The signal pulses will be broadened through the fiber and so we need a very sharp cutoff filters with demultiplexers. The in-line fiber filter becomes the appropriate filters. Most of these demultiplexers used after erbium-doped fiber amplifier (EDFA) which need a sharp wavelength bandpass filter (BPF) to filter out wavelengths other than the signal wavelength Exact analysis of in-line fiber filter is occurred by derive a polynomial expression for the dopant ratios of GeO2 and P2O5. The refractive index of silica doped either GeO2 or P2O5 is defined as a function of both the wavelength and the dopant ratio. The calculations of the dopant ratios of the pair materials (GeO2 and P2O5) are done at a specific filter cutoff wavelength (λc) for both high pass filter (HPF) and low pass filter (LPF). Two in-line fiber filters (HPF and LPF) are cascaded to make a sharp bandpass filter (BPF). The length of in-line fiber filter must be very short to overcome the weak of the propagation constsnt throuh it. The normalized frequency is discused. A theoritical 8-channels demultiplexer is done. The design of demultiplexer becomes very easy.

The performance of optical fiber directional coupler depends upon the material dispersion and the change of temperature. Usually, the directional coupler is designed by assuming that it is used with a specific temperature such as 300 o K and a specific wavelength such as 1.55μm. The directional coupler is used with different wavelengths and temperatures than that the specific values. Therefore, with neglect the effect of temperature and change of operating wavelength there are errors in the performance of directional coupler as a power divider (error of output powers ), power combiner (error of output powers) and as a bandpass filter (error of passband wavelengths). The propagation constants of the two fibers and the coupling coefficient between the two fibers are functions of the operating temperature and operating wavelength. The errors for as a power divider (as a combiner) are greater than the error for as a bandpass filter. As the germania ratio increases, the error value increases. Also as the absolute value of the difference between the wavelength and the reference wavelength increases, the error becomes more. Also, this occurs with the temperature. While as the coupler length (L) increases, the error of output power either increases or decreases. These errors can be minimized at certain operating and structure parameters of the optical fiber directional coupler.