Birefringent hollow core fibers

Hollow core photonic crystal fiber (HC-PCF), fabricated according to a nominally non-birefringent design, shows a degree of un-controlled birefringence or polarization mode dispersion far in excess of conventional non polarization maintaining fibers. This can degrade the output pulse in many applications, and places emphasis on the development of polarization maintaining (PM) HC-PCF. The polarization cross-coupling characteristics of PM HC-PCF are very different from those of conventional PM fibers. The former fibers have the advantage of suffering far less from stressfield fluctuations, but the disadvantage of a higher loss figure and the presence of interface roughness induced modecoupling which increases in strength as birefringence reduces. Close to mode anti-crossing events of one polarization mode, the PM HC-PCF is characterized by high birefringence, a high polarization dependent loss and an increased overlap between the polarization modes at the glass interfaces. The interplay between these effects leads to a wavelength for optimum polarization maintenance, λPM, which is detuned from the wavelength of highest birefringence. By a suitable fiber design involving antiresonance of the core-surround geometry, λPM may coincide with a low-loss wavelength for the signal carrying polarization mode.