Brillouin gain spectrum measurements in fibres are presented, where a single source is used to generate the pump and probe signals, giving highly reliable results

Niklès, Marc

Robert, Philippe

Thévenaz, Luc

Brillouin gain spectrum measurements in fibres are presented, where a single source is used to generate the pump and probe signals, giving highly reliable results.THEVESCI-STI-LTPaper MD5-

Infoscience - École polytechnique fédérale de Lausanne

Brillouin Gain Spectrum Measurementsusing a Single Laser SourceMarc NiklèsEPFL Swiss Federal Institute of Technology, Metrology Lab.1015 Lausanne, SWITZERLANDTel: +41-21-6934774 Fax: +41-21-6932614 E-mail: nikles@methp 1 .epftchLuc ThévenazEPFL Swiss Federal Institute of Technology, Metrology Lab.1015 Lausanne, SWITZERLANDTel: +41-21-6934774 Fax: +41-21-6932614 E-mail: thevenaz@methp 1 .epfLchPhilippe RobertEPFL Swiss Federal Institute of Technology, Metrology Lab.1015 Lausanne, SWITZERLANDTel: +41-21-6932607 Fax: +41-21-6932614 E-mail: Phlllppe.Robert@deqm.epfl.chAbstractBrillouin gain spectrum measurements in fibres are presented, where a singlesource is used to generate the pump and probe signals, giving highly reliableresults.Brillouin Gain Spectrum Measurements using a Single Laser SourceMarc Njklès, Luc Thévenaz, Philippe RobertEPFL Swiss Federal Institute of Technology, Metrology Lab.1015 Lausanne, SWITZERLANDTel: +41-21-6934774 Fax: +41-21-6932614 E-mail: nlkles@methpl.epfl.chIntroductionStimulated Brillouin scattering (SBS) is a non-linear process that occurs in optical fibres at muchlower pump power than most other non-linear effects. The pump wave is scattered into abackward-propagating Stokes wave that experiences a frequency shift lying in the 12-13 GHzrange at 1300 nm. This shift is proportional to the acoustic velocity within the fibre, making SBSan efficient tool for sensing applications, e.g. for monitoring strains in installed fibres [1].For these applications the determination of the spontaneous Brillouin gain spectrum (BGS) is akey measurement, because it contains important information, such as the Brillouin frequencyshift VB and linewidth AVB, that depends on environmental quantities.Many techniques have been proposed to date to perform this gain spectrum measurement[2,3,4]. All these methods require two distinct sources to generate the pump and probe signals.This results in several experimental constraints that are difficult to meet, such as simultaneouspower and frequency stability of the two lasers, low jitter and narrow linewidth, accuratetunability within the Brillouin gain band. The reported measured tvB using these techniquesare at least twice larger than the predicted value for fused silica and this discrepancy is so farnot fully understood.In this paper the measure of spontaneous Brillouin gain spectrum using a single source ispresented. With this method the probe signal is generated by modulating the pump wave usinga microwave generator and a LiNbO3 guided-wave modulator, so that the probe wave actuallycorresponds to the lower sideband. This scheme offers many advantages: any laser frequencydrift is automatically compensated and no tunability of the source is required. The Brillouingain curve can be scanned with ideal accuracy and stability by sweeping the frequency of themicrowave generator.Measurements performed using this method show an excellent agreement with those obtainedusing two-sources techniques. In addition the optical scheme of the method can be modified, sothat the access to one fibre end is necessary to perform the measurement.Generation of the probe signalAs mentioned above, the traditional measurement scheme using a probe and a pump signal canbe improved and simplified by using an electrooptic modulator (EOM). In this case the probesignal is one of the generated sidebands. Fig. Ia shows the optical frequency sidebandsgenerated by intensity modulating a CW light using a LiNbO3EOM. Among the carrier andharmonics only the lower first sideband is of use. However, by properly setting the DC biasapplied to the EOM electrodes, the carrier v0 can be fully suppressed and the power transferredinto the first harmonics maximised, as shown in Fig. lb. The unused upper sideband cannot beeasily filtered out and be distinguished from the probe signal in the detection stage, but it onlycauses an intensity offset that needs to be subtracted before processing the data.A major issue is the EOM modulation bandwidth that must be in excess of the Brillouinfrequency shift (13 GHz), but such devices are now widely available. However, anothermodulation scheme was also tested that has the advantage to require a much smaller EOMbandwidth. When the DC bias of the EOM is set to suppress the carrier, the first ordersidebands are separated by twice the modulation frequency m• Provided that this frequency ishalf the Brilouin frequency (2fm yB)1 the upper sideband corresponds to the pump and thelower to the probe.a) b)V0n,-l’%h tW’‘m2f0,A5 10 15 20 25 30Frequency, GHzFig. 1 Optical spetra of CW light modulated using a EOM.a) Carrier and first oder sidebands b) Suppression of the carrier by properly setting the DC bias.Two-end measurementThe experimental set-up used to measure the BGS is schematically shown in Fig. 2. The lightdelivered by a 1.32 J.Lm diode-pumped YAG laser is split by a 3 dB coupler. One output servesas a pump for the fibre under test, and the other is launched into the EOM, in order to generatethe probe signal. The probe signal is then fed back into the output end of the fibre under test.An in-line isolator is inserted into the loop before the EOM to prevent interferences to occur. Asthe modulation frequency is swept through the BGS, the relative output level of the probe isrecorded, and yields the gain curve after a simple data processing. Polarisation controller areused to match the polarisations of the pump and probe waves, in order to maximise themeasurement contrast.Fig. 2 Expermental setup for BGS measurements using a EOM to generate the probe signal.Fig. 3 shows the BGS of a 25 km dispersion-shifted single mode fibre. It clearly shows two peaksseparated by 260 MHz, which attest the presence of two guided acoustic modes (LOom) thatCU 5 10 15 20Frequency, GHz25 30Optical IsolatorPump Laser Test fiberRFGeneratormay exist in this type of fibre (Ge02-doped core) [41. Since the linewidth of the sidebands arevery narrow (<20 kHz), the resolution of the measurement is excellent. Moreover, the use of asingle laser greatly decreases the frequency jitters and consequently improves the resolution onthe linewidth determination. The measured FWHM of the BGS is 43 MHz, which is nearly twicethe value in bulk silica (22 MHz at 1.3 jim) showing a remarkable fibre homogeneity for such afibre length.CuCu0EFig. 3 Brillouin gain spectrum of a 25 km dispersion-shifted single mode fibre.Single-end measurementThe set-up in Fig. 2 requires that the fibre far end is not remote. Fig. 4 shows a simpler schemethat requires only one fibre end to be accessible. After passing through the EOM the modulatedlight is launched into the test fibre. When the light propagates in the forward direction, thecarrier pumps the medium, while the whole light experiences the fibre loss. A small part of thelight is reflected at the fibre far end through Fresnel reflection. The lower sideband acts as aprobe signal and is thus amplified by the SBS process, the undepleted part of the pump beingjust attenuated.As mentioned above, the necessary EOM bandwidth may be reduced by properly setting theDC bias. In fact, when the carrier is suppressed, the upper sideband pumps the medium in theforward direction while the lower sideband plays the role of the probe after being reflected atthe fibre end. When the frequency separation of the sidebands lies within the BGS, the probe isamplified whereas the undepleted pump is attenuated. By sweeping the modulation frequencyFrequency, GHzEOModulatorCouplerRFGeneratorCleavedendTest FiberETFig. 4 Experimental setup for single-end Brillouin gain spectrum measurements.of the EOM, and monitoring the relative level of the output signal on the detector, we can scanthe BGS. The signal to noise ratio can be improved by modulating the EOM RF driving signal inorder to perform a lock-in detection. Fig. 5 shows the BGS of a 12.8 km OCVD single-modefibre. In this modified scheme the pump depletion can in most cases not be neglected, so thatthe BGS is obtained using a more complicated data processing.CuG)EFig. 5 Brillouin gain spectrum of a 12.8 km OCVD single-mode fibreConclusionThe results obtained using this method confirm the latest measurements [3] that the Brillouingain linewidth in optical fibre (43 MHz) is very close to twice that observed in bulk silica(22 MHz). The inherent high stability of the jitter-free probe signal gives a good level ofconfidence to this observation and this should hopefully clarify the further discussions.The generation of the probe signal using a modulator may bring many more advantages inaddition to those presented here, such as frequency-domain coding for a more sophisticatedand sensitive detection and time-domain coding to address distinct positions along the fibre.These more advanced modulation schemes are currently investigated.AcknowledgementsThe authors are grateful to Swiss PU for their kind support and valuable discussions and toP.A. Nicati at Stanford University for fruitful comments and help.References1. T. Horiguchi, T. Kurashima, M. Tateda, “A Technique to Measure Distributed Strain in OpticalFibers”, IEEE Photonics Tech. Lett. 2, pp.352-354, (1990).2. N. Shibata, R.-G. Waarts, R. P. Braun, “Brillouin-gain spectra for single-mode fibers having puresilica, GeO2-doped cores”, Opt. Lett. 12, pp.269-271, (1987)3. T.-O. Tsun, A. Wada, T. Sakai, R. Yamauchi, “Novel Method using White Spectral Probe Signals toMeasure Brillouin Gain Spectra of Pure Silica Core Fibers”, Electron. Lett. 28, pp.247’-249, (1992).4. N. Shibata, K. Okamoto, Y. Azuma, “Longitudinal acoustic modes and Brillouin-gain spectra forGe02-doped core single-mode fibers”, J. Opt. Soc. Am. B, 6, pp.1167-ll7’4, (1989).Frequency, GHz

Brillouin Gain Spectrum Measurement using a Single Laser Source

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Brillouin Gain Spectrum Measurement using a Single Laser Source

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