Optical wavelength converting device

Claim 1

An optical wavelength converting device, comprising: a ferroelectric substrate having an upper surface of which the normal is inclined to a C-crystal axis toward an X-Y crystalline plane defined as (001) crystalline plane in Miller indices, spontaneous polarization of the ferroelectric substrate being directed toward a +C-crystal axis direction of the C-crystal axis; a plurality of inverted-polarization layers arranged at regular intervals in an upper surface region of the ferroelectric substrate, polarization of the inverted-polarization layers being directed toward a -C-crystal axis direction of the C-crystal axis, and the inverted-polarization layers being formed by exchanging positive ions included in the ferroelectric substrate for H⁺ ions; and an optical waveguide crossing alternate rows of the inverted-polarization layers and the ferroelectric substrate positioned between the inverted-polarization layers, a part of fundamental waves transmitting through the optical waveguide being converted into second harmonic waves, and a refractive index of the optical waveguide being higher than that of the ferroelectric substrate to confine the fundamental and second harmonic waves in the optical waveguide.

Claim 5

A device according to any one of claims 1 to 4 in which the difference in a refractive index between the inverted-polarization layers and the ferroelectric substrate positioned in the optical waveguide is 0.005 or less to prevent a transmission loss of the fundamental and second harmonic waves passing through the optical waveguide from being increased.

Claim 6

A device according to any one of claims 1 to 5 in which the regular intervals of the inverted-polarization layers range from 1 µm to 10 µm.

Claim 7

A device according to any one of claims 1 to 5 in which the regular intervals of the inverted-polarization layers range from 3 µm to 5 µm to obtain blue light consisting of the second harmonic waves on condition that a quasi-phase-matching condition Λ = λ f /{2*(N2ω-Nω)} is satisfied, where the symbol Λ denotes the regular intervals,the symbol λ f denotes a wavelength of the fundamental waves, the symbol N2ω denotes an effective refractive index of the optical waveguide for the second harmonic waves, and the symbol Nω denotes an effective refractive index of the optical waveguide for the fundamental waves.

Claim 8

A device according to any one of claims 1 to 7 in which the ferroelectric substrate is made of ferroelectric substance selected from the group consisting of pure LiTaO₃ crystal, LiTaO₃ crystal doped by MgO, Nb, or Nd, pure LiNbO₃ crystal, and LiNbO₃ crystal doped by MgO, Ta, or Nd.

Claim 9

A device according to any one of claims 1 to 8 in which each of the inverted-polarization layers is formed in a stripe shape.

Claim 10

A device according to any one of claims 1 to 9, additionally including: a plurality of gratings periodically arranged on the optical waveguide at grating intervals in parallel to the inverted-polarization layers, a first refractive index of the gratings being equal to N1; and a covering layer arranged between the gratings to produce a periodic structure, a second refractive index of the covering layer being equal to N2 which differs from the first refractive index N1 of the gratings to form a diffraction grating in the periodic structure, and the fundamental waves which are not converted into the second harmonic waves being reflected by the diffraction grating.

Claim 11

A device according to any one of claims 1 to 10, additionally including: a first electrode arranged on the ferroelectric substrate and the inverted-polarization layers positioned in the +C-crystal axis direction to the optical waveguide; a second electrode arranged on the ferroelectric substrate and the inverted-polarization layers positioned in the -C-crystal axis direction to the optical waveguide; and an electric source for applying an electric potential difference between the first and second electrodes, electric field being induced through the optical waveguide between the first and second electrodes applied the electric potential difference by the electric source to change a first refractive index of the inverted-polarization layers in the optical waveguide and a second refractive index of the ferroelectric substrate in the optical waveguide, increase or decrease of the first refractive index being opposite to that of the second refractive index to produce a diffraction grating formed of periodic change of the first and second refractive indexes in the optical waveguide, and the fundamental waves which are not converted into the second harmonic waves being reflected by the diffraction grating.

Claim 12

A device according to any one of claims 1 to 11, additionally including: a plurality of second inverted-polarization layers arranged in parallel to the inverted-polarization layers at second regular intervals to cross the optical waveguide, polarization of the inverted-polarization layers being directed toward the -C-crystal axis direction, and the second inverted-polarization layers being formed by exchanging positive ions included in the ferroelectric substrate for H⁺ ions; and a first electrode arranged on the ferroelectric substrate and the second inverted-polarization layers positioned in the +C-crystal axis direction to the optical waveguide; a second electrode arranged on the ferroelectric substrate and the second inverted-polarization layers positioned in the -C-crystal axis direction to the optical waveguide; and an electric source for applying an electric potential difference between the first and second electrodes, electric field being induced through the optical waveguide between the first and second electrodes applied the electric potential difference by the electric source to change a first refractive index of the second inverted-polarization layers in the optical waveguide and a second refractive index of the ferroelectric substrate in the optical waveguide, increase or decrease of the first refractive index being opposite to that of the second refractive index to produce a diffraction grating formed of periodic change of the first and second refractive indexes in the optical waveguide, and the fundamental waves which are not converted into the second harmonic waves being reflected by the diffraction grating.

Claim 13

A method of manufacturing an optical wavelength converting device, comprising the steps of: preparing a ferroelectric substrate having an upper surface of which a normal line is inclined to a C-crystal axis toward an X-Y crystalline plane defined as (001) crystalline plane in Miller indices, spontaneous polarization of the ferroelectric substrate being directed toward a +C-crystal axis direction; arranging first masks on the upper surface of the ferroelectric substrate at regular intervals; immersing the ferroelectric substrate with the first masks in a phosphoric acid solution to exchange positive ions of the ferroelectric substrate not covered with the first masks for H⁺ ions of the phosphoric acid solution, a plurality of proton exchange regions arranged at the regular intervals being formed in an upper region of the ferroelectric substrate not covered with the first masks; thermally processing the ferroelectric substrate and the proton exchange regions to diffuse the H⁺ ions densified in the proton exchange regions into the ferroelectric substrate at a first diffusion speed and diffuse heavy ions of the ferroelectric substrate into the proton exchange regions at a second diffusion speed lower than the first diffusion speed, the proton exchange regions being charged with negative electricity, the ferroelectric substrate surrounding the proton exchange regions being charged with positive electricity, and an electric field having a component directed in a -C-crystal axis direction being induced because of a difference in electricity between the proton exchange regions and the ferroelectric substrate to form inverted-polarization kernels having inverted polarization directed in the -C-crystal axis direction in boundary regions between the proton exchange regions and the ferroelectric substrate; continuing to thermally process the ferroelectric substrate and the proton exchange regions to grow the inverted-polarization kernels, the proton exchange regions and the ferroelectric substrate positioned under the proton exchange regions being changed to a plurality of inverted-polarization regions arranged at the regular intervals of which inverted polarization is directed in the -C-crystal axis direction opposite to the C-crystal axis direction; and forming an optical waveguide which crosses alternate rows of the inverted-polarization layers and the ferroelectric substrate positioned between the inverted-polarization layers, fundamental waves transmitting through the alternate rows of the optical waveguide being converted into second harmonic waves, and a refractive index of the optical waveguide being higher than that of the ferroelectric substrate to confine the fundamental and second harmonic waves in the optical waveguide.

Claim 17

A method according to any one of claims 13 to 16 in which the phosphoric acid solution utilized in the step of immersing the ferroelectric substrate includes is a dilute phosphoric acid solution obtained by mixing phosphoric acid with lithium base at a weight ratio of 20/80 of lithium base/phosphoric acid.

Claim 19

A method according to any one of claims 13 to 18 in which the step of thermally processing the ferroelectric substrate includes heating up the ferroelectric substrate and the proton exchange regions at a rising rate of 10 °C or more per a second until the ferroelectric substrate and the proton exchange regions reach a thermal processing temperature ranging from 500 °C to 600 °C, and the step of continuing to thermally process includes maintaining the ferroelectric substrate and the proton exchange regions at the thermal processing temperature for a thermal processing time limited to less than sixty seconds.

Claim 20

A method according to any one of claims 13 to 19 in which the step of forming an optical waveguide includes: thermally processing the ferroelectric substrate and the inverted-polarization layers at a temperature ranging from 450 °C to 550 °C for one hour or more to reduce a difference in a refractive index between the inverted-polarization layers and the ferroelectric substrate to 0.005 or less; arranging a second mask having an opening on the upper surface of the ferroelectric substrate and the inverted-polarization layers, the opening being positioned to cross alternate rows of the ferroelectric substrate and the inverted-polarization layers; and immersing the ferroelectric substrate with the second mask in another phosphoric acid solution to exchange positive ions of the ferroelectric substrate and the inverted-polarization layers positioned just under the opening of the first mask for H⁺ ions of the phosphoric acid solution, the alternate rows of the ferroelectric substrate and the inverted-polarization layers positioned just under the opening of the first mask being changed to the optical waveguide.

Claim 23

A method according to any one of claims 13 to 22 in which the first masks utilized in the step of arranging first masks are made of metal material selected from the group consisting of Ta, Au, Pt, Ti, Ag, W, and Ti₂O₅.

Claim 25

A method according to any one of claims 13 to 24 in which the regular intervals at which the first masks are arranged in the step of arranging first masks is limited to 10 µm or less.

Claim 26

A method according to any one of claims 13 to 25 in which each of the first masks arranged in the step of arranging the first masks is formed in a stripe shape to form each of the inverted-polarization layers in a stripe shape.

Claim 31

An apparatus according to any one of claims 27 to 30, addititionally including an optical system for converging the coherent light radiated from the semiconductor laser at an incident end facet of the optical waveguide while maintaining the coherent light polarized in a transverse electric mode.