The growing demand for faster data exchange between data centers calls for compact, high-speed optical transmitters. Researchers from Japan developed an integrated optical transmitter that mitigates the destabilizing effects of unwanted optical feedback, allowing stable laser operation without an optical isolator. The device operates across a wavelength range of approximately 67 nm and demonstrates data transmission at 180 Gbit/s, offering a promising approach to compact, high-capacity optical communication systems.
The growing use of generative artificial intelligence (AI) and cloud services is increasing data exchange between data centers, creating a need for faster, more energy-efficient, compact optical transmitters. One promising approach is to integrate silicon photonics-based tunable lasers, which generate light at different wavelengths, with thin-film lithium niobate (TFLN) modulators, which encode data onto light at high speeds. However, integrating lasers and modulators built on different platforms can introduce unwanted optical reflections from the modulator back into the laser. These reflections can destabilize the laser and disrupt data transmission.
Conventional systems use optical isolators to block backward-travelling light and protect the laser from unwanted reflections. Researchers, led by Professor Tomohiro Kita of the Department of Applied Physics at Waseda University in Japan, developed an integrated transmitter that controls wavelength-dependent optical feedback through cross-platform spectral co-design, allowing the laser to operate stably without an optical isolator.
The study was published in the Journal of Lightwave Technology on October 07, 2026. The research team also included Shotaro Hirata, Shingo Takano, Yu Kataoka, and Ryo Shimizu, researchers at Sumitomo Osaka Cement Co., Ltd., Japan, and Dr. Yuya Yamaguchi of the National Institute of Information and Communications Technology (NICT), Japan.
“This architecture provides a promising pathway toward compact, isolator-free optical transmitters for next-generation photonic integrated circuits and high-speed optical communication systems,” says Kita.
The transmitter combines an external-cavity wavelength-tunable laser, formed by a semiconductor gain chip and a silicon photonics circuit containing two silicon nitride ring resonators, with a TFLN modulator incorporating an asymmetric Mach–Zehnder interferometer (AMZI). The two ring resonators form a Vernier filter that selects the lasing wavelength, while the TFLN modulator encodes high-speed data onto the optical signal.
The AMZI has a wavelength-dependent transmission response. By co-designing this response with that of the SiN Vernier filter, the researchers shape the wavelength-dependent optical feedback returning toward the laser and mitigate its destabilizing effects. This cross-platform spectral co-design enables stable single-mode laser operation without an optical isolator.
The transmitter can operate across a broad range of wavelengths, approximately 67 nm, while maintaining a stable laser output even without active temperature control. The researchers also demonstrated data transmission at 180 Gbit/s, the highest data rate investigated in the present experiment, with test results showing that the optical signals remained distinguishable at these high speeds.
By eliminating the need for an optical isolator, the design could simplify the optical transmitter’s design and potentially reduce its footprint, making it easier to integrate different photonic technologies into compact devices. Such a design could be particularly useful for high-capacity optical connections between data centers, including AI data centers, which need to transfer large amounts of data quickly.
"The technology is particularly relevant to high-capacity optical interconnects between data centers, including AI data centers. Its broad wavelength tunability makes it suitable for wavelength-division multiplexing, in which multiple data channels at different wavelengths are transmitted through a single optical fiber," says Kita.
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Reference
Authors: Tomohiro Kita 1 , Shotaro Hirata 2 , Shingo Takano 2 , Yu Kataoka 2 , Yuya Yamaguchi 3 , and Ryo Shimizu 2
Affiliations:
1 Department of Applied Physics, Waseda University, Japan
2 Sumitomo Osaka Cement Co., Ltd., Japan
3 National Institute of Information and Communications Technology (NICT), Japan
About Waseda University
Located in the heart of Tokyo, Waseda University is a leading private research university that has long been dedicated to academic excellence, innovative research, and civic engagement at both the local and global levels since 1882. The University has produced many changemakers in its history, including eight prime ministers and many leaders in business, science and technology, literature, sports, and film. Waseda has strong collaborations with overseas research institutions and is committed to advancing cutting-edge research and developing leaders who can contribute to the resolution of complex, global social issues. The University has set a target of achieving a zero-carbon campus by 2032, in line with the Sustainable Development Goals (SDGs) adopted by the United Nations in 2015.
To learn more about Waseda University, visit https://www.waseda.jp/top/en
About Professor Tomohiro Kita from Waseda University
Professor Tomohiro Kita leads the Optical Integration Device Laboratory at Waseda University’s Faculty of Science and Engineering. Specializing in silicon photonics and photonic integrated circuits, his research focuses on developing advanced technologies for next-generation communications and sensing applications. He actively explores LiDAR systems for autonomous vehicles and terahertz wave transceivers to support Beyond 5G and 6G networks. Kita earned his Ph.D. from JAIST and previously held positions at Tohoku University. His work combines chip design with experimental evaluation, aiming to create smaller, more precise devices that enhance societal safety and convenience through innovative light and radio wave manipulation.
Journal of Lightwave Technology
Experimental study
Not applicable
Spectrally Engineered Isolator-Free Silicon Photonic Transmitter Based on a SiN External-Cavity Laser and Thin-Film Lithium Niobate Modulator
7-Oct-2026
Nothing to declare