Light Coupling

To incorporate optical fibres into an optical circuit various coupling arrangements are available which are detailed below.

5.2.2.1 The Gradient Index Lens

Gradient index (GRIN) lenses offer the simplest method of collimating a laser beam. A gradient index lens is a cylinder of glass whose refractive index varies radially. In conventional lenses imaging is a result of discrete refractions occurring at the boundaries of homogenous media of different refractive index. By using materials in which the refractive index varies in some controlled way it is possible to form images by continuous refraction.

Combining surface refraction with continuous refraction provides a number of advantages over conventional lens systems. The more notable of these are:

* correction of abberations without complex, multi-element systems of aspherics * simplification of the geometry of lenses

* formation of real images at the lens surface

The gradient index lenses available are manufactured from SELFOC® (a registered trademark of the Nippon Sheet Glass Company) which is a radial gradient index material. These are rod lenses combining refraction at the plane end surfaces with continuous refraction within the rod. They are highly suitable for coupling the output of a diode laser into an optical fibre.

In SELFOC® material, the refractive index varies parabolically as a function of radius. The index variation may be expressed as [166]:

«r “ "00 f 1 - ^ 2) 5 ' 12

where

n,. = refractive index at a distance r from the optical axis noo = design index on the optical axis

A = a positive constant

As a result of this parabolic index variation a ray incident on the front surface follows a sinusoidal path along the rod lens. The period of this sinusoidal path is called the pitch of the lens and is an important parameter in gradient index imaging. It is given by

2it

P - — 5.13

Knowing the pitch of gradient index rod material it is possible to achieve various imaging characteristics simply by varying the length of the lens. Imaging properties of common fractional pitch lens types are shown in figure 5.3.

5.2.2.2 The Coupling Sphere

Of the various techniques currently used to couple optical fibres to each other (and to diode lasers and detectors) the ball lens is the one which allows the researcher to interact with the beam in the coupling process. Using this method, spheres are arranged such that the fibre end (or input/output device) is located at the focal point of the sphere. The output of the beam is then a collimated beam. If two spheres are arranged in axial alignment with each other the beam will be transferred from one focal point to the other as is shown in figure 5.4. This is exactly analogous to a pair of thin lenses

relaying the object to the image space through a collimator/focusing lens combination. By enlarging the coupling beam, translational alignment sensitivity is reduced.

Spheres are much easier to manufacture than thin lenses in these small diameters. In the same way, they are easy to align in experimental applications. Because of their simplicity, ball lens coupling systems tend to achieve better coupling efficiencies than other methods. Using communication fibres, coupling efficiencies up to 95% (0.4dB insertion loss) have been achieved [167].

All of these spheres are coated on both hemispheres with a single layer MgF2 antireflection coating. Because of the high index of these spheres, this results in a very effective coating with low reflectivity at the specified wavelengths.

5.2.2.3 Laser to Fibre Source Coupler

The source coupler makes use of a tilt coupling method to focus the laser beam onto the fibre which is located at the image plane of a lens system. The method is based on precision control of the angle between the laser beam and the receiver lens. The source coupler is comprised of two baseplates each having axial bore. One of the baseplates is adapted to receive a lens holder which carries a lens and a fibre. The other baseplate is attached to the laser. A resilient member, such as a rubber ’O’ ring, is sandwiched between the baseplates. The screws can then be adjusted by a screwdriver to alter the angular orientation of one baseplate relative to the other. By monitoring the output of the fibre, the coupler can be adjusted until the output is optimised. The coupling arrangement is shown in figure 5.5. If the lens is tilted by an angle relative to the laser beam, the focused laser spot on the focal plane of the lens will be displaced relative to the receiver lens axis by an amount given by the equation

z - f t a n d 5.14

where f is the focal length of the lens and 6 is the tilt angle.

5.2.2.4 The Fibre Collimator

Light leaving an optical fibre diverges to a great extent leading to an obvious loss of power. The beam therefore needs to be collimated. The fibre collimator is very similar in construction to the fibre optic coupler except that no aligning is necessary. The theoretical loss of this device is reported as 0.6 dB [168],

5.2.2.5 Conclusions

Both the coupling sphere and gradient index lens can be used to couple light into and out of an optical fibre as shown. In fact the gradient index lens is usually incorporated in the commercially available ’off the shelf couplers. However, these couplers are too bulky for use in light modulated accelerometers and custom made couplers would need to be manufactured.