This paper has been written as a key note address and backdrop to the 2005 SPIE Technologies for Optical Countermeasures II Conference. The paper uses as a topic the problem of protecting civil aircraft from the Man Portable Air Defence missile Systems (MANPADS). The paper examines the economic background of the airline industry and the effects such a successful attack could have. It then addresses the various motives, means, and opportunities that the terrorists have to use MANPADS to progress attacks against civil aircraft. In reviewing the various mitigation options available to defeat or deny MANPAD engagements, the paper identifies key technology areas available for exploitation. It then focuses on the optical countermeasure technologies used in providing aircraft platform self protection. Finally, the paper summarises and concludes that whilst a lot has and can be done to militate against the MANPAD threat there is not yet an exportable, affordable and robust countermeasures technology for large scale commercial systems and operations.

In the whole scope of developing a countermeasure system against infrared guided missiles, the development of adequate laser sources is one of many aspects, however certainly the most demanding. The article describes from a directed infrared countermeasure (DIRCM) system aspect the functional and operational basics relevant for countering strategies. DIRCM techniques can be categorized into jamming and damaging countermeasure techniques. The resulting requirements for the countermeasure laser sources are briefly explained. Jam laser source types with advantages and drawbacks as well as possible damage laser sources, including the estimation of maturity and their specific features, are described. The FLASH countermeasure demonstrator was developed to validate the concept of having a closed loop jamming and damaging system. The laser setup for the DIRCM demonstrator FLASH is briefly described and the conducted tests including laser performance are explained.

The area of laser sensing is vast. This review will be limited to some examples of laser sensing of relevance for defense and security applications especially those of relevance to countermeasures. Examples from the areas ISR (Intelligence, Surveillance and Reconnaissance), target acquisition/recognition, warning/protection and some miscellaneous applications such as positioning, tagging, communication and weapon guidance will be given. The laser sensor can itself have a multifunction capability and will probably not be the only sensor in a system. Future possibilities of powerful multisensor systems using new device technology will be discussed.

Nonlinear optical conversion of 500 mJ pulses from a Nd:YAG laser to the mid-infrared is demonstrated in a two-step architecture. Using a type 2 phase matched KTiOPO₄-based master-oscillator/power-amplifier (MOPA) architecture for conversion to 2 μm, 140 mJ signal at 2.08 μm with M² = 2.3 and 80 mJ idler at 2.18 μm were obtained. Using 58 mJ of the signal beam to pump a ZnGeP₂-based MOPA, we have obtained 21 mJ in the 3-5 μm range with M² ≈ 15.

Efficient generation with good beam quality and high output power in the 3-5 μm wavelength range is desired and a 2 μm-pumped Optical Parametric Oscillator is a promising design for this purpose. The low quantum defect of a resonant laser pumping of the Ho ⁵I₇ manifold using 1.9 μm radiation leads to an efficient 2.09 μm laser. Due to a long fluorescence lifetime and a good energy storage capability, the system is well-suited for either increased average power at high repetition rates (more than tens of kHz) or high energy output at low repetition rates (several tens of Hz). For both repetition rate ranges, we propose to evaluate critical components of the system such as the Tm pump laser source. Performances of a Ho:YAG laser pumped either by a 50 W Tm:fiber laser or by a 20 W Tm:YLF laser are compared. Efficiencies of the modulation devices are reported for an AOM and a RTP Q-switch cell.

Adaptive Optics (AO) is a critical underpinning technology for future optical countermeasures, laser delivery, target illumination and imaging systems. It measures and compensates for optical distortion caused by transmission through the atmosphere, resulting in the ability to deploy smaller lasers and identify targets at greater ranges. AO is also well established in ground based astronomy, and is finding applications in free space optical communications and ophthalmology. One of the key components in an AO system is the wavefront modifier, which acts on the incoming or outgoing beam to counter the effects of the atmosphere. BAE SYSTEMS ATC is developing multi-element Deformable Bimorph Mirrors (DBMs) for such applications. A traditional bimorph deformable mirror uses a set of edge electrodes outside the active area in order to meet the required boundary conditions for the active aperture. This inflicts a significant penalty in terms of bandwidth, which is inversely proportional to the square of the full mirror diameter. We have devised a number of novel mounting arrangements that reduce dead space and thus provide a much improved trade-off between bandwidth and stroke. These schemes include a novel method for providing vertical displacement at the periphery of the aperture, a method for providing a continuous compliant support underneath the bimorph mirror, and a method for providing a three point support underneath the bimorph. In all three cases, there is no requirement for edge electrodes to provide the boundary conditions, resulting in devices of much higher bandwidth. The target is to broaden the use of these types of mirror beyond the current limits of either low order/low bandwidth, to address the high order, high bandwidth systems required by long range, horizontal path applications. This paper will discuss the different mirror designs, and present experimental results for the most recently assembled mirrors.

Laser device technology has developed rapidly in the laser few years and it is used in many applications spanning the civil and military sectors. The military operational use of laser technology has been responsible for highly significant improvements in many weapon systems, such as guidance accuracy. Recent developments in solid-state laser technology, in terms of its efficiency and size, have lead to an interest in the use of this technology for countermeasure applications. This paper considers the current status of laser technology and its potential application to countermeasure systems. This review outlines approaches to defining a laser-based system so that the device is compatible with the whole system.

Specific developments on technological steps involved in the fabrication of antimony based diode lasers have been carried out. Evident improvements of the performances have been achieved. High power, low threshold, room temperature (RT) and continuous wave (CW) operation laser diodes emitting around 2.4μm, based on the InGaAsSb / AlGaAsSb materials system, have been realised. with a 100μm wide and 1mm long cavity laser, a maximum output power of 720mW has been obtained at 285K.

We have developed high-power lasers, which are based on an Al-free active region at 915 nm. The laser structure has very low internal losses of 0.5 cm^-1, a very low transparency current density of 86 A/cm², and a high internal quantum efficiency of 86%. Based on these good results, we have realised narrow-aperture, index-guided tapered lasers which deliver 1 W CW with and M² beam quality factor of 3.0 using both the 1/e² and standard-deviation methods. We have also fabricated index-guided tapered lasers with a Clarinet shape, which deliver 0.65 W CW with an M² beam quality factor of less than 1.5 at 1/e², and less than 2.5 using the standard deviation method.

This paper studies the Man-Portable Air Defence System (MANPADS) threat against large commercial aircraft using flight profile analysis, engagement modelling and simulation. Non-countermeasure equipped commercial aircraft are at risk during approach and departure due to the large areas around airports that would need to be secured to prevent the use of highly portable and concealable MANPADs. A software model (CounterSim) has been developed and was used to simulate an SA-7b and large commercial aircraft engagement. The results of this simulation have found that the threat was lessened when a escort fighter aircraft is flown in the 'Centreline Low' position, or 25 m rearward from the large aircraft and 15 m lower, similar to the Air-to-Air refuelling position. In the model a large aircraft on approach had a 50% chance of being hit or having a near miss (within 20m) whereas escorted by a countermeasure equipped F-16 in the 'Centerline Low' position, this was reduced to only 14%. Departure is a particularly vulnerable time for large aircraft due to slow climb rates and the inability to fly evasive manoeuvres. The 'Centreline Low' escorted departure greatly reduced the threat to 16% hit or near miss from 62% for an unescorted heavy aircraft. Overall the CounterSim modelling has showed that escorting a civilian aircraft on approach and departure can reduce the MANPAD threat by 3 to 4 times.

Countermeasures consisting of obscurants and decoys can be used separately or in combination in attempting to defeat an attack on an Armoured Fighting Vehicle (AFV) by an IR SACLOS missile system. The engagement can occur over a wide range of conditions of wind speed, wind direction and the AFV route relative to the SACLOS firing post. The countermeasures need to be evaluated over the full set of conditions. Simulation with a man in the loop can be expensive and very time consuming. Without using a man in the loop, a fully computer based simulation can be used to identify the scenarios in which defeat of the SACLOS system may be possible. These instances can be examined in more detail using the same simulation application or by using the conditions in a more detailed modelling and simulation facility. An IR imaging tracker is used instead of the man in the loop to simulate the SACLOS operator. The missile is guided onto the target by either the clear view of the AFV or by the AFV position predicted by the tracker while the AFV is obscured. The modelled scenarios feature a typical AFV modelled as a 3D object with a nominal 8 -12 μm signature. The modelled obscurant munitions are hypothetical but based on achievable designs based on current obscurant material performance and dissemination methods. Some general results and conclusions about the method are presented with a view of further work and the use of decoys with the obscurant to present a reappearing alternative target.

Understanding and predicting laser beam propagation effects in the atmosphere is of importance for laser countermeasures and related applications. Turbulence effects cause beam wander, beam broadening and intensity scintillations reducing e.g. the power in bucket and the tracking accuracy. Modelling laser beam propagation in turbulence using successive phase screens provides an efficient tool for performance predictions. In this work phase screens are used to model laser beam propagation over land and sea. Different phase screens generators utilising the Kolmogorov or von Karman spectra were considered. Critical parameters using phase screens include the number of screen applied along the propagation path, inner- and outer scale size, variations in the structure parameter and spatial frequencies. Effects such as beam wander, angle-of-arrival fluctuations and intensity scintillations are discussed. The simulated results are compared with experimental data recorded at different ranges, various turbulence strengths and for single- and double paths. A generic example describing laser countermeasure against an infrared homing missile in a naval scenario is presented.

Cryptography in communications is of increasing importance and several sophisticated cryptographic algorithms have been developed. Quantum mechanical principles have been employed yielding cryptographic keys to generate ciphertext, especially applicable to optical communications, and which are believed to be unbreakable. In optical communications, the quantum ciphertext is the bit-by-bit exclusive-Or (XOR) operation of the quantum key and the text. Thus, an all-optical XOR gate is required at the transmitter, which for compactness and efficiency it should be simple and monolithically integrable. In this paper we review the quantum ciphertext generation, and we present a novel XOR gate that requires a single wavelength, in contrast to other solutions that require additional optical signals for control and synchronization. Simulation results support the viability of our novel all-optical XOR gate at data rates up to 40 Gbps.

An original compact test method of studying the influence of a modified Nd:YAG laser beam irradiation at the unusual wavelength of λ = 1.3 μm on IR-Ge-windows is investigated: optical parameters such as transmission loss, surface temperature during the laser irradiation, morphological deformation, and damage thresholds are measured in real time and compared with theoretical simulations. To study the thermal-mechanical relationship of the laser-matter interaction, an original pyrometer array is developed for the temperature-profile measurement and an original deformation experimental set-up, including a "line generator", is introduced. The damage behaviour of germanium at the wavelength of λ = 1.3 μm is also presented in this paper.

A numeric model has been developed to identify the critical components and parameters in improving the output beam quality of a flashlamp pumped Q-switched Nd:YAG laser with a folded Porro-prism resonator and polarization output coupling. The heating of the laser material and accompanying thermo-optical effects are calculated using the finite element partial differential equations package FEMLAB allowing arbitrary geometries and time distributions. The laser gain and the cavity are modeled with the physical optics simulation code GLAD including effects such as gain profile, thermal lensing and stress-induced birefringence, the Pockels cell rise-time and component aberrations. The model is intended to optimize the pumping process of an OPO providing radiation to be used for ranging, imaging or optical countermeasures.

Laser labelling inside glass induces micro-cracks by high energy densities in the focus. The micro-cracks reduce the mechanical stability of glass. Light scattering allows the observer to perceive the cracks as white pixels. Coloured marking of glass in this manner is not possible. Coloured marking inside glass by changing the oxidation state of the metal ions locally in the focus does not weaken the mechanical properties of the glass. Two kind of glass systems, lime-natron-silicate and borosilicate with 0.5 % mass-content of doping are investigated. The simultaneous presence of donators and acceptors allows a transition of electrons between polyvalent ions, and can lead to permanent colour-centres inside the glass, due to the fact that the absorption of the polyvalent ions is changed by the laser-induced conversion process. For this purpose a 3 ω Nd:YAG (wavelength λ_L = 355 nm, pulse duration t = 10 to 80 ns) and a Ti:Sapphire solid-state laser (wavelength λ_L = 810 nm, pulse duration t = 200 fs) are used. The radiation parameters and the chemical composition of the glass (mainly doping) are the dominant factors to generate coloured marking. The transmittance as a function of the fluence and the change of the absorption coefficient is measured and gives a statement of the colourshade. Further the difference between lime-natron-silicate and borosilicate glass (same doping variety) is examined. Actually mauve, yellow, red-brown an grey colouring can be produced. Cracks in the microstructure of glass can also be the cause for brown colour-centres generating.

We present an exact treatment of wave propagation in some inhomogeneous thin films with highly space-dependent dielectric constant. It is based on a space transformation which replaces the physical space by the optical path. In the new space, the dispersion equation is that of a normal progressive wave. We will show that the dispersion properties of such films are plasma- or waveguide-like, the characteristic frequency being determined by the spatial characteristics of the dielectric constant's variations only. The theory is scalable, so that it can be applied in any wavelength range: optical, IR, radiofrequency, etc. depending only on the characteristic space scales. Several applications will be presented, concerning the reflection properties of such films (broadband anti-reflection, or dichroic coatings) or to the propagation and transmission through the film. We will show that depending on the type of space dependence, an incident wave can either propagate or tunnel through such films. We will investigate the behaviour of the light group-velocity and tunneling time inside or through such films. Though we can reproduce the phase-time saturation corresponding to the Hartman effect, analysis of the group velocity in the tunneling case shows no sign of superluminal propagation. A strong frequency dependence can be obtained in some situations, which allows to anticipate a strong reshaping of brodband laser pulses.

Modern table-top laser systems are capable of generating ultrashort optical pulses with sufficiently high intensity to induce nonlinear optical effects in many of the materials that are used in the construction of optical components. In this paper we discuss the interaction of such pulses with two types of dielectric filters: (a) dielectric stacks composed of a sequence of two dielectric layers with quarter-wave optical thickness and (b) rugate filters, i.e. filters with a refractive index profile that is sinusoidally modulated on the length scale of an optical wavelength. Our simulations were performed using the finite difference time domain (FDTD) technique to numerically integrate the Maxwell curl equations for both the electric and magnetic fields. This technique enables the reflection of an optical pulse from a multilayer dielectric stack to be accurately simulated and also allows the incorporation of dispersion and nonlinearity through an auxiliary differential equation. We present computer simulations of optical pulse propagation through dielectric filters for pulses with pulsewidths in the range 5-100 fs with peak intensities up to ~10 TW/cm². At low intensities the reflective properties of the dielectric filters determined using FDTD simulations are directly comparable to the results calculated using the characteristic matrix method, while at high intensities, optical nonlinearity in the dielectric layers is responsible for a decrease in the reflectance of the filter and causes stretching and distortion of the reflected pulses.

We report on sub-μm structuring of semiconductors, dielectrics, polymers and metals using a laser scanning microscope. A commercial Ti:Sapphire oscillator laser (20 nJ/pulse; 90 MHz; 150 fs) was coupled into a laser scanning microscope FemtOcut (JenLab GmbH). High numerical aperture objectives were applied to obtain fluences in the range of a few J/cm² which are well above the ablation threshold. Such a cost-effective and reliable system compared to amplified lasers systems (μJ or mJ/pulse) is adapted for material manufacturing and can be of prime interest for specific applications in security like counterfeiting.

Organic materials exhibiting strong two-photon absorption cross-sections and subsequent up-converted fluorescence have been targeted for use in a variety of applications including optical data storage, nondestructive imaging, frequency up-converted lasing, and microfabrication. In order for these materials to be useful in practical application they must either be coupled with a liquid solvent or doped into a solid host material. The purpose of this study is to examine effects of different host environments on the nonlinear photophysical properties of AF-455, a recently developed organic two-photon absorber. We present results of experiments using both emission and absorption methods to characterize the linear and nonlinear response of AF-455 dissolved in solvents of varying polarity and doped in a polymer (PMMA) matrix.

Rapid progress in recent years in the development of high power ultrashort pulse laser systems has opened up a whole new vista of applications and computational challenges. Amongst those applications that are most challenging from a computational point of view are those involving explosive critical self-focusing with concomitant explosive growth in the generated light spectrum. Moreover, new experimental developments in the field of extreme nonlinear optics will require more rigorous propagation models beyond those existing in the current literature. Specific applications areas chosen for illustration in this paper include atmospheric light string propagation and nonlinear self-trapping in condensed media. These examples exhibit rather different aspects of intense femtosecond pulse propagation and demonstrate the robustness and flexibility of the unidirectional Maxwell propagator. A novel aspect of our approach is that the pulse propagator is designed to faithfully capture the light-material interaction over the broad spectral landscape of relevance to the interaction. Moreover the model provides a seamless and physically self-consistent means of deriving the many ultrashort pulse propagation equations presented in the literature.

Laser ranging and burst illumination imaging (BIL) in the atmospheric transmission window at 1.5μm are made difficult by speckle effects which are observed when a rough surface is illuminated by a laser beam with a coherence length greater than the characteristic surface feature size. For the narrowband pulsed lasers currently used, this length is of the order of a few millimetres which leads to observable speckle effects for many common surfaces. In this context we describe progress towards the development a short-coherence length laser source operating at 1.5μm and based on optical parametric amplification of broadband seed pulses from a modelocked femtosecond erbium-doped fibre laser. Our optical parametric amplifier (OPA) system comprises a compact actively Q-switched 1047nm Nd:YLF laser, operating at ~1kHz repetition frequency, to which a 54MHz femtosecond 1.55μm Er:fibre laser is synchronised. The fibre laser produces bandwidth-limited 100fs pulses which are stretched by chromatic dispersion in a spool of SMF28 fibre to match the 3.5ns duration of the Q-switched pulses. Pulses from the Nd:YLF and Er:fibre lasers act as the pump and seed respectively for an OPA based on an aperiodically-poled crystal of MgO:PPLN containing a single linearly chirped grating. The chirp grating enables broadband parametric amplification across a wavelength range comparable with the spectral bandwidth of the seed pulses, amounting to ~150nm in the wings of the spectrum. Early results from this system have demonstrated output energies of 2.55μJ and a single-pass gain ~51dB and are expected to be increased with continued development of the project.

When propagating in air intense femtosecond laser pulses organize into intense self-guided light pulses. In turn, such self-guided pulses generate in their wake extended thin plasma columns. We show that these plasma columns act like antennas able to radiate electromagnetic pulses.

Femtosecond lasers have been widely used in laboratories for years and are now suitable for industrial applications and new military ones. Due to their very short pulse duration, they have the capability to generate intense electric fields and plasmas in targeted materials. We present here a novel scheme of missile counter-measure that is using such an intense laser source to disrupt the operation of IR guidance systems. Classical lasers for missile defense are based on thermal effects on the target whereas photons are used as the kill vehicle [1,2]. In femtosecond countermeasure, the average power is quite low, but the very intense field creates ionization effects than can damage sensitive optics and also plasma that can be used as active decoys against IR homing electronics. As the recent systems are compact and portable, an airport protection scheme is proposed to eliminate manpads threats in the vicinity of a civilian airport.

Conventionally optical parametric oscillators (OPOs) have been used in high-resolution absorption-spectroscopy as narrow-band tuneable sources where the measurement resolution is determined by the OPO output linewidth, rather than the wavelength resolution of the detector. In contrast, the absorption spectroscopy of gases and other media has for many years been carried out using instruments such as Fourier-transform infrared (FTIR) spectrometers or high-resolution diffraction-grating-based tuneable monochromators. These techniques commonly utilise broadband thermal sources with highly-divergent illumination beams limiting their use in remote sensing or fibre delivery applications. The work presented here reports a new approach to FTIR spectroscopy based around a novel Ti:sapphire pumped, signal-resonant OPO that uses a 10mm crystal of aperiodically-poled lithium niobate (APPLN) as the gain medium producing an idler output covering a 3.2-3.85μm tuning range with a typical full-width-half-maximum bandwidth of 85nm. Methane was used to demonstrate the technique since the OPO tuning range almost completely covers the strongest mid-infrared absorption lines of methane from 3.0-3.7μm (limited only by the available resonator optics). A double-beam Michelson interferometer was built around the OPO idler beam using a helium-neon laser as the second beam to self-calibrate each trace. Course tuning of the OPO resulted in the measurement of absorption data across the 3.2-3.85μm tuning range using methane held at pressures ranging from 2000mbar down to 25mbar. A maximum resolution of around 1cm^-1 was achieved using a simple rapidly scanning mirror assembly indicating that with further development this approach could yield very high-resolution measurements.

This paper reports a novel approach based on femtosecond laser processing to design micro-mechanical sensors such as force and displacement sensors. The basic concept is to combine integrated optics and mechanical functions in a single piece of glass. It differs from previous micro displacement sensor works in that the measured variable is optical rather than electrically based (strain-resistive, piezo-electric, etc.). Furthermore, a single process is used to define both the optical and the mechanical features. This significantly simplifies the overall fabrication and eliminates alignment issues associated with sequential fabrication processes.

We achieved the generation of unipolar and bipolar electrical picosecond pulses by using semiconductor switches in linear mode. Electrical pulses of hundreds volts to several thousands volts were obtained: in example unipolar pulses of about 11kV with duration of 300ps (full width at half maximum) were obtained and bipolar pulses of 3kV peak-to-peak with cycle duration of 450ps were done. Besides the linear mode running of these doped silicon semiconductors permitted to synchronise several generators and it allows adjustment of the bipolar pulse spectrum by using the frozen wave generator principle. So we demonstrated the feasibility of high voltage ultra-wideband electrical pulse generation with less optical energy than already published in the linear mode.

During the European project, BIGBAND, we have developed 1.55 μm quantum dash Fabry-Perot lasers based on InP using a ridge waveguide operating in continuous wave at room temperature. These devices have not only reached a high power of 50mW per facet but also have shown an ultra low relative intensity noise (RIN) of -162 dB/Hz ± 1.6 dB/Hz in 0.1-13 GHz range for the first year. At the end of the project we have succeeded in obtaining a very low RIN of -160 dB/Hz ± 2 dB/Hz over a wide bandwidth from 50 MHz to 18 GHz. This paper deals with the analysis of the experimental results, obtained with quantum dash Fabry Perot lasers, especially those relating to noise performances and also the first results of reliability demonstrating the suitability of these new devices for microwave optical links.

We present a sol-gel derived photonic structure of beryllium oxide. A new low temperature synthesis route has been developed to produce beryllium oxide from beryllium alkoxide. The beryllium alkoxide was prepared from beryllium metal, methyl mercury and butyl alcohol as starting materials. The samples have been mathematically modelled and their IR reflectance characteristics in the 2-18 μm wavelengths rang has been measured.

The envelope function approach for the electric and magnetic fields of the light wave in photonic crystals is proposed. We used this approach to investigate the light propagation in disordered photonic crystals and its reflection/refraction at the boubdary. We showed that small long-range distortion of the crystal lattice can explain pecularities of the translittance spectrum of photonic crystals.

Important applications of the Terahertz/Submillimeter/Nearmillimeter/Millimeter/Far Infrared have been known for many years and a number of scientific laboratory and field instruments that approach fundamental limits have been developed. More recently, a number of 'public' applications for the non-specialist have been heavily promoted. In spite of this, no 'public' application has come to fruition and been widely adopted. With specific examples, we will show that advances in technology and scientific understanding are poised to change this. A particular emphasis will be placed on distinguishing between those opportunities for which there is a clear path to a 'public' application and those for which fundamentally unknown phenomenology or technological breakthroughs will be required.

A new mathematical method, the Phase Distribution Model, is devised for the calculation of attenuation and scattering of THz radiation in random materials. The accuracy of the approximation is tested by comparison with exact calculations and with experimental measurements on textiles and specially constructed phantoms.

In this paper we describe the capability of passive millimeter-wave imaging in obstacle width and clearance measurement for automobile applications. To confirm the capabilities, a passive millimeter-wave imaging (PMMWI) test system has been developed. This test system employs the Dicke configuration, which includes a 150mm diameter antenna, a PIN switch, a reference terminator, a 50dB RF amplifier, a 76-77GHz BPF, a W-band detector, a lock-in-amplifier, an elevation and azimuth scanning stage, and a PC for scan control and data acquisition. A van and sedans are utilized as targets. The vehicle width and the clearance between two vehicles measured from PMMWIs show fairly good agreement with real values. Therefore, the capability of PMMWI in width and clearance measurement is confirmed. In addition, the experimentally obtained PMMWIs of vehicles represent the vehicle shapes. This result indicates that obstacle distinction is possible by PMMWI.

This paper describes the compromise necessary between image sampling and thermal sensitivity in the design of passive millimetre wave imaging systems. The use of linear arrays of receivers in fast (f/0.5) conical scanned systems, for real time imagery at 35GHz and 94GHz, presents particular difficulties. Analysis is presented which shows that it is not possible, with a single row of receivers, to achieve both high optical efficiency which equates to thermal sensitivity and good sampling of the image. Two methods are discussed for overcoming this limitation. In the first method, good efficiency and sampling are achieved simultaneously with multiple rows of receivers; however this method uses more receivers than predicted by basic theory. In the second method, the focal length in image space is increased by introducing an additional mirror in a Cassegrain configuration. This allows a single row of receivers to sample the image at the Nyquist rate, but results in an efficiency loss. The choice of which method to use will depend on the application. If the application requires the spatial resolution to be maximized and there is a high contrast scene, the Cassegrain method could be chosen. However, if high efficiency and resolution are both required a multiple row solution would be preferred. A 90GHz imager which uses 5 rows is discussed and contrasted to an alternative design that uses the Cassegrain approach.

During recent year's research on radiometric signatures, non-imaging, of the exhaust jet of missiles and imaging, on small vehicles in critical background scenarios were conducted by the mmW/submmW-group at FGAN-FHR. The equipment used for these investigations was of low technological status using simple single channel radiometers on a scanning pedestal. Meanwhile components of improved performance are available on a cooperative basis with the Institute for Applied Solid State Physics (Fraunhofer-IAF). Using such components a considerable progress concerning the temperature resolution and image generation time could be achieved. Emphasis has been put on the development of a demonstrator for CWD applications and on an imaging system for medium range applications, up to 200 m. The short range demonstrator is a scanning system operating alternatively at 35 GHz or 94 GHz to detect hidden materials as explosives, guns, knifes beneath the clothing. The demonstrator uses a focal plane array approach using 4 channels in azimuth, while mechanical scanning is used for the elevation. The medium range demonstrator currently employs a single channel radiometer on a pedestal for elevation over azimuth scanning. To improve the image quality, methods have been implemented using a Lorentzian algorithm with Wiener filtering.

This paper describes the further development of a scene simulation model by the introduction of multiple reflections and subjects with backgrounds. Inclusion of these features enables more realistic imagery to be created, reproducing known phenomenology in passive millimetre wave imagery, such as specular reflections of objects in concrete or tarmac surfaces and the appearances of darker cavity areas in security scanning images. Examples of these are shown at 94 GHz together with other more general images of subjects and their backgrounds.

We have developed several millimeter/submillimeter/terahertz systems to study active and passive imaging and associated phenomenology. For measuring the transmission and scattering properties of materials, we have developed a dual rotary stage scattering system with active illumination and a Fourier Transform spectrometer. For imaging studies, we have developed a system based on a 12-inch diameter raster-scanned mirror. By interchange of active sources and both heterodyne and bolometric detectors, this system can be used in a variety of active and passive configurations. The laboratory measurements are used as inputs for, and model calibration and validation of, a terahertz imaging system performance model used to evaluate different imaging modalities for concealed weapon identification. In this paper, we will present examples of transmission and scattering measurements for common clothing as well as active imaging results that used a 640 GHz source and receiver.

In this paper we present the modeling, design and fabrication of high-speed photonic modulators for use at high GHz, namely millimeter wave (MMW), frequencies based on the electro-optic materials, such as LiNbO₃. To accurately design the traveling wave MMW modulators rigorous EM numerical tools are used to determine the propagation characteristics of both the optical and MMW waveguides. Extensive studies have been made to achieve an optimal design, which includes a close refractive index match between optical and MMW wave and a reduction of MMW propagation loss. The designed devices have been fabricated and tested with a modulation up to 135GHz.

In this paper, antennas that combine transitions from microstrip line / coplanar waveguide (CPW) to horn antenna in a single unit are presented. Conventional single layer microstrip patch antennas inherently suffer narrow operation bandwidth; to widen the frequency bandwidth, stacked patch antennas are used and high gain is achieved from the horn antenna. Here, microstrip line / CPW directly feeds the bottom patch while the top patch couples parasitically to the bottom patch. For -10 dB return loss, 25% bandwidth is achieved for both microstrip line to horn antenna (MSLTHA) at center frequency f₀=17.5 GHz and for CPW to horn antenna (CPWTHA) at f₀=97 GHz. The designs were optimized using 3D Finite Element Method (FEM) software HFSS by Ansoft Corporation. The optimal design of MSLTHA has been fabricated and characterized. The return loss and far field radiation pattern are measured and has been found in very good agreement with the simulation results.

We review our recent progress made on the development of widely-tunable monochromatic THz sources implemented based on difference-frequency generation (DFG) in GaSe, ZnGeP₂, and GaP. Using a GaSe crystal the output wavelength was tuned in the range of from 66.5 μm (150 cm^-1) to 5664 μm (1.77 cm^-1), i.e. into the mm-wave region, with the peak power as high as 389 W. Such a record-high peak power corresponds to a conversion efficiency of about 0.1%. On the other hand, based on DFG in ZnGeP₂ and GaP crystals the output wavelengths were tuned in the wide ranges with high peak powers. There is an obvious advantage of using GaP over GaSe and ZnGeP₂, i.e. crystal rotation is no longer required for achieving wavelength tuning. Instead, one just needs to tune the wavelength of one mixing beam within a narrow bandwidth.

Passive millimeter-wave imagers have shown significant potential for use in applications that require penetration through atmospheric obscurations such a fog and smoke. However, the large apertures required to achieve sufficient diffraction-limited resolution in such systems often prohibit their use for many applications. One possible technique to circumvent this limitation is to use sparse-aperture imaging techniques. To date, such systems have not been realized because they require a high number of phase-sensitive, low-noise detectors spread over a large physical area. Collection and correlation processing of the data from this large array of sensors has not been practical using available technologies. Herein, we present the potential of optical upconversion detectors for sparse aperture imaging. The optical signals generated in such detectors preserve the phase information of the detected signal up until photodetection and may be easily routed to a central processor using low-loss optical fiber. Potential architectures for sparse aperture imagers using optical upconversion are discussed and compared to more traditional down-converted approaches. In addition, experimental results demonstrating the viability of such imagers are presented.

Millimeter wave passive radiometric imager based on frequency scanning antenna is suggested. As a frequency scanning antenna, the periodically perturbed dielectric image waveguide is considered. In frequency range 30÷38 GHz, the scan of the main lob over 40° has been realized. Parallel multibeam forming is realized by means of the set of narrow bandwidth resonant modulators, each of them controlled by the own modulating frequency.

The U.S. Army Night Vision and Electronic Sensors Directorate and the U.S. Army Research Laboratory have developed a terahertz-band imaging system performance model for detection and identification of concealed weaponry. The MATLAB-based model accounts for the effects of all critical sensor and display components, and for the effects of atmospheric attenuation, concealment material attenuation, and active illumination. The model is based on recent U.S. Army NVESD sensor performance models that couple system design parameters to observer-sensor field performance using the acquire methodology for weapon identification performance predictions. This THz model has been developed in support of the Defense Advanced Research Project Agencies' Terahertz Imaging Focal-Plane-Array Technology (TIFT) program and is presently being used to guide the design and development of a 0.650 THz active/passive imaging system. This paper will describe the THz model in detail, provide and discuss initial modeling results for a prototype THz imaging system, and outline plans to validate and calibrate the model through human perception testing.

Real time, passive mm wave and Terahertz imaging generally requires arrays of receivers in the system focal plane, resulting in high component count and cost. This papers gives an alternative approach based on image data fusion where low frame rate Terahertz imagery is combined with video frame rate optical data to give an apparently video rate Terahertz data stream.

This paper discusses the state-of-the-art in through-the-wall surveillance technology (TWS) and examines the detector performances as a function of TWS application. This state-of-the-art is the most exhaustive possible, from the centimetre non-imaging system, to the submillimetre imaging system. This is followed by a review of the various functions and the results which can be expected from a TWS system depending on the situation and the applications (law enforcement and civil security). Then, different key parameters of a TWS imaging system are studied and discussed according to the applications. Reasons for the choice of wavelength are considered. This appears to be the most important parameter for improving the sensitivity (image contrast), the spatial resolution and the size of the system. Detector performances (noise equivalent power or noise temperature) are also examined as a function of the detection mode (heterodyne or direct) and the imaging system (passive or active).

We present theoretical and experimental results of surface-emitted THz-wave difference frequency generation (DFG) in 2 dimensional (D) periodically poled lithium niobate (PPLN) crystal. The two orthogonal periodic structures compensate the phase mismatch in two mutually perpendicular directions of the optical and THz-wave propagation. The tunable 1.5 - 1.8 THz wave generation with impulse power of 0.1 mW and repetition rate 1 MHz is obtained. The opportunity of the power enhancement using an interference of THz fields generated by the both forward and backward propagating optical waves is investigated. The power and radiation pattern of generated of THz-wave are calculated in far-field approximation. Analysis of THz-wave DFG in 2D PPLN and its correlation with experimental data is presented.