Scientists report the fabrication of a nonreciprocal optical resonator with a small length footprint of 290 µm on a silicon-on-insulator substrate. The device achieves unidirectional optical transmission with an isolation ratio of up to 19.5 dB near the telecommunications wavelength of 1,550 nm in a homogeneous external magnetic field.

https://udel.edu/~hujuejun/My%20Papers/Journal%20Papers/On-chip%20optical%20isolation%20in%20monolithically%20integrated%20non-reciprocal%20optical%20resonators.pdf

On-chip optical isolation in monolithically integrated non-reciprocal optical resonators

The application of reciprocity principles in optics has a long history that goes back to Stokes, Lorentz, Helmholtz and others. Moreover, optical applications need to be seen in the context of applications of reciprocity in particle scattering, acoustics, seismology and the solution of inverse problems, generally. In some of these other fields vector wave propagation is, as it is in optics, of the essence. For this reason the simplified approach to light wave polarization developed by, and named for, Jones is explored initially to see how and to what extent it encompasses reciprocity. The characteristic matrix of a uniform dielectric layer, used in the analysis of interference filters and mirrors, is reciprocal except when the layer is magneto-optical. The way in which the reciprocal nature of a characteristic matrix can be recognized is discussed next. After this, work on the influence of more realistic attributes of a dielectric stack on reciprocity is reviewed. Some of the numerous technological applications of magneto-optic non-reciprocal media are identified and the potential of a new class of non-reciprocal components is briefly introduced. Finally, the extension of the classical reciprocity concept to systems containing components that have nonlinear optical response is briefly mentioned.

https://iopscience.iop.org/article/10.1088/0034-4885/67/5/R03/pdf

Reciprocity in optics

Silicon-based large-scale photonic integrated circuits are becoming important, due to the need for higher complexity and lower cost for optical transmitters, receivers and optical buffers. In this paper, passive technologies for large-scale photonic integrated circuits are described, including polarization handling, light non-reciprocity and loss reduction. The design rule for polarization beam splitters based on asymmetrical directional couplers is summarized and several novel designs for ultra-short polarization beam splitters are reviewed. A novel concept for realizing a polarization splitter–rotator is presented with a very simple fabrication process. Realization of silicon-based light non-reciprocity devices (e.g., optical isolator), which is very important for transmitters to avoid sensitivity to reflections, is also demonstrated with the help of magneto-optical material by the bonding technology. Low-loss waveguides are another important technology for large-scale photonic integrated circuits. Ultra-low loss optical waveguides are achieved by designing a Si3N4 core with a very high aspect ratio. The loss is reduced further to <0.1 dB m−1 with an improved fabrication process incorporating a high-quality thermal oxide upper cladding by means of wafer bonding. With the developed ultra-low loss Si3N4 optical waveguides, some devices are also demonstrated, including ultra-high-Q ring resonators, low-loss arrayed-waveguide grating (de)multiplexers, and high-extinction-ratio polarizers. Newly developed photonic components could allow large-scale photonic integrated circuits to be built on silicon substrates. Daoxin Dai from Zhejiang University, China, alongside co-workers from the University of California, USA, have proposed several new optical technologies for use in photonic integrated circuits, which substitute or work alongside electrical circuits in optical devices. The researchers have designed new ultrashort polarization-handling devices that split high-intensity beams of light, and a ring optical isolator that reduces reflections. The team have also created a new waveguide based on silicon nitride that can guide optical waves with a minimal loss of energy. These new technologies will allow scientists to construct higher performance, more compact optical devices.

/pdf/passive-technologies-for-future-large-scale-photonic-4doamz9zqi.pdf

Passive technologies for future large-scale photonic integrated circuits on silicon: polarization handling, light non-reciprocity and loss reduction

Magnetooptical garnets combine high Faraday rotation with low optical losses in the near infrared region where optical communication via glass ?ber is established. In this spectral range garnets are the only materials discussed to realize nonreciprocal devices as optical isolators and circulators. Although such devices are available as microoptical components, practical versions of their integrated counterparts are still lacking. Numerous concepts have been developed theoretically many of which are tested experimentally. This paper presents an overview of the state of the art of the applications of garnet films in integrated optics. Also the technique of combining garnets with semiconductor materials is shortly discussed.

Applications of magneto-optical waveguides in integrated optics: review

Many novel materials and device designs have been proposed as photonic analogs to electrical diodes over the last four decades. This paper seeks to revisit these materials and designs as advanced technologies may enable experimental realization that was not possible upon conception of several of these designs. The background behind integration challenges, including waveguide birefringence, fabrication tolerances, garnet/semiconductor mismatch, and optimized interfaces will hopefully spark new ideas that will finally enable the realization of integrated optical isolators and circulators.

Integrated Magneto-Optical Materials and Isolators: A Review

An integrated optical isolator is demonstrated based on efficient nonreciprocal conversion from a fundamental TM mode to a deep TE radiation mode away from the cutoff. The isolator is realized using a single-mode rib channel waveguide in Ce-substituted yttrium iron garnet, which has a very large Faraday rotation. 27 dB isolation is obtained at a wavelength of 1535 nm.

Integrated optical isolator based on efficient nonreciprocal radiation mode conversion

Ce-substituted yttrium iron garnet films are epitaxially grown in situ on (111)-oriented substrates of Gd3Sc2Ga3O12 garnet by conventional rf diode sputtering for application to magneto-optic waveguide devices in optical communication systems The lattice constant of the substrates is larger than that of the previously used (Gd,Ca)3(Ga,Mg,Zr)5O12 garnet substrates whose lattice constant is smaller than that of the films The lowest ever reported propagation losses of 97 dB/cm in the TE mode and 58 dB/cm in the TM mode are obtained for a film thickness of 05 μm at λ=155 μm

Ce-substituted yttrium iron garnet films prepared on Gd3Sc2Ga3O12 garnet substrates by sputter epitaxy

Ce-substituted yttrium iron garnet films are epitaxially grown in situ on (111)-oriented substrates of (Gd, Ca, Mg)3(Ga, Zr)5O12 garnet by conventional rf diode sputtering for application to magneto-optic waveguide devices in optical communication systems. Pure argon or argon containing a small amount of oxygen is used as the sputter gas. After sputtering, the films are annealed in a nitrogen atmosphere. By optimizing this process, film propagation loss can be reduced while maintaining a large Faraday rotation. A propagation loss of 13 dB/cm and a Faraday rotation of -3900°/ cm have been obtained at a wavelength of 1.55 µ m.

Preparation of Ce-substituted yttrium iron garnet films for magneto-optic waveguide devices

PROBLEM TO BE SOLVED: To provide an optical coupling component capable of allowing the light from an optical fiber drawn in parallel with a board surface to be entered to a light receiving element of which an upper surface is faced to the board surface. SOLUTION: This optical coupling component 10A is manufactured by integrally molding amount hole 11 capable of holding a point of an optical fiber, and a concave mirror part 12 capable of changing the course of the light (a) emitted from a point of the optical fiber mounted in the mount hole 11 by 90 degree, and focusing the same to allow the light to be entered into a light receiving element 30 on a board 40, with a light transmitting member such as a transparent plastic material, glass, silicon and the like.

Optical coupling parts

An inexpensive polarization independent optical nonreciprocal circuit which does not require a use of expensive birefringent crystals, optical branching circuits, and a phase adjustment. The optical nonreciprocal circuit is formed by at least one lens through which a light propagating between two waveguides passes; and a polarization rotation circuit for sequentially applying a nonreciprocal rotation and a reciprocal rotation to a first half of the light propagating between two waveguides, and sequentially applying a reciprocal rotation and a nonreciprocal rotation to a second half of the light propagating between two waveguides, so that the first half and the second half of a light propagating forward are set to have an identical polarization to form an even guided mode and coupled, while the first half and the second half for a light propagating backward are set to have opposite polarizations to form an odd non-guided mode and dissipated.

Shintaku Toshihiro

Papers

Integrated optical isolator based on efficient nonreciprocal radiation mode conversion

Ce-substituted yttrium iron garnet films prepared on Gd3Sc2Ga3O12 garnet substrates by sputter epitaxy

Preparation of Ce-substituted yttrium iron garnet films for magneto-optic waveguide devices

Optical coupling parts

Polarization independent optical nonreciprocal circuit based on even mode to odd mode conversion