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We propose a novel scheme for the expansion and comb densification of gain-switched optical frequency combs (GS-OFC). The technique entails mutual injection locking of two gain-switched lasers with a common master to generate a wider bandwidth OFC. Subsequently, the OFC is further expanded and/or densified using a phase modulator with optimum drive conditions. We experimentally demonstrate the generation of an OFC with 45 highly correlated lines separated by 6.25 GHz with an expansion factor ~3. In addition, operating in comb densification mode, the channel spacing of the OFC is tuned from 6.25 GHz to 390.625 MHz. Finally, a detailed characterization of the lines, across the entire expanded comb, is reported highlighting the excellent spectral purity with linewidths of ~40 kHz, a relative intensity noise better than −152 dB/Hz, and a high degree of phase correlation between the comb lines. The proposed method is simple, highly flexible and the architecture is suitable for photonic integration, all of which make such an OFC extremely attractive for the employment in a multitude of applications.

This paper contains two main areas of research: First, this work outlines a novel, highly sensitive strain sensor design that should support various levels of deformation, depending on the substrate type used. Physical implementations in this work have focused on proving its large deformation capabilities, and simulations have been used to assess its more general electromagnetic response. The other part of this paper focusses on exploring other effects that will impact the sensing of strain of resolutions below 10 με, which is a capability achieved by other aerospace-grade strain sensor technologies. These effects are limited to mechanical swelling and sensor orientation in the azimuth and elevation planes, as these appear to be unexplored and highly relevant issues to the topic of chipless RFID-based strain sensing. From this exploration, it is apparent that the effects of mechanical swelling and sensor orientation (amongst others) will need to be addressed in any real-life implementation of the sensor, requiring a strain resolution below 10 με.

A custom-designed gain-switched frequency comb (GSFC) source was passively coupled to a medium finesse (F &asymp, 522) cavity in off-axis configuration for the detection of ammonia (14NH3) in static dry air. The absorption of ammonia was detected in the near infrared spectral region between 6604 and 6607 cm&minus, 1 using a Fourier transform detection scheme. More than 30 lines of the GSFC output (free spectral range 2.5 GHz) overlapped with the strongest ro-vibrational ammonia absorption features in that spectral region. With the cavity in off-axis configuration, an NH3 detection limit of &sim, 3.7 ppmv in 20 s was accomplished in a laboratory environment. The experimental performance of the prototype spectrometer was characterized, advantages, drawbacks and the potential for future applications are discussed.

Chipless Radio Frequency Identification (RFID) has been used in a variety of remote sensing applications and is currently a hot research topic. To date, there have been a large number of chipless RFID tags developed in both academia and in industry that boast a large variation in design characteristics. This review paper sets out to discuss the various design aspects needed in a chipless RFID sensor. Such aspects include: (1) Addressing strategies to allow for unique identification of the tag, (2) Sensing mechanisms used to allow for impedance-based response signal modulation and (3) Sensing materials to introduce the desired impedance change when under the influence of the target stimulus. From the tabular comparison of the various sensing and addressing techniques, it is concluded that although many sensors provide adequate performance characteristics, more work is needed to ensure that this technology is capable/robust enough to operate in many of the applications it has been earmarked for.