GB2289770A - Writing bragg gratings in photosensitive waveguides - Google Patents

Abstract

A Bragg reflection grating is written in an optical fihre 11 or other waveguide using e.g. ultraviolet light incident laterally via a phase grating 10. The light does not flood the grating, hut is confined to a small area tracked along the grating in the axial direction of the fibre. The light intensity and/or tracking speed may be modulated to provide chirp according to a desired recipe. Shown are argon ion laser 12, telescope 13, aperture 14, mirror 15, stepper drive 17 and cylindrical lens 16. <IMAGE>

Description

BRAGG GRATINGS IN WAVEGUIDES This invention relates to the creation of Bragg gratings in photosensitive optical waveguides. It is known that such a grating can be created by illuminating the photosensitive waveguide from the side, writing the lines simultaneously with an interferometrically generated grating fringe pattern of light. Such a fringe pattern can be created using two-beam interferometry, or as a localised fringe pattern generated in the vicinity of a diffraction grating, typically a phase grating, through which light is caused to pass.

In the case of Bragg gratings whose lines are created simultaneously from an interference fringe pattern generated in the vicinity of a diffraction grating, one of the problems in creating a narrow-band reflection grating of high reflectivity lies in obtaining sufficient light of sufficient uniformity over a sufficient length of the waveguide. In this context it will be understood that if the intensity of the light, measured as a function of position along the waveguide, varies significantly, for instance with a Gaussian distribution, then the photo-induced refractive index changes produced in the waveguide will also vary as a function of position. In consequence, though the physical pitch of the grating is uniform, the effective pitch, being the product of physical pitch with effective refractive index, will be non-uniform, with the result that the spectral selectivity of the reflector is impaired.

The present invention is directed to the circumventing or at least ameliorating these Gaussian light distribution occasioned problems.

This affords the possibility of creating narrow-band Bragg reflection gratings of high reflectivity in optical waveguides, and also provides a manufacturing method that is relatively easily adapted to the production of chirped gratings conforming to desired recipes.

According to the present invention there is provided a method of creating a Bragg grating in a photosensitive optical waveguide, which grating is created by traversing a beam of electro-magnetic radiation along the waveguide, which beam is incident upon the waveguide through a diffraction grating located adjacent the waveguide and oriented to have diffracting elements extending at an angle to the waveguide axis.

If the beam intensity and speed of traversal are both kept constant, a substantially uniform Bragg grating results whereas, by varying either or both of these parameters in a controlled manner, a chirped Bragg grating can be created to a desired recipe.

There follows a description of the creation, by a method embodying the invention in a preferred form, of a Bragg reflection grating in an optical fibre waveguide. The description refers to the accompanying drawings, in which: Figure 1 is a schematic diagram of the apparatus employed to create the grating, Figure 2 is a schematic diagram of apparatus employed to analyse the spectral properties of the grating so formed, and Figure 3 is a plot of the spectral properties of the grating.

The writing of the Bragg reflection grating required the use of a diffraction grating. In this instance the diffraction grating took the form of a phase grating (phase contrast diffraction grating). This grating was created in a plasma enhanced chemical vapour deposited silica layer deposited upon a silica substrate. For this purpose the deposited silica layer was coated with a layer of chromium that was itself created with a layer of electron beam lithography resist. A grating 50mm in length was created in the chromium layer by electron beam lithography. With the particular equipment employed it was not found possible to scan the electron beam over the full 50mm without incurring significant distortions or discontinuities, to the linear scan, and so a mechanical step and repeat procedure was employed to create the full length of grating in sections. Such mechanical stepping can itself introduce large discontinuities but precautions were taken to reduce this to minimal levels by careful attention to the calibration of the electron beam scan field and correction of scan distortions after the manner set out by C Dix et al., "High accuracy electron-beam grating lithography for optical and optoelectronic devices J. Vac. Sci. Technol. B, 1992, 10(6), pp 2662.

The electron beam lithography resist was patterned and developed to provide an etch mask for the underlying chromium layer. Then the chrome layer was etched to provide a mask for the silica layer that underlied it, and finally the chromium mask layer was etched away to leave the required phase grating, which is depicted at 10 in Figure 1.

To use this phase grating to create a Bragg reflection grating in a length 11 of single mode optical fibre according to a method embodying the present invention in a preferred form, the phase grating 10 is located almost in contact with fibre 11 with the grating lines extending transversely of the fibre axis, preferably at right angles to that axis. An argon ion laser 12 providing a frequency doubled ultraviolet light output has this output directed on to the phase grating 10 via a telescope 13, and aperture 14, a mirror 15, and a cylindrical lens 16. The mirror 15 is attached to a stepper drive 17 by means of which the mirror can be translated in a controlled manner in the axial direction of the fibre 11 so as to track the light beam across the phase grating 10 in the axial direction of the fibre 11. For monitoring purposes, the fibre 11 was spliced at 18 to a 3dB fibre coupler 19 so that light from an ELED 20 could be launched into the fibre and the reflection monitored on a spectrum analyser 21. This monitoring arrangement is suitable for monitoring the creation of the Bragg reflector, but does not have the spectral resolution to measure the bandwidth of that reflector. This can be measured, as depicted in Figure 2, by replacing the ELED 20 with a tuneable diode laser source 22 which is scanned under computer control in 0.001 nm (125 MHz) spectral steps across the bandwidth of the Bragg reflector while the reflected and transmitted powers are measured on two optical power meters 23. While bandwidth measurements are made, the 3dB fibre coupler 19 and the fibre 11 (with its Bragg grating represented by lines 24) are preferably kept in a temperature controlled housing 25 in order to minimise errors in the results attributable to the effects of temperature upon the Bragg grating.

Figure 3 depicts the results obtained in respect of the creating of a Bragg grating in boron/germania doped single mode optical fibre housing a core-cladding index difference of 7x10-3 and a core diameter of 7lim. The Bragg reflector was created using a phase grating 40mm long in association with a light source emitting at 244nm and providing 50mW of cw power in a spot which at the phase grating measured approximately 3mm long (in the axial direction of the fibre) by 125Rm wide (to match the fibre diameter).

The spot was traversed the full length of the grating in a single pass at a uniform speed of lOmm per minute. Traces (a), (b) and (c) of Figure 3 are respectively the measured transmission, measured reflection and calculated (assuming rectangular intensity profile and uniform exposure) reflection spectral characteristics of the reflector, and show it to have a peak reflectivity of about 50% and a spectral line width of about 0.03nm. In respect of the other Bragg gratings made in the same way, with written lengths in the range 40 to 50mm, reflectivities of up to about 70% were obtained. For purposes of comparison, it may be noted that the paper by Archambault et al entitled 'High reflectivity and narrow bandwidth fibre gratings written by single excimer pulse', Electronics Letters, Vol. 29, 1993, pp 28-29 quotes, for the exemplified narrow bandwidth reflector, a bandwidth of 0.05 nm and a peak reflectivity of only 7%.

A particular feature of creating Bragg reflection gratings by the method of the present invention is that, by choosing to vary the rate of traversal of the beam scanning, or the intensity of the light during the scan (or by varying both), it is possible to write chirp and shading into the grating according to a specific recipe.

While the specific description has related exclusively to the creation of Bragg reflectors in optical fibres, it will be evident that this method is also applicable to the creation of Bragg reflectors in other forms of optical waveguide such as those created in or on planar substrates.

Claims (7)

CLAIMS;

1. A method of creating a Bragg grating in a photosensitive optical waveguide, which grating is created by traversing a beam of electro-magnetic radiation along the waveguide, which beam is incident upon the waveguide through a diffraction grating located adjacent the waveguide and oriented to have diffracting elements extending at an angle to the waveguide axis.

2. A method as claimed in claim 1, wherein the intensity of the beam of electro-magnetic radiation is modulated during the traversal.

3. A method as claimed in claim 1 or 3 wherein the rate of traversal of the beam of electro-magnetic radiation is modulated during the traversal.

4. A method as claimed in claim 1, 2 or 3, wherein the beam of electromagnetic radiation is a beam of ultraviolet light.

5. A method of creating a Bragg grating substantially as hereinbefore described with reference to Figure 1 of the accompanying drawings.

6. An optical waveguide provided with a Bragg grating by the method claimed in any preceding claim.

7. An optical waveguide as claimed in claim 6, which waveguide is an optical fibre.