Tantalum oxide anti-reflective thin films for C-band travelling-wave semiconductor optical amplifiers

The work in this thesis describes an approach to suppressing cavity resonances in Travelling-Wave Semiconductor Optical Amplifiers (TW-SOA) by application of a single layer anti-reflective thin film, tuned for C-Band (1530-1565 nm) emission. A 215 nm layer of tantalum oxide (TaOx) is applied to both facets of AlGaInAs/InP ridge waveguide (RWG) Fabry-Pérot (FP) laser diodes (LD) via electron-beam (e-beam) evaporation. E-beam evaporation allows a dielectric layer, thicker than 200 nm, to be deposited onto many LD facets in a cost-effective and time efficient manner. This deposition technique is chosen as it allows the refractive index of the antireflection coating (ARC) to be tuned to produce a TaOx layer of the required refractive index, nfilm, by exploiting the relationship between nfilm and chamber pressure during evaporation. Monitoring the power-current characteristics and emission spectrum from the devices before and after coating demonstrates the successful realisation of a TW-SOA where lasing is no longer present. Remaining cavity resonances show gain ripple, Gr, values across the C-Band gain spectrum of ≤0.1 dB with a maximum of 0.5 dB at injection currents over three times the original laser threshold current. These values are competitive when compared to commercially available SOAs operating across the same wavelength region. The remaining FP fringes observed in the Amplified Spontaneous Emission (ASE) spectrum of the TW-SOA are consistent with geometric mean facet reflectivity, √ (R1 R2), values as low as 10-4 and 10-5 across the C-Band wavelength range (1530-1565 nm) of interest. The results in this thesis show promise for the future use of TaOx thin films to improve the performance of Distributed Feedback (DFB), and Distributed Bragg Reflector (DBR) laser diodes, or as an effective antireflection coating for different types of semiconductor optical amplifiers

Design and Characterisation of Multi-Mode Interference Reflector Lasers for Integrated Photonics

InAs quantum dot ridge waveguide lasers comprising single-port multi-mode-interference-reflectors (MMIR) and single-cleaved reflectors are designed, fabricated, and characterised, to demonstrate capability for optoelectronic-integrated-circuits. Simulations of an MMIR show high values of fundamental mode reflectivity ($> 80\% )$ and good selectivity against higher order modes. Deep-etched MMIR lasers fabricated with 0.5 mm long cavities have a threshold current of 24 mA, compared to 75 mA for standard Fabry–Perot cleaved–cleaved FP-RWG lasers of the same length, both at 25 °C, and 56 mA compared to 102 mA at 55 °C. MMIR lasers exhibit stable ground state operation up to 50 °C and show promise as small footprint sources for integrated photonics

Quick fabrication VCSELs for characterisation of epitaxial material

A systematic analysis of the performance of VCSELs, fabricated with a decreasing number of structural elements, is used to assess the complexity of fabrication (and therefore time) required to obtain sufficient information on epitaxial wafer suitability. Initially, sub-mA threshold current VCSEL devices are produced on AlGaAs-based material, designed for 940 nm emission, using processing methods widely employed in industry. From there, stripped-back Quick Fabrication (QF) devices, based on a bridge-mesa design, are fabricated and this negates the need for benzocyclcobutane (BCB) planarisation. Devices are produced with three variations on the QF design, to characterise the impact on laser performance from removing time-consuming process steps, including wet thermal oxidation and mechanical lapping used to reduce substrate thickness. An increase in threshold current of 1.5 mA for oxidised QF devices, relative to the standard VCSELs, and a further increase of 1.9 mA for unoxidised QF devices are observed, which is a result of leakage current. The tuning of the emission wavelength with current increases by ~0.1 nm/mA for a VCSEL with a 16 μm diameter mesa when the substrate is unlapped, which is ascribed to the increased thermal resistance. Generally, relative to the standard VCSELs, the QF methods employed do not significantly impact the threshold lasing wavelength and the differences in mean wavelengths of the device types that are observed are attributed to variation in cavity resonance with spatial position across the wafer, as determined by photovoltage spectroscopy measurements

Realisation of multi-mode reflector lasers for integrated photonics

The epitaxial growth of III-V materials on silicon is an alternative approach to combining silicon photonics with the active laser source. Substantial progress has been made to reduce the defects created at the III-V / Si interface to a level that has a negligible impact on laser operating current and lifetime, providing quantum dot gain materials are utilized [1], [2]. A number of issues remain for the integration of III-V structures with silicon, not least that of reducing the footprint and ensuring the fabrication required is as simple as possible. While the laser reflectors can be fabricated in the silicon here we focus on using the III-V material, which removes the need to have the III-V / Silicon interface and its associated losses within the laser cavity

Gain measurements on vertical cavity surface emitting laser material using segmented contact technique

We report direct measurements of the optical gain profile for a vertical cavity surface emitting laser (VCSEL) epitaxial structure, by characterising the transverse electric (TE) in-plane net modal gain using the segmented contact method

InAs quantum dot-based one- and two-port multimode interference reflectors for integrated photonic devices: design, fabrication, and evaluation

1-port and 2-port multi-mode interference reflectors (MMIR) are excellent components for Photonic Integrated Circuits, being highly reflective and easy to fabricate. We demonstrate InAs-Quantum-Dot MMIR lasers, where the high reflectivity is particularly advantageous, with lower threshold current than Fabry-Perot ridge lasers with the same cavity length e.g. 6mA compared to 46-mA. The threshold current density of the 1-mm MMIR laser is equivalent to the Fabry-Perot laser with a 3-mm cavity length. MMIRs have a higher optical slope efficiency, indicating mirror reflectivity above 85%