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We investigate the operation of directionally bistable semiconductor-ring lasers as all-optical flip-flops. We demonstrate fast switching between the two lasing directions by injection of optical pulses acting as set and reset control signals with switching times as fast as 20 ps, delay times as short as 60 ps, and switching energy of 150 fJ.

Micro-ring lasers, with a minimum bend radius of 10 $\mu{\rm m}$ , are presented that exhibit directional bistability and output powers up to 1.5 mW. In order to achieve devices with low threshold currents, mW output powers and directional bistability, the confilicting design constraints on the device are highlighted. Optimized adiabatic waveguide bends, bi-level evanescent couplers, and deeply etched ridge waveguide designs are presented in order to meet the stringent device design requirements. The optimized micro-ring laser devices exhibit directional extinction ratios of 25 dB and side-mode suppression ratios ${>}{\rm 25}~{\rm dB}$ .

We report on the detailed characterization of ultrashort pulses emitted from a 1.5- AlGaInAs/InP semiconductor passively mode-locked laser, operating at a repetition frequency of 35 GHz. Both the temporal and phase profiles of the pulses are retrieved using a sonogram technique that utilizes a highly-sensitive two-photon absorption waveguide detector. The system enables full characterization of pulses with energy as low as 10 fJ and peak power level of 5 mW, which is only inaccessible by a limited number of high-sensitivity measurement approaches. We show that the pulses exhibit a prevailing positive linear chirp across a wide range of biasing conditions. Its high sensitivity to the gain section current proves the dominant contribution of the gain conditions to the group delay characteristics of the emitted pulses.

In this paper, we present an experimental and theoretical study of passive mode-locking in semiconductor Fabry-Pérot, quantum-well, lasers operating at around 1550 nm and producing picosecond pulses at a repetition frequency of 40 GHz. The different regimes that occur as the reverse bias voltage applied to the saturable absorber (SA) section or the bias current injected into the amplifier section are characterized both in the time and frequency/wavelength domains. Our results reveal that the lasers display spectral competition between the gain of the amplifier section and the absorption of the SA, with variations of the lasing wavelength up to 25 nm as the bias conditions are changed. These wavelength variations result from the thermal drift of the SA band-edge due to Joule heating by the generated photocurrent and from the competition between two possible lasing regions placed either at the amplifier gain peak or near the band-edge of the SA. The experimental observations are satisfactorily reproduced and explained in the framework of a Traveling Wave Model complemented by a time-domain description of the semiconductor susceptibility. © 2011 IEEE., This work was supported by the U.K. Engineering and Physical Sciences Research Council Financial under project EP/E065112/1–High Power, High Frequency Mode-locked Semiconductor Lasers, Spain's Ramón y Cajal Program, and the project TEC2009-14581-C02-01

We report theoretically and experimentally on excitability in semiconductor ring lasers in order to reveal a mechanism of excitability, general for systems close to Z 2 -symmetry. The global shapes of the invariant manifolds of a saddle in the vicinity of a homoclinic loop determine the origin of excitability and the features of the excitable pulses. We show how to experimentally make a semiconductor ring laser excitable by breaking the Z 2 -symmetry in a controlled way. The experiments confirm the theoretical predictions.

We report on the design and experimental evaluation of AlGaInAs/InP multiquantum well epistructures for mode-locked emission at 1.5 μm. We show that mode-locked lasers fabricated on an optimized three quantum well active region with a low optical confinement factor deliver pulses with increased peak power and stability over a much wider biasing range than those fabricated using a standard five quantum well design. Sonogram measurements indicate that sub-ps symmetrical pulses with a substantially reduced linear blue chirp are generated up to nearly three times the laser current threshold.

We demonstrated room-temperature continuous-wave (RTCW) operation of electrically driven in-plane emitting micro-disc lasers with radius of 10 μm. The disk cavity was fabricated using high resolution electron beam lithography based on hydrogen silsesquioxane (HSQ) and BCl3-Cl2-Ar inductively coupled plasma etching. By using circular bump or notch defects on the circumference as output coupling as well as mode control mechanisms, single-mode operation for large bump defect lasers with sidemode suppression ratio larger than 30 dB was observed, with highly directional output emission.

This chapter is concerned with the design, fabrication, and characterization of deeply-etched III–V semiconductor ring lasers (SRL) whose radius is scaled down to sub-10 µm values. The laser threshold performances are experimentally investigated for different laser geometries. Compact racetrack-shape SRLs are shown to exhibit robust unidirectional bistable operation with direct extinction ratio and single mode rejection ratio of 20 and 30 dB, respectively.

We report on room-temperature continuous-wave operation and single-mode lasing of microdisk and microring lasers with radii as small as 7 mum. The waveguide sidewall roughness was minimized by an optimized fabrication process using hydrogen silsesquioxane e-beam resist and Cl 2-CH 3-H 2 inductively coupled plasma etching. The devices show unidirectional bistability between the counterpropagating modes for radii larger than 30 mu m and a strong hybrid output polarization for radii smaller than 15 mum with a transverse-magnetic component of approximately 30%.

We have successfully designed and fabricated 1.55-mu m semiconductor ring lasers with novel parabolic mirror retro-reflector cavities. The parabolic mirror is designed by means of ray-tracing and finite-difference time-domain to optimize its shape and position. The fabricated devices, with a range of sizes down to an equivalent circular ring radius of 16 mu m, operate in continuous-wave mode at room temperature. Threshold current of 22 mA and output power up to 140 mu W have been achieved for the smallest device. These devices are suitable for high-speed all-optical signal processing and digital photonic functions.

A monolithic semiconductor ring laser is operated as an all-optical set-reset flip-flop triggered by external optical pulses. Bit-error-rate measurements of set-reset switchings under the injection of a pseudo-random-bit-sequence have been performed, showing reliable operation up to 10 Gb/s.

A compact semiconductor mode-locked laser (MLL) is presented that demonstrates strong passive Q-switched mode-locking over a wide range of drive conditions. The Q-switched frequency is tunable between 1 and 4 GHz for mode-locked pulses widths around 3.5 ps. The maximum ratio of peak to average power of the pulse-train is >120, greatly exceeding that of similarly sized passively MLLs.

We experimentally and theoretically study the characteristics of semiconductor ring lasers bidirectionally coupled by a single bus waveguide. This configuration has, e.g., been suggested for use as an optical memory and as an optical neural network motif. The main results are that the coupling can destabilize the state in which both rings lase in the same direction, and it brings to life a state with equal powers at both outputs. These are both undesirable for optical memory operation. Although the coupling between the rings is bidirectional, the destabilization occurs due to behavior similar to an optically injected laser system., Comment: 5 pages, 7 figures

In recent years there has been a high level of interest in the properties of fast semiconductor saturable absorbers operating at λ ∼ 1.5»m. The applications for these include the fabrication of passively modelocked lasers[1] and wavelength conversion of optical data streams through cross-absorption modulation. To enable these effects to be utilised at the highest repetition rates, very fast absorption recovery times are required, in materials that can be conveniently integrated with optical gain elements. Several methods for achieving very short absorption recovery times have been demonstrated such as the inclusion of high densities of crystal defects in the material and the use of intersubband transitions. However, it is not possible to integrate these with gain elements operating at the same wavelength without the use of expensive and technologically demanding epitaxial regrowth processes.

We demonstrate electrically-pumped room-temperature continuous-wave lasing in InP-based micro-disc lasers with threshold current of 6.5mA. A non-concentric hole inside the disc is used to extract the light and control the lasing mode.

Fast saturable absorber recovery times have been obtained in AlGaInAs/InP multiple quantum wells using the cross absorption modulation method for wavelength conversion. We report sub-picosecond pulse generation using a 40 GHz passively mode locked laser.

We demonstrate electrically-pumped room-temperature continuous-wave lasing in InP-based microdisks lasers with threshold current of 5mA. A defect on the disc circumference is used to extract the light and to achieve high quality single mode operation.

Semiconductor Ring Lasers (SRLs) are a novel class of semiconductor lasers whose active cavity is characterized by a circular geometry. SRLs have attracted attention due to the possibility of monolithical integration of thousands of them on the same chip in a cheap and reliable way. SRLs are interesting for applications that rely on the presence of two counter-propagating modes inside the optical cavity. For instance, fully symmetric coupled SRLs have been proposed as candidates for the realisation of small and fast all-optical memories. At the same time, a wealth of nonlinear and stochastic dynamics have been predicted and observed in symmetric SRLs which is a consequence of the underlying Z2-symmetry of the device. However, unavoidable fabrication defects, material roughness and chip-cleaving break the device symmetry in an uncontrolled and unpredictable way, which may result in a deterioration of the device's performance in applications such as all-optical signal-processing. Despite their importance, the effects of symmetry breaking in SRLs remain unaddressed. In this contribution we investigate theoretically and experimentally the stochastic dynamics of SRLs with weakly broken Z2-symmetry . We show how the symmetry of an SRL can be experimentally manipulated using the reflection from a cleaved facet of a multi-mode optical fibre and a control electrode on the bus waveguide. The experiments are performed on an InP-based multi-quantum well SRL operating in single-longitudinal mode regime. The power at the CCW output is collected using a fast photodiode connected to an oscilloscope with a sampling rate of 4.0 ns. For a not-too-weak symmetry breaking, we reveal that SRLs become excitable and therefore can emit large, deterministic power bursts as a response to stochastic fluctuations. The origin of excitability is explained by investigating the topology of the invariant manifolds of an asymptotic two-dimensional phase space model with broken Z2-invariance. The results of the experiments confirm the prediction of the theory.

A monolithic semiconductor ring laser (SRL) operates as all-optical Flip-Flop. We report on ps-scale temporal response and Bit-Error-Rate experiments.

All-optical digital signal processing can provide future photonic systems with new capabilities to be exploited in fiber optic communication systems, all-optical networks, and optical computation applications. A practical device enabling signal processing must be monolithic, and its operation must be proved under severe test conditions in terms of the optical driving signal and the functionality offered.

Semiconductor ring lasers (SRLs) are a particular class of lasers whose cavity has a circular geometry. From a theoretical point of view, SRL attracted interest due to their intrinsic Z2 symmetry; on the other hand, the bistability between two counter-rotating modes makes SRLs ideal candidates to efficiently store bits of information in an all-optical way [1,2].

It is previously demonstrated that modulation bandwidth of semiconductor lasers can be enhanced by modulating the master laser in an optical injection locking (OIL) system [1]. Semiconductor ring laser (SRL) is a promising device for integrated OIL configuration, as the SRL operates only in the direction receiving injection, therefore does not feedback into the master laser [2]. In this paper we report bandwidth enhancement of SRL to ≫20GHz in a master-slave OIL configuration with the SRL as the slave laser.

This paper demonstrates all-optical digital logic gates using bistable monolithic semiconductor ring lasers (SRLs). With the flexible multi-functional experimental setup, we have experimentally demonstrated the application of SRLs to realize logic functions including: AND, OR, NOR, XOR and all-optical non-retum-to-zero (NRZ) to retum-to-zero (RZ) format conversion. Using bistable semiconductor ring lasers to fulfil all-optical logic functions have the advantages of high output extinction ratio, low switching energy, integration and the potential logic unit in the future.

A key advantage that semiconductor ring lasers have over Fabry-Perot (FP) devices for relatively high power applications, is that they do not suffer from Catastrophic Optical Damage (COD) to the cleaved facets of the device bar[1]. However, the directional coupler (DC) section in the ring geometry, equivalent to the facet conditions in a FP laser, cannot be modulated as easily as the FP facets to provide the equivalent to high and low reflectivity coatings, that allow increased quantum efficiency. The symmetric nature of the coupling in ring lasers produces both bidirectional or (fluctuating) bistable directional lasing of the device[2]. In order to produce unidirectional, stable lasing in ring lasers a snail geometry, as shown in Fig.2(a), is considered here. In this case the laser has one cleaved output facet, and the input waveguide is curved until it couples with itself. The benefit is that low coupling between the waveguides in the DC may be easily designed, where the normal ring and bus waveguide geometry equivalent, requiring high coupling values, is more sensitive to fabrication tolerances, and requires long interaction lengths, increasing the device footprint. As a theoretical result, we have calculated the fundamental mode thresholds and output power for Semiconductor Snail Laser (SSL) as a function of device parameters, i.e. coupler efficiency and output facets reflectivities. The coupler efficiency (δ) is the fraction of light transmitted between the waveguides in the DC. We have used a scattering matrix formulation and an analytical approximation to the optical susceptibility for the medium description [3], in order to determine the “optimal” coupling efficiency that maximizes the output power (see Fig.1.) Lasing action and good emission properties were therefore theoretically predicted.

The frequency response and alpha factor of monolithic semiconductor ring lasers (SRLs) with 150 micron radius are measured using an all-optical modulation technique, yielding a -3 dB, frequency of 11 GHz and alpha factor of 2.4.

A monolithic semiconductor ring laser (SRL) is demonstrated to operate as an all-optical flip-flop that is triggered by optical pulses of 400 ps and 5 ps duration, showing a response time of 130 ps.

We fabricated micro-sized semiconductor ring and disk lasers. Devices operate continuous wave down to a radius of 7 mum, with robust directional switching exhibited to a radius of 30 mum. Additionally, polarisation rotation is reported.

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.

Room temperature CW operation is achieved in novel semiconductor ring lasers with parabolic mirror retro-reflector cavities downsized to equivalent ring radius of 16 μm. These devices are suitable for high speed digital photonic functions.

A monolithic semiconductor ring laser (SRL) is demonstrated as a fully all-optical flip-flop that can be triggered by optical pulses of 400 ps duration, showing 130 ps response time.

We report oil 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 oil this design with an equivalent circular ring radius of 16 mu m. These devices operate in continuous wave at room temperature with a threshold current of only 22mA and all emission wavelength of 1.55 mu m.

Novel all-optical implementations for logic AND and XOR gates are experimentally demonstrated at 2.5 Gbit/s based on cavity-enhanced four-wave-mixing (CE-FWM) combined with optical injection locking phenomena in monolithically integrated semiconductor ring lasers (SRLs). An all-optical AND gate with simultaneous wavelength conversion has been achieved using CE-FWM. An XOR function is demonstrated using two parallel SRLs, each performing one part of the XOR Boolean equation A ⊕ B=AB+AB. Error-free operation with extinction ratio higher than 10 dB confirms the suitability of SRLs for all-optical logic applications.

Novel all-optical implementations for logic AND and XOR gates are experimentally demonstrated at 2.5Gb/s based on cavity-enhanced four-wave-mixing (CE-FWM) combined with optical injection locking phenomena in monolithically integrated semiconductor ring lasers (SRLs). All-optical AND gate with simultaneous wavelength conversion have been achieved using the CE-FWM. XOR function is demonstrated using two parallel SRLs, each performing one part of XOR Boolean equation. Error-free operation with extinction ratio higher than 10dB confirms the suitability of SRLs for all-optical logic applications.

We experimentally and theoretically study coupled semiconductor ring lasers. We report on the destabilization of unidirectional operation for large coupling strengths and the appearance of a coupling-induced bidirectional state, both undesirable for optical memory operation.

We report on the integration of semiconductor ring lasers with tunable directional couplers to modulate the Q-factor of the ring cavity. Active Q-switching is demonstrated with 120 ps pulses, up to frequencies of 1.8 GHz.

A monolithic semiconductor ring laser is operated as an all-optical Flip-Flop triggered by 4ps optical pulses. Bit-Error-Rate measurements of Set-Reset switchings under the injection of a Pseudo-Random-Bit-Sequence at 5 and 10 Gb/s have been performed.

We present both an experimental and theoretical investigation of multistable states in a single-longitudinal mode and single transverse mode semiconductor ring laser (SRL). Our experiments have been performed on an InP-based multiquantum-well SRL with a racetrack geometry and a free-spectral-range of 53.6 Ghz. The power emitted from the chip is collected with a multimode fiber and detected with a 2.4 GHz photodiode connected to an oscilloscope. We show how the operation of the device can be steered to either monostable, bistable or multistable dynamical regimes in a controlled way. The diverse multistable dynamical regimes are shown to be organized in well reproducible sequences [Gelens et al., Phys. Rev. Lett. 102, 193904 (2009)]. These sequences are demonstrated to match the bifurcation diagrams of an asymptotic two-dimensional Z 2 -symmetric model for SRLs. Apart from predicting the different measured multistable time series, we demonstrate how the stochastic transitions between multistable states take place by analyzing the phase space in this model.

A monolithic semiconductor ring laser is operated as an all-optical Flip-Flop triggered by 4ps optical pulses. Bit-Error-Rate measurements of Set-Reset switchings under the injection of a Pseudo-Random-Bit-Sequence at 5 and 10 Gb/s have been performed.

Semiconductor Ring Lasers (SRLs) are interesting devices from the technological point of view because they do not require cleaved facets or gratings in order to achieve the necessary optical feedback for lasing [1]. In SRL, light extraction is accomplished via evanescent coupling from the waveguide into several output waveguides.

A set of passively mode-locked Distributed Bragg Reflector lasers on a single chip, are shown to exhibit stable mode-locking across a 40nm bandwidth. Integration of two gratings in a single device shows dual band locking.

Semiconductor Ring Lasers (SRLs) usually exhibit regimes of unidirectional operation, where bistability between clockwise (CW) and counter-clockwise (CCW) emission occurs [1]. In this regime, the direction of emission of the SRL can be set by injection of optical pulses through the waveguides that define the light-extraction sections, which offers a great potential for applications such as all-optical memories, switching and gating [1]. One of the limiting factors for such applications is the switching time needed for the device to reverse its emission direction, as it sets the maximum speed at which the device can be operated.