Fiber optic data distribution systems utilizing variable tap ratio optical couplers

Brian E. Kincaid Lockheed Palo Alto Research Laboratory, Palo Alto, Cali­fornia 94304. Received 13 April 1977.

Fiber optic data bus systems have generated considerable recent interest and have been discussed by a number of au­thors.1-6 Two major data bus configurations have been de­veloped; the T system,4 which is a serial configuration with access couplers connected along a single bus line; and the star system,3 which has only one coupler and parallel lines going to each terminal. The star system has been shown to be the best approach for a system with a large number of termi­nals1,3,6; however, the T system requires less total cable, with its attendant penalties of cost, weight, and volume, and therefore offers the best approach for certain small sys­tems.1,5

One approach to the T data bus system involves a fixed tap ratio (ratio of power tapped off to the terminal detector to power incident into the coupler) for the T couplers, inde-

Fig. 1. Variable tap ratio T coupler. The component elements are: (A) fiber-bundle bus line cable; (B) connectors; (C) scrambler rod; (D) prism 1; (E) prism 2; (F) coupling apertures; (G) reflective coatings; (H) terminal detectors; (I) terminal source; and (J) source-coupling fiber bundle. For clarity, the detectors arid prism 2 are

shown in a displaced position from prism 1.

pendent of the number of terminals in the system.1,3 How­ever, the optimum performance of a T system is realized when the coupler tap ratios are set according to the number of ter­minals in the system.2 The practical disadvantage of this optimum tap ratio approach is that different tap ratio couplers have to be constructed for systems with different numbers of terminals. This letter describes a fiber-bundle T coupler having a continuously variable tap ratio which is adjustable over a wide range.

The variable tap ratio T coupler is shown in Fig. 1. The bus line cables are interfaced with glass scrambler rods which have a high index core (equal in diameter to the bus lines) with a low index cladding. The scrambler rods interface with a right angle prism (prism 1). The hypotenuse is coated with alu­minum and has a clear circular coupling aperture etched in the aluminum coating. Prism 2 also has a coated hypotenuse with the same size coupling aperture. Light entering from either scrambler rod is reflected from the coating of prism 1 to couple into the other scrambler rod, except for the portion incident upon the coupling aperture, which travels through prism 2 to one of the two terminal detectors. Translation of prism 2 reduces the effective coupling area and thereby in­creases the tap ratio Ly. The minimum value of L T is achieved when the two coupling apertures are aligned with one another and is set by the ratio of the coupling area to the area of the prism hypotenuse. All optical interfaces are matched to eliminate reflection losses. The terminal source is coupled to a small fiber bundle, which is bifurcated and fits into slots in the top of prism 1 such that the source light is efficiently

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coupled to the scrambler rod cores. The scrambler rod length is chosen so the terminal source input uniformly illuminates the bus line cables. The coupler is designed so the scrambler rods fit into fiber optic bulkhead connectors in the terminal unit. Therefore, only one bus line packing fraction loss is encountered for a through signal from bus line to bus line.

Laboratory versions of this coupler were built and tested. The relevant parameters are as follows: bus line fiber-bundle cable diameter of 2.54 mm, numerical aperture (NA) of 0.56, and attenuation of 0.6 dB/m; scrambler rod core diameter of 2.54 mm; scrambler rod length of 22.5 mm; scrambler rod NA of 0.56; prism entrance/exit face size of 3.2 mm X 3.2 mm; coupling aperture of 1.1 mm diam. Loss measurements were taken using full NA launching conditions for the bus line cable and the source coupling fiber bundle. These measurements were made with standard techniques and were normalized with respect to the power input into the coupler from the bus line or source coupling cable, as applicable. The results of the loss measurements are as follows: LsI = 0.5 dB; LIF = 1.5 dB; and LTF = 0.5 dB. LsI is the coupler insertion loss from the optical input port to the bus line port; LIF is the fixed throughput insertion loss of the coupler (independent of tap ratio); and LTF is the fixed tap off loss (independent of tap ratio). The fixed throughput insertion loss LIF does not in­clude the bus line cable packing fraction loss because the packing fraction loss is, in the strict sense, not a coupler loss but is dependent on the bus line cable employed. The mea­sured tap ratio L T was continuously variable from 11.2 dB to over 30 dB. The minimum value of the tap ratio is approxi­mately equal to the ratio of the coupling aperture area to that of the hypotenuse area of the prism. The minimum tap ratio calculated from the area ratios is 11.8 dB; the difference be­tween the measured and calculated values is attributable to greater light intensity at the center of the prism hypotenuse face. The coupler also exhibited a high degree of isolation between its optical input and output ports of over 35 dB. Exclusive of packing fraction, the connector loss LC is low for a fiber-bundle, scrambler rod connection with index match­ing. The only loss mechanisms are size mismatch, lateral and angular misalignment, and imperfect index matching. The experimentally measured value for Lc was 0.05 dB.

It is anticipated that these loss results can be improved with prisms that are more closely matched in size to the bus line cable diameter. Also, the fiber-bundle cables used have a high NA; and, therefore, the coupler should exhibit better performance with lower NA cable. One possible approach to lower the coupler insertion loss is the use of graded index Selfoc lenses between the scrambler rods and prism 1 so the light entering the prism has a lower divergence angle. An­other possible improvement involves the use of square cross-section scrambler rods (along with square format fiber-bundle bus line cable ends), which would more effi­ciently interface with the square entrance/exit faces of prism 1.

The availability of a variable tap ratio coupler allows the optimization of a T system using only one standard coupler unit. For the case of a bidirectional, N terminal T system, it has been shown3 that the worst case system loss is between the optical input port of terminal 1 and the output port of terminal N — 1. With all tap ratios equal, the worst case loss L in dB may be written as

where Lsi= insertion loss from input port to bus line port; LQ- connector loss;

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LPF= packing fraction loss of signal incident on bus line cable;

Lip= fixed throughput insertion loss of coupler (in­dependent of tap ratio);

LIT= insertion loss due to power tapped to coupler output port;

= -10 log (1 - 10-LT/10); LT- tap ratio of coupler; and

LTF- fixed tap off loss (independent of tap ratio). Differentiation of Eq. (1) with respect to LT and setting the result equal to zero yields the optimum tap ratio as a function of N. Other possible system configurations may be optimized using LT as a parameter. For example, the variable tap ratio coupler may be used in a tapered system, in which the tap ratio is different for each terminal. The greatest advantage of ta­pered tap ratios is realized in a unidirectional system, in which case the tap ratios increase down the bus line and are set such that a minimum allowable signal is tapped off at each termi­nal.

To realize the full advantage of the optimum T system approach using variable couplers, it is critical to minimize the fixed throughput losses (2Lc + LIF + LPF) of the coupler. Several improvements have been suggested which would lower the throughput losses, as reported here. In summary, a variable tap ratio T coupler has been described which allows optimum system performance using only one standard cou­pler.

References 1. M. K. Barnoski, Appl. Opt. 14, 2571 (1975). 2. A. F. Milton and A. B. Lee, Appl. Opt. 15, 244 (1976). 3. M. C. Hudson and F. L. Thiel, Appl. Opt. 13, 2540 (1974). 4. A. F. Milton and L. W. Brown, IEEE J. Quantum Electron. QE-9,

642 (1973). 5. H. F. Taylor, W. M. Caton, and A. L. Lewis, NELC Report TR1930

(August 1974). 6. J. R. Baird and J. E. Shaunfield, AFAL Report TR-74-314 (De­

cember 1975).