CA2714847A1 - Optical limiting method and devices based on self-formed standing waves with continuous multiple layer structure - Google Patents

Abstract

This invention relates to an optical limiting method and the devices designed based on this method. More particularly, this invention relates to an method of forming and utilizing self-formed standing waves with continuous multiple layer structure for beams with different incident angles from field of view in the nonlinear optical absorption, scattering, refraction, or photorefraction, photosensitivity material. This invention also relates to the stationary and portable optical limiters designed based on this method. These limiters are used to protect sensitive optical sensors, components and human eyes from light beam damage, dazzle, and so on.

Description

Background of the Invention Mane attempts have been made for providing method and devices to reduce optical transmission to desired low \alue for high incident power. and to keep a good transmission for lovN incident power. I he main target of these attempts is to protect sensitive optical sensors. components and human eyes from laser damage. This kind of devices has mane other applications. such as for laser power regulation or stabilization.
for optical data signal level restoration and so on ("Handbook of Optics", Vol. IV. 2d ed..
edited by M. Bass. J. M. Enoch. E. W. V. Stryland. W. L. Wolfe. McGraw-Hill.
New York. 2001. p. 19.1). If these devices have enough low limiting threshold, they can even be used for non-laser light. such as to view the object in dazzling background or emitting strong light, thus to act as sun visor for night driving. welding helmet and the like.

Since advantages of simplicity. high speed, compactness. and lo~N cost. the passive optical limiters attract more interests than the active control ones.
There are various principles oi' passive optical limiting. They are nonlinear absorption, scattering, refraction. and photorefraction, photosensitivity. etc ("Handbook of Optics", Vol. IV. 2d ed.. edited by M. Bass. .1. M. Enoch. F. NV. V. Stryland. W. L. Wolfe. McGraw-Hill. Nety York. 2001, p. 19.2-19.9; .1. S. Shirk. "Protecting the War Fighter's Vision in a Laser-Rich, Battlefield Environment'. Optics & photonics News, Vol. 11. No. 4, p.19-23, 2000).
The nonlinear absorption is produced by intensive irradiance. It absorbs light energy and reduces the high light power. The nonlinear absorption includes reverse saturable. two-photon and free carrier absorptions and relates to semiconductors, organic and organornetallic materials. The nonlinear scattering is due to strong light induced creation of scatter centers. These centers scatter the light rays of the transmitted beam and attenuate the high light power. The nonlinear scattering materials usually consist of a material with large refractive nonlinearity and a linear material which is suspended in the former or inversely. The indexes of two materials are matched normally. At high intensities, the indexes are not matched again and the materials become scattering. These materials include nonlinear photonic crystals and carbon-black suspensions, etc. The nonlinear refraction is generally caused by high intensity induced thermal change of refractive index. Usually the largest index change is at the beam focal point by heating and following material expansion, resulting in beam defocusing. Such self-defocusing bends the light rays from their transmission directions and reduces the high light power.
A typical nonlinear refraction material is semiconductor. Photorefractive materials change their index when exposed to light. and their index change is in proportion to the intensity gradient of light. They include photorefractive crystals. such as Lithium Niobate.
The photosensitive materials change their optical property, such as polarization, when illuminated by light. A typical material is liquid crystals. The liquid crystals have an irradiance-dependent birefringence, which is used for optical limiting.

In order to get ideal limiting results, the used materials must have large nonlinear optical absorption, scattering, refraction, or photorefractive, photosensitive coefficients, and intensity of the stimulating light must be high. Since the said coefficients of existing materials are not large enough, the power of incident light must be high.
There is a commonly used method to raise the stimulating light intensity and so to lower the limiting threshold. It is to focus the incident beam by lens. Beam focusing raises the intensity greatly. However. due to optical diffraction, the shrink of waist size of the focusing beam has a limitation. In visible region. for a laser beam with excellent space coherence. the minimum waist size is larger than a few microns. Therefore, the available optical limiters are just effective for high or relatively high light powers.
New method and devices for limiting low power light, such as low power CW laser and even non-laser light are needed.

Summary of the Invention The goal of this invention is to create a method for passive optical limiting, and to provide limiter structure designs based on this method. This method can lower the optical limiting threshold. Furthermore. this method can be used to construct various types of optical limiters. including nonlinear optical absorption, scattering, refraction. along with photorefraction. photosensitivity and so on, thus to satisfy different optical limiting requirements. These optical limiters have characteristics of simple structure, ordinary elements and compact size. They not only can he installed in various optical equipments and systems as a component. but also can be made as a portable goggle type of device for individual use.

The said method of the invention is based on forming and utilizing the standing waves formed by spherical partial reflecting, circlewise cut amplitude dividing, or circlewise cut partial reflecting the beams with different incident angles from whole field of view. These standing waves are formed in the nonlinear optical absorption.
scattering.
refraction, or photorefraction, photosensitivity material, and are made have intensity distribution with periodic continuous multiple layer structure.

Speaking in detail, the parallel beam with each incident angle from field of view is converged by lens or mirror. The converged beam is then divided into two separate beams by spherical partial reflection, circlewise cut amplitude division, or circlewise cut partial reflection (beam dividing may be before beam converging). Afterwards.
two separate beams are guided to travel along opposite or near opposite directions, and are connected at their focal points in the nonlinear optical absorption, scattering. refraction.

or photorefraction. photosensitivity material. When using spherical partial reflection, two meeting beams are overlapped entirely. When using circlewise cut amplitude division. the solid angle of one of each pair of divided beams is circlewise cut by a diaphragm to make remain area of the solid angle can be overlapped entirely by another divided beam of the same pair. When using circlewise curt partial reflection, the solid angle of each transmitted beam passing through the partial reflecting coating and output (compound) lens is circlewise cut by a diaphragm to only let the beam part emerging from the entirely overlapped area pass. In these entirely overlapped areas. the standing waves are formed.
These standing waves have intensity distribution with periodic continuous multiple layer structure. Realize same process for all parallel beams with different incident angles from the whole field of view. Then utilize the features of the formed standing waves, including higher light intensity at anti-node. ununilormity of intensity distribution.
large intensity gradient, periodic or quasi periodic continuous multiple layer structure of intensity distribution, and so on, to lower the optical limiting threshold and enhance the optical limiting performance.

In other words, the above said method is to create two identical focal curved surfaces or focal planes in the nonlinear optical absorption, scattering, refraction, or photorefraction, photosensitivity material. and to make them coincide with each other. Or the above said method is to create two identical image fields for all parallel beams with different incident angles from field of view in the nonlinear optical absorption. scattering, refraction. or photorefraction. photosensitivity material, and to make them coincide with each other.

Every pair of two separate beams must have same frequency, same polarization, and appropriate amplitude ratio (the ratio value depends on concrete limiter structure and is discussed underneath). If an incident beam has a relatively wide bandwidth, its two separate beams must have same spectral distribution. Thus a special optical interference field containing many standing waves is formed in the above said optical limiting material, that is, the nonlinear optical absorption, scattering, refraction, or photorefractive, photosensitive material. Each standing wave corresponds to one parallel beam from the field of view, and has a periodic continuous multiple layer structure. For an incident beam with a relatively wide bandwidth. if the phase difference between its two separate beams at their connected focal points is near zero, a thin quasi periodic continuous multiple layer structure may still be formed in the optical limiting material due to optical interference.

It is known that in a standing wave consisting of two beams with almost equal amplitude value, the light intensity at anti-nodes is 4 times as high as the light intensity of each component beam. Because the coefficients of the above said optical limiting materials rise with increase of light intensity, and most of them are nonlinear, the standing wave will enhance these effects around its anti-nodes. In addition.
the respond time of these effects usually reduces with increase of the light intensity, so the standing wave will shorten the respond time of these effects too in some situations.
For nonlinear optical scattering and refraction materials, the ununiformity of intensity distribution in standing wave (the light intensity varies from zero at a node to a maximum at an anti-node in a distance of 4) will make distributions of scattering centers and refractive index become more un-uniform. It will enlarge optical scattering and self-defocusing.
Furthermore. for nonlinear optical refraction materials, the standing wave will make the index distribution have a period continuous multilayer structure. This structure satisfies Bragg reflection requirement just for the beam induced it ("Handbook of Optics". Vol. III, 2d ed., edited by M. Bass, J. M. Enoch. E. W. V. Stryland, W. L. Wolfe. McGraw-Hill, New York, 2001. p. 22.1-22.4). Thus, a mirror like multilayer reflector is formed. This mirror like reflector is much more efficient than the normal interference pattern or grating for optical limiting. The interference pattern or grating used for optical limiting is induced by two-beam coupling in the nonlinear refractive or photorefractive materials.
Using such interference pattern or grating to limit the light is like using a lattice to block light. The light leaks from the holes of the lattice and deteriorates the limiting result.
However. the standing wave with continuous multiple layer structure can work like a multilayer reflector or mirror with no holes. It will limit the intensity of the incident light greatly. At last, in the standing wave the light intensity gradient is extremely large because the intensity changes from about zero to maximum within a short distance of .
This extremely large intensity gradient will dramatically increase the limiting results of the effects depending on the light intensity gradient. such as in the photo refractive crystals and photosensitive liquid crystals.

The said method of the invention is realized through setting up optical focusing, dividing, cutting. deflecting, and adjusting paths by using appropriate combinations of ordinary optical elements. including lens, mirror. prism, diaphragm, etc, and using the above said optical limiting materials. The preferred embodiments of limiter structure design for realizing the said method will be described in the underneath.
Obviously. these embodiments are not the all limiter structures which can be designed based on the said method. By using existing optical design knowledge, these embodiments may be alternated, modified to fit different practical needs. Furthermore, basing on the said method and using the existing design knowledge, the other limiter structures may be worked out. Therefore, the applicant of this invention reserves the right of all alternatives.
modifications. and equivalent arrangements of the limiter structure embodiments described underneath. The applicant also reserves the right of any optical limiter structure designs which base on the said method.

The aforementioned aspects and advantages of the invention will be appreciated from the following descriptions of preferred embodiments and accompanying drawings wherein:

Fig.] is the schematic optical structure of the limiter with spherical partial reflection as the first preferred embodiment of the limiter according to the invention.
Fig.2 is the schematic optical structure of the container used for the limiter shown in Fig. I.

Fig.3 is a schematic diagram illustrating reflected non-parallel beam by spherical partial reflection. A special standing wave is formed as interference.

Fig.4 is a schematic diagram illustrating reflected parallel beam with tilting incident angle by spherical partial reflection. A special standing wave is formed as interference.

Fig.5 is the schematic modified optical structure of the limiter with spherical partial reflection shown in Fig. I. A layer for avoiding self-defocusing and a pre-limiter are added.

Fig.6 is the schematic modified optical structure of the container shown in Fig.2.
A layer for avoiding self-defocusing and a pre limiter are added.

Fig.7 is another schematic modified optical structure of the limiter with spherical partial reflection shown in Fig.l. The optical limiting material filled in the container is nematic liquid crystals.

Fig.8 is a schematic diagram illustrating nematic liquid crystal director alignment in relaxed phase state (left tiled row). and in the reorient state driven by electric field induced via standing wave (right tilted row).

Fig.9 is a schematic diagram illustrating the ratio of the cross section areas separated by interval of 4 for the focusing beam.

Fig.10 is a schematic diagram illustrating two special standing waves formed in the limiter shown in Fig. 11, which correspond to the parallel normal incident beam.

Fig.lI is the schematic optical structure of the limiter with circlewise cut amplitude division as the second preferred embodiment of the limiter according to the invention.

Fig.12 is a schematic diagram illustrating two special standing waves formed in the limiter shown in Fig. 11, which correspond to the parallel tilting incident beam.

Fig.].') is a schematic diagram illustrating two special standing waves formed in the limiter shown in Fig. ll. which correspond to the non-parallel and normal incident beam.

Fig.14 is the schematic optical structure of another limiter with circlewise cut amplitude division as the third preferred embodiment of the limiter according to the invention.

Fig. 1 is the schematic left side view, taken on the dash line 122-122 in Fig.14 of the third preferred embodiment of the limiter according to the invention.

Fig.16 is the schematic optical structure of the limiter with circlewise cut partial reflection as the fourth preferred embodiment of the limiter according to the invention.
Fig. 17 is the schematic modified optical structure of the limiter with circlewise cut partial reflection shown in Fig.] 6. The solid angle of the entirely overlapped beam part is widened by enlargement of input lens aperture.

Fig.l8 is a schematic diagram illustrating the special standing wave formed in the limiter shown in Fig. 16. which corresponds to the non-parallel tilting incident beam.

Fig. 19 is the first schematic image erecting scheme for first preferred embodiment shown in Fig. I of the limiter according to the invention. Two right-angle prisms are used.
Fig.20 is the schematic top side view. taken on the dash line 176-176 in Fig.]
9 of the first schematic image erecting scheme for the first preferred embodiment of the limiter according to the invention.

Fig.21 is the second schematic image erecting scheme for the first preferred embodiment shown in Fig. 1 of the limiter according to the invention. An Abbe's prism is used.

Fig.22 is the third schematic image erecting scheme for the first preferred embodiment shown in Fig. I of the limiter according to the invention. An inverting lens is used.

Fig.23 is the fourth schematic image erecting scheme for the first preferred embodiment shown in Fig.1 of the limiter according to the invention. Two right-angle prisms are used. With position change of input compound lens, the field of view is increased.

Fig.24 is the schematic top side view, taken on the dash line 221-221 in Fig.23 of the fourth schematic image erecting scheme for the first preferred embodiment of the limiter according to the invention.

Fig 25 is the first schematic image erecting scheme for second preferred embodiment shown in Fig. 11 of the limiter according to the invention. An inverting lens is used.

Fig.26 is the second schematic image erecting scheme for the second preferred embodiment shown in Fig. 11 of the limiter according to the invention. A right-angle prism is used.

Fig.27 is the schematic top side view, taken on the dash line 256-256 in Fig.26 of the second schematic image erecting scheme for the second preferred embodiment of the limiter according to the invention.

Fig.28 is the first schematic image erecting scheme for fourth preferred embodiment shown in Fig.16 of the limiter according to the invention. Two right-angle prisms are used.

Fig.29 is the schematic top side view, taken on the dash line 272-272 in Fig.28 of the first schematic image erecting scheme for the fourth preferred embodiment of the limiter according to the invention.

Fig.30 is the second schematic image erecting scheme for the fourth preferred embodiment shown in Fig.16 of the limiter according to the invention. An Abbe's prism is used.

Fig.31 is the third schematic image erecting scheme for the fourth preferred embodiment shown in Fig.16 of the limiter according to the invention. An inverting lens is used.

Fig.32 is the fourth schematic image erecting scheme for the fourth preferred embodiment shown in Fig. 16 of the limiter according to the invention. Two right-angle prisms are used. With position change of input lens, the field of view is increased.

Fig.-33 is the schematic top side view. taken on the dash line 324-324 in Fig.32 of the fourth schematic image erecting scheme for the fourth preferred embodiment of the limiter according to the invention.

Fig.34 is the fifth schematic image erecting scheme for the fourth preferred embodiment shown in Fig.16 of the limiter according to the invention. Four mirrors are used.

Fig.35 is the schematic top side view, taken on the dash line 350-350 in Fig.34 of the fifth schematic image erecting scheme for the fourth preferred embodiment of the limiter according to the invention.

Fig.36 is the schematic detailed structure of the solid material 342 used in the fifth schematic image erecting scheme for the fourth preferred embodiment in the Fig.34.
Fig.37 is the schematic detailed structure of the container 342 filled with liquid material used in the fifth schematic image erecting scheme for the fourth preferred embodiment in the Fig.34.

Detailed Description of the Invention Fig.] illustrates the schematic optical structure of first preferred embodiment of the optical limiter according to the invention. A converging input compound lens comprising lenses 10 and 18 focuses a parallel incident beam into a solid above said optical limiting material 12. The focal point of the beam falls on the back surface 14 of the solid material. The back surface 14 is spherical and its sphere center is at optical center of the input compound lens. There is a partial reflecting coating with an intensity reflection ratio of. such as 90%, on the back surface 14. Thus most of the incident light returns hack against its incident direction. A small part of the beam passes the output compound lens comprising lenses 16 and 20. and is converged to become a parallel beam again. The complemental lens 18 is used to curve the image field of the lens 10. Unlike the normal correction. here the focal curved surface of the lens 10 needs to be spherical, and to match the back surface 14 of the solid material. In fact, any simple lens has a curved image field with concave focal surface tilting to the lens optical center, and there is a mature aberration correcting technology for modifying such field curvature ("Handbook of Optics". Vol. I. 2d ed.. edited by M. Bass, J. M. Enoch, E. W.
V. Stryland, W. L. Wolfe. McGraw-Hill, New York, 2001, p. 33.1-33.6). Similarly, a complemental lens 20 is used to make the focal curved surface of the lens 16 to match that of the input compound lens. and two focal curved surfaces are coincided. Because how to modify the focal curved surface of a lens may adopt the existing technology, here it is not discussed in detail.

If using liquid above said optical limiting material, the liquid material is placed into a transparent container 22 shown in Fig.2. In this situation, the said partial reflecting coating is on the inside back surface 24 of the container. The shape and location of the inside hack surface 24 of the container are same as those of the back surface 14 of the solid material.

In order to reduce energy loss and arbitrary light disturbance. all other optical surfaces of the lenses, the solid material, and the container are coated with high antireflection films for the desired wavelength range. The other optical surfaces of the solid material and the container are spherical too and have a common sphere center with their partial reflecting surfaces.

Thus, each of the reflected beams has same frequency, same polarization, roughly equal amplitude" and opposite direction with its corresponding incident beam.
The phase difference between the reflected beam and its corresponding incident beam is zero at reflecting coating and is fixed within their coherence length. For a beam with narrow bandwidth, especially for a laser beam, its coherence length AL is long according to the equation AL A~ (1) where A is the central wavelength, A2, is the bandwidth ("Handbook of Optics", Vol. I.
2d ed.. edited by M. Bass, J. M. Enoch, E. W. V. Stryland, W. L. Wolfe, McGraw-Hill, New York. 2001. p. 4.3. p.11.8). As a reference, for a beam with the bandwidth of full visible region from 0.4 pm to 0.76 m, its coherence length is 1.6 m. For a beam with the bandwidth being 1% of the full visible region. its coherence length is 160 [till. The coherence length of most laser beams is much over 1 Omm. Thus, for a narrow bandwidth beam especially a laser beam, a long conoid standing wave is formed in the traveling path of the incident beam, which overlaps the incident beam completely within the coherence length. In this spacial interference field, light intensity distributes in the manner of continuous multiple concentric spherical layers, and the interval between contiguous layers is just half of the wavelength of the incident beam.

A special attention must be given to reflection phase change. In normal light reflection, there is a phase change of 180 for one of two polarization components, which depends on the refractive indexes of the media on two sides of the reflecting interface and the incident angle ("Handbook of Optics". Vol. 1, 2d ed., edited by M. Bass.
J. M. Enoch, E. W. V. Stryland, W. L. Wolfe, McGraw-Hill, New York, 2001. p. 5.1-5.7).
These two polarization components will interfere with the incident beam and form two standing waves. They intervene to each other with interval of 4 . If amplitudes of two components are same or almost same, the distribution of the compounding intensity will become smooth as superposition of two standing waves. The periodic multilayer structure of intensity distribution will disappear or almost disappear. Therefore, measures must be adopted to eliminate it.

How to change or eliminate the phase difference between two polarization components is also an intensive study topic in Optics, and has got plenty results. The simplest way is to insert a half-wave plate made of birefringent crystal into the traveling path of the incident light at a suitable position ("handbook of Optics". Vol.
I. 2d ed., edited by M. Bass, J. M. Enoch. E. W. V. Stryland. W. L. Wolfe, McGraw-Hill, New York. 2001, p. 5.22-5.25), such as at the input end of the limiter. Thus, one polarization component of the incident beam, such as the extraordinary ray. travels slower or faster than the ordinary ray by half wave length. resulting in a phase difference of 180 being produced. When these two components are reflected, another phase difference of 180 is produced. making the total phase difference become 0 or 360 . However, as a convergent beam enters the reflective coating with different angles. each extraordinary ray splits into another two polarization components. The polarization direction of one is parallel with the incident plane. and another is perpendicular with the incident plan. Thus, part of the extraordinary ray does not have the same phase change as the other part. The same thing happens for the ordinary ray. As result, not all reflected rays have 0 or 360 phase difference with their corresponding incident rays. It means two standing waves are still formed. However, by choosing indexes of used limiting material and reflecting coating. and crystal axis direction of the half-wave plate properly, the intensities of two standing waves may differ large, resulting in a periodic distribution of the compounding intensity with relatively large difference between the anti-node and node. In addition, half-wave plate is wavelength dependence. For different wavelengths, it retards phase differently from 180 . This makes the length of conoid standing wave reduced, or in an exact word, the intensity distribution of the standing wave is modulated.

A better way is to deposit special constructed reflecting coating on the solid material back surface 14 or the container inside back surface 24. Usually such coating has multilayer structure. Such as metal/dielectric interferometer mirrors, total-internal-reflection phase retarders with 1, 2. or even 4 internal reflections, and so on (`Handbook of Optics"', Vol. I. 2d ed., edited by M. Bass, .1. M. Enoch, E. W. V.
Stryland, W. L.
Wolfe, McGraw-Hill, New York. 2001, p. 42.98-42.100). These mirrors and retarding films can maintain phase difference of being or near 0 or 360 for parallel and perpendicular polarization components in the vicinity of the designed incident angle, typically 0 or 45 . Furthermore, they permit wavelength to change within a certain range.
Another thing needing consideration is the shift of the image field of input compound lens. The real light source or object is not located at infinity distance. The moving of light source or the object. or the light sources or objects at different distances will make the image field shift or thicken. The lens equation is 1 = l + -1 (2) .S';
f S' where f is the lens focal length. s and s' are distances from object and image to lens optical center respectively ("Handbook of Optics", Vol. I. 2d ed., edited by M. Bass, J. M.
Enoch, E. W. V. Stryland, W. L. Wolfe. McGraw-Hill, New York. 2001. p. 1.55).
For example, if f= 5 cm. and object distance s = 4 in, 10 m, 100 m and 1000 m. the image distance s'= 5.06329 cm , 5.02513 cm, 5.0025 cm. and 5.0003 cm respectively, resulting in an image field shift or thickening of 0.6299 mm.

First this is not a problem for the applications with fixed light source, such as in some optical equipments and systems, because the input compound lens may be moved or specially designed to let its image field fall on the reflecting coating.
Second, for the most important application - the protection from laser damage. such as in the battlefield or laboratory, it is not a problem too. It is known that the extremely high brightness of the laser beam originates its extremely small divergence angle, that is, almost all of laser beams (for ones still with high brightness) are very good parallel beams.
Therefore, almost all of the laser beams entering the limiter can be focused on the reflecting coating well no matter how far they come from. Of course, for the strong non-laser light sources located at finite distances. the problem of image field shift or thickening must be considered.

This problem can be solved by the said method of the invention pleasantly. See Fig.3, a non-parallel beam comes from a light source at a finite distance.
After converged by input compound lens, it becomes beam 26. its focal point does not fall on the reflecting surface 28. that is, not on the back surface 14 of the solid material or the inside back surface 24 of the container. Its virtual focus is at point 30. This beam is reflected by the surface 28 and focused at point 32. It then diverges into beam 34, and interferes with the beam 26. It is known that in an interference field of two spherical waves with same frequency. polarization. amplitude. fixed phase difference. and within the coherence length, a conoid standing wave with intensity distribution of periodic continuous multilayer structure can still be formed along the connecting line of two sphere centers ("Handbook of Optics". Vol. 1. 2d ed.. edited by M. Bass. J. M. Enoch. E. W.
V. Stryland, W. L.. Wolfe. McGraw-Hill. New York. 2001. p. 2.11-2.14). These layers may not be spherical, but are still continuous within a certain solid angle. The angle value depends on the distance between two sphere centers. There are continuous multiple layers of intensity distribution between two sphere centers too. They are hyperboloids of revolution and are continuous within the whole coherence length. In brief, for the strong non-laser light beam comes from a finite distance, if its bandwidth is narrow enough, and its source is not located too near to the limiter, the standing wave with periodic continuous multilayer structure is still formed. This standing wave overlaps its corresponding incident beam completely within the coherence length. Of course, the output compound lens is designed to be moveable to adapt to object distance change.

As the reflecting coating has a high reflectance. the brightness of the objects in field of view is reduced for observer. Therefore the input compound lens is designed to have larger aperture and longer focal length than those of the output compound lens. Thus, the cross section area of the output beam is smaller than that of the input beam, which compensates the intensity loss for the objects. For example, if the radius of the input compound lens is 3.16 times the radius of the output one, the output intensity will be 10 times as high as that of the input one. The diameter and focal length of the input compound lens depends concrete applications, usually is from 0.5 cnl to 50 cm and 1 cm to 100 cm respectively. The thickness of the used solid above said optical limiting material, or the container filled with liquid above said optical limiting material is from I
m to 5 cm.

Thus, different types of optical limiters can be constructed by using different solid or liquid materials with nonlinear optical absorption. scattering, refraction, or photo refractive, photosensitive effects as the solid material 12 in Fig. I.
or as the liquid material filled in the container 22 in Fig.2.

First, the optical limiting process of the limiter using solid or liquid nonlinear optical absorption materials is described here. Making a solid nonlinear absorption material, such as a glass containing semiconductor nanocrystals with two-photon absorption, as the material 12, which has the same shape and reflecting coating as the material 12. or filling the container 22 with a liquid nonlinear optical absorption material.
such as the organic dye solution with reverse saturable absorption. Such type of limiter is built. If a high power laser beam enters the limiter, it is focused on the reflecting coating 36. that is, the partial reflecting coating 14 in Fig. I or 24 in Fig.2, at point 38 as shown in Fig.4. and is reflected partially. As mentioned above, a special standing wave forms in the traveling way of the incident beam and overlaps the incident beam completely within the coherence length.

Supposing the incident beam has the amplitude of unit 1. the amplitude of reflected beam is 0.9 because of 90% intensity reflectance. Then the maximum compounding intensity is (1 + 0.9~ = 3.797 at anti-node. and is (i - 0.9 0.0026 at node. Thus the light intensity at anti-node increases to about 4 times as high as the intensity of the original incident beam. The intensity at node drops from I to 0.0026. As the intensity drop amount at node is about quarter of the increasing amount at anti-node.
the net and cumulate intensity amount in total absorption range is raised. The optical absorption coefficients of used materials increase with the rise of light intensity nonlinearly, thus the absorption increasing at anti-node is over 4 times.
Therefore. the high power light will be absorbed more than that in the situation without the standing wave. Thus. comparing with the limiter under same working conditions, that is.
same limiting material and same light intensity, the limiter invented here has lower optical limiting threshold.

For a non-laser incident beam located at a finite distance, if its bandwidth is narrow enough. as shown in the Fig.'), a special standing wave with periodic continuous multilayer structure of the intensity distribution is formed too. Thus similarly as mentioned above, the high power light is absorbed more than that in the situation without the standing wave, making the limiter invented here have lower optical limiting threshold.

Second, the optical limiting process of the limiter using solid or liquid nonlinear optical scattering materials is described in sequence. Making a solid nonlinear optical scattering material, such as a nonlinear photonic crystal with fluence dependent transmission as the material 12, which has the same shape and reflecting coating as the material 12, or filling the container 22 with a liquid nonlinear optical scattering material, such as the black carbon suspension with the scattering induced by heating carbon particles. Such type of limiter is built.

Many nonlinear optical scatterings depend on high intensity induced ununiformity of optical properties of the material, such as the distribution ununiformity of refractive index and density. For example, a typical nonlinear photonic crystal consists of ordered array of tiny holes or channels in a glass. These holes or channels are filled with a nonlinear material whose refractive index initially matches that of the glass.
The high intensity makes the index matching lost, resulting in strong scattering and reduced transparency. In the black carbon suspension, high intensity produces plasma formation and scattering from the bubbles induced by a rapid heating of absorbing carbon particles.

Similarly as mentioned above, if a high power laser beam enters the limiter, it is focused on the reflecting coating and reflected partially. Then the special standing wave is formed. It has been known that in such a standing wave, the intensity is increased to 3.797 times at anti-node. and reduced to 0.0026 times at node if intensity reflectance is 90%. Thus, the scattering increases nonlinearly at anti-node with intensity rise. Since the intensity drop at node is about quarter of the intensity increase at anti-node, the net and cumulate intensity amount in total scattering range is raised. It will increase the total scattering amount. Furthermore. the distribution of the index changed holes/channels or density changed bubbles becomes un-uniform too. Such ununiformity is large because it is from almost zero to over 4 in a very short distance of A , which will cause additional scattering. Thus, the high power light is scattered more than that in the situation without the standing wave. Comparing with the limiter under same working conditions, that is, same material and same light intensity, the limiter invented here has lower optical limiting threshold. Same thing is for the non-laser beam, if it has enough narrow bandwidth and is not located too near to the limiter.

Next, the optical limiting process of the limiter using solid or liquid nonlinear optical refraction materials is described underneath. Making a solid nonlinear optical refraction material, such as semiconductor ZnSe exhibiting both of nonlinear absorption and refraction effects, as the material 12, which has the same shape and reflecting coating as the material 12, or filling the container 22 with a liquid nonlinear optical refraction material, such as pararosanilin dye in liquid media with third-order nonlinear refraction (G. Vinitha, & A. Ramalingam, "Third-order optical nonlinearities and optical-limiting properties of a Pararosanilin dye in liquid and solid media", Laser Physics.
Vol. 18, No. 9, P. 1070-1073, 2008). Such type of limiter is built.

When a high power laser beam enters the limiter, it is focused on the reflecting coating 14 or 24 and reflected partially. Then the special standing wave is formed.
Because the intensity reflectance is 90%. the maximum compounding intensity at anti-node is 1460 times as high as that at node since .797 - =1460 as mentioned above. Thus 0.0026 the induced refractive index change at anti-node is much larger than that at node (it may happen first that the light energy is absorbed much more at anti-node than at node). As a result, the distribution of refractive index exhibits periodic continuous multiple layer structure. The interval between every two contiguous layers is - (see Fig.4).
This kind of structure is very like periodic multilayer reflector consists of two materials. which has very high reflectance if the index difference of two materials is large and especially the number of periods is large. For a periodic multilayer reflector consists of two materials which refractive index is homogeneous, the highest reflectance occurs whenever nõ dõ
n,, d,, are each equal to an odd multiple of 4 , where nõ , n,, , dõ and d,, are indexes of two materials A. B. and thicknesses of the layers made of A and B respectively.
The maximum intensity reflectance for normal incident is given by n /n+(n/n J- 0) where n,,, . n, and N are indexes of two contiguous media beside the reflector and number of periods respectively ("Handbook of Optics", Vol. I, 2d ed., edited by M.
Bass, J. M.
Enoch. E. W. V. Stryland, W. L. Wolfe, McGraw-Bill, New York, 2001, p. 42.34-42.42).
In the standing wave, the refractive index is not homogeneous in each layer.
The related calculation is somewhat different. However, the intensity reflectance of such standing wave will he very high. One reason is that the standing wave has very large intensity gradient, another reason is that getting a large number of periods N is easy in the standing wave.

For a beam comes from a finite distance, some more things need to be considered compared with the situations using nonlinear absorption and scattering. It is known that a conoid standing wave may also be formed if the focal point of the beam falls behind the reflecting coating. And a periodic continuous multilayer structure is produced as shown in Fig.3. However, the value of the solid angle and the length of the conoid standing wave depend on the distance between points 30 and 32. and the beam coherence length. Unlike nonlinear absorption and scattering, the layers in the multilayer structure must be continuous. Otherwise, light reflectance is reduced obviously like a multilayer reflector, that is, a mirror breaks at somewhere. Therefore. short distance between points 30 and 32, and long coherence length are preferred.

In order to obtain these conditions, the nonlinear optical refractive materials with (In do minus index change, that is. with minus or -- , are better candidates, where do .
dT dl dT and dl are increments of refractive index, temperature and intensity respectively.
Thus, the distance between points 30 and 32 is reduced or even the focal point of the beam falls in front of the reflecting surface. As indicated above, the layers between the centers of two spherical waves in an interference induced multilayer structure are continuous within the whole coherence length. Thus, the distance between two sphere centers is allowed longer. Using bandpass filter, the bandwidth of incident beam may be reduced. To get white light feeling, the filter having three narrow transmission regions for tree primary colors may be used.

Unlike the normal limiting using nonlinear optical refraction effect, the self-defocusing must be avoided, which destroys the formation of the special standing wave.
One method is to build an additional layer of linear material on top of the reflecting coating as shown in Fig.5 and Fig.6. In Fig.5 and 6, 40 and 46 are the added layers. and 42 and 48 are the reflecting coatings. In these added layers, the beam intensity is allowed to be very high. Because these layers are linear optical materials. the self-defocusing does not happen. The thickness of these layers depends on the possible power range of the incident beams, and the values of the material nonlinear refractive coefficients. An additional optical limiter 44 in Fig.5 or 50 in Fig.6 may be inserted into the optical path to pre-limit the incident power below a certain value. In some cases, two or more than two additional optical limiters may be inserted into the optical path to get larger power pre-limiting. The refractive index of the layers 40 and 46 are selected to near or match that of the used solid or liquid nonlinear optical refractive materials, or the partial reflecting coating. Thus, the beam focal point still falls on the partial reflecting coating, and additional reflection phase change is eliminated, or the amount of the reflection part with undesired phase change is reduced.

Therefore, the high power light will be limited much more than that in the situation without the standing wave. Thus, comparing with the limiter using same optical refractive material and under the same incident intensity, the limiter invented here has much lower limiting threshold.

Next. the optical limiting process of the limiter using solid or liquid photorefractive materials is described underneath. Making a solid photorefractive material, such as lithium niobate crystal doped with Fe. as the material 12, which has the same shape and reflecting coating as the material 12, or filling the container 22 with a liquid photorefractive material, such as dye-doped liquid crystals (lam Choon Khoo, Min-Yi Shih, M. V. Wood. B. D. Guenther. Pao Hsu Chen. F. Simoni. S. S.
Slussarenko, 0.
Francescangeli, L. Lucchetti, "Dye-doped Photorefractive Liquid Crystals for Dynamic and Storage Holographic Grating Formation and Spatial Modulation", Proceedings of IEEE, Vol. 87, Issue 11, P. 1897-1911, 1999). Such type of limiter is built.

When a parallel light beam enters this limiter, the special standing wave is produced. As emphasized above, in the standing wave and if the intensity reflectance is 90 10. the maximum compounding intensity at anti-node is 1460 times as high as that at node within a very short distance 4 . It corresponds to a very large intensity gradient.
Most of photorefractive crystals. including photorefractive liquid crystals are intensity gradient dependence. Thus the material refractive index changes much larger at anti-node than at node. As a result, the distribution of refractive index forms a periodic continuous multilayer structure. This structure satisfies the Bragg reflection requirements. So in the material, a periodic multilayer reflector is formed. Because the index difference between anti-node and node may be large. and the number of periods is large too because of large intensity gradient and large photorefractive coefficients of the materials, this kind of reflector has very high reflectance, and so a very low limiting threshold.

Most of' the details about structure design and limiting process of this type of limiters are like those discussed for using nonlinear optical refraction material, such as inserting additional limiters to pre-limit high light power. Here they are not repeated. As photorefractive materials have very large photorefractive coefficients, there is a possibility to use these limiters to limit non-laser light. In the following, this possibility is discussed, with using these limiters as the sun visor for driving at night as an example.

It is difficult to view the object in blazing background, such as the road ahead for drivers facing oncoming car light at night. Most cars use filament or arc headlamps with power around 50 w. For a typical low beam headlamp. its beam cross section areas at the distances between 20 m and 30m from the car are about 10 in x 3 m, corresponding to a intensity of 0.167 mw/cm' The light intensity increases when the car comes nearer.
Supposing the light energy of the beam distributes in the whole visible region from 0.4 pun to 0.76 pm. In order to get enough long coherence length, a bandpass filter with a bandwidth of 5110 of the whole visible region is used. The bandwidth central is at wavelength of 0.55 m. According to the above coherence length calculation equation (1), the passed beam has a coherence length of 16.8 pm. In addition, According to the above lens equation (2), when the car is at the distance of 20 in, the image field shift is 125 m.
When this beam is reflected from the partial reflecting coating, the initial phase difference between the incident beam and the reflected beam is zero. Thus a short special standing wave with a thickness of half coherence length, that is, 8.4 m is formed. In this conoid standing wave, there are 30 reflecting layers because 8.4 pm/ (0.5 x 0.55 pin) _ 30.

n ln-(n/n The above multilayer reflector equation (3). that is, R,,,,,, _ n /n, +(nõln) may be used here for a rough estimation. Choosing N = 30, n = 1.5 17,= 1 and 17,=1.5.
then for n,,=1.501, 1.51, 1.55. 1.6, 1.65 and 1.7. the R,,,,, is 0.048. 0.146.
0.688, 0.946, 0.991 and 0.9985 respectively. As reference, if choosing n = 1.5 n, = 1, nõ
=1.5, n,, =1.51, but N is 30, 100, 300 and 1000, the R is 0.146, 0.4899, 0.952 and 0.999995 respectively. By focusing the incident beam from the car. the beam intensity may be increased far over -mw/cm ' within its coherence length range. The photorefractive sensitivity is from about -mw/em ' for the photorefractive materials ("Handbook of Optics". Vol. II, 2d ed.. edited by M. Bass, .1. M. Enoch, E. W. V. Stryland, W. L. Wolfe, McGraw-Hill, New York, 2001, p. 39.12). Thus to get intensity reflectance over 90% for the incident beam from a car is possible. For limiting blazing light from a car, an intensity drop of 90% is already useful. One may visualize the scene as a 50 w lamp becomes 5 w one.

Using bandpass filter reduces the brightness of the objects further. In the case of the filter transmission width is 5% of the visible region, the passing energy is about 10%
of that in the visible region (the light energy has an un-uniform distribution within the visible region, and the maximum value is at 0.55 m). This may be compensated by reducing the reflectance of the partial reflecting coating or increasing the aperture size of the input compound lens. Using bandpass filter with three transmission regions around tree primary colors can increase the objet brightness and produce white color feeling.

Therefore, like the limiters discussed above, this type of limiters can limit the high power light much more than that in the situation without the standing wave.
Comparing with the limiter using same photorefractive material and under the same incident intensity, the limiter invented here has much lower optical limiting threshold, and provides the very precious possibility for limiting non-laser light.

At last, the optical limiting process of the limiter using solid or liquid photosensitive materials is described underneath. Making a solid photosensitive material as the material 12, which has the same shape and reflecting coating as the material 12, or filling the container 22 with a liquid photosensitive material, such as dye-doped nernatic liquid crystals (I. C. Khoo, M. V. Wood, M. Y. Shih, and P. H. Chen, "Extremely Nonlinear Photosensitive Liquid Crystals for Image Sensing and Sensor Protection", Optics Express. Vol. 4. No. 11. p. 432-442, 1999). Such type of limiter is built.

Similarly, the structure design is for forming a special standing wave in the path of the incident beam. and for utilizing the features of this standing wave to limit the beam power. As many details about structure design and limiting process for this type of limiter are like those discussed for the above limiters, including phase change eliminating, high power pre-limiting, and so on. These details are not discussed repeatedly.
Only some special considerations about the structure design for using nematic liquid crystals is described underneath.

Two polarization filters 52 and 54 are inserted into the optical path as shown in Fig.7. One is in front of the container 56. that is, the container 22. and another is behind the container 56. The alignment of the liquid crystals in the container is chosen so that its relaxed phase is a twisted one, the twist angle may be one of 45 , 90 180 , 200 and 270 ('Handbook of Optics". Vol. II. 2d ed.. edited by M. Bass, J. M. Enoch, F. W. V.
Stryland, W. L. Wolfe, McGraw-Hill. New York, 2001, p.14.14). Here, considering the 90 twist angle. In order to get this relaxed phase alignment, two inside surfaces of the container 56, including the completed partial reflecting coating, are treated using existing technology, such as rubbing or tilting deposition. Probably. an additional transparent layer needs to be deposited on top of the completed partial reflecting coating for getting the relaxed phase. In this situation, the refractive index of the additional layer must be as same as or near that of the liquid crystal or the reflection coating. Thus, the additional phase change can be avoided, or the amount of the reflected light with undesired phase change can be reduced. The directions of two polarization filters are crossed (oriented at 90 to one another) and match the liquid crystals alignment. The direction of the polarizer 52 may be selected arbitrarily in the plane perpendicular to the primary optical axis of the limiter. Here, making the direction of the polarizer 52 vertical and that of the polarizer 54 horizontal. The alignment of the liquid crystals needs to match the polarizer directions.
Thus the beam passes through the first polarizer 52, and then its polarization direction rotates 90 following the liquid crystal directors, and passes through the second polarizer 54. The most of the beam reaching the partial reflecting coating is reflected hack, and returns into the liquid crystals again. If the beam power is enough low, the returned beam rotates its polarization direction reversely following the liquid crystal directors, and passes the first polarizer. In Fig.8, the left tilted row indicates the schematic director alignment in the relaxed phase. Sheets 58 and 60 are the first and second polarization filters respectively.

If beam power is high, as described above, in the formed standing wave the maximum compounding intensity at anti-node is 1460 times as high as that at node within a short distance - , if the intensity reflectance is 90%. It is a very large intensity gradient.
and is much larger than that produced in a focusing beam which was utilized by I.C.
Khoo and etc (see above reference). In a focusing beam, the light intensity gradient is proportional to the ratio of the beam cross section areas separated by a certain distance.
From Fig.9, one can see that in a focusing beam, the ratio of the cross section areas separated by interval of is equal to 'R2 , R, = R, + AR and sin 0 = (4 x OR)/
A . The 4 yrR,_ -angle 0 is determined by the f-number N / of the input compound lens (N f/d, where f and d are focal length and diameter of the entrance pupil of the input compound lens).
Usually, N 1 is larger than 1, thus 0 is less than 26 . Substituting 0 = 23.57 , thus sin 0 =
0.4. the ratio becomes I + 0.:A + 0.1), . Even at the waist of the laser beam, the radius R1 R, of the waist is larger than several m (" I-landbook of Optics", Vol. IV. 2d ed., edited by M. Bass, .l. M. Enoch. E. W. V. Stryland. W. L.,. Wolfe. McGraw-Hill. New York, 2001. p.
28.22). Supposing R, A . the ratio is equal to 1.21. Thus. the intensity gradient in the special standing wave is more than 1207 times as high as that in the focusing beam even with a large solid angle of 47 .

Such high intensity gradient will reorient the directions of the liquid crystal directors. The intensity changes along the light traveling direction periodically. It generates a photo-induced periodic space charge distribution in the liquid crystals. and produces a periodic space electric field distribution. This periodic space electric field reorients the liquid crystal directors, and making them form a periodic alignment.
Because the electric field strength is very large in each periodic interval, the most of the directors are aligned parallel to the light traveling direction. The right tilted row in Fig.8 indicates the schematic reoriented alignment of the directors. It just shows a very short range of , and the number of the liquid crystal directors in that range is much less than the real number. The length of a typical liquid crystal molecule is 30 angstroms.
Therefore. 40 to 50 liquid crystal molecules may align in the range of a quarter of wavelength of 0.55 m. In each periodic interval of electric field, the most directors are aligned parallel to the field direction as high field strength. In this state, the liquid crystal directors do not reorient the polarization direction of the light, so the light polarized at the first polarizer is absorbed at the second polarizes. and the beam is blocked.
Very few directors located around anti-nodes and nodes are aligned perpendicularly to the field direction. As the lengths of the ranges with almost zero field at anti-node and node are much less than the wavelength, the directors in these ranges do not affect the light reorientation.

In the work did by I. C. Khoo and etc (see above reference), the dye-doped liquid crystals can limit incident CW laser beam with low power of several mw. They used focusing lens to produce light intensity gradient in the liquid crystals.
Because the light intensity gradient produced by standing wave is much higher than that produced by a focusing lens, the limiter invented here will provide much lower optical limiting threshold, and provides great possibility for limiting non-laser light, such as being used as the sun visor for driving at night.

Fig.] I illustrates the schematic optical structure of second preferred embodiment of the optical limiter according to the invention. A converging input lens 62 focuses the parallel incident beam into a partial reflecting mirror 64. The light intensity reflectance of the mirror 64 is 37.5%. The transmitted part of the beam enters a right-angle prism 66.
After one total internal reflection, this transmitted part is reflected by another partial reflecting interface 68, and goes into a solid above said optical limiting material 70. 70 may also be a transparent container filled with a liquid above said optical limiting material. The focal point of the transmitted part of the beam falls on the middle plane of the solid material 70 or the container 70. The light intensity reflectance of the interface 68 is 60%. The reflected part of the incident beam from the mirror 64 enters a penta prism 72 ("Handbook of Optics', Vol. 11, 2d ed., edited by M. Bass, .1. M. Enoch, E.
W. V.

Stryland, W. L. Wolfe. McGraw-Hill, New York, 2001, p. 4.13-4.14). After two total internal reflections (the penta prism has larger refractive index), this reflected part goes into the solid material 70 or the container 70 from an opposite side. Then, this reflected part travels through the partial reflecting interface 68 and is converged by output lens 74 to become a parallel beam again. The right-angle prism 75 has same refractive index with the prism 66, and is connected to the prism 66 without air gap. The focal point of this reflected part of the beam falls on the middle plane of the solid material 70 or the container 70 too, and is connected to the focal point of the transmitted part of the beam.
The optical path lengths of the transmitted part and the reflected part are made to be same when their focal points meet at the middle plane of the solid material 70 or the container 70. Realize the same process for all parallel incident beams with different incident angles from the field of view. Thus two focal planes are created and are coincided with each other in the solid material 70 or the container 70. In other words, two identical image fields for all parallel incident beams with different incident angles from total field of view are created. and are coincided with each other in the above said optical limiting material.

If a laser beam is incident, two special standing waves arc formed in the solid material 70 or the container 70 as shown in Fig. 10. In Fig. 10. 76 are the connected focal points of the transmitted and reflected parts of the incident beam, and 78 is the focal plane of the input lens 62. In the Fig.] 0, the reflected part of the beam is overlapped by the transmitted part of the beam completely. It means that the incident high power beam can be limited effectively. However. for the incident beam which traveling direction is not along the primary optical axis of the device. its two divided beam parts can't overlap each other completely when they meet again. Thus in the un-overlapped area the special standing wave is not formed, resulting in the un-overlapped part of the beam is not limited and leaks out the limiter. For a high power beam, even a fraction of its energy passing the limiter directly may cause a serious damage. If this problem persists, the related limiting scheme is only effective for limiting the beam with single incident direction, and so becomes no use in practice. The desired scheme is one that can limit high power beams with any possible incident angle from the total field of view. Limiting the beams with different incident angles is much more difficult than limiting the beam just with single incident angle. Furthermore, the scheme for limiting the beams with different incident angles needs to be simple and convenient. without losing the simplicity and compactness of the passive device.

By circlewise cutting the solid angle of one of two divided beams, this problem can be solved ideally. In Fig. 11. a diaphragm 80 is used to circlewise cut the solid angle of the reflected part of the tilting incident beam. As result, the passing part of the tilting incident beam is overlapped well as shown in Fig.12. In Fig.12, the reflected part 82 of the tilting incident beam has a reduced solid angle cut by the diaphragm, which is smaller than that of the transmitted part 84 of the same beam. Thus the special standing wave is formed in this overlapped solid angle. As result, all of the output lights through the lens 74 have been treated by the limiting process. In Fig.12, 86 is the focal plane of the input lens 62, 88 is the primary optical axis of the limiter. The diaphragm size is chosen in a way that the reflected part of the tilting beam reaching the edge of the desired image field may be overlapped effectively by the transmitted part of the same beam.

During the limiting process, some lights of the transmitted part of the beam will return to the right-angle prism from the solid material 70 or the container 70, and go out through the lens 74. Its origins are related to the structure of the limiter and the optical effect utilized, such as back scattering from induced scattering centers, reflection from the generated multilayer reflectors" and so on. These noise lights degrade the function of the limiter. In order to block these noise lights, a quarter-wave plate 90 and a polarizer 92 are used. The polarization direction of the polarizer 92 is placed at 45 to the optical axis direction of the quarter-wave plate 90. Thus, the linear polarization light emerging from the polarizer 92 is divided into two polarization components when passed through the quarter-wave plate. These two polarization components combine to become a circular polarization light. When this circular polarization light returns back by reflection, scattering and so on, it passes through the quarter-wave plate and becomes linear polarization light again. but with a further 90 rotation of the polarization direction ("Handbook of Optics", Vol. 1. 2d ed.. edited by M. Bass. J. M. Enoch, E. W.
V. Stryland.

W. L. Wolfe. McGraw-Hill, New York, 2001, p.5.22-5.26). Thus, the returned light can't pass the polarizer 92 again.

However. the two polarization components produced by the polarizer 92 and quarter-wave plate 90 has a 90 phase difference, which reduce the intensity gradient of the standing wave formed in the solid material 70 or the container 70 (see above discussions about reflection phase change as the reference). Therefore, another polarizer 94 and a three-quarter-wave plate 95 are used. The polarization direction of the polarizer 94 is parallel to that of the polarizer 92, and is at 45 to the optical axis direction of the three-quarter-wave plate 95. Thus. the linear polarization light emerging from the polarizer 94 is divided into two polarization components when passed through the three-quarter-wave plate 95. These two polarization components have a 270 phase difference.
When these two polarization components traveling from left to right meet two polarization components traveling from right to left, the two components which directions are parallel to the polarizer direction have zero phase difference.
The two components which polarization directions are perpendicular to the polarizer direction have 360 or 0 phase difference. Therefore, the standing wave formed in the solid material 70 or the container 70 has largest intensity gradient.

To block the noise lights returning to the right-angle prism from the solid material 70 or the container 70. and to get largest intensity gradient of the standing wave may adopt other methods according to the existing optical design technology.

In order to eliminate or reduce the reflection phase changes caused by reflecting surfaces, interfaces of the mirror and prisms, special constructed reflecting coatings maintaining phase difference of being or near 0 or 360 for parallel and perpendicular polarization components introduced above are deposited on these surfaces and interfaces.
For decreasing cost, some of these surfaces and interfaces may not have these special reflecting coatings if the induced cumulated reflection phase change between two polarization components is, equates with or near 0 or 360 . For example, the two reflecting surfaces of the penta prism may not have these special coatings.

In order to reduce energy loss and arbitrary light disturbance, all optical surfaces of the lenses, prisms, mirror, wave-plates, polarizers and the solid material or the container, excepting above said reflecting surfaces and interface, are coated with high antireflection films for the desired wavelength range.

Thus, each of the reflected beam parts from the mirror 64 has same frequency, same polarization, roughly equal amplitude, and opposite or near opposite direction with its corresponding one of the transmitted beam parts through the mirror 64. The phase difference between each pair of the reflected and transmitted parts is zero at middle plane of the solid material 70 or the container 70, and is fixed within their coherence length.
Thus, for a narrow bandwidth beam especially a laser beam, two long special standing waves are formed in the traveling paths for each other. The passing part through the diaphragm of the reflected part of the incident beam is overlapped by the transmitted part of the same beam completely within the range of the coherence length. In this spacial interference field, light intensity distributes in the manner of multiple continuous layers, and the interval between two contiguous layers is just half of the wavelength of the incident beam.

For a non-laser light beam which source is located at finite distances, if its coherence length is enough long, two standing waves may still be formed like shown in Fig.13. In Fig.13 the focal points of the reflected and transmitted parts of the incident beam are located at 98 and 100 respectively. Of course, the output lens is designed to be moveable to adapt to object distance change.

As partial reflecting. diaphragm cutting and polarizer filtering, the brightness of the objects in field of view is reduced for observer. Therefore the input lens 62 has larger aperture and longer focal length than those of the output lens 74. For example, if the light intensity reflectances of the mirror 64 and interface 68 are 37.5% and 60%
respectively, the diaphragm cutting ratio is 33%, as the transmittance of the polarizer is 50%, the ratio of output/input energy is 5%. Thus. making the radius of the input lens is 4.47 times the radius of the output lens, the output intensity will be 20 times as high as that of the input one. The diameter and focal length of the input lens depend on the concrete applications.

Usually they are from 0.5 cm to 50 cm and 1 cm to 100 cm respectively. The sizes of mirror and prisms are matched with the input lens. The thickness of the used solid above said optical limiting material, or the container filled with liquid above said optical limiting material is from 1 m to 5 cm. In order to reduce the weight, the prisms may be replaced by appropriate combinations of mirrors.

Thus, different types of optical limiter can be constructed by using solid or liquid materials with different nonlinear optical absorption, scattering, refraction, or photorefractive. photosensitive effects as the solid material 70 in Fig. 11, or as the liquid material filled in the container 70 in Fig.11. The details about structure modification and limiting process discussion for these limiters, such as adding polarizers for liquid crystals material. adding additional pre-limiters, one may see the above descriptions for the first embodiment shown in Fig.], '15. 6 and 7 as the reference.

Fig. 14 illustrates the schematic optical structure of third preferred embodiment of the optical limiter according to the invention. A converging input lens 102 focuses the parallel incident beam into an abbe's prism 104 ("Handbook of Optics", Vol.
II, 2d ed., edited by M. Bass. J. M. Enoch. E. W. V. Stryland, W. L. Wolfe, McGraw-Hill, New York. 2001, p.4.7), and reaches the partial reflecting interface 106. The light intensity reflectance of the interface 106 is 37.5%. The transmitted part of the beam enters another reflecting interface 108, and then is reflected by a pair of mirrors l 10 and 112. In Fig.14.
the mirror 1 12 is behind the mirror 110, and the two mirrors are perpendicular to each other, with a structure like a roofing prism (see Fig.15, which is a schematic left side view, taken on the dash line 122-122 in Fig.14). These two mirrors invert the beam, and deviate the beam to the mirror 114. The intensity reflectances of the reflecting interface 108, mirrors 110, 112 and 114 are 100%. After further reflection by partial reflecting mirror 116, this transmitted part goes into a solid above said optical limiting material 118.
1 18 may also be a transparent container filled with a liquid above said optical limiting material. The intensity reflectance of the partial reflecting mirror 116 is 60%. The focal point of the transmitted part falls on the middle plane of the solid material 118 or the container 118. The reflected part of the incident beam from the interface 106 enters two roofing surfaces of the Abbe's prism, and is inverted and deviated to the reflecting interface 108. The reflections on two roofing surfaces are total internal reflections ("Handbook of Optics' Vol. 11. 2d ed., edited by M. Bass. J. M. Enoch, E. W.
V.
Stryland. W. L. Wolfe. McGraw-Hill, New York, 2001, p. 4.7-4.8). Then this reflected part goes into the solid material 1 18 or the container 118 on an opposite side. Afterwards, this reflected part travels through the partial reflecting mirror 116 and is converged by output lens 120 to become a parallel beam again. The prism 122 has same refractive index with the Abbe's prism 104, and is connected to the prism 104 without air gap. The focal point of the reflected part of the beam falls on the middle plane of the solid material 118 or the container 118 too, and is connected to the focal point of the transmitted part of the beam. The optical path lengths of the transmitted part and the reflected part of the beam are made to be same when their focal points meet at the middle plane of the solid material 118 or the container 118. Realize same process for all parallel incident beams with different incident angles from the field of view. Thus two focal planes are created and are coincided with each other in the solid material 118 or the container 118. In other words, two identical image planes for all parallel beams with different incident angles from field of view are created and are coincided with each other in the solid material 118 or the container 118.

If a laser beam is incident, two special standing waves are formed in the solid material 118 or the container 1 18 same as shown in Fig. 10. For limiting a tilting incident beam well. a diaphragm 126 is used to cut the solid angle of the reflected part of the tilting incident beam. As result. the passing part through the diaphragm of the tilting incident beam can he overlapped well as shown in Fig.12. Also, the diaphragm size is chosen in a way that the passing part of the reflected part of the tilting beam reaching the edge of the desired image field may be overlapped completely by the transmitted pat of the same beam.

In order to block the noise lights returning from the solid material 118 or the container 118, a quarter-wave plate 128 and a polarizer 130 are used. The polarization direction of the polarizer 130 is placed at 45 to the optical axis direction of the quarter-wave plate 128. Thus, the light becomes linear polarization light first and then circular polarization light when passes the polarizer 130 and quarter-wave plate 128.
When this circular polarization light returns back by reflection, scattering and so on, it passes through the quarter-wave plate and becomes linear polarization light again with a 90 rotation of the polarization direction, and thus can't pass the polarizer 130.
In order to compensate 90 phase difference between two polarization components produced by the polarizer 130 and quarter-wave plate 128, another polarizer 134 and a three-quarter-wave plate 132 are used. The polarization direction of the polarizer 134 is placed parallel to that of the polarizer 130. and is at 45 to the optical axis direction of the three-quarter-wave plate 132. Thus. the two polarization components passing through the polarizer 134 and the three-quarter-wave plate 132 have a 270 phase difference. When the reflected part light traveling from left to right meet the transmitted part light traveling from right to left, the two components which polarization directions are parallel to the polarizer direction have zero phase difference, and the two components which polarization directions are perpendicular to the polarizer direction have 360 or 0 phase difference.
Therefore, the special standing wave formed in the solid material 1 18 or the container 118 has largest intensity gradient.

To block the noise lights returning to the mirror 116 from the solid material or the container 1 l 8. and to get largest intensity gradient of the standing wave may adopt other methods according to the existing optical design technology.

In order to eliminate or reduce the reflection phase changes caused by reflecting surfaces, interfaces of the mirrors and prisms, special constructed reflecting coatings maintaining phase difference of being or near 0 or 360 for parallel and perpendicular polarization components introduced above are deposited on these surfaces and interfaces.
For decreasing cost, some of these surfaces and interfaces may not have these special reflecting coatings if the induced cumulated reflection phase change between two polarization components is, equates with or near 0 or 360 . For example, the two roofing surfaces of the Abbe's prism, the two reflecting surfaces of the mirrors 110 and 112. may not have these special coatings.

In order to reduce energy loss and arbitrary light disturbance, all optical surfaces of the lenses, prisms, mirrors. wave-plates, polarizers and the solid material or the container, excepting above said reflecting surfaces and interface. are coated with high antireflection films for the desired wavelength range.

Thus, each of the reflected beam parts from the interface 106 has same frequency, same polarization, roughly equal amplitude, and opposite or near opposite direction with its corresponding one of the transmitted beam parts through the interface 106.
The phase difference between each pair of the reflected and transmitted parts is zero at middle plane of the solid material 118 or the container 118, and is fixed within their coherence length.
Thus. for a narrow bandwidth beam especially a laser beam, two long special standing waves are formed in the traveling paths for each other. The passing part through the diaphragm of the reflected part of the incident beam is overlapped by the transmitted part of the same beam completely within the range of the coherence length. In this spacial interference field, light intensity distributes in the manner of multiple continuous layers.
and the interval between two contiguous layers is just half of the wavelength of the incident beam.

For a non-laser light beam which source is located at finite distance, if its coherence length is enough long, two special standing waves may still be formed like shown in Fig.13. Of course, the output lens is designed to be moveable to adapt to object distance change. As partial reflecting. diaphragm cutting and polarizer filtering, the brightness of the objects in field of view is reduced for observer. Therefore the input lens 102 has larger aperture and longer focal length than those of the output lens 120. For example. if the light intensity reflectances of the interface 106 and the surface 116 are 37.5% and 60% respectively, the diaphragm cutting ratio is 33%, as the transmittance of the polarizer is 50%. the ratio of output/input energy is 5%. Thus. making the radius of the input lens is 4.47 times the radius of the output lens, the output intensity will be 20 times as high as that of the input one.

The diameter and focal length of the input lens depend on the concrete applications. Usually they are from 0.5 cm to 50 cm and I cm to 100 cm respectively.

The sizes of mirror and prisms are matched with the input lens. The thickness of the used solid above said optical limiting material 118, or the container 118 filled with liquid above said optical limiting material is from I pm to 5 cm. In order to reduce the weight, the Abbe's prism 104 may be replaced by suitable combination of mirrors, and without using the fitting prism 122. Vice versa, the mirror pairs of 110 and 112, 114 and 116 may be replaced by a roofing prism and a right-angle prism respectively to increase device stability and assembling simplification.

Thus, different types of optical limiter can be constructed by using solid or liquid materials with different nonlinear optical absorption, scattering, refraction, or photorefractive, photosensitive effects as the solid material 118 in Fig.14, or as the liquid material filled in the container 118 in Fig.14. The details about further structure modification and limiting process discussion for these limiters, such as adding polarizers for liquid crystals material, adding pre-limiters, one may see the above descriptions for the first embodiment shown in Fig. 1, 2. 5, 6 and 7 as the reference.

Fig.16 illustrates the schematic optical structure of fourth preferred embodiment of the optical limiter according to the invention. A converging input lens 136 focuses the parallel incident beam into a solid above said optical limiting material 138.
138 may also be a transparent container filled with a liquid above said optical limiting material. The focal point of the beam falls on the back surface 140 of the solid material 138 or the inside back surface 140 of the container 138 (the container is not drawn in detail here). A
partial reflecting coating with light intensity reflectance of, such as 90%, is coated on the back surface 140 of the solid material 138 or the inside back surface 140 of the container 138. Unlike the first preferred embodiment, the surface 140 is plane here, and is located at the focal plane of the lens 136. Thus the most part of the beam returns by partial reflection. The returned part of the beam interferes with the incident beam.
The transmitted part of the incident beam goes to the lens 142, and is converged to become a parallel beam again.

In order to eliminate or reduce the reflection phase changes caused by partial reflection, the special constructed reflecting coating, which maintains a phase difference of being or near 0 or 360 for parallel and perpendicular incident polarization components introduced above, is deposited on the back surface 140 of the solid material l 38 or the inside back surface 140 of the container 138.

If a laser beam is incident, most part of the beam returns back into the solid or liquid limiting material as shown in Fig.16. If the beam has a tilting incident angle, the returned beam can't overlap the whole incident beam, resulting in part of the returned beam interferes with part of the incident beam. That is, the standing wave is not formed in whole solid angle of the incident beam. The un-overlapped part of the incident beam will not be limited, and then leaks out the lens 142. This problem is also solved by using a diaphragm 146. Place the diaphragm at a proper position. At that position, only the overlapped part of the incident beam, that is, the part interfered with the returned part of the beam can pass. In addition, at that position the overlapped part can pass with maximum amount. The position and the aperture size of the diaphragm usually can be determined by drawing an optical path chart. By enlarging input lens aperture size, the ratio of the overlapped part of the beam to the whole of the beam can be increased. It is shown in Fig.17. The locating position and aperture size of the diaphragm 150 are also changed to match input lens enlargement.

As returned part of the beam has same frequency, same polarization, roughly equal amplitude, and zero phase difference with the incident beam at the reflecting coating, the special standing wave is formed in their interference area. In the standing wave, the light intensity distribution exhibits a periodic continuous multilayer structure.
As described above. such type standing wave can produce enhanced light absorption and scattering in nonlinear optical absorption and scattering materials. It can also produce high and extremely high reflection in nonlinear optical refraction and photorefractive materials by self-formed multilayer reflector. And at last, it can produce extremely large light blocking in photosensitive nematic liquid crystals. Of course, two polarizers must be used, and two inside surfaces of the container 138 must be treated to produce relaxed phase when using photosensitive nematic liquid crystals, which is like that described for the first embodiment in Fig.7. And pre-limiters may be used in some situations to pre-lower the power of incident beam.

Thus only the part of the beam which has been treated by the optical limiting process can pass through the limiter.

In order to reduce energy loss and arbitrary light disturbance. all optical surfaces of the lenses, the solid material 138 or the container 138 filled with liquid material.
excepting the said partial reflecting coating. are coated with high antireflection films for the desired wavelength range.

For a non-laser light beam which source is located at finite distance, if its coherence length is enough long. the special standing wave may still be formed like shown in Fig. 18. In Fig. 18. the virtual focus of the tilting incident beam 152 is at point 154. The tilting beam 152 is reflected by the reflecting coating 156, that is.
the back surface 140 of the solid material 138 or the inside back surface 140 of the container 138, and focused at point 158. It then diverges into beam 160. It is known that in an interference field of two spherical waves with same frequency, polarization, amplitude, fixed phase difference. and within the coherence length, the special standing wave with intensity distribution with periodic continuous multilayer structure can be produced between two sphere centers, or outside the region between two sphere centers.
Between two sphere centers, the produced layers are hyperboloids of revolution and are continuous within the whole coherence length. Outside the region between two sphere centers, the continuous layers are just within a certain solid angle which axis is along the connecting line of two sphere centers. The value of the solid angle depends on the distance between two sphere centers. Therefore, if the bandwidth of the tilting incident beam is narrow enough, and the beam is not from a position located too near to the limiter, the special standing wave having continuous periodic multiple layer structure is still formed. Of course. the output lens is designed to he moveable to adapt to object distance change.

As partial reflecting, diaphragm cutting, the brightness of the objects in field of view is reduced for observer. Therefore the input lens may have larger aperture and longer focal length than those of the output lens 142. For example, if the light intensity reflectance of the partial reflecting coating is 90%, the diaphragm cutting ratio is 50%.
the total ratio of outputlinput energy is 5%. Thus, making the radius of the input lens is 4.47 times the radius of the output one, the output intensity will be 20 times as high as that of the input one.

The diameter and focal length of the input lens depends on the concrete applications, usually. they are from 0.5 cm to 50 cm and 1 cm to 100 cm respectively.
The size of output lens is matched with the input lens. The thickness of the used solid above said optical limiting material 138, or the container 138 filled with liquid above said optical limiting material is from I m to 5 cm. In order to reduce the device size, especially in the case of using short focal length lens, the input and output lenses may adopt Fresnel lenses.

Thus, different types of optical limiter can be constructed by using solid or liquid materials with different nonlinear optical absorption, scattering, refraction, or photorefractive, photosensitive effects as the solid material 138 in Fig.] 6, or as the liquid material filled in the container 138 in Fig.16. The details about further structure modification and limiting process discussion for these limiters, one may see the above descriptions for the first embodiment shown in Fig. 1, 2, 5, 6 and 7 as the reference.

One important application of optical limiter is to protect human eyes. Such kind of application needs the limiter to provide erect image for observer, including providing erect image for some imaging optical instruments, with performing ideal optical limiting function. The preferred embodiments of the limiter structure design providing erect image will he described in the underneath. The principle of these designs is using lens, prism, mirror and so on to invert the images limited by the above described limiters, that is, above four preferred embodiments of the of the optical limiter according to the invention. By using existing optical design knowledge. these embodiments may be alternated, modified to fit different practical needs. The applicant of this invention reserves the right of all alternatives, modifications, and equivalent arrangements of the limiter structure embodiments described underneath.

In the following descriptions, the stress is laid on inverting inverted image.
The structure design and optical process about light power limiting is omitted.
For detailed consideration about light power limiting, one may refer the above descriptions of related preferred embodiment of the optical limiter according to the invention.

Fig.19 shows first schematic image erecting scheme for the first preferred embodiment shown in Fig.l and 2 of the optical limiter according to the invention. In Fig.19. the input compound lens consisting of lens 162 and 164 converges the parallel incident beam. The image is inverted 180 in vertical plane by right-angle prism 166, and is inverted 180 in horizontal plane by right-angle prism 168. Afterwards, the image is selectively limited by the solid above said limiting material 170 or the container 170 filled with liquid above said limiting material, and goes out through the output compound lens consisting of lens 172 and 174. The partial reflecting coating of the solid material 170 or the container 170 is located at the focal curved surface of the input compound lens.

In order to eliminate or reduce the reflection phase changes caused by the reflections on prism surfaces. the special constructed reflecting coatings.
which maintains a phase difference of being or near 0 or 360 for parallel and perpendicular polarization components introduced above, are deposited on these surfaces. Considering the phase changes on surfaces of each right-angle prism are complementary, these special coatings may not be needed. This scheme is like the optical structure of one most popular binoculars with double right-angle prism design. Thus, plenty of modification designs for binoculars with double right-angle prism design may be adopted to improve the imaging performance of this optical limiter. Fig.20 is a schematic top side view, taken on the dash line 176-176 in Fig.19.

Fig.21 shows second schematic image erecting scheme for the first preferred embodiment shown in Fig.] and 2 of the optical limiter according to the invention. In Fig21, the input compound lens consisting of lens 176 and 178 converges the parallel incident beam. Then the beam enters an Abbe's prism. The beam is inverted 180 in vertical plane by reflections on two roof surfaces 180 and 182 (182 is behind 180), and is inverted 180 in horizontal plane by reflections on surfaces 184, 180. 182 and 186 in sequence. Afterwards, the beam is selectively limited by the solid above said optical limiting material 188 or the container 188 filled with liquid above said optical limiting material, and goes out through the output compound lens consisting of lens 190 and 192.
The partial reflecting coating of the solid material 188 or the container 188 is located at the focal curved surface of the input compound lens.

In order to eliminate or reduce the reflection phase changes caused by the reflections on prism surfaces, the special constructed reflecting coatings, which maintains a phase difference of being or near 0 or 360 for parallel and perpendicular polarization components introduced above- are deposited on these surfaces. Considering the phase changes on each pair of surfaces. that is. the roof surfaces 180 and 182, the surfaces 184 and 186 are complementary. these special coatings may not needed. This scheme is like the optical structure of another most popular binoculars with Abbe's prism design. Thus, plenty of modification designs for binoculars with Abbe's prism design may be adopted to improve the imaging performance of this optical limiter.

Fig.22 shows third schematic image erecting scheme for the first preferred embodiment shown in Fig.1 and 2 of the optical limiter according to the invention. In Fig.22, the objective lens 194 focuses the parallel beam on its focal plane 196. The input compound lens consisting of lens 198 and 200 converges the divergent beam from the plane 196 on its spherical image surface 202, that is, the back surface of the solid above said optical limiting material 204 or the inside back surface of the container 204 filled with the liquid above said optical limiting material. The partial reflecting coating is on the surface 202. The beam is selectively limited by the solid limiting material 204 or the liquid limiting material in the container 204. The beam then goes out through the output compound lens consisting of lens 206 and 208. During this process, the image is inverted.
and then inverted again becoming an erect image.

Fig.23 shows fourth schematic image erecting scheme for the first preferred embodiment shown in Fig.] and 2 of the optical limiter according to the invention. In Fig.23, the right-angle prism 210 inverts beam 180 in vertical plane. Then the input compound lens consisting of lens 212 and 214 converges the parallel incident beam. The beam is inverted 180 in horizontal plane by another right-angle prism 216.
Afterwards, the beam is selectively limited by the solid above said optical limiting material 218 or the container 218 filled with liquid above said optical limiting material, and goes out through the output compound lens consisting of lens 220 and 222. The partial reflecting coating of the solid material 218 or the container 218 tilled with liquid material is located at the focal curved surface of the input compound lens.

In order to eliminate or reduce the reflection phase changes caused by the reflections on prism surfaces. the special constructed reflecting coatings, which maintains a phase difference of being or near 0 or 360 for parallel and perpendicular polarization components introduced above, are deposited on these surfaces. Considering the phase changes on surfaces of each right-angle prism are complementary, these special coatings may not be needed. This scheme uses double right-angle prism design. So it keeps the imaging advantages of one most popular binoculars, and plenty of modification designs for this kind of binoculars may be adopted to improve the imaging performance of this optical limiter. Fig.24 is a schematic top side view, taken on the dash line 221-221 in Fig.23.

In order to reduce the device weight or to adapt different focal length of the input compound lens, two right-angle prisms may be replaced by two pairs of mirrors, such as two pairs of mirrors A, B and C, D. The locations and directions of the mirror pairs of A, B and C, D are same as four reflecting surfaces of two prisms 210 and 216 respectively.
The input compound lens may be placed between A and B, or B and C, or C and D, or even after D. In every situation, the spherical focal surface of the input compound lens coincides with the back surface of the solid material 218 or the inside back surface of the container 218. Thus, the limiter can have different field of view.

Fig 25 shows first schematic image erecting scheme for the second preferred embodiment shown in Fig. 1 1 of the optical limiter according to the invention. As most details of the structure design are same as those described for Fig.I 1. here only difference between Fig.25 and Fig. 1 I is indicated. Moving output lens 74 to make output beam from interface 68 focused on the image plane 226. Chen another lens 230 is used to converge the divergent beam from plane 226 becoming a parallel beam again. Thus, the inverted image is inverted again to become erect image.

Fig.26 shows second schematic image erecting scheme for the second preferred embodiment shown in Fig. l1 of the optical limiter according to the invention.
The converging input lens 232 focuses the parallel incident beam into the partial reflecting mirror 234. The transmitted part of the beam enters the right-angle prism 236.
After two total internal reflections, this transmitted part goes into the solid above said optical limiting material 238, or the container 238 filled with a liquid above said optical limiting material. The focal point of the transmitted part of the beam falls on the middle plane of the solid material 238 or the container 238. Then. this part travels through the partial reflecting interface 241. passes another right-angle prism 252, and then is converged by output lens 254 to become a parallel beam again. The light intensity reflectance of the interface 241 is 60%. The reflected part of the incident beam from the mirror 234 enters the penta prism 240. After one total internal reflection, this reflected part is reflected partially by the interface 241, and then goes into the solid material 238 or the container 238 from an opposite side. The fitting prism 256 has same refractive index with the prism 240, and is connected to the prism 240 without air gap. The focal point of the reflected part of the beam falls on the middle plane of the solid material 238 or the container 238 too, and is connected to the focal point of the transmitted part of the beam.
The optical path lengths of the transmitted part and the reflected part are made to be same when their focal points meet at the middle plane of the solid material 238 or the container 238.
Realize the same process for all parallel incident beams with different incident angles from the field of view. Thus, the image is inverted by two right-angle prisms in vertical and horizontal planes, and becomes erect image when output.

In Fig.26, 242 and 244 are polarizer and quarter-wave plate. 246 and 248 are another polarizer and three-quarter-wave plate, and 250 is the diaphragm. They are used for blocking returned light. eliminating reflection phase change and circlewise cutting the solid angle of the transmitted part of the beam. Fig.27 is a schematic top side view, taken on the dash line 256-256 in Fig.26.

The above third preferred embodiment shown in Fig.14 of the optical limiter according to the invention provides already an erect image, so there is no modification for it.

Fig.28 shows first schematic image erecting scheme for the fourth preferred embodiment shown in Fig. 16 and 17 of the optical limiter according to the invention. In Fig.28. the input lens 260 converges the parallel incident beam. The beam is inverted 180 in vertical plane by right-angle prism 262, and is inverted 180 in horizontal plane by right-angle prism 264. Afterwards, the beam is selectively limited by the solid above said optical limiting material 266 or the container 266 filled with liquid above said optical limiting material, and goes out through the output lens 268 to become a parallel beam again. 270 is the diaphragm, which only let the beam part overlapped by the reflected part of the incident beam pass (see the description related to the Fig.16 and 17).
The partial reflecting coating of the solid material 266 or the container 266 is located at the focal plane of the input lens.

In order to eliminate or reduce the reflection phase changes caused by the reflections on prism surfaces, the special constructed reflecting coatings, which maintains a phase difference of being or near 0 or 360 for parallel and perpendicular polarization components introduced above, are deposited on these surfaces. Considering the phase changes on surfaces of each right-angle prism are complementary. these special coatings may not be needed. This scheme is like the optical structure of one most popular binoculars with double right-angle prism design. Thus, plenty of modification designs for binoculars with double right-angle prism design may be adopted to improve the imaging performance of this optical limiter. Fig.29 is a schematic top side view, taken on the dash line 272-272 in Fig 28.

Fig.30 shows second schematic image erecting scheme for the fourth preferred embodiment shown in Fig.16 and 17 of the optical limiter according to the invention. In Fig.30, the input lens 280 converges the parallel incident beam. Then the beam enters an Abbe's prism. The beam is inverted 180 in vertical plane by reflections on two roof surfaces 282 and 284 (284 is behind 282), and is inverted 180 in horizontal plane by reflections on surfaces 286. 282. 284 and 288 in sequence. Afterwards, the beam is selectively limited by the solid above said optical limiting material 290 or the container 290 filled with liquid above said optical limiting material, and goes out through the output lens 292 to become parallel beam again. 294 is the diaphragm, which only lets the beam part overlapped by reflected part of the incident beam pass. The partial reflecting coating of the solid material 290 or the container 290 is located at the focal plane of the input lens.

In order to eliminate or reduce the reflection phase changes caused by the reflections on prism surfaces, the special constructed reflecting coatings, which maintains a phase difference of being or near 0 or 360 for parallel and perpendicular polarization components introduced above, are deposited on these surfaces. Considering the phase changes on each pair of surfaces, that is. the roof surfaces 282 and 284, the surfaces 286 and 288 are complementary, these special coatings may not be needed. This scheme is like the optical structure of another most popular binoculars with Abbe's prism design.
Thus. plenty of modification designs for binoculars with Abbe's prism design may be adopted to improve the imaging performance of this optical limiter.

Fig.31 shows third schematic image erecting scheme for the fourth preferred embodiment shown in Fig.16 and 17 of the optical limiter according to the invention. In Fig.31. the objective lens 300 focuses the parallel beam on its focal plane 302. The input lens 304 converges the divergent beam from the plane 302 on its image plane 307, that is, the back surface of the solid above said optical limiting material 306 or the inside back surface of the container 306 filled with the liquid above said optical limiting material.
The partial reflecting coating is on the back surface 307 of the solid material 306 or the inside back surface 307 of the container 306. The beam is selectively limited by the solid limiting material 306 or the liquid limiting material in the container 306.
The beam then is converged and goes out through the output lens 308 to become parallel beam again.
310 is the diaphragm. During this process, the image is inverted, and then inverted again becoming an erect image.

Fig.32 shows fourth schematic image erecting scheme for the fourth preferred embodiment shown in Fig.16 and 17 of the optical limiter according to the invention. In Fig. 32. the right-angle prism 312 inverts beam 180 in vertical plane. Then the input lens 314 converges the incident parallel beam. The beam is inverted 180 in horizontal plane by another right-angle prism 316. Afterwards. the beam is selectively limited by the solid above said optical limiting material 318 or the container 318 filled with liquid above said optical limiting material. The beam is converged and goes out through the output lens 320.
322 is the diaphragm. The partial reflecting coating of the material 318 or the container 318 filled with liquid material is located at the focal plane of the input lens. Fig.33 is a schematic top side view, taken on the dash line 324-324 in Fig.32.

In order to eliminate or reduce the reflection phase changes caused by the reflections on prism surfaces, the special constructed reflecting coatings.
which maintains a phase difference of being or near 0 or 360 for parallel and perpendicular polarization components introduced above, are deposited on these surfaces. Considering the phase changes on surfaces of each right-angle prism are complementary. these special coatings may not he needed. This scheme uses double right-angle prism design. So it keeps the imaging advantages of one most popular binoculars, and may adopt plenty of modification designs for binoculars with double right-prism design to improve the imaging performance of this optical limiter.

In order to reduce the device weight or to adapt different focal length of the input lens, two right-angle prisms may be replaced by two pairs of mirrors, such as two pairs of mirrors A. B and C. D. The locations and directions of mirrors A, B and C. D
are same as four reflecting surfaces of two prisms 312 and 316 respectively. The input lens may be placed between A and B, or B and C, or C and D. or even after D. In every situation, the focal plane of the input lens coincides with the hack surface of the solid material 318 or the inside back surface of the container 318. Thus, the limiter can have different width of field of view.

At last, Fig.34 shows fifth schematic image erecting scheme for the fourth preferred embodiment shown in Fig.16 and 17 of the optical limiter according to the invention. In Fig.34, the mirrors 330 and 332 invert parallel incident beam 180 in horizontal plane. Then the mirror 334 inverts the beam 90 in vertical plane.
Afterwards, the input lens 338 (see Fig.35) converges the parallel incident beam, and another mirror 336 inverts the beams 90 in vertical plane further. An optical pre-limiter 340 pre-limits the beam from the mirror 336 to lower its power under a certain value. 342 is a solid above said optical limiting material. 342 may also be a transparent container filled with liquid above said optical limiting material. The back surface of the solid material 342 or the inside back surface of the container 342 coincides with the focal plane of the input lens 338. The detailed structure of the solid material 342 or the container 342 is shown in Fig. 36 or 37. Afterwards, the beam is limited selectively by the solid limiting material 342 or the liquid limiting material filled in the container 342. The beam then is converged and goes out through the output lens 344. 346 is the diaphragm, which is used to cut the unlimited part of the tilting incident beams through the output lens. The partial reflecting coating is on the back surface of the solid material 342 or the inside back surface of the container 342. Fig.35 is a schematic top side view, taken on the dash line 350-350 in Fig.34.

In order to eliminate or reduce the reflection phase changes caused by the reflections on mirror reflecting surfaces, the special constructed reflecting coatings, which maintains a phase difference of being or near 0 or 360 for parallel and perpendicular polarization components introduced above, are deposited on these surfaces.
Considering the phase changes on each pair of surfaces, that is. mirrors 330 and 332 or 334 and 336, are complementary, these special coatings may not be needed. This scheme is like that of binoculars using double right-angle prism design. So it keeps the imaging advantages of one most popular binoculars, and the plenty of modification designs for binoculars with double right-prism design may be adopted to improve the imaging performance of this optical limiter.

The mirrors 330 and 332 may be replaced by a right-angle prism. In order to adapt different focal length of the input lens, the input lens 338 may be placed between mirrors 330 and 332, or 332 and 334, or even after 336. In every situation, the focal plane of the input lens must coincide with the back surface of the solid material 342 or the inside back surface of the container 342. Thus, the limiter can have different width of field of view.
Fig.36 is the detailed structure of the solid material 342. It is made of a solid above said optical limiting material. A partial reflecting coating is deposited on its back surface 360. The light intensity reflectance of the reflecting coating is from 20% to 98%.
in the case described in Fig.34, the reflectance is 90%. The reflecting coating is plane, and is located at the focal plane of the input lens 338. The reflecting coating is the special constructed reflecting coating, which maintains a phase difference of being or near 0 or 360 for parallel and perpendicular polarization components introduced above.
If needed.
on top of the reflecting coating. an additional layer may be deposited to avoid self-defocusing of the high power beams, which happens around the beam focal points (this layer is not drawn in Fig.36). The refractive index of this additional layer must be same as or near that of the material 342 or the reflecting coating under it to eliminate the induced reflection phase change or reduce the amount of the light with induced reflection phase change. In any situation, the reflecting coating is kept to be located at the focal plane of the input lens 338.

Fig.37 is the detailed structure of the transparent container 342. It is filled with a liquid above said optical limiting material. A partial reflecting coating is deposited on its inside back surface 362. The light intensity reflectance of the reflecting coating is from 20% to 98%, in the case described in Fig.-34, the reflectance is 90%. The reflecting coating is plane, and is located at the focal plane of the input lens 338. The reflecting coating is the special constructed reflecting coating, which maintains a phase difference of being or near 0 or 360 for parallel and perpendicular polarization components introduced above. If needed, on top of the reflecting coating, an additional layer may be deposited to avoid self-defocusing of the high power beams, which happens around the beam focal points (this layer is not drawn in Fig.37). The refractive index of this additional layer must be same as or near that of the liquid material filled in the container 342 or the reflecting coating under it to eliminate the induced reflection phase change or reduce the amount of the light with induced reflection phase change. In any situation, the reflecting coating is kept to be located at the focal plane of the input lens 338.

For using photosensitive nematic liquid crystals. two polarizers 364 and 366 are placed on both sides of the container. The alignment of the nematic liquid crystals in the container is chosen so that its relaxed phase is a twisted one, the twist angle may be one of 45 , 90 , 180 . 200 and 270 . Here, considering the 90 twist angle. In order to get this relaxed phase alignment. two inside surfaces of the container 342, including the completed partial reflecting coating, are treated using existing technology, such as rubbing or tilting deposition. Probably, an additional transparent layer needs to be deposited on top of the completed partial reflecting coating for getting the relaxed phase.
In this situation, the refractive index of the additional layer must be as same as or near that of the liquid crystal or the reflection coating. Thus, the additional phase change can be avoided, or the amount of the reflected light with undesired phase change can be reduced. The directions of two polarization filters are crossed and match the liquid crystals alignment. The direction of the polarizer 52 may be selected arbitrarily in the plane perpendicular to the primary optical axis of the limiter. Here, making the direction of the polarizer 364 vertical and that of the polarizer 366 horizontal. The alignment of the liquid crystals needs to match the polarizer directions.

Us each of the above described preferred embodiments offering erect image, including those shown in Fig.14. Fig.19, Fig.21. Fig.22, Fig.23, Fig.25.
Fig.26, Fig.28, Fig.30, Fig.31, Fig.32 and Fig.34 as an optical limiter. Then mount a pair of two identical such optical limiters side-by side and align them to point accurately in the same direction.
and allow the viewer to us both eyes when viewing distant objects, the portable optical limiter for individual use to protect observer eyes are built. Most of them are sized to he held using both hands. The location positions of the output lens are moveable to let observer to view the objects at different distances.

In all of the above described preferred embodiments, the input lenses, the output lenses and the objective lenses may he modified by using additional compensation lens to correct their aberrations. In addition, in all of the above described preferred embodiments, the light intensity rellectances of the partial reflecting coatings, partial reflecting surfaces and the partial reflecting interfaces may be changed according to the concrete requirements of the applications, for example, from 10% to 98%.

Claims (39)

1. An optical limiting method based on forming and utilizing the standing waves formed by spherical partial reflecting, circlewise cut amplitude dividing, or circlewise cut partial reflecting the beams with different incident angles from field of view in nonlinear optical absorption, scattering, refraction, or photorefraction, photosensitivity material, said standing waves are made have intensity distribution with periodic continuous multiple layer structure.

2. The optical limiting method of claim 1, wherein said spherical partial reflecting is using spherical partial reflecting coating to reflect backwards the convergent beams correspond to the parallel beams with different incident angles from field of view to form standing waves.

3. The optical limiting method of claim 1, wherein said circlewise cut amplitude dividing is using partial reflecting coating to divide amplitudes of the convergent beams correspond to the parallel beams with different incident angles from field of view, guiding every pair of two divided beams to meet at their focuses with same optical path length, and using diaphragm to circlewise cut the solid angle of one of every pair of divided beams to form the standing wave which entirely occupies the remain area of said cut solid angle.

4. The optical limiting method of claim 1, wherein said circlewise cut partial reflection is using partial reflecting coating to reflect the convergent beams correspond to the parallel beams with different incident angles from field of view to form standing waves by entire or partial overlap, and using a diaphragm to circlewise cut the solid angle of every transmitted beams passing through the partial reflecting coating and output lens to only let the beam parts emerge from the standing waves pass.

5. The optical limiting method of claim 1. wherein said utilizing the standing waves is utilizing the features of formed standing waves, including higher light intensity at anti-nodes, ununiformity of intensity distribution, large intensity gradient, intensity distribution with periodic continuous multiple layer structure.

6. The optical limiting devices designed based on the optical limiting method of claim 1. or 2. or 3, or 4.

7. The optical limiting device designed based on the optical limiting method of claim 2, which comprises an input compound lens, a said spherical partial reflecting coating, an output compound lens and a solid said nonlinear optical absorption, scattering, refraction, or photorefraction, photosensitivity material, the back surface of said solid material is spherical and said spherical partial reflecting coating is coated on this back surface, the focal curved surfaces of said input compound lens and said output compound lens coincide with said spherical partial reflecting coating, said output compound lens is moveable to adapt to object distance change.

8. The optical limiting device designed based on the optical limiting method of claim 2, which comprises an input compound lens, a transparent container, a said spherical partial reflecting coating, an output compound lens and a liquid said nonlinear optical absorption, scattering, refraction, or photorefraction, photosensitivity material, the inside back surface of said transparent container is spherical and said spherical partial reflecting coating is coated on this inside back surface, said liquid material is tilled in the said transparent container, the focal curved surfaces of said input compound lens and said output compound lens coincide with said spherical partial reflecting coating, said output compound lens is moveable to adapt to object distance change.

9. The optical limiting device of claim 7 or 8. wherein said focal curved surfaces of input compound lens and output compound lens are modified to be spherical and coincide with the back surface of said solid material or the inside back surface of said transparent container.

10. The optical limiting device of claim 7 or 8. wherein said partial reflecting coating is a specially constructed reflecting coating which maintains phase difference of being or near 0° or 360° for parallel and perpendicular polarization components in the vicinity of the normal incident angle and within a certain wavelength range.

11. The optical limiting device modified based on the limiting devices of claims 7, 9 and 10, or 8, 9 and 10, which character is that the bandpass filter having narrow transmission region is used to increase coherence length of non-laser incident beam, or the filter having three narrow transmission regions for tree primary colors is used to increase coherence length of incident non-laser beam and to get white light feeling.

12. The optical limiting device modified based on the limiting devices of claims 7. 9 and 10, or 8, 9 and 10, which character is that a layer of transparent linear material is deposited on the top of the said spherical partial reflecting coating to avoid self-defocusing.

13. The optical limiting device modified based on the limiting devices of claims 7, 9 and 10, or 8, 9 and 10, which character is that one or more than one additional optical limiters are used to pre-limit the incident beam power to lower than a desired value.

14. The optical limiting device modified based on the limiting devices of claims 8, 9, 10, or and 11, or and 12, or and 13, which character is that two inside surface of the said transparent container are treated to make nematic liquid crystals in relaxed phase state. and two polarizers are used to produce linear polarized light and block the high power light.

15. The optical limiting device designed based on the optical limiting method of claim 3, which comprises an input lens, a said partial reflecting coating, a right-angle prism, a penta prism, a said diaphragm, an output lens, and a solid nonlinear optical absorption, scattering, refraction, or photorefraction, photosensitivity material, or a transparent container filled with liquid nonlinear optical absorption, scattering, refraction, or photorefraction, photosensitivity material, said every pair of two divided beams are guided by said right-angle prism and penta prism respectively to meet at their focuses with same optical path length, said focuses of every pair of two divided beams fall on the middle plane of said solid material or said transparent container, use said diaphragm to circlewise cut the solid angle of one of said every pair of divided beams to form the standing wave which entirely occupies the remain area of said cut solid angle, said output lens is moveable to adapt to object distance change, said prisms may be replaced by appropriate combinations of mirrors.

16. The optical limiting device designed based on the optical limiting method of claim 3, which comprises an input lens, a said partial reflecting coating, an Abbe's prism, a roof prism and a right-angle prism, a said diaphragm, an output lens, and a solid nonlinear optical absorption, scattering, refraction, or photorefraction, photosensitivity material, or a transparent container filled with liquid nonlinear optical absorption, scattering, refraction, or photorefraction, photosensitivity material, said every pair of two divided beams are guided by said Abbe's prism, and said roof prism and right-angle prism respectively to meet at their focuses with same optical path length, said focuses of every pair of two divided beams fall on the middle plane of said solid material or said transparent container, use said diaphragm to circlewise cut the solid angle of one of said every pair of divided beams to form the standing wave which entirely occupies the remain area of said cut solid angle, said output lens is moveable to adapt to object distance change. said prisms may be replaced by appropriate combinations of mirrors.

17. The optical limiting device of claim 15 or 16, wherein said pai-tial reflecting coating is a specially constructed reflecting coating which maintains phase difference of being or near 0° or 360° for parallel and perpendicular polai-ization components in the vicinity of the designed incident angle and within a certain wavelength range.

18. The optical liniiting device modified based on the limiting devices of claims 15 and 17, or 16 and 17, which character is that a quarter-wave plate and a polarizer are used to block noise light going backwards. and a three-quarter-wave plate and another polarizer are used to eliminate induced phase difference by said quarter-wave plate and polarizer.

19. The optical limiting device modified based on the limiting devices of claims 15.
17 and 18, or 16. 17 and 18, which character is that the bandpass filter having narrow transmission region is used to increase coherence length of non-laser incident beam, or the filter having three narrow transmission regions for tree primary colors is used to increase coherence length of incident non-laser beam and to get white light feeling.

20. The optical limiting device modified based on the limiting devices of claims 15, 17. 18 or and 19, or 16, 17, 18 or and 19, which character is that using a layer of transparent linear material to avoid self-defocusing, which middle plane is at the focal plane of all said pairs of two divided beams.

21. The optical limiting device modified based on the limiting devices of claims 15, 17, 18 or and 19, or 16, 17, 18 or and 19, which character is that one or more than one additional optical limiters are used to pre-limit the incident beam power to lower than a desired value.

22 . The optical limiting device modified based on the limiting devices of claims 15, 17, 18 or and 19, 21, or 16, 17, 18 or and 19, 21, which character is that two inside surface of the said transparent container are treated to make nematic liquid crystals in relaxed phase state, and two polarizers are used to produce linear polarized light and block the high power light.

23. The optical limiting device designed based on the optical limiting method of claim 4, which comprises an input lens, a said partial reflecting coating, an output lens, a said diaphragm, and a solid said nonlinear optical absorption, scattering, refraction, or photorefraction, photosensitivity material, or a transparent container filled with the said nonlinear optical absorption, scattering, refraction, or photorefraction, photosensitivity material, the back surface of said solid material or the inside back surface of the said transparent container is plane and said partial reflecting coating is coated on this back surface or inside back surface, the focal plane of said input lens coincides with said partial reflecting coating, said output lens is moveable to adapt to object distance change.

24. The optical limiting device of claim 23, wherein said partial reflecting coating is a specially constructed reflecting coating which maintains phase difference of being or near 0° or 360° for parallel and perpendicular polarization components in the vicinity of the designed incident angle and within a certain wavelength range.

25. The optical limiting device modified based on the limiting devices of claims 23 and 24, which character is that the bandpass filter having narrow transmission region is used to increase coherence length of non-laser incident beam, or the filter having three narrow transmission regions for tree primary colors is used to increase coherence length of incident non-laser beam and to get white light feeling.

26. The optical limiting device modified based on the limiting devices of claims 23 and 24, which character is that a layer of transparent linear material is deposited on the top of the said partial reflecting coating to avoid self defocusing.

27. The optical limiting device modified based on the limiting devices of claims 23 and 24, which character is that one or more than one additional optical limiters are used to pre-limit the incident beam power to lower than a desired value.

28. The optical limiting device modified based on the limiting devices of claims 23, 24 or and 25, or and 27, which character is that two inside surface of the said transparent container are treated to make nematic liquid crystals in relaxed phase state, and two polarizers are used to produce linear polarized light and block the high power light.

29. The optical limiting device modified based on the limiting devices of claim 7, or 8, or 9, or 10, or 11, or 12, or 13, or14, which character is that two right-angle prisms are used to offer erect image, said input compound lens may be placed in the way that the incident beam goes through it first, or in the way that the incident beam goes through a said right-angle prism first, said prisms may be replaced by appropriate combinations of mirrors.

30. The optical limiting device modified based on the limiting devices of claim 7, or 8, or 9, or 10, or 11, or 12, or 13, or14, which character is that an Abbe's prism is used to offer erect image. said Abbe's prism may be replaced by appropriate combination of mirrors.

31. The optical limiting device modified based on the limiting devices of claim 7, or 8, or 9, or 10, or 11, or 12, or 13, or14, which character is that a inverting lens is used to offer erect image.

32. The optical limiting device modified based on the limiting devices of claim 15, or 17, or 18, or 19, or 20, or 21, or 22, which character is that a inverting lens is used to offer erect image.

33. The optical limiting device modified based on the limiting devices of claim 15, or 17, or 18_ or 19. or 20, or 21. or 22, which character is that an additional right-angle prism is used to offer erect image. said additional right-angle prism may be replaced by appropriate combination of mirrors.

34. The optical limiting device modified based on the limiting devices of claim 23. or 24, or 25, or 26, or 27, or 28, which character is that two right-angle prisms are used to offer erect image, said input lens may be placed in the way that the incident beam goes through it first, or in the way that the incident beam goes through a said right-angle prism first, said two right-angle prisms may be replaced by appropriate combinations of mirrors.

35. The optical limiting device modified based on the limiting devices of claim 23, or 24, or 25, or 26, or 27, or 28. which character is that an Abbe's prism is used to offer erect image, said Abbe's prism may be replaced by appropriate combination of mirrors.

36. The optical limiting device modified based on the limiting devices of claim 23, or 24, or 25, or 26, or 27, or 28, which character is that a inverting lens is used to offer erect image.

37. The optical limiting device modified based on the limiting devices of claim 23, or 24, or 25, or 26. or 27, or 28. which character is that two pairs of two mirrors are used to offer erect image, said input lens may be placed in the way that the incident beam goes through it first, or in the way that the incident beam goes through one, or two or three, or even four said mirror(s) first to get different field of view, one of said two pairs of two mirrors may be replaced by a right angle prism.

38. The optical limiting device modified based on the limiting devices of claim 29, or 30. or 31, or 32, or 33, or 34, or 35, or 36. or 37, which character is that using each of said limiting devices offering erect image as an optical limiter, mount a pair of two identical such optical limiters side-by side and align them to point accurately in the same direction, and allow the viewer to us both eyes when viewing distant objects to build portable optical limiter for individual use, said output lenses are moveable to let observer to view the objects at different distances.

39. The optical limiting devices of claims from 7 to 37, the apertures and focal lengths of said input lenses or input compound lenses are from 0.5 cm to 50 cm and 1 cm to 100 cm respectively, the sizes of said prisms, mirrors, output lenses or output compound lenses are matched the sizes of their corresponding input lenses or input compound lenses in each individual device, the thicknesses of said solid nonlinear optical absorption, scattering, refraction, or photorefractive, photosensitive material, or the thicknesses of said transparent container filled with nonlinear optical absorption. scattering, refraction, or photorefractive.

photosensitive material are from 1 µm to 5 cm, light intensity reflectances of said partial reflecting coatings are from 10% to 98%.