Intracavity Saturable Absorber Research Articles

Photonic integrated spiking neuron chip based on a self-pulsating DFB laser with a saturable absorber

We proposed and experimentally demonstrated a simple and novel photonic spiking neuron based on a distributed feedback (DFB) laser chip with an intracavity saturable absorber (SA). The DFB laser with an intracavity SA (DFB-SA) contains a gain region and an SA region. The gain region is designed and fabricated by the asymmetric equivalent π-phase shift based on the reconstruction-equivalent-chirp technique. Under properly injected current in the gain region and reversely biased voltage in the SA region, periodic self-pulsation was experimentally observed due to the Q-switching effect. The self-pulsation frequency increases with the increase of the bias current and is within the range of several gigahertz. When the bias current is below the self-pulsation threshold, neuronlike spiking responses appear when external optical stimulus pulses are injected. Experimental results show that the spike threshold, temporal integration, and refractory period can all be observed in the fabricated DFB-SA chip. To numerically verify the experimental findings, a time-dependent coupled-wave equation model was developed, which described the physics processes inside the gain and SA regions. The numerical results agree well with the experimental measurements. We further experimentally demonstrated that the weighted sum output can readily be encoded into the self-pulsation frequency of the DFB-SA neuron. We also benchmarked the handwritten digit classification task with a simple single-layer fully connected neural network. By using the experimentally measured dependence of the self-pulsation frequency on the bias current in the gain region as an activation function, we can achieve a recognition accuracy of 92.2%, which bridges the gap between the continuous valued artificial neural networks and spike-based neuromorphic networks. To the best of our knowledge, this is the first experimental demonstration of a photonic integrated spiking neuron based on a DFB-SA, which shows great potential to realizing large-scale multiwavelength photonic spiking neural network chips.

Influence of transverse carrier diffusion on two-dimensional optical rogue waves in broad-area semiconductor lasers with a saturable absorber

The statistics and dynamics of two-dimensional rogue waves in a broad-area semiconductor laser with an intracavity saturable absorber are numerically investigated under the effect of transverse carrier diffusion. We show that lateral diffusion of carriers alters the statistics of rogue waves by enhancing their formation in smaller ratios of carrier lifetimes in active and passive materials while suppressing them when the ratio is larger. The temporal dynamics of the emitted rogue waves is also studied and show that the finite nonzero transverse carrier diffusion coefficient gives them a longer duration. To further approach a realistic experimental situation, we also investigate the statistics and dynamics of rogue waves by simulating a circular disk-shape pump which replaces the flat pump profile typically used in numerical simulations of broad-area lasers. We show that a finite pump shape reduces the number of emitted rogue waves per unit area both below and above the laser threshold for every carrier lifetime ratio. The temporal width of the emitted rogue waves is also shown to decrease as a consequence of removing the nonphysical effects of the infinite flat pump on carrier dynamics.

Spatiotemporal Mode-Locking in Lasers with Large Modal Dispersion.

Dissipative nonlinear wave dynamics have been investigated extensively in mode-locked lasers with single transverse mode, whereas there are few studies related to three-dimensional nonlinear dynamics within lasers. Recently, spatiotemporal mode locking (STML) was proposed in lasers with small modal (i.e., transverse-mode) dispersion, which has been considered to be critical for achieving STML in those cavities because the small dispersion can be easily balanced. Here, we demonstrate that STML can also be achieved in multimode lasers with much larger modal dispersion, where we find that the intracavity saturable absorber plays an important role for counteracting the large modal dispersion. Furthermore, we observe a new STML phenomenon of passive nonlinear autoselection of single-mode mode locking, resulting from the interaction between spatiotemporal saturable absorption and spatial gain competition. Our work significantly broadens the design possibilities for useful STML lasers thus making them much more accessible for applications, and extends the explorable parameter space of the novel dissipative spatiotemporal nonlinear dynamics that can be achieved in these lasers.

Control of spatiotemporal rogue waves by harmonic pump modulation in a semiconductor laser with a saturable absorber

Through numerical simulations, statistical and dynamical properties of extreme events in a broad-area semiconductor laser with intracavity saturable absorber are investigated. By inclusion of a diffusion coefficient for the field, formation of rogue waves in a state of extended turbulence is studied and shown to be affected by harmonic perturbations. In particular, we propose a control technique based on periodic modulation of the pump parameter which can either drive the state of the system closer to or away from the chaotic attractors respectively enhancing or suppressing the generation of rogue waves. By statistical and dynamical analysis of the events in terms of intensity and optical gain, we show that when the system is under resonant modulation with frequency close to that of the dominant oscillations in the turbulent state (which is equal to the relaxation oscillation frequency typical of semiconductor lasers), more rogue waves are triggered with larger intensities and shorter lifetimes. On the other hand, off-resonant modulations restrain the formation of rogue waves where they appear in lower intensities and longer lifetimes. An example of special cases where the proposed scheme can completely forbid or allow the emission of rogue waves is also presented.

Pulse dynamics in SESAM-free electrically pumped VECSEL.

Self-starting pulsed operation in an electrically pumped (EP) vertical-external-cavity surface-emitting-laser (VECSEL) without intracavity saturable absorber is demonstrated. A linear hemispherical cavity design, consisting of the EP-VECSEL chip and a 10% output-coupler, is used to obtain picosecond output pulses with energies of 2.8 pJ and pulse widths of 130 ps at a repetition rate of 1.97 GHz. A complete experimental analysis of the generated output pulse train and of the transition from continuous-wave to pulsed operation is presented. Numerical simulations based on a delay-differential-equation (DDE) model of mode-locked semiconductor lasers are used to reproduce the pulse dynamics and identify different laser operation regimes. From this, the measured single pulse operation is attributed to FM-type mode-locking. The pulse formation is explained by strong amplitude-phase coupling and spectral filtering inside the EP-VECSEL.

Improved Phase Locking of Laser Arrays with Nonlinear Coupling.

An arrangement based on a degenerate cavity laser for forming an array of nonlinearly coupled lasers with an intracavity saturable absorber is presented. More than 30 lasers were spatially phase locked and temporally Q switched. The arrangement with nonlinear coupling was found to be 25 times more sensitive to loss differences and converged five times faster to the lowest loss phase locked state than with linear coupling, thus providing a unique solution to problems that have several near-degenerate solutions.

Dynamics of single- and multicontact external cavity semiconductor lasers under intracavity frequency sweeping: effects of material and laser parameters

We present numerical analysis of the dynamic behavior of a single- and multicontact Fabry–Perot semiconductor laser frequency-swept by means of an ultrafast intracavity filter. The interrelated roles of the tuning speed, the filter bandwidth, the cavity length, and the self-phase modulation in laser dynamics are analyzed. Regular and irregular mode hopping, as well as fundamental and second harmonic sliding frequency mode locking and chaotic laser operation are identified as important dynamic regimes. The relation between the tuning dynamics asymmetry and the self-phase modulation in the laser is studied. The operation of a multicontact laser with an intracavity saturable absorber in a combined sliding-frequency and passive mode locking regime is shown; the dynamic asymmetry in the laser behavior is shown to be reduced but not eliminated by the presence of the absorber.Graphical abstract

Multisoliton hybrid generation of fiber lasers with anomalous dispersion

We have performed a numerical simulation study of a hybrid regime of anomalous-dispersion-fiber lasers in which passive mode locking occurs simultaneously with the regime of regular undamped spikes induced by an intracavity saturable absorber. Stabilization and reproducibility of the laser radiation is provided by a fiber optical delay line due to which each subsequent spike is formed from the seed radiation of the previous one. The transient evolution and steady-state generation are analyzed. It is found that the number of ultrashort pulses in the steady-state generation depends on the initial conditions of the transient process. In the investigated generation, the spatial-temporal concentration of radiation energy is due to both passive mode locking and $Q$-switch operation. The hybrid regime is of interest for obtaining reproducible high-energy light pulses.

Dynamic cavity effects in topological insulator Bi2Te3 based passive Q-switched solid state laser

In this work, a comprehensive investigation of dynamic evolution for pulsed solid state laser using topological insulator as saturable absorber (SA) is reported. Without material damage, dynamic transition from Q-switched (QS) state to continuous wave (CW) state in the laser cavity is observed when the pumping rate to the Nd3+:YAG laser crystal increases. With a sufficiently large pumping rate, the initiated laser spiking damps and relaxes to CW state similar to a laser system without intra-cavity SA. Formulated by well-known laser rate equations, analysis of the perturbations around fixed points reveals the instability spectrum that is able to explain the observed transition in experiments. Theoretically, the parameters space to support QS operation are given in terms of the ratio of saturation intensity of laser gain media over the saturation intensity of SA, pumping speed, the ratio of relaxation time of laser gain media over cavity lifetime and the modulation depth of the SA. The result of this work suggests a simple approach to understand passive Q-switched (PQS) laser with SAs of low saturation intensity and guidelines to the design of laser system employing low-saturation materials as SA, such as two-dimensional TI material.

Epsilon-near-zero medium for optical switches in a monolithic waveguide chip at 1.9 μm

Abstract A saturable absorber is a building block for integrated ultrafast photonics and passive optical circuits. However, options currently available suffer from the bottlenecks of the necessity for fine control of the material preparation, large optical losses, and compatibility. This paper presents a complementary metal–oxide–semiconductor (CMOS)-compatible alternative based on an epsilon-near-zero (ENZ) medium, in which the real part of the dielectric constant vanishes. Excellent nonlinear optical modulations, including low linear optical losses, low bleaching threshold, moderate optical amplitude modulation, and high modulation speed of indium tin oxide (ITO) in its ENZ region are achieved. The use of ITO as an intracavity saturable absorber for optical switches of integrated waveguide chip lasers at 1.9 μm has been realized. A stable mode-locked waveguide laser with a repetition rate of 6.4 GHz and an average output power of 28.6 mW is achieved via carefully adjusting the intracavity three-surface interferometer (TSI). This work may pave the way for integrated photonics and electro-optics using a CMOS-compatible ENZ medium.

Localized optical micromotors based on twin laser cavity solitons

It has recently been shown that vertical-cavity surface-emitting lasers with an intracavity saturable absorber are capable of forming binary localized structures called twin laser cavity solitons (LCSs). Apart from their asymmetric intensity distribution, they can spontaneously rotate about their center of mass with a frequency that is controllable via a bifurcation parameter representing the ratio of carrier lifetimes in the amplifier and in the absorber. Moreover, circles of maximum phase are found to form around the center of the binary structure which start rotation once the twin LCSs begin to do so. In this paper, we propose to use the fact that these phase circles continually shrink on the center of mass to trap microparticles at the center and subsequently rotate them through the dipole force provided by the asymmetric intensity distribution. This finding can be regarded as a first step in the realization of a localized microdimensional optical motor potentially useful in the manipulation of microparticles and nanofluids.

Multipulse instabilities of a femtosecond SESAM-modelocked VECSEL.

Optically pumped passively modelocked vertical external-cavity surface-emitting lasers (VECSELs) can generate pulses as short as 100 fs with an intracavity semiconductor saturable absorber mirror (SESAM). Very stable soliton modelocking can be obtained, however, the high-Q-cavity, the short gain lifetime, and the kinetic-hole burning can also support rather complex multipulse instabilities which we analyze in more details here. This onset of multipulse operation limits the maximum average output power with fundamental modelocking and occurs at the roll-over of the cavity round trip reflectivity. Unfortunately, such multipulse operation sometimes can mimic stable modelocking when only limited diagnostics are available.

RESEARCH AND DEVELOPMENT OF DIODE-PUMPED PASSIVELY Q-SWITCHED Nd:YVO4 LASERS USING Cr4+:YAG CRYSTAL AS SATURABLE ABSORBER

The results in research and development of diode-pumped passively Q-switched Nd:YVO4 lasers are presented with Cr4+:YAG crystal used as an intra-cavity saturable absorber. Using a-cut Nd:YVO4 laser crystals, the passively Q-switched lasers have been successfully developed and provided stable single shortest pulses of 40 ns at the repetition rate of 18.5 kHz, the maximal average output power of 180 mW was achieved corresponding to the optical efficiency of 9% and the slope efficiency of 16%. The dependences of average output power, repetition rate, pulse width and peak power on pump diode power for different transmissions of output mirror are also investigated.

Laser diode structures with a saturable absorber for high-energy picosecond optical pulse generation by combined gain-and Q-switching

The performance of gain-switched Fabry–Perot asymmetric-waveguide semiconductor lasers with a large equivalent spot size and an intracavity saturable absorber was investigated experimentally and theoretically. The laser with a short (∼20 μm) absorber emitted high-energy afterpulse-free optical pulses in a broad range of injection current pulse amplitudes; optical pulses with a peak power of about 35 W and a duration of about 80 ps at half maximum were achieved with a current pulse with an amplitude of just 8 A and a duration of 1.5 ns. Good quality pulsations were observed in a broad range of elevated temperatures. The introduction of a substantially longer absorber section leads to strong spectral broadening of the output without a significant improvement to pulse energy and peak power.

Ultra low-energy switch based on a cavity soliton laser with pump modulation

We study the effects of pump modulation in a cavity soliton laser consisting of a vertical cavity surface emitting laser with an intra-cavity saturable absorber. We show that a drifting soliton experiences enhanced mobility features by modulating the pump at the resonance frequency, and the effects are even larger below resonance. In particular, pump modulation reduces the rest time of the soliton in the initial stage of the motion and it increases its drift velocity in this regime. Moreover, pump modulation allows a decrease in the switching energy of the soliton to an amount equal to 36 photons. These results indicate that pump modulation is a promising way for the use of a cavity soliton laser as a fast optical buffer and an ultra low-energy optical switch.

Passive mode-locking of a fiber laser operating in the regime of undumped regular spiking

Computer simulation is used to investigate a new regime of oscillation of fiber lasers in which passive mode-locking takes place simultaneously with the regime of regular undumped spiking induced by an intracavity saturable absorber. Such a superposition regime takes place when part of the output radiation of the laser operating in the regime of spiking is propagated through a fiber-optic delay line and is coupled back into the laser cavity in a time interval equal to the time interval between adjacent spikes. The advantages of the proposed regime of oscillation relative to other means of achieving passive mode-locking in fiber lasers are discussed. The proposed regime is of interest for obtaining reproducible high-energy light pulses.

Dual-comb modelocked lasers: semiconductor saturable absorber mirror decouples noise stabilization.

In this paper we present the stabilization of the pulse repetition rate of dual-comb lasers using an intracavity semiconductor saturable absorber mirror (SESAM) for passive modelocking and an intracavity birefringent crystal for polarization-duplexing to obtain simultaneous emission of two modelocked beams from the same linear cavity sharing all components. Initially surprising was the observation that the cavity length adjustments to stabilize one polarization did not significantly affect the pulse repetition rate of the other. We gained insight in the underlying physics using both a semiconductor and Nd:YAG laser gain material with the conclusion that the pulse arrival timing jitter of the two beams is decoupled by the uncorrelated time delay from the saturated SESAM and becomes locked with sufficient but not too much pulse overlap. Noise stabilization is in all cases still possible for both combs. The dual-comb modelocked laser is particularly interesting for the semiconductor laser enabling the integration of gain and absorber layers within one wafer (referred to as the modelocked integrated external-cavity surface emitting laser--MIXSEL).

Spatiotemporal Chaos Induces Extreme Events in an Extended Microcavity Laser.

Extreme events such as rogue waves in optics and fluids are often associated with the merging dynamics of coherent structures. We present experimental and numerical results on the physics of extreme event appearance in a spatially extended semiconductor microcavity laser with an intracavity saturable absorber. This system can display deterministic irregular dynamics only, thanks to spatial coupling through diffraction of light. We have identified parameter regions where extreme events are encountered and established the origin of this dynamics in the emergence of deterministic spatiotemporal chaos, through the correspondence between the proportion of extreme events and the dimension of the strange attractor.

Excitable solitons in a semiconductor laser with a saturable absorber

Self-pulsing cavity solitons may exist in a semiconductor laser with an intracavity saturable absorber. They show locally the passive $Q$-switching behavior that is typical of lasers with saturable absorbers in the plane-wave approximation. Here we show that excitable cavity solitons are also possible in a suitable parameter range and characterize their excitable dynamics and properties.

All-optical Q-switching limiter for high-power gigahertz modelocked diode-pumped solid-state lasers.

Passively modelocked diode-pumped solid-state lasers (DPSSLs) with pulse repetition rates in the gigahertz regime suffer from an increased tendency for Q-switching instabilities. Low saturation fluence intracavity saturable absorbers - such as the semiconductor saturable absorber mirrors (SESAMs) - can solve this problem up to a certain average output power limited by the onset of SESAM damage. Here we present a passive stabilization mechanism, an all-optical Q-switching limiter, to reduce the impact of Q-switching instabilities and increase the potential output power of SESAM modelocked lasers in the gigahertz regime. With a proper cavity design a Kerr lens induced negative saturable absorber clamps the maximum fluence on the SESAM and therefore limits the onset of Q-switching instabilities. No critical cavity alignment is required because this Q-switching limiter acts well within the cavity stability regime. Using a proper cavity design, a high-power diode-pumped Yb:CALGO solid-state laser generated sub-100 fs pulses with an average output power of 4.1 W at a pulse repetition rate of 5 GHz. With a pulse duration of 96 fs we can achieve a peak power as high as 7.5 kW directly from the SESAM modelocked laser oscillator without any further external pulse amplification and/or pulse compression. We present a quantitative analysis of this Kerr lens induced Q-switching limiter and its impact on modelocked operation. Our work provides a route to compact high-power multi-gigahertz frequency combs based on SESAM modelocked diode-pumped solid-state lasers without any additional external amplification or pulse compression.