CN111399118A - An integrated polarizing beam splitter based on thin-film lithium niobate waveguide - Google Patents

Abstract

The invention discloses an integrated polarization beam splitter based on a thin-film lithium niobate waveguide. In the present invention, a 1×2 multi-mode interference coupler, a multi-segment phase shifter and a 2×2 multi-mode interference coupler are successively cascaded to form a Mach-Zehnder interferometer structure. The 1×2 multimode interference coupler is composed of a first tapered waveguide, a second tapered waveguide, a third tapered waveguide and a multimode waveguide. The 2×2 multimode interference coupler is composed of a first tapered waveguide, a second tapered waveguide, a third tapered waveguide, a fourth tapered waveguide and a multimode waveguide. The multi-stage phase shifter is composed of a first interference arm and a second interference arm. The first interference arm is sequentially connected by the first connecting waveguide, the first phase-shifting waveguide, the second connecting waveguide, the second phase-shifting waveguide and the third connecting waveguide. The second interference arm is sequentially connected by the fourth connection waveguide, the third phase shift waveguide, the fifth connection waveguide, the fourth phase shift waveguide and the sixth connection waveguide. The invention has the characteristics of high extinction ratio, low loss, simple structure, simple design and simple processing.

Description

An integrated polarizing beam splitter based on thin-film lithium niobate waveguide

technical field

The invention specifically relates to an integrated polarization beam splitter based on a thin-film lithium niobate waveguide, which is suitable for optical fiber communication, on-chip optical communication, optical sensing systems, and quantum optical systems that require polarization beam splitting, polarization beam combining, and polarization filtering. application.

Background technique

Lithium niobate has strong electro-optic effect, strong nonlinear effect and good thermal stability, so it is widely used in integrated optical devices such as electro-optic modulators and wavelength converters. In recent years, with the maturity of thin-film lithium niobate processing and preparation technology, ultra-small integrated optical devices based on thin-film lithium niobate waveguides have developed rapidly. Due to the high refractive index difference and small mode spot size of thin-film lithium niobate waveguides, these waveguides have strong birefringence characteristics, so most of the integrated optical devices based on thin-film lithium niobate waveguides have strong polarization correlation and can only work under single polarization conditions. Therefore, a polarization beam splitter needs to be used to separate different polarization states in the input signal, or to filter out polarization states other than the working polarization. In addition, the thin-film lithium niobate waveguide has good polarization-maintaining properties, and different polarization states do not generate crosstalk during transmission. Therefore, for optical communication systems, it is possible to load signals in different polarization states and combine the signals with a polarization beam splitter, thereby doubling the channel capacity without increasing the wavelength channel. For optical sensing systems, since different polarization states have different sensitivities to changes in environmental parameters, a polarization beam splitter can be used to monitor multiple environmental parameters simultaneously. For quantum optical systems, polarization beam splitters can be used to achieve quantum entanglement between different polarization states.

At present, most of the design ideas of the integrated polarization beam splitter are based on the asymmetric coupler structure. By adjusting the structural parameters of the coupler, one polarization state satisfies the phase matching condition, while the other polarization state is phase mismatched, so that the different polarization states are matched. state separation. However, lithium niobate is an anisotropic material, and x-cut films are usually required to ensure maximum electro-optic modulation efficiency and wavelength conversion efficiency, resulting in thin-film lithium niobate waveguides with different transmission directions usually have different effective refractive indices, This greatly increases the design difficulty of asymmetric couplers. Therefore, there is a need for a new technical solution for realizing an integrated polarizing beam splitter based on thin-film lithium niobate waveguides.

SUMMARY OF THE INVENTION

The purpose of the present invention is to propose an integrated polarization beam splitter based on a thin-film lithium niobate waveguide. By cascading polarization-insensitive multimode interference couplers and multi-segment phase shifters, and controlling the transmission direction and transmission length of the thin-film lithium niobate waveguide in the phase shifter, the phase shift between the TE fundamental mode and the TM fundamental mode is realized. Independent control of the transmission phase in the transmitter, which in turn separates the TE polarization state from the TM polarization state in the input light.

The integrated polarization beam splitter based on thin-film lithium niobate waveguide proposed in the present invention is composed of a 1×2 multi-mode interference coupler (I), a multi-segment phase shifter (II), and a 2×2 multi-mode interference coupler (III) Cascaded in turn to form a Mach-Zehnder interferometer structure. Wherein, the input waveguide (0) is connected to the first tapered waveguide (31) in the 1×2 multimode interference coupler (I), and the second tapered waveguide (31) in the 1×2 multimode interference coupler (I) 32) and the third conical waveguide (33) are respectively connected with the first connecting waveguide (11) and the fourth connecting waveguide (14) in the multi-segment phase shifter (II), and the The three-connected waveguide (13) and the sixth-connected waveguide (16) are respectively connected to the first tapered waveguide (41) and the second tapered waveguide (42) in the 2×2 multimode interference coupler (III), 2× 2. The third tapered waveguide (43) and the fourth tapered waveguide (44) in the multimode interference coupler (III) are respectively connected to the first output waveguide (1) and the second output waveguide (2). The multi-segment phase shifter (II) is composed of two interference arms, wherein a first connection waveguide (11), a first phase shift waveguide (21), a second connection waveguide (12), a second phase shift waveguide (22), The third connecting waveguide (13) is sequentially connected to form the first interference arm (II-1), the fourth connecting waveguide (14), the third phase-shifting waveguide (23), the fifth connecting waveguide (15), and the fourth phase-shifting waveguide (24) The sixth connecting waveguide (16) is connected in sequence to form a second interference arm (II-2).

In the present invention, after the input light is transmitted from the input waveguide (0) to the polarization-insensitive 1×2 multimode interference coupler (I), it is divided into two beams with the same intensity and phase, and enters the first interference arm ( II-1) and the second interference arm (II-2). The optical signal input to the first interference arm (II-1) passes through the first connecting waveguide (11), the first phase-shifting waveguide (21), the second connecting waveguide (12), the second phase-shifting waveguide (22), The third connecting waveguide (13) enters the 2×2 multi-mode interference coupler (III) through the first tapered waveguide (41); the optical signal input to the second interference arm (II-2) passes through the fourth connecting waveguide in sequence (14), the third phase-shifted waveguide (23), the fifth connecting waveguide (15), the fourth phase-shifting waveguide (24), the sixth connecting waveguide (16), and enter 2 through the second tapered waveguide (42) ×2 multimode interference coupler (II). The optical signal mainly propagates along the y direction in the first phase-shift waveguide (21) and the third phase-shift waveguide (23), so the TE fundamental mode transmitted therein is extraordinary light, and the TM fundamental mode transmitted therein is ordinary light. The optical signal mainly propagates along the z-direction in the second phase-shift waveguide (22) and the fourth phase-shift waveguide (24), so the TE fundamental mode and the TM fundamental mode transmitted therein are both ordinary light. By controlling the length difference between the first phase-shift waveguide (21) and the third phase-shift waveguide (23), and the length difference between the second phase-shift waveguide (22) and the fourth phase-shift waveguide (24), the TE fundamental mode can be After passing through the multi-stage phase shifter (II), a -90° phase difference is generated between the first interference arm (II-1) and the second interference arm (II-2), and the TM fundamental mode is passed through the multi-stage phase shifter. After the shifter (II), a +90° phase difference is generated between the first interference arm (II-1) and the second interference arm (II-2). After entering the polarization-insensitive 2×2 multimode interference coupler (III), the TE fundamental mode in the input light is completely coupled to the first output waveguide (1) due to a -90° phase difference. After entering the polarization-insensitive 2×2 multimode interference coupler (III), the TM fundamental mode in the input light is fully coupled to the second output waveguide (2) due to a +90° phase difference.

The beneficial effects that the present invention has are:

(1) By adjusting the transmission direction and transmission length of the thin-film lithium niobate waveguide, the transmission phases of the TE fundamental mode and the TM fundamental mode in the multi-segment phase shifter can be independently controlled, so as to realize the TE polarization state and the TM polarization state. completely separated.

(2) It has the advantages of simple structure, simple design and easy processing.

(3) It has excellent properties such as high extinction ratio (greater than 40dB), low loss (less than 0.9dB), large operating bandwidth (greater than 200nm) and large processing tolerance (waveguide width tolerance -10nm to +10nm).

Description of drawings

1 shows a schematic diagram of an integrated polarizing beam splitter based on a thin-film lithium niobate waveguide according to the present invention;

In the figure: I, 1×2 multimode interference coupler, II, multi-segment phase shifter, III, 2×2 multimode interference coupler, II-1, the first interference arm, II-2, the second interference arm , 0, input waveguide, 1, first output waveguide, 2, second output waveguide, 11, first connection waveguide, 12, second connection waveguide, 13, third connection waveguide, 14, fourth connection waveguide, 15, The fifth connecting waveguide, 16, the sixth connecting waveguide, 21, the first phase-shifting waveguide, 22, the second phase-shifting waveguide, 23, the third phase-shifting waveguide, 24, the fourth phase-shifting waveguide.

Figure 2 is a schematic diagram of the structure of a 1×2 multimode interference coupler (I);

In the figure: 31, the first tapered waveguide, 32, the second tapered waveguide, 33, the third tapered waveguide, 34, the multimode waveguide.

FIG. 3 is a schematic structural diagram of a 2×2 multimode interference coupler (III);

In the figure: 41, the first tapered waveguide, 42, the second tapered waveguide, 43, the third tapered waveguide, 44, the fourth tapered waveguide, 45, the multimode waveguide.

Figure 4 is a schematic cross-sectional view of a single-mode lithium niobate waveguide;

In the figure: 51, lithium niobate core layer, 52, silicon dioxide substrate layer and upper cladding layer

Fig. 5 is the transmittance spectrum simulation curve of each output port.

Detailed ways

The present invention will be further described below with reference to the accompanying drawings and an embodiment of an integrated polarizing beam splitter based on a thin-film lithium niobate waveguide.

A thin-film lithium niobate waveguide based on Lithium-Niobate On Insulator (LNOI) material is selected, and its core layer is a lithium niobate material with a thickness of 700 nm; its substrate layer is a silicon dioxide material with a thickness of 2 μm; The upper cladding layer is made of silica material with a thickness of 2 μm. In this embodiment, the sidewall inclination angle of all waveguide structures is 70°, and the width of all single-mode thin-film lithium niobate waveguides in the multi-segment phase shifter (II) is 300 nm.

The transmission directions of the input waveguide (0), the first output waveguide (1), and the second output waveguide (2) are all in the y direction. The first phase-shift waveguide (21) and the third phase-shift waveguide (23) are both composed of a curved waveguide and a straight waveguide that transmits in the y-direction. The second phase-shift waveguide (22) and the fourth phase-shift waveguide (24) are both composed of a curved waveguide and a straight waveguide that transmits in the z-direction. The length difference between the first phase shift waveguide (21) and the third phase shift waveguide (23) is −13.79 μm, and the length difference between the second phase shift waveguide (22) and the fourth phase shift waveguide (24) is +13.64 μm. The radius of all the curved waveguides in the first interference arm (II-1) and the second interference arm (II-2) is 25 μm.

In the 1×2 multimode interference coupler (I), the opening width of the first tapered waveguide (31), the second tapered waveguide (32), and the third tapered waveguide (33) are all 700 nm and 2 μm in length. The multimode waveguide (34) in the 1×2 multimode interference coupler (I) has a width of 4.5 μm and a length of 9.3 μm. Opening widths of the first tapered waveguide (41), the second tapered waveguide (42), the third tapered waveguide (43), and the fourth tapered waveguide (44) in the 2×2 multimode interference coupler (III) Both are 700 nm, and both are 2 μm in length. The multimode waveguide (45) in the 2×2 multimode interference coupler (III) has a width of 4.5 μm and a length of 20.4 μm.

The above-mentioned embodiments are used to explain the present invention, rather than limit the present invention. Within the spirit of the present invention and the protection scope of the claims, any modifications and changes made to the present invention all fall into the protection scope of the present invention.

Claims (6)

1. An integrated polarizing beam splitter based on a thin-film lithium niobate waveguide, characterized in that it is composed of the following components: 1×2 multimode interference coupler (I), multi-segment phase shifter (II) and 2×2 2 multi-mode interference couplers (III) are cascaded in sequence to form a Mach-Zehnder interferometer structure; the 1×2 multi-mode interference coupler (I) is composed of a first tapered waveguide (31), a second tapered waveguide (32) ), a third tapered waveguide (33) and a multimode waveguide (34); the first tapered waveguide (31) in the 1×2 multimode interference coupler (I) is connected to the input waveguide (0); The 2×2 multi-mode interference coupler (III) is composed of a first tapered waveguide (41), a second tapered waveguide (42), a third tapered waveguide (43), a fourth tapered waveguide (44) and A multimode waveguide (45) is formed; the third tapered waveguide (43) and the fourth tapered waveguide (44) in the 2×2 multimode interference coupler (III) are respectively connected with the first output waveguides (1), The second output waveguide (2) is connected; the multi-stage phase shifter (II) is composed of a first interference arm (II-1) and a second interference arm (II-2); the first interference arm (II- 1) It consists of a first connection waveguide (11), a first phase-shift waveguide (21), a second connection waveguide (12), a second phase-shift waveguide (22), and a third connection waveguide (13) connected in sequence; the The second interference arm (II-2) consists of a fourth connecting waveguide (14), a third phase-shifting waveguide (23), a fifth connecting waveguide (15), a fourth phase-shifting waveguide (24), and a sixth connecting waveguide (16). ) are connected in sequence. 2. An integrated polarization beam splitter based on thin film lithium niobate waveguide according to claim 1, characterized in that the input light is transmitted from the input waveguide (0) to a polarization-insensitive 1×2 multimode interference coupler After (I), it is divided into two beams with the same intensity and phase, and enters the first interference arm (II-1) and the second interference arm (II-2); wherein the input of the first interference arm (II-1) The optical signal sequentially passes through the first connection waveguide (11), the first phase-shift waveguide (21), the second connection waveguide (12), the second phase-shift waveguide (22), and the third connection waveguide (13), and passes through the first connection waveguide (13). The tapered waveguide (41) enters the 2×2 multi-mode interference coupler (III); the optical signal input to the second interference arm (II-2) passes through the fourth connecting waveguide (14) and the third phase-shifting waveguide (23) in turn , the fifth connecting waveguide (15), the fourth phase-shifting waveguide (24), the sixth connecting waveguide (16), and enter the 2×2 multimode interference coupler (III) through the second tapered waveguide (42); light The signal propagates mainly along the y direction in the first phase-shift waveguide (21) and the third phase-shift waveguide (23), so the TE fundamental mode transmitted in it is extraordinary light, and the TM fundamental mode transmitted in it is ordinary light; light; The signal propagates mainly along the z-direction in the second phase-shift waveguide (22) and the fourth phase-shift waveguide (24), so the TE fundamental mode and the TM fundamental mode transmitted in them are both ordinary light; by controlling the first phase-shift waveguide ( 21) The length difference with the third phase-shift waveguide (23), and the length difference between the second phase-shift waveguide (22) and the fourth phase-shift waveguide (24), can make the TE fundamental mode pass through the multi-segment phase shifter ( After II), a -90° phase difference is generated between the first interference arm (II-1) and the second interference arm (II-2), and at the same time, after passing through the multi-stage phase shifter (II), the TM fundamental mode is A +90° phase difference is generated between the first interference arm (II-1) and the second interference arm (II-2); the TE fundamental mode in the input light enters the polarization-insensitive 2×2 multimode interference coupler ( III), it is completely coupled to the first output waveguide (1) due to a -90° phase difference; after entering the polarization-insensitive 2×2 multimode interference coupler (III), the TM fundamental mode in the input light is Fully coupled to the second output waveguide (2) due to a +90° phase difference. 3. An integrated polarizing beam splitter based on a thin-film lithium niobate waveguide according to claim 1 or 2, characterized in that the first phase-shifting waveguide (21) and the third phase-shifting waveguide (23) are both formed by bending The waveguide is composed of a straight waveguide that transmits in the y-direction; the second phase-shift waveguide (22) and the fourth phase-shift waveguide (24) are both composed of a curved waveguide and a straight waveguide that transmits in the z-direction. 4. The integrated polarizing beam splitter based on thin film lithium niobate waveguide according to claim 3, characterized in that the length difference between the first phase-shift waveguide (21) and the third phase-shift waveguide (23) , and the length difference between the second phase-shift waveguide (22) and the fourth phase-shift waveguide (24) can ensure that the TE fundamental mode produces a -90° phase difference, and at the same time ensures that the TM fundamental mode produces a +90° phase difference. 5. An integrated polarizing beam splitter based on a thin-film lithium niobate waveguide according to claim 3, characterized in that the TE fundamental mode input in the input waveguide (0) can be transmitted to the first output waveguide (1). ). 6. An integrated polarizing beam splitter based on thin film lithium niobate waveguide according to claim 1, characterized in that the TM fundamental mode input in the input waveguide (0) can be transmitted to the second output waveguide (2) ).

Images