Classifications

• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B27/00—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00
  • G02B27/09—Beam shaping, e.g. changing the cross-sectional area, not otherwise provided for
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B19/00—Condensers, e.g. light collectors or similar non-imaging optics
  • G02B19/0004—Condensers, e.g. light collectors or similar non-imaging optics characterised by the optical means employed
  • G02B19/0009—Condensers, e.g. light collectors or similar non-imaging optics characterised by the optical means employed having refractive surfaces only
  • G02B19/0014—Condensers, e.g. light collectors or similar non-imaging optics characterised by the optical means employed having refractive surfaces only at least one surface having optical power
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B19/00—Condensers, e.g. light collectors or similar non-imaging optics
  • G02B19/0033—Condensers, e.g. light collectors or similar non-imaging optics characterised by the use
  • G02B19/0047—Condensers, e.g. light collectors or similar non-imaging optics characterised by the use for use with a light source
  • G02B19/0052—Condensers, e.g. light collectors or similar non-imaging optics characterised by the use for use with a light source the light source comprising a laser diode
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B27/00—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00
  • G02B27/09—Beam shaping, e.g. changing the cross-sectional area, not otherwise provided for
  • G02B27/0927—Systems for changing the beam intensity distribution, e.g. Gaussian to top-hat
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B27/00—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00
  • G02B27/09—Beam shaping, e.g. changing the cross-sectional area, not otherwise provided for
  • G02B27/0938—Using specific optical elements
  • G02B27/0944—Diffractive optical elements, e.g. gratings, holograms
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S5/00—Semiconductor lasers
  • H01S5/005—Optical components external to the laser cavity, specially adapted therefor, e.g. for homogenisation or merging of the beams or for manipulating laser pulses, e.g. pulse shaping

Definitions

• the invention relates to the shaping and quality-improvement of the intensity distributions of fields emitted by multimode lasers and other spatially partially coherent light sources.
• Multimode lasers can therefore be considered as primary sources of spatially partially coherent light [F. Gori, Opt. Commun. 34, 301 (1980); A. Starikov ja E. Wolf, J. Opt. Soc. Am. 72, 923 (1982); S. Lavi, R. Prochaska and E. Keren, Appl. Opt. 27, 3696 (1988)].
• the intensity distribution of a laser beam across a plane perpendicular to the propagation direction is an important property in nearly all industrial applications of lasers.
• the beam shape of a pulsed excimer laser is typically far from ideal: sharp intensity fluctuations can be observed, the beam is not rotationally necessarily symmetric but strongly elliptic, and the intensity distribution may vary from pulse to pulse.
• the far-field distribution of a multimode laser beam is, to a good approximation, of the same Gaussian form as the far-field distribution of a single-mode laser.
• the fundamental difference is that the multimode beam is far from being diffraction-limited, i.e., its spread is larger than that of a single-mode beam with the same wavelength and initial size.
• a propagating multimode high-power laser beam often exhibit strong local intensity fluctuations not seen in high-quality single-mode laser beams.
• a Gaussian intensity distribution is not always ideal. In many laser applications one prefers an intensity distribution, which is uniform within a certain regic , such as a circle or a square, at a plane perpendicular to the propagation direction. For example, square-shaped beams are desirable in laser beam of patterns consisting of square pixels, while circular- shaped uniform beams are useful in laser drilling of different materials. Other shapes are useful as well: in laser fusion experiments a spherical object is illuminated by beams arriving from different directions, and in the optimum case each beam should illuminates a half-sphere uniformly. This requires a circular beam with the intensity distribution growing according to a cosine law from the center towards the edged and finally drops rapidly to zero.
• the beams emanating from high-power edge-emitting semiconductor lasers also often consists of a large number of transverse modes.
• the special feature of these lasers of the the beam is spatially partially coherent in the direction of the light-emitting waveguide but (nearly) coherent in the opposite direction.
• the beam quality is poor in the direction ofthe waveguide: strong local oscillations are observed, which one wishes to smooth out.
• Bright semiconductor light sources not based on pure stimulated emission are also under development.
• One example is the resonant-cavity light-emitting diode (RC-LED), which is an intermediate for between a laser and a light-emitting diode (LED).
• RC-LED resonant-cavity light-emitting diode
• Tjhe emitted radiation consists of a large number coherent cavity modes, an the superposed field is globally incoherent, or quasihomogeneous.
• Tjhe emitted radiation consists of a large number coherent cavity modes, an the superposed field is globally incoherent, or quasihomogeneous.
• a partially coherent, quasi-collimated light fields is obtained! but the intensity distribution in, e.g., the far field is not ideal.
• the beam is collimated (imaged) with a lens such that the far-field (image-plane) intensity distribution is approximately the image of the source surface.
• the lens aperture cuts off the high spatial frequencies in the angular spectrum of the primary field. Therefore a low-pass- filtered image is obtained, which usually does not have the desired form.
• the beam emanating from the end face of a multimode optical fiber is a spatially partially coherent field, which other requires shaping.
• the task of shaping the intensity distribution of a coherent light beam either in the. far field or at some finite distance from the source can in principle be performed using tradiational refractive optics: one places an aspheric refractive surface in front of the source, the surface shape being optimized such that the energy distribution in the target plane is of the desired form [P. W. Rhodes and D. L. Shealy, Appl. Opt. 19, 3545 (1980)].
• the obtained surface is rotationally symmetric, it can be fabricated for example by the diamond turning technique. If the refractive surface is not rotationally symmetric, its fabrication using present-day technology is difficult.
• Diffractive optics J. Turunen and F. Wyrowski, eds., Diffractive Optics for Industrial and Commercial Applications (Wiley- VGH, Berlin, 1997), in the following "Diffractive Optics”] has proved to be an excellent solution to many coherent laser beam shaping problems: an originally Gaussian intensity profile can be transformed into an almost arbitrary (for example, uniform or edge-enhanced) intensity distribution in the far field or at a finite distance by inserting on the beam path a surface-microstructured globally flat element, which modulates the phase, the amplitude, or both (“Diffractive Optics" , chapter 6).
• Diffractive optics offers a solution also the realization of above-mentioned rotationally nonsymmetric intensity distributions: since the microstructure is fabricated by microlithogrphic technology, the spefici form of the microstructure is not important from fabrication point of view. Nevertheless, the optical function of the element is still be analogous with that of an aspheric lens, so the problems with the sensitivity of the output profile to variations in the incident intensity distribution or alignment of the optical axes do not disappear. In diffractive optics it is possible to reduce the effects of these errors by including in the microstructure some controlled scattering, but the price to be paid is a reduction of conversion efficiency ( "Diffractive Optics" , chapter 6) .
• the starting point of the design of conventional diffractive beam shaping elements is the assumption of perfect spatial coherence [W. B. Veldkamp ja C. J. Kastner, Appl. Opt. 21, 879 (1982); C.-Y. Han, Y. Ishii ja K. Murata, Appl. Opt. 22, 3644 (1982); M. T. Eis an, A. M. Tai ja J. N. Cederquist, Appl. Opt. 28, 2641 (1989); N. Roberts, Appl. Opt. 28, 31 (1989)].
• US A 4410237 represents prior art in shaping fully coherent laser beams.
• the assumed diffractive structure is non-periodic.
• US A 6157756 represents prior art an shaping a fully coherent laser beam into a laser line with a large divergence angle.
• the fiber grating is periodic, but not microstructred, and its operation does not rely on partial coherence.
• US A 4790627 discloses a method to shape spatially incoherent, wideband laser beams in laser fusion experiments. The main goal is to reduce the aberrations Of the laser system using a shape- variant absorber and pattern projection.
• US A 4521075 is concerned with essentially the same problem, but discloses a method that involves echelon gratings to convert a spatially coherent wideband bam into a wideband but essentially spatially incoherent beam.
• This invention discloses a method to shape intensity distributions of multimode optical fields using diffractive optics ["Diffractive Optics”].
• the invention is based on essentially periodic diffractive elements and the use of the partial spatial coherence of a multimode beam, i.e., in a property of light that was previously considered a problem.
• the invention solves the above mentioned problems of prior art. It is characterized in that the shape of the transformed intensity distribution is independent; on the transverse alignment with respect to the incident bean and on reasonable deviations of the incident beam shape from the shape assumed in design.
• the partial spatial coherence is employed as disclosed below. :
• the intensity distribu- tion is an interference pattern: if the beams are equally intense, fringes wijth bright maxima
• the main idea is that the partial coherence of the incident field facilitates the use of periodic diffractive elements, which split the incident beam into several beams, in multimode beam shaping. This discovery may be viewed, in a sense, as an extension of the above-described observation on two-beam interference.
• W GSM (x ⁇ , X2) exp [- + xf) /w ] exp [- (x x - x 2 f /2 ⁇ 2 ] , (1)
• WQ the 1/e 2 half- width of the intensity profile
• ⁇ 0 the rms width of the desgree of coherence at the source plane
• the angle ⁇ in figure 2 is the above mentioned 1/e 2 half width of the far-field intensity distribution.
• a Gaussian Schell-model beam behaves as a spherical wave with a radius of curvature R(z).
• R(F) oo, i.e., the wave front is planar.
• equations (l)-(3) allows us to govern also this geometry by searching for Fourier-plane values of the beam and coherence widths is such a way the beam width and coherence area match with those of the incident beam at the plane of the lens.
• Using in addition the known law of spherical-wave transformation by a thin lens one can find the output beam parameters.
• the procedure can be extended to propagate the Gaussian Schell-model beam though an arbitrary paraxial lens system [A. T. Friberg ja J. Turunen, J. Opt. Soc. Am. A 5, 713 (1988)].
• Figure 4 illustrates a geometry in which a Gaussian Schell-model beam hits a periodic diffractive element, which splits a plane wave into a number of beams propagating in slightly different directions.
• the element is periodic in one or two directions and, as an ordinary diffraction grating, it produces diffraction orders with propagation directions given by the grating equation.
• the grating periods d x and d y in x and y directions are typically chosen such that the separations ⁇ x « ⁇ /d x and ⁇ y « ⁇ /d y are less than the far-field divergence angles ⁇ x and ⁇ y in x and y directions.
• Figure 6 illustrates numerical simulations based on equation (7) for the intensity distributions at the plane 302 of figure 3.
• the goal is to transform an originally Gaussian intensity distribution into a distribution with a flat top by using a diffractive element that would transform a fully coherent plane wave into nine equal-efficiency diffraction orders m — —4, . . . , +4.
• the optimum is d « 1 mm in figure 5a and d w 0.5 mm in figure 5b, i.e., a reduction in the degree of coherence reduces the optimum grating period because it increases the beam width wp. It should be noted that the total energy is the same in all cases: reduction of d widens the beam while simultaneously decreasing its top intensity.
• the period d is the most important tool influencing the beam shape (also the number of orders M has a smaller influence). It is of advantage to optimize d -separately in x and y directions whenever the source is anisotropic, i.e., its intensity distribution is periodic.
• Figure 5 illustrates such a situation, observed in a plane perpendicular to the beam propagation direction. Because the source is anisotropic, so is its far-field diffraction pattern, but a proper choice of grating periods in x and y directions transforms the far-field pattern into a rotationally symmetric shape. If necessary, a different number of beams may be used in the two orthogonal directions. As illustrated in the numerical simulations of figure 6, an element capable of transforming a Gaussian beam into a uniform-intensity beam produces a set of Gaussian beams propa ⁇
• the partially coherent beam is divided into several beams that propagate into slightly different directions such that its intensity distribution does not spread appreciably, and the beams interfere only partly. Therefore the intensity fluctuations tend to average out and the superposed beam is more homogeneous than the original beam.
• the method is suitable, for example, in improving the quality of individual excimer laser pulses and to obtain a better pulse-shape repeatability. It is also suitable for the homogenization of multimode semiconductor laser beams (as illustrated in figure 6).
• Figure 8 illustrates the imaging of several discrete, mutually uncorrelated light sources into the observation plane.
• the sources may be either lasers or LEDs. If the imaging lens is diffraction-limited and does not appreciably truncate the angular spectra of the sources, we obtain an image (801) of the source array. In practice a slightly wider distribution (802) is obtained. However, often one prefers a more or less continuous intensity distribution instead of a discrete array, for example a square or a rectangular uniformly illuminated region. This can be achieved by methods presented in the invention: the image of each source is multiplied in x and y directions such that the empty spaces between the discrete sources are filled. The images of different sources may overlap because the sources are mutually uncorrelated. Thus no interference is produced and the result is an incoherent sum of different intensity distributions (803) .
• Drawing 1 Prior art.
• the intensity distribution of the laser beam (101) is shaped with the aid of an aspheric lens (102) such that the desired distribution arises at the plane (103).
• Drawing 2 Propagation of a Gaussian Schell-model beam in free space: w(z) is the 1/e 2 half- width of the intensity distribution, ⁇ (z) is the spatial coherence widtrl of the beam, and R(z) is its radius of wave front curvature.
• Drawing 3 Fourier transformation of a Gaussian Schell-model source by a thin lens (301) into the plane (302) .
• Drawing 4 Shaping of a Gaussian Schell-model beam by means of a thin lens (401) and a periodic diffractive element (403).
• Drawing 5 Interference of spatially partially coherent beams in a geometry of the type illustrated in Drawing 3 if the grating produces a two-dimensional array of diffraction orders (the ellipses). The center points of the ellipses denote the spatial frequencies of the diffraction orders. After superposition these mutually partially correlated fields form an almost constant-intensity region within the shown circular area.
• Curves 601 and 605: d 10 mm.
• Curves 602 and 606: d 1 mm.
• Curves 603 and 607: d 0.5 mm.
• Curves 604 and 608: D 0.25 mm.
• Drawing 7 Homogenization of a multimode semiconductor laser (701) beam with a diffractive beam splitter, (a) The intensity distribution (702) on the screen (703) is non-uniform.
• the diffractive element (704) produces a set (here three for clarity) of beams propagating in slightly different directions.
• the intensity distributions of all individual beams is of the type (702) but the superposition of the spatially partially coherent beams produces a homogenized beam (705).
• Drawing 8 Combination of several mutually uncorrelated light beams emitted by independent light sources into an approximately flat-top pattern in the image plane of the source.

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