Spectrometer is used to measure the numerical aperture of photonic crystal fibers.The method overcomes the limitation that the traditional measurement can only measure the numerical aperture in some fixed wavelengths,Therefore the parameters rebated with wavelength can be better studied such as:the mode field area cut-off wavelength and so on.The measured results agree well with the theoretical calculation ones.According to the measured numerical aperture,the mode field area of fiber is calculated and consi...