Single-mode broadband coupler employs the bandwidth-extending Technique (Asymmetrical Technique) to extend bandwidth up to+/-40nm at 1310/1550nm channels by changing the wavelength characteristics. In this broad range, the accurate coupling ratio can be obtained. Moreover, this kind of couplers can satisfy the requirements of Low Excess Loss, Low PDL, etc. Single-mode Dual Window Couplers have already been widely used in broadband light source, test instruments and broadband optic-fiber networks.