Ultra-Broadband Add-Drop Filter/Switch Circuit Using Subwavelength Grating Waveguides

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An ultrabroadband add-drop filter/switch circuit is designed and demonstrated by integrating a pair of subwavelength grating waveguides in a $2\times 2$ Mach–Zehnder interferometer configuration using silicon photonics technology. The subwavelength grating is designed such that its stopband and passband are distinguished by a band-edge wavelength $\lambda _{\text{edge}} \sim$ 1565 nm, separating C and L bands. The stopband ( $\lambda ) is filtered at the drop port of the device, whereas the passband ( $\lambda >\lambda _{\text{edge}}$ ) is extracted either in cross port or in bar port. The device is designed to operate only in TE polarization. Experimental results exhibit a nearly flat-top band exceeding 40 nm for both stopband and passband. The stopband extinction at cross- and bar ports are measured to be $>$ 35 dB with a band-edge roll-off exceeding 70 dB/nm. Wavelength independent directional coupler design and integrated optical microheaters at different locations of the Mach–Zehnder arms for thermo-optic phase detuning are the key for stopband filtering at the drop port and switching of passband between cross- and bar ports with flat top response. Though the insertion loss of fabricated subwavelength grating waveguides are negligibly small, the observed passband insertion loss is $\sim$ 2 dB, which is mainly due to the combined excess loss of two directional couplers. Experimental results also reveal that the passband switching between cross- and bar ports of the device has been possible with an extinction of $>$ 15 dB by an electrical power consumption of $P_\pi \sim$ 54 mW. A switching time of 5 $\mu$ s is estimated by analyzing the transient response of the device. The passband edge could also be detuned thermo-optically at a rate of 22 pm/mW.