Solid state optical fiber amplifiers, such as erbium-doped material, have advantages as gain media for use in lasers. A fiber pulsed laser with subpicosecond pulses would be useful in high speed communications systems.
However, optical fibers have shortcomings due to fiber birefringence. Except for expensive polarization-maintaining or single-polarization fibers, optical fibers modify the state of polarization of light passing through them because of physical irregularity, thermal or mechanical stress, or other environmental factors. This birefringence is typically time-varying and unpredictable. As a result of such birefringence, fiber lasers, such as ring fiber lasers, are unstable in terms of frequency, polarization, amplitude, or power, unless they use extremely expensive and rare polarization-maintaining or single-polarization fibers. Fiber lasers are adversely affected by polarization-mode competition, polarization-mode dispersion, and polarization gain dependence and may manifest mode hopping and polarization instability. Furthermore, multiple pass fiber lasers producing standing waves are adversely affected by spatial hole burning resulting in mode hopping and mode instability.
Various mode-locked fiber lasers have been proposed for producing externally timed pulses. Because of the above-discussed fiber birefringence, such lasers are typically not polarization stable.
U.S. Pat. No. 5,303,314 to Irl. N. Duling, III et al., U.S. patent application filed Apr. 28, 1995 by Ronald D. Esman, James Dexter, Irl. N. Duling and David G. Cooper and having Navy Case No. 75,937 and entitled "A Polarization-Stable Laser," and Cooper et al., "Widely Tunable Polarization-Stable Fiber Lasers," IEEE Jnl of Selected Topics in Quantum Electronics," vol 1, No. 1, pp. 14-21, which references are incorporated herein by reference, describe polarization stable cw lasers in which the gain medium can be made of single-mode optical fiber. Light passes through the gain medium in both directions, each direction having an orthogonal polarization state to the other. The light passing in opposite directions, especially through the gain medium, remains orthogonal because of reciprocal birefringence. Such lasers are not ideal for providing passive mode-locking because of ".chi..sub.3 nonlinearity". As the light intensity changes, the index of refraction changes, and so the polarization state evolves differently for light of differing intensity propagating through the fiber. Therefore, light passing through the gain medium in opposite directions does not necessarily have orthogonal polarization with respect to each other, and polarization-selecting elements in the lasers mentioned above will reject light not in that orthogonal polarization state. The advantages of this design for cw lasers are not fully utilized in passive mode-locking lasers.