This PDF file contains the front matter associated with SPIE Proceedings Volume 6997, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and the Conference Committee listing.

We report the realization of GaSb-based optically pumped vertical-external-cavity surface-emitting lasers (VECSELs) emitting at 2.25 μm which are capable of multiple-Watt output power. VECSEL structures were grown on GaSb-substrates by molecular beam epitaxy. SiC heat spreaders were capillary bonded onto the surface of the VECSEL chip in order to facilitate efficient heat removal. A continuous-wave output power of more than 3.4 W was recorded at a heat sink temperature of -10 °C. At room temperature (20 °C) we still obtained more than 1.6 W output power. A beam propagation factor in the range of M²≤5 was measured at maximum output power. In adjusting the fundamental mode diameter on the VECSEL chip to the pump spot diameter the beam quality could be further improved resulting in a beam propagation factor of M²~1.5. Furthermore, initial results on a GaSb-based dual-chip VECSEL are reported, capable of delivering a maximum output power of 3.3 W for a heat sink temperature of 20 °C and an emission wavelength of 2.25 μm.

An innovative combination of concepts, namely microphotoluminescence (μPL) mapping, focused ion beam (FIB) microscopy, micro-Raman spectroscopy, and high-speed thermal imaging, was employed to reveal the physics behind catastrophic optical damage (COD), its related temperature dynamics, as well as associated defect and near-field patterns. μPL mapping showed that COD-related defects are composed of highly nonradiative complex dislocations, which start from the output facet and propagate deep inside the cavity. Moreover, FIB analysis confirmed that those dark line defects are confined to the active region, including the quantum wells and partially the waveguide. In addition, the COD dependence on temperature and power was analyzed in detail by micro-Raman spectroscopy and real-time thermal imaging. For AlGaInP lasers in the whole spectral range of 635 to 650 nm, it was revealed that absorption of stimulated photons at the laser output facet is the major source of facet heating, and that a critical facet temperature must be reached in order for COD to occur. A linear relationship between facet temperature and near-field intensity has also been established. This understanding of the semiconductor physics behind COD is a key element for further improvement in output power of AlGaInP diode lasers.

This work relates to combining a phase corrected array of tapered laser diodes, emitting at λ = 975 nm, coherently using the Talbot effect. Diffractive coupling of semiconductor lasers by use of the Talbot effect provides a means for coherent beam addition of multiple elements in laser diode arrays and makes possible a very compact external cavity. We have used, in this work, fully index guided tapered laser diodes. They contain a ridge waveguide, which acts as a modal filter, and a tapered section of increasing width, which provides high power. We have realized arrays of several emitters (N=10), which are not optically coupled to each other. First, to improve the beam quality of the array, a phase correcting micro system, achieving collimation in the fast axis, correction of the wave front tilts in both directions and also a slow axis collimation, was added. The FWHM divergences of the array were reduced from 34 ° to 0.17 ° in the fast-axis and from 3.5 ° to 0.7 ° in the slow-axis at 6A, 3.7 W. Then, to be close to diffraction limit, we have combined this corrected array coherently using the Talbot effect. We have obtained quasi-monolobe slow axis far field profile for the in phase mode with a central peak divergence of only 0.27 ° at 1.5 A, 315 mW under CW operation and of only 0.20 ° at 2.5 A, 787 mW under pulsed operation.

The principle of the operation of a Gunn laser is based on the band to band recombination of impact ionized non-equilibrium electron-hole pairs in propagating high field space-charge domains in a Gunn diode, which is biased above the negative differential resistance threshold and placed in a Fabry-Perot or a vertical micro cavity (VCSEL). In conventional VCSEL structures, unless specific measures such as the addition of oxide apertures and use of small windows are employed, the lack of uniformity in the density of current injected into the active region can reduce the efficiency and delay the lasing threshold. In a vertical-cavity structured Gunn device, however, the current is uniformly injected into the active region independently of the distributed Bragg reflector (DBR) layers. Therefore, lasing occurs from the entire surface of the device. The light emission from Gunn domains is an electric field induced effect. Therefore, the operation of Gunn-VCSEL or F-P laser is independent of the polarity of the applied voltage. Red-NIR VCSELs emitting in the range of 630-850 nm are also possible when Ga_1-xAl_xAs (x < 0.45) is used the active layer, making them candidates for light sources in plastic optical fibre (POF) based short-distance data communications. Furthermore the device may find applications as an optical clock and cross link between microwave and NIR communications. The operation of a both Gunn-Fabry-Perot laser and Gunn-VCSEL has been demonstrated by us recently. In the current work we present the potential results of experimental and theoretical studies concerning the applications together with the gain and emission characteristics of Gunn-Lasers.

We study mid-infrared quantum-cascade lasers (QCL's) with a pair of triply harmonic resonant levels. Potential applications of such designs are discussed based on the resonant third-order nonlinear susceptibility χ⁽³⁾ at the third-harmonic (TH) frequency, χ⁽³⁾(3ω; ω, ω, ω), and that at fundamental mode (FM) frequency χ⁽³⁾( ω; ω, ω,-ω). Mode overlap and the phase mismatch effect are considered in the TH power evaluation. In addition to single-photon processes, resonant two-photon processes are included in the evaluation of χ⁽³⁾( ω; ω, ω,-ω), which results in the enhancement of the positive nonlinear (Kerr) refractive index, and thus induce stronger self-phase modulation (SPM). It is concluded that QCL's with multiple-resonance design are not only favorable for tunable light emission in the near- to mid-infrared region, but are also promising candidates for spectrum broadening by SPM.

We investigate two types of 405 nm (In, Al)GaN test laser structures (TLSs), one of them grown on SiC substrates, the other grown on low dislocation density freestanding GaN substrates. Measuring the lasing spectra of these structures, we observe an individual behavior depending on the substrate. TLSs on GaN substrates show a broad longitudinal mode spectrum above threshold, whereas TLSs on SiC are lasing only on one mode with various jumps of the laser emission at certain currents. Estimating the gain of each longitudinal mode with the Hakki-Paoli method, we find minute variations of the gain for TLSs on GaN substrate. In contrary, TLSs on SiC substrate show much larger fluctuations of the gain for individual longitudinal modes. Using a rate equation model with nonlinear gain effects, we simulate the longitudinal mode spectrum of both types of TLSs. Once we modify the gain of each longitudinal mode as observed in the gain measurements, the simulated spectra resemble the SiC or GaN substrate TLS spectra.

The Detuning Loading Effect, i.e., the effects of the modulation performance on the position of the lasing mode relative to the Bragg reflection peak, is investigated in a Modulated Grating Y-branch laser. By proper adjustment of the lasing mode position, simultaneous chirp reduction and modulation bandwidth enhancement can be obtained. The lasing mode position is also crucial for side mode suppression ratio and output power.

The Transfer Matrix Method (TMM) is the standard method for simulating resonators with internal reflections occurring in Distributed Feedback (DFB) lasers and other laser types. A restriction of this method is that it cannot be applied to two dimensions or to time-dynamic simulations. We present a new Finite Element approach which can be treated as a generalization of the TMM in two or three dimensions. Furthermore, it can be used for time-dynamic problems as well as for large, tapered structures. We apply it to the time-dynamic simulation of the optical wave in DFB lasers and show numerical results.

We consider a system of two identical, but possibly detuned, spatially separated semiconductor lasers that are mutually coupled via their optical fields. In a rate equation approach this system can be modeled by a set of delay differential equations, where the delay takes into account the propagation time of the light from one laser to the other. The delay introduces a complicated structure the compound laser modes (CLMs) whose interaction may lead to complicated dynamics. In this paper we present a bifurcation study of the CLM structure for the relevant system parameters, including the pump current and the detuning. Initially stable CLMs can destabilizes in Hopf bifurcations that lead to complicated dynamics on different time scales. In particular, we concentrate on the dynamics near the boundary of locked dynamics of the two lasers. Depending on the pump current we find different scenarios for the route to locking as a function of the detuning between the two lasers.

The constraints on dilute-nitride Semiconductor Optical Amplifiers (SOAs) for multi-wavelength amplification have been evaluated. SOAs have been fabricated by angling the facets of a GaInNAs/GaAs edge emitting laser using gas enhanced focused ion beam etching. The original laser has been characterized in terms of its temperature dependence and net modal gain. A full width half maximum (FWHM) of 40nm has been found at 298K. Good temperature stability has also been found with a value of 0.35nm/K for the lasing wavelength. The good temperature stability of the device has been explained in terms of the role that the monomolecular recombination plays in the temperature dependence of the device. The monomolecular recombination has been reported temperature independent having two key effects; reduction of the temperature performance and reduction of the dynamic performance in terms of an increase in the threshold current and a decrease of the high speed potential. Iodine gas enhanced focused ion beam etching (GAE-FIB) has been used for the fabrication of the SOA, the iodine gas increasing the etching rate by a factor of 2.5. The fabrication has been completed in two steps; in the first one the facets have been angled and in the second step a cross-section procedure has been employed for smoothing of the facets. Once the SOA has been fabricated its potential for simultaneous multiple channel amplification has been studied. A flat gain spectrum over a range of 40nm has been obtained. This value and the wavelength range have good agreement with the net modal gain measured in the original laser device. In addition, minimum channel interspacing has been achieved with no wavelength degradation.

We investigate theoretically the identification of the external-cavity roundtrip time of an external-cavity semiconductor laser (ECSL). The time-delay identification is performed by analyzing the laser-intensity time series with conventional techniques based on the autocorrelation function or mutual information. We find that a weak feedback rate and a time-delay close to the laser's intrinsic relaxation-oscillation period are two conditions leading to difficult delay identification. This arduous time-delay identification is of particular interest for the security improvement of chaos-based communications schemes using ECSLs.

Chaotic wavelength transmitters based on a DBR laser submitted to optoelectronic feedback with periodic time delay are considered. We investigate the retrieval of the periodic time delay function from experimental time series. Square-wave and sinusoidal modulations are considered for the frequency clock of a delay module based on a First-In First-Out memory. It is shown that the period of the time delay can be extracted from experimental data by using the mutual information function. Different values of the nonlinearity are considered. Applying a modified filling factor method the periodic time delay function is retrieved in the sinusoidal modulation case for different periods and modulation depths.

We report the first experimental observation of multi-pulses excitability in a 1.55 μm injection-locked bulk semi-conductor laser. Several temporal waveforms are presented showing different excitability orders. We complete these observations by drawing the excitability areas in the map "detuning-injected power" of the injected laser. We show for the first time synchronization on excitable pulses between a receiver and a transmitter, using a cascade of optically injected systems (a master, a transmitter and a receiver).

There has been much effort devoted to frequency conversion technology due to strong demand for optical communication systems. A frequency converter converts an incoming optical carrier of one frequency to an outgoing optical carrier of another frequency while preserving the quality of carried data. The all-optical approach is promising for such a purpose because not only the complexity and power consumption of a converter are much reduced but also the flexibility and reconfigurability are greatly improved. However, most proposed methods, such as applying cross gain modulation in semiconductor optical amplifiers, suffer from the need of a probe or a pump beam besides the incoming optical carrier, making systems complicated and costly. In this study, we propose to use semiconductor lasers as frequency converters instead, where no probe or pump beam is necessary. When a semiconductor laser is subject to an incoming optical carrier, equivalently an external optical injection, it can enter into period-one dynamics through Hopf bifurcation. By taking advantage of the dynamics, tens of gigahertz of frequency conversion can be achieved, which can be continuously and dynamically tuned by controlling the injection level and frequency. The conversion efficiency and transmission efficiency can also be varied through the change of both injection parameters. Their behaviors as functions of the parameters, however, are opposite to each other. High conversion efficiency is observed to achieve under low levels of injection, where strong filtering of frequency may not be necessary and significant signal amplification can be achieved. Low bit-error-rate and a 3-dB penalty are also observed, suggesting the quality of carried data is preserved.

All-optical flip-flops draw more and more attention as potential parts of all-optical packet or burst switching schemes. Recently several schemes for such all-optical flip-flops have been proposed, e.g. mutually coupled laser diodes, Mach-Zehnder interferometers with fed back output and ring lasers. All flip-flops are based on bistable behaviour and on the possible switching between the two stable states using short optical pulses. Previously, we have shown numerically that a DFB laser diode in which a CW signal is injected can exhibit a bistability in the laser output power and the amplification of the input power. A condition for this bistability was that the injected light is not reflected inside the laser diode. The wavelength of the injected light must therefore be not too close to the Bragg wavelength and the laser must be AR coated. In this contribution, we present additional modelling results as well as some experimental results for λ/4-shifted DFBs. We show numerically that for a certain bias current the bistability only occurs when the carrier lifetime and the series resistance of the laser diode are not too small, such that sufficient spatial hole burning can exist inside the laser. Experimental results show that bistability is observed in some lasers but not in others, something which may be related to the series resistance and carrier lifetime. We will also discuss the dynamic, all-optical flip-flop operation, which is possible by injecting short pulses on either facet of the laser.

We study the behaviour of a multi-transverse-mode vertical-cavity surface-emitting laser subject to optical feedback in which the optical modes are coupled through the external round-trip. Starting from a delayed partial differential equation description of the spatial optical mode profiles and the carrier diffusion, we first use eigenfunction expansion techniques to resolve the spatial dependence. The resulting system of delay differential equations is then amenable to a full nonlinear bifurcation analysis by means of numerical continuation techniques. As illustration, we present bifurcation diagrams of a two-mode VCSEL in the plane of feedback strength versus feedback phase. In this way, we identify a number of changes in the structure and bifurcations of the VCSEL's dynamics. In particular, we find coexisting stable steady state solutions, which bifurcate to stable in-phase and anti-phase periodic solutions with vastly differing frequencies. We show how these periodic solutions give rise to quasiperiodic and chaotic laser dynamics.

Two edge-emitting lasers coupled through polarization-rotated optical injection exhibit square-wave oscillations provided the roundtrip time from laser to laser and back is sufficiently large. If the mutual coupling between the lasers is relatively weak, the two plateaus of the square-waves exhibit different durations even though the total period remains close to the roundtrip time. This asymmetry progressively disappears as the feedback strength is increased. The experimental observations are confirmed by numerical simulations. The simulations also reveal that the square-wave regimes appear through a series of complex bifurcations and that a sufficiently large roundtrip time is needed.

We report an essential progress towards the development of efficient GaInNAs-based semiconductor disk lasers operating at 1220 nm spectral range. The gain mirrors were fabricated by molecular beam epitaxy using a radio frequency plasma source for incorporating the nitrogen. The typical structure consisted of a 30-pair GaAs/AlAs distributed Bragg reflector and 10 GaInNAs quantum wells with relatively low content of nitrogen. The growth parameters and the composition of the structures have been optimized to reduce the detrimental effect of nitrogen on the emission efficiency. We have achieved a maximum output power of 3.5 W and a differential efficiency of 20%.

Chirped multilayer (N=10) QD lasers with 2-, 3- and 5-layer of longer-, medium-, and shorter-wavelength QD stacks, respectively, were grown in this work. Low threshold current density and high saturated modal gain were achieved in this specially designed QD structure. Empirical gain-current analysis was performed on this chirped multilayer QD structure for the first time. It was consistent with our spectral observations and provided valuable information on carrier recombination in chirped multilayer QD structure. Two novel spectral characteristics were discovered also for the first time. First, simultaneous two-wavelength lasing around threshold was observed under particular gain-loss condition at this specific multilayer structure of QD stacking numbers. Second, at cryogenic temperature, simultaneous two-wavelength lasing emissions switched from longer-wavelength lasing first to shorter-wavelength lasing first with increasing current injection. Non-uniform carrier distribution among chirped multilayer QD structure is evident at low temperature below 200 K from our analysis.

The comparison of relative intensity noise (Rin) shows improved performances, for quantum dash laser (QD) compared to the ones of bulk medium structures. We introduced a statistical measurement through a coupling parameter that reveals the impact of strong damping on the competition between modes or the so-called partition noise. The existence of a strong damping in QD laser prevents the relaxation frequency from being observed in the coupling parameter, which makes the noise to appear as if the laser lines were inhomogeneous. However the method also enables the characterization of the coupling strength between modes, showing again differences between QD and bulk structures.

A Semiconductor Saturable Absorber Mirror utilising the electroabsorption effect in a self-biased stack of extremely shallow quantum wells or in a bulk semiconductor is proposed and analysed theoretically and numerically. The saturation flux and recovery time of the proposed device when operated with picosecond incident pulses are shown to compare very favourably with existing all-optical constructions.

In this contribution we report on radio-frequency and in particular time-domain studies to develop a better understanding of mode-locked quantum dot (QD) two-section lasers emitting at 1.3 μm. Based on substantial investigations of the optical pulsewidth evolution showing pulsewidths well below 4 ps, we will present the measured dependences of the optical pulsewidth as well as the pulse-to-pulse timing jitter on gain current, absorber bias voltage and RF power. Based on these results we will discuss the shortening of the pulsewidth, the corresponding RF spectra evolution as well as the pulse-to-pulse rms timing-jitter evolution within a selected range of operating parameters.

We review the analysis of loss and optical confinement in etched photonic crystal (PhC) vertical-cavity surface-emitting lasers (VCSELs). The lossy PhC structure is modeled using a waveguide approach employing the scalar Helmholtz equation with complex refractive indices. Theoretical results are developed, analyzed, and compared to experimental measurements of spectral mode splitting, modal loss, and slope efficiency of fabricated VCSELs. This model is shown to design accurately single-mode photonic crystal VCSELs.

In this article, we report on long wavelength (1.27 μm) single-mode micro-structured photonic crystal strained InGaAs quantum wells VCSELs for optical interconnection applications. Single fundamental mode room-temperature continuous-wave lasing operation was demonstrated for devices designed and processed with different two-dimensional etched patterns. The conventional epitaxial structure was grown by Metal-Organic Vapor Phase Epitaxy (MOVPE) and contains fully doped GaAs/AlGaAs DBRs, one oxidation layer and three strained InGaAs quantum wells. The holes were etched half-way through the top-mirror following various designs (triangular and square lattices) and with varying hole's diameters and pitches. We obtained up to 1.7 mW optical output power and more than 30 dB Side-Mode Suppression Ratio (SMSR) at room temperature and in continuous wave operation. Systematic static electrical, optical and spectral characterization was performed on wafer using an automated probe station. Numerical modeling using the MIT Photonic-Bands (MPB [1]) package of the transverse modal behaviors in the photonic crystal was performed using the plane wave method in order to understand the index-guiding effects of the chosen patterns, and to further optimize the design structures for mode selection at the given wavelength.

GaAs-based VCSELs emitting near 1.3 μm are realized using highly strained InGaAs quantum wells and a large detuning of the cavity resonance with respect to the gain peak. The VCSELs have an oxide aperture for current and optical confinement and an inverted surface relief for suppression of higher-order transverse modes. The inverted surface relief structure also has the advantage of suppressing oxide modes that otherwise appear in VCSELs with a large detuning between the cavity resonance and the gain peak. Under large signal, digital modulation, clear and open eyes and error free transmission over 9 km of single mode fiber have been demonstrated at the OC-48 and 10 GbE bit rates up to 85°C. Here we review these results and present results from a complementary study of the RF modulation characteristics, including second order harmonic and third order intermodulation distortion, relative intensity noise (RIN), and spurious free dynamic range (SFDR). RIN levels comparable to those of single mode VCSELs emitting at 850 nm are demonstrated, with values from -140 to -150 dB/Hz. SFDR values of 100 and 95 dB•Hz^2/3 were obtained at 2 and 5 GHz, respectively, which is in the range of those required in radio-over-fiber systems.

In this work we report on an experimental investigation of the nonlinear dynamics of a 850 nm multitransverse mode vertical-cavity surface-emitting laser (VCSEL) when subject to high-frequency current modulation. Different frequencies and modulation amplitudes are applied to the VCSEL. Regular periodic dynamics - with periods equal to the modulation period or twice the modulation period - and irregular pulsating dynamics are obtained. Different dynamical behaviors are illustrated by using power time traces, radio-frequency spectra and bifurcation diagrams. Our results show that irregular pulsating dynamics in multimode VCSELs subject to large-signal current modulatton can be obtained due to the competition between different transverse modes.

Semiconductor disk lasers have attracted a lot of interest in the last few years due to high output power combined with good beam quality and possible wavelength engineering. One of the disadvantages is the need for external optical pumping by edge-emitting semiconductor lasers that increase packaging effort and cost. Therefore, semiconductor disk lasers with monolithically integrated pump lasers would be of high interest. We report on a novel design and experimental realization to monolithically integrate pump lasers with a semiconductor disk laser in a one-step epitaxial design. By careful design of integrated pump lasers and stacking sequence, it is possible to efficiently excite vertical emitter areas with different mesa sizes. First results are shown at 1060 nm emission wavelength with high output power out of mesa diameters of 100 μm to 400 μm. The devices can be conveniently characterized on a wafer level using dry-etched pump laser facets. In pulsed operation 1.7W out of a 100 μm diameter mesa and 2.5W out of a 200 μm diameter mesa are demonstrated. Additionally, more than 0.6W in cw operation using a 400 μm structure were achieved. In summary, an innovative approach for truly monolithic integration of a semiconductor disk laser with pump lasers has been pioneered.

We present experiments measuring the complex degree of coherence of a Broad-Area Vertical-Cavity Surface-Emitting Laser (BA-VCSEL) when it is driven into a regime of spatially incoherent emission. This high-power, spatially incoherent emission regime is quite uncommon for semiconductor lasers but can be useful in e.g. illumination and projection systems, as the low degree of spatial coherence may help to reduce speckle. The near-field coherence properties are measured for different positions in the VCSEL's aperture using a 180 degrees reversing-wavefront Michelson interferometer. We give evidence that the coherence area is much smaller than the VCSEL's aperture and that the intensity fluctuations across the coherence area are small, therefore allowing the BA-VCSEL to be considered a quasi-homogeneous source. We explain the reason for the relatively small coherence radius of about 1.4 μm based on the mode size in a planar cavity together with the thermal gradient within the VCSEL aperture.

We experimentally and numerically report on polarization switching (PS) mechanism which involves a two-mode limit cycle dynamics in a vertical-cavity surface-emitting laser (VCSEL) subject to orthogonal optical injection from a master laser (ML). The VCSEL (slave laser, SL) emits a horizontal linearly polarized (LP) fundamental mode, without optical injection. The VCSEL is injected by a vertically polarized light from ML. Dynamical characteristics of the VCSEL are investigated as a function of optical injection parameters, i.e., injection strength and frequency detuning between master and slave lasers. We experimentally resolve an injection parameter region for which, as the injection strength is increased for fixed detunings, a limit cycle dynamics in both non-injected and injected modes is abruptly excited. For larger injection strengths, the VCSEL switches from the two-mode to a single-mode limit cycle dynamics which involves only the injected mode. Using continuation methods, we numerically identify two torus bifurcation mechanisms, namely TR₁ and TR₂, which support such a switching scenario. We show that both TR₁ and TR₂ originate from a particular Hopf bifurcation which plays a key role in the polarization dynamics of the injected VCSEL. Furthermore, our results reveal that the newly observed switching dynamics are generic features of VCSEL two-mode systems.

According to a generally known rule of thumb, a stable single-fundamental-mode operation is achieved in standard VCSELs with relatively uniform radial active-region gain profiles. However, in analogous detuned oxide-confined VCSELs, lasing thresholds of higher-order modes may surprisingly be lower than that of the fundamental one. The above unusual VCSEL behavior is explained with the aid of the comprehensive self-consistent simulation. It has happened to be a result of a strong wavelength dependence of the active-region optical gain in highly detuned oxide-confined VCSELs, because of which longer-wavelength higher-order cavity modes may exhibit much higher modal gain values than that of the fundamental one. For the 10-μm-diameter mesa top-emitting 1.3-μm GaInNAs/GaAs QW VCSEL design, the optical active region gain spectrum exhibits its maximum for the wavelength distinctly lower than that, for which the VCSEL cavity and the DBR mirrors have been designed. As a result, the transverse LP₇₁ mode, whose wavelength (1291.1 nm) is close enough to the maximal optical gain, has happened to be the lowest-threshold mode (2221 cm^-1). For the LP₇₁ threshold voltage, the fundamental LP₀₁ mode (1300.7 nm) manifests lower threshold (1432 cm^-1), as expected, but it is still considerably higher than its available modal gain (958 cm^-1).

We report on a novel mirror design for micro-ring semiconductor ring lasers that uses those mirrors to connect straight waveguide sections in a closed loop. The nearly parabolic mirrors are designed by ray-tracing to optimize their shape. The efficiency of the designs is verified with FDTD simulations, with a simulated coupling efficiency of 98%. We have successfully fabricated devices based on this design with an equivalent circular ring radius of 16μm. These devices operate in continuous wave at room temperature with a threshold current of only 22mA and an emission wavelength of 1.55μm.

In this paper, numerical investigation is performed for a 1.55μm InGaAsP-InP microring laser as a function of the bus waveguide reflectivity, the injection current and the phase of the backreflected field. The nascent nonlinear instabilities are identified utilizing a multimode rate equation model, originating from the continuous injections of each clockwise to the counterclockwise mode and inverse. The resulted time series are filtered using a 40GHz electrical low pass filter in order to omit the mode beatings. Chaos data analysis revealed high-dimensional chaos by means of the correlation dimension and the metric entropy calculation with continuously testing surrogate data. With increasing the bus waveguide reflectivity, period-doubling and quasiperiodic route to chaos was found and the dimension was found to follow a linear increase. The same dimension increase was found with increasing the injection current, with the system experiencing sudden transitions from chaos to limit cycles. With altering the phase of the backreflected field the dynamics were found to transit from limit cycle (Δφ=0→π/2) to chaos, maintained chaotic (Δφ=π/2→2π/3) and finally returning to periodic states (Δφ=2π/3→2π). Furthermore, the dynamics are investigated with calculating the standardized moments.

We review theoretical results on the dynamics of solitary single longitudinal mode and single transversal mode semiconductor ring lasers. These analyses are based on a rate equation model for the slowly varying envelopes of the counter-propagating fields in the ring cavity which has been proposed by Sorel et al. [Opt. Lett. 27, 1992 (2002); IEEE J. Quantum Electron. 39, 1187 (2003)]. The model shows several operating regimes. The lasers are found to operate bidirectionally up to twice the threshold, where unidirectional operation starts. Just above threshold, the lasers operate in a regime where the two counterpropagating modes are continuous wave, while as the injected current is increased, a regime appears where the intensities of the two counterpropagating modes undergo alternate sinusoidal oscillations. To understand these dynamical features, we discuss a reduction of this basic rate equation model derived by Van der Sande et al. [accepted for publication in J. Phys. B (2008)]. The reduction has been achieved using asymptotic methods based on the typical relative scaling of the dynamical time scales of the system. Physical conditions for the emergence of the operating regimes are assessed quantitatively in terms of nonlinear (saturation processes) and linear coupling (backscattering) between the counter-propagating modes.

We present a theoretical and experimental research of the linewidth enhancement factor or α factor in multiple-longitudinal mode semiconductor lasers. In this work, several methods originally developed for single-mode lasers have been adapted for their use with multiple-longitudinal mode lasers and applied to each mode of several Fabry-Perot lasers at a time. The concepts of material LEF and device LEF are compared and discussed.

For many applications a frequency stabilized beam source with high output power and a good beam quality is needed. Tapered lasers and amplifiers can provide a high output power, whereas they have a slightly lower beam quality than ridge lasers. In a single mode fiber (SMF) coupled module, the beam quality provided by the module is predetermined by the fiber. The technological progress of tapered lasers should allow a high enough coupling efficiency to give SMF coupled modules using a tapered laser or amplifier the potential for a higher output power than modules using a ridge laser. It will be shown how this potential can be exploited by using different coupling systems for example with cylindrical lenses either crossed or in combination with rotational lenses. The advantages, problems and coupling results of those systems will be illustrated. Two approaches of frequency stabilization will be shown. To stabilize a tapered amplifier the external cavity has been set up by a fiber bragg grating on the backside of the amplifier. A volume holographic grating, which is written in the fast axis collimation lens of the coupling system, was used to stabilize a tapered laser.

We use the traveling wave model for simulating and analyzing nonlinear dynamics of complex semiconductor ring laser devices. This modeling allows to consider temporal-spatial distributions of the counter-propagating slowly varying optical fields and the carriers, what can be important when studying non-homogeneous ring cavities, propagation of short pulses or fast switching. By performing numerical integration of the model equations we observe several dynamic regimes as well as transitions between them. The computation of ring cavity modes explains some peculiarities of these regimes.

This paper aims at revisiting the basic Lorenz-Haken equations with two-fold harmonic-expansion approaches, yielding new analytical information on both the transient and the long term characteristics of the system pulse-structuring. First, we extend the well-known Casperson Hendow-Sargent weak-sideband analysis to derive a general formula that gives the value of the transient frequencies, characteristic of the laser relaxing towards its long-term state, either stable or unstable. Its validity is shown to apply with a remarkable precision at any level of excitation, both beyond and below the instability threshold. Second, we put forward a strong-harmonic expansion scheme to analyse the system long-term solutions. Carried up to third order in field amplitude, the method allows for the derivation of a closed form expression of the system eigen-frequency (derived here for the first time in three decades of laser dynamics) that naturally yields an iterative algorithm to build, analytically, the regular pulsing solutions of the Lorenz-Haken equations. These solutions are constructed for typical examples, extending well beyond the boundary region of the instability domain, inside which the laser field amplitude undergoes regular pulsations around zero-mean values.

This paper aims at revisiting some of the self-pulsing properties of the integro-differential "Maxwell-Bloch" equations that describe single-mode inhomogeneously broadened, including semi-conductor and fibre lasers, focusing mainly on the dynamic gain contour, which is shown to undergo deep modifications during pulse build-up. First, we extend the well-known Casperson Hendow-Sargent weak side-band approach and delimit its applicability with respect to dynamic gain profiling. In particular, we demonstrate, both numerically and analytically that the small-sideband method only describes centre-line saturation which occurs in the small periodic-oscillations regime. In the strong self-pulsing regime, however, lateral saturation along the gain curve occurs as demonstrated with numerical simulations. The small side-band approach fails to describe such gain structuring during pulse build-up. Second, we put forward a strong-harmonic expansion method which reveals quite adapted for the description of the dynamic gain profiles. Extended to third-order in field amplitude and to second order in population inversion, the method allows for the extraction of analytical expressions that retrieve the lateral saturation effects with a remarkable precision.

The aim of this paper is to design measuring method for determining applicability of picked laser diode in the optical wireless communications. Chosen laser diode must operate to not debase received optical signal quality and dynamics in optical wireless link. We are focusing on the processes in the optical links which are influenced by thermal effects. Varying laser diode's operating temperature and thermal fluctuations in atmospheric transmission media, in which the information carried by laser beam is propagating, are considered as the most significant thermal events in optical wireless communications. As mentioned before, the influence of operating temperature for laser diode's light emission has to be taken into account. The operation temperature affects on the physical properties of laser diode. Transmitting laser beam is collimated by transmitting lens but in case of laser diode's operating temperature vary the change in irradiation characteristics can occurs as an issue in optical wireless links because the distance between laser diode and transmitting lens can't be adapted by the reason of laser beam divergence settings. Atmospheric turbulence influence on spectrum and laser beam geometry of laser beam is investigated. In terms of results' evaluation we can determine when detected signal is acceptable or it doesn't fulfill conditions for signal in optical wireless communications.

Optical communication systems have been widely preferred for network communications, especially for Datacoms Local Area Network links. The optical technology is an excellent candidate for on-board systems due to the potential weight saving and EMC immunity. According to the short length of the link and a cost saving, Vertical Cavity Surface Emitting Laser (VCSEL) and multimode fiber are the best solution for gigabit systems. In this context, we propose a modeling of 850nm VCSEL based on the rate equations analysis to predict the optical interconnect performances (jitter, bit error rate). Our aim is to define the operation conditions of VCSEL under large signal modulation in order to maximize the Extinction Ratio (current I_OFF below threshold) without affecting link performances. The VCSEL model is developed to provide large signal modulation response. Biasing below threshold causes stochastic turn-on delay. Fluctuations of this delay occur, due to the spontaneous emission. This leads to additional turn-on jitter. These stochastic effects are included in the model by adding the Langevin photon and electron noise sources. The VCSEL behavior under high-speed modulation is studied to observe the transient response and extract the resonance frequency, overshoot and turn-on delay. The associated jitter is evaluated with the standard deviation of the turn-on delay probability density function. Simulations of stochastic and deterministic jitters are realized under different conditions of modulation (OFF current levels). Comparing simulations with measurement results carried out on VCSEL and a short haul gigabit link validates the approach.

Multi-mode VCSEL arrays are candidates for compact illumination modules with applications as flash light illumination for the detailed imaging of fast moving objects (> 100 m/s) or for time-of-flight cameras with modulation frequency in the > 10 MHz domain. Rise and fall times have to be as short as a few nanoseconds, while the optical output power has to be in the order of one Watt. In this paper we investigate, for multi-mode VCSEL arrays, the dependency of the far-field pattern on the drive current and on the distance to a reflecting surface. We demonstrate that, with the help of diffusing elements, the current dependency of the far-field pattern can be reduced. We have realized an illumination unit with modulation frequencies of up to 80 MHz and an optical output of 1 Watt.

We study experimentally the polarization bistability of a single transverse and polarization mode vertical-cavity surface-emitting laser (VCSEL) induced by orthogonal optical injection. As the master laser frequency is scanned near the resonance frequency of the depressed linearly polarized fundamental-mode, and for a fixed master laser power, the VCSEL exhibits two successive polarization switchings. Pure frequency-induced polarization bistability is found because both polarization switchings exhibit bistable regions. The width of both bistable regions is analyzed as a function of the master laser power and for different values of the VCSEL current. The hysteresis width fluctuates around a constant level while the injected power is smaller than a certain value. Further increase of the master laser power leads to a smaller constant value of the hysteresis width. The value of that transition value is independent on the VCSEL current. Power-induced polarization bistability - by fixing the frequency of the injected light- is found near that transition value. The character of the power-hysteresis cycles change from clockwise to anticlockwise as the master laser frequency moves away from the frequency of the depressed linearly polarized fundamental-mode. Qualitatively similar results are found for different VCSELs with wavelength around 1550 nm.

We study a semiconductor laser subject to filtered optical feedback from two separate filters. This work is motivated by an application where two fiber gratings are used to stabilize the output of a laser source. Specifically, we consider the structure of the external filtered modes (EFMs), which are the basic cw-states of the system. The system is modelled by a set of four delay differential equations with two delays that are due to the travel times of the light in each of the external cavities. Here, each filter is approximated by a Lorentzian and we assume that there is no interaction between the two filters. We derive a transcendental equation for the EFMs as a function of the widths, detunings and the feedback strengths of the two filters. With continuation techniques we investigate how the number of EFMs changes with parameters. In particular, we consider the equation for its envelope. This allows us to determine regions in the plane of the two detunings that correspond to one, two or three EFM components - disjoint closed curves that are traced out by the EFMs as a function of the feedback phase.

We consider a semiconductor laser device, where the active region consists of parallel stripes in the longitudinal direction. In the composite cavity model, the stripes are coupled via the transversal modes of the entire compound laser device. By calculating the spatial mode profiles we accurately account for the frequency detuning between the modes as well as for the gain and coupling of the individual modes, which are determined by spatial overlap integrals of the mode profiles. In particular, we show the nonlinear dependence of these quantities on the geometry of the laser device. The temporal dynamics of the composite cavity modes are described by corresponding rate equations. Bifurcation analysis of these rate equations, which are coupled to the spatial mode equations, unravels the dynamics of a twin-stripe laser. We identify different locking regions as well as regions with possibly chaotic dynamics.

We show that, in their unstable regime of operation, the "Maxwell-Bloch" equations that describe light-matter interactions and dynamics inside a bad-cavity-configured laser carry the same resonance properties as any externally driven mechanic or electric oscillator. This finding demonstrates that the non-linearly coupled Laser equations belong to the same universal family of forced oscillatory systems. The primary difference is that while mechanical or electrical systems are put into resonance with an external sinusoidal force with constant amplitude, the resonance-curve of the laser equations is described exclusively in terms of linear pump scans, for fixed cavity and material decay rates. In both cases however, the damping factors play the same fundamental role. In addition, the basic phase factor between the external excitation mechanism and the mechanical or electric oscillator response is shown to play the same essential role in the dynamic response of the "Maxwell-Bloch" equations with respect to the external driving pump level. Dephasing mechanisms occur between successive-order components of an adapted strong-harmonic expansion that describes the regular self-pulsing solutions of light-matter interactions inside a bad-cavity configured laser cavity.

We have analyzed experimentally and theoretically the modal properties of a semiconductor ring laser and the wavelength jumps that occur in connection with directional switching above threshold. A transfer matrix analysis allow us to explain the transfer function measurements when amplified spontaneous emission in the cavity is accounted for. Moreover the transfer matrix analysis permits to determine the threshold condition for the laser modes, which split in two branches due to the symmetry breaking imposed by the output coupler and output waveguides. The wavelength jumps displayed by the device above threshold are interpreted with the frequency splitting and threshold difference between these two branches of solutions, together with the redshift of the material gain.

We study the nonlinear dynamics of a vertical-cavity surface-emitting laser (VCSEL) subject to a repetitive optical pulse injection numerically using the SFM model. In our study, a linearly polarized slave laser is optically injected by a train of optical pulses from a master laser, where the polarization of the master laser is orthogonal to the polarization of the solitary slave laser (x-polarized). By varying the strength and the repetition frequency of the injected pulses, different dynamical states, including regular pulsations, period-doubled pulsations, chaotic pulsations, periodic oscillations, quasi-periodic oscillations, and chaotic oscillations, are found. Instead of having only one polarization mode at the slave laser output, both the y- and x-polarized modes are observed for the pulsation and oscillation states. While the pulsation states with y-polarization follow a period-doubling route to chaotic pulsations, the oscillation states with the x-polarization undergo a quasi-periodic route to chaos oscillations. Then, with adequate strength of the injection, the x-polarized mode will be suppressed (i.e. polarization switching) and eventually the slave laser will lock to the master laser with higher injection strength. Also, the switching points, the boundary of the injection-locked, and the regions of the chaotic states are found to be strongly influenced by the repetition frequency of the injection pulses and the detuning frequency between the two lasers.

The nonlinear dynamics of a semiconductor laser (slave laser) injected by optical pulses with high repetition rate are investigated experimentally. The pulses for injection are generated from a laser (master laser) subjected to either an optoelectronic feedback or an optical feedback. The repetition rates of the pulses are controlled by varying the delay time and the feedback strength of the feedback loop. By injecting the repetitive optical pulses of different intensities and repetition frequencies into another laser (slave laser), rich dynamical states including regular pulsations, frequency beatings, and chaotic pulsations are observed. Moreover, frequency-locked states with different winding number, the ratio of the main pulsation frequency of the slave laser and the repetition frequency of the injected pulses, are also found. Compared to a laser subject to a sine modulated optical injection, the linewidths of the high-order microwave components in the output spectrum of the slave laser are substantially narrower for the laser under repetitive optical pulse injection.

We analyze a rate equation model in the Langevin formulation for the two modes of the electric field and the carrier density, modelling the spontaneous emission noise in a semiconductor ring laser biased in the bidirectional regime. We analytically investigate the influence of complex backscattering coefficient when the two modes are reinterpreted in terms of mode-intensity sum (I-Spectrum) and difference (D-spectrum). The D-spectrum represents the energy exchange between the two counterpropagating modes and it is shaped by the noisy precursor of a Hopf bifurcation influenced mainly by the conservative backscattering. The I-Spectrum reflects the energy exchange between the total field and the medium and behaves similarly to the standard relative intensity noise for single-mode semiconductor lasers. Good agreement between analytical approximation and numerical results is found.

We report on the theoretical analysis and fabrication of a novel type of vertical-cavity surface-emitting laser (VCSEL) that provides selection of a certain higher-order transverse mode. This selection is based on a spatial variation of the threshold gain by adding an antiphase layer with an etched relief structure. The field intensity profile emitted from this laser is calculated numerically as well as with an analytical approach. The main factors that control the selected mode such as the threshold gain, the confinement factor, and the phase parameter are calculated as a function of the active aperture, aiming to achieve single higher-order transverse mode emission. For a given aspect ratio of a rectangular oxide aperture, the threshold gain difference between the selected and neighboring modes is maximized via the relief diameter and the size of the aperture. The fabrication process involves selective etching of the antiphase layer, passivation of the relief, oxidation of an AlAs layer to the desired aperture after reaching this layer using wet-chemical etching. N- and p-metalization processes are applied, followed by polyimide passivation. Finally, bondpad metalization is carried out for electrical contacting. Mode selection is successfully achieved. Attractive applications for such devices are found in optical manipulation of micro-particles such as sorting and separation.

In this work we present a characterization of the chirp parameters of a commercially available ILM by means of measurements made with a high-resolution optical spectrum analyzer. Particularly, we will use the FM/AM method for the characterization of the transient chirp parameter and will compare results with the standard Fibre Transfer Function method. The FM/AM method will be applied to frequencies down to 100 MHz due to the capabilities of the high-resolution optical spectrum analyzer. We will see that, for the studied ILM, the transient chirp parameter varies in such a way that it changes from positive to negative values when the bias voltage applied to the device changes from 0 to -1,5 volts. Moreover, we will also characterize the adiabatic chirp of the device, which is a parameter difficult to measure due to the small value it has compared with directly modulated lasers. In this case, the possibility of measuring optical frequencies with extremely high resolution will simplify the measurement and will provide accurate values for this parameter.

There is a growing demand for precise gyroscopes and atomic clocks for positioning, flight navigation systems and aerospatial applications. One of the prerequisites for atomic optical pumps is a laser diode with high power (a few 10mW), narrow linewidth (<2MHz), and beam qualities (M²<1.5). Another important factor for aerospatial applications is a very high reliability performance of the laser devices. With an aim to address these issues, we have laid down the technological foundation and further developed ridge waveguide distributed feedback (DFB) laser diodes with an emission wavelength of 852nm corresponding to the D2 cesium transition in atomic clocks. The epitaxy is based on an Al free active region with a GaInP large optical cavity and a single compressive strained GaInAsP quantum well. Fabricated DFB uncoated lasers have shown wavelength emission at 852.12nm with an output optical power of 40mW, a SMSR >30dB at the D₂ line, at 37°C. Low self-heterodyne linewidths of 0.8MHz and 1.2MHz were measured respectively at 20mW 12°C and 40mW 37°C. With this uncoated diode, we have realized saturation spectra of cesium atoms to determine the resolution and the stability of the laser diode working on Cs. The saturation spectrum of the D₂ line of ¹³³Cs was recorded with a resolution close to the natural line width. Preliminary studies of reliability were the measurement of catastrophical optical mirror damage (COMD) for different anti-reflection (AR) coatings. We obtained a COMD density of 19MW/cm².

For broad ridge (Al,In)GaN laser diodes, which are inevitable for high output power applications in the UV and blue spectral range, filaments or higher order lateral modes build p, which influence the far-field beam quality. We investigate the lateral profile of the optical laser mode in the waveguide experimentally by temporal and spectral resolved scanning near-field optical microscopy measurements on electrically pulsed driven laser diodes and compare these results with one-dimensional simulations of the lateral laser mode in the waveguide. We present a model that describes the optical mode profile as a superposition of different lateral modes in a refractive index profile which is modified by carrier- and thermal-induced effects. In this way the mode dynamics on a nanosecond to microsecond time scale can be explained by thermal effects.

We have used the stochastic Monte-Carlo method to determine the carrier transport studies in the bulk GaInNAs material. We have incorporated phonon and impurity scattering processes and explicitly considered the role of the nitrogen impurities as scattering centers. We show that in the expression of the relaxation times it is the perturbed rather than the free electron density of states that should be incorporated. This is derived from the Green's functions and the many impurity Anderson model and yields an enhanced scattering rate. The nitrogen impurities can also act as centers with an infinite scattering cross-section when their broadening becomes infinitely small. We show that the increase of the electron effective mass in GaInNAs system is more important than the non-parabolicity parameter in the decrease of mobility. Monte-Carlo calculations take into account the total scattering rate, which is significantly enhanced due to nitrogen scattering. The average electric field and the average energy are found to decrease with increasing N concentration and increase of the effective mass.

We have developed two Al-free active region laser structures, which have a high maximum wall-plug efficiency (WPE) of 69% on an uncoated 2 mm x 100 μm single emitter broad area (BA) laser. Both structures include a Large Optical Cavity (LOC) with an optimized doping profile. One structure contains improved interfaces between material layers, and the other one an optimized strain compensated quantum well.

We propose in this communication an experimental study of the relaxation oscillations behavior in mode-locked lasers. The semiconductor self-pulsating laser diode is composed by two gain sections, without saturable absorber. It is made of bulk structure and designed for optical telecommunication applications. This specific device allows two different regimes of optical modulation: the first one corresponds to the resonance of the relaxation oscillations and the second one, to the mode-locking regime at FSR value. This singular behavior leads us to characterize the self-pulsations which are coexisting in the laser and to describe two regimes of output modulation: the first one appears thanks to the resonance of the oscillation relaxation and the other one corresponds to the FSR of the Fabry-Perot laser at 40 GHz.

Nonlinear dynamics of semiconductor lasers have recently attracted much attention in the area of microwave photonics. By invoking the nonlinear dynamics of an optically injected laser diode, high-speed microwave oscillation can be generated using the period-one oscillation state. The oscillation is harnessed for application as a photonic microwave source in radio-over-fiber (RoF) systems. It is advantageous over conventional direct current modulation because it alleviates the modulation bandwidth limitation and naturally generates single sideband signals. The method is thus applicable to wireless communication systems even when the subcarrier frequency increases to 60 GHz. Because RoF is usually incorporated with standard wireless schemes that involve frequency division multiplexing (FDM), we investigate the performance of the optical injection system under simultaneous current injection of multiple data streams. Frequency mixings and competition for locking among subcarriers result in intermodulation distortion (IMD). The relative weightings of different channels should be optimized to ensure acceptable signal qualities. The results illustrate the feasibility of applying the optical injection system for FDM RoF transmission at high subcarrier frequencies.

It is shown theoretically that broadening the optical confinement layer in monolithic mode-locked semiconductor lasers may suppress Q-switching instability, by increasing the carrier transport time, and lead to emission of shorter, more stable optical pulses.

In the plane wave approximation, we study spatio-temporal dynamics of a semiconductor class B laser driven by a coherent injected field in a Fabry-Perot configuration. Below the lasing threshold, we manage to reduce the dynamics to a single evolution equation for the carrier density, to analytically compute the stationary field configurations and to predict their stability. The numerical simulations, performed by implementing an efficient and accurate split-step code, perfectly agree with the analytical results.

We present an alternative method of determining the oscillating state of a laser and demonstrate the suitability of relative intensity noise (RIN) measurements for this purpose. The experiments were carried out using a two-section DFB laser. Optical and RIN spectra have been recorded and correlated subsequently. The variation of the maxima of the RIN spectra have been evaluated with respect to intensity and position in the frequency domain. Varying the frequency, a distinct transition in the above mentioned parameters can be observed, wich can be correlated clearly to the mode degeneracy at the transition and a dominating oscillating mode below and above. This delivers a conclusive means of determining the lasing state from RIN spectra.

The thermal oxidation of an Al-rich AlGaAs buried layer is a common established technique used to improve the performances of some optoelectronic devices, like VCSEL or optical waveguides, in terms of electro-optical confinement. This oxidation technique is usually proceeding laterally, which allows achieving good results but leads to some difficulties on the control of the shape and size of the oxidized areas. In this work, a new technology to oxidize GaAs/AlAs epitaxial structures which avoids these limitations is presented. This method consists of an oxidation through the top of the sample, allowing in consequence a total control of the shape of oxidation by means of photolithography. For this purpose the method has two steps: first, the intentional creation of defects in the top GaAs layer, in order to make it possible the oxidant species diffusion through this material, and second the planar oxidation of the AlAs layer. In this paper this technique is thoroughly studied: different methods to create defects in the GaAs layer have been analysed, and the optimization of the procedure has been achieved leading to a uniform oxidation and a reduced lateral oxidation spreading. Finally a comparison between the experiments and simulations has been realized in order to provide an explanation for this type of vertical oxidation. This innovating technique allows addressing separately the electrical and optical operating aspects of optoelectronic devices, thus opens to novel structures with controlled transverse optical behaviour.

Monolithic Semiconductor Ring Lasers (SRLs) are promising devices for all-optical memory and all-optical switching applications, as they can operate in a directional bistable regime where only one directional mode (clockwise or anti-clockwise) is active at one time. The unidirectional bistable regime can be naturally associated to a binary logic, and the SRL represents an elementary digital memory cell that can be written all-optically, realising the function of an all-optical flip-flop. In fact, the direction of operation can be switched by injecting an external optical signal pulse into the SRL through one of the 4 input/output ports. Directional switching of the SRL-based all-optical flip-flop has been demonstrated by injecting optical pulses with 5 ps duration into one of the four input/output ports. The required switching energy is around 100 fJ, and the swiching time is between 100 and 200 ps. The same function has been demonstrated by injecting 400 ps pulses as optical trigger.

The optical spectrum of monolithic Semiconductor Ring Lasers (SRLs) is measured simultaneously for both lasing directions with a grating-based OSA, in the regimes of bidirectional and unidirectional operation. In the unidirectional operation regime the SMSR is larger than 25 dB, and the directional extinction ratio (i.e., the ratio of the power emitted in the two opposite directions) is larger than 20 dB. The influence of the current injected in the active output waveguides that act as SOAs is outlined. In the unidirectional regime the linewidth of the SRL is measured by an heterodyne technique, revealing linewidth values around 2 MHz.