Three - University of Arkansas Physics Department

Image transfer by mutually
pumped phase conjugators
Richard J. Anderson, Edward J. Sharp, Gary L. Wood, and Gregory J. Salamo
Cross talk is observed during the transient time of the photorefiactive grating formation in a mutually
pumped phase conjugator. We show that this feature can be used to transfer pictorial information from
one location to another. The transfer is instantaneous and is demonstrated at a resolution of 6
lines/mm. 0 1996 Optical Society ofAmerica
1. Introduction
Multiwave mixing in photorefractive crystals has
been used for a number of applications in optical
image processing. The photorefractive properties
of energy exchange and phase conjugation have led
to demonstrations of image amplification, edge enhancement,
addition and subtraction of images, restoration
of aberrated images, correlation and convolution
of images, conversion of incoherent images
into coherent images, and image transfer from one
location to another.' Sending spatial information
from one location to another can be accomplished by
many different techniques. One technique is to use
photorefractive crystals as a medium for the transfer
of images between two laser beams or between an
incoherent source and a laser beam.%' For example,
in the case of the photorefractive incoherenttocoherent
converter described in Ref. 6, which is
based on a self-pumped phase conjugator, the image
transfer was demonstrated for both low-power cwlaser
beams and for white light. This particular
device exhibited a spatial resolution of -40 line
pairs/mm, and the typical time to transfer a single,
two-dimensional image was 140 ms at 1 ~/cm2.
This slow response time is typical for photorefractive
materials and corresponds to the time to write or
erase a photorefractive grating.
R. J. Anderson is with the National Science Foundation, Washington,
D.C. 20550. E. J. Sharp and G. L. Wood is with the U.S.
Army Research Laboratory, Fort Belvoir, Virginia 220604838.
G. J. Salamo is with the Department of Physics, University of
Arkansas, Fayetteville, Arkansas 72701.
Received 21 February 1995; revised manuscript received 26
September 1995.
0003-6935/96/050854-06$06.00/0
o 1996 Optical Society ofAmerica
854 APPLIED OPTICS / Vol. 35, No. 5 / 10 February 1996
While the concept of image transfer between light
sources by use ofphotorefractive crys:.:!; -nc be us22
to transfer information from one locatiozi to ~i~ltl,;~;
it is both alignment sensitive and slow. In this
paper we present a new technique to accomplish this
transfer that is based on a mutually pumped phase
conjugator (MPPC) that is self-aligning and instantaneous.
Previous studiess have shown that temporal
information at one location could instantaneously be
transferred to another location by the MPPC. In
this paper we show, for the first time to our knowledge,
that pictorial information can also bt: transferred.
In addition we characterize the quality of
the image transfer.
2. Double Phase Conjugation
Anew type of phase conjugator, unique to phol orefrao:
tive crystals, called the mutually pumped phase
conjugator (MPPC), has been demonstratvd in a
variety of geometrie~.~~~ In these devices, two beams
are incident (usually on opposite faces of the crystal)
and overlap in some region of the cryst:,l. The
beams may be derived from differ en^ LC!- - n 9 iong
*
as the laser wavelength is nominally the Sam.: (;.e, two He-Ne lasers, for example). These conjugators
can be classified by the number of internal reflections
the beams experience before conjugation:
n~ne,~J~Jl one,12 two,13 or three.14 There have alsd
been MPPC's demonstrated that are based on the
semilinear mirror design.16J6 In all these devices,
two phaeeconjugate outputs (double phase conjugation)
are produced simultaneously by the interaction
of the two mutually incoherent beams of the same
wavelength within the photorefractive crystal.
Since MPPC's differ in geometry rather than In their
physical mechanism, they have been shown to share
many common characteristics: (1) the conjugation
of two beams occur simultaneously, (2) the conjugat-
'