That鈥檚 why researchers with 麻豆原创’s and the University of California, Los Angeles, have developed new technology to make data transfer over optical fiber communication faster and more efficient.

Their new development, a novel class of optical modulators, is detailed in a new study published recently in the journal . Modulators can be thought of as like a light switch that controls certain properties of data-carrying light in an optical communication system.

鈥淐arrying torrents of data between internet hubs and connecting servers, storage elements, and switches inside data centers, optical fiber communication is the backbone on which the digital world is built,鈥� says Sasan Fathpour, the study鈥檚 co-author and CREOL professor. 鈥淭he basic constituents of such links, the optical fiber, semiconductor laser, optical modulator and photoreceiver, all place limits on the bandwidth and the accuracy of data transmission.鈥�

Fathpour says particularly the dispersion of optical fibers, or signal distortion over long distances, and noise of semiconductor lasers, or unwanted signal interference, are two fundamental limitations of optical communication and signal processing systems that affect data transmission and reliability.

He says their research has invented a unique class of optical modulators that simultaneously address both limitations by taking advantage of phase diversity, or varied timing of signals, and differential operations, or comparison of light signals.

By doing so, the researchers have created an advanced 鈥渓ight switch鈥� that not only controls data transmission but does so while comparing the amount and timing of data moving through the system to ensure accurate and efficient transmission.

鈥淒ubbed four-phase electrooptic modulators, the circuit is demonstrated on thin-film lithium niobate, which is an ultracompact platform for integrated photonic applications, including optical communication,鈥� Fathpour says.

The concepts of phase diversity and differential operation existed before this research and have been explored by the UCLA team, he says.

鈥淭he problem is that off-the-shelf optical components and existing modulator architectures are not capable of achieving these two operations simultaneously,鈥� Fathpour says. 鈥淭he compactness of the thin-film lithium niobate platform allows tight integration of several components on the same small chip and helped shaping up the concept of four-phase electrooptic modulators.鈥�

Bahram Jalali, a distinguished professor emeritus and Fang Lu Chair in Engineering in the Electrical and Computer Engineering Department at UCLA, says the concept originated from 25 years of research into time-stretch instruments, an optical slow-motion technique that stands as the most effective method for capturing ultrafast single-shot events.

鈥淚nvented at UCLA in the 1990s, time-stretch technology has yielded breakthroughs, such as the creation of the world鈥檚 fastest spectrometers, cameras, lidars, velocimeters, oscilloscopes, and more, ultimately uncovering optical rogue waves and the introduction of innovative blood screening microscopes, among other advancements,鈥� Jalali says. 鈥淭his new electrooptic 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.鈥�

How the Research Was Performed

The four-phase electrooptic modulator was analyzed within the context of a time-stretch system used for analyzing signal processing, and a comprehensive analytical model was developed to explain its operation. The technology was also optimized for electro-optic bandwidth and modulation efficiency using simulation tools for fine-tuning.

The application of the four-phase electrooptic modulator in optical communication was also explored. It was shown that the four-phase electrooptic modulator can eliminate common mode noise and dispersion, and simulation results demonstrated its ability to improve signal quality and power budget in optical communication systems.

Ehsan Ordouie 鈥�23PhD was a doctoral student in optics and photonics when the research was conducted and is the study鈥檚 lead author. He worked on mathematical modeling, device simulations, chip design, fabrication and more.

He says the innovative device enables both phase diversity and differential operations on a single photonic integrated circuit, thereby canceling the dispersion penalty, or signal quality degradation, and noise in optical communication links.

鈥淥ur 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,鈥� Ordouie says. 鈥淎lthough 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. 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.鈥�

Collaborators

Study co-authors also included Tianwei Jiang and Tingyi Zhou with the University of California, Los Angeles; 麻豆原创 optics and photonics doctoral students Farzaneh Juneghani ’21MS and Mahdi Eshaghi ’21MS;聽and former graduate research assistant Milad Vazimali 鈥�22PhD.

Researcher Credentials

Fathpour received his doctoral degree in electrical engineering from the University of Michigan, Ann Arbor. He joined the faculty of the College of Optics and Photonics in 2008.

Ordouie was a graduate student when the research was conducted and received his doctoral degree in optics and photonics from 麻豆原创 in 2023.

The work was conducted by Ph.D. student Jeff Chiles in professor Sasan Fathpour鈥檚 research group and was recently published in聽鈥淥ptica,鈥� a peer-reviewed publication from The Optical Society that emphasizes high-impact scientific and technology innovations in optics and photonics.

A key aspect of light is the wavelength of the electromagnetic wave associated with it.聽 The visible color range of light 380 to 700 nanometer wavelengths; the invisible light that is shorter wavelengths is ultraviolet, and the longer wavelengths is infrared.聽 The infrared spectrum has several divisions, the most widely studied of which are the near- and short-infrared regimes.聽 These wavelengths are the realm of optical-fiber telecommunications (telecom), without which the internet and high-speed telecom would not exist.

The longer, mid-infrared wavelengths 鈥� loosely defined as 3 to 5 micrometers 鈥� are another range covering bands of low-loss optical transmission in the atmosphere. This range also covers the absorption spectrum of vibrational modes of organic molecules, whose signature absorption resonances can be used for environmental monitoring, biological sensing, etc. Also, water absorbs light strongly at certain wavelength bands in the mid-infrared and the corresponding photoablation process in tissues has potential applications in skin surgery and dentistry.

Mid-infrared has been one of the least-developed wavelength regimes, however. It is a difficult range due to certain physical limitations, and hence both optical sources and detectors that can operate at these wavelengths are very difficult to achieve. Progress has been made in this regard in the past decade or so and decent quantum-cascade lasers and uncooled photodetectors have been developed.

Another key photonic device for system applications is optical modulators and that is what CREOL researchers have demonstrated for the first time. Optical modulators are, in essence, light switches needed to turn light off and on in order to transmit data in a communication link at very high speeds.

Standard waveguide and modulator platforms developed for shorter wavelengths do not work properly at mid-infrared wavelengths for a variety of reasons. 聽聽To achieve mid-infrared optical modulators, Chiles bonded a thin film (220 nanometers thick) of silicon to a lithium niobate wafer by a novel technique. Lithium niobate is an exotic compound of聽niobium,聽lithium and聽oxygen and has a strong electro-optic effect useful for ultrafast modulators. The silicon-on-lithium-niobate (SiLN) was then used as a platform to fabricate optical waveguides and modulators at CREOL鈥檚 nanofabrication facility, an advanced 麻豆原创 multiuser laboratory equipped with state-of-the-art tools such as electron-beam lithography. The fabricated modulators were then characterized by pumping light in and out of the photonic chips at the mid-infrared wavelength of 3.39 micrometers showing satisfactory performances.

The work is funded under the Office of Naval Research Young Investigator Program and the National Science Foundation CAREER Program.

鈥淥ne of the best things about our research group is our willingness to keep pushing the boundaries in photonics,鈥� Chiles said. 鈥淚t鈥檚 a lot of work to create a new photonic platform. We could have settled with the conventional material platforms already available, but they still had many unsolved problems and missing features. The SiLN platform enables new mid-infrared devices that were not practical to make before. The whole project took place at CREOL 鈥� the concept, design, fabrication and measurement, and the result is a testament to the excellent facilities we have available.鈥�

Chiles holds a 麻豆原创 Trustees Doctoral Fellowship and won the IEEE Photonics Society 2014 Summer Topical Meeting student paper award on related research.

鈥淲e had previously demonstrated the first lithium-niobate thin films on silicon wafers to drastically improve the performance of modulators in the near- and short-infrared regimes,鈥� Fathpour said. 鈥淲hat Jeff has brilliantly done is to flip the argument over and put silicon thin films on lithium-niobate wafers. It is probably not the only way to get modulators in the mid-infrared, but this approach has several advantages including the possibility of doing nonlinear optics on the same chip and taking advantage of the strong electro-optic effect in lithium niobate yet achieving very compact devices鈥�.

The researchers are doing additional work to characterize the devices at higher speeds. The challenge is the availability of high-speed photodetectors in the mid-infrared. They also would like to investigate the nonlinear optical properties of the demonstrated waveguide platform for attaining light at wavelengths other than the input of the laser source, a phenomenon known as wavelength conversion.

Awards totaling $39.9 million will help 149 university researchers at 84 academic institutions purchase state-of-the-art research equipment, which will benefit science education, medical training and the preparation of troops.

鈥淥ur strategy of focusing on research that stimulates the local and state economies has served us very well in competing for these federal awards,鈥� said MJ Soileau, vice president for research and commercialization at 麻豆原创. 鈥淥ur faculty have consistently shown that they can compete with the best and the equipment purchased with these awards will position them even better for future funding. This is particularly important since most contracts and grants are for specific work to be done and have little or no money for purchase of major capital equipment.鈥� 聽聽聽

The money coming to 麻豆原创 will purchase major equipment for projects in the Center for Research and Education in Optics and Lasers|College of Optics and Photonics (CREOL|COP) and the Institute for Simulation and Training in partnership with the College of Nursing.聽 聽聽

The CREOL|COP projects include:

Professor Greg Welch from the College of Nursing, IST and the College of Engineering and Computer聽Science received $178,000 from the Office of Naval Research to purchase specialized humanoid robots with computer-rendered or rubber 鈥渟kin鈥� faces that allow the robots to聽perform as surrogate humans. The work he is conducting, along with professors Arjun Nagendran in IST and Charles Hughes in CECS, will create an聽integrated platform for testing and developing these surrogate humans and associated computer graphics and animation to assist in training for the聽military, healthcare and teaching.聽

The Army Research Office, Office of Naval Research, and Air Force Office of Scientific Research fund the projects. They received 735 proposals requesting $278 million in support for research equipment.

麻豆原创, the University of Illinois and Rutgers University were the only universities to receive five awards.聽

The other Florida universities receiving awards this year are the University of Miami and Florida Atlantic University, with one award each.

The work, led by professor Sasan Fathpour, was recently published in Optics Express, a peer-reviewed publication that emphasizes scientific and technology innovations in optics and photonics.

It is hard to imagine our modern society without electronic microchips. Microelectronics heavily owes its success to silicon 鈥� an extraordinary material with unique properties 鈥� on which up to billions of tiny devices, mostly transistors,聽are routinely integrated. The choice for integrated photonic materials has not been as clean-cut as integrated electronics. Integrated photonics is the technology of integrating several photonic devices such as lasers, optical switches, modulators and detectors on a single chip for using light as a means to transmit data over the Internet, between equipment racks of data centers and supercomputers, and perhaps on boards of future laptops and other mobile devices. 聽聽

Despite decades of research, there appears to be no ideal photonic material that can play the unifying role that silicon has been dominantly playing for decades in microelectronics. For example, some materials are good for laser light generation, while some are good for electro-optics (an effect useful for switches and modulators). Silicon itself is a good passive optical material to move light around a chip and there are ways to make modulators on it but the performance of the devices is not great because the material is not electro-optic.聽

Lithium niobate (an exotic compound of聽niobium,聽lithium and聽oxygen) has a strong electro-optic effect and the vast majority of ultrafast modulators that drive the Internet are based on it.

“Lithium niobate devices are bulky and expensive,” Fathpour said. “If thin films of lithium niobate are developed and the geometrical cross-section of the devices on the films can be reduced to submicron dimensions, the field of integrated photonics can move toward a more unifying platform.”

In addition, if such achieved miniaturized photonic devices are made on silicon wafers that already house electronic circuits, ultrafast photonics and electronics can be seamlessly merged. Such a hybrid platform can pave the path toward using optics to transmit data between microprocessors, graphic and memory chips of future personal computers, game consoles, laptops and tablets.聽

Fathpour and his team’s breakthrough results make such a hybrid versatile platform closer to reality. For the first time, they have managed to bond thin films less than half a micrometer thick of lithium niobate to silicon wafers. The films themselves may have applications other than photonics (microelectromechanical systems, microwave聽filters for cell phones and piezoelectric transducers, to name a few).聽 For now, Fathpour鈥檚 group is more focused on demonstrating basic integrated photonic devices such as low-loss microring resonators and high-performance optical modulators.

Dr. Payam Rabiei, a research scientist at CREOL, and three graduate students have been working for more than a year to make this happen. They have managed to demonstrate electro-optic modulators whose driving voltage is several times less than the best commercial devices. 聽This has become possible because the dimensions of the devices are less than a micrometer in width and height, compared to tens of microns in conventional devices.聽 Having smaller devices means less voltage, and less real estate on thin films means less consumption of expensive lithium niobate wafers.

Unlike, conventional lithium niobate waveguides (such as light pipes) that can be hardly bent, the 麻豆原创 researchers have demonstrated optical rings with diameters less than a third of a millimeter for the first time on the material. All of these will allow integrating several miniaturized modulators and other photonic devices on a single chip for advanced communication formats. The work is funded under the Office of Naval Research Young Investigator Program.

鈥淭he advantage of our novel platform is beyond optical telecommunication applications,鈥� Fathpour said. 鈥淟ithium niobate is one of the best known nonlinear optical materials but with the lack of small and efficient waveguides, nonlinear photonic chips that can manipulate the color of light, and quantum optics chips that allow quantum-mechanical interaction of light and matter have been hard to achieve.鈥� 聽聽

He said he expects these nonlinear and quantum optical chips can become a reality in the near future.

His group plans to start working on building such chips in addition to pursuing their research on advanced photonic devices for optical telecommunication applications.

Sasan Fathpour, an assistant professor in the College of Optics & Photonics, and Jennifer Pazour, an assistant professor in Industrial Engineering & Management Systems, were recognized for showing 鈥渆xceptional promise for doing creative research鈥� early in their tenure tracks, according to the Navy.

Only two other universities 鈥� Massachusetts Institute of Technology and University of Texas 鈥� had two recipients chosen for the Young Investigator Awards.

Fathpour was awarded a three-year, $680,000 grant to study novel nonlinear integrated photonic devices on silicon.

The research involves working on fast, low-power optical modulators that are more compact than existing devices. The modulators are used to turn on and off the light generated by semiconductor lasers in order to send data over the Internet and other communication links. One day the devices may be used to optically connect the microelectronic chips of a laptop or to send huge amounts of 3-D video data to a TV, Fathpour said.

Fathpour said the research has multiple engineering and fundamental scientific aspects, ranging from telecommunication devices that are 10 to 100 times more compact than the state of the art to new ways of efficiently manipulating the wavelength of light on microchips. He said he is thankful to his research team, particularly to Dr. Payam Rabiei, who played a critical role in achieving the initial results that led to the success of the research proposal.

Fathpour has been at 麻豆原创 four聽years, and his group started working on this project last summer. He earned his Ph.D. in electrical engineering from the University of Michigan. Last year, he received the National Science Foundation CAREER award, another grant designed for tenure-track faculty.

Pazour received a three-year, $509,000 grant to study the design of sea-based logistic delivery systems, which operate in challenging environments that include the need for increased security measures, synchronization of sea-based logistics with land operations, the absence of permanent infrastructure, and individual logistic-transport needs.

Pazour is developing models and algorithms to evaluate and improve naval logistics.

鈥淏ecause of the wear and tear of identification information that can occur during transport, the difficulty of real-time transmission due to network bandwidth limitations, and the lack of scanning equipment at the receiving end, logistic decisions have to be made in an opaque network with imperfect visibility,鈥� Pazour said.

Pazour earned her bachelor鈥檚 degree from South Dakota School of Mines and Technology, and her masters and Ph.D. from the University of Arkansas.

This is Fathpour鈥檚 third NSF Award, but Dean Bahaa Saleh said the CAREER Award is the foundation鈥檚 most prestigious recognition in support of junior faculty members.

Fathpour鈥檚 prize-winning research, Mid-Infrared Photonic Devices and Integrated Circuits on Silicon, involves working with wavelengths beyond visible light that can interact with interesting molecular vibrations of organic materials.聽 One distinction in his study is that the base of silicon chips he works with is made of sapphire, which is a difficult process but very rewarding for achieving miniaturized devices.

鈥淪ilicon on sapphire is a new platform for photonic engineering,鈥� he said. 鈥淭hese are very exotic wafers.鈥�

Someday the photonics research may prove valuable in laser medicine applications such as ophthalmic microsurgery, facial surgery, and tooth cleaning, as well as in biochemical sensing and optical ranging,聽 he said.

The NSF award is for $400,000 over five years.

Fathpour has been at 麻豆原创 three 聽years, and he started working on this project two years ago. He earned his Ph.D. in Electrical Engineering from the University of Michigan.

In a well-attended presentation Wednesday afternoon at Frontiers in Optics 2009, Sasan Fathpour, assistant professor of integrated photonics & energy solutions at the 麻豆原创’s CREOL (麻豆原创 College of Optics and Photonics), outlined the benefits and challenges facing engineers attempting to make integrated photonic components more environmentally friendly.

Computing-related power usage represented 15 percent of the total power consumption in the US in 2007, he said, and most of the power used by large data centers is consumed by its communications equipment. The annual power requirements of such massive centers, including Internet giant Google, represents seven times the power generation of the Hoover Dam.

Integrated circuits have become faster and faster in recent years by adding more and more transistors to chips – some have nearly 2 billion.

In a related FiO talk on the road to building an exascale supercomputer, Jeffrey Kash of IBM Research said progress in chip clock speeds has recently slowed because engineers have essentially reached the upper limits of on-chip electronics. To make more powerful computing systems now, they are not making more powerful microprocessors, just using more of them (dual core, quad-core etc.). Also, electrical packaging is running out of pin space. Increasing the role of optics could help with both problems, he said.

Under the high optical intensities required by today’s chips, silicon begins “soaking up” photons in a process called two-photon absorption, or TPA. That process leads to more electrons being freed to gobble up even more photons (free-carrier absorption). The loss of photons is the main problem to be overcome in silicon devices that use nonlinear optics to perform optical amplification, lasing, wavelength conversion and switching.

Fathpour and his CREOL colleagues have demonstrated that the optical energy lost to TPA can be converted into electrical power used to drive the chip. They came to that conclusion by taking a concept used in photovoltaic (PV) processes – optical absorption – and applying it to silicon.

Their process, called the two-photon photovoltaic effect (TPPV), collects 40 percent of the carriers lost through TPA, Fathpour said, harvesting their energy. TPPV is the nonlinear equivalent of the single-photon PV effect exploited in solar cells.

“This is the first observation of the two-photon PV effect in any material,” Fathpour said.

TPPV can also be used to harvest energy in Raman amplifiers, in optically powered sensors for fiber optic networks, and in compound semiconductors, as the free-carrier absorption effect in III-V materials is typically the same as in silicon. It could also lead to all-silicon optically powered sensors, which could be beneficial for applications where sparks generated by electrical wiring would be dangerous, such as in coal mines and near airliner fuel tanks, Fathpour said.

For more information, visit: or see the article on green silicon photonics cover article by Fathpour and colleagues in the June 2009 issue of Optics and Photonics News.

Source: photonics.com, by Melinda Rose, senior editor, Oct. 16, 2009

Objective: The objective of this program is to demonstrate InGaAs/GaAs quantum dot laser grown on dislocation filters on silicon substrates that will be used as a pump source for cladding-pumped Si Raman amplifiers integrated on the same chip.

Intellectual merit: The intellectual merit is the first demonstration of high-performance hybrid III-V on Si lasers and their integration with Si Raman amplifiers.

Broader impacts: The broader impacts are the interdisciplinary nature of the program, its undergraduate and underrepresented minorities and K-12 outreach. The proposed program provides a broader impact to science and society beyond technical merits. It combines optoelectronics with microelectronics, physics, and materials science and engineering, thereby extends optoelectronics to real applications. The proposed program outlines interdisciplinary research for graduate and undergraduate students. Outreach projects for women and underrepresented minorities are highlighted. The program proposes to expose middle- and high-school students to scientific and engineering disciplines.

For more information visit the National Science Foundation (NSF) .