WO2005019888A1 - Reseau de bragg planaire - Google Patents

Reseau de bragg planaire Download PDF

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Publication number
WO2005019888A1
WO2005019888A1 PCT/AU2004/001132 AU2004001132W WO2005019888A1 WO 2005019888 A1 WO2005019888 A1 WO 2005019888A1 AU 2004001132 W AU2004001132 W AU 2004001132W WO 2005019888 A1 WO2005019888 A1 WO 2005019888A1
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WO
WIPO (PCT)
Prior art keywords
grating
waveguide
core
variation
planar
Prior art date
Application number
PCT/AU2004/001132
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English (en)
Inventor
Stanislav Petrovich Tarnavskii
Original Assignee
Redfern Integrated Optics Pty Ltd
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 Redfern Integrated Optics Pty Ltd filed Critical Redfern Integrated Optics Pty Ltd
Publication of WO2005019888A1 publication Critical patent/WO2005019888A1/fr

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    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
    • G02B6/12—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind
    • G02B6/122—Basic optical elements, e.g. light-guiding paths
    • G02B6/124—Geodesic lenses or integrated gratings

Definitions

  • the present invention relates to planar waveguide Bragg gratings for photonic applications.
  • Bragg gratings have a number of important applications in optical components, such as lasers, sensors and dispersion-compensation devices.
  • a Bragg grating formed in a photonic waveguide reflects a characteristic wavelength or range of wavelengths centred at a characteristic wavelength, referred to as the Bragg wavelength l_.
  • the period of the grating A may be constant, in which case a narrow range of wavelengths are reflected, or the period may change throughout the grating and cause reflections over a range of wavelengths.
  • Bragg gratings can be formed in optical fibers and planar waveguides.
  • a planar waveguide also referred to as a planar lightwave circuit (PLC) is an optical waveguide formed with thin transparent films on a generally-planar substrate. Planar waveguides confine optical power within a region referred to herein as the core.
  • the core has a higher refractive index than surrounding material and is generally configured as either a channel waveguide, or rib waveguide.
  • the grating in the core can be defined by exposure to an optical interference pattern.
  • the grating can be "written" by exposing the core to a periodic pattern of contrasting light and dark bands or fringes, the period of which determines the period of the grating to be written, at a wavelength which causes refractive index changes in the core material (e.g. UV light).
  • An alternative method of fabricating a planar grating involves etching a corrugated profile into the surface of the core before coating the core with a layer of lower refractive index material (the cladding layer). The surface corrugation can be etched into the core by photolithographically-defined etching.
  • the photolithography also includes an optical exposure step in which a layer of photoresist is exposed to a grating pattern of contrasting light and dark fringes.
  • the fringe pattern can be formed by optical interference of two coherent beams. If there is any variation in the period of the interference fringes along the length of the grating, there will be a corresponding variation in the period of the grating along its length. In applications where a constant grating period is required, the grating period variations result in a phase error for the grating. It has been noticed by the inventor that gradual variations in the period of the interference fringes, referred to as a chirp in the grating period, inadvertently arise when the pattern is created by interference of two spherical beams on a planar wafer.
  • the variation in the period of the fringe pattern on the wafer increases with distance from the centre of the interference pattern and, depending on the optics, the period can change parabolically with increasing radius from the centre.
  • One solution is to use additional optics to create interfering beams which have a planar wavefront. Whilst this solution is technically feasible, it requires the use of large mirrors and lenses which can be particularly sensitive to atmospheric contamination.
  • Another alternative is to use only the centre of the interference pattern and to move the wafer to where the exposure is required. However, this is inconvenient to do when the time required for each exposure is significant.
  • a planar photonic waveguide comprising a Bragg grating formed in a planar waveguide core and having a grating period which varies gradually across its length, wherein the waveguide core has dimensions which vary gradually along its length, the width variation having an effect on phase error of the grating which at least partially cancels an effect that the period variation has on the phase error.
  • the variation in dimensions may comprise a variation in a width of the core along its length.
  • the grating may comprise a surface-corrugation grating.
  • the grating may comprise a UV-written grating.
  • a method of fabricating a planar photonic waveguide Bragg grating with reduced phase error comprising: defining a Bragg grating structure in a layer of material to be formed into a planar waveguide core, the grating structure having a grating period which varies gradually over the layer; and fabricating a waveguide core in a portion of the core layer in which the
  • the step of defining the Bragg grating structure may include a step in which the grating structure is defined by an interference fringe pattern in which a separation between fringes varies gradually over an exposure area.
  • a planar photonic waveguide comprising a Bragg grating formed in a planar waveguide core and having a thickness which varies gradually across its length, wherein the waveguide core has a width which varies gradually along its length, the width variation having an effect on phase error of the grating which at least partially cancels an effect that the thickness variation has on the phase error.
  • a planar photonic waveguide comprising a Bragg grating formed in a planar waveguide core and having an average refractive index which varies gradually across its length, wherein the waveguide core has dimension which vary gradually along its length, the variation in dimensions having an effect on phase error of the grating which at least partially cancels an effect that the refractive index variation has on the phase error.
  • an external cavity laser incorporating a planar photonic waveguide as defined in the first aspect of the invention.
  • Figure 1 is a cross-sectional view through a wafer on which a planar photonic waveguide is formed with a surface corrugation Bragg grating.
  • Figure 2 is a schematic illustration of a holographic interference exposure arrangement for exposing a grating pattern in a layer of photoresist on a wafer.
  • Figure 3 is a measured plot of the variation in grating period ⁇ produced across a wafer using the arrangement shown in Fig. 2.
  • Figure 4 shows plan views of five waveguides at different positions across a wafer. The waveguides each have a tailored width profile designed to compensate for the variation in grating period ⁇ shown in Fig. 3.
  • Figure 5 A shows an enlarged plan view of one of the waveguides 142 shown in Figure 4.
  • Figure 5B shows an enlarged plan view of another of the waveguides 144 shown in Figure 4.
  • a planar photonic waveguide 10 having a core 20 in which a Bragg grating 30 is formed.
  • the core is formed on a buffer layer 40 which is in turn formed on a wafer of silicon 50.
  • the core 20 is also covered with a cladding layer 60.
  • the core 20 has a higher refractive index than both the buffer layer 40 and cladding layer 60 in order to confine optical power within the core.
  • the Bragg grating is in the form of a periodic series of peaks 70 and troughs 80, referred to collectively as a surface corrugation, formed in an upper surface of the core.
  • the surface corrugation has a nominal period ⁇ 0 .
  • surface corrugations are formed in the core layer before the cladding layer has been deposited by exposing a layer of photoresist 90 on a wafer 100 to an optical pattern 110 created by two interfering coherent beams of light 120.
  • the interference can be arranged to produce a regular series of light and dark interference fringes with a nominal period ⁇ 0 corresponding to the period of the grating to be written.
  • the optics of such exposure systems do not always produce beams with a planar wavefront.
  • the period of the interference pattern 110 tends to vary to some extent from the nominal value ⁇ 0 .
  • Fig. 3 there is shown a graph 130 of ⁇ versus position along the x-axis of an interference pattern of the type 110 shown in Fig. 2 in which the interfering beams have generally-spherical wavefronts.
  • ⁇ is a parabolic function of x and there is a similar parabolic dependence along the y-axis (not shown).
  • the Bragg wavelength ⁇ of a grating is a function of both the effective refractive index n e _> of the waveguide and the grating period A.
  • n eff is a function of the waveguide dimensions and refractive index.
  • n e ff can be altered by controlling the thickness and width of the waveguide core.
  • the waveguide width can be defined with an appropriate mask. As ⁇ changes across the exposure area, each grating needs to be compensated to a different extent. It is therefore necessary to individually tailor the width profile of each waveguide in order to compensate for the values of ⁇ associated with the waveguide location on the wafer.
  • Figures 4 and 5 show plan view width profiles for partial waveguides 140-144 at five different locations across the x-axis of a wafer for the particular function ⁇ (x) plotted in Fig. 3.
  • a grating region 150 is defined in a central portion of each waveguide. Pairs of dashed lines 160 correlate each grating region 150 with a corresponding range 170 of the function ⁇ (x) in Fig. 3.
  • the width profile of each grating region 150 is dependent on the shape of the function ⁇ (x) over the same region of the wafer. In order to design the width profile of each waveguide it is necessary to first measure the variation in in ⁇ for all waveguide locations i.e. the function ⁇ (x,y).
  • the measurement of ⁇ (x,y) can be carried out by exposing and developing a grating structure in a layer of photoresist and measuring A over the wafer.
  • Fig. 5 there are shown enlarged plan views of two of the waveguides 142 and 144 shown in Fig. 4.
  • Each waveguide comprises a grating region 150 with varying width and two constant-width waveguides regions 180 and 190 projecting from each end of the grating region.
  • the two constant-width waveguides regions 180 and 190 are devoid of gratings.
  • a dashed line 200 parallel to each waveguide 142 and 144 shows the shape of prior art waveguides which do not compensate for the measured function ⁇ (x,y).
  • the waveguide width variation ⁇ W(x,y) in the grating region (width being measured relative to the dashed line where the waveguide width is W 0 ) is calculated to compensate the grating for ⁇ (x,y) according to equation (2).
  • n eJf AA(x,y) AW(x,y) - (2) dn tf A dW
  • ⁇ W m x -W 0
  • W max is the maximum width of the waveguide in the grating region
  • n ejj and -_ - can each be calculated using beam propagation modelling software.
  • the value of ⁇ can be positive, negative or zero. In other words, the maximum waveguide width in the grating region can be greater than, less than or equal to W 0 .
  • the value of ⁇ is zero in the embodiment shown in Fig. 5A and negative in Fig. 5B. It is the variation of the width ⁇ W(x,y) over the grating region which compensates for the variation in grating period.
  • the value of W max should be no greater than the threshold width for single mode propagation. If W 0 is less than the threshold width, Wthreshoid, for single mode propagation then W max can be greater than W 0 but less than W thresho i d while maintaining single mode confinement.
  • the grating 150 shown in Fig. 5 A is centred on the x-axis of the wafer.
  • the width profile W(x) is symmetric about the x-axis as the function ⁇ (x,y) is also symmetric about the x-axis.
  • the left-hand constant-width waveguide region 180 has the same width as the right-hand constant- width waveguide region 190.
  • the left and right constant-width waveguide regions 180 and 190 necessarily have different widths.
  • the width variation ⁇ W(x,y) over each of the grating regions 150 shown in Figures 4, 5 A and 5B is achieved by creating one waveguide edge 210 which is curved and an opposite waveguide edge 220 which is straight.
  • the same width variation ⁇ W(x,y) can be achieved by making both waveguide edges 210 and 220 curved.
  • the required curvature for each waveguide edge is less than for when only one waveguide edge is curved.
  • the technique of varying the width profile of the grating region can be used to compensate for variations in refractive index or thickness of the waveguide core along its length. The reason is that the refractive index and thickness of the core each have an effect on the effective refractive index of the grating. If these effects are consistent and can be quantified, they can be compensated by varying the width of the waveguide in a manner analagous to that described above.

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

Abstract

Un guide d'ondes photonique planaire (142) comprend une région de réseau de Bragg planaire (150) qui compense la variation graduelle de la période du réseau, l'indice de réfraction ou l'épaisseur sur toute la longueur du réseau planaire. La largeur W(x) du guide d'ondes varie sur toute la longueur de la région du réseau (150). La variation de la largeur est destinée à avoir un effet opposé sur l'indice de réfraction du réseau par rapport à la variation graduelle d'autres paramètres du réseau.
PCT/AU2004/001132 2003-08-25 2004-08-25 Reseau de bragg planaire WO2005019888A1 (fr)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
US49748603P 2003-08-25 2003-08-25
US60/497,486 2003-08-25

Publications (1)

Publication Number Publication Date
WO2005019888A1 true WO2005019888A1 (fr) 2005-03-03

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Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5805750A (en) * 1997-02-21 1998-09-08 International Business Machines Corporation Optical wavefront correction for variable groove depth grating waveguide couplers
GB2377031A (en) * 2001-06-29 2002-12-31 Bookham Technology Plc Optical waveguide having chirped diffraction grating with varying refractive index
WO2003102646A2 (fr) * 2002-05-30 2003-12-11 Massachusetts Institute Of Technology Guide d'ondes optiques a reseau non-uniforme sur les parois laterales
US6801689B1 (en) * 2002-04-30 2004-10-05 Intel Corporation Correcting the phase of waveguide bragg gratings

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5805750A (en) * 1997-02-21 1998-09-08 International Business Machines Corporation Optical wavefront correction for variable groove depth grating waveguide couplers
GB2377031A (en) * 2001-06-29 2002-12-31 Bookham Technology Plc Optical waveguide having chirped diffraction grating with varying refractive index
US6801689B1 (en) * 2002-04-30 2004-10-05 Intel Corporation Correcting the phase of waveguide bragg gratings
WO2003102646A2 (fr) * 2002-05-30 2003-12-11 Massachusetts Institute Of Technology Guide d'ondes optiques a reseau non-uniforme sur les parois laterales

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