Magnetic diffraction grating materials, being developed to provide a simple means of deflecting light in a two dimensional, solid state fashion are discussed. The most promising material, for several applications, appears to be bismuth substituted iron garnet films in epitaxial form. Calculations indicate that deflection efficiency greater than 60% is possible in the near infrared region of the spectrum. Within the field of view of the deflector, measurements predict that 105 resolvable spots can be expected. Applications include: (1) general purpose deflection of free laser light, (2) image processing of extended sources such as transparencies, (3) programmable lensing, and (4) fiber optic matrix switching

Drawczak, J. A.

Hewitt, F. G.

Nelson, G. L.

Torok, E. J.

English

NASA Technical Reports Server

14ANIPULATION OF LIGHT WITH. Iv~AGN~~TO-OPTIC 
STRIPE DONAIN FILMS 
E. J. Torok, F. G. Hewitt, J. A. Krawczak, G. L. Nelson 
Sperry Univac, DSD, St. Paul, Minnesota 
Work sponsored in part by the Army Research Office (ARO), Research Triangle Park, 
North Carolina. 
ABSTRACT 
Magnetic diffraction grating materials are currently being developed to provide a 
simple means of deflecting light in a two-dimensional, solid-state fashion. 
promising material, for several applications, appears to be bismuth substituted iron 
garnet films in epitaxial form. 
greater than 6% is possible in the near-infrared region of t e spectrum. Within the 
expected. Applications include 1) general purpose deflection of free laser light, 
2) 
lensing, and 4) 
The most 
Calculations indicate that deflection efficiency 
field of view of the deflector, measurements predict that 10 sh resolvable spots can be 
image processing of extended sources such as transparencies, 3) programable 
fiber optic matrix switching. 
MAGS€YTO-OPTIC LIGHT DEFLECTOR 
The active component of this deflector is a dynamically alterable, solid-state 
phase diffraction grating that is the energetically favored domain structure of pro- 
perly configured Fgnetic materials. A description of the d-eflector follows. 
Diffraction of a light beam occurs as a result of periodic variations in the wave 
amplitude or phase across a wave n o m 1  surface. 
introduce a periodic 180' phase variation in an incident optical field, through mag- 
netic birefringence. A stripe domain is a long, straight region of uniform width in 
which the magnetization is nearly constant. 
micron to 30 microns or more while the length can extend to several centimeters$ 
Stripe domains in a given sample m y  or magnot have equal width, depending on sample 
properties and on imposed magnetic fields. 
Magnetic stripe domain arrays can 
Typically, the width can vary from .5 
Consider a linear array of stripe domains in a magnetic platelet as depicted in 
Adjacent stripes have z" components of magnetization that are antiparallel figure 1. 
and usually&ve a+continuous component in the x y plane. 
effect the Ex and E components of an incident optical field suffer clockwise rotation 
in odd-numbered strxpes and counterclockwise rotation in even-numbered stripes. 
differential rotation provides an electric component that is orthogonal to the inci- 
dent polarization and has 180 alternations (parallel and antiparallel to y^) that mtch 
the spatial period of the domains. The process operates uniformly for all incident 
polarizations, including random. 
Because of the Faraday 
This 
In the far-field emerging light adds constructively at angles En, given by 
34 1 
https://ntrs.nasa.gov/search.jsp?R=19820008049 2020-03-21T09:50:22+00:00Z
sin On = - 7  nX 
A 
where n is the order number, X the incident wavelength, and -A the grating period. 
the special case where each stripe has the same width, even orders are suppressed. 
Magnetic apodization at the transitions between even and odd stripes discourages 
higher orders, as well. 
In 
The nth odd-order power diffraction efficiency for a square phase grating is 
found from 
with 01 the op ical absorption coefficient, t the material thickness, and F the Fara- 
day rotation.' Figure 2 shows the potential total efficiency of several candidate 
crystalline deflector materials in th? Eisible and infrared region of the spectrum, 
based on reported values for F and a. - 
In order to alter the grating in a solid-state dynamic fashion, reliance is made 
on the strong coupling of the material mgnetization and applied magnetic fields 
either coplanar with, or perpendicular to, the stripe domains. 
in which a magnetic grating can be field programmed to deflect light. 
ular field changes the stripe width, periodicity, or both, causing linear deflection 
and perhaps a shuffling of light amongst the various allowed orders. 
applied in the plane, and collinear with the domains, then the stripe width varies 
with H-I, increasing the diffraction angle with increases in field intensity. 
ly, if the field is applied to a general direction in the plane,the grating is re- 
established collinear t o  the field, resulting in azimuthal deflection. 
materials the field strength necessary to cause deflection in the annular field of 
view is less than that required for  Lorentz deflection of electron beams in CKI's. 
Thus, microsecond switching speeds are possible with watt level electrical power. 
There are several ways 
A perpendic- 
If the field is 
Final- 
For some 
GARNET 
Of the deflector candidates contrasted in figure 2, rare earth iron garnet is 
presently the preferred material for a variety of reasons. 
as epitaxial films up to 100 microns thick grown on gadolinium gallium garnet, com- 
monly employed as magnetic bubbl memory substrates. Stable stripe domain arrays of 
been observed. Curie points to 550 K insure that the magnetization is nearly constant 
over a wide temperature range, including room ambient. The applied fields required to 
manipulate the stripe domains are on the order of 100 oe., derivable from computer 
controlled Helmholtz pairs or strip lines in close pgoximity to the crystal. With 
bismuth doping the Farad y rotation can reach 50,oOO /cm in the visible and 12,oOo0/ 
em in the near-infrared? The optical absorption embits a local minimum at the .81 
micron fiber-optic wavelength and a large window of near-zero absorption at wave- 
lengths greater than 1.2 microns. 
duce the absorption at important wavelengths.8 From experimental data obtained with 
It is readily obtainable 
sufficient quality to provide 10 9 rfjsolvable spots in the annular field of view have 
Calcium do ing has been successfully used to re- 
342 
a Faraday hysteresigraph and a spectrophotometer on thin epitaxial samples it is 
possible to calculate the maXirmrm deflection efficiency of thick samples or of thin 
samples operated inside an optical resonant cavity. 
and the results are shown in figure 3. Work is currently under way to experimentally 
evaluate these expectations and to develop the necessary crystal growth facilities to 
exploit the potential of bismuth garnet for light deflector applications. 
The calculation h s  been done 
APPLICATIONS 
Agile light deflectors based on magneto-optics offer unique solvLtions to a number 
of optical switching and processing problems. 
dar at 1.06 microns and 10.6 microns, focal & n e  array scanners, and optical cormnuni- 
cations between moving platforms. 
deformed by spatially varying fields, adaptive optic processing of images of extended 
objects can also be performed. In guided wave comrrmnications, bismuth garnet, in its 
present form, serves quite well as a high fanout fiber optic switch. 
Obvious applications include laser ra- 
Because stripe domain arrays can be plastically 
Adaptive Optics 
In a stripe domain grating the periodicity, stripe width, and orientation need 
not be constants over the aperture. Spatially varying magneticfieldscan be utilized 
to locally modify the deflection of incident light from pointlike sources or from ex- 
tended sources such as collimated beams or transparencies. By properly tailoring the 
grating, dynamically alterable compressors or expanders and image rotators can be 
generated. 
One-dimensional compressors or expanders are constructed from the domain pattern 
Both the stripe density and that bas the basic spatial modulation seen in figure ba. 
grating orientation are functions of local field. 
image distortions. 
This would be useful for correcting 
Gratings obtained by converting the linear array of a film with some in plane 
magnetic component, into the radial array of figure kb, provide a means for continuous 
distortionless image rotation if the grating has the appropriate radial stripe density 
gradient. 
density is inversely proportional to radius. Then the impressed image rotation angle, 
B, is found to be 
A geometric optic analysis reveals that the gradient is such that the stripe 
with s the distance to the post grating image plane and c a quantity that depends on 
field mgnitude and stripe widths. In addition, the image experiences a rmgnification 
of see B. Continuous rotation occurs with intensity changes in the field. 
In order to initialize the grating to a radial mode it is necessary to bring a 
point pole into approximate contact with the film surface. 
field causes the grating to assume the desired form but with constant stripe density. 
The symmetry in the pole's 
343 
In practice the pole can be supplied by a polished ferrite needle. 
that develops the proper stripe density was found to match the tangential field pro- 
duced by an extended magnetic polepiece. Thus, it has been possible to observe both 
the rotation andothe scaling properties of the radial domain image processor. Rota- 
tions to near 90 were recorded. 
form, high speed rotation is expected.. 
Arrays formed from domains that are concentric annuli about a fixed center may 
act like a Fresnel zone plate; i.e., collimated light focuses on the zone axis when 
the domain widths satisfy the pertinent Fresnel relations. 
pinges 
axis. In either case, dynamic control of the domains implies dynamic lensing. Most 
likely, garnet materials for this application are of the bubble type because they 
have only a perpendicular magnetization component. 
The inplane field 
Since the required polepiece is available in ferrite 
If the incident light im- 
on just a sector of this array as in figure 4c, focusing occurs off the optic 
Fiber Optic Switch 
Useful deflection efficiency at the present fiber optic wavelengths, along with 
Information-bearing light, propa- 
magnetic control of the intrinsic grating in garnet films is the basis for a d t i -  
port fiber optic switchboard as seen in figure 5. 
gating in any or all of the elements of the input fiber 2-dimensionalmatrix is 
collimated by gradient index lenses (GRIN). 
just one deflector element which steers it to one fiber in the focal plane of the 
output lens. The switchboard has high fanout with just a single level of optical 
switching. 
Light from a given fiber lens falls on 
Greater than 100 X 100 input or output arrays can be accomodated. 
Figure 6 shows a basic version of a fiber optic switch that utilizes only the 
inner circumference of the deflector field of view. 
selected output fibers that are arranged at the appropriate positions on the 1/0 face 
of a GRIN lens. 
tation of the stripes has been established by the field coils to direct input light 
to the selected tap, all coil current can be removed until another tap fiber selec- 
tion is required. 
Input bus light is tapped by 
This version provides a set-and-forget feature; i.e., once the orien- 
Meaurements and observations to d.ate indicate that the 
other desirable features. 
ticularly attractive for fiber optics. 
fanout of 10. 
model of a 1 X 3 switch. 
second select time. 
and is dispersive, allowing two-way operation of multiwavelength carriers. 
attributes provide significant systems potential. 
switch has a number of 
Physical size can be as low as 1 ern3 as shown in figure 1, a working 
It is insensitive to incident polarization, making it par- 
Crosstalk can be held to < - 20 db. with a 
Field coil switching is approximately 1 watt with micro- 
As a diffraction grating it satisfies the reciprocity theorem 
These 
One useful systems device is an optical switch that can access just one output or 
simultaneously access all outputs, on electrical command. 
the basic stripe domain fiber optic switch can accomplish this because of its adaptive 
optic capability, With reference to figure 4b, if the stripe grating is switched to 
the constant periodicity radial format discussed under adaptive optics, the deflection 
space is a thin annulus at the same deflection angle as for the linear grating. Since 
the tap fibers are arranged to intercept light at this angle, they are uniformly il- 
l-crminated with the light propagating in the bus fiber. 
It was demonstrated that 
Thus, all tap fibers are 
344 
simultaneously selected. 
the optic axis, just behind the mirror. 
miltiselect star coupler mode should occur in microseconds. 
This can be implemented by placing a point magnetic pole on 
Switching between single select and the 
Because of its high fanout the garnet fiber optic switch may have use as a resi- 
due arithnetic adder for optical computing. 
modulo 5 adder in which input fiber light represents one addend and the grating se- 
lected output represents the other. 
Figuse 8 is a schematic of a possible 
Magnetic stripe dormin arrays in bismuth iron garnet epitaxial films are alter- 
able in orientation and grating constant with externally applied magnetic fields 
generated by high speed deflection circuitry. 
grating, is thus diffracted in a two-dimensional solid-state fashion. Favorable 
deflection efficiencies and local control of the grating suggest a number of applica- 
tions. 
optical processing. 
Optical radiation, incident upon the 
Further work may lead to compact devices that are applicable to robotics and 
1. 
2. 
3. 
4. 
5. 
6. 
7. 
8. 
Johansen, T. R.; Norman, D. I.; Torok, E. J.: 
in a Faraday Effect Light Deflector. JAP, Vol. 42, No. 4, I%-, 1971, 1715. 
Scott, G. B.; Lacklison, D. E.: 
Bismuth Substituted Iron Garnets. 
Variation of Stripe Domain Spacing 
Magneto-optic Properties and Applications of 
IEEE Trans. on Mag., Vol. Mag-12, No. 4, Ju ly  
1976, 292. 
Fleming, D. L.; Lund, R. E.: 
SPIE, Vol. 128, 1977, 230. 
Magneto-optic Bubble Domain Spatial Light Modulator. 
Dimmock, J. 0.; Huorwitz, C. E.; Reed, T. B.: 
Birefringence and Faraday Rotation in EUO. 
Infrared Transmission Magnetic 
APL, Vol. 14, No. 2, 15 Jan., 1969. 
Ahn, K. Y.; Suits, J. C.: IEEE Trans. onbg., Vol. MAG-3, Sept., 1967. 
Shafer, A. W.; Torrance, J. B.; Penney, T.: 
Conductivity, and Stoichiometry in EUO. 
C. D. Graham, Jr. (ed.), AIP, 1971. 
Correlations of Infrared Absorption, 
Magnetism and Magnetic Materials, 
Soref, R. A.; Kestigian, M.: 
Domain IWtimode Optical Switches. 
80-25, 1980. 
Development of Liquid Crystal and Magnetic Stripe- 
Rome Air Development Center, Report SRC-CR- 
Nelson, G. L.; Harvey, W. A.: Optical Absorption Reduction in Bi L Fe 
Magneto-Optical Materials, JAF, February 1982. 1 u2 5012 Garnet (To be published) 
345 
INCIDENT LIGHT 
F i g u r e  1.- Magneto-opt ic  d i f f r a c t i o n  g r a t i n g .  
2.0 4.0 6.0 8.0 10.0 12.0 
WAVELENGTH, MICRONS 
F i g u r e  2 . -  C a l c u l a t e d  d i f f r a c t i o n  e f f i c i e n c y  o f  s t r i p e  
domain m a t e r i a l s .  
346 
SO 
DEFLECTION 
EFFICIENCY IN  04 
40 
I I I I I I 
WAVELENGTH, MICRONS 
1 
0.7 0.8 0.9 1 .o 1.1 1.2 1.3 
Figure 3.- Calculated diffraction of optimized garnet film. 
Figure 4.- Adaptive optic configurations. (a) Compression/expansion. 
( b )  Image rotator. (e) Fresnel zone plate. 
347 
OUTPUT FIBERS 
INPUT FIBERS 
t 
IGHT DEFLECTOR 
LEMENTS 
ELECTRICAL 
CONNECTIONS 
MIRROR 
\ 
GRIN LENSES 
F i g u r e  5 . -  N X M s w i t h b o a r d .  
GARNET 
SUBSTRATE 
TAP F 
/ 
GRADIENT INDEX 
LENS INTERFACE 
STRIPE DOMAINS 
BUS FIBER 
F i g u r e  6 . -  F i b e r  o p t i c  s w i t c h .  
348 
F i g u r e  7 . -  Operational model  of 1 x 3 garnet I ’ iber  o p t i c  s w i t c h .  
349 
OUTPUT FIBERS 
2 
I 
A 
A 
A 
I 
t 1 
sc2 
~~~* 
A 
TRANSMISSIVE 
STAR COUPLERS 
4 
I 
1 2 3 
INPUT FIBERS 
FIBER OPTIC 
SWITCH 
4 
Figure 8.- Residue arithmetic modulo 5 adder. 
350  


Manipulation of Light with Magneto-optic Stripe Domain Films

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