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.
Colorimetric sensors are ubiquitous tools that are deployed in various research, academic and on-field diagnostic settings. High accuracy and quick readout capability of these sensors are key for biochemical sensing and medical diagnostics. In this article, we introduce ColorX, a novel colorimetric sensor built by modifying a genetic commercially available fitness tracker. The inbuilt components of a fitness tracker such as a heart-rate sensor, LEDs, photodiodes, transimpedance amplifiers provide a compact and portable form factor. ColorX leverages the bluetooth from the fitness tracker for wireless data transmission to smart phones. As a device validation strategy, ColorX was validated by measuring Nitrates in water. In all our experiments ColorX matched the technical performance of a standard benchspectrophotometer. ColorX was able to achieve excellent sensitivity in measuring nitrates as low as 10mg/ml.
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.
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.
We consider four different laser arrangements with the nonlinear loops of Kerr type, and discuss square wave pulse operation using modeling based on delay differential equation (DDE) approach. We reduce DDE models to 1D maps, which enable square wave operation and analyze numerically the possible dynamical scenarios of the square wave evolution.
Long cavity fibre-based wavelength sweeping lasers are promising devices with a wide range of potential applications ranging from communications to life sciences. For example, Fourier Domain Mode-Locked (FDML) lasers, which are commonly used for Optical Coherence Tomography (OCT) imaging applications are long cavity lasers incorporating an intra-cavity resonator driven in resonance with the cavity round trip time. The coherence properties of such swept sources are of major importance as they define the image quality. The purpose of this work is to analyze the mechanism that deteriorates the coherence of long lasers. In our experiment, the laser included a 100nm wide semiconductor optical amplifier at 1310nm and a fibre cavity that could vary from 20m to 20km. the laser emission wavelength was controlled using a fibre based intra-cavity filter with a bandwidth of 10GHz. Near the lasing threshold and/or for fast carrier decay rate, we observed the appearance of periodic power dropouts with stable Nozaki-Bekki holes (NBH) that circulate in the laser cavity. As a function of the injection current, the laser could operate in various regimes including bi-stability between NBH and stable (cw) operation, unstable NBH or chaotic operation. Such behavior indicates that the interplay between the injection current and carrier decay rate can lead to highly coherent emission of a long cavity laser.
Fiber to planar waveguide interconnects and planar beam modifiers are crucial for the implementing efficient silicon photonic devices for communication applications. In this work, broadband performance in the 1300nm- 1600nm region is ensured by appropriately controlling the spatial variation of the effective index of silicon to realize a beam splitter (1×2) and a quarter wave GRIN lens. Sub wavelength (λ/10) 1D periodic array of low-index air holes in the high index host silicon, along the propagation direction of the optical beam, is repeated, with decreasing or increasing periodicities in the transverse direction to form a 2D photonic crystal(PhC) structure to realize a beam splitter or GRIN lens respectively. The lowest wavelength of 1300nm is used as the design wavelength to ensure less than λ/10 periodicities for all useful wavelength ranges. A decreasing transverse periodicity of 1D hole arrays from the horizontal axis at the centre along the direction of propagation results in formation of 2 high index wave guiding structures towards the edges of the crystal separated by low effective index central region thus leading to formation of a beam splitter. On the other hand increasing transverse periodicity of the defect array leads to lowering of effective index gradually to the edges of the device resulting in the formation of a GRIN lens. FEM analysis of the propagation of electromagnetic field through these structures show that GRIN lens focuses the input beam to a mode field diameter (MFD) of 1.5μm and that MFD of each output arm of the 1×2 splitter is 1.75μm. The decrease in intensities at the focal point of the GRIN lens with increasing input wavelength in the 1300-1600nm is found to be within 6% and that in the two arms of the beam splitter is found to be less than 13%.
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