Semiconductor lasers are known. Because of their spectral and beam properties, as well as their capability to be directly modulated at very high rates, semiconductor lasers have typically been used as components for many optoelectronic applications (e.g., optical telecommunication). Directly modulated lasers, however, typically suffer from high distortion near the resonance frequency. Accordingly, it is desirable to set the laser resonance frequency such that it greatly exceeds a highest desired radio frequency (RF) frequency of a particular application.
In general, the bandwidth of a directly modulated laser is largely defined by the relaxation resonance frequency, which is intimately tied to the photon density in the gain region. It is known to use optical injection locking (OIL) in directly modulated lasers to increase the relaxation resonance frequency, to reduce nonlinear distortion and to reduce frequency chirp. OIL typically uses the output of one laser (master) to optically lock another laser (slave), which may still be directly modulated. OIL may overcome the dependence of the relaxation resonance frequency on gain section current, to produce a significant increase in the resonance frequency (with resonance frequencies as high as about 50 GHz). In general, injecting a laser with a frequency detuned from that laser's optimal frequency may enhance the modulation response by roughly the difference between the two frequencies. Because the bandwidth of directly modulated lasers is largely defined by relaxation resonance frequency, this characteristic of OIL has generated significant interest as it allows directly modulated lasers to achieve much higher bandwidths. For example, OIL has been used in single sideband modulation and optical oscillators.