How does Bragg reflections occurs through propgation in PCF?

I need an explanation for this:
I know that for low index core photonic crystal fiber, the propagation ocuurs for the modes in the photonic bandghap using bragg reflections.
Please Could any one expalin how does this occur? or what meant by bragg reflections?
please any one has any expalanation for this, it will be very helpful.

Well, there are two kind of PCFs, the index guiding and the bandgap guiding. I suppose that by low index PCF you mean the bandgap guiding, where the transversal leaking is prevented due to the full bandgaps of the periodic cladding.

Could you expalin this in details (low index guding)?

Index guiding: the refractive index of the defect core is higher than the refractive index of hole cladding (commonly air). Propagation is based on total internal reflection.

Bandgap guiding: the refractive index of the defect core is lower than the refractive index of hole cladding. Propagation is based on the photonic bandgap effect.

If you still need more details I can give you some pictures

i know what you told me in the above message.
My question
How the propagation occures in the low index core depending on the photonic bandgap effect. ?

Well, the defect core, which has a lower refractive index, operates like a cavity for the wavelengths into the photonic bandgap regime. In normal conditions (without the photonic crystal structure in the fiber), light can not propagate through the core because it has a lower refractive index than the cladding.
Photonic crystals are periodic structures that do not allow certain wavelenghts to propagate through them, working like reflectors. Bragg reflectors are almost the same thing. So, if light gets into the core, it is trapped into it because it reflects to the photonic crystal "walls"(bandgap effect), and so it propagates through core even if core has a lower refractive index than cladding.
Hope I explained well.

Thanks alot for your time.
Do you interested in photonic crystal fiber modelling or not? What is your interest?

Well, Iam interested in photonic crystal devices in general. My last year project (undergraduate studies) was on that topic. Although, my job is in a totally different area now, I try to keep some kind of contact with Photonic crystals...(mostly I read than work on that issue). Hope some day I manage to work again on PCs.

i hope tha we can manage work together in this field