Focused ion beam (FIB) etching technology is a highly efficient post-processing technique with the functionality to perform sputter etching and deposition of metals or insulators by means of a computer-generated mask. The high resolution and the ability to remove material directly from the sample in-situ make FIB etching the ideal candidate for device prototyping of novel micro-size photonic component design. Furthermore, the fact that arbitrary profile can be etched directly onto a sample without the need to prepare conventional mask and photolithography process makes novel device research with rapid feedback from characterisation to design activities possible. In this paper, we present a concise summary of the research work in Cambridge based on FIB technology. We demonstrate the applicability of focussed ion beam post processing technology to active photonic devices research. Applications include the integration of advanced waveguide architectures onto active photonic components. We documents details on the integration of lens structure on tapered lasers, photonic crystals on active SOA-integrated waveguides and surface profiling of low-cost gain-guided vertical-cavity surface-emitting lasers. Furthermore, we discuss additional functions of FIB in the measurement of buried waveguide structures or the integration of total-internal-reflection (TIR) mirror in optical interconnect structures.
KEYWORDS: Etching, Thermal effects, Near field optics, Mirrors, High power lasers, Waveguides, Geometrical optics, Semiconductor lasers, Laser processing, Diffraction
This paper approaches the problem of catastrophic optical mirror damage from a geometrical waveguide point of view. Instead of engineering the characteristics of the semiconductor material at the facet of the laser using quantum-well intermixing or other sophisticated wafer growth technique, a simple intra-cavity diverging lens concept is proposed and demonstrated to be capable of effectively expanding the lateral optical mode in order to counter the effect of SHB and thermal lensing effect, thereby reducing the risk of COMD. The Gaussian output beam profile is maintained throughout the whole of the current range tested, showing that expanding the nearfield at facet using integrated lens does not compromise the brightness of the laser. A key finding in this work is that the diverging effect on an optical mode is a thoroughly scalable effect that can be engineered by varying the etch-depth of the integrated lens. Fabrication of the lens is compatible with existing laser manufacturing process flow in that it can be easily implemented either by post-processing technology or by an additional lithographical step. This opens up new possibility in device design, with the beam width along the lateral direction being a parameter that can be optimized in isolation.
A novel transmitter for directed-beam infra-red wireless that utilizes a combination of both gain-guiding and index guiding mechanism to ameliorate the shortcoming of the state-of-the-art technology is proposed and demonstrated. The 3mm long tapered laser consists of an index-guided ridge straight section and a gain-guided tapered section with a full angle of 6°. By implementing multiple contact with a sufficiently high inter-contact resistance, discrete switching between different angles can be obtained by selectively pumping current to different contact (gain-guiding effect), while fine-tuning of a given angle can be achieved by adjusting the injection current of nearby contacts (index-guiding effect). The tapered laser's metal cover is removed using focus ion beam etching technique to form three separate sections: base section as filter, left section and right section for beam steering. The device is biased by current pulses of 1μs width and 0.1% duty cycle. With a 1.6A injection current, an output power of 2W is achieved, which would be suitable to overcome large free space optical loss and facilitate the use of transmitting methods. The beam profile steered by 3.2° and -5.4° from the central lobe when injection current is limited to the left and right section respectively is measured. It is also observed that as injection current increases, the beam profile is steered towards the central position. This is because as the injection current increases, the local refractive index is decreased, thereby shifting the beam profile towards the opposite direction.
KEYWORDS: Semiconductor lasers, Etching, Near field, Laser resonators, Fresnel lenses, High power lasers, Diodes, Lens design, Near field optics, Monochromatic aberrations
We demonstrate, for the first time, a monolithic integrated lens for wide aperture gain-guided tapered laser beam quality enhancement by compensating the quadratic phase curvature. The 3mm long tapered laser with an output aperture of 170μm adopted in this design consists of a gain-guided tapered section and an index-guided ridge section and operated at 980nm. The lens design is implemented by focus ion beam etching (FIBE) technique, whereby the laser diode is mounted p-side up in order to facilitate the etching process. The lens is located 600μm away from the junction of the tapered and ridge sections, and is 40μm wide and 300μm long with a focal length of 800μm. The laser diode is characterised by light-current characteristics together with near- and far- field measurements before and after etching. The device is biased by current pulses of 1μs width and 0.1% duty cycle. Light-current measurement shows a drop of 10.5% in threshold current from 380mA to 340mA after the inclusion of lens. This is an evidence that the lens effectively equalised the curved phase in order to reduce the laser cavity loss by improving the coupling efficiency of backward travelling wave at the output facet. Throughout the whole current range tested, the width of near-field at waist is broadened by an average of 36% after the inclusion of lens. By successfully compensating the quadratic phase curvature of the mode, the beam divergence in the far-field is significantly narrowed by an average of 28.5%. M2 factor is improved by an average of 12%.
Enhanced mode selection is facilitated by incorporating compact two dimensional photonic crystal gratings into Fabry-Perot lasers. The gratings are etched directly through the active layer and interact strongly with the guided mode. A high mode-selectivity 2D grating which is significantly wider than the ridge waveguide structure is investigated. The mode selectivity for the structure is -37 dB. The enhanced selectivity afforded by the broad 2D grating is attributable to a novel and robust photonic bandgap mechanism: transverse mode beating.
We describe a photonic bandgap polarization selector based on a photonic crystal placed at junction of two 90° intersecting waveguides to form an ultra-compact device. The photonic crystal consists of 7 layers of a triangular lattice with a radius to pitch ratio (r/a) of 0.24 and a lattice constant of 0.386μm. The PBG is orientated so that the light is incident and collected at 45° to the Γ-K crystallographic direction. Modeling of the PBG shows that TM polarized light is strongly reflected while TE light passes largely into the crystal. Measurements of the fibre-to-fibre transmitted power of the device for each polarization show that the TM collected power is ~6dB higher than the TE light for equal input polarization powers. Further evidence of the strong reflection of TM light comes from an equivalent sample without a 2-D lattice at the waveguide junction. In these samples, no TM light is detected at the output. Furthermore, by taking into account the TE and TM gains within the active waveguides, the TM to TE polarization selection of the PBG is estimated to be up to 22dB.
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