Magneto-optical modulator for superconducting digital output interface

We propose an ultrafast magneto-optic (MO) modulator for the SFQ-to-optical digital interface. Our MO modulator is based on the Faraday effect and consists of a microwave microstrip line (MSL) with a polarization-sensitive MO active medium and a fiber-optic cw light delivery. The light modulation occurs in parallel to the magnetic field and perpendicular to the rf signal propagation. The low characteristic impedance of MSL, together with the superconducting ground plane, ensures that the magnetic-field component of the electromagnetic signal is uniform and effectively “focused” across the length of the modulator. For several different MO devices of the above geometry, we have numerically calculated magnetic-field distributions inside the MO material and verified that the H field was uniform over the width of the top electrode. The input modulation current was assumed to be 1 mA-the realistic upper current-output value for the Nb-based SFQ circuit. Taking EuSe as the MO material at 4.2 K, we obtained H=2.51 Oe for a device with 100-μm-wide top electrode and characteristic impedance of 4.4 Ω. The H magnitude could be further increased to as much as 60 Oe for a macroscopic device with the 5-mm-long optical interaction distance, yielding the 36° phase retardation and ~10% modulation depth in the single-pass-type device. The most desired configuration for the MO modulator was found to be a Mach-Zehnder design. The Mach-Zehnder interferometer increases the device sensitivity, making it very attractive for direct, SFQ-to-optical digital I/O interface