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1. An optical sensor comprising an optical fibre in a continuous loop, which extends, in use, from a light source (28) to a photo detector (29) , the optical fibre comprising an inner light-transmitting core (20) and an outer cladding (21) , which normally ensures total internal reflection of light transmitted along the optical fibre core from the source to the detector, a portion (22,23) of the optical fibre being configured to allow light to escape from the light transmitting core at a sensing region in dependence on the optical properties of a surrounding medium at the sensing region.

2. A sensor according to claim 1, wherein the optical fibre has a bend (23) at, or immediately upstream of.-, in the direction of light transmission along the conductor, the sensing region (22,23) .

3. A sensor according to claim 1 or claim 2, in which the configuring is provided by stripping away a portion (22) of the cladding (21) to expose an area of the core (20) .

4. A sensor according to claims 2 and 3, wherein the portion (25) of the cladding (20) is stripped away on the concave side of the bend (23B) and the cladding (21) on the convex side of the bend is provided with an inwardly facing rough reflecting surface (24) .

5. A sensor according to claim 2, in which the optical fibre is flexible and the cladding (21) is translucent, the configuring being provided by providing the conductor with a sharp bend (23) (as herein defined) .

6. A sensor according to claim 5, wherein there are two sharp bends adjacent to one another.

7. A sensor according to claim 6, wherein the two bends are in different planes.

8. A sensor according to any one of the preceding claims, in which the core (20) is made of polymethyl methacrylate, and the cladding (21) is made of a fluorinated polymer.

9. A liquid sucker comprising a tubular body (10,11) carrying a sensor according to any one of the preceding claims, the sensing region (23) of the optical fibre being adjacent to the tip (12) of the tubular body through which the liquid is, in use, sucked.

10. A sucker according to claim 9, in which the wall of the tubular body incorporates at least one duct (13) accommodating parts (14,15) of the optical fibre extending, in use, from the light source (28) and back to the detector (29) .

11. A sucker according to claim 9 or claim 10, in which the portion of the optical fibre, where the sensing region (22,23) is provided, lies in a groove (19) in the inner or outer wall surface of the tubular body (11) .

12. A sucker according to any one of claims 9 to

11. in which the optical fibre extends between a light source (28) and a detector (29) , a conditioned output of the detector being connected to a vacuum pump (31) whereby the pump is started when light is lost from the optical fibre at the sensing region and the light received by the detector (29) consequentially falls below a threshold, and vice versa.