Doublet Pulses Research Articles

Optical UWB doublet pulse generation using multiple nonlinearities of single SOA

All-optical ultra-wideband (UWB) doublet pulse generation based on the multiple nonlinear effects of a single semiconductor optical amplifier (SOA) is demonstrated. First, a monocycle pulse is obtained by means of cross-gain modulation and self gain-saturation in the SOA, and a Gaussian pulse at conjugated light is obtained using four-wave mixing of the SOA. Secondly, the Gaussian pulse at the conjugated light is combined with the monocycle pulse by proper time delay to form a doublet pulse. The central frequency and 10 dB bandwidth of the doublet radio frequency are 5 and 6.25 GHz, respectively. The scheme is simple since only a single SOA is required.

All-Optical 500-Mb/s UWB Transceiver:An Experimental Demonstration

We propose and demonstrate experimentally, for the first time, a prototype for all-optical ultra-wideband (UWB) transceiver at 500 Mb/s. We report 1) UWB pulse optimization that takes into account the transmitter RF front end and the U.S. federal communications commission (FCC) spectral mask, 2) a new approximate match filter receiver using optical signal processing, and 3) modulation at 500 Mb/s. Our previous optimization of UWB pulse shapes was based only on the FCC spectral mask, without taking into account the frequency response of the RF components (amplifier and antenna) in the UWB transmitter. Here, we modify our pulse optimization technique to ensure that the equivalent isotropic radiated power (EIRP) from the transmitter meets FCC specifications. For the RF hardware used, we achieve 63.6% efficiency over the FCC mask, which yields an 11.6- and a 5.9-dB improvement over Gaussian monocycle and doublet pulses, respectively. We also introduce simple optical signal processing at the receiver that allows the incoming RF signal to be matched against a square pulse whose duration is adapted to the channel. The exact matched filter would require a new optimized pulse that would include not only hardware frequency response but channel effects that vary with antenna placement as well. The proposed approximation allows a simple variation of the pulse duration: an increase to account for pulse expansion in the channel but an upper limit to combat multipath effects. Finally, we demonstrate the optimized pulse and approximate match filter receiver at 500 M/s. We attain a 10-6bit error rate at a 65-cm separation line of sight (LOS) link with simple on-off keying and no forward error correction.

UWB Pulse Generation and Distribution Using a NOLM Based Optical Switch

An approach is proposed in this paper to generate ultra-wideband (UWB) pulses based on cross-phase modulation (XPM) effects in nonlinear optical loop mirror (NOLM). The NOLM can be considered as an optical switch which is controlled by pump to generate positive and negative Gaussian pulses, which are then delayed by SMF and combined to generate both Gaussian monocycle and doublet pulses. These two pulse shapes can be switched at a high speed by controlling the on-off of one of the probe. In addition, the SMF acting as a chromatic dispersion media in generating the UWB pulses can also be used to distribute the UWB signals from the central station (CS) to remote access points (AP). Different distance between CS and remote APs has no effects on the generated UWB pulses by adopting a wavelength tunable laser as probe.

Methods for Ultra-Wideband Pulse Generation Based on Optical Cross-Polarization Modulation

Optical methods for different type ultra-wideband (UWB) pulse generation based on cross-polarization modulation (CPM) are proposed and demonstrated in this paper. Two polarity-reverse pulses can be obtained by CPM and birefringence time delay to form a monocycle pulse. A semiconductor optical amplifier (SOA) is placed after the monocycle pulse process for doublet pulse generation. These two kinds of pulses can be employed in single-band impulse radio UWB (IR-UWB) systems. Two kinds of multi-band UWB pulses can be generated based on monocycle pulse train with proper apodization profiles, realized by hybrid photonic microwave filter and synchronous polarization modulation respectively. Experimental results show that these pulses can be used in multi-band UWB (MB-UWB) over fiber systems.

Optical Generation of Binary Phase-Coded Direct-Sequence UWB Signals Using a Multichannel Chirped Fiber Bragg Grating

A novel technique to generate binary phase-coded, direct-sequence ultra-wideband (DS-UWB) signals in the optical domain is proposed and demonstrated. In the proposed system, the wavelengths from a laser array are modulated by a Gaussian pulse, which is sent to a multichannel optical frequency discriminator, to generate a UWB monocycle or doublet pulse sequence with a predetermined phase-code pattern. By tuning the wavelengths of the laser array, or by tuning the states of polarization of the wavelengths, the generated pulse shape and code pattern can be changed. The key device in the system is the multichannel dispersive chirped fiber Bragg grating (FBG), which functions, in combination with a dispersive fiber, as a multichannel frequency discriminator with a step-increased group-delay response, to ensure the generated UWB sequence to have uniform time spacing among the chips and to compensate for the fiber-induced chromatic dispersion. The proposed scheme is experimentally demonstrated. A multichannel chirped FBG is designed and fabricated. Binary phase-coded DS-UWB signals with different code patterns are experimentally generated.

Optical generation of binary-phase-coded, direct-sequence ultra-wideband signals by polarization modulation and FBG-based multi-channel frequency discriminator

In this paper a novel optical generation approach for binary-phase-coded, direct-sequence ultra-wideband (UWB) signals is experimentally demonstrated. Our system consists of a laser array, a polarization modulator (PolM), a fiber Bragg grating (FBG), a length of single mode fiber, and a photo detector (PD). The FBG, designed based on the superimposed, chirped grating, is used as the multi-channel frequency discriminator. The input electronic Gaussian pulse is modulated on the optical carrier by the PolM and then converted into UWB monocycle or doublet pulses sequence by the multi-channel frequency discriminator. The PolM is used so that the desired binary phase code pattern could be simply selected by adjusting the polarization state of each laser, rather than tuning the laser wavelengths. The desired UWB shape, monocycle or doublet, could be selected by tuning the FBG. Based on our proposed approach, four-chip, binary-phase-coded, DS-UWB sequences with different pulse shapes and code patterns are experimentally demonstrated. The impact of the fiber dispersion on the generated UWB pulses is also discussed in our paper.

High-Order Ultrawideband Pulse Generation from NRZ-DPSK Signals

A simple method to generate ultrawideband (UWB) doublet and triplet from nonreturn-to-zero (NRZ) differential phase shift keying (DPSK) signals is proposed and experimentally demonstrated. The proposed configuration consists of a Mach–Zehnder modulator (MZM) to generate NRZ-DPSK signals, a section of single-mode fibre to form a microwave bandpass filter, which is used to generate doublet pulses, and a Gaussian optical bandpass filter (OBF), which serves as a frequency discriminator to generate higher-order UWB pulses. A pair of polarity-reversed triplet pulses is achieved by locating the optical carrier at the positive and negative linear slopes of the OBF, where the OBF detuning is 0.12 nm and -0.2 nm, respectively. The spectra of the pair of UWB triplets have a central frequency of 5 GHz and 5.6 GHz, and have a -10 dB bandwidth of 6.9 GHz and 8.1 GHz, respectively. The UWB pulses remain doublet shape when the light wavelength is located at the peak of the OBF. The spectrum of the doublet has a central frequency of 5.6 GHz and a -10 dB bandwidth of 6.9 GHz.

Ultra-Wideband Waveform Generator Based on Optical Pulse-Shaping and FBG Tuning

We propose and demonstrate experimentally a prototype for ultra-wideband (UWB) waveform generator based on optical pulse shaping. The time-domain pulse shape is written in the frequency domain, and a single-mode fiber performs frequency-to-time conversion. A U.S. Federal Communications Commission (FCC)-compliant power efficient pulse shape is inscribed in the frequency domain by a fiber Bragg grating (FBG) with an excellent match between optimized and measured pulses. Two other popular UWB pulse shapes (Gaussian monocycle and doublet pulses) are achieved by proper tuning of two FBG-based variable optical filters. A balanced photodetector removes an unwanted rectangular pulse superimposed on the desired waveform, assuring compliance at low frequency.

A Novel Composite Method for Ultra-Wideband Doublet Pulses Generation

A novel composite approach to generate ultra-wideband (UWB) doublet pulses in optical domain through an electrical Gaussian pulse is proposed and demonstrated in this letter. Two pulses with reversed polarity generated by polarization modulation are fed into a differential group delay device and a semiconductor optical amplifier (SOA) wherein the optical pulse profile is modified. Doublet pulses are obtained after optical-electrical detection due to gain saturation and recovery mechanism in the SOA. The fractional bandwidth of doublet pulses exceeds 180%, fulfilling the UWB requirements.

Optical Arbitrary Waveform Generation Based on Sampled Fiber Bragg Gratings

Two approaches are proposed for generating optical arbitrary waveforms using sampled fiber Bragg gratings (FBGs). The manipulation of the optical spectrum of an input pulse is achieved by a sampled FBG which is fabricated using a novel technique based on the reconstruction-equivalent-chirp (REC) method. The key advantage of the REC technique is that it can produce FBGs with various phase profiles using a fabrication system having only micrometer precision. Two examples for the generation of Gaussian doublet pulses are demonstrated, which would find applications in ultra-wideband over fiber systems.

An Electrically Switchable Optical Ultrawideband Pulse Generator

A novel electrically switchable optical ultrawideband (UWB) pulse generator that is capable of generating both Gaussian monocycle and Gaussian doublet pulses by using a polarization modulator (PolM) and a fiber Bragg grating is proposed and experimentally demonstrated. The polarity and the shape of the generated UWB pulses can be electrically switched by adjusting the voltages applied to two arbitrary wave plates (AWPs), which are incorporated at the input and the output of the PolM to adjust the polarization state of the lightwaves. The key component in the UWB pulse generator is the PolM, which is a special phase modulator that can support both transverse electric and transverse magnetic modes but with opposite phase modulation indexes. Depending on the polarization state of the incident lightwave to the PolM that is linearly polarized and aligned to one principle axis of the PolM or circularly polarized, UWB monocycle or doublet pulses are generated. The polarity of the UWB pulses can be electrically switched by adjusting the voltages applied to the AWPs. The proposed system is implemented, and the generation of UWB monocycle and doublet pulses is experimentally demonstrated. Gaussian monocycle and doublet pulses with a fractional bandwidth of about 150% and 160% are experimentally generated. The proposed electrically switchable optical UWB pulse generator has the potential for applications in UWB communications and radar systems that employ pulse polarity modulation and pulse shape modulation schemes.

Robust manipulation of electron spin coherence in an ensemble of singly charged quantum dots

Using the recently reported mode locking effect we demonstrate a highly robust control of electron spin coherence in an ensemble of (In,Ga)As quantum dots during the single spin coherence time. The spin precession in a transverse magnetic field can be fully controlled up to 25 K by the parameters of the exciting pulsed laser protocol such as the pulse train sequence, leading to adjustable quantum beat bursts in Faraday rotation. Flipping of the electron spin precession phase was demonstrated by inverting the polarization within a pulse doublet sequence.

All-Fiber Ultrawideband Pulse Generation Based on Spectral Shaping and Dispersion-Induced Frequency-to-Time Conversion

An approach to generating and distributing ultrawideband (UWB) pulses based on optical spectral shaping and frequency-to-time conversion using all-fiber components is proposed and demonstrated. In the system, the spectrum of an ultrashort pulse from a mode-locked fiber laser is spectrally shaped by an all-fiber spectrum shaper, to produce a monocycle- or a doublet-shaped power spectrum. Thanks to the frequency-to-time conversion in a dispersive fiber, time-domain optical pulses exhibiting the user-defined shape of the optical power spectrum are obtained. Experiments based on the proposed approach are carried out. UWB monocycle or doublet pulses are generated

All-optical UWB impulse generation based on cross-phase modulation and frequency discrimination

All-optical ultra-wideband (UWB) impulse generation based on optical cross-phase modulation (XPM) and frequency discrimination is proposed and experimentally demonstrated. In the proposed configuration, an optical pulse train from a femtosecond pulse laser that is spectrum sliced to get the required pulse width is used as a pump, which is then injected into a nonlinear fibre together with a continuous-wave (CW) probe to create XPM. A fibre Bragg grating (FBG) is used as a frequency discriminator. By locating the cross-phase-modulated probe at the linear or the quadrature slopes of the FBG reflection spectrum, UWB monocycle or doublet pulses are generated.

Switchable optical UWB monocycle and doublet generation using a reconfigurable photonic microwave delay-line filter

Optically switchable Ultra-Wideband (UWB) monocycle and doublet pulse generation using an optically reconfigurable photonic microwave delay-line filter is proposed and demonstrated. The microwave filter can be reconfigured as a two- or three-tap microwave filter with coefficients of (1, -1) or (1, -2, 1). The function of the two- or three-tap microwave filter is equivalent to an operation of a first- or second-order difference, which can be approximated as a first- or second-order derivative. When a Gaussian pulse is inputted to the two- or three-tap microwave delay-line filter, a Gaussian monocycle or doublet pulse is generated. The proposed photonic microwave delay-line filter is implemented using a polarization modulator (PolM), a length of polarization maintaining fiber (PMF), and a balanced photo-detector (BPD). In the experiment, Gaussian monocycle and doublet pulses with a fractional bandwidth of about 170% and 130% are generated. The switchability of the proposed UWB pulse generator in pulse shape and polarity is also experimentally demonstrated.

Ultrawideband Impulse Radio Signal Generation Using a High-Speed Electrooptic Phase Modulator and a Fiber-Bragg-Grating-Based Frequency Discriminator

We propose a novel approach to generating ultrawideband impulse radio signals in the optical domain. The proposed system consists of a laser source, an electrooptic phase modulator (EOPM), a fiber Bragg grating (FBG), and a photodetector (PD). The light source is phase modulated by an electrical Gaussian pulse train via the EOPM. The optical phase modulation to intensity modulation conversion is achieved by reflecting the phase modulated light at the slopes of the FBG that serves as a frequency discriminator. Electrical monocycle or doublet pulses are obtained at the output of the PD by locating the wavelength of the optical carrier at the linear or the quadrature slopes of the FBG reflection spectrum. The use of the proposed configuration to implement pulse polarity and pulse shape modulation in the optical domain is discussed. Experimental measurements in both temporal and frequency domains are presented

Feedback-controlled stimulation enhances human paralyzed muscle performance

Chronically paralyzed muscle requires extensive training before it can deliver a therapeutic dose of repetitive stress to the musculoskeletal system. Neuromuscular electrical stimulation, under feedback control, may subvert the effects of fatigue, yielding more rapid and extensive adaptations to training. The purposes of this investigation were to 1) compare the effectiveness of torque feedback-controlled (FDBCK) electrical stimulation with classic open-loop constant-frequency (CONST) stimulation, and 2) ascertain which of three stimulation strategies best maintains soleus torque during repetitive stimulation. When torque declined by 10%, the FDBCK protocol modulated the base stimulation frequency in three ways: by a fixed increase, by a paired pulse (doublet) at the beginning of the stimulation train, and by a fixed decrease. The stimulation strategy that most effectively restored torque continued for successive contractions. This process repeated each time torque declined by 10%. In fresh muscle, FDBCK stimulation offered minimal advantage in maintaining peak torque or mean torque over CONST stimulation. As long-duration fatigue developed in subsequent bouts, FDBCK stimulation became most effective ( approximately 40% higher final normalized torque than CONST). The high-frequency strategy was selected approximately 90% of the time, supporting that excitation-contraction coupling compromise and not neuromuscular transmission failure contributed to fatigue of paralyzed muscle. Ideal stimulation strategies may vary according to the site of fatigue; this stimulation approach offered the advantage of online modulation of stimulation strategies in response to fatigue conditions. Based on stress-adaptation principles, FDBCK-controlled stimulation may enhance training effects in chronically paralyzed muscle.

An approach to ultrawideband pulse generation and distribution over optical fiber

We propose a novel approach to generating and distributing ultrawideband (UWB) pulse signals over optical fiber. The proposed system consists of a single-wavelength laser source, an electrooptic phase modulator (EOPM), a length of single-mode fiber (SMF), and a photodetector (PD). The combination of the EOPM, the SMF link, and the PD forms an all-optical microwave bandpass filter, which is used to generate a UWB signal with a spectrum meeting the regulation of the Federal Communication Commission. Gaussian doublet pulses are obtained at the receiver front-end, which can provide several gigahertz bandwidths for applications in high-bit-rate UWB wireless communications. Experimental results measured in both temporal and frequency domains are presented.

Pulse trains produced by phase-modulation of ultrashort optical pulses: tailoring and characterization

In this paper, creation of pulse doublets and pulse trains by spectral phase modulation of ultrashort optical pulses is investigated. Pulse doublets with specific features are generated through step-like and triangular spectral phase modulation, whereas sequences of pulses with controllable delay and amplitude are produced via sinusoidal phase modulations. A temporal analysis of this type of tailored pulses is exposed and a complete characterization with the SPIDER technique (Spectral Phase Interferometry for Direct Electric-field Reconstruction) is presented.

Directional guidance method for the blind using time separation pitch discrimination of triple pulse signals

A guidance system for the blind, using time separation pitch (TSP) was simulated. Listeners estimated a simulated straight line course perpendicular to and bisecting a line between two horizontally separated transmitters, one transmitting a pulse doublet and the other, a single pulse. The course was represented by the coincidence of the single pulse with the doublet's center, forming a triplet of identical broadband, evenly spaced pulses. In test signals simulating deviations from the course, interpulse intervals were varied by changing the timing of the second pulse. The subjects' ability to discriminate pulse separations in triplets was measured. Next, subjects manipulated the timing of the second pulse of test signals to produce a triplet containing two equal intervals. Subjects' ability to discriminate and manipulate signals suggests that a trained listener could correct veering by observing changes in TSP.