WO1987007255A1

WO1987007255A1 - Optical fibre apparatus and method

Info

Publication number: WO1987007255A1
Application number: PCT/GB1987/000344
Authority: WO
Inventors: Robin David Birch; Luksun Li; David Neil Payne; George Wylangowski
Original assignee: Robin David Birch; Luksun Li; David Neil Payne; George Wylangowski
Priority date: 1986-05-20
Filing date: 1987-05-20
Publication date: 1987-12-03
Also published as: GB2192289A; GB8711925D0; EP0270591A1; GB8612189D0

Abstract

Method of manufacture of optical fibres having at least one integral metal region (A) therein comprises the steps of fabricating a preform (P) having at least one aperture (A) therein, introducing a metallic medium into said aperture, subjecting said preform (P) to a reduced ambient pressure substantially to remove entrapped air therefrom and drawing an optical fibre containing a metal region therein from the preform.

Description

OPTICAL FIBRE APPARATUS AND METHOD This invention relates to methods of manufacture of optical fibres incorporating metal regions and to communications and sensing devices incorporating such optical fibres.

For optical fibre communication systems and fibre sensors, a number of optical devices are required. In particular, phase or amplitude modulators play a central role in both fields, being one of the most commonly-required devices, and both integrated optics and fibre versions are known. Of the latter, acousto-optic (or fibre stretching) and magneto-optic (Faraday-effect) devices have been demonstrated. However, owing to the very small electro-optic coefficients in glass, it is difficult to construct a fibre electro-optic device using either the Kerr effect or electrostriction. Another possibility, the Pockels effect, is absent in glasses, being normally only found in crystals. The Kerr-effect is quadratic, that is, the phase shift produced is proportion to electric field intensity squared. Despite the low Kerr coefficient, it is therefore possible to construct a fibre Kerr-modulator, provided a sufficiently high electric field can be maintained over the core region for lengths of several metres. In this way the small electro-optic effect can be compensated by the long fibre interaction length. Thus a pair of long, parallel metal plates with fibre sandwiched between forms the basis of a Kerr-effect fibre-modulator.

However, for practical voltage levels the electric field produced by such electrodes, will be limited by their relatively large separation, the minimum spacing being equal to the fibre diameter. Moreover, it is difficult to maintain the cumbersome, high-voltage electrodes parallel over long distances and they have a large capacitance. Furthermore, if polarisation-maintaining fibre is used, aligning the optical axes of the fibre and the direction of the electric field is a difficult problem. Each of these problems can be overcome by the simple expedient of incorporating the metal electrodes into the fibre. In this way large electric field intensities are obtainable for low applied voltage, since the electrode spacing is no longer limited by the fibre diameter. The electrodes can be disposed very accurately within a few microns of the core for virtually any fibre length. The birefringent axes of the fibre and the electrodes are automatically aligned for all interaction lengths. The present invention relates to the manufacturing methods for such a composite metal/glass fibre.

Fibres incorporating metal/glass composites can be used for magnetic field sensing if a magnetostrictive metal is incorporated. Several metals and alloys exhibit magnetostrictive effects, e.g. nickel. If an optical fibre incorporating magnetostrictive material in the cladding is subjected to a magnetic field, it will experience a strain which can be detected as an optical phase change in the output light. The phase change can be conventionally detected by either interferometric or polarimetric means. In the case of an interferometric magnetic field sensor, the output from the metal/glass composite fibre is caused to interfere with that from a reference arm in any one of number of interferometer designs e.g. Mach-Zehnder. Phase changes produced in the sensing fibre can be detected as a change in the intensity of the interference pattern thus produced.

Furthermore, a metal/glass composite fibre can provide a novel polarimetric magnetic-field sensor. The presence of the two solid metal sectors on either side of the core leads to a large thermal stress, owing to the mismatch in expansion coefficients between the metal and glass. Birefringence in the core results, the magnitude of which depends on the thermally-induced strain in the metal sectors. In the case of a magnetostrictive metal, this strain can be perturbed by the presence of magnetic fields, leading to a change in birefringence. Light launched into the fibre at 45° to the birefringent axes will have an output state of polarisation which is particularly sensitive to changes in birefringence and this fact can be used to measure magnetic fields.

The birefringence induced in composite metal/glass fibres is also useful for constructing polarisation-maintaining fibres, which are normally fabricated from glasses with different thermal expansion coefficients. The large difference in expansion coefficients between metal and glass could provide a route to still higher birefringence.

According to the present invention there is provided a method of manufacture of optical fibres having at least one integral metal region therein comprising the steps of fabricating a preform having at least one aperture therein, introducing a metallic medium into said aperture, subjecting said preform to a reduced ambient pressure substantially to remove entrapped air therefrom and drawing an optical fibre containing a metal region therein from said preform. The invention will now be particularly described with reference to the accompanying drawings in which:-

Figure 1 is a cross-section through an optical fibre with a metal inclusion in its core region Figure 2 illustrates the drawing of such a fibre Figure 3a shows a sleeving tube preform containing a number of optical fibres

Figure 3b is a cross-section through an optical fibre drawn from this preform

Figure 4 illustrates preform for drawing an optical fibre Figure 5 is an assembly incorporating the preform of Figure 4; and

Figure 6 is an optical fibre drawn from this preform Figure 7 shows a method of injecting metal into a fibre Figure 8 is a fibre incorporating coaxial electrodes. Referring to the drawings, Figure 1 shows the cross-section of an optical fibre having a core A and cladding B. The diameter of the core is of diameter Φ. Metal regions M are spaced at distance D from the core.

Two methods can be employed to make a composite metal/glass single-mode fibre preform, which can subsequently be pulled into fibre's. Note that the following descriptions refer to high-silica preform materials, since these are commonly used for the fabrication of optical fibres. However, other glasses could be chosen for their thermal and mechanical compatibility with the metal sections.

Holes H, running parallel to the core, are drilled into a normal single-mode fibre preform P (for example fabricated by the MCVD process) using a diamond drill. (see Figure 2). The drilled holes are thoroughly cleaned and acid etched.The holes are either drilled blind or one end E of the preform is heated and collapsed to form a seal. A metal or alloy in a variety of forms (e.g., chip, powder) with a melting temperature less than about 1900°C is introduced into the holes, which are subsequently evacuated. The preform is heated to the melting point of the metal and shaken slightly to remove any air bubbles which may be trapped at the metal-glass interfaces.

The preform is then drawn into a conventional fibre using standard drawing techniques.

Referring to Figures 3a and 3b, an alternative approach to making a composite metal /glass preform is to place the metal or alloy inside a glass tube T which has one end sealed. The tube is normally of silica to ensure compatibility with commonly available fibre preforms. Following the above procedure the metal is melted and bubbles are removed. These metal/glass tubes together with a conventional single-mode fibre preform P and a number of silica rods R can then be placed inside a sleeving tube S as shown in Figure 3a. Composite metal/glass fibres can be drawn from this preform with a cross-section similar to that shown in Figure 3b.

If low melting-point metals or alloys are suitable for the application in mind, the metal can be introduced into the fibre by constant-pressure pumping.

In this method, the fibre is drawn from a preform made so that it has holes disposed on either side of the core. The metal is then heated to its melting point and pumped at high pressure into the holes in the fibre.

This method is most suited for metals with a melting point lower than the degradation temperature of the fibre coating (400°C for polyimide). Moreover, practical considerations (e.g. pressure available, and time taken to fill) limit the length of fibre that can be filled to a few hundred metres.

A preform similar to that used for the producti on of si ngl e-mode tel ecommuni cations fi bre was used. The preform, shown in Figure 4, was manufactured by the MCVD process with a one to six ratio of core to outer diameter and was drawn down to an outer diameter K of 4mm. The core was germanium-doped silica and had a refractive index 0.014 higher than that of the cladding. Two sides of the preform were polished slightly so that a flat surface F for the electrodes could be obtained. The core was of diameter 0.45mm and the distance between the flat faces was 3mm. The preform, together with four silica rods, was then assembled inside a sleeving tube (Figure 5). The diameter L of the rods was 8mm and the sheathing tube S had an inner diameter S₁ of 21mm and an outer diameter s₂ of 28mm.

A fibre with the cross-section shown in Figure 6 was drawn from the preform, giving two axe-head shaped holes H disposed symmetrically either side of the core A. The spacing E₂ between the holes was ~20μm and the diameter F₁ of the fibre ~135μm. A syringe SG was filled with gallium/indium alloy (76% Ga + 28% In), which is liquid at room temperature, and air bubbles removed by placing the syringe inside a vacuum chamber. The fibre FI was bonded to the syringe needle V using epoxy adhesive E (Figure 7). Pressure was applied to the metal using a plunger PL controlled by a DC motor-driven lead screw N. Some 30 metres of fibre could be readily filled with metal in this way.

The fibre exhibited birefringence (B ~ 4x10^-5) corresponding to a beat length Lp~15mm at a wavelength of 633nm. Note that in this case the birefringence resulted from the intrinsic fibre stresses and not from the presence of the metal, since this remained liquid. The electrical resistance of each electrode was measured to be 300 ohm per metre.

An a.c. electrical signal was applied to the electrodes using a pair of gold wires (diameter -15μm), inserted through the fibre electrode holes into the liquid Gain alloy. The applied driving signal was variable up to 67 volts (r.m.s.) and had a frequency range from several hundred Hz to 1MHz. For an applied signal of 67V_rms the phase shift between the two orthogonally-polarised modes of the fibre was measured to be 4.3°/meter. This corresponds closely to a previously reported value of the electro-optic Kerr coefficient for silica. (K ~ 5.0x10^-16mV^-2). which is liquid at room temperature, and air bubbles removed by placing the syringe inside a vacuum chamber. The fibre was bonded to the syringe needle using epoxy adhesive (Figure 7). Pressure was applied to the metal using a plunger controlled by a DC motor-driven lead screw. Some 30 metres of fibre could be readily filled with metal in this way.

The fibre exhibited birefringence (B ~ 4x10^-5) corresponding to a beat length Lp~15mm at a wavelength of 633nm. Note that in this case the birefringence resulted from the intrinsic fibre stresses and not from the presence of the metal, since this remained liquid. The electrical resistance of each electrode was measured to be 100 ohm per metre. An a.c. electrical signal was applied to the electrodes using a pair of gold wires (diameter ~15μm), inserted through the fibre electrode holes into the liquid Gain alloy. The applied driving signal was variable up to 67 volts (r.m.s.) and had a frequency range from several hundred Hz to 1MHz. For an applied signal of 67.V_rms the phase shift between the two orthogonally-polarised modes of the fibre was measured to be 4.3°/meter. This corresponds closely to a previously reported value of the electro-optic Kerr coefficient for silica (K ~ 5.0x10^-16mV^-2). In an alternative embodiment two holes parallel to the core are ultrasonically-drilled in a normal single-mode preform. The hole size and spacing on either side of the core is chosen from mechanical considerations and to ensure that the optical field does not significantly penetrate the electrodes, which would introduce increased losses. The holes are cleaned thoroughly and pure chips or a billet of indium or other metal is placed into the holes. A vacuum is applied to the holes and the preform is heated to melt the metal. The preform is shaken slightly to make sure there are no bubbles trapped at the metal-glass interfaces. The heat is then removed, and the preform cooled to allow the metal to solidify. A composite metal /glass fibre can then be drawn from this preform (e.g. Figure 1).

This method has the advantage that long lengths of metal/glass optical fibre can be manufactured. If a magnetostrictive metal is used, the long length would lead to a highly-sensitive magnetic-field sensor.

A magnetic field sensing fibre may be fabricated using nickel or other magneto-strictive metal.

In accordance with different embodiments of the invention The core of the preform used can be circular or non-circular, e.g. elliptical. The latter can be used to create fibre birefringence. Metals or alloys with melting temperature below 1900°C can be used in combination with silica-based preforms, since these are normally drawn into fibre at temperatures in this region.

Obviously, if lower softening-point glasses are used, lower melting point metals will have to be employed.

To avoid significant evanescent-field interaction between the core and the metal, the distance between the edge of the core and the metal should be larger than twice the core diameter. (Figure 1). The number of metal sectors can be one or more.

During fibre drawing, the preform can be spun to achieve rotating electrodes.

Core materials will usually be silica-based. However, compound glasses of various kinds can also be used, and glasses with higher Kerr coefficients than silica are known. If the core region of the fibre is left hollow, it can be subsequently filled with liquids, or a material which will solidify to give a single crystal. In both these cases, a much higher electro-optic effect will be present. One or more cores can be offset from the centre, and placed within a pair of co-axial electrodes (Figure 8). This may have some electrical advantages.

Since the electrodes represent an electrical transmission line, and the core an optical transmission line, it is possible to envisage travelling wave phase-modulators. In this case it is necessary to design the electrical signal to propagate synchronously (i.e. at the same speed) as the optical wave.

It is an advantage to decrease the electrical resistance of the electrodes, especially if long lengths of fibre are used in a Kerr-effect modulator. Copper or silver electrodes are therefore desirable.

Claims

1. A method of manufacture of optical fibres having at least one integral metal region therein comprising the steps of fabricating a preform having at least one aperture therein, characterised by the steps of introducing a metallic medium into said aperture, subjecting said preform to a reduced ambient pressure substantially to remove entrapped air therefrom and drawing an optical fibre containing a metal region therein from said preform.

2. A method of manufacture of optical fibres having at least one integral metal region therein as claimed in claim 1 characterised in that said aperture is a hole formed by drilling said preform with a diamond drill.

3. A method of manufacture of optical fibres having at least one integral metal region therein as claimed in claim 1 characterised in that said aperture comprises a glass tube.

4. A method of manufacture of optical fibres having at least one integral metal region therein as claimed in any one of the preceding claims characterised in that said metal is introduced into said aperture by a process of pumping.

5. A method of manufacture of optical fibres having at least one integral metal region therein as claimed in any one of the preceding claims characterised in that the preform is spun duri ng the drawi ng process .

6. A method of manufacture of optical fibres substantially as herein described with reference to and as shown in the accompanying drawings.

7. A fibre optic device incorporating a fibre produced by a method as claimed in any one of the preceding claims and having a plurality of elongated metal electrodes embedded therein.