The velocity measurement and the length measurement sensors using a longitudinal mode-controlled erbium-doped fiber(EDF) lsaer as their sources are demonstrated.
The longitudinal mode controlled erbium-doped fiber laser was realized using a mode selective mechanism of induced gain and absorption gratings in the gain medium and the saturable absorber. The gain medium and the saturable absorber were 900ppm 0.6m-long and 80ppm 2m-long EDF's, respectively. This fiber laser had two stable oscillating modes whose states of polarization were the same and linear. The frequency difference between two lasing modes were 11.95MHz which corresponded to twice the free spectral range of the 17.2m-long fiber cavity. The output wavelength of this laser was 1.53㎛, the linewidth of the longitudinal mode beat signal was less than 30Hz. The output power was ~10μW.
Using the laser described above, we constructed a laser Doppler velocimeter. Both the amplitude and the maximum velocity of a harmonically oscillating mirror at the frequency of 50Hz were measured. The accuracy in the measurements of the amplitude and the maximum velocity were 0.04㎛ and 12㎛/s, respectively.
A coherent optical frequency domain reflectometer(OFDR) for measuring the length of a fiber was demonstrated. In order to modulate the frequency of the laser, a part of the laser cavity(3m) was wound around a cylindrical PZT transducer. The output frequency excursion obtained from this fiber laser, at a modulation frequency of 10~1000Hz range, was about 2.7MHz/V. We measured the length of a fiber spool(2km) by using the OFDR. The resolution of the length measurement was ~30m.
