WO2012097982A1 - Filtre optique - Google Patents

Filtre optique Download PDF

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Publication number
WO2012097982A1
WO2012097982A1 PCT/EP2012/000215 EP2012000215W WO2012097982A1 WO 2012097982 A1 WO2012097982 A1 WO 2012097982A1 EP 2012000215 W EP2012000215 W EP 2012000215W WO 2012097982 A1 WO2012097982 A1 WO 2012097982A1
Authority
WO
WIPO (PCT)
Prior art keywords
multimode
optical filter
waveguide
filter according
interferometers
Prior art date
Application number
PCT/EP2012/000215
Other languages
English (en)
Inventor
Yufang Hu
Graham Reed
Original Assignee
University Of Surrey
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 University Of Surrey filed Critical University Of Surrey
Publication of WO2012097982A1 publication Critical patent/WO2012097982A1/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
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/26—Optical coupling means
    • G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
    • G02B6/2804—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals forming multipart couplers without wavelength selective elements, e.g. "T" couplers, star couplers
    • G02B6/2808—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals forming multipart couplers without wavelength selective elements, e.g. "T" couplers, star couplers using a mixing element which evenly distributes an input signal over a number of outputs
    • G02B6/2813—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals forming multipart couplers without wavelength selective elements, e.g. "T" couplers, star couplers using a mixing element which evenly distributes an input signal over a number of outputs based on multimode interference effect, i.e. self-imaging
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/26—Optical coupling means
    • G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
    • G02B6/293—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means
    • G02B6/29344—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means operating by modal interference or beating, i.e. of transverse modes, e.g. zero-gap directional coupler, MMI

Definitions

  • the present invention relates to optical filters, and particularly to optical filters comprising multimode interferometers.
  • Multimode interferometers have been used for a large variety of photonic devices, including power splitters/combiners, couplers, switches, and multiplexers. MMI- based devices are very suitable for practical use, largely due to their relative ease of fabrication, low sensitivity to fabrication error, and low dependence on temperature, wavelength and polarization.
  • Figure la is a plan-view of a lxl MMI 10 with one single-mode input 12 and one single-mode output waveguide 16 joining the multimode waveguide 14 at the mid-point across the width of the single-mode/multimode waveguide interface, i.e. the optical axes of the single mode input and output waveguides and the multimode waveguide are coincident. Only the raised, thicker parts of the device are shown for clarity. Figures lb and lc are cross-sections through the single-mode and multimode parts of the device, respectively.
  • the MMI is formed using silicon-on-insulator (SOI) methods, with a layer of silicon (in which the core of the waveguide is formed) adjacent to a layer of insulator 18 (for example silicon dioxide, Si0 2 ) which acts as the cladding; however, other material systems are also applicable.
  • insulator 18 for example silicon dioxide, Si0 2
  • Cladding e.g. air, Si0 2 , etc
  • Si0 2 silicon dioxide
  • the single-mode waveguide has a width of w3 ⁇ 4 M and the multimode waveguide has a width of W mi and a length of ZMMI-
  • the device has a refractive index of n r in the core and n c in the cladding for the operating wavelength, ⁇ .
  • p represents the self-image order and is a positive integer
  • ⁇ is the operating wavelength
  • n r is the refractive index of waveguide core
  • H ff is the effective multimode waveguide width given by:
  • the self-image distance U is linearly dependent on operating wavelength ⁇ , and is more sensitive to wavelength change at higher self-image order, i.e. larger value of p.
  • the wavelength corresponding to different self-image order p appears periodic.
  • the transmittance of the device can be modulated by varying the input wavelength, with the transmission maxima appearing periodically as a function of wavelength.
  • the M I 10 may therefore work as a coarse wavelength filter, and indeed great effort has been made to design devices based on single MMIs. However, their performance in this role is limited.
  • EP-A-1705504 discloses an interferometer comprising two MMI devices coupled in series and located adjacent to or between reflectors that couple the light diffused from the first MMI device into the second MMI device after a travelling over a different path length. The reflected and diffused light interferes with light in the second MMI device. A complex and fine-tuned arrangement of reflectors is however unnecessary as disclosed herein.
  • the present inventors have realised there is significant potential to combine a plurality of MMIs within a single device for the realization of improved device performance, e.g. higher extinction ratio, narrower wavelength transmission linewidth, and larger free spectral range (FSR).
  • improved device performance e.g. higher extinction ratio, narrower wavelength transmission linewidth, and larger free spectral range (FSR).
  • an optical wavelength filter based on multiple cascaded MMIs.
  • the optical filter comprises an optical waveguide, which itself comprises an input for receiving photons; a first multimode interferometer coupled to the input; a second multimode interferometer, connected in series to the first multimode interferometer; and an output, coupled to the second multimode interferometer, for providing a filtered output.
  • the first multimode interferometer and the second multimode interferometer can be connected via a suitable waveguide structure, and are preferably optically separate apart from that waveguide structure.
  • the first and second multimode interferometers may have the same or different physical characteristics.
  • the second multimode interferometer may have a different effective refractive index, width, length, height, or etch depth, or be constructed from a different material.
  • the second multimode interferometer has a length, width or height which is an integer multiple of the length, width or height of the first multimode interferometer (or vice versa). That is, the length of the second multimode interferometer is an integer multiple of the length of the first multimode interferometer (or vice versa); the width of the second multimode interferometer is an integer multiple of the width of the first multimode interferometer (or vice versa); or the height of the second multimode interferometer is an integer multiple of the height of the first multimode interferometer (or vice versa).
  • the length and width of the multimode interferometers may be related to one another according to equations (1) or (la), and (2) above, where ⁇ is the design wavelength.
  • each multimode interferometer comprises a portion of the waveguide having at least one relatively larger dimension, relative to the corresponding at least one dimension of portions between the multimode interferometers.
  • the multimode interferometers may be wider than the portions interconnecting them, as well as the input and output.
  • one or more of the input, the output, and the portions connecting the multimode interferometers are fundamental-mode waveguides, i.e. single-mode.
  • the optical filter may comprise multiple groups of one or more multimode interferometers, known as "stages". Each multimode interferometer of a particular stage has the same physical characteristics. Each stage may have the same number of multimode interferometers.
  • the waveguide can be arranged linearly (i.e. in a straight line), or can comprise one or more curved portions. In the latter arrangement, the waveguide can be folded back upon itself, to decrease the footprint of the overall device.
  • the optical filter may be manufactured using a range of materials and methodologies.
  • the filter comprises a layer of light-carrying material, wherein the waveguide comprises a relatively thicker portion of the light-carrying layer.
  • This type of waveguide is generally known as a "rib waveguide".
  • the present invention is equally applicable to strip waveguides or a planar waveguide with alternative light-guiding structures.
  • the light-carrying material may be any optical material, such as silicon, Si0 2 , germanium, UNb0 3 , GaAs, InP, etc. BRIEF DESCRIPTION OF THE DRAWINGS
  • Figures la, lb and lc show alternative views of a conventional single multimode optical filter
  • FIGS. 2a and 2b show alternative designs of an optical filter according to embodiments of the present invention
  • Figure 3 is a graph of the simulated transmission spectra of devices according to Figures la, 2a and 2b;
  • Figures 4a, 4b and 4c show optical filters according to further embodiments of the present invention;
  • Figure 5 is a graph of the simulated transmission spectra of devices according to Figures la, 4a, 4b and 4c;
  • Figures 6a and 6b show optical filters according to yet further embodiments of the invention, including a filter in a folded arrangement
  • Figure 7 is a graph of the simulated transmission spectra of devices according to Figures 4c and 6a.
  • FIG 2a is a plan view of an optical filter 100 according to an embodiment of the present invention.
  • the filter 100 comprises a single-mode input waveguide 102, connected to the first multimode waveguide 112.
  • a further single-mode waveguide 103 couples the first multimode waveguide 112 to a second multimode waveguide 112', and this second multimode waveguide is coupled to a single- mode output 104 which outputs a wavelength-filtered optical signal.
  • the filter has a similar silicon-on- insulator (SOI) construction to those shown in Figures la to lc (although other optical methods and materials could also be used).
  • SOI silicon-on- insulator
  • a layered structure comprising a layer of silicon on a layer of insulator (e.g. Si0 2 ).
  • the waveguide shown in Figure 2a comprises a relatively thicker portion of the silicon layer, and this may be covered in an insulating cladding (e.g. air, Si0 2 etc).
  • an insulating cladding e.g. air, Si0 2 etc.
  • a third multimode waveguide 112" is coupled in series between the second multimode waveguide 112' and the output 104.
  • any number of waveguides may be connected in this way.
  • each of the multimode waveguides 14, 112, 112', 112" has the same length as well as other physical characteristics, i.e. the same material, same width, same height.
  • the number of multimode waveguides having the same length is referred to as the "order" of the filter: thus, filter 10 is first order; filter 100 is second order; and filter 100' is third order.
  • the fidelity of self- imaging varies with the self-image order. Indeed, such a change in fidelity corresponds to a beating of the modulation depth in the curve of transmittance versus Z! MMI .
  • Each of the filters described above has multimode waveguides with the same physical characteristics, i.e. the same material, same height, same width and same length.
  • multimode waveguides with different physical characteristics may be combined intelligently to create an optical filter with a desired overall transmission characteristic.
  • the length is varied for simplicity; however, one or more of many alternative characteristics may be varied in order to change the effective index of one multimode waveguide as compared to another.
  • the effective index of the modes of a particular multimode waveguide depends on the length, width, height, etch depth and material (i.e. refractive index) of the multimode waveguide. Therefore any one or more of those characteristics may be changed in order to alter the effective index.
  • Each multimode waveguide or group of multimode waveguides having the same physical characteristics is referred to as a "stage" of the filter.
  • Figures 4a, 4b and 4c show further optical filters according to embodiments of the present invention in which multimode waveguides with different lengths have been combined.
  • Figure 4a shows an optical filter 200 comprising a single-mode input waveguide 202, connected to a first multimode waveguide 212, having a length Lp.
  • a further single-mode waveguide couples the first multimode waveguide 212 to a second multimode waveguide 214 having a length 2L P , and this second multimode waveguide is coupled to a single-mode output 204.
  • Figure 4b shows a similar optical filter 200' where the second multimode waveguide 216 has a length 3L P ; and Figure 4c shows a further optical filter 200" which combines three multimode waveguides 212, 214, 216, having lengths L P , 2L Pl and 3L P , respectively.
  • filter 10 (dash- dot line); filter 200 (dash line); filter 200' (dot line); and filter 200" (solid line).
  • Such a multiple-stage structure gives substantial reduction of spectral bandwidth, while keeping the free spectral range the same as a single lx l MI.
  • the increase of extinction ratio is also shown, although it is not as efficient as the multiple-order structure and some side peaks are shown.
  • the three-stage structure (Fig. 4c) has the optimum spectral response (solid curve in Fig. 5) with a -3 dB spectral bandwidth of 10 nm, an extinction ratio of 10 dB, a side peak suppression ratio of 7 dB, and a free spectral range of 60 nm.
  • the optical filter 300 comprises a single-mode input waveguide 302, connected to a first stage of three multimode waveguides 312, each having a length L P , connected in series via respective single-mode waveguides. These are in turn connected to a second stage of three multimode waveguides 314, each having a length 2L Pf connected in series via respective single-mode waveguides; and then to a third stage of three multimode waveguides 316, each having a length 3L P This final stage of multimode waveguides is coupled to a single-mode output 304.
  • This device achieves excellent filter characteristics, as can be seen in Figure 7.
  • This graph shows the progression from a three-stage, first-order filter as shown in Figure 4c (solid curve), to a three-stage, second-order filter (dashed curve), to the three-stage, third- order filter 300 shown in Figure 6a (dotted curve).
  • the latter filter achieves the best separation of wavelengths.
  • the filters have been arranged linearly, that is to say, in a straight line. However, they may also be folded back on themselves to conveniently reduce the overall length of the device.
  • Such a "folded" design of filter is shown in Figure 6b, where a three-stage, third-order filter 400 has been folded back on itself several times by virtue of several curved portions 420.
  • the turning radius of the curved portion r can be chosen to reduce optical losses as a result of the curve.
  • a straight line achieves the minimum optical loss; however, simulation has shown that a turning radius rof 5 pm results in acceptable losses while allowing a tight curve to be used.
  • the devices described above have employed a silicon-on-insulator construction; that is to say, a layer of silicon on an insulator (typically Si0 2 , although others can be used), and a thicker portion of the silicon layer used as a waveguide.
  • silicon-on-insulator construction that is to say, a layer of silicon on an insulator (typically Si0 2 , although others can be used), and a thicker portion of the silicon layer used as a waveguide.
  • germanium may be employed instead of silicon.
  • the waveguide may comprise a planar waveguide with substantially any light-guiding structure, including a strip waveguide.
  • the multimode interferometers disclosed herein have a multimode nature which provides wavelength dependent self-imaging of the fundamental mode input field. This means the multimode interferometers support an appropriate spectrum of modes—including relevant higher order modes.
  • the wavelength dependence of the phase coefficients of the modes in the mode spectrum excited in the particular multimode interferometer leads to the wavelength dependence of the axial distance along the multimode interferometer at which self-imaging occurs.
  • the length and width of each multimode interferometer in the optical filter are dictated according to equation (1), where p may take different values for each multimode interferometer in the filter.
  • the value of the design wavelength ⁇ i.e.
  • the wavelength intended to be passed by each multimode interferometer) for each multimode interferometer in these embodiments may be the same, to the extent that a consistent filtering operation is achieved by the combination of multimode interferometers.
  • the value of design wavelength ⁇ may be the same for each multimode interferometer to within 5 nm. In some applications, the value of design wavelength ⁇ may be the same for each multimode interferometer to within 3 nm.
  • the single-mode waveguides disclosed herein are arranged so that only the single, or fundamental mode propagates therein.
  • the present invention thus provides an optical filter, for use in a range of devices, in which two or more multimode waveguides are combined into a single filter.
  • the filter can be designed to achieve many different transmission characteristics by adapting the physical characteristics and/or number of multimode waveguides.

Abstract

La présente invention se rapporte à un filtre optique qui est destiné à être utilisé dans divers dispositifs et dans lequel deux guides d'ondes multimodes ou plus sont combinés pour former un seul filtre. Ledit filtre peut être conçu pour présenter de nombreuses caractéristiques de transmission différentes grâce à l'adaptation des caractéristiques physiques (telles que la longueur, la largeur ou la hauteur) et/ou du nombre de guides d'ondes multimodes.
PCT/EP2012/000215 2011-01-19 2012-01-18 Filtre optique WO2012097982A1 (fr)

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GB201100926A GB2488308A (en) 2011-01-19 2011-01-19 Optical Filter
GB1100926.3 2011-01-19

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Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2016086043A1 (fr) * 2014-11-24 2016-06-02 Massachusetts Institute Of Technology Procédés et appareil pour imagerie spectrale
US10718668B2 (en) 2017-08-08 2020-07-21 Massachusetts Institute Of Technology Miniaturized Fourier-transform Raman spectrometer systems and methods
US10983003B2 (en) 2019-02-11 2021-04-20 Massachusetts Institute Of Technology High-performance on-chip spectrometers and spectrum analyzers
US11041759B2 (en) 2018-06-28 2021-06-22 Massachusetts Institute Of Technology Systems and methods for Raman spectroscopy
CN114200588A (zh) * 2021-11-16 2022-03-18 武汉光迅科技股份有限公司 一种光解复用组件结构和封装方法

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JP2018194665A (ja) * 2017-05-17 2018-12-06 日本電信電話株式会社 波長分離素子

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EP1705504A1 (fr) 2005-03-23 2006-09-27 Avanex Corporation Interféromètre optique avec guides d'onde multimodes and réflecteurs externes

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EP1705504A1 (fr) 2005-03-23 2006-09-27 Avanex Corporation Interféromètre optique avec guides d'onde multimodes and réflecteurs externes

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Cited By (11)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2016086043A1 (fr) * 2014-11-24 2016-06-02 Massachusetts Institute Of Technology Procédés et appareil pour imagerie spectrale
US10240980B2 (en) 2014-11-24 2019-03-26 Massachusetts Institute Of Technology Methods and apparatus for spectral imaging
US10571335B2 (en) 2014-11-24 2020-02-25 Massachusetts Institute Of Technology Methods and apparatus for spectral imaging
US10718668B2 (en) 2017-08-08 2020-07-21 Massachusetts Institute Of Technology Miniaturized Fourier-transform Raman spectrometer systems and methods
US11313725B2 (en) 2017-08-08 2022-04-26 Massachusetts Institute Of Technology Miniaturized Fourier-transform Raman spectrometer systems and methods
US11041759B2 (en) 2018-06-28 2021-06-22 Massachusetts Institute Of Technology Systems and methods for Raman spectroscopy
US11885684B2 (en) 2018-06-28 2024-01-30 Massachusetts Institute Of Technology Systems and methods for Raman spectroscopy
US10983003B2 (en) 2019-02-11 2021-04-20 Massachusetts Institute Of Technology High-performance on-chip spectrometers and spectrum analyzers
US11885677B2 (en) 2019-02-11 2024-01-30 Massachusetts Institute Of Technology High-performance on-chip spectrometers and spectrum analyzers
CN114200588A (zh) * 2021-11-16 2022-03-18 武汉光迅科技股份有限公司 一种光解复用组件结构和封装方法
CN114200588B (zh) * 2021-11-16 2024-04-02 武汉光迅科技股份有限公司 一种光解复用组件结构和封装方法

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