Manufacturing Technology for Low Cost Couplers

AbstractThe widespread use of fiber optic couplers has been significantly inhibited by theirhigh cost. This paper presents a review of three generic types of fiber optic couplers,concentrating on their manufacturability. It is this ease of manufacture that ultimatelydetermines the coupler cost and availability in large quantity. Fused biconical tapercouplers prove to be quite versatile with excellent optical characteristics but less adap-table to mass production. A detector /tap coupler is described that has much improved man -ufacturability and includes a detector in the coupler itself, thereby saving connectorcosts. Finally, an active bidirectional coupler is discussed that incorporates a sourceand detector in the coupler body, thereby eliminating the need for all external device con-nections. This compact and rugged device is easily manufactured in large quantity while atthe same time having excellent optical characteristics.I. IntroductionIn the past several years, a variety of basic coupler techniques have been developedas commercial products and marketed. These couplers are used in a number of applications,including full or half duplex transmission on a single fiber optic line, multiwavelengthtransmission on a fiber, parallel or star data distribution, serial bus taps, line moni-tors, etc. One of the primary factors inhibiting widespread use of these couplers up untilthe present has been their very high cost and the lack of high volume manufacturing capa-bility.In this paper we summarize the results of an extensive development effort aimed pri-marily at producing fiber optic couplers that are easily manufactured in medium to largequantity and therefore are low in cost. Two of these coupler types include active compo-nents, either a detector or a source and detector, in the coupler itself, thereby alsosav-ing the user additional connector or splice costs. We begin the discussion with the morestandard fused biconical taper coupler that does not include active devices.II. Fused Coupler TechnologyPrinciple of operationDevices based on the fused biconical taper process1'2 are by far the most commonly a-vailable couplers at this time. The basic fabrication technique requires the fusion of twoor more fibers using a flame or other heat source while at the same time pulling on the fi-bers so as to form a biconical taper region. The physical basis for the coupling is asfollows: light is incident on the descending taper initially, and as the taper is tra-versed gradually, almost all of the light is coupled out of the higher index core region.However, due to the large index difference between the cladding and the surrounding air,all of this light is still trapped in the composite fused glass region. With the properminimum fiber diameter at the center of the taper region, it is generally possible to coup-le most of the light out of the original core, while still not losing light to radiationfrom the cladding. Upon entering the up taper region this light is now recaptured by thehigher index fiber core regions and divided equally among the cores of the output fibers.Thus, the maximum coupling that can be achieved with this simple fusion technique approach-