Tunable Local Oscillator Research Articles

Burst Mode Receiver for 112 Gb/s DP-QPSK with parallel DSP

A burst mode 112 Gb/s DP-QPSK digital coherent optical receiver with parallel DSP suitable for implementation in a CMOS ASIC with a 218.75 MHz clock speed is presented. The receiver performance is validated in a five channel 50 GHz grid WDM burst switching experiment using a commercially available wavelength tunable laser as the local oscillator. A new equalizer initialization scheme that overcomes the degenerate convergence problem and ensures rapid convergence is introduced. We show that the performance of the tunable local oscillator is commensurate with burst mode coherent reception when differential decoding in employed and that required parallel DSP implementation does not seriously impair the polarization and frequency tracking performance of a digital coherent receiver under burst mode operation. We report a burst acquisition time of less than 200 ns.

Monolithically integrated dual-quadrature receiver on InP with 30 nm tunable local oscillator

We demonstrate a monolithically integrated dual-quadrature coherent receiver with greater than 30 nm widely-tunable SG-DBR local oscillator, signal input SOAs, a 90° optical hybrid and four 10 GHz photodetectors. With 20 Gb/s NRZ-QPSK, we demonstrate a required OSNR of 10 dB for a BER of 10(-3) at four different wavelengths.

Sounding of Titan’s atmosphere at submillimeter wavelengths from an orbiting spacecraft

An investigation of the capabilities and science goals of a submillimeter-wave heterodyne sounder onboard a Titan orbiter is presented. Based on a model of Titan’s submillimeter spectrum, and including realistic instrumental performances, we show that passive limb observations of Titan’s submillimeter radiation would bring novel and unique information on the dynamical and chemical state of Titan’s atmosphere, particularly in the so far poorly probed 500–900 km region. The 300–360, 540–660 and 1080–1280 GHz spectral ranges appear especially promising, and could be explored with an instrument equipped with a tunable local oscillator system. Vertical temperature profiles can be determined up to ∼1200 km using rotational lines of CH 4, CO, and HCN. Winds can be measured over the 200–1200 km altitude range with an accuracy of 3–5 m/s from Doppler shift measurements of any strong optically thin line. Numerous molecular species are accessible, including H 2O, NH 3, CH 3C 2H, CH 2NH, and several nitriles (HC 3N, HC 5N, CH 3CN, and C 2H 3CN). Many of them are expected to be detectable in a large fraction of the atmosphere and in some cases at all levels, providing an observational link between stratospheric and thermospheric chemistry. Isotopic variants of some of these species can also be measured, providing new measurements of H, C, N, and O isotopic ratios. Mapping of the thermal, wind, and composition fields, best achieved from a polar orbit and with an articulated antenna, would provide a new view of the couplings between chemistry and dynamics over an extended altitude range of Titan’s atmosphere. Additional science goals at Saturn and Enceladus are briefly discussed.

Development of 1.5 THz waveguide NbTiN superconducting hot electron bolometermixers

We present a characterization of a 1.5 THz waveguide niobium titanium nitride (NbTiN)superconducting hot electron bolometer (HEB) mixer which can be pumped by acommercial solid state tunable local oscillator (LO) source. The NbTiN HEB mixer is madefrom a 12 nm thick NbTiN thin film deposited on a quartz substrate at room temperature.A gold electrode is formed in situ on the NbTiN thin film without breaking vacuum toensure good contact. The uncorrected DSB receiver noise temperature is measured to be1700 K at 1.5 THz, whereas the mixer noise temperature is derived to be 1000 K aftercorrections for losses of the input optics and the intermediate frequency (IF) amplifierchain. The required LO power absorbed in the HEB mixer is evaluated to be340 nW by using an isothermal technique. The IF gain bandwidth is supposed to beabout 1.3 GHz or higher. The present results show that good performance can beobtained at 1.5 THz even with a relatively thick NbTiN film (12 nm), as in the case of0.8 THz. In order to investigate the cooling mechanism of our HEB mixers, wehave conducted performance measurements for a few HEB mixers with differentmicrobridge sizes both at 1.5 and 0.8 THz. The noise performance of the NbTiNHEB mixers is found to depend on the length of the NbTiN microbridge. Theshorter the microbridge is, the lower the receiver noise temperature is. This mayimply a contribution of the diffusion cooling in addition to the phonon cooling.

High-resolution broadband (>100 cm^-1) infrared heterodyne spectro-radiometry using an external cavity quantum cascade laser

Broadband thermal infrared heterodyne spectro-radiometry using an external cavity quantum cascade laser as a tunable local oscillator has been performed over a frequency range of more than 100 cm(-1) at a central frequency of 1190 cm(-1). Heterodyne spectro-radiometry is demonstrated for two local oscillator tuning modes: broadband tuning for transmission and emission spectroscopy of broadband absorbers (Freon 12), and broadband frequency selection in combination with fine continuous frequency tuning for high-resolution (0.021 cm(-1)) transmission spectroscopy (N(2)O). In each case concentration retrievals are performed and analyzed. The spectroradiometer noise level is demonstrated to be twenty two and eight times the fundamental shot-noise limit in the two scanning modes respectively.

Low-Cost, Continuously Tunable, Millimeter-Wave Photonic LO Generation Using Optical Phase Modulation and DWDM Filters

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> We report on a low-cost technique for the photonic generation of wideband continuously tunable millimeter-wave local oscillator signals. It is based on sideband filtering using an optical phase modulator and thin-film dense wavelength- division-multiplexing<?Pub _nolinebreak=""?> filters. The generated millimeter-wave signal exhibits low phase noise, and its frequency covers the <formula formulatype="inline"><tex Notation="TeX">$W$</tex> </formula>- and <formula formulatype="inline"><tex Notation="TeX">$F$</tex> </formula>-bands, from 75 to 140 GHz. </para>

Full characterization and analysis of a terahertz heterodyne receiver based on a NbN hot electron bolometer

We present a complete experimental characterization of a quasioptical twin-slot antenna coupled small area (1.0×0.15μm2) NbN hot electron bolometer (HEB) mixer compatible with currently available solid state tunable local oscillator (LO) sources. The required LO power absorbed in the HEB is analyzed in detail and equals only 25nW. Due to the small HEB volume and wide antenna bandwidth, an unwanted direct detection effect is observed which decreases the apparent sensitivity. Correcting for this effect results in a receiver noise temperature of 700K at 1.46THz. The intermediate frequency (IF) gain bandwidth is 2.3GHz and the IF noise bandwidth is 4GHz. The single channel receiver stability is limited to 0.2–0.3s in a 50MHz bandwidth.

Physical Modeling and Frequency-Response Analysis of a High-Temperature Superconducting Terahertz Photomixer

A physical modeling and a rigorous theoretical analysis consisting of the two-temperature model and the generalized transmission-line model is used to evaluate a photo-induced terahertz continuous-wave voltage from a dc current-biased high-temperature superconducting (HTS) transmission line by optical heterodyne photomixing. The electrical and optical frequency-response analyses show the amplitude of this voltage increases with increasing beat frequency and decreasing optical frequency of laser beams. Its maximum frequency is found to be limited by the gap frequency of the HTS material, which is consistent with the available experiments reported in the literature. The developed model along with our numerical simulation reveal the ways to produce a coherent traveling-wave and high-power terahertz signal with proper choice of an HTS material, bias condition, geometrical configuration, and parameters of the transmission line and characteristics of two laser beams. This HTS photomixer can play an important role in terahertz transceivers as broadly tunable local oscillators with low-noise/low-power consumption characteristics.

Frequency shift filtering technique

A heterodyne continuous time active filter is described. Control over the operating has been achieved without external components, through a tunable local oscillator. It has been possible to build a single chip filter that may be tuned from very low frequencies up to 70 kHz in a single range.

An all solid-state superconducting heterodyne receiver at terahertz frequencies

A superconducting hot-electron bolometer mixer-receiver operating from 1 to 1.26 THz has been developed. This heterodyne receiver employs two solid-state local oscillators each consisting of a Gunn oscillator followed by two stages of varactor frequency multiplication. The measured receiver noise temperature is 1350 K at 1.035 THz and 2700 K at 1.26 THz. This receiver demonstrates that tunable solid-state local oscillators, supplying only a few micro-watts of output power, can be used in terahertz receiver applications.

ディジタル放送 ＯＦＤＭ用低位相雑音局部発振器の設計と搬送波のＳ／Ｎ特性検討

Orthogonal frequency division multiplexing (OFDM) is a suitable digital modulation method for terrestrial digital broadcasting systems. However, the phase noise in an OFDM receiver must be reduced, because the phase noise disturbs the orthogonality among the OFDM sub-carriers. This report describes the design of a wideband tunable local oscillator with low phase noise characteristics for OFDM receivers. We have developed a balanced oscillator with an L-C parallel resonance circuit between the transistor emitter ports, which has a wide band frequency range of 1.15-2.2 GHz and low phase noise performances of-66 dBc/Hz (1-kHz offset) or less. We also calculated the SNR at the output of the tuner of OFDM sub-carriers by using this balanced oscillator, and the results show that the SNRs of 33dB or more are achievable even for the 1-kHz sub-carrier spacing system.

Performance evaluation of coherent subcarrier multiplexed optical distribution systems with common local oscillators

Coherent subcarrier multiplexed local distribution systems using a common local oscillator (LO) in which the LO is provided by a distribution centre instead of locally generated at the subscriber premise are described. Both single and multiple optical carrier systems are considered. Because of the low path loss in a local optical network, a common LO system is possible with a LO shared by many subscribers, and indeed provides many advantages. First, a simplified receiver is achievable which eliminates a tunable LO, an automatic frequency control (AFC) circuit, a polarisation controller, and an optical coupler. Secondly, the system cost is decreased due to the reduction of components. Thirdly, laser phase noise and intermediate frequency (IF) drift are expected to be minimised. Fourthly, because no active optical components are placed at the subscriber premises, the system reliability is increased. Fifthly, the common LO system can be expanded with optical amplifiers without increasing the number of LO and simultaneous amplification of the message, and LO signals are achievable. By sharing high power, highly frequency stabilised, and very narrow linewidth LO lasers provided by the centre, it is expected that a simple and reliable coherent optical distributing network can be constructed by using common LO.

A coherent photonic wavelength-division switching system for broad-band networks

A coherent photonic wavelength-division (WD) switching system, utilizing a coherent wavelength switch ( lambda switch), is proposed. In the proposed coherent lambda switch, the tunable wavelength filter function is accomplished using coherent optical detection with a wavelength tunable local oscillator. The coherent photonic WD switching system has the following features; (1) low crosstalk switching for dense WDM signal, and (2) large line capacity capability. Design considerations show that 32 wavelength division channels can be available with a coherent lambda switch. It is also shown that a broadband metropolitan-area-network with over 1000 line capacity is possible, using a multistage connection in the coherent lambda switches. The switching function of the coherent lambda switch is demonstrated in a two-channel wavelength-synchronized switching experiment, using 8-GHz-spaced, 280-Mb/s optical FSK signals.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Star-coupler-based optical cross-connect switch experiments with tunable 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/sup -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 approximately=5170. Therefore, it is capable of covering over 1000 channels of 2.9 GHz each. >

Submillimeter backward wave oscillators

Summary form only given. Lewis Research Center is conducting a program to develop a series of backward-wave oscillators (BWOs) for use as voltage tunable local oscillators in spectrometers deployed above the Earth's atmosphere. The project goal is to produce an oscillator with an operating frequency of 2 THz. The BWO program has produced a number of successful design innovations. These include a microfabricated circuit on a diamond substrate, an optical output coupler, and a high-impedance slow wave structure. Experiments being conducted on the BWO are briefly described. >

A tunable CO2laser sideband spectrometer

We describe a high-resolution spectrometer for the 9-11 \mu m wavelength region, which is tunable over 23 GHz wide regions below and above each of some 80 CO 2 laser lines. An electrooptic modulator mixes the frequency-stabilized CO 2 laser output (frequency f L ) with tunable microwave signals (frequency f M ) to produce tunable, spectrally pure ( \pm1 \times 10^{-8} ) sidebands in the IR, whose frequencies f_{L} \pm f_{M} are readily determined with high precision ( \pm2 \times 10^{-7} ). The design of traveling wave and resonant modulators and associated tuners is given and discussed. The usefulness of the instrument for high resolution absorption spectroscopy and as a tunable local oscillator for heterodyne spectroscopy is demonstrated.

Doppler-independent selective heterodyne radiometry for detection of remote species

A multiple-frequency heterodyne radiometer is proposed for the detection of radiation from known species moving with unknown velocities. Expressions for the signal-to-noise ratio and the minimum detectable total power are obtained for sinusoidal signals and for Gaussian signals with both Gaussian and Lorentzian spectra. In distinction to the conventional system, knowledge of absolute line rest frequencies and a stable, tunable local oscillator are not required. The system may find use in the detection of certain remote species such as interstellar molecules and pollutants. A number of potential applications are examined.

Three-Frequency Nonlinear Heterodyne Detection 1: cw Radar and Analog Communications

Though heterodyne detection provides a valuable technique for detecting small signals, the conventional system has several inherent disadvantages. In applications such as communications and radar, obtaining a reasonably high signal-to-noise ratio (SNR) requires (1) a good knowledge of the velocity of the transmitter or target, (2) a stable yet tunable local oscillator, and (3) a target or source that presents a minimum of frequency broadening. These conditions are frequently not adhered to by real systems, particularly in the ir and optical, giving rise to detection capabilities far below optimum. Calculations are presented for the use and operation of a three-frequency nonlinear heterodyne system that eliminates many of the stringent conditions required for conventional heterodyne detection, while maintaining its near-ideal SNR. The technique, which is similar in principle to heterodyne radiometry, makes use of a two-frequency transmitter and a nonlinear second detector and is particularly useful for signal acquisition; for signals of unknown Doppler shift, in fact, performance is generally superior to that of the conventional system. Although primary emphasis is on the ir and optical because of the large Doppler shifts encountered there, application of the principle in the microwave and radiowave is also discussed. For cw radar and analog communications, the SNR, power spectral density (PSD), and minimum detectable power (MDP) are obtained and compared with the standard configuration. Both sinewave and Gaussian input signals are treated. A variety of specific cases are discussed including the optimum performance case, the typical radar case, and the AM and FM communications case. Evaluation of the technique for pulsed radar and digital communications applications (both in the absence and in the presence of the lognormal atmospheric channel) is reserved for Part 2 of this paper [Appl. Opt. 14, 680 (1975)].

Analysis of Laser Measurement of High-Altitude Aircraft Emissions

Accurate prediction of the environmental impact of stratospheric aircraft operation requires highly sensitive and specific measurement of effluents which have long residence times and possible catalytic interactions with the ambient atmosphere. Calculations have been performed to analyze three narrow bandwidth tunable laser measurement techniques for high altitudes where molecular absorption lines are narrow. Two ended absorption measurements using state-of-the-art tunable infrared diode lasers can provide sensitivities of ^ 0.02-0.2 ppm over 10 m for O3, CO, NO, H2O, and SO2 with minimal interference effects. Heterodyne radiometry of radiation from a hot near wake is capable of measuring initial wake concentrations, while heterodyne radiometry of absorption of solar radiation is sensitive to < 0.01 ppm of NO and CO in the far wake. These heterodyne measurements, with tunable diode laser local oscillators, will require development of milliwatt per mode diode lasers. Remote (1 km) resonance fluorescence measurements using dye lasers are capable of measuring to 0.15 ppm NO2. Sensitivities can be improved to close to ambient levels during wake penetration.

A direct conversion v.h.f. receiver

A direct conversion radio receiver is described which is characterized by many fewer spurious responses than the conventional superheterodyne, and lends itself to integration in monolithic form. The major design problem derives from the need for an accurate two-phase tunable local oscillator. A circuit is described which uses a servo-controlled phase shifting network, and maintains the two outputs within ±2° of quadrature from 66 to 95 MHz. Complete a.m. and s.s.b. receivers are described, and problems of a.g.c. discussed. The expected relative freedom from spurious responses was confirmed experimentally.