October 1, 1991 / Vol. 16, No. 19 / OPTICS LETTERS 1487

Preparation of frequency-doubling fibers under UV excitation

I. C. S. Carvalho and W Margulis

Department of Physics, Pontificia Universidade Cat6lica do Rio de Janeiro, C.P 38071, 22452 Rio de Janeiro RJ, Brazil

B. Lesche

Physics Institute, Universidade Federal do Rio de Janeiro, P.O. Box 68528, Rio de Janeiro, Brazil

Received May 29, 1991

Germanium-doped silica fibers were prepared for frequency doubling of 1.06-,um wavelength radiation underthe simultaneous presence of the first, second, and fourth harmonics of the laser light. It was found that the

preparation rate for frequency doubling was dramatically increased by the presence of the fourth harmonic.The experiments also showed that a metastable excited state plays a significant role in the process.

It is now well established that glass optical fibers canfrequency double an IR input after a long prepara-tion process, whereby a periodic grating is written inthe fiber.`3 The time taken for this process tosaturate is many hours if only IR light is used towrite the grating or several minutes if second-harmonic radiation is also used.4 It has also beenreported that exposing glass fibers to green or toblue light before preparation can lead to the creationof defects that help in accelerating the grating for-mation, even in the absence of an external electricfield.5'6 Furthermore, it is known that the gratingof a prepared fiber can be erased in many ways, in-cluding the injection of green7'8 or UV radiation.9

Although there is still no consensus as to the exactmechanism in operation, it is generally believed thatthe preparation process involves a multiphoton pro-cess with the mixing in the fiber of the harmonicsof the laser radiation. In this Letter we describea study of the role played by the fourth harmonicof the laser light in the preparation process forfrequency doubling in fibers, with the help of0.266-Am radiation from a frequency-quadrupledNd:YAG laser.

The experimental setup is illustrated in Fig. 1. Acw Q-switched (at a 1.2-kHz repetition rate) andmode-locked (at 100 MHz) Nd-YAG laser was used.After frequency doubling in a KTP crystal, a smallpart of the w and 2cs light was separated by a beamsplitter and used for preparation (which we refer toas the preparation beam). The remaining greenlight, after passing through a A/2 plate, was fre-quency doubled in a 4-cm-long KDP crystal. The 4colight produced was selected by a UG 11 filter (F),combined with the preparation beam in a semi-transparent gold mirror on a quartz substrate (BS),and directed to the fiber. The total average powercoupled through a 5-cm focal-length quartz lens intothe fiber was typically 1 mW (0.1-W peak power) ofUV light, 30-130 mW of IR light, and 10-100 ,uW ofgreen light at 532 nm. The powers at C9 and 2cswere measured at the output of the fiber. The focusof the lens was optimized for the IR input, and since

the lens was not achromatic the coupling of the UVradiation was done preferentially into claddingmodes. The fiber used was doped with Ge (no P),with a 6.7-ttm core diameter, An = 0.0037, a mini-mum dispersion of 1.359 ,tm, and a cutoff wave-length of 1.099 lkm. The relative delay between thepreparation and the UV beams was monitored withpicosecond resolution with a high-speed photo-conductive switch and adjusted by altering thepath lengths.

The absorption of 266-nm light in the Ge-dopedcore of the fiber is large, and the amount of UV lightpropagating in the core is significantly reduced af-ter a few centimeters. The cladding of the fiber,however, does not have Ge, and the 266-nm radia-tion can be coupled into cladding modes and excitethe core. The 15-cm-long pieces of fiber used in theexperiments were kept straight and secure in thincapillary glass tubes to ensure that the UV excita-tion would be relatively uniform along the fiber. Itis found that the Ge in the fiber core under exposureto UV gives rise to two luminescence bands withspectral peaks at 350 and 420 nm. The lifetime ofthe main peak at 420 nm is =80 /,ts (FWHM), andonly the core (owing to the Ge doping) producesradiation at this wavelength.'0 Therefore, in orderto quantify the exposure dose of the core to UVlight, the linearity of the system was tested in afiber preform for various UV powers, and thereafterthe blue signal was monitored in a photomultiplieras a measure of the fraction of the UV propagatingin the cladding that actually excited the core.

A comparison was carried out of the preparationrate for frequency doubling of fibers subjected to (i)only the preparation beam (green and IR light), (ii)long time exposure to UV followed by illuminationwith the preparation beam, and (iii) exposure to UV,green, and IR together. Figure 2 illustrates the re-sult of such a comparison, carried out for differentpieces of fiber. It was found that the preparationrate increases by more than 2 orders of magnitudewhen all beams illuminate the fiber together, evencompared with the case of the fiber previously ex-

0146-9592/91/191487-03$5.00/0 © 1991 Optical Society of America

1488 OPTICS LETTERS / Vol. 16, No. 19 / October 1, 1991

KTP J AI_

Fig. 1. Experimental setup for frequency doubling of op-tical fibers under UV excitation.

1-2

Z -4

W /

0100U)

0~~~~~~~~~~~010 5.1~~~~~~0 t

0 )o 1 2'0 3'0TIME (min)

Fig. 2. Comparison of the growth rate for second-harmonic generation (SHG) in fibers subjected to simulta-neous illumination with oi, 2w, and 4w (top curve),previous exposure for 20 min with 4wo followed by prepara-tion with co and 2w only (middle curve), and preparationwith w and 2w) only (bottom curve). The inset shows thetop and bottom traces plotted on a linear scale (same units).

posed to UV It was also found that after a few sec-onds in the case with UV light, and a few minutes inthe case without UV light, the growth curve of thesecond-harmonic signal generated by the fiber be-comes linear in time.

The growth rate under excitation with a), 2w, and4w is so great that it is easy to reach true saturation ina few minutes. Figure 3 illustrates the total greenpower that leaves a fiber (prepared with 20 pW of 2wsradiation, 130 mW of w) radiation, and 2 x 10' W of4w radiation), and as can be seen the green lightgenerated rapidly exceeds the injected signal.After 9 min the second harmonic reaches a maxi-mum, and no increase is observed after this time.It is clear from Fig. 3 that before saturation sets in,the second-harmonic light generated grows at a con-stant rate in spite of the fact that the total green

signal increases by 30 times. This shows that forthe optical power levels used here it is not the greenthat sets a limit for the preparation rate. It wasalso found that after frequency doubling had beeninduced by seeding, preparation could be contin-ued by injection of o) and 4w) alone (for sufficientIR powers).

The conclusion drawn from the experiments isthat the effect of the UV light is not simply to createdefects. The preparation process involves a rela-tively short-lived excitation as an essential step, andthis excitation can be achieved efficiently by absorp-tion of 4wo photons. The excitation alone would resultin erasure of the grating,9 and therefore the co and2w) fields prevent erasure and organize the grating.

Since a short-lived excitation exists and the ex-cited centers can be made to contribute for fre-quency doubling only as long as the co and 2w) beamsare injected soon after the passage of the UV light,

02-

3 0

O sseedg b ti 6 9

T I ME (m in)Fig. 3. Growth in time of second-harmonic (SH) lightleaving a fiber. After a few minutes the frequency-doubled light generated by the fiber exceeds the injectedseed green by 30 times.

10

5-zW * simultaneouscc:0 delay 10 if * delay 2

0

T I M E (min)Fig. 4. Second-harmonic generation growth rate of a sin-gle fiber prepared with various delays between the UV ex-citation and the injection of w and 2w for a time lag of zero(simultaneous), 400 ,us (delay 1), and 1 ms (delay 2).

October 1, 1991 / Vol. 16, No. 19 / OPTICS LETTERS 1489

it is important to determine the lifetime of theexcitation, in the hope of identifying the center in-volved in the preparation. The arrival of the co and2co pulses was delayed by as much as 9.7 ns after thepassage of the UV pulses without any noticeablereduction in the preparation rate. This indicatesthat the lifetime of the excited state is much greaterthan 10 ns. An upper limit for the lifetime of theexcited state was also determined. The Q-switchingrate was increased to 5 kHz, and a chopper was usedwith an opening time of 1 ms and a duty cycle of0.25. For every opening five trains of pulses illumi-nated the fiber. The geometry of the arrangementwas such that by moving the chopper alone it waspossible to have both beams illuminating the fibertogether (zero delay) or to introduce a delay betweenthe UV excitation and the injection of co and 2wolight. Figure 4 shows a comparison of the second-harmonic generation growth rate of a fiber preparedwith zero delay, and then 400 ,us, and finally 1 msbetween the last UV train and the first preparationtrain for every chopper opening. The procedurewas repeated three times. It was found that thesecond-harmonic generation growth rate is similarfor both delay values and is larger when Co, 2co, and4cs illuminate the fiber together. It can be con-cluded that the lifetime of the excited state involvedis much smaller than 1 ms. It was assumed herethat a single time constant governed the time evolu-tion of the process, and it should be possible to ver-ify this assumption in a more detailed experiment.

The results can be summarized as follows. Thepreparation of fibers for frequency doubling can begreatly enhanced by illuminating the fiber with266-nm radiation together with the fundamentaland second harmonic of the laser. An important ef-fect of the UV light is to excite centers with a char-acteristic lifetime 10 ns << r << 1 ms. It can bespeculated that the center involved is that respon-sible for the blue-light emission with a lifetime of80 As. The blue-light signal seen since early fre-quency-doubling experiments in fibers11 is an indica-tion that the fourth harmonic plays a role in thepreparation process even without the direct injec-tion of UV light. It is therefore conceivable that thecreation of the x(') in frequency-doubling fibers re-

quires not only the organization by ca and 2w), butalso excitation with UV. In this case, the completedescription of the preparation process is quite com-plex because in normal situations the input fieldsboth generate the 4wo excitation and organize thegrating. Throughout the experiments the UV lightmaintained no fixed phase relation to the Co and 2cwwaves. The results reported here should be takeninto account in models to describe second-harmonicgeneration in fibers that rely on the interference offew photons of relatively low energy.

The authors thank the Brazilian Agencies Con-selho Nacional de Pesquisas, Coordenadoria para oAperfeigoamento de Pessoal de Ensino Superior, andFundagdo de Amparo a Pesquisa do Estado do Rio deJaneiro for financial support and A. S. L. Gomes forlending us the KDP crystal.

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