Photonic and Optoelectronic Polymers - American Chemical Society

Chapter 4

High-Power Polymer Optical Fiber Amplifiers in the Visible Region

Downloaded by NORTH CAROLINA STATE UNIV on October 8, 2012 | http://pubs.acs.org Publication Date: September 1, 1997 | doi: 10.1021/bk-1997-0672.ch004

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1,2

1,2

Takeyuki Kobayashi , Akihiro Tagaya , Shiro Nakatsuka , Shigehiro Teramoto , Eisuke Nihei , Keisuke Sasaki , and Yasuhiro Koike 1

1

1

1,2

1

Department of Material Science, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223, Japan Kanagawa Academy of Science and Technology, 3-2-1 Sakato, Takatsu-ku, Kawasaki 213, Japan

2

High-power (1200 W) and high-gain (36 dB, approximately 4000 times) amplification has been achieved in a graded-index (GI) polymer optical fiber amplifier (POFA) with 0.3 W input signal under optimized amplification conditions suggested from theoretical analysis. High gain of more than 25 dB (320 times) is expected through the wavelength region of 540-610 nm using Rhodamine 6G, Rhodamine B, and Rhodamine 101-doped GI POFA. The amplification at 649-nm wavelength where POF has a low loss window of transmission was achieved in an Oxazine 4 perchlorate-doped GI POFA. The construction of optical fiber network systems is proceeding rapidly in recent years due to the emergence of an advanced information-oriented society. As longhaul communication media in such a multimedia society, conventional wire cables have been replaced by single-mode silica optical fibers because high-speed data transmission with a capacity of the order of giga bits/s (bps) is required. In shortdistance communication, many junctions and connections of optical fibers would be necessary. The small core of the single-mode optical fiber requires highaccuracy alignment in the fiber connection and junction, which increases the cost of the whole system. Recently, a high bandwidth and low loss graded-index (GI) type polymer optical fiber (POF) having a large core diameter (such as 500 μπι or more) was prepared at Keio University (1-4). The GI POF is one of the promising candidates to solve this problem of short-distance communication because of its easy processing and handling. Data communication in the GI POF network is expected to be carried out at a visible wavelength because the GI POF has a low loss region (around 650 nm wavelength). Therefore, optical amplifiers, couplers, and switches in such a wavelength region for the GI POF network are necessary. © 1997 American Chemical Society

In Photonic and Optoelectronic Polymers; Jenekhe, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.

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PHOTONIC AND OPTOELECTRONIC POLYMERS

Very recently, a GI type organic dye-doped polymer optical fiber amplifier (POFA) which has a gain in the visible region was proposed and demonstrated for the first time at Keio University (5, 6). High-power (more than 620 W) and highgain (more than 620 times, 28 dB) nanosecond-pulse-light-amplification in a short length (0.5-2.0 m) of the GI POFA is possible due to the extremely large absorption and emission cross sections of the dye (approximately 10,000 times larger than those of rare-earth elements) and a large core diameter (more than 500 μιη) (7, 8). As far as we know, such a high-power amplification with the signal and pump pulses of several nanosecond duration in the visible region was realized for the first time. Doping of organic dyes in the POF is possible because of its low heat-drawing temperature (180-220 "C) compared with a silica optical fiber whose heat-drawing temperature is more than 1000 "C. The availability of many organic dyes that can be used as dopants result in potential amplification over the whole visible region. These advantages offer attractive applications such as an amplifier in the GI POF network, a booster for a laser diode or other light sources, and so on. In this paper, optimization of amplification conditions (signal wavelength, fiber length, and dye concentration) to achieve maximum gain in the GI POFA is described. In addition, amplification performance of GI POFA over a wide spectral range in the visible region is shown by theoretical analysis based on dye density distribution and cross section measurements. Actual amplification in the GI POFA at several wavelengths is demonstrated and discussed. Preparation of POFA Details of the preparation of the POFA have been described previously (5-8). In this section, the preparation of GI POFA is briefly described. A series of methyl methacrylate (MMA) solutions of organic dyes ranging in concentrations from 0.01 to 20 ppm were prepared, in which specified amounts of η-butyl mercaptan, benzyl η-butyl phthalate and dimethyl sulfoxide were dissolved. The η-butyl mercaptan is a chain transfer agent for controlling the molecular weight of polymer, the benzyl η-butyl phthalate is for obtaining the graded-index distribution (9), and the dimethyl sulfoxide is for enhancing the solubility of organic dyes in MMA. The organic dye/MMA solution, a t-butyl peroxy isopropylcarbonate initiator, was placed in a PMMA tube with an outer diameter of 10 mm and an inner diameter of 5 mm. The tube was placed in a furnace at 90-95 Polymerization was carried out for 24 hours followed by heat treatment at 110 V, for 60 hours. A preform rod with a diameter of 10 mm prepared by this process was heatdrawn into a fiber at 190-250 *C by taking up reel. The preform rod moved down with a constant velocity Vi (ca. 10 mm/min) and was heat-drawn by drive roll with a velocity V2 (ca. 5 m/min). A GI POFA with a desired diameter can be obtained by controlling the ratio of V2 to Vi. An example of actual organic dye density distribution in a GI POFA preform rod is shown in Figure 1, where Rp is the radius of the preform rod and r is



4. KOBAYASHI ET AL.

High-Power Polymer Optical Fiber Amplifiers

distance from the center axis. The dye density distribution was determined by two techniques: (1) fluorescence technique; (2) absorbance technique. In the first technique, the dye density was calculated from the fluorescence intensity measured with a fluorescence microscope. In the second technique, the dye density was obtained from the absorbance at 532 nm wavelength measured in the radial direction of disk-like sample (thickness: 0.1-1.0 mm) and the absorption cross section. From these two techniques, it was experimentally confirmed that the GI POFA had a quadratic dye density distribution which provides efficient amplification compared with homogeneous dye density distribution. Figure 1 also shows a typical example of refractive index distribution in the GI POFA preform rod measured by an interferometric technique (10, 11), where n and η mean the refractive indices at the center axis and at distance r, respectively. As shown in the curve of Figure 1, the preform rod has a cladding region coming from the PMMA tube and a quadratic index profile in the core region. The normalized refractive index distribution of the GI POFA was almost the same as that of this preform rod (12). 0

Theoretical Analysis In order to analyze the amplification in a GI POFA, we assume the model shown in Figure 2 where the decay of the vibrational energy in the first excited singlet state Si is extremely fast. Thus the population densities of each level are described as follows: (1)

N (t, z) s 0 3

Ν

τ

= Ν (ΐ λ

9

(2)

Ζ) + ΛΓ (Γ, Ζ) 2

where Ni is the population density of level i at a position ζ along the fiber at a time r and #7 is the total density of dyes in the fiber. The rate equation for level 2 is written as

(3)

α

a

Here, P (t, z) and P (t, z) are the pump and the signal power densities, σ and o are absorption cross sections at the pump and the signal wavelengths, of is the emission cross section at the signal wavelength, y and y are frequencies for the pump and the signal, and h means Planck's constant. The signal and pump pulses p

5

ρ

p

s


s

49


50


0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 (r/Rp) Figure 1. Dye density and refractive index distributions of Rhodamine B-doped GI POFA preform rod prepared from MMA solution of 20 ppm-Rhodamine B. Plots and solid line denote the dye density and refractive index values, respectively.

1/T

V

: fast p h o n o n decay

2

1

»0 α

Figure 2. Schematic representation of amplifier model. σ , absorption cross section at pump wavelength; o , absorption cross section at signal wavelength; σΛ emission cross section at signal wavelength; % lifetime of level 2; r , lifetime of level 3. ρ

a

s

v


4. KOBAYASHI ET AL.


propagate in the positive direction of the fiber axis, according to: e

= (a N (t, z)-a:N (t, z))P (t, z) s

2

l

(4)

s


dz dz Taking into account the radial distributions of the optical power density and the dye density, the functions of power density distribution ψ (r) and the dye density distribution Θ (r) are defined as I (t,z,r)-P (t,z)W(r) StP

(6)

SiP

n (t,z, r) = N (t,z)0(r) u

0)

ia

By integrating the overlap between the optical power density and the dye density distributions, equations (3)~(5) become ^2(',*)

=

2 ^ , ( 1 ,

dt

z)P jt,z) p

Av

y

d

r

_ N (t,z) 2

Jo

p

τ

_ 2tt(a iV (f, z)-o?N (t, z))P (t, z) * s

e

2

1

s

hv

e

Q

(

r

)

w

(

r

)

r

d

r

·*°

s

^ ^ -

0

( g )

a

= 2x{o N (t, z)-o N (t, z))P {t, z)f °e(r)W(r)rdr s

2

s

x

s

= - 2 π σ ^ , ( ί , z)P (t, z)f °0{r)W(r)rdr p

(9)

0

(10)

o

where a is the core radius. Here, ψ (r) is defined as the Gaussian function, in which 95 % of the total optical energy is in the core region. Θ (r) is defined as the quadratic function that is similar to the refractive index profile. Equations (8)~(10) are solved by numerical integration, where At = 20 ps and Δζ = 0.01 mm. 0

Cross Section Measurements α

Absorption Cross Section. Absorption cross section σ (λ) is related to absorbance ABS(A) by equation (11).


51

52


ABSjk) v

' 0.4343^ Here, N\ means the dye density and ζ means the optical path. The absorption cross section of a variety of dyes in bulk PMMA was obtained by measuring ABS(A), dye density, and optical path.


Emission Cross Section. In general, quantum yield of fluorescence is defined as

J

k^,

CE(X\dk*\

(12)

he where Κ(λ) and Α(λ) are fluorescence and absorption intensities at wavelength λ, and Ε(λ) is the line shape function. The emission cross section σ"(λ) is related to £(A)by

σ

ΐ

{

λ

)

^Εψ 8jvcn τ

=

(13)

Therefore, the emission cross section for a variety of organic dyes can be obtained from the measured quantum yield, fluorescence spectrum, and lifetime data. Figure 3 shows the absorption and emission cross section spectra obtained for Rhodamine 101 perchlorate (R101) in bulk PMMA. These cross sections (o = 3.2 χ 10' m , σ max = 2.3 x 10~ m ) are approximately 10,000 times lager than those of Er* in Ge0 -Si0 glass (o max = 8.0 χ 10' m , o max = 7.0 χ 10' m ) (13). The optical amplification in the GI POFA was simulated using these cross section data. Further details of the cross section measurements were described elsewhere (7, 8). a

20

2

m a x

20

2

β

+

25

2

2

a

2

25

e

2

Amplification Performance of POFA Optimization for maximum signal gain. Calculated signal gain against fiber length in a R101-doped GI POFA is shown in Figure 4 for a series of launched pump powers. Such a calculation enables us to optimize fiber length and dye concentration for each launched pump power to achieve maximum signal gain. Signal gain against signal wavelength. Calculated signal gain versus signal wavelength for a variety of organic dye-doped GI POFA are shown in Figure 5. These signal gains were calculated under the optimized conditions (fiber length and dye concentration) mentioned in the previous section. High gain of more than 25


4. KOBAYASHI ET AL.

High-Power Polymer Optical Fiber Amplifiers 53


4.0

450

500

550 600 Wavelength ( nm )

650

700

Figure 3. Absorption and emission cross section spectra of Rhodamine 101 perchlorate in bulk PMMA.

50

0

10

20 30 Fiber length ( cm )

40

50

Figure 4. Signal gain versus fiber length for Rhodamine 101 perchlorate-doped GI POFA at a series of launched pump powers. Rhodamine 101 perchlorate concentration = 1.0 ppm. Core diameter = 500 μιη. Launched signal power (at 598 nm, FWHM = 3.5 ns) = 1.0 W. Pump wavelength (FWHM = 6.0 ns) = 532 nm.



54


520

540

560 580 600 Wavelength (nm)

620

Figure 5. Calculated signal gain versus signal wavelength for Rhodamine Β (RB), Rhodamine 6G (R6G), DCM, Rhodamine 101 (R101), and Sulforhodamine 101 (SlOl)-doped GI POFA. Launched signal power (FWHM = 3.5 ns) = 1.0 W. Launched pump power (at 532 nm, FWHM = 6.0 ns) = 10 kW. Core diameter = 500 μπι.

YAG

BS

DL

ML

GIPOFA

4 ML

AT

{>• BS

MO

SP

PM

Figure 6. Experimental setup for investigating output spectra of the GI POFA. YAG, frequency-doubled Q-switched Nd:YAG laser; DL, dye laser; BS, beamsplitter; ML, mirror; AT, attenuator; MO, microscope objective; SP, spectroscope; PM, photomultiplier tube.


4. KOBAYASHI ET AL.


dB (320 times) can be obtained throughout the spectral region of approximately 70nm wavelength (540-610 nm) by using Rhodamine 6G, Rhodamine Β (RB), and RIOl-doped GI POFA. In particular, a high gain of 35 dB (3200 times) can be achieved around 580 nm. These results show the possibility of amplification covering the whole visible region in the GI POFA by doping with a variety of organic dyes.


Amplification Experiments Amplification experiment was carried out under the optimum conditions (signal wavelength, fiber length, and dye concentration) in order to achieve the maximum signal gain of 35 dB by using RB-doped GI POFA. The setup for the amplification experiment is shown in Figure 6. The pump source was a frequency-doubled Q-switched Nd:YAG laser at 532-nm wavelength and the signal source was a dye laser at 580-nm wavelength. The pump and the signal lights were combined by means of a beamsplitter and were coaxially launched into the RB-doped GI POFA. After passing through a spectroscope, the output from the RB-doped GI POFA with a launched signal power of 0.3 W was detected by a photomultiplier tube connected to an oscilloscope. Full width at half maximum for the pump and signal lights were approximately 6.0 ns and 3.5 ns, respectively. The launched pump power and signal power were estimated from the pump and signal coupling efficiencies of the RB-doped GI POFA. The signal gain of the RB-doped GI POFA at 580-nm wavelength plotted against the launched pump power at 532 nm wavelength is shown in Figure 7. The maximum signal gain of 36 dB (4000 times) was obtained with the launched pump power of 3.5 kW, in which 0.3 W of input signal was amplified up to 1200 W. As far as we know, such a high-power amplification of the signal and the pump pulses of several nanosecond duration in the visible region is realized for the first time. Agreement between the experimental data and the calculated values shows the validity of the optimization based on the theoretical analysis. Other results of the amplification experiments are summarized in Table I. It should be pointed out that the amplification at 649 nm wavelength where GI POF has a low loss region was achieved in an Oxazine 4 perchlorate-doped POFA. Except for the case of RB-doped GI POFA, higher gains than the results shown in Table I are expected because the amplification conditions have not been optimized (signal wavelength, fiber length, and dye concentration) yet. Conclusions Polymer optical fiber amplifiers which have a gain in the visible region have been demonstrated for the first time at Keio University. The high-gain (36 dB, approximately 4000 times) and high-power (1200 W) amplification was achieved experimentally under the optimized amplification conditions based on a theoretical analysis using the absorption and emission cross section data. A high gain of more than 25 dB (320 times) is expected throughout the spectral region of


55

56


l 3000

-! 30 ι 3 1 0.3 - 0.03

Outpu t sign al power(W


ι 300

ι 0.003 0.0003 0.5 1 1.5 2 2.5 3 3.5 Launched pump power (kW)

Figure 7. Signal gain versus launched pump power for Rhodamine B-doped GI POFA. Launched signal power (at 580 nm, FWHM = 3.5 ns) = 0.3 W. Pump wavelength - 532 nm. Fiber length = 90 cm. Core diameter = 300 μπι. Assumed dye concentration = 0.17 ppm.

Table I.

The experimental results of gain measurements Signal Wavelength (nm) Fiber Length (m)

Dye

Gain (dB/times)

Rhodamine Β

36/4000 28/620

580 591

0.9 1.0

Rhodamine 6G

26/400

572

1.2

Rhodamine 101 P. C.

13/20

598

2.2

Pyrromethene 567

14/25

567

1.5

Perylene red

20/100

597

1.6

Oxazine 4 P.C.

18/63

649

1.0


4. KOBAYASHI ET AL.

High-Power Polymer Optical Fiber Amplifiers 57

approximately 70-nm wavelength (540-610 nm) by using Rhodamine 6G, RB, and RIOl-doped GI POFA. Amplification at 649-nm wavelength where GI POF has a low loss region, on the other hand, was achieved in the Oxazine 4 perchloratedoped GI POFA. These results demonstrate the potential of GI POFA to cover the whole visible spectral region.


Literature Cited (1) Koike, Y.; Ishigure, T.; Nihei, E. J. Lightwave Technol. 1995, 13, 1475. (2) Ishigure, T.; Nihei, E.; Koike, Y. Appl. Opt. 1994,33,4261. (3) Ishigure, T.; Nihei, E.; Koike, Y. In Conference on Lasers and Electro-Optics in Europe (CLEO EUROPE 94), Technical Digest, 1994, CThD5. (4) Koike, Y. In Third Int. Conference on Plastic Optical Fibers and Applications (POF 94), Conference Proceedings, 1994. (5) Tagaya, Α.; Koike, Y.; Kinoshita, T.; Nihei, E.; Yamamoto, T.; Sasaki, K. Appl. Phys. Lett. 1993, 63, 883. (6) Tagaya, A.; Koike, Y.; Nihei, E.; Teramoto, S.; Fujii, K.; Yamamoto, T.; Sasaki, K. Appl. Opt. 1995, 34, 988. (7) Tagaya, Α.; Teramoto, S.; Yamamoto, T.; Fujii, K.; Nihei, E.; Koike, Y.; Sasaki, K. IEEE J. Quantum Electron. 1995, 31, 2215. (8) Tagaya, A.; Kobayashi, T.; Nakatsuka, S.; Nihei, E.; Sasaki, K.; Koike, Y. Jpn. J. Appl. Phys., submitted. (9) Koike, Y. In Proc. ECOC'92, 1992. (10) Koike, Y.; Sumi, Y.; Ohtsuka, Y. Appl. Opt. 1986, 25, 3356. (11) Ohtsuka, Y.; Koike, Y. Appl. Opt. 1980, 19, 2866. (12) Koike, Y. Polymer, 1991, 32, 1737. (13) Barnes, W. L.; Laming, R. I.; Tarbox, E. J.; Morkel, P. R. IEEE J. Quantum Electron., 1991, 27,1004.


Photonic and Optoelectronic Polymers - American Chemical Society

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