IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 12, NO. 5, MAY 2000
`
`561
`
`Bidirectional Wavelength Add–Drop Multiplexer
`Using Multiport Optical Circulators and Fiber Bragg
`Gratings
`
`Jungho Kim and Byoungho Lee, Member, IEEE
`
`Abstract—We propose and experimentally demonstrate a new
`structure of bidirectional wavelength add–drop multiplexer using
`multiport optical circulators and fiber Bragg gratings. It has the
`sufficient suppression of the unwanted light caused by Rayleigh
`backscattering and optical reflection and is economical for the ef-
`fective use of multiport optical circulators.
`Index Terms—Add–drop multiplexer, bidirectional
`trans-
`mission, fiber gratings, optical networks, wavelength-division
`multiplexing.
`
`I. INTRODUCTION
`
`T HE EXPLOSIVE growth of data traffic drives the de-
`
`ployment of wavelength-division-multiplexing (WDM)
`technology. Single fiber bidirectional ring networks (SFBRN’s)
`attract a great deal of attention due to their cost-effectively
`enhanced capacity and possibility of self-healing character-
`istic [1]. A bidirectional wavelength add–drop multiplexer
`(B-WADM) is one of the important components to implement
`SFBRN’s. Several different structures of B-WADM using a
`WDM multiplexer/demultiplexer or an arrayed-waveguide
`grating (AWG) have been proposed and demonstrated [2]–[4].
`In this paper, we propose a reconfigurable B-WADM using
`multiport optical circulators (OC’s) and fiber Bragg gratings
`(FBG’s). The proposed B-WADM has a good filtering shape
`and sufficiently suppresses the relative intensity noise (RIN)
`caused by Rayleigh backscattering and optical
`reflection
`owing to the filtering characteristic of FBG’s. In addition, it is
`cost-effective because it makes an effective use of multiport
`OC’s.
`
`II. CONFIGURATION OF B-WADM
`Fig. 1(a) shows the schematic diagram of the proposed
`B-WADM that can switch three optical channels in each
`direction. The reflective center wavelength of
`the FBG
`i (i = 1; 2; . . . ; N ) is designed to match the optical channel
`i. Three optical channels in each direction enter port 2 of
`each six-port OC, and they are reflected by their corresponding
`FBG’s that are connected to port 3 of the six-port OC. If
`these gratings are chirped, they can simultaneously provide
`
`Manuscript received December 14, 1999; revised February 1, 2000. This
`work was supported in part by the Ministry of Science and Technology of
`Korea through Korea Telecom and KAIST.
`The authors are with the School of Electrical Engineering, Seoul National
`University, Seoul 151-742, Korea (e-mail: byoungho@plaza.snu.ac.kr).
`Publisher Item Identifier S 1041-1135(00)03597-7.
`
`(a)
`
`(b)
`
`(a) Schematic diagram of the proposed B-WADM that can switch three
`Fig. 1.
`optical channels in each direction. (VA: variable attenuator.) (b) Schematic
`diagram of the proposed B-WADM that can switch only one optical channel
`in each direction.
`
`dispersion compensation for the reflected optical channels.
`Because only two FBG’s are connected to port 4 of the OC,
`one optical channel passes through two FBG’s and enters a
`2  2 optical switch (OSW). The other two optical channels
`are reflected again by their respective FBG’s and sent to the
`next port of the OC. In the same manner, other optical channels
`are separated and enter a 2  2 OSW, respectively. A 2  2
`OSW decides the operation state between “pass through” and
`“add–drop” of each optical channel. Optical channels leaving
`each 2  2 OSW (“pass through” or “added” channels) are
`combined by a 1 N coupler and exit the B-WADM through
`the other six-port OC. If a wavelength multiplexer (MUX)
`replaces a 1 N coupler, it reduces the insertion loss but adds
`cost to the system. Erbium-doped fiber amplifiers (EDFA’s)
`for compensating the loss of the module and the fiber link and
`optical attenuators for optical power equalization are inserted
`in the B-WADM.
`
`1041–1135/00$10.00 © 2000 IEEE
`
`Capella 2012
`Ciena/Coriant/Fujitsu v. Capella
`IPR2015-00816
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`562
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`IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 12, NO. 5, MAY 2000
`
`Fig. 2. Experimental setup of the proposed B-WADM. (TLS: Tunable light
`source.)
`
`the RIN
`transmission system,
`In a general bidirectional
`induced by Rayleigh backscattering and optical reflection may
`degrade the receiver sensitivity. In addition, the maximum gain
`of the optical amplifier used in a bidirectional transmission
`system is limited by the multiple path reflection noise caused
`by Rayleigh backscattering and optical reflection. However,
`in our proposed B-WADM, these effects can be suppressed
`sufficiently by using FBG’s. When the counterpropagating
`optical channels are in different wavelengths, the unwanted
`optical signals induced by Rayleigh backscattering and optical
`reflection pass through the FGB’s that are connected to port 3
`of a six-port OC and are filtered out by a light absorber.
`In the proposed B-WADM, the six-port OC with corre-
`sponding FBG’s can be regarded as a wavelength demultiplexer
`(DEMUX). Compared with conventional DEMUX such as
`an AWG and a multilayer interference-filter-based filter, this
`DEMUX has the good filtering shape that is ascribed to the
`filtering characteristics of FBG. Furthermore, because two
`six-port OC’s simultaneously include the function of separating
`two unidirectional transmission optical signals from the bidi-
`rectional transmission optical signals, we do not need dummy
`three-port OC’s used in the previous structures [3], [4].
`The proposed B-WADM has good design flexibility. Fig. 1(b)
`shows the schematic diagram of the proposed B-WADM that
`can switch only one optical channel in each direction. The
`FBG’s and light absorber that are connected to port 4 of a
`six-port OC suppress the unwanted optical signals induced
`by Rayleigh backscattering and optical reflection. If a 2  2
`OSW is in bar state, all optical channels pass through. Only
`one optical channel whose wavelength matches with the FBG
`can be dropped or added when a 2  2 OSW is in cross-state.
`Compared with the structures of B-WADM using a WDM
`multiplexer/demultiplexer or an AWG, it may be the fittest for a
`B-WADM node that switches only one optical channel because
`it has a low insertion loss and a low cost.
`
`III. EXPERIMENTAL RESULTS
`Fig. 2 shows the experimental setup. Since the configuration
`of the proposed B-WADM is symmetric and there is no mutual
`influence between counterpropagating signals in the system, we
`perform experimental demonstration on only one side of the
`B-WADM. The average insertion loss of a six-port OC (E-Tek
`PIFC2610TER01) is 1.0 dB, and the isolation between adjacent
`ports of the OC’s is >50 dB, respectively. The 2  2 OSW’s
`used here are optomechanical switches with an insertion loss of
`0.7 dB and a crosstalk of <60 dB. A 1  4 coupler with the av-
`erage insertion loss of 7 dB is used to combine the demultiplexed
`
`Fig. 3. Optical spectra obtained at port OUT. The resolution bandwidth of
`the optical spectrum analyzer is 0.1 nm. (a) When all optical channels pass
`through, (b) when the optical channel 3 is dropped, and (c) when both the
`optical channel 2 and 3 are dropped.
`
`optical channels. The reflective center wavelengths of FBG’s
`are 1 = 1554:24 nm, 2 = 1555:84 nm, and 3 = 1557:44
`nm with 1.6-nm channel spacing. The 3-dB bandwidth and re-
`flectivity of the FBG’s are 0.4 nm and 25 dB, respectively. We
`use an FC/APC connector as a light absorber to prevent optical
`signals arriving at the light absorber from being reflected.
`Three tunable laser diodes whose wavelengths are matched
`with the reflective center wavelengths of the FBG’s are
`launched into port IN. To demonstrate the feasibility of
`suppressing the unwanted signals caused by Rayleigh backscat-
`tering from the counterpropagating light and optical reflection
`with different wavelengths, another optical channel b for
`simulating the unwanted back-reflected (copropagating with
`1, 2, and 3) signal is added. The wavelength of b is
`1556.64 nm, which is in the middle of optical channel 2 and
`3. The input optical power of each optical channel is around
`5 dBm at port IN.
`Fig. 3(a)–(c) shows the optical spectra obtained at the port
`OUT for different connection states of two OSW’s. In Fig. 3(a),
`the crosstalk between channel b and the other channels is
`about 20 dB when the input optical power of b is the same
`as that of 1, 2, or 3. In general, the reflection induced by
`Rayleigh backscattering in a long optical fiber (>20 km) is
`about 32 dB [5]. Therefore, the crosstalk of 20 dB may
`be sufficient for suppressing the RIN caused by Rayleigh
`backscattering. Moreover, using narrower reflection spectra
`of FBG’s or larger channel spacing can further enhance the
`suppression of b. The crosstalk between channel b and
`the other channels is due to the nonideal rolloff of the FBG
`spectrums. Even if the center wavelengths of the FBG’s are
`not matched with b, a small portion of b light is reflected
`by the nonideal rolloff of the FBG’s. As the number that b is
`reflected by the rolloff o fthe FBG’s increases, optical channel
`b is more suppressed. Fig. 3(c) shows the optical spectra
`obtained at OUT when only channel 1 passes through. In this
`case, the crosstalk between optical channel b and pass-through
`
`

`

`KIM AND LEE: BIDIRECTIONAL WAVELENGTH ADD–DROP MULTIPLEXER
`
`563
`
`Fig. 4(a)–(c) shows the dropped signals at different locations.
`The optical spectrum of the dropped channel 1 is obtained by
`disconnecting port 5 of a six-port OC from the 1  4 coupler. In
`Fig. 4(c), the power level of optical channel 3 is near the noise
`level of OSA. The reason is that the optical signal 3 that is re-
`flected by the nonideal rolloff of FBG’s twice arrives at port 5
`of the multiport OC. In Fig. 4, the heterodyne crosstalk between
`the dropped channel and the other channels is about 20 dB.
`In WDM systems, less than 15-dB heterodyne crosstalk may
`induce a power penalty less than 0.5 dB at a BER of 109 [7].
`Increasing the steepness of the rolloff of the FBG and the reflec-
`tivity of the FBG would improve the add–drop performances.
`
`IV. SUMMARY
`A new structure of B-WADM using multiport OC’s and
`FBG’s is proposed. It has good filtering characteristics and
`uses multiport OC effectively. In addition, it has good design
`flexibility. In our experiment, the suppression of the unwanted
`light caused by Rayleigh backscattering counterpropagating
`signals is more than 20 dB, but this can be further reduced
`using the FBG’s of narrower reflective bandwidth or larger
`channel spacing.
`
`REFERENCES
`[1] C. H. Kim, C. H. Lee, and Y. C. Chung, “Bidirectional WDM self-
`healing ring network on simple bidirectional add–drop amplifier mod-
`ules,” IEEE Photon. Technol. Lett., vol. 10, no. 9, pp. 1340–1342, 1998.
`[2] K.-P. Ho and S.-K. Liaw, “Eight-channel bidirectional WDM add/drop
`multiplexer,” Electron. Lett., vol. 34, no. 10, pp. 947–948, 1998.
`[3] C. H. Kim, C. H. Lee, and Y. C. Chung, “A novel bidirectional add–drop
`amplifier,” IEEE Photon. Technol. Lett., vol. 10, no. 8, pp. 1118–1120,
`1998.
`[4] Y. Zhao, X. J. Zhao, J. H. Chen, and F. S. Choa, “A novel bidirec-
`tional add–drop module using waveguide grating routers and wavelength
`channel matched fiber gratings,” IEEE Photon. Technol. Lett., vol. 11,
`no. 9, pp. 1180–1182, 1999.
`[5] N. Henmi, Y. Aoki, S. Fujita, Y. Sunohara, and M. Shikada, “Rayleigh
`scattering influence on performance of 10 Gb/s optical receiver with
`Er-doped optical fiber preamplifier,” IEEE Photon. Technol. Lett., vol.
`2, no. 4, pp. 277–278, 1990.
`[6] K.-P. Ho, “Analysis of homodyne crosstalk in optical networks using
`Gram-Charlier series,” J. Lightwave Technol., vol. 17, no. 2, pp.
`149–154, 1999.
`[7] K.-P. Ho and S.-K. Liaw, “Demultiplexer crosstalk rejection require-
`ments for hybrid WDM system with analog and digital channels,” IEEE
`Photon. Technol. Lett., vol. 10, no. 5, pp. 737–739, 1998.
`
`Fig. 4. Optical spectra of dropped optical channels at different locations. The
`resolution bandwidth of the optical spectrum analyzer is 0.1 nm. (a) Dropped
`optical channel 3, (b) dropped optical channel 2, and (c) dropped optical
`channel 1.
`
`channel 1 is about 60 dB because optical channel b that is
`reflected by the nonideal rolloff FBG’s three times arrives at
`the port OUT. Therefore, we cannot see b in Fig. 3(c) because
`the power level of optical channel b is lower than the noise
`level of the optical spectrum analyzer (OSA).
`Since the optical channel 1 is reflected by three individual
`FBG 1’s, the reflective center wavelengths of the FBG’s should
`be very identical. Otherwise, the small wavelength misalign-
`ment may induce a large power penalty and signal-level degra-
`dation. In our experiment, the problem of wavelength misalign-
`ment does not occur seriously because the 3-dB bandwidth of
`the FBG used in the experiment is as wide as 0.4 nm. In ad-
`dition, the reflectivity of three individual FBG 1’s should be
`very high to prevent the problems of the homodyne crosstalk
`from the optical channel 1. According to the theoretical anal-
`ysis of the homodyne crosstalk, a homodyne crosstalk level of
`less than 30 dB may induce a power penalty less than 1 dB
`at a bit error rate (BER) of 109 [6]. To avoid the problem of
`homodyne crosstalk in our system, more than 30-dB reflectivity
`of FBG’s should be used.
`
`