Optical signal-electric signal converter

An optical signal to electrical signal converter of the present invention comprises an optical waveguide propagating a modulated optical signal therethrough; a pair of electrodes disposed at positions opposite to each other sandwiching the optical waveguide with in a region where an electric field reaches that is generated in the optical waveguide when the optical signal propagates through the optical waveguide; and a resonator coupled to the pair of electrodes, the resonator receiving for excitation an electrical signal induced at the pair of electrodes by the electric field.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Section 371 of International Application No. PCT/JP03/007831, filed Jun 19, 2003, which was published in the Japanese language on Dec. 31, 2003, under International Publication No. WO 2004/001500 A1, the disclosure of which is incorporated herein by reference.

1. Technical Field

The present invention relates to an optical signal to electrical signal converter using a nonlinear optical effect.

2. Background Art

Conventionally, as a device for converting an optical signal to an electrical signal there have been widely used an electronic tube represented by a photo-multiplier and a semiconductor photodetector represented by a photodiode. The electronic tube is a device which detects an optical signal utilizing the “external photo-electric effect” and the semiconductor photodetector detects an optical signal utilizing the “internal photo-electric effect” in the semiconductor.

The electronic tube has high detection sensitivity (signal amplification factor) and is often used still currently in uses for physics and chemistry, however, is large and needs a high-voltage power source for operation. Therefore, the electric tube is mostly not used in uses as a photodetector for optical communications.

In contrast, because the semiconductor photodetector is small and consumes a little electric power, it is used in a wide range of fields including the optical communication field. Among semiconductor photodetectors, pin-type photodiodes (pin-PD) are inexpensive and are used for various uses. However, avalanche photodiodes (APD) capable of responding at a high speed are used for high-speed optical communications. In recent years, pin-type photodiodes with an improved response speed have been also developed. Then, the current situation is such that these types of semiconductor photodetectors can be used with substantially no problem at the current communication speeds (bandwidth <60 GHz band).

However, there is a problem that the semiconductor photodetector can not respond sufficiently in an ultra-high frequency band where the communication speed exceeds 100 GHz. This is because the response speed of the semiconductor photodetector is limited by the mobility of electric carriers generated by the input of an optical signal.

For the pin-type photodiode, pairs of an electron and a hole are generated when light is incident on a light-absorbing layer of the photodiode. The mobility of the hole is smaller than that of the electron. The delay time that determines the response speed of the photodiode is limited by the drift speed of holes. In this manner, the response speed of the semiconductor photodetector is determined by factors including the carrier mobility inherent to the semiconductor material, the voltage applied and the drift length. However, even when these parameters are further increased, it is considered difficult to improve the response speed up to a response speed with which an optical signal modulated at a speed exceeding 100 GHz can be accurately detected.

The present invention was conceived in order to solve the above problem and the primary objective thereof is to provide an optical signal to electrical signal converter capable of converting an optical signal modulated at a high speed into an electrical signal.

DISCLOSURE OF THE INVENTION

An optical signal to electrical signal converter of the present invention comprises an optical waveguide for receiving and propagating a modulated optical signal; and a pair of electrodes disposed at positions opposite to each other sandwiching the optical waveguide within a region where an electric field applies, said electric field being generated in the optical waveguide by a nonlinear optical effect when the optical signal propagates through the optical waveguide.

In a preferred embodiment, the optical signal to electrical signal converter further comprises a resonator coupled to the pair of electrodes. The resonator is capable of be excited by an electrical signal induced at the pair of electrodes by the electric field.

In a preferred embodiment, the optical signal contains a side band signal corresponding to a modulation frequency fm.

In a preferred embodiment, the optical waveguide is formed on a dielectric substrate or in the dielectric substrate, with the electrodes being supported by the dielectric substrate.

In a preferred embodiment, at least a portion of the optical waveguide and at least a portion of the dielectric substrate are formed from a nonlinear optical material and generate the electric field by an optical rectifying effect when the optical signal propagates through the optical waveguide.

In a preferred embodiment, the optical signal to electrical signal converter further comprises an electromagnetic wave radiating device coupled to the resonator and radiates the electrical signal as a radio signal.

In a preferred embodiment, the resonator and the electromagnetic radiating device are integrated with the substrate.

In a preferred embodiment, the resonator and the electrodes are connected by micro strip lines formed on the dielectric substrate.

In a preferred embodiment, the modulation frequency of the optical signal is 10 GHz or higher.

In a preferred embodiment, the optical signal to electrical signal converter further comprises a light beam input portion coupled to the optical waveguide.

In a preferred embodiment, the nonlinear optical material is a material selected from a group consisting of lithium niobate (LiNbO3), lithium tantalate (LiTaO3)-based material, potassium titanyl phosphate (KTiOPO4)- based material, rare earth-calcium oxyborate (RECa4O(BO3)3, RE: a Rare Earth element)-based material, DAST (4-dimethylamino-N-methyl-4-stilbazorium-toxyrate) and 3RDCVXY (dicyanovinyl termination-dimethyl substitution-diazo).

In a preferred embodiment, the optical waveguide has a periodic polarization inversion structure where the polarization direction is different from the polarization direction in the other portion.

In a preferred embodiment, the optical signal to electrical signal converter further comprises a resistor connecting electrically the pair of electrodes with each other.

In a preferred embodiment, the optical signal to electrical signal converter further comprises a housing accommodating the dielectric substrate.

BEST MODE FOR CARRYING OUT THE INVENTION

In the present invention, an optical signal is converted into an electrical signal without utilizing drifts of carriers excited by incident light, but by utilizing a nonlinear optical effect. Therefore, the response speed is not limited by the drift speed of the majority of carriers.

In the following, the principle of the operation of the optical signal to electrical signal converter according to the present invention will be described.

The polarization of a material having a nonlinear optical effect is represented by the following Eq. 1.
D=εE+PNLEq. 1
where D is an electric displacement vector (the electric flux density), ε is a dielectric constant, E is an electric field and PNLis nonlinear polarization.

As represented by Eq. 1, the electric displacement vector D is normally a sum of the nonlinear polarization PNLand the product of the dielectric constant ε and the electric field E. The term for the nonlinear polarization PNLcan be represented by the following Eq. 2 taking into consideration only the quadratic nonlinear optical effect.
PNL=χ(2)E·EEq. 2
where χ(2)is the quadratic nonlinear polarizability.

The light incident on the nonlinear optical material is assumed to be represented as the sum of two (2) electric fields E1and E2represented in the following Eq. 3.
E1=E01cos(ω1t−κ1r+φ1),
E2=E02cos(ω2t−κ2r+φ2)  Eq. 3
where ω1and ω2are frequencies of light beams, t is the time, κ1and κ2are wave numbers of the light and φ1and φ2are phases of the light.

Using Eq. 3, the square of the electric field E in Eq. 2 is represented as follows.
E·E=(E1+E2)·(E1+E2)=E012cos2(ω1t−κ1r+φ1)+2E01E02cos(ω1t−κ1r+φ1)·cos(ω2t−κ2r+φ2)+E022cos2(ω2t−κ2r+φ2)  Eq. 4

Term A of Eq. 5 is a term for optical rectification. Term B and Term C represent generation of the secondary harmonics, Term D represents generation of the summed frequency, and Term E represents generation of the differential frequency.

According to the present invention, using the effect represented by Term E among the nonlinear optical effects represented in Eq. 5, an optical signal is converted into an electrical signal. This point will be described in detail as follows.

For a light beam (having the central frequency of 1.5 μm) modulated with a signal having the central frequency of, for example, 26 GHz band by an optical modulating device, a peak called “side band” is generated at a position 0.19 nm away from the central frequency. In general, representing the frequency of the modulating signal as fmHz, the frequency λsbat which the side band is generated is represented as the following Eq. 6.
λsb=λC+Δλ
Δλ=λC−CλC/(C+fmπ)=fmλC2/(C+fmλC)   Eq. 6

C: the light velocity

λc: the central frequency of the light beam

fm: the frequency of the modulating signal

The optical signal to electrical signal converter of the present invention carries out the conversion into a modulated signal by the generation of the differential frequency (Eq. 7) between the wavelength λsbof this side band and the central wavelength λc.
ωm=ωsb=ωC
That is,
1/λm=1/λsb−1/λC=fm/CEq. 7

ωm: the angular frequency of the modulating signal

ωsb: the angular frequency of the side band

ωC: the angular frequency of the central frequency

ωm: the wavelength of the modulating signal

ωsb: the wavelength of the side band

ωC: the central wavelength

Here, for convenience, the case where two light beams each having a frequency (wavelength) different from each other are input into a nonlinear optical material is considered. However, the case where one light beam having one frequency (wavelength) is input may be considered similarly to the above case.

Now, a preferable embodiment of the optical signal to electrical signal converter according to the present invention will be described.

FIRST EMBODIMENT

First, referring toFIG. 1, the configuration of the optical signal to electrical signal converter of the embodiment will be described.

The optical signal to electrical signal converter of the embodiment has a dielectric substrate101formed from a nonlinear optical material, an optical waveguide102formed on the upper face of the substrate101and a pair of electrodes103and104provided at positions opposite to each other sandwiching the optical waveguide101on the upper face of the substrate101.

An optical signal to be detected is incident on the input portion102aof the optical waveguide102and propagates through the optical waveguide102from the left to the right in the figure. At this time, an electric field is generated by the differential frequency generation effect among the nonlinear optical effects. The pair of electrodes103and104is provided within a region where the electric field generated in the optical waveguide102reaches.

According to the composition of the embodiment, variation of the electric field generated when the optical signal propagates through the optical waveguide102from the left to the right in the figure can be detected through the electrodes103and104. As has been described, this electric field is formed in the optical waveguide and in the vicinity of the optical waveguide by the differential frequency generation of the nonlinear optical effect. In order to convert the optical signal into an electrical signal by generating such a differential frequency, the optical signal inputted needs to be a signal modulated such that the optical signal has a side band signal.

In the embodiment, lithium niobate (LiNbO3) substrate may be preferably used as the dielectric substrate101. The material of the substrate101is not limited to lithium niobate (LiNbO3), and lithium tantalate (LiTaO3), potassium titanyl phosphate (KTiOPO4), rare earth-calcium oxyborate (RECa4O(BO3)3, RE: a Rare Earth element), DAST (4-dimethylamino-N-methyl-4-stilbazorium-toxyrate) or 3RDCVXY (dicyanovinyl termination-dimethyl substitution-diazo) may also be used.

Next, a method for manufacturing the optical signal to electrical signal converter shown inFIG. 1will be described.

First, ultrasonic cleaning is applied to the substrate101in a liquid such as distilled water, acetone or alcohol. Thereafter, ultrasonic cleaning is applied to the substrate101also in acetic acid for one minutes or less. Again, ultrasonic cleaning is applied to the substrate101in a liquid such as distilled water, acetone or alcohol.

Next, a resist mask for defining the position and the shape of the optical waveguide102is formed on the upper face of the substrate101using a photo-lithography method. Thereafter, a Ti film is deposited on the resist mask using an electron beam deposition method. The thickness of the Ti film is set at, for example, 40-50 nm.

Next, the portion of the Ti film except the area where the optical waveguide102is to be formed on is removed using a lift-off method. In this manner, a Ti film patterned to define the area where the optical waveguide is formed. The method to form the Ti film is not limited to the electron beam deposition method, and sputtering such as the RF magnetron sputtering method may be used.

Next, the substrate102on which the patterned Ti film is present on the surface is loaded into a tube furnace and Ti is diffused in the surface region of the substrate102. The tube furnace has a heater and a quartz tube heated by this heater. The substrate101is set on a quartz boat placed in the quartz tube. As the ambient gas in the quartz tube, Ar gas containing steam and having the humidity of 80% or more is used for the first five hours of the diffusion process. After the first five hours, the ambient gas is switched to O2gas containing steam and having the humidity of 80% or more and the substrate101is heated for around one hour. The temperature for heating is set at, for example, around 1,000° C. The reason why the substrate101is heated in the O2atmosphere for the last one hour of the Ti diffusion process is in order to compensate the oxygen defects generated in the substrate101.

In this manner, the optical waveguide102is formed on the substrate101. The method for forming the optical waveguide102is not limited to the Ti diffusion method, and methods in which transition metals such as V (vanadium), Ni (nickel) and Cu (cupper) are respectively diffused may be used. Otherwise, a method in which protons exchange is carried out by dipping the substrate101in the melted salt of benzoic acid for around 24 hour may be employed.

When an organic nonlinear optical material such as DAST (4-dimethylamino-N-methyl-4-stilbazorium-toxyrate) or 3RDCVXY (dicyanovinyl termination-dimethyl substitution-diazo) is used for the substrate101, it is preferable to form the optical waveguide using a refractive index variation method (photo-bleaching method) employing illumination of a UV light beam.

The width and the depth of the optical waveguide102are both around 5 μm in this embodiment. However, the width and the depth of the optical waveguide102are optimized with the wavelength of the optical signals to be guided.

Next, the electrodes103and104extending along the optical waveguide102are formed. More specifically, first, an aluminum film is deposited on the upper face of the substrate101formed with the optical waveguide102, using the electron beam deposition method. The material for the electrode is not limited to aluminum, and simple substances or alloys of platinum, gold, titanium, germanium etc. may be used. After depositing the metal film or a film of another conductive material, the electrodes103and104can be formed by patterning the conductive film using various methods. The patterning of the electrodes103and104may be carried out using the lift off method.

It is preferable to form a thin film made of SiO2, HfO2or SiN, that works as a protective film, over the whole area of the upper face of the substrate101before forming the electrodes103and104.

Next, a terminal resistor105(50 Ω) is connected respectively with one end of each of the electrodes103and104such that the terminal resistor105bridges the electrodes103and104and the converter shown inFIG. 1is completed. Relaxation of the difference in the phase velocity between an optical signal and an electrical signal can be achieved by the terminal resistor105as the electrodes of a traveling-wave-type optical modulator.

The effective nonlinear optical constant deffof a nonlinear optical material is proportional quadratically to the power of a generated electrical signal as represented in Eq. 8.
P=Adeff2L2P1P2[(sinx)/x]2/n1n2n3λ3Eq. 8

deff: nonlinear optical constant

L: the length of a crystal

P1: the power of an input light beam1

P2: the power of an input light beam2

n1: refractive index to the input light beam1

n2: refractive index to the input light beam2

n3: refractive index to an output (a square of the dielectric constant)

λ3: the wavelength of the output

The above input light beam1is a signal at the central frequency of an optical signal inputted into the waveguide and the input light beam2is a signal of the side band.

From the above, it is preferable to form the optical waveguide using a material having a high effective nonlinear optical constant deff. Generally, organic nonlinear optical materials have higher effective nonlinear optical constants deffthan inorganic nonlinear optical materials. Therefore, the detection sensitivity for optical signals is more improved and conversion efficiency from an optical signal into an electrical signal is more enhanced when an organic nonlinear optical material is used. The effective nonlinear optical constant deffof LiNbO3crystal that is one of the inorganic crystal having a relatively high effective nonlinear optical constant is around 30 pm/V. In contrast, the effective nonlinear optical constant deffof DAST crystal that is one of organic crystals is 1,000 pm/V that is a high value of 30 times as high as or higher than the effective nonlinear optical constant deffof LiNbO3. Therefore, DAST is preferably used as the material for the substrate or the optical waveguide of the embodiment.

An effective nonlinear optical constant deffvaries depending on the direction of incidence of a light beam. Therefore, the direction of incidence of a light beam is preferably in the x-y plane when a lithium-niobate-based or a lithium-tantalate-based crystal is used.

For not only the niobate-based nonlinear optical crystal but also effective nonlinear optical crystals, each of the equations representing the incident angle of a light beam to a crystal and the effective nonlinear optical constant deff, using the crystal system (the point group and the space group) that the crystal has is different between the two kinds of crystals. Therefore, it is necessary to select an angle at which the effective nonlinear optical constant deffbecomes maximal according to the kind of the crystal.

For example, for a LiNbO3crystal, because the crystal is a uniaxial crystal and has a point group32, Eq. 9 representing the crystalline angle and the effective nonlinear optical constant is represented as follows.
deff=d11cos 2θsin 3∅  Eq. 9
where θ is the angle formed by the z-axis and a component projected onto the x-z plane of the dielectric principal axis in the incident direction of the light beam, and ∅ is the angle formed by the x-axis and a component projected onto the x-z plane of the dielectric principal axis in the incident direction of the light beam.

The optical signal to electrical signal converter according to the present invention can further reduce the difference in the phase velocity between an optical signal and an electrical signal by introducing a polarization inversion structure into the optical waveguide section. The optical signal to electrical signal converter can also carry out pseudo matching of velocities by introducing the polarization inversion structure. Then, a higher effective nonlinear optical constant deffcan be obtained than the case where matching of velocities is carried out by the incident angle of the light beam into the crystal. The sensitivity can be improved and the conversion efficiency from an optical signal to an electrical signal can be made higher by introducing the polarization inversion structure.

SECOND EMBODIMENT

Next, referring toFIG. 2, a second embodiment equipped with an optical waveguide having a structure in which polarization is periodically inverted will be described.

The basic structure of the embodiment is almost same as the structure of the converter shown inFIG. 1except an optical waveguide203having a polarization inversion structure202. That is, an optical signal to electrical signal converter of the embodiment has a substrate201, the optical waveguide203formed on the upper face of the substrate201and a pair of electrodes204and205provided at positions opposite to each other on the upper face of the substrate201. When an optical signal incident upon the input portion203awhich is located at an end of the optical waveguide203propagates through the optical waveguide203from the left to the right in the figure, an electric field is generated by the differential frequency generation effect among the nonlinear optical effects. The pair of electrodes204and205is provided within a region where the electric field generated in the optical waveguide202reaches. One end of each of the electrodes204and205is connected with each other by a terminal resistor206.

The material, the size and the manufacturing method of this converter are basically same as those described for the first embodiment. Because the different point of the embodiment from the first embodiment is that the polarization inversion structure202is manufactured, this point will be described as follows.

In the embodiment, first, metal electrodes are deposited on the substrate201using the electron beam deposition method. More specifically, comb-type electrodes are formed on the upper face of the substrate201and a front-face electrode is formed on the back face of the substrate201. As the material for the metal electrodes, simple substances or alloys of aluminum, platinum, gold, titanium, germanium, nickel or etc. is preferably used. The comb-type electrodes are manufactured by patterning the electrodes with photolithography and etching techniques after depositing the metal film on the substrate201. However, the comb-type electrodes may be formed using the lift off method after forming a patterned resist mask on the substrate and depositing a metal film.

After the comb-type electrodes have been completed, the direction of the polarization in a specific area of the optical wave guide202is inverted against the direction of the polarization in the other area by forming an electric field between the electrodes on the upper face of the substrate201and the electrode on the back face of the substrate201.

The polarization inversion period Λ=2LCis calculated from the following Eq. 10 or Eq. 11.
LC=λm/2(ng−nm)  Eq. 10
LC=½fm(1/vm−1/vg)  Eq. 11
where LCis the coherence length, ngis the refractive index of a light beam, nmis the refractive index of an electric wave, vgis the group velocity of the light beam, vmis the phase velocity of the electric wave, λmis the wavelength of an electromagnetic wave and fmis the frequency of the electric wave.

In the embodiment, a He—Ne laser beam is used as the optical signal and, because fmis 26 GHz, vmis 6.4×107m/s, vgis 1.36×108m/s, the coherence length is 2.4 mm and the polarization inversion period is 4.7 mm.

After forming the polarization inversion structure202, ultrasonic cleaning is applied to the substrate201in a liquid such as distilled water, acetone or alcohol. The manufacturing process thereafter is same as the process described for the first embodiment.

In each of the above embodiments, the arrangement for detecting an electrical signal through the electrodes is not limited especially. The electrical signal may be amplified using a known high-sensitivity detection circuit. However, because the electrical signal converted from an optical signal is very weak in the optical signal to electrical signal converters in the above embodiments, it is preferable to equip a mechanism for amplify the very weak signal easily.

Description will be given below of an embodiment in which the electrical signal converted from an optical signal is amplified using a resonator.

THIRD EMBODIMENT

Now, a third embodiment of the optical signal to electrical signal converter according to the present invention will be described. In the embodiment, a polarization inversion structure is introduced into the optical waveguide section as well as an antenna (electromagnetic wave radiation device) is connected to the electrodes through a dielectric resonator. The periodic polarization inversion structure in the embodiment has the same structure as the structure in the second embodiment.

Referring toFIG. 3, the converter of this embodiment will be described.

The converter has a substrate301accommodated in the housing309. Similarly to the second embodiment, an optical waveguide302having the periodic polarization inversion structure is formed on the substrate301. A pair of electrodes303aand303bis formed positions opposite to each other sandwiching the optical waveguide302on the upper face of the substrate301. When an optical signal is incident on the input portion located at an end of the optical waveguide302propagates through the optical waveguide302from the left to the right in the figure, an electric field is generated by the nonlinear optical effect. The pair of electrodes303aand303bis provided within a region where the electric field generated in the optical waveguide302reaches.

A dielectric resonator304equipped with an electromagnetic wave radiation mechanism306is provided on a section located on an optical outgoing side308. A terminal resistor305(50 Ω) is formed on an optical incidence side307.

Next, referring toFIG. 4(a) toFIG. 4(c), the configuration of the converter of the embodiment will be described more specifically.FIG. 4(a) is a perspective view showing the main portion, in which the resonator is removed from the optical signal to electrical signal converter of the embodiment.FIG. 4(b) is a cross-sectional view of the main portion along the line A-A′ inFIG. 4(a) andFIG. 4(c) is a cross-sectional view of the main portion along the line B-B′ inFIG. 4(a).

As shown inFIG. 4(b), a polarization inversion structure403is formed in the optical waveguide302on the substrate301accommodated in the housing309. The polarization inversion structure403is a structure in which areas where the polarization direction of the material of the substrate is inverted are arranged periodically. The cycle of the arrangement is set at a length equal to the coherent length of an optical signal input. The optical waveguide302in the embodiment is designed to propagate an optical signal having the wavelength of 633 nm.

The housing is a case made of metal covering the bottom face and the side faces of the substrate301and cutouts are formed in portions each corresponding respectively to the sections for the optical signals to enter and go out of the substrate301. In order to reduce the influence of the external electromagnetic waves, the housing309preferably has a shape such that the upper face of the substrate301is covered with a cover section (not shown).

As shown inFIG. 4(c), the electrodes303(303aand303b) are formed along the optical waveguide302and can detect a very weak electric field generated in the optical waveguide302. A portion of each of the electrodes303aand303bin the embodiment is buried inside the substrate301from the upward. However, the electrodes may not have such a structure. For example, electrodes obtained by patterning a metal film deposited on the upper face of the substrate301may be used.

Next, referring toFIG. 5(a) andFIG. 5(b), a dielectric resonator section and an electromagnetic field radiation mechanism will be described.FIG. 5(a) shows the schematic composition of the dielectric resonator.FIG. 5(b) is a cross-sectional view of the dielectric resonator along the line A-A′ inFIG. 5(a).

A dielectric resonator304comprises a metal housing501for insulating the electromagnetic field inside the resonator from the exterior and a high-dielectric-constant dielectric503arranged inside the housing501. A material having a relatively low dielectric constant (for example, a material having a commercial name, Teflon (registered trade mark)) is inserted between the high-dielectric-constant dielectric503and the housing501. The high-dielectric-constant dielectric503is surrounded and held by this low-dielectric-constant material.

The high-dielectric-constant dielectric503of the embodiment is separated into two sections and a slit505is formed in the spacing between the two sections in order to enhance the electric field inside the resonator. Furthermore, the housing501is provided with a slit504at a position opposite to the slit505of the high-dielectric-constant dielectric503in order to radiate electromagnetic waves.

As shown inFIG. 6, the high-dielectric-constant dielectric503of the dielectric resonator304is electromagnetically coupled to micro strip lines502aand502b.FIG. 6is a perspective view showing the converter of the embodiment shown inFIG. 3. As can be seen fromFIG. 6, one end of each of the micro strip line502aand502bis respectively formed on each of alumina substrates602aand602band is connected respectively with a branch line of the electrodes303aand303bthrough bonding wires605aand605b.

An electrical signal induced at the electrodes303aand303bwhen an optical signal propagates through the optical waveguide is transmitted to the micro strip lines502aand503athrough the bonding wires605aand605bshown inFIG. 6and is guided to the inside of the dielectric resonator304. The various parameters of the dielectric resonator304are designed such that this electrical signal resonates inside the dielectric resonator304.

In order to accumulate efficiently the energy of the electrical signal using the resonance inside the dielectric resonator304, it is preferable to provide a switch for selectively open and close the slit505, to the dielectric resonator304. Because leak of the electromagnetic field to the exterior is suppressed and the Q value of the resonator is increased while this switch is closed, the energy of the electrical signal is amplified. The electrical signal having the energy amplified in the resonator is radiated to the exterior of the resonator as an electromagnetic wave by opening the switch.

The dielectric constant becomes drastically low relative to the inside of the resonator at the slit505while the slit505is open. Therefore, the electromagnetic field of the electrical signal propagated through the high-dielectric-constant dielectric503becomes drastically enhanced at the portion at the slit505. Because the slit504of the housing501is arranged in the vicinity of the portion where the electric field is enhanced in this manner, the electrical signal propagated through the micro strip lines502aand502bis converted into an electromagnetic wave and is radiated from the slit504to the exterior of the resonator.

The dielectric resonator304in the embodiment is designed to resonate in TM11δ mode. For example, when an electrical signal at 26 GHz is radiated as an electromagnetic wave, the length of the slit504is set at approximately 3 mm and the width of the slit504is set at 0.6 mm.

Assuming the dielectric constant of vacuum ε0is 24 and the relative dielectric constant at the slit504εris 1, the electric flux density D is equal to εrε0E and is constant both inside the dielectric resonator304and in the portion at the slit504. Therefore, the electric field E is enhanced to a magnitude 24 times as strong as the original magnitude when the electric field goes out of the inside of the resonator having a high relative dielectric constant to the exterior of the resonator through the slit504. Because the electric field energy is accumulated inside the dielectric resonator304, the generated electric field energy itself can be enhanced by the resonator. In the embodiment, the Q value of the dielectric resonator304can be set at around 2,000.

When the substrate301is formed from a DAST crystal, the electric field generated between the electrodes when an optical signal propagates through the optical waveguide is around 80 μV/m. However, the electric field can be enhanced to a magnitude exceeding 2,000 times as strong as the original magnitude by using the resonator of the embodiment.

The high-dielectric-constant dielectric503is formed from, for example, a MgYiO3—CaTiO3-based ceramic. The cross section of the high-dielectric-constant dielectric503is 1 mm×1 mm and the length in the longitudinal direction of the high-dielectric-constant dielectric503is 5 mm.

The housing501in the embodiment has a shape of, for example, a rectangle having a cross section of 3 mm×3 mm and the length in the longitudinal direction of 15 mm. The housing501has a structure in which PTFE is filled with between the housing501and the high-dielectric-constant dielectric503.

Dielectric ceramic materials represented by Zr—TiO4.BaTiO3may be used for the high-dielectric-constant dielectric503. Because the dielectric constant differs according to the dielectric material, the dimensions of the housing501and the dimensions of the dielectric resonator304need to be changed.

The resonator in the embodiment is designed to resonate in TM11δ mode. However, the resonator may be designed to resonate in another mode such as TE10mode. Furthermore, in the embodiment, though the slit504working as the electromagnetic wave radiation mechanism is provided to the housing501of the resonator304, the amount of electric wave radiation can be increased by providing a conductive electrode in a state where the electrode is not grounded to the resonator housing. The slit504provided to the housing501of the resonator304operates as a slot antenna. However, the slit504may be operated as a dielectric antenna by further mounting another dielectric resonator to the slit504.

As described above, according to the optical signal to electrical signal converter of each of the above embodiments, an optical signal can be converted into an electrical signal by inputting the optical signal into an optical waveguide and propagating the optical signal through the optical waveguide. Then, the electrical signal is amplified by the resonator. Therefore, an ultra-high-speed-modulated light beam can be accurately detected.

Furthermore, the converter can be made compact by providing the optical waveguide, the electrodes and the antenna integrated on the base material. Therefore, when information is transmitted and received between various communication devices or between electric devices, it is easy to incorporate the converter into any of those devices.

As described above, according to the embodiments of the present invention, it is also easy to control communication devices and electric devices, by each other by radio, that can easily convert an optical signal sent through optical signal transmission means such as an optical fiber, into a radio signal. Furthermore, dissemination of “net home appliances” that are the existing home appliances controlled by radio can be further facilitated.

According to the above embodiments, an optical signal is detected using the optical waveguide. Therefore, the sensitivity and the conversion efficiency can be enhanced due to the pseudo velocity matching. Especially, when the periodic polarization inversion structure is used, the converter can be applied to the detection of an optical signal having an arbitrary wavelength and at an arbitrary frequency by properly designing the polarization inversion period, the optical waveguide and the dielectric resonator.

Needless to say, the various materials and the device compositions used in the present invention are not limited to the materials and compositions used in the embodiments described above. The optical signal to electrical signal converter of the present invention can be realized using materials other than the above-described dielectric materials and the nonlinear optical materials.

INDUSTRIAL APPLICABILITY

According to the present invention, high-speed responses can be achieved because an optical signal can be converted into an electrical signal without utilizing the drift of electric charges (carriers). Furthermore, a high-efficiency conversion from an optical signal to an electrical signal is realized by amplifying with a resonator an electrical signal converted from an optical signal and radiating the electrical signal from an antenna as an electromagnetic wave. According to the present invention, an optical signal to electrical signal converter that is small-sized and capable of high-speed operation can be provided.