1. A medical probe, comprising: a cannula (20) having a multi-cavity insert (30), and at least two essentially straight separated optical fibers (10) located in a multi-fiber cavity (45； 47) of said multi-cavity insert (30), wherein the shape and size of said multi-fiber cavity (45； 47) are adapted to said at least two optical fibers (10), so that the orientation of said at least two optical fibers (10) is mutually fixed. 2. A probe according to claim 1, wherein said at least two optical fibers (10) comprise source and detector fibers for detecting fluorescence. A probe according to claim 1, wherein said at least two optical fibers (10) are placed opposite each other in order to have the smallest distance between the cores. A probe according to claim 1, wherein said multi-fiber cavity (47) of said multi-cavity insert (30) contains three optical fibers (10). A probe according to claim 1, wherein said cannula (20) and said multi-cavity insert (30) have beveled ends. A probe according to claim 5, wherein said multi-cavity insert (30) is beveled at an angle that is smaller than the bevel angle of said cannula (20). 7. A probe according to claim 1, wherein said multifibre cavity (45； 47) is at least partially coated with a metal coating or a coating having a low intrinsic fluorescence. A probe according to claim 1, wherein said probe comprises a biopsy needle, an ablation needle, or a catheter. An optical measuring system comprising a medical probe according to claim 1 and an optical console with which said medical probe can be connected, said optical console with1. Медицинский зонд, содержащий:канюлю (20), имеющую мультиполостную вставку (30), ипо меньшей мере два по существу прямых разделенных оптических волокна (10), расположенных в мультиволоконной полости (45； 47) упомянутой мультиполостной вставки (30),причем форма и размер упомянутой мультиволоконной полости (45； 47) приспособлены к упомянутым по меньшей мере двум оптическим волокнам (10), так что ориентация упомя
