Illumination – Revolving
Patent
1992-01-31
1994-06-07
Husar, Stephen F.
Illumination
Revolving
362234, 362240, 362259, 362268, 359318, F21V 704
Patent
active
053195280
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention relates to light sources, and relates particularly, but not exclusively, to light sources comprising solid state laser diodes.
(2) Description of the Prior Art
Laser diodes are compact, robust, efficient and relatively inexpensive sources of laser light. It has therefore been proposed to use laser diodes as light energy sources in many applications in place of previously used gas lasers or solid state lasers such as Nd:YAG which are large and not easily portable. Such applications include body implantable angioplasty probes, ophthalmic treatments, contact laser surgery, etc.
However, the power output available from a single laser diode is limited to a few watts. Furthermore, each diode emits from an elongated high aspect ratio "stripe" into a relatively large cone angle. The cone angle, i.e. the numerical aperture of the emitted beam, is smaller in the direction parallel to the long axis of the laser stripe (hereinafter called the x-axis) than in the direction perpendicular to the long axis of the stripe (y-axis).
In view of the power limitation of a single laser diode, for many applications it is necessary to combine the outputs from a number of laser diodes. In any such system, the light from a number of laser diodes must be efficiently transmitted to a target area, which typically may have an aspect ratio which is lower than that of the laser stripe and which also may have a certain maximum acceptance cone or solid angle for efficient transmission of light to the target. For example, in applications such as angioplasty devices where light energy is transmitted through an optical fibre, the target area, i.e. the end of the fibre, is round, and the acceptance solid angle is the same in both axes and corresponds to the maximum numerical aperture of an incident beam which can be efficiently coupled into the fibre. A further example is treatment of the retina in the eye, where the light energy must be focussed onto a predetermined target area on the retina via the iris which imposes a maximum acceptance solid angle.
Problems arise in practice in seeking to combine light from a plurality of laser diode sources such that the required power may be delivered to a predetermined target area within the constraints imposed by the efficient acceptance solid angle associated with the target or indeed with the delivery optics of the source itself.
There has been a number of approaches to overcome the problem. It has been proposed to combine the beams from two laser diodes using a polarising beam combiner and to focus the combined beam onto the end of an optical fibre. It has also been proposed to minimise the diameter of the fibre in such an arrangement by anamorphically demagnifing the image of the laser stripe in its long dimension up to a point where the numerical apertures of the combined laser beam in both dimensions match the acceptance numerical aperture of the fibre. However, for either of these proposals the maximum power for each fibre is equivalent to that from only two laser diodes, and to achieve more power it is necessary to bundle a plurality of such fibres together. In certain applications this may not be an efficient solution in view of image to target size mis-match and in view of the complex fibre optics required. The bundling of a number of fibres together in this way also results in a loss of brightness and at high power can be prone to thermally induced damage. It has also been proposed to combine beams from a number of lasers of different wavelengths using wavelength selective mirrors. However, this is unsatisfactory in practice, in view in particular of the extreme temperature sensitivity of the wavelength of laser diodes. A further approach has been to form the end of optical fibres into an oblong shape to match more closely the laser diode stripe, but such fibres are expensive to produce and the coupling of light energy into each fibre is not particularly efficient.
A still further attempt at providing a more powerfu
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Diomed Limited
Husar Stephen F.
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