Abstract: A catheter adapted to be inserted into a blood vessel is disclosed which comprises a flexible tube-like sheath including a fluid passage therein and a radially expansive and contractive balloon disposed about the peripheral surface of the distal end portion of said sheath and communicating with said fluid passage. The catheter is characterized in that a light guide formed by extrusion for transmitting illumination light to the leading end of the catheter has disposed therein, an image guide for transmitting an image from the distal end of the catheter, and a transparent liquid guide passage opening at the distal end of the catheter, are guide being disposed substantially in parallel with each other within the flexible tube-like sheath whereby the catheter includes the concurrent function of an endoscope.

Abstract: An optical waveguide for illumination comprising a core made of a transparent polymer having a glass transition temperature not higher than 50.degree. C. and a cladding made of a polymer having a lower refractive index than that of the core, which has good light transmission property and flexibility and is useful as an illumination element of an optical sensor.

Abstract: A plastic optical fiber which comprises a core consisting of an amorphous polymer containing a plasticizer and a cladding consisting of a transparent polymer which substantially prevents cross-migration of said plasticizer therein and has a refractive index lower than the core polymer, and which is preferably produced by a process which comprises heat polymerizing a monomer which is polymerized to afford said amorphous polymer and contains said plasticizer, a polymerization initiator and a chain transfer agent, melt spinning said amorphous polymer to form a core and simultaneously cladding the core with said transparent polymer.