Square-law Detector Research Articles

Significant improvements in optical power budgets of real-time optical OFDM PON systems

Based on a comprehensive theoretical optical orthogonal frequency division multiplexing (OOFDM) system model rigorously verified by comparing numerical results with end-to-end real-time experimental measurements at 11.25Gb/s, detailed explorations are undertaken, for the first time, of the impacts of various physical factors on the OOFDM system performance over directly modulated DFB laser (DML)-based, intensity modulation and direct detection (IMDD), single-mode fibre (SMF) systems without in-line optical amplification and chromatic dispersion compensation. It is shown that the low extinction ratio (ER) of the DML modulated OOFDM signal is the predominant factor limiting the maximum achievable optical power budget, and the subcarrier intermixing effect associated with square-law photon detection in the receiver reduces the optical power budget by at least 1dB. Results also indicate that, immediately after the DML in the transmitter, the insertion of a 0.02nm bandwidth optical Gaussian bandpass filter with a 0.01nm wavelength offset with respect to the optical carrier wavelength can enhance the OOFDM signal ER by approximately 1.24dB, thus resulting in a 7dB optical power budget improvement at a total channel BER of 1 × 10(-3).

Effects of clock frequency stability on digital microwave radiometer performance

[1] A new generation of digital microwave radiometer back ends replaces conventional analog square-law detectors with a high-speed digital signal processing (DSP) stage to enhance their performance. The enhancements can include frequency subbanding for improved spectral resolution and mitigation of radio frequency interference, complex cross-correlation of orthogonally polarized signals for full Stokes polarimetry, and high-order moment (kurtosis) detection for improved radio frequency interference detectability. In each of these cases, the quality of the performance enhancement can be degraded by the nonideal realization of the desired DSP algorithm. In particular, instability in the master clock, which times the cadence of the DSP stage, can affect the performance. Analytical expressions are developed to model critical radiometer performance criteria as functions of the clock frequency stability. Experimental results are presented to validate the expressions. These results can be used to derive clock frequency stability requirements for future digital microwave radiometers.

Design and Characterization of $W$-Band SiGe RFICs for Passive Millimeter-Wave Imaging

This paper describes the design and measurement of a wideband (W-band) passive radiometer chip developed in a standard 0.12-?m SiGe BiCMOS technology (IBM8HP, ft /fmax = 200/265 GHz). Design equations, simulations, and measurements are presented for a 94-GHz square-law detector and wideband low noise amplifier, and an 80-110-GHz single-pole double-throw switch. A total-power radiometer is presented, which can achieve a temperature resolution of ? 0.69 K (30-ms integration time) with periodic calibration or chopping above 10 kHz. A switched Dicke radiometer chip is also presented, which addresses the 1/f noise of the total-power radiometer, and can achieve a temperature resolution of 0.83 K with a 30-ms integration time. This performance is comparable to current III-V imaging modules, and demonstrates, to our knowledge, the first implementation of a SiGe or CMOS W -band radiometer on a single chip.

Continuous-wave Terahertz Spectroscopy System Based on Photodiodes

Photodiodes have been commonly used for generation of continuous terahertz (THz) waves. In this paper, we proposed the use of photodiodes also for detection of THz waves in order to realize CW THz spectroscopy system based on 1.55-m flber-optics. We experimentally demonstrated two detection schemes based on the square-law detection and downconversion, and compared them with respect to sensitivity and dynamic range at 260{420GHz.

Envelope-Law and Geometric-Mean STAP Detection

Two detectors for space-time adaptive processing (STAP) are proposed here. These are variants that use envelope-law and geometric-mean (GM) (or logarithmic) processing, both being well-known concepts from conventional constant false alarm rate (CFAR) square-law radar detection. The variants are based on normalized adaptive matched filter (NAMF) STAP processing, and their CFAR property is established. Threshold setting for the detectors for specified false alarm probability (FAP) is accomplished using fast simulation based on importance sampling. Performance analyses of these detectors reveal almost indistinguishable loss in detection probability in homogeneous Gaussian interference compared with conventional square-law STAP detector versions. In addition, they exhibit robust detection performance in the presence of interfering targets in the training data for both nonfluctuating as well as fluctuating target models. Comparisons are made with the corresponding envelope-law and GM variants of the adaptive matched filter (AMF) detector previously proposed in a recent paper.

A measuring square-law diode detector

A general approach, circuit diagrams, and test results of a square-law detector with improved metrological characteristics are presented. The detector is based on a “diode + current-to-voltage converter” scheme. The measured characteristics are as follows: the dynamic range is 40 dB, and quadraticity factor is 2.00 ± 0.05. The working frequency band of a particular detector is from 0.001 to 2 GHz. The proposed approach is suitable for detectors of most kinds (series, parallel, single- and two-cycle, etc.), operating in the radio frequency band.

IF Amplifier Section in 90 nm CMOS Technology for SoC Microwave Radiometers

In this letter a novel design solution in 90 nm CMOS technology is proposed for the IF amplifier, low-pass filter and square-law power detector of a system-on-chip (SoC) microwave radiometer. To minimize the number of off-chip components (just a single capacitor) the IF filter is based on an active g <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> C configuration, whereas the power detector exploits the MOS transistor non-linearity, i.e., avoiding the need for Schottky diodes. The fabricated IF chip features a sensitivity of 3 mV/nW, a linearity range of about 25 dB around the -60 dBm level, a noise equivalent bandwidth of 58 MHz and a current consumption of only 1.8 mA at 1.2 V supply. The core area is within a rectangle of 220 mum times 540 mum. The proposed design solution is also compatible with SiGe BiCMOS processes and can be regarded as a further step toward the realization of microwave radiometric sensors fully integrated on silicon.

Terahertz Imaging Detectors in CMOS Technology

Square-law power detection circuits with on-chip antennas and amplifiers are presented for the detection of 0.65-THz radiation in a low-cost 0.25-μm CMOS technology. The circuit architecture combines metal-insulator-metal (MIM) coupling capacitors with NMOS transistors to facilitate self-mixing in a resistive mixer. A low-frequency (quasi-static) and a high-frequency (non-quasi-static) analysis of the broad-band circuit is presented. Current and voltage readout techniques of non-amplified detectors are compared, and exhibit a measured responsivity of 5.3 mA/W and 150 V/W respectively. A monolithic integrated 3×5 pixel focal-plane array has been used for single-pixel and multi-pixel imaging of concealed objects at 0.65 THz.

Detection Properties of Thin-Film Slot-Antenna-Coupled GaAs Schottky Barrier Diodes

GaAs Schottky barrier diodes (SBDs) coupled with a thin-film slot antenna were fabricated using a microfabrication technique, and video detection and mixing properties were investigated at 94 GHz. In the video detection, the bias voltage dependence of detected voltage showed almost the same variation with the calculated value derived from the theory of square-law detection. It was confirmed that detected voltage was obtained from the nonlinearity of the current–voltage (I–V) characteristic of the device. A Si extended hemispherical lens was coupled to the device to improve device responsivity. The increase in detected voltage was 22.2 dB upon using a lens with a radius of 10 mm, and a noise equivalent irradiance (NEI) of 1.2×10-10 W/(cm2·Hz1/2) was obtained. In the mixing, IF signals were observed up to a harmonic number of 7 using the thin-film GaAs SBD.

MIMO signaling over the MMF optical broadcast channel with square-law detection

This paper proposes an architecture for using multiple-input multiple-output techniques for a multimode fiber broadcast channel, allowing simultaneous transmission of unique streams to different users on the same fiber while using square-law detection. The resulting system throughput scales nearly linearly with the number of transmitters and receivers. The paper also proposes a training scheme appropriate for use with square-law detection.

Multilevel modulations and digital coherent detection

In this paper, we experimentally investigate the performance of several polarization-division-multiplexed (PDM) two- and three-level modulation formats employing digital coherent detection for high-speed and high spectral-efficiency optical transmission. 112 Gb/s PDM-NRZ-QPSK, 112 Gb/s PDM-RZ-QPSK and 114 Gb/s PDM-RZ-8PSK signals have been experimentally demonstrated by using the combination of Mach–Zehnder and phase modulators in a serial configuration with binary electrical drive signals, where the performance of both NRZ-QPSK and RZ-QPSK under tight filtering has been studied and compared. Furthermore, we generate the first optical 8-QAM signal by using a novel 8-QAM modulator, which consists of a parallel I-Q modulator followed by a phase modulator, all driven with binary electrical signals. Because single-ended photo detection is used in the coherent receiver, a novel DSP algorithm is proposed to mitigate the distortion caused by direct square-law detection of the signal component. By employing single-ended detector coherent detection, several spectrally-efficient long-haul DWDM transmissions experiments have been demonstrated without using Raman amplification or optical dispersion compensation. These includes 20 × 112 -Gb / s PDM-RZ-QPSK transmission over 1540-km of standard single mode fiber (SSMF) at a spectral efficiency (SE) of 2 bit/s/Hz, a record capacity of 17 Tb/s within the C-band optical bandwidth (4.025 THz) over 660 km of ultra-low-loss fiber using 161 × 114 -Gb / s PDM-RZ-8PSK formats at a SE of 4.2 bit/s/Hz, and hybrid 10 × 112 Gb / s PDM-RZ-QPSK and 10 × 44 Gb / s PDM-NRZ-QPSK transmission with 25-GHz channel spacing over 1600 km of SSMF at a SE of 2.8 bit/s/Hz.

Differential preamble detection in packet-based wireless networks

A novel hypothesis-based preamble detection method for uncoordinated, high-density packet-based communication over an additive white Gaussian noise channel is proposed and analyzed. Received samples are observed over a window of length equal to that of the preamble and a metric is computed for each sample shift of the window. A metric exceeding a noise dependent pre-computed threshold flags the presence of a preamble. The preamble sequence consists of concatenated sections of spreading sequences whose length is at most the coherence time of the channel. These sections are then differentially combined. A differential correlation-based detection is employed to locate the boundaries of the preamble. A theoretical framework is developed to provide exact analytical solutions for missing and falsely detecting a preamble using matrix analysis of quadratic Gaussian statistics. Furthermore, the robustness of the proposed methodology in a two path channel is studied. The effects of frequency and timing offsets on the system performance is evaluated. Simulation results are presented to validate the analytical expressions. Additionally, a performance comparison of the proposed differential detection scheme with that of a noncoherent square-law detector is presented.

High-Spectral-Efficiency 114-Gb/s Transmission Using PolMux-RZ-8PSK Modulation Format and Single-Ended Digital Coherent Detection Technique

Employing spectrally efficient PolMux-RZ-8PSK modulation format and single-ended digital coherent detection technique, we demonstrate 8 times 114-Gb/s DWDM transmission over 640 km of SSMF on a 25-GHz grid with record spectral efficiency of 4.2 bit/s/Hz, utilizing no optical dispersion compensation or Raman amplification. We show that the distortion caused by the direct square-law detection of the signal component with single-ended photo detection can be effectively mitigated by a novel DSP algorithm.

Sb-Heterostructure Millimeter-Wave Detectors With Reduced Capacitance and Noise Equivalent Power

InAs/AlSb/GaSb backward diodes are used for millimeter-wave square-law power detection. A new heterostructure design with low capacitance, low resistance, and high curvature coefficient as compared to previous designs is presented. Voltage sensitivity, which is directly proportional to curvature coefficient, is improved by 31% as compared to prior reports of devices with similar barrier thicknesses. The junction capacitance is also reduced by 24% to 13 . The improved sensitivity and decreased junction capacitance originate from the incorporation of a p-type-doping plane with sheet concentration of in the n-InAs cathode layer. The combination of low resistance (and thus Johnson noise) and high sensitivity results in an estimated noise equivalent power of 0.24 at 94 GHz for a conjugately matched source, whereas the reduced capacitance facilitates wideband matching and increases the detector cutoff frequency. These modified Sb-based detectors have promise for improving the performance of passive millimeter-wave and submillimeter-wave imaging systems.

A numerical method for array sensor noise field calculation in detection performance optimization

Different from the generally adopted criteria of minimizing the sidelobe level and maximizing the array gain in the weighting design of a sonar array, an approach was proposed recently to optimize the shading weights with the aim of maximizing the deflection coefficient in the square-law detector, which in essence suppresses the self-noise by including the noise information at array sensors in the optimization procedure. When several noise sources are present and/or an analytical expression of the noise transfer function is not available, the sensor noise needs to be measured in forming the sensor noise response cross correlation matrix required in the optimization which is very demanding when an array with large number of sensors is considered. To avoid the tedious work of noise measurement, a numerical method is developed in this paper. In this method, the main self-noise sources are assigned with different positions and strengths and the noise field at the array sensors is calculated via either the finite element method or the boundary element method. Tank experiment validated the effectiveness of this method. By using this method, the detection performance of a sensor array can be predicted before it is physically built and tested in under practical conditions.

The IR antenna pair coupled sensor element and its potential application in wavefront sensing

A single IR antenna-coupled square-law sensor does not preserve the phase of the radiation field. We considered an IR radiation detector element in which the square-law sensor is coupled with a pair of IR dipole antennas, the simplest coherently interconnected IR antenna array with the potential of preserving phase information. The circuit equivalent model is employed for analyzing the response features of an element to a tilted incident plane wave. The analytical results show that the response of such an element contains the phase difference information of incident radiation field at the positions of the two antennas and can be characterized by its induced phase shift, Δ , and the “visibility” of its oscillation profile, κ . In addition, the numerical computations for the case of shifted square-law detectors shows that Δ and κ are strongly affected by, and therefore can be controlled by the electric and geometrical parameters of the antenna element system. Furthermore, a method of using the response measurement of such elements to retrieve the wavefront phase information is also provided.

Analogue kurtosis detector for radio-frequency interference in microwave radiometers

An analogue system has been developed for radio frequency interference (RFI) detection in microwave radiometers. The detector measures the higher order moments of the signal and the kurtosis measurement is used to determine the presence of RFL The hardware comprises a square-law detector and a high-speed analogue multiplier. Because the circuit uses only analogue components at radio and/or intermediate frequencies, it can easily augment conventional radiometer architectures used in both airborne and spaceborne instruments.

Microwave Photonic Signal Detection Using Phase-Matched Optical Rectification in an AlGaAs Waveguide

We demonstrate a novel approach for detecting microwave-modulated optical signals through velocity-matched optical rectification in second-order optical nonlinear materials. Optical intensity modulation is down-converted to microwave frequency (5-20 GHz) using a traveling-wave GaAs electrooptic modulator as an optical square-law detector. Phase matching between microwave and optical waves is achieved through a velocity-matched slow-wave asymmetric coplanar-strip electrode structure.

Channel Optimized Predistortion in Self-Heterodyne Direct Conversion Architecture for OFDM Systems

This paper presents a novel technique to compensate intermodulation distortion of a self-heterodyne direct conversion OFDM system in multipath propagation environments. A self-heterodyne direct conversion system has an advantage that simple receivers can be built that are completely immune to any phase noise or frequency offset. This system, however, has a disadvantage that the nonlinear square-law detector at the receiver of the self-heterodyne direct conversion system gives rise to second order intermodulation distortion. In this study, channel estimation is performed using a training sequence and then the predistortion coefficients with regard to estimated channel parameters are derived to compensate the receiver nonlinearity. Transmit power distribution is employed to overcome multipath fading channels as well. Computer simulation demonstrates that the proposed approach improves the BER performance of the self-heterodyne direct conversion OFDM system in a multipath fading channel. This scheme gives advantage to multi-carrier systems that are much more sensitive to frequency and phase error than single-carrier systems.

Bit-Error Probability for Direct Detection of Optical RZ Signal Degraded by ASE Noise and Timing Jitter

We analyze theoretically the interplay between optical return-to-zero signal degradation due to timing jitter and additive amplified-spontaneous-emission noise. The impact of these two factors on the performance of a square-law direct detection receiver is also investigated. We derive an analytical expression for the bit-error probability and quantitatively determine the conditions when the contributions of the effects of timing jitter and additive noise to the bit error rate can be treated separately. The analysis of patterning effects is also presented