JPH09258285A - Optical frequency shifter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make a frequency shifter suitable for optical measurement by using one of the electrodes which generate low frequency surface acoustic waves(SAW) and emitting plural outgoing light beams having low frequency differences. SOLUTION: The shifter is formed by a single channel optical waveguide 34, a three branching channel optical waveguide 38 which has a multimode optical waveguide 41A, branching multimode optical waveguides 41B and 41C and light emitting channel optical waveguides 40A, 40B and 40C, all of which are formed on the surface layer of an LiNbO3 substrate. In the vicinity of the branching section of the waveguide 38, an inter-digital transducer(IDT) is formed which generates SAW whose mutual operating length with the light waves, that propagate in an optical waveguide with a frequency fSAW, becomes L.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical frequency shifter, and more particularly, to high stability and high stability for use in optical measurement and the like.
The present invention relates to an integrated optical frequency shifter having a compact structure and a simple structure.

[0002]

2. Description of the Related Art Conventionally, an optical frequency shifter is composed of an optical crystal having an acousto-optic effect, or an optical integrated circuit integrated on a substrate having an acousto-optic effect.

FIG. 5 (1982 Ultrasonics Symposium Pr
Oceedings, pp422-425) and FIG. 6 (JP-A-60-16).
6921) shows a conventional optical frequency shifter composed of an optical integrated circuit.

The optical frequency shifter shown in FIG. 5 forms a cross-channel optical waveguide 12 in which linear optical waveguides are crossed on a Y-cut LiNbO ₃ (lithium niobate) substrate 10 having an acousto-optical effect, and at the same time, cross-channels are formed. An interdigital finger electrode (interdigital transducer (IDT)) 14, which is an interdigital transducer that generates a surface acoustic wave (SAW), is provided near the intersection of the optical waveguides 12. The interdigital electrode 14 is connected to an oscillator (not shown) that generates a high frequency.

In FIG. 5, the cross channel optical waveguide 1
A light wave having a frequency f _opt is incident from one end P0 of
When a high frequency is applied to 14 to generate a SAW having a frequency f _saw and the light wave and the SAW are caused to interact with each other, the other end P1 of the cross channel optical waveguide 12 has a frequency f of 0th order light.
Non-diffracted light of _opt is obtained, and the cross channel optical waveguide 12
Diffracted light of frequency f _opt + f _saw shifted by the frequency f _saw of SAW is obtained at the other end P2. In this case, in order to satisfy the Bragg condition, it is necessary to generate SAW having a frequency of several hundred MHz.

The optical frequency shifter shown in FIG. 6 has a structure in which two asymmetric two-branch channel optical waveguides are coupled to each other, and each optical waveguide 16 has three emission ends P1, P2, and P3. _{Interdigitated} electrodes 18 and 20 for generating _SAWs of frequencies f _saw1 and f _saw2 are arranged.

In FIG. 6, one end P0 of the optical waveguide 16
Frequency f_optOf the interdigital electrodes 18, 2
High frequency is applied to each of 0_saw1, F_saw2S
Each AW is generated, and the light wave and these SAWs are made to interact with each other.
The output ends P1 and P of the optical waveguide 16
2 and P3 have frequency f_opt+ F_saw1, F_opt+ F
_saw2 , F_optLight is obtained. In this case, the frequency f
_saw1, F_saw2Two cross-finger electrodes that generate a SAW of 1
Since 8 and 20 are provided, the circumference of the two light waves emitted is
Wave number difference (| f_saw1−f_saw2｜) can be reduced
You.

[0008]

The Klein-Cook parameter Q, which is an index of diffraction efficiency, is defined by the following equation.

Q = (2πλL) / (Λ ² cos θ) (1) where λ is the wavelength of the light wave in the optical waveguide, Λ is the wavelength of the SAW, L is the interaction length between the light wave and the SAW, θ Is the diffraction angle.

Here, when Q >> 1 (about 20 to 100), Bragg diffraction, and when Q << 1 (about 0.1), Raman-
It becomes eggplant diffraction.

The optical frequency shifter shown in FIG. 5 is designed to operate by Bragg diffraction, and it is necessary to set the SAW frequency to several hundred MHz in order to cause Bragg diffraction. By doing so, the maximum efficiency in one diffraction operation can be 100%, but there is a problem that the oscillator and the signal processing circuit are complicated because the SAW frequency needs to be increased.

Further, a practical optical frequency shifter for optical measurement is required to be able to obtain outgoing light having an optical frequency difference of about several tens MHz with a simple structure.

However, in order to reduce the optical frequency difference between the two emitted lights, two IDTs are arranged as shown in FIG. 6 and several hundreds of M different in frequency from each other by several tens MHz.
There is a problem that the structure becomes complicated because two oscillators that oscillate at about Hz are required.

The present invention has been made in order to solve the above-mentioned problems, and one electrode for generating SAW having a low frequency is used to emit a plurality of emitted light having a low frequency difference. An object of the present invention is to provide a highly stable and compact optical frequency shifter suitable for optical measurement, which has a simple structure.

[0015]

In order to achieve the above object, an optical frequency shifter of the present invention comprises a single channel optical waveguide, an electrode for generating a surface acoustic wave, and one end of the channel optical waveguide. In addition, the propagating light wave is formed so as to be incident substantially parallel to the wavefront of the surface acoustic wave, and the intensities diffracted by the interaction with the surface acoustic wave are substantially equal to each other. It is configured by a three-branch optical waveguide having a three-branched optical waveguide for propagating light, and an emission light channel optical waveguide coupled to the other end of each of the three-branched optical waveguides. .

The optical frequency shifter of the present invention uses an acousto-optic medium and has 0.1 <2πLλ _O / n ₁ Λ ² cos θ _i <1.
The electrode and the three-branch optical waveguide can be configured so as to satisfy the conditions of 0 and 0 <πΔn ₁ L / λ ₀ <2.

Next, the principle of the present invention will be described. A surface acoustic wave (SAW) propagating at a wavelength Λ, a wave number K _saw , and an angular frequency Ω _s by defining xyz three-dimensional coordinates as shown in FIG.
With respect to wavelength λ _{O in} vacuum, wave number k _O , electric field intensity E _O
The (O) light wave is made incident on the wavefront of the SAW at an angle θ _i , and the light wave and the SAW are allowed to interact with each other at an interaction length L to obtain S
When the light wave is diffracted by the AW, the electric field intensity E (r, t) of the diffracted light that is the emitted light is expressed by the superposition of the m-th order diffracted light as shown in equation (2).

[0018]

[Equation 1]

Where r is the coordinate of the SAW traveling direction, t
Is time, m is the order of diffracted light, E _m (z) is the electric field strength in the z-axis direction of m-th order diffracted light, and ω _O is the angular frequency of incident light in a vacuum.

Further, the electric field intensity E _m (z) of the m-th order diffracted light in the equation (2) is expressed by the following (3) which expresses the coupling of many light waves.
It is obtained by solving the differential equation of the equation.

[0021]

[Equation 2]

Where α is (n ₁ Λ sin θ _i ) /
λ _O and Q are the above Klein · Cook parameters represented by 2πLλ _O / n ₁ Λ ² cos θ _i , and Δn ₁ is the SAW.
Of the refractive index n ₁ of the acousto-optic medium induced by E,
_{m + 1} (z) and E _m-1 (z) are the electric field strengths in the z-axis direction of the diffracted light of the m + 1st order and the m−1th order, respectively.

The refractive index n (r, t) of the acousto-optic medium at the point (r, t) is expressed as follows. n (r, t) = n 1 + Δn 1 sin (Ω s t-K saw · r) ··· (4) Here, as the light wave is propagated is substantially parallel to incident on SAW wavefront, 0.1 <Q <10 (preferably,
1 ≦ Q ≦ 2) and 0 <πΔn ₁ L / λ ₀ <2 (preferably πΔn ₁ L / λ ₀ ≈1), the high-order diffracted light of ± 2nd order or more is sufficiently small, There is a solution in which the electric field intensity of the non-diffracted light and the electric field intensity of the ± 1st-order diffracted light expressed by the equation (2) become substantially equal, that is, the electric field intensity becomes 1: 1: 1. The wavelength of the light wave, the wavelength of the SAW, the light wave and the SA
There are innumerable combinations of solutions depending on the interaction length L with W and the strength of SAW, but any combination may be used as long as it satisfies the relationships of the expressions (2) and (3).

The range of the parameter Q is set to 0.1 as described above.
In the range of <Q <10, an intermediate diffraction phenomenon between the Bragg diffraction (Q >> 1) and the Raman-Nass diffraction (Q << 1) can be obtained, which is an intermediate between the Raman-Nass diffraction and the Bragg diffraction. If the light is diffracted in a typical region, the SA with a frequency lower than the SAW frequency used when diffracting under the Bragg condition is used.
It is possible to diffract using W, and thus S
By lowering the AW frequency, the configuration of the signal processing system can be simplified when the optical frequency shifter is used for optical measurement or the like. Further, in the optical frequency shifter, the frequency of the incident light is shifted by the SAW frequency due to diffraction,
By shifting the frequency by the intermediate diffraction between the Bragg diffraction and the Raman-Nass diffraction as described above, it is possible to obtain a plurality of lights having a small frequency difference.

Further, from the above equation expressing the parameter Q,
Since the interaction length L is proportional to the parameter Q, the interaction length L can be made smaller than in the case of Bragg diffraction. This interaction length L is S of the electrode that generates the SAW.
Since it corresponds to the width in the direction orthogonal to the AW traveling direction, the electrode can be made smaller by reducing the interaction length L, and as a result, the optical frequency shifter can be made smaller.

In the three-branch optical waveguide of the present invention, the propagating light wave is incident substantially parallel to the SAW wavefront, that is, the angle θ formed between the propagating light wave and the SAW wavefront.
_{Since i} is formed so as to be substantially 0, the above (3)
In the formula, α≈0. Therefore, for example, Q = 1, πΔn ₁
When L / λ ₀ = 1 the above equation (3) is as follows.

[0027]

(Equation 3)

The optical frequency shifter of the present invention utilizes both non-diffracted light and diffracted light when it is actually used in the optical heterodyne measurement method or the like. Therefore, the optical frequency shifter is designed to have a diffraction efficiency of 100%. It is not necessary to do so, and the output of the optical waveguide branched into three of the three branched optical waveguides is 33% at most.
Should be obtained.

[0029]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention in which the optical frequency shifter of the present invention is applied to an optical fiber laser Doppler velocimeter will be described in detail below with reference to the drawings.

As shown in FIG. 3, this embodiment has an optical frequency shifter 30 formed on a substrate 32 of LiNbO ₃ having an acousto-optic effect.

As shown in FIGS. 1 and 3, the optical frequency shifter 30 has a TiN layer formed on the surface layer of a substrate 32 made of LiNbO _3.
A single-channel optical waveguide 34, a multi-mode optical waveguide 41A, and a three-branch channel optical waveguide 38 including branch multi-mode optical waveguides 41B and 41C, and an outgoing light channel optical waveguide 4
It is equipped with 0A, 40B and 40C. Note that these optical waveguides can be formed by a method such as proton exchange instead of thermal diffusion of Ti.

The channel optical waveguide 34 is composed of a single mode optical waveguide having a width Wo capable of propagating a single mode light wave (guided light), and one end P0 thereof is a semiconductor laser or the like for emitting coherent light. It is located on one side of the substrate 32 so that the light of the frequency f _opt from the constructed coherent light source 42 is incident.

At the other end of the channel optical waveguide 34, a tapered optical waveguide 36D having a width gradually expanded from Wo to W at a constant inclination angle from the narrow side to the wide side is formed.
Are combined.

A multimode optical waveguide 41A having a width W and a predetermined length capable of propagating a multimode light wave is coupled to the wide side of the tapered optical waveguide 36D. To the other end of the multimode optical waveguide 41A, a tapered optical waveguide 36A whose width is gradually reduced from W to Wo at a constant inclination angle from the wide side to the narrow side is coupled. One end of an outgoing light channel optical waveguide 40A is coupled to the narrow side of the tapered optical waveguide 36A.

In order to effectively obtain the non-diffracted light and the ± 1st-order diffracted light, from the substantially central portion of the multimode optical waveguide 41A, θ _m (≉λ) with respect to the multimode optical waveguide 41A.
_O / n ₁ Λ), two branch multimode optical waveguides 41B and 41C having a width W branch symmetrically with respect to the multimode optical waveguide 41A.

The branched multi-mode optical waveguide 41B has a width Wo capable of propagating a single-mode light wave through the tapered optical waveguide 36B having the same shape as the tapered optical waveguide 36A and a channel light for outgoing light. Waveguide 40
A channel optical waveguide 40B for outgoing light, which is bent so as to be parallel to A, is coupled to form a branched multimode optical waveguide 41.
C has a width Wo capable of propagating a single-mode light wave through the tapered optical waveguide 36C having the same shape as the tapered optical waveguide 36A, and is parallel to the channel optical waveguide 40A for emission light. The bent outgoing light channel optical waveguide 40C is coupled.

In the vicinity of the branch portion of the three-branch channel optical waveguide 38, an IDT 44 for forming a SAW having a frequency f _saw and an interaction length L with a light wave propagating in the optical waveguide is formed. An oscillator 46 that generates a high frequency is connected to the IDT 44.

Here, the reason why the width of the three-branch channel optical waveguide 38 is widened to W is to prevent the occurrence of diffraction broadening and the excitation of multimode light waves. Further, since the channel optical waveguide and the three-branch channel optical waveguide are coupled by the tapered optical waveguide, the tapered optical waveguide portion also acts as an interaction region between the SAW and the light wave, so that the three-branched channel optical waveguide is lengthened. The interaction length L can be increased without doing so.

One ends of the polarization maintaining optical fibers 48A, 48B and 48C are coupled to one ends of the outgoing light channel optical waveguides 40A, 40B and 40C of the optical frequency shifter 30, respectively. The other ends of the polarization maintaining optical fibers 48B and 48C are composed of gradient index lenses such as SELFOC lenses, and irradiation measurement probes 50B and 50C that emit the incident light as parallel light are coupled to the other ends. Has been done.

A condenser lens 52 is arranged on the light wave emission side of the irradiation measurement probes 50B and 50C, and the light emitted from the irradiation measurement probes 50B and 50C is condensed on a light condensing portion (second focal point). To be done. As a result, the light emitted from the irradiation measurement probes 50B and 50C intersect at the angle 2φ at the converging portion. The irradiation measurement probe 50B,
50C and the condensing lens 52 act as a condensing device that condenses light on the measured region.

At the first focal position of the condenser lens 52, a light-receiving measurement probe 54 which is a ball lens and acts as an incident device is arranged. One end of a single mode optical fiber 56 is coupled to the light receiving measurement probe 54.

The other end of the polarization maintaining optical fiber 48A and the other end of the single mode optical fiber 56 are connected to the photodetector 58 so that the photoelectric conversion surface of the photodetector 58 for heterodyne detection on the photoelectric conversion surface is irradiated with light. Are combined. Photodetector 5
Reference numeral 8 is connected to a Doppler frequency analyzer 60 which is a calculation device equipped with a computer for calculating the flow velocity from the signal photoelectrically converted by the photodetector 58.

Next, the operation of this embodiment will be described.
You. In FIG. 3, one end P0 of the channel optical waveguide 34
To frequency f_optIncident light wave, and high frequency to the IDT44
And the frequency f_sawInteract with the SAW
If this is done, an intermediate value between Bragg diffraction and Raman-Nass diffraction
Due to the diffraction phenomenon, the SAW frequency f_sawShifted by ±
First-order diffracted light is obtained. ± 1st order diffracted light is branched multi-mode
The non-diffracted light propagates through each of the optical waveguides 41B and 41C.
It propagates through the multimode optical waveguide 41A. As a result,
Zero-order light from the other end P1 of the light output channel optical waveguide 40A
Frequency f which is _optNon-diffracted light of
The other ends P1 and P2 of the channel optical waveguides 40B and 40C
Frequency f_opt+ F_saw, F_opt−f_sawDiffracted light
Is ejected.

The ± first-order diffracted light having frequencies f _opt + f _saw and f _opt −f _saw emitted from the optical frequency shifter 30 is collimated from the irradiation measuring probes 50B and 50C via the polarization maintaining optical fibers 48B and 48C. And is condensed by the condensing lens 52 on the condensing portion, that is, intersected.

When the scattering particles pass through the condensing portion, the passing scattering particles scatter the irradiated light in all directions. A part of the backscattered light from the passing scattering particles is again collected by the condenser lens 5.
The light is collimated through 2 and is incident on the photodetector 58 through the light receiving measurement probe 54 and the single mode optical fiber 56. Non-diffracted light is incident on the photodetector 58 via the polarization maintaining optical fiber 48A.

Emitted from the irradiation measuring probe 50B
After the light wave is backscattered by scattering particles,
Single-mode optical fiber 56 via fixed probe 54
FIG. 4 shows the relationship between the wavenumber vectors when the light enters the. Figure
In 4 (1), K_iaIs the wave number of the light incident on the scattering particles.
Ktor, K_saIs the backscatter, that is, in the negative direction of the x-axis
Is the wave vector of the scattered light, and V is the velocity vector
Torr and V_x, V _yIs the x and y components of the velocity vector
is there. In addition, when the crossing angle of the light incident on the scattering particles is 2φ,
Therefore, the light emitted from the irradiation measurement probe 50B
Angle between wave and scattered light, ie wave vector K_iaAnd x
The angle formed with the axis is φ.

Here, the wave number vector K _a obtained by subtracting the wave number vector K _ia from the wave number vector K _sa shown in FIG. 4A.
Corresponds to the Doppler frequency shift f _da , which is the amount of frequency transition received by scattering by the scattering particles.

This Doppler frequency shift f _da is expressed by using the following vector inner product using the velocity vector V (= iV _x + jV _y ), the wave number vector K _ia , and the wave number vector K _sa . In _{f da = V · K a /} (2π) = V · (K sa -K ia) / (2π) ··· (6) where the vector K _sa, the approximate size of the K _ia and K , F _da is represented by the following equation. _{f da = (iV x + jV} y) {- iK- (iKcosφ + jKsinφ)} / (2π) = (iV x + jV y) {- iK (1 + cosφ) + jKsinφ} / (2π) = {- (1 + cosφ) V x K + sinφV y K} / (2π) = { - (1 + cosφ) V x + sinφV y} / λ ··· (7) also holds the same relationship as described above for emitted from the irradiation measurement probe 50C lightwave, Doppler frequency shift f _db Can also be derived in the same manner as above, and is therefore expressed as f _db = {− (1 + cosφ) V _x −sin φV _y } / λ (8).

Therefore, the frequencies of the light waves reaching the photoelectric conversion surface of the photodetector 58 are expressed as follows. Scattered light by the light wave from the irradiation measurement probe 50B f _opt + f _saw + f _da = f _opt + f _saw + {-(1 + cos φ) V _x + sin φV _y } / λ (9) By the light wave from the irradiation measurement probe 50C Scattered light f _opt −f _saw + f _db = f _opt −f _saw + {− (1 + cos φ) V _x −sin φV _y } / λ (10) Non-diffracted light from the polarization-maintaining optical fiber 48A (local light) f _{opt Since} the three light waves of scattered light and local light emitted by the light waves emitted from the irradiation measurement probes 50B and 50C are combined and interfered at the photoelectric conversion surface of the photodetector 58, the following three types of beat signals are obtained. Is obtained.

[0050]

(Equation 4)

By computing these beat signals with a computer provided in the Doppler frequency analyzer 60, the following vector components (V _x , V _y ) of the flow velocity can be obtained.

Irradiation measuring probes 50B, 50C
The backscattered light due to the light wave emitted from the above acts as a signal light of intensity P _s after entering the single-mode optical fiber 56, and the non-diffracted light emitted from the outgoing light channel optical waveguide 40A has a local intensity of P _Lo . Since it acts as a light emission,
The beat signal obtained by the interference also gives the component V _y of the flow velocity perpendicular to the optical axis as in the following expression (15).

[0053]

(Equation 5)

That is, in the conventional optical fiber laser Doppler velocimeter, only the V _y component can be measured, but in the present embodiment, the two-dimensional vector component (V _x , V _y ) is measured. It can be measured.

In the above-described embodiment, the backscattered light emitted from the irradiation measuring probes 50B and 50C and scattered by the scattering particles interferes with the local light, so that the intensity of the local light is increased. Further, the S / N can be improved.

In the above, a pair of irradiation measurement probes is used.
An optical system equipped with a measuring probe for receiving light is scattered in the x-axis direction.
A two-dimensional vector placed at a position where it can receive scattered light
The example of measuring the components was explained, but the same as this optical system.
Can receive light scattered in the z-axis direction by another optical system of one configuration
3D vector components if further arranged at different positions
(V _x, V_y, V_z) Can be measured.

In the above, Li is used as the acousto-optic material.
An example using NbO ₃ has been described, but LiTa
Other materials having an acousto-optic effect such as O ₃ and ZnO may be used.

The frequency f_opt+ F_saw, F_opt−f
_sawOf the frequency f_optLocal light
Light, but frequency f_opt+ F_saw, F_optMeasure the light of
Irradiate the area, frequency f_opt−f_sawOf the local light
May be the frequency f_opt−f _saw, F_optMeasuring light
Irradiate the area and frequency f_opt+ F_sawLight as local light
Is also good.

[0059]

As described above, according to the present invention, one electrode for generating a surface acoustic wave having a low frequency is used,
Since a plurality of emitted lights having a small frequency difference are emitted, it is possible to provide an optical frequency shifter that is suitable for optical measurement and has a highly stable and compact structure and a simple structure.

[Brief description of drawings]

FIG. 1 is an enlarged view of a vicinity of a branch portion of an optical frequency shifter of the present invention.

FIG. 2 is a diagram showing a light diffraction direction of an optical frequency shifter of the present invention.

FIG. 3 is an optical fiber using the optical frequency shifter of the present invention.
It is a block diagram of a laser Doppler velocity meter.

FIG. 4 is a diagram showing a wave number relationship of the optical fiber laser Doppler velocimeter of the present embodiment.

FIG. 5 is a configuration diagram of a conventional optical frequency shifter.

FIG. 6 is a configuration diagram of a conventional optical frequency shifter.

[Explanation of symbols]

 30 Optical Frequency Shifter 32 Substrate 34 Channel Optical Waveguide 38 3 Branched Channel Optical Waveguide 40A, 40B, 40C Channel Optical Waveguide for Emitting Light

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Tadashi Ichikawa, Aichi-gun, Nagakute-cho, Aichi-gun, Nagakage 1 41 of Yokomichi, Toyota Central Research Institute Co., Ltd. (72) Inventor, Satoshi Kato Nagakute-cho, Aichi-gun 1 in 41 Chuo-do, Toyota Central Research Institute Co., Ltd. (72) Inventor, Hiroshi Ito, Nagakute-cho, Aichi-gun, Aichi Prefecture 1-chome, 1 in Toyota Central Research Co., Ltd.

Claims (2)

[Claims] 1. A single-channel optical waveguide, an electrode for generating a surface acoustic wave, and an optical wave which is coupled to one end of the channel optical waveguide and propagates so as to be substantially parallel to the wavefront of the surface acoustic wave. Three-branch optical waveguide having three branched optical waveguides that are formed so as to be incident on the optical waveguide and propagate the ± 1st-order diffracted light and the undiffracted light that have substantially the same intensity diffracted by the interaction with the surface acoustic wave An optical frequency shifter comprising: a waveguide; and an outgoing light channel optical waveguide coupled to the other end of each of the three-branching optical waveguides. 2. Using an acousto-optic medium, L is an interaction length of a propagating light wave and a surface acoustic wave, λ _O is a wavelength of the light wave in a vacuum, n ₁ is a refractive index of the acousto-optic medium, and Λ is a surface. When the wavelength of the elastic wave, θi, the angle between the propagating light wave and the wavefront of the surface acoustic wave, and Δn ₁ are changes in the refractive index of the acousto-optic medium induced by the surface acoustic wave, the three-branch optical waveguide The waveguide and 0.1 <2πLλ _O / n ₁ Λ ² cos θ _i <10
The optical frequency shifter according to claim 1, configured to satisfy the condition of 0 <πΔn ₁ L / λ ₀ <2.