Dr. Jacob Khurgin (Johns Hopkins University)
Coherent Frequency Combs in Mid-Infrared and THz Produced By Self Frequency Modulated Quantum Cascade Lasers
For many applications Optical Frequency Combs (OFCs) require a high degree of temporal coherence and thus narrow linewidth1 as well as wide bandwidth (i.e. many spectral lines. Commonly OFCs are generated in some nonlinear media from a monochromatic narrow linewidth laser sources or from a mode-locked laser pulses but in the all-important mid-infrared (MIR) and terahertz (THz) regions of spectrum OFCs can be generated intrinsically (i.e. without any intracavity mode-lockers) by the free-running quantum cascade lasers (QCLs) with high efficiency These combs do not look anything like conventional OFCs as the phases of each mode are different and in temporal domain the OFC is a combination of amplitude- and phase-modulated signals rather than a short pulse. Despite this fact the experimental evidence suggests that the linewidth of the QCL OFC is just as narrow as that of a QCL operating in the single mode. While universally acknowledged, this observation is not fully understood. In this work we rigorously prove the narrowness of the QCL OFC linewidth by deriving the expression for the Schawlow-Townes linewidth and obtain an analytical expression for the maximum potential bandwidth of the frequency modulated comb naturally occurring in free running QCL’s. The bandwidth is shown to critically depend on the flatness of the gain spectrum and the cavity length and less so on pump current. The results firmly establish that the performance of QCL frequency combs can be on par with combs generated by other means.
Expanding Dynamic Range (Linearizing) of Electro Optic Modulators by All Optical Means
Analog photonic systems are crucial for expanding RF photonics applications and higher-order coherent digital systems. The increasing demand for high-performance RF photonic links in 5G and other applications necessitates highly linear transmitters. A key challenge is the inherent nonlinearity of Mach-Zehnder modulators (MZMs), which limits the spurious-free dynamic range (SFDR). Numerous MZM linearization techniques have been explored, including electrical, optical, and mixed methods. Electrical linearization suffers from bandwidth limitations and high power consumption. Mixed methods, often employing multiple modulators, require precise control of numerous voltages and may not compensate for second harmonic distortion. In this talk I highlight the work on development of integrated rugged all-optical linear modulators using three different linearization schemes: (1) Ring Assistant MZI (RAMZI), (2) Grating Assisted MZI (GAMI) and (3) Combined Dual Output MZI. The modulators have been realized using Si, III-V and LiNbO3 platforms and have show record high FDR results.
Bio – Jacob B. Khurgin, a professor of electrical and computer engineering, is known for his diverse and eclectic research in the areas of optics, electronics, condensed matter physics, and telecommunications. Khurgin earned his BS and MS in Optics from the Institute of Fine Mechanics and Optics in St. Petersburg, Russia in 1977 and 1979, respectively. He immigrated to the United States in 1980 and spent eight years working as a researcher at Philips Laboratories in New York. He earned a PhD in Electro-Physics from New York University in 1987 and joined Johns Hopkins in 1988.
This is an in-person seminar. If you opt to join via zoom use meeting ID 817 6524 8204 Passcode 014783