Characterization of the Phase and Amplitude Modulation of Injection-Locked VCSELs at 1550 nm Using Coherent Optical Demodulation

Experimental measurements of the optical amplitude and phase modulation from an injection-locked VCSEL at 1550 nm are reported. The use of a coherent optical demodulation technique, using a 90° optical hybrid, balanced photodiodes, and a high-speed oscilloscope, has enabled the complete modulation response to be recorded as a function of time. The VCSEL's response to current modulation was examined under various experimental parameters: over a wide frequency range, from kilohertz to gigahertz, for small and large current modulation and for different bias currents. Low-frequency (<;1 MHz) current modulation leads to temperature modulation, where the injection-locked VCSEL acts predominantly as a phase modulator, with a maximum depth of phase modulation, β, of 0.34 π rad. In the high-frequency (>100 MHz) electronic modulation regime, the VCSEL produces significant amplitude and phase modulation, with a maximum depth of amplitude modulation, m, of 35% and β = 0.47 π rad. The VCSEL's thermal response is much more efficient (requires less RF power) in producing phase modulation than its electronic response. The detailed experimental data on the optical phase modulation response of an injection-locked VCSEL are reported for the first time here and are useful for its many applications.