Low-loss access coupler for multimode optical fiber distribution networks B. S. Kawasaki and K. 0. Hill

Department of Communications, Communications Re­search Centre, Ottawa, Ontario K2H 8S2. Received 25 March 1977.

The two main network topology systems for the distribu­tion of optical communication signals using single-strand multimode fiber are the tree distribution system and the star system. For networks with many terminals the tree distri­bution scheme provides advantages of flexibility in the number and location of the distribution paths or drops and minimizes the amount of fiber used in comparison with a star system. However, a tree network can suffer from an inef­fective utilization of the total optical power launched in the trunk feeder if there are many lossy access junctions along the trunk which are encounter in series. The tree scheme can be effectively utilized only if the excess loss above furcation loss at each access junction is made sufficiently small.

Recently, two relatively simple methods for producing low-loss access couplers for multimode fibers have been demonstrated. Ozeki and Kawasaki1 used twin biconical tapered sections of multimode optical fiber joined by an op­tical epoxy to produce a directional coupler, and Barnoski and Friedrich2 fused two sections of multimode fiber side-by-side to form a low-loss junction. In both these structures, the excess loss was of the order of 1 dB. In this Letter, we dem­onstrate how a structure which is essentially a combination of these two techniques overcomes the inherent limitations of each and results in a coupler with excess loss that is an order of magnitude lower than has been reported thus far.

Figure 1 is a schematic of the coupler. Two biconical tapers of multimode fiber are formed and fused together along one side in a single fabrication process step. Initially, two sections of fiber are twisted or wound around one another and put under spring tension in a clamping jig. An oxy-butane mi-crotorch flame is then used to soften and fuse the two fibers. At the same time, the spring elongates the fibers in the soft­ened region to form the twin biconical tapers. The twist causes the two tapered sections to stay together during the pulling process. The total length of the resulting biconical section in the experiments reported here is approximately 1 cm. For all the couplers described in this study, we used as the starting material a Coming silica step-index fiber having an 85-μm core diameter, a 20-μm cladding thickness, and a numerical aperture of 0.175. The input and output fiber arms were left approximately ½-m in length to facilitate evaluation of the coupler.

To evaluate the couplers, we illuminated the input arm (port 1) with a He-Ne beam coupled into the fiber with a X50 microscope objective. Oil-bath cladding mode strippers were mounted on ports 1, 3, and 4; and the power coupled from

1794 APPLIED OPTICS / Vol. 16, No. 7 / July 1977

Fig. 1. Schematic of coupler.

Table I. Power Measurements and Performance of Couplers

Fig. 2. Far-field mode patterns from ports 3 (left) and 4 (right) of access coupler.

ports 3 and 4 was measured. The input power to the coupler was then measured by breaking the input arm at a point lo­cated downstream from the mode stripper. Table I shows the measured power levels and the calculated excess insertion loss for several couplers with various values of the coupling ratio P4/P1. The values of excess insertion loss are very low with the better couplers having values between 0.1 dB and 0.2 dB. We stress that these are typical results; virtually all the cou­plers we made showed similarly efficient coupler action.

The efficient coupling action can be understood by con­sidering qualitatively how the coupler works and how it overcomes the limitations of the structures described in Refs. (1) and (2). As the light in port 1 enters the narrowing tapered section, the higher order modes are forced to radiate out of the core area to be guided as cladding modes. The light can cross the fused boundary between the two biconical sections and is, therefore, guided in the over-all structure. As the light propagates beyond to the region of increasing tapers associ­ated with ports 3 and 4, the cladding modes propagate at gradually decreasing angles to the fiber axis and are recap­tured by the tapered core section to again become core modes. The coupling action is much less lossy in this structure than

in the coupler of Ref. 1 because of the high optical quality of the air-cladding interface. In the previous coupler, this boundary was formed by thermal epoxy, which produced boundary interfaces that were not so smooth as the fire-pol­ished surfaces formed by the fabrication technique used in making the present devices. Indeed, in the present coupler, it is difficult to discern the coupling region from a cursory examination of the scattered light alone. The coupling action is more efficient than in the device of Ref. 2 because of the expanding taper section. In this region of the device, light which is propagating in the cladding region can be recaptured by the core because the effect of an increasing taper is to re­duce the propagation angle of the light.3 In the previous structure of Ref. 2, there was no taper and thus no efficient mechanism to enable the core region to recapture light, which was initially radiated into the cladding region.

The present coupler shows one other significant difference from the device of Ref. 1. Considerable mode mixing occurs in the present device. The previous coupler showed that only the high order modes took part in the coupling, and these modes remained clearly defined in the output arms of the coupler. Figure 2 shows the far-field mode pattern from ports 3 and 4 of coupler c in Table I. We can see that again the low-order modes are predominantly in port 3 rather than port 4 as expected, but port 4 shows fairly uniform filling of the modes. We suspect that this effect is caused by mode mixing due to the twist asymmetry in the device. This result was unexpected but fortuitous when the use of these couplers in series as a part of a tree distribution network is considered.

In summary, we have shown that access couplers for mul-timode fibers can be readily made in the form of fused bi-conical taper sections. The devices were made using a very typical step-index optical fiber that can be readily mated into an optical distribution system. Excess insertion loss of 0.1-0.2 dB has been demonstrated to be easily attainable, and reasons have been given for this significant improvement in perfor­mance.

References 1. T. Ozeki and B. S. Kawasaki, Appl. Phys. Lett. 28, 528 (1976). 2. M. K. Bamoski and H. R. Friedrich, Appl. Opt. 15, 2629 (1976). 3.. T. Ozeki and B. S. Kawasaki, Electron. Lett. 12, 407 (1976).

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