Spectral characteristics of asymmetric directional couplers in graded index channel waveguides analyzed by...

We present a detailed analysis of the spectral characteristics of asymmetric directional couplers (ADC's) formed by K(+) -Na(+) ion exchange in BK7 glass and compare the results obtained by the use of normal-mode theory with those obtained by the use of the coupled-mode approach. Maximum power transfer is observed to occur at the wavelength at which the propagation constants of the perturbed component arms are equal. This is attributed to the strong coupling inherent in these devices. Strong coupling and asymmetry are observed to result in unequal confinement of the normal modes, leading to reduce power transfer, even when both arms of the coupler are synchronized. Additionally, significant polarization dependence is observed because of birefringence induced by both the K(+) -Na(+) exchange process and the chosen device structure. Polarization extinction ratios of 11.9 and 26.1 dB are obtained for the arms of the directional couplers. ADC's are observed to operate as bandpass filters, and by adjustment of structural parameters, the peak transmission wavelength can be tuned, with spectral bandwidths of 25-55 nm and coupling lengths of 6-12 mm. Over 90% of the input power in the TE polarization at 1.07 µm can be transferred to the second arm of the coupler, whereas power either in the TM polarization or at wavelengths outside the passband is relatively unaffected. We also show that within the bandpass, where the two arms are phase matched, exact normal-mode analysis yields the same results as the quasi-normal-mode approach, in which the normal modes are expressed in terms of the individual modes.