Method of forming optical fibre gratings

An optical fibre is fixed between a clamp and a piezo-electric translation stage. An argon ion laser emitting at 514.5 nm is used to write Bragg gratings in the fibre. A different longitudinal stress is applied to the fibre before optically writing each Bragg grating. The fibre when unstressed will have a Bragg grating of different peak reflectivity corresponding to the number of different applied stresses.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a method of forming two or more Bragg gratings in 
an optical fibre waveguide. 
2. Related Art 
In this specification the term "optical" is intended to refer to that part 
of the electromagnetic spectrum which is generally known as the visible 
region, together with those parts of the infra-red and ultraviolet (UV) 
regions at each end of the visible region which are capable of being 
transmitted by dielectric optical waveguides such as optical fibres. 
There is considerable interest in exploiting photosensitivity in 
germanosilicate optical fibres for application in the areas of optical 
communications and sensors. The first reported permanent optically-induced 
changes of the refractive index of optical fibres was by K. O. Hill, Y. 
Fujii, D. C. Johnson and B. S. Kawasaki, "Photosensitivity in Optical 
Fibre Waveguides: Application to Reflection Filter Fabrication" Appl. 
Phys. Lett, 32, 647 (1978). In their experiment, coherent radiation at 
514.5 nm, reflected from the fibre ends, generated a standing wave in the 
fibre which induced a periodic refractive index change along its length. 
This formed a high reflectivity Bragg grating in the fibre which peaked at 
the wavelength of the incident beam. Since then, numerous studies into the 
grating growth mechanism and photosensitive fibres have been carried 
out--see for example D. K. W. Lam, B. K. Garside, "Characterization of 
Single-Mode Optical Fibre Filters" Appl. Phys, Lett, 20, 440 (1981) and J. 
Stone, J. Appl. Phys., 62, 4371 (1987). However, the mechanism which 
results in the perturbation to the refractive index of the fibre core is 
not fully understood. The spectral region where the fiber is 
photosensitive has been found to range from the UV to around 700 nm. 
The potential applications of fibre gratings are numerous. For example, in 
telecommunications applications, tunable integrated fibre gratings, 
externally written with a UV laser, may be used for spectral control of 
fibre lasers. B. S. Kawasaki, H. O. Hill, D. C. Johnson and Y. Fujii, in 
an article entitled "Narrow-band Bragg reflectors in optical fibres", 
Optics Letters Vol 3 No. 2 August 1978, pp 66-68, note that an important 
property of the grating formation process is the extent to which the 
filter response can be tailored. For example, one method of forming a 
complex filter is to superimpose two or more simple band-stop 
characteristics in the same fibre by illuminating the fibre with different 
wavelengths of light either simultaneously or consecutively. 
Another known method of forming the Bragg gratings is by side-writing the 
gratings by interfering two coherent radiation beams at an appropriate 
angle. The pitch of the grating is determined by the angle of intersection 
of the two beams, so different grating pitches can be formed by adjusting 
this angle. 
SUMMARY OF THE INVENTION 
According to the present invention a method of forming two Bragg gratings 
in an optical fibre is characterized in that a different longitudinal 
stress is applied to the fibre before optically writing each grating, all 
the gratings having the same Bragg condition at the time of writing. 
The present invention provides a method of writing two or more Bragg 
gratings without the need for multiple wavelength illumination. 
The method of the present invention exploits the fact that optical fiber 
can, theoretically, be linearly strained by up to 20%. If a 
photorefractive fibre, length 1, is illuminated by a light from a laser of 
wavelength .lambda..sub.0, this will result in a grating of period of 
about .lambda..sub.0 /2n.sub.aff, where n.sub.aff is the fiber mode 
refractive index. If the fibre is now stretched by .DELTA.l, then, when 
illuminated, a grating of the same pitch, i.e. the same Bragg condition, 
as before will be written. When the fibre is allowed to relax to its 
unstressed, normal length after writing, the pitch of this second grating 
will be slightly smaller than the first grating. For the case of a 
reflection filter, the second grating has a peak wavelength which is 
smaller than the writing wavelength. This can be extended to providing 
several different pitch gratings in the same fibre. 
If, for example, there are two gratings of different period in the fibre; 
then, if it is assumed that they have some relative phase relationship, 
the index modulation in the fibre is effectively given by the 
superposition of the two index modulations. This is given by 
EQU n.sub.aff (z)=A.sub.1 cos ((k.sub.1 +k.sub.2)Z) cos ((k.sub.1 -.sub.2)Z) 
where k.sub.1 and k.sub.2 are the wave numbers of the two gratings, Z is 
the propagation direction, and A.sub.1 is the amplitude of the refractive 
index perturbation. At present, it is the second modulation term that is 
of interest and it can be assumed that the first term, which is a high 
frequency term, is a constant. (This high frequency term can in principle 
be used as a short wavelength reflection filter). The index modulation is 
therefore now given by 
EQU n.sub.aff (Z)=A.sub.2 cos ((k.sub.1 -k.sub.2)Z) 
From this expression, it can be seen that, by choosing the periods of the 
two optically-written gratings, a resultant grating of any period can be 
generated. The frequency difference grating written in the fibre is of 
particular interest in applications such as SHG, polarization conversion 
and mode conversion, as it allows the necessary phase matching conditions 
to be met for these processes. The actual operating wavelength depends 
only on the difference in the values of k.sub.1 and k.sub.2, and not on 
the actual write wavelength itself. For example, a simple calculation 
shows that the fibre would have to be stretched by approximately 2% if it 
is to be used for phase matching in SHG. Even smaller changes in fibre 
length would be required for polarization and mode convertors. These fibre 
length changes should be easily attainable in the fibre currently being 
used in experiments. 
It may also be possible to write reflection gratings for use at the 
telecommunications bandwidth of 1.3-1.5 .mu.m if the fibre can be 
stretched by approximately 10%. This is still within the theoretically 
predicted change; but, due to defects in the manufacture of the fibre, it 
is not clear whether it is possible to do this. Assuming that it may be 
done, this would allow high reflectivity, small bandwidth gratings to be 
written in the fibre. It would also be possible to write several gratings 
in the fibre, which would allow pulse generation and shaping of incident 
laser light. 
A convenient method of applying the different longitudinal stresses to the 
fibre to produce the different strains, is to clamp one end of the fibre, 
and to apply the stress by means of a piezo-electric translation stage 
clamped to the other end of the fibre. Clearly, other stressing means may 
be used such as a clamped micrometer attached to the fibre end instead of 
the piezo-electric translation stage. 
Other writing techniques can be used, for example wrapping the fibre around 
a cylinder, the stress being applied to the fibre by varying the radius of 
the cylinder by a piezo-electric expander. Also the fibre could be coated 
with a piezo-electric cladding, and the strain could be changed by varying 
the applied voltage. 
The invention is applicable to external grating writing methods, as well as 
to gratings written by launching an optical signal down the fibre.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
The experimental apparatus used to write gratings in single mode 
germanosilicate fibres at 514.5 nm is shown in FIG. 1. A fibre 2 with a 
radius of 0.9 .mu.m and a .DELTA.n of 0.012 has one end 4 enclosed in a 
glass ferrule 5 and clamped with a clamp 6. The other end 8 of the fiber 2 
is connected to a piezo-electric translation stage 10, which allows the 
length of the fibre, about equal to 50 cm in this case, to be changed by 
up to by 20 .mu.m. The gratings are written by coupling an argon ion laser 
12 lasing at 514.5 nm into the end 4 of the fibre 2 via a partial 
reflector 14 and a lens 16. The signal exiting the end 8 of the fibre 2 
during writing of a grating is focussed onto a photodetector 18. The 
increasing signal reflected by the grating as it is written into the fibre 
2 exits the end 4 of the fibre 2, and is focussed by the lens 16 and 
reflected to a photodetector 20 by the partial reflector 14. 
During the writing and reading of the grating, the polarization of the 
input and monitor beams are carefully controlled. 
The reflectance profile of the fibre 2 after writing the gratings is 
obtained by launching 0.5 mW of 514.5 nm light into the fibre, and then 
stretching the fibre using the piezo-electric translation stage 10. 
FIG. 2 shows the low power reflectance/transmittance of a typical grating 
formed in the fibre after 250 mW, from the single mode argon ion laser 12, 
is launched into it for approximately two minutes. This gives the 
reflectance/transmittance profile of the grating as the Bragg condition of 
the grating linearly changes with strain. From this data, the grating was 
found to have a peak relativity of 70% and a bandwidth of 482 MHz. The 
profile of the grating shown in FIG. 2 is similar to the sinc.sup.2 
reflection profile normally associated with Bragg reflectors. 
By changing the strain applied to the fibre 2 before writing a grating, a 
further three gratings can be optically written in the same fibre, each 
with a peak wavelength separated by 46 GHz. By varying the strain applied 
to the fibre, the four gratings written in the fibre can be scanned 
through. 
FIG. 3 shows the transmittance and reflectance, as a function of applied 
strain, for a probe signal of 514.5 nm from the argon laser 12 for a 
strain range which scans through two of the four gratings.