can easily be derived by our theory. It is found that the outputimpedance intrinsically behaves as a series RC circuit (for lowsubstrate resistance) or a “shifted” series RC circuit (for very highsubstrate resistance) at low frequencies, and a parallel RC circuitat high frequencies. It is this inherent triple characteristic of theoutput impedance that causes the dip points described in [1](type-II dip) and [2, 3] (type-I dip).

ACKNOWLEDGEMENT

This work was supported by the National Science Council of theR.O.C. under Contract NSC90-2218-E-260-007.

REFERENCES

1. H. Hjelmgren and A. Litwin, Small-signal substrate resistance effect inRF CMOS identified through device simulations, IEEE Trans ElectronDevices 48 (2001), 397–399.

2. S.-S. Lu, C. Meng, T.-W. Chen, and H.-C. Chen, A novel interpretationof transistor S-parameters by poles and zeros for RF IC circuit design,IEEE Trans Microwave Theory Techniques 49 (2001), 406–409.

3. H.-Y. Tu, Y.-S. Lin, P.-Y. Chen, and S.-S. Lu, An analysis of theanomalous dip in scattering parameter S22 of InGaP/GaAs heterojunc-tion bipolar transistors (HBTs), IEEE Trans Electron Devices 49 (2002),accepted for publication.

4. Y. Aoki and Y. Hirano, High-power GaAs FETs, High Power GaAsFET Amplifiers (1993), 81.

5. P.R. Gray and R.G. Meyer, Analysis and design of analog integratedcircuits, Wiley, New York, 1993, pp. 579–584.

6. R.A. Minasian, Simplified GaAs M.E.S.F.ET. model to 10 GHz, Elec-tron Lett 13 (1977), 549.

© 2003 Wiley Periodicals, Inc.

MULTIWAVELENGTH FIBER LASERUSING A SAGNAC LOOP FILTER

Gautam Das1 and John W. Y. Lit1,2

1 Guelph Waterloo Physics InstituteDepartment of PhysicsUniversity of WaterlooWaterloo, Ontario, N2L 3G1, Canada2 Department of Physics and ComputingWilfrid Laurier UniversityWaterloo, Ontario, N2L 3C5, Canada

Received 19 July 2002

ABSTRACT: A Sagnac interferometer has been used in the ringcavity to produce a multiwavelength fiber laser. The results showthat an elliptical-core erbium-doped fiber can produce more stablelasing lines compared to a normal erbium-doped fiber. The principleof operation is based on polarization-dependent gain phenomena inerbium-doped fiber. The number of lasing lines and stability may beadjusted by using a polarization controller to control the polariza-tion of the wave inside the cavity. © 2003 Wiley Periodicals, Inc.Microwave Opt Technol Lett 36: 200 –202, 2003; Published online inWiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.10719

Key words: Sagnac loop filter; multiwavelength fiber laser; laser tun-ing; elliptical fiber; polarization-dependent gain

INTRODUCTION

Erbium-doped fiber lasers, which can simultaneously generatemultiple wavelengths, are of great interest because they may serveas simple and compact sources for applications in wavelength-division multiplexed (WDM) communications and sensor systems.The basic requirements in multiwavelength fiber lasers (MWFL)are uniform output powers for the lasing lines and precise linespacing, stability, and tunability. An erbium-doped fiber is a ho-mogeneous gain medium. To obtain multiple wavelengths, weneed inhomogeneous gain to reduce the mode competition be-tween the lasing lines. Different techniques have been proposed forroom temperature operation to produce inhomogeneous gain inEDF, and thus multiple wavelengths in lasers, by exploiting po-larization hole burning [1], using elliptical core erbium-doped fiber[2, 3], and also cooling the EDF to liquid nitrogen temperature [4]and using a comb filter [5, 6].

In this paper, we propose and demonstrate a simple techniqueto obtain multiple wavelengths by using a Sagnac loop filter [7, 8]in a unidirectional ring cavity. Here we present results obtainedwith two different types of erbium-doped fibers as the gain me-dium: (a) single-mode erbium-doped fiber (SM-EDF) and (b)polarization-maintaining erbium-doped fiber (PM-EDF). Our re-sults show that polarization-anisotropic gain behavior [9, 10] inerbium-doped fiber can give stable multiple lasing lines at roomtemperatures. It also shows that the use of a PM-EDF can increasethe stability of the lines, power, and tuning capabilities.

EXPERIMENTAL SETUP

Figure 1 shows the experimental setup for the multiwavelengthfiber laser, as follows: a 3-dB coupler, a polarization-maintainingfiber (PMF, bow-tie) with a beat length of about 4 mm at 1550 nm,and a polarization controller (PC2) form the Sagnac interferome-ter, which acts as a comb filter in the laser cavity. The intensitytransfer function of the Sagnac interferometer, and thus the spacingbetween filter transmission peaks, depend on the loop birefrin-gence. Hence, by adjusting the length of the PMF, we can change

Figure 5 Effect of C�ds on the anomalous dip points of S22

200 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 36, No. 3, February 5 2003

the spacing of the lasing lines, for example, to make the lines tofall on the ITU grid [8]. In the ring we have an erbium-doped fiberas gain medium, which in laser 1 is a single mode erbium-dopedfiber (SM-EDF) of length 9 m, numerical aperture 0.17, anddoping concentration of 1400 ppm. In laser 2, the gain medium isa single mode elliptical core fiber (PM-EDF) with minor and majordiameters of 3.8 and 14.8 �m, respectively, length 6.56 m, numer-ical aperture 0.15, and doping concentration 2800 ppm. A polar-ization-independent optical isolator is used to generate unidirec-tional propagation in the ring cavity, in order to avoid spatial holeburning effects in the gain medium. A 980-nm laser diode with amaximum power of 72 mW is used to pump the erbium-dopedfiber through a 980/1550-nm wavelength division multiplexer. Thepolarization controller (PC1) in the ring can vary the number oflasing lines by changing the polarization of the wave inside thecavity. A 10% coupler is used to extract the output, which is fedinto an optical spectrum analyzer through a polarization-indepen-dent isolator.

RESULTS AND DISCUSSION

First, we consider an SM-EDF as the gain medium in the ringcavity. A PMF of length 4 m is used in the Sagnac loop, whichproduces a filter of spacing of about 1.6 nm. Figure 2(a) shows onelasing line with more than 1-mW power. Figure 2(b) shows twolasing lines of approximately equal powers (� �3.25 dBm) witha separation of 1.61 nm. The single line is stable for more than 30minutes while the two lines are stable for only several minutes. Ingeneral, line stability decreases when the number of lines in-creases. The intensity fluctuations in both cases are less than 0.5dBm. The side lobe intensities are about �20 dBm. A maximumof four lines has been obtained with very small output powers (��30 dBm) and they are stable for only several tenths of a second.When the length of the PMF decreases, the maximum number ofobtainable lines decreases, while their separation increases. Thus,when the length of the PMF is less than 3 m, a maximum numberof two lines with separation of more than 2.25 nm are obtained.

Next we consider a PM-EDF as the gain medium in the ringcavity. Figure 3(a), (b), and (c) are obtained with the lengths of thePMF in the Sagnac loop section equal to 3.79 m, 7.6 m, and 8 m,respectively; the line separations in the three cases are 1.61 nm, 0.8nm, and 0.76 nm, respectively. In all the above cases, the lines arestable over 10 minutes, and the output power fluctuations are lessthan 0.5 dBm. The side lobe intensities are less than �20 dBm. In

each of the cases, changing the polarization controller PC1 mayvary the number of lines emitted. As expected, when the spacingbetween the lines increases, the number of lasing lines decreases.When the number of lines emitted decreases, their stability in-creases; the output power fluctuations decrease. The lasing thresh-old for all five lines is 20 mW. The 3-dB bandwidth of each lineis 0.06 nm, limited by the resolution bandwidth of the spectrumanalyzer for all lasing lines.

By using a polarization analyzer at the output, the polarizationstates of the lasing lines are found to be elliptical with differentazimuth angles. Thus different lasing lines occupy different spatialpositions within the fiber core with maximum ellipticity. An ellip-tical EDF has more prominent polarization anisotropic gains thana normal SM-EDF [9] when it is used as the gain medium. Withregard to phase changes caused by strain and temperature, anelliptical fiber suffers much smaller changes when compared withother type of polarization-maintaining fibers [11]. Therefore, anelliptical PM-EDF makes a more stable multiwavelength fiberlaser. To study the effects of the PM-EDF in the ring, we tookthree different lengths of PM-EDF: 8.56 m, 7.56 m, and 6.56 m,with �8.0 m of PMF in the Sagnac loop section; in all cases theseparations between the lines were 0.76 nm. So, the length of thePM-EDF has no effect on the spacing of the lasing lines, whichdepends solely on the length of the PMF in the loop.

CONCLUSION

A fiber laser has been experimentally studied. It can producemultiple lines at room temperature that may be changed by adjust-

Figure 1 Experimental setup of the multiwavelength fiber laser. EDF:Erbium-doped fiber; PMF: polarization maintaining fiber; PC1 & PC2:polarization controller; OI: polarization independent optical isolator

Figure 2 Output spectrum from the laser using a SM-EDF: (a) one lasingline of more than 1-mW power; (b) two lasing lines of 1.61-nm separation.[Color figure can be viewed in the online issue, which is available atwww.interscience.wiley.com.]

MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 36, No. 3, February 5 2003 201

ing a polarization controller. Compared with other types of polar-ization-maintaining fibers, a fiber with an elliptical core mayincrease the stability of the laser lines.

ACKNOWLEDGMENT

This research is supported in part by the National Science andEngineering Research Council of Canada and by the CanadianInstitute of Photonics Innovations.

REFERENCES

1. J. Sun, J. Qiu, and D. Huang, Multiwavelength erbium-doped fiberlasers exploiting polarization hole burning, Opt Commun 182 (2000),193–197.

2. G. Das and J.W.Y. Lit, L-band multiwavelength erbium-doped fiberlasers using an elliptical fiber, IEEE Photon Technol Lett 14 (2002),606–608.

3. G. Das and J.W.Y. Lit, Room temperature multiwavelength operationof an elliptical core erbium-doped fiber laser, Microwave Opt TechnolLett 33 (2002), 184–186.

4. X.P. Dong, S. Li, K.S. Chiang, M.N. Ng, and B.C.B. Chu, Multiwave-length erbium-doped fiber laser based on a high-birefringence fiberloop mirror, Electron Lett 36 (2000), 1609–1610.

5. J. Chow, G. Town, B. Eggleton, M. Ibsen, K. Sugden, and I. Bennion,Multiwavelength generation in an erbium-doped fiber laser using in-fiber comb filters, IEEE Photon Technol Lett 8 (1996), 60–62.

6. H.L. An, X.Z. Lin, E.Y.B. Pun, and H.D. Liu, Multi-wavelengthoperation of an erbium-doped fiber ring laser using a dual-pass Mach-Zehnder comb filter, Opt Commun 169 (1999), 159–165.

7. X. Fang and R.O. Claus, Polarization-independent all-fiber wave-length-division multiplexer based on a Sagnac interferometer, Opt Lett20 (1995), 2146–2148.

8. N.J.C. Libatique and R.K. Jain, A broadly tunable wavelength-select-able WDM source using a fiber Sagnac loop filter, IEEE PhotonTechnol Lett 13 (2001), 1283–1285.

9. B. Srinivasan, S. Gupta, and R.K. Jain, Polarization anisotropic gainbehavior in elliptical-core rare-earth-doped fibers, Doped Fiber De-vices and Systems, SPIE 2289 (1994), 51–55.

10. V.J. Mazurczyk and J.L. Zyskind, Polarization dependent gain inerbium doped-fiber amplifiers, IEEE Photon Technol Lett 6 (1994),616–618.

11. F. Zhang and J.W.Y. Lit, Temperature and strain sensitivity measure-ments of high-birefringence polarization-maintaining fibers, Appl Opt32 (1993), 2213–2218.

© 2003 Wiley Periodicals, Inc.

MULTI-CHANNEL LONG-DISTANCEOPTICAL FIBER TRANSMISSION USINGDISPERSION-INDUCED MICROWAVETRANSMISSION WINDOWS

C. Gutierrez-Martınez,1 P. Mollier,2 H. Porte,2

I. Zaldıvar-Huerta,1 L. Carcano-Rivera,1 J. P. Goedgebuer2

1 Instituto Nacional de AstrofısicaOptica y Electronica (INAOE)Lab. de Microondas-GTM72 000 Puebla, Pue., Mexico2 GTL-CNRS TELECOMUMR CNRS 6603Georgia Tech Lorraine2-3, rue Marconi57070 Metz Cedex, France

Received 23 July 2002

ABSTRACT: In this paper, the model and implementation of an op-tical transmission system using a multi-longitudinal mode opticalsource and a dispersive optical channel are described. Such a systemexhibits multi-bandpass transmission windows, as a result of a com-bination of the wideband optical spectrum and chromatic dispersionon the optical channel. As shown in this paper, the band-pass win-dows are centered at frequencies that depend on the spectral freerange of the optical source and optical channel length. In the tele-communications domain, band-pass transmission windows can beused for multiplexing analog and/or digital signals, as an alternativeway for transmitting baseband and modulated subcarriers, throughoptical-fiber links. To show such a potential application, the multi-

Figure 3 Output spectrum from the laser using a PM-EDF with differentlengths of PMF in the Sagnac loop: (a) length of the PMF � 3.79 m, lineseparations � 1.61 nm; (b) length of PMF � 7.6 m, line separations � 0.8nm; (c) length of PMF � 8 m, line separations � 0.76 nm. [Color figurecan be viewed in the online issue, which is available at www.interscience.wiley.com.]

202 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 36, No. 3, February 5 2003