Simultaneous measurement of concentration and temperature in liquid sample using multi-mode interference fiber comb

Optical frequency combs (OFCs) have attracted attention as optical frequency rulers due to their tooth-like discrete spectra together with their inherent mode-locking nature and phase-locking control to a frequency standard. Based on this concept, their applications until now have been demonstrated in the fields of optical frequency metrology. However, if the utility of OFCs can be further expanded beyond their application by exploiting new aspects of OFCs, this will lead to new developments in optical metrology and instrumentation. Previously, we reported a fiber sensing application of OFCs based on a coherent link between the optical and radio frequencies, enabling high-precision refractive index (RI) measurement based on frequency measurement in radio-frequency (RF) region. Our technique encoded an RI change of a liquid sample into a repetition frequency of OFC by a combination of an intracavity multi-mode-interference fiber sensor and wavelength dispersion of a cavity fiber. Then, the change in refractive index was read out with an RI resolution of 4.88 × 10^-6 RIU and an RI accuracy of 5.35 × 10^-5 RIU by measuring the repetition frequency in RF region based on a frequency standard. However, the temperature instability of a sample limits the performance because a refractive index is a function of temperature. In this paper, we demonstrate simultaneous measurement of concentration and temperature in a sample by measuring RI-dependent repetition frequency shift and optical spectrum shift.