WO2004099837A1 - Compensation of group delay dispersion in a switchable add-drop filter. - Google Patents

Compensation of group delay dispersion in a switchable add-drop filter. Download PDF

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Publication number
WO2004099837A1
WO2004099837A1 PCT/SE2004/000602 SE2004000602W WO2004099837A1 WO 2004099837 A1 WO2004099837 A1 WO 2004099837A1 SE 2004000602 W SE2004000602 W SE 2004000602W WO 2004099837 A1 WO2004099837 A1 WO 2004099837A1
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Prior art keywords
main filter
compensator
group delay
filter
delay dispersion
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PCT/SE2004/000602
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French (fr)
Inventor
Marcin Swillo
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Phoxtal Communications Ab
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Publication of WO2004099837A1 publication Critical patent/WO2004099837A1/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
    • G02B6/2935—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
    • G02B6/29353—Mach-Zehnder configuration, i.e. comprising separate splitting and combining means in a light guide with a wavelength selective element in at least one light guide interferometer arm, e.g. grating, interference filter, resonator
    • 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
    • G02B6/29355—Cascade arrangement of interferometers
    • 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
    • G02B6/29382—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 including at least adding or dropping a signal, i.e. passing the majority of signals
    • G02B6/29383—Adding and dropping
    • 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
    • G02B6/29392—Controlling dispersion
    • G02B6/29394—Compensating wavelength dispersion
    • 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
    • H04B10/2513—Arrangements specific to fibre transmission for the reduction or elimination of distortion or dispersion due to chromatic dispersion
    • H04B10/2519—Arrangements specific to fibre transmission for the reduction or elimination of distortion or dispersion due to chromatic dispersion using Bragg gratings
    • 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
    • G02B6/29398—Temperature insensitivity
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04J—MULTIPLEX COMMUNICATION
    • H04J14/00—Optical multiplex systems
    • H04J14/02—Wavelength-division multiplex systems
    • H04J14/0201—Add-and-drop multiplexing
    • H04J14/0202—Arrangements therefor
    • H04J14/0204—Broadcast and select arrangements, e.g. with an optical splitter at the input before adding or dropping

Definitions

  • the present invention refers to a method and a device for Compensation of group delay dispersion in a switchable add- drop filter.
  • a switchable optical add drop filter consists of passive filters and a 2x2 thermal switch.
  • the passive filters is based on an unbalanced Mach-Zender interferometer with a phase difference of JI and two identical longitudinal Bragg- • gratings. Each grating is placed in the respective arms of the MZ interferometer.
  • the add drop filter is used to pass light of a certain frequencies, to catch a certain frequency and add or to drop frequencies. The add drop filter will described in more detail below.
  • Such add drop filters may be used in tele- and data communication systems.
  • Optical communications based on high bit rate transmission needs compensation for chromatic dispersion.
  • the chromatic dispersion results in broadening of optical pulses, which finally cause neighbouring bits to overlap.
  • the present invention solves the problem by creating such compensation.
  • the present invention refers to a method for compensation of group delay dispersion in an add-drop filter comprising a passive main filter comprising a Mach-Zender interferometer with two identical Bragg gratings placed in the 'respective arms of the Mac-Zender interferometer, and is characterized in, that after the main filter, in the travelling direction of introduced light, group delay dispersion from the main filter is compensated for by a compensator in the form of a reflector for light leaving the main filter, where the band- width of reflection of the compensator is larger than that of the main filter and where the reflection spectrum of the compensator is substantially 100% for the wavelength range defined by the main filter.
  • the invention relates to a device of the kind and with the features as stated in claim 5.
  • FIG. 1 shows an optical filter based on a Mach-Zender interferometer and two Bragg gratings according to prior art
  • FIG. 2 shows a switchable optical add-drop filter with a thermal switch according to prior art
  • FIG. 4a and 4b shows an example of reflection and group delay spectrum for single Bragg grating waveguide, where 4a is a narrow band filter and 4b is a wide band filter
  • FIG. 5 shows a switchable add-drop filter with a 2x2 switch according to prior art
  • FIG. 6 shows an add-drop filter with a device for compensation of group delay dispersion in the switchable add-drop filter according to the present invention
  • an optical filter which consists of a passive filter 1.
  • the passive filter is based on an unbalanced Mach-Zender interferometer 2 with a phase difference of Ji and two identical longitudinal Bragg-gratings 3, 4. Each grating is placed in the respective arms of the MZ interferometer.
  • the MZ interferometer is created by two identical evanescent couplers 5, 6 with single mode waveguides. The JI phase shift and the Bragg gratings are written by UV exposure .
  • the structures are fabricated with silica-on-silicon planar technology by using Plasma Enhaced Chemical Vapor Deposition (PECVD) for deposition of thin films and then by lithografi- cally pattering the surface to be eventually etched.
  • PECVD Plasma Enhaced Chemical Vapor Deposition
  • a low temperature process results in fabrication of unannealed low loss channel waveguides devices in temperatures below 300 °C with losses of about 0.1 dB/cm at 1.55 ⁇ m.
  • the longitudinal Bragg gratings are photo-induced into the core material by UV illumination through a phase mask with a period of about 1 ⁇ m and surpressed zero- and higher than the first diffraction orders.
  • a 5 mm long grating filter exposed with a total energy flow of about 3 KJ/cm 2 exhibits a -3dB bandwidth of 0.2 nm with a change of the refractive index of 2x10 "3 .
  • the phase shift equal to JI, for the light guided in single mode waveguide fabricated according to the this process, can be obtained by a photo induced change of the refractive index 2x10 "3 on the length of a waveguide of 375 ⁇ m.
  • the couplers have a splitting ratio of 50/50 for the operating wavelength X x .
  • both Bragg gratings have a maximum reflection and the guided light, whicjh is coming from the input port 7, is reflected to Port2 8.
  • thermal 2x2 thermal switch 9 To make a switchable add-drop filter, shown in figure 2, based on the MZ interferometer described there is a thermal 2x2 thermal switch 9. In figure 2 there is a JI phase shift in one arm. This gives an unbalanced MZ interferometer. However, one may well make an add-drop filter without the JI phase shift.
  • the switching mechanism is based on a MZ interferometer 10 with a heating plate 11 close to one arm of the MZ interferometer. The temperature of the plate 11 is con- trollede by current flow through the plate. A thermally induced change of the refractive index causes the change of phase of the guided light.
  • the full switching of light coming from the input port 12 from being guided to the drop port 13 instead of to the output port 14 is obtained when the phase difference in the two arms of the MZ interferometer 10 is JI.
  • the light with the wavelength ⁇ i incoming from input port 12 is reflected by the filter, then passes through a reflrctor 15 and is transmitted to the 2x2 thermal switch 10.
  • the light is either transmit- ted directly to the drop port 13 or passes through a reflector 16 and thereby back to the filter 1 and to the output port 14.
  • incoming light from add port 17 with the same wavelength can be added to output port 14.
  • the switchable optical add-drop filter enable exchanging of optical signals having the same wavelength.
  • the Bragg gratings of the reflectors 15, 16 are made as described above.
  • V group (d ⁇ I d ⁇ ) ⁇ ⁇ , if the width of frequency band of the
  • v group z/ t gr oupr where z is a propagation distance and t groUp is a group delay.
  • the group delay is responsible for delay time of the optical pulse passing through the waveguide device. This group delay caused by an optical device is given by:
  • the key parameter responsible for changing the shape of an optical pulse is the first derivative of the group delay in the frequency domain. This parameter is called group delay dispersion (GDD) and can be presented in the following form:
  • the group delay is significantly changing within the frequency band of the pulse, then the group delay should be compensated for each frequency within the band of the pulse. Then the optical pulse passing through the waveguide device will not be distorted.
  • Figure 4b illustrates the group delay for reflected light for waveguide with a Bragg grating filter for a much wider bandwidth. As one can see, by comparison the figure 4a and 4b, the changes of group delay in the center region of the reflected band are smaller, if the filter 1 has wider bandwidth.
  • the add-drop filter presented in figure 5 consists of three identical filters: one main filter 1 and two reflectors 15, 16. These two reflectors are used in order to avoid waveguide bends, which would have increased the total size of the de- w vice. Since, the device presented in figure 5 thus contains three filters, the signal which is transmitted to the output port is reflected four times from these gratings. In this way, the group delay dispersion for this device is four times higher comparing to single reflection from grating. Addition-
  • the add-drop filter switch 10 is design to work in optical network, where more than one identical filter switch with the same operating channel is already connected. It means that the total group delay will be proportional to the number of identical add-drop filter switches connected to the 0 same optical node. If this system is used in high bit rate transmission, should exhibit constant group delay (zero group delay dispersion) in the desired wavelength range, due to fluctuation of the operating wavelength or differences of light wavelengths emitted by different sources. 5
  • the total group delay dispersion • in the device presented in figure 5 can be significantly decreased if a first and second reflector acts as a group delay dispersion compensator 18, 19, please see figure 6.
  • the add-drop filter is a switchable add-drop filter having an input port 12, an output port 14, an add port 17 and a drop port 13, a main filter 1 comprising one Mach Zender interferometer and comprising a 2X2 switch or a thermal 2X2 switch 10, where the respective ends of the interferometer and of the switch are connected to 50/50 couplers, in that a first compensator 18 is located between the main filter and a first end of the switch and where a second compensator 19 is located between the main filter and a second end of the switch, where the compensators 18,19 are arranged to compensate for the group delay dispersion of both ends of the main filter 1.
  • the group delay compensators 18, 19 are working only for the desired wavelength range, which is previously reflected from the main filter 1. In this way, the total bandwidth of these two components 18, 19 is determined by the main filter 1, whereas the total group delay is a sum of group delays of separate components. It means that the bandwidth of reflection for the compensator can be larger than that of the main filter, and the reflection spectrum for the compensator has to be nearly 100% only for the wavelength range corresponding to main filter. The flattening of group delay is possible if the group delay dispersions in the compensator and main filter have opposite signs. According to the present solution the compensator can be realized by the superposition of two or more Bragg gratings.
  • the compensator is made by superposition of Bragg gratings with central wavelength: ⁇ 0 + ⁇ and ⁇ 0 - ⁇ if only two Bragg gratings are used.
  • the two, total length of the grating and ⁇ depends o the grating strength.
  • Figure 7 shows the example of final shape of the grating.
  • FIG. 7 The dashed line in figure 7 represents the apodization, which was made in gratings ⁇ 0 + ⁇ and ⁇ 0 - ⁇ before superposition.
  • Figure 8 shows the reflection and group delay spectrum for a compensator created from two gratings: ⁇ 0 + ⁇ and ⁇ 0 - ⁇ .
  • the group delay for the compensator and the main filter has opposite dispersions in the central part of reflection band, whereas the reflection bandwidth of the compensator is much wider than for the main filter.
  • the final group delay presented in figure 9 is calculated as a sum of group delay for the main filter and the compensator.
  • the group delay for the main filter and the group delay for the compensator together forms the final group delay according to the solid curve. This gives a GDD of substantially zero around the center frequency, thus giving that a full compensation has been achieved.
  • the group delay in the desired wavelength range is constant (zero group delay dispersion) . If the switch is passing the light the two output ports 13, 14, or the new optical signal is adding from the add port 17 to output port 14, the light is passing through the second compensator 19 and also is reflected from the main filter 1. Due to the symmetry of the main filter, the group delay of the second compensator 19 has to be the same as for the first one 18.
  • the second compensator is introducing a group delay which is compensated by the second reflection from the main filter. This will give zero group delay dispersion for the desired wavelength range reflected from the main filter if the light is transmitted from input to output port. For a wavelength range out of the reflection band of the main filter, the dispersion introduced by main grating is negligible and doesn't need any compensation.
  • optical filters have been de- scribed.
  • the present invention can be used in any configuration of add-drop filters where the group delay dispersion in a Bragg grating can be compensated for by a compensator according to the invention.

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Optics & Photonics (AREA)
  • Chemical & Material Sciences (AREA)
  • Dispersion Chemistry (AREA)
  • Electromagnetism (AREA)
  • Engineering & Computer Science (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Signal Processing (AREA)
  • Optical Communication System (AREA)
  • Optical Modulation, Optical Deflection, Nonlinear Optics, Optical Demodulation, Optical Logic Elements (AREA)
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Abstract

Method for compensation of group delay dispersion in an add-drop filter comprising a passive main filter (1) comprising a Mach-Zender interferometer with two identical Bragg gratings (3,4) placed in the respective arms of the Mac-Zender interferometer. The invention is characterized in, that after the main filter (1), in the travelling direction of introduced light, group delay dispersion from the main filter is compensated for by a compensator (18;19) in the form of a reflector for light leaving the main filter, where the bandwidth of reflection of the compensator is larger than that of the main filter and where the reflection spectrum of the compensator (18,19) is substantially 100% for the wavelength range defined by the main filter (1).

Description

Compensation of group delay dispersion in a s itchable add- drop filter.
The present invention refers to a method and a device for Compensation of group delay dispersion in a switchable add- drop filter.
A switchable optical add drop filter consists of passive filters and a 2x2 thermal switch. The passive filters is based on an unbalanced Mach-Zender interferometer with a phase difference of JI and two identical longitudinal Bragg- • gratings. Each grating is placed in the respective arms of the MZ interferometer. The add drop filter is used to pass light of a certain frequencies, to catch a certain frequency and add or to drop frequencies. The add drop filter will described in more detail below.
Such add drop filters may be used in tele- and data communication systems.
Optical communications based on high bit rate transmission needs compensation for chromatic dispersion. The chromatic dispersion results in broadening of optical pulses, which finally cause neighbouring bits to overlap.
Without compensation the quality of transmitted signal deteriorates and can not be detected properly.
The present invention solves the problem by creating such compensation. The present invention refers to a method for compensation of group delay dispersion in an add-drop filter comprising a passive main filter comprising a Mach-Zender interferometer with two identical Bragg gratings placed in the 'respective arms of the Mac-Zender interferometer, and is characterized in, that after the main filter, in the travelling direction of introduced light, group delay dispersion from the main filter is compensated for by a compensator in the form of a reflector for light leaving the main filter, where the band- width of reflection of the compensator is larger than that of the main filter and where the reflection spectrum of the compensator is substantially 100% for the wavelength range defined by the main filter.
Further, the invention relates to a device of the kind and with the features as stated in claim 5.
In the following the present invention is described in more detail, partly in connection with an embodiment of the inven- tion together with the attached drawings, where
- Figure 1 shows an optical filter based on a Mach-Zender interferometer and two Bragg gratings according to prior art
- Figure 2 shows a switchable optical add-drop filter with a thermal switch according to prior art
- Figure 3 shows the effect of broadening the optical pulse passing through the waveguide device
- Figure 4a and 4b shows an example of reflection and group delay spectrum for single Bragg grating waveguide, where 4a is a narrow band filter and 4b is a wide band filter
- Figure 5 shows a switchable add-drop filter with a 2x2 switch according to prior art - Figure 6 shows an add-drop filter with a device for compensation of group delay dispersion in the switchable add-drop filter according to the present invention
- Figure 7 "gives an example of refractive index distribution along the wavegiude for the group delay compensator
- Figure 8 shows reflection and group delay spectrum for a compensator containing two superimposed gratings
- Figure 9 shows group delay and reflection spectrum of the main filter plus group delay for the compensator.
In figure 1 an optical filter is shown, which consists of a passive filter 1. The passive filter is based on an unbalanced Mach-Zender interferometer 2 with a phase difference of Ji and two identical longitudinal Bragg-gratings 3, 4. Each grating is placed in the respective arms of the MZ interferometer. The MZ interferometer is created by two identical evanescent couplers 5, 6 with single mode waveguides. The JI phase shift and the Bragg gratings are written by UV exposure .
The structures are fabricated with silica-on-silicon planar technology by using Plasma Enhaced Chemical Vapor Deposition (PECVD) for deposition of thin films and then by lithografi- cally pattering the surface to be eventually etched. A low temperature process results in fabrication of unannealed low loss channel waveguides devices in temperatures below 300 °C with losses of about 0.1 dB/cm at 1.55 μm. SiH4 and N2Oare used as main precursors for the deposition 'of undoped silica layers and GeH is used for doping the core material to in- crease its refractive index and increase its UV photosensi- tivity . The longitudinal Bragg gratings are photo-induced into the core material by UV illumination through a phase mask with a period of about 1 μm and surpressed zero- and higher than the first diffraction orders. As an example, a 5 mm long grating filter exposed with a total energy flow of about 3 KJ/cm2 exhibits a -3dB bandwidth of 0.2 nm with a change of the refractive index of 2x10"3 . The phase shift equal to JI, for the light guided in single mode waveguide fabricated according to the this process, can be obtained by a photo induced change of the refractive index 2x10"3 on the length of a waveguide of 375 μm.
The couplers have a splitting ratio of 50/50 for the operating wavelength Xx . For this wavwlength both Bragg gratings have a maximum reflection and the guided light, whicjh is coming from the input port 7, is reflected to Port2 8.
To make a switchable add-drop filter, shown in figure 2, based on the MZ interferometer described there is a thermal 2x2 thermal switch 9. In figure 2 there is a JI phase shift in one arm. This gives an unbalanced MZ interferometer. However, one may well make an add-drop filter without the JI phase shift. The switching mechanism is based on a MZ interferometer 10 with a heating plate 11 close to one arm of the MZ interferometer. The temperature of the plate 11 is con- trollede by current flow through the plate. A thermally induced change of the refractive index causes the change of phase of the guided light. The full switching of light coming from the input port 12 from being guided to the drop port 13 instead of to the output port 14 is obtained when the phase difference in the two arms of the MZ interferometer 10 is JI. The light with the wavelength λi incoming from input port 12 is reflected by the filter, then passes through a reflrctor 15 and is transmitted to the 2x2 thermal switch 10. Depending on the positon of the -switch 10 the light is either transmit- ted directly to the drop port 13 or passes through a reflector 16 and thereby back to the filter 1 and to the output port 14.
When the light is transmitted from input port 12 to drop port 13 incoming light from add port 17 with the same wavelength can be added to output port 14. In this way the switchable optical add-drop filter enable exchanging of optical signals having the same wavelength.
When light with the wave length λi is transmitted from input port 12 to output port 14 by the filter 1 and the switch 10, the incoming light from add port 17 remains in drop port 13.
Other wave lengths, except for λi , are passing through the filter 1 and are transmitted directly to output port independently of the position of the switch 10.
The Bragg gratings of the reflectors 15, 16 are made as described above.
The top of an optical pulse propagating in an optical waveguide travels at a group velocity (vgrθup) given by:
V group (dβ I dώ) ω=ω~ , if the width of frequency band of the
pulse is negligible compared to the carrier frequency of the pulse Do, where D(D) is a propagation constant of a guided mode, where DDDD~D..and where D is a wavelength of light. For an optical waveguide device the group velocity can be described as: vgroup=z/ tgroupr where z is a propagation distance and tgroUp is a group delay. The group delay is responsible for delay time of the optical pulse passing through the waveguide device. This group delay caused by an optical device is given by:
= d group dω ω~ω- r where U.is a phase shift of light passing
through the device.
The key parameter responsible for changing the shape of an optical pulse is the first derivative of the group delay in the frequency domain. This parameter is called group delay dispersion (GDD) and can be presented in the following form:
GDD = d φ ω-ω. dω2 For zero group delay dispersion (GDD=0, meaning that tgrθuP =const) , the optical pulse is passing through the device without broadening of its shape. The effect of the pulse broadening for GDD>0 is presented in Figure 3.
If the group delay is significantly changing within the frequency band of the pulse, then the group delay should be compensated for each frequency within the band of the pulse. Then the optical pulse passing through the waveguide device will not be distorted.
Figure 4a shows the transmission and group delay for a single Bragg grating waveguide. As one can see the group delay dispersion is zero (group delay=const) only at the center wavelength. Figure 4b illustrates the group delay for reflected light for waveguide with a Bragg grating filter for a much wider bandwidth. As one can see, by comparison the figure 4a and 4b, the changes of group delay in the center region of the reflected band are smaller, if the filter 1 has wider bandwidth.
5
The add-drop filter presented in figure 5, consists of three identical filters: one main filter 1 and two reflectors 15, 16. These two reflectors are used in order to avoid waveguide bends, which would have increased the total size of the de- w vice. Since, the device presented in figure 5 thus contains three filters, the signal which is transmitted to the output port is reflected four times from these gratings. In this way, the group delay dispersion for this device is four times higher comparing to single reflection from grating. Addition-
15 ally, the add-drop filter switch 10 is design to work in optical network, where more than one identical filter switch with the same operating channel is already connected. It means that the total group delay will be proportional to the number of identical add-drop filter switches connected to the 0 same optical node. If this system is used in high bit rate transmission, should exhibit constant group delay (zero group delay dispersion) in the desired wavelength range, due to fluctuation of the operating wavelength or differences of light wavelengths emitted by different sources. 5
The use of wide band reflection gratings in order to minimize the total group delay dispersion is not acceptable for densely spaced transmitted channels.
30 This dependence, between flatness of the group delay and the reflection bandwidth can be avoided if gratings are used to compensate for the group delay dispersion, which is generated by the main filter. It is our idea that this method can be applied to a switchable add-drop filter.
According to the invention the total group delay dispersion • in the device presented in figure 5 can be significantly decreased if a first and second reflector acts as a group delay dispersion compensator 18, 19, please see figure 6.
According to one important embodiment the add-drop filter is a switchable add-drop filter having an input port 12, an output port 14, an add port 17 and a drop port 13, a main filter 1 comprising one Mach Zender interferometer and comprising a 2X2 switch or a thermal 2X2 switch 10, where the respective ends of the interferometer and of the switch are connected to 50/50 couplers, in that a first compensator 18 is located between the main filter and a first end of the switch and where a second compensator 19 is located between the main filter and a second end of the switch, where the compensators 18,19 are arranged to compensate for the group delay dispersion of both ends of the main filter 1.
In the present solution the group delay compensators 18, 19 are working only for the desired wavelength range, which is previously reflected from the main filter 1. In this way, the total bandwidth of these two components 18, 19 is determined by the main filter 1, whereas the total group delay is a sum of group delays of separate components. It means that the bandwidth of reflection for the compensator can be larger than that of the main filter, and the reflection spectrum for the compensator has to be nearly 100% only for the wavelength range corresponding to main filter. The flattening of group delay is possible if the group delay dispersions in the compensator and main filter have opposite signs. According to the present solution the compensator can be realized by the superposition of two or more Bragg gratings. If the central frequency of the main filter is ω0, which corresponds to wavelength λ0, then the compensator is made by superposition of Bragg gratings with central wavelength: ω0+Δω and ω0-Δω if only two Bragg gratings are used. The two, total length of the grating and Δω depends o the grating strength. Figure 7 shows the example of final shape of the grating.
The dashed line in figure 7 represents the apodization, which was made in gratings ω0+Δω and ω0-Δω before superposition. Figure 8 shows the reflection and group delay spectrum for a compensator created from two gratings: ω0+Δω and ω0-Δω .
As one can see in figure 8 and figure 4a, for wavelengths around 1545nm, the group delay for the compensator and the main filter has opposite dispersions in the central part of reflection band, whereas the reflection bandwidth of the compensator is much wider than for the main filter. In this way the group delay from main filter can be compensated without changing the bandwidth of the reflected light, please see figure 9. The final group delay presented in figure 9 is calculated as a sum of group delay for the main filter and the compensator.
As can be seen in figure 9 the group delay for the main filter and the group delay for the compensator together forms the final group delay according to the solid curve. This gives a GDD of substantially zero around the center frequency, thus giving that a full compensation has been achieved. Finally in the add-drop filter presented in figure 6, if the reflected light from main filter 1 is switched to the drop port 13 then the group delay in the desired wavelength range is constant (zero group delay dispersion) . If the switch is passing the light the two output ports 13, 14, or the new optical signal is adding from the add port 17 to output port 14, the light is passing through the second compensator 19 and also is reflected from the main filter 1. Due to the symmetry of the main filter, the group delay of the second compensator 19 has to be the same as for the first one 18. In this way the second compensator is introducing a group delay which is compensated by the second reflection from the main filter. This will give zero group delay dispersion for the desired wavelength range reflected from the main filter if the light is transmitted from input to output port. For a wavelength range out of the reflection band of the main filter, the dispersion introduced by main grating is negligible and doesn't need any compensation.
It is apparent that the present invention solves the problem addressed in the opening part of the present application.
Above several embodiments of optical filters have been de- scribed. However, the present invention can be used in any configuration of add-drop filters where the group delay dispersion in a Bragg grating can be compensated for by a compensator according to the invention.
Thus, the present invention shall not be deemed restricted to any specific embodiment, but can be varied within the scope of the claims.

Claims

Claims.
1. Method for compensation of group delay dispersion in an add-drop filter comprising a passive main filter (1) compris-
5 ing a Mach-Zender interferometer with two identical Bragg gratings (3,4) placed in the respective arms of the Mac- Zender interferometer, c h a r a c t e r i z e d in, that after the main filter (1), in the travelling direction of introduced light, group delay dispersion from the main filter
10 is compensated for by a compensator (18; 19) in the form of a reflector for light leaving the main filter, where the bandwidth of reflection of the compensator is larger than that of the main filter and where the reflection spectrum of the compensator (18,19) is substantially 100% for the wavelength
15 range defined by the main filter (1) .
2. Method according to claim 1, c h a r a c t e r i z e d i n that, the compensator (18,19) is caused to have two or more superimposed Bragg gratings (20, 21; 22, 23) . 0
3. Method according to claim 1 or 2, c h a r a c t e r i z e d i n, that the group delay dispersion of the compensator (18,19) is caused to be such that the group delay dispersion of the compensator plus the group delay dispersion 5 of the main filter (1) is substantially zero around the center frequency given by the main filter (1) .
4. Method according to claim 1, 2 or 3, c h a r a c t e r i z e d i n, that the add-drop filter is a switacable
30 add-drop filter having an input port (12), an output port (14), an add port (17) and a drop port (13), a main filter (1) comprising one Mach Zender interferometer and a 2X2 switch (10) , where the respective ends of the interferometer (1) and of the switch (10) are connected to 50/50 couplers, where the group delay dispersion of both ends of the main filter (1) is compensated for by a first compensator (18) located between the main filter and a first end of the switch and where a second compensator located between the main filter and a second end of the switch.
5. Device for compensation of group delay dispersion in an add-drop filter comprising a passive main filter (1) comprising a Mach-Zender interferometer with two identical Bragg gratings (3,4) placed in the respective arms of the Mac- Zender interferometer, c h a r a c t e r i z e d in, that after the main filter (1) , in the travelling direction of introduced light, a compensator (18; 19) in the form of a reflector for light leaving the main filter is present, which compensator is arranged to compensate for group delay dispersion from the main filter (1) and in that the bandwidth of reflection of the compensator (18; 19) is larger than that of the main filter (1) and where the reflection spectrum of the compensator is substantially 100% for the wavelength range defined by the main filter.
6. Device according to claim 5, c h a r a c t e r i z e d i n that, the compensator (18,19) comprises two or more superimposed Bragg gratings (20, 21; 22, 23) .
7. Device according to claim 5 or 6, c h a r a c t e r i z e d i n, that the group delay dispersion of the compensator (18,19) is caused to be such that the group delay dispersion of the compensator plus the group delay dispersion of the main filter (1) is substantially zero around the center frequency given by the main filter.
8. Device according to claim 5, 6 or 7, c h a r a c t e r i z e d i n, that the add-drop filter is a switchable add-drop filter having an input port (12), an output port (14), an add port (17) and a drop port (13), a main filter (1) comprising one Mach Zender interferometer and comprising a 2X2 switch (10) , where the respective ends of the interferometer (and of the switch are connected to 50/50 couplers, in that a first compensator (18) is located between the main filter and a first end of the switch and where a second compensator (19) is located between the main filter and a second end of the switch, where the compensators (18,19) are arranged to compensate for the group delay dispersion of both ends of the main filter (1) .
PCT/SE2004/000602 2003-05-05 2004-04-20 Compensation of group delay dispersion in a switchable add-drop filter. WO2004099837A1 (en)

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SE0301286A SE525760C2 (en) 2003-05-05 2003-05-05 Compensation of so-called group delay dispersion in a switchable so-called add-drop filter.
SE0301286-1 2003-05-05

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Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US6137604A (en) * 1996-12-04 2000-10-24 Tyco Submarine Systems, Ltd. Chromatic dispersion compensation in wavelength division multiplexed optical transmission systems
EP1065813A2 (en) * 1999-06-30 2001-01-03 Marconi Communications Limited Optical system

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US6137604A (en) * 1996-12-04 2000-10-24 Tyco Submarine Systems, Ltd. Chromatic dispersion compensation in wavelength division multiplexed optical transmission systems
EP1065813A2 (en) * 1999-06-30 2001-01-03 Marconi Communications Limited Optical system

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