Fiber lasers and their applications

The use of optical fiber amplifiers in optical telecommunication systems has opened up many new possibilities with the amplifier's large gain and wide spectral bandwidth. They include ultra-long distance signal transmission without repeaters, and multi channel optical frequency division multiplexing. Very short optical pulses, optical soliton pulses, and wavelength tunable laser sources with extremely narrow linewidth are also of great interest as light source for broadband optical communication. More recently, new applications of the optical fiber amplifiers to sensors have been intr~duced. '-~ They are in forms of various types of fiber lasers whose output optical frequencies or temporal properties are determined by the optical pathlengths of laser cavities. The magnitude of external pertubations that change the optical pathlength of the cavity can be measured by monitoring the output characteristics of the laser. To achieve good performance from the fiber laser sensors, the longitudinal mode and polarization properties of the fiber laser have to be understood in detail. One of the good exam les is a polarimetric fiber laser sensor','where the frequency difference between two eigen polarization components of the laser output has a linear dependence on the net birefringence of the fiber laser cavity. External perturbations such as longitudinal strain, lateral stress, and temperature change can easily be measured with such a fiber laser sensor. Since the sensor has a frequency readout with an inherently linear scale factor, complicated electronic signal processing can be avoided. This is an important feature for practical sensors that has been pursued since the early stage of sensor development. Another example utilizes mode locked operation with short optical pulse output? The timing of the pulses is determined by the phase difference in a fiber interferometer that is part of the laser cavity. Therefore, the output is in time interval that can be conveniently measured. Rotation rate sensing can be accomplished with this approach. Other examples include fiber ring laser sensors using either rare-earth doped fiber amplifiers4 or stimulated Brillouin scattering amplifier? The operating principles and applications of several fiber laser sensors will be described. Some of the recent activities including new results obtained with a fast semiconductor amplifier instead of a slow fiber amplifier in a fiber laser cavity will be pres,ented. The properties of fiber laser output as a function of linear and circular birefringence in the cavity will also be briefly discussed.