Lj Sargent

A simple device to allow enhanced bandwidths at 850 nm in multimode fibre links for gigabit LANs

Summary form only given. There is currently much interest in gigabit/s datacommunication links for computer LANs. However, due to the low modal bandwidth of installed multimode fibre (MMF), the achievable link lengths are limited. At an operating wavelength of 1300 nm, greater link lengths can be achieved with mode conditioning patch cords which generate an offset launch into the MMF. However, at the alternative operating wavelength of 850 nm, there is currently no equivalent low cost commercial method available that allows restricted mode launch to be achieved. Therefore, there is considerable interest in developing techniques for generating conditioned launch in MMF which can be implemented in a cost-effective manner. A device has been previously reported that used focused ion beam etching to suppress the emission at the centre of the output aperture. However, this technique requires additional processing steps and results in non planar devices. This paper therefore demonstrates a simpler approach for enhancing the performance of these links.

Investigation of polarization-pinning mechanism in deep-line-etched vertical-cavity surface-emitting lasers

Recently, it has been shown that the etching of deep trenches in close proximity to GaAs vertical-cavity surface-emitting laser (VCSEL) apertures causes the linearly polarized TE emission to be pinned in a direction parallel to the line etch. In this letter, we show that etching introduces compressive strain or relaxes tensile strain through the creation of free interfaces. An anisotropic variation of strain is the origin of the polarization pinning effect. We report on the enhancement of polarization pinning by postannealing after etching. Photoluminescence and Raman measurements of the VCSEL wafer were taken before and after etching and annealing. The observed shift in the Fabry–Perot mode was used to model the strain, giving 4×108 dyn/cm2, or 0.05%, compressive strain perpendicular to the etch.

Design and performance analysis of deep-etch air/nitride distributed Bragg reflector gratings for AlInGaN laser diodes

The key parameters in the fabrication of deep-etch high-order λ/4 Bragg gratings for short-wavelength nitride-based lasers are investigated. Calculations indicate that, for an air-gap thickness of 1.73 μm and single-spot Gaussian beam profile, the reduction in grating reflectivity due to light diffraction in the air gaps is only 17% with respect to a first-order structure with 0.1 μm air gaps. Scanning electron microscopy and microphotoluminescence characterizations confirm the validity of the numerical predictions and show that the 28%–38% reflectivity obtained from prototype focused-ion-beam-etched air/nitride gratings is mainly limited by imperfections and material disorder due to etching. Improving the etching technique would, therefore, allow standard lithographic fabrication of reduced-threshold GaN lasers.

High performance multimode fibre link using ring-lasing vertical cavity surface emitting lasers

High performance laser sources are reported which avoid bandwidth collapse without requiring careful control of launch. By using VCSEL sources with ring-shaped beams, high performance gigabit transmission is achieved even for alignment tolerances of +/-10 microns.

Polarization pinning of a VCSEL array

Polarization control is reported for a 2-D independently addressable VCSEL array. Polarization pinning for emitters on the same die at several orientations is achieved by etching trenches close to the contact metallization of each emitter in the array using focused ion beam etching. Polarization extinction ratios in excess of 50 are achieved over a wide range of orientations and bias currents. No significant effects are observed on threshold current subsequent to the etching of polarization pinning trenches.

Gaussian beam profile and single transverse mode emission from previously multimode gain-guided VCSEL using novel etch

A simple model is used to design and optimize masks that when etched onto the aperture of a normally multi-mode gain guided vertical-cavity surface-emitting laser, VCSEL, cause the device to lase in a single transverse mode. After etching, Gaussian beam profiles for the LP01 mode are achieved over the entire operating current range with greater than 16 dB modal suppression. To date single mode emission at 2 mW has been achieved with pinning of the nearfield width. The modelling shows that the etching introduces a spatial filter on the higher order modes. The power in the fundamental mode remains constant before and after etching.

Investigation of 2-D-lattice distributed reflector lasers

The introduction of a deep-etched hexagonal lattice of round holes around a short section of the ridge waveguide of a Fabry-Perot laser can be used to create single-mode lasers suitable for high-speed data communications. A detailed characterization of the operating characteristics of the two-dimensional-lattice laser is presented. In addition, modeling of gain, reflectivity, and waveguide properties is used to explain experimentally observed results.

Mode-Controlled Vertical Cavity Surface Emitting Lasers for Bandwidth Enhancement of Multimode Fibre Links

A GaAs Vertical Cavity Surface Emitting Laser (VCSEL) that generates controlled modes offset from the center is described. The device is modulated with a 27-1 pseudo-random bit sequence and its output is transmitted along a 1 km length of multimode fiber (MMF). Open eyes are obtained for data rates as high as 1.4Gb/s. The transmission bandwidth increases by a factor of 4 over over-filled launch (OFL). This enhancement is stable against environment influences on the fiber.

Spatial emission control of vertical cavity surface emitting lasers to provide bandwidth gain in multimode fibre links using a simple alignment technique

Etched VCSEL sources are reported which avoid bandwidth collapse in multimode fibre using a simple coupling technique to control the launch. These devices have allowed for a better over-filled launch bandwidth for alignment tolerances of /spl plusmn/7 microns.

Eye-opening lattice work

The 2-D lattice laser is a promising low-cost device for high-speed optical data communications applications.