Isolator-free optical transmitter could shrink data center interconnects

Data centers are racing to keep up with the explosive growth of generative artificial intelligence and cloud services, and the optical links that carry data between these facilities have become a critical bottleneck. Now, researchers in Japan have built an integrated optical transmitter that can run stably without an optical isolator, the bulky component that conventional systems rely on to protect lasers from their own reflected light. The device operates across a wavelength range of roughly 67 nanometers and demonstrated data transmission at 180 gigabits per second, the highest rate investigated in the experiment, offering a pathway toward more compact, high-capacity optical communication systems.

The work, led by Professor Tomohiro Kita of the Department of Applied Physics at Waseda University, was published in the Journal of Lightwave Technology on October 7, 2026. The research team also included Shotaro Hirata, Shingo Takano, Yu Kataoka, and Ryo Shimizu of Sumitomo Osaka Cement Co., Ltd., Japan, along with Dr. Yuya Yamaguchi of the National Institute of Information and Communications Technology (NICT), Japan. The study was an experimental investigation of a transmitter that combines two photonic platforms that normally do not like to share a chip.

The core idea behind the new transmitter is to merge silicon photonics-based tunable lasers, which generate light at different wavelengths, with thin-film lithium niobate (TFLN) modulators, which encode data onto that light at very high speeds. Each platform brings a distinct strength: silicon photonics excels at compact, wavelength-selective filtering, while TFLN is prized for fast, efficient electro-optic modulation. But integrating lasers and modulators built on different platforms introduces a stubborn problem. Unwanted optical reflections from the modulator travel back into the laser, and these reflections can destabilize the laser and disrupt data transmission.

In conventional systems, that problem is solved with an optical isolator, a device that blocks backward-travelling light and shields the laser from its own reflections. Isolators, however, are typically bulky, difficult to integrate with chip-scale photonics, and add to the size and complexity of the transmitter. Eliminating them has long been a goal for engineers trying to shrink optical interconnects, particularly for the dense, high-throughput links that AI data centers demand.

The new device attacks the feedback problem at its source by shaping it spectrally. 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 that incorporates 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 crucial trick lies in the AMZI’s wavelength-dependent transmission response. By co-designing this response with that of the silicon nitride Vernier filter, the researchers shaped the wavelength-dependent optical feedback returning toward the laser and mitigated its destabilizing effects. This cross-platform spectral co-design, the team reports, enables stable single-mode laser operation without an optical isolator. In other words, rather than blocking the reflected light with a separate component, the transmitter engineers the interaction between the two photonic platforms so that the feedback no longer wrecks the laser’s stability.

The experimental results point to practical benefits beyond stability. The transmitter can operate across a broad range of wavelengths, approximately 67 nanometers, while maintaining a stable laser output even without active temperature control. That combination of wide tunability and thermal tolerance matters for real deployments, where temperature swings and channel switching are routine. The researchers also demonstrated data transmission at 180 gigabits per second, 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 transmitter’s architecture 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 enormous amounts of data quickly. The technology is also relevant to wavelength-division multiplexing, in which multiple data channels at different wavelengths travel through a single optical fiber, and the transmitter’s broad wavelength tunability makes it suitable for that role, according to Kita.

“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 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.”

As data centers continue to multiply and the traffic between them swells, approaches like this one, which trade added components for clever spectral engineering, could help determine how small, fast, and energy-efficient the next generation of optical interconnects becomes. The demonstration remains an experimental result at the highest data rate investigated in the present experiment, but it sketches a concrete route toward transmitters that pack laser, filter, and modulator onto integrated platforms without the isolator that has long been considered indispensable.

Subject of Research: An integrated isolator-free silicon photonics and thin-film lithium niobate optical transmitter for high-speed data center interconnects

Article Title: Isolator-free optical transmitter could shrink data center interconnects

Article References: Isolator-free optical transmitter could shrink data center interconnects. Original publication (Bibliographic details unavailable.)

Image Credits: AI Generated

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Keywords: optical transmitter, silicon photonics, thin-film lithium niobate, optical isolator, data centers, laser stability, wavelength-division multiplexing, photonic integrated circuits, optical feedback, high-speed data transmission, Isolator-free, optical