WO1994022038A1

WO1994022038A1 - Improvements to optical waveguides

Info

Publication number: WO1994022038A1
Application number: PCT/AU1994/000125
Authority: WO
Inventors: Steven James Frisken
Original assignee: Telstra Corporation Limited
Priority date: 1993-03-15
Filing date: 1994-03-15
Publication date: 1994-09-29

Abstract

An optical waveguide, and a method of forming same, are disclosed. The waveguide is formed by self-focusing effects in a photorefractive material, to provide a low cost, simple waveguide, for use e.g. as a beam expander or concentrator. The material is exposed to a source of coherent radiation for a period and at an intensity such that a tapered waveguide is formed. The material may be an epoxy resin.

Description

IMPROVEMENTS TO OPTICAL WAVEGUIDES Technical Field

The present invention relates to optical systems, in particular to systems using optical fibres. Background Art

Effective low-cost coupling of light into single mode optical fibres and the incorporation of bulk devices into optical fibres remains a major obstacle to the widespread adoption of single mode fibre to the customer in communications and sensor applications. Embedding devices into the fibre by expanding the size of the mode is a promising technique for eliminating costly lenses and alignment systems. This technique relies on the fact that the larger the spot size, the less the fibre suffers from diffraction and so two fibres can be separated by distances of up to a few millimetres without introducing large losses. The angular tolerances for alignment of such a beam are much relaxed in comparison to the case of a beam expanded using the more common approach of gradient index lenses. It is important that such a transition to a large mode size be made adiabatically to ensure full coupling of light into the larger mode. This involves a tapering of the waveguide, and either up-tapering or down- tapering can be employed. Several techniques have been explored to produce a tapered waveguide including core diffusion and physical tapers of the whole fibre.

It is desired to provide an efficient, low cost coupling device, and a method of making such a device which at least partly overcomes the deficiencies of the prior art. Summary of Invention

According to one aspect the present invention provides a method for producing a optical waveguide, said waveguide being formed in a photorefractive material, comprising exposing said material to a source of coherent radiation for a period and at an intensity such that a tapered waveguide is formed.

Preferably, an optical fibre is embedded in the material and a laser source is used to provide coherent radiation to excite the fundamental mode of the fibre.

According to another aspect the present invention provides an optical waveguide, said waveguide being formed from a photorefractive material by exposing said material to a source of coherent radiation for a period and intensity such that a tapered waveguide is formed within said material.

Many materials exhibit a permanent change of refractive index upon exposure to intense light. In general these photorefractive effects are small, but can have applications such as in the fields of holography, fibre gratings and second harmonic generation. In some materials it is possible to cause refractive index changes of more than 1%, though often this requires some form of development after exposure. The present invention is predicated upon the discovery that a useful tapered waveguide may be produced by utilising the photorefractive effect in materials. In particular, a material with a positive refractive index change, that is, a self focusing material, can be induced by stimulation with a coherent optical source to form a tapered waveguide. Stimulation induces the photorefractive effect, which alters the refractive index in stimulated regions of the waveguide so as to form a taper of material with a greater refractive index to surrounding material, thereby forming a waveguide. This has been observed to be essentially a permanent effect.

Thus, the present invention provides a relatively simple and effective method of forming a waveguide, such as a beam expander, and a novel class of devices produced by the method. Brief Description of Drawings

An illustrative embodiment of the invention will now be described with reference to the accompanying figures, in which: Figure 1 illustrates diffraction from a fibre in a medium;

Figure 2 illustrates formation of a tapered waveguide;

Figure 3 illustrates an experimental arrangement for performing a preferred method according to the present invention;

Figure 4 is an intensity measurement plotting of an illustrative waveguide according to the present invention; and

Figure 5 illustrates an application for devices made using the inventive method. Description

We will initially consider the theory which is believed to describe the method according to the present invention. It is emphasised, however, that the invention is not to be considered limited to the validity or otherwise of this explanation.

Photorefractive media may be defined as those in which the refractive index is altered by the propagation of radiation. Various materials exhibit this property, as would be understood by those skilled in the art, for example epoxy materials, and various glasses such as certain hydrogenated germanium doped silicates.

Let us consider how an optical waveguide can be self-induced by a sufficiently strong pump laser. An optical fibre is inserted into a photorefractive medium and the fundamental mode of the fibre is excited. At the fibre end the light diffracts, and spreads with a radius ω given by: ω(z)=ω₀[l+(λz/πω₀ ²)₂]^1/2

where z is the distance propagated, λ is the wavelength of light and c^ is the initial 1/_e ² intensity spot radius. There is a small waist before the light diffracts with an asymptotic cone having an angle q = λ/πω₀. This can be seen in Figure

1. In the area of the waist where the light is most intense, the refractive index will change most rapidly. If the refractive index change is positive (a self-focusing material) this region will become more strongly guiding until the refractive index change saturates. The fibre has effectively grown into the waist region of the initial fibre, and so the process can continue. Further into the material the refractive index is raised slightly over a larger area by this initial exposure. A core with a slightly larger effective area will be produced as the waveguide grows into the photorefractive material. As the core size increases the beam divergence reduces correspondingly so that the rate of taper reduces with increasing distance. This is illustrated in Figure 2. All of these processes occur simultaneously, increasing the complexity of the analysis, but it is clear that a tapered waveguide can be expected. This is different to the case of self-induced fibres in nonlinear material, where the nonlinearity is considered to be instantaneous or having a finite relaxation time. The present case is equivalent to the transient stage of a self-induced nonlinear waveguide with an infinite relaxation time, or in other words produces a permanent effect.

An experimental arrangement is shown in Figure 3. The photorefractive material was prepared with a standard telecommunications optical fibre embedded in it. UV exposure was used to cure a 1 mm thick sample of a epoxy. The length of the sample was varied up to 18 mm in length. A diode-pumped 532 nm frequency doubled YAG laser coupled approximately 20 mW of CW light into the fundamental mode of the fibre and higher order modes were stripped by a loop of the appropriate radius.

The formation of an induced, up-tapered waveguide can be recognised by observing the far-field pattern of the 532 nm light. The far-field spot was reduced by a factor of up to 10 before the induced fibre became multimoded at the interface and interference rings could be observed. This corresponds to a ten fold increase in the fundamental spot size at 532 nm. The far-field pattern was then measured at 1550 nm using the knife-edge technique at a distance of 33 mm. For an induced fibre of length 5 mm and assuming a Gaussian shape, the far-field 1/e2 intensity cone occurred at an angle of 0.014 radians. This result is shown in Figure 4. The calculated increase in the near field spot size for light of 1550 nm wavelength was approximately 5-fold, with a 1/e intensity spot diameter of 50 μm for the expanded waveguide. This is consistent with the visible light (532 nm) measurement when the V-values of the modes at the respective wavelengths are taken into consideration.

Attenuation of the longest tapers was measured to be less than 0.5 dB/mm at 1550 nm, which includes material absorption, reflection losses and light lost to the waveguiding region. Measurements at 980 nm found negligible loss, so it is likely that the attenuation at 1550 nm is material and wavelength dependent.

Depending upon the curing time, the formation of the tapered waveguide took place on a time scale varying from a few seconds to several hours. The time of the reaction was found to be linear with respect to the 532 nm intensity, and so it is probably a single photon absorption process which is responsible for the refractive index change.

There was no observable photorefractivity noted when a high-power (50 mW) 980 nm laser was used as the pump, suggesting that these devices will be stable to radiation in the communications wavelengths of 1300 nm and 1550 nm. The waveguides were left for several days and then remeasured and it was found that some relaxation had taken place, which in general improved slightly the far-field pattern by reducing the power in the wings. This led to a slightly large 1/e near field spot measurement. After 5 weeks, there were only slight changes to the observed far-field patterns, possibly due solely to measurement variations. The waveguides were then tested at high temperatures, with no degradation being observed at 1550 nm for temperatures of up to 200°C. Above this temperature, some cracking and bulk deformation of the material was observed, though the waveguide still appeared to be intact in some cases. At temperatures over 150°C there was a browning of the epoxy, implying an increased absorption in the visible.

Tapers produces according to the inventive method provide an extremely simple and effective means of incorporating devices such as filters, polarisers and Faraday rotators into fibres. The tapers show a good degree of stability, although further lifetime testing would be essential. This will in turn depend upon the material selected, as would be understood by the skilled worker in the art. From the measured spot size we would expect a separation of 5 mm in an index matched medium to be possible with an insertion loss of less than one dB. Standard fibre at this separation will give an insertion loss of 28 dB.

Other applications of this technology can be envisaged. The incorporation of dyes, saturable absorbers or nonlinear elements into the epoxy allows a simple method of producing waveguides with properties tailored to specific applications. The photorefractive effect in this material can be further utilised to produce fibre gratings or other holographic effects.

The photorefractive material used was a readily available epoxy, LOCTITE 358 UV curable adhesive.

The applications of the present inventive method, and devices produced thereby, are broad. Figure 5 illustrates the use of tapered waveguides as beam expanders and concentrators.

The beam expander consists of an input fibre, (with an angled end-face to reduce back reflection if necessary), which is embedded in a photorefractive material of low loss at the wavelength of interest. A gap of the dimensions required for the bulk element is in the middle of the device, followed by a second photorefractive material with fibre embedded in it. The two fibres are orientated so as to maximise the capture of the light which has passed through (or reflected from) the bulk device. To create a beam expander, the above device must now be correctly prepared by exposure to light as described above. The light can be at any wavelength appropriate to the photorefractive material, and preferably

(but not uniquely) the fundamental mode should only be excited at that wavelength. Each fibre is pumped with light to create the self-induced up-taper, with the light being blocked at the mid region of the device if required. Some material dependent processing may be required to stabilise or to develop the induced refractive index change.

It will be apparent that variations and additions are possible within the spirit and scope of this disclosure.

Claims

1. A method for producing a optical waveguide, said waveguide being formed in a photorefractive material, comprising exposing said material to a source of coherent radiation for a period and at an intensity such that a tapered waveguide is formed.

2. A method according to claim 1, wherein an optical fibre is embedded in the material and a laser source is used to provide coherent radiation to excite the fundamental mode of the fibre.

3. A method according to claim 2, wherein said fibre is single mode.

4. A method according to any one of the preceding claims, wherein the material exhibits an increase in refractive index after exposure to coherent radiation.

5. A method according to any one of the preceding claims, wherein the source of coherent radiation used has a different output wavelength to that intended for operative use of said waveguide.

6. A method according to claim 5, wherein the wavelength of the source is less than half the operative wavelength.

7. A method according to claim 4, wherein the material is an epoxy resin.

8. An optical waveguide, said waveguide being formed from a photorefractive material by exposing said material to a source of coherent radiation for a period and intensity such that a tapered waveguide is formed within said material.

9. A waveguide according to claim 8, wherein an optical fibre is embedded in the material and a laser source is used to provide coherent radiation to excite the fundamental mode of the fibre.

10. A waveguide according to claim 9, wherein said fibre is single mode.

11. A waveguide according to any one of claims 8 to 10, wherein the material exhibits an increase in refractive index after exposure to coherent radiation.

12. A waveguide according to any one of claims 8 to 11 , wherein the source of coherent radiation used has a different output wavelength to that intended for operative use of said waveguide.

13. A waveguide according to claim 12, wherein the wavelength of the source is less than half the operative wavelength.

14. A waveguide according to claim 1 1 , wherein the material is an epoxy resin.

15. A waveguide according to any one of claims 8 to 14, wherein said waveguide forms a beam expander.

16. A waveguide according to any one of claims 8 to 14, wherein said waveguide forms a beam concentrator.

17. A method of forming a waveguide substantially as hereinbefore described with reference to the accompanying figures.

18. An optical waveguide substantially as hereinbefore described with reference to the accompanying figures.