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.
Refractive index measurement is important for evaluation of liquid materials, optical components, and bio sensing. One promising approach for such measurement is use of optical fiber sensors such as surface plasmonic resonance or multi-mode interference (MMI), which measure the change of optical spectrum resulting from the refractive index change. However, the precision of refractive index measurement is limited by the performance of optical spectrum analyzer. If such the refractive index measurement can be performed in radio frequency (RF) region in place of optical region, the measurement precision will be further improved by the frequency-standard-based RF measurement. To this end, we focus on the disturbance-to-RF conversion in a fiber optical frequency comb (OFC) cavity. Since frequency spacing frep of OFC depends on an optical cavity length nL, frep sensitively reflects the external disturbance interacted with nL. Although we previously demonstrated the precise strain measurement based on the frep measurement, the measurable physical quantity is limited to strain or temperature, which directly interacts with the fiber cavity itself. If a functional fiber sensor can be installed into the fiber OFC cavity, the measurable physical quantity will be largely expanded. In this paper, we introduce a MMI fiber sensor into a ring-type fiber OFC cavity for refractive index measurement. We confirmed the refractive-index-dependent frep shift.