Method for impressing gratings within fiber optics

A method of establishing a dielectric periodic index of refraction phase grating upon the core of an optical waveguide by intense angled application of several transverse beams of ultraviolet light, enabling the establishment of a distributed, spatially resolving optical fiber strain gauge.

BEST MODE FOR CARRYING OUT THE INVENTION 
FIG. 1 shows a schematic diagram of the spatially resolving optical fiber 
strain gauge 13. The gauge 13 includes an optical waveguide 15 or fiber 
operative to transmit a single or lowest order mode of injected light. 
The core 19 of waveguide 15 is preferably a Germanium-doped silica or glass 
filament. The core 15 contains a series of variable spacing Bragg 
reflection gratings 16 written, impressed or otherwise applied by 
application of a variable two-beam ultraviolet (less than 300 nanometer) 
interference pattern. These periodic gratings 16 or refractive index 
perturbations are permanently induced by exposure to intense radiation. 
FIGS. 2A through 2C shows the establishment of different wavelength 
gratings 16 corresponding to respective locations on core 19. 
Each of selected gratings 16 is formed by transverse irradiation with a 
particular wavelength of light in the ultraviolet absorption band of the 
core material associated with a position in a structural component 22. 
This procedure establishes a first order absorption process by which 
gratings 16 each characterized by a specific spacing and wavelength can be 
formed by illuminating core 19 from the side with two coplanar, coherent 
beams incident at selected and complementary angles thereto with respect 
to the axis of core 19. The grating period is selected by varying the 
selected angles of incidence. Thus, a permanent change in the refractive 
index is induced in a predetermined region of core 19, in effect creating 
a phase grating effective for affecting light in core 19 at selected 
wavelengths. 
As indicated in FIG. 1 the optical waveguide 15 and core 19 are attached or 
embedded in a section of structural component 22, in particular a plate 
for example. Core 19 contains characteristic periodic refractive index 
perturbations or gratings 16 in regions A, B and C thereof. A broadband 
light source 33 or tunable laser is focused through lens 33' onto the 
exposed end of core 19. A beam splitter 34 serves to direct the return 
beam from core 19 toward a suitable readout or spectrometer 37 for 
analysis. Alternatively, a transmitted beam passing out of the end 19' of 
core 19 could be analyzed. 
The spectrum of the reflected light intensities from strain gauge 13 is 
shown in FIG. 3. A complementary tranmitted spectrum is also established 
passing out of the end 19' of core 19. The spectrum contains three 
narrowband output lines centered at respective wavelengths: lambda.sub.A, 
lambda.sub.B and lambda.sub.C. These output signals arise by Bragg 
reflection or diffraction from the phase gratings 16 at respective regions 
A, B and C. In this example, regions A and C of structural component 22 
have been strained by deformation, causing a compression and/or dilation 
of the periodic perturbations in the fiber core. 
As a result, the corresponding spectral lines are shifted as shown in FIG. 
3 to the dotted lines indicated. The respective wavelength differences 
delta lambda.sub.A and delta lambda.sub.C are proportional to strain in 
respective regions A and C. 
FIG. 4 illustrates the formation of periodic perturbations or gratings 16 
in a region of fiber core 19 in response to exposure of core 19 to intense 
transverse ultraviolet radiation. Grating spacings.DELTA. a and .DELTA.c 
are controlled by the incidence angle of incident interfering beams 99 and 
beam 101. As can be seen, the angles of incidence of beams 99 are 
complements (i.e. their sum equals 180 degrees) to each other with respect 
to the axis of core 19. The incident pair of beams 99 can be derived from 
a single incident beam 101 passing in part through a beam splitter 103 and 
reflecting from spaced parallel reflectors 105. By increasing the 
separation between reflectors 105 and correspondingly varying the angles 
of incidence of beam 101, the angles of incidence of beams 99 upon core 19 
can be controlled. Accordingly, the fringe spacing in grating 16 is varied 
as desired along the length of core 19, to permit a determination of 
strain or temperature corresponding to location along gauge 13. 
Several spacings can be superimposed or colocated by this technique for the 
response set forth below. 
Sensitivity to external perturbations upon structural component 22 and thus 
also upon core 19 depends upon the Bragg condition for reflected 
wavelength. In particular, the fractional change in wavelength due to 
mechanical strain or temperature change is: 
##EQU1## 
q is the thermooptic coefficient, which is wavelength dependent; .alpha. 
is the expansion coefficient; 
.epsilon. is the axial or longitudinal strain; 
lambda.sub.i is the wavelength reflected by the grating at location i along 
the core 19; 
n is the refractive index of the optical waveguide; and 
.DELTA.T is the change in temperature. 
This relationship suggests a way to compensate for temperature changes 
along the length of the fiber sensor. In particular, if superimposed 
gratings of different spacings are provided, each of the two gratings will 
be subject to the same level of strain, but the fractional change in 
wavelength of each grating will be different because q is wavelength 
dependent. 
Accordingly, each pair of superimposed gratings will display a 
corresponding pair of peaks of reflected or transmitted intensity. 
Accordingly, the shifts of these peaks due to a combination of temperature 
and strain can be subtracted. The shifts in these peaks due to strain will 
be the same in magnitude. Accordingly, any remaining shift after 
subtraction is temperature related. Thus, when it is desired to know the 
strain difference as between several locations possibly subject to a 
temperature difference, the temperature factor can be compensated. 
The relationship therefore permits compensation for temperature variation 
during measurement, since the photoelastic and thermoptic effects are 
wavelength dependent. In other words, by superimposing two or more 
gratings at each location of interest, two or more spectral lines are 
established at each point of measurement. Strain will affect both lines 
equally; temperature will not. Thus, sufficient information is available 
to permit determination of the magnitude of strain and the temperature 
difference. 
The information above is likely to cause others skilled in the art to 
conceive of other variations in carrying out the invention addressed 
herein, which nonetheless are within the scope of the invention. 
Accordingly, reference to the claims which follow is urged, as those 
specify with particularly the metes and bounds of the invention.