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WO2003062870A1 - Structure de reseau de diffraction lamellaire dont l'efficacite est independante de la polarisation - Google Patents

Structure de reseau de diffraction lamellaire dont l'efficacite est independante de la polarisation Download PDF

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Publication number
WO2003062870A1
WO2003062870A1 PCT/US2003/002364 US0302364W WO03062870A1 WO 2003062870 A1 WO2003062870 A1 WO 2003062870A1 US 0302364 W US0302364 W US 0302364W WO 03062870 A1 WO03062870 A1 WO 03062870A1
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WO
WIPO (PCT)
Prior art keywords
grating
prism
face
wavelength
exit
Prior art date
Legal status (The legal status 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 status listed.)
Ceased
Application number
PCT/US2003/002364
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English (en)
Inventor
Robert Frankel
John Hoose
Evgeny Popov
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Chromaplex Inc
Original Assignee
Chromaplex Inc
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
Priority claimed from US10/062,228 external-priority patent/US6724533B2/en
Application filed by Chromaplex Inc filed Critical Chromaplex Inc
Publication of WO2003062870A1 publication Critical patent/WO2003062870A1/fr
Anticipated expiration legal-status Critical
Ceased legal-status Critical Current

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Classifications

    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B5/00—Optical elements other than lenses
    • G02B5/18—Diffraction gratings

Definitions

  • the invention relates to lamellar gratings, and more particularly to lamellar gratings having a substantially polarization-independent diffraction efficiency over a wavelength range useful for optical telecommunication applications.
  • Fiber optic telecommunication systems are increasingly used to provide high-bandwidth transmission of information to homes and businesses.
  • This transmission method is referred to as wavelength division multiplexing/demultiplexing (WDM/D).
  • the international telecommunications union (ITU) standards body has proposed a channel allocation grid with 100 GHz channel spacing ( ⁇ 0.81 nm at a 1550 nm wavelength) on even 100 GHz intervals, counting nominally in both directions from a center frequency of 193.1 THz. Newer systems are being designed to reduce the channel spacing to 50 GHz or less.
  • the total wavelength range over which these devices are designed to operate is increasing.
  • WDM is a general term applied to the separation and integration of information carried on these optical channels.
  • the technologies involved in WDM/D require efficient handling of the optical signals propagating over fiber optic cables, and being routed through various devices that deliver the high bandwidth signals to the end customer.
  • Wavelength-selective optical elements such as interference filters, fiber Bragg gratings, arrayed waveguide gratings (AWG), and free space gratings, e.g., surface relief diffraction gratings, are employed for this purpose. Many of these wavelength-selective components have a polarization-sensitive response.
  • the gratings can be ruled gratings, holographic gratings or etched gratings.
  • Gratings may employ a crystalline substrate, such as a Si wafer, that can be processed by conventional semiconductor processing techniques.
  • grating structures can be formed in Si by preferential etching using an aqueous solution of KOH to expose the Si (111) planes, or by (non-preferential) reactive ion etching which allows the formation of arbitrary grating characteristics.
  • optical components used with optical fibers should be made polarization independent, thereby reducing costs and complexity of the fiber-optic communications system. Moreover, the optical components should be highly efficient to extend the range of data transmission through optical fibers.
  • the invention is directed, among other things, to lamellar immersion gratings with a high dispersion for wavelength-dispersive applications, such as wavelength filtering, wavelength tuning and wavelength multiplexing/demultiplexing for optical communication systems.
  • the gratings are designed to provide high efficiency single mode (or single propagating order) diffraction in both TM and TE polarizations with a diffraction efficiency that is essentially wavelength-independent over a selected communication channel, such as the C-band between 1530 and 1565 nm.
  • the high dispersion design makes possible small standalone mux/demux components and integrated subcomponents that can be easily manufactured.
  • the lamellar grating is implemented in Si, which is transparent in the infrared portion of the spectrum of interest.
  • the lamellar structure can be fabricated by standard semiconductor processing techniques, such as wet etching, ion beam etching or reactive ion etching.
  • the grating can be operated in reflection or in transmission. If the grating is operated in reflection, it may be operated either in Littrow, or in non-Littrow, where the light exits the grating at an angle substantially different from the entrance angle.
  • the non-Littrow configuration has advantages in certain applications, for instance when double pass operation of the grating is desired.
  • the lamellar structure may be composed of alternating "teeth" and grooves which together define a grating period.
  • the grooves would be etched, leaving the "teeth" which are hence formed of the semiconductor material.
  • the grooves can be filled with a dielectric or a metal having a refractive index different from that of the substrate or "teeth.”
  • the "teeth” can also be coated with a material having a different dielectric constant or refractive index. It will be understood that the actual diffraction properties of the grating will depend on the dimensions and the refractive indices of the substrate and groove fill material.
  • the gratings are preferably operated in first order. However, the gratings can also be operated at higher diffraction orders. A grating will typically operate in a low diffraction order if the grating period is comparable to, e.g., within a factor of 2, of the wavelength of the light propagating in the medium in which the grating is formed.
  • a lamellar grating is formed or disposed on a surface of a prism with a high index of refraction, such as Si.
  • the grooves of the grating are filled with a material having a lower index of refraction, such as polymer or glass (n ⁇ 1.5).
  • a second prism is attached to the grating surface of the first prism. This arrangement has small dimensions due to the large index of the semiconductor (Si) prism and can advantageously be incorporated in an optical connector.
  • Fig. 1 is a cross-sectional view of a lamellar reflection grating having alternating metal and glass regions;
  • Fig. 2 shows the diffraction efficiency for the grating of Fig. 1;
  • Fig. 3 is an embodiment of a lamellar reflection grating with alternating silicon and gold regions implemented as an immersion grating in Littrow configuration
  • Fig. 4 is a cross-sectional view of the immersion grating of Fig. 3;
  • Fig. 5 shows the diffraction efficiency for TE and TM modes as a function of height-to-width ratio for the grating of Fig. 4;
  • Fig. 7 is another embodiment of a lamellar transmission grating with alternating silicon and glass regions operating in transmission;
  • Fig. 8 is a cross-sectional view of the transmission immersion grating of Fig. 7;
  • Fig. 9 shows the diffraction efficiency for the grating of Fig. 7;
  • Fig. 10 shows schematically a multiplexing/demultiplexing (DeMUX) connector employing the grating structure of Fig. 7.
  • DeMUX multiplexing/demultiplexing
  • the invention is directed to a lamellar volume grating operating in reflection and transmission and providing substantially equal diffraction efficiency TE and TM polarization directions.
  • the lamellar volume grating described herein can be employed in an optical multiplexer/demultiplexer arrangement.
  • Fig. 1 shows schematically in cross-section an exemplary lamellar grating structure 10, nominally rectangular, or quasi-rectangular.
  • the raised portions 12 can be provided with a sufficiently thick gold coating 14 so as to be deemed opaque over the intended optical wavelength range. Alternatively, the raised portions 12 could be made entirely from gold.
  • the height-to-width ratio d 2 /h of the grooves is 2.4 and the groove frequency is approximately 1200 grooves/mm.
  • Fig. 2 shows the diffraction efficiencies for the 1 st order TE mode 22 and the 1 st order TM mode 24.
  • the diffraction efficiencies for these TE and TM modes are approximately identical and substantially independent of wavelength between 1.52 and 1.58 ⁇ m, which corresponds to the C-band in optical communication.
  • a grating of this type can be manufactured using conventional semiconductor manufacturing techniques.
  • the term "substantially independent" is to be understood as representing a diffraction efficiency that is within ⁇ 5% to +10% of an average value in the wavelength range of interest.
  • Fig. 3 shows a lamellar immersion grating structure 30 operating in Littrow configuration.
  • the grating 34 can fabricated in crystalline silicon, and the grooves 35 can be filled and/or coated with a dielectric or a metal, such as gold.
  • the exemplary grating device 30 has the form of a prism 32, with light 36 entering the prism through the prism entrance face 39 at approximately normal incidence. This arrangement hence can substantially reduce and even eliminate polarization- dependence of the reflectivity at the entrance face 39.
  • the grating profile of the rectangular volume grating 34 can be fabricated directly on one of the prism faces, for example, by reactive ion etching.
  • the grating structure can be fabricated on a separate semiconductor wafer, for example, a Si wafer, that is subsequently bonded to the prism face 31, either with the surface of the wafer with the formed grating structure or with the unprocessed wafer surface facing the prism face 31.
  • Methods for bonding materials for example, by using an index-matching adhesive, are known in the art. Wafer bonding techniques, which rely on van der Waals forces between two flat surfaces in close contact, can also be used.
  • the entering light beam 36 is diffracted by the grating 34, preferably in a low order, such as in first order, and exits through the prism face 39 as diffracted beam 38 also at approximately normal incidence.
  • the grating is used in reflection and the grating surface (facing downward in Fig. 4) is coated or filled with a conductive material 44, for example gold.
  • a suitable h/d ! ratio was selected by calculating the diffraction efficiency for the TE and TM mode for 1 st order diffraction as a function of h/d ! . It was experimentally observed that an advantageous operating point over the wavelength range of interest has an h d value which is moved to a lower h/di value by one maximum of the TM curve 54 from the value where the TE 52 and the TM 54 diffraction efficiencies overlap.
  • the diffraction efficiency is > .8 and substantially wavelength-independent for both the TE and TM polarization directions.
  • Fig. 7 depicts a different embodiment of a grating device 70, wherein a lamellar immersion grating 78 operating in transmission is formed inside a double prism structure 76, 77.
  • the double prism structure can advantageously be made of a material with a high refractive index, such as silicon with a refractive index of 3.5.
  • the grooves of the lamellar grating can be filled with a dielectric material, such as glass (n ⁇ 1.5), or another material with a low refractive index.
  • Polarization- dependence of the optical losses at the entrance face 72 and exit face 74 of the double prism structure 76, 77 can be eliminated by passing the incident light 71 and the diffracted light 75 through the entrance and exit faces at approximately normal incidence.
  • the prism faces 72, 74 can be oriented so that the incident and/or diffracted light passes through the entrance and exit faces of the prism at non-normal incidence, causing the transmission to be different for TE and TM modes, as is known from electromagnetic theory.
  • Fig 8 shows a detailed cross-sectional view of the lamellar grating of the double prism structure of Fig. 7 that provides a high polarization-independent diffraction efficiency.
  • the grooves in the illustrated exemplary embodiment are filled with a material with an index of refraction of approximately 1.5, for example glass. • However, other materials, such as a polymer, Si 3 N , and the like can also be used.
  • the h/d ratio was selected by calculating the diffraction efficiency as a function of h d for the TE and TM mode in 1 st order and selecting as an operating point an h/d value just below the h/d value where the TE and the TM diffraction efficiencies intersect for the wavelength range of interest.
  • the grating profile depicted in Fig. 8 with the double prism structure of Fig. 7 the diffraction efficiencies of both TE and TM polarizations approach 100% over this wavelength range and are essentially independent of the wavelength.
  • the double prism grating structure depicted in Fig. 7 can advantageously be incorporated in a fiber connector 100 with built-in wavelength multiplexing and demultiplexing capability (DeMUX).
  • DeMUX wavelength multiplexing and demultiplexing capability
  • an optical beam containing a plurality of wavelengths is transmitted to the device by a pre- designated optical fiber 102.
  • the divergence of the beam depends on the numerical aperture of the input fiber 102 whose end is located in the vicinity of the focal point of the collimating lens 109 which has sufficient numerical aperture to accept the diverging beam from the optical fiber 102.
  • the beam thus is substantially collimated by the lens and then passes through the entrance face 104 and impinges on the lamellar grating 108.
  • the individual wavelengths within the incident beam are diffracted and angularly separated by the lamellar grating 108 according to their wavelengths.
  • the spatially separated beams then pass through the exit face 102 and are focused by the focusing lens 109' onto exit ports, for example, a series of spaced-apart optical fibers 112a, 112b, detectors, or other suitable optical/electrical devices known in the art.
  • the aforementioned beam path is reversed and the beam would be propagating in the reversed directions when compared to the demultiplexer mode.
  • all the different wavelengths from the series of optical fibers 112a, 112b are collected by the lens 109' and diffracted by the lamellar grating 108 with specific angular orientations according to the individual wavelengths.
  • the diffracted beams are then combined into a series of substantially overlapping collimated beams regardless of their wavelengths. These beams are essentially one beam that contains all the wavelengths and are focused onto the optical fiber 102 by focusing lens 109.
  • the aberrations are higher than in the case where two lenses are used, there may be situations where imaging into single mode devices is not required and therefore larger focal spots may be accommodated.
  • optical fibers as well as inputs/outputs of optical integrated circuits can be arranged in the focus spots.
  • a currently employed DeMUX device with a free space grating has a length of approximately 15-25 cm and a diameter of 2.5 - 5 cm, whereas an arrayed waveguide device measures 10 x 10 x 2 cm, in both cases without the connector.
  • the DeMUX device depicted in Fig. 10 can be fabricated with dimensions of approximately 2 cm x 2 cm x 1 cm.

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Optics & Photonics (AREA)
  • Diffracting Gratings Or Hologram Optical Elements (AREA)
  • Optical Couplings Of Light Guides (AREA)

Abstract

Réseau de diffraction volumique lamellaire fonctionnant en mode de réflexion et de transmission, dont l'efficacité de diffraction est pratiquement indépendante de la longueur d'onde et de la polarisation. Ce réseau de diffraction lamellaire présente un profil approximativement rectangulaire (10) dont le rapport entre la hauteur et la largeur des dents (12) est supérieur à 2. Ce réseau de diffraction opère, de préférence, dans un premier ordre et peut être mis en application sous forme de réseau à immersion. Ce réseau de diffraction volumique lamellaire peut être, en particulier, utilisé dans un ensemble optique de multiplexage/démultiplexage faiblement dimensionné.
PCT/US2003/002364 2002-01-23 2003-01-23 Structure de reseau de diffraction lamellaire dont l'efficacite est independante de la polarisation Ceased WO2003062870A1 (fr)

Applications Claiming Priority (4)

Application Number Priority Date Filing Date Title
US35106702P 2002-01-23 2002-01-23
US60/351,067 2002-01-23
US10/062,228 US6724533B2 (en) 2002-01-31 2002-01-31 Lamellar grating structure with polarization-independent diffraction efficiency
US10/062,228 2002-01-31

Publications (1)

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WO2003062870A1 true WO2003062870A1 (fr) 2003-07-31

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Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP1341026A3 (fr) * 2002-02-28 2004-11-24 Canon Kabushiki Kaisha Séparateur de faisceaux et appareil optique l'utilisant
CN111708113A (zh) * 2020-08-24 2020-09-25 苏州大学 低偏振高衍射效率金属反射浸没光栅及光学系统

Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US6275630B1 (en) * 1998-11-17 2001-08-14 Bayspec, Inc. Compact double-pass wavelength multiplexer-demultiplexer
US6449066B1 (en) * 1999-04-29 2002-09-10 Kaiser Optical Systems, Inc. Polarization insensitive, high dispersion optical element
US20020176124A1 (en) * 2001-05-23 2002-11-28 Wise Kent Lawson Linear axially transmissive photon-diffracting device

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US6275630B1 (en) * 1998-11-17 2001-08-14 Bayspec, Inc. Compact double-pass wavelength multiplexer-demultiplexer
US6449066B1 (en) * 1999-04-29 2002-09-10 Kaiser Optical Systems, Inc. Polarization insensitive, high dispersion optical element
US20020176124A1 (en) * 2001-05-23 2002-11-28 Wise Kent Lawson Linear axially transmissive photon-diffracting device

Non-Patent Citations (2)

* Cited by examiner, † Cited by third party
Title
ARNS ET AL.: "Volume phase gratings for spectroscopy, ultrafast laser compressors and wavelength division multiplexing", SPIE PROCEEDINGS, vol. 3779, 1999, pages 313 - 323, XP002963366 *
ROUMIGUIERES ET AL.: "On the efficiencies of rectangular-groove gratings", J. OPT. SOC. AM., vol. 66, August 1976 (1976-08-01), pages 772 - 775, XP002963365 *

Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP1341026A3 (fr) * 2002-02-28 2004-11-24 Canon Kabushiki Kaisha Séparateur de faisceaux et appareil optique l'utilisant
US6900939B2 (en) 2002-02-28 2005-05-31 Canon Kabushiki Kaisha Polarization insensitive beam splitting grating and apparatus using it
CN111708113A (zh) * 2020-08-24 2020-09-25 苏州大学 低偏振高衍射效率金属反射浸没光栅及光学系统

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