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We experimentally demonstrate the feasibility of implementing a cost effective all-optical fractional orthogonal frequency division multiplexing (AO-FrOFDM) receiver using an arrayed waveguide grating (AWG). The all-optical fractional Fourier transform at the receiver is implemented by modifying the second slab coupler from a conventional all-optical discrete Fourier transform AWG. The open eye diagrams obtained from the experimental results indicate that 12 × 10 Gbit/s DBPSK AO-FrOFDM signals were successfully demultiplexed.

A geometrical-optics approach is proposed to increase the accuracy in photometric measurements, using a point-of-care testing (POCT) LED-based sensor. Due to stray-light effects, the measurement accuracy depends on the dimension of the CMOS area, where the radiation is detected. We propose two image processing approaches and evaluate the influence of the sensor area. In addition, we demonstrate that with the same measurement, both absorption coefficient and refractive index can be determined, measuring the beam attenuation and the spot-size enlargement due to ray refraction.

A PAM4-OCDM system with optical multi-/demultiplexing and electrical pre-/post-processing is proposed for short-reach applications. We experimentally demonstrate, for the first time, a power-efficient 4 OC x 10 GSymbol/s PAM4-OCDM system. The four PSK-OCs are simultaneously generated using a single light source and a passive multiport optical encoder and received by a single optical decoder and cascaded DSP. The effectiveness of the electrical-domain amplitude level pre-tuning and post-equalizer are demonstrated, considering different values of shot and beat noises.

We develop a suitable geometrical-optics approach and demonstrate that it is possible to measure the optical density (OD) of bacterial cultures using a light emitting diode (LED)-based photometer. We measure both attenuation and spot-size variation, and we compensate for diffraction and stray-light impairment related to the incoherent source and large detection area. The approach is validated for different concentrations of two bacterial species, Escherichia coli and Staphylococcus aureus, that present different shapes and clustering organization.

Real-time image data processing tools are proposed to enhance the spectrophotometric absorbance accuracy, using a three-wavelength LED source. The measurements of the beam spot-size allow us to determine the ray displacement in the liquid cuvette and then the corresponding refractive index.

We furnish a general procedure for the geometric phase hologram (GPH) design, starting from the desired output phase profile and polarization states, and extending the theory of polarization gratings (PG) to arbitrary phase profiles. As an example, we consider the case of liquid crystal (LC) devices.

We present two novel, low-cost and power efficient WSSs, that act simultaneously as polarization beam splitter and frequency demultiplexer. Both architectures have 2N output ports, and N frequency channels are demultiplexed in the upper or lower fiber array, according to their polarization states.

Wavelength selective switches (WSS) play a key role in current optical networks, and are the core components of most reconfigurable optical add-drop multiplexers (ROADM), that enable fast, inexpensive, automated reconfiguration in wavelength division multiplexing (WDM)-based systems. In a routing node, a WSS can selectively forward certain wavelengths at its outputs, allowing optical broadcasting or multicasting solutions, with variable bandwidth. In a 1×N configuration, the incoming WDM signal is angularly dispersed by a static grating onto a liquid crystal on silicon (LCoS)-based spatial light modulator (SLM), which independently directs and filters the different frequency channels to the N output ports. In most configurations, light travels twice through the diffractive grating, doubling the intrinsic losses; in addition, the input polarization state should be carefully controlled, and two WSSs are required in each ROADM for polarization-multiplexed transmission. We present two other WSS architectures, with larger efficiency, that act simultaneously as polarization beam splitter and frequency demultiplexer, using polarization gratings (PG), that periodically vary the polarization state of the input light. The PGs are used either to frequency and polarization disperse the input light into two sets of diffraction orders, with orthogonal polarization states, replacing the static grating. Otherwise, the LCoS array is used in a PG configuration, to steer the frequency channels for each state of polarization toward 2N output ports.

An experimental comparison of all optical-orthogonal frequency division multiplexing (AO-OFDM) transmission is presented, using at the receiver either a fiber Bragg grating (FBG) or an arrayed waveguide gratings (AWG). System performances are analyzed, without dispersion compensation or optical time gating.

We numerically evaluate the performance of optical vectorial linear modulators, which consist of asymmetric power splitter, phase shifters and phase modulators (PM); 2N-level quadrature amplitude modulated (QAM) signals are generated by using only N single-drive two-level (binary) RF signals. We also demonstrate that a proper architecture design, with phase and amplitude modulators, largely increases the constellation size. Finally, we show that a more compact layout can be achieved with an arrayed waveguide grating (AWG) configuration. The influence of the insertion losses, phase distortion and power imbalance are also investigated.

We report on the transmission experiment of seven 12.5-GHz spaced all optical-orthogonal frequency division multiplexed (AO-OFDM) subcarriers over a 35-km fiber link, using differential quadrature phase shift keying (DQPSK) modulation and direct detection. The system does not require chromatic dispersion compensation, optical time gating at the receiver (RX) or cyclic prefix (CP), achieving the maximum spectral efficiency. We use a wavelength selective switch (WSS) at the transmitter (TX) to allow subcarrier assignment flexibility and optimal filter shaping; an arrayed waveguide grating (AWG) AO-OFDM demultiplexer is used at the RX, to reduce the system cost and complexity.

We propose a hybrid solution to exploit the capabilities of the optical orthogonal frequency-division multiplexing (OFDM) for passive optical network applications implemented by the electronic generation of the OFDM signal at the transmitter and by the all-optical demultiplexing of the OFDM subcarriers at the remote node by means of a passive element, such as suitable arrayed waveguide grating, performing the fast Fourier transform (FFT) in the optical domain. Drastic simplicity at the optical network unit is achieved with neither coherent detection nor any high-speed electronics, allowing an effective exploitation of all the available transmission spectra. By means of simulations and preliminary experimentation, we demonstrate 4 $\times$ 10-Gb/s transmission over more than 40-km standard single-mode fiber, the main limitation to the number of subcarriers being the employed device's actual electronic bandwidth.

Two new arrayed waveguide grating (AWG) architectures are presented, for flat-top multiplexing and for the generation of Nyquist sinc -shaped optical pulses. We demonstrate that an AWG, with a number of grating arms double with respect to a conventional configuration, and an input y -branch, is equivalent to a synchronized router, composed of a Mach–Zehnder interferometer and an AWG in tandem. The proposed device is more compact, with reduced losses and dispersion sensitivity, and a more flat-top frequency behavior. To generate and process a sinc -shaped optical pulse train, a new AWG is used, that implements the fractional Fourier transform (FrFT), followed by a dispersion compensating fiber. We use the time-lens effect and the chirped modulation introduced by a fully passive FrFT-AWG to optically transform a sinc -shaped transfer function into a sin c-shaped impulse response, and vice versa. Detailed AWG design guidelines for wavelength division multiplexing and orthogonal frequency division multiplexing multiplexing are also provided, evidencing the different roles played by the two slab couplers, in a signal processing framework.

We present a highly integrated add/drop multiplexer, where the contra-directional coupling is realized by phase matching two photonic crystal waveguides. The device band structure, the corresponding transmission and drop spectra, and the coupling length are carefully analysed. Different device configurations are discussed and by tailoring the coupling factor, we optimize the frequency response of the filter, obtaining a μ-sized channel selector, characterized by a very high drop efficiency.

Polarization gratings are nondepolarizing polarization elements that alter the polarization state of the transmitted light in a periodic way, yielding a polarization-dependent diffraction. Using both the Jones and Stokes formalisms, we demonstrate that the polarization states of the higher order waves do not depend on the polarization state of the incoming light and that they are fully characterized by the polarization grating vector. We furnish a general model for polarization gratings that can be advantageously used for their design and show that a chain of three polarization gratings is a achromatic in-line polarimeter. In addition, we show that polarization gratings can be used as a tool to measure polarization mode dispersion.

We have proposed and experimentally demonstrated all-optical multiplexing (MUX)-format conversion from Nyquist optical time division multiplex (OTDM) to Nyquist wavelength division multiplex (WDM). The system is simply configured with a straight-type phase modulator (PM) driven by a sinusoidal wave and an optical Nyquist filter. In the theoretical investigation, it is proved that the single Nyquist signal is completely converted to Nyquist WDM signal, which consists of two half-baud-rate signals with different carrier frequencies. The theoretical modulation voltage for the phase modulator is slightly lower than Vπ: 0.913 Vπ, and it is experimentally verified. In the experimental demonstrations, 50 Gbaud to 25 Gbaud x 2 and 25 Gbaud to 12.5 Gbaud x 2 conversions are successfully demonstrated with quite low optical signal-to-noise ratio (OSNR) penalties. In addition, cascaded MUX-format conversion is also demonstrated; 50 Gbaud Nyquist signal is converted to four channels of 12.5 Gbaud Nyquist signals.

Different AWG configurations are described, to implement the discrete Fourier transform and the discrete fractional Fourier transform for all-optical OFDM systems. Phased array switches and polarization multiplexers are presented, and new schemes for optical modulators.

We propose and experimentally demonstrate a novel technique for chromatic dispersion (CD) monitoring and adaptive compensation in an 8 x 12.5 Gbit/s all-optical orthogonal frequency-division multiplexing (AO-OFDM) system by using two pilot symbols and a virtually imaged phased array (VIPA) for a tunable CD compensator. The two pilot symbols are added to the first and the last sub-channels of the OFDM signal, and their relative time delay is detected and used for CD estimation at the CD monitoring circuit. The monitored CD value is fed to VIPA for CD compensation. In the experiments, the relative time delay between the two pilot symbols was successfully observed, and the adaptive CD compensation drastically improved the bit-error-rate (BER) from over 10(-5) to under 10(-9). The estimated CD values showed less than 10 ps/nm difference from the values measured by a photonic dispersion analyzer, which is accurate enough since the AO-OFDM system can keep BER

An arrayed waveguide grating (AWG) has a very well-known configuration, composed of two slab couplers and a grating array; during the past decades, the device has been extensively investigated as frequency (de)multiplexer ((D)MUX), and many different approaches have been proposed to obtain a flat-top response. In the present paper, we separately analyse the functionalities of the input/output slab couplers, showing that a proper parameter selection can shape the filter transmittance. In addition, the device can be used in all-optical orthogonal frequency division multiplexing (OFDM), dense OFDM (D-OFDM) and interleaved OFDM (IL-OFDM) systems. A single slab coupler with 4 input/output ports can be designed as a 90° hybrid, using different tapering widths to compensate the loss non-uniformity due to the diffraction effect.

A novel hybrid all-optical mode-division multiplexing and code division multiplexing architecture for flexible and scalable access networks is presented. We successfully demonstrate, for the first time, an asynchronous on-off keying modulation, 2 mode x 4 code x 10 Gbps transmission over 42-km link, using a set of single-mode and two-mode fibers, without dispersion compensation. The four phase-shift keyed optical codes are generated at a single wavelength, by a multiport encoder/decoder, and we use an optical mode multiplexer/demultiplexer in the remote node and at the central office. We also experimentally evaluate the mode crosstalk tolerance considering different access span distances for the LP(01) and LP(11) modes.

The properties of the ordinary and extraordinary components of paraxial beams propagating along the optical axis of uniaxially anisotropic crystals are investigated. From this analysis, a general method descends for obtaining analytical expressions of optical fields. As an example, the propagation of a Gaussian beam is described.

Spectral changes occurring to laser radiation originated from amplified stimulated emission are evaluated as a function of the number of transits performed inside a Gaussian optical cavity. It is shown that the spectral shifts on the optical axis are quite different from those already established for Gaussian Schell-model beams and that pulsed lasers deliver pulses whose peak frequency slightly changes from the beginning to the end of the pulse.

The twenty-seven papers in this special section are devoted to optical signal processing. Some of the topics covered include: waveform generation; coherent photonic processing; A/D conversion; spatially resolved detection; active optical filters; high spatial resolution; and boundless-range optics.

We investigate the performance and furnish design guideline for a 1/spl times/N router realized as a tree of Mach-Zehnder interferometers, with free-spectral ranges that follow a logarithmic progression. We show that the crosstalk can be reduced and the channel passband flattened by inserting all-pass (AP) filters in the interferometers arms, with a suitable choice of the coupling ratios. The proposed device is inherently low loss and the chromatic dispersion introduced by the AP filters can be easily compensated. Two different realizations are investigated, and the power penalties induced by losses and dispersion are evaluated numerically.

We experimentally demonstrate an 8 x 12.5 Gbit/s all-optical orthogonal frequency-division multiplexing (AO-OFDM) system using arrayed waveguide gratings (AWGs), which perform discrete Fourier transform (DFT) and inverse DFT (IDFT) of a signal directly in the optical domain. The experimental results show that frequency orthogonality of OFDM sub-channels is degraded in the AWG due to the slab-diffraction effect. To restore the frequency orthogonality and improve the system performance, we propose and demonstrate a waveform reshaping scheme, that improve the bit-error-rate (BER) from 10(-4) to 10(-6). We also experimentally investigate the influence of frequency mismatch between the OFDM signal and AWG at the receiver. The measured BER shows a serious degradation from 10(-6) to 10(-4) in case of ± 1.88 GHz frequency mismatch. To keep the BER under 10(-5), the frequency mismatch should be smaller than ± 0.5 GHz ( ± 4% of the channel spacing).

The propagation of Hermite-Gauss beams in absorbing media has been studied. The beam attenuation has been demonstrated to increase with the beam order index. As a consequence, during the propagation in absorbing media, the degree of global coherence increases for the whole class of beams that can be expanded into a series of Hermite-Gauss functions. Numerical simulations are presented. An application to multimode lasers is also considered.

We describe a innovative OFDM scheme based on orthogonal chirped subcarriers, that corresponds to the fractional Fourier transform (FrFT) of the input signal. The FrFT can be electronically implemented with a complexity equivalent to the conventional fast Fourier transform (FFT); on the other hand, the planar device that implements the FrFT in the optical domain is similar to the passive arrayed waveguide grating (AWG) device that performs the FFT. We analyze the spectral efficiency, the peak-to-average power ratio (PAPR) and the frequency offset sensitivity of a FrFT-based optical OFDM system, and make an accurate comparison with the standard FFT-based implementation.

A twofold generalization of the optical schemes that perform the discrete Fourier transform (DFT) is given: new passive planar architectures are presented where the 2 × 2 3 dB couplers are replaced by M × M hybrids, reducing the number of required connections and phase shifters. Furthermore, the planar implementation of the discrete fractional Fourier transform (DFrFT) is also described, with a waveguide grating router (WGR) configuration and a properly modified slab coupler.

We have developed a new 8-chip, 320 Gchip/s encoder/decoder with eight input/output ports, that can be used in 40-Gb/s PON networks. The device has been to multiplex four asynchronous 40 Gb/s users, using DPSK modulation. The transmission over 50 km has been successfully demonstrated for the first time.

A block-ciphered (M-ary) optical code division multiplexing (OCDM) can provide larger security than a conventional OCDM system based on bit ciphering. We propose a polarization multiplexed (POL-MUX) M-ary OCDM system and demonstrated 2.5 Gbps, POL-MUX 256 ( = 16X16)-ary OCDM transmission using a single multi-port optical encoder/decoder (E/D). We show that this architecture reduces the number of required optical codes and enhances the system confidentiality.

In a passive optical network signalling and monitoring are very important architecture functions. The modulation of a booster amplifier laser diode pump can assist this function without corruption of downlink signals towards subscribers.

Two-dimensional (2-D) transducer arrays are potentially able to generate real-time volumetric images of internal organs of the human body, and much work has been done on the subject in recent years. A 2-D array with high resolution and low grating lobe level requires a prohibitively large number of elements for existing technology. A successful solution to reduce the number of elements, without sacrificing the above mentioned characteristics, is to select a limited number of elements in a random way or combining transmitting and receiving apertures with element spacing greater than one-half of a wavelength. In this work, the effect of the human body attenuation on the performances of these so-called sparse arrays is investigated. We analytically demonstrate that, for continuous wave excitation and under paraxial approximation, the medium losses can be modeled as a Gaussian weighting function, acting off-axis in the observation plane. The variance of this weighting function decreases with the covered distance. Radiation patterns computed with both this simple model and with a more exact expression, are presented for sparse and dense 2-D arrays under continuous and pulsed wave operation. Comparisons between the results obtained with and without attenuation also are shown.

A novel implementation of quantum-noise optical cryptography is proposed, which is based on a simplified architecture that allows long-haul, high-speed transmission in a fiber optical network. By using a single multiport encoder/decoder and 16 phase shifters, this new approach can provide the same confidentiality as other implementations of Yuen's encryption protocol, which use a larger number of phase or polarization coherent states. Data confidentiality and error probability for authorized and unauthorized receivers are carefully analyzed.

We demonstrate that fast tunable optical switches and tunable slow-light devices can be designed as optical cavities entangled with quantum dots. We derive the analytical conditions for switching and tuning the filter transmittance, considering both dipole-induced transparency and electromagnetic-induced transparency effects.

In this paper, through an input-output formulation based on a full-quantum mechanical treatment of light matter interaction, some regimes to detect dipole induced transparency and electromagnetic induced transparency have been given. The main novelty of this paper is that the possibility to achieve total reflectivity from a system composed of many cavities could be obtained increasing the number of cavities, relaxing at the same time the condition for coupling a quantum state and a cavity mode. The main applications of this study are related to switching and all optical buffers, mainly in the field of quantum information processing.

We design slow-light PhC structures based on the coupling of point defects. The performance of different enhanced coupled cavity waveguides are analyzed by using the tight binding (TB) approximation and the plane wave expansion (PWE) method. We show that light can be further slowed down by coupling high-Q cavities and that the delay line performance depends on the cavity symmetry. A group velocity of about 10-4c can be achieved and the bit length is 300 times smaller than the corresponding parameter of a fiber delay line.

We demonstrated, for the first time, a field trial of 160 (16 lambda x 10) Gbit/s, fine granularity, DWDM-based optical packet switching and transmission by newly-developed burst-mode EDFAs and an optical packet switch prototype with multiple all-optical label processors. We achieved 64 km field transmission and switching of 160 (16 lambda x 10) Gbit/s DWDM-based optical packets encapsulating almost 10 Gbit/s IP packets with error-free operation (IP-packet-loss-rate10(-6) and bit-error-rate10(-9)).

We analyze the behaviour of a quantum dot embedded in a photonic crystal add-drop filter and in a direct coupled photonic crystal microcavity for optical signal processing applications. The joint effect of dipole induced transparency and electromagnetic induced transparency, considering a cascade configuration of the quantum dot, are used to modify the behaviour of the whole system. The mathematical model and a number of different conditions for tuning the frequency response of the filters are derived for application in all-optical switching and wavelength tunable devices.

Hybrid wavelength division multiplexing/optical code division multiple access (WDM/OCDMA) passive optical network (PON), where asynchronous OCDMA traffic transmits over WDM network, can be one potential candidate for gigabit-symmetric fiber-to-the-home (FTTH) services. In a cost-effective WDM/OCDMA network, a large scale multi-port encoder/decoder can be employed in the central office, and a low cost encoder/decoder will be used in optical network unit (ONU). The WDM/OCDMA system could be one promising solution to the symmetric high capacity access network with high spectral efficiency, cost effective, good flexibility and enhanced security. Asynchronous WDM/OCDMA systems have been experimentally demonstrated using superstructured fiber Bragg gratings (SSFBG) and muti-port OCDMA en/decoders. The total throughput has reached above Tera-bit/s with spectral efficiency of about 0.41. The key enabling techniques include ultra-long SSFBG, multi-port E/D with high power contrast ratio, optical thresholding, differential phase shift keying modulation with balanced detection, forward error correction, and etc. Using multi-level modulation formats to carry multi-bit information with single pulse, the total capacity and spectral efficiency could be further enhanced.

We analyze the behavior of a quantum dot embedded in an add-drop filter for optical signal processing applications. The joint effects of dipole induced transparency and electromagnetic induced transparency, which shift the transparency peak induced by a dipole in an add-drop filter due to vacuum Rabi splitting, have been considered. The mathematical model and some different conditions for tuning the transparency peak are derived for application to all optical switching and wavelength tunable devices.

The use of transparent free-space optics (FSO) allows the adoption of DWDM transmission enhancing the capacity of optical wireless link exploiting the full capacity of optical fibers. 40 Gbps data rates were obtained by time-multiplexing four 10 Gbit/s electrical sequences with a pattern length of 2. On field test on a real 60 m long transparent FSO link was performed so two devices to allow the necessary movements of the fibre and the lens series were assembled. An error free conditions with BER and Q factor measurements up to 40 Gb/s, for different interference power level, has been achieved. The achieved experimental results show that transparent FSO technique has the ability to provide high bit rate transmissions in the last hundred of meters.

We present an all-optical switch based on joint effects of dipole induced transparency (DIT) and electromagnetic induced transparency (EIT), that forbids the transparency induced by a dipole in an add-drop filter due to the vacuum Rabi shifting. The switching speed is very high, since only quantum interference between probe and pump signals is involved.

We designed a multiport two-dimensional (2D) photonic crystal (PhC) multiplexer based on contra-directional coupling and standing-wave resonators. The device is composed of PhC filters with up to 12 cavities in the same micro-patterned structure: adjacent strips with different periods contain a single cavity, and by selecting the proper lattice constants we tune the dropped frequency of each cavity. Each filter has 1.16 nm bandwidth, at a central wavelength 1550 nm, and the channels are 10 nm spaced. The variations in the local lattice period are very small, and the mismatch between two adjacent structures does not affect the propagation of the beam in the bus channel. The PhC structure has been analyzed by using the plane wave expansion method and the device performance has been investigated with the finite-difference time-domain technique. The transmittance of each drop filter is about 74%, with a non-uniformity between channels less than 5%. The overall longitudinal dimension of the multiplexer is less than 20 μm.

Free Space Optics is one key solution for addressing the last hundred meters of broadband requerements. Dense wavelength division multiplexing experiments operating at bit rates from 155 Mbit/s up to 40 Gbit/s is reported.

We design an ultra compact add/drop multiplexer by contra-directional coupling two photonic crystal waveguides. We analyse different device configurations and evaluate their performances, evidencing a trade-off between the filter selectivity and the device dimension. The design guidelines for an optimal channel selector are given and a highly selective /spl mu/-sized device is presented, that transmits about 99% of the input power to the drop port.

The propagation of the radial and azimuthal components of a paraxial beam along the optical axis of a uniaxially anisotropic crystal is investigated, deriving the paraxial wave equations and solving the associated boundary value problem. As a particular case, we obtain the propagation equations of optical vortices and give an integral representation of their solution; we show that each vortex of topological charge m propagates independently from the others, both in isotropic and anisotropic media.

We investigate the paraxial propagation along the optical axis of a uniaxially anisotropic crystal of a general paraxial beam whose boundary Cartesian components possess cylindrical symmetry. This property allows us to obtain expressions whose dependence on the azimuth angle phi (in cylindrical coordinates) is fully described and very simple. We also find that the beam loses its boundary cylindrical symmetry during propagation, as a consequence of medium anisotropy. Further, these expressions elucidate the way in which the anisotropy changes the state of polarization. As an example, we discuss the case of a Gaussian beam focused into the crystal by a thin spherical lens.

Angular spectra of reflected and transmitted fields, induced by an arbitrary electromagnetic beam passing through the planar interface between a homogeneous medium and a uniaxially anisotropic medium, are derived and related to the incident medium. By using these formulas, we obtain the expressions for paraxial and slightly nonparaxial fields. The reflected paraxial field is related to the incident one by means of Fresnel relations; the transmitted paraxial field is the superposition of an ordinary and an extraordinary beam, multiplied by the Fresnel coefficient. We find that the nonparaxial corrections, owing to the medium discontinuity, are larger than their free-propagation counterparts and that they are very simply related to the paraxial solutions of the incident beam. The case of two homogeneous media with different refractive indices is also discussed. The general expressions obtained are applied to the case of a nonparaxial Gaussian beam.