WO2003089969A2 - Waveguide optical filters with multiple spectral bands - Google Patents

Waveguide optical filters with multiple spectral bands Download PDF

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Publication number
WO2003089969A2
WO2003089969A2 PCT/CA2003/000561 CA0300561W WO03089969A2 WO 2003089969 A2 WO2003089969 A2 WO 2003089969A2 CA 0300561 W CA0300561 W CA 0300561W WO 03089969 A2 WO03089969 A2 WO 03089969A2
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Prior art keywords
filter
gratings
optical
coupled
grating
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PCT/CA2003/000561
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French (fr)
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WO2003089969A3 (en
Inventor
Sophie La Rochelle
Radan Slavik
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Universite Laval
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Publication of WO2003089969A2 publication Critical patent/WO2003089969A2/en
Publication of WO2003089969A3 publication Critical patent/WO2003089969A3/en

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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
    • G02B6/24—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
    • G02B6/29346—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 wave or beam interference
    • G02B6/29358—Multiple beam interferometer external to a light guide, e.g. Fabry-Pérot, etalon, VIPA plate, OTDL plate, continuous interferometer, parallel plate resonator
    • G02B6/29359—Cavity formed by light guide ends, e.g. fibre Fabry Pérot [FFP]
    • 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
    • G02B6/02—Optical fibres with cladding with or without a coating
    • G02B6/02057—Optical fibres with cladding with or without a coating comprising gratings
    • G02B6/02076—Refractive index modulation gratings, e.g. Bragg gratings
    • G02B6/0208—Refractive index modulation gratings, e.g. Bragg gratings characterised by their structure, wavelength response
    • G02B6/02085—Refractive index modulation gratings, e.g. Bragg gratings characterised by their structure, wavelength response characterised by the grating profile, e.g. chirped, apodised, tilted, helical
    • 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
    • G02B6/29316—Light guides comprising a diffractive element, e.g. grating in or on the light guide such that diffracted light is confined in the light guide
    • G02B6/29317—Light guides of the optical fibre type
    • G02B6/29319—With a cascade of diffractive elements or of diffraction operations
    • 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
    • G02B6/29346—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 wave or beam interference
    • G02B6/2935—Mach-Zehnder configuration, i.e. comprising separate splitting and combining means
    • G02B6/29352—Mach-Zehnder configuration, i.e. comprising separate splitting and combining means in a light guide
    • 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
    • G02B6/27—Optical coupling means with polarisation selective and adjusting means
    • G02B6/2706—Optical coupling means with polarisation selective and adjusting means as bulk elements, i.e. free space arrangements external to a light guide, e.g. polarising beam splitters
    • 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
    • G02B6/2938—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 for multiplexing or demultiplexing, i.e. combining or separating wavelengths, e.g. 1xN, NxM
    • G02B6/29386—Interleaving or deinterleaving, i.e. separating or mixing subsets of optical signals, e.g. combining even and odd channels into a single optical signal
    • 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
    • G02B6/29397—Polarisation insensitivity

Definitions

  • the present invention relates to optical devices and, more particularly, to optical filters with multiple spectral bands.
  • the optical filter includes a series of uniform gratings with phase shift to approximate linear chirped gratings.
  • This patent also teaches a series of in-line Fabry-Perot filters with a phase discontinuity between them to obtain a large number of narrower peaks in transmission. The resulting spectral response has non- uniform rejection and narrow stop-bands.
  • the aim of this approach is to form a phase-shifted (DFB) structures periodic in the spectral domain. The result of such an approach is even sharper transmission peaks.
  • AWGs Arrayed' Waveguide Gratings
  • TFF thin film filters
  • an optical filter with multiple spectral bands is realized in an optical waveguide by the superposition of chirped gratings slightly shifted along the propagation axis of the waveguide.
  • a Fabry-Perot filter can be realized with two chirped gratings but the spectral response of this type of device has severe limitations in its useable bandwidth. It is therefore not suitable for filtering optical signals that are not monochromatic. It is herein proposed to realize a filter using several superimposed chirped gratings (more than two) of different coupling strength to achieve the required performance, in terms of bandwidth and dispersion, for applications to high bit rate and spectrally dense optical communication systems.
  • This filter relies on multiple Fabry-Perot coupled cavities to tailor the filter response to given specifications.
  • the well known Fabry-Perot resonator can be used as a building structure, but is realized through the use of superimposed distributed reflectors rather than reflectors separated in space.
  • the multiple Fabry- Perot coupled resonators structure is then proposed in order to flatten the pass band of the single Fabry- Perot peaks.
  • the square shape, of the passband is obtained by controlling the strength of the various superimposed reflectors.
  • an optical broadband filter comprising at least three chirped gratings in an optical medium having a propagation axis, the chirped gratings being superimposed with a longitudinal shift therebetween along the propagation axis of the medium, thereby creating multiple coherent coupled-cavities providing for a . desired broadband spectral response.
  • a broadband optical filter having a desired spectral response comprising the steps of providing an optical medium, and providing multiple coherently coupled resonant cavities by superposing a plurality of chirped gratings in said medium with a selected longitudinal shift along a propagating axis, between the gratings.
  • the filter can be made either polarization insensitive or polarization selective.
  • apodized gratings are used to improve the uniformity of the device performance, more specifically in terms of its free spectral range (FSR) , amplitude shape and group delay ripples.
  • nonlinearly chirped gratings are used to compensate for the intrinsic dispersion of the waveguide thus further improving the FSR uniformity.
  • the non-linear chirp may be realized .by chirp in the grating period or by variations in the average refractive index along the linearly chirped grating.
  • This device can also be used for frequency referencing, as frequency selective element in fiber lasers or optical sources, as optical encoder/decoder, as filter or sensor in optical sensor system, etc...
  • the filter comprises successive Fabry-Perot filters with coherent coupling between them.
  • the filter can be provided in the form of coupled Fabry-Perot filters with no phase discontinuity between them.
  • the present invention is characterized by: 1) the use of superimposed chirped FBG to realize a periodic; and 2) the use of coupled FP cavities and to vary the strength of each reflector to obtain the desired shape (e.g. flat-top) for the transmission peaks.
  • the present invention has the following advantages:
  • the filter can be realized in optical waveguides, for example an optical fiber, it presents low loss when compared to other technologies .
  • the proposed filter is PMBF while still offering the possibility of designing shape and dispersion.
  • the proposed filter can be used in dense wavelength division multiplexed systems to separate adjacent channels (interleavers) with low-loss.
  • the filter can then be followed by existing technologies to separate individual channels.
  • the filter can be used as an all fiber/waveguide polarizer for which few alternatives exist.
  • Fig. 1 Two chirped gratings spatially separated by distance d and their superimposed product representing the required modulation of the waveguide effective index.
  • the grating period ⁇ z is wavelength of the reflected light, n ef is the guided mode effective refractive index and z is the propagation axis;
  • Fig. 2 Transmission of a fiber Fabry-Perot filter with two superimposed chirped gratings a) whole wavelength band and b) close-up on a few transmission peaks for the two principal modes of polarization.
  • the two gratings parameters were a 3.9 cm length, a 15.19 nm/cm Bragg wavelength chirped, a photo-induced index change of 1.54xl0 ⁇ 3 for the each FBG in the optical fiber core and longitudinal shift of 1 mm between the gratings;
  • Figure 3 Calculation of the transmitted passbands of a filter made with 5, 6 and 7 superimposed gratings corresponding to 4, 5 and 6 coupled cavities respectively.
  • A) transmission of the peaks B) detail showing also the variation in the group delay for 6 superimposed gratings and C) detail showing also the group delay for 7 superimposed gratings.
  • the required index modulation depth is (2.5, 4.5, 5.8, 5.8, 4.5, 2.5)xl0 ⁇ 4 for each grating in B) and (1.8, 3.5, 4.7, 5.0, 4.7, 3.5, 1.8)xl0 ⁇ 4 for each grating in C) .
  • Grating lengths of 7 cm, with a 3.625 nm/cm Bragg wavelength chirp and a 1mm longitudinal shift were considered;
  • Figure 4 Schematic illustration of an apodized grating
  • Figure 5 Polarization splitters with superimposed chirped gratings photo-induced in an optical fiber a) with photo-induced birefringence and b) using a polarization maintaining fiber with an elliptical core.
  • the trace in a) and the upper trace in b) are measured using a polarized source polarized predominantly along a principle axis of the filter, lower trace in b) is measured with dominantly unpolarized light source;
  • Fig. 6 is a schematic illustration of a two stage cascaded filter in accordance with a second embodiment of the present invention.
  • Fig. 7 is a graph of the amplitude and group delay of an experimentally realized cascaded filter. DESCRIPTION OF THE PREFERRED EMBODIMENTS
  • Interferometric Fabry-Perot devices are well known components that present a periodic spectral transmission function.
  • Single cavity Fabry-Perot is composed of two partially reflecting mirrors spaced by a given distance that will determine the spectral period. These devices typically present very narrow transmission peaks when good out-of-band isolation is required. To obtain square-like transmission peaks several cavities placed in series are required. For example, a device with three cavities will require four mirrors. Precise control of the length, and phase, of each cavity is required to obtain a good spectral response. Such multiple cavities are routinely used, for example, in thin-film filter design.
  • the present invention concerns the realization of coupled-cavity filters using superimposed chirped Bragg gratings.
  • the coupled- cavities can be implemented in a waveguide (optical fiber, planar waveguide, channel waveguide, photonic bandgap) by photosensitivity or etching; or in bulk materials by photosensitivity.
  • the paired lattices can be realized by using the methods typically used for uniting Bragg lattices.
  • the phase-mask method could be used.
  • the proposed implementation allow the realization of multi-channel filters covering a wide spectral range with transmission and dispersion characteristics that make them suitable for channel selection and interleaving in optical communication applications and sensor systems. To achieve the required performance, several cavities must be realized (at least two cavities which mean at least three superimposed gratings with well controlled coupling strengths and phase) .
  • an optical filter is made by the superposition of several chirped Bragg grating filters in an optical waveguide or photosensitive material with a longitudinal shift along the waveguide propagation axis, between the gratings.
  • Gratings refer to periodic modification of the effective index of a waveguide to create a frequency selective filter.
  • Chirped gratings refer to gratings with a period that varies along the propagation axis of the waveguide (Fig. 1) . The grating period at a given point determines the reflected wavelength or frequency of this segment.
  • the chirp of the grating period can be either continuous or discontinuous along the propagation axis.
  • the superposition of two chirped gratings displaced longitudinally along the propagation axis creates resonant cavities similar to Fabry-Perot structures.
  • the resulting filter therefore has multiple transmission peaks with frequency spacing between the peaks that is called the free spectral range (FSR) .
  • the FSR is determined by the amount of longitudinal shift between the grating portions having the same period, the effective index of the waveguide and the dispersion of the grating structure.
  • the width of the transmission peak and the extinction ratio is related to the grating coupling coefficient, a function of the effective index modulation and its overlap with the guided mode.
  • the finesse of the filter is defined as the spectral width of the transmitted peak divided by the spectral spacing of successive peaks.
  • Fabry-Perot filters have been realized in our laboratory in optical fiber with a finesse of up to 250, free spectral ranges close to 50 and 100 GHz and covering a spectral width of up to 60 nm.
  • Figure 2 shows the transmission of the filter.
  • the filter was made by writing of overlapping chirped Bragg grating in a photosensitive optical fiber exposed to the interference pattern of a UV laser. Similar gratings could be made in planar and channel waveguides. Fabrication techniques could include the realization of etched gratings through photolithographic techniques, index gratings induced through electro- optic or acousto-optic effect or by successive exposure of photosensitive material to an interference pattern or through a specifically designed phase-mask.
  • the transmission peaks of the filter presented in Figure 2 has a Lorentzian shape (sharp peak) that is detrimental to the transmission of optical signal or sources that are not monochromatic. Amplitude and group delay variations across the peak passband distort the transmitted signal. Furthermore, if the gratings have uniform coupling coefficient along the z-axis, variations of the device FSR are observed due to ripples in the amplitude and phase response of the chirped grating. Another cause of the FSR variation is the dispersion of the effective index of the waveguide that causes a slow drift of the FSR with optical frequency.
  • an optical filter is realized by using several (more than two) superimposed chirped Bragg gratings, with improved properties in the transmission and reflected bands.
  • the filter can be realized in bulk photosensitive materials or waveguides, including optical fiber or planar waveguides.
  • the filter characteristics display several peaks with improved uniformity of the FSR.
  • the superposition of a large number of gratings creates multiple coupled cavities.
  • the transmission peaks of the filter can then be tailored to meet specific requirements regarding filter bandwidth and dispersion by adjusting the coupling coefficient or index change associated with each grating. Examples of the calculated responses in transmission for such filters are shown in Figure 3, similar characteristics are obtained for the reflected bands .
  • Apodization refers to any method employed to modify the coupling coefficient of the grating along the propagation axis. Typically, the coupling coefficient will be maximum near the center of the grating and will decrease gradually towards the edge of the grating as shown in Figure 4. Variations of the FSR related to the waveguide dispersion could be compensated by using gratings with nonlinear chirp so that the cavity length varies with wavelength. The resulting filter could be used both in transmission and reflection, for example as interleavers in optical communication systems.
  • Polarization splitters for optical communications systems or fiber lasers or fiber sensor system. Filters for frequency referencing.
  • Optical frequency encoders/decoders for optical communications systems, for example code division multiplexed systems, or for fiber sensor systems.
  • FP lattice coupled cavities
  • IIR infinite response filter
  • any finite input response (FIR) filter has no inherent group delay ripple and good stopband isolation, but need a large number of stages to achieve an acceptable passband transmission shape.
  • the proposed structure is a cascaded filter composed of generally lattice FIR filter (for example a Mach-Zehnder filter) and the lattice coupled FP filter.
  • the FIR filter ensures good stopband isolation whereas the lattice coupled FP filter ensures a square-like passband.
  • An example of an experimentally realized interleaver for 50 GHz DWDM is shown in Fig. 7.

Abstract

A coupled-cavities filter made by the superposition of several chirped Bragg grating filters in a photosensitive material or waveguide, the chirped Bragg gratings being longitudinal shifted along the light propagation axis.

Description

WAVEGUIDE OPTICAL FILTERS WITH MULTIPLE SPECTRAL BANDS
BACKGROUND OF THE INVENTION Field of the Invention
The present invention relates to optical devices and, more particularly, to optical filters with multiple spectral bands. Description of the Prior Art United States Patent No. 6,198,863 issued on
March 6, 2001, to Lealman et al . discloses an optical filter in the form of a well known Fabry-Perot resonator, but realized with distributed reflectors
(Bragg gratings) separated in a waveguide region. The optical filter includes a series of uniform gratings with phase shift to approximate linear chirped gratings. This patent also teaches a series of in-line Fabry-Perot filters with a phase discontinuity between them to obtain a large number of narrower peaks in transmission. The resulting spectral response has non- uniform rejection and narrow stop-bands. The aim of this approach is to form a phase-shifted (DFB) structures periodic in the spectral domain. The result of such an approach is even sharper transmission peaks.
Other technologies that offer optical filters used as multiplexers/demultiplexers includes Arrayed' Waveguide Gratings (AWGs) and thin film filters (TFF) . The performance of both technologies is limited for close channel spacing. AWGs can be used as periodic multiple-band filters (PMBF) but they suffer from high throughput loss and cross-talk. The fabrication of these devices is complicated further by thermal and polarization sensitivity that require compensation techniques. Thin film filters (TFF) offer the possibility to be optimized in terms of shape and dispersion, but the complicated designs, requiring a large number of layers, are not practical for close channel spacing. TFF are not appropriate as PMBFs. SUMMARY OF THE INVENTION
It is therefore an aim of the present invention to provide an optical filter adapted to provide spectrally flatter and steeper peaks in transmission. It is also an aim of the present invention to provide a filter which is optimized in terms of bandwidth and dispersion.
It is a further aim of the present invention to provide an optical filter having a relatively large number of transmission peaks with a relatively uniform and wide stop band.
It is a further aim of the present invention to optimize both the amplitude, the dispersion and rejection characteristics of filters in order to satisfy the requirement of optical filters for WDM applications .
Therefore, in accordance with the present invention, there is provided an optical filter with multiple spectral bands. The filter is realized in an optical waveguide by the superposition of chirped gratings slightly shifted along the propagation axis of the waveguide. A Fabry-Perot filter can be realized with two chirped gratings but the spectral response of this type of device has severe limitations in its useable bandwidth. It is therefore not suitable for filtering optical signals that are not monochromatic. It is herein proposed to realize a filter using several superimposed chirped gratings (more than two) of different coupling strength to achieve the required performance, in terms of bandwidth and dispersion, for applications to high bit rate and spectrally dense optical communication systems. This filter relies on multiple Fabry-Perot coupled cavities to tailor the filter response to given specifications. The well known Fabry-Perot resonator can be used as a building structure, but is realized through the use of superimposed distributed reflectors rather than reflectors separated in space. The multiple Fabry- Perot coupled resonators structure is then proposed in order to flatten the pass band of the single Fabry- Perot peaks. The square shape, of the passband is obtained by controlling the strength of the various superimposed reflectors.
In accordance with another general aspect of the present invention, there is provided an optical broadband filter comprising at least three chirped gratings in an optical medium having a propagation axis, the chirped gratings being superimposed with a longitudinal shift therebetween along the propagation axis of the medium, thereby creating multiple coherent coupled-cavities providing for a . desired broadband spectral response.
In accordance with a further general aspect of the present invention, there is provided method of providing a broadband optical filter having a desired spectral response, the method comprising the steps of providing an optical medium, and providing multiple coherently coupled resonant cavities by superposing a plurality of chirped gratings in said medium with a selected longitudinal shift along a propagating axis, between the gratings.
According to a further general aspect of the present invention, the filter can be made either polarization insensitive or polarization selective. According to a still further general aspect of the present invention, apodized gratings are used to improve the uniformity of the device performance, more specifically in terms of its free spectral range (FSR) , amplitude shape and group delay ripples.
According to a still further general aspect of the present invention, nonlinearly chirped gratings are used to compensate for the intrinsic dispersion of the waveguide thus further improving the FSR uniformity. The non-linear chirp may be realized .by chirp in the grating period or by variations in the average refractive index along the linearly chirped grating. This device can also be used for frequency referencing, as frequency selective element in fiber lasers or optical sources, as optical encoder/decoder, as filter or sensor in optical sensor system, etc...
According to another general aspect of the present invention, the filter comprises successive Fabry-Perot filters with coherent coupling between them. The filter can be provided in the form of coupled Fabry-Perot filters with no phase discontinuity between them.
In summary, the present invention is characterized by: 1) the use of superimposed chirped FBG to realize a periodic; and 2) the use of coupled FP cavities and to vary the strength of each reflector to obtain the desired shape (e.g. flat-top) for the transmission peaks.
As will be seen hereinafter, the present invention has the following advantages:
Since the filter can be realized in optical waveguides, for example an optical fiber, it presents low loss when compared to other technologies .
The proposed filter is PMBF while still offering the possibility of designing shape and dispersion. - The proposed filter can be used in dense wavelength division multiplexed systems to separate adjacent channels (interleavers) with low-loss. The filter can then be followed by existing technologies to separate individual channels.
The improved performance in terms of FSR uniformity makes it a promising technology for filters in superfluorescent source or laser. In a polarization selective configuration, the filter can be used as an all fiber/waveguide polarizer for which few alternatives exist. BRIEF DESCRIPTION OF THE DRAWINGS
Having thus generally described the nature of the invention, reference will now be made to the accompanying drawings, showing by way of illustration a preferred embodiment thereof, and in which:
Fig. 1: Two chirped gratings spatially separated by distance d and their superimposed product representing the required modulation of the waveguide effective index. The grating period
Figure imgf000007_0001
λz is wavelength of the reflected light, nef is the guided mode effective refractive index and z is the propagation axis;
Fig. 2: Transmission of a fiber Fabry-Perot filter with two superimposed chirped gratings a) whole wavelength band and b) close-up on a few transmission peaks for the two principal modes of polarization. The two gratings parameters were a 3.9 cm length, a 15.19 nm/cm Bragg wavelength chirped, a photo-induced index change of 1.54xl0~3 for the each FBG in the optical fiber core and longitudinal shift of 1 mm between the gratings;
Figure 3 : Calculation of the transmitted passbands of a filter made with 5, 6 and 7 superimposed gratings corresponding to 4, 5 and 6 coupled cavities respectively. A) transmission of the peaks B) detail showing also the variation in the group delay for 6 superimposed gratings and C) detail showing also the group delay for 7 superimposed gratings. If the gratings are realized in an optical fiber, the required index modulation depth is (2.5, 4.5, 5.8, 5.8, 4.5, 2.5)xl0~4 for each grating in B) and (1.8, 3.5, 4.7, 5.0, 4.7, 3.5, 1.8)xl0~4 for each grating in C) . Grating lengths of 7 cm, with a 3.625 nm/cm Bragg wavelength chirp and a 1mm longitudinal shift were considered;
Figure 4 : Schematic illustration of an apodized grating; Figure 5 : Polarization splitters with superimposed chirped gratings photo-induced in an optical fiber a) with photo-induced birefringence and b) using a polarization maintaining fiber with an elliptical core. The trace in a) and the upper trace in b) are measured using a polarized source polarized predominantly along a principle axis of the filter, lower trace in b) is measured with dominantly unpolarized light source;
Fig. 6 is a schematic illustration of a two stage cascaded filter in accordance with a second embodiment of the present invention; and
Fig. 7 is a graph of the amplitude and group delay of an experimentally realized cascaded filter. DESCRIPTION OF THE PREFERRED EMBODIMENTS
Interferometric Fabry-Perot devices are well known components that present a periodic spectral transmission function. Single cavity Fabry-Perot is composed of two partially reflecting mirrors spaced by a given distance that will determine the spectral period. These devices typically present very narrow transmission peaks when good out-of-band isolation is required. To obtain square-like transmission peaks several cavities placed in series are required. For example, a device with three cavities will require four mirrors. Precise control of the length, and phase, of each cavity is required to obtain a good spectral response. Such multiple cavities are routinely used, for example, in thin-film filter design.
The present invention concerns the realization of coupled-cavity filters using superimposed chirped Bragg gratings. The coupled- cavities can be implemented in a waveguide (optical fiber, planar waveguide, channel waveguide, photonic bandgap) by photosensitivity or etching; or in bulk materials by photosensitivity. In the case of UV photo-exposition, the paired lattices can be realized by using the methods typically used for uniting Bragg lattices. Alternatively, the phase-mask method could be used. The proposed implementation allow the realization of multi-channel filters covering a wide spectral range with transmission and dispersion characteristics that make them suitable for channel selection and interleaving in optical communication applications and sensor systems. To achieve the required performance, several cavities must be realized (at least two cavities which mean at least three superimposed gratings with well controlled coupling strengths and phase) .
In accordance with the present invention, an optical filter is made by the superposition of several chirped Bragg grating filters in an optical waveguide or photosensitive material with a longitudinal shift along the waveguide propagation axis, between the gratings. Gratings refer to periodic modification of the effective index of a waveguide to create a frequency selective filter. Chirped gratings refer to gratings with a period that varies along the propagation axis of the waveguide (Fig. 1) . The grating period at a given point determines the reflected wavelength or frequency of this segment. The chirp of the grating period can be either continuous or discontinuous along the propagation axis. The superposition of two chirped gratings displaced longitudinally along the propagation axis creates resonant cavities similar to Fabry-Perot structures. The resulting filter therefore has multiple transmission peaks with frequency spacing between the peaks that is called the free spectral range (FSR) . The FSR is determined by the amount of longitudinal shift between the grating portions having the same period, the effective index of the waveguide and the dispersion of the grating structure. The width of the transmission peak and the extinction ratio is related to the grating coupling coefficient, a function of the effective index modulation and its overlap with the guided mode. The finesse of the filter is defined as the spectral width of the transmitted peak divided by the spectral spacing of successive peaks. Fabry-Perot filters have been realized in our laboratory in optical fiber with a finesse of up to 250, free spectral ranges close to 50 and 100 GHz and covering a spectral width of up to 60 nm. Figure 2 shows the transmission of the filter. The filter was made by writing of overlapping chirped Bragg grating in a photosensitive optical fiber exposed to the interference pattern of a UV laser. Similar gratings could be made in planar and channel waveguides. Fabrication techniques could include the realization of etched gratings through photolithographic techniques, index gratings induced through electro- optic or acousto-optic effect or by successive exposure of photosensitive material to an interference pattern or through a specifically designed phase-mask.
The transmission peaks of the filter presented in Figure 2 has a Lorentzian shape (sharp peak) that is detrimental to the transmission of optical signal or sources that are not monochromatic. Amplitude and group delay variations across the peak passband distort the transmitted signal. Furthermore, if the gratings have uniform coupling coefficient along the z-axis, variations of the device FSR are observed due to ripples in the amplitude and phase response of the chirped grating. Another cause of the FSR variation is the dispersion of the effective index of the waveguide that causes a slow drift of the FSR with optical frequency.
According to a preferred embodiment of the present invention, an optical filter is realized by using several (more than two) superimposed chirped Bragg gratings, with improved properties in the transmission and reflected bands. The filter can be realized in bulk photosensitive materials or waveguides, including optical fiber or planar waveguides. The filter characteristics display several peaks with improved uniformity of the FSR. The superposition of a large number of gratings creates multiple coupled cavities. The transmission peaks of the filter can then be tailored to meet specific requirements regarding filter bandwidth and dispersion by adjusting the coupling coefficient or index change associated with each grating. Examples of the calculated responses in transmission for such filters are shown in Figure 3, similar characteristics are obtained for the reflected bands . To improve the spectral uniformity of the filter, we propose the use of apodized gratings and nonlinear chirped gratings to compensate for the dispersion. Apodization refers to any method employed to modify the coupling coefficient of the grating along the propagation axis. Typically, the coupling coefficient will be maximum near the center of the grating and will decrease gradually towards the edge of the grating as shown in Figure 4. Variations of the FSR related to the waveguide dispersion could be compensated by using gratings with nonlinear chirp so that the cavity length varies with wavelength. The resulting filter could be used both in transmission and reflection, for example as interleavers in optical communication systems. It is also contemplated to realize polarization-splitting filters using superimposed chirped gratings (two or more) . If the gratings are written in a birefringent waveguide or if the waveguide birefringence is subsequently tuned following the realization of the gratings, it becomes possible to select or separate orthogonal polarizations (Figure 5) . Several Fabry-Perot filters with multiple peaks have been realized experimentally. The filters covered a bandwidth of up to 60 nm with peaks spaced by 50 and 100 GHz and with a finesse of up to 250. Numerical simulations have been performed to calculate the spectral response of filters with multiple cavities. The results show that the required coupling coefficients (or index changes) are compatible with current technologies like for example, the exposure a photosensitive optical fiber to UV radiation. The features of the present invention could be used in:
Filters for WDM and DWDM optical communication systems or fiber sensor systems or fiber lasers or other optical sources. - Interleavers for WDM and DWDM optical communication systems.
Filters for 2.5 Gbit/s and 160 Gbit/s optical communication systems.
Polarization splitters for optical communications systems or fiber lasers or fiber sensor system. Filters for frequency referencing.
Optical frequency encoders/decoders for optical communications systems, for example code division multiplexed systems, or for fiber sensor systems. Although the proposed lattice coupled cavities FP (Fabry-Perot) filter can have a near- square amplitude response, even with a very small number of stages (which is typical for any infinite response filter (IIR) ) , it may suffer from low isolation in the stop-band and group delay variations in the passband. On the contrary, any finite input response (FIR) filter has no inherent group delay ripple and good stopband isolation, but need a large number of stages to achieve an acceptable passband transmission shape.
According to a further aspect of the present invention, it is proposed to combine the above- described coupled-cavity filter with another filter in order to improve its characteristics (see Fig. 6) . The proposed structure is a cascaded filter composed of generally lattice FIR filter (for exemple a Mach-Zehnder filter) and the lattice coupled FP filter. The FIR filter ensures good stopband isolation whereas the lattice coupled FP filter ensures a square-like passband. An example of an experimentally realized interleaver for 50 GHz DWDM is shown in Fig. 7.

Claims

CLAIMS :
1. An optical broadband filter comprising at least three chirped gratings provided an optical broadband filter comprising at least three chirped gratings in an optical medium having a propagation axis, the chirped gratings being superimposed with a longitudinal shift therebetween along the propagation axis of the medium, thereby creating multiple coherent coupled-cavities providing for a desired broadband spectral response.
2. An optical coupled-cavity filter as defined in claim 1, wherein said coherent coupled-cavities provide for spectrally flat and steep peaks in transmission.
3. An optical coupled-cavity filter as defined in claim 1, wherein said chirped gratings have different coupling strengths selected to obtain flat top broadband transmission peaks.
4. An optical coupled-cavity filter as defined in claim 1, wherein at least some of the gratings have non-linear chirps to compensate for dispersion.
5. An optical coupled-cavity filter as defined in claim 4, wherein the non-linear chirps are realized by chirps in the grating period.
6. An optical coupled-cavity filter as defined in claim 4, wherein the non-linear chirps are realized by variations in the average refractive index of the device along linearly chirped gratings.
7. An optical coupled-cavity filter as defined in claim 1, wherein said chirped gratings include apodize gratings in order to improve the spectral uniformity of the filter.
8. An optical coupled-cavity' filter as defined in claim 4, wherein the chirps of the gratings are continuous along the propagation axis.
9. A multi-stage cascaded or lattice filter comprising a finite input response (FIR) filter combined in cascade with a coupled-cavity filter as defined in claim 1.
10. A multi-stage cascaded or lattice filter as defined in claim 9, wherein said FIR filter includes a Mach-Zehnder filter.
11. A method of providing a broadband optical filter having a desired spectral response, the method comprising the steps of providing an optical medium, and providing multiple coherently coupled resonant cavities by superposing a plurality of chirped gratings in said medium with a selected longitudinal shift along a propagating axis, between the gratings.
12. A method as defined in claim 11, further comprising the step of selecting the strength and the waveguide characteristics of each grating.
13. A method as defined in claim 12, comprising the step of selecting the longitudinal shift between the gratings, the strength and the waveguide characteristics of each grating so as to obtain a filter having spectrally flat and steep peaks in transmission.
14. A method as defined in claim 12, wherein the optical filter has multiple transmission peaks with frequency spacing between peaks, said frequency spacing being termed free spectral range (FSR) , and wherein the method further comprises the step of adjusting the amount of longitudinal shift between grating portions of a same period in accordance with a desired FSR.
15. A method as defined in claim 11, wherein each grating has a coupling coefficient, and wherein the method further comprises the step of tailoring the' transmission peak of the filter to meet specific requirements regarding filter bandwidth and dispersion by adjusting the coupling coefficient associated with each grating.
16. A method as defined in claim 11, further comprising the step of introducing apodization of each chirped grating.
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