High-Speed VCSELs with Strong Confinement of Optical Fields and Carriers

We present the design, fabrication, and performance of our latest generation high-speed oxide-confined 850-nm verticalcavity surface-emitting lasers. Excellent high-speed properties are obtained by strong confinement of optical fields and carriers. Highspeed modulation is facilitated by using the shortest possible cavity length of one half wavelength and placing oxide apertures close to the active region to efficiently confine charge carriers. The resulting strong current confinement boosts internal quantum efficiency, leading to low threshold currents, high wall-plug efficiency, and state-of-the-art high-speed properties at low bias currents. The temperature dependent static and dynamic performance is analyzed by current-power-voltage and small-signal modulation measurements

High Speed VCSELs and VCSEL Arrays for Single and Multicore Fiber Interconnects

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

Quasi-Single Mode VCSELs for Longer-Reach Optical Interconnects

The vertical-cavity surface-emitting laser (VCSEL) is the standard light source in optical interconnects connecting racks in supercomputers and datacenters hosting services such as online data sharing, storage and processing. VCSELs have numerous advantages such as low power consumption, fast direct modulation at low currents, circular output beam for efficient fiber coupling and low-cost fabrication. However, today's commercial optical interconnects, operating at around 10 Gbit/s over up to 300 m of multimode fiber, have insufficient speed and reach for future datacenters. The goal of this work has been to extend the maximum reach of GaAs-based 850 nm VCSEL-based optical interconnects. Recently developed high-speed VCSELs featuring strained InGaAs quantum wells and multiple oxide layers are highly transverse multimode with large root-mean-square (RMS) spectral widths around 1 nm. This leads to problems with chromatic and modal fiber dispersion, limiting the maximum reach to around 100 m at 25 Gbit/s. By using quasi-single mode VCSELs with a side-mode suppression ratio of ~15-20 dB and RMS spectral widths around 0.3 nm, the reduced dispersive effects enable extended reach at high data rates. Two different techniques to reduce the spectral width are investigated. By using a small oxide aperture, the number of modes guided by the VCSEL waveguide is significantly reduced. A 3 um oxide aperture VCSEL was used to transmit 22 Gbit/s over 1100 m of OM4 fiber. However, small aperture devices have high resistance and relatively low output power. The spectral width of larger aperture VCSELs can be reduced by etching a shallow surface relief that induces a mode selective loss to suppress higher order modes. In effect, the etched feature acts as an integrated mode filter. A 6 um oxide aperture surface relief VCSEL enabled transmission at 20 Gbit/s over 2000 m of fiber, setting a new bit-rate-distance product record for directly modulated 850 nm VCSEL links. Benefits and drawbacks of both methods are discussed and compared

25 Gbit/s transmission over 500 m multimode fibre using 850 nm VCSEL with integrated mode filter

An integrated mode filter in the form of a shallow surface relief was used to reduce the spectral width of a high-speed 850 nm vertical-cavity surface-emitting laser (VCSEL). The mode filter reduced the RMS spectral width from 0.9 to 0.3 nm for a VCSEL with an oxide aperture as large as 5 mu m. Because of reduced effects of chromatic and modal fibre dispersion, the mode filter significantly increases the maximum error-free (bit error rate < 10(-12)) transmission distance, enabling transmission at 25 Gbit/s over 500 m of multimode OM3+ fibre

High-speed 850 nm VCSELs operating error free up to 57 Gbit/s

Error-free transmission is demonstrated at bit rates up to 57 Gbit/s back-to-back, up to 55 Gbit/s over 50 m fibre and up to 43 Gbit/s over 100 m fibre using an oxide-confined 850 nm high-speed vertical cavity surface-emitting laser with a photon lifetime optimised for high-speed data transmission

Silicon-Integrated Hybrid-Cavity 850-nm VCSELs by Adhesive Bonding: Impact of Bonding Interface Thickness on Laser Performance

The impact of bonding interface thickness on the performance of 850-nm silicon-integrated hybrid-cavity vertical-cavity surface-emitting lasers (HC-VCSELs) is investigated. The HC-VCSEL is constructed by attaching a III–V “half-VCSEL” to a dielectric distributed Bragg reflector on a Si substrate using ultrathin divinylsiloxane-bis-benzocyclobutene (DVS-BCB) adhesive bonding. The thickness of the bonding interface, defined by the DVS-BCB layer together with a thin SiO2 layer on the “half-VCSEL,” can be used to tailor the performance, for e.g., maximum output power or modulation speed at a certain temperature, or temperature-stable performance. Here, we demonstrate an optical output power of 2.3 and 0.9 mW, a modulation bandwidth of 10.0 and 6.4 GHz, and error-free data transmission up to 25 and 10 Gb/s at an ambient temperature of 25 and 85 °C, respectively. The thermal impedance is found to be unaffected by the bonding interface thickness

GaAs High-Contrast Gratings with InGaP Sacrificial Layer for Multi-Wavelength VCSEL Arrays

We report on highly reflective suspended GaAs high-contrast gratings (HCGs) using an InGaP sacrificial layer. A high reflectivity approaching 100% was observed both in direct reflectivity measurement and by low threshold currents in fabricated multi-wavelength HCG-VCSEL arrays

High-speed 850 nm VCSELs with 28 GHz modulation bandwidth for short reach communication

We present results from our new generation of high performance 850 nm oxide confined vertical cavity surface-emitting lasers (VCSELs). With devices optimized for high-speed operation under direct modulation, we achieve record high 3dB modulation bandwidths of 28 GHz for similar to 4 mu m oxide aperture diameter VCSELs, and 27 GHz for devices with a similar to 7 mu m oxide aperture diameter. Combined with a high-speed photoreceiver, the similar to 7 mu m VCSEL enables error-free transmission at data rates up to 47 Gbit/s at room temperature, and up to 40 Gbit/s at 85 degrees C

22 Gb/s error-free data transmission beyond 1 km of multi-mode fiber using 850 nm VCSELs

The first error-free data transmission beyond 1 km of multi-mode fiber at bit-rates exceeding 20 Gb/s is demonstrated using a high modulation bandwidth, quasi-single mode (SMSR similar to 20 dB) 850 nm VCSEL. A VCSEL with small similar to 3 mu m aperture shows quasi-single mode operation with a narrow spectral width. The top mirror reflectivity of the VCSEL is optimized for high speed and high output power by shallow etching. A combination of narrow spectral width and high optical power reduces the effects of fiber dispersion and fiber and connector losses and enables such a long transmission distance at high bit-rates

20 Gbit/s error-free operation of 850 nm oxide-confined VCSELs beyond 1 km of multimode fibre

Error-free transmission over 1.1 km of OM4 multimode fibre is demonstrated at 20 Gbit/s bit rate using a narrow spectral width, high-speed 850 nm vertical-cavity surface-emitting laser