1016 OPTICS LETTERS / Vol. 18, No. 12 / June 15, 1993

Heat scan: a simple technique to study gratings in fibers

W. Margulis, I. C. S. Carvalho, and P. M. P. Gouvea

Pontificia Universidade Cat6lica do Rio de Janeiro, C.P. 38071, 22453-900, Rio de Janeiro, RI, Brazil

B. Lesche

Physics Institute, Universidade Federal do Rio de Janeiro, C.P. 68528, 20000, Rio de Janeiro, RI, Brazil

Received March 5, 1993

A simple and powerful technique to characterize gratings in fibers is described. It is based on scanning a smallheating element along the fiber so as to disturb the phase relationship of the waves in a controlled way.

Efficient frequency doubling in Ge-doped opticalfibers' has attracted much attention because findingthe explanation for the process is still a challengeand because it has important potential applications.Although the microscopic details of the creation of asecond-order nonlinear susceptibility are still muchan open question, it is now generally believed that aperiodic grating is written in the fiber through theinteraction of the fundamental and second-harmonic(SH) waves during the preparation process.2 Theextension of the grating (several centimeters) isvery long compared with its period (-30-40 ,um),and a nearly constant phase relationship must existfor thousands of periods to account for the highconversion efficiencies observed. As a consequence,any small disturbance in the relative phase of thewaves or in the period of the grating can dramaticallyreduce the conversion efficiency. This fact makesit possible to use frequency-doubling fibers asdistributed temperature or stress sensors and hasbeen exploited as a means to study second-harmonicgeneration (SHG) in fibers.3 A moving oven, whichraised the temperature of the rear part of a fibersufficiently to prevent SHG, gave the informationnormally obtained by cutback experiments in anondestructive way. The disturbance can also beintroduced intentionally by heating the fiber locally,thereby altering the phase of the waves. Thismechanism has been used in planar waveguidesas a way to modulate and tune an interferometricmultiplexer.4 In the case of frequency-doublingfibers, even if the temperature necessary to annealthe grating is not reached,6 local heat can causea (temporary) phase change that results in totalor partial destructive interference of the SH signalalong the fiber. In this Letter we describe a simpleand powerful technique to characterize frequency-doubling gratings in fibers by scanning a smallheating element along the fiber. We call it the heat-scan technique, and we discuss some studies andapplications.

Preliminary tests were carried out by moving a100-W incandescent lamp close to the setup with thefrequency-doubling fiber or alternatively by warm-ing a copper base on which the fiber rested. Forevery temperature increase of 3VC, the conversion

efficiency dropped to -83% of the original value,over the range 22-40'C, a result similar to that ofRef. 3. We also observed that, when the laboratorywas cooler than it had been when the fiber hadbeen prepared, it was advantageous to heat the fiberto improve the phase-matching condition. Althoughthese results are hardly surprising, they show thatlong-term comparisons of the conversion efficiencyshould be carried out with careful control of thetemperature conditions. Furthermore, they indicatethat annealling experiments, in which the conversionefficiency of a frequency-doubling fiber is monitoredafter a high-temperature heat treatment, must becarried out with the fiber back at the temperatureprevailing during preparation.

The experimental setup used for the heat scan is asfollows. A piece of fiber was prepared for frequencydoubling, and its conversion efficiency was monitoredwhile the fiber lay straight along a wooden ruler. A1-W, 1O0-0 resistor connected to a 0-16-V dc powersupply was placed on the ruler transverse to the fiberto heat a small region adjacent to it. In this way, ashort length of fiber under the resistor was heated toa temperature that could be conveniently adjusted.The temperature of the heating element was mea-sured with a contacting thermocouple and found toreach the steady state a few minutes after the voltagebias had been altered. Although the temperaturecould be brought from 220C (room temperature) to:1000C, it is suspected that the temperature of theheated part of the fiber was always considerablylower than 1000C because of the geometry of thearrangement and the presence of the primary coating.The fiber used in these experiments was a Ge-dopedfiber with a 6.7-Aum core diameter, a 1.099-pum cutoffwavelength, and a 1.359-,gm minimum dispersionwavelength. The laser system was a Nd:YAG laseroperating Q switched and mode locked. The averagepower used for preparation was typically 130 mW offundamental and 0.1 mW of SH seed- radiation.

The initial test was carried out with the conversionefficiency monitored as a voltage bias of 13.5 V wasswitched on and off. The resistor was placed --10 cmfrom the input end of the fiber. Figure 1 shows a plotof the time evolution of the SH power generated bythe prepared fiber. A sharp drop in the conversion

0146-9592/93/121016-03$6.00/0 © 1993 Optical Society of America

June 15, 1993 / Vol. 18, No. 12 / OPTICS LETTERS 1017

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Fig. 1. Time evolution of the SH power generated by aprepared fiber as the 13.5-V bias of the resistor is switchedon and off. The resistor is placed 10 cm from the fiberinput end.

efficiency followed after the supply was turned on,indicating that the optimal phase arrangement of thegrating was disturbed by the heating element. Theefficiency dropped considerably, even though a smallpart of the fiber was heated. This resulted from thefact that the SH radiation created before the resistorinterfered destructively with the SH generated afterthe resistor and not from the fact that a small partof the grating in the fiber had the wrong periodicityfor phase matching. As the resistor became evenwarmer, the conversion efficiency increased again,and later it dropped once more, slowly approachinga well-defined steady-state value. This time evo-lution can be interpreted as arising from the factthat the temperature reached by the heating elementproduced a phase change that exceeded 2wr. Forthis particular voltage (thermal power), the phaseshift was close to 3w. As the power supply wasturned off, the temperature dropped and the con-version efficiency returned by a reversed path tothe original value. This shows that the heat-scantechnique is reversible and nondestructive. Earliermeasurements5 had shown that thermal anneallingof the frequency-doubling gratings for this fiber takesplace at -150'C, and here even the warmest partof the fiber is far from that value. Figure 1 alsoshows that when the power supply is turned ononce again the steady-state value of the conversionefficiency that is reached is the same as that obtainedbefore, i.e., corresponding to a phase shift close to3wr. It should be noted that the linear expansioncoefficient of fused quartz is small (-0.5 X 10-6 K-1 ),and the variation of the fiber length caused by thethermal expansion cannot account for the large phaseshifts obtained. The most likely physical origin ofthe process exploited by the present technique is asignificant change in the dispersion characteristics ofthe fiber when it is heated.

In order to characterize in detail the phase shift,we altered the voltage applied to the resistor andmeasured the steady-state amplitude of the SHGefficiency, as illustrated in Fig. 2. The steady-statevalue of the conversion efficiency was found to be

a periodic function of the thermal power (which isa quadratic function of the voltage). The same pe-riod was obtained when the resistor was placed atvarious points of the fiber. From such a plot it waspossible to choose the phase shift desired and adjustthe voltage accordingly. For example, a 27r shiftcorresponded to -1.2 W of power, i.e., -11 V. Forthe voltage of 13.5 V used in the initial test (1.82 W ofthermal power), the phase shift was found to be closeto 37r. The relatively large amplitude modulation ofthe periodic curve does not introduce a large error inthe phase determination. This modulation may becaused by the fact that the phase shift introduced bythe resistor is not abrupt, and the deviation from theideal case is greater for higher thermal powers. Theregion of the fiber that does not contribute efficientlyto the SH signal is longer for higher temperatures,leading to the general behavior observed.

The net phase shift caused by a temperature in-crease AT is3

AX = (27/A2s,)ALAT8, (1)

where a = (dn2 ,,dT)IT0 - (dnj,/dT)IT0 and AL is thelength of fiber heated. There is great discrepancyin the values of 3 in the literature.3'7'8 In order todetermine that value for the particular fiber used inthis research, an interferometric experiment was car-ried out. Low-power fundamental and SH radiationwere coupled into a piece of fiber, and the infraredradiation leaving the fiber was frequency doubled ata KTP crystal. The SH light thus produced inter-fered with that injected into the fiber. A 16.5-mm-long part of the fiber was fastened to a metal piecethat was heated to a controlled temperature. As thetemperature changed, a periodic variation in the out-put power was produced as a result of interference.Since the heated length and the temperature wereknown, Eq. (1) was then used to determine the valueof 131, which was found to be 1.3 ± 0.1 X 10-6 K-1.With this result and Fig. 2, it is possible to estimatethat the length of the fiber heated by the resistorin the heat-scan experiments was -5 mm, somewhatwider than the part of the resistor in contact with thefiber (-2 mm). The greatest error in this estimatecomes from uncertainty in the temperature of the

101

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Fig. 2. Frequency-doubling conversion efficiency as afunction of thermal power dissipated by the resistor. Thephase shift that is due to heat can be inferred directly.

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1018 OPTICS LETTERS / Vol. 18, No. 12 / June 15, 1993

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POSITION (cm)Fig. 3. Solid curve: frequency-doubling conversion effi-ciency as a function of the position of the heating element.Dashed curve: amplitude of the second-order nonlinearsusceptibility read by 32 mW of infrared average power.The phase shift introduced was 7r rad.

heated part of the fiber in the case of the heat-scanexperiments.

Having characterized the phase shift caused bythe heating element, it is possible for us to studythe grating by the heat-scan technique. The resis-tor was scanned along the frequency-doubling fiber,and the thermal power was kept constant at 0.7 W,corresponding to a w phase shift, i.e., a signal rever-sal. The dramatic effect of the scan can be seen inFig. 3 (solid curve), which shows how the SH powerchanges as a function of the position of the resistor foran average infrared reading power of 32 mW. Whenthe heating element was close to 10 cm from theinput end of the fiber, the SH power went through aminimum, indicating that the SH powers generatedbefore and after the resistor were nearly equal andhad opposite phases. It is possible to derive a mapof the grating amplitude from this result. Assumingperfect phase matching, neglecting all effects that aredue to dispersion and self- and cross-phase modula-tion, and assuming that the resistor introduces onlya 7r phase shift, we can express the SH signal at theoutput of the fiber as

P2.(X) = b f A(x')I dx' -fL A(x')I,,,dx']

where x is the position of the resistor, L is thefiber length, A is the grating amplitude, and b is aconstant. It follows that

A(x) = 12. ax2bI,, ax

and therefore the absolute value of the derivativeof the square root of the solid curve in Fig. 3 isproportional to the actual amplitude of (2) along thefiber. The result of such calculation is also shownin Fig. 3 (dashed curve). We found that the gratingamplitude peaked at approximately 10 cm from theinput end of the fiber.

The measurement shown in Fig. 3 was carried outwhen the preparation of the fiber had saturated. Incontrast to the sidelight technique, which relies onthe measurement of the SH signal from the side ofthe fiber,' the heat-scan technique monitors the SHat the output end; i.e., it depends on the integratedguided radiation along the fiber. The present tech-nique is therefore sensitive and makes it possible forone to investigate the time evolution of the gratingamplitude and position as the preparation processtakes place from its beginning. Systematic investi-gations can also be carried out on the role played byself- and cross-phase modulation in the grating struc-ture. The heat-scan technique as described abovemakes use of a heating element that limits the spatialresolution to the millimeter level. It is possible,however, to scan the periodic grating with a very fine(of the order of a few micrometers) heat source andimprove the spatial resolution. Naturally the heat-scan technique can also be used to characterize oralter the properties of Bragg gratings in fibers.

Finally, it can be speculated that, with the controlof the phase of the waves provided by the heat-scan technique, it should be possible to overcome thegreatest difficulty of adding several gratings in seriesin order to obtain a higher conversion efficiency. Inthis arrangement several pieces of fiber should beprepared with a high-power laser and spliced sequen-tially to be read by a weak laser beam. Every spliceregion should have a heating element arranged suchthat the phase shift can be adjusted for maximumconversion efficiency. This would be a convenientway to phase match a series of macroscopic gratings.

It is a pleasure to acknowledge the help of allthe members of the Optoelectronics Laboratory (theDepartment of Physics and the TelecommunicationsCenter, Pontificia Universidade Cat6lica do Rio deJaneiro) and the partial financial support of the Con-selho Nacional de Pesquisas, the Coordenadoria parao Aperfeigoamento de Pessoal de Ensino Superior,and Telebras.

References

1. U. Osterberg and W. Margulis, Opt. Lett. 11, 517 (1986);12, 57 (1987).

2. R. H. Stolen and H. W. K. Tom, Opt. Lett. 12, 585(1987).

3. E. M. Dianov, P. G. Kazanskii, and D. Yu. Stepanov,Sov. J. Quantum Electron. 19, 575 (1989).

4. K. D. Fitchew, H. Toba, K. Oda, and N. Takato, Opt.Lett. 12, 781 (1987).

5. I. C. S. Carvalho, P. M. P. Gouv~a, W. Margulis, and B.Lesche, in Quantum Electronics Laser Science, Vol. 11of 1991 OSA Technical Digest Series (Optical Society ofAmerica, Washington, D.C., 1991), p. 38.

6. T. J. Driscoll and N. M. Lawandy, in Annual Meeting,Vol. 23 of 1992 OSA Technical Digest Series (OpticalSociety of America, Washington, D.C., 1992), p. 50.

7. D. E. Gray, ed., American Institute of Physics Handbook(McGraw-Hill, New York, 1972), pp. 6-29.

8. I. H. Malitson, J. Opt. Soc. Am. 55, 1205 (1965).