Investigations of semiconductor laser modulation dynamics and field fluctuations
Active-layer photomixing is an optical modulation technique to probe the fundamental modulation response of a semiconductor laser. By heterodyning two laser sources with a tunable frequency difference in the device's active region, the gain, and hence the optical output, is modulated at the bea...
| Summary: | Active-layer photomixing is an optical modulation technique to probe the fundamental modulation response of a semiconductor laser. By heterodyning two laser sources with a tunable frequency difference in the device's active region, the gain, and hence the optical output, is modulated at the beat frequency of the sources. Using an equivalent circuit model for the laser diode, the optical modulation is shown to be decoupled from the electrical parasitics of the laser structure. The fundamental modulation response of the laser can thereby be studied independently of the parasitic response, which would otherwise mask the fundamental response. The photomixing technique is used on GaAs/GaAlAs lasers at room temperature, liquid nitrogen and liquid helium temperature, and it is verified that the modulation response appears ideal to millimeter-wave frequencies.
Application of the active-layer photomixing technique led to the discovery and explanation of a new effect called the "gain lever." It enhances the modulation efficiency of a semiconductor laser with a quantum well active layer. By inhomogeneously pumping the device, regions with unequal differential gain are created. If the laser is above threshold, then the overall modal gain is clamped, and by modulating the section with larger differential gain, the output power can be modulated with greater than unity quantum efficiency.
The fundamental coupling between intensity noise and phase noise in semiconductor laser light is investigated. This coupling, described by the [alpha] parameter, causes the well-known linewidth enhancement, but also implies the fluctuations are correlated. By the technique of "amplitude-phase decorrelation," the intensity noise can be passively reduced by the ratio 1/(1 + [alpha](2)). Using a Michelson interferometer as a frequency discriminator, intensity noise from a DFB laser is reduced below its intrinsic level up to a factor of 28.
A balanced homodyne detection scheme is used to study the noise reduction in relation to the photon shot noise floor. The decorrelated intensity noise can be reduced to within a dB of the shot noise level. Reduction below shot noise may be inhibited by uncorrelated phase noise in the lasing mode. |
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