WO1993002018A1

WO1993002018A1 - Light transmitting device having regions of differing refractive index

Info

Publication number: WO1993002018A1
Application number: PCT/AU1992/000354
Authority: WO
Inventors: Simon Poole; Mark Sceats
Original assignee: The University Of Sydney; Telstra Corporation Limited
Priority date: 1991-07-15
Filing date: 1992-07-15
Publication date: 1993-02-04

Abstract

A method of forming periodic regions of differing refractive index within a glass light transmitting device, typically an optical fibre, and in which the device is formed in a manner to create weak chemical bonds that have a weak binding energy relative to that of oxygen-linking bonds in the device. Tensile stress is established within a portion of the device containing the weak bonds at a level which permits preservation of at least a majority of the weak bonds. The device after its formation is exposed to optical irradiation at the spaced-apart periodic regions, the irradiation having an energy level which is sufficient to break the weak chemical bonds and thereby effect a stress level reduction and consequential refractive index increase in the exposed periodic regions of the device.

Description

LIGHT TRANSMITTING DEVICE HAVING REGIONS OF DIFFERING REFRACTIVE INDEX

TECHNICAL FIELD

This invention relates to light transmitting devices which incorporate periodic regions of differing refractive index and to a method of producing such devices. The invention has particular application to optical fibres, including multi-core fibres, and is hereinafter described in the context of such fibres. However, it will be understood that the invention may have broader application, for example to planar waveguides and other light transmitting devices. BACKGROUND ART

Optical fibres having localised regions of differing refractive index are known in the art and have various applications. For example, they may be employed for effecting relative dispersion of light of different wavelengths and for enhancing or creating non-linear optical properties that may be used in the control of optical information. Additionally, they may be employed as filters, by reflecting and transmitting light of specific wavelengths, and they may be used to provide coupling between modes of a fibre or between the cores of a multi-core fibre. Furthermore, they may be employed as sensors or modulators in devices which are employed for measuring changes in such physical characteristics as strain and temperature.

Localised regions of differing refractive index have been produced in planar waveguides and optical fibres, using prior art techniques, by shaping (e.g. corrugating) a surface which is located at or near the light propagating region of the waveguides and fibres. Various techniques have been employed for this purpose, including photo or electron beam lithography and, in the case of fibres, the required surface has been produced after removing the relatively thick cladding at the required locations and/or by using a D-shaped fibre. Also, regions having different refractive indices have been produced in planar waveguides and fibres, again using prior art procedures, by^* doping the waveguides and fibres using such techniques as ion implantation, electron beam irradiation and in-diffusion processes. Here again, in the case of fibres it has been a requirement that the cores be located close to the surface of the fibres in order to facilitate core penetration. Furthermore, disclosures have recently been made in respect of a technique for modifying the properties of optical fibres, in which it has been proposed that localised regions of the fibre be exposed to UV radiation for the purpose of modifying the structure within the core of the fibre. DISCLOSURE OF THE INVENTION

The present invention is directed to a development of the latter approach and it flows from a discovery made by the inventors that, by creating weak chemical bonds

(i.e., "wrong" bonds) within a light transmitting device, forming the device in a manner to induce tensile stress within the or each portion of the device containing the weak bonds, whilst preserving the weak bonds, and exposing localised regions of the device to optical radiation, the exposed regions are caused to exhibit a reduction in stress level and a consequential increase in the level of refractive index. Therefore, the present invention provides a method of forming regions of differing refractive index within a glass light transmitting device and which comprises the steps of:

(a) forming the device in a manner to create weak chemical bonds that have a weak binding energy relative to that of oxygen-linking bonds in the device,

(b) establishing a tensile stress within at least a portion of the device containing the weak bonds, during its formation, at a level which permits preservation of at least a majority of the weak bonds, and

(c) exposing spaced-apart regions of the device to optical irradiation having an energy level which is sufficient to break the weak bonds and thereby effect a stress level reduction and consequential refraction index increase in the exposed regions of the device.

Oxide glasses are amorphous materials which consist of a network of one or more types of glass-forming atoms such as Si, Ge, B, P and As bonded to a number of oxygen atoms. Additional, conditional, glass-forming atoms such as Te, Se, Mo, W, Bi, Al, Ga and V may also be incorporated. Generally, these various glass-forming atoms are characterised by being linked to one another through an intermediate oxygen atom and the linking bonds are referred to herein as "oxygen-linking bonds". However, it is possible that the atoms may be directly bonded without an intermediate oxygen atom and such bonds are referred to herein as "weak chemical bonds", "weak bonds" or "wrong bonds".

In breaking the weak bonds at the irradiated sites, stress reduction occurs with a rearrangement of the lattice structure in the portion of the device containing the weak bonds. This in turn results in an increase in the refractive index as a consequence of the stress-optic effect. The weak bonds may be broken under the influence of either single or multi-photon absorption.

The invention also provides a glass light transmitting device per se. having regions of differing refractive index, when formed by the above defined method. The device may comprise a length of optical fibre of any desired configuration, a planar waveguide or other light transmitting device.

When the device comprises an optical fibre the weak bonds may be created during formation of the preform by depositing the core and/or cladding under oxygen deficient conditions and/or by doping the portion of the fibre which is to contain the weak bonds with at least one dopant material that is selected to contribute to the weak chemical binding. Preferably, the dopant material(s) also has/have the property of enhancing differential contraction of the core and/or cladding during drawing of the fibre. That is, the dopant is 01

- 4 - preferably selected to aid in the creation of tensile stress in the core and/or the cladding at a level which is as high as possible (typically in the order of 5MPa to 600MPa) but which is not so high as to cause breakage of a significant number of the weak bonds during the fibre drawing process.

Whilst the regions of differing refractive index may be located in the core or the cladding or both the core and the cladding of an optical fibre, in most applications of the invention such regions will be located in the core. In such case the cladding of the fibre may be formed in the usual way from Siθ2, optionally with 2O5 and F dopants, and the core may be formed as a Geθ2 doped Siθ2 structure. Such cladding and core structures have Si-0 and Ge-0 bonds and, as above mentioned, the weak bonds (or "wrong" bonds) , for example Ge-Ge, Si-Si or Ge-Si bonds, may be created by reducing the pressure of O2 carrier gas during vapour deposition of the core and/or the cladding in the fibre preform. The weak bonds might alternatively or additionally be created by substituting O2 with an oxygen deficient carrier gas and/or by depositing the Ge doped regions at a higher-than-optimum temperature which encourages the formation of a Ge sub-oxide. Alternatively or additionally, the weak bonds may be created by doping or co-doping the core and/or the cladding with aluminium, boron, titanium, lead, bismuth, arsenic, selenium, phosphorous or such other dopant as will contribute to the creation of weak bonds. Materials that have been found to be particularly suitable for doping the stress-producing regions include ^B2°3' ~⁵2°5' ^A-~2°3 ^anc- ^Geθ2' either individually or in various combinations.

External photo-irradia ion of the device may be effected by direct single-beam irradiation from a laser source and by either masking the fibre or by periodically advancing the fibre relative to the source. This approach is suitable for the production of a grating - 5 - having millimetric periodicity. Gratings having micrometric or sub-micrometrie periodicity may be produced by interferometric processes.

Internal photo-irradiation may be effected by propagating light through the fibre in a manner so as to create standing waves with nodes positioned to cause irradiation at the required spacings.

The irradiation will typically be effected at an energy level in the range 2eV to 7eV. The effect of the photo-irradiation may be increased by optimising the initial stress levels of the regions containing the weak bonds. This may be achieved either by optimising the stresses generated when doping the regions containing the weak bonds or, alternatively, by introducing other regions within the fibre which act on the regions containing the weak bonds so as to optimise the induced stresses within those regions. Such stresses may be introduced in a circularly symmetric form or in an asymmetric manner, the latter being known from prior art literature to form highly birefringent fibres based on the stress-optic effect.

The invention will be more fully understood from the following description of a preferred method of producing a grating in an optical fibre, the description being provided with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:

Figure 1 shows a schematic representation of a modified chemical vapour deposition process for forming a fibre preform,

Figure 2 shows a schematic representation of a method of drawing a fibre from a preform,

Figure 3 shows a schematic representation of an arrangement for side writing or photo-irradiating a length of fibre, and

Figure 4 shows a schematic representation of an alternative arrangement for side writing or photo-irradiating a length of fibre. 018

MODE FOR CARRYING OUT THE INVENTION

The fibre itself is produced by joining two 100cm lengths of OPTIQ-100 (trade mark) fused quartz tubing having outside and inside diameters of 20mm and 16mm respectively and by soaking the tubing for 12 hours in a PYRONEG (trade mark) solution. The tube is thoroughly rinsed with demineralised water and acetone and is then dried with a flow of nitrogen. The cleaned and dried tube is placed in a glass working lathe, as indicated schematically in Figure 1, and the exterior of the tube is wiped with acetone.

The tube is rotated in the lathe and warmed with a natural gas torch which is passed along the exterior of the tube. The interior of the .tube is etched with a flow of 0₂ saturated with SFg and the 0₂ flow is maintained throughout ensuing depositions. The temperature of the torch is adjusted to approximately 1700°C and a Siθ2/ 2-*5/F cladding region is deposited using the conventional modified chemical vapour deposition (MCVD) process with the sources of Si0₂, P2O5 and F being SiCl₄, POCI3 and SFg respectively. The cladding region is deposited over 15 passes of the torch.

Introduction of SiCl₄, POCI3 and SFg is terminated and the torch temperature is reduced to approximately 1200-C. Then, SiCl₄ and (2-6 wt.%) GeCl₄ are introduced to deposit a Siθ2/Geθ2 frit.

One end of the tube is then sealed by melting and the tube is removed from the lathe.

A pre-prepared solution of aluminium chloride (AICI3) (approximately 1.0 wt.%) is poured into the tube and the tube is stored for approximately 12 hours to allow the frit to be wet by the solution. The solution is prepared by dissolving the AICI3 in water. Excess solution is drained from the tube, the sealed end is cut from the tube and the exterior of the tube is cleaned with acetone prior to re-mounting the tube in the lathe. - 7 -

When remounted, the tube in the lathe is heated at a temperature below 1000-C with a flow of 0₂ being introduced to convey free water from within the tube. Thereafter, the torch temperature is increased to approximately 2000°C to sinter the frit and collapse the preform. Optionally, a slight negative pressure may be maintained during the collapsing process to ensure that the core of the final fibre will be elliptical in shape if a birefringent fibre is required. The preform is removed from the lathe and its refractive index is measured for classification purposes.

Optical fibre is drawn from the preform in the conventional manner, using a fibre drawing/coating system as shown schematically in Figure 2. The level of stress within the fibre may be altered as required by changing the fibre drawing conditions.

Weak bonds that exist within the preform core as a consequence of co-doping with the Ge and Al are preserved during drawing of the fibre and creation of tensile stress which is established during contraction of the core and cladding. The bonds are subsequently broken by photo-irradiating the side of the fibre, and it has been established that such irradiation causes a reduction in the tension in the glass network and a consequential increase in the refractive index of the glass (due to the stress-optic effect) in the irradiated regions.

Photo-irradiation of the core may be effected in the manner illustrated in Figure 3, using an Excimer laser which provides a 248nm output. The fibre is caused to traverse through the laser beam in incremental steps and irradiation is effected by way of the illustrated mask.

Alternatively, photo-irradiation of the core may be effected holographically in the manner illustrated schematically in Figure 4, using an Excimer pumped dye laser which provides a frequency doubled 488nm output to an interferometer. The interferometer comprises a cylindrical lens, beam splitter and mirror surfaces arranged to provide for beam interference and, thus, the - 8 - required interference pattern at the side of the optical fibre that is to be irradiated.

It is desirable that the fibre be subjected to single pulse exposures, to obviate problems associated with lack of stability in the irradiating system, but in any case an energy level in the order of 2 to 7 eV will typically be required to effect breaking of a significant number of the weak bonds for the purpose of achieving significant increases in the refractive index at the irradiated sites. An energy density of approximately

10mJ/cιrr is typically required to achieve a significant index change.

Optical fibres produced in accordance with the prescribed method have exhibited localised refractive index increases in the order of 10 .

It is known that fibre polarisation rocking filters, which show a strong dependence of transmission within wavelengths for a given input polarisation state, can be fabricated by perturbing a birefringent fibre in such a manner that the perturbation beat length matches the birefringent beat length between the two polarisation modes. The perturbations act to couple light launched on one polarisation state of the fibre into the other polarisation state, with the coupling being highly wavelength dependent because it only occurs where the birefringent beat length is equal to the perturbation beat length and the birefringent beat length is highly dispersive (wavelength-dependent) . These perturbations can be induced by suitably irradiating the fibre using an Excimer laser operating at 248nm to produce a modulation of the fibre birefringence as a consequence of the stress relief which is derived from the above described procedure. Using this technique on a fibre with a beat length of approximately 6.5mm at 1.55μm, polarisation coupling efficiencies of greater than 50% at 1500nm have been achieved, this corresponding to a localised refractive index change of 0.5 x 10 .

Claims

THE CLAIMS

1. A method of forming regions of differing refractive index within a glass light transmitting device and which comprises the steps of: (a) forming the device in a manner to create weak bonds that have a weak binding energy relative to that of oxygen-linking bonds in the device,

2. The method as claimed in claim 1 wherein the device comprises an optical fibre and wherein the core of the fibre is deposited during formation of a preform to provide for the creation of the weak bonds within the core.

3. The method as claimed in claim 1 wherein the device comprises an optical fibre and wherein the cladding of the fibre is deposited during formation of a preform to provide for creation of the weak bonds within the cladding.

4. The method as claimed in claim 2 or claim 3 wherein the portion of the fibre which contains the weak bonds is deposited under oxygen deficient conditions.

5. The method as claimed in claim 2 wherein the core is deposited as a doped-silicate core, wherein the dopant is selected from any one or more of aluminium, titanium, lead, bismuth, arsenic, selenium, phosphorous, boron, and germanium.

6. The method as claimed in claim 2 wherein the core is deposited as a germano-silicate core co-doped with a dopant selected from aluminium, titanium and lead.

7. The method as claimed in claim 2 wherein the preform is formed by the modified chemical vapour deposition process, wherein the cladding is deposited at least predominantly as a Siθ2 layer, wherein the core is deposited as a Siθ2~ eθ2 frit and wherein the core frit is wetted with a solution containing an aluminium salt.

8. The method as claimed in any one of the preceding claims wherein the optical irradiation is effected by exposing the device externally to photo-irradiation.

9. The method as claims in claim 8 wherein the device is exposed to an interference pattern having a sub-micrometric periodicity.

10. An optical fibre having periodic regions of differing refractive index and produced by a method which comprises the steps of: forming the core and/or the cladding of the fibre in a manner to create weak chemical bonds that have a weak binding energy relative to that of oxygen-linking bonds in the fibre, establishing a tensile stress in the or each portion of the fibre containing the weak bonds during forming of the fibre, the tensile stress being limited such that at least a majority of the weak bonds are preserved, and exposing periodic regions of the fibre to optical irradiation at an energy level which is sufficient to break the weak bonds and thereby effect a stress level reduction and consequential refractive index increase in the periodic regions.

11. The optical fibre as claimed in claim 10 wherein the core alone is formed in a manner to create the weak chemical bonds and wherein the core is formed as a doped silicate core with the dopant comprising one or more of aluminium, titanium, lead, bismuth, arsenic, selenium, phosphorous, boron and germanium.

12. The optical fibre as claimed in claim 10 wherein the cladding is composed at least predominantly of Siθ2 and the core comprises Siθ2 co-doped with Geθ2 and a dopant selected from aluminium, phosphorous, boron or a mixture thereof.