Optical: systems and elements – Optical modulator – Light wave directional modulation
Reexamination Certificate
1999-03-22
2001-10-23
Lester, Evelyn A (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave directional modulation
C359S286000, C359S287000, C359S305000, C359S285000
Reexamination Certificate
active
06307665
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an acousto-optic modulator, and more particularly, to an acousto-optic modulator equipped with a transducer having a structure in which impedance matching is easy.
2. Description of the Related Art
With a recent increase in the demand for multimedia, various large screen display devices appeared in order to overcome the defects of an existing cathode ray tube (CRT) and cope with the multimedia. However, existing image display means such as the CRT or a liquid crystal display (LCD) are difficult to manufacture, and their resolution is degraded, as they become larger. Thus, there is a limit in the use of the CRT and LCD. A projector is an image display device capable of projecting a picture onto a large screen using the CRT or LCD. However, the projector also has many problems and technical limitations.
Another image display device for a large screen is a laser projector for directly projecting laser light having picture information onto a screen. The laser projector can realize a large screen, has both high contrast and optical efficiency, has no distortion or color errors, has both a luminance and contrast independent of distance, and is applied to large screen HDTV (high definition television). Generally, the laser projector uses a xenon (Xe) lamp, helium-neon (He—Ne) laser, and argon (Ar) laser as an optical source. However, krypton-argon (Kr—Ar) laser has received much attention for simplification of the system.
In general, a color display system is comprised of an optical generation unit, an optical modulation unit, an image signal generation unit, a scanning unit, and a screen unit. A predetermined beam is generated by the optical generation unit, and is incident upon the optical modulation unit. The optical modulation unit modulates the incident beam according to an image signal provided from the image signal generation unit. Here, the modulated beam has pixel-unit information. The scanning unit sequentially and continuously scans the screen unit with the modulated beam, such that an image is displayed on the screen unit.
An acousto-optic modulator (AOM), an electrooptic modulator (EOM), or an acousto-optic tunable filter (AOTF) is included as an optical modulator in the optical modulation units currently in use to make laser beam spots scan the screen unit. Among them, the AOM is the most frequently used at present, and has a simple driving circuit. A typical optical modulation system uses three AOMs. In a laser projection display system, a high-output laser beam is used as an optical source to form a high definition image on a large screen, and an AOM used in this laser projection display system must have a structure which can sufficiently resist high output.
FIG. 1
is a perspective view showing the structure of a conventional AOM.
Referring to
FIG. 1
, in the conventional AOM, a transducer
12
for generating acousto-elastic waves by an electrode
14
is provided on one side of an ultrasonic medium
10
. The transducer
12
is coupled to the ultrasonic medium
10
by an adhesive layer
13
including a silver (Ag) layer, and the electrode
14
made of gold (Au) is installed on the upper surface of the transducer
12
. Accordingly, a structure is formed in which the transducer
12
is interposed between the electrode
14
and the adhesive layer
13
, and this structure acts as a capacitor.
An acousto-elastic wave absorbing element (not shown) for preventing reflection of ultrasonic waves is installed on the side of the medium opposite to the transducer
12
. The ultrasonic medium
10
is made of a material selected from the group consisting of fused quartz, PbMoO
4
, TeO
2
, Te glass, and Schwer-Flint glass (SF
4
). A crystal material, PbMoO
4
or TeO
2
, is suitable for relatively high frequencies, and a glass material is used mainly for low frequencies since it is cheap, but has a large propagation loss at a high frequency. A light incident/ emitting surface
10
A or
10
B passing laser light is optically polished An antireflection layer is generally deposited on the light incident/emitting surface
10
A or
10
B since on account of a high refractive index of the ultrasonic medium
10
there is a large reflection loss when laser light enters or is emitted.
FIG. 2
is a top view of the electrode
14
installed on the transducer
12
of the conventional AOM.
Referring to
FIG. 2
, a single electrode is used as the electrode
14
in the conventional AOM. When the length (l) of the electrode is 10 mm and the mean height ((h
1
+h
2
)/2) thereof is 0.55 mm, the area (A) of the electrode is 5.5 mm
2
.
Meanwhile, impedance matching must be performed to use the AOM together with a driving circuit. The impedance matching has the following objectives: (1) to compensate for frequency mismatching of transducer due to the thickness error so as to resonate with the frequency of a driving circuit; and (2) to effectively transmit RF power by performing impedance matching between the driving circuit and the transducer.
When an impedance Rs of 50 &OHgr; is required by a 150 MHz AOM, the desired impedance cannot be obtained with one transducer employed as in the conventional AOM.
To be more specific, the relationship between the impedance (Rs and a capacitance (C
0
) in the structure of the AOM can be generally expressed by the following Equation 1:
R
s
=
1
ω
o
C
o
(
1
)
wherein &ohgr;
0
indicates angular frequency and is equal to 2&pgr;f
0
(where, f
0
is 1.1 &lgr; s; fs indicates a center frequency and is equal to 150 MHz in the case of the 150 MHz AOM.
Also, the capacitance (C
0
) and the area (A) of the electrode have a relationship expressed by the following Equation 2:
C
0
=
ϵ
0
ϵ
γ
A
l
(
2
)
wherein ∈
0
indicates a dielectric constant in a vacuum and is equal to 8.8542 &lgr;0
−12
F/m, ∈
&ggr;
indicates relative permittivity and is equal to 38.6 in the case of a transducer made of an LiNbO
3
(hereinafter, abbreviated to LN) single crystal of 36° Y cut, and l indicates the distance between electrodes, i.e., the thickness of the dielectric, and is expressed by V/2f
0
. Here, V indicates an acoustic speed in the LN single crystal and is equal to 7300 m/s in case of the transducer made of an LN single crystal of 36° Y cut.
The area of an electrode to render an impedance of 50&OHgr; can be obtained with respect to the given length of an electrode by Equations 1 and 2. Alternatively, the impedance can also be calculated from the area of the electrode.
The impedance in the conventional AOM having such an electrode structure as shown in
FIG. 2
is calculated to be 11.30&OHgr; by applying the above-described relationship.
As described above, there is a difference between the desired impedence of 50&OHgr; and an impedance value obtained when one transducer is employed as in the conventional AOM. The difference therebetween serves as a significant disadvantage in impedance matching between an AOM and a driving circuit, so that RF power cannot be effectively transmitted.
SUMMARY OF THE INVENTION
To solve the above problem, it is an objective of the present invention to provide an acousto-optic modulator (AOM) for use in a laser projection display (LPD) system, which can be easily impedance-matched with a driving circuit.
Accordingly, to achieve the above objective, there is provided an acousto-optic modulator comprising: an ultrasonic medium for controlling light from an optical source through diffraction; two transducers each having one electrode formed on one side thereof, the electrodes being for generating an acousto-elastic wave; and a conductive adhesive layer interposed between the ultrasonic medium and each of the sides of the transducers opposite to the sides on which the electrodes are installed, in order to adhere each of the transducers to the ultrasonic medium.
The ultrasonic medium is formed of a TeO
2
single crystal, each of the transducers is formed of LiNbO
3
single crystal, and the electrode
Kim Yong-hoon
Lee Hang-woo
Burns Doane Swecker & Mathis L.L.P.
Lester Evelyn A
Samsung Electronics Co,. Ltd.
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