Quasi-Static Mode Behavior of Multiple Transverse-Mode VCSELs Under High-Speed Direct Modulation

We have investigated the transverse-mode behavior of 850 nm VCSELs using time-average lasing spectra and eye patterns. Measurements were done at varying DC bias currents, bitrates, and modulation amplitudes. Bitrates range from 0.5 to 10 Gb/s and pseudo-random bit sequence of 2
¹⁵
-1 was applied to the VCSEL with ~6 major modes. DC bias currents were 4, 6, and 8 mA. The amplitude of modulation current is adjusted so that the off-state current is higher than 2 times the threshold current. Correlation coefficients and normalized differences were calculated at each bias current for quantitative analysis using 2.5 Gb/s DC spectrum as a reference. Measurement results show that lasing spectra are determined by DC bias current and almost independent of bitrates and modulation amplitudes. The correlation coefficients are nearly unity and the normalized differences are small. Also, the correlation coefficients and optical-eye shapes remain almost unchanged even with multimode-fiber-VCSEL misalignment. The results mean that transverse modes respond to the modulation current in a synchronized manner when the off-state current is much higher than threshold. This behavior together with the DC-bias determined lasing spectra is called the quasi-static mode behavior. This quasi-static behavior leads to linear relationship between the optical modulation and the modulation current amplitudes, and to optical-eye shapes invariant with fiber-VCSEL misalignment. The quasi-static behavior is weakened as the bitrate is decreased below ~2 Gb/s and modulation current amplitude is increased so that the off-state current decreases near to the threshold current.