WO1996036893A1

WO1996036893A1 - Coherence filter

Info

Publication number: WO1996036893A1
Application number: PCT/SE1996/000639
Authority: WO
Inventors: Nils Hugo Leopold Abramson
Original assignee: Nils Hugo Leopold Abramson
Priority date: 1995-05-18
Filing date: 1996-05-15
Publication date: 1996-11-21
Also published as: SE9501844D0; SE9501844L

Abstract

A device is disclosed which, using destructive interference, stops coherent light from lasers but lets through incoherent light. The device comprises four identical gratings (G1, G2, G3, G4). A part of a parallel laser light beam at an entering position (A) is diffracted by the gratings. Another part of the beam passes right through the gratings and interferes with the diffracted beam at an exiting position (B). Regardless of the laser light wavelength, symmetry will make the undiffracted and the diffracted beams meet at the fourth grating (G4) under the same angle as they left the first grating (G1). Thus destructive interference can occur at the fourth grating (G4) across the entire beam width.

Description

COHERENCE FILTER

The present invention relates to a device that blocks coherent light of lasers, regardless of laser light wavelength, using destructive interference, and lets through ordinary incoherent light.

Background of the invention

For the protection of eyes, or instruments, it is often important to be able to filter out the coherent light from strong lasers, while ordinary light passes through. Such a filter can be an ordinary interference filter (or etalon) that by constructive interference reflects almost all light of one certain wavelength. Consequently, it stops by destruc¬ tive interference almost all light of that same single wave-length from passing through.

Description of related art

Grating interferometers have been described in literature, by for instance E.N. Leith and B.J. Chang in "Image Formation with an Achromatic Interferometer", Opt. Commun. 23,217 (1977). Their main accomplishment has been to produce interference fringes in white light of almost zero coherence. To get such a result, all the light passing directly through is stopped. Entering light is diffiracted in a symmetrical way both upwards and downwards by a first grating. The two beams pass a second and a third grating, whereupon they intersect at the axis of symmetry. Because the two beams are rnirror-symmetric, they have identical pathlengths and produce interference even for incoherent light. Thus, this concept is not useful as a coherence filter but is used to produce interference fringes at an output of the interferometer.

A diffraction coherence filter that can differentiate between coherent and incoherent radiation is known from US 4,958,892. This filter is based on Bragg interference principles and comprises a plurality of spaced-apart lower-hierarchy optical elements, containing a series of interference structures, which form a higher-hierarchy compound optical structure in a manner such that mutual constructive interference of light occurs as a function of incident light wavelength. However, since the interference is wavelength dependent, this filter cannot be used to block coherent light of all wavelengths.

Summary

It is an object of the present invention to provide a filter that stops all coherent light within a wide range of wavelengths.

It is a further object of the invention to provide a filter that stops all coherent light within a wide range of wavelength and within a wide angle of incidence.

It is another object of the invention to provide a filter that, using the same inventive concept, conversely stops all incoherent light within a wide range of wavelength and within a wide incident angle.

These objects are achieved by a de\ice having the characterizing features in claim 1 or 2. Fuither features and improvements of the invention, are given in the dependent claims.

According to the invention, an optical filter for blocking coherent light within a wide wavelength region is provided. The optical filter comprises optical means having an input diffracting some incoming light beams and passing other light beams through undiffracted, and an output providing destructive interference between said diffracted light beams and said undiffracted light beams, blocking all coherent light beams from exiting through said output. Depending on the implementation of the filter, it could be used to block coherent light and let incoherent light through, or the other way around, blocking incoherent light. Brief Description of the Drawings

For a more complete understanding of the present invention and for further objects and advantages thereof, reference is now made to the following description taken in conjunction with the accompanying drawings, in which:

fig 1 shows a coherence filter according to a preferred embodiment of the invention;

fig 2 shows a coherence filter according to a second embodiment of the invention;

fig 3 shows a coherence filter according to a third embodiment of the invention; and

fig 4 shows a coherence filter according to a fourth embodiment of the invention.

Detailed Description of Embodiments

Fig 1 shows a coherence filter according to a preferred embodiment of the invention. The filter comprises optical means 1 comprising an input 2, an output 3 and therebetween intermediate means 4. The input 2 diffracts incoming light beams and the output 3 provides destructive interference between said diffracted light beams, thus blocking all coherent light beams from exiting through said output 3.

In the preferred embodiment, the filter is an interferometer made up of four identical gratings as described in Fig. 1. The input 2 thus comprises a first grating G_l5 The intermediate means 4 comprises a second G₂ and a third grating G₃, while the output comprises a fourth grating G₄. The intermediate means may also comprise a compensating glass block C. Each grating diverges light of a certain wavelength at a certain angle and the light rays would travel in exactly the same path if time were to move backwards (the rule of reciprocity).

One parallel beam of red light arrives at an entering position A, whereupon it is diffracted upwards by the first grating

passes an intermediate position D. is again diffracted by the second grating G₂ so that it fravels in the original direction, is diffracted downwards by the third grating G₃ until it hits the fourth grating G₄ which diffracts the beam so that at an exiting position B it is again parallel and collinear to the original beam at the entering position A. All the gratings are identical and separated by one same distance 1.

Another part of the entering beam passes straight through the gratings G|-G₄, whereupon it at the exiting position B interferes with the diffracted beam. Because of the symmetry of the device the two beams at the fourth grating G₄ intersect at the same angle as the two beams leave the first grating G| . The result is that at the fouith grating G there could be either constructive or desfructive interference over the whole beam width. Destructive interference means that no light passes through the filter if the two beams are out of phase by 180° at the exiting position B. Thus, in this filter according to the invention, all coherent light will be reflected due to consntictive interference in the reflective mode, whereas all incoherent light will pass through due to constructive interference in the transmission mode.

The only reason for this result is the symmetry of the device. Therefore, the same reasoning could be made for light of any colour. Blue light will be less diffracted at all gratings and thus passes an intermediate position E, but the end result will be the same as for red light. Thus, this grating interferometer is independent of colour and can be so adjusted that it produces destructive interference even for white light. It will stop all light if the direct beam and the diffracted beam are of the same power. This could be arranged by adjusting the efficiency of the gratings or by placing an absoiption filter in one of the beams. However, there is a difference in pathlength for the diffracted light and the light passing directly through. If the coherence length of the light is shorter than this differ¬ ence, there will be no destructive interference and no blocking.

There is, however, one more problem to be solved. An ordinary interference filter works properly only for light over a certain angle. This angle becomes smaller the larger the separation is between the two reflecting surfaces. As we need a separation large enough to distinguish between light of different coherence length, our device will be quite angle-dependent. The result will be that we can only stop coherent light from a very limited area of sight, e.g. from small objects at large distances. However, this restriction disappears if the radii of curvature are identical for the interfering wave fronts. This would be the case if we could arrange so that the pathlength difference appears to be zero to light rays (but not to lightwaves).

One way to accomplish this could be to place a glass block C in the longer path. This increases the pathlength measured in wavelengths but decreases the pathlength as measured by trigonometric calculations of the rays. (This is why an object appears closer when seen through water.) By choosing the right length and shape of the glass block, the filter could become almost insensitive to the angles of incidence of the incoming light rays. There would be some error left caused by the dispersion of the glass block. This would, however, be a small second order effect that probably could be compensated for. The glass block could also be used to adjust the phases at the fourth grating G , by varying angles, thickness, shape or material of the glass block. This adjustment could also be achieved by displacing one or more grating(s).

The set-up of Fig. 1 shows the preferred embodiment of the filter according to the invention. The second and third gratings G₂, G₃ could be replaced by one single grating with a higher frequency. The pathlength difference could be in the order of less than a millimetre and the whole filter could consist of a sandwich of thin layers. Because of the symmetry of the device, a semi-transparent mirror placed somewhere between G and G₃ could eliminate the use of G₃ and G₄, their function taken over by G₂ and Gi, or G₂ could be replaced by a reflection grating.

Fig 2 shows a coherence filter according to a second embodiment of the invention. In this embodiment, the input 2 comprises a first reflecting surface Si, the output 3 comprises a second reflecting surface S₂ and the intermediate means 4 comprises a material

The two reflecting surfaces S|, S₂ of the interference filter are separated by a distance D that, measured in wavelengths, has the same value for all wavelengths. This will be the case if, for instance the material M* by which the two reflecting surfaces S I, S2 are separated in a certain manner slows down red light more than blue, the result then being that the number of waves between the two reflecting surfaces is constant and independent of wavelength. The thickness of the separating material Mi will then determine the limit in coherence length for the light that is blocked.

Fig 3 shows a coherence filter according to a third embodiment of the invention. In this embodiment, the input 2 comprises a first polarisation filter F- , the output 3 comprises a second polarisation filter F₂ and the intermediate means 4 comprises a material M₂. The two crossed polarisation filters F| , F₂ could be separated by the material M₂ that rotates light of all wavelengths by the same angle. The thickness of the separating material M₂ will also in this case determine the limit in coherence length for the light that is blocked.

Fig 4 shows a coherence filter according to a fourth embodiment of the invention. In this case, the function is reversed and the filter blocks incoherent light and lets coherent light through. The configuration is basically the same as in the preferred embodiment, with small alterations. The optical-means 1 comprises an input 2 comprising a first grating Gi, intermediate means 4 comprising a second and a third grating G₂, G₃ and a compensating glass block C, and an output 3 comprising a fourth grating G₄. In this embodiment, the optical means also comprises a phase shift means 5. The phase shift means 5, here comprised in the input 2, may for instance be comprised in either the output 3 or the inteimediate means 4 instead, and its function is to change the phase of the incoming coherent light beams so that constructive interference is substituted for desπtictive interference and vice versa. Thus, all incoherent light will be reflected due to consntictive interference in the reflective mode, whereas all coherent light will pass through due to consntictive interference in the transmission mode.

Claims

1. Optical filter for blocking coherent light within a wide wavelength region, characterized by optical means ( 1 ) having an input (2) diffracting some incoming light beams while other incoming light beams pass through undiffracted, and an output (3) providing destructive interference between said diffracted light beams and said undiffracted light beams, blocking all coherent light beams from exiting through said output (3).

2. Optical filter for blocking incoherent light within a wide wavelength region, characterized by optical means ( 1 ) having an input (2) diffracting some incoming light beams while other incoming light beams pass through undiffracted, and an output (3) providing destructive interference between said diffracted light beams and said undiffracted light beams, blocking all incoherent light beams from exiting through said output (3).

3. Optical filter according to Claim 1 or 2, characterized by intermediate means (4) in said optical means ( 1) provided between said input (2) and said output (3) and providing different pathways for light beams of different wavelengths.

4. Optical filter according to Claim 3, characterized in that said input (2) comprises an optical grating (Gi).

5. Optical filter according to anyone of the preceding claims, characterized in that said output (3) comprises an optical grating (G₄).

6. Optical filter according to anyone of the preceding claims, characterized in that said intermediate means (4) comprises at least one grating (G₂, G₃).

7. Optical filter according to anyone of claim 1 -5, characterized in that said intermediate means (4) comprises at least two gratings (G₂, G₃) and in that a compensating glass block (C) is provided between the two gratings (G;, G₃) to compensate for differences in path length.

8. Optical filter according to Claim 1 or 2, characterized by intermediate means (4) in said optical means ( 1) provided between said input (2) and said output (3) providing equal path lengths, measured in wavelengths, for light of all wavelengths.

9. Optical filter according to Claim 8, characterized in that said intermediate means (4) comprises a material (Mi) that decelerates light of different wavelengths to different extents thus providing equal path lengths, measured in wavelengths, for light of all wavelengths.

10. Optical filter according to Claim 1 or 2, characterized in that said input (2) and said output (3) together comprise two crossed polarisation filters (Fi, F₂) separated by a material (M ) that rotates light of all wavelengths by the same angle.