Svetlen Tonchev

In early July, we unexpectedly lost our dear friend, mentor, and colleague Svetlen Tonchev, one of the world’s foremost experts in diffraction gratings. With a scientific career spanning over 50 years, his contributions to optical physics were profound and far-reaching. From Bulgaria to France to Rochester, Svetlen’s expertise elevated generations of scientists and left a lasting impact on the global science community. Our friend will forever be in our hearts and minds, and his loss is heavy for the scientific community.

Svetlen began his career in Bulgaria at the Laboratory of Optics and Spectroscopy, ISSP-BAS, where he earned his PhD and pioneered work in integrated optics and diffraction gratings. He developed numerous patented waveguide-based devices, introduced novel methods for optical waveguide fabrication in lithium niobate and lithium tantalate, and proposed an original technique for holographically recording blazing gratings. ^{1 2}

In France, Svetlen spent more than 15 years as a visiting professor at the Hubert Curien Laboratory, Jean Monnet University, Saint-Étienne. There, he shared his deep knowledge of grating exposure, etching, and characterization while fostering a culture of scientific integrity and collaboration. He contributed to groundbreaking work on resonant diffraction gratings and surface wave phenomena, often in collaboration with colleagues from the Moscow Institute of General Physics.

Two notable publications highlight his impact:

• The first experimental demonstration of resonant reflection from a metal grating supporting the long range plasmon mode. ³
• A novel method for writing a grating array with an integer number of periods on a cylindrical surface using a single-phase mask exposure. ⁴

In Rochester, Svetlen became a vital part of Richardson Gratings Lab, now MKS, where he continued advancing diffraction grating technology. More than a colleague, he became family—present for holidays, weddings, and life’s milestones. His scientific legacy is immeasurable and his personal warmth unforgettable. Whenever there was a question, his response was always, “Yes, please.” Working alongside Svetlen was an everyday gift, and his impact on our lives personally and professionally will be measured in lifetimes.

Colleagues at the Hubert Curien Laboratory, including university president Florent Pigeon and members of ISSP-BAS, remember Svetlen as a brilliant interdisciplinary scientist, a generous friend, and a man of wit and wisdom. His passing leaves a profound intellectual and emotional void.

At Richardson Gratings, MKS, we will always remember Svetlen in the light that he created, in reflection or transmission, Svetlen was our rainbow maker. In his honor, we will continue the pursuit of scientific discovery, standing on the shoulders of a giant.

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