EP1444796A1 - Egaliseur spectral dynamique mettant en oeuvre un miroir holographique programmable - Google Patents
Egaliseur spectral dynamique mettant en oeuvre un miroir holographique programmableInfo
- Publication number
- EP1444796A1 EP1444796A1 EP02793239A EP02793239A EP1444796A1 EP 1444796 A1 EP1444796 A1 EP 1444796A1 EP 02793239 A EP02793239 A EP 02793239A EP 02793239 A EP02793239 A EP 02793239A EP 1444796 A1 EP1444796 A1 EP 1444796A1
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- European Patent Office
- Prior art keywords
- lens
- holographic
- equalizer
- mirror
- equalizer according
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Classifications
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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
- G02B6/24—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
- G02B6/29304—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 diffraction, e.g. grating
- G02B6/29305—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 diffraction, e.g. grating as bulk element, i.e. free space arrangement external to a light guide
- G02B6/2931—Diffractive element operating in reflection
-
- 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
- G02B6/29304—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 diffraction, e.g. grating
- G02B6/29305—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 diffraction, e.g. grating as bulk element, i.e. free space arrangement external to a light guide
- G02B6/29311—Diffractive element operating in transmission
-
- 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
- G02B6/29379—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 characterised by the function or use of the complete device
- G02B6/29395—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 characterised by the function or use of the complete device configurable, e.g. tunable or reconfigurable
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/25—Arrangements specific to fibre transmission
- H04B10/2507—Arrangements specific to fibre transmission for the reduction or elimination of distortion or dispersion
- H04B10/25073—Arrangements specific to fibre transmission for the reduction or elimination of distortion or dispersion using spectral equalisation, e.g. spectral filtering
Definitions
- the field of the invention is that of telecommunications by optical fiber. More specifically, the invention relates to a dynamic spectral equalizer 5 making it possible, in the context of a multi-channel transmission system, to equalize the spectral power density of the transmitted signal.
- wavelength multiplexing in English DWDM for "Dense Wavelength Division Multiplexing" is more and more frequently used in the field of optical telecommunications. It in fact makes it possible to increase the data transfer rate through a single-mode fiber, by simultaneously propagating the light coming from several spectrally distinct laser sources, but of equal powers, through the optical fiber.
- Each laser source is associated with a propagation channel in the fiber.
- the transmission system does not present spectral ripples, that is to say that it has a spectral density of flat power over the width of the transmission band considered.
- the spectral power density is not flat, because the power per channel, formed by a narrow band around a central wavelength, is not constant.
- optical amplifiers there are several types of optical amplifiers. Among the most widespread, there may be mentioned semiconductor optical amplifiers (in English, SOA for “Semiconductor Optical Amplifier”), non-linear amplifiers such as Raman amplifiers, and erbium-doped fiber amplifiers (in English, EDFA for "Erbium Doped Fiber Amplifier”).
- SOA semiconductor optical amplifier
- non-linear amplifiers such as Raman amplifiers
- erbium-doped fiber amplifiers in English, EDFA for "Erbium Doped Fiber Amplifier”
- SOA and Raman type amplifiers can operate on bandwidths sufficient to cover most of them - the bands S, C and -L (remember that the S band corresponds to wavelengths substantially between 1480 nm and 1520 nm, the C band corresponds to the wavelengths substantially between 1525 nm and 1565 nm, and the L band corresponds to the wavelengths substantially between 1570 nm and 1620 nm).
- S, C and -L reference that the S band corresponds to wavelengths substantially between 1480 nm and 1520 nm
- the C band corresponds to the wavelengths substantially between 1525 nm and 1565 nm
- the L band corresponds to the wavelengths substantially between 1570 nm and 1620 nm.
- Raman amplifiers have the drawback of generating large variations in gain over a wide spectral band (of the order of a few hundred nanometers) dependent on the charge of the channel. In order to reduce these gain variations, it is necessary to flatten the spectral power density of the transmitted signal
- equalization by individual channels consists in separating or demultiplexing the channels, in adjusting the power separately. channels, then to-recombinei-or remultiplex. canals ;
- dynamic spectral equalization by Fourier filtering consists in cutting the gain curve of the optical system considered into five to ten windows with a width of 3 to 6 nm. Individual windows are then adjusted regardless of the number of channels they include. Technologies that allow filtering of
- thermo-optical Mach-Zehnder devices tunable acousto-optical filters (in English AOTF for "Acousto-Optic Tunable Filters”), and Bragg gratings electrically switchable (in English ESBG for "Electronically Switchable Bragg Gratings”).
- tunable acousto-optical filters in English AOTF for "Acousto-Optic Tunable Filters”
- Bragg gratings electrically switchable in English ESBG for "Electronically Switchable Bragg Gratings”
- thermo-optical devices of the Mach-Zehnder type have the drawback of exhibiting great heat dissipation in the substrate of the waveguides, and a low reconfiguration speed.
- thermo-optical Mach-Zehnder filter is a temperature-controlled Mach-Zehnder interferometer with waveguide.
- the optical path of the arms of the interferometer is controlled by modifying the temperature of the refractive material of the arms.
- the beams are then combined using a coupler with two outputs. Each output supports only one wavelength under certain conditions of constructive interference, and different wavelengths can be adjusted by differences in optical path by changing the temperature of the refractive material.
- Filters of the AOTF type exploit the Bragg effect produced when acoustic waves are created in a refractive material in a way collinear to the direction of light propagation. Acoustic waves are created by placing the material in a radio frequency (RF) field, They in turn create compression and expansion zones which give rise to a modulation of the refractive index, thus forming a periodic structure of Bragg.
- RF radio frequency
- ESBG type networks also exploit the Bragg effect. They can be produced using holographic liquid crystal technologies dispersed in polymer (also called holo-PDLC).
- holographic liquid crystal technologies dispersed in polymer (also called holo-PDLC).
- This technology enables thick phase holograms to be produced in a polymer substrate. by a process which allows the control of the diffractive bandwidth and the central wavelength of the system.
- the diffractive structure can be removed by applying an electric field. It is thus possible to control the coupling between the guide and the substrate, while minimizing the insertion losses and the loss of polarization of PDL type (in English "Polarization Dependent Loss", for "polarization dependent loss”).
- PDL type in English "Polarization Dependent Loss", for "polarization dependent loss”
- An equalization technique using ESBG 10 type networks typically consists of cascading a plurality of waveguides. All the wavelengths of an incident beam cross all of the Bragg gratings thus cascades, and undergo a corresponding power attenuation.
- ESBG-based equalizers therefore generally have very limited bandwidths. Indeed, to increase the bandwidth, it is necessary
- a first drawback of such solutions is that they have a dependence on polarization and on temperature.
- these isolated channel approaches do not allow large spectrum ranges to be treated as well. only isolated wavelengths, which is however necessary in the context of long-distance and metropolitan networks.
- the invention particularly aims to overcome these drawbacks of the prior art.
- an object of the invention is to provide a dynamic spectral equalization technique making it possible to equalize optical signals having a plurality of distinct wavelengths.
- Another objective of the invention is to implement such an equalization technique, which is rapid and suitable for wide spectral bands.
- the invention also aims to implement such a spectral equalization technique which is independent of the polarization of the incident beam.
- the invention also aims to implement such a technique - spectral equalization which is - independent of temperature -.
- the invention also aims to provide such a technique which has low reconfiguration times.
- Another object of the invention is to provide such an equalization technique which is suitable for any type of optical communication network, and in particular long distance networks, metropolitan networks, and submarine networks.
- a dynamic spectral equalizer comprising: means for demultiplexing an incident beam of at least two multiplexed wavelengths, comprising at least a first element dispersive optics, so as to form a spatial multiplex of said at least two wavelengths; means for attenuating the spectral power associated with at least one wavelength of said spatial multiplex, comprising at least one programmable semi-transparent holographic mirror, so as to form an equalized spatial multiplex; means for multiplexing said equalized spatial multiplex, comprising at least one second dispersive optical element, so as to form an equalized beam of at least two multiplexed wavelengths.
- the invention is based on a completely new and inventive approach to dynamic spectral equalization, based on the combination of free space optics with high dispersive power and a semi-transparent holographic programmable mirror.
- a spectral equalization technique therefore proposes an innovative solution consisting in implementing, on the one hand a multiplexing / demultiplexing technique, and on the other hand a programmable semi-transparent holographic mirror to attenuate an isolated wavelength or a wavelength band of a wavelength multiplex conveyed as an optical signal.
- Such an approach advantageously allows adaptation of the equalization device to changes in one or more wavelengths of the multiplex. It also allows faster response times than according to techniques known in the prior art.
- said first and second dispersive optical elements are combined.
- the dynamic spectral equalizer thus designed is indeed more compact.
- said holographic mirror is optically recorded in liquid crystal dispersed in polymer (PDLC), so as to form a holo-PDLC.
- PDLC polymer
- holo-PDLCs contain droplets of liquid crystals with an electro-optical effect, and that their periodic structure can be modified (or go from an active state to an inactive state) by applying an electric field.
- said holographic mirror is a holographic network thick in reflection.
- said thick holographic network in reflection is
- a spatial "chirp" that is to say a spatial variation of the network period substantially in the form of a ramp
- a spatial "chirp" that is to say a spatial variation of the network period substantially in the form of a ramp
- said holographic mirror comprising at least two strata, the direction of propagation of said spatial multiplex incident on said holographic mirror is substantially perpendicular to said strata.
- the attenuation of the wavelengths, induced by the holographic mirror is insensitive to the polarization of the spatial multiplex.
- said dispersive optical element is a thick phase holographic network.
- Such a dispersive optical element can also be of any other nature capable of performing a function of multiplexing and demultiplexing the beam of wavelengths.
- such a dynamic spectral equalizer further comprises: - at least one input port of said incident beam of at least two wavelengths multiplexed in said equalizer; at least a first output port of said equalized beam of at least two multiplexed wavelengths of said equalizer.
- said input port and said first output port are combined.
- said at least one holographic mirror comprises at least two electrodes making it possible to electrically control the reflectivity of at least certain zones of said mirror.
- the fraction of energy of the incident wavelength which is reflected by the mirror it is possible to control the fraction of energy of the incident wavelength which is reflected by the mirror, so as to achieve an equalization which is adapted as a function of each of the lengths of the incident beam.
- such an equalizer further comprises: an input optical fiber (F in ) conveying said incident beam of - - at least two wavelengths multiplexed towards said input port -; - a first lens (L1), located so that said input port is in the object focal plane of said first lens; a second lens (L2), located so that said holographic mirror is in the object focal plane of said second lens, and that the object focal plane of said second lens is coincident with the image focal plane of said first lens; an output optical fiber (F out ) receiving, from said first output port, said equalized beam of at least two multiplexed wavelengths.
- a 4-f system is thus produced, namely a double diffraction imaging system.
- said dispersive optical element is located in the image focal plane of said first lens (L1) and in the object focal plane of said second lens (L2).
- said dispersive optical element is a lens, comprising two prisms and a thick holographic grating of non-inclined phase
- said input optical fiber is placed on the optical axis of said equalizer.
- such an equalizer further comprises a three-port circulator, making it possible to transmit said incident beam of at least two wavelengths multiplexed from said input optical fiber (F in ) to said input port and to transmit said equalized beam of at least two wavelengths multiplexed from said output port to said output optical fiber (F out ).
- a circulator thus makes it possible to block the passage of the equalized beam from the output port towards the input optical fiber, and therefore isolates the input optical fiber from the output optical fiber.
- said dispersive optical element is implemented in a configuration in reflection, and said first and second lenses are combined.
- the concepts of object focal plane and image focal plane of the lens are linked to the only direction of light travel: in other words, the object focal plane (respectively image) of the lens, when light passes through it from the input port to the dispersive optical element, corresponds to the image focal plane (respectively object) of this same lens, when light passes through it from the dispersive optical element towards the holographic mirror.
- said dispersive optical element is located between said first and second lenses, near said first lens.
- Such a displacement of the dispersive optical element makes it possible to add, in the imaging plane, an angular multiplex to the spatial multiplex.
- said dispersive optical element and said first lens are replaced by a single holographic lens, chosen so that the axial radius of a wavelength of said beam of at least two lengths d multiplexed waves pass through the focal point of said holographic lens.
- the angular multiplex is located around the perpendicular to the holographic mirror, that is to say that at least one of the wavelengths of the multiplex beam arrives on the holographic mirror perpendicularly to the latter.
- the focal point of the holographic lens coincides with the focal point of the second lens (L2).
- such an equalizer further comprises a second output port, making it possible to receive at least one wavelength of said spatial multiplex transmitted by said holographic mirror.
- said dispersive optical element is a thick holographic network of non-inclined phase and said input optical fiber is placed at a distance from the optical axis of said equalizer.
- Such non-inclined holographic networks have in fact several technological advantages compared to networks with inclined layers, in particular an insensitivity to changes in thickness.
- said spatial multiplex is projected onto said holographic mirror by a first mirror (Ml), and said equalized spatial multiplex transmitted by said holographic mirror is directed towards said second lens by a second mirror ( M2).
- At least one of the first and second mirrors is a prism with total internal reflection.
- said input and output optical fibers are placed symmetrically with respect to said optical axis.
- such an equalizer further comprises an isolator making it possible to isolate said input optical fiber from said equalized beam by at least two multiplexed wavelengths.
- said holographic mirror is placed in a virtual focal plane, image of the image focal plane of said second lens by said first mirror (Ml), -
- said first and second mirrors respectively form an angle of substantially 45 ° relative to said optical axis, and said holographic mirror is placed along said optical axis.
- said holographic mirror is a holographic mirror with inclined layers, and it is placed at a distance from a virtual focal plane, image of the focal plane image of said second lens by said first mirror, of so that said spatial multiplex reflected by said holographic mirror is not reinjected into said input optical fiber.
- said equalizer further comprises a second output port, making it possible to receive at least one wavelength of said spatial multiplex reflected by said holographic mirror.
- FIG. 1 presents a block diagram of a first embodiment of a dynamic spectral equalizer according to the invention
- - Figure 2 illustrates a folded version of the spectral equalizer of Figure 1
- FIG. 3 describes a third embodiment of the invention, in which the switchable thick hologram is placed along the optical axis
- Figure 4 shows a fourth embodiment of the invention, in which the dispersive optical element has been moved relative to the embodiment of Figure 1
- FIG. 5 illustrates a fifth embodiment of the invention, implementing a holographic lens
- FIG. 6 shows an example of a dispersive optical element which can be implemented in a dynamic spectral equalizer according to the invention
- - - - - - Figures 7 and 8 illustrate the diffraction efficiency of a dispersive optical element of the invention, as a function of the wavelength
- Figures 9 and 10 show the spatial dispersion spectrum of a dispersive optical element of Figure 6, as a function of the wavelength
- Figures 11 and 12 illustrate the diffraction efficiency of a phase hologram that can be implemented in a dynamic spectral equalizer of the invention
- FIG. 13 shows the diffraction efficiency of a thick phase grating in reflection which can be implemented in a dynamic spectral equalizer of the invention
- FIG. 14 is a sectional view of a hologram in chirped reflection and pixelated which can be implemented in a dynamic spectral equalizer of the invention.
- the general principle of the invention is based on the combination of free space optics with high dispersive power and a thick network in "chirped" reflection, acting as a programmable semi-transparent mirror, used to attenuate wavelengths isolated or wavelength bands.
- a dynamic spectral equalizer receives from an input port (typically an optical fiber) a multiplex of wavelengths, conveying data over a plurality of wavelengths ⁇ ⁇ .
- an input port typically an optical fiber
- the beam is imaged in a basic configuration thanks to a 4-f system, possibly having an enlargement factor.
- system 4-f is meant here and throughout the rest of the document a system comprising two lenses, in which the image focal plane of the first lens is coincident with the focal plane object of the second lens.
- 4-f system performs double diffraction imaging.
- the wavelength multiplex is transformed into a spatial multiplex by means of a diffractive optical element (preferably a thick grating) located in the Fourier plane (that is to say in the image focal plane of the first lens and in the object focal plane of the second lens of the aforementioned system 4-f).
- This spatial multiplex illuminates the thick gom usable hologram.
- diffraction structure recorded in the holographic medium is such that the hologram operates in a similar way to a mirror and presents a modulation of continuous spatial period (or "chirp"), in order to compensate for the variation in wavelength along the 'axis of dispersion.
- Electrodes are spatially distributed on the switchable thick hologram and allow to locally control the efficiency of the hologram, which behaves like a pixelated spatial light modulator (SLM for Spatial Light Modulator).
- SLM pixelated spatial light modulator
- the different data streams carried by an isolated wavelength ⁇ ; or by a band of wavelengths, focus on different pixels of the switchable thick hologram, and the fraction r ; energy associated with the wavelength ⁇ ; reflected by the switchable thick holo-PDLC can be adjusted using a voltage applied to the pixel over which the wavelength ⁇ ; is focused.
- the reflected wavelengths then pass through the dispersive optical element, which acts as a dispersion compensator, and the different wavelengths are all reinjected into the output port (typically an optical fiber).
- the wavelengths transmitted by the switchable thick hologram can be reinjected into another port (for example another optical fiber), using a symmetrical optical system with the one described above.
- a first embodiment of a dynamic spectral equalizer according to the invention is presented in relation to FIG. 1.
- the equalizer in Figure 1 receives a DWDM comb from an input optical fiber F in .
- This incident beam of multiplexed wavelengths is sent to the optical fiber F via a three-port circulator C.
- the output 1 of the optical fiber F is in the focal plane object of a first lens L1.
- the DWDM comb is transformed from Fourier by the first lens L1 on a dispersive optical element D located in the focal plane -image thereof.
- the effect of the dispersive optical element D is to transform the wavelength multiplex (or DWDM comb) into an angular multiplex.
- This angular multiplex, coming from the dispersive optical element D, is then transformed into a spatial multiplex by a second lens L2, which is positioned so that the dispersive optical element D is in its image focal plane.
- the spatial multiplex from the second lens L2 focuses in the focal plane of the latter, and illuminates the switchable thick hologram H.
- the wavelengths reflected in the equalizer in the form of an equalized spatial multiplex are projected onto the dispersive optical element D via the second lens L2, which transforms the spatial multiplex equalized by the hologram H into an equalized angular multiplex.
- the equalized angular multiplex is in turn transformed back into a wavelength multiplex equalized by the dispersive optical element D.
- the equalized wavelength multiplex coming from the dispersive optical element D is focused on the optical fiber F by the first lens L1, and each wavelength of the multiplex is reinjected into the optical fiber F with an efficiency of proportional coupling to the energy fraction r ; reflected by the switchable thick hologram H.
- the incoming and outgoing wavelengths in the equalizer of FIG. 1 are separated by the three-port circulator C, the outgoing wavelengths (and therefore equalized) being sent to the output optical fiber F out , and isolated from the input optical fiber F in .
- FIG. 2 a second embodiment of the invention, in which the dispersive optical element D of FIG. 1 is implemented in a configuration in reflection.
- Such an alternative embodiment has the advantage of allowing a significant gain in compactness, the dynamic spectral equalizer thus designed being much less bulky than that presented in FIG. 1.
- a beam of multiplexed wavelengths is incident on the dynamic spectral equalizer of FIG. 2 by the input optical fiber F in , and transmitted to an optical fiber F via a circulator with three ports C.
- the input port in the multiplexed wavelength incident beam equalizer corresponds to the output 1 of the optical fiber F, and is located in the object focal plane of a lens L
- the incident multiplex is transformed from Fourier by the lens L on a reflecting dispersive optical element D, located in the image focal plane of the lens L.
- the reflecting dispersive optical element D transforms the wavelength multiplex into an angular multiplex, and reflects all of the wavelengths towards the lens L.
- the latter transforms the incident angular multiplex into a spatial multiplex, which illuminates a holographic semi-transparent controllable mirror H, located in the object focal plane of the lens L, that is to say in the same plane as the input port of the equalizer.
- the holographic mirror H reflects, for each of the wavelengths ⁇ [of the spatial multiplex, a fraction of the associated energy, as a function of the voltage applied to the holographic mirror H, and of the point of impact of the wavelength on the holographic mirror H.
- the beam reflected at least partially by the holographic mirror H, in the form of an equalized spatial multiplex is transformed back into an angular multiplex by the lens L, which it crosses before illuminating the reflecting dispersive optical element D.
- the latter transforms the equalized angular multiplex into an equalized beam of multiplex wavelengths, which it reflects in the direction of the lens L.
- the lens L then focuses the multiplex equalized in wavelengths on the output 1 of the optical fiber F.
- the circulator C transmits the equalized beam of multiplexed wavelengths to the output optical fiber F out , and blocks its passage to the input optical fiber F in .
- FIG. 3 presents a third embodiment of the invention, in which the switchable thick hologram (or controllable semi-transparent holographic mirror) H is placed along the optical axis of the dynamic spectral equalizer.
- This configuration is such that the wavelengths equalized by the hologram H are injected into an output optical fiber F out distant from the input optical fiber F in without these two fibers being connected by a circulator.
- the equalizer in FIG. 3 has a first lens L1, a dispersive optical element D, and a second lens L2, which perform functions similar to those of the equalizer in FIG. 1, and which will therefore not be described here more. in detail.
- the wavelengths of the spatial multiplex coming from the second lens L2 are projected onto the holographic mirror H by a first mirror Ml (preferably a prism with total internal reflection) which makes an angle of 45 ° relative to the optical axis of the equalizer.
- a first mirror Ml preferably a prism with total internal reflection
- the wavelengths of the spatial multiplex transmitted by the switchable thick hologram H are fed back into the equalizer by means of a second mirror (preferably a prism with total internal reflection) which also makes an angle of 45 ° relative to the optical axis of the equalizer.
- the mirror M2 can also have an angle other than 45 °.
- the equalized spatial multiplex is transformed back into equalized multiplex of wavelengths by the optical system made up of the dispersive optical element D and of the first and second lenses -L 1 and L2.
- the equalized wavelengths coming from the first lens L 1 focus on the output optical fiber F out placed symmetrically with the input optical fiber F in with respect to the optical axis of the equalizer .
- the wavelengths reflected by the hologram H, after having passed through the optical system (D, L1, L2) are, by construction, reinjected into the input optical fiber F in .
- Such a drawback can easily be overcome, for example by placing an insulator at the end of the input optical fiber F in , or by directing the reflected beam towards a control optical fiber, not shown in the figure. 3, by means of a three-port circulator also placed at the end of the input optical fiber F in .
- the variant embodiment illustrated in FIG. 4 differs from the equalizer presented in FIG. 1 in that the dispersive optical element D is moved from the focal plane of the first and second lenses L1 and L2, to be brought closer to the first lens L1.
- Such a configuration adds, in the imaging plane, an angular multiplex (around a non-zero angle relative to normal to the semi-transparent programmable holographic mirror H) to the spatial multiplex.
- the arrows shown in dotted lines between the dispersive optical element D and the hologram H represent the axial rays corresponding to two wavelengths of the multiplex considered, ⁇ m and ⁇ ⁇ , with ⁇ m ⁇ _.
- the optical assembly 'comprising the first lens L1 and - 1-' dispersive optical element - is replaced 'by a single element, namely a holographic lens HL.
- the holographic lens HL is preferably chosen so that the axial radius of one of the wavelengths of the angular multiplex passes through the focal point PF of the holographic lens.
- the angular multiplex is now around the perpendicular to the holographic mirror of the holo-PDLC H type.
- FIG. 5 shows the axial radii associated with two wavelengths ⁇ j and ⁇ 2 of the multiplex considered.
- the axial radius associated with the wavelength ⁇ x is shown in solid lines between the holographic lens HL and the second lens L2 on the one hand, and between the second lens L2 and the holographic mirror H on the other hand.
- the axial radius associated with the wavelength ⁇ ⁇ is represented in dotted lines between the holographic lens HL and the second lens L2 on the one hand, and between the second lens L2 and the holographic mirror H on the other hand, ⁇ j is the smallest wavelength of the comb DWDM entering the equalizer, and ⁇ ⁇ is the next wavelength in the comb.
- the main characteristics of the dispersive optical element D are its spectral bandwidth (including polarization effects), its efficiency and its dispersive power.
- An ideal dispersive optical element D would have the following characteristics: - a great dispersive power, so as to be able to spatially separate the imaged spots corresponding to the impact points of the different wavelengths of the multiplex to be equalized on the holographic mirror of holo type PDLC H; an efficiency substantially equal to 100% over the wavelength band considered; insensitivity to polarization of the beam of multiplexed wavelengths.
- Phase Holographie Gratings have characteristics close to those of the ideal dispersive optical element.
- HPV networks are optically recorded by placing a photosensitive film several tens of microns thick in the interference region of two light beams The interference pattern is recorded in the volume of the film as a generally sinusoidal modulation of the refractive index.
- DCG Dichromated gelatin
- photopolymers are almost ideal materials for recording HPV-type networks, as explained by RRA Syms, in “Practical Volume Holography", Clarendon Press, Oxford, 1990. Indeed, their diffraction efficiency can be greater than 95%.
- DCG-based networks have lifespans of at least 20 years, if they have adequate sealing conditions.
- HPV gratings diffract light according to the classical equation of gratings.
- n is the average refractive index of the medium
- ⁇ B is the angle of incidence and diffraction inside the grating, measured with respect to the strata (also called Bragg angle)
- ⁇ g is the length Bragg wave (in a vacuum)
- ⁇ is the lattice period.
- the energy diffracted by the grating is maximum when the pair wavelength and angle of incidence of the incident light satisfies the Bragg condition.
- a beam whose characteristics deviate slightly from the Bragg conditions can be effectively diffracted according to the network parameters.
- Kogelni's coupled wave theory H. Kogelnik,
- HPV networks are also sensitive to the polarization of incident light.
- the above equations are valid for a polarization of the TE type light. If the incident light is polarized in the TM plane, the ⁇ parameter must be corrected as follows: As long as the angle between the incident and diffracted beams is not close to 90 °, the diffraction efficiency hardly varies according to the state of polarization.
- non-inclined HPV networks are preferably used, which have several technological advantages compared to HPV networks whose strata are inclined, such as, for example, insensitivity to changes in thickness.
- the invention is of course also applicable to any other type of dispersive optical element, and in particular to networks of the HPV type having inclined strata.
- we will focus in the rest of the document to describe the case of non-inclined HPV networks. It will be easy for a person skilled in the art to deduce therefrom the characteristics of a dynamic spectral equalizer according to the invention implementing any other type of dispersive optical element.
- a first possible adaptation of the assembly of FIG. 1 consists in moving the input optical fiber F in from the optical axis, as illustrated for example in FIG. 3.
- a second possible adaptation consists in maintaining the input optical fiber F in on the optical axis, as illustrated in FIG. 1, and in using a combination of two prisms and of a non-inclined HPV network as dispersive optical element D. Such a combination, illustrated in FIG. 6, is called grayism.
- Such a grism comprises a first prism PI, a VPH type network denoted VPHG in FIG. 6, and a second prism P2.
- the non-inclined network VPHG has strata F perpendicular to the faces of the network.
- the dotted line L crossing the whole of grayism represents a beam of light.
- Figures 7 and 8 present numerical simulation results of the diffraction efficiency distribution for two HPV networks with different spatial periods (3 and 4 microns respectively).
- the thickness of the photosensitive film is 50 microns for the two gratings
- the index average refraction is 1.51
- the modulation of refractive index ⁇ n is substantially equal to 0.015 and differs for each of the two networks.
- FIGS. 9 and 10 present the simulated spatial dispersion characteristics of these networks when they are placed in the Fourier plane of an assembly 4-f, for example of the type of assembly of FIG. 1.
- the results of FIG. 9 are obtained with a focal length of 100 mm.
- the results of FIG. 10 are obtained with a focal distance of 75 mm.
- the distance between two spots associated with two wavelengths juxtaposed on the holographic mirror H is approximately equal to 10.4 microns.
- Such a thick hologram generates a predetermined wavefront by means of diffractive structures recorded in a holographic medium.
- An important feature of thick holograms is that the efficiency with which the wavefront is generated is highly dependent on the wavelength and the angle of incidence of the light relative to the hologram.
- a switchable thick hologram is a thick hologram whose diffraction efficiency can be controlled electrically between 0% and 100%.
- a switchable thick hologram is used so as to reproduce the effect of a mirror whose reflectivity at the Bragg condition can be varied between substantially 0% and 100%.
- This hologram is optically recorded in liquid crystal dispersed in polymer (in English PDLC for "Polymer Dispersed Liquid Crystal”) during a one-step process, making it possible to form a holo-PDLC.
- polymer in English PDLC for "Polymer Dispersed Liquid Crystal”
- PDLC materials allow the recording of phase holograms in reflection presenting high diffraction efficiencies. Switching voltages can become as small as 50 True for frequencies of 1-2 kHz, for example by adding a surfactant to the PDLC material.
- a sample is prepared by applying a mixture formed from a monomer, a liquid crystal, a binding monomer, a co-initiator, a photo-initiator dye and a surfactant between two plates of glasses separated by spacers of appropriate thickness, as detailed in the rest of the document.
- the glass plates are covered with bands of indium tin oxide (in English ITO for "indium tin oxide”) forming pixelated electrodes.
- the sample is then placed in the interference region of two beams of coherent light and a photopolymerization process is induced by the distribution of optical intensity. In areas of high illumination, the concentration of liquid crystal droplets (LC) will be small, while areas of low illumination will be rich in liquid crystal droplets.
- the interference pattern is recorded as a change in the concentration of liquid crystal droplets in the PDLC material.
- the hologram is stored in the form of a refractive index modulation in the holographic medium.
- the difference in refractive index of the liquid crystal droplets and the polymer can be controlled by the voltage applied to the ITO electrodes.
- the diffraction efficiency of a thick phase hologram depends on the modulation of the refractive index, this efficiency can be controlled by the voltage applied to the electrodes.
- An important factor determining the effect of the PDLC medium on the light which illuminates it is the size of the liquid crystal droplets.
- the droplets act as Rayleigh diffusers. If the droplet size is much smaller than the wavelength of the incident light (for example for a droplet size less than 100 nm for the near infrared), the PDLC medium becomes optically isotropic (i.e. i.e. there is no diffusion) in the direction collinear to the applied field with a net refractive index determined by the refractive index of the polymer and that of the liquid crystal droplets.
- the size of the liquid crystal droplets is a function of the rate of polymerization of the PDLC system: the higher this speed, the smaller the liquid crystal droplets.
- the droplet size In order to produce good quality phase holograms, the droplet size must be small enough so that the holo-PDLC acts as a phase-shifting, non-diffusing medium.
- Sutherland et al. (US 5, 942, 157) reported the recording of holograms in PDLC materials with liquid crystal droplets whose size is in the range 30-50 nm, which is suitable for the production of phase holograms having a high diffraction efficiency.
- the switchable thick hologram used in this invention should act as a programmable semi-transparent mirror.
- the diffraction structure is therefore formed by strata parallel to the faces of the holographic medium.
- This type of hologram is called a reflection hologram.
- holograms in reflection have the important characteristic of being insensitive to polarization.
- the diffraction efficiency, ⁇ is given by:
- n is the average refractive index of the holographic medium
- An is the modulation of refractive index
- ⁇ is the deviation of the wavelength from the Bragg wavelength
- d is the thickness of the hologram.
- This graph shows that it is impossible to cover a wide range of wavelengths by keeping the period of the network constant.
- the inventors of the present application have envisaged d 'introduce a spatial "chirp" into the holographic network H.
- spatial “chirp” is meant here, and throughout the rest of the document, a spatial variation of the period of the holographic network H, substantially along a ramp.
- a chirped reflection network can be recorded by placing the PDLC sample in the interference region of two divergent beams.
- Figure 14 gives a schematic representation of a thick chirped, switchable, pixelated hologram, produced with such a recording montage.
- Such a hologram has six electrodes E, a common ground CG, two glass plates G, and chirped strata P.
- the double arrows ⁇ , to ⁇ 6 represented in FIG. 14, incident on the six electrodes E, represent six lengths d waves of the DWDM comb supplying the equalizer of the invention, each having an impact point on a different pixel from the hologram H.
- Each of these wavelengths will therefore be reflected differently by the hologram H, as a function of the voltage applied to the electrode E corresponding to the point of impact of the wavelength ⁇ j .
- the fringes of a chirped network are slightly inclined with respect to each other.
- the angle between two adjacent fringes being less than 10 "3 degrees, we can consider, by approximation, that the fringes are parallel to each other, as developed by SM Schultz, EN Glytsis and TK Gaylord in” Design of a high-efficiency volume grating coupler for line focusing "(in French” Design of a high-efficiency volume network coupler for line focusing "), Applied Optics, 1998.
- This gradient of inclination of the fringes can be minimized by adding an angular multiplex to the spatial multiplex, that is to say by compensating for the spatial variation in wavelength along the direction of dispersion by a spatial variation of the angle of incidence of the wavelengths.
- Such compensation can be achieved by moving the multiplexing dispersive optical element D from the focal plane of the assembly 4f, as illustrated in the assemblies of FIGS. 4 and 5.
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- Electromagnetism (AREA)
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Abstract
Description
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Application Number | Priority Date | Filing Date | Title |
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FR0114756 | 2001-11-14 | ||
FR0114756A FR2832227B1 (fr) | 2001-11-14 | 2001-11-14 | Egaliseur spectral dynamique mettant en oeuvre un miroir holographique semi-transparent programmable |
PCT/FR2002/003906 WO2003043233A1 (fr) | 2001-11-14 | 2002-11-14 | Egaliseur spectral dynamique mettant en oeuvre un miroir holographique programmable |
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EP02793239A Withdrawn EP1444796A1 (fr) | 2001-11-14 | 2002-11-14 | Egaliseur spectral dynamique mettant en oeuvre un miroir holographique programmable |
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US (1) | US20050018960A1 (fr) |
EP (1) | EP1444796A1 (fr) |
FR (1) | FR2832227B1 (fr) |
WO (1) | WO2003043233A1 (fr) |
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AU2002950003A0 (en) * | 2002-07-05 | 2002-09-12 | Edith Cowan University | A multi-function opto-vlsi processor for intelligent optial networks |
AU2003304614A1 (en) * | 2003-11-17 | 2005-07-05 | France Telecom S.A. | Spectral equaliser with chromatic dispersion compensation |
WO2012037445A2 (fr) | 2010-09-17 | 2012-03-22 | Drexel University | Nouvelles applications pour carbone alliforme |
US9625878B2 (en) * | 2009-03-10 | 2017-04-18 | Drexel University | Dynamic time multiplexing fabrication of holographic polymer dispersed liquid crystals for increased wavelength sensitivity |
US8107167B2 (en) * | 2009-05-04 | 2012-01-31 | The Regents Of The University Of Michigan | Spatial-dispersion-free spectral combining of pulsed high peak power fiber laser beams |
US9752932B2 (en) | 2010-03-10 | 2017-09-05 | Drexel University | Tunable electro-optic filter stack |
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US4834474A (en) * | 1987-05-01 | 1989-05-30 | The University Of Rochester | Optical systems using volume holographic elements to provide arbitrary space-time characteristics, including frequency-and/or spatially-dependent delay lines, chirped pulse compressors, pulse hirpers, pulse shapers, and laser resonators |
GB9616598D0 (en) * | 1996-08-07 | 1996-09-25 | Univ Cambridge Tech | Active holographic spectral equalizer |
JP3196742B2 (ja) * | 1998-11-11 | 2001-08-06 | 日本電気株式会社 | 液晶光学素子及びその製造方法 |
JP2000275675A (ja) * | 1999-03-26 | 2000-10-06 | Nec Corp | 液晶光学素子およびその製造方法およびその駆動方法 |
US6538775B1 (en) * | 1999-09-16 | 2003-03-25 | Reveo, Inc. | Holographically-formed polymer dispersed liquid crystals with multiple gratings |
US6498872B2 (en) * | 2000-02-17 | 2002-12-24 | Jds Uniphase Inc. | Optical configuration for a dynamic gain equalizer and a configurable add/drop multiplexer |
US6678445B2 (en) * | 2000-12-04 | 2004-01-13 | Jds Uniphase Corporation | Dynamic gain flattening filter |
US20020109917A1 (en) * | 2001-02-14 | 2002-08-15 | Sagan Stephen F. | Polarization insensitive variable optical attenuator |
-
2001
- 2001-11-14 FR FR0114756A patent/FR2832227B1/fr not_active Expired - Fee Related
-
2002
- 2002-11-14 US US10/490,898 patent/US20050018960A1/en not_active Abandoned
- 2002-11-14 WO PCT/FR2002/003906 patent/WO2003043233A1/fr not_active Application Discontinuation
- 2002-11-14 EP EP02793239A patent/EP1444796A1/fr not_active Withdrawn
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WO2003043233A1 (fr) | 2003-05-22 |
FR2832227A1 (fr) | 2003-05-16 |
US20050018960A1 (en) | 2005-01-27 |
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