3800 APPLIED OPTICS / Vol. 25, No. 21 / 1 November 1986 Control of wavelength selectivity of power transfer in fused biconlcal monomode directional couplers D. C. Johnson and K. O. Hill Department of Communications, Communications Re- search Centre, P.O. Box 11490, Station H, Ottawa, Ontario K2H, 8S2. Received 5 February 1986. 0003-6935/86/213800-04$02.00/0. Fiber-optic directional couplers fabricated using the fuse- pull-and-taper technique 1 can be used as wavelength multi- plexing and demultiplexing devices in fiber-optic links. Typically, in such applications, it is necessary to fabricate the directional coupler with either a 0 or 100% coupling ratio at preselected design wavelengths of operation. In this let- ter we describe briefly some physical factors that determine the coupling ratio of a fused biconical coupler and control the wavelength dependence of the device. A novel technique is also presented to vary the coupling ratio of a coupler postfa- brication and simultaneously translate in wavelength the characteristic sinusoidal response curve depicting the wave- length dependence of its coupling ratio. This technique may find application in multiplexing and demultiplexing by pro- viding a practical and simple means for adjusting the depen- dence on wavelength of the coupling ratio to compensate for any device variabihty introduced during fabrication. Recently, significant progress has been made in under- standing the operation of the fused-biconical directional coupler.2-5 The structure of a simple four-port fused cou- pler consists of two fiber bicones in coplanar contact fused together for a few millimeters along their length. The cross section at the center of the fused bicones is approximately dumbbell in outline. To describe the coupler's operation, it is useful conceptually to partition the coupler along its length into three distinct sections; an entrance down-taper- ing cone section of decreasing radial dimension; a central
Control of wavelength selectivity of power transfer in fused biconlcal monomode directional couplers D. C. Johnson and K. O. Hill
Department of Communications, Communications Research Centre, P.O. Box 11490, Station H, Ottawa, Ontario K2H, 8S2. Received 5 February 1986. 0003-6935/86/213800-04$02.00/0. Fiber-optic directional couplers fabricated using the fuse-
pull-and-taper technique1 can be used as wavelength multiplexing and demultiplexing devices in fiber-optic links. Typically, in such applications, it is necessary to fabricate the directional coupler with either a 0 or 100% coupling ratio at preselected design wavelengths of operation. In this letter we describe briefly some physical factors that determine the coupling ratio of a fused biconical coupler and control the wavelength dependence of the device. A novel technique is also presented to vary the coupling ratio of a coupler postfa-brication and simultaneously translate in wavelength the characteristic sinusoidal response curve depicting the wavelength dependence of its coupling ratio. This technique may find application in multiplexing and demultiplexing by providing a practical and simple means for adjusting the dependence on wavelength of the coupling ratio to compensate for any device variabihty introduced during fabrication.
Recently, significant progress has been made in understanding the operation of the fused-biconical directional coupler.2-5 The structure of a simple four-port fused coupler consists of two fiber bicones in coplanar contact fused together for a few millimeters along their length. The cross section at the center of the fused bicones is approximately dumbbell in outline. To describe the coupler's operation, it is useful conceptually to partition the coupler along its length into three distinct sections; an entrance down-tapering cone section of decreasing radial dimension; a central
Fig. 2. Wavelength dependence of the coupling ratio of a fused biconical coupler with an interaction length of 2.75 beat lengths at 632.8 nm.
Fig. 1. Wavelength dependence of the coupling ratio of a fused biconical coupler with an interaction length one-half of the beat
length at 632.8 nm.
section of approximately constant radial dimension; and an exit up-tapering cone section. It is now known2-4 that as light in the core of a monomode fiber enters the down-tapering cone, the light mode field becomes detached from the fiber core at some point along the taper (core-mode cutoff). The light is then guided by the boundary between the glass of the cladding and external medium encapsulating the coupler (which may be air or a potting material). The coupling of light takes place primarily in the central section, which is a composite waveguide consisting of a glass core having a dumbbell cross-sectional shape and a cladding provided by the external surrounding medium. Light entering this composite waveguide excites preferentially the symmetric and antisymmetric modes of the waveguide structure. It is the interference or beating phenomenon that takes place between these two modes, as they propagate with different characteristic phase velocities through the central section, that causes light power to transfer back and forth from one side of the composite waveguide to the other. The beating of the symmetric and antisymmetric modes in the central region of the coupler is also the principal physical mechanism underlying the observed characteristic sinusoidal dependence on wavelength of the coupling ratio. After passing through the central section, the light enters the up-taper cones where recapture of light in the fiber cores of the two output fiber arms occurs approximately in proportion to the relative strengths of the light fields at the two fiber cores. The above qualitative description of the coupler operation has been developed into a quantitative theoretical model
that successfully describes the principal operating characteristics of the fused coupler.6
Experimentally, the parameter most readily controlled during the coupler fabrication process is the physical length that the coupler is pulled. The usual procedure is to monitor at a particular wavelength (typically the He-Ne wavelength of 632.8 nm) the amount of light power transferred to the coupler fiber arm as the fused and heat-softened fiber bi-cones are lengthened. Clearly, as the fused coupler is elongated, the light power will cycle back and forth between the coupled and through-fiber-arm-side of the composite waveguide. It is advantageous to define a quantity called the beat length, which is the physical elongation in length that results in the light power transferring completely from the through to the coupled arm and back again to the through arm. A fabricated coupler can then be described as having been pulled through a specific number of beat lengths at the monitoring wavelength. Note that lengthening of the taper decreases the cross-sectional diameter of the composite waveguide thus decreasing the elongation length required for a subsequent cycle in the light power transfer. The fabrication process is stopped after the fused coupler is pulled through the desired number of beat lengths. The very nature of the process, however, makes it difficult to stop the taper lengthing at exactly the right point.
The period of the characteristic sinusoidal curve depicting the variation of the coupling ratio with wavelength depends strongly on the number of beat lengths the coupler has been pulled through.
Figure 1 shows the sinusoidal dependence on wavelength of the coupling ratio for a coupler in which fabrication was stopped at a length corresponding to one-half of the beat length [i.e., light completely transferred over to the coupled arm at the monitoring wavelength (632.8 nm)]. The measurements show that the one-half beat length condition actually occurs for a wavelength of 690 nm. The coupler was fabricated from two pieces of Corning telecommunications fiber (8-μm core diameter, 125-μm cladding diameter) whose cladding diameters had been etched to 36 μm before coupler fabrication. The coupling ratio changes from 100 to 0% for light separated in wavelength by ~210 nm. In Fig. 1, the coupling ratio is expressed as a percentage of the light power emanating out of the coupled arm to the sum of the powers out of the coupled and through arms. The measurements thus ignore loss; during fabrication the loss of the coupler is measured at the monitoring wavelength to be <10%.
A more rapid variation in coupling ratio can be attained by fabricating the coupler with a longer length. This effect is shown in Fig. 2 for a coupler in which the fabrication process
Fig. 3. Wavelength dependence of the coupling ratio for a fused coupler which is straight and then bent at the coupler waist.
Fig. 4. Wavelength dependence of the coupling ratio for a splitter that has mode conversion loss in the taper.
bending of the composite waveguide provides a simple technique for adjusting the location in wavelength of the 0 to 100% coupling ratio points.
In the discussion above, the wavelength dependence of the coupling ratio of the fused coupler is considered to result from beating of the symmetric and antisymmetric modes of the composite waveguide in the central region of the coupler. However, the tapers of the bicones can also make in certain conditions a contribution to the wavelength dependence of the coupling ratio. Recently, we reported a new wavelength-dependent loss mechanism in bicone tapers. 12 Since a bi-cone taper is one-half of a fused biconical coupler, this wavelength-dependent loss mechanism should also occur in fused couplers. In the case of a simple bicone, the transmission of light through the bicone is found to oscillate sinusoidally with the wavelength of the light. 12 The effect is attributed to the conversion in the taper of the HE11 local mode to the HE12 local mode.
Since the HE11 and HE12 local modes are coupled through the tapers, the amount of power in the HE11 local mode oscillates sinusoidally as the light propagates through those regions of the bicone where coupling is strong enough, that is, whenever the coupling affinity coefficient
K2/[K2 + (ΔB/2)2],
was stopped after 2.75 beat lengths. In this case the cladding of the Corning fiber was etched only to 65 μm before commencing coupler fabrication. The coupling ratio now changes from 100 to 0% for light separated in wavelength by only 70 nm. At long wavelengths in Fig. 2, the oscillation in coupling ratio with wavelength does not attain 0 or 100%. We attribute this effect to the fact that the coupling coefficient becomes slightly polarization dependent for a fused coupler pulled through a few beat lengths.7 Since the measurements were made using unpolarized white light and the coupling coefficient is polarization dependent, the points of 0 or 100% coupling occur at different wavelengths for the two light polarizations. Even much more rapid changes in coupling ratio with wavelength for very long (30-cm) couplers have been demonstrated.8
Using the techniques described above, wavelength multiplexers or demultiplexers can be made. The procedure is to determine empirically the correct fabrication conditions that yield a coupler with 0 and 100% coupling ratios at the desired wavelengths.9 A difficulty with this fabrication process is that the 0 and 100% coupling ratio points may be separated by the specified number of wavelengths but not be positioned exactly at the appropriate wavelengths. It would be advantageous to have a means for shifting the characteristic sinusoidal coupling ratio response curve of the coupler so that the peaks and valleys are located at the proper wavelengths. Note that a shift in the sinusoidal response curve of the coupling ratio with wavelength is concomitant with a change in the coupling ratio at any fixed wavelength. Thus techniques which vary the coupling ratio of a fused directional coupler at a fixed wavelength can also be applied for adjusting the wavelength dependence of the coupling ratio. Two techinques are available for varying the coupling ratio of fabricated fused biconical taper couplers, either the refractive index of the external media4 surrounding or encapsulating the coupler proper can be adjusted or the fused coupler can be bent or flexed at its waist.2,10
It has been shown that at a fixed wavelength the coupling ratio of a fused directional coupler can be varied from 0 to 90% by suitable selection of the refractive index of the material encapsulating the coupler.4 Thus, by adjusting the refractive index of the external medium, the sinusoidal curve of the coupling ratio vs wavelength can be finely tuned.11 A difficulty with this technique is that not all fused couplers are sensitive to the external refractive index. Fused directional couplers with a composite waveguide having a figure-of-eight cross section and small radial dimensions are the most sensitive to the refractive index of the external medium. For some fused couplers, the cross section of the composite waveguide is such that the light fields extend only weakly beyond the glass-external medium boundary, and, therefore, the refractive index of the external medium has no effect at all on the coupling ratio.
The other approach for varying the coupling ratio is by bending or flexing the fused coupler at the waist of the composite waveguide.2,10 Figure 3 shows the wavelength dependence of the coupling ratio for a fused coupler that has been pulled through a one-half beat length at 632.8 nm. The sinusoidal curve of coupling ratio vs wavelength is shifted by ~200 nm by bending. For simplicity, the insets in Fig, 3 depict bending of the fused coupler by holding one end of the coupler fixed and moving the other end in a transverse direction to the coupler's longitudinal axis. However, in practice, we found that a more effective procedure to bend the coupler is to move the free end in a longitudinal direction toward the fixed end. The coupler then buckles in the vicinity of the center of the composite waveguide into the S shape. Such
on a local mode basis is >0.1; ΔB is the difference in the propagation constants of the HE11 and HE12 local modes, and K denotes the strength of the coupling between the two modes.
The loss arises in the up-taper region of the bicone, since only the portion of the light which is in the HE11 local mode will be launched back into the HE11 mode of the monomode fiber. This loss is also wavelength-dependent since the fraction of power in the HE11 mode, in the up-taper recapture region, is wavelength dependent. Figure 4 illustrates the wavelength dependence of the coupling ratio of a splitter fabricated from Corning sensor fiber (4.5-μm core diameter, 75-μm cladding diameter). The wavelength-dependent mode conversion loss in the tapers is the short period sinusoidal oscillation which is superimposed on top of the long period sinusoidal oscillation due to beating of the symmetric and antisymmetric modes in the composite waveguide. This observation is the first reported of mode conversion loss in a fused directional coupler. The reason the effect has not been observed before is that the tapers in the bicones must be sufficiently steep to induce significant mode conversion. 13
Most low-loss fused couplers are made by etching the fibers prior to the fuse-pull-and-taper stage to reduce their starting cladding diameters. The resultant taper in the completed coupler is usually not as steep as in the splitter used here to obtain the results illustrated in Fig. 4, which was fabricated from unetched fiber. This conclusion is consistent with the earlier observation that the mode conversion loss is not present in bicones which have been fabricated from etched fiber. 12
References 1. B. S. Kawasaki, K. 0. Hill, and R. G. Lamont, "Biconical-Taper
Single-Mode Fiber Coupler," Opt. Lett. 6, 327 (1981). 2. K. O. Hill, D. C. Johnson, and R. G. Lamont, "Efficient Cou
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4. R. G. Lamont, D. C. Johnson, and K. O. Hill, "Power Transfer in Fused Biconical-Taper Single-Mode Fiber Couplers: Dependence on External Refractive Index," Appl. Opt. 24, 327 (1985).
5. K. 0. Hill, D, C. Johnson, and R. G. Lamont, "Wavelength Dependence in Fused Biconical Taper Splitters: Measurement and Control," in Technical Digest, Fifth International Conference on Integrated Optics and Optical Fiber Communications, Eleventh European Conference on Optical Communications, Venice (Instituto Internazionale Delle Comunicazioni, Genova, Italy, 1985), Vol. 1, p. 567.
6. F. P. Payne, C. D. Hussey, and M. S. Yataki, "Modelling Fused Single-Mode-Fibre Couplers," Electron. Lett. 21, 461 (1985).
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8. M. S. Yataki, M. P. Varnham, and D. N. Payne, "Fabrication and Properties of Very-Long Fused-Taper Couplers," in Technical Digest, Conference on Optical Fiber Communication (Optical Society of America, Washington, DC, 1985), paper WK3.
9. C. M. Lawson, P. M. Kopera, T. Y. Hsu, and V. P. Tekippe, "Inline Single-Mode Wavelength Division Multiplexer/Demultiplexer," Electron. Lett. 20, 963 (1984).
10. B. S. Kawasaki, M. Kawachi, K. 0. Hill, and D. C. Johnson, "A Single-Mode-Fiber Coupler with a Variable Coupling Ratio," lEEE/OSA J. Lightwave Technol. LT-1, 176 (1983).
11. H. A. Roberts, "Single Mode Fused Wavelength Division Multiplexer," Proc. Soc. Photo-Opt. Instrum. Eng. 574, 100 (1985).
12. D. T. Cassidy, D. C. Johnson, and K. 0. Hill, "Wavelength Dependent Transmission of Monomode Optical Fiber Tapers," Appl. Opt. 24, 945 (1985).
13. W. J. Stewart and J. D. Love, "Design Limitation on Tapers and Couplers in Single Mode Fibers," in Technical Digest, Fifth International Conference on Integrated Optics and Optical Fibre Communications, Eleventh European Conference on Optical Communications, Venice (Instituto Internazionale Delle Comunicazioni, Genova, Italy, 1985), Vol. 1, p. 559.
