Er3+-doped fiber ring laser for gyroscope application.

Fiber-laser sensors have potential advantages over conventional fiber-optic interferometric sensors owing to their simplicity in both optical configuration and electronic signal processing." A fiber ring laser is a good candidate for rotation-rate sensing because of the same advantages, and such a fiber-ring-laser gyroscope was demonstrated by use of Brillouin gain in the fiber.4'5 The Brillouin fiber-ring-laser gyroscope, however, still requires complex signal processing and has the problems associated with limited dynamic range and Kerr-effect-induced error. A fiber-ring-laser gyroscope built with a rare-earthdoped fiber amplifier has the potential to become practical, but it has not been seriously investigated because of a few basic problems, including gain competition between the two laser signals oscillating in opposite directions. 6' 7 In this Letter we describe an Er3+-doped fiber ring laser that operates bidirectionally in multiple longitudinal modes with stable output power in both directions. The ring laser produces a beat note whose frequency is linearly proportional to the rotation rate. In steady state the phase and the state of polarization of each longitudinal mode at any arbitrary position in a fiber ring laser should have the same value after one round trip. If a single-mode fiber cavity with random birefringence seen by clockwise(CW-) propagating light is represented by a Jones matrix A for one round trip, the same cavity seen by counterclockwise(CCW-) propagating light is represented by the transpose of A, i.e., AT, in the laboratory frame in the absence of nonreciprocal elements in the cavity.8 Then the steady-state laser condition can be expressed by the following eigenvalue equations: