WO2022090728A1 - Spectromètre à puce photonique - Google Patents

Spectromètre à puce photonique Download PDF

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Publication number
WO2022090728A1
WO2022090728A1 PCT/GB2021/052807 GB2021052807W WO2022090728A1 WO 2022090728 A1 WO2022090728 A1 WO 2022090728A1 GB 2021052807 W GB2021052807 W GB 2021052807W WO 2022090728 A1 WO2022090728 A1 WO 2022090728A1
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WO
WIPO (PCT)
Prior art keywords
optical
photonic chip
light
slab waveguide
waveguide core
Prior art date
Application number
PCT/GB2021/052807
Other languages
English (en)
Inventor
Robert R Thomson
Calum Ross
Original Assignee
Heriot-Watt University
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Heriot-Watt University filed Critical Heriot-Watt University
Publication of WO2022090728A1 publication Critical patent/WO2022090728A1/fr

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Classifications

    • 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
    • G02B6/12007—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 forming wavelength selective elements, e.g. multiplexer, demultiplexer
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
    • G01J3/02—Details
    • G01J3/0205—Optical elements not provided otherwise, e.g. optical manifolds, diffusers, windows
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
    • G01J3/02—Details
    • G01J3/0256—Compact construction
    • G01J3/0259—Monolithic
    • 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
    • G02B6/42—Coupling light guides with opto-electronic elements
    • G02B6/4201—Packages, e.g. shape, construction, internal or external details
    • G02B6/4204—Packages, e.g. shape, construction, internal or external details the coupling comprising intermediate optical elements, e.g. lenses, holograms
    • G02B6/4215—Packages, e.g. shape, construction, internal or external details the coupling comprising intermediate optical elements, e.g. lenses, holograms the intermediate optical elements being wavelength selective optical elements, e.g. variable wavelength optical modules or wavelength lockers

Definitions

  • the present disclosure relates to a monolithic photonic chip for an optical spectrometer, an optical spectrometer device including the monolithic photonic chip, an optical spectrometer system including the optical spectrometer device, and a method of manufacturing the monolithic photonic chip.
  • chip-based optical spectrometers which allow several components to be pre-aligned on a single substrate.
  • chip-based optical spectrometers are often more robust and compact than their traditional counterparts.
  • chip-based optical spectrometers are known which are based on conventional arrayed waveguide gratings (AWG’s) in which broadband light is coupled into an array of single-mode optical waveguides, wherein different optical waveguides have different path lengths such that the individual spectral components are resolved by constructive interference after a subsequent period of free-space propagation.
  • AWG’s having multimode waveguides generally perform poorly due to modal dispersion in the multi-mode waveguides.
  • AWG having multi-mode waveguides are generally not suitable where the spectral analysis of multimode light is required. It is also known to feed a conventional AWG photonic chip having single-mode waveguides with light via a photonic lantern.
  • the photonic lantern divides light from a multi-mode fibre (MMF) into multiple single-mode fibres which are aligned across a width of an input slab region of the AWG.
  • MMF multi-mode fibre
  • using an AWG in combination with a photonic lantern in this way results in many spectra overlapping at the output of the AWG requiring the use of complex cross-dispersion compensation techniques to spatially resolve the individual spectral components.
  • a monolithic photonic chip for an optical spectrometer comprising: an optical input and an optical output; and a slab waveguide core for guiding light from the optical input to the optical output, wherein the photonic chip defines a relative spatial relationship between the optical input, the optical output and a dispersive element so that, in use, light enters the photonic chip at the optical input, the dispersive element receives light from the optical input via the slab waveguide core, the dispersive element spatially disperses the received light in the slab waveguide core, and the spatially dispersed light exits the photonic chip at the optical output.
  • Such a monolithic photonic chip for an optical spectrometer may be more versatile than an AWG.
  • the slab waveguide core may be configured to support a plurality of guided optical modes at visible wavelengths in a direction transverse to the slab waveguide core.
  • the slab waveguide core may be configured to support a plurality of guided optical modes at infra-red wavelengths in a direction transverse to the slab waveguide core.
  • the slab waveguide core may be configured to support a plurality of guided optical modes at wavelengths in the near infra-red region of the electromagnetic spectrum in a direction transverse to the slab waveguide core.
  • the slab waveguide core may be configured to support a plurality of guided optical modes for all wavelengths for which the slab waveguide core is transparent.
  • the slab waveguide core may be a multimode slab waveguide core at visible wavelengths.
  • the slab waveguide core may be a multimode slab waveguide core at infra-red wavelengths.
  • the slab waveguide core may be a multimode slab waveguide core at wavelengths in the near infra-red region of the electromagnetic spectrum.
  • the slab waveguide core may be a multimode slab waveguide core at all wavelengths for which the slab waveguide core is transparent.
  • Such a photonic chip supports the propagation of multiple modes within the slab waveguide core and may be fed with light from a multi-mode fibre (MMF).
  • MMF multi-mode fibre
  • a photonic chip may exhibit less modal dispersion because light is confined in the slab waveguide core only in a direction transverse to the dispersion provided by the dispersive element. Consequently, the photonic chip may function as a spectrometer which is well suited to applications requiring the spectral resolution of multimode light, such as in astronomy.
  • the photonic chip may be configured for passive alignment.
  • the photonic chip may be low-cost.
  • the photonic chip may have a high degree of robustness.
  • the slab waveguide core may have a thickness of greater than or equal to 20 p.m, a thickness of greater than or equal to 50 p.m, a thickness of greater than or equal to 100 pm, a thickness of greater than or equal to 200 pm, a thickness of greater than or equal to 1 mm, a thickness of greater than or equal to 2 mm, or thickness of greater than or equal to 5 mm.
  • the dispersive element may comprise a diffraction grating.
  • the dispersive element may comprise a transmissive diffraction grating.
  • the dispersive element may comprise a volume Bragg grating (VBG).
  • VBG volume Bragg grating
  • the dispersive element may comprise a reflective diffraction grating.
  • the dispersive element may be configured to reduce the divergence of light received from the optical input in the slab waveguide core.
  • the dispersive element may be configured to collimate divergent light received from the optical input in the slab waveguide core.
  • the dispersive element may be configured to focus divergent light received from the optical input to the optical output via the slab waveguide core.
  • the photonic chip may define a relative spatial relationship between the optical input, the optical output, the dispersive element and at least one of a collimating element or a focussing element.
  • the photonic chip may define a relative spatial relationship between the optical input, the optical output, the dispersive element and at least one of a collimating element, a focussing element, or a re-direction element.
  • the collimating element may be configured to reduce the divergence of light received from the optical input in the slab waveguide core and/or to collimate divergent light received from the optical input in the slab waveguide core.
  • the focussing element may be configured to increase the convergence of the spatially dispersed light in the slab waveguide core or to focus the spatially dispersed light in the slab waveguide core to the optical output.
  • the re-direction element may be configured to receive light via the slab waveguide core from a first direction and re-direct the received light in the slab waveguide core along a second direction, wherein the second direction is different from the first direction.
  • the second direction may be opposite to the first direction.
  • the photonic chip may define the collimating element and/or the focussing element.
  • the photonic chip may define at least one of the collimating element, the focussing element or the re-direction element.
  • the collimating element and/or the focussing element may be formed separately from the photonic chip.
  • At least one of the collimating element, the focussing element and the re-direction element may be formed separately from the photonic chip.
  • the collimating element may comprise a collimating mirror.
  • the focusing element may comprise a focusing mirror.
  • the re-direction element may comprise a planar mirror.
  • the collimating mirror may be defined by, or provided at, a first edge of the photonic chip.
  • the focusing mirror may be defined by, or provided at, a second edge of the photonic chip.
  • the first and second edges of the photonic chip may be the same edge of the photonic chip or different edges of the photonic chip.
  • the photonic chip may define a slot or a notch at an edge of the photonic chip, wherein the slot or notch terminates at the optical input of the slab waveguide core and wherein the slot or the notch is configured to receive an end portion of a multi-mode fibre (MMF).
  • MMF multi-mode fibre
  • the slot or notch may define a cylindrical tunnel for receiving the end portion of the MMF.
  • a thickness of the slab waveguide core may match a diameter of a core of the MMF.
  • the cylindrical tunnel may have a diameter which is greater than a thickness of the slab waveguide core.
  • the photonic chip may define a passive alignment feature for aligning a carrier for a MMF relative to the photonic chip, wherein the carrier defines a passive alignment feature for receiving an end portion of a MMF.
  • the combination of the passive alignment feature defined by the photonic chip and the passive alignment feature defined by the carrier may allow the carrier and therefore also the MMF to be aligned passively relative the optical input.
  • the photonic chip may define an optical transition for reforming a spatial distribution of light in the slab waveguide core.
  • the optical transition may be configured so that a width of the spatial distribution of the light in the slab waveguide core at an optical output of the optical transition is less than a width of the spatial distribution of the light in the slab waveguide core at an optical input of the optical transition.
  • the optical transition may be configured to split light in the slab waveguide core into a plurality of 2D optical waveguides defined in the slab waveguide core, wherein the 2D optical waveguides are aligned relative to one another at different positions across a thickness of the slab waveguide core.
  • Each of the 2D optical waveguides of the plurality of 2D optical waveguides may comprise a single-mode optical waveguide.
  • the optical transition may comprise a photonic lantern.
  • a thickness of the slab waveguide core may be greater than a diameter of a core of the MMF.
  • the photonic chip may define a gap in the slab waveguide core.
  • the gap may have a first end and a second end opposite the first end, wherein the first end of the gap is closer to the optical input than the second end of the gap.
  • the slab waveguide core may define a first light transmitting end surface at the first end of the gap and a second light transmitting end surface at the second end of the gap.
  • the first light transmitting end surface may be configured to receive light through the slab waveguide core from the optical input and to transmit the received light across the gap to the second light transmitting end surface back into the slab waveguide core.
  • One or both of the first and second light transmitting end surfaces may define a lens profile.
  • the relative spatial relationship between the optical input, the optical output, the collimating element, the dispersive element and the focusing element may be consistent with, and/or define, a Czerny-Turner spectrometer configuration.
  • the photonic chip may define one or more location features for locating the photonic chip relative to a base arrangement and/or relative to a cover arrangement.
  • the photonic chip may comprise an optical substrate.
  • the slab waveguide core may be defined by the optical substrate.
  • the slab waveguide core may be defined by first and second outer surfaces of the optical substrate.
  • the first and second outer surfaces of the optical substrate may be planar and/or parallel to one another.
  • the photonic chip may comprise a cladding layer on only one side of the optical substrate.
  • the photonic chip may comprise a first cladding layer on a first side of the optical substrate and a second cladding layer on a second side of the optical substrate.
  • the optical substrate may comprise, or be formed from, fused silica.
  • the optical substrate may be transparent for visible wavelengths.
  • the optical substrate may be transparent at infra-red wavelengths, for example at wavelengths in the near infra-red region of the electromagnetic spectrum.
  • the photonic chip may be formed by exposing selected areas of the optical substrate to light so as to define the relative spatial relationship between the optical input, the optical output, and the dispersive element.
  • the photonic chip may be formed by exposing selected areas of the optical substrate to light so as to define and/or form at least one of the optical input, the optical output, and the dispersive element.
  • the photonic chip may be formed by exposing selected areas of the optical substrate to light so as to define and/or form the optical input, the optical output, and one or more passive alignment features for aligning the dispersive element relative to the optical input and the optical output.
  • the photonic chip may be formed by removing substrate material from at least one of the selected areas of the optical substrate after exposure of the selected areas to light so as to form at least one of the optical input of the slab waveguide core, the optical output of the slab waveguide core, and the dispersive element.
  • the photonic chip may be formed by etching substrate material from at least one of the selected areas of the optical substrate after exposure of the selected areas to light so as to form at least one of the optical input, the optical output, and the dispersive element.
  • the photonic chip may be formed by exposing at least one of the selected areas of the optical substrate to a chemical etchant after exposure of the selected areas to light.
  • the photonic chip may be formed by mechanically and/or thermally cleaving substrate material from at least one of the selected areas of the optical substrate after exposure of the selected areas to light. Exposing the selected areas of the optical substrate to light may comprise writing the selected areas of the optical substrate with at least one of coherent light, laser light, UV light, or pulses of light, such as ultrashort pulses of light.
  • the photonic chip may be manufactured using a highly automated process.
  • the photonic chip may be written using ultrafast laser inscription (ULI).
  • ULI ultrafast laser inscription
  • the photonic chip may be written using ULI in high quality fused silica glass.
  • the photonic chip may be defined using lithography.
  • the manufacturing process may be suitable for wafer-scale production when optimised.
  • the photonic chip may define the dispersive element.
  • the photonic chip may define the dispersive element in the slab waveguide core.
  • an optical spectrometer device comprising the monolithic photonic chip as described above.
  • the optical spectrometer device may comprise a MMF aligned with the optical input of the photonic chip.
  • the optical spectrometer device may comprise a detector array or an image sensor such as a charge coupled device (CCD) or complementary metal oxide semiconductor (CMOS) line image sensor aligned relative to the optical output of the photonic chip.
  • CCD charge coupled device
  • CMOS complementary metal oxide semiconductor
  • the optical spectrometer device may comprise an optical fibre array aligned relative to the optical output of the photonic chip.
  • the dispersive element may be formed separately from the photonic chip, and the photonic chip may define one or more alignment features for passively aligning the separately formed dispersive element relative to the slab waveguide core.
  • an optical spectrometer device comprising: the monolithic photonic chip as described above; and a separately formed dispersive element.
  • the separately formed dispersive element may be reflective.
  • the separately formed dispersive element may comprise a reflective diffraction grating.
  • the separately formed dispersive element may be located at an edge of the photonic chip.
  • the optical spectrometer device may comprise a MMF aligned with the optical input of the photonic chip.
  • the optical spectrometer device may comprise a detector array or an image sensor such as a charge coupled device (CCD) or complementary metal oxide semiconductor (CMOS) line image sensor aligned relative to the optical output of the photonic chip.
  • CCD charge coupled device
  • CMOS complementary metal oxide semiconductor
  • the optical spectrometer device may comprise a carrier for the MMF, wherein the photonic chip defines a passive alignment feature for aligning the carrier relative to the photonic chip and wherein the carrier defines a passive alignment feature for receiving an end portion of a MMF.
  • an optical spectrometer device comprising: the monolithic photonic chip as described above; and a MMF aligned with the optical input of the photonic chip.
  • the optical spectrometer device may comprise a detector array or an image sensor such as a charge coupled device (CCD) or complementary metal oxide semiconductor (CMOS) line image sensor aligned relative to the optical output of the photonic chip.
  • CCD charge coupled device
  • CMOS complementary metal oxide semiconductor
  • an optical spectrometer device comprising: the monolithic photonic chip as described above; and a detector array or an image sensor such as a charge coupled device (CCD) or complementary metal oxide semiconductor (CMOS) line image sensor aligned relative to the optical output of the photonic chip.
  • CMOS complementary metal oxide semiconductor
  • the optical spectrometer device may comprise a MMF aligned with the optical input of the photonic chip.
  • the optical input, the optical output, and the dispersive element may be arranged in a Czerny-Turner spectrometer configuration.
  • an optical spectrometer system comprising: the optical spectrometer device as described above, wherein the photonic chip defines one or more location features for locating the photonic chip relative to a base arrangement and/or relative to a cover arrangement; and a base arrangement defining one or more location features, wherein each of the one or more location features of the base arrangement is complementary to a corresponding location feature of the photonic chip, and wherein each of the one or more location features of the base arrangement engages a corresponding location feature of the photonic chip.
  • the base arrangement may comprise a base member and one or more base member spacer elements.
  • the base member may define one or more location features, wherein each of the one or more base member spacer elements is complementary to a corresponding location feature of the base member and each of the one or more base member spacer elements is complementary to a corresponding location feature of the photonic chip.
  • Each base member spacer element may engage a corresponding location feature of the base member and a corresponding location feature of the photonic chip.
  • the optical spectrometer system may comprise a cover arrangement defining one or more location features, wherein each of the one or more location features of the cover arrangement is complementary to a corresponding location feature of the photonic chip, and wherein each of the one or more location features of the cover arrangement engages a corresponding location feature of the photonic chip.
  • the cover arrangement may comprise a cover member and one or more cover member spacer elements.
  • the cover member may define one or more location features, wherein each of the one or more cover member spacer elements is complementary to a corresponding location feature of the cover member and each of the one or more cover member spacer elements is complementary to a corresponding location feature of the photonic chip.
  • Each cover member spacer element may engage a corresponding location feature of the cover member and a corresponding location feature of the photonic chip.
  • a method of manufacturing the monolithic photonic chip for an optical spectrometer as described above comprising: providing an optical substrate, the optical substrate defining the slab waveguide core; exposing selected areas of the optical substrate to light so as to define the relative spatial relationship between the optical input, the optical output, and the dispersive element.
  • the method may comprise exposing selected areas of the optical substrate to light so as to define and/or form at least one of the optical input, the optical output, and the dispersive element.
  • the method may comprise exposing selected areas of the optical substrate to light so as to define and/or form the optical input, the optical output, and one or more passive alignment features for aligning the dispersive element relative to the optical input and the optical output.
  • Exposing the selected areas of the optical substrate to light may comprise writing the selected areas of the optical substrate with at least one of coherent light, laser light, UV light, or pulses of light, such as ultrashort pulses of light.
  • the method may comprise exposing the selected areas of the optical substrate to light so as to cause, initiate, or at least partially define, a change in an optical property of at least one of the selected areas of the optical substrate.
  • the method may comprise exposing the selected areas of the optical substrate to light so as to cause, initiate, or at least partially define, a change in a refractive index of at least one of the selected areas of the optical substrate.
  • the method may comprise exposing the selected areas of the optical substrate to light so as to cause, initiate, or at least partially define, a change in etchability of at least one of the selected areas of the optical substrate.
  • the method may comprise exposing the selected areas of the optical substrate to light so as to cause, initiate, or at least partially define one or more cracks in the optical substrate in at least one of the selected areas of the optical substrate.
  • the method may comprise removing substrate material from at least one of the selected areas of the optical substrate after exposure of the selected areas to light so as to form at least one of the optical input, the optical output, and the dispersive.
  • the method may comprise etching substrate material from at least one of the selected areas of the optical substrate after exposure of the selected areas to light so as to form at least one of the optical input, the optical output, and the dispersive element.
  • the method may comprise exposing at least one of the selected areas of the optical substrate to a chemical etchant after exposure of the selected areas to light.
  • the method may comprise mechanically and/or thermally cleaving substrate material from at least one of the selected areas of the optical substrate after exposure of the selected areas to light.
  • the slab waveguide core may be defined by first and second outer surfaces of the optical substrate.
  • the photonic chip may comprise a cladding layer on only one side of the optical substrate.
  • the photonic chip may comprise a first cladding layer on a first side of the optical substrate and a second cladding layer on a second side of the optical substrate.
  • the optical substrate may comprise, or be formed from, fused silica.
  • the method may utilise several laser processing windows to induce different glass modifications which are suited for each of the components of the photonic chip.
  • These laser processing windows include controlled material densification, chemical etchability enhancement and micro-crack formation amongst others.
  • the different types of modification allow each of the individual components of the photonic chip to be fabricated in a single run on a single substrate in a fully automated process.
  • a monolithic photonic chip comprising: an optical input; a slab waveguide core extending from the optical input; and a slot or a notch defined at an edge of the photonic chip, wherein the slot or notch terminates at the optical input, and wherein the slot or the notch is configured to receive an end portion of a MMF.
  • a monolithic photonic chip comprising: a slab waveguide core, wherein the photonic chip defines a VBG in the slab waveguide core.
  • an optical spectrometer system comprising: a monolithic photonic chip having a slab waveguide core and one or more location features; a base arrangement defining one or more location features, wherein each of the one or more location features of the base arrangement is complementary to a corresponding location feature of the photonic chip, and wherein each of the one or more location features of the base arrangement engages a corresponding location feature of the photonic chip.
  • the base arrangement may comprise a base member and one or more base member spacer elements.
  • the base member may define one or more location features, wherein each of the one or more base member spacer elements is complementary to a corresponding location feature of the base member and each of the one or more base member spacer elements is complementary to a corresponding location feature of the photonic chip.
  • Each base member spacer element may engage a corresponding location feature of the base member and a corresponding location feature of the photonic chip.
  • the optical spectrometer system may comprise a cover arrangement defining one or more location features, wherein each of the one or more location features of the cover arrangement is complementary to a corresponding location feature of the photonic chip, and wherein each of the one or more location features of the cover arrangement engages a corresponding location feature of the photonic chip.
  • the cover arrangement may comprise a cover member and one or more cover member spacer elements.
  • the cover member may define one or more location features, wherein each of the one or more cover member spacer elements is complementary to a corresponding location feature of the cover member and each of the one or more cover member spacer elements is complementary to a corresponding location feature of the photonic chip.
  • Each cover member spacer element may engage a corresponding location feature of the cover member and a corresponding location feature of the photonic chip.
  • a monolithic photonic chip for an optical spectrometer, an optical spectrometer device including the monolithic photonic chip, an optical spectrometer system including the optical spectrometer device, and a method of manufacturing the monolithic photonic chip will now be described by way of non-limiting example only with reference to the drawings of which:
  • FIG. 1A is a schematic of an optical spectrometer system
  • FIG. 1 B is an exploded view of the optical spectrometer system of FIG. 1 A;
  • FIG. 2 is a schematic of a monolithic photonic chip of the optical spectrometer system of FIG. 1A;
  • FIG. 3 is a detailed schematic of a slot or a notch formed in an edge of the monolithic photonic chip of FIG. 2 for the passive alignment of a MMF relative to the monolithic photonic chip;
  • FIG. 4A is a side view of the monolithic photonic chip of FIG. 2 illustrating the propagation of multiple modes within a slab waveguide core of the monolithic photonic chip;
  • FIG. 4B shows a refractive index profile of the monolithic photonic chip of FIG. 2 and illustrates the effective refractive indices of different guided modes of the monolithic photonic chip;
  • FIG. 5 illustrates a photonic lantern for use with the monolithic photonic chip of FIG. 2;
  • FIG. 6A is a perspective view of an edge portion of the monolithic photonic chip of FIG. 2, wherein the edge portion of the monolithic photonic chip defines a gap having a first light transmitting end surface at a first end of the gap and a second light transmitting end surface at a second end of the gap, wherein the first light transmitting end surface has a curved lens profile for magnifying light from an MMF onto the second light transmitting end surface;
  • FIG. 6B is a cross-section taken through the gap of the monolithic photonic chip of FIG. 6A;
  • FIG. 7A shows a first step of a method for manufacturing the monolithic photonic chip of FIG. 2;
  • FIG. 7B shows a second step of the method for manufacturing the monolithic photonic chip of FIG. 2;
  • FIG. 7C shows a third step of the method for manufacturing the monolithic photonic chip of FIG. 2;
  • FIG. 7D shows a fourth step of the method for manufacturing the monolithic photonic chip of FIG. 2;
  • FIG. 7E shows a fifth step of the method for manufacturing the monolithic photonic chip of FIG. 2;
  • FIG. 7F shows a sixth step of the method for manufacturing the monolithic photonic chip of FIG. 2.
  • FIG. 8 shows an alternative spectrometer configuration for an alternative monolithic photonic chip for use in the optical spectrometer system of FIG. 1 A.
  • the optical spectrometer system 2 includes an optical spectrometer device generally designated 4, a base arrangement generally designated 6, and a cover arrangement generally designated 8.
  • the optical spectrometer device 4 includes a monolithic photonic chip 10, a multi-mode optical fibre (MMF) 12 and a detector array in the form of a line array sensor 14.
  • MMF multi-mode optical fibre
  • the photonic chip 10 is formed from an optical substrate in the form of a monolithic fused silica substrate which defines a slab waveguide core 20.
  • the photonic chip 10 has an optical input 22 and an optical output 24.
  • the photonic chip 10 defines a collimating element in the form of a metal-coated parabolic collimating mirror 26 defined at an edge of the monolithic fused silica substrate, a dispersive element in the form of a volume Bragg grating (VBG) 28 defined in the slab waveguide core 20, and a focusing element in the form of a metal-coated parabolic focusing mirror 30 defined at a second edge of the monolithic fused silica substrate.
  • VBG volume Bragg grating
  • the photonic chip 10 defines a slot or a notch 40 at an edge of the photonic chip 10, wherein the slot or notch 40 terminates at the optical input 22 and wherein the slot or the notch 40 is configured to receive an end portion of the MMF 12.
  • the slot or notch 40 defines a cylindrical tunnel for receiving the end portion of the MMF 12.
  • a thickness of the slab waveguide core 20 is selected so as to match a diameter of a core of the MMF 12 and the cylindrical tunnel has a diameter which is greater than a thickness of the slab waveguide core 20.
  • the end portion of the MMF 12 is inserted into the cylindrical tunnel and the MMF 12 is pushed towards the optical input 22 of the slab waveguide core 20 until the end face of the MMF 12 abuts the optical input 22.
  • the cylindrical tunnel defines the position of the end face of the MMF 12 and ensures high angular and positional constraint of the MMF 12 relative to the photonic chip 10.
  • the MMF 12 is then attached to the photonic chip 10, for example using adhesive or epoxy.
  • the line array sensor 14 is aligned relative to optical output 24 and is also attached to the photonic chip 10, for example using adhesive or epoxy.
  • the slab waveguide core 20 is defined by first and second generally planar outer surfaces 51 , 52 of a fused silica substrate 50 which confine light within the slab waveguide core 20 by total internal reflection (TIR).
  • the optical substrate has a thickness a which is selected so as to match a diameter of a core of the MMF 12 (not shown in FIG. 4A), thereby preserving the etendue from the object plane of the optical input 22 to the image plane of the optical output 24.
  • a more nuanced but important feature of the photonic chip 10 is that the slab waveguide core 20 supports many more modes than the MMF 12. Specifically, the slab waveguide core 20 supports approximately M modes where M is given by:
  • the refractive index profile of the slab waveguide core 20 is shown as a heavy solid line.
  • Only the lower order modes of the slab waveguide core 20 are excited when light from the MMF 12 is coupled into the slab waveguide core 20 i.e. only the lower order modes of the slab waveguide core 20 which have an effective refractive index which matches the effective refractive index of a MMF mode.
  • the modes excited in the slab waveguide core 20 experience essentially the same dispersion at the VBG 28. Consequently, mode-dependent dispersion is minimised and a high spectral resolution can be achieved.
  • the underpopulation of modes ensures that dispersion by the VBG 28 is similar across all guided modes that are excited, facilitating near diffraction limited resolving power of multi-mode light.
  • Use of a slab waveguide core 20 as defined by the optical substrate 50 confines the light to a plane which means simple one-dimensional or cylindrical optics can be used. Additionally, the image plane geometry is perfectly suited for interfacing with an inexpensive line sensor array 14.
  • the mirrors 26, 30 In order to collimate the light well, the mirrors 26, 30 have a profile which is an off-axis section of a parabola to direct the beam in the desired direction. If the radius of curvature of the mirrors 26, 30 is different in the two orthogonal axes, then the mirrors 26, 30 are toroidal or biconic, if the profiles are parabolic. In principle, biconic mirrors completely remove spherical aberration and astigmatism from the reflected beam.
  • the VBG 28 is formed directly into the slab waveguide core 20.
  • the VBG 28 extends over the full thickness of the slab waveguide core 20 so that the VBG 28 diffracts light propagating across the full thickness of the slab waveguide core 20. Efficiencies of >90% are expected with a grating period as small as 1 pm.
  • the base arrangement 6 includes a base member 60 and a plurality of base member spacer elements 62.
  • the base member 60 defines a plurality of location features in the form of alignment holes 63, each alignment hole 63 being configured to receive a corresponding base member spacer element 62.
  • the cover arrangement 8 includes a cover member 64 and a plurality of cover member spacer elements 66.
  • the cover member 64 defines a plurality of location features in the form of alignment holes 67, each alignment hole 67 being configured to receive a corresponding cover member spacer element 66.
  • the photonic chip 10 also defines one or more location features in the form of alignment holes 70, each alignment hole 70 being configured to receive a corresponding one of the base member spacer elements 62 and a corresponding one of the cover member spacer elements 66 so as to locate the photonic chip 10 relative to the base member 60 and the cover member 64.
  • the base member 60 and the cover member 64 protect the optical surfaces 51 , 52 of the photonic chip 10 which guide the light by TIR to ensure that the optical surfaces 51 , 52 remain contact free.
  • the line sensor array 14 is attached to the base member 60 and the cover member 64 so that, in effect, the alignment of the photonic chip 10 relative to the base member 60 and the cover member 64 also ensures that the line sensor array 14 is aligned relative to the optical output 24 of the photonic chip 10.
  • the slab waveguide core 20 In use, as indicated by the rays shown in FIG. 2, light enters the slab waveguide core 20 from the MMF 12 at the optical input 22.
  • the slab waveguide core 20 confines the light in the Y-direction whilst allowing the light to diverge within the slab waveguide core 20 in the X-Z plane as the light propagates towards the collimating mirror 26.
  • the collimating mirror 26 then collimates the light in the X-Z plane.
  • the collimated light propagates through the slab waveguide core 20 to the VBG 28, whereupon the VBG 28 spatially disperses the collimated light across a range of different directions in the X-Z plane according to the optical spectrum of the collimated light.
  • the spatially dispersed light propagates through the slab waveguide core 20 to the focusing mirror 30 which focuses different wavelengths of the light through the slab waveguide core 20 to different positions in X across the line array sensor 14 at the optical output 24.
  • the photonic chip 10 defines a Czerny-Turner spectrometer configuration consisting of a source plane, a collimating mirror, a dispersive element, a focusing mirror and a sensor plane.
  • the spectrometer system 2 is fully compatible with a multi-mode fibre input.
  • the spectral resolution of a spectrograph is determined by several factors including the grating line density, the exit focal length, the sensor pixel geometry and the object width, often defined by an input aperture slit.
  • the object width is equal to the diameter of the core of the MMF 12 which can be as wide as the thickness of the slab waveguide core 20. In this scenario, the object width would likely limit the spectral resolution. Consequently, in a variant of the photonic chip 10 described with reference to FIGS. 1A to 4B, the photonic chip 10 may define an optical transition in the slab waveguide core 20 in the form of an integrated photonic lantern 80 as shown in FIG.
  • the photonic lantern 80 is configured so that a width of the spatial distribution of the light in the slab waveguide core 20 at an optical output 82 of the photonic lantern 80 is less than a width of the spatial distribution of the light in the slab waveguide core 20 at the optical input 22 of the slab waveguide core 20.
  • the photonic lantern 80 is configured to split light in the slab waveguide core 20 into a plurality of 2D optical waveguides 84 in the slab waveguide core 20.
  • Each of the 2D optical waveguides 84 of the plurality of 2D optical waveguides 84 comprises a single-mode optical waveguide 84.
  • the photonic lantern 80 adiabatically splits multimode light received from the MMF 12 into the 2D optical waveguides 84.
  • the photonic lantern 80 operates by spatially tapering light from the core of the MMF 12 into the single-mode 2D optical waveguides 84.
  • the single-mode 2D optical waveguides 84 are arranged into a vertical line, or quasi slit at the optical output 82 of the photonic lantern 80, which is then imaged by the collimating mirror 26, the VBG 28 and the focusing mirror 30 to the optical output 24 of the slab waveguide core 20. Since each 2D optical waveguide 84 is single-moded, the optical spectrometer device 4 may be considered to be diffraction limited and the spectral resolution is no longer limited by the object width. In other words, use of the photonic lantern 80 decreases the object width without sacrificing core size in order to improve the spectral resolution of the optical spectrometer device 4.
  • each 2D optical waveguide 84 may be configured to support fewer transverse modes than the slab waveguide core 20.
  • each 2D optical waveguide 84 may be “few-moded”.
  • the optical spectrometer system 2 may include a fibre optic lantern (not shown) connected between the MMF 12 and the integrated photonic lantern 80, wherein the fibre optic lantern reformats the optical output from the MMF 12 into a multi-core optical fibre and wherein each core of the multi-core optical fibre is single-moded and aligned with an input of a corresponding one of the 2D optical waveguides 84 in the slab waveguide core 20.
  • a fibre optic lantern (not shown) connected between the MMF 12 and the integrated photonic lantern 80, wherein the fibre optic lantern reformats the optical output from the MMF 12 into a multi-core optical fibre and wherein each core of the multi-core optical fibre is single-moded and aligned with an input of a corresponding one of the 2D optical waveguides 84 in the slab waveguide core 20.
  • a lens profile may be written into the optical substrate 50 at the input to magnify the core of the MMF 12 onto the slab waveguide core 20 in order to preserve the etendue and allow the MMF 12 to be reimaged at the optical output 24. For example, as shown in FIGS.
  • the photonic chip 10 may define a gap 90 in the slab waveguide core 20, wherein the gap 90 has a first end 91 and a second end 92 opposite the first end 91 , wherein the first end 91 of the gap 90 is closer to the optical input 22 than the second end 92 of the gap 90.
  • the slab waveguide core 20 may define a first light transmitting end surface 93 at the first end 91 of the gap 90 and a second light transmitting end surface 94 at the second end 92 of the gap 90, wherein the first light transmitting end surface 93 is configured to receive light through the slab waveguide core 20 from the optical input 22 and to transmit the received light across the gap 90 to the second light transmitting end surface 94 back into the slab waveguide core 20. As shown in FIG.
  • the first light transmitting end surface 93 is curved so to as define a lens profile for magnifying the core of the MMF 12 onto the slab waveguide core 20.
  • the collimating mirror 26 may be configured to receive divergent light through the slab waveguide core 20 from the second light transmitting end surface 94.
  • the lens profile defined by the first light transmitting end surface 93 may be curved in only one dimension (i.e. the lens profile may be cylindrical) so as to image the light in the Y-Z plane only. Such a cylindrical lens profile would allow the light to continue to diverge in the slab waveguide 20 in the X direction for collimation by the collimating mirror 26.
  • the lens profile defined by the first light transmitting end surface 93 may be curved in two dimensions.
  • the second light transmitting end surface 94 may be curved so as to define a 1 D or a 2D lens profile.
  • the photonic chip 10 is manufactured by ultrafast laser inscription (ULI).
  • the method begins with the fused silica substrate 50 as shown in FIG. 7A.
  • the upper and lower surfaces of the fused silica substrate 50 are masked with a transparent etch stop protective coating 95 as shown in FIG. 7B.
  • a first focused laser beam 96 of ultrashort optical pulses such as a Bessel beam is then used to define the VBG 28 as shown in FIG. 7C.
  • ULI relies on non-linear photon absorption induced by focused ultrashort laser pulses to permanently modify the local refractive index of the fused silica substrate 50 and it is this modification of the local refractive index which is used to inscribe the VBG 28 directly into the slab waveguide core 20.
  • Highly efficient 1 st order VBGs can be written in the slab waveguide core 20 by translating the fused silica substrate 50 relative to the first focused laser beam 96 and/or translating the first focused laser beam 96 relative to the fused silica substrate 50.
  • the laser processing time can be reduced significantly by writing with a Bessel beam.
  • the VBG 28 extends across the full thickness of the slab waveguide core 20 so that, in use, the collimated light is diffracted by the VBG 28 across the full thickness of the slab waveguide core 20. Diffraction efficiencies of >90% are expected with a grating period as small as 1 pm.
  • a second focused laser beam 98 of ultrashort optical pulses is directed through the transparent etch stop protective coating 95 to write selected three-dimensional regions 97 of the fused silica substrate 50 corresponding to the slot or notch 40, the collimating mirror 26 and the focusing mirror 30 to enhance the chemical etchability of the fused silica substrate 50 in the selected three-dimensional regions 97.
  • the second focused laser beam 98 may have different ULI parameters to the first focused laser beam 96 of ultrashort optical pulses which is used to write the VBG 28.
  • Biconic mirrors are difficult to manufacture using traditional methods but by laser inscribing the surface directly, freeform profiles can be fabricated with ease.
  • the mirrors 26, 30 may be curved along one dimension only. In such a case, a Bessel beam with an elongated focus might be used to increase the glass processing rate and reduce the time required for inscription.
  • ULI may also be used to modify the local refractive index to inscribe the photonic lantern 80 if required.
  • a third focused laser beam 99 of ultrashort optical pulses is used to ablate the etch stop protective coating 95 above and below the selected three- dimensional regions 97.
  • the third focused laser beam 99 may have different ULI parameters to the first and second focused laser beams 96, 98 of ultrashort optical pulses.
  • the slot or notch 40, and the parabolic profiles of the collimating mirror 26 and the focusing mirror 30 are then formed at the edges of the photonic chip 10 as shown in FIG. 7E by exposing the coated fused silica substrate 50 to a chemical etchant which dissolves the material of the fused silica substrate 50 in the selected three-dimensional regions 97 up to 1000 times faster than the surrounding pristine glass of the fused silica substrate 50.
  • a chemical etchant which dissolves the material of the fused silica substrate 50 in the selected three-dimensional regions 97 up to 1000 times faster than the surrounding pristine glass of the fused silica substrate 50.
  • the optical substrate 50 is submerged in an etchant which removes the laser written material.
  • the surfaces of the collimating mirror 26 and the focusing mirror 30 can then be coated with a reflective metal (not shown) by physical vapour deposition. It may be that the surface roughness of the collimating mirror 26 and the focusing mirror 30 is of suboptimal quality after the etching process. In such a case, the glass of the substrate 50 may be polished prior to coating by physical or thermal means. This may include fine polishing by lapping or thermal reflow by CO 2 laser heating or flame brushing.
  • first, second and third laser beams 96, 98 and 99 may be different.
  • the first, second and third laser beams 96, 98 and 99 may have different optical powers, pulse widths, pulse repetition rates, wavelengths, and/or beam profiles.
  • the method of manufacturing of the photonic chip 10 ends with the removal of the etch stop protective coating 95 as shown in FIG. 7F.
  • ULI may be used to cause, initiate, or at least partially define one or more cracks in one or more selected areas of the fused silica substrate 50 and the method may comprise mechanically and/or thermally cleaving material of the fused silica substrate 50 from the one or more selected areas of the fused silica substrate 50 after exposure of the selected areas to light to fabricate the slot or notch 40, and to fabricate the parabolic profiles of the collimating mirror 26 and the focusing mirror 30 directly at the edges of the photonic chip 10.
  • ULI may also be used to enhance the chemical etchability of the fused silica substrate and/or to cause, initiate, or at least partially define one or more cracks in the fused silica substrate for fabrication of the gap 90 and the profiles of the first and second light transmitting end surfaces 93, 94 described with reference to FIGS. 6A and 6B if required.
  • the photonic chip 10 takes full advantage of the direct ULI technique and includes both modified refractive index elements and chemically etched elements so that the slot or notch 40, the optical input 22, the optical output 24, the profile of the collimating mirror 26, the VBG 28, and the profile of the focusing mirror 30 are written into a single monolithic fused silica substrate 50 in a single run in a fully automated process. If required, the photonic lantern 80 and/or the gap 90 and the profiles of the first and second light transmitting end surfaces 93, 94 may also be written into the monolithic fused silica substrate 50 in the same run.
  • Etching the fused silica substrate 50 may be detrimental to the quality of the fused silica glass and therefore also to the propagation of light through the slab waveguide core 20.
  • the etch stop protective coating 95 is transparent at the laser writing wavelength, allowing the slot or notch 40, the optical input 22, the optical output 24, the profile of the collimating mirror 26, the VBG 28, and the profile of the focusing mirror 30 to be written with the coating 95 applied.
  • a coating 95 may from protect the surfaces of the fused silica glass from exposure to the chemical etchant between the selected areas corresponding to the slot or notch 40, the optical input 22, and the mirrors 26, 30 to thereby preserve the quality of the fused silica glass and minimise the scattering of light as it propagates through the slab waveguide core 20 between the slot or notch 40, the optical input 22, and the mirrors 26, 30.
  • the quality of the outer surfaces 51 , 52 of the fused silica substrate 50 may not be adversely affected by chemical etching. Under these circumstances, the etch stop protective coating 95 may not be required. Moreover, the upper and lower extents of the VBG 28 may be located sub-surface within the fused silica substrate 50.
  • the optical spectrometer device 4 may include a dispersive element of any kind.
  • the optical spectrometer device 4 may include a diffraction grating of any kind.
  • the optical spectrometer device 4 may include a transmissive diffraction grating.
  • the optical spectrometer device 4 may include a reflective diffraction grating e.g. a reflective diffraction grating which is formed separately from the photonic chip 10 and which is aligned with the slab waveguide core 20 at an edge of the photonic chip 10.
  • the optical spectrometer device 4 may include a transmissive collimating element, for example a collimating lens.
  • the collimating element may be formed by ULI or may be formed separately from the photonic chip 10 and then aligned with the slab waveguide core 20, for example at an edge of the photonic chip 10.
  • the optical spectrometer device 4 may include a transmissive focusing element, for example a focusing lens.
  • the focusing element may be formed by ULI or may be formed separately from the photonic chip 10 and then aligned with the slab waveguide core 20, for example at an edge of the photonic chip 10.
  • FIG. 8 shows an alternative configuration for an alternative optical spectrometer device which includes an alternative monolithic photonic chip.
  • the alternative monolithic photonic chip includes an optical input 122, an optical output 124 and a slab waveguide core 120 for guiding light from the optical input 122 to the optical output 124, wherein the alternative monolithic photonic chip defines a relative spatial relationship between the optical input 122, the optical output 124 and a dispersive element in the form of a separately formed curved reflective diffraction grating 128 which is aligned with an edge of the alternative monolithic photonic chip.
  • the alternative optical spectrometer device includes a MMF 112 aligned with the optical input 122 of the alternative monolithic photonic chip and a detector array in the form of a line image sensor 1 14 aligned with the optical output 124 of the alternative monolithic photonic chip.
  • the optical input 122 receives light from the MMF 112.
  • the received light diverges in the slab waveguide core 120.
  • the curved reflective diffraction grating 128 receives the divergent light from the optical input 122 via the slab waveguide core 120 and spatially disperses and focuses the received light to the line image sensor 1 14 at the optical output 124 via the slab waveguide core 120.
  • the photonic chip 10 may include one or more re-direction elements such as one or more planar mirrors for re-directing light in the slab waveguide core 20.
  • Each re-direction element may be configured to receive light via the slab waveguide core from a first direction and re-direct the received light in the slab waveguide core along a second direction, wherein the second direction is different from the first direction.
  • the second direction may be opposite to the first direction.
  • the dispersive element may be formed separately from the photonic chip 10, and the photonic chip 10 may define one or more alignment features for passively aligning the dispersive element relative to the slab waveguide core 20.
  • At least one of the collimating element, the dispersive element, the focusing element and the re-direction element may be formed separately from the photonic chip 10, and the photonic chip 10 may define one or more alignment features for passively aligning at least one of the collimating element, the dispersive element, the focusing element and the re-direction element relative to the slab waveguide core 20.
  • passive alignment features or guides can be written into the fused silica substrate 50 by ULI for alignment of the component relative to the optical input 22 of the slab waveguide core 20 and the optical output 24 of the slab waveguide core.
  • the base member spacer elements and/or the cover member spacer elements may be formed from fused silica glass.
  • the base member spacer elements and/or the cover member spacer elements may be formed by ULI.
  • the photonic chip may comprise a cladding layer on only one side of the slab waveguide core.
  • the photonic chip may comprise a first cladding layer on a first side of the slab waveguide core and a second cladding layer on a second side of the slab waveguide core.
  • the photonic chip may define a passive alignment feature for aligning a carrier for the MMF relative to the photonic chip so that the MMF is aligned with the optical input of the slab waveguide core.

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  • General Physics & Mathematics (AREA)
  • Spectroscopy & Molecular Physics (AREA)
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  • Engineering & Computer Science (AREA)
  • Microelectronics & Electronic Packaging (AREA)
  • Optical Integrated Circuits (AREA)

Abstract

L'invention concerne une puce photonique monolithique (10) pour un spectromètre optique (4), comprenant une entrée optique (22) et une sortie optique (24), et un cœur de guide d'ondes en plaque (20) pour guider la lumière de l'entrée optique (22) à la sortie optique (24). La puce photonique (10) définit une relation spatiale relative entre l'entrée optique (22), la sortie optique (24) et un élément dispersif (28) de sorte que, en utilisation, la lumière entre dans la puce photonique (10) à l'entrée optique (22), l'élément dispersif (28) reçoit la lumière de l'entrée optique (22) par l'intermédiaire du cœur de guide d'ondes en plaque (20), l'élément dispersif (28) disperse spatialement la lumière reçue dans le cœur de guide d'ondes en plaque (20), et la lumière dispersée spatialement sort de la puce photonique (10) à la sortie optique (24). Le coeur de guide d'ondes en plaque (20) peut être configuré pour supporter une pluralité de modes optiques guidés à des longueurs d'onde visibles ou infrarouges dans une direction transversale au coeur de guide d'ondes en plaque. Un dispositif de spectromètre optique (2) comprend la puce photonique monolithique (10) et un élément dispersif (28) formé intégralement ou séparément. Le dispositif de spectromètre optique (2) peut comprendre une fibre multimode (12) alignée par rapport à l'entrée optique (22) et/ou un réseau de détecteurs (14) ou un capteur d'images aligné par rapport à la sortie optique (24) de la puce photonique (10).
PCT/GB2021/052807 2020-10-30 2021-10-29 Spectromètre à puce photonique WO2022090728A1 (fr)

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GBGB2017266.4A GB202017266D0 (en) 2020-10-30 2020-10-30 Photonic chip spectrometer
GB2017266.4 2020-10-30

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Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20030197862A1 (en) * 2000-12-13 2003-10-23 Cohen Mitchell S. Multimode planar spectrographs for wavelength demultiplexing and methods of fabrication
US7034935B1 (en) * 2003-03-24 2006-04-25 Mpb Technologies Inc. High performance miniature spectrometer
WO2010140998A1 (fr) * 2009-06-02 2010-12-09 Vladimir Yankov Nanospectromètre intégré optique et procédé de fabrication de celui-ci

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20030197862A1 (en) * 2000-12-13 2003-10-23 Cohen Mitchell S. Multimode planar spectrographs for wavelength demultiplexing and methods of fabrication
US7034935B1 (en) * 2003-03-24 2006-04-25 Mpb Technologies Inc. High performance miniature spectrometer
WO2010140998A1 (fr) * 2009-06-02 2010-12-09 Vladimir Yankov Nanospectromètre intégré optique et procédé de fabrication de celui-ci

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