CN115016154A - Polarization filtering type intensity modulator based on lithium niobate thin film - Google Patents

Abstract

The invention relates to a lithium niobate film-based polarization filtering type intensity modulator, which is integrated on the same chip and has the functions of filtering and intensity modulation. Including an optical waveguide, an intensity-controlled center electrode, and an intensity-controlled ground electrode. The optical waveguide layer is improved by depositing silicon nitride on the lithium niobate layer in the optical waveguide layer to form a silicon nitride loading strip layer; the optical waveguide is divided into a Y waveguide beam splitter and a Y branch beam combiner from a functional structure, a filtering metal silver coating layer is added on a straight waveguide at the front end of the Y waveguide beam splitter to form the optical waveguide with a polarization filtering function, the filtered optical wave is equally divided into two parallel waveguides by the Y waveguide beam splitter, an intensity control center electrode is arranged between the two parallel waveguides, the outer sides of the two parallel waveguides are respectively provided with an intensity control ground electrode, and the intensity control center electrode and the intensity control ground electrode form an amplitude controller electrode for intensity modulation; after the two paths of light are subjected to intensity modulation, the two paths of light are combined and output by a Y-branch beam combiner.

Description

A Polarization Filtered Intensity Modulator Based on Lithium Niobate Thin Film

technical field

The invention belongs to the field of integrated optics, and relates to a polarization filtering type intensity controller based on a lithium niobate film (LNOI), which can be applied to optical communication, optical computing, microwave photonic technology and the like.

Background technique

As a new material, lithium niobate (LN) has many excellent optical properties, including large electro-optic coefficient, nonlinear optical effect, wide optical transparent window, good temperature stability, low thermo-optic coefficient, etc. Therefore, in the past In the past ten years, lithium niobate has developed rapidly in the field of integrated optics. The newly developed new single-crystal lithium niobate film retains the inherent advantages of lithium niobate to a large extent. The thickness of the overall structure is in the order of microns. The difference between the refractive index (n≈2.1) and the refractive index of silicon dioxide (n≈1.4) is as high as 0.7, so the waveguide made of the LNOI structure has a strong ability to bind light, and the optical mode field can be limited in a small range, thus Miniaturization and integration of devices can be achieved. Optical waveguide is the basic optical element in integrated optics, the channel for optical signal transmission, and one of the main structures of various complex optical devices. The waveguide structures of lithium niobate films mainly include lithium niobate ridge waveguides and loaded strip ridge waveguides. Using the single crystal lithium niobate film as the substrate, a ridge structure is formed on the surface of LN by etching and deposition methods, which can effectively confine light (Δn≈0.7) to form a small optical mode field (the mode field size is about 1μm2 ). Compared with the traditional bulk material lithium niobate waveguide, this ridge waveguide, as a new type of optical waveguide, has a smaller bending radius, smaller waveguide cross-section, shorter device length and better modulation field lower modulation voltage and higher modulation bandwidth. Loaded strip waveguide is an important type of optical waveguide on LNOI. It achieves light confinement by covering the LNOI surface with a layer of loading strip material. Loaded strip waveguides enable hetero-integration of lithium niobate with other materials, combining the excellent properties of both materials.

Due to the high refractive index difference between the waveguide layer and the cladding layer of the lithium niobate-silicon nitride waveguide material, the optical field can be well confined in the waveguide to transmit, so that the size of the device is greatly reduced. The difference leads to a large birefringence effect, and photonic devices have to face polarization-related problems, that is, the performance of optical devices is affected by the incident polarization state, and there are problems such as polarization mode dispersion and polarization-related loss. Therefore, solving the polarization dependence of devices is an important challenge for optical devices. Therefore, one of the solutions to solve the polarization dependence of optical devices is to use polarization-independent optical devices, that is, optical devices whose polarization state has no effect on performance, so as to reduce the performance loss of the device.

The traditional polarization device and modulation device are separated, which is no different from increasing the length and transmission loss of the whole system, and the current polarizer usually uses shallow etched waveguides or subwavelength grating waveguides to realize the function of polarization selection. Such polarizers are generally large in size and complex in structure.

SUMMARY OF THE INVENTION

In order to solve the problem of separation of the traditional intensity modulation device and the polarization filter device, the present invention proposes a polarization filter type intensity modulator which combines the polarization filter and the intensity modulation function. The specific scheme adopted in the present invention is to make a metal covering layer structure on the surface of the optical waveguide, when the electromagnetic wave is transmitted in the optical waveguide, a tube wall current will be formed, and the magnetic field formed by the tube wall current has a suppressing effect on the magnetic field of the incident electromagnetic wave, thereby Only the electric field is allowed to pass through. Therefore, the metal-clad optical waveguide polarizer can absorb the TM mode and allow the TE mode to pass through, thereby realizing polarization filtering. Then, the optical waveguide is equally divided into two parallel paths by using a Y-type waveguide beam splitter, and an intensity controller electrode structure is arranged between the two optical waveguides. The intensity controller electrode is controlled by the intensity located between the two optical waveguides. It is composed of the center electrode of the device and the ground electrodes of the two intensity controllers located outside the two-way optical waveguide, so as to realize intensity modulation and form a polarization filter-type intensity modulator.

The specific technical solutions are as follows:

It comprises an optical waveguide (9), an intensity control center electrode (3) and an intensity control ground electrode (4), the optical waveguide (9) is made of an optical waveguide layer (1), and the optical waveguide layer (1) is from bottom to top It comprises an LNOI lithium niobate film and a silicon dioxide protective layer (5), wherein the LNOI lithium niobate film comprises a silicon dioxide buffer layer (8) and a lithium niobate layer (7) from bottom to top, and is characterized in that :

The intensity modulator is integrated on the same chip and has the functions of filtering and intensity modulation;

The optical waveguide layer (1) is improved, and the improvement lies in that the lithium niobate layer (7) adopts X-cut Y-type lithium niobate, and is deposited on the lithium niobate layer (7) Silicon nitride forms a silicon nitride loading strip layer (6);

The optical waveguide (9) is functionally divided into a Y-waveguide beam splitter (10) and a Y-branch beam combiner (11), and a filter metal silver coating is added on the straight waveguide at the front end of the Y-waveguide beam splitter (10). The layer (2) constitutes an optical waveguide with polarization filtering function, the filtered light wave is equally divided into two parallel waveguides by the Y-waveguide beam splitter (10), and the intensity control center electrode (3) is placed between the two parallel waveguides , an intensity control ground electrode (4) is arranged on the outside of the two parallel waveguides, the intensity control center electrode (3) and the intensity control ground electrode (4) form an amplitude controller electrode for intensity modulation; after the two paths of light are modulated, Combined output is performed by the Y branch combiner (11).

The silicon nitride thickness is 350 nm.

The thickness of the filter metal silver cladding layer (2) is 14 nm.

The lengths of the intensity control center electrode (3) and the intensity control ground electrode (4) are both 1.7 cm; the waveguide width of the optical waveguide (9) is 1.7 μm; the intensity control center electrode (3) is associated with each intensity control The distance between the ground electrodes (4) is 8 μm, and the half-wave voltage is 2V.

The Y-waveguide beam splitter (10) adopts cosine curve bending to transition from straight waveguides to two parallel waveguides, and the Y branch beam combiner (11) adopts cosine curve bending to combine the two parallel waveguides into straight waveguides .

The beneficial effects of the present invention are: (1) The waveguide structure of the present invention is a isomerization of thin film lithium niobate and silicon nitride, so that the intensity modulator has the advantages of low loss, high modulation efficiency, high integration, etc., and has potential economic and application value , which can be widely used in the fields of optical detection and optical communication; (2) the present invention is a polarization filter type intensity modulator, that is, it can perform polarization filtering and intensity regulation on the light field at the same time according to requirements, avoiding device separation and realizing device functional integration.

Description of drawings

1 is a schematic structural diagram of a polarization filter type intensity modulator based on a lithium niobate film (LNOI) of the present invention;

FIG. 2 is a schematic cross-sectional structure diagram of the electrode modulation part of the polarization filter type intensity modulator based on the lithium niobate film (LNOI) of the present invention;

In the figure 1. Optical waveguide layer, 2. Filter metal silver coating, 3. Intensity control center electrode, 4. Intensity control ground electrode, 5 Silica protective layer, 6. Silicon nitride loading strip layer, 7. Niobic acid Lithium layer, 8. Silica buffer layer, 9. Optical waveguide, 10. Y-waveguide beam splitter, 11. Y branch beam combiner.

Detailed ways

The present invention will be further described below with reference to the accompanying drawings and specific embodiments, but the present invention is not limited to this embodiment.

A schematic structural diagram of a polarization filter type intensity modulator based on a lithium niobate film (LNOI) in this embodiment is shown in FIG. 1 . It includes the optical waveguide layer 1 constructed on the LNOI film, the filtering metal silver cladding layer 2, the central electrode 3 of the intensity controller and the two ground electrodes 4 of the intensity controller located outside the two optical waveguides.

The LNOI film is composed of a lithium niobate layer 7 and a SiO2 buffer layer 8. The lithium niobate layer 7 adopts X-cut Y-type lithium niobate, and plasma electrochemical deposition (PECVD) is used on the lithium niobate layer 7. technology to deposit 350nm thick silicon nitride, and then etch through inductively coupled plasma (ICP) equipment to form a single-mode waveguide, and deposit silicon nitride to form a lithium niobate heterogeneous single-mode waveguide, which constitutes the optical waveguide 9. The upper layer of the waveguide continues to deposit a 500nm silicon dioxide protective layer 5 to protect the optical waveguide, and then the front end of the Y-branch beam splitter 10 on the front side of the optical waveguide is coated with a silver coating by magnetron sputtering to filter the input light wave. When the electromagnetic wave is transmitted in the optical waveguide, a tube wall current will be formed, and the magnetic field formed by the tube wall current has a suppressing effect on the magnetic field of the incident electromagnetic wave, so that only the electric field is allowed to pass. Therefore, the TM mode can be absorbed, and the TE mode can be passed through, thereby realizing polarization filtering. The filtered light wave is divided into two paths by the Y-waveguide beam splitter 10, and the intensity is modulated by the intensity control center electrode 3 between the two paths of optical waveguides and the two intensity control ground electrodes 4 located outside the two paths of light guides. Under the action of the electrodes, a phase difference is generated between the light waves passing through the two waveguides, and then the Y branch beam combiner performs interference output. The intensity after interference is related to the phase difference between the two light waves. In this way, filtering and intensity modulation of the light waves are realized.

The manufacturing process of the present invention is simple, the lithium niobate layer 7 adopts X-cut Y transmission, and the optical waveguide 9 uses the plasma-enhanced chemical vapor deposition (PECVD) technology to deposit nitrogen with a thickness of 350 nm on the lithium niobate layer 7 Silicon is deposited, and then the waveguide is formed by inductively coupled plasma ICP etching. In summary, the present invention proposes a polarization filtering intensity modulator based on lithium niobate thin film (LNOI), which can well realize polarization filtering and intensity modulation of light. Due to the lithium niobate-silicon nitride loading strip structure Due to the high refractive index difference between the waveguide layer and the cladding layer, the optical field can be well bound and transmitted in the waveguide, so that the size of the device is greatly reduced, taking into account the high electro-optic modulation efficiency of lithium niobate, low propagation loss and processing. The advantages of mature technology and compatibility with the mature CMOS technology at this stage are realized. At the same time, the functional on-chip integration of filters and intensity modulators is realized, which avoids device separation. It has potential economic and application value and is expected to be widely used in the field of integrated optics. application.

Claims (5)

1. A polarization filtering type intensity modulator based on a lithium niobate thin film, which comprises an optical waveguide (9), an intensity control center electrode (3) and an intensity control ground electrode (4), wherein the optical waveguide (9) is made of an optical waveguide layer (1), the optical waveguide layer (1) comprises an LNOI lithium niobate thin film and a silica protective layer (5) from bottom to top, the LNOI lithium niobate thin film comprises a silica buffer layer (8) and a lithium niobate layer (7) from bottom to top, and the polarization filtering type intensity modulator is characterized in that:

the intensity modulator is integrated on the same chip and has the functions of filtering and intensity modulation;

the optical waveguide layer (1) is improved, and the improvement is that the lithium niobate layer (7) adopts X-cut Y-type lithium niobate, and a silicon nitride loading strip layer (6) is formed on the lithium niobate layer (7) by depositing silicon nitride;

the optical waveguide (9) is divided into a Y waveguide beam splitter (10) and a Y branch beam combiner (11) from the functional structure, a filtering metal silver coating (2) is added on a straight waveguide at the front end of the Y waveguide beam splitter (10) to form an optical waveguide with a polarization filtering function, the filtered optical wave is equally divided into two parallel waveguides by the Y waveguide beam splitter (10), an intensity control center electrode (3) is arranged between the two parallel waveguides, the outer sides of the two parallel waveguides are respectively provided with an intensity control ground electrode (4), and the intensity control center electrode (3) and the intensity control ground electrode (4) form an amplitude controller electrode for intensity modulation; the two paths of light are subjected to intensity modulation and then are combined and output by a Y-branch beam combiner (11).

2. The lithium niobate thin film based polarization-filtering intensity modulator of claim 1, wherein: the silicon nitride thickness was 350 nm.

3. The lithium niobate thin film based polarization-filtering intensity modulator of claim 1, wherein: the thickness of the filtering metal silver coating (2) is 14 nm.

4. The lithium niobate thin film based polarization-filtering intensity modulator of claim 1, wherein: the length of the intensity control center electrode (3) and the length of the intensity control ground electrode (4) are both 1.7 cm; the waveguide width of the optical waveguide (9) is 1.7 μm; the distance between the intensity control center electrode (3) and each intensity control ground electrode (4) is 8 mu m, and the half-wave voltage is 2V.

5. The lithium niobate thin film based polarization-filtering intensity modulator of claim 1, wherein: the Y waveguide beam splitter (10) adopts cosine curve bending to transit from the straight waveguide to the two parallel waveguides, and the Y branch beam combiner (11) adopts cosine curve bending to combine the two parallel waveguides into the straight waveguide.

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