Time-Reversed Replication of a Wave Front

This invention relates to an apparatus and method for creating a time-reversed replica of an incident wave front.

For any electromagnetic wave propogating through a light-scattering medium having no permanent magnetism, there can exist in principle a time-reversed replication of that wave. Thus an appropriately patterned but irregular wave front can travel through an inhomogeneous or light-scattering medium and emerge as a coherent uniform wave front, provided it is a replica, reversed in time, of a coherent beam that was originally deformed by the same homogeneous medium.

It is well known that time-reversed waves can be generated by non-linear effects in an irradiated medium. A nearly time-reversed wave of a ruby laser beam whose phase front has been deformed by an inhomogeneous medium has been produced by stimulated Brillouin scattering in the backward direction. However, time reversed waves produced by Brillouin scattering have been found to be downshifted slightly in frequency. Stimulated Brillouin scattering has also been utilized to restore coherency of a laser beam amplified by an inhomogeneous amplifying medium. It is theorized that distortion of images transmitted in multimode optical fibers will be reduced with time-reversed wave fronts generated by parametric mixing in an acentric crystal. This mixing process should produce a time-reversed wave front which, when redirected through the multimode optical fibers, will restore a close facsimile of the originally transmitted wave. However, limitations will be placed on the beam-acceptance angles in this process due to phase matching requirements on waves which can mix efficiently in the acentric crystal. The present invention eliminates both frequency shift and phase matching requirements associated with prior time-reversed wave generation techniques, and a more accurate time-reverse replication is obtained that with either stimulated Brillouin scattering or parametric mixing. In addition, the apparatus disclosed herein allows time-reversed wave fronts to be generated with less input power than was heretofore possible.

According to the present invention, a method and apparatus is provided which will generate a time-reversed replica of an image-bearing wave by superimposing the image bearing wave on two counter-propagating waves present in a transparent medium, the counter-propagating waves being at substantially the same frequency. The apparatus consists of a substantially monochromatic light source irradiating a medium having inhomogeneities in its refractive index, an example of such a medium being glass treated with hydrofluoric acid. The source or incident light passing through the inhomogeneous medium is directed to the time-reversed wave-front generation device of the present invention. This device consists of a homogeneous, transparent, non-linear medium, such as liquid carbon disulfide, which has counter-propagating light passing therethrough, the light being generated by a laser having a substantially monochromatic output. The source light when striking the non-linear medium containing the counter-propagating waves is partially reflected back along the path of the incoming source light, the reflected light being time-reversed with respect to the source light. This time-reversed reflected wave upon striking the inhomogeneous medium is reformed into light having wave front characteristics the same as the originally transmitted source light. This light is then isolated by a beam splitter which reflects a portion of the reconstructed light wave to a viewing means such as a camera. The time-reversed replication of the incident optical wave is induced by a process of non-linear refraction caused by an interaction of the incident optical wave with the counter-propagating pump laser waves within the non-linear medium. Applications of the device can be found in spectroscopy, microscopy, the correction of beam distortions within optical fibers, and correction of beam distortions caused by transmission through an inhomogeneous or light-scattering medium.

FIG. 1 is an apparatus for generation of a time-reversed replica of a wave front by non-linear refraction according to the present invention;

FIG. 2 is an alternative apparatus showing the use of a beam splitter in place of one of the laser components.

As required, a detailed illustrative embodiment of the invention is disclosed herein. This embodiment exemplifies the invention and is currently considered to be the best embodiment for that purpose. However, it is to be recognized that various non-linear mediums and techniques for counter-propagating a laser beam through a non-linear medium to implement the principles of the invention can be utilized. Accordingly, the specific apparatus disclosed is representative in providing a basis for the claims which define the scope of the present invention.

As above discussed, the invention provides an optical image replication apparatus in which a time-reversed replica of a wave front is generated by non-linear refraction. The replication is generated when an image-bearing wave is superimposed on two counter-propagating waves, at or near the same frequency, in a transparent non-linear medium. A substantially monochromatic source or incident light beam passing through a phase distorting medium creates an irregular wave front. When this irregular wave front intersects a homogeneous, transparent, non-linear medium having counter-propagating, substantially monochromatic, light waves passing therethrough, it is reflected as a time-reversed wave through non-linear refraction within the medium. When the reflected wave irradiates the phase-distorting medium, it emerges as an ordered wave front having characteristics substantially identical to that of the incident light.

Referring to FIG. 1, a monochromatic light source 10, which could be a laser, is positioned so a that light beam projected therefrom will strike a beam splitter 12 which transmits a part of the light and reflects a part of the light to an appropriately positioned light source camera 14. A phase-distorting medium 16 is positioned in the path of the light transmitted through the beam splitter 12. The phase-distorting medium could be any light-scattering, preferably low-absorbing medium and is exemplified herein by clouded glass such as that commonly utilized for shower doors. Other examples of light scattering mediums include optical fibers, jet engine exhausts and light amplifiers. A focusing lens 18 focuses light radiating from the phase-distorting medium 16 onto a homogeneous, substantially transparent, non-linear medium which in this specific embodiment is liquid carbon disulfide. The thus focused light is referred to as the probe beam 19. A second monochromatic light source 26, referred to as a pump laser (i.e., its beam is activating) is directed so that its output beam irradiates the non-linear medium 20 at a location such that it intersects the probe beam 19. It is important that the pump laser output beam is counter-propagated within the non-linear medium 20. The counter-propagating effect in this embodiment is obtained by a mirror 28 positioned so that light from the pump laser 26 is reflected back into the pump laser 26. Although in this embodiment the mirror 28 is shown immediately adjacent the non-linear medium 20, it could just as well be spaced therefrom. As the probe beam 19 intersects the counter-propagating pump laser beam within the non-linear medium 20, a third order non-linear susceptibility causes a time-reversed replica of the probe beam 19 to be formed. A significant portion of the probe beam 19 is reflected back along its originally propagated path, through the focusing lens 18, the phase-distorting medium 16 and from the beam splitter 12 into an image-recording camera 30. According to the principles of the present invention, it is important that the counter-progagating waves within the non-linear medium 20 be of substantially the same frequency.

Alternatively, a single monochromatic light source in conjunction with a beam splitter could be used in lieu of the monochromatic light source 10 and pump laser 26, light reflected from the beam splitter constituting a light source beam and light transmitted through the beam splitter constituting a pump laser beam. Such an arrangement is shown in FIG. 2 in which a pair of beam splitters 26' and 26" intercepts light from the laser source 10 to direct a portion of that light to irradiate the non-linear medium 20 such that it intersects the probe beam 19, in the manner described above with respect to the output of monochromatic light source 26 of that figure. Of course, in this embodiment the apple image 32' is placed forward of the beam splitter 26', but rearwardly of the beam splitter 12. Operation in all other respects is identical to that with respect to the apparatus of FIG. 1.

Although the radiation in this embodiment is a monochromatic light source, any monochromatic (i.e., narrow frequency range) wave (e.g., acoustic, electromagnetic, plasma) could be utilized provided a similar type monochromatic wave is counter-propagated through a non-linear medium.

In operation, the monochromatic light source 10 propagates an image which for illustrative purposes is shown at 32 as an apple. The image intersects the beam splitter 12, a reflected portion being recorded by the light source camera 14 as shown at 34. This image 34 is a record of the image shown at 32. The portion of the light source 10 output passing through the beam splitter 12 irradiates a phase distorting medium 16 which reorders the various wave fronts forming the image shown at 32 and radiates light with wave fronts unrecognizable as such an image. This radiated beam is focused by the focusing lens 18 and is directed in the form of a probe beam 19 into the non-linear medium 20. The pump laser 26 radiates the non-linear medium 20, the light passing therethrough being reflected by the mirror 18 thereby providing counter-propagating waves within the non-linear medium 20 having substantially the same frequency. The probe beam 19 intersects the counter-propagating waves within the non-linear medium 20 and a portion thereof is reflected back along the original path of the probe beam 19, into the focusing lens, and onto the phase distorting medium 16. Light emerging from the phase-distorting medium is reflected by the beam splitter 12 to the image-recording camera 30, the light forming an image as shown at 36. Thus one can appreciate that with the apparatus above described, the original image shown at 34, after having passed through a wave distorting medium 16, can be converted into a time-reversed replica of which, when again passing through the phase distorting medium 16, is converted into an ordered wave front corresponding to the original image 34 as shown at 36. Without the non-linear medium 20 and the counter-propagating pump-laser wave fronts, such a replication of the original image would not occur. Illustrative of this point, a reflecting mirror 40 (shown in phantom) positioned in the path of the probe beam 19 would cause formation of an image at the recording camera 30 which would be unrecognizable with respect to the original image shown at 32 as shown at 42.

In a specific application of the above embodiment, a ruby laser at a wave length of 6943 angstroms was utilized to create the counter-propagating waves within a carbon disulfide non-linear medium 20. It was found that a probe beam 19 power of approximately 4% of the pump laser output power produced execellent results. The output of the monochromatic light source 10 need not have any particular wavelength relationship to the output of the pump laser 26. However, if the light source has a frequency of W.sub.1 and the probe beam has a frequency of W.sub.2, then the time-reversed replica will have a frequency if 2W.sub.1 -W.sub.2.

The apparatus of the present invention could be utilized in conjunction with an optical quantum amplifier for increasing the brightness of lasers. A conventional optical quantum amplifier usually induces a directivity deterioration of an incoming laser beam due to static or dynamic inhomogeneities in the refractive index of its amplifying medium. Using the apparatus of the present invention, high directivity of the amplified laser beam can be obtained when optical quality of the quantum amplifier is low by substituting the quantum amplifier for light-scattering medium of the above described embodiment. The output of the quantum amplifier, after being reflected back from the non-linear medium, will pass through the quantum amplifier and emerge with the same wave front as the entering laser beam to be amplified. This emerging radiation can then be isolated by a beam splitter according to the above described embodiment, thereby providing an amplified laser beam having the same directivity as the incoming laser beam.