We review the temporal dynamics of the laser output spectrum and polarization state of 1.55？μm wavelength single-mode (SM) vertical-cavity surface-emitting lasers (VCSELs) induced by external optical beam injection. Injection of an external continuous-wave laser beam to a gain-switched SM VCSEL near the resonance wavelength corresponding to its main polarization-mode output was critical for improvement of its laser pulse generation characteristics, such as pulse timing-jitter reduction, linewidth narrowing, pulse amplitude enhancement, and pulse width shortening. Pulse injection of pulse width shorter than the cavity photon lifetime into the SM VCSEL in the orthogonal polarization direction with respect to its main polarization mode caused temporal delay of the polarization recovery after polarization switching (PS), and its delay was found to be the minimum at an optimized bias current. Polarization-mode bistability was observed even in the laser output of an SM VCSEL of a standard circularly cylindrical shape and used for all-optical flip-flop operations with set and reset injection pulses of very low pulse energy of order of the 3.5~4.5？fJ. 1. Introduction Dynamical laser output of the vertical-cavity surface-emitting lasers (VCSELs) under injection of an external laser beam has been investigated widely for potential application to pulse timing-jitter reduction, polarization switching, all-optical flip-flop operation, and long-distance fiber transmissions [1–15]. VCSELs are known to be low power-consuming optical signal sources compared to the existing edge-emitting laser diodes (LDs) and to be potentially useful all-optical logic gate devices in a two-dimensional array. Beside the conventional signal source application of the stand-alone VCSELs for relatively short-distance optical communications or interconnects, the injection locking of an external optical beam to the VCSELs allows new application areas of the VCSELs possible and improves fiber transmission properties of the VCSELs’ output. Weak continuous-wave (CW) laser beam injection to a gain-switched VCSEL lowered its pulse timing jitter [2, 3]. Laser pulse beam injection of an orthogonal or circular polarization into a VCSEL caused a high-speed polarization switching (PS) [4–7] or spin-induced polarization oscillation [8, 9]. The PS and induced bistability mechanism of the VCSEL output under a laser beam injection of orthogonal polarization to the VCSEL’s main polarization mode were used for demonstration of all-optical flip-flop operation and optical buffer memory application [10–13]. An
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