Optical Fiber Communications Laboratory
High-capacity optical communication through linear and nonlinear channels including free space and optical fiber using synergy of advanced optical and electronic techniques.
... More than fiber, more than communication: photonic integration, computing, imaging and
...Beyond publication: innovation, patenting and commercialization...
Optical Communication
Free-Space Optical Communication
Our work in free-space optical communication focuses on wavefront correction using techniques such as 1) electronic wavefront correction via coherent detection and digital signal processing and 2) few-mode optical pre-amplification.
Space-Division Multiplexing
Our lab was the first to demonstrate long-distance transmission of using few-mode fibers, which enables space-division multiplexing to increase capacity beyond the single-mode fiber capacity limit. Our lab developed the theory of the few-mode EDFA and experimentally demonstrated the first amplified mode-division multiplexed transmission in few-mode fibers in collaboration with NEC and Corning. We introduced the concept of supermode fibers, which is also referred to as strongly-coupled multi-core fibers. We developed techniques for frequency-domain equalization for electronic mode demultiplexing.
Digital Coherent Optical Communication
Coherent optical communication experienced a remarkable revival in recent years. Our contributions include the first introduction of the MIMO concept to coherent optical communication for polarization demultiplexing, use of infinite-impulse response filters for digital dispersion compensation, orthogonal wavelength-division multiplexing and electronic wavefront correction for free-space coherent optical communications. Our lab's most noteworthy contribution is fiber nonlinearity compensation using digital backward propagation. With digital backward propagation, all deterministic linear and nonlinear impairments can be compensated, pushing the envelope for fundamental capacity at high transmission powers. A large number of recording-setting transmission experiments employ digital backward propagation.
Microwave Photonics
Our group takes advantage of nonlinear dynamics in multi-section semiconductor lasers for application in analog fiber-optic links and subcarrier-multiplexed (SCM) optical networks. Particular areas that we have made significant contributions include optical generation of microwave/millimeter-wave (MMW) signals, high-gain and low noise figure analog fiber-optic links using few-mode fibers, and optical phased-array beamforming for massive MIMO.
Clock Recovery and Regeneration
To realize all-optical retiming, reshaping and reamplification (3R regeneration), clock recovery at the line rate is a necessary function that must be realized in the optical domain. Our approach to all-optical clock recovery also exploits nonlinear dynamics in multi-section semiconductor lasers, namely, coherent injection locking of self-pulsation two-section gain-coupled DFB lasers. Our group demonstrated record speed in all-optical clock recovery at 180 Gb/s, which still remain as the highest speed in all-optical clock recovery at the line rate.
In the area of optical regeneration, our group has made fundamental contributions especially for regeneration of phase-modulated signals. Prior to our pioneering work on phase regeneration, all-optical regeneration was limited to regeneration of intensity levels. It was not clear whether signals encoded in the optical phase could be regenerated from a fundamental perspective. Using the phase-sensitive amplification processes, we proposed and demonstrated regeneration of the two phase levels of a DPSK signal.
Photonic Integration Circuits
We design PDKs in house and fabricate large-scale photonic integrated circuits for optical communication, optical computing, and imaging. Our group is among only a selected few university labs that have completed dedicated runs at commercial foundries.
Optical Computing
Computing has been dominated by electronics. Research in optical computing must find solutions that complement and supplement electronics in real world applications. Our current research focuses on photonic tensor accelerators for large-scale tensor operations. We demonstrated the first optical computing architecture that exploits all degrees of freedom of light, including amplitude, phase, polarization, wavelength as well as spatial modes for scalability. We also demonstrated the first floating-point operation that is essential for optical computing to play any role in training neural networks, in inference of large neural networks, and possibly in scientific computing.
Coherent Beam Combining/Multi-Plane Light Conversion
Multi-Plane Light Conversion (MPLC) can perform unitary transformation of the electric field of light. It consists of a series of phase masks separated by (free-space) diffraction. We have used MPLC not only to do mode (de)multiplexing but also a myriad of other functions related to optical communication such as ultrawide-band hybrid mixing, multi-mode hybrid mixing and simultaneous wavelength and mode (de)multiplexing. Another area of application for MPLC that we are pursuing is coherent beam combining using non-mode selective MPLCs.
Imaging
Our research on image originated from single-shot digital holography using 90-degree optical hybrids frequency used in coherent optical communication. Built upon digital holography, our current research focuses on optical diffraction tomography (ODT), which can be thought of as taking holography in multiple angles and reconstructing the object using these holograms. We have developed some of the most advanced, physics-based and optimized based inversion algorithms for ODT.