A waveguide optical phase modulator with ultra-low residual amplitude modulation

Abstract

This invention discloses an ultra-low residual amplitude modulation waveguide-optical phase modulator, comprising a first electrode layer and a second electrode layer parallel to each other. An α-phase waveguide channel and a substrate layer are disposed between the first and second electrode layers. The optical axes of the α-phase waveguide channel and the substrate layer are both perpendicular to the plane of the first electrode layer. The difference between the thermo-optical coefficient of the α-phase waveguide channel and the thermo-optical coefficient of the substrate layer is less than a preset thermo-optical coefficient difference threshold. Part of the side of the α-phase waveguide channel is covered by the substrate layer. The electric field between the first and second electrode layers uniformly covers the entire α-phase waveguide channel and the substrate layer. The two ends of the α-phase waveguide channel serve as the input and output ends of the light beam, respectively. This invention, by changing the electric field application method and thermo-optical coefficient matching, fundamentally maintains the consistency of the interference phase, thereby significantly reducing the sensitivity of RAM signals to voltage and ambient temperature fluctuations.

Description

Technical Field

[0001] This invention belongs to the field of electro-optic phase modulator technology, specifically relating to an electro-optic phase modulator that can suppress residual amplitude modulation caused by waveguide-substrate optical interference. Background Technology

[0002] Electro-optic phase modulators have wide applications in high-precision optical measurement, optical frequency stabilization, laser locking, and optical communication. In practical applications, to obtain high-purity phase modulation signals, a modulation electric field is typically applied to a crystal material exhibiting electro-optic effects (such as lithium niobate) to change its refractive index, thereby achieving phase modulation of light. However, during the modulation process, the output optical signal of the modulator is often accompanied by residual amplitude modulation (RAM). This modulation component introduces unnecessary intensity perturbations, reducing the system's measurement sensitivity and stability, and becoming one of the key interference sources in precision optical measurements.

[0003] In waveguide-type electro-optic phase modulators, there is interference between the light propagating in the waveguide region and the light propagating in the substrate region. This interference effect causes intensity modulation of the output light, thus forming a RAM signal. Due to the differences in the propagation path, the properties of the transmitted materials, and the environmental response between the two light components, the interference phase of the two light components is easily affected by external disturbances (such as electric fields and temperatures), causing the RAM signal to fluctuate with time and operating conditions.

[0004] To reduce RAM signals caused by interference, existing technologies reduce RAM signals at the interference source by employing annealed proton exchange (APE) waveguides. These waveguides utilize the propagation loss of light in specific polarization states (such as ordinary light) to weaken the intensity of light interference, thus partially suppressing the RAM signal at its source. However, even with some suppression of the light interference intensity, a small amount of residual interference light inevitably remains. These residual portions will still cause phase changes due to differences in the response of the waveguide and substrate to electric fields or temperatures, leading to non-zero RAM signals, the magnitude of which varies with external parameters.

[0005] Therefore, relying solely on increasing propagation loss to suppress interference intensity is insufficient to achieve long-term stable, low-drift RAM signal output.

[0006] In existing technologies, the design of electro-optic phase modulator electrodes is mostly based on electro-optic efficiency, focusing on making the electric field in the waveguide region uniform and aligned with the crystal axis to achieve higher modulation efficiency. However, from the perspective of residual amplitude modulated RAM, focusing solely on the electric field distribution in the waveguide region is insufficient. To ensure that the optical interference characteristics between the substrate and waveguide regions are unaffected by the electric field, the electric field distributions in the substrate and waveguide regions need to be consistent. This ensures that the RAM signal caused by interference is not affected by the electric field.

[0007] Furthermore, in the existing technology, for the design of low residual amplitude modulation RAM for electro-optic phase modulators, no clear requirements have been put forward for the phase state of the waveguide material or its thermo-optic coefficient matching with the substrate, nor has the key role of the consistency of the thermo-optic coefficients of the two in suppressing the influence of temperature fluctuations on low residual amplitude modulation RAM been recognized.

[0008] Therefore, it is necessary to start by ensuring the uniformity of electric field intensity and the consistency of thermal properties of materials inside and outside the waveguide, optimize the device structure and waveguide process, and fundamentally suppress interference phase disturbances during the modulation process in order to achieve ultra-low residual amplitude modulation performance. Summary of the Invention

[0009] This invention addresses the phase drift problem caused by optical interference between the waveguide and the substrate in traditional structures by proposing a waveguide-optical phase modulator with ultra-low residual amplitude modulation. By changing the electric field application method and improving the thermal properties of the waveguide material, the interference phase fluctuation is fundamentally suppressed, ensuring the long-term stability of the modulated output.

[0010] To achieve the above objectives, the present invention is implemented through the following technical solution:

[0011] An ultra-low residual amplitude modulation waveguide optical phase modulator includes a first electrode layer and a second electrode layer that is not coplanar and parallel to the first electrode layer. An α-phase waveguide channel and a substrate layer are disposed between the first electrode layer and the second electrode layer. The optical axis of the α-phase waveguide channel and the optical axis of the substrate layer are both perpendicular to the plane where the first electrode layer is located. The difference between the thermo-optic coefficient of the α-phase waveguide channel and the thermo-optic coefficient of the substrate layer is less than a preset thermo-optic coefficient difference threshold.

[0012] Part of the side surface of the α-phase waveguide channel is covered by a substrate layer.

[0013] The electric field between the first electrode layer and the second electrode layer uniformly covers the entire α-phase waveguide channel and the substrate layer. The two ends of the α-phase waveguide channel serve as the input and output ends of the light beam, respectively.

[0014] As described above, the substrate layer uses Z-cut lithium niobate crystal or magnesium oxide-doped lithium niobate crystal.

[0015] When a voltage is applied to the first electrode layer and the second electrode layer, the electric field formed between the first electrode layer and the second electrode layer is a uniform electric field, and the direction of the uniform electric field is parallel to the optical axis of the substrate layer.

[0016] As described above, the α-phase waveguide channel is formed by an annealing proton exchange process and embedded in the substrate layer.

[0017] As described above, the α-phase waveguide channel is semi-cylindrical, the rectangular side of the α-phase waveguide channel is perpendicular to the optical axis of the α-phase waveguide channel, and the semi-cylindrical curved surface of the α-phase waveguide channel is covered by a substrate layer.

[0018] As described above, a first buffer layer is provided on the side of the first electrode layer that is close to the α-phase waveguide channel, and a second buffer layer is provided on the side of the second electrode layer that is close to the α-phase waveguide channel.

[0019] As mentioned above, both the first and second buffer layers are made of silicon dioxide.

[0020] Compared with the prior art, the beneficial effects of the present invention are:

[0021] To avoid modulation phase differences between the waveguide and substrate regions caused by voltage variations, this invention introduces a pair of symmetrically arranged metal electrodes on the top and bottom sides of the device, employing a full-coverage layout so that the waveguide layer and substrate region are simultaneously situated between these parallel planar electrodes. Under ideal electrostatic approximation conditions, the electric field is uniformly distributed in the space between the electrodes, and the electric field strength experienced by the waveguide and substrate tends to be consistent, fundamentally eliminating the disturbance of the interference phase caused by electric field differences due to modulation voltage.

[0022] Furthermore, this invention employs an optical waveguide with an α-phase crystal structure in its structural design, whose thermo-optic coefficient is highly matched and identical to that of the lithium niobate substrate. This structure ensures that the refractive indices of the waveguide and substrate regions change synchronously under temperature fluctuations, maintaining a stable phase relationship between the light in the waveguide and substrate regions. This significantly reduces the risk of thermally induced interference phase drift, thereby significantly reducing the sensitivity of the residual amplitude modulation RAM to temperature changes, enhancing the device's robustness to environmental changes, and significantly reducing the temperature-dependent variation of the residual amplitude modulation RAM. This approach emphasizes the crucial role of thermo-optic coefficient matching in reducing the impact of residual amplitude modulation RAM. This method eliminates the need for external temperature control, effectively minimizing the influence of environmental temperature disturbances on the residual amplitude modulation RAM.

[0023] In summary, this invention achieves dual optimization of electric field uniformity and thermo-optical consistency by constructing a symmetrical electrode structure and introducing an α-phase waveguide channel with thermo-optical coefficient matching. It reduces the RAM signal by mitigating the response of the interference term to electric field or temperature. By changing the electric field application method and thermo-optical coefficient matching, it fundamentally maintains the consistency of the interference phase. This further reduces the RAM signal beyond existing techniques that reduce it from the interference light source, potentially significantly reducing the RAM signal's sensitivity to voltage and ambient temperature fluctuations, and significantly reducing RAM signal intensity variations caused by waveguide-substrate optical interference. This structure does not rely on an additional active control system and can achieve long-term stable ultra-low RAM signal output in a passive state, making it suitable for scenarios with extremely high modulator stability requirements, such as laser frequency stabilization, atomic clocks, and coherent spectroscopy. Attached Figure Description

[0024] The accompanying drawings, which are included to provide a further understanding of the invention and form part of this application, illustrate exemplary embodiments of the invention and, together with their description, serve to explain the invention and do not constitute an undue limitation thereof. In the drawings:

[0025] Figure 1 This is a schematic diagram of the cross-sectional structure of the electro-optic phase modulator in an embodiment of the present invention;

[0026] Among them, 101-first electrode layer, 102-second electrode layer, 201-first buffer layer, 202-second buffer layer, 3-α phase waveguide channel, and 4-substrate layer. Detailed Implementation

[0027] To facilitate understanding and implementation of the present invention by those skilled in the art, the present invention will be further described in detail below with reference to embodiments. The embodiments described herein are for illustration and explanation only and are not intended to limit the present invention.

[0028] Example 1:

[0029] This embodiment provides a waveguide-optical phase modulator with ultra-low residual amplitude modulation (RAM). Its core innovation lies in constructing a spatially uniform electric field structure parallel to the waveguide's z-axis (optical axis) and employing an α-phase waveguide channel material. This effectively suppresses the residual amplitude modulation (RAM) caused by interference between the light propagating in the waveguide region and the light propagating in the substrate region. To achieve this function, the specific structure of the electro-optical phase modulator is as follows: Figure 1 As shown:

[0030] The system includes a first electrode layer 101 and a second electrode layer 102 that is not coplanar or parallel to the first electrode layer 101. The first electrode layer 101 and the second electrode layer 102 form a parallel plate electrode. An α-phase waveguide channel 3 and a substrate layer 4 are disposed between the first electrode layer 101 and the second electrode layer 102. The crystal z-axis (i.e., optical axis) of the α-phase waveguide channel 3 and the crystal z-axis (i.e., optical axis) of the substrate layer 4 are both perpendicular to the plane where the first electrode layer 101 is located. The difference between the thermo-optical coefficient of the α-phase waveguide channel 3 and the thermo-optical coefficient of the substrate layer 4 is less than a preset thermo-optical coefficient difference threshold. Part of the side surface of the α-phase waveguide channel 3 is covered by the substrate layer 4. In this embodiment, the α-phase waveguide channel 3 is semi-cylindrical. The rectangular side of the α-phase waveguide channel 3 is perpendicular to the optical axis of the α-phase waveguide channel 3. The semi-cylindrical curved surface of the α-phase waveguide channel 3 is covered by the substrate layer 4 (in this embodiment, the α-phase waveguide channel 3 is disposed in the upper region of the substrate layer 4). The electric field between the first electrode layer 101 and the second electrode layer 102 uniformly covers the entire α-phase waveguide channel 3 and the substrate layer 4 (that is, the first electrode layer 101 and the second electrode layer 102 both fully cover the entire substrate layer 4 along the direction parallel to the rectangular side of the α-phase waveguide channel 3). The semi-circular planes at both ends of the α-phase waveguide channel 3 serve as the input end and the output end of the light beam, respectively.

[0031] Furthermore, substrate 4 employs a Z-cut lithium niobate crystal ( ) or lithium niobate crystals doped with magnesium oxide ( Both the first electrode layer 101 and the second electrode layer 102 are metal electrodes. In this embodiment, the first electrode layer 101 is disposed on the top of the electro-optic phase modulator, and the second electrode layer 102 is disposed on the bottom of the electro-optic phase modulator. The first electrode layer 101 and the second electrode layer 102 form an approximately parallel plate structure. When a voltage is applied to the first electrode layer 101 and the second electrode layer 102, the electric field formed between the first electrode layer 101 and the second electrode layer 102 is a uniform electric field, and the direction of the uniform electric field is parallel to the crystal Z-axis (i.e., the optical axis) of the substrate layer 4. The intensity of the uniform electric field is approximately equal in both the optical axis direction and the direction perpendicular to the optical axis (i.e., the extension direction of the first electrode layer 101 and the second electrode layer 102) (in the experiment, the fluctuation of the uniform electric field intensity in both the optical axis direction and the direction perpendicular to the optical axis of the substrate layer 4 is within the preset fluctuation threshold range), ensuring that the electric field intensity in the α-phase waveguide channel 3 and the substrate layer 4 is equal everywhere, thereby obtaining a high photoelectric efficiency.

[0032] In this embodiment, the rectangular side surface of the α-phase waveguide channel 3 abuts against the bottom surface of the first electrode layer 101, which is determined by the characteristics of the annealed proton exchange process. The formation process of the annealed proton exchange waveguide involves immersing the surface of a lithium niobate crystal into a proton source, where Li ions in the lithium niobate crystal... + With ion H +The PE layer is formed through exchange and then thermally annealed, allowing it to diffuse into deeper regions of the substrate layer 4. This diffusion process determines that the cross-sectional shape of the α-phase waveguide channel 3 is approximately semi-circular.

[0033] The α-phase waveguide channel 3 is formed by an annealing proton exchange process and embedded in the substrate layer 4. The lattice constant of the α-phase waveguide channel 3 is similar to that of the substrate layer 4, thereby giving the α-phase waveguide channel 3 a thermo-optic coefficient that is highly consistent with that of the substrate layer 4. This allows the α-phase waveguide channel 3 and the surrounding substrate layer 4 to change refractive index synchronously under temperature variations, avoiding thermal drift due to interference optical path difference.

[0034] In this embodiment, the term "α-phase waveguide" refers to the waveguide formed by controlled proton exchange and thermal annealing in a lithium niobate crystal (… ) or lithium niobate crystals doped with magnesium oxide ( An optical waveguide channel formed in the substrate with a crystal structure in the α-phase state. The α-phase structure has a lattice constant and thermo-optic coefficient similar to the crystal in substrate layer 4, and is usually formed in regions with low exchange ratios (such as...). (x < 0.12). This structure can keep the thermal response characteristics in the waveguide and the outer substrate consistent, significantly reducing the accumulation and drift of non-zero interference phase caused by the mismatch of thermo-optic coefficients inside and outside the waveguide (i.e. the RAM signal generated therefrom), and greatly reducing the fluctuation of residual amplitude modulated RAM caused by changes in ambient temperature.

[0035] The polarization direction of the input light to the electro-optic phase modulator needs to be aligned with the optical axis of the electro-optic phase modulator (i.e., the crystal's Z-axis). To suppress cross-sectional reflections during coupling, the incident angle is mostly oblique octave. The above-mentioned limitations on the input light are standard operating procedures in the field of electro-optic phase modulators.

[0036] Example 2:

[0037] To prevent the metal electrodes (i.e., the first electrode layer 101 and the second electrode layer 102) from absorbing or scattering the propagating light, a waveguide optical phase modulator with ultra-low residual amplitude modulation based on Embodiment 1 is provided with buffer layers between the first electrode layer 101 and the α-phase waveguide channel 3, and between the second electrode layer 102 and the substrate layer 4. A first buffer layer 201 is provided on the side of the first electrode layer 101 that is close to the α-phase waveguide channel 3, and a second buffer layer 202 is provided on the side of the second electrode layer 102 that is close to the α-phase waveguide channel 3. In this embodiment, the rectangular side of the α-phase waveguide channel 3 abuts against one side of the first buffer layer 201.

[0038] In this invention, the substrate layer 4 is only covered on the semi-cylindrical curved surface of the α-phase waveguide channel 3. If the substrate layer 4 is also covered on the rectangular side surface of the α-phase waveguide channel 3, on the one hand, the area of ​​the substrate layer 4 is increased, and the possibility of light propagating from the substrate layer 4 is increased, which may lead to a larger residual amplitude modulation RAM; on the other hand, the refractive index difference between the α-phase waveguide channel 3 and the substrate layer 4 is smaller than the refractive index between the α-phase waveguide channel 3 and the added buffer layer, resulting in a weaker ability to confine the fundamental mode light.

[0039] Both the first buffer layer 201 and the second buffer layer 202 are made of silicon dioxide. Silicon dioxide has good transparency and insulation properties. Since the refractive index of silicon dioxide is about 1.5, which is lower than that of lithium niobate crystal (about 2.2), when light propagates from the high-refractive-index lithium niobate crystal to the low-refractive-index silicon dioxide, total internal reflection will occur at the interface, confining most of the light in the lithium niobate crystal, with only a very small portion of the light transmitting to the silicon dioxide. Therefore, in this embodiment, the buffer layer is made of silicon dioxide, which not only spatially increases the distance between the α-phase waveguide channel 3 and the metal electrodes (i.e., the first electrode layer 101 and the second electrode layer 102), but also uses the total internal reflection effect to isolate the light field between the metal electrodes and the α-phase waveguide channel 3, causing the light energy reaching the metal electrodes to be greatly attenuated, thereby significantly reducing the light absorption of the metal electrodes. This effectively shields the light absorption loss of the first electrode layer 101 and the second electrode layer 102 (i.e., reduces metal absorption loss) and improves the stability of the electro-optic phase modulator.

[0040] The structure of this invention has significant technical advantages in the following two aspects:

[0041] Firstly, the first electrode layer 101 and the second electrode layer 102 fully cover and vertically symmetrically arrange the internal and external light fields of the α-phase waveguide channel 3, ensuring the high uniformity of the modulation electric field in space, greatly reducing the phase difference between the propagating light between the α-phase waveguide channel 3 and the substrate layer 4 caused by the non-uniform electric field, and effectively suppressing the RAM signal caused by the non-uniformity of the electric field.

[0042] Secondly, the waveguide used is the α-phase waveguide channel 3, which keeps the waveguide and the substrate layer 4 synchronized in terms of temperature response, significantly reducing RAM signal instability caused by thermo-optical mismatch.

[0043] In summary, this invention achieves effective intervention in the RAM signal source mechanism in waveguide-type electro-optic phase modulators through dual optimization of structure and materials. It provides an electro-optic phase modulator that can achieve ultra-low residual amplitude modulation RAM output without external active feedback control, and has good temperature stability and voltage response consistency. It is suitable for applications with high requirements for suppressing additional amplitude modulation in phase modulation, such as high-precision laser frequency stabilization and modulation transfer spectroscopy.

[0044] The specific embodiments described herein are merely illustrative of the spirit of the invention. Those skilled in the art to which this invention pertains may make various modifications or additions to the described specific embodiments or use similar methods to substitute them, without departing from the spirit of the invention or exceeding the scope defined by the appended claims.

Claims

1. A waveguide electro-optic phase modulator of ultra-low residual amplitude modulation comprising a first electrode layer (101), characterized in that, The application further comprises a second electrode layer (102) which is not coplanar with the first electrode layer (101), an alpha phase waveguide channel (3) and a substrate layer (4) are arranged between the first electrode layer (101) and the second electrode layer (102), the optical axis of the alpha phase waveguide channel (3) and the optical axis of the substrate layer (4) are both perpendicular to the plane where the first electrode layer (101) is located, and the difference between the thermo-optic coefficient of the alpha phase waveguide channel (3) and the thermo-optic coefficient of the substrate layer (4) is less than a preset thermo-optic coefficient difference threshold value. Part of the side surface of the alpha phase waveguide channel (3) is covered by the substrate layer (4), The electric field between the first electrode layer (101) and the second electrode layer (102) uniformly covers the entire alpha phase waveguide channel (3) and the substrate layer (4), and the two ends of the alpha phase waveguide channel (3) are respectively used as the input end and the output end of the light beam. When a voltage is applied to the first electrode layer (101) and the second electrode layer (102), the electric field formed between the first electrode layer (101) and the second electrode layer (102) is a uniform electric field, and the direction of the uniform electric field is parallel to the optical axis of the substrate layer (4).

2. The waveguide electro-optic phase modulator of claim 1, wherein, The substrate layer (4) is a z-cut lithium niobate crystal or a lithium niobate crystal doped with magnesium oxide.

3. The waveguide electro-optic phase modulator of claim 1, wherein, The alpha phase waveguide channel (3) is formed by an annealing proton exchange process and is embedded in the substrate layer (4).

4. The waveguide electro-optic phase modulator of claim 3, wherein, The alpha phase waveguide channel (3) is a semi-cylindrical shape, the rectangular side surface of the alpha phase waveguide channel (3) is perpendicular to the optical axis of the alpha phase waveguide channel (3), and the semi-cylindrical curved surface of the alpha phase waveguide channel (3) is covered by the substrate layer (4).

5. The waveguide electro-optic phase modulator of claim 1, wherein, A first buffer layer (201) is arranged on the side of the first electrode layer (101) which is close to the alpha phase waveguide channel (3), and a second buffer layer (202) is arranged on the side of the second electrode layer (102) which is close to the alpha phase waveguide channel (3).

6. The waveguide electro-optic phase modulator of claim 5, wherein, The first buffer layer (201) and the second buffer layer (202) are both made of silicon dioxide.

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