Ultrasound detection using a tunable low beat-frequency Er3+-doped DBR fiber laser

A compact low-beat-frequency dual-polarization distributed Bragg reflector (DBR) fiber laser for high-frequency ultrasound detection has been demonstrated. The laser was fabricated in high germanium concentration, small-core erbium-doped fiber with very small birefringence. Induced birefringence to the fiber during the UV inscription process is small (~10-7) because of the small fiber core (4.2-m) and consequently the laser beats at a low frequency of ~20 MHz, making frequency down-conversion unnecessary. The beat frequency can be adjusted by controlling the side-exposure time of the UV light irradiating the gain cavity, providing a simple approach to multiplex a large number of DBR fiber lasers of different frequencies in series using frequency division multiplexing (FDM) technique.