Douglas R. Moore

Production, performance, and reliability of fused couplers

It has been twenty years since the first fused fiber optic coupler was made and remarkable progress has been achieved during this period on the production, performance, and reliability of these devices. Several million fused couplers have now been produced and placed into service. Applications have included test instrumentation, both outside plant and central office telephony applications, cable TV systems, sensors, optical amplifiers, DWDM systems, and submarine repeaters, to mention a few. To meet the increased production requirements, significant work has been done to make the process more flexible and thereby more amenable to automation and computerization. Modern fused coupler operations are highly automated and can make a large variety of products by simply changing a computer file. Similar improvements have also been made in high volume testing so that sophisticated tests of a number of parameters can be performed in a matter of minutes. As the volume and variety of the products have increased over the years, so has the performance. Improvements have been made in reducing excess loss, polarization effects, and wavelength dependence. In addition, higher port count devices (1 X 3, 1 X 4, etc.) have been developed. Numerous studies have been conducted over the years to determine the reliability of these devices. The introduction of glass-to-glass sealing techniques to isolate the fused region by bonding the fibers directly to the quartz substrate significantly improved the stability and long term reliability of these devices. The state-of-the-art production, performance, and reliability of fused coupler technology will be reviewed in this paper.

Optimization of tap couplers made by the FBT process

Tap couplers with small coupling ratios (1-10%) play a critical role in monitoring optical fiber systems such as optical amplifiers. They are often used in a feedback control loop and hence must exhibit extreme stability. Very small changes in the operating characteristics of the taps can be interpreted as changes in laser power giving rise to instability in the amplifier gain. Tap couplers made by the fused biconical taper (FBT) process are inherently stable with regard to temperature and, thus, this is usually not a concern. Of much greater significance are the wavelength dependence and polarizastion sensitivity of the tap ratio. Wavelength dependence can be minimized by introducing a mismatch in the propagation constants between the two fibers, but it is also influenced to a lesser extent by the coalescence of the fibers. On the other hand, polarization sensitivity is primarily dependent on coalescence and only somewhat dependent on propagation constant mismatch. As a consequence, it is necessary, in practice, to trade off wavelength dependence against polarization sensitivity in order to optimize the overall stability of the tap coupler. We report here the results of such an optimization study, consistently yielding taps with less than 0.5 dB total change in the insertion loss of the tap leg due to all effects.

Wavelength-Dependent Coupling in Single-Mode Fused Biconical Taper Couplers

Many fiber optic systems in operation today require low loss single-mode couplers to passively split or combine light between two or more single-mode fibers. At present, this need is best met by standard fused biconical taper couplers because of their ease of fabrication and very stable performance over a wide range of operating conditions. However, as single-mode fiber technology transitions from todays predominately long haul applications to tomorrows local area networks (LANs) and subscriber loops, more sophisticated single-mode couplers will be required. For example, many of these systems will likely incorporate wavelength division multiplexing techniques and, as such, will require several types of wavelength dependent couplers. In anticipation of this growing need, three different types of single-mode, wavelength-dependent couplers have been developed at Gould. A discussion of the wavelength dependent coupling in these fused couplers is presented in this paper.

Recent advances in fused fiber optic coupler technology

Refinement and new product development of splitters made by the fused biconical taper process are discussed relative to the emerging technologies of optical fiber amplifiers and practical fiber to the curb telecommunication systems.

Optical performance of recladded couplers

In the fused biconical taper process (FBT) the mode diameter of the optical fields within the fibers are spread to the extent that the surrounding medium acts as the fiber cladding. Usually this medium is air whose optical properties are benign resulting in stable devices. However, if the coupler is recladded with a material other than air, the optical fields spread further into the cladding region and the behavior of the coupler becomes substantially more sensitive to variations in the refractive index of the cladding material. In this paper we report the results of a study of recladded FBT couplers. Experimental results will be presented and compared to the predictions of a model previously used to describe the coupling characteristics of FBT couplers.

1xN broadband fused biconical taper fiber couplers

The fused biconical taper process has been the technology of choice for fabricating a variety of passive fiber optic couplers. These couplers exhibit excellent optical and environmental characteristics. Most recently, this process has been used to fabricate tree and star couplers of various configurations: 1x3, 3x3, 1x4, 4x4 and 1x7 for both local loop and sensing applications. In this paper we report the development of broadband 1x3 and 1x4 couplers for applications requiring simultaneous transmission in both the 1300 and 1550 nm wavelength regions. The wavelength response, insertion loss and optical uniformity of these broadband couplers will be discussed. Their environmental performance will also be presented.

Single-mode MxN star couplers fabricated using fused biconical taper techniques

Over the last few years the fused biconical taper (FBT) process has been the technology of choice for fabricating passive fiber optic couplers. The FBT couplers can be made with a wide variety of fiber types and exhibit excellent optical and envirornnental characteristics. Recently much attention has been given to the fabrication of compact MxN singlemode star couplers for future local subscriber loop applications. Some of the systems being considered utilize passive star architectures which require singlemnode star couplers to split the light to a multiple number of ports or residences. In this paper we report the fabrication of true lxN and MxN singlemnode tree and star couplers using the fused biconical taper techniques. In addition to good optical uniformity and insertion loss the fused couplers offer the same environmental stability and long term reliability that are characteristic of standard FBT couplers. A discussion of the optical and environmental data will be presented.

High Isolation Wavelength Division Multiplexers/Demultiplexers

In this paper we report the development of a high isolation WDM component for use in a variety of applications incorporating wavelength division multiplexing and demultiplexing techniques. Wavelength isolation in excess of 30dB is demonstrated over bandwidths of 40nm in both the 1300 and 1550nm wavelength regions. The basic operation and performance of the high isolation WDM component will be discussed.