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We measure the spectral emission and the small-signal modulation frequency response of our 980 nm VCSEL arrays across OM2 multiple-mode optical fiber (MMF) patch cords and for the first time across free space (air) via optical fiber collimators (small focusing triplet lenses with optical fiber connectors) separated by 0.4 m up to 10 m. The VCSELs in our arrays are electrically in parallel but optically uncoupled, with an aim toward mitigating scintillation and speckle loss mechanisms in high bit rate free space data streams. We measure the center (mean) wavelength and the root-mean-square spectral width of our VCSEL arrays as a function of forward bias current, both across MMF and across free space. We find the spectral results are nearly identical within fractions of a nanometer. We similarly measure the S21 (s-parameter) small signal modulation frequency response of our arrays from 0.05 to 40 GHz, across fiber and across free space, and extract and plot the -3 dB bandwidths as functions of forward bias current. We achieve bandwidths up to 25 and 30 GHz for 3-element arrays with 9 and 7 micrometer oxide aperture diameters (Φ), respectively, and find the bandwidths are nearly identical when comparing data transfers across OM2 MMF patch cords versus across the MMF plus a section of free space.
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