A subcarrier demultiplexing scheme based on a coefficient-switchable microwave photonic filter in radio-over-fiber systems

A flexible method for subcarrier demultiplexing in radio-over-fiber (RoF) systems is proposed and experimentally demonstrated. The subcarrier demultiplexer is realized by a coefficient-switchable microwave photonic filter, which is based on biasing Mach–Zehnder modulator (MZM) at different transmission slopes. The 2.4 GHz and 2.7 GHz subcarrier signals both modulated with 100 Mbps on-off keying (OOK) data are transmitted and demultiplexed in the RoF system. The experimental result shows that each of the two subcarriers can be individually selected out with the other one well depressed. The proposed subcarrier demultiplexing scheme is simple, feasible, cost-effective and has good potential applications in the multiplexed RoF systems.     

Multifrequency super-thin cloaks

Bandwidth and thickness have become the most troublesome problems for EM cloaks. In this paper, we propose to solve the two problems using connected patches based on the microwave network model. By covering an obstacle with combined connected patches, cloaking effect can be achieved at multiple frequencies so as to expand the operating band. As an explicit example, a dual-band super-thin cloak using two different connected patch unit cells is demonstrated. Cloaking effect can be achieved at 3.50 GHz and 4.14 GHz simultaneously with an 8 dB transmission enhancement. The cloak design method provides a new route to broadening the bandwidth of thin EM cloaks.     

Temperature dependence of microwave resistances of n++np++ Si X band IMPATT diode

The change of both microwave negative resistance (R) and its parasitic series resistance (R_s) on the rise of junction temperature in the range of 100–220 °C of HP n⁺⁺np⁺⁺ Si IMPATT [M. Mitra, M. Das, S. Kar, S.K. Roy, IEEE Trans. Electron. Dev. 40 (1993) 1890] diode at X band (8–12 GHz) have been simulated. The studies followed by Gummel–Blue Technique [H.K. Gummel, J.L. Blue, IEEE Trans. Electron. Dev. 14 (1967) 569] show that for a constant experimental bias current of 25 mA [Mitra et al., 1993], for which the space charge effect is not prominent, the values of negative conductance and negative resistance degrade taking into account the changes in the ionization rates and drift velocities due to rise of temperature. Also observed that the critical series resistance increases with the increase of temperature up to 2.23 Ω, slightly higher than the realistic limit of 2 Ω [Mitra et al., 1993].     

Novel attributes of AlGaN/AlN/GaN/SiC HEMTs with the multiple indented channel

In this paper, a high performance AlGaN/AlN/GaN/SiC High Electron Mobility Transistor (HEMT) with the multiple indented channel (MIC-HEMT) is proposed. The main focus of the proposed structure is based on reduction of the space around the gate, stop of the spread of the depletion region around the source–drain, and decrement of the thickness of the channel between the gate and drain. Therefore, the breakdown voltage increases, meanwhile the elimination of the gate depletion layer extension to source/drain decreases the gate–source and gate–drain capacitances. The optimized results reveal that the breakdown voltage and the drain saturation current increase about 178% and 46% compared with a conventional HEMT (C-HEMT), respectively. Therefore, the maximum output power density is improved by factor 4.1 in comparison with conventional one. Also, the cut-off frequency of 25.2 GHz and the maximum oscillation frequency of 92.1 GHz for the MIC-HEMT are obtained compared to 13 GHz and 43 GHz for that of the C-HEMT and the minimum figure noise decreased consequently of reducing the gate–drain and gate–source capacitances by about 42% and 40%, respectively. The proposed MIC-HEMT shows a maximum stable gain (MSG) exceeding 24.1 dB at 3.1 GHz which the greatest gain is yet reported for HEMTs, showing the potential of this device for high power RF applications.     

Microwave behavior of ferrites prepared via sol–gel method

Ni-ferrites were prepared at different temperatures via sol–gel method. The electromagnetic properties of these materials, namely permittivity and permeability were measured in the 0.1–13 GHz frequency range. Following a mathematical procedure, microwave absorption diagrams were constructed including the dependence of the microwave absorption of ferrite layer on microwave frequency and layer thickness. The permeability spectra broaden and the microwave absorption improves at 9–10 GHz with increase of annealing temperature.     

Photonic generation of millimeter-wave ultra-wideband signal using phase modulation to intensity modulation conversion and frequency up-conversion

We propose and experimentally demonstrate the generation of a pair of polarity-reversed 24 GHz millimeter-wave (MMW) ultra-wideband (UWB) monocycles. The scheme is realized by using delay interferometer (DI) based phase modulation to intensity modulation (PM–IM) conversion and carrier suppression modulation (CSM) based frequency up-conversion. The phase modulation is realized by using either electro-optic phase modulator (EOPM) or cross phase modulation (XPM) in semiconductor optical amplifier (SOA), which is an all-optical approach to obtaining baseband UWB signals, respectively. After frequency up-converted by using CSM in a Mach–Zehnder modulator (MZM), a pair of polarity-reversed 24 GHz MMW-UWB signals complying with the Federal Communication Committee (FCC) requirements is generated. The bi-phase modulation (BPM) of 24 GHz MMW-UWB signals can also be realized by electrically switching the bias voltage of delay interferometer.     

A filterless optical millimeter-wave generation based on frequency octupling

A novel method of a filterless optical millimeter-wave (MMW) signal generation with frequency octupling using four nested Mach–Zehnder modulators (MZMs) is proposed for Radio-over-fiber systems. By symmetrically biasing the MZMs and using two RF driving signals with 90 deg phase delay, a cost-effective, high-quality and filterless optical millimeter-wave at 80 GHz with an optical harmonic distortion suppression ratio exceeding 40 dB is obtained. The proposed system is insensitive to the MZM bias drift, which demonstrates a relatively higher stability. So it is a viable solution for the future ultra-high frequency MMW applications.     

Realization of compatible stealth material for infrared,laser and radar based on one-dimensional doping-structure photonic crystals

To inhibit the radiant infrared energy between 8 and 14 μm, which is the infrared atmospheric window, and decrease the echo power of detecting laser and radar, to achieve compatible stealth, a doping structural one-dimensional photonic crystal (1-D PC) with Ge, ZnSe and Si was fabricated; and then combine it with radar absorbing material (RAM) to make a compound. After that, the reflection spectra of this compound was tested, and the result shows a high average reflectance (89.5%) in 8–14 μm waveband, and a reflective valley (39.8%) in the wavelength of 10.6 μm, which is the wavelength of CO₂ laser; and the reflectance in radar band shows that at high frequency, especially between 7.8 and 18 GHz, the radar power is strongly absorbed by this material and the reflected energy attenuate over 10 dB within the range from 11.1 GHz to 18.3 GHz, even 24.5 dB to the most in the frequency of 14.6 GHz.     

High speed ultra short pulse fiber ring laser using photonic crystal fiber nonlinear optical loop mirror

A scheme to generate high speed optical pulse train with ultra short pulse width is proposed and experimentally studied. Two-step compression is used in the scheme: 20 GHz and 40 GHz pulse trains generated from a rational harmonic actively mode-locked fiber ring laser is compressed to a full width at half-maximum (FWHM) of ~ 1.5 ps using adiabatic soliton compression with dispersion shifted fibers (DSF). The pulse trains then undergo a pedestal removal process by transmission through a cascaded two photonic crystal fiber (PCF)-nonlinear optical loop mirrors (NOLM) realized using a double-ring structure. The shortest output pulse width obtained was ~ 610 fs for 20 GHz pulse train and ~ 570 fs for 40 GHz pulse train. The signal to noise ratio of the RF spectrum of the output pulse train is larger than 30 dB. Theoretical simulation of the NOLM transmission is conducted using split-step Fourier method. The results show that two cascaded NOLMs can improve the compression result compared to that for a single NOLM transmission.     

High LO-to-RF isolation 94 GHz coplanar monolithic down-converter for FMCW radar sensor applications

This paper presents a 94 GHz monolithic down-converter with low conversion loss and high local oscillator (LO)-to-RF isolation using the 0.1 μm T-gate metamorphic high electron-mobility transistor (MHEMT) technology. The down-converter consists of a one-stage amplifier and a single-balanced mixer based on the high-directivity tandem coupler structure using the air-bridge crossovers, thereby amplifying the RF signals and maximizing the LO-to-RF isolation by using an inherent S₁₂ isolation characteristic of the amplifier and good phase balance of the tandem coupler. The fabricated one-stage amplifier using a 30 μm × 2 MHEMT shows a small signal gain of 7 dB at 94 GHz. The single-balanced mixer comprising two 20 μm × 2 MHEMT Schottky diodes and the tandem coupler with an additional λ/4-length line exhibits the conversion loss less than 7.8 dB and the LO-to-RF isolation higher than 30 dB in a RF frequency range of 91–96 GHz. Two circuits designed both for a 50 Ω impedance system are integrated into the down-converter of a 2.6 × 2.5 mm² chip size, and it shows a low conversion loss of ∼1 dB at 94 GHz and excellent LO-to-RF isolation above 40 dB in a frequency range of 90–100 GHz. This is the best isolation among the W-band monolithic down-converters reported to date.     

Investigation of enhancement mode HfO2 insulated N-polarity GaN/InN/GaN/In0.9Al0.1N heterostructure MISHEMT for high-frequency applications

In this paper, we examined normally-OFF N-polar InN-channel Metal insulated semiconductor high-electron mobility transistors (MISHEMTs) device with a relaxed In_0.9Al_0.1N buffer layer. In addition, the enhancement-mode operation of the N-polar structure was investigated. The effect of scaling in N-polar MISHEMT, such as the dielectric and the channel thickness, alter the electrical behavior of the device. We have achieved a maximum drain current of 1.17 A/mm, threshold voltage (V_T) =0.728 V, transconductance (g_m) of 2.9 S mm⁻¹, high I_ON/I_OFF current ratio of 3.23×10³, lowest ON-state resistance (R_ON) of 0.41 Ω mm and an intrinsic delay time (τ) of 1.456 Fs along with high-frequency performance with f_t/ f_max of 90 GHz/109 GHz and 180 GHz/260 GHz for T_CH =0.5 nm at V_ds =0.5 V and 1.0 V. The numerically simulated results of highly confined GaN/InN/GaN/In_0.9Al_0.1N heterostructure MISHEMT exhibits outstanding potential as one of the possibility to replace presently used N-polar MISHEMTs for delivering high power density and frequency at RF/power amplifier applications.     

Simultaneous demodulation and dispersion compensation of WDM DPSK channels using optical ring resonator

We have introduced and comprehensively analyzed a novel scheme of simultaneous demodulation and dispersion compensation of wavelength division multiplexed (WDM) non-return-to zero (NRZ) differential phase shift keying (DPSK) optical link using an optical ring resonator (ORR) based filter. Using extensive numerical simulation we have demonstrated the transmission of 10.7 Gb/s WDM DPSK channels having 50 GHz and 100 GHz spacing over 400 km of unrepeatered reach at 20 dB optical-signal-to-noise-ratio (OSNR) to achieve a bit error rate (BER) of 10^? 3.     

RF behavior of triple-frequency high power fusion gyrotron

The RF behavior of high power, triple frequency (170-, 127.5-, and 85 GHz) gyrotron for fusion application is presented in this paper. The operating mode selection is discussed in detail for each corresponding frequencies and TE_34,10, TE_25,8 and TE_17,5 modes are selected as the operating mode for 170 GHz, 127.5 GHz and 85 GHz operation of the device, respectively. The interaction cavity geometry and beam parameters are finalized by the cold cavity analysis and beam-wave interaction simulations. Considering the beam parameters and the beam launching positions in cavity (beam radius), the design of Magnetically Tunable MIG (MT-MIG) is also presented. Results of MT-MIG confirm the beam launching with desired beam parameters at the beam radius corresponding to the selected operating modes for all three frequencies. The CVD diamond window is also designed for RF power transmission. The beam-wave interaction simulations confirm more than 1 MW power at all three frequencies (170-, 127.5-, and 85 GHz).     

V-band directional tandem coupler of monolithic uniplanar structure

We present a uniplanar coplanar-waveguide 3-dB tandem coupler operating at V-band frequencies. The uniplanar structure is monolithically fabricated by using two-section parallel-coupled lines and air-bridge crossovers replacing the conventional multilayer or bonded structures. Due to an optimized tandem structure and non-bonded crossovers minimizing the parasitic components, a maximum coupling of 2.5 dB is measured at 62 GHz with a 2 dB bandwidth of 83%, while a high directivity factor of 33 dB is simultaneously obtained at 58–62 GHz. Over the entire design frequency range of 30–90 GHz, we achieve good phase unbalance of 90 ± 6.0°, as well as return loss and isolation lower than −23 and −16 dB, respectively.     

Coupled frequency-doubling optoelectronic oscillator based on polarization modulation and polarization multiplexing

A coupled frequency-doubling optoelectronic oscillator (OEO) is proposed and experimentally demonstrated, which is constructed based on the perfect combination of polarization modulation and polarization multiplexing. A fundamental microwave signal at 9.95 GHz or a frequency-doubled microwave signal at 19.9 GHz is generated with a wavelength-independent sidemode-suppression ratio (SMSR) as high as 78 dB obtained. The phase noise of the generated 19.9-GHz signal is ? 103.45 dBc/Hz at 10-kHz frequency offset, indicating a good short-term stability. The proposed scheme is simple and flexible, which can find applications in radars and wireless communications.     

Synthesis and microwave absorbing properties of FeNi alloy incorporated ordered mesoporous carbon–silica nanocomposite

Ordered mesoporous carbon–silica/FeNi nanocomposite were prepared by a sol–gel method and following sintering process. The electromagnetic parameters were measured in the 0.5–18 GHz range. Compared with ordered mesoporous carbon–silica composite, the permittivity of ordered mesoporous carbon–silica/FeNi nanocomposite decreases, while the permeability almost remains unchanged. The optimal reflection loss of ordered mesoporous carbon–silica/FeNi nanocomposite can reach ?45.6 dB at 11.1 GHz for a layer thickness of 3.0 mm. The enhanced microwave absorption of the mesoporous carbon–silica/FeNi nanocomposite is due to better balance between the complex permittivity and permeability, geometrical effect, as well as multiple reflections by the ordered mesoporous structure.     

Electromagnetic wave absorption properties of ε-Fe3N/Y2O3 nanocomposites derived from Y2Fe17 intermetallic compound

The electromagnetic wave absorption properties of ε-Fe₃N/Y₂O₃ nanocomposites were characterized in a frequency range of 0.05–20.05 GHz. The imaginary part of relative permeability μ_r″ exhibited “twin peak” dispersion and μ_r″ value retained high over a 0.5–10 GHz range. The real part (ε_r′) and imaginary part (ε_r″) of relative permittivity almost kept a low constant in a region of 0.5–10 GHz, respectively. As a result, the resin composites with 51 vol% ε-Fe₃N/Y₂O₃ powders exhibited excellent electromagnetic wave absorption properties (RL<−20 dB) in a frequency range of 0.6–4.4 GHz, with a thickness of 3.3–19.3 mm. A minimum reflection loss of −55 dB was observed at 1.8 GHz with an absorber thickness of 7.05 mm.     

Microfabrication of solenoid-type RF SMD chip inductors with an Al2O3 core

We investigated micron size, high-performance, and solenoid-type radio-frequency surface-mounted device (SMD) chip inductors with a low-loss Al₂O₃ core for a GHz drive microwave circuit application. Copper coils with a diameter of 27 μm were used and the chip inductors fabricated in this study are 0.86 × 0.46 × 0.45 mm³. The high-frequency characteristics of the inductance (L), quality factor (Q), and impedance (Z) of the developed inductors were measured using a RF impedance/material analyzer (HP4291B with HP16193A test fixture). The developed inductors have a self-resonant frequency of 3.7–5.2 GHz and exhibit L of 15–34 nH. The inductors have Q of 38–49 over the frequency ranges of 900 MHz–1.7 GHz. The calculated data obtained from the equivalent circuit and the derived equation of Q described the high-frequency data of L, Q, and Z of the inductors developed quite well.     

Sub-THz continuous wave generation scheme using high-order harmonics modulated lightwave

We propose a sub-THz continuous wave (CW) generation scheme using a high-order harmonics modulated lightwave (HML) to reduce an electronic dependency of a conventional double sideband suppressed carrier (DSB-SC) scheme. The electronic dependency should be overcome to increase frequency tunability of the conventional DSB-SC scheme. This is because the frequency of a local oscillator (LO), f_LO, should be one-half frequency of the frequency of a desired sub-THz CW in the conventional DSB-SC scheme. The proposed scheme is formed by adding an optical feedback loop to the conventional DSB-SC scheme. In order to verify our proposed scheme, a 120 GHz CW is generated using the LO with f_LO = 20 GHz. Based on our experimental results, we have found that the frequency of the LO can be reduced by our proposed scheme up to one-sixth (20 GHz) of 120 GHz. The 120 GHz CW generated by the proposed scheme has 52 dB higher photomixed output power with narrow spectral linewidth than that of the 120 GHz CW generated by the conventional DSB-SC scheme using the LO with f_LO = 20 GHz. Consequently, our proposed scheme can be helpful to reduce the electronic dependency of the conventional DSB-SC scheme.     

Low pitch LWIR QWIPs: Performance level and image quality

We present recent results obtained on 15 μm pitch LWIR QWIP arrays at Sofradir. Based on experimental data gathered on several QWIP wafers, the performance (NETD) at the system level has been estimated. We show that, in spite of the small pitch, values as low as 50 mK can be achieved for rather closed optical systems (f/2.5) and for operating temperatures (74 K) compatible with available compact cryo-coolers.We also demonstrate that specific pixel configurations can be designed to investigate the pixel-to-pixel optical crosstalk. Such measurements can help to better understand the limitations set by the geometry of the pixel on the Modulation Transfer Function (MTF). In particular, we show that the optical crosstalk due to photon transfer through the inter-pixel space is rather small for unthinned devices.