Optical: systems and elements – Optical modulator – Light wave directional modulation
Reexamination Certificate
2001-07-20
2004-08-03
Dang, Hung Xuan (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave directional modulation
C359S313000, C359S285000, C359S286000, C359S287000
Reexamination Certificate
active
06771412
ABSTRACT:
BACKGROUND OF THE INVENTION
General
Coherent light radiation is converted, amplified and generated to obtain light signals for use in telecommunications, spectroscopy, medicine and so on and to obtain wavelength-division-multiplexing (WDM) signals for use in telecommunications. Unfortunately, the lasers used as sources of coherent light usually have an almost fixed wavelength that can be changed in a relatively narrow wavelength range. An optical parametric converter comprising an optical non-linear crystal (LiNbO
3
, KTP etc) can convert light from a laser having a fixed wavelength into coherent light having one or more different wavelengths. An optical parametric converter uses one light signal as a feeder, or pump, to convert, or amplify, a second light signal. When a pump light signal with frequency &ohgr;
p
and a second light signal with frequency &ohgr;
s
are launched in specific directions into a non-linear crystal, those signals interact to generate a signal with frequency &ohgr;
s
+&ohgr;
p
or |&ohgr;
s
−&ohgr;
p
|. Known optical parametric converters require a powerful coherent light radiation pump, such as, for example, a powerful gas or solid state laser. These lasers are large, heavy, and costly and have a short life.
In the field of acoustooptics, there are known optical devices in which an acoustic wave interacts with a light wave. In most known acoustooptic devices, a light wave's wavelength is preserved. Usually, the spatial distribution of a light wave changes as a result of its interaction with an acoustic wave. However, there are known acoustooptic devices in which the wavelength of a light wave is expressly changed. One such device is based on an acoustooptic Bragg cell. In this device, the frequency of a light wave transmitted through the device is equal to &ohgr;+&OHgr;, where &ohgr; is the frequency of the light wave and &OHgr; is the frequency of the acoustic wave. That is, in such a device, the interaction of a light wave with frequency &ohgr; and an acoustic wave with frequency &OHgr; produces a light wave with frequency &ohgr;+&OHgr;.
Another device using converters to directly convert light waves in a fiber has been described. A converter with properties analogous to this device has been disclosed by Risk, et. al. in U.S. Pat. No. 4,872,738 issued Oct. 10, 1989. Like the acoustooptic Bragg cell device, the frequency of the output wave in the Risk device is &OHgr;+&ohgr;.
In Russian Patent #2085984 entitled
Parametric Amplifier and Converter for a Wavelength of Electromagnetic Radiation,
V. P. Torchigan disclosed an acoustooptic device that increases the frequency of an output light wave.
The theoretical considerations of the interaction of an ultrasonic wave and a light wave in a modified Bragg cell in the form of a truncated pyramid have been discussed in a paper by Torchigin.
Enlarged shift of light frequency in a modified Bragg cell.
Torchigin, V. P. and Torchigin, A. V. Pure Appl. Opt. 7 (1998) 763-782.).
An acousto-optic cell (AO cell) with constant cross-section is a well known acousto-optic device that is used widely for switching, modulation, filtering light radiation, light beam deflection and scanning.
In an acousto-optic device, under the proper conditions, the spatial distribution of a light wave may be changed as a result of the interaction of a light wave with an acoustic wave. Such a change in the spatial distribution of a light wave may occur, for example, when a light wave, which usually propagates in free space, is launched in an AO cell, and the light wave interacts with the acoustic wave over a distance of about 1 cm with an interaction time of less than 0.1 ns.
Compared to free space optical configurations that require relatively large amounts of space and materials, integrated optics decrease the size and increase the reliability of acousto-optic devices. Such is the case, for example, in devices in which light waves are propagated in a planar waveguide of a few micrometers thickness.
In the present invention, the same structure acts as a light guide for the light wave and a sound guide for the acoustic wave.
Theoretical
Theoretical considerations are helpful in understanding and explaining the beam deflection used in the novel devices discovered and disclosed by the inventor.
Consider a waveguide in which both an acoustic wave and a light wave are launched. When the wavelength of the light wave is less than the wavelength of the acoustic wave by a factor of about 100, there is, usually, no noticeable interaction between the waves in the waveguide. However, if the light wave has a “whispering gallery” type waveguide mode, its wavelength along the axis of the waveguide can increase significantly enough that the interaction between the waves can be very strong. In addition, varying the cross-section of the waveguide results in other advantages.
Since an AO cell with variable cross-section is considered to be a waveguide, the means for entering light radiation in an AO cell are similar to the means used for entering light radiation into waveguides. There are known means for entering light radiation through an end face of a waveguide and various coupling devices for entering light radiation through a side surface of a waveguide. Both types of means are used in the present invention. The type of coupling used depends on the specific application.
In general, in this disclosure, an AO cell is a sound guide. Typically, the sound guides used in this invention are wave guides suitable for functioning as both sound guides and light guides.
In a conical waveguide (usually called a focon or focusing cone) fabricated from a homogeneous linear isotropic optical medium, even a very small change in the direction of an input beam incident on the wide face, or base, of the focon, results in a significant change in the direction of an output beam reflected from the wide face, or base, of that focon.
Since a modified AO cell with variable cross-section along its axis is a waveguide with variable cross-section, the known peculiarities of the propagation of light waves in a waveguide with variable cross-section are applicable.
Geometrical Optics Approximation
Consider an input beam of light that enters a focon at a point on the base near the side surface of the focon. If the direction of the input light beam lacks a radial component (that is, the projection of the input light beam on a radius that extends from the center of the base of the focon through the entry point of the light beam equals zero) and the light beam first strikes the side of the cone at a point just below the base, the trajectory of the beam in the focon looks like a conical spiral with decreasing intervals between its coils as the beam travels from the base towards the narrow end of the focon.
The beam spirals inside the cone to a point R, where R is the point of maximal penetration of the beam into the cone. The distance from R to the vertex O of the cone is determined by the equation OR=&rgr;[1−Sin &agr;], where OR is the length of the straight line drawn from the vertex of the cone to the point of maximal penetration, where A is the point at which the input beam first strikes the side surface of the cone, &rgr; is the length of the straight line drawn from the vertex of the cone to the point A, and &agr; is the angle that the input light beam makes with the straight line OA.
Once the beam reaches maximal penetration, the beam reverses direction relative to the axis of the focon and travels towards the base. That is, the point of maximal penetration is also a return point. The angle that an exit beam makes with the straight line OB drawn from the vertex to the point B, where B is the point at which the beam travelling within the focon last strikes the side surface of the cone, is the same angle &agr; that the incident input light beam made with the straight line OA. The location of exit point B of the output beam varies dependent on the angle &agr;. That is, there exists an angle &psgr; such that &D
Costa John G.
Martinez Joseph P
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