Brendan Roycroft

Discretely Tunable Semiconductor Lasers Suitable for Photonic Integration

A sequence of partially reflective slots etched into an active ridge waveguide of a 1.5 mum laser structure is found to provide sufficient reflection for lasing. Mirrors based on these reflectors have strong spectral dependence. Two such active mirrors together with an active central section are combined in a Vernier configuration to demonstrate a tunable laser exhibiting 11 discrete modes over a 30 nm tuning range with mode spacing around 400 GHz and side-mode suppression ratio larger than 30 dB. The individual modes can be continuously tuned by up to 1.1 nm by carrier injection and by over 2 nm using thermal effects. These mirrors are suitable as a platform for integration of other optical functions with the laser. This is demonstrated by monolithically integrating a semiconductor optical amplifier with the laser resulting in a maximum channel power of 14.2 dBm from the discrete modes.

A Novel Two-Section Tunable Discrete Mode Fabry-PÉrot Laser Exhibiting Nanosecond Wavelength Switching

A novel widely tunable laser diode is proposed and demonstrated. Mode selection occurs by etching perturbing slots into the laser ridge. A two-section device is realized with different slot patterns in each section allowing Vernier tuning. The laser operates at 1.3 mum and achieves a maximum output power of 10 mW. A discontinuous tuning range of 30 nm was achieved with a side mode suppression greater than 30 dB. Wavelength switching times of approximately 1.5 ns between a number of wavelength channels separated by 7 nm have been demonstrated.

Practical Design of Lensed Fibers for Semiconductor Laser Packaging Using Laser Welding Technique

We propose a set of guidelines for the practical design of lensed fiber for the optical coupling of semiconductor lasers in butterfly packages using laser welding. These guidelines have optimized the tradeoff between coupling efficiency and alignment tolerance. Moreover, a radius of curvature of 11 mum is shown to be optimal for semiconductor lasers whose divergence angles range from 5deg to 30deg. To experimentally evaluate the design, lensed fibers were assembled by a Nd:YAG laser welding technique in conventional butterfly packages and their coupling efficiencies were 28%-72% without horizontal misalignment compensation.

A Multiwavelength Low-Power Wavelength-Locked Slotted Fabry–Pérot Laser Source for WDM Applications

Wavelength-locking of a multiwavelength stabilized slotted Fabry-Peacuterot (SFP) laser to a single-mode laser source is experimentally demonstrated. The SFP resonates at channels spaced by ~8 nm between 1510 and 1565 nm over a wide range of temperatures and drive currents. Under low-power (<-20 dBm) external optical injection, wavelength-locking with a sidemode suppression ratio (SMSR) >25 dB is achieved. A locking width of >25 GHz and SMSR >30 dB can be achieved for each locked wavelength channel at injection power >-16 dBm

InP-Based Active and Passive Components for Communication Systems at 2 μm

Progress on advanced active and passive photonic components that are required for high-speed optical communications over hollow-core photonic bandgap fiber at wavelengths around 2 μm is described in this paper. Single-frequency lasers capable of operating at 10 Gb/s and covering a wide spectral range are realized. A comparison is made between waveguide and surface normal photodiodes with the latter showing good sensitivity up to 15 Gb/s. Passive waveguides, 90° optical hybrids, and arrayed waveguide grating with 100-GHz channel spacing are demonstrated on a large spot-size waveguide platform. Finally, a strong electro-optic effect using the quantum confined Stark effect in strain-balanced multiple quantum wells is demonstrated and used in a Mach-Zehnder modulator capable of operating at 10 Gb/s.

Design, Characterization, and Applications of Index-Patterned Fabry–Pérot Lasers

The design and measured performance characteristics of a range of index-patterned diode laser sources are presented. These devices incorporate slotted regions etched into the laser ridge waveguide, which are formed in the same fabrication step as the ridge, thus avoiding the requirement for complex lithography and regrowth steps. We first demonstrate that the index profile of single and multimode devices can be obtained directly from an inverse problem solution based on a perturbative calculation of the threshold gain of the longitudinal modes of the cavity. Measurements of temperature stability, linewidth, and modulation bandwidth of single-mode devices obtained in this way are presented. It is then shown that the design of multimode devices including two-color and pulsed mode-locked devices designed to support a discrete comb of modes is also possible. We finally demonstrate a tunable source based on a multisection design defined using etched features. This device is shown to have wide tunability with narrow linewidth modes and fast wavelength switching speed.

Fast Wavelength Switching Lasers Using Two-Section Slotted Fabry&#8211;P&#201;rot Structures

Fast wavelength switching of a two-section slotted Fabry-Perot laser structure is presented. The slot design enables operation at five discrete wavelength channels spaced by 10 nm by tuning one section of the device. These wavelengths operate with sidemode suppression ratio in excess of 35 dB, and switching times between these channels of approximately 1 ns are demonstrated

Fabrication of GaN-Based Resonant Cavity LEDs

This article describes a fabrication process for GaN based resonant cavity (RC) LEDs. The devices emit through the transparent sapphire substrate. An AlGaN/GaN DBR forms the bottom mirror, while a PdAg-based p-contact metallisation serves as top mirror. The PdAg mirrors have been characterized optically by reflectometry, and electrically by circular TLM measurements. A specific contact resistivity of 1.0 × 10 -2 Ωcm 2 has been measured, without any annealing of the contacts. For the GaN to PdAg interface a reflectivity of 60% has been measured for a wavelength of 510 nm. At 20 mA bias the RC-LEDs show a forward voltage of 3.5 V and an optical output of 330 μW through the back of the wafer.

Experimental Characterisation of GaN-Based Resonant Cavity Light Emitting Diodes

For the first time, 2λ and 3λ GaN-based microcavities are demonstrated, consisting of a monolithically grown distributed Bragg reflector (DBR) as the first mirror and a metal as the second mirror. The metal acts both as a mirror and electrical contact, emission being through the sapphire substrate. No epitaxial lift-off or dielectric mirrors are required. Reflectance measurements and angle-resolved electroluminescence measurements clearly show the presence of a resonant cavity at wavelengths around 520 nm for the samples presented. At measurement angles off axis, the resonance moves to shorter wavelengths, in agreement with simple theory. Reflectance measurements indicate an effective penetration depth of the optical field into the DBR of approximately 3λ. Design and epitaxy were developed within the EU-funded AGETHA consortium.

Integration of AlInGaAs-MQW Fabry–Pérot Lasers With Emission at Two Wavelength Ranges via Quantum-Well Intermixing

We demonstrate ridge waveguide lasers based on AlInGaAs multiple quantum wells emitting at 1434 and 1541 nm on the same laser bar using quantum-well intermixing with dielectric capping layers. The internal quantum efficiencies are measured to be 61% and 72% and the internal losses are 49 and 23 cm-1 for lasers with intermixing promoted and inhibited, respectively. The characteristic temperatures are found to be approximately 50 K for lasers emitting around 1433 nm and 75 K for those emitting around 1541 nm.