An angularly tunable unpolarized narrowband optical bandpass filter

Abstract

The present application belongs to the technical field of guided-wave optics, and specifically relates to an angle-tunable non-polarized narrow-band optical band-pass filter. The optical filter can pass transverse electric waves and transverse magnetic waves at the same time, the passband center wavelengths of the transverse electric waves and the transverse magnetic waves monotonously and continuously change with the incident angle, and the two center wavelengths remain coincident during the tuning process; the passband full width at half maximum of the two kinds of waves has a polarization-independent narrow-band optical band-pass filtering effect; and the specific implementation comprises the following steps: a composite transparent body of a column and a planar optical waveguide; the column is cut along the longitudinal axis at a certain angle, and the planar optical waveguide is prepared on a cutting surface according to the transmission waveband; the planar optical waveguide is composed of thin films with different refractive indexes and thicknesses, and the two cutting bodies are pasted and sealed along the cutting surface to form a composite body; and wide-band antireflection films are coated on the two end surfaces of the composite body. The transmittance of the band-pass filter exceeds 99% in the tuning range, and the band-pass filter has important application value in optical communication, laser technology, precise spectroscopy and the like.

Description

Technical Field

[0001] This invention belongs to the field of waveguide optics technology, specifically relating to an angle-tunable non-polarized narrowband optical bandpass filter. Background Technology

[0002] Optical filters with varying bandwidths are essential optical components in optical communication and spectroscopy. Many techniques exist for generating optical filters, such as optical thin films, interference filters, and guided-mode resonant filters based on waveguide grating structures. These optical filtering techniques are primarily designed based on the transmission of polarized light, and are therefore polarization-dependent; that is, the transverse electric wave (TE) and transverse magnetic wave (TM) of the filter have different spectral passbands. A transverse electric wave (TE wave) is a light wave whose polarization direction is perpendicular to the plane formed by the normal to the planar waveguide and the wave vector direction of the light wave. A transverse magnetic wave (TM wave) is a light wave whose polarization direction is parallel to the plane formed by the normal to the planar waveguide and the wave vector direction of the light wave.

[0003] The optical bandpass filter involved in this invention can simultaneously form a narrow bandpass filtering effect for two polarized waves. The center wavelengths of the passbands of the transverse electric and transverse magnetic waves change monotonically and continuously within a certain range with the incident angle, and the two center wavelengths remain coincident during the tuning process. The full width at half maximum (FWHM) of the passbands for both waves is about 10 nanometers (nm), thus forming a polarization-independent narrow bandpass optical bandpass filtering effect.

[0004] As is well known, the period of a waveguide grating-based narrowband optical bandpass filter is generally subwavelength, which increases the technical complexity and difficulty in device fabrication, such as fabrication accuracy. Therefore, the fabrication cost is higher than that of a non-grating uniform structure, and the fabrication process is also more complex, thus affecting the device performance and yield. Summary of the Invention

[0005] The purpose of this invention is to provide a tunable, non-polarized narrowband optical bandpass filter that is simple to manufacture, has excellent performance, and is easy to use.

[0006] The non-polarized narrowband optical bandpass filter designed in this invention is based on the optical bandpass filtering principle of prism-coupled guided mode resonance. The center wavelengths of its transverse electric and transverse magnetic waves monotonically and continuously change with the incident angle within a certain range (e.g., 61-63 degrees), and the two center wavelengths remain coincident during tuning. The full width at half maximum (FWHM) of both waves is approximately 10 nm (e.g., 5 nm to 12 nm, preferably 8-11 nm), thus providing polarization-independent narrowband optical bandpass filtering. Its structure is as follows: Figure 1As shown. Specifically, it includes: a collimated incident beam, a composite transparent body consisting of two cut bodies formed by cutting a cylinder and a planar optical waveguide, hereinafter referred to as the composite body; wherein: the two cut bodies are two symmetrical parts obtained by cutting the cylinder along the longitudinal axis at an angle A, and the planar optical waveguide is located between the opposite cut surfaces of the two cut bodies. The planar optical waveguide is composed of a coupling layer-waveguide layer-coupling layer; the waveguide layer is composed of thin films with different refractive indices and thicknesses.

[0007] In the fabrication process, the cylinder is first cut at an angle A along its longitudinal axis to obtain two symmetrical cut bodies. A planar optical waveguide is then fabricated on the cut surface (sloping surface) of one of the cut bodies according to the transmission band. The planar optical waveguide is composed of thin films with different refractive indices and thicknesses. The two cut bodies are then bonded together along the cut surface, fixed, and sealed to form a composite. Broadband antireflection coatings are deposited on both ends of the composite. During use, the light beam is transmitted along the longitudinal axis of the composite. Angle A satisfies: A = 90° - θ, where θ is greater than the critical angle θc between the cut body and the waveguide coupling layer: θc = asin(n4 / n3). Here, n4 and n3 are the refractive indices of the cut body and the waveguide coupling layer, respectively.

[0008] In this invention, the collimated incident beam is a collimated beam.

[0009] In this invention, the antireflective coatings on both ends are broadband antireflective coatings deposited according to the designed center wavelength.

[0010] In this invention, the refractive index of the material of the cylindrical (such as a cylindrical or square) cut body is higher than the refractive index of the coupling layer of the planar optical waveguide.

[0011] In this invention, the planar optical waveguide is composed of a basic structure of coupling layer-waveguide layer-coupling layer; the waveguide layer is composed of thin films with different refractive indices and thicknesses.

[0012] In this invention, the column is a cylindrical or square column, and the cut surfaces of the two cut surfaces are inclined planes. When the column is a cylindrical column, its cut surface is elliptical, and when the column is a square column, its cut surface is rectangular.

[0013] In this invention, the high and low refractive indices of the two low-refractive-index coupling layers and waveguide layers are relative terms.

[0014] The optical bandpass filter proposed in this invention is based on prism-coupled guided-mode resonance filtering technology. Compared with the guided-mode resonance technology of grating structure, the device structure of this invention does not contain a grating, so there is no need for the complex process of fabricating subwavelength grating structures, which greatly simplifies the device fabrication process and reduces costs, and is conducive to improving device quality.

[0015] The optical bandpass filter proposed in this invention is polarization-independent, meaning the polarization state of the incident light does not affect the filter's transmission efficiency; in other words, the optical bandpass filter of this invention is unrestricted by the polarization state of the incident light. The overall light transmittance can reach over 99%. In use, the center wavelength of the filter passband can be tuned simply by fine-tuning the incident angle of the light, thus possessing wide applicability.

[0016] Advantages of this invention:

[0017] 1. It can achieve the coincidence of the passband centers of transverse electric waves and transverse magnetic waves passing through the filter, realizing polarization-independent full passability;

[0018] 2. Its peak transmittance can reach over 99%. The full width at half maximum (FWHM) of the filter is on the order of 10 nanometers.

[0019] 3. The structure of the present invention is a planar optical waveguide transmission structure based on prism coupling. Compared with the structure of grating coupling, its biggest advantage is that it does not use grating. Therefore, there is no need for the complicated process of fabricating subwavelength gratings. It can be completed by simply using a certain coating process and assembly.

[0020] 4. Another advantage of the present invention is that by adjusting the incident angle of the collimated light, the two passbands can be tuned simultaneously within a certain wavelength range, which greatly increases the flexibility and applicability of the filter. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the structure of the tunable non-polarized narrowband optical bandpass filter of the present invention.

[0022] Figure 2 for Figure 1 The transmission passbands of transverse electric and transverse magnetic waves in the filter structure shown.

[0023] Figure 3 for Figure 1 The average passband transmittance of transverse electric and transverse magnetic waves in the filter structure shown.

[0024] Figure 4 for Figure 1 The peak value of the average passband transmittance of transverse electric and transverse magnetic waves in the filter structure shown varies with the incident angle.

[0025] Figure 5 for Figure 1 The wavelength values ​​corresponding to the peak values ​​of the average passband transmittance of transverse electric and transverse magnetic waves in the filter structure shown vary with the incident angle.

[0026] Figure 6 for Figure 1The diagram shows the variation of the transverse electric wave and transverse magnetic wave transmittance passbands, as well as the bandwidth (full width at half maximum) of the average of the two passbands, with the incident angle in the filter structure shown.

[0027] The diagram is labeled as follows: 1 is the collimated incident beam, 2 and 8 are the end-face antireflection coatings, 3 and 7 are cylindrical or square prism cut bodies, 4 and 6 are low refractive index coupling layers, and 5 is the waveguide layer. Detailed Implementation

[0028] Based on the foregoing description, a non-polarized narrowband optical bandpass filter with tunable angle is constructed, as shown in the following figure. Figure 1 As shown, select a working wavelength band, such as 600-700nm. The cylinder can be constructed using two base corners ( Figure 1 The A-angle in the waveguide is composed of a 60-degree equilateral triangular prism made of ZF6 glass with a refractive index of 1.750. The low-refractive-index coupling layers 4 and 6 in the planar waveguide structure are made of fused silica with a refractive index of 1.462 and a thickness controlled at 350 nm. The waveguide layer 5 consists of five thin films made of three materials: a high-refractive-index Ti3O5 layer with a refractive index of 2.352 and a thickness of 172.6 nm; a low-refractive-index alumina (Al2O3) layer with a refractive index of 1.660 and a thickness of 130.1 nm; and a spacer layer with a refractive index of 1.605, made of nitrile glass (N-SK2) with a thickness of 400 nm.

[0029] The waveguide layer structure is distributed according to the refractive index of the material in the order of high-low-interval-low-high, controlling the thickness of each layer as described above. Using the waveguide theory of biprism coupling, the optical transmittance of different polarized light waves in the above structure can be calculated. The calculation method can be found in: Optics Express 25(11), 12121-12130(2017), IEEE Photonics Technology Letters, 28(23), 2705-7, 2016, and related cited literature.

[0030] The transmittance spectra of incident light in two polarization states (i.e., transverse electric wave TE and transverse magnetic wave TM) were calculated by the filter with an incident angle of 63.13 degrees in the waveguide layer. The results are as follows: Figure 2 As shown, the filter exhibits bandpass filtering for both polarized light, with each passband possessing 100% transmittance. As can be seen, the passband centers of the two polarized light states coincide, thus demonstrating polarization-independent filtering, i.e., non-polarization filtering. The only difference lies in the passband bandwidth (full width at half maximum) of the two polarization states. The TE wave bandwidth is approximately 4.4 nm, and the TM wave bandwidth is approximately 7.9 nm, both less than 10 nm, classifying them as narrow bandwidths. The above calculations take into account material dispersion in the corresponding wavelength bands.

[0031] The non-polarized form is the average of the two polarization states. Figure 3 yes Figure 2 The average transmittance of the passbands in the two polarization states is shown in the figure. As can be seen from the figure, the obtained average value is also a well-shaped passband with high transmittance, reaching up to 99.9%.

[0032] Figure 4 and Figure 5 This refers to the peak transmittance of the average values ​​of the two polarization passbands and the variation of the wavelength corresponding to the peak transmittance with the incident light angle. Figure 5 As shown in the figure, the peak transmittance wavelength of the average passband changes from 0.7269 μm to 0.6614 μm with the incident angle from 61.93° to 63.33°, a range of 65.5 nm. This change exhibits a linear relationship. It is evident that the peak wavelength of the passband can be linearly tuned by changing the incident angle within a certain range. Furthermore, from... Figure 4 As can be seen, the transmittance change is very small with this angle tuning. The transmittance remains above 99% throughout the entire angle tuning range. This also shows that the overlap of the two polarization passband spectra is well maintained.

[0033] Figure 6 The diagram shows the variation of two polarization passbands and their average passband bandwidth (full width at half maximum) with the incident angle. It can be seen that the bandwidth is mostly less than 10 nm within the angular tuning range, thus maintaining its narrow bandwidth characteristic.

[0034] Therefore, the optical bandpass filter constructed in this invention has polarization-independent tunability, and maintains stable transmission efficiency and bandwidth performance. Furthermore, the device is very simple to use and fabricate.

Claims

1. An angularly tunable non-polarizing narrowband optical bandpass filter based on the principle of optical bandpass filtering by prism-coupled guided-mode resonance, characterized in that, The passband center wavelengths of the transverse electric wave and the transverse magnetic wave monotonously and continuously change with the incident angle, and the two center wavelengths keep coinciding during the tuning process, the full width at half maximum of the passband of the two kinds of waves is 5-12 nm, thereby forming a polarization-independent narrowband optical bandpass filtering effect; specifically comprising: a collimated incident light beam, a composite transparent body composed of two cutting bodies and a planar optical waveguide, hereinafter referred to as a composite body; wherein: The two cutting bodies are two symmetrical parts obtained by cutting a column along the longitudinal axis at an angle A, and the planar optical waveguide is prepared on a cutting surface according to the transmission band; the planar optical waveguide is composed of a coupling layer-a waveguide layer-a coupling layer; the waveguide layer is composed of 5 thin films and contains three materials, and the waveguide layer structure is distributed according to the size of the material refractive index as high-low-separation-low-high; the two cutting bodies are pasted and sealed along the cutting surface; the planar optical waveguide and the two cutting bodies form a composite body, which is still a column; broadband antireflection films are prepared on the two end faces of the composite body; Wherein, the angle A satisfies: A = 90°-θ, θ is greater than the critical angle θc of the cutting body and the waveguide coupling layer, θc = asin(n4 / n3), wherein n4 and n3 are the refractive indexes of the cutting body and the waveguide coupling layer, respectively. The material refractive index of the cutting body of the column is higher than the refractive indexes of the two coupling layers of the planar optical waveguide.

2. The angle-tuneable non-polarizing narrow-band optical bandpass filter according to claim 1, characterized in that The broadband antireflection films on the two end faces are broadband antireflection film layers coated according to the designed center wavelength.

3. The angle-tuneable non-polarizing narrow-band optical bandpass filter according to claim 1 or 2, characterized in that The column is a circular column or a square column; when the column is a circular column, the cutting surface is an ellipse, and when the column is a square column, the cutting surface is a rectangle.

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