S. O’Brien

Theory of improved spectral purity in index patterned Fabry-Pérot lasers

The spectral purity of a ridge waveguide Fabry-Perot laser can be improved by patterning the effective refractive index seen by an optical mode propagating in the cavity. Here we present a transmission matrix calculation to first order in the effective index step from which we derive the threshold condition as a function of cavity mode index. This approach enables us to solve the inverse problem relating the index pattern along the cavity to the threshold gain modulation in wavenumber space. Quasiperiodic index patterns are constructed, which lead to improved spectral purity at a predetermined wavelength.

All-optical memory based on the injection locking bistability of a two-color laser diode.

The device we consider is a multi-quantum well InP/InGaAlAs two-colour Fabry-Perot laser [Eblana Photonics] that incorporates slotted regions in the laser ridge waveguide [1]. A hysteresis loop of the output power in the uninjected mode as a function of the power injected in the long-wavelength primary mode is shown in Fig. 1 (left). Insets show the co-existing steady states associated with the bistability. Pulsed modulation of the injected power leads to switching between these states, as shown in Fig. 1 (centre).

Refractive femtosecond laser beam shaping for two-photon polymerization

Three dimensional microstructure fabrication by two-photon polymerization is an established technique that normally uses single beam serial writing. Recently the use of a micro-optical element, to give multipoint beam delivery, was reported to give a degree of parallel processing. The authors describe an alternative approach to parallel processing using an axicon lens. This is a refractive element that, in combination with a high power microscope objective, efficiently transforms the laser beam from a Gaussian spot to an annulus. The authors demonstrate that the beam can polymerize a three dimensional shape, with nanoscale resolution. The use of more sophisticated refractive beam shaping is also discussed.

Wavelength switching dynamics of two-colour semiconductor lasers with optical injection and feedback

The wavelength switching dynamics of two-colour semiconductor lasers with optical injection and feedback are presented. These devices incorporate slotted regions etched into the laser ridge waveguide for tailoring the output spectrum. Experimental measurements are presented demonstrating that optical injection in one or both modes of these devices can induce wavelength bistability. Measured switching dynamics with modulated optical injection are shown to be in excellent agreement with numerical simulations based on a simple rate equation model. We also demonstrate experimentally that time-delayed optical feedback can induce wavelength bistability for short external cavity lengths. Numerical simulations indicate that this two-colour optical feedback system can provide fast optical memory functionality based on injected optical pulses without the need for an external holding beam.

Antiphase dynamics in a multimode semiconductor laser with optical injection

A detailed experimental study of antiphase dynamics in a two-mode semiconductor laser with optical injection is presented. The device is a specially designed Fabry-Perot laser that supports two primary modes with a THz frequency spacing. Injection in one of the primary modes of the device leads to a rich variety of single and two-mode dynamical scenarios, which are reproduced with remarkable accuracy by a four dimensional rate equation model. Numerical bifurcation analysis reveals the importance of torus bifurcations in mediating transitions to antiphase dynamics and of saddle-node of limit cycle bifurcations in switching of the dynamics between single and two-mode regimes.

Bistability in an injection locked two color laser with dual injection

A two color Fabry-Perot laser subjected to optical injection in both modes is examined experimentally and theoretically. The theoretical analysis predicts a bistability between locked states due to a swallow-tail bifurcation, which is unique to the dual injection system. This bistability is confirmed experimentally and used as the basis for an all optical memory element with switching times below 500 ps.

Comb transmission filters defined by phase-shifted superstructure Bragg gratings.

We present a design method and numerical results describing the construction of distributed feedback grating filters that support discrete combs of transmission resonances. These filter designs define open superstructure grating resonators with transmission channels that can be placed at predetermined frequencies, such as those defined by the wavelength division multiplexing grid or by a secondary frequency comb source. Focusing on a specific example with 40 GHz channel spacing, we optimize an active structure that defines three low-threshold lasing modes. How our design approach relates to filter synthesis techniques based on cascaded grating resonators is also discussed.

Timing characterization of 100 GHz passively mode-locked discrete mode laser diodes

We report on the characterization of the timing stability of passively mode-locked discrete mode diode laser sources. These are edge-emitting devices with a spatially varying refractive index profile for spectral filtering. Two devices with a mode-locking frequency of 100 GHz are characterized. The first device is designed to support a comb of six modes and generates near Fourier limited 1.9 ps pulses. The second supports four primary modes resulting in a sinusoidal modulation of the optical intensity. Using a cross-correlation technique, we measured a 20 fs pulse to pulse timing jitter for the first device, while, for the second device, a mode-beating (RF) linewidth of 1 MHz was measured using heterodyne mixing in a semiconductor optical amplifier. Comparison of these results with those obtained for an equivalent Fabry-Perot laser indicates that the spectral filtering mechanism employed does not adversely affect the timing properties of these passively mode-locked devices.

Design of waveguide-integrated semiconductor laser sources for optical frequency comb generation

A numerical study of threshold gain and modal dispersion in integrated semiconductor laser optical frequency comb sources is presented. We consider an example device where one of the cleaved facets of the laser is replaced by a short Bragg grating section and show that as many as 16 modes can be selected at the first harmonic of the underlying Fabry-Perot cavity. An intracavity approach to limiting the grating-induced dispersion that can be implemented directly through the grating profile is demonstrated.

Wavelength switching performance of single-?and dual-contact two-mode semiconductor lasers with current modulation

The wavelength switching performance of single-?and dual-contact two-mode semiconductor lasers is studied. These devices are characterized experimentally in terms of switching time, frequency chirp, contrast ratio and current modulation amplitude. Models based on multimode rate equations are used for reproducing these experimental results with good agreement between theory and experiment. The benefits of a dual-contact device are shown with respect to a single-contact device.