Photonic processing for wideband cancellation and spectral discrimination of RF signals

Photonic signal processing is used to implement common mode signal cancellation across a very wide bandwidth utilising phase modulation of radio frequency (RF) signals onto a narrow linewidth laser carrier. RF spectra were observed using narrow-band, tunable optical filtering using a scanning Fabry Perot etalon. Thus functions conventionally performed using digital signal processing techniques in the electronic domain have been replaced by analog techniques in the photonic domain. This technique was able to observe simultaneous cancellation of signals across a bandwidth of 1400 MHz, limited only by the free spectral range of the etalon

Laser detection using liquid crystal polarization modulators

Lasers can be identified by their relatively long coherence lengths using interferometry. A Mach-Zehnder interferometer incorporating liquid crystal polarization modulators is demonstrated as a means of low-cost, robust laser detection. Temporal modulations, as a signature of coherence, can be induced by modulating polarization changes in liquid crystal modulators using low voltages. Sensitivities of <10 nW can be achieved. The suitability as a means of laser detection is discussed

Developing concurrent coding:An unconventional encoding scheme applied to visible light communications

An unconventional encoding scheme called concurrent coding has recently been demonstrated and shown to offer interesting features and benefits in comparison to conventional techniques, e.g., robustness against burst errors and improved efficiency of transmitted power. This concept has been demonstrated for the first time with optical communications, where standard light-emitting diodes have been used to transmit information encoded with concurrent coding. The technique successfully transmits and decodes data despite unpredictable interruptions to the transmission causing significant dropouts to the detected signal. The technique also shows how it is possible to send a single block of data in isolation with no presynchronization required between transmitter and receiver, and no specific synchronization sequence appended to the transmission. Our work also demonstrates the successful use of multithreaded (overlaid) concurrent codes for the first time

Technique for passive scene imaging of gas and vapor plumes using transmission-waveband modulation

A new approach to locating gas and vapor plumes is proposed that is entirely passive. By modulating the transmission waveband of a narrow-band filter, an intensity modulation is established that allows regions of an image to be identified as containing a specific gas with absorption characteristics aligned with the filter. A system built from readily available components was constructed to identify regions of NO. Initial results show that this technique was able to distinguish an absorption cell containing NO gas in a test scene

A new encoding scheme for visible light communications with applications to mobile connections

A new, novel and unconventional encoding scheme called concurrent coding, has recently been demonstrated and shown to offer interesting features and benefits in comparison to conventional techniques, such as robustness against burst errors and improved efficiency of transmitted power. Free space optical communications can suffer particularly from issues of alignment which requires stable, fixed links to be established and beam wander which can interrupt communications. Concurrent coding has the potential to help ease these difficulties and enable mobile, flexible optical communications to be implemented through the use of a source encoding technique. This concept has been applied for the first time to optical communications where standard light emitting diodes (LEDs) have been used to transmit information encoded with concurrent coding. The technique successfully transmits and decodes data despite unpredictable interruptions to the transmission causing significant drop-outs to the detected signal. The technique also shows how it is possible to send a single block of data in isolation with no pre-synchronisation required between transmitter and receiver, and no specific synchronisation sequence appended to the transmission. Such systems are robust against interference -- intentional or otherwise -- as well as intermittent beam blockage

Laser detection utilizing coherence

Lasers are an unnatural occurrence, rendered almost impossible in nature due to the laws of thermodynamics. Thus, the presence of laser radiation is always accompanied by an intent for that laser such as sensing, targeting, range finding etc. Detection of laser radiation is therefore important as it may be a precursor to impending action. Laser warning receivers have been around for decades and have been aligned with the type of laser threat. In the last few years new threats have appeared in the form of low-cost diode lasers with dangerously high power levels (several Watts for a few hundred US dollars) and an ever expanding range of wavelengths. Protecting against such threats requires its detection, analysis and classification. In this paper we will discuss the types of technologies that have been used to detect lasers and the properties they can discern. We then focus on the developments in the detection of coherence properties and its ability to detect weak continuous wave (CW) laser sources

Photoacoustic spectroscopy for remote detection of liquid contamination

The remote detection and identification of liquid chemical contamination is a difficult problem for which no satisfactory solution has yet been found. We have investigated a new technique, pulsed indirect photoacoustic spectroscopy (PIPAS), and made an assessment of its potential for operation at stand-off ranges of order 10m. The method involves optical excitation of the liquid surface with a pulsed laser operating in the 9-11μm region. Pulse lengths are of order 3μs, with energy ∼300μJ and repetition rates ∼200Hz. Rapid heating of the liquid by the laser pulse produces acoustic emission at the surface, and this is detected by a sensitive directional microphone to increase the signal-to-noise ratio and reduce background clutter. The acoustic pulse strength is related to the liquid's absorption coefficient at the laser wavelength; tuning allows spectroscopic investigation and a means of chemical identification. Maximum coverage rates have been examined, and further experiments have examined the specificity of the technique, allowing a preliminary assessment of false-alarm and missed-signal rates. The practical aspects of applying the technique in a field environment have been assessed

Temporal and spectral dispersion of an optical source using a micromirror array-based streak camera

The digital micromirror device (DMD) is an array of tilting micromirrors, capable of high frame rates that are made possible by rapid changes of angular state. The rapid angular change of the mirrors is used to sweep an optical signal across a camera sensor, resulting in a version of a streak camera, capable of temporal dispersion of an optical signal. Using a single pixel or single line of pixels can produce a continuous temporal track, whereas a two-dimensional array of mirrors scans an intensity envelope across discrete diffraction orders. Temporal resolutions of 10nS have been achieved and used to measure laser pulse widths. Combining with a diffraction grating oriented orthogonally to the temporal dispersion enables temporal and spectral dispersion to be obtained simultaneously

Microwave properties of an inhomogeneous optically illuminated plasma in a microstrip gap

The optical illumination of a microstrip gap on a thick semiconductor substrate creates an inhomogeneous electron-hole plasma in the gap region. This allows the study of the propagation mechanism through the plasma region. This paper uses a multilayer plasma model to explain the origin of high losses in such structures. Measured results are shown up to 50 GHz and show good agreement with the simulated multilayer model. The model also allows the estimation of certain key parameters of the plasma, such as carrier density and diffusion length, which are difficult to measure by direct means. The detailed model validation performed here will enable the design of more complex microwave structures based on this architecture. While this paper focuses on monocrystalline silicon as the substrate, the model is easily adaptable to other semiconductor materials such as GaAs