An optical sensor arrangement (1;100) for measuring an observable, the arrangement comprising: a light generation circuit (2; 102) for generating a first light component of a first frequency and a second light component of a second frequency; a optical resonance circuit (3; 103) including at least a first optical resonator (34; 134) having a first optical length, an entry port (35; 135) and a drop port (36; 137), where the first optical length is variable depending on the observable, wherein the at least one light generation circuit is coupled to the optical resonance circuit such that the first and second light components are fed into the optical resonance circuit and the first light component is fed to the entry port of the first optical resonator via a first optical path (32; 132) and the second light component is not fed into the first optical resonator; an optical coupler coupled to the optical resonance circuit, where the drop port of the first optical resonator is coupled to the optical coupler via a continuation part (37; 136) of the first optical path and the coupler is configured for superposing the first light component and the second light component and for synchronizing a phase noise between the first and second light component; a detector unit (5; 105) coupled to the optical coupler and receiving the superposed first and second light components and configured for outputting an output signal representing a frequency difference between the first and second frequencies of the first and second light components, respectively, wherein the output signal is configured to include a signal with a beat frequency of the superposed first and second optical light components, wherein the observable is a substance which causes the refractive index of the first optical resonator to change and thereby causes a shift of the optical length of the first optical resonator. a light generation circuit (2; 102) for generating a first light component of a first frequency and a second light component of a second frequency; a optical resonance circuit (3; 103) including at least a first optical resonator (34; 134) having a first optical length, an entry port (35; 135) and a drop port (36; 137), where the first optical length is variable depending on the observable, wherein the at least one light generation circuit is coupled to the optical resonance circuit such that the first and second light components are fed into the optical resonance circuit and the first light component is fed to the entry port of the first optical resonator via a first optical path (32; 132) and the second light component is not fed into the first optical resonator; an optical coupler coupled to the optical resonance circuit, where the drop port of the first optical resonator is coupled to the optical coupler via a continuation part (37; 136) of the first optical path and the coupler is configured for superposing the first light component and the second light component and for synchronizing a phase noise between the first and second light component; a detector unit (5; 105) coupled to the optical coupler and receiving the superposed first and second light components and configured for outputting an output signal representing a frequency difference between the first and second frequencies of the first and second light components, respectively, wherein the output signal is configured to include a signal with a beat frequency of the superposed first and second optical light components, wherein the observable is a substance which causes the refractive index of the first optical resonator to change and thereby causes a shift of the optical length of the first optical resonator.