O 1 : Estilos de vida.
4.2. DISCUSIÓN DE RESULTADOS
By carrying out this investigation into the transition dynamics of a QD laser under gain conditions during laser operation, a greater understanding of the carrier dynamics can be gained. Taken in conjunction with the work done on the gain recovery dynamics of QD material in semiconductor optical amplifier configurations and its behaviour as an absorber, there now exists a clearer picture of the recovery characteristics of the material. We showed that the higher differential gain of ES1 and the rapid state filling coupled with increased relaxation times to the GS result in the production of ultrashort pulses from ES1 which were measured to be less than 24ps in duration. These pulses were produced earlier in time than pulses from the GS.
From the perspective of ultrafast laser physics, the realisation that the different states emit light on different time scales has important implications. Because the states are behaving effectively as separate lasers (albeit operating from the same charge reservoir), it does not seem possible to engage both the GS and ES1 in the same mode-locking interactions. Thus the wider bandwidths hoped for are not yet accessible. Despite this, the design of QD material in which there exists more of a spectral overlap between GS and ES1 emission could result in a significant increase in ‘cross talk’ between these interacting states. This could thus have the potential to enable the exploitation of considerably wider spectral bandwidths in mode locking that could be “designed” to facilitate the generation of significantly shorter pulses. It should be noted here that in order for shorter pulses to result, there would need to also be some attempt to better control the dispersion in the system.
There are a number of steps that could be taken to further the findings of my investigations into this type of material structure. Autocorrelation measurements carried out with a more sensitive instrument would give a high resolution time-resolved picture of the pulses produced, possibly revealing additional temporal
features in the pulse profiles that may have become hidden due to the limited resolution of the measurement technique employed here.
Additionally, through the use of laser quality filter elements placed after the QD laser, it should be possible to severely attenuate the light from one quantum energy level. For example, the use of a filter blocking light below 1100nm would remove visible spectral and temporal contributions from ES2. By then operating the device at high pump powers, it would be possible to examine the pulse profile solely due to ES1 contributions. Likewise, the employment of a filter that blocks wavelengths above 1125nm would allow the study of ES2 in isolation.
In this chapter I have outlined an investigation into the transition dynamics from the different states in QD material with a view to engaging an enlarged bandwidth of two states for mode-locked operation. As mentioned previously, another important factor limiting the performance of QD lasers is the large positive chirp brought about by the carrier-induced slow self-phase modulation caused by a pulse passing through the device. Intracavity-based strategies to overcome the high amounts of positive frequency chirp in QD mode-locked lasers are very important if QD lasers are to be able to fulfil their potential. In Chapter 5 I will describe a preliminary investigation made into an external cavity based mode-locked QD laser. This more versatile cavity design allowed the inclusion of various elements designed to improve the performance of the device through stabilisation and feedback and intensities within the cavity.
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