Gain Characterisation of 1.3μm GaAs Quantum Dot lasers

Gain characterisation of a laser device is of fundamental importance to assist in the physical understanding of laser materials. Not only does it determine important parameters such as threshold, material loss and transparency current density, but is also a vital source of information regarding the evolution of states as a function of current density and temperature. The differential gain (dg/dn) is of key importance in determining the dynamic performance of a laser. Hence, the important role of gain characterisation has driven researchers to devise improved techniques for spectral gain measurement.
This thesis discusses the gain characterisation of 1.3μm quantum dot, commercial Innolume material and bi-layer laser devices. Initially, different gain measurement techniques are reviewed. High resolution spectroscopy and variable stripe length methods are analysed and compared in detail. A technical review is presented for the first time for the commonly used Hakki and Paoli, segmented contact and a new �integrated mode filter� method for gain measurement.
Then the Hakki and Paoli method is used to perform high current density analysis of the gain spectrum of 1.3μm Innolume, quantum dot laser material under continuous wave drive conditions. The device is characterised with and without self-heating effects. The elimination of self-heating effects is achieved by using a longitudinal mode as a junction temperature monitor to keep the junction temperature constant. This allowed an unambiguous study of the continuous wave gain spectrum at average dot occupancy levels up to ~8 e-h pairs per quantum dot. A negative differential gain is observed in both cases. This is shown to be predominantly due to the free carrier effects. As a result, free-carrier related negative differential gain is observed for the first time.
A variant to the segmented contact method, which utilises an integrated amplifier and mode filter is demonstrated for the first time. The measurement of the gain/absorption spectrum is critically compared under identical data acquisition conditions as for the integrated mode filter and segmented contact methods. By driving the amplifier section, it is possible to achieve ~3-dB of signal amplification. As a result the measurement of the gain spectrum is achieved over a broader spectral range. Further, it is shown that the integrated amplifier method enables gain measurements at lower current densities as compared to the standard technique.
Lastly, the effect of inhomogeneous line width on the lasing line width of ~ 1.3μm quantum dot lasers is studied, as the line width of the transmitter is one of the key factors to determine the dispersion limit for optical communication systems. Two samples, with different inhomogeneous line width are compared under conditions where it is hoped that the effects of homogeneous line width and spectral hole burning are maintained at a constant level. This allows the effects of inhomogeneous line width alone to be studied. A ~30% reduction in inhomogeneous line width is shown to have a significant impact in reducing the lasing line width.