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Four schemes for the generation of chaos in vertical-cavity surface-emitting lasers (VCSELs) by optical feedback and/or optical injection have been evaluated experimentally. The experimental results show that the preferred scheme is to effect chaotic optical injection into a stable operating VCSEL. This is an effective method to generate a flat broad-bandwidth chaotic signal with the bandwidth of the chaotic signal being enhanced by more than four times compared with that obtained using optical feedback. In addition, the chaotic signal generated by chaotic optical injection has a flat power spectrum. If a continuous wave (CW) signal is injected into a chaotic VCSEL, the bandwidth of chaos can also be enhanced, but less than that using the preferred scheme. Moreover, the spectrum in this case is not very flat. For CW optical injection into a CW VCSEL, the power spectrum of the chaos has many peaks and so is not suitable for applications in chaotic optical communication and random number generation. Injecting chaotic light into a chaotic VCSEL provides little enhancement of the chaos bandwidth.