In this paper, we experimentally and theoretically analyse the formation and interaction of dark solitons in a long laser. The laser includes a semiconductor optical amplifier (SOA), centred around 1300nm, an intracavity filter and a fibre cavity whose length can vary from 20m to 20km. Near the lasing threshold the laser exhibits slowly evolving power dropouts the circulate the cavity. These dropouts are associated with the formation of Nozaki-Bekki Holes (NBH), also referred to as dark solitons. We observe both experimentally and numerically that the core of these holes exhibit chaotic dynamics and emit short light pulses. These pulses are found to be blue shifted with respect to the frequency of the dark solitons and therefore travel with a faster group velocity. These pulses are strongly damped, as they are detuned with respect to the filter transmission, but they may lead to the creation of new dark solitons. These pulses also play a major role in the development of optical turbulence when the filter is set at a frequency above 1310nm. In this case, the laser displays numerous dark solitons per round trip and the fast travelling pulses act as an interaction between the solitons, which can lead to the development of defect mediated turbulence.
This paper aims to characterise, both experimentally and theoretically, the dynamics which occur during the turn on transient of a long cavity semiconductor laser. The laser comprised of a semiconductor optical amplifier (SOA), centered around 1300nm, a tuneable narrow bandwidth filter, for wavelength selectivity, a polarisation controller, an output coupler and multiple single mode fibre isolators to ensure the unidirectional propagation of light within the ring cavity. The bias current driven to the SOA was periodically switched on and off in order to examine the laser dynamics within each cavity round trip. It is observed that the laser intensity builds up in a step-wise manner, with each step corresponding to one cavity round trip. By examining the space-time diagrams of the lasers intensity during the turn on, it is seen that the laser will initially randomly oscillate before transitioning into a semi-stationary state. After a certain amount of round trips the laser may develop one or more localised structures, characterised by their short and fast drops of intensity. In this paper we also aim to not only explain the formation of these localised structures but also expand on their development by examining the phase evolution of their electric field.
We present here a combined theoretical and experimental study to investigate the influence of external optical feedback in a semiconductor swept-source laser. The applied feedback is shown to transfer the coherence between the subsequent modes and retain it along the full sweep. As a result, the technique can act as a solution to the de-coherence during the mode-hops observed in this kind of swept-source lasers thus noticeable increasing the image quality of Optical Coherence Tomography systems.
In this paper, we will discuss the properties of long cavity frequency sweeping lasers and demonstrate various scenarios of coherence deterioration in such lasers. The long cavity lasers are known to demonstrate a rich variety of dynamical regimes including the formation of localised structures and transition to turbulence. The interest to frequency sweeping long cavity lasers has recently increased due to their application for imaging and sensing. For these applications, the stability of the laser is an important parameter as it directly influences its coherence and therefore, the quality of the obtained images.1 Our laser consists of a fiber based ring cavity resonator including a semiconductor optical amplifier as a gain medium and a Fabry-Perot tunable filter. Experimentally, we considered different laser configurations which has allowed us to study the influence of the cavity length, frequency sweeping speed and the detuning. We considered the dynamical regimes of the laser operating at a static (fixed output frequency) and quasi { static regimes. The study shows that the laser can be stable or unstable and demonstrate localised structures stable over multiple roundtrips. We also show the connection of the dynamics observed in the static, quasi-static and synchronisation regimes of long cavity lasers. Numerically, we used a model based on a system of delayed differential equations. The numerical simulation showed excellent agreement with the experimental data. We also show the formation of dark pulses, both periodic and nonperiodic, and showed that they are closely connected to Nozaki-Bekki holes previously predicted in the complex Ginzburg-Landau equation.
In this paper we study, both experimentally and theoretically, the turn on transient dynamics observed in a long (20m) cavity laser. The laser consists of a ring cavity based on a single mode fiber with unidirectional propagation of light. The gain is provided by a semiconductor optical amplifier (SOA) centered around 1300nm and wavelength selection is provided by a tunable narrow transmission bandwidth Fabry-Perot filter. At high bias current and when the filter transmission sets the laser to operate in an anomalous dispersion regime, the laser exhibits only chaotic oscillations, while in a normal dispersion regime, the laser can exhibit stable operation. At a bias current close to the threshold the laser always exhibits multiple dropouts. In order to record the lasing build up dynamics, the bias current driven to the SOA is periodically switched from the off-state to a high current level. The lasing build up occurs at each roundtrip via a step-wise increase of the laser intensity. The laser intensity is widely oscillating during the first steps and approaches a stationary state after a large number of roundtrips. Recording of the phase evolution of the electric field during each step demonstrates the linewidth narrowing at each subsequent roundtrip. Theoretically, we describe the system by a set of delay differential equations and observe similar behavior. While typically a semiconductor laser exhibits relaxation oscillations before reaching the stable lasing regime, which is associated with class B lasers, our study shows that the long cavity laser demonstrates a different mechanism of lasing build up.
We show theoretically that optical feedback can be used to phase lock the successive modes of multi-section frequency-swept source lasers as a means to increase the coherence length. The time-gated feedback technique can be applied to transfer the coherence between the subsequent modes to retain the coherence along the full sweep or to synchronise two independent swept sources. In analogy with CW lasers, we derive an Adler equation describing the locking conditions. When the constant feedback is applied, the laser can operate in a self-mixing, mode-locking or chaotic regime, depending on the sweeping speed. In order to verify the theoretical results, we have developed an experimental set up and performed initial measurements with optical feedback.
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