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.