CN108663822B - Point diffraction light source based on nanowire waveguide - Google Patents
Point diffraction light source based on nanowire waveguide Download PDFInfo
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- CN108663822B CN108663822B CN201810359717.3A CN201810359717A CN108663822B CN 108663822 B CN108663822 B CN 108663822B CN 201810359717 A CN201810359717 A CN 201810359717A CN 108663822 B CN108663822 B CN 108663822B
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B27/00—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00
- G02B27/42—Diffraction optics, i.e. systems including a diffractive element being designed for providing a diffractive effect
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
- G02B6/12—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind
- G02B6/122—Basic optical elements, e.g. light-guiding paths
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/24—Coupling light guides
- G02B6/26—Optical coupling means
Abstract
The invention discloses a point diffraction light source based on a nanowire waveguide. The invention simultaneously solves the problems of lower pinhole diffraction wavefront energy and smaller optical fiber diffraction wavefront numerical aperture. The method is technically characterized in that a strongly-limited optical field is obtained through the nanowire waveguide, and then near-ideal spherical waves are generated through end face diffraction. The diffraction wavefront with larger numerical aperture is obtained by utilizing the extremely small section of the nanowire waveguide, and the measurement range is larger. The single-mode optical fiber is used as a link between the nanowire waveguide and the space light, so that the single-mode transmission property of the optical fiber enables the shape of the diffraction wavefront to be stable while stronger light energy is obtained, the quality of the diffraction wavefront cannot be influenced by incident wavefront aberration, and better repeatability is achieved. And because the cladding of the nanowire waveguide is in the micron order, the shear rate of the two interference wavefronts is small, and the dense interference fringes are not generated. The invention has flexible structure and provides a convenient and reliable high-precision point diffraction light source for wavefront detection.
Description
Technical Field
The invention belongs to the field of light beam wavefront detection, and particularly relates to a point diffraction light source based on a nanowire waveguide, which is used for a diffraction reference interferometer.
Background
The diffraction reference interferometer uses the near ideal spherical wavefront generated by diffraction as a reference wavefront, is not limited by the precision of a reference standard element, and becomes a warping technology in a high-precision optical detection technology. The key point is that the near-ideal spherical wavefront is generated by the diffraction of an optical field with a tiny cross section, and the size of the optical field is equivalent to the wavelength. For the diffraction reference interferometer of the visible light wave band, the generation of the diffraction reference wavefront mainly has two modes, one mode is that the diffraction reference wavefront passes through a pinhole etched on a metal film layer, and the other mode is that the diffraction reference wavefront passes through the diffraction of an optical field in an optical fiber on the end face. Both the two have certain problems, the energy of the diffraction wavefront generated by the pinhole on the metal film layer is weak, the adjustment is difficult, the quality of the diffraction wavefront is influenced by the error introduced by the optical element before the residual part of the diffraction light due to the limited thickness of the metal film layer, and the numerical aperture of the diffraction wavefront is small due to the large core diameter of the optical fiber, so that the measurement range of the interferometer is greatly limited. The invention provides a point diffraction light source based on a nanowire waveguide, which can limit a light field in wavelength or even sub-wavelength order, so that a small light field section can be obtained, and a sufficiently small size and a proper excitation condition can be selected, so that the generation of a high-order mode is inhibited, the transmission of the high-order mode is cut off, the single-mode transmission of the light field in the light field is stable, the reference wavefront generated by the light field diffraction is stable and high in quality, the influence of the quality of an optical element is avoided, and the measurement precision of an interferometer can be improved. And by selecting a proper structure and a proper coupling mode, higher light energy utilization rate can be obtained. The invention combines the traditional interference detection technology with the novel micro-nano optical waveguide, injects new activity to the high-precision wavefront detection of the diffraction reference interferometer, and has important engineering significance.
Disclosure of Invention
The invention aims to overcome the defects of the prior art and provides a point diffraction light source based on a nanowire waveguide.
The invention comprises an optical fiber coupler, a single mode optical fiber and a nanowire waveguide; and (3) coupling the spatial light with the wavelength of 532nm into the single-mode optical fiber by using an optical fiber coupler, transmitting the spatial light into the nanowire waveguide, and diffracting at the emergent end face of the waveguide to generate a near-ideal spherical wave.
The nanowire waveguide is mainly a rectangular nanowire waveguide structure formed by three parts, namely a silicon dioxide cladding S3a, a silicon substrate S3b and a silicon nitride core layer S3 c. The structure on the silicon nitride chip of the nanowire waveguide mainly comprises three parts of the size of the silicon nitride chip, a bent waveguide part and a coupler for coupling light into the waveguide. The refractive index of the silicon nitride core material was 2, while the refractive index of the cladding silica material was 1.46. In order to meet the single-mode stable transmission condition in the waveguide and generate diffraction wavefront with large numerical aperture, the size of the silicon nitride core layer is designed to be 250 nm. The curvature radius of the curved part of the silicon nitride nanowire waveguide is determined by the bending loss of the waveguide and the optical field distribution deformation of the mode spot, and the curvature radius of the curved part of the waveguide is designed to be 5 mu m under the condition of negligible loss and small deformation. The coupler of the optical fiber and the nanowire waveguide is designed by adopting a Y-shaped coupler, so that high coupling efficiency can be achieved, and the processing is easy.
The silicon nitride core layer has a small section, meets the characteristic of single-mode transmission, and has small transmission loss of a basic mode; the shape of the mode spot is very close to a circle, the distribution of the diffraction field is also uniform circular diffraction spot, and the mode spot can be used as a high-quality diffraction spherical wave to be used as a reference wave front for interference detection.
The nanowire waveguide is a rectangular nanowire waveguide structure with a silicon dioxide substrate and a silicon nitride core layer formed on a substrate by methods of film deposition, doped atom replacement, epitaxial growth, oxidation and the like. Carrying out etching post-treatment such as spin coating of electronic glue, exposure, development, electron beam etching, cleaning, photoresist removal and the like to obtain a rectangular nanowire waveguide structure; finally, the end faces are cleaved and polished.
The invention has the following beneficial effects:
according to the point diffraction light source based on the nanowire waveguide, the single-mode optical fiber is used as a link between the nanowire waveguide and the space light, so that the strong light energy is obtained, the single-mode transmission property of the optical fiber enables the shape of the diffraction wavefront to be stable, the quality of the diffraction wavefront cannot be influenced by the incident wavefront aberration, and the point diffraction light source has better repeatability. The silicon nitride nanowire waveguide limits an optical field in wavelength or even sub-wavelength magnitude, a small optical field section can be obtained, meanwhile, the size which is small enough and proper excitation conditions can be selected, the generation of a high-order mode is inhibited, the transmission of the high-order mode is cut off, and accordingly, the reference spherical wave with ultrahigh precision and large numerical aperture is formed by diffraction at the emergent end of the waveguide. And the Y-shaped coupler is designed, so that high coupling efficiency can be achieved, and the light intensity of the diffracted wavefront is greatly enhanced.
Drawings
FIG. 1 is a schematic diagram of a point-diffraction light source structure based on nanowire waveguides;
FIG. 2 is a schematic diagram of a nanowire waveguide structure;
FIG. 3 is a schematic diagram of a silicon nitride core layer structure;
FIG. 4A shows mode field distributions of a silicon nitride nanowire waveguide with a cross-sectional side length of 250nm at a wavelength of 532nm, respectively;
FIG. 4B shows far field diffraction wavefront distributions at 532nm wavelength for silicon nitride nanowire waveguides with 250nm cross-sectional side lengths, respectively;
FIG. 5 is a scanning electron microscope viewing of a fabricated curved nanowire waveguide;
FIG. 6 is a scanning electron microscope image of the processed Y-coupler;
FIG. 7 is a block diagram of a point-diffraction light source based on nanowire waveguides for use in a diffractive reference interferometer for detecting large numerical aperture spherical mirror shapes.
Detailed Description
Fig. 1 is a schematic structural diagram of a point diffraction light source based on a nanowire waveguide, which includes an optical fiber coupler S1, a single-mode optical fiber S2, and a nanowire waveguide S3; the optical fiber coupler S1 is adopted to couple spatial light with the wavelength of 532nm into the single-mode optical fiber S2, and then the spatial light is transmitted into the nanowire waveguide S3, and the near-ideal spherical wave is generated by diffraction on the emergent end face of the waveguide.
Fig. 2 is a schematic diagram of a nanowire waveguide structure. The nanowire waveguide is mainly a rectangular nanowire waveguide structure formed by three parts, namely a silicon dioxide cladding S3a, a silicon substrate S3b and a silicon nitride core layer S3 c. As shown in fig. 3, the silicon nitride core layer structure of the nanowire waveguide mainly includes three designs of the silicon nitride core layer size, the curved waveguide portion, and the coupler optically coupled into the waveguide.
The size of the cross section of the silicon nitride core layer is designed to meet the conditions of single-mode transmission and low transmission loss of a fundamental mode; and the shape of the mode spot is required to be very close to a circle, so that the distribution of the diffraction far field is also uniform circular diffraction spot, and the mode spot can be used as a high-quality diffraction reference spherical wave for interference detection.
Firstly, under the condition of satisfying single-mode transmission, the influence of the cross-sectional dimension of the silicon nitride core layer smaller than 250nm on the transmission characteristic needs to be analyzed. For the fundamental mode, the transmission loss of the fundamental mode increases as the sectional size of the waveguide decreases, and at a sectional size of 150nm, the loss is too large and the fundamental mode does not exist. And as the size of the waveguide cross section is reduced, the asymmetry of the mode spot is more obvious, and the size of the mode spot is not reduced, which is caused by the existence of a strong evanescent field in the cladding. In the simulation range, the far-field diffraction spot of the 150nm section is minimum, and the effect of increasing the numerical aperture of the diffracted wavefront is not achieved by further reducing the size of the section of the waveguide. Therefore, the cross-sectional dimension of the waveguide is not as small as possible, which has an effect on the symmetry of the spot and the efficiency of light transmission, and ultimately determines the wavefront quality of the diffraction field. Therefore, in order to satisfy the single-mode stable transmission condition in the waveguide and generate the diffraction wavefront with a large numerical aperture, the size of the silicon nitride core layer can be designed to be 200 nm-250 nm.
Table 1 shows the effect of different silicon nitride core layer cross-sectional dimensions on the fundamental mode of transmission.
| 150nm | 200nm | 250nm | 300nm | 400nm | |
| Waveguide loss (dB/cm) | 30 | 0.80 | 0.02 | -0.012 | 0.0034 |
| Far field diffraction spot size (°) | 20 | 25 | 30 | 30 | 30 |
Secondly, during the two-wavefront interference detection, in order to avoid the point diffraction light source device from blocking the wavefront to be detected, the optical coupling end and the light emitting end of the nanowire waveguide are designed to be perpendicular to each other, as shown in fig. 3, the nanowire waveguide has a section of bend, and therefore the bend radius R of the nanowire waveguide needs to be designed. The curvature radius of the silicon nitride nanowire waveguide bending part is determined by waveguide bending loss and mode spot optical field distribution deformation. The radius of curvature of the curved portion of the waveguide is designed to be 5 μm.
Finally, a coupler for coupling light from an optical fiber into a waveguide needs to be designed. The coupler design of the optical fiber and the nanowire waveguide adopts a Y-shaped coupler in consideration of the easy processing and packaging of the coupler and the high coupling efficiency, as shown in FIG. 3.
And processing the nanowire waveguide and cutting and polishing the end face according to the designed parameters.
The nanowire waveguide is a rectangular nanowire waveguide structure with a silicon dioxide substrate and a silicon nitride core layer formed on a substrate by methods of film deposition, doped atom replacement, epitaxial growth, oxidation and the like. Carrying out etching post-treatment such as spin coating of electronic glue, exposure, development, electron beam etching, cleaning, photoresist removal and the like to obtain a rectangular nanowire waveguide structure; finally, the end faces are cleaved and polished.
Example 1
And (3) coupling the space light with the wavelength of 532nm into the single-mode optical fiber by using an optical fiber coupler, transmitting the space light into the nanowire waveguide, and diffracting the emergent end face of the waveguide to generate a near-ideal spherical wave. The planar waveguide structure of the nanowire waveguide mainly comprises three parts, namely a silicon nitride core layer size, a bent waveguide part and a coupler for coupling light into the waveguide. The cross section size of the silicon nitride nanowire waveguide is designed to be 250nm, the single-mode transmission characteristic is met, and the transmission loss of a basic mode is small and is only 0.02 dB/cm. The mode field and far-field diffraction wave front distribution of the silicon nitride nanowire waveguide with the cross-sectional dimension of 250nm at the wavelength of 532nm are respectively shown in FIGS. 4A and B. Therefore, the shape of the mode spot is very close to a circle, the diffraction field distribution is also uniform circular diffraction spot, and the mode spot can be used as a high-quality diffraction spherical reference wave for interference detection. The curvature radius of the curved part of the silicon nitride nanowire waveguide is determined by the bending loss of the waveguide and the optical field distribution deformation of the mode spot, and the curvature radius of the curved waveguide part is designed to be 5 mu m under the condition of negligible loss and small deformation. Fig. 5 is a scanning electron microscope observation image of the processed curved nanowire waveguide. The coupler of the optical fiber and the nanowire waveguide is designed by adopting a Y-shaped coupler, so that high coupling efficiency can be achieved, and the processing is easy. The scanning electron microscope observation image of the finished coupler is shown in fig. 6.
Example 2
The invention is applied to the construction of a point diffraction interferometer for detecting the surface shape of a spherical mirror with a large numerical aperture, and the example is described as follows:
FIG. 7 is a schematic diagram of a system principle of a point diffraction light source based on a nanowire waveguide for detecting a large numerical aperture spherical mirror surface shape by a point diffraction interferometer. The point diffraction light source is placed at the image point of the wavefront to be measured, the ideal reference wavefront generated by the point diffraction light source is interfered with the wavefront to be measured, interference fringes are obtained on the CCD detector through the imaging mirror, and then the high-precision measurement of the surface shape of the spherical mirror to be measured is achieved.
Claims (3)
1. A point diffraction light source based on a nanowire waveguide, characterized by comprising a fiber coupler (S1), a single mode fiber (S2), a nanowire waveguide (S3); adopting an optical fiber coupler (S1) to couple spatial light with the wavelength of 532nm into a single-mode optical fiber (S2), further transmitting the spatial light into a nanowire waveguide (S3), and diffracting at the emergent end face of the waveguide to generate near-ideal spherical waves;
the nanowire waveguide is a rectangular nanowire waveguide structure formed by a silicon dioxide cladding S3a, a silicon substrate S3b and a silicon nitride core layer S3 c; the structure on the silicon nitride chip layer S3c of the nanowire waveguide comprises three parts of the design of the size of the silicon nitride chip layer, the design of a bent waveguide part and the design of a coupler for coupling light into the waveguide; the refractive index of the silicon nitride core layer material is 2; the refractive index of the silica cladding S3a was 1.46; the size of the silicon nitride core layer, namely the design of length and width is 250 nm; the curvature radius of the silicon nitride nanowire waveguide bending part is determined by waveguide bending loss and mode spot optical field distribution deformation, and the curvature radius of the waveguide bending part is designed to be 5 microns; the coupler design of the optical fiber and the nanowire waveguide adopts a Y-shaped coupler.
2. The nanowire-waveguide-based point-diffraction light source of claim 1, wherein the mode spot is circular in shape and the diffraction field distribution is also a uniform circular diffraction spot.
3. The point diffraction light source based on the nanowire waveguide as claimed in claim 1, wherein the nanowire waveguide is a rectangular nanowire waveguide structure formed by a silicon dioxide substrate and a silicon nitride core layer on a substrate through thin film deposition, doped atom replacement, epitaxial growth and oxidation methods; carrying out spin coating of electronic glue, exposure, development, electron beam etching, cleaning and photoresist removal, and carrying out etching post-treatment to obtain a rectangular nanowire waveguide structure; finally, the end faces are cleaved and polished.
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| JPH0973026A (en) * | 1995-09-05 | 1997-03-18 | Mitsubishi Heavy Ind Ltd | Integrated type optical connecting structure |
| CN102297725A (en) * | 2011-05-18 | 2011-12-28 | 中国科学院长春光学精密机械与物理研究所 | Device and method for detecting reference spherical wave deviation in visible point diffraction interferometer |
| CN107024734A (en) * | 2017-05-10 | 2017-08-08 | 北京航空航天大学 | A kind of sub-wavelength spot light bored based on micro-nano fiber and preparation method thereof |
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2018
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| US5420688A (en) * | 1992-12-14 | 1995-05-30 | Farah; John | Interferometric fiber optic displacement sensor |
| US5355237A (en) * | 1993-03-17 | 1994-10-11 | The United States Of America As Represented By The Administrator Of The National Aeronautics And Space Administration | Wavelength-division multiplexed optical integrated circuit with vertical diffraction grating |
| JPH0973026A (en) * | 1995-09-05 | 1997-03-18 | Mitsubishi Heavy Ind Ltd | Integrated type optical connecting structure |
| CN102297725A (en) * | 2011-05-18 | 2011-12-28 | 中国科学院长春光学精密机械与物理研究所 | Device and method for detecting reference spherical wave deviation in visible point diffraction interferometer |
| CN107024734A (en) * | 2017-05-10 | 2017-08-08 | 北京航空航天大学 | A kind of sub-wavelength spot light bored based on micro-nano fiber and preparation method thereof |
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