Three - University of Arkansas Physics Department

Self-trapping of two-dimensional optical beams and light-induced
waveguiding in photorefractive InP at telecommunication wavelengths
M. ~hauvet,~) S. A. Hawkins, and G. J. Salamo
Depariment of Physics, University of Arkansas, Fayetteville, Arkansas 72701
M. Segev
Department 4 Electrical Engineering and Advanced Center for Photonia and Optoelectronic Materials,
Princeton University, Princeton, New Jersey 08544
D. F. Bliss and G. Bryant
Rome Laboratory, United States Air Force, Hanscom Air Force Base, Massachuserts 01 731
(Received 16 December 1996; accepted for publication 10 March 1997)
We demonstrated an experimental observation of self-trapping and self-deflection of a
two-dimensional optical beam by the photorefractive effect at telecommunication wavelengths
under an applied dc field. Self-trapping is effective for an intensity range related to the
intensity-temperature resonance known for two-wave mixing in 1nP:Fe. The photorefractive index
change giving rise to the trapping is measured at while the photorefractlve space-charge field
is measured at about 50 kV/cm, ten times higher than the applied field. We show experimentally that
this index change creates a waveguide that can be used to guide a second beam at 1.55 pm.
O 1997 American Institute of Physics. [S0003-695 1 (97)01019-X]
A photorefractive soliton'-9 is created when a photoinduced
index change exactly compensates for the diffraction
of the beam. In this sense, the beam is able to create its own
waveguide and offers potential for applications in the field of
all-optical switching and beam steering. These effects have
been extensively studied in ferroelectric oxide and sillenite
oxide crystals for visible wavelengths. For the near infrared
wavelengths used in telecommunications, 1nP:Fe crystals
have already demonstrated interesting photorefractive
properties10-13 and self-trapping of a one-dimensional beam
has been reported.14 In this letter, we report the first observation
of trapping of a two-dimensional Gaussian beam at
1.3 pm. The conditions for the beam trapping are presented
and the index change and associated space charge field making
possible the formation of the induced optical waveguide
are measured. The self-induced waveguide at 1.3 pm is then
shown to guide a second beam at the telecommunications
wavelength 1.55 pm.
For the experiment, the beam from a Nd:YAG laser at
1.3 pm is collimated and focused with a 5 cm focal length
lens on the entrance face of an 1nP:Fe crystal whose temperature
is stabilized at 297 K. As shown in Fig. 1, the electric
field, ELis applied along (1 lo), while the beam propagates
along (1 1 O), and is polarized either horizontally or vertically
at 45" from (110).
Figure 2 shows an example of 2D trapping using this
configuration. The beam size at the entrance face of the crystal
has a diameter of about 55 pm and diverges to a 170 pm
diameter at the exit face when no field is applied to the
crystal. The 1nP:Fe crystal length is 1 cm in the direction of
the propagation. When a 12 kV/cm field is applied to the
crystal, the beam is trapped and the beam diameter at the exit
face is elliptical and reduced to about 55 pm by 65 pm. Our
experiments show that for exactly the same value of all parameters,
the beam is trapped to the same diameter for either
a'~lectronic mail: chauvet@comp.uark.edu
a 1 or a 0.5 cm crystal length, supporting the conclusion that
the trapped diameter is the same throughout uli. -r;r,tal. 'To
show that an efficient waveguide is formed in the crystal, we
have also propagated a 1.55 pm low ~qtensity
(=lm~/cm~) laser beam collinear to the
1.3 pm(l W/cmZ) trapped laser beam. To observe only the
1.55 pm beam, a color filter is placed after the crystal. The
diffracted 1.55 pm beam at the crystal exit face without the
1.3 pm induced waveguide is shown on Figs. 3(a) and 31~).
When both beams are present and vertically polarized, the
1.55 pm beam is observed to be guided along the same direction
as the 1.3 pm beam [Fig. 3(b)]. However, when the
polarization of the 1.3 pm beam is changed to horizontal and
the 1.55 pm beam is kept vertical, the 1.55 pm beam is
guided on the side of the 1.3 pm trapped beam [Fig. 3(d)].
While in both cases the 1.55 pm beam is we can see
that when the two beams have orthogonal polarizations, the
1.55 pm beam is more efficiently guided and has a near
circular profile. The background noise is an ordcr of rnagnitude
below the guided light and is due to an .irnperfcct
launching of the light in the waveguide gi,;,,, to cladding
modes.
FIG. 1. Apparatus and crystal orientation for observing tiyo-dimensional
trapping of a 1.3 pm beam and measuring the index change. ti-hall-wave
plate; P-polarizer; C-InP crystal.
Appl. Phys. Lett. 70 (19), 12 May 1997 0003-6951/97/70 19)/2499/3/$10.00 O 1997 American Institute ui ;;,;.sirs 2494
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