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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.

All-optical pulse reamplification, reshaping (2R), and retiming (3R) using a monolithic bistable semiconductor ring laser (SRL) is demonstrated for the first time. The regeneration performance of the SRL is characterized with an all-optical settable switching threshold, achieving significant increases in the extinction ratio (ER) of the output pulse for input ER as low as 1 dB. For retiming, a rectangular retiming window generated by a clean clock signal is used to eliminate the timing jitter in the input pulse. For input pulse with peak-to-peak timing jitter as high as ~ 12% of the bit period, the timing jitter in the retimed output pulse is reduced to < 2% of the bit period. The pulsewidth of the final regenerated data can be controlled by changing the width of the retiming window. The SRL is, therefore, shown to have a "hard" digital performance in both amplitude and time domain suitable for all-optical 3R.

A set of distributed Bragg Reflector mode-locked semiconductor lasers are presented that generate picosecond pulses over a spectral range of 40 nm on a single chip. The same technology is used to realize an integrated laser with two separate Bragg gratings coupled into a common gain and saturable absorption section. This laser is shown to lock together the two spectral slices from each grating in a single pulse train.

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.

Optical injection locking and cavity enhanced four-wave mixing in semiconductor ring lasers have been used to generate data modulated millimeter (mm)-wave optical signals. The scheme is shown to have multigigahertz (multi-GHz) modulation bandwidth. The 4-Gb/s data is transferred directly from an intensity modulated optical signal onto an mm-wave optical signal with the mm-wave frequency tunable in steps of 62.5 GHz and with flexible radio-frequency modulation formats over the optical carrier. Bit-error-rate and eye-diagram measurements confirm excellent signal quality.

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%.

The all-optical response of a semiconductor ring laser (SRL) to two optical injections is characterized. Once the lasing direction is locked by one optical injection, the SRL direction of operation can be switched by another optical injection into the counterpropagating direction. The switching process manifests a typical bistable hysteresis loop, with its width and switching thresholds variable by the first injection power. Extremely sharp transition has been measured which confirms the potential of the SRL for all-optical regeneration applications.

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.

We report the observation of room-temperature optical gain at 1.3 mu m in electrically driven dilute nitride vertical cavity semiconductor optical amplifiers. The gain is calculated with respect to injected power for samples with and without a confinement aperture. At lower injected powers, a gain of almost 10 dB is observed in both samples. At injection powers over 5 nW, the gain is observed to decrease. For nearly all investigated power levels, the sample with confinement aperture gives slightly higher gain.

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 novel approach for resolution of contentions between asynchronous optical packets, exploiting a self-synchronizing mechanism by means of dynamic all-optical tunable delay line based on stimulated Brillouin scattering in fibers, is reported. Time delay between contending optical packets is mapped into the output timing of a contention-detecting pulse produced by the tunable delay line. The output pulse from the delay line triggers an integrated all-optical flip-flop that generates a synchronized gating signal driving a semiconductor-based all-optical switch, for low-priority packets deflection. Correct operation of the proposed scheme without bit loss or packet fragmentation is demonstrated, and error-free operation for the switched packets is reported under different cases of relative delay between incoming optical packets. The scheme operates entirely in the photonic domain and exploits integrated optical switches and a passive optical-delay line for minimized node latency. Extension to symmetric operation allowing priority swapping between the input packets is also proposed.

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

A frequency-domain model for semiconductor ring lasers (SRLs) is presented. Every aspect of the lasing characteristics of SRLs can be reproduced. Several SRLs are fabricated and tested. Good agreement between theory and experiment is demonstrated.

We measured the absorption recovery times in reverse biased AlInGaAs multiple quantum well material designed to emit at around 1.5 μm wavelength. Absorption recovery times as low as 2.5 ps were found at -4V bias, with values below 5 ps consistently found for biases above 3 V. The short absorption recovery times obtained under reverse bias were confirmed by using cross-absorption modulation in the material to demonstrate wavelength conversion of a 10 GHz pulse train, showing both up and down conversion of the incident pulses.

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.

Semiconductor ring lasers (SRL) have received growing interest because of their potential as compact and coherent light source. Although intensive effort has been invested into the bistability of SRL under continuous wave operation, very little research has been conducted on the direct modulation of SRL[1]. We report the direct modulation of SRL at bit rates up to 1.5Gb/s. The experimental results show that the DC bias current is the key factor for direct modulation.

We report on the use of active waveguides to compensate for losses in reconfigurable delay lines based on ring resonators. Pulse delay in both transparency and amplification regimes is demonstrated.

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.

All-optical digital logic AND and XOR gates based-on cavity-enhanced four-wave-mixing in an integrated semiconductor ring laser are demonstrated at 2.5Gb/s. Error free operation with ER >10dB is achieved.

An optical static random access memory cell is experimentally demonstrated using a semiconductor ring laser set-reset flip-flop integrated with four SOAs at each output. Read and write operations at 1Gb/s are achieved.

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

RF signals for Radio over fibre systems (RoF) can be generated optically by mode-locking (ML) in semiconductor lasers [1]. External optical injection at non-lasing cavity modes of semiconductor ring laser (SRL) generates strong cavity enhanced four-wave mixing(FWM)[2], that result in generation of optical waves modulated by high purity RF frequencies as a result of the mode-locking by way of such FWM. This makes the SRL very attractive for the generation of RF or millimetre wave modulated optical signals that may be used in RoF systems. We report, for the first time, the generation and wideband modulation of RF optical signals using a combination of optical injection locking (OIL) and FWM in the SRL. We demonstrate that ordinary intensity modulated optical data up to 4Gb/s can be converted onto RoF carrier, with the RF frequency widely tunable up to hundreds of GHz, with excellent data quality. We further demonstrate that this scheme is versatile in generating various modulation formats by simple filtering.

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.

We review and compare recent results on the design and fabrication of semiconductor microring and microdisk lasers for applications in all-optical switching and signal processing. The design of the optical cavities will be analyzed in detail with a strong emphasis on the evaluation of the various loss mechanisms. Both lithographic and etching technologies were thoroughly optimized to ensure high-resolution patterning and deep and vertical waveguide sidewalls. The majority of the fabricated devices exhibited continuous wave operation down to 7 μm and strong unidirectional bistability down to device footprints as small as 30 μm. While both microdisks and microrings show similar behaviour for medium radii (30-60 μm), in smaller devices the microdisk geometry shows much lower losses and better performance.

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.

Reconfigurable all-optical multicast at multi-gigabit data rates has been realised using simultaneous multi-wavelength conversion based on cavity-enhanced four-wave-mixing in a monolithically integrated semiconductor ring laser. The scheme provides high efficiency, low power penalty, and cost-effectiveness.

In this paper, we propose and experimentally demonstrate an all-optical non-return-to-zero (NRZ) to return-to-zero (RZ) format conversion using semiconductor ring laser (SRL). A 155Mb/s NRZ optical data signal is injected into the counterclockwise (CCW) direction of the SRL and an optical logic clock signal is injected to the clockwise (CW) propagation direction. The power and wavelength on both sides are adjusted so that during the 'high' level of the clock, the SRL is held to the CW direction regardless of the logic value of the corresponding data bit. The SRL will only be switched to the CCW direction when the input data is 'high' and when the clock level is 'low'. Therefore the NRZ format signal is converted to RZ format signal, with a duty cycle decided by the duty cycle of the optical clock. Extinction ratio of > 10 dB has been achieved and the scheme can also be used for all-optical 3R. Although the speed of the signal is yet limited, higher speed can be achieved with faster SRL devices.

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.

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., This research was founded by he EU-FP6 ‘IOLOS’ project.

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.