Fabrication and amplified spontaneous emission

of Eu(DBM)3Phen doped step-index polymer

optical fiber

H A O L I A N G 1 , 2 , Z H I Q I A N G Z H E N G 3 , Z E N G C H A N G L I 2 , J I E X U 2 ,

B I A O C H E N 2 , H U I Z H A O 2 , Q I J I N Z H A N G 1 , 2 , * A N D H A I M I N G 3

1Structure Research Laboratory, University of Science and Technology of China, Hefei, Anhui 230026,

P.R. China2Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei,

Anhui 230026, P.R. China3Department of Physics, University of Science and Technology of China, Hefei, Anhui 230026, P.R. China

(*author for correspondence: E-mail: zqjm@ustc.edu.cn)

Received 14 November 2003; accepted 11 January 2005

Abstract. A step-index polymer optical fiber (SI POF) with a core of poly(methyl methacrylate) (PMMA)

containing Eu(DBM)3Phen has been fabricated by a preform technique. Fluorescence analysis showed

that there is not obvious change in the local environment around Eu3+with the increase of Eu(DBM)3Phen

doping concentration. Amplified spontaneous emission (ASE) phenomenon of the doped SI POF by end-

pumping with an Ar+-laser of 457.9 nm at ambient temperature was observed. The threshold of the pump

power for the onset of ASE is found to be 120 mW for the doped SI POF with a length of 50 cm, the

length is also found to be the optimal length for maximum ASE power under the input power of 200 mW.

The maximum quantum conversion efficiency of the fiber is about 40%.

Key words: amplified spontaneous emission, luminescence, polymer optical fiber, rare earth

1. Introduction

Polymer optical fibers (POFs) have attracted much attention in past decadesbecause POFs have some unique characteristics, such as flexibility, easy tohandle and relative low cost in coupling due to their large core diameter. Thesecharacteristics make them especially suitable to be a transmission medium inlocal area networks (LANs) and short-haul optical communications (Zubiaet al. 2001, Kuriki et al. 2002). With the development of POF, increasingresearch activities have been also carried out in the field of active polymeroptical fibers, most of which aim at making polymer optical fiber amplifiers orlasers (Koeppen et al.1997; Zhang et al. 1998a, b; Kuriki et al. 2002).

Laser material used in the active POF includes organic dyes and rare earthions. Generally speaking, rare earth ions are more stable than organic dyesand have been used widely in silica optical fiber amplifiers and lasers.

Optical and Quantum Electronics (2004) 36:1313–1322 � Springer 2005

DOI 10.1007/s11082-005-0407-1

However, rare earth ions are insoluble in organic matrixes, such as a poly-mer. In order to solve this problem, two methods have been adopted (Zhanget al. 1996): First, the rare earth complexes or organic salts were made andthen mixed with the polymers; second, the rare earth ion-containing mono-mers were synthesized and then polymerized. Here, the first method wasadopted and the incorporated rare earth complexes is Eu(DBM)3Phen. Asone of the typical rare earth complexes, it is easily synthesized and showsexcellent solubility in organic polymer host with high fluorescence intensity.In addition to that, the emission of Eu3+ at 613 nm (5D0 fi 7F2) is close tothe low-loss windows (650 nm) of PMMA-made POF.

In this paper, a preform fabrication technique was presented and usedto prepare a step-index polymer optical fiber (SI POF) doped withEu(DBM)3Phen. Thermal stability and fluorescence properties of the dopingmaterial were analyzed. To evaluate the potential of this SI POF to be a fiberamplifier, amplified spontaneous emission (ASE) experiment was carried outfor it is an indication of populations inversion which shows the potential ofthe material to be a laser medium. The ASE phenomenon of the doped SIPOF by end-pumping with an Ar+-laser at 457.9 nm at ambient temperaturewas studied under different conditions. Discussion is made based on theexperimental observations.

2. Experimental

Eu2O3 (99.99%) was purchased from Shanghai Yaolong Non-Ferrous Co.Ltd. and used without further purification. Methyl methacrylate (MMA) waspurified according to standard procedures. 2,2-Azoisobutyronitrile (AIBN)and dibenzoylemethane (DBM) were recrystallized from methanol beforeuse. Other chemicals were analytical grade.

Eu(DBM)3Phen was synthesized according to the procedure reported be-fore (Melby et al. 1964), its molecular structure is shown in the inset ofFig. 3. The central Eu3+ ion is bound to three dibenzoylmethane (DBM)ligands. 1,10-phenanthroline (Phen) acts as a synergic shielding ligand, whichcan reduce the rate of nonradiative decays and enhance the fluorescenceintensity of the complex strongly (Janos et al.1997).

Eu(DBM)3Phen doped PMMA was made by bulk polymerization. First,15 ml purified MMA, 0.02 g 2,2-Azoisobutyronitrile (AIBN) as an initiator,35 ll n-butyl mercaptan as a chain-transfer agent, and a specified amount ofEu(DBM)3Phen were mixed in a vessel. The above solution was filtratedthrough a microporous filter (d ¼ 0.20 lm) before being injected into amodel, and then the thermal polymerization of the filled model was carriedout in a water bath at 50�C for 48 h under 6 atm nitrogen and additionallyheated at 75�C until solidification was fulfilled.

1314 H. LIANG ET AL.

In order to fabricate Eu(DBM)3Phen doped SI POF, the preform can bealso made by the procedure of bulk polymerization described above. Thepreform with a diameter of 10 mm prepared by this process was then heat-drawn into an optical fiber at 180�C by a taking up spool. The diameter ofthe Eu(DBM)3Phen doped SI POF can be controlled by adjusting the ratio ofthe preform moving velocity and driving roll velocity. The cladding (a resinwith low refractive index GU101) was coated during the drawing process.

Thermal stability of the materials was studied by thermal gravimetricanalysis (TGA) using a TGA-50H Shimadzu thermogravimetric analyzer,under nitrogen atmosphere, at heating rate of 10�C min)1. Differentialscanning calorimetry (DSC) curves were obtained with a Shimadzu DSC-50at 5�C min)1 to 150�C in a nitrogen atmosphere.

The excitation and fluorescence emission spectra of Eu(DBM)3Phen dopedPMMA were recorded on a Shimadzu RF-5301PC spectrofluorophotometer(the sample was cut to 2 mm thickness and polished).

The experimental arrangement for ASE of Eu(DBM)3Phen doped SI POFis shown in Fig. 1.

The pump beam from an Ar+-laser operating at 457.9 nm wavelength waslaunched into the doped polymer optical fiber by a microscope objective. Anoptical attenuator was used to adjust the incident intensity. The fluorescencespectrum was recorded by a spectrum analyzer. The input power of laserbeam and the output power of fluorescence were measured by a dual-channeloptical power meter.

The amount of pump power coupled into the fiber was estimate by mea-suring the coupling efficiency of the pump beam in a short section of the samefiber, in which the pump absorption was negligible. The efficiency was about50%. The absorbed pump power was then calculated from the measuredincident and output pump powers (the latter was, in fact, negligible). Thediameter of the fiber sample used in this work was 400 lm. The Eu3+ contentof the doped SI POF is 4000 ppm-wt. The refractive index of the core was1.468. The index difference between the core and cladding region was 0.057.

LFMOAT

Power meter

Spectrum analyzer

Eu3+ doped SI POF

Ar+ laser

457.9nm

Fig. 1. Experimental arrangement for ASE AT: optical attenuator, MO: microscope objective, LF:

500 nm high-pass filter.

FABRICATION AND AMPLIFIED SPONTANEOUS EMISSION 1315

Therefore the numerical aperture of POF is 0.41. The attenuation of the POFat the signal wavelength (613 nm) is about 3.2 dB/m. The POF ends weremounted in a special clamp and polished mechanically.

3. Results and discussion

3.1. FABRICATION OF PMMA PREFORM

Bulk polymerization of methyl methacrylate is a highly exothermic one.Because viscosity increases early in the reaction, the problem of heattransfer leads to difficulty in control and a development of localized ‘‘hotspots’’ and ‘‘runaways’’ (Fred. W. Billmeyer, Jr 1970). This may result inundesirable mechanical properties. In order to reduce heat evolution andavoid autoacceleration (also referred to as Trommsdor effect) that takesplace during the polymerization, in general a syrup of partially polymerizedmethyl methacrylate is used. However, we have found even in this way, theformation of bubbles cannot be prevented also. It is a serious problem infabricating polymer optical fiber preforms, especially in the drawing processof the fiber. So we adopted another method to solve the problem. Thepolymerization was carried out in a cylinder under pressure. At the sametime, the amount of initiator and the reaction temperature should beproperly controlled. It is very effective to prevent bubbles formation evenwithout prepolymerization. A preform fabricated in this way shows goodprocessability in fiber drawing.

3.2. THERMAL ANALYSIS

Thermal analysis curves of PMMA and Eu(DBM)3Phen doped PMMA (theconcentration of Eu-ion is 5000 ppm-wt) are shown in Fig. 2. TGA wascarried out to measure the thermal stability of PMMA and Eu(DBM)3Phendoped PMMA. The glass transition temperature Tg was determined by themeans of differential scanning calorimetry (DSC).

It can be found from Fig. 2a that the degradation temperature for Eu(-DBM)3Phen doped PMMA is about 210�C, while the pure PMMA has adegradation temperature of 290�C. The decrease of thermal stability is due tothe lower degradation temperature of Eu(DBM)3Phen (210�C). From theDSC results in Fig. 2b, the glass transition temperature (Tg) of PMMA andEu(DBM)3Phen doped PMMA are 100.5 and 103�C, respectively.

A significant change of Tg has always been observed when rare earth ionswere incorporated into the polymer matrix, especially at a higher doping

1316 H. LIANG ET AL.

concentration (10%) (Twomey et al. 1991, 1993; Zhang et al. 1998c; Bianet al. 2002; Parra et al. 2002).

The interactions between the rare earth ions and the polymer chains, suchas coordination between the rare earth ions and the polymer functionalgroups, the formation of the cross-linking points, are accounted for thisphenomenon. In the case shown in Fig. 2b, there is only a slight change of Tg.There are two reasons for this slight change. One is the relatively low dopingconcentration of Eu(DBM)3Phen (the content of Eu(DBM)3Phen is 3.3wt%). Another one is the chemical structure of Eu(DBM)3Phen. Rare earthions typically prefer to eightfold or ninefold coordination (Koeppen et al.

0 100 200 300 400 500 600 700

0

20

40

60

80

100

Wei

ght (

%)

Temperature (0C)

Temperature (0C)

PMMA Eu(DBM)3Phen dopedPMMA Eu(DBM)3Phen

20 40 60 80 100 120 140 160-1.0

-0.8

-0.6

-0.4

-0.2

0.0

Hea

t flo

w (

mW

/mg) PMMA

Eu(DBM)3Phen Doped PMMA

(a)

(b)

Fig. 2. Thermal analysis curves of PMMA and Eu(DBM)3Phen doped PMMA. (a) TGA thermograms;

(b) DSC thermograms.

FABRICATION AND AMPLIFIED SPONTANEOUS EMISSION 1317

1997). As showed in the inset of Fig. 3, the Eu3+ ion is completely coordi-nated by six oxygen atoms of three dibenzoylmethane ligands and twonitrogen of one 1,10-phenanthroline ligand. So it does not need to coordinatewith the polymer functional groups.

The degradation temperature of Eu(DBM)3Phen is far above the glasstransition temperature of PMMA and fiber heating draw temperature(180�C), so the preform fabricated in this way shows good thermo-stabilityduring the processing and fiber drawing.

3.3. FLUORESCENCE OF EU(DBM)3PHEN DOPED PMMA

The excitation and fluorescence emission spectra of a typical Eu(DBM)3PhendopedPMMAis shown inFig. 3 (the concentrationofEu-ion is 4000 ppm-wt).

The excitation spectrum Fig. 3a was obtained by monitoring the emissionat 613 nm which corresponds to the hypersensitive 5D0 fi 7F2 transition.The emission spectrum Fig. 3a was recorded form 550 to 750 nm underexcitation at 396 nm. Four emission peaks are centered at 579, 591, 613,652 nm, and can be assigned to the 5D0 fi 7F0,

5D0 fi 7F1,5D0 fi 7F2,

5D0 fi 7F3, respectively. The presence of only one 5D0 fi 7F0 line indicatesthat the Eu3+ ion occupies only a single site and a single chemical envi-ronment exists around it (Brito et al. 1999). The 5D0 fi 7F2 transition at613 nm dominates all other transitions, emission intensity of this transitionstrongly depend on the chemical environment which the Eu ion is located.The much stronger intensity of 5D0 fi 7F2 than those of other transitionsindicates the ligand field surrounding the Eu ion is highly polarizable (Wonget al. 2002). It also indicated that Eu(DBM)3Phen doped PMMA has goodcolor purity and the Eu3+ ion is in a site without a center of inversion (Britoet al. 2000).

200 250 300 350 400 450 5000

200

400

600

800

1000

Wavelength (nm) Wavelength (nm)

Inte

nsi

ty (

a.u

)

Inte

nsi

ty (

a.u

)

550 600 650 700 7500

200

400

600

800

1000

5D0-

7F0 5

D0-7F1 5

D0-7F3

5D0-7F2

O

ON

N

Eu

3

(a) (b)

Fig. 3. Excitation and fluorescence emission spectra of Eu(DBM)3Phen doped PMMA (a) excitation

spectrum monitored at 613 nm (b) emission spectrum excited at 396 nm. The inset is the chemical

structure of Eu(DBM)3Phen.

1318 H. LIANG ET AL.

For a free Eu3+ ion, all the (4fn) intra-configurational transitions areforbidden by the Laporte rule (Kumar et al. 2002). However, the ligand fielddue to the surrounding ions perturbs the free ion levels, causing energy statesof different configurations to mix, so that transitions between the differentlevels of the 4f n configurations become allowed. Even slight differences in thebonding parameters cause variations in the strength of the ligand field andhence in the energy of free ion levels. In other words, the fluorescencespectrum is quite sensitive to the local environment of the ion. 5D0 fi 7F2

transition is a hypersensitive one (DJ ¼ 2) and its intensity is very sensitive tothe local environment. The 5D0 fi 7F1 transition is usually used as a refer-ence because it is allowed by magnetic dipole and its intensity is independentof the environment. The relative intensity ration (R) of 5D0 fi 7F2 to5D0 fi 7F1 transition (also called monochromaticity) is commonly used as ameasure of the rare earth site symmetry (Kumar et al. 2002). Table 1 showsthe relation between R and doped concentration.

Compared with R values of Eu3+ doped glass which varies from 0.90 to7.04 in different host (Kumar et al. 2002), the Eu(DBM)3Phen PMMA showsa larger R value which indicated a more asymmetry in the vicinity of Eu3+.Because the variation of the R is very sensitive to the structural change in thevicinity of Eu3+ ions, it can be used to reflect the local structure and com-position (Nageno et al. 1994). However, it was found in Table 1 that thevalue of R ratio did not show significant change with the increasing of Eu3+

content. This indicates that there is not obvious change in the local envi-ronment around Eu3+ with the increase of doping concentration. Thechemical structure of Eu(DBM)3Phen can be accounted for this phenome-non. The Eu3+ is coordinated by four bidentate ligands: three DBM mole-cules and one Phen molecule. The ligands can form a sheath around theEu3+. In addition to shield the ion from high-energy vibrations of the hostand prevent the ion–ion interactions, the high bulkiness of the sheath canalso protect the local environment around the Eu3+. Due to the excellentsolubility and the shielding effect of the ligands, local environment changearound Eu3+ can not be found even at a high doping concentration(10000 ppm). The high doping concentration is important for a short-length-amplification device. But we found the loss of POF would increase dramat-ically while the doping concentration exceeds 4000 ppm. That’s the reason weselected 4000 ppm as the doping concentration of Eu3+ in the ASE experi-ment.

Table 1. Relation between R and doped concentration

Eu 3+

concentration (ppm-wt)400 800 1000 2000 3000 4000 5000 10000

R 10.514 10.656 10.507 10.669 10.559 10.256 10.197 10.563

FABRICATION AND AMPLIFIED SPONTANEOUS EMISSION 1319

3.4. AMPLIFIED SPONTANEOUS EMISSION (ASE) OF EU(DBM)3PHEN DOPED POF

The experimental arrangement for ASE has been shown in Fig. 1. The SIPOF was end-pumped by an Ar+ laser. The advantage of end-pumping isthat the pump power can be well confined in the core area and propagatesdiffraction-free. Fig. 4 shows a plot of the measured output power versusabsorbed pump power for Eu3+ doped SI POF with a length of 50 cm.

From Fig. 4, it can be found that a nonlinear relationship exists betweeninput and output power. At low pump power, the populations of groundstate are greater than that of excitation state, the output consists mostly ofspontaneous emission and the growth occurs linearly at a low rate. With theincrease of pump power, the population of excitation state would increasealso. When the pump power attains to a certain value that is required by thepopulation inversion, the spontaneous emission is amplified which is calledamplified spontaneous emission (ASE). Beyond this point, the output be-comes mostly ASE and increase nonlinearly. The threshold of absorbedpump power for the onset of ASE is about 120 mW. When the absorbedpump power is higher than 250 mW, the increase in the speed of the outputpowers drops, which corresponds to self-saturation. This kind of self-saturation could result from a nonlinear coupling of the optical waves frompumping light and ASE. The result in Fig. 4 shows the population inversioncan take place in a Eu3+ doped SI POF, and this character makes the dopedPOF be of use as an amplifying medium. Related work is still undergoing.

The experimental results of the ASE power versus length of doped SI POFare shown in Fig. 5. The input power is 200 mW.

From Fig. 5, we can see that that ASE power is gradually increasing andthen reducing when the fiber length is increasing from 30 to 80 cm. This

0 50 100 150 200 250 300 3500.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Flu

ores

cenc

e po

wer

(mW

)

Input power(mW)

Fig. 4. Output power versus absorbed pump power.

1320 H. LIANG ET AL.

experimental result is identical with theoretical anticipation (Wysocki et al.1994): There is an optimal length for ASE power. The output increases withlength until to a maximum quantum conversion efficiency. Beyond thisoptimal length, the output decreases gradually. Because for shorter fibers, thegain is insufficient to amplify the spontaneous emission to high power levels,but for longer fibers, a lossy region exists at forward fiber end which absorbsthe forward ASE power. Obviously, 50 cm length is roughly the optimallength for maximum ASE power and the maximum quantum conversionefficiency is about 40%.

4. Conclusion

In summary, a preform technique has been used to fabricate Eu(DBM)3Phendoped SI POF. The polymerization being carried out under pressure isproved to be an efficient way to prevent bubbles formation. Related prop-erties of the doped material were discussed, such as thermo stability, fluo-rescence, etc. ASE phenomenon of doped SI POF by end-pumping with anAr+-laser at 457.9 nm at ambient temperature was observed. The thresholdof absorbed pump power for the onset of ASE of a 50 cm length doped SIPOF is about 120 mw. The results show that Eu(DBM)3Phen doped SI POFis of potential application in making polymer optical fiber amplifiers.

Acknowledgements

This work was supported by the National Natural Science Foundationof China (No. 50025309, and No. 90201016) and State Key Project of

30 40 50 60 70 800.0

0.1

0.2

0.3

0.4

0.5

AS

E P

ower

(mW

)

Length(cm)

Fig. 5. ASE power versus length of doped SI POF.

FABRICATION AND AMPLIFIED SPONTANEOUS EMISSION 1321

Fundamental Research (No. RE-05-02). The authors gratefully acknowledgethe financial support and wish to express their thanks to the referees forcritically reviewing the manuscript and making important suggestions.

References

Bian L.J., X.F. Qian, J. Yin, Q.H. Lu, L. Liu and Z.K. Zhu, Polym Test 21 841, 2002.

Brito, H.F., O.L. Malta and J.F.S. Menezes, J. Alloy. Comp. 303 336, 2000.

Brito, H.F., O.L. Malta, L.R. Souza, J.F.S. Menezes and C.A.A. Carvalho. J. Non-Cryst. Solids 247 129,

1999.

Fred. W. Billmeyer, Jr. Textbook of Polymer Science 2nd, 356, John Wiley, New York, 1970.

Janos, E., B. Andrea, K. Attila and K. Laszlo, J. Lumines. 72 570, 1997.

Koeppen, C., S. Yamada, G. Jiang and A.F. Garito, J. Opt. Soc. Am. B. 14 155, 1997.

Kumar, A., D.K. Rai, and S.B. Rai, Spectroc. Acta Pt. A. 58 2115, 2002.

Kuriki, K., Y. Koike and Y. Okamoto. Chem. Rev. 102 2347, 2002.

Melby, L., N. Rose, E. Abramson and J.C. Caris. J. Am. Chem. Soc. 86 5117, 1964.

Nageno, Y., H. Takebe, K. Morinaga and T. Izumitani. J. Non-Cryst. Solids. 169 228, 1994.

Parra, D.F., H.F. Brito, J.D.R. Matos and L.C. Dias, J. Appl. Polym. Sci. 83 2716, 2002.

Twomey, C.J., and S.H. Chen, J. Polym. Sci. Pt. B-Polym. Phys. 29 859, 1991.

Twomey, C.J., S.H. Chen, T. Blanton, A. Schmid, and K.L. Marshall, J. Polym. Sci. Pt. B-Polym. Phys 31

647, 1993.

Wong, K.S., T. Sun, X.L. Liu, J. Pei and W. Huang, Thin Solid Films 417 85, 2002.

Wysocki, P., M. Digonnet, B. Kim and J. Lightwave. Technol. 12 550, 1994.

Zhang, Q., H. Ming and Y. Zhai. J. Appl. Polym. Sci. 62 887, 1996.

Zhang, Q., P. Wang, X. Sun, Y. Zhai, P. Dai, B. Yang, M. Hai and J.P. Xie. Appl. Phys. Lett. 72 407,

1998a.

Zhang, Q., P. Wang and Y. Zhai. J. Appl. Polym. Sci. 67 1431, 1998b.

Zhang, Q., W. liu and P. Dai. Polymer, 39 3787, 1998b.

Zubia, J., and J. Arrue, Opt. Fiber Technol. 7 101, 2001.

1322 H. LIANG ET AL.