Passively Mode Locked Semiconductor Lasers For All Optical Applications

The recent increase of internet traffic is creating demand for higher bandwidth in telecommunication networks. In order to satisfy this ever increasing demand for bandwidth, it is necessary to investigate new devices and technologies for all-optical signal processing that allow increasing the transmission data rate and the capacity for the current and future optical networks. Optical time division multiplexing (OTDM) is a widely deployed technique that allows increasing the bit rate and capacity of optical networks. In OTDM networks the regeneration and the demultiplexing of the data channels are two common and important functions normally carried out. However, they require a clock signal, which is usually implemented by optoelectronics components, making a system expensive, bulky and difficult to implement. In order to provide a solution to this issue, the focus of this thesis is to investigate all-optical clock recovery by using external injection locking of passively semiconductor mode-locked lasers. In particular, quantum-dash mode-locked laser diodes (QDash-MLLDs) are studied. These lasers can generate optical pulses with durations in the order of picoseconds and femtoseconds using only DC-bias with no need for external modulation. Besides, they are attractive due to their simplicity of operation, low power consumption, fast carrier dynamics and compactness. Furthermore, they provide a narrow radio frequency beating linewidth, resulting in a small amount of phase noise and low timing jitter. In this thesis, all-optical clock recovery of data signals at base bit rate (40 Gb/s) and high bit rates (up to 320 Gb/s) was achieved using QDash-MLLDs. The recovered clocks from the different data input signals considered in this thesis feature low values of timing jitter, which are compliant with the minimum requirements for practical applications. Furthermore, the recovered clocks at high speed are used to demultiplex signals to tributaries of 40 Gb/s, achieving error free performance. Finally, investigation of the QDash-MLLD dynamics demonstrated that the laser provides a very fast locking time (25 ns) when synchronised to data signals which enables it as a solution to optical burst/packet switched networks. All these results contribute to demonstrate that the laser is an extremely reliable, cost-effective and a green solution for all-optical signal processing.

This thesis investigates the dynamics of passively mode-locked semiconductor lasers, with a focus on the influence of optical feedback on the noise characteristics. The results presented here are important for improving the performance of passively mode-locked semiconductor lasers and, at the same time, are relevant for understanding delay-systems in general. The semi-analytic results developed are applicable to a broad range of oscillatory systems with time-delayed feedback, making the thesis of relevance to various scientific communities. Passively mode-locked lasers can produce pulse trains and have applications in the contexts of optical clocking, microscopy and optical data communication, among others. Using a system of delay differential equations to model these devices, a combination of numerical and semi-analytic methods is developed and used to characterize this system.

In this dissertation, self-assembled InAs/InGaAs quantum dot Fabry-Pérot lasers and mode-locked lasers are investigated. The mode-locked lasers investigated include monolithic and curved two-section devices, and colliding pulse mode-locked diode lasers. Ridge waveguide semiconductor lasers have been designed and fabricated by wet etching processes. Electroluminescence of the quantum dot lasers is studied. Cavity length dependent lasing via ground state and/or excited state transitions is observed from quantum dot lasers and the optical gain from both transitions is measured. Stable optical pulse trains via ground and excited state transitions are generated using a grating coupled external cavity with a curved two-section device. Large differences in the applied reverse bias voltage on the saturable absorber are observed for stable mode-locking from the excited and ground state mode-locking regimes. The optical pulses from quantum dot mode-locked lasers are investigated in terms of chirp sign and linear chirp magnitude. Upchirped pulses with large linear chirp magnitude are observed from both ground and excited states. Externally compressed pulse widths from the ground and excited states are 1.2 ps and 970 fs, respectively. Ground state optical pulses from monolithic mode-locked lasers e.g., two-section devices and colliding pulse mode-locked lasers, are also studied. Transformed limited optical pulses (~4.5 ps) are generated from a colliding pulse mode-locked semiconductor laser. The above threshold linewidth enhancement factor of quantum dot Fabry-Pérot lasers is measured using the continuous wave injection locking method. A strong spectral dependence of the linewidth enhancement factor is observed around the gain peak. The measured linewidth enhancement factor is highest at the gain peak, but becomes lower 10 nm away from the gain peak. The lowest linewidth enhancement factor is observed on the anti-Stokes side. The spectral dependence of the pulse duration from quantum dot based mode-locked lasers is also observed. Shorter pulses and reduced linear chirp are observed on the anti-Stokes side and externally compressed 660 fs pulses are achieved in this spectral regime. A novel clock recovery technique using passively mode-locked quantum dot lasers is investigated. The clock signal (~4 GHz) is recovered by injecting an interband optical pulse train to the saturable absorber section. The excited state clock signal is recovered through the ground state transition and vice-versa. Asymmetry in the locking bandwidth is observed. The measured locking bandwidth is 10 times wider when the excited state clock signal is recovered from the ground state injection, as compared to recovering a ground state clock signal from excited state injection.

This timely book combines theory, applications, and projections on ultrafast diode lasers (UDL). A comprehensive treatment of UDLs from basic physical principles to applications in optical fiber communications and ultrafast electronics.

Semiconductor Monolithic Passively Mode Locked Lasers (MMLLs) have been demonstrated and have a variety of applications including sensing, in optical communications and optical clock generation. The major advantage that these semiconductor light sources possess is their compact size, high efficiency, low cost and robustness. Also the ability to access different wavelengths using various semiconductor materials is a big advantage. The inability to get these lasers to operate at low repetition rates and the limited peak output power of the pulses are the two major limitations of these lasers. The repetition rate is inversely proportional to the semiconductor laser cavity length. The lowest repetition rate in current MMLLs is around 1GHz. This rate is limited by the complexity involved in the fabrication of centimeter long semiconductor laser cavities. The material loss, dispersion and carrier radiative recombination lifetime also limit the output repetition rate. Lower repetition rate lasers can be used in low frequency integrated optical circuits and also for imaging especially for Two-Photon Microscopy (TPM). TPM works by exciting florescence dyes using two photons instead of one. This requires pulsed lasers with high peak power and high energy per pulse. TPM is currently done by using expensive and bulky Ti:Sapphire mode locked lasers that can produce subpicosecond pulses at a repetition rate of order of 100 MHz. The possible use of semiconductor lasers for this application can transform this field by dramatically reducing the cost of imaging and allowing for dramatically smaller sized and more mobile imaging solutions. One potential way to reduce the repetition rate of the lasers without increasing the physical cavity length is to incorporate a slow light photonic crystal structure inside the lasers cavity. Such a laser cavity will have a group index that is much larger than the material refractive index thereby giving a longer optical path length for the same physical length of the device. The incorporation of the photonic crystal will also allow the possibility to do dispersion engineering within the laser cavity and to enable pulse compression. Both these effects can increase the two-photon excitation efficiency for TPM. In this work we highlight the design and progress towards the development of a monolithic semiconductor photonic crystal passively mode locked laser for two-photon microscopy.

This invaluable book provides a comprehensive treatment of the design and application of Mode Locked Lasers and Short Pulse Generation. With the advances in semiconductor laser and fiber laser technologies in the 1980s to now, these devices have been made compact, refined, and developed for a wide range of applications including further scientific studies.Semiconductor mode-locked lasers are stable pulse sources and can be made over a range of wavelengths where laser operation is feasible. Rare earth doped fiber lasers or planar waveguides extend this range further and can provide compact pulsed sources. The principles of operation, analysis, design and fabrication of these sources are described. Recent results on high repetition rate and high-power pulse generation from these compacts sources are also described, together with current and future directions of application of these types of laser sources.Mode-Locked Lasers: Introduction to Ultrafast Semiconductor and Fiber Lasers is self-contained and unified in presentation. It can be used as an advanced text by graduate students and by practicing engineers. It is also suitable for non-experts who wish to have an overview of mode-locked lasers and pulse generation. The explanations in the book are detailed enough to capture the interest of the curious reader and complete enough to provide the necessary background to explore the subject further.