Thermally tunable EDFA gain equalizer using point symmetric cascaded Mach-Zehnder coupler

In this paper, a thermally tunable EDFA gain equalizer filter based on point symmetric cascaded Mach-Zehnder (CMZ) filter based two mode interference (TMI) coupler is presented with its mathematical model. Transmission characteristics of these CMZ couplers are analyzed and compared with Y symmetric CMZ couplers by using this model. For EDFA gain equalizer, point symmetric CMZ circuit is chosen due to its higher wavelength flattening width than Y symmetric CMZ circuit. The ripples of equalized EDFA gain spectrum are formulated and estimated from the equalized gain spectrum of point symmetric CMZ filters. It is found that 2 stage point symmetric CMZ coupler with binomial coupler distribution (2PB CMZ) using Δn = 5% provides gain equalized width of 35 nm with ripple of 0.4-0.6 dB and bending loss of 0.24 dB and device length is ∼15 times lower than that of the existing EDFA gain equalizer based CMZ filter. It is also seen that if during the fabrication process, waveguide core width w is increased or decreased by 0.1 μm (in percentage ∼±6.6%), the power imbalance of TMI based 2PB CMZ filter is slightly increased by ∼8% in comparison to that based on directional coupler (DC) by 40%. Low power thermooptic structure of varying gap between two waveguide cores with silicon trench just below the heater is used and it requires ∼1.5 times less heating power than the conventional structure for thermal tuning of EDFA gain equalization.