A liquid chromatography spectrophotometer, arranged to accept a sample fluid from a chromatographic separation column of a liquid chromatography system for measuring the light absorption or determining the concentration of a substance in the sample fluid, the liquid chromatography system comprising: an optical fiber arrangement for inducing coupling among propagation modes of light, said arrangement comprising: a first optical fiber, comprising a non-circular cross section multimode optical fiber having an input end for receiving light and an output end for emitting light, with a coupling inducing section extending from said input end to said output end, wherein the coupling inducing section comprises a plurality of regions; a holder comprising a surface, said holder arranged for mounting the first optical fiber thereon; and fastening means arranged to secure the first optical fiber on said holder, wherein the coupling inducing section is arranged along a path in said holder, said path comprising open bends such that no region of the coupling inducing section is arranged to overlap any other region, and every region of the coupling inducing section is fixated in relation to the surface of the holder; a light source; a monochromator arranged to receive light emitted by the light source and project said light onto the input end of the first optical fiber; a beam splitter arranged to receive light from the first optical fiber, wherein the beam splitter is configured to split said light into a first portion of light that is transmitted in a second optical fiber, and a second portion of light that is transmitted in a third optical fiber, wherein the second optical fiber and the third optical fiber have a non-circular cross section; a flow cell arranged to accept the sample fluid flow and allow light from the third optical fiber to irradiate the flow cell such that light from the flow cell is transmitted in a fourth optical fiber; and optical detectors to detect light from the second optical fiber and the fourth optical fiber to provide a reference value and a sample value. an optical fiber arrangement for inducing coupling among propagation modes of light, said arrangement comprising: a first optical fiber, comprising a non-circular cross section multimode optical fiber having an input end for receiving light and an output end for emitting light, with a coupling inducing section extending from said input end to said output end, wherein the coupling inducing section comprises a plurality of regions; a holder comprising a surface, said holder arranged for mounting the first optical fiber thereon; and fastening means arranged to secure the first optical fiber on said holder, wherein the coupling inducing section is arranged along a path in said holder, said path comprising open bends such that no region of the coupling inducing section is arranged to overlap any other region, and every region of the coupling inducing section is fixated in relation to the surface of the holder; a first optical fiber, comprising a non-circular cross section multimode optical fiber having an input end for receiving light and an output end for emitting light, with a coupling inducing section extending from said input end to said output end, wherein the coupling inducing section comprises a plurality of regions; a holder comprising a surface, said holder arranged for mounting the first optical fiber thereon; and fastening means arranged to secure the first optical fiber on said holder, wherein the coupling inducing section is arranged along a path in said holder, said path comprising open bends such that no region of the coupling inducing section is arranged to overlap any other region, and every region of the coupling inducing section is fixated in relation to the surface of the holder; a light source; a monochromator arranged to receive light emitted by the light source and project said light onto the input end of the first optical fiber; a beam splitter arranged to receive light from the first optical fiber, wherein the beam splitter is configured to split said light into a first portion of light that is transmitted in a second optical fiber, and a second portion of light that is transmitted in a third optical fiber, wherein the second optical fiber and the third optical fiber have a non-circular cross section; a flow cell arranged to accept the sample fluid flow and allow light from the third optical fiber to irradiate the flow cell such that light from the flow cell is transmitted in a fourth optical fiber; and optical detectors to detect light from the second optical fiber and the fourth optical fiber to provide a reference value and a sample value.