Optical waveguides – With optical coupler – Input/output coupler
Patent
1996-08-01
1998-04-07
Lee, John D.
Optical waveguides
With optical coupler
Input/output coupler
385 33, 385 40, 250281, G02B 632
Patent
active
057374627
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to radiation coupling arrangements. In particular, it relates to a method and an apparatus for coupling laser radiation into an optical guide.
There are many practical applications in which high power laser beams are required to be transmitted via optical guides. For example, lasers used in surgical applications are employed together with optical fibre cables.
There are a number of technical difficulties associated with the coupling of high power, pulsed laser radiation with optical fibre cables. One such difficulty arises when the laser beam is focused into the fibre. Since many fibres have core diameters less than 1 mm, very large peak power densities of laser radiation may be produced on the end face of the fibres. This can result in significant damage to the fibres and, therefore, a rapid deterioration in the optical transmission characteristics of the fibres.
To date, a number of techniques have been developed to facilitate the coupling into optical fibres of radiation outputs of high power pulsed lasers. One such technique involves the use of tapered fibres. This type of fibre is manufactured in such a way that one end has a considerably larger diameter than the other end (eg 1 cm and 1 mm respectively). When a laser beam is focused onto the larger diameter end, power densities on the face of the fibre are considerably reduced. However, these tapered fibres are very expensive and are not ideally suited to certain applications (eg laser surgery) as the fibres are expensive and the cost of replacing the fibres after each use is likely to be prohibitive.
In a typical laser-to-fibre guide coupling arrangement the output laser beam is normally focused by a lens onto the fibre(s). The focal point of the lens is set to be some distance in front of the fibre in order that a diverging beam enters the fibre. If the focal point of the lens is situated within the fibre, very high laser power densities will be produced at the focus within the fibre material resulting in a high probability of fibre damage.
In the known coupling arrangement described, a problem arises at very high laser powers in the following way. There is a limiting laser power density above which the air through which the laser beam travels becomes ionised and produces a plasma ("laser-induced plasma"). Typical power densities required to produce a plasma in air are of the order 10.sup.10 watts per cm.sup.2. Power densities of this order a readily obtainable when the output beams of certain pulsed lasers such as Nd:YAG (Q-switched) are focused using conventional optical lenses. Once a plasma is produced, plasma shielding prevents further laser radiation from entering the fibre.
It is the purpose of the present invention to provide an arrangement for coupling the output of a laser, especially a high power pulsed laser beam, into an optical guide in which the problem of producing a laser-induced plasma is eliminated or at least reduced in effect.
According to the present invention in a first aspect there is provided an optical apparatus including a laser producing in use an output laser beam and an optical guide for guiding radiation of the output laser beam when incident thereon and characterised by means for depleting of free charge carriers a region in a path to be traversed by the output laser beam between the laser and the guide.
According to the present invention in a second aspect there is provided a method of coupling laser radiation from a laser to an optical guide using the apparatus of the first aspect which includes directing the output laser beam produced by the laser onto the guide whilst maintaining, in a region in the path traversed by the output laser beam, a depletion of free charge carriers.
The said means for depleting may comprise means for producing in the said region an electric field.
The said means for depleting may comprise an electrode to which an electrical potential may be applied from a high voltage source. The applied potential may exceed 1 kV, and desirably exceeds 5 kV. The applied pot
REFERENCES:
patent: 5149406 (1992-09-01), Mullen et al.
Perks Richard Marc
Whitehouse Andrew Ian
Williams Aled Wyn
British Nuclear Fuels PLC
Lee John D.
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