For the first time to our knowledge, high-strength $(>30\mathrm{dB})$ first-order Bragg grating waveguides were fabricated in bulk fused silica glass in a single-scanning step by modulating a high-repetition-rate femtosecond fiber laser with an external acousto-optic modulator. The modulation induced a waveguide segmentation by delivering controlled bursts of laser pulses to define an array of partially overlapped refractive index voxels. With appropriate choice of modulation frequency and sample scanning speed, low loss waveguides could be formed at high writing speeds to yield sharp Bragg spectral resonances tunable over the $1300\text{to}1550\mathrm{nm}$ telecom band. Effects of acousto-optic modulation duty cycle on propagation loss and grating strength are characterized. This modulation method offers facile control and integration of multiwavelength Bragg grating devices to enhance overall functionality of optical circuits in three-dimensional geometries.