Silicon photonic devices with either silicon or silicon nitride waveguides have increasingly been used in many applications besides communications, especially as sensors in label-free biosensing, where guided light signals are affected by biorecognition molecules immobilized on the surface. The coating of protein (i.e., bioreceptors) by biochemical process on the waveguide surface is a crucial step in creating a functionalized device that can be used for biosensing. As a conventional method that uses 3-aminopropryltriethoxysilane (APTES) and glutaraldehyde (GA), the APTES-GA method has the limitation of using a GA crosslink, of which the two functional groups can bind to nonspecific proteins, causing irregular binding. In this study, we proposed a new coating technique to avoid such problem by applying APTES silanization with 1-ethyl-3-(3-dimethyl aminopropyl)-carbodiimide (EDC)-N-hydroxysuccinimide (NHS) protein crosslink, denoted by the APTES-(EDC/NHS) method. The EDC/NHS reaction was shown to be able to immobilize protein in ordered orientation due to consistent arrangement between a carboxylic group of protein molecules and an amine group of covalent-linked APTES on surface. By applying APTES silanization, we circumvented the use of hazardous cleaning agent in the conventional EDC/NHS technique. Several surface characterization techniques were carried out to assess and compare the two biocoating techniques, including scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), spectroscopic ellipsometry (SE), and atomic force microscopy (AFM). On silicon, the results of antihuman TNF-alpha antibody coating showed that the proposed APTES-(EDC/NHS) technique has better repeatability in terms of less roughness of the coated protein at 1.5 nm compared with 6.3 nm, due to the ordered arrangement of coated antibody molecules. On a silicon nitride waveguide device, the proposed APTES-(EDC/NHS) technique exhibits dense antibody immobilization on a waveguide in SEM images due to stable amide bond formation via EDC/NHS crosslink mechanism. The specificity of the immobilized antibodies was confirmed by enzyme-linked immunosorbent assays (ELISA), with an average optical density at 450 nm of 0.175 ± 0.01 compared with 0.064 ± 0.009 of negative control. The proposed technique also reduced the overall process time since proteins are crosslinked to the silanized waveguide surface in a single step.

Millimeter wave fixed wireless systems in future backhaul and access network applications can be affected by weather conditions. The losses caused by rain attenuation and antenna misalignment due to wind-induced vibrations have greater impacts on the link budget reduction at E-band frequencies and higher. The current International Telecommunications Union Radiocommunication Sector (ITU-R) recommendation has been widely used to estimate rain attenuation, and the recent Asia Pacific Telecommunity (APT) report provides the model to estimate the wind-induced attenuation. This article provides the first experimental study of the combined rain and wind effects in a tropical location using both models at a frequency in the E band (74.625 GHz) and a short distance of 150 m. In addition to using wind speeds for attenuation estimation, the setup also provides direct antenna inclination angle measurements using the accelerometer data. This solves the limitation of relying on the wind speed since the wind-induced loss is dependent on the inclination direction. The results show that the current ITU-R model can be used to estimate the attenuation of a short fixed wireless link under heavy rain, and the addition of wind attenuation via the APT model can estimate the worst-case link budget during high wind speeds.

We present a method to achieve vector modulation of lightwave using an electroabsorption modulator intregrated laser. The laser section is used for direct phase modulation followed by amplitude modulation of the electroabsorption modulator section in a cascade manner, which is an alternative method to conventional IQ modulator. To allow improved phase demodulation, we propose a digital pre-compensation technique for a transmitter in optical coherent system for the first time, which includes phase-balanced differential phase shift keying encoding and digital domain pre-equalization filtering. The phase-balanced encoding technique applies parallel DC-balance coding with phase-balanced symbol mapping, which results in bounded cumulative phase and constant statistical characteristic of the modulating signal in differential M-ary phase shift keying (M-PSK) and M-ary quadrature amplitude modulation (M-QAM) formats. The digital pre-equalization filter also compensates for the phase response of the laser section. Experimental results show that the pre-digital compensation technique allows the improved complex signal demodulation compared to conventional DQPSK modulation in a back-to-back setup.

We demonstrate frequency response measurement technique of photodiodes using standard Mach-Zehnder Modulator (MZM). The technique is based on heterodyne detection principle where two-tone light, whose frequency separation is in Microwave frequency range, is used to stimulate the photodiode. The MZM, with typical on-off extinction ratio of 30 dB in optical domain, is used to generate two-tone light, instead of 50 dB extinction ratio of the previously proposed high ER MZM system. The converted RF signal shows extinction ratio of 40 dB, thus accurate power of the required component can be measured using a wideband RF power sensor. The measured frequency responses of two tested photodiodes indicate agreement with their specifications.

We propose a novel method to measure chirp of an electro-absorption modulator at any frequency using optical homodyne quadrature demodulation technique. Chirp parameters are directly obtained in the small-signal vector space, enabling practical chirp assessment for advanced modulation optical transmission.

We demonstrate that our recently proposed chirp measurement technique can be applied to the measurement and visualization of the small-signal dynamic chirp characteristics of modulators in vector space for the first time. Both discrete modulators and those integrated in externally modulated laser modules can be characterized by this method. The results agree with existing method to the first order approximation, whilst dynamic chirp change can also be measured during amplitude transition.