Patent Application
20050147343
This application claims priority to prior application JP 2003-397880, the disclosure of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to optical elements used for optical communication, optical wiring, or optical storage, optical integrated devices each formed by integrating the optical elements, optical information transmission systems, and manufacturing methods thereof.
2. Description of the Related Art
As optical communication and optical storage have been widely spread in recent years, optical elements used therefor have been increasingly in demand. In particular, following the practical use of wavelength multiplexing of optical communication, the use of optical elements for metro and access networks have been increased Instead of the use thereof for a so-called backbone network system which is primarily formed using optical fibers. Therefore, as optical elements used in add-drop multiplexers for optical signals, instead of elements formed in combination of bulk materials, use has been increasingly made of planar waveguides which are reduced in size and made of thin optical material films and which can be easily integrated with various elements.
The planar waveguide is generally formed of a quartz substrate or a silicon substrate and an amorphous material such as silicon oxide provided thereon. As a method for forming a thin film, for example, use is made of a flame deposition method in which a thin film is formed by depositing a material using flame treatment or a vapor-phase growth method such as CVD. The thin film thus obtained is formed into a predetermined shape by a reactive ion etching method or the like, so that an optical element is obtained.
In order to positively control light such as modulation and switching of optical signal, it is necessary to use a physical effect which is obtained through the interaction between an exterior input signal such as electricity or heat and a material forming an optical element. As a silicon-based planar waveguide, for example, an optical switch using a thermooptic effect and comprising a directional coupler provided with a heater, and a switch formed in combination with MEMS have been known. However, unfortunately, both switches have slow speed of response, such as a microsecond level, an optical switch using a thermooptic effect consumes a large amount of electric power, and an MEMS type switch has a complicated structure and is expensive.
Because of high speed of response, low power consumption due to voltage drive system, and a simple structure, an electrooptic effect in which the refractive index is changed by the interaction between an electric field and a material has been used for an optical modulator. In order to form an optical modulator using LiNbO3, a Mach-Zehnder waveguide is formed on a single crystal LiNbO3 substrate by the use of a Ti diffusion method and is combined with electrodes. When a voltage is applied to the electrodes, the refractive index of the waveguide can be changed, and as a result, ON/OFF operation of an optical signal can be performed. However, since it is required to use a single crystal substrate, the modulator described above is expensive. Further, since the length of the waveguide must be increased due to a small electrooptic effect of LiNbO3, the size of the element is extremely and problematically increased to several centimeters.
Pb1-xLax(ZryTil-y)1-x/4O3 (PLZT) as a transparent ceramic has a larger electrooptic coefficient by approximately two orders of magnitude than that of a LiNbO3 single crystal forming an optical modulator which is recently used. Therefore, the size of an optical element may be decreased, and consequently, reduction in cost, lower power consumption, and higher speed of response can be expected. Accordingly, investigation of sol-gel processes for forming thinner films of PLZT have been carried out (see K, D, Preston and G. H, Haertling, Appl. Phys. Lett. 60 (1992), p. 2831, and K. Nashimoto, K. Haga, M. Watanabe, S. Nakamura and E. Osakabe, Appl. Phys. Lett. 75 (1999), p. 1054).
However, in order to form a thin film having a high transmittance and a high electrooptic effect, epitaxial growth must be performed, and a single crystal substrate must also be used as an underlying material. Accordingly, waveguides other than silicon-based waveguides are not easily formed on a substrate. In addition, a film-forming process in accordance with a sol-gel method must be performed for a long time in order to obtain a film having a thickness required for an optical element. Therefore, the cost is disadvantageously increased.
Electrooptic materials such as UNbO3 and PLZT are ferroelectric materials, and the properties thereof appear when the particular crystal structures of the individual materials are formed. Therefore, when an electrooptic material is used for an optical element, it has been believed that a single crystal substrate thereof must be used or that an electrooptic material must be epitaxially grown on a single crystal substrate.
Hereinafter, the formation of nanophotonic devices which can realize the integration between light and electronics on one chip will be further desired as a significant innovative technique. In order to realize the devices described above, a technique in which an LSI such as a central processing unit (CPU) or a memory and an active optical element such as an optical switch are formed on the same chip must be developed. In addition, a technique in which an electrooptic material such as PLZT is deposited with high crystallinity on a silicon or a quartz substrate must also be developed.
On the other hand, as a novel ceramic film-forming method, aerosol deposition (AD method) has been developed. The AD method is a film-forming method by an impact consolidation phenomenon and utilizes a phenomenon of collision and coalescence of ultra fine particles. Compared to a related thin film-forming method, the AD method is expected to realize high film-forming rate and a low process temperature (see Jun Akedo and Maxim Lebedev, “MATERIA” 41, (2002), P. 459 and Jun Akedo and Maxim Lebedev, Jpn. J. Appl. Phys. 38, (1999), P. 5397). Moreover, in the AD method, the properties of a film are independent of an underlayer provided thereunder, and therefore, a material for the substrate can be optionally selected.
A technique disclosed in Japanese Unexamined Patent Application Publication No. 2001-3180 is a film-forming technique in accordance with the AD method and is characterized in that ultra fine fragile particles provided on a substrate are pulverized by applying a mechanical impact so that the above ultra fine fragile particles are bonded to each other or that the above ultra fine fragile particles are boned to each other and are bonded to the substrate. As a result, the bonding between the ultra fine particles is realized, and without applying heat, a film having a high density and a high strength can be formed.
A technique disclosed in Japanese Unexamined Patent Application Publication No. 2002-235181 relates to the structure formed in accordance with the AD method and is characterized in that the structure is formed of a polycrystal material in which no crystal orientations exist and in which grain boundaries made of glass layers are not substantially present. In addition, this structure is characterized in that the average crystal particle diameter is 50 nm or less and in that the compactness is 99% or more. The average crystal particle diameter disclosed in Japanese Unexamined Patent Application Publication No. 2002-235181 is the size of the crystal particle calculated in accordance with the Scherrer method of X-ray diffraction analysis.
A technique disclosed in Japanese Unexamined Patent Application Publication No. 2002-20878 is a film-forming technique in accordance with the AD method in which ultra fine particles are supplied onto a substrate surface by spraying and is characterized in that at least a part of this spraying flow is obliquely incident on the substrate surface. As a consequence, a film can be formed in which the ultra fine particles therein are sufficiently bonded to each other, the texture is dense, the surface is smooth, and the density is uniform.
A technique disclosed in Japanese Unexamined Patent Application Publication No. 2001-38274 is a technique for forming a planarized film composed of ultra fine particles in accordance with the AD method and is characterized in that the film formation is carried out by performing at least one planarizing step for planarizing the surface of a film formed by deposition.
As described above, the development of techniques for piezoelectric materials and the like in accordance with the AD method has significantly advanced. However, the application of the AD method to optical elements has not been studied and, with respect to PZT and PLZT thin films, the transparency thereof have only been reported as of today. The transparency of the film obtained by the AD method is 30 dB/mm or more in terms of the transmission loss, and therefore, it is impossible to apply this film for optical elements.
It is therefore an object of the present invention to provide a molded body having a high transparency formed by the AD method and is also to provide an optical element formed using the molded body.
In addition, it is another object of the present invention to provide a method for manufacturing a highly transparent molded body by the AD method and a manufacturing apparatus which is optimally used for the manufacturing method.
Furthermore, it is still another object of the present invention to provide an optical integrated device incorporating an optical element which is formed from the molded body formed by the AD method.
In order to achieve the objects described above, the molded body of the present invention is a molded body formed by an impact consolidation phenomenon in which a mechanical impact is applied to ultra fine fragile particles supplied onto a substrate so that the ultra fine fragile particles are pulverized and are bonded to each other, and when a scattering central radius of the molded body is represented by d, d≦15 nm holds.
The scattering central radius is the radius of a part of the molded body, such as a pore (void) or a different phase, having a refractive index different from that of a primary component of the molded body. In the present invention, since this scattering central radius d is set so that d≦15 nm holds, the scattering central radius is sufficiently smaller than a light wavelength, and therefore, a transparent molded body can be obtained by an impact consolidation method.
In accordance with a first aspect of the present invention, there is provided an optical element produced by applying a mechanical impact to ultra fine fragile particles supplied onto a substrate so that the ultra fine fragile particles are pulverized and bonded to each other in accordance with an impact consolidation phenomenon to form a molded body. In the molded body described above, d6/λ4<4×10−5 nm2 holds, in which d (nm) is the average radius of a pore or a different phase contained in the molded body, which has a refractive index different from that of a primary component of the molded body, and in which λ (nm) is the wavelength of light transmitting through the molded body.
In the present invention, It is first disclosed that the transparency of a molded body formed by an impact consolidation phenomenon is generated by Rayleigh scattering. Rayleigh scattering occurs when the size of a scatter is sufficiently smaller than the wavelength of light. In the case, the scatter is assumed to be placed in a uniform electromagnetic filed which spatially vibrates, and the Intensity of scattering can be defined by the wavelength and the radius of the scatter. The average radius d (nm) and the light wavelength λ (nm) transmitting through the molded body are set so that d6/λ4<4×10−5 nm2 holds. Therefore, the light scattering central radius can be sufficiently smaller than the light wavelength. As a result, a transparent optical element can be obtained by an impact consolidation phenomenon.
In accordance with a second aspect of the present invention, there is provided an optical element produced by applying a mechanical impact to ultra fine fragile particles supplied onto a substrate so that the ultra fine fragile particles are pulverized and bonded to each other to form a molded body. In the molded body described above, d6/λ4<4×10−5 nm2 nm2 holds, in which d (nm) is the average radius of a pore, a different phase, or the like contained in the molded body, which has a refractive index different from that of a primary component of the molded body, and in which λ (nm) is the wavelength of light transmitting through the molded body.
According to the second aspect of the present invention, since the average radius d (nm) and the light wavelength λ (nm) transmitting through the molded body are set so that d6/λ4<4×10−5 nm2 holds, the light scattering central radius can be more sufficiently smaller than the light wavelength. As a result, a more transparent optical element can be obtained by an impact consolidation phenomenon.
In accordance with a third aspect of the present invention, there is provided a method for forming a molded body by applying a mechanical impact to ultra fine fragile particles supplied onto a substrate so that the ultra fine fragile particles are pulverized and bonded to each other to form the molded body, and comprising a step for selectively removing particles having weak bonding forces therebetween.
According to the present invention, since the particles having weak bonding forces therebetween are selectively removed, the content of a component forming the light scattering centers can be reduced, and the scattering central radius can also be decreased. As a consequence, a transparent optical element can be obtained by an impact consolidation phenomenon.
In the method for manufacturing a molded body according to the third aspect of the present invention, the particles having weak bonding forces therebetween may be a part of the ultra fine fragile particles, which is insufficiently pulverized, and/or a phase different from a primary component of the molded body.
According to the present invention, since the particles having weak bonding forces therebetween, namely, the part of the ultra fine fragile particles which is insufficiently pulverized and/or the different phase from the primary component of the molded body is removed, the content of a component forming the light scattering centers can be reduced, and the scattering central radius can also be decreased. Consequently, a transparent optical element can be obtained by an impact consolidation phenomenon.
Since the ultra fine fragile particles, which are insufficiently pulverized, are larger than fine crystal particles of the primary component of the molded body, flying particles are not likely to collide against places which are located behind the insufficiently pulverized ultra fine fragile particles. As a consequence, pores are likely to be formed. Since these large and coarse particles are formed by insufficient pulverization, new fresh surfaces are not easily formed, and as a result, the bonding force to the film surface tends to be decreased as compared to that between the fine crystal particles which primarily form the film. In addition, the different phase, which is the other factor of forming scattering centers, also has a weak bonding force to the film surface because the composition and the crystal structure of the different phase are different from those of the fine crystal particles primarily forming the film.
In the method for manufacturing a molded body according to the third aspect of the present invention, at least one of a material, particle diameter, coagulation state, supplying speed, hardness, and incident angle of the ultra fine fragile particles may be a factor of the step of selectively removing the particles having weak bonding forces therebetween, in which the incident angle described above is with respect to the substrate.
According to the present invention, by changing at least one of a material, particle diameter, coagulation state, supplying speed, hardness, and incident angle of the ultra fine fragile particles, in which the incident angle is with respect to the substrate, the balance between the speed of removing the particles having weak bonding forces therebetween and the speed of depositing fine crystal particles primarily forming the film is controlled so that the particles having weak bonding forces therebetween can be selectively removed. Accordingly, the content of a component forming the light scattering centers can be reduced, and the scattering central radius can also be decreased. As a result, a transparent optical element can be obtained by an impact consolidation phenomenon.
In the method for manufacturing a molded body, described above, the step of selectively removing particles having weak bonding forces therebetween may be performed by ultra fine particles supplied from at least one nozzle different from a nozzle supplying the ultra fine fragile particles, which are formed into a primary component of the molded body, onto the substrate.
According to the present invention, since the nozzle for supplying the ultra fine fragile particles which are formed into a primary component of the molded body and the nozzle for supplying the ultra fine particles to remove the particles having weak bonding forces therebetween are separately provided, deposition conditions for forming the molded body and particle-removing conditions can be optimized, respectively. Accordingly, the content of a component forming the light scattering centers can be reduced, and the scattering central radius can also be decreased. Thus, a transparent optical element can be obtained by an impact consolidation phenomenon.
In the method for manufacturing a molded body described above, the ultra fine particles for removing the particles having weak bonding forces therebetween are preferably dry ice (CO2).
According to the present invention, since dry ice (CO2) is used as the ultra fine particles for removing the particles having weak bonding forces therebetween, the particle-removing particles composed of dry ice having high volatility are not incorporated into the film. Accordingly, light scattering is not caused by the particle-removing particles incorporated into the film. Thus, a transparent optical element can be obtained by an impact consolidation phenomenon.
The method for forming a molded body, described above, may further comprise the step of irradiating the molded body with electromagnetic waves in order to selectively decrease bonding forces between the particles having weak bonding forces therebetween. Thus, the selective removal thereof Is promoted.
In the present invention, by irradiating the molded body with electromagnetic waves such as microwaves or infrared laser in order to selectively decrease the bonding forces between the particles having weak bonding forces therebetween, a temperature thereof can be selectively increased, so that the bonding forces can be further decreased. Thus, the selective removal of the particles having weak bonding forces therebetween can be promoted.
The optical element according to the first or the second aspect of the present invention may comprise a lower electrode, a waveguide, and an upper electrode provided In that order on the waveguide. In which the waveguide is formed from the molded body.
In the optical element of the present invention, since the waveguide is formed on the lower electrode using an impact consolidation phenomenon, the properties of the waveguide are not depend on the type and the structure of a lower electrode material. As a consequence, the lower electrode material can be freely selected in view of the optimum optical design.
In accordance with a fourth aspect of the present invention, there is provided an optical integrated device comprising a first optical element and a second optical element integrated therewith on a substrate, in which the first optical element is formed from a molded body which is formed by applying a mechanical impact to ultra fine fragile particles supplied onto the substrate so that the ultra fine fragile particles are pulverized and bonded to each other, and in which the second optical element is at least one selected from the group consisting of a laser, an electrooptic converter, a photoelectric converter, an optical amplifier, an optical waveguide, an optical filter, and the like. In the optical integrated device described above, d6/λ4<4×10−5 nm2 holds, in which d (nm) is the average radius of a pore (void) or a different phase contained in the molded body, which has a refractive index different from that of a primary component of the molded body, and in which λ (nm) is the wavelength of light transmitting through the molded body.
According to the present invention, the integration of a plurality of optical elements, which has been difficult due to different manufacturing processes of the plurality of optical elements, can be performed at room temperature on any type of underlying material by an impact consolidation phenomenon. Thus, an optical integrated device can be realized.
In accordance with a fifth aspect of the present invention, there is provided an optical integrated device comprising an optical element and an electronic circuit integrated therewith on a substrate, in which the optical element is formed from a molded body which is formed by applying a mechanical impact to ultra fine fragile particles supplied onto the substrate so that the ultra fine fragile particles are pulverized and bonded to each other, and in which the electronic circuit is at least one of a central processing unit (CPU), a memory, and the like. In the optical integrated device described above, d6/λ4<4×10−5 nm2 holds, in which d (nm) is the average radius of a pore (void) or a different phase contained in the molded body, which has a refractive index different from that of a primary component of the molded body, and in which λ (nm) is the wavelength of light transmitting through the molded body.
According to the present invention, the integration of an optical element and an electronic circuit on the same substrate, which has been difficult due to different manufacturing processes thereof, can be performed at room temperature on any type of underlying material by an impact consolidation phenomenon. Thus, an optical integrated device can be realized.
In accordance whit a sixth aspect of the present invention, there is provided an optical information transmission system comprising an optical element formed from a molded body which is formed by applying a mechanical impact to ultra fine fragile particles supplied onto a substrate so that the ultra fine fragile particles are pulverized and bonded to each other. In the optical information transmission system described above, a light wave having a wavelength of 800 nm or more is transmitted through the optical element.
According to the sixth aspect of the present invention, since the light wavelength transmitting the molded body which forms the optical information transmission system is set to 800 nm or more, the light scattering central radius in the molded body can be sufficiently smaller than the light wavelength. Thus, an optical information transmission system having a small waveguide loss can be obtained.
In accordance whit a seventh aspect of the present invention, there is provided an optical information transmission system comprising an optical integrated device composed of a first optical element and a second optical element integrated therewith on a substrate, in which the first optical element is composed of a molded body formed by applying a mechanical impact to ultra fine fragile particles supplied onto the substrate so that the ultra fine fragile particles are pulverized and bonded to each other, and in which the second optical element is at least one selected from the group consisting of a laser, an electrooptic converter, a photoelectric converter, an optical amplifier, an optical waveguide, an optical filter, and the like. In the optical information transmission system described above, a light wave having a wavelength of 800 nm or more is transmitted through the optical integrated device.
According to the seventh aspect of the present invention, since the light wavelength transmitting the molded body, which forms the optical information transmission system, is set to 800 nm or more, the light scattering central radius in the molded body can be sufficiently smaller than the light wavelength. Thus, an optical information transmission system having a small waveguide loss can be obtained.
In accordance whit an eighth aspect of the present invention, there is provided an optical information transmission system comprising an optical integrated device composed of an optical element and an electronic circuit integrated therewith on a substrate, in which the optical element is formed from a molded body which is formed by applying a mechanical impact to ultra fine fragile particles supplied onto the substrate so that the ultra fine fragile particles are pulverized and bonded to each other, and in which the electronic circuit is at least one of a central processing unit (CPU), a memory, and the like. In the optical information transmission system described above, a light wave having a wavelength of 800 nm or more is transmitted through the optical Integrated device.
According to the eighth aspect of the present invention, since the light wavelength transmitting the molded body, which forms the optical information transmission system, is set to 800 nm or more, the light scattering central radius in the molded body can be sufficiently smaller than the light wavelength. Thus, an optical information transmission system having a small waveguide loss can be obtained.
In accordance with a ninth aspect of the present invention, there is provided an optical element produced by applying a mechanical impact to ultra fine fragile particles supplied onto a substrate so that the ultra fine fragile particles are pulverized and bonded to each other to form a molded body, wherein: d6/λ4<4×10−5 nm2 holds, d (nm) being the average radius of a pore or a different phase contained in the molded body, which has a refractive index different from that of a primary component of the molded body, λ (nm) being the wavelength of light transmitting through the molded body, and d6/λ4<4×10−5 nm2 holds, D (nm) being an average radius of the primary component of the molded body, λ (nm) being the wavelength of light transmitting through the molded body.
In accordance with a tenth aspect of the present invention, there is provided an optical element, wherein: d6/λ4<4×10−5 nm2 holds, d (nm) being an average radius of a pore or a different phase contained in a molded body, which has a refractive index different from that of a primary component of the molded body, λ (nm) being a wavelength of light transmitting through the molded body, and d6/λ4<4×10−5 nm2 holds, D (nm) being an average radius of the primary component of the molded body, λ (nm) being the wavelength of light transmitting through the molded body.
With respect to the ninth and tenth aspects of the present invention, the aerosol deposition (AD method) has an advantage in which an functional oxide having a high property can be deposited with a low cost. In order to achieve the transparency, the following two structures are required.
First, it is necessary to reduce scattering when a substance such as a pore (void) or a different phase or the like, which has a refractive index different from that of the molded body, is contained therein. In order to reduce the scattering point, it is required that d6/λ4<4×10−5 nm2 holds to maintain the transparency, d (nm) being an average radius of a pore or a different phase contained in a molded body, which has a refractive index different from that of a primary component of the molded body, λ (nm) being a wavelength of light transmitting through the molded body
Second, in a material having a high optical anisotropy, scattering of a grain boundary becomes large so that the transparency is decreased. | this case, in order to reduce the scattering point, it is required that d6/λ4<4×10−5 nm2 holds to maintain the transparency, D (nm) being an average radius of the primary component of the molded body, λ (nm) being the wavelength of light transmitting through the molded body.
Both cases can be introduced base upon Rayleigh scattering.
As mentioned above, according to the present invention, a mechanical impact is applied to ultra fine fragile particles so that the ultra fine fragile particles are pulverized and bonded to each other to form a molded body. As a consequence, an optical element having an improved transparency can be formed from the molded body.
In order to enable a molded body to be practically used for an optical element, which molded body is formed by applying a mechanical impact to ultra fine fragile particles so that the ultra fine fragile particles are pulverized and bonded to each other, intensive research and many experiments were carried out, and as a result, an element having a high transparency as compared to that realized in the past can be finally obtained. In addition, through the consideration of the results in view of the light scattering theory, the present invention was completed.
Hereinafter, example 1 will be described in detail together with the principle of the present invention.
Referring to
PLZT as a transparent ceramic having a large electrooptic effect was used as a film-forming material and was deposited on a glass substrate. N2 was used as a carrier gas, and three samples were formed by changing the incident angle of the nozzle with respect to substrate, the gas flow rate, the growth rate, and the vibration frequency of the vibrator. The film-forming conditions are shown in Table 1. In the composition represented by Pb1-xLax(ZryTiz)1-x/4O3 x, y, and z were set to 0.09, 0.65, and 0.35, respectively. The average particle diameter of the powdered raw material was 0.7 μm. The composition has a dielectric region which is called Relaxor and has a perovskite crystal structure. The composition had a large quadratic electrooptic coefficient and was generally applied to an optical device.
In order to remove the influence of the surface roughness on the transmittance, the surface of the sample was polished. The film thickness of each sample was 4 μm. The degrees of transparency of the three samples were considerably different from each other, sample No. 1 was most transparent, and the degrees of transparency of sample Nos. 2 and 3 were decreased in that order. From the cross-sectional FE-SEM images, it was found that, in sample Nos. 1 and 2 which had a high degree of transparency, ultra fine particles having a diameter of 100 nm or less are densely bonded to each other. However, it was also found that, in sample No. 3 having a low degree of transparency, a large amount of particles having a diameter of approximately 0.3 μm is present in the film.
The transmittance was measured by the use of a spectrophotometer (UV-365, manufactured by Shimadzu Corporation). Transmittance 41 of sample No. 1 was increased as the wavelength was increased and was approximately saturated at a wavelength of 1 μm or more. On the other hand, transmittances 42 and 43 of sample Nos. 2 and 3, respectively, were not saturated even at a wavelength of 2 μm.
The inventors of the present invention first disclosed that the difference in transmittance among the samples was caused by the difference in pore (void) diameter and the presence or not of a phase (hereinafter referred to as a “different phase” in some cases) having a different composition. Hereinafter, with reference to the observation results of the images of composition distribution obtained by transmission electron microscope (TEM) and EDX, the disclosure will be described.
In sample No. 1 having a high transmittance shown in
In addition, the refractive index of the different phase differs from that of a primary material of the film, thereby causing light scattering. In order to evaluate the uniformity of the film, the EDX image of element distribution along the cross-section was observed.
Next, in order to find the reason in which the considerable difference in transmittance was generated by different forming methods, analysis was carried out In accordance with the light scattering theory.
Based on the film-forming conditions for sample Nos. 1, 2, and 3 described above, samples having different thicknesses were formed by changing the film-forming time, and the transmittance spectra thereof were measured.
The transmittance was measured using a spectrophotometer (UV-365, manufactured by Shimadzu Corporation). Lines 71, 72, 73, and 74 show the transmittances measured at wavelengths of 500, 600, 800, and 1,550 nm, respectively. The transmittance is decreased as the film thickness is increased, and the thickness dependence of the transmittance is increased as the wavelength is decreased. The decrease in transmittance of PLZT, which has no optical absorption in the wavelength region of this measurement, is primarily caused by the Fresnel reflection at the interface and scattering. The change in transmittance with the film thickness can be represented by equation (1) based on the Lambert-Beer law.
T=(1−Rp)(1−Rps)(1−Rs)Exp(−β′t) (1)
In the above equation (1), Rp represents the Fresnel reflection at the surface of the PLZT; Rps represents the Fresnel reflection at the interface between the PLZT and the substrate; Rs represents the Fresnel reflection at the surface of the substrate; β represents the excitation coefficient; and t represents the thickness of the PLZT. Among those described above, the transmission loss caused by the Fresnel reflection can be prevented by the addition of an antireflection layer or the like, and the decrease in excitation coefficient generated by scattering caused by grain boundaries or the like becomes important.
Through intensive research carried out by the inventors of the present invention, it was found that, in sample No. 1 having a high transmittance, the excitation coefficient β is determined by Rayleigh scattering. In Rayleigh scattering, when the size of a scatter is sufficiently smaller than a light wavelength the scatter is assumed to be placed in a uniform electromagnetic filed which spatially vibrates, and the excitation coefficient in this case can be represented by equation (2) (see K. Miyauchi and G Toda, “Opto-Ceramics” p. 33 (1984), by Gihoudo Shuppan K. K.).
β=Σm|((128π5/3λ4)*di6″((M2−1)(M2+2))2) (2)
In equation (2), m| represents the number of scattering centers; λ represents the wavelength: di represents a radius of the scattering center; and M represents the difference in refractive index between the scatter and a medium.
Since the wavelength dependence of the refractive index of PLZT is small, M can be kept constant. Therefore, when the proportional relationship between β and λ−4 is shown, it can be verified that, in sample No. 1 having a high transmittance, the change in excitation coefficient β with the wavelength is caused by Rayleigh scattering.
Lines 81, 82, and 83 correspond to samples Nos. 1, 2, and 3, respectively. The solid line indicates an approximate line obtained from data of each sample, and in sample Nos. 2 and 3 each having large pores and different phases in the film, the proportional relationship between the excitation coefficient β and λ−4 is not observed, and hence the attenuation of transmitting light cannot be explained by Rayleigh scattering.
In sample No. 1 having a high transmittance, in which the pore diameter is small and no different phase is present, the proportional relationship between the excitation coefficient β and λ−4 holds, and therefore, it is clearly understood that the attenuation of light is caused by Rayleigh scattering.
Accordingly, in order to decrease the excitation coefficient β, increase in wavelength λ of light to be used, decrease in radius d of the scattering center, and decrease in refractive index difference M between the scatter and the medium are effective. That is, when the radius d of the scattering center can be sufficiently decreased as compared to the wavelength λ, a practically usable and transparent optical element can be realized.
Based on the experimental results described above, the relationships are summarized. In
The radius d was 15 nm which was obtained from the TEM image. Since the transmittance loss which can be applied to an optical element is 20 dB/mm, the range of d6/λ4<4×10−5 nm2 or less. In addition, since a desirable transmittance loss used for an optical element is 5 dB/mm, the range of d6/λ4 is 1×10−5 nm2 or less.
In addition, in an optical information transmission system using the optical element formed in accordance with the method of the present invention, a usable light wavelength has a lower limit. According to this example, the wavelength dependence of the transmission loss of the film optimally formed in accordance with this film-forming method is as shown in
The transmittance loss is rapidly decreased as the wavelength is increased. In the optical information transmission system using the optical element formed in accordance with this film-forming method, in order to obtain a practical transmission loss, the wavelength of light to be used for transmitting information must be set to 800 nm or more.
The mechanism which can considerably decrease the number of pores and different phases in the film by the incident angle effect can be construed as described below. When a powdered raw material having an average particle diameter of 0.7 μm is allowed to collide against the substrate, a mechanical impact is applied to the powder, so that fragile oxide particles were pulverized and bonded to each other to form a molded body. However, due to a non-uniform speed, shape, diameter, coagulated state of flying particles, large and coarse particles were also formed by insufficient pulverization and were bonded to the surface of the film.
The presence of large and coarse particles can be confirmed by the SEM image of sample No. 3 shown in
By increasing the incident angle of fine particles, the incident particles are allowed to have a kinetic momentum in an oblique direction, and therefore, the effect of removing the particles bonded to the surface of the film is increased. Accordingly, when the incident angle is appropriately selected, particles having weak bonding forces therebetween can be selectively removed. When the incident angle is excessively increased, the removing effect is also excessively increased, and as a result, the film cannot be formed In Example 2, when the incident angle was increased to 40° or more, the film was hardly formed by deposition.
The selective removal of particles having weak bonding forces therebetween by the control of the incident angle, described in Example 2, is merely one example of a method for manufacturing an optical element having a high transmittance. When at least one of a material, particle diameter, coagulation state, speed, and hardness of the ultra fine particles is changed, the selective removal described above can be performed. Hereinafter, Examples will be further described.
Two nozzles for spraying powder were provided unlike the case of Example 1, one nozzle was used for forming a thin film while the other nozzle was used for spraying particles to selectively remove particles having weak bonding forces. A gas cylinder 1200 containing a nitrogen gas was connected to a glass bottle 1201 through a pipe. After a powdered raw material 1202 was placed in the glass bottle 1201, and the inside thereof was evacuated to approximately 20 Torr through an exhaust pipe 1203, nitrogen was fed as a carrier gas while the flow rate thereof was controlled. When the glass bottle 1201 was vibrated by a vibrator 1204, powdered aerosol was generated and was then carried to a film-forming chamber 1206 through a pipe 1205 with the carrier gas.
The film-forming chamber 1206 was evacuated to a predetermined level by a vacuum pump 1207. The powder was sprayed from a nozzle 1208 onto a substrate 1209 to thereby form a thin film. The substrate 1209 rotated, and a nozzle 1210 was provided at a position opposite to the nozzle 1208 with respect to the center of the substrate 1209 for spraying particles thereto for selectively removing particles having weak bonding forces. A gas cylinder 1211 containing a nitrogen gas is connected to a glass bottle 1212 through a pipe.
After a particle-removing powder 1213 was placed in the glass bottle 1212, and the inside thereof was evacuated to approximately 20 Torr through an exhaust pipe 1214, nitrogen was fed as a carrier gas while the flow rate thereof was controlled. When the glass bottle 1212 was vibrated by a vibrator 1215, powdered aerosol was generated and was then carried to the film-forming chamber 1206 through a pipe 1216 with the carrier gas. At the center of the film-forming chamber, a partition 1217 was provided for preventing the powdered raw material and the particle-removing powder from being mixed with each other.
As the powdered raw material and the particle-removing material, PLZT and alumina were used, respectively, and the film was formed on the glass substrate. The rotation speed of the substrate was set to 20 rpm. The incident angle of the nozzle with respect to the substrate was 0° and that of the particle-removing nozzle was 30°. N2 was used as a carrier gas, the flow rate of PLZT was set to a predetermined value of 8 l/min, and the film formation was carried out by changing the flow rate of alumina. In the composition represented by (Pb1-xLax)(ZryTiz)1-x/4O3 x, y, and z were set to 0.09, 0.66, and 0.35, respectively. The average particle diameter of the powdered raw material PLZT was 0.7 μm, and the average particle diameter of the particle-removing alumina was 1 μm.
When the gas flow rate of alumina was zero, that is, when the film formation was performed only by the use of the powdered raw material, the transmittance of the PLZT film was as low as 42%. The thickness of the film was 4 μm. When the flow rate of alumina was increased, the transmittance was gradually increased, and when the flow rate was increased to 4 l/min, the transmittance was increased to 70%. In this example, as the ultra fine particles used for particle removal, alumina having a particle diameter of 1 μm was used. However, the present invention is not limited thereto, and materials such as fine silica particles or fine diamond particles may also be used. In addition, although the material is not changed, when the particle diameter, coagulation state, speed, or hardness thereof is changed, particles having weak bonding forces therebetween can be selectively removed, and as a result, an optical element having a high transmittance can be formed.
In Example 4, unlike the case of Example 3, dry ice was used as particles for selectively removing particles having weak bonding forces. A gas cylinder 1300 containing a nitrogen gas was connected to a glass bottle 1301 through a pipe. After a powdered raw material 1302 was placed in the glass bottle 1301, and the inside thereof was evacuated to approximately 20 Torr through an exhaust pipe 1303, nitrogen was fed as a carrier gas while the flow rate thereof was controlled.
When the glass bottle 1301 was vibrated by a vibrator 1304, powdered aerosol was generated and was then carried to a film-forming chamber 1306 through a pipe 1305 with the carrier gas. The film-forming chamber 1306 was evacuated by a vacuum pump 1307 to a predetermined evacuation level. The powder was sprayed from a nozzle 1308 onto a substrate 1309 to thereby form a thin film. The substrate 1309 rotated, and a nozzle 1310 was provided at a position opposite to the nozzle 1308 with respect to the center of the substrate 1309 for spraying fine particles of dry ice (CO2) to selectively remove particles having weak bonding forces. A gas cylinder 1311 containing a CO2 gas was connected to the film-forming chamber 1306 through a pipe 1312. When being fed from the nozzle 1310 into the film-forming chamber 1306 at a low pressure, the CO2 gas was adiabatically expanded and condensed, thereby forming fine particles. At the center of the film-forming chamber, in order to prevent the fine dry ice particles and the powdered raw material from being mixed with each other, a partition 1313 was provided.
PLZT was used as a material for forming the film, and the film was formed on the glass substrate. The rotation speed of the substrate was set to 20 rpm. The incident angle of the film-forming nozzle and that of the dry ice nozzle with respect to the substrate were both 0°. N2 was used as a carrier gas of the powdered raw material, the gas flow rate of PLZT was set to 8 l/min, and the gas flow rate of the dry ice was set to 11 l/min. In the composition represented by (Pb1-xLax)(ZryTz)1-x/4O3 x, y, and z were set to 0.09, 0.65, and 0.35, respectively. The average particle diameter of the powdered raw material PLZT was 0.7 μm. When the film formation was carried out only using the powdered raw material, that is, when the formation was performed without spraying the dry ice, the transmittance of the PLZT film was as low as 42%. The film thickness was 4 μm. When the dry ice was sprayed, the transmittance was increased to 72%. Since having high volatility, the dry ice was vaporized immediately after removing particles having weak bonding forces therebetween, and therefore, the particle-removing particles were not incorporated into the film. Accordingly, light scattering will not be caused by the particle-removing particles, and consequently, an optical element having a high transmittance can be obtained.
In Example 5, unlike the case of Example 3, laser light was concentrated on a place of a substrate onto which particle-removing particles for selectively removing particles having weak bonding forces are sprayed. A gas cylinder 1400 containing a nitrogen gas was connected to a glass bottle 1401 through a pipe. After a powdered raw material 1402 was placed in the glass bottle 1401, and the inside thereof was evacuated to approximately 20 Torr through an exhaust pipe 1403, nitrogen was fed as a carrier gas while the flow rate thereof was controlled. Powdered aerosol was generated when the glass bottle 1401 was vibrated by a vibrator 1404 and was then carried to a film-forming chamber 1406 through a pipe 1405 with the carrier gas.
The film-forming chamber 1406 was evacuated by a vacuum pump 1407 to a predetermined evacuation level. The powder was sprayed from a nozzle 1408 onto a substrate 1409 to thereby form a thin film. The substrate 1409 rotated, and a nozzle 1410 was provided at a position opposite to the nozzle 1408 with respect to the center of the substrate 1409 for spraying a powder to selectively remove particles having weak bonding forces. A gas cylinder 1411 containing a nitrogen gas was connected to a glass bottle 1412 through a pipe. After a particle-removing powder 1413 was placed in the glass bottle 1412, and the inside thereof was evacuated to approximately 20 Torr through an exhaust pipe 1414, nitrogen was fed as a carrier gas while the flow rate thereof was controlled.
When the glass bottle 1412 was vibrated by a vibrator 1415, powdered aerosol was generated and was then carried to the film-forming chamber 1406 through a pipe 1416. Laser light radiating from a light source 1418 was concentrated on a place of the substrate 1409 onto which the particles are sprayed from the nozzle 1410. At the center of the film-forming chamber, a partition 1417 was provided so as to prevent the powdered raw material and the particle-removing powder from being mixed with each other.
PLZT was used as a material for forming the film, zirconia was used as a particle-removing material, and the film was formed on the glass substrate. The rotation speed of the substrate was set to 20 rpm. The incident angle of the film-forming nozzle with respect to the substrate was 0°, and that of the particle-removing nozzle was 15°, N2 was used as a carrier gas of the powdered raw material, the gas flow rate of PLZT was set to a predetermined rate of 8 l/min, and the gas flow rate of zirconia was set to a predetermined rate of 2 l/min. As the laser light, argon laser having a wavelength of 488 nm was used, and the power density at an irradiated surface was changed from 0 to 2 mW/mm2, and the transmittance of the film was measured. In the PLZT composition represented by (Pb1-xLax)(ZryTiz)1-x/4O3 x, y, and z were set to 0.09, 0.65, and 0.35, respectively. The average particle diameter of the powdered raw material PLZT was 0.7 μm, and the average particle diameter of the zirconia used as the particle-removing particles was 0.5 μm.
When the radiation power of the laser was 0, the transmittance of the PLZT film was as low as 51%, and the film thickness was 4 μm. When the radiation power of the laser was increased, the transmittance was gradually increased, and at a power density of 2 mW/mm2, the transmittance was increased to 73%.
In this example, as an electromagnetic wave, visible laser light having a wavelength of 488 nm was used. However, the present invention is not limited thereto, and for example, ultraviolet rays, visible rays, infrared rays, and microwaves may also be used. When the selective removal of particles having weak bonding forces therebetween is promoted by the use of electromagnetic waves, mechanical energy of the particle-removing particles can be decreased. As a consequence, generation of defects and stress caused by the mechanical impact applied to fine crystals for forming the molded body can be suppressed, and as a result, the electrooptic properties of the molded body can be improved.
Next, an example of an optical modulator using the optical element of the present invention will be described.
First, an ITO film having a thickness of 200 nm was formed as a lower electrode on a glass substrate by sputtering. The substrate was heated to 300° C., and the formation was performed by a magnetron sputtering method using a mixed gas of argon and oxygen.
On the ITO lower electrode, a lift-off resist having a Mach-Zehnder waveguide shape was formed. Subsequently, a PLZT film 2 μm thick was formed as a core layer using the film-forming apparatus shown in
Light having a wavelength of 1.55 μm was introduced into the light modulator thus formed using a round-end optical fiber, and when a voltage of 10 V was applied to the electrodes, a degree of modulation of 15 dB was obtained.
When the wavelength dependence of the transmission loss of the light modulator of this example was measured, the waveguide loss was 20 dB or less at a wavelength of 800 nm or more, and therefore, it was found that the light modulator described above can be used in practice. In addition, as the wavelength for measurement was increased, the waveguide loss was decreased, and as a result, the loss became 5 dB at a wavelength of 1.55 μm.
In this example, a lift-off method was used for the formation of the PLZT core layer. However, after a film for the PLZT core layer is formed, and a desired shape of the waveguide is formed using a resist, the film may be etched by a reactive ion etching or the like to form the PLZT core layer. Further, in this example, by changing the shape of the waveguide, an optical switch may also be formed.
Since the properties of the optical element of the present invention are independent of an underlying material, the element can be advantageously formed on a polycrystal material having a dfferent crystal structure such as ITO.
Moreover, since being formed regardless of a substrate and an underlying layer, an optical element can be formed on a substrate provided with an optical element such as a laser, electrooptic converter, photoelectric converter, optical amplifier, optical waveguide, or optical filter, or on a substrate provided with an electronic circuit such as a CPU or memory. Thus, an optical integrated device can be formed.
Continuous light emitted from a laser 1501 is concentrated by a microlens 1502 and is then incident on a light modulator 1503. The light modulator 1503 is composed of a Mach-Zehnder waveguide 1504 formed by the same process as that described in Example 6 and electrodes 1505 and is placed on a substrate which is provided with a laser beforehand. A modulation signal generation circuit 1506 is electrically connected to the electrodes 1505, and a high frequency signal driven by voltage is supplied to the electrodes. By the voltage thus applied, the refractive index of the waveguide is changed, and thereby, the optical signal can be modulated. The optical signal thus modulated by the optical modulator is converted into an electric signal by a photoelectric converter 1507.
When a signal of 10 MHz was input into the optical information transmission system of this example, and the wavelength dependence of SIN was measured, S/N was 16 dB or more at a wavelength of 800 nm. Thus, it was found that the optical information transmission system can be used in practice.
The film-forming method of the optical element is similar to that of the example 1. Specifically, a thin film is formed by the use of the film-forming apparatus shown in
PZT having a high optical anisotropy was used as a film-forming material and was formed on a glass substrate so as to be transparent. In the composition represented by Pb(ZryTiz)O3, y, and z were set to 0.52, and 0.48, respectively.
The transmittance was measured by the use of the spectrophotometer (UV-365, manufactured by Shimadzu Corporation) in a similar manner with the example 1. The transmittance of the sample was increased with the wavelength and was almost saturated at wavelengths of 1 μm or more.
While the present invention has thus far been disclosed in conjunction with several embodiments or examples thereof, it will be readily possible for those skilled in the art to put the present invention into practice in various other manners.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2003-397880 | Nov 2003 | JP | national |
| 2004-338997 | Nov 2004 | JP | national |
