We recently applied time domain near infrared diffuse optical spectroscopy (TD-NIRS) to monitor
hemodynamics of the cardiac wall (oxy and desoxyhemoglobin concentration, saturation,
oedema) on anesthetized swine models. Published results prove that NIRS signal can provide
information on myocardial hemodynamic parameters not obtainable with conventional diagnostic
clinical tools.1 Nevertheless, the high cost of equipment, acquisition length, sensitivity to
ambient light are factors limiting its clinical adoption.
This paper introduces a novel approach, based on the use of wavelength and code division
multiplexing, applicable to TD-NIRS as well as diffuse optical imaging systems (both topography
and tomography); the approach, called WS-CDM (wavelength and space code division mltiplexing),
essentially consists of a double stage intensity modulation of multiwavelength CW laser
sources using orthogonal codes and their parallel correlation-based decoding after propagation
in the tissue; it promises better signal to noise ratio (SNR), higher acquisition speed, robustness
to ambient light and lower costs compared to both the conventional systems and the more recent
spread spectrum approach based on single modulation with pseudo-random bit sequences
(PRBS).2 Parallel acquisition of several wavelengths and from several locations is achievable.
TD-NIRS experimental results guided Matlab-based simulations aimed at correlating different
coding sequences, lengths, spectrum spreading factor, with the WS-CDM performances
on such tissues (achievable SNR, acquisition and reconstruction speed, robustness to channel
inequalization, ...).
Simulations results and preliminary experimental validation confirm the significant improvements
that WS-CDM could bring to diffuse optical imaging (not limited to cardiac functional
imaging).
The phase response of a commercial saturable absorber based on semiconductor quantum wells embedded in a resonant
cavity is investigated. The nonlinear absorption change is accompanied by a variation of the spectral phase characteristic.
Also, a nonlinear change in the refractive index of the material, induced by the modified carrier density, produces a weak
shift in the resonant wavelength of the cavity. These effects can be exploited to realize an optically-controllable phase
shifter. Simulations based on a nonlinear model are also carried out in order to investigate the effect of the various cavity
parameters and phase response of the device under different operating conditions. The results from this characterization
and numerical analysis show that such device can have the potential for practical applications in telecom systems,
including dynamic dispersion compensation, tunable nonlinear effects compensation, and nonlinear signal processing
and all-optical regeneration of phase-modulated optical signals.
Cardiovascular diseases are the main cause of death in industrialized countries. Worldwide, a large number of
patients suffering from cardiac diseases are treated by surgery. Despite the advances achieved in the last decades
with myocardial protection, surgical failure can still occur. This is due at least in part to the imperfect control
of the metabolic status of the heart in the various phases of surgical intervention. At present, this is indirectly
controlled by the electrocardiogram and the echographic monitoring of cardiac mechanics as direct measurements
are lacking. Diffuse optical technologies have recently emerged as promising tools for the characterization of
biological tissues like breast, muscles and bone, and for the monitoring of important metabolic parameters such
as blood oxygenation, volume and flow. As a matter of fact, their utility has been demonstrated in a variety of
applications for functional imaging of the brain, optical mammography and monitoring of muscle metabolism.
However, due to technological and practical difficulties, their potential for cardiac monitoring has not yet been
exploited. In this work we show the feasibility of the in-vivo determination of absorption and scattering spectra
of the cardiac muscle in the 600-1100 nm range, and of monitoring myocardial tissue hemodynamics by time
domain near-infrared spectroscopy at 690 nm and 830 nm. Both measurements have been performed on the
exposed beating heart during open chest surgery in pigs, an experimental model closely mimicking the clinical
cardio-surgical setting.
In the perspective of a future all-optical communication network optical shift register will play an important role
especially for what concerns several binary functions, such as serial to parallel conversion and cyclic operations, that are
involved in techniques allowing error detection and correction as parity check, or cyclic redundancy check. During the
last decades, several attempts of realizing circulating memories or shift register in the optical domain were made, with
some limits in terms of functionality, number of bit to be stored (under three), scalability or photonic integrability.
In this paper, we present a new approach to realize a circulating optical shift register consisting on an SOA-based optical
buffer (OB) and a bit selecting circuit (BSC). The OB is potentially integrable and is able to store a finite number of bit
at high bit rate. The BSC returns consecutive bits at a lower clock rate, achieving proper shift register function. The bit
selection is realized by means of four wave mixing (FWM) in a Kerr medium, and the sequence cancellation is allowed
to enable new sequence storing. Experimental validation of the scheme for fB=59MHz and fB=236MHz shows optical
signal to noise ratio per bit penalty of 5.6dB at BER=10-9.
A novel solution for all-optical packets buffering in OPS nodes is proposed. Variable delays are performed by exploiting
a low-loss optically controlled fiber-based loop configuration. XGM in SOAs allows polarization and wavelength
independent operation in the whole C-band. A packet delay resolution of 5 μs is obtained as well as a storage time of
50 μs with moderate signal degradation. Performances are evaluated in terms of bit error rate measurements for 10 Gb/s
NRZ data payload, providing an OSNR penalty lower than 3 dB after 10 circulations. The proposed solution is
particularly attractive in slotted OPS nodes architectures where packet contention would be managed entirely in the
optical domain.
A 2x2 cross/bar optically-driven switch is implemented with a single semiconductor optical amplifier. The switch
exploits nonlinear polarization rotation experienced by two input signals in the amplifier in presence of a control pump
light. The two input data signals travel in opposite directions inside the amplifier. In absence of the control light, the lowpower
input signals do not experience nonlinear effects inside the amplifier; when the pump light is applied, both the
input data signals experience cross-phase modulation, which reflects in nonlinear polarization rotation for the output
signals due to polarization-dependent carriers modulation in the semiconductor amplifier. Polarizes are then used in the
output paths in order to discriminate the output packets for the two possible cases of control pump signal in the ON and
OFF state. Bit error rate measurements demonstrate error-free operation for both the possible switch configurations. By
letting the input signals to travel the amplifier in opposite directions this architecture enables operation with data packets
at the same wavelength. The switch speed is limited by the carriers recombination time in the amplifier, in the order of
few hundreds of ps. Semiconductor technology allows implementation of compact, cost-effective, and low-power
operating all-optical devices.
Optical technologies represent the main bet for future communication systems. Among the others, digital subsystems for
optical processing are of great interest thanks to their intrinsic properties in terms of bandwidth, transparency, immunity
to the electromagnetic interference, cost, power consumption, as well as robustness in hostile environment. Key basic
functions are represented by logic gate, logic function, flip-flop memories, optical random access memories, etc..
Research in this field is in its very early stages even if some interesting techniques have been already theoretically
addressed and experimentally demonstrated. In this paper we review the state of the art for all-optical flip-flop including
different approaches such as fiber based, semiconductor amplifier based or waveguide and micro-ring solutions. Best
result will be highlighted in terms of transition speed, switching energy, complexity and power consumption. Finally our
best results are discussed.
The main techniques for high bandwidth optical digital sampling are briefed. Potentialities and drawbacks of
optical solutions are investigated and compared.
A modular photonic interconnection network based on a combination of basic 2×2 all-optical nodes where a photonic
combinatorial network manages the packet contention, is presented. The proposed architecture is synchronous, can
operate Optical Time Division Multiplexing (OTDM) packets up to 160Gb/s and exhibits self-routing capability and
very low switching latency.
In such a scenario, OTDM has to be preferred to Wavelength Division Multiplexing (WDM), because in the former case
the instantaneous packet power carries the information related to only one bit, making more simpler the signal processing
based on instantaneous nonlinear interactions between packets and control signals. Moreover OTDM can be utilized in
interconnection networks without caring about the propagation impairments, since these networks are characterized by a
very limited size (< 100m). Finally, in such a limited domain, the packet synchronization can be solved at the network
boundary in the electronic domain, without the need of complex optical synchronizers. The 2×2 switching element is
optically managed by exploiting a photonic combinatorial network able to carry out contention detection, and to drive the
contention resolution and the switching controller blocks. The implementation of such photonic combinatorial network
is based on semiconductor devices, making the solution very promising in terms of compactness, stability, and power
consumption. The network performances have been investigated for bit streams at 10 Gb/s in terms of Bit Error Rate
(BER) and Contrast Ratio. Moreover, the suitability of the 2×2 photonic node architecture exploiting the above
mentioned combinatorial network, has been verified up to 160 Gb/s, demonstrating the potentialities of photonic digital
processing in the next generation broad-band and flexible interconnection networks.
KEYWORDS: Receivers, Optical filters, Electronic filtering, Interference (communication), Electroluminescence, Optical components, Photodiodes, Optical fibers, Chemical elements, Signal to noise ratio
We investigate, both theoretically and experimentally, how the use of an all-Optical Decision Element (ODE) in front of a
conventional receiver improves, in Return-to-Zero (RZ) systems, the receiver performance when the signal bandwidth exceeds
the bandwidth of the available opto-electronic components.
A theoretical analysis of the ODE behavior shows the field of applicability of the investigated solution. The experimental
evaluation of the performance improvement in an RZ system is realized using an ODE based on two cascaded Nonlinear
Optical Loop Mirrors. Benefits in terms of Bit Error Rate for different signal bandwidths and for different received Optical
Signal-to-Noise Ratio (OSNR) are presented. Substantial agreement of the experimental results with the theoretical analysis is
obtained. The impact of the ODE in the presence of relevant thermal noise at the receiver is also considered.
The ODE can extend the use of common band-limited receivers to wide-bandwidth signals, and can be an alternative
solution to the development of wide-band receivers.
An all-optical and ultra-fast combinatorial network based on semiconductor optical amplifiers and able to detect packet
contention in a 2x2 photonic node is demonstrated. The signal at the output of the combinatorial network has a contrast
ratio higher than 8.4 dB. The combinatorial network is used for demonstrating the feasibility of a 2x2 photonic node at
160 Gb/s.
Wavelength tunable and ultra stable pulse generation at 10GHz is experimentally demonstrated using a non-PM, regeneratively mode-locked fiber ring laser. Less than 2.0 ps, nearly transform-limited, sech2-shaped pulses are directly generated with supermode noise suppression ratio over 70dB and RMS timing jitter value less than 162 fs. Dispersion management has been exploited in order to obtain the shortest pulse duration at 1.1ps. The mechanisms for the super-mode noise suppression are also discussed.
Wavelength conversion with high contrast ratio and low OSNR penalty has been achieved by using a resonant vertical-cavity all-optical switch based on saturable absorption in multiple-quantum-wells. The device was grown by MBE on InP substrate. It comprised a 19.5 pairs n+-Ga0.47In0.53As/InP bottom DBR, 28 Ga0.47In0.53As QWs, and a 50% reflective top dielectric mirror. We carried out conversion experiments between a wavelength-tunable modulated pump signal and a CW beam with a wavelength matching the Fabry-Perot resonance of the switch. Using a 622 Mb/s modulated pump with an average power of only 6-dBm we have demonstrated a 15 dB extinction ratio for the converted signal. The wavelength conversion process exhibited a weak dependence on the pump signal wavelength; we have achieved wavelength conversion in a range of 20 nm. BER/OSNR measurements on the wavelength converted data signal indicated a maximum OSNR penalty (at a BER=10-9) of about 2.5 dB, with respect to the input pump data, over the entire conversion range. Error free operation was observed up to 2 Gb/s when device performance degraded due to its long absorption recovery time. However, with further optimization, the device recovery time could be reduced to the picosecond range, extending its application to much higher date rates.
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.
We report on a simple self-starting diode-pumped passively mode-locked Er-doped fibre laser based on two semiconductor saturable absorber mirrors (SESAMs), generating sub-picosecond stable optical pulses. Pulses duration between 350 and 650 fs (FWHM) was observed for pulses central wavelengths ranging between 1540 nm and 1570 nm. The cavity basic frequency was 3.7 MHz, and stable operation up to the third harmonic (11.1 MHz) was observed when the output power of the 980 nm diode pump was increased to its maximum value of ~300 mW. The maximum average output power was 19.45 mW, which corresponded to a pulse energy of ~4 nJ. Noise characterization of the mode-locked laser source was performed, in order to estimate the phase noise of the output pulses in terms of timing jitter. All the fiber components in the cavity were polarization maintaining in order to increase long-term stability of the laser operation.
Wavelength conversion with high contrast ratio and low OSNR penalty has been achieved by using a resonant vertical-cavity all-optical switch based on saturable absorption in multiple-quantum-wells. The device was grown by MBE on InP substrate. It comprised a 19.5 pairs n+-Ga0.47In0.53As/InP bottom DBR, 28 Ga0.47In0.53As QWs, and a 50% reflective top dielectric mirror. We carried out conversion experiments between a wavelength-tunable modulated pump signal and a CW beam with a wavelength matching the Fabry-Perot resonance of the switch. Using a 622 Mb/s modulated pump with an average power of only 6-dBm we have demonstrated a 15 dB extinction ratio for the converted signal. The wavelength conversion process exhibited a weak dependence on the pump signal wavelength; we have achieved wavelength conversion in a range of 20 nm. BER/OSNR measurements on the wavelength converted data signal indicated a maximum OSNR penalty (at a BER=10-9) of about 2.5 dB, with respect to the input pump data, over the entire conversion range. Error free operation was observed up to 2 Gb/s when device performance degraded due to its long absorption recovery time. However, with further optimization, the device recovery time could be reduced to the picosecond range, extending its application to much higher date rates.
The virtually unlimited bandwidth of optical fibers has caused a great increase in data transmission speed over the past decade and, hence, stimulated high-demand multimedia services such as distance learning, video-conferencing and peer to peer applications. For this reason data traffic is exceeding telephony traffic, and this trend is driving the convergence of telecommunications and computer communications. In this scenario Internet Protocol (IP) is becoming the dominant protocol for any traffic, shifting the attention of the network designers from a circuit switching approach to a packet switching approach. A role of paramount importance in packet switching networks is played by the router that must implement the functionalities to set up and maintain the inter-nodal communications. The main functionalities a router must implement are routing, forwarding, switching, synchronization, contention resolution, and buffering. Nowadays, opto-electronic conversion is still required at each network node to process the incoming signal before routing that to the right output port. However, when the single channel bit rate increases beyond electronic speed limit, Optical Time Division Multiplexing (OTDM) becomes a forced choice, and all-optical processing must be performed to extract the information from the incoming packet.
In this paper enabling techniques for ultra-fast all-optical network will be addressed. First a 160 Gbit/s complete transmission system will be considered. As enabling technique, an overview for all-optical logics will be discussed and experimental results will be presented using a particular reconfigurable NOLM based on Self-Phase-Modulation (SPM) or Cross-Phase-Modulation (XPM). Finally, a rough experiment on label extraction, all-optical switching and packet forwarding is shown.
Experimental measurement of in-band Four-Wave-Mixing (FWM) power in non-zero dispersion fiber is presented. A comparison with known methods shows how the proposed procedure is more accurate in frequently interesting cases.
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