While true that a birefringence of 0.001 is larger than 0.0001 or 0.00001, one would not normally consider any of these to be "highly birefringent". Please note the table of common birefringent materials in the attached link to the "Birefringence" Wikipedia page: even weakly-birefringent materials like ice, magnesium fluroide, and sapphire have a stronger birefringence than 0.001. Most listed materials possess a birefringence of >0.01, with some crystals (calcite, BBO, and rutile) having a birefringence of >0.1 -- certainly these would be considered "highly birefringent".

https://en.wikipedia.org/wiki/Birefringence#Common_birefringent_materials

Hi Vincent --

Of course, you are correct. Naturally, it is the product of the birefringence and the propagation length that determines the amount of phase shift between the orthogonally-polarized ordinary and extraordinary waves, so certainly a birefringence of 0.001 may be considered high when the propagation distance is especially long, as may be the case with fibers. (Given the numbers in the original question, I suspect you are probably correct about the unstated context, given the magnitudes of the numbers in question. For anyone interested, the attached article on polarization-maintaining fibers discusses the basics of such devices, including typical values one could expect for the birefringence.) If, however, you are hoping to design a waveplate or phase retarder, a birefringence of 0.001 isn't going to get you far :).

I also find it worthwhile to mention that the amount of phase shift between the two orthogonally-polarized waves in a birefringent medium is also dependent on the wavenumber, so certainly the standard for what would be considered "highly birefringent" in the infrared regime compared to the xray regime, especially given typically material responses in these parts of the electromagnetic spectrum, would differ significantly.

Ah, the importance of perspective strikes again! Hope this is useful to someone, ~Eric

https://www.rp-photonics.com/polarization_maintaining_fibers.html

Hi ,---Eric Cunningham and  Vincent  Lecoeuche

I simulate a pcf structure that exhibits only single polarization. I know from research article that if it occur single polarization then if call highly polarized. Why it is called highly birefrengent ?

I get the equation to calculate the birefringence 

birefringence = |nx-ny|

Hi -Vincent Lecoeuche,

I am novice and learning .This question may be silly for anyone . 

My question is that " Why absolutely single polarization fiber is called birefringence fiber " ?

(Can I inbox you about this?)

I hope it is ok to ask here too. I'm looking for the source giving quantatively numbers for o- and e-rays refraction indexes in the ice crystal depending on the wavelength?

https://www.researchgate.net/post/Are_broadband_refractive_indexes_of_o-and_e-rays_in_the_ice_crystal_known