Makoto Hosoda

Femtosecond time-resolved optical polarigraphy:?imaging of the propagation dynamics of intense light in a medium

A time-resolved imaging technique for visualizing ultrafast propagation dynamics of intense light pulses in a medium has been demonstrated. The method probes the instantaneous birefringence induced by a pulse in the medium. Through consecutive femtosecond snapshot images of intense femtosecond laser pulses propagating in air, ultrafast temporal changes in the two-dimensional spatial distribution of the optical pulse intensity were clearly seen.

Femtosecond snapshot imaging of propagating light itself

An ultrafast imaging technique has been developed to visualize directly a light pulse that is propagating in a medium. The method, called femtosecond time-resolved optical polarigraphy (FTOP), senses instantaneous changes in the birefringence within the medium that are induced by the propagation of an intense light. A snapshot sequence composed of each femtosecond probing the pulse delay enables ultrafast propagation dynamics of the intense femtosecond laser pulse in the medium, such as gases and liquids, to be visualized directly. Other examples include the filamentation dynamics in CS2 liquid and the propagation dynamics in air related to the interaction with laser breakdown plasma. FTOP can also be used to extract information on the optical Kerr constant and its decay time in media. This method is useful in the monitoring of the intensity distribution in the nonlinear propagation of intense light pulses, which is a frequently studied subject in the field of physics regarding nonlinear optics and laser processing.

Frequency domain spectroscopy of free-space terahertz radiation

We demonstrate frequency domain spectroscopy of free-space terahertz (THz) radiation by using a vibrating optical delay and a rf spectrum analyzer. When the timing of the pump pulses is repeatedly varied at several Hz and an optical chopper at a frequency of several kHz simultaneously modulates the pump pulse, the spectrum analyzer directly obtains the THz spectrum as a sideband of the chopping frequency. This method can be used to measure in real time the transient change in the THz spectrum at a specific frequency corresponding to the water absorption.

Visualization of ultrafast dynamics of laser pulse propagation in the atmosphere using a novel measurement method: FTOP (femtosecond time-resolved optical polarigraphy)

In the field of laser processing, it is important to monitor the beam quality such as the spatial- and time-distribution. A novel time-resolved imaging technique named FTOP (Femtosecond Time-resolved Optical Polarigraphy) for visualizing the ultrafast propagation dynamics of intense light pulses in a medium has been proposed and demonstrated. FTOP is used to monitor the 3D intensity distribution of the pump pulse focused in a medium by the probe pulse. Femtosecond snapshot images can be created with a high spatial resolution by imaging only the polarization components of the probe pulse; these polarization components change due to the instantaneous birefringence induced by the pump pulse in the medium. Ultrafast temporal changes in the 2D spatial distribution of the optical pulse intensity were clearly visualized in consecutive images by changing the delay between the pump and probe. We observe that several filaments appear and then come together before the vacuum focus due to nonlinear effects in air. We also prove that filamentation dynamics such as the formation position and the propagation behavior are complex and are strongly affected by the pump energy. The results collected clearly show that this method FTOP succeeds for the first time in directly visualizing the ultrafast dynamics of the self- modulated nonlinear propagation of light.

All-Solid-State Cr:LiSAF Femtosecond Lasers

Two types of all-solid-state Cr:LiSAF femtosecond lasers are demonstrated. One laser aims at generation of ultrashort pulses. Transform-limited 30-fs pulses were generated from this diode-pumped, Kerr-lens mode-locked laser with a time-bandwidth product of 0.32 by using a wide gain bandwidth of the Cr:LiSAF crystal. The other aim is shortening the laser size. By using a novel geometry for dispersion compensation of femtosecond pulses, the laser length was much reduced to 38 cm. This configuration also enabled the laser to operate at variable repetition rates from 163 to 235 MHz keeping the pulse width less than 90 fs. In addition, a very compact, i.e., about 25 x 16 cm laser size was achieved by using F8 material for the dispersion-compensation prism pair, and the laser generated 143-fs pulses at a 262-MHz repetition rate.