New fabrication technique of fluorocarbon polymer—coated hollow

waveguides by liquid—phase coating for medical applications

Yuji Kato, Mitsuo Osawa, and Mitsunobu MiyagiDepartment of Electrical Communications, Faculty of Engineering,

Tohoku University

Masaru Aizawa, Shin—ichi Abe, and Shinji OnoderaInstitute for Chemical Reaction Science,Tohoku University, Sendai, 980 JAPAN

ABSTRACT

A new method to fabricate dielectric—coated hollow waveguides has been proposed based on allliquid—phase techniques. Long silver tubes with an extremely smooth inner surface have beenfabricated by using a new process of silver mirror reaction and loss reduction of short silverwaveguides has been made by liquid flow—coating of a fluorocarbon polymer.

1. INTRODUCTION

Low—loss and non—toxic thin waveguides have been strongly required for minimally invasivemedicine. Hollow waveguides have an advantage over solid core fibers in their high power capability1.Sapphire hollow waveguides can be used for transmission of CO1 laser light2. Dielectric—coated hollow

waveguides can transmit Er:YAG, CO, and CO2 laser light simultaneously or independently when thethickness of dielectric layer is properly chosen3. So far, several methods have been employed tofabricate the waveguides48. In order to realize the waveguides with a small—core diameter, liquid—phase8'9 coating of dielectric inside a pipe is very promising.

In this paper, we present some experimental results on a new fabrication technique offluorocarbon—coated silver waveguides by using a method of all liquid—phase coating. A method ofsilver mirror reaction has been newly developed to coat a silver layer with an ultra—smooth surfaceinside a pyrex tube. Loss reduction has been made for short silver waveguides by liquid flow—coatingof a fluorocarbon polymer at the infrared wavelengths where the absorption of the polymer isrelatively small.

2. FABRICATION AND TRANSMIUANCE OF SILVER HOLLOW WAVEGUIDES

Silver is the most suitable material as a metal to realize dielectric—coated waveguides with smalllosses for the infrared. We first prepare a pyrex tube, and silver is coated inside the pyrex tube byusing a method of silver mirror reaction. When a conventional method of silver mirror reaction whose

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condition is summarized in Table 1 is used, there are many small reduced silver particles inside thepipe which cause the scattering losses. Therefore, we have developed a new process of the silverdeposition. Improvement of plating condition is as follows: A concentration of solution is changedas summarized in Table 2.

Table 1. Conventional plating condition of silver.

Plating (1) a: AgNO3 O.875g/6Occ +NH3JJ,

b: KOH O.625g/6OccSolution

c: a+b +NH3

Reducing (2) Grape Sugar 25g/HNO3 O.625g/?

=boi1ing +C2H5OH 12.5ccCleaning Distilled Water

Temprature Room Temprature

Table 2. Improved plating condition of silver.

Plating (1) a: AgNO3 7'.-'1O.5g/6Occ +NH3

b: KOH 5'-'7.5g/6OccSolution

c: a+b +NH3Reducing (2) Grape Sugar lOOg/t

HNO3 2.5g/=boiling +C2H5011 50cc

Cleaning Distilled Water+CH3COCH3

Temprature Room Temprature

Next, the deposition is conducted as schematically shown in Fig. 1 by separating plating solution(1) from reducing solution (2) and the reaction is conducted in a dark room.

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PlatingSolution

Pump

ReducingSolution

Fig. 1. Schematic view of silver deposition by the improved method of silver mirrorreaction.

Figures 2 (a) and (b) show the inner surface profiles of the silver tubes with a diameter of0.8mm fabricated by the conventional and improved methods. The smoothness of inner surface isdrastically improved by employing the new deposition method.

100 200 300Axial Distance (ii m)

(a)

Pyrex Tube

+0.2E

+0.1CE 0.0C)

U)0

—0.20

— I I I —

E- . :t . -i;--I I ••,•I

400 0 100Axial

200Distance

300 400(ti m)

(b)Fig. 2. Inner surface profiles of silver tubes by using the conventional (a) and

improved (b) methods of silver mirror reaction.

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Figure 3 shows loss spectra of silver hollow waveguides fabricated by the conventional andimproved methods of silver mirror reaction. By noting that the loss of waveguide fabricated by usingthe improved method becomes smaller than that fabricated by the conventional method at shorterwavelengths, it is clear that the surface roughness substantially affects the transmission losses.

:ffveflflaIcø2:11IaP" . Improved

c:I I I I I I I

2 4 6 8 10 12Wavelength (tim)

Fig. 3. Loss spectra of silver hollow waveguides with a diameter of 0.8mm and20cm long fabricated by the conventional and improved methods of silvermirror reaction. Waveguides are excited by a Gaussian beam whosedivergence angle is 4.3° at FWHM.

3. LIQUID FLOW-COATING OF FLUOROCARBON POLYMER

In order to coat a dielectric based on the liquid—phase method, we have found that anamorphous fluorocarbon polymer (CYTOP® from Asahi Glass Company) is suitable as a dielectric9in the waveguides for Er:YAG, CO, CO2 laser light, for the refractive index (1.34) of CYTOP (FCP)is near the optimum refractive index of 1.41 and the absorption coefficient is relatively small at thewavelength except for 7—9 tm. The FCP layer is able to be formed by dipping a substrate intoperfluoro solution including perfluorocarbon polymer at a constant speed and by vaporizing thesolution at certain temperature10. As the thickness of FCP layer formed by one cycle is very thin, thesame process is repeated many times. Finally the deposited FCP layer is annealed at highertemperature than the temperature employed in each process.

In order to deposit a FCP layer inside a silver tube, we have developed a liquid flow—coatingmethod, i.e., perfluoro solution including FCP is flowed by using a pump and is vaporized inside anelectric furnace. The liquid flow coating apparatus is schematically shown in Fig. 4 and the fabricationconditions of each process is summarized in Table 3.

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Furnace

Thermoregulator

Fig. 4. Schematic view of liquid flow—coating apparatus.

Table 3. Conditions of each process to coat a FCP layer.

SolutionConcentrationBoiling Point of Solvent

Perfiuoro Solution2 or 5 wt.%100 or 180 °C

Including FCP

Flow Speed 6'.'200cm/minNumber of Flow 1'240Heating 40.'120 °C-l0rnin

100--'180 °C-lh(each coating)

(final coating)

Figures 5 (a) and (b) show loss spectra of silver hollow waveguides with a 0.8mm innerdiameter and 10cm long with (FCP/Ag) and without (Ag) a FCP layer. The loss reduction issignificantly made by coating the FCP layer except for the absorption band of 7—9tm. This absorptionis caused due to the C—F bond. It is preferable to use a solution with high concentration of FCP andhigh boiling point in order to obtain low—loss waveguides. From the results, it is easily expected thatthe method based on the liquid flow—coating can be applied to fabricate waveguides with smaller corediameter. At the present stage, the loss reduction has not been sufficiently performed, which may becaused by the thickness irregularity or by thin thickness of FCP layer.

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Solution

Silver CoatedPyrex Tube

PeristalticPump

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3

-o

0c.ci)

02 12

(a)

3

C0(0:3CU)

0 '2 4 6 8 10 12

Wavelength (1um)(b)

Fig. 5. Loss spectra of silver hollow waveguides (0.8mmx10cm) with and withouta FCP layer. The fabrication conditions used in the process are (a) 2wt.%(concentration of FCP), 1000C (boling point of solvent), 115cm/mm (flowspeed), and 240 (number of flow) and (b) Swt.%, 180° C, 12cm/mm, and 30,

respectively.

4. CONCLUSION

A new fabrication method has been proposed based on all liquid phase techniques andwaveguides with smaller losses have been obtained by coating a FCP layer inside a silver tube. The

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4 6 8 10Wavelength (,am)

I• I I ;• N.•

I I I I I I I

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method is also developed to obtain long silver hollow pipes with an ultra—smooth inner surface.

5. ACKNOWLEDGEMENT

This research has been supported by a Scientific Research Grant—in—Aid (02402036) from theMinistry of Education, Science, and Culture of Japan.

6. REFERENCES

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7. M. B. Levy and K. D. Laakmann, "Flexible waveguide for CO2 laser surgery," Proc. Soc. Photo—Opt. Instrum. Eng., vol. 605, pp. 57—58, 1986.

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