WO1998048303A1

WO1998048303A1 - Method of manufacture of an optical grating in an optical fibre

Info

Publication number: WO1998048303A1
Application number: PCT/GB1998/001139
Authority: WO
Inventors: Raymond John Chaney; Nigel Joseph Copner; Tingdi Liao
Original assignee: Renishaw Plc
Priority date: 1997-04-19
Filing date: 1998-04-20
Publication date: 1998-10-29

Abstract

An optical grating is created within the core of an optical fibre by exposing the fibre to ultra-violet light of uniform intensity, but cyclically varying polarisation angle. The extent to which the incident light interacts with the fibre core to change its refractive index depends upon the relative alignment of the polarisation vector of the incident light and the fibre core, and thus only part of the uniform intensity of incident light causes a change in refractive index of the core. The periodicity of the variation in polarisation angle may be linear or non-linear; alternatively individual regions of varying refractive index may be created by exposing a single region of the fibre to a field of uv light with a varying polarisation angle, and then stepping the relative displacement of the incident light and the fibre to create further regions as desired.

Description

METHOD OP MANUFACTURE OF AN OPTICAL GRATING IN AN OPTICAL FIBRE

The present invention relates to the manufacture of a grating within an optical fibre. It is known to manufacture optical fibres whose light-conducting core includes a plurality of regions (usually equi-spaced) whose refractive indices differ to that of the remainder of the core. Light travelling along the core, may at each of these regions, be refracted, either back along the fibre, or out of the fibre all together. Collectively, the regions of differing refractive index are known colloquially as " a grating", since their effect upon light travelling along the fibre is analogous in many respects to that of a conventional phase or ronchi grating to incident light. The manufacture of such gratings within optical fibres by the selective exposure of regions of the fibre to incident ultra-violet light is known, for example, from US 4,474,427, US 4,807,950 or US 5,104,209, the contents of which are hereby incorporated by reference.

The present invention proposes an alternative manufacturing technique in which an optical fibre is exposed to a uniform intensity of incident ultra-violet light along the entire longitudinal region of the fibre in which the grating is to be provided, wherein the polarisation of the incident ultra-violet light varies cyclically in the direction of the length of the fibre.

In one preferred embodiment, the pitch of the resulting grating is equal to the pitch of the cyclic variation in polarisation, and the polarisation vector of the incident light is rotating at a constant rate with displacement along the fibre.

The resultantly configured grating within the fibre may be used for a variety of purposes. For example an optical fibre with such a grating may be used to stabilise the emitted frequency of a laser diode (as described in our co- pending U.K. applications GB 9700417.0 and 9707953.7). A second independent aspect of the present invention provides an alternative laser diode configuration employing a grating within an optical fibre for frequency stabilisation of the diode, in which the emission surface of the diode, and the two ends of the optical fibre are configured so that no reflection occurs at these surfaces which would cause light either to pass back along the fibre, or into the diode. This has the effect of converting the diode and fibre apparatus from what was previously an optical gain medium (i.e. the diode) with a resonant cavity (i.e. the fibre with the Bragg grating) into a unitary optical gain medium.

Embodiments of the invention will now be described, by way of example, and with reference to the accompanying drawings in which:

Fig 1 is a schematic perspective view of a wavefront of ultra-violet light incident upon the light-conducting core of an optical fibre;

Figs 2A and 2B are graphical illustrations of the variation in polarisation of the incident ultra-violet light with displacement along the length of the fibre and the resultant variation in refractive index of the fibre core with displacement along the fibre respectively;

Fig 3 is a schematic view illustrating the effect of the incidence of light having different polarisation states upon the core of the optical fibre;

Fig 4 is a schematic representation of an apparatus for generating a polarisation distribution illustrated in Fig 2A;

Fig 5 shows a schematic view of a first embodiment of optical fibre and diode configuration;

Fig 6 shows a schematic view of a second embodiment of optical fibre and diode configuration; and

Figs 7A and 7B are graphs illustrating the operation of the embodiments of Figs 5 and 6. Referring now to Fig 1, an optical fibre includes an inner, light-conducting core 10 (which may be elliptical, polarisation preserving), and an outer layer 12. The fibre core 10 is photosensitive, having (for example) been doped with germanium, hydrogen or E_V ⁺A1₂0₃. To provide a grating within the light conducting core 10, the fibre is exposed to incident ultra-violet radiation 14 (although any wavelength which has the effect of changing the core refractive index may be used) , whose intensity is uniform with displacement D along the direction of the length of the fibre core.

Referring now to Fig 2A, in a first embodiment, the polarisation of the incident ultra-violet radiation varies cyclically with displacement D in the direction of the length of the fibre, and it can be seen from Fig 2A that the polarisation angle of the incident light varies linearly within a single period P of the cyclic variation. The effect of different polarisation angles of incident ultra-violet light upon the core 10 of the fibre is illustrated in Fig 3. The greatest level of interaction of incident ultra-violet light with the fibre core 10 occurs when the angle of the polarisation vector V of the incident light is such that the vector V extends parallel to the direction of the fibre core 10; in the illustrated example, when the polarisation state of the incident light is 0 °.

Conversely, the lowest level of interaction of the incident light with the fibre core 10 occurs when the angle of the polarisation vector is exactly perpendicular to the direction in which the fibre core 10 extends; in this example ±90° Between the two extremes the extent of interaction will vary with the variation in the polarisation angle.

Typically the exposure may be performed by pulsing of the light, which enables greater control of the extent to which the refractive index of the core is altered; continuous exposure is also possible. In the event that the fibre is over-exposed, the unwanted grating may be erased by heating, by immersion in a suitable bath of chemicals, or by exposure to uniform non-polarised radiation.

Referring to Figs 2A and 2B, it can be seen that the regions of the fibre core which have the highest modulation in refractive index n as a result of the incidence of the ultra-violet radiation lie at values of displacement D along the length of the fibre core which correspond to the incidence of ultra-violet light whose polarisation angle is 0! Conversely, regions of the fibre core having the lowest modulation in refractive index lie at displacements D where the polarisation angle of the incident ultra-violet radiation is ±90 °. Between these two extremes, the refractive index of the fibre core varies sinusoidally, thus providing an ideal Bragg grating within the fibre core.

In a modification of the embodiment of Figs 2A and 2B a short length of fibre can be exposed to a single cycle variation of polarisation of incident light. The fibre and light source are then stepped relative to each other, whereupon a further exposure takes place; this cycle is repeated to generate the requisite periodic structure. This " step and repeat" method of generating a grating differs from similar such prior art methods in that an entire "grating pitch" of the fibre is exposed, but by virtue of the polarisation variation of the incident light, only approximately half of the "pitch" undergoes a change in refractive index.

Referring now to Fig 4, incident ultra-violet light having linear cyclic variation in polarisation angle may be generated by use of an interferometer having a laser source 40 whose output beam 42 is split into sub-beams 44A,B at beamsplitter 46, and which, after passing through a pair of quarter wave plates 48A,B respectively are recombined at a further beamsplitter 50. Beamsplitter 50 is constructed such that the angle of the reflective face 52 interacts with the sub-beams 44A,B to create an interference beam 60, within which the polarisation of the resulting interference beam varies as shown in Fig 3B. The period P of the cyclic variation in polarisation depends upon the interfering angle of the beams; the period of the cyclic variation incident upon the fibre core may be further adjusted by adjusting the angle of the fibre relative to the interference beam 60.

In an alternative set-up for generating a cyclically varying polarisation field, the interfering light beams are directed off non-planar surfaces to generate a "chirped" polarisation field. This chirped field will have an increasing or decreasing (i.e. non-linear) periodicity, and the variation in the angle of the polarisation vector with displacement along the fibre may, for example, be substantially quadratic or cubic.

Optical fibres configured with gratings have a wide range of applications, such as, for example, use in the stabilisation of laser diodes. Various diode-fibre set-ups are described in Figs 5 to 7.

Referring now to Fig 5, a laser diode 110 generates an output beam 112 (travelling parallel to the optical axis A' ) which is subsequently focused by means of a lens 114 (a ball lens may be used) into the injection end 116 of a monomode, polarisation preserving optical fibre 118. The fibre 118 includes a plurality of regions 120 whose refractive indices differ to that of- the remainder of the fibre (typically up to a difference in refractive index of approximately one part in ten thousand) , and are spaced apart by a distance which is equal to the desired output wavelength of the laser diode (and thus which corresponds to the desired output frequency) . Collectively, the regions 114 are referred to as a Bragg "grating" 122. Together, the fibre 110 and grating 122 operate to stabilise the frequency of light emitted from the diode, and to reduce the frequency bandwidth over which such light is emitted. This occurs because when the diode emits a wavefront of light corresponding to the desired output frequency, a proportion of the aforementioned wavefront is reflected at the grating 122 back into the diode 110 (since the spacing between adjacent regions 120 is equal to the wavelength which corresponds to the desired output frequency) . The reflected part of the wavefront results in the diode emitting a greater proportion of photons with the requisite frequency/wavelength, and the bandwidth of the emitted light beam reduced and centralised about the desired frequency/wavelength. N.B. The reflectivity of grating 122 at the optimum desired frequency for the laser diode 110 is 90%, thereby permitting the emission of laser light for use in, e.g., metrological devices, such as interferometers.

The output face 124 of the diode 110 is cleaved at an angle relative to the output direction of the laser light 112, as are the surface 126 at the injection end 116 of the fibre

118, and the surface 128 at the remote end 130 of the fibre 118. Configuring of these surfaces provides the following benefits:

1. Light which is emitted from the diode 110 and reflected at grating 122 is prevented from being reflected back up the fibre at the injection end 116;

2. Light which is reflected at the grating 122 and passing out of the injection end 116 of the fibre back into the diode is prevented from being reflected back up the fibre at the emission surface 124 of the diode;

3. Light which is passing through the grating 122 (i.e. is unreflected at the grating) is prevented from being reflected back up the fibre at surface 128 at the remote end 130 of the fibre;

4. Light which is emitted from diode 110 is prevented from being reflected back into the diode at surface 126 at the injection end 116 of the fibre; and

5. Light is prevented from being reflected between the front and rear surfaces of the diode.

It should be noted that the surfaces 124 and 126 may be cleaved in opposite directions, as are the surfaces 126 and 128. This ensures that adjacent cleaved surfaces do not co-operate to form a weak resonant cavity such as an etalon. Thus, the only surfaces within the diode apparatus which have a reflective action in the direction of the optical axis A' are the rear reflective surface 132 of the diode, and the Bragg grating 122. This greatly reduces the probability of laser light resonating within one of a number of possible cavities. Rather, there is now only a single cavity created by the rear reflective surface 132 of the diode and the grating 122 of the fibre.

Referring now to Figs 7A and 7B, the reflectivity of the Bragg grating with variation in frequency is illustrated.

It can be seen from Fig 7A, that the Bragg grating behaves as a distributed Bragg reflector with a relatively narrow bandwidth, centred about frequency f₂. Referring to Fig 7B, the optical gain of the device with variation in frequency is illustrated, and the longitudinal modes of the overall cavity (defined by the reflective surface 132 and the grating 122) are shown. The combination of the optical gain characteristics and the distributed Bragg reflector characteristics mean that a unique mode of operation of the laser diode is selected with this configuration of apparatus, since the grating 122 will not reflect a significant quantity of light at any mode of operation of the laser diode other than f₂. This configuration therefore prevents mode-hopping. In a modification of the apparatus of Fig 5, the emission surface of the laser diode is coated with an anti- reflective coating, as is illustrated in Fig 6. This has the equivalent effect to the apparatus of Fig 5, but may, in certain circumstances, be more economical to manufacture.

In the embodiments of Figs 5 and 6 it is not essential to cleave all the diode/fibre surfaces, or to provide anti- reflective coatings in conjunction with the cleaved ends; substantial benefits will accrue from cleaving or coating one or more surface.

Claims

1. A method of creating a periodic variation in refractive index of a core of an optical fibre, comprising the steps of: exposing the fibre to a uniform intensity of incident electromagnetic radiation wherein the core of the fibre reacts to the incidence of radiation by undergoing a change in refractive index; varying the polarisation of the incident light along the length of the fibre.

2. A method according to claim 1 wherein the polarisation of the incidence light varies linearly with displacement along the length of fibre.

3. A method according to claim 1 wherein the polarisation of the incident light varies non-linearly with displacement along the fibre.

4. A method according to claim 2 or claim 3 wherein the polarisation is varied cyclically along the fibre to create the periodic variation.

5. A method according to claim 1 further comprising the steps of: displacing the fibre relative to the incident light, in the direction of the length of the fibre, and by an amount corresponding to a single pitch of the periodic variation; and repeating the method steps of claim 1.

6. A method according to any one of the preceding claims wherein the radiation is U.V. light.

7. A method according to any one of the preceding claims wherein the periodicity of the periodic variation is nonlinear.