In order to share access infrastructure between mobile and access network, integration of wireless services on optical access network with Radio over Fibre (RoF) technologies is a potential solution for operators. In this paper, we propose an overlay Universal Mobile Telecommunication System (UMTS) or third generation (3G) signal transmission on a Gigabit Passive Optical Network. Main advantages of this transmission scheme are the use of only one laser to feed the signal and one photodiode to receive it. A trade-off between extinction ratio and laser linearity has been found and the penalty induced by the 3G signal overlay is small. We demonstrate a 1x32 PON over 20km, which can distribute UMTS signal for remote base station, and binary signal at 1.25Gbit/s for others users.

The proof-of-concept demonstration of an microwave photonics cross-connect for telecom satellite repeaters is reported. The system includes optical distribution of a high-purity local oscillator at 26 GHz, frequency down-conversion from 30 to 4 GHz, and optical cross-connection of microwave signals.

This paper investigates the use of optical spectrum slicing techniques for radio over fibre networks. We show that the transmission of complex-modulated data is possible by directly modulating a Super-luminescent Light Emitting Diode (SLED) with an Intermediate Frequency (IF) signal. This signal can then be up-converted to a higher frequency (18 GHz) using a Mach-Zehnder modulator. This has the advantage that a number of WDM channels can be up-converted using a single device. We demonstrate that due to dispersion the system performance over a given distance decreases as the width of the slice increases. This runs counter to the usual trend observed in spectrum slicing systems whereby, an increase in slice width is required to increase SNR. It is shown that although optical suppressed carrier modulation is used the length dependant nulls can be observed due to the width of the slices. It is seen that the intensity noise of the source and the required optical amplification stage degrades the signal by introducing intensity noise on the received signal. We detail the achievable performance of such a system with the potential to be used with multiple WDM channels.

We propose and demonstrate the use of a fiber Bragg grating operated in transmission to build a simple, low-cost and efficient optical carrier processor that permits to enhance the fiber-optic transmission of RF signals. It simultaneously achieves an improvement in the detected electric power level by partially attenuating the optical carrier and the capability of reconfiguring the link frequency response avoiding the carrier suppression effect due to the fiber-optic chromatic dispersion.

Semiconductor based terahertz sources and lasers allow applications in biomolecular spectroscopy and imaging. We present typical experimental set-ups to study liquid and water samples. Water has a very high absorption coefficient in the THz spectral range. It can be measured in transmission if a high signal-to-noise ratio is available otherwise by reflection measurements. We show THz imaging examples of dried lactalbumin drops and a THz spectrum of a thin film of the nucleobase guanine.

Terahertz Time-Domain Spectroscopy (THz-TDS) is used to investigate water and soot contaminations in oils, exhibiting different dilution modes. For synthetic polyglycol oils, the water is dissolved due to the polar behavior of the oil, whereas in non polar mineral oils the water-oil compound forms an emulsion. This behavior is modeled with an effective medium approximation (EMA) formalism. Small soot agglomerates are remaining in suspension when mineral oils are polluted with soot particles. In this case, the absorption spectrum is dominated by scattering effects. Due to the small particle size of the soot agglomerates compared to the THz wavelength, coherent scattering is the dominant process.

We present the Raman spectra of sulfur-containing biomolecules and the spectra measured in far-infrared region with the use of terahertz (THz) time-domain spectroscopic technique. The spectral features in THz and Raman spectra are different among sulfur-containing di- and tetrapeptides in powder. We are discussing the possibility to observe and to assign the low-frequency THz and Raman lines to the disulfide bridges.

In this paper a simplified "1.5D" modeling approach is presented which can be used to characterize and optimize an entire active millimeter wave imaging system for concealed weapon detection. The method uses Huygens' Principle to compute one field component on selected planes of the imaging set-up. The accuracy of the method is evaluated by comparing it to a rigorous 2D method of moments approach. The model includes the effects of lenses, diffusers, mirrors, object and any other component present in the system. The approach allows fast determination of the influence of each of the system components on the image projected onto the sensor, including effects such e.g. speckle. Also, the effectivity of different speckle reduction techniques, e.g. using a Hadamard diffuser or a multifrequency approach are evaluated in this paper.

We demonstrate a semi-confocal THz imaging system based on a THz gas laser operating at 2.52 THz (118 μm) with power up to tens of milliwatts. Our ultimate goal is the development of a fully confocal THz microscope. We discuss the stability of the system and present THz images obtained on different objects to illustrate the potential of the system.

THz technology makes possible imaging of phenomena, inaccessible to both visible and infrared radiation, but the imaging is still in its early stages of development. This paper draws attention to the aspects of speckle reduction to improve the image quality. Because all existing THz sources are coherent - speckle is an ultimate limiting factor of the free-space imaging techniques. Speckle arises when coherent light scattered from a rough surface is detected by an intensity detector with a finite aperture, hiding the image information. This problem is of special importance for THz imaging, because surface roughness is closer to the object dimension as in optical imaging. The reduction of speckle is highly desirable and we propose here a Hadamard matrix solution. Hadamard diffuser for mm-wave frequency range have been designed, built and tested. We report 50% speckle reduction measurements using a free-space vector network analyzer over the full W-band (75-110 GHz). The advantage of the mm-wave Hadamard technology over optical: the diffuser doesn't have to be moved (vibrated) any more to accomplish the technology of speckle reduction. Temporal optical effect is substituted here by spatial quasi-optical: Hadamard coding in each scan pixel. Second method delivers realistic system parameters for the speckle reduction with polychromatic light for aviation security.

A 0.2 A/W responsivity waveguide-uni-travelling carrier photodiode with a -3 dB electrical frequency response > 108 GHz is demonstrated. Up to -5 dBm electrical power at 110 GHz, and 28 mA photocurrent (DC excitation) were detected. The photodiode was also integrated with an antenna to permit a record breaking emission of up to 148 μW at 457 GHz and 25 μW at 914 GHz.

The paper reviews the state-of-the-art in high-frequency photodetectors for THz generation and compares optical heterodyne THz generation with existing electrical and optical approaches.

In this work, we present new results of numerical analyses for the radiation properties of our plasmon-resonant photomixer. The photoresponse of plasmon excitation is first calculated from the Dyakonov-Shur theory, which is input to the electromagnetic simulator based on a frequency-dependent finite differential time-domain method. This procedure allows us to distinguish the structure-dependent transmittance property from the responsivity of plasmon excitation.

CW-photomixing semiconductor devices have hardly exceeded an output power of 10 μW around 1 THz. Availability of a few mW, however, would stimulate the demand for THz-imaging, -scanning, and spectroscopy. Increasing the poor power conversion efficiency from the optical pump to THz-output is most desirable. On the other hand, the thermal threshold "per pixel" is limited to about 100 mW of pump laser power. So both limits have to be pushed towards higher performance. In this paper we report on arrays of photomixing devices to overcome the thermal threshold limit. If each individual photomixer in the array can be driven to the same thermal threshold power, the overall THz output can be larger by a factor N\times M for an array. The power of directed emission, however, can be increased even by a factor (N x M)² compared to the individual device. In addition, by adjusting the two laser beams slightly noncollinear, a directional control of the emitted THz-beam is achieved. The angular difference of the incident beams is enhanced by the ratio of the THz-wavelength (≈300 μm) and the optical wavelength (≈0.85 μm) with regard to direction of the emitted THz-beam. Thus, a full steering of the THz beam can be achieved by tuning this angle by less than 1 degree (17.5 mrad).

In this paper we show that the real emission frequency of a photoconductive dipole antenna operating in conjunction with photomixers is not its natural resonant frequency, but a frequency where the antenna's input resistance is the highest and the corresponding mismatch with the source is the smallest. We also introduce a new kind of antenna that offers a much higher input resistance to the photomixer and hence enhances the efficiency of the continuous-wave system.

In this paper we discuss three different techniques to generate THz radiation at room temperature with semiconductor lasers. The state of the art is to generate the THz radiation by photomixing of two optical modes in a photoconducting antenna. We show that the complexity of this setup can be reduced by using a laser system emitting two colors simultaneously. Optical power is in excess availiable by using an optical amplifier. In a second approach we demonstrate, that the complexity and cost of the setup can be significantly reduced, by generating the THz radiation directly in the two color laser. This is realized by making use of nonlinear processes in the carrier system. While all these methods generate cw radiation, we finally suggest a third approach to generate pulsed THz radiaton with semiconductor lasers. This THz source can be used in THz time domain systems.

InN, a novel semiconductor material, is used as THz surface emitter. The material is irradiated with fs-laser pulses at 1060 nm and 800 nm and the emitted ultrashort THz pulses are measured by phase sensitive detection. Pulsforms, amplitudes and spectra are compared to the THz emission of p-doped InAs, the standard material for THz surface emission.

The concept of an electron gun for generating pulses with a duration <10 fs at energies suitable for electron diffraction experiments is presented. The principle is based on an rf cavity oscillating in the TM₀₁₀ mode. Laser pulses for photoemission are injected at a well-defined phase of the rf oscillation such that electrons with different initial velocities and different time delays arrive at a target within a very small temporal window. Coulomb broadening is prevented by reducing the number of electrons to the level of a single electron per pulse while increasing the repetition rate to the MHz range. The fs-electron pulses generated will advance the time-resolution of electron diffraction experiments to the level of a vibrational period of molecules.

Dielectric mirrors are widely used in optical setups for spectral regions such as UV, visible, as well as IR. Yet, for the rapidly growing field of terahertz spectroscopy dielectric multilayer optics are sparsely utilized. But with low-loss materials high quality THz optics can be obtained. We present two approaches for the realization of highly effective dielectric THz mirrors. First, four thin slices of high-resistivity silicon and five common polypropylene (PP) foils were alternately stacked together to obtain a broad reflection band. This stop-band blueshifts with increasing angles of incidence. But due to the high index step between Si and PP a band from 0.32 to 0.375 THz always remains the stopband for all incidence angles and both the s- and p-polarization. The measurement data obtained in reflection and transmission geometry are reproduced well by numerical simulations. With a minor change of the layer sequence a microresonator is obtained which reveals a sharp transmission peak at around 0.3 THz within the reflection band. The second material system consists of ceramic laminates of alumina (A) and alumina-zirconia (AZ). Measurements on 12.5 pairs of A/AZ layers yield a strong stop-band from 0.3 to 0.37 THz at normal incidence, which again match numerical simulations. The big advantage of the ceramic mirror is the rugged, quasimonolithic design of the sintered multilayer structure.

The stable two-mode operation of a 4-sections semiconductor laser emitting at 1.55 μm is demonstrated and analysed. The two-mode operation only depends on the current feed in the Bragg section. The characterization of the two-mode laser operation exhibits the possibility of terahertz wave generation by photomixing using this device. An interpretation of the two-mode regime involving a saturable absorber is discussed.

We report here on experiments with surface plasmon excitation and propagation along corrugated and smooth aluminum surface in the terahertz frequency range. Plasmon excitation by a picosecond terahertz pulse is shown to be a transient process and plasmon propagation sufficiently changes its measured time profile. Plasmon duration and life-time were defined and plasmon propagation lengths on smooth and corrugated surface were measured. Plasmon propagation length on flat surface turned out to be much smaller than the Drude model predicts.

The applicability of moth-eye structures to THz components is investigated. With the help of RCWA and effective medium theory, optimal structural parameters for one-dimensional and two-dimensional periodical surface-relief gratings are deduced. The required structural parameters are in such order of magnitude that they can be manufactured by ultra-precision machining directly into the surface of the substrate material. Benefiting is that plastic materials, which are preferred materials in THz spectral region, can be accurately manufactured by ultra-precision machining. The application of the moth-eye structures follows directly the primary shaping of the components by conventional manufacturing methods like turning and milling so that no additional materials are necessary. A comparison between several structures fabricated on planar plastic probes is given.

Moderately cooled, fast and diffraction-limited THz bolometer is proposed and its theoretical model is developed. Unlike thermal bolometers, the radiation rapidly heats only electrons in narrow gap bipolar semiconductor without the semiconductor lattice inertial heating. In conditions determined this heating changes generation and recombination processes, that leads to the carrier concentration decrease and semiconductor resistance rise. This rise creates the device output signal. According to this model the SHEB on base of narrow gap mercury cadmium telluride (MCT) semiconductor at temperature of T = 78 K can have detectivity D* ~ (0.3-2)10⁷ cmHz^1/2/W in the range of (0.01-1.5) THz.

A sub-ps optical sampler based on Four Wave Mixing (FWM) in 250 meter-long Highly Non-Linear Fibre (HNLF), has been implemented. Its accuracy resolving ps soliton pulses has been estimated exploiting a commercial oscilloscope and a commercial autocorrelator.

Spatio-temporal characteristics of phase-conjugate processes on the basis of a stimulated scattering or three-wave mixing are considered. On an instance of phase conjugation for a dipolar spin wave pulse at microwave frequency, conjugate mirror effective reflectance application in case of instantaneous nonlinearity is shown.