Many new generation lasers, having a spectrum which is out of the sensitivity  response of typical camera detectors require different technologies to be examined. Tomographic knife-edge technology offers a feasible solution. Multiple  knife-edges scanning from different directions provide beam profiles dependent on scanning angle. By reconstruction techniques used in tomography,  an image-like profile can be reconstructed and analyzed. Various detectors  are used to provide a wide spectral range measurement capability.

 Originally, Wilhelm Röntgen discovered X-rays and enabled insights of  the human body. In October 1963,  Oldendorf received a US patent for a  “radiant energy apparatus for investigating selected areas of interior objects  obscured by dense material”. But the  story did not stop there. In 1972, Godfrey Hounsfield found  a solution when he invented computerized tomography (CT) scans, thereby  a technology for creating an image of  sectors of the human body was created by using a rotating ring to project X-rays  at different angles and reconstructing  the inner image of the human body.  Based on this original advancement  in tomography, an inspiration for processing profiles of lasers perpendicular  to the propagation axis was developed  by Duma Optronics. This technology –  computerized knife-edge tomography  (CKET) generates profiles by mechanically scanning across the beam. The  scanning is performed in an orthogonal  direction to beam’s propagation, thusshowing the profile at one location along  the propagation axis.  Similar to CT scanners, which use a  rotating ring to take images from different angles, the CKET generates different profiles from multiple knife-edges  slanted at numerous angles relative to  the center of the beam. A computer then  combines these profiles to create a 3D  image of the scanned laser location.  This creates image-like footprints of the  laser power distribution at the specific  scanned location and allows reconstruction of the beam’s power distribution  along this area.  The described instrument will generate the profile by using a single detector, sensitive to the appropriate wavelength. The mechanical scanner will  obstruct the light striking the detector  as a function of its rotational speed. The  amount of light reaching the detector is  modulated by mechanical means while  adequate data processing measures the  laser beam profile. Our company developed an enabling mechanical scanning  technology, presenting a 3D reconstruction of the beam based upon a combination of computer and knife edge  tomography.  The result is an instrument capable of 3D measuring of minute beams  (down to one micron) and currently up  to 10 mm, without any optical magnification or reduction. The outlines of this  technology create clear power distribution along the laser propagation axis  at wavelengths where mosaic images  devices are not available. For example,  a new 10 mm single detector size of  indium gallium arsenide enables measurement of small and large beams at a  wavelengths range starting from 600 nm  up to 2.7 µm in special cases. Sub-micron measuring resolution is achieved  by this technology.

Fig. 1 Multiple scanning knife, each generating a different profile (Source: Duma  Optronics)

Beam profiling and beam measurement importance 

A laser propagating through space has  a different width and spatial intensity  distribution along its propagation path  continuously changing as a function of  its laser cavity, divergence, interaction  with optical elements and electronics  driver characteristics. Beam profile  intensity distribution is an important  parameter that indicates how a laser  beam will behave in an application and  will dictate the overall system performance in a specific setup. Although  existing theory accurately predicts laser  propagation in a real-world involving  engineering specification, it is crucial  for researchers, system designers, and  laser manufacturers to be able to measure accurately these parameters. ISO  standard 11146 defines approaches to  be used in measuring such beams.

Beam profile definition 

Laser beam profile in a perpendicular  direction to its propagation axis is not  defined, and in theory extends to infinity. The commonly used definition of  beam width is the width at which the  beam intensity is 1/e2 (13.5 %) of its  peak value. This value is derived from  the propagation of a Gaussian beam and  accurately describes the beams distribution for lasers operating in the fundamental TEM00 mode. However, many lasers are close  enough to a Gaussian approximation  and applying this simple definition is  a common practice in the industry.  Another more accurate definition is  found as well in the IS011146 standard  which specifies the beam width at the  second moment. The point of the second moment is a value that is calculated  from the raw intensity data and it is very  sensitive to measurement noise as well  as to random laser noise. A third way is calculated from the beams integral and  it is free of noise problems and known  as the knife edge method. Fig. 2 describes a beam profile at a  certain position at a cross-section perpendicular to the propagation axis. Fig. 3 describes the propagation in space of a  laser beam and the embedded Gaussian  concept.

Fig. 2 Beam profile definition (Source: Duma Optronics)

Fig. 3 Embedded Gaussian concept (Source: Duma Optronics)

Measuring beam width technologies 

There are four main types of beam-profiling measurement instruments, and a  proprietary technology introduced by  Duma Optronics completes the lineup  by introducing a technology that combines between knife edge technology  and a camera-based system. The basic  four are: camera-based systems, knife  edge scanners, slit scanners, and pinhole  scanners. Each has specific advantages  and disadvantages. Different measurement techniques may result in slightly  different results. As rule of thumb measuring pulsed laser is best when performed with CCD beam profilers which  offer superior performance when compared with CMOS technology, on the  other hand measuring cw lasers is best  using knife edge technologies especially  tomographic reconstruction as offered  by Duma.

Scanning knife edge in-depth 

Knife edge profilers use an aperture  large enough to pass the entire beam.  The aperture has one sharp, straight  edge (knife edge). As the aperture traverses the beam, the system measures  the portion of the beam that is not  blocked by the blade (see Fig. 1) and plots  the differential (rate of change in intensity) vs. position of the power through  the aperture. As the knife edge passes  through the beam the system approximately calculates the beam size and a  sophisticated electronic circuit samples  across the beam 12,000 times per sweep,  to be further processed to yield over  1,000 useful points per profile regardless of beam size. Very small beams in  the micron region are sampled with  lower resolution. This auto zooming  procedure offers highest possible accuracy independent of beam size. This is  advantageous when compared to a slit  or pinhole scans: The beam intensity is  not limited by the size of the pinhole or  slit; resolution is not limited by the size  of the aperture, allowing beams of a few  microns in diameter to be measured.  Moreover, accurate power measurement  is also provided since at some point the  full unobstructed power incidents the  detector surface. A special power scope  function will analyze power stability at  high bandwidth of a few megahertz and  will display in real time calibrated power  values. The scanning technology is advantageous for its wide dynamic range of  beams from less than 3 µm up to 9 mm.  Using special sensitive detectors, it can  measure beams up to a wavelength of  2.8 µm.

Using CKET technology for M2 measurements We would like to focus our point of  interest on CKET technologies, which  offer a broader wavelengths sensitivity compared to camera systems, and  are responsive from deep UV to far IR.  Furthermore, due to their flexibility in  direct measurement of micron beams up  to large beams, they are more suitable to  implement into M2 systems where there  is a need to measure the laser beam along  the propagation axis, scanning various  beam sizes. The CKET solution is free of  pixelization or pixel size, which usually  limits the performance of camera-based  systems.  The beam propagation factor M2 is a  common single parameter that characterizes the whole beam as it propagates  through space. According to ISO standard  11146, this parameter could be defined  by several measurement techniques based  on beam profiling along several points  of the propagating beam. The standard  defines several measurement techniques,  all of which are based on beam profiling  measurements using devices such as cameras, knife edge and slits.Multiple instruments have been  developed to measure the propagation  factor and M2 on production lines and  in the laboratory. There are two main  measurement requirements: One is the  measurement of focused beams, and the  second is measurement of collimated  laser beams. For the latter, the fundamental operating principle is the focusing of the incoming laser beam by a lens,  creating a waist position and a divergent beam on its two opposite sides. By  scanning and calculating the beam waist  W0 at a focal point and the divergence  in a far region many times the Raleigh  range, one can determine an accurate  value for M2 measurement. For focused  beams, the principle is similar and the  focal point of the system is scanned  along with its divergence angle to create the focal beam characteristics such  as beam size, depth of focus, M2, etc. A  popular technique for M2 measurement  is to scan the beam along its propagation  axis including the waist position area  by a mechanical linear motor and measuring the beam distribution at several  locations by a beam profiler or by the  CKET technology. A dedicated software  will reconstruct the beam propagation  and its M2 value.  Frequently on certain applications,  especially in high power ones, the M2 term is replaced by the beam parameter  product (BPP) i.e. the product of beam  radius at the beam waist and the far field  beam divergence angle. The M2 factor, as  will be shown on the following formulas,  will also include the wavelength. The  best possible beam quality is a diffraction-limited Gaussian beam having an  M2 equal to 1. M2 = θ · π · W0 _______ λ wherein: BPP = θ · W0.M2 could be derived from the second  formula by dividing the BPP by that  of an ideal Gaussian beam at the same  wavelength. An intuitive and user-friendly software will not only calculate the tomographic features along the scanning  aperture, but will also provide full 3D  reconstruction of laser beam propagation through space.  A typical measurement cycle, as  shown by Fig. 4, will display W and V  directions along the propagation axis,  as well as 2D/3D reconstruction. Moreover, the software will continuously  show the folding optics location in real  time. A full set of accessories will enable  measurement of low and high power as  well as various wavelengths up to 2.7 µm.

Fig. 4 2D/3D M2 reconstruction of real measurement (Source: Duma Optronics)

Conclusions 

In conclusion, adapting this technology to well-known knife-edge beam  profiling enables laser beam profiling  characterization perpendicular to the  propagation axis, providing an imagelike laser footprint with adaptable resolution from less than a micron to large  beams. Moreover, this technology creates image-like beam profiling over a  wide spectral range and special beam  sampling accessories will allow measurements of high power lasers as well.

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