Cross Gain Research Articles

Enabling cost-effective wireless THz communication through semiconductor optical amplifier’s direct/cross gain modulation for seamless integration of hybrid fiber-coaxial connectivity

We propose a cost-effective THz wireless communication scheme for future femtocell networks that leverages either direct gain modulation (DGM) or cross gain modulation (XGM) of a semiconductor optical amplifier (SOA) to convert coaxially or optically carried data into intensity-modulated THz waves, respectively. This architecture, based on an SOA and a uni-traveling-carrier photodiode (UTC-PD), has advantages over typical THz modulation schemes involving a resonant tunneling diode or an electro-optic modulator combined with a UTC-PD. Consequently, it facilitates the seamless convergence of hybrid fiber-coaxial networks to THz radio access networks with low cost, straightforward monolithic integration, and a more compact footprint. Furthermore, we experimentally verified the possibility of non-return-to-zero on–off keying wireless communication over 0.5 m in the 300 GHz band, achieving real-time transmission of 4 Gbit/s for coaxial access via an SOA’s DGM and 10 Gbit/s for fiber access via the SOA’s XGM. Notably, these data rates were attained by simple non-coherent THz intensity demodulation based on a Fermi-level managed barrier diode while meeting the 7% hard-decision forward error correction (HD-FEC) threshold (3.4×10−3). Finally, we conducted an analysis of the merits and drawbacks of the proposed THz communication scheme based on the combination of an SOA and a UTC-PD, discussed bottlenecks that hinder further rate improvement, and explored possibilities for optimization. In conclusion, this scheme is crucial for realizing future ultra-high-speed wireless networks that seamlessly connect to fiber-coaxial networks.

Revisiting the luminescence properties of Er3+, Tm3+ and Ho3+ doped Bi4Ge3O12 rod-type crystal fibers for the laser application

BGO single crystals doped with Er3+, Tm3+ and Ho3+ rare-earth ions were prepared in the form of rod-type crystal fibers via the micro-pulling down technique to perform an extensive investigation of their visible, near- and mid-infrared luminescence properties and to really decide on their interest as solid-state laser media in the various spectral domains. Revisiting and completing the absorption spectra already available in the literature led to the derivation of more reliable “effective” radiative lifetimes and branching ratios. Registration of the fluorescence decays of the most important emitting levels versus dopant concentrations led for the first time to an unambiguous determination of “intrinsic” emission lifetimes – corresponding to negligible contributions from inter-ionic energy-transfers - and non-radiative multiphonon relaxation rates. Such data also allowed for the determination of the dominant phonon energy which comes into play in the electron-phonon coupling responsible for these multiphonon relaxation processes and for its lattice vibration origin, i.e. a high phonon energy of the order of 820 cm−1 corresponding to the coupling between bending vibrations of (GeO4)4- tetrahedra and vibrations corresponding to motions of these (GeO4)4- tetrahedra against the Bi3+ ions for which the rare-earth dopant ions substitute. Using the above data and different methods for the calibration of the registered emission spectra in cross section unit and for the derivation of gain cross section curves finally show that mid-infrared emissions of Er:BGO, Tm:BGO and Ho:BGO around 1.55 μm, 1.9 μm and 2 μm, respectively, occur with comparable gain cross sections as in the most important reference mid-infrared laser materials, although over slightly shifted thus complementary spectral domains. It is shown that potentialities also exist with Tm:BGO in the blue, deep-red and near-infrared spectral domains around 470 nm, 800 nm and 1.45 μm, by pumping the crystals with conventional pump sources and using some suitable co-dopants to avoid detrimental self-terminating processes.

Wideband High-Gain Low-Profile Endfire Linearly/Circularly Polarized Antennas Based on Magnetic Dipole Directors for Millimeter-Wave Applications

Wideband high-gain low-profile endfire linearly polarized (LP)/circularly polarized (CP) antennas for 5G millimeter-wave (mmWave) applications are presented in this letter. The basic structure of the antennas include a stripline fed horn as the feeder and multiple magnetic diploe directors (MDDs) printed on a single layer substrate. The stripline fed horn exhibits a wide bandwidth and stable endfire radiation, and magnetic dipole directors help further improvement the endfire gain. A LP antenna based on a LP feeder and vertically polarized (VP) MDDs (supporting VP resonance) working at the 5G mmWave band is first designed. Then, on this basis, CP antenna based on a CP feeder and dual-polarized MDDs (supporting two orthogonal polarized resonances) working at the same band is designed. The two proposed antennas with the low profile of 0.14λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> are fabricated and measured, both of which show good impedance matching, high gain, and low cross polarization within the 5G mmWave band.

Spectral properties and high-efficiency broadband laser operation of Tm:CaY0.9Gd0.1AlO4 crystal

We report on spectral properties and a high-efficiency broadband laser operation of Tm:CaY0.9Gd0.1AlO4 (Tm:CYGA) crystal. The polarized absorption and emission spectra, gain cross sections and fluorescence lifetime are systematically studied, presenting a broad absorption and emission spectra and thus a smooth gain band. Furthermore, under an absorbed pump power of 4.35 W, a maximum continuous-wave (CW) laser output power of 1.43 W at 1967.8 nm is obtained, corresponding to a slope efficiency of 42.2 %. The beam quality factors (M2) in X-axis and Y-axis are measured to be 1.011 and 1.027, respectively. A tunable Tm:CYGA laser with a tuning range of 284.7 nm from 1837.5 to 2122.2 nm is achieved employing a birefringent filter (BF). The results indicate that the Tm:CYGA crystal is an excellent laser medium for generating high-efficiency broadband CW laser, indicating its potential for ultrashort pulse generation in the 2-μm spectral range.

Спектральные и люминесцентные свойства Yb-=SUP=-3+-=/SUP=- в растворах CCl_4-GaCl_3-Yb-=SUP=-3+-=/SUP=-

The absorption and luminescence spectra and the luminescence lifetime of the excited state 2F5/2 of Yb3+ ions in CCl4 – GaCl3 – Yb3+ solutions have been measured. Information was obtained on the Stark splitting of levels, on the absorption and stimulated emission cross sections, and on the quantum yield of Yb3+ luminescence. The spectral dependences of the gain cross section are calculated for different relative inverse populations of the 2F5/2 excited state. Based on an analysis of the spectral and laser properties of Yb3+, it was concluded that the use of CCl4 – GaCl3 – Yb3+ solutions as the active medium of a liquid ytterbium laser is promising. Keywords: carbon tetrachloride, ytterbium, absorption, luminescence, lifetimes, Stark splitting, gain cross section

Simulative analysis of 1 Tb/s all-optical half-adder using quantum dot semiconductor optical amplifiers

Abstract All-optical half adder using quantum dot semiconductor optical amplifier is proposed and analyzed using numerical simulations for two different unsaturated gains. Cross gain modulation (XGM) and cross phase modulation (XPM) in Mach–Zehnder interferometer structure generate CARRY and SUM, respectively, of the half adder. The performance of the device is optimized with respect to input pump powers and effect of amplified spontaneous emission (ASE) noise is also investigated. We have found extinction ratio (ER) as high as 16.48 dB for SUM and 12.95 dB for CARRY which ensures efficient performance of the proposed half adder. The quality factor Q is also high (24.58 dB for SUM and 18.47 dB for CARRY). These confirm clear transmissions of the bits with low bit error rate (BER).

Growth process of Yb, Er: Na0.04K0.96Y(WO4)2 crystals and luminescence properties in the near infrared range

The top seed solution growth (TSSG) method was used to grow the x mol% Yb, 5 mol%Er: Na0.04K0.96Y(WO4)2(x = 1, 3, 5, 7) (Yb, Er: NKYW) crystals. The structure and physical properties of the crystals were investigated using TG-DTA and X-ray diffraction (XRD) characterization. The absorption and luminescence intensities of crystals doped with various concentrations of rare earth ions were studied. Inside the crystal, the luminescence mechanism is based on Yb3+ ions as the sensitizer and Er3+ ions as the activator. The x mol% Yb, 5 mol%Er: NKYW (x = 1, 3, 5, 7) crystal spectral intensity parameters Ω2, Ω4, and Ω6 were calculated using the Judd-Ofelt theory. The absorption cross section, emission cross section, and gain cross section assisted analysis concluded that the 3 mol% Yb, 5 mol%Er: NKYW crystal has high laser emission potential. This research helps to advance the development of high- performance laser materials.

Near-infrared luminescence of Er3+ doped Na0.04K0.96Y(WO4)2 single crystals

Single crystals of the compositions Na0.04K0.96Y(WO4)2 (NKYW) and x mol% Er: Na0.04K0.96Y(WO4)2 (x = 1, 3, 5, 7) (Er: NKYW) have been successfully grown by the top-seeded solution growth (TSSG) method. The growth process parameters have been investigated in detail. X-ray diffraction (XRD) and Raman spectroscopy measurements showed that the K+ and Na+ cations occupied equivalent lattice sites and the crystal structure remained monoclinic. The spectral properties and energy transfer mechanisms of the single crystals doped with different concentrations of Er3+ ions were investigated. According to the Judd-Ofelt theory, the intensity parameters of the 5 mol% Er: NKYW crystal were calculated to be Ω2 = 1.77 × 10−20 cm2, Ω4 = 3.15 × 10−20 cm2, Ω6 = 3.08 × 10−20 cm2, and the radiative transition probability of 4I13/2 → 4I15/2 was 541.25 s−1. The gain cross section of the 5 mol% Er: NKYW crystal at 1542 nm was determined to be 2.63 × 10−20 cm2, indicating that high-quality Er: NKYW single crystals have great potential for laser applications.

Efficient few-cycle Yb-doped laser oscillator with Watt-level average power.

So far, the operation of ultrafast bulk laser oscillators based on Yb-doped gain materials and directly emitting few-cycle pulses have been restricted to low optical-to-optical efficiencies and average output powers of only a few milliwatt. This performance limitation can be attributed to the commonly-applied standard collinear pumping scheme in which the optical pump is transmitted through a dichroic mirror whose spectral transmission and dispersion properties severely perturb the oscillating pulse when its optical spectrum extends towards the pump wavelength. In this study, we report on a novel pumping scheme relying on cross polarization that overcomes this challenge. In our concept, the pump transmitting mirror is highly transmissive for the pump light in p-polarization, while it is highly reflective for the laser light in s-polarization over a broad wavelength range, even covering the pump wavelength and beyond. In contrast to a standard thin-film polarizer featuring similar polarization dependent properties, it provides a low and flat dispersion profile over a broad spectral range for the s-polarization. Implementing this pumping scheme in a soft-aperture Kerr-lens mode-locked bulk laser oscillator based on the gain material Yb:CALGO, we achieve clean 22-fs soliton pulses at 729 mW of average output power and an optical-to-optical efficiency of 25%. In a second configuration optimized for the highest average output power, we demonstrate a high optical-to-optical efficiency of 36.6%, which was obtained for 31-fs pulses at 1.63 W of average output power. In a third configuration we experimentally confirm the limiting effect of a dichroic mirror commonly used in the standard collinear pumping scheme. All the results presented here and obtained in the first and second configuration generate pulses with a center wavelength ranging from 1030 nm to 1056 nm, well within the spectral region of high gain cross sections of Yb:CALGO. While this initial demonstration was realized using a commercial diffraction-limited fiber laser as pump source, the pump geometry appears also well suited for pumping with laser diodes coupled into multimode fibers. This novel approach opens up new opportunities for compact and cost-efficient high-power few-cycle bulk laser oscillators based on Yb-doped gain materials and can be applied to any gain material with small quantum defect.

Novel Rational Approximated Fractional Order Lead Compensator

This paper proposes a simplified and analytical method for designing fractional order lead compensator (FOLC). In the proposed method, the structure of fractional order lead compensator considered is more generalized. The fractional operator of the FOLC is approximated to its equivalent rational form using binomial expansion to lower the computation complexity of the designing approach. Then, steady-state gain, gain cross over frequency and phase margin are used as the design specifications of the FOLC. The proposed method also considers the magnitude and phase of the plant to be controlled. Phase margin criteria ensure the stability of the designed compensator. The compensator is designed strictly adhering to gain and phase margin criteria. The proposed methodology is validated with four different illustrative examples through simulation. Integral absolute error (IAE) and integral square error (ISE) are considered as the performance indices. The simulation results demonstrate that the proposed scheme gives a better transient response, IAE, and ISE as compared to the existing FOLC design techniques.

Spectroscopic properties of Yb3+-doped fluorophosphate glasses with low phosphate content

A series of new fluorophosphate glasses of 5Ba(PO3)2–10BaF2–18.5CaF2–10MgF2–18.5SrF2–(38–x)AlF3–xYbF3 (x = 0.5, 1, 2, 5, 8, 10) composition was synthesized. Experimental characterization, including absorption, luminescence, and luminescence kinetics measurements, was performed. Based on experimental data, the optimal concentration of Yb3+ ions, providing the highest gain, was established. To characterize potential laser performance of the studied glasses several parameters, including pump saturation, minimal pump intensity, and minimal fraction of excited ions needed to balance gain and absorption, were calculated. Dependence of gain cross section on the excited state population was determined to characterize amplifying properties of the glasses.

Spectroscopic investigation of dysprosium doped BaF2 single crystals pertaining to the 3 µm mid-IR laser potential

We present the results of a comprehensive spectroscopic investigation pertaining to laser potential evaluation of the 3-micron Dy3+ mid-IR transition in the low-maximum phonon energy host barium fluoride (BaF2). This investigation involved absorption, fluorescence, and decay time measurements, recorded for a range of temperatures. Laser-relevant parameters such as absorption and stimulated-emission cross sections, quantum-efficiencies, and radiative lifetimes were determined for room temperature (300 K) and liquid nitrogen temperature. The peak stimulated emission cross section was found to be 0.45×10−20 cm2 at room temperature and 1.58×10−20 cm2 at 77 K. The gain cross sections, predictive of laser potential, were also derived. Finally, an examination of the nature of the non-radiative decay rates as a function of temperature was performed, showing the degree to which multi-phonon relaxation and other non-radiative pathways affect the overall fluorescence behavior.

Reduced-Surface-Wave-Inspired Circular Microstrip Antenna for Concurrent Improvement in Radiation Characteristics

Classical microstrip patch antenna fabricated on a dielectric substrate suffers from poor gain, efficiency, and high cross polarization due to the automatic excitation of TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> surface wave. It also radiates strong fields along the horizon which is susceptible to multiple-input–multiple-output (MIMO) or array configurations. In view of this, a separate class of circular microstrip antenna known as the reduced surface wave (RSW) circular patch antenna has been proposed with a pair of shorting strips. Unlike earlier RSW-inspired antennas that mainly confirmed their ability for the successful reduction in the mutual coupling between array elements, here, and the enhancement of radiation properties is considered.

Design of FOPID controller for higher order continuous interval system using improved approximation ensuring stability

This contribution deals with the design of a fractional-order proportional-integral-derivative (FOPID) controller through reduce-order modeling for continuous interval systems. First, a higher order interval plant (HOIP) is considered. The reduced-order interval plant (ROIP) for considered HOIP is derived by multipoint Padé approximation integrated with Routh table. Then, FOPID controller is designed for ROIP to satisfy the phase margin and gain cross over frequency. Thus obtained FOPID controller is implemented on HOIP also to validate the performance of designed FOPID on HOIP. A single-input-single-output (SISO) test system is taken up to elaborate the entire process of controller design. The outcomes affirm the validity of the designed FOPID controller. The designed FOPID controller produced stable results retaining the phase margin and gain cross-over frequency when implemented on HOIP. The results further proved that FOPID controller is working efficiently for ROIP and HOIP.

Ultrahigh Green and Red Optical Gain Cross Sections from Solutions of Colloidal Quantum Well Heterostructures.

We demonstrate amplified spontaneous emission (ASE) in solution with ultralow thresholds of 30 μJ/cm2 in red and of 44 μJ/cm2 in green from engineered colloidal quantum well (CQW) heterostructures. For this purpose, CdSe/CdS core/crown CQWs, designed to hit the green region, and CdSe/CdS@CdxZn1-xS core/crown@gradient-alloyed shell CQWs, further tuned to reach the red region by shell alloying, were employed to achieve high-performance ASE in the visible range. The net modal gain of these CQWs reaches 530 cm-1 for the green and 201 cm-1 for the red, 2-3 orders of magnitude larger than those of colloidal quantum dots (QDs) in solution. To explain the root cause for ultrahigh gain coefficient in solution, we show for the first time that the gain cross sections of these CQWs is ≥3.3 × 10-14 cm2 in the green and ≥1.3 × 10-14 cm2 in the red, which are two orders of magnitude larger compared to those of CQDs.

Спектрально-люминесцентные свойства Yb-=SUP=-3+-=/SUP=- в растворах SO-=SUB=-2-=/SUB=-Cl-=SUB=-2-=/SUB=--GaCl-=SUB=-3-=/SUB=--Yb-=SUP=-3+-=/SUP=-

The spectra of absorption, luminescence, and lifetime of Yb3+ in SO2Cl2 – GaCl3 – Yb3+ solutions were measured. Information is obtained on the Stark splitting of energy levels, absorption and stimulated emission cross sections of Yb3+. An analysis of the spectral dependence of the gain cross sections indicates that the SOCl2 – GaCl3 – Yb3 solution is promising as the active medium of a diode-pumped liquid ytterbium laser.

Sine Cosine Algorithm Assisted FOPID Controller Design for Interval Systems Using Reduced-Order Modeling Ensuring Stability

The focus of present research endeavor was to design a robust fractional-order proportional-integral-derivative (FOPID) controller with specified phase margin (PM) and gain cross over frequency (ωgc) through the reduced-order model for continuous interval systems. Currently, this investigation is two-fold: In the first part, a modified Routh approximation technique along with the matching Markov parameters (MPs) and time moments (TMs) are utilized to derive a stable reduced-order continuous interval plant (ROCIP) for a stable high-order continuous interval plant (HOCIP). Whereas in the second part, the FOPID controller is designed for ROCIP by considering PM and ωgc as the performance criteria. The FOPID controller parameters are tuned based on the frequency domain specifications using an advanced sine-cosine algorithm (SCA). SCA algorithm is used due to being simple in implementation and effective in performance. The proposed SCA-based FOPID controller is found to be robust and efficient. Thus, the designed FOPID controller is applied to HOCIP. The proposed controller design technique is elaborated by considering a single-input-single-output (SISO) test case. Validity and efficacy of the proposed technique is established based on the simulation results obtained. In addition, the designed FOPID controller retains the desired PM and ωgc when implemented on HOCIP. Further, the results proved the eminence of the proposed technique by showing that the designed controller is working effectively for ROCIP and HOCIP.

Fabrication, microstructures, and optical properties of Yb:Lu2O3 laser ceramics from co-precipitated nano-powders

The Yb:Lu2O3 precursor made up of spherical particles was synthesized through the co-precipitation method in the water/ethanol solvent. The 5 at% Yb:Lu2O3 powder is in the cubic phase after calcination at 1100 °C for 4 h. The powder also consists of spherical nanoparticles with the average particle and grain sizes of 96 and 49 nm, respectively. The average grain size of the pre-sintered ceramic sample is 526 nm and that of the sample by hot isostatic pressing grows to 612 nm. The 1.0 mm-thick sample has an in-line transmittance of 81.6% (theoretical value of 82.2%) at 1100 nm. The largest absorption cross-section at 976 nm is 0.96×10−20 cm2 with the emission cross-section at 1033 nm of 0.92×10−20 cm2 and the gain cross sections are calculated with the smallest population inversion parameter β of 0.059. The highest slope efficiency of 68.7% with the optical efficiency of 65.1% is obtained at 1033.3 nm in quasi-continuous wave (QCW) pumping. In the case of continuous wave (CW) pumping, the highest slope efficiency is 61.0% with the optical efficiency of 54.1%. The obtained laser performance indicates that Yb:Lu2O3 ceramics have excellent resistance to thermal load stresses, which shows great potential in high-power solid-state laser applications.

Laser emission at 0.89 µm in Nd3+-doped NaYF4 nanocrystalline glass ceramics

Nanocrystalline glass ceramics with different concentrations of Nd3+-doped NaYF4 are prepared by melt quenching. Different characterization methods show that the cubic NaYF4 crystalline phase has been formed. Based on the absorption and emission spectra of samples, the absorption (σabs) and emission (σsem) cross sections of Nd3+ (4I9/2 ↔ 4F3/2) in the NaYF4 nanocrystalline glass ceramics are analyzed. The absorption and emission spectra show typical absorption and emission peaks, suggesting the possibility of concentration quenching. The values of σabs and σsem reach 2.04 × 10−21 cm2 and 1.93 × 10−21 cm2, respectively, which are comparable to those of other glasses, and a positive value of the gain cross section (σg) for 0.89 µm is obtained when the population inversion is chosen as 0.2. The results show that the studied material has promising applications in waveguide lasers and optical amplifiers.

Linearly tapered meander line broadband monopole tag antenna with T‐match for Ultra High Frequency Radio Frequency Identification applications

In this paper, a compact, broadband linearly tapered meandered monopole tag antenna for UHF-RFID is designed and optimized using particle swarm optimization (PSO) algorithm. An inductive T-match network is utilized for impedance matching with capacitive Higgs-4 chip. The optimization goal of PSO was conjugate matching and in consequence the maximization of read range. Equivalent circuit of the proposed tag antenna is derived using ADS software to validate its impedance characteristics. The performance of the proposed tag in terms of tag power sensitivity, read range, realized gain, and differential radar cross section has been experimentally characterized. To check the tolerance of the designed tag to various object platforms, its read range performance is also verified on objects like wood, fiber, plastic, and so forth. Furthermore, read pattern of the proposed tag has been measured and found to have figure of eight in E-plane and omnidirectional in H-plane. Experimental results reveal that the proposed tag covers 865-867 MHz (ETSI band, Europe) and 902-928 MHz (FCC band, United States) both major RFID bands with a read range of 10 and 12 m, respectively. The proposed tag has 2060 mm3 of volumetric size with maximum measured readable distance of 12 m with EIRP of 3.28 W.