Rate-equation theory of a feedback insensitive unidirectional semiconductor ring laser.

For our recently designed continuous-wave and single-frequency ring laser with intra-cavity isolator, we have formulated a rate-equation theory which accounts for two sources of mutual back-scattering between the clockwise and counterclockwise modes, i.e. induced by side-wall irregularities and due to inversion-grating-induced spatial hole burning. With this theory we first confirm that for a ring laser without intra-cavity isolation, from sufficiently large pumping strength on, the inversion-grating-induced bistable operation (i.e. either clockwise or counterclockwise) will overrule the back-reflection-induced coupled-mode operation (i.e. both clockwise and counterclockwise). We then analyze the robustness of unidirectional operation in case of intra-cavity isolation against the intra-cavity back-reflection mechanism and grating-induced mode coupling and derive for this case an explicit expression for the directionality in the presence of external optical feedback, valid for sufficiently strong isolation. The predictions posed in the second reference remain unaltered in the presence of the mode coupling mechanisms here considered.

Model and experimental validation of a unidirectional phase modulator.

A unidirectional phase modulator consisting of tandem phase modulators is studied in detail for use as an integral part of an integrated optical isolator. The effects of non-linearity and residual amplitude modulation in the modulators, as well as the effect of the RF driving signals are captured in a phenomenological model for the first time. The model has been verified experimentally using a device realized in a generic InP based photonic integration platform and is used to study the operating range of the device. Design parameters of the modulator are derived such that modulation side bands in the forward propagating light are less than 40 dB, while isolation is maximized.

Coupled cavity laser based on anti-resonant imaging via multimode interference.

We report the experimental demonstration of two coupled laser cavities via self-imaging interference in a multimode waveguide. The coupling is optimized by considering images formed by two coherent phase-delayed signals at the input of a 3×3 splitter. As a result, the complex transfer coefficients of the coupling element can be chosen to increase the mode selectivity of the coupled system. A demonstration is given by the successful fabrication of a tunable laser with a side-mode suppression ratio (SMSR) up to 40 dB and a 6.5 nm tuning range. The laser delivers milliwatts of output power to a lensed fiber and is fully compatible with processes supporting vertically-etched sidewalls.

Feedback phase sensitivity of a semiconductor laser subject to filtered optical feedback: experiment and theory.

We investigate experimentally and theoretically the dynamics of a semiconductor laser subject to filtered optical feedback. Depending on the feedback strength we find dynamical regimes with different dependence on the feedback phase. In particular, the influence of the feedback phase on cw emission and on frequency oscillations is characterized experimentally. We also measure the dependence of the filter mirror distance on the frequency oscillations. In general, good agreement between experiment and theory is found.

New role for nonlinear dynamics and chaos in integrated semiconductor laser technology.

Using an integrated colliding-pulse mode-locked semiconductor laser, we demonstrate the existence of nonlinear dynamics and chaos in photonic integrated circuits (PICs) by demonstrating a period-doubling transition into chaos. Unlike their stand-alone counterparts, the dynamics of PICs are more stable over the lifetime of the system, reproducible from batch to batch and on faster time scales due to the small sizes of PICs.

Frequency versus relaxation oscillations in a semiconductor laser with coherent filtered optical feedback.

We investigate the dynamics of a semiconductor laser subject to coherent delayed filtered optical feedback. A systematic bifurcation analysis reveals that this system supports two fundamentally different types of oscillations, namely relaxation oscillations and external roundtrip oscillations. Both occur stably in large domains under variation of the feedback conditions, where the feedback phase is identified as a key quantity for controlling this dynamical complexity. We identify two separate parameter regions of stable roundtrip oscillations, which occur throughout in the form of pure frequency oscillations.

Plasmon-assisted two-slit transmission: Young's experiment revisited.

We present an experimental and theoretical study of the optical transmission of a thin metal screen perforated by two subwavelength slits, separated by many optical wavelengths. The total intensity of the far-field double-slit pattern is shown to be reduced or enhanced as a function of the wavelength of the incident light beam. This modulation is attributed to an interference phenomenon at each of the slits, instead of at the detector. The interference arises as a consequence of the excitation of surface plasmons propagating from one slit to the other.

SOA-based all-optical switch with subpicosecond full recovery.

We investigate all-optical switching in a multi-quantum-well semiconductor optical amplifier-based nonlinear polarization switch using optical pulses with duration of 200 fs at a central wavelength of 1520 nm. We show full recovery of the switch within 600 fs, in both the gain and absorption regime. We discuss the switching and recovery mechanisms using numerical simulations that are in qualitatively good agreement with our experimental data.

Sub-picosecond pulse generation employing an SOA-based nonlinear polarization switch in a ring cavity.

We demonstrate the generation of sub-picosecond optical pulses using a semiconductor optical amplifier (SOA) and a linear polarizer placed in a ring-laser configuration. Nonlinear polarization rotation in the SOA serves as the passive mode-locking mechanism. The ring cavity generates pulses with duration below 800 fs (FWHM) at a repetition rate of 14 MHz. The time -bandwidth product is 0.48. Simulation results in good agreement with the experimental results are presented.

Dielectric structures with bound modes for microcavity lasers.

Cavity modes of dielectric microspheres and vertical cavity surface emitting lasers, in spite of their high Q, are never exactly bound, but have a finite width due to leakage at the borders. We propose types of microstructures that sustain three-dimensionally bound modes of the radiation field when dissipation is neglected. Unlike photonic crystals, the photonic systems that we consider here rely on periodicity in only one or two dimensions. In particular, we discuss a cavity composed of two crossed vertical layers combined with a periodic structure of horizontal layers. The layers have an anisotropic dielectric tensor, which could be obtained by making air holes in the vertical and horizontal directions within isotropic material. We calculate cavity resonance frequencies and spontaneous emission rates. The simplicity of this laser geometry allows an analytical study of light propagation and amplification in three dimensions.

Unnested islands of period doublings in an injected semiconductor laser.

We present a theoretical study of unnested period-doubling islands in three-dimensional rate equations modeling a semiconductor laser subject to external optical injection. In this phenomenon successive curves of period doublings are not arranged in nicely nested islands, but intersect each other. This overall structure is globally organized by several codimension-2 bifurcations. As a consequence, the chaotic region existing inside an unnested island of period doublings can be entered not only via a period-doubling cascade but also via the breakup of a torus, and even via the sudden appearance of a chaotic attractor. In order to fully understand these different chaotic transitions we reveal underlying global bifurcations and we show how they are connected to codimension-2 bifurcation points. Unnested islands of period doublings appear to be generic and hence must be expected in a large class of dynamical systems.

Sudden chaotic transitions in an optically injected semiconductor laser.

We study sudden changes in the chaotic output of an optically injected semiconductor laser. For what is believed to be the first time in this system, we identify bifurcations that cause abrupt changes between different chaotic outputs, or even sudden jumps between chaotic and periodic output. These sudden chaotic transitions involve attractors that exist for large regions in parameter space.

Near fields and far fields generated by sources in the presence of dielectric structures with cylindrical symmetry.

A full-vectorial integral equation method is presented for calculating near fields and far fields generated by sources in the presence of general finite-sized dielectric structures with cylindrical symmetry. The method is relevant for modeling of a class of antenna designs and some optical components with cylindrical symmetry, e.g., vertical-cavity surface-emitting lasers, microdisk lasers, and light-emitting diodes.

Mode density inside an omnidirectional mirror is heavily directional but not small.

We show that ominidirectional reflection is not a sufficient signature of a photonic bandgap. Although dramatic angular redistribution takes place, the mode density of the electromagnetic field is hardly altered within the ominidirectional reflection range but rather has characteristics typical of a waveguide. The strikingly large polarization anisotropy is due to the huge dielectric contrast but not to a photonic bandgap.

Excitability and coherence resonance in lasers with saturable absorber.

We show that a laser with a saturable absorber, described by the Yamada model, displays excitability just below threshold. A small perturbation, for example, a small input pulse, can trigger a single high output pulse, after which the system relaxes back to the off state. In order to study possible applications, such as pulse reshaping and clock recovery, approximate expressions are given for the excitability threshold and the delay between input and output pulses. Under the influence of optical noise, the system displays coherence resonance: below threshold the laser produces pulse trains with minimal jitter for a particular optimal noise level. This all-optical coherence resonance allows direct experimental verification.

Stability of a diode laser with phase-conjugate feedback.

An exact analysis is presented of the steady-state stability of a semiconductor laser subjected to feedback from a phase-conjugate mirror. Reduced stability occurs at low feedback whenever the effective external delay time is an integer multiple of the relaxation oscillation period. The role of a finite response time of the mirror is to enhance drastically the steady-state stability.

Prevention of coherence collapse in diode lasers by dynamic targeting.

We numerically show that a laser that would suffer from coherence collapse if precautions were not taken can be made to operate with a small linewidth and a stable maximum output power by application of a new dynamic targeting technique.

Saturation-induced frequency shift in the noise spectrum of a birefringent vertical-cavity surface emitting laser.

We show by means of analytical and numerical calculations that saturation, in combination with the linewidth-enhancement factor and the finite rate of spin-flip processes, causes a redshift of the spontaneous-emission peak with respect to the off-lasing-mode frequency in the optical spectrum of a quantum-well vertical-cavity surface emitting laser.