JP2008241770A - Optical device containing nonlinear optical single crystal - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a small optical device having a wavelength conversion function in addition to wavelength selection. <P>SOLUTION: In the optical device including an input waveguide which guides first light at least having first wavelength λ<SB>n</SB>and at least has a first effective refractive index n<SB>n</SB>at least to the first wavelength λ<SB>n</SB>, an output waveguide which guides second light at least having second wavelength λ<SB>n'</SB>and has a second effective refractive index to at least the second wavelength λ<SB>n'</SB>and a ring waveguide which is positioned between the input waveguide and the output waveguide to guide the first light and the second light, the first wavelength λ<SB>n</SB>and the second wavelength λ<SB>n'</SB>satisfy relation λ<SB>n'</SB>=λ<SB>n</SB>/2, the ring waveguide is formed on a nonlinear optical single crystal and has polarization reversal structure of prescribed circumference L and a prescribed period Λ. Λ=λ<SB>n</SB>/2×[1/(n<SB>n'</SB>- n<SB>n</SB>)], L=K×λ<SB>n</SB>/n<SB>n</SB>(K: an integer). <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a small optical element having a wavelength conversion function in addition to wavelength selection, and more particularly to an optical element including a nonlinear optical single crystal.

  Currently, in a high-density wavelength division multiplexing optical communication, a wavelength selection filter that selectively extracts an optical signal having a specific wavelength from a large number of optical signals having different wavelengths is used. As the amount of information communication in the field of optical communication in recent years has increased significantly, further increase in capacity and function of optical communication has been demanded. The channel spacing or central wavelength spacing of multiplexed optical signal wavelengths. There is a demand for an optical communication element that can further narrow down the wavelength or select and demultiplex or multiplex a necessary wavelength from signals multiplexed with a narrow-band wavelength.

As such an optical communication element, there is a wavelength selection filter capable of controlling a resonance wavelength (see, for example, Patent Document 1).
FIG. 3 is a schematic diagram of the wavelength selection filter described in Patent Document 1.
As shown in the figure, the wavelength selection filter 3000 includes a linear input waveguide 3100, a linear output waveguide 3200 arranged in parallel in the width direction of the input waveguide 3100, and an input waveguide 3100. The distance between the planar resonator (ring resonator) 3300 existing between the ring resonator 3300 and the output waveguide 3200 and the ring resonator 3300 is variable, and the resonance wavelength of the ring resonator 3300 is caused by the interaction with the ring resonator 3300. Is configured from a resonance wavelength adjusting structure 3400 that changes the resonance frequency from the resonance wavelength of the ring resonator 3300 alone.

In the wavelength selection filter 3000, a resonance wavelength adjustment structure 3400 is disposed in the vicinity of the ring resonator 3300. By adjusting the distance between the ring resonator 3300 and the resonance wavelength adjustment structure 3400, the ring is adjusted. An optical signal having a resonance wavelength different from the resonance of the resonator 3300 alone can be transferred to the output waveguide 3200 via the ring resonator 3300 and output from the drop port P3.
For example, when the resonance wavelength of the ring resonator 3300 alone is the wavelength lambda _1, the resonance wavelength of the ring resonator 3300 by adjusting the distance between the ring resonator 3300 and the resonance wavelength adjustment structure 3400 to lambda _n When changed, optical signals of wavelengths λ ₁ , λ ₂ ,..., Λ _n−1 other than the resonance wavelength λ _n propagate toward the through port P 2 of the input waveguide 3100. Also, when the optical signal of wavelength lambda _n matching resonance wavelength lambda _n above the add port P4 of the output waveguide 3200 is input, the optical signal of the wavelength lambda _n is input waveguides via a ring resonator 3300 The light is guided to the through port P2 of the waveguide 3100.

As described above, according to the wavelength selection filter 3000, optical signals of various wavelengths are selectively demultiplexed to the drop port P3 only by providing one resonance wavelength adjusting structure 3400 for one ring resonator 3300. Or multiplexed to the through port P2.
However, in the technique described in Patent Document 1, although the resonance wavelength can be changed as appropriate, the change range can be limited to the wavelength of the optical signal input at the input port P1. Therefore, the wavelength λ ₁ , λ ₂ ,..., Λ _{n of the} optical signal input to the input port P1 cannot be converted to, for example, the wavelength λ _n / 2.

On the other hand, in addition to diversification such as high output and miniaturization of lasers for optical communication, the development of technology using laser light is remarkable. Among them, the development of a wavelength conversion element using a ferroelectric single crystal having a nonlinear optical constant has attracted attention. If such a wavelength conversion element is used, the wavelength of the existing laser light source can be converted into light having a desired wavelength range using the fundamental light as the basic light. As such a wavelength conversion element, a technique of forming a periodically poled structure in a stoichiometric composition lithium tantalate single crystal is known (see, for example, Patent Documents 2 and 3).
FIG. 4 is a perspective view of a wavelength conversion element made of a cylindrical ferroelectric single crystal described in Patent Document 3. As shown in FIG.
As shown in the figure, the cylindrical ferroelectric single crystal 4000 has a periodic polarization inversion structure 4100 having a predetermined period in a direction perpendicular to the polarization direction.

Such a wavelength conversion element 4000 can emit green light or mid-infrared light according to the period of the periodically poled structure 4100. For example, in the generation of green light, an average output of 4.4 watts is possible, and application to small-sized and low power consumption laser displays and liquid crystal projectors is expected. Further, in the generation of mid-infrared light, it can be used as a light source for medical treatment, environmental measurement, and spectroscopic analysis by combining with a small solid-state laser.
However, according to the techniques described in Patent Documents 2 and 3, the element length of the wavelength conversion element requires about 30 mm in order to satisfy a predetermined wavelength conversion efficiency. Although the laser light source is smaller than the conventional one, there is a limit to further miniaturization.
JP 2005-128442 A JP 2002-90785 A JP 2005-156634 A

  The present invention has been made in view of the above circumstances, and an object thereof is to provide an optical element that has a wavelength conversion function in addition to wavelength selection and that can be miniaturized. is there.

  As a result of intensive studies to achieve the above object, the inventors have provided a ring waveguide having a polarization inversion structure having a predetermined circumference and a predetermined period on a nonlinear optical single crystal. The knowledge that it can be achieved was obtained.

The present invention has been completed based on these findings, and is as follows.
(1) is guided first light having at least a first wavelength lambda _n, an input waveguide having at least a first effective refractive index n _n with respect to the at least first wavelength lambda _n, at least a second wavelength lambda _{n 'second} light having a is waveguide, said at least a second wavelength lambda _n' and an output waveguide having at least a second effective refractive index n _{n 'with} respect to the input waveguide And an optical element including a ring waveguide that guides the first light and the second light.
The first wavelength λ _n and the second wavelength λ _{n ′} satisfy the relationship λ _{n ′} = λ _n / 2,
The ring waveguide is formed on a nonlinear optical single crystal and has a polarization inversion structure having a predetermined circumference L and a predetermined period Λ satisfying the following relationship.
Λ = λ _n / 2 × [1 / (n _{n ′} −n _n )]
L = K × λ _n / n _n (K: integer)
(2) Another ring waveguide is further included between the ring waveguide and the output waveguide, and the circumference L ′ of the other ring waveguide is L ′ = K ′. The optical element according to (1), wherein λ _{n ′} / n _{n ′} (K ′: integer) is satisfied.
(3) The circumference L ′ of the other ring waveguide is further satisfying L ′ = (K ″ +0.5) × λ _n / n _n (K ″: integer). Optical element.
(4) The non-linear optical single crystal is selected from the group consisting of a lithium niobate having a congruent composition, a lithium niobate having a stoichiometric composition, a lithium tantalate having a congruent composition, and a lithium tantalate having a stoichiometric composition. The optical element according to (1), characterized in that
(5) The optical element according to (4), wherein the nonlinear optical single crystal is a Z-cut substrate, and a waveguide mode of the first light and the second light is a TE mode. .

According to the optical element of the present invention, the ring waveguide having the periodically poled structure is provided between the input / output waveguides. Since the period of polarization inversion satisfies the predetermined condition, the ring waveguide can convert the wavelength of light (wavelength λ _n ) guided through the input waveguide (effective refractive index n _n ). Further, since the circumference of the ring waveguide satisfies the above predetermined condition, the wavelength-converted light (wavelength λ _{n ′} ) resonates in the ring waveguide, and then the output waveguide (effective refractive index n _{n ′).} ) And a wavelength can be selected. The light output in the output waveguide is resonated in the ring waveguide, and a desired conversion efficiency can be achieved. As described above, since a separate element can be manufactured on a single nonlinear optical single crystal, it can be downsized.

  Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings, but the present invention is not limited thereto. In addition, the same referential mark is attached | subjected to the same element and it avoids that description overlaps.

(First embodiment)
FIG. 1 is a schematic diagram showing a first embodiment of an optical element according to the present invention.
The optical element 100 according to the first embodiment of the present invention is located on, for example, a ferroelectric single crystal 110. The ferroelectric single crystal 110 is not particularly limited as long as it has a nonlinear optical effect for wavelength conversion, but preferably has a large nonlinear optical constant d value. Specifically, the ferroelectric single crystal 110 includes congruent composition lithium niobate (CLN), congruent composition lithium tantalate (CLT), stoichiometric composition lithium niobate (SLN), and stoichiometric composition tantalate. Lithium (SLT). In particular, CLN and CLT are desirable because large-scale crystal substrate manufacturing techniques have already been established. CLN, CLT, SLN and SLT may contain 0.1 to 3.0 mol% of an element selected from the group consisting of Mg, Zn, In and Sc. Thereby, optical damage can be reduced.
For example, when using LN (including CLN and SLN) and LT (including CLT and SLT) as the ferroelectric single crystal 110, it is desirable to use a Z-cut substrate from the viewpoint of the efficiency of the optical element.

  The optical element 100 includes an input waveguide 120, an output waveguide 130, and a ring waveguide 140 positioned between the input waveguide 120 and the output waveguide 130. The input waveguide 120, the output waveguide 130, and the ring waveguide 140 may be ridge-type waveguides or diffusion waveguides in which Ti or protons are diffused. A ridge-type waveguide is preferable because it has a large light confinement effect. The width of the waveguide depends on the wavelength to be guided, but may be 2 μm to 6 μm, for example. The input waveguide 120, the output waveguide 130, and the ring waveguide 140 are all made of a ferroelectric single crystal 110.

The input waveguide 120 is preferably linear in order to totally reflect light to be guided, but may have any shape as long as the total reflection condition is satisfied. The input waveguide 120 is guided first light having at least a first wavelength lambda _n, having at least a first effective refractive index n _n for at least a first wavelength lambda _n. In FIG. 1, the input waveguide 120 shows a state in which the first light is guided from left to right.

The output waveguide 130 is preferably linear, like the input waveguide 120, but may be any shape as long as the total reflection condition is satisfied. Output waveguide 130 has at least _'the second light is guided with at least a second wavelength lambda _n' second wavelength lambda _n of at least a second effective refractive index n _n with respect to _'. Here, the first wavelength λ _n and the second wavelength λ _{n ′} satisfy the relationship λ _{n ′} = λ _n / 2. In FIG. 1, the output waveguide 130 shows a state in which the second light is guided from right to left toward the second light.
Note that the first effective refractive index n _n and the second effective refractive index effective refractive index represented by n _{n ',} a waveguide (or ridge-type waveguide) and modal index of the wave resonator guided (Modal index).

The ring waveguide 140 guides the first light and the second light. In FIG. 1, the ring waveguide 140 shows a state in which the first light and the second light are guided in the counterclockwise direction. The ring waveguide 140 has a domain-inverted structure 150 having a predetermined period Λ. The predetermined period Λ satisfies the relationship Λ = λ _n / 2 × [1 / (n _{n ′} −n _n )]. Thereby, the ring waveguide 140 can convert the wavelength of the first light into the second light. The circumference L of the ring waveguide 140 satisfies the relationship L = K × λ _n / n _n (K: integer). As a result, the second light whose wavelength is converted in the ring waveguide 140 is guided and resonated in the ring waveguide 140.

  Ring waveguide 140 has regions 160 and 170 that optically couple with input waveguide 120 and output waveguide 130, respectively. The ring waveguide 140, the input waveguide 120, and the output waveguide 130 are provided with a distance of, for example, 200 nm in the region 160 and the region 170, respectively.

Next, the operation of the optical element 100 will be described.
First light having wavelengths λ ₁ , λ ₂ ,..., Λ _n (n is an integer of 1 or more) propagates from the left to the right in FIG. Such light can be, for example, light from an arbitrary light source such as an optical fiber or a semiconductor laser focused by a lens. Of the first light propagating through the input waveguide 120, the light having the wavelength λ _n is optically coupled to the ring waveguide 140 in the region 160. As a result, only the light having a wavelength lambda _n of the first light, in the region 160, is branched into the ring waveguide 140, other wavelengths _{λ 1, λ 2, ...,} λ n-1 of the light Propagates through the input waveguide 120.

The first light demultiplexed into the ring waveguide 140 propagates through the ring waveguide 140. Meanwhile, the first light is quasi-phase-matched with the second harmonic by the periodically poled structure 150 formed in the ring waveguide 140, and the first light becomes the second light having the wavelength λ _{n ′} . And wavelength conversion. The wavelengths λ _n and λ _{n ′} satisfy the relationship λ _{n ′} = λ _n / 2. Therefore, in the ring waveguide 140, the second light propagates with a first light and a wavelength lambda _{n 'having} a wavelength lambda _n. The second light guided and resonated in the wavelength-converted ring waveguide 140 can achieve a desired conversion efficiency only by propagating through the ring waveguide 140 a plurality of times. Thus, the optical element 100 can be dramatically reduced in size.

Next, the second light out of the first light and the second light propagating through the ring waveguide 140 is optically coupled with the output waveguide 130 in the region 170 and is demultiplexed from the right in FIG. Propagates to the left. In this way, wavelength conversion and wavelength selection can be achieved.
For example, when Z-cut LN and LT are used as the ferroelectric single crystal 110, the optical waveguide mode is a TE mode (ie, the electric field direction of the light and the Z axis) from the viewpoint of the efficiency of the optical element 100. Parallel to the direction) may be employed. When a material other than LN or LT is used as the ferroelectric single crystal 110, the ring waveguide 140 and the first light appropriately select the direction condition with respect to the crystal axis of the ferroelectric single crystal 110 (ie, the electric field direction of light). However, it can be set so that the nonlinear constant is maximized.

  According to the first embodiment of the present invention, by providing the periodically poled structure 150 in the ring waveguide 140, wavelength conversion and wavelength selection can be achieved simultaneously. Further, such an optical element 100 can be made extremely small as compared with a conventional wavelength conversion element, and by combining such a new optical element with an existing element such as a modulator, a novel Functional fusion type optical device can be expected.

(Second Embodiment)
FIG. 2 is a schematic view showing a second embodiment of the optical element according to the present invention.
The optical element 200 according to the second embodiment of the present invention is located on the ferroelectric single crystal 110 similarly to the optical element 100.

  The optical element 200 includes an input waveguide 120, an output waveguide 210, a ring waveguide 140 positioned between the input waveguide 120 and the output waveguide 210, and another ring waveguide 220. . The input waveguide 120, the output waveguide 210, the ring waveguide 140, and the ring waveguide 220 may be ridge-type waveguides or diffusion waveguides in which Ti, protons, or the like are diffused. . The width of the waveguide depends on the wavelength to be guided, but may be 2 μm to 6 μm, for example. The input waveguide 120, the output waveguide 210, the ring waveguide 140, and the ring waveguide 220 are all made of a ferroelectric single crystal 110. The input waveguide 120 and the ring waveguide 140 are the same as those in FIG.

The output waveguide 210 is preferably linear, similar to the output waveguide 130 of FIG. 1, but may be any shape as long as the total reflection condition is satisfied. Output waveguide 210 has at least _'the second light is guided with at least a second wavelength lambda _n' second wavelength lambda _n of at least a second effective refractive index n _n with respect to _'. Here, the first wavelength λ _n and the second wavelength λ _{n ′} satisfy the relationship λ _{n ′} = λ _n / 2. In FIG. 2, the output waveguide 210 shows the second light guided from the left to the right.

Ring waveguide 220 is optically coupled to ring waveguide 140 in region 230. The circumference L ′ of the ring waveguide 220 satisfies the relationship L ′ = K′λ _{n ′} / n _{n ′} (K ′: integer). Under this condition, the ring waveguide 220 can guide at least the second light. More preferably, the circumference L ′ of the ring waveguide 220 further satisfies the relationship L ′ = (K ″ +0.5) × λ _n / n _n (K ″: integer). Under this condition, the first light does not propagate through the ring waveguide 220, so that only the second light among the first light and the second light propagating through the ring waveguide 140 is reliably transmitted to the ring waveguide 220. Can be demultiplexed. As a result, a decrease in wavelength conversion efficiency due to leakage of the first light from the ring waveguide 140 to the ring waveguide 220 is suppressed, so that high efficiency (theoretically 100%. However, efficiency is propagation loss, In addition, it should be understood that the coupling may be limited by the coupling loss in the input / output waveguides 120 and 210 and the ring waveguides 140 and 220 between the first light and the second light. Is about 20% at the maximum). In FIG. 2, the ring waveguide 220 shows a state in which the second light propagates in the clockwise direction. The second light propagating through the ring waveguide 220 is optically coupled with the output waveguide 240 in the region 240.

Next, the operation of the optical element 200 will be described.
First light having wavelengths λ ₁ , λ ₂ ,..., Λ _n (n is an integer of 1 or more) propagates from the left to the right in FIG. Such light can be, for example, light from an arbitrary light source such as an optical fiber or a semiconductor laser focused by a lens. Of the first light propagating through the input waveguide 120, the light having the wavelength λ _n is optically coupled to the ring waveguide 140 in the region 160. As a result, only the light having a wavelength lambda _n of the first light, in the region 160, is branched into the ring waveguide 140, other wavelengths _{λ 1, λ 2, ...,} λ n-1 of the light Propagates through the input waveguide 120.

The first light demultiplexed into the ring waveguide 140 propagates through the ring waveguide 140. Meanwhile, the first light is quasi-phase-matched with the second harmonic by the periodically poled structure 150 formed in the ring waveguide 140, and the first light becomes the second light having the wavelength λ _{n ′} . And wavelength conversion. The wavelengths λ _n and λ _{n ′} satisfy the relationship λ _{n ′} = λ _n / 2. Therefore, in the ring waveguide 140, the second light propagates with a first light and a wavelength lambda _{n 'having} a wavelength lambda _n. Thus, since the first light can achieve a desired conversion efficiency (theoretically 100%) only by propagating through the ring waveguide 140 a plurality of times, compared to the conventional bulk type wavelength conversion element, The optical element 200 can be dramatically reduced in size.

  Then, at least a second light of the first light and the second light propagating through the ring waveguide 140 is optically coupled with another ring waveguide 220 in the region 230 and is demultiplexed. The The second light demultiplexed into the ring waveguide 220 can propagate in a direction different from the propagation direction in the ring waveguide 140, for example, clockwise in FIG. The second light then optically couples with the output waveguide 210 in region 240 and propagates from left to right in FIG. In this way, wavelength conversion and wavelength selection can be achieved.

  Unlike the optical element 100 of FIG. 1, the optical element 200 can control the waveguide direction of light to be demultiplexed. More importantly, the optical element 200 can propagate only the second light to the output waveguide 210 theoretically with 100% efficiency by the ring waveguide 220 (ie, the first light Leakage into the output waveguide 210 can be theoretically suppressed with 100% efficiency). In the optical element 200 according to the present invention, in addition to the effect of the optical element 100, by providing another ring waveguide 220, the light guiding direction can be arbitrarily controlled, and the second light can be controlled. Therefore, it is advantageous in device design.

  In the first and second embodiments, the input waveguide 120, the ring waveguide 140, the ring waveguide 220, and the output waveguides 130 and 210 are all provided on the ferroelectric single crystal 110. However, according to the present invention, the ring waveguide 140 may be provided on the ferroelectric single crystal 110 and may be quasi-phase matched to the light guided through the input waveguide 120. 120, another ring waveguide 220, and output waveguides 130 and 210 are not necessarily provided on the ferroelectric single crystal 110. These may be provided on another substrate and optically and mechanically coupled. By doing in this way, the design according to a user's hope can be made arbitrarily.

  In the first and second embodiments, the case where the second harmonic is generated in the ring waveguide 140 has been described. However, the present invention is not limited to this. If the technique of the present invention is used, those skilled in the art can easily conceive that the optical elements 100 and 200 are appropriately designed so that the third harmonic generation, the difference frequency generation, and the optical parametric oscillation occur in the ring waveguide 140. Can do. In particular, in the case of optical parametric oscillation of the optical elements 100 and 200, the oscillation threshold can be greatly reduced by the large Q factor due to the ring resonator and the large confinement effect due to the ridge-type waveguide. Thereby, an optical parametric oscillator on the chip can also be realized. Further, since the optical elements 100 and 200 do not require the mirror required for the conventional bulk type OPO element, the manufacturing process can be simplified.

  The optical elements 100 and 200 of the first and second embodiments can be manufactured using any existing technique.

  According to the present invention, wavelength conversion and wavelength selection can be achieved simultaneously by providing a periodically poled structure in a ring waveguide on a ferroelectric single crystal substrate. Such an optical element can be made extremely small as compared with a conventional wavelength conversion element, and by combining such a new optical element with an existing element such as a modulator, a novel function fusion type optical The device can be expected.

FIG. 1 is a schematic diagram showing a first embodiment of an optical element according to the present invention. Schematic diagram showing a second embodiment of the optical element according to the present invention. Schematic diagram of wavelength selective filter described in Patent Document 1 The perspective view of the wavelength conversion element which consists of a cylindrical ferroelectric single crystal described in patent document 3

Explanation of symbols

100, 200: Optical element 110: Ferroelectric single crystal 120: Input waveguide 130, 210: Output waveguide 140: Ring waveguide 150: Polarization inversion structure 160, 170, 230, 240: Region 220: Another one another Two ring waveguides 3000: Wavelength selection filter 3100: Input waveguide 3200: Output waveguide 3300: Ring resonator 3400: Resonance wavelength adjusting structure 4000: Wavelength conversion element 4100: Periodic polarization inversion structure

Claims (5)

It is guided first light having at least a first wavelength lambda _n, the input waveguide having at least a first effective refractive index n _n for at least a first wavelength lambda _n, at least a second wavelength _'is guided a second light having a said at least a second wavelength lambda _n' lambda _n and an output waveguide having at least a second effective refractive index n _{n 'with} respect to the input waveguide and said output In an optical element including a ring waveguide that is positioned between a waveguide and guides the first light and the second light,
The first wavelength λ _n and the second wavelength λ _{n ′} satisfy the relationship λ _{n ′} = λ _n / 2,
The ring waveguide is formed on a nonlinear optical single crystal and has a polarization inversion structure having a predetermined circumference L and a predetermined period Λ satisfying the following relationship.
Λ = λ _n / 2 × [1 / (n _{n ′} −n _n )]
L = K × λ _n / n _n (K: integer) Another ring waveguide is further included between the ring waveguide and the output waveguide, and the circumference L ′ of the other ring waveguide is L ′ = K′λ _{n ′.} 2. The optical element according to claim 1, wherein / n _{n ′} (K ′: integer) is satisfied. The optical element according to claim 2, wherein a circumference L ′ of the other ring waveguide further satisfies L ′ = (K ″ +0.5) × λ _n / n _n (K ″: integer). .   The nonlinear optical single crystal is selected from the group consisting of a lithium niobate having a congruent composition, a lithium niobate having a stoichiometric composition, a lithium tantalate having a congruent composition, and a lithium tantalate having a stoichiometric composition. The optical element according to claim 1. The nonlinear optical single crystal is a Z-cut substrate,
The optical element according to claim 4, wherein a waveguide mode of the first light and the second light is a TE mode.

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