M.Sc Thesis

M.Sc StudentEyal Ori
SubjectHigh-Speed 1.5 micron quantum dot lasers and their
temperature stability
DepartmentDepartment of Electrical and Computer Engineering
Supervisor PROFESSOR EMERITUS Gad Eisenstein


High speed semiconductor lasers are in strong demand in the rapidly increasing optical communication and data com networks. Low-dimensional nanostructure lasers are expected to substitute their quantum well (Qwell) counterparts since they are superior, in principle, in every important aspect, first and foremost in efficiency and speed. Efforts have been devoted during the past years to achieve nanostructure lasers with broad modulation bandwidth, low frequency chirp, low sensitivity to temperature, and nearzero linewidth enhancement factor (also known as α-factor). Particularly, 1.55-μm InP-based quantum dash (Qdash) / dot (Qdot) lasers hold a great promise for long-haul transmission and short range links (in data centers) compared to 1.3-μm Qdot laser sources grown on GaAs substrate.

In this dissertation, we investigate first the static properties of InP-based quantum dot semiconductor lasers operating under constant injection current and at different temperatures. Investigation of dynamic properties such as modulation bandwidth, differential gain, maximum bitrate, and temperature stability follow in a later part.

Through the rate equation analysis of the modulation response in a semi-analytical approach, it is found that the modulation bandwidth of the quantum dot laser is strongly limited by the finite carrier capture, and relaxation rates which are also responsible for the large damping factor. This approach is somewhat sufficient for a qualitative approximate understanding of the dynamic properties of a Qdot laser. However, it neglects transport effects as well as the changes in the energy bands due to the coulomb forces between injected carriers that adds an important contribution to the various relaxation rates. The more advanced models were used to properly design high speed lasers but the experimental results can be explained using the simpler approach.

It is well known that achieving simultaneously a wide 3-dB modulation bandwidth and chirp-free operation in conventional (quantum well) lasers, is very challenging an often not achievable. Moreover, even a small α-factor cannot be maintained at high current densities where the large bandwidths are accessible. Thus, it is important to properly understand the underlying physical mechanism of QD lasers that can be leveraged for enhanced dynamical performance. Low chirp operation is of course crucial for transmission over any but the shortest distance in dispersive fibers.

Qdot lasers hold the promise to provide several of the most important requirements for directly modulated lasers: high speed, low chirp and most important - temperature insensitivity. This thesis address mainly the speed and temperature sensitivity issues. Linewidth properties of CW lasers are also described.