Optics: measuring and testing – Lamp beam direction or pattern
Reexamination Certificate
1999-09-10
2001-11-06
Font, Frank G. (Department: 2877)
Optics: measuring and testing
Lamp beam direction or pattern
Reexamination Certificate
active
06313910
ABSTRACT:
1 BACKGROUND OF INVENTION
This invention relates to an apparatus for measurement of optical beams, specifically to an apparatus for the real-time measurement of optical beam intensity profile, diameter and centroid in single or multiple planes; for the measurement of beam divergence and axial angular deviation; and for the determination of the beamwaist position location, and M-squared value for focused optical beams
2 DESCRIPTION OF THE INVENTION
2.1 Problem Addressed
This invention addresses measurement of optical beams. It enables real-time measurement of optical beam parameters for comparison with specifications. Measurable beam parameters are:
a) Optical beam intensity profile, diameter and centroid. From measurement of these parameters in multiple planes separated in the beam propagation direction, the following may be derived:
b) The direction, x-y-z centroid location and diameter of a focused waist along the z-axis, derived from a fit of the beam measurements in multiple planes to standard beam propagation formulae.
c) Beam angular divergence, derived from the variation of diameter with distance along the beam propagation axis.
d) Beam angular deviation from the nominal beam axis, from the measurement apparatus axis, or from a defined axis, derived from the locus of the beam centroids.
e) Beam M
2
(pronounced “M-Squared”), a quality parameter for focused laser beams, derived from a fit of the beam measurements in multiple planes to standard beam propagation formulae. M
2
is a measure of how closely a laser beam approaches the ideal TEM
00
Gaussian propagation profile for a single-mode laser beam. Beams that are closer to the ideal may be focused to a smaller spot.
FIG. 1
a
illustrates a propagating optical beam in the focus (beamwaist) region and illustrates the conventional x-y-z orthogonal axes, the z axis being the beam propagation axis. In practice, the z axis may be the actual beam propagation axis, the nominal beam propagation axis, the measurement apparatus axis, or another defined axis).
FIG. 1
b
illustrates a simple diverging beam for which the axial angular deviation and angular divergence is to be measured.
Addendum 1 addresses the definition and derivation of beam diameter, beam centroid, beam divergence and M
2
from measurements of beam intensity versus position.
This invention may be employed for real-time measurement and hence adjustment of coherent and incoherent optical beams. An exception is beam M
2
measurement, which only applies to coherent beams.
This invention comprises a generic and a specific approach which allows the measurement of the listed parameters. A specific important embodiment allows real-time operation, defined here as an update rate greater than 1 Hz. That is an update rate fast enough to allow real-time positional, angular and/or beam shape adjustment of the optical and/or fiber optic beam assemblies and instruments from which the optical beams emanate.
In the manufacture and adjustment of single or multiple optical and/or fiber optic beam assemblies that must give optical beams with specific values of parameters selected from the list above, this apparatus can assist the real-time adjustment of said assemblies to give beams of the desired specifications. If a beam cannot be adjusted to the desired specifications, then the measurement of one or more of the parameters listed above, can be an important diagnostic for correction of the problem(s).
Optical is defined here as the region of the electromagnetic spectrum between the deep UV around 10 nanometers and the far infrared around 100 microns.
Fiber optic assemblies include but are not limited to: single-mode fibers, multi-mode fibers, fiber lasers, and/or coherent or incoherent fiber bundles, and assemblies employing such fibers, which either emit or accept optical beams.
Optical assemblies include but are not limited to: coherent laser sources (Including but not limited to: single chip semiconductor lasers, gas lasers, pumped solid state lasers, any of the multiple means of generating a laser beam), incoherent sources (Including but not limited to: arc lamps, discharge lamps, filament lamps, LEDs, solid state emitters, discharge lamps, etc.), complex assemblies or instruments which optically shape the output from one or more of these sources in order to create single or multiple beams shaped and/or focused for a particular purpose.
2.2 Prior Art
2.2.1 Single Plane Measurement
A variety of measurement techniques are available that determine laser beam diameter from measurements of beam intensity versus position. An apparatus for measuring beam diameter 2W(z) in an x-y plane at position z, typically consists of either:
a) Some form of photosensitive camera onto which the beam falls. or:
b) Some form of aperture, typically a slit or knife-edge, which moves, or can be resized, orthogonal to the beam propagation axis (conventionally termed the z-axis) and is interposed between the beam and a photosensitive detector and changes the portion of the beam falling upon the photosensitive detector.
The signal output from the detector is analyzed in order to determine beam intensity profile I(x,y) versus x,y position in the plane of measurement. The beam diameter and centroid position is then determined from I(x,y). See Addendum 1 for definitions.
The basic techniques employ an apertured photosensitive detector from which an electrical signal proportional to the received intensity may be derived. As outlined in the ISO 11146 standard (Ref. 1), and illustrated in
FIG. 2
, these are:
1) Scanning a slit or a pinhole across the profile in the x-y plane, directly measures the profile. As the slit or pinhole aperture scans across the intensity profile, the intensity transmitted through the aperture and measured by the photosensitive detector is a measure of the intensity profile in the scan direction, and is passed to processing electronics and software. The slit width should be less than ten times the beam diameter; if this is not the case mathematical deconvolution of the slit width from the measured profile, or use of an empirical correction formula based on the ratio of slit width to measured width should be employed. See, e.g., Ref. 5.
2) Scanning a knife-edge across the profile in the x-y plane, measures the integral intensity profile. As the knife-edge aperture scans across the intensity profile, the intensity transmitted past the knife-edge and measured by the photosensitive detector is a measure of the integral of the intensity profile in the scan direction. The signal from the photosensitive detector is passed to processing electronics and software for differentiation and subsequent analysis.
It should be noted that when a high percentage of the beam profile, (preferably, but not necessarily, close to 100%), passes through a slit, then the slit can be treated as a pair of opposed knife edges and the scan profile is then the integral of the intensity profile followed by the reverse integral of the intensity profile. This approach is well-known, and may be employed in the apparatus described in this patent.
3) Measurement of total energy passed by a series of circular apertures of different diameters placed in the same x-y plane at the same z. The apertures are sequentially placed between the beam and the detector, each aperture being centered on the beam in a manner to maximize the transmitted signal. The total energy passing through each aperture is then a measure of the integral energy passing through that aperture diameter. The difference between the energy transmitted by sequential apertures is a measure of the differential radial intensity. The radial intensity profile may therefore be reconstructed as the differential of the measured transmitted intensity versus aperture radius. A variation on this technique employs a variable diameter iris aperture.
4) Beam imaging onto an array of detectors in the x-y plane, e.g. a camera, each pixel or resolution element of which constitutes a staring apertured detector. The intensity measured by each pixel is a measure of the I(x,y) inte
Garvey Steven E.
MacGregor Andrew D.
Dataray, Inc.
Font Frank G.
John Smith-Hill, Smith-Hill and Bedell
Stafira Michael P.
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