Design and Dynamics of Discrete Mode Diode Lasers


Research Overview

In these pages we describe our groups' work on the design and dynamical properties of semiconductor lasers with tailored spectra. Our research in this area has developed from an original solution to the problem of obtaining single-mode emission in Fabry-Perot laser resonators that we found in 2005. Our laser design approach is formulated as an inverse problem solution for the structure of an intracavity grating spectral filter. This approach has enabled us to design both single-mode and multiwavelength edge-emitting devices with low-density grating filters defined by etched features in the laser ridge waveguide.

Here we first use the example of the single mode device to describe how the inverse problem solution is formulated. We also discuss how our design approach can be extended to the case of open grating resonators. These distributed feedback filters will allow us to adapt our laser designs to an integrated optics platform. We then review our work on the nonlinear dynamics of dual-mode devices with saturable absorption, optical injection and delayed feedback. The final section  below highlights our recent results on passively mode-locked devices that support discrete combs of lasing modes.

 Mode Selection in Fabry-Perot and Open Grating Resonators

The Fabry-Perot laser comprises an active gain medium and two external mirrors providing feedback for oscillation. Discrete mode lasers employ a spatially varying refractive index profile to filter the spectral output of the Fabry-Perot cavity. We have developed an approach to this design problem that directly relates the index pattern in real space to the threshold gain modulation in wavenumber space. The optical spectrum of a single-mode device designed using our approach is shown in Fig. 1. (a), where one can see that we have obtained excellent spectral purity in excess of 50 dB. For comparison, the spectrum of a plain Fabry-Perot laser fabricated on the same bar is shown in Fig. 1 (b).

 Here we use the example of a single mode device to  illustrate how our solution is formulated. We also show how our design approach can be applied to open grating resonators that support combs of transmission resonances. These filter designs can define on-chip mode-locked devices with tailored combs of phase-locked lasing modes.


Fig.1: (a) Optical spectrum of a single mode index patterned device at twice threshold. (b) Spectrum at twice threshold of an equivalent Fabry-Perot laser

Nonlinear Dynamics of Dual-Mode Devices

Semiconductor lasers have very rich nonlinear dynamical properties, and we have recently completed multimode extensions of the three classic paradigms in this field, namely, devices with optical injection, time-delayed feedback, and saturable absorption. Fig. 2 illustrates the complex dynamical behaviour we have observed in our injection experiments.

In this section we highlight results obtained in each of these three systems. Our most recent results concern a novel multimode  self-pulsing instability that we found in a dual-mode two-section device. In the optically injected dual-mode system we have demonstrated an all-optical memory element based on the injection locking bistability, and we have also found regular bursting dynamics and on-off intermittency. Here we also highlight numerical simulations demonstrating how optical feedback can induce wavelength bistability in dual-mode devices, with possible applications as fast switching memory elements.



Fig. 2: Optical spectra of a dual-mode device as the frequency detuning of the injected signal from the long-wavelength primary mode is varied

Mode-locking of Semiconductor Lasers with Intracavity Grating Spectral Filters

In some of our recent work our goal was to demonstrate passive mode-locking of discrete combs of Fabry-Perot modes. Fig. 3 opposite illustrates  an example device where four primary modes are selected at twice the fundamental mode spacing of the cavity. Mode-locking is induced in these devices by applying a reverse bias voltage to the saturable absorber section. In the example shown we have found that the intensity output in the mode-locked state is essentially sinusoidal. Where larger numbers of modes are selected we have obtained near transform-limited picosecond pulsed output. We also found significantly reduced RF and optical phase-noise in these devices as compared to their Fabry-Perot equivalents.

Our most recent numerical work addressed the question of finding the ideal wavelength location for the grating filter in order to maximise the stable mode-locking region and tunability of these devices. Here we present details of the optimisation method that we developed, which was based on experimentally calibrated models for the semiconductor susceptibility, and a delay-differential model of the device dynamics.

Fig. 3: Two-section mode-locked device designed to support four primary modes. The device generates quasi-sinusoidal intensity output at 160 GHz

Dr. Stephen O'Brien

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