Injection locking has been actively researched for its possibility to improve laser performance for both digital and analog applications. When a modulated follower laser (also termed "slave" laser) is locked to the master laser, its nonlinear distortion and frequency chirp may be reduced. As well, the resonance frequency can increase to several times higher than its free running case. In this paper, we show that the frequency response (S21) of an injection-locked laser is similar to a parasitic-limited laser with a high resonance frequency. The S21 was studied experimentally and the condition to achieve a flat, enhanced frequency response was identified. For analog applications, a record 112 dB-Hz2/3, single-tone third harmonic spur-free dynamic range of a 1.55 um VCSEL was demonstrated. An improvement was attained for a wide injection parameter space. Furthermore, the RIN of the VCSEL was found to be 10 dB lower at 2 GHz for certain injection condition. In a 50 km 2.5 Gb/s digital link, a 2 dB power penalty reduction at 10-9 bit-error-rate was also demonstrated. As a novel application, an injection-locked uncooled tunable VCSEL was shown to have a reasonable modulation performance in a wide ambient temperature range. The VCSEL was locked to a designated wavelength and the injection compensated the temperature-induced performance degradation. This concept can be extremely attractive for low-cost DWDM transmitters.