1. Field of the Invention
The present invention relates to a lens.
2. Description of the Related Art
Lenses are used in many different optical devices in order to focus or collimate light. In particular applications the components employed in such optical devices are small and have dimensional tolerances that lead to device-by-device variations in the heights of the components above a reference plane, thereby leading to optical alignment problems.
For example, in an optical modulator the height of the components may vary above the base of the device or above a silicon wafer substrate or temperature controller. The final component to be positioned and glued into place in such devices is usually a collimating lens which needs to be aligned with a 0.2 to 0.3 μm tolerance. Whilst it is, in principle, possible to adjust the height of the lens by varying the glue thickness, doing so would introduce undesirable temperature dependence since the glue will typically have a higher coefficient of thermal expansion (CTE) than the other components in the device. Accordingly, it is common to use a standard glue thickness of 15 to 20 microns and to select a lens with an appropriate optical axis height to achieve the best optical alignment.
The lenses employed in such applications are generally in the shape of a spherical ball which has been squashed inwardly to create centrally disposed concave cavities on opposite side faces, such that the optical axis passes through the centres of the cavities. A bottom surface of the lens is then ground to form a planar base at a particular distance from the optical axis. In practice, a number of different bins of lenses are provided, with each bin including lenses ground to a particular height. Thus, during manufacture of the optical device, a lens is chosen from the bin providing the closest height to that desired for good optical alignment. This method therefore requires a large inventory of lenses which have been ground to particular heights in case they are required.
It is therefore an object of the present invention to provide a lens which addresses at least some of the above-aforementioned problems.