Optical: systems and elements – Optical modulator – Light wave directional modulation
Reexamination Certificate
2002-10-11
2004-06-01
Dang, Hung Xuan (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave directional modulation
C359S245000, C359S249000
Reexamination Certificate
active
06744551
ABSTRACT:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable
REFERENCE TO A SEQUENCE LISTING, A TABLE, OR A COMPUTER PROGRAM LISTING COMPACT DISK APPENDIX
Not Applicable
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to electrooptic materials and real-time electrooptic devices and more particularly, to techniques for reducing or eliminating optical damage effects in electrooptic materials so they can be used to effectively control, with electric signals, attributes of a light beam traversing the material.
2. Description of the Related Art
From an application perspective, there are two types of electrooptic effects: direct and indirect. The direct electrooptic effect is the change of index of refraction of an optical transmission medium induced by an electric field that is directly controlled by an electric voltage signal applied on a pair of electrodes enclosing the medium. The indirect electrooptic effect involves the change of index of refraction induced by electric field that is generated by a nonelectric process, such as by light illumination, as in a photorefractive process. It has been established that, in a photorefractive process, a light beam can cause a redistribution of the electrons and ions, hence creating a non-uniform internal electric field in the medium. Such a non-uniform internal electric field also causes changes of refractive index, affecting the light propagation inside the medium. The goal of photorefractive research efforts and photorefractive application devices has been to find a way to utilize the non-uniform distribution of the index of refraction induced by incident light beams. Thus, even though in the photorefractive process, indirect electrooptic effects constitute an important contributing factor, the photorefractive process and the direct electrooptic effects are two essentially different physical processes. The most important differences are that (1) the photorefractive process is a light-light interaction process, while the direct electrooptic process is control of a light beam through an external electric field, and (2) in physical implementations, the photorefractive process involves electrodeless electrooptic effects, whereas, for direct electrooptic effects, there must be at least a pair of electrodes to generate the electric field.
Direct electrooptic effects have been used for the construction of light beam deflectors, modulators, spectral filters, optical switches, multiplexers, and optical computing devices. According to convention, the phrase, “electrooptic device,” is used solely for those devices that utilize the direct electrooptic effect, while for the devices involving indirect electrooptic effects, substantially different terminologies are used, such as those used in photorefractive devices. Thus, unless noted otherwise, the present specification uses the terms, “electrooptic effects” and “electrooptic devices,” to indicate the physical process involving direct electrooptic processes.
For practical applications of the direct electrooptic effects, one of the most important requirements for an electrooptic material is a high electrooptic coefficient. As is well known, when an electric field is applied to an appropriate electrooptic material in the appropriate direction, the change of the index of refraction is
n
(
E
)−
n
(0)=
a
1
E
+(½)
a
2
E+
(1)
where n(E)−n(0) is the change in the index of refraction due to the electric field E, and a
1
, a
2
, . . . , are the first order and second order electrooptic coefficients for the material. According to general convention, Equation (1) can also be written as
n
(
E
)−
n
(0)=(½)
n
(0)
3
·r·E
+(½)
n
(0)
3
·R·E
2
(2)
where r and R are first order and second order electrooptic coefficients, respectively, and are generally complex high-rank tensors. Note that, for the construction of direct electrooptic devices, a critically important difficulty is the fact that for all known electrooptic materials, electrooptic coefficients are very small. For example, in the prior art, the best and the most popular electrooptical materials for direct electrooptic devices are the crystals ADP, KDP (KH
2
PO
4
), lithium niobate (LiNbO
3
), and lithium tantalate (LiTaO
3
). Lithium niobate may be considered as having the largest linear electrooptical coefficient among all the qualified materials. Lithium niobate has a linear electrooptic coefficient r
33
=35.8 pm/V, or 35.8×10
−6
mm/kV. When electric field E is as strong as 1 kV/mm, the change in the index of refraction is only approximately 1.85×10
−4
, which is too small for many potential applications.
On the other hand, some materials have been found that exhibit much larger electrooptic coefficients. Certain single crystal materials, such as SBN (Sr
1-x
Ba
x
Nb
2
O
6
, where x is the percentage composition, in the range of 0.25<x<0.75), have an electrooptic coefficient 30 to 100 times larger than that of the above-identified materials. In the present specification, these materials with unusually large electrooptic coefficients are called “high sensitivity electrooptic materials,” in contrast to the conventional electrooptic materials of the prior art, which are called “low sensitivity electrooptic materials.” There is no clear demarcation line for differentiating the high sensitivity from the low sensitivity electrooptic materials. However, high sensitivity electrooptic materials may be roughly defined as those materials that have an electrooptic coefficient at least five times greater than that of the average coefficient of conventional electrooptic materials.
In the 1960s, it was found that the incident light, the light whose attributes are being intentionally modified by applying a voltage signal to the electrodes through direct electrooptic effects, causes these high-sensitivity electrooptic crystals to incur a kind of internal “optical damage.” Optical damage is a phenomenon that, under light illumination, the index of refraction of an originally homogeneous crystal material becomes spatially inhomogeneous. It was also found that the optical damage becomes apparent only after illumination over a relatively extended period of time, measured in seconds or greater. As a consequence of the optical damage, when a light beam passes through such a material, the well-behaved incident light beam becomes severely distorted. Because of this effect, all materials vulnerable to optical damage effects were effectively excluded from use for control of incident light beam attributes through direct electrooptic effects.
In recent years, the underlying cause of optical damage in electrooptic materials has been essentially explained. It was discovered that the light-induced change of the index of refraction is due to the light-induced redistribution of electric charge inside the material. A light-induced inhomogeneous electric charge distribution inside the material produces a strong inhomogeneous internal electric field which, in turn, causes an inhomogeneous distribution of index of refraction.
As indicated, those materials having the most pronounced optical damage have been excluded from use for control of incident light beam attributes through use of direct electrooptic effects because of the severe detrimental effects caused by the incident light beam. Unfortunately, those electrooptic materials with severe optical damage are often the best direct electrooptic materials because they have the largest electrooptic coefficients, which is by far the most important property.
BRIEF SUMMARY OF THE INVENTION
The object of the present invention is to provide a method and device for reducing or eliminating optical damage in high sensitivity electrooptic materials so that they can be employed in the dynamic control of light beam attributes.
The present invention provides an effective means for reducing or eliminating the optical damage effects so that high sensitivity electrooptic materi
Altman & Martin
Dang Hung Xuan
Martinez Joseph
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