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^oK 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.