Doping eliminates hydrogen loading

A new inherently photosensitive fiber slated for launch at the Optical Fiber Communication Conference (OFC ?97, Feb. 16?21, Dallas, TX) will allow fiber Bragg grating manufacturers to produce the structures without the time-consuming ste¥of hydrogen loading.

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Doping eliminates hydrogen loading

Kristin Lewotsky

A new inherently photosensitive fiber slated for launch at the Optical Fiber Communication Conference (OFC ?97, Feb. 16?21, Dallas, TX) will allow fiber Bragg grating manufacturers to produce the structures without the time-consuming ste¥of hydrogen loading.

Fiber Bragg gratings are generated when an ultraviolet (UV) laser creates a standing wave pattern in photosensitive fiber. The UV exposure triggers a permanent, periodic change in the refractive index of the fiber material that replicates the standing wave pattern.

Most fiber Bragg gratings are made from standard telecommunications fiber, which contains germanium at dopant levels of a few percent. This impurity renders the fiber weakly responsive to ultraviolet exposure, but the degree of photosensitivity is generally insufficient for the fiber Bragg grating fabrication process. To successfully write Bragg gratings, manufacturers increase photosensitivity by implanting hydrogen in the fiber.

In hydrogen loading, small amounts fiber are placed in a high-pressure (1400 to 2500 psi), high-temperature (80!C to 100!C) hydrogen environment for as long as six days. A grating structure can then be written in the fiber, but to ensure permanence, the finished grating must be annealed. This process, however, tends to shift the gratings spectrally. Because hydrogen tends to diffuse out of hydrogen-loaded fiber over time, the technique is inherently a small-quantity batch process. The fiber must be prepared in lengths of a few meters, and gratings written soon after.

Intrinsic photosensitivity

A photosensitive fiber developed by SpecTran Specialty Optics (Avon, CT) should allow manufacturers to avoid such pitfalls. Called PhotoSil, and now in the final stages of development and testing, the product is a single-mode, codoped fiber compatible with standard telecommunications fiber. It requires no hydrogen loading and remains stably photosensitive over time.

OPhotoSil will enable customers to more accurately fabricate the Bragg grating and more precisely hit target center wavelength,O says Douglas Norton, senior applications engineer at SpecTran. OThey don?t have to be concerned with drift that takes place with hydrogen-loaded fibers.O

As of this writing, SpecTran engineers were still optimizing the final specifications, investigating the relative benefits of doped cladding designs, depressed cladding designs, and variations in co-dopants. The program has produced eight different fiber designs, the one constant being a germanium concentration increased several times over that of conventional fiber.

Fiber designs now in beta testing with customers have produced gratings with reflectivities as high as 99%. The material response to UV illumination is rapid; gratings with the above reflectivity have been written in as little as two minutes. Norton predicts that the response of the final design will be even faster. The fiber also allows the production of gratings with cladding modes more highly suppressed than those in conventional fiber gratings. In tests, engineers have recorded high-reflectivity-grating cladding modes below 1 dB (see figure).

OThe key thing to note with this fiber,O says Norton, Ois that it achieves this sort of performance and remains compatible with single-mode fiber.O Plans call for a family of PhotoSil fibers, tailored to the various telecommunications wavelengths.

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Fiber Bragg gratings written in intrinsically photosensitive fiber can offer better cladding-mode suppression than gratings written in conventional hydrogen-loaded silica fiber. Tests have revealed cladding modes below 1 dB.

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