Formation of gratings in polymer-coated optical fibers

Recognizing the rate-determining nature of the coating removal and recoating steps, applicants have demonstrated that with proper combination of low absorbing polymer, glass and low intensity radiation, UV-induced gratings can be side-written into polymer coated fibers without removing the polymer, thus permitting up the possibility of high speed fabrication of fiber gratings.

FIELD OF THE INVENTION 
This invention relates to methods for forming gratings, such as Bragg 
gratings, in optical fibers and, in particular, to a method for forming 
photo-induced gratings in polymer-coated optical fibers without removing 
the polymer. 
BACKGROUND OF THE INVENTION 
The dominant method for writing photo-induced gratings in optical fibers is 
side-writing with ultraviolet (UV) light through the fiber cladding. An 
optical fiber having a photosensitive glass core and a surrounding 
cladding is exposed to ultraviolet light having an intensity which varies 
periodically along a length of the fiber. The periodically varying 
intensity pattern is typically provided by applying a UV beam to an 
optical phase grating as described in Anderson et al U.S. Pat. No. 
5,327,515 issued Jul. 5, 1994 which is incorporated herein by reference. 
Alternatively, the intensity pattern can be provided by an amplitude mask 
or by interfering a pair of coherent UV beams as described in W. H. Glenn 
et al U.S. Pat. No. 4,725,110 issued Feb. 16, 1988, incorporated herein by 
reference. In each of these conventional techniques, the source of UV 
light is typically a high intensity Excimer laser. 
Surprisingly, the rate-determining step in conventional fiber grating 
manufacture is not writing the grating but rather removing and 
subsequently reapplying the protective polymer coating that the fiber was 
provided at manufacture. These coatings are needed to protect the 
sensitive fiber from contamination and mechanical damage, but typical 
coatings significantly absorb UV radiation and interfere with grating 
formation. Moreover the coating would be damaged by UV laser beams. Thus, 
an initial step in conventional grating formation is striping the polymer 
coating, as by soaking the fiber in hot sulfuric acid. A new coating must 
be applied and cured after the grating is formed. The coating removal and 
reapplication steps consume more than half the time required to write a 
grating in the conventional process. 
SUMMARY OF THE INVENTION 
Recognizing the rate-determining nature of the coating removal and 
recoating steps, applicants have demonstrated that with proper combination 
of low absorbing polymer, glass and low intensity radiation, UV-induced 
gratings can be side-written into polymer coated fibers without removing 
the polymer, thus permitting high speed fabrication of fiber gratings.

DETAILED DESCRIPTION 
Referring to the drawings, FIG. 1 is a flow diagram depicting the steps in 
forming a grating in polymer-coated fiber. The grating can be a Bragg 
grating or a long period grating. As shown in block A of FIG. 1, the first 
step is to provide an optical fiber waveguide having a polymer coating 
with low ultraviolet absorbtion polymer. The optical fiber, as is well 
known, comprises an inner core of relatively high refractive index and an 
outer cladding. The inner core is made of UV photosensitive glass, such as 
gcrmanosilicate, so that a grating can be written by UV radiation. The 
outer polymer coating should be of low UV absorbing polymer such as an 
aliphatic poly(meth)acrylate, a silsesquioxane, a vinyl ether, or an alkyl 
substituted silicone. 
Advantageously, the fiber is sensitized to UV radiation as by treating the 
fiber with deuterium D.sub.2. This preferably involves placing the fiber 
in a D.sub.2 gas environment, advantageously at an elevated pressure and 
temperature, so that D.sub.2 will diffuse through the polymer, the 
cladding and into core. Typical treatment conditions are 3500 psi, 
50.degree.-70.degree. C. for 3 days. The treatment enhances the 
sensitivity of the UV photosensitive core so that the grating can be 
written at lower intensity. 
FIG. 2, which shows typical apparatus for practicing the method, includes a 
typical fiber 20 comprising a core 21, a cladding 22 surrounding the core, 
and an outer polymer coating 23. 
Block B shows that the next step is to expose the fiber by side writing 
through the polymer and cladding, a pattern of UV radiation corresponding 
to the desired grating. Successive radiation intensity peaks are spaced 
apart by the desired grating spacing. The grating pattern can be defined 
by a mask along the fiber such as an amplitude mask or a phase mask 
schematically illustrated in FIG. 2. UV light from laser 24 passes through 
mask 25, the polymer coating 23, and the cladding 22 to write a pattern of 
index changes along the photosensitive core 21. Alternatively, the pattern 
can be defined by two interfering beams of UV radiation. The UV radiation 
should be at a sufficiently low intensity that it does not seriously 
damage the polymer coating. 
Methods for forming gratings in polymer-coated fiber can be better 
understood by consideration of the following specific examples. 
EXAMPLE 1 
A methylsilsesquioxane-coated fiber treated with D.sub.2 was exposed to UV 
light from a KrF excimer laser. An amplitude mask was used to produce long 
period gratings. The fiber was held taut next to the amplitude mask, and 
its side adjacent to the mask was exposed to the UV radiation. At 130 
mJ/cm.sup.2 1 dB loss developed at the selected wavelength after exposure 
for approximately 5 minutes. Examination of the fiber surface showed some 
physical damage to the polymer with periodicity comparable to the 
amplitude mask, but the coating remained intact and the damage appeared 
superficial. Decreasing the power to 100 mJ/cm.sup.2 resulted in 0.5 dB 
loss, and minor damage to the surface of the polymer. 
EXAMPLE 2 
In this example, using a similar methylsilsesquioxane coated fiber, UV 
laser pulses at 242 nm were obtained from a frequency-doubled dye laser 
(which was pumped by a KrF excimer laser). The radiation was defocused to 
decrease the fluence. In the first attempt, the focal point was moved 2 
inches behind the fiber. After .about.3 minutes of exposure at 20 mw there 
was no evidence of a grating impressed in the fiber core. In the second 
attempt, the focal point was moved to within 1 inch of the fiber. A weak 
reflector (.about.3%) was observed. In subsequent attempts a 10% reflector 
was grown in less than 1.5 minutes and an .about.70% reflector was grown 
after 6 minutes of exposure. In none of these cases was damage to the 
polymer coating detected. 
It is to be understood that the above-described embodiments are 
illustrative of only a few of the many possible specific embodiments of 
the invention. Numerous and varied other arrangements can be made by those 
skilled in the art without departing from the spirit and scope of the 
invention.