which generates hard copies of the data. 
   When the laser monitoring device is used in a closed loop fashion, a signal is automatically sent to a laser mode or power adjust control driver device  28  that ramps the power level of the laser source  10  or modifies the spatial intensity distribution profile of the laser beam by driving the adjust mechanisms of the laser source  10 . 
     FIGS. 2(A)-2(C)  illustrate cross-sectional examples of three different spatial intensity profiles found with typical laser beams. 
     FIG. 3  more specifically illustrates the light beam path of a typical primary laser beam  30  as it strikes the 90 degree bending mirror  12  and the small portion of stray light  32  is scattered off mirror  12 , captured by the lens  18 , and the rays  34  are focused and directed toward the photo detector  20 . The mirror  12  is housed by the beam transport tube  36 , which provides a wall enclosing the beam  30 , and a mirror capture cover plate  40 . The beam transport tube  36  surrounds the scattered light  32 ,  34 . 
   Historically, all optical surfaces possess some degree of roughness or imperfection due to limitations of the manufacturing processes. In some cases, the roughness or imperfections may be at the atomic or molecular level but most often it is in the form of scratches and gouges having width and depth measurements in the range of angstroms to microns. The surface defects can be randomly or regularly located depending on the manufacturing process. The surface defects produce light scattering as a light beam strikes the surface. Now referring to  FIG. 4 , with long established conventional manufacturing processes, the defects are uniform in size and shape, and uniformly dispersed and located on the optical surface, such that a small portion of the bulk of the scattered light is directionally scattered and propagates in a uniform pattern that is linearly proportional to the intensity distribution or mode pattern of the primary optical beam striking the mirror. The spatial intensity distribution striking lens  18  tracks the spatial intensity distribution of the primary beam  30 . Therefore, a custom made mirror, such as a diffraction grating, is no longer required. 
     FIG. 4  is intended to clearly illustrate the above-reference scattering phenomenon as it applies to the embodiments of the present invention. When mirror  12  is exposed to a light, a portion of the light is scattered off by surface defects. For clarity, three identical scattering defect sources  48  are illustrated in  FIG. 4 . The light photons can scatter in all directions in a hemispherical envelope  50  but a few of the light photons  52  scatter off and remain relatively parallel to each other for some distance. This group or envelope of parallel rays  52  are collected and focused by lens  18 . Rays  52 ′ converge at a focus point of the lens  18  and pass through a small hole known as a pin-hole spatial filter  54 . The spatial filter  54  blocks all extraneous and non-parallel rays  56  while other rays  58  completely pass by the lens  18 . The rays  52 ′ that pass through the spatial filter  54  are relatively free of scattered extraneous non-contributing rays  56 ,  58  which are considered background noise herein. The rays  52 ′ passing through filter  54  are then imaged onto photo detector  20  by a second lens  60 . Photo detector  20  converts the photon energy into an electrical signal that is proportional to the radiant power energy in the primary beam when a single detector element is used and the spatial intensity profile of the primary beam of light when a photonic detector array is used. 
     FIG. 5  further illustrates the relationship between the primary beam  30  and the parallel scattering rays  46  and the projected spatial profile  62  which moves through lens  64  to the detector  20 . In this configuration, a spatial filter is not used since lens  64  has a very short focal length. More specifically, the spatial filter is not necessary because extraneous non-parallel rays suffer from spherical aberration caused by lens  64  and said rays do not focus well and, therefore, fall outside the imaging envelop of the detector  20 . Thus, said extraneous rays do not substantially contribute to background noise at the true image plane on the detector  20 . Accordingly, resolution of the spatial intensity profile remains high. The primary laser beam  30  has a Gaussian profile  66  and  66 ′ (also see  FIG. 2(A) ). The Gaussian profile  66  is illustrated striking a surface of mirror  12  causing scattering  46  to take place. In conventional practice, scattering randomly changes the spatial distribution of the beam  30 . However, the spatial distribution does not change within a very small envelope considering that only a small portion of photons scatter off in non-random angle patterns but remain parallel to several adjacent photons where the several adjacent photons are sufficient in number to represent the profile  62  of the original primary beam  30 . 
   This is further illustrated in  FIG. 6  where the cross section of a donut mode laser  68  (also see  FIG. 2(B) ) and scattering profile  70  is shown with a high intensity outer shell and low intensity central section as it propagates toward the lens  64 . The only photons that preserve the original profile  70  are those that remain relatively parallel and therefore pass through the lens  64  and form the donut mode profile  74  on detector  20 .  FIGS. 7A and 7B  illustrate two actual images produced by the embodiments of the present invention.  FIG. 7A  is a two dimensional image of the spatial intensity distribution (see  FIGS. 2B and 6 ) of a 4,000 watt CO2 laser beam.  FIG. 7(B)  is a three dimensional image of the same beam. 
   Although the invention has been described in detail with reference to several embodiments, additional variations and modifications exist within the scope and spirit of the invention as described and defined in the following claims.