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In this paper, the influences of the relative intensity noise (RIN), in particular the relaxation oscillation noise, of an erbium distributed feedback (DFB) fibre laser has been investigated theoretically and experimentally for use a polarimetric ultrasonic sensor. We show that the relaxation oscillation noise and the ultrasonic signal both induce modulation sidebands on the polarization beat frequency of the DFB laser ultrasonic sensor. We present a novel demodulation algorithm for the polarimetric DFB fibre laser ultrasonic sensor, which can effectively eliminate the influences of the intensity noise and slowly varying drift of the polarization beat frequency.

The experimental proof-of-concept demonstration of an end-pumped Yb:KGW laser with tunable spectral and temporal output characteristics is reported. For the first time, tuning of both of these characteristics in a solid-state laser is achieved simultaneously using a single electrothermally-actuated MOEMS micromirror with controllable tilt angle in two dimensions. In continuous-wave mode, a wavelength tuning range of 22 nm with < 0.3 nm linewidth was achieved by using static MOEMS mirror tilt about one axis to select the intracavity wavelength dispersed by a prism. A wavelength tuning range of 15.3 nm with < 1.1 nm linewidth was achieved in Q-switched operation, by using simultaneous static tilt and resonant scanning of the MOEMS mirror about orthogonal axes. In this operational mode, a pulse repetition rate of 2.06 kHz was obtained with pulse durations varying from 460 ns to 740 ns over the addressable range of laser wavelengths.

Two MEMS thin film aluminium nitride ultrasound transducer array designs for photoacoustic sensing applications are presented. They are characterized using the pitch and catch technique and photoacoustic excitation. The results of both approaches are subsequently compared.

This paper introduces a directional MEMS microphone designed for hearing aid applications appropriate to low frequency hearing impairment, inspired by the hearing mechanism of a fly, the female Ormia ochracea. It uses both piezoelectric and capacitive sensing schemes. In order to obtain a high sensitivity at low frequency bands, the presented microphone is designed to have two resonance frequencies below the threshold of low frequency hearing loss at approximately 2 kHz. One is around 500 Hz and the other is slightly above 2 kHz. The novel dual sensing mechanism allows for optimization of the microphone sensitivity at both frequencies, with a maximum open-circuit (excluding pre-amplification) acoustic response captured via differential piezoelectric sensing at approximately – 46 dB (V) ref. 94 dB (SPL) at the resonance frequencies. The corresponding minimum detectable sound pressure level is just below -12 dB. The comb finger capacitive sensing was employed due to a lower electrical response generated from a ground referenced single-ended output by the piezoelectric sensing at the first resonance frequency compared to the second resonance frequency. The capacitive sensing mechanism, connected to a charge amplifier, generates a -28.4 dB (V) ref. 94 dB (SPL) acoustic response when the device is excited at either of the two resonance frequencies. Due to the asymmetric geometry and the 400 µm thick substrate, the device is predicted to perform as a bi-directional microphone below 3 kHz, which is shown by the measured directional polar patterns.

The influence of custom microphone housings on the acoustic directionality and frequency response of a multi - band bio - inspired MEMS microphone is presented. The 3.2mm by 1.7mm piezoelectric MEMS microphone, fabricated by a cost - effective multi - user process, has four frequency bands of operation below 10 kHz, with a desired first order directionality for all four bands. 7x 7x2.5 mm 3 3D - printed bespoke housings with varying acoustic access to the backside of the microphone membrane are investigated through simulation and experiment with respect to their influence on the directionality and frequency response to sound stimulus. Results show a clear link between directionality and acoustic access to the back cavity of the microphone. Further, there was a change in direction of the first order directionality with reduced height in this back cavity acoustic access. The required con figuration for creating an identical directionality for all four frequency bands is investigated along with the influence of reducing the symmetry of the acoustic back cavity access. This work highlights the overall requirement of considering housing geome tries and their influence on acoustic behavior for bio - inspired directional microphones

Directional MEMS microphones inspired by the parasitoid fly Ormia ochracea have been studied since the discovery that the micro-scale tympana structure of this female fly can amplify and locate narrow band mating calls from its host. This presentation will concentrate on the first piezoelectric Ormia-inspired MEMS microphone that operates in a low range of frequency bands overlapping with human vocal frequencies, and as such is suitable for hearing aid applications. Including two plates performing as Ormia’s two tympana, the entire region in motion in our microphone is about 3.2 mm × 1.42 mm × 10 μm. Compared to other piezoelectric Ormia-inspired designs, our design transfers the working frequency band from over 10 kHz to below 3 kHz due to its asymmetric structure and an S-type rotational cantilever. Furthermore, it provides a unidirectional response around two resonance frequencies below 3 kHz. The open-circuit acoustic response of the device is approximately 3.9 mV/Pa at 464 Hz which is close to human vocal frequencies, with a maximum value of 9.9 mV/Pa at 2275 Hz, which is near the frequency region where the human auditory system is most sensitive. The new microphone, coupled with a custom-built preamplifier, has a noise floor of 10 μV/√Hz at 1 kHz.