CN107065233A - A kind of electric light tunable filter based on sub-wavelength high-contrast grating - Google Patents

Abstract

The invention discloses an electro-optical tunable filter based on a sub-wavelength high-contrast grating, and relates to the field of optoelectronic technology. The electro-optic tunable filter of the present invention is sequentially stacked with a substrate, an interdigitated sub-wavelength grating, a transparent conductive layer and an electrode from bottom to top, wherein: the exposed part of the substrate surface is provided with an electro-optic material layer flush with the transparent conductive layer, and the two The electrodes are connected with an external circuit to form an electric field between the grating ridges, and the refractive index of the interdigitated subwavelength grating material is much higher than that of the substrate material, the photoelectric material layer material and the transparent conductive layer material. The invention has a reasonable structural design, can not only obtain high-quality factor resonance and adjustable filtering spectral range through reasonable selection of grating parameters, but also can tune the filtering wavelength through voltage modulation, and then adjust the refractive index of the electro-optical material based on the electric field effect. Compared with mechanical tunable filters, the tuning accuracy and tuning speed are greatly improved. The invention has simple structure and easy-to-obtain materials, and is favorable for large-scale production and application.

Description

An electro-optic tunable filter based on subwavelength high-contrast grating

technical field

The invention belongs to the technical field of optoelectronics, and specifically relates to a filter capable of realizing the adjustable filtering function of the optical communication band through an external voltage.

Background technique

In today's digital age, optical communication technology is developing at an unimaginable speed, and wavelength division multiplexing (Wavelength division multiplexer, WDM) has greatly increased the capacity of optical communication systems. With the application of wavelength division multiplexing (WDM) technology, optical communication technology has developed rapidly. In the future, optical communication technology will develop in a more intelligent and efficient direction. As a key device in wavelength division multiplexing technology, tunable optical filters have received more and more attention and research. The tunable optical filter is a wavelength selection device, and its function is to select an optical signal of a specific wavelength from an input optical signal containing different wavelengths as required. Therefore, the tunable optical filter can be used to dynamically allocate wavelength signals in different optical channels, or to selectively filter wavelengths at the receiving end.

At present, the tunable filter that is more mature in research is the fiber grating tunable filter, which realizes wavelength tuning through temperature changes or mechanical devices, and belongs to a manual tunable filter. However, the temperature-tuned filter has a slow tuning speed, and the mechanically-tuned filter has very strict requirements on mechanical stability and accuracy, so the improvement of tuning filter accuracy is limited.

In order to overcome the shortcomings of the above fiber grating tunable filter, a tunable active filter controlled by a circuit, such as a liquid crystal tunable filter, was proposed. Morris HR et al. published "Imaging Spectrometers for Fluorescence and Raman Microscopy" :Acousto-Optic and LiquidCrystal Tunable Filters"("Imaging Spectrometers for Fluorescence and Raman Microscopy: Acousto-Optic and Liquid Crystal Tunable Filters"); and Electro-Optic Tunable Filters Based on Ti:LiNbO ₃ Waveguide Gratings, Zhang DL et al published "Electro-optically tunable super-broadband filter based on long periodgrating in Ti:LiNbO ₃ waveguide"("Electro-optical tunable ultra-broadband filter based on Ti:LiNbO ₃ waveguide long-period grating structure"). The liquid crystal tunable filter mentioned above is based on the principle of FP cavity interferometer and the frequency selection principle of the electronically controlled birefringence effect of the liquid crystal. The electronically controlled tunable filter is realized by applying a voltage across the liquid crystal layer. However, the liquid crystal tunable filter The device has the disadvantages of low transmittance (about 25%), large half-maximum width, and slow tuning speed. However, the electro-optic tunable filter based on Ti:LiNbO ₃ waveguide grating mentioned above has the problem of high driving voltage, which limits its practical application, and the manufacturing process is complicated, which increases the manufacturing cost.

Compared with optical fiber-structured gratings, waveguide-structured gratings are more advantageous in material selection, geometric parameter selection, and type selection of gratings and waveguides in the field of tunable optical filters, and can achieve a wider filtering spectral range, and Lower insertion loss. In 2004, the Chang-Hasnain CJ research group of the University of California, Berkeley proposed a subwavelength grating device with high reflection within a broad spectrum, see "Broad-band mirror (1.12-1.62μm) using a subwavelength grating" published by Mateus, CFR et al. "Broadband (1.12-1.62μm) Mirrors Made Using Subwavelength Gratings"). Subsequently, the research group proposed a single-layer subwavelength high refractive index difference grating (High-contrastgrating, HCG), see the article "A surface-emitting laser incorporating a high-index-contrast subwavelength grating" published by Michael CYHuang et al. "("A Surface-Emitting Laser Containing Subwavelength High-Contrast Gratings"). Sub-wavelength high-contrast grating (High-contrast grating, HCG) is a kind of grating structure with low refractive index material completely surrounding the high refractive index grating medium structure, and the grating period is much smaller than the wavelength. Unlike the general grating, the HCG structure can Two unique properties are produced: one is that broadband high reflectivity or high transmittance can be obtained (reflectivity or transmittance>99%); the other is that resonance with high quality factor can be produced (Q>10 ⁵ ). Recently, subwavelength high-contrast gratings have been used in applications such as quantum cavities, vertical-cavity surface-emitting lasers (VCSELs), and optomechanical nanooscillators. Because subwavelength high-contrast grating (High-contrast grating, HCG) has the characteristics of high quality factor, CuiX et al. published "High-index-contrast subwavelength grating reflectors and filters"("Reflection based on subwavelength high refractive index difference grating Filters and Filters"), the theoretical analysis of the filter quality factor can reach 10 ⁴ , the fly in the ointment is that the filter can only achieve a specific wavelength filtering effect.

In view of the above, in order to realize the practicality of optical communication and the flexibility and controllability of optical communication systems, how to design an optical filter that can flexibly tune the filtering wavelength, and then provide a tunable optical filter to reduce The operating cost of the system, on this basis, has better performance such as tunability, tuning range and precision, so as to improve the performance of the entire optical communication network, which has become a technical problem to be solved in this field.

Contents of the invention

The present invention can overcome the deficiencies of the prior art, and provides an electro-optical tunable filter based on a sub-wavelength high-contrast grating that can be modulated by an external voltage. Compared with the traditional manual tunable filter, the present invention has higher precision It also solves the shortcomings of the existing electro-optic tunable filters such as low transmittance and large driving voltage, and realizes tunable filtering in the optical communication band.

To achieve the above object, the present invention provides the following technical solutions:

An electro-optic tunable filter based on a sub-wavelength high-contrast grating, characterized in that it includes: a substrate, and an interdigitated sub-wavelength grating is provided on the surface of the substrate, and a transparent conductive layer is provided on the surface of the interdigitated sub-wavelength grating , the surface of the substrate not covering the part of the interdigitated subwavelength grating is provided with an electro-optic material layer; the surface of the transparent conductive layer is provided with electrodes electrically connected to the two fingers of the interdigitated subwavelength grating respectively; wherein: the The two electrodes are connected to an external circuit so that an electric field is formed between the grating ridges in the interdigitated subwavelength grating. The thickness of the electro-optical material layer is equivalent to the sum of the thickness of the interdigitated subwavelength grating and the transparent conductive layer, and the The refractive index of the wavelength grating material is greater than that of the substrate material, the photoelectric material layer material and the transparent conductive layer material.

According to a specific embodiment of the present invention, the ratio of the refractive index of the grating material in the present invention to the refractive index of other structural layer materials should be greater than 1.75.

As a preferred embodiment, the material of the substrate in the present invention is silicon dioxide.

As a preferred embodiment, the material of the grating of the present invention is silicon, because silicon has a high refractive index, has relatively small absorption loss, and has zero dispersion for infrared to terahertz bands.

Define the refractive index of the grating material as n ₁ , the refractive index of the substrate material as n ₂ , and the incident wavelength as λ, then the period Λ of the interdigitated sub-wavelength grating of the present invention should satisfy: λ/n ₁ <Λ<λ/n ₂ ; The thickness of the interdigitated sub-wavelength grating should be as small as possible to reduce the difficulty in process.

In the present invention, the material of the transparent conductive layer should be transparent to the infrared band and have high conductivity, and transparent conductive film glass can be used, such as indium tin oxide (ITO), tin dioxide coating (FTO), zinc oxide-based thin film (AZO) .

In the present invention, the electrode material can be any suitable material, usually one or more of gold, silver, copper, platinum, nickel, aluminum, and the two electrodes are used as lead-out electrodes, which are connected with an external control circuit so that between the two electrodes Loaded with voltage.

As a preferred embodiment, the electro-optic coefficient of the electro-optic material layer should be greater than 200pm/V to achieve the purpose of greatly reducing the driving voltage. Specifically, electro-optic polymers can be used. Preferably, the electro-optic material layer should also have temperature stability.

In the present invention, the grating ridges of the interdigitated sub-wavelength grating (2) are equidistantly arranged in parallel, and according to a specific embodiment of the present invention, the cut plane of the grating ridges is rectangular.

Compared with the prior art, the present invention has the following beneficial effects:

1. The electro-optic tunable filter based on the sub-wavelength high-contrast grating provided by the present invention has a reasonable structural design, and the filter wavelength is tuned through voltage modulation, and then the refraction index of the electro-optic material is regulated based on the action of the electric field. Compared with The mechanically tunable filter greatly improves the tuning accuracy and tuning speed; in addition, the present invention optimizes the performance of the filter through reasonable selection of materials, wherein: the present invention selects electro-optic materials with large electro-optic coefficients to greatly reduce the driving Voltage, and due to the rapid photoelectric effect, the tuning rate of the device can be improved; silicon is selected as the grating material, which has the advantages of extremely low infrared light absorption loss and insensitivity to temperature while meeting a high refractive index, which can effectively reduce the device Due to the influence of loss and temperature on the device, silicon dioxide (SiO ₂ ) is selected as the substrate, which is compatible with the CMOS process and has the advantage of being easy to integrate.

2. The electro-optic tunable filter based on the sub-wavelength high-contrast grating provided by the present invention can obtain resonance with a high quality factor by reasonably selecting the grating parameters (grating period Λ, grating thickness t _g , duty cycle), thereby realizing Filtering function, and, by designing the grating parameters, the filter spectral range can be adjusted, and by selecting the appropriate grating thickness, it can be designed as a reflective or transmissive filter.

3. The electro-optic tunable filter based on the sub-wavelength high-contrast grating provided by the present invention has a simple structure, easy-to-obtain materials, greatly reduces costs, and is conducive to large-scale production and application.

Description of drawings

Fig. 1 is a schematic structural diagram of an electro-optic tunable filter based on a sub-wavelength high-contrast grating provided by the present invention;

Fig. 2 is a schematic cross-sectional view of the structure shown in Fig. 1;

Fig. 3 is the simulation diagram of the reflectivity contour line of embodiment 1 of the present invention;

Fig. 4 is the reflection spectrum simulation figure of embodiment 1 of the present invention;

Fig. 5 is the reflection spectrum simulation figure of embodiment 2 of the present invention;

Fig. 6 is the reflection spectrum simulation figure of embodiment 3 of the present invention;

Among them: 1 is a substrate, 2 is a grating, 3 is an electrode, 4 is an electro-optical material layer, and 5 is a transparent conductive layer.

detailed description

The present invention is described in detail below in conjunction with accompanying drawing by specific embodiment:

As shown in Figure 1, an electro-optic tunable filter based on a sub-wavelength high-contrast grating includes: a substrate 1, and an interdigitated sub-wavelength grating 2 is arranged on the surface of the substrate 1, and the interdigitated sub-wavelength grating 2 The surface is provided with a transparent conductive layer 5, and the surface of the substrate 1 that does not cover the part of the interdigitated subwavelength grating is provided with an electro-optical material layer 4, that is, the interdigitated subwavelength grating 2 and the transparent conductive layer 5 are both in phase with the electro-optic material layer 4. Contact; the surface of the transparent conductive layer 5 is provided with electrodes 3 electrically connected to the two fingers of the interdigitated subwavelength grating 2; wherein: the two electrodes 3 are connected to an external circuit so that the interdigitated subwavelength grating An electric field perpendicular to the electro-optic material layer 4 is formed between the grating ridges in 2, the thickness of the electro-optic material layer 4 is equivalent to the sum of the thicknesses of the interdigitated subwavelength grating 2 and the transparent conductive layer 5, and the interdigitated subwavelength grating material The refractive index is greater than the refractive index of the substrate material, the photoelectric material layer material and the transparent conductive layer material.

In the embodiment of the present invention, the material of the substrate 1 is preferably silicon dioxide, the material of the grating 2 is preferably silicon, the material of the transparent conductive layer 5 is indium tin oxide, and the electro-optic material layer 4 is preferably an electro-optic polymer;

Wherein: the ratio of the refractive index of the material of the interdigitated sub-wavelength grating 2 to the material of the substrate 1 and the refractive index of the electro-optic material layer 4 is greater than 1.75, which can be reasonably set according to the knowledge in this field and actual needs;

Define the refractive index of the grating material as n ₁ , the refractive index of the substrate material as n ₂ , and the incident wavelength as λ, then the period Λ of the interdigitated sub-wavelength grating of the present invention should satisfy: λ/n ₁ <Λ<λ/n ₂ ;

The thickness of the interdigitated sub-wavelength grating should be as small as possible to reduce the difficulty in process.

Based on the electro-optic effect of the electro-optic material, the present invention can tune the electric field intensity through an external voltage, and then control the refractive index of the material of the electro-optic material layer 4, so as to change the resonant wavelength of the sub-wavelength high-contrast grating filter, and realize electronically adjustable filtering. The present invention adopts the electro-optic material with high electro-optic coefficient, preferably the electro-optic coefficient of the electro-optic material is greater than 200. On the one hand, the driving voltage can be reduced, and on the other hand, the photoelectric effect of the electro-optic material acts quickly, which improves the tuning speed of the device.

The working principle of the present invention is as follows:

The incident light is incident perpendicular to the grating. After the incident light irradiates the sub-wavelength high-refractive index grating, the guided mode of the grating is excited, and two low-order modes are excited in the arrayed waveguide. The interference effect of the subwavelength high-contrast grating (HCG) will exhibit high reflection characteristics, high transmission characteristics and high quality factor resonance characteristics. After selecting the appropriate grating structure parameters, when the incident wavelength reaches the resonance wavelength and satisfies the resonance condition, the resonance filtering effect will appear. Through the modulation of the external voltage, the refractive index of the electro-optical material around the sub-wavelength high-contrast grating (HCG) is changed, and the resonance wavelength is changed accordingly, thereby realizing the electronically controlled and tunable filtering characteristics.

The present invention will be further described below in conjunction with specific embodiment:

Example 1:

In this embodiment, the simulation parameters are set as follows: in this embodiment, the material of the substrate 1 is silicon dioxide, and the material of the grating 2 is silicon; the grating period Λ is 650 nm, the duty ratio is 0.5, and the grating thickness is 1100 nm; The refractive index of the substrate material is 3.48, the refractive index of the substrate material is 1.44, and the refractive index n ₀ of the electro-optical material layer material (when the driving voltage is zero) is 1.60.

In this embodiment, TM polarized light is incident, and since the thickness of the transparent conductive layer 5 is very small, the influence of the thickness of the transparent conductive layer 5 can be ignored in the simulation process. In this embodiment, the rigorous coupled wave analysis method is used for simulation, and the reflectance contour map of the electro-optic tunable filter based on the sub-wavelength high-contrast grating shown in FIG. 3 is drawn by using Matlab simulation software.

The gradient color in Figure 3 represents the reflectivity. According to the figure, the lighter the color, the closer the reflectivity is to 1, and the darker the color, the closer the reflectivity is to 0. The white dotted line area in Figure 3 is the high quality factor resonance area of the sub-wavelength high-contrast grating. Select an appropriate grating thickness t _g , satisfying that the ratio of the grating thickness to the grating period (t _g /Λ) is within the range of the white dotted line in Figure 3, so that the resonance wavelength of the electro-optic tunable filter of this embodiment is estimated to be between 1510 nm and 1580nm range.

In this embodiment, TM polarized light is incident, and the simulation is carried out by the strict coupled wave analysis method, and the reflection spectrum shown in Figure 4 is obtained by using the Matlab simulation software. Spectrum magnification. The black dotted line area in the figure is the filtering wavelength area of the electro-optic tunable filter. It can be seen from the figure that the center wavelength of the electro-optic tunable filter in this embodiment is about 1514.5 nm.

Example 2:

In this embodiment, the simulation parameters are set as follows: in this embodiment, the material of the substrate 1 is silicon dioxide, and the material of the grating 2 is silicon; the grating period Λ is 650 nm, the duty ratio is 0.5, and the grating thickness is 1100 nm; The refractive index of the substrate material is 3.48, the refractive index of the substrate material is 1.44, and the refractive index of the electro-optical material layer material is 1.65 (the driving voltage is not zero at this time); the electro-optic material selection electro-optic coefficient r ₃₃ is an electro-optic polymer of 300pm/V.

In this embodiment, TM polarized light is incident, and the simulation is carried out according to the strict coupled wave analysis method, and the reflection spectrum shown in Figure 5 is drawn by using Matlab simulation software, wherein the small picture in the upper right corner is the reflection of the filter wavelength region of the electro-optic tunable filter Spectrum magnification.

According to the formula given by the Pockel effect:

Δn is the change in refractive index of the material of the electro-optic material layer, n ₀ is the refractive index of the electro-optic material layer when the driving voltage is zero, and it can be obtained that the driving voltage applied at this time is 25V.

The black dotted line area in FIG. 5 is the filtering wavelength area of the electro-optic tunable filter. It can be seen from the figure that the central wavelength of the electro-optic tunable filter in this embodiment is about 1526 nm.

Example 3:

In this embodiment, the simulation parameters are set as follows: in this embodiment, the material of the substrate 1 is silicon dioxide, and the material of the grating 2 is silicon; the grating period Λ is 650 nm, the duty ratio is 0.5, and the grating thickness is 1100 nm; The refractive index of the substrate material is 3.48, the refractive index of the substrate material is 1.44, and the refractive index of the electro-optic material layer material is 1.70 (the driving voltage is not zero at this time); the electro-optic material selection electro-optic coefficient r ₃₃ is an electro-optic polymer of 300pm/V.

In this embodiment, TM polarized light is incident, and the simulation is carried out according to the strict coupled wave analysis method, and the reflection spectrum shown in Figure 6 is drawn by using Matlab simulation software, wherein the small picture in the upper right corner is the reflection of the filter wavelength region of the electro-optical tunable filter Spectrum magnification.

According to the formula given by the Pockel effect:

Δn is the change in refractive index of the material of the electro-optic material layer, n ₀ is the refractive index of the electro-optic material layer when the driving voltage is zero, and it can be obtained that the driving voltage applied at this time is 50V.

The black dotted line area in FIG. 6 is the filtering wavelength area of the electro-optic tunable filter. It can be seen from the figure that the center wavelength of the electro-optic tunable filter in this embodiment is about 1535 nm.

In summary, according to the simulation results, it can be obtained that by changing the driving voltage so that the refractive index of the electro-optic material layer 5 changes by 0.1, the tunable filtering range of the center wavelength of the electro-optic tunable filter can be realized from 1514.5nm to 1535nm and, as the refractive index of the electro-optic material layer 5 increases, the resonant wavelength increases accordingly, so that the resonant wavelength can be modulated by controlling the electric field intensity, thereby realizing the function of electro-optic tunable filtering.

The above is a detailed introduction and description of a method for realizing the adjustable filtering of the optical communication band based on the sub-wavelength high-contrast grating provided by the present invention. In this paper, a specific example is used in the simulation to illustrate the principle and implementation of the present invention. Elaboration and analysis, the parameters designed in the simulation can be changed appropriately in the actual engineering design. The description of the above embodiments is only used to help understand the method of the present invention and its core idea; at the same time, for scholars or technical personnel who study this field, according to the ideas provided by the present invention, they will understand the structure in terms of specific implementation plans and application ranges. With optimization, improvements to the invention are possible. In summary, the content of the above specific embodiments should not be construed as limiting the present invention.

Claims (5)

1. a kind of electric light tunable filter based on sub-wavelength high-contrast grating, it is characterised in that including：Substrate (1), substrate (1) surface is provided with interdigitated sub-wave length grating (2), and interdigitated sub-wave length grating (2) surface is provided with transparency conducting layer (5), substrate (1) does not cover the surface of interdigitated sub-wave length grating part and is provided with electro-optical material layer (4)；The transparency conducting layer (5) surface is provided with two interdigital electrodes (3) electrically communicated respectively with interdigitated sub-wave length grating (2)；Wherein：Described two Electrode (3) is connected with external circuit with so that forming electric field, electro-optical material layer between grating ridge in interdigitated sub-wave length grating (2) (4) thickness of thickness and both interdigitated sub-wave length grating (2) and transparency conducting layer (5) and suitable, and interdigitated Asia ripple The refractive index of long grating material is more than the refractive index of backing material, photoelectric material layer material and electrically conducting transparent layer material.

2. a kind of electric light tunable filter based on sub-wavelength high-contrast grating according to claim 1, it is characterised in that The material of grating is silicon.

3. a kind of electric light tunable filter based on sub-wavelength high-contrast grating according to claim 1, it is characterised in that The material of the substrate is silica.

4. a kind of electric light tunable filter based on sub-wavelength high-contrast grating according to claim 1, it is characterised in that The material of the transparency conducting layer is transparent conducting film glass.

5. a kind of electric light tunable filter based on sub-wavelength high-contrast grating according to claim 1, it is characterised in that The grating ridge equidistant parallel arrangement of the interdigitated sub-wave length grating (2).