  We developed an optical phased array using an optical frequency comb and
demonstrated its proof-of-principle. Optical phased arrays have been actively
developed in recent years as a technology that can control the wavefront of
light without any mechanical devices like phased array radar. Conventional
optical phased arrays have been implemented using optical integrated circuits,
but it has been difficult to achieve broadband operation with simple control.
This is because control and calibration of a large number of phase modulators
are required for each wavelength, and the dispersion of the waveguide makes
whole bandwidth phase control of ultrashort pulses difficult. In contrast, we
have developed a novel optical phased array that realizes wavefront control of
ultrashort pulses generated by mode-locked laser by phase control of the comb,
using high controllability of the comb and an optical array antenna with
free-space optics. This is achieved by simply controlling the ratio of the two
radio frequencies of the comb to realize a broadband optical phased array while
suppressing environmental fluctuations. Experiments demonstrated broadband
optical dot scanning at an optical frequency by forming an optical dot pattern
and suppressing the environmental fluctuation by controlling the comb
frequency. This innovative optical technology enables direct control of
wavefronts by optical frequencies, i.e. controlling transverse modes by
longitudinal modes.
