Gel-glass photochromic optical delay generator

Gel-glass photochromic optical delay generator

M. L6pezAmoa, D. vyb, F. del Monteb, I. Matfasa, P. Datta a and J.M. Ot6na

a) Dpto. de TecnologIa Fotónica. ETSI Telecomunicación, Ciudad Universitaria. 28040 Madrid.

b) Instituto de Ciencia de Materiales. CSIC. Serrano 1 15. 28006 Madrid. Spain. ABSTRACT

Variable delays in fiber optic optical transmission have been achieved by using photochromic-doped sol-gel silica glasses. Gaps between two commercial fibers in V-groove

connectors, when filled with these materials, produce variable propagation times. Variations

of the optical propagation time depending on the input power, wavelength and dopant concentration are shown.

1. INTRODUCTION

Polymers and glasses have both been used as substrate materials for immobilization of photosensitive dopants. Usually, organic dopants are photochemically and thermally unstable'.

This is a limitation for photoactive molecules, since the enhanced reactivity of their excited states leads to rapid aging of these materials. Polymeric substrates may interact not only with

the doping molecule but also with external and internal impurities and quenchers. Compared to polymers, silica glasses provide better optical and photochemical properties, thus being good candidates for developing passive and active photonic waveguides for optical interconnections,

where ruggedness, high chemical and thermal stability, low optical absorption in the visible and infrared range, and easiness of deposition on different substrates are often required. Since

19842, the sol-gel process is being used as a powerful method for the preparation of silica glasses doped with organic molecules3, including photoactive ones.

Application of the sol-gel process to fiber optics has been previously proposed and demonstrated utilizing thin layers of sol-gel on dip-coated fibers, unclad silica fibers and

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porous core silica glass fibers". Silica gel-glasses obtained by the sol-gel process are chemically

and optically very similar to the optical fiber itself. This opens the possibility of using doped

sol-gel glasses for modifying the properties of the fiber light throughput. In this work, the behavior of hybrid fiber/gel-glass devices doped with photochromic materials is analyzed.

Power meter or spectrum analyzer

V-groove adapter

Multiline Focusing system

Figure 1: Experimental set-up for a gel-glass photochromic optical delay generator

2. EXPERIMENTAL SETUP Optical waveguides between two multimode 50/125 jhm fiber optic pigtails have been prepared using Fujikura SPL-9 13K-CT commercial fiber optic removable connectors. These

connectors feature a 140 m deep, 200 jm wide V-groove for aligning the fiber ends. The

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grooves were filled up with photochromic-doped sol-gel materials varying the dopant concentration (Figure 1). The first fiber was coupled to a 10 mW Ar multiline laser, whereas

the second fiber was connected to an Anritsu ML9001A optical power meter or to an Advantest TQ8345 optical spectrum analyzer. The remaining ends of both fibers were installed

in the V-groove connector. These fiber ends in different devices were separated from tens of

m to 2 mm. The doped silica matrix formed upon the sol-gel process protects the trapped molecules from environmental agents. The matrix refractive index (1.443 at 633 nm) is quite close to the fiber core index (1.462), therefore nearly index matching is achieved at the interfaces with the

1

0.9 0.8 0.7 C

0

0 0 °05 .0' 0.4

0.3 0.2 0.1

450

500

550

600

650

700

Wavelength (nm) Figure 2. Photochromic behavior of a sol-gel thin-film doped with (1) with UV irradiation. Absorption intensities after a) 10 s, b) 20 s, c) 30 s, d) 40 s, e) 50 s, and f) 180 s are shown.

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fiber ends. Losses were further decreased by preparing the fiber ends with a Fujikura CT-03 precision fiber cleaver. Experimental details of the sol-gel process have been previously published2. The dopant photochromic material employed in this work was the spiropyran dye 5-bromo-8-methoxy-6nitro- 1 ',3 ',3 -trimethylspiro[2H- 1 -benzopyran-2,2 '-indoline] (1). The spectral behavior of a

sot-gel silica thin-film doped with (1) is shown in Figure 2.

3. EXPERIMENTAL RESULTS

The doped waveguide is colorless before irradiation. External irradiation at 365 nm,

obtained from a 6 W UV commercial fluorescent iamp develops a reddish color in the photochromic dopant (Figure 2). Any optical signal sent through the fiber within the affected wavelength range will be therefore attenuated accordingly.

w

0Q. 0.

0

-76

2

4

6

8

10

12

14

16

18

20

Time (s) Figure 3: Optical output power time response of the structure for two gap lengths and two wavelengths: a) 514.5 nm, 110 .m long, -64 dBm final output power. b) 488.0 nm, 2 mm long, -63 dBm final output power. c) 488.0 nm, 110 m long, -65.5 dBm final output power. d) 514.5 nm, 2 mm long, -59.5 dBm final output power.

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The color achieved with the UV irradiation in this material can be reversed with visible light within the absorption band, i.e. , with the same optical signal attenuated by the colored material. When using Ar lines, this band corresponds to the two main lines at 514.5 nm and 488.0 nm, and the less powerful intermediate line 496.5 nm. Depending on the gap length, the UV irradiation time and power, and the power and wavelength of the fiber optical throughput,

a variable optical delay generator is obtained by bleaching. Moreover, recoloring of the photochromic dopant is also achieved using the shortest (448 nm) visible wavelength of the

Ar laser through the fibers. Delays ranging from ms to several seconds have been achieved by varying the gap between

the fiber ends (i.e. the transition zone length). Figure 3 shows the behavior of two devices

00 0

0 13 dBNs

O.E12 0 /Lm

Wavelength

0.E17 0 /Lin

Figure 4: Delay generator optical output power response at room temperature for an input signal of 514.5 nm. -75 dBm corresponds to the noise threshold of the fiber optic spectrum analyzer. SPIEVo!. 2449/217 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 10/15/2015 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx

Output power [dBm]

2

4

6

8

10

12

14

16

18

20

Time[s] Figure 5: Optical output power time response of the fiber structure at room temperature for different input powers and wavelengths and for a gap length of 2 mm.

with different gap lengths, 110 /2m and 2 mm respectively, for two different input wavelengths. The delay induced in the transmitted power increases, as expected, with device

length, and show different rise times for each wavelength. Figure 4 shows the behavior of a 2 mm structure when an optical power of -60.5 dBm at 514.5 nm is applied. An optical delay of 3 s is obtained in the propagation from the input to the output. This output is connected to the optical spectrum analyzer.

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Different behaviors are achieved depending on the input power and wavelength. Figure 5 shows the output optical power, from UV-saturated probes using different input powers at two

wavelengths. As expected, the higher the input power, the shorter the delay time is. On the other hand, delays at 5 14.5 nm are shorter than those at 488 nm for the same optical power, as the absorption coefficient for the first wavelength is larger. Measurements before 2 s are not shown because correspond to the noise power threshold of the detector.

Notice that a clear exponential behavior is shown for each curve, where the y axis represents the transmitted power in a logarithmic scale (dBm). The dynamic range of each curve also depends on the utilized wavelength, as expected from Figure 2.

4. DISCUSSION A simple model of photochromic bleaching may account for the light intensity variations

with time. The model assumes that the light power reaching every section of the doped gelglass is roughly the same. This approximation obviously holds for low dopant concentrations

and/or low coloring levels. In this low power model, the Lambert-Beer law applies: =

If

lO

(2)

where I and are the actual and final (bleached material) light intensities respectively, a is

the absorption coefficient at the corresponding wavelength, c is the dopant concentration and

L is the gap length. Let us rewrite c as N/V, the number of dopant molecules per unit volume. Within the above

mentioned assumption (i.e., for a constant photon density input), N performs an exponential

decay with time by bleaching:

N = N0exp(-kt)

(3)

Substituting (3) into (2), an exponential behavior of time with log I is predicted:

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log!1-log! =

aNL

(4)

exp(-kt)

Figures 6 and 7 show this behavior. It can be seen that the predicted exponential decay is fairly well followed, except for the shortest time values, where the dopant concentration is still

too high for the absorption of the first layers to be negligible. The slope is a function of the light intensity, as expected, whereas the intercept depends on the initial optical density, i.e. the dopant concentration and the laser wavelength. The opposite situation (not shown in this work) would have been produced for high dopant

concentrations. In this case, the gel-glass would be opaque in the first stages, and would become transparent afterwards, thus giving a neat switching in light transmission. Samples with

10.0

c2

ED L..

00

1.0

b2

C

a2

0.1

514.5 nm

0

100

200

300

400

Time (s) Figure 6. Fitting of eq. (4) for two different doping concentrations (a and b) at three different bleaching powers each (1, 2, and 3). The laser wavelength was 514.5 nm and the gap length was 2 mm in all cases.

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much higher dopant concentration are currently being developed. A more involved transmission model, taking into account the photon density evolution along the bleaching process, is also under study. Even the simple model shown above allows reaching some conclusions. Light impinging

the gel-glass from the input fiber effectively "digs" a bleached hole in the material, thus making a waveguide which ultimately couples the input and the output fiber. This fact, along with the option of recoloring the material from the fiber itself (using shorter wavelengths, see

above), leads to the possibility of preparing simple all-optic couplers and routers based on photochromic gel glasses.

10.0

a

a3

a)

0

1.0

G)

b3 0.1

a) 514.5 nm b) 488.0 nm 0

40

80

120

160

200

Time (s) Figure 7. Fitting of eq. (4) for two different wavelengths (a and b) at three different bleaching powers each (1, 2, and 3). The doping concentration was the same and the gap length was 2 mm in both cases.

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Wavelength affects the optical behavior in a non-straightforward manner. Indeed, both a and k in eq. (4) are functions of X, thus determining the intercept and the slope of the lines in

figures 6 and 7.

In conclusion, a novel and easy method of preparing optical waveguides offering the possibility of generating different optical delays depending on the wavelength, optical input power and waveguide length has been demonstrated. Based on previous experiments, the response time can be substantially reduced by reducing the waveguide length and employing alternative matrix trapping conditions and/or other dopants. More complicated waveguiding and routing structures utilizing this material are possible. Therefore, the simple demonstrated optical transition of sol-gel potentially offers capabilities not only for optical delay generation,

but for optical wavelength analysis or optical signal modulation. Application of the device utilizing different dopants for fiber optics sensors is also under study.

5. ACKNOWLEDGEMENTS This work has been supported by the CICYT grants: Mat-90/0791, TIC- 92-0052-C02 and

TIC-93-0638.

6.REFERENCES

1. K. W. Beeson, K.A. Horn, M. McFarland, C. Wu and J.T. Yardley, Integrated Optics and optoelectronics 2, SPIE Proceedings 1374, 176, 1990.

2. D. Avnir, D. Levy and R. Reisfeld. "The nature of silica cage as reflected by spectral changes and enhancement photostability of trapped Rhodamine 6G", J. Phys. Chem. 88, 5956, 1984. 3. D. Levy. "Sol-Gel glasses for optics and electrooptics" , J. Non-Cryst Solids. 147-148, 508, 1992.

4. D. Yuan-Chieh. "The development offiber optic chemical sensors based on porous optical fibers and sol-gel immobilization techniques", Ph. D. thesis, Rutgers The State University of New Jersey, 1993.

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