US3893749A - Process for the determination of an assembly having isotropic oblique reflection in an extensive spectral region and assemblies obtained by this process - Google Patents

Process for the determination of an assembly having isotropic oblique reflection in an extensive spectral region and assemblies obtained by this process Download PDF

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US3893749A
US3893749A US409160A US40916073A US3893749A US 3893749 A US3893749 A US 3893749A US 409160 A US409160 A US 409160A US 40916073 A US40916073 A US 40916073A US 3893749 A US3893749 A US 3893749A
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mirror
plate
thickness
light
anisotrophy
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Michael Ferray
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Societe dOptique Precision Electronique et Mecanique SOPELEM SA
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Societe dOptique Precision Electronique et Mecanique SOPELEM SA
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    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B5/00—Optical elements other than lenses
    • G02B5/30—Polarising elements
    • G02B5/3083—Birefringent or phase retarding elements
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B5/00—Optical elements other than lenses
    • G02B5/08—Mirrors

Definitions

  • a reflecting assembly for polarized white light includes at least one mirror associated with a birefringent compensator preventing the reflected light from having a large phase anisotropy.
  • the mirror or mirrors are metallic and covered with a protective silica layer having a geometric thickness of from 0 to about 400A.
  • the birefringent compensator is a crystalline plate of predetermined thickness depending upon the thickness of the protective layer. Known compensators may be used in place of the birefringent plate.
  • the present invention relates to a process for the determination of a reflecting assembly which does not affect the state of polarization of an oblique incident wave, in an extensive spectral region. It likewise relates to the assemblies having isotropic oblique reflection thus obtained.
  • the invention applies in particular to all instruments using polarized light when it is necessary to bend the beam for reasons of space. This is particularly the case in modern optical microscopes using polarized light, in which it is an advantage to introduce auxiliary systems such as zoom or a pupillary relay between the objective and the eye-piece, without increasing the height of the instrument as a result. Up to now, it has been impossible to bend the beam without disturbing the state of polarization of the incident wave.
  • any oblique reflection of a polarized wave on a metallic surface is anisotropic. This is due to the inequality of the reflection coefficients corresponding to the states of polarization parallel (polarizatiomp) and perpendicular (polarizationzs) to the plane of incidence. These coefficients generally have a complex expression of the form re" and may differ either by their modulus r or by their phase qb.
  • the inequality in the moduli leads to a rotation of the direction of incident polarization which can easily be com pensated by rotation of the polarizer, while the inequality in the phases leads to transforming a rectilinear incident vibration into an elliptical reflected vibration.
  • a total-reflecting prism constitute a solution; it does not have any anisotropy of amplitude but the anisotropy of phase is considerable (about 51 for a glass of incidence 1.6) and substantially constant in the visible spectrum.
  • FIG. 1 is a table of the values in degrees of the anisotropy of phase for an aluminum mirror coated with silica having a thickness of about IOOOA',
  • FIG. 2 is a table of values of the coefficients of the index of aluminum and silver
  • FIG. 3 is a series of curves showing in full line curves of the differences in phase depending on wave length for an aluminum mirror receiving a beam of polarized white light at an incidence of 45 corresponding to thicknesses of the protective layer from O to 1020A and in broken lines curves relating to quartz plates of various thickness;
  • FIG. 4 is a table of values for a bare aluminum mirror showing anisotropy of phase, the anisotrophy of phase in degrees introduced by a quartz plate and the residue of compensation;
  • FIG. 5 is a series of curves over the visible spectrum of the value in degrees of the residue of compensation for a bare aluminum mirror
  • FIG. 6 is a table of values similar to FIG. 4 for a bare silver mirror
  • FIG. 7 is a table of values for an aluminum mirror covered with a protective layer of silica A thick;
  • FIG. 8 is a diagrammatic showing of polarized light reflected from a mirror and passing through a crystalline birefringent plate.
  • FIG. 9 is a showing similar to that of FIG. 8 in which the crystalline birefringent plate is replaced by an equivalent compensator.
  • the table of FIG. 1 gives, in its first line, the values in degrees of the anisotropy of phase introduced by such a mirror for an incidence of 45 and for the whole of the visible spectrum. In order to retain zero anisotropy in the middle of the spectrum, it would be necessary to use a 60-micron-wave quartz plate which produces a birefringence of 360 for this wavelength.
  • the second line of the table indicates, in degrees, the anisotropy of phase introduced by this quartz plate depending on the wavelengths of the spectrum.
  • the third line of the table gives the compensation residue, that is to say the difference in phase existing after reflection of the beam of the mirror and passage through the quartz plate.
  • the birefringent compensator only provides real compensation in a very narrow zone about the middle of the spectrum; on the contrary, it increases the defect as soon as there is a very slight movement away from this median wavelength. It would be the same if a precise correction were aimed at on another wavelength.
  • the object of the present invention is to permit the constitution of an assembly associating a birefringent compensator with one or more mirrors, so that this assembly can be used in polarized light in an extensive spectral region.
  • the invention relates to a process for the determination of the characteristics of the elements of this assembly and likewise relates to the assemblies thus constituted,
  • the invention applies to an assembly consisting of at least one metallic mirror, which may or may not be covered with a thin layer of transparent dielectric, with which there is associated a crystalline compensator device equivalent to a thin plate.
  • the thickness of the dielectric layer of the mirror or mirrors and the thickness of the compensating crystal plate are determined depending on one another, working out by calculation.
  • the variation in the difference in the complex coefficient phases of reflection on the mirror or mirrors relating to the polarization parallel to and to the polarization perpendicular to the plane of incidence, and this for different thickness of the dielectric. depending on the wavelength, in the spectral region under consideration.
  • the final selection of the pair of thicknesses being made taking into consideration the minimum thickness of dielectric compatible with the mechanical behaviour of the mirror when this factor is determinant, or taking into consideration the maximum permissible residue of compensation when the use of the mirror allows the corresponding thicknesses of dielectric to be accepted.
  • r, (rp or rs) is the modulus of the complex reflection coefficient for the polarization p or s.
  • b (d p or d) s) is the phase of the complex reflection coefficient for the polarization p or s.
  • r1, (rlp or rls) is the real reflection coefficient airlayer for the polarization p or s, r2, (r or ra is the modulus of the complex reflection coefficient metal-layer for the polarization p or s.
  • a (up or as) is the phase of the complex reflection coefficient metal-layer for the polarization p or s.
  • the graph of FIG. 3 gives, in full lines, such a network of curves relating to an aluminium mirror receiving a beam of polarized white light at an incidence of 45.
  • the various curves correspond to various thick ness of the protective layer of silica, varying from 0 (bare mirror) to 1020 A, this last thickness corresponding to conventional practice for such protected mirrors.
  • D is the thickness of the crystal plate, is the wavelength of the light
  • N is the extraordinary index of the crystal
  • N is the ordinary index of the crystal.
  • the anisotropy of phase varies substantially as the inverse of the wavelength, and for each thickness of crystal, the curve of the anisotropy of phase depend ing on the wavelength has a hyperbolic shape.
  • the graph of FIG. 3 gives, in broken lines, a network of curves relating to quartz plates of various thickness.
  • EXAMPLE I A bare aluminium mirror is used, that is to say not covered with dielectric.
  • the table in FIG. 4 gives, in its first line, the values in degrees of the anisotropy of phase introduced by the mirror under an incidence of 45, for the whole of the visible spectrum.
  • the thickness of the quartz plate which brings a precise correction for the median wavelength of the visible spectrum, namely about 5500 A will then be sought.
  • the second line of the table indicates in degrees the anisotropy of phase introduced by this quartz plate 2.17 microns thick
  • the third line of the table gives the residue of compensation, that is to say the difference in phase existing after reflection of the beam on the mirror and passage through the quartz plate.
  • the residue of compensation varies with the angle of incidence, but it may be noted that it always remains small.
  • the graph of FIG. 5 gives, over the extent of the visible spectrum, the values in degrees of this residue of compensation for this same bare aluminium mirror compensated by a quartz plate of 2.17 microns, and for incidences from 2230 to 60.
  • EXAMPLE 2 This example is illustrated by the table of FIG. 6 and relates to a bare silver mirror, without dielectric protection.
  • the thickness D 3.92 microns of the quartz plate was determined in such a manner as to ensure precise compensation for the median wavelength of the spectrum.
  • the third line of the table gives, in degrees, the residues of compensation.
  • aluminium might be preferred to silver for the mirror because the residues of compensation are ultimately greater than in the previous example of a bare aluminium mirror.
  • EXAMPLE 3 This example is illustrated by the first part of the table of FIG. 7 and relates to an aluminium mirror covered with a protective layer of silica I50 A thick, which corresponds to an optical thickness of 220 A; it gives the residues of compensation here resulting from its associated with a quartz plate 3.39 microns thick. It will be seen that the residues of compensation are higher than in Example I but still remain broadly acceptable for numerous application.
  • EXAMPLE 4 This example is illustrated by the second part of the table of FIG. 7 and gives, under the same conditions as before, the residue of compensation resulting from the association of an aluminium mirror covered with 300 A of silica (optical thickness 440 A) with a quartz plate 4.45 microns thick.
  • the residues of compensation increase at the same time as the thickness of the protective layer of silica is increased.
  • the final selection will therefore be made depending on the preponderant requirements in the intended application. If an attempt is made to minimize the residual anisotropy of phase in the mirror and compensating plate assembly it would be necessary to accept limitation of the protective dielectric layer to very low values, even to use a bare mirror, which necessitates special conditions for mounting the mirror in the instrument.
  • the mechanical strength of the mirror which is the preponderant element the minimum thickness of dielectric to be deposited would be fixed first, the thickness of the compensating quartz plate would be determined, and the corresponding residues of compensation could be deduced therefrom.
  • the invention is not strictly limited to the examples which have been described but likewise covers the equivalent modes of embodiment.
  • a suitably adjusted compensator for example of the Babinet Soleil type as generally indicated at 13 in FIG. 9.
  • the examples described relate to the correction of a single mirror, it is likewise possible to compensate a plurality of mirrors by means of a single plate; in this case the thickness of the compensating crystal plate would be multiplied by the number of mirrors, as would the residue of compensation.
  • Optical assembly for the oblique reflection of a beam of polarized light covering a wide spectrum of wave lengths comprising a plane metallic mirror, a protective layer of transparent dielectric on said mirror having an optical thickness of between and 600A, the beam of light impinging on and being reflected by the mirror and protective layer and crystalline birefringent compensating means receiving the reflected light having birefringent compensation equivalent to that of a crystalline plate having a thickness for which the anisotropy of phases created by said means compensates exactly for the anisotrophy of phases created by said mirror for the means wave length of the spectrum of the beam of light.
  • Method of compensating for the anisotrophy of phases created by the reflection of a polarized beam of light covering a wide spectrum of wave lengths by a plane metallic mirror protected by a layer of transparent dielectric having a thickness of O to 600A the steps of directing the reflected beam of light through a birefringent compensating crystalline plate, adjusting the thickness of the plate whereby the anisotrophy of phases created by the plate compensates for the anisotro' phy of phases created by the mirror for the mean wave length of the spectrum of the beam of light, determining the adjusted thickness of the plate by measuring the anisotrophy of phases created by the mirror as a function of wave length, preparing a set of curves of the variation of the anisotrophy of phases of the birefringent plate as a function of the wave length and for a series of thicknesses for the plate and then finding the thickness of the plate from the curve providing exact compensation for the mean wave length of the spectrum 3.
  • Method of compensating for the anisotrophy of phases created by the reflection of a polarized beam of light covering a wide spectrum of wave lengths by a plane metallic mirror protected by a layer of transparent dielectric having a thickness of O to 600A the steps of directing the reflected beam of light through a birefringent compensating crystalline plate, adjusting the thickness of the protective layer as a function of the thickness of the plate whereby the anisotrophy of phases created by the mirror compensates for anisotrophy of phases created by the plate for the mean wave length of the spectrum of the beam light, determining the adjusted thickness of the layer by measuring the variation of the anisotrophy of phases of the plate as a function of wave length, preparing a set of curves of the variation of anisotrophy of phases for the mirror as a function of wave length and for a series of thicknesses of the layer and then finding the thickness of the layer from the curve providing exact compensation for the mean wave length of the spectrum,

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Optics & Photonics (AREA)
  • Optical Elements Other Than Lenses (AREA)
  • Surface Treatment Of Optical Elements (AREA)
  • Polarising Elements (AREA)
US409160A 1972-11-20 1973-10-24 Process for the determination of an assembly having isotropic oblique reflection in an extensive spectral region and assemblies obtained by this process Expired - Lifetime US3893749A (en)

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FR7241106A FR2257914B1 (xx) 1972-11-20 1972-11-20

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US (1) US3893749A (xx)
JP (1) JPS5849843B2 (xx)
AT (1) AT349787B (xx)
CH (1) CH594300A5 (xx)
DE (1) DE2354562C2 (xx)
FR (1) FR2257914B1 (xx)
GB (1) GB1403636A (xx)

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4332476A (en) * 1979-04-17 1982-06-01 Stenberg Johan E Method and apparatus for studying surface properties
EP1408355A1 (en) * 2002-10-10 2004-04-14 Lucent Technologies Inc. Polarization compensated optical tap

Families Citing this family (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPS5546706A (en) * 1978-09-29 1980-04-02 Canon Inc Phase difference reflecting mirror
JPS61208710A (ja) * 1985-03-14 1986-09-17 フアナツク株式会社 スイツチ構造

Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2464141A (en) * 1946-07-13 1949-03-08 Eastman Kodak Co Mirror with low thermal expansion support
US3774986A (en) * 1971-11-05 1973-11-27 Co Gen Electricite Recording superimposed holograms

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2464141A (en) * 1946-07-13 1949-03-08 Eastman Kodak Co Mirror with low thermal expansion support
US3774986A (en) * 1971-11-05 1973-11-27 Co Gen Electricite Recording superimposed holograms

Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4332476A (en) * 1979-04-17 1982-06-01 Stenberg Johan E Method and apparatus for studying surface properties
EP1408355A1 (en) * 2002-10-10 2004-04-14 Lucent Technologies Inc. Polarization compensated optical tap
US20040070830A1 (en) * 2002-10-10 2004-04-15 Carver Gary E. Polarization independent optical taps
US6807004B2 (en) 2002-10-10 2004-10-19 Lucent Technologies Inc. Polarization independent optical taps

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Publication number Publication date
ATA959173A (de) 1978-09-15
DE2354562C2 (de) 1984-02-23
GB1403636A (en) 1975-08-28
FR2257914B1 (xx) 1976-04-30
JPS4984261A (xx) 1974-08-13
CH594300A5 (xx) 1978-01-13
DE2354562A1 (de) 1974-06-06
FR2257914A1 (xx) 1975-08-08
AT349787B (de) 1979-04-25
JPS5849843B2 (ja) 1983-11-07

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