TUNABLE FIBER ATTENUATORS FORCHANNEL POWER EQUALIZATIONIN WDM SYSTEMSZ. Chen,1, * K. S. Chiang,1 K. O. Lee,1 and K. P. Lor11 Optoelectronics Research Centre and Departmentof Electronic EngineeringCity University of Hong KongHong Kong, P.R. China

Recei ed 26 January 2000

ABSTRACT: A tunable fiber attenuator is proposed for channel power( )equalization in wa¨elength-di ision-multiplexed WDM optical commu-

nication systems. This fiber attenuator consists of two identical fiberBragg gratings, which are subject to bending in opposite directions by anelectrically controlled mechanical dri er. The attenuation le¨el at thedesired wa¨elength depends on the amount of bending applied to thegratings. A de¨ice with a continuous adjustment of attenuation o¨er arange of 32 dB and a dri ing ¨oltage of less than 2 V has beendemonstrated. The performance of these tunable attenuators for channelpower equalization in a two-channel WDM system has been measured.

( )Excellent spectral flattening qry 0.1 dB has been achie ed for staticchannel power equalization. Q 2000 John Wiley & Sons, Inc.Microwave Opt Technol Lett 26: 1]4, 2000.

Key words: optical communications; optical filters; optical attenuators;optical fibers; fiber Bragg gratings

1. INTRODUCTION

Ž .In wavelength-division-multiplexed WDM optical communi-cation systems, power equalization is usually required inreconfigurable photonic networks, where different channel

w xsignals travel along different paths 1 . Channel power equal-ization can be achieved with an adjustable fiber attenuatorover a fixed narrow-wavelength region. While band-rejectionfilters can be made easily with fiber Bragg gratings or long-period gratings to provide fixed attenuation, it is necessary toincorporate an amplitude-modulation mechanism into suchfilters to turn them into adjustable attenuators. For example,an adjustable fiber attenuator can be made by variation ofthe central wavelength of a Bragg grating or a long-period

w xfiber grating 2]4 . However, such methods may create prob-lems in WDM systems because several channel signals can beaffected simultaneously when one channel is attenuated.

w xStarodubov et al. 1 have presented a fiber amplitudemodulator for channel power equalization in WDM systems,which consists of two identical long-period fiber gratings and

Ž .an electromechanical driver loudspeaker . A major disadvan-tage of their device as an attenuator is that it has muchstronger temperature dependence compared with fiber-Bragg-grating-based devices. In this paper, we propose anovel fiber-Bragg-grating-based tunable attenuator, whichuses a simple principle to adjust the power of the wavelengthchannel. It is an all-fiber attenuator, and can offer manyadvantages: low cost, compatibility with fibers, polarizationinsensitivity, and a wide, adjustable attenuation range.

2. PRINCIPLE OF OPERATION

We consider two identical fiber Bragg gratings, which areconnected in series with sufficient separation and glued on

*Present address: School of Electrical and Electronic Engineering,Nanyang Technological University, Singapore 639798.Contract grant sponsor: University Grants Committee, Hong Kong Gov-ernment

the two sides of a cantilever beam. The transmission spec-Ž .trum from the two gratings is shown in Figure 1 a . As the

gratings are identical, they have the same Bragg wavelength,and therefore, only one rejection band appears in the spec-trum. By bending the cantilever beam, one grating is bent

Ž .inward compressed , while the other one is bent outwardŽ .tensioned . As a result, the Bragg wavelengths of the twogratings shift in opposite directions. When the amount of

Ž . Ž .bending increases, as shown in Figure 1 b and c , the tworejection bands of the gratings split, and eventually becomeseparated. As can be seen from Figure 1, the transmission atthe original Bragg wavelength changes continuously fromnearly complete rejection to nearly complete transmissionwhen the amount of bending applied to the gratings increasescontinuously. This mechanism provides an efficient means forcontrolling the attenuation level at the Bragg wavelength ofthe straight gratings. If the channel spacing is much widerthan the bandwidth of the Bragg grating, the attenuatorshould only work for a particular channel, and has littleinfluence on the other channels.

3. EXPERIMENTS

The fiber Bragg gratings used in our experiments were fabri-Ž .cated with photosensitive fibers from QPS . To form a Bragg

Žgrating in the fiber, a section of the fiber with the jacket.removed was exposed to 248 nm UV radiation from a KrF

excimer laser through a phase mask. The exposure conditionswere typically 100]150 mJrpulse, with a pulse repetition rateof 10 Hz. The transmission spectrum was actively monitoredduring the fabrication.

Two Bragg gratings with nearly identical transmissionspectra were fabricated. They were connected in series, and

Figure 1 Illustration of the expected transmission spectra of twoconcatenated identical fiber Bragg gratings mounted on the opposite

Ž . Ž . Ž .sides of a beam subject to: a no bending, b small bending, c largebending

MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 26, No. 1, July 5 2000 1

glued on the two sides of a flexible beam made of PMMA. Aloudspeaker was attached to the beam, and was used as anelectromechanical driver to bend the beam. When the beamwas bent, one grating was compressed, while the other wastensioned.

Figure 2 shows the transmission spectra from the concate-nated gratings measured with an optical spectrum analyzerand an erbium-doped fiber ASE source. Without bending, theBragg wavelengths of the two gratings were almost the same

Figure 2 Measured transmission spectra of two concatenated iden-tical Bragg gratings mounted on the opposite sides of a beam subject

Ž . Ž . Ž .to: a no bending, b small bending, c large bending

Ž .1551.6 nm , and the two rejection bands of the gratingsŽ .overlapped nearly completely, as shown in Figure 2 a . When

bending was applied to the beam, the two rejection bands ofthe gratings shifted in opposite directions, as shown in FigureŽ . Ž .2 b and c , in good agreement with the theoretical predic-

tion. It can be seen from Figure 2 that attenuation over 20dB was achieved at the 1551.6 nm wavelength. The attenua-tor had a FWHM passband of 0.4 nm, an adjustable attenua-tion range over 20 dB, and an insertion loss less than 1 dB.The full spectral width of the attenuator was approximately1.8 nm. All channels outside the spectral width of the attenu-ator were practically unaffected.

In general, the sideband suppression from the resonancevaried with the modulation depth, and a value of 9 dB is

Ž .shown in Figure 2 c . The sideband suppression could beimproved by using gratings with larger reflectivities at theBragg wavelengths. Figure 3 shows the measured transmis-sion spectra of two concatenated saturated Bragg gratingsthat had much deeper rejection bands. The attenuator pro-vided attenuation at the 1562.4 nm wavelength, and had anFWHM passband width of 0.6 nm and an adjustable attenua-tion range over 32 dB. The excess loss was less than 4 dB.The sideband suppression from the resonance was over 36dB. The full spectral width of the device was approximately 3nm, which is broader than the one shown in Figure 2.

Typical variation of the attenuation with the voltage ap-plied to the electromechanical driver is shown in Figure 4.The attenuation level easily can be controlled electrically. Acontinuous adjustment of attenuation over a range as high as32 dB was achieved with less than 2 V by using our experi-mental setup.

The performance of these novel tunable attenuators forchannel power equalization in a WDM system was measured

Ž .with a two-channel system. As shown in Figure 5 a , theŽ .channel at 1551.6 nm right was 9 dB higher in power than

Ž .the channel at 1548.6 nm left . By using the tunable fiberattenuator shown in Figure 2, the power levels of the twochannels were equalized to within qry 0.1 dB, as shown in

Ž .Figure 5 b . Similar experiments were performed with theattenuator shown in Figure 3. The results are presented inFigure 6, which shows that the power levels of the channels at1556.2 and 1548.6 nm were equalized to within qry 0.1 dBfrom an initial difference of 25 dB. It is obvious from theseresults that excellent spectral flattening in WDM systems canbe achieved with the present tunable fiber attenuators.

A concern with the present fiber attenuator is the genera-tion of high reflection from the Bragg gratings. This problemcan be partly overcome by using blazed fiber Bragg gratings.The introduction of a properly tilted angle in the writing ofthe grating can cause the reflected light to couple into thefiber cladding. The light in the cladding is absorbed by thejacket of the fiber, and thus is removed from the reflectionspectrum. The attenuators demonstrated in this paper, infact, were made of low-reflection blazed fiber Bragg gratings,which had a reflection of about y5 dB. By using a fiber with

w xa photosensitive cladding 5 , back reflection could be re-duced further.

4. CONCLUSION

We have demonstrated a novel electrically tunable fiberBragg grating attenuator. This device is based on bending twoidentical Bragg gratings in opposite directions to control theamount of transmission at the Bragg wavelength of the grat-ings. A continuous adjustment of attenuation over a range of

MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 26, No. 1, July 5 20002

Figure 3 Measured transmission spectra of two concatenated satu-rated Bragg gratings mounted on the opposite sides of a beam which

Ž . Ž . Ž .is subject to: a no bending, b small bending, c large bending

Figure 4 Measured attenuation as a function of the applied voltage

Figure 5 Demonstration of static channel power equalization in aŽ . Ž .two-channel WDM system. a No channel equalization is applied. b

The right channel at 1551.6 nm is equalized against the left referencechannel at 1548.6 nm to within qry 0.1 dB

32 dB has been achieved with less than 2 V. With thisattenuator, channel equalization to an accuracy within qry0.1 dB has been demonstrated with a two-channel system thathas an initial power imbalance as high as 25 dB. The presentattenuator is particularly suitable for channel power equaliza-tion in WDM systems. New applications in optical filters,optical switches, and optical sensors are also expected.

MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 26, No. 1, July 5 2000 3

Figure 6 Demonstration of static channel power equalization in aŽ . Ž .two-channel WDM system. a No channel equalization is applied. b

The right channel at 1556.2 nm is equalized against the left referencechannel at 1548.6 nm to within qry 0.1 dB

REFERENCES

1. D.S Starodubov, V. Grubsky, A. Skorucak, J. Feinberg, K.-M.Feng, J.-X. Cai, and A.E. Willner, Novel fiber amplitude modula-tors in dynamic channel power equalization in WDM systems,Proc Opt Fiber Commun Conf, 1998, paper PD8-1.

2. A.M. Vengsarkar, J.R. Pedrazzani, J.B. Judkins, P.J. Lemaire, N.S.Bergano, and C.R. Davidson, Long-period fiber-grating-based gain

Ž .equalizers, Opt Lett 21 1996 , 336]338.3. V. Bhatia, T.D. Alberto, K.A. Murphy, R.O. Claus, and C.P.

Nemarich, Comparison of optical fiber long-period and BraggŽ .grating sensors, SPIE Proc 2718 1996 , 110]121.

4. S.K. Liaw, K.P. Ho, and S. Chi, Dynamic power-equalized EDFAmodule based on strain tunable fiber Bragg gratings, IEEE Pho-

Ž .ton Technol Lett 11 1999 , 797]799.5. I. Riant, L. Gasca, P. Sansonetti, G. Bourrent, and J. Chesnoy,

Gain equalization with optimized slanted Bragg grating on adaptedfiber for multichannel long-haul submarine transmission, Proc OptFiber Commun Conf, 1999, paper THJ6-1.

Q 2000 John Wiley & Sons, Inc.

ON EXTRACTING THE RADIATIONCENTER REPRESENTATION OFANTENNA RADIATION PATTERNS ON ACOMPLEX PLATFORMTao Su1, C. Ozdemir,2 and Hao Ling11 Department of Electrical and Computer EngineeringUniversity of Texas at AustinAustin, Texas 787122 Dept. of Electrical EngineeringMersin University, Turkey

Recei ed 2 February 2000

ABSTRACT: Using full-wa¨e computational electromagnetics data, wedemonstrate that the radiated field from an antenna on a complexplatform can be sparsely represented by a radiation center model. Anextraction scheme based on the matching-pursuit algorithm is de¨ised toextract the model parameters from multifrequency, multiaspect radiationdata. To accelerate the parameterization procedure, the pre¨iously de¨el-oped antenna synthetic aperture radar imaging algorithm is used topro¨ide an initial estimate of the radiation center position during thesearch. It is shown that the resulting radiation center model pro¨ides asparse, physical representation of the antenna]platform interaction.Q 2000 John Wiley & Sons, Inc. Microwave Opt Technol Lett 26:4]7, 2000.

Key words: radiation center model; antenna]platform interaction;matching-pursuit algorithm

I. INTRODUCTION

In antenna analysis and design, the mounting platform canhave a significant effect on the antenna radiation characteris-tics. In addition to the forward problem of characterizing theantenna radiation in the presence of the platform, also ofimportance is the development of a sparse model to describethe radiation physics in antenna]platform interaction. To-ward this end, we have previously proposed a point radiatormodel to represent the complex radiation patterns resulting

w xfrom antenna]platform interactions 1, 2 . In this model, weassume that the platform scattering due to the primary an-tenna radiation can be approximately described by a set of‘‘radiation centers’’ on the platform. This concept is similar tothe scattering center model widely used in radar scattering,where the high-frequency scattering from a complex targetoften can be modeled efficiently by a sparse set of point

w xscatterers on the target 3]6 . To extract the radiation centermodel from a multifrequency, multiaspect antenna radiationpattern, we have proposed a parameterization scheme basedon microwave imaging. The procedure entails first generating

Ž .the 3-D antenna synthetic aperture radar ASAR imagery ofthe platform, and then parameterizing the resulting image bya collection of point radiators via the CLEAN algorithm. Itwas shown that, once such a representation is obtained, wecan rapidly reconstruct antenna radiation patterns over fre-quencies and aspects with good fidelity. Thus, such a modelcan be used for the real-time reconstruction of complexantenna patterns in high-level system simulations. Further-more, the resulting radiation center information can be usedto pinpoint cause-and-effect in platform scattering, and toprovide design guidelines for antenna placement and opti-mization. However, the method was based on a Fourier-based

Contract grant sponsor: Office of Naval Research

Contract grant number: N00014-98-1-0178

MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 26, No. 1, July 5 20004