WO2003079063A1 - Optical tunable microwave filter using higher mode - Google Patents

Optical tunable microwave filter using higher mode Download PDF

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Publication number
WO2003079063A1
WO2003079063A1 PCT/KR2003/000518 KR0300518W WO03079063A1 WO 2003079063 A1 WO2003079063 A1 WO 2003079063A1 KR 0300518 W KR0300518 W KR 0300518W WO 03079063 A1 WO03079063 A1 WO 03079063A1
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WO
WIPO (PCT)
Prior art keywords
mode
fiber
modes
signal
signals
Prior art date
Application number
PCT/KR2003/000518
Other languages
French (fr)
Inventor
Woo-Young Choi
Kwang-Hyun Lee
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Yonsei University
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Publication date
Application filed by Yonsei University filed Critical Yonsei University
Priority to AU2003215936A priority Critical patent/AU2003215936A1/en
Publication of WO2003079063A1 publication Critical patent/WO2003079063A1/en

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Classifications

    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B5/00—Optical elements other than lenses
    • G02B5/20—Filters
    • 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
    • G02B6/2804—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals forming multipart couplers without wavelength selective elements, e.g. "T" couplers, star couplers
    • G02B6/2861—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals forming multipart couplers without wavelength selective elements, e.g. "T" couplers, star couplers using fibre optic delay lines and optical elements associated with them, e.g. for use in signal processing, e.g. filtering
    • 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
    • G02B6/032—Optical fibres with cladding with or without a coating with non solid core or cladding
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
    • G02B6/14—Mode converters

Definitions

  • the present invention relates, in general, to a optical tunable microwave filter using a higher mode and, more particularly, to a filter that can construct a plurality of delay lines using a single short dispersion compensation fiber through two mode converters.
  • components required to manufacture a filter are delay lines for changing the phase of a signal.
  • most filters which have been proposed until now, use high-dispersion fibers 5 as shown in Fig. 1. That is, a signal output from a laser diode 1 is modulated and amplified in order by an RF modulator 2 and an amplifier 3, respectively, and the modulated signal is divided into several signals by a divider 4 (in the ratio of 1 :8 in Fig. 1). Further, several paths 8 for allowing these signals to pass therethrough are formed, and the high-dispersion fibers 5 having different lengths are connected to respective paths 8, thus constructing a plurality of delay lines. In this way, signals passing through respective delay lines are detected by photodiodes which are detectors 6, and characteristics of the filter can be obtained using detected frequency characteristics. Further, the signals detected by the detectors 6 are output through a 8: 1 RF power combiner 7.
  • one high-dispersion fiber constructs one delay line. Further, since a detector is necessary for each path, as many detectors as the number of required delay lines are necessary. Further, in order to construct required delay lines, high- dispersion fibers each with a length greater than several tens of meters are required, which acts as an unfavorable factor in the size and cost of a filter.
  • an object of the present invention is to provide a optical tunable microwave filter using a higher mode that can construct a plurality of delay lines using a single short dispersion compensation fiber and a single detector through two mode converters.
  • Dispersion Compensation Fiber one of wavelength division multiplexing transmission methods attempted to transmit a signal at a speed higher than 10 Gbps, wherein a DCF having a dispersion value opposite to a typical optical fiber is arranged in the middle of a transmission path, thus compensating for dispersion
  • Single Mode Fiber optical fiber having a single propagation mode in which used wavelengths can be transmitted, the optical fiber being characterized in that the diameter of a core thereof is very small to be less than 10 ⁇ m and a single light propagation mode exists, so signal loss is small and there is no signal transformation (distortion), thus enabling signals to be transmitted over a long distance.
  • Hollow Optical Fiber optical fiber having a hollow center portion
  • Group Velocity velocity at which a signal wave is propagated, and the reciprocal of the rate of change of the phase constant with respect to angular frequency in a specific mode
  • Group Index for a given mode propagating in a medium of refractive index n, index value obtained by dividing the velocity of light in a vacuum by the group velocity of the mode
  • Delay line means for introducing a time delay
  • Fig. 1 is a view showing the construction of a conventional filter
  • Fig. 2 is a view showing the construction of a filter according to the present invention.
  • Fig. 3 is a view showing the construction according to another embodiment according to the present invention.
  • the present invention provides a filter comprised of first and second mode converters 30 and 30', a single mode fiber 12, a dispersion compensation fiber 16, a laser diode 10, a modulator 20, and a receiving unit 40.
  • the filter of the present invention comprises the laser diode 10 for generating a carrier signal; the modulator 20 for modulating the carrier signal generated by the laser diode 10; the first mode converter 30 for receiving power of the signal modulated by the modulator 20 and dispersing the signal power, that is, the power of the signal transmitted in a single mode LPOl in the single mode fiber 12, into two modes LPOl and LP02 in the dispersion compensation fiber 16; the second mode converter 30' for receiving signals of two modes LPOl and
  • the LP02 having a time delay therebetween through the first mode converter 30 and converting the signals of the two modes LPOl and LP02 into a signal of a single mode LPOl in the single mode fiber 12; and the receiving unit 40 for receiving the signal converted by the second mode converter 30'.
  • the first and second mode converters 30 and 30 ' are hollow optical fibers 14 and 14 ' that each couple the single mode fiber 12 and the dispersion compensation fiber 16 and each have a radius of 3.4 ⁇ m and an empty space with a radius of 0.6 ⁇ m in its center.
  • a Transverse Electric (TE) mode, a Transverse Magnetic (TM) mode and a Hybrid mode exist in an optical fiber.
  • the TE mode is a mode in which a H-field (magnetic field component) exists, not an E- field (electric field component), in the direction of propagation of electromagnetic waves.
  • the TM mode is a mode in which an E-field (electric field component) exists, not a H-field (magnetic field component), in the direction of propagation of electromagnetic waves.
  • the hybrid mode is a mode in which an E- field and a H-field exist in the direction of propagation of electromagnetic waves, differently from the TE and TM modes. Refractive indexes of the modes can be obtained through characteristic equations, which are obtained by applying boundary conditions to a wave equation and are different from each other according to the TM. TE and Hybrid modes.
  • LPmn of the LP mode m is an index included in the characteristic equations obtained by applying the boundary conditions to the wave equation, and n is the sequence of solutions obtained by applying a given index m to the characteristic equations.
  • the laser diode 10 generates a carrier signal
  • the generated carrier signal is intensity-modulated by the modulator 20.
  • the power of the modulated signal is transmitted in only a single mode existing in the single mode fiber 12, that is, the LPO l mode, and the transmitted signal power is input to the first mode converter 30.
  • the first mode converter 30 is formed in such a way that the single mode fiber 12 and the dispersion compensation fiber 16 are coupled through the hollow optical fiber 14, and the function of the first mode converter 30 is described below.
  • the first mode converter 30 serves to disperse the power of the signal transmitted only in the LPO l mode. As the power of the modulated signal, transmitted only in the LPOl mode within the single mode fiber 12, passes through the first mode converter 30, the shape of a mode changes to a ring shape. This ring-shaped LPOl mode is dispersed into two modes existing in the dispersion compensation fiber 16, that is, modes LPOl and LP02, in the power ratio of 50:50. As described above, the first mode converter 30 serves to allow the signal being propagated in the single mode LPOl to be dispersed into two modes LPOl and LP02 and propagated. Further, as described above, the mode is changed to be ring-shaped while the signal power passes through the hollow optical fiber 14. The reason for this is to more effectively perform mode coupling to the LP02 mode existing in the dispersion compensation fiber 16.
  • Signals transmitted in the two modes LPOl and LP02 dispersed in this way pass through the dispersion compensation fiber 16 with different group indexes arid group velocities in the dispersion compensation fiber 16, which represents that a time delay occurs due to a velocity difference between two modes even though the signals are propagated in two modes LPOl and LP02 by the same length.
  • the signals of two modes LPOl and LP02 having a time delay therebetween pass through the second mode converter 30'.
  • the second mode converter 30 ' converts the signals of two modes LPOl and LP02 again into a signal of the single mode LPOl existing in the single mode fiber 12.
  • the second mode converter 30' is the hollow optical fiber 14' for coupling the single mode fiber 12 and the dispersion compensation fiber 16, similar to the first mode converter 30.
  • group velocities of respective modes LPOl and LP02 have values differing from those measured before the wavelength of the laser is varied, due to mode characteristics in the dispersion compensation fiber 16.
  • These different group velocities vary a time delay between the two modes LPO l and LP02 at the end of the dispersion compensation fiber 16. That is, since the time delay is the reciprocal of a filter period, filter characteristics can be easily adjusted.
  • filters suitable for application fields can be manufactured using different mode converters. That is, a power coupling degree (efficiency) between the modes LPO l and LP02 is controlled by adjusting a diameter of a hollow optical fiber and a diameter of an empty space formed therein. Therefore, the diameter of the hollow optical fiber (including the diameter of the empty space formed therein) is changed, so the shape of a manufactured filter (not shown) is also changed.
  • other filter characteristics can be obtained using a plurality of laser diodes. That is. as shown in Fig. 3, if plural laser diodes 10 having different wavelengths are connected, as many LPO l modes propagating different wavelengths as the number of laser diodes 10 exist in the single mode fiber 12. Further, respective LPOl modes propagating different wavelengths are connected to the dispersion compensation fiber 16 through the first mode converter 30. The respective LPOl modes propagating different wavelengths are coupled to the LP02 mode in the dispersion compensation fiber 16. Further, the modes LPO l and LP02 in the dispersion compensation fiber 16 have group indexes corresponding to their wavelengths, so they have different group velocities.
  • a time delay occurs due to this velocity difference, and a signal, converted into a signal of the single mode LPO l existing in the single mode fiber 12 through the second mode converter 30', is received by the receiving unit 40.
  • other filter characteristics differing from the conventional filter characteristics can be obtained using a plurality of laser diodes having different wavelengths.
  • the present invention provides a optical tunable microwave filter using a higher mode, which can construct a plurality of delay lines using a single short dispersion compensation fiber through two mode converters, differently from conventional filters, and tuning of the filter can be easily performed. Further, the present invention is advantageous in that it can obtain other filter characteristics differing from conventional filter characteristics by adjusting a diameter of a hollow optical fiber, which is a mode converter, or connecting a plurality of laser diodes having different wavelengths.

Abstract

Disclosed is an optical tunable microwave filter using higher mode. The filter comprises a laser diode producing carrier signals; a modulator modulating the signals produced by the laser diode; a first mode converter being input power of the modulated signals by the modulator and dispersing the power of the input signals, namely the power of the signal with a mode (LP01) in a single mode fiber, into two modes (LP01, LP02) in a dispersion compensation fiber; a second mode converter being input two modes (LP01, LP02) signals having time delay through the first mode converter and converting the input modes (LP01, LP02) signals into a mode (LP01) existing in the single mode fiber; a receiver receiving the signals converted by the second mode converter.

Description

OPTICAL TUNABLE MICROWAVE FILTER USING HIGHER MODE
Technical Field
The present invention relates, in general, to a optical tunable microwave filter using a higher mode and, more particularly, to a filter that can construct a plurality of delay lines using a single short dispersion compensation fiber through two mode converters.
Background Art
Generally, components required to manufacture a filter are delay lines for changing the phase of a signal. In order to construct these delay lines, most filters, which have been proposed until now, use high-dispersion fibers 5 as shown in Fig. 1. That is, a signal output from a laser diode 1 is modulated and amplified in order by an RF modulator 2 and an amplifier 3, respectively, and the modulated signal is divided into several signals by a divider 4 (in the ratio of 1 :8 in Fig. 1). Further, several paths 8 for allowing these signals to pass therethrough are formed, and the high-dispersion fibers 5 having different lengths are connected to respective paths 8, thus constructing a plurality of delay lines. In this way, signals passing through respective delay lines are detected by photodiodes which are detectors 6, and characteristics of the filter can be obtained using detected frequency characteristics. Further, the signals detected by the detectors 6 are output through a 8: 1 RF power combiner 7.
However, in the conventional filter having the above construction, one high-dispersion fiber constructs one delay line. Further, since a detector is necessary for each path, as many detectors as the number of required delay lines are necessary. Further, in order to construct required delay lines, high- dispersion fibers each with a length greater than several tens of meters are required, which acts as an unfavorable factor in the size and cost of a filter.
Therefore, a method of constructing a plurality of delay lines using a single short fiber and a single detector is required.
Disclosure of the Invention
Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide a optical tunable microwave filter using a higher mode that can construct a plurality of delay lines using a single short dispersion compensation fiber and a single detector through two mode converters.
Terms used in the description of the present invention are defined in brief below. Higher mode: modes except for a main mode with a lowest cutoff frequency
Dispersion Compensation Fiber (DCF): one of wavelength division multiplexing transmission methods attempted to transmit a signal at a speed higher than 10 Gbps, wherein a DCF having a dispersion value opposite to a typical optical fiber is arranged in the middle of a transmission path, thus compensating for dispersion
Single Mode Fiber: optical fiber having a single propagation mode in which used wavelengths can be transmitted, the optical fiber being characterized in that the diameter of a core thereof is very small to be less than 10 μm and a single light propagation mode exists, so signal loss is small and there is no signal transformation (distortion), thus enabling signals to be transmitted over a long distance.
Hollow Optical Fiber: optical fiber having a hollow center portion Group Velocity: velocity at which a signal wave is propagated, and the reciprocal of the rate of change of the phase constant with respect to angular frequency in a specific mode
Group Index: for a given mode propagating in a medium of refractive index n, index value obtained by dividing the velocity of light in a vacuum by the group velocity of the mode Delay line: means for introducing a time delay
Brief Description of Drawings
Fig. 1 is a view showing the construction of a conventional filter; Fig. 2 is a view showing the construction of a filter according to the present invention; and
Fig. 3 is a view showing the construction according to another embodiment according to the present invention.
Best Mode for Carrying Out the Invention
Hereinafter, embodiments of the present invention will be described in detail with reference to the attached drawings.
<embodiment> Construction
The entire construction of a filter of the present invention is described with reference to Fig. 2.
The present invention provides a filter comprised of first and second mode converters 30 and 30', a single mode fiber 12, a dispersion compensation fiber 16, a laser diode 10, a modulator 20, and a receiving unit 40.
The filter of the present invention comprises the laser diode 10 for generating a carrier signal; the modulator 20 for modulating the carrier signal generated by the laser diode 10; the first mode converter 30 for receiving power of the signal modulated by the modulator 20 and dispersing the signal power, that is, the power of the signal transmitted in a single mode LPOl in the single mode fiber 12, into two modes LPOl and LP02 in the dispersion compensation fiber 16; the second mode converter 30' for receiving signals of two modes LPOl and
LP02 having a time delay therebetween through the first mode converter 30 and converting the signals of the two modes LPOl and LP02 into a signal of a single mode LPOl in the single mode fiber 12; and the receiving unit 40 for receiving the signal converted by the second mode converter 30'.
In this case, the first and second mode converters 30 and 30' are hollow optical fibers 14 and 14' that each couple the single mode fiber 12 and the dispersion compensation fiber 16 and each have a radius of 3.4μm and an empty space with a radius of 0.6μm in its center.
The modes LPO l and LP02 of the present invention are described below. Generally, a Transverse Electric (TE) mode, a Transverse Magnetic (TM) mode and a Hybrid mode exist in an optical fiber. In this case, the TE mode is a mode in which a H-field (magnetic field component) exists, not an E- field (electric field component), in the direction of propagation of electromagnetic waves. The TM mode is a mode in which an E-field (electric field component) exists, not a H-field (magnetic field component), in the direction of propagation of electromagnetic waves. Further, the hybrid mode is a mode in which an E- field and a H-field exist in the direction of propagation of electromagnetic waves, differently from the TE and TM modes. Refractive indexes of the modes can be obtained through characteristic equations, which are obtained by applying boundary conditions to a wave equation and are different from each other according to the TM. TE and Hybrid modes.
Meanwhile, if refractive indexes of a core and a cladding of an optical fiber are almost the same, there are modes in which the refractive indexes of the above modes become almost equal at the time the refractive indexes thereof are obtained. These modes are integrally called a Linearly polarized (LP) mode. Further, in LPmn of the LP mode, m is an index included in the characteristic equations obtained by applying the boundary conditions to the wave equation, and n is the sequence of solutions obtained by applying a given index m to the characteristic equations. That is, LPO l is a mode having a refractive index corresponding to a first value of solutions obtained by applying m =0 to the characteristic equation of LPmn, and LPOl is a mode having a refractive index corresponding to a second value of solutions obtained by applying m =0 to the characteristic equation of LPmn. Operation
An operating principl of the construction of the present invention as described above is described with reference to Fig. 2. First, if the laser diode 10 generates a carrier signal, the generated carrier signal is intensity-modulated by the modulator 20. In this case, the power of the modulated signal is transmitted in only a single mode existing in the single mode fiber 12, that is, the LPO l mode, and the transmitted signal power is input to the first mode converter 30. The first mode converter 30 is formed in such a way that the single mode fiber 12 and the dispersion compensation fiber 16 are coupled through the hollow optical fiber 14, and the function of the first mode converter 30 is described below.
The first mode converter 30 serves to disperse the power of the signal transmitted only in the LPO l mode. As the power of the modulated signal, transmitted only in the LPOl mode within the single mode fiber 12, passes through the first mode converter 30, the shape of a mode changes to a ring shape. This ring-shaped LPOl mode is dispersed into two modes existing in the dispersion compensation fiber 16, that is, modes LPOl and LP02, in the power ratio of 50:50. As described above, the first mode converter 30 serves to allow the signal being propagated in the single mode LPOl to be dispersed into two modes LPOl and LP02 and propagated. Further, as described above, the mode is changed to be ring-shaped while the signal power passes through the hollow optical fiber 14. The reason for this is to more effectively perform mode coupling to the LP02 mode existing in the dispersion compensation fiber 16.
Signals transmitted in the two modes LPOl and LP02 dispersed in this way pass through the dispersion compensation fiber 16 with different group indexes arid group velocities in the dispersion compensation fiber 16, which represents that a time delay occurs due to a velocity difference between two modes even though the signals are propagated in two modes LPOl and LP02 by the same length. In this way, the signals of two modes LPOl and LP02 having a time delay therebetween pass through the second mode converter 30'. The second mode converter 30' converts the signals of two modes LPOl and LP02 again into a signal of the single mode LPOl existing in the single mode fiber 12. The signal, converted into the signal of the LPO l mode existing in the single mode fiber 12 through the second mode converter 30', is received by the receiving unit 40. In this case, the second mode converter 30' is the hollow optical fiber 14' for coupling the single mode fiber 12 and the dispersion compensation fiber 16, similar to the first mode converter 30. Through the above operation, the characteristics of the filter can be obtained by measuring frequency response characteristics while increasing a modulation frequency of the signal received by the receiving unit 40.
Meanwhile, a tuning method of the filter proposed in the present invention is described below. If a laser wavelength of the laser diode 10 is varied, group velocities of respective modes LPOl and LP02 have values differing from those measured before the wavelength of the laser is varied, due to mode characteristics in the dispersion compensation fiber 16. These different group velocities vary a time delay between the two modes LPO l and LP02 at the end of the dispersion compensation fiber 16. That is, since the time delay is the reciprocal of a filter period, filter characteristics can be easily adjusted.
<Another embodiment
Another embodiment proposed in the present invention is described below.
First, filters suitable for application fields can be manufactured using different mode converters. That is, a power coupling degree (efficiency) between the modes LPO l and LP02 is controlled by adjusting a diameter of a hollow optical fiber and a diameter of an empty space formed therein. Therefore, the diameter of the hollow optical fiber (including the diameter of the empty space formed therein) is changed, so the shape of a manufactured filter (not shown) is also changed.
Second, other filter characteristics can be obtained using a plurality of laser diodes. That is. as shown in Fig. 3, if plural laser diodes 10 having different wavelengths are connected, as many LPO l modes propagating different wavelengths as the number of laser diodes 10 exist in the single mode fiber 12. Further, respective LPOl modes propagating different wavelengths are connected to the dispersion compensation fiber 16 through the first mode converter 30. The respective LPOl modes propagating different wavelengths are coupled to the LP02 mode in the dispersion compensation fiber 16. Further, the modes LPO l and LP02 in the dispersion compensation fiber 16 have group indexes corresponding to their wavelengths, so they have different group velocities. A time delay occurs due to this velocity difference, and a signal, converted into a signal of the single mode LPO l existing in the single mode fiber 12 through the second mode converter 30', is received by the receiving unit 40. As described above, other filter characteristics differing from the conventional filter characteristics can be obtained using a plurality of laser diodes having different wavelengths.
Industrial Applicability As described above, the present invention provides a optical tunable microwave filter using a higher mode, which can construct a plurality of delay lines using a single short dispersion compensation fiber through two mode converters, differently from conventional filters, and tuning of the filter can be easily performed. Further, the present invention is advantageous in that it can obtain other filter characteristics differing from conventional filter characteristics by adjusting a diameter of a hollow optical fiber, which is a mode converter, or connecting a plurality of laser diodes having different wavelengths.

Claims

Claims
1 . A optical tunable microwave filter using a higher mode, the filter having two mode converters, a single mode fiber, a dispersion compensation fiber, at least one laser diode, a modulator and a receiving unit, comprising. the diode for generating at least one carrier signal; the modulator for modulating the carrier signal generated by the laser diode; the first mode converter for receiving power of the signal modulated by the modulator and dispersing the signal power, namely, the power of the signal transmitted in a single mode (LPOl) in the single mode fiber, into two modes
(LPOl, LP02) in the dispersion compensation fiber; the second mode converter for receiving signals of two modes (LPO l and LP02) having a time delay therebetween through the first mode converter and converting the signals of the two modes (LPOl , LP02) again into a signal of a single mode (LPO 1 ) existing in the single mode fiber; and the receiving unit for receiving the signal converted by the second mode converter.
2. The optical tunable microwave filter according to claim 1, wherein the first and second mode converters are hollow optical fibers that each couple the single mode fiber and the dispersion compensation fiber and each have a radius of
3.4μm and an empty space with a radius of 0.6μm in its center.
3. The optical tunable microwave filter according to claim 1 or 2, wherein the first mode converter is constructed so that the power is dispersed into the respective modes LPOl and LP02 in the ratio of 50: 50 in the first mode converter.
4. The optical tunable microwave filter according to claim 1 or 2, wherein each of the first and second mode converters is constructed so that a power coupling degree (efficiency) between the modes (LPO l and LP02) is controlled by adjusting a diameter of the hollow optical fiber and a diameter of the empty space formed therein.
5. The optical tunable microwave filter according to 1 , wherein the laser diode for generating the carrier signal is implemented to be plural in number, thus enabling filters having different characteristics to be manufactured.
PCT/KR2003/000518 2002-03-18 2003-03-18 Optical tunable microwave filter using higher mode WO2003079063A1 (en)

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KR100428073B1 (en) 2004-04-28
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