Distributed antenna systems based on radio over fibre technology (fibre DAS) are becoming increasingly popular for providing cellular radio coverage within large buildings such as shopping centres, airports and corporate office blocks. The main limitation with fibre DAS, however, is its lack of flexibility for providing tailored radio coverage in multi-service or multi-operator environments. A new architecture, switched DAS, is proposed for these situations, which leads to both capital and operational cost savings as well as much greater operational flexibility

An unbalanced Mach Zehnder interferometer (UMZ) is used here for up-conversion of a microwave subcarrier, with IQ modulation at 30 and 200 Msymb/s. Both QPSK and QAM-16 are investigated, and a low BER of 10^-9 is demonstrated after detection of the up-converted microwave subcarrier for an optical power of -1dBm delivered by a directly modulated laser diode.

Radio Over Fibre (RoF) techniques have received a great deal of interest in recent years with a number of manufacturers now offering production RoF systems. However, in many cases component cost is still a critical limiting factor to the wide spread adoption of the technology. In this paper we evaluate the performance of Wavelength Division Multiplexed (WDM) RoF links constructed using spectrum sliced broadband sources as the optical transmitter. The main limitation in spectrum sliced system is the excess beat noise produced by the incoherent nature of the optical source. For digital transmission links this has a severe effect on the achievable transmission bit rate as the signal-to-noise ratio (SNR) of such limits is approximately the ratio of the optical slice bandwidth to electrical bandwidth. In radio over fibre systems, due to the relatively narrowband nature of most radio signals, it is common that although the radio signal may be at a high frequency, its electrical bandwidth is relatively narrow allow for an acceptable SNR to be achieved with relatively narrow optical slice widths. This paper will define the performance limits of such systems, demonstrating that acceptable transmission of radio frequency (RF) signals is possible over fibre distances typical in access networks.

A heterogeneous wireless/wireline optical transmission link using a reflection type electroabsorption transceiver (R-EAT) is presented. Simultaneous transmission of full-duplex broadband wireless LAN (WLAN) channels and 1Gb/s base band data is experimentally demonstrated. The system link employs sub-carrier multiplexing (SCM) and two optical channels for full duplex transmission of various analog WLAN channels and downlink digital base band data. The developed link architecture is suitable for simultaneous transmission of broadband wireline and wireless signals, it enables the coexistence and interoperability between wireline and wireless access technologies. The developed R-EAT component employed in this wireline/wireless access system, features "single-chip-component" base stations in access networks with star type topology where only a single optical fiber is used for bidirectional optical transmission. The R-EAT can be used within the optical C-band (1530- 1560nm) and is suitable for (D)WDM networks. Bit error rate measurements demonstrate the capabilities of the R-EAT for 1Gb/s base band transmission. The analog performance for WLAN transmission is characterised by a spurious free dynamic range (SFDR) of more than 75dB and 90dB for uplink and downlink transmission, respectively. The link gain for uplink and downlink transmission is -42dB and -37dB, respectively. The demonstrates the analog performances of the R-EAT for being used in wireless access networks such as W-LAN.

Various circuits dedicated to high spectral purity signal transmission using fiber optics are presented. Three application types are investigated: signal transmission of ultra stable oscillators at 10 MHz, IF distribution at 874 MHz and microwave synthesized signals at 3.5 GHz. The receiver circuit is an optically synchronized oscillator, which provides a good signal conditioning far from the carrier while maintaining the high input signal quality close to the carrier

In this paper, we propose a novel approach to realizing all-optical microwave filtering using a broadband light source and fiber Bragg gratings (FBGs) with identical reflectivities. The filter tapping coefficients are determined by the spectrum profile of the broadband optical source. Since the FBGs have identical reflectivities, the characteristics of the FBGs with high uniformity is possible, which simplifies implementation and reduces implementation error. In addition, the spectrum profile of the broadband source can be controlled using an optical comb filter. Experiments are carried out and the results agree well with the theoretical analysis.

In this work we report on a reliable and low-cost frequency-response characterization method for high-bandwidth photodiodes. Using the photomixing technique, we were able to experimentally characterize the electrical response of commercial devices up to 60GHz using both DFB and low-cost Fabry-Perot laser sources.

Different architectures for all-optical microwave filtering using fiber Bragg gratings (FBGs) have been proposed in the last few years. Most of the research efforts have been focused on the basic structures and synthesis of wideband FBG-based all-optical microwave filters with possible reconfigurability and tunability, where all FBGs are assumed to have identical time delay for all the frequency components of the incident light. However, when the filter is proposed to work at very high frequency range, the dispersion effects induced by the FBGs have to be addressed. In this paper, a theoretical model is established to study the dispersion effects in the FBG-based all-optical microwave filter. Simulations and experiments are carried out. The results show that the effective coefficients of the filter are not constant, but a function of the modulating frequency, which will degrade the performance of the all-optical microwave filters and limit the highest operating frequency. To reduce the dispersion effects, we propose to use properly-apodized short FBGs with relatively flat reflection spectra and group delay responses.

In this work we demonstrate that the normal photomixing scheme, i.e. one built around a two-mode laser (or a mode-locked laser) as the source, and a fast photodiode acting as the optical mixer of the two modes, can be used also to perform the electrical demodulation of an incoming weak signal at the same frequency. In particular, we consider a mode spacing c/2L in the range of mm-waves, typically 60GHz for a practical WLAN communication system. With optical powers in the range of mW’s (or 0dBm) for each mode, and an optical power amplifier boosting powers up to 8-10 dBm, the two modes can be photomixed on a high frequency photodiode and obtain an electrical signal with power of about 0dBm at the carrier frequency of 60GHz. Now, if an electrical signal, with a frequency slightly different from 60GHz, is applied to the photodiode output, electrical mixing with the photomixing carrier takes place and demodulation of the weak signal is performed, down to the baseband.

Narrow band resonant reflection of a polarized focused beam from a single-crystal silicon grid is demonstrated at terahertz frequencies with an efficiency close to 100 %. A spectral width of less than 7 μm at a resonance wavelength of 570 μm is achieved with a 4 mm-waist beam which overlaps with only ~10 grid periods. This compact reflection filter is a scale model of what can be expected at optical frequencies, but also represents a new type of compact terahertz device.

A travelling wave structure (similar in concept to a distributed amplifier) has been applied to a series cascade of Fabry-Perot laser diodes. The resulting travelling wave semiconductor cascade laser demonstrates a number of advantages over both conventional single laser diodes and also simple "lumped" cascades of them. These include improved quantum efficiency, enhance intensity modulation response and also broadband, lossless impedance matching (due to the use of a distributed structure to interconnect the discrete laser diodes). A travelling wave semiconductor laser (TW-SCL) based on four Fabry-Perot laser diodes has been designed and fabricated on microstrip. Measured results show a good return loss (10 dB or more) across the modulation bandwidth (of 2 GHz). In addition, the outputs of the individual devices have been optically combined in order to measure the intensity modulation response of the TW-SCL. Measured results indicate that varying the relative phase shifts of the lasers (through different fibre lengths) can be used to modify the modulation response.

Lumped circuit and 3D-electromagnetic models are presented for a photo-induced plasma that induces local changes in the dielectric properties of a coplanar waveguide transmission line to switch millimeter waves, propagating along transmission lines on BCB coated high resistive silicon substrates. Measurements up to 110 GHz are compared with the various developed models. The insertion loss of these transmission lines was only 1 dB/mm at 100 GHz. The modulation of S₂₁ was about 40 dB at 110 GHz for an optical power of 60 mW focused on one slot between signal and ground. S₁₁ could only be fitted with a lumped circuit model when introducing a frequency dependent impedance was introduced.

Single side band (SSB) or suppression carrier (SC) techniques are especially useful in radio on fiber communication systems. These techniques are used to eliminate the effect of the dispersion of the fiber. In this paper, we present a detailed study of a design of a new single side band suppression carrier modulator (SSB-SC) in which the basic element is a Bragg photonic micro cavity. This new scheme is composed of a dual Mach-Zehnder modulator (D-MZM) and a standard Mach-Zehnder modulator (MZM). Also, we analyze the feasibility of this integrated modulator with a Silicon on Insulator (SOI) technology. The COMSIS¹ optical simulator has been used to analyze the optical operation of the modulator. Macleod one Dimensional (1-D) optical simulator² is required to calculate the transmission and reflection spectra in the planar Fabry-Perot (F-P) double micro cavity with deep Si/SiO₂ Bragg reflectors.

This work reports on the fabrication, optimisation and characterisation of monolithic mode-locked lasers (MLLs) and colliding-pulse mode-locked (CPM) lasers with repetition rate in the range 10-60 GHz. The devices consist of double section split-contact ridge waveguide lasers fabricated in GaAs/AlGaAs double quantum well (DQW) material. For CPM devices, the saturable absorber section was fabricated with a coplanar ground-signal-ground (G-S-G) pad structure. The optimum saturable absorber size for efficient mode-locking is found experimentally. The fabricated devices are characterised in terms of operating regimes (Continuous Wave, Self-Pulsation, Mode-Locking) and the mode-locking signals were observed in the frequency domain using an external fast photodiode. In the case of CPM devices a mm-wave signal could also be extracted directly from the saturable absorber section of the laser using a microwave probe.

Using a tunable semiconductor laser diode and an array of diffractive optical elements (DOEs), a time-continuous, free-space optical analog to digital converter (ADC) with five bits of resolution was experimentally evaluated. The signal to be A/D-converted was fed to the tuning sections of a grating coupled twin-guide sampled reflector (GCSR) laser diode, giving a quasi-continuous tuning range of 10 nm that spanned 40 longitudinal modes. The 32 central modes were mapped to specific digital output values by first converting wavelength to deflection angle using a diffraction grating and then focusing on an array of beam splitting DOEs. Each DOE element generated a five-spot digital code word in the detector plane. Using Gray code, only one code bit changed value at a time. Thus, the beam could straddle two adjacent DOE elements without large read out errors. Furthermore, the grating components of the elements in the DOE array were all in-phase to keep the spot focused when such straddling occurs. The SNR of a converted 10 MHz sine signal covering 23 modes was 21 dB, mainly limited by tuning hysteresis. This SNR corresponds to 3.2 effective bits. The laser's analog tuning bandwidth was found to be 45 MHz, probably limited by the carrier lifetime in the passive tuning sections, but we also measured the ADC performance at 100 MHz. As the studied ADC system is time-continuous, the sampling was done in the digital domain.

We show that diode-pumped solid-state lasers can generate tunable high-purity microwave signals. In the case of a single-axis cavity containing an adjustable linear phase anisotropy, orthogonal linear eigenstates oscillate with a continuously tunable frequency difference. The maximum beat frequency is fixed by the laser cavity length and can reach a few tens of GHz. In order to reach the THz range, insertion of a double refraction crystal inside the laser cavity creates a two-axis laser that allows one to choose independently the frequencies of the two eigenstates. In this case the maximum beat frequency is fixed by the active medium gain bandwidth which is of a few THz for an Er:Yb:glass active medium. We show that doubling the two frequencies emitted by such a two-axis laser at 1.55 mum yields a source of tunable cw THz beat notes suitable for photomixing in GaAs-based THz emitters. Moreover, the beat notes generated by diode-pumped solid-state lasers can be phase-locked to microwave local oscillators. In particular, we show that a single-axis Er:Yb:glass laser provides a beat note continuously tunable from 0 to 20 GHz with a 170 muHz line width. The phase noise of such a source is measured to be lower than -130 dBc/Hz at 100 kHz offset from the carrier.

Methods for the photonic generation of stable millimetre-wave reference signals are examined and compared. In particular, the generation of optical comb lines with microwave frequency separation in an amplified fibre ring and by sideband generation with an optical phase modulator are reported. Two comb lines at a millimetre-wave difference frequency can be selected using optical filters and heterodyned. The fibre ring can produce comb lines over a broad range of up to about 1 THz, whereas the sideband generation scheme is limited to frequencies of about 160 GHz. Both methods produce stable, low-phase-noise millimetre-wave signals useful as phase/frequency references. The transmission of such reference signals through optical fibre links of up to 9 km is also investigated. Differential dispersion effects can cause a power penalty in the received millimetre-wave signal, through the interaction of chromatic dispersion and SPM/XPM effects in the fibre, and through differential polarisation changes in the fibre causing non-alignment of the two optical fields at the photodiode. For the transmission of phase reference signals, the effects of differential dispersion, both chromatic and PMD, will cause phase variations in the received millimetre-wave signal, with the PMD effect being more serious due to its stochastic behaviour.

A photonic microwave signal generator is assembled utilizing self-oscillation and bidirectional modulation in a LiNbO₃-phase modulator. The generated frequency is determined by the round trip of an outer and an inner fiber cavity, in addition to a narrow bandpass electrical filter in the feedback loop. A Faraday mirror reflects the light in the outer fiber cavity, and the polarization shift enhances the stability of the oscillation. Light is detected in the inner cavity, using a photodetector followed by a filter and a high power electrical amplifier. The modulator is thereby driven at several V_π by the detected output signal. In our experiment, the oscillation is locked on 10 GHz, but generate harmonic frequencies which are extracted through narrowband Brillouin amplification. The output of the system is thus only dominated by two frequencies in the optical domain, separated by arbitrary harmonic of the oscillator frequency. This technique is shown for generation of 60 GHz microwaves, limited mainly by the amount of phase-shift that can be achieved in the modulator. It is a simple technique that does not require an electrical signal generator, or any electronics faster than 10 GHz.

The combination of the optical and microwave techniques offers new approaches for the generation of microwave signals. That is made possible by the very high bandwidth of the optical devices and the very low attenuation of the optical fiber. The main general advantage of these approaches is the generation of very high frequencies with low noise. For the optical stabilization of microwave oscillators a clean reference signal is optically transmitted. However, instead of the millimeter wave signal, one of its subharmonics is optically transmitted and the millimeter wave signal is generated utilizing the subharmonic signal as a reference. This way the optical components become much simpler and cheaper. In this procedure a crucial point is the frequency division at millimeter waves. For this purpose a new method is introduced: the superharmonic injection locking of oscillators. Utilizing combined injection and phase locking techniques the frequency of the microwave oscillator is stabilized while its noise is significantly reduced.

Using a novel optical frequency multiplication technique, microwave signal carriers up to 20-GHz are delivered to a significantly simplified remote radio access unit fed by a multimode fibre link having modal bandwidth below 1-GHz, as well as standard single mode fibre. Measurement results show that the remotely generated carriers have very narrow linewidths below 20-Hz, and exhibit much lower phase noise (< -90 dBc/Hz) than even a commercially available high frequency electronic signal generator. Thus by using optical frequency multiplication, existing in-building silica multimode fibre infrastructure, and the emerging polymer optical fibres may be used to not only transport fixed data services such as gigabit Ethernet but also to transparently distribute in-doors (and for short links), signals of present WLANs as well as future broadband WLAN services leading to significant system-wide cost reduction. It also enables the radio signal processing to be consolidated in a single central site, which is beneficial for advanced signal processing such as needed in multiple-input multiple output (MIMO) systems.

A recent study initiated by the European Space Agency aimed at identifying the most promising technologies to significantly improve on the generation of coherent electromagnetic radiation in the THz regime. The desired improvements include, amongst others, higher output powers and efficiencies at increasingly higher frequencies, wider tunability and miniaturization. The baseline technologies considered revolve around Photomixing and novel laser based technologies compared to all electronic techniques. Some of the most significant findings will be presented together with technological developments and experimental results selected for medium to short term development. These technologies include advanced p-i-n photomixer with superlattice structures and, THz quantum cascade lasers. Recent results achieved in these fields will be put into the potential perspective for the respective technology in the future.

Photomixing is a flexible and efficient method of providing both local oscillator signals for heterodyne receivers and high frequency phase reference signals. Ultrafast, 70 GHz bandwidth, = 1.55 m, photodiodes from u2t Photonics AG have been incorporated into mm-wave waveguide mounts. The photomixers utilise a thin gold probe to couple power into the waveguide and a gold-on-quartz choke filter to deliver photodiode bias. A method of rapidly characterizing the frequency response of these photomixers using spontaneous-spontaneous beating of light from an EDFA is described. Recent work has been directed at increasing the degree of integration of the photodiode, waveguide probe and choke filter to reduce the frequency dependence of the output power. A simplified photomixer block manufacturing process has also been introduced. A combined probe and filter structure, impedance matched to both the coplanar output line on the photodiode chip and to 0.4 height milled waveguide, is presented. This matching is achieved over the W-band with a fixed waveguide backshort. We present modelled and experimental results showing the increased efficiency and smoother tuning. Subsequent integration steps could use the InP photodiode substrate to support the waveguide probe and rf filter. The design and frequency response of such a probe is presented.

This paper will report on continuous (sub-)millimetre-wave (cw) generation by photomixing two laser signals at around 1.55μm in ultrafast InAlAs/InGaAs(P) travelling-wave photodetectors (TW-PD). At first TW-PD's are characterised up to 160GHz mm-wave frequency using conventional MMW equipment. Further, photonic THz transmitters with integrated planar slot and bow-tie antenna structures for quasi optical free space coupling will be presented. Resonant slot antenna coupled TW-PD exhibit sufficient power for replacing the 460GHz solid-state-LO of an SIS-based heterodyne receiver while the equivalent device with integrated bow-tie antenna operates in a very broadband frequency range between 20 and 620GHz. Next we report on waveguide coupled TW-PD's and show experimental results in a wide frequency range, up to 1THz. Finally, several low-cost strategies for enhancing the available power as well as the efficiency will be presented.

We develop device models of a terahertz (THz) photomixer based on a high-electron mobility transistor (HEMT) structure utilizing the excitation of electron plasma oscillations in the HEMT channel by the electrons and holes photogenerated by optical signals. We use hydrodynamic equations both for electrons in the channel and for photoelectrons and photoholes in the absorption layer, or hydrodynamic equations for electrons in the channel combined with a kinetic description of the photogenerated carriers, and the Poisson equation for the self-consistent electric field. The models are used for an analytical as well as numerical (based on an ensemble Monte Carlo particle technique) analysis of the HEMT-photomixer operation in the THz frequency range.

Two-dimensional (2-D) electron plasma in a submicron channel of a high-electron mobility transistor (HEMT) can make resonant oscillation in the terahertz range. The gate bias potential V_gs can control the resonant frequency fr, which offers the possibility of tunable coherent terahertz oscillators. The terahertz plasma-wave excitation can be performed by means of interband photoexcitation in a manner of laser-photomixed difference-frequency (Δf) generation. The 2-D electron plasma in the electron channel is excited by the terahertz Δf component of the photoexcited carriers. Since the photoelectrons perturb the surface density of 2-D electrons, strong photoexcitation dynamically modulates the fr, resulting in considerable resonant-spectral broadening. This effect was modeled analytically in the 2-D plasma hydrodynamic equation. The modulation depth of the density of 2-D electrons by the photoelectrons deeply relates to the resonant intensity and f_r. In order to validate the analytical calculation, the plasma-wave resonance was experimentally observed for a 0.15-μm gate-length InGaP/InGaAs/GaAs pseudomorphic HEMT in the terahertz range. At the modulation depth of 30%, the resonance was clearly observed with a double peak (the peak at 1.9/5.8 THz corresponding to the fundamental /third harmonic resonance). On the contrary, under a low modulation depth condition, the plasma resonant intensity decreased. Observed resonant frequencies support the analytical calculation.