Lensed-taper launching couplerfor small-bore, infrared hollow fibers

Yuji Matsuura, Hajime Hiraga, You Wang, Yuji Kato, Mitsunobu Miyagi, Shin-ichi Abe,and Shinji Onodera

A new type of launching coupler for small-bore, hollow fibers, consisting of a lens and a tapered hollowwaveguide, is proposed to increase the alignment tolerance between an input laser beam and small borefibers. First, we designed the structural dimensions of the coupler by using a ray-tracing method.Then, a series of experiments employing tapered hollow waveguides made of Pyrex glass was performedto investigate the effectiveness of the new coupler. It is shown that the coupler has a high efficiency withattenuation of around 0.5 dB, especially when the inside of the taper section is coated with a polymer andsilver film. In addition, we also show that the coupler has great tolerance for the transverse displace-ment of a waveguide axis, which gives a 0.1-dB loss increase for a 300-mm displacement. © 1997Optical Society of America

Key words: Hollow waveguide, infrared fiber optics, infrared laser.

1. Introduction

Beam delivery systems with hollow fibers are appro-priate for infrared laser applications owing to theirefficiency and high-power capability.1,2 For infraredhollow fibers, it was recently shown that hollowwaveguides with small bore sizes less than 700 mmoffer the advantages of flexibility and output beamquality.3,4 It has been reported simultaneously thatglass hollow waveguides with bore sizes as small as250 mm were successfully fabricated by using aliquid-phase deposition technique.3,5 These small-bore fibers, however, have the disadvantage of lesstolerance for optical alignment between an incidentlaser beam and the fibers. Even a misalignment assmall as 10 mm can cause significant coupling lossand can result in fatal damage of the fiber’s inputend. For small-bore fibers, therefore, a conventionallaunching system with only a lens needs careful andprecise optical alignment. In addition, since a con-ventional delivery system with a small-bore fibertends to be sensitive to shock and vibrations, it must

Y. Matsuura, H. Hiraga, Y. Wang, Y. Kato, and M. Miyagi arewith the Department of Electrical Communications, Tohoku Uni-versity, Sendai 980-77, Japan. S. Abe and S. Onodera are withthe Institute for Chemical Science, Tohoku University, Sendai980-77, Japan.

Received 24 January 1997.0003-6935y97y307818-04$10.00y0© 1997 Optical Society of America

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contain some kind of absorber. Otherwise, the inci-dent end of the fiber can be easily damaged by theenergy of the laser beam that is misaligned with thefiber’s bore.

As an alternative to a conventional lens system, wepropose a launching coupler for small-bore hollowfibers that consists of a lens and a tapered hollowwaveguide. The purpose of this launching system isnot only to increase the tolerance for optical align-ment but also to suppress high-order modes excitedin hollow fibers that are due to misalignment. Sincethese high-order modes have high attenuation coef-ficients, they sometimes cause heat-induced damagein the fibers.

Although launching couplers with a taperedwaveguide are commonly used as optical communi-cation components,6 for hollow fibers, as far as weknow, this is the first investigation of a launchingcoupler that is composed of a lens and a hollow taperwaveguide. Here we report the results of a ray-tracing calculation and loss measurements of alensed-taper coupler and show the effectiveness ofthe new launching device.

2. Design of the Launching Coupler

First we design the configuration of the launchingoptics that leads to maximum efficiency. Thelaunching coupler is composed of a plano–convex lensand a tapered waveguide as shown in Fig. 1.

To design the coupler, we used a conventional ray-tracing program on a personal computer because the

wave-optics theory, which is generally used to eval-uate hollow fibers, is not easily applicable to highlymultimode guides such as a tapered waveguide.With the ray-tracing program, the laser beam is sim-ulated by a ray bundle that propagates through thetapered waveguide section according to geometrical-optics laws as described elsewhere in detail.7,8 Inour case, to simplify the calculation, we assumed aconstant reflection coefficient at the waveguide innersurface that is independent of incident angle. Wedetermined the reflection coefficient by calculatingthe average of coefficients at a range of possible inci-dent angles that were calculated in advance by aray-tracing simulation. For our calculation of thereflection coefficients, we used the refractive indicesof materials from the literature.9 The total numberof rays considered in the ray-tracing calculation isaround 25,000. We determined the bore size of thetaper’s input end as 1400 mm and the output end as700 mm, which is the same as the size of hollow fibersconnected to the coupler. We also assumed a lenswith a focal length of 100 mm and a focused spot sizeof 600 mm located at the input end of the taperedsection.

Figure 2 shows calculated attenuations of thelensed-taper couplers that are made of fused silicaglass as a function of transverse displacement of thecoupler’s input end from the optical axis. For all thelengths, the lensed-taper launching system showslarge tolerance for optical alignment, and the atten-uations increase by less than 0.1 dB because of500-mm displacement from the optical axis. Al-though Fig. 2 shows that the coupler with the short-

Fig. 1. Schematic view of a launching coupler composed of a lensand a tapered waveguide.

Fig. 2. Calculated attenuation of launching couplers with a ta-pered waveguide. For a polymeryAg-coated waveguide, we as-sumed a refractive index of 1.5 and a thickness of 1.44 mm.

est taper section has the lowest attenuation, the shorttaper has a large beam divergence from the coupler’soutput end, which excites lossy, high-order modes inthe hollow fiber connected to the lensed-taper cou-pler. Consequently, the losses of the tapered guideand the fiber compensate each other.

Figure 2 also shows the attenuation of a lensed-taper waveguide with silver and polymer inner coat-ings. The loss difference between coated anduncoated waveguides results from the different re-flection coefficients of the guide’s inside wall andtherefore the waveguide with the polymer and silvercoating that is a reflection enhancement film showsthe lowest loss. The dielectric layer has an assumedrefractive index of 1.5 and a thickness of 1.44 mm thatis optimized for the CO2 laser wavelength of 10.6 mm.

3. Experiment

We fabricated the lensed-taper launching couplermade of a Pyrex glass tube by using a glass blowingtechnique. We confirmed by a microscopic measure-ment that the shape of the tapered section is a linearhorn. The inner diameter of the input end is 1400mm and the output end is 700 mm. The lengths ofthe taper sections are 50 and 100 mm. The focallength of the lens embodied in the coupler is 100 mmand it produces a focused spot size of 570 mm for a4.5-mm-diameter input laser beam. Although theoptimum spot size of an incident beam for the 700-mmbore hollow fiber is 450 mm,10 the larger size of 570mm was used in the experiment to simulate a mis-coupling condition. In the experiment, the inputend of the tapered section is located at the beam waistof the focused beam.

Figure 3 shows the measured attenuations of thelaunching coupler system as well as the attenuationsof a conventional coupler consisting of a lens and astraight waveguide with a bore size of 700 mm. In aconventional system without a taper, the attenuationincreases rapidly when the waveguide axis deviatestransversely. Even for a small displacement of 100mm, the loss increases around 0.8 dB in a conven-tional system, whereas the launching system with atapered section shows only a 0.1-dB increase. Thisis the first advantage of the new launching scheme.

Fig. 3. Measured attenuation of launching couplers with andwithout a Pyrex glass taper.

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In addition, the input ends of a nontaperedwaveguide can be seriously damaged even for 5 W ofinput power when the displacement is 100 mm,whereas no damage was found in the tapered launch-ing couplers. This is the second advantage of alensed taper and is an important factor for a launch-ing system of high-powered laser light. From theresults in Fig. 3, we confirm the validity of the ray-tracing method since the measured attenuations areclose to the calculated losses shown in Fig. 2. It isalso seen that the coupler with the shorter tapershows lower losses as expected when the hollow fiberis not connected to the coupler.

Next we investigated the total attenuation of thedelivery system that consists of a launching couplerand a hollow fiber with a 700-mm bore and a 1-mlength connected to the end of a tapered waveguide.The fiber is a glass hollow waveguide with silver andcyclic-olefin polymer films inside the bore, which wasespecially designed for CO2 laser delivery. The at-tenuation loss of the fiber is 0.5 dBym for CO2 laserlight at the optimum coupling condition. The otherspecific properties of the fiber will be reported else-where. First we compared the losses of two deliverysystems having different taper lengths, i.e., 50 and100 mm. In contrast to the result of a coupler with-out a connected fiber ~Fig. 3!, when the fiber is con-nected to the taper, the total loss of the system witha 100-mm taper is 1.9 dB, which is lower than the lossof the system with a shorter, 50-mm taper, that is, 2.4dB. This is because of the larger beam divergencefrom the taper guide’s output end, which exciteslossy, high-order modes in the fiber.

Taking this result into consideration, we chose thetaper length of 100 mm as the proper size and fabri-cated a low-loss, lensed-taper coupler. The couplerincludes a Pyrex taper waveguide with silver andfluorocarbon polymer ~FCP! inner coatings. We de-posited the silver layer by using a conventional plat-ing method and the FCP layer by using a liquid-phasecoating method.11

Figure 4 shows the effect of the FCP and silvercoatings inside the tapered waveguide with a lengthof 100 mm. In comparison with L 5 100 mm, theuncoated Pyrex taper shown in Fig. 3 demonstrates

Fig. 4. Measured attenuation of a launching coupler with a taperwaveguide that is coated with silver and FCP film.

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that the loss of the coupler alone is drastically re-duced by the coatings. Furthermore, the total loss ofthe delivery system including the hollow fiber is alsoreduced and shows high tolerance for misalignment.However, the loss of the coated taper is four timeshigher than that expected from the calculation pos-sibly because the FCP and silver coatings are notuniform and the polymer layer is not thick enough toprovide the maximum interference effect. We arenow researching improvement of the optical qualityof coatings by modifying the fabrication process.With regard to coupler losses with the fiber con-nected, the loss induced by the tapered coupler isestimated to be around 0.5 dB, which we derived bysubtracting the losses of the taper alone ~0.4 dB! andthe hollow fiber ~0.5 dB! from the loss of the couplerwith the fiber ~1.4 dB!. Although this coupling lossis not small at this time, we believe that the couplingloss can be reduced by optimizing the coupling con-dition including the focal length of the lens.

4. Conclusion

We have proposed a new type of launching coupler forsmall-bore, hollow fibers consisting of a lens and atapered hollow waveguide to increase the alignmenttolerance between an input laser beam and smallbore fibers. First we designed the structural dimen-sions of the coupler by using a ray-tracing method.Then we performed experiments using tapered hol-low waveguides made of Pyrex glass to investigatethe effectiveness of the new coupler. We haveshown that the coupler has high efficiency with at-tenuation of around 0.5 dB especially when the insideof the taper section is coated by polymer and metalfilms. In addition, we have also shown that the cou-pler has large tolerance for transverse displacementof a waveguide axis, that is, 0.1-dB loss increase for a300-mm displacement.

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