Photonic Time-Stretch Enabled High Throughput Microwave and MM-Wave Interferometry Applied to Fibre Grating Sensors and Non-Contact Measurement

The research presented in this thesis is focused towards developing real-time, high-speed applications, employing ultrafast optical microwave generation and characterisation techniques. This thesis presents a series of experiments wherein mode-locked laser pulses are utilised. Photonics-based microwave and MM-Wave generation and detection are explored and employed for applications pertaining to fibre grating sensors and non-contact measurement. The application concepts leverage techniques from optical coherence tomography and non-destructive evaluation of turbid media. In particular, I use the principle of dispersion-induced photonic Time-Stretch to slow down high-speed waveforms to speeds usable by state-of-the-art photo-detectors and digital signal processors. The concept of photonic time-stretch is applied to map instantaneous microwave frequency to the time instant of the signal, which in turn is related to spatial location as established by the space-wavelength-time conversions. The experimental methods applied throughout this thesis is based upon Michelson interferometer architecture. My original contribution to knowledge is the realisation of Photonics-based, single tone, and chirped microwave and MM-Wave pulse generation applied to deciphering physical strain profile along the length of a chirped fibre Bragg grating employed in a Michelson interferometer configuration. This interrogation scheme allows intra-grating high-resolution, high-speed, and temperature independent strain measurement. This concept is further extended to utilise photonic generation of microwave pulses to characterise surface profile information of thin film and thin plate infrared transparent slides of variable thickness setup in a Michelson interferometer architecture. The method basis for photonically generated high-frequency microwave signals utilises the principle of photonic Time-Stretch. The research was conducted in the Photonics Lab at the University of Kent. In addition, the photonically generated microwave/ MM-Wave pulses is utilised as a potential broadband frequency-swept source for non-contact measurement of turbid media. Investigation of the proof-of-concept based on an MM-Wave coherence tomography set-up is implemented at Vrije Universiteit Brussel (VUB), Department of Electronics and Informatics (ETRO)

Analysis on spatial and temporal resolution in photonic time stretch frequency domain reflectometry based fully distributed fiber Bragg grating sensors

We propose and experimentally demonstrate fully distributed strain sensing along the length of a linearly chirped fibre Bragg grating with simultaneously high spatial and temporal resolution based on photonic time stretch frequency domain reflectometry (PTS-FDR). The distributed strain information is reconstructed from the instantaneous RF frequency of a temporal interference waveform via short-time Fourier transform analysis. The utility of the proposed method is characterized by analysing the spatial and temporal resolution obtained. An ultrafast strain measurement at a speed of 50 MHz with a high spatial resolution of 31.5 µm over a gauge length of 25 mm and a strain resolution of 9.1 µ? have been achieved

High spatial and temporal resolution interrogation of fully distributed chirped fiber Bragg grating sensors

A novel interrogation technique for fully distributed linearly chirped fiber Bragg grating (LCFBG) strain sensors with simultaneous high temporal and spatial resolution based on optical time-stretch frequency-domain reflectometry (OTS-FDR) is proposed and experimentally demonstrated. LCFBGs is a promising candidate for fully distributed sensors thanks to its longer grating length and broader reflection bandwidth compared to normal uniform FBGs. In the proposed system, two identical LCFBGs are employed in a Michelson interferometer setup with one grating serving as the reference grating whereas the other serving as the sensing element. Broadband spectral interferogram is formed and the strain information is encoded into the wavelength-dependent free spectral range (FSR). Ultrafast interrogation is achieved based on dispersion-induced time stretch such that the target spectral interferogram is mapped to a temporal interference waveform that can be captured in real-Time using a single-pixel photodector. The distributed strain along the sensing grating can be reconstructed from the instantaneous RF frequency of the captured waveform. High-spatial resolution is also obtained due to high-speed data acquisition. In a proof-of-concept experiment, ultrafast real-Time interrogation of fully-distributed grating sensors with various strain distributions is experimentally demonstrated. An ultrarapid measurement speed of 50 MHz with a high spatial resolution of 31.5 μm over a gauge length of 25 mm and a strain resolution of 9.1 μϵ have been achieved

Ultrafast Interrogation of Fully Distributed Chirped Fibre Bragg Grating Strain Sensor

A novel ultrafast and high spatial-resolution interrogation method for fully distributed chirped fibre Bragg grating sensors based on photonic time-stretch frequency-domain reflectometry is presented. Real-time interrogation at measurement speed of 50 MHz with a spatial resolution of 35 µm was experimentally demonstrated

Letter to the Editor

Background Disaster, whether man made or natural, may occur at any place or time. This study was conducted to assess the preparedness of hospitals in handling emergencies as per District Disaster Management Plan (DDMP) at Mangalore, a coastal city on the Western coast of Karnataka. Method A cross sectional study was conducted in 12 hospitals of Mangalore city, located at the Southwestern coast of India in April 2009, using a semi-structured proforma. All surveyed hospitals were included in the DDMP. The respondents were hospital administrators. Results Though all the hospitals surveyed were aware about the existence of DDMP in the district of Dakshina Kannada, 6 (50%) were unaware that their hospitals were included in the same plan. Out of 12 hospitals, 4 (33.3%) said that they had got a letter from DDMP, spelling out their responsibilities. Only 6 (50%) hospitals had a contingency plan for emergency. Mock drill was conducted only by 6 (50%) hospitals. Six (50%) hospitals had blood bank, 5 (41.6%) had trauma center and 8 (66.6%) had burns ward available for emergency. Half of them had more than 2 ambulances and 10(83.3%) had sufficient stock of medicines. Extra beds for emergency were available in 11(91.7%) hospitals with maximum number of 42 beds in one hospital. Conclusion; Most hospitals in Mangalore were not well prepared to manage emergencies in disasters. Facilities like burns ward, blood bank and ambulance services need to be enhanced

Broadband frequency swept millimeter-wave source based on cascaded temporal optical pulse shaping

A new approach to generate high-quality broadband frequency swept millimeter-wave (mm-wave) waveforms based on photonics-assisted frequency up conversion of a low-frequency chirped drive signal has been proposed. This is made possible by using an unbalanced temporal pulse shaping system involving two cascaded Mach-Zehnder modulators both biased at the minimum transmission points. The proposed approach is verified by simulations. A frequency swept mm-wave pulse with its instantaneous frequency spanning from 40 to 100 GHz and a chirp rate of 66.7 GHz/ns is generated thanks to 40 times frequency up conversion. Two-fold improvement on the time-bandwidth product of the generated waveform is also achieved thanks to the cascaded modulation scheme

Data-efficient high-throughput fiber Bragg grating sensors using photonic time-stretch compressive sensing

In this paper, we demonstrate the first application of photonic compressive sensing technique in a data-efficient interrogation system for high-throughput distributed FBG sensors. In particular, reconstruction of a wide bandwidth chirped temporal waveform has been achieved using compressive sensing with optical integration. This enables data-compressed high-throughput interrogation of FBG sensors for dynamic non-uniform strain sensin

All-optical random sequence generation for compressive sensing detection of RF signals

Photonic compressive sensing is a promising data compression method and has been successfully applied in high-speed RF signal detection with greatly reduced requirement for receiver bandwidth. A key challenge is due to the electronic bottleneck in high-speed random sequence generation and mixing. In this work, we propose and experimentally demonstrated for the first time all-optical random sequence generation and mixing for compressive sensing detection of RF signals. The technique is based on photonic time stretch involving cascaded Mach-Zehnder Interferometers (MZIs) for spectral domain random mixing. In a proof-of-concept experiment, successful detection of 1 GHz RF signal with 25% compression ratio using only 50 MHz detection bandwidth has been demonstrated