Photo-acoustic imaging is a promising modality for deep tissue imaging with high spatial resolution in the field of biology and medicine. High penetration depth and spatial resolution of the photo-acoustic imaging is achieved by means of the advantages of optical and ultrasound imaging, i.e. tightly focused beam confines ultrasound-generated region within micrometer scale and the ultrasound can propagate through tissues without significant energy loss. To enhance the detection sensitivity and penetration depth of the photo-acoustic imaging, highly sensitive ultrasound detector is greatly desired. In this study, we proposed a novel ultrasound detector employing optical frequency comb (OFC) cavity. Ultrasound generated by the excitation of tightly focused laser beam onto a sample was sensed with a part of an OFC cavity, being encoded into OFC. The spectrally encoded OFC was converted to radio-frequency by the frequency link nature of OFC. The ultrasound-encoded radio-frequency can therefore be directly measured with a high-speed photodetector. We constructed an OFC cavity for ultrasound sensing with a ring-cavity erbium-doped fiber laser. We provided a proof-of-principle demonstration of the detection of ultrasound that was generated by a transducer operating at 10 MHz. Our proposed approach will serve as a unique and powerful tool for detecting ultrasounds for photo-acoustic imaging in the future.
Optical Frequency combs can be used as a tool for fully controlling the phase and frequency information of light waves, i.e., “optical synthesizer”. It provides powerful tools not only in frequency metrology as “ultraprecise frequency ruler” but also in broad area since light wave can be used to its full extent with an extremely wide dynamic range. Frequency-traceable length measurement using frequency combs provides direct realization of the definition of meter, remote calibration using a GPS technology, and precise measurements of wide range of lengths by taking advantage of high dynamic range in frequency measurements. In this paper, ultrahigh-precision length metrology using fiber-based optical frequency combs are presented. By precisely controlling the frequency and phase of the combs, self-correction of air refractive index and noise cancellation in fiber path in interferometer are demonstrated. Heterodyne interferometry of 61- m path-length based on two-color optical frequency combs is developed for air-refractive-index correction. Measured two-color optical-path-differences agreed with calculations with 10−11 for 10-hour. Corrected distance variation agreed with thermal expansion of base-plate. A fiber-based optical frequency comb interferometer with 168-m-length reference path was stabilized to nm-level with fiber noise cancellation technique using a single frequency CW laser. Extremely wide range interferometric fringe scanning of 3.3-m path length
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Optical Measurement Systems for Industrial Inspection IX