Nonlinear Polarization Rotation
When an intense optical pulse propagates in an optical fiber which is not polarization-maintaining, there can be a nonlinear (i.e., intensity-dependent) change in the polarization state. This is usually not exactly a rotation of a linear polarization direction, but rather the change to some elliptical polarization state. The physical cause of these effects is related to self-phase modulation and cross-phase modulation as well as to some (uncontrolled) birefringence of the fiber. If the pulses then pass a polarizer, the power throughput effectively also depends on the optical power.
Self-phase modulation and cross-phase modulation can change the polarization state, essentially since they introduce polarization-dependent changes of optical phase. The details of the polarization evolution can be rather complicated; they depend on the input polarization state and possibly on the random birefringence of the fiber (which is usually not known and depends on the temperature). Therefore, it is not easy to accurately investigate that with modeling.
Use for Passive Mode Locking
While nonlinear polarization rotation can be disturbing e.g. in systems with fiber amplifiers, it is also often utilized for passive mode locking of fiber lasers (→ mode-locked fiber lasers). Here, a typical configuration contains some fiber polarization controller or a set of waveplates, which can be adjusted such that the maximum transmission (minimum loss) at the polarizer occurs for the highest possible optical intensity. The configuration then serves as an artificial saturable absorber. As the Kerr effect is very fast, that artificial absorber is very fast, and its strength is adjustable with the polarization controllers. This makes mode locking with nonlinear polarization rotation a powerful technique. However, there are some severe disadvantages, particularly for use in industrial lasers:
- The optimum polarization settings can drift with temperature, so that readjustment may often become necessary.
- It is difficult to experimentally quantify and reproduce the achieved modulation depth and saturation power.
A more environmentally stable mode-locking technique with nonlinear polarization rotation can be implemented by using a highly birefringent (polarization-maintaining) fiber and Faraday rotators . That technique, however, is less easily implemented in the form of an all-fiber setup.
The technique of mode locking with nonlinear polarization rotation is sometimes also called polarization additive-pulse mode locking. Additive-pulse mode locking has originally been invented for bulk lasers. Here, interference occurs between beams which have propagated different ways in a laser setup, where they experienced different amounts of Kerr nonlinearity. On the other hand, nonlinear polarization rotation can be interpreted as interference of different polarization components which have traveled the same path. A difference is that cross-phase modulation can also occur in the case of nonlinear polarization rotation.
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See also: Kerr effect, cross-phase modulation, optical activity, mode locking, passive mode locking, additive-pulse mode locking, fiber lasers
and other articles in the categories fiber optics and waveguides, nonlinear optics, light pulses