Polarization rotation electro-optical modulator

Abstract

The polarization rotation electro-optical modulator comprises an electro-optical polarization rotation structure, a polarization selection structure and a waveguide structure, and the electro-optical polarization rotation structure is achieved through domain inversion of lithium niobate; the polarization selection structure is realized through a proton exchange waveguide or a metal film; the waveguide structure comprises a proton exchange waveguide and a film lithium niobate waveguide; a metal positive electrode and a metal negative electrode are prepared on the two sides of the proton exchange waveguide, an electric field between the positive electrode and the negative electrode generates electro-optical polarization rotation through the proton exchange waveguide, and the distance between the positive electrode and the negative electrode is larger than the waveguide width. The polarization state is rotated by adopting an electro-optical polarization rotation method, and the electro-optical intensity modulation is realized by matching with the polarization selection waveguide; according to the scheme, only one waveguide is needed, complex structures such as Y-shaped coupling brought by two waveguides can be reduced, and meanwhile the size is reduced; an interference scheme is not adopted, so that the device is not sensitive to the refractive index, and the problem of zero drift is avoided.

Description

TECHNICAL FIELD

[0001] The present application relates to an electro-optical modulator, in particular to a polarization rotation electro-optical modulator. BACKGROUND

[0002] As a core device in optical interconnection, optical communication, optical sensing and other systems, the electro-optical modulator is responsible for the key function of electrical-optical signal conversion. At present, the more practical scheme mainly has two kinds, one is the silicon-based electro-optical modulator based on silicon-on-insulator (SOI), and the other is the electro-optical modulator based on lithium niobate. Due to the different electro-optical modulation mechanisms, the modulation speed of the lithium niobate electro-optical modulator is much higher than that of the silicon-based modulator, and therefore it is regarded as the main scheme of the next generation of electro-optical modulators.

[0003] In recent years, the electro-optical modulator based on thin film lithium niobate has developed rapidly. Since the waveguide thickness and width on the thin film lithium niobate sheet can reach microns or even sub-microns, the half-wave voltage of the modulator can be greatly reduced, which is consistent with the voltage of the traditional silicon-based electronic chip. Therefore, the current high-speed lithium niobate modulator is made of thin film lithium niobate. The electro-optic effect of thin film lithium niobate can adjust the refractive index of the lithium niobate waveguide through voltage, thereby adjusting the phase of the light passing through the waveguide. However, if intensity modulation is required, a waveguide and two Y-type coupling waveguides must be added to split the light through the Y-type structure so that the light in the two waveguides interferes. This configuration not only increases the complexity of the structure, increases the width and length of the period, but also causes a random drift in the refractive index of the two waveguides due to temperature, optical breakdown and other effects, resulting in a difficult-to-overcome zero drift problem, which interferes with the reliability of the electro-optical modulator.

[0004] The electro-optical polarization rotation method in lithium niobate uses room temperature electric field polarization method to make specific period domain inversion in lithium niobate. Under the action of electro-optic effect, the optical axis of positive and negative domains rotates in the opposite direction. When the domain thickness meets the thickness of the half-wave plate, the domain will mirror rotate the polarization of light along the optical axis. After multiple rotations of positive and negative domains, the polarization of light is rotated to be perpendicular to the initial state. SUMMARY

[0005] The purpose of the present application is to provide a polarization rotation electro-optical modulator, which rotates the polarization of light through the electro-optic effect by designing the domain structure of lithium niobate, and realizes electro-optical intensity modulation by designing a polarization selection waveguide.

[0006] Technical scheme: The present application comprises an electro-optical polarization rotation structure, a polarization selection structure and a waveguide structure, the electro-optical polarization rotation structure is realized by domain inversion of lithium niobate; the polarization selection structure is realized by proton exchange waveguide or metal thin film; the waveguide structure comprises a proton exchange waveguide and a thin film lithium niobate waveguide.

[0007] The positive electrode and the negative electrode are prepared on both sides of the proton exchange waveguide, and an electric field between the positive electrode and the negative electrode generates an electro-optic polarization rotation through the proton exchange waveguide. When the voltage is 0, there is no polarization rotation, the light in the waveguide is transmitted along the waveguide, and the modulation intensity is 1; when the voltage is large enough, the polarization rotation is 90 degrees, the light in the waveguide cannot be transmitted, and the modulation intensity is 0.

[0008] The positive electrode and the negative electrode are both metal electrodes, and an electric field formed between the positive electrode and the negative electrode after applying positive and negative voltages generates an electro-optic polarization rotation through the proton exchange waveguide.

[0009] The distance between the positive electrode and the negative electrode is wider than the width of the waveguide.

[0010] The proton exchange waveguide only supports Z-direction polarization, and Y-direction polarized light is dissipated in the waveguide.

[0011] The domain inversion is made by a room temperature electric field polarization process.

[0012] The polarization selection structure is realized by thin film lithium niobate, and the waveguide adopts a metal layer to realize polarization selection of the waveguide. The polarization state perpendicular to the metal surface is absorbed or transmitted, and the polarization state parallel to the metal surface is reflected and exists in the waveguide for transmission.

[0013] The waveguide has a positive electrode and a negative electrode on both sides, and an electric field is generated in the waveguide to generate an electro-optic deflection.

[0014] The polarization selection structure is realized by thin film lithium niobate, and the waveguide is tightly attached to the positive electrode and the negative electrode made of metal on both sides, which simultaneously functions as voltage application and polarization selection; the polarization state of 1550nm light coupled into the waveguide is horizontal. When the voltages of the positive electrode and the negative electrode are both 0, the polarization state does not change, the horizontally polarized light is reflected by the metal layer and transmitted in the waveguide, and the modulation intensity is 1; when the voltage of the positive electrode is changed to 3V, the light in the waveguide is rotated to a vertical state and is absorbed by the metal layer, so it cannot pass through the waveguide, and the modulation intensity is 0.

[0015] The waveguide is provided below a substrate made of an optical material with a lower refractive index than lithium niobate, so that the refractive index of the waveguide is higher than that of the substrate.

[0016] Beneficial effects: The application rotates the polarization state by the method of electro-optic polarization rotation, and realizes electro-optic intensity modulation by cooperating with the polarization selection waveguide; this scheme only needs one waveguide, which can reduce the complex structure such as Y-type coupling caused by two waveguides, and at the same time, the volume is reduced; since the interference scheme is not used, the refractive index is not sensitive, so the zero drift problem is avoided. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1 It is a structural schematic diagram of the embodiment 1 of the application;

[0018] Figure 2 is a structural schematic diagram of embodiment 2 of the present application;

[0019] Figure 3 is a structural schematic diagram of embodiment 3 of the present application. DETAILED DESCRIPTION

[0020] The present application is further described below with reference to the accompanying drawings.

[0021] The polarization-rotating electro-optical modulator of the present application comprises an electro-optical polarization-rotating structure, a polarization-selecting structure and a waveguide structure, the electro-optical polarization-rotating structure is realized by domain inversion of lithium niobate; the polarization-selecting structure is realized by proton-exchange waveguide or metal thin film; the waveguide structure comprises proton-exchange waveguide and thin-film lithium niobate waveguide.

[0022] The design of the electro-optical polarization-rotating structure should make the thickness L c of the domain satisfy the following formula:

[0023]

[0024] wherein λ is the wavelength of light, n o and n e are the O light and E light refractive indexes of lithium niobate respectively. After passing through a positive domain with a length of L c , the polarization state of light rotates by an angle of 6θ, and after passing through a negative domain with a length of L 51 , the polarization state of light rotates by an angle of -6θ, and so on.

[0025]

[0026] wherein γ c is the electro-optic coefficient, is the electric field intensity. After passing through a negative domain with a length of L Z , the polarization state of light rotates by an angle of -6θ, and so on.

[0027] The above process is described by the complex amplitude of the light field in Z and Y directions as follows:

[0028]

[0029] Δβ=(β Y -β c )-G (5)

[0030] wherein β is the wave vector,

[0031]

[0032] G is the wave vector of PPLN compensation:

[0033]

[0034] When Λ=2L cAt this time, Δβ = 0, (3) and (4) become:

[0035]

[0036] After differentiating (8), (9) is brought in to obtain:

[0037]

[0038] The general solution of the above formula is:

[0039] A Z = C (e iKx ± e -iKx ) (11)

[0040] That is, a sine or cosine function. Taking the cosine function as an example,

[0041] A Z = Ccos (Kx) (12)

[0042] From the derivation of formulas (3) to (11), formula (12) is obtained, and the appropriate waveguide length can be obtained according to formula (12). For example, in order to make the light field in the Z direction all converted into the light field in the Y direction, A Z = 0, that is, Kx = π / 2, or the length The size of K can be calculated according to formula (6) according to the voltage size.

[0043] Embodiment 1

[0044] As Figure 1 shown, the polarization rotation electro-optical modulator of the embodiment includes an electro-optical polarization rotation structure, a polarization selection structure, and a waveguide structure, and the electro-optical polarization rotation structure is realized by domain inversion of lithium niobate; the polarization selection structure is realized by a proton exchange waveguide. The proton exchange waveguide only supports Z direction polarization, and Y direction polarized light is dissipated and cannot be transmitted in the waveguide. The proton exchange waveguide is obtained by using the common method of mask making and benzoic acid exchange to form a strip-shaped buried waveguide. The domain thickness is calculated according to formula (1), the domain inversion structure is made by room temperature electric field polarization process, and positive electrode 1 and negative electrode 2 are prepared on both sides of waveguide 3.

[0045] Positive electrode 1 and negative electrode 2 are both metal electrodes, and an electric field formed between the positive and negative electrodes after applying positive and negative voltages generates electro-optical polarization rotation through the proton exchange waveguide. When the voltage is 0, there is no polarization rotation, the light in waveguide 3 is transmitted along waveguide 3, and the modulation intensity is 1; when the voltage is large enough, the polarization rotation is 90 degrees, and the light in waveguide 3 cannot be transmitted, and the modulation intensity is 0.

[0046] The width of a proton-exchange waveguide is typically 5μm-15μm, with the distance between the positive and negative electrodes slightly wider than the width of waveguide 3. Taking a commonly used λ = 1550nm as an example, the domain thickness is calculated to be 10.6μm according to formula (1). Taking a proton-exchange waveguide width of 8μm as an example, with a distance between the positive and negative electrodes of 10μm, and using a voltage V = 3V as the highest modulation voltage, according to... The waveguide length can be calculated to be approximately 10 mm.

[0047] Example 2

[0048] like Figure 2 As shown, the polarization rotation electro-optic modulator of this embodiment adopts a thin-film lithium niobate form, similar to that of Embodiment 1. Embodiment 2 also features a domain inversion structure distributed in waveguide 3. Since the width and thickness of the thin-film lithium niobate are relatively small, they may affect the refractive indices no and ne. Therefore, it is necessary to calculate the effective refractive index using simulation methods, and then calculate the domain thickness. The methods for calculating the effective refractive index described above can be implemented using methods such as Comsol and FDTD, which are commonly used methods.

[0049] In this embodiment, waveguide 3 employs a metal layer 5 to achieve polarization selectivity. Polarization states perpendicular to the metal surface are absorbed or transmitted, while polarization states parallel to the metal surface are reflected and propagate within waveguide 3. The metal layer 5 may also have, for example, a wire grid structure to further enhance polarization selectivity. A substrate 6 is disposed beneath the metal layer 5. Positive electrode 1 and negative electrode 2 are located on both sides of waveguide 3. These electrodes apply positive and negative voltages, generating an electric field within waveguide 3, resulting in electro-optic deflection.

[0050] Taking a thin-film lithium niobate waveguide with a width of 10 μm and a thickness of 10 μm as an example, this waveguide size has a large aperture, which is beneficial for coupling, and the refractive index is approximately the same as that of the bulk. According to formula (1), with λ = 1550 nm, a domain thickness of 10.6 μm, a positive and negative electrode spacing of 20 μm, and a maximum modulation voltage of V = 3 V, according to The waveguide length can be calculated to be approximately 20 mm. When 1550 nm light with a horizontal polarization state is coupled into the waveguide, the polarization state remains unchanged when both the positive and negative electrode voltages are 0. The horizontally polarized light is reflected by the metal layer and propagates in the waveguide with a modulation intensity of 1. When the positive electrode voltage is changed to 3 V, the light in the waveguide is rotated to a vertical state, absorbed by the metal layer, and cannot pass through the waveguide, resulting in a modulation intensity of 0.

[0051] Example 3

[0052] The polarization rotation electro-optical modulator of the embodiment also uses thin film lithium niobate. Different from the embodiment 2, the positive electrode 1 and the negative electrode 2 of the embodiment 3 are both made of metal and closely contact the waveguide 3, and simultaneously play the role of applying voltage and polarization selection. The substrate 6 is arranged below the waveguide 3. The substrate 6 of the embodiment is made of optical material such as quartz and sapphire, which has a lower refractive index than lithium niobate, so that the refractive index of the waveguide is higher than the substrate 6. The 1550nm light with a horizontal polarization state is coupled into the waveguide 3. When the voltage of the positive electrode and the negative electrode is both 0, the polarization state does not change. The horizontally polarized light is absorbed by the metal electrode and cannot be transmitted, and the modulation intensity is 0. When the voltage of the positive electrode is changed to 3V, the light in the waveguide is rotated to the vertical state, is reflected by the metal electrode, passes through the waveguide, and the modulation intensity is 1.

Claims

1. A polarization-rotating electro-optic modulator, characterized by, The application relates to an electro-optical polarization rotation structure, a polarization selection structure and a waveguide structure, wherein the electro-optical polarization rotation structure is realized through domain reversal of lithium niobate; the polarization selection structure is realized through a proton exchange waveguide or a metal film; and the waveguide structure comprises a proton exchange waveguide and a thin film lithium niobate waveguide.

2. The polarization-rotating electro-optic modulator of claim 1, wherein, The positive and negative electrodes are arranged on both sides of the proton exchange waveguide, and an electro-optical polarization rotation is generated through the proton exchange waveguide.

3. The polarization-rotating electro-optic modulator of claim 2, wherein, The positive and negative electrodes are metal electrodes.

4. The polarization-rotating electro-optic modulator of claim 2, wherein, The spacing between the positive and negative electrodes is wider than the width of the waveguide.

5. The polarization-rotating electro-optic modulator of claim 2, wherein, The proton exchange waveguide only supports Z-direction polarization, and Y-direction polarized light is dissipated in the waveguide.

6. The polarization-rotating electro-optic modulator of claim 1, wherein, The domain reversal is realized through a room-temperature electric field polarization process.

7. The polarization-rotating electro-optic modulator of claim 1, wherein, The polarization selection structure is realized through a thin film lithium niobate, and a metal layer is used to realize polarization selection of the waveguide.

8. The polarization-rotating electro-optic modulator of claim 7, wherein, The positive and negative electrodes are arranged on both sides of the waveguide, and an electric field is generated in the waveguide to generate electro-optical deflection.

9. The polarization-rotating electro-optic modulator of claim 1, wherein, The polarization selection structure is realized through a thin film lithium niobate, and the positive and negative electrodes made of metal are arranged on both sides of the waveguide to simultaneously realize voltage application and polarization selection; and 1550nm light in a horizontal polarization state is coupled into the waveguide.

10. The polarization-rotating electro-optic modulator of claim 9, wherein, A substrate is arranged below the waveguide, and the substrate is made of an optical material with a refractive index lower than that of lithium niobate.

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