Higher speed VCSELs at different wavelengths and adapted to different modulation formats are needed to meet demands for higher optical interconnect capacity and bandwidth density. We discuss VCSEL dynamics and speed limitations, recent progress on high-speed VCSELs from 850 to 1060 nm, and less conventional VCSEL designs for potentially more significant speed improvements. We also discuss the interconnect capacity enabled by equalization, pulse shaping, higher order modulation formats, digital signal processing, and wavelength division multiplexing.
The hybrid vertical-cavity laser is a potential low current, high-efficiency, and small footprint light source for silicon photonics integration. As part of the development of such light sources we demonstrate hybrid-cavity VCSELs (HC-VCSELs) on silicon where a GaAs-based half-VCSEL is attached to a dielectric distributed Bragg reflector on silicon by adhesive bonding. HC-VCSELs at 850 nm with sub-mA threshold current, >2 mW output power, and 25 Gbit/s modulation speed are demonstrated. Integration of short-wavelength lasers will enable fully integrated photonic circuits on a silicon-nitride waveguide platform on silicon for applications in life science, bio-photonics, and short-reach optical interconnects.
Vertical-cavity surface-emitting lasers and multi-mode fibers is the dominating technology for short-reach optical interconnects in datacenters and high performance computing systems at current serial rates of up to 25-28 Gbit/s. This is likely to continue at 50-56 Gbit/s. The technology shows potential for 100 Gbit/s.
We present a vertical-cavity surface-emitting laser (VCSEL) where a GaAs-based “half-VCSEL” is attached to a
dielectric distributed Bragg reflector on silicon using ultra-thin divinylsiloxane-bis-benzocyclobutene (DVS-BCB)
adhesive bonding, creating a hybrid cavity where the optical field extends over both the GaAs- and the Si-based parts of
the cavity. A VCSEL with an oxide aperture diameter of 5 μm and a threshold current of 0.4 mA provides 0.6 mW
output power at 845 nm. The VCSEL exhibits a modulation bandwidth of 11 GHz and can transmit data up to 20 Gbps.
Our recent work on high speed 850 nm VCSELs and VCSEL arrays is reviewed. With a modulation bandwidth approaching 30 GHz, our VCSELs have enabled transmitters and links operating at data rates in excess of 70 Gbps (at IBM) and transmission over onboard polymer waveguides at 40 Gbps (at University of Cambridge). VCSELs with an integrated mode filter for single mode emission have enabled transmission at 25 Gbps over >1 km of multimode fiber and a speed-distance product of 40 Gbps·km. Dense VCSEL arrays for multicore fiber interconnects have demonstrated 240 Gbps aggregate capacity with excellent uniformity and low crosstalk between the 40 Gbps channels.
We present a GaAs-based VCSEL structure, BCB bonded to a Si3N4 waveguide circuit, where one DBR is substituted by
a free-standing Si3N4 high-contrast-grating (HCG) reflector realized in the Si3N4 waveguide layer. This design enables
solutions for on-chip spectroscopic sensing, and the dense integration of 850-nm WDM data communication transmitters
where individual channel wavelengths are set by varying the HCG parameters. RCWA shows that a 300nm-thick Si3N4
HCG with 800nm period and 40% duty cycle reflects strongly (<99%) over a 75nm wavelength range around 850nm. A
design with a standing-optical-field minimum at the III-V/airgap interface maximizes the HCG’s influence on the
VCSEL wavelength, allowing for a 15-nm-wide wavelength setting range with low threshold gain (<1000 cm-1).