Tetsuya Mizumoto

We present recent progress in cryogenic optical modulators for data links from cryogenic to room temperature environments, which can have applications in emerging cryogenic classical and quantum computing systems. Two efficient solutions based on III-V alloys and Ce:YIG are presented, utilizing the electro-optic and magneto-optic effects, respectively.

In this paper, an on-chip optical circulator on the InP-membrane-on-Si (IMOS) platform is demonstrated. The circulator is composed of two multi-mode interferometers (MMIs), four polarization converters (PCs), and a Cerium-doped Yttrium Iron Garnet (Ce:YIG) die. The Ce:YIG die is adhesively bonded on the InP membrane via a 125-nm thin bonding layer. Nonreciprocal phase shift (NRPS) is employed in the presence of a transverse magnetic field. The device works as a 4-port optical circulator with a maximum optical isolation of 27.0 dB and a minimum optical isolation of 18.6 dB for the TE mode. The measured optical isolation bandwidth is 0.12 nm. Two methods are proposed in order to improve the optical isolation bandwidth.

Superconducting circuits can operate at higher energy efficiencies than their room-temperature counterparts and have the potential to enable large-scale control and readout of quantum computers. However, the required interface with room-temperature electronics creates difficulties in scaling up such cryogenic systems. One option is to use optical fibres as a medium in conjunction with fast optical modulators that can be efficiently driven by electrical signals at low temperatures. However, as superconducting circuits are current operated with low impedances, they interface poorly with conventional electro-optical modulators. Here we report an integrated current-driven modulator that is based on the magneto-optic effect and can operate at temperatures as low as 4 K. The device combines a magneto-optic garnet crystal with a silicon waveguide resonator and integrates an electromagnet to modulate the refractive index of the garnet. The modulator offers data rates of up to 2 Gbps with an energy consumption below 4 pJ per bit of transferred information, which could be reduced to less than 50 fJ per bit by replacing dissipative electrodes with superconductors and optimizing the geometric parameters.

Thermomagnetic recording is a technique used as a writing process for magneto-optical (MO) drives. Despite their significant advantages, such as rewritability, nonvolatility, reliability, and large cycling endurance, MO drives are rarely used today because of the complex drive systems that must deal with magnetic field and lightwave simultaneously. This study reports on the light-induced thermomagnetic recording of a ferromagnetic thin-film CoFeB on a Si photonic platform. Lightwave guided in the Si waveguide evanescently coupled to the thin-film magnet and underwent optical absorption, resulting in heating and a decrease in coercive force. Therefore, we observed magnetization reversal with an applied magnetic field for both continuous and modulated light pulses using a magneto-optical Kerr effect microscope, and the light-induced thermomagnetic recording was experimentally demonstrated on a Si photonic platform. The proposed scheme enables the realization of on-chip MO memories on the Si photonic platform in which neither bulky free-space optics nor mechanical rotation systems are required.

Optical isolators are necessary components in photonic integrated circuits to ensure system stability by blocking the back-reflected light waves. In this study, a transverse electric mode magneto-optical isolator based on an asymmetric microring resonator without additional polarization rotators in series is demonstrated. Cerium-substituted yttrium iron garnet was integrated with silicon-on-insulator substrates by wafer bonding to break the Lorentz reciprocity on-chip after applying a unidirectional magnetic field. The isolator shows a 22 dB isolation ratio and a 4.3 dB insertion loss at an operation wavelength of 1572.62 nm.

We firstly demonstrate light-induced thermomagnetic recording of a ferromagnetic thin-film CoFeB placed on a silicon waveguide. The magnetization reversal is observed when light propagates in the waveguide and evanescently heats up the thin-film magnet.

In photonic integrated circuits (PICs), optical isolators and circulators are essential to prevent lasers from backreflections and to reroute the light flow. In this paper, an integrated polarization-independent device that can be operated as an optical isolator or an optical circulator, based on an InP membrane on silicon platform, is demonstrated. A cerium-doped yttrium iron garnet die is adhesively bonded on a Mach–Zehnder interferometer, in combination with four polarization converters. The device shows maximum optical isolations of 27.0 dB for transverse-electric (TE)-mode input and 34.0 dB for transverse-magnetic TM-mode input. The device also works as a four-port optical circulator. Optical isolations of at least 18.6 dB and 16.4 dB are measured between each circulator port pair for TE- and TM-mode input, respectively. This work could remove the optical interfaces between laser and isolator for robust production. It also provides a step forward toward a multifunctional and high-density PIC.

This paper extends our previous work on the microring resonator-type serial-to-parallel conversion scheme aimed for all-optical label processing on the silicon photonic platform. In the previously proposed scheme, the architecture of the delay lines was silicon wires. This resulted in considerable propagation loss, confining multibit operation of serial-to-parallel conversion to a label packet of 2 bits. However, during a practical application, propagation loss at the delay lines has to be improved to support label packets comprising of a greater number of bits. The purpose of this article is twofold. First, we confirm that propagation loss can be lowered to sub-dB/cm values by introducing rib waveguides via simulations and experiment. Second, we demonstrate multibit serial-to-parallel conversion on a 4-bit label “1101” by utilizing a cascaded microring resonator device of which the straight sections of the delay lines are of rib waveguide architecture. In order to focus on free-carrier dispersion effect of silicon during operation, we left out a temporal delay between the pump pulses and probe bits to be extracted. As a result, serial-to-parallel conversion was viable even at peak pump power levels as low as ∼100 mW. We hereby verify that the proposed serial-to-parallel conversion scheme incorporated with rib waveguide delay lines is a realistic solution in realizing a photonics-based solution to all-optical label processing.

As an indispensable component in the photonic integrated circuits, the design and fabrication of optical isolators, particularly in the transverse electric (TE) polarized mode, is a long-standing challenge. Herein, we present a TE mode magneto-optical isolator using adiabatic tapered waveguides to realize conversions between designated modes. The isolator exhibits an ultranarrow structure of 1.27 μm × 1498 μm. We demonstrate that the device functions under a TE mode input with a maximum isolation ratio of 15 dB and an insertion loss of 5 dB at a wavelength of 1537.3 nm.

Silicon waveguide optical isolators were fabricated by direct bonding of magneto-optical (MO) garnet. The technique allowed efficient MO phase shift owing to the use of single-crystalline garnet and negligibly thin interlayer on the silicon core layer. A Mach–Zehnder interferometer (MZI) provided optical isolation utilizing the MO phase shift. High isolation, wide bandwidth, and temperature-insensitive operations had been demonstrated by tailoring the MZI design. Also, transverse electric (TE)–transverse magnetic (TM) mode converters were integrated to control operating polarization. In this paper, we reviewed these progresses on silicon waveguide optical isolators.