Polarization Effects of Mechanical Impacts on Dispersion Compensating Modules

Novel methods and apparatus used to measure high-speed state of polarization changes are developed. Knowledge of the effects of mechanical impacts on the state of polarization will benefit the reliability of optical communication systems. The impact creates a high-speed but continuous motion of the state of polarization over the Poincar´e sphere. The maximum speed at which the state of polarization changes due to an impact is shown to be higher than what has been reported previously. The investigation into the state of polarization changes led to the discovery of the repeatability and elasticity of state of polarization changes due to mechanical impacts. The repeatability and elasticity allow novel measurements of important polarization effects in optical fibres such as high-speed polarization mode dispersion and rotation vector measurements

Germanium on silicon photonics

Silicon photonics technologies have the potential to overcome the bandwidth limitations inherent in electrical interconnect technology. Modulation technology which is efficient both in terms of size and energy is required if silicon photonics are to replace electronics for interconnect communications. Silicon germanium technologies have the potential to not only improve the performance of current semiconductor devices but to also extend the reach of semiconductor technology into new areas such as development of a room temperature THz laser. A novel process that allows easy fabrication of Ohmic contacts to moderately doped n-type Germanium has been developed. This process has the potential to allow the realization of new devices which have been previously hampered by non-Ohmic contacts or dopant segregation problems. This work reported in this thesis also includes the design and fabrication of Ge/SiGe QCSE devices. Thin barrier QCSE designs have been put forward as a potential way to produce a more energy efficient modulator. Simulations of the devices show that a design with 16 nm Ge QWs and 8 nm SiGe barriers can provide effective modulation covering the entire optical communications C band with less than 3 V DC offset and achieve a contrast ratio across the band of over 3 dB. It was also shown that despite the thin barriers the wavefunctions remain well confined to the QWs suggesting that even thinner barriers are possible. MQW structures with thin barriers were grown and photodiodes fabricated from them. While the wafers did not have barriers as thin as designed they were thinner than devices previously demonstrated. From photocurrent measurements it was shown that these MQW structures were able to effectively modulate light near the 1550 nm wavelength with better performance than devices found in the literature

High-Efficiency Ge-on-Si SPADs for Short-Wave Infrared

High efficiency, Ge-on-Si single-photon avalanche diode (SPAD) detectors operating in the short-wave infrared region (1310 nm - 1550 nm) at near room temperature have the potential to be used for numerous emerging applications, including quantum communications, quantum imaging and eye-safe LIDAR applications. In this work, planar geometry Ge-on-Si SPAD designs demonstrate a significant decrease in the dark count rate compared to previous generations of Ge-on-Si detectors. 100 μm diameter microfabricated SPADs demonstrate record low NEPs of 2.2×10-16 WHz-1/2, and single-photon detection efficiencies of 18% for 1310 nm at 78 K. The devices demonstrate single-photon detection at temperatures up to 175 K

High performance planar germanium-on-silicon single-photon avalanche diode detectors

Single-photon detection has emerged as a method of choice for ultra-sensitive measurements of picosecond optical transients. In the short-wave infrared, semiconductor-based single-photon detectors typically exhibit relatively poor performance compared with all-silicon devices operating at shorter wavelengths. Here we show a new generation of planar germanium-on-silicon (Ge-on-Si) single-photon avalanche diode (SPAD) detectors for short-wave infrared operation. This planar geometry has enabled a significant step-change in performance, demonstrating single-photon detection efficiency of 38% at 125 K at a wavelength of 1310 nm, and a fifty-fold improvement in noise equivalent power compared with optimised mesa geometry SPADs. In comparison with InGaAs/InP devices, Ge-on-Si SPADs exhibit considerably reduced afterpulsing effects. These results, utilising the inexpensive Ge-on-Si platform, provide a route towards large arrays of efficient, high data rate Ge-on-Si SPADs for use in eye-safe automotive LIDAR and future quantum technology applications

Geiger Mode Ge-on-Si Single-Photon Avalanche Diode Detectors

High efficiency single photon avalanche detectors (SPADs) based on the Ge-on-Si material system are a promising emerging technology for high sensitivity optical detection in the short-wave infrared region. Here we demonstrate record single photon detection efficiencies of 38% at 1310nm with an operating temperature of 125K. This was achieved using a novel planar geometry which allowed us to achieve an NEPs of 3×10 −16 WHz −1/2 and reduced afterpulsing when compared to InGaAs/InP based SPADs operated in nominally identical conditions

Geiger Mode Ge-on-Si Single-Photon Avalanche Diode Detectors

Ge-on-Si single-photon avalanche diode (SPAD) detectors have demonstrated a high single-photon detection efficiency of 38% at a wavelength of 1310 nm when operated at a temperature of 125 K. These devices exhibit reduced afterpulsing compared to InGaAs/InP SPADs under nominally identical operating conditions

3D LIDAR imaging using Ge-on-Si single–photon avalanche diode detectors

We present a scanning light detection and ranging (LIDAR) system incorporating an individual Ge-on-Si single-photon avalanche diode (SPAD) detector for depth and intensity imaging in the short-wavelength infrared region. The time-correlated single-photon counting technique was used to determine the return photon time-of-flight for target depth information. In laboratory demonstrations, depth and intensity reconstructions were made of targets at short range, using advanced image processing algorithms tailored for the analysis of single–photon time-of-flight data. These laboratory measurements were used to predict the performance of the single-photon LIDAR system at longer ranges, providing estimations that sub-milliwatt average power levels would be required for kilometer range depth measurements

Variation of Sidewall Passivation on Sub-um Selectively Grown Ge-on-Si Devices Towards Single Photon Avalanche Detectors

Developing single photon avalanche diodes (SPADs) at short-wave infrared (SWIR) wavelengths beyond 1000 nm has attracted interest lately. Numerous quantum technology applications such as light detection and ranging (LIDAR), imaging through obscurants and quantum communications require sensitivity in this region. In quantum communications, operation at the telecoms wavelengths of 1310 nm and 1550 nm is essential. Ge-on-Si SPADs offer potential for lower afterpulsing and higher single photon detection efficiencies in the SWIR in comparison with InGaAs/InP SPADs, at a lower cost due to Si foundry compatibility. In this study, Ge-on-Si devices are fabricated on silicon-on-insulator (SOI) substrates, with a separate absorption, charge and multiplication layer (SACM) geometry and a lateral Si multiplication region. This Si foundry compatible process will allow for future integration with Si waveguides and optical fibres. The Ge is selectively grown inside sub-μm wide SiO2 trenches, reducing the threading dislocation in comparison with bulk Ge; a typical process for integrated Ge detectors. Here we deliberately exposed Ge sidewalls with an etch-back technique, to allow a passivation comparison not normally carried out in selectively grown devices planarised by chemical-mechanical polishing. Reduced dark currents are demonstrated using thermal GeO2 passivation in comparison to plasma-enhanced chemical-vapourdeposition SiO2. The improved passivation performance of GeO2 is verified by activation energy extraction and density of interface trap (Dit) calculations obtained from temperature-dependent capacitance-voltage (CV) and conductance-voltage (GV) measurements. This highlights the benefit of optimal surface passivation on sub-μm wide selectively grown Ge-on-SOI photodetector devices, potentially critical for waveguide integrated SPADs

Decoupling the dark count rate contributions in Ge-on-Si single photon avalanche diodes

Single Photon Avalanche Diodes (SPADs) are semiconductor devices capable of accurately timing the arrival of single photons of light. Previously, we have demonstrated a pseudo-planar Ge-on-Si SPAD that operates in the short-wave infrared, which can be compatible with Si foundry processing. Here, we investigate the pseudo-planar design with simulation and experiment to establish the spatial contributions to the dark-count rate, which will ultimately facilitate optimisation towards operation at temperatures compatible with Peltier cooler technologies

Simulation and Design Optimization of Germanium-on-Silicon Single Photon Avalanche Diodes

Single photon avalanche diodes (SPADs) are semiconductor photodiode detectors capable of detecting individual photons, typically with sub-ns precision timing. We have previously demonstrated novel pseudo-planar germanium-on-silicon SPADs with absorption into the short-wave infrared, which promise lower costs and potentially easier CMOS integration compared to III-V SPADs. Here we have simulated the dark count rate of these devices, using a custom solver for McIntyre’s avalanche model and a trap assisted tunnelling generation model. Calibration and fitting have been performed using experimental data and the results have highlighted areas in which the technology can be optimised