An apparatus for two-dimensional spectral spreading based on planar optical waveguide

Abstract

The application provides a device for two-dimensional spectrum spreading based on a planar optical waveguide. The device comprises: a light to be measured, a collimating system, an in-coupling grating, an out-coupling grating, a vertical grating, a waveguide and a detector, wherein the in-coupling grating and the out-coupling grating are symmetrically arranged inside the waveguide; the collimating system converts the light to be measured into parallel light; the parallel light enters the waveguide under the diffraction of the in-coupling grating and is transmitted inside the waveguide; the light is emitted from the waveguide under the action of an out-coupling grating with the same dispersion; a vertical grating in another direction is used to spread the light emitted from the waveguide in the vertical direction; the focused light reaches a light detector array to obtain a two-dimensional spectrum. The device combines the planar waveguide with the diffraction grating, maximally realizes the miniaturization of the spectrum, and can obtain high-resolution spectrum detection or a large spectrum detection range.

Description

Technical Field

[0001] This invention relates to the field of optical communication technology, and in particular to a device for two-dimensional spectral expansion based on a planar optical waveguide. Background Technology

[0002] As spectroscopy plays an increasingly important role in various industries, the research and demand for spectrometers have become particularly important and urgent. Driven by the pursuit of miniature spectrometers, various miniature spectroscopic systems have gradually emerged since the 1990s. In 2021, Cai Weiwei, a special researcher at Shanghai Jiao Tong University, Yang Zongyin, a researcher at Zhejiang University, and others categorized miniature spectrometers into four main types: dispersive optics, narrowband filters, Fourier transform optics, and computational reconstruction optics.

[0003] Dispersive spectrometers typically consist of one or more diffraction gratings, an optical path, and a detector array. Light is dispersed in different directions in space by the diffraction gratings. Generally, these spectrometers offer ultra-high resolution, a wide spectral range, and mature technology. However, due to their bulky dispersive elements and long optical path lengths, miniaturization is difficult. Although advancements in micro- and nanotechnology have led to increasingly smaller component sizes in these systems, the trade-offs between device size and optical path length, and between the quality of optical components and the fabrication process of the detector array, still prevent a perfect balance between high resolution and small size.

[0004] Narrowband filter spectrometers, compared to dispersive spectrometers, offer advantages such as planar device design and the elimination of long distances, avoiding the limitations imposed by the distance between dispersive elements and the detector array, thus enabling a more compact structure. However, the miniaturization of narrowband spectrometers is still limited by the size of the detector array and filters. Tunable filter differential spectrometers analyze spectra in a time-series manner, sacrificing time response rate, making them unsuitable for real-time, high-speed spectral analysis. While using narrowband filter arrays and linearly variable filters can offer some improvement, allowing for the simultaneous measurement of multiple spectral components and rapid spectral detection, this approach increases the number of detectors required. Furthermore, achieving wide-range wavelength detection necessitates additional moving parts, which contradicts the original intention of miniaturized spectrometers.

[0005] In recent years, with the continuous development of optical frequency comb applications in spectroscopy, the spectral resolution requirements for time-resolved broadband measurement technology have reached several pm. To address this, the Virtual Image Phase Array (VIPA) spectrometer has emerged. It is a novel dispersive device, shaped like a double-coated glass plate, possessing stronger dispersive capabilities than a grating. However, its output beam suffers from periodic spectral aliasing, preventing full utilization of its high-resolution characteristics. Although later researchers added a grating after the VIPA to achieve two-dimensional dispersion, thus solving the problem of periodic spectral overlap, the VIPA still has extremely high requirements for the input beam, requiring it to be focused to the order of tens of micrometers to ensure proper incident light into the VIPA's input window.

[0006] The development and application potential of miniature spectrometers is enormous, not only standing out in applications such as smartphones, satellites, drones, and wearable devices, but also opening new avenues for agriculture, mining, medicine, and public consumption. While the aforementioned grating-dispersive miniature spectrometers, including tunable filter-type and Fourier transform-type based on microelectromechanical systems (MEMS), have achieved commercial applications, challenges remain, such as the trade-off between miniaturization and performance, and the stringent requirements for the input beam in VIPA technology. Therefore, this paper proposes a miniature spectrometer based on a combination of waveguide plane and dispersive software. This model integrates the advantages of dispersive spectrometers to some extent, overcomes the limitation of not being able to achieve a more compact structure, and is less demanding on the input beam. Furthermore, with the maturity of grating technology, using pure grating dispersive elements to achieve two-dimensional spectral analysis significantly reduces the requirements for manufacturing processes and costs. In addition, the Bragg diffraction characteristics based on gratings can also solve the problem of high loss due to multi-order diffraction in VIPA.

[0007] Currently, although there are many types of miniature spectrometers in existing technologies, none have achieved the desired high-resolution miniaturized spectrometer. The complexity of the structure, the strictness of the manufacturing process, and the strict requirements for the light beam mean that these spectrometers can only be applied in a certain field. Summary of the Invention

[0008] Embodiments of the present invention provide a device for two-dimensional spectral expansion based on planar optical waveguides to achieve effective spectral analysis of input light.

[0009] To achieve the above objectives, the present invention adopts the following technical solution.

[0010] A device for two-dimensional spectral unfolding based on a planar optical waveguide includes: a light to be measured, a collimation system, an input grating, an output grating, a vertical grating, a waveguide, and a detector array, wherein the input grating and the output grating are symmetrically arranged inside the waveguide;

[0011] The light to be measured enters the collimation system, which converts the light into parallel light. The parallel light enters the waveguide for transmission under the diffraction of the coupling grating. The light undergoes total internal reflection inside the waveguide. Under the action of the coupling grating, the light exits from the inside of the waveguide in a direction parallel to the incident light. Based on the position of the light reaching the coupling grating in the horizontal direction, different wavelengths are distinguished for the first time, and a horizontally unfolded spectrum is obtained.

[0012] A vertical grating perpendicular to the output direction of the light is used to perform vertical spectral expansion of the light emitted from the waveguide. After focusing, the light reaches the photodetector array to obtain a two-dimensional spectrum.

[0013] Preferably, the vertical grating expands the wavelength of light emitted from the waveguide in an orthogonal direction and separates the superimposed frequencies in the free spectral range based on the planar waveguide spectrum in the vertical direction, so that the output angle of light of different wavelengths is different and the focusing position after passing through the lens is also different, thus obtaining a spectrum expanded in the vertical direction.

[0014] Preferably, the collimation system is a paraxial lens, used to convert the light to be measured into a beam of parallel light or nearly parallel light.

[0015] Preferably, when the beam converted into parallel light is incident on the coupling grating, the beam changes its propagation direction under the diffraction effect of the coupling grating and begins to propagate in the waveguide. The diffraction effect of the coupling grating is as follows:

[0016] Λsinθ i =mλ i (m=0,1) (1)

[0017] Where Λ is the grating constant, θ i λ is the diffraction angle, m is the diffraction order, and λ is the diffraction order. i It is the incident wavelength.

[0018] Preferably, after light is coupled into the waveguide, the diffraction angles of light rays of different wavelengths are different, resulting in different propagation periods T of the light rays within the waveguide. i As shown in the following formula:

[0019] T i =2htanθ i (3)

[0020]

[0021] h is the waveguide thickness, n1 is the cladding refractive index, n2 is the refractive index of the waveguide material at the minimum wavelength, and θ i The diffraction angle is the smallest wavelength in the spectral beam.

[0022] Preferably, the position of the beam reaching the coupling grating is obtained according to the propagation path l of the beam in the waveguide. Taking the endpoint of the coupling grating closest to the coupling grating as the origin, let b = mod(mod(l,T(λ)). i Then the position of the beam at the coupling grating is:

[0023]

[0024] λ i It refers to the wavelength of the input light.

[0025] In the horizontal direction, different wavelengths are first distinguished according to the position of the light reaching the coupling grating, and a horizontally unfolded spectrum is obtained;

[0026] The grating tilt angles of the input and output gratings are symmetrical about the z-axis, the fringe surface structures of the input and output gratings are symmetrical, and the periods of the input and output gratings are the same.

[0027] As can be seen from the technical solutions provided by the embodiments of the present invention above, the method and apparatus for two-dimensional spectral expansion based on planar optical waveguides of the present invention can achieve a more compact structure of the spectrometer by utilizing the small size and planarization of the planar waveguide; by utilizing existing mature grating technology, the performance impact caused by immature processes is avoided; at the same time, by utilizing the total internal reflection of the waveguide and the dispersion characteristics of the grating, through reasonable design, more light beams can be efficiently focused into the optical waveguide for transmission, improving optical efficiency, and there are basically no requirements for the input light beam.

[0028] Additional aspects and advantages of the invention will be set forth in part in the description which follows, and will become apparent from the description or may be learned by practice of the invention. Attached Figure Description

[0029] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the following description of the embodiments will be briefly introduced. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0030] Figure 1 A structural diagram of a device for two-dimensional spectral expansion based on a planar optical waveguide, provided for an embodiment of the present invention;

[0031] Figure 2 A side view of a planar waveguide provided in an embodiment of the present invention;

[0032] Figure 3 A top view of a planar waveguide provided in an embodiment of the present invention;

[0033] Figure 4 This is a schematic diagram of the design of an input grating and an output grating provided in an embodiment of the present invention;

[0034] Figure 5 A structural diagram based on planar waveguide spectrum is provided for an embodiment of the present invention;

[0035] Figure 6 A planar waveguide spectral structure based on a transmission grating is provided in this embodiment of the invention;

[0036] Figure 7 This invention provides a prism-based planar waveguide spectral structure. Detailed Implementation

[0037] Embodiments of the present invention are described in detail below, examples of which are shown in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention.

[0038] Those skilled in the art will understand that, unless specifically stated otherwise, the singular forms “a,” “an,” “the,” and “the” used herein may also include the plural forms. It should be further understood that the term “comprising” as used in this specification means the presence of the stated features, integers, steps, operations, elements, and / or components, but does not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof. It should be understood that when we say an element is “connected” or “coupled” to another element, it can be directly connected or coupled to the other element, or there may be intermediate elements. Furthermore, “connected” or “coupled” as used herein can include wireless connections or couplings. The term “and / or” as used herein includes any and all combinations of one or more of the associated listed items.

[0039] It will be understood by those skilled in the art that, unless otherwise defined, all terms used herein (including technical and scientific terms) have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. It should also be understood that terms such as those defined in general dictionaries should be understood to have the same meaning as in the context of the prior art, and should not be interpreted in an idealized or overly formal sense unless defined as herein.

[0040] To facilitate understanding of the embodiments of the present invention, the following will provide further explanation and description with reference to the accompanying drawings and several specific embodiments. These embodiments do not constitute a limitation on the embodiments of the present invention.

[0041] A structural diagram of a device for two-dimensional spectral expansion based on a planar optical waveguide provided in this embodiment of the invention is shown below. Figure 1 As shown, it includes: the light under test (source), collimating system (collimating lens), incoupled-grating (incoupled-grating1), outcoupled-grating (outcoupled-grating2), planar waveguide, vertical grating (grating3), cylindrical lens, and detector array.

[0042] The working process of the above device includes: the light to be measured enters the collimation system, converting the light into collimated parallel light. The parallel light enters the waveguide for transmission under the diffraction of the coupling grating. The light undergoes total internal reflection transmission inside the waveguide. Due to the wavelength selectivity of the coupling grating, different wavelengths have different transmission angles inside the waveguide, resulting in different transmission periods. Until it encounters the coupling grating, the light exits in a direction parallel to the incident light under the symmetrical effect of the coupling grating and coupling grating structures. Since the transmission periods of different wavelengths are different, the positions of different wavelengths reaching the coupling grating are also different. Therefore, different wavelengths are first distinguished by position in the horizontal direction, which is called the one-dimensional expansion of the spectrum in the horizontal direction. According to the relationship between the propagation period and the position of arrival, the wavelengths will periodically overlap, that is, different wavelengths will overlap at the same position. Therefore, in order to further expand it, a grating that can expand the wavelength in the orthogonal direction is added after the coupling grating, that is, the superposition frequencies in the free spectral range (FSR) based on the planar waveguide spectrum are separated in the vertical direction. By designing the parameters of this grating, high resolution can be obtained in the vertical direction. Then, a cylindrical lens is used to focus the light at a vertical position. Since the output angle is different for each wavelength at this moment, different wavelengths will be imaged at different positions on the detector array. The wavelength information of the spectrum can be obtained from the position information on the detector array. At this time, the spectrum obtained by unfolding it in both the horizontal and vertical directions is called a two-dimensional spectrum.

[0043] The structure of the coupling grating and waveguide in the aforementioned device jointly determines the measurable wavelength range. Because it is easily implemented, the device structure readily enables wide-range wavelength measurements. The dispersion degree of the coupling grating determines the propagation period of different wavelengths within the waveguide, resulting in positional variations at the coupling grating. These positional variations, in turn, determine the resolution of the horizontal spectrum. Therefore, the parameter design of the coupling grating and the waveguide structure jointly determine the resolution of the horizontal dispersive spectrum. The grating used for orthogonal dispersion completely determines the dispersion degree in the vertical direction, thus achieving high resolution. It is evident that the device has a simple structure, is easy to implement, and readily achieves a wide range and high resolution. There are no strict requirements for the input beam's entry point; after collimation, parallel light can propagate efficiently within the waveguide under the diffraction effect of the coupling grating.

[0044] The functions of each component in the above device are described below.

[0045] Collimation system: The collimation system can be approximated as an ideal paraxial lens, which mainly converts the light to be measured into parallel light.

[0046] Coupled grating: A side view of a planar waveguide provided in an embodiment of the present invention is shown below. Figure 2 As shown, the top view is as follows Figure 3 As shown, the design schematic of the coupling-in and coupling-out gratings is as follows: Figure 4 As shown in the attached diagram. When the beam of light, converted into parallel light, is incident on the coupling grating, as... Figure 2 and 3 As shown, under the diffraction effect of the coupling grating, the beam changes its propagation direction and begins to propagate in the waveguide. The diffraction effect of the coupling grating is as follows:

[0047] Λsinθ i =mλ i (m=0,1) (1)

[0048] Where Λ is the grating constant, θ i λ is the diffraction angle, m is the diffraction order, and λ is the diffraction order. i It is the incident wavelength. Here, a volume grating is used. The volume grating does not have higher-order diffraction orders, only 0th and 1st (or -1st) diffraction orders. When the incident light meets the following conditions, the incident light satisfies the Bragg diffraction condition, and the diffraction efficiency at this wavelength can theoretically reach 100%.

[0049] 2Λsinθ m =λ(m=0,1) (2)

[0050] Waveguide transmission system: When light is coupled into a waveguide, for parallel light with the same angle, the diffraction angle depends only on the wavelength. As shown in equation (1), different wavelengths have different diffraction angles, resulting in different propagation periods T within the waveguide. The propagation period is shown in the following equation:

[0051] T i =2htanθ i (3)

[0052] Where h is the waveguide thickness; and to ensure that more light beams can propagate inside the waveguide, the diffraction angle of the smallest wavelength in the spectral beam should be greater than the 'critical angle' to allow total internal reflection propagation inside the waveguide. Therefore, the diffraction angle should also satisfy the following condition:

[0053]

[0054] Where n1 is the cladding refractive index, and n2 is the refractive index of the waveguide material at the minimum wavelength. (See attached image) Figure 2 As shown, different wavelengths propagate within the waveguide with different propagation periods, and all meet the conditions for total internal reflection. It can be seen that by designing the waveguide and the coupling grating, more light beams can be confined within the waveguide for transmission as much as possible, which to some extent improves the incident efficiency of light.

[0055] Output grating: Based on the propagation period and the waveguide structure, i.e., the propagation path l of the beam within the waveguide, the final position of the beam reaching the output grating can be obtained. Taking the endpoint of the output grating closest to the input grating as the origin, let b = mod(mod(l,T(λ)). i Then the position of the beam at the coupling grating is:

[0056]

[0057] As attached Figure 2 As shown, different wavelengths arrive at the coupling grating at different emission positions, thus achieving horizontal wavelength separation. Similarly, due to the relationship between the waveguide structure and the propagation period, the wavelength arrival positions exhibit a certain periodicity, resulting in spectral aliasing in the vertical direction at these locations. This is why a vertical dispersion grating is subsequently added. (See attached image) Figure 3 As shown, in order to ensure that wavelength separation in the horizontal direction is only position-dependent, the periodic structures of the coupling-in and coupling-out gratings are symmetrical.

[0058] Based on the imaging principle of waveguides, and combining k-vector theory and recording theory, the propagation characteristics of a beam inside a waveguide are closely related to the periods of the coupling-in and coupling-out gratings. Therefore, to achieve the desired imaging effect, the grating period must be rationally designed. The grating period depends on the incident angle and incident wavelength of the object light and reference light waves. Considering the waveguide structure and requirements, to ensure that the horizontal wavelength separation is only position-dependent and meets the waveguide optical system's requirements for beam propagation direction, the incident states of the reference light and signal light of the two coupled gratings are shown in the attached figure. Figure 4 As shown. The analysis is as follows:

[0059] θ R =θ′ S -π (6)

[0060] θ′ R =π-θ s (7)

[0061]

[0062] The final analysis yields the following results:

[0063]

[0064] As can be seen from equation (9), in order to obtain the desired beam propagation effect, the grating tilt angles of the input and output gratings are symmetrical about the z-axis. Since the grating vector is perpendicular to the fringe plane, the fringe planes of the input and output gratings are also structurally symmetrical. Furthermore, since the period, as shown in equation (10), depends only on the angle θ between the incident angle and the fringe plane, and the wavelength λ of the reference light, it can be concluded that the periods of the input and output gratings are the same. Therefore, the periodic structures of the input and output elements are symmetrical.

[0065]

[0066] Vertical dispersion grating: In order to further increase the resolution of the spectrum, a diffraction grating perpendicular to the direction of the coupling grating needs to be added after the coupling grating to further separate wavelengths that overlap in the vertical direction. Figure 5 A structural diagram based on planar waveguide spectrum provided for an embodiment of the present invention is shown in the attached figure. Figure 5 As shown, light undergoes one-dimensional separation in the horizontal direction after passing through the planar waveguide structure, resulting in a horizontally separated spectrum. At this point, there is an aliasing frequency in the vertical direction, meaning that the wavelength output positions periodically overlap. Therefore, a grating 3 perpendicular to the output direction is added for secondary separation, resulting in different wavelength output angles and different focusing positions after passing through the lens.

[0067] Focusing Lens and Detector Array System: An embodiment of the present invention provides a planar waveguide spectral structure based on a transmission grating, as shown below. Figure 6 As shown, the spectral structure of the planar waveguide based on the prism is as follows: Figure 7 As shown. After two unfolding operations, the angles of the vertical dispersion gratings output at different wavelengths are different. Then, the gratings are focused onto the detector array by a focusing system. The wavelength information is mapped to the position information to obtain a two-dimensional spectrum.

[0068] In summary, the device of this invention combines miniaturization with high resolution; it combines a waveguide planar imaging system with a diffraction grating, utilizing the integrability and ease of fabrication of planar waveguides to maximize spectral miniaturization; at the same time, it integrates the high dispersion and high resolution characteristics of the grating, combining the two to optimize both miniaturization and high resolution.

[0069] The device in this invention reduces the restrictions on the input light beam; however, a comparison with VIPA's technology shows that the restrictions on the light beam also limit the development of spectral analysis to some extent.

[0070] However, in this structure, apart from the need to collimate the beam, there are basically no additional input limitations, which maximizes the range of applications.

[0071] The device of this invention expands the wavelength range of the measurable spectrum. As can be seen from the above design, the range of measurable wavelengths is mainly related to the structure of the waveguide. Since the waveguide manufacturing process is relatively mature, it is easy to achieve a wide spectral measurement range by optimizing the waveguide structure, which is very meaningful for spectral analysis.

[0072] The device in this embodiment of the invention solves the problem of multiple diffraction orders and high loss in VIPA by utilizing the Bragg diffraction of a grating; only the 0th and 1st (or -1st) order diffraction exists at wavelengths that satisfy the Bragg condition, and efficient beam diffraction propagation can be achieved through the design of the grating.

[0073] Those skilled in the art will understand that the accompanying drawings are merely schematic diagrams of one embodiment, and the modules or processes shown in the drawings are not necessarily essential for implementing the present invention.

[0074] As can be seen from the above description of the embodiments, those skilled in the art can clearly understand that the present invention can be implemented by means of software plus necessary general-purpose hardware platforms. Based on this understanding, the technical solution of the present invention, or the part that contributes to the prior art, can be embodied in the form of a software product. This computer software product can be stored in a storage medium, such as ROM / RAM, magnetic disk, optical disk, etc., and includes several instructions to cause a computer device (which may be a personal computer, server, or network device, etc.) to execute the methods described in various embodiments or some parts of the embodiments of the present invention.

[0075] The various embodiments in this specification are described in a progressive manner. Similar or identical parts between embodiments can be referred to mutually. Each embodiment focuses on describing the differences from other embodiments. In particular, for apparatus or system embodiments, since they are basically similar to method embodiments, the description is relatively simple; relevant parts can be referred to the descriptions in the method embodiments. The apparatus and system embodiments described above are merely illustrative. The units described as separate components may or may not be physically separate. The components shown as units may or may not be physical units; that is, they may be located in one place or distributed across multiple network units. Some or all of the modules can be selected to achieve the purpose of this embodiment according to actual needs. Those skilled in the art can understand and implement this without creative effort.

[0076] The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A device for two-dimensional spectral expansion based on a planar optical waveguide, characterized in that, include: The device includes a light under test, a collimation system, an input grating, an output grating, a vertical grating, a waveguide, and a detector array, wherein the input grating and the output grating are symmetrically arranged inside the waveguide. The light to be measured enters the collimation system, which converts the light into parallel light. The parallel light enters the waveguide for transmission under the diffraction of the coupling grating. The light undergoes total internal reflection inside the waveguide. Under the action of the coupling grating, the light exits from the inside of the waveguide in a direction parallel to the incident light. In the horizontal direction, based on the position of the light reaching the coupling grating, different wavelengths are distinguished for the first time, and a horizontally unfolded spectrum is obtained. A vertical grating perpendicular to the output direction of the light is used to perform vertical spectral expansion of the light emitted from the waveguide. After focusing, the light reaches the photodetector array to obtain a two-dimensional spectrum. The vertical grating expands the wavelength of light emitted from the waveguide in an orthogonal direction and separates the superimposed frequencies in the free spectral range based on the planar waveguide spectrum in the vertical direction, so that the output angle of light of different wavelengths is different and the focusing position after passing through the lens is also different, thus obtaining a spectrum expanded in the vertical direction. Based on the propagation path of the light beam within the waveguide The position where the beam reaches the coupling grating is obtained. Taking the endpoint of the coupling grating closest to the coupling grating as the origin, let b = Then the position of the beam at the coupling grating is: It refers to the wavelength of the input light. This refers to the period of the wavelength of the input light; In the horizontal direction, different wavelengths are first distinguished according to the position of the light reaching the coupling grating, and a horizontally unfolded spectrum is obtained; The tilt angles of the input and output gratings are symmetrical about the z-axis, which is the axis in the waveguide plane along the direction of light propagation, pointing from the position of the input grating to the position of the output grating. The fringe surface structures of the input and output gratings are symmetrical, and the periods of the input and output gratings are the same.

2. The apparatus according to claim 1, characterized in that, The collimation system is a paraxial lens, used to convert the light to be measured into a beam of parallel light or nearly parallel light.

3. The apparatus according to claim 2, characterized in that, When the beam of light, converted into parallel light, is incident on the coupling grating, the beam changes its propagation direction due to the diffraction effect of the coupling grating and begins to propagate in the waveguide. The diffraction effect of the coupling grating is as follows: in It is the grating constant. The diffraction angle, For diffraction orders, It is the incident wavelength.

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