VCSEL design breaks laser power, speed limits

November 20, 2020 //By Jean-Pierre Joosting
New VCSEL design breaks laser power and speed limits
Novel vertical-cavity surface-emitting laser (VCSEL), the workhorses of datacenters and optical sensors, demonstrates record-fast temporal bandwidth.

Researchers at the George Washington University have developed a new design of VCSEL that demonstrates record-fast temporal bandwidth. This was achieved by combining multiple transverse coupled cavities, which enhances optical feedback of the laser.

VCSELs are a vital class of semiconductor laser diodes that are 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, known as 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 known as "slow-light," thus extending the temporal laser bandwidth (speed) 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.

Fast, powerful compact lasers: a novel VCSEL for next-generation datacenters and sensors. Image courtesy of Volker Sorger/GWU.

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