Superlattice electro-optical thin film material and composite film layer and electro-optical component comprising same
By alternating growth of dielectric material layers and barium titanate layers on a substrate, the performance limitations of existing electro-optic materials have been solved, enabling high-performance and miniaturized electro-optic modulators that improve bandwidth and reduce half-wave voltage.
Patent Information
- Application Number
- CN202411311813.2
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2024-09-19
- Publication Date
- 2026-03-20
AI Technical Summary
Existing electro-optic materials, such as lithium niobate thin films, have low electro-optic coefficients, which limits the bandwidth improvement and half-wave voltage reduction of electro-optic modulators. They also have poor compatibility with CMOS technology, making it difficult to achieve direct epitaxial growth. Epitaxial growth of barium titanate thin films suffers from performance degradation.
Superlattice electro-optic thin film materials, consisting of periodically alternating dielectric material layers and barium titanate layers, are formed on a substrate by epitaxial growth. The dielectric material is selected from scandates or strontium tantalum lanthanum aluminate. Superlattice electro-optic thin films are prepared by combining pulsed laser deposition and other techniques.
The electro-optic performance of the electro-optic modulator has been improved, with the electro-optic coefficient approaching or exceeding the bulk level. This has enabled the miniaturization and performance enhancement of the electro-optic modulator, increased the phase change by at least one order of magnitude, and improved compatibility with CMOS technology.
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Abstract
Description
Technical Field
[0001] This application relates to the field of materials technology, and more particularly to a superlattice electro-optic thin film material, as well as composite films and electro-optic devices containing the same. Background Technology
[0002] Electro-optic materials are widely used in technologies involving electro-optic signal conversion, such as optical transmission, optical computing, and on-chip light sources. Electro-optic modulators are key components for data transmission and processing using a hybrid of electronic and optical elements. The critical parameters of electro-optic modulators are half-wave voltage and bandwidth. The electro-optic properties of the material directly affect the performance parameters of the electro-optic modulator. Considering miniaturization, manufacturing cost, and energy efficiency, high-performance novel electro-optic thin-film platforms are an effective solution for achieving performance breakthroughs on on-chip devices.
[0003] Currently, lithium niobate thin films (LNOI, γ-ray distillate) on insulating substrates with high electro-optic coefficients are available. 33 = 30 pm / V) is considered a next-generation electro-optic functional platform. However, this electro-optic material still has many shortcomings. For example, the electro-optic coefficient of lithium niobate thin films is still relatively low, which limits the bandwidth improvement and half-wave voltage reduction of electro-optic modulators. The compatibility of lithium niobate with complementary metal-oxide-semiconductor (CMOS) technology is difficult to achieve, making direct epitaxial growth challenging and requiring the use of high-tech and costly single-crystal bonding processes. Furthermore, while bulk barium titanate materials possess excellent electro-optic properties, exhibiting an ultra-high clamped electro-optic coefficient component γ of 550 pm / V in the 1550 nm band... 42 However, barium titanate films grown epitaxially exhibit significant performance degradation and cannot reach the bulk level.
[0004] Therefore, there is a need for an electro-optic thin film material that has improved electro-optic performance and is easy to manufacture. Summary of the Invention
[0005] The following is an overview of the subject matter described in detail herein. This overview is not intended to limit the scope of protection of this application.
[0006] In one aspect, this application provides a superlattice electro-optic thin film material, which is composed of periodically alternating dielectric material layers and barium titanate layers. In each cycle, each barium titanate layer has m unit cells, and each dielectric material layer has n unit cells, where m is in the range of 5-50 and n is in the range of 1-20; wherein the material used for the dielectric material layer is selected from scandates or strontium tantalum aluminate.
[0007] In this application, the end of the cycle can be either the barium titanate layer or the dielectric material layer.
[0008] In one embodiment, the scandate is selected from one of GdScO3, SmScO3, TbScO3, NdScO3, HoScO3, ErScO3, TmScO3, YbScO3, LuScO3, PrScO3, LaScO3, and YScO3.
[0009] In one embodiment, the total thickness of the superlattice electro-optic thin film material is in the range of 50 nm to 1000 nm.
[0010] In this application, the total thickness d of the superlattice electro-optic thin film material is equal to the number of periods W of the superlattice structure × (m × 0.4 nm + n × 0.4 nm). That is, in the barium titanate layer, the thickness of each unit cell is 0.4 nm; in the dielectric material layer, the thickness of each unit cell is also 0.4 nm.
[0011] On the other hand, this application provides a composite film layer, including a substrate and the aforementioned superlattice electro-optic thin film material located on the substrate.
[0012] In one embodiment, the dielectric material layer and the barium titanate layer are alternately grown on the substrate by an epitaxial growth method.
[0013] In one embodiment, the epitaxial growth method is pulsed laser deposition, molecular beam epitaxy, radio frequency magnetron sputtering, or sol-gel method.
[0014] In one embodiment, the material used for the substrate is selected from one of strontium tantalum lanthanum aluminate, GdScO3, SmScO3, TbScO3, NdScO3, HoScO3, ErScO3, TmScO3, YbScO3, LuScO3, PrScO3, LaScO3, YScO3, SiO2, MgO, Al2O3, and silicon-on-insulator (SOI).
[0015] In one embodiment, the composite film layer further includes a buffer layer located between the substrate and the dielectric material layer.
[0016] In this application, the material of the buffer layer can be selected from one of strontium tantalum aluminate, lanthanum tantalum aluminate, GdScO3, SmScO3, TbScO3, NdScO3, HoScO3, ErScO3, TmScO3, YbScO3, LuScO3, PrScO3, LaScO3, and YScO3.
[0017] In another aspect, this application also provides an electro-optic device comprising the aforementioned superlattice electro-optic thin film material.
[0018] In one embodiment, the electro-optical device includes one of an electro-optical modulator, an electro-optical switch, an electro-optical phase shifter, and a wavelength division multiplexer / mode division multiplexer.
[0019] In this application, depending on the principle, the electro-optic modulator may include a Mach-Zehnder electro-optic modulator, a resonant ring electro-optic modulator, etc.
[0020] The superlattice electro-optic thin film material of this application can also be used in integrated photonic chip systems based on these electro-optic modulators, and its function can be used for optical computing, optical communication, etc.
[0021] The electro-optic composite film based on the barium titanate superlattice structure of this application solves the problem of deterioration in the electro-optic performance of traditional barium titanate films, enabling the electro-optic coefficient of barium titanate films to approach or even exceed the bulk level, such as the γ-ray coefficient of the composite film of this application. 42 The coefficient can reach greater than 500 pm / V, thus realizing the miniaturization of electro-optical components based on barium titanate thin films.
[0022] Furthermore, compared with currently used lithium niobate films, the phase change of the electro-optic composite film based on the barium titanate superlattice structure of this application is increased by at least one order of magnitude under the same electric field strength.
[0023] Other features and advantages of this application will be set forth in the following description, and will be apparent in part from the description, or may be learned by practicing the application. Other advantages of this application can be realized and obtained by means of the solutions described in the description and the accompanying drawings. Attached Figure Description
[0024] The accompanying drawings are used to provide an understanding of the technical solutions of this application and constitute a part of the specification. They are used together with the embodiments of this application to explain the technical solutions of this application and do not constitute a limitation on the technical solutions of this application.
[0025] Figure 1A This is a schematic diagram of the structure of the composite film layer provided according to Embodiment 1 of this application; Figure 1B This is a schematic diagram of the structure of the composite film layer provided according to Embodiment 3 of this application; Figure 2A The XRD pattern of the composite film provided according to Example 1 of this application; Figure 2B The XRD reciprocal space diagram of the composite film layer provided according to Embodiment 2 of this application; Figure 3A A low-resolution scanning transmission electron microscope (STEM) photograph of a cross-section of the composite film layer provided in Embodiment 1 of this application; Figure 3BAtom-resolved high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) photograph of a cross-section of the composite film layer provided in Embodiment 2 of this application; Figure 4 A schematic diagram of the structure of a testing device used to test the electro-optic properties of the composite films provided in the embodiments of this application; Figure 5A To illustrate the electro-optic response loops of the composite film layer provided in Embodiment 1 of this application under different bias electric fields when the AC signal electric field strength is constant; and Figure 5B The graph shows the linear electro-optic response loops of the composite film layer provided according to Embodiment 2 of this application under different AC signal electric fields when the bias signal electric field strength is constant. Detailed Implementation
[0026] To make the objectives, technical solutions, and advantages of this application clearer, the embodiments of this application are described in detail below. It should be noted that, unless otherwise specified, the embodiments and features described in these embodiments can be arbitrarily combined with each other.
[0027] This application provides a superlattice electro-optic thin film material, which is composed of periodically alternating dielectric material layers and barium titanate layers. In each cycle, each barium titanate layer has m unit cells, and each dielectric material layer has n unit cells, where m is in the range of 5-50 and n is in the range of 1-20; wherein the material used for the dielectric material layer is selected from scandates or strontium tantalum aluminate.
[0028] This application also provides a composite film layer, including a substrate and a superlattice electro-optic thin film material located on the substrate.
[0029] The following describes in detail the preparation method of the composite film layer containing superlattice electro-optic thin film material of this application. Unless otherwise specified, the materials used in the examples are all commercially available.
[0030] Example 1 This embodiment provides a composite film layer, such as Figure 1A As shown, the composite film may include a substrate 1, a plurality of dielectric material layers 3 and a barium titanate layer 4 that are alternately grown on the substrate 1.
[0031] The method for preparing the composite film may include the following steps: (1) Provide BaTiO3 target, GdScO3 target and strontium tantalum lanthanum aluminate (LSAT) single crystal substrate; use silver paste to attach the substrate to the heating module, and use adhesive to attach BaTiO3 target and GdScO3 target to the target holder respectively.
[0032] (2) Use a molecular pump assembly to extract the gas in the cavity to achieve a vacuum of 10. -4 Pa, after heating the substrate to 770°C using a heating module, oxygen is introduced to maintain the oxygen pressure in the cavity at 5 Pa, and the laser spot size is set to 3 cm after focusing. 2 The energy density is 1.1 J / cm³. 2 The laser is turned on so that a focused laser with a pulse frequency of 3 Hz is irradiated on the GdScO3 target, generating gadolinium scandate ion afterglow, which is then sputtered onto the substrate surface. The growth state and growth rate of the thin film on the substrate surface are observed by a reflective electron beam system, and the number of laser pulses is controlled to grow a 2 nm thick GdScO3 epitaxial layer on the strontium tantalum lanthanum aluminate (LSAT) substrate (along the
[001] direction).
[0033] (3) Lower the substrate temperature to 660°C, maintain the oxygen pressure inside the cavity at 5 Pa, and maintain the laser energy density at 1.1 J / cm². 2 The pulse frequency is 3 Hz, and the target stage is moved to focus the laser pulse onto the barium titanate target. The growth state and growth rate of the thin film on the substrate surface are observed by a reflective electron beam system, and the number of laser pulses is controlled to grow a 6 nm thick BaTiO3 epitaxial layer.
[0034] (4) Repeat steps (2) and (3) 20 times in this example until the superlattice film as a whole reaches about 160 nm.
[0035] (5) Introduce oxygen into the cavity to make the oxygen pressure in the cavity reach 20 kPa, and anneal the substrate and the superlattice thin film layer grown on it at 660°C for 10 minutes. Then, cool the cavity at a rate of 5°C / min to obtain the composite film layer.
[0036] Example 2 This embodiment provides a composite film layer, which may include a substrate, a plurality of dielectric material layers and a barium titanate layer grown alternately on the substrate.
[0037] The method for preparing the composite film may include the following steps: (1) Provide BaTiO3 target, GdScO3 target and strontium tantalum lanthanum aluminate (LSAT) single crystal substrate. Use silver paste to attach the substrate to the heating module, and use adhesive to attach the BaTiO3 target and GdScO3 target to the target holder respectively.
[0038] (2) Use a molecular pump assembly to extract the gas in the cavity to achieve a vacuum of 10. -4 After heating the substrate to 770°C using a heating module, oxygen was introduced to maintain the oxygen pressure inside the cavity at 5 Pa. The laser spot size was set to 3 cm after focusing. 2The energy density is 1.1 J / cm³. 2 The laser is turned on, and a focused laser with a pulse frequency of 3 Hz is used to irradiate the GdScO3 target, generating gadolinium scandate ion afterglow, which is then sputtered onto the substrate surface. The growth state and growth rate of the thin film on the substrate surface are observed through a reflective electron beam system, and the number of laser pulses is controlled to grow a 1 nm thick GdScO3 epitaxial layer.
[0039] (3) Lower the substrate temperature to 660°C, maintain the oxygen pressure inside the cavity at 5 Pa, and maintain the laser energy density at 1.1 J / cm². 2 The pulse frequency is 3 Hz, and the target stage is moved to focus the laser pulse onto the barium titanate target. The growth state and growth rate of the thin film on the substrate surface are observed by a reflective electron beam system, and the number of laser pulses is controlled to grow a 6 nm thick BaTiO3 epitaxial layer.
[0040] (4) Repeat steps (2) and (3) 20 times in this embodiment until the superlattice film reaches approximately 140 nm.
[0041] (5) Introduce oxygen into the cavity to make the oxygen pressure in the cavity reach 20 kPa, and anneal the substrate and the superlattice thin film layer grown on it at 660°C for 10 minutes. Then, cool the cavity at a rate of 5°C / min to obtain the composite film layer.
[0042] Example 3 This embodiment provides a composite film layer, such as Figure 1B As shown, the composite film may include a substrate 1, a buffer layer 2 located on the substrate 1, and a plurality of dielectric material layers 3 and barium titanate layers 4 that are alternately grown in sequence on the buffer layer 2.
[0043] The method for preparing the composite film may include the following steps: (1) Provide BaTiO3 target, PrScO3 target, GdScO3 target and strontium tantalum lanthanum aluminate (LSAT) single crystal substrate. Use silver paste to attach the substrate to the heating module, and use adhesive to attach the BaTiO3 target, PrScO3 target and GdScO3 target to the target holder respectively.
[0044] (2) Use a molecular pump assembly to extract the gas in the cavity to achieve a vacuum of 10. -4 After heating the substrate to 750°C using a heating module, oxygen was introduced to maintain the oxygen pressure inside the cavity at 5 Pa. The laser spot size was set to 3 cm after focusing. 2 The energy density is 1.1 J / cm³. 2The laser is turned on, and a focused laser with a pulse frequency of 3 Hz is used to irradiate the PrScO3 target, generating praseodymium scandate ion afterglow, which is then sputtered onto the substrate surface. The growth state and growth rate of the thin film on the substrate surface are observed through a reflective electron beam system, and the number of laser pulses is controlled to grow a 20 nm thick PrScO3 epitaxial buffer layer.
[0045] (3) Raise the substrate temperature to 770°C, maintain the oxygen pressure in the cavity at 5 Pa, and maintain the laser energy density at 1 J / cm². 2 The pulse frequency is 3 Hz. The target stage is moved so that the laser pulse is focused on the GdScO3 target. The growth state and growth rate of the thin film on the substrate surface are observed by a reflective electron beam system. The number of laser pulses is controlled to grow a 2 nm thick GdScO3 epitaxial layer on the PrScO3 epitaxial buffer layer.
[0046] (4) Lower the substrate temperature to 660°C, maintain the oxygen pressure in the cavity at 5 Pa, and maintain the laser energy density at 1.1 J / cm². 2 The pulse frequency is 3 Hz. The target stage is moved so that the laser pulse is focused on the barium titanate target. The growth state and growth rate of the thin film on the substrate surface are observed by a reflective electron beam system. The number of laser pulses is controlled to grow a 6 nm thick BaTiO3 epitaxial layer on the GdScO3 layer.
[0047] (5) Repeat steps (3) and (4) 20 times in this embodiment until the superlattice film reaches approximately 160 nm.
[0048] (5) Introduce oxygen into the cavity to make the oxygen pressure in the cavity reach 20 kPa, and anneal the substrate and the thin film layer grown on it at 660°C for 10 minutes. Then, cool the cavity at a rate of 5°C / min to obtain the composite film layer.
[0049] This application reduces the degree of lattice mismatch between the substrate layer (LSAT) and the superlattice structure by first growing a PrScO3 epitaxial buffer layer on the substrate before epitaxially growing the superlattice structure, which is more conducive to the growth quality of the subsequent superlattice structure.
[0050] Performance testing 1. X-ray diffraction (XRD) analysis XRD analysis was performed on the composite films obtained in Examples 1 and 2, and the results are as follows: Figure 2A and 2B As shown, where Figure 2A In the graph, the horizontal axis represents 2θ, where θ is the angle between the X-rays and the surface of the superlattice thin film sample, and the vertical axis represents the X-ray intensity. From... Figure 2AThe film peaks of the ultra-clean lattice film, as well as the multi-level reflection peaks generated by superlattice interface diffraction, can be observed. This indicates that the film has formed a good superlattice periodic structure in the out-of-plane direction, and a clear epitaxial interface is formed between different materials. Figure 2B This is the XRD reciprocal space diagram of the thin film (103) crystal plane in Example 2. The lattice parameters of the c-axis outside the thin film plane and the a-axis inside the thin film plane can be obtained through the thin film (103) crystal plane; at the same time Figure 2B This also indicates that in Example 2, the thin film follows epitaxial growth on the substrate, and the epitaxial stress of the substrate relaxes during the thin film growth process.
[0051] 2. Transmission electron microscopy analysis Transmission electron microscopy analysis was performed on the composite films obtained in Examples 1 and 2, and the results are as follows: Figure 3A and Figure 3B As shown, where Figure 3A A low-resolution image of a cross-section of a single-crystal thin film (100) with c-axis orientation
[001] . Figure 3B This is a magnified HAADF-STEM image of the region at atomic resolution. From... Figure 3A and Figure 3B It can be seen that the thin film has good epitaxiality, clear superlattice interface, no defects, no dislocations, and high film quality, showing a good superlattice structure.
[0052] 3. Electro-optical performance testing Such as Figure 4 The exemplary electro-optic performance testing equipment shown is used to test the composite films obtained in Examples 1 and 2. Figure 4 As shown, the testing equipment may include a laser light source 10, a dichroic plate 11, a half-wave plate 12 for testing the optical path, a microscope objective 13, a sample 14, a variable temperature stage 15, a displacement stage 16, a quarter-wave plate 17, a polarizer 18, and a photodetector 19, as well as modules for providing test signal generation and reception, including a lock-in amplifier 20 and a high-voltage amplifier 21, wherein the sample 14 includes a thin-film electrode 30 prepared from the composite film layer of this application.
[0053] The exemplary electro-optic performance testing device provided in this application can have three testing modes to achieve different performance measurements: the electro-optic loop is measured by a constant amplitude AC test signal provided by a lock-in amplifier and a DC bias signal provided by a high voltage amplifier to determine the electro-optic performance under different bias electric fields; a constant DC electric field is provided by a high voltage amplifier and a variable amplitude AC test signal is provided by a lock-in amplifier to measure the electro-optic coefficient of the thin film under a specific bias voltage; by changing the angle between the thin film electrode 30 and the in-plane crystal axis, the electro-optic coefficient component value of the thin film can be calculated by measuring the equivalent electro-optic coefficient value of the thin film.
[0054] Before testing the composite film of this application, the testing equipment was calibrated using a lithium niobate film on a commercially available insulating substrate, and the results met the requirements.
[0055] The electro-optical performance testing process of this application is carried out at room temperature (20°C) and may include the following steps: (1) Bottom polishing of the substrates of the composite films in Examples 1 and 2. The substrates of the composite films were attached bottom-up onto a cylindrical polishing block. A smooth polyurethane polishing cloth was used, and a diamond abrasive polishing slurry with a particle size of 1400~7000 nm was used for rough polishing. The rough polishing time was set to 2-4 hours, and the polishing disc speed was set to 100 r / min. A smooth polyurethane polishing cloth was then used, and a diamond abrasive polishing slurry with a particle size of 3000~500 nm was used for fine polishing. The rough polishing time was set to 2-4 hours, and the polishing disc speed was set to 200 r / min. Finally, the bottom of the substrate was made optically flat.
[0056] (2) A patterned electrode is deposited on the surface of the composite film. The electrode pattern is fabricated using photolithography or a mask, and a patterned electrode such as gold or platinum is deposited using a magnetron deposition apparatus. In Example 1, the direction of electrode 30 is along the in-plane crystal axis of sample 14. <001> The direction is at an angle θ, with the angle ranging from 0° to 90°. During the measurement, electrode 30 is connected to the high-voltage amplifier 21 using a wire binding machine, or a probe can be used to connect the electrode.
[0057] A constant-amplitude AC test signal is provided by a lock-in amplifier; a stepped-rising and falling DC bias signal is provided by a high-voltage amplifier. This step enables the measurement of the electro-optic loop to determine the maximum bias electric field that achieves electro-optic performance saturation, as shown in the results. Figure 5A As shown.
[0058] (3) The high-voltage amplifier provides a constant DC electric field, and the lock-in amplifier provides a variable amplitude AC test signal. Under a specific bias electric field (which can be 0V, i.e., no bias), the phase delay generated by the electro-optic functional layer is linearly related to the amplitude of the AC signal provided by the lock-in amplifier, i.e., it satisfies the Pockels electro-optic effect, as shown in the figure. Figure 5B As shown. By measuring thin-film electrodes with different angles θ between them and the in-plane crystal axis, the equivalent electro-optic coefficient values of the thin film at different electrode angles are obtained, and the electro-optic coefficient component values of the thin film can be calculated. From Figure 5B It can be seen that the electro-optic coefficient component γ in Example 1 42 It is 264 pm / V, the electro-optic coefficient component γ of Example 2 42 It is 605 pm / V.
[0059] These results demonstrate that the composite film based on the barium titanate superlattice structure of this application solves the problem of deterioration in the electro-optic performance of traditional barium titanate films, enabling the electro-optic coefficient of barium titanate thin films to approach or even exceed its bulk level, such as the γ-ray coefficient of the composite film of this application. 42 The coefficient can reach greater than 500 pm / V, or in other words, when an electric field is applied to the (110) crystal plane and its equivalent crystal plane, the effective electro-optic coefficient of the (001) crystal plane and its equivalent crystal plane with the light transmission direction can be at least greater than 350 pm / V, or when an electric field is applied to the (110) crystal plane and its equivalent crystal plane, the effective electro-optic coefficient of the (110) crystal plane and its equivalent crystal plane with the light transmission direction can be at least greater than 150 pm / V.
[0060] Although embodiments of this application have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting this application. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of this application.
Claims
1. A superlattice electro-optic thin film material, characterized in that, The superlattice electro-optic thin film material consists of periodically alternating dielectric material layers and barium titanate layers. In each cycle, each barium titanate layer has m unit cells, and each dielectric material layer has n unit cells, where m is in the range of 5-50 and n is in the range of 1-20. The material used for the dielectric material layer is selected from scandates or strontium tantalum aluminate.
2. The superlattice electro-optic thin film material according to claim 1, characterized in that, The scandates are selected from one of GdScO3, SmScO3, TbScO3, NdScO3, HoScO3, ErScO3, TmScO3, YbScO3, LuScO3, PrScO3, LaScO3, and YScO3.
3. The superlattice electro-optic thin film material according to claim 1 or 2, characterized in that, The total thickness of the superlattice electro-optic thin film material is in the range of 50 nm to 1000 nm.
4. A composite film layer, characterized in that, It includes a substrate and a superlattice electro-optic thin film material according to any one of claims 1-3 located on the substrate.
5. The composite film layer according to claim 4, characterized in that, The dielectric material layer and the barium titanate layer are grown alternately on the substrate by an epitaxial growth method.
6. The composite film layer according to claim 5, characterized in that, The epitaxial growth method is pulsed laser deposition, molecular beam epitaxy, radio frequency magnetron sputtering, or sol-gel method.
7. The composite film layer according to any one of claims 4-6, characterized in that, The material used for the substrate is selected from one of strontium tantalum lanthanum aluminate, GdScO3, SmScO3, TbScO3, NdScO3, HoScO3, ErScO3, TmScO3, YbScO3, LuScO3, PrScO3, LaScO3, YScO3, SiO2, MgO, Al2O3, and silicon on the insulating layer.
8. The composite film layer according to claim 4, characterized in that, The composite film layer also includes a buffer layer located between the substrate and the dielectric material layer.
9. An electro-optical component, characterized in that, Including the superlattice electro-optic thin film material according to any one of claims 1-3.
10. The electro-optical component according to claim 9, characterized in that, The electro-optic components include one of an electro-optic modulator, an electro-optic switch, an electro-optic phase shifter, and a wavelength division multiplexer / mode division multiplexer.