The polarization properties of single-mode optical fibers are theoretically modeled with use of the Jones formalism. The fiber is described as an elliptical birefringent plate. The properties predicted by this model are discussed and lead to the development of a simple experimental method to extract the parameters that describe a real fiber. A magneto-optic method that measures the beat length of the fiber is also presented and gives a more complete description of the fiber. The validity of the model is then clearly established. Finally, the wavelength dependence of the parameters characteristic of the fiber is experimentally investigated.

Measurement of the elliptical birefringence of single-mode optical fibers.

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.

Fiber lasers and their applications

By controlling the birefringence via twisting of a distributed feedback fibre laser, single frequency as well as single polarisation operation is ensured, with no reduction in output power. A significant narrowing of the optical linewidth is also observed when operating in the single polarisation regime.

Single polarisation twisted distributed feedback fibre laser

A comprehensive model for steady-state analysis of polarization-mode competition in fiber distributed feedback (DFB) lasers is presented. Effects of polarization-dependent grating nonuniformities, polarization-dependent grating strength, coupling between the linear polarization states due to twist or Faraday rotation, back reflections, cross saturation from serially multiplexed lasers, as well as spatially and polarization-dependent gain hole burning are covered by the model. Regimes of single and dual polarization operation are identified for different types of polarization imperfections in the cavity. The output powers of the individual modes and the magnitudes of the hole-burning mechanisms are also calculated anti discussed.

Modeling of polarization-mode competition in fiber DFB lasers

An Erbium-doped fibre ring laser hybrid mode-locked with single-wall carbon nanotubes (SWNT) and nonlinear polarisation evolution (NPE) without an optical isolator has been investigated for various cavity conditions. Precise control of the state of polarisation (SOP) in the cavity ensures different losses for counter-propagating optical fields. As the result, the laser operates in quasi-unidirectional regime in both clockwise (CW) and counter-clockwise (CCW) directions with the emission strengths difference of the directions of 22 dB. Furthermore, by adjusting the net birefringence in the cavity, the laser can operate in a bidirectional generation. In this case, a laser pumped with 75 mW power at 980 nm generates almost identical 790 and 570 fs soliton pulses with an average power of 1.17 and 1.11 mW. The operation stability and pulse quality of the soliton pulses in both unidirectional regimes are highly competitive with those generated in conventional ring fibre lasers with isolator in the cavity. Demonstrated bidirectional laser operation can find vital applications in gyroscopes or precision rotation sensing technologies.

Isolator-free switchable uni- and bidirectional hybrid mode-locked erbium-doped fiber laser.

We describe a new form of an all-fiber-optic gyroscope that utilizes a fiber laser. The Sagnac interferometer is used as a loop reflector with a modulated reflectivity to produce actively mode-locked optical pulses. The rotation-induced phase shift is obtained from the timing shift of the pulses. Experimental results compare well with the theoretical predictions.

Mode-locked fiber laser gyroscope.

Mode Locked Fiber Laser Gyroscope

The characteristics of the output polarization angle and the polarization-mode beat frequency of a twisted fiber laser are investigated. A theoretical model is used to describe successfully the polarization properties.

HJ Jeong

Papers

Mode-locked fiber laser gyroscope.

Mode Locked Fiber Laser Gyroscope

Polarization properties of a twisted fiber laser.