Two different optical fiber bus networks are compared and demonstrated experimentally as means of gathering information from four wavelength-division-multiplexed photonic sensors. Both topologies include distributed amplification, which allows one to overcome the limitation in the maximum number of sensors that can be multiplexed in a single structure. Results obtained with a dual-bus topology are compared with those achieved with a single-bus network

Amaya Unanua

Datta

David Benito

Elisa Achaerandio

Hernandez-Lorenzo

Jose Miguel López-Higuera

López-Amo

Manuel López-Amo

Silvia Abad

Vizoso

Wagner

Optics Letters

English

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Single and double distributed optical amplifier fiber bus networks with wavelength-division multiplexing for photonic sensors

Abad Valtierra, Silvia

López Amo, Manuel

López Higuera, José Miguel

Benito Pertusa, David

Unanua, Amaya

Achaerandio, Elisa

UCrea

June 15, 1999 / Vol. 24, No. 12 / OPTICS LETTERS 805Single and double distributed optical amplifierfiber bus networks withwavelength-division multiplexing for photonic sensorsSilvia Abad and Manuel López-AmoDepartamento de Ingenierı́a Eléctrica y Electrónica, Universidad Pública de Navarra, Campus de Arrosadı́a s/n, 31006 Pamplona, SpainJose Miguel López-HigueraEscuela Técnica Superior de Ingenieros Industriales y Telecomunicación, Grupo de Ingenierı́a Fontónica,Universidad de Cantabria, Avenida Los Castros s/n, 39005 Santander, SpainDavid Benito, Amaya Unanua, and Elisa AchaerandioDepartamento de Ingenierı́a Eléctrica y Electrónica, Universidad Pública de Navarra, Campus de Arrosadı́a s/n, 31006 Pamplona, SpainReceived January 28, 1999Two different optical f iber bus networks are compared and demonstrated experimentally as means of gatheringinformation from four wavelength-division-multiplexed photonic sensors. Both topologies include distributedamplif ication, which allows one to overcome the limitation in the maximum number of sensors that canbe multiplexed in a single structure. Results obtained with a dual-bus topology are compared with thoseachieved with a single-bus network.  1999 Optical Society of AmericaOCIS codes: 060.4230, 060.2370, 060.2410, 060.2300, 060.4250.The proliferation of the so-called smart materials thatare employed in aeronautics, medical applications,and intelligent buildings is increasing the number ofsensors that need to be dealt with in a single structure.It is in these fields that photonic sensors exhibit theiradvantages, especially their low weight and small size.Bus architectures are widely used in data and sensornetworks because of their simple cabling requirements,especially when a large number of sensors (or stations)must be multiplexed. The main disadvantage of thesetopologies is the power loss that is introduced by thecouplers, which limits the number of sensors that canbe connected to the structure and causes a difference inthe power that is received from each sensor, dependingon the number of couplers that the signal must cross toreach it.Overcoming the limitation imposed by the bus struc-tures by use of optical amplif ication has been pro-posed1 and has been demonstrated in both lumped2 anddistributed3 configurations. The utilization of ampli-fying fiber between couplers for this purpose is par-ticularly appealing. Thus the gain of each segmentof fiber counterbalances the attenuation that is intro-duced by the coupler located after it. This distributedamplification yields constant power in each sensor anda minor overall loss, allowing a greater number of sen-sors to be multiplexed in each structure. On the otherhand, the use of erbium-doped fiber introduces an im-portant source of noise, amplified spontaneous emis-sion (ASE), that has to be accounted for.In this Letter we demonstrate experimentally thesingle- and dual-bus structures shown in Fig. 1; simu-lations of these structures have already been pub-lished.4 The sensors that are used here provideamplitude modulation in response to the curvature in-troduced by a temperature-sensitive element.5 Eachsensor is identified by a different wavelength, assigned0146-9592/99/120805-03$15.00/0by the fiber Bragg grating (FBG) located after it. Thetotal insertion losses of each sensor and the FBG are6 dB. We locate the operation wavelengths of the FBGin the f lat region of the erbium gain profile to providegood equalization among channels 1539.60, 1547.77,1552.23, and 1560.09 nm. The FBG’s bandwidths areapproximately 0.5 nm.The signal is provided by a tunable laser source withan output power of 210 dBm and a 0.2-nm linewidth.The pump scheme is a three-level diagram and usesa 980-nm laser that delivers 60 mW of power intothe bus. We select the pump power and the lengthFig. 1. (a) Wavelength-division-multiplexed optical fibersingle-bus network with distributed gain for sensors.(b) Equivalent dual network, including a nonamplifyinglower bus. WDM’s, wavelength-division multiplexers;S1–S4, sensors. 1999 Optical Society of America806 OPTICS LETTERS / Vol. 24, No. 12 / June 15, 1999of the active fiber segments to provide 1-dB gain, whichapproximately compensates for the loss from the fol-lowing coupler. We immerse all the free terminationson the bus in refractive-index-matching gel to avoid un-wanted ref lections.In the single topology [Fig. 1(a)] the same bus is usedto carry both methods of propagation. The incomingpower propagates from left to right, and the modulatedsignal travels from right to left. The 50:50 coupler onthe header divides these two signals and introducesa 6-dB loss. If this loss is to be avoided, the couplershould be replaced with a circulator, although thissolution would increase the complexity and the priceof the system. In Fig. 2 the four return signals in thesingle configuration are superimposed upon the ASEpedestal. We obtained these results by applying thetechnique for allocating the signal wavelength to thesensor position to equalize the received signal powerthat was described in Ref. 4. The amount of noisein each channel, the signal power, and the signal-to-noise ratio (SNR) are shown in Table 1. The averagesignal power is ,241.5 dBm, with a difference of1.7 dB between extreme channels. Had we used apassive structure, this difference would have been avalue of ,3.6 dB (taking into account a 0.6-dB lossin each coupler). The SNR values are close to 13 dB,which might be too low for some applications, especiallythose with amplitude modulation. Replacement ofthe header coupler with a circulator would increasethe received signal power to 6 dB, but it would alsoapproximately double the final ASE level, yielding onlya 3-dB improvement in the final SNR values.To increase the values of SNR that are obtained withthe single structure we propose the use of the double-bus topology presented in Fig. 1(b),4 in which the upperbus is built with erbium-doped fiber and the lower oneuses standard fiber. In this case the return signalfrom the sensors is collected in the lower bus formedby couplers with a 5% coupling ratio. We selectedthis low coupling ratio to avoid high losses in thenonamplifying lower bus. The results are presentedin Fig. 3 and Table 1. In the double configuration theASE noise level coming from the active fiber is highlyattenuated when it passes through the 90:10 and the95:5 couplers. Although the losses in the lower busare not compensated for by active fiber segments,the signal power received from each channel at themonitoring point is very close (within 1 dB) to thatobtained in the single-bus topology. This fact is due tothe absence of the 50:50 header coupler, which produceshigh losses in the single-bus topology. The reductionof the ASE level translates into an increment of 20 dBin the SNR values, which turn out to be greater than32 dB. We gave the first positions a slightly highergain to minimize the effects of the greater attenuationsuffered by the signals in the lower bus. After weapply the same allocation technique that we used inthe single configuration, the power difference betweenchannels has a maximum value of 1 dB.In conclusion, we have compared experimentallytwo different wavelength-division-multiplexed data-gathering networks based on distributed optical am-plif ier buses. The improvement in the equalizationamong channels when distributed amplification isused has been confirmed. We have also experimen-tally shown the advantage in terms of signal-to-noisevalues of the double distributed optical amplif ier fiberbus network versus the single-bus topology, as waspredicted based on a previously reported theoreticalresult.4Fig. 2. Output power of the single distributed opticalamplifier bus network, as measured by the spectrumanalyzer.Table 1. Signal Power, Noise Level, and SNR for theSingle- and the Double-Bus TopologiesSingle Bus Double BusWavelength Signal Noise SNR Signal Noise SNR(nm) (dBm) (dBm) (dB) (dBm) (dBm) (dB)1539.60 241.0 253.7 12.7 241.7 274.0 32.31547.77 240.9 254.2 13.3 241.7 274.6 32.91552.23 241.7 255.0 13.3 241.9 274.9 33.01560.09 242.6 255.6 13.0 240.9 275.5 34.6Fig. 3. Output power of the double distributed opticalamplifier bus network, as measured by the spectrumanalyzer.June 15, 1999 / Vol. 24, No. 12 / OPTICS LETTERS 807Financial support through Spanish Education Min-istry projects TIC98-0397-C03 and TIC95-631-C04 andby the Gobierno de Navarra is acknowledged. Wethank P. Urquhart, R. Hernandez, A. Loayssa, I.Matı́as, and F. Arregui for their help and discussionon this subject. M. Lopez-Amo’s e-mail address ismla@upna.es.References1. J. L. Wagner, C. W. Hodgson, M. J. F. Digonnet, andH. J. Shaw, J. Lightwave Technol. 15, 1681 (1997).2. B. Vizoso, I. Matı́as, M. López-Amo, and M. A. Muriel,Opt. Quantum Electron. 27, 847 (1995).3. M. López-Amo, L. T. Blair, and P. Urquhart, Opt. Lett.18, 1159 (1993).4. R. Hernandez-Lorenzo, M. López-Amo, and P. Urquhart,J. Lightwave Technol. 16, 485 (1998).5. P. Datta, I. Matı́as, C. Aramburu, A. Bakas, M. López-Amo, and J. M. Otón, Microwave Opt. Technol. Lett. 11,93 (1996).

Single and double distributed optical amplifier fiber bus networks with wavelength-division multiplexing for photonic sensors

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