Delays in optical data transmission cause inconvenient lag times in communications and interfere with immersive experiences like gaming. Two factors that adversely affect the speed and reliability of optical communication are signal distortion over long distances and signal interference from noise. Optical modulators, which control certain properties of the light that carries the data, can contribute to transmission delays. To address this issue, researchers at University of Central Florida, College of Optics and Photonics (UCF CREOL) and UCLA developed modulators that compare the amount and the timing of data moving through the system to ensure accurate, efficient transmission. A 3D schematic of the four-phase electro-optic modulator developed by researchers at UCF CREOL and UCLA. The modulator compares the amount and timing of data to ensure accurate efficient transmission. Courtesy of UCF/CREOL. The new class of modulators allows phase diversity (varied timing of signals) and differential operations (comparison of light signals) to be implemented concurrently on a single photonic integrated circuit (PIC). The modulator’s capacity for phase diversity compensates for a degradation in signal quality. The modulator’s capacity for differential operations enables it to overcome intensity noise and other common mode fluctuations and cancel the noise in optical communication links. The researchers fabricated the new modulator, which they call a four-phase electro-optic modulator, in thin-film lithium niobate (TFLN) in a small footprint. The circuit comprises two nested interferometric structures (Mach-Zehnder modulators). “Dubbed four-phase electro-optic modulators, the circuit is demonstrated on thin-film lithium niobate, which is an ultracompact platform for integrated photonic applications, including optical communication,” said Sasan Fathpour, a professor of optics and photonics at CREOL. According to Fathpour, off-the-shelf optical components and existing modulator architectures are not capable of achieving phase diversity and differential operations simultaneously. “The compactness of the thin-film lithium niobate platform allows tight integration of several components on the same small chip and helped [in] shaping up the concept of four-phase electro-optic modulators,” he said. The four-phase electro-optic modulator is augmented by two dispersive optical fiber elements and fiber-optic delay lines for time-stretching and synchronization, respectively. Time-stretch technology is an optical, slow-motion technique invented at UCLA in the 1990s to capture ultrafast, single-shot events. It has led to breakthroughs, such as the creation of the world’s fastest spectrometers, cameras, lidars, velocimeters, oscilloscopes, and more, UCLA professor Bahram Jalali said. UCF CREOL researchers Ehsan Ordouie and professor Sasan Fathpour have developed new technology to improve optical communication. Courtesy of UCF/CREOL. “This new electro-optic modulator architecture culminated from the quest to create improved methods for encoding ultrafast data onto a laser beam to enable time-stretch instruments with high bandwidth and high sensitivity,” Jalali said. The researchers analyzed the four-phase electro-optic modulator within the context of a time-stretch system used to analyze signal processing. They developed a comprehensive analytical model of the modulator and optimized the electro-optic bandwidth and efficiency of the modulator using simulation tools to fine-tune the technology. The team demonstrated applications of the modulator in time-stretch data acquisition and in optical communication. Experimental results showed that the modulator could eliminate common mode noise and dispersion. In simulations, the researchers demonstrated that the modulator could improve signal quality and the power budget in optical communication systems. “Our experiments demonstrate that this approach eliminates the inherent nulls in the frequency response, which is a significant advancement for photonic time-stretch systems and coherent optical communication systems,” researcher Ehsan Ordouie said. The new modulator’s ability to mitigate noise and limitations in bandwidth in optical communications makes it worth the price of a more complicated architecture, according to the team. “Although the proposed modulator is more complex than standard ones, leading to a larger chip size and potentially lower fabrication yield, we believe that the advantages of phase diversity and differential operations justify the added complexity,” Ordouie said. “This breakthrough represents a noteworthy advancement in the practical implementation of photonic systems, and opens up new possibilities for faster and more efficient data communication and acquisition.” The research was published in Nature Communications (www.doi.org/10.1038/s41467-023-41772-y).
