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 con sists of a microwave microstrip line (MSL) with a polarization-sensitive MO active medium and a fiber-optic cw light delivery, The light modulation oc curs in parallel to the magnetic field and perpendicular to the rf signal p ropagation. The low characteristic impedance of MSL, together with the supe rconducting ground plane, ensures that the magnetic-field component of the electromagnetic signal is uniform and effectively "focused" across the leng th 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 t op electrode. The input modulation current was assumed to be 1 mA-the reali stic upper current-output value for the Nb-based SFQ circuit. Taking EuSe a s the MO material at 4.2 K, we obtained H = 2.51 Oe for a device with 100-m um-wide top electrode and characteristic impedance of 4.4 a. The H magnitud e could be further increased to as much as 60 Oe for a macroscopic device w ith the 5-mm-long optical interaction distance, yielding the 36 degrees pha se retardation and similar to 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 devic e sensitivity, making it very attractive for direct, SFQ-to-optical digital I/O interface.