JPH11202271A - Optical waveguide element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical waveguide element having a structure capable of simultaneously realizing low driving voltage characteristics and low propagation loss characteristics. SOLUTION: This element is provided with an SrTiO3 monocrystalline semiconductor substrate 2 to be a lower electrode to which the impurity element of from 0.001 wt.% to 0.1 wt.% is doped, epitaxial or single orientation optical waveguide 1, provided on the SrTiO3 monocrystalline semiconductor substrate 2, and an upper electrode 7 of a conductive or semiconductive thin film provided on the optical waveguide 1. In this case, this element can modulate, switch or deflect a light beam made incident to the optical waveguide by impressing a voltage between the upper electrode 7 and the lower electrode 2.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical waveguide device having an optical waveguide and electrodes for deflecting, switching or modulating a laser beam incident on the optical waveguide by an electro-optic effect. The present invention is particularly applicable to optical waveguides applicable to all optoelectronics including laser printers, digital copiers, optical deflecting elements for facsimile, optical switches and optical modulators for optical communications and optical computers, pickups for optical discs, etc. It relates to a waveguide element.

[0002]

2. Description of the Related Art As a laser beam light deflector used in a laser beam printer, a digital copier, a facsimile, etc., a rotating polygon mirror called a polygon mirror for deflecting a beam from a gas laser or a semiconductor laser, and a rotating polygon mirror. A lens composed of an fθ lens that condenses a laser beam reflected by a rotary polygon mirror on an image forming surface such as a photoconductor in a state of linear movement at a constant velocity is typically used. The light deflecting device using such a polygon mirror has a problem in durability because the polygon mirror is rotated at a high speed by a motor, and generates noise,
There is also a problem that the light scanning speed is limited by the number of rotations of the motor.

On the other hand, as a solid-state laser beam light deflector, there is a light deflector using an acousto-optic effect.
In particular, optical waveguide devices are expected. This optical waveguide device is being studied for application to a printer or the like as a laser beam light scanning device for solving the drawbacks of a laser beam light scanning device using a polygon mirror. This optical waveguide type optical deflection element has an optical waveguide made of LiNbO ₃ , ZnO, or the like, and means for coupling (incident) a laser light beam into the optical waveguide. It is equipped with a comb-shaped electrode for exciting surface acoustic waves to be deflected by the optical effect, and a means for outputting the deflected light beam from the optical waveguide. A lens or the like is added to the element. However, the optical deflection element using the acousto-optic effect generally has a problem of an upper limit of the laser deflection speed due to the deflection speed limit, and there is a limit in application to an image forming apparatus such as a laser printer, a digital copying machine, and a facsimile. .

On the other hand, a technique using an oxide ferroelectric material having an electro-optic effect having a higher modulation speed than the acousto-optic effect is disclosed in, for example, "A. Yariv, Optical Electronic
s, 4th ed. (New York, Rinehart and Winston, 1991) 33
6 to 339 ”are known. As such an element, there is a bulk element using a ceramic or a single crystal. However, since the dimensions are large and the driving voltage is considerably high, a practical deflection angle cannot be obtained. In addition, a prism-type domain inversion light deflection element or a prism-type electrode light deflection element in which prisms are arranged in a cascade using a LiNbO ₃ single crystal wafer on which a Ti diffusion type optical waveguide or a proton exchange type optical waveguide is manufactured is referred to as “Q.Chen”. , et al., J. Lightwave Tech.vo
l.12 (1994), p. 1401 ”(Reference 1) and JP-A-1-248.
No. 141, for example. However, LiNbO
_{(3) Since} the electrode spacing of about 0.5 mm, which is the thickness of the single-crystal wafer, is still required, the driving voltage is still high. In the above-mentioned reference 1, a deflection angle of only about 0.2 degrees can be obtained even with a driving voltage of ± 600 V. Therefore, there is a problem that a practical deflection angle cannot be obtained.

[0005]

On the other hand, the present inventor has an oxide optical waveguide having an electro-optical effect provided on a conductive substrate, and a light source for inputting a light beam into the optical waveguide. Then, a prism type optical deflecting element which solves the problem of the driving voltage by using a thin film optical waveguide provided with an electrode for deflecting the light beam in the optical waveguide by an electro-optic effect was invented. No. 5797.

However, the electromagnetic field distribution of the laser light propagating through the optical waveguide leaks into the substrate. The absorption coefficient of a substrate having a practical resistivity is large, and in many cases, the exudation component is strongly absorbed by carriers in the conductive substrate, so that the propagation loss in the thin film optical waveguide is caused by the scattering of the optical waveguide itself. In addition to the loss, the absorption becomes several tens of dB / cm, which is not practical. In general, in a device having a coplanar electrode arrangement, a cladding layer made of SiO ₂ is inserted between a metal electrode on the optical waveguide and the optical waveguide, thereby preventing seepage of an electromagnetic field into the metal electrode and absorbing propagation light. There are ways to avoid this. However, the relative permittivity of the oxide optical waveguide material having the electro-optic effect ranges from tens to thousands, and the relative permittivity of SiO ₂ is 3.
9, the effective voltage applied to the thin-film optical waveguide is only several percent or less of the applied voltage because a series capacitor is formed as an equivalent circuit in the thin-film optical waveguide structure on the conductive substrate. However, there has been a problem that the driving voltage eventually increases significantly.

An object of the present invention is to provide an optical waveguide device,
An object of the present invention is to provide a structure capable of simultaneously solving low drive voltage characteristics and low propagation loss characteristics. Another object of the present invention is to make the optical waveguide device usable for various deflection elements, switching elements, or modulation elements.

[0008]

The object of the present invention is to provide SrT doped with 0.001% by weight to 0.1% by weight of an impurity element.
iO ₃ single crystal semiconductor substrate or 0.0
Epitaxial or highly oriented ferroelectric thin film optical waveguide with electro-optic effect fabricated on a single crystal substrate having an epitaxial or highly oriented SrTiO ₃ semiconductor thin film doped on a surface of from 01% to 0.1% by weight When,
A conductive or semiconductive upper electrode on the thin film optical waveguide is provided, and a doped SrTiO ₃ single crystal semiconductor substrate serving as a lower electrode or a portion having a different refractive index by applying a voltage between the thin film and the upper electrode. It is achieved by generating, deflecting, switching, or modulating a laser beam in response to a voltage.

In the present invention, the element which can be used as an impurity dopant of a doped SrTiO ₃ single crystal semiconductor substrate as a lower electrode substrate or an epitaxially or highly oriented doped SrTiO ₃ semiconductor thin film is a SrTiO ₃ Sr site. Alternatively, any element having an ionic radius capable of substituting a Ti site and having a valence different from that of Sr or Ti may be used. Desirably, the Sr site takes 12 coordination with oxygen ions. Sc, Lu, Yb, Tm, Er, H
o, Y, Dy, Tb, Bi, Gd, Na, Eu, Sm,
Zn, Nd, Pr, Ce, La, In, K, Tl, R
Al, As, V, Ni, G capable of coordinating 6 with oxygen ions for Ti sites such as b and Cs
a, Sb, Co, Fe, Ta, Rh, Nb, Cr, M
n, Bi, Ru, In, Sc, Sn, Pu, Np, L
u, Yb, U, Tm, Er, Pa, Ho, Y, Dy,
Tb, Tl, Gd, Eu, Sm, Pm, Am, Nd, P
r, Ce, La, Th, Ac, etc., and more preferably, Sc, Y, L which is an element of Group III of the periodic table.
a, Ce, Pr, Nd, Pm, Sm, Eu, Gd, T
b, Dy, Ho, Er, Tm, Yb, Lu, Ac, A
V, Nb, T which are elements of l, Ga, In, or group V
a, Pa, As, Sb, Bi are selected, and more preferably, La or Nb can be used.

The amount of the impurity dopant is 0.001.
% To 0.1% by weight dope, preferably 0.005%
A weight percent to 0.05 weight percent dope is effective. The resistivity of the SrTiO ₃ semiconductor to which such doping is performed is 10 ⁶ Ω · cm or less, and preferably 10 ⁴ Ω · cm or less is effective in terms of RC time constant and voltage drop.
The light absorption coefficient of 100 or less, preferably 20 or less is effective from the viewpoint of the propagation loss of the optical waveguide.

A material which can be used as a substrate of an epitaxial or highly oriented SrTiO ₃ semiconductor thin film doped between 0.001% by weight and 0.1% by weight with an impurity element provided between a substrate and an optical waveguide is SrTiO. ₃ , Ba
TiO ₃ , BaZrO ₃ , LaAlO ₃ , ZrO ₂ , Y ₂ O ₃
8% -ZrO _2, MgO, oxides such as MgAl ₂ O _4,
Single semiconductor such as Si, Ge, diamond, etc., AlA
s, AlSb, AlP, GaAs, GaSb, InP,
InAs, InSb, AlGaP, AlLnP, AlG
aAs, AlInAs, AlAsSb, GaInAs,
II such as GaInSb, GaAsSb, InAsSb
IV compound semiconductor, ZnS, ZnSe, ZnT
e, CaSe, Cdte, HgSe, HgTe, CdS
Although it is possible to use II-VI based compound semiconductors, etc., it is often advantageous to use SrTiO ₃ for the film quality of the oxide thin film optical waveguide disposed on the top.

The thin-film optical waveguide material is selected from oxides. Specifically, in the case of the ABO ₃ type perovskite type, for example, BaTiO ₃ is used as a tetragonal system, orthorhombic system or pseudo-cubic system.
_{3, PbTiO 3, Pb 1-} x La x (Zr y Ti 1-y) 1-x / 4
O ₃ (PZT, PLT, PLZ depending on the values of x and y
T), Pb (Mg _1/3 Nb _2/3 ) O ₃ , KNbO _3, etc., a ferroelectric material represented by a hexagonal system such as LiNbO ₃ , LiTaO ₃ etc., and a tungsten bronze type Sr _x
Ba _1-x Nb ₂ O ₆ , Pb _x Ba _1-x Nb ₂ O _6, etc. In addition, Bi ₄ Ti ₃ O ₁₂ , Pb ₂ KNb ₅ O ₁₅ , K ₃ L
i ₂ Nb ₅ O _15, but is selected from such addition or substituted derivatives have similar perovskite structure with respect to doped SrTiO ₃ SrTiO ₃ semiconductor thin film single crystal semiconductor substrate or doped, the difference in lattice constant for these Pb _1-x La _x (Zr) having a higher refractive index than those of 2.399 and a high electro-optic coefficient.
_y Ti _1-y ) _{1-x / 4} O ₃ (0 <x <0.3, 0 <y <1.
0) is most desirable. The thickness of the thin film optical waveguide is usually set between 0.1 μm and 10 μm, but this can be appropriately selected depending on the purpose.

The upper electrode is made of Al, Ti, Cr, Ni, C
u, Pd, Ag, In, Sn, Ta, W, Ir, Pt,
It is possible to use various metal electrodes or alloys such as Au, or transparent oxide electrodes such as ITO or Al-doped ZnO having a smaller refractive index than the optical waveguide. When a cladding layer having a smaller refractive index than that of the optical waveguide is provided between the optical waveguide and the upper electrode, any material can be used for the upper electrode, but the drive voltage is increased. It is desirable to use a transparent oxide electrode such as The resistivity of the conductive or semiconductive thin film used as the upper electrode is 10 ⁶ Ω ·
cm or less, desirably 10 ⁴ Ω · cm or less is effective. However, if the resistivity is such that the voltage drop is negligible, it can be used as the upper electrode.

The thin film optical waveguide is a gas phase selected from electron beam evaporation, flash evaporation, ion plating, Rf-magnetron sputtering, ion beam sputtering, laser ablation, MBE, CVD, plasma CVD, MOCVD and the like. It is produced by a solid phase growth method of a thin film produced by a growth method and a wet process such as a sol-gel method or a MOD method.

In the optical waveguide device having the above-mentioned structure, a portion having a different refractive index is generated by applying a voltage between a conductive or semiconductive substrate or a thin film serving as a lower electrode and an upper electrode to generate a laser beam. The beam is deflected, switched, or modulated according to the voltage.

Next, the basic principle of the optical waveguide device of the present invention will be described in detail with reference to FIGS. FIGS. 1 and 2 show a general form of a slab type optical waveguide used in the optical waveguide element of the present invention. Here, the light is waveguide 1
1 in the z-direction. The waveguide 11 has a refractive index and a relative dielectric constant represented by n ₁ and ε ₁ , a medium 12 having a refractive index and a relative dielectric constant represented by n ₂ and ε ₂ , and a medium 12 having n ₃ and ε ₃ . Between the medium 13 having a refractive index and a relative dielectric constant. In FIG. 1, the x direction is a direction perpendicular to the surfaces of the media 12, 13 and the optical waveguide 11, and the y direction is defined as a direction perpendicular to the x direction and the z direction. As shown in FIG. 2, it is assumed that the coordinates of the boundary between the medium 13 and the optical waveguide 11 are x = 0, and the thickness of the optical waveguide 11 is d.

At this time, expj [ωt-β
The wave equation of the light wave propagating in [z] is as follows.

Δ ² (E _z , H _z ) / δ _x ² + δ ² (E _z , H _z ) / δ _y ² + χ _i ² (E _z , H _z ) / δ _z ² = 0. _{^{] (χ i 2 = k i}} 2 -β 2, k i 2 = ω 2 μ 0 ε i = k 0 2 n i 2, i =
1, 2, 3) where ω is the angular frequency of the light wave, μ ₀ is the magnetic permeability of vacuum, j
Is an imaginary number, and β is a propagation constant.

If the electromagnetic field is uniform in the y direction, e
omitting xpj [ωt-βz], E z, by placing a _{H z αF (x), [} 1] equation becomes wave equation as follows. d ² F (x) / _dy ² + χ _i F (x) = 0 [2]

Therefore, all electromagnetic field components are represented by exponential functions or trigonometric functions, and a uniform electromagnetic field in one direction can be expressed by a TE mode (E _z = 0) and a TM mode (H _z = 0).
And the electromagnetic field component is as follows:

Here, for the TE mode, the electromagnetic field in the region of the medium 12 (hereinafter, referred to as II region) and the region of the medium 13 (hereinafter, referred to as III region) is | x | = ｜
, III region: E _y3 = E ₃ exp (−γ ₃ x), x> 0... [3] I region: E _y1 = E ₁ cos (k _x x + φ ₃ ), − d <x <0... [4] II region: E _y2 = E ₂ exp {γ ₂ (x + t)}, x <−d... [5], and it can be seen that the electromagnetic field seeps into the substrate. . In addition,
The region of the optical waveguide 11 is an I region. _{Here, γ 3 = k 0 (N} 2 -n 3 2) 0.5 ··· [6] k x = k 0 (n 1 2 -N 2) 0.5 ··· [7] γ 2 = k 0 (N ² −n ₂ ² ) ^0.5 ... [8] From the boundary condition that the electric field components E _y and H _z are continuous at x = 0, E ₃ = E ₁ cos φ ₃ ... [9] tan φ ₃ = Γ ₃ / k _x ··· [10] Since the same applies to x = -d, the boundary condition is applied, and E ₂ = E ₁ cos (k _x d -φ ₃ ) ··· [11] tan (k _x t−φ ₃ ) = γ ₂ / k _x [12] From these relationships,

K _x d = (m + 1) π-tan ^-1 (k _x / γ ₂ ) -tan ^-1 (k _x / γ ₃ ) [13]

Here, m is a mode number (m = 0,
1, 2,. . . . . ). In addition to obtaining the electromagnetic field distribution by such an analytical method, BPM (Beam Propag
ation method) can be used to determine the electromagnetic field distribution.

Here, referring to FIG. 3, S having a refractive index of 2.40 is used.
A refractive index of 2.56 with a thickness of 600 nm on an rTiO ₃ substrate
In the _{Pb 1-x La x (Zr} y Ti 1-y) 1-x / 4 O 3 thin film optical waveguide structure provided, T at a wavelength of 633nm
FIG. 3 shows a schematic diagram of an E ₀ mode intensity distribution. The calculated value in this case is that 3.6% of the total light intensity is the substrate (the medium 12 in FIG. 1).
). At this time, the component that permeates the substrate due to the light propagation in the optical waveguide due to the light absorption of the substrate is absorbed, resulting in a propagation loss. The light intensity is the square of the amplitude represented by the formulas [3], [4], and [5].
The ratio I _s / I ₀ of the intensity in the substrate is as follows as the integrated value of the region II with respect to the sum of the integrated values of the regions I, II and III.

[0025]

(Equation 1)

At this time, in addition to scattering due to the surface of the optical waveguide and grain boundaries in the optical waveguide, and loss due to absorption of the optical waveguide itself, propagation loss caused by substrate absorption is expressed as follows. _{-10 · log (I / I in} ) = (10 - α · δ · z) ··· [15]

As described above, the propagation loss caused by the substrate absorption is determined by the absorption coefficient α of the substrate, and the relationship between the propagation loss and the thickness of the optical waveguide is as shown in FIG. Therefore, it is desirable that the absorption coefficient of the substrate is small. Also, it can be seen that the smaller the thickness of the thin film optical waveguide, the smaller the absorption coefficient required to obtain the same propagation loss.

FIG. 5 shows the refractive index at a wavelength of 633 nm.
SrTiO ₃ substrate of _{40 Pb 1-x La x (} Zr y Ti
_1-y ) The refractive index of the _{1-x / 4} O ₃ thin film optical waveguide and TE ₀ , TE ₁ ,
Shows the relationship between the TE ₂ mode of cut-off thickness. P
_{b 1-x La x (Zr} y Ti 1-y) 1-x / 4 O 3 thin film refractive index of which varies depending on the composition, ranges from about 2.45 2.70, being considered many 2 .49 to 2.65. When the refractive index is 2.45, the optical waveguide thickness is 300 n, which is larger than the cut-off film thickness in the TE ₀ mode.
m or more is required. Further, at a higher refractive index, the cut-off film thickness becomes smaller, and a smaller value is possible as the optical waveguide film thickness.
It is difficult to put an optical waveguide device having the following film thickness into practical use from the viewpoint of light coupling efficiency.

Therefore, a film thickness of 300 on the SrTiO ₃ substrate
FIG. 6 shows the relationship between the refractive index and the propagation loss of the thin-film optical waveguide of nm. Propagation loss due to substrate absorption as an optical waveguide element needs to be 20 dB / cm or less, preferably 10 dB / cm or less. Therefore, when the refractive index is 2.65 or less, the absorption coefficient is 20 or less, preferably 10 or less. It is. On the other hand, in order to achieve single mode, the thickness of the optical waveguide is T
It is necessary to below ₁ mode cutoff thickness of E, but the cut-off thickness of TE ₁ mode at 2.50, which is one of the refractive index to be considered many of which are 634 nm. T
In the case of the E ₀ single mode waveguide, this film thickness can be applied to a limited region having a refractive index of 2.50 or less.

Considering such a case, FIG. 7 shows the result of obtaining the relationship between the refractive index and the propagation loss of the thin film optical waveguide having a thickness of 600 nm on the SrTiO ₃ substrate. In order to obtain a propagation loss of 20 dB / cm or less as an optical waveguide element, the refractive index is in a range of 2.65 or less, and in order to have a propagation loss of 20 dB / cm or less, an absorption coefficient is 100 or less.
In order to make it less than dB / cm, the absorption coefficient needs to be 50 or less. As compared with the case of a thin film optical waveguide having a film thickness of 300 nm, as described above, the smaller the film thickness of the thin film optical waveguide has the same propagation loss. It can be seen that the absorption coefficient required to obtain must be smaller.

Next, FIG. 8 shows Pb _{1 -x} La _x (Z on a SrTiO ₃ substrate having a refractive index of 2.40 at a wavelength of 1310 nm.
r _y Ti _1-y ) _{1-x / 4} O ₃ Thin film optical waveguide refractive index and TE ₀ ,
The relationship between the cut-off film thickness in the TE ₁ and TE ₂ modes is shown. At long wavelengths, the cut-off film thickness increases, and when the refractive index is 2.45, the optical waveguide film thickness is about 600 nm or more, which is larger than the cut-off film thickness of the TE ₀ mode, and is one of the frequently studied refractive indices. Since the cut-off film thickness of the TE ₁ mode at a certain 2.50 is 1311 nm, the film thickness of the optical waveguide is 1300 to achieve the single mode.
nm or less is required. FIG. 9 shows the relationship between the refractive index and the propagation loss of the thin film optical waveguide having a thickness of 600 nm on the SrTiO ₃ substrate. Propagation loss as an optical waveguide element is 20 dB
/ Cm, preferably 10 dB / cm or less, so that the absorption coefficient is about 20 or less, preferably 10 or less when the refractive index is in the range of 2.65 or less. Wavelength 63
Compared to the case of 3 nm, the same film thickness of 600 nm
It can be seen that the value is almost equal to the value of the film thickness of 300 nm which is the same cut-off condition of the TE ₀ mode than the value of.

Therefore, at a wavelength of 633 nm, a film thickness of 600 nm
The absorption coefficient to reduce the loss even to the extent
From 00 or less, a condition of a thin film optical waveguide that can provide a propagation loss of 20 dB / cm or less appears. The absorption coefficient is 100 or less, preferably 50 or less, and more preferably 2 or less.
0 or less is required.

On the other hand, when the absorption coefficient decreases, that is, when the transparency increases, the resistivity generally tends to increase. Therefore, the deflection speed, switching speed, or modulation speed in the structure in which the thin film optical waveguide element is sandwiched between the substrate and the upper electrode is the allowable range of the substrate resistivity at a frequency response of 3 db point f determined by the RC time constant of the following equation. To consider.

F = 1 / (2π · RC) = 1 / (2π · ρ · _ds / S × ε ₀ · ε _r · S / d _w ) = 1 / (2π · ρ · ε ₀ · ε _{r × d s / d w)} ··· [16]

Here, R is the resistance of the substrate, ρ is the resistivity of the substrate, d _s is the thickness of the substrate, C is the capacitance of the optical waveguide, and ε ₀ is the dielectric constant of a vacuum of 8.854 × 10 ^{−. 14} (F / cm), ε _r
Is the relative permittivity, d _w is the thickness of the optical waveguide, and S is the electrode area. The limit of the thickness is 50μm about the thinness is for 50μm and d _s of the substrate, the relationship between the resistivity and the frequency response when the thickness of the optical waveguide was generally 1μm to d _w because it is about 1μm 10 Shown in The relative permittivity of the ferroelectric material is
Since it is from 0 to several thousand, the optical waveguide element is 1 kHz.
In order to exhibit the above response, the resistivity of the substrate must be 10 ⁶ Ω · c
m or less is required. When the response of the optical waveguide element is 1 kHz or less, it is the level of the response speed of the mechanical element,
The significance of an element using the electro-optical effect, which is originally high speed, is lost.

When a voltage for deflection, switching, or modulation is applied to a structure in which a thin film optical waveguide element is sandwiched between a substrate and an upper electrode, the thin film optical waveguide often has a voltage of 10 ^-7 A / cm. ^Two or more leak currents flow. For this reason, when the substrate resistance is large, the voltage drop on the substrate becomes remarkable, and the voltage applied to the thin film optical waveguide decreases. The voltage drop ΔV at the substrate will be examined based on the following equation.

ΔV = IR = j · S × ρ · d _s / S = j × ρ · d _s [17]

Here, I is the leak current, j is the leak current density, R is the resistance of the substrate, ρ is the resistivity of the substrate, _ds is the thickness of the substrate, and S is the electrode area. The thickness of the substrate is 500 μm
FIG. 11 shows the relationship between the resistivity and the voltage drop when d _{s is set} to 500 μm. In the structure of the thin-film optical waveguide element on the conductive substrate, low-voltage driving is possible, so that the driving voltage is at least 100 V or less. In order to make the voltage drop ΔV in the substrate negligible to 1% or less, ie, 1 V or less, with respect to this drive voltage, the substrate has a leakage current density of 10 ⁻⁷ A / cm ² and a resistivity of 10 ⁸ Ω ·
cm is required. In many cases, the driving voltage is 10 V or less, so that the voltage drop ΔV is 1% or less and 0.1 V or less, and the leakage current density is 10 ⁻⁷ A /
The resistivity of the substrate in cm ² is desirably 10 ⁷ Ω · cm or less. The leak current density is 10 ⁻⁵ A / cm ^2.
In many cases, the resistivity of the substrate should be 10 ⁶ Ω · cm or less. However, these leak current densities are the current densities in a steady state, and the current densities at the moment of charge and discharge of the optical waveguide are much higher. Therefore, at least the resistivity is required to be 10 ⁶ Ω · cm or less.

[0039]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An optical waveguide device according to a first embodiment of the present invention will be described with reference to FIGS. FIG.
2 is a top view of the optical waveguide device according to the present embodiment,
FIG. 13 is a side view thereof. In this embodiment, Nb 0.02% -doped S having a resistivity of 1 × 10 ² Ω · cm, an absorption coefficient of 50, a refractive index of 2.40, and a thickness of 500 μm.
An electro-optic coefficient of 50 pm / V, a refractive index of 2.56, and a rTiO ₃ (100) single crystal conductive lower electrode substrate
900 nm-thick epitaxial PZT (52/48)
By growing a thin film optical waveguide and further forming a prism type electrode, a prism type EO deflection element as shown in FIGS. 11 and 12 was produced.

The PZT (52/48) optical waveguide layer was formed by solid phase epitaxial growth using a sol-gel method. First, anhydrous lead acetate Pb (CH ₃ COO) ₂ , zirconium isopropoxide Zr (OiC ₃ H ₇ ) ₄ ,
And titanium isopropoxide Ti (OiC ₃
As H _{7) 4} starting materials, after the 2-methoxy-ethanol in the dissolution, distillation was carried out with refluxing, 0 in the final Pb concentration.
A 6M precursor solution for PZT (52/48) was obtained. Further, this precursor solution was spin-coated on an Nb-doped SrTiO ₃ substrate. All of the above operations were performed in an N ₂ atmosphere.

Next, the temperature was raised at 20 ° C./sec in an O ₂ atmosphere, maintained at 350 ° C., maintained at 650 ° C., and finally, the electric furnace was turned off and cooled. As a result, a first-layer PZT thin film having a thickness of 100 nm was grown by solid phase epitaxial growth. By repeating this process eight more times, an epitaxial PZT thin film having a total thickness of 900 nm was obtained. The crystallographic relationship is unidirectional PZT (100) // N
b-SrTiO ₃ (100), in-plane orientation PZT [00]
1] // Nb—SrTiO ₃ [001] was obtained. After poling this PZT thin film optical waveguide,
On a PZT thin film optical waveguide, 15 prism-shaped upper electrode arrays of 200 μm in height and 200 μm in height with a base of 100 nm made of an ITO thin film formed by a Rf sputtering method with a thickness of 200 nm were formed.
An O deflection element was manufactured. Also, Nb-doped SrTiO ₃
Ohmic contact to the substrate was obtained by In.

Here, when an electric field is generally applied to a material having an electro-optic effect, the refractive index change due to the electric field in a crystal structure having no center of symmetry is as follows: Δn = n ⁰ −n = −aE−bE ² - ... [18] among the primary term is called the Pockels (Pockels) effect, generally described as ^{follows, Δn = -1 / 2rn 3 E} ··· [19] second order terms car This is called the (Kerr) effect and is generally expressed as follows. Δn = − ／ Rn ³ E ² [20]

In practice, as the electric field is increased, the refractive index changes in such a manner that the Kerr effect, which is a secondary electro-optical effect, gradually overlaps the Pockets effect, which is a primary electro-optical effect, as the electric field is increased. When such an electro-optic effect is used, a ferroelectric material having a crystal structure without a center of symmetry and having a high coefficient is used, and an oxide ferroelectric material as described above is typical. As in the case where a local electric field is applied to such a ferroelectric, a decrease in the refractive index of that portion occurs.

In this embodiment, an Nb-doped SrTiO ₃ (100) substrate, which is a lower electrode provided with a triangular prism-shaped electrode 7 at the position shown in FIGS. And the upper electrode IT
When a voltage V is applied between the prism and the O electrode, a refractive index change of Δn = − ／ · r · n ³ · (V / d) (21) occurs, and the length of the prism is reduced by L. And the width is W

Θ = −Δn × L / W = １ ／ · r · n ³ · (V / d) · (L / W) [22]

The following deflection occurs. The following is a description of a prism type optical deflection element using a ferroelectric material having a Kerr effect, which is a secondary electro-optic effect. θ = １ ／ · r · n ³ · (V / d) ² · (L / W) [23]

First, in order to evaluate the optical waveguide characteristics,
Laser light of 633 nm was introduced into the PZT thin-film optical waveguide 1 of the optical waveguide device according to the present embodiment by the prism coupling, and the scattered light intensity distribution in the TE ₀ mode in the light propagation direction was measured by an optical fiber. When the light propagation loss was calculated from the slope of the relationship between the logarithm of the scattered light intensity and the light propagation distance, the light propagation loss of the PZT optical waveguide 1 according to the present embodiment was 17.5 dB / cm, which is a practical level. showed that.

Next, a laser beam having a wavelength of 633 nm is applied to the prism type EO deflecting element of this embodiment with a width of 1 mm.
After that, the light was introduced into the PZT thin-film optical waveguide 1 via the prism 5. The applied laser beam 6 is applied with a voltage between the lower Nb-doped SrTiO ₃ substrate electrode and the ITO upper prism electrode 7 to generate different refractive indexes in the lower portion of the prism electrode and in the other portions. Deflected. The deflected laser beam was emitted from the end face as an outgoing beam 8, and the deflection angle was determined from the displacement of the laser spot position on the projection plane.

On the other hand, when the deflection speed was measured, the frequency response was 31.0 kHz. Nb-doped S of the present embodiment
The relative dielectric constant of the PZT thin-film optical waveguide grown on the rTiO ₃ substrate was measured. As a result, it was 1000. Therefore, the frequency response determined by the RC time constant from equation [16] was 32.4 kHz, which was almost the same as the actually measured value. . The leak current density is 2 × 10 ⁻⁷ A / cm ² , and the equation [16]
The required voltage drop ΔV at the substrate was 1 × 10 ⁻⁶ V, which was a negligible level. As described above, the optical waveguide device according to the present embodiment worked effectively.

Next, a first comparative example of this embodiment will be described. In this comparative example, the resistivity is 5 × 10 ⁻² Ω.
Cm, absorption coefficient 170, refractive index 2.40, thickness 5
00 μm Nb 0.5% doped SrTiO ₃ (100)
An electro-optic coefficient of 50 is formed on a single crystal conductive lower electrode substrate.
An epitaxial PZT (52/48) thin film optical waveguide having a pm / V, a refractive index of 2.56, and a film thickness of 900 nm was grown. The PZT (52/48) optical waveguide layer was produced by solid phase epitaxial growth using a sol-gel method in the same manner as in the first embodiment.

However, as in the first embodiment, T
E ₀ mode propagation loss where measurements were made of, it was not possible to attenuate a large propagation loss is obtained. Since the light propagation loss due to substrate absorption is 22 dB / cm according to the simulation, the propagation loss of the PZT thin film optical waveguide of this comparative example is considered to be 25 dB / cm or more in consideration of the propagation loss due to scattering of the PZT thin film optical waveguide. .

Next, a second comparative example of this embodiment will be described. In this comparative example, the resistivity is 1 × 10 ⁷ Ω
Cm, absorption coefficient 2, refractive index 2.40, thickness 50
0 μm Nb 0.0005% doped SrTiO ₃ (10
0) An epitaxial PZT (52/48) thin film optical waveguide having an electro-optic coefficient of 50 pm / V, a refractive index of 2.56, and a thickness of 900 nm was grown on a single crystal conductive lower electrode substrate. The PZT (52/48) optical waveguide layer was produced by solid phase epitaxial growth using a sol-gel method in the same manner as in the first embodiment. A prism type EO deflecting element was manufactured by forming a prism type electrode on the surface of the PZT thin film optical waveguide in the same manner as in the first embodiment.

When the light propagation loss was determined in the same manner as in the first embodiment, the light propagation loss of the PZT optical waveguide of this comparative example was 4.5 dB / cm, which is a characteristic that is in a practical level. Next, in the same manner as in the first embodiment, a laser beam having a wavelength of 633 nm is applied to the prism type EO deflecting element of this comparative example.
The laser beam was deflected by introducing the beam through a prism into a PZT thin film optical waveguide and applying a voltage between the lower Nb-doped SrTiO ₃ substrate electrode and the ITO upper prism electrode. When the deflection angle was determined from the displacement of the laser spot position on the projection plane emitted from the end face, deflection of 1.00 degree was confirmed when 20 V was applied. However, when the deflection speed was measured, the frequency response reached only 0.30 Hz. Nb-doped SrTiO ₃ of this comparative example
The relative permittivity of the PZT thin film optical waveguide grown on the substrate is 10
Since it is 00, the frequency response determined by the RC time constant from Equation [16] is 0.32 Hz, which is almost the same as the actually measured value. Further, the leak current density is 1 × 10 ⁻⁷ A / cm ² , and the voltage drop ΔV on the substrate obtained from Expression [16]
Was a relatively large level of 0.05 V.

Next, an optical waveguide device according to a second embodiment of the present invention will be described. In this embodiment, 0.005% Nb-doped S having a resistivity of 5 × 10 ⁴ Ω · cm, an absorption coefficient of 5, a refractive index of 2.40 and a thickness of 50 μm.
An epitaxial PZT (52/48) thin film optical waveguide having a refractive index of 2.49 and a film thickness of 1500 nm was grown on an rTiO ₃ (100) single crystal conductive lower electrode substrate.
The PZT optical waveguide layer was formed by solid phase epitaxial growth using a sol-gel method in substantially the same manner as in the first embodiment. A prism type EO deflecting element was manufactured by forming a prism type electrode on the surface of the PZT thin film optical waveguide in the same manner as in the first embodiment.

When the light propagation loss of the PZT thin film optical waveguide of the present embodiment was determined, it was 6.1 dB / cm, which is a characteristic which is in a practical level. When the frequency response of the deflection of the laser beam is measured in the same manner as in the first embodiment, 1.1 is obtained.
kHz. Nb-doped SrTiO of the present embodiment
₃ The relative permittivity of the PZT thin film optical waveguide grown on the substrate is 1
Since it is 000, the frequency response determined by the RC time constant from equation [16] is 1.1 kHz, which is in agreement with the actually measured value. Further, the leak current density was 8 × 10 ⁻⁸ A / cm ² , and the voltage drop ΔV at the substrate obtained from the equation [16] was 2 × 10 ⁻⁵ V, which was a negligible level.

Next, an optical waveguide device according to a third embodiment of the present invention will be described. In the present embodiment, a lower portion of a single crystal Nb 0.01% doped SrTiO ₃ (100) single crystal having a resistivity of 1 × 10 ³ Ω · cm, an absorption coefficient of 30, a refractive index of 2.40, and a thickness of 500 μm. An epitaxial PLZT (9/65/35) thin film optical waveguide having a refractive index of 2.49 and a thickness of 600 nm was grown on the electrode substrate. The PLZT optical waveguide layer was produced by solid phase epitaxial growth using a sol-gel method in substantially the same manner as in the first embodiment. The crystallographic relationship is that of a single orientation PLZT (1
00) // Nb-SrTiO ₃ (100), in-plane orientation P
Structure of LZT [001] // Nb-SrTiO 3 [001] was obtained. A prism type EO deflection element was manufactured by forming a prism type electrode on the surface of the PLZT thin film optical waveguide in the same manner as in the first embodiment.

The PLZT thin-film optical waveguide according to the present embodiment has a light propagation loss of 16.8 dB / cm, which is a characteristic which is on a practical level. Also, when the deflection speed of the laser beam was measured, the frequency response showed 1.4 kHz. PZT grown on Nb-doped SrTiO ₃ substrate of the present embodiment
Since the relative permittivity of the thin film optical waveguide is 1500, the expression [1]
6], the frequency response determined by the RC time constant is obtained.
4 kHz, which was in agreement with the actually measured value. Further, the leak current density was 9 × 10 ⁻⁷ A / cm ² , and the voltage drop ΔV at the substrate determined from the equation [16] was 4.5 × 10 ⁻⁵ V, which was a negligible level.

Next, an optical waveguide device according to a fourth embodiment of the present invention will be described. In this embodiment, La 0.1% doped Sr having a resistivity of 1 × 10 ⁴ Ω · cm, an absorption coefficient of 16, a refractive index of 2.40 and a thickness of 50 μm.
On the lower electrode substrate of TiO ₃ (100) single crystal conductive,
An epitaxial PZT (85/15) thin film optical waveguide having a refractive index of 2.49 and a thickness of 1000 nm was grown. PZ
The T optical waveguide layer was formed by solid phase epitaxial growth using a sol-gel method in substantially the same manner as in the first embodiment. The crystallographic relationship is unidirectional PZT (100) // L
a-SrTiO ₃ (100), in-plane orientation PZT [00]
1] // La-SrTiO ₃ [001] was obtained. A prism type EO deflecting element was manufactured by forming a prism type electrode on the surface of the PZT thin film optical waveguide in the same manner as in the first embodiment.

The light propagation loss of the PZT thin-film optical waveguide of the present embodiment was 8.4 dB / cm, which is a characteristic which is on a practical level. Next, as in the first embodiment, a laser beam having a wavelength of 633 nm is introduced into the PZT thin film optical waveguide through the prism to the prism type EO deflecting element of the present embodiment, and the lower La-doped SrTiO ₃ substrate electrode is used. The frequency response was 7.0 kHz where the laser beam was deflected by applying a voltage between the electrode and the ITO upper prism electrode.
showed that. The relative permittivity of the PLZT thin-film optical waveguide grown on the La-doped SrTiO ₃ substrate of this embodiment is 500
Therefore, when the frequency response determined by the RC time constant was obtained from Expression [16], it was 7.2 kHz, which was almost consistent with the actually measured value. Further, the leak current density was 3 × 10 ⁻⁷ A / cm ² , and the voltage drop ΔV at the substrate obtained from the equation [16] was 1.5 × 10 ⁻⁵ V, which was a negligible level.

In the first to fourth embodiments, the prism type EO deflecting element has been described. Needless to say, the idea of the present invention is a Bragg reflection type switch, a total reflection type switch, and a directional coupling switch. , Mach-Zehnder interference switch, phase modulation element, mode conversion element, wavelength filter element, etc., can be similarly applied to all optical waveguide elements using the EO effect, and these thin-film optical waveguide elements also have low driving voltage characteristics and low propagation. A structure is provided that can simultaneously solve the loss characteristics.

[0061]

As described above, according to the present invention, a thin-film optical waveguide device having an electro-optical effect capable of simultaneously solving low drive voltage characteristics and low propagation loss characteristics can be realized. The optical waveguide device of the present invention can be used for all optical waveguide devices utilizing the electro-optic effect, including various deflection elements, switching elements, and modulation elements.

[Brief description of the drawings]

FIG. 1 is a diagram for explaining the principle of an optical waveguide device according to the present invention.

FIG. 2 is a diagram showing an electromagnetic field distribution in the optical waveguide device of the present invention.

FIG. 3 is a diagram showing an electric field distribution in the optical waveguide device of the present invention.

FIG. 4 shows SrT of various absorption coefficients at a wavelength of 633 nm.
iO ₃ is a diagram showing the relationship between transmission loss and film thickness of the PZT optical waveguide (refractive index = 2.56) on a substrate (refractive index = 2.40).

FIG. 5 is a diagram showing the relationship between the refractive index and the cut-off film thickness of a PLZT optical waveguide on a SrTiO ₃ substrate (refractive index = 2.40) at a wavelength of 633 nm.

FIG. 6 shows SrT of various absorption coefficients at a wavelength of 633 nm.
iO ₃ is a diagram showing the relationship between refractive index and propagation loss of the PLZT optical waveguide having a thickness of 300nm on the substrate (refractive index = 2.40).

FIG. 7 shows SrT of various absorption coefficients at a wavelength of 633 nm.
FIG. 9 is a diagram showing the relationship between the refractive index and the propagation loss of a 600 nm-thick PLZT optical waveguide on an iO ₃ substrate (refractive index = 2.40).

FIG. 8 is a diagram showing the relationship between the refractive index and the cut-off film thickness of a PLZT optical waveguide on a SrTiO ₃ substrate (refractive index = 2.40) at a wavelength of 1310 nm.

FIG. 9 shows Sr of various absorption coefficients at a wavelength of 1310 nm.
600 nm thickness on TiO ₃ substrate (refractive index = 2.40)
FIG. 4 is a diagram showing a relationship between a refractive index and a propagation loss of the PLZT optical waveguide of FIG.

FIG. 10 is a diagram showing a relationship between a substrate frequency and a driving frequency based on an RC time constant.

FIG. 11 is a diagram showing a relationship between a substrate resistivity and a voltage drop.

FIG. 12 is a top view of the EO prism type deflection element of the optical waveguide device according to the first embodiment of the present invention.

FIG. 13 is a side view of the EO prism type deflection element of the optical waveguide device according to the first embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Thin-film optical waveguide 2 Conductive substrate 3 Air or cladding layer 5 Incident prism 6 Incident beam 7 Prism electrode 8 Outgoing beam 9 Laser light source 10 Lens

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Nagagabe Eiji 430 Nakai-cho, Nakai-cho, Ashigara-kami, Kanagawa Prefecture Inside Fuji Xerox Co., Ltd. (72) Inventor Hiroro Moriyama 430 Sakai, Nakai-cho, Ashigara-gun, Kanagawa Green Tech-Nakai Fuji Xerox Corporation

Claims (10)

[Claims] (1) an impurity element containing from 0.001% by weight to 0.1% by weight;
A SrTiO ₃ single crystal semiconductor substrate serving as a lower electrode doped with weight%, an epitaxial or mono-oriented ferroelectric thin film optical waveguide formed on the surface of the single crystal semiconductor substrate, and formed on the optical waveguide An optical waveguide device comprising: a conductive thin film or a semiconductive thin film upper electrode. 2. The optical waveguide device according to claim 1, wherein the single-crystal semiconductor substrate has a surface on which an impurity element is doped with 0.001 impurity element.
An optical waveguide device having an epitaxial or unidirectionally oriented SrTiO ₃ semiconductor thin film doped with 1 wt% to 0.1 wt%. 3. The optical waveguide device according to claim 1, wherein said impurity element is a group III or V group element. 4. The optical waveguide device according to claim 1, wherein said impurity element is Nb. 5. The optical waveguide device according to claim 1, wherein said impurity element is La. 6. The optical waveguide device according to claim 1, wherein the single-crystal semiconductor substrate has a resistivity of 10 ⁶ Ω · cm or less. 7. The optical waveguide device according to claim 1, wherein a light absorption coefficient of said single crystal semiconductor substrate is 100 or less. 8. An optical waveguide device according to any one of claims 1 to 7, wherein the ferroelectric thin film optical _{waveguide, Pb 1-x La x (} Zr y T
i _1-y ) _{1-x / 4} O ₃ (0 <x <0.3, 0 <y <1.0)
An optical waveguide device, characterized in that: 9. The optical waveguide device according to claim 1, wherein said upper electrode is an oxide having a refractive index smaller than that of said optical waveguide. 10. The optical waveguide device according to claim 1, wherein a voltage is applied between said upper electrode and said lower electrode to modulate a light beam incident on said optical waveguide. An optical waveguide device characterized by switching or deflecting.