SPIE Proceedings [SPIE Symposium on Integrated Optics - San Jose, CA (Saturday 20 January 2001)] Rare-Earth-Doped Materials and Devices V - Plastic optical amplifier using europium

Plastic optical amplifier using europium complex

Doogie Oha, Namwoong Songb, Jang-Joo Kim*a

aDepartment of Materials Science and Engineering, Kwangju Institute of Science and Technology,

1 Oryong-dong, Buk-gu, Kwangju 500-712, Korea

bDivision of Quantum Metrology, Korea Research Institute of Standards and Science, P. O. Box 102, Yusong-gu, Taejon, Korea

ABSTRACT Potential of polymer optical amplifier doped with europium complex has been analyzed for practical use in visible range. Europium tris(2-thenoyltrifluoroacetonate)-1,10-phenanthroline (Eu(TTA)3phen) was used as the amplification dopant and poly(methylmethacrylate) (PMMA) as matrix. Spectroscopic properties of the dopant such as metastable excited state lifetime, stimulated emission cross section, and stimulated absorption cross section were obtained using the photoluminescence spectroscopy, ultraviolet visible spectrophotometry and time-resolved spectroscopy. Lifetime of 5D0 metastable state is 0.9 ms, which is longer than usual rare earth complex. Its emission cross section is comparable to erbium ions and absorption cross section is 4 orders of magnitude higher than bare rare earth ions. Optical amplifier was fabricated by the dip-coating method. The refractive index profile of the polymer optical amplifier was designed to manifest a single mode structure for the optimization of amplification performance. Amplification characteristics were simulated with respect to pump power, amplifier length, and number density of Eu(TTA)3phen. The simulations showed that optical gains are saturated above some maximum point. More than 30 dB optical gain can be achieved with 5 m long amplifier at 300 mW pump power. Keywords: polymer optical fiber amplifier, europium complex, metastable lifetime, numerical simulation, pump power, organic ligand, optical gain, dip-coating, time-resolved spectroscopy

1. INTRODUCTION Optical amplification is a key element in optical communications. Rare earth ions such as erbium or praseodymium are doped into silica to obtain optical amplification. It is only recently that polymer amplifiers attract attention. While bare rare-earth ions are directly doped into glass material, they are encapsulated by organic ligands before being incorporated into polymer host. Their insulating ligands block vibronic oscillations of host material which give rise to deterious multiphonon relaxation of 4f electronic transition. They also inhibit the interaction between europium ions when the europium ion density is high. Metal to metal interaction results in reducing the metastable state lifetime due to energy transfer between metal ions such as upconversion phenomenon. Up to now, several trials applying polymers to the optical amplifier have been made. At first, Neodymium was used for amplification material in polymer amplifier because of its good amplification properties. A NdCl3-doped-photolime-gelatin-thin-film optical amplifier was demonstrated.1 Since then, neodymium doped plastic optical fibers and waveguides have been suggested.2-5 Research on organic dye doped plastic optical fiber amplifiers began in the early 90s.6 Koike’s group obtained the optical gain of 33 dB using rhodamine B- doped plastic optical fiber.7 In experiments adopting organic dye molecules, however, the amplification measurements were carried out by using pulsed laser generating high peak power as the pump source. They also neglected the amplified spontaneous emission that becomes the optical noise, and degrades the performance of optical amplifiers because of the short metastable lifetime of organic dye. By these reasons, these experiments cannot evaluate the amplifier performance appropriately.7-8 Recently, rare-earth complexes have been investigated in the optical application fields. They can be easily dissolved in polymer matrixes so that the fabrication process of the rare-earth doped plastic optical amplifier is easily handled. Optical amplifier using samarium complexes are simulated.9 And the samarium complex doped plastic optical fiber was fabricated.10 Neodymium doped polymer optical fiber was also simulated using side pumping technique.5 Lately, neodymium doped polymer optical fiber have been

Rare-Earth-Doped Materials and Devices V, Shibin Jiang, Editor,Proceedings of SPIE Vol. 4282 (2001) © 2001 SPIE · 0277-786X/01/$15.00 1

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fabricated and the fluorescence of optical fiber was observed when pumped by a CW dye laser.11 The previously investigated optical fiber amplifiers have multimode structure causing weak intensity of pump power at the core region. In this presentation, Eu(TTA)3phen was used as the amplification dopant for plastic optical fiber amplifier. Polymer optical fiber was fabricated using PMMA doped with Eu(TTA)3phen with single mode structure which can solve the problem of low pump intensity of the previously investigated multimode polymer optical fiber amplifiers. Spectroscopic properties of Eu(TTA)3phen was measured and analyzed. Amplification characteristics were simulated using the spectroscopic properties of Eu(TTA)3phen in order to analyze the performance of the fabricated amplifier. Optical loss of the fiber was measured.

2. EXPERIMENTAL Fabrication process is composed of the preparation of preform and fiber drawing. Fiber preform was fabricated as follows. The solution of methylmethacrylate (MMA), 0.2 wt% benzoyl peroxide, 0.2 wt% n-butyl mercaptan, and 5 wt% triphenyl phosphate was poured into the rotating glass tube at 80 °C. MMA was purified before use to remove impurities and the inhibitor. The inhibitor was removed by mixing MMA with NaOH aqueous solution and separating two liquid. Then MgSO4 was added to remove water by hydration. After hydrated MgSO4 was separated from MMA, vacuum distillation was carried out to remove remaining impurities. Benzoyl peroxide and n-butyl mercaptan were added to the purified MMA as the initiator and the chain transfer agent, respectively. Chain transfer agent was added for the improvement of thermal properties of fiber. Molecular weight of PMMA was controlled by the amount of initiator and chain transfer agent. Triphenyl phosphate was also added to lower the glass transition temperature of the polymerized PMMA bulk. One day reaction at 80 °C produced a PMMA tube with the inner diameter of 3 mm, which serve as the cladding region. Then, MMA solution mixed with 0.4 wt% Eu(TTA)3phen was poured into PMMA tube for the core fabrication. The poured solution was maintained for several minutes and drained. Due to the adhesive force between inner wall of PMMA and MMA solution, thin film was formed on the inner wall. This film was polymerized to become solid film. The fabricated preform was vacuum dried at 120 °C for 24 hours in order to remove unreacted monomer that can cause the void problem in the drawing process. The dried preform was heat drawn to optical fiber. Simultaneously, the hollow region of the preform was collapsed. Drawing process was carried out below 250 °C to prevent the decomposition of Eu(TTA)3phen. Fig. 1 shows the core formation and drawing process.

FIG. 1. (a) Dip-Coating method for core region (b) Fiber drawing process Spectroscopic properties of Eu(TTA)3phen were measured. To measure the metastable state lifetime, PMMA films doped with Eu(TTA)3phen were formed by dissolving both materials in cyclohexanone followed by spin-coating on substrates. The concentration of the Eu(TTA)3phen in PMMA was systematically varied from 0.01 to 10 wt% to study the concentration quenching effect. Lifetime of the Eu(TTA)3phen was determined from the transient photoluminescence (PL).

Monomer Discharge

PMMA Tube

Heater

Capstan

Proc. SPIE Vol. 42822


The third harmonic (355 nm) of the pulsed Nd:YAG laser was used as the excitation source for the experiment. Spontaneous emission was detected with a photomultiplier tube at the wavelength of 614 nm which corresponds to the emission peak (the 5D0 → 7F2 transition of Eu3+) of Eu(TTA)3phen. Stimulated absorption cross section of Eu(TTA)3phen was determined from the absorption spectrum of the Eu(TTA)3phen doped PMMA, which was measured by a Perkin-Elmer spectrophotometer. The Photoluminescence spectrum of the Eu(TTA)3phen was measured using an Acton Research Spectrapro-300 monochromator with the excitation wavelength of 355 nm which was selected by a Spectrapro-150 monochromator from a xenon lamp. The photoluminescence spectrum was used to estimate the stimulated emission cross section.

FIG. 2. Setup for the measurement of optical loss of fiber. To characterize the fabricated fiber amplifier, optical loss was measured. Experimental setup is displayed in Fig. 2. Rhodamine B dye laser pumped by Ar laser was used for the signal light source. Wavelength was tuned to 614 nm wavelength. Lockin amplifier and chopper were used to detect the signal. To get the signal intensities, cutback method was employed.

3. RESULTS AND DISCUSSION 3.1. Spectroscopic properties of Eu(TTA)3phen The structure of Eu(TTA)3phen is shown in Fig 3. Different from erbium ions used for erbium doped fiber amplifier, europium complex exhibits the four level system, which has advantages over three level system for population inversion.

FIG. 3. Chemical structure and energy level of europium complex.

Eu3+

HC CC

O O

CF3

S

-

3

NN

Argon Laser Dye Laser Chopper Photodiode

Lockin Amplifier

3

0

1

2

ττττ32

φφφφpσσσσ03

φφφφsσσσσ21 ττττ21 ττττ20

ττττ10

Europium tris(2-thenoyltrifluoroacetonate)-1,10-phenanthroline Four level structure for europium complex

Proc. SPIE Vol. 4282 3


The metastable state lifetime of the Eu(TTA)3phen was obtained from the temporal decay profile of the emission intensity of Eu(TTA)3phen in PMMA as shown in Fig. 4. The temporal profiles are not fitted to single exponential decay but fitted to bi-exponential decay. The origin of the bi-exponential decay is not clear yet and further study is required for the clarification. The lifetime dependence on the doping concentration of Eu(TTA)3phen in PMMA is displayed in Fig. 4. The lifetime of the Eu(TTA)3phen is about 900 µs. This value is high compared with other europium complexes and there is no significant decrease in lifetime up to 10 wt%. This indicates that no significant concentration quenching takes place to the high concentration in the system. Concentration quenching of lifetime relaxing the excited state ions degrades the amplifier performance. Eu(TTA)3phen has four ligands that effectively block the coupling of the excited europium ions to the phonon of the matrix, which is the harmonic overtone of C-H vibration energy of PMMA bulk. Also each europium ion is completely coordinated with eight atoms of the surrounding ligands (6 oxygen atoms and 2 nitrogen atoms) and does not need to cluster with another ion to fill the coordination. Therefore the concentration quenching does not take place up to the solubility limit.

FIG. 4. (a) Decay profile of time-resolved photoluminescence (b) Metastable state lifetimes with respect to Eu(TTA)3phen concentration

FIG. 5. Photoluminescence spectrum of Eu(TTA)3phen doped PMMA Photoluminescence spectrum of Eu(TTA)3phen is shown in Fig. 5. This spectrum shows the transitions from the 5D0 state to the four 7F0, 1, 2, 3 state. Most of the emitted photon is the electronic transition from the 5D0 state to 7F2 state centered at 614

Time (ms)

0 1 2 3 4

Inte

nsity

0.0

0.5

1.0

10 wt% 5 wt% 2.5 wt% 1.25 wt%0.625 wt%0.313 wt%0.156 wt%0.078 wt%0.039 wt%

Wavelength (nm)

550 600 650

Inte

nsity

(a.

u.)

0.0

0.5

1.0

Concentration (wt%)

0.1 1 10

Life

time

( µs)

0

300

600

900

1200

(a) (b)



nm. Generally, the transition strengths of lanthanide ions are not influenced by the environment. However, hypersensitive transition which is very sensitive to the environmental situation depends on the ligand and matrix. Intensity ratio (5D0 → 7F2) / (

5D0 → 7F1) of Eu(TTA)3phen is higher than the bare Eu3+ ion. This fact indicates that the bonding nature of Eu3+ to ligand has sizable amount of covalency in this complex, though not sufficient for drawing any conclusion regarding comparative covalency of the metal-ligand bond. Higher intensity of 5D0 → 7F2 can improve the amplification properties of the optical amplifier. Peak width can be known from the spectrum. The width of the transition from 5D0 to 7F2 is 8 nm, which is very narrow due to the shielded 4f transition of the lanthanide complex. The measured lifetime is the quantum efficiency times the radiative lifetime. The quantum efficiency of Eu(TTA)3 was measured to be 0.56.12 This value can be assumed to be of Eu(TTA)3phen. Therefore, the radiative lifetime of 5D0 state of Eu(TTA)3phen is 1.61 ms. Then, the emission cross-section at 614 nm is calculated as 6.5×10-21 cm2. This value is comparable with erbium ions doped inorganic glasses. The absorption cross-section of Eu(TTA)3phen is easily calculated from the UV-visible spectrum (which is not shown). The obtained value is 7.1×10-17 cm2 at 355 nm which is about 4 order of magnitude higher than bare rare-earth ions. This high absorption cross-section is due to organic ligands that have very large oscillator strength between π → π* transition. The large absorption cross-section and long lifetime can lower the threshold pumping power that is required for the population inversion between the ground state and the excited state. 3.2. Numerical Simulation Four level structure of europium complex for amplification modeling can be assumed as two level. The amplification processes of optical amplifiers are expressed as

∫

∫

ΨΘ−

−

−ΨΘ=

0

0

0

12

2

0

12

)()(),()),(),((2

),()()(

),(),(2),(

a

s

sas

es

a

p

pap

rdrrrh

ztPztNztN

ztNrdrrr

h

ztPztN

dt

ztdN

νσσπ

τνπσ

(1)

21 NNN += , (2)

∫ ΨΘ−= 0

012 )()(),()),(),((2),( a

sas

es

s rdrrrztPztNztNdz

ztdP σσπ , (3)

∫ ΨΘ−= 0

01 )()(),(),(2),( a

pap

p rdrrrztPztNdz

ztdPπσ , (4)

where N1 and N2 are the number density of ground state and excited state, respectively, σp

a stimulated absorption cross section of pump light, σs

e stimulated emission cross section of signal light, τ metastable state lifetime, Ps signal intensity of 614 nm, Pp pump intensity of 355 nm. Using the steady state assumption and neglecting the signal absorption due to the four level nature of europium complex, these equations can be simplified. Overlap factor, 2π∫Θ(r)Ψ(r)rdr, was calculated as 0.572 assuming the gaussian profile of guided mode. Experimentally measured spectroscopic parameters were used for simulations. Number density of Eu(TTA)3phen is 3.58×1018 cm-3. Optical loss of bulk sample, 2.8 dB per meter at 614 nm was applied as loss of optical fiber. The amplified spontaneous emission was ignored. This assumption would work well because the metastable state lifetime of Eu(TTA)3phen is very long. All simulations were carried out using 1 µW as signal power. Optical gains of 10 m fiber length of optical amplifier with respect to the pump power are shown in Fig. 6. (a). No net optical gains are obtained up to 100 mW (20 dBm). The absorption curve of the pump power shows that 100 mW pump



power is absorbed completely within 10 m. If pump power is a few hundred milliwatt, it is possible to obtain the considerable optical gain because the active region where the amplification takes place extends to a few meter. The pump power of a few hundred miliwatt is much smaller than that of about hundred watt required for 30 dB gain of organic dye doped polymer optical fiber amplifiers. This fact indicates that small core dimension of the fabricated optical fiber amplifier confines the pump power more efficiently than multimode polymer fiber amplifier does. In the curve of absorption ratios of the pump power, it is interesting to note that optical gain saturates above the pump power of 30 dBm. The gain saturation is due to the unabsorbed power that is released from the output end of an amplifier. The pump power causing the gain saturation is low because of the high value of the stimulated absorption cross section of europium complex. The change of optical gains with respect to an amplifier length is shown in Fig. 6 (b) when pumped with the power of 300 mW. Optical gains increase with the fiber length. The gain saturation near 11 m can be understood by the complete absorption of the pump power at that position. Beyond 11 m, there are few excited states of europium complex. Therefore, the 11 m is an optimized length for the 300 mW pump power. Optimized lengths for different pump powers are shown in the Fig. 7 (a). Only a few hundred milliwatt is required if the amplifier length is optimized to a few meters. As explained above, optical gains are considerable at this conditions. Optimized length to the corresponding pump power is shorter than glass optical fiber amplifier. Complete absorption of pump power is also due to the large absorption cross section.

FIG. 6. (a) Amplification with respect to the pump power. Core radius of optical fiber is 2 µm, length 10 m, number density of europium complex, 3.58×1018 cm-3 (b) Amplification with respect to the fiber length. Core radius of optical fiber is 2 µm, number density of europium complex, 3.58×1018 cm-3, pump power, 300 mW When all of the Eu(TTA)3phen are in excited states, optical gains are proportional to the doping concentration of Eu(TTA)3phen. The relation Ps/Ps0 = exp( σeNz ), indicates that increasing the doping concentration arouses the similar effect as the increase of the amplifier length. In Fig. 7 (b) the change of optical gains with respect to the doping concentration of Eu(TTA)3phen is displayed. 40 dB of optical gain can be obtained by the 5 meter of amplifier doped with 1×1019 cm-3 of Eu(TTA)3phen. A gain saturation appears at the region near 5×1019 cm-3. High doping concentration of europium complex can result in aggregation phenomenon. Eu(TTA)3phen can be doped into PMMA matrixes up to 0.5 wt % not seriously degrading the fiber performance because the structure of organic ligands of europium complex is similar to the polymer matrix and the lifetime quenching of europium ion does not appear.

Pump power (dBm)

0 20 40 60

Gai

n (d

B)

-40

-20

0

20

40

Una

bsor

bed

pow

er r

atio

0.0

0.5

1.0

Gainunabsorbed power ratio

Length (m)

0 5 10 15

Gai

n (d

B)

0

10

20

30

Una

bsor

bed

pow

er r

atio

0.0

0.5

1.0

GainUnabsorbed power ratio

(a) (b)



FIG. 7. (a) Optimized fiber length with respect to the pump power. Core radius of optical fiber is 2 µm, number density of europium complex, 3.58×1018 cm-3 (b) Amplification with respect to the number density of europium complex. Core radius of optical fiber is 2 µm, fiber length, 5 m, pump power, 300 mW

4. CONCLUSION The properties of Eu(TTA)3phen are proved to have some advantages for the optical amplifier application. Its long lifetime and large stimulated emission cross section relevant to the amplification performance come from the blocking effect of the organic ligand to prevent the excited state from decaying nonradiatively by multiphonon relaxation due to high energy C-H vibration of PMMA. High intensity of main electronic transition centered at 614 nm originates from the covalent nature of ligand to metal bond. Lifetime of 5D0 metastable state is 0.9 ms, which is longer than usual rare earth complex. Its stimulated emission cross section is comparable to commercial EDFA and stimulated absorption cross section would show better performance reducing threshold pump power to get optical gain. Optical amplifier was fabricated by newly designed dip-coating method that has benefit for fabrication process. Simulation results of amplification characteristics showed that high optical gain can be obtained by small pump power because of well-confined pump intensity compared with the previous dye doped polymer fiber amplifier. More than 30 dB optical gain can be achieved with the amplifier modeled as less than 10 m of fiber length and 300 mW of pump power.

ACKNOWLEDGMENTS This work was supported by the Brain Korea 21 project. The authors are grateful to S. H. Park who is the member of Photonic Glasses Lab., POSTECH for optical loss measurement.

REFERENCES 1. R. T. Chen, M. Lee, S. Natarajan, C. Lin, Z. Z. Ho, and D. Robinson, “Single-Mode Nd3+-doped graded-index polymer

waveguide amplifier,” IEEE Phot. Tech. Lett. 5, pp. 1328-1331, 1993. 2. S. Lin, R. J. Feuerstein, and A. R. Mickelson, “A study of neodymium-chelate-doped optical polymer waveguides,” J.

Appl. Phys. 79, pp. 2868-2874, 1996. 3. Q. Zhang, H. Ming, and Y. Zhai, “Preparation and fluorescence of Nd3+ doped poly(methyl methacrylate) fibre,”

Polymer International 41, pp. 413-417, 1996. 4. D. An, Z. Yue, and R. T. Chen, “Dual functional polymeric waveguide with optical amplification and electro-optic

modulation,” Appl. Phys. Lett. 72, pp. 2806-2897, 1998. 5. K. Fujii, “Possibility of plastic optical fiber amplifier by CW-side excitation,” Opt. Comm. 171, pp. 81-84, 1999.

Pump power (dBm)

-20 0 20 40 60 80

Opt

imiz

ed le

ngth

(m

)

0

10

20

30

Number Density (cm-3)

1018 1019 1020

Gai

n (d

B)

0

20

40

Una

bsor

bed

pow

er r

atio

0.0

0.5

1.0

GainUnabsorbed power ratio

(a) (b)



6. A. Tagaya, Y. Koike, T. Kinoshita, E. Nihei, T. Yamamoto, and K. Sasaki, “Polymer optical fiber amplifier,” Appl. Phys. Lett. 63, pp. 883-884, 1993.

7. A. Tagaya, S. Teramoto, E. Nihei, K. Sasaki, and Y. Koike, “High-power and high-gain organic dye-doped polymer optical fiber amplifiers: novel techniques for preparation and spectral investigation,” Appl. Opt. 36, pp. 572-578, 1997.

8. G. D. Peng, P. Chu, Z. Xiong, T. W. Whitbread, and R. P. Chaplin, “Dye-doped step-index polymer optical fiber for broadband optical amplification,” J. Light. Tech. 14, pp. 2215-2223, 1996.

9. C. Koeppen, S. Yamada, G. Jiang, and A. F. Garito, “Rare-earth organic complexes for amplification in polymer optical fibers and waveguides,” J. Opt. Soc. Am. B 14, pp. 155-162, 1997.

10. Q. Zhang, X. Sun, and P. Wang, “Preparation of graded index plastic rods doped with Sm3+ by interfacial-gel polymerization,” Polymer International 45, pp. 185-190, 1998.

11. K. Kuriki, T. Kobayashi, N. Imai, T. Tamura, S. Nishihara, A. Tagaya, Y. Koike, and Y. Okamoto, “Fabrication and properties of polymer optical fibers containing Nd-Chelate,” IEEE Phot. Tech. Lett. 12, pp. 989-991, 2000.

12. R. A. Gudmundsen, O. J. Marsh, and E. Matovich, “Fluorescence of europium thenoyltrifluoroacetonate. II. Determination of absolute quantum efficiency,” J. Chem. Phys. 39, pp. 272-274, 1963.



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SPIE Proceedings [SPIE Symposium on Integrated Optics - San Jose, CA (Saturday 20 January 2001)] Rare-Earth-Doped Materials and Devices V - Plastic optical amplifier using europium