A coherent mechanical submillimeter frequency shifter

Highly correlated transmit and local oscillator signals for use in a narrowband heterodyne system are derived from a single submillimeter source. One component is Doppler shifted in frequency by a mechanically rotating device before transmission. The frequency translation does not lead to significant spectral deterioration. Under the condition of only a small differential delay between transmit and local oscillator paths, most phase and frequency instabilities of the submillimeter source are eliminated at the intermediate frequency. The paper describes the frequency shifter's mode of operation and a practical 637GHz design. Based on this design we simulate the effect of various imperfections in the frequency translation process on the output spectrum. Preliminary measurements at 140GHz are included.     

A compact 220 GHz heterodyne receiver module with planar Schottky diodes

This letter presents the development of a compact 220 GHz heterodyne receiver module for radars application in which a novel low pass wide stop band intermediate frequency (IF) filter is integrated. The planar Schottky anti-parallel mixing diode based subharmonic mixer (SHM) is used as the receiver’s first stage. The diode is flip-chip mounted on a 50 μm thick quartz substrate. The accurate modeling of the self and mutual inductance of the diode’s air-bridges are discussed. The measured conversion loss (CL) of the SHM has a minimum value of 6.2 dB at 210.5 GHz, and is lower than 8.4 dB in the frequency range 209.4–219.6 GHz with a 10 mW input power from a local oscillator (LO). The LO chain consists of a 110 GHz passive tripler, two Ka-band amplifiers and a Ka-band active tripler. The tested minimum double side band (DSB) noise temperature of the integrated 220 GHz heterodyne receiver is 725 K at 205.2 GHz and lower than 1550 K in the frequency range 199–226 GHz.     

Star-coupler-based optical cross-connect switch experiments withtunable receivers

An optical cross-connect switch using the star-coupler-based frequency-division-multiplex technique are discussed. Two specific tunable receivers have been implanted. The first is a heterodyne receiver with a tunable laser as the local oscillator (LO) and the second is a tunable filter followed by a direct-detection receiver. In the heterodyne receiver, the tunable LO was a monolithic three-section multiple-quantum-well distributed Bragg laser capable of a 1000-GHz tuning range. Receiver sensitivity was measured to be -38 dBm at 1 Gb/s (BER=10^-10). The power margin in the system substantiated feasibility for a 400×400 switch. In the tunable-filter receiver, the tunable filter is a tunable two-stage optical fiber Fabry-Perot filter design consisting of a narrowband filter followed by a wideband filter. The tuning of the filters is computer controlled, and the combined filter has a tuning range of 15000 GHz with a finesse of ≈5170. Therefore, it is capable of covering over 1000 channels of 2.9 GHz each     

GaAs optoelectronic mixer operation at 4.5 GHz

Optoelectronic heterodyne detection of high-frequency intensity modulation signals using a GaAs photoconductive mixer is reported. Flat response for optical modulation frequencies up to 4.5 GHz is observed for downconversion to a 500-MHz intermediate frequency using a low power local oscillator. Heterodyne responsivity was enhanced significantly over direct detection with the same photoconductive detector due to improved contact performance under RF bias.     

The optical response of epitaxial lift-off HEMT's to 140 GHz

We present measurements on the optical frequency response of epitaxial lift-off (ELO) 1.0-μm InP high-electron mobility transistors (HEMTs) to 140 GHz using electrooptic sampling and heterodyne techniques. Our picosecond sampling measurements established that the lift-off devices exhibited substantial optical response to 140 GHz. Heterodyne measurements made at 60 and 94 GHz later confirmed these findings. A novel three wave mixing technique was used to extend the heterodyne bandwidth to 130 GHz. In these experiments, millimeter waves were generated in our optically driven HEMT's and launched into waveguides. These lift off devices can be major additions to future millimeter wave integrated optoelectronic systems either as high frequency optical detectors or as optically driven tunable millimeter wave sources     

Evaluation of a simplified Doppler frequency shifter

Mechanical Doppler frequency shifting of a millimeter or submillimeter wave signal provides a means to implement a sensitive and highly coherent heterodyne receiver in a laboratory system. The rotary shifter, which we have previously described, is a precision-machined device suitable for use down to submillimeter wavelengths. We found at 140GHz, however, that the coherence of such a system is not affected by mechanical imprecision of the frequency shifter, and that the system dynamic range is only moderately affected by mechanical imprecision and by certain design simplifications. We have therefore built several versions of simpler and less precise Doppler frequency shifters and report here on their performance at 140GHz.     

System-on-Packaging with Electroabsorption Modulator for a 60-GHz Band Radio-Over-Fiber Link

A system-on-packaging (SoP) with an electroabsorption modulator (EAM) for a 60 GHz band radio-over-fiber (RoF) link is described. The system consists of an EAM device, a microstrip filter, and a low noise amplifier (LNA). The microstrip filter was used to achieve impedance matching between the EAM device and the LNA and to reject the local oscillator (LO) frequency of the heterodyne system. The frequency response and the effect of the EAM bias voltage were measured for a simple RF/optical link. A 60 GHz band RoF link with 2.5 GHz intermediate frequency (IF) was prepared to measure the transmission characteristics of the 16 QAM data.     

Minimally cooled heterojunction laser heterodyne detectors in metalorganic vapor phase epitaxially grown Hg1-xCdxTe

By taking advantage of Auger suppression techniques, the leakage currents of room temperature infrared detectors operating in the LWIR band can be greatly reduced. At present, these detectors suffer from large 1/f noise and hence the improvement in the detectivity resulting from the reduced leakage currents can only be realized at high frequencies. However, this is not a problem for heterodyne systems which employ intermediate frequencies above 40 MHz. A thermo-electrically cooled Auger suppressed infrared detector operated at 260K has been studied as a heterodyne detector. The device was operated with the application of sufficient local oscillator power to double its dark current (about 0.3 mW) and a NEP of 2 x 10^-19 W Hz⁻¹ was deduced from heterodyne measurements. The frequency response is presently limited by a combination of the detector capacitance and the series resistance to about 70 MHz and ways to reduce this series resistance are considered.     

8.4--A 3.39-micron infrared optical heterodyne communication system

An optical heterodyne communication system is described which employs a separate stable laser local oscillator at the receiver. The theoretical advantage of quantum-limited reception has been realized, demonstrating an improvement in receiver sensitivity of more than 40 dB over that of a conventional photodetector receiver. The fundamental sources of noise in the system are identified as laser oscillator frequency noise, atmospheric phase noise, atmospheric amplitude noise, and quantum noise. The quantitative characteristics of these noise sources are analyzed as they influence the operation of AM and FM laser communications.     

Production of high quality Nb/Al-oxide/Nb SIS trilayers for submillimeter wave mixers

Owing to a very sharp nonlinearity in the quasiparticle currentvoltage characteristic, which fortuitously occurs on the scale of a few millivolts rather than a few volts as with semiconductor devices, superconductor/insulator/superconductor (SIS) tunnel junctions are the most sensitive detectors for heterodyne mixing at millimeter and submillimeter wavelengths. They can also provide sources of coherent local oscillator power at very high frequencies; more broadly, they have a number of interesting applications as fast, low-power logic elements and as detectors at optical wavelengths. For submillimeterwave mixers, in many ways the most demanding of these applications, the Nb/Al-oxide/Nb material system has emerged as the system of choice to frequencies of ～ 700 GHz and beyond. Production of SIS devices requires careful attention to a number of critical microfabrication issues, and I describe here some of the insights gained from developing a process for high-quality niobium trilayers that successfully yielded small-area junctions with unusually low sub-gap leakage current.     

Optical FSK heterodyne detection experiments using semiconductor laser transmitter and local oscillator

Frequency modulated semiconductor laser signals are demodulated by optical heterodyne detection using an independently temperature stabilized semiconductor laser local oscillator and a square-law detector followed by electrical frequency discrimination circuitry. Short-term and long-term beat frequency stability of the free-running laser transmitter and the local oscillator are delineated. Direct frequency modulation characteristics of the laser transmitter are studied by observing the intermediate frequency spectra. Frequency modulation-demodulation of pulse pattern signals at 100-200 Mbit/s is demonstrated. Reduction of the intermediate frequency fluctuation to less than 5 MHz is realized by employing a feedback of the RF frequency discriminator output to the local oscillator injection current.     

Submillimeter-wave heterodyne spectroscopy with a compact solid state radiometer

A compact, solid state submillimeter-wave heterodyne radiometer has been developed and was used to measure spectral characteristics of a water vapor jet in a space simulation chamber. Features of the 557 GHz water vapor line profile were observed in significantly greater detail than in previous experiments through an increased sensitivity and improved frequency resolution (600 kHz). The local oscillator of the radiometer consisted of a frequency multiplication chain which was driven by an InP Gunn oscillator at 92.6 GHz, and which contained a frequency tripler and harmonic mixer in cascade. The front end of the receiver had a noise temperature of 4500 K (DSB) at 555 GHz, consumed 3 W and weighed 3 kg. This advance in technology is particularly relevant to submillimeter-wave radiometry from a space-based platform.     

High Current Responsivity and Wide Modulation Bandwidth Terahertz Detector Using High-Electron-Mobility Transistor for Wireless Communication

A high-current-responsivity terahertz (THz) detector was fabricated using a broadband bow-tie antenna and an InAlAs/InGaAs high-electron-mobility transistor (HEMT) with a short gate length. High-current responsivity can be achieved by using a short gate length; the resulting high transconductance exhibited ballistic transport in the channel. We fabricated the HEMT detector with a 50-nm-long channel; the transconductance was 1.2 S/mm and the subthreshold slope was 120 mV/dec, yielding a high-current responsivity (～5 A/W) and a cutoff frequency of 460 GHz. We also measured the modulation bandwidth of the THz detector using a heterodyne mixing technique with a uni-traveling carrier photodiode (UTC-PD) for providing the radio frequency (RF) and a frequency multiplier as a local oscillator. The intensity of the intermediate signal (IF) was measured by changing the frequency of the UTC-PD; very high bandwidths of up to 26 GHz were obtained. The experimental results agree well with electromagnetic simulations, which indicate that the bandwidth is determined by the external circuit. The conversion gain from RF to IF was ?2 dB in the heterodyne mixing by using the HEMT detector.     

Polarisation-diversity receiver for coherent FSK communications

A polarisation-insensitive optical heterodyne receiver, in which two signals with orthogonal polarisations are independently detected and demodulated, and subsequently combined, has been demonstrated for FSK-modulated signals at 50 Mbit/s with a sensitivity of ?55.5 dBm (Pe=10?9). The receiver uses the entire received signal and local oscillator signal with only one photodiode in each branch. Differences between the two PINFET detectors result in a ±ldBm sensitivity variation for all polarisation states of the received signal.     

Noise and mixing properties of high-Tc Josephson junctions at W-band frequencies

We report on the noise and Josephson mixing properties of high-T_c superconductor (HTS) Josephson junctions. Direct radiation measurements and heterodyne mixing experiments in the frequency range 45–141 GHz have been performed by using YBa₂Cu₃O_7−x (YBCO) step-edge junctions (SEJ) on LaAlO₃ and MgO and bicrystal junctions (BCJ) on MgO substrates. Junctions with current voltage characteristics (CVC) close to predictions of the resistivity shunted junction (RSJ) model were mounted into a high sensitive radiometer system. From linewidth measurements we calculated an effective noise temperature of our junctions. In heterodyne mixing experiments we obtained conversion efficiencies around −14 dB in the 11 GHz intermediate frequency (IF) band under the radiation of two monochromatic signals. In the fundamental mixing regime we observed response at IF at working temperatures up to 72 K. The measured receiver and mixer noise temperature of the Josephson mixer at 94 GHz local oscillator (LO) frequency, an IF of 1.4 GHz and at a working temperature of 10 K was 4700 and 3400 K, respectively.     

Field effect transistor as heterodyne terahertz detector

A theory of nonlinear response of the channel of a field effect transistor subjected to two terahertz beams (measured signal and local oscillator) with the close frequencies has been developed. It is shown that electric current flowing in the transistor channel drastically increases heterodyne efficiency. Also, it is demonstrated that such a heterodyne detector is capable of operating effectively with very high intermediate frequencies up to 10 ÷ 100 GHz.     

A low noise heterodyne receiver for astronomical observations operating around 0.63 mm wavelength

A heterodyne receiver is described in which an InSb hot electron bolometer is used as a mixer. The local oscillator power is obtained by doubling the frequency of a backward wave oscillator. The receiver operates between 460 and 500 GHz (0.65–0.6 mm). Noise temperatures amount typically to 1000 K.     

Exploration of Terahertz Imaging with Silicon MOSFETs

We summarize three lines of development and investigation of foundry-processed patch-antenna-coupled Si MOSFETs as detectors of THz radiation: (i) Exploiting the pinciple of plasma-waved-based mixing in the two-dimensional electron gas of the transistors’ channels, we demonstrate efficient detection at frequencies as high as 9 THz, much above the transit-time-limited cut-off frequencies of the devices (tens of GHz). Real-time imaging at 600 GHz with a 12 × 12 detector array is explored. (ii) Given the limited THz power usually available for applications, we explore imaging with enhanced sensitivity in heterodyne mode. We show that real-time operation of a 100 × 100-pixel heterodyne camera should be possible at 600 GHz with a better dynamic range (30 dB) than for direct power detection (20 dB), even if only a quarter-milliwatt of local-oscillator power, distributed radiatively over all detector pixels, is available. (iii) Finally, we present an all-electronic raster-scan imaging system for 220 GHz entirely based on CMOS devices, combining the CMOS detectors with an emitter circuit implemented in a 90-nm CMOS process and delivering radiation with a power on the 100- μW scale. Considering progress in the field, we anticipate that the emitter concept of oscillator-based power generation with on-chip frequency multiplication will carry well into the sub-millimeter-wave regime.     

Analysis of SIS heterojunctions for use as mm-wave mixers exhibiting coversion gain

We apply the quantum formulation of heterodyne mixer theory to SIS heterojunctions (junctions between dissimilar superconductors). Conversion gain is predicted over a wide range of mm-wave frequencies in the 3-port Y-mixer model by exploiting the naturally occurring region of negative conductance in the DC I-V characteristic. In the signal frequency range 50–250 GHz this region persists in the pumpedjunction I–V characteristic for local oscillator power <1 nW and leads to a negative conductance at the mixer's IF port.     

Modulation characteristic of waveguide-type optical frequency combgenerator

We report the modulation characteristics of a waveguide-type optical frequency comb generator (WG-OFCG) with the advantages of compactness, high modulation index and low driving power. The characteristics of the optical frequency comb (OFC) generated when the modulation index exceeds 2π are discussed. The power of the modulation sidebands was measured by the optical heterodyne method with an external-cavity laser as a local oscillator. The dependence of the modulation index of the WG-OFCG on the modulation frequency up to 40 GHz was measured. The generation span of the OFC at the modulation frequency detuned from an integer multiple of the free spectral range (FSR) is discussed