Breaking the Power Limit of Lasers

Researchers at the George Washington Univer­sity have developed a new design of vertical-cavity surface-emitting laser (VCSEL) that demons­trates record-fast temporal bandwidth. This was possible by combining multiple transverse coupled cavities, which enhances optical feedback of the laser. VCSELs have emerged as a vital approach for realizing energy-efficient and high-speed optical inter­connects in data centers and super­computers.

Fast, powerful compact lasers: a novel design of vertical-cavity surface-emitting laser (VCSEL) for next-generation datacenters and sensors. (Source: V. Sorger, GWU)

VCSELs are a vital class of semi­conductor laser diodes accompanying a monolithic laser resonator that emits light in a direction perpen­dicular to the chip surface. This class of lasers is gaining market importance given their compact size and high opto­electronic performance. As minia­turized lasers, they are used as an optical source in highspeed, short-wavelength communi­cations and optical data networks. Dense traffic and high-speed trans­mission are key require­ments for smart sensor applications in automotive or in data communi­cations, which are enabled by compact and high-speed VCSELs. However, the 3-dB bandwidth, the speed limit of VCSELs, is limited by thermal effects, parasitic resistance, capa­citance and nonlinear gain effects.

Direct modulation of VCSELs cannot exceed about 30 GHz due to nonlinear optical ampli­fication effects known as gain relaxation oscil­lations. This invention introduces a revo­lutionary novel VCSEL design. Since feedback inside the laser needs to be carefully managed, researchers intro­duced a multi-feedback approach by combining multiple coupled cavities. This allowed them to strengthen the feedback, thus extending the temporal laser bandwidth beyond the known limit of the relaxa­tion oscil­lation frequency. The innovation is ground-breaking because the direct feedback from each cavity only needs to be moderate and can be controlled precisely via the coupled cavities, allowing for a higher degree of design freedom. Following this coupled cavity scheme, a resulting modulation bandwidth in the 100 GHz range is expected.

“Here we introduce a paradigm-shift in laser design. We utilize a novel coupled cavities approach to carefully control the feedback to the laser achieved by signi­ficantly slowing the laser light down. This coupled cavity approach adds a new degree of freedom for laser design, with opportunities in both funda­mental science and technology”, said Volker Sorger, associate professor of electrical and computer engi­neering at the George Washington Univer­sity. “This invention is timely since demand for data services is growing rapidly and moving towards next generation communi­cation networks such as 6G, but also in automotive as proximity sensor or smart phone’s face ID. Furthermore, the coupled cavity system paves a way for emerging appli­cations in quantum information processors such as coherent Ising machines.” (Source: GWU)

Reference: E. Heidari et al.: Hexagonal transverse-coupled-cavity VCSEL redefining the high-speed lasers, Nanophot. 9, 0437 (2020); DOI: 10.1515/nanoph-2020-0437

Link: Nanophotonics Lab. (V. Sorger), Dept. of Electrical and Computer Engineering, George Washington University, Washington, USA • Omega Optics Inc., Austin, USA

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