Fig. 1. Experiment setup. A tunable laser (SDL 8610) provides the probe signal and is modulated using a Mach-Zehnder interferometer. The optical attenuator is used to adjust the signal power. PM OC and fibers are used to couple the light into the VCSEL cavity. Time domain measurements are carried out using a fast photo-receiver and oscilloscope. An EDFA is used to amplify the reflected signal. The optical spectra and power levels are monitored by OSA and power meter respectively. (M-Z: Mach-Zehnder interferometer, PM: polarization maintaining, EDFA: Erbium-doped fiber amplifier, OSA: optical spectrum analyzer.)

Fig. 2. Optical spectra taken by sweeping the tunable laser frequency at various VCSEL biases. Spectra are taken when the VCSEL is biased around threshold (1.15mA) and the asymmetric profile indicating it is lasing, while symmetric profile indicating it is in the amplifier regime.

Fig. 3. Measurements of delay for a RF-modulated optical signal (f = 2.8 GHz) at various VCSEL bias conditions around threshold. The probe laser power is fixed at zero detuning, P = 100 nW. The reference waveform in red is taken when the VCSEL is off. Delays increase with increasing VCSEL bias current.

Fig. 4. Measured delay waveforms at modulation frequencies 1, 2, and 3 GHz with VCSEL biased at 0.9I_th, and fixed signal power 200 nW. The dotted lines are measured data, while the solid lines are fitted data.