JP2012145696A - Optical device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical device that is improved in insertion loss of light.SOLUTION: An optical device includes: a substrate; an optical guide formed on the substrate to guide light; and a ridge portion where a recess having a predetermined depth is formed on both side of the optical guide by digging down part of the substrate. The recess is formed to have a predetermined depth that gradually becomes deeper toward the edge thereof. As light is guided in the direction of the edge of the recess, the spot size of the light to be guided gradually becomes larger. Thus, coupling loss of light at the boundary of the ridge portion and a planar portion formed in series with the ridge portion is reduced.

Description

  The present invention relates to an optical device with good manufacturability and low insertion loss.

  In recent years, high-speed and large-capacity optical communication systems have been put into practical use. A high speed, small size, low cost, and highly stable optical modulator required for incorporation in such a high speed, large capacity optical communication system will be described as an example of an optical device.

As an optical modulator that meets such demands, a light modulator such as lithium niobate (LiNbO ₃ ) is used for a substrate having a so-called electro-optical effect (hereinafter abbreviated as an LN substrate) whose refractive index changes by applying an electric field. There is a traveling wave electrode type lithium niobate optical modulator (hereinafter abbreviated as an LN optical modulator) in which a waveguide and a traveling wave electrode are formed. This LN optical modulator is applied to a large capacity optical communication system of 2.5 Gbit / s and 10 Gbit / s because of its excellent chirping characteristics. Recently, application to a 40 Gbit / s ultra-high capacity optical communication system is also being studied.

  Hereinafter, an LN optical modulator using the electro-optic effect of lithium niobate that has been put to practical use or has been proposed will be described.

(First prior art)
FIG. 10 shows a schematic perspective view of a so-called ridge-type LN optical modulator configured using a z-cut LN substrate disclosed in Patent Document 1 as a first conventional optical device. Here, FIG. 11 is a schematic top view of FIG. 10, and FIG. 12 is a schematic cross-sectional view taken along lines AA ′ and BB ′ of FIG.

  An optical waveguide 3 is formed on the z-cut LN substrate 1. The optical waveguide 3 is an optical waveguide formed by thermally diffusing metal Ti at 1050 ° C. for about 10 hours, and constitutes a Mach-Zehnder interference system (or Mach-Zehnder optical waveguide). Accordingly, two interaction optical waveguides 3a and 3b, that is, two arms of the Mach-Zehnder optical waveguide are formed at a portion where the electrical signal and light of the optical waveguide 3 interact (referred to as an interaction portion).

A SiO ₂ buffer layer 2 is formed on the upper surface of the optical waveguide 3, and a traveling wave electrode 4 is formed on the upper surface of the SiO ₂ buffer layer 2 via a conductive layer 5. The conductive layer 5 is a conductive layer for suppressing a temperature drift caused by the pyroelectric effect peculiar to the LN optical modulator manufactured using the z-cut LN substrate 1, and usually a Si conductive layer is used. As the traveling wave electrode 4, a coplanar waveguide (CPW) having one central conductor 4a and two ground conductors 4b and 4c is used. Normally, the traveling wave electrode 4 is formed of Au, which is a noble metal material. The center conductor 4a takes various values, but in many cases is about 7 μm, and the gap between the center conductor 4a and the ground conductors 4b and 4c also takes various values, but is often about 15 μm. For simplification of description, the Si conductive layer 5 for suppressing temperature drift shown in FIG. 10 is omitted in FIGS. 11 and 12. In the following, the Si conductive layer 5 is omitted and discussed. Reference numeral 6 denotes a high-frequency electric signal (or microwave) feeding line, which is a high-frequency connector or a microwave line. Reference numeral 7 denotes an output line for a high-frequency electric signal, which normally uses an electrical termination, but may be a high-frequency connector or a microwave line.

  Further, in the region shown as I in FIG. 11, the high-frequency electrical signal propagating through the center conductor 4a and the ground conductors 4b and 4c interacts with the light propagating through the two interaction optical waveguides 3a and 3b. This is called a high-frequency electrical signal interaction unit.

  In the first prior art, the recesses 9a, 9b and 9c (or ridges 8a and 8b) are formed by digging the z-cut LN substrate 1 by etching or the like. Here, 12 is a side wall of the ridge portion (here, 8a).

  By adopting this ridge structure, it is possible to realize excellent characteristics in the effective refractive index (or microwave effective refractive index), characteristic impedance, modulation band, driving voltage, etc. of a high-frequency electric signal (or microwave). . In FIG. 12, the depths (or heights of the ridges 8a and 8b) H of the recesses 9a, 9b and 9c are emphasized, but in actuality, the depth is about several μm, and the central conductor Compared with the thickness of 4a and the ground conductors 4b and 4c of several tens of micrometers, the value is small. Note that the depths of the recesses 9a, 9b, and 9c (or the heights of the ridges 8a and 8b) H have a substantially constant value in each part.

  Next, the operation of the LN optical modulator according to the first prior art will be described. In order to operate this LN optical modulator, it is necessary to apply a DC bias voltage and a high-frequency electric signal between the center conductor 4a and the ground conductors 4b and 4c.

  The voltage-light output characteristic shown in FIG. 13 is the voltage-light output characteristic of the LN optical modulator, and Vb is the DC bias voltage at that time. As shown in FIG. 13, the DC bias voltage Vb is normally set at the midpoint between the peak and bottom of the light output characteristic. In the first prior art, a DC bias voltage is also applied to the interaction part I where the high-frequency electrical signal and light interact, so that a capacitor is provided at the electrical terminal (not shown) provided at the output part of the high-frequency electrical signal. Therefore, it is necessary to provide the function of the bias T.

FIG. 14 is a diagram in which the central conductor 4a, the ground conductors 4b and 4c, the SiO ₂ buffer layer 2, and the conductive layer 5 are omitted from FIG. FIGS. 15A and 15B show cross-sectional views in the vicinity of the optical waveguide 3b taken along lines AA ′ and BB ′ in FIG. Width of the top portion 13 of the interaction unit I in the ridge portion 8a as shown in FIG. 15 (a) is the W _1, the distance between the side walls 12 of the edge and the ridge portion of the optical waveguide (3b in this case) is a finite value U. FIG. 15B, which is a cross section taken along the line BB ′, has a planar structure that does not have a ridge structure.

FIGS. 16A and 16B are obtained by adding electric field distributions 14 and 15 of light propagating through the optical waveguide 3b to FIGS. 15A and 15B. As can be seen from FIG. 16A, the light 14 propagating through the optical waveguide 3b is affected by the side wall 12 of the ridge portion and is more strongly confined in the horizontal direction. Therefore, the mode field diameter (twice the spot size) _WRx is smaller than the mode field diameter _WPx of the light 15 propagating through the optical waveguide 3b of FIG. As a result, an increase in insertion loss due to the difference in spot size occurs at the boundary portion C between the ridge structure and the planar structure shown in FIG. Since there are several boundary portions such as C in the light propagation direction, the insertion loss may increase by several dB.

(Second prior art)
FIG. 17 shows an optical modulator as a second prior art optical device, which has a high frequency electrical signal interaction unit III in which a high frequency electrical signal interacts with the interaction optical waveguides 3a and 3b, and a DC bias voltage. Is provided with a DC bias interaction portion IV applied to the interaction optical waveguides 3a and 3b, and is called a bias separation type structure. In this structure, in order to eliminate the bias T required in the first prior art, the high-frequency electrical signal interaction unit III has an electrical terminal (not shown) having only a resistor, and a DC bias is newly provided. The DC bias interaction unit IV is applied. An example thereof is disclosed in Patent Document 2.

  Cross-sectional views taken along the line DD ′ and the line EE ′ in FIG. 17 are shown in FIGS. 18A and 18B, respectively. Here, as described above, 4a is a central conductor, and 4b and 4c are ground conductors. Reference numerals 9a, 9b, and 9c are concave portions of the DC bias interaction portion, and form ridge portions 8a and 8b. 11a and 11b are DC bias electrodes. As described above, reference numeral 12 denotes a side wall of the ridge portion 8a in the high frequency electrical signal interaction portion I through which the high frequency electrical signal propagates.

FIG. 19 is a diagram in which the central conductor 4a, the ground conductors 4b and 4c, the SiO ₂ buffer layer 2, and the conductive layer 5 are omitted in FIG. The cross-sectional structures taken along the line DD ′ and the line EE ′ in FIG. 19 are the cross sections taken along the lines AA ′ and BB ′ of the first prior art shown in FIGS. Is the same. Further, in the boundary F between the ridge structure and the planar structure shown in FIG. 19, an increase in insertion loss due to the difference in spot size is also the same as in the first prior art.

(Third prior art)
As a configuration for solving the problems of the first conventional technique and the second conventional technique described above, a connection type optical waveguide structure as an optical device of the third conventional technique disclosed in Patent Document 3 is shown. A perspective view and a top view of the third prior art are shown in FIGS. 20 and 21, respectively. This optical waveguide structure includes first, second, and third optical waveguides 111, 112, and 113. 101 is a core of a buried optical waveguide 111 which is a first optical waveguide, 102 is a cladding thereof, 104 is an InP substrate, 105 is a core of a second optical waveguide 112, 103 is a cladding provided above and below the cores 101 and 105, Reference numeral 106 denotes a clad made of a low refractive index medium material provided on the left and right sides of the second optical waveguide 112, which is air here.

  In FIG. 21, 120 is light propagating through the embedded optical waveguide 111 which is the first optical waveguide, and its spot size is relatively large, for example, 4 μm. On the other hand, 122 is light propagating through the second optical waveguide 112, and the second optical waveguide 112 has a high mesa optical waveguide structure, and its spot size is defined by the width of the high mesa structure. Or as small as 1 μm.

  Therefore, if the first optical waveguide 111 and the second optical waveguide are directly joined (Butjoin), the coupling loss of light becomes large. In view of this, the third prior art is configured to perform spot conversion. Reference numeral 107 denotes left and right clads provided in the third optical waveguide 113. The spot conversion optical waveguide matches the spot size of the first optical waveguide to the spot size of the second optical waveguide by changing the width of the clad 107. Has a role. It can be seen from the propagating light 121 that the mode field diameter of the light propagating through the third optical waveguide changes.

  The third prior art has a structure in which the width of the lateral cladding in the ridge portion is made different in the light propagation direction. Thereby, it is possible to reduce the insertion loss of light, which has been a problem in the first and second prior arts. However, since this structure needs to form a wide ridge portion, for example, an area that is narrow in area such as the inner side of the multiplexing point (or branching point) of the Y branch of the optical waveguide, such as DQPSK or DP-QPSK. When a ridge portion is formed in an arrayed optical waveguide formed in the vicinity, it is difficult to ensure a sufficient lateral cladding (top portion of the ridge portion) in terms of area.

JP-A-4-288518 JP 2008-122786 A Japanese Patent No. 3877973

  In an optical device having a ridge portion and a planar portion in the prior art, the ridge portion has a substantially constant cross-sectional structure, and a mode field of light propagating through the optical waveguide of the ridge portion and light propagating through the optical waveguide of the planar portion. Since the diameters differed greatly, light coupling loss occurred at the boundary between the ridge portion and the planar portion.

  Also, in the structure in which the width of the lateral cladding in the ridge portion is made different with respect to the light propagation direction, the insertion loss of light can be reduced, but, for example, in the vicinity of the inside of the Y branching multiplexing point (or branching point) of the optical waveguide When the ridge portion is formed in a narrow area such as DQPSK or DP-QPSK having a single electrode configuration, and in an arrayed optical waveguide formed close to each other, a sufficient lateral cladding is provided. It was difficult to ensure (the top of the ridge).

  In order to solve the above-mentioned problem, an optical device according to claim 1 of the present invention includes a substrate, an optical waveguide for guiding light formed on the substrate, and the substrate on both sides of the optical waveguide. In the optical device in which a concave portion having a predetermined depth formed by digging a part is formed to form a ridge portion, the predetermined depth of the concave portion is formed to become gradually shallower toward an end of the concave portion, The spot size of the guided light gradually increases as the light is guided in the direction of the end of the recess.

  In order to solve the above problem, an optical device according to claim 2 of the present invention is the optical device according to claim 1, wherein the predetermined depth of the recess is zero at an end of the recess. It is a feature.

  In order to solve the above-mentioned problem, an optical device according to a third aspect of the present invention is the optical device according to the first aspect, wherein the end of the recess reaches the end of the substrate and has a predetermined depth. It is characterized by being formed.

  In order to solve the above problem, an optical device according to a fourth aspect of the present invention is the optical device according to the first or second aspect, wherein the optical waveguide is a Mach-Zehnder optical waveguide, and the branch point of the Y branch waveguide Alternatively, the concave portion is formed to become gradually shallower toward the multiplexing point.

  In order to solve the above problem, an optical device according to claim 5 of the present invention is characterized in that in the optical device according to any one of claims 1 to 4, the substrate is made of lithium niobate. Yes.

  In the optical device according to the present invention, the depth of the concave portion of the ridge portion (or the height of the ridge portion) is made deeper in the ridge portion (the height of the ridge portion is increased), and is made shallower (the height of the ridge portion is reduced as it approaches the planar portion). Since the small spot size in the lateral direction in the ridge portion can be gradually increased, the light coupling loss at the boundary portion between the ridge portion and the planar portion can be greatly reduced. In the present invention, it is not necessary to increase the width of the top portion of the ridge portion, and it is only necessary to make the depth of the concave portion of the ridge portion (or the height of the ridge portion) different in position. ) And an area-like narrow area such as an arrayed optical waveguide.

The top view which shows schematic structure of the optical device concerning embodiment of this invention (A) Sectional view taken along line JJ 'in FIG. 1, (b) Sectional view taken along line KK' in FIG. The figure explaining the manufacturing process of embodiment of this invention The figure explaining the manufacturing process of embodiment of this invention The figure explaining the principle of this invention The figure explaining the principle of this invention The figure explaining the principle of this invention Modification of optical device according to embodiment of the present invention Another embodiment of an optical device according to an embodiment of the present invention The perspective view which shows schematic structure about the optical device of 1st prior art The top view which shows schematic structure about the optical device of 1st prior art (A) A sectional view taken along the line AA 'in FIG. 11, (b) A sectional view taken along the line BB' in FIG. The figure explaining the principle of operation of an optical modulator The top view which shows schematic structure about the optical waveguide of the optical device of 1st prior art (A) Enlarged view of a sectional view of the ridge structure portion of the first prior art optical device, (b) Enlarged view of the sectional view of the planar structure portion of the first prior art optical device. (A) An enlarged view of a sectional view of the ridge portion structure of the first prior art optical device and a field distribution of light propagating through the optical waveguide, (b) a planar structure portion of the first prior art optical device. Expanded cross-sectional view and field distribution of light propagating in optical waveguide The top view which shows schematic structure about the optical device of the 2nd prior art (A) Cross-sectional view taken along line DD ′ of FIG. 17, (b) Cross-sectional view taken along line EE ′ of FIG. The top view which shows schematic structure about the optical waveguide of the optical device of the 2nd prior art The perspective view which shows schematic structure about the optical device of the 3rd prior art The top view which shows schematic structure about the optical waveguide of the optical device of a 3rd prior art

  Hereinafter, embodiments of the present invention will be described. However, since the same reference numerals as those in the related art shown in FIGS. 10 to 21 correspond to the same functional units, description of the functional units having the same reference numerals is omitted here. To do.

(Embodiment)
FIG. 1 is a schematic top view of an optical modulator as an optical device according to an embodiment of the present invention. It is to be noted that the object of the present invention is to suppress an increase in insertion loss due to optical coupling, and in the following discussion, the central conductor 4a, the ground conductors 4b and 4c, the SiO ₂ buffer layer 2 and the conductive layer 5 are omitted. To do. 2A and 2B are cross-sectional views taken along lines JJ ′ and KK ′ in FIG.

  The present embodiment is an optical device 50 having a ridge structure having a recess similar to the first and second conventional techniques. In addition to the recesses 9a to 9c having a predetermined depth, the recesses 9a 'to 9c' whose depth gradually decreases toward the Y branch branching point (branch point) are provided. Here, the regions of the recesses 9a ′ to 9c ′ are defined as the spot conversion region V, and the region from the end of the spot conversion region V to the substrate end is defined as the planar region VI.

Here, G ₁ , G ₂ , G ₃ , G ₁ ′, G ₂ ′, G ₃ ′ are gaps in the ridge portion, and H ₁ , H ₂ , H ₃ , H ₁ ′, H ₂ ′, H ₃ 'Is the depth of the recess (the height of the ridge). 2A and 2B, the gaps G of the three ridge portions and the depth H of the concave portions are shown to be equal, but may be different from each other.

The important point of the present invention is that the recess depths H _{1 to} H ₃ and H ₁ ′ to H ₃ ′ in FIGS. 2A and 2B are different from each other (the heights of the ridge portions 8a and 8b). And the heights of the ridge portions 8a 'and 8b' are different from each other). That is, in FIG. 2A corresponding to the line JJ ′ in FIG. 1, the depth of the recess is deep, but in FIG. 2B corresponding to the line KK ′, the depth of the recess becomes shallow. The structure is close to the mold.

  Next, the manufacturing method and operation principle of the present invention will be described with reference to FIGS. 3A and 3B are cross sections corresponding to FIGS. 2A and 2B (that is, cross-sectional views taken along lines JJ ′ and KK ′ in FIG. 1, respectively). .

On the LN substrate 1, dry etching masks 20a, 20b, 20c, and 20d for forming a ridge portion are formed. The dry etching masks 20a, 20b, 20c, and 20d are formed by gradually narrowing a gap between adjacent masks toward a predetermined position. The mask gaps G _M1 ′, G _M2 ′, and G _M3 ′ in the cross section along the line KK ′ are narrower than the mask gaps G _M1 , G _M2 , and G _M3 in the cross section along the line JJ ′. In this embodiment, the mask gap is made zero or sufficiently small near the multiplexing point 3c (branch point 3d) of the Y branching portion. Dry etching is performed on the substrate in this state.

In dry etching, the ion beam is not completely perpendicular to the dry etching masks 20a, 20b, 20c, 20d and the LN substrate 1, so that the side wall (for example, 12) of the ridge formed as a result of the inclination is inclined. As shown in FIG. 5 (only the ridge portion 8a is shown as an example in FIG. 5), the angle of the ridge portion 8a is not 90 degrees but has an inclination θ (usually 60 to 75 degrees). Therefore, if the mask gap is narrow, the depth of the recess is shallow, and if the mask gap is wide, the depth of the recess is deep. It shows the relationship between the mask gap G _M and the recess depth H (H ') in Fig.

The mode after the etching is as shown in FIGS. As shown in FIG. 4B, when the gap G _M ′ (where G _M1 ′, G _M2 ′, and G _M3 ′ are equal to each other and G _M ′) is narrow, the depth H ′ ( Here, H ₁ ′, H ₂ ′, and H ₃ ′ are equal to each other, and H ′ is shallow. On the other hand, when the wide gap G _M as shown in FIG. 4 (a), the depth H of the concave portion becomes deeper.

When the dry etching masks 20a, 20b, 20c, and 20d are removed from the modes shown in FIGS. 4A and 4B, the light having ridge portions having different recess depths shown in FIGS. It can be a device. The mask gap _G M1 _{~G M3,} 'a gap _G 1 _~G 3 of the ridge _{portion, G 1'~G 3' G M1} '~G M3 and is often each substantially equal.

FIG. 7 shows a relationship between the depth of the recess and the mode field diameter. The depth H of the concave portion is shallow (mask gap G _M is narrow) and the spot size is large, the propagation of the depth H of the concave portion is zero waveguide if (mask gap G _M is zero) of (3a, 3b) The spot size of the light is the same as the planar value. On the other hand, if the depth H of the concave portion is deep (mask gap G _M is wide), since the lateral direction of the spot size of light propagating through the optical waveguide is confined by the side wall of the ridge portion decreases.

  That is, according to the present invention, the light guided from the ridge structure portion having a predetermined depth (as shown in FIG. 2A) to the planar region VI can be propagated near the end of the recess so that the propagation loss can be reduced. A spot conversion region V that is gradually shallower is provided.

  Thus, in the present invention, it is not necessary to increase the width of the top portion of the ridge portion as in the prior art, and it is only necessary to change the depth of the concave portion of the ridge portion (the height of the ridge portion) depending on the location. There is an advantage that the present invention can be applied to a small area such as a branch point 3c (combining point 3d) of the Y branching portion (multiplexing portion) of the optical waveguide or an arrayed optical waveguide.

  Note that the spot conversion region V is not limited to the position described above, and may be applied to a part of the input / output optical waveguide, for example, as shown in the optical device 51 of FIG.

(Each embodiment)
The present invention has a feature that enables efficient spot size conversion of light between the ridge structure portion and the planar structure portion even in a narrow area. The idea of the present invention is to have a spot conversion region where the depth of the recess gradually decreases at the boundary between the ridge structure portion and the planar structure portion. Therefore, for example, even when all the optical waveguides have a ridge structure, when the depth of some of the recesses is shallow and the spot size of the shallow region is larger than the spot size of the complete ridge structure, the present invention It belongs to thought.

  Further, it goes without saying that the present invention is applicable not only to a Mach-Zehnder optical waveguide type optical device but also to an optical device having a single optical waveguide. FIG. 9 is a top view of an optical device 60 to which the present invention is applied. The optical device 60 includes concave portions 9d and 9e having a predetermined depth on the substrate 1 'on which the optical waveguide 3 is formed, and the concave portions 9d' and 9e 'extending to 9d and 9e and gradually becoming shallow as in the above embodiment. A spot conversion area V is provided.

  The present invention can also be applied to a bias separation type optical modulator. Further, the present invention does not depend on the electrode structure, and it goes without saying that it can be applied to various electrode structures such as a CPW structure, an asymmetric coplanar strip (ACPS) structure, or a symmetric coplanar strip (CPS) structure.

1, 1 ': z-cut LN substrate (LN substrate)
2: SiO ₂ buffer layer (buffer layer)
3: Mach-Zehnder optical waveguide (optical waveguide)
3a, 3b: Interaction optical waveguide constituting the Mach-Zehnder optical waveguide 3c, 3d: Combined point and branch point of Y branch 4: Traveling wave electrode 4a: Center conductor 4b, 4c: Ground conductor 5: Si conductive layer 6: High frequency Electric signal feeder 7: High-frequency electric signal output line 8a, 8b: Ridge part 8a ', 8b' in the high-frequency electric signal interaction part I: Ridge part 9a, 9b, 9c in the spot conversion region V: High-frequency electric signal mutual Concave part 9d, 9e in action part I: Concave part 9a ', 9b', 9c ', 9d', 9e ': Concave part in spot conversion region V 11a, 11b: DC bias electrode 12: Side wall of ridge part 13: Top part of ridge part 14, 15: Electric field distribution of light 20a, 20b, 20c, 20d: Dry etching mask 50, 51, 60: Optical device 101: Co of first optical waveguide 102: Cladding of the first optical waveguide 103: Upper and lower claddings 104: InP substrate 105: Core of the second optical waveguide 106: Left and right claddings of the second optical waveguide 107: Left and right claddings of the third optical waveguide 111 : First optical waveguide 112: Second optical waveguide 113: Third optical waveguide 120, 121, 122: Light field distribution I and III: High frequency electric signal interaction part IV: DC bias interaction part V : Spot conversion region VI: Planar region C, F: Boundary portion between ridge structure portion and planar structure portion G ₁ , G ₂ , G ₃ : Gap of ridge portion G ₁ ′, G ₂ ′, G ₃ ′: Spot conversion Gap G _M1 , G _M2 , G _M3 : Mask gaps G _M1 ′, G _M2 ′, G _M3 ′: Mask gaps H ₁ , H of spot conversion region V ₂ , H ₃ : Depth of recess in ridge structure (height of ridge)
H ₁ ′, H ₂ ′, H ₃ ′: Depth of concave portion of spot conversion region V (height of ridge portion)
U: Distance between interaction optical waveguide and ridge side wall

Claims (5)

A substrate, an optical waveguide for guiding light formed on the substrate, and a recess having a predetermined depth formed by digging a part of the substrate on both sides of the optical waveguide to form a ridge portion In the optical device
The predetermined depth of the recess is formed to become gradually shallower toward the end of the recess, and the spot size of the guided light gradually increases as the light is guided in the direction of the end of the recess. An optical device characterized by an increase in size.   The optical device according to claim 1, wherein the predetermined depth of the recess is zero at an end of the recess.   The optical device according to claim 1, wherein an end of the recess reaches the end of the substrate and has a predetermined depth.   3. The optical waveguide according to claim 1, wherein the optical waveguide is a Mach-Zehnder optical waveguide, and the concave portion is formed to be gradually shallower toward a branching point or a combining point of the Y branching waveguide. Optical device.   The optical modulator according to any one of claims 1 to 4, wherein the substrate is made of lithium niobate.

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