Many fiber Bragg grating interrogation systems uses ratiometric wavelength measurement based on an edge filter. In any ratiometric scheme a 3dB coupler is a vital component which splits the signal and makes the system ratiometric. All commercial 3dB couplers exhibit a wavelength dependency and polarization dependent loss. In this paper the effects of the wavelength dependency and polarization dependent loss of the 3dB coupler in a ratiometric wavelength measurement system are investigated using both simulation and experimental techniques. The ratio response of the system is simulated considering the wavelength dependency of the coupler and is compared with that of a response with a wavelength independent coupler. A comparison study of the polarization induced ratio fluctuation and corresponding errors in wavelength with a polarization insensitive 3 dB coupler (very low PDL) and an ordinary 3 dB coupler is also presented. The results show that the 3 dB coupler has a significant influence on the ratio response and accuracy of a ratiometric wavelength measurement system.