EA025868B1 - Dispersing element for a spectrometer - Google Patents

Abstract

The proposed invention is related to optical spectrometry and optical instruments integrated on a crystal, and is industrially applicable to measurement of spectral composition of radiation, in particular, in analyzers of liquid and gaseous media, in the field of biomedicine, in the field of analysis of hyperspectral data. The essence of the invention: resonators in the massif used as a dispersing element in a spectrometer consist of two components: first, a ridged photon-crystal waveguide where the period of structure does not vary; second, the resonator includes a fragment of an additive material having an area of several periods of the photon crystal. As the two components are combined, a defect is formed where a resonant mode can be excited. The invention makes it possible to broaden functional capabilities of the dispersing element in a spectrometer and thereby to improve the spectrometer quality and to increase Q-factor of the resonator in the spectrometer to values of ~10.

Description

The alleged invention relates to optical spectrometry and integrated on-chip optical instrumentation and is industrially applicable for measuring the spectral composition of radiation in analyzers of liquid and gaseous media, in the field of biomedicine, for example, body-worn analyzers, when mass and dimensional characteristics of devices are important, in the field of analysis of hyperspectral data, for example, when solving problems of remote sensing of the Earth.

A dispersing element for a spectrometer is known in which a concave holographic diffraction grating is used to separate light by wavelength. The concave diffraction grating provides better focusing, greater brightness and less aberration over a wide range of wavelengths. (8protection1), which is related to Lobodaris FGTacIop dgaIpd, He \\ 1ey-Raskagb Sotrapu, EP 0322654 B1, IPC OOP 3/02, 03/30/1994).

The disadvantages of the known dispersing element for the spectrometer are the following circumstances:

low spectral resolution of light analysis;

limited operating frequency range of the diffraction grating due to the geometry and characteristics of the material;

the possibility of inducing interference due to exposure of the diffraction grating to spurious radiation sources.

A dispersing element for a spectrometer is known in which an attempt is made to improve the parameters of the previous invention. To increase the spectral resolution of the analysis of light radiation to ~ 1 nm, a spectrometer based on a micro-ring resonator combined with a diffraction grating was implemented. (Kuooki, V.V.S., L. Syep, apb M. Yrkop, 8b-ft rs8o1i1yup sauyu epkapseb t1sg8res1gote1eg, Orisk Exp, 18 (1): pp. 102-107 (2010)).

The disadvantages of the known dispersing element for the spectrometer are the following circumstances:

the complexity of manufacturing the device;

limited operating frequency range of the diffraction grating, due to the geometry and characteristics of the material.

A known dispersing element for a spectrometer in which a Fabry-Perot interferometer is used. The Fabry-Perot interferometer creates an interference pattern of the input light. The image of the interference pattern is captured by the detector, which is connected to the processor. The processor performs image processing of the interference pattern to determine information about the spectral composition of the input light. (RaTu-reto! Goipeg (gashGogt 8res1gote1eg, Rai1 Liceu, I8 Ra1ep1 20110228279 A1, MPZH ON 3/45, 09/22/2011).

The disadvantages of the known dispersing element for the spectrometer are the following circumstances:

the limited working frequency range of the Fabry-Perot interferometer, which makes it necessary to scan the working frequency in time and, thus, slows down the analysis of the spectral composition.

The closest in its technical essence and the achieved result is a dispersing element for the spectrometer, which consists in the formation on the crystal of an array of coupled photonic crystal (FC) resonators, each of which is tuned to a different resonant frequency. The resonator used in the spectrometer consists of a comb-type FC waveguide in which the period of the structure changes. A change in the period of the PC structure of the waveguide forms a defect in which the resonance mode can be excited.

A spectrometer based on such a dispersing element combines the best qualities of Fabry-Perot spectrometers (their high resolution) and diffraction grating spectrometers (their speed). In addition, the advantage of such spectrometers is their compactness and the potential for integrating photonic and electronic functionality on one substrate. Using a spectrometer based on FC resonators, the discrete spectrum of the input optical signal can be estimated with very high resolution at different points. The spectrum of the signal between discrete points can be obtained by scanning the resonance in a narrow wavelength range using, for example, a temperature change or charge injection. A FC resonator can have a quality factor of more than 10 ⁶ , which determines the resolution of up to one millionth wavelength. This is much better than the resolution of spectrometers based on diffraction gratings and, more importantly, this is achieved on a chip whose size is about 1 mm ² . In a known dispersing element for a spectrometer, the working resonant frequency of each FC resonator can be changed mechanically, by temperature, by injection of charges, or by nonlinear optical processes. Waveguide splitters are used to supply an input signal to each resonator. The spectrum of the unknown input signal is obtained by collecting the signal from each resonator in the array (1n1dta1eb papoeat sauyu aggau 8res1tote1eg, Natuatb Co11ede,

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I8 Ra! Ep! 20130176554 Α1, UN IGC 3/26, 07/11/2013).

The disadvantages of the known dispersing element for the spectrometer are the following circumstances:

the difficulty of introducing a nonlinear optical material into the working region of the resonator; the complexity of the implementation of the electronic pumping of the resonator. In the paper | δα1ιιηίάΙ. V., e! A1., Sotras! e1esίτο-ries toyi1a! from op 8Schsop-op-1P8i1a1ot 8i81n1a1e8 i8shd sauyez νίθι u11ta-8ta11 toya uo1ite8, Oris8 Exrge88, 15 (6): p. 3140-3148 (2007)] describes the possibility of horizontal electron pumping of a PC resonator. However, such an implementation substantially limits the quality factor of the resonator to ~ 10 ² ;

the two previous drawbacks lead to the fact that the previously proposed dispersing element for the spectrometer has a small range of variation of the working resonant frequency of the FC resonators.

The basis of the invention is the task of expanding the functionality of the dispersing element in the spectrometer and, thus, improving the quality of the spectrometer, as well as increasing the quality factor of the resonator in the spectrometer to values of ~ 10 ⁵ .

The problem is solved due to the fact that the dispersing element for the spectrometer consists of an array of photonic crystal resonators, each of which is tuned to a different resonant frequency and includes a photonic crystal comb waveguide with holes, according to the invention, the holes are equally spaced from each other and have the same radius, a waveguide symmetrically relative to its axis of symmetry is a fragment of an elliptical shape made of a material that matches the material of the waveguide, with a width in the range from half the value d about two values of the width of the waveguide and a length in the range from 2 to 30 periods of the photonic crystal.

In addition, a photonic crystal waveguide is located on the substrate. A layer of additional material is applied over the fragment. A fragment of the complementary material is made of optically non-linear material, for example, chalcogenide glass. The holes are filled with air.

Using the P and N type regions, a vertical electronic pumping of the resonance chamber is realized.

According to this invention, the resonator used in the array of resonators in the spectrometer consists of two components. Firstly, from a comb FC waveguide in which the period of the structure does not change. Secondly, the resonator contains a fragment of the complementary material with an area of several periods of the photonic crystal. When two components are combined, a defect is formed in which the resonance mode can be excited.

This resonator structure simplifies the creation of devices with nonlinear properties. The second component of the structure can be used to introduce an optically nonlinear material directly into the nanoresonator region. In addition, the two-component structure of the resonator makes it possible to realize vertical electronic pumping of the resonance chamber without significantly reducing the Q-factor of the resonator.

The invention is illustrated by drawings, where in FIG. 1 shows a FC resonator: 1 - FC waveguide; 2 - a fragment of the complementary material; in FIG. 2 shows a top view of the FC resonator: 1 - FC waveguide; 2 - a fragment of the complementary material; 3 - holes in the FC waveguide 1;

in FIG. 3 shows a side view of the FC resonator: 1 - FC waveguide; 2 - a fragment of the complementary material; 3 - holes in the FC waveguide 1; 4 - a layer of material over a fragment of the complementary material 2; 5 layer of material (substrate) under the FC waveguide 1;

in FIG. 4 shows the distribution of the electromagnetic field component Eu in a vertical plane passing through the axis of the waveguide;

in FIG. 5 shows the distribution of the component of the electromagnetic field Eu in the horizontal plane directly above the elliptical fragment (in quartz);

in FIG. 6 shows the following graphs: dotted line — values of Eu along the line of intersection of two planes shown in FIG. 3 and 4, the dashed line is the Eu value along the center line directly below the PC waveguide (in quartz), the solid line is the function co8 (nx / a) exp (-n, x ² ) at σ = 0.23; a = 0.34 μm;

in FIG. 7 shows the dependence of the Q factor of the resonator on the ellipse parameter A in FIG. 2 for various values of B;

in FIG. 8 shows the dependence of the resonant frequency on the parameter of the ellipse A in FIG. 2 for various values of B;

in FIG. 9 shows the dependence of the Q factor of the resonator on the ellipse parameter A in FIG. 2 for several shear values of an elliptical fragment in the transverse direction and in the longitudinal directions;

in FIG. 10 shows an example of the geometry of the P and N type zones for the implementation of vertical electron pumping.

In order to illustrate the proposed approach to the creation of FC resonators, the structure shown in FIG. 2. The first component of this resonator is a comb FC waveguide. The waveguide consists of silicon and is located on a quartz substrate. Waveguide Holes

- 2 025868 have the same radius, are equally spaced from each other and can be filled with air or any material. The parameters of the structure, which is given as an example, are as follows: the FC waveguide (n = 3.46) lies on the substrate (n = 1.45). The width of the FC waveguide is 6 = 0.5 μm, the thickness 1 ,,, = 0.26 μm. The round holes have a radius of K = 75 nm and are filled with air; the period of the PC structure is a = 0.34 μm. A fragment of an elliptical shape (parameters of the ellipse A and B) (n = 3.46) lies on the substrate (n = 1.45). Fragment thickness C = 100 nm. With such parameters of the FC waveguide, a band gap is created for radiation with a predominant TE polarization in the range from 1.4 to 1.7 μm. The second component of the resonator is an elliptical silicon fragment located on a quartz substrate.

To create a high-Q resonator, it is necessary to reduce the radiation of the resonant mode into space. This is achieved by optimizing the shape of the envelope of the resonant mode. The distribution spectrum of the electromagnetic field directly above the waveguide determines the distribution of the field in the far zone. This spectrum consists of two peaks.

Energy is dissipated from the resonator through the light cone of the waveguide, which is located between the spectral peaks. Consequently, the width of the spectral peaks determines the scattering loss in the resonator. Therefore, it makes sense to form a resonant mode with an envelope that corresponds to the Gauss function. The shape of the envelope of the resonance mode depends, in particular, on the imaginary part of the wave vector of the periodic structure. The Gaussian shape of the envelope is provided by the PC waveguide, in which γ varies linearly from period to period of the photonic crystal. It can be shown that a resonant mode with an envelope in the form of a Gaussian function can be realized by a quadratic change in the material filling factor (CMM) of the FC waveguide. The elliptical shape of the defect of the resonator proposed in this work makes it possible to reduce the SCM of the FC waveguide in a quadratic dependence on the center to the edges of the resonator. In further calculations, the thicknesses of the PC of the waveguide and the defect were assumed to be 260 and 100 nm, respectively. Such thicknesses make it possible to achieve an optimal change in the SCM in the cavity region.

A resonator with the ellipse parameters A = 6.8 μm (20 holes under the ellipse) and B = 0.5 μm was calculated. At the edges of the resonator, 5 more holes were located, i.e. the total cavity length was (20 + 5x2) x 0.34 = 10.2 μm.

A good correspondence between the distributions Eu and the analytic function in FIG. 6 indicates the Gaussian shape of the envelope of the resonance mode. Assuming a linear dependence of γ on x, we can obtain

In FIG. Figures 7 and 8 show the results of modeling the proposed resonator for various parameters of the elliptic fragment. For all calculated resonators, 5 more holes were located at the edges, not lying under the elliptic fragment. From FIG. 7 shows that with an increase in the length of the resonator, the Q factor increases. For a resonator with a length of 12.24 μm (36 holes - of which 26 holes are under an elliptical fragment, i.e., A = 8.84 μm) and B = 0.5 μm, the Q factor was ~ 1.4 × 10 ⁵ .

The minimum value of A, at which the resonance mode with a Q factor of more than 10 ³ , is still excited, is 3.4 μm (10 holes under the ellipse), B = 0.65 μm. The quality factor of such a resonator was 2.5 × 10 ³ . The values of B at which the maximum figure of merit of the resonator is achieved are different for each value of A. The optimal values of B are 650, 600, 550, and 500 nm for values of A equal to 3400, 4080, 6800, and 8840 nm, respectively. The optimal value of B for each A corresponds to the frequency of the excited resonant mode. The horizontal gray line in FIG. 7 corresponds to 1.525 μm. This is the value of the resonant frequency for A = 3400 nm, B = 650 nm. Other optimal ratios of values A and B in FIG. 7 and 8 also roughly correspond to the value of this resonant frequency. Changing the value of B can be used to adjust the resonant frequency of the resonator in the array. For example, with A = 4.08 μm and a change in the value of B in the range from 0.4 to 0.7 μm, the value of the resonant frequency varies from 1.485 to 1.535 μm.

In FIG. Figure 9 shows the results of modeling errors of horizontal displacement of two components of the resonator relative to each other. The longer the cavity, the more precisely the two components must be aligned. In this case, the ellipse shift error in the direction perpendicular to the waveguide axis is most critical. Modern technologies make it possible to combine structural layers with an accuracy of ~ 10 nm. From FIG. Figure 9 shows that with a transverse displacement of only 20 nm, the Q factor of the resonator does not exceed 10 ³ for A = 4.08 μm. The longitudinal direction is less critical to the errors of combining the components of the resonator. FIG. 4 also shows that with a longitudinal displacement of 100 nm, the value of the Q factor of the resonator changes slightly. Such an asymmetry of permissible errors along the X and ям axes is useful, for example, when using the relatively inexpensive matching technology described in [Au, Α. ΝαποίιηρππΙ Шодгарыю \ νί11ι <60 fr ousg1au rgseMop / A. Ai, K.O. AaNpbsu. Α.-Ό. S, X. S, Κ.δ. ASh1at8 // Arr11e6 Pb81C8 A. - 2012. V. 106 (4), P. 767-772]. When using this combination technology, the average error in one of the axes does not exceed 60 nm, while the error in the other axis is less than 10 nm.

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FIG. 10 shows the vertical electron pump geometry of a resonance chamber. This geometry allows you to arrange the zone P and N so that they have a minimal impact on the quality factor of the resonator.

The FC resonator described by this invention can be calculated for different wavelength ranges. For example, for the wavelength range used in telecommunications (1.30-1.65 microns). It can also be an optical wavelength range (0.39-0.75 microns).

Claims (6)

CLAIM 1. Dispersing element for the spectrometer, consisting of an array of photonic crystal resonators, each of which is tuned to a different resonant frequency and includes a photonic-crystal comb waveguide with holes, characterized in that the holes are equidistant from each other and have the same radius, symmetrically on the waveguide axes of symmetry, there is a fragment of an elliptical shape made of a material that coincides with the material of the waveguide, with a width in the range from half the magnitude to two magnitudes of the width and the length in the range from 2 to 30 periods of the photonic crystal. 2. Dispersant element according to claim 1, characterized in that the photonic crystal waveguide is located on the substrate. 3. Dispersant element according to claim 1, characterized in that a layer of additional material is applied above the fragment. 4. Dispersant element according to claim 3, characterized in that the layer of additional material is made of optically non-linear material, for example, of chalcogenide glass. 5. Dispersant element according to claim 1, characterized in that the holes are filled with air. 6. Dispersant element according to claim 1, characterized in that vertical electron pumping of the resonant chamber is implemented using the P and N type regions.