RU2531127C2 - Photonic crystal waveguide for selective transmission of optical radiation - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: photonic crystal waveguide with a hexagonal shape comprises cladding and a hollow core in which a multilayer of capillaries is inserted. The period and diameter of channels of the multilayer of capillaries is close or much smaller than the radiation wavelength of the required spectral range. The diameter of capillaries of the cladding is always greater than the diameter of channels of the multilayer.

EFFECT: enabling selection of spectral components with a width smaller than 200 nm from optical radiation flux of a wideband source within the entire visible wavelength range.

2 cl, 10 dwg

Description

The invention relates to the field of nanotechnology, intended for the production of optical fiber used for various purposes, including the transmission of information, in nano- and optoelectronics, as well as photonics.

Photonic crystal waveguides are a new type of optical waveguide, the potential capabilities of which are significantly higher, which is the case with a standard optical fiber. This is achieved due to the unusual shell structure around the core of the optical waveguide in the form of a photonic crystal lattice.

Various types of photonic crystal waveguides are used in optical applications and, in particular, in solving problems associated with controlling the spectral characteristics of an optical system. In particular, optical devices capable of selectively transmitting optical radiation in the required spectral ranges can be used in photovoltaics to concentrate a powerful radiation flux over a small area of the receiver without reducing the conversion efficiency and preventing high heating of semiconductor elements by filtering spectral components that are converted into electrical energy with the smallest efficiency.

The South Korean patent KR No. 20082002130 and the Chinese patent CH No. 101561535 are known. Also known is a hollow photonic crystal fiber, US patent US No. 8306379, having waveguide properties. The shell region surrounding the air core consists of an array of microcapillaries; the structure has a step 5 times larger than the wavelength. The step of the structure is less than 10 μm, the wall thickness in the range of 200 nm is 1 μm, the structure is of the kagome type. The disadvantage of such solutions is the wide transmission spectrum or the presence of several transmission maximums of optical radiation in the visible range, as well as the low intensity of transmitted radiation due to the small ratio of the areas of the hollow core and the structural shell.

Closest to the invention is the solution described in US patent US No. 2012/0206726, where optical waveguides formed as a defect in a photonic crystal are used to isolate narrow bands from the emission spectrum from broadband sources. The device is an optical waveguide embedded in the structure of a dielectric with periodically located holes. The transmission spectrum of the formed photonic crystal waveguide is defined by an array of hollow air channels and the distance between the channels. A significant disadvantage of this solution is the dependence of the transmittance of the structure at the maximum on the central wavelength of the maximum (decrease in the transmittance of the structure with a decrease in the transmittance maximum wavelength). It should also be noted the complexity of the design, and therefore the difficulty in achieving reproducibility and manufacturing accuracy.

The objective of the invention is the narrowing of the passband of the photonic crystal waveguide in the target spectral range while maintaining the highest possible intensity of optical radiation.

This is achieved by the fact that in a photonic crystal waveguide having a cross section in the direction perpendicular to the axis of the waveguide, with a periodically arranged array of holes, and the central part is a hollow core into which a multilayer of capillaries with a period and diameter close to or many shorter wavelengths from the required spectral range of maximum transmittance.

In addition, the diameter of the capillaries of the shell of the waveguide can be made larger than the diameter of the channels of the multilayer.

The essence of the claimed invention lies in the fact that due to the micro- and nanostructuring of the shell and the hollow core of the fiber, they form the frequency profile of the dispersion and the spatial profile of the distribution of the electromagnetic field. The period and diameter of the channels of the multicapillary structure of the waveguide are close to or much less than the wavelengths of visible or infrared radiation. The waveguide effect in such structures arises due to the creation of a wide forbidden zone for radiation propagating along such a structure.

The proposed photonic crystal waveguide contains a hollow core surrounded by a periodic array of multicapillaries, which is surrounded by thin-walled capillaries of larger diameter. For structural strength, monolithic glass racks are laid outside.

The technical result of the invention is the creation of a simple, cheap design, because it is made of multicomponent glasses, and not of quartz, which allows to obtain narrow transmission bands of less than 200 nm while maintaining the highest possible intensity of optical radiation.

The propagation of electromagnetic radiation in a photonic crystal waveguide occurs due to the periodic structure of the sheath, because light rays reflected from regions with different refractive indices will interfere with each other, amplifying or attenuating depending on the ratio of the wavelength and the period of the structure.

Certain colors (or frequencies) stand out from white light due to interference. In a photonic crystal, the interference suppresses (prohibits propagation) for a whole range of wavelengths - in this case “forbidden zones” arise. Such forbidden modes (wavelengths) are localized in the central part of the fiber along its entire length.

Thus, there is no need to create a certain difference in the refractive indices between the core and the sheath, as in the case of classical optical fiber - the choice of material for the inner part is unlimited. Moreover, hollow fibers, where light propagates inside the air cavity, have an advantage over classical fibers with a stepwise change in the refractive index - the fiber core there should always have a higher refractive index than the cladding.

The advantage of hollow fibers is in the infinitesimal dispersion, since light propagates in a practically dispersionless medium - air.

Figure 1 shows a method of stacking glass capillaries in a hexagonal bag.

Figure 2 schematically shows the geometry of the internal structure of a photonic crystal waveguide for dividing a wide spectrum of optical radiation into narrow ranges, where 1 is a series of glass posts for strength and rigidity, 2 is a thin-walled glass tube, the outer layer of the structural shell, 3 is a multicapillary array , geometry and parameters provide the necessary narrowing of the bandwidth.

Figure 3 shows a micrograph of a cross section of a photonic crystal waveguide for dividing a wide spectrum of optical radiation into narrow ranges.

Figure 4 shows the transmission spectra of some samples of photonic crystal waveguides with different sizes of the internal structure:

a - the diameter of the hollow core is 125 μm, the diameter of the pylons in layer 1 and the capillaries in the layers 2 is 22 μm, the diameter of the capillaries in the multicapillary layer is 3–3 μm, the wall thickness of the capillaries in the layer 3 is 0.33 μm;

b - the diameter of the hollow core is 137 μm, the diameter of the pylons in the layer 1 and the capillaries in the layers 2 is 24.2 μm, the diameter of the capillaries in the multicapillary layer is 3 - 3.3 μm, the wall thickness of the capillaries in the layer 3 is 0.37 μm;

c - the diameter of the hollow core is 147 μm, the diameter of the pylons in layer 1 and the capillaries in layers 2 is 25.9 μm, the diameter of the capillaries in the multicapillary layer 3 is 3.5 μm, the wall thickness of the capillaries in the layer 3 is 0.39 μm;

g - the diameter of the hollow core - 151 μm, the diameter of the piles in layer 1 and the capillaries in layers 2 - 26.6 μm, the diameter of the capillaries in the multicapillary layer 3 - 3.6 μm, the wall thickness of the capillaries in the layer 3 - 0.4 μm;

d - the diameter of the hollow core - 152 μm, the diameter of the racks in layer 1 and the capillaries in layers 2 - 26.8 μm, the diameter of the capillaries in the multicapillary layer 3 - 3.6 μm, the wall thickness of the capillaries in the layer 3 - 0.41 μm;

e - the diameter of the hollow core is 157 μm, the diameter of the piles in the layer 1 and the capillaries in the layers 2 is 27.7 μm, the diameter of the capillaries in the multicapillary layer 3 is 3.73 μm, the wall thickness of the capillaries in the layer 3 is 0.42 μm;

g - the diameter of the hollow core is 182 μm, the diameter of the beads in layer 1 and the capillaries in layers 2 is 32.1 μm, the diameter of the capillaries in the multicapillary layer 3 is 4.3 μm, and the wall thickness of the capillaries in the layer 3 is 0.49 μm.

The present patent describes the design of hollow core photonic-crystal waveguides, which make it possible to isolate spectral components with a width of 100-200 nm from a stream of optical radiation from a broadband source within the entire visible wavelength range while maintaining the highest possible intensity of optical radiation.

Round thin-walled glass capillaries are used as the starting material for the production of photonic-crystal waveguides with a hollow core. Glass capillaries are preliminarily made from molten glass according to classical fiber technology by drawing on a plant consisting of a furnace, a spinneret assembly and a drawing mechanism. When heated (not more than 1000 ° C), the glass block softens, and the shape, size and subsequent configuration of the product are formed by a die, a die assembly and the operation of the drawing mechanism.

After receiving thin-walled capillaries, they are placed in a package (Figure 1), and the geometry of the structural shell of the waveguide is formed by several stackings and constrictions. If necessary, they carry out alternate drawing of several structural elements of intermediate sizes from large diameter capillaries, and the whole structure is formed from multicore elements.

At the center of symmetry of the photonic-crystal waveguide with a hollow core, the frequency of the air channels is violated - thus forming a hollow lattice defect. The formation of the hollow core is carried out at the stage of assembly of the structure by replacing one or more glass capillaries with a guide sleeve of the same geometry.

The main geometric parameter of the structure that affects the spectral characteristics of the waveguide is the wall thickness of the capillaries in the structural shell. The wavelengths of the transmittance maxima of the hollow core photonic crystal waveguide depend on the wall thickness of the capillaries of the cladding and the refractive index of the material from which the waveguide is made [1]:

, $${\lambda }_{\max }=\frac{2d}{2j+\mathrm{one}}\sqrt{n_2^2-n_{\mathrm{one}}^2}$$

,

where d is the wall thickness of the shell capillaries, n ₁ is the refractive index of the medium filling the waveguide structure (in this case, air, i.e., n ₁ is unity), n ₂ is the refractive index of the glass of which the waveguide is made.

The geometry of the cross section of the waveguide is schematically depicted in Figure 2. In the particular case discussed below, the preform of the structure was formed on the basis of certain geometric relationships. These dimensions of the structure are given for a detailed demonstration of the technological process. At the stage of preform formation, its geometry may vary.

Example

The structure of the blank for drawing the waveguide includes one row of glass posts (1) with a diameter of 1.85 mm, two rows of glass capillaries (2) with an outer diameter of 1.85 mm, one row of multicapillaries of a hexagonal shape (3). Multicapillaries (3) consist of 37 glass capillaries with a diameter of 250 μm, the wall thickness of the capillaries is 28 μm.

From the workpiece (Figure 2) are pulled waveguides (Figure 3) with a diameter of the hollow core of 125-182 μm. Each waveguide sample has its own transmission spectrum (Figure 4).

Figure 5 shows some examples of transmission spectra of PCF, the geometric parameters of which are given below. These spectra and the corresponding geometric parameters of the waveguides are provided solely for the purpose of detailed illustration of the operation of the device. The geometric parameters of the structures can be arbitrarily varied during production in order to obtain the required spectral characteristics of the optical device.

Literature

1. Zheltikov A.M. "The colors of thin films, antiresonance phenomena in optical systems, and the ultimate loss of eigenmodes of hollow fibers." - Advances in Physical Sciences, vol. 178, No. 6, pp. 619-620.

Claims (2)

1. Photonic crystalline waveguide having a cross section perpendicular to the axis of the waveguide with a hexagonal shape with a periodically arranged array of holes, characterized in that the central part is a hollow core into which a multilayer of capillaries with a period and diameter close to or much shorter than the length is inserted waves from the required spectral range of maximum intensity. 2. The photonic crystal waveguide according to claim 1, characterized in that the diameter of the capillaries of the waveguide sheath is larger than the diameter of the channels of the multilayer.

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