Optics: measuring and testing – By light interference
Reexamination Certificate
2001-06-08
2004-01-27
Glick, Edward J. (Department: 2882)
Optics: measuring and testing
By light interference
Reexamination Certificate
active
06683691
ABSTRACT:
This application claims priority to German Patent Application No. 100 28 756.6, filed Jun. 9, 2000. The entire contents of the German Patent Application are hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to methods and arrangements for spatially resolved and time-resolved characterization of ultrashort laser pulses.
2. Related Art
For laser applications in the fields of working materials, spectroscopy, medicine, sensors and measurement technology, there is a demand for reliable and compact laser sources whose radiation must be suitably shaped and brought to the area of the specific interaction. In particular in the case of collimation, focusing or homogenization of high-performance lasers, laser components and complex laser systems must be optimized or adapted and interference must be eliminated for their stabilization.
In addition to the spectral and time profiles, other relevant measured variables include the intensity and phase distributions of the wave package as a function of location, which are variable over time and have not typically been measured by previously known arrangements for time-resolved characterization of ultrashort pulsed lasers in the subpicosecond range (see L. Sarger, J. Oberlé: “How to Measure the Characteristics of Laser Pulses” in: Claude Rullière: Femtosecond Laser Pulses, Springer Verlag, Berlin 1998, 177-201).
Although autocorrelation measurements of the first order (interferometer) only provide information about the coherence length of the pulse, information about the pulse period can also be derived in good approximation from autocorrelation functions of a higher order by means of non-linear optical interactions. The autocorrelation function of the second order can be obtained with a two-photon process such as second harmonic generation (SHG).
The known interferometric measurement methods can be divided into collinear and non-collinear methods with which autocorrelation functions can be measured in chronological order on the basis of changes in optical path length in one interferometer arm, and time-integrated detection of intensity is possible for different axial positions of two superimposed component beams or simultaneously by generation of conical or conically superimposed beam bundles and time-integrated detection of intensity for different transverse positions in a single-shot method.
In the non-collinear arrangement with axial symmetrical superpositioning of planar partial waves, interference patterns with the following spatial frequency are formed perpendicular to the direction of propagation:
v
=
2
sin
(
α
/
2
)
λ
(&lgr;=central wavelength of the wave package; &agr;=angle of the component beams to the optical axis).
These characteristic interference structures known from the production of holographic gratings, for example, i.e., rings in the case of radial symmetry, are typically imaged on a CCD camera and are resolved only if allowed by the magnification of the imaging system and the size of the camera pixels.
Therefore, rings are not observed with many arrangements that use a large angle, and there is automatic averaging, which would otherwise have to be performed by data analysis.
The averaged curve of the intensity distribution corresponds to an autocorrelation function from which the pulse period can be determined by using known mathematical procedures.
The only arrangement for measurement with spatial resolution known so far uses a complicated and expensive system of a so-called SPIDER type (SPIDER=spectral phase interferometry for direct electric-field reconstruction), where a spatial resolution is achieved in only one direction in space through the width of the gap of a spectrometer arrangement, where the other axis corresponds to the spectral coordinate (L. Gallmann, D. H. Sutter, N. Matuschek, G. Steinmeyer, U. Keller, C. Iaconia, I. A. Walmsley: “Spatially Resolved Amplitude and Phase Characterization of Ultrashort Optical Pulses Using SPIDER,” CLEO 2000, San Francisco 2000, Technical Digest, Paper CFE1, 583-584).
Another known, relatively compact design uses a combination of cylindrical lenses and a Fresnel biprism, which can be regarded as a special case of an axicon, to generate two focused component beams that are superimposed (P. O'Shea, R. Trebino: “Extremely Simple Intensity-and-Phase Ultrashort-Pulse Measurement Device with No Spectrometer, Thin Crystal or Delay Line,” CLEO 2000, San Francisco 2000, Technical Digest, Paper CFE6, 587-588).
This utilizes the spatial splitting of the beam in non-linear frequency conversion in a thick SHG crystal as a function of the respective spectral component. This arrangement is wavelength-specific, i.e., it cannot be used universally for any desired wavelength ranges, and it also does not yield spatial resolution. In addition, because of their thickness and dispersion, the refractive components that are used cause a deformation of the pulses in time even in front of the non-linear optical crystal, which thus represents another disadvantage of such arrangements.
Japanese Patent 9304189 A2 also describes an arrangement in which the crystal plane of a non-linear crystal in a single-shot correlator is designed with prismatic faces, but only as a macroscopic individual element.
SUMMARY OF THE INVENTION
The object of the present invention is to develop generic methods and an arrangement with which the disadvantages of the state of the art described herein can be avoided, and with which a spatially resolved and time-resolved measurement of the intensity of ultrashort laser pulses is achieved in single-shot operation with a design that is both compact and simple and is based on a correlator technique.
The object of this invention is achieved by methods according to the features of claims
1
and
8
and by an arrangement according to the features of claim
13
.
In particular, according to a method of the present invention, for spatially resolved and time-resolved characterization of ultrashort laser pulses, a spatially resolved non-collinear measurement of the autocorrelation function of the first or higher order is performed by means of a matrix of beam shaping individual components such that local splitting of the beam into a beam matrix of conical component beams is performed, with each component beam representing the spatially integrated information over the partial face of the matrix through which it passes, the spatial resolution is thus determined by the matrix geometry, and the interference pattern produced in space by each component beam in a certain plane imaged on a matrix camera provides an autocorrelation function of the first order or a higher order. Accordingly, by using non-linear interactions in a suitable medium, the coherence time or pulse period of individual laser pulses or trains of several laser pulses can be determined as a function of the location.
According to a preferred embodiment of the present invention, individual elements of a beam shaping matrix are formed by micro-optical components such as thin-film micro-axicons which are advantageously characterized by a low dispersion and low absorption and thus a low susceptibility to destruction with respect to high powers. Depending on the embodiment, the influence of neighboring elements is to be taken into account as a function of the pulse period (change in the angular components contributing to interference due to differences in transit time with very short pulses). This influence can also be utilized to gain additional information on the pulse properties.
In another embodiment of the present invention, a suitable arrangement of highly reflective micro-axicons may be used, making it possible to prevent dispersive effects even more effectively.
In yet another embodiment of the present invention, the individual elements of the beam shaping matrix are designed so that the conical component beams form small angles to the optical axis such that the lengths of the interference zones on the respective optic
Elsaesser Thomas
Griebner Uwe
Grunwald Ruediger
Hartmann Hans-Juergen
Jueptner Werner Paul Otto
Artman Thomas R
Birch & Stewart Kolasch & Birch, LLP
Glick Edward J.
Max-Born-Institut
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