An optical detector for a detection of a chemical compound, the optical detector comprising - a first optical fiber element, wherein the first optical fiber element is designed to guide at least one optical signal and at least one modified optical signal; - a second optical fiber element, wherein the second optical fiber element is adapted to communicate with the first optical fiber element, wherein the second optical fiber element is designed to receive the at least one optical signal provided by the first optical fiber element and to emit the at least one modified optical signal to the first optical fiber element, wherein the second optical fiber element has a core and a cladding, wherein the cladding surrounds the core, wherein a partition of the second optical fiber element comprises a first material which is adapted to be at least partially converted into a second material upon a chemical reaction with at least one chemical compound to be detected, wherein the core is chemically stable with respect to the at least one chemical compound, wherein the first material is comprised by the cladding, wherein the first material is designated to be converted into the second material in a manner that the second material exhibits a change in volume compared to the first material, wherein the at least one modified optical signal depends on a conversion of the first material into the second material by the chemical compound; and - an evaluation device, wherein the evaluation device is adapted to determine at least one property related to the chemical compound by evaluating the at least one modified optical signal provided to the evaluation device by the first optical fiber element; wherein the at least one optical signal which is received by the second optical fiber element experiences a different volume of the cladding surrounding the core of the second optical fiber element, whereby the at least one modified optical signal is generated, depending on the conversion of the first material into the second material by the chemical compound, characterized in that the second optical fiber element is or comprises a single-mode fiber, wherein, apart from the first material, the cladding is chemically stable with respect to the at least one chemical compound, and in that the second optical fiber element comprises a polarization-maintaining optical fiber comprising at least one stress rod, wherein the first material is comprised by the at least one stress rod located in the cladding of the polarization-maintaining optical fiber. - a first optical fiber element, wherein the first optical fiber element is designed to guide at least one optical signal and at least one modified optical signal; - a second optical fiber element, wherein the second optical fiber element is adapted to communicate with the first optical fiber element, wherein the second optical fiber element is designed to receive the at least one optical signal provided by the first optical fiber element and to emit the at least one modified optical signal to the first optical fiber element, wherein the second optical fiber element has a core and a cladding, wherein the cladding surrounds the core, wherein a partition of the second optical fiber element comprises a first material which is adapted to be at least partially converted into a second material upon a chemical reaction with at least one chemical compound to be detected, wherein the core is chemically stable with respect to the at least one chemical compound, wherein the first material is comprised by the cladding, wherein the first material is designated to be converted into the second material in a manner that the second material exhibits a change in volume compared to the first material, wherein the at least one modified optical signal depends on a conversion of the first material into the second material by the chemical compound; and - an evaluation device, wherein the evaluation device is adapted to determine at least one property related to the chemical compound by evaluating the at least one modified optical signal provided to the evaluation device by the first optical fiber element;