Optical fibre feedthrough

An optical fibre feedthrough in which a polarization maintaining (PM) fibre is sealed within a metallic sleeve by a glass seal. The seal applies asymmetric stresses to the fibre which is orientated so that they reinforce its PM properties. This arrangement reduces the manufacturing tolerances needed to avoid destroying the PM properties of the fibre associated with symmetric stress seals. It has application to packages for opto-electronic components. A double optical fibre feedthrough is also disclosed.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to optical fibre feedthroughs. 
2. Related Art 
A device package that incorporates an optical or opto-electronic component 
has an aperture for a feedthrough feeding one or more optical fibres that 
conduct light to or from inside of the package. One approach is to seal 
the optical fibres hermetically within a metallic sleeve with glass 
solder, which sleeve is, in turn, mounted in the aperture. 
Generally `single mode` optical fibre supports two orthogonally polarized 
He.sub.11 modes. Because of the near-degeneracy of the propagation 
constants B.sub.X and B.sub.y, for the x and y axes respectively, any 
small perturbations along the length of the fibre lifts this degeneracy 
and thus the state of polarization (SOP) will evolve unpredictably. Any 
linearly polarized light injected into the fibre will thus become 
elliptically polarized after a short distance. 
Many applications, e.g. lithium niobate components and coherent optical 
transmission systems, require a stable SOP output from the fibre. One 
solution is to use High Birefringence Polarization Maintaining (HB-PM) 
fibre. 
PM fibres maintain the state of polarization by introducing a large 
intrinsic birefringence arising from asymmetry in the core or stress 
effects due to the fibre cladding. The latter of the two methods includes, 
for example, the formation of high stress regions by means of circular 
rods along the length of the fibre either side of the core. When launched 
into one or other of the principal modes, light will propagate unchanged 
along the length of the fibre provided no external perturbations exceed 
the internal intrinsic stresses, otherwise the SOP will be unpredictable. 
When using circular profile tubes it is imperative that the fibre is 
concentric within the tube, that the tube wall thickness is uniform and 
that no air bubbles or contaminants interfere with the glass seal. Any 
significant deviation from a symmetrical structure, both longitudinally 
and axially, will destroy the symmetry of the stresses external to the 
fibre and could therefore act to destroy the fibres PM properties. Keeping 
the symmetry accurate to such an extent can pose difficult problems if 
large-quantity production is envisaged. 
BRIEF SUMMARY OF THE INVENTION 
The present invention provides an optical fibre feedthrough assembly 
comprising a metallic sleeve, at least one polarization maintaining fibre, 
and a glass seal completely surrounding the or each fibre and sealing the 
or each fibre to the sleeve, the seal applying asymmetric stresses to the 
or each fibre, and the or each fibre being orientated so that the stresses 
act to reinforce the polarization maintaining properties of that fibre. 
Because the feedthrough is designed to apply stresses to the fibre via the 
glass seal reinforcing the polarization maintaining properties of the 
fibre, small manufacturing deviations from the ideal will not result in a 
net stress to the fibre acting to destroy its polarization maintaining 
properties. 
The asymmetric stresses can be achieved by use of a metallic sleeve having 
an asymmetric inner cross-section. These may be made by metal injection 
moulding due to the difficulty of machining a metallic sleeve with an 
asymmetric inner cross-section to have a circular external profile. 304L 
and 316L stainless steels are expected to be suitable materials for such 
moulding. In this case the outer cross-section is preferably circular to 
simplify fixing within the aperture of the device package. Alternatively 
rod elements can be introduced into an otherwise circularly symmetric seal 
to provide the required asymmetric stresses. These rod elements could be 
standard, bare, single-mode fibres, for example.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
Referring to FIG. 1, a feedthrough assembly 2 comprises a metallic sleeve 4 
through which is threaded a polarization maintaining (PM) optical fibre 6. 
The sleeve 4 has a tubular section 8, a frusto-conical section 10 
providing a taper to a second tubular section 12 flattened at its end to 
produce an elliptical cross-sectioned section 14. The PM fibre 6 
terminates about 0.5 mm beyond the narrow end of section 14 of the sleeve 
8. 
The section 8 has a throughhole 16 through which epoxy glue 18 is injected 
to hold the fibre 6 in place after the termination end of the fibre 6 is 
sealed by means of glass solder 20 fed into the sleeve 4 via an aperture 
22 in the tapered section 10. 
The fibre 6 has its outer protective coatings stripped, and if needed, the 
end formed into a lens before it is threaded into a spacer 24 of brass, 
silica glass or other suitable material. The fibre 6 and spacer 24 are 
then threaded into the sleeve 4 so that the spacer 24 abuts the tapered 
section 10 for positive location and the end of the fibre 6 protrudes the 
desired distance from the elliptical end section 14. The spacer 24 
supports the fibre 6 centrally within the sleeve 4 at the front sections 
(10, 12, 14). The sleeve 4 is heated by small electric coils wound round 
ceramic tubes (not shown) into which the feedthrough assembly is placed. 
The front sleeve sections 10, 12 and 14 are heated by one heater coil, the 
tubular section 8 being heated by a second to give background heat to the 
larger diameter portion 8. Glass solder is heated to its flow temperature 
and introduced into the sleeve 4 via the aperture 22 until the front 
sections 10, 12 and 14 are filled, generally by capillary action. Enough 
is introduced to produce an even meniscus 26 around the protruding section 
of the fibre 6 which has the effect of centering the fibre 6 within the 
tube section 14. 
Alternative methods of forming the seal may be employed. Glass solder in 
the form of a preform can be threaded on to the fibre for subsequent 
heating, for example. 
The section 14 is then viewed end-on to check that the fibre is centrally 
located and that the high stress regions are orientated to within 
3.degree. of short axis of the elliptical end-section 14. The glass solder 
is then allowed to cool slowly. Some glass withdraws into the tube section 
14 on cooling. Sufficient solder should be introduced before cooling to 
ensure a meniscus remains after cooling. Finally, glue is fed in through 
throughhole 16 to fix the unsealed portion of fibre 6 within the tube 4 to 
relieve strain of the glass sealed portion of the fibre 6 during 
subsequent handling. The rear of the tube 4 may be crimped provided care 
is taken not to affect the fibre. 
Various types of glass may be chosen for the glass solder. Generally it is 
desirable to use a solder which flows at a temperature to avoid fibre 
enbrittlement so one with a glass transition temperature in the range 
400.degree. C. to 480.degree. C. 
The most consistent results have been obtained using "OI" PP-200 Glass, 
expansion coefficient 125.times.10.sup.-7 /.degree. C. Poisson's ratio 
0.22 and glass transition temp 290.degree. C. manufactured by 
Owens-Illinois Inc., USA. This company is apparently no longer trading but 
the glass is obtainable as a single billet from Garner Glass, 177, South 
Indian Hill Blvd., Claremount, Calif., USA. 
Good fibre preparation is essential for achieving a satisfactory seal of 
the fibre 6 within the sleeve 4. The outer protective coating must be 
stripped off to expose the bare optical fibre with its reflective 
cladding. This is then cleared using a solvent, for example acetone, until 
all traces of the coating have been removed. 
The feedthrough is then tested for hermeticity using helium, the seal being 
considered hermetic if the detected leak rate does not exceed 10.sup.-8 
atmospheres cm.sup.3 s.sup.-1 Helium, MIL SPEC 883. 
The seal can be seen to consist of three coaxial cylinders with each 
differing properties: modulus of elasticity E, Poisson's ratio .nu., and 
coefficient of thermal expansion .alpha.c. Due to the heat treatment 
required to flow the sealing glass the subsequent cooling will result in 
radial, .sigma..sub..gamma., circumferential (hoop), .sigma..sub.100 , and 
axial, .sigma..sub.z, stresses being set up in the seal, these can be 
compressive or tensile in each of the three regions, independently. The 
resultant properties are dependent on the physical properties of the 
individual components and their physical size, giving a great many 
permutations. To obtain a hermetic seal these should be "largely 
compressive", although acceptable seals may be produced with slightly 
tensile hoop stress. A major benefit of having the glass capillary in 
compression is that the formation, and propagation, of cracks in the seal 
due to tensile stresses are reduced giving excellent long term hermetic 
properties. 
The constitutive equations relating stress to strain for an elastic 
material, in polar coordinates, are: 
EQU .epsilon..sub..gamma. =.epsilon..sub.f +E.sup.-1 [.sigma..sub..gamma. 
-.nu.(.sigma..sub..phi. +.sigma..sub.z)] (1) 
EQU .epsilon..sub..phi. =.epsilon..sub.f +E.sup.-1 [.sigma..sub..phi. 
-.nu.(.sigma..sub..gamma. +.sigma..sub.z)] (2) 
EQU .epsilon..sub.z =.epsilon..sub.f +E.sup.-1 [.sigma..sub.z 
-.nu.(.sigma..sub.r +.sigma..sub..phi.)] (3) 
and the free strain, .epsilon..sub.f, is given by, 
EQU .epsilon..sub.f =.alpha..DELTA.T 
where .alpha. is the linear thermal expansion coefficient and .DELTA.T is 
the temperature change. 
By applying appropriate boundary conditions, and selecting known values for 
E, .nu. and .epsilon..sub.f, it is possible to solve numerically for the 
three component stresses. This analysis is described in detail in the 
applicant's co-pending published application EP87310670 which is hereby 
incorporated by this reference. The above calculation will give results 
which should not be regarded as exact because it is assumed that the 
materials are elastic, whereas the hot glass capillary is actually 
viscoelastic. 
Referring now to FIG. 2 there is shown the end view of the fibre 
feedthrough assembly 2 of FIG. 1 showing the orientation of the PM fibre 6 
within the oval cross-sectional tube section 14. The fibre is standard 
commercially available PM silica fibre having a core region 30 and a 
cladding 32 in which are located two stressing rods 34 one either side of 
the core 30. 
The metal of tube 14, and glass are again chosen so that the forces acting 
on the glass seal 20 after formation are largely compressive. 
The correct orientation of the high stress regions in relation to the tubes 
asymmetry was determined by trial and error, although a complicated stress 
analysis could be carried out based on work by P. L. Chu and R. A. Sammut, 
"Analytical method for calculation of stresses and material birefringence 
in polarization-maintaining optical fibre," J. Lightwave Technol. , vol. 
LT-2, no. 5, Dp. 650-662. 1984. Two suggested trial orientations are to 
align the high stress regions of the PM fibre parallel to either of the 
natural axes of asymmetry. 
Other structures may be used to obtain such asymmetric stresses, for 
example a feedthrough of circular internal cross-section with rod inserts 
either side of the fibre positioned on or at 90.degree. to the high stress 
axis of the fibre, the position depending on the coefficient of expansion 
of the insert. 
Referring now to FIG. 3 a sleeve of a fibre feedthrough assembly according 
to the present invention is as shown in FIG. 1 except that the termination 
end has a circular cross-section 36, which is the only part of the sleeve 
4 shown. Sealed within the tube section 36 are two PM fibres 38 and 40. 
The non-circularly symmetric arrangement of the fibres 38 and 40 within 
the tube section 36 create asymmetric stresses within the glass seal 26. 
The PM fibres are arranged, again by use of a video camera, to align the 
high stress regions of the fibres 38 and 40. 
The non-circularly symmetric arrangement of the fibres 38 and 40 within the 
tube section 36 create asymmetric stresses within the class seal 26 thus 
alleviating the need for an asymmetric tube profile. The fibres are 
mounted so that their stress regions are parallel (vertically in the 
orientation of the FIG. 3) thus aligning the fast and slow axes of each 
fibre to the other. Other alignments of the fibres, for example each fibre 
rotated through 90.degree., may also work. 
Because two fibres have to fit inside the tube section 36, standard 900 
.mu.m coated fibre cannot be used: fibre with 250 .mu.m diameter sleeving 
must be used instead. The same standard of preparation is required as for 
the single fibre assembly, but a steel spacer is fitted over one of the 
fibres to maintain a 250 .mu.m centre spacing between the fibres at the 
rear of the tube section 36. PTFE sleeving is then slid over the double 
fibre tail to achieve the same dimensions as for the single 900 .mu.m 
coated fibre. The protruding ends of the fibres are laid in accurately 
machined `V` grooves to give a fibre centre-to-centre spacing of 250.+-.2 
.mu.m. The high stress regions are aligned vertically, with the aid of a 
video camera, to within .+-.3.degree.0 by rotating the fibres. When 
orientation and fibre protrusion are satisfactory the assembly is heated 
and molten glass applied through the hole at back of narrow tube section 
until it appears at the front to form a meniscus around both fibres 
described before with reference to FIGS. 1 and 2. 
Testing of the assemblies was carried out by temperature modulating an 80 
mm length of the fibre tail over a 20.degree. C. range whilst monitoring 
the output SOP for light launched on one of the principal modes. The 
output SOP is plotted on a Poincare sphere, and for a perfect fibre 
(undegraded) will be on the equator. Any degradation will be shown as a 
circle describing the varying states of polarization. The larger the size 
of the circle the greater the degradation of the fibres properties. This 
can be related to the extinction ratio of the fibre (i.e. ratio of the 
power in the two modes) by 10 log[tan.sup.2 (z/4)] where z is the angle in 
degrees that the circle subtends at the centre of the sphere. 
To give an extinction ratio of the fibre &gt;25 dB requires a maximum z angle 
of the order of 13.degree.. Because of measurement noise, the minimum 
circle that could be accurately measured is 2.degree.. This corresponds to 
an extinction ratio of about 40 dB. 
Early results, for single fibre assemblies, using round tubes with poor 
uniform tube wall thickness and varying concentricity gave widely varying 
results from no degradation of the fibre to little or no extinction ratio. 
Asymmetric versions gave more predictable results with a more consistent 
extinction ratio between different feedthroughs (See FIG. 4), but if the 
fibre was a long way off centre or the tube wall thickness too variable, 
the degradation could still be significant (see FIG. 5). A bare fibre 
result is shown in FIG. 6 for comparison. 
When the concentricity is strictly controlled with the round version and 
the glass seal forms a good meniscus a fibre feedthrough can be made 
keeping the extinction ratio within the specification of &gt;25 dB. The ones 
that fail can usually be spotted during manufacture by poor centring of 
the fibre or a non-uniform glass seal. Assemblies that fail can be 
reheated and allowed to find an equilibrium state, more glass being added 
if necessary. These have been found to be within specification when 
retested. A potential problem with reheating is that the epoxy, used at 
the rear of the feedthrough to hold the outer sleeving, which can creep 
forward contaminating the glass seal. 
The double fibre assemblies of the type shown in FIG. 3 have proved to be 
more consistent than the single fibre assemblies, and the only failure to 
date has been due to both fibres being held to near the top of the 
feedthrough. Aligning the double fibres to a lithium niobate wafer has 
shown the coupling to be within the insertion loss limits specified 
indicating the spacing to be within tolerance. 
Transmission tests of the fibre tails showed no extra losses are introduced 
when sealed into a fibre feedthrough according to the present invention. 
Laser welding of a bush, as normally done during fibre-package attachment, 
resulted in no further degradation of the fibres PM properties within the 
accuracy of the testing apparatus. 
Referring now to FIGS. 7 to 9 there are shown examples of fibre feedthrough 
end sleeves 42, 44, and 46 respectively in which the asymmetry of their 
cross-section provides the required asymmetric stresses. These have the 
advantage of having circular outer cross-sections for ease of fixing to a 
bush before welding to the aperture in a device package. They may be made 
by metal injection moulding of stainless steel, for example. 
Each end face 48 of the metal sleeves 42 and 46 has alignment markers 50 to 
aid visual alignment of the fibre stress axis to the planes of symmetry of 
the feedthrough 42, 44 and 46. 
Referring now to FIG. 10, a further manner of obtaining asymmetric stresses 
to a PM fibre 51 in a feedthrough 52 of circular inner and outer 
cross-section is by the incorporation of rodlike elements 54 orientated 
parallel to the PM fibre 5. For example, the elements 54 may comprise bare 
standard, non-PM fibres.