Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
2000-02-03
2002-09-17
Lester, Evelyn A (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S288000, C359S289000, C359S299000, C359S350000
Reexamination Certificate
active
06452710
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to an optical element useful, for example, in the field of optoelectronics and photonics, such as optical communications and optical information processing, and to an optical control method and method of manufacturing an optical element using such an optical element.
BACKGROUND OF THE INVENTION
In the field of optoelectronics and photonics, much research is being carried out on light and optical control methods which attempt to modulate the intensity (amplitude) or frequency (wavelength) of light without using electronic circuitry, by using the change of transmissivity or refractive index caused by irradiating an optical element produced by fabricating an optical material or optical composition with light.
When the properties of light are used to perform parallel optical logic computing and image processing, “spacial light modulators” for performing certain types of modulation such as varying the optical intensity distribution of the beam cross-section are of great importance, and optical control techniques are also expected to find application in this area.
Phenomena to which light and optical control methods are expected to be applied are saturable absorption, nonlinear refraction, nonlinear optical effects such as the photorefractive effect and photochromic phenomena, and these are now attracting wide attention.
A phenomenon is known wherein light in a first wavelength region excites absorption of light in a second wavelength region different from the first wavelength region without an accompanying change of molecular structure. This phenomenon may be referred to as “excited state absorption”, “induced absorption” or “transient absorption.”
In one example of an attempt to produce excited state absorption, in Japanese Patent Laid-open publication No. Sho 53-137884, an optical conversion technique is reported wherein a liquid or solid containing a porphyrin compound and an electron acceptor is irradiated with at least two kinds of light of differing wavelength, and the information contained in light of one wavelength is transferred to light of the other wavelength. In Japanese Patent Laid-open publication No. Sho 55-100503 and Japanese Patent Laid-open publication No. Sho 55-108603, a propagation light is selected corresponding to the time-dependent variation of excited light, using the difference in the spectrophotometer spectrum between the ground state and excited state of organic compounds such as porphyrin derivatives.
In Japanese Patent Laid-open publication No. Sho 61-129621, a radiant energy transmission control method is disclosed comprising a step wherein a first photon flux is introduced into a barium crown glass fiber doped with uranium oxide without attenuation, the first photon flux is attenuated by introducing a second photon flux, energy level
2
of the fiber is populated, part of the first photon flux is absorbed to populate energy level
3
and part of energy level
3
then returns to energy level
2
so as to further attenuate the first photon flux.
Japanese Patent Laid-open publication No. Sho 63-89805 discloses a plastic optical fiber that contains an organic compound such as a porphyrin dielectric in its core having an absorption that corresponds to a transition to a triplet state higher than the triplet state excited by light. Japanese Patent Laid-open publication No. Sho 63-236013 discloses an optical functional device which performs switching wherein crystals of a cyanine dye such as cryptocyanine are irradiated with light of a first wavelength to excite the molecules, the molecules are irradiated with light of a second wavelength different from that of the first wavelength, and light of the second wavelength is either transmitted or reflected according to the optical excitation state due to light of the first wavelength. In Japanese Patent Laid-open publication No. Sho 64-73326, a light signal modulating medium is disclosed comprising a photo-induced electron shift material such as a porphyrin dielectric in a matrix material which is irradiated by light of a first and second wavelength, and optical modulation is performed using the difference of absorption spectrum between the excited and ground states of the molecule.
As optical apparatus used in this prior art, Japanese Patent Laid-open publication No. Sho 55-100503, Japanese Patent Laid-open publication No. Sho 55-108603 and Japanese Patent Laid-open publication No. Sho 63-89805 disclose a device structure wherein a light propagating optical fiber is wrapped around an exciting light source (for example, a flash lamp). In Japanese Patent Laid-open publication No. Sho 53-137884 and Japanese Patent Laid-open publication No. Sho 64-73326, a device is disclosed wherein the whole of a propagation part corresponding to signal light in a light-responsive optical element is irradiated with a control light which is not converged, and is in fact diverged by a means such as a projecting lens.
Other methods in the prior art perform modulation of light using the refractive index distribution due to a thermal effect. In the aforesaid prior art, studies are also being carried out on methods to modulate light using the refractive index distribution due to the thermal effect.
In Japanese Patent Laid-open publication No. Sho 59-68723, an optical modulator is disclosed wherein an electrical signal passes through a heat emitting resistor, and the wavefront of a light flux is modified due to a refractive index distribution in a liquid medium in which a refractive index distribution is produced by heat from the heat emitting means. It is described that a cycle is performed of the order of kHz or msec from formation to extinction of the refractive index distribution. Further, in Japanese Patent Laid-open publication No. Sho 60-130723, a method is disclosed for converting near infra-red control light into heat energy in a heat absorbing layer, transmitting this heat to a thermal effect medium via a near infrared light reflecting layer and visible light reflecting layer, and converting the wavefront of a light flux incident on the visible light reflecting layer using the refractive index distribution produced in the thermal effect medium.
However, in these methods for modulating light using refractive index distribution due to the aforesaid thermal effect, there is a long heat propagation path until a thermal effect is produced, and as the heat is propagated while the surface area of an increased temperature part increases relative to the surface area of the control beam, the volume and heat capacity of the propagation path increases, the usage efficiency of energy supplied from the control beam is low, and a high speed response cannot be expected.
In all of these prior art techniques, very high-density optical power is required to cause a change of transmissivity or refractive index that is sufficient for practical purposes. The response to optical irradiation is slow, fine adjustment of the optical system is necessary, and there is a large variation in the control light output if there is a slight change in the optical system. For these reasons, a practical system has not yet been developed.
To resolve the above problems in the prior art, the following two inventions have been disclosed relating to optical control methods and optical control devices which aim to induce an optical response of sufficient magnitude and speed from a photoresponsive optical element using as low a power as possible. Japanese Patent Laid-open publication No. Hei 8-286220 discloses an optical control technique wherein control light is made incident on an optical element comprising a photoresponsive composition, and intensity modulation and/or light flux density modulation of a signal light which passes through the optical element is performed by reversibly varying the transmissivity and/or refractive index of the signal light in a wavelength region different from that of the control light. The control light and signal light are respectively converged and irradiated to the optical element, and the o
Hiraga Takashi
Moriya Tetsuo
Tanaka Norio
Tsujita Koji
Ueno Ichiro
Lester Evelyn A
National Institute of Advanced Industrial Science and Technology
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