We recorded and observed, for the first time, three-dimensional image of femtosecond light pulse propagation as continuous moving picture using light-in-flight recording by holography. We present the moving pictures of collimated and converging light pulses and some images extracted from them. We also discussed inherent feature appearing in the images. Such a discussion is essential to determine the actual shape of the propagating light pulse. This technique provides the means for observation of a temporally and spatially continuous moving picture of light itself and also enables the analysis of various kinds of ultrafast phenomena.

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. Here, we report a fiber sensing application of OFCs based on a coherent link between the optical and radio frequencies, enabling high-precision refractive index measurement based on frequency measurement in radio-frequency (RF) region. Our technique encodes a refractive index 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 is read out by measuring the repetition frequency in RF region based on a frequency standard. Use of an OFC as a photonic RF converter will lead to the development of new applications in high-precision fiber sensing with the help of functional fiber sensors and precise RF measurement.

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We present a simple and highly sensitive optical detection method based on two-beam interferometry for application to coherent nonlinear optical microscopy (CNOM). The theoretical sensitivity of this method is higher than that of conventional spectral interferometry (SI). We experimentally applied this technique to stimulated parametric emission (SPE) microscopy and achieved a high sensitivity that is only 4 or 5 dB lower than that of a theoretical shot noise limit. In order to validate the practical applicability of this technique, we demonstrated a noise reduction experiment in the observation of a plant cell with an SPE microscope.

A mode-locked fiber comb equipped with a multimode interference fiber sensor functions as a high-precision refractive-index (RI) sensor benefitting from precise RF measurement. However, its dynamic range and repeatability are hampered by the inherent characteristics of nonlinearpolarization-rotation mode-locking oscillation. In this article, we introduce saturable-absorber-mirror mode-locking for RI sensing with a wide dynamic range and high repeatability. While the RI dynamic range was expanded to 41.4 dB due to high robustness against cavity disturbance, the self-starting capability without the need for polarization control improves the RI sensing repeatability to 1.10 × 10 −8 for each mode-locking activation. The improved dynamic range and repeatability will be useful for enhancing the performance of RI sensing.