Tunable, coherent optical comb source via on-chip bidirectional coupling.

A tunable comb source is demonstrated through gain switching on a three-sectioned photonic integrated circuit (PIC). The PIC consists of two mutually coupled lasers connected by a passive waveguide. One of these is a tunable, two-section, single mode laser. The second laser is a simple Fabry-Perot cavity laser which can be phase-locked with the single mode laser via bidirectional coupling. Frequency combs are produced by gain switching the Fabry-Perot laser by applying a high-power radio frequency signal. Combs are generated with line spacings ranging from 3.5 to 8 GHz. The on-chip bidirectional coupling causes the comb to also be generated in the two-section device. Despite the lack of on-chip optical isolation between the lasers, the resulting combs are stable. Numerical simulations using a delay-differential model reproduce the results and reveal the important role played by the short delay times inherent to on-chip integration in this stability.

Optical information processing using dual state quantum dot lasers: complexity through simplicity.

We review results on the optical injection of dual state InAs quantum dot-based semiconductor lasers. The two states in question are the so-called ground state and first excited state of the laser. This ability to lase from two different energy states is unique amongst semiconductor lasers and in combination with the high, intrinsic relaxation oscillation damping of the material and the novel, inherent cascade like carrier relaxation process, endows optically injected dual state quantum dot lasers with many unique dynamical properties. Particular attention is paid to fast state switching, antiphase excitability, novel information processing techniques and optothermally induced neuronal phenomena. We compare and contrast some of the physical properties of the system with other optically injected two state devices such as vertical cavity surface emitting lasers and ring lasers. Finally, we offer an outlook on the use of quantum dot material in photonic integrated circuits.

Universal generation of devil's staircases near Hopf bifurcations via modulated forcing of nonlinear systems.

The discrete circle map is the archetypical example of a driven periodic system, showing a complex resonance structure under a change of the forcing frequency known as the devil's staircase. Adler's equation can be seen as the direct continuous equivalent of the circle map, describing locking effects in periodic systems with continuous forcing. This type of locking produces a single fundamental resonance tongue without higher-order resonances, and a devil's staircase is not observed. We show that, with harmonically modulated forcing, nonlinear oscillations close to a Hopf bifurcation generically reproduce the devil's staircase even in the continuous case. Experimental results on a semiconductor laser driven by a modulated optical signal show excellent agreement with our theoretical predictions. The locking appears as a modulation of the oscillation amplitude as well as the angular oscillation frequency. Our results show that by proper implementation of an external drive, additional regions of stable frequency locking can be introduced in systems which originally show only a single Adler-type resonance tongue. The induced resonances can be precisely controlled via the modulation parameters.

Dynamics of on-chip asymmetrically coupled semiconductor lasers.

We investigate the dynamics of asymmetrically coupled semiconductor lasers on photonic integrated circuits in experiment and theory. The experimental observations are explained using a rate-equation model for coupled lasers incorporating a saturable coupling waveguide. We perform a bifurcation analysis of the coupled laser dynamics, focusing on the effects of the coupling phase and the dynamical difference between passive and saturable coupling waveguides. For a passive waveguide, we find a bifurcation scenario closely resembling the well-known optical injection setup, which is largely insensitive to the coupling phase. When the coupling waveguide is saturable, the dynamics become increasingly complex and unpredictable, with a strong phase-dependence. Our results show the possibility of a simple layout for reproducible laser dynamics on a chip.

Harmonic frequency locking and tuning of comb frequency spacing through optical injection.

We show, both experimentally and theoretically, that a slave laser injected with an optical frequency comb can undergo two distinct locking mechanisms, both of which decrease the output optical comb's frequency spacing. We report that, for certain detuning and relative injection strengths, slave laser relaxation oscillations can become undamped and lock to rational frequencies of the optical comb spacing, creating extra comb tones by nonlinear dynamics of the injected laser. We also study the frequency locking of the slave laser at detunings in between the injected comb lines, which add the slave laser's frequency to the comb. Our results demonstrate the effect of the α parameter and stability of the locked states and indicate how the frequency of the relaxation oscillations affect both of these locking mechanisms. These optical locking mechanisms can be applied to regenerate or multiply optical combs.

Slowed gastric emptying and improved oral glucose tolerance produced by a nanomolar-potency inhibitor of calcium-activated chloride channel TMEM16A.

Interstitial cells of Cajal, which express the calcium-activated chloride channel transmembrane member 16A (TMEM16A), are an important determinant of gastrointestinal (GI) motility. We previously identified the acylaminocycloalkylthiophene class of TMEM16A inhibitors, which, following medicinal chemistry, gave analog 2-bromodifluoroacetylamino-5,6,7,8-tetrahydro-4H-cyclohepta[b]thiophene-3-carboxylic acid o-tolylamide (TMinh-23) with 30 nM half-maximal inhibitory concentration. Here, we tested the efficacy of TMinh-23 for inhibition of GI motility in mice. In isolated murine gastric antrum, TMinh-23 strongly inhibited spontaneous and carbachol-stimulated rhythmic contractions. Pharmacokinetic analysis showed predicted therapeutic concentrations of TMinh-23 for at least 4 h following a single oral or intraperitoneal dose at 10 mg/kg. Gastric emptying, as assessed following an oral bolus of phenol red or independently by [99mTc]-diethylenetriamine pentaacetic acid scintigraphy, was reduced by TMinh-23 by ∼60% at 20 min. Interestingly, there was little effect of TMinh-23 on baseline whole-gut transit time or time to diarrhea induced by castor oil. Consequent to the delay in gastric emptying, TMinh-23 administration significantly reduced the elevation in blood sugar in mice following an oral but not intraperitoneal glucose load. These results provide pharmacological evidence for involvement of TMEM16A in gastric emptying and suggest the utility of TMEM16A inhibition in disorders of accelerated gastric emptying, such as dumping syndrome, and potentially for improving glucose tolerance in diabetes mellitus/metabolic syndrome and enhancing satiety in obesity.-Cil, O., Anderson, M. O., Yen, R., Kelleher, B., Huynh, T. L., Seo, Y., Nilsen, S. P., Turner, J. R., Verkman, A. S. Slowed gastric emptying and improved oral glucose tolerance produced by a nanomolar-potency inhibitor of calcium-activated chloride channel TMEM16A.

Two-color bursting oscillations.

Neurons communicate by brief bursts of spikes separated by silent phases and information may be encoded into the burst duration or through the structure of the interspike intervals. Inspired by the importance of bursting activities in neuronal computation, we have investigated the bursting oscillations of an optically injected quantum dot laser. We find experimentally that the laser periodically switches between two distinct operating states with distinct optical frequencies exhibiting either fast oscillatory or nearly steady state evolutions (two-color bursting oscillations). The conditions for their emergence and their control are analyzed by systematic simulations of the laser rate equations. By projecting the bursting solution onto the bifurcation diagram of a fast subsystem, we show how a specific hysteresis phenomenon explains the transitions between active and silent phases. Since size-controlled bursts can contain more information content than single spikes our results open the way to new forms of neuron inspired optical communication.

All-optical switching with a dual-state, single-section quantum dot laser via optical injection.

An all-optical switching mechanism via optical injection of an InAs/GaAs quantum dot laser is presented. Relative state suppression in excess of 40 dB is achieved, and experimental switching times of the order of a few hundred picoseconds are demonstrated.

Coherence and incoherence in an optical comb.

We demonstrate a coexistence of coherent and incoherent modes in the optical comb generated by a passively mode-locked quantum dot laser. This is experimentally achieved by means of optical linewidth, radio frequency spectrum, and optical spectrum measurements and confirmed numerically by a delay-differential equation model showing excellent agreement with the experiment. We interpret the state as a chimera state.

Characterization of time-resolved laser differential phase using 3D complementary cumulative distribution functions.

An experimental method for characterizing the time-resolved phase noise of a fast switching tunable laser is discussed. The method experimentally determines a complementary cumulative distribution function of the laser's differential phase as a function of time after a switching event. A time resolved bit error rate of differential quadrature phase shift keying formatted data, calculated using the phase noise measurements, was fitted to an experimental time-resolved bit error rate measurement using a field programmable gate array, finding a good agreement between the time-resolved bit error rates.

Transition from unidirectional to delayed bidirectional coupling in optically coupled semiconductor lasers.

We investigate the transition from unidirectional to delayed bidirectional coupling of semiconductor lasers. By tuning the coupling strength in one direction we show how the locking region evolves as a function of the detuning and coupling strength. We consider two representative values of the relaxation oscillation damping: one where the relaxation oscillations are very underdamped and one where they are very overdamped. Qualitatively different dynamical scenarios are shown to emerge for each case. Several features of the delayed bidirectional system can be seen as remaining from the unidirectional system while others clearly arise due to the delayed coupling and are similar to effects seen in delayed feedback configurations.

Phase-locked mutually coupled 1.3 microm quantum-dot lasers.

Fabry-Perot InAs quantum-dot lasers grown on GaAs substrates are mutually coupled with a delay of several nanoseconds. Stable phase-locked output with narrow linewidth is obtained when the frequency detuning between the two lasers is less than 4 GHz. This simple locking scheme could find application in a variety of photonics applications.