Since LPFGs couple copropagating modes , their resonances can only be observed in transmission spectra .
Because the period of an LPFG is much larger than the wavelength , LPFGs are relatively simple to manufacture .
Since LPFG couple copropagating modes , their resonances can only be observed in transmission spectra .
Because the period of an LPFG is much larger than the wavelength , LPFGs are relatively simple to manufacture .