Performance of PAM-4 VCSEL for short reach 100 Gb/s per lane applications up to 85°C

We report on development and characterization of 850 nm vertical-cavity surface-emitting lasers (VCSELs) having a -3dB modulation bandwidth above 24 GHz with a flat frequency response at temperatures up to 85 degrees C. Aperture size is optimized for a high relaxation oscillation frequency with a narrow spectral width and low relative intensity noise. Two types of VCSELs (Gen 1 and Gen 2) with different epitaxial designs are fabricated with an optimized aperture size. Large-signal modulation at 53 GBd PAM-4 (106 Gb/s) is performed for eye diagram and TDECQ measurements. The Gen 1 VCSEL is capable of 53 GBd PAM-4 modulations at temperatures up to 70 degrees C, but performance is insufficient at 85 degrees C. The Gen 2 VCSEL with a stronger optical confinement achieves higher modulation bandwidth with an extremely suppressed resonance peak in frequency response, leading to reduction in TDECQ compared to the Gen 1 VCSEL. TDECQ below 4.5 dB are verified at temperatures up to 85 degrees C without any pre-emphasis in the transmitter. Also, we use a pre-emphasis with 3-tap feed forward equalizer to improve the TDECQ by 2 dB. Furthermore, after the transmission over 100 m multimode fiber (OM5), the TDECQ keeps below 3.0 dB even at 85 degrees C. These results demonstrate the capability of 850 nm VCSELs for 100 Gb/s per optical lane short-reach interconnects operating over a wide temperature range.