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Timestamp: 2013-05-23 00:33:49
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Matched Legal Cases: ['Application No. 2', 'Application No. 2', 'Application No. 2', 'Application No. 2', 'Application No. 4', 'Application No. 4', 'Application No. 50', 'Application No. 50', 'Application No. 52', 'Application No. 52', 'Application No. 63', 'Application No. 63']

Patent US5347608 - Optical waveguide and optical waveguide device - Google PatenteSuche Bilder Maps Play YouTube News Gmail Drive Mehr » Erweiterte Patentsuche | Webprotokoll | Anmelden Erweiterte Patentsuche PatenteOptical device including an optical waveguide composed of a LiTaO.sub.3 monocrystalline substrate and a LiNbO.sub.3 monocrystalline thin film waveguide formed on the surface of the substrate, with the lattice length of the LiNbO.sub.3 monocrystalline thin film waveguide and the LiTaO.sub.3 monocrystalline...http://www.google.de/patents/US5347608?utm_source=gb-gplus-sharePatent US5347608 - Optical waveguide and optical waveguide device Ver�ffentlichungsnummerUS5347608 APublikationstypErteilung Anmeldenummer08/068,102 Ver�ffentlichungsdatum13. Sept. 1994Eingetragen28. Mai 1993 Priorit�tsdatum30. Juni 1992 ErfinderMasanori NakamuraYoshikazu SakaguchiUrspr�nglich Bevollm�chtigterIbiden Co., Ltd. US-Klassifikation385/130385/122385/8385/2385/4Internationale KlassifikationG02F1/01G02B6/13G02B6/136G02F1/225G02B6/12 UnternehmensklassifikationG02B6/12004G02F1/225G02B6/136 Europ�ische KlassifikationG02B6/12DG02B6/136G02F1/225ReferenzenPatentzitate (5)Nichtpatentzitate (22) Referenziert von (8)Externe LinksUSPTO USPTO-Zuordnung EspacenetOptical waveguide and optical waveguide deviceUS 5347608 A Zusammenfassung Optical device including an optical waveguide composed of a LiTaO.sub.3 monocrystalline substrate and a LiNbO.sub.3 monocrystalline thin film waveguide formed on the surface of the substrate, with the lattice length of the LiNbO.sub.3 monocrystalline thin film waveguide and the LiTaO.sub.3 monocrystalline substrate being matched with each other, and an electrode provided on at least one of the branched sections of the waveguide extending along the optical axis thereof for changing a refractive index of the waveguide. The optical device having the optical waveguide has excellent electro-optic effects and a resistance to optical damage as well as an advanced modulation efficiency and a stable amplitude modulation.
What is claimed is: 1. A single mode optical waveguide comprising: a LiTaO.sub.3 monocrystalline substrate, and a LiNbO.sub.3 monocrystalline thin film waveguide formed on a surface of said LiTaO.sub.3 monocrystalline substrate, said thin film waveguide having Mach-Zehnder type branched sections on a part of the thin film waveguide, and including at least an Na content in a range of 0.1 to 14.3 mol % and a Mg content in a range of 0.8 to 10.8 mol %, so that lattice lengths of the LiNbO.sub.3 monocrystalline thin film waveguide and the LiTaO.sub.3 monocrystalline substrate are matched with each other with the following relationship being satisfied; in the case of TM mode, 1.9&amp;lt;(T+0.7)/&#955;&amp;lt;5.7 (T&amp;gt;0) in the case of TE mode, 0.29&amp;lt;(T+0.04)/&#955;&amp;lt;1.19 (T&amp;gt;0) where T(μm) represents a thickness of the LiNbO.sub.3 monocrystalline thin film waveguide and λ(μm) a wavelength of the guided wave.
2. The single mode optical waveguide as claimed in claim 1, wherein the shape of the optical waveguide satisfies the following relationship; in the case of TM mode, W&#8806;(4&#955;-0.5) in the case of TE mode, W&#8806;(0.04.lambda..sup.3 +0.1.lambda..sup.2)/&#916;T+2.5.lambda. where W(μm) represents a width of the waveguide, ΔT(μm) an etching depth, and λ(μm) a wavelength of the guided wave.
3. The optical waveguide as claimed in claim 1, wherein the LiNbO.sub.3 monocrystalline thin film waveguide formed on the surface of the LiTaO.sub.3 monocrystalline substrate is an inverted ridge type waveguide.
4. The optical waveguide as claimed in claim 1, wherein the LiNbO.sub.3 monocrystalline thin film waveguide formed on the surface of the LiTaO.sub.3 monocrystalline substrate is a ridge type waveguide.
5. The optical waveguide as claimed in claim 1, wherein the branched LiNbO.sub.3 monocrystalline thin film is formed on a (0001) face of the surface of the LiTaO.sub.3 monocrystalline substrate.
6. The optical waveguide as claimed in claim 1, wherein a lattice length of an axis of the LiTaO.sub.3 monocrystalline substrate is matched with a lattice length of an axis of the LiNbO.sub.3 monocrystalline thin film.
7. The optical waveguide as claimed in claim 1, wherein a lattice length of the LiNbO.sub.3 monocrystalline thin film is in a range of 99.81 to 100.07% of a lattice length of the LiTaO.sub.3 monocrystalline substrate for matching the lattice lengths with each other.
8. A waveguide device comprising an LiTaO.sub.3 monocrystalline substrate, an LiNbO.sub.3 monocrystalline thin film waveguide formed on a surface of said LiTaO.sub.3 monocrystalline substrate, said thin film waveguide having Mach-Zehnder type branched sections on a part of the thin film waveguide, and an electrode for changing a refractive index of the waveguide provided on at least one of the branched sections, and including at least an Na content in a range of 0.1 to 14.3 mol % and a Mg content in a range of 0.8 to 10.8 mol %, so that lattice lengths of the LiNbO.sub.3 monocrystalline thin film waveguide and the LiTaO.sub.3 monocrystalline substrate are matched with each other with the following relationship being satisfied; in the case of TM mode, 1.9&amp;lt;(T+0.7)/&#955;&amp;lt;5.7 (T&amp;gt;0) in the case of TE mode, 0.29&amp;lt;(T+0.04)/&#955;&amp;lt;1.19 (T&amp;gt;0) where T(μm) represents a thickness of the LiNbO.sub.3 monocrystalline thin film waveguide and λ (μm) a wavelength of the guided wave.
10. The waveguide device as claimed in claim 8, wherein the LiNbO.sub.3 monocrystalline thin film waveguide formed on the surface of the LiTaO.sub.3 monocrystalline substrate is an inverted ridge type waveguide.
11. The waveguide device as claimed in claim 8, wherein the LiNbO.sub.3 monocrystalline thin film waveguide formed on the surface of the LiTaO.sub.3 monocrystalline substrate is a ridge type waveguide.
12. The waveguide device as claimed in claim 8, wherein the branched LiNbO.sub.3 monocrystalline thin film is formed on a (0001) face of the surface of the LiTaO.sub.3 monocrystalline substrate.
13. The waveguide device as claimed in claim 8, wherein a lattice length of an axis of the LiTaO.sub.3 monocrystalline substrate is matched with a lattice length of an axis of the LiNbO.sub.3 monocrystalline thin film.
14. The waveguide device as claimed in claim 8, wherein a lattice length of the LiNbO.sub.3 monocrystalline thin film is in a range of 99.81 to 100.07% of a lattice length of the LiTaO.sub.3 monocrystalline substrate for matching the lattice lengths with each other.
In an optical integrated circuit, such as optical matrix switch, optical modulator and the like, a Ti diffused LiNbO.sub.3 channel waveguide is generally utilized. This comes from the reason that, LiNbO.sub.3 has a relatively large electro-optic constant among other stable inorganic crystals and gives an advanced effect to the device using the electro-optic effect.
However, for the Mach-Zehnder optical modulator proposed by NORMANDIN et al., as reported in "Appl. Phys. Lett.," vol.34, No. 3, pp.200(1979) by R. Normandin et al., and "J. Opt. Commun.," vol.9, No. 1 pp. 19(1988) by M. Rottschalk et al., (1) an LiNbO.sub.3 optical waveguide produced by a Ti diffusion method suffers large optical damage due to its Ti content and accordingly a waveguide for visible light is not available, (2) an LiNbO.sub.3 optical waveguide produced by a proton exchange method, after the waveguide has been formed, has a different crystalline characteristic from that of a virgin LiNbO.sub.3, and together with the other reason, an electro-optic constant is smaller than that of a bulk crystal. Accordingly, when such an optical waveguide is used as the optical device for an optical directional coupler and an optical modulator, etc., then a problem has arisen in an inability to realize the electric-power-saving and the minituarization because a larger switching voltage and a longer effective length are required.
SUMMARY OF THE INVENTION An object of the invention is to remove the aformentioned difficulties in an optical device such as an optical waveguide switch, optical modulator and the like, particularly Mach-Zehnder type optical device and its component, i.e., optical waveguide
Further, the optical waveguide according to the invention not only prevents diffusion of impurities, but also exhibits waveguide characteristics with no degradation after a temperature thereof rises up to around a Curie point of LiNbO.sub.3 because heat resistance is obtained by lattice matching between the substrate and the waveguide. In the conventional Ti diffused LiNbO.sub.3 waveguide or the proton exchanged LiNbO.sub.3 waveguide, when the temperature rises up to the Curie point of the LiNbO.sub.3, the Ti or proton is sufficiently diffused to change a waveguide mode profile or to eliminate a presence of the waveguide mode. However, according to the invention, such adverse effect is prevented.
The optical waveguide of the invention, which is constituted on the basis of the knowledge and the concept as described, is characterized in that, an LiNbO.sub.3 monocrystalline thin film waveguide is formed on a surface of an LiTaO.sub.3 monocrystalline substrate, concurrently a branch section is provided on a part of the thin film waveguide, and a lattice length of the LiNbO.sub.3 monocrystalline thin film waveguide is matched to that of the LiTaO.sub.3 monocrystalline substrate.
Further, it is an object of the invention to provide an optical waveguide device characterized in that, an LiNbO.sub.3 monocrystalline thin film waveguide is Formed a surface of an LiTaO.sub.3 monocrystalline substrate, concurrently branch sections are provided on a part of the thin film waveguide, and a lattice length of the LiNbO.sub.3 monocrystalline thin film waveguide is matched to that of the LiTaO.sub.3 monocrystalline substrate, and at least one of the branch sections of the waveguide is provided with an electrode for changing a refractive index of the waveguide.
Operation Mode of the Invention The optical waveguide according to the invention is characterized in that an electrode for changing a refractive index (propagation constant) this waveguide is provided on a Y shaped branch of the LiNbO.sub.3 monocrystalline thin film waveguide which is formed in or on the LiTaO.sub.3 monocrystalline substrate, and a lattice length of the LiTaO.sub.3 monocrystal substrate is matched to that of the LiNbO.sub.3 monocrystalline thin film waveguide.
A face (0001) of the LiNbO.sub.3 monocrystalline waveguide must be formed to be laminated on a face (0001) of the LiTaO.sub.3 monocrystalline substrate.
In the above constitution, the matching of the lattice lengths of laminated monocrystals means that lattice length of the LiNbO.sub.3 monocrystalline thin film is substantially coincident with that of an LiTaO.sub.3 monocrystalline substrate; namely, is adjusted to be in a range of 99.81 to 100.07%, preferably 99.92 to 100.03% of the lattice length of the LiTaO.sub.3 monocrystalline substrate.
With matching lattice lengths of both monocrystals as mentioned above, when a liquid phase epitaxial growth of the LiNbO.sub.3 monocrystal is performed on the LiTaO.sub.3 monocrystalline substrate, an occurrence of crystal defect is prevented, and as a result there is formed an LiNbO.sub.3 monocrystalline thin film waveguide having an electro-optic effect equivalent to that of an LiNbO.sub.3 monocrystalline bulk.
When the LiTaO.sub.3 monocrystal is used as a substrate, the LiNbO.sub.3 monocrystalline thin film exhibits an extremely superior optical characteristic if the lattice length of the LiNbO.sub.3 monocrystalline thin film is matched with that of the substrate monocrystal, and furthermore there can be formed a thick film which has not been produced in the conventional technique.
A reason why the LiNbO.sub.3 monocrystalline thin film exhibits such extremely advanced optical characteristics is that, the LiNbO.sub.3 monocrystalline thin film and the LiTaO.sub.3 monocrystal substrate are obtained by lattice match therebetween and formed unitary with each other to greatly reduce their strain and crystal defects, to thereby upgrade a crystalline characteristic, and as a result, a high qualitative film is formed without any micro cracks, etc.
In the construction mentioned above, a method of lattice matching both laminated monocrystals may preferably be a method which the inventors have proposed in the International Patent Application Number PCT/JP/90/01207. Namely, (1) a method of containing Na and Mg in the LiNbO.sub.3 monocrystal; (2) a method of varying a ratio of Li and Nb in a range of 41/59 to 56/44; and (3) a method of reducing the lattice length of the LiTaO.sub.3 monocrystalline substrate by doping Ti, etc. These described methods are satisfactorily in conformity with the method which the inventors have proposed in the specification.
(1) It is known in general that the lattice length of the LiTaO.sub.3 monocrystalline substrate is larger than that of the LiNbO.sub.3 monocrystal. If the LiNbO.sub.3 monocrystal to be laminated is replaced with or doped by Na or Mg, that is to enlarge the lattice length of the LiNbO.sub.3 monocrystal, then the lattice lengths of both monocrystals can be matched with each other without optical damage.
The LiNbO.sub.3 monocrystalline thin film is preferably contained therein with Na and Mg, the contents of which may preferable be Na of 0.1 to 14.3 mol %, and Mg of 0.8 to 10.8 mol % for the LiNbO.sub.3 monocrystal. The reason why the preferable content ranges of Na and Mg are decided is due to the fact that in case the Na content is less than 0.1 mol %, then irrespective of added quantity of Mg, the lattice length of the LiNbO.sub.3 monocrystal is not enlarged enough to match that of the LiTaO.sub.3 monocrystalline substrate, and when Na content exceeds 14.3 mol %, then the lattice length becomes too large. Thus in both cases the lattice match between LiNbO.sub.3 monocrystal and the LiTaO.sub.3 monocrystal becomes difficult. In case the Mg content is less than 0.8 mol %, then the optical damage can not be sufficiently prevented, and if it exceeds 10.8 mol %, then crystal of magnesium niobate is deposited, whereby Mg contents outside the described range are not preferable.
(2) According to the invention, a method of lattice-match between the LiNbO.sub.3 monocrystal thin film and the LiTaO.sub.3 monocrystalline substrate by changing a mol ratio of Li and Nb in LiNbO.sub.3 may use the liquid phase epitaxy technique. In this case, it is advantageous to utilize a composition mainly consisting of K.sub.2 O, V.sub.2 O.sub.5, Li.sub.2 O and Nb.sub.2 O.sub.5. Such reason is that K.sub.2 O and V.sub.2 O.sub.5 act as a melting agent (flux). By using the K.sub.2 O and V.sub.2 O.sub.5 as a flux, supply of Li from the flux is prevented, thereby the mol ratio of Li and Nb in the LiNbO.sub.3 to be separated can be varied by changing a composition ratio of the Li.sub.2 O and Nb.sub.2 O.sub.5 in the raw material.
As a conclusion, the change of the mol ratio of the Li and Nb mol % provides variation of the lattice length "a" axis. The control of the composition ratio of the Li.sub.2 O and Nb.sub.2 O.sub.5 in the raw material provides a control the lattice length of the "a" axis of the LiNbO.sub.3 monocrystalline thin film, thus the lattice match between the LiNbO.sub.3 monocrystal thin film and the LiTaO.sub.3 monocrystalline substrate can be achieved.
(3) According to the invention, another lattice-match technique may be used to achieve the matching by reducing the lattice length of the "a" axis of the LiTaO.sub.3 monocrystalline substrate layer. In this method, Ti may preferably be contained in the LiTaO.sub.3 monocrystal, because Ti atom or ion acts to reduce the lattice length of the "a" axis of the LiTaO.sub.3 monocrystalline substrate layer.
The Ti atom or ion in a range from 0.2 to 30 mol % for LiTaO.sub.3 may preferably be contained in the LiTaO.sub.3. The reason is that, if the Ti content is less than 0.2 mol %, then the lattice length of the LiTaO.sub.3 is not reduced enough to match to that of the LiNbO.sub.3, and if it exceeds 30 mol %, then, to the contrary, the lattice length is reduced too much, and therefore in both cases, cases, the lattice match between the LiTaO.sub.3 monocrystalline substrate and the LiNbO.sub.3 monocrystalline thin film is not achieved.
BRIEF DESCRIPTION OF THE DRAWING FIGS. 1(a) and 1(b) illustrate a schematic perspective view of an embodiment of an optical modulator according to the invention and a schematic perspective view of another embodiment of the invention, respectively;
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A method of producing the waveguide will now be described.
Referring to FIG. 2(a) illustrating a method of forming the inverted ridge type optical waveguide 2, grooves 4 are formed in a LiTaO.sub.3 monocrystalline substrate 1 and then a thin LiNbO.sub.3 monocrystalline film 5 is grown on the substrate 1 with their lattices being matched with each other, after which a surplus portion of the thin film 5 is removed, and the LiNbO.sub.3 monocrystal is remained only in the groove 4, thus the waveguide 2 is formed.
Referring to FIG. 2(b) illustrating a method of forming the ridge type waveguide 2, a thin LiNbO.sub.3 monocrystalline film 5 is grown on a LiTaO.sub.3 monocrystalline substrate 1 with their lattices being matched with each other, and then a surplus portion is removed by masking waveguide portions of the LiNbO.sub.3 monocrystalline film 5 with Ti, etc., and dry-etching the film to form the waveguide 2.
Another method of forming a thin LiNbO.sub.3 monocrystalline film 5 on the LiTaO.sub.3 monocrystalline substrate with their lattice being matched, includes step of contacting the LiTaO.sub.3 monocrystalline substrate with a melted compound consisting of lithium oxide, vanadium pentoxide, niobium pentoxide, sodium oxide and magnesium oxide.
The invention is to provide further the other aspects; namely, by the inverted ridge type waveguide in the LiTaO.sub.3 monocrystalline substrate 1, as shown in FIG. 1(a); or by the ridge type waveguide on the substrate as shown in FIG. 1(b), a Mach-Zehnder optical interferometer is constituted as the optical device used for the optical modulator or the optical switch.
The optical modulator as described above is constructed to apply an electric field along the optical axis of the LiNbO.sub.3 monocrystalline thin film waveguide whose lattice is matched with that of the LiTaO.sub.3 monocrystalline substrate; such "optical modulator" is an optical device having a long range service life with a stable amplitude modulation, exhibiting a remarkably high efficiency for the optical modulation.
The optical modulator has a LiTaO.sub.3 monocrystalline substrate 1 and a branch type LiNbO.sub.3 monocrystalline waveguide 2 formed on the vicinity of a (0001) face of the LiTaO.sub.3 monocrystalline substrate 1, and at least a part of the waveguide is provided with two branched sections.
The waveguide described above must be formed to laminate the (0001) face of the branch type LiNbO.sub.3 monocrystalline waveguide on the (0001) face of the LiTaO.sub.3 monocrystalline substrate 1. The lattice length of the "a" axis of the LiTaO.sub.3 monocrystalline substrate is matched with that of the "a" axis of the LiNbO.sub.3 monocrystalline thin film waveguide.
Electrodes 7-1 and 7-2 having suitable constructions are provided on the LiNbO.sub.3 monocrystalline waveguides 2-1 and 2-2, and adopted for inducing a change of a refraction index of the waveguide by the electro-optic effect of the LiNbO.sub.3 monocrystalline waveguide to control a phase of a guided light electrically and intensity of an output light with voltage applied to the electrodes.
In the construction as described above, the guided light is separated into two lights at the Y branch of an input-side, and guided to the optical waveguides 2-1 and 2-2. At least one-side of the branched optical waveguides 2-1 and 2-2 is provided thereon with a suitably constructed electrode, and a phase difference Δφ (this is a phase difference generated between the two waveguides due to variation, such as an increase or a decrease of a propagation constant of the waveguide because an electric field concentration, is produced beneath the electrode when the voltage is applied to the electrode) is produced between the two waveguides by applied voltage. The two guided lights having the phase difference with each other are allowed to be interfered at an output-side branch, the intensity of output light is varied depending on the phase difference Δφ. In case where each of the Y branched waveguides are formed with electrodes, then the guided lights respectively receive phase variations of Δφ and Δφ in the branched waveguides, accordingly the phase difference between the waveguides becomes 2Δφ, this indicates two times the phase difference can be obtained with a high efficiency compared to the case that the electrode is provided only on either-side waveguide. The phase difference produced between the two waveguides is given in the following equation, ##EQU1## where Vπ represents a half wavelength voltage (a voltage where a phase difference becomes π/2), "1" an effective length of the electrode, "d" an interval between the electrodes, Γ a reduction coefficient of the applied voltage, λ a wavelength of the guided light, "r" an electro-optical constant of the LiNbO.sub.3 monocrystalline thin film, and V an applied voltage.
When a light with power P.sub.i is coupled, an output light P.sub.o satisfies the following equation, ##EQU2## where r.sub.p represents a power distribution ratio of each waveguide.
As a method of producing the LiNbO.sub.3 monocrystalline thin film there can be used; namely, one comprising the steps of forming a groove in the waveguide formed portion of the LiTaO.sub.3 monocrystalline substrate 1, forming the LiNbO.sub.3 monocrystalline thin film while being allowed to receive lattice match, thereafter removing a surplus portion so that the LiNbO.sub.3 monocrystal 5 remains only in the groove 4, thereby forming the waveguide 2; and the other comprising the steps of forming the LiNbO.sub.3 monocrystalline film on the LiTaO.sub.3 monocrystalline substrate 1 while both being allowed to receive lattice match, thereafter removing a surplus portion by using Ti and the like, and dry-etching on the waveguide formed portion, thus forming the waveguide 2.
The method of forming the LiNbO.sub.3 monocrystalline thin film while being allowed to receive lattice match, is achieved by contacting the LiTaO.sub.3 monocrystalline substrate 1 with a melted body consisting of lithium oxide--vanadium pentoxide--niobium pentoxide--sodium oxide--magnesium oxide.
An electrode 7-1 is provided as a means for changing a refractive index on the LiNbO.sub.3 monocrystalline waveguide 2 produced in the above described process. An electrode 7-2 may preferably be produced by coating metal film such as aluminium or gold by evaporation, plating or sputtering.
If the LiNbO.sub.3 waveguide includes an Na content in a range of 0.1 to 14.3 mol % and an Mg content in a range of 0.8 to 10.8 mol %, then the relationships in the following equations are preferable,
1.9&amp;lt;(T+0.7)/&#955;&amp;lt;5.7
0.29&amp;lt;(T+0.04)/&#955;&amp;lt;1.19
W&#8806;(4&#955;-0.5)(&#955;.sup.2 10T+2.0)
W&#8806;(0.04.lambda..sup.3 +0.1 &#955;.sup.2)/&#916;T+2.5.lambda.
The range described above is a particular condition for the LiNbO.sub.3 waveguide which is lattice matched to the LiTaO.sub.3 substrate using Na and Mg.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT EXAMPLE 1 (1) 2 mol % of MgO with respect to a theoretical quantity of LiNbO.sub.3 which could be separated from a melted body composition was added to a mixture consisting of Na.sub.2 CO.sub.3 22 mol %, Li.sub.2 CO.sub.3 28 mol %, V.sub.2 O.sub.5 40 mol % and Nb.sub.2 O.sub.5 10 mol %, a resultant mixture was put into a platinum crucible, and the mixture in the cruciblents was heated up to 1100 atmosphere within an epitaxial growth forming system. The resultant obtained melt in the crucible was stirred at 100 rpm for 12 hours by means of a propeller.
(2) A (0001) face of an LiTaO.sub.3 monocrystal having a thickness of 2 mm was polished to prepare a substrate. Next, a portion of the surface of the LiTaO.sub.3 monocrystal substrate to be formed with the waveguide as shown in FIG. 1 was patterned by a photolithography technique and masked by a Ti strip line formed by a lift-off method. Further, a groove having a width of 10 μm and a depth of 3.5 μm was formed by an Ar plasma etching technique and the Ti mask was peeled off.
(3) Next, the melt was gradually cooled down to 915 rate of 60 heated at 915 rpm for 4 minutes in the melt. A growth rate of the LiNbO.sub.3 was 1 mm per minute.
(4) The substrate 1 was taken up from the melt, and rotated at 1000 rpm for 30 seconds to shake away a surplus melt medium from the substrate. Thereafter, the substrate was gradually cooled to room temperature at a cooling rate of 1 monocrystalline thin film containing Na and Mg of about 4 μm thickness was formed on the substrate.
(5) The Na and Mg contents contained in the thus obtained LiNbO.sub.3 monocrystalline thin film are 3 mol % and 2 mol % respectively. A lattice length ("a" axis) of the thin film was 5.156, and an extraordinary refractive index measured by a guided light with a wavelength of 0.83 μm was 2.163.+-001.
(7) Further, this optical waveguide was kept at 1000 hours in a water vapor atmosphere, and was coupled to the laser with the wavelength of 1.55 μm at TM mode. Its waveguide mode was a single one with a propagation loss of less than 1 dB per cm. A deterioration with age of an emitting light was not changed at least for 24 hours as a result of measurement.
(8) A silica buffer layer was placed on the optical waveguide obtained in the above process by sputtering technique, and an aluminium electrode was formed on the waveguide by the photolithography and vapor deposition techniques. An electro-optic constant (r 33) of this optical waveguide was 30 pm/V, which was substantially equal to a value of a bulk LiNbO.sub.3 monocrystal as a result of a practical measurement.
EXAMPLE 2 (1) 5 mol % of MgO with respect to a theoretical quantity of LiNbO.sub.3 which could be separated from the melted body composition was added to a mixture consisting of Li.sub.2 CO.sub.3 44.3 mol %, V.sub.2 O.sub.5 46.4 mol %, Nb.sub.2 O.sub.5 9.3 mol %, and Na.sub.2 CO.sub.3 27.2 mol % relative to Li.sub.2 CO.sub.3, a resultant mixture was put into a platinum crucible, and the mixture was heated up to 1050 under the air atmosphere within the epitaxial growth forming system. The resultant obtained melt in the crucible was stirred at 100 rpm for 20 hours by means of a propeller.
(2) A (0001) face of an LiTaO.sub.3 monocrystal with a thickness of 1 mm was polished to prepare a substrate. Next, a portion where a waveguide 2 was formed, as shown in FIG. 1, was patterned by the photolithography technique and was masked with Ti formed by a lift-off method. Further, a groove with a width of 1.5 μm and a depth of 0.8 μm was formed by the Ar plasma etching technique and the Ti mask was peeled off.
(3) Next, the melt was gradually cooled down to 938 rate of 60 preliminarily heated at 915 rotated at 35 rpm for 2.5 minutes in the melt. A growth rate of the LiNbO.sub.3 was 1.6 μm per minute.
(4) The substrate 1 was taken up from the melt, and rotated at 1000 rpm for 30 seconds to shake away surplus melt from the substrate. Thereafter, the substrate was gradually cooled to the room temperature at a cooling rate of 1 with sodium and magnesium contents of about 4 μm thickness was formed on the substrate.
(6) Further, the optical waveguide was kept at 1000 in the water vapor atmosphere, and was coupled by at laser beam at TM mode with a wavelength of 0.515 μm, where its waveguide mode was a single one with a propagation loss of less than 1 dB per cm. It has been found as a result of measurement that a deterioration with age of an emitting light was not changed at least during the time of 24 hours.
(7) A silica buffer layer was placed on the optical waveguide 2 obtained in the above process by sputtering technique, and an aluminium electrode was formed on the waveguide by the photolithography and vapor deposition technique. An electro-optic constant (r33) of this optical waveguide was 30 pm/V, which was substantially equal to a value of a bulk LiNbO.sub.3 monocrystal as a result of a practical measurement.
EXAMPLE 3 (1) 5 mol % of MgO with respect to a theoretical quantity of LiNbO.sub.3 which could be separated from the melted body composition was added to a mixture consisting of Li.sub.2 CO.sub.3 40.5 mol %, V.sub.2 O.sub.5 50.5 mol %, Nb.sub.2 O.sub.5 9.0 mol %, and Na.sub.2 CO.sub.3 23.4 mol % relative to Li.sub.2 CO.sub.3, a resultant mixture was put into a platinum crucible, and the mixture in the crucible was heated up to 1050 and the crucible melted under the air atmosphere within the epitaxial growth forming system. The resultant obtained melt in the crucible was stirred at 100 rpm for 19 hours by means of a propeller.
(2) A (0001) face of an LiTaO.sub.3 monocrystal having a thickness of 2 mm was polished to prepare a substrate.
(3) Next, the melt was gradually cooled down to 915 rate of 60 preliminarily heated at 938 the melt for 2.5 minutes with being rotated at a rotating speed 20 rpm. A growth rate of the LiNbO.sub.3 was 1.6 μm per minute.
(4) The substrate 1 was taken up from the melt as described above, rotated at a rotating speed of 1000 rpm for 30 seconds to shake away a surplus melt from the substrate. Thereafter, the substrate was gradually cooled to the room temperature at cooling rate of 1 result, an LiNbO.sub.3 monocrystalline thin film with Na and Mg contents of about 4 μm thickness has been formed on the substrate.
(5) The Na and Mg contents contained in the thus obtained LiNbO.sub.3 monocrystalline thin film were 3 mol % and 2 mol % respectively. A lattice length ("a" axis) the thin film was 5.756A, a extraordinary refractive index measured by a guided light with a wavelength of 0.83 μm was 2.163.+-001.
(9) Further, this optical waveguide was kept at 1000 hours in a water vapor atmosphere, and was coupled to the laser with the wavelength of 1.55 μm at TM mode. Its waveguide mode was a single one with a propagation loss of less than 1 dB per cm. A deterioration with age of an emitting light was not changed at least for 24 hours as a result measurement.
(10) A silica buffer layer was placed on the optical waveguide 2 obtained in the above process by sputtering technique, and an aluminium electrode was formed on the waveguide by the photolithography and vapor deposition technique. An electro-optic constant (r33) of this optical waveguide was 30 pm/V, which was substantially equal to a value of a bulk LiNbO.sub.3 monocrystal as a result of a practical measurement.
EXAMPLE 4 (1) 5 mol % of MgO with respect to a theoretical quantity of LiNbO.sub.3 which could be separated from a melted body composition was added to a mixture consisting of Li.sub.2 CO.sub.3 39.7 mol %, V.sub.2 O.sub.5 46.0 mol %, Nb.sub.2 O.sub.5 14.3 mol %, and Na.sub.2 CO.sub.3 14.5 mol % relative to Li.sub.2 CO.sub.3, a resultant mixture was put into a platinum crucible, and the mixture in the crucible was heated up to 1050 and melted under the air atmosphere within the epitaxial growth forming system. The resultant obtained melt in the crucible was stirred at 100 rpm for 21 hours by means of a propeller.
(2) For a substrate, a (0001) face of an LiTaO.sub.3 monocrystal with a thickness of 1 mm was polished.
(3) Next, the melt was gradually cooled down to 940 rate of 60 preliminarily heated at 940 1 minute into the melt while with being rotated at 25 rpm. A growth rate of the LiNbO.sub.3 was 1 μm per minute.
(4) The substrate 1 was taken up from the melt, and rotated at 1000 rpm for 30 seconds to shake away a surplus melt from the substrate. Thereafter the substrate was gradually cooled to a room temperature at a cooling rate of 1 containing Na and Mg of about 1 μm thickness was formed on the substrate.
(5) The Na and Mg contents contained in the thus obtained LiNbO.sub.3 monocrystalline thin film were 3 mol % and 2 mol % respectively. A lattice length ("a" axis) of the thin film was 5.156A, an extraordinary refractive index measured by an incident light with a wavelength of 0.83 μm was 2.163.+-001.
(9) Further, this optical waveguide was kept at 1000 hours in a water vapor atmosphere, and was coupled to the laser with the wavelength of 0.515 μm at TM mode. Its waveguide mode was a single one with a propagation loss of less than 1 dB per cm. A deterioration with age of an emitting light was not changed at least for 24 hours as a result of measurement.
(10) A silica buffer layer was placed on the optical waveguide obtained in the above process by sputtering technique, and an aluminium electrode was formed on the waveguide by the photolithography and vapor deposition techniques. An electro-optic constant (r 33) of this optical waveguide was 30 pm/V, which was substantially equal to a value of a bulk LiNbO.sub.3 monocrystal as a result of a practical measurement.
COMPARATIVE EXAMPLE 1 (1) Ti was formed by a photolithography and a sputtering techniques with a thickness of approximately 100Å on a Z face position of an LiNbO.sub.3 monocrystal plate having a C axis in a thickness direction and having a size of about 5 such Z face being the same as when the waveguide was formed in the embodiment 1, and thereafter thermal treated at 1000 hours in the water vapor atmosphere, Ti was thermal diffused. Thus, an optical waveguide was formed.
COMPARATIVE EXAMPLE 2 (1) Ti was formed by a photolithography and a sputtering techniques with a thickness of approximately 100Å on a Z face position of an LiNbO.sub.3 monocrystal plate having a C axis in a thickness direction and having a size of about 5 Z face being the same as when the waveguide was formed in the embodiment 1, and thereafter thermal treated at 1000 water vapor atmosphere, Ti was thermal diffused. Thus, an optical waveguide was formed.
(3) This waveguide was kept at 1000 vapor atmosphere, and again coupled to a laser beam with the wavelength of 1.55 μm at the TM mode. In this process, no output intensity of the guided light was found.
COMPARATIVE EXAMPLE 3 (1) For a substrate, a (0001) face of an LiTaO.sub.3 monocrystal with a thickness of 2 mm was polished, thereafter as shown in FIG. 1, a groove was formed on a portion where the waveguide 2 was formed. The size of the groove and the like were made the same as in the example 1.
(2) A mixture consisting of Li.sub.2 CO.sub.3 50 mol %, V.sub.2 O.sub.5 40 mol %, and Nb.sub.2 O.sub.5 10 mol % was heated to 1000 made a melt.
(3) Next, this melt was gradually cooled to 915 rate of 60 preliminary heated at 915 melt and rotated at 30 rpm for 4 minutes. A growth rate of the LiNbO.sub.3 was 1 μm per minute.
(4) The substrate was taken up from the melt described above, and then rotated at 1000 rpm for 30 seconds to shake away a surplus melt from the substrate. Thereafter, the substrate gradually cooled to the room temperature at a cooling rate of 1 LiNbO.sub.3 monocrystalline thin film with a thickness of about 4 μm was formed on the substrate.
(6) A surplus portion of the LiNbO.sub.3 monocrystalline thin film was etched by the ion beam form a ridge type LiNbO.sub.3 monocrystalline waveguide. A shape of the waveguide had a width of 10 μm with an etching depth of 1 μm.
(8) A silica buffer layer was placed on the optical waveguide 2 obtained in the above process by sputtering technique, and aluminium electrode was formed on the waveguide by the photolithography and vapor deposition technique. The optical waveguide was measured of its electro-optic constant, which was 3 pm/V, which was found to be equal to 1/10 the value of the bulk LiNbO.sub.3 monocrystal.
COMPARATIVE EXAMPLE 4 (1) For a substrate 1, a (0001) face of an LiTaO.sub.3 monocrystal of a thickness of 2 mm was polished for preparation, and chemical etched, thereafter, as shown in FIG. 1, a groove was formed on a portion where the waveguide 2 is formed. A size of the groove and the like were made the same design as in Example 1.
(2) A mixture material consisting of Li.sub.2 CO.sub.3 50 mol %, V.sub.2 O.sub.5 40 mol %, and Nb.sub.2 O.sub.5 10 mol % was heated to 1000 to form a melt.
(3) Next, this melt was gradually cooled to 915 rate of 60 preliminary heated at 915 into the melt and rotated at 30 rpm for 4 minutes to be gradually cooled. A growth rate of the LiNbO.sub.3 was 1 μm per minute.
(4) The substrate 1 was taken up from the melt, and then rotated at 1000 rpm for 30 seconds to shake away a surplus melt from the substrate. Thereafter the substrate was gradually cooled to the room temperature at a cooling rate of 1 monocrystalline thin film with a thickness of about 4 μm has been formed on the substrate material.
(6) A surplus portion of the LiNbO.sub.3 monocrystalline thin film was etched by the ion beam to form a ridge type LiNbO.sub.3 monocrystalline waveguide. A shape of the waveguide had a width of 10 μm with an etching depth of 1 μm.
(8) When this optical waveguide was kept at 1000 the water vapor atmosphere, a crack occurred on the film.
COMPARATIVE EXAMPLE 5 (1) A Z face of an LiNbO.sub.3 monocrystalline substrate having a C axis in a thickness direction and having a size of about 5 thickness of 0.5 mm was formed with Ta mask of a thickness of approximately 1 μm a width of 3 μm, and pattern by a photolithography, RIE techniques.
The resultant of the above process was dipped in H.sub.4 P.sub.2 O.sub.7 and treated at 250 the proton-exchanged waveguide was formed.
(3) This waveguide was kept at 1000 vapor atmosphere and again coupled to a laser beam with the wavelength of 1.55 μm at a TM mode. In this process, no output intensity of the guided light was found.
As apparent from a result shown in Table 1, the optical waveguide according to the invention can exhibit a superior characteristic than the conventional optical waveguide. More specifically, in the waveguide of the invention, since the lattice length is matched between the LiNbO.sub.3 monocrystalline waveguide 2 and the LiTaO.sub.3 crystal 1, then lattice defects are not formed in the LiNbO.sub.3 monocrystalline waveguide 2. Further, unlike the case of the diffused optical waveguides such as the Ti diffusion LiNbO.sub.3 optical waveguide and the proton exchanged optical waveguide and the like, in the optical waveguide according to the invention, impurity to form the waveguide is not diffused, thus resulting in an upgraded anti-annealing characteristic.
EXAMPLE 5 (1) 2 mol % of MgO with respect to a theoretical quantity of LiNbO.sub.3 which could be separated from a melted body composition was added to a mixture consisting of Na.sub.2 CO.sub.3 22 mol %, Li.sub.2 CO.sub.3 28 mol %, V.sub.2 O.sub.5 40 mol % and Nb.sub.2 O.sub.5 10 mol %, a resultant produced mixture was put into a platinum crucible, and heated up to 1100 system.
(2) A (0001) face of an LiTaO.sub.3 monocrystal with a thickness of 2 mm was polished. Next, a groove was formed by the ion beam etching on a portion where the waveguide 2 was formed, as shown in FIG. 1. The groove was of a rectangular shape having a width of 8 μm with a depth of 1 μm, and a branch angle was 1.5.degree..
The melt was gradually cooled down to 915 60 at 915 for 8 minutes. A growth rate of the LiNbO.sub.3 was 1 μm per minute.
(3) The substrate 1 was taken up from the melted body, and a surplus melt was shaken away by rotating the by substrate at 1000 rpm for 30 seconds. Thereafter the substrate gradually cooled to the room temperature at cooling rate of 1 monocrystalline thin film with Na and Mg contents of a thickness of about 5 μm was formed on the substrate.
(4) The Na and Mg contents in the obtained LiNbO.sub.3 monocrystalline thin film were 3 mol % and 2 mol % respectively. A lattice length ("a" axis) of the thin film was 5.156, a extraordinary refractive index measured by a guided light with a wavelength of 0.83 μm was 2.164.+-001.
(5) Further, a surplus portion was removed by grinding the LiNbO.sub.3 monocrystalline thin film. Thus, an embedded type LiNbO.sub.3 monocrystalline waveguide with a depth of 4 μm was formed.
(6) A silica buffer layer was placed on the optical waveguide 2 obtained in the above process by sputtering technique, and aluminium electrode 7 was formed on the waveguide by the photolithography and vapor deposition techniques. An electro-optic constant (r 33) of this optical waveguide was 30 pm/V, which was substantially equal to a value of a bulk LiNbO.sub.3 monocrystal as a result of a practical measurement. The electrode shape was a planer electrode construction having a lengthy portion of 15 mm covering the waveguide, and an interval between the electrodes was 20 μm.
EXAMPLE 6 (1) A (0001) face of an LiTaO.sub.3 monocrystal of a thickness of 2 mm was polished, and then chemically etched. As is the method in the example 1, the resultant was dipped for 5 minutes into a cooled melt. A growth rate of the LiNbO.sub.3 was 1 μm per minute.
(2) A substrate 1 was taken up from the melt, and a surplus melt was shaken away by rotating the substrate at 1000 rpm for 30 seconds. Thereafter, substrate was gradually cooled to the room temperature at a cooling rate of 1 with Na and Mg contents of a thickness about 5 μm was obtained on the substrate material.
(3) An adjustment of film thickness was performed by grinding the LiNbO.sub.3 monocrystalline thin film to produce an LiNbO.sub.3 monocrystalline thin film having a thickness of 2.8 μm. Next, a ridge type LiNbO.sub.3 monocrystalline waveguide having a width of 5 μm with a level difference of 1.8 μm was formed by the photolithography and ion beam etching technique.
EXAMPLE 7 (1) The Mach-Zehnder type optical modulator was produced as is the case of the example 2, but except of a waveguide size and an electrode size.
This waveguide size meant a size of a ridge type LiNbO.sub.3 monocrystalline waveguide of, a width 6 μm, thickness 2.5 μm, and level difference 2.0 μm.
EXAMPLE 8 In the Mach-Zehnder type optical modulator produced by the same method as is the case of example 3, the half wavelength voltage (a voltage for changing a phase difference by π) was measured.
COMPARATIVE EXAMPLE 6 (1) Ti with a thickness of approximately 100Å was formed by a photolithography and a sputtering techniques on a Z face portion of an LiNbO.sub.3 monocrystalline plate having a C axis in as a thickness direction and having a size of about 5 mm, a position on such Z face being the same as when the waveguide was formed in the example 1, and thereafter was subjected to thermal treated at 950 thermal diffused, thus the optical waveguide was formed.
COMPARATIVE EXAMPLE 7 (1) A (0001) face of an LiTaO.sub.3 monocrystal of a thickness of 2 mm was polished, thereafter a groove was formed by the ion beam etching on a portion where the waveguide 2 shown in FIG. 1 was formed. The groove was a rectangle shape having a width of 8 μm and depth of 1 μm with a branch angle of 1.5.degree..
(2) A mixture consisting of Li.sub.2 CO.sub.3 50 mol %, V.sub.2 O.sub.5 40 mol %, and Nb.sub.2 O.sub.5 10 mol % was heated to 1000 formed a melt, and thereafter this melt was gradually cooled to 915 substrate 1 was preliminary heated at 915 then dipped into the melted body for 8 minutes in a process of being gradually cooled and being rotated at 100 rpm. A growth rate the LiNbO.sub.3 was 1 μm per minute.
(3) The substrate 1 was taken up from the melt, and rotated at 1000 rpm for 30 seconds to shake away a surplus melt from substrate. Thereafter, the substrate gradually cooled to the room temperature at a cooling rate of 1 film with a thickness of about 8 μm was formed on the substrate material.
(4) A surplus portion of the LiNbO.sub.3 monocrystalline thin film was etched by the ion beam to form a ridge type LiNbO.sub.3 monocrystalline waveguide 2.
As is apparent from the examples hereinbefore described, the optical modulator according to the invention has an advanced modulation efficiency and a stable amplitude modulation. Since the lattice lengths of the LiNbO.sub.3 monocrystalline waveguide 2 and the LiTaO.sub.3 monocrystal 1 was matched with each other, then no lattice defect arised in the LiNbO.sub.3 monocrystalline waveguide 2, a refractive index of the LiNbO.sub.3 monocrystalline waveguide 2 was not changed largely from an original value even when a variation of the refractive index due to an electric field was repeated.
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