Single-mode distributed feedback lasing using photonic bandgaps to suppress degenerate modes.

We present a method for achieving single-mode distributed feedback lasing, using photonic bandgaps (PBGs) to suppress degenerate lasing modes. These PBGs arise from direct-Bragg coupling and exchange-Bragg coupling due to a passive waveguide placed in proximity to a uniform-grating-active-waveguide lasing structure. Analytic solutions at lasing threshold for this dual-waveguide, four-port structure reveal a high normalized gain margin (e.g., ΔαL = 0.86) accompanied by a low power flatness (e.g., F = 0.001) along the active region, outperforming the λ/4-shifted DFB laser structure. This single-mode lasing mechanism is compelling for both III/V and III/V-on-silicon platforms.

Downstream hysteresis-shape control of a bistable optical signal created by an upstream nonlinear photonic resonator.

We experimentally demonstrate the control of the hysteresis shape of a bistable optical signal at a location downstream of the nonlinear photonic resonator within which the bistability is generated; this shape control mechanism is performed without any signal sent upstream to the bistable system. The downstream shape-control element consists of a fiber-optic polarization controller and linear polarizer in series, and the nonlinear resonator generates an optical signal whose bistable branches exhibit different states of power and polarization. The diversity of demonstrated hysteresis shapes include the canonical counter-clockwise (CCW) shape (S-shape), the clockwise (CW) shape (inverted S-shape), and butterfly shapes; since all shapes originate from the same bistable signal, all shapes exhibit the same switching input powers. Moreover, the shape-selection process enhances the bistable switching contrast to 20 and 21 dB for the CCW and CW shapes, respectively. The shape-selection technique is demonstrated using a Fabry-Pérot semiconductor optical amplifier, is applicable to other nonlinear photonic resonators, and provides flexibility to future combinational and sequential all-optical signal processing applications.

Distributed feedback lasing based on a negative-index metamaterial waveguide.

This Letter lays the foundation of a new type of distributed feedback (DFB) laser whose optical feedback is due to the evanescent coupling between an active positive-index material (PIM) waveguide and a lossy negative-index metamaterial (NIM) waveguide. Active PIM-NIM coupled-mode equations are presented and solved to characterize the dispersion relation, resonant optical gain, and lasing. The photonic bandgap of this grating-less DFB laser does not depend on a Bragg wavenumber, but depends on the difference between the wavenumbers of the PIM and NIM waveguides; controlling this wavenumber difference allows for single-mode lasing and, ultimately, single-mode broadband lasing.

Reflection and transmission of electromagnetic waves at a temporal boundary.

We consider propagation of an electromagnetic (EM) wave through a dynamic optical medium whose refractive index varies with time. Specifically, we focus on the reflection and transmission of EM waves from a temporal boundary and clarify the two different physical processes that contribute to them. One process is related to impedance mismatch, while the other results from temporal scaling related to a sudden change in the speed of light at the temporal boundary. Our results show that temporal scaling of the electric field must be considered for light propagation in dynamic media. Numerical solutions of Maxwell's equations are in full agreement with our theory.

Propagation of few-cycle pulses in nonlinear Kerr media: harmonic generation.

We apply our recently developed time-transformation method for studying the propagation of few-cycle optical pulses inside a nonlinear Kerr medium after taking into account that changes in the refractive index vary with the electric field as E2 and not by its average over an optical cycle. Our technique correctly predicts carrier-wave shocking and generation of odd-order harmonics inside a Kerr medium, the two features found earlier with directly solving Maxwell's equations using the finite-difference time-domain (FDTD) methods. We extend our method to study the impact of a finite response of the Kerr nonlinearity on harmonic generation and to include chromatic dispersion that cannot be ignored for ultrashort pulses. We show that nonlinear effects can help in controlling the width of an ultrashort pulse, even though it cannot propagate as a fundamental soliton. Our time-transformation method provides an alternative to the FDTD technique, as it deals with the electric field directly but does not require step size to be a small fraction of the wavelength, resulting in much faster computation speeds.

Nonlinear pulse propagation: a time-transformation approach.

We present a time-transformation approach for studying the propagation of optical pulses inside a nonlinear medium. Unlike the conventional way of solving for the slowly varying amplitude of an optical pulse, our new approach maps directly the input electric field to the output one, without making the slowly varying envelope approximation. Conceptually, the time-transformation approach shows that the effect of propagation through a nonlinear medium is to change the relative spacing and duration of various temporal slices of the pulse. These temporal changes manifest as self-phase modulation in the spectral domain and self-steepening in the temporal domain. Our approach agrees with the generalized nonlinear Schrödinger equation for 100 fs pulses and the finite-difference time-domain solution of Maxwell's equations for two-cycle pulses, while producing results 20 and 50 times faster, respectively.

Assessment of VCSEL thermal rollover mechanisms from measurements and empirical modeling.

We use an empirical model together with experimental measurements for studying mechanisms contributing to thermal rollover in vertical-cavity surface-emitting lasers (VCSELs). The model is based on extraction of the temperature dependence of threshold current, internal quantum efficiency, internal optical loss, series resistance and thermal impedance from measurements of output power, voltage and lasing wavelength as a function of bias current over an ambient temperature range of 15-100 °C. We apply the model to an oxide-confined, 850-nm VCSEL, fabricated with a 9-µm inner-aperture diameter and optimized for high-speed operation, and show for this specific device that power dissipation due to linear power dissipation (sum total of optical absorption, carrier thermalization, carrier leakage and spontaneous carrier recombination) exceeds power dissipation across the series resistance (quadratic power dissipation) at any ambient temperature and bias current. We further show that the dominant contributors to self-heating for this particular VCSEL are quadratic power dissipation, internal optical loss, and carrier leakage. A rapid reduction of the internal quantum efficiency at high bias currents (resulting in high temperatures) is identified as being the major cause of thermal rollover. Our method is applicable to any VCSEL and is useful for identifying the mechanisms limiting the thermal performance of the device and to formulate design strategies to ameliorate them.

Spectral and temporal changes of optical pulses propagating through time-varying linear media.

We present universal formulas for the spectral and temporal output optical fields from a linear traveling-wave medium whose refractive index changes during its propagation within the medium. These formulas agree with known changes in central wavelength and energy that are associated with adiabatic wavelength conversion (AWC). Moreover, they reveal new changes to the optical pulses that have not been noticed, such as pulse compression and spectral broadening. Most significantly, we find that AWC alters the pulse power, pulse chirp, and pulse delay. All of these effects depend on whether the central wavelength is blueshifted or redshifted, the first sign of asymmetry to be reported for AWC. These findings impact the applications of AWC to optical signal processing in microphotonic and nanophotonic structures as well as in lightwave systems.

Efficient adiabatic wavelength conversion in Gires-Tournois resonators.

We compare theoretically the performance capabilities of Fabry-Perot and Gires-Tournois resonators when used for adiabatic wavelength conversion. It is shown that the Gires-Tournois device will exhibit superior performance and is able to convert the wavelength of optical pulses with >74% efficiency while nearly preserving their temporal duration.

Spectral broadening in ultrafast semiconductor optical amplifiers induced by gain dynamics and self-phase modulation.

We investigate experimentally the self-phase modulation (SPM) induced by gain dynamics on picosecond pulses in semiconductor optical amplifiers whose gain recovery is enhanced by amplified spontaneous emission (ASE). The observed pulse spectra are highly asymmetric at low drive currents but become more symmetric with increasing current, owing to the ASE-induced reduction in the gain-recovery time down to 9 ps. Furthermore, the amount of spectral broadening is shown to saturate with drive current. We show that a variety of spectral-lobe strengths is selectable, while maintaining a nearly constant small-signal gain, a feature desirable for all-optical signal processing applications.

Remote optical control of an optical flip-flop.

We experimentally demonstrate control of a holding-beam-enabled optical flip-flop by means of optical signals that act in a remote fashion. These optical-control signals vary the holding-beam power by means of cross-gain modulation within a remotely located semiconductor optical amplifier (SOA). The power-modulated holding beam then travels through a resonant-type SOA, where flip-flop action occurs as the holding-beam power falls above and below the switching thresholds of the bistable hysteresis. Control is demonstrated using submilliwatt pulses whose wavelengths are not restricted to the vicinity of the holding beam. Benefits of remote control include the potential for controlling multiple flip-flops with a single pair of optical signals and for realizing all-optical control of any holding-beam-enabled flip-flop.