Optical: systems and elements – Optical modulator – Light wave directional modulation
Reexamination Certificate
2000-05-01
2002-10-22
Lester, Evelyn A (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave directional modulation
C359S279000, C359S315000, C359S319000, C359S578000, C359S619000, C359S627000, C385S003000, C385S014000, C349S202000
Reexamination Certificate
active
06469822
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an optical phased-array that electronically steers a beam of light.
Phased-array is an array of plurality of phase-controlled element. By adjusting the phase relationship among the electromagnetic waves (or other waves such that sonic wave) radiated from each phase-controlled element, the electromagnetic waves radiated from each phase-controlled element become in-phase in a given direction (or at a give position), thus, a constructive interference is formed, and therefore, the phase-array produces a high intensity beam in that direction. In other directions, the electromagnetic waves from each phase-controlled element do not meet the in-phase condition, and are cancelled out with each other due to the interference, therefore, the radiation from the phased-array is close to zero. The geometric dimension of the phased-array (i.e. the aperture) determines the resolution of the phased-array (i.e. the width of the beam). The number of the phase-controlled element is related to the intensity of the beam. The significant advantage of a phased-array device is that the phase relationship among the electromagnetic waves radiated from each phase-controlled element can be adjusted electronically, therefore, the beam can be steered at extremely high speed.
For the prior art, to ensure that the phased-array radiates only one high intensity beam in the given direction, and that the radiation in other directions is close to zero, the distance between the phase-controlled elements (i.e. the center-to-center distance of the adjacent phase-controlled elements) must be less than half of the wavelength for which the phased-array is concerned (details will be in the following).
It is well known that light is also an electromagnetic wave. In the frequency range of light, the wavelength of the visible light is around 0.4 to 0.7 micrometer, the wavelength of infrared is around 0.7 to several hundreds micrometer, and the wavelength of the ultraviolet is around 0.4 to 0.04 micrometer. Now, let's use the 0.5 micrometer wavelength visible light as an example in the discussion of the prior optical phased-array technology. As mentioned above, to ensure that the phased-array radiates only one high intensity beam in the given direction while the intensity of the radiation is close to zero in other directions, the center-to-center distance between the phase-controlled elements has to be less than 0.25 micrometer. Thus, the size of the phase-controlled element itself must also be less than 0.25 micrometer. At present, the light source as the phase-controlled element, which is phase controllable, and is small enough in size does not exist yet. Therefore, the phased-array in optical frequency is to use a coherent beam passing through many space-phase-modulators to create many beams with particular phase relationship among them, i.e. each phase-modulator produces one beam with a given phase. Here, each phase-modulator is one phase-controlled element as mentioned above. Phase-modulator consists of two electrodes and electro-optical material between the two electrodes. The refractive index of the electro-optical material can be alerted in a certain rang according to the electrical field between the two electrodes, which alerts the optical path length as a beam of light travel trough the phase modulator, and therefore results phase modulation (i.e. phase shifting). The electrical field between the two electrodes is controlled by adjusting the electrical potential on the two electrodes with a controller.
For the sack of the convenience, in this document, when the structure of the phased-array and phase-controlled element are concerned, width means the dimension in the direction perpendicular to the boresight of the phased-array (or simply called as dimension), the thickness means the dimension along the boresight of the phased-array.
Referring now to
FIG. 1
, a cross-section view of a prior art optical phased-array device. It consists of a controller
11
and an array of optical phase-modulators
12
(it will be called as array of phase-modulators in the following text for simplicity). The array of phase-modulators consists of plurality of phase-modulators. Since the electro-optical material
13
is the liquid-crystal, the phase-modulator array
12
possesses also a front window
14
and a rear window
15
. Window
14
and window
15
are usually flat plates, and parallel to each other. They are transparent in the optical frequency rang that they are working with. Each phase-modulator has a control electrode, denoted as
17
0
,
17
1
, . . . ,
17
9
, collectively referred as control electrodes
17
. Phase-modulator consists of control electrode
17
, common electrode
16
and liquid-crystal
13
. Common electrode
16
and control electrode
17
are transparent in optical frequency range concerned.
FIG. 1
illustrates the cross-section view of a one dimensional phase-modulator array. Control electrode
17
are plurality of parallel strip electrodes. The width of the strip electrode is denoted as w. The spacing between the electrodes is denoted as p. The center-to-center distance between adjacent electrodes is denoted as d. d=p+w. The incident light
18
enters the phase-modulator array
12
from the rear window
15
. Light is phase-modulated by each modulator, and the emitted light from each modulator becomes in-phase in direction &thgr;, thus, a beam
19
is generated in direction &thgr;. The
46
represents the wavefront. The control lines
20
, between the phase-modulator array
12
and controller
11
is for carrying the control signal. The prior art requires that the center-to-center distance d to be less than the wavelength in order to ensure that the phased-array produces only one beam in the given direction. Otherwise, there will be other beams in other direction also, which is not desirable. Therefore, prior art has to limit the width of the phase-modulator w (
FIG. 1
) to be less than the wavelength. Because of this, it produces the following problems:
1. For a give aperture of a phased-array, since the phase-modulator is very small, the required number of the phase-modulator will be very large. For example, for the wavelength of 0.5 micrometer, 20,000 rows of phase-modulator will be needed for each center meter aperture. This makes the structure of the phased-array device very complex, cost, and difficult to fabricate.
2. The spacing p between the electrodes is limited by the insulation requirement and fabrication process. For a given material and fabrication technology, the minimum p achievable can be regarded as a constant. Obviously, the smaller the width of the phase-modulator w, the larger the portion of the aperture that is occupied by the spacing, and therefore, the lower the filling rate. For example, at wavelength of 0.5 micrometer, assuming w and p are all 0.5 micrometer, then 50% of the aperture area is wasted, only half of the incident light is useful.
3. When the dimension of the phase-modulator (i.e. w. Same in the following) is very small, the light entering the phase-modulator significantly diverges due to diffraction, part of the light will enter neighboring phase modulators, which disturbs the light emitted from each phase-modulator, and only a part of light emitted actually possesses the correct phase. Since the thickness of the phase-modulator is much large than the width (e.g. the thickness is larger than 10 &mgr;m), the diverging of the light due to diffraction is very significant.
4. When the dimension of the phase-modulator is very small, the width of the electrode is also very small. Since the thickness of the phase-modulator is much larger than its width, i.e. the distance between the electrodes are much larger than the width of the electrode itself, the infringing effect will significantly affect the uniformity of the electrical field within the phase-modulator. Besides, since the distance between the electrodes is much larger than the width of the electrodes, the electrical field of neighboring phase
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