Optical fibre reflective diffraction grating devices

An optical device comprising a length of optical fibre of predetermined convex configuration supported by fibre mounting and/or attachment means, the convex outer part of the optical fibre having a portion thereof removed therefrom closely adjacent to or even just entering the core of the optical fibre to produce a substantially flat surface therealong on which a reflective diffraction grating of predetermined form is provided according to the function requirements of the device.

BACKGROUND OF THE INVENTION 
This invention relates to optical devices and relates more specifically to 
optical devices for the qualitative and/or quantitative reflection of 
light travelling along an optical fibre. Such optical devices may be used 
as sensors (e.g. temperature sensors), reflectors or mirrors (e.g. 
partially reflective) or they may be used in or for guided wavelength 
filters, so-called external cavity mode stabilisation of injection lasers 
and phase match non-linear interactions. 
SUMMARY OF THE INVENTION 
According to a first feature of the present invention there is provided an 
optical device comprising a length of optical fibre of predetermined 
convex configuration supported by fibre mounting and/or attachment means, 
the convex outer part of the optical fibre having a portion thereof 
removed therefrom closely adjacent to or even just entering the core of 
the optical fibre to produce a substantially flat surface therealong on 
which a reflective diffraction grating of predetermined form is provided 
according to the function requirements of the device. 
In carrying out the first feature of the present invention the optical 
lines or corrugations of the diffraction grating will be disposed 
orthogonally relative to the direction of light propagation along the 
optical fibre so that a beam of light having a wavelength .lambda.g=2V, 
where V equals the grating period, will undergo Bragg diffraction as it 
impinges on the grating and the diffracted beam will be guided by the 
optical fibre back along the incident path. The percentage of light 
reflected may be controlled by the number and/or depth of the corrugations 
and/or the number and optical density of the lines of the diffraction 
grating, as the case may be. The reflected percentage of the light could 
be 100% and since the mode of operation is diffractive the device exhibits 
strong wavelength selectivity with only a narrow band of wavelengths 
centred on .lambda.g being strongly reflected whilst all other wavelengths 
pass through the grating without significant attenuation thereof. The 
wavelength response of the device may include side lobes giving reduced 
reflectivity at wavelengths corresponding to multiples of .lambda.g but 
these are of relatively small amplitude or intensity and the depth of the 
grating corrugations or the optical density o the grating lines can be 
adjusted in order to reduce the side lobe intensities to insignificant 
levels. 
The diffraction grating may be produced by applying photoresist material to 
the aforesaid substantially flat surface of the optical fibre and then 
exposing the material to a suitable interference pattern derived from 
laser means. Following the development of the photoresist material the 
grating is formed as a depth modulation of the photoresist and it may be 
used in this form or it may alternatively be transferred into the optical 
fibre itself by the use of an ion-beam milling technique. 
Alternatively, the diffraction grating may be produced by exposing 
photochromic material dissolved in a suitable matrix and applied to the 
aforesaid flat surface of the optical fibre to interfering laser beams 
which accordingly produce a modulation of the refractive index of the 
photochromic material along the grating surface. 
According to a second feature of the present invention a plurality of such 
optical devices may be optically interconnected to provide inter alia a 
series of partially reflective mirrors eminently suitable for 
incorporation in optical hydrophones or other optical systems for sensing 
strain or deformation of optical fibres according to our British Patent 
No. 2136113B in which optical fibre means is arranged to be subjected to 
fibre deforming forces during use of the system and means is provided for 
producing a coherent signal for transmission along the optical fibre 
means, in which the optical fibre means comprises at points along its 
length respective discontinuities from which a light signal being 
transmitted along the optical fibre means will be partially reflected back 
along the fibre means and combined with the light signal being transmitted 
down the optical fibre means so that heterodyning occurs between the 
interfering signals and in which the combined light signals are applied to 
demodulation means which provides an output indicative of the acoustic or 
other deforming force acting on the optical fibre means. 
The discontinuities provided along the length of the optical fibre may be 
provided in accordance with the second feature of the present invention by 
a plurality of optical devices of the form described above and introduced 
at predetermined points along the length of the optical fibre common to 
all the optical devices. 
For the fabrication of an optical sensing arrangement according to the 
present invention an optical fibre mounting and/or attachment means is 
provided having formed in a surface thereof a plurality of curved grooves 
for receiving spaced apart relatively short lengths or portions of a 
common optical fibre, the short lengths of the optical fibre then being 
secured as by adhesive within the curved grooves so that the short lengths 
of optical fibre have a convex configuration after which the grooved 
surface of the mounting and/or attachment means is polished or pared away 
to remove part of the outer convex surface of the lengths of optical fibre 
within the curved grooves closely adjacent to or just entering the core of 
the optical fibre to produce substantially flat surfaces therealong and 
diffraction gratings are then provided on the plurality of flat surfaces 
to define reflective points along the optical fibre and the fibre mounting 
and/or attachment means is finally divided up into separate parts so that 
a plurality of optical devices are effectively distributed along the 
length of the common optical fibre. 
The optical fibre mounting and/or attachment means may comprise a plurality 
of grooved blocks of glass or silica, for example, bonded together prior 
to winding the optical fibre around the bonded structure so that portions 
of the fibre engage the curved grooves in which they may be cemented as by 
UV setting cement for example. 
It may here be mentioned that the second feature of the present invention 
enables a plurality of partially or totally reflective discontinuities to 
be introduced into an optical fibre by a single operation and without the 
need to sever and rejoin the ends of the optical fibre after insertion of 
suitably reflective components or reflective coatings between the fibre 
ends. The latter known techniques introduce undesirable optical losses and 
complexity (e.g. the need for precision alignment of the fibre ends etc.) 
into the production of optical hydrophones of the kind having partially 
reflective discontinuities along a sensing fibre. 
By way of example the present invention will now be described with 
reference to the accompanying drawings in which:

DETAILED DESCRIPTION OF THE INVENTION 
Referring to the drawings, FIG. 1 shows an optical hydrophone according to 
our British Patent No. 2136113B. The optical fibre sensor arrangement of 
the hydrophone comprises an optical fibre 1 having at predetermined 
intervals along its length partially reflective discontinuities provided 
by optical devices ODl to ODN. In operation of the hydrophone as is more 
fully described in the above-mentioned patent a light signal is launched 
into one end of the optical fibre 1 from coherent light signal generating 
means 21 and light reflected back along the optical fibre from the 
discontinuities interferes with the transmitted light to produce signals 
which can be processed (e.g. demodulated) by receiver/detector means 22 to 
derive an indication of acoustic forces acting on the optical fibre 1 
along its length. 
Referring now to FIG. 2 of the drawings, the optical device, such as the 
device ODl in FIG. 1, comprises a monomode optical fibre 1 provided with a 
core 2 and cladding 3. A portion 4 of the optical fibre 1 is accommodated 
within a curved groove or slot 5 formed in a mounting or support block 
structure 6 of glass or silica for example. The groove or slot 5 which 
will be of a predetermined radius (e.g. 0.5 to 1.5 m) may, for example, be 
produced by the use of a diamond saw-blade. To perform the cutting 
operation the block structure 6 may be mounted on a pivotable arm of 
adjustable length (not shown). By selecting the length of the arm and the 
position of the pivot relative to the saw-blade a groove of predetermined 
depth and curvature may be cut. By the use of an adhesive cement 7 (e.g. 
UV setting epoxy resin) the optical fibre portion 4 is secured to the 
convex surface 8 of the block structure 6 so that the fibre portion 4 is 
of convex configuration and the adhesive cement also fills the groove or 
slot 5 as shown. 
In order to produce a diffraction grating 9 on the optical fibre portion 4 
the outer surface of the mounting block structure 6, as viewed in the 
drawing, will have previously been polished or pared away to a sufficient 
depth to produce a substantially flat surface 10 on the fibre portion 4. 
The polishing may extend through the cladding 3 of the optical fibre to 
within approximately 1 .mu.m of the fibre core 2 and it may even just 
penetrate the core itself. 
Having produced the flat polished surface 4 the surface may then be coated 
with a photo-resist (e.g. positive photo-resist marketed under the product 
name Shipley AZ1350) which is then exposed to a two beam interference 
pattern derived from a laser (e.g. argon laser in the case of the specific 
positive photo-resist mentioned above). 
After developing the exposed photo-resist the grating 9 is formed as a 
depth modulation which may, if desired, be transferred into the optical 
fibre material by means of an ion-beam milling procedure. 
As an alternative method of producing the grating 9 photochromic material 
dissolved in a suitable matrix may be applied to the flat optical fibre 
surface 10 and then exposed to the two beam laser interference pattern in 
order to produce modulation of the refractive index of the photochromic 
material. In this way diffraction lines or stripes of varying optical 
density are produced to produce periodic variation of the refractive 
index. 
In fabricating the diffraction grating of the optical device described it 
will be arranged that the grating period V=.lambda.g/2 where .lambda.g is 
the wavelength of the light within the fibre which is required to be 
reflected (e.g. selectively) back along the fibre. 
In order to provide a plurality of optical devices of the form shown in 
FIG. 2 spaced at intervals along the optical fibre 1 without severing the 
optical fibre and in one simultaneous overall operation thereby 
simplifying the fabrication whilst keeping optical losses within the fibre 
to a negligible level, a plurality of grooved block mounting structures 6 
may be bonded together as shown in FIG. 3. The curved grooves 5 of the 
structure are arranged in the upper surface 11 of the bonded structure 12 
as shown. The optical fibre 1 is then wound around the bonded block 
structure 12 so that portions 4 of the optical fibre 1 at the appropriate 
locations along the fibre length engage the curved grooves 5. The optical 
fibre portions 4 are then cemented into the grooves 5, as by using UV 
setting epoxy resin cement 7. 
As can be appreciated, the curved convex outer surfaces of the optical 
fibre portions 4 protrude above the surface 11 of the bonded structure 12. 
These protruding surfaces of the fibre and the upper surface of The block 
are then polished or pared down to provide substantially flat surfaces 
along the fibre as shown at 10 in FIG. 2 at which diffraction grating 9 
(FIG. 2) are formed on each surface in the manner already described with 
reference to FIG. 2. However, it may here be mentioned that the 
reflectivities of the respective diffraction gratings could be varied by 
exposing the photo-resist or photochromic material, as the case may be, to 
the interfering pattern from the laser beams through a suitably graded 
filter. 
The block structure 12 is finally divided into separate blocks 6 which will 
be distributed along the optical fibre 1. 
It will of course be appreciated that the block structure 12 could take 
alternative forms and could, alternatively, be of unitary form and 
subsequently cut into sections after fabrication of the optical device 
assembly has been completed. 
Although in the foregoing description the optical devices are partially 
reflective, in other applications, such as wavelength filtering the 
reflective percentage will be high (e.g. 100%) in order to ensure that a 
particular wavelength or narrow band of wavelengths will be reflected back 
along the fibre whilst other wavelengths pass through the grating.