WO2013024818A1

WO2013024818A1 - Frequency shifter and frequency shifting method

Info

Publication number: WO2013024818A1
Application number: PCT/JP2012/070502
Authority: WO
Inventors: 信太郎久武; 小林　哲郎
Original assignee: 国立大学法人大阪大学
Priority date: 2011-08-15
Filing date: 2012-08-10
Publication date: 2013-02-21

Abstract

This frequency shifter (10) comprises a substrate (20) having a polarization reversal structure, a modulating electrode (30) provided on the surface of the substrate (20), and a waveguide (40) provided to the substrate (20). Supplying an alternating-current electric field to the modulating electrode (30) causes diffracted light to be generated from incident light that is incident on the waveguide (40), the diffracted light having a frequency that is shifted from the frequency of the incident light. The width of the waveguide (40) is preferably greater than that of the modulating electrode (30). The thickness of the substrate (20) is preferably no greater than 0.3 mm.

Description

Frequency shifter and frequency shift method

The present invention relates to a frequency shifter and a frequency shift method.

A frequency shifter that shifts the frequency of light is used in optical equipment such as an optical communication system, an optical measuring device, and a spectroscopic device. To date, various frequency shift techniques have been proposed.

An acousto-optic frequency shifter using an acousto-optic effect is known as an example of a frequency shifter (see, for example, Patent Document 1). The acousto-optic frequency shifter utilizes the light diffraction phenomenon caused by the sound wave (dense / dense wave) propagating in the crystal, and can theoretically realize a shift efficiency of 100%. However, the shift amount of the acousto-optic frequency shifter is about several hundred MHz to 1 GHz and is not so large. This is because the propagation loss of the sound wave in the crystal becomes very large in the high frequency region, and the wavelength of the sound wave is smaller than the wavelength of the light wave in the high frequency region of about 1 GHz or more, so that diffraction does not occur.

Also, as another frequency shifter, a frequency shifter using sideband (Single Side Band: SSB) modulation is known (see, for example, Patent Document 2). In the SSB modulation frequency shifter, incident light is divided into two waveguides, and one of the lights is shifted in phase by π / 4 by a phase modulator. The two high-frequency phase modulators are respectively supplied with modulation signals (ie, sinΩt and cosΩt) that are out of phase with each other by π / 2, and phase-modulate the two light waves and combine them. At this time, if the depth of the phase modulation is appropriately selected, the fundamental frequency component can be suppressed and only the first sideband frequency component can be extracted, and it can function as a frequency shifter. Such an SSB modulation type frequency shifter uses the electro-optic effect, and thus can operate at high speed. For example, an SSB frequency shifter can obtain a frequency shift amount of 1 to 100 GHz or more, which is much larger than a frequency shifter using an acoustooptic effect.

However, in the SSB frequency shifter, even if the first sideband frequency component having the largest energy transfer amount is used among the sideband frequency components generated by the phase modulation, the theory of the conversion efficiency (shift efficiency) is used. The maximum value is only 34%, and a conversion efficiency higher than that cannot be obtained.

The inventor of the present application has been researching a frequency shifter that can realize a shift amount of 1 GHz to 100 GHz and a relatively high conversion efficiency by a method other than the SSB method (for example, Non-Patent Documents 1 and 2). reference). In the frequency shifters disclosed in

Non-Patent Documents

1 and 2, the frequency-shifted diffracted light is generated by supplying an AC electric field to an electro-optic crystal having a domain-inverted structure and performing Bragg diffraction. .

JP-A-7-270734 JP 2007-11125 A

As described above, the frequency shifter of Non-Patent

Document

1 or 2 can perform a relatively large frequency shift. However, in order to perform Bragg diffraction in the frequency shifter of Non-Patent

Document

1 or 2, Q is required to be sufficiently larger than 10 as described in Non-Patent Document 1. Q is expressed as follows.
Q = 2πλ ₀ L / (nΛ ² )
Here, λ ₀ represents the wavelength of incident light, L represents the interaction length, n represents the refractive index of the electro-optic crystal, and Λ represents the period of the periodically poled structure.

Further, the incident Bragg angle θ _B is expressed as follows.
θ _B = arcsin (λ ₀ / (2nΛ))

In order to realize a Q value sufficiently larger than 10, a design that reduces the period Λ of the periodically poled structure is required. In this case, it may be difficult to produce a periodically poled structure. Further, the incident Bragg angle θ _B increases as the period Λ decreases. In this case, in order to interact with incident light with a large incident Bragg angle θ _B at a constant length, it is necessary to increase the width of the modulation electrode, which reduces design freedom. Sometimes.

Also, in order to realize a Q value sufficiently larger than 10, a design that increases the interaction length L is required. In this case, in order to propagate the light wave diffracted in the crystal over a long distance, it is necessary to use a relatively thick crystal. However, when a thick crystal is used, the distance between the electrodes arranged so as to sandwich the crystal becomes long, and as a result, the modulation efficiency decreases, and a large modulation power may be required to obtain a predetermined operation. . In the frequency shifters of

Non-Patent Documents

1 and 2, modulated power of about several hundred W is supplied. As described above, the frequency shifter of Non-Patent

Document

1 or 2 may not be able to easily perform frequency shift.

The present invention has been made in view of the above problems, and an object thereof is to provide a frequency shifter and a frequency shift method that can easily realize a relatively large frequency shift. Another object of the present invention is to provide a frequency shifter and a frequency shift method with improved design flexibility.

A frequency shifter according to the present invention is a frequency shifter including a substrate having a domain-inverted structure, a modulation electrode provided to extend in a predetermined direction on the surface of the substrate, and a waveguide provided on the substrate. By supplying an alternating electric field to the modulation electrode, diffracted light having a frequency obtained by shifting the frequency of the incident light is generated from the incident light incident on the waveguide.

In one embodiment, the waveguide is multimode at least in the width direction.

In one embodiment, the thickness of the substrate is 0.3 mm or less.

In one embodiment, the incident light is incident on the waveguide inclined with respect to the predetermined direction.

In one embodiment, the incident light is inclined with respect to the predetermined direction at an angle other than a Bragg angle and is incident on the substrate.

In one embodiment, the angle of the incident light with respect to the predetermined direction is set so that Bragg diffraction is performed.

In one embodiment, the modulation electrode forms an electric field extending from the front surface to the back surface of the substrate.

In one embodiment, the period in the direction orthogonal to the predetermined direction in the polarization inversion structure is equal to or greater than a half wavelength of the incident light.

In one embodiment, the domain-inverted structure has a synthetic domain-inverted structure obtained by synthesizing at least two domain-inverted structures.

In one embodiment, the domain-inverted structure includes a first region having a first domain-inverted structure and a second region having a second domain-inverted structure different from the first domain-inverted structure.

A frequency shift method according to the present invention provides a frequency shifter including a substrate having a domain-inverted structure, a modulation electrode provided to extend in a predetermined direction on the surface of the substrate, and a waveguide provided on the substrate. And a step of supplying an alternating electric field to the modulation electrode and generating diffracted light having a frequency obtained by shifting the frequency of the incident light from incident light incident on the waveguide.

A frequency shifter according to the present invention includes a substrate having a domain-inverted structure and a modulation electrode provided to extend in a predetermined direction on the surface of the substrate, and supplies an alternating electric field to the modulation electrode. Thus, diffracted light having a frequency obtained by shifting the frequency of the incident light is generated from incident light incident on the substrate, and the incident light is inclined with respect to the predetermined direction at an angle other than a Bragg angle. Incident on the substrate.

The frequency shift method according to the present invention includes a step of preparing a frequency shifter including a substrate having a domain-inverted structure and a modulation electrode provided to extend in a predetermined direction on the surface of the substrate, and an AC electric field applied to the modulation electrode. Generating diffracted light having a frequency obtained by shifting the frequency of the incident light from incident light incident on the substrate, and in the step of generating the diffracted light, Is incident on the substrate at an angle other than the Bragg angle with respect to the predetermined direction.

According to the present invention, a relatively large frequency shift can be easily realized. In addition, according to the present invention, it is possible to provide a frequency shifter with improved design freedom.

FIG. 3 is a schematic diagram of an embodiment of a frequency shifter according to the present invention. (A)-(h) is a schematic diagram which shows an example of the manufacturing method of the frequency shifter shown in FIG. It is a schematic diagram of the frequency shifter of this embodiment. It is a schematic diagram of the frequency shifter of this embodiment. It is a figure which shows the measurement result of the frequency shift using the frequency shifter of this embodiment. It is a graph which shows the change of the shift efficiency with respect to the square root of modulation power. It is a schematic diagram of a modulation index measuring device that measures the modulation index of the frequency shifter of the present embodiment. It is a graph which shows the change of the modulation index with respect to the square root of modulation power. It is a graph which shows the change of the normalization modulation index with respect to the thickness of a substrate. It is a schematic diagram of the frequency shifter of this embodiment. It is a typical top view of the frequency shifter shown in FIG. (A) is a schematic diagram which shows the relationship of the wave vector for reference, (b) is a schematic diagram which shows the relationship of the wave vector in the frequency shifter of this embodiment. It is a schematic diagram which shows an example of the relationship of the wave vector in the frequency shifter of this embodiment. (A) is a graph which shows the modulation depth dependence of the normalization power in the frequency shifter of this embodiment, (b) is a graph which shows an output spectrum. It is a schematic diagram which shows an example of the relationship of the wave vector in the frequency shifter of this embodiment. (A) is a graph which shows the modulation depth dependence of the normalization power in the frequency shifter of this embodiment, (b) is a graph which shows an output spectrum. (A) is a schematic diagram for demonstrating the relationship of the wave number vector when each wave number vector of the input wave and modulation wave in the frequency shifter of this embodiment is collinear, (b) is a part of (a) It is an enlarged view. It is a schematic diagram for demonstrating the walk-off in the frequency shifter of this embodiment. It is a typical perspective view of the frequency shifter of this embodiment. It is a schematic diagram of the frequency shifter of this embodiment. (A)-(f) is a schematic diagram which shows the manufacturing method of the frequency shifter shown in FIG.

Hereinafter, embodiments of a frequency shifter and a frequency shift method according to the present invention will be described with reference to the drawings. However, the present invention is not limited to the following embodiments.

[Embodiment 1]
FIG. 1 shows an embodiment of a frequency shifter according to the present invention. The frequency shifter 10 of the present embodiment includes a substrate 20 having a domain-inverted structure, a modulation electrode 30 provided on the surface of the substrate 20 so as to extend in a predetermined direction, and a waveguide 40 provided on the substrate 20. ing.

The substrate 20 includes a dielectric, a semiconductor, an organic material, etc. having an electro-optic effect. As an example, the substrate 20 is made of a ferroelectric crystal. The ferroelectric is, for example, lithium tantalate. The polarization inversion structure of the substrate 20 is produced by, for example, periodically inverting the polarization of a predetermined region of the ferroelectric member. In FIG. 1, a region indicated by a black line indicates a region where polarization is inverted.

The modulation electrode 30 extends in the y direction. Here, the z direction orthogonal to each of the x direction and the y direction is parallel to the normal direction of the main surface of the substrate 20.

An alternating current (modulation signal) is applied to the modulation electrode 30. Here, the alternating current changes sinusoidally. For example, alternating current is microwave. In this case, it is also possible to apply a voltage to the modulation electrode 30 using a commercially available amplifier. For example, an alternating current obtained by amplifying a signal from the synthesizer by an amplifier (for example, a semiconductor amplifier) is applied to the modulation electrode 30. Note that the width of the modulation electrode 30 is preferably relatively wide in order to use the spatial characteristics of light traveling in the substrate 20 (here, the waveguide 40) in the x direction (using a diffraction phenomenon). For example, the width of the modulation electrode 30 is not less than 0.5 mm and not more than 1.5 mm. Such a modulation electrode 30 is also called a microstrip line.

The waveguide 40 exhibits a higher refractive index than other regions of the substrate 20. Here, the waveguide 40 is provided on one entire main surface of the substrate 20, and the waveguide 40 is a planar type. For example, the thickness (depth) of the waveguide 40 is about 0.8 μm. The waveguide 40 propagates light in a state where light is confined in the z direction (thickness direction of the substrate 20). The waveguide 40 propagates light in the length direction (y direction). The waveguide 40 is multimode at least in the width direction (x direction). The waveguide 40 is preferably single mode with respect to the thickness direction (z direction).

In the frequency shifter 10, the polarization inversion structure of the substrate 20 is disposed obliquely with respect to the direction in which the modulation electrode 30 extends. Such a periodic polarization inversion structure is also called an oblique periodic polarization inversion structure. In the present specification, the direction in which the polarization is reversed at the shortest distance in the periodically poled structure may be referred to as a polarization reversal direction.

By supplying an alternating electric field to the modulation electrode 30, diffracted light having a frequency obtained by shifting the frequency of the incident light is generated from the incident light incident on the substrate 20. In the frequency shifter 10, the frequency of the incident light is shifted according to the AC frequency applied to the modulation electrode 30. For example, the frequency of light shifts beyond 10 GHz. When the AC frequency applied to the modulation electrode 30 is 10 GHz, the waveguide 40 emits diffracted light having a frequency shifted by ± N × 10 GHz of the frequency of the incident light. However, N is an integer. As described above, the light in the waveguide 40 is confined in the z direction and is diffracted in the xy plane. An alternating current having a frequency of several hundred GHz may be applied to the modulation electrode 30.

In the frequency shifter 10 of the present embodiment, since the substrate 20 has a periodic polarization inversion structure, the phase velocity of the alternating current (modulated wave) supplied to the modulation electrode 30 and the waveguide 40 (substrate 20) travel. Although the difference from the group velocity of light waves is relatively large, the speed can be matched in a pseudo manner, and incident light can be efficiently modulated.

Although the modulation electrode 30 is in contact with the waveguide 40 here, an insulating layer is provided between the modulation electrode 30 and the waveguide 40 in order to reduce light loss due to the modulation electrode 30 as will be described later. Is preferably provided. Although not particularly shown in FIG. 1, in the frequency shifter 10, a ground electrode is provided on a main surface different from the main surface on which the modulation electrode 30 is provided on the two main surfaces of the substrate 20. The modulation electrode 30 forms an electric field extending from the front surface to the back surface of the substrate 20.

Since the frequency shifter 10 of the present embodiment is provided with the waveguide 40, the interaction length of the frequency shifter 10 can be made relatively long even if the substrate 20 is relatively thin. For example, the thickness of the substrate 20 is 0.3 mm or less, and as an example, the thickness of the substrate 20 is 0.1 mm. Even in this case, an interaction length of 30 mm or more can be realized. In addition, since the substrate 20 is thin as described above, relatively high shift efficiency can be realized even if the power applied to the modulation electrode 30 is relatively low.

Hereinafter, an example of a method for manufacturing the frequency shifter 10 will be described with reference to FIG. First, as shown in FIG. 2A, a substrate 20 is prepared. The substrate 20 includes a ferroelectric material. For example, the substrate 20 is z-cut coincidence melting type LiTaO ₃ (Congruent Lithium Tantalate: CLT). In FIG. 2, the arrow in the substrate 20 indicates the direction of polarization.

As shown in FIG. 2B, a resist R is applied on the substrate 20. For example, the resist R is applied to the surface of the substrate 20 by spin coating at 1000 rpm for 5 seconds and 3000 rpm for 30 seconds. Next, baking is performed at 90 ° C. for about 40 minutes. Thereafter, a mask M to which the polarization inversion pattern is transferred is formed on the resist R. Using the mask M, UV exposure is performed for about 30 to 40 seconds to form a polarization inversion resist pattern RP as shown in FIG.

As shown in FIG. 2 (d), polarization inversion is performed. The polarization inversion is performed using, for example, a lithium chloride aqueous solution E (for example, 500 g of lithium chloride per 600 ml of water) as a liquid electrode. Note that the polarization inversion may be performed using an electrode in which silver is quick-coated or vapor-deposited on both surfaces.

As shown in FIG. 2E, the resist pattern RP is removed. Thereafter, the waveguide 40 is formed on the substrate 20. Here, as shown in FIG. 2 (f), proton exchange is performed on the surface of the substrate 20 to form a high refractive index region, and as a result, as shown in FIG. Is formed.

For example, when proton exchange is performed on the CLT used as the substrate 20, the substrate 20 is immersed in a benzoic acid (C ₆ H ₅ COOH) melt, whereby Li ⁺ and H ⁺ are exchanged, and H _x Li _1-x TaO ₃ is produced. Generally, the higher the temperature and the longer the time, the deeper the region where proton exchange takes place. For example, proton exchange is performed at 240 ° C. for 4 hours. In place of benzoic acid, phthalic acid, inphthalic acid or pyrophosphoric acid may be used.

After proton exchange, it is preferable to perform an annealing treatment as necessary. In general, the waveguide 40 may be lost due to proton exchange and the electro-optic constant γ ₃₃ may be reduced. However, the electro-optic constant γ ₃₃ can be recovered together with the loss of the waveguide 40 by annealing. For example, the annealing process is performed at 400 ° C. for 30 minutes.

As shown in FIG. 2H, the modulation electrode 30 is formed on the substrate 20. The modulation electrode 30 is made of silver, for example. As described above, the frequency shifter 10 shown in FIG. 1 can be manufactured.

In the frequency shifter 10 shown in FIG. 1, the modulation electrode 30 is provided directly on the waveguide 40, but the present invention is not limited to this. The modulation electrode 30 may be provided on the waveguide 40 via another member.

FIG. 3 shows a schematic diagram of the frequency shifter 10 of the present embodiment. The frequency shifter 10 shown in FIG. 3 has the same configuration as the frequency shifter 10 described above with reference to FIG. 1 except that an insulating layer 50 is further provided between the modulation electrode 30 and the waveguide 40. In order to avoid redundancy, duplicate descriptions are omitted. For example, the thickness of the insulating layer 50 is about 0.1 μm.

The insulating layer 50 is made of, for example, silicon oxide (SiO ₂ ). The silicon oxide layer 50 is formed, for example, by sputtering a SiO ₂ target after forming the waveguide 50. For example, in this case, the refractive index n _f of the waveguide 40 is 2.262, the refractive index n _c of the insulating layer 50 is 1.467. In addition, when the wavelength of incident light is 514.5 nm, the waveguide 40 can be made into single mode by making the depth of the waveguide 40 into 0.8 micrometer or less.

Here, the measurement result of the frequency shift by the frequency shifter 10 will be described with reference to FIGS. FIG. 4 shows a schematic diagram of the frequency shifter 10. Here, the interaction length Ld is equal to the length of the substrate 20. The length Ld of the substrate 20 is 34 mm, and the thickness t is 0.1 mm. The period of the domain-inverted structure in the substrate 20 (distance along the domain-inverted direction of the domain-inverted structure) Λ _p is 60 μm, and the inclination θ _t of the domain-inverted structure with respect to the y-direction is 0.567 °. The polarization inversion length (polarization inversion pitch along the y direction) Lu is 3.03 mm.

The width w of the modulation electrode 30 is 0.5 mm, and the length of the modulation electrode 30 in the extending direction (y direction) is 34 mm, which is equal to the length Ld of the substrate 20. Further, an alternating current with a frequency of 16.25 GHz is applied to the modulation electrode 30 at about 2.7 W.

Light having a wavelength of 514.5 nm is incident on the waveguide 40 from an Ar laser. In the domain-inverted structure provided on the substrate 20, the period Λ _{x in the} x direction perpendicular to the y direction in which the modulation electrode 30 extends is equal to or more than a half wavelength of the incident light. Here, incident light is incident at a Bragg angle. For example, incident light is incident on the x direction side with an inclination of θ with respect to the y direction, and the incident angle θ is 0.11 °. In this case, the diffracted light is emitted from the frequency shifter 10 while being inclined to the x direction side at the same emission angle θ (that is, 0.11 °) with respect to the y direction. In this way, the light wave traveling in the domain-inverted structure interacts with the modulated wave (alternating current), and diffracted light whose frequency is shifted by the frequency component of the modulated wave with respect to the frequency of the incident light is emitted at the Bragg angle. The Note that the emitted light having a shifted frequency is spatially separated from light having a frequency equal to the frequency of the incident light.

FIG. 5 shows the measurement results of the frequency shifter 10 shown in FIG. In FIG. 5, the horizontal axis indicates the space, and the vertical axis indicates the frequency. In the frequency shifter 10, since the light is dispersed in the diffraction grating, the difference between the two plotted points in the vertical axis direction corresponds to the frequency shift amount of the light wave. From FIG. 5, it is understood that the frequency of the first-order diffracted light is shifted by 16.25 GHz with respect to the zero-order diffracted light having the same frequency as the incident light. Here, the modulation power is adjusted so that both a component having a frequency of incident light and a component having a frequency shifted frequency are measured.

FIG. 6 shows the change in shift efficiency with respect to the square root of the modulation power. As understood from FIG. 6, a shift efficiency of about 68% can be realized with a modulation power of about 2.7 W. Such a frequency shifter 10 can be driven continuously as well as pulsed. Note that in Non-Patent Document 1, a magnetron that performs pulse driving is used as a modulation signal source.

As described above, the thickness of the substrate 20 is preferably 0.3 mm or less. Hereinafter, changes in the modulation index according to the thickness of the substrate 20 will be described with reference to FIGS.

FIG. 7 shows a schematic diagram of the modulation index measuring apparatus 100. Here, the modulation index of the phase modulator D is measured. The phase modulator D has the same configuration as the above-described frequency shifter except that the polarization inversion direction defined by the polarization inversion structure is parallel to the direction in which the modulation electrode extends.

The modulation index measurement apparatus 100 includes a light source 110 that emits light incident on the phase modulator D, an AC power source 120 that applies an AC electric field to the modulation electrode of the phase modulator D, and the light emitted from the phase modulator D. A diffraction grating 130 for spatially decomposing frequency components, a Fourier transform lens 140, and an imaging unit 150 are provided. Here, the light source 110 is an Ar laser that emits light having a wavelength of 514.5 nm, and the imaging unit 150 is a CCD (Charged Coupled Device) camera. A modulation index is obtained by fitting a sideband image picked up by the image pickup unit 150.

FIG. 8 shows the change of the modulation index with respect to the square root of the modulation power. A plurality of types of phase modulators having different thicknesses of the substrate 20 and the lengths of the modulation electrodes, and types of phase modulators without a waveguide (that is, bulk type) are also measured.

In any case, the modulation index increases as the square root of the modulation electrode increases. When the waveguide 40 is provided, the thinner the substrate 20, the higher the modulation index. In particular, when the thickness of the substrate 20 is 0.3 mm or less, a modulation index larger than that of a bulk type having a thickness of 0.5 mm can be obtained.

FIG. 9 shows changes in the normalized modulation index with respect to the thickness of the substrate 20. Here, the polarization inversion period is corrected with respect to the measurement result shown in FIG. 8 and is normalized according to the length of the modulation electrode 30. From FIG. 9, it can be understood that the normalized modulation index when the thickness of the substrate 20 is 0.1 mm is 3.4 times that of the bulk type, which results in about 10 times the power efficiency. As described above, the thickness of the substrate 20 is preferably 0.3 mm or less. However, as will be appreciated by those skilled in the art, the preferred thickness of the substrate 20 varies depending on various conditions.

In the above description, the pattern of the domain-inverted structure of the substrate 20 is one, but the present invention is not limited to this.

FIG. 10 shows a schematic diagram of the frequency shifter of the present embodiment. The frequency shifter 10 illustrated in FIG. 10 includes a shift unit 10a and a shift 10b. The shift unit 10a includes a substrate 20a having a domain-inverted structure and a waveguide 40a provided on the substrate 20a. The shift unit 10b includes a substrate 20b having a polarization inversion structure different from that of the substrate 20a, and a waveguide 40b provided on the substrate 20b. A modulation electrode 30 is provided over the

substrates

20a and 20b. The waveguide 40b is configured to communicate with the waveguide 40a.

In this specification, the shift unit 10a, the substrate 20a, and the waveguide 40a may be referred to as a first shift unit, a first substrate, and a first waveguide, respectively, and the shift unit 10b, the substrate 20b, and the waveguide 40b may be referred to as the first shift unit, the first substrate, and the first waveguide, respectively. It may be called a 2nd shift part, a 2nd board | substrate, and a 2nd waveguide. Further, the polarization inversion structure of the substrate 20a and the region provided with the polarization inversion structure are referred to as a first polarization inversion structure and a first region, respectively, and the polarization inversion structure of the substrate 20b and the region provided with the polarization inversion structure are respectively provided. Sometimes referred to as a second domain-inverted structure and a second region.

The first domain-inverted structure of the first substrate 20a is different from the second domain-inverted structure of the second substrate 20b. For example, the polarization inversion direction of the first polarization inversion structure is different from the polarization inversion direction of the second polarization inversion structure. Here, the polarization inversion direction defined by the first polarization inversion structure has a + component in the x direction in addition to the y direction component, and the polarization inversion direction defined by the second polarization inversion structure is in the y direction. In addition to the component, it has a-component in the x direction. However, the period of the first domain-inverted structure is preferably equal to the period of the second domain-inverted structure.

Here, the diffraction direction in the frequency shifter 10 shown in FIG. 10 will be described with reference to FIG. In FIG. 11, the modulation electrode 30 is omitted.

For example, when light having a wave vector whose x-direction component is positive and whose y-direction component is positive is incident on the first waveguide 40a of the first substrate 20a, this light has a negative x-direction component and a y-direction component. The light is diffracted and emitted as light represented by a positive wave vector. After entering the second waveguide 40b of the second substrate 20b, this light is diffracted and emitted as light represented by a wave vector whose x-direction component is positive and whose y-direction component is positive. Incident light is frequency-shifted twice as compared with the incident light before entering the first substrate 20 a, thereby shifting the incident light by twice the frequency of the alternating current applied to the modulation electrode 30. For example, when alternating current with a frequency of 16.25 Hz is supplied to the modulation electrode 30, the frequency shifter 10 shown in FIG. 10 emits diffracted light having a frequency shifted by 32.5 GHz with respect to the frequency of incident light.

In the above description, the x-direction component in the polarization inversion direction defined by the first polarization inversion structure has the opposite sign to the x-direction component in the polarization inversion direction defined by the second polarization inversion structure. However, the present invention is not limited to this. The component in the x direction in the polarization inversion direction defined by the first polarization inversion structure may have the same sign as the component in the x direction in the polarization inversion direction defined by the second polarization inversion structure. For example, the angle between the direction in which the modulation electrode 30 extends and the polarization inversion direction defined by the second polarization inversion structure is larger than the polarization inversion direction defined by the first polarization inversion structure, and the second waveguide The diffraction of 40b may be performed in the same direction as the first waveguide 40a.

Although not shown in FIGS. 10 and 11, at least one of the shift portion 10a and the shift portion 10b is an insulating layer between the modulation electrode 30 and the waveguide 40 as described above with reference to FIG. 50 may be further included. In FIG. 10, the frequency shifter 10 includes the two

shift units

10a and 10b. However, the present invention is not limited to this. The frequency shifter 10 may include three or more shift units.

In the above description and

Non-Patent Documents

1 and 2, the frequency shift was performed by Bragg diffraction, but the present inventor has found that the frequency shift can be performed without Bragg diffraction. As described above, the frequency shifter 10 generates diffracted light having a frequency shifted from the frequency of incident light based on the modulation current (radio wave). Such a frequency shift by the frequency shifter 10 includes an input wave (incident light), a refractive index distribution wave (a wave of a refractive index distribution generated in a domain-inverted structure to which an AC electric field is applied), and a generated wave (diffracted light). It can be considered that this is caused by the mixing of the three waves.

First, let us consider a state where the input wave and the modulated wave are collinear. Further, the frequency of the input wave is f ₀ . From the frequency f ₀ of the input wave, focusing on the shifted component of the frequency by the frequency + f _m of the modulation wave. The difference between the wave vector of this component, the wave vector of the input wave, and the sum of the wave vector of the modulated wave is

It is expressed. Here, c is shown a speed of light in vacuum, n _g represents the group refractive index, n _m is the refractive index for the modulation wave.

It is possible to compensate so that Δk becomes zero by the periodic polarization inversion. This is also called quasi phase matching. The polarization inversion period at this time is

It is represented by Here, v _g represents the group velocity of the light wave, v _m denotes the phase velocity of the modulating wave.

Here, for reference, the relationship between wave number vectors in normal phase modulation will be described with reference to FIG. k ₀ represents the wave vector of the input wave, and k _m represents the wave vector of the modulated wave. In the present specification, underlined symbols indicate vectors. In a typical phase modulation, as shown in FIG. 12 (a), quasi-phase matching is achieved by periodically poled structure with a wave vector k _0, k _m and the wave vector K in the same direction. In this case, from the viewpoint of the momentum conservation law, the coupling between the zeroth-order component and the first-order component is
_{_{_{k 1 = k 0 + k m}}} + K
It is expressed. The wave number vector k ₀ defines the traveling direction of the incident light having the frequency f ₀ , and the wave number vector k ₁ defines the traveling direction of the light having the frequency f ₁ (= f ₀ + f _m ).

The combination of the primary component and the secondary component is
_{_{_{k 2 = k 1 + k m}}} + K
It is expressed. As long as there is no group velocity dispersion, the other orders have the same relationship. The same applies to the negative order, and the coupling between the 0th-order component and the −1st-order component is
_{_{_{k -1 = k 0 - k m}}} - K
It is expressed. The amplitude of the n-order component in a situation where the n-order component and the (n ± 1) -order component are combined is expressed as J _n (Δθ). Here, J _n () is a Bessel function, and Δθ is a modulation index. For this reason, energy cannot be transferred to a specific sideband component with 100% efficiency. Therefore, in the conventional phase modulation scheme must be withdrawn in such a filter the desired sideband components, the highest energy transfer efficiency is 34% of the component frequencies with respect to the input wave is shifted by ± f _m. Of course, the energy transfer efficiency to higher frequency components is even lower. All so-called SSB frequency shifters shift the frequency in this way.

In contrast, in the frequency shifter 10 of the present embodiment, an angle is provided between the direction of the wave vector K of the periodically poled structure and the direction of the wave vector k ₀ of the input wave and the wave vector k _m of the modulated wave. Thus, the number of modes for energy coupling is limited, and efficient energy transfer to a target frequency component is realized. FIG. 12B is a schematic diagram showing the relationship between wave vectors. Wave vector K periodically poled structure, the angle (θ ₁ ≠ 0) with respect to wave vector k _0, k _m collinear input wave and the modulated wave is inclined by. Note that the wave vector of the refractive index distribution wave is expressed by a composite vector of the wave vector K and the wave vector k _m. Refractive index distribution wave and the input wave, so as to satisfy the phase matching condition between the three waves of the diffracted wave _{_{(k 1 = k 0 + k}} m + K), | K | by determining a theta _1, wave vector A situation can be created in which only k ₀ and k ₁ are combined. Therefore, theoretically, the energy of the input wave (f ₀ , k ₀ ) can be moved 100% to the component (f ₁ = f ₀ + f _m , k ₁ ) combined therewith.

By the way, (f ₀ , k ₀ ) and (f ₁ , k ₁ ) are in a coupled state, and (f ₀ , k ₀ ) and (f ₋₁ , k ₋₁ ) are in a coupled state. a _{_{condition, K = 2πf m (n m}} -n g) / c and the direction of the wave vector K is the same as the direction of wave vector k _0, k _m. This is exactly the same condition as described above with reference to FIG. Therefore, in theory, it can be realized binding of only the desired mode by tilting the wave vector K with respect to the collinear wavevector k _0, k _m.

In FIG. 12B, the input wave and the modulated wave are collinear, but they are not necessarily collinear. Situation in which the frequency is shifted to the -f _m can also be realized by a similar concept.

Although details will be described later, on the basis of such a concept, only the components of (f ₀ , k ₀ ), (f ₁ , k ₁ ), and (f ₂ , k ₂ ) are coupled. Cascade type energy transfer is also possible. However,
_{_{_{f 1 = f 0 + f m}}} ,
f ₂ = f ₁ + f _m = f ₀ + 2f _m
It is. In this case, the polarization inversion structure has two wave vectors _{_{K 1, K 2, (f}} 0, k 0) and (f _{_1,} k ₁₎ wave vector in the energy coupling between K ₁ + k _m quasi-phase matching according to the refractive index distribution wave represented by (quasi-phase-matching: QPM ) using, (f _{_1,} k ₁₎ and (f _{_2,} k ₂₎ is the energy coupling between the wave vector K ₂ + k utilizing QPM by refractive index distribution waves indicated by _m. Here, + has been described a frequency shift to 2f _m, it can be carried out similarly frequency shift to -2f _m. By such cascaded movement of light energy, energy can be efficiently transferred to a component whose frequency is shifted by an integral multiple of the modulation frequency.

Here, with reference to FIG. 13, energy transfer from the (f ₀ , k ₀ ) component to the (f ₀ + f _m , k ₁ ) component will be described. _{_{| K 0 |, | k m}} |, | k 1 |, | K | is expressed as follows.
| K ₀ | = k ₀ = 2πn ₀ f ₀ / c,
_{_{| K m | = k m =}} 2πn m f m / c,
| K ₁ | = k ₁ = 2π (n ₀ f ₀ + _ng f _m ) / c,
| K | = K = 2π / Λ,
Here, Λ is the period of the periodically poled structure. When the period Λ is determined, the angle θ ₁ that satisfies the condition of QPM and the angle φ ₁ formed by the input wave and the diffracted wave are determined as follows.

When an Ar laser is used as the light source of incident light and a LiTaO ₃ crystal is used as the nonlinear optical medium, θ ₁ = ± 1.58 rad, φ ₁ = ± 0 .0, assuming that the modulation frequency is f _m = 16 GHz and Λ = 10 μm. 023 rad.

FIG. 14A shows a simulation result based on a beam propagation method (BPM). Here, the electro-optic constant γ ₃₃ is 32.2 pm / V, the AC power applied to the modulation electrode 30 is 3 W, the thickness t of the substrate 20 is 0.1 mm, and the length of the substrate 20 is (Device length) Ld is 34 mm. When the amplitude Δn of the refractive index change due to the modulation wave is πλ ₀ / (4Ld), the energy of the input wave (f _{_0,} k ₀₎ is, + f _m just shifted frequency (f _{_1,} k ₁₎ component 100% Move with efficiency.

FIG. 14B shows the spectrum of the emitted light. In FIG. 14B, the broken line indicates the frequency component of the incident light. The solid line is (f _{_1,} k ₁₎ component, this frequency is + f _m shift with respect to the frequency of the input wave.

The wave vector relationship has been described above with reference to FIGS. 12 to 14. However, energy transfer from the wave vector k ₀ to the wave vector k ₁ occurs, and energy transfer from the wave vector k ₁ to the wave vector k ₂ occurs. May occur.

Referring to FIG. 15, with the energy transfer from the wave vector k ₀ to the wave vector k _1, illustrating the relationship between the wave vector of the case where the energy transfer from the wave vector k ₁ to the wave vector k ₂ is generated. In this case, energy is transferred in a cascade type and frequency shifted by + 2f _m with respect to the frequency of the input wave component. Here, + the frequency shift to 2f _m be described, as described above, it is also possible frequency shift to -2f _m.

Wave vector k _m and wave vector K ₁ and quasi-phase matching method using the refractive index distribution wave represented by the wavevector k _{m1 'is} a composite vector of (Quasi Phase Matching: QPM) by (f _{_0,} k ₀₎ component and ( f ₁ , k ₁ ) are energetically coupled to each other, and by a quasi-phase matching method using a refractive index distribution wave indicated by a wave vector k _{m2 ′} which is a composite vector of the wave vector k _m and the wave vector K ₂ ( It is possible to create a situation where the (f ₁ , k ₁ ) component and the (f ₂ , k ₂ ) component are energetically coupled. However, | K ₁ | = K ₁ = 2π / Λ ₁ , | K ₂ | = K ₂ = 2π / Λ ₂ . When Λ ₁ and Λ ₂ are determined, θ ₁ , φ ₁ , θ ₂ , and φ ₂ are determined so as to satisfy the respective pseudo phase matching conditions. Here, θ ₁ and φ ₁ are obtained as described above. On the other hand, θ ₂ and φ ₂ can be obtained from the following equations.

When Λ ₁ = 10 μm and Λ ₂ = 20 μm under the same conditions as described above, θ ₁ = 1.58 rad, φ ₁ = 0.023 rad, θ ₂ = 1.60 rad, φ ₂ = 0.034 rad It becomes. The substrate 20 (see, for example, FIG. 1) is formed with a combined polarization inversion structure corresponding to a combined vector of wave number vectors K ₁ and K ₂ indicating _two polarization inversion structures.

FIG. 16A shows a simulation result based on BPM. The energy transfer cascade is understood that the moving energy efficiency of 100% _{_{(f 2 = f 0 + 2f}} m, k 2) to component. Although Λ ₁ and Λ ₂ are different here, Λ ₁ and Λ ₂ may be equal to each other.

FIG. 16B shows the spectrum of the emitted light. In FIG. 16B, the broken line indicates the frequency component of the incident light. The solid line is (f _{_2,} k ₂₎ component, this frequency is + 2f _m shift with respect to the frequency of the input wave.

In addition, although the connection of three wave vector was demonstrated here, this invention is not limited to this. Four or more wave vectors may be combined.

As described above, in theory, the component of the frequency f ₀ can be moved to a component of the component or f ₀ -f _m of f ₀ + f _m, the energy selectively. However, in reality, since the interaction length and the like are finite, the wave number used for coupling includes uncertainty, and in the case of selectively shifting the frequency to other unintended modes (particularly, + fm _). May easily cause energy transfer to −f _m or + f _m when the frequency is selectively shifted to −f _m .

This problem is very small wavenumber difference _{_{_{k 1 -k -1 = 4πn g f}}} m / c with wave vector k ₁ and k _-1 are compared to normal three-wave mixing, discrimination of the components due to a difference in wavenumber This is due to the difficulty of making it. For example, when the above numerical values are used, k ₁ -k ₀ is about 800. On the other hand, since the device length is generally finite, the interaction length cannot be made infinitely long. For example, assuming the interaction length of the

frequency shifter

10 and 30 mm, uncertainty of k _m becomes about 400, is the same order as the previous wave number difference.

Here, the relationship of the wave vector will be described with reference to FIG. FIG. 17A is a schematic diagram showing a relationship between wave number vectors when the wave number vectors of the input wave and the modulated wave in the frequency shifter of the present embodiment are collinear, and FIG. FIG. As shown in FIGS. 17A and 17B, even if the phase matching condition for combining the wave vector k ₀ and the wave vector k ₁ is set, the wave vector k There may be a coupling between ₀ and the wave vector k ₋₁ .

Therefore, the frequency shifter 10, as shown in FIG. 18, it is preferable not to co-linear to wave vector k ₀ of the input wave with wavenumber vector k _m of the modulation wave. For example, if the sum of the wave vector K + k _m and the wave number vector k ₀ is the wave vector K are determined so that the wave vector k _1, also the angle between the wave vector k ₁ and the wave vector k _m incident When designed to be approximately equal to the angle (alpha), the wave vector k _-1 will have a multiple of the angular (2.alpha) with wavenumber vector k _m, the wave vector k _m of wave vector k _-1 The walk-off angle with respect to is doubled compared to the wave vector k ₁ . Since the size of the modulation electrode 30 is finite, as a result of the large walk-off angle, the interaction length between the wave vector k ₋₁ and the wave vector k ₀ is limited, and the wave vector k ₀ to the wave vector k ₋₁ is limited. Energy transfer is suppressed.

It should be noted that the phenomenon in which two waves move away with propagation and the degree of spatial overlap between the two waves interacting with propagation decreases is also called walk-off. Since the diameter of the light beam and the width of the modulation electrode 30 in the frequency shifter 10 are finite, there is a difference in the traveling direction of the wave vector of the generated wave and the modulated wave (because the generated wave and the modulated wave are not collinear). As the wave propagates, the degree of spatial overlap of the two waves decreases. In FIG. 18, the angle formed by the nth-order wave vector and the n + 1-order wave vector differs from the angle formed by the n-th wave vector and the n−1th wave vector (where n is an integer), n Since the degree of walk-off between the next wave vector and the n + 1-th wave vector is different from the degree of walk-off between the n-th wave vector and the n−1-th wave vector, Energy transfer is performed efficiently.

As described above, without the wave vector k ₀ of the input wave to the collinear with wavenumber vector k _m of the modulation wave, it is preferred that intersect at an angle both small. Specifically, the incident light is preferably incident at an incident angle α obliquely with respect to the direction in which the modulation electrode 30 extends.

In the frequency shifter 10 shown in FIG. 1, the waveguide 40 is formed on the entire main surface of the substrate 20, but the present invention is not limited to this. When viewed from the normal direction of the main surface of the substrate 20, it may be provided in a partial region of the substrate 20. However, as shown in FIG. 19, the width wo (the length along the x direction) of the waveguide 40 is preferably larger than the width w of the modulation electrode 30.

For example, when the width wo of the waveguide 40 is smaller than the width w of the modulation electrode 30, all the propagation modes in the waveguide 40 interact with the modulation wave, and the above-described walk-off causes energy coupling limitation. There may not be. On the other hand, when the width wo of the waveguide 40 is larger than the width w of the modulation electrode 30, with respect to a specific propagation mode in the waveguide 40, there is no overlap between the modulated wave electric field and the light wave according to the propagation. As a result, it is possible to prevent energy coupling with other light using the modulated wave.

Here, as described above, the incident angle α may be a Bragg angle, and the diffraction of the frequency shifter 10 may be a Bragg diffraction. For example, the angle of incident light with respect to the extending direction of the modulation electrode 30 shown in FIG. 1 may be set so that Bragg diffraction is performed. Alternatively, the incident angle α may not be a Bragg angle, and the diffraction of the frequency shifter 10 may not be a Bragg diffraction.

[Embodiment 2]
In FIG. 20, the schematic diagram of the frequency shifter 10 of this embodiment is shown. The frequency shifter 10 includes a substrate 20 having a domain-inverted structure and a modulation electrode 30 provided on the surface of the substrate 20. The frequency shifter 10 shown in FIG. 20 has the same configuration as the above-described frequency shifter except that the substrate 20 is not provided with a waveguide, and redundant description is omitted for the purpose of avoiding redundancy. To do.

In the frequency shifter 10 of the present embodiment, the incident light is incident on the substrate 20 with an inclination other than the Bragg angle with respect to the direction in which the modulation electrode 30 extends. Also in this case, as described above with reference to FIGS. 17 and 18, it is possible to suppress energy from moving to an unintended mode. Further, in the frequency shifter 10, by performing diffraction different from Bragg diffraction, Q may be 10 or less, and the degree of freedom in designing the frequency shifter 10 can be improved.

In addition, although the detailed description which overlaps in order to avoid redundancy is abbreviate | omitted, the frequency shifter 10 of this embodiment may also be provided with the two

shift parts

10a and 10b as above-mentioned with reference to FIG. Alternatively, also in the frequency shifter 10 of the present embodiment, as described above with reference to FIG. 15, the substrate 20 includes a synthesis corresponding to a synthesized vector of wave number vectors K ₁ and K ₂ indicating two polarization inversion structures. A domain-inverted structure may be formed.

The frequency shifter 10 shown in FIG. 20 is manufactured as follows, for example. Hereinafter, a method of manufacturing the frequency shifter 10 will be described with reference to FIG.

First, as shown in FIG. 21A, a substrate 20 is prepared. The substrate 20 includes a dielectric having nonlinear optical characteristics. For example, the substrate 20 is z-cut coincidence melting type LiTaO ₃ (Congruent Lithium Tantalate: CLT).

As shown in FIG. 21B, a resist R is applied on the substrate 20. For example, the resist R is applied by spin coating the surface of the substrate 20 for 5 seconds at 1000 rpm and 30 seconds at 3000 rpm. Next, baking is performed at 90 ° C. for about 40 minutes. Thereafter, a mask M to which the polarization inversion pattern is transferred is formed on the resist R. Using the mask M, UV exposure is performed for about 30 to 40 seconds to form a polarization-inverted resist pattern RP as shown in FIG.

As shown in FIG. 21 (d), polarization inversion is performed. The polarization inversion is performed using a lithium chloride aqueous solution E (for example, 500 g of lithium chloride per 600 ml of water) as a liquid electrode. Note that the polarization inversion may be performed using an electrode in which silver is quick-coated or vapor-deposited on both surfaces.

As shown in FIG. 21 (e), the resist pattern RP is removed. Thereafter, as shown in FIG. 21F, the modulation electrode 30 is formed on the substrate 20. The modulation electrode 30 is made of silver, for example. As described above, the frequency shifter 10 shown in FIG. 20 can be manufactured.

According to the present invention, a relatively large frequency shift can be easily realized. Moreover, according to the present invention, the degree of freedom in designing the frequency shifter can be improved. By using such a frequency shifter, it is possible to improve the resolution in optical measurement. The frequency shifter of the present invention can be applied to high-accuracy characteristic evaluation of ultrashort light pulses. It can also be used for precise frequency control in the next generation coherent optical communication system.

10 frequency shifter 20 substrate 30 modulation electrode 40 waveguide 50 insulating layer

Claims

A substrate having a domain-inverted structure;
A modulation electrode provided to extend in a predetermined direction on the surface of the substrate;
A frequency shifter comprising a waveguide provided on the substrate,
A frequency shifter that generates diffracted light having a frequency obtained by shifting the frequency of the incident light from incident light incident on the waveguide by supplying an alternating electric field to the modulation electrode.
The frequency shifter according to claim 1, wherein the waveguide is multimode at least in the width direction.
The frequency shifter according to claim 1 or 2, wherein a width of the waveguide is larger than a width of the modulation electrode.
The frequency shifter according to any one of claims 1 to 3, wherein the thickness of the substrate is 0.3 mm or less.
The frequency shifter according to any one of claims 1 to 4, wherein the incident light is inclined with respect to the predetermined direction and enters the waveguide.
6. The frequency shifter according to claim 5, wherein the incident light is inclined with respect to the predetermined direction at an angle other than a Bragg angle and is incident on the substrate.
The frequency shifter according to claim 5, wherein an angle of the incident light with respect to the predetermined direction is set so that Bragg diffraction is performed.
The frequency shifter according to any one of claims 1 to 7, wherein the modulation electrode forms an electric field extending from the front surface to the back surface of the substrate.
The frequency shifter according to any one of claims 1 to 8, wherein a period in a direction orthogonal to the predetermined direction in the domain-inverted structure is equal to or more than a half wavelength of the incident light.
10. The frequency shifter according to claim 1, wherein the domain-inverted structure has a synthetic domain-inverted structure obtained by synthesizing at least two domain-inverted structures.
11. The domain inversion structure according to claim 1, wherein the domain inversion structure includes a first region having a first domain inversion structure and a second region having a second domain inversion structure different from the first domain inversion structure. The described frequency shifter.
Providing a frequency shifter comprising a substrate having a domain-inverted structure, a modulation electrode provided to extend in a predetermined direction on the surface of the substrate, and a waveguide provided on the substrate;
And supplying an alternating electric field to the modulation electrode and generating diffracted light having a frequency obtained by shifting the frequency of the incident light from incident light incident on the waveguide.