Morteza Sheikhsofla

Performance assessment of lower VHF band for short-range communication and geolocation applications

The focus of this paper is to characterize near-ground wave propagation in the lower very high frequency (VHF) band and to assess advantages that this frequency band offers for reliable short-range low-data rate communications and geolocation applications in highly cluttered environments as compared to conventional systems in the microwave range. With the advent of palm-sized miniaturized VHF antennas, interest in low-power and low-frequency communication links is increasing because (1) channel complexity is far less in this frequency band compared to higher frequencies and (2) significant signal penetration through/over obstacles is possible at this frequency. In this paper, we quantify the excess path loss and small-scale fading at the lower VHF and the 2.4 GHz bands based on short-range measurements in various environments. We consider indoor-to-indoor, outdoor-to-indoor, and non-line-of-sight outdoor measurements and compare the results with measurements at higher frequencies which are used in conventional systems (i.e., 2.4 GHz). Propagation measurements at the lower VHF band are carried out by using an electrically small antenna to assess the possibility of achieving a miniaturized, mobile system for near-ground communication. For each measurement scenario considered, path loss and small-scale fading are characterized after calibrating the differences in the systems used for measurements at different frequencies, including variations in antenna performance.

Coherent pulse stacking extension of CPA to 9ns effectively-long stretched pulse duration

Coherent pulse stacking with a 9ns effectively-long burst of equal amplitude chirped pulses into a single pulse using a compact cascade of four Gires-Tournois interferometers is experimentally demonstrated with a fiber chirped pulse amplification system.

Coherent pulse stacking amplification — Extending chirped pulse amplification by orders of magnitude

A new technique of time-domain pulse combining — coherent pulse stacking amplification — is enabling nonlinearity-free energy extraction at the stored energy limit from rare-earth doped fiber based ultrashort pulse amplification systems.

Interferometer design and controls for pulse stacking in high power fiber lasers

In order to develop a design for a laser-plasma accelerator (LPA) driver, we demonstrate key technologies that enable fiber lasers to produce high energy, ultrafast pulses. These technologies must be scalable, and operate in the presence of thermal drift, acoustic noise, and other perturbations typical of an operating system. We show that coherent pulse stacking (CPS), which requires optical interferometers, can be made robust by image-relaying, multipass optical cavities, and by optical phase control schemes that sense pulse train amplitudes from each cavity. A four-stage pulse stacking system using image-relaying cavities is controlled for 14 hours using a pulse-pattern sensing algorithm. For coherent addition of simultaneous ultrafast pulses, we introduce a new scheme using diffractive optics, and show experimentally that four pulses can be added while a preserving pulse width of 128 fs.

Indoor wave propagation modeling at low-VHF band

In this paper the feasibility and advantages of HF-VHF band for short-range and near-ground ad hoc communication in complex environment is investigated. The propagation environment is modelled by dielectric layers of vertical dielectric walls and horizontal ceilings, as well as metallic frames around the buildings. For such scenarios, a low frequency approximation known as the Generalized Rayleigh-Gans (GRG) approximation is proposed for the calculation of scattered fields by the dielectric walls. To model scattering from metallic frames the method of mathematical modal expansion for connect loops of metallic frames in conjunction with a macro-modeling approach is used to estimate the induced currents for elementary sources such as short dipoles or plane waves. The macro-model captures the variations of the modal coefficients as a function of geometrical parameters and the incident wave attributes.

Indoor wave propagation simulations at HF using rayleigh-gans approximation

Summary form only given. In this paper indoor wave propagation issues at HF band is considered. A salient feature for wave propagation at this band is the fact that the size of objects is smaller or comparable to the wavelength and therefore high frequency methods such as ray-tracing can no longer be applied. On the other hand there are other approximations and simplifying feature exist that can be utilized to achieve accurate models as discussed below. Analysis of wave propagation in a complex indoor environment composed of many dielectric walls is encountered often for many military and civilian applications such as wireless mobile broadband communications, and inter-floor wave propagation. Thus, it is important to understand the complex indoor propagation mechanisms, including attenuation, scattering, the ground effect, etc., to assess the performance of such radios in an urban or indoor setting. The premise of low frequency radios is in low attenuation and low multi-path fading and their major drawback for mobile applications is in the size of the antenna. With the advent of antenna miniaturization methods techniques this limitation is being removed (Oh, J., J. Choi, F. T. Dagefu, and K. Sarabandi, “Extremely Small Two-Element Monopole Antenna for HF band Applications” IEEE Transactions on Antennas and Propagation, in press 2013) . Depending on the frequency and the size of the buildings, it is possible to use many full-wave methods such as FDTD at HF bands in order to obtain accurate results. However, these methods quickly lose their advantage for problems with large computational domains and in situations where statistical parameters of wave propagation are to be determined. As mentioned before, high frequency methods are computationally fast, but not appropriate at the desired HF and VHF bands where most interior wall dimensions are small compared to the wavelength. and hence low frequency approximations such as Rayleigh-Gans approximations can be utilized to find the scattered field. In such solutions, the fields inside the walls can easily be calculated using the appropriate polarizability tensor from which the fields in the near-field and far-field regions can easily be computed. Finally, multiple scattering can be analyzed by following an iterative procedure. Initially all the polarization currents for all walls in line-of-sight of the transmitter are computed, then the secondary scattered field is computed for all walls in the line-of-sight of the previous iteration lit walls. This process is continued until a convergence is reached and then fields are computed. In these computations the closed form of the dyadic Greens functions is used to achieve the electric field at desired receiving points.

Multi-mJ ultrashort pulse coherent pulse stacking amplification in a Yb-doped 85μm CCC fiber based system

Multi-mJ 81ns effectively-long burst of chirped pulses is amplified through fiber amplification system based on 85μm Yb-doped Chirally-Coupled-Core fiber and coherently stacked into a single pulse. 5.4mJ energy extraction at 1kHz repetition rate is demonstrated.

Multi-mJ energy extraction using Yb-fiber based coherent pulse stacking amplification of fs pulses (Conference Presentation)

We report multi-mJ energy (>5mJ) extraction from femtosecond-pulse Yb-doped fiber CPA using coherent pulse stacking amplification (CPSA) technique. This high energy extraction has been enabled by amplifying 10’s of nanosecond long pulse sequence, and by using 85-µm core Yb-doped CCC fiber based power amplification stage. The CPSA system consists of 1-GHz repetition rate mode-locked fiber oscillator, followed by a pair of fast phase and amplitude electro-optic modulators, a diffraction-grating based pulse stretcher, a fiber amplifier chain, a GTI-cavity based pulse stacker, and a diffraction grating pulse compressor. Electro-optic modulators are used to carve out from the 1-GHz mode-locked pulse train an amplitude and phase modulated pulse burst, which after stretching and amplification, becomes equal-amplitude pulse burst consisting of 27 stretched pulses, each approximately 1-ns long. Initial pulse-burst shaping accounts for the strong amplifier saturation effects, so that it is compensated at the power amplifier output. This 27-pulse burst is then coherently stacked into a single pulse using a multiplexed sequence of 5 GTI cavities. The compact-footprint 4+1 multiplexed pulse stacker consists of 4 cavities having rountrip of 1 ns, and one Herriott-cell folded cavity - with 9ns roundtrip. After stacking, stretched pulses are compressed down to the bandwidth-limited ~300 fs duration using a standard diffraction-grating pulse compressor.

10mJ Energy Extraction from Yb-doped 85µm core CCC Fiber using Coherent Pulse Stacking Amplification of fs Pulses

81ns effectively-long burst of chirped pulses is amplified to 10mJ with low nonlinearity in a Yb-doped 85µm core CCC-fiber based system, and coherently stacked with a multi-GTI arrangement, and compressed into a single <540fs pulse.

Multiplexed Coherent Pulse Stacking of 27 Pulses in a 4+1 GTI Resonator Sequence

Coherent stacking of 27 equal-amplitude pulses is achieved in 5-GTI sequence, with 4 cavities having rountrip of 1 ns, and one cavity - 9ns. Compression of effectively ~27ns long stretched-pulses down to 330fs is demonstrated.