Fiber Optic Coatings Present Challenges
(by Michael Joseph - Coating Manager, OptoSigma Corporation)

Optical coatings are widely used in telecommunications, illumination, image transmission, and sensing. In some cases, applying thin-film antireflection coatings to the tips and sometimes sides of a fiber is necessary to improve transmission of a light through the fiber, as well as to reduce Fresnel back-reflections. For example, many applications require that the light from a semiconductor laser be coupled into an optical fiber; however, if a significant amount of light reflects from the tip back into the laser, laser instability can result. Antireflection coatings on the fiber tips can eliminate this problem.

In a successful fiber-coating system, the coating chamber is set up with certain special features. The most important requirement is a broadband, high current, low voltage ion gun or a suitable plasma system to densify the coating. An ion-assist process produces dense films on cold substrates such as connectorized fibers or cemented achromatic lenses. In most cases, coating manufacturers are able to deposit coatings onto substrates heated 300oC. Because many connectorized fibers and fiber bundles require epoxy for mounting purposes, they cannot be exposed to temperatures higher than 90oC and, in certain cases, even lower temperatures.

In addition to densifying the coating, an ion gun also will clean the surface prior to coating, thus improving adhesion of the coating to the fiber tips. The ion gun used must be capable of handling oxygen gases, because oxide coating is the most-robust coating that can be applied to a fiber. A 50:50 mixture of argon and oxygen in the ion gun has been found to be a good combination (other ratios are used as well). As a result of the ion-cleaning capability in such a system, in most cases there is no need to do any manual cleaning of the fiber tips before loading into the chamber.

Coating design and tooling

To successfully produce a low-reflectance coating, the designer must consider the properties of coating materials used in the design. Magnesium fluoride is the most commonly used low-index coating material. Even though magnesium fluoride will adhere to the fiber tips as well as to other layers, an oxide layer such as silicon dioxide will produce a more-robust coating. This difference in adhesion is a result of the stress ensuing from the deposition of a "cold" magnesium fluoride layer; even with ion assist, the stress may be detrimental to the long-term durability of the coating. 

Another area critical to the successful production of fiber coatings is the tooling used to hold fibers in the coating chamber. Because most fibers will have to be coiled and packed into the tool, it is important that the fiber remain unstressed. This requirement might necessitate tooling in a standard coating chamber that reduces the distance from the source to the substrate. Another approach is to have tooling in the form of a drum such as one proposed by Deposition Sciences (Santa Rosa, CA).1 Even in such cases the number of fibers that can be coated is limited by the amount of epoxy in the fiber bundles. Even cured epoxy will outgas in a vacuum chamber, with most of the gas being water vapor absorbed from ambient air. Such outgassing of the epoxy impacts the adhesion of the coating to the fiber, the packing density of the coating, and the refractive index of the deposited films.

It is important that the coating materials be characterized on the coating tool using all the relevant coating-process parameters. These parameters include the temperature of the substrate (cold in this case), the evaporation rate, and pressures in the chamber, including the pressure contributed by the ion-gun gas flow. Experience has shown that the refractive indices of a "cold" coated film are usually less than that of a hot film, even with the use of ion assist (see table). In many instances, they are also inhomogeneous. With care, however, both narrow and broadband antireflection coatings can be consistently produced on fibers (see figure).

 

Hot (Ta2O5M21) vs. cold (Ta2O5CF2) substrates

Wavelength (nm)

Ta2O5M21
Refractive index

Ta2O5CF2
Wavelength (nm)

Refractive index

415.50

2.2587

403.5

2.2139

451.51

2.2313

441.3

2.1795

494.42

2.1991

485.4

2.1306

552.88

2.1859

551.5

2.1181

625.05

2.1623

633.4

2.0852

724.25

2.1475

756.2

2.0745

856.19

2.1156

935.6

2.0533

1076.84

2.1287

1234.8

2.0325

 

Chamber pumping

Some ion guns will become unstable if the pressure in the chamber is too high during operation. In addition, the higher pressure also can lower the refractive index by making the film more porous and inhomogeneous. If the film is porous, it will absorb moisture in the air and the coating performance will change along with making the film weak. If an existing chamber is to be used, it is important that the true pumping speed be evaluated by flowing various gases and observing the pressure over an extended period of time. 

One way to improve the pumping speed is to use some form of Meissner trap. A Meissner trap will freeze the water in the chamber, improving the quality of the film. A cold chamber can be pumped down to a pressure of 2.0x10-6 Torr in less than 40 minutes with a Meissner trap; even after several coating runs in the chamber. The pressure reached in the same time without a Meissner trap was 1.5x10-5 Torr.

Temperature control of the fiber during coating is another parameter that must be monitored. If the fibers have epoxy or plastic jackets, as in most cases, they cannot be exposed to high temperature. Usually, the temperature controller of the coating system is programmed to a temperature between 35o and 55oC; however, the heat from the electron-beam gun and ion gun will raise the temperature of the fiber during coating. If the temperature of the fiber exceeds a safe limit during the coating, the ion and electron guns must be shut off to permit the fiber to cool down.

Preparation of the fiber bundle

When a fiber bundle is assembled, any epoxy that is not fully cured can create havoc in the coating chamber. Under evacuation, such epoxy can ooze out of the assembly. In addition, any trace of any fluid left over from the assembly process can cause the coating-chamber equipment to malfunction. Therefore, the finished fiber bundles must be baked at a low temperature (less than the softening temperature of the epoxy) in air or vacuum.

Depositing coating onto fibers has been done successfully by several companies that have addressed challenges unique to the process. Once a process has been developed that contains the appropriate steps, such coatings can be produced routinely and repeatably.

Reference
1. D.Z. Rogers, "Practical Fiber Coating Technology," Fiber Optic Product News (Dec. 1999).

 

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