GB2213142A

GB2213142A - Manufacture of optical fibres

Info

Publication number: GB2213142A
Application number: GB8824696A
Authority: GB
Inventors: Noorallah Nourshargh
Original assignee: General Electric Co PLC
Current assignee: General Electric Co PLC
Priority date: 1987-10-23
Filing date: 1988-10-21
Publication date: 1989-08-09
Also published as: GB8824696D0; GB8724892D0

Abstract

A method of fabricating a wavelength selective optical fibre comprising the step of pulling a fibre preform to form a fibre, whilst simultaneously applying periodic perturbations to that fibre still in a softened condition, so as to introduce corresponding periodic variations in either the fibre refractive index or the fibre core radius or both.

Description

Manufacture of Optical Fibres This invention relates to a method of fabricating optical fibres in which either the core radius or the refractive index or both vary periodically, having equally spaced perturbations.

If the periodicity of the perturbations in the fibres is chosen correctly, extremely wavelength-sensitive behaviour can be induced in the fibres. For example, a fibre with a periodic refractive index variation along its length can be made to reflect a very narrow band of wavelengths whilst transmitting all other wavelengths forming a fibre grating. Similarly, a sinusoidal variation of the fibre core radius along its length can be used to couple a narrow band of wavelengths either to another guided mode (in a multimode fibre) or to radiation modes (in a monomode fibre) as described in IEE Proceedings Viol .134 PT.J No.3 pp.197-202.

Furthermore, by varying the periodicity of the perturbation, the affected waveband can be centred on to any desired wavelength.

Wavelength selective fibres have a wide range of applications and ay be used in wavelength division multiplexers, sensors and reflectors (such as cavity lasers). Two or more fibres with different perturbation periodicities can be combined to form directional couplers or Mach Zehnder interferometers.

In all of these applications, such fibre gratings have the extremly important advantage of being suitable for direct fusion splicing to conventional fibres, thereby eliminating the problems of alignment and packaging associated with the use of conventional grating structures.

Fibre gratings with a periodically varying refractive index are commonly made using a high power Ar+ laser; interferometric techniques are used to induce the variation of refractive index in the fibre core by taking advantage of the photorefractive properties of Germanium Oxide (O2). Only short lengths of fibre grating (of the order of im) can be manufactured by the process and the range of periodicities which can be produced is very limited.

The technique employed in the manufacture of fibre gratings with a periodically varying core radius can produce a wider range of periodicities but is extremely difficult to use, is very time consuming, and can only produce very short lengths (of the order of 1cam) of grating. By this process, nearly all of the fibre cladding is polished away, the fibre is coated with photoresist and the photoresist is exposed holographically. The fibre may then be plasma etched.

It is an object of this invention to provide a method of fabricating fibre gratings of any length with precise control of the periodicity of the perturbations over an extremely wide range.

The invention provides a method of fabricating a wavelength selective optical fibre comprising the step of pulling a fibre preform to form a fibre, whilst simultaneously applying periodic perturbations to that fibre still in a softened condition, so as to introduce corresponding periodic varations in either the fibre refractive index or the fibre core radius or both.

The prturbations may consist of vibrations applied to the fibre, e.g. axial or transverse vibrations. Both the frequency and the amplitude of the vibration have a direct influence on the optical performance of the fibre. The frequency of the vibration determines the periodicity of the fibre and hence its operating wave-length.

The amplitude of the vibration, on the other hand, determines the degree of perturbation of the fibre and affects the length of the fibre which must be used; the greater the perturbation, the shorter the required length of fibre. In practice, the magnitude of the perturbation is often kept below a small fraction of 1% in order to prevent the fibre becoming lossy.

Various nethods of vibrating the fibre may be used. The source of the variations can be electromechanical, electromagnetic or piezoelectric.

The variations nay be introduced either before or after the fibre is introduced into the primary coating furnace. Vibrating the fibre before it is coated has the advantage that the source of vibrations will be close to the hot zone of the preform and will therefore imprint the periodic variation on the fibre more efficiently.

Alternatively, the vibrations may be imparted to the fibre without making physical contact with it. By using acoustic waves through a fluid medium, acoustic vibrations may be transmitted to the fibre e.g. by air friction.

As a further alternative, electrostatic charge may be sprayed onto the fibre which is then passed through an alternating electric field. If an A.C. voltage is applied to a parallel plate capacitor through which the fibre passes, for example, this will create an alternating electrostatic force on the fibre and cause vibration.

As an alternative, the periodic perturbations are introduced thermally by periodically softening further the said fibre as it is pulled from the fibre preform.

Example of apparatus and of methods in accordance with the invention will now be described with reference to the accompanying drawings in which: Figures 1, 2(a) and 2(b) show the application of transverse and axial vibration to the fibre; Figure 3 shows the application of acoustic waves to the fibre to induce transvrse and axial vibration; Figure 4 shows the application of an alternating electrostatic force to the fibre to induce transverse and axial vibration; Figures 5(a) and 5(b) show the forms of the perturbation induced by the vibration of the fibres; and Figures 6(a), 6(b) and 6(c) show alternative ways of inducing perturbations in the fibre by softening the fibre with a laser.

In Figure 1, an optical fibre preform 1 is stretched using a standard fibre pulling tower to form an optical fibre 2 whilst either axial or transverse vibrations are applied to the stretched preform.

Figure 2 shows that the vibration may be applied using a pair of pulley wheels 3 around which the fibre 2 is fed, one (Fig. 2(b)) or both (Figure 2 (a)) of the wheels 3 being vibrated either axially or transversely.

The fibre 2 may pass through an coustic cavity 4 as shown in Figure 3. The acoustic signal in the cavity generates periodic perturbations in the fibre 2 which may be transverse or axial depending on whether the acoustic waves travel parallel or perpendicular to the fibre. For optimum operation, an acoustic cavity must be used whose dimensions are designed such that the fibre is placed at the node of the standing wave pattern. The periodicity and depth of the gratings formed are determined by the frequency and intensity of the acoustic signal respectively; both can be readily adjusted. The efficiency of the process may be improved by introducing a suitable fluid of predetermined viscosity and pressure into the cavity.

Alternatively, as shown in Figure 4, electrostatic force may be used to vibrate the fibre. Electrostatic charge 5 is sprayed onto the fibre 2 using a sharp pointed conductor. Applying an A.C.

voltage to a parallel plate capacitor, through which the fibre passes, will then create an alternating electrostatic force on the fibre and thus cause vibration.

Figure 5 shows schematically two possible periodic perturbations that might arise. In Fig. 5(a) the fibre radius varies sinusoidally between a minima r and a maximum R; in Fig. 5(b) the refractive index varies sinusoidally between a minimum nl and a maximum n2. For practical applications, the pitch/9 of the perturbation will have to be similar in magnitude to the desired centre wavelength of the grating (of the order of 1 pom). If the fibre pulling speed is lm/s, the vibration needed will be 1MHz.

The required frequency can be reduced by pulling the fibre at a lower speed. For example, if the pulling speed is reduced to less than Im/min, one can use an audio frequency to achieve the desired grating pitch. The amplitudes of the periodic perturbations is chosen depending on the application but will typically be of the order of 1X.

Instead of vibrating the fibre, similar periodic perturbations of radius and/or refractive index may be achieved by softening the fibre as it is pulled from the fibre preform, as shown in Figure 6.

A modulated CO2 laser is focussed onto the fibre. The output power of the laser is adjusted such that it softens the fibre.

As shown in Fig. 6(a), repeated applications of C02 laser pulses combined with the tension applied to the fibre due to pulling will result in a periodic variation of the fibre geometry and/or refractive index. The modulation frequency and the output power of the laser can be adjusted to produce a periodic perturbation of the fibre with the desired pitch and amplitude.

In order to achieve a more uniform heating of the fibre, it is preferred to use two or more laser beams (Figure 6 (b)) and/or reflectors (6, Figure 6 (c)) postioned around the fibre.

Claims

1. A method of fabricating a wavelength selective optical fibre comprising the step of pulling a fibre preform to form a fibre, whilst simultaneously applying periodic perturbations to that fibre still in a softened condition, so as to introduce corresponding periodic variations in either the fibre refractive index or the fibre core radius or both.

2. A method according to Claim 1, in which the periodic perturbations consist of vibrations applied to the fibre.

3. A method according to Claim 2, in which the source of the vibrations is electromechanical, electromagnetic or piezoelectric.

4. A method according to Claim 2 or 3, in which the vibrations are applied directly to the fibre by the same means as is used to pull the fibre.

5. A method according to Claim 2, in which the vibrations are applied by subjecting the fibre to acoustic waves transmitted through a fluid medium.

6. A method according to Claim 2, in which the vibrations are applied by first spraying the fibre with an electrostatic charge and then passing the fibre through an alternating electric field.

7. A method according to Claim 1, in which the periodic perturbations are introduced thermally by periodically softening further the said fibre as it is pulled from the fibre preform.

8. A method according to Claim 7, in which the further softening is achieved by means of at least one modulated laser beam directed onto the fibre.

9. A method of fabricating a wavelength sensitive fibre, substantially as described herein with reference to Figures 1 to 6 of the accompanying drawings.

10. A method of fabricating a wavelength sensitive fibre, substantially as described herein with reference to Figures 5 and 6 of the accompanying drawings.