This invention relates to an optical waveguide element and an optical modulator used in the field of optical communication and optical instrumentation. More particularly, the invention relates to an optical waveguide element having a ridge structure, and to an optical modulator using this optical waveguide element.
As the use of the Internet spreads, the amount of data communicated is rapidly increasing, making the optical fiber communication very important. In the optical fiber communication, electric signals are converted into optical signals, and the optical signals are transmitted through optical fibers. The optical fiber communication is characterized in that the signals are transmitted in the broad band, with a small loss, and are not affected by noise.
Known as systems for converting electric signals into optical signals are the direct modulation system using a semiconductor laser and the external modulation system using optical modulators. The direct modulation system need not use optical modulators and its running cost is low, but cannot achieve high-speed modulation. This is why the external modulation system is used in high-speed, long-distance data communication.
In practically used optical modulators, an optical waveguide is formed by titanium (Ti) diffusion in the vicinity of a surface of a single-crystal lithium niobate substrate. High-speed optical modulators of 40 Gb/s or more are commercially available. However, these high-speed optical modulators have the drawback of having a length as long as approximately 10 cm.
Japanese Patent Application Laid-Open Nos. 2014-006348, 2014-142411 and 2015-014716 disclose Mach-Zehender optical modulators, each having a sapphire single-crystal substrate and a lithium niobate film epitaxially formed on the substrate, 2 μm or less thick, c-axis orientated and shaped like a ridge. Any optical modulator that has a lithium niobate film is much smaller than, and can be driven at a lower voltage than, the optical modulator having a lithium niobate signal-crystal substrate.
The conventional optical waveguide element 400 is disadvantageous in that the propagation loss is large in TM fundamental mode. The reason why a propagation loss of the element 400 is large in TM fundamental mode is the coupling of TM fundamental mode to TE higher-order mode. In TE higher-order mode, light propagates to, for example, the slab parts 4 located outside the ridge part 3, not restricted by the ridge part 3. Hence, the propagation loss in TM fundamental mode can be reduced by suppressing the coupling of TM fundamental mode to TE higher-order mode.
The conventional optical waveguide element 400 is disadvantageous, also in that the propagation loss in TM fundamental mode increases if the ridge width W decreases.
Moreover, the propagation loss abruptly increases even if the shape of the ridge part 3 differs only a little from the design shape. In such a case, the propagation loss may increase due to a variation in manufacturing processes.
It is therefore an object of the present invention to provide a ridge-shaped optical waveguide element that suppresses the coupling of TM fundamental mode to TE higher-order mode, thereby to reduce the propagation loss in TM fundamental mode, and to provide an optical modulator having the optical waveguide element.
Another object of the present invention is to provide a ridge-shaped optical waveguide element that can reduce the propagation loss in TM fundamental mode even if the ridge with W is decreased, and to provide an optical modulator having the optical waveguide element.
Still another object of this invention is to provide a ridge-shaped optical waveguide element that constantly has a small propagation loss, not affected by dimensional changes, if any, during the manufacture, and to provide an optical modulator having the optical waveguide element.
The profound study of the inventors hereof have found that the conventional ridge-shaped optical waveguide has a large propagation loss in TM fundamental mode because TM fundamental mode is coupled to TE slab mode. In TM mode, the main part of the electric field is perpendicular to the major surface of the substrate 1 (i.e., vertical direction in
To reduce the coupling to TE slab mode, the optical waveguide element may be so shaped that the effective refractive index of TE slab mode is lower than the effective refractive index of TM fundamental mode. This invention is based on this technical finding. An optical waveguide element according to the invention comprises a substrate and a waveguide layer made of lithium niobate and formed on the substrate. The waveguide layer has a slab part having a predetermined thickness and a ridge part protruding from the slab part. The optical waveguide element is characterized in that the maximum thickness of the slab part is 0.05 times or more, but less than 0.4 times the wavelength of the light propagating in the ridge part.
In this invention, the effective refractive index for TE slab mode sufficiently decreases. The TM fundamental mode and the TE slab mode are therefore scarcely coupled to each other, and the propagation loss in the TM fundamental mode can be suppressed. Further, if the optical waveguide element according to this invention is used in an optical modulator, the value of VπL can fall within a desirable range.
In this invention, the width of the ridge part is preferably 0.1 times or more, but less than 1.0 time, the wavelength of the light propagating in the ridge part. The invention can, therefore, provide a single-mode optical waveguide element that confines light at the ridge part.
In this invention, the slab part may have a uniform thickness. This prevents the processing of the waveguide layer from becoming complicated.
In this invention, the thickness of the ridge part is preferably 0.5 times or more, but less than 2.0 times, the wavelength of the light propagating in the ridge part. This can prevent the optical waveguide element from operating in multimode, and light can be efficiently confined at the ridge part.
In this invention, the sides of the ridge part are preferably inclined by 70° or more. This means that the sides of the ridge part need not be completely vertical. Even if the sides of the ridge part are inclined, the optical waveguide element can acquire desirable characteristics so long as the inclination angle is 70° or more.
In this invention, the slab part may have inclining parts located at the sides of the ridge part, respectively, and gradually thinned away from the ridge part. This prevents the propagation loss from abruptly changing even if the shape of the ridge part differs a little from the design shape. The propagation loss can therefore remain small.
In this case, the maximum thickness of the inclining part is preferably 0.1 times or more and less than 0.37 times the wavelength of the light propagating in the ridge part, the width of the ridge part is preferably 0.3 times or more, but less than 1.2 times, the wavelength of the light propagating in the ridge part, and the thickness of the ridge part is preferably 0.5 times or more, but less than 2.0 times, the wavelength of the light propagating in the ridge part. Then, the optical waveguide element does not operate in a mixed mode, and can efficiently confine light at the ridge part. The “mixed mode” is mixture of TM mode and TE mode. If the optical waveguide element operates in the mixed mode, the coupling loss between the waveguide and an optical fiber will increase. If the element is used in an optical modulator, the extinction ratio will decrease, the insertion loss will increase, and VπL will increase. Hence, the optical waveguide element should be operated in almost pure TM mode.
An optical modulator according to this invention is characterized in that it has an optical waveguide having an optical waveguide element according to the invention. The optical modulator, which has a high-performance optical waveguide element, has a low insertion loss and a large extinction ratio.
In this invention, the coupling of TM fundamental mode (m=0) to TE higher-order mode (TE slab mode, TE m=1 mode etc.) can be suppressed. This invention can therefore provide an optical waveguide element that can stably have a low propagation loss, and also an optical modulator using the optical waveguide element.
Further, this invention can provide an optical waveguide element that can stably have a low propagation loss, not influenced by dimensional changes, if any, occurring during the manufacture of the element, and can provide an optical modulator having this optical waveguide element.
Moreover, this invention can provide an optical modulator which has an optical waveguide element according to the invention and which can therefore has a small insertion loss and VπL of small value.
The above and other objects, features and advantages of this invention will become more apparent by reference to the following detailed description of the invention taken in conjunction with the accompanying drawings, wherein:
Embodiments of the present invention will be described with reference to the accompanying drawings. This invention is not limited to the embodiments described below. The components described below may include some that can be anticipated by any person with ordinary skill in the art, be substantially identical to those known to such a person, or may be used in any possible combination. The drawing is schematic, and the relation between the thickness and planer size of each component shown may differ from the actual one, so long as the advantage of the invention can be achieved in any embodiment.
The waveguide layer 2 is made mainly of lithium niobate (LiNbO3). Lithium niobate has a large electro-optical constant, and is material for optical devices such as the optical modulator. The waveguide layer 2 is a lithium niobate film having the composition of LixNbAyOz, where A is an element other than Li, Nb and O, x is 0.5 to 1.2, preferably 0.9 to 1.05, y is 0 to 0.5, and z is 1.5 to 4, preferably 2.5 to 3.5. Examples of the element A include K, Na, Rb, Cs, Be, Mg, Ca, Sr, Ba, Ti, Zr, Hf, V, Cr, Mo, W, Fe, Co, Ni, Zn, Sc, and Ce, alone or in combination.
The slab part 4 has thickness T2 so that the effective refractive index for TM fundamental mode may be larger than the effective refractive index for TE slab mode.
More specifically, if the thickness T2 of the slab part 4 is reduced to satisfy 0.05≤T2/λ<0.4, where λ is the wavelength of light propagating in the ridge part 3, the effective refractive index for TE slab mode can be smaller than the effective refractive index for TM fundamental mode even if the ridge width W1 is made smaller to some degree. If the effective refractive index for TE slab mode is smaller than the effective refractive index for TM fundamental mode, the coupling to TM fundamental mode will greatly decrease, reducing the propagation loss in TM fundamental mode.
In this embodiment, the slab part 4 has a substantially uniform thickness T2. Therefore, the ridge part 3 has a simple one-step shape. This is because more complicated processing is required to forma ridge part having two or more steps. A structure having parts gently tapered from the root of the ridge part to the left and right slab parts, as seen in cross section, will be described in connection with the second embodiment.
Preferably, the width W1 of the ridge part 3 should be:
0.1≤W1/λ<1.0.
If the width W1 of the ridge part 3 has this value, the optical waveguide element 100A can be driven, substantially in the single mode, and the light can be fully confined at the ridge part 3. If the width W1 of the ridge part 3 is less than 0.1λ, the light cannot be fully confined at the ridge part 3. If the width W1 of the ridge part 3 is equal to, or greater than, 1λ, the element 100A may operate in multimode.
If the width W1 of the ridge part 3 is decreased, the effective refractive index for TM fundamental mode will decrease. In this case, the thickness T2 of the slab part 4 should therefore be reduced. This means that even if the following relation is satisfied, the effective refractive index for TM fundamental mode will not always exceed the effective refractive index for TE slab mode, and also that T2/λ should be decreased if the ridge with W1 is small.
0.05≤T2/λ<0.4
On the other hand, the thickness T1 of the ridge part 3 should preferably be:
0.5≤T1/λ≤2.0.
More preferably, the thickness T1 of the ridge part 3 should be:
0.6≤T1/λ≤1.5.
This is because if the ridge part 3 is too thin, the light is weakly confined in the waveguide layer 2 and the waveguide may not perform its function. Conversely, if the ridge part 3 is too thick, the manufacture processing will become difficult, and the element 100A will be more likely to operate in multimode.
The substrate 1 is not limited in material if it has a smaller refractive index than the lithium niobate film. Nonetheless, the substrate should preferably be one on which a lithium niobate film can be formed as epitaxial film. In view of this, a sapphire single-crystal substrate or a silicon single-crystal substrate is desirable. The crystal orientation of the single-crystal substrate is not limited to any particular one. Lithium niobate film can be easily formed as c-axis orientated epitaxial film on single-crystal substrates of various crystal orientations. Since the c-axis orientated lithium niobate film has three-fold symmetry, the single-crystal substrate on which it is formed should desirably have the same symmetry. Hence, it is preferable that the substrate has c-plane if it is a sapphire single-crystal substrate, or (111) plane if it is a silicon single-crystal substrate.
The term “epitaxial film”, as used herein, refers to a film having the crystal orientation of the underlying substrate or film. The crystal of an epitaxial film is uniformly oriented along the X-axis and Y-axis on the film surface and along the Z-axis in the thickness direction. For example, an epitaxial film can be confirmed by first measuring the peak intensity at the orientation position by 2θ-θ X-ray diffraction and secondly observing poles.
More specifically, first, in the 2θ-θ X-ray diffraction measurement, all the peak intensities except for the target plane must be 10% or less, preferably 5% or less, of the maximum peak intensity on the target plane. For example, in a c-axis oriented epitaxial lithium niobate film, the peak intensities except for a (00L) plane are 10% or less, preferably 5% or less, of the maximum peak intensity on the (00L) plane. (00L) is a general term for (001), (002), and other equivalent planes.
Secondly, poles must be observed in the measurement. Under the condition where the peak intensities are measured at the first orientation position, only the orientation in a single direction is proved. Even if the first condition is satisfied, in the case of nonuniformity in the in-plane crystalline orientation, the X-ray intensity is not increased at a particular angle, and poles cannot be observed. Since LiNbO3 has a trigonal crystal system, single-crystal LiNbO3 (014) has 3 poles. For the lithium niobate film, it is known that crystals rotated 180 degrees about the c-axis are epitaxially grown in a symmetrically-coupled twin crystal state. In this case, three poles are symmetrically-coupled to form six poles. When the lithium niobate film is formed on a single-crystal silicon substrate having a (100) plane, the substrate has fourfold symmetry, and 4×3=12 poles are observed. In the present invention, the lithium niobate film epitaxially grown in the twin crystal state is also considered to be an epitaxial film.
It is desirable to form the lithium niobate film by a film forming method such as sputtering, CVD or sol-gel process. If the c-axis is orientated perpendicular to the major surface of the single-crystal substrate, an electric field is applied parallel to the c-axis, thereby changing the optical refractive index in proportion to the intensity of the electric field. If the single-crystal substrate is sapphire, a lithium niobate film is formed by epitaxial growth directly on the sapphire single-crystal substrate. If the single-crystal substrate is a silicon substrate, a lithium niobate film is formed by epitaxial growth on the cladding layer (not shown) formed on the substrate. The cladding layer (not shown) is made of material which has a smaller refractive index than lithium niobate film and which well undergoes epitaxial growth. If the cladding layer (not shown) is made of Y2O3, a lithium niobate film of high quality can be formed.
As is known in the art, the lithium niobate film may be formed by polishing a lithium niobate single-crystal substrate, reducing the thickness thereof. This method is advantageous in that the polished substrate acquires the characteristic as single crystal, and can therefore be used in the present invention.
Since the ridge part 3 has a trapezoidal cross section, the width of its upper surface shall be defined as ridge width W1. The inclination angle θA should be as close to 90° as possible, but may be at least 70°. If the inclination angle θA is 70° or more, the optical waveguide element 101A can operate in pure TM mode not mixing with TE mode, provided that the ridge width W1 has an appropriate value. If the inclination angle θA is less than 70°, optical waveguide element 101A can hardly operate in pure TM mode not mixing with TE mode at all.
As exemplified in the modification of
The slab part 4 can be so shaped in some cases, depending on the condition of etching the waveguide layer 2. As exemplified in the modification of
The operating principle of the optical modulator 200A will be explained. As shown in
The waveguide layers 2 provided in the optical waveguides 10a, 10b and 10c have the same shape as described with reference to
0.05≤T2/λ≤0.4,
where λ is the wavelength of the light propagating through the ridge part 3. The effective refractive index for TM fundamental mode can therefore be larger than the effective refractive index for TE slab mode. Hence, the propagation loss in the TM fundamental mode can be greatly reduced.
If the thickness T2 of the slab part 4 is less than 0.05λ, the electric field applied to the ridge part 3 will become weak, hardly modulating the light sufficiently, even if a voltage is applied between the first electrodes 7a and 7b, on one hand, and the second electrodes 8a, 8b and 8c, on the other. This inevitably degrades VπL.
Preferably, the ridge width W1 should have the following value:
0.1≤W1/λ≤1.0.
If the ridge width W1 should have this value, the optical modulator 200A can operate, substantially in the single mode, and the light can be fully confined in the ridge part 3.
As described above, in the optical waveguide element 100A and the optical modulator 200A, both according to the present invention, the slab part 4 has thickness T2 of 0.05λ, or more, but less than 0.4λ. The effective refractive index for TM fundamental mode can be larger than the effective refractive index for TE slab mode. Therefore, the coupling of TM fundamental mode to TE higher-order mode can decrease, thereby it is possible to reduce the propagation loss in TM fundamental mode.
The second embodiment of this invention will now be described.
The flat part 4a of the slab part 4 has thickness that is almost uniform, i.e., thickness T3. The inclining part 4b of the slab part 4 is gradually thinned away from the ridge part 3, and has the maximum thickness T2. Hence, the maximum thickness T2 each slab part 4 has is the maximum thickness of the inclining part 4b in this embodiment.
In this embodiment, the width (ridge width) W1 of the ridge part 3 should preferably be:
0.3≤W1/λ≤1.2.
If the ridge width W1 satisfies this relation, the optical waveguide element 100B can operate in almost pure TM mode, not in TM-TE mixed mode, if an appropriate maximum thickness T2 is set for the inclining part 4b. If the ridge width W1 is less than 0.3λ, however, the light will be insufficiently confined in the ridge part 3. If the ridge width W1 exceeds 1.2λ, the main component of TE m=1 mode will exist not in the inclining part 4b, but in the ridge part 3, and the optical waveguide element 100B will inevitably operate in TM-TE mixed mode even if the maximum thickness T2 of the inclining part 4b is changed.
In this embodiment, too, the thickness T1 of the ridge part 3 should preferably be:
0.5≤T1/λ≤2.0,
more suitably,
0.6≤T1/λ≤1.5.
If the ridge part 3 is too thin, the light will be insufficiently confined in the ridge part 3, and the optical waveguide element 100B will probably cease to function as an optical waveguide. If the ridge part 3 is too thick, it will be difficult to manufacture the optical waveguide element 100B.
It is most desirable that the sides of the ridge part 3 should be perpendicular to the substrate 1 and the upper surface of the ridge part 3 should be horizontal to the substrate 1. Nevertheless, this invention is not limited to this configuration. The ridge part 3 may be chamfered at corners as shown in
The inclining part 4b is gradually thin away from the ridge part 3, and incline by the inclination angle θB of 45° or less. The inclination angle θB may be constant as shown in
The flat part 4a is that part of the slab part 4, which has an almost uniform thickness. Their thickness T3 should preferably satisfy the following relation:
T3/λ<0.37
Then, the effective refractive index for TE slab mode can be smaller than the effective refractive index for TM fundamental mode even if the ridge width W1 is reduced to some extent. If the effective refractive index for TE slab mode is smaller than the effective refractive index for TM fundamental mode, the coupling of TM fundamental mode to TE slab mode is much suppressed, reducing the propagation loss in TM fundamental mode.
The difference between the maximum thickness T2 of the inclining part 4b and the thickness T3 of the flat part 4a is not limited so long as T2>T3. However, the difference between the thickness T2 and thickness T3 is desirably as follows:
T2−T3≥0.05 μm.
The width W2 of the inclining part 4b is not limited. However, it may range from 0.5 μm to 50 μm in most cases.
The optical waveguide element 100B according to this embodiment has an inclining part 4b between the ridge part 3 and the flat part 4a. The optical waveguide element 100B may be more likely to operate in TM-TE mixed mode, i.e., mixture of TM fundamental (m=0) mode and TE higher-order mode. If it operates in TM-TE mixed mode, problems will arise, such as decrease in extinction ratio, increase in insertion loss and the rising of VπL. The TM-TE mixed mode is due to a condition is satisfied that the effective refractive index for TM fundamental (m=0) mode is almost equal to the effective refractive index for TE higher-order mode such as TE, m=1 mode.
In order to make the optical waveguide element 100b operate not in the mixed mode, the maximum thickness T2 of the inclining part 4b should better have a value falling within the following range:
0.1≤T2/λ≤0.37.
If the maximum thickness T2 of the inclining part 4b falls within this range, the effective refractive index for TE slab (m=1) mode is smaller than the effective refractive index for TM fundamental (m=0) mode. Then, the optical waveguide element 100b operates not in the mixed mode, but in almost pure TM mode.
The main component of the electric field extends vertically in the TM mode, and the main component of the electric field extends horizontally in the TE mode. At least four wave guiding modes exist, TM, m=0 mode, TE, m=0 mode, TM, m=1 mode, and TE, m=1 mode. The lithium niobate film is material exhibiting birefringence. If the film is c-axis orientated, it has refractive index ne in the vertical direction of the electric field (i.e., refractive index for extraordinary light), which is smaller than the refractive index no in the horizontal direction of the electric field (i.e., refractive index for ordinary light). Usually, the effective refractive index for the m=0 mode is larger than the effective refractive index for the m=1 mode. Due to this birefringence, however, the effective refractive index for TM, m=0 mode is almost equal to the effective refractive index for TE, m=1 mode resulting in the mixed mode in some cases.
In the optical waveguide element 100B according to this embodiment, the light component of TM, m=0 mode is confined in the ridge part 3 as seen from
Some of the inventors hereof profoundly studied how to suppress the coupling of TM fundamental mode to TE higher-order mode, and invented the two-step ridge structure shown in
In the two-step ridge structure, however, the propagation loss abruptly increases as shown in
The optical waveguide element 100B according to this embodiment prevents the propagation loss from changing abruptly contrary to the two-step ridge structure having a ridge shaped so specifically as sown in
First, as shown in
An example of an optical modulator using this optical waveguide element 100B according to this embodiment will be described below.
As may be seen from
In this embodiment, a sapphire substrate is used as substrate 1 and a lithium niobate film is formed on the major surface of the substrate. The waveguide layer 2 constitutes optical waveguides 10a and 10b, each composed of a ridge part 3 and an inclining part 4b. A buffer layer 13 is formed on the ridge part 3 of the optical waveguide 10a, and a first electrode 7a is formed on the buffer layer 13. Similarly, a buffer layer 13 is formed on the ridge part 3 of the optical waveguide 10b, and a first electrode 7b is formed on the buffer layer 13. Second electrodes 8a, 8b and 8c are spaced apart, with the first electrodes 7a and 7b located among them, and contact the upper surfaces of the slab parts 4 of the waveguide layer 2. Dielectric layers 14 are formed, each contacting the lower surface of the associated buffer layer 13 and the sides of the associated ridge part 3.
The operating principle of the optical modulator 200B will be explained. As shown in
The waveguide layers 2 provided in the optical waveguides 10a, 10b and 10c have the same shape as described with reference to
The maximum thickness T2 of the inclining part 4b, the width W1 of the ridge part 3, and the thickness T1 of the ridge part 3 are set as follows:
0.1≤T2/λ≤0.37,
0.3≤W1/λ≤1.2,
0.5≤T1/λ≤2.0,
where λ is the wavelength of the light propagating through the ridge part 3.
The optical modulator 200B can therefore efficiently confine the light at the ridge part with a small propagation loss, while being prevented from operating in the mixed mode. Thus, the optical modulator 200B can have a small insertion loss, a high extinction ratio and low VπL.
The multimode interference branching waveguide 150 is characterized in that the propagation loss is larger in the m=1 mode than in the m=0 mode.
In this multimode interference branching waveguide 150, the m=0 mode component is branched, without being attenuated, into two optical waveguides 10a and 10b, and the m=1 mode component is attenuated and scarcely output from the optical waveguides 10a and 10b. The waveguide 150 can therefore be handled in the same way as a single-mode interference branching waveguide, though the light propagates through it in both TM, m=0 mode and TM, m=1 mode.
As specified above, the optical waveguide element 100B according to this embodiment has a slab part 4 that has an inclining part 4b around the ridge part 3. Therefore, the propagation loss would not greatly change even if dimensional changes occur during the manufacture of the element 100B. The optical waveguide element 100B has, but a small propagation loss, even if dimensional changes occur during the manufacture.
It is apparent that the present invention is not limited to the above embodiments, but may be modified and changed without departing from the scope and spirit of the invention.
In the optical waveguide element 100B according to the second embodiment, the slab part 4 has a flat part 4a. According to this invention, however, the slab part 4 need not have a flat part 4a. As in the modification shown in
In any one of the embodiments described above, the waveguide layer 2 that is a lithium niobate film orientated in the c-axis, and guides light in TM, m=0 mode. Nonetheless, this invention can be applied to the case where the c-axis is orientated in a plane, as will be explained below.
If the c-axis is orientated in a plane, the electro-optical effect increases in TE mode. The optical waveguide element is operated in TE, m=0 mode. If the c-axis is orientated in plane, the refractive index ne (for extraordinary light) the electric field has in the horizontal direction (parallel to the c-axis) is smaller than the refractive index no (for ordinary light) the electric field has in the horizontal vertical. After all, the in-plane orientation of the c-axis is equivalent to TM and TE inverted in the vertical orientation, and this invention works well. If the c-axis is orientated in plane, the effective refractive index for TE, m=0 mode is almost equal to the effective refractive index for TM, m=1 mode in some cases. The optical waveguide element may therefore operate in the mixed mode. However, the element can be prevented from operating in the mixed mode if the maximum thickness of the inclining part, the width of the ridge part and the thickness of the ridge part are set to the values identical to those for the vertical orientation.
The optical modulators 200A and 200B shown in
As shown in
The Mach-Zehender optical waveguide 10 is an optical waveguide having the structure of the Mach-Zehender interferometer. The optical waveguide 10 has first and second optical waveguides 10a and 10b. The optical waveguides 10a and 10b are branched from one input optical waveguide 10i by a multimode interference branching waveguide 10c, and are combined by a multimode interference branching waveguide 10d, forming an output optical waveguide 10o. Input light Si is therefore branched by the multimode interference branching waveguide 10c into two light beams. The light beams propagate through the optical waveguides 10a and 10b, respectively, and are then combined by the multimode interference branching waveguide 10d into modulated light So. The modulated light So is output from the output optical waveguide 10o.
The signal electrode 27 is positioned between the first ground electrode 28 and the second ground electrode 29 as viewed from above the optical modulator 200C. One end 27e of the signal electrode 27 is the signal input terminal. The other end 27g of the signal electrode 27 is connected to the first and second ground electrodes 28 and 29 via a terminal resistor 22. The signal electrode 27 and the first and second ground electrodes 28 and 29 function as coplanar traveling-wave electrodes. The signal electrode 27 and the first ground electrode 28 are double-layer electrodes. The lower layer 27b of the signal electrode 27, indicated by broken lines, overlaps the first optical waveguide 10a as viewed from above the optical modulator 200C. Similarly, the lower layer 28b of the first ground electrode 28, indicated by broken lines, overlaps the second optical waveguide 10b as viewed from above the optical modulator 200C.
An electric signal (modulation signal) is input to the end 27e of the signal electrode 27. Since the first and second optical waveguides 10a and 10b are made of material having electro-optical effect, such as lithium niobate, the electric field applied to the first and second optical waveguides 10a and 10b changes the refractive indices of the optical waveguides 10a and 10b to +Δn and −Δn, respectively. As a result, the phase difference between the two optical waveguides 10a and 10b changes. An optical signal modulated by this change in phase difference is output from the output optical waveguide 10o.
The optical modulator 200C according to this embodiment has one signal electrode 27 and is single-drive type. Therefore, the first ground electrode 28 can therefore have a sufficient area, and the optical modulator 200C can operate at high frequencies. Further, since the second ground electrode 29 opposes the first ground electrode 28 across the signal electrode 27, the radiation loss can be reduced. The optical modulator 200C can therefore acquire a good high-frequency characteristic.
Simulation was conducted to see how the effective refractive index N for TE slab mode changes in the optical waveguide element 100A shown in
As shown in
An optical waveguide element 100A of the type shown in
As
An optical waveguide element 100A of the type shown in
As
Simulation was conducted to see how the effective refractive index N changes in the optical waveguide element 100A shown in
As shown in
Simulation was conducted to see how the effective refractive index N changes in the optical waveguide element 100A shown in
As shown in
An optical modulator 200A of the type shown in
As shown in
Simulation was conducted to see whether the optical waveguide element 101A shown in
As seen from
Simulation was conducted to see in which waveguide mode the optical waveguide element 100B having the structure shown in
As seen from
W1/λ=0.39 to 1.16
T2/λ=0.26 to 0.35.
To prevent the optical waveguide element 100B from operating in the mixed mode, the effective refractive index (TM, m=0 mode) must be larger than the effective refractive index (TE, m=1 mode), namely:
Index(TM,m=0 mode)>index(TE,m=1 mode).
In the m=1 mode, the light is confined mainly at the inclining part 4b, and the effective refractive index for the m=1 mode can be reduced by decreasing the maximum thickness T2 of the inclining part 4b. In the m=0 mode, the light is confined mainly at the ridge part 3, and the effective refractive index changes only a little even if the maximum thickness T2 is decreased. Hence, if the maximum thickness T2 is decreased, satisfying the following relation:
Effective refractive index(TM,m=0 mode)>effective refractive index(TE,m=1 mode).
As may be seen from
The effective refractive index for the slab mode is smaller than the effective refractive index for the m=1 mode. Therefore, if the effective refractive index (for TM, m=0 mode) is larger than the effective refractive index (for TE, m=1 mode), the following relation is automatically satisfied:
Effective refractive index(TM,m=0 mode)>effective refractive index(TE slab mode).
The TM, m=0 mode would not, therefore, be coupled to the TE slab mode to increase the propagation loss.
Simulation was conducted to see how the optical waveguide element 100B having the structure of
As seen from
T2/λ=0.26 to 0.35
T3/λ=0.06 to 0.26.
As seen from
An optical waveguide element was produced, in which T2/λ=0.29 and T3/λ=0.19, and was evaluated for physical properties. The propagation loss was as small as 1 dB/cm or less. The output light had linear polarization degree of 20 dB or more. This proves that the operating mode did not mix with the TE mode.
Several samples of the optical modulator 200B shown in
As seen from
This application is a continuation of U.S. Ser. No. 15/906,280 filed on Feb. 27, 2018, now U.S. Pat. No. 10,203,583, which is a continuation of U.S. Ser. No. 15/236,633 filed on Aug. 15, 2016, now U.S. Pat. No. 9,939,709
Number | Name | Date | Kind |
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20160377953 | Feng | Dec 2016 | A1 |
Number | Date | Country | |
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20190079366 A1 | Mar 2019 | US |
Number | Date | Country | |
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Parent | 15906280 | Feb 2018 | US |
Child | 16185337 | US | |
Parent | 15236633 | Aug 2016 | US |
Child | 15906280 | US |