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May 22, 2023

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image: (a) Optical microscope image of the fabricated isolator; (b) Cross-section of the SiN loaded LNOI waveguide; (c) the traveling wave electrode alignment to the waveguide; and (d) SEM image of the fabricated SiN loaded LNOI waveguide. view more

Credit: OES

A new publication from Opto-Electronic Science; DOI 10.29026/oes.2023.220022 considers spatio-temporal isolator in lithium niobate on insulator.

Integrated photonics is making strides towards hosting an increasing range of functionalities on a chip. Examples include information processing and computation as well as optical sensing and ranging applications. This has spurred advances in integrated laser light sources, needed for photonic chips to become truly autonomous devices. Thus, on-chip isolation likewise becomes important for suppressing feedback detrimental to their operation.

Nonreciprocal optical devices can be realized using three methods: magnetic biasing, optical nonlinearity, and spatiotemporal modulation. Magnetic biasing is inherently broadband but requires lossy magneto-optical materials. Nonlinear non-reciprocal devices are achievable monolithically in certain materials, yet their operation is complicated by dependence on input power. Conversely, isolators that leverage spatiotemporal modulation have no such power scaling issues, and can readily be integrated monolithically, particularly on platforms with excellent electro-optical characteristics, such as lithium niobate on insulator (LNOI).

In this contribution, nonreciprocal operation is achieved by using spatiotemporal modulation of two cascaded travelling wave phase shifters. The microwave signal applied to the modulators and delay line ensures that their effect on forward-propagating light cancels out so that its spectral signature remains unchanged. However, reverse-propagating optical power is spectrally dispersed to the sidebands, which are then suppressed by a ring resonator filter, enabling an optical isolation of 27 dB.

Lithium niobate, owing to its broad spectral transparency, high power handling capabilities, and strong nonlinear as well as electro-optic properties, for decades remained a staple material in nonlinear optics and optical fiber communications. The relatively recent emergence of thin-film lithium niobate-on-insulator (LNOI), as a direct analogue of silicon on insulator photonics, has enabled the creation of lithium niobate waveguides with tight mode confinements, that likewise allow for wafer-scale densely integrated photonics. Recent standout achievements on LNOI include efficient electro-optical frequency combs as well as modulators operating at CMOS voltage levels, however, long term prospects for LNOI photonics are extensive, and include fully integrated LiDAR, optical neural networks, or RF signal processing devices to name a few. Key prerequisites for such developments are emerging techniques of heterogeneous integration of on-chip coherent light sources, which for their stable operation must be isolated from feedback from the rest of the circuit. To address this issue the research group of distinguished professor Arnan Mitchell from RMIT university realize integrated isolators in the LNOI waveguide platform.

Their device, depicted in the micrograph of Figure 1 (a), was fabricated using a loaded LNOI waveguide approach, in which refractive index contrast for light confinement is achieved not by etching out lithium niobate, but by processing a silicon nitride layer that is deposited on top of the LNOI wafer. The isolator design is based on a tandem modulator approach in which two identical travelling wave phase modulator sections are connected in series and separated by a looping delay line. The modulators are driven at the same harmonic signal frequency but with a shift in phase, so that for forward propagating light the two modulators counteract each other, and the 1550 nm wavelength carrier light exits the device unchanged. On the other hand, for reverse propagation, this balance set by the delay line and modulating signal phase offset is not valid, hence the action of both modulators in cumulative and spectrally disperses the carrier energy into multiple sidebands. Device input is filtered through a racetrack resonator that is matched to the carrier frequency but designed to reject any modulation-induced spectral sidebands.

Testing the device by injecting the optical carrier in backward and forward directions revealed the non-reciprocal operation of the device. The blue curve in Figure 2 (a) shows the transmission spectrum of the racetrack resonator, which allows optical carriers with the same resonant frequency to pass through. Red curve shows the case when backward light was modulated but without being filtered by racetrack resonator. The original frequency of backward carriers was suppressed, and its power was transferred to sidebands. Figure 2 (b) shows the backward light was modulated and filtered by racetrack resonator, which means the light was strongly spectrally dispersed and is redirected without being able to pass the ring filter. Figure 2 (c) shows in forward propagation, most of the optical power remains confined in the carrier and only minimal power is transferred to the sidebands. Isolation strength was quantified by measuring the power transmission ratio between forward and reverse operation, resulting in an isolation of 27 dB. This result is among the highest spatiotemporal modulation-based isolation ratios achieved on any platform to date. The demonstrated suppression of the reverse propagating light makes such isolators suitable for the integration with III-V laser diodes and Erbium doped gain sections in the thin-film lithium niobate on insulator waveguide platform.

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The Integrated Photonics and Applications Centre (InPAC) was established in 2020 at RMIT University in Australia and, under the leadership of distinguished Prof Arnan Mitchell, hosts 9 research staff and 20 PhD students. Their goal is to create impactful integrated photonic technologies covering a range of platforms, including silicon, silicon nitride as well as lithium niobate. The centre brings together teams that strive to develop and refine photonic chip simulation and design, fabrication, and interfacing capabilities as well as application-oriented teams that work on data communication, biomedical detection, as well as defence and precision sensing.

A key focus for the centre is to build expertise and in-house capabilities to perform the entire integrated photonic device creation workflow. Using the state-of-the art fabrication tools available at RMIT's Micro Nano Research Facility 2,500 square metre cleanroom, the device platform development teams continuously work to devise modular building blocks for custom photonic chips to address the needs identified by application-oriented teams, collaborating researchers, and industry partners. Conversely, data communication, biomedical detection, and defence and precision sensing teams serve as bridge between applied photonic integrated circuit research and its translation to real-world applications. Through this approach, InPAC seeks to make integrated photonics technologies more readily available to a broader worldwide community, including researchers and small, specialised industries.

InPAC website:

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Opto-Electronic Science (OES) is a peer-reviewed, open access, interdisciplinary and international journal published by The Institute of Optics and Electronics, Chinese Academy of Sciences as a sister journal of Opto-Electronic Advances (OEA, IF=9.682). OES is dedicated to providing a professional platform to promote academic exchange and accelerate innovation. OES publishes articles, reviews, and letters of the fundamental breakthroughs in basic science of optics and optoelectronics.

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Huang HJ, Balčytis A, Dubey A, Boes A, Nguyen TG et al. Spatio-temporal isolator in lithium niobate on insulator. Opto-Electron Sci 2, 220022 (2023). doi: 10.29026/oes.2023.220022

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image: (a) Optical microscope image of the fabricated isolator; (b) Cross-section of the SiN loaded LNOI waveguide; (c) the traveling wave electrode alignment to the waveguide; and (d) SEM image of the fabricated SiN loaded LNOI waveguide. # # # # # # # # # # # # # # # # # # More information: Editorial Board: Contact Us: Twitter: WeChat: # # # # # # 2 # # # # # # Disclaimer: