WO2007014773A1 - Optical system for attenuating and imaging an optical beam for a subsequent intensity measurement - Google Patents

Optical system for attenuating and imaging an optical beam for a subsequent intensity measurement Download PDF

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Publication number
WO2007014773A1
WO2007014773A1 PCT/EP2006/007662 EP2006007662W WO2007014773A1 WO 2007014773 A1 WO2007014773 A1 WO 2007014773A1 EP 2006007662 W EP2006007662 W EP 2006007662W WO 2007014773 A1 WO2007014773 A1 WO 2007014773A1
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WO
WIPO (PCT)
Prior art keywords
optical
attenuating
optical system
optical beam
arrangement
Prior art date
Application number
PCT/EP2006/007662
Other languages
French (fr)
Inventor
Alois Herkommer
Bernhard Weigl
Holger Muenz
Original Assignee
Carl Zeiss Laser Optics Gmbh
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Publication date
Application filed by Carl Zeiss Laser Optics Gmbh filed Critical Carl Zeiss Laser Optics Gmbh
Publication of WO2007014773A1 publication Critical patent/WO2007014773A1/en

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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
    • G02B27/10—Beam splitting or combining systems
    • G02B27/14—Beam splitting or combining systems operating by reflection only
    • G02B27/145—Beam splitting or combining systems operating by reflection only having sequential partially reflecting surfaces
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J1/00—Photometry, e.g. photographic exposure meter
    • G01J1/02—Details
    • G01J1/04—Optical or mechanical part supplementary adjustable parts
    • G01J1/0407—Optical elements not provided otherwise, e.g. manifolds, windows, holograms, gratings
    • G01J1/0411—Optical elements not provided otherwise, e.g. manifolds, windows, holograms, gratings using focussing or collimating elements, i.e. lenses or mirrors; Aberration correction
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J1/00—Photometry, e.g. photographic exposure meter
    • G01J1/02—Details
    • G01J1/04—Optical or mechanical part supplementary adjustable parts
    • G01J1/0407—Optical elements not provided otherwise, e.g. manifolds, windows, holograms, gratings
    • G01J1/0418—Optical elements not provided otherwise, e.g. manifolds, windows, holograms, gratings using attenuators
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J1/00—Photometry, e.g. photographic exposure meter
    • G01J1/02—Details
    • G01J1/04—Optical or mechanical part supplementary adjustable parts
    • G01J1/0407—Optical elements not provided otherwise, e.g. manifolds, windows, holograms, gratings
    • G01J1/0477—Prisms, wedges
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J1/00—Photometry, e.g. photographic exposure meter
    • G01J1/42—Photometry, e.g. photographic exposure meter using electric radiation detectors
    • G01J1/4257—Photometry, e.g. photographic exposure meter using electric radiation detectors applied to monitoring the characteristics of a beam, e.g. laser beam, headlamp beam
    • 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
    • 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
    • G02B27/095—Refractive optical elements
    • G02B27/0972—Prisms
    • 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
    • G02B27/10—Beam splitting or combining systems
    • G02B27/108—Beam splitting or combining systems for sampling a portion of a beam or combining a small beam in a larger one, e.g. wherein the area ratio or power ratio of the divided beams significantly differs from unity, without spectral selectivity
    • 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
    • G02B27/10—Beam splitting or combining systems
    • G02B27/14—Beam splitting or combining systems operating by reflection only
    • G02B27/144—Beam splitting or combining systems operating by reflection only using partially transparent surfaces without spectral selectivity

Definitions

  • the invention relates to an optical system for attenuating and imaging an optical beam for a subsequent intensity measurement of said optical beam.
  • the invention generally relates to optical intensity measurements of high power light sources or high power optical systems which are, for example, used in laser machining or laser annealing of substrates.
  • An attenuation based on a limiting or structured physical aperture, for example a grid, is often not desired, since the optical beam to be measured may have a substructure, which may be subject to change and therefore, as different portions of the beam are blocked by the aperture, will modify the measurement results.
  • the polarization and the incidence angle of the optical beam at the measurement location may be subject to change, either due to changes of the light source, or due to changes in the subsequent optical system.
  • Lopez discloses a variable attenuator for optical transceivers such as laser range finders for ordnance control.
  • a polarization rotation device such as a mechanically rotatable half-wave plate or an electro-optical cell, operates in combination with a beam splitter polarizer to attenuate the transmitted laser beam by a continuously variable amount without further attenuating the received laser beam.
  • This known optical attenuator is based on polarization dependent attenuation thus suffering from the disadvantage mentioned before related to a polarization dependent attenuation.
  • Bennett et al. disclose an optical variable attenuator which includes four wedges, each of the wedges having two surfaces (entrance and exit face) defined by an angle of convergence.
  • the attenuation of the optical beam is dependent on the incidence angle of the optical beam on each of these wedges.
  • the incidence angles of the optical beam on each wedge is large, i.e. the optical beam enters each wedge in glancing fashion.
  • Marrs et al. Another optical system for attenuating high power laser beams is disclosed by Marrs et al. (US 4,747,673) which comprises an annular beam dump through which the beam passes diametrically and in which the beam passes successively through a pair of pivoting transmissive and reflective elements. At each element a portion of the beam is reflected toward the dump and a portion is transmitted, the relative energy in each portion being determined by the angle of the elements to the beam so that adjustment of this angle determines the amount of attenuation.
  • the systems according to Bennett et al. and according to Marrs et al. are dependent on and sensitive to the incidence angle of the optical beam, giving rise to the problems mentioned above related to an angle dependent attenuation.
  • an optical system for attenuating and imaging an optical beam for a subsequent intensity measurement of the optical beam comprising an input aperture for said optical beam, a collimating optical element for collimating said optical beam propagating divergently from said input aperture, an attenuating optical arrangement for attenuating the intensity of said optical beam propagating from said collimating optical element, said attenuating optical arrangement being arranged to modify said optical beam such that a beam portion with an intensity lower than the intensity of the optical beam is generated, said attenuating optical arrangement being insensitive to a polarization state of said optical beam, and a detector for measuring said intensity of said beam portion propagating from said attenuating optical arrangement.
  • the optical system according to the present invention comprises a collimating optical element, for example a collimation lens, in order to create the same incidence angle of all optical beam portions on the following attenuating optical arrangement independent of the input angle of the optical beam before being collimated.
  • the attenuating optical arrangement is designed and arranged to be not sensitive to the polarization state of the optical beam, and further modifies the optical beam such that a beam portion with low intensity is generated. The beam portion with low intensity is then directed to the detector for measuring the intensity of the beam portion of low intensity from which measurement the intensity of the original input optical beam can be derived.
  • the attenuating optical arrangement comprises at least one wedge through which the optical beam passes, wherein the at last one wedge has an entrance face and an exit face inclined with respect to the entrance face.
  • the attenuation of the optical beam is accomplished by at least one partial reflection at the entrance face and at least one partial reflection at the exit face, wherein the reflected parts of the optical beam are passed to the detector.
  • the attenuating optical arrangement preferably comprises at least two wedges.
  • One of the at least two wedges has an orientation with respect to the other of the at least two wedges, which is off-set by at least approximately 90° with respect to the direction of propagation of the beam portion of low intensity of the optical beam between the at least two wedges.
  • the attenuating optical arrangement preferably comprises an even number of wedges.
  • the attenuating optical arrangement comprises at least one group of a first partially reflective optical element and a second partially reflective optical element, which form a retro- reflector-like arrangement.
  • the reflected portion of the optical beam which is reflected at the optical elements is used as the beam portion of low intensity which is passed to the detector.
  • the attenuating optical arrangement preferably comprises two groups or an even number of groups larger than two, wherein each of the groups has two optical elements which form a retro-reflector-like arrangement.
  • the optical elements forming a retro-reflector-like arrangement of one group are angularly off-set with respect to the optical elements of the other group of optical elements, which also form a retro-reflector-like arrangement, by an angle of at least approximately 90°.
  • the attenuating optical arrangement comprises at least one diffuser plate for attenuating the optical beam.
  • Fig. 1 shows a schematic drawing of an embodiment of an optical system for attenuating and imaging an optical beam for a subsequent intensity measurement of the optical beam according to the present invention to illustrate the principles of the present invention
  • Fig. 2 shows a perspective view of two wedges forming an attenuating optical arrangement used in an optical system of the present invention
  • Fig. 3 shows a side view of a specific optical system for attenuating and imaging an optical beam for a subsequent intensity measurement according to the present invention
  • Fig. 4 shows the optical system in Fig. 3 in a side view, wherein the plane of drawing is rotated with respect to the plane of drawing in Fig. 3 by an 90°;
  • Fig. 5 shows a perspective view of the optical system of Fig. 3 and
  • FIG. 4 shows another embodiment of an optical system for attenuating and imaging an optical beam for a subsequent intensity measurement which is modified with respect to the embodiment of Figs. 3 - 5;
  • Fig. 7a) and 7b) show another embodiment of an optical system for attenuating and imaging an optical beam for a subsequent intensity measurement, wherein Fig. 7a) and Fig. 7b) are partial cross-sectional views with the plane of cross-section in Fig. 7b) being angularly off-set with respect to the plane of cross-section in Fig. 7a) by 90°;
  • Fig. 8 shows a portion of the embodiment in Figs. 7a) and 7b);
  • Fig. 9 shows still another embodiment of an optical system for attenuating and imaging an optical beam for a subsequent intensity measurement.
  • Fig. 1 shows an optical system 10 for attenuating and imaging an optical beam 12 for a subsequent intensity measurement of the optical beam 12.
  • the optical system 10 comprises, in the order of the propagation direction of the input optical beam 12, an input aperture 14 and a collimating optical element 16 for collimating the optical beam 12 propagating divergently from the input aperture 14.
  • the collimating optical element 16 is, for example, a collimation lens which creates a parallel optical beam 12a from the divergent input optical beam 12.
  • the optical system 10 further comprises an attenuating optical arrangement 18 for attenuating the intensity of the optical beam 12a propagating from the collimating optical element 16.
  • the attenuating optical arrangement 18 is arranged to split the optical beam 12a into a first beam portion and a second beam portion 12b with lower intensity than the first beam portion, wherein the attenuating optical arrangement 18 is insensitive to a polarization state of the optical beam 12a.
  • the optical system 10 further comprises a focusing element 20 for focusing the second beam portion 12b with low intensity onto a detector 22.
  • the detector 22 measures the intensity of the second beam portion 12b propagating from the attenuating optical arrangement 18.
  • the attenuating optical arrangement 18 comprises two wedges 24 and 26 made of a material which is transparent to the optical beam 12a and 12b, respectively.
  • the collimating optical element 16 creates the same input angle of the optical beam 12a on the wedges 24 and 26 independent of the input angle of optical beam 12.
  • the wedge 24 has an entrance face 28 and an exit face 30, and the wedge 26 has an entrance face 32 and an exit face 34.
  • the optical beam 12a passes first through wedge 24 and then through wedge 26.
  • the entrance face 28 and the exit face 30 are inclined with respect to each other and form a wedge angle 36, which is, for example, 7°.
  • the entrance face 32 and the exit face 34 of the wedge 26 are also inclined with respect to each other and form a wedge angle 38, which preferably is identical with the wedge angle 36.
  • the wedge angles 36 and 38 are shown in Fig. 2.
  • Attenuation of the optical beam 12, 12a is achieved by a single or double reflection, or in general, by at least one reflection at the exit face 30 and at least one reflection at the entrance face 28 of the wedge 24 inside the wedge 24, as shown in Fig. 2.
  • the reflected portion of the optical beam 12a is then used as the second beam portion 12b with low intensity which is directed or imaged onto the detector 22 by focusing element or focusing lens 20.
  • the input angles of the optical beam 12a with respect to the wedges 24 and 26 are kept small in order to avoid a strong sensitivity to angle or polarization.
  • two wedges like wedges 24 and 26 are used, wherein the wedge 26 is oriented with respect to the wedge 24 with an offset of 90° with respect to the direction of propagation (optical axis) of the reflected portion 12b of the optical beam 12 between the two wedges 24 and 26, as shown in Fig. 2.
  • the wedge direction 42 of the wedge 26 is rotated with respect to the wedge direction 44 of the wedge 24 by 90°, in order to make the optical system 10 completely insensitive to the input beam polarization of the input optical beam 12a.
  • the attenuating optical arrangement comprises an even number of wedges, for example two as in the preferred embodiment, or four or six, etc.
  • the collimating optical element 16 is preferably arranged in a distance from the input aperture 14 which is the focal length f (Fig. 1) of the collimating optical element 16.
  • the focusing optical element 20 can also be arranged in a distance from the detector 22 which is the focal length f of the focusing optical element 20.
  • the focusing optical element 20 directs the attenuated beam 12b to the detector 22 independent of the initial input angle of the input optical beam 12.
  • the input aperture 14 is imaged onto the plane of the detector 22.
  • the wedges 24 and 26 are two identical CaF-wedges with a wedge angle of 7° and are used in double reflection in order to attenuate the optical beam 12.
  • the transmitted optical beam 12c can be used for other purposes than processing the substrate 40, and can also be blocked by apertures or guided out of the optical system 10 in order to avoid stray light.
  • Fig. 6 shows another embodiment of an optical system 10' which differs from the optical system 10 by the fact that the input beam 12 does not extend in two dimensions transverse to the direction of propagation, but only in one dimension, for example in case that the input beam 12 is a line focus.
  • the focusing optical element 20' is cylindrical or toroidal rather than spherical as in case of focusing element 20 of optical system 10.
  • the focusing lens 20' does not image the focused input optical beam 12 on the detector 22, which would create high local intensity on the detector 22, but distributes the intensity of the optical beam 12b 1 over some area on the detector 22.
  • the wedges 24 and 26 may be coated or uncoated in order to tune the reflection coefficient of the wedges 24 and 26.
  • the collimating optical element 16 may be spherical, aspherical or cylindrical, and the focusing element 20 or 20' may also be spherical, aspherical, or cylindrical.
  • the material of all optical elements used in the optical system 10 or 10' is selected for high optical power stability (e.g. CaF for UV-light).
  • the detector 22 may be a photodiode or any kind of light sensitive device.
  • Figs. 7a) and 7b) show an optical system 50 for attenuating and imaging an optical beam 52 for a subsequent intensity measurement of the optical beam 52 according to another embodiment.
  • the optical system 50 is not only insensitive to a polarization of the optical beam 52, but is also insensitive to the state of collimation of the optical beam 52, in particular in cases, where the cross-sectional size of the optical beam 52 is large.
  • the optical system 50 comprises, in the order of the propagation direction of the input optical beam 52, an input aperture 54 and a collimating optical element 56 for collimating the optical beam 52 propagating divergently from the input aperture 54.
  • the collimating optical element 56 for example a collimation lens, again creates a parallel optical beam 52a from the divergent input optical beam 52.
  • the optical system 50 further comprises an attenuating optical arrangement 58 for attenuating the intensity of the optical beam 52a propagating from the collimating optical element 56.
  • the attenuating optical arrangement 58 comprises a first group 60 of a first optical element 62 and a second optical element 64.
  • the optical element 62 and 64 are configured as plane-parallel plates, for example made of glass, which are partially reflective and partially transmissive.
  • the plates preferably are thin, for example have a thickness of about 1 mm.
  • the optical element 62 has a surface 66 and the optical element 64 has a surface 68.
  • the surfaces 66 and 68 form a retro-reflector-like arrangement which means that the surfaces 66 and 68 form an angle with one another which is preferably about 90°, but may be vary in a range from about 70° to about 110°.
  • the surface 66 is oriented with respect to the optical beam 52a propagating from the collimating optical element 56 such that the optical beam 52a is incident on the surface 66 at an incident angle of about 45°, wherein the incident angle may vary according to the orientation of the optical element 62.
  • the optical beam 52a is partially reflected at the surface 66 (reflected part 52b) and partially transmitted through the optical element 62 (transmitted part 52c).
  • the reflected part 52b of the optical beam 52 is then directed to the surface 68 of the optical element 64 and again partially reflected (reflected part 52d) and partially transmitted (transmitted part 52e).
  • the transmitted beam parts 52c and 52e can be destroyed in a beam dump as described with respect to the above embodiment.
  • the attenuating optical arrangement 58 further comprises a second group 70 of a third optical element 72 and a fourth optical element 74.
  • the third and fourth optical elements 72 and 74 are also configured as plane-parallel plates, which are partially reflective and partially transmissive.
  • the third optical element 72 has a surface 76
  • the fourth optical element 74 has a surface 78.
  • the third and fourth optical elements 72 and 74 again form a retro-reflector-like arrangement with one another, and, in particular, form an angle of about 90° with respect to one another.
  • the angle between the third and fourth optical element 72 and 74 and their individual orientation is adjustable.
  • the reflected part 52d propagating from the second optical element 74 is incident on the surface 76 of the optical element 72 and partially reflected (reflected part 52f) and partially transmitted (transmitted part 52g).
  • the third optical element 72 is arranged such that it is not in the same plane as the optical elements 53 or 56 so that the transmitted part 52g does not pass through the optical element 53, 56.
  • the reflected part 52f of the optical beam 52 is directed to the fourth optical element 74 and partially reflected at the surface 78 (reflected part 52h) and partially transmitted (transmitted part 52i).
  • the reflected part 52h is then directed to a focusing element 80 which focuses the optical beam having a low intensity due to its attenuation by the attenuating optical arrangement 58, onto a detector 82.
  • partial reflection and partial transmission of the optical is described with respect to the front surfaces of the optical elements 62, 64 and 72, 74, partial reflection and transmission can instead occur at the back surfaces only, or at the front as well as at the back surfaces.
  • the optical system 50 is not only insensitive to polarization, but also insensitive to incidence angles at the respective surfaces 66, 68 and 76, 78, as will be described with respect to Fig. 8.
  • Fig. 8 shows one of the retro-reflector-like arrangements, for example the first group 60.
  • the optical system 50 is particularly useful for optical beams, which are not or cannot be perfectly collimated, i.e. have an angle spread as it is the case for optical beams having a large cross-sectional size.
  • the provision of the pupil stop 51 mainly blocks diffracted light and the input aperture 54 which is a field stop, mostly blocks stray light.
  • the aforementioned apertures and further the large splitting angles between the reflected beam parts (for example reflected part 52b) and the transmitted parts (for example transmitted part 52c) render the optical system 50 insensitive for stray light and diffracted light.
  • Fig. 9 shows still another embodiment of an optical system 90 for attenuating and imaging an optical beam 92 for a subsequent intensity measurement of the optical beam 92 by a detector 94.
  • the optical system 90 is similar or identical with the optical system 50 so that reference is made to the description above.
  • the optical system 90 comprises an attenuating optical arrangement 104, which is simply formed by a diffuser plate 106 which diffuses the optical beam 92a propagating from the collimating optical element 102.
  • the detector 94 is arranged behind the diffuser plate 106 such that a beam portion of low intensity reaches the detector 94, while the remaining portion of the optical beam 92a passes by the detector 94 and may be absorbed in a beam dump as previously described.
  • imaging here means that a part of the light is scattered by the diffuser plate 106 in direction to the detector 94, so that the imaging is diffuse in contrast to the afore-going embodiments.

Abstract

An optical system for attenuating and imaging an optical beam for a subsequent intensity measurement of said optical beam comprises an input aperture for the optical beam, a collimating optical element for collimating the optical beam propagating divergently from the input aperture, an attenuating optical arrangement for attenuating the intensity of the optical beam propagating from the collimating optical element, the attenuating optical arrangement being arranged to modify the optical beam such that a beam portion with an intensity lower than the intensity of the optical beam is generated, the attenuating optical arrangement being insensitive to a polarization state of the optical beam, and a detector for measuring the intensity of the beam portion propagating from the attenuating optical arrangement.

Description

OPTICAL SYSTEM FOR ATTENUATING AND IMAGING AN OPTICAL BEAM FOR A SUBSEQUENT INTENSITY MEASUREMENT
BACKGROUND OF THE INVENTION
The invention relates to an optical system for attenuating and imaging an optical beam for a subsequent intensity measurement of said optical beam.
The invention generally relates to optical intensity measurements of high power light sources or high power optical systems which are, for example, used in laser machining or laser annealing of substrates.
In the measurement of the intensity of an optical beam having a high intensity, for example of more than 50 W, the problem arises that such high intensities can damage or saturate the detector used for the intensity measurements. A common detector in use is, for example, a photodiode. Thus, when measuring high intensities of high power optical beams, it is usually required to attenuate the intensity of the optical beam in order to avoid a damage or saturation of the detector in use. Several concepts have been taken into account to attenuate the intensity of an optical beam. An attenuation based on a limiting or structured physical aperture, for example a grid, is often not desired, since the optical beam to be measured may have a substructure, which may be subject to change and therefore, as different portions of the beam are blocked by the aperture, will modify the measurement results.
Further, in most cases the polarization and the incidence angle of the optical beam at the measurement location may be subject to change, either due to changes of the light source, or due to changes in the subsequent optical system.
Lopez (US 4,530,600) discloses a variable attenuator for optical transceivers such as laser range finders for ordnance control. A polarization rotation device, such as a mechanically rotatable half-wave plate or an electro-optical cell, operates in combination with a beam splitter polarizer to attenuate the transmitted laser beam by a continuously variable amount without further attenuating the received laser beam. This known optical attenuator is based on polarization dependent attenuation thus suffering from the disadvantage mentioned before related to a polarization dependent attenuation.
Bennett et al. (US 4,398,806) disclose an optical variable attenuator which includes four wedges, each of the wedges having two surfaces (entrance and exit face) defined by an angle of convergence. The attenuation of the optical beam is dependent on the incidence angle of the optical beam on each of these wedges. In particular, the incidence angles of the optical beam on each wedge is large, i.e. the optical beam enters each wedge in glancing fashion.
Another optical system for attenuating high power laser beams is disclosed by Marrs et al. (US 4,747,673) which comprises an annular beam dump through which the beam passes diametrically and in which the beam passes successively through a pair of pivoting transmissive and reflective elements. At each element a portion of the beam is reflected toward the dump and a portion is transmitted, the relative energy in each portion being determined by the angle of the elements to the beam so that adjustment of this angle determines the amount of attenuation. Thus, the systems according to Bennett et al. and according to Marrs et al. are dependent on and sensitive to the incidence angle of the optical beam, giving rise to the problems mentioned above related to an angle dependent attenuation.
Therefore, there is still a need to provide an optical system for attenuating and imaging an optical beam for subsequent intensity measurement in which the attenuation is not sensitive to the substructure of the input beam, not sensitive to the polarization of the input beam and also not sensitive to the incidence angle.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an optical system for attenuating and imaging an optical beam for a subsequent intensity measurement of the optical beam in which the attenuation is not sensitive to the substructure of the input beam.
It is another object of the present invention to provide an optical system for attenuating and imaging an optical beam for a subsequent intensity measurement of the optical beam, in which the attenuation is not sensitive to the polarization of the optical beam.
It is another object of the present invention to provide an optical system for attenuating and imaging an optical beam for a subsequent intensity measurement of the optical beam, in which the attenuation is not sensitive to the incidence angle.
It is another object of the present invention to provide an optical system for attenuating and imaging an optical beam for a subsequent intensity measurement of the optical beam, in which the attenuation is not sensitive to the state of collimation of the optical beam and/or which is suited for optical beams having a large cross- sectional size. It is another object of the present invention to provide an optical system for attenuating and imaging an optical beam for a subsequent intensity measurement of the optical beam, in which the attenuation is insensitive to stray light and light diffracted from optical elements of the system.
According to the present invention, an optical system for attenuating and imaging an optical beam for a subsequent intensity measurement of the optical beam is provided, comprising an input aperture for said optical beam, a collimating optical element for collimating said optical beam propagating divergently from said input aperture, an attenuating optical arrangement for attenuating the intensity of said optical beam propagating from said collimating optical element, said attenuating optical arrangement being arranged to modify said optical beam such that a beam portion with an intensity lower than the intensity of the optical beam is generated, said attenuating optical arrangement being insensitive to a polarization state of said optical beam, and a detector for measuring said intensity of said beam portion propagating from said attenuating optical arrangement.
The optical system according to the present invention comprises a collimating optical element, for example a collimation lens, in order to create the same incidence angle of all optical beam portions on the following attenuating optical arrangement independent of the input angle of the optical beam before being collimated. Further, the attenuating optical arrangement is designed and arranged to be not sensitive to the polarization state of the optical beam, and further modifies the optical beam such that a beam portion with low intensity is generated. The beam portion with low intensity is then directed to the detector for measuring the intensity of the beam portion of low intensity from which measurement the intensity of the original input optical beam can be derived.
In one embodiment, the attenuating optical arrangement comprises at least one wedge through which the optical beam passes, wherein the at last one wedge has an entrance face and an exit face inclined with respect to the entrance face. The attenuation of the optical beam is accomplished by at least one partial reflection at the entrance face and at least one partial reflection at the exit face, wherein the reflected parts of the optical beam are passed to the detector.
In a refinement of this embodiment, the attenuating optical arrangement preferably comprises at least two wedges. One of the at least two wedges has an orientation with respect to the other of the at least two wedges, which is off-set by at least approximately 90° with respect to the direction of propagation of the beam portion of low intensity of the optical beam between the at least two wedges.
In another refinement, the attenuating optical arrangement preferably comprises an even number of wedges.
In another embodiment of the optical system according to the invention, the attenuating optical arrangement comprises at least one group of a first partially reflective optical element and a second partially reflective optical element, which form a retro- reflector-like arrangement. In this case, the reflected portion of the optical beam which is reflected at the optical elements, is used as the beam portion of low intensity which is passed to the detector.
In a refinement of this embodiment, the attenuating optical arrangement preferably comprises two groups or an even number of groups larger than two, wherein each of the groups has two optical elements which form a retro-reflector-like arrangement. The optical elements forming a retro-reflector-like arrangement of one group are angularly off-set with respect to the optical elements of the other group of optical elements, which also form a retro-reflector-like arrangement, by an angle of at least approximately 90°.
In still another embodiment of the optical system according to the present invention, the attenuating optical arrangement comprises at least one diffuser plate for attenuating the optical beam. Further advantages or features will become apparent from the following description and the accompanying drawings in which preferred embodiments of the invention are shown.
BRIEF DESCRIPTION OF THE DRAWINGS
Preferred exemplary embodiments will be described hereinafter with reference to the accompanying drawings, in which:
Fig. 1 shows a schematic drawing of an embodiment of an optical system for attenuating and imaging an optical beam for a subsequent intensity measurement of the optical beam according to the present invention to illustrate the principles of the present invention;
Fig. 2 shows a perspective view of two wedges forming an attenuating optical arrangement used in an optical system of the present invention;
Fig. 3 shows a side view of a specific optical system for attenuating and imaging an optical beam for a subsequent intensity measurement according to the present invention;
Fig. 4 shows the optical system in Fig. 3 in a side view, wherein the plane of drawing is rotated with respect to the plane of drawing in Fig. 3 by an 90°;
Fig. 5 shows a perspective view of the optical system of Fig. 3 and
Fig. 4; Fig. 6 shows another embodiment of an optical system for attenuating and imaging an optical beam for a subsequent intensity measurement which is modified with respect to the embodiment of Figs. 3 - 5;
Fig. 7a) and 7b) show another embodiment of an optical system for attenuating and imaging an optical beam for a subsequent intensity measurement, wherein Fig. 7a) and Fig. 7b) are partial cross-sectional views with the plane of cross-section in Fig. 7b) being angularly off-set with respect to the plane of cross-section in Fig. 7a) by 90°;
Fig. 8 shows a portion of the embodiment in Figs. 7a) and 7b); and
Fig. 9 shows still another embodiment of an optical system for attenuating and imaging an optical beam for a subsequent intensity measurement.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Fig. 1 shows an optical system 10 for attenuating and imaging an optical beam 12 for a subsequent intensity measurement of the optical beam 12.
The optical system 10 comprises, in the order of the propagation direction of the input optical beam 12, an input aperture 14 and a collimating optical element 16 for collimating the optical beam 12 propagating divergently from the input aperture 14. The collimating optical element 16 is, for example, a collimation lens which creates a parallel optical beam 12a from the divergent input optical beam 12.
The optical system 10 further comprises an attenuating optical arrangement 18 for attenuating the intensity of the optical beam 12a propagating from the collimating optical element 16. As will be described in more detail hereinafter, the attenuating optical arrangement 18 is arranged to split the optical beam 12a into a first beam portion and a second beam portion 12b with lower intensity than the first beam portion, wherein the attenuating optical arrangement 18 is insensitive to a polarization state of the optical beam 12a.
The optical system 10 further comprises a focusing element 20 for focusing the second beam portion 12b with low intensity onto a detector 22. The detector 22 measures the intensity of the second beam portion 12b propagating from the attenuating optical arrangement 18.
The attenuating optical arrangement 18 comprises two wedges 24 and 26 made of a material which is transparent to the optical beam 12a and 12b, respectively.
The collimating optical element 16 creates the same input angle of the optical beam 12a on the wedges 24 and 26 independent of the input angle of optical beam 12.
The wedge 24 has an entrance face 28 and an exit face 30, and the wedge 26 has an entrance face 32 and an exit face 34. The optical beam 12a passes first through wedge 24 and then through wedge 26.
The entrance face 28 and the exit face 30 are inclined with respect to each other and form a wedge angle 36, which is, for example, 7°. The entrance face 32 and the exit face 34 of the wedge 26 are also inclined with respect to each other and form a wedge angle 38, which preferably is identical with the wedge angle 36. The wedge angles 36 and 38 are shown in Fig. 2.
Attenuation of the optical beam 12, 12a is achieved by a single or double reflection, or in general, by at least one reflection at the exit face 30 and at least one reflection at the entrance face 28 of the wedge 24 inside the wedge 24, as shown in Fig. 2. The reflected portion of the optical beam 12a is then used as the second beam portion 12b with low intensity which is directed or imaged onto the detector 22 by focusing element or focusing lens 20.
Each reflection will attenuate the optical beam 12a according the Fresnel reflection coefficient (e.g. for a reflective index of n = 1.5 of the material of the wedge 24 the reflection coefficient is about 4 % for an uncoated exit face 30 and entrance face 28). The wedge angle 36 or 38 is useful in order to separate or to split the attenuated reflected optical beam 12b from the transmitted optical beam 12c (cf. Fig. 3). The optical beam 12c is used for e.g. material processing of a substrate 40 with high intensity.
Preferably, the input angles of the optical beam 12a with respect to the wedges 24 and 26 are kept small in order to avoid a strong sensitivity to angle or polarization.
In order to completely avoid a sensitivity to polarization, two wedges like wedges 24 and 26 are used, wherein the wedge 26 is oriented with respect to the wedge 24 with an offset of 90° with respect to the direction of propagation (optical axis) of the reflected portion 12b of the optical beam 12 between the two wedges 24 and 26, as shown in Fig. 2. In other words, the wedge direction 42 of the wedge 26 is rotated with respect to the wedge direction 44 of the wedge 24 by 90°, in order to make the optical system 10 completely insensitive to the input beam polarization of the input optical beam 12a.
For this reason, it is further preferred if the attenuating optical arrangement comprises an even number of wedges, for example two as in the preferred embodiment, or four or six, etc.
The collimating optical element 16 is preferably arranged in a distance from the input aperture 14 which is the focal length f (Fig. 1) of the collimating optical element 16. The focusing optical element 20 can also be arranged in a distance from the detector 22 which is the focal length f of the focusing optical element 20. The focusing optical element 20 directs the attenuated beam 12b to the detector 22 independent of the initial input angle of the input optical beam 12. Thus, the input aperture 14 is imaged onto the plane of the detector 22.
Figs. 3 - 5 show a detailed embodiment of the optical system 10, which comprises as the collimating optical element 16 and the focusing optical element 20 two identical lenses which are made from CaF (calcium fluoride) for collimation and focusing, both of which having a focal length of f = 25 mm and f = 25 mm, thus creating a 1:1 imaging of the input aperture 14 onto the detector 22. The wedges 24 and 26 are two identical CaF-wedges with a wedge angle of 7° and are used in double reflection in order to attenuate the optical beam 12.
The transmitted optical beam 12c can be used for other purposes than processing the substrate 40, and can also be blocked by apertures or guided out of the optical system 10 in order to avoid stray light.
Fig. 6 shows another embodiment of an optical system 10' which differs from the optical system 10 by the fact that the input beam 12 does not extend in two dimensions transverse to the direction of propagation, but only in one dimension, for example in case that the input beam 12 is a line focus.
For such an application, it can be desirable that the focusing optical element 20' is cylindrical or toroidal rather than spherical as in case of focusing element 20 of optical system 10. The focusing lens 20', in this case, does not image the focused input optical beam 12 on the detector 22, which would create high local intensity on the detector 22, but distributes the intensity of the optical beam 12b1 over some area on the detector 22. For a similar reason, it might be desirable not to image exactly the input aperture 14 onto the detector 22, but to introduce some defocus in order to achieve some smoothing of the input distribution of the optical beam 12. This can be achieved by choosing the distance from the plane of the input aperture 14 to the collimating optical element 16, and/or the distance between the focusing element 20' and the detector 22 not exactly equal to the focal length of the corresponding element 16 or 20'.
Other variations may have different focal lengths for the collimating optical element 16 and the focusing element 20' in order to achieve a certain magnification or demagnification of the input plane of the input aperture 14 onto the detector 22.
Further, the wedges 24 and 26 may be coated or uncoated in order to tune the reflection coefficient of the wedges 24 and 26.
The collimating optical element 16 may be spherical, aspherical or cylindrical, and the focusing element 20 or 20' may also be spherical, aspherical, or cylindrical.
The material of all optical elements used in the optical system 10 or 10' is selected for high optical power stability (e.g. CaF for UV-light).
The detector 22 may be a photodiode or any kind of light sensitive device.
Figs. 7a) and 7b) show an optical system 50 for attenuating and imaging an optical beam 52 for a subsequent intensity measurement of the optical beam 52 according to another embodiment.
The optical system 50 is not only insensitive to a polarization of the optical beam 52, but is also insensitive to the state of collimation of the optical beam 52, in particular in cases, where the cross-sectional size of the optical beam 52 is large. The optical system 50 comprises, in the order of the propagation direction of the input optical beam 52, an input aperture 54 and a collimating optical element 56 for collimating the optical beam 52 propagating divergently from the input aperture 54. In this embodiment, there is also arranged a pupil stop 51 and a positive lens 53 upstream of the input aperture 54. The collimating optical element 56, for example a collimation lens, again creates a parallel optical beam 52a from the divergent input optical beam 52.
The optical system 50 further comprises an attenuating optical arrangement 58 for attenuating the intensity of the optical beam 52a propagating from the collimating optical element 56.
The attenuating optical arrangement 58 comprises a first group 60 of a first optical element 62 and a second optical element 64. The optical element 62 and 64 are configured as plane-parallel plates, for example made of glass, which are partially reflective and partially transmissive. The plates preferably are thin, for example have a thickness of about 1 mm.
The optical element 62 has a surface 66 and the optical element 64 has a surface 68. The surfaces 66 and 68 form a retro-reflector-like arrangement which means that the surfaces 66 and 68 form an angle with one another which is preferably about 90°, but may be vary in a range from about 70° to about 110°.
The surface 66 is oriented with respect to the optical beam 52a propagating from the collimating optical element 56 such that the optical beam 52a is incident on the surface 66 at an incident angle of about 45°, wherein the incident angle may vary according to the orientation of the optical element 62.
The optical beam 52a is partially reflected at the surface 66 (reflected part 52b) and partially transmitted through the optical element 62 (transmitted part 52c). The reflected part 52b of the optical beam 52 is then directed to the surface 68 of the optical element 64 and again partially reflected (reflected part 52d) and partially transmitted (transmitted part 52e).
The transmitted beam parts 52c and 52e can be destroyed in a beam dump as described with respect to the above embodiment.
The attenuating optical arrangement 58 further comprises a second group 70 of a third optical element 72 and a fourth optical element 74. The third and fourth optical elements 72 and 74 are also configured as plane-parallel plates, which are partially reflective and partially transmissive.
The third optical element 72 has a surface 76, and the fourth optical element 74 has a surface 78.
The third and fourth optical elements 72 and 74 again form a retro-reflector-like arrangement with one another, and, in particular, form an angle of about 90° with respect to one another. The angle between the third and fourth optical element 72 and 74 and their individual orientation is adjustable.
The reflected part 52d propagating from the second optical element 74 is incident on the surface 76 of the optical element 72 and partially reflected (reflected part 52f) and partially transmitted (transmitted part 52g). The third optical element 72 is arranged such that it is not in the same plane as the optical elements 53 or 56 so that the transmitted part 52g does not pass through the optical element 53, 56.
The reflected part 52f of the optical beam 52 is directed to the fourth optical element 74 and partially reflected at the surface 78 (reflected part 52h) and partially transmitted (transmitted part 52i). The reflected part 52h is then directed to a focusing element 80 which focuses the optical beam having a low intensity due to its attenuation by the attenuating optical arrangement 58, onto a detector 82.
It is to be understood that while partial reflection and partial transmission of the optical is described with respect to the front surfaces of the optical elements 62, 64 and 72, 74, partial reflection and transmission can instead occur at the back surfaces only, or at the front as well as at the back surfaces.
The attenuating optical arrangement 58 of the optical system 50 is insensitive to polarization of the optical beam 52 due to the fact that two or an even number of groups of optical elements are provided, wherein each group forms a retro-reflector- like arrangement, wherein at least two groups of retro-reflector-like optical arrangements are off-set with one another by at least approximately 90°, as shown in Figs. 7a) and 7b) for the groups 60 and 70.
The optical system 50, however, is not only insensitive to polarization, but also insensitive to incidence angles at the respective surfaces 66, 68 and 76, 78, as will be described with respect to Fig. 8.
Fig. 8 shows one of the retro-reflector-like arrangements, for example the first group 60.
There are shown two rays B and A which are incident on the optical element 62 at an angle ΦA and φB, wherein φβ > φA.
After reflection at the optical element 62, the rays A and B are incident on the optical element 64 at an angle ψA and ψβ, wherein now ψA > ψB. This means, according to Fresnel's law of reflection which reveals an angle dependency of the reflectivity of an optical element, a larger reflectivity for a higher incidence angle on the first optical element 62 is compensated by a lower reflectivity for the lower incidence angle on the second optical element 64.
Thus, the optical system 50 is particularly useful for optical beams, which are not or cannot be perfectly collimated, i.e. have an angle spread as it is the case for optical beams having a large cross-sectional size.
Further, the provision of the pupil stop 51 mainly blocks diffracted light and the input aperture 54 which is a field stop, mostly blocks stray light. The aforementioned apertures and further the large splitting angles between the reflected beam parts (for example reflected part 52b) and the transmitted parts (for example transmitted part 52c) render the optical system 50 insensitive for stray light and diffracted light.
Fig. 9 shows still another embodiment of an optical system 90 for attenuating and imaging an optical beam 92 for a subsequent intensity measurement of the optical beam 92 by a detector 94.
With respect to a pupil stop 96, a positive lens 98, a field stop 100 (input aperture) and a collimating optical element 102, the optical system 90 is similar or identical with the optical system 50 so that reference is made to the description above.
The optical system 90 comprises an attenuating optical arrangement 104, which is simply formed by a diffuser plate 106 which diffuses the optical beam 92a propagating from the collimating optical element 102. The detector 94 is arranged behind the diffuser plate 106 such that a beam portion of low intensity reaches the detector 94, while the remaining portion of the optical beam 92a passes by the detector 94 and may be absorbed in a beam dump as previously described. In the context of this embodiment, imaging here means that a part of the light is scattered by the diffuser plate 106 in direction to the detector 94, so that the imaging is diffuse in contrast to the afore-going embodiments.

Claims

1. An optical system for attenuating and imaging an optical beam for a subsequent intensity measurement of said optical beam, comprising
an input aperture for said optical beam,
a collimating optical element for collimating said optical beam propagating divergently from said input aperture,
an attenuating optical arrangement for attenuating the intensity of said optical beam propagating from said collimating optical element, said attenuating optical arrangement being arranged to modify said optical beam such that a beam portion with an intensity lower than the intensity of the optical beam is generated, said attenuating optical arrangement being insensitive to a polarization state of said optical beam,
a detector for measuring said intensity of saidbeam portion propagating from said attenuating optical arrangement.
2. The optical system of claim 1, wherein said attenuating optical arrangement comprises at least one wedge through which said optical beam passes, wherein said at least one wedge has an entrance face and an exit face inclined with respect to said entrance face, and wherein attenuation of said optical beam is accomplished by at least one partial reflection at said entrance face and at least one partial reflection at said exit face, wherein the reflected parts of said optical beam are used as said beam portion.
3. The optical system of claim 2, wherein said attenuation is accomplished by at least one reflection at said exit face and at least one reflection at said entrance face inside said at least one wedge.
4. The optical system of claim 2, wherein said attenuating optical arrangement comprises at least two wedges, wherein one of said at least two wedges has an orientation with respect to the other of said at least two wedges, which is offset by at least approximately 90° with respect to the direction of propagation of said beam portion of said optical beam between said at least two wedges.
5. The optical system of claim 4, wherein said at least two wedges have at least approximately identical wedge angles.
6. The optical system of claim 2, wherein said attenuating optical arrangement comprises an even number of wedges, wherein said number is at least two.
7. The optical system of claim 2, wherein said at least one wedge is oriented with respect to said optical beam such that said optical beam enters said at least one edge at least approximately at small angles.
8. The optical system of claim 2, wherein said at least one wedge has a wedge angle smaller than 20°.
9. The optical system of claim 8, wherein said wedge angle is smaller than 10°.
10. The optical system of claim 1, wherein said attenuating optical arrangement comprises at least one group of a first partially reflective optical element and a second partially reflective optical element, said first and second optical elements form a retro-reflector-like arrangement, and wherein a reflected portion of said optical beam reflected at said first and second optical elements is used as said beam portion passed to said detector.
11. The optical system of claim 10, wherein said first and second optical elements form an angle with respect to one another in a range from about 70° to about 110°, and preferably form an angle of about 90°.
12. The optical system of claim 11, wherein said angle is adjustable.
13. The optical system of claim 10, wherein said first and second optical elements are plane plates.
14. The optical system of claim 10, wherein said at least one group of said first and second optical elements is a first group, said attenuating optical arrangement further comprising a second group of a third partially reflecting optical element and a fourth partially reflecting element, said third and fourth optical elements forming a retro-reflector-like arrangement, said second group being rotated with respect to said first group by an angle of at least approximately 90°.
15. The optical system of claim 1, wherein said attenuating optical arrangement comprises at least one diffuser plate.
16. The optical system of claim 1, further comprising a focusing optical element for focusing said beam portion of said optical beam on said detector.
17. The optical system of claim 16, wherein said detector is positioned in a distance of about the focal length of said focusing optical element from said focusing optical element.
18. The optical system of claim 1, wherein said input aperture is positioned in a distance of about the focal length of said collimating optical element from said collimating optical element.
19. The optical system of claim 16, wherein said focusing optical element is chosen from the group comprising a spherical, an aspherical and a cylindrical lens.
20. The optical system of claim 1, wherein said collimating optical element is chosen from the group comprising a spherical, an aspherical and a cylindrical lens.
PCT/EP2006/007662 2005-08-02 2006-08-02 Optical system for attenuating and imaging an optical beam for a subsequent intensity measurement WO2007014773A1 (en)

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CN108594444A (en) * 2018-03-28 2018-09-28 浙江师范大学 The method for generating Mathieu light beams based on film amplitude modulation and axicon lens phase-modulation

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