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.
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.
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.
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.
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.
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.
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.
We analyze the properties of a unidirectional class-A ring laser containing a nonlinear amplifying loop mirror (NALM). The NALM is a Sagnac interferometer consisting of an amplifier and a Kerr-type nonlinear element, and has a reflectivity that periodically varies with the intra-cavity power. To model the dynamics of these lasers, we use the approach based on Delay Differential Equations (DDEs) that has been successfully applied to describe the properties of passively mode-locked semiconductor lasers. The proposed model allows us to investigate mode locking operation in this laser. The analysis of this DDE model for mode-locked operation was performed numerically and analytically in the limit of large cavity round trip times. We demonstrate that mode-locked pulses are born though a modulational instability of the steady state solutions when the pseudo- continuous branch crosses the imaginary axis. These asymmetric pulses always co-exist with the stable laser-off solution. Hence, they can be viewed as temporal cavity solitons having similar properties with localized structures observed in bistable spatially-extended systems.
A new material pairing is presented for the realisation of sub-wavelength graings in this work and has been used to realise high contrast gratings which operate at wavelengths of 10 μm and greater. The chosen material pairing overcomes the absorption issue which prevents the popular Si/SiO2 pairing from being useful at wavelengths above 6 μm. The obstacles that exist with the currently used grating materials for this wavelength range are described and it is outlined how the chosen materials overcome these issues. It is numerically demonstrated that gratings utilising these materials are capable of wideband high reflectivity. The gratings were fabricated using standard optical photolithography only and it is shown experimentally that the spectral response of gratings which were fabricated show good agreement with theoretically predicted performance
In this work, a low cost optical pH sensing system that allows for small volume sample measurements was developed. The system operates without the requirement of laboratory instruments (e.g. laser source, spectrometer and CCD camera), this lowers the cost and enhances the portability. In the system, an optical arrangement employing a dichroic filter was used which allows the excitation and emission light to be transmitted using a single fibre thus improving the collection efficiency of the fluorescence signal and also the ability of inserting measurement. The pH sensor in the system uses bromocresol purple as the indicator which is immobilised by sol-gel technology through a dip-coating process. The sensor material was coated on the tip of a 1 mm diameter optical fibre which makes it possible for inserting into very small volume samples to measure the pH. In the system, a LED with a peak emission wavelength of 465 nm is used as the light source and a silicon photo-detector is used to detect the uorescence signal. Optical filters are applied after the LED and in front of the photo-detector to separate the excitation and emission light. The fluorescence signal collected is transferred to a PC through a DAQ and processed by a Labview-based graphic-user-interface (GUI). Experimental results show that the system is capable of sensing pH values from 5.3 to 8.7 with a linear response of R2=0.969. Results also show that the response times for a pH changes from 5.3 to 8.7 is approximately 150 s and for a 0.5 pH changes is approximately 50 s.
We describe the lasing characteristics of a compact tunable laser source formed by the butt-coupling of a reflective indium phosphide optical amplifier to an SU8 waveguide coupled to few-mode photonic crystal reflector. The short cavity length ensured that only a single longitudinal mode of the device could overlap with each photonic crystal reflection peak.
Depending on device and operating parameters, the emission of lasers based on InAs quantum dot (QD) material may come from the ground state (GS) only, from the first excited state (ES) only or simultaneously from both states. When the emission is from the ES only, optical injection at the GS frequency can completely suppress the ES output and instead, phase-locked emission from the GS can be obtained. We report on a variety of non-linear phenomena obtained when the frequency of the master laser is varied revealing two antiphase, dual-state excitable regimes.
A set of differential equations with distributed delay is derived for modeling of multimode ring lasers with intracavity chromatic dispersion. Analytical stability analysis of continuous wave regimes is performed and it is demonstrated that sufficiently strong anomalous dispersion can destabilize these regimes.
Fourier Domain Mode Locked laser is a novel fast frequency swept source used in Optical Coherence Tomography. The laser has a unidirectional all-fiber ring cavity that incorporates a semiconductor optical amplifier, a tunable Fabry-Perot filter and a fiber delay forming the cavity of up to 20km long. Our numerical modeling based on a set of delay differentiation equations is in excellent agreement with the experimental results that employed real-time intensity and phase characterisation techniques. We show that FDML lasers display a sequence of bifurcations that can co-exist within a sweep and lead, in particular, to the formation of Nozaki-Bekki holes.
Multiple reference optical coherence tomography (MR-OCT) applies a unique low-cost solution to enhance the scanning depth of standard time domain OCT by inserting an partial mirror into the reference arm of the interferometric system. This novel approach achieves multiple reflections for different layers and depths of an sample with minimal effort of engineering and provides an excellent platform for low-cost OCT systems based on well understood production methods for micro-mechanical systems such as CD/DVD pick-up systems. The direct integration of a superluminescent light-emitting diode (SLED) is a preferable solution to reduce the form- factor of an MR-OCT system. Such direct integration exposes the light source to environmental conditions that can increase fluctuations in heat dissipation and vibrations and affect the noise characteristics of the output spectrum. This work describes the impact of relative intensity noise (RIN) on the quality of the interference signal of MR-OCT related to a variety of environmental conditions, such as temperature.
Quantum dot lasers readily lase from multiple distinct energy states. We examine the influence of optical injection into the ground state (GS) when the free-running operation is the first second excited state (ES) only and demonstrate the existence of an injection-induced bistability between GS emission and ES emission. There is a consequent hysteresis loop in the lasing output. Experimental and numerical investigations are in excellent agreement.
This work presents an investigation of room temperature ultra-fast carrier dynamics in a p-doped dash-in-a-well structure emitting at 1.5 μm using single colour heterodyne pump-probe spectroscopy. This technique enabled simultaneous access to the gain and refractive index dynamics in various operational conditions including both the absorption and gain regime. Comprehensive analysis of the timescales related to carrier relaxation and escape processes in addition to the ’dynamical’ linewidth enhancement factor are presented and compared with results obtained from similar un-doped materials. The direct influence of the p-doping on the carrier dynamics is also discussed.
A novel, time-resolved interferometric technique is presented allowing the reconstruction of the complex electric field output of a fast frequency swept laser in a single-shot measurement. The power of the technique is demonstrated by examining a short cavity swept source designed for optical coherence tomography applications, with a spectral bandwidth of 18 THz. This novel analysis of the complete electric field reveals the modal structure and modal evolution of the device as well as providing a time-resolved real-time characterization of the optical spectrum, linewidth and coherence properties of a dynamic rapidly swept laser.
The unique carrier processes in quantum dot lasers mean that lasing can be achieved at the ground state (GS) transition wavelength or at the excited state (ES) transmission wavelength or indeed simultaneously at both wavelengths. The details depend on the device characteristics and control parameters such as the pumping current and temperature. When the lasing is from the ES only one can induce all-optical switching between the two states via optical injection into the GS. The high damping of the relaxation oscillations in these devices allows for very fast switching times, with sub-nanosecond transitions easily obtained. Such ultrafast switching times are vastly superior to those obtained with conventional semiconductor lasers and make these devices very attractive for all-optical switching applications. The interplay of the two states leads to a new dynamic regime. Near the boundary of stable locking for the injected GS, deep GS intensity dropouts are observed. Further, each dropout in the GS coincides with a burst in the ES output.
We analyse the dynamical behaviour of a Fourier domain mode locked laser experimentally and theoretically. Heterodyne measurements of laser dynamics allows some insight into the frequency behaviour of the laser which coupled with theoretical arguments from previous work allow for a clear interpretation of the observations. Direct simulations using a delay differential equation model in full FDML mode display excellent agreement with the experimental results.
In recent years quantum dot lasers (QDLs) and optically injected QDLs in particular, have provided a new arena for the demonstration of many intriguing non-linear phenomena. We show here that slow optothermal relaxations may significantly affect the output dynamics in QDLs for both the free running and the external injection configurations. In particular, square-wave intensity drop-outs and pulsations can be obtained reminiscent of Fitzhugh-Nagumo excitable dynamics.
An experimental and theoretical analysis of the dynamics of a Fourier domain mode locked laser, currently one of the fastest swept source lasers applied in optical coherence tomography, is performed. A novel time- resolved technique to measure the laser output electric field allows access to the phase dynamics of the laser and thus the coherence properties. A delay-differential equation model for the laser is used to analyse the system theoretically and via direct simulation. Numerical simulations of the laser output are in excellent agreement with experimentally measured data.
We analyse the dynamical behaviour of a short cavity OCT swept-source laser experimentally and theoretically. Mode-hopping, sliding frequency mode-locking and chaos are all observed during the laser sweep period. Hetero- dyne measurements of laser dynamics allows some insight into the behaviour of the laser, while interferometric techniques allow the full phase reconstruction of the laser electric field. A delay differential equation enables modelling of the laser output, and laser parameters can be altered to provide optimisation conditions for future laser designs.
In this work, the optical properties and emission dynamics of core-shell InGaAs/GaAs nanopillars (NPs) have been in-
vestigated using low-temperature photoluminescence (PL) and time-resolved photoluminescence (TRPL). These novel
structures have recently attracted much interest within the silicon photonics scientific community due to their potential
employment as gain medium for monolithically integrated lasers on silicon substrates. The optimization of the emission
properties of these heterostructures is essential to obtain full compatibility with silicon photonics and requires an accurate
tailoring of the pillar geometry (i.e. size, pitch) and composition. Therefore it is critical to gain deeper insight into the
optical and dynamical properties of different NP designs if optimal device performance is to be achieved. The experimental
characterization, carried out on a number of different NP structures with different geometries and compositions, shows that
the time evolution of the emission peak exhibits a strong excitation-dependent blue-shift which can be attributed to the
band-filling effect. Measured emission decay times were strongly geometry-dependent and varied from nanoseconds to
tens of picoseconds. In addition, a dramatic reduction of the decay time was observed for the highest indium concentration
due to the dominant contribution of the strain-induced non-radiative recombination processes.
We describe the technique allowing for generation of low-noise wider frequency combs and pulses of shorter duration in quantum-dot mode-locked lasers. We compare experimentally noise stabilization techniques in semiconductor mode-locked lasers. We discuss the benefits of electrical modulation of the laser absorber voltage (hybrid mode-locking), combination of hybrid mode-locking with optical injection seeding from the narrow linewidth continues wave master source and optical injection seeding of two coherent sidebands separated by the laser repetition rate.
The behaviour of a dual state quantum dot laser undergoing optical injection is analysed. Depending on the injection current the free-running operation of the slave can be ground state only emission, both ground state and excited state emission or excited state emission only. Complete suppression of the excited state in the dual state lasing regime via sufficiently strong optical injection of the ground state is presented. Injection induced switching between two longitudinal modes of the ground state is also presented in the ground state only regime.
A technique has been developed for the measurement of pulse trains demonstrating a dynamical behaviour (i.e. not ideally periodic). Existing techniques in this area (e.g. FROG, SPIDER or other heterodyne methods) require very stable pulse trains, or large averaging times, and so are limited when applied to even slowly varying pulse trains. The technique presented involves mixing the comb under test (CUT) with a reference optical frequency comb (OFC) which has a known spectral intensity profile. Mixing these signals on a photodiode results in a series of radio frequency (RF) beat tones. The phase properties of these beat tones can be used to measure the spectral phase between adjacent modes in the CUT, allowing the full complex spectrum of the CUT to be measured simultaneously with one single real time oscilloscope acquisition. With the spectral properties of the comb known, the pulse train can be reconstructed in the temporal domain. By applying this technique to very small sections of the beating signal ( tens of nanoseconds), a time resolved picture of the pulse train behaviour can be obtained. Dynamic signals generated in a LiNbO3 modulator driven by a modulated RF signal have been measured. This technique is well suited to studying the combs produced by mode-locked semiconductor lasers. Quantum dot mode-locked laser combs can be characterised, and pulse train instabilities measured.
Quantum dot lasers have been shown to have greatly enhanced stability in the feedback configuration thanks to a high damping of the relaxation oscillations and they display different dynamics to those of conventional semiconductor lasers. For high feedback levels in conventional devices one obtains Low Frequency Fluctuations: sharp dropouts in intensity and subsequent gradual build-ups. Standard low frequency fluctuation-like traces are conspicuous by their absence in studies of feedback with quantum dot devices. We experimentally examine single mode quantum dot lasers at high feedback levels with a long delay and observe regular pulse-trains with a period equaling the external cavity round-trip time where each pulse features a distinctive broad trailing edge plateau. The distinctive pulse shape is very similar to the recently published strong pulse-asymmetry in two-section, passively mode-locked quantum dot lasers where this asymmetry was shown to result from the creation of different modal groups. We attribute the pulses in our experiment to the same phenomenon: each pulse corresponds to a simultaneous excitation of a number of the external cavity modes. We consider a model tailored specifically for quantum dot lasers with strong optical feedback and find it reproduces the experimentally observed trains extremely well.
We report on the development of monolithic two-section dilute nitride passively mode-locked ridge-waveguide lasers.
The dilute nitride material system can cover a wide wavelength range from 1.2 μm to 1.6 μm, while enabling fabrication
on low-cost GaAs substrates. The laser structure comprised 3 GaInNAs quantum wells embedded within GaAs
waveguide and AlGaAs claddings. To achieve mode-locking at 40 GHz repetition rate the laser chips consisted of a 950
μm long gain section and a 90 μm long reverse biased absorber section with a ridge width of 3.5 μm. The mode-locked
laser output exceeded 3 mW per as-cleaved facet with 80 mA current in the gain region and a reverse voltage of 3.8 V
applied to the saturable absorber. The corresponding pulse width was 3.4 ps.
To study the effect of increasing the number of N-related recombination traps present in the proximity of the quantum
wells, we have compared the performance of lasers employing GaAsN or GaAs as quantum well barriers. Time-resolved
photoluminescence measurements revealed that the material comprising GaAsN barriers exhibited a photoluminescence
lifetime of 12 ps with a reverse bias of 5 V. For similar reverse bias, the photoluminescence lifetime for material
comprising GaAs barriers was 108 ps.
Quantum-dot mode-locked lasers are injection-locked by coherent two-tone master sources. With optical injection
the slave laser optical spectrum becomes narrowed and tunable via the master wavelength. Frequency-resolved
Mach-Zehnder gating measurements performed to characterize slave laser pulses showed significantly improved
pulse time-bandwidth product (TBP) with optical injection. Measurements of the modal optical linewidths of
the injected laser demonstrated phase locking of all the slave laser modes to the master laser, which improved
significantly the device timing jitter. Integrated over a 20 kHz-80 MHz range timing jitter values of 210 fs were
achieved for small injection powers, close to the best reported results for the hybrid mode-locking of similar
QD-MLLs.
We study quantum dot mode locked lasers (QD MLL) under optical injection. For the experimental study we
use slave lasers two-section monolithic InAs/GaAs QD devices with a repetition rate of about 9.4 GHz, emitting
at 1.3 μm. A frequency resolved Mach-Zehnder gating (FRMZG) technique was utilised for the experimental
study of the pulse intensity, phase and chirp. For numerical simulations we use a modified delay-differential
model. We show experimentally improvement of the laser performance under injection and provide numerical
locking ranges obtained with DDEBIFTOOL package.
Modal optical linewidths of a passively mode-locked and optical injection locked quantum dot laser are studied.
For the free-running case the modal linewidth is in the order of tens of MHz and demonstrates a parabolic dependence
on the mode optical frequency. The slope of the parabola, as was predicted theoretically, is proportional
to the radio-frequency (RF) linewidth, which provides a direct measurement of the timing jitter. With optical
injection the slave laser optical spectrum becomes narrowed and tunable via the master wavelength. Frequency
resolved Mach-Zehnder gating measurements performed to characterize slave laser pulses showed significantly
improved pulse time-bandwidth product with optical injection. Measurements of the modal optical linewidths
of the injected laser have shown phase locking of the slave laser modes to the master laser in the vicinity of the
injection wavelength. However, far from this wavelength modal linewidth of the slave laser increases to greater
than that of the free running case, leading to increase of the RF linewidth and timing jitter with single-tone
injection.
Mode-locked semiconductor lasers have great promise for many emerging applications if they can be made sufficiently
robust. Typical performance requirements are 1-10 ps pulses with timing jitter ~1 ps and 1-10 GHz repetition rates with
peak powers 10-100 mW. Here we consider pulse width and timing jitter in passively mode-locked two-section InAs
quantum-dot lasers emitting at 1310 nm. We have identified two distinct, extensive mode-locked regions with robust
short pulses and low timing jitter. A record combination of 2 ps pulses and 20 fs/cycle timing jitter (500 fs, from 1-100
MHz), with 100 mW peak power per facet is demonstrated. The implications for practical systems design are discussed.
Quantum-dot semiconductor lasers have several distinctive features when compared with bulk and quantum-well devices. The phase-amplitude coupling of such devices was predicted to be near zero, but several experiments have shown this not to be generally the case. Here, we review several experimental investigations and their theoretical underpinnings.
We analyse the stochastic polarization fluctuations in a vertical cavity surface emitting laser (VCSEL) under the influence of electro-optical feedback and show that the dynamics can be modeled as a bistable system with time-delayed memory. Assuming an asymmetric potential, we show the existence of a regime in which the systems dynamic displays excitability. We calculate the relevant residence time distributions and correlation times and compare our system to a well known discrete model for excitability. Finally, we present experimental data that demonstrates excitable behaviour in the polarization dynamics of a VCSEL and, in particular, show the appearance of coherence resonance.
Ultrafast intraband carrier dynamics strongly influence many important characteristics in bulk and quantum well lasers and amplifiers through Spectral-Hole Burning (SHB) leading to nonlinear gain effects. In Quantum Dot (QD) devices, where the inter-level relaxation times can be even longer than the intraband relaxation times in conventional devices, SHB effects should also be substantial. A number of promising applications of QD amplifiers in high-speed optical processing (Cross-Gain Modulation, for instance) are based on features of the carrier dynamics in QD structures. In the present paper, based on a density matrix approach, we develop a theory of SHB-based nonlinear gain in QD lasers and amplifiers, which can affect such important characteristics as the modulation bandwidth in QD lasers and the saturation power and pulse energy in QD amplifiers. We give an expression for the nonlinear gain in QD devices, and show how it depends, particularly, on the capture/escape and relaxation/excitation rates.
We analyse the dynamics of a self-pulsating semiconductor laser with optical feedback. Without re-injection of light the laser displays periodic oscillations. At very weak feedback levels we observe an amplitude instability whose frequency increases with the feedback level until the laser enters the low frequency fluctuation regime
commonly observed in cw lasers with optical feedback. We show that such behaviour can be observed within the framework of the Lang Kobayashi equations for self-pulsating semiconductor lasers.
We analyse the sensitivity of quantum dot semiconductor lasers to optical feedback. While bulk and quantum well semiconductor lasers are usually extremely unstable when submitted to back reflection, quantum dot semiconductor lasers exhibit a reduced sensitivity. Using a rate equation approach, we show that this behaviour is the result of a relatively low but nonzero line-width enhancement factor and strongly damped relaxation oscillations.
An analysis of the transverse and longitudinal mode structure of broad area quantum dot lasers emitting at 1060 nm is presented. In particular, temperature is shown to play an important role in the stabilisation of the transverse mode structure of the devices. In addition, the investigation of the interaction between these transverse modes, through the measurement of the spatial intensity correlation, shows that the laser retains some modal properties in the unstable regime. Finally, measurements of spectral correlations between longitudinal mode groups display a strong dependency on their respective transverse mode structures indicating the importance of spatial overlap.
The polarisation dynamics of vertical cavity surface emitting lasers
(VCSELs) in the bistable regime is well described by Kramers theory
for noise induced transitions. By employing feedback, a memory mechanism can be introduced, which make the dynamics of the system
non-Markovian. Here we analyse theoretically and experimentally the
residence time distribution of the bistable systems in the presence
of noise and time-delayed feedback, using an opto-electronic feedback cycle for a VCSEL. We demonstrate and explain various non-exponential features of the residence time distribution using a continuous as well as a two-state model. Additionally we compare the results to an electronic Schmitt trigger, which represents an experimental realization of the two-state model.
We analyse the sensitivity of quantum dot semiconductor lasers to optical. While bulk and quantum well semiconductor lasers are usually extremely unstable when submitted to back reflection, quantum dot semiconductor lasers exhibit a reduced sensitivity. Using a rate equation approach, we show that this behaviour is the result of a relatively low but nonzero line-width enhancement factor and of strongly damped relaxation oscillations.
We analyse theoretically and experimentally the residence time distribution of bistable systems in the presence of noise and time-delayed feedback. The feedback provides a memory mechanism for the system which leads to non-Markovian dynamics. We demonstrate and explain various non-exponential features of the residence time distribution using a two-state as well as a continuous model. The experimental results are based on a Schmitt Trigger where the feedback is provided by a computer generated delay loop and on a semiconductor laser with opto-electronic feedback.
Universal self-organisation on surfaces of semiconductors upon deposition of a few non-lattice-matched monolayers using MOCVD or MBE lead to the formation of quantum dots. Their electronic and optical properties are closer to those of atoms than of solids.
We have demonstrated for QD-lasers a record low transparency current density of 6A/cm2 per dot layer at 1.16 μm, high-power of 12W, an internal quantum efficiency of 98%, and an internal loss below 1.5 cm-1. Relaxation oscillations indicate the potential for cut-off frequencies larger than 10 GHz.
GaAs-based QD-lasers emitting at 1.3 μm exhibit output power of 5 W and single transverse mode operation up to 300 mW. At 1.5 μm again an output power of 5 W has been obtained for first devices showing a transparency current of 700 A/cm2.
Single mode lasers at 1.16 and 1.3 μm show no beam filamentation, reduced M2, sensitivity to optical feedback by 30 db and α-parameter as compared to quantum well lasers.
Passive mode locking of 1.3 μm lasers up to 20 GHz is obtained.
Thus GaAs-lasers can now replace InP-based ones at least in the range up to 1.3 µm, probably up to 1.55 μm.
Microcavity semiconductor lasers are important devices from both practical and fundamental viewpoints. Practically, these lasers/resonators are excellent candidates for the next generation of all-optical network components, including switches and filters, because of their size and low power consumption. We will present a novel packaging scheme which further facilitates these applications. This scheme involves the bonding of the optically pumped micro-resonator to a piece of multi-mode fiber. The laser is optically pumped directly and the emission is collected through another multi-mode fiber. This raises the possibility for 'all fiber' packaging schemes where the micro-resonator is sandwiched between two pieces of optical fiber. The pump and signal light can be injected in at one end and the output collected at the other. This illustrates the potential that these devices have for all optical network applications. In addition, the dynamic properties of these lasers are not well understood because the low level of laser light (order of nanoWatts) makes experimental analysis difficult. We will present experimental results that highlight some of the future challenge, which will have to be overcome if these devices are to realise their potential.
We describe the different mechanisms to generate waves in the transverse section of lasers. Our analysis, based on the Maxwell-Bloch equations, is compared to recent experimental results.
KEYWORDS: Semiconductor lasers, Near field, Vertical cavity surface emitting lasers, Profiling, Polarization, Laser damage threshold, Near field optics, High power lasers, Numerical simulations, Physics
We describe the different mechanisms to generate waves in the transverse section of lasers. Our analysis, based on the Maxwell-Bloch equations, is compared to recent experimental results.
We present a simple broad area semiconductor laser which uses a current spreading layer to modify the transverse gain profile. The device exhibits excellent spatial coherence to total output powers of 2.5 W under pulsed operation. Devices have been focused down to a spot size of approximately 5 micrometers FWHM at 2.5 W with the beam profile and position remaining stable over the entire range of operation. Under CW operation, thermal effects reduce spatial coherence leading to a significantly increased spot size and loss of beam stability. This work demonstrates the advantages of modifying the transverse gain profile and how it can be used to produce high brightness devices required for single mode fiber coupling.
We analyze the microwave dynamics of a semiconductor lasers with optical feedback in the Low Frequency Fluctuation regime. Our experimental measurements, which are based on a statistical approach, are at variance with the solution of the model commonly used to describe this instability. We then discuss the possible reason for the differences and develop an analogy with the turn on transient of a semiconductor laser.
We analyze the near and far field intensity distributions of large aperture native oxide confined AlGaAs/GaAs VCSELs. We demonstrate single mode highly divergent emission when the cavity is red shifted with respect to the quantum well gain peak. As the gain peak is detuned red of the cavity, the VCSEL operates in multiple less divergent modes.
We experimentally investigate the radio frequency noise spectrum of electrically pumped VCSELs during polarization switching. It is found that the relaxation oscillation and beating frequency of polarization modes may lock before the switching if the frequencies are comparable. We compare our experimental results with the existing models.
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