  Acousto-optic imaging (AOI) is a non-invasive method that uses acoustic
modulation to map the light fluence inside biological tissue. In many AOI
implementations, ultrasound pulses are used in a time-gated measurement to
perform depth-resolved imaging without the need for mechanical scanning.
However, to achieve high axial resolution, it is required that ultrasound
pulses with few cycles are used, limiting the modulation strength. In this
Letter, we developed a new approach to pulse-based AOI in which a coded
ultrasound transmission is used. In coded-transmission AOI (CT-AOI), one may
achieve an axial resolution that corresponds to a single cycle, but with a
signal-to-noise ratio (SNR) that scales as the square root of the number of
cycles. Using CT-AOI with 79 cycles, we experimentally demonstrate over 4-fold
increase in SNR in comparison to a single-cycle AOI scheme.
